Experimental and Theoretical Mechanistic Investigation on the Catalytic CO₂ Hydrogenation to Formate by a Carboxylate-Functionalized Bis(<i>N</i>‑heterocyclic carbene) Zwitterionic Iridium(I) Compound

The bis-imidazolium salt, 1,1-bis­(<i>N</i>-methylimidazolium) acetate bromide, is a convenient precursor for the synthesis of zwitterionic iridium­(I) [Ir­(cod)­{(MeIm)₂CHCOO}] and cationic iridium­(III) [IrH­(cod)­{(MeIm)₂CHCOO}]⁺ compounds (MeIm = 3-methylimidazol-2-yliden-1-yl) having a carboxylate-functionalized bis­(NHC) ligand. The [Ir­(cod)­{(MeIm)₂CHCOO}] compound catalyzes the hydrogenation of CO₂ to formate in water using NEt₃ as base, reaching turnover numbers of approximately 1500. Reactivity studies have shown that activation of the catalyst precursor involves the reaction with H₂ in a multistep process that under catalytic conditions results in the formation of a dihydrido iridium­(III) octahedral [IrH₂(H₂O)­{(MeIm)₂CHCOO}] species stabilized by the κ³-<i>C</i>,<i>C′</i>,<i>O</i> coordination of the ligand. DFT studies on the mechanism were carried out to elucidate two possible roles of the base. In the first one, NEt₃ neutralizes only the produced formic acid, whereas in the second it assists the proton transfer in heterolytic cleavage of the H₂ molecule. Although this base-involved mechanism is more favorable in that it exhibits a lower energy span for the overall reaction, the energy barrier obtained from kinetic experiments suggests that both mechanisms could be operative under the experimental reaction conditions

Zwitterionic Rhodium and Iridium Complexes Based on a Carboxylate Bridge-Functionalized Bis-N-heterocyclic Carbene Ligand: Synthesis, Structure, Dynamic Behavior, and Reactivity

A series of water-soluble zwitterionic complexes featuring a carboxylate bridge-functionalized bis-N-heterocyclic carbene ligand of formula [Cp*M^IIICl­{(MeIm)₂­CHCOO}] and [M^I(diene)­{(MeIm)₂­CHCOO}] (Cp* = 1,2,3,4,5-pentamethylcyclopentadienyl; M = Rh, Ir; MeIm = 3-methylimidazol-2-yliden-1-yl; diene = 1,5-cyclooctadiene (cod), norbornadiene (nbd)) were prepared from the salt [(MeImH)₂­CHCOO]Br and suitable metal precursor. The solid-state structure of both types of complexes shows a boat-shaped six-membered metallacycle derived of the κ²C,C′ coordination mode of the bis-NHC ligand. The uncoordinated carboxylate fragment is found at the <i>bowsprit</i> position in the Cp*M^III complexes, whereas in the M^I(diene) complexes it is at the <i>flagpole</i> position of the metallacycle. The complexes [Rh^I(diene)­{(MeIm)₂CHCOO}] (diene = cod, nbd) exist as two conformational isomers in dichloromethane, <i>bowsprit</i> and <i>flagpole</i>, that interconvert through the boat-to-boat inversion of the metallacycle. An inversion barrier of ∼17 kcal·mol^–1 was determined by two-dimensional exchange spectroscopy NMR measurements for [Rh^I(cod)­{(MeIm)₂­CHCOO}]. Reaction of zwitterionic Cp*M^III complexes with methyl triflate or tetrafluoroboric acid affords the cationic complexes [Cp*M^IIICl­{(MeIm)₂­CHCOOMe}]⁺ or [Cp*M^IIICl­{(MeIm)₂­CHCOOH}]⁺ (M = Rh, Ir) featuring carboxy and methoxycarbonyl functionalized methylene-bridged bis-NHC ligands, respectively. Similarly, complexes [M^I(diene)­{(MeIm)₂­CHCOOMe}]⁺ (M = Rh, Ir) were prepared by alkylation of the corresponding zwitterionic M^I(diene) complexes with methyl triflate. In contrast, reaction of [Ir^I(cod)­{(MeIm)₂­CHCOO}] with HBF₄·Et₂O (Et = ethyl), CH₃OTf, CH₃I, or I₂ gives cationic iridium­(III) octahedral complexes [Ir^IIIX­(cod)­{(MeIm)₂­CHCOO}]⁺ (X = H, Me, or I) featuring a tripodal coordination mode of the carboxylate bridge-functionalized bis-NHC ligand. The switch from κ²C,C′ to κ³C,C′,O coordination of the bis-NHC ligand accompanying the oxidative addition prevents the coordination of the anions eventually formed in the process that remain as counterions

Mechanistic Insights into Transfer Hydrogenation Catalysis by [Ir(cod)(NHC)₂]⁺ Complexes with Functionalized N‑Heterocyclic Carbene Ligands

The synthesis of unbridged biscarbene iridium­(I) [Ir­(cod)­(MeIm∩Z)₂]⁺ complexes having N- or O-functionalized NHC ligands (∩Z = 2-methoxybenzyl, pyridin-2-ylmethyl, quinolin-8-ylmethyl) is described. The molecular structures of the complexes show an antiparallel disposition of the carbene ligands that minimize the steric repulsions between the bulky substituents. However, the complexes were found to be dynamic in solution, due to the restricted rotation about the C­(carbene)–Ir bond that results in two interconverting diasteromers having different dispositions of the functionalized NHC ligands. A rotational barrier of around 80 kJ mol^–1 (298 K) has been determined by 2D EXSY NMR spectroscopy. The iridium­(III) dihydride complex [IrH₂(MeIm∩Z)₂]⁺ (∩Z = pyridin-2-ylmethyl) has been prepared by reaction of the corresponding iridium­(I) complex with molecular hydrogen. These complexes efficiently catalyzed the transfer hydrogenation of cyclohexanone using 2-propanol as a hydrogen source and KOH as base at 80 °C with average TOF values of 117–155 h^–1 at 0.1 mol % iridium catalyst loading. All of the catalyst precursors showed comparable activity independent of both the wingtip type at the NHC ligands and the counterion. Mechanistic studies support the involvement of diene free bis-NHC iridium­(I) intermediates in these catalytic systems. DFT calculations have shown that a MPV-like concerted mechanism (Meerwein–Ponndorf–Verley mechanism), involving the direct hydrogen transfer at the coordination sphere of the iridium center, might compete with the well-established hydrido mechanism. Indirect evidence of a MPV-like mechanism has been found for the catalyst precursor having NHC ligands having with a pyridin-2-ylmethyl wingtip

ONO Dianionic Pincer-Type Ligand Precursors for the Synthesis of σ,π-Cyclooctenyl Iridium(III) Complexes: Formation Mechanism and Coordination Chemistry

The σ,π-cyclooctenyl iridium­(III) pincer compounds [Ir­(κ³-pydc-X)­(1-κ-4,5-η-C₈H₁₃)] (X = H (<b>1</b>), Cl, Br) have been prepared from [Ir­(μ-OMe)­(cod)]₂ and the corresponding 4-substituted pyridine-2,6-dicarboxylic acids (H₂pydc-X) or, alternatively, from their lithium salts (X = H) and [Ir­(cod)­(CH₃CN)₂]­PF₆. Deuterium labeling studies in combination with theoretical calculations have shown that formation of <b>1</b> involves a metal-mediated proton transfer in the reactive intermediate [Ir­(κ²-Hpydc)­(cod)], through the solvent-stabilized hydrido complex [IrH­(κ³-pydc)­(cod)­(CH₃OH)], followed by olefin insertion. The formation of this hydrido intermediate results from solvent-assisted proton transfer through a hydrogen-bonding network, forming an eight-membered metallacycle. In contrast, reaction of [Ir­(μ-OMe)­(cod)]₂ with iminodiacetic acid derivatives, RN­(CH₂COOH)₂, gave the stable iridium­(I) mononuclear [Ir­{κ²-MeN­(CH₂COOH)­(CH₂COO)}­(cod)] (R = Me) complex having a free carboxymethyl group and the tetranuclear complex [Ir₄{κ⁴-PhN­(CH₂COO)₂}₂(cod)₄] (R = Ph) with doubly deprotonated ligands. The molecular structure of the related cyclooctene complex [Ir₄{κ⁴-PhN­(CH₂COO)₂}₂(coe)₈] has been determined by X-ray analysis. Reaction of <b>1</b> with monodentate N- and P-donor ligands gave the compounds [Ir­(κ³-pydc)­(1-κ-4,5-η-C₈H₁₃)­(L)] (L = py, BnNH₂, PPh₃, PMe₃). Reaction of <b>1</b> with the short-bite bis­(diphenylphosphino)­methane (dppm) afforded the mononuclear <b>1-dppm</b>, with an uncoordinated P-donor atom, or the dinuclear complex <b>1</b>_<b>2</b><b>-dppm</b> as a function of the molar ratio used. Similarly, the dinuclear complexes <b>1</b>_<b>2</b><b>-dppe</b> and <b>1</b>_<b>2</b><b>-dppp</b> have been prepared using 1,2-bis­(diphenylphosphino)­ethane (dppe) and 1,3-bis­(diphenylphosphino)­propane (dppp) as bridging ligands. The diphosphine-bridged dinuclear assemblies have been obtained as two diastereoisomers in a 1:1 ratio due to the chirality of the mononuclear building block. The single-crystal X-ray structures of <b>1-py</b> and <b>1-dppm</b> are reported

ONO Dianionic Pincer-Type Ligand Precursors for the Synthesis of σ,π-Cyclooctenyl Iridium(III) Complexes: Formation Mechanism and Coordination Chemistry

The σ,π-cyclooctenyl iridium­(III) pincer compounds [Ir­(κ³-pydc-X)­(1-κ-4,5-η-C₈H₁₃)] (X = H (<b>1</b>), Cl, Br) have been prepared from [Ir­(μ-OMe)­(cod)]₂ and the corresponding 4-substituted pyridine-2,6-dicarboxylic acids (H₂pydc-X) or, alternatively, from their lithium salts (X = H) and [Ir­(cod)­(CH₃CN)₂]­PF₆. Deuterium labeling studies in combination with theoretical calculations have shown that formation of <b>1</b> involves a metal-mediated proton transfer in the reactive intermediate [Ir­(κ²-Hpydc)­(cod)], through the solvent-stabilized hydrido complex [IrH­(κ³-pydc)­(cod)­(CH₃OH)], followed by olefin insertion. The formation of this hydrido intermediate results from solvent-assisted proton transfer through a hydrogen-bonding network, forming an eight-membered metallacycle. In contrast, reaction of [Ir­(μ-OMe)­(cod)]₂ with iminodiacetic acid derivatives, RN­(CH₂COOH)₂, gave the stable iridium­(I) mononuclear [Ir­{κ²-MeN­(CH₂COOH)­(CH₂COO)}­(cod)] (R = Me) complex having a free carboxymethyl group and the tetranuclear complex [Ir₄{κ⁴-PhN­(CH₂COO)₂}₂(cod)₄] (R = Ph) with doubly deprotonated ligands. The molecular structure of the related cyclooctene complex [Ir₄{κ⁴-PhN­(CH₂COO)₂}₂(coe)₈] has been determined by X-ray analysis. Reaction of <b>1</b> with monodentate N- and P-donor ligands gave the compounds [Ir­(κ³-pydc)­(1-κ-4,5-η-C₈H₁₃)­(L)] (L = py, BnNH₂, PPh₃, PMe₃). Reaction of <b>1</b> with the short-bite bis­(diphenylphosphino)­methane (dppm) afforded the mononuclear <b>1-dppm</b>, with an uncoordinated P-donor atom, or the dinuclear complex <b>1</b>_<b>2</b><b>-dppm</b> as a function of the molar ratio used. Similarly, the dinuclear complexes <b>1</b>_<b>2</b><b>-dppe</b> and <b>1</b>_<b>2</b><b>-dppp</b> have been prepared using 1,2-bis­(diphenylphosphino)­ethane (dppe) and 1,3-bis­(diphenylphosphino)­propane (dppp) as bridging ligands. The diphosphine-bridged dinuclear assemblies have been obtained as two diastereoisomers in a 1:1 ratio due to the chirality of the mononuclear building block. The single-crystal X-ray structures of <b>1-py</b> and <b>1-dppm</b> are reported

ONO Dianionic Pincer-Type Ligand Precursors for the Synthesis of σ,π-Cyclooctenyl Iridium(III) Complexes: Formation Mechanism and Coordination Chemistry

The σ,π-cyclooctenyl iridium­(III) pincer compounds [Ir­(κ³-pydc-X)­(1-κ-4,5-η-C₈H₁₃)] (X = H (<b>1</b>), Cl, Br) have been prepared from [Ir­(μ-OMe)­(cod)]₂ and the corresponding 4-substituted pyridine-2,6-dicarboxylic acids (H₂pydc-X) or, alternatively, from their lithium salts (X = H) and [Ir­(cod)­(CH₃CN)₂]­PF₆. Deuterium labeling studies in combination with theoretical calculations have shown that formation of <b>1</b> involves a metal-mediated proton transfer in the reactive intermediate [Ir­(κ²-Hpydc)­(cod)], through the solvent-stabilized hydrido complex [IrH­(κ³-pydc)­(cod)­(CH₃OH)], followed by olefin insertion. The formation of this hydrido intermediate results from solvent-assisted proton transfer through a hydrogen-bonding network, forming an eight-membered metallacycle. In contrast, reaction of [Ir­(μ-OMe)­(cod)]₂ with iminodiacetic acid derivatives, RN­(CH₂COOH)₂, gave the stable iridium­(I) mononuclear [Ir­{κ²-MeN­(CH₂COOH)­(CH₂COO)}­(cod)] (R = Me) complex having a free carboxymethyl group and the tetranuclear complex [Ir₄{κ⁴-PhN­(CH₂COO)₂}₂(cod)₄] (R = Ph) with doubly deprotonated ligands. The molecular structure of the related cyclooctene complex [Ir₄{κ⁴-PhN­(CH₂COO)₂}₂(coe)₈] has been determined by X-ray analysis. Reaction of <b>1</b> with monodentate N- and P-donor ligands gave the compounds [Ir­(κ³-pydc)­(1-κ-4,5-η-C₈H₁₃)­(L)] (L = py, BnNH₂, PPh₃, PMe₃). Reaction of <b>1</b> with the short-bite bis­(diphenylphosphino)­methane (dppm) afforded the mononuclear <b>1-dppm</b>, with an uncoordinated P-donor atom, or the dinuclear complex <b>1</b>_<b>2</b><b>-dppm</b> as a function of the molar ratio used. Similarly, the dinuclear complexes <b>1</b>_<b>2</b><b>-dppe</b> and <b>1</b>_<b>2</b><b>-dppp</b> have been prepared using 1,2-bis­(diphenylphosphino)­ethane (dppe) and 1,3-bis­(diphenylphosphino)­propane (dppp) as bridging ligands. The diphosphine-bridged dinuclear assemblies have been obtained as two diastereoisomers in a 1:1 ratio due to the chirality of the mononuclear building block. The single-crystal X-ray structures of <b>1-py</b> and <b>1-dppm</b> are reported

Unsaturated Iridium(III) Complexes Supported by a Quinolato–Carboxylato ONO Pincer-Type Ligand: Synthesis, Reactivity, and Catalytic C–H Functionalization

The unsaturated σ,π-cyclooctenyl iridium­(III) pincer compound [Ir­(κ³-hqca)­(1-κ-4,5-η-C₈H₁₃)] (<b>1</b>) has been prepared by the reaction of [Ir­(cod)­(CH₃CN)₂]­BF₄ with lithium 8-oxidoquinoline-2-carboxylate (Li₂hqca) and obtained as two isomers derived from the relative disposition of the pincer and the σ,π-cyclooctenyl ligands. Compound <b>1</b> can be prepared as a single isomer by reaction of 8-hydroxyquinoline-2-carboxylic acid (H₂hqca) with [Ir­(μ-OMe)­(cod)]₂. Reaction of [Ir­(μ-OH)­(coe)₂]₂ with H₂hqca gave the square-pyramidal iridum­(III) complex [IrH­(κ³-hqca)­(coe)] (<b>3</b>). This compound exists as dinuclear assemblies [IrH­(κ³-hqca)­(coe)]₂ in noncoordinating solvents and as the corresponding labile mononuclear solvates in more polar solvent solutions. The dimerization of <b>3</b> was established by ¹H-DOSY NMR spectroscopy and an ESI⁺ mass spectrum and supported by DFT calculations. Reaction of <b>3</b> with pyridine gave the adduct [IrH­(κ³-hqca)­(coe)­(py)] (<b>4</b>) and the bis-pyridine complexes [IrH­(κ³-hqca)­(R-py)₂] (R = H (<b>6</b>), 2-Me (<b>7</b>)) by replacement of the coe ligand. Compound <b>4</b> was transformed into the bromo derivative [IrBr­(κ³-hqca)­(coe)­(py)] (<b>5</b>) by reaction with <i>N</i>-bromosuccinimide. Carbonylation of <b>4</b> gave the cyclooctenyl complex [Ir­(κ³-hqca)­(1-κ-C₈H₁₅)­(CO)­(py)] (<b>8</b>), which is stable only under a carbon monoxide atmosphere. The pincer complexes were active in the catalytic borylation of arenes under thermal conditions

Enhanced Hydrogen-Transfer Catalytic Activity of Iridium N‑Heterocyclic Carbenes by Covalent Attachment on Carbon Nanotubes

Oxidized multiwall carbon nanotubes (<b>CNT</b>) were covalently modified with appropriate hydroxyl-ending imidazolium salts using their carboxylic acid groups. Characterization of the imidazolium-modified samples through typical solid characterization techniques, such as TGA or XPS, allows for the determination of 16 wt % in <b>CNT-1</b> and 31 wt % in <b>CNT-2</b> as the amount of the imidazolic fragments in the carbon nanotubes. The imidazolium-functionalized materials were used to prepare nanohybrid materials containing iridium N-heterocyclic carbene (NHC)-type organometallic complexes with efficiencies as high as 95%. The nanotube-supported iridium–NHC materials were active in the heterogeneous iridium-catalyzed hydrogen-transfer reduction of cyclohexanone to cyclohexanol with 2-propanol/KOH as hydrogen source. The iridium hybrid materials are more efficient than related homogeneous catalysts based on acetoxy-functionalized Ir–NHC complexes with initial TOFs up to 5550 h^–1. A good recyclability of the catalysts, without any loss of activity, and stability in air was observed

Iridium(I) Complexes with Hemilabile N-Heterocyclic Carbenes: Efficient and Versatile Transfer Hydrogenation Catalysts

A series of neutral and cationic rhodium and iridium(I) complexes based on hemilabile O-donor- and N-donor-functionalized NHC ligands having methoxy, dimethylamino, and pyridine as donor functions have been synthesized. The hemilabile fragment is coordinated to the iridium center in the cationic complexes [Ir(cod)(MeImR)]⁺ (R = pyridin-2-ylmethyl, 3-dimethylaminopropyl) but remains uncoordinated in the complexes [MBr(cod)(MeImR)], [M(NCCH₃)(cod)(MeImR)]⁺ (M = Rh, Ir; R = 2-methoxyethyl and 2-methoxybenzyl) and [IrX(cod)(MeImR)] (X = Br, R = pyridin-2-ylmethyl; X = Cl, R = 2-dimethylaminoethyl, 3-dimethylaminopropyl). The structure of [IrBr(cod)(MeIm(2-methoxybenzyl))] has been determined by X-ray diffraction. The iridium complexes are efficient precatalysts for the transfer hydrogenation of cyclohexanone in 2-propanol/KOH. A comparative study has shown that cationic complexes are more efficient than the neutral and also that complexes having O-functionalized NHC ligands provide much more active systems than the corresponding N-functionalized ligands with TOFs up to 4600 h^–1. The complexes [Ir(NCCH₃)(cod)(MeImR)]⁺ (R = 2-methoxyethyl and 2-methoxybenzyl) have been successfully applied to the reduction of several unsaturated substrates as ketones, aldehydes, α,β-unsaturated ketones, and imines. The investigation of the reaction mechanism by NMR and MS has allowed the identification of relevant alkoxo intermediates [Ir(OR)(cod)(MeImR)] and the unsaturated hydride species [IrH(cod)(MeImR)]. The β-H elimination in the alkoxo complex [Ir(O<i>i</i>Pr)(cod)(MeIm(2-methoxybenzyl))] leading to hydrido species has been studied by DFT calculations. An interaction between the β-H on the alkoxo ligand and the oxygen atom of the methoxy fragment of the NHC ligand, which results in a net destabilization of the alkoxo intermediate by a free energy of +1.0 kcal/mol, has been identified. This destabilization facilitates the β-H elimination step in the catalytic process and could explain the positive effect of the methoxy group of the functionalized NHC ligands on the catalytic activity

Steric Effects in the Oxidative Addition of MeI to a Sulfido-Bridged ZrRh₂ Early–Late Heterobimetallic Compound

Reaction of the early–late heterobimetallic compound [Cp^tt₂Zr­(μ₃-S)₂{Rh­(CO)}₂(μ-dppm)] (<b>1</b>) with MeI affords the unusual oxidative addition product [Cp^tt₂Zr­(μ₃-S)₂Rh₂(μ-CO)­(μ-dppm)­(I)­(COCH₃)] (<b>4</b>), showing the presence of a bridging carbonyl and a terminal acetyl ligand. The optimized structure of <b>4</b> by DFT calculations is further substantiated by the spectroscopic data of the ¹³CO-labeled complex <b>4*</b>. The same reaction carried out on the structurally related <i>gem</i>-dithiolate dinuclear complexes [Rh₂(μ-S₂CR₂)­(CO)₂(PPh₃)₂] (<b>2</b>, <b>3</b>) gives the one-center oxidative addition acetyl products [Rh₂(μ-S₂CR₂)­(COCH₃)­(I)­(CO)­(PPh₃)₂] (R₂ = −(CH₂)₅–, <b>5</b>; R = Bn (benzyl), <b>6</b>). Reaction of <b>3</b> with molecular iodine affords the dinuclear compounds [Rh₂(μ-S₂CBn₂)­(I)₂(μ-CO)­(PPh₃)₂] (<b>7</b>) and [Rh₂(μ-S₂CBn₂)­(I)₂(CO)₂(PPh₃)₂] (<b>8</b>), the transannular oxidative addition product, in a 40:60 ratio. Compound <b>7</b> was also formed in the reaction of <b>3</b> with MeI after a prolonged reaction time. The molecular structures of compounds <b>5</b> and <b>7</b> have been determined by X-ray analyses. A natural bond orbital (NBO) analysis has shown an analogous bonding scheme in the “Rh₂(μ-CO)­P₂” subunit of complexes <b>4</b> and <b>7</b>. Although both complexes can be formally described as composed of a Rh­(III)–Rh­(III) unit, assuming a ketonic character of the bridging carbonyl ligand, the natural charges on the rhodium atoms and the WBI indexes for the Rh–Rh and CO bonds points to electronically unsaturated metal centers bridged by a carbonyl ligand lacking ketonic character and a significant metal–metal interaction. In addition, DFT calculations on the reaction pathway leading to the formation of <b>4</b> evidenced the key role of the bulky “Cp^tt₂Zr” fragment in the course of the reaction