Synthesis of conformational isomers, thermally induced isomerism and crystal structure of cis-di-isothiocyanatobis(N-methylpropane-1,3-diamine)nickel (II)

Two isomers, (1) and (1a), of [NiL₂(NCS) ₂](L = N-methylpropane-1,3-diamine) have been synthesised. Isomer (1) undergoes an irreversible endothermic solid-state phase transition (383–413 K, ΔH= 8.1 kJ mol^–1) transforming into species (1a). This isomerisation also takes place on crystallisation from methanol, ethanol, chloroform, or acetonitrile. Species (1a) has been characterised by X-ray crystallography. The crystals are monoclinic, space group P2₁/c with four formula units in a unit cell of dimensions a = 8.136(2), b= 13.057(2), c = 15.235(3)Å and β= 90.75(1)°. The structure has been solved by Patterson and successive Fourier syntheses and refined by block-diagonal least-squares calculations to R = 0.048 and R′ = 0.061 for 2 311 reflections. The metal atom co-ordination is nearly octahedral with the two thiocyanate groups in cis position. The overall configuration and conformation of the complex molecule is Λ_pp (two chairs fold in the same rotational direction). Isomer (1) probably exists in the Λ_pa form (two chairs fold apart from each other)