Base Metal Bifunctional Catalysis

80%以上的商业化学过程涉及催化剂，这些催化剂减少了能源消耗，提高了反应选择性，并有助于过程强化，例如，通过将多个步骤结合为一个步骤。虽然商品化学品制造通常使用多相固体催化剂，但生产价值更高的化学品往往涉及分子催化剂，可以合理地“调整”分子催化剂，以提供所需产品的高产率。尽管贵金属(Ru、Rh、Ir、Pd、Pt)络合物催化剂在过去的50年里得到了广泛的发展，但人们一直在大力开发更有效的催化剂，其中包括毒性较小、稀土含量较高的贱金属(Mn、Fe、Co、Ni、Cu)。在贱金属催化方面，已报道的一些最大的成功包括金属配位配体能够提供比金属中心所提供的更多的机械作用的络合物。在几个突出的例子中，伯胺或氨基配体能够作为氢键供体或有助于激活E-H键的内碱(E=H、B、C、N、O、Si)。在模拟自然的催化剂中，硫酸盐配体也可以参与双功能E-H键的活化，尽管配位的硫醇S-H配对通常比它的N-H配位更酸性。最后，氧化还原活性配体可以作为电子源或受体，与贱金属中心协同工作，实现多电子转换。在这项提案中，我们概述了一项挑战11名研究生和本科生的计划，使用具有五种不同贱金属的简单SNS配体来探索催化反应中金属-配体协作性的基本性质。在过去的资助期间，我们开发了易于制备的SnS、氨基和硫酸盐配体，并研究了它们的配位化学，以比较‘Hard’N和‘Soft’S内碱在一些催化反应中的效率。重点展示了用于胺-硼烷脱氢的双功能铁催化剂、用于醇脱氢的钴催化剂和用于烯烃硅氢加成反应的锰催化剂。通过亚胺C-C键的形成，在Fe、Co和Ni-SnS硫酸盐络合物上发生内部氧化还原反应，生成氧化还原的非无害的N2S2配体。初步研究表明，这一可逆过程可能参与了M(N2S2)的催化循环，多个稳定的氧化还原态(Fe为3个，Co为4个)的存在开启了依赖于氧化还原的催化活性和选择性的激发可能性。下一个资助期的目标包括通过改变辅助配体的金属和性质来调整供体的碱性，并探索配基供体的协作性和氧化还原活性。通过实验和计算相结合的方法对双功能催化进行的这些基础研究，最终将使我们能够开发和优化新的“借氢”串联催化过程。

More than 80% of commercial chemical processes involve catalysts that decrease energy use, increase reaction selectivity and contribute to process intensification, e.g., by combining multiple steps into one. While commodity chemical manufacturing typically employs heterogeneous solid catalysts, production of higher value chemicals often involves molecular catalysts that can be rationally `tuned' to provide high yields of the desired product. Although precious metal (Ru, Rh, Ir, Pd, Pt) complex catalysts have been broadly developed over the last 50 years, there has been an intensive drive to develop more effective catalysts that include base-metals (Mn, Fe, Co, Ni, Cu) that are less toxic and more earth abundant. Some of the greatest reported successes in base metal catalysis include complexes in which the metal-coordinated ligand is able to provide an additional mechanistic role to those offered by the metal center. In several prominent examples a primary amine or amido ligand is able to function as a hydrogen bond donor or internal base that contributes to the activation of E-H bonds (E = H, B, C, N, O, Si). In catalysts modeled after Nature, the thiolate ligand can also participate in bifunctional E-H bond activation although the coordinated thiol S-H partner is typically more acidic than its N-H counterpart.  Finally, a redox-active ligand can serve as an electron source or acceptor, working in tandem with the base-metal center to achieve multi-electron transformations. In this proposal we outline a program that will challenge 11 graduate and undergraduate students, using simple SNS ligands with five different base-metals to probe the fundamental properties of metal-ligand cooperativity in catalytic reactions. Over the last funding period we developed easily prepared SNS amido and thiolate ligands and investigated their coordination chemistry to compare the efficiency of `hard' N vs. `soft' S internal bases for a number of catalyzed reactions. Highlights include demonstration of bifunctional Fe catalysts for amine-borane dehydrogenation, Co catalysts for alcohol dehydrogenation and Mn catalysts for alkene hydrosilylation. An internal redox reaction on Fe, Co, and Ni SNS thiolate complexes afforded a redox non-innocent N2S2 ligand via imine C-C bond formation. Initial studies suggest that this reversible process may be involved in the M(N2S2) catalytic cycles and the presence of multiple stable redox states (3 for Fe, 4 for Co) opens up the exciting possibility of redox-dependent catalytic activity and selectivity. Objectives for the next funding period include tuning donor basicity by changing the metal and nature of the ancillary ligands and probing the cooperativity of ligand donors and redox activity. These fundamental investigations of bifunctional catalysis by a combination of experimental and computational approaches will eventually allow us to develop and optimize new `hydrogen-borrowing' tandem catalysis processes.