Reactions of cyclopentadienyl-amidinate titanium imido compounds with CO2: cycloaddition-extrusionvs.cycloaddition-insertion

Abstract

A combined experimental and DFT study of the reactions of cyclopentadienyl-amidinate titanium imido complexes with CO₂ is reported. Cycloaddition reactions of the aryl imido compounds Ti(η-C₅R₄Me)(NAr){R²C(NR¹)₂} (R = H or Me; R¹, R² = SiMe₃, Ph or ⁱPr, Me) with CO₂ gave the corresponding N,O-bound carbamate complexes Ti(η-C₅R₄Me){N(Ar)C(O)O}{R²C(NR¹)₂}. These reacted further with CO₂ by insertion into the Ti–N(Ar) bond to afford the new dicarboxylates Ti(η-C₅R₄Me){OC(O)N(Ar)C(O)O}{R²C(NR¹)₂} in which the original TiNAr bond has been completely cleaved. The X-ray structures of two of these have been determined. The CO₂insertion reactions of the para-substituted phenyl carbamate complexes Ti(η-C₅Me₅){N(-4-C₆H₄X)C(O)O}{MeC(NⁱPr)₂} (X = Me, CF₃ or NMe₂) were first order with respect to both carbamate complex and CO₂ and the pseudo first order rate constants were effectively independent of the para substituent. The corresponding tert-butyl imido compounds Ti(η-C₅R₄Me)(N^tBu){R²C(NR¹)₂} also reacted with CO₂ to form N,O-bound carbamate complexes, Ti(η-C₅R₄Me){N(^tBu)C(O)O}{R²C(NR¹)₂}. However, these did not insert a further molecule of CO₂ and instead extruded ^tBuNCO to form the crystallographically characterized oxo-bridged dimers [Ti(η-C₅R₄Me)(μ-O){R²C(NR¹)₂}]₂. These reactions proceeded via transient terminal oxo intermediates, one of which was trapped by the addition of TolNCO (Tol = p-tolyl). DFT (B3PW91) calculations on Ti(η-C₅H₅)(NR){MeC(NMe)₂} (R = Me, Ph, 4-C₆H₄Me, 4-C₆H₄NMe₂, 4-C₆H₄CF₃) reacting with CO₂ showed that the second CO₂insertion is thermodynamically favoured over isocyanate extrusion, and that the rates of the two processes are similar. Calculations on Ti(η-C₅R₅)(N^tBu){MeC(NⁱPr)₂} (R = H or Me) showed that increasing the steric bulk increases the thermodynamic favourability of the isocyanate extrusion process and significantly raises the activation barrier for the second CO₂insertion, making the latter process impossible.