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 electr onic configuration is formally (d(xy))(2)(d(xz),d(yz))(3) for both 1 a nd 2, but these two complexes give rise to near-infrared porphyrin(pi) -->ferric(d) charge transfer bands (NIR-CT) with very different MCD in tensities. These intensity differences are correlated with the symmetr y 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 intens e 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 ultraviol et and visible wavelengths. It is argued that these low intensities ar e the result of a novel (d(xz),d(yz))(4)(d(xy))(1) ground state for wh ich the MR-CT transition is formally symmetry forbidden. Complex 3 thu s provides an explanation for the unusual EPR and MCD properties of lo w-spin ferric forms of heme d, which have similar EPR spectra to compl ex 3. It is concluded that these properties are a consequence of a reo rdering of the energies of the ferric d-orbitals by the axial ligands for the porphyrinate complex, or by the macrocycle for chlorin complex es, in each case leading to a predominantly (d(xz),d(yz))(4)(d(xy))(1) ground state.