Molybdenum(III) chloride tetrahydrothiophene (tht) complexes in the catalytic polymerization and cyclotrimerization of alkynes: Structures and reactivities of the possible intermediates [MoCl3(tht)(2)(PhC CR)] (R=Me or Et)

The binuclear complexes [Mo2Cl6(tht)(3)] (both the C-2. and C-s isomers, tht = tetrahydrothiophene) have been found to be active catalysts for the selective polymerization and cyclotrimerization of a variety of alkynes. The mononuclear complex [MoCl3(tht)(3)] shows similar behaviour, leading to the postulate that the active species in all cases is mononuclear. Two unique molybdenum(III) alkyne complexes, [MoCl3(tht)(2)L] (L = PhC=CMe or PhC=CEt) have been isolated and structurally characterized. The structural parameters for these complexes suggest that the alkynes behave as four-electron donors. These complexes are also catalytically active, and the alkyne L is incorporated into the product cyclotrimers and polymers suggesting they are the first intermediates in the formation of active catalysts from the original thioether complexes.