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 CO 2 . Mechanistic and DFT computational studies are also described. Reaction of [Ti(η-C 5 R 4 Me)(N t Bu)Cl(py)] (R = Me, H) with Li[Me 3 SiNC(Ph)N(CH 2 ) n NMe 2 ] ( n = 2, 3) afforded the corresponding (dimethylamino)ethylidene or -propylidene functionalized benzamidinate complexes [Ti(η-C 5 R 4 Me)(N t Bu){Me 3 SiNC(Ph)N(CH 2 ) n NMe 2 }] (R = Me, n = 2 ( 1 ), 3 ( 2 ); R = H, n = 2 ( 3 ), 3 ( 4 )). The n- propyl functionalized amidinate complex [Ti(η-C 5 H 4 Me)(N t Bu){Me 3 SiNC(Ph)NCH 2 CH 2 Me}] ( 5 ) was also prepared for comparative purposes. The NMe 2 donors in 3 and 4 coordinate relatively weakly, and in solution the activation Gibbs free energy for NMe 2 dissociation for 3 is ca. 10 kJ mol - 1 higher than that for 4 . All five compounds react with CO 2 to form the corresponding N,O-bound carbamate complexes [Ti(η-C 5 R 4 Me){N t BuC(O)O}{Me 3 SiNC(Ph)NR‘}] (R = H, Me; R‘ = CH 2 CH 2 NMe 2, CH 2 CH 2 CH 2 NMe 2, CH 2 CH 2 Me), which, in turn, extrude t BuNCO (above −25 °C for R = H and at room temperature for R = Me) to form the μ-oxo-bridged dimeric complexes [Ti 2 (η-C 5 R 4 Me) 2 (μ-O) 2 {Me 3 SiNC(Ph)NR‘} 2 ], two of which have been structurally characterized. Only the C 5 Me 5 carbamate complexes [Ti(η-C 5 Me 5 ){N t BuC(O)O}{Me 3 SiNC(Ph)N(CH 2 ) n NMe 2 }] can be isolated and (for n = 3) structurally characterized. The reaction of 4 and 5 with CO 2 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 t BuNCO from [Ti(η-C 5 H 4 Me){N t BuC(O)O}{Me 3 SiNC(Ph)NR‘] at −25 °C is twice as fast for R‘ = CH 2 CH 2 NMe 2 as for CH 2 CH 2 CH 2 NMe 2 and CH 2 CH 2 Me. 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 N t Bu/CO 2 coupling reaction.