REVIEW www.advancedscience.com Metal–Organic Polyhedra as Building Blocks for Porous Extended Networks Akim Khobotov-Bakishev, Laura Hernández-López, Cornelia von Baeckmann, Jorge Albalad, Arnau Carné-Sánchez,* and Daniel Maspoch* versatility,[4–9] which makes them attractive Metal–organic polyhedra (MOPs) are a subclass of coordination cages that candidates for applications such as gas can adsorb and host species in solution and are permanently porous in sorption/separation,[10–14] heterogeneous solid-state. These characteristics, together with the recent development of catalysis[15–18] and sensing.[19,20] The chem- their orthogonal surface chemistry and the assembly of more stable cages, istry of porous materials has matured significantly over the past decade, thanks have awakened the latent potential of MOPs to be used as building blocks for to the introduction of supramolecular the synthesis of extended porous networks. This review article focuses on chemistry as a synthetic tool for intro- exploring the key developments that make the extension of MOPs possible, ducing pre-organized subunits within highlighting the most remarkable examples of MOP-based soft materials and extended materials.[21–23] Thus, researchers crystalline extended frameworks. Finally, the article ventures to offer future have developed an extensive library of synthetic (and post-synthetic) strategies perspectives on the exploitation of MOPs in fields that still remain ripe toward that, unlike standard one-step syntheses, the use of such unorthodox molecular porous platforms. aim to control the assembly of elaborated architectures with well-defined chemistry in a stepwise fashion.[24–26] Although these multistep approaches are generally tedious, 1. Introduction they generate materials with unparalleled molecular precision at one or more porous domains, owing to the greater levels of pre- The development of new materials with benchmark physical organization during self-assembly than those seen in one-step properties is imperative for the chemical industry. To this end, syntheses. researchers must gain fundamental insights into self-assembly The judicious assembly of pre-synthesized molecular build- and create predictive design principles for structure-function re- ing blocks into porous architectures has enabled an unprece- lationships. Advances in both areas have recently been reported dented degree of control over the structure and composition for porous materials, for which establishing the relationship of porous materials, as exemplified by the success of reticular between physical properties and pore chemistry is crucial.[1–3] chemistry to engineer metal–organic frameworks (MOFs) with Porous materials have garnered attention from both industry unprecedented features such as hierarchical porosity.[27,28] Hi- and academia due to their outstanding structural diversity and erarchically porous materials possess a complex structure with multiple micro-, meso- and macroporous domains that exhibit highly distinct chemistries and functionalization.[29,30] These ma- A. Khobotov-Bakishev, L. Hernández-López, C. von Baeckmann, J. Albalad, A. Carné-Sánchez, D. Maspoch terials have attracted interest for their unique performance dur- Catalan Institute of Nanoscience and Nanotechnology (ICN2) ing controlled transport of substrates throughout their frame- CSIC and The Barcelona Institute of Science and Technology works, which is heavily influenced by the degree of order in Campus UAB, Bellaterra, Barcelona 08193, Spain their 3D structures.[31,32] Among the most promising—albeit E-mail: arnau.carne@icn2.cat; daniel.maspoch@icn2.cat underexplored—routes to generate multiple well-defined porous J. Albalad Centre for Advanced Nanomaterials and Department of Chemistry domains is the stepwise assembly of intrinsically porous build- The University of Adelaide ing blocks (i.e., the spatial polymerization of preformed porous North Terrace, Adelaide, South Australia 5000, Australia cages).[33] This methodology arguably offers the highest level D. Maspoch of hierarchical control, in which the pore’s inner chemistries Catalan Institution for Research and Advanced Studies (ICREA) (windows, diameter, polarity) and outer chemistries (function- Pg. Lluís Companys 23, Barcelona 08010, Spain alization, directionality) are strategically pre-established to con- The ORCID identification number(s) for the author(s) of this article fer the resultant material with at least two well-defined chemi- can be found under https://doi.org/10.1002/advs.202104753 cal domains.[34] Furthermore, the better defined the reactivity of © 2022 The Authors. Advanced Science published by Wiley-VCH GmbH. these porous building blocks, the greater the precision at which This is an open access article under the terms of the Creative Commons they can be interconnected for specific applications without dis- Attribution License, which permits use, distribution and reproduction in turbing their core functionalities.[35] any medium, provided the original work is properly cited. Metal–organic polyhedra (MOPs) have become one of the most DOI: 10.1002/advs.202104753 attractive classes of building blocks for the growth of hierarchi- Adv. Sci. 2022, 9, 2104753 2104753 (1 of 19) © 2022 The Authors. Advanced Science published by Wiley-VCH GmbH www.advancedsciencenews.com www.advancedscience.com cally controlled, metal–organic, porous materials.[36,37] MOPs are solid-state.[48,49] This characteristic stems from the strong metal– a subclass of coordination cages, which present the distinctive ligand (typically, metal–carboxylate) coordination bonds that sus- feature of being permanently porous in solid state.[38] The molec- tain their structure. Therefore, MOPs can be classified as retic- ular nature of MOPs and their hybrid metal–organic surface en- ular materials, owing to the presence of strong and directional dow these materials with a rich orthogonal surface reactivity that bonds that lead to permanently porous materials. Accordingly, is accessible both in solution and in the solid-state. Furthermore, one can subject MOPs to reticular chemistry, such as the sec- the number and location of the different functional groups on ondary building unit (SBU) approach, to generate discrete poly- the surface of the MOP can be precisely known. This enables hedral architectures, as exemplified by the assembly of M(II) pad- very fine control over their processability, stability, and chemical dlewheel clusters with bent dicarboxylate ligands. This paddle- reactivity while maintaining their porosity.[39] The unique com- wheel can be conceptualized as a 4-connected (4-c) square planar bination of intrinsic porosity with a well-defined surface reactiv- unit. Accordingly, the assembly of M(II) paddlewheels with bent ity is what makes MOPs ideal candidates to be used as porous ligands can be anticipated by considering the angle between the monomeric units for the assembly of extended porous networks. two carboxylate groups. The most representative MOPs obtained The use of MOPs as building blocks for porous networks with these building blocks are the lantern-type (general formula presents several advantages such as the possibility to confer ex- M2 L4 ; 0 ° angle between carboxylic groups), the octahedral-type tended materials with permanent porosity regardless of the fi- (general formula M12 L12 ; 90 ° angle between carboxylic groups), nal degree of order of these materials, since both the function- and the cuboctahedral-type (general formula M24 L24 ; 120 ° an- alization and integrity of the cage—particularly for those with gle between carboxylic groups).[37] This approach has been pro- strong metal–ligand coordination bonds—are thoroughly main- gressively expanded to other metallic clusters of higher nuclear- tained during the polymerization step.[40] This in turn paves the ity, including iron- or zirconium-capped trimers, and calixarene- way for the controlled embedding of porosity into soft matter capped tetranuclear clusters.[46,50,51] The defined position of each such as gels or self-assembled monolayers. Alternatively, the pos- building block in a final MOP structure enables the use of reticu- sibility to precisely locate reactive sites on the polyhedral surface lar chemistry to functionalize, with atomic precision, the MOP’s of MOPs with accurate knowledge of their number, location, and internal cavity and external surface. For example, the 5-position of orientation enables the use of MOPs as pre-synthesized super- the benzene ring of the 1,3-benzenedicarboxylic acid (BDC) has molecular building blocks that can be assembled into highly con- been used to functionalize the surface of cuboctahedral MOPs nected crystalline nets via reticular chemistry.[41] This approach with up to 24 functional groups of diverse nature (Figure 1, was recently employed to target such nets, which could not be orange).[52] In the case of reactive moieties, the scope of the MOP readily accessed through conventional syntheses.[42] surface functionality can be further expanded through covalent Theoretically, the hierarchical assembly of MOPs should post-synthetic modification reactions. Such control over the sur- offer tremendous advantages over more conventional one- face chemistry of MOPs enables tuning of their physicochemi- pot syntheses in reticular crystalline frameworks as well as cal properties such as solubility, reactivity, and chemical stability, in soft/amorphous materials. However, the use of MOPs making MOPs a modular porous unit with unique features for as monomers in subsequent self-assembly/polymerization re- hierarchical self-assembly. actions has been long hindered by three deficiencies: lack of solubility in desired solvents, poor stability, and limited 3. MOPs as Porous Building Blocks for reactivity.[43] Fortunately, all three issues are being addressed by Hierarchical Self-Assembly researchers, who have engineered a new generation of MOPs that combine high structural stability with surfaces amenable The use of MOPs as molecular porous building blocks for to functionalization.[44–47] Moreover, the synthetic strategies to bottom-up assembly of extended materials rests on three pillars: assemble such MOPs have been flourishing: indeed, MOPs as- stoichiometric reactivity, stability, and solubility. Each of these sembled through coordination, supramolecular, and/or covalent three requirements ensures that the final extended material is ob- interactions have recently been reported. These MOPs are be- tained through the linkage of pre-designed MOPs. Moreover, the ing harnessed to synthesize an exciting variety of porous mate- hierarchical assembly of soluble MOPs with well-defined reactive rials that are well-poised to advance the field of porous networks. sites enables a better reaction design and rationalization of the In this context, this review article is poised to guide the lector structure/properties of the finally accessed porous network. The through the main factors that contributed to bracing MOP toward aforementioned pre-requisites can be inherently present in the becoming excellent reticular building blocks for the assembly of as-synthetized MOPs thanks to the properties that their molec- complex porous extended architectures, from soft matter to crys- ular constituents confer to them, or can be introduced a posteri- talline frameworks. ori through post-synthetic modification. In the following section, we describe the different routes explored to develop robust, solu- ble, and reactive MOPs that can subsequently be used as building 2. The Building Block: Metal–Organic Polyhedra blocks in the synthesis of hierarchical porous networks. As a subclass of coordination cages, MOPs employ directional metal–ligand coordination bonds that can be exploited for the de- 3.1. The Surface Reactivity of MOPs sign and synthesis of discrete molecular architectures that pos- sess internal cavities. However, MOPs differ from other coor- MOPs inherit distinctive elements from both MOFs and co- dination cages in that their cavities are preserved upon activa- ordination cages, such as open metals sites and organic pen- tion or desolvation, making them permanently porous in the dant groups. Therefore, MOPs are potentially reactive through Adv. Sci. 2022, 9, 2104753 2104753 (2 of 19) © 2022 The Authors. Advanced Science published by Wiley-VCH GmbH www.advancedsciencenews.com www.advancedscience.com Figure 1. Schematic representation of the reactive sites on the surface of MOPs: functional pendant groups on the organic ligand (highlighted in orange), coordinated ligands on the open metal sites (highlighted in blue), and counter-ions in charged MOPs (highlighted in green). their exposed organic functional groups and/or their metallic and coordination chemistries, MOPs can also participate in two nodes. Remarkably, both surface-reactive organic groups and other supramolecular interactions: electrostatic interactions and metal sites generally exhibit orthogonal reactivity, meaning that H-bonding. Electrostatic charge has been introduced into MOPs they can be targeted independently or in tandem. Surface organic by using intrinsically charged clusters, as has been done with a groups have been employed to endow MOPs with both covalent zirconocene cluster,[58] and by using charged organic functionali- and/or coordination reactivities. For example, covalent reactiv- ties such as sulfonate groups (Figure 1, green).[59] Charged MOPs ity has been introduced to MOPs by derivatizing their surfaces can then undergo metathesis for tuning of surface chemistry or with amines, alcohols, alkyne, alkene, aldehyde or dithiobenzoate induction of self-assembly. Similarly, the possibility to function- groups,[53] among other moieties, thereby enabling subsequent alize the surface of MOPs with H-bond acceptor/donor groups chemistry with the resultant MOPs, including condensations, opens the door to guiding their assembly through complemen- click chemistry, olefin metathesis or polymerizations (Figure 1, tary H-bonding interactions. orange). Similarly, MOPs have been functionalized with organic MOPs can be linked through various supramolecular and co- functional groups with coordination capabilities such as amines, valent interactions provided that their structural integrity is not hydroxyls, and carboxylates. Thus, further coordination towards compromised by such polymerization reactions. The design of additional metal ions allows the use of coordination chemistry in such supramolecular or covalent polymerization reactions bene- MOP surfaces. In both cases, reactive groups have typically been fits from precise knowledge of the orientation, number, and loca- introduced onto the surface of the MOP through direct synthesis tion of the reactive sites on the polyhedral surface. Therefore, the by selecting ligands bearing such reactive moieties as pendant geometrical features of the reactive sites, and the type of linkage groups.[52] However, in some cases, such as with free amines and chemistry, can be strategically combined to dictate the structural carboxylic acids, functionalized MOPs could not be directly syn- features of the resultant network at the molecular scale (connec- thesized, thus highlighting a demand for alternative approaches tivity and periodicity), the mesoscale (hierarchical porosity), and based on using protecting groups.[54] Interestingly, MOPs func- the macroscale (shape and mechanical properties). We address tionalized with coordinating groups can undergo supramolecular these topics in the following sections. polymerization via self-condensation reactions or upon coordina- tion with additional metal ions.[42,55] The reactivity of open metal sites in MOPs has been exploited 3.2. Stability mainly for M(II) paddlewheel-based MOPs (Figure 1, blue). Pad- dlewheel clusters have a well-established reactivity at their axial In this section, we focus on the stability of MOPs in solution. sites, which can undergo ligand-exchange reactions with N-based Here, we define the stability of a MOP as the conservation ligands such as pyridines, amines, or imidazoles without com- of a distinguishable 0D entity throughout the entire “cage-to- promising the equatorial M(II)-carboxylate coordination.[56,57] Ac- network” reaction (i.e., without any disassembly or re-assembly cordingly, these open metal sites can be used as anchoring points of the MOP constituents). The synthesis of MOPs relies on to functionalize the surface of paddlewheel-based MOPs or to the reversibility of the coordination bonds that sustain their link them with (monodentate or bidentate) N-based ligands. structure.[60–62] However, this reversibility is also a source of This chemistry has been explored mainly for Cu(II)-based MOPs concern when assessing their stability, because they can dis- (named Cu-MOPs), although it has recently been expanded to sociate in the presence of competing coordinating molecules Rh(II)-based MOPs (named Rh-MOPs).[38] In addition to covalent such as water (i.e., hydrolysis) or additional ligands (i.e., ligand Adv. Sci. 2022, 9, 2104753 2104753 (3 of 19) © 2022 The Authors. Advanced Science published by Wiley-VCH GmbH www.advancedsciencenews.com www.advancedscience.com Figure 2. Main strategies to increase MOP stability. a) Shielding of the metallic nodes with hydrophobic or hydrophilic polymeric chains. b) Use of metal ions with strong intermetallic bonds [i.e., Mo(II), Rh(II), Ru(II) or Cr(II)]. c) Increasing metal–ligand coordination bond using strongly coordinating hard-metal ions in combination with hard bases (O-donor ligands) (i.e., zirconium clusters). d) Introduction of chelating groups. exchange).[63,64] Therefore, strategies to augment the robustness the Cu(II) paddlewheel.[65] The same approach was later used by of MOPs in solution focus on either shielding the metallic node, Yin et al. using poly(N-isopropylacrylamide) (PNiPAM) as a poly- using metal ions with strong intermetallic bonds, increasing the meric chain instead of PEG.[66] coordination strength of the metal–ligand coordination bond, or Hydrophobic shielding also has been used to modulate the using chelating groups (Figure 2). permeability of water molecules into a MOP structure.[67] For The most frequently used metallic node in MOP chemistry is example, Ghosh et al. demonstrated that attaching hydropho- the Cu(II) paddlewheel. However, both the chemical stability and bic moieties to the exposed surface of a cuboctahedral Cu- hydrolytic stability of Cu-MOPs are compromised by the lability MOP enhanced its hydrolytic stability.[67] The authors synthe- of the Cu-carboxylate bond, especially its tendency to undergo sized neutral cages based on 12 Cu(II) paddlewheels and 24 lig- both hydrolysis and ligand exchange.[59] Consequently, the scope ands, with a naphthalene diimide core and different pendant of reactivity of Cu-MOPs toward coordination and covalent poly- amino acids (alanine, valine, isoleucine, and phenylalanine) that merization reactions may be limited. Among the most success- each provided a different degree of hydrophobicity to the MOP. ful approaches to increase the stability of Cu-MOPs is to preclude They then tested the stability of the resultant MOPs heteroge- the interaction of external agents with the labile nodes, whereby neously in aqueous media under alkaline, acidic, oxidizing, re- the high surface density of functional groups on the MOP is ex- ducing, and buffered conditions. Owing to the blocking of the ploited to shield the vulnerable Cu-carboxylate bonds from hy- paddlewheel units, the MOPs bearing the most hydrophobic drolysis or ligand exchange. To this end, a surrounding organic groups fully retained their structures in solid state, even after shell is attached to the rigid organic backbone of the MOP (Fig- several weeks under the test conditions. However, the potential ure 2a). Depending on the shell used, the shielding can be either of this approach for solubilized MOPs, in which the diffusion hydrophilic or hydrophobic. of water molecules toward the MOP core is less hindered, is The protective effect of densely grafted hydrophilic polymers unknown.[67] was first suggested by Zhou et al., who covalently attached The aforementioned examples demonstrate the potential of polyethylene glycol chains (exhibiting 5 kDa as molecular weight, shielding strategies to protect Cu(II) paddlewheel units in PEG5k) to a cuboctahedral Cu-MOP (named pi-CuMOP; also MOPs and consequently, to enhance the stability of MOPs to- known as Cu(pi)) formulated as [Cu2 (pi)2 ]12 (in which pi refers ward hydrolysis in more aggressive media (i.e., alkaline, acidic, to the ligand 5-(prop-2-ynoxybenzene)-1,3-dicarboxylic acid), via etc.). However, the effects of organic shielding on the reac- a copper(I)-catalyzed, azide-alkyne cycloaddition.[65] The resul- tivity of MOPs in polymerization reactions, particularly those tant structure, denoted as PEG5k-CuMOP, demonstrated supe- based on the reactivity of open metal sites, remain unclear. rior stability to the starting material, as demonstrated in a 24 Contrariwise, covalent polymerization reactions using poly- h aqueous dialysis experiment. The authors attributed this im- meric/macromolecular ligands can be used to link MOPs to- provement to intermolecular aggregation between soluble enti- gether, maintaining the hydrophobic/hydrophilic shielding of ties, which prevented the water molecules from interacting with open metal sites. Adv. Sci. 2022, 9, 2104753 2104753 (4 of 19) © 2022 The Authors. Advanced Science published by Wiley-VCH GmbH www.advancedsciencenews.com www.advancedscience.com Given the widespread use of the 4-c paddlewheel cluster in However, the use of ligands with strong coordinating chelating the synthesis of MOPs, researchers have sought to generate an moieties is challenging, as it hinders the necessary reversible equivalent SBU made of metal ions with a stronger metal–metal bond formation involved in the synthesis of structured metal– bond strength, as a way to maximize overall robustness, includ- organic assemblies such as MOPs. Interestingly, Nitschke and ing resistance to hydrolysis (Figure 2b). For instance, paddle- Errington et al. developed an elegant solution: the synthesis of wheel units containing intermetallic bonds such as those based the chelating pyridyl-imine moiety in situ by reversible covalent on Mo(II), Ru(II), Cr(II), or Rh(II), have been described to afford chemistry.[77,78] However, metal–organic cages based on pyridyl- more robust MOPs in the solid-state, as reflected by their large imine ligands have been studied mainly in solution; their stability surface areas (order of magnitude: 1000 m2 g–1 ).[44,45,49,68,69] In and porous properties in the solid-state have only recently begun particular, Rh-MOPs are highly stable in solution under various to be investigated.[79,80] In 2017, Zhang et al. introduced an al- conditions, including in the presence of coordinating ligands, ternative, strong-coordinating, chelate-based cluster for the syn- at high temperature, and in aqueous solutions at extreme pH thesis of robust MOPs.[81] They reported the synthesis of a tetra- levels.[38,70,71] The outstanding stability of Rh-MOPs stems from hedral Ti(IV)-based MOP (named Ti-MOPs) assembled from a the cooperative strength of the Rh-carboxylate coordination bond naphthalene ligand functionalized with adjacent carboxylic and and the Rh-Rh intermetallic bond, which lock the equatorial coor- phenol groups. In this Ti-MOP, three different ligands chelate dination sites at room temperature, thereby preventing the cage the Ti(IV) ion, thereby generating a distorted octahedral coor- from undergoing ligand-exchange reactions. dination geometry. Due to the strong chelate-Ti(IV) coordina- Another strategy to improve the stability of coordination com- tion, the MOP was stable in water. Alternatively, Tezcan et al. pounds is to increase the strength of the metal–ligand coordina- recently introduced the use of hydroxamates as charged chelat- tion bond.[72] The Zr-O coordination bond has a high dissociation ing groups for the synthesis of Fe(III)-based MOPs (named Fe- energy (≈776 kJ·mol–1 ), making it exceptionally resilient to hy- MOPs). Interestingly, they observed a tetrahedral geometry in drolysis in a wide pH range (pH 1-10).[73] Furthermore, metallic the Fe(III)-hydroxamate node, suggesting that this moiety could centers with high coordination numbers can generate high nu- be used to rationally design other MOPs.[82] Given these exam- clearity clusters, thereby yielding remarkably stable M-L nodes. ples, such existing porous chelate-based MOPs and others that For this, Zr(IV) clusters are regularly used to provide chemical can be potentially made of highly stable chelates (e.g., metal– stability—especially hydrolytic stability—to metal–organic mate- catecholates) hold great potential for their future use in subse- rials (typically: MOFs).[74] Recently, Zr(IV)-carboxylate chemistry quent self-assembly processes. was implemented to synthesize Zr(IV)-based MOPs (named Zr- MOPs), which exhibit a similar stability profile to their extended MOF counterparts. 3.3. Solubility Most Zr-MOPs reported in the literature are based on trinuclear zirconocene nodes and synthesized in situ. This Owing to their discrete molecular structure, MOPs have remark- 3-connected pyramidal cluster was also employed by Yuan able solubility in diverse solvents while maintaining their poros- et al. to build-up a series of cationic tetrahedral cages, for ity. Accordingly, some MOPs exhibit liquid processability, which which they used 1,4-benzenedicarboxylic acid (pBDC) and can be exploited to perform chemical processes in homogeneous 1,3,5-benzenetricarboxylic acid (BTC) as organic ligands to conditions under stoichiometric control. This is highly advanta- obtain the MOPs {[Cp3 Zr3 μ3 -O(μ2 -OH)3 ]4 [pBDC]6 }4+ (named geous for hierarchical assembly of MOPs into porous networks, ZrT-1, with Cp being cyclopentadienyl) and {[Cp3 Zr3 μ3 -O(μ2 - because self-assembly can be controlled at the molecular level, OH)3 ]4 [BTC]4 }4+ (named ZrT-2), respectively.[46] By subjecting thereby avoiding possible sources of anisotropy (e.g., partial re- the extended carboxylate ligands; 4,4′-biphenyldicarboxylic acid activity preclusion or aggregation-induced defects) in the final (BPDC) and1,3,5-tris(4-carboxyphenyl)benzene (BTB) to reticu- material. Additionally, the solvent in which the MOP is soluble lar chemistry, the authors obtained two larger MOPs formu- can strongly influence the self-assembly process and/or the per- lated as {[Cp3 Zr3 μ3 -O(μ2 -OH)3 ]4 (BPDC)6 }4+ (named ZrT-3) and formance/application of the obtained polymeric network, as ob- {[Cp3 Zr3 μ3 -O(μ2 -OH)3 ]4 (BTB)4 }4+ (named ZrT-4), respectively. served for hydrogels. Therefore, the solubility profile of the MOP These ZrT-1 to ZrT-4 MOPs showed to be stable in methanol must be understood and controlled before the assembly process. (MeOH) and dimethyl sulfoxide (DMSO). More recently, Su et In this regard, MOPs can be readily functionalized—either by di- al. modified pBDC with an amino group to obtain the ligand rect synthesis or by post-synthetic modification—with functional 2-aminobenzene-1,4-dicarboxylic acid (2-NH2 -pBDC) and then groups that enhance their solubility. used it to construct a Zr-MOP of formula {[Cp3 Zr3 μ3 -O(μ2 - The solubilization of MOPs, as for any molecule, is regulated OH)3 ]4 [2-NH2 -pBDC]6 }4+ .[75] The authors evaluated the stabil- by the balance of the interaction energies of intermolecular at- ity of this tetrahedral MOP (named NH2 -ZrMOP; also known traction in the solid-state and of solvation in solution. Therefore, as UMOP-1-NH2 ) under an aqueous environment from pH= the surface of MOPs is critical to solubilization, as it can be used 2 to pH= 10, after which point the NH2 -Zr-cage began to to hinder inter-MOP interactions (e.g., by using bulky groups to decompose.[75] Interestingly, their finding is consistent with the generate steric hindrance) and/or augment MOP-solvent interac- behavior reported for Zr-based MOFs.[72] tions (e.g., by using hydrophilic or hydrophobic moieties to pro- An alternative method to introduce robust metal nodes into mote solubility in water or organic solvents, respectively). MOP structures entails the use of ligands that form chelates with Among the first strategies for surface-functionalizing of MOPs metal ions, as the chelating effect has been observed to increase to tune their solubility is the use of pendant functional groups. the stability of coordination complexes in aqueous media.[76] Interestingly, these organic terminal groups in bridging ligands Adv. Sci. 2022, 9, 2104753 2104753 (5 of 19) © 2022 The Authors. Advanced Science published by Wiley-VCH GmbH www.advancedsciencenews.com www.advancedscience.com are largely responsible for the solubility of MOPs. A representa- and, for the amide-functionalized cages, ascertained the influ- tive example of this is the cuboctahedral MOP family, based on ence of the ligands on the solubility of the resulting cages. 5-functionalized BDC bridging ligands. Yaghi et al. published the They found that introduction of alkane- and diphenyl-groups first report of a cuboctahedral Cu-MOP (named H-CuMOP; also into the parent MOP enhanced solubility in volatile organic known as MOP-1), having the formula [Cu2 BDC2 ]12 , which did solvents such as dichloromethane, chloroform, tetrahydrofuran, not bear any functional group on its surface.[36] Therefore, the ag- and acetone. Alternatively, our group previously developed a gregation of this H-CuMOP during its synthesis cannot be easily protection/deprotection strategy to synthesize the first-ever (re- reverted, as solvation is hindered by the strong H-CuMOP pack- ported) example of a MOP bearing free carboxylic groups on its ing interactions, which preclude the solvent molecules and the surface.[54] The synthesized cuboctahedral Rh-MOP of formula MOP surface from interacting. Later, the same group reported a [Rh2 (BTC)2 ]12 (named COOH-RhMOP) was highly soluble in po- surface-modified isostructural MOP related to H-CuMOP, whose lar solvents such as DMF. surface was functionalized with 24 aliphatic dodecyloxy chains by Zhou et al. described a unique mechanism for tuning substituting BDC with the 5-dodecoxybenzene-1,3-dicarboxylic the solubility of cuboctahedral Cu-MOPs, based on iso- acid (5-C12 -BDC).[83] This functionalized MOP with formula merization of pendant azobenzene groups at the MOP [Cu2 (5-C12 -BDC)2 ]12 was highly soluble in polar and apolar apro- surface.[87] First, they linked Cu(II) ions to the ligand 5-(2,4- tic organic solvents (e.g., dimethylformamide (DMF) and chloro- dimethylphenyl)diazinyl)benzene-1,3-dicarboxylic acid (L1) to form, respectively). This approach has been expanded to myriad obtain a cuboctahedral Cu-MOP, with the formula [Cu2 (L1)2 ]12 functional groups to confer the cuboctahedral Cu-MOP family (named srMOP-1). The azobenzene moieties of the as-made with a truly broad solubility profile. For example, Zaworotko et srMOP-1, which the authors identified as the trans-isomer, in- al. synthesized hydroxyl-functionalized cuboctahedral Cu-MOPs duced strong inter-MOP interactions through 𝜋-𝜋 interactions, that exhibited high solubility in alcoholic solvents, DMF, and thereby limiting the solubility of trans-srMOP in organic sol- hot acetonitrile.[84] Alternatively, Zhou et al. reported that tert- vents such as MeOH or chloroform. Next, the authors irradiated butyl functionalized MOPs were highly soluble in polar solvents srMOP-1 with UV-light to trigger the trans-to-cis isomerization such as MeOH and DMF.[59] The same authors were the first to of the azobenzene groups, which disrupted the inter-MOP introduce charged groups on the surface of cuboctahedral Cu- interactions and consequently, making cis-srMOP-1 soluble in MOPs by employing BDC ligands functionalized with sodium organic solvents. Their photo-switching mechanism holds great sulfonate at the 5-position (5-SO3 -BDC) to yield a MOP with the potential to control the release of active molecules as well as for formula Na24 [Cu2 (5-SO3 -BDC)2 ]12 (named SO3 Na-CuMOP).[59] spatiotemporal control, in situ, of the amount of soluble MOP The exposed sulfonate groups rendered a negatively charged available for polymerization reactions. MOP that inhibited MOP aggregation due to electrostatic repul- To date, the role of pendant organic groups on the solubil- sion. Additionally, they used hydrophilic counter-cations such ity of MOPs has been investigated according to their intrinsic as Na(I) to solubilize the SO3 Na-CuMOP in water, albeit it hy- characteristics or covalent reactivity. However, MOPs bearing pH- drolyzed after short incubation times. Conversely, the function- responsive groups can undergo alternative chemical modifica- alization of MOPs with strongly hydrophobic groups (e.g., triiso- tions that are based on their protonation state and consequently, propylsilyl groups) enables their solubilization in highly apolar can be selectively pH-triggered. These processes alter the surface solvents such as diethyl ether or benzene.[85] Examples of ligands chemistry of MOPs and, consequently, affect their solubility. We used, implementing this effect, include again 5-functionalized exploited this concept to tune, in situ, the solubility of a hydroxyl- ligands such as 5-C12 -BDC and 5-hydroxy-1,3-dicarboxylic acid functionalized Rh-MOP of formula [Rh2 (5-OH-BDC)2 ]12 (named (5-OH-BDC), which have been combined with highly stable Mo- OH-RhMOP).[70] We showed that the 24 hydroxyl groups at the or Rh-based cuboctahedral cages. A similar strategy has been periphery of OH-RhMOP could be stoichiometrically deproto- used in Zr-MOPs. For example, the solubility of Zr-based tetra- nated with NaOH to afford a negatively charged MOP of for- hedral cages in organic solvents such as DMF can be increased mula Na24 [Rh2 (5-OH-BDC)2 ]12 (named ONa-RhMOP). We ob- by derivatizing them with pendant NH2 groups.[75] Similarly, served that ONa-RhMOP was soluble in aqueous solution, which Zhou et al. reported a series of geometrically distinct, lantern-, is consistent with the ability of water molecules to solvate charged octahedral- and cuboctahedral-type MOPs based on a quadruple- species in the presence of hydrophilic counter-ions. We further bonded Mo(II) paddlewheel combined with diverse ligands of demonstrated the crucial role of counter-cations in dictating the various sizes, bending angles, and organic pendant groups. The solubility of the charged MOP. We discovered that, analogously authors attributed the solubility of the resultant products to the to what had previously been reported in inorganic nanoparticles influence of the pendant groups, observing distinct solubility pro- or other metal–organic cages, the sodium ions in ONa-RhMOP files for isostructural cages that differed only in the functionality could be substituted with organic cations such as cetyltrimethy- of such surface moieties.[68] lammonium (CTA), which afforded a lipophilic MOP of formula Interestingly, solubility provided by the pendant group in CTA24 [Rh2 (5-OH-BDC)2 ]12 that was highly soluble in chloroform MOPs can be further tailored through covalent post-synthetic (Figure 3). modification. Bloch et al. demonstrated this phenomenon with The aforementioned examples reflect the remarkable role of two cuboctahedral cages, of the formulas M24 (5-OH-BDC)24 and the pendant groups in defining the solubility profile of MOPs. M24 (5-NH2 -BDC) (where M = Cr, Mo, Cu and 5-NH2 -BDC = The polarity, hydrophilicity/hydrophobicity, charge, and counter- 5-amino-1,3-benzenedicarboxylic acid), which they functional- ions of these groups govern the solvation of MOPs by sol- ized with esters and amides, respectively.[86] The authors em- vent molecules and consequently, their solubilization. Neverthe- ployed a notably large library of functional groups in their study less, despite the predominant role of these groups in dictat- Adv. Sci. 2022, 9, 2104753 2104753 (6 of 19) © 2022 The Authors. Advanced Science published by Wiley-VCH GmbH www.advancedsciencenews.com www.advancedscience.com Figure 3. Schematic illustration of the control over solubility dependent on the pendant organic groups. ing the solubility of MOPs, the metallic nodes also contribute can be used to further tune solubility. For example, Bloch et to solubility. For instance, the trinuclear zirconocene cluster al. used ion-exchange to increase the solubility in methanol of found in Zr-MOPs is inherently positively charged which in- a MOP with the formula Cl4 {[CpZrμ3 -O(μ2 -OH)]3 [Me2 -pBDC]6 } hibits strong inter-MOP interactions, thereby facilitating sol- (named Me2 -ZrMOP) with Me2 -BDC being 2,5-dimethyl-1,4- ubilization of these MOPs in polar solvents.[88] Additionally, benzenedicarboxylic acid, by exchanging the initial chloride with these metallic cluster undergo cation-exchange reactions that trifluoromethanesulfonate as counter-cation.[88] Adv. Sci. 2022, 9, 2104753 2104753 (7 of 19) © 2022 The Authors. Advanced Science published by Wiley-VCH GmbH www.advancedsciencenews.com www.advancedscience.com An indirect strategy to modify the solubility of MOPs is to tively, the possibility of functionalizing MOPs with strong coor- leverage the coordination chemistry of their metallic nodes to lo- dinating groups (e.g., carboxylic acids) while avoiding MOP self- calize axial ligands toward their periphery, as these ligands ulti- condensation has enabled the assembly of MOPs via coordina- mately modify the overall surface chemistry of MOPs and con- tion using bridging metal ions/clusters.[42] sequently, their solubility. Our group, together with the team of Furukawa, have demonstrated the potential of this approach by functionalizing cuboctahedral Rh-MOPs with N-donor lig- 4.1.1. Self-Condensation of Functional MOPs ands containing diverse functional groups.[38] Specifically, we found that the solubility of [Rh2 (BDC2 )]12 (named H-RhMOP) The self-condensation of MOPs is based on the fact that they can and OH-RhMOP could be modulated by functionalizing their combine open metal sites and coordinating groups from their 12 surface di-rhodium axial sites with distinct N-donor ligands. constituent organic ligands. Zaworotko et al. exploited this fea- We observed that the solubility of the resulting functionalized ture to assemble MOPs into crystalline networks for the first Rh-MOP correlated with the hydrophilicity/hydrophobicity of time.[95] They showed that cuboctahedral Cu-MOPs with reac- the N-donor ligand used to functionalize them. For example, tive surface groups self-polymerized through sulfonate-Cu(II) or L/D-proline as axial ligand afforded water-soluble Rh-MOPs, methoxy-Cu(II) coordination into 3D or 1D networks, respec- whereas 4-(trifluoromethyl)pyridine as axial ligand yielded MOPs tively. These networks are distinguished by the fact that their soluble in halogenated or aprotic organic solvents such as cages are held together chiefly by a strong metal–ligand coordi- dichloromethane or tetrahydrofuran. Remarkably, the coordina- nation bond, rather than by the weak, intermolecular, van der tive solubilizer method applied to Rh-MOPs does not imply any Waals interactions typically found in previously reported MOP irreversible structural modification in the cage, as the coordi- packing structures. Niu et al. extended this approach by isolating nating ligand can be removed by ligand exchange or by proto- the target MOP before its self-condensation reaction to achieve a nating the donor group, in either case without compromising higher degree of control over the final framework structure.[55] the structural stability of the cages.[70] Therefore, this strategy To this end, they synthesized a cuboctahedral Cu-MOP of for- does not prevent the use of functionalized MOPs in subsequent mula [Cu2 (5-OH-BDC)2 ]12 (named OH-CuMOP) functionalized coordination-driven polymerization reactions (vide infra). with 24 hydroxyl groups. The reactivity of this OH-CuMOP was Importantly, the surfaces of some MOPs are functionalized modulated by the solvent in which it was solubilized. For most with capping ligands that fulfill the coordination requirements MOPs, solvent molecules tend to coordinate to open metal sites, of the MOP’s constituent metal ions. Regarding the potential im- thereby influencing their surface chemistry. Indeed, bulky sol- pact of these ligands on the solubility profile of MOPs, we be- vents such as dimethylacetamide and DMSO imposed steric hin- lieve that they may have been overlooked in the literature, as in- drance around the MOP, which precluded the self-condensation dicated by the fact that they have been previously employed to reaction. However, when the authors dissolved OH-CuMOP in tune the solubility of related non-porous Pd(II)-based discrete less-coordinating and bulky solvents such as methanol, it spon- metal–organic assemblies.[89] This strategy may be applicable to taneously assembled into a 3D structure bridged via Cu(II)- permanently porous Zr-MOPs, which contain on their surface phenolic hydroxyl coordination of adjacent MOPs. Interestingly, three capping Cp moieties per Zr(IV) cluster. Each of these Cp lig- they were able to exploit the presence of small quantities of ands can be functionalized or exchanged by other capping ligands DMSO in the methanolic reaction mixture to modulate the steric to tune the surface chemistry and solubility of Zr-MOPs. Simi- hindrance around the MOPs, partially masking some of its open larly, the solubility of calixarene-capped MOPs could be modu- metal sites and reducing the connectivity of the OH-CuMOP, lated by tuning exposed functional group of the macrocyclic cap- thereby enabling synthesis of layered 2D networks.[55] This exam- ping ligand.[90–94] ple highlights how previously unconsidered parameters, such as the used solvent, can heavily influence the surface chemistry of 4. Assembly of MOPs into Porous Networks MOPs and their self-assembly reactivity. An alternative manner to control the assembly of functional 4.1. Coordination-Driven Polymerization MOPs is to temporally occlude their latent reactivity by using protecting groups. For example, Bloch et al. reported that self- The reactivity of exposed open metal sites in MOPs has been har- polymerization of a protected NH2 -functionalized, lantern-type nessed to drive the assembly of MOPs into crystalline as well Cu-MOP of formula [Cu2 (L2)2 ]12 (where L2 is 3,3′-((2,5-diamino- as amorphous porous networks. Accordingly, M(II) paddlewheel- 1,3-phenylene)bis(ethyne-2,1-diyl))benzoic acid) could be trig- based MOPs, with accessible reactive metal sites, are excellent gered by unmasking reactive groups in solution.[96] However, candidates for coordination-driven MOP polymerization reac- their strategy demanded that the protected MOP withstand the tions. This approach benefits from the fact that the equatorial deprotection conditions, which for Cu(II)-based cages, might carboxylate-M(II) coordination bonds of M(II) paddlewheels are limit the scope of compatible protecting groups. Indeed, the more robust than their axial coordination sites. Thus, one can authors showed that, although the protected MOP could not target such reactive sites to polymerize MOPs without com- withstand the high temperature or strong acids required to re- promising their backbone structure. This can be achieved by move di-tert-butyl dicarbonate protecting groups, it was stable two different types of coordination reaction: self-condensation to the alkaline conditions required to remove protecting groups of MOPs functionalized with coordinating moieties on their sur- such as fluorenylmethyloxycarbonyl. Interestingly, the authors face; and linkage of MOPs via ditopic N-donor ligands. Alterna- correlated the self-polymerization rate of protected MOP to the Adv. Sci. 2022, 9, 2104753 2104753 (8 of 19) © 2022 The Authors. Advanced Science published by Wiley-VCH GmbH www.advancedsciencenews.com www.advancedscience.com Figure 4. Schematic representation of the assembly of MOPs using rigid N-ditopic linkers, highlighting the generated interMOP cavity (orange sphere). amount of base added to the reaction media, as the latter de- derlying 12-connected (12-c) fcu topology (Figure 4).[98] Later, Su termined the rate of formation of the reactive groups (i.e., pen- et al. employed the same strategy to tune the structure and com- dant amines). The coordination polymers that they obtained were position of MOFs with fcu topology, using a Cu-MOP precursor amorphous, nanoscopic (size: < 50 nm), spherical nanoparti- bearing free amino groups at its surface, of formula [Cu2 (5-NH2 - cles. However, they found all the coordination polymers to be BDC)2 ]12, (NH2 -CuMOP; also known as MOP-15). Thus, they as- permanently porous to N2 and CO2 , with great uptake values sembled the NH2 -CuMOP with bipyridine to obtain an fcu MOF directly correlating to the polymerization rate. The authors rea- in which the inter-MOP cavities were derivatized with pendant soned that at higher self-polymerization rates, the MOPs pack amino groups.[56] The same authors also demonstrated that the less efficiently, thereby generating larger pore volumes in the inter-MOP space could be systematically expanded by increasing coordination-polymer products. the length of the pyridine based ligand, from pyrazine to 1,2- di(pyridin-4-yl)ethane.[57] Furthermore, given that the polyhedral shape of a MOP pre- 4.1.2. Assembly of MOPs through N-Based Ditopic Ligands cursor determines the final topology of the MOF, the dimension- ality of the assembled structure can also be reduced by employing N-donor ligands exhibit an attractive balance between coordina- MOPs with fewer vertices or paddlewheel units in their structure. tion strength and directionality for M(II) paddlewheel axial sites, For example, Klosterman et al. used a lantern type Cu-MOP as thus making pyridine-, amine- and imidazole-based ditopic lig- precursor to synthesize a 1D coordination polymer.[101] The above ands suitable for coordination-driven polymerization of MOPs. examples illustrate how strategic choice of MOP and ligand pre- The use of these ligands to link MOPs has enabled the scope of cursors enables the design of dimensionality, framework topol- MOP-based networks to widen, as it precludes the standard pre- ogy, and/or inter-MOP chemical space of MOP-to-MOF struc- requisite that only inherently reactive MOPs can be used. Addi- tures. tionally, the geometry and the degree of flexibility of N-based lig- As previously stated, the solubility of a MOP in a given reaction ands can be harnessed to direct the distance, symmetry, and pe- medium strongly influences the assembly of MOPs into extended riodicity between adjacent MOPs, which in turn shape the final structures. However, Lah et al. demonstrated that MOPs could assembled material. Thus, this approach has successfully been also be used in solid state providing excellent templates for gen- employed both with rigid ligands, to assemble MOPs into crys- erating hollow MOF structures.[102] They implemented this ap- talline MOFs, and with flexible ligands, to produce amorphous proach by soaking single crystals of OH-CuMOP in a methanolic soft materials. solution containing N-donor rigid ligands (dabco, pyrazole, and Crystalline MOFs: Zhou et al. and Chun et al. were the bipy). Under these reaction conditions, two processes were cou- first ones to show the feasibility of crosslinking previously pled: (i) solubilization of surface MOP units from the MOP crys- synthesized and isolated MOPs through ditopic N-based lig- tal into the methanol phase; and (ii) the in situ cross-linking of ands to obtain MOFs.[97,98] By applying the supermolecular the solubilized MOPs with the ligand, which generated a shell of building block approach to MOPs with well-defined polyhedral the pcu MOF on the MOP crystals. As the reaction progressed, shapes, and rigid or straight ligands, researchers have been the authors obtained hollow single crystals of the pcu MOF. Later, able to anticipate the topology of the resultant structures.[99,100] Choe et al. demonstrated that single crystals of the same MOP For example, the linkage of octahedral Cu-MOPs of formula could be transformed into a pcu MOF through a single-crystal [Cu2 (CDC)2 ]12 (where CDC is 9H-carbazole-3,6-dicarboxylic acid) to single-crystal process comprising immersion of OH-CuMOP through 4,4′-bipyridine (bipy), generated a 6-connected (6-c) pcu crystals into a DMF/DMSO solution containing dabco.[103] The network,[97] whereas the connection of cuboctahedral Zn(II)- success of this approach relies on the fact that the fcc packing based MOPs (named Zn-MOPs) of formula [Zn2 (5-Me-BDC)2 ]12 ] structure of the MOP units in the OH-CuMOP crystal is related to (where 5-Me-BDC is 5-methyl-1,3-dicarboxylic acid) through 1,4- the position of the MOP units in the fcu MOF. Thus, slight rota- diazabicyclo[2.2.2]octane (dabco), resulted in a MOF with an un- tion and translations of the MOP units in the MOP crystal enable Adv. Sci. 2022, 9, 2104753 2104753 (9 of 19) © 2022 The Authors. Advanced Science published by Wiley-VCH GmbH www.advancedsciencenews.com www.advancedscience.com accommodation of dabco ligands inside the final MOF structure. namic light scattering, for both kinetic analysis of the gelation Interestingly, by quenching the reaction at intermediate times be- process itself as well structural characterization of the colloidal fore the completion of the single-crystal to single-crystal transfor- network.[39] The structural data were important, as the struc- mation, the authors were able to isolate MOP@MOF superstruc- ture of these gels determines their viscoelastic properties. In tures. Subsequent selective removal of the MOP core through this sense, the authors discovered that the gel network depends methanol washings enabled synthesis of single crystalline MOF mainly on the concentration of the initial C12 -RhMOP(bix)12 pre- hollow structures. Further MOP growth on the core shell struc- cursor. Additionally, through mechanistic analysis, the authors tures, followed by MOF synthesis and subsequent MOP removal, determined that the main parameters governing the gel net- enabled synthesis of matryoshka and double-shell hollow struc- work structure (density, correlation length, and degree of branch- tures. ing) are established at the stage in which the colloids aggregate Soft Porous Materials: By assembling MOPs into amorphous through attractive coordinating forces, just before percolation. soft materials, researchers can design porosity into materials Based on this discovery, the authors devised a novel methodology that are typically non-porous, such as amorphous coordina- to selectively confer gels with a continuous and gradual change tion polymers. Compared to crystalline networks, these mate- in mechanical properties. Their strategy is based on using cen- rials have the advantage of a higher degree of processability, trifugal force to create a gradient of pre-gelating, reactive colloidal as their porosity does not rely on long-range order. Interest- particles, which are then incubated at high temperature to in- ingly, MOPs have been polymerized into soft porous materials duce gelation. Since the concentration of reactive colloidal gels through use of short N-based flexible ligands. Chun et al. were varies across the height of the gel, so do the mechanical proper- the first to employ this approach, in which they polymerized ties of each final gel. More recently, Furukawa et al. also showed Cu-MOPs into amorphous coordination polymers using flexi- an alternative way to trigger the gelation of C12 -RhMOP(bix)12 via ble diamine ligands, such as ethylenediamine, xylylenediamine, acid-triggered detachment of bix.[40] They found that addition of and diaminoheptane.[104] Crucially, all assembled coordination stoichiometric amounts of trifluoroacetic acid (TFA) to a DMF polymers exhibited permanent porosity, as assessed by N2 , H2, solution of C12 -RhMOP(bix)12 triggered gelation at room temper- and CO2 adsorption measurements at low temperature. Fur- ature. By replacing TFA with the photoacid 8-hydroxypyren-1,3,6- thermore, the amorphous coordination polymers displayed bi- trisulfonic acid (pyranine), they were able to synthesize porous modal porosity: the authors ascribed contributions from the mi- gels upon light irradiation at room temperature, thereby enabling croporous regime to the inner MOP cavity, and from the meso- photo-patterning of the gel. porous regime to the inter-MOP voids. Following the flexible The long aliphatic chains on the surface of the C12 -RhMOP ditopic ligand approach, Furukawa et al. synthesized intrinsi- used to synthesize MOP-based gels ensure the solubility of the cally porous amorphous coordination polymers by assembling MOP precursor. However, these chains are detrimental to the C12 -RhMOP with the imidazole-based ligand 1,4-bis(imidazole- porosity of the derived aerogels, as they occupy free volume in 1-ylmethyl)benzene (bix).[105] This assembly process was dictated the final assembly, thus hindering gas diffusion. Nevertheless, by the quantity of bix ligand added to the C12 -RhMOP solution. as explained above, MOPs can also become soluble through re- The stepwise addition of bix to the C12 -RhMOP solution triggered activity of their axial site. Accordingly, our group, together with a polymerization reaction that followed a nucleation-elongation Furukawa et al., developed a methodology to assemble the highly mechanism to yield spherical nanoparticles. Alternatively, addi- porous but insoluble H-RhMOP into colloidal particles and gels tion of C12 -RhMOP to a solution containing excess bix (12 mo- (Figure 5b).[106] First, the long aliphatic imidazole, 1-dodecyl-1H- lar eq. relative to MOP) in a single portion, resulted in forma- imidazole, was attached to the open metal sites of H-RhMOP tion of a kinetically trapped molecule in which all the Rh(II) ax- to make it soluble in organic solvents. Next, bix was added to ial sites of the C12 -RhMOP were occupied by bix ligands mon- the functionalized H-RhMOP, which triggered its self-assembly odentate coordination, to give a MOP of formula [Rh2 (5-C12 - into colloidal particles via ligand exchange reaction. Remarkably, BDC)2 (bix)]12 (named C12 -RhMOP(bix)12 ). Self-assembly of such this strategy enabled us to increase the porosity of the derived C12 -RhMOP(bix)12 in solution was triggered by heating the solu- aerogels, which showed bimodal porosity and BET surface areas tion at 80 °C. Under these conditions, a fraction of the monoden- (SBET ) of up to 540 m2 g–1 . Moreover, Furukawa et al. were able tate bix was detached from the surface of the MOP, generating to augment the porosity of the H-RhMOP-derived gels through vacancies that enabled crosslinking of the MOPs via formation a post-synthetic gel maturation strategy.[107] They found that, of bidentate bix bridges. This in turn generated a colloidal sus- upon incubation at 80 °C, the colloidal gel network became more pension that, upon further incubation at 80 °C, assembled into a porous due to greater inter-MOP crosslinking, reaching an SBET colloidal gel in situ. In terms of functional porosity, both forms of value of 758 m2 g–1 . More importantly, this work highlights how the coordination polymer (colloidal particles and supramolecular molecular scale phenomena (e.g., MOP crosslinking) span mul- gels) outperformed the initial C12 -RhMOP, as assessed by N2 and tiple length scales up to the macroscale (e.g., gel densification) to CO2 sorption measured at 77 and 195 K, respectively. Specifically, influence the final properties (e.g., gas adsorption) of functional N2 adsorption measurements displayed a negligible adsorption products. Alternatively, MOP-based hydrogels can be prepared by of 0.17 mol(N2 )/mol(C12 -RhMOP) for the discrete C12 -RhMOP, employing the counteraction solubilization approach. For exam- and uptakes of 8.70, and 18.61 mol(N2 )/mol(C12 -RhMOP) for the ple, Furukawa et al. reacted the water-soluble ONa-RhMOP with colloidal particles and supramolecular gel, respectively.[105] bix in a mixture of acetonitrile and water to obtain a colloidal Furukawa et al. studied the gelation of kinetically trapped gel.[108] Subsequent water exchange treatment afforded a hydro- porous building blocks into colloidal gels by time-resolved dy- gel with no traces of organic solvent. Adv. Sci. 2022, 9, 2104753 2104753 (10 of 19) © 2022 The Authors. Advanced Science published by Wiley-VCH GmbH www.advancedsciencenews.com www.advancedscience.com Figure 5. a) Schematic illustration of the assembly of MOPs as porous monomers through flexible ditopic linkers to form amorphous supramolecular polymers with intrinsic porosity arising from the MOP units. b) Photograph of a supramolecular colloidal gel resulting from this assembly. Reproduced with permission.[] Copyright 2019, Wiley-VCH. 4.1.3. Assembly of MOPs through Coordination with Additional prevents the Cu(II) ions from replacing the Rh(II) ions in the Metal Ions MOP structure, and prevents the HINA from detaching from the MOP in the presence of Cu(II). Therefore, the stepwise assem- Recently, our group introduced a novel methodology that dif- bly of MOPs functionalized with well-defined points of extension fers from the approaches that we have discussed so far, such through hierarchic or orthogonal interactions provides the oppor- as self-condensation of MOPs, or linking of MOPs via rigid tunity to construct multi-component MOF structures with a pre- ditopic ligands. Our approach consists of introducing pendant defined topology and sequence of its different constituent build- and available carboxylic acid groups onto the periphery of a ing blocks. cuboctahedral Rh-MOP, and then harnessing the reactivity of these groups to assemble COOH-RhMOPs with additional metal ions.[42] The chemistry of Rh-MOPs enabled us to precisely po- 4.2. Assembly of MOPs through Supramolecular sition carboxylic acid groups on the 24 edges or the 12 vertices Non-Coordinative Bonds of their cuboctahedral surfaces, through a covalent or coordina- tion post-synthetic route, respectively. The Rh(II) axial sites of the The supramolecular reactivity of MOPs is not limited to coordina- OH-RhMOP were used to anchor 12 isonicotinic acid (HINA) tion chemistry: their surfaces can be functionalized with organic molecules onto its vertices via Rh-pyridine coordination to ob- moieties able to establish alternative non-covalent interactions tain a MOP with formula [Rh2 (5-OH-BDC)2 (HINA)]12 (named such as 𝜋-𝜋 stacking, H-bonding, and electrostatic or hydropho- OH-RhMOP(HINA)12 ), unaltering the carboxylic acids. Topolog- bic/hydrophilic interactions. Indeed, such interactions are com- ically, the resulting OH-RhMOP(HINA)12 can be described as a monly observed in as-made MOP packing structures.[109] How- 12-c cuboctahedral supermolecular building block. We prepared ever, the deliberate use of non-coordinating supramolecular in- the analogous 24-connected (24-c) rhombicuboctahedral super- teractions for the hierarchical construction of MOP-based net- molecular building block COOH-RhMOP, following our own works is currently in its infancy. Bloch et al. recently provided the protocol, entailing use of stoichiometric protecting groups. Both first example of electrostatically driven assembly of MOPs.[88] The COOH-functionalized MOPs were soluble in organic solvents authors used two oppositely charged MOPs as building blocks and, owing to the low chemical affinity of carboxylic acid groups to synthesize the first intrinsically porous salt. Reaction of the for the Rh(II) axial sites, did not undergo any self-condensation. positively charged Me2 -ZrMOP with a negatively charged Cu- However, addition of Cu(II) ions to OH-RhMOP(HINA)12 and MOP of formula [Cu24 (5-SO3 -BDC)12 ]24− (named SO3 -CuMOP) COOH-RhMOP triggered their self-assembly into crystalline net- gave rise to amorphous powders, in which the charge is bal- works of the topologies (4,12)-c ftw and (3,24)-c rht, respectively. anced exclusively by the presence of charged MOPs (i.e., 6 Me2 - The final accessed topology for each of the COOH-functionalized ZrMOPs for 1 SO3 -CuMOP). The amorphous porous salt ex- MOPs agreed with the outcome that we had expected upon con- hibited greater porosity than its building blocks, due to the re- sidering the connectivity and geometry of each of the COOH- moval of non-porous counter-ions from the original structure, RhMOPs. Interestingly, Cu(II) adopts an uncommon coordina- displaying a SBET of 496 m2 g–1 for the amorphous porous salt, tion environment, forming a Cu(II) trimer in the rht structure against the SBET of 416 m2 g–1 and non-porous of the individ- to satisfy the steric and connectivity requirements of the (24-c) ual Me2 -ZrMOP and SO3 -CuMOP, respectively (Figure 7b). In- COOH-RhMOP. From a chemical perspective, the assembly of terestingly, the presence of large counter-ions in each of the up to 4 different molecular components into a single compo- charged precursor MOPs, such as tetraethylammonium for the sitionally complex structure, such as in the ftw network, is en- anionic SO3 -CuMOP or triflate for the cationic Me2 -ZrMOP, en- abled by a clear gradient in coordination strength of the differ- abled the authors to isolate a crystalline salt of formula X16 [Me2 - ent coordination bonds present in the multi-component struc- ZrMOP]2 [SO3 -CuMOP] (where X is H+ or tetraethylammonium ture (Figure 6). This hierarchy in coordination strength is what cations) (Figure 7). The authors suggested that the use of larger Adv. Sci. 2022, 9, 2104753 2104753 (11 of 19) © 2022 The Authors. Advanced Science published by Wiley-VCH GmbH www.advancedsciencenews.com www.advancedscience.com Figure 6. a) Fragments of the structure of RhCu-ftw-MOF, highlighting the connectivity of OH-RhMOP(HINA)12 (12-c SBB) through the 4-c Cu2 paddle- wheel unit. b) Structure of RhCu-ftw-MOF, highlighting the generated interMOP cavity (orange sphere). c) Illustration of the ftw topology. Figure 7. a) A portion of the crystal structure of the doubly porous salt X16 [Me2 -ZrMOP]2 [SO3 -CuMOP]. Grey and green polygons represent the cuboc- tahedral SO3 -CuMOP and the tetrahedral Me2 -ZrMOP, respectively. b) N2 adsorption isotherm at 77 K of the porous salt (red), Me2 -ZrMOP (blue), and SO3 -CuMOP (black). Reproduced with permission.[88] Copyright 2020, American Chemical Society. counter-ions likely slowed down the metathesis reaction, en- more, the POM-MOP network is permanently porous in the solid abling formation of the crystalline product. Later, they expanded state exhibiting an SBET up to 425 m2 g–1 , making the POM units their approach to create a large library of novel porous salts, by highly accessible within the network. combining an array of positively charged and negatively charged MOPs.[110] 4.3. Covalent Polymerization Interestingly, Ohba et al. used a similar strategy to demon- strate that bulky counter-ions could be used to control assem- Covalent bonds are amongst the strongest chemical bonds. bly of charged MOPs.[111] They assembled Me2 -ZrMOP with the Therefore, their use to link MOPs holds great potential to de- polyoxometalates (POMs) of the formula [SiW12 O40 ]4− . Consis- velop robust MOP-based networks. Additionally, the mature field tent with the charge of each building block, the resultant POM- of polymer science can be used as a guide to develop synthetic MOP crystalline assembly has a MOP/POM ratio of 1:1. Further- methodologies that enable shaping of MOP-based networks into Adv. Sci. 2022, 9, 2104753 2104753 (12 of 19) © 2022 The Authors. Advanced Science published by Wiley-VCH GmbH www.advancedsciencenews.com www.advancedscience.com Figure 8. Schematic representation of the covalent polymerization and the corresponding cross-linking reactions. Highlighted in red are the newly formed covalent bonds and in blue the participating functionality on the bridging ligand. Photos of green leaves: Reproduced with permission.[116] Copyright 2020, Royal Society of Chemistry. Phtos of blue gels: Reproduced with permission.[119] Copyright 2019, Royal Society of Chemistry. diverse functional macroscopic materials such as gels, mono- NH2 -ZrMOP with acyl chloride ligands (6 to 10-membered car- liths, or thin films. However, there are few literature reports on bon chains) to cross-link the MOPs through amide bonds.[112] using covalent bonds to assemble MOPs into porous networks, They ran the reaction under heterogeneous conditions, with the underlining the difficulty of this approach. Possible drawbacks NH2 -ZrMOP precursor in its solid crystalline state. Interest- include the stability and/or solubility of the MOPs (vide supra) ingly, the crystalline packing and the intrinsic microporosity of under the required coupling conditions (i.e., organic solvents at the MOPs were retained after condensation. Therefore, this pi- high temperatures in the presence of potentially coordinating oneering example illustrates that Zr-MOPs are sufficiently ro- reagents). Interestingly, most of the literature examples are con- bust to withstand amide-bond formation, even when this en- densation (formation of amides, urethanes, or imines)[112–116] or tails in situ release of HCl. However, the heterogeneous reac- olefin-based cross-linking (polymerization or metathesis)[117–121] tion conditions demanded long reaction times (up to 3 days) (Figure 8). and offered only limited control over the processability and the macroscopic form of the final MOP network. Intriguingly, to gain greater control over amide-bond formation and to reduce dif- 4.3.1. Cross-Linking Based on Condensation Processes fusion pathways, Guo et al. used a water-soluble NH2 -ZrMOP as precursor.[113] They reacted NH2 -ZrMOP with trimesoyl chlo- The first example of cross-linking MOPs through covalent chem- ride through an interfacial polymerization method to produce istry was provided by Choe et al. in 2017.[112] The authors reacted Zr-MOP-based thin films. The resultant films exhibited excel- Adv. Sci. 2022, 9, 2104753 2104753 (13 of 19) © 2022 The Authors. Advanced Science published by Wiley-VCH GmbH www.advancedsciencenews.com www.advancedscience.com lent water permeability, dye rejection, and strong antibacterial ac- the NH2 -CuMOP with methacrylate to make it reactive toward tivity, making them suitable for nanofiltration. Similarly, Zhang butyl methacrylate, using UV-light to yield a cross-linked mem- et al. fabricated MOP-based membranes by condensing NH2 - brane with a MOP content of up to 10%. The MOP within ZrMOP with aldehydes to form polyimine networks.[114] They the membrane was much more stable against hydrolysis than ran the reaction under homogeneous conditions, in a mixture the initial MOP precursor, due to the polymeric shielding ef- of organic solvents, and in the presence of the triamine tris(2- fect (vide supra), which enabled the use of these membranes aminoethyl)-amine and terephthalaldehyde as co-monomers to as adsorbents of organic dyes in aqueous media.[118] The same form the hyper-cross-linked networks. The membranes were eas- authors used an alkene-functionalized MOP of formula [Cu2 (5- ily obtained upon solvent removal and exhibited high perme- (3-butene-1-yloxy)BDC2 ]12 (named Bt-CuMOP) as monomer in ability and anionic dye rejection. The membranes also had en- self-polymerizing reactions, and as co-monomer in polymeriza- hanced mechanical properties, stemming from the highly cross- tion reactions with polystyrene.[119] In the latter case, the authors linked nature of their network. Alternatively, Volkmer et al. used azobisisobutyronitrile and heat as radical initiators, which showed that Cu-MOPs functionalized with hydroxyl groups could led to a final composite exhibiting a MOP content of 88% (1 be grafted with polymer chains through the formation of ure- MOP polymerized with 10 styrene molecules). Both strategies thane bonds.[115] Likewise, Yan et al. used this chemical ap- yielded covalently linked MOP-networks with greater hydrolytic proach to crosslink cuboctahedral Rh-MOPs into polyurethane stability and better sorption properties relative to the initial Bt- networks.[116] They synthesized urethane-reactive MOPs with the CuMOP. Additionally, the polystyrene co-polymerization strategy formula [Rh2 (BDC)m (5HOC4 -BDC)n ]12 (named HOC4 -RhMOP) enabled the synthesis of easy-to-shape gels, thus enhancing the by introducing 5-(4-hydroxybutoxy)-1,3-benzenedicarboxylic acid processability of the MOP-networks. Similarly, Zhao et al. sub- (5-HOC4 -BDC) into the synthesis of H-RhMOP with different ra- jected an amine-functionalized Zr-MOP to post-synthetic, UV- tios (m = 16, n = 8; m = 8, n = 16 and m = 0, n = 24). They light mediated cross-linking.[120] The authors reacted single crys- found that the ratio between the reactive ligand and BDC dic- tals of NH2 -ZrMOP with acryloyl chloride to obtain an acrylated tated the reactivity of HOC4 -RhMOP. In all cases, upon react- NH2 -ZrMOP (named AA-ZrMOP),[121] which they then reacted, ing HOC4 -RhMOP with a mixture containing isophorone diiso- as soluble precursor, with acrylate-terminal polyethylene oxide cyanate, polytetramethylene glycol, and 1,4-butanediol, they ob- precursors. This cross-linking reaction gave rise to self-standing tained elastic MOP-polymeric materials, which they named Elas- thin membranes with a MOP content of up to 3%. The use of the toMOPs. Interestingly, they correlated the mechanical properties AA-ZrMOP as cross-linking agent in the polymerization reaction of the ElastoMOPs (MOP content: 6.5% to 7.8% w/w) to the type later enabled synthesis of membranes with great thermal stabil- of MOP precursor used: those ElastoMOPs with a higher percent- ity and CO2 permeability. The above examples demonstrate that age of hydroxyl groups at their surfaces exhibited better mechani- covalent linking can be used to enhance the robustness of synthe- cal properties, due to their greater degree of cross-linking. Finally, sized superstructures and thus is ripe for further investigation. inspired by nature, the authors used Elasto-MOP to build a leaf that exhibited biometric movement upon exposure to vapor (see Figure 7, right). 5. Perspectives The longstanding potential of functional MOPs as building 4.3.2. Cross-Linking Based on Olefins blocks for the design and assembly of porous materials is finally beginning to bear fruits, as reflected in an increas- There are various examples of MOPs undergoing olefin-based ing number of reports of their use in hydrogels,[122] hybrid cross-linking chemistry. For instance, Shimizu et al. cross- composites,[123] salts,[89] hairy dendrimers,[124] and MOFs.[42] In- linked a cuboctahedral Cu-MOP that was partially functional- deed, beyond their porosity, MOPs boast an impressive array ized with alkyl chains containing terminal alkene groups [5- of other exploitable structural properties, including tunable sol- (dec-9-en-1-yloxy)1,3-benzenedicarboxylic acid (C = C9 -BDC) and ubility, high connectivity, and well-defined peripheral points of 5-(octyloxy)1,3-benzenedicarboxylic acid (C18 -BDC)], through extension.[71,106,125] However, their practical usage has remained olefin metathesis using a Grubbs catalyst.[117] The degree of limited due to a lack of robust functionalized MOPs and a poor crosslinking between the alkene-functionalized MOPs with the compatibility with extension chemistry. Fortunately, the develop- ideal formula [Cu2 (C = C9 -BDC)(C18 -BDC)]12 (named C = C- ment of robust MOPs, either through direct synthesis or post- CuMOPs) could be controlled by the amount of catalyst used. synthetic modification, has enabled researchers to harness the The physicochemical properties of the obtained metathesized surface chemistry of MOPs to modulate their solubility and to networks related to their cross-linking degree, which varied from open up alternative extension pathways that, whilst demanding 20% to 80% (depending on the catalyst loading). Thus, net- more aggressive conditions than in other methods, neverthe- works with a higher degree of cross-linking were insoluble in less afford materials with a higher degree of specialization and organic solvents and had superior mechanical properties (i.e., structural integrity.[108,112,126] However, there is ample room for greater hardness and lower elastic modulus). Additionally, the improvement in the design and synthesis of stable functional MOP-networks showed greater adsorption than did the isolated MOPs and their subsequent incorporation into extended materi- C = C-CuMOPs, most likely due to the appearance of addi- als. Herein, we provide the lector with our insight into the future tional voids at the inter-MOP space. Another compelling cou- of the field and highlight the most promising pathways for the pling method using olefins to connect MOPs is radical poly- synthesis of previously inaccessible materials with cutting-edge merization, as realized by Sun et al.[118,119] They derivatized properties. Adv. Sci. 2022, 9, 2104753 2104753 (14 of 19) © 2022 The Authors. Advanced Science published by Wiley-VCH GmbH www.advancedsciencenews.com www.advancedscience.com The first pathway, and a rather straightforward one, is to in- these particular macromolecules. Thus, if researchers could fur- vest more in developing new MOP cages with directional func- ther optimize the stepwise reactivity of the orthogonal surfaces tionalities poised for extension chemistry,[54,96] whether by di- of MOPs and achieve stoichiometric control over the total degree rect synthesis or post-synthetic modification. As we previously of extension, then they could potentially combine some exciting mentioned, the design of functional MOPs with reactive sur- porous macromolecular materials with unprecedented degrees faces had long been hampered by stability issues, which have of control. recently been overcome with the advent new synthetic and post- Finally, we believe that the internal functionality of MOPs synthetic methodologies. To date, researchers have not had any has been underexploited for developing unique extended materi- incentive to add new functionalities to MOPs per se; however, als. Unlike other archetypical porous frameworks, those assem- there is now a vast landscape of potential pathways for the bled from functionalized MOPs are built from supermolecular preparation of extended MOP-based materials. The importance building blocks with accessible cavities positioned for host-guest of further developing this pathway is illustrated by the recent chemistry, and thus, can be used to periodically space important creation of carboxylate-tagged MOP cages, in which incorpora- substrates within the solid-state. Although the host-guest chem- tion of such a simple, yet challenging, moiety at the periphery istry of MOPs is presently not as developed as this approach de- of a Rh-based cage has laid the first stone toward the forma- mands, the same principles could be extrapolated from the more tion of crystalline, hierarchically porous, MOF architectures with mature host-guest chemistry of coordination cages. Indeed, co- unusual topologies and well-defined hybrid metal sites.[42] In- ordination cages have rich pathways for encapsulation of target- deed, although there is only one reported example of this use solvated guests within their cavities.[135–137] In fact, such encap- of carboxylates, it demonstrates the potential for a single func- sulation stabilizes the guest molecules and can even enhance tional group to massively expand the potential catalog of syn- their reactivity.[138] Thus, once researchers extend this concept thetically feasible MOPs: thus, thousands of previously inacces- to MOPs, they will be able to arrange critical substrates within sible hierarchical MOF structures become available through the a periodic solid-state lattice, thereby exposing them to heteroge- assembly of COOH-tagged preformed cages, without the need neous conditions, including solid-gas phase processes, strongly for any uncontrollable metal–ligand exchange reactions or any demanded by industry.[139] This approach could be further en- chemical-etching.[30,127] We anticipate the same effects for other, riched by ordered combination of different MOPs into porous equally relevant functional groups (acrylates, azide-alkyne, se- networks to pave the way for the spatial organization of varied quential nucleotides, etc.), once they are used in a similar way functional guests. on MOP surfaces, whether individually or collectively, to gen- In summary, the use of MOPs in bottom-up synthesis of erate novel soft matter and crystalline extended architectures. porous materials, mostly overlooked, is now a thriving topic in Thus, there is a need to re-explore the chemistry behind the ba- materials science. Recent advances in MOP surface chemistry, sic cage design bearing new horizons in mind. Furthermore, and the development of soluble MOP platforms that can with- most MOP-extension chemistry, whether reported or simply pos- stand aggressive thermal and chemical conditions, together have tulated, is based on building a single type of cage for simplic- led to a new generation of highly pre-organized molecular build- ity. However, as clearly exemplified in the field of multicompo- ing blocks with exciting structural properties. This progress is nent MOFs,[128] there is a latent possibility of building complex influencing a diverse array of materials, from soft matter to crys- architectures from multiple molecular building blocks, whether talline frameworks, for which researchers are achieving unparal- nodes or ligands.[129,130] Therefore, the same principles could be leled levels of structural control thanks to their intrinsic cavities, applied to assemble multiple MOP cages, with distinct geome- open metal sites, and orthogonally reactive surfaces. Our review tries and functionality, to obtain extended elaborated architec- represents merely the tip of the iceberg of the potential of MOPs tures through one or more assembly approaches, including H- to contribute to nearly any self-assembled material, whether dis- bonds, dynamic covalent bonds, or metal-mediated coordination. crete or extended, which is built from well-defined, specialized, Pioneering work on the co-crystallization of varied MOPs reflects molecular building blocks. the potential of this approach toward fine tuning of the physico- chemical properties of MOP-based materials.[125,131] Another topic that we consider promising for the future is the molecular nature of MOP surfaces, which can enable stoichio- Acknowledgements metric control over the total number of appended functionali- A.K.-B. and L.H.-L. contributed equally to this work. This work was sup- ties, such that the final degree of cross-linking among the cages ported by the Spanish MINECO (project RTI2018-095622-B-I00) and the could be controlled. Indeed, although in this review we have fo- Catalan AGAUR (project 2017 SGR 238). L.H.-L. acknowledges the sup- cused on periodically extended materials, the molecular nature port from the Spanish State Research Agency (PRE2019-088056). It was also funded by the CERCA Program/Generalitat de Catalunya. ICN2 is sup- of MOPs offers vast potential to explore assembly of finite super- ported by the Severo Ochoa program from the Spanish MINECO (Grant structures through stoichiometric, limited-growth steps.[132,133] No. SEV-2017-0706). The project that gave rise to these results received At the intersection of molecular materials and periodic materi- the support of a fellowship (LCF/BQ/PR20/11770011) from “La Caixa” als lies an alluring world of finite architectures (e.g., dendrimers, Foundation (ID 100010434). stepwise polymers, and molecular superstructures) with excep- tional properties.[134,135] To the best of our knowledge, there is scarcely any representation of coordination-based porous materi- als of those types.[135] MOPs, with their tunable solubility and fi- Conflict of Interest nite functionalities, can operate in the same molecular regime as The authors declare no conflict of interest. Adv. Sci. 2022, 9, 2104753 2104753 (15 of 19) © 2022 The Authors. Advanced Science published by Wiley-VCH GmbH www.advancedsciencenews.com www.advancedscience.com Keywords [32] L. Bu, M. R. Nimlos, D. J. Robichaud, S. Kim, Catal. 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Then, he completed his M.Sc. in Biomedicine at the University of Barcelona in 2018. His research focuses on developing methods to assemble metal–organic polyhedral into novel functional materials. Laura Hernández-López is currently pursuing a Ph.D. in Chemistry at the Catalan Institute of Nanoscience and Nanotechnology under the supervision of Prof. Daniel Maspoch and Dr. Arnau Carné. She received her B.Sc. degree in Nanoscience and Nanotechnology at Autonomous University of Barcelona (UAB) in 2017. In 2019, she got her M.Sc. in Applied Materials Chemistry from the Uni- versity of Barcelona (UB). Her research interest lies in understanding the reactivity and assembly of discrete metal–organic polyhedral in solution. Cornelia von Baeckmann is a postdoctoral researcher in the group of Prof. Daniel Maspoch (Catalan Institute of Nanoscience and Nanotechnology) and was rewarded an Erwin-Schrödinger Fellowship which she will start in 2022. She completed her B.Sc. degree in 2015 and her M.Sc. degree in chemistry in 2016 and performed her Ph.D. studies at the Institute of Inorganic Chemistry and Functional Mate- rials at the University of Vienna where she worked on the synthesis and application of functionalized mesoporous silica nanoparticles. She received her Ph.D. degree with distinction in 2021. Her current research focuses on the designed synthesis of novel metal–organic porous materials. Adv. Sci. 2022, 9, 2104753 2104753 (18 of 19) © 2022 The Authors. Advanced Science published by Wiley-VCH GmbH www.advancedsciencenews.com www.advancedscience.com Jorge Albalad is an ARC-funded postdoctoral researcher under the supervision of Prof. Christian J. Doonan and Christopher J. Sumby at the University of Adelaide (Australia). He received his B.Sc. de- gree in Chemistry at the Autonomous University of Barcelona (UAB) in 2014. Then, he completed his M.Sc. in Industrial Chemistry and Introduction to Chemical Research (2015) and his Ph.D. in Chem- istry (2019) at the same university, conducting his research at the Catalan Institute of Nanoscience and Nanotechnology. His research focuses on the post-synthetic modification of metal–organic enti- ties, with particular emphasis on the influence of molecular-level transformations to the macroscopic properties. Arnau Carné-Sánchez is a “La Caixa” Junior Leader fellow at the Catalan Institute of Nanoscience and Nanotechnology. He received his B.Sc. degree in Chemistry from the Universitat de Barcelona (UAB) in 2008. In 2014, he got his Ph.D. in Chemistry from the UAB under the supervision of Prof. Daniel Maspoch and Dr. Inhar Imaz. In 2014, he joined Prof. Susumu Kitagawa and Prof. Shuhei Furukawa’s group in iCeMS at Kyoto University (Japan) under a JSPS postdoctoral fellowship. He is currently re- searching the synthesis and reactivity of nano-sized molecular porous materials. Daniel Maspoch is an ICREA Research Professor at the Institut Català de Nanociència i Nanotecnolo- gia (ICN2). He received his B.S. degree at the Universitat de Girona and his Ph.D. degree at the Uni- versitat Autònoma de Barcelona & Institut de Ciència de Materials de Barcelona. He worked as a post- doctoral fellow at Northwestern University. His research interests include reticular materials (MOFs, COFs, and MOPs) and delivery systems. Adv. Sci. 2022, 9, 2104753 2104753 (19 of 19) © 2022 The Authors. Advanced Science published by Wiley-VCH GmbH