Reversible carbon-hydrogen bond insertion/reductive elimination in (.eta.5-pentamethylcyclopentadienyl)(trimethylphosphine)iridium complexes. Use in determining relative metal-carbon bond energies and thermally activating methane
Reversible carbon-hydrogen bond insertion/reductive elimination in (.eta.5-pentamethylcyclopentadienyl)(trimethylphosphine)iridium complexes. Use in determining relative metal-carbon bond energies and thermally activating methane
复制标题
(η5-五甲基环戊二烯基)(三甲基膦)铱配合物中的可逆碳氢键插入/还原消除。
DOI：
10.1002/chin.198421310
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发表时间：
1984
期刊：
影响因子：
--
通讯作者：
R. Bergman
中科院分区：
文献类型：
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作者：
M. Wax;J. Stryker;J. M. Buchanan;C. Kovac;R. Bergman
[Cp[n5-C5Me5; L = PMe，]对应于在140 ℃下Δ G反应=-2.0千卡/摩尔。合理地假设反应1的AS 0接近于0，环己烷中的二级CH键能为94.5 kcal/mol，正戊烷中的初级CH键能为98 kcal/mol，我们由此计算出初级络合物2中的金属-碳键能比6中的二级金属-碳键能强5.5 kcal/mol。在全氟烷烃溶剂中，在4个大气压的CH4下，二氢化物1的尝试光解没有检测到甲烷活化（与Graham对（C5Me5）Ir（CO）2的甲烷活化光解的观察相反），这可能是因为1在这些溶剂中的溶解度非常低。在环辛烷中的光解，一种假定的中间体（5-C5Me5）（PMe3）Ir的CH插入的"缓慢"底物，在CH4下，仅得到先前所述的1-十二烷基环辛基络合物（8）。然而，我们能够在可逆条件下以热和高产率实现甲烷活化，这是通过利用以下假设实现的：即双脱氧甲基络合物7甚至比伯烷基络合物如2在化学上更稳定。因此，如方案II所示，在密封的Pyrex容器中，在20大气压的CH4下，在140 - 150 ℃的温度下，加热环辛烷溶剂中的邻位环己基络合物6 14小时，导致邻位甲基络合物7的58%产率（NMR）7（NMR 1.87（dd，J = 2.0，0.7 Hz，C5 Me5），1.22（d，J = 9.8 Hz，PMe3），0.71（d，J = 5.8 Hz，IrMe），-17.22（d，J = 35.9 Hz，IrH））沿着8%二氢化物1。尝试通过结晶或色谱法分离氯甲基络合物证明是困难的，正如在其他情况下一样，1，因此用CHCl3处理该物质，将其转化为相应的氯甲基络合物9，其可以通过常规方法纯化和表征。8.从起始的顺式环己基络合物6分离得到9的产率为（5）Benson，S. W. "热化学动力学"; Wiley：纽约，1976年。(6)正如一位裁判所指出的那样，由于键离解能是针对气相定义的，所以这个计算也假定方程1中化合物的溶解热和汽化热不会显著地干扰平衡常数。没有充分解决这一问题的适当数据。尽管人们通常认为结构相关的有机金属配合物的升华热是相同的，但其他数据表明，有机金属配合物的升华热随结构的相对细微的变化而变化。6 "11 * 鉴于这种情况，以及我们目前的粗略数据，我们宁愿作出以下假设：（正如我们对系统熵变的贡献所做的那样）升华热的贡献近似是相加的-也就是说，that the difference差异in heats热of vaporizationbetween蒸发between之间n-pentane戊烷and chanchane环己烷（约1.0kcal/mol）被络合物6和2的升华热的估计差近似平衡。(a)Adedeji，F.一、布朗，DL S.;康纳，J. A.;
[Cp'n5-C5Me5; L= PMe,] corresponds to AGreaction=-2.0 kcal/mol at 140 C. Making the reasonable assumption that AS0 for reaction 1 is close to 0 and using 94.5 kcal/mol for the secondary CH bond energy in cyclohexane and 98 kcal/mol for the primary CH bond energy in/¡-pentane, 5 we calculate6 from this Gthat the metal-carbon bond energy in primary complex 2 is 5.5 kcal/mol stronger than the secondary metal-carbon bond in 6. Attempted photolysis of dihydride 1 in perfluoroalkane solvents under 4 atm of CH4 gave no detectable methane activation (in contrast toGraham’s observations2'on the methane-activating photolysis of (C5Me5) Ir (CO) 2), perhaps because of the very low solubility of 1 in these solvents. Photolysis in cyclooctane, a “slow” substrate for CH insertion of the presumed intermediate (5-C5Me5)(PMe3) Ir, under CH4 gave only the previously observed1 hydridocyclooctyl complex (8). However, we were able to achieve methane activation thermally and in high yield under reversible conditions by taking advantage of the presumption that the hydridomethyl complex 7 would be thermodynamically more stable even than primary alkyl complexes such as 2. Thus, as shown in Scheme II, heating hydridocyclohexyl complex 6 in cyclooctane solvent in a sealed Pyrex vessel under 20 atm of CH4 at tem-peratures between 140 and 150 C for 14 h led to a 58% yield (NMR) 7 of hydridomethyl complex 7 (NMR 1.87 (dd, J= 2.0, 0.7 Hz, C5Me5), 1.22 (d, J= 9.8 Hz, PMe3), 0.71 (d, J= 5.8 Hz, IrMe),-17.22 (d, J= 35.9 Hz, IrH)) along with 8% of dihydride 1. Attempted isolation of the hydridomethyl complex by crystallization or chromatography proved difficult, as it has in other cases, 1 and so the material was treated with CHC13, converting it to the corresponding chloromethyl complex 9, which could be purified and characterized by conventional means. 8 The isolated yield of 9 from starting hydridocyclohexyl complex 6 was (5) Benson, S. W.“Thermochemical Kinetics”; Wiley: New York, 1976.(6) As pointed out by a referee, this calculation also assumes, since bond dissociation energies are defined for the gas phase, that the heats of solution and vaporization of the compounds in eq 1 do notperturb the equilibrium constant significantly. Data appropriate to resolve this question fully are not available. Althoughit has often been assumed that heats of sublimation of structurally related organometallic complexes are identical, other data indicate that heats of sublimation of organometalliccomplexes vary with relatively subtle changes in structure. 6”'11* Given this situation, and the presently crude level of our data, we would prefer to make the assumption (as we did for contributions to the entropy change for the system) that contributions to heats of sublimation are approximately additive—that is, that the difference in heats of vaporizationbetween n-pentane and cyclohexane (about 1.0 kcal/mol) is approximately balanced by the estimated difference in heats of sublimation of complexes 6 and 2.(a) Adedeji, F. A.; Brown, DL S.; Connor, J. A.;