Electron Transfer Reactions in Three-Nuclear Ti Complexes with π-Acceptor Ligands

Abstract

Multinuclear transition metal complexes bridged by ligands with extended π-electronic systems such as hexaphenyl-5,6,11,12,17,18-hexaaza-tribiscyclopentadienyl titaniumnaphtylene [(Cp2Ti)3HATN(Ph)6] show a variety of complicated electronic transitions and electron transfer reactions [1]. Therefore, they are intensively investigated as artificial analogues to a number of natural systems capable of converting light into chemical energy by translocating charges over large distances or to generate light efficiently [1]. While a systematic understanding of the photochemistry and electrochemistry has been atained for binuclear complexes [1], much less is known about trinuclear and multinuclear complexes [2]. Trinuclear titanocene(II) complexes include formally three titanocene(II) fragments with two d-electrons, i.e. they contain six potential d-electrons. In this study the complex has been characterised by electrochemical measurements and absorption spectroscopy. The voltammogramm of [(Cp2Ti)3HATN(Ph)6] shows three reduction waves and six oxidation waves. Solution spectra of [(Cp2Ti)3HATN(Ph)6] and of the electrochemically formed oxidation products show electronic transition in the UV, visible and the NIR ranges. The existence of a single occupied molecular orbital (SOMO) populated by an electron transferred from one of the Ti(II) centers to the HATN(Ph)6 ligand gives rise to several characteristic electronic transitions from which the most prominent is the intervalence charge transfer (IVCT) transition between the Ti(III) center and the remaining Ti(II) centers. Another transition related to the SOMO is the ligand-centered transition from the SOMO to the lowest unoccupied molecular orbital (LUMO). When oxidized to [(Cp2Ti)3HATN(Ph)6]+, the SOMO orbital becomes the LUMO’. If the complex is oxidized to [(Cp2Ti)3HATN(Ph)6]2+, one electron is extracted from the Ti(II). For the reduced form [(Cp2Ti)3HATN(Ph)6]-, the Ti(III) accepted an electron an no IVCT can be observed. In further reduction the “SOMO” is fully occupied and becomes the new HOMO'. In the last reduction an electron is accepted to the one orbital of the Ti(II) and attaining a mixed valence states again. Finally, the orbital diagram in the accurate energy scale was constructed from the assignment of the transitions, the spectra and the formal potentials obtained from the voltammetric data. From this diagram it becomes evident that removal of an electron from the orbitals of the ligand does not change the energetic position of the Ti orbitals, whereas removal of an electron from one of the Ti centers affects the energy of all orbitals in the complex. Figure 1: Structure of [(Cp2Ti)3HATN(Ph)6].