Metallacycle expansion by alkyne insertion. Chemistry of a new family of ruthenium organometallics

Abstract

Acetylene and phenylacetylene reacts with carbonylchloro[4-methyl-6-((R-imino)methyl)phenolato-C,O]bis(triphenylphosphine)ruthenium(II), Ru(RL¹)(PPh₃)₂(CO)Cl (3), affording the inserted product carbonylchloro[2-vinyl-4-methyl-6-((R-imino)methyl)phenolato-C,O]bis(triphenylphosphine)ruthenium(II), Ru(RL²,X)(PPh₃)₂(CO)Cl (4), in virtually quantitative yield. The X-ray structures of 4b (R = X = Ph) and 4g (R = Et, X = H) have revealed the presence of distorted-octahedral RuC₂P₂ClO coordination spheres. In the conversion 3 → 4, the Ru(C,O) chelate ring expands from four-membered to six-membered. The insertion of phenylacetylene is regiospecific, and a reaction model implicating initial Ru-O cleavage and steric control is proposed. The Ru-O bond in 4 is significantly shorter (by ~0.14 Å) and stronger than that in 3. This is reflected in the lowering of the ruthenium(III)-ruthenium(II) reduction potential by ~200 mV. The uncoordinated Schiff base moiety in 4 is present in the hydrogen-bonded iminium-phenolato zwitterionic form, as revealed by the N···O distance as well as by IR and NMR data.