COPPER(I) COMPLEXES WITH PYRIDYL-CONTAINING AND IMIDAZOYL-CONTAINING TRIPODAL TETRADENTATE LIGANDS AND THEIR REACTIONS WITH DIOXYGEN

Continuing investigations of copper(I) complex reactions with dioxygen have led us to utilize imidazoyl-containing ligands, with the potential to reveal more about these donor ligand groups of significance in copper-containing proteins. The tripodal tetradentate ligands BPIA (bis((2-pyridyl)methyl)((1-methylimidazol-2-yl)methyl)amine) and BIPA (bis(1-methylimidazol-2-yl)methyl)((2-pyridyl)methyl)amine) have been synthesized. These are close analogues of TMPA (=tris((2-pyridyl)methyl)amine), where we previously showed that [(TMPA)Cu(RCN)]+ (1a) (R = Me, Et) reacts (-80-degrees-C) with O2 reversibly, to form the trans-(mu-1,2-peroxo)dicopper(II) complex [{(TMPA)Cu}2(O2)]2+ (1b). Copper(I) complexes of BPIA and BIPA have been synthesized, and the X-ray structure of [(BPIA)2Cu2]2+ (2a') was determined; for C36H36Cu2F6N10O6S2, a = b = 14.057(4) angstrom, c = 41.755(6) angstrom, tetragonal space group I42d, and Z = 8. The structure is a dimer, and each Cu(I) ion is coordinated in a trigonal pyramidal N4 environment. Dinuclear complex formation comes about as a result of ligation of the imidazolyl group of the BPIA donor to the adjacent Cu(I) ion. Conductivity measurements reveal that the cuprous complexes are mononuclear in nitrile or dimethylformamide solvents, thus formulated as [(BPIA)Cu(S)]+ (2a) and [(BIPA)Cu(S)]+ (3a) (S = solvent). Substitution of one or two imidazolyl donors for pyridyl groups in 1a, on going to 2a or 3a, has little effect on redox properties. Cyclic voltammetry measurements indicate that 1a-3a undergo quasi-reversible one-electron oxidation processes, occurring close to -600 mV versus Ag/AgNO3. Low-temperature (-80-degrees-C) oxygenation of 2a does result in the generation of a stable dioxygen adduct, formulated as [{(BPIA)Cu}2(O2)]2+ (2b) on the basis of manometric measurements (Cu/O2 = 2:1 in EtCN at -80-degrees-C), its EPR silence, and the close similarity of its UV-vis spectrum (lambda(max) = 535 nm, epsilon = 11 500 M-1 cm-1) compared to that of 1b. By contrast, 3a reacts rapidly with O2 at -80-degrees-C to give an intensely red-colored solution (lambda(max) = 330 nm), but the putative O2 adduct is unstable, as judged by the bleaching of the solution within minutes. The results are compared to ligand/copper complex systems containing pyridyl and/or quinolyl donors.