Insertion reactions of metal carbonyl anions with methyl formate in the gas phase as revealed by 13C- and D-labeling

The gas-phase reactions of coordinatively unsaturated metal carbonyl anions (M(CO)⁻_n, M = Cr, Mn, Fe, Co; n = 0–3 and Co(CO)_nNO⁻, n = 0–2) with unlabeled and D- and ¹³C-labeled methyl formate have been studied with Fourier transform ion cyclotron resonance mass spectrometry. The reactions proceed in most instances by loss of one or more CO molecules from the collision complex. In the reactions of the dicarbonyl and tricarbonyl anions with H¹³COOCH₃, part of the eliminated carbon monoxide molecules contain the label revealing the occurrence of initial insertion of the metal center into the bonds adjacent to the carbonyl function of the substrate with formation of five- or six-coordinate intermediates, respectively. In addition, the Mn(CO)⁻₃, Fe(CO)⁻₂, and Co(CO)⁻₂ ions react by the loss of methanol and a [C,H₂,O] neutral species. The D- and ¹³C-labeling show that methanol is expelled in a reductive elimination from a five- or six-coordinate species, whereas the [C,H₂,O] loss is a more complex process possibly involving the competing losses of formaldehyde and CO + H₂. In the reaction of Fe(CO)⁻₃ with H¹³COOCH₃, a facile consecutive exchange of all three CO ligands of the reactant ion for ¹³CO is observed. This novel reaction appears to involve initial insertion into the H¹³COOCH₃-bond followed by facile hydrogen shifts from the formyl ligand to a CO ligand prior to the loss of unlabeled methyl formate.