Department of Chemistry, Indian Institute of Technology Bombay, Powai, Mumbai400076, India.
Inorg Chem. 2023 Feb 6;62(5):2342-2358. doi: 10.1021/acs.inorgchem.2c04119. Epub 2023 Jan 23.
In this study, we have explored the catalytic reactivities of four PNP-pincer supported Fe(II) complexes, namely, [(PNP)FeH(CO)] (), [(PNP)FeH(CO)(BH)] (), [(PNP)FeH(CO)] (), and [(PNP)FeH(BH)] () (PNP = MeN{CHCH(PPr)} and PNP = HN{CHCH(PPr)}) toward reductive CO hydrogenation for formate production. Our density functional theory and ab initio complete active space self-consistent field study have identified three fundamental steps in this catalytic transformation: (i) anchoring of the CO molecule in the vicinity of the metal using noncovalent interactions, (ii) catalyst regeneration via H cleavage, and (iii) formate rebound step leading to catalytic poisoning. The variations in the catalytic efficiency observed among these catalysts were attributed to either easing of steps (i) and (ii) or the hampering step (iii). This can be achieved in various chemical/non-chemical ways, for instance, (a) incorporation of strong-field ligands such as CO facilitating single-state reactivity and eliminating two-state reactivity that generally enhances the rate and (b) inclusion of Lewis acids such as LiOTf and strong bases found to either avoid catalytic poisoning or ease the H-H cleavages, to enhance the rate of reaction (c) evading mixing of excited open-shell singlet states to the ground closed-shell singlet state that hampers the catalytic regeneration. We have probed the role of oriented external electric fields (OEEFs) in the entire mechanistic profile for the best and worst catalyst, and our study suggests that imposing OEEFs opposite to the reaction axis (-axis) fastens the catalytic regeneration step and, at the same time, hampers catalytic poisoning. The application of OEEFs is found to regulate the energetics of various spin states and can hamper two-state reactivity, therefore increasing the efficiency. Thus, this study provides insights into the CO hydrogenation mechanism where the role of bases/Lewis acid, ligand design, spin states, and electric field in a particular direction has been established and is, therefore, likely to pave the way forward for a new generation of catalysts.
在这项研究中，我们探索了四种 PNP-钳式支持的 Fe(II) 配合物的催化反应活性，即 [(PNP)FeH(CO)] ()、[(PNP)FeH(CO)(BH)] ()、[(PNP)FeH(CO)] () 和 [(PNP)FeH(BH)] ()（PNP = MeN{CHCH(PPr)} 和 PNP = HN{CHCH(PPr)})，它们在还原 CO 加氢生成甲酸盐的反应中具有催化活性。我们的密度泛函理论和从头算完全活性空间自洽场研究确定了这个催化转化中的三个基本步骤：（i）使用非共价相互作用将 CO 分子锚定在金属附近，（ii）通过 H 断裂进行催化剂再生，（iii）甲酸盐回弹步骤导致催化中毒。这些催化剂之间观察到的催化效率变化归因于步骤 (i) 和 (ii) 的简化或步骤 (iii) 的阻碍。这可以通过各种化学/非化学方法来实现，例如，（a）引入强场配体如 CO，促进单态反应并消除通常会提高速率的双态反应，（b）包含路易斯酸如 LiOTf 和强碱，发现可以避免催化中毒或简化 H-H 断裂，从而提高反应速率，（c）避免激发开壳 singlet 态与阻碍催化再生的基态闭壳 singlet 态混合。我们研究了外加电场（OEEFs）在最佳和最差催化剂的整个机理中的作用，我们的研究表明，施加与反应轴（-轴）相反的外加电场（OEEFs）可以加快催化再生步骤，同时阻碍催化中毒。外加电场的应用被发现可以调节各种自旋态的能量，并且可以阻碍双态反应，从而提高效率。因此，这项研究提供了对 CO 加氢反应机制的深入了解，其中已经确定了碱/Lewis 酸、配体设计、自旋态和特定方向的电场在其中的作用，因此可能为新一代催化剂铺平道路。
