DIMERIZATION OF TRIMETHYLSILYLPROPYNE ON A TRIOSMIUM CLUSTER - FORMATION, CRYSTAL-STRUCTURE AND REARRANGEMENT OF OS3H[MU(3)-C(SIME(3))C(ME)CC(SIME(3))CH2](CO)(8) COMPLEX

Trimethylsilylpropyne reacts with Os-3(CO)(10)(C8H14), (C8H14 = cycloo ctene) to give Os-3(mu(3)-Me(3)SiC(2)Me)(mu-CO)(CO)(9) (3). On heating this complex in hydrocarbon solvents, the allenyl compound Os3H[mu(3) -C(SiMe(3))CCH2](CO)(9) (4) is formed. Reaction of the excess Me(3)SiC drop CMe with the alkyne compound 3 in refluxing hexane first gives t he red compound Os-3[C-4(SiMe(3))(Me)(SiMe(3))-Me](CO)(9) (5), which i s then smoothly converted to the yellow compound Os3H[mu(3)-C(SiMe(3)) C(Me)CC(SiMe(3))CH2](CO)(8) (6). The reaction of excess Me(3)SiC drop CMe with the vinyl compound Os3H(mu-CH=CH2)(CO)(10) (7), proceeds anal ogously giving 5 and then 6. Compounds 5 and 6 were characterized by I R and H-1 NMR spectroscopy, and 6 also by X-ray diffraction. Cluster 6 crystallizes in the space group P2(1)/n, with a = 9.196(2), 6 = 29.49 5(2), c = 10.233(2) Angstrom, beta = 99.48(2)degrees, Z = 4. The molec ular structure involves a triangular array of osmium atoms and sigma, pi-coordinated organic ligand, formed by linear coupling of two alkyne molecules. This coupling is accompanied by the dehydrogenation of a m ethyl group and 1,2 shift of a trimethylsilyl group of one of the alky ne molecules. When the reaction of Me(3)SiC drop CMe with 3 was conduc ted in refluxing heptane, as well as compound 6, the formation of comp ound Os3H[mu(3)-CHC(SiMe(3))CC(Me)CH(SiMe(3))](CO)(8) (8) was indicate d. In a separate experiment, it was established that, in refluxing hep tane, 6 --> 8 rearrangement takes place. Allenyl compound 4, in contra st to alkyne compound 3, does not react with Me(3)SiC drop CMe in refl uxing hexane, and only under more severe conditions in refluxing hepta ne, does the formation of 6 and 8 take place in very low yields. These results show that compounds 3 and 5 but not 4 are the true intermedia tes in the reaction of trimethylsilylpropyne dimerization on the trios mium duster.