Spin equilibrium in five-co-ordinate Lewis-base adducts of cobalt(<scp>II</scp>) Schiff-base complexes with a S₂N₃ligand environment. Molecular and crystal structure of [CoL⁴(mim)] at 293 and 103 k [H₂L⁴=N,N′-bis(5-mercapto-3-methyl-1-phenylpyrazol-4-ylmethylene)-o-phenylenediamine, mim =N-methylimidazole]

Four novel five-co-ordinate Lewis-base adducts of cobalt(II) Schiff-base complexes with a S2N3 ligand environment have been prepared and their variable-temperature magnetic susceptibilities determined (T= 77–300 K). The χco–1vs. T plots revealed a minimax behaviour characteristic of an S=½⇌ spin equilibrium. The corresponding magnetic moments decrease in a gradual manner on lowering the temperature. The thermodynamic parameters for the equilibrium were estimated from the magnetic moment data and ΔH° values were found to lie in the range of 273–757 cm–1. Decrease of the in-plane ligand field causes a decrease of the transition temperature and stabilization of the high-spin relative to the low-spin ground state. The X-ray single-crystal structural determinations for [CoL4(mim)][H2L4=N,N′-bis(5-mercapto-3-methyl-1-phenylpyrazol-4-ylmethylene)-o-phenylenediamine, mim =N-methylimidazole] were carried out at 293 (µeff= 3.6 µB) and 103K (µeff= 2.3 µB). The co-ordination geometry around the cobalt centre revealed a distorted trigonal bipyramid with one N and one S atom of the metallocycle as axial ligands, and the other N, S atoms and the N-imidazole nitrogen situated in an equatorial plane. Comparison of the molecular structures at different temperatures indicates an occurrence of some configurational and conformational changes along with variations in metal–ligand bond lengths associated with a spin transition. Lowering the temperature results in increase of one of the N–Co–S valence angles by 7.2°, an increase in the Co–N(imidazole) bond length by 0.023 Å but decreases in the other metal–ligand bonds by 0.02–0.05 Å.