Resonance Raman Spectroscopic Study of Phenoxyl Radical Complexes

Resonance Raman (RR) spectroscopy has been employed to study coordinated phenoxyl radicals (M = Ga, Sc, Fe) which were electrochemically generated in solution by using 1,4,7-triazacyclononane-based ligands containing one, two, or three p-methoxy or p-tert-butyl N-substituted phenolates, i.e., 1,4,7-tris(3,5-di-tert-butyl-2-hydroxybenzyl)-1,4,7-triazacyclononane (³L^but), 1,4,7-tris(3-tert-butyl-5-methoxy-2-hydroxybenzyl)-1,4,7-triazacyclononane (³L^met), 1,4-bis(3-tert-butyl-5-methoxy-2-hydroxybenzyl)-7-ethyl-1,4,7-triazacyclononane (²L^met), and 1-(3-tert-butyl-5-methoxy-2-hydroxybenzyl)-4,7-dimethyl-1,4,7-triazacyclononane (¹L^met). A selective enhancement of the vibrational modes of the phenoxyl chromophores is achieved upon excitation in resonance with the π → π* transition at ca. 410 nm. The interpretation of the spectra was supported by quantum chemical (density functional theory) calculations which facilitate the vibrational assignment for the coordinated phenoxyl radicals and provide the framework for correlations between the RR spectra and the structural and electronic properties of the radicals. For the uncoordinated phenoxyl radicals the geometry optimization yields a semiquinone character which increases from the unsubstituted to the p-methyl- and the p-methoxy-substituted radical. This tendency is indicated by a steady upshift of the ν_8a mode which predominantly contains the C_ortho−C_meta stretching coordinate, thereby reflecting strengthening of this bond. The calculated normal-mode frequencies for these radicals are in a good agreement with the experimental data constituting a sound foundation for extending the vibrational analysis to the 2,6-di-tert-butyl-4-methoxyphenoxyl which is the building block of the macrocyclic ligands ³L^met, ²L^met, and ¹L^met. The metal-coordinated radical complexes reveal a similar band pattern as the free radicals with the modes ν_8a and ν_7a (CO stretching) dominating the RR spectra. These two modes are sensitive spectral indicators for the structural and electronic properties of the coordinated phenoxyl radicals. A systematic investigation of complexes containing different ligands and metal ions reveals that two parameters control the semiquinone character of the phenoxyls:  (i) an electron-donating substituent in the para position which can accept spin density from the ring and (ii) an electron-accepting metal ion capable of withdrawing excess electron density, introduced by additional electron-donating substituents in ortho positions. It appears that both effects, which are reflected by (i) the frequency of the mode ν_8a and (ii) the frequency difference of the modes ν_8a and ν_7a, balance an optimum electron density distribution in the phenoxyl radical. Along similar lines, it has been possible to interpret the RR spectral changes between the Fe monoradical, [Fe(³L^met)]^+•, and diradical, [Fe(³L^met)]^2+••, complexes. Both the parent as well as the radical complexes of Fe exhibit a phenolate-to-iron charge transfer band >500 nm. Excitation in resonance with this transition yields a selective enhancement of the vibrational modes of the coordinated phenolates which reveal a significantly more complex band pattern than the coordinated phenoxyls. For a large number of phenolate modes, distinct differences in frequencies and relative intensities were found between the parent and the monoradical Fe complexes implying that oxidation of one phenolate affects the structures and electron density distributions in the ground and excited states of the remaining phenolates. These results are discussed in relation to the structure of the copper-coordinated tyrosyl radical in the active site of galactose oxidase.