Intramolecular C–H Oxidative Addition to Iridium(I) in Complexes Containing a <i>N</i>,<i>N</i>′‑Diphosphanosilanediamine Ligand

The iridium­(I) complexes of formula Ir­(cod)­(SiNP)⁺ (<b>1</b>^<b>+</b>) and IrCl­(cod)­(SiNP) (<b>2</b>) are easily obtained from the reaction of SiMe₂{N­(4-C₆H₄CH₃)­PPh₂}₂ (SiNP) with [Ir­(cod)­(CH₃CN)₂]⁺ or [IrCl­(cod)]₂, respectively. The carbonylation of [<b>1</b>]­[PF₆] affords the cationic pentacoordinated complex [Ir­(CO)­(cod)­(SiNP)]⁺ (<b>3</b>⁺), while the treatment <b>2</b> with CO gives the cation <b>3</b>⁺ as an intermediate, finally affording an equilibrium mixture of IrCl­(CO)­(SiNP) (<b>4</b>) and the hydride derivative of formula IrHCl­(CO)­(SiNP–H) (<b>5</b>) resulting from the intramolecular oxidative addition of the C–H bond of the SiCH₃ moiety to the iridium­(I) center. Furthermore, the prolonged exposure of [<b>3</b>]­Cl or <b>2</b> to CO resulted in the formation of the iridium­(I) pentacoordinated complex Ir­(SiNP–H)­(CO)₂ (<b>6</b>). The unprecedented κ³<i>C</i>,<i>P</i>,<i>P</i>′ coordination mode of the [SiNP–H] ligand observed in <b>5</b> and <b>6</b> has been fully characterized in solution by NMR spectroscopy. In addition, the single-crystal X-ray structure of <b>6</b> is reported

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

Labile Rhodium(I)–N-Heterocyclic Carbene Complexes

The neutral square-planar complexes Rh­(acac)­(IPr)­(η²<i>-</i>olefin) have been prepared from [Rh­(μ-Cl)­(IPr)­(η²<i>-</i>olefin)]₂ (IPr = 1,3-bis-(2,6-diisopropylphenyl)­imidazol-2-carbene; olefin = cyclooctene, ethylene) and sodium acetylacetonate (acac). Protonation of the acetylacetonato complexes with triflic acid opens the way to the formation of the putative bare [Rh-IPr]⁺ fragment that has been stabilized at low temperature by labile ligands such as triflate, cyclooctene, and acetonitrile to generate Rh­(OTf)­(IPr)­(η²<i>-</i>coe), [Rh­(IPr)­(η²<i>-</i>coe)­(NCCH₃)₂]­OTf, and [Rh­(IPr)­(NCCH₃)₃]­OTf complexes. The derivative [Rh­(IPr)­(η²<i>-</i>coe)­(NCCH₃)₂]­OTf was further characterized by an X-ray diffraction analysis

Rhodium(I)-N-Heterocyclic Carbene Catalyst for Selective Coupling of <i>N</i>‑Vinylpyrazoles with Alkynes via C–H Activation

The complex [Rh­(μ-Cl)­(<i>I</i>Pr)­(η²<i>-</i>coe)]₂ {<i>I</i>Pr = 1,3-bis­(2,6-diisopropylphenyl)­imidazol-2-carbene, coe = <i>cis</i>-cyclooctene} efficiently catalyzes the coupling of alkynes and <i>N</i>-vinylpyrazole via C–H activation, leading to Markovnikov-selective butadienylpyrazole derivatives under mild conditions. A straightforward approach to cross-conjugated acyclic trienes is also operative through a one-pot alkyne dimerization-hydrovinylation tandem reaction. The proposed mechanism involves C–H activation of vinylpyrazole directed by nitrogen coordination to the metallic center. Subsequent alkyne coordination, insertion, and reductive elimination steps lead to the coupling products. Several key intermediates participating in the catalytic cycle have been detected and characterized, including a κ-N, η²-CC coordinated vinylpyrazole complex and a Rh^III-hydride-alkenyl species resulting from the C–H activation of the vinylpyrazol

Hydroxo–Rhodium–N-Heterocyclic Carbene Complexes as Efficient Catalyst Precursors for Alkyne Hydrothiolation

The new Rh–hydroxo dinuclear complexes stabilized by an N-heterocyclic carbene (NHC) ligand of type [Rh­(μ-OH)­(NHC)­(η²-olefin)]₂ (coe, IPr (<b>3</b>), IMes (<b>4</b>); ethylene, IPr (<b>5</b>)) are efficient catalyst precursors for alkyne hydrothiolation under mild conditions, presenting high selectivity toward α-vinyl sulfides for a varied set of substrates, which is enhanced by pyridine addition. The structure of complex <b>3</b> has been determined by X-ray diffraction analysis. Several intermediates relevant for the catalytic process have been identified, including Rh^I-thiolato species Rh­(SCH₂Ph)­(IPr)­(η²-coe)­(py) (<b>6</b>) and Rh­(SCH₂Ph)­(IPr)­(η²-HCCCH₂Ph)­(py) (<b>7</b>), and the Rh^III-hydride-dithiolato derivative RhH­(SCH₂Ph)₂(IPr)­(py) (<b>8</b>) as the catalytically active species. Computational DFT studies reveal an operational mechanism consisting of sequential thiol deprotonation by the hydroxo ligand, subsequent S–H oxidative addition, alkyne insertion, and reductive elimination. The insertion step is rate-limiting with a 1,2 thiometalation of the alkyne as the more favorable pathway in accordance with the observed Markovnikov-type selectivity

Hydroxo–Rhodium–N-Heterocyclic Carbene Complexes as Efficient Catalyst Precursors for Alkyne Hydrothiolation

The new Rh–hydroxo dinuclear complexes stabilized by an N-heterocyclic carbene (NHC) ligand of type [Rh­(μ-OH)­(NHC)­(η²-olefin)]₂ (coe, IPr (<b>3</b>), IMes (<b>4</b>); ethylene, IPr (<b>5</b>)) are efficient catalyst precursors for alkyne hydrothiolation under mild conditions, presenting high selectivity toward α-vinyl sulfides for a varied set of substrates, which is enhanced by pyridine addition. The structure of complex <b>3</b> has been determined by X-ray diffraction analysis. Several intermediates relevant for the catalytic process have been identified, including Rh^I-thiolato species Rh­(SCH₂Ph)­(IPr)­(η²-coe)­(py) (<b>6</b>) and Rh­(SCH₂Ph)­(IPr)­(η²-HCCCH₂Ph)­(py) (<b>7</b>), and the Rh^III-hydride-dithiolato derivative RhH­(SCH₂Ph)₂(IPr)­(py) (<b>8</b>) as the catalytically active species. Computational DFT studies reveal an operational mechanism consisting of sequential thiol deprotonation by the hydroxo ligand, subsequent S–H oxidative addition, alkyne insertion, and reductive elimination. The insertion step is rate-limiting with a 1,2 thiometalation of the alkyne as the more favorable pathway in accordance with the observed Markovnikov-type selectivity

Labile Rhodium(I)–N-Heterocyclic Carbene Complexes

The neutral square-planar complexes Rh­(acac)­(IPr)­(η²<i>-</i>olefin) have been prepared from [Rh­(μ-Cl)­(IPr)­(η²<i>-</i>olefin)]₂ (IPr = 1,3-bis-(2,6-diisopropylphenyl)­imidazol-2-carbene; olefin = cyclooctene, ethylene) and sodium acetylacetonate (acac). Protonation of the acetylacetonato complexes with triflic acid opens the way to the formation of the putative bare [Rh-IPr]⁺ fragment that has been stabilized at low temperature by labile ligands such as triflate, cyclooctene, and acetonitrile to generate Rh­(OTf)­(IPr)­(η²<i>-</i>coe), [Rh­(IPr)­(η²<i>-</i>coe)­(NCCH₃)₂]­OTf, and [Rh­(IPr)­(NCCH₃)₃]­OTf complexes. The derivative [Rh­(IPr)­(η²<i>-</i>coe)­(NCCH₃)₂]­OTf was further characterized by an X-ray diffraction analysis

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