Lewis base behavior of bridging nitrido ligands of titanium polynuclear complexes

The Lewis base behavior of mu3-nitrido ligands of the polynuclear titanium complexes [{Ti(eta5-C5Me5)(mu-NH)}3(mu3-N)] (1) and [{Ti(eta5-C5Me5)}4(mu3-N)4] (2) to MX Lewis acids has been observed for the first time. Complex 1 entraps one equivalent of copper(I) halide or copper(I) trifluoromethanesulfonate through the basal NH imido groups to give cube-type adducts [XCu{(mu3-NH)3Ti3(eta5-C5Me5)3(mu3-N)}] (X=Cl (3), Br (4), I (5), OSO2CF3 (6)). However, the treatment of 1 with an excess (> or = 2 equiv) of copper reagents afforded complexes [XCu{(mu3-NH)3Ti3(eta5-C5Me5)3(mu4-N)(CuX)}] (X=Cl (7), Br (8), I (9), OSO2CF3 (10)) by incorporation of an additional CuX fragment at the mu3-N nitrido apical group. Similarly, the tetranuclear cube-type nitrido derivative 2 is capable of incorporating one, two, or up to three CuX units at the mu3-N ligands to give complexes [{Ti(eta5-C5Me5)}4(mu3-N)(4-n){(mu4-N)CuX}n] (X=Br (11), n=1; X=Cl (12), n=2; X=OSO2CF3 (13), n=3). Compound 2 also reacts with silver(I) trifluoromethanesulfonate (> or = 1 equiv) to give the adduct [{Ti(eta5-C5Me5)}4(mu3-N)3{(mu4-N)AgOSO2CF3}] (14). X-ray crystal structure determinations have been performed for complexes 8-13. Density functional theory calculations have been carried out to understand the nature and strength of the interactions of [{Ti(eta5-C5H5)(mu-NH)}3(mu3-N)] (1') and [{Ti(eta5-C5H5)}4(mu3-N)4] (2') model complexes with copper and silver MX fragments. Although coordination through the three basal NH imido groups is thermodynamically preferred in the case of 1', in both complexes the mu3-nitrido groups act as two-electron donor Lewis bases to the appropriate Lewis acids.