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首页> 外文期刊>Organometallics >Electrophilic methylplatinum complexes: A theoretical study of the mechanism of C-C and C-H bond formation and activation
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Electrophilic methylplatinum complexes: A theoretical study of the mechanism of C-C and C-H bond formation and activation

机译：亲电子甲基铂配合物：C-C和C-H键形成和活化机理的理论研究

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The reductive elimination of methane or ethane from the five-coordinate intermediate model complexes [PtHMe2L2](+), or [PtMe3L2](+) respectively, and the corresponding C-H or C-C bond activation from the alkane complexes [PtMe(CH4)L-2](+) or [PtMe(C2H6)L-2](+), respectively, have been studied by carrying out extended Huckel molecular orbital (EHMO) calculations and density functional theory (DFT) calculations on both the ground-state and transition-state structures with L = NH3 or PH3. The EHMO calculations on trans-[PtL2-Me-3](+), L = PH3, show that the regular trigonal-bipyramidal (TBP) structure has an orbitally degenerate ground state and should undergo distortion to either the square-pyramidal (SP) or pinched trigonal-bipyramidal (PTBP) structure. In the PTBP structure, two methyl groups are in close proximity (C-Pt-C ca. 70 degrees) and tilted away from each other. Although the tilting leads to a close Pt ... HC contact, no attractive agostic Pt ... H bonding is indicated. The DFT calculations predict that C-H reductive elimination and oxidative addition are much easier than C-C reductive elimination and oxidative addition, but there is no major difference between the activation energies when L = NH3 or PH3. However, the platinum(IV) complexes are relatively more stable when L = NH3 than when L = PH3 compared to the platinum(II) alkane complexes, and so the activation energies for C-H or C-C oxidative addition are calculated to be lower for the NH3 complexes. The platinum(IV) complexes with ligands L mutually cis or trans are most stable in the SP or PTBP stereochemistry, respectively. In the platinum(II) alkane complexes, the stereochemistry with ligands L mutually trans is preferred. The oxidative-addition/reductive-elimination reactions occur by a concerted mechanism, probably with a PTBP complex on the reaction coordinate. For C-H reductive elimination, the methane remains coordinated to platinum through the C-H a complex. For C-C reductive elimination, the transition state is a C-C sigma complex but in the final ethane complex the binding is as a C-H a complex. For methane complexes, the binding is eta(3) but one platinum C-H contact is shorter than the other, while for ethane complexes, the binding is usually eta(4) through two eclipsed platinum C-H sigma-complex interactions, but one appears much stronger than the other. The weaker of these sigma-complex interactions is just strong enough to overcome the tendency of ethane to adopt the staggered conformation (ca. 3 kcal mol(-1)). Activation of the C-C bond of ethane is likely only in systems where the much easier C-H activation is rapid and reversible. [References: 33]
机译：分别从五配位中间体模型配合物[PtHMe2L2]（+）或[PtMe3L2]（+）中还原去除甲烷或乙烷，并从烷烃配合物[PtMe（CH4）L-中相应地去除C​​H或CC键2]（+）或[PtMe（C2H6）L-2]（+）分别通过对基态和基态进行扩展的Huckel分子轨道（EHMO）计算和密度泛函理论（DFT）计算得到研究。 L = NH3或PH3的过渡态结构。对反式[PtL2-Me-​​3]（+）的EHMO计算，L = PH3，表明规则的三角双锥（TBP）结构具有轨道简并基态，并且应变形为方锥（SP ）或三棱锥双锥（PTBP）结构。在PTBP结构中，两个甲基非常接近（C-Pt-C大约70度）并且彼此倾斜。尽管倾斜导致紧密的Pt ... HC接触，但未显示出吸引人的偏执的Pt ... H键。 DFT计算预测，C-H还原消除和氧化加成比C-C还原消除和氧化加成要容易得多，但是当L = NH3或PH3时，活化能之间没有重大差异。然而，与铂（II）烷烃配合物相比，当L = NH3时，铂（IV）配合物相对稳定，而当L = PH3时，则相对稳定，因此计算出的CH3或CC氧化加成的活化能较低。复合体。在SP或PTBP立体化学中，分别具有顺式或反式配体L的铂（IV）配合物最稳定。在铂（II）烷烃络合物中，优选与配体L相互反式的立体化学。氧化加成/还原消除反应是通过协同机制发生的，在反应坐标上可能带有PTBP配合物。对于C-H还原消除，甲烷通过C-H络合物保持与铂的配位。对于C-C还原消除，过渡态为C-Cσ络合物，但在最终的乙烷络合物中，结合形式为C-H络合物。对于甲烷络合物，结合是eta（3），但一个铂CH接触比另一个短，而对于乙烷络合物，结合通常是通过两个黯淡的铂CH sigma-络合物相互作用而成为eta（4），但其中一个似乎更强比其他。这些sigma-complex相互作用的弱点刚好足以克服乙烷采用交错构象（约3 kcal mol（-1））的趋势。乙烷的C-C键的活化仅在快速且可逆的C-H活化容易的系统中才可能发生。 [参考：33]

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