REACTIONS OF THE TRIGONAL-BIPYRAMIDAL COBALT(I) HYDRIDE [(P(CH2CH2PPH2)3)COH] WITH 1-ALKYNES - SYNTHESIS AND REACTIVITY OF ACETYLIDE, ALKENYL, AND VINYLIDENE COMPLEXES

The reactions of HC==CR (R = CO2Et, Ph, n-C5H11) with the Co(I) hydride [(PP3)CoH] are influenced by a number of factors, including stoichiometry, alkyne substituent, and temperature (PP3 ≡ P-(CH2CH2PPh2)3). The only 1-alkyne that reacts with 1 at room temperature is ethyl propiolate. The stoichiometric reaction yields a mixture of three components, namely the starting hydride, the σ-acetylide [(PP3)Co(C=CCO2Et)j (3), and the σ-alkenyl [(PP3)Co{gem-C(CO2Et)=CH2}] (4). Ethyl acrylate forms in an amount corresponding to that of the σ-acetylide. When a 10-fold excess of HC≡CCO2Et is used, 1 quantitatively converts into 3 while 1 equiv of alkyne is hydrogenated to alkene. No reaction is observed between 1 and excesses of HC≡CPh or HC≡CC5H11 in both room-temperature and refluxing THF solutions. In contrast, a reaction occurs with phenylacetylene in toluene at reflux temperature to give the σ-acetylide [(PP3)Co(C≡CPh)] (5). The reactions between 1 and HC≡CR (R = CO2Et, Ph) in a 1:100 ratio in toluene at reflux temperature consume all of the organic reagent, which is converted into cyclic and linear oligomers. The termination products of the catalysis cycles are the Co(I) σ-acetylides 3 and 5, which, in turn, catalyze themselves the oligomerization of 1-alkynes. The redox behavior of the Co(I) σ-acetyíides 3 and 5 have been investigated by electrochemical methods. Upon one-electron oxidation, 3 and 5 are transformed into the corresponding Co derivatives. The phenylacetylide 5 is protonated by strong protic acids to give [(PP3)Co(C-C(H)Phj]BPh4 (8), which represents the first cobalt vinylidene complex. The chemistry and redox properties of the latter compound have been investigated, particularly the one-electron reduction, some ligand-substitution reactions, and the reactions with nucleophiles. The carbethoxyacetylide 3 reacts with protic acids to give the vinylphosphonium complex [|(Ph2PCH2CH2)2P(CH2CH2PPh2))}Co{C=C-(H)C(O)OEt)]BPh4 (9). Unlike the vinylidene complex 8, 9 is not deprotonated by bases but adds H- from NaBH4 to give the alkenyl complex [(PP3)Co{(E)-CH=C(H)CO2Et}] (11). Compound 11 reacts with H+ under nitrogen, liberating ethyl acrylate and forming the dinitrogen derivative [(PP3)Co(N2)]BPh4. © 1990, American Chemical Society. All rights reserved.