Reactivity of titanium pyrazolates towards CO2, CS2 and N2O

Abstract

The tetravalent titanium pyrazolate Ti(pz^Me₂)₄ inserts two molecules of carbon dioxide instantly at ambient temperature and 1 bar heteroallene, forming the complex Ti(CO₂·pz^Me₂)₂(pz^Me₂)₂ (pz^Me₂ = 3,5-dimethylpyrazolato). CO₂ insertion is reversible as proven by thermogravimetric analysis (TGA). Solid Ti(CO₂·pz^Me₂)₂(pz^Me₂)₂ exhibits an exceptionally high stability at ambient atmosphere but converts in solution over time via deoxygenation into the oxo-bridged species [Ti(CO₂·pz^Me₂)₂(pz^Me₂)]₂(μ-O) and [Ti(μ-O)₂(μ-pz^Me₂)₄(Ti{CO₂·pz^Me₂}{pz^Me₂})₂]. Treatment of Ti(pz^Me₂)₄ with CS₂ results in a mono-insertion to Ti(CS₂·pz^Me₂)(pz^Me₂)₃, which is reversible at 110 °C. Applying similar conditions, trivalent Ti(pz^tBu₂)₃ inserts also two molecules of CO₂ to afford Ti(CO₂·pz^tBu₂)₂(pz^tBu₂) displaying complete de-insertion at 70 °C. At elevated temperatures compound Ti(CO₂·pz^tBu₂)₂(pz^tBu₂) does neither display redox activity nor engages in a deoxygenation reaction. While tetravalent Ti(pz^Me₂)₄ does not react with N₂O under ambient conditions, trivalent Ti(pz^tBu₂)₃ converts into oxo-bridged [Ti(pz^tBu₂)₃]₂(μ-O) via initial N₂O insertion and subsequent fast release of N₂.