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)Å. 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)Å 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 Cu₂^II,III–Cu₂^II,III and Cu₂^II,III–Cu₂^III,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 (A_av. = 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.