Electronic properties and redox chemistry of N confused metalloporphyrins

Abstract

Here we study the effect of metals on the characteristic Soret band of N-confused porphyrins. We used DFT calculations to study how this low-lying region of the spectrum of the NCP-2H isomer is affected by the introduction of transition metals with various (d2 , d3 , d4 and d6) d-electron configurations. The spin ground state of these complexes is mostly dependent on the number of unpaired electrons, both with and without the presence of an axial ligand. The analysis of the electronic distribution and spin density showed that these unpaired electrons are often harbored by the N-confused porphyrin ring instead of on the metal. Time-dependent DFT results indicated that the aromatic system of porphyrin is disrupted in the N-confused isomer: instead of the typical large Soret band, this now gives rise to two peaks of much lower intensity. Most metallo-porphyrins exhibited similar optical properties, with the HOMO/LUMO orbitals showing a mixed metal/porphyrin character. The only exception was the Rh metalloporphyrin that exhibited a ligand-to-metal charge transfer band with increasing intensity as function of the ligand field. This suggests Rh is the only metal whose orbitals are higher in energy than the ligand's, indicating that it is the only system where the redox processes occur on the metal