Catalytic alkyne metathesis and stoichiometric metal–alkylidyne formation from Ntriple bond; length of mdashMo(OR)3 complexes promoted by Lewis acids

Nitride complex Ntriple bond; length of mdashMo(OCMe2CF3)3 is synthesized in 78% yield on a multigram scale. Although the irreversible but sluggish conversion of terminal molybdenum–nitride complexes Ntriple bond; length of mdashMo(OCMe(CF3)2)3 (1) and Ntriple bond; length of mdashMo(OC(CF3)3)3(NCMe) (2) to their propylidyne analogs EtCtriple bond; length of mdashMo(OR)3(DME) (OCMe(CF3)2 (3), OC(CF3)3 (4)) via metathesis with 3-hexyne occurs, the analogous reaction with Ntriple bond; length of mdashMo(OR)3 (ORdouble bond; length as m-dashOCMe2CF3 (5), OCMe3 (6)) complexes does not occur under similar reaction conditions. However, the kinetic barrier to alkylidyne formation from 5 and 6 with internal alkynes can be overcome through the addition of simple Lewis acids, including MgBr2, MgI2 and BPh3 in specific instances. Although this typically leads to accelerated decomposition of the alkylidyne complex so formed, the combination of metal–nitride complex plus exogenous Lewis acid frequently leads to alkyne metathesis of the test substrates 1-phenyl-1-propyne and 1-phenyl-1-butyne under milder conditions than possible in the absence of Lewis acid, in some cases at room temperature. The interaction of solvent, ancillary alkoxide ligands, and Lewis acid is complex and was not predicted a priori. New benzylidyne complexes 4-MeOC6H4Ctriple bond; length of mdashMo(OC(CF3)3)3(MeOC6H4CN) (22%), 4-Ph–C6H4Ctriple bond; length of mdashMo(OC(CF3)3)3(4-PhC6H4CN) (46%) were isolated in low yield via the nitride-to-alkylidyne route upon reaction with suitable diarylalkynes. Several related alkylidyne complexes were formed but could not be separated cleanly from the alkyne reagents used.