Radical Anion Complexes of Tris(1,3-diphenyltriazenido)aluminum

Abstract

Electrochemical studies of Al(dpt)3 (Hdpt = 1,3-diphenyltriazene) by cyclic voltammetry in THF solution reveal three successive pseudo reversible one-electron reduction waves (E112 = -1.50, -1.84, and -2.16 V). The chemical reduction of Al(dpt)3 by sodium metal in THF allows for the isolation of the radical anion complexes [Na(THF)x]n[Al(dpt)3], n = 1 ( l ) , 1 (2), and 3 (3). Characterization by EPR, NMR, UV-visible, and X-ray photoelectron (XP) spectroscopy, in addition to the X-ray structural determination of [PPN] [Al(dpt)3] (4), supports the formation of the f i s t homologous series of ligand-centered aluminum(II1) radical anion complexes. Analogous electrochemical reduction series are observed for the p-methyland p-methoxy-substituted triazenides. The dependence of the complex reduction potentials is discussed with respect to the UV-visible spectra of the unreduced complex and the ligand's Hammett substituent constant (a). In contrast, irreversible electrochemical reduction (-1.5 to -2.2 V) occurs for the pentafluoroand p-fluoro-, p-chloro-, and p-bromo-substituted triazenido complexes. Irreversible reduction also occurs for the alkyl and aryloxide compounds Al(R)z(dpt) (R = 'Bu, 'Bu), Al('Bu)(dpt)z, Al(BHT)z(dpt), and Al(BHT)(dpt)z (BHT-H = 2,6-di-tert-butyl-4-methylphenol). Ab initio molecular orbital calculations have been carried out on the model compounds Al(HNNNH)3 and [Al(HNNNH)3I3-. The identity of the frontier molecular orbitals and calculated structures are considered in relation to experimental data.