Synthesis, structure, and redox properties of dicopper(II,II) complexes of N-(2-hydroxyphenyl)salicylamide and homologues. Facile oxidation to dicopper-(II,III) and -(III,III) species; crystal structure of bis(tetrabutylphosphonium) bis{[(2-oxidobenzoyl)(2′-oxidophenyl)amido(3–)-NO2,µ-O2′]cuprate(II)}

Abstract

Binuclear copper (II) complexes of N-(2- hydroxyphenyl)salicylamide (H3L1) and its homologues with a substituent on the ‘2-hydroxyphenyl’ moiety (5-Me, H3L2; 5-Cl, H3L3; 5-NO2, H3L4) have been obtained as tetrabutylphosphonium salts [PBu4]2[CU2L2]. The structure of [ PBu4]2[Cu2L12] was solved by single-crystal X-ray crystallography, which demonstrated that two copper ions are bridged by the phenolic oxygen of the ‘aminophenol’ moiety with a Cu ⋯ Cu separation of 3.035(2)A. Each copper ion adopts an essentially planar configuration, and the Cu–O and Cu–N bond distances involving the salicylamide moiety are relatively short [1.838(6) and 1.915(6)A respectively]. Cryomagnetic investigations (80–300 K) revealed a significant antiferromagnetic spin exchange though the bridging oxygen, the 2J value (energy separation between the spin singlet and triplet states) being –393.4 to –448.8 cm–1. The cyclic voltammogram of each complex showed two oxidation waves attributable to Cu2II,III–Cu2II,III and Cu2II,III–Cu2III,III processes at significantly low potentials. The dicopper(II,III) species of the L2 complex was generated by electrochemical and chemical (with Br2) oxidation and identified by e.s.r. investigations. The mixed-valence complex in dichloromethane showed an e.s.r. signal with a four-line hyperfine structure (Aav. = 79 ×10–4cm–1) at g= 2.10 at room temperature, indicating that the unpaired electron is localized on one of the copper ions on the e.s.r. time-scale. The mixed-valence complexes each showed an intense absorption in the visible region which is tentatively assigned to the intervalence transitions.