Protonation of a Linear Oxo-Bridged Biiron Unit without Rehybridization of the Bridging Oxygen: Structure of the (mu-Hydroxo)bis-(tetraphenylporphyrinato)iron(III) Cation

摘要

The recognition that oxo- and hydroxo-bridged diiron active sites are widespread biological units has led to intense activity in defining their chemistry. The transformation of an oxo to a hydroxo bridge is an obligatory or proposed step in the reaction pathways of a great variety of iron and copper redox enzymes, particularly the oxidases and is even a part of fee more simple process of dioxygen binding in hemerythrin. Structural responses, changes in spin coupling, pK_a values, and rates of protonation are all of particular interest. Structurally, singly bridged M-O-M units tend towards linearity because of pi bonding and steric effects. Upon protonation, they become more bent as the oxygen atom rehybridizes. We now report a diiron(III) complex inwhich this does not occur Protonation of the nearly linear mu-oxo bridge in [(tpp)Fe- O-Fe(tpp)] (tpp = tetraphenylporphyrinate) leaves the bridge angle essentially unchanged. The resulting mu-hydroxo-briuged cation, [(tpp)Fe-O(H)-Fe(tpp)]~+. provides only the second known example of a diiron complex with a single hydroxo bridge unsupported by ancillary ligands. Two additional features are notable. Antiferromagnetic coupling is greatly attenuated and the self-exchange rate of proton transfer for this strong acid is slow on the NMR timescale.