RESONANCE RAMAN-SPECTROSCOPIC STUDY OF PHENOXYL RADICAL COMPLEXES

Resonance Raman (RR) spectroscopy has been employed to study coordinat ed phenoxyl radicals (M = Ga, Sc, Fe) which were electrochemically gen erated in solution by using 1,4,7-triazacyclononane-based ligands cont aining one, two, or three p-methoxy or p-tert-butyl N-substituted phen olates, i.e., ert-butyl-2-hydroxybenzyl)-1,4,7-triazacyclononane (L-3( but)), 5-methoxy-2-hydroxybenzyl)-1,4,7-triazacyclononane (L-3(met)), y-2-hydroxybenzyl)-7-ethyl-1,4,7-triazacyclononane L-2(met), and ydrox ybenzyl)-4,7-dimethyl-1,4,7-triazacyclononane (L-1(met)). A selective enhancement of the vibrational modes of the phenoxyl chromophores is a chieved upon excitation in resonance with the pi --> pi transition at ca. 410 nm. The interpretation of the spectra was supported by quantu m chemical (density functional theory) calculations which facilitate t he vibrational assignment for the coordinated phenoxyl radicals and pr ovide the framework for correlations between the RR spectra and the st ructural and electronic properties of the radicals. For the uncoordina ted phenoxyl radicals the geometry optimization yields a semiquinone c haracter which increases from the unsubstituted to the p-methyl- and t he p-methoxy-substituted radical. This tendency is indicated by a stea dy upshift of the nu(8a) mode which predominantly contains the C-ortho -C-meta stretching coordinate, thereby reflecting strengthening of thi s bond. The calculated normal-mode frequencies for these radicals are in a good agreement with the experimental data constituting a sound fo undation for extending thr vibrational analysis to the 2,6-di-tert-but yl-4-methoxyphenoxyl which is the building block of the macrocyclic li gands L-3(met), L-2(met), and L-1(met). The metal-coordinated radical complexes reveal a similar band pattern as the free radicals with the modes nu(8a), and nu(7a) (C=O stretching) dominating the RR spectra. T hese two modes are sensitive spectral indicators for the structural an d electronic properties of the coordinated phenoxyl radicals. A system atic 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 posit ion which can accept spin density from the ring and (ii) an electron-a ccepting metal ion capable of withdrawing excess electron density, int roduced by additional electron-donating substituents in ortho position s. It appears that both effects, which are reflected by (i) the freque ncy of the mode nu(8a) and (ii) the frequency difference of the modes nu(8a) and nu(7a), balance an optimum electron density distribution in the phenoxyl radical. Along similar lines, it has been possible to in terpret the RR spectral changes between the Fe monoradical, [Fe(L-3(me t))](+.), and diradical, [Fe(L-3(met))(2+..), complexes. Both the pare nt as well as the radical complexes of Fe exhibit a phenolate-to-iron charge transfer band >500 nm. Excitation in resonance with this transi tion yields a selective enhancement of the vibrational modes of the co ordinated phenolates which reveal a significantly more complex band pa ttern than the coordinated phenoxyls. For a large number of phenolate modes, distinct differences in frequencies and relative intensities we re found between the parent and the monoradical Fe complexes implying that oxidation of one phenolate affects the structures and electron de nsity distributions in the ground and excited states of the remaining phenolates. These results are discussed in relation to the structure o f the copper-coordinated tyrosyl radical in the active site of galacto se oxidase.