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 [Ir2(CO)3(dppm)2] (dppm = Ph2PCH2PPh2) with dimethyl acetylenedicarboxylate (DMAD) first yields [Ir2(CO)2(μ-η1:η1-DMAD)(dppm)2] (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, [Ir2(CO)2(μ-η2:η2-DMAD)(dppm)2] (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 [Ir2(CO)2(μ-η2:η2-HFB)(dppm)2] (5) was isolated. Compound 2 reacts readily with PMe3 to yield [Ir2(CO)(PMe3)(μ,-CO)(μ-η1:η1-DMAD)(dppm)2], and with CH3OSO2CF3 to yield [Ir2(CH3)(CO)2(μ-η1:η1-DMAD)(dppm)2][SO3CF3], whereas 3 reacts with neither reagent. Both 2 and 3 react with HBF4•OEt2 to yield the respective alkyne-bridged hydrides, [Ir2H(CO)2(μ-η1:η1-DMAD)(dppm)2][BF4] and [Ir2H(CO)2(μ-η2:η2-DMAD)(dppm)2][BF4], in which the gross structural features and the alkyne coordination mode of the precursor are retained in each case. The latter species rearranges readily at ambient temperature, via migratory insertion, to give the vinyl-bridged product, [Ir2(CO)2(μ-η1:η2-RC=C(H)R)(dppm)2][BF4] (R = CO2Me); however, the former is inert under these conditions, yielding the above vinyl species together with other decomposition products only upon reflux in benzene for several hours. Protonation of the perpendicular hexafluoro-2-butyne adduct also yields the corresponding vinyl product, together with decomposition products. The structure of 3, as the methylene chloride disolvate, was established by X-ray analysis. Crystal data are as follows. 3•2CH2Cl2: C60H54Cl4O6P4Ir2, monoclinic, P2/c, a = 26.088(5) Å, b = 9.896(4) Å, c = 23.954(3) Å, β = 109.27(1)°, Z = 4, R(F) = 0.038, Rw(F2) = 0.0997 (all data). Key words: diiridium alkyne complexes, parallel and perpendicular alkyne coordination.