The reversible binding of CO and O2 to dinuclear copper(I) complexes is described in chemical systems which mimic to a significant extent a number of properties of the copper-containing dioxygen carrier protein hemocyanin (Hc). Various dicopper(I) complexes have been synthesized; these utilize neutral dinucleating ligands L, in which two tridentate PY2 units (PY2 = bis[2-(2-pyridyl)ethyl]amine) are connected by a variable alkyl chain -(CH2)n- (L = Nn, n = 3-5). These include [Cu2(L)]2+ (1, L = Nn, n = 3-5), which are tricoordinate dicopper(I) complexes, and [Cu2(L)(L')2]2+ (3, L = Nn and L' = CH3CN; 4, L' = CO; 5, L' = PPh3), which are bisadducts possessing tetracoordinate Cu(I) moieties. All of these dicationic compounds are isolated as either perchlorate or hexafluorophosphate salts. The binding of CO to 1 is reversible, as indicated by the ability to regenerate 1 by the application of a vacuum to dichloromethane solutions of [Cu2(N3)(CO)2]2+ (4) at room temperature. The structure of the bis(carbonyl) adduct [Cu2(N3)(CO)2](C1O4)2 (4a(C1O4)2) has been determined by X-ray crystallographic studies, and the results are described. Complex 4a(C1O4)2 crystallizes in monoclinic space group P2l/n with a = 10.255 (2) Å, b = 25.943 (3) Å, c = 14.767 (3) Å, β = 94.94 (1)°, and Z = 4 and was refined to R = 0.0781. Both complexes 1 and 3 serve as precursors to the dioxygen adducts [Cu2(Nn)(O2)]2+ (2), which form and are stable only at low temperatures, i.e., -80°C, in dichloromethane solutions. These species are characterized by strong and multiple electronic neutral absorptions in the visible region, including a prominent band in the 350-360-nm range (e = 14000-21 400 M-1 cm-1). The reaction of 1 with O2 is reversible, and the application of a vacuum to the dioxygen adduct (2) formed removes the bound O2 and regenerates 1. The recovered dioxygen can be identified both qualitatively and quantitatively (a gravimetric method is described), and the vacuum cycling can be followed spectrophotometrically over several cycles. In addition, saturating a -80°C solution of the dioxygen complexes with carbon monoxide results in the displacement of the O2 ligand with the resulting formation of the bis(carbonyl) adducts [Cu2(Nn)(CO)2]2+ (4). Carbonyl cycling, where 1 reacts with O2 to produce 2, dioxygen is displaced by CO to give 4, and 4 is decarbonylated to regenerate 1, can also be followed spectrophotometrically over several cycles. Manometric measurements indicate that the stoichiometry of the reaction of 1 or 3 with O2 at -80°C is Cu:O2 = 2:1, and a variety of other evidence (e.g. X-ray absorption spectroscopy, UV-vis CT bands, presence of d-d bands) lead to the conclusion that complexes [Cu2(Nn)(O2)]2+ (2) are best described as peroxodicopper(II) complexes, formed by reactions of O2 with 1 in an intramolecular process. Other characteristics of these (Cu2-O2)2+ species are described, including EPR (silent), 1H NMR (normal), and magnetic behavior (essentially diamagnetic). The electrochemical properties (cyclic voltammetry) of 1 and 2 are also reported. A novel bent ε-ε2:ε2-peroxo-di-copper(II) structure is proposed for [Cu2(L)(O2)]2+ (2), consistent with the physical properties observed as in line with the recent important structure described by Kitajima and co-workers (J. Am. Chem. Soc. 1992, 114, 1277-1291). Comparison of [Cu2-(Nn)(O2)]2+ (2) with other types of (Cu2-O2) complexes are made, including when L is a m-xylyl group (L = XYL-X), where complexes like 2 are seen to be intermediates in the hydroxylation reactions which occur when [Cu2(XYL-H)]2+ (le) is reacted with O2. The biological relevance of the studies is discussed, and a structure similar to that occurring in [Cu2(L)(O2)]2+ (2) is suggested to occur in oxyhemocyanin. Contribution from the Lehrstuhl fttr Anorganische Chemie I, Ruhr-Universitat, D-4630 Bochum, Germany, Institut fttr Anorganische und Analytische Chemie der Universitat, D-7800 Freiburg, Germany, and Anorganisch-Chemisches Institut der Universitat, D-6900 Heidelberg, Germany.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Inorganic Chemistry