Department of Chemistry, The University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-3290, USA.
J Am Chem Soc. 2011 Aug 10;133(31):12274-84. doi: 10.1021/ja204851x. Epub 2011 Jul 19.
Syntheses of the olefin hydride complexes [(POCOP)M(H)(olefin)][BAr(f)(4)] (6a-M, M = Ir or Rh, olefin = C(2)H(4); 6b-M, M = Ir or Rh, olefin = C(3)H(6); POCOP = 2,6-bis(di-tert-butylphosphinito)benzene; BAr(f) = tetrakis(3,5-trifluoromethylphenyl)borate) are reported. A single-crystal X-ray structure determination of 6b-Ir shows a square-pyramidal coordination geometry for Ir, with the hydride ligand occupying the apical position. Dynamic NMR techniques were used to characterize these complexes. The rates of site exchange between the hydride and the olefinic hydrogens yielded ΔG(++) = 15.6 (6a-Ir), 16.8 (6b-Ir), 12.0 (6a-Rh), and 13.7 (6b-Rh) kcal/mol. The NMR exchange data also established that hydride migration in the propylene complexes yields exclusively the primary alkyl intermediate arising from 1,2-insertion. Unexpectedly, no averaging of the top and bottom faces of the square-pyramidal complexes is observed in the NMR spectra at high temperatures, indicating that the barrier for facial equilibration is >20 kcal/mol for both the Ir and Rh complexes. A DFT computational study was used to characterize the free energy surface for the hydride migration reactions. The classical terminal hydride complexes, M(POCOP)(olefin)H, are calculated to be the global minima for both Rh and Ir, in accord with experimental results. In both the Rh ethylene and propylene complexes, the transition state for hydride migration (TS1) to form the agostic species is higher on the energy surface than the transition state for in-place rotation of the coordinated C-H bond (TS2), while for Ir, TS2 is the high point on the energy surface. Therefore, only for the case of the Rh complexes is the NMR exchange rate a direct measure of the hydride migration barrier. The trends in the experimental barriers as a function of M and olefin are in good agreement with the trends in the calculated exchange barriers. The calculated barriers for the hydride migration reaction in the Rh complexes are ∼2 kcal/mol higher than for the Ir complexes, despite the fact that the energy difference between the olefin hydride ground state and the agostic alkyl structure is ∼4 kcal/mol larger for Ir than for Rh. This feature, together with the high barrier for interchange of the top and bottom faces of the complexes, is proposed to arise from the unique coordination geometry of the agostic complexes and the strong preference for a cis-divacant octahedral geometry in four-coordinate intermediates.
报告了烯烃氢化物配合物[(POCOP)M(H)(烯烃)][BAr(f)(4)]（6a-M，M=Ir 或 Rh，烯烃=C(2)H(4)；6b-M，M=Ir 或 Rh，烯烃=C(3)H(6)；POCOP=2,6-双（二-叔丁基膦基）苯；BAr(f)=四（3,5-三氟甲基苯基）硼酸酯）的合成。6b-Ir 的单晶 X 射线结构测定表明 Ir 的配位几何为正方形金字塔形，氢化物配体占据顶点位置。动态 NMR 技术用于表征这些配合物。氢化物和烯烃氢之间的位点交换速率得出ΔG(++)=15.6（6a-Ir）、16.8（6b-Ir）、12.0（6a-Rh）和 13.7（6b-Rh）kcal/mol。NMR 交换数据还确定，丙烯配合物中氢化物的迁移仅产生源于 1,2-插入的伯烷基中间体。出乎意料的是，在高温下，在 NMR 光谱中未观察到正方形金字塔配合物的顶面和底面的平均化，这表明 Ir 和 Rh 配合物的面平衡势垒均>20 kcal/mol。DFT 计算研究用于描绘氢化物迁移反应的自由能表面。对于 Rh 和 Ir，计算出游离端氢化物配合物M(POCOP)(烯烃)H是全局最小值，与实验结果一致。在 Rh 乙烯和丙烯配合物中，形成桥接物种的氢化物迁移过渡态（TS1）的能量高于配位 C-H 键原位旋转的过渡态（TS2），而对于 Ir，TS2 是能量表面上的最高点。因此，只有对于 Rh 配合物，NMR 交换速率才是氢化物迁移势垒的直接度量。实验势垒随 M 和烯烃的变化趋势与计算交换势垒的变化趋势非常吻合。尽管 Ir 与 Rh 相比，烯烃氢化物基态和桥接烷基结构之间的能量差大 4 kcal/mol，但 Rh 配合物中氢化物迁移反应的计算势垒比 Ir 配合物高约 2 kcal/mol。这种特征，加上配合物顶面和底面之间的高交换势垒，据推测是由桥接配合物的独特配位几何和四配位中间体中强烈倾向于顺式空缺八面体几何结构引起的。
