Electron Spin Resonance, Magnetic, and X-ray Crystallographic Studies of a Binuclear, Imidazolate Bridged Copper(II) Complex, [(TMDT)₂Cu₂(im)(ClO₄)₂](ClO₄)

Abstract

The syntheses of the mononuclear [(TMDT)Cu(NO3)](PF6), [(TMDT)Cu(l-MeIm)](ClO4)2, and binuclear [(TMDT)2Cu2(im)(ClO4)2](ClO4) complexes, where TMDT=1,1,7,7-tetramethyldiethylenetriamine, 1-MeIm is 1-methyl-imidazole, and is the imidazolate ion, are described. Single-crystal X-ray diffraction studies of the binuclear complex revealed the geometry of the imidazolate bridged dicopper(II) center. Each copper is pentacoordinate, with a bridging imidazolate and three TMDT nitrogen atoms forming an approximate plane and a perchlorate oxygen atom in the apical position of a distorted tetragonal pyramid. The bridging ligand is nearly perpendicular to both copper coordination planes and steric interactions restrict rotation about the Cu-N 1 and Cu-N3 bonds, which have respective distances of 1.944 (12) and 1.966 (14) A. The two copper mean coordination planes intersect at an angle of 148.7°. Two weakly coordinated, axial perchlorate ligands, the two N-H protons of the TMDT ligands, and the C2-H proton of te bridging imidazolate ligand all occupy space on one side (inside the shallow “V”) of the binuclear complex. The Cu-Cu distance is 5.935 (4) A. Variable temperature (4.2-295 K) magnetic susceptibility data for the bridged complex show an antiferromagnetic exchange interaction with a coupling constant (H=-2JS1-S2) J=-25.80 (16) cm-1. The exchange interaction is also apparent in the powder ESR spectrum which has a weak AM=2 transition exhibiting well-resolved copper hyperfine coupling and a characteristic line shape for the ΔM=1 transition. Temperature and pH dependent ESR and optical spectroscopic studies of frozen 50% aqueous Me2SO solutions of all three complexes have also been carried out and show that the bridged cation [(TMDT)2Cu2(im)(ClO4)2]+ exists as a major species in solution over the fairly narrow range 8.5 ≲ pH ≲ 9.5. At low pH the bridge is broken and eventually free cupric ion forms; hydroxide ion is capable of breaking the bridge in alkaline solutions. The magnetic and ESR spectral properties of the binuclear complex are in excellent agreement with those of bovine erythrocyte superoxide dismutase in which copper(II) is substituted for zinc(II). © 1978, American Chemical Society. All rights reserved.