Titanium tert-Butoxyimido Compounds
Résumé
The synthesis and molecular and electronic structures of the first tert-butoxyimido complexes of titanium (TiNOtBu functional group) are reported, featuring a variety of mono- or poly-dentate, neutral or anionic N-donor ligands. Reaction of Ti(NMe2)2Cl2 with tBuONH2 gave good yields of Ti(NOtBu)Cl2(NHMe2)2 (1). Compound 1 serves as an excellent entry point into new tert-butoxyimido complexes by reaction with a variety of fac-N3 donor ligands, namely, Me3[9]aneN3 (trimethyl-1,4,7-triazacyclononane), HC(Me2pz)3 (tris(3,5-dimethylpyrazolyl)methane), or Me3[6]aneN3 (trimethyl-1,3,5-triazacyclohexane) to give Ti(NOtBu)(Me3[9]aneN3)Cl2 (2), Ti(NOtBu){HC(Me2pz)3}Cl2 (3), or Ti(NOtBu)(Me3[6]aneN3)Cl2 (4) in good yield. It was found that 4 could be converted into Ti(NOtBu)Cl2(py)3 (5) in very good yield by reaction with an excess of pyridine. Compound 5 is effective in a range of salt metathesis reactions with lithiated amide or pyrrolide ligands, and reacts with Li2N2Npy, Li2N2NMe, LiNpyrNMe2, or Li2N2pyrNMe to give Ti(N2Npy)(NOtBu)(py) (6), Ti(N2NMe)(NOtBu)(py) (7), Ti(NpyrNMe2)(NOtBu)Cl(py)2 (9), or Ti(N2pyrNMe)(NOtBu)(py)2 (10) in moderate to good yields (N2Npy = (2-NC5H4)C(Me)(CH2NSiMe3)2; N2NMe = MeN(CH2CH2NSiMe3)2; NpyrNMe2 = Me2NCH2(2-NC4H3); N2pyrNMe = MeN{CH2(2-NC4H3)}2). Compounds 7, 9, and 10 reacted with 2,2′-bipyridyl by pyridine exchange reactions forming Ti(N2NMe)(NOtBu)(bipy) (8), Ti(NpyrNMe2)(NOtBu)Cl(bipy) (11), and Ti(N2pyrNMe)(NOtBu)(bipy) (12). Ten tert-butoxyimido compounds, namely, 1-6, 11, and 12, have been structurally characterized revealing approximately linear Ti-N-OtBu linkages with Ti-N distances [range 1.686(2)-1.734(2) Å] that are generally intermediate between those in the homologous alkylimido and phenylimido analogues, and shorter than in the diphenylhydrazido counterparts. Density functional theory (DFT) studies on the model compounds Ti(NR)Cl2(NHMe2)2 (1_R; R = OMe, Me, Ph, NMe2) confirmed this trend and found that the destabilizing effect of the −OMe oxygen 2pπ lone pair on one of the Ti-N π-bonds in 1_OMe is comparable to that of the occupied phenyl ring π orbitals in the phenylimido homologue 1_Ph but much less than for the −NMe2 nitrogen lone pair in 1_NMe2.