Pendant arm functionalized benzamidinate titanium imido compounds:: Experimental and computational studies of their reactions with CO2

The syntheses of cyclopentadienyl titanium tert-butylimido complexes containing pendant arm functionalized amidinate ligands are reported, together with their reactions with CO2. Mechanistic and DFT computational studies are also described. Reaction of [Ti(eta-C5R4Me)((NBu)-Bu-t)Cl(py)] (R = Me, H) with Li[Me3SiNC(Ph)N(CH2)(n)NMe2] (n = 2, 3) afforded the corresponding (dimethylamino)ethylidene or -propylidene functionalized benzamidinate complexes [Ti(eta-C5R4Me)((NBu)-Bu-t){Me3SiNC(Ph)N(CH2)(n)NMe2}] (R = Me, n = 2 (1), 3 (2); R = H, n = 2 (3), 3 (4)). The n-propyl functionalized amidinate complex [Ti(eta-C5H4Me)((NBu)-Bu-t){Me3SiNC(Ph)NCH2CH2Me}] (5) was also prepared for comparative purposes. The NMe2 donors in 3 and 4 coordinate relatively weakly, and in solution the activation Gibbs free energy for NMe2 dissociation for 3 is ca. 10 kJ mol(-1) higher than that for 4. All five compounds react with CO2 to form the corresponding N,O-bound carbamate complexes [Ti(eta-C5R4Me)-{(NBuC)-Bu-t(O)O}{Me3SiNC(Ph)NR'}] (R = H, Me; R' = CH2CH2NMe2, CH2CH2CH2NMe2, CH2CH2Me), which, in turn, extrude (BuNCO)-Bu-t (above -25 degrees C for R = H and at room temperature for R = Me) to form the mu-oxo-bridged dimeric complexes [Ti-2(eta-C5R4Me)(2)(mu-O)(2){Me3SiNC(Ph)NR'}(2)],two of which have been structurally characterized. Only the C5Me5 carbamate complexes [Ti(eta-C5Me5){(NBuC)-Bu-t(O)O}{Me3SiNC(Ph)N(CH2)(n)NMe2}] can be isolated and (for n = 3) structurally characterized. The reaction of 4 and 5 with CO2 is spontaneous at -78 degrees C, whereas the corresponding reaction of 3 does not occur at significant rates below -35 degrees C. The rate of extrusion of tBuNCO from [Ti(eta-C5H4Me){(NBuC)-Bu-t(O)O}{Me3SiNC(Ph)NR'] at -25 degrees C is twice as fast for R' = CH2CH2NMe2 as for CH2CH2CH2NMe2 and CH2CH2Me. DFT calculations have modeled the cycloaddition/extrusion reactions of 3-5 in conjunction with mechanistic and variable-temperature NMR experiments. The results show that the favored route is for the cycloaddition reaction to take place on 16-valence-electron, non pendant arm chelated isomers of 3 and 4 and that the relative ease of accessing these intermediates in fact controls the Ti=(NBu)-Bu-t/CO2 coupling reaction.