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

[ReOMe2(bipy)(CH3CN)][PF6], prepared in situ, reacted with alkynes (ethyne, propyne, 2-butyne, 3-hexyne, phenylacetylene, and diphenylacetylene) to gi ve the cationic alkyne complexes trans-[ReOMe2(bipy)(alkyne)][PF6] (trans-M e-Re-Me). H-1-NMR studies indicated that a cis isomer was formed initially. Kinetics studies showed that the isomerizations were first order in the al kyne complex. The observed rate constants depended on the steric bulk of th e alkyne with bulkier alkynes producing smaller k(obs) values. An Eyring pl ot for the isomerization of [ReOMe2(bipy)(2-butyne)][PF6] yielded Delta H-d ouble dagger = 21(1) kcal mol(-1) and Delta S-double dagger = 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 o f [ReOMe2(bipy)(CH3CN)][PF6] with dimethyl acetylenedicarboxylate afforded the metallacycle [Re{C[C(O)OMe]C(Me)C(O)(OMe)}(O)Me(bipy)][PF6] via inserti on of the alkyne into a Re-CH3 bond. Trans-[ReOMe2(RCCH)(bipy)][PF6] reacte d with PMe3 or PPh3 to form the ylide complexes cis-[ReOMe2(bipy) {C(R)CH(P R3')}][PF6] (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 Spectrosc opic and X-ray crystallographic studies suggest that the ylide complexes ca n be described as organometallic analogs of resonance-stabilized phosphoniu m ylides. The structures of trans-[ReOMe2(bipy)(PhCCPh)][PF6], [Re{C[C(O)OM e]C(Me)C(O)(OMe)}(O)Me(bipy)][PF6], cis-[ReOMe2(bipy){C(H)CH(PPh3)}][PF6] a nd cis-[ReOMe2(bipy){C(Ph)CH(PMe3)}][PF6] were determined by X-ray crystall ography. (C) 2000 Elsevier Science S.A. All rights reserved.