PI-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 (porphyr inato)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 exp ected effect from the strong pi-acceptor character of the axial tert-b utyl isocyanide ligands, the X-ray structure of the complex shows that the porphyrinate ring is strongly ruffled. The spectroscopic properti es of these complexes suggest the possibility of ''blurring'' of the d efinitions of the electron configurations of low-spin Fe(III) macrocyc les having (d(xy))(1) electronic ground states, with the extreme possi bilities being low-spin Fe(III)-(macrocycle)(2-), with the unpaired el ectron localized in the d(xy) orbital of the metal, and low-spin Fe(II )-(macrocycle)(1-.), with the unpaired electron localized on the macro cycle. EPR spectroscopy of the TPP and OEP complexes shows that the g- values (g perpendicular to = 2.20-2.28, g parallel to = 1.94-1.83) are consistent with an electron configuration that is (d(xz)d(yz))(4)(d(x y))(1), the purest (d(xy))(1) ground state system with the most comple te quenching of orbital angular momentum discovered thus far (Sigma g( 2) as small as 13.5). Proton NMR spectra of [OEPFe(t-BuNC)(2)]ClO4 in CD2Cl2, recorded over the temperature range -100 to +37 degrees C, als o support the (d(xy))(1) ground state, where ruffling of the porphyrin ate ring makes it possible for unpaired electron spin delocalization t o the 3a(2u)(pi) orbital of the porphyrinate ring. This orbital has ve ry large electron density coefficients at the meso positions and hence explains the very large negative contact shift of the meso-H; its siz e indicates considerable (similar to 19%) spin delocalization from low -spin Fe(III) to the 3a(2u)(pi) orbital by porphyrin --> Fe pi donatio n. Mossbauer and IR spectral data are also consistent with the (d(xy)) (1) ground state.