Anne Hickey - Evidence for the Existence of Terminal Scandium Imidos: Mechanistic Studies Involving Imido-Scandium Bond Formation and C-H Activation Reactions.

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  Publication Details (including relevant citation   information): Wicker,Benjamin F., Fan,Hongjun,   Hickey,Anne K., Crestani,Marco G., Scott,Jennifer, Pink,Maren,   Mindiola,Daniel J., J. Am. Chem. Soc.,   2012, 134 (49), pp 20081-20096

  Abstract: The anilide-Me complex   (PNP)Sc(NH[DIPP])Me (1) [PNP- =   bis(2-diisopropylphosphino-4-tolyl)amide, DIPP =   2,6-diisopropylphenyl] eliminates methane (kavg = 5.13 × 10-4   M-1s-1 at 50°) in the presence of pyridine to generate the   transient Sc imido (PNP)Sc:N[DIPP](NC5H5) (A-py), which rapidly   activates the C-H bond of pyridine in 1,2-addn. fashion to form   the stable pyridyl complex (PNP)Sc(NH[DIPP])(η2-NC5H4) (2).   Mechanistic studies suggest the C-H activation process to be 2nd   order overall: 1st order in Sc and 1st order in substrate   (pyridine). Pyridine binding precedes elimination of methane, and   α-H abstraction is overall-rate-detg. [the kinetic isotope effect   (KIE) for 1-d1 conversion to 2 was 5.37(6) at 35° and 4.9(14) at   50°] with activation parameters ΔH⧧ = 17.9(9) kcal/mol and ΔS⧧ =   -18(3) cal/(mol K), consistent with an associative-type   mechanism. No KIE or exchange with the anilide proton was obsd.   when 1-d3 was treated with pyridine or thermolyzed at 35 or 50°.   The post-rate-detg. step, C-H bond activation of pyridine,   revealed a primary KIE of 1.1(2) at 35° for the intermol. C-H   activation reaction in pyridine vs. pyridine-d5. Complex 2   equilibrated back to the imide A-py slowly, as the isotopomer   (PNP)Sc(ND[DIPP])(η2-NC5H4) (2-d1) converted to   (PNP)Sc(NH[DIPP])(η2-NC5H3D) over 9 days at 60°. MO anal. of A-py   suggested that this species possesses a fairly linear Sc imido   motif (169.7°) with a very short Sc-N distance of 1.84 Å.   Substituted pyridines can also be activated, with the rates of   C-H activation depending on both the steric and electronic   properties of the substrate. [on SciFinder(R)]

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