title: Evidence for the existence of terminal scandium imidos: mechanistic studies involving imido-scandium bond formation and C-H activation reactions.
abstract: The anilide-methyl complex (PNP)Sc(NH[DIPP])(CH(3)) (1) [PNP(-) = bis(2-diisopropylphosphino-4-tolyl)amide, DIPP = 2,6-diisopropylphenyl] eliminates methane (k(avg) = 5.13 × 10(-4) M(-1) s(-1) at 50 °C) in the presence of pyridine to generate the transient scandium imido (PNP)Sc═N[DIPP](NC(5)H(5)) (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])(η(2)-NC(5)H(4)) (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-d(1) 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-d(3) 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-d(5). Complex 2 equilibrated back to the imide A-py slowly, as the isotopomer (PNP)Sc(ND[DIPP])(η(2)-NC(5)H(4)) (2-d(1)) converted to (PNP)Sc(NH[DIPP])(η(2)-NC(5)H(3)D) 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 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.
authors.affiliation: Department of Chemistry, Indiana University, Bloomington, Indiana 47405, United States.
authors.forename: Benjamin F
authors.initials: BF
authors.lastname: Wicker
authors.forename: Hongjun
authors.initials: H
authors.lastname: Fan
authors.forename: Anne K
authors.initials: AK
authors.lastname: Hickey
authors.forename: Marco G
authors.initials: MG
authors.lastname: Crestani
authors.forename: Jennifer
authors.initials: J
authors.lastname: Scott
authors.forename: Maren
authors.initials: M
authors.lastname: Pink
authors.forename: Daniel J
authors.initials: DJ
authors.lastname: Mindiola
mesh_terms: D007094:Imides / Q000737:chemistry*; D007700:Kinetics; D008968:Molecular Conformation; D009942:Organometallic Compounds / Q000138:chemical synthesis / Q000737:chemistry*; D011725:Pyridines / Q000737:chemistry; D012538:Scandium / Q000737:chemistry*
chemical_list: D007094:Imides; D009942:Organometallic Compounds; D011725:Pyridines; D012538:Scandium