A kinetic study of the cleavage of the ironcarbon σ bond in η 5-C 5H 5Fe(CO) 2R by halogenated acetic acids

Abstract

The rates of the reaction of η 5-C 5H 5Fe(CO) 2R (R = alkyl and aryl) with CF 3CO 2H to give η 5-C 5H 5Fe(CO) 2OC(O)CF 3 and RH wre investigated in organic solvents, mostly at 25°C, by infrared spectroscopic, manometric and volumetric techniques. When R = alkyl, the cleavage in CH 2Cl 2 is first order in η 5-C 5H 5Fe(CO) 2R and first order in the CF 3CO 2H monomer at acid concentrations ⪆0.1 M, but first order in η 5-C 5H 5Fe(CO) 2R and second order in the CF 3CO 2H monemer at lower acid concentrations. The dependence of the second-order rate constant on R follows the order C 5H 5 > CH 5Si(CH 3) 3 (>110) > CH 3 (32) > n-C 4H 9 (15) > C 2H 5 (11) > CH 2C(CH 3) 3 (6.2) > CH 2CH 2C 6H 5 (5.3) > CH(CH 3)C 6H 5 (∼1.2) ⪆ CH 2C 6H 5 (1.0) > CH(CH 3) 2. The isotope effect, k 2/ k D, for the cleavage of η 5-C 5H 5Fe(CO) 2CH 3 by CF 3CO 2H and CF 3CO 2D is 4.7. Solvent influence on the rate of the FeCH 3 bond scission in CH 2Cl 2, CH 2ClCH 2Cl and C 6H 6 is very small. A mechanism is proposed which involves the formation of an Fe-H-OC(O)CF 3 hydrogen-bridged 1 1 adduct of the reactants in a reversible step. This adduct then affords [η 5-C 5H 5Fe(CO) 2(R)H][CF 3CO 2H·O 2CCF 3] with the assistance of a second molecule of CF 3CO 2H. Reductive elimination of RH and coordination to iron of CF 3CO 2 − complecte the reaction. The corresponding cleavage of a given FeR bond by CHCl 2CO 2H is substantially slower than that by CF 3CO 2H; for the aryl complexes it follows the order R = p-C 6H 4CH 3 > p-C 6H 4F > C 6H 5 > p-C 6H 4Cl, with p ∼ −5.4.