No abstract

Treatment of fluorenone, 2,3-diphenylindenone, tetraphenylcyclopentadienone, or 2,5-diethyl-3,4-diphenylcyclopentadienone with ((trimethylsilyl)ethynyl)lithium gives, after hydrolysis, the analogous alkynol; subsequent addition of dicobalt carbonyl and then fluoroboric acid yields the corresponding fluorenyl, indenyl, or cyclopentadienyl cation stabilized by complexation to a tricarbonylcobalt moiety. Variable-temperature NMR data on these cluster cations, and on their bis(diphenylphosphino)methane derivatives, reveal that the barrier to migration of the cationic center between cobalt cluster vertices increases in the order fluorenyl < indenyl < cyclopentadienyl and suggest that the cations with more antiaromatic character have the greatest need for charge delocalization onto the metal center. Replacement of a Co(CO)3 cationic fragment by an Fe(CO)3 unit yields the mixed-metal species [((fluorenyl)CCSiMe3)FeCo(CO)6] (22) and [((2,3-diphenylindenyl)CCSiMe3)FeCo(CO)6] (27). In these structural models for the cationic complexes, the Fe−C(9) distance in 22 is 2.626(11) Å, while in the indenyl system 27 the Fe−C(1) distance is 2.347(7) Å, again indicating that the 8π indenyl cation interacts more strongly with the metal center than does the 12π fluorenyl cation.

Direct irradiation of ((trimethylsilyl)ethynyl)pentamethyldisilane in hydrocarbon solution affords a mixture of reactive intermediates which have been detected and identified using laser flash photolysis techniques and trapped as the methanol adducts in steady state irradiation experiments. Flash photolysis of air-saturated hexane solutions of the disilane allows detection of transient species assigned to 1,1-dimethyl-3,3-bis(trimethylsilyl)-1-silaallene and dimethylsilylene, along with a non-decaying species assigned to 1,1-dimethyl-2,3-bis(trimethylsilyl)-1-silacyclopropene. These were identified on the basis of their UV absorption spectra and reactivity toward various reagents. The 1-silaallene is a minor photoproduct, but it is readily observable in transient absorption experiments because it is relatively long-lived (τ ) 2.4 µs in air-saturated solution) and absorbs strongly, with absorption maxima considerably to the red of those of the 1-silacyclopropene. The 1-silaallene exhibits characteristic silene reactivity; it reacts with methanol, acetic acid, acetone (k ) 10 6 -10 8 M -1 s -1 ) and oxygen (k ∼ 10 8 M -1 s -1 ), exhibits low reactivity toward aliphatic dienes and tert-butyl alcohol, and decays with second-order kinetics in the absence of quenchers. With methanol and acetic acid, it has been shown that reaction in hexane occurs with the monomeric ROH species only. Steady state irradiations in the presence of methanol afford methanol-addition products consistent with the formation of the silaallene, silacyclopropene, and silylene, along with bis(trimethylsilyl)acetylene as the major product.

A series of compounds containing alkynylsilane and arylsilane groups, including oligomers and polymers, were used as transition-metal ligands. The alkyne groups could be complexed by dicobalt hexacarbonyl. Alternatively, complexation of the aryl groups by chromium tricarbonyl readily took place. The molecular weights of these complexes ranged up to 155 000 for a silicone polymer derived complex. Mixed-metal clusters containing Cr/Mo and Cr/Co could be made by modifying the chromium derivatives with Mo2Cp2(CO)6 and Co2(CO)8, respectively. The model compound 20 shows an interesting self-assembly of the metals (Co and Cr, respectively) in the crystal.

Laser flash photolysis of nitrogen-saturated hexane solutions of pentamethyl(phenylethynyl)disilane affords a mixture of three transient species and a product which is stable on the millisecond time scale. These are assigned to the triplet state of the disilane (τ ∼ 5 μs), 1,1-dimethyl-3-phenyl-3-(trimethylsilyl)-1-silaallene (τ ∼ 25 μs), dimethylsilylene (τ ∼ 100 ns), and 1,1-dimethyl-2-phenyl-3-(trimethylsilyl)-1-silacyclopropene (τ > 100 ms), respectively, on the basis of their UV spectra and reactivity toward various reagents. While previous work has shown that the 1-silaallene is formed in less than 25% yield, it can be readily observed in transient absorption experiments in spite of extensive spectral overlap with the alkynyldisilane triplet and the 1-silacyclopropene. Absolute rate constants for reaction of the silaallene with methanol, acetone, acetic acid, and oxygen in hexane solution are reported and compared to data for a related compound.