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 (k(avg) = 5.13 x 10(-4) M(-1)s(-1) at 50 degrees 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]) (eta(2)-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 alpha-hydrogen abstraction is overall-rate-determining [the kinetic isotope effect (KIE) for 1-d(1) conversion to 2 was 5.37(6) at 35 degrees C and 4.9(14) at 50 degrees C] with activation parameters Delta H-double dagger = 17.9(9) kcal/mol and Delta S-double dagger = -18(3) cal/(mol K), consistent with an associative-type mechanism. No KIE or exchange with the anilide proton was observed when 1-d(3) was treated with pyridine or thermolyzed at 35 or 50 degrees C. The post-rate-determining step, C-H bond activation of pyridine, revealed a primary KIE of 1.1(2) at 35 degrees C for the intermolecular C-H activation reaction in pyridine versus pyridine-d(5). Complex 2 equilibrated back to the imide A-py slowly, as the isotopomer (PNP)Sc(ND[DIPP])(eta(2)-NC5H4) (2-d(1)) converted to (PNP)Sc(NH[DIPP])(eta(2)-NC5H3D) over 9 days at 60 degrees 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 angstrom. Substituted pyridines can also be activated, with the rates of C-H activation depending on both the steric and electronic properties of the substrate.