RESONANCE RAMAN AND INFRARED-SPECTRA OF AZIDOIRON(III) AND AZIDOMANGANESE(III) PORPHYRINS

Infrared and resonance Raman (RR) frequencies of ligand modes have been measured for a series of five-coordinate azidoiron(III) and azidomanganese(III) porphyrins and their N-15(3), (NN2)-N-15-N-14 (Fe, Mn) and Fe-54 (Fe) isotopically-substituted analogues. The RR spectra of Fe porphyrins are dominated by the v(Fe-N3) stretching mode (approximately 420 cm-1) when excitation is made in resonance with porphyrin pi-pi* transitions (406.7-514.5 nm). In contrast, the delta(N3) bending (approximately 630 cm-1) and v(as) (N3) asymmetric stretching (approximately 2040 cm-1) modes are the only ligand vibrations which occur in the RR spectra of azido-Mn porphyrins. These two modes are enhanced with visible excitations between a split Soret band, suggesting the presence of a new charge-transfer transition in this region that involves azide pi-electrons. The polarized nature of these bands (rho = 0.25) implies that the effective symmetry of the bound N3 ligand is C(s). A band at 349 cm-1 in the IR spectrum of N3Mn (III)TMP (TMP = tetramesitylporphyrin), which shifts to 344 cm-1 upon (NN2)-N-15-substitution, is assigned to v(Mn-N3). This frequency is much lower than v(Fe-N3), indicating a substantially weaker Mn(III)-N3 bond. A nearly complete set of the azido-bound vibrational frequencies afforded by RR spectra of thin films of N3Fe(III)OEP (OEP = octaethylporphyrin) at 30 K has permitted accurate band assignments based on the observed isotope shifts and the results of normal coordinate analysis carried out on a Fe-N-N-N fragment. This anlysis has also shown that the mixed isotope splitting (NN2)-N-15-N-14 of the asymmetric intra-azide stretch is due to a non-equivalence in bonding within the azide molecule.