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 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-d1) 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.苯胺基-甲基配合物(PNP)Sc(NH[DIPP])(CH₃)(1)，其中PNP⁻为双(2-二异丙基膦基-4-甲苯基)胺根配体(bis(2-diisopropylphosphino-4-tolyl)amide)，DIPP为2,6-二异丙基苯基(2,6-diisopropylphenyl)。该配合物在吡啶存在下可消除甲烷（50℃时平均速率常数k_avg=5.13×10⁻⁴ M⁻¹·s⁻¹），生成瞬态钪亚胺基配合物(PNP)Sc=N[DIPP](NC₅H₅)(A-py)。该瞬态物种以1,2-加成方式快速活化吡啶的C-H键，得到稳定的吡啶基配合物(PNP)Sc(NH[DIPP])(η²-NC₅H₄)(2)。机理研究表明，该C-H活化过程整体为二级反应：对钪为一级，对底物吡啶亦为一级。吡啶配位先于甲烷消除步骤，而α-氢夺取为整体决速步；1-d₁转化为2的动力学同位素效应(Kinetic Isotope Effect, KIE)在35℃时为5.37(6)，50℃时为4.9(14)，对应的活化参数ΔH‡=17.9(9) kcal/mol，ΔS‡=-18(3) cal/(mol·K)，这与缔合型反应机理相符。当1-d₃与吡啶反应或在35℃、50℃下热解时，未观测到KIE现象或苯胺基质子的交换反应。决速步之后的步骤为吡啶的C-H键活化：在35℃下以吡啶与吡啶-d₅为底物进行分子间C-H活化反应时，测得其一级动力学同位素效应为1.1(2)。配合物2可缓慢逆向平衡为亚胺基配合物A-py：同位素异构体(PNP)Sc(ND[DIPP])(η²-NC₅H₄)(2-d₁)在60℃下经过9天可转化为(PNP)Sc(NH[DIPP])(η²-NC₅H₃D)。对A-py的分子轨道分析表明，该物种的钪亚胺基结构具有近乎线性的键角（169.7°），Sc-N键长极短，仅为1.84 Å。取代吡啶同样可被活化，其C-H活化速率取决于底物的空间位阻与电子性质。