Kinetic studies of organochromium (III) cations and their hydrido analog

Abstract

<p>The kinetics of the reaction of the complex (CH(,3))(,2)CHCr(H(,2)O)(,5)('2+) with molecular oxygen, in aqueous perchloric acid, were investigated primarily by spectrophotometry. The reaction follows the rate law: -dRCr('2+)/dt = kRCr('2+)('3/2)O(,2)('0), and its rate is independent of H(,3)O('+) in the region 0.003 to 0.10M; k is 0.49M('-1/2)s('-1) at 25(DEGREES)C and (mu) = 1.0M. A free-radical chain mechanism is proposed in which one of the propagating reactions involves S(,H)2 attack by the isopropylperoxy radical on the chromium center of the substrate, displacing the isopropyl radical, and forming the unstable (not isolable) isopropylperoxochromium(III) ion. An alternative free-radical chain mechanism, which differs in the identity of the chain-carrying free-radicals, was eliminated from consideration by the effect of Cu(,aq)('2+) and ionic strength on the reaction rate. The principal products of the autoxidation--acetone and Cr(H(,2)O)(,6)('3+)--are accounted for by rapid protonolysis of the isopropylperoxochromium(III) intermediate;The complex HOCH(,2)CH(,2)Cr(H(,2)O)(,5)('2+) was prepared in dilute aqueous perchloric acid, by the photochemical generation of (beta)-hydroxyethyl radicals in the presence of Cr(,aq)('2+). Using flash photolysis, the kinetics of its reaction with H(,3)O('+) were found to obey predominately (if not entirely) a rate law of the form: -dRCr('2+)/dt = kH(,3)O('+)RCr('2+), where k has a value of (1.43 (+OR-) 0.02) x 10('4)M('-1)s('-1) at 24(DEGREES)C and (mu) = 0.05M. The products of the reaction are ethylene and Cr(H(,2)O)(,6)('3+), consistent with a mechanism where the rate-limiting step is assigned to a proton-assisted beta-elimination of a water molecule to form a highly unstable (pi)-bonded C(,2)H(,4)Cr(H(,2)O)('3+) intermediate;The complex HCr(H(,2)O)(,5)('2+) was made by the UV flash photolysis^of aqueous chromium(II) perchlorate. Its reaction with H(,3)O('+) (to^form H(,2) and Cr(H(,2)O)(,6)('3+)) follows the rate law: -dHCr('2+)/dt = kH(,3)O('+)Cr(,aq)('2+), with k equal to (9.7 (+OR-) 1.0) x 10('3)M('-1)s('-1) at 26(DEGREES)C and (mu) = 0.20M. Activation parameters of (DELTA)H('(DBLDAG)) = 6.3 (+OR-) 0.2 kcal/mol and (DELTA)S('(DBLDAG)) = -19.1 (+OR-) 0.7 cal/mol (DEGREES)K were measured. The deuterated complex DCr(D(,2)O)(,5)('2+) shows a large kinetic isotope effect in its reaction with D(,3)O('+)(k(,H)/k(,D) = 4.8), which is similar in magnitude to the acidolysis of the CH(,3)Cr(D(,2)O)(,5)('2+) ion by D(,3)O('+)(k(,H)/k(,D) = 6.3), supportive of a mechanism where O-H bond breaking is rate-limiting for both complexes. Protonolysis of HCr(H(,2)O)(,5)('2+) is pictured as occurring via a nonlinear three-center transition state in which the H-Cr bond is still intact;A description of a conventional microsecond flash photolysis instrument is presented in the Appendix, with a brief guide to its usage;('1)DOE Report IS-T-956. This work was performed under ContractW-7405-Eng-82 with the Department of Energy.</p>