ELECTRONIC-STRUCTURE OF THE TRIS(1,3-DIPHENYLTRIAZENIDO)ALUMINUM RADICAL-ANION - A THEORETICAL AND EXPERIMENTAL ESEEM AND EPR STUDY

Multifrequency ESEEM (electron spin echo envelope modulation) and EPR spectroscopies are employed to elucidate the electronic structure of t he monoradical anion of tris(1,3-diphenyltriazenido)aluminum, [Al(dpt) (3)](-), which is obtained by the chemical reduction of Al(dpt)(3). An alysis of the EPR and ESEEM spectra obtained for various isotope-label ing combinations, including N-15 isotope labeling of either the 1,3 (f lanking) or 2 (central) positions of the diphenyltriazenide ligands, a llows for the detailed study of the electronic structure of the radica l anion. EPR spectra of the N-15-labeled derivatives clearly indicate that the radical anion complex is ligand-centered with strong hyperfin e couplings to the nitrogen nuclei of a single triazenide ligand obser ved. However, detailed analysis of the strong hyperfine couplings exhi bited in the EPR spectra is prevented by the lack of resolution, A mul tifrequency ESEEM spectroscopy study was undertaken in order to resolv e the weakly coupled nuclei obscured in the EPR spectra by the inhomog eneous line broadening. A very weak coupling to the aluminum center is manifested in the ESEEM spectra, further supportive of the ligand-cen tered nature of the radical anion complex. Also evident from the ESEEM spectra are three distinguishable types of nitrogen nuclei: weak, ani sotropically coupled central and flanking nitrogens and weak, isotropi cally coupled (ca. 1 Gauss) flanking nitrogens. Ab initio calculations were performed in order to aid in the assignment of the nuclear hyper fine couplings observed in the ESEEM and EPR spectra to specific nitro gen positions on the triazenide rings. Together, the EPR, ESEEM, and n b initio calculations give a coherent picture of the electron spin den sity distribution and bonding in the radical anion [Al(dpt)(3)](-). Re duction of the aluminum complex results in localization of the electro n onto the p-pi molecular orbitals of one of the three diphenyltriaze nide ligands.