Competitive Carbon-Sulfur vs Carbon-Carbon Bond Activation of 2-Cyanothiophene with [Ni(dippe)H]2

The processes of C C and C S bond cleavage have been studied with the homogeneous organometallic compound [Ni(dippe)H](2) (1). When 1 is reacted with 2-cyanothiophene at room temperature, cleavage of the nitrile-substituted C S bond occurs, forming the Ni-metallacycle complex (dippe)Ni(kappa(2)-S,C-SCH=CHCH=C(CN)) (2a), which has been fully characterized by NMR spectroscopy and X-ray diffraction. 2a was converted to the C CN cleavage product (dippe)Ni(CN)(2-thiophenyl) (3) when heated in solution. On closer inspection, four other intermediates were observed by P-31 NMR spectroscopy at low temperature. Structures for the intermediates were elucidated through a combination of independent synthesis, theoretical calculations, chemical characterization, and experimental precedent. A kinetic product (dippe)Ni(kappa(2)-S,C-SC(CN)=CHCH=CH) (2b) was formed from cleavage of the nonsubstituted C S bond, as well as a Ni(0) eta(2)-nitrile intermediate, (dippe)Ni(eta(2)-C,N-2-cyanothiophene) (4), and a dinuclear mixed Ni(0) Ni(II) product (6b). A complete DFT analysis of this system has been carried out to reveal comparative details about the two bond cleavage transition states.