A New Look at an Old Reaction: The Potential Energy Surface for the Thermal Carbonylation of Mn(CO)5CH3. The Role of Two Energetically Competitive Intermediates on the Reaction S
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- 作者：Agnes Derecskei-Kovacs and Dennis S. Marynick
- 刊名：Journal of the American Chemical Society
- 出版年：2000
- 出版时间：March 8, 2000
- 年：2000
- 卷：122
- 期：9
- 页码：2078 - 2086
- 全文大小：121K
- 年卷期：v.122,no.9(March 8, 2000)
- ISSN：1520-5126
文摘
A detailed theoretical study of the carbonyl insertion reaction Mn(CO)5CH3 + CO 
Mn(CO)5(COCH3) is presented using gradient corrected density functional theory. As has been well-documentedexperimentally, this reaction proceeds through a two-step mechanism. In the first step, a stereochemicallywell-defined intermediate is formed via migration of the methyl group to a cis carbonyl. In the second step,the incoming nucleophile attacks the intermediate to form product. Two stable intermediates have been locatedon the potential energy surface. Both are formally Mn(CO)4(COCH3); however, in one case the intermediateis stabilized by a strong agostic interaction between a methyl group hydrogen and the metal, and in the secondcase the acyl group distorts to form an Mn-O bond and thus acts as an 
2 (dihapto) ligand. The transitionstates between the reactant and the intermediates have been located. In addition, the transition states for COattack of each intermediate have also been characterized. A detailed kinetic analysis of two possible reactionchannels demonstrates that the solvent unassisted mechanism proceeds via CO attack on the agostic intermediate,even though the dihapto intermediate is lower in energy. Our calculated energetics (both activation energy andoverall exothermicity) are in excellent agreement with experiment. We have also investigated some aspects ofthe photodecarbonylation of Mn(CO)5(COCH3) to yield Mn(CO)5CH3. This reaction has been proposed toproceed via the dihapto intermediate, and we confirm this result on the basis of a comparison of calculated vsobserved CO stretching frequencies of the experimentally characterized intermediate. Therefore, the thermalcarbonylation of Mn(CO)5CH3 and the photodecarbonylation of Mn(CO)5(COCH3) proceed along differentreaction channels. Some additional comments on the role of solvent in the photodecarbonylation of Mn(CO)5(COCH3) are included. Specifically, we find (again on the basis of a comparison of calculated vs experimentallyobserved vibrational frequencies) that the dihapto intermediate in the photodecarbonylation experiments isunsolvated, even in coordinating solvents such as THF.
Mn(CO)5(COCH3) is presented using gradient corrected density functional theory. As has been well-documentedexperimentally, this reaction proceeds through a two-step mechanism. In the first step, a stereochemicallywell-defined intermediate is formed via migration of the methyl group to a cis carbonyl. In the second step,the incoming nucleophile attacks the intermediate to form product. Two stable intermediates have been locatedon the potential energy surface. Both are formally Mn(CO)4(COCH3); however, in one case the intermediateis stabilized by a strong agostic interaction between a methyl group hydrogen and the metal, and in the secondcase the acyl group distorts to form an Mn-O bond and thus acts as an 2 (dihapto) ligand. The transitionstates between the reactant and the intermediates have been located. In addition, the transition states for COattack of each intermediate have also been characterized. A detailed kinetic analysis of two possible reactionchannels demonstrates that the solvent unassisted mechanism proceeds via CO attack on the agostic intermediate,even though the dihapto intermediate is lower in energy. Our calculated energetics (both activation energy andoverall exothermicity) are in excellent agreement with experiment. We have also investigated some aspects ofthe photodecarbonylation of Mn(CO)5(COCH3) to yield Mn(CO)5CH3. This reaction has been proposed toproceed via the dihapto intermediate, and we confirm this result on the basis of a comparison of calculated vsobserved CO stretching frequencies of the experimentally characterized intermediate. Therefore, the thermalcarbonylation of Mn(CO)5CH3 and the photodecarbonylation of Mn(CO)5(COCH3) proceed along differentreaction channels. Some additional comments on the role of solvent in the photodecarbonylation of Mn(CO)5(COCH3) are included. Specifically, we find (again on the basis of a comparison of calculated vs experimentallyobserved vibrational frequencies) that the dihapto intermediate in the photodecarbonylation experiments isunsolvated, even in coordinating solvents such as THF.