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 } 6 Co 7 (OH) 12 ](NO 3 ) 2 (CF 3 SO 3 ) 6 ·10H 2 O ( 1 ), [{Co(tacn)(OH) 2 } 6 Ni 7 (OH) 12 ](NO 3 ) 2 (CF 3 SO 3 ) 6 ·10H 2 O ( 2a ), [{Co(tacn)(OH) 2 } 6 Ni 7 (OH) 12 ](BNPP) 8 ·6CH 3 NO 2 ·8H 2 O [ 2b ; BNPP = bis( p -nitrophenyl)phosphate], [{Co(tacn)(OH) 2 } 12 Ni 16 (OH) 26 (OH 2 ) 2 ](SO 4 ) 4 (CF 3 SO 3 ) 10 ·30H 2 O ( 3a ), [{Co(tacn)(OH) 2 } 12 Ni 16 (OH) 26 (OH 2 ) 2 ](SO 4 ) 8 (CF 3 SO 3 ) 2 ·44H 2 O ( 3b ), [{Co(tacn)(OH) 2 } 2 Co 2 (OH) 2 (OH 2 ) 4 ](SO 4 )(CF 3 SO 3 ) 2 ·4H 2 O ( 4 ), [{Co(tacn)(OH) 2 } 2 Ni 2 (OH) 2 (OH 2 ) 4 ](SO 4 )(CF 3 SO 3 ) 2 ·4H 2 O ( 5 ), and [{Co(tacn)(OH) 2 } 4 Cu 4 (OH) 6 ](ClO 4 ) 6 ·5H 2 O ( 6 ), where tacn is 1,4,7-triazacyclononane. The peripheral of each metal hydroxo cluster plane is chemically protected by the coordination of {Co III (tacn)(OH) 2 } + groups to prevent further hydrolysis. These clusters were synthesized by the reaction of an equimolar amount of [Co(tacn)(OH 2 ) 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 Co 7 (OH) 12 core in 1 . Moreover, the arrangement of M 7 (OH) 12 in 1 and 2a or 2b and Cu 4 (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.