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Kinetics and Thermodynamics of Hydrogen Atom Transfer to Superoxometal Complexes

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journal contribution
posted on 05.11.1997, 00:00 by Andreja Bakac
Dicationic metal superoxo complexes, L(H2O)MOO2+ (M = Rh, Cr; L = (NH3)4, (H2O)5, [14]aneN4) react with rhodium hydrides L‘(H2O)RhH2+ (L‘ = (NH3)4, [14]aneN4, and meso-Me6-[14]aneN4) to yield L(H2O)MOOH2+ and L‘(H2O)2Rh2+. The rhodium(II) product is rapidly converted to L‘(H2O)RhOO2+ in a reaction with molecular oxygen (k ∼ 108 M-1 s-1), or to a binuclear superoxo product in the absence of O2. When the central metal atom and nonparticipating ligands are identical in the two reactants (M = Rh, L = L‘), the O2 insertion into Rh−H bond becomes catalytic, L‘RhH2+ + O2 → L‘RhOOH2+ (L‘RhOO2+ = catalyst). The hydrogen transfer step exhibits a large kinetic isotope effect (kH/kD = 7.6 for the reaction of CraqOO2+ with ([14]aneN4)(H2O)RhH2+, ΔHH = 20.3 ± 1.8 kJ mol-1, ΔSH = −136 ± 6 J mol-1 K-1, and ΔHD = 24.7 ± 4.3 kJ mol-1, ΔSD = −138 ± 15 J mol-1 K-1), but the isotopic composition of the solvent has no effect on rates, kH2O/kD2O = 1. Steric effects play a major role in the kinetics. From the available thermochemical data, the energy of the CraqOO−H2+ bond was calculated to be ∼330 kJ/mol.