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 [Ci1] 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) Å3, 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 = KD[(TEED)2Cu2Br2]2[O2], with kD = (5.50 ± 0.18) X 102 M-2 s-1 at 25.0 oC, ΔHD* = (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.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Inorganic Chemistry