Interpretation of EXAFS spectra for sitting-atop complexes with the help of computational methods

The metallation of tetrapyrroles is believed to proceed via a sitting-atop (SAT) complex, in which some of the pyrrole nitrogen atoms are protonated and the metal ion resides above the ring plane. No crystal structure of such a complex has been presented, but NMR and EXAFS (extended X-ray absorption fine structure) data has been reported for Cu2+ in acetonitrile, which have been interpreted as the observation of a SAT complex. However, this interpretation has been challenged and other investigations have shown that there are many possible SAT structures. We have recently developed a method to combine quantum mechanical (QM) calculations and EXAFS fits (EXAFS/QM), which in principle is a standard EXAFS fit that employs all multiple-scattering information in an optimum and self-consistent way and uses the QM calculations to ensure that the obtained structures are chemically reasonable. By this approach, we show that out of the 15 putative SAT complexes, structures with the copper ion coordinating to two cis pyrrolenine nitrogen atoms and two or three acetonitrile molecules fit the experimental EXAFS spectrum best. However, an equally good fit can be obtained also by a mixture of the reactant and product complexes.