Catalytic alkyne metathesis and stoichiometric metal–alkylidyne formation from NMo(OR)3 complexes promoted by Lewis acids

Nitride complex NMo(OCMe2CF3)3 is synthesized in 78% yield on a multigram scale. Although the irreversible but sluggish conversion of terminal molybdenum–nitride complexes NMo(OCMe(CF3)2)3 (1) and NMo(OC(CF3)3)3(NCMe) (2) to their propylidyne analogs EtCMo(OR)3(DME) (OCMe(CF3)2 (3), OC(CF3)3 (4)) via metathesis with 3-hexyne occurs, the analogous reaction with NMo(OR)3 (OROCMe2CF3 (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-MeOC6H4CMo(OC(CF3)3)3(MeOC6H4CN) (22%), 4-Ph–C6H4CMo(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.

Molybdenum–nitride complexes NMo(OC(CF3)nMe3−n)3 (n = 0–3) are pre-catalysts for metathesis of simple alkynes. Simple Lewis acids can accelerate the rate of alkyne metathesis dramatically, depending on the identities of the solvent, Lewis acid, and alkoxide ligands. In some cases (n = 2, 3) metal–alkylidyne complexes can be isolated. Figure optionsDownload as PowerPoint slideHighlights
► Molybdenum–nitride complexes react with alkynes to form alkylidyne analogs.
► Molybdenum–nitride complexes are pre-catalysts for alkyne metathesis.
► Alkyne metathesis catalyzed by nitride complexes is accelerated by Lewis acids.