Reversible C–H Activation in Zirconaaziridine Species: Characterization and Bonding of a Bridging (Amino)alkylidene Complex Active in Alkyne Hydroaminoalkylation

Abstract

Efforts to synthesize a low-coordinate zirconaaziridine complex supported by a bis(ureate) ligand resulted in the formation of bridging aminoalkylidene 2. Complex 2 is a rare example of an aminoalkylidene complex featuring a μ(η2:η2)-N,C binding mode. This complex was fully characterized by single-crystal X-ray diffraction, NMR spectroscopy, and LIFDI-MS. Analysis of the natural localized molecular orbitals (NLMOs) showed a σ-bonding interaction between the alkylidene carbon and each zirconium center with significant delocalization of the Zr1–C1 σ bond into Zr2, thereby explaining the significant differences in the Zr–C bond lengths observed in the solid state. A solution-phase equilibrium between complex 2 and the putative zirconaaziridine species was supported by evaluating the reactivity of 2 in alkyne hydroaminoalkylation. The reaction of diphenylacetylene with complex 2 produced a five-membered metallacycle product, a key intermediate in alkyne hydroaminoalkylation. This reaction was greatly accelerated in the presence of pyridine, suggesting that the presence of neutral donors is essential to favor the formation of reactive zirconaaziridine complexes. In addition, the catalytic activity of complex 2 in the benchmark hydroaminoalkylation reaction of diphenylacetylene with N-benzylaniline was established.