C−C Bond Formation between Vinylidene and Alkynyl Ligands at Ruthenium(II) Leading to either Enynyl or Dienynyl Complexes

Abstract

The Ru(II) complex mer,trans-(PNP)RuCl2(PPh3) (1) reacts with either phenylacetylene or p-tolylacetylene to give fac,cis-(PNP)RuCl2{CCH(R)} (R = Ph (2), p-tolyl (3)) in refluxing tetrahydrofuran (THF) and mer,trans-(PNP)RuCl2{CCH(R)} (R = Ph (4), p-tolyl (5)) in a refluxing THF/EtOH mixture (1:2 v:v) [PNP = CH3CH2CH2N(CH2CH2PPh2)2]. The fac,cis vinylidene complexes 2 and 3 react with an excess of LiC⋮CPh converting to the σ-alkynyl−η3-enynyl complexes anti,mer-(PNP)Ru(C⋮CPh){η3-PhC3CH(Ph)} (8) and anti,mer-(PNP)Ru(C⋮CPh){η3-PhC3CH(p-tolyl)} (9), respectively. Conversely, treatment of the mer,trans vinylidene isomers with an excess of LiC⋮CPh, followed by addition of a primary alcohol, exclusively gives the σ-alkynyl−η-dienynyl complexes mer-(PNP)Ru(C⋮CPh){η-PhCC(C⋮CPh)CHCH(R)} (R = Ph, 12; R = p-tolyl, 13). Single-crystal X-ray analyses have been carried out on 8 and 13. In both compounds the coordination geometry around the Ru atom approximates an octahedron with three positions taken by a mer PNP ligand and one position taken by a phenylethynyl group. The coordination sphere around the metal center is completed by an η3-1,4-diphenylbut-3-en-1-ynyl ligand in 8 and by a 1-p-tolyl-3-(phenylethynyl)-4-phenylbuta-1(E),3(Z)-dien-4-yl ligand in 13. The latter ligand essentially uses the C1 carbon atom to bind the metal, although a weak bonding interaction may be envisaged also with the alkynyl substituent in the 3-position. A single-crystal X-ray analysis has also been carried out on the octahedral fac,cis-(PNP)RuCl2(CO) complex obtained by treatment of the vinylidenes 2 and 3 with molecular oxygen. The influence of the bonding mode of the PNP ligand on the different reactivity shown by the fac and mer vinylidene complexes toward LiC⋮CPh is discussed in light of a multiform experimental study (X-ray structure determinations, multinuclear NMR spectroscopy, deuterium- and p-tolyl-labeling experiments, independent reactions with isolated compounds).