Dioxygen—Copper Reactivity. Reversible Binding of O₂ and CO to a Phenoxo-Bridged Dicopper(I) Complex

A chemical system possessing features that mimic certain structural properties and the O₂ binding behavior of the active site of the copper-containing dioxygen carrier hemocyanin (He) is presented. A phenolic dinucleating ligand possessing two tridentate py2 units (py2 = bis(2-(2-pyridyl)ethyl)amine) (XYL-OH) forms a phenoxo-bridged dicopper(I) complex, [Cu₂(XYL-0-)]PF₆(1). At -80 °C in dichloromethane solution, 1 reacts with O₂(Cu:O₂= 2:1) to give an intensely purple-colored dioxygen adduct, [Cu₂(XYL-0-)(O₂)]PF₆(2), having charge-transfer absorptions in the visible region at 505 (e 6000) and 610 (sh) nm (e 2100 (M'Cm)“¹) with additional features at 385 (e 2900), 790 (e 700), and 925 nm (e 600 (M.cm)^_1). The binding of O₂to 1 is reversible, and the O₂ ligand can be removed in vacuo to regenerate 1. This vacuum cycling can be followed spectrophotometrically over several cycles. Carbon monoxide and triphenylphosphine react with 1 to form the bis adducts 3,[cu2(xyl-0-)(c0)2]pf6(3a) and [Cu₂(XYL-0-)(PPh₃)₂]PF₆(3b), respectively; the reaction with carbon monoxide is also reversible. The dioxygen adduct 2 reacts with CO and/or PPh₃, displacing the bound O₂ ligand and producing the adducts 3; this behavior further substantiates that the reaction of 1 with O₂ is an equilibrium process. Carbonyl cycling, where 1 reacts with O₂to produce 2, O₂is displaced by CO to produce 3a, and 3a is decarbonylated to regenerate 1, can also be followed spectrophotometrically over several cycles. In both cycling procedures, a decomposition product is observed and identified as the phenoxo and hydroxo doubly bridged dicopper(II) complex [Cu₂(XYL-0)(OH)]²⁺. Crystallographic studies have been completed for both 1 and 3b. 1 crystallizes in the monoclinic space group P2_1/n with Z = 8 (2 per asymmetric unit) and a = 13.861 (4) Å, b = 13.482 (8) Å, c = 16.956 (5) Å, and 0 = 98.20 (2)°. Complex 3b crystallizes in the triclinic space group P with Z = 2 and a = 13.410 (3) Å, b = 14.867 (3) Å, c = 18.990 (4) Å, a = 102.35 (2)°, 0 = 91.71 (2)°, and y ~ 98.68 (2)°. While 1 contains two phenoxo-bridged tetracoordinate Cu(I) ions (Cu—Cu = 3.62-3.72 Å) where each copper atom is also coordinated to the py2 tridentate group, in 3b one pyridine donor from each py2 unit remains uncoordinated such that each Cu(I) atom is bound by two nitrogen donors, the P atom from a PPh₃ ligand, and the bridging phenoxo oxygen atom (Cu-Cu = 3.992 A).