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

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═N[DIPP](NC5H5) (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 ...