Sawatlon Boodsarin, Surawatanawong Panida
Department of Chemistry and Center of Excellence for Innovation in Chemistry, Faculty of Science, Mahidol University, Bangkok 10400, Thailand.
Dalton Trans. 2016 Oct 14;45(38):14965-78. doi: 10.1039/c6dt02431a. Epub 2016 Aug 23.
The catalytic dehydrogenation and hydrogenation of N-heterocycles have potential applications in organic hydrogen storage. Recently, Fe(HPNP)(CO)(H)(HBH3) (cp1) and Fe(HPNP)(CO)(H)(Br) (cp2), the iron(ii) complexes supported by bis(phosphino)amine pincer (Fe-PNP) (PNP = N(CH2CH2P(i)Pr2)2), have been reported to be the starting complexes which can catalyze the dehydrogenation and hydrogenation of N-heterocycles. The active species were proposed to be the trans-dihydride complexes, Fe(HPNP)(CO)(H)2 (cp4) and Fe(PNP)(CO)(H) (cp3), which can be interconverted. Here, our density functional study revealed that the N-heterocyclic substrate plays a role in the formation of cp4 from cp1, while the tert-butoxide base assists with the formation of cp3 from cp2. The mechanism for cp3 catalyzed dehydrogenation of a 1,2,3,4-tetrahydroquinoline (THQ) substrate to quinoline (Q) involves two main steps: (i) dehydrogenation of THQ to 3,4-dihydroquinoline (34DHQ) and (ii) dehydrogenation of 34DHQ to Q. In each dehydrogenation step, the proton is transferred from the substrate to the N of the PNP ligand of cp3. An ion-pair complex between Fe-PNP and the deprotonated substrate is then formed before the hydride at the adjacent C is transferred to Fe. Notably, the isomerization of 34DHQ to 14DHQ or 12DHQ is not necessary, as the bifunctionality of Fe-PNP in cp3 can stabilize the ion-pair complex and facilitate direct dehydrogenation of the C3-C4 bond in 34DHQ. On the other hand, the mechanism for hydrogenation of Q involves the initial formation of 14DHQ, which can easily isomerize to 34DHQ with the assistance of a tert-butoxide base. Finally, 34DHQ is dehydrogenated to THQ. As the overall energy barriers for cp3 catalyzed dehydrogenation of THQ (+27.6 kcal mol(-1)) and cp4 catalyzed hydrogenation of Q (+23.8 kcal mol(-1)) are only slightly different, reaction conditions can be conveniently adjusted to favor either the dehydrogenation or hydrogenation process. Insights into the role of metal-ligand cooperativity in Fe-PNP complexes in promoting the dehydrogenation and the hydrogenation of N-heterocycles should benefit the development of efficient catalysts for organic hydrogen storage.
N-杂环的催化脱氢和氢化在有机储氢方面具有潜在应用。最近，据报道，由双（膦基）胺钳形配体（Fe-PNP）（PNP = N(CH2CH2P(i)Pr2)2）支撑的铁（II）配合物Fe(HPNP)(CO)(H)(HBH3)（cp1）和Fe(HPNP)(CO)(H)(Br)（cp2）是可催化N-杂环脱氢和氢化的起始配合物。推测活性物种为可相互转化的反式二氢化物配合物Fe(HPNP)(CO)(H)2（cp4）和Fe(PNP)(CO)(H)（cp3）。在此，我们的密度泛函研究表明，N-杂环底物在由cp1形成cp4的过程中起作用，而叔丁醇盐碱有助于由cp2形成cp3。cp3催化1,2,3,4-四氢喹啉（THQ）底物脱氢生成喹啉（Q）的机理涉及两个主要步骤：（i）THQ脱氢生成3,4-二氢喹啉（34DHQ）和（ii）34DHQ脱氢生成Q。在每个脱氢步骤中，质子从底物转移到cp3的PNP配体的N上。然后在相邻C上的氢化物转移到Fe之前，形成Fe-PNP与去质子化底物之间的离子对配合物。值得注意的是，34DHQ异构化为1,4-DHQ或1,2-DHQ并非必要，因为cp3中Fe-PNP的双功能性可以稳定离子对配合物并促进34DHQ中C3-C4键的直接脱氢。另一方面，Q氢化的机理涉及首先形成1,4-DHQ，其在叔丁醇盐碱的帮助下可轻松异构化为3,4-DHQ。最后，3,4-DHQ脱氢生成THQ。由于cp3催化THQ脱氢（+27.6 kcal mol(-1)）和cp4催化Q氢化（+23.8 kcal mol(-1)）的整体能垒仅略有不同，因此可以方便地调整反应条件以有利于脱氢或氢化过程。深入了解Fe-PNP配合物中金属-配体协同作用在促进N-杂环脱氢和氢化中的作用，应有助于开发用于有机储氢的高效催化剂。
