RADICAL-ANION COMPLEXES OF TRIS(1,3-DIPHENYLTRIAZENIDO)ALUMINUM

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 (E(1/2) = -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 (I), 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 first homologous series of ligand-centered aluminum(III) radical anion complexes. Analogous electrochemical reduction series are observed for the p-methyl- and 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 (sigma). In contrast, irreversible electrochemical reduction(-1.5 to -2.2 V) occurs for the pentafluoro- and p-fluoro-, p-chloro-, and p-bromo-substituted triazenido complexes. Irreversible reduction also occurs for the alkyl and aryloxide compounds Al(R)(2)(dpt) (R = (i)Bu, (i)Bu), Al((i)Bu)(dpt)(2), Al(BHT)(2)(dpt), and Al(BHT)(dpt)(2) (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)(3)](3-). The identity of the frontier molecular orbitals and calculated structures are considered in relation to experimental data.