Dioxygen-binding kinetics and thermodynamics of a series of dicopper(I) complexes with bis[2-(2-pyridyl)ethyl] amine tridendate chelators forming side-on peroxo-bridged dicopper(II) adducts

Copper-dioxygen interactions are of interest due to their importance in bio logical systems as reversible O-2-carriers, oxygenases, or oxidases and als o because of their role in industrial and laboratory oxidation processes. H ere we report on the kinetics (stopped-flow, -90 to 10 degreesC of O-2-bind ing to a series of dicopper(I) complexes, [Cu-2(Nn)(MeCN)(2)](2+) (1(Nn))(- (CH2)(n)- (n = 3-5) linked bis[2-(2-pyridyl)ethyl]amine, PY2) and their clo se mononuclear analogue, [(MePY2)Cu(MeCN)](+) (3. which form mu-eta (2):eta (2)-peroxodicopper(II) complexes [Cu-2(Nn)-(O-2)](2+) (O-2)](2+) (2(Nn)) a nd [{(MePY2)Cu}(2)(O-2)](2+) (4), respectively. The overall kinetic mechani sm involves initial reversible (k(+,open)/k(-.open)) formation of a nondete ctable intermediate O-2-adduct [Cu-2(Nn)(O-2)](2+) (open), suggested to be a Cu-I. . . Cu-II-O-2(-) species, followed by its reversible closure (k(+,c losed)/k(-.closed)) to form 2(Nn). At higher temperatures (253 to 283 K), t he first equilibrium lies far to the left and the observed rate law involve s a simple reversible binding equilibrium process (K-on,K-high = (k(+,open) /k(-,open))(k(+,closed))). From 213 to 233 K the slow step in the oxygenati on is the first reaction (k(on,low) = k(+,open)), and first-order behavior (in 1(Nn) and O-2) is observed. For either temperature regime, the DeltaH d ouble dagger for formation of 2(Nn) are low (DeltaH double dagger = -11 to 10 kJ/mol; k(on,low) = 1.1 x 10(3) to 4.1 x 10(3) M-1 s(-1), k(on,high) = 2 .2 x 10(3) to 2.8 x 10(4) M-1 s(-1)), reflecting the likely occurrence of p reequilibria, The DeltaH degrees ranges between -81 and -84 kJ mol(-1) for the formation of 2(Nn), and the corresponding equilibrium constant (K-1) in creases (3 x 10(8) to 5 x 10(10) M-1; 183 K) going from n = 3 to 5, Below 2 13 K, the half-life for formation of 2(Nn) increases with, rather than bein g independent of, the concentration of 1(Nn), probably due to the oligomeri zation of 1(Nn) at these temperatures. The Or reaction chemistry of 3 in CH 2Cl3 is complicated, including the presence of induction periods, and could not be fully analyzed. However, qualitative comparisons show the expected slower intermolecular reaction of 3 with Or compared to the intramolecular first-order reactions of 1(Nn), Due to the likelihood of the partial dimeri zation of 3 in solution, the t(1/2) for the formation of 4 remains constant with increasing complex concentration rather than decreasing. Acetonitrile significantly influences tho kinetics of the O-2 reactions with 1(Nn) and 3. For 1(N4), the presence of MeCN inhibits the formation of a previously ( Jung et al, J. Am. Chem. Sec. 1996, 118, 3763-3764) observed intermediate. Small amounts of added MeCN considerably slow the oxygenation rates of 3, i nhibit its full formation to J, and increase the length of the induction pe riod. The results for 1(Nn) and their mononuclear analogue 3 are presented, and they are compared with each other as well as with other dinucleating d icopper(I) systems.