Copper(I)/O(2)Chemistry with Imidazole Containing Tripodal Tetradentate Ligands Leading to mu-1,2-Peroxo-Dicopper(II) Species

Abstract

Cuprous and cupric complexes with the new imidazolyl containing tripodal tetradentate ligands {L-MIm, (1H-imidazol-4-yl) -N,N-bis((pyridin-2-yl)methyl)methanamine, and L-EIm, 2-(1H-imidazol-4-yl)-N,N-bis((pyridin-2-yl)methyl)ethanamine}, have been investigated to probe differences in their chemistry, especially in copper(I)-dioxygen chemistry, compared to that already known for the pyridyl analogue TMPA, tris(2-pyridyl)methyl)amine. Infrared (IR) stretching frequencies obtained from carbon monoxide adducts of [(L-MIm)Cu-I](+) (1a) and [(L-EIm)Cu-I](+) (2a) show that the imidazolyl donor is stronger than its pyridyl analogue. Electrochemical data suggest differences in the binding constant of Cu-II to L-EIm compared to TMPA and L-MIm, reflecting geometric changes, Oxygenation of [(L-MIm)Cu-I](+) (1a) in 2-methyltetrahydrofuran (MeTHF) solvent at -128 degrees C leads to an intensely purple colored species with a UV-vis spectrum characteristic of an end-on bound peroxodicopper(II) complex [{(L-MIm)Cu-II}(2)(mu-1,2-O-2(2-))](2+) (1b(P)) {lambda(max)=528 nm}, very similar to the previously well characterized complex [{(TMPA)Cu-II}(2)(mu-1,2-O-2(2-))](2+) {lambda(max)=520 nm (epsilon=12 000 M-1 cm(-1)), in MeTHF; resonance Raman (rR) spectroscopy: nu(O-O)=832 (Delta(O-18(2))=-44) cm(-1)}. In the low-temperature oxygenation of 2a, benchtop (-128 degrees C) and stopped-flow (-90 degrees C) experiments reveal the formation of an initial superoxo-Cu(II) species [(L-EIm)Cu-II(O-2(center dot-))](+) (2b(S)), lambda(max)=431 nm in THF). This converts to the low-temperature stable peroxo complex [{(L-EIm)Cu-II}(2)(mu-1,2-O-2(2-))](2+) (2b(P)) {rR spectroscopy: nu(O-O)=822 (Delta(O-18(2))=-46) cm(-1)}. Complex 2b(P) possess distinctly reduced Cu-O and O-O stretching frequencies and a red-shifted UV-vis feature {to lambda(max)=535 nm (epsilon=11 000 M-1 cm(-1))} compared to the TMPA analogue due to a distortion from trigonal bipyramidal (TBP) to a square pyramidal ligand field. This distortion is supported by the structural characterization of related ligand-copper(II) complexes: A stable tetramer cluster complex [(mu(2)-LEIm-)(4)-(Cu-II)(4)](4+), obtained from thermal decomposition of 2b(P) (with formation of H2O2), also exhibits a distorted square pyramidal Cu(II) ion geometry as does the copper(II) complex [(L-EIm)Cu-II(CH3CN)](2+) (2c), characterized by X-ray crystallography and solution electron paramagnetic resonance (EPR) spectroscopy.