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 Co^III₂Co^II trinuclear and Co^II₄ 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 Co^III complexes, the hydrazonate anion (Lpy^–) coordinates to a Co^III center in three different coordination modes: a bidentate κ²N¹,N² mode with the E-form (in 1) and two kinds of tridentate modes with the respective configuration, Z-κ³N¹,N²,N⁴ and E-κ³O,N³,N⁴, in 2a and 2b. The former tridentate mode was sterically favored in the Co^III 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 Co^II ion with the κ²O,N³ mode, forming the Co^III–Co^II–Co^III trinuclear complex (3). In the novel tetranuclear Co^II complex (4), the E-Lpy^– hydrazonates bridged two Co^II ions in two different modes: μ-κ²O,N¹:κ³O,N³,N⁴ and μ-κ²N¹,N²:κ³O,N³,N⁴, where all chelate coordinations formed a five-membered ring with the Co^II ion.