A Computational Investigation of the Insertion of Carbon Dioxide into Four‐ and Five‐Coordinate Iridium Hydrides

Recently, it has been demonstrated that the insertion of CO 2 into iridium hydrides is a crucial step in the catalytic conversion of CO 2 and H 2 into formic acid. We and others have elucidated the mechanism by which CO 2 inserts into six‐coordinate iridium(III) trihydrides supported by pincer ligands; these complexes are very active catalysts for CO 2 hydrogenation. However, it has also been demonstrated that five‐coordinate iridium(III) dihydrides can react with CO 2 and catalyze both thermal and electrochemical CO 2 hydrogenation. In this work, we study the mechanism of CO 2 insertion into pincer‐supported five‐coordinate iridium(III) dihydrides and four‐coordinate iridium(I) hydrides using density functional theory. The mechanisms differ slightly between the two cases. Insertion into the five‐coordinate species is a multistep process involving initial CO 2 precoordination, whereas insertion into the four‐coordinate species proceeds via a single step with no prior CO 2 coordination. Both of these mechanisms are different from the pathway that was recently proposed for CO 2 insertion into six‐coordinate iridium(III) trihydrides. In addition, a complete pathway for catalytic CO 2 hydrogenation using a five‐coordinate iridium(III) dihydride has been calculated.