Crystal and molecular structure of bis(μ-bromo)bis(n,n,n',n'-tetraethyletliylenediamine)dicopper(I) and the kinetics of its oxidation by dioxgen in nitrobenzene¹

An X-ray structural determination of the air-sensitive product of reaction of equimolar N,N,N^', N^'-tetraethylethylenediamine, (C₂H₅)₂N(CH₂)₂N(C₂Hs)₂ (TEED), with Copper(I) bromide in dioxygen-free methylene chloride has revealed discrete molecular dimers containing 4-coordinate copper(I) centers in bis(μ-bromo)bis(N,N,N^', N^'-tetraethylethylenediamine)-dicopper(I), (TEED)₂Cu₂Br₂. The crystals are triclinic (space group Pi [C_i¹] No. 2]), with a = 7.860 (2)^Å, b = 9.109 (3)^Å, c = 19.773 (8) Å,α= 101.04 (3)^o, β = 100.68 (3),γ = 90.13 (2)^o, V= 1364.4 (8) ų, and Z = 2. The principal features of the molecular structure are distorted tetrahedral copper R centers with Cu-Cu = 2.606 (6) Å, Cu-Br(bridge) = 2.467 (12) Å and Cu-Br(bridge)-Cu = 63.9 (1)^o. Manometric, cryoscopic, and spectral measurements established that the primary product of reaction of (TEED)₂Cu₂Br₂ with dioxygen is the dimeric species (TEED)₂Cu₂Br₂G, which appears to contain one oxo bridge and two bromo bridges between its 5-coordinate copper(II) centers. The rate law for the reaction of excess (TEED)₂Cu₂Br₂ with dioxygen in nitrobenzene is d[(TEED)₂Cu₂Br₂o]/dt = K_D[(TEED)₂Cu₂Br₂]²[O₂], with k_D = (5.50 ± 0.18) X 10² M-² s-¹ at 25.0 ^oC, ΔH_D* = (12.9 ± 0.3) kcal mol-¹, and ΔS* =-(3 ± 2) cal deg-¹ mol-¹ at 25.0 ^oC. A mechanism involving a weak intermediate complex (TEED)₂Cu₂Br₂o₂ is suggested. The kinetic data are compared with those observed previously for oxidation of dimeric chloro(pyridine)copper(I) complexes in which the primary products are tetrameric dioxocopper(II) species. Cryoscopic and manometric measurements indicate that the primary oxidation product slowly polymerizes and that its coordinated TEED ligands are subject to slow oxidation by excess dioxygen in aprotic solvents. Both of these processes lessen the effectiveness of the primary oxidation product as an initiator for the oxidative coupling of 2,6-dimethylphenol by dioxygen. They can be prevented by rapid reaction of the primary oxidation product with carbon dioxide, which produces the paramagnetic μ-carbonato complex μ-CO₃)-cis-Br₂(TEED)₂Cu₂, an active, stable, oxidative coupling initiator.