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

• Published in: Journal of the American Chemical Society 2012-12, Vol.134 (49), p.20081-20096
• Main Authors: Wicker, Benjamin F, Fan, Hongjun, Hickey, Anne K, Crestani, Marco G, Scott, Jennifer, Pink, Maren, Mindiola, Daniel J
• Format: Article
• Language: English
• Subjects: Imides - chemistry; Kinetics; Molecular Conformation; Organometallic Compounds - chemical synthesis; Organometallic Compounds - chemistry; Pyridines - chemistry; Scandium - chemistry
• ISSN: 0002-7863; 1520-5126
• DOI: 10.1021/ja307279r

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 × 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-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])­(η2-NC5H4) (2-d 1) converted to (PNP)­Sc­(NH­[DIPP])­(η2-NC5H3D) 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.