The gas-phase reactions of metal porphyrins with diazoacetate esters

The Mn(III), Fe(III) and Co(III) complexes of tetraphenylporphyrin were allowed to react with ethyl and t-butyl diazoacetate in an ion trap mass spectrometer. The manganese system produces only adducts, but the iron and cobalt systems give addition with loss of N2 to produce carbene-like species. All the reactions are fast and approach the collision-controlled limit. Fragmentation of the iron and cobalt carbene species follow three major pathways: (a) alkene loss from the ester to give a carboxylic acid (which can subsequently decarboxylate to give CH2 complexed to the metal porphyrin), (b) homolytic cleavage of the ester OR bond with loss of CO2 and an alkyl radical to produce CH complexed to the metal porphyrin, and (c) alcohol loss to give CCO complexed to the metal porphyrin. Computational data from density functional theory (B3LYP) are consistent with the observed reactivity trends and indicate that all the carbene complexes prefer a MN insertion structure where the carbene carbon bonds to the metal and one of the porphyrin nitrogens (metalnitrogen bond is lost). The MN insertion structures are generally more than 25 kcal/mol more stable than the conventional metal carbene structures, MC, at the B3LYP level and should dominate their reactivity.

Graphical abstractFigure optionsDownload full-size imageDownload high-quality image (222 K)Download as PowerPoint slideHighlights► Gas-phase reactions of metal porphyrins with diazoacetates to give carbenes. ► Fragmentation gives novel metal species including CH and CCO complexes. ► Computations indicate strong preference for bridging in metal carbenes. ► Bridging will have critical effect on activity of these cationic catalysts.