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Cite this: DOI: 10.1039/c5ce01581b

Received 6th August 2015,Accepted 3rd December 2015

DOI: 10.1039/c5ce01581b

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From pink to blue and back to pink again:changing the CoIJII) ligation in a two-dimensionalcoordination network upon desolvation†

Diana Chisca,a Lilia Croitor,a Eduard B. Coropceanu,b Oleg Petuhov,b

Svetlana G. Baca,a Karl Krämer,c Shi-Xia Liu,c Silvio Decurtins,c Hector J. Rivera-Jacquez,d Artëm E. Masunovde and Marina S. Fonari*a

Heating of a pink two-dimensional CoIJII) coordination network

{[Co2IJμ2-OH2)IJbdc)2IJS-nia)2IJH2O)IJdmf)]·2IJdmf)·(H2O)}n (1) built from

1,4-benzenedicarboxylic acid (H2bdc) residues and thio-

nicotinamide (S-nia) ligands initiates a single-crystal-to-single-

crystal transition accompanied by removal of both coordinated

and co-crystallized solvents. In the dry blue form, [CoIJbdc)IJS-nia)]n(dry_1), the CoIJII) centers changed from an octahedral to a square

pyramidal configuration.

Porous coordination polymers (PCPs)1 including metalorganic–frameworks (MOFs)2 has become a rapidly growingarea of chemistry in the past decades.3 This is mainly due tothe intriguing topological architectures and potentialapplications of MOFs in fields such as gas storage, catalysis,separation, ion exchange, and molecular magnetism.4 Thesesolids not only possess regular porosity with high porevolume, but contain tunable organic groups within themolecular framework. This allows an easy modulation of thepore size. MOFs that show a structural response to externalstimuli such as guest sorption, temperature, or mechanicalpressure are of particular interest.5

In addition to the rigid three-dimensional (3D) polymericcoordination networks, the flexible two-dimensional (2D)structures are attracting considerable attention.6 Many fasci-nating examples have been documented since Zaworotko'sseminal work.7 This work highlighted the superstructural di-versity in the laminated solids, the possibilities of the ratio-nal design of both hydrophilic and hydrophobic surfaces,and their common inherent ability to mimic clays by interca-lation of a wide range of organic guest molecules. The breath-ing behaviour, the ability of the metal sites in the regulargrids to work as catalytically active centers, the preference inCO2 gas capture and gas stepwise adsorption were reported.8

Coordination layers were proposed as a source of crystallinesheets with nanometer thickness for molecular sieving. Theirability to achieve high proton conductivity and high watersorption under low humidity conditions has been demon-strated. A novel strategy to design and synthesize homochiralPCPs9 and non-linear optical (NLO) materials from layeredPCPs was disclosed.10

The unusual properties of 2D coordination networksprompted us to introduce pyridine-n-aldoxime/dioxime li-gands as pillars or chelating agents. These bulky metallo-chelate corner fragments in carboxylic networks can affordpotentially porous structures that are able to accommodatesmall molecules in the crystal lattices.11

Here we combine 1,4-benzenedicarboxylic acid (H2bdc)with thionicotinamide (S-nia), resulting in a CoIJII)-based 2Dcoordination network that undergoes a single-crystal-to-single-crystal (SC–SC) transition in the solid state upondesolvation. Our choice is based on the following: 1) struc-tural similarity of S-nia to the pyridine-n-aldoximes, previouslyexplored by us;10,11 2) all reported data are restricted toorganic solids;12 3) this molecule is one of the commerciallyavailable analogs of nicotinamide, and 4) it presents opportu-nities for the generation of hydrogen-bonded networks for 2Dstacked layers. The latter would support Kitagawa's idea ofinventing “metallo-amino acid” ensembles.13

CrystEngCommThis journal is © The Royal Society of Chemistry 2015

a Institute of Applied Physics Academy of Sciences of R. Moldova, Academy str., 5,

MD2028, Chisinau, Moldova. E-mail: fonari.xray@phys.asm.md;

Fax: +373 22 725887; Tel: +373 22 738154b Institute of Chemistry Academy of Sciences of R. Moldova, Academy str., 3,

MD2028, Chisinau, Moldovac Department of Chemistry and Biochemistry, University of Bern, Freiestrasse 3,

3012-Bern, SwitzerlanddNanoScience Technology Center, Department of Chemistry, Department of

Physics, and Florida Solar Energy Center, University of Central Florida, 12424

Research Parkway, Ste. 400, Orlando, Florida 32826, USAeDepartment of Condensed Matter Physics, National Research Nuclear University

MEPhI, Kashirskoye shosse 31, Moscow, 115409, Russia

† Electronic supplementary information (ESI) available: General information,synthetic procedures, IR spectra, XRPD, TGA, DSC, figures of crystal packing, de-tails of periodical optimizations and spectral predictions. CCDC 1416671,1416672 and 1430303. For ESI and crystallographic data in CIF or otherelectronic format see DOI: 10.1039/c5ce01581b

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The reaction of CoIJCH3COO)2·4H2O with H2bdc andS-nia in a solvent mixture of CH3OH, dmf, and H2Oresulted in pink block-shaped crystals with the composi-tion {[Co2IJμ2-OH2)IJbdc)2IJS-nia)2IJH2O)IJdmf)]·2IJdmf)·(H2O)}n (1)(Fig. 1a) (Details are given in the ESI†).

Compound 1 crystallizes in the triclinic space group P1̄. Inthe asymmetric unit, there are two CoIJII) atoms that occupygeneral positions and four half bdc ligands all lying aboutindependent inversion centers. The components are associ-ated in the layered structure with the binuclear [Co2IJμ2-OH2)IJRCOO)2] metal cluster as a secondary building unit(SBU).2 The SBU contains two crystallographically distinctCoIJII) cations that are both found in a distorted NO5-octahedral environment. The triple bridge between the metalcenters is provided by two syn,syn-bidentate bridging carboxy-lato groups and one water molecule (Fig. 2a). For Co(1), thecoordination core is composed of oxygen atoms from two bis-bidentate bridging bdc2− anions, one bridging water moleculeand one terminal water molecule, and one dmf molecule; thepyridine nitrogen atom stems from the S-nia ligand. ForCo(2), the coordination core comprises four bdc2− anions(two bis-bidentate bridging and two bis-monodentate), abridging water molecule, and the pyridine nitrogen atom ofthe S-nia ligand. The Co⋯Co separation within the SBU is3.579(5) Å. The aggregation of the SBU in the (4,4) 2D net-work with a sql topology6 occurs via the bdc2− bridgesresulting in a rhombohedral pattern with diagonal dimen-sions of 11.383 Å × 11.483 Å (Fig. 2b).

For the two terminal S-nia ligands, one is situated in adangling position being inclined to the layer plane, while thesecond S-nia ligand is upraised perpendicularly to the layeredcoordination skeleton. The layers stack along the crystallo-graphic a axis, while each rhombohedral vacancy in the layeris partially closed by NHIJNH2)⋯OIJCOO) hydrogen bondsfrom the dangling S-nia ligands from the adjacent layer(Fig. S2, Table S1†).13 Despite this partial blocking, the sol-vent molecules (H2O and dmf) occupy the hydrophilic andhydrophobic regions in the intra- and inter-layer space, andare held in place by OH⋯O hydrogen bonds and stackinginteractions between the solvated dmf and coordinated bdc2−

residues (Fig. S3†). The volume occupied by solventmolecules as calculated by PLATON comprises 735 Å3 or31.5% of the total unit cell volume. Delicate heating of 1 at105 °C for 4 h in vacuum resulted in the desolvated product[CoIJbdc)IJS-nia)]n (dry_1). The SC–SC transition is

accompanied by a colour change from pink to dark-blue(Fig. 1b). It involves a rearrangement of the coordination en-vironment and substitution of solvent molecules with a lessvolatile ligand, similar to the cases reported previously.14–17

For CoIJII) compounds, a SC–SC transition is often accompa-nied by a colour change due to variation in the CoIJII) coordi-nation polyhedron which was, however, not alwayssupported by the single crystal data.15–17 Fortunately, dry_1retained its crystallinity despite essential deterioration ofthe crystal quality. This enabled us to collect the diffractiondata and to obtain an acceptable structural model (see theESI†). The structure of dry_1 was also solved in the triclinicspace group P1̄ with the unit cell volume reduced by ~10%compared to 1 (2124 Å3 vs. 2333 Å3). The b axis iscontracted, while the a and c axes are slightly elongated.The unit cell contour changes from oblique to nearly or-thogonal. All solvent molecules were removed, includingthose coordinated to the metal centers in 1. The asymmetricunit of the dark-blue dry_1 is formed by the bdc2− ligandslying about the inversion centers in network B and associ-ated with Co(1B); as well as those bonded to Co(1A) andCo(2A) in network A, so the asymmetric unit has three com-plete Co(II) atoms, two complete bdc2− ligands and two halfbdc2− ligands. The CoIJII) coordination sphere changes froman octahedral to a square pyramidal shape (Fig. 3a), al-though the SBU remains binuclear. Thus, the binuclearSBU transforms from [Co2IJμ2-OH2)IJRCOO)2] into the paddle-wheel [Co2IJμ2-RCOO)4]. Unlike in 1, there are two chemi-cally identical but crystallographically non-equivalent layersin dry_1. As a result, one of the two binuclear SBUs (shownin Fig. 3a) is sitting in a general position and another oneis residing on an inversion center.

Each CoIJII) atom is coordinated by four oxygen atoms fromfour bidentate bridging bdc2− residues and one nitrogenatom from the terminal S-nia ligand. The Co⋯Co separationswithin the paddle-wheel SBUs are 2.784 and 2.735 Å. Thecrystal retains the 2D network, but its structure is modified.Two nearly identical layered motifs have an almost idealsquare-grid (4,4) sql topology with diagonal dimensions of15.513 Å × 15.586 Å (Fig. 3b). All the S-nia ligands appearperpendicular to the layers forming the pillars. The layerspack in the AABAAB mode (Fig. S3†), with significant inter-digitation of the adjacent layers, where the phenyl rings ofS-nia and bdc2− overlap. Thus, the absence of solvent mole-cules modifies both coordination geometry and local connec-tivity of the SBUs, similar to the case reported by Kitagawaet al. for catena-{[Zn2IJbdc)2IJbpb)]·2.5IJdmf)·0.5IJH2O)} (bpb =2,3-difluoro-1,4-bisIJ4-pyridyl)benzene).18

After blue dry_1 was soaked in methanol for one day atroom temperature, the sample changed colour back to pink,producing dry_1s (Fig. 1c). The crystallinity was retained, butthe crystal quality did not allow us to solve the structure. Theunit cell for dry_1s was found to be triclinic with a = 11.11 Å,b = 15.73 Å, c = 18.08 Å, α = 73.81°, β = 86.67°, γ = 76.43°, andV = 2955.16 Å3. This corresponds to ~39% increase of the unitcell volume compared to dry_1, and ~27% increase compared

Fig. 1 Photographs of single crystals of compounds 1 (a), dry_1 (b)and dry_1s (c).

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to the initial 1. Based on the reported molecular volume of67 Å3 for methanol,16a the solvent capacity of the dry_1 formcould be estimated as 12 methanol molecules per unit cell

(the same as the total number of solvent molecules in theunit cell of 1). We performed semiempirical quantum chem-ical calculations of dry_1s starting from 1 and replacing

Fig. 3 (a) View of a paddle-wheel binuclear SBU (network A) and (b) a fragment of the 2D coordination network in dry_1. H-atoms are omittedfor clarity.

Fig. 2 View of (a) a binuclear CoIJII) SBU and (b) a fragment of the 2D coordination network in 1; the co-crystallised solvent molecules and H atomsare omitted for clarity.

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both water and dmf with methanol molecules (see theESI†). An acceptable agreement with the experimental latticeparameters was obtained, with 10–18 methanol moleculesper unit cell.

In order to analyse the nature of the colour change upontransition from 1 to dry_1, Time-Dependent Density Func-tional Theory (TDDFT), which gives accurate spectral predic-tions of organic chromophores and coordination com-plexes,19 was used. The electronic absorption spectra werepredicted for the isolated dinuclear complexes taken fromthe crystal structures of 1 and dry_1. The electronic absorp-tion spectra for these complexes are shown in Fig. S5a andb,† respectively. As follows from the detailed electronic struc-ture analysis (see the ESI†), both binuclear complexes absorbin the 420 to 500 nm range due to the presence of metal to li-gand charge-transfer (MLCT) states. However, in dry_1, twonew absorption peaks appear in the 650–800 nm range,which results in the colour change. These new absorptionpeaks are also dominated by MLCT states, but have consider-ably lower excitation energies due to destabilization of metald-orbitals, directed toward the ligands. This destabilization isdue to the shorter metal to ligand distances in CoIJII) com-plexes with lower coordination number (average Co–O dis-tances are 2.26 Å and 2.02 Å in 1 and dry_1, respectively).Therefore, the pink colour of dry_1s supports our hypothesisof octahedral CoIJII) coordination when resolvated withmethanol.

The limits of thermal stability of 1 were estimated bythermogravimetric analysis. The weight loss occurs in severalconsecutive steps corresponding to the gradual evacuation ofthe solvents and the network degradation (details are givenin the ESI†). The N2 sorption isotherm was recorded fordry_1 (Fig. 4). The compound hardly adsorbs N2 at 77 K,which leads to a Brunauer–Emmett–Teller (BET) surface areaof 43.3 m2 g−1. The isotherm hysteresis loop indicates thepresence of mesopores, which correlates with the pore vol-ume distributions with respect to pore radii. The BET con-stant is equal to 3 and indicates a low affinity of nitrogen tothe test sample. The total pore volume constitutes 0.166cm3 g−1, due to the presence of mesopores. The isotherm in-dicates that adsorption occurs in several steps; such a

behaviour is characteristic for samples with an ordered area,but with different energy adsorption centers.

The magnetic susceptibility data for compounds 1 anddry_1 are displayed as plots of χmT vs. T (Fig. 5). For 1, uponcooling, the χmT values remain constant at 6.8 cm3 K mol−1

down to about 150 K, followed by a distinct decrease to 0.4cm3 K mol−1 at 1.9 K. The high temperature value is muchhigher than the spin-only value of 3.75 cm3 K mol−1 for twononinteracting spins with S = 3/2 (g = 2.0). Clearly, the metalion exhibits a significant orbital contribution, which ishowever quite typical for CoIJII)20 and the range of experimen-tal values compares well with the literature data.20b Theshape of the curve is typical for an exchange coupled CoIJII)system and reflects contributions from both strong spin–or-bit coupling on the 4T1 ground state (Oh notation) and weakantiferromagnetic exchange interaction; there is no levellingof χmT down to 1.9 K. A method of choice to model the datais the empirical Rueff's approach,21 which considers in itsequation contributions from zero-field splitting (D parameter)and magnetic exchange interactions (J parameter); a dummyfactor α weights the two parameters in the equation. Areasonable fit based on the values for the Curie constantC = 3.4 cm3 K mol−1, g = 2.7, |D/k| = 58(4) K, J/k = −4.5IJ1) K(α = 1.8) is shown in Fig. 5a. These parameter values comparewell with the literature values.20 In contrast, dry_1 showsvery different magnetic characteristics (Fig. 5b). At roomtemperature, the χmT value is only 5.2 cm3 K mol−1 and iscontinuously decreasing on lowering the temperature. Whilereflecting on the same theoretical model, the D and J parame-ters will certainly differ from those of 1, but it is doubtfulthat based on this curve shape a physically reliable ratio be-tween them can be deduced and so no fitting procedure hasbeen applied. Still and importantly, the structural change isclearly underlined by the compelling change in the magneticsusceptibility.

In conclusion, the 2D coordination polymer {[Co2IJμ2-OH2)IJbdc)2IJS-nia)2IJH2O)IJdmf)]·2IJdmf)·(H2O)}n (1) shows a dy-namic behaviour in response to the guest removal/inclusion.It exhibits a SC–SC transition under thermal stress accompa-nied by changes in colour, coordination geometry of theCoIJII) centers, changes of the 2D network, and magnetic prop-erties. The colour changes from pink to blue and back to

Fig. 5 (a) Thermal variation of χmT for 1, including the fit and (b) fordry_1 (Figures are scaled per CoIJII)2 units).

Fig. 4 Nitrogen adsorption-desorption isotherm (77 K) and porevolume distribution by radii for dry_1.

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pink again indicate the higher stability of the CoIJII) octahe-dral coordination against the pentagonal one.

Acknowledgements

The authors acknowledge the financial support from the pro-ject SCOPES (IZ73Z0_152404/1). Computational work wassupported by the Russian Science Foundation, project no. 14-43-00052, base organization Photochemistry Center of theRussian Academy of Sciences. Computer time was providedby The Stokes Advanced Research Computing Center, Univer-sity of Central Florida (UCF), the National Energy ResearchScientific Computing Center (NERSC) and the Extreme Sci-ence and Engineering Discovery Environment (XSEDE). Theauthors are indebted to Dr. G. Volodina and Dr. J. Hauser forhelp with powder diffraction and low temperature X-rayexperiments.

Notes and references

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