MECHANISTIC STUDIES OF CARBON-DIOXIDE INSERTION INTO METAL-HYDRIDES AND EXTRUSION FROM THE CORRESPONDING METAL FORMATES UTILIZING GROUP-6 METAL-CARBONYL DERIVATIVES AS REACTION PROBES
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Mechanistic aspects of the CO3 insertion into [Cr(CO)5H]- to yield [Cr(CO)5O2CH]- have been investigated by the examination of the CO2 exchange reaction: [Cr(CO)5O2CH]- + 13CO2 [Cr(CO)5O213CH]- + CO2. Kinetic measurements show the reaction to be first order in metal substrate concentration and zero order in [13CO2], and the rate is retarded by added [CO], The activation parameters were determined to be H* = 18.9 0.7 keal/mol and S* = 12.0 2.9 eu. The value of kH/kD was found to be 1.13. The mechanism of this process is dissociative in character, proceeding through the following: (1) CO loss with formation of an intermediate O, O-dihapto species, [Cr(CO)4(2-O2CH)]-; (2) rearrangement to an H, O-dihapto chelate (rate-determining step); (3) decarboxylation and recarbonylation; and (4) recarboxylation with 13CO2 in the reverse direction. Consistent with this mechanism, [Cr(CO)4(2-O2CH)]- has been synthesized and identified by IR spectroscopy. To further investigate the decarbonylation step of the mechanism, the energetics of CO loss for a [Cr(CO)5O2CR]- derivative were determined from the reaction of [Cr(CO)5O2CCH3]- with P(OCH3)3 to afford cis-[Cr(CO)4(P(OCH3)3)O2CCH3]- + CO. The reaction was found to be first order in metal substrate concentration and to have a small dependence on [P(OCH3)3], The activation parameters for the phosphite-independent pathway were determined to be H* = 18.0 2.2 keal/mol and S* = -8.8 8.3 eu. These data coupled with the activation parameters for the decarboxylation/recarboxylation reaction afford a well-defined reaction pathway for the C-H bond-forming reaction of CO2 and [Cr(CO)5H]-. The experimental results are discussed in relation to recent theoretical calculations on the carbon dioxide insertion reaction. 1990, American Chemical Society. All rights reserved.
