We developed syntheses of the "constrained geometry" (cg) zirconaaziridines Me₄C5SiMe₂N(tBu)Zr-(2 ²)-[N(Ph)CH(Ph)](PMe₂R) (R = Me, Ph) by analogy with the well-known Cp₂-ligated zirconaaziridines Cp₂Zr-(η ²)[N(Ph)CH(Ph)](PR₃). cgZr-(η²)-[N(Ph)CH(Ph)](PMe ₂Ph) has been structurally characterized by X-ray crystallography. Comparison of its solid-state structure with that of its Cp₂ analogue Cp ₂Zr-(η²)-[N(Ph)CH(Ph)](PMe₂Ph) reveals that the Zr–P bond length in the cg complex is significantly longer than in the Cp₂ complex.

Treatment of these zirconaaziridines with unsaturated electrophiles such as diphenylacetylene results in insertion. The product of diphenylacetylene insertion into Me₄C₅SiMe₂N(tBu)Zr-(η ²)-[N(Ph)CH(Ph)](PMe₃) has been characterized by X-ray crystallography. The reaction of Me₄C₅SiMe₂N( tBu)Zr-(η²)-[N(Ph)CH(Ph)](PMe₂R) with diphenylacetylene is several orders of magnitude faster than the reaction of Cp₂Zr-(η²)-[N(Ph)CH(Ph)](PMe₂R) with diphenylacetylene. Kinetic data for these irreversible reactions indicate that PMe₃ dissociation must occur prior to insertion. We have proposed a rate law for the insertion reactions and used kinetic simulations along with the integrated form of the rate law to generate good fits to the experimental data.

We have measured the PMe₃ dissociation rate constants for both Me₄C5SiMe₂N(tBu)Zr-(η ²)-[N(Ph)CH(Ph)](PMe₃) and Cp₂Zr-(η ²)[N(Ph)CH(Ph)](PMe₃), providing quantitative ligand binding data for directly analogous cg and Cp₂ complexes. PMe₃ dissociation from Me₄C₅SiMe₂N(tBu)Zr-(η ²)-[N(Ph)CH(Ph)](PMe₃) is faster (k ₁ = 0.204(7) s^-1) than from Cp₂Zr-(η ²)-[N(Ph)CH(Ph)](PMe₃) (k₁' = 0.0013(1) s^-1).

Early transition metal η²-imine complexes (metallaaziridines) are well-documented as reaction intermediates for the coupling of organic molecules to form a variety of desirable functionalized products. The structures of zirconaaziridines are well-understood, as are their reactions with unsaturated electrophiles. There are few examples of titanaaziridines in the literature that have been either isolated or structurally characterized, though they are implicated as intermediates in many synthetic transformations. The synthesis of new titanaaziridines would fill a gap in the structural data for early transition-metal imine complexes, as well as potentially reveal new and useful modes of reactivity.

We have synthesized the titanaaziridines (CpR¹)₂ Ti-(η²)-[N(R²)CH(R ³)](PMe₃) (R¹= H, Me) from the reactions of (CpR¹)₂Ti(PMe₃)₂ with imines. Preliminary studies of their reactivity towards unsaturated organic compounds suggest that alkynes and cyclic carbonates displace the imine, while aldehydes insert into the Ti–C or Ti–N bonds.