Dinitrogen Cleavage and Functionalization with Carbon Dioxide in a Dititanium Dihydride Framework.

Abstract

The activation and functionalization of dinitrogen (N2) with carbon dioxide (CO2) are of great interest and importance but highly challenging. We report here for the first time the reaction of N2 with CO2 in a dititanium dihydride framework, which leads to N-C bond formation and N-N and C-O bond cleavage. Exposure of a dinitrogen dititanium hydride complex {[(acriPNP)Ti]2(μ2-η1:η2-N2)(μ2-H)2} (1) (acriPNP = 4,5-bis(diisopropylphosphino)-2,7,9,9-tetramethyl-9H-acridin-10-ide) to a CO2 atmosphere at room temperature rapidly yielded a nitrido/N,N-dicarboxylamido complex {[(acriPNP)Ti]2(μ2-N)[μ2-N(CO2)2]} (2, 28%) and a diisocyanato/dioxo complex {[(acriPNP)Ti]2(NCO)2(μ2-O)2} (3, 52%) with release of H2. When the reaction of 1 with CO2 (1 atm) was carried out at -50 °C, complex 2 was selectively formed in 82% yield within 5 min. Heating 2 at 80 °C under 1 atm CO2 for 30 min afforded 3 in 67% yield. When 1 was allowed to react with 1.5 equiv of CO2 at room temperature, an isocyanato/nitrido/oxo complex {[(acriPNP)Ti]2(NCO)(μ2-N)(μ2-O)} (4) was exclusively formed in 89% yield within 5 min. The reaction of 4 with CO2 at room temperature almost quantitatively yielded the dioxo/diisocyanato complex 3 within 5 min. The mechanistic details were clarified by the 15N- and 13C-labeled experiments and density functional theory (DFT) calculations, providing unprecedented insights into the reaction of N2 with CO2. A titanium-mediated cycle for the synthesis of trimethylsilyl isocyanate Me3SiNCO from N2, CO2, and Me3SiCl using H2 as a reducing agent was also established.