Preparation, Characterization, and Electrochemical Activation of a Model [Cp*Rh] Hydride

Assessment of the applicability of DFT methods to [Cp*Rh]-catalyzed hydrogen evolution processes

The present computational study provides a benchmark of density functional theory (DFT) methods in describing hydrogen evolution processes catalyzed by [Cp*Rh]-containing organometallic complexes. A test set was composed of 26 elementary reactions featuring chemical transformations and bonding situations essential for the field, including the emerging concept of non-innocent Cp* behavior. Reference values were obtained from a highly accurate 3/4 complete basis set and 6/7 complete PNO space extrapolated DLPNO-CCSD(T) energies. The performance of lower-level extrapolation procedures was also assessed. We considered 84 density functionals (DF) (including 13 generalized gradient approximations (GGA), nine meta-GGAs, 33 hybrids, and 29 double-hybrids) and three composite methods (HF-3c, PBEh-3c, and r2SCAN-3c), combined with different types of dispersion corrections (D3(0), D3BJ, D4, and VV10). The most accurate approach is the PBE0-DH-D3BJ (MAD of 1.36 kcal mol-1) followed by TPSS0-D3BJ (MAD of 1.60 kcal mol-1). Low-cost r2SCAN-3c composite provides a less accurate but much faster alternative (MAD of 2.39 kcal mol-1). The widely used Minnesota-family M06-L, M06, and M06-2X DFs should be avoided (MADs of 3.70, 3.94, and 4.01 kcal mol-1, respectively).

Protonation of Dppbz- and Binap-Ligated Rhodathiaboranes Yielding Hydron, Hydride, and Hydrogen Exchange

The reaction of the 11-vertex rhodathiaborane [8,8,8-(H)(PPh3)2-3-(NC5H5)-nido-7,8-RhSB9H10] (1) with 1,2-bis(diphenylphosphine)benzene (dppbz) and (S)-(-)-2,2'-bis(diphenylphosphino)-1,1'-binaphthyl (binap) affords [1,1-(η2-dppbz)-3-(NC5H5)-closo-1,2-RhSB9H8] (2) and [1,1-(η2-binap)-3-(NC5H5)-closo-1,2-RhSB9H8] (3). These 11-vertex closo-rhodathiaborane chelates result from PPh3 ligand substitution at the rhodium center and a nido-to-closo structural cluster transformation driven by H2 loss. Treating compounds 2 and 3 with triflic acid (TfOH) leads to the formation of cationic clusters 4 and 5. The hydrons bind to the polyhedral clusters, acquiring hydride character and providing chemical nonrigidity that manifests through metal vertex-to-thiaborane pseudo-rotations and concomitant hydron tautomerisms. The resulting cations react with hydrogen to form mixtures of hydrons, hydrogen, and hydridorhodathiaboranes in equilibrium. The reaction products are the result of heterolytic cleavage of the H-H bond, with full participation of the clusters and the addition of hydrogen atoms to the cages. In these reactions, there is a conversion of electrons and hydrons into hydrogen, and hydrogen into hydride ligands, demonstrating that these boron-based metal compounds act as electron reservoirs, capable of promoting multielectron processes.

Isolation, Characterization and Reactivity of Key Intermediates Relevant to Reductive (Electro)catalysis with Cp*Rh Complexes Containing Pyridyl-MIC (MIC=Mesoionic Carbene) Ligands

In recent years, metal complexes of pyridyl-mesoionic carbene (MIC) ligands have been reported as excellent homogeneous and molecular electrocatalysts. In combination with group 9 metals, such ligands form highly active catalysts for hydrogenation/transfer hydrogenation/hydrosilylation catalysis and electrocatalysts for dihydrogen production. Despite such progress, very little is known about the structural/electrochemical/spectroscopic properties of crucial intermediates for such catalytic reactions with these ligands: solvato complexes, reduced complexes and hydridic species. We present here a comprehensive study involving the isolation, crystallographic characterization, electrochemical/spectroelectrochemical/theoretical investigations, and in-situ reactivity studies of all the aforementioned crucial intermediates involving Cp*Rh and pyridyl-MIC ligands. A detailed mechanistic study of the precatalytic activation of [RhCp*] complexes with pyridyl-MIC ligands is presented. Intriguingly, amphiphilicity of the [RhCp*]-hydride complexes was observed, displaying the substrate dependent transfer of H+ , H or H- . To the best of our knowledge, this study is the first of its kind targeting intermediates and reactive species involving metal complexes of pyridyl-MIC ligands and investigating the interconversion amongst them.

Computational Insights into the Influence of Ligands on Hydrogen Generation with [Cp*Rh] Hydrides

This work reports a computational investigation of the effect of ancillary ligands on the activity of an Rh catalyst for hydrogen evolution based on the [Cp*Rh] motif (Cp* = η⁵-pentamethylcyclopentadienyl). Specifically, we investigate why a bipyridyl (bpy) ligand leads to H₂ generation but diphenylphosphino-based (dpp) ligands do not. We compare the full ligands to simplified models and systematically vary structural features to ascertain their effect on the reaction energy of each catalytic step. The calculations based on density functional theory show that the main effect on reactivity is the choice of linker atom, followed by its coordination. In particular, P stabilizes the intermediate Rh-hydride species by donating electron density to the Rh, thus inhibiting the reaction toward H₂ generation. Conversely, N, a more electron-withdrawing center, favors H₂ generation at the price of destabilizing the hydride intermediate, which cannot be isolated experimentally and makes determining the mechanism of this reaction more difficult. We also find that the steric effects of bulky substituents on the main ligand scaffold can lead to large effects on the reactivity, which may be challenging to fine-tune. On the other hand, structural features like the bite angle of the bidentate ligand have a much smaller impact on reactivity. Therefore, we propose that the choice of linker atom is key for the catalytic activity of this species, which can be further fine-tuned by a proper choice of electron-directing groups on the ligand scaffold.

Mechanistic roles of metal- and ligand-protonated species in hydrogen evolution with [Cp*Rh] complexes

Protonation reactions involving organometallic complexes are ubiquitous in redox chemistry and often result in the generation of reactive metal hydrides. However, some organometallic species supported by η5-pentamethylcyclopentadienyl (Cp*) ligands have recently been shown to undergo ligand-centered protonation by direct proton transfer from acids or tautomerization of metal hydrides, resulting in the generation of complexes bearing the uncommon η4-pentamethylcyclopentadiene (Cp*H) ligand. Here, time-resolved pulse radiolysis (PR) and stopped-flow spectroscopic studies have been applied to examine the kinetics and atomistic details involved in the elementary electron- and proton-transfer steps leading to complexes ligated by Cp*H, using Cp*Rh(bpy) as a molecular model (where bpy is 2,2'-bipyridyl). Stopped-flow measurements coupled with infrared and UV-visible detection reveal that the sole product of initial protonation of Cp*Rh(bpy) is [Cp*Rh(H)(bpy)]+, an elusive hydride complex that has been spectroscopically and kinetically characterized here. Tautomerization of the hydride leads to the clean formation of [(Cp*H)Rh(bpy)]+. Variable-temperature and isotopic labeling experiments further confirm this assignment, providing experimental activation parameters and mechanistic insight into metal-mediated hydride-to-proton tautomerism. Spectroscopic monitoring of the second proton transfer event reveals that both the hydride and related Cp*H complex can be involved in further reactivity, showing that [(Cp*H)Rh] is not necessarily an off-cycle intermediate, but, instead, depending on the strength of the acid used to drive catalysis, an active participant in hydrogen evolution. Identification of the mechanistic roles of the protonated intermediates in the catalysis studied here could inform design of optimized catalytic systems supported by noninnocent cyclopentadienyl-type ligands.

Rhodium(III)-Catalyzed Anti-Markovnikov Hydroamidation of Unactivated Alkenes Using Dioxazolones as Amidating Reagents

The amide is one of the most prevalent functional groups in all of pharmaceuticals, and for this reason, reactions that introduce the amide moiety are of particular value. Intermolecular hydroamidation of alkenes remains an underexplored method for the synthesis of amide-containing compounds. The majority of hydroamidation procedures exhibit Markovnikov regioselectivity, while current methods for anti-Markovnikov hydroamidation are somewhat limited to activated alkene substrates or radical processes. Herein, we report a general method for the intermolecular anti-Markovnikov hydroamidation of unactivated alkenes under mild conditions, utilizing Rh(III) catalysis in conjunction with dioxazolone amidating reagents and isopropanol as an environmentally friendly hydride source. The reaction tolerates a wide range of functional groups and efficiently converts electron-deficient alkenes, styrenes, and 1,1-disubstituted alkenes, in addition to unactivated alkenes, to their corresponding linear amides. Mechanistic studies reveal a reversible rhodium hydride migratory insertion step, leading to exquisite selectivity for the anti-Markovnikov product.

Efficient Nicotinamide Adenine Dinucleotide Regeneration with a Rhodium-Carbene Catalyst and Isolation of a Hydride Intermediate

Regeneration of nicotinamide adenine dinucleotide (NADH) has been the primary interest in the field of enzymatic transformation, especially associating oxidoreductases given the stoichiometric consumption. The synthesized carbene-ligated rhodium complex [(η⁵-Cp*)Rh(MDI)Cl]⁺ [Cp* = pentamethylcyclopentadienyl; MDI = 1,1'-methylenebis(3,3'-dimethylimidazolium)] acts as an exceptional catalyst in the reduction of NAD⁺ to NADH with a turnover frequency of 1730 h^-1, which is over twice that of the higher catalytic activity of the commercially available catalyst [Cp*Rh(bpy)Cl]⁺ (bpy = 2,2'-bipyridine). Offsetting the contentious atmosphere currently taking place over the specific intermediate of the NADH regeneration, this study presents pivotal evidence of a metal hydride intermediate with a bis(carbene) ligand: a stable form of the rhodium hydride intermediate, [(η⁵-Cp*)Rh(MDI)H]⁺, was isolated and fully characterized. This enables thorough insight into the possible mechanism and exact intermediate structure in the NAD⁺ reduction process.

Remote Oxidative Activation of a [Cp*Rh] Monohydride

Half-sandwich rhodium monohydrides are often proposed as intermediates in catalysis, but little is known regarding the redox-induced reactivity accessible to these species. Herein, the bis(diphenylphosphino)ferrocene (dppf) ligand has been used to explore the reactivity that can be induced when a [Cp*Rh] monohydride undergoes remote (dppf-centered) oxidation by 1e^- . Chemical and electrochemical studies show that one-electron redox chemistry is accessible to Cp*Rh(dppf), including a unique quasi-reversible Rh^II/I process at -0.96 V vs. ferrocenium/ferrocene (Fc^+/0 ). This redox manifold was confirmed by isolation of an uncommon Rh^II species, [Cp*Rh(dppf)]⁺ , that was characterized by electron paramagnetic resonance (EPR) spectroscopy. Protonation of Cp*Rh(dppf) with anilinium triflate yielded an isolable and inert monohydride, [Cp*Rh(dppf)H]⁺ , and this species was found to undergo a quasireversible electrochemical oxidation at +0.41 V vs. Fc^+/0 that corresponds to iron-centered oxidation in the dppf backbone. Thermochemical analysis predicts that this dppf-centered oxidation drives a dramatic increase in acidity of the Rh-H moiety by 23 pK_a units, a reactivity pattern confirmed by in situ ¹ H NMR studies. Taken together, these results show that remote oxidation can effectively induce M-H activation and suggest that ligand-centered redox activity could be an attractive feature for the design of new systems relying on hydride intermediates.

Evidence for Charge Delocalization in Diazafluorene Ligands Supporting Low-Valent [Cp*Rh] Complexes

Ligands based upon the 4,5-diazafluorene core are an important class of emerging ligands in organometallic chemistry, but the structure and electronic properties of these ligands have received less attention than they deserve. Here, we show that 9,9'-dimethyl-4,5-diazafluorene (Me₂ daf) can stabilize low-valent complexes through charge delocalization into its conjugated π-system. Using a new platform of [Cp*Rh] complexes with three accessible formal oxidation states (+III, +II, and +I), we show that the methylation in Me₂ daf is protective, blocking Brønsted acid-base chemistry commonly encountered with other daf-based ligands. Electronic absorption spectroscopy and single-crystal X-ray diffraction analysis of a family of eleven new compounds, including the unusual Cp*Rh(Me₂ daf), reveal features consistent with charge delocalization driven by π-backbonding into the LUMO of Me₂ daf, reminiscent of behavior displayed by the workhorse 2,2'-bipyridyl ligand. Taken together with spectrochemical data demonstrating clean conversion between oxidation states, our findings show that 9,9'-dialkylated daf-type ligands are promising building blocks for applications in reductive chemistry and catalysis.

Incorporation of [Cp*Rh] and [Cp*Ir] Species into Heterobimetallic Complexes via Protonolysis Reactivity and Dioximato Chelation

The synthesis of multimetallic compounds can enable the placement of two or more metals in close proximity, but efforts in this area are often hindered by reagent incompatibilities and a lack of selectivity. Here, we show that organometallic half-sandwich [Cp*M] (M = Rh, Ir) fragments (where Cp* is η⁵-pentamethylcyclopentadienyl) can be cleanly installed into metallomacrocyclic structures based on the workhorse diimine-monooxime-monooximato ligand system. Six new heterobimetallic compounds have been prepared to explore this synthetic chemistry, which relies on in situ protonolysis reactivity with precursor Ni(II) or Co(III) monometallic complexes in the presence of suitable [Cp*M] species. Solid-state X-ray diffraction studies confirm installation of the [Cp*M] fragments into the metallomacrocycles via effective chelation of the Rh(III) and Ir(III) centers by the nascent dioximato site. Contrasting with square-planar Ni(II) centers, the Co(III) centers prefer octahedral geometry in the heterobimetallic compounds, promoting bridging ligation of acetate across the two metals. Spectroscopic and electrochemical studies reveal subtle influences of the metals on each other's properties, consistent with the moderate M'···M distances of ca. 3.6-3.7 Å in the modular compounds. Taken together, our results show that heterobimetallic complexes can be assembled with organometallic [Cp*M] fragments on the diimine-dioximato platform.

Immobilization of Rh(I) precursor in a porphyrin metal-organic framework - turning on the catalytic activity

Two model porphyrin metal-organic frameworks were used for the incorporation of Rh(i) species by a post-synthetic metallation under mild conditions. As a result, new rhodium MOFs (Rh/MOFs), Rh/PCN-222 and Rh/NU-1102, were synthesized and structurally characterized. To illustrate the potential of this catalytic platform, we use Rh/MOFs as phosphine-free heterogeneous catalysts in the hydrogenation of unsaturated hydrocarbons under mild reaction conditions (30 °C and 1 atm H2). We found that for our Rh/MOFs an activation step is required during the first run of the catalytic process. The presence of Rh-CO moieties allowed us to monitor the activation pathway of the catalyst under a H2 atmosphere, by in situ Diffuse Reflectance Infrared Fourier Transform Spectroscopy (DRIFTS). After activation, the catalyst remains highly active during the subsequent catalytic cycles. This simple post-synthetic modification approach presents new possibilities for the utilization of Rh-based catalytic systems with robust porphyrin-based MOFs as supports.

Metal-ligand cooperative κ1-N-pyrazolate Cp*RhIII-catalysts for dehydrogenation of dimethylamine-borane at room temperature

3,5-Dimethylpyrazole (Pz*H) in well-defined Cp*RhIII (Cp* = 1,2,3,4,5-pentamethylcyclopentadienyl) complexes, or as an additive to [Cp*RhCl2]2 enhances catalytic activity in the dehydrogenation of dimethylamine-borane (DMAB) at room-temperature. Mechanistic studies indicate that the Lewis acidic RhIII-centre and dangling N-atom of the Pz* fragment operate cooperatively in accepting a hydride and proton from DMAB, respectively, leading directly to dimethylamino-borane and a RhIII-H complex. The rate limiting step involves protonation of the RhIII-H by the proximal NH fragment of the Pz*H moiety.

Systematic Trends in the Electrochemical Properties of Transition Metal Hydride Complexes Discovered by Using the Ligand Acidity Constant Equation

Understanding the thermodynamics of paramagnetic transition metal hydride complexes, especially of the abundant 3d metals, is important in the design of electrocatalysts and organometallic catalysts. The pK_a^MeCN([MHL_n]⁺/[ML_n) of paramagnetic hydrides in MeCN are estimated for the first time using the ligand acidity constant (LAC) equation where contributions to the pK_a^MeCN from each ligand are simply added together, with the sum corrected for effects of charge and 5d metals. The pK_a^LAC-MeCN([MHL_n]⁺/ML_n) of over 200 hydride complexes MHL_n are used, along with their electrochemical potentials from the literature, in an uncommonly applied thermochemical cycle in order to reveal systematic trends in the redox couples M^III/II and M^V/IV (M = Cr, Mo, W), Mn^II/I, Re^VI/V and Re^IV/III, M^III/II and M^IV/III (M = Fe, Ru, Os), and M^III/II and M^II/I (M = Co, Rh, and Ir) and allow the estimation of the bond dissociation free energies BDFE(MH) of the unoxidized hydrides MHL_n and the prediction of the electrochemical potential for their oxidation. Density functional theory (DFT) calculations are used to validate the pK_a^LAC-MeCN values of hydrides of W^III, Mn^II, Fe^III, Ru^III, Co^II, and Ni^III. When a pK_a^LAC-MeCN is less than zero for a given complex [MHL_n]⁺, the oxidation of MHL_n is irreversible due to proton loss from the oxidized complex to the solvent. When pK_a^LAC-MeCN ≫ 0, the oxidation is reversible when there is no gross change in the coordination geometry upon a change in the redox state. Twenty paramagnetic hydrides prepared in bulk all have pK_a^LAC-MeCN > 8.

Synthesis and Reactivity Studies of a [Cp*Rh] Complex Supported by a Methylene-Bridged Hybrid Phosphine-Imine Ligand

[Cp*Rh] complexes (Cp* = η ⁵-pentamethylcyclopentadienyl) supported by bidentate chelating ligands are useful in studies of redox chemistry and catalysis, but little information is available for derivatives bearing "hybrid" [P,N] chelates. Here, the preparation, structural characterization, and chemical and electrochemical properties of a [Cp*Rh] complex bearing the κ²-[P,N]-2-[(diphenylphosphino)methyl]pyridine ligand (PN) are reported. Cyclic voltammetry data reveal that [Cp*Rh(PN)Cl]PF₆ (1) undergoes a chemically reversible, net two-electron reduction at -1.28 V vs. ferrocenium/ferrocene, resulting in generation of a rhodium(I) complex (3) that is stable on the timescale of the voltammetry. However, ¹H and ³¹P{¹H} NMR studies reveal that chemical reduction of 1 generates a mixture of products over a 1 h timescale; this mixture forms as a result of deprotonation of the methylene group of 1 by 3 followed by further reactivity. The analogous complex [Cp*Rh(PQN)Cl]PF₆ (2; PQN = κ²-[P,N]-8-(diphenylphosphino)quinoline) does not undergo self-deprotonation or further reactivity upon two-electron reduction, confirming the reactivity of the acidic backbone methylene C-H bonds in the PN complexes. Comparison of the electrochemical properties 1 and 2 also shows that the extended conjugated system of PQN contributes to an additional ligand-centered redox event for 2 that is absent for 1.

Structural and chemical properties of half-sandwich rhodium complexes supported by the bis(2-pyridyl)methane ligand

[Cp*Rh] complexes (Cp* = pentamethylcyclopentadienyl) supported by bidentate chelating ligands are a useful class of compounds for studies of redox chemistry and catalysis. Here, we show that the bis(2-pyridyl)methane ligand, also known as dipyridylmethane or dpma, can support [Cp*Rh] complexes in the formally +iii and +ii rhodium oxidation states. Specifically, two new rhodium complexes ([Cp*Rh(dpma)(L)]ⁿ⁺, L = Cl^-, CH₃CN) have been isolated and structurally characterized, and the properties of the complexes have been compared with those of [Cp*Rh] complexes bearing the related dimethyldipyridylmethane (Me₂dpma) ligand. Complex [Cp*Rh(dpma)(NCCH₃)]²⁺ displays a quasireversible rhodium(iii/ii) reduction by cyclic voltammetry; related electron paramagnetic resonance (EPR) spectroscopic studies confirm access to the unusual rhodium(ii) oxidation state. Further reduction to the formally rhodium(i) oxidation state, however, is followed by deprotonation of dpma, as observed in electrochemical studies and chemical reduction experiments. This reactivity can be understood to occur as a consequence of the presence of doubly benzylic protons in the dpma ligand, since use of the analogous Me₂dpma enables reduction to rhodium(i) without involvement of ligand deprotonation. These findings highlight the important role of the ligand backbone substitution pattern in influencing the stability of highly-reduced complexes, a key class of metal species for study of electron and proton management in catalysis.

Single-Electron Redox Chemistry on the [Cp*Rh] Platform Enabled by a Nitrated Bipyridyl Ligand

[Cp*Rh] complexes (Cp* = pentamethylcyclopentadienyl) are attracting renewed interest in coordination chemistry and catalysis, but these useful compounds often undergo net two-electron redox cycling that precludes observation of individual one-electron reduction events. Here, we show that a [Cp*Rh] complex bearing the 4,4'-dinitro-2,2'-bipyridyl ligand (dnbpy) (3) can access a distinctive manifold of five oxidation states in organic electrolytes, contrasting with prior work that found no accessible reductions in aqueous electrolyte. These states are readily generated from a newly isolated and fully characterized rhodium(III) precursor complex 3, formulated as [Cp*Rh(dnbpy)Cl]PF₆. Single-crystal X-ray diffraction (XRD) data, previously unavailable for the dnbpy ligand bound to the [Cp*Rh] platform, confirm the presence of both [η⁵-Cp*] and [κ²-dnbpy]. Four individual one-electron reductions of 3 are observed, contrasting sharply with the single two-electron reductions of other [Cp*Rh] complexes. Chemical preparation and the study of the singly reduced species with electronic absorption and electron paramagnetic resonance spectroscopies indicate that the first reduction is predominantly centered on the dnbpy ligand. Comparative cyclic voltammetry studies with [NBu₄][PF₆] and [NBu₄][Cl] as supporting electrolytes indicate that the chloride ligand can be lost from 3 by ligand exchange upon reduction. Spectroelectrochemical studies with ultraviolet (UV)-visible detection reveal isosbestic behavior, confirming the clean interconversion of the reduced forms of 3 inferred from the voltammetry with [NBu₄][PF₆] as supporting electrolyte. Electrochemical reduction in the presence of triethylammonium results in an irreversible response, but does not give rise to catalytic H₂ evolution, contrasting with the reactivity patterns observed in [Cp*Rh] complexes bearing bipyridyl ligands with less electron-withdrawing substituents.