Low-temperature MCD studies of low-spin ferric complexes of tetramesitylporphyrinate: Evidence for the novel (d(xz),d(yz))(4)(d(xy))(1) ground state which models the spectroscopic properties of heme d

Low-temperature MCD spectra were recorded for the bis 4-(dimethylamino)pyridine (1), bis(1-methylimidazole) (2), and bis(4-cyanopyridine (3) complexes of ferric tetramesitylporphyrinate. The ground state electronic configuration is formally (d(xy))(2)(d(xz),d(yz))(3) for both 1 and 2, but these two complexes give rise to near-infrared porphyrin(pi)-->ferric(d) charge transfer bands (NIR-CT) with very different MCD intensities. These intensity differences are correlated with the symmetry imposed on the ferric ion by different relative orientations of the ligand planes in these two complexes. Near perpendicular ligand planes in 1 result in effective 4-fold symmetry at the ferric ion and intense NIR-CT MCD while parallel ligand planes in 2 give rise to relatively weak NIR-CT MCD. Complex 3 has perpendicularly orientated ligands but displays an axial EPR spectrum with unusual g-values and anomalously weak MCD transitions not only in the NIR but also across the ultraviolet and visible wavelengths. It is argued that these low intensities are the result of a novel (d(xz),d(yz))(4)(d(xy))(1) ground state for which the MR-CT transition is formally symmetry forbidden. Complex 3 thus provides an explanation for the unusual EPR and MCD properties of low-spin ferric forms of heme d, which have similar EPR spectra to complex 3. It is concluded that these properties are a consequence of a reordering of the energies of the ferric d-orbitals by the axial ligands for the porphyrinate complex, or by the macrocycle for chlorin complexes, in each case leading to a predominantly (d(xz),d(yz))(4)(d(xy))(1) ground state.