RESONANCE RAMAN-SPECTRA OF HIGH-SPIN AND LOW-SPIN FERRIC PHENOLATES - MODELS FOR DIOXYGENASES AND NITRILE HYDRATASE

A series of ferric phenolate complexes with Schiff base or methylamine substitutents ortho to the phenolic oxygen have been investigated. These include complexes with N, N′-ethylenebis((o-hydroxyphenyl)glycine), e.g., Na[Fe(EHPG)]; N-[2-(o-salicylideneamino)ethyl](o-hydroxyphenyl)glycine, e.g., [Fe(EHGS)(X)n], where (X)n = OH−, H2O, or (CN−)3; and N1,N4-di-salicylidenetriethylenetetramine, e.g., [Fe((sal)2trien)](PF6). Although the majority of the complexes contained high-spin Fe(III), two low-spin species were generated: [Fe((sal)2trien)](PF6) at 90 K and [Fe(EHGS)(CN)3]3−. Upon excitation within the phenolate → Fe(III) CT band, the resonance Raman spectra of both the high- and low-spin complexes are dominated by a set of phenolate vibrational modes at approximately 615, 630, 895, 1120, 1280, 1330, 1450, 1475, 1560, and 1600 cm−1. The Schiff base containing ligands exhibit an additional resonance-enhanced vibration due to v(C=N) at ~ 1640 cm−1. The approximate doubling in the number of resonance-enhanced modes compared to the number in the Raman spectra of iron tyrosinate proteins is a consequence of the lower symmetry of ortho-relative to para-substituted phenolates. A correlation between Fe-O(phenolate) bond length with the intensity and/or frequency of the ~ 600-cm−1 Fe-O stretching mode is proposed to explain the variability in the energy and intensity of the v(Fe-O) mode in iron tyrosinate proteins. The primary consequence of the formation of a low-spin state is to shift the phenolate → Fe(III) CT band from ~480 to ~650 nm, on the basis of the corresponding shift in the enhancement maximum of the ferric phenolate vibrational modes. © 1990, American Chemical Society. All rights reserved.