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 N-2 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 O-R bond with loss of CO2 and an alkyl radical to produce CH complexed to the metal porphyrin, and (c) alcohol loss to give C=C=O 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 M-N insertion structure where the carbene carbon bonds to the metal and one of the porphyrin nitrogens (metal-nitrogen bond is lost). The M-N insertion structures are generally more than 25 kcal/mol more stable than the conventional metal carbene structures, M=C, at the B3LYP level and should dominate their reactivity. (c) 2012 Elsevier B.V. All rights reserved.