Repository logo

Browsing by Author "George, Darren S. A."

Now showing 1 - 2 of 2
  • Thumbnail Image
    Item
    Sulfur–carbon bond formation and bond cleavage in alkynyl-bridged heterobinuclear complexes of rhodium and iridium
    (2000) George, Darren S. A.; Hilts, Robert; McDonald, Robert; Cowie, Martin
    The phenylacetylide-bridged heterobinuclear complexes [RhIr(CO)2(μ-η1:η2-C2Ph)(dppm)2][X] (X=BF4, SO3CF3; dppm=Ph2PCH2PPh2) (1) react with carbon disulfide to give several products. At temperatures between −60 and −80°C the first product, [RhIr(CO)(η2-CS2)(μ-CO)(μ-η1:η2-C2Ph)(dppm)2][X] (2), is the result of CS2 coordination at Ir. Upon warming, two products are formed as a result of condensation of two CS2 groups. In [RhIr(CO)(μ-η1:η3-CC(Ph)SCSCS2)(μ-CO)(dppm)2][X] (3), the resulting C2S4 fragment has also condensed at the β-carbon of the acetylide group to give a heteroatom-substituted vinylidene group. The other identified product, [RhIr(CO)(C2S4)(μ-C2Ph)(μ-CO)(dppm)2][X] (4), is very similar to 3 apart from the absence of coupling of the C2S4 moiety and the acetylide group. Compound 3 appears to be formed independently of 4, but also slowly transforms into 4 by cleavage of a C-S bond. The reaction of 1 with nBuNCS at −80°C yields [RhIr(CO)(η2-SCNnBu)(μ-CCPh)(μ-CO)(dppm)2][X] (5), analogous to 2, and upon warming this rearranges to the isothiocyanate-bridged product [RhIr(CCPh)(CO)2(μ-SCNnBu)(dppm)2][X] (6). Compound 6 undergoes S-C bond cleavage to yield [RhIr(CCPh)(CO)(CNnBu)(μ-S)(μ-CO)(dppm)2][X] (7), slowly at ambient temperature or within hours under reflux. Although no simple adducts analogous to 5 and 6 were observed with tBuNCS, refluxing 1 in the presence of an excess of this substrate yields [RhIr(CCPh)(CO)(CNtBu)2(μ-S)2(dppm)2][X] (9) as the major product along with smaller amounts of [RhIr(CCPh)(CO)(CNtBu)(μ-S)(μ-CO)(dppm)2][X] (8), analogous to compound 7. Refluxing 1 in the presence of excess nBuNCS also yields some of the bis-n-butylisocyanide product, analogous to 9. The X-ray structures of compounds 3 (SO3CF3 − salt), 7 (BF4 − salt) and 9 (BF4 − salt) are reported.
  • Thumbnail Image
    Item
    An unusual example of allyl-to-alkynyl migration in a phenylacetylide-bridged heterobinuclear complex of rhodium and iridium
    (1999) George, Darren S. A.; Hilts, Robert; McDonald, Robert; Cowie, Martin
    The reactivity of the alkynyl-bridged complex [RhIr(CO)2(μ2-η1:η2-C2Ph)(dppm)2][X] (X = BF4 (1a), SO3CF3 (1b); dppm = Ph2PCH2PPh2) with electrophiles has been demonstrated. Protic acids HX (X = BF4, SO3CF3) first yield the oxidative-addition products [RhIr(X)(CO)2(μ-H)(μ-C2Ph)(dppm)2][X], which under carbon monoxide result in displacement of the weakly coordinating BF4- or SO3CF3- anions and subsequent conversion to the vinylidene-bridged [RhIr(CO)4(μ-CC(H)Ph)(dppm)2][X]2. Reaction of 1 with allyl halides yields the allyl vinylidene-bridged compounds [RhIr(Y)(CO)(μ-CC(Ph)CH2CHCH2)(μ-CO)(dppm)2][X] (Y = Br (5), Cl (6)), by coupling of the alkynyl and allyl groups at the β-position of the alkynyl moiety. NMR studies at low temperatures show coordination of allyl halide at Ir at −80 °C, followed by allyl halide loss and subsequent oxidative addition at −50 °C. The oxidative-addition intermediates, [RhIr(η1-CH2CHCH2)(CO)2(μ-Y)(μ-C2Ph)(dppm)2][X] (Y = Br (9), Cl (10)), rearrange to the allylvinylidene products (5 and 6) at ambient temperature. Although halide removal from compounds 5 and 6, using AgBF4, does not result in destabilization of the allylvinylidene fragment, resulting instead in replacement of halide by fluoborate ion, the reaction of 1 with allyl halide in the presence of a silver salt does not lead to coupling of the allyl and alkynyl moieties, but gives [RhIr(η3-C3H5)(CO)(μ-C2Ph)(μ-CO)(dppm)2][X]2 (13). Addition of halide ion to this η3-allyl complex at ambient temperature again leads to formation of 5 or 6. On the basis of these results a mechanism is proposed for the allyl/alkynyl coupling reaction.