molecules Article A Reversible Phase Transition of 2D Coordination Layers by B–H···Cu(II) Interactions in a Coordination Polymer Lei Gan 1 , Pol G. Fonquernie 1 , Mark E. Light 2 , Gantulga Norjmaa 3 , Gregori Ujaque 3 , Duane Choquesillo-Lazarte 4 , Julio Fraile 1 , Francesc Teixidor 1 , Clara Viñas 1 and José G. Planas 1, * 1 Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), 08193 Bellaterra, Spain 2 Department of Chemistry, University of Southampton, Highfield, Southampton SO17 1BJ, UK 3 Departament de Química, Universitat Autònoma de Barcelona, 08193 Cerdanyola del Valles, Catalonia, Spain 4 Laboratorio de Estudios Cristalográficos, IACT, CSIC-Universidad de Granada, Av. de las Palmeras 4, E-18100 Armilla, Granada, Spain * Correspondence: jginerplanas@icmab.es Academic Editors: Igor B. Sivaev, Narayan S. Hosmane and Bohumír Grűner   Received: 19 July 2019; Accepted: 1 September 2019; Published: 3 September 2019 Abstract: Materials that combine flexibility and open metal sites are crucial for myriad applications. In this article, we report a 2D coordination polymer (CP) assembled from CuII ions and a flexible meta-carborane-based linker [Cu2 (L1)2 (Solv)2 ]•xSolv (1-DMA, 1-DMF, and 1-MeOH; L1: 1,7-di(4-carboxyphenyl)-1,7-dicarba-closo-dodecaborane). 1-DMF undergoes an unusual example of reversible phase transition on solvent treatment (i.e., MeOH and CH2 Cl2 ). Solvent exchange, followed by thermal activation provided a new porous phase that exhibits an estimated Brunauer-Emmett-Teller (BET) surface area of 301 m2 g−1 and is capable of a CO2 uptake of 41 cm3 g−1 . The transformation is reversible and 1-DMF is reformed on addition of DMF to the porous phase. We provide evidence for the reversible process being the result of the formation/cleavage of weak but attractive B–H···Cu interactions by a combination of single-crystal (SCXRD), powder (PXRD) X-ray diffraction, Raman spectroscopy, and DFT calculations. Keywords: metal-organic frameworks; open metal sites; boron hydrides; carborane; reversible phase transition 1. Introduction Porous Coordination Polymers (CPs) or Metal-Organic Frameworks (MOFs) are a class of porous crystalline materials formed by the assembly of metal ions or metal clusters with different types of bridging organic linkers or ligands [1–4]. Open metal sites (OMS) in these materials, where Lewis base molecules can coordinate or interact, are known to play an important role in a variety of applications, ranging from catalysis [5–7] to molecules storage [8,9], separation [10–12], or sensing [13,14]. Such interactions often involve molecules including H2 O, CO, CO2 , H2 , CH4 , N2 , or H2 among others and undergo dynamic processes that are difficult to probe directly by spectroscopic experiments such as infrared or Raman spectroscopy [15–17]. Among the MOFs that can easily generate OMS, Cu(II)-based paddlewheel MOFs have been extensively studied and exhibit structural diversity and high porosity [18–20]. Solvent molecules bound to the apical position of the Cu2 -paddlewheel motif (vide infra) can be removed to generate unsaturated Cu(II) OMS. Among the various strategies for MOFs activation (removal of pore-filling and metal coordinated solvent), only thermal activation, which consists of applying heat energy and vacuum, has so far been able to dissociate coordinating Molecules 2019, 24, 3204; doi:10.3390/molecules24173204 www.mdpi.com/journal/molecules Molecules 2019, 24, 3204 2 of 18 solvent and generate OMS [15,21,22]. In a series of papers [23–26], N. C. Jeong and coworkers have nicely demonstrated that the Cl atoms of dichloro- or trichloro methane can weakly coordinate to the open metal sites of well-known Cu(II) MOFs such as HKUST-1 (Copper benzene-1,3,5-tricarboxylate) or Cu-MOF-2 (Copper 1,4-benzene-dicarboxylate). Such weak coordination of chloromethanes leads to what the authors termed a chemical route to active open metal sites by removing coordinating solvent. They proved the ability of chloromethanes to remove precoordinated solvent molecules from OMS, by in situ Raman spectroscopy, and therefore leading to MOF activation without the need for thermal activation. Such a mild activation in solution opens new avenues for potential applications of MOFs [24,27]. Icosahedral boranes and carboranes ([B12 H12 ]2− and 1,n-C2 B10 H12 (n = 2,7 or 12); Scheme 1) are an interesting class of commercially available and exceptionally stable 3D-aromatic boron-rich clusters that possess material-favorable properties such as thermal and chemical stability and hydrophobicity [28–30]. The neutral carboranes are remarkably robust boron clusters with two carbon atoms and possess 26 electrons for 12 vertices. The delocalized electron density is not uniform through the cage, giving rise to extraordinary differences in the electronic effects of the cluster [31]. This unusual electronic structure is often highlighted by regarding carboranes as inorganic three-dimensional “aromatic” analogs of arenes [32]. Such properties make icosahedral carborane clusters valuable ligands for CPs or MOFs. For example, some of us have reported the synthesis and electronic and magnetic properties of purely inorganic CPs based on the monocarboxylic acid of ortho-carborane [33–35]. Mirkin and co-workers explored the use of di-, tri- and tetra-carboxylic acid derivatives of para-carborane (I to IV in Scheme 1) for CPs synthesis, providing a series of CPs exhibiting unprecedented stabilities with respect to thermal degradation, inherited from the carborane moiety [36–42]. Jin and co-workers also constructed CPs based on the dicarboxylic acid derivatives of para- but also of meta-carborane linkers (V in Scheme 1), and studied their adsorption and luminescence properties [43,44]. Dicarboxylic and tricarboxylic derivatives of the smaller carborane closo-1,10-C2 B8 H10 were also incorporated into porous CPs [38,45]. We have recently designed flexible carborane-based ligands for dynamic MOFs (VI and VII in Scheme 1) [46,47]. In addition to water stability [47], the spherical shape of the carborane moiety in these flexible linkers seems to have a noticeable influence in the dynamic behavior of the MOFs. Thus, we previously reported an unusual reversible 3D to 2D transformation of a Cobalt-based MOF, incorporating a carborane-based dipyridine ligand [48]. We argued that contrary to classical flat aromatic ligand-based MOFs, spherical-shape linkers such as carboranes, can access extensive conformational space by a combination of low-energy torsion of the substituents and by the spherical core of the ligand. Such spherical carborane-based ligands can facilitate multiple supramolecular contacts that are of a different nature to those found in conventional planar carbon-based ligands. These include B–H···A (A = H [49–51], I [52], π [53,54]) interactions or B–H···Metal [55–62] (agostic) interactions in molecular systems. The latter type of interaction is well documented in icosahedral cages and is considered a preliminary step in many B–H activation reactions and in hydrogenation or hydroboration [62–64]. The hydridic nature of the H atoms in boranes and carboranes make these BH moieties suitable for metal coordination [65,66]. During the preparation of this manuscript, it has been reported a first example of a B–H···Cu(II)-based MOF from [B12 H12 ]2− [67]. Here we report the synthesis of a new family of Cu-paddlewheel-based 2D coordination polymers incorporating a dicarboxylate m-carborane ligand [Cu2 (L1)2 (Solv)2 ]•xSolv (L1: 1,7-di(4-carboxyphenyl)-1,7-dicarba-closo-dodecaborane; Scheme 1) and an unprecedented reversible phase transition through formation/cleavage of a weak but attractive B–H···Cu(II) interactions. We provide evidence for the observed reversible process by a combination of single-crystal (SCXRD), powder (PXRD) X-ray diffraction, Raman spectroscopy and DFT calculations. This reversible transformation is achieved by solvent-guest exchange under ambient conditions in one direction and in DMF solutions at room temperature in the other. The transformation is mediated by B–H···Cu(II) interactions, when generating open metal sites during solvent-exchange. Molecules 2019, 24, 3204 3 of 18 Molecules 2019, 24, x FOR PEER REVIEW 3 of 18 Scheme 1.1. Graphical Graphical representation representation of ofthe thecarborane carboraneisomers isomers(closo-C (closo-C2 B 10H12 2B10 12)) and their derivatives derivatives used as linkers to form CPs. 2. Results and 2. Results and Discussion Discussion Reaction Reaction of of Cu(NO Cu(NO33))22,, and and L1 L1 in in dimethylformamide dimethylformamide (DMF), (DMF), dimethylacetamamide dimethylacetamamide (DMA) (DMA) or or methanol (MeOH) at 80 ◦ C for 48 h afforded greenish crystals for methanol (MeOH) at 80 C for 48 h afforded greenish crystals for [Cu2(L1)2(Solv)2]•xSolv (1-Solv = 1- o [Cu 2 (L1) 2 (Solv) 2 ]•xSolv (1-Solv = 1-DMF, 1-DMA or 1-MeOH) in good yield. IR spectrum showed DMF, 1-DMA or 1-MeOH) in good yield. IR spectrum showed the characteristic broad B-H stretching the characteristic broad B-H stretching bands −1 ), and the C=O vibration of the carboxylate bands from from the the carborane carborane (in (in the the range range2617–2531 2617–2531cm cm−1), and the C=O vibration of the carboxylate groups (Figure S1). groups (Figure S1).Single-crystal Single-crystalX-ray X-ray diffraction diffraction (Figure (Figure 1 and 1 and Tables Tables S1 and S1, S2) S2) revealed revealed a 2D networka 2D network for all the compounds. Phase purity was confirmed for all the compounds. Phase purity was confirmed by elemental analysis and powder X-ray by elemental analysis and powder X-ray diffraction diffraction (PXRD) (PXRD) for for 1-DMF 1-DMF and and 1-DMA 1-DMA (Figure (Figure S2). Crystals for S2). Crystals for 1-MeOH 1-MeOH were were onlyonly stable stable in in the the solvent and a rapid phase transition was observed when the crystals solvent and a rapid phase transition was observed when the crystals were dried in air (vide infra). were dried in air (vide infra). The The basic basic unit unit ofof 1-Solv 1-Solv isis aa Cu Cu22-paddlewheel -paddlewheel motif motif ofof [Cu [Cu22(COO) (COO)44]] units units (Figure (Figure 1). The Cu–Cu 1). The Cu–Cu distances distances in in the the paddlewheel paddlewheel units units are are 2.647, 2.647, 2.642 2.642 and and 2.620 2.620 Å Å forfor 1-DMA, 1-DMA, 1-DMF, 1-DMF, and and 1-MeOH, 1-MeOH, respectively. respectively. The two copper atoms share four L1 linkers at the basal positions and one oxygen atom The two copper atoms share four L1 linkers at the basal positions and one oxygen atom from from the solvent (DMA, DMF or MeOH for 1-DMA, 1-DMF, and 1-MeOH, respectively) occupy the the solvent (DMA, DMF or MeOH for 1-DMA, 1-DMF, and 1-MeOH, respectively) occupy the apical apical positions positions (Figure (Figure 1A). Cu–OOC and 1a). Cu–OOC and Cu–O Cu–Osolv bondlengths solvbond lengthsrange range fromfrom 1.952 1.952 to to 1.971 1.971 Å Å and and 2.081 2.081 toto 2.137 2.137 Å, Å, respectively. respectively. The The carborane carborane L1 L1 linker linker shows shows aa V-shape (OOC–CBcentroid–COO ≈≈ V-shape(OOC–CBcentroid–COO 111–115 ◦ ◦ 111–115°)) and and twotwo noncoplanar noncoplanar phenyl phenyl ringsrings (78–87 (78–87°). ). Thus, Thus, the the bent bent L1 L1 linkers linkers and and noncoplanar noncoplanar phenyl rings in 1-Solv adopt a 4 4 -grid topology by bending alternately above and below the plane phenyl rings in 1-Solv adopt a 4 -grid topology by bending alternately above and below the plane 4 containing containing the the paddlewheel paddlewheel [Cu [Cu22(COO) (COO)4]4 ]units unitstotoproduce producea avery verycorrugated corrugated 2D2D layers. layers. AsAs shown shown in in Figure 1b, the 2D 4 4 net consists of chair-like units, similar to that found in other related V-shape Figure 1b, the 2D 4 net consists of chair-like units, similar to that found in other related V-shape 4 ditopic linkers [68–70]. ditopic linkers [68–70].TableTable1 1summarizes summarizes thethe parameters parameters forforeach each of the of the structures structures in this in this workwork and and in comparison with some related structures where there in comparison with some related structures where there is a 1,3-benzene moiety instead of a m- is a 1,3-benzene moiety instead of a m-carborane unit [68–70]. In each 4 4 unit instructures, our structures, two consecutive carborane clusters are carborane unit [68–70]. In each 44 unit in our two consecutive carborane clusters are above above the plane containing the paddlewheel the plane containing the paddlewheel [Cu2(COO)4]2 units, and [Cu (COO) 4 ] units, and the other two the other two are below the plane. Theare below the plane. The corrugated layers in 1-Solv are thick corrugated layers in 1-Solv are thick (~15-16 Å, Table 1) and as can (~15-16 Å, Table 1) be and as can seen be seen in Figure 1b,inthey Figure 1B, create they create nanoscale channels (see colored dotted rectangles in Figure nanoscale channels (see colored dotted rectangles in Figure 1b). A more detailed analysis of the 1B). A more detailed analysis of the structures shows that the nanoscale channels are formed by the structures shows that the nanoscale channels are formed by the particular bridging coordination of particular bridging coordination of the the V-shape V-shape LLCB (COO) linker. Each carborane linker bridges two different paddlewheel units in a CB(COO)22 linker. Each carborane linker bridges two different paddlewheel units in a way way that result in aa single-strand that result in single-strand helix helix chain chain (Figure (Figure 1),1), generating generating the the above-mentioned above-mentioned channels. channels. These 1D helical chains are alternately connected with each other through the paddlewheel units, constructing the observed 2D networks. The handedness of the 21 helicity for all descriptions in this Molecules 2019, 24, 3204 4 of 18 These 1D helical chains are alternately connected with each other through the paddlewheel units, constructing the observed 2D networks. The handedness of the 21 helicity for all descriptions in this paper are determined using a method similar to the supramolecular tilt chirality method (STC) [71]. In brief, given a helix in front of the 21 screw axis inclining to the right, the assemblies can be defined to be right-handed, or inclining to the left are defined to be left-handed. Quite interestingly, the pitches of the helixes are different in each of the new structures. The pitch decreases in the order 1-DMA (16.2) > 1-DMF (14.4) > 1-MeOH (12.5) and correlate with the shorter of the two diagonal paddlewheel to paddlewheel distances in each 44 chair-like unit (Table 1). Such changes in the pitch suggest that a single 2D sheet would stretch or shrink, depending on the solvent. The analysis of data in Table 1 reveals that the larger diagonal paddlewheel to paddlewheel distances in each 44 chair-like unit vary between 28–30 Å, whereas the variation for the shorter diagonal distances is much smaller (13–20 Å). Interestingly, the corresponding distances for the non-carborane containing CPs, e.g., those having a 1,3-benzene moiety instead of a m-carborane unit are longer (ca. 20 × 28 Å) [68–70]. A shorter m-carborane dicarboxylic ligand (1,7-dihydroxycarbonyl)-1,7-dicarba-closo-dodecaborane) provided related Cu-Paddlewheel-based 2D structures with smaller 44 nets and shorter diagonal paddlewheel to paddlewheel distances (13–16 Å; Table 1) [44]. As mentioned above, the shorter of the two diagonal paddlewheel to paddlewheel distances in each 44 chair-like unit corresponds to the pitch of the single-strand helix chains in the structures (Figure 1B). Larger pitches (or diagonal distances) for the non-carborane CPs favor interpenetration in all structures reported in the literature [68–70]. However, no interpenetration is observed for the structures reported in this work. As mentioned above, the pitch in 1-Solv increases with the size of the solvent molecules. The latter is followed by a concomitant decrease in the layer thickness, as it happens, for example, when one stretches or compresses a coil. Thus, the 1-DMA and 1-DMF layer thicknesses are 15.4 and 15.0 Å, respectively, whereas that for 1-MeOH is 16.3 Å. However, this does not seem to affect the interlayer distances as these are all very close (Table 1) and similar to other related 2D Cu(II) coordination polymers [72]. Stacking of the 2D layers gives rise to the 3D structures of the compounds. A comparison of the 3D structures shows only lateral displacements or sliding of the 2D layers as shown in Figure 2. Layers in 1-DMA and 1-DMF arrange in a way to maximize the nanoscale channels, whereas those in 1-MeOH are occluded. Table 1. Selected crystal parameters for compounds in this work and related 2D structures. 44 nets Layer Interlayer Coordination Polymer SGa Db Diagonal P (Å)d Thicknesse Distances (Å) Distances (Å)c (Å) 1-DMA (this work) P-1 1.262 16.2 × 29.0 16.2 15.41 6.94 1-DMF (this work) P-1 1.355 14.4 × 29.4 14.4 14.95 6.86 1-MeOH (this work) P21 /c 1.367 12.5 × 29.8 12.5 16.29 7.06 PELFOEf C2/c 1.448 19.7 × 27.6 19.7 - interpenetrated ([CuL1 ·DMA]DMF) MAPLATf C2/c 1.353 19.9 × 27.6 19.9 - interpenetrated ([CuL1 ·DMA]H2O) OFEHOYf C2/c 1.245 19.6 × 27.7 19.6 - interpenetrated ([CuL2 ·DMF]DMF) NEYRIUf C2/c 1.261 12.8 × 13.8 12.8 10.313 10.187 ([CuL3 ·MeOH]MeOH) NEYREQf P21 /n 12.7 × 14.0 12.7 9.827 10.454 ([CuL3 ·MeOH]DMF) a Space groups. b Calculated density. c Paddlewheel to paddlewheel distances in each in each 44 chair-like unit. d Pitches of the 21 helices. e Measured from H atom to H atom in B-H bonds; See also Figure 2. f CSD Refcodes. L1 : 1,3-di(4-carboxyphenyl)benzene; L2 : 50 -methyl-1,10 :30 ,1”-terephenyl-4,4”-dicarboxylic acid; L3 : 1,7-dicarba-closo-dodecaborane-1,7-dicarboxylato. Molecules 2019, 24, 3204 5 of 18 Molecules 2019, 24, x FOR PEER REVIEW 5 of 18 Figure 1. A Figure 1. comparative A comparative view of the view crystal of the structures crystal of 1-DMA, structures 1-DMF, of 1-DMA, and and 1-DMF, 1-MeOH. 1-MeOH.(a) View of (A) View theofCu2-paddlewheel units with L1 coordination. (b) Two perpendicular views of the Cu2-paddlewheel units with L1 coordination. (B) Two perpendicular views of the extended the extended structures showing structures showing thethe 2D2D44 networks; 1D1D 44 networks; helical chains helical with chains right with hand right (green) hand or left (green) hand or left (orange) hand (orange) areare indicated indicatedforfor 1-DMF. 1-DMF.H atoms H atomsareare omitted omittedforfor clarity. Color clarity. code: Color B pink; code: C grey; B pink; C grey;O red; N blue, O red; N blue, CuCuorange. orange. Thermogravimetric Thermogravimetric analysis analysis of of 1-DMA 1-DMA and 1-DMF and 1-DMF from fromroomroom temperature temperature to to 600600°C◦shows C shows a a continuous continuous drop drop until 400400 until °C◦(Figure C (Figure S3), so so S3), that evaporation that evaporation of of thethe free and free andcoordinated coordinated solvent solvent cannot cannot bebe distinguished distinguished fromfromthethedecomposition decomposition of of thetheframework, framework, making making thethe thermal thermal activation activation difficult. difficult. Thus,Thus,solvent exchange solvent exchange was carried was carried outout successively successively withwith methanol methanol (MeOH; (MeOH; 2 days) 2 days) and and dichloromethane dichloromethane (CH 2Cl2 (CH for2 for 2 Cl 2 days; see Experimental 2 days; see Experimental for details) for 1-DMF. for details) Treatment for 1-DMF. Treatmentof large 1- of large DMF 1-DMF crystals withwith crystals MeOH MeOH showed showed a clear macroscopic a clear macroscopic transformation transformation in the sizesize in the andand color of of color thethe crystals crystals to togive givea new a new phase phase 10 -MeOH 1′-MeOH (Figure (Figure 3 and 3 andvideos videos ininthethe supporting supporting information). information). Large Large platelets platelets broke brokeinto small into smallpieces, pieces,followed followed withwitha color change a color changefromfrom greenish greenish to sky to skyblue (Figure blue (Figure3b).3b). Immersion Immersion of 10 -MeOH of 1′-MeOH crystals in CH crystals Cl2 2provoked in 2CH Cl2 provoked a color change a color from from change sky blue skyto deep blue to navy deep (1′- navy CH 0 -CH (12Cl 2), 2 consistent with the Cl2 ), consistent withpresence of OMS the presence of OMS[24] [24] but nobutfurther change no further changeof morphology of morphology of crystals of crystals was was observed observed (Figure (Figure3c). In In 3c). contact contact with with air, air,deep deepnavy navycolored coloredcrystalline 10 -CH crystalline1′-CH 2Cl 2 turns 2 Cl 2 turns within within seconds seconds into sky into skyblue color. blue TGA color. TGA curves curves forfor airair dried driedsamples samples ofof 0 1′-MeOH-air 1 -MeOH-air and and 10 -CH 1′-CH 2ClCl 2-air show -air show 2 2 very similar profiles that are significantly different to that for 1-DMF; 1′-MeOH-air and 1′-CH2Cl2-air show a three-step loss, one at 75-98, 280 and another at 350 °C (Figure S4). Molecules 2019, 24, 3204 6 of 18 very similar profiles that are significantly different to that for 1-DMF; 10 -MeOH-air and 10 -CH2 Cl2 -air show a three-step loss, one at 75-98, 280 and another at 350 ◦ C (Figure S4). Molecules 2019, 24, x FOR PEER REVIEW 6 of 18 Molecules 2019, 24, x FOR PEER REVIEW 6 of 18 Figure Figure2.2. View Viewofof 2.View the ofthe packing packing the structures structures packing for for structures for 1-Solv 1-Solv1-Solv = DMA, (Solv(Solv (Solv == DMA, DMF,DMF, DMA, and and MeOH) and MeOH) DMF, showing MeOH) showing the the staking showing the staking of 2D of layer 2D layer structures structures in which in the which various the various colors colors differentiates differentiates the contiguous staking of 2D layer structures in which the various colors differentiates the contiguous layers.the contiguous layers. layers. Coordinated Coordinated solvent Coordinated solvent molecules solventaremolecules omitted are molecules are omitted for clarity. for omittedLayerfor clarity. Layer thickness clarity. thickness Layer(a) (a) (a) and and Interlayer and Interlayer thickness distance distance (b) Interlayer (b) are only distance (b) are only represented represented in 1-DMA. in 1-DMA. See Table See 1 Table for values1 for forvalues all thefor all the structures. are only represented in 1-DMA. See Table 1 for values for all the structures. structures. Figure Figure 3. Figure 3. Left: 3. Left: Photographs Photographs Photographsand and SEM andSEM images SEMimages showing imagesshowing crystals showingcrystals color crystalscolor and colorand morphology andmorphology morphology of as ofof made asas made made 1- 1- DMF (a), DMF (a), 1-DMF after (a),after 2 days after22days in methanol daysininmethanol (b) methanol(b) and (b)and after andafter 2 days after22days in daysin dichloromethane indichloromethane (c), in the solvent dichloromethane (c), in the solvent and and after and after after being being exposed being exposed to exposed to air. to air. Right: air. Right: PXRD Right: PXRD patterns PXRD patterns for patterns for 1-DMF, for 1-DMF,11′-MeOH-air, 1-DMF, and 0 -MeOH-air, and 1′-MeOH-air, 0 -CH22Cl and11′-CH 1′-CH Cl22-air. -air. -air. 2 2 Remarkably, Remarkably, PXRD PXRD datadata for for 1′-MeOH-air 1′-MeOH-air andand 1′-CH 1′-CH22Cl Cl22-air -air show show that that both both correspond correspond to to the the same same phase and those are markedly different to that for 1-DMF (Figure 3). The PXRD data is phase and those are markedly different to that for 1-DMF (Figure 3). The PXRD data is consistent consistentwith withaaphase phasetransition transitiontriggered triggeredby byimmersing immersing1-DMF 1-DMFin inMeOH, MeOH,the thenew newcrystalline crystallinephase phase remaining remaining stable stable in in CHCH22Cl Cl22.. Quite Quite interestingly, interestingly, the the PXRD PXRD pattern pattern for for 1′-MeOH-air 1′-MeOH-air clearly clearly differs differs from from that for the calculated pattern of 1-MeOH (Figure S5). As mentioned above, crystalsfor that for the calculated pattern of 1-MeOH (Figure S5). As mentioned above, crystals for1-MeOH 1-MeOH were were only only stable stable in in the the solvent solvent andand aa rapid rapid phase phase transition transition was was observed observed when when the the crystals crystals were were Molecules 2019, 24, 3204 7 of 18 Remarkably, PXRD data for 10 -MeOH-air and 10 -CH2 Cl2 -air show that both correspond to the same phase and those are markedly different to that for 1-DMF (Figure 3). The PXRD data is consistent with a phase transition triggered by immersing 1-DMF in MeOH, the new crystalline phase remaining stable Molecules in2019, CH224, Clx2 FOR . Quite PEER interestingly, REVIEW the PXRD pattern for 10 -MeOH-air clearly differs from that 7 offor 18 the calculated pattern of 1-MeOH (Figure S5). As mentioned above, crystals for 1-MeOH were only dried in stable in air; the multiple solvent and single crystal a rapid measurement phase transitiontrials showed a when was observed fast change of symmetry the crystals were dried on solvent in air; loss. Thesingle multiple new symmetry appears to trials crystal measurement be a centered showed aversion fast changeof theoforiginal symmetry cellonwith a doubling solvent loss. The of the newa parameter, diffraction was too poor to yield more than a rough symmetry appears to be a centered version of the original cell with a doubling of the a parameter, backbone structure with very high R values. PXRD diffraction wasof the too latter poor to crystals yield more exposed than atorough air confirmed backbonethat 1-MeOH structure withtransformed very high Rinto values.1′-MeOH- PXRD airthe of (Figure latterS5). Unfortunately, crystals exposed towe aircould not obtain confirmed that the1-MeOHSCXRD structure ofinto transformed 10 -MeOH-air either 1′-MeOH-air or 1′- (Figure CH2Cl S5). 2-air even bywe Unfortunately, using could synchrotron not obtain theradiation SCXRDmeasurements. structure of either 0 Nevertheless, 1 -MeOH-air elemental 0 or 1 -CHanalyses 2 Cl2 -air correlate even by using with 1′-MeOH-air synchrotron andmeasurements. radiation 1′-CH2Cl2-air Nevertheless,being [Cu2elemental (L1)2(H2O)(2MeOH)] analyses correlate [73] withand 0 -MeOH-air 1[Cu 2(L1)2(H2O)(CH and 102-CHCl2)],2 Cl respectively. -air being Overall [Cu (L1) the(H data suggest O)(2MeOH)] the successive [73] and [Cu exchange (L1) (H of O)(CHCu-bound 2 2 2 2 2 2 2 2 2 )], Cl DMF by MeOH respectively. and the Overall thendata CHsuggest 2Cl2 to provide the deep the successive navy colored exchange 1′-CH2Cl of Cu-bound DMF2-wet byhaving MeOHOMS and then(see Raman CH Cl discussion to provide bellow), the deep that navy on exposure colored 1 0 -CH to air Cl rapidly -wet changes having OMS to (seesky blue. Raman As mention discussion in the bellow), 2 2 2 2 introduction, such chemical route to active OMS species have that on exposure to air rapidly changes to sky blue. As mention in the introduction, such chemical been widely investigated and showed that owing route to active to OMS the weakspeciesLewis havebasicity of CHinvestigated been widely 2Cl2, it spontaneously and showed dissociated that owingfrom to the theweakmetal and Lewis thus provided basicity of CH2open Cl2 , itmetal sites at room spontaneously temperature dissociated from [21,23,25,26]. the metal andHowever,thus provided we note open that whereas metal sites theroom at colortemperature change from deep navy However, [21,23,25,26]. to sky blue weon notemoist that water whereas coordination the color change usually occurs from deepwithin navy minutes to sky blue or hours on moistin the reported water examplesusually coordination [25,74], occurs color changewithinfrom 1′-CH minutes or2Cl 2-wet in hours to the 1′-CH 2Cl2-air reported takes place examples withincolor [25,74], secondschange (videfrom 10 -CH infra). Based2 Clon the to 2 -wet 10 -CH2 Cl observed color 2 -airchanges takes place and within Ramanseconds data bellow, (vide we hypothesize infra). Based on the 1′-CH 2Cl2-wet observed being color [Cu2(L1) changes and2]xCH Raman 2Cldata 2, where bellow,theweCHhypothesize 0 2Cl2 solvent 1molecules-CH2 Cl2 -wet are partially being [Cuor 2 totally (L1) 2 ]xCHuncoordinated. Cl 2 2 , where the CH Cl 2 2 solvent molecules are partially or totally uncoordinated. 0 -CH 2Cl2-air 11′-CH 120◦ C°Cunder 2 2 -air was activated activatedatat120 under dynamic dynamic high high vacuum vacuum for 20forh,20affording h, affording a deep a navy deep navy solid, presumably corresponding to desolvated [Cu 0 solid, presumably corresponding to desolvated [Cu2 (L1) 2 ]2(L1) 2] (1′-activated), (1 -activated), containing containing OMS. OMS. The The latter latter is is porous porous to N2 to at N 772 at K 77 (BET K (BET surface surface area:area: 301 301m2 ) m also to CO2 (41 cm3 g3−1 2) and also to CO2 (41 cm and g−1STP) STP)at at273 273 K K and and 11 bar. bar. N N22 sorption-desorption isotherms exhibited a typical reversible type-I behavior with with an uptake 125 cm of 125 3 cm /g (Figure 4). The pore width distribution was 6–7 Å (Figure 4 and 3 and Figure Figure S8), S8), suggesting suggesting that that the micropores from micropores from 1 -activated0 1′-activated might correlate with the observed nanoscale channels in 1-solv (Figure 1). (Figure 1). Interestingly, Interestingly, CO CO22 sorption capacity of 11′-activated 0 -activated is twice that measured for the related related (but shorter) (but shorter) carborane-dicarboxylated carborane-dicarboxylated Cu Cu22-paddle wheel CP CP [44]. [44]. 0 -CH Cl Figure 4. N Figure 4. N22 adsorption isotherm atat 77 77 K K for foractivated activated11′-CH 22 22-air. The inset shows the calculated calculated pore-size pore-size distribution distribution using using aa 2D-NLDFT 2D-NLDFT Heterogeneous HeterogeneousSurface Surfacemodel. model. Having determined that [Cu2(L1)2] (1′-activated) retains porosity, we evaluated the possible structural reversibility. Indeed, 1-DMF could be obtained by immersing samples of 1′-activated, 1′- CH2Cl2-air or 1′-MeOH-air into DMF at room temperature (Figure S6). Such a reversible transformation under ambient conditions indicates a rather weak interaction between layers in 1′ structures. Intrigued by this reversible phase transition, we further investigated the process by Raman spectroscopy in order to identify the possible changes in the paddlewheel units in 1. Molecules 2019, 24, 3204 8 of 18 Having determined that [Cu2 (L1)2 ] (10 -activated) retains porosity, we evaluated the possible structural reversibility. Indeed, 1-DMF could be obtained by immersing samples of 10 -activated, 10 -CH2 Cl2 -air or 10 -MeOH-air into DMF at room temperature (Figure S6). Such a reversible transformation under ambient conditions indicates a rather weak interaction between layers in 10 structures. Intrigued by this reversible phase transition, we further investigated the process by Raman spectroscopy in order to identify the possible changes in the paddlewheel units in 1. Therefore, we measured the Raman spectra for 1-DMF, 10 -MeOH-air, 10 -CH2 Cl2 -air, and 10 -CH2 Cl2 -wet. Molecules 2019, 24, x FOR PEER REVIEW 8 of 18 A comparison of the complete and selected Raman spectra for all the above compounds is provided in Figure S9 and Figure 5, respectively. Stretching modes ofof Cu–Cu vibrations −1 provided in Figures S9 and 5, respectively. Stretching modes Cu–Cu vibrations(240–150 (240–150cm cm−1)) were were clearly clearly observed observed in in all all the the samples samples as as broad broad bands. Whereas Cu–Cu bands. Whereas Cu–Cu vibration vibration for for 1-DMF 1-DMF appeared appeared at at 162 cm −1 , that for 10 -CH Cl -wet appeared at 207 cm−1 , that is, blue-shifted ca. 50 cm−1 . Such blue-shift 162 cm , that for 1′-CH2Cl2-wet appeared at 207 cm , that is, blue-shifted ca. 50 cm . Such blue-shift −1 2 2 −1 −1 displacement displacementcorrelatescorrelateswell wellwith thatthat with observed observedfor HKUST-1 on chemical for HKUST-1 activationactivation on chemical [26]. As mentioned [26]. As earlier, exposure of 1 0 -CH Cl -wet to air quickly provided 10 -CH Cl -air. Interestingly, after the sample mentioned earlier, exposure 2 2 of 1′-CH2Cl2-wet to air quickly provided 2 2 1′-CH2Cl2-air. Interestingly, was afterexposed the sample to ambient was exposed air (within to ambient1 minute) a color1change air (within minute) from navy a color to skyfrom change blue navy was noticed and to sky blue the Cu–Cu vibration observed for 1 0 -CH Cl -wet at 207 cm−1 , was red-shifted and split into two bands was noticed and the Cu–Cu vibration observed 2 2 for 1′-CH2Cl2-wet at 207 cm , was red-shifted and −1 at approximately split into two bands 180 atand 162 cm−1 in180 approximately 10 -CH and2162 Cl2 -air cm−1(Figure in 1′-CH 5).2ClThe latter remained unaltered with 2-air (Figure 5). The latter remained longer exposure to air. Two bands are also observed unaltered with longer exposure to air. Two bands are also observed in the Raman in the Raman spectrum of 10 -MeOH-air, spectrumatof182 1′- and 162 cm −1 . The observed spontaneous red shift on exposing 1 0 -CH Cl -wet to air MeOH-air, at 182 and 162 cm−1. The observed spontaneous red shift on 2 exposing 2 1′-CHconsistent is with 2Cl2-wet to air aislengthening consistent with of theaCu···Cu lengtheningdistances on dissociation of the Cu∙∙∙Cu distances of weakly oncoordinated dissociationCH of2 Cl 2 and coordination weakly coordinated of CH2Cl2 and coordination of other better bonding2 species, such as e.g., H2O from airis[23]. other better bonding species, such as e.g., H O from air [23]. The above data in agreement The above with data ispreviously in agreement reported data and proof with previously reportedthat data DMF-coordinated and proof thattoDMF-coordinated Cu in 1 can be effectively to Cu in 1 and can successively dissociated by MeOH and CH Cl [24]. It is noteworthy be effectively and successively dissociated by MeOH and CH2Cl2 [24]. It is noteworthy that no other 2 2 that no other changes in the Raman signals were observed in the region for the organic moiety changes in the Raman signals were observed in the region for the organic moiety of the linker (Figuresof the linker (Figure 5 and Figure S9). The 5 andcharacteristic double banddouble S9). The characteristic of νsymband − (COOof) νvibration for the bridged carboxylates at around 1400 sym(COO−) vibration for the bridged carboxylates at and 1420 cm −1 (Figure 5 and Figure S9) remains mostly unchanged, demonstrating that there is no around 1400 and 1420 cm (Figure 5 and Figure S9) remains mostly unchanged, demonstrating that −1 significant change in change there is no significant the coordination mode of mode in the coordination the carboxylates from the of the carboxylates fromCuthe 2 -paddlewheel Cu2-paddlewheel units during the transformation. units during the transformation. There Thereis, however, a clear achange is, however, in the B-H clear change vibrations, in the suggesting B-H vibrations, that the suggesting change in the Cu···Cu distances also affects the carborane moieties. that the change in the Cu∙∙∙Cu distances also affects the carborane moieties. Thus, B-H vibrations Thus, B-H vibrations appear as broad −1 appearbands as broad at 2569, bands 2582 and 2617 at 2569, 2582cm and in 26171-DMFcm−1 and change in 1-DMF andinto three broad change bands into three at 2566, broad bands2597 at and 2625 cm −1 in 10 -CH Cl -wet (Figure 5). A clear change was observed when 10 -CH Cl -wet was 2566, 2597 and 2625 cm in 1′-CH2Cl2-wet (Figure 5). A clear change was observed when 1′-CH2Cl2- −1 2 2 2 2 exposed wet was to the air,to exposed providing 10 -CH2 Cl21′-CH the air, providing -air. Thus, new B-H vibrations appear at 2581, 2599, 2607, 2632 2Cl2-air. Thus, new B-H vibrations appear at 2581, 2599, and −1 0 2607,2667 2632 cmandfor 1 -CH 2667 cm2−1Clfor2 -air. As observed 1′-CH 2Cl2-air. As in the Cu–Cu in observed vibration the Cu–Cu region, a nearly region, vibration identicala Raman nearly spectrum to that of 1 0 -CH Cl -air is observed in the case of 10 -MeOH-air. identical Raman spectrum2 to2that of 1′-CH2Cl2-air is observed in the case of 1′-MeOH-air. Figure 5. Successive changes in the Raman spectrum of 1-DMF after sequential exposure to methanol (MeOH), dichloromethane (CH22Cl22)) and dichloromethane (CH and ambient ambient air air (See (See full full spectra in the SI). A deep understanding of the vibrational features described in the present work has been obtained by density functional theory (DFT) calculations (See the experimental section for details). In order to simplify the calculations, we used model structures consisting of four monosubstituted carborane-based ligands bonded to a Cu2-paddlewheel unit [Cu2(LCB)4(Solv)2] (LCB = 1-(4- carboxyphenyl)-1,7-dicarba-closo-dodecaborane; Solv = DMF, MeOH, H2O, C2B10H12 and none; see Molecules 2019, 24, 3204 9 of 18 A deep understanding of the vibrational features described in the present work has been obtained by density functional theory (DFT) calculations (See the experimental section for details). In order to simplify the calculations, we used model structures consisting of four monosubstituted carborane-based ligands bonded to a Cu2 -paddlewheel unit [Cu2 (LCB )4 (Solv)2 ] (LCB = 1-(4-carboxyphenyl)-1,7-dicarba-closo-dodecaborane; Solv = DMF, MeOH, H2 O, C2 B10 H12 and none; see Table 2, model compounds). We computed bond distances, vibrational frequencies and energies for paddlewheel units having DMF, MeOH, H2 O, none and/or a discrete m-carborane moiety bonded to the Cu atoms at the apical positions (Table 2). Calculation for the Cu–Cu vibrational frequencies for 1-DMF resulted in a frequency of 207 cm−1 , a value significantly higher than the experimental value (162 cm−1 ). This disagreement in the Cu–Cu calculated values was also observed in models for HKUST-1 [25]. The calculations, however, correlate well with the observed raman shifts (vide infra), that is, the vibrational frequencies increases as the coordination strength of Lewis base molecules decreases and viceversa. The calculated Raman spectra show a good agreement with the experimental data (Figure S10). Regardless of the expected shift between the experimental and calculated Raman spectra, the model provides reliable results in the range 300–3200 cm−1 and allowed identification of all the experimental bands (Figure S10). Quite remarkably, the relatively sharp band at 2581 cm−1 in 10 -CH2 Cl2 -air and 10 -MeOH-air (Figure 5) can be attributed to a B–H···Cu(II) agostic interaction, by comparison with our models where at least one hydride of a m-carborane molecule is coordinating to one of the Cu(II) atoms of the paddlewheel unit. In addition, the observed weak band at 2667 cm−1 seems to correlate with one of the B–H hydrides in close proximity (although not bonded) to the Cu(II) atoms in the model. Comparison of the B–H Raman region for 10 -CH2 Cl2 -air and 10 -MeOH-air (Figure 5) with that for various models (Figure S11) indicates that interactions between 2D layers involve one or perhaps two B–H···Cu(II) interaction per paddlewheel unit. It is possible that not all Cu(II) atoms in each paddlewheel unit are interacting with a hydride, leaving this open to coordinate one water molecule from the air or even remain unsaturated (OMS). In fact, the corrugated shape of the 2D layers might complicate the match between all OMSs and available hydrides between layers. Further evidence for such B–H···Cu(II) interactions is obtained from the DFT calculations on our model compounds. An energy comparison of our DFT models (Table 2) reveals a clear stabilization of the fully open metal site model [Cu2 (LCB )4 ] when interacting with one or two B–H hydrides. Thus, stabilization energies of 34.4, 26.7 or 13.6 Kcal/mol were obtained for [Cu2 (LCB )4 ] interacting with one H2 O and one m-carborane molecule, two m-carborane molecules or one OMS and one m-carborane molecule, respectively. In addition, calculated Cu–Cu vibrational frequencies for our models (Table 2) show the same trend that was observed experimentally (Figure 5), that is, a blue-shift (from lower to higher frequencies) on generation of open metal sites (or removal of DMF by CH2 Cl2 from 1-DMF) and red-shift displacements (from higher to lower frequencies) on solvent or B-H coordination. Assuming that the observed trend is correct, we hypothesize that the experimentally observed Cu–Cu bands at 162 and 180 cm-1 for 10 -CH2 Cl2 -air and 10 -MeOH-air correspond to B–H···Cu–Cu···H–B and B–H···Cu–Cu(OMS) species, although we cannot exclude the formation of partially hydrated species. In fact, it seems difficult that all Cu atoms are coordinated to a hydride atom, due to the high corrugation of 2D layers. It is also possible that multiple (B–H)x ···Cu interactions are taking place. This will certainly stabilize the binding enthalpy of such interactions [75]. A reversible coordination of B–H to a Cu(I) complex has been previously reported for a molecular system [75]. 2D CPs or MOFs present the unique feature of structural isomerism by sliding of layers involving the supramolecular interactions between them and the included solvent [76,77]. However, to our knowledge, no such mediated B–H···Metal transformations have been observed previously in coordination polymers or MOFs. The observed lower stability of crystals for 1-MeOH than the corresponding ones for 1-DMF (Figure 1), when exposed to air, can be explained by a comparable binding energy for B–H···Cu and Me(H)O···Cu interactions. molecular system [75]. 2D CPs or MOFs present the unique feature of structural isomerism by sliding of layers involving the supramolecular interactions between them and the included solvent [76,77]. However, to our knowledge, no such mediated B–H∙∙∙Metal transformations have been observed previously in coordination polymers or MOFs. The observed lower stability of crystals for 1-MeOH than Molecules the24,corresponding 2019, 3204 ones for 1-DMF (Figure 1), when exposed to air, can be explained 10 of 18 by a comparable binding energy for B–H∙∙∙Cu and Me(H)O∙∙∙Cu interactions. 2. Calculated TableTable 2. Calculated Energies, Cu-Cu Energies, BondBond Cu-Cu Lengths and vibrational Lengths frequencies and vibrational for the frequencies for Model the Model compounds [Cu2 (L compounds CB 2)(L [Cu 4 (Solv) 2 ] (LCB CB)4(Solv) = 1-(4-carboxyphenyl)-1,7-dicarba-closo-dodecaborane; 2] (LCB = 1-(4-carboxyphenyl)-1,7-dicarba-closo-dodecaborane;Solv = Solv = DMF,DMF, MeOH, H MeOH, 2 O, C B H and none). See experimental section for details. H22O,10C2B1210H12 and none). See experimental section for details. Cu-CuCu-Cu Vibrational Vibrational Cu-CuCu-Cu ModelCompounds Model Compounds a Distances Energy Energya Distances Frequencies Frequencies (Å) (Moleculesatat (Molecules thethe Apical Apical (Kcal/mol) (cm−1 ) −1 Positions) (Kcal/mol) (Å) (cm ) Positions) Exp. Calc. Exp. Calc. Exp. Calc. Exp. Calc. −41.6 2.641b b 2.668 b b 162162 207 −41.6 2.641 2.668 207 Molecules 2019, Molecules 2019, 24, 24, xx FOR FOR PEER PEER REVIEW REVIEW 10 of 10 of 18 18 Molecules Molecules 2019, 2019, 24,24, x FOR x FOR PEER PEER REVIEW REVIEW 1010ofof1818 (2Molecules (2 Molecules of of DMF) DMF) c c −41.0 −41.0 −41.0 −41.0 −41.0 2.620 2.620 2.620 2.620c cc 2.597 2.597 2.597 2.597 - -- - 194 194 194 194 −41.0 2.620 2.597 - 194 (2 Molecules Molecules ofMeOH) ofof MeOH) (2(2 Molecules (2Molecules Molecules MeOH) ofofMeOH) MeOH) 00 000 ----- 2.507 2.507 2.507 2.507 - ---- 223 223 223 223 2.507 223 (None) (None) (None) (None) (None) −26.7 −26.7 −26.7 −26.7 ----- 2.561 2.561 2.561 2.561 - ---- 206 206 206 206 −26.7 2.561 206 (2 (2 Molecules Molecules of ofof m-Carborane) m-Carborane) (2(2 Molecules (2Molecules Moleculesof ofm-Carborane) m-Carborane) m-Carborane) −13.6 −13.6 −13.6 −13.6 ----- 2.536 2.536 2.536 2.536 - ---- 214 214 214 214 −13.6 2.536 214 (1(1Molecule of Moleculeof m-Carborane) ofm-Carborane) of (1(1 (1 Molecule Molecule Molecule of m-Carborane) m-Carborane) m-Carborane) −34.4 −34.4 −34.4 −34.4 ----- 2.577 2.577 2.577 2.577 - ---- 192 192 192 192 −34.4 2.577 192 (1 (1 Molecule Molecule (1Molecule Molecule of of ofm-Carborane m-Carborane m-Carborane (1 (1 Molecule of of m-Carborane m-Carborane 1 11molecule 1 molecule molecule of of H 2O) of moleculeof 1 molecule H 2H ofHO) H 22O) 2O)O) a SeeDFT aa aSee aSee See DFT DFT DFT SeeDFT calculation calculation calculation DFTcalculation section section section calculationsection calculation for sectionfor section for details. for details. details. fordetails. b For details. For for details. For 1-DMF. b Forb1-DMF. bbFor 1-DMF. 1-DMF. For1-DMF. For 1-DMF. For For 1-MeOH. c For 1-MeOH. c c c cFor 1-MeOH. 1-MeOH. For1-MeOH. 1-MeOH. See 3. Materials Materials and Methods 3.3.3.Materials and Materialsand Methods andMethods Methods 3.1. 3.1. Characterization Characterization 3.1.Characterization and and Characterizationand Methods Methods andMethods Methods 3.1. Attenuated Attenuated total total Attenuatedtotal reflection reflection totalreflection Fourier Fourier reflectionFourier transformed transformed Fouriertransformed infra-red infra-red transformedinfra-red (ATR-FTIR) (ATR-FTIR) infra-red(ATR-FTIR) spectra spectra (ATR-FTIR)spectra spectrawere were were recorded recorded wererecorded recorded Attenuated using using a PerkinElmer usingaaaPerkinElmer PerkinElmer Spectrum Spectrum PerkinElmerSpectrum SpectrumOne One One spectrometer spectrometer Onespectrometer equipped equipped spectrometerequipped with with equippedwith a Universal withaaaUniversal Universal UniversalATR ATR ATR sampling sampling ATRsampling sampling using accessory. accessory. Spectra Spectra accessory.Spectra were were Spectrawere collected collected werecollected with with2222cm with collectedwith cm cm −1 spectral resolution in the 4000–650 cm−1 range. −1 spectral −1−1 cm resolution spectralresolution in resolutionin the inthe 4000–650 the4000–650 4000–650cm cm cm −1 range. −1−1 range. accessory. spectral range. Elemental Elemental analyses analyses Elementalanalyses were were analyseswere obtained obtained wereobtained obtainedbyby by using using a Thermo usingaaaThermo byusing Thermo (Carlo (Carlo Thermo(Carlo Erba) Erba) (CarloErba) Flash Flash Erba)Flash 2000 2000 Flash2000 Elemental Elemental 2000Elemental Analyser, Analyser, ElementalAnalyser, Analyser, Elemental configured configured configured for for forwt% wt% wt% CHN. CHN. CHN. Thermogravimetric Thermogravimetric Thermogravimetric Analysis Analysis Analysis (TGA) (TGA) (TGA) was was was performed performed performed in in inN NN2, on an nSTA 449 , ,on onan annSTA nSTA449449 449 configured for wt% CHN. Thermogravimetric Analysis (TGA) was performed in N2, on an nSTA 22 o Molecules 2019, 24, 3204 11 of 18 3. Materials and Methods 3.1. Characterization and Methods Attenuated total reflection Fourier transformed infra-red (ATR-FTIR) spectra were recorded using a PerkinElmer Spectrum One spectrometer equipped with a Universal ATR sampling accessory. Spectra were collected with 2 cm−1 spectral resolution in the 4000–650 cm−1 range. Elemental analyses were obtained by using a Thermo (Carlo Erba) Flash 2000 Elemental Analyser, configured for wt% CHN. Thermogravimetric Analysis (TGA) was performed in N2 , on an nSTA 449 F1 Jupiter-Simultaneous TGA-DSC or SDT Q600 V8.3 Build 101 instruments (heating rate: 10 ◦ C/min; temperature range: 25 ◦ C to 600 ◦ C). Gas sorption-desorption (CO2 /273 K and N2 /77 K) measurements were performed using an ASAP2020 surface area analyzer. Samples were first degassed at 120 ◦ C for 20 h. Powder X-ray Diffraction (PXRD) was recorded at room temperature on a Siemens D-5000 diffractometer with Cu Kα radiation Field-emission(λ = 1.54056 Å, 45kV, 35mA, increment = 0.02o ). Morphological features were examined first by optical microscopy and next by scanning electron microscopy (SEM) with a QUANTA FEI 200 FEGESEM microscope. Raman spectra were recorded using an LabRam HR800 (Horiba Jobin-Yvon) dispersive mircro-raman spectrometer coupled with a CCD detector. A solid state laser with a 532 nm wavelength was used as the excitation source and the spectra were measured in a backscattering configuration through an Olympus BXFM objective. Excitation of the samples was performed by focusing the laser beam on crystalline samples with a laser power of 0.5 mW with a 50x magnifying objective lens. 3.2. Materials 1,7-di(4-carboxyphenyl)-1,7-dicarba-closo-dodecaborane ligand was synthesized according to the literature procedure [78]. CPs syntheses were done in air. All chemicals were commercially available and used as received. Synthesis of [Cu2 (L1)2 (DMF)2 ]·2DMF·H2 O (1-DMF). Cu(NO3 )2 ·6H2 O (48.7 mg, 0.167 mmol) was mixed with H2 L1 (64.1 mg, 0.167 mmol) in 4 mL of DMF. This mixture was sonicated until all solids were uniformly dissolved, followed by heating at 80 ◦ C for 48 h. Greenish crystals of 1-DMF were collected and washed with DMF (60 mg, 59.8%). IR (ATR; selected bands; cm−1 ): 2605, 2570 (BH); 1672 (C=O from DMF); 1617 (C=O from carboxylate). Elemental analysis (%) calculated for Cu2 (LCB (COO)2 )2 (DMF)4 (H2 O): C 43.96, H 5.53, N 4.66; Found: C 43.91, H 5.58, N 5.20. Synthesis of [Cu2 (L1)2 (DMA)2 ]·DMA·H2 O (1-DMA). Cu(NO3 )2 ·6H2 O (48.7 mg, 0.167 mmol) was mixed with H2 L1 (64.1 mg, 0.167 mmol) in 4 mL of DMA. This mixture was sonicated until all solids were uniformly dissolved, followed by heating at 80 ◦ C for 48 h. Greenish crystals of 1-DMA were collected and washed with DMA (55.0 mg, 56.2%). IR (ATR; selected bands; cm−1 ): 2604 (BH); 1646 (C=O from DMA); 1602 (C=O from carboxylate). Elemental analysis (%) calculated for Cu2 (LCB (COO)2 )2 (DMA)3 (H2 O): C 45.12, H 5.59, N 3.59; Found: C 44.52, H 5.51, N 4.02. [Cu2 (L1)2 (MeOH)2 ]·4MeOH (1-MeOH). Cu(NO3 )2 ·6H2 O (48.7 mg, 0.167 mmol) was mixed with H2 L1 (64.1 mg, 0.167 mmol) in 4 mL of MeOH. This mixture was sonicated until all solids were uniformly dissolved, followed by heating at 80 ◦ C for 24 h. Light-blue crystals of 1-MeOH were collected, washed with MeOH and stored in this solvent (46 mg, 50.8%). General method for chemical activation of 1-DMF. As-synthesized greenish crystals of 1-DMF (10 mg) were immersed in methanol (10 mL), provoking 1-DMF brake into smaller sky blue crystals of 10 -MeOH. MeOH was further exchanged once a day for 2 days and replaced by CH2 Cl2 (10 mL). The latter provided deep blue crystals for 10 -CH2 Cl2 -wet after 10 min. The entire process was performed while maintaining the crystals fully covered with the solvents. When deep blue crystals for 10 -CH2 Cl2 -wet were dried on filter paper at room temperature a very fast color change was observed to sky blue crystals for 10 -CH2 Cl2 -air. See videos for transformation and color changes. Molecules 2019, 24, 3204 12 of 18 10 -MeOH-air: IR (ATR; selected bands; cm−1 ): 2596, 2579 (BH); 1610 (C=O from carboxylate). Elemental analysis (%) calculated for Cu2 (LCB (COO)2 )2 (H2 O)(2MeOH): C 41.93, H 4.76; Found: C 41.99, H 4.70. 10 -CH2 Cl2 -air: IR (ATR; selected bands; cm−1 ): 2596, 25 (BH); 1610 (C=O from carboxylate). Elemental analysis (%) calculated for Cu2 (L1)2 (H2 O)(CH2 Cl2 ): C 39.84, H 4.05; Found: C 39.71, H 4.26. Synthesis of 1-DMF from 10 -Solv-air. Crystals for freshly made 10 -Solv-air (10 mg; Solv = MeOH or CH2 Cl2 ) were suspended in DMF (10 mL) in a capped glass vial and left at room temperature for 3 days. No dissolution of the crystals was observed during this time. The obtained crystals for 1-DMF were then filtered, washed with DMF and stored in the same solvent. See Figure S10 for powder X-ray diffraction. 3.3. Crystallography Measured crystals were prepared under inert conditions immersed in perfluoropolyether or paratone as protecting oil for manipulation. Suitable crystals were mounted on MiTeGen MicromountsTM , and used for data collection. Crystallographic data for 1-DMA were collected with a Bruker D8 Venture diffractometer, processed with APEX3 program [79] and corrected for absorption using TWINABS [80]. The structures were solved by direct methods and subsequently refined by correction of F2 against all reflections. All non-hydrogen atoms were refined with anisotropic thermal parameters by full-matrix least-squares calculations on F2 [81]. Hydrogen atoms were not located in difference Fourier maps and included as fixed contributions riding on attached atoms with isotropic thermal displacement parameter 1.2 (C–H, B–H) or 1.5 (O–H) times those of the respective atom. 1-DMA was treated as a two component non-merohedral twin. The twin matrix describes a rotation of 179.8◦ around the [1-1 0] direction, given by the matrix (0-1 0 -1 0 0 0 0-1). The structure of 1-DMA was solved using direct methods with only the non-overlapping reflections of component 1. The structure was refined using the HKLF 5 routine with all reflections of component 1 (including the overlapping ones), resulting in a BASF value of 0.2356(12). The structure exhibits disorder of the coordinated DMA molecule, which was successfully refined using a two-site model with 0.63:0.37 occupancy ratio. The contribution of the disordered solvent molecules to the diffraction pattern could not be rigorously included in the model and were consequently removed with the SQUEEZE routine of PLATON [82] that suggest the presence of 2 DMA molecules that have not been included in the refined structure but considered for calculation of empirical formula, formula weight, density, linear absorption coefficient and F(000). Crystallographic data for 1-DMF and 1-MeOH were collected on a Rigaku AFC12 4-circle goniometer equipped with a HyPix 6000HE (Hybrid Photon Counting) detector mounted at the window of an FR−E+ SuperBright molybdenum (Mo Kα1/Kα2 = 0.71073Å) rotating anode generator with VHF Varimax optics (70 µm focus) operating at 2.475 kW (45 kV, 55 mA). Data reduction was performed using the CrysAlisPro (Rigaku, V1.171.40.18b, 2018) software and the structure was solved by Intrinsic Phasing using the ShelXT (Sheldrick, 2015) structure solution program and refined by Least Squares using version 2016/6 of ShelXL (Sheldrick, 2015). In the case of 1-MeOH the crystal was a non-merohedral twin and data integrated from one component were used for the refinement. Discarding severely overlapped reflections has led to a low completeness. A summary of crystal data is reported in Table S1. 3.4. Computational Details The geometry of all computed systems was optimized by dispersion-corrected (D3) [83] density functional theory (DFT) using the hybrid B3LYP functional [84–86] as implemented in the Gaussian 09 [87]. The Cu atom was described using the scalar relativistic Stuttgart-Dresden SDD pseudopotential and its associated double-ζ basis set, [88] complemented with a set of f polarization functions [89]. The 6–31G(d) basis sets was used for the rest of atoms [90]. All stationary points were characterized and confirmed by frequency analysis. Molecules 2019, 24, 3204 13 of 18 Calculations were performed on molecular structures according to the general formula [Cu2 (LCB (COO)2 )2 (Solv)2 ] where LCB (COO)2 is 1,7-di(4-carboxyphenyl)-1,7-dicarba-closo-dodecaborane. Several Solv were considered occupying two coordination sites to investigate their effect on the properties of the system. The Solv molecules computed were DMF, H2 O, MeOH and closo-dodecaborane. The electronic state of the system was initially investigated for Solv = DMF and MeOH. The closed shell singlets were calculated to be 35.8 kcal/mol and 35.4 kcal/mol higher in energy than the triplets for the complexes with DMF and MeOH as Solv, respectively. Nevertheless, the open shell singlets states were calculated to be lower in energy than the triplet state by 0.6 kcal/mol for both complexes. This is in agreement with previous DFT studies in the literature for copper paddlewheel-based systems [91–95]. Thus, all the structures presented in this work were calculated as open shell singlets. 4. Conclusions In summary, we report the syntheses of 2D 1-Solv (Solv: DMF, MeOH or CH2 Cl2 ), a novel family of Cu2 -paddlewheel CPs that incorporate the flexible ligand 1,7-di(4-carboxyphenyl)-1,7-dicarba-closo- dodecaborane (L1). 1-DMF undergoes a reversible phase transition on solvent exchange and provided a crystalline material that is porous to N2 and CO2 . The combination of the experimental and calculated data supports the spontaneous release of the solvent from 1-DMF, with the consequent generation of OMSs. Sliding of the layers allows the OMSs to be in close proximity to readily available hydride atoms of the many present from the carborane moieties, and thus promote the formation of B–H···Cu(II) interactions. The observed phase transition on solvent loss is accompanied by new Raman modes in the B–H and Cu–Cu region that are in agreement with the formation of B–H···Cu(II) interactions. There is enough flexibility in the solid to move the paddlewheels relative to each other so that the Cu centers can interact with the many available hydride atoms from the carborane moieties in another paddlewheel unit. Such B–H···Cu(II) interactions can be easily disturbed in the presence of a strongly coordinating solvent such as DMF and provide the starting 1-DMF structure at room temperature. The present work adds unprecedented knowledge to the possible reasons for boranes or carboranes acting to stabilize flexible MOFs [48] but it also discloses another possible mechanism for constructing new flexible architectures or hydride/MOF composites. Supplementary Materials: The following are available online. Table S1: Crystal and structure refinement data for 1-Solv (Solv = DMA, DMF or MeOH), Table S2: Selected distances (Å) and angles (◦ ) for 1-Solv (Solv = DMA, DMF or MeOH), Figure S1. FT-IR spectra of L1 ligand, 1-DMA, 1-DMF, 10 -MeOH-air and 10 - CH2 Cl2 -air, Figure S2: Comparison of calculated and experimental PXRD fot 1-DMF and 1-DMA, Figure S3: Figure S3. TGA diagram of as made 1-DMA (red) and 1-DMF (green), Figure S4: TGA diagrams of 10 -MeOH-air (black) and 10 -CH2 Cl2 -air (red), Figure S5: PXRD comparison of calculated 1-MeOH, 1-MeOH-air and 10 -MeOH-air, Figure S6: PXRD patterns showing reversible transformation from different samples to 1-DMF, Figure S7: CO2 adsorption isotherm at 273 K for activated 10 - CH2 Cl2 -air, Figure S8: Pore Distribution by Density Functional Theory Model: CO2 -DFT Slit Geometry for activated 10 - CH2 Cl2 -air, Figure S9: Comparison of Experimental Raman Spectra of 1-DMF, 10 -DCM-wet, 10 -DCM-air and 10 -MeOH-air, Figure S10: Comparison of Experimental and Calculated Raman for 1-DMF with [Cu2 (LCB)4 (DMF)2 ] (LCB = 1-(4-carboxyphenyl)-1,7-dicarba-closo-dodecaborane), Figure S11: Comparison of Experimental and Calculated Raman for 10 -CH2 Cl2 -air and 10 -MeOH-air with [Cu2 (LCB)4 (Solv)2 ] (LCB = 1-(4-carboxyphenyl)-1,7-dicarba-closo-dodecaborane; Solv = H2 O, C2 B10 H12 and/or none (Open Metal Site)), computational details, Video S1: DMF plus MeOH, Video S2: DCM-wet plus DMF, Video S3: DCM-wet plus air. Author Contributions: Writing and original draft preparation, J.G.P.; sample preparation and characterization L.G. and P.G.F.; single crystal X-ray data collection and/or refinement M.E.L. and D.C.-L.; sorption data measurement and interpretation, J.F.; DFT calculations, G.N. and G.U.; editing, data analysis and interpretation, L.G., G.N., G.U., F.T., C.V. and J.G.P. Funding: This research was funded by MEC grant CTQ2016-75150-R and the Generalitat de Catalunya (2017/SGR/1720) and the Spanish MINECO through the Severo Ochoa Centers of Excellence Program, under Grant SEV-2015-0496. Acknowledgments: L.G.: P.G., F.T., C.V. and J.G.P. thank MEC grant CTQ2016-75150-R and the Generalitat de Catalunya (2017/SGR/1720) for financial support. ICMAB acknowledges the support of the Spanish MINECO through the Severo Ochoa Centers of Excellence Program, under Grant SEV-2015-0496. L. Gan and G. Norjmaa are enrolled in the UAB PhD program. 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Dalton Trans. 2016, 45, 11944–11948. [CrossRef] Sample Availability: Samples of the compounds are available from the authors. © 2019 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (http://creativecommons.org/licenses/by/4.0/).