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Hydrogenation Catalysis Based on Functional Pincer Ligands Lukas Alig‡, Maximilian Fritz‡, Sven Schneider*

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Hydrogenation Catalysis Based on Functional Pincer Ligands Lukas Alig‡, Maximilian Fritz‡, Sven Schneider* This document is the Accepted Manuscript version of a Published Work that appeared in final form in Chemical Reviews, copyright © 2018 American Chemical Society after peer review and technical editing by the publisher. To access the final edited and published work see DOI: 10.1021/acs.chemrev.8b00555. First-Row Transition Metal (De)hydrogenation Catalysis based on Functional Pincer Ligands Lukas Alig‡, Maximilian Fritz‡, Sven Schneider* Universität Göttingen, Institut für Anorganische Chemie, Tammannstrasse 4, D-37077 Göttingen, Germany ABSTRACT: The use of 3d metals in de-/hydrogenation catalysis has emerged as a competitive field with respect to ‘tradi- tional’ precious metal catalyzed transformations. The introduction of functional pincer ligands that can store protons and/or electrons as expressed by metal-ligand cooperativity and ligand redox-activity strongly stimulated this development as conceptual starting point for rational catalyst design. This reviews aims at providing a comprehensive picture of the utilization of functional pincer ligands in first-row transition metal hydrogenation and dehydrogenation catalysis and re- lated synthetic concepts relying on these such as the hydrogen borrowing methodology. Particular emphasis is put on the implementation and relevance of cooperating and redox-active pincer ligands within the mechanistic scenarios. CONTENTS 3.4. Hydrogenation of Nitriles 1. Introduction 3.5. Hydrogenation of Esters 1.1. Metal-ligand Cooperativity (MLC) 3.6. Hydrogenation of CO2 1.2. Redox active ligands 3.6.1. Pyridyl-Based Pincer Catalysts 2. Privileged Pincer Platforms 3.6.2. Diphosphinoamine Pincer Catalysts 2.1. Aminopincer Platforms 3.7. Hydrogenation of Olefins 2.1.1. Diphosphinoamines and Related Ligands 3.7.1. Pyridyl-Based Pincer Catalysts 2.1.2. BPI and Boxmi Ligands 3.7.2. Aminodiphosphine and Related Pincer 2.1.3. Aminophenol Pincer Ligands Catalysts 2.1.4. Dihydrazonopyrroles 3.8. Hydrogenation of Alkynes 2.2. Pyridyl Pincer Ligands 4. Dehydrogenation 2.2.1. Diphosphinopyridine and Related Lig- 4.1. (Acceptorless) Alcohol Dehydrogenation ands 4.1.1. Dialkylamino Pincer Catalysts 2.2.2. Pyridinediimines and Related Ligands 4.1.2. Pyridyl-Based and Related Pincer Cata- 2.2.3. Terpyridine lysts 2.2.4. Pybox 4.2. Formic Acid Dehydrogenation 2.2.5. Pyridyldicarbenes 4.3. Dehydrogenation of Nitrogen-Containing Sub- 2.2.6. Iminopyridine and Related Systems strates 2.2.7. Bipyridine Based Pincer Ligands 4.3.1. Amineboranes 2.2.8. Azaphenyl Ligands and Related Systems 4.3.2. N-Heterocycles 2.2.9. Bis(pyrazolyl)pyridine 4.3.3. Dehydrogenative C-N Coupling 2.2.10. Phosphaalkene Pincer Ligands 4.3.4. Dehydrogenative Aminomethylation 3. Hydrogenation 5. Hydrogen Borrowing Methodology 3.1. Hydrogenation of Ketones and Aldehydes 6. Outlook 3.1.1. Pyridyl-Based and Related Pincer Cata- lysts 3.1.2. Dialkylamino Pincer Catalysts 3.1.3. Asymmetric Hydrogenation of Ketones 3.2. Hydrogenation of Imines and N-Heterocycles 3.3. Hydrogenation of Amides 1 INTRODUCTION many other application.2,32,51–60 However, the term ‘pincer ligand’ has since been considerably widened to a point First-row transition metal compounds have emerged in re- where a precise definition is blurred by the inclusion of cent years as homogeneous catalysts for numerous organic many meridionally binding, mono-, di- and tri-anionic and transformations,1–19 such as de-/hydrogenation,18,20–33 and even neutral tridentate ligands that cover a vast range of cen- other hydroelementation34–38 reactions that were tradition- tral and flanking coordinating groups and ‘backbones’ link- ally the realm of precious metal catalysis. The development ers connecting them. This structural and electronic variabil- of base metal hydrogenation catalysts also inspired sustain- ity allows for versatile fine-tuning of chemical properties, able concepts for the controlled release and fixation of dihy- which strongly contributed to the success of the pincer lig- drogen within chemical energy storage schemes.32,39–46 Due and concept.54,61–64 Importantly, the conceptual utilization of to the generally lower toxicity compared with 4d and 5d functional pincer ligands decisively stimulated base metal metal complexes,47 the use of 3d metals is less environmen- catalyzed de-/hydrogenation catalysis. tally harmful meeting several principles of Green Chemistry. Furthermore, 3d metals are considerably more abundant in Scheme 1. Schematic representations of typical hydrogenation cy- the continental crust than their heavier 4d/5d homologues, cles of olefins and carbonyl groups with and without formal metal redox changes.65,81 ranging from scandium (16 ppm) to iron (43200 ppm), the fourth most abundant element (Figure 1).48 In contrast, some 4d metals (Ru, Rh, Pd) and most of the 5d metals (in partic- ular Re, Os, Ir and Pt) are rare elements with limited deposits suitable for today’s mining technology. Figure 1. Abundance of mid to late transition metals in the earth’s continental crust (concentration in ppm).48 The increasing demand and cost for noble metal precursors The parameters that contribute to the activity of 3d metal hy- sparked a rising interest in their replacement for more abun- drogenation catalysts with respect to their heavier homo- dant homogeneous catalysts. However, the robustness (ex- logues were recently discussed by Zell and Langer.65 Within pressed in the turnover number TON) and the activity (ex- a strongly simplified picture, two ‘conventional’ mechanis- pressed in the turnover frequency TOF) of base metal cata- tic scenarios are well established for hydrogenation of ole- lysts often lacks behind the performance of noble metal cat- fins and carbonyl groups, respectively (Scheme 1): The di- alysts. The emergence of base metal catalysis was therefore hydride mechanisms (right) imply substrate binding, step- strongly driven by the development of sophisticated ligand wise hydrogen transfer to the substrate and H2 oxidative ad- systems as an enabling methodology. The proportional cost dition, associated with formal metal ±2 redox steps. Within of such complex ligand platforms for 3d metal catalysis with monohydride mechanism (left), metal redox events are high activity and selectivity, should not be underestimated. avoided by H2 heterolysis with proton transfer from a dihy- Besides cost considerations, the field of base metal catalysis drogen ligand to alkyl (olefin hydrogenation) or alkoxy (car- is therefore also driven by catalytic transformations that bonyl hydrogenation) ligands. These considerations define have not been realized with precious metal catalysts before. the relevant parameters that determine the thermochemical framework for comparing 3d vs 4/5d metal catalysts, i.e. (1) ‘Pincer’ ligands were introduced by Shaw in 1976.49 Ac- substrate binding constants (2) M–H vs. M–C/O bond cording to the early definition by van Koten and Albrecht, strength as expressed by bond dissociation free energies pincer ligands are tridentate ligands with a central, s-donat- (BDFE) for homolytic processes and M–H hydricity (DGH-) ing phenyl moiety that carries two lateral amino- or phos- and pKa for hydride or proton transfer, respectively, and (3) phinomethyl CH2ER2 (E = N, P) side groups in ortho-posi- acidity of the dihydrogen intermediate as expressed by 50 66–68 tion. As a result of the chelating, rigid binding mode, tran- pKa(M–H2). These parameters will be discussed in the sition metal complexes often exhibit high thermal stability next sections in the context of two ligand design principles and well defined reactivity at the remaining coordination that have been instrumental in the emergence of first-row sites, which has been extensively exploited in catalysis and 2 metal catalysis, i.e. the introduction of redox active pincer in identical coordination environment. No clear trend along 27,69–74,78,142–145 ligands and metal-ligand the group could be found, but the pKa differences between cooperativity.26,27,30,32,39,46,51,77,93 iron and ruthenium are generally small within 1-2 units while osmium compounds are less acidic accounting for This review aims at covering the application of functional stronger 5d M–H bonding.67 In fact, Morris’ increment pincer ligands in 3d metal catalyzed de-/hydrogenation. The methods for estimating M–H acidities does not discriminate term ‘functional ligand’ has been used for a variety of dif- between first- and second-row metals.82 ferent phenomena of active ligand participation both in the first and second coordination sphere around the metal.78 M–H hydricities have a strong dependence on the metal and II + Within this review on de-/hydrogenation catalysis we will ligand sphere. Within a homologous M H(diphosphine)2 focus on redox active and cooperating pincer ligands to dis- group 10 series, the nickel compound exhibited considerably cuss the relevance of reversible reservoirs for protons and/or higher hydricity in the order DGH-(Ni) >> DGH-(Pt) > DGH- electrons in pivotal organic applications of proton coupled (Pd) by more than 40 kcal mol–1, which renders nickel(II) by electron transfer. Both principles have been separately re- far the weakest hydride donor.83 The same general trend is 26,27,30,32,39,46,51,69-74,77,78,93,142–145 III + viewed. However, the direct observed for example for M H2(diphosphine)2 and I 84–87 comparison offers additional insights for catalyst design. M H(diphosphine)2 (M = Co, Rh), or computed hy- dricity values for MH(CO)5 (M = Mn, Re; DDGH- = 11 Scheme 2. Simplified, schematic representations
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