Complexes of ditopic carbo- and thio-carbohydrazone ligands – mononuclear, 1D chain, dinuclear and tetranuclear examples

Abstract

Ligands based on carbo- and thio-carbohydrazone cores, modified with pyridine, carboxylate and oxime ends, have been examined. They display a tautomeric versatility based on the flexible nature of the hydrazone linkages, leading to varied coordination motifs. Examples of mononuclear (Co(II), Ni(II)), dinuclear (Co(III)), 1D chain (Cu(II)) and square [2 × 2] grid (Ni(II)) complexes are obtained. Ferromagnetic (Cu(II)) and antiferromagnetic (Ni(II)) exchange is observed, with spin coupling in the Ni(II)₄ square grids propagated through the μ-O and μ-S bridges. Weak antiferromagnetic exchange (J = −6.0 cm⁻¹) is observed for the μ-O bridged grid, despite the large Ni–O–Ni angles (137–141°), while for the μ-S bridged grids much stronger exchange is observed (J = −148 cm⁻¹, −198 cm⁻¹). This is much larger than expected based on the Ni–S–Ni bridge angles (151-169°), and is associated with the soft (less polarizing than oxygen) nature of the sulfur bridge, which would allow for much more efficient transmission of spin exchange than observed in the μ-O bridged case. Structures and variable temperature magnetic data are included, and spin exchange is analyzed using normal Heisenberg exchange models. No examples involving oxime (NO) bridging are reported, which reflects the positioning of the N,O and N,S donor combinations in each ligand, and the preferred coordination through these donor atoms.