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Density Functional Theory Studies on the Mechanism of the Reduction of CO2 to CO Catalyzed by Copper(l) Boryl Complexes.

The detailed reaction mechanism for the reduction of CO to CO catalyzed by (NHC)Cu(boryl) complexes (NHC = N-heterocyclic carbene) was studied with the aid of DFT by calculating the relevant intermediates and transition state structures. Our DFT calculations show that the reaction occurs through CO...

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Published in:Journal of the American Chemical Society Vol. 128; no. 49; pp. 15637 - 15644
Main Authors: Haitao Zhao, Zhenyang Lin, Marder, Todd B.
Format: Article
Published: American Chemical Society 12/13/2006
Subjects:
Density functionals
Reaction mechanisms (Chemistry)
Copper compounds
Functional analysis
Conditions & laws of chemical reaction
Physical & theoretical chemistry
Online Access:View this record in EBSCOhost
Summary:The detailed reaction mechanism for the reduction of CO to CO catalyzed by (NHC)Cu(boryl) complexes (NHC = N-heterocyclic carbene) was studied with the aid of DFT by calculating the relevant intermediates and transition state structures. Our DFT calculations show that the reaction occurs through CO insertion into the CuB bond to give a CuOC(O)-boryl species (i.e., containing CuO and CB bonds), and subsequent boryl migration from C to O, followed by o-bond metathesis between pinB-Bpin (Bpin, pin = pinacolate = OCMeCMeO) and (NHC)Cu(OBpin). The overall reaction is exergonic by 38.0 kcal/mol. It is the nucleophilicity of the CuB bond, a function of the very strong σ-donor properties of the boryl ligand, rather than the oxophilicity of boron, which determines the direction of the CO insertion process. The boryl migration from C to O, which releases the product CO, is the rate-determining step and involves the ‘vacant’ orbital orbital on boron. The (NHC)Cu(boryl) complexes show unique activity in the catalytic process. For the analogous (NHC)Cu(alkyl) complexes, the CO insertion into the CuC bond giving a copper acetate intermediate occurs with a readily achievable barrier. However, the elimination of CO from the acetate intermediate through a methyl migration from C to O is energetically inaccessible.