Kinetics and mechanism of oxidation of aromatic hydrocarbons by Ru(trpy)(bpy)O2+

Abstract

The kinetics of oxidation of p-/sup -/O/sub 2/CC/sub 6/H/sub 4/CH(CH/sub 3/)/sub 2/,p-/sup -/O/sub 2/CC/sub 6/H/sub 4/CH/sub 2/CH/sub 3/,and p-/sup -/O/sub 2/CC/sub 6/H/sub 4/CH/sub 3/ by Ru(trpy)(bpy)O/sup 2 +/ (trpy is 2,2',2''-terpyridine; bpy is 2,2'-bipyridine) to the corresponding ..cap alpha.. alcohols in water and of C/sub 6/H/sub 5/CH(CH/sub 3/)/sub 2/ and C/sub 6/H/sub 5/CH/sub 3/ by Ru(bpy)/sub 2/(py)O/sup 2 +/ in acetonitrile have been studied. The following conclusions are drawn from kinetics data obtained spectrophotometrically: (1) Rate constants increase with increasing alkyl substitution; for the carboxylates in water at 24.3/sup 0/C, k = 12.2 +/- 1.2, 3.4 +/- 0.3, and 0.43 +/- 0.04 M/sup -1/ s/sup -1/ in the order shown above. (2) Rate constants decrease dramatically for the reactions in acetonitrile; k(24.3/sup 0/C) = 0.026 +/- 0.003 M/sup -1/ s/sup -1/ for isopropylbenzene. (3) In water, rate constants are independent of added O/sub 2/ or of changes in ionic strength. (4) In acetonitrile the added nucleophiles water, tert-butyl alcohol, or bromide ion enter the rate law directly in terms first order in added nucleophile. From the temperature dependence of k for the oxidation of p-/sup -/O/sub 2/CC/sub 6/H/sub 4/CH(CH/sub 3/)/sub 2/, ..delta..H/sup + +/ = 7 +/- 1 kcal/mol andmore » ..delta..S/sup + +/ = -32 +/- 4 eu. It is concluded that the redox step for the reactions involves a two-electron, hydride ion transfer step. The reactions occur by a template mechanism in that oxo group transfer from Ru to the substrate does not occur and the added oxygen atom must come from the solvent, p-/sup -/O/sub 2/CC/sub 6/H/sub 4/CH(CH/sub 3/)/sub 2/ (-H:/sup -/; +H/sub 2/O) ..-->.. p-/sup -/O/sub 2/CC/sub 6/H/sub 4/C(OH)(CH/sub 3/)/sub 2/. The solvent or added nucleophile (in acetonitrile) is directly involved in the redox step, apparently by assisting the loss of the hydride ion by electron pair donation.« less