π-Acid ligands in iron(III) porphyrinates. Characterization of low-spin bis(tert-butylisocyanide)(porphyrinato)iron(III) complexes having (d(xz),d(yz))⁴(d(xy))¹ ground states

The synthesis and characterization of isocyanide complexes of (porphyrinato)iron(III) species, [(Porph)-Fe(t-BuNC)2]ClO4, Porph = OEP, TPP, are reported. The crystal structures of [(TPP)Fe(t-BuNC)2]ClO4 and [(OEP)Fe(t-BuNC)2]ClO4 have been determined. Consistent with the expected effect from the strong π-acceptor character of the axial tert-butyl isocyanide ligands, the X-ray structure of the complex shows that the porphyrinate ring is strongly ruffled. The spectroscopic properties of these complexes suggest the possibility of 'blurring' of the definitions of the electron configurations of low-spin Fe(III) macrocycles having (d(xy))1 electronic ground states, with the extreme possibilities being low-spin Fe(III)-(macrocycle)2-, with the unpaired electron localized in the d(xy) orbital of the metal, and low-spin Fe(II)-(macrocycle)(1-·), with the unpaired electron localized on the macrocycle. EPR spectroscopy of the TPP and OEP complexes shows that the g-values (g(perpendicular to) = 2.20-2.28, g(parallel with) = 1.94-1.83) are consistent with an electron configuration that is (d(xz),d(yz))4(d(xy))1, the purest (d(xy))1 ground state system with the most complete quenching of orbital angular momentum discovered thus far (Σg2 as small as 13.5). Proton NMR spectra of [OEPFe(t-BuNC)2]ClO4 in CD2Cl2, recorded over the temperature range -100 to +37°C, also support the (d(xy))1 ground state, where ruffling of the porphyrinate ring makes it possible for unpaired electron spin delocalization to the 3a(2u)(π) orbital of the porphyrinate ring. This orbital has very large electron density coefficients at the meso positions and hence explains the very large negative contact shift of the meso-H; its size indicates considerable (~ 19%) spin delocalization from low-spin Fe(III) to the 3a(2u)(π) orbital by porphyrin → Fe π donation. Mossbauer and IR spectral data are also consistent with the (d(xy))1 ground state.