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Concept
Metal carbon dioxide complex
Summary
Metal carbon dioxide complexes are coordination complexes that contain carbon dioxide ligands. Aside from the fundamental interest in the coordination chemistry of simple molecules, studies in this field are motivated by the possibility that transition metals might catalyze useful transformations of CO2. This research is relevant both to organic synthesis and to the production of "solar fuels" that would avoid the use of petroleum-based fuels.
Carbon dioxide binds to metals in only a few ways. The bonding mode depends on the electrophilicity and basicity of the metal centre. Most common is the η2-CO2 coordination mode as illustrated by Aresta's complex, Ni(CO2)(PCy3)2, which was the first reported complex of CO2. This square-planar compound is a derivative of Ni(II) with a reduced CO2 ligand. In rare cases, CO2 binds to metals as a Lewis base through its oxygen centres, but such adducts are weak and mainly of theoretical interest. A variety of multinuclear complexes are also known often involving Lewis basic and Lewis acidic metals, e.g. metallacarboxylate salts (C5H5)Fe(CO)2CO2−K+. In multinuclear cases (compounds containing more than one metal), more complicated and more varied coordination geometries are observed. One example is the unsymmetrical compound containing four rhenium centres, [(CO)5ReCO2Re(CO)4]2. Carbon dioxide can also bind to ligands on a metal complex (vs just the metal), e.g. by converting hydroxy ligands to carbonato ligands.
Transition metal carbon dioxide complexes undergo a variety of reactions. Metallacarboxylic acids protonate at oxygen and eventually convert to metal carbonyl complexes:
[LnMCO2]− + 2 H+ → [LnMCO]+ + H2O
This reaction is relevant to the potential catalytic conversion of CO2 to fuels.
N-heterocyclic carbene (NHC) supported CuI complexes catalyze carboxylation of organoboronic esters. The catalyst forms in situ from CuCl, an NHC ligand, and KOtBu. Copper tert-butoxide can transmetallate with the organoboronic ester to generate the CuI-C bond, which intermediate can insert into CO2 smoothly to get the respective carboxylate.
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