Speciality Grand Challenges in Organometallic Catalysis

Abstract

The interaction between a metal center (M) and a molecular moiety (substrate) is the basis of most catalytic processes. The chemical environment surrounding M can equally be a set of suitable ligands (Coordination Catalysis) (Crabtree, 2014), a set of properly engineered/functionalized ligands anchored onto a solid support (Single-Site Surface Coordination Catalysis) (Copéret et al., 2016), a small cluster of metal atoms as well as a lattice of a material (Heterogeneous Catalysis) (Friend and Xu, 2017), and an enzymatic framework (Biocatalysis) (Schwizer et al., 2018) (Figure 1). If at least one of the M-environment interactions involves an M–R bond (where R C and H), all types of catalysis listed above are by definition Organometallic Catalysis. Furthermore, even in the absence of a M–R bond in the starting molecule/material, the catalytic process may be still defined as of organometallic nature if a M–R fragment forms in any step of the catalytic cycle. These simple considerations clearly indicate the generality and importance of organometallic catalysis. Relevant examples of organometallic catalysis, for each of category illustrated above, are very well known and reported in the textbooks (Drauz et al., 2012; Bochmann, 2014). The success of organometallic catalysis may be ascribed to the capability of a metal to activate lowenergy reaction pathways along which the deformed substrate, stabilized through coordination at a properly designed LnM-fragment, is induced to react in a novel and original way. This explains why some reactions are exclusive of coordination/organometallic complexes. In this respect, a classical example is the reductive elimination, which is one of the fundamental steps of organometallic catalytic cycles (Hartwig, 1998; Chen et al., 2017; Chu and Nikonov, 2018; Wolczanski, 2018). It involves the release of R–X from a (LnMXR) complex, where oxidation state, coordination number and electron of the metal center are reduced by two units. As a result of this propensity to activate a substrate by opening low-energy reaction pathways, the activity of organometallic catalysts can be so high that a <10−6 M active metal concentration is sufficient for carrying out the reaction efficiently: in these cases, catalyst separation and recovery from the products might even be avoided, as it occurs in some industrial polymerization processes (Stürzel et al., 2016). This notwithstanding, catalyst recovery is still necessary is many cases, and typically more easily achievable with heterogenous rather than molecular systems. For this reason, industrially relevant molecular catalysts are often heterogenized onto suitable supports, as mentioned above, leading to heterogeneous catalysts with similar (ideally the same) activity and selectivity to the molecular counterpart, but with the additional advantage of being easy to separate from the reaction environment and recycle (Schwarz et al., 1995; McNamara et al., 2002; Witzke et al., 2020). Selectivity is another strong suit of organometallic catalysis, which can be achieved by tailoring the chemical environment of the active metal by proper selection/combination of ligands. As a matter of fact, chemical, regio–and stereo–selectivity approaching 100% have been obtained for many reactions of industrial relevance, even in non-enzymatic systems. Importantly, the effectiveness of organometallic catalysts stems also from possible M–environment cooperativity. The latter may involve ligands, which may be redox active or bear a dandling functionality (a base, an acid, etc.), support, other metallic centers (both in Edited and reviewed by: Frank Hollmann, Delft University of Technology, Netherlands