Cobalt Phosphino-α-Iminopyridine-Catalyzed Hydrofunctionalization of Alkenes: Catalyst Development and Mechanistic Analysis

A family of CoCl₂(PNpy) complexes were prepared, where PNpy = 2-iminopyridyl-phosphine ligands derived from aminoalkyl and aminoaryl phosphines and 2-keto- and 2-formylpyridines. Reduction of CoCl₂(PNpy) complexes in the presence of PPh₃ gave CoH(PNpy)(PPh₃) and CoMe(PNpy)(PPh₃), which were active for hydrofunctionalization of alkenes. According to DFT calculations, the CoMe(PNpy)(PPh₃) complexes are best described as Co(II) derivatives of the anion [PNpy]^-, with a labile PPh₃ coligand. Metalation of Na[^Ph2P^C2N^Hpy] gave the dimers [CoCl(^Ph2P^C2N^Hpy)]₂. Monomeric complexes catalyze hydrosilylation of 1-octene with Ph₂SiH₂, with the CoCl₂(^iPr2P^C3N^Hpy)/2NaBEt₃H system exhibiting the highest rate and selectivity for anti-Markovnikov product. In situ NMR studies established the following: (i) silanes protonolyze catalyst precursors to give the Co-silyl complexes Co(SiR₃)(^Ph2P^C6H4N^Phpy)(PPh₃), (ii) alkenes compete with PPh₃ to give Co(SiHPh₂)(^Ph2P^C6H4N^Phpy)(η²-alkene), (iii) ethylene inserts into the Co-Si bond to give Co(CH₂CH₂SiR₃)(^Ph2P^C6H4N^Phpy)(PPh₃).