Axial imidazole binding strengths in porphyrinoid cobalt(III) complexes as studied by tandem mass spectrometry.

The Co(II) complexes of twelve meso-tetraaryl-porphyrins, -chlorins, and chlorin analogues containing non-pyrrolic heterocycles were synthesized and converted in situ to the corresponding Co(III) complexes coordinated to one or two imidazoles. Electrospray ionization tandem mass spectrometry (ESI-MS/MS) in conjunction with the energy-variable collision-induced dissociation (CID) technique was used to compare the relative gas-phase binding strength of the axially coordinated imidazoles to the octahedral and square planar Co(III) porphyrinoid complex ions. The observed binding energies of these ligands were rationalized in terms of the effects of porphyrinoid core structure and meso-substitution on the electron density on the central Co(III) centers. Some of these trends were supported by DFT-based computational studies. The study highlights to which extend porphyrins vary from chlorins and chlorin analogues in their coordination abilities and to which extraordinary degree meso-thienyl-substituents influence the electronic structure of porphyrins. The study also defines further the scope and limits CID experiments can be used to interrogate the electronic structures of metalloporphyrin complexes.