The binding of O2 to copper(I) and its subsequent reactivity is important in a variety of chemical and biological processes. We previously studied functional models of hemocyanins (arthropodal and molluscan O2 carriers) involving a class of O2 adducts [Cu2(Nn)(O2)]2+ (2) (Karlin, K. D.; et al. Inorg. Chem. 1992, 31, 1436-1451), where Nn are dinucleating ligands which employ two tridentate bis(2-pyridylethyl)amine (PY2) moieties, connected by a variable alkyl chain. Here, we report studies of O2 reactivity using R-substituted PY2 tridentate ligands and derived mononuclear Cu(I) precursor complexes [Cu(RPY2)]+ (3a, R = Me; 3b, R = PhCH2; 3c, R = Ph). The X-ray structure of the nearly “T-shaped” complex 3c(PF6) (C20H21CuN3F6P) is reported; it crystallizes in the space group P21/n, a = 10.037 (4) Å, b = 14.406 (4) Å, c = 15.210 (5) Å, β = 105.99 (3)°, Dcalcd = 1.61 g/cm3, Z = 4. Electrochemical studies indicate reversible one-electron oxidations in DMF, with E1/2 values varying between +80 and +150 mV vs NHE, depending on R. Above 0 °C, complexes 3 react with O2 in a stoichiometry of Cu/O2 = 4:1 (manometry) giving oxo-bridged dicopper(II) products [Cu2(RPY2)2(0)]2+ (4). EPR spectra are consistent with the bridged dinuclear formulation; i.e., 4a, c are EPR silent while 4b exhibits a typical “half-field” (g = 4) line in CH2C12. In methanol, the complexes break up and exhibit normal tetragonal spectra. Oxo complexes 4 react with water (reversibly, for 4a, b) to give dihydroxo-bridged complexes [Cu2(RPY2)2(OH)2]2+ (5), with “parallel-planar” structures; EPR spectroscopic and other physical measurements are consistent. However, low-temperature (-80 °C) oxygenation of complexes 3 results in the formation of O2 adducts [Cu2(RPY2)2(O2)]2+ (6), as indicated by manometry (Cu/O2 = 2:1) and EPR (“silent”) and UV-vis (i.e., strong 350-360-nm absorption) spectroscopies. The latter indicates a close similarity of these complexes to 2, with their proposed bent (μ-n2:n2-peroxo)dicopper(II) structures. Prolonged exposure of [Cu(PhCH2PY2)]+ (3b) to O2 causes a novel N-dealkylation reaction, with oxidative cleavage of the ligand producing H-PY2 and benzaldehyde (20% yield). Isotope labeling studies reveal that the source of oxygen in the PhCHO product is O2. The relationship of the chemistry presented here to other copper-dioxygen chemistry and copper proteins is discussed.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Inorganic Chemistry