Electrochemical synthesis and crystal structure studies of defective dicubane tetranuclear hydroxo and carboxamide ligand bridged cobalt(II)–cobalt(III) complexes with carboxamides produced from unprotected hydroxyaromatic carboxylic acids

Abstract

Two new tetraanionic tetradentate carboxamide ligands ( L ) have been synthesized from unprotected hydroxyaromatic carboxylic acids using a benign method. Mixed valance tetranuclear cobalt complexes, [Co 4 (μ 3 -OH) 2 (μ 2 -L) 2 (CH 3 CN) 6 ] have been synthesized by electrochemical oxidation of cobalt metal in acetonitrile solution. X-ray analysis revealed that cobalt complexes consist of discrete mixed valence (Co 2 II -Co 2 III ) defective dicubane molecular units with distorted octahedral geometry around each cobalt atom. • Benign synthesis of two new carboxamide ligands, H 4 L from hydroxyaromatic acids. • Electrochemical synthesis of their mixed valance tetranuclear cobalt complexes. • X-ray crystal structures of the ligands and [Co 4 (μ 3 -OH) 2 (μ 2 -L) 2 (CH 3 CN) 6 ] complexes. • The cobalt ions are coordinated by the intact ligands, in deprotonated form, L 4− . • Distorted octahedral geometry around each Co, in discrete (Co 2 II –Co 2 III ) defective dicubane molecular units. The benign synthesis of two new carboxamide ligands L 1 = 1,2-bis(2-hydroxybenzamido)-4,5-dimethylbenzene (H 4 Me 2 hybeb) and L 2 = 1,2-bis(2-hydroxy-3-naphthamido)-4,5-dimethylbenzene (H 4 Me 2 hyneb) from unprotected hydroxyaromatic carboxylic acids and the electrochemical synthesis of their mixed valance tetranuclear cobalt complexes, [Co 4 (μ 3 -OH) 2 (μ 2 -Me 2 hybeb) 2 (CH 3 CN) 6 ] ( 1 ), and [Co 4 (μ 3 -OH) 2 (μ 2 -Me 2 hyneb) 2 (CH 3 CN) 6 ] ( 2 ) by electrochemical oxidation of cobalt metal in acetonitrile solution is reported. The carboxamide ligands act as tetraanionic tetradentate ligands. The crystal structures of the two ligands L 1 and L 2 and of complex 1 have been determined in detail by X-ray crystallography. Complexes 1 and 2 consist of discrete mixed valence (Co 2 II –Co 2 III ) defective dicubane molecular units, where each cobalt atom attains a distorted octahedral geometry.