Titanium Hydrazido and Imido Complexes: Synthesis, Structure, Reactivity, and Relevance to Alkyne Hydroamination

Treatment of Ti(NMe2)2(dpma) (1) with aniline results in the protonation of the dimethylamido
ligands, which are retained as dimethylamines, and generation of a titanium imido complex Ti(NPh)(NHMe2)2(dpma) (2) in 94% yield. The monomeric imido 2 is converted to the reactive dimeric μ-imido [Ti(NPh)(dpma)]2 (3) on removal of the labile dimethylamine donors. The dimer 3 is converted to monomeric terminal
imido complexes in the presence of added donors, e.g., 4,4‘-di-tert-butyl-2,2‘-bipyridine (But-bpy) and DME.
Compounds 1−3 exhibit the same rate constant for 1-phenylpropyne hydroamination by aniline and are all
kinetically competent to be involved in the catalytic cycle. Attempts to use 1 as a catalyst for hydroaminations
involving 1,1-dimethylhydrazine resulted in only a few turnovers under the best conditions. Consequently,
the chemistry of 1 with hydrazines to generate hydrazido complexes was scrutinized for comparison with
the imido species. Through these studies, titanium hydrazido complexes including Ti(η2-NHNC5H10)2(dpma)
(5), Ti(η2-NHNMe2)2(dpma) (6), and [Ti(μ:η1,η2-NNMe2)(dpma)]2 (7) were characterized. In addition, a terminal
hydrazido(2-) complex was available by addition of But-bpy to 1 prior to 1,1-dimethylhydrazine addition,
which provided Ti(η1-NNMe2)(But-bpy)(dpma) (8). Compound 8 was structurally characterized and compared
to Ti(NPh)(But-bpy)(dpma) (4b), an imido derivative with the same ancillary ligand set. Compound 8 has
a nucleophilic β-nitrogen consistent with a hydrazido(2-) formulation, as determined by reaction with MeI
to form the ammonium imido complex [Ti(NNMe3)(But-bpy)(dpma)]I (9). Analogous pyridinium imido
complexes [Ti(N-1-pyridinium)(But-bpy)(dpma)]+ (10) are available by addition of 1-aminopyridinium iodide
to 1. From the investigations, some conclusions regarding the activity of titanium pyrrolyl complexes in
hydroamination were drawn. The lack of conversion of the bis[μ-hydrazido(2-)] 7 to monomeric species in
the presence of donor ligands is put forth as one explanation for the poor hydrazine hydroamination activity
of 1. This problem was combated in the synthesis of Ti(NMe2)2(dap)2, which is an active catalyst for hydrazine
hydroamination of alkynes.