Resonance Raman Spectroscopic Core-Size Correlations for the Crystallographically Defined Complexes Fe(II)(OEP), Fe(II)(OEC), Fe(III)(OEP)(NCS), [Fe(III)(OEP)(N-MeIm)2](1+), and [Fe(III)(OEP)(DMSO)2](1+)

Article abstract of DOI:10.1021/ic00119a017

Resonance Raman (rR) spectroscopy of metalloporphyrins can provide detailed information about the oxidation state, coordination number, and spin state of the central metal ion. These data have been of particular value in the study of natural and model heme systems. Core-size correlations, plots of the porphyrin-ring center to pyrrole nitrogen (Ct-N) distances versus rR frequencies, are frequently utilized to assess structural properties or to confirm spectral assignments. However, in most cases, such plots for β-pyrrole-substituted porphyrins are limited by the use of Ct-N distances derived from the crystal structures of meso-tetraphenylporphyrins. Because of well-documented structural and spectroscopic differences in these two classes of porphyrins, the utility of such core-size correlations may be impaired. Here we report the crystal structuresof three new octaethylporphyrin (OEP) complexes: (1) Fe(OEP)(NCS).C7H8,triclinic, P1-, a = 11.669(2) ?, b = 12.151(2) ?, c = 14.251(3)?, α = 73.77(1)°, β = 87.41(1)°, γ =82.07(1)°, Z = 2, T = -130(1)°C; (2) [Fe(OEP)(N-MeIm)2][Ag(N-MeIm)2][PF6]2, P1-, a = 10.577(2) ?, b = 11.322(2) ?, c = 13.205(2) ?, α = 66.61(2)°, β = 82.43(2)°, γ = 77.51(2)°, Z = 1, T = -100(1)°C; (3) [Fe(OEP)(DMSO)2][PF6], monoclinic, C2/c, a = 21.877(4) ?, b = 14.680(3) ?, c= 14.593(3) ?, β = 93.43(3)°, Z = 4, T = -100(1)°C. These three constitute a set of ferric compounds that are a high-spin pentacoordinate, a low-spin hexacoordinate, and a high-spin hexacoordinate, respectively. The rR properties of these three compounds together with rR data of two crystallographically defined intermediate-spin, tetracoordinate ferrous complexes, Fe(OEP) and Fe(OEC), are presented. The latter is a chlorin, i.e., a dihydroporphyrin, complex. The present rR data for these crystallographically characterized complexes permit the first rigorous test of earlier Ct-N vs ν correlations in that the spectroscopic and structural parameters are from the same compounds in the same crystalline phase and yield a new core-size correlation for β-pyrrole-substituted metalloporphyrins. With the present analysis as a foundation, rR spectroscopic studies may be extended to simple complexes in solution as well as heme proteins in the determination of core sizes andin facilitating spectral assignments.