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)}Co(OH)](NO)(CFSO)·10HO (), [{Co(tacn)(OH)}Ni(OH)](NO)(CFSO)·10HO (), [{Co(tacn)(OH)}Ni(OH)](BNPP)·6CHNO·8HO [; BNPP = bis(-nitrophenyl)phosphate], [{Co(tacn)(OH)}Ni(OH)(OH)](SO)(CFSO)·30HO (), [{Co(tacn)(OH)}Ni(OH)(OH)](SO)(CFSO)·44HO (), [{Co(tacn)(OH)}Co(OH)(OH)](SO)(CFSO)·4HO (), [{Co(tacn)(OH)}Ni(OH)(OH)](SO)(CFSO)·4HO (), and [{Co(tacn)(OH)}Cu(OH)](ClO)·5HO (), where tacn is 1,4,7-triazacyclononane. The peripheral of each metal hydroxo cluster plane is chemically protected by the coordination of {Co(tacn)(OH)} groups to prevent further hydrolysis. These clusters were synthesized by the reaction of an equimolar amount of [Co(tacn)(OH)] and cobalt, nickel, or copper salt at pH values in the range of 6.0-12.0. The structure of the cation in compounds , or , , and is relevant to the surface structure of the cobalt phosphate and nickel borate oxygen-evolution catalysts; in particular, the Co(OH) core in . Moreover, the arrangement of M(OH) in and or and Cu(OH) in represents the solid-state structures of the (111) face of the cubic CoO or NiO and the (002) plane of Cu(OH), respectively. Extended X-ray absorption fine structure spectra of an aqueous solution of , , , and 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.