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

A combined experimental and DFT study of the reactions of cyclopentadienyl-amidinate titanium imido complexes with CO(2) is reported. Cycloaddition reactions of the aryl imido compounds Ti(eta-C(5)R(4)Me)(NAr){R(2)C(NR(1))(2)} (R = H or Me; R(1), R(2) = SiMe(3), Ph or (i)Pr, Me) with CO(2) gave the corresponding N,O-bound carbamate complexes Ti(eta-C(5)R(4)Me){N(Ar)C(O)O}{R(2)C(NR(1))(2)}. These reacted further with CO(2) by insertion into the Ti-N(Ar) bond to afford the new dicarboxylates Ti(eta-C(5)R(4)Me){OC(O)N(Ar)C(O)O}{R(2)C(NR(1))(2)} in which the original Ti=NAr bond has been completely cleaved. The X-ray structures of two of these have been determined. The CO(2) insertion reactions of the para-substituted phenyl carbamate complexes Ti(eta-C(5)Me(5)){N(-4-C(6)H(4)X)C(O)O}{MeC(N(i)Pr)(2)} (X = Me, CF(3) or NMe(2)) were first order with respect to both carbamate complex and CO(2) and the pseudo first order rate constants were effectively independent of the para substituent. The corresponding tert-butyl imido compounds Ti(eta-C(5)R(4)Me)(N(t)Bu){R(2)C(NR(1))(2)} also reacted with CO(2) to form N,O-bound carbamate complexes, Ti(eta-C(5)R(4)Me){N((t)Bu)C(O)O}{R(2)C(NR(1))(2)}. However, these did not insert a further molecule of CO(2) and instead extruded (t)BuNCO to form the crystallographically characterized oxo-bridged dimers Ti(eta-C(5)R(4)Me)(mu-O){R(2)C(NR(1))(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(eta-C(5)H(5))(NR){MeC(NMe)(2)} (R = Me, Ph, 4-C(6)H(4)Me, 4-C(6)H(4)NMe(2), 4-C(6)H(4)CF(3)) reacting with CO(2) showed that the second CO(2) insertion is thermodynamically favoured over isocyanate extrusion, and that the rates of the two processes are similar. Calculations on Ti(eta-C(5)R(5))(N(t)Bu){MeC(N(i)Pr)(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 CO(2) insertion, making the latter process impossible.