1407965-79-0Relevant articles and documents
Evidence for the existence of terminal scandium imidos: Mechanistic studies involving imido-scandium bond formation and c-h activation reactions
Wicker, Benjamin F.,Fan, Hongjun,Hickey, Anne K.,Crestani, Marco G.,Scott, Jennifer,Pink, Maren,Mindiola, Daniel J.
, p. 20081 - 20096 (2012)
The anilide-methyl complex (PNP)Sc(NH[DIPP])(CH3) (1) [PNP - = bis(2-diisopropylphosphino-4-tolyl)amide, DIPP = 2,6-diisopropylphenyl] eliminates methane (kavg = 5.13 × 10-4 M-1s-1 at 50 °C) in the presence of pyridine to generate the transient scandium imido (PNP)Sc=[DIPP](NC 5H5) (A-py), which rapidly activates the C-H bond of pyridine in 1,2-addition fashion to form the stable pyridyl complex (PNP)Sc(NH[DIPP])(η2-NC5H4) (2). Mechanistic studies suggest the C-H activation process to be second order overall: first order in scandium and first order in substrate (pyridine). Pyridine binding precedes elimination of methane, and α-hydrogen abstraction is overall-rate-determining [the kinetic isotope effect (KIE) for 1-d1 conversion to 2 was 5.37(6) at 35 C and 4.9(14) at 50 C] 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 observed when 1-d3 was treated with pyridine or thermolyzed at 35 or 50 °C. The post-rate-determining step, C-H bond activation of pyridine, revealed a primary KIE of 1.1(2) at 35 °C for the intermolecular C-H activation reaction in pyridine versus pyridine-d5. Complex 2 equilibrated back to the imide A-py slowly, as the isotopomer (PNP)Sc(ND[DIPP])(η2-NC5H4) (2-d 1) converted to (PNP)Sc(NH[DIPP])(η2-NC 5H3D) over 9 days at 60 °C. Molecular orbital analysis of A-py suggested that this species possesses a fairly linear scandium imido motif (169.7 ) with a very short Sc-N distance of 1.84 A?. Substituted pyridines can also be activated, with the rates of C-H activation depending on both the steric and electronic properties of the substrate.
