Correlating Thermodynamic and Kinetic Hydricities of Rhenium Hydrides

Molecular catalyst and co-catalyst systems based on transition metal complexes for the electrochemical oxidation of alcohols

Molecular catalysts allow deeper study of underlying mechanisms relative to heterogeneous systems by offering a discrete active site to monitor. Mechanistic study with knowledge of key intermediates subsequently enables the development of design principles through an understanding of how improved reactivity or selectivity can be achieved through modification of the catalyst structure. The co-catalytic inclusion of redox mediators (RM), which are small molecules that can aid in the transfer of protons and electrons, has been shown to improve product conversion and selectivity in many molecular systems, through intercepting key intermediates to direct reaction pathways. The primary focus for the majority of molecular electrocatalysts has been on optimizing design for reductive reactions, such as the hydrogen evolution reaction (HER), the oxygen reduction reaction (ORR), and the carbon dioxide reduction reaction (CO2RR). By comparison, there has been much less focus on key oxidative reactions by molecular species, apart from the oxygen evolution reaction (OER). The focus of this review is to highlight molecular catalyst systems optimized for the electrochemical oxidation of alcohols. The electrochemical alcohol oxidation reaction (AOR) can serve a role in synthesizing value-added chemicals and can serve as the counterpart to the CO2RR by releasing electricity from energy-rich molecules. State-of-the-art molecular systems for the AOR are divided between single-site catalysts and co-catalytic systems with redox mediators. The AOR is contextualized as an energy relevant reaction, an overview of the area is provided, foundational improvements in catalyst systems are highlighted, and future development principles for incorporating redox mediators are suggested.

Reactivity of metal hydrides with CO2: going beyond formate with a high-valent cationic pentahydride Mo(vi) complex

The cationic molybdenum pentahydride complex [MoH5(depe)2]+ (depe = 1,2-bis(diethylphosphino)ethane) is shown to undergo two consecutive reactions with carbon dioxide. In the initial, room-temperature process, classical insertion of CO2 into a metal-hydride bond is observed, resulting in the formation of the expected formate complex, [MoH2(HCOO)(depe)2]+. Further reactivity is triggered at temperature above 100 °C. Complete conversion into two new complexes is indeed observed, resulting from the formal cleavage of a C-O bond of carbon dioxide, [MoH(CO)2(depe)2]+ and [MoO(HCOO)(depe)2]+. Unprecedented in the absence of ligand assistance, such metal hydride reactivity has been comprehensively studied by a combination of experimental and computational means with the aim of elucidating the underlying mechanism that governs this process.

Lewis Acid-Tethered (cAAC)-Copper Complexes: Reactivity for Hydride Transfer and Catalytic CO2 Hydrogenation

We present a series of borane-tethered cyclic (alkyl)(amino)carbene (cAAC)-copper complexes, including a borane-capped Cu(I) hydride. This hydride is unusually hydridic and reacts rapidly with both CO2 and 2,6-dimethylphenol at room temperature. Its reactivity is distinct from variants without a tethered borane, and the underlying principles governing the enhanced hydricity were evaluated experimentally and theoretically. These stoichiometric results were extended to catalytic CO2 hydrogenation, and the borane-tethered (intramolecular) system exhibits ~3-fold enhancement relative to an intermolecular system.

Linear Free Energy Relationships Associated with Hydride Transfer From [(6,6'-R2-bpy)Re(CO)3H]: A Cautionary Tale in Identifying Hydrogen Bonding Effects in the Secondary Coordination Sphere

Six rhenium hydride complexes, [(6,6'-R2-bpy)Re(CO)3H] (bpy = 2,2'-bipyridine, R = OEt, OMe, NHMe, Me, F, Br), were synthesized. These complexes insert CO2 to form rhenium formate complexes of the type [(6,6'-R2-bpy)Re(CO)3{OC(O)H}]. All the rhenium formate species were characterized using X-ray crystallography, which revealed that the bpy ligand is not coplanar with the metal coordination plane containing the two nitrogen donors of the bpy ligand but tilted. A solid-state structure of [(6,6'-Me2-bpy)Re(CO)3H] determined using MicroED also featured a tilted bpy ligand. The kinetics of CO2 insertion into complexes of the type [(6,6'-R2-bpy)Re(CO)3H] were measured experimentally and the thermodynamic hydricities of [(6,6'-R2-bpy)Re(CO)3H] species were determined using theoretical calculations. A Brønsted plot constructed using the experimentally determined rate constants for CO2 insertion and the calculated thermodynamic hydricities for [(6,6'-R2-bpy)Re(CO)3H] revealed a linear free energy relationship (LFER) between thermodynamic and kinetic hydricity. This LFER is different to the previously determined relationship for CO2 insertion into complexes of the type [(4,4'-R2-bpy)Re(CO)3H]. At a given thermodynamic hydricity, CO2 insertion is faster for complexes containing a 6,6'-substituted bpy ligand. This is likely in part due to the tilting observed for systems with 6,6'-substituted bpy ligands. Notably, the 6,6'-(NHMe)2-bpy ligand could in principle stabilize the transition state for CO2 insertion via hydrogen bonding. This work shows that if only the rate of CO2 insertion into [(6,6'-(NHMe)2-bpy)Re(CO)3H] is compared to [(4,4'-R2-bpy)Re(CO)3H] systems, the increase in rate could be easily attributed to hydrogen bonding, but in fact all 6,6'-substituted systems lead to faster than expected rates.

Kinetic Studies of CO2 Insertion into Metal-Element σ-Bonds

ConspectusDespite the plethora of metal catalyzed reactions for CO2 utilization that have been developed in academic laboratories, practical systems remain elusive. The understanding of the elementary steps in catalysis is a proven method to improve catalytic performance. In many catalytic cycles for CO2 utilization, the insertion of CO2 into a metal-element σ-bond, such as hydrides, alkyls, amides, or hydroxides, is a crucial step. However, despite the many demonstrations of CO2 insertion, there are a paucity of kinetic studies, and information about the reaction mechanism has been predominantly elucidated from computational investigations. In this Account, kinetic studies on CO2 insertion into late transition metal-element σ-bonds performed by my group are summarized, along with their implications for catalysis.A common pathway for CO2 insertion into a metal hydride involves a two-step mechanism. The first step is nucleophilic attack on CO2 by the hydride to generate an H-bound formate, followed by rearrangement to form an O-bound formate product. Kinetic studies on systems in which both the first and second steps are proposed to be rate-determining, known as inner-sphere and outer-sphere processes, respectively, show that insertion rates increase as (i) the ligand trans to the hydride becomes a stronger donor, (ii) the ancillary ligand becomes more electron-donating, and (iii) the Dimroth-Reichardt parameter of the solvent increases. However, the magnitude of these effects is generally smaller for inner-sphere processes because there is less buildup of charge in the key transition state. For similar reasons, the presence of Lewis acids only increases the rate of outer-sphere processes. These results suggest it may be possible to experimentally differentiate between inner- and outer-sphere processes.The insertion of CO2 into a metal-alkyl bond results in the formation of a C-C bond, which is important for the generation of fuels from CO2. For square planar Group 10 complexes, the presence of a strong donor ligand trans to the alkyl group is critical for kinetically promoting insertion. Further, the nucleophilicity of the alkyl ligand directly impacts the rate of CO2 insertion via an SE2 mechanism, as does the steric bulk of the complex, and the reaction solvent. In contrast to the relatively slow rates of insertion observed for metal alkyls, CO2 insertion is rapid for metal hydroxides and amides. Although kinetics trends could be determined for hydroxides, reactions with amides are too fast for quantitative studies.Overall, the rates of insertion correlate with the nucleophilicity of the element in the metal-element σ-bond, so amide > hydroxide > hydride > alkyl. Due to the related pathways for insertion, similar trends in ligand and solvent effects are observed for insertion into different metal-element σ-bonds. Thus, the same strategies can be used to control the rates of insertion across systems. Differences in the magnitude of solvent and ligand effects are caused by variation in the amount of charge build-up on the metal in the rate-determining transition state. Likely, given that CO2 is related to organic molecules such as aldehydes, ketones, and amides, the results described in this Account are general to a wider range of substrates.

Electrocatalytic formate and alcohol oxidation by hydride transfer at first-row transition metal complexes

The electrocatalytic oxidation of carbon-based liquid fuels, such as formic acid and alcohols, has important applications for our renewable energy transition. Molecular electrocatalysts based on transition metal complexes provide the opportunity to explore the interplay between precise catalyst design and electrocatalytic activity. Recent advances have seen the development of first-row transition metal electrocatalysts for these transformations that operate via hydride transfer between the substrate and catalyst. In this Frontier article, we present the key contributions to this field and discuss the proposed mechanisms for each case. These studies also reveal the remaining challenges for formate and alcohol oxidation with first-row transition metal systems, for which we provide perspectives on future directions for next-generation electrocatalyst design.

Kinetic Isotope Effects and the Mechanism of CO2 Insertion into the Metal-Hydride Bond of fac-(bpy)Re(CO)3H

The 1,2-insertion reaction of CO2 into metal-hydride bonds of d6-octahedral complexes to give κ1-O-metal-formate products is the key step in various CO2 reduction schemes and as a result has attracted extensive mechanistic investigations. For many octahedral catalysts, CO2 insertion follows an associative mechanism in which CO2 interacts directly with the coordinated hydride ligand instead of the more classical dissociative mechanism that opens an empty coordination site to bind the substrate to the metal prior to a hydride migration step. To better understand the associative mechanism, we conducted a systematic quantum chemical investigation on the reaction between CO2 and fac-(bpy)Re(CO)3H (1-Re-H; bpy = 2,2'-bipyridine) starting with the gas phase and then moving to THF and other solvents with increased dielectric constants. Detailed analyses of the potential energy surfaces (PESs) and intrinsic reaction coordinates (IRCs) reveal that the reaction is enabled in all media by an initial stage of making a 3c-2e bond between the carbon of CO2 and the metal-hydride bond that is most consistent with an organometallic bridging hydride Re-H-CO2 species. Once CO2 is bent and anchored to the metal-hydride bond, the reaction proceeds by a rotation motion via a cyclic transition state TS2 that interchanges Re-H-CO2 and Re-O-CHO coordination. The combined stages provide an asynchronous-concerted pathway for CO2 insertion on the Gibbs free energy surface with TS2 as the highest energy point. Consideration of TS2 as a rate-determining TS gives activation barriers, inverse KIEs, substituent effects, and solvent effects that agree with the experimental data available in this system. An important new insight revealed by the analyses of the results is that the initial stage of the reaction is not a hydride transfer step as has been assumed in some studies. In fact, the loose vibration of the TS that can be identified for the first stage of the reaction in solution (TS1) does not involve the Re-H stretching vibrational mode. Accordingly, the imaginary frequency of TS1 is insensitive to deuteration, and therefore, TS1 leads to no significant KIE.

Reduction of CO to Methanol with Recyclable Organic Hydrides

The reaction steps for the selective conversion of a transition metal carbonyl complex to a hydroxymethyl complex that releases methanol upon irradiation with visible light have been successfully quantified in acetonitrile solution with dihydrobenzimidazole organic hydride reductants. Dihydrobenzimidazole reductants have been shown to be inactive toward H2 generation in the presence of a wide range of proton sources and have been regenerated electrochemically or photochemically. Specifically, the reaction of cis-[Ru(bpy)2(CO)2]2+ (bpy = 2,2'-bipyridine) with one equivalent of a dihydrobenzimidazole quantitatively yields a formyl complex, cis-[Ru(bpy)2(CO)(CHO)]+, and the corresponding benzimidazolium on a seconds time scale. Kinetic experiments revealed a first-order dependence on the benzimidazole hydride concentration and an unusually large kinetic isotope effect, inconsistent with direct hydride transfer and more likely to occur by an electron transfer-proton-coupled electron transfer (EΤ-PCET) or related mechanism. Further reduction/protonation of cis-[Ru(bpy)2(CO)(CHO)]+ with two equivalents of the organic hydride yields the hydroxymethyl complex cis-[Ru(bpy)2(CO)(CH2OH)]+. Visible light excitation of cis-[Ru(bpy)2(CO)(CH2OH)]+ in the presence of excess organic hydride was shown to yield free methanol. Identification and quantification of methanol as the sole CO reduction product was confirmed by 1H NMR spectroscopy and gas chromatography. The high selectivity and mild reaction conditions suggest a viable approach for methanol production from CO, and from CO2 through cascade catalysis, with renewable organic hydrides that bear similarities to Nature's NADPH/NADP+.

Thermodynamic and Kinetic Activity Descriptors for the Catalytic Hydrogenation of Ketones

Activity descriptors are a powerful tool for the design of catalysts that can efficiently utilize H2 with minimal energy losses. In this study, we develop the use of hydricity and H- self-exchange rates as thermodynamic and kinetic descriptors for the hydrogenation of ketones by molecular catalysts. Two complexes with known hydricity, HRh(dmpe)2 and HCo(dmpe)2, were investigated for the catalytic hydrogenation of ketones under mild conditions (1.5 atm and 25 °C). The rhodium catalyst proved to be an efficient catalyst for a wide range of ketones, whereas the cobalt catalyst could only hydrogenate electron-deficient ketones. Using a combination of experiment and electronic structure theory, thermodynamic hydricity values were established for 46 alkoxide/ketone pairs in both acetonitrile and tetrahydrofuran solvents. Through comparison of the hydricities of the catalysts and substrates, it was determined that catalysis was observed only for catalyst/ketone pairs with an exergonic H- transfer step. Mechanistic studies revealed that H- transfer was the rate-limiting step for catalysis, allowing for the experimental and computation construction of linear free-energy relationships (LFERs) for H- transfer. Further analysis revealed that the LFERs could be reproduced using Marcus theory, in which the H- self-exchange rates for the HRh/Rh+ and ketone/alkoxide pairs were used to predict the experimentally measured catalytic barriers within 2 kcal mol-1. These studies significantly expand the scope of catalytic reactions that can be analyzed with a thermodynamic hydricity descriptor and firmly establish Marcus theory as a valid approach to develop kinetic descriptors for designing catalysts for H- transfer reactions.

Theory-guided development of homogeneous catalysts for the reduction of CO2 to formate, formaldehyde, and methanol derivatives

The stepwise catalytic reduction of carbon dioxide (CO2) to formic acid, formaldehyde, and methanol opens non-fossil pathways to important platform chemicals. The present article aims at identifying molecular control parameters to steer the selectivity to the three distinct reduction levels using organometallic catalysts of earth-abundant first-row metals. A linear scaling relationship was developed to map the intrinsic reactivity of 3d transition metal pincer complexes to their activity and selectivity in CO2 hydrosilylation. The hydride affinity of the catalysts was used as a descriptor to predict activity/selectivity trends in a composite volcano picture, and the outstanding properties of cobalt complexes bearing bis(phosphino)triazine PNP-type pincer ligands to reach the three reduction levels selectively under different reaction conditions could thus be rationalized. The implications of the composite volcano picture were successfully experimentally validated with selected catalysts, and the challenging intermediate level of formaldehyde could be accessed in over 80% yield with the cobalt complex 6. The results underpin the potential of tandem computational-experimental approaches to propel catalyst design for CO2-based chemical transformations.

Synthesis and Surface Attachment of Molecular Re(I) Complexes Supported by Functionalized Bipyridyl Ligands

Eleven 2,2'-bipyridine (bpy) ligands functionalized with attachment groups for covalent immobilization on silicon surfaces were prepared. Five of the ligands feature silatrane functional groups for attachment to metal oxide coatings on the silicon surfaces, while six contain either alkene or alkyne functional groups for attachment to hydrogen-terminated silicon surfaces. The bpy ligands were coordinated to Re(CO)₅Cl to form complexes of the type Re(bpy)(CO)₃Cl, which are related to known catalysts for CO₂ reduction. Six of the new complexes were characterized using X-ray crystallography. As proof of principle, four molecular Re complexes were immobilized on either a thin layer of TiO₂ on silicon or hydrogen-terminated silicon. The surface-immobilized complexes were characterized using X-ray photoelectron spectroscopy, IR spectroscopy, and cyclic voltammetry (CV) in the dark and for one representative example in the light. The CO stretching frequencies of the attached complexes were similar to those of the pure molecular complexes, but the CVs were less analogous. For two of the complexes, comparison of the electrocatalytic CO₂ reduction performance showed lower CO Faradaic efficiencies for the immobilized complexes than the same complex in solution under similar conditions. In particular, a complex containing a silatrane linked to bpy with an amide linker showed poor catalytic performance and control experiments suggest that amide linkers in conjugation with a redox-active ligand are not stable under highly reducing conditions and alkyl linkers are more stable. A conclusion of this work is that understanding the behavior of molecular Re catalysts attached to semiconducting silicon is more complicated than related complexes, which have previously been immobilized on metallic electrodes.

[ReH3 (PPh3 )4 ] - A Key Compound in the Rhenium Hydride Chemistry

The chemistry of the rhenium trihydrido complex [ReH₃ (PPh₃ )₄ ] (1) has been reinvestigated. An improved synthesis and the solid-state structure of the compound as well as several reactions are reported. The solid-state structure of 1 is similar to that of [TcH₃ (PPh₃ )₄ ] having a capped-octahedral coordination sphere. The PPh₃ ligands surround the Re atom in a trigonal-pyramidal mode with a short apical Re-P bond (2.300(2) Å) and three longer basal bonds (2.429(2)-2.449(2) Å). Reactions of 1 with monodentate phosphines such as PMe₃ or PBu₃ give the mono-substituted complexes [ReH₃ (PPh₃ )₃ (PMe₃ )] (2) and [ReH₃ (PPh₃ )₃ (PBu₃ )] (3) under retention of the apical PPh₃ ligand and substitution of one of the basal PPh₃ ligands. The stability of the phosphine trihydride complexes decreases in the order PPh₃ >PMe₃ >PBu₃ . Treatment of [ReH₃ (PPh₃ )₄ ] with trityl hexafluorophosphate in CH₃ CN does not result in a hydride abstraction, but gives the tetrahydrido cation [ReH₄ (NCCH₃ )(PPh₃ )₃ ]⁺ (4), while reactions with nitriles give unstable azavinylidene complexes of the composition [ReH₂ (PPh₃ )₃ (NC(H)R)] (5). They are formed by an insertion of the nitrile into a Re-H bond. The solid-state structure of the methyl derivative [ReH₂ (PPh₃ )₃ (NC(H)CH₃ )] (5 a) was determined showing a linear Re-N-C unit with rhenium-nitrogen and nitrogen-carbon double bonds, while the N=CH-C bond is clearly bent with an angle of 124°. Two previously unknown polymorphs of [ReH₅ (PPh₃ )₃ ] were isolated from reactions of 1 with HOC₆ H₃ (CH₃ )₂ and thiourea after prolonged heating in toluene and characterized by IR spectroscopy and X-ray diffraction.