H2-release from alcohols, diols, and compounds with amino functionality promoted by titanium(II) sandwich complex, [Cp2Ti]: a theoretical approach

Abstract

The role of Ti(II) sandwich complex, [Cp2Ti], generated by the combination of [Cp2TiCl2] and two equivalents of nBuLi, in situ, in toluene has been investigated theoretically for the release of H2 from a series of liquid organic hydrogen carriers (LOHCs), mainly alcohols or diols of low molecular weights. The compounds with hydroxyl functionality considered here are methanol, ethanol, isopropanol, methanediol, and ethylene glycol. Satisfactory results have also been obtained on applying the same mechanism to two other compounds with amino functionality, viz., methanediamine and ethylene diamine. The exploration has been carried out using density functional theory (DFT). Our findings reveal that the reaction is initiated with the formation of an adduct of the Ti center of sandwich complex, [Cp2Ti], with O/N atom of the compound under investigation. From the adduct thus formed, H2-release is possible via two routes. In one of the paths, O/N–H bond breaks first, followed by the cleavage of C–H bond, and in the other path, C–H bond breaking takes place prior to the scission of O/N–H bond. Our results show that the second path is thermodynamically and kinetically more preferable for the release of H2. Thus, [Cp2Ti] can be highly effective in releasing one equivalent of H2 from alcohols and compounds with amino functionalities, and more importantly, removal of two equivalents of H2 from methanediol is possible using [Cp2Ti], which is significant in terms of hydrogen storage purpose.