Preparation and crystal structure of a binuclear cobalt（Ⅱ）three－dimensional supramolecular complex based on 2，6－pyridine dicarboxylic acid

LI Yan－zhou，LUO Jie，2，YUAN Jing，ZHAO Jun－wei ＊

（1.College of Chemistry and Chemical Engineering，Henan Key Labratory of Polyoxometalate Chemsitry，Henan University，Kaifeng475004，Henan，China； 2.Basic Experimental Teaching Center，Henan University，Kaifeng475004，Henan，China）

In the fields of supramolecular chemistry and material chemistry，attention has been increasingly paid to metal organic complexes，due to their intriguing molecular topologies and crystal packing motifs as well as their versatile applications in catalysis，sorption，optoelectronics，magnetism and molecular recognition［1］.Among various metal organic complexes，metal carboxylate materials are particularly interesting，because，with the assistance of multiple carboxyl groups as linkers between inorganic moieties，they can be assembled into fascinating structures.A large number of metal complexes containing monocarboxylic acid ligands have been structurally characterized and studied［2］.Moreover，polycarboxylic acids as organic ligands have also been developed［3］.Recently，the design and synthesis of organic inorganic hybrid metal carboxylate materials have been mostly concentrated on the reactions of transition－metal（TM）／lanthanide（Ln）salts with N－h(huán)eterocyclic carboxylic ligands.Since 2010，we have conducted systematic investigations on the reactions of TM／Ln cations with N－h(huán)eterocyclic carboxylic ligands to construct novel TM／Ln carboxylate complexes.Those investigations are based on the following considerations.Firstly，flexible N－h(huán)eterocyclic carboxylic ligands with N and O donor atoms can offer good possibility for constructing novel structures.Secondly，according to the hard soft acid base rule，TM cations have a strong affinity to both N－and O－donors while Ln cations have a strong affinity to O－donors rather than N－donors.Viewing these considerations，we successfully synthesized two novel one dimensional（1D）copper bpdc complexes｛［CuⅡ2（Hbpdc）2］Cl2｝2·2H2O and CuⅠ（H2bpdc）Cl（H2bpdc＝ 2，2′－bipyridyl－5，5′－dicarboxylic acid）viaa onepot reaction as well as a three dimensional（3D）3d－4fheterometallic polymer Pr4（H2O）9Cu3.5Cl0.5（bpdc）6.5（OH）2·5H2O in 2010and 2011［4－5］.Very recently，we reported a metal organic complex built up from ferrous sulfate chains and 2，2′－bipyridyl－5，5′－dicarboxylic acid ligands［Fe（H2bpdc）］SO4［6］.Noticing that 2，6－pyridine dicarboxylic acid（H2PDCA）displays a wide range of coordination modes in various complexes［7－8］，in the present research we adopt H2PDCA to construct a 3Dsupramolecular cobalt（Ⅱ）complex［Co2（PDCA）2（H2O）5］·2H2O （1）（CCDC：925868）.In this paper，we report its synthesis and crystal structure.

1 Experimental

1.1 Physical measurements

All reagents were obtained from commercial resources and used without further purification.Elemental analyses（C，H，and N）were performed with a Perkin－Elmer 240Celemental analyzer.A Nicolet 170 SXFT－IR spectrometer was performed to record the infrared（IR）spectrum of as－synthesized product pelleted with KBr in the wavenumber range of 4 000－400cm－1.

1.2 Synthesis of 1

CoCl2·6H2O （0.462g，1.942mmol），2，6－pyridine dicarboxylic acid（0.145g，0.868mmol），TbCl3（0.109g，0.411mmol），K8［α－SiW11O39］·13H2O （0.550g，0.171mmol）and KCl（0.300g，4.024 mmol）were suspended in H2O（15mL）.Resultant mixture was stirred for 6h，followed by heating in a water bath（95℃）for 2has well as cooling and filtering.Resultant filtrate was slowly evaporated at room temperature and dried in air to afford red columnar crystals suitable for X－ray diffraction（XRD）analysis within 3days.Yield：ca.40% （based on 2，6－pyridine dicarboxylic acid）.Anal.calcd.（%）for C14H20Co2N2O15：C，29.29；H，3.51；N，4.88.Found（%）：C，29.46；H，3.73；N，4.60.Unexpectedly，though K8［α－SiW11O39］·13H2O，TbCl3，and KCl were used as the starting materials in the reaction system，they were not observed in1.Relevant paralleling experiments indicate that it is unable to obtain1in the absence of K8［α－SiW11O39］·13H2O，TbCl3，and KCl，which suggests that they might play a synergistic role with other components in the reaction system.Similar phenomena have also been observed in our previous researches［4，6］.

1.3 X－ray crystallography

XRD intensity data for1were collected with a Bruker APEX－II CCD diffractometer at 296（2）K（graphite－monochromated Mo Kαradiation（λ＝0.071 073nm））.Cell constants and an orientation matrix for data collection were obtained from least－squares refinements of the setting angles in the range of 2.27≤θ≤25.00°.Routine Lorentz polarization and an empirical absorption correction were applied to intensity data.The structure was solved by direct methods and the heavy atoms were found by full－matrix leastsquares methods with the SHELXTL－97program package［9］.The remaining atoms were found from successive full－matrix least－squares refinements onF2and Fourier syntheses.Hydrogen atoms associated with water molecules can be located from the difference of relevant Fourier map.Positions of the hydrogen atoms attached to carbon and nitrogen atoms were geometrically placed.All hydrogen atoms were refined isotropically in a riding mode while the default SHELXTL parameters were cited.The crystallographic data and structural refinements for 1 are listed in Table 1.

Table 1 Crystallographic data and structural refinements of 1

2 Results and discussion

2.1 Crystal structure

Single－crystal XRD datum indicates that1crystallizes in the space groupP21／cand its asymmetrical molecular unit consists of 1neutral［Co2（PDCA）2（H2O）5］unit and 2lattice water molecules（Fig.1）.There are two crystallographically unique Co2＋cations and two PDCA2－ligands in the asymmetrical structural unit.Although two Co2＋cations employ the octahedral geometry，their coordination environments are obviously different.Namely，the distorted octahedral Co1cation is defined by five water molecules and a carboxyl oxygen atom from a PDCA ligand.Four water molecules built up the basal plane［Co－O：0.205 47（18）－0.217 00（16）nm］of the distorted octahedral Co1cation，while one water molecule and one carboxyl oxygen atom from a PDCA ligand occupy two axial positions［Co－O：0.208 57（16）－0.208 70（14）nm］.The severely distorted octahedral Co2cation is established by four carboxyl oxygen atoms［Co－O：0.211 28（14）－0.221 60（15）nm］and two nitrogen atoms from two PDCA ligands［Co－N：0.201 60（16）－0.202 36（16）nm］.Both PDCA2－ligands also exhibit distinct coordination modes：one adopts the tridentate coordination manner to bond the Co2cation and another utilizes the tetradentate coordination manner to link the Co1and Co2cations .

Fig.1 View of the asymmetrical structural unit of 1with selected numbering scheme

It should be mentioned that the design and assembly of metal－involved supramolecular architectures are currently of great interest in the field of supramelocular chemistry and crystal engineering，because they can provide novel topology and functional materials［10］.From the viewpoint of supramelocular chemistry，supramolecular architecture is also present in1.Namely，adjacent asymmetrical molecular units are interconnected together through hydrogen－bond interactions between the coordination water molecules（O13and O9）and the carboxylic oxygen atoms（O5and O6）of PDCA2－ligands forming an extended 1Dsupramolecular chain（Fig.2）.The 1Dsupramolecular chains are further combined with each other by means of hydrogen－bond interactions between lattice water or coordination water molecules and other carboxylic oxygen atoms of PDCA2－ligands giving rise to the final extended 3Dsupramolecular network（Fig.3）.The O－H…O distances are in the range of 0.263 9（2）－0.306 2（3）nm.

Fig.2 The 1Dsupramolecular chain constructed by the coordination water molecules（O13and O9）and the carboxylic oxygen atoms（O5and O6）of PDCA2－

2.2 IR spectrum

In the low wavenumber region，the combined vibration modes of carboxylic groups and pyridine ring deformation appear.In general，the carboxylic groups are expected to give rather intense bands from asymmetric（1 500－1 630cm－1）and symmetric（1 350－1 460cm－1）stretching vibration［11－12］.Therefore，the strong absorption band at 1618cm－1is assigned to the asymmetric stretching vibration of carboxylic groups［denoted asνas（CO－2）］while the absorption band at 1 388cm－1corresponds to the symmetric vibration of carboxylic groups［denoted asνs（CO－2）］.Usually，the difference（Δν）betweenνas（CO－2）andνs（CO－2）in the IR spectrum can be used to derive information regarding the bonding modes of carboxylic anions［13］.Namely，theΔνof 230cm－1indicates that the carboxylic groups in1exhibit monodentate coordination mode，which well agrees with corresponding XRD result.The strong absorption peak at ca.1 700 cm－1does not appear，which suggests that the two carboxylic groups are dehydrogenized.Both the asymmetric stretching vibration band and the symmetric vibration band of carboxylic groups split into two peaks，which indicates that the two carboxylic groups are not equivalent.Besides，in the high wavenumber region of the IR spectrum，the strong broad peak at 3 200－3 600cm－1are attributed to water molecules.

Fig.3 The 3Dsupramolecular architecture of 1

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