| Abstrakt: |
A combined experimental and DFT study of the reactions of cyclopentadienyl-amidinate titanium imido complexes with CO2is 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 CO2gave the corresponding N,O-bound carbamate complexes Ti(η-C5R4Me){N(Ar)C(O)O}{R2C(NR1)2}. These reacted further with CO2by 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, CF3or NMe2) were first order with respect to both carbamate complex and CO2and the pseudo first order rate constants were effectively independent of the parasubstituent. The corresponding tert-butyl imido compounds Ti(η-C5R4Me)(NtBu){R2C(NR1)2} also reacted with CO2to 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 CO2and instead extruded tBuNCO to form the crystallographically characterized oxo-bridged dimers [Ti(η-C5R4Me)(μ-O){R2C(NR1)2}]2. These reactions proceeded viatransient 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 CO2showed 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)(NtBu){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. [ABSTRACT FROM AUTHOR] |
