Pendant Arm Functionalized Benzamidinate Titanium Imido Compounds: Experimental and Computational Studies of Their Reactions with CO2

Abstract

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(η-C5R4Me)(NtBu)Cl(py)] (R = Me, H) with Li[Me3SiNC(Ph)N(CH2)nNMe2] (n = 2, 3) afforded the corresponding (dimethylamino)ethylidene or -propylidene functionalized benzamidinate complexes [Ti(η-C5R4Me)(NtBu){Me3SiNC(Ph)N(CH2)nNMe2}] (R = Me, n = 2 (1), 3 (2); R = H, n = 2 (3), 3 (4)). The n-propyl functionalized amidinate complex [Ti(η-C5H4Me)(NtBu){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(η-C5R4Me){NtBuC(O)O}{Me3SiNC(Ph)NR‘}] (R = H, Me; R‘ = CH2CH2NMe2, CH2CH2CH2NMe2, CH2CH2Me), which, in turn, extrude tBuNCO (above −25 °C for R = H and at room temperature for R = Me) to form the μ-oxo-bridged dimeric complexes [Ti2(η-C5R4Me)2(μ-O)2{Me3SiNC(Ph)NR‘}2], two of which have been structurally characterized. Only the C5Me5 carbamate complexes [Ti(η-C5Me5){NtBuC(O)O}{Me3SiNC(Ph)N(CH2)nNMe2}] can be isolated and (for n = 3) structurally characterized. The reaction of 4 and 5 with CO2 is spontaneous at −78 °C, whereas the corresponding reaction of 3 does not occur at significant rates below −35 °C. The rate of extrusion of tBuNCO from [Ti(η-C5H4Me){NtBuC(O)O}{Me3SiNC(Ph)NR‘] at −25 °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 TiNtBu/CO2 coupling reaction.