This is the peer reviewed version of the following article:

Grancha T., Carné-Sánchez A., Zarekarizi F., Hernández-López
L., Albalad J., Khobotov A., Guillerm V., Morsali A., Juanhuix J.,
Gándara F., Imaz I., Maspoch D.. Synthesis of Polycarboxylate
Rhodium(II) Metal–Organic Polyhedra (MOPs) and their use as
Building Blocks for Highly Connected Metal–Organic Frameworks
(MOFs). Angewandte Chemie - International Edition, (2021). 60.
: 5729 - . 10.1002/anie.202013839,

which has been published in final form at
https://dx.doi.org/10.1002/anie.202013839. This article may be
used for non-commercial purposes in accordance with Wiley
Terms and Conditions for Use of Self-Archived Versions.
Synthesis of Polycarboxylate Rh(II)-Metal-Organic Polyhedra
and their use as Building Blocks for Highly-Connected Metal-
Organic Frameworks
Thais Grancha,[a] Arnau Carné-Sánchez,*[a] Farnoosh Zarekarizi,[c] Laura Hernández-López,[a]
Jorge Albalad,[a] Akim Khobotov,[a] Vincent Guillerm,[a] Ali Morsali,[c] Judith Juanhuix,[d] Felipe
Gándara,[e] Inhar Imaz,[a] and Daniel Maspoch*[a][b]
[a]   Dr. T. Grancha, Dr. A. Carné-Sánchez, L. Hernández-López, Dr. J. Albalad, A. Khobotov, Dr. V. Guillerm, Dr. I. Imaz, Prof. Dr. D. Maspoch.
      Catalan Institute of Nanoscience and Nanotechnology (ICN2), CSIC and The Barcelona Institute of Science and Technology, Campus UAB,
      Bellaterra, 08193 Barcelona, Spain.
      E-mail: Arnau.carne@icn2.cat, Daniel.maspoch@icn2.cat.
[b]   Prof. Dr. D. Maspoch.
      ICREA, Pg. Lluís Companys 23, 08010 Barcelona, Spain.
[c]   Dr. F. Zarekarizi, Dr. A. Morsali.
      Department of Chemistry, Faculty of Sciences, Tarbiat Modares University, P.O. Box: 14115-4838, Tehran, Islamic Republic of Iran.
[d]   Dr. J. Juanhuix.
      ALBA Synchrotron, 08290 Cerdanyola del Vallès, Barcelona, Spain.
[e]   Dr. F. Gándara.
      Department of New Architectures in Materials Chemistry, Materials Science Institute of Madrid – CSIC, Sor Juana Inés de la Cruz 3, Madrid
      28049, Spain.


          Supporting information for this article is given via a link at the end of the document.



Abstract: Use of preformed metal-organic polyhedra (MOPs) as supermolecular building blocks (SBBs) for
the synthesis of metal-organic frameworks (MOFs) remains underexplored due to lack of robust
functionalized MOPs. Herein we report the use of polycarboxylate cuboctahedral Rh(II)-MOPs for
constructing highly-connected MOFs. Cuboctahedral MOPs were functionalized with carboxylic acid
groups on their 12 vertices or 24 edges through coordinative or covalent post-synthetic routes,
respectively. We then used each isolated polycarboxylate Rh(II)-MOP as 12-c cuboctahedral or 24-c
rhombicuboctahedral SBBs that, upon linkage with metallic secondary building units (SBUs), afford
bimetallic highly-connected MOFs. The assembly of a pre-synthesized 12-c SBB with a 4-c paddle-wheel
SBU, and a 24-c SBB with a 3-c triangular Cu(II) SBU gave rise to bimetallic MOFs having ftw (4,12-c) or rht
(3,24-c) topologies, respectively.

Metal-organic polyhedra (MOPs) have emerged as powerful building blocks for the creation of extended
porous solids as they are intrinsically porous and exhibit high symmetry, connectivity and well-defined
peripheral points of extension.1-3 These characteristics have been recently exploited for the synthesis of
porous amorphous soft materials, wherein MOPs are connected through flexible N,N’-based linkers using
the peripheral axial positions of their constituent paddle-wheel units.4 Additionally, porous crystalline
salts have been assembled from the electrostatic interaction of oppositely charged MOPs. 5 In metal-
organic frameworks (MOFs), MOPs are used for the rational construction of highly-connected nets
following the well-known supermolecular building block (SBB) approach.6,7 In this approach, the in-situ
assembly of the MOP is traditionally harnessed to guide formation of a targeted MOF with a given
topology.8-13

An alternative, but far less-explored, route for the formation of MOFs is the use of pre-synthesized,
isolated MOPs as SBBs. Zhou and Chun first showed the feasibility of this route. They applied a similar
approach to that used for the synthesis of MOP-based soft materials but, instead of flexible N,N’-based
linkers, they used the rigid 4,4’-bipyridine (bipy) and 1,4-diazabicyclo[2.2.2]octane linkers to bridge Cu(II)-
and Zn(II)-based octahedral and cuboctahedral MOPs and form 6-connected (6-c) pcu and 12-c fcu MOFs,
respectively.14,15 Later, two more 12-c fcu MOFs made from the assembly of cuboctahedral MOPs with
bipy were reported.16,17 However, no one has yet demonstrated the ability to chemically control the
formation and isolation of MOPs with multiple peripheral coordinating sites (e.g. carboxylic acid groups),
and then use them as SBBs for the synthesis of MOFs. This is mainly due to the difficulty of controlling the
formation of the MOP, rather than extended coordination networks, when the precursor MOP linkers are
functionalized with additional coordinating groups; the challenge of preventing any cross-linking reactions
between MOPs that are functionalized with available coordinating sites;18,19 and the lack of stability and
solubility of the isolated MOPs under the conditions commonly used for MOF synthesis.20

Herein we report the design, synthesis and isolation of two types of polycarboxylate MOPs, and their
subsequent use as SBBs to build highly-connected bimetallic MOFs. We chose cuboctahedral Rh(II)-based
MOP as our platform for poly-carboxylate MOPs owing to its high stability, symmetry, and multiple sites
for functionalization with carboxylic acid groups.21,22 We began by synthesizing the two polycarboxylate
MOPs, each of which by a different route (Figure 1). Firstly, we synthesized a cuboctahedral MOP
functionalized with twelve carboxylic acid groups by using a coordinative route, in which the twelve
vertices of the cuboctahedron are functionalized by coordination of isonicotinic acid (HINA) in each
peripheral axial position of the twelve constituent Rh(II) paddle-wheel units (Figure 1, top). Next, we
prepared a cuboctahedral MOP functionalized with 24 carboxylic acid groups, using a covalent route
based on selective carboxylic group protection.23 In it, the 24 edges of the cuboctahedron are
functionalized with carboxylic acid groups using a protective synthetic strategy (Figure 1, bottom). 24 We
anticipated that the different number and location of carboxylic acid groups on the periphery of both
Rh(II)-MOPs, which define a 12-c cuboctahedral SBB and a 24-c rhombicuboctahedral SBB, could be used
to anticipate their structural outcome upon assembly with metallic secondary building units (SBUs). To
demonstrate the utility of these SBBs for constructing MOFs, we combined both a 12-c SBB with a paddle-
wheel 4-c SBU, and a 24-c SBB with a triangular 3-c Cu(II) SBU to construct atomically-precise bimetallic
MOFs having (4,12-c) ftw and (3,24-c) rht topologies, respectively.

                                                  -Figure 1-

We began with the coordinative synthesis of the 12-c cub-octahedral SBB. To this end, we first prepared
the cuboctahedral Rh(II)-MOP with formula [Rh2(OH-bdc)2(H2O)(DMA)]12 (hereafter named OH-RhMOP;
where OH-bdc is 5-hydroxy-1,3-benzenedicarboxylate and DMA is N,N-dimethylacetamide) following a
previously reported protocol.21 Then, the twelve DMA linkers coordinating each peripheral axial position
of their twelve paddle-wheel units were substituted with HINA linkers, through addition of 12 mol. eq. of
HINA to a DMF/methanol solution of OH-RhMOP. This reaction generated [Rh2(OH-bdc)2(H2O)(HINA)]12
(here-after named OH-RhMOP(HINA)12), as determined by UV−Vis spectroscopy and single-crystal X-ray
diffraction (SCXRD) (Figures S1-S2a and Figure 1,top). Note that, to prepare this SBB, instead of using an
N,N’-based linker that would connect the MOPs to form an amorphous soft material4 or a MOF with fcu
topology,25 we selected HINA because it bears both pyri-dine and carboxylic acid groups. These
coordinating groups enable HINA to coordinate to the peripheral axial positions of each paddle-wheel unit
through its pyridine moiety, while leaving the carboxylic acid group free, as it cannot replace either the
Rh(II)-pyridine or the Rh(II)-(OH-bdc) coordination bonds. Topologically, OH-RhMOP(HINA)12 forms a
cuboctahedral SBB with twelve available carboxylic acid groups (Figure 1,top).
Next, we synthesized the 24-c rhombicuboctahedral SBB via the covalent route. We began by preparing
the Rh(II)-MOP with formula [Rh2(COOH-bdc)2(H2O)(DMF)]12 (hereafter named COOH-RhMOP), following
our own previously reported protocol entailing the use of stoichiometric protecting groups. 24 Crystals of
COOH-RhMOP suitable for SCXRD were obtained by slow diffusion of diethyl ether vapors into a DMF
solution of this MOP. Formation of the cuboctahedral Rh(II)-MOP functionalized with 24 free carboxylic
acid groups on its edges was confirmed by SCXRD. Topologically, COOH-RhMOP can be considered as a
rhombicuboctahedral SBB with 24 available carboxylic acid groups (Figure 1,bottom).

Once isolated the 12-c and 24-c polycarboxylate Rh(II)-MOPs, we then evaluated their use as SBBs in the
construction of highly-connected bimetallic nets, by reacting them with Cu(II)-based SBUs. We considered
that 24-c SBB geometry/connectivity is not compatible with square planar nodes (4-c Cu paddle-wheel
clusters) to form a (4,24)-c net, as the only binodal topology comprising 24-c nodes of
rhombicuboctahedra directionality is the edge-transitive, (3,24)-c rht net. It is known that, in presence of
geometry mismatch,26 certain clusters have tendency to undergo structural changes and form
unprecedented clusters, including Cu(II). Therefore, we confidently challenged the classical paddle-wheel
formation in the presence of our COOH-MOP, aiming for the discovery of a novel 3-c node comprising 3
carboxylates, distinct from the tetrazolate based trimer commonly used for the formation of rht-MOFs.11
Thus, following this SBB principle, we first assessed our polycarboxylate 24-c rhombicuboctahedral MOP
for the formation of a bimetallic RhCu-rht-MOF. Solvothermal reaction of COOH-RhMOP with copper(II)
nitrate in DMF/ethanol (3:1) at 85 °C for 24 h afforded RhCu-rht-MOF, in which each Rh(II)-MOP is linked
through an in-situ-synthesized triangular 3-c Cu(II) SBU (Figure 2). This SBU of formula Cu3O(OOC)3
comprises three Cu(II) ions that are interconnected through three bridging bidentate carboxylate moieties
and one central oxygen atom (Figure 2a and S3).

Ideally, the structure of RhCu-rht-MOF is a MOP-based three-dimensional (3D) framework in which each
Rh(II)-MOP unit is connected to twelve neighboring polyhedra through the 24 available carboxylate
groups (Figures 2 and S4). With this connectivity, we would expect a Rh/Cu ratio of 1:1, meaning that a
positively charged (+12) framework should be formed. However, EDX analysis indicated a Rh/Cu molar
ratio of 1:0.3. Remarkably, the amount of Cu(II) in these crystals increased after 1 week of incubation in a
concentrated solution of copper(II) acetate in DMF/EtOH. Afterwards, both EDX and the copper
occupancy factor number (extracted from the SCXRD structure) revealed a Rh/Cu molar ratio of 1:0.75
(Figure S10), which is identical to the molar ratio for the bulk sample that we had synthesized in water
(Figures S11-15). Altogether these values suggested that only eighteen carboxylic acid groups from each
Rh-MOP SBB were coordinating to Cu(II) ions to afford neutrality to the framework, leading to charge-
induced Cu(II) defects in the overall structure and giving a formula for RhCu-rht-MOF of
[Rh24(btc)24Cu18O6].

                                                 -Figure 2-

The overall framework of RhCu-rht-MOF comprises three different types of microporous cages (A-C,
Figure S5). Cage A is a cuboctahedron composed of 24 isophthalate units from btc linkers and twelve Rh2
paddle-wheels, with an inner sphere diameter of ca. 15.0 Å (Figure S6). Each triangular window in cage A
is shared with another cage, which can be defined as a truncated tetrahedron (t-Td, Cage B). Cage B is
formed by four Cu3O trimers (one in each face of the t-Td) and twelve Rh2 paddle-wheels, and has an inner
sphere diameter of ca. 12.0 Å (Figure S7). Finally, cage A connects with 6 truncated octahedra (t-Oh, cage
C) through its square-shaped windows. Cage C comprises eight Cu3O trimers (one in each face of the t-
Oh) and 24 Rh2 paddle-wheels and has an inner sphere diameter of ca. 16.8 Å (Figure S8). Thus, the overall
network in RhCu-rht-MOF is constructed by the packing of cages A, B and C at a ratio of 1:2:1, respectively.
This generates an open framework in which the total accessible volume is 71 %.27

Once stable polycarboxylate MOPs are isolated, then they can rationally be connected to other well-
known metallic SBUs as a strategy to increase the variety of accessible hierarchical highly-connected nets.
Accordingly, we sought to connect our 12-c OH-RhMOP(HINA)12 with a 4-c paddle-wheel SBU to guide the
formation of an edge-transitive net exhibiting an underlying (4,12-c) ftw topology (Figure 3). To this end,
we slowly diffused DMF/methanol solutions of stoichiometric amounts of HINA-OHRhMOP and Cu(NO3)2
in an H-shaped tube standing at RT for 3 weeks. This yielded a homogenous crystalline sample suitable for
SCXRD analysis. In this structure, hereafter called RhCu-ftw-MOF-1, each Rh(II)-MOP is linked through 4-
c Cu(II) paddle-wheel SBUs to eighteen other neighboring Rh(II)-MOPs, adopting the expected (4,12)-c ftw
underlying topology (Figures 3 and S16). Note that the topology can be alternately regarded as the (4,5)-
c xae net, considering the two types of paddle-wheels (Rh, 5-c and Cu, 4-c) SBUs as nodes. RhCu-ftw-
MOF-1 shows three different types of cages (D-F). Cage D is a cuboctahedron composed of 24 isophthalate
units from OH-bdc linkers and twelve Rh2 paddle-wheels, with an inner sphere diameter of ca. 15.0 Å
(Figure S18). Each triangular window in cage D is shared with cage E, which can be defined as a
rhombicuboctahedron. Cage E is formed by six Cu2(OOC-py)4 and 24 Rh2 paddle-wheel units, and has an
inner sphere diameter of ca. 18.4 Å (Figure S19). Each square window in cage D is connected with six
prismatic cages (cage F) that fill the gaps between cages D and E. Cage F is formed by eight Rh 2 paddle-
wheels and has an inner sphere diameter of ca. 11 Å (Figure S20). This structure leads to an open
framework in which the total accessible volume is 72 %.27

Interestingly, the larger cages E of RhCu-ftw-MOF-1 are decorated with the outer OH groups of the initial
OH-RhMOP(HINA)12 SBB (Figure 3d). This feature paves the way to using the external functionalization of
isolated MOP SBBs to modulate the pore functionalization of the resulting MOFs. We proved this concept
by synthesizing an identical SBB in which we substituted the outer OH groups of the OH-RhMOP(HINA)12
with H atoms. Accordingly, we followed the aforementioned coordinative route except that the Rh(II)-
MOP was prepared with bdc instead OH-bdc (see SI). Then, by combining the new SBB with Cu(II) ions, we
prepared the isostructural RhCu-ftw-MOF-2, in which we decorated the E cages with H atoms instead of
OH groups (Figures S22, S27-28, S31-32).

The three MOFs that we have presented here exhibit reversible transformations between crystalline and
amorphous states upon solvent guest removal and upon immersion in either water (for RhCu-rht-MOF;
Figure S15) or methanol (for both RhCu-ftw-MOFs; Figures S23-24).28 However, unlike most flexible MOFs
and coordination polymers, which lose their porosity upon activation-triggered amorphization,
amorphous RhCu-rht-MOF and both RhCu-ftw-MOFs exhibit permanent porosity. We attributed this
feature to the fact that, similarly to the case of previously reported amorphous soft porous materials, 29
amorphization does not entail collapse of the Rh(II)-MOPs that remain accessible upon activation. Indeed,
gas-sorption measurements revealed that all three MOFs are microporous to N2, showing BET areas (ABET)
of 700 m2/g (RhCu-rht-MOF), 400 m2/g (RhCu-ftw-MOF-1) and 680 m2/g (RhCu-ftw-MOF-2) (Figures S33,
S35-36). Moreover, CO2 uptakes at 1 bar and 298 k were 2.5 (RhCu-rht-MOF) 1.1 (RhCu-ftw-MOF-1) and
1.3 mmol/g (RhCu-ftw-MOF-2) (Figures S34, S37).

In summary, we have reported the synthesis and structure of two types of robust polycarboxylate Rh-
MOPs, each by a different route. We demonstrated that once isolated, these MOPs can be used as SBBs
to build highly-connected MOFs through coordination of their available peripheral carboxylate groups
towards metallic SBUs. The well-located and highly-directional carboxylate groups in Rh-MOPs allow to
predict their assembly by means of reticular chemistry. It also allows introducing with atomic precision
the elusive Rh(II) paddle-wheel into highly connected MOF structures. We believe that the results here
presented will enrich the repertoire of strategies for synthesis of MOFs, including those incorporating
inert metal ions.30

Acknowledgements

This work was supported by the Spanish MINECO (project RTI2018-095622-B-I00) and the Catalan AGAUR
(project 2017 SGR 238). It was also funded by the CERCA Pro-gram/Generalitat de Catalunya. ICN2 is
supported by the Severo Ochoa program from the Spanish MINECO (Grant No. SEV-2017-0706). A.C.S. and
T.G. thank the Spanish MINECO for Juan de la Cierva fellowships (IJCI-2016-29802 and FJCI-2017-31598).
F.G. acknowledges funding from the Spanish Research Agency (AEI, CTQ2017-87262-R, EUR2019-103824).
The project that gave rise to these results received the support of a fellowship (LCF/BQ/PR20/11770011)
from ”la Caixa” Foundation (ID 100010434).

Keywords: coordination chemistry • metal-organic framework • metal-organic polyhedra • rhodium •
self-assembly

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Figure 1. Schematic illustration showing both coordinative (top) and covalent (bottom) routes to
polycarboxylate Rh-MOP SBBs. CCDC numbers of OH-RhMOP(HINA)12 and COOH-RhMOP can be found in
the Supporting Information.
Figure 2. a) Fragments of the structure of RhCu-rht-MOF, highlighting the connectivity of COOH-RhMOPs
(24-c SBB) through 3-c Cu3 unit. b) Structure of RhCu-rht-MOF, highlighting the inner cavity of COOH-
RhMOP (blue) and the cavity C (pink). c) Augmented representation of the rht topology. CCDC number
of RhCu-rht-MOF can be found in the Supporting Information.
Figure 3. a) Fragments of the structure of RhCu-ftw-MOF-1, highlighting the connectivity of OH-
RhMOP(HINA)12 (12-c SBB) through 4-c Cu2 paddle-wheel unit. b) Structure of RhCu-ftw-MOF-1,
highlighting the inner cavity of HINA-OHRhMOP (blue) and the cavity E (orange). c) Illustration of the ftw
topology. d) Outer functionalization of the starting Rh-MOP (pink spheres) decorates the inner surface of
cage E in the resulting MOF. CCDC number of RhCu-ftw-MOF-1 can be found in the Supporting
Information.