Molecular Structures and Redox Properties of Homoleptic Aluminum(III) Complexes with Azobisphenolate (azp) Ligands

To elucidate the oxidation behavior of the 2,2 '-azobisphenolate (azp) ligand, a series of homoleptic 1:2 Al-III complexes of four azp derivatives (L1) with 5,5 '-dichloro-, 5,5 '-dimethyl-, 5,5 '-di-t-butyl-, 3,3 ',5,5 '-tetramethyl-substituents and of one imino derivative (L2) were synthesized and obtained as TPP[Al(L)(2)]center dot solvent (TPP = tetraphenylphosphonium ion). The X-ray crystal structure analyses showed that the two ONO-tridentate ligands were meridionally coordinated to a central Al-III ion in an almost perpendicular manner to give a homoleptic octahedral coordination structure in all the Al-III complexes. The proton nuclear magnetic resonance spectra suggested that all the Al-III complexes retained the homoleptic coordination structure in solution. From the cyclic voltammetry measurements in dichloromethane solutions, all the Al-III complexes with the azp ligands showed two partially reversible oxidation waves, and an additional reversible or partially reversible reduction wave. The substitution effects on the first oxidation and reduction peak potentials were revealed in the Al-III complexes with the azp ligands. On the other hand, the imino complex showed a partially reversible oxidation wave accompanying a film deposition. The density functional theory (DFT) calculations indicated that the molecular orbital (MO) coefficients of the frontier MOs in the Al-III complexes were present on the ligands and were absent on the Al-III ion. These results confirmed that the azp ligands are susceptible to oxidation and can give a relatively stable oxidation species depending upon substituent effects.

Automated summary

A series of Al(III) complexes of azp derivatives were synthesized and their oxidation behavior was investigated. The results showed that the azp ligands are easily oxidized and can form relatively stable oxidation species depending on substituent effects.