Selective head-to-tail dimerization of phenylacetylene catalyzed by a diruthenium μ-methylene complex

The μ-methylene complex [Ru2(μ-CH2)(CO)4(μ-dppm)2] (1) in toluene solution at 75 °C is the first known catalyst for the selective dimerization of PhCCH or of PhCCD to give the head-to-tail dimer PhCCC(Ph)CH2 or PhCCC(Ph)CD2 respectively, probably by a mechanism involving alkenyl–alkynyl group coupling at the diruthenium center. After many turnovers, a new diruthenium complex was detected and identified as [Ru2{μ-η2-PhCCH}(CO)4(μ-dppm)2] (2) which was not an active catalyst for the dimerization reaction, and the organic product PhCCMe was also detected at this stage. Complex 2 was prepared more readily by reaction of PhCCH with [Ru2(μ-CO)(CO)4(μ-dppm)2] (3) in toluene at 75 °C. The reaction of 1 with a large excess of PhCCH gave further oligomerization and the major products were identified as an octamer and hexamer of PhCCH. Complex 1 was a catalyst for the dimerization of 1-hexyne to a mixture of dimerization products E-BuCCCHCHBu and BuCCC(Bu)CH2 in a ratio of about 2:1, and also for polymerization of HCCH, but it failed to react with the alkynes PhCCMe, PhCCPh or 3-hexyne at 75 °C in toluene.