Metal carbon dioxide complex

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.^[1]

Structural trends

Illustrative structures of transition metal carbon dioxide complexes, from the left: Ni(η²-CO₂)(PCy₃)₂, Rh(η¹-CO₂)ClL₄ (L₄ = (diars)₂), the metallacarboxylic ester CpFe(CO)₂(η¹:η¹-CO₂)Re(CO)₅, and another dimetalla-ester.

Carbon dioxide binds to metals in only a few ways. The bonding mode depends on the electrophilicity and basicity of the metal centre.^[2] Most common is the η²-CO₂ coordination mode as illustrated by 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 (C₅H₂)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.

Reactions

Transition metal carbon dioxide complexes undergo a variety of reactions. Metallacarboxylic acids protonate at oxygen and eventually convert to carbonyl complexes:

[L_nMCO₂]⁻ + 2 H⁺ → [L_nMCO]⁺ + H₂O

This reaction is relevant to the potential catalytic conversion of CO₂ to fuels.^[3]

General references

1. "Carbon Dioxide as Chemical Feedstock" Edited by Michele Aresta. Wiley-VCH, Weinheim, 2010. ISBN 978-3-527-32475-0.
2. Gibson, Dorothy H. (1996). "The Organometallic Chemistry Carbon Dioxide". Chem. Rev 96: 2063–2095. doi:10.1021/cr940212c.
3. Eric E. Benson, Clifford P. Kubiak, Aaron J. Sathrum and Jonathan M. Smieja "Electrocatalytic and homogeneous approaches to conversion of CO₂ to liquid fuels" Chem. Soc. Rev., 2009, 38, 89–99. doi:10.1039/b804323j