Ir Cl Research Articles

Organometallic complexes as preferred precursors to form molecular Ir(pyalk) coordination complexes for catalysis of oxygen evolution

Our previously reported ‘blue solution’ oxygen-evolution catalyst consists of an isomeric mixture of coordination complexes containing the (pyalk)IrIV–O–IrIV(pyalk) core unit and is thus entirely molecular but only when formed from organometallic precursors such as Cp*Ir(pyalk)Cl or Ir(pyalk)(CO)2 (pyalk = (2-pyridyl)-2-propanolate). We now show that attempts to form it from such obvious coordination precursors as Na[Ir(pyalk)Cl4] or Na[IrCl2(pyalk)(O2CPh)2], under a variety of conditions, always fail in our hands, leading to a mixture of molecular ‘blue solution’ species and IrOx nanoparticles, rather than the purely homogeneous catalyst formed from organometallic precursors. The loss of the pyalk ligand during the oxidative activation is associated with the nanoparticle generation. External chelating ligands also failed to stop the nanoparticle formation. This work implies the paradoxical conclusion that organometallic complexes are effective as catalyst precursors, even when coordination complexes are the catalytically active species, since the inner-sphere organometallic ligands, although ultimately lost or degraded, may nevertheless have a stabilizing effect sufficient to suppress undesirable nanoparticle production pathways in the activation of the catalyst precursor. A key aspect of the present study is that organometallic complexes in general may be useful catalyst precursors even if the organometallic ligands are lost in the activation process.

Iridium‐Catalyzed [4+3] Cyclization of ortho‐Tosylaminophenyl‐Substituted para‐Quinone Methides with Vinylic Oxiranes/Vinyl Aziridines

AbstractAn Iridium‐catalyzed [4+3] cyclization of ortho‐tosylaminophenyl‐substituted para‐quinone methides with vinylic oxiranes or vinyl aziridines has been successfully developed. In the presence of [Ir(cod)Cl]2 and a suitable base, this unprecedented cascade reaction occurs readily in good yield (up to 97%) and high diastereoselectivity (dr>20 : 1), providing a highly efficient synthetic approach to synthetically valuable seven‐membered heterocyclic frameworks.

Synthesis and Isolation of an Anionic Bis(dipyrido-annulated) N-Heterocyclic Carbene CCC-Pincer Iridium(III) Complex by Facile C-H Bond Activation.

Meridional tridentate N-heterocyclic carbene (NHC)-based pincer ligands contribute to a substantial growth in modern organometallic chemistry in both homogeneous catalysis and luminescence materials. Among all NHC-based pincer ligands, the dianionic LX2-type CCC-pincer ones constitute the smallest subcategory owing to their limited ligand frameworks suitable for complexation. This work reports a one-pot, high-yield synthesis of a homoleptic anionic all-carbon bis-pincer iridium(III) complex (4) directly from a bis(aryl)-substituted dipyrido-annulated (dpaAr2) imidazolium salt and [Ir(COD)Cl]2 via a cascade of deprotonation/C-H activation processes. Both experimental complexation chemistry and computational mechanistic investigation suggest that the large bite angle and π-rich character of the dpaAr2 NHC are responsible for its facile complexation as a dianionic LX2-type CCC-pincer ligand precursor. The all-carbon ligated iridium(III) complex (4) bearing a π-conjugated ligand scaffold showed remarkably low oxidation potentials, which allows future investigations in its redox chemistry and photophysical properties.

A Proton-Responsive Pyridyl(benzamide)-Functionalized NHC Ligand on Ir Complex for Alkylation of Ketones and Secondary Alcohols.

A Cp*Ir(III) complex (1) of a newly designed ligand L1 featuring a proton-responsive pyridyl(benzamide) appended on N-heterocyclic carbene (NHC) has been synthesized. The molecular structure of 1 reveals a dearomatized form of the ligand. The protonation of 1 with HBF4 in tetrahydrofuran gives the corresponding aromatized complex [Cp*Ir(L1 H)Cl]BF4 (2). Both compounds are characterized spectroscopically and by X-ray crystallography. The protonation of 1 with acid is examined by 1 H NMR and UV-vis spectra. The proton-responsive character of 1 is exploited for catalyzing α-alkylation of ketones and β-alkylation of secondary alcohols using primary alcohols as alkylating agents through hydrogen-borrowing methodology. Compound 1 is an effective catalyst for these reactions and exhibits a superior activity in comparison to a structurally similar iridium complex [Cp*Ir(L2 )Cl]PF6 (3) lacking a proton-responsive pendant amide moiety. The catalytic alkylation is characterized by a wide substrate scope, low catalyst and base loadings, and a short reaction time. The catalytic efficacy of 1 is also demonstrated for the syntheses of quinoline and lactone derivatives via acceptorless dehydrogenation, and selective alkylation of two steroids, pregnenolone and testosterone. Detailed mechanistic investigations and DFT calculations substantiate the role of the proton-responsive ligand in the hydrogen-borrowing process.

Oxidative Coordination versus C3 -C(O)Me Bond Cleavage in Acetylacetonate Iridium Complexes.

Iridabicycles [Ir{κ3 -N,C,O-(pyC(H)=C(C(O)Me)2 }(Cl)(L-L)](L-L=cod (cod=1,5-cyclooctadiene), 1 a; bipy (bipy=2,2'-bipyridine), 1 b) have been obtained by oxidative coordination of 3-(pyridine-2-yl-methylene)pentane-2,4-dione L1, to the complexes [{Ir(μ-Cl)(cod)}2 ] and [{Ir(μ-Cl)(coe)2 }2 ] (coe=cis-cyclooctene), the latter in the presence of bipy. Remarkably, cleavage of the C3 -C(O)Me bond of L1 has instead been achieved in the reaction with [Ir(Cl)(dmb)2 ] (dmb=2,3-dimethylbutadiene), yielding a compound formulated as [Ir{κ2 -N,C-(pyC(H)C(C(O)Me))}(CO)(μ-Cl)(Me)]2 , 2. Treatment of dimer 2 with DMSO or PMe3 produced the complexes[Ir{κ2 -N,C-(pyC(H)C(C(O)Me)}(CO)Cl(Me)L] (L=DMSO, 3 a; PMe3 , 3 b). Plausible mechanisms for the reactions leading to complexes 1 and 2 are proposed by means of DFT calculations.

Efficient Iridium Catalysts for Formic Acid Dehydrogenation: Investigating the Electronic Effect on the Elementary β-Hydride Elimination and Hydrogen Formation Steps

We report herein a series of Cp*Ir complexes containing a rigid 8-aminoquinolinesulfonamide moiety as highly efficient catalysts for the dehydrogenation of formic acid (FA). The complex [Cp*Ir(L)Cl] (HL = N-(quinolin-8-yl)benzenesulfonamide) displayed a high turnover frequency (TOF) of 2.97 × 104 h-1 and a good stability (>100 h) at 60 °C. Comparative studies of [Cp*Ir(L)Cl] with the rigid ligand and [Cp*Ir(L')Cl] (HL' = N-propylpypridine-2-sulfonamide) without the rigid aminoquinoline moiety demonstrated that the 8-aminoquinoline moiety could dramatically enhance the stability of the catalyst. The electron-donating ability of the N,N'-chelating ligand was tuned by functionalizing the phenyl group of the L ligand with OMe, Cl, and CF3 to have a systematical perturbation of the electronic structure of [Cp*Ir(L)Cl]. Experimental kinetic studies and density functional theory (DFT) calculations on this series of Cp*Ir complexes revealed that (i) the electron-donating groups enhance the hydrogen formation step while slowing down the β-hydride elimination and (ii) the electron-withdrawing groups display the opposite effect on these reaction steps, which in turn leads to lower optimum pH for catalytic activity compared to the electron-donating groups.

Oxidative addition reactions of an iridium(I) complex containing phosphine-NHC

The phosphane functionalized N-heterocyclic carbene ligand 3-[2-(diphenylphosphino)ethyl]-1-ethylimidazol-2-ylidene was reacted with the [Ir(COD)Cl]2 metal precursor to give the homoleptic mononuclear cationic complex [Ir(NHC-PPh2)2][PF6] 2a in good yield. 2a undergoes oxidative addition and addition reactions with small molecules like H2, O2, S8, I2, CH3I and CO to afford the novel Ir(III) complexes [Ir(H)2(NHC-PPh2)2][PF6] 3a, [Ir(η2-O2)(NHC-PPh2)2][PF6] 4a, [Ir(η2-S2)(NHC-PPh2)2][PF6] 5a, [Ir(I)2(NHC-PPh2)2][I3] 6a, [Ir(CH3)(I)(NHC-PPh2)2][I] 7a, and [Ir(CO)(NHC-PPh2)2][PF6] 8a, respectively. All the new Ir(I) and Ir(III) complexes have been characterised by 31P, 1H and 13C NMR spectroscopy and single-crystal X-ray analysis.

The catalytic activity of new iridium(I) N-heterocyclic carbene complexes for hydrogen transfer reaction of ketones

In this paper, the reaction of [Ir(COD)Cl]2 with in situ prepared Ag–N-heterocyclic carbene (NHC) complexes yields a series of [IrCl(COD)(NHC)] complexes. All compounds were fully characterized by 1H NMR, 13C NMR, and FT–IR spectroscopy. The manuscript focused on the preparation of new Ir–NHC complexes, characterization and catalytic behavior. A series of hydrogenation transfer reactions were performed to reveal the effects of the Ir–NHC complexes. The new Ir–NHC complexes of benzimidazole-2-ylidene are effective catalysts for the transfer of hydrogenation of different ketones, using i-PrOH as the source of hydrogen in the presence of KOH. The reactions were conducted at a substrate/catalyst/base (S/C/base) molar ratio of 1:0.001:2. Although all of the complexes are active catalysts for the transfer hydrogenation of ketones, moderate yields were obtained with acetylnaphthalene and conversion was not observed with very substituted ketones such as 2′,3′,4′,5′,6′-pentamethylacetophenone. It was observed that for transfer hydrogenation reactions Ir–NHC catalysts were more active, compared to Ru–NHC catalyzed studies performed by our team.

N-Heterocyclic Carbene Complexes of Nickel, Palladium, and Iridium Derived from Nitron: Synthesis, Structures, and Catalytic Properties

The mesoionic compound (1,4-diphenyl-1,2,4-triazol-4-ium-3-yl)phenylazanide, commonly referred to as Nitron, has been employed as a “crypto-NHC” to afford 1,2,4-triazolylidene compounds of nickel, palladium, and iridium. Specifically, Nitron reacts with NiBr2, PdCl2, and [Ir(COD)Cl]2 to afford the N-heterocyclic carbene complexes (NitronNHC)2NiBr2, (NitronNHC)2PdCl2, and (NitronNHC)Ir(COD)Cl, respectively. The lattermost compound reacts with (i) CO to afford the dicarbonyl compound (NitronNHC)Ir(CO)2Cl and (ii) CO, in the presence of PPh3, to afford the monocarbonyl compound (NitronNHC)Ir(PPh3)(CO)Cl. Structural studies on (NitronNHC)Ir(COD)Cl and (NitronNHC)Ir(CO)2Cl indicate that NitronNHC has a stronger trans influence than does Cl; furthermore, IR spectroscopic studies on (NitronNHC)Ir(CO)2Cl indicate that NitronNHC is electronically similar to the structurally related Enders carbene but is less electron donating than imidazol-2-ylidenes with aryl substituents. Significantly, the NitronNHC ligand affords catalytic systems, as illustrated by the ability of (NitronNHC)Ir(CO)2Cl to effect (i) the dehydrogenation of formic acid, (ii) aldehyde hydrosilylation, (iii) dehydrocoupling of hydrosilanes and alcohols, and (iv) ketone reduction via transfer hydrogenation.

Preparation and antitumor application of N-phenylcarbazole/triphenylamine-modified fluorescent half-sandwich iridium(III) Schiff base complexes.

Four N-phenylcarbazole/triphenylamine-appended half-sandwich iridium(III) salicylaldehyde Schiff base complexes ([(η5-Cpx)Ir(O^N)Cl]) were prepared and characterized. The complexes exhibited similar antitumor activity to cisplatin and effectively inhibited the migration of tumor cells. Furthermore, the complexes showed favourable hydrolytic activity, while remaining relatively stable in the plasma environment, which facilitated the binding of serum proteins and transport through them. These complexes could decrease the mitochondrial membrane potential, catalyze the oxidation of nicotinamide adenine dinucleotide, induce an increase in intracellular reactive oxygen species (ROS), and eventually result in apoptosis. Aided by their suitable fluorescence property, laser confocal detection showed that the complexes followed an energy-dependent mechanism for their cellular uptake, effectively accumulating in the lysosome and leading to lysosomal damage. In summary, the half-sandwich iridium(III) salicylaldehyde Schiff base complexes could induce lysosomal damage, increase intracellular ROS, and lead to apoptosis, which contributed to their antitumor mechanism of oxidation.

A highly active Cp*Ir complex with an anionic N,N-donor chelate ligand catalyzes the robust regeneration of NADH under physiological conditions

Anionic N,N-donor ligand chelate iridium complex [Cp*Ir(pba)Cl] 3 was developed, and exhibited the highest activity for NADH regeneration so far and stable chemoenzymatical coordinate catalytic performance for acetophenone enantioselective hydrogenation.

Regioselective B(3)-H bond activation based on an o-carboranyl dithiocarboxylate ligand and its derivatives.

Herein, we describe the synthesis of tetraphenylphosphonium o-carboranyl dithiocarboxylate (Ligand 1) and methyldithiocarboxyl-o-carborane (Ligand 2). The complexes [Cp*M(o-C2B10H11CS2)Cl] (M = Ir (3); Rh (4)) and [Cp*M(o-C2B10H11CS2)2] (M = Ir (5); Rh (6)) have been synthesized based on Ligand 1. The selective B-H bond activation of Ligand 2 has also been explored, leading to the synthesis of the B-H activated complex [Cp*Ir(o-C2B10H10CS2CH3)Cl] (7) and four of its substituted derivatives (8, 9, 10 and 11). All of these compounds have been characterised through single-crystal X-ray diffraction, NMR, IR spectroscopy and elemental analysis.

Understanding the Deactivation Pathways of Iridium(III) Pyridine-Carboxiamide Catalysts for Formic Acid Dehydrogenation.

The degradation pathways of highly active [Cp*Ir(κ2 -N,N-R-pica)Cl] catalysts (pica=picolinamidate; 1 R=H, 2 R=Me) for formic acid (FA) dehydrogenation were investigated by NMR spectroscopy and DFT calculations. Under acidic conditions (1 equiv. of HNO3 ), 2 undergoes partial protonation of the amide moiety, inducing rapid κ2 -N,N to κ2 -N,O ligand isomerization. Consistently, DFT modeling on the simpler complex 1 showed that the κ2 -N,N key intermediate of FA dehydrogenation (INH ), bearing a N-protonated pica, can easily transform into the κ2 -N,O analogue (INH2 ; ΔG≠ ≈11 kcal mol-1 , ΔG ≈-5 kcal mol-1 ). Intramolecular hydrogen liberation from INH2 is predicted to be rather prohibitive (ΔG≠ ≈26 kcal mol-1 , ΔG≈23 kcal mol-1 ), indicating that FA dehydrogenation should involve mostly κ2 -N,N intermediates, at least at relatively high pH. Under FA dehydrogenation conditions, 2 was progressively consumed, and the vast majority of the Ir centers (58 %) were eventually found in the form of Cp*-complexes with a pyridine-amine ligand. This likely derived from hydrogenation of the pyridine-carboxiamide via a hemiaminal intermediate, which could also be detected. Clear evidence for ligand hydrogenation being the main degradation pathway also for 1 was obtained, as further confirmed by spectroscopic and catalytic tests on the independently synthesized degradation product 1 c. DFT calculations confirmed that this side reaction is kinetically and thermodynamically accessible.

Dehydrogenative Diboration of Alkynes Catalyzed by Ir/CO/tBuNC System

Catalytic dehydrogenative diboration (DHDB) of alkyne with HBpin was achieved using [Ir(COD)Cl]2 and other related Ir precursors under CO atmosphere. The selectivity for DHDB over hydroboration was higher in less polar solvents and under higher CO pressure. It was further improved when catalytic amount of tBuNC was added to the reaction. It was possible to achieve DHDB of both terminal and internal alkynes with selectivity for DHDB of up to 9:1 under the best conditions. Some DHDB products were isolated on the preparative scale.

Hexafluoroacetylacetonate (hfac) as ligand for pentamethylcyclopentadienyl (Cp*) rhodium and iridium complexes: Some surprising results, including an Ir3Na1O4 cubane structure

Attempts to prepare hexafluoroacetylacetonate (hfac) piano stool complexes of pentamethylcyclopentadienyl Cp* iridium and rhodium led to a surprising array of unexpected products. The desired complex, Cp*M(hfac)Cl, was obtained following the reaction of sodium hexafluoroacetylacetonate (hfacNa) with [Cp*MCl2]2 in dichloromethane. Variations of this synthetic method resulted in twelve unique crystallographically identified products, eleven of which contain the hfac ligand or a trifluoroacetylacetonate (TFA) ligand, either coordinated to the metal or as a non-coordinating anion. Five dinuclear Cp* iridium hydroxo-bridged products with various fluorinated non-coordinating anions were obtained. The most intriguing two complexes are trinuclear Cp*IrIII hydroxo-bridged clusters that have, at their core, an Ir3NaO4 cubane structure. Attempts to devise rational syntheses of the hydroxo-bridged cluster compounds were not successful. Generation of the TFA moieties likely occurred following degradation of Cp*Ir(hfac)Cl. The reaction between [Cp*MCl2]2 and 1,1,1-trifluoroacetylacetonate (tfac) proceeded as expected to give Cp*M(tfac)Cl with no observed side product formation, indicating that the metal-coordinated tfac moiety is significantly more stable than the metal-coordinated hfac moiety.

Iridium complexes with ligands of 1,8-Naphthyridine-2-carboxylic acid derivatives-preparation and catalysis

Complexation of 1,8-naphthyridine(Np)-2-carboxylic derivatives L1-L3 [L1 = Np-2-COOH, L2 = Np-2-CONH2, L3 = Np-2-CONHCH2Py] with [Ir(COD)(µ-OMe)]2 yielded the corresponding complexes [Ir(COD)(Ln)] (1~3, n = 1~3, respectively). The potential tridentate L3 behaves as a bidentate donor in the complex 3. Treatment of L1 with [Ir(COD)Cl]2 under nitrogen atmosphere gave a Ir(III) hydride complex [Ir(COD)(L1)HCl] (4). However, carrying out the reaction in the presence of oxygen rendered a Ir(III) dichloride species [Ir(COD)(L1)Cl2] (5). All these complexes were characterized by spectroscopic analyses and X-ray single crystal determination. Catalytic activity of iridium complexes in amination of amines with alcohols was screened. It appears that iridium amido complexes 2 and 3 show excellent catalytic activity on amination of anilines with alcohols in the presence of Cs2CO3 at 120 °C.

Crystal structures of [μ2-(Ra ,Sa ,3aR,7aR)-1,3-bis-(2,7-di-cyclo-hexyl-naphthalen-1-yl)octa-hydro-1H-benzo[d]imidazolidin-2-yl-idene]chlorido-(η4-1,5-cyclo-octa-diene)iridium di-chloro-methane monosolvate and [μ2-(Sa ,Sa ,3aR,7aR)-1,3-bis-(2,7-di-cyclo-hexyl-naphthalen-1-yl)octa-hydro-1H-benzo[d]imidazolidin-2-yl-idene]chlorido-(η4-1,5-cyclo-octa-diene)iridium.

The title compounds, [Ir(C51H64N2)Cl(C8H12)]·CH2Cl2, (I), and [Ir(C51H64N2)Cl(C8H12)], (II), represent the first two examples of hexa-hydro-benzo-imidazole-based N-heterocyclic carbene (NHC) iridium complexes. The diastereomeric complexes differing only in their axial chirality, which could be separated via column chromatography, show noticeable differences in their 1H NMR spectra. Compound (I) crystallizes in the monoclinic system (P21) with two independent complexes and two half-occupied di-chloro-methane mol-ecules in the asymmetric unit, while compound (II) crystallizes in the ortho-rhom-bic system (P212121) with one complex in the asymmetric unit. The fused five-membered N-heterocycles of NHCs show unusually high backbone torsion angles of -34.1 (5) and -30.9 (5)° for (I) and -31.5 (7)° for (II), but the Ir-Ccarbene bond lengths of 2.046 (6) and 2.021 (6) Å for (I) and 2.045 (8) Å for (II) present typical NHC-Ir bond lengths. The solvent mol-ecule in the crystal of (I) was found to be highly disordered and its contribution to the scattering was masked using the solvent-masking routine smtbx.mask in OLEX2 [Dolomanov et al. (2009 ▸). J. Appl. Cryst. 42, 339-341]. The solvent contribution is not included in the reported chemical formula and other crystal data.

Phosphorescent Iridium(III) Complexes with a Dianionic C,C',N,N'-Tetradentate Ligand.

To prepare new phosphorescent iridium(III) emitters, 2-phenyl-6-(1-phenyl-1-(pyridin-2-yl)ethyl)pyridine (H2L) has been designed and its reactions with [Ir(μ-Cl)(η4-COD)]2 (1, COD = 1,5-cyclooctadiene) have been studied. The products obtained depend on the refluxing temperature of the solvent. Thus, complexes Ir(κ4-C,C',N,N'-L)Cl(CO) (2), [Ir(η4-COD)(κ2-N,N'-H2L)][IrCl2(η4-COD)] (3), and [Ir(μ-Cl)(κ4-C,C',N,N'-L)]2 (4) have been formed in 2-ethoxyethanol, propan-2-ol, and 1-phenylethanol, respectively. Complex 4 reacts with K(acac) to give the acetylacetonate derivative Ir(κ4-C,C',N,N'-L)(acac) (5). Complexes 2 and 5 are efficient blue-green and green emitters of classes [6tt+1m+2m] and [6tt+3b], respectively. They display lifetimes in the range of 1.1-4.5 μs and high quantum yields (0.54-0.87) in both PMMA films and 2-MeTHF at room temperature.

A Visible-Light Promoted Amine Oxidation Catalyzed by a Cp*Ir Complex.

Through a rapid screening of Cp*Ir complexes based on a turn‐on type fluorescence readout, a [Cp*Ir(dipyrido[3,2‐a : 2’,3’‐c]phenazine)Cl]+ complex was found to catalyze the blue‐light promoted dehydrogenation of N‐heterocycles under physiological conditions. In the dehydrogenation of tetrahydroisoquinolines, the catalyst preferentially yielded the monodehydrogenated product, accompanying H2O2 generation. We surmise that this mechanism may be reminiscent of flavin‐dependent oxidases.

Synthesis, characterisation and comparative study of the hydroxyl, acrylate and vinyl-ether terminated cyanobiphenyl bridged with different spacer lengths

ABSTRACT Three homologous series hydroxyl, acrylate and vinyl-ether terminated cyanobiphenyl compounds have been synthesised, termed as nOCB-OH, nOCB-AE and nOCB-VE (n = 2–12), respectively. The vinyl-ether terminated cyanobiphenyl compounds were synthesised by employing environmentally benign, highly efficient iridium complexes [Ir(cod)Cl]2 catalyser. The chemical structures and liquid crystalline properties of these compounds were characterised by nuclear magnetic resonance (1H NMR and 13C NMR), Fourier transform infrared spectroscopy (FT-IR), differential scanning calorimetry (DSC) and polarised optical microscopy (POM). Furthermore, the structure-properties relationships were systematically investigated with the aid of molecular modelling at the B3LYP/6-31G* level of theory.