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

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.