Reactivity of titanium pyrazolates towards CO2, CS2 and N2O.

The tetravalent titanium pyrazolate Ti(pzMe2)4 inserts two molecules of carbon dioxide instantly at ambient temperature and 1 bar heteroallene, forming the complex Ti(CO2·pzMe2)2(pzMe2)2 (pzMe2 = 3,5-dimethylpyrazolato). CO2 insertion is reversible as proven by thermogravimetric analysis (TGA). Solid Ti(CO2·pzMe2)2(pzMe2)2 exhibits an exceptionally high stability at ambient atmosphere but converts in solution over time via deoxygenation into the oxo-bridged species [Ti(CO2·pzMe2)2(pzMe2)]2(μ-O) and [Ti(μ-O)2(μ-pzMe2)4(Ti{CO2·pzMe2}{pzMe2})2]. Treatment of Ti(pzMe2)4 with CS2 results in a mono-insertion to Ti(CS2·pzMe2)(pzMe2)3, which is reversible at 110 °C. Applying similar conditions, trivalent Ti(pztBu2)3 inserts also two molecules of CO2 to afford Ti(CO2·pztBu2)2(pztBu2) displaying complete de-insertion at 70 °C. At elevated temperatures compound Ti(CO2·pztBu2)2(pztBu2) does neither display redox activity nor engages in a deoxygenation reaction. While tetravalent Ti(pzMe2)4 does not react with N2O under ambient conditions, trivalent Ti(pztBu2)3 converts into oxo-bridged [Ti(pztBu2)3]2(μ-O) via initial N2O insertion and subsequent fast release of N2.