The synthesis and characterization of the cationic cobalt(I) arene complex, [(dppf)Co(ν6-C7H8)][BArF4] (dppf = 1,1′-bis(diphenylphosphino)ferrocene; BArF4 = B[(3,5-(CF3)2)C6H3]4) from an air-stable cobalt precursor is described. Dissolution in benzene-d6 or tetrahydrofuran (THF) resulted in rapid arene substitution and generated [(dppf)Co(ν6-C6H6)][BArF4] or [(dppf)Co(THF)2][BArF4]. The latter compound was characterized by a combination of X-ray diffraction and magnetometry and established an S = 1 cobalt(I) derivative. The isolated bis(phosphine)cobalt complexes were evaluated as precatalysts for carbon-carbon bond-forming reactions. The [2 + 2] cycloaddition of internal alkynes and olefins was observed with cobalt precatalyst loadings of 0.25 mol % with high chemoselectivity. The catalytic method was compatible with Lewis basic functional groups, an advantage over in situ-generated catalysts that rely on excess trialkyl aluminum activators. The cationic bis(phosphine)cobalt arene complex was also an effective catalyst precursor for the hydrovinylation of isoprene with ethylene. In both C-C bond-forming reactions, the corresponding cobalt(0) complex, [(dppf)Co(COD)] (COD = 1,5-cyclooctadiene), was inactive, providing strong evidence of the role of cobalt(I) during catalysis. In both catalytic reactions, deuterium crossover experiments provide experimental evidence of the role of metallacyclic intermediates during turnover.
All Science Journal Classification (ASJC) codes
- Physical and Theoretical Chemistry
- Organic Chemistry
- Inorganic Chemistry