Diiridium Complex Research Articles

Structural Diversity in Cyclometalated Diiridium(III) Complexes with Bridging syn and anti μ2‐Oxamidato and μ2‐Dithioxamidato Ligands

AbstractSix new diiridium complexes containing 2‐methyl‐6‐phenylpyridyl as the cyclometalating ligand with a μ2‐oxamidato or a μ2‐dithioxamidato ligand as the bridge have been synthesized in 60–73 % yields. These complexes were revealed by multinuclear NMR spectroscopy to contain inseparable mixtures of diastereomers (rac, ΔΔ/ΛΛ and meso, ΔΛ) with bridges in anti and syn configurations. The remarkable variety of isomers present was confirmed by X‐ray crystallography on single crystals grown from mixtures of each complex. In one complex with a N,N’‐bis(4‐trifluoromethylphenyl)‐μ2‐oxamidato bridge, two single crystals of anti and syn isomers were structurally determined. Two single crystals of the μ2‐dithioxamidato bridge complex were found to contain rac and meso forms of the syn isomer. Hybrid DFT computations on the four isomers of each diiridium complex revealed negligible energetic preferences for one isomer despite the methyl groups in the 2‐methyl‐6‐phenylpyridyl cyclometalating ligands being close to the neighboring methyl groups and the bridge, thus supporting the experimental findings of isomer mixtures. Two distinct broad emissions with maxima at 522–529 nm and at 689–701 nm observed in these complexes in dichloromethane are attributed to mixed metal‐ligand to ligand charge transfer (MLLCT) excited states involving the pyridyl and bridge moieties respectively with the aid of electronic structure computations.

Efficient near-infrared (NIR) polymer light-emitting diode (PLED) based on the binuclear [(C^N)2Ir-(bis-N^O)-Ir(C^N)2] complex with aggregation-induced phosphorescent enhancement (AIPE) character

In this work, for the obtained NIR-emitting dinuclear [(iqbt)2Ir-Ln-Ir(iqbt)2] complexes (n = 1–3, 1–3; Hiqbt = 1-(benzo[b]-thiophen-2-yl)-isoquinoline) with the bridging linkages of three flexible bis-N^O-Schiff-base ligands H2Ln, their obvious aggregation-induced phosphorescent enhancement (AIPE) behaviors in powder-form solid or MeCN–H2O mixture are observed, respectively. Moreover, based on the AIPE-switched NIR phosphorescence (Φem = 0.11 and τ = 0.27 μs) of the diiridium complex 1 without electron-donating substituents in the aggregation state, its solution-processed NIR-PLED is reliably realized, exhibiting the attractive electroluminescent performance with both ηEQEmax = 1.08% and almost negligible efficiency-roll-off.

Spectroscopic Characterization of μ-η1:η1-Peroxo Ligands Formed by Reaction of Dioxygen with Electron-Rich Iridium Clusters.

Although oxygen is a common ligand in supported metal catalysts, its coordination has been challenging to elucidate. We now characterize a diiridium complex that has been previously shown by X-ray diffraction crystallography to incorporate a μ-η1:η1-peroxo ligand. We observe markedly enhanced intensity at 788 cm-1 in the Raman spectrum of this complex, which is a consequence of bonding of the peroxo ligand but does not shift upon 18O labeling. Electronic structure calculations at the density functional theory level suggest that this increase in Raman intensity results from bands associated with rocking of CH2 substituents directly attached to P(Ph)2 groups coupling with the O-O band. These results provide part of the foundation for understanding oxygen ligands on a silica-supported tetrairidium carbonyl cluster stabilized with bulky electron-donating phosphine ligands [p-tert-butyl-calix[4]arene(OPr)3(OCH2PPh2) (Ph = phenyl; Pr = propyl)]. Reaction of the cluster with O2 also led to the growing in of a Raman band at 788 cm-1, similar to that in the diiridium complex and also assigned to the bonding of a bridging peroxo ligand. Infrared spectra recorded as the supported cluster reacted in sequential exposures to (i) H2, (ii) O2, (iii) H2, and (iv) CO indicate that two bridging peroxo ligands were bonded irreversibly per tetrairidium cluster, replacing bridging carbonyl ligands without altering either the cluster frame or the phosphine ligands. X-ray absorption near edge and infrared spectra include isosbestic points signifying a stoichiometrically simple reaction of the cluster with O2, and mass spectra of the effluent gas show that CO2 formed by oxidation of one terminal CO ligand per cluster as H2 (and not H2O) formed, evidence that hydride ligands had been present on the cluster following treatment (i). The understanding of how O2 reacts with the metal polyhedron provides a foundation for understanding of how oxidation catalysis may proceed on the surfaces of noble metals.

Successive Activation of C–H and C–O Bonds of Vinyl Ethers by a Diphosphine and Hydrido-Bridged Diiridium Complex

The reaction of [(Cp*Ir)2(μ-dmpm)(μ-H)][OTf] (2) [Cp* = η5-C5Me5, dmpm = bis(dimethylphosphino)methane] with 2,3-dihydrofuran gives [(Cp*IrH)2(μ-dmpm){μ-(2,3-dihydrofuranyl)}][OTf] (3) in an isolated yield of 70% via the C–H bond activation at the 5-position of 2,3-dihydrofuran. Complex 3 is slowly converted into [(Cp*Ir)2(μ-dmpm)(μ-C=C(H)CH2CH2OH)][OTf] (4) quantitatively via the proton-mediated C–O bond activation. The reaction of 2 with ethyl vinyl ether gives [(Cp*Ir)2(μ-dmpm)(μ-CH=CH2)][OTf] (5) in the isolated yield of 64% via the successive activation of C–H and C–O bonds.

A Single‐Anion‐Based Red‐Emitting Cationic Diiridium(III) Complex Bearing a Pyrimidine‐Based Bridging Ligand for Oxygen Sensing

Compared with double‐anion‐based cationic diiridium complexes, the study of single‐anion‐based cationic diiridium complexes as emitters is largely unexplored. In this work, two methods were used to prepare single‐anion‐based cationic diiridium complex Ir‐Ir‐1 bearing 2‐pyrimidine‐benzimidazole as bridging ligand, both sides of which are symmetrical structures comprising 2‐phenylpyridine as cyclometalated ligand. It shows efficient red phosphorescence emission with redshift of the emission wavelength by 85 nm relative to the corresponding monoiridium complex Ir‐1. Moreover, it exhibits long emission lifetime and high oxygen sensitivity, which were attributed to the intraligand charge‐transfer excited state. Hence, Ir‐Ir‐1 was used as a oxygen probe to monitor intracellular O2 content by time‐resolved luminescence imaging.

Pyridazine-bridged cationic diiridium complexes as potential dual-mode bioimaging probes.

A novel diiridium complex [(N^C^N)2Ir(bis-N^C)Ir(N^C^N)2Cl]PF6 (N^C^N = 2-[3-tert-butyl-5-(pyridin-2-yl)phenyl]pyridine; bis-N^C = 3,6-bis(4-tert-butylphenyl)pyridazine) was designed, synthesised and characterised. The key feature of the complex is the bridging pyridazine ligand which brings two cyclometallated Ir(iii) metal centres close together so that Cl also acts as a bridging ligand leading to a cationic complex. The ionic nature of the complex offers a possibility of improving solubility in water. The complex displays broad emission in the red region (λem = 520–720 nm, τ = 1.89 μs, Φem = 62% in degassed acetonitrile). Cellular assays by multiphoton (λex = 800 nm) and confocal (λex = 405 nm) microscopy demonstrate that the complex enters cells and localises to the mitochondria, demonstrating cell permeability. Further, an appreciable yield of singlet oxygen generation (ΦΔ = 0.45, direct method, by 1O2 NIR emission in air equilibrated acetonitrile) suggests a possible future use in photodynamic therapy. However, the complex has relatively high dark toxicity (LD50 = 4.46 μM), which will likely hinder its clinical application. Despite this toxicity, the broad emission spectrum of the complex and high emission yield observed suggest a possible future use of this class of compound in emission bioimaging. The presence of two heavy atoms also increases the scattering of electrons, supporting potential future applications as a dual fluorescence and electron microscopy probe.

H–H and N–H Bond Cleavages of Dihydrogen and Ammonia by a Bifunctional Imido (NH)-Bridged Diiridium Complex: A DFT Study

The mechanisms of H–H and N–H bond cleavages of dihydrogen and ammonia mediated by the diiridium μ2-imido complex [(Cp*Ir)2(μ2-H)(μ2-NH)]+ (A; Cp* = η5-C5Me5) were theoretically investigated with the density functional theory (DFT) method. Both oxidative addition and metal–ligand cooperation modes have been studied for H–H and N–H bond cleavages, respectively. The H–H bond cleavage most likely occurs through competitive oxidative addition and metal–ligand cooperation, and both cleavage modes have similar overall free energy barriers (24.3 and 25.7 kcal/mol, respectively). The ligand-assisted N–H bond heterolytic cleavage mechanism is proposed for the NH3 reaction, and the general oxidative addition pathway can be reasonably ruled out, as it is kinetically unfavorable.

Aqueous NMR Signal Enhancement by Reversible Exchange in a Single Step Using Water-Soluble Catalysts.

Two synthetic strategies are investigated for the preparation of water-soluble iridium-based catalysts for NMR signal amplification by reversible exchange (SABRE). In one approach, PEGylation of a variant N-heterocyclic carbene provided a novel catalyst with excellent water solubility. However, while SABRE-active in ethanol solutions, the catalyst lost activity in >50% water. In a second approach, synthesis of a novel di-iridium complex precursor where the cyclooctadiene (COD) rings have been replaced by CODDA (1,2-dihydroxy-3,7-cyclooctadiene) leads to the creation of a catalyst [IrCl(CODDA)IMes] that can be dissolved and activated in water—enabling aqueous SABRE in a single step, without need for either an organic cosolvent or solvent removal followed by aqueous reconstitution. The potential utility of the CODDA catalyst for aqueous SABRE is demonstrated with the ∼(−)32-fold enhancement of 1H signals of pyridine in water with only 1 atm of parahydrogen.

Oxidative Addition of the N–H Bond of Ammonia to Iridium Bis(phosphane) Complexes: A Combined Experimental and Theoretical Study

The chloro complex [IrCl(cod)(dppe)] (dppe = bis(diphenylphosphane)ethane; cod = 1,5-cyclooctadiene) was found to react with gaseous ammonia, affording the amido-bridged diiridium complex [{Ir(μ-NH2)H(dppe)(NH3)}2][Cl]2 (1), whose molecular structure has been solved by X-ray methods. The related cationic complexes [{Ir(μ-NH2)H(dppp)(NH3)}2][BF4]2 (3) and [{Ir(μ-NH2)H(dppb)(NH3)}2][BF4]2 (4) (dppp = bis(diphenylphosphane)propane; dppb = bis(diphenylphosphane)butane) are only accessible from the reactions of NH3(g) with the cationic starting materials [Ir(cod)(dppp)][BF4] and [Ir(cod)(dppb)][BF4], respectively. The formation of these species comes from an oxidative addition of an N–H bond of ammonia to the metals. The main structural difference between complex 2 and 3/4 relies on the relative stereochemistry of both ammonia and hydrido ligands; their cisoidal disposition in dppe complex [{Ir(μ-NH2)H(dppe)(NH3)}2][BF4]2 (2) directed the reactivity toward dppm (dppm = bis(diphenylphosphane)methane), generatin...

Nucleophilicity and P–C Bond Formation Reactions of a Terminal Phosphanido Iridium Complex

The diiridium complex [{Ir(ABPN2)(CO)}2(μ-CO)] (1; [ABPN2](-) = [(allyl)B(Pz)2(CH2PPh2)](-)) reacts with diphenylphosphane affording [Ir(ABPN2)(CO)(H) (PPh2)] (2), the product of the oxidative addition of the P-H bond to the metal. DFT studies revealed a large contribution of the terminal phosphanido lone pair to the HOMO of 2, indicating nucleophilic character of this ligand, which is evidenced by reactions of 2 with typical electrophiles such as H(+), Me(+), and O2. Products from the reaction of 2 with methyl chloroacetate were found to be either [Ir(ABPN2)(CO)(H)(PPh2CH2CO2Me)][PF6] ([6]PF6) or [Ir(ABPN2)(CO)(Cl)(H)] (7) and the free phosphane (PPh2CH2CO2Me), both involving P-C bond formation, depending on the reaction conditions. New complexes having iridacyclophosphapentenone and iridacyclophosphapentanone moieties result from reactions of 2 with dimethyl acetylenedicarboxylate and dimethyl maleate, respectively, as a consequence of a further incorporation of the carbonyl ligand. In this line, the terminal alkyne methyl propiolate gave a mixture of a similar iridacyclophosphapentanone complex and [Ir(ABPN2){CH═C(CO2Me)-CO}{PPh2-CH═CH(CO2Me)}] (10), which bears the functionalized phosphane PPh2-CH═CH(CO2Me) and an iridacyclobutenone fragment. Related model reactions aimed to confirm mechanistic proposals are also studied.

Direct Synthesis and Bonding Properties of the First μ(2)-η(2),η(2)-Allyl-Bridged Diiridium Complex.

The direct synthesis of the first μ(2)-η(2),η(2)-allyl-bridged diiridium complex ([2](+)), bearing the uncommon counterion [IrCl2(COD)](-) ([3](-)), is described. Both bridging moieties in [2](+), namely, allyl and acetate, are introduced in a single reaction step from [{IrCl(COD)}2] (1) and allyl acetate. A combination of X-ray crystallography and density functional theory calculations reveals pronounced metal-allyl π-back-bonding.

P–H activation of secondary phosphanes on a parent amido diiridium complex

Selected secondary phosphanes (H-PR2; R = Ph, Cy, (i)Pr) smoothly react with a parent amido-bridged diiridium cyclooctadiene complex affording mixed amido/(bis)phosphido dinuclear species. A careful investigation of the reaction profile, carried out by experimental and theoretical tools, revealed that, after an initial amido/phosphido exchange, at low temperatures a second molecule of secondary phosphane adds to the dinuclear system through an oxidative addition process leading to a hydrido amido/bis(phosphido) mixed-valence complex [Ir(III)/Ir(I)]. These species rearrange above -10 °C into the most stable isomer that arises from a migration of the hydrido moiety to one of the =CH fragments of a coordinated cod molecule, a transformation facilitated by the formation of an intermetallic bond. Further heating of these species reductively eliminates ammonia affording bis(phosphido)-metal-metal bonded complexes.

ChemInform Abstract: Solvent‐Free Preparation of N,N′‐Dialkylbenzenediamines.

A diiridium complex, stabilized by a bis(N-heterocyclic carbene) ligand, was found to be an effective precatalyst for the N,N′-dialkylation of o-, m-, and p-benzenediamine in excellent yields. Benzyl and primary aliphatic alcohols were used as alkylating reagents. Furthermore, the reactions were carried out without using any organic solvent, offering an environmentally benign process.

Terminal and Bridging Parent Amido 1,5‐Cyclooctadiene Complexes of Rhodium and Iridium

The ready availability of rare parent amido d(8) complexes of the type [{M(μ-NH2)(cod)}2] (M=Rh (1), Ir (2); cod=1,5-cyclooctadiene) through the direct use of gaseous ammonia has allowed the study of their reactivity. Both complexes 1 and 2 exchanged the di-olefines by carbon monoxide to give the dinuclear tetracarbonyl derivatives [{M(μ-NH2)(CO)2}2 ] (M=Rh or Ir). The diiridium(I) complex 2 reacted with chloroalkanes such as CH2Cl2 or CHCl3, giving the diiridium(II) products [(Cl)(cod)Ir(μ-NH2)2Ir(cod)(R)] (R=CH2Cl or CHCl2) as a result of a two-center oxidative addition and concomitant metal-metal bond formation. However, reaction with ClCH2CH2Cl afforded the symmetrical adduct [{Ir(μ-NH2)(Cl)(cod)}2] upon release of ethylene. We found that the rhodium complex 1 exchanged the di-olefines stepwise upon addition of selected phosphanes (PPh3, PMePh2, PMe2Ph) without splitting of the amido bridges, allowing the detection of mixed COD/phosphane dinuclear complexes [(cod)Rh(μ-NH2)2Rh(PR3)2], and finally the isolation of the respective tetraphosphanes [{Rh(μ-NH2)(PR3)2}2]. On the other hand, the iridium complex 2 reacted with PMe2 Ph by splitting the amido bridges and leading to the very rare terminal amido complex [Ir(cod)(NH2)(PMePh2)2]. This compound was found to be very reactive towards traces of water, giving the more stable terminal hydroxo complex [Ir(cod)(OH)(PMePh2)2]. The heterocyclic carbene IPr (IPr=1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) also split the amido bridges in complexes 1 and 2, allowing in the case of iridium to characterize in situ the terminal amido complex [Ir(cod)(IPr)(NH2)]. However, when rhodium was involved, the known hydroxo complex [Rh(cod)(IPr)(OH)] was isolated as final product. On the other hand, we tested complexes 1 and 2 as catalysts in the transfer hydrogenation of acetophenone with iPrOH without the use of any base or in the presence of Cs2CO3, finding that the iridium complex 2 is more active than the rhodium analogue 1.

Solvent-Free Preparation of N,N′-Dialkylbenzenediamines

A diiridium complex, stabilized by a bis(N-heterocyclic carbene) ligand, was found to be an effective precatalyst for the N,N′-dialkylation of o-, m-, and p-benzenediamine in excellent yields. Benzyl and primary aliphatic alcohols were used as alkylating reagents. Furthermore, the reactions were carried out without using any organic solvent, offering an environmentally benign process.

The Dehydrogenation of Alcohols through a Concerted Bimetallic Mechanism Involving an Amido‐Bridged Diiridium Complex

Financial support from the CONSOLIDER INGENIO-2010 program under the projects MULTICAT (CSD2009-00050) and Factoria de Cristalizacion (CSD2006-0015), and the DGA-ESF are acknowledged. P.G.O. acknowledges financial support from the CSIC “JAE-Doc” program, a contract co-funded by the ESF.

The Dehydrogenation of Alcohols through a Concerted Bimetallic Mechanism Involving an Amido‐Bridged Diiridium Complex

The Dehydrogenation of Alcohols through a Concerted Bimetallic Mechanism Involving an Amido‐Bridged Diiridium Complex

Trapping of a Doubly Unsaturated Dinuclear Iridium(II) Sulfonylimido Complex with Phosphine and Lewis Acidic Group 11 and 12 Metals

Isolation of the doubly unsaturated mono(sulfonylimido)-bridged diiridium(II) complex [(Cp*Ir)2(μ2-NMs)] (5; Cp* = η5-C5Me5, Ms = SO2Me), which would be generated by 2-fold dehydrochlorination of the sulfonylamido-bridged diiridium(III) complex [(Cp*IrCl)2(μ2-H)(μ2-NHMs)] (2), was attempted. Treatment of 2 with 2 equiv of a base in the presence of trimethylphosphine afforded the mono(phosphine) adduct [(Cp*Ir)2(μ2-NMs)(PMe3)] (6), while dehydrochlorination of 2 with dimethylzinc and silver acetate led to the formation of the mixed-metal clusters [(ZnCl2)(Cp*Ir)2(μ2-NMs)] (7) and [(AgCl)2(μ2-Cl)2Ag2{(Cp*Ir)2(μ2-NMs)}2] (8), respectively. The detailed structures of 6–8, containing the imido-bridged diiridium(II) core of 5, have been determined by X-ray analysis.

Tandem C–F and C–H Bond Activation in Fluoroolefins Promoted by a Bis(diethylphosphino)methane-Bridged Diiridium Complex: Role of Water in the Activation Processes

The diiridium complex [Ir2(CO)3(μ-H)(depm)2]+ (1) reacts with vinyl fluoride, 1,1-difluoroethylene, trifluoroethylene, and tetrafluoroethylene, undergoing C–F bond activation in all cases, in addition to C–H activation in the incompletely substituted fluoroolefins. Reaction of 1 with vinyl fluoride readily undergoes geminal C–F/C–H activation, resulting in the bridging vinylidene product, [Ir2(H)(CO)3(μ-C═CH2)(depm)2]+ (2). Compound 1 reacts with 1,1-difluoroethylene at subambient temperature to give minor amounts of [Ir2(CO)3(κ1:η2-C≡CH)(depm)2]+ (4), resulting from the loss of 2 equiv of HF from the fluoroolefin complex, along with a mixture of two isomers of [Ir2(C(F)═CH2)(CO)3(μ-CF2CH2)(depm)2]+ (5a/5b), in which 2 equiv of the olefin has been incorporated. Compound 1 also reacts with trifluoroethylene at −30 °C, giving a 1:1 mix of isomers of the trifluoroethylene-bridged species [Ir2(H)(CO)3(μ-CFHCF2)(depm)2]+ (7a/7b), and warming this mixture above −15 °C converts both isomers to two products, [Ir2...

H–H and N–H Bond Cleavage of Dihydrogen and Ammonia with a Bifunctional Parent Imido (NH)-Bridged Diiridium Complex

Hydrogenation and protonation of parent imido complexes have attracted much attention in relation to industrial and biological nitrogen fixation. The present study reports the structure and properties of the highly unsaturated diiridium parent imido complex [(Cp*Ir)(2)(μ(2)-H)(μ(2)-NH)](+) derived from deprotonation of a parent amido complex. Because of the Lewis acid-Brønsted base bifunctional nature of the metal-NH bond, the parent imido complex promotes heterolysis of H(2) and deprotonative N-H cleavage of ammonia to afford the corresponding parent amido complexes under mild conditions.