Versatility of coordination modes of N'-(pyridin-2-ylmethylene) picolinoylhydrazidate in the mononuclear cobalt(III) and polynuclear cobalt(II) complexes

Molecular and crystal structures of the mononuclear cobalt(III) complexes bearing N'-(pyridin-2-ylmethylene) picolinoylhydrazidate (Lpy-), [Co(Lpy)3] (1) and [Co(Lpy)2]BF4 (2a and 2b), as well as those of the Lpy-bridged CoIII2CoII trinuclear and CoII4 tetranuclear complexes, [Co3(Lpy)4(H2O)2](BF4)4 (3) and [Co4(Lpy)4Cl2(CH3CN)2] (BF4)2 (4), were determined by the single-crystal X-ray diffraction method, which revealed a variety of coordination and bridging modes of the multidentate Lpy- ligand. The protonated free hydrazone, HLpy, exists as an E-form (in regard to the imine configuration) in the solid state and in solution. In the mononuclear CoIII complexes, the hydrazonate anion (Lpy-) coordinates to a CoIII center in three different coordination modes: a bidentate kappa 2N1,N2 mode with the E-form (in 1) and two kinds of tridentate modes with the respective configuration, Z-kappa 3N1,N2,N4 and E-kappa 3O,N3,N4, in 2a and 2b. The former tridentate mode was sterically favored in the CoIII complexes, resulting in the five- and six-membered chelate rings, with a conversion of the imine configuration during the complexation. Complex 2a could also serve as a complex ligand toward a CoII ion with the kappa 2O, N3 mode, forming the CoIII-CoII-CoIII trinuclear complex (3). In the novel tetranuclear CoII complex (4), the ELpy- hydrazonates bridged two CoII ions in two different modes: mu-kappa 2O,N1:kappa 3O,N3,N4 and mu-kappa 2N1,N2:kappa 3O,N3,N4, where all chelate coordinations formed a five-membered ring with the CoII ion.