Departamento de Química de Coordinación y Catálisis Homogénea, Instituto de Ciencia de Materiales de Aragón, CSIC-Universidad de Zaragoza, E-50009 Zaragoza, Spain.
J Am Chem Soc. 2010 Jul 7;132(26):9111-21. doi: 10.1021/ja102479h.
The structure, coordination properties, insertion processes, and dynamic behavior in solution of the five-coordinate complexes [IrXH(biPSi)] (biPSi = kappa-P,P,Si-Si(Me){(CH(2))(3)PPh(2)}(2); X = Cl (1), Br (2), or I (3)) have been investigated. The compounds are formed as mixtures of two isomers, anti and syn, in slow equilibrium in solution. The equilibrium position depends on the halogen and the solvent. Both isomers display distorted square-based pyramidal structures in which the vacant position sits trans to silicon. The equatorial plane of the syn isomer is closer to the T structure due to distortions of steric origin. The small structural differences between the isomers trigger remarkable differences in reactivity. The syn isomers form six-coordinate adducts with chlorinated solvents, CO, P(OMe)(3), or NCMe, always after ligand coordination trans to silicon. The anti isomers do not form detectable adducts with chlorinated solvents and coordinate CO or P(OMe)(3) either trans to silicon (kinetic) or trans to hydride (thermodynamic). NCMe coordinates the anti isomers exclusively at the position trans to hydride. Qualitative and quantitative details (equilibrium constants, enthalpies, entropies, etc.) on these coordination processes are given and discussed. As a result of the different coordination properties, insertion reagents such as acetylene, diphenylacetylene, or the alkylidene resulting from the decomposition of ethyl diazoacetate selectively insert into the Ir-H bond of 1-syn, not into that of 1-anti. These reactions give five-coordinate syn alkenyl or alkyl compounds in which the vacancy also sits trans to silicon. Acetylene is polymerized in the coordination sphere of 1. The nonreactive isomer 1-anti also evolves into the syn insertion products via anti<-->syn isomerizations, the rates of which are notably dependent on the nature of the insertion reactants. H(2) renders anti<-->syn isomerization rates of the same order as the NMR time scale. The reactions are second order (k(obs) = k(anti<-->syn)[H(2)]) and do not involve H(2)/IrH hydrogen atom scrambling. A possible isomerization mechanism, supported by MP2 calculations and compatible with the various experimental observations, is described. It involves Ir(V) intermediates and a key sigma Ir-(eta(2)-SiH) agostic transition state. A similar transition state could also explain the anti<-->syn isomerizations in the absence of oxidative addition reactants, although at the expense of high kinetic barriers strongly dependent on the presence of potential ligands and their nature.
已研究了五配位配合物[IrXH(biPSi)]（biPSi=κ-P，P，Si-Si{（CH(2))(3)PPh(2)}(2)；X=Cl(1)，Br(2)，或 I(3)）的结构、配位性质、插入过程和溶液中的动态行为。这些化合物以反式和顺式两种异构体的混合物形式形成，在溶液中缓慢平衡。平衡位置取决于卤素和溶剂。两种异构体都呈现出扭曲的四方锥结构，其中空位置位于硅原子的反式。由于立体位阻的影响，顺式异构体的赤道平面更接近 T 结构。异构体之间的微小结构差异导致反应性的显著差异。顺式异构体与氯化溶剂、CO、P(OMe)(3)或 NCMe 形成六配位加合物，总是在硅原子反式配位后形成。反式异构体与氯化溶剂不形成可检测到的加合物，并且 CO 或 P(OMe)(3)的配位要么是硅原子反式（动力学），要么是氢化物反式（热力学）。NCMe 仅在氢化物反式位置配位反式异构体。给出并讨论了这些配位过程的定性和定量细节（平衡常数、焓、熵等）。由于配位性质的不同，插入试剂如乙炔、二苯乙炔或乙基重氮乙酸酯分解产生的亚烷基选择性地插入到 1-顺式的 Ir-H 键中，而不是插入到 1-反式的 Ir-H 键中。这些反应在空位也位于硅原子反式的情况下，生成五配位顺式烯基或烷基化合物。乙炔在 1 的配位球中聚合。非反应性异构体 1-反式也通过反式-顺式异构体转化为顺式插入产物，其速率明显取决于插入反应物的性质。H(2)使反式-顺式异构体转化速率与 NMR 时间尺度相同。反应为二级（k(obs)=k(anti<-->syn)[H(2])），不涉及 H(2)/IrH 氢原子重排。描述了一种可能的异构化机制，该机制得到了 MP2 计算的支持，并与各种实验观察结果一致。它涉及 Ir(V)中间体和关键的 sigma Ir-(eta(2)-SiH)桥接过渡态。类似的过渡态也可以解释在没有氧化加成反应物的情况下的反式-顺式异构体转化，尽管需要依赖于潜在配体及其性质的高动力学势垒。
