Rare-earth mediated dihydrogen activation and catalytic hydrogenation☆
Graphical abstract
Dihydrogen activation by well-defined rare-earth metal complexes is reviewed, which is organized in two parts depending on the different mechanisms, namely σ-bond metathesis and non-σ-metathesis.
Introduction
Dihydrogen (H2) is an excellent source of energy for which the only side consumption product is water (H2O).1,2 This advantage puts H2 as one of the most promising candidates for renewable energy sources to phase out fossil fuels and control global greenhouse gases emission.3 In academic research, catalytic hydrogenation of unsaturated organic molecules using H2 as a hydrogen source is an indispensable tool for organic chemists to construct complex molecules.4, 5, 6 This methodology plays a pivotal role in modern organic and bioorganic chemistry. The significance of catalytic hydrogenation was highlighted by multiple Nobel Prizes in Chemistry, e.g., Sabatier in 1912, and Knowles & Noyori in 2001.
A communal key step of all these applications of H2 is the cleavage of H–H bond, which is a non-polar σ-bond with relatively high bonding energy (436 kJ/mol).7 Enormous research effort has been invested in understanding and delivering controlled H–H bond cleavage of H2. From a perspective of synthetic chemists, cleaving the H–H bond by structurally well-defined metal complexes provides invaluable insight to understanding metal catalyzed hydrogenation. As the foundation of designing high performance catalysts, metal-mediated H–H bond activation, thus is always an active research area.
To date, both d- and f-block metals have been reported to deliver H–H bond activation, and these processes usually undergo three types of mechanism. For late transition-metals, oxidative addition is the most dominant mechanism, during which the metal center experiences a two-electron transfer process (Scheme 1(a)).8 When late transition-metals are bound with N-, O- and S-donor ligands which own a lone pair or are assisted by additional bases, heterolytic H2 activation might be the alternative pathway (Scheme 1(b)).9 For redox-inactive early transition-metals, σ-bond metathesis mechanism plays a major role, which is generally considered to proceed via a [2σ + 2σ] four-membered ring transition state between a M−C bond and the H–H bond (Scheme 1(c)).10
Rare-earth (RE) metals have attracted much attention for their distinct properties in a variety of catalytic transformations.11, 12, 13, 14, 15, 16 In general, rare-earth metals have high coordination numbers, thus are feasible to coordinate and activate incoming organic substrates. Compared to transition-metals, rare-earth metals are more sensitive to the presence of functional groups in substrates and adventitious moisture and oxygen. On the other hand, the variety of ionic radii of 17 rare-earth metals provides a spectrum of finely tunable reactivities. In combination with steric/electronic design on supporting ligands, rare-earth metal complexes could provide otherwise unavailable reactivity and selectivity in catalysis. Recently, rare-earth metal complexes have exhibited excellent activity in inert bond activation, such as C–H activation,17,18 which have been reviewed. On the other hand, H–H bond activation mediated by rare-earth metal complexes has not been reviewed on its own right, albeit its long history linked with rare-earth hydride complexes19 and intriguing new trends such as applications of rare-earth metal-based frustrated Lewis pairs (FLPs).20,21
In this review, we will cover the development of rare-earth metal complexes mediated H–H activation, as well as their applications in catalysis. According to the different mechanisms of H–H bond cleavage, we categorize these reports in two types: (1) dihydrogen activation based on traditional σ-bond metathesis; (2) recently emerged non-σ-metathesis.
Section snippets
Dihydrogen activation based on σ-bond metathesis
In this section, we provide a snapshot of H2 activation by rare-earth metal alkyl complexes via σ-bond metathesis mechanism, particularly review the seminal development related to rare-earth metal catalyzed hydrogenations. This is a well-documented research area; thus, due to the limitation of volume, we will not provide a comprehensive coverage of all relevant reports. In this term, interested readers are referred to the reviews or book chapters for more information about the detailed
Non σ-metathesis dihydrogen activation
In contrast with the widespread σ-bond metathesis, H–H activations mediated by rare-earth metal not via a σ-bond metathesis only emerged very recently. This new trend is enabled by recent advances in the synthesis of novel rare-earth metal complexes, such as rare-earth metal terminal imides, alkylidenes and phosphinidenes, as well as rare-earth metal based frustrated Lewis pairs. In this section, we will provide a comprehensive coverage of these non-σ-metathesis dihydrogen activation.
Conclusions and outlook
In this review, we provide a snapshot of σ-bond metathesis mechanism of H2 activation by rare-earth metal complexes since the first case was reported in 1982,26 and applications of the resultant RE-H complexes in hydrogenation of unsaturated organic molecules. More importantly, we summarize a new trend of non-metathesis rare-earth metal mediated H2 activation, which emerged very recently and is the gathering momentum, including:
- (1)
RE = E (E = N, C, P) double bond activation system (2017);53, 54, 55
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