Base-Free Transfer Hydrogenation of Ketones Using Cp*Ir(pyridinesulfonamide)Cl Precatalysts

Syntheses and crystal structures of 4-benzyl-1-ethyl-1,2,4-triazolium bromide and its corresponding NHC complexes of rhodium and iridium

The syntheses and crystal structures of a triazolium salt, 4-benzyl-1-ethyl-1,2,4-triazolium bromide, C11H14N3 +·Br- (2), and the corresponding N-heterocyclic carbene complexes, (4-benzyl-1-ethyl-1,2,4-triazol-5-yl-idene)chlorido-[(1,2,5,6-η)-cyclo-octa-1,5-diene]rhodium(I), [RhCl(C8H12)(C11H13N3)] (3), (4-benzyl-1-ethyl-1,2,4-triazol-5-yl-idene)[(1,2,5,6-η)-cyclo-octa-1,5-diene](tri-phenyl-phosphane)iridium(I) tetra-fluorido-borate, [Ir(C8H12)(C11H13N3)(C18H15P)]BF4 (5), and (4-benzyl-1-ethyl-1,2,4-triazol-5-yl-idene)[(1,2,5,6-η)-cyclo-octa-1,5-diene](tri-cyclo-hexyl-phosphane)iridium(I) tetra-fluorido-borate dicholoro-methane sesquisolvate, [Ir(C8H12)(C11H13N3)(C18H33P)]BF4·1.5CH2Cl2 (6), are presented. Complexes 2 and 6 crystallize in the monoclinic space group P21/c, complex 3 in the triclinic space group P1 and complex 5 in the triclinic space group P1 with two mol-ecules in the asymmetric unit. The three metal complexes 3, 5, and 6 have a distorted square-planar geometry around the metal ions. The N1-C1-N3 bond angle in the triazolium salt 2 is 107.1 (2)° and is observed in the range of 102.2 (3) to 103.8 (5)° in the NHC ligands in complexes 3, 5, and 6. The two substituent 'wing tips' in the NHC ligand (N-ethyl and N-benz-yl) are oriented in an anti-arrangement in compounds 2 and 3, a syn-arrangement in compound 6, and both syn and anti-arrangements in the two independent ion pairs in compound 5. All structures exhibit non-classical hydrogen-bonding inter-actions with the most acidic hydrogen atoms in complexes 2 and 3 playing critical roles in the orientations of structural units.

[(1,2,5,6-η)-Cyclo-octa-1,5-diene](1-ethyl-4-iso-butyl-1,2,4-triazol-5-yl-idene)(tri-phenyl-phosphane)iridium(I) tetra-fluorido-borate di-chloro-methane hemisolvate

A new triazole-based N-heterocyclic carbene IrI cationic complex with a tetra-fluorido-borate counter-anion and hemi-solvating di-chloro-methane, [Ir(C8H12)(C8H15N3)(C18H15P)]BF4·0.5CH2Cl2, has been synthesized and structurally characterized. There are two independent ion pairs in the asymmetric unit and one di-chloro-methane solvent mol-ecule per two ion pairs. The cationic complex exhibits a distorted square-planar conformation around the IrI atom, formed by a bidentate cyclo-octa-1,5,diene (COD) ligand, a tri-phenyl-phosphane ligand, and an N-heterocyclic carbene (NHC). There are several close non-standard H⋯F hydrogen-bonding inter-actions that orient the tetra-fluorido-borate anions with respect to the IrI complex mol-ecules. The complex shows promising catalytic activity in transfer hydrogenation reactions. The structure was refined as a non-merohedral twin, and one of the COD mol-ecules is statistically disordered.

[(1,2,5,6-η)-Cyclo-octa-1,5-diene](1-ethyl-4-isobutyl-1,2,4-triazol-5-yl-idene)(tri-phenyl-phosphane)rhodium(I) tetra-fluorido-borate

A new, cationic N-heterocyclic carbene RhI complex with a tetra-fluorido-borate counter-anion, [Rh(C8H12)(C8H15N3)(C18H15P)]BF4, has been synthesized and structurally characterized. There are two independent ion pairs in the asymmetric unit. Each complex cation exhibits a distorted square-planar conformation around the RhI atom. Bond lengths and bond angles are as expected for an Rh-NHC complex. There are several close, non-standard C-H⋯F hydrogen-bonding inter-actions between the ions. One of the tetra-fluorido-borate anions shows statistical disorder of the F atoms.

(4-Butyl-1-ethyl-1,2,4-triazol-5-yl-idene)[(1,2,5,6-η)-cyclo-octa-1,5-diene](tri-phenyl-phosphane)iridium(I) tetra-fluorido-borate

The title compound, [Ir(C8H12)(C8H15N3)(C18H15P)]BF4, a new triazole-based N-heterocyclic carbene iridium(I) cationic complex with a tetra-fluorido-borate counter-anion, crystallizes with two cations and two anions in the asymmetric unit of space group Pc. The Ir centers of the cations have distorted square-planar conformations, formed by a bidentate (η2 + η2) cyclo-octa-1,5-diene (COD) ligand, an N-heterocyclic carbene and a tri-phenyl-phosphane ligand with the NHC carbon atom and P atom being cis. In the extended structure, non-classical C-H⋯F hydrogen bonds, one of which is notably short (H⋯F = 2.21 Å), link the cations and anions. The carbon atoms of one of the COD ligands are disordered over adjacent sites in a 0.62:0.38 ratio.

Effect of Ligand Substituents on Spectroscopic and Catalytic Properties of Water-Compatible Cp*Ir-(pyridinylmethyl)sulfonamide-Based Transfer Hydrogenation Catalysts

Transition-metal-based hydrogenation catalysts have applications ranging from high-value chemical synthesis to medicinal chemistry. A series of (pyridinylmethyl)sulfonamide ligands substituted with electron-withdrawing and -donating groups were synthesized to study the influence of the electronic contribution of the bidentate ligand in Cp*Ir piano-stool complexes. A variable-temperature NMR investigation revealed a strong correlation between the electron-donating ability of the substituent and the rate of stereoinversion of the complexes. This correlation was partially reflected in the catalytic activity of the corresponding catalysts. Complexes with electron-withdrawing substituents followed the trend observed in the variable-temperature NMR study, thereby confirming the rate-determining step to be donation of the hydride ligand. Strongly electron-donating groups, on the other hand, caused a change in the rate-determining step in the formation of the iridium-hydride species. These results demonstrate that the activity of these catalysts can be tuned systematically via changes in the electronic contribution of the bidentate (pyridinylmethyl)sulfonamide ligands.

BICAAC-Derived Covalent and Cationic Ir(I) Complexes: Application of Ir(BICAAC)Cl(COD) Complexes as Catalysts for Transfer Hydrogenation and Hydrosilylation Reactions

The ambiphilic bicyclic (alkyl)(amino)carbenes (Me/iPrBICAAC) upon reaction with [IrCl(COD)]2 smoothly afford mononuclear Ir(I) complexes that have been spectroscopically and structurally characterized. These complexes exhibit good catalytic activity for transfer hydrogenation (TH) of 4-chlorobenzaldehyde using isopropyl alcohol (iPrOH), with turnover frequency values ranging between 6269 and 8093 h-1. Choosing the covalent complex Ir(MeBICAAC)Cl(COD) as a catalyst, a wide array of carbonyls and imines functionalized with electron-withdrawing and electron-donating substituents have been surveyed and afforded their reduced products in moderate-to-good yields. No detachment of the BICAAC unit from the Ir center was observed upon prolonged heating of Ir(MeBICAAC)Cl(COD) in toluene-d8 or isopropyl alcohol-d8, which evidenced good thermal stability of the catalyst. Complex Ir(MeBICAAC)Cl(COD) was also found to be catalytically active for the hydrosilylation of a variety of aldehydes using triethylsilane (Et3SiH).

(4-Butyl-1-ethyl-1,2,4-triazol-5-yl-idene)[(1,2,5,6-η)-cyclo-octa-1,5-diene](tri-phenyl-phosphane)rhodium(I) tetra-fluorido-borate

In the title triazole-based N-heterocyclic carbene rhodium(I) cationic complex with a tetra-fluorido-borate counter-anion, [Rh(C8H12)(C8H15N3)(C18H15P)]BF4, which crystallizes with two cations and two anions in the asymmetric unit, the Rh center has a distorted square-planar coordination geometry with expected bond distances. Several nonclassical C-H⋯F hydrogen-bonding inter-actions help to consolidate the packing. Two of the F atoms of one of the anions are disordered over adjacent sites in a 0.814 (4):0.186 (4) ratio.

[(1,2,5,6-η)-Cyclo-octa-1,5-diene](1-ethyl-4-isopropyl-1,2,4-triazol-5-yl-idene)(tri-phenylphos-phane)iridium(I) tetra-fluorido-borate di-chloro-methane sesquisolvate

The synthesis and crystal structure of a new triazole-based N-heterocyclic carbene iridium(I) cationic complex with a tetra-fluorido-borate counter-anion and solvating di-chloro-methane, [Ir(C8H12)(C7H13N3)(C18H15P)]BF4·1.5CH2Cl2, is reported. The IrI center of the cationic complex has a distorted square-planar conformation, formed by a bidentate cyclo-octa-1,5-diene (COD) ligand, an N-heterocyclic carbene, and a triphenylphosphane ligand. There are weak hydrogen-bonding inter-actions between C-H groupings of the iridium complex and F atoms of the [BF4]- counter-ions. The atoms of the COD ligand are disordered over two sets of sites in a 0.65:0.35 ratio and two of the F atoms of the anion are disordered over adjacent sites in a 0.6:0.4 ratio. One of the di-chloro-methane solvent mol-ecules is disordered about an inversion center with 0.5 occupancy.

[(1,2,5,6-η)-Cyclo-octa-1,5-diene](4-isopropyl-1-methyl-1,2,4-triazol-5-yl-idene)(tri-phenyl-phos-phane)iridium(I) tetra-fluorido-borate di-chloro-methane 0.8-solvate

A new triazole-based N-heterocyclic carbene iridium(I) cationic complex with a tetra-fluorido-borate counter-anion, [Ir(C8H12)(C18H15P)(C6H11N3)]BF4·0.8CH2Cl2, has been synthesized and structurally characterized. The central IrI atom of the cationic complex has a distorted square-planar coordination environment, formed by a bidentate cyclo-octa-1,5-diene (COD) ligand, an N-heterocyclic carbene, and a tri-phenyl-phosphane ligand. The crystal structure comprises C-H⋯π(ring) inter-actions that orient the phenyl rings; non-classical hydrogen-bonding inter-actions between the cationic complex and the tetra-fluorido-borate anion are also present. The complex crystallizes in a triclinic unit cell with two structural units and an incorporation of di-chloro-methane solvate mol-ecules with an occupancy of 0.8.

[(1,2,5,6-η)-Cyclo-octa-1,5-diene](4-isopropyl-1-methyl-1,2,4-triazol-5-yl-idene)(tri-cyclo-hexyl-phosphane-κP)iridium(I) tetra-fluorido-borate di-chloro-methane monosolvate

The title compound, [Ir(C8H12)(C6H11N3)(C18H33P)]BF4·CH2Cl2, a triazole-based N-heterocyclic carbene iridium(I) cationic complex with a tetra-fluorido-borate counter-anion, crystallizes with one di-chloro-methane solvent mol-ecule per formula unit. The IrI atom of the cationic complex has a distorted square-planar coordination environment, defined by a bidentate cyclo-octa-1,5-diene (COD) ligand, an N-heterocyclic carbene, and a tri-cyclo-hexyl-phosphane ligand. The complex crystallizes in a C-centered monoclinic unit cell and has an unusually high number of eight formula units.

(4-Butyl-1-methyl-1,2,4-triazol-5-yl-idene)[(1,2,5,6-η)-cyclo-octa-1,5-diene](tri-phenyl-phosphane)iridium(I) tetra-fluorido-borate

A new triazole-based N-heterocyclic cationic carbene iridium(I) complex with a tetra-fluorido-borate counter-anion, [Ir(C8H12)(C7H13N3)(C18H15P)]BF4, has been synthesized and structurally characterized. The IrI atom of the cationic complex has an expected square-planar coordination environment with unexceptional bond lengths. There are several close F⋯H contacts between the cations and the anions in the range 2.36-2.58 Å, stabilizing the orientation of the out-sphere [BF4 -] counter-anion. In the crystal, C-H⋯π(ring) inter-actions are observed that orient the phenyl rings of the tri-phenyl-phosphane ligands.

(4-Benzyl-1-methyl-1,2,4-triazol-5-yl-idene)[(1,2,5,6-η)-cyclo-octa-1,5-diene](tri-phenyl-phosphane-κP)iridium(I) tetra-fluorido-borate

A new triazole-based N-heterocyclic carbene iridium(I) cationic complex with a tetra-fluorido-borate counter-anion, [Ir(C10H11N3)(C8H12)(C18H15P)]BF4, has been synthesized and structurally characterized. The cationic complex exhibits a distorted square-planar environment around the IrI ion. One significant non-standard hydrogen-bonding inter-action exists between a hydrogen atom on the N-heterocyclic carbene ligand and a fluorine atom from the counter-ion, BF4 -. In the crystal, π-π stacking inter-actions are observed between one of the phenyl rings and the triazole ring. Both inter-molecular and intra-molecular C-H⋯π(ring) inter-actions are also observed.

(Chlorido/bromido)[(1,2,5,6-η)-cyclo-octa-1,5-diene](4-isopropyl-1-methyl-1,2,4-triazol-5-yl-idene)rhodium(I)

A new triazole-based neutral RhI complex, [Rh(Cl0.846Br0.154)(C6H11N3)(C8H12)], has been synthesized and structurally characterized. The RhI atom has a distorted square-planar coordination environment, formed by a bidentate cyclo-octa-1,5-diene (COD) ligand, an N-heterocyclic carbene and a halide ligand that shows substitutional disorder (Cl:Br = 0.846:0.154). No significant inter-molecular inter-actions other than van der Waals forces are found in the crystal structure. Diffraction data indicated a two-component inversion twin with a ratio of 0.95 (5):0.05 (5).

Synthesis, Structure, and Catalytic Hydrogenation Activity of [NO]-Chelate Half-Sandwich Iridium Complexes with Schiff Base Ligands

A series of N,O-coordinate iridium(III) complexes with a half-sandwich motif bearing Schiff base ligands for catalytic hydrogenation of nitro and carbonyl substrates have been synthesized. All iridium complexes showed efficient catalytic activity for the hydrogenation of ketones, aldehydes, and nitro-containing compounds using clean H₂ as reducing reagent. The iridium catalyst displayed the highest TON values of 960 and 950 in the hydrogenation of carbonyl and nitro substrates, respectively. Various types of substrates with different substituted groups afforded corresponding products in excellent yields. All N,O-coordinate iridium(III) complexes 1-4 were well characterized by IR, NMR, HRMS, and elemental analysis. The molecular structure of complex 1 was further characterized by single-crystal X-ray determination.

[(1,2,5,6-η)-Cyclo-octa-1,5-diene](1-ethyl-3-isopropyl-1,3-imidazol-2-yl-idene)(tri-phenyl-phosphane)rhodium(I) tetra-fluorido-borate

A new N-heterocyclic cationic rhodium(I) complex with a tetra-fluorido-borate counter-anion, [Rh(C8H14N2)(C8H12)(C18H15P)]BF4, has been prepared and structurally characterized. The cationic complex exhibits a distorted square-planar environment around the rhodium(I) ion. Two connections are made from rhodium(I) to the carbon atom of an N-heterocylic carbene ligand and to the phospho-rus atom of a tri-phenyl-phosphane ligand. The remaining two coordination sites are made via a bidentate inter-action from the two olefinic bonds of cyclo-octa-diene to the rhodium(I) ion. The compound includes an out-sphere tetra-fluorido-borate counter-anion. Within the crystal of the compound exist several weak inter-molecular C-H⋯F inter-actions.

Artificial imine reductases: developments and future directions

Biocatalytic imine reduction has been a topic of intense research by the artificial metalloenzyme community in recent years. Artificial constructs, together with natural enzymes, have been engineered to produce chiral amines with high enantioselectivity. This review examines the design of the main classes of artificial imine reductases reported thus far and summarises approaches to enhancing their catalytic performance using complementary methods. Examples of utilising these biocatalysts in vivo or in multi-enzyme cascades have demonstrated the potential that artIREDs can offer, however, at this time their use in biocatalysis remains limited. This review explores the current scope of artIREDs and the strategies used for catalyst improvement, and examines the potential for artIREDs in the future.

A New Mechanism of Metal-Ligand Cooperative Catalysis in Transfer Hydrogenation of Ketones

We report iridium catalysts IrCl(η⁵-Cp*)(κ²-(2-pyridyl)CH₂NSO₂C₆H₄X) (1-Me, X = CH₃ and 1-F, X = F) for transfer hydrogenation of ketones with 2-propanol that operate by a previously unseen metal-ligand cooperative mechanism. Under the reaction conditions, complexes 1 (1-Me and 1-F) derivatize to a series of catalytic intermediates: Ir(η⁵-Cp*)(κ²-(C₅H₄N)CHNSO₂Ar) (2), IrH(η⁵Cp*)(κ²-(2-pyridyl)CH₂NSO₂Ar) (3), and Ir(η⁵-Cp*)(κ³-(2-pyridyl)CH₂NSO₂Ar) (4). The structures of 1-Me and 4-Me were established by single-crystal X-ray diffraction. A rate-determining, concerted hydrogen transfer step (2 + R₂CHOH ⇄ 3 + R₂CO) is suggested by kinetic isotope effects, Eyring parameters (ΔH ^≠ = 29.1(8) kcal mol^-1 and ΔS ^≠ = -17(19) eu), proton-hydride fidelity, and DFT calculations. According to DFT, a nine-membered cyclic transition state is stabilized by an alcohol molecule that serves as a proton shuttle.

Transfer Hydrogenation from 2-propanol to Acetophenone Catalyzed by [RuCl2(η6-arene)P] (P = monophosphine) and [Rh(PP)2]X (PP = diphosphine, X = Cl−, BF4−) Complexes

The reduction of ketones through homogeneous transfer hydrogenation catalyzed by transition metals is one of the most important routes for obtaining alcohols from carbonyl compounds. The interest of this method increases when opportune catalytic precursors are able to perform the transformation in an asymmetric fashion, generating enantiomerically enriched chiral alcohols. This reaction has been extensively studied in terms of catalysts and variety of substrates. A large amount of information about the possible mechanisms is available nowadays, which has been of high importance for the development of systems with excellent outcomes in terms of conversion, enantioselectivity and Turn Over Frequency. On the other side, many mechanistic aspects are still unclear, especially for those catalytic precursors which have shown only moderate performances in transfer hydeogenation. This is the case of neutral [RuCl2(η6-arene)(P)] and cationic [Rh(PP)2]X (X = anion; P and PP = mono- and bidentate phosphine, respectively) complexes. Herein, a summary of the known information about the Transfer Hydrogenation catalyzed by these complexes is provided with a continuous focus on the more relevant mechanistic features.

Cooperation of cerium oxide nanoparticles and soluble molecular catalysts for alcohol oxidation

Nano-cerium oxide and organometallic catalysts cooperate in anaerobic and aerobic alcohol oxidations.

Catalytic hydrogenation of carbonyl and nitro compounds using an [N,O]-chelate half-sandwich ruthenium catalyst

A series of N,O-chelate half-sandwich ruthenium complexes have been synthesized, which exhibited high activity for the catalytic hydrogenation of carbonyl and nitro compounds in aqueous solution.

Base free N-alkylation of anilines with ArCH2OH and transfer hydrogenation of aldehydes/ketones catalyzed by the complexes of η5-Cp*Ir(iii) with chalcogenated Schiff bases of anthracene-9-carbaldehyde

The condensation of anthracene-9-carbaldehyde with 2-(phenylthio/seleno)ethylamine results in Schiff bases [PhS(CH₂)₂C[double bond, length as m-dash]N-9-C₁₄H₉](L1) and [PhSe(CH₂)₂C[double bond, length as m-dash]N-9-C₁₄H₉] (L2). On their reaction with [(η⁵-Cp*)IrCl(μ-Cl)]₂ and CH₃COONa at 50 °C followed by treatment with NH₄PF₆, iridacycles, [(η⁵-Cp*)Ir(L-H)][PF₆] (1: L = L1; 2: L = L2), result. The same reaction in the absence of CH₃COONa gives complexes [(η⁵-Cp*)Ir(L)Cl][PF₆] (3-4) in which L = L1(3)/L2(4) ligates in a bidentate mode. The ligands and complexes were authenticated with HR-MS and NMR spectra [¹H, ¹³C{¹H} and ⁷⁷Se{¹H} (in the case of L2 and its complexes only)]. Single crystal structures of L2 and half sandwich complexes 1-4 were established with X-ray crystallography. Three coordination sites of Ir in each complex are covered with η⁵-Cp* and on the remaining three, donor atoms present are: N, S/Se and C^-/Cl^-, resulting in a piano-stool structure. The moisture and air insensitive 1-4 act as efficient catalysts under mild conditions for base free N-alkylation of amines with benzyl alcohols and transfer hydrogenation (TH) of aldehydes/ketones. The optimum loading of 1-4 as a catalyst is 0.1-0.5 mol% for both the activations. The best reaction temperature is 80 °C for transfer hydrogenation and 100 °C for N-alkylation. The mercury poisoning test supports a homogeneous pathway for both the reactions catalyzed by 1-4. The two catalytic processes are most efficient with 3 followed by 4 > 1 > 2. The mechanism proposed on the basis of HR-MS of the reaction mixtures of the two catalytic processes taken after 1-2 h involves the formation of an alkoxy and hydrido species. The real catalytic species proposed in the case of iridacycles results due to the loss of the Cp* ring.

[(1,2,5,6-η)-Cycloocta-1,5-diene](1-ethyl-3-isopropyl-1,3-imidazol-2-ylidene-κC2)(triphenylphosphane-κP)iridium(I) tetrafluoridoborate

In the title compound, [Ir(C8H12)(C8H14N2)(C18H15P)]BF4, the cationic complex has the anticipated square-planar geometry. The asymmetric unit comprises the iridium complex and one tetrafluoridoborate anion. The space group is non-centrosymmetric, with the Flack parameter of 0.007 (3) well determined, and confirms the hand for the complex cation. This compound shows promising catalytic activity in transfer hydrogenation reactions.

Enhanced Catalytic Activity of Iridium(III) Complexes by Facile Modification of C,N-Bidentate Chelating Pyridylideneamide Ligands

A set of aryl-substituted pyridylideneamide (PYA) ligands with variable donor properties owing to a pronounced zwitterionic and a neutral diene-type resonance structure were used as electronically flexible ligands at a pentamethylcyclopentadienyl (Cp*) iridium center. The straightforward synthesis of this type of ligand allows for an easy incorporation of donor substituents such as methoxy groups in different positions of the phenyl ring of the C,N-bidentate chelating PYA. These modifications considerably enhance the catalytic activity of the coordinated iridium center toward the catalytic aerobic transfer hydrogenation of carbonyls and imines as well as the hydrosilylation of phenylacetylene. Moreover, these PYA iridium complexes catalyze the base-free transfer hydrogenation of aldehydes, and to a lesser extent also of ketones. Under standard transfer hydrogenation conditions including base, aldehydes are rapidly oxidized to carboxylic acids rather than reduced to the corresponding alcohol, as is observed under base-free conditions.