Formation of metallacyclobutene complexes via the addition of hydrazines to ruthenium vinylidene complexes

The reaction between monosubstituted hydrazines and Ru(II) vinylidene complexes [RuCl(P∩N)₂-(=C=CH(R))]BPh₄ (where R = Ph, n-Bu and P∩N = 3,5-dimethyl[1-((2-diphenylphosphino)ethyl)]pyrazole (Me ₂PyP)) afforded metallacyclobutene derivatives of the general formula [RuCl(κ¹P-Me₂PyP)(κ²P,N-Me ₂PyP)(κ²N,C-NH₂N′(R ¹)C(CH₂R²))]BPh₄ (where R ¹, R² = Me, Ph, n-Bu). The new metallacyclic ruthenium complexes contain a four-membered ring (-Ru=C-N-N-) and one pendant P,N-donor ligand, Me₂PyP, bound through the P-donor atom. The solid-state and solution-state structures of these complexes were confirmed through X-ray crystallographic analysis and extensive 2D NMR spectroscopic studies. The reactivity of [RuCl(Me₂PyP)₂(=C=CH(R))]BPh₄ toward amines and disubstituted hydrazines was also investigated. The Me ₂PyP ligand exhibited hemilabile behavior on reaction of [RuCl(Me₂PyP)₂-(=C=CH(R))]BPh₄ with aniline. The mechanism of the formation of the four-membered metallacycle was investigated, and it was found that the formation of N-C bonds from hydrazines and vinylidene was only possible when facilitated by displacement of the N-donor atom of the P,N-donor ligand Me₂PyP by the -NH₂ group of the monosubstituted hydrazines.