Unprecedented Addition of Tetrahydroborate to an Osmium–Carbon Triple Bond

The complexes [OsHCl(≡CPh)(IPr)(PR3)]OTf (IPr = 1,3-bis(2,6-diisopropylphenyl)imidazolylidene; OTf = CF3SO 3; PR3 = PiPr3 (1), PPh3 (2)) react with NaBH4 in tetrahydrofuran to give OsH 2Cl(η2-H-BCH2Ph)(IPr)(PR3) (PR3 = PiPr3 (3), PPh3 (4)). The mechanism of addition has been studied by means of isotopic labeling experiments and DFT calculations. The results indicate that the boron ligand is generated by hydroboration of the Os≡C triple bond of 1 and 2 and subsequent B-H bond activation of the resulting alkylborane. The original hydride ligand does not participate in the hydroboration process. However, it plays a secondary role in the B-H bond activation. © 2014 American Chemical Society.
Financial support from the Spanish MINECO (Projects CTQ2011-23459, CTQ2010-20714-C02-01/BQU) and Consolider Ingenio 2010 (CSD2007-00006), the DGA (E35), and the European Social Fund (FSE) is acknowledged. J.J.F.C. acknowledges support via a predoctoral fellowship from the DGA.