Coordination of Hydrazone Derivatives of 2,2'-Bipyridine-6-carbaldehyde. Electronic and Structural Aspects of the Iron(II), Cobalt(II) and Nickel(II) Complexes

Abstract

Iron(II), cobalt(II) and nickel(II) [MN₆]²⁺ type complexes of the tridentate terimine ligands 2,2′-bipyridine-6-carbaldehyde phenylhydrazone ( bph ) and 2,2′-bipyridine-6-carbaldehyde 2-pyridylhydrazone ( bpyh ) have been prepared. The electronic spectrum of the [NiN₆]²⁺ species indicates that the field strength of these ligands is near that at the iron(II) singlet/quintet crossover. Magnetic and Mossbauer spectral data reveal that salts of [Fe( bph )₂]²⁺ are essentially low spin, while those of [Fe( bpyh )₂]²⁺ are high spin. Salts of [Co( bph )₂]²⁺ have strongly temperature-dependent magnetic moments which indicate a thermally induced doublet ↔ quartet spin transition in the metal atom. The structure of [Ni( bph )₂][ClO₄]₂ reveals meridional coordination of the tridentate units, the azomethine nitrogen of the hydrazone moiety being bound to the metal atom. There is significant twisting (8.3°) of the two pyridyl rings in each ligand molecule about the interring bridge. The [NiN₆]²⁺ fragment is tetragonally compressed and the mean Ni-N distance is 2.09 Ǻ. [Ni( bph )₂][ClO₄]₂ is monoclinic, space group C2/c, Z = 4, a 23.949(3), b 7.868(1), c 21.303(3)Ǻ, β 117.95(2)°.