Convenient syntheses of novel ruthenium catalysts bearing N-heterocyclic carbenes

The 16-electron ruthenium(II) complexes Cp*Ru[(R)]Cl (Cp*=η5-C5Me5; R=Cy (ICy), 1a; Mes (IMes), 1b) containing N-heterocyclic carbenes are easily accessible in quantitative yields from [Cp*Ru(OMe)]2 (Me=CH3) and the corresponding 1,3-diorganylimidazolium chloride by methanol elimination. Compounds 1a–b can also be prepared in 75–80% yield by treating the commercially available polymeric ruthenium(III) compound [Cp*RuCl2]n with the free 1,3-diorganylimidazolin-2-ylidenes in 1 to 1.5 molar amounts. 1a reacts with CO, PPh3, pyridine and ethyl diazoacetate (EDA) affording the 18-electron derivatives Cp*Ru(ICy)(L)Cl (L=CO, 2; PPh3, 3; py, 4; CHCO2Et, 5). The mixed dicarbene complex 5 is the first isolable ruthenium cyclopentadienyl species bearing a CHCO2Et moiety. Compounds 1a–b catalyze the carboncarbon coupling of terminal alkynes HCCR (R=Ph, SiMe3, tBu, p-Tol) under mild conditions, with the selectivity strongly depending on the substituent R.