Binding, release, and functionalization of CO2 at a nucleophilic oxo anion complex of titanium

Abstract

The titanium oxo anion complex [(Et₂O)₂Li][OTi(N[^tBu]3,5-Me₂C₆H₃)₃] ([(Et₂O)₂Li][1]) reacts with CO₂ in diethyl ether to form the carbonate complex ([Li][O₂COTi(N[^tBu]3,5-Me₂C₆H₃)₃])₆ ([Li][2]). The solid-state structure of complex [Li][2] is a hexamer with a hexagonal prismatic core comprised of six lithium cations bridged by the carbonate functionality. In the monomeric subunits of [Li][2], the carbonate ligand is bound κ¹- to the titanium metal center and pseudo κ²- to the lithium countercation. The hexameric structure persists in benzene solutions as determined by ¹H DOSY NMR techniques. The binding of CO₂ in complex [Li][2] is reversible and can be effected by the introduction of the lithium sequestration reagent 12-crown-4 to diethyl ether solutions of [Li][2]. Complex [Li][2] is readily functionalized with Me₃SiOS(O)₂CF₃ to yield the silyl carbonate complex Me₃SiOC(O)OTi(N[^tBu]3,5-Me₂C₆H₃)₃ (3), the solid-state structure of which is presented. Functionalization with pivaloyl chloride results in the rapid loss of CO₂ and formation of the pivalate complex ^tBuC(O)OTi(N[^tBu]3,5-Me₂C₆H₃)₃ (4).