Synthesis, X-ray crystallographic, and reactivity studies of rhenium(V) alkyne complexes

Abstract

[ReOMe 2(bipy)(CH 3CN)][PF 6], prepared in situ, reacted with alkynes (ethyne, propyne, 2-butyne, 3-hexyne, phenylacetylene, and diphenylacetylene) to give the cationic alkyne complexes trans-[ReOMe 2(bipy)(alkyne)][PF 6] ( trans-MeReMe). 1H-NMR studies indicated that a cis isomer was formed initially. Kinetics studies showed that the isomerizations were first order in the alkyne complex. The observed rate constants depended on the steric bulk of the alkyne with bulkier alkynes producing smaller k obs values. An Eyring plot for the isomerization of [ReOMe 2(bipy)(2-butyne)][PF 6] yielded Δ H ‡=21(1) kcal mol −1 and Δ S ‡=6(3) eu. The isomerization mechanism was proposed to involve the rearrangement of a five-coordinate intermediate formed by dissociation of a ReN bond. Treatment of [ReOMe 2(bipy)(CH 3CN)][PF 6] with dimethyl acetylenedicarboxylate afforded the metallacycle [ Re{C[C(O)OMe]C(Me)C(O )(OMe)}(O)Me(bipy)][PF 6] via insertion of the alkyne into a ReCH 3 bond. Trans-[ReOMe 2(RCCH)(bipy)][PF 6] reacted with PMe 3 or PPh 3 to form the ylide complexes cis-[ReOMe 2(bipy){C(R)CH(PR′ 3)}][PF 6] (R=H, R′=Me or Ph; R=Ph, R′=Me). In each case, a trans isomer ( trans-MeReMe) of the ylide complex was formed initially, which isomerized to the cis isomer. Spectroscopic and X-ray crystallographic studies suggest that the ylide complexes can be described as organometallic analogs of resonance-stabilized phosphonium ylides. The structures of trans-[ReOMe 2(bipy)(PhCCPh)][PF 6], [ Re{C[C(O)OMe]C(Me)C(O )(OMe)}(O)Me(bipy)][PF 6], cis-[ReOMe 2(bipy){C(H)CH(PPh 3)}][PF 6] and cis-[ReOMe 2(bipy){C(Ph)CH(PMe 3)}][PF 6] were determined by X-ray crystallography.