KINETICS AND THERMODYNAMICS OF FORMATION OF COPPER DIOXYGEN ADDUCTS - OXYGENATION OF MONONUCLEAR COPPER(I) COMPLEXES CONTAINING TRIPODAL TETRADENTATE LIGANDS

Copper-dioxygen (O2) interactions are of interest in chemical as well as biological transformations involving reversible O2-binding, oxygenation, or oxidation of organic substrates. In this report, we describe the complete kinetics and thermodynamics of O2-reactions with a series of mononuclear copper(I) complexes containing N4-tripodal tetradentate donor ligands. The latter include tris[(2-pyridyl)methyl]amine (TMPA) and corresponding ligands with one (BPQA), two (BQPA), or three (TMQA) 2-quinolyl groups substituting for 2-pyridyl donors. [(TMPA)Cu(CH3CN)]+ (1a) is known to react with O2 in EtCN or CH2Cl2 at -80-degrees-C to form a dinuclear (II) complex, [{(TMPA)Cu}2(O2)]2+ (1c), with a trans-mu-1,2-peroxo dicopper(II) structure, as determined by X-ray crystallography (Tyeklar, Z.; et al. J. Am. Chem. Soc. 1993, 115, 2677-2689). [(BPQA)Cu]+ (2a) and [(BQPA)Cu]+ (3a) also form O2-adducts, but [(TMQA)Cu]+ (4a) is unreactive. Variable-temperature (-90-degrees-C) to room temperature), multiwavelength (359-776 nm), stopped-flow data were collected. The numerical analysis shows that all three reacting Cu(I) species 1a-3a follow the same reaction mechanism, involving the initial reversible formation (k1/k-1) of 1:1 Cu:O2 adducts [LCu(O2)]+ (1b-3b), which react reversibly (k2/k-2) with starting Cu(I) species 1a-3a to form 2:1 complexes [(LCU)2(O2)]2+ (1c-3c), respectively. However, considerable differences exist in detail, depending on the ligand. Thus, the 1:1 adduct 1b (k1 = 2 x 10(4) M-1 s-1, K1 = k1/k-1 = 1.9 x 10(3) M-1 at -90-degrees-C) is observed spectroscopically below -70-degrees-C (lambda(max) = 410 nm, epsilon = 4000 M-1 cm-1) prior to the transformation (k(ter) = k1k2/k-1 = 6 x 10(7) M-2 s-1, -90-degrees-C) to the final stable dinuclear product 1c. In the reaction of [(BPQA)Cu]+ (2a) with O2, the 1:1 intermediate was not observable and the stable 2:1 adduct [{(BPQA)Cu}2(O2)]2+ (2c) formed rapidly, having spectroscopic properties very similar to those for 1c. In the case of [(BQPA)Cu]+ (3a), both 1:1 and 2:1 adducts formed and were spectroscopically characterized. Here, the thermodynamically stable product is the 1:1 adduct [{(BQPA)Cu}(O2)]+ (3b) (lambda(max) = 378 nm, epsilon = 8000 M-1 cm-1; k1 = 18 M-1 s-1, K, = k1/k-1 = 3 x 10(3) M-1 at -90-degrees-C), but the kinetics are such that there is an ''overshoot'', and the 2:1 adduct [{(BQPA)Cu}2(O2)]+ (3c) forms initially (lambda = 545 nm, epsilon = 5500 M-1 cm-1) as an intermediate. The temperature-dependent data allow for thermodynamic analyses, showing that Cu(n)O2 (n = 1 or 2) bonding is strong at reduced temperatures with favorable negative enthalpies, DELTAH-degrees congruent-to -34 kJ mol-1 for 1:1 adducts and -50 to -80 kJ mol-1 for 2:1 adducts. However, room-temperature instabilities are caused by strongly negative reaction entropies, DELTAS-degrees congruent-to 200 J K-1 mol-1, for 2:1 adducts. The kinetic and thermodynamic parameters are compared with corresponding data for cobalt(II)- and iron(II)-porphyrin complexes as well as that for iron and copper proteins. The synthetic copper-dioxygen adducts form with rate constants comparable to or exceeding those of iron and cobalt, and the present investigation shows that Cu/O2 interactions can indeed be studied in detail.