| Summary: | The mechanism of the carbonylation of diazomethane in the presence of iron–carbonyl–phosphine catalysts has been investigated by means of DFT calculations at the M06/def-TZVP//B97D3/def2-TZVP level of theory, in combination with the SMD solvation method. The reaction rate is determined by the formation of the coordinatively unsaturated doublet-state Fe(CO)<sub>3</sub>(P) precursor followed by the diazoalkane coordination and the N<sub>2</sub> extrusion. The free energy of activation is predicted to be 18.5 and 28.2 kcal/mol for the PF<sub>3</sub> and PPh<sub>3</sub> containing systems, respectively. Thus, in the presence of less basic P-donor ligands with stronger <inline-formula><math display="inline"><semantics><mi>π</mi></semantics></math></inline-formula>-acceptor properties, a significant increase in the reaction rate can be expected. According to energy decomposition analysis combined with natural orbitals of chemical valence (EDA–NOCV) calculations, diazomethane in the Fe(CO)<sub>3</sub>(phosphine)(<i>η</i><sup>1</sup>-CH<sub>2</sub>N<sub>2</sub>) adduct reveals a <inline-formula><math display="inline"><semantics><mi>π</mi></semantics></math></inline-formula>-donor–<inline-formula><math display="inline"><semantics><mi>π</mi></semantics></math></inline-formula>-acceptor type of coordination.
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