A self-assembled supramolecular complex is reported to catalyze alkyl-alkyl reductive elimination from high-valent transition metal complexes such as gold(III) and platinum(IV), the central ...bond-forming elementary step in many catalytic processes. The catalytic microenvironment of the supramolecular assembly acts as a functional enzyme mimic, applying the concepts of enzymatic catalysis to a reactivity manifold not represented in biology. Kinetic experiments delineate a Michaelis-Menten-type mechanism, with measured rate accelerations (Kcat/Kuncat)up to 1.9 × 10⁷ (here Kcat and Kuncat are the Michaelis-Menten enzymatic rate constant and observed uncatalyzed rate constant, respectively). This modality has further been incorporated into a dual catalytic cross-coupling reaction, which requires both the supramolecular microenvironment catalyst and the transition metal catalyst operating in concert to achieve efficient turnover.
The chemistry of vanadium has seen remarkable activity in the past 50 years. In the present review, reactions catalyzed by homogeneous and supported vanadium complexes from 2008 to 2018 are ...summarized and discussed. Particular attention is given to mechanistic and kinetics studies of vanadium-catalyzed reactions including oxidations of alkanes, alkenes, arenes, alcohols, aldehydes, ketones, and sulfur species, as well as oxidative C–C and C–O bond cleavage, carbon–carbon bond formation, deoxydehydration, haloperoxidase, cyanation, hydrogenation, dehydrogenation, ring-opening metathesis polymerization, and oxo/imido heterometathesis. Additionally, insights into heterogeneous vanadium catalysis are provided when parallels can be drawn from the homogeneous literature.
Catalysis is a ubiquitous element of the modern economy, representing a cornerstone of many sectors including energy, materials, and pharmaceuticals. Within this realm, 80% of reactions are carried ...out by heterogeneous catalysts due to their advantageous physical and chemical characteristics. In some heterogeneous systems, the reaction rate can be optimized through rational catalyst design, in which support materials can be tailored structurally and stereoelectronically for targeted purposes to better facilitate the catalytic reaction. To this end, metal–organic frameworks (MOFs) have garnered recent attention due to their high porosity, crystallinity, and chemical tunability, which can be used to derive structure–activity relationships in the design of catalysts. In this Perspective, we survey examples of catalytically active guest metals on the inorganic nodes of MOFs. This is done with the intention of providing design guidelines for tailoring chemical reactivity and insights into the opportunities for future growth in this field. Strategies for rational design in MOF catalysis include electronic tuning of the metal node in the MOF support, installing promoter ions onto the node to effect the reactivity of grafted metals, modulation of the spatial environment around the transition metal using the metal node structure, and achieving site uniformity in supported catalysts. By highlighting these efforts, we seek to underscore the role of MOF nodes as nonspectator supports for catalytic metal complexes and provide future directions for rational catalyst design by tuning reactivity on these anchoring platforms.
Supramolecular assembly 1 catalyzes a bimolecular aza-Prins cyclization featuring an unexpected transannular 1,5-hydride transfer. This reaction pathway, which is promoted by constrictive binding ...within the supramolecular cavity of 1, is kinetically disfavored in the absence of 1, as evidenced by the orthogonal reactivity observed in bulk solution. Mechanistic investigation through kinetic analysis and isotopic labeling studies indicates that the rate-limiting step of the transformation is the encapsulation of a transient iminium ion and supports the proposed 1,5-hydride transfer mechanism. This represents a rare example of such an extreme divergence of product selectivity observed within a catalytic metal–ligand supramolecular enzyme mimic.
This study offers a detailed mechanistic investigation of host–guest encapsulation behavior in a new enzyme–mimetic metal–ligand host and provides the first observation of a conformational selection ...mechanism (as opposed to induced fit) in a supramolecular system. The Ga4L4 host described features a C 3-symmetric ligand motif with meta-substituted phenyl spacers, which enables the host to initially self-assemble into an S 4-symmetric structure and then subsequently isomerize to a T-symmetric tetrahedron for better accommodation of a sufficiently large guest. Selective inversion recovery 1H NMR studies provide structural insights into the self-exchange behaviors of the host and the guest individually in this dynamic system. Kinetic analysis of the encapsulation–isomerization event revealed that increasing the concentration of guest inhibits the rate of host–guest relaxation, a key distinguishing feature of conformational selection. A comprehensive study of this simple enzyme mimic provides insight into analogous behavior in biophysics and enzymology and aims to inform the design of efficient self-assembled microenvironment catalysts.
The field of Surface Organometallic Chemistry (SOMC) aims to blend the positive attributes of homogeneous and heterogeneous catalysis. Significant insight into heterogeneous systems has been gained ...over the years through the synthesis, characterization, and application of well-defined surface organometallic catalysts, predominantly supported on silica and alumina. Considerable research efforts have focused on the application of homogeneous methods to the synthesis and characterization of these systems. Homogeneous catalysis has thrived on its ability to electronically and sterically tune ligands to yield desired reactivity and selectivity. Efforts in SOMC, however, have only recently turned to harnessing the stereoelectronic diversity of potential inorganic support materials beyond silica and alumina in order to exert similar control on the reactivity of the organometallic active site. The support material is intrinsically linked to electronic structure and reactivity of heterogeneous organometallic systems in the same way that ligands exert control over homogeneous catalyst systems. The ability to tune the reactivity of heterogeneous catalysts by changing the support is of great value, and it is anticipated that this will represent an area of significant growth in the field. In this Perspective, the use and future of nontraditional catalyst supports, such as sulfated metal oxides, modified silicas, and redox active supports are discussed.
The scope and mechanism of the microenvironment-catalyzed C(sp3)−C(sp3) reductive elimination from transition metal complexes Au(III), Pt(IV) is explored. Experiments detailing the effect of ...structural perturbation of neutral and anionic spectator ligands, reactive alkyl ligands, solvent, and catalyst structure are disclosed. Indirect evidence for a coordinatively unsaturated encapsulated cationic intermediate is garnered via observation of several inactive donor-arrested inclusion complexes, including a crystallographically characterized encapsulated Au(III) cation. Finally, based on stoichiometric experiments under catalytically relevant conditions, a detailed mechanism is outlined for the dual supramolecular and platinum-catalyzed C–C coupling between methyl iodide and tetramethyltin. Determination of major platinum species present under catalytic conditions and subsequent investigation of their chemistry reveals an unexpected interplay between cis–trans isomerism and the supramolecular catalyst in a Pt(II)/Pt(IV) cycle, as well as several off-cycle reactions.
Heteroleptic copper(
i
) bis(phenanthroline) complexes with surface anchoring carboxylate groups have been synthesized and immobilized on nanoporous metal oxide substrates. The species investigated ...are responsive to the external environment and this work provides a new strategy to control charge transfer processes for efficient solar energy conversion.
The ground- and excited-state properties of redox- and photo-active Cu(
i
) complexes immobilized in nanoporous AAO are responsive to pore diameter.
Isolated Pd atoms supported on high surface area MnO2, prepared by the oxidative grafting of (bis(tricyclohexylphosphine-palladium(0)), catalyze (>50 turnovers, 17 h) the low temperature (≤325 K) ...oxidation of CO (7.7 kPa O2, 2.6 kPa CO) with results of in situ/operando and ex situ spectroscopic characterization signifying a synergistic role of Pd and MnO2 in facilitating redox turnovers.
Isolated Pd atoms supported on high surface area MnO
, prepared by the oxidative grafting of (bis(tricyclohexylphosphine-palladium(0)), catalyze (>50 turnovers, 17 h) the low temperature (≤325 K) ...oxidation of CO (7.7 kPa O
, 2.6 kPa CO) with results of
/
and
spectroscopic characterization signifying a synergistic role of Pd and MnO
in facilitating redox turnovers.
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