Synthesis of Water-Soluble Planar Cobalt(II), Nickel(II), and Copper(II) Hydroxo Clusters Using a (1,4,7-Triazacyclononane)cobalt(III) Complex as a Hydrolysis-Terminating Group

We report on a group of planar cobalt(II), nickel(II), and copper(II) hydroxo clusters that have a definite composition and are water-soluble: [{Co(tacn)(OH)2}6Co7(OH)12](NO3)2(CF3SO3)6·10H2O (1), [{Co(tacn)(OH)2}6Ni7(OH)12](NO3)2(CF3SO3)6·10H2O (2a), [{Co(tacn)(OH)2}6Ni7(OH)12](BNPP)8·6CH3NO2·8H2O [2b; BNPP = bis(p-nitrophenyl)phosphate], [{Co(tacn)(OH)2}12Ni16(OH)26(OH2)2](SO4)4(CF3SO3)10·30H2O (3a), [{Co(tacn)(OH)2}12Ni16(OH)26(OH2)2](SO4)8(CF3SO3)2·44H2O (3b), [{Co(tacn)(OH)2}2Co2(OH)2(OH2)4](SO4)(CF3SO3)2·4H2O (4), [{Co(tacn)(OH)2}2Ni2(OH)2(OH2)4](SO4)(CF3SO3)2·4H2O (5), and [{Co(tacn)(OH)2}4Cu4(OH)6](ClO4)6·5H2O (6), where tacn is 1,4,7-triazacyclononane. The peripheral of each metal hydroxo cluster plane is chemically protected by the coordination of {CoIII(tacn)(OH)2}+ groups to prevent further hydrolysis. These clusters were synthesized by the reaction of an equimolar amount of [Co(tacn)(OH2)3]3+ and cobalt, nickel, or copper salt at pH values in the range of 6.0-12.0. The structure of the cation in compounds 1, 2a or 2b, 4, and 5 is relevant to the surface structure of the cobalt phosphate and nickel borate oxygen-evolution catalysts; in particular, the Co7(OH)12 core in 1. Moreover, the arrangement of M7(OH)12 in 1 and 2a or 2b and Cu4(OH)6 in 6 represents the solid-state structures of the (111) face of the cubic CoO or NiO and the (002) plane of Cu(OH)2, respectively. Extended X-ray absorption fine structure spectra of an aqueous solution of 1, 2a, 4, and 5 exhibit well-resolved peaks at the first and second coordination spheres due to the M-O and M···M distances, respectively; the solution-state bond distances were estimated, and they agreed well with the bond distances in the solid-state structures.