Base-catalyzed insertion of dioxygen into rhodium-hydrogen bonds: kinetics and mechanism.
The reaction between molecular oxygen and rhodium hydrides L(OH)RhH(+) (L = (NH(3))(4), trans-L(1), and cis-L(1), where L(1) = cyclam) in basic aqueous solutions rapidly produces the corresponding hydroperoxo complexes. Over the pH range 8 < pH < 12, the kinetics exhibit a first order dependence on [OH(-)]. The dependence on [O(2)] is less than first order and approaches saturation at the highest concentrations used. These data suggest an attack by OH(-) at the hydride with k = (1.45 +/- 0.25) x 10(3) M(-1) s(-1) for trans-L(1)(OH)RhH(+) at 25 degrees C, resulting in heterolytic cleavage of the Rh-H bond and formation of a reactive Rh(I) intermediate. A competition between O(2) and H(2)O for Rh(I) is the source of the observed dependence on O(2). In support of this mechanism, there is a significant kinetic isotope effect for the initial step, L(1)(OH(D))RhH(D)(+) + OH(D)(-) k(1)/k(-1) L(1)(OH(D))Rh(I) + H(D)(2)O, k(1H)/k(1D) = 1.7, and k(-1H)/k(-1D) = 3.0. The activation parameters for k(1) for trans-L(1)(OH)RhH(+) are DeltaH(++) = 64.6 +/- 1.3 kJ mol(-1) and DeltaS(++) = 40 +/-4 J mol(-1) K(-1).
Ames Laboratory, Iowa State University, Ames, Iowa 50011, USA.
Ewa
E
Szajna-Fuller
Andreja
A
Bakac
D000641:Ammonia / Q000737:chemistry*; D002384:Catalysis; D006859:Hydrogen / Q000737:chemistry*; D006860:Hydrogen Bonding; D006863:Hydrogen-Ion Concentration; D007700:Kinetics; D009942:Organometallic Compounds / Q000737:chemistry; D010084:Oxidation-Reduction; D010100:Oxygen / Q000737:chemistry*; D012238:Rhodium / Q000737:chemistry*; D013816:Thermodynamics
D009942:Organometallic Compounds; D000641:Ammonia; D006859:Hydrogen; D012238:Rhodium; D010100:Oxygen