Parallel vs. perpendicular alkyne coordination in binuclear complexes. The first examples of reactivity differences in isomers differing in their alkyne coordination modes

The reaction of (Ir,(CO),(dppm),) (dppm = Ph,PCH,PPh,) with dimethyl acetylenedicarboxylate (DMAD) first yields (I~~(co)~(~,-~': ~~-DMAD)(~~~~)~) (2) in which the alkyne is bound parallel to the metal-metal axis and the diphosphines are bound in a trans arrangement at both metals. This metastable isomer slowly rearranges to the stable form, (I~~(CO),(~,-~~:~~-DMAD)(~~~~)~) (3), in which the alkyne is now bound perpendicular to the metals and the diphosphines are bent back in a cis arrangement at both metals. The analogous species can be prepared by substituting hexafluoro-2-butyne (HFB) for DMAD; however, for the HFB adduct the isomer having the parallel geometry is seen only as a transient species; only (I~~(CO)~(~,-~~: ~~-HFB)(~~~~)~) (5) was isolated. Compound 2 reacts readily with PMe, to yield (Ir,(CO)(PMe,)(p,-CO)(p,- Tl:TL-~MAD)(dppm)2), and with CH,OSO,CF, to yield (Ir2(CH,)(CO)2(p,-T':T'-DMAD)(dppm)2)(S0,C~,), whereas 3 reacts with neither reagent. Both 2 and 3 react with HBF4.0Et2 to yield the respective alkyne-bridged hydrides, (Ir2H(CO)2(p,-Tl:$- DMAD)(dppm),:I(BF,) and (I~~H(CO),(~,-~~:~~-DMAD)(~~~~)~~(BF,), in which the gross structural features and the alkyne RCsumC : La rCaction du (Ir,(CO),(dppm),) (dppm = Ph2PCH2PPh2) avec 1'acCtylknedicarboxylate de dimCthyle (ADDM) donne en premier lieu le (I~,(CO),(~,-~':~~-ADDM)(~~~~),) (2) dans lequel I'alcyne est lit5 parallklement B l'axe metal-mktal et les diphosphines sont liCs selon un arrangement trans par rapport aux deux atomes mktalliques. Cet isomkre mCtastable se transpose lentement pour donner l'isomkre stable (I~,(CO),(~,-~~:~~-ADDM)(~~~~)~) (3), dans lequel l'alcyne se retrouve lik perpendiculairement aux deux atomes mCtalliques et les diphosphines sont ramenCs B un arrangement cis au niveau des deux atomes mCtalliques. On peut prCparer les espkces analogues en rempla~ant I'ADDM par l'hexafluoro-2-butyne (HFB), cependant dans l'adduit HFB, l'isomkre ayant la gComCtrie parallkle semble uniquement une espkce transitoire; on a seulement isole le (I~,(CO),(~,-~~:~~-HFB)(~~~~),) (5). Le composC 2 rCagit facilement avec PMe, pour donner (Ir,(CO)(PMe,)(p,- CO)(~,-~':~'-ADDM)(~~~~),), et avec CH,OSO,CF, pour donner (I~,(CH,)(CO),(~,-~~:~~-ADDM)(~~~~),) (so,cF,), tandis que le composC 3 ne rkagit pas avec aucun de ces rCactifs. Les composks 2 et 3 reagissent avec le HBF,.OEt, pour conduire respectivement aux hydrures avec le groupe alcyne en position de pont : (I~~H(co),(~,-~,~:~'-ADDM)(~~~~),)(BF,) et (I~~H(CO),(~,-~~:~~-ADDM)(~~~~)~:~(BF~), dans lesquels les caractCristiques structurales principales et le mode de coordination de l'alcyne du prkcurseur sont retenus dans chacun des case. La dernikre espkce se transpose facilement B la temperature ambiante, via une insertion migratoire, pour donner le produit avec un groupe vinyle en position de pont (I~~(co)~(~,-~~:~~-RC=C(H)R)(~~~~)~)(BF~)

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