"PCP"-pincer-ligated iridium complexes have been found to be highly effective catalysts for the dehydrogenation of alkanes. We report a computational and experimental study of the effect on catalytic activity resulting from systematically varying steric crowding by the substitution of methyl groups for the phosphino tert-butyl groups of ((PCP)-P-R4)Ir ((PCP)-P-R4 = k(3)-C6H3-2,6-(CH2PR2)(2); R = Bu-t or Me). DFT calculations for ((PCP)-P-R4)Ir species (R-4 = Bu-t(4) or (Bu3Me)-Bu-t) indicate that the rate-determining step in the n-alkane/1-alkene transfer dehydrogenation cycle is beta-H elimination by ((PCP)-P-R4)Ir(n-alkyl)(H). It is calculated that the transition state for this step is ca. 10 kcal/mol lower for ((PCP)-P-tBu3Me)Ir than for ((PCP)-P-tBu4)Ir (relative to the corresponding free ((PCP)-P-R4)Ir). However, this catalytically favorable effect is calculated to be partially offset by the strong binding of 1-alkene to ((PCP)-P-tBu3Me)Ir in the resting state, so the overall barrier is thus lower by only ca. 4 kcal/mol. Further Me-for-Bu-t substitutions have a smaller effect on the transition states, and the calculated energy of the olefin-bound resting states is lowered by comparable amounts; therefore these additional substitutions are predicted to have little overall favorable effect on catalytic rates. ((PCP)-P-tBu3Me)IrH4 has been synthesized and isolated, and its catalytic activity has been investigated. It is indeed found to be a more active catalyst precursor than ((PCP)-P-tBu3Me)IrH4 for alkane transfer dehydrogenation. ((PCP)-P-tBu3Me)IrH4 was also synthesized and as a catalyst precursor is found to afford somewhat lower activity than ((PCP)-P-tBu3Me)IrH4. However, synthetic precursors of ((PCP)-P-tBu3Me)IrH4 tended to yield dinuclear clusters, while complex mixtures were observed during catalysis that were not amenable to characterization. It is therefore not clear if the lesser catalytic activity of((PCP)-P-tBu3Me)Ir vs ((PCP)-P-tBu3Me)Ir derivatives is due to the energetics of the actual catalytic cycle or due to deactivation of this catalyst via the facile formation of clusters.
"PCP"配位的
铱复合物已被发现是
烷烃脱氢反应极其高效的催化剂。我们报告了一项计算与实验结合的研究,旨在探讨当通过甲基取代
磷(-tert-butyl)基团来系统地改变空间拥挤性时,催化剂活性所受到的影响。具体而言,我们研究了(((PCP)-P-R4)Ir复合物(其中((PCP)-P-R4) = k(3)-
C6H3-2,6-(CH2PR2)(2),R= Bu-t或Me)的催化性能。
密度泛函理论(DFT)计算表明,在n-
烷烃/1-烯烃转移脱氢循环中,速率决定步骤是(((PCP)-P-R4)Ir(n-烷基)(H) 通过β-H消除作用形成烯烃。计算结果显示,对于由Bu-t和Me取代组成的复合物,关键过渡态的能量约为降低10千卡/摩尔,这一数值确保了更高的催化效率。然而,这种优势部分被烯烃与催化剂结合的强相互作用所抵消,导致整体能垒仅降低约4千卡/摩尔。
进一步的Me取代Bu-t对过渡态的影响较小,并且烯烃结合状态下能垒的降低幅度相似。因此,这些后续的取代对整体催化活性的提升有限。我们成功合成了(((PCP)-P-tBu3Me)IrH4,并对其催化性能进行了评估。实验证实,相较于((PCP)-P-tBu4)IrH4,其作为催化剂前体更为活跃。
此外,((PCP)-P-tBu3Me)IrH4的合成也取得了成功,但其催化活性不及预期。有趣的是,在催化过程中,某些合成前体易于形成双核簇,导致催化剂性能下降。由于实验中观察到的复杂混合物难以表征,因此无法明确判断这一现象是由于催化剂内在的失活还是由于簇形成的副反应导致。
综上所述,通过取代反应调控
配体的空间位阻,能够在一定程度上优化
铱复合物的催化性能,但同时也需关注可能产生的副反应对催化活性的潜在影响。