Regioselective Generation of Single-Site Iridium Atoms and Their Evolution into Stabilized Subnanometric Iridium Clusters in MWW Zeolite

Preparation of supported metal catalysts with uniform particle size and coordination environment is a challenging and important topic in materials chemistry and catalysis. In this work, we report the regioselective generation of single-site Ir atoms and their evolution into stabilized subnanometric Ir clusters in MWW zeolite, which are located at the 10MR window connecting the two neighboring 12MR supercages. The size of the subnanometric Ir clusters can be controlled by the post-synthesis treatments and maintain below 1 nm even after being reduced at 650 °C, which cannot be readily achieved with samples prepared by conventional impregnation methods. The high structure sensitivity, size-dependence, of catalytic performance in the alkane hydrogenolysis reaction of Ir clusters in the subnanometric regime is evidenced.     

Chemoselective Hydrogenation of Functionalized Nitroarenes and Imines by Using Carbon Nanofiber‐Supported Iridium Nanoparticles

The reaction of three types of carbon nanofibers (CNFs; platelet: CNF‐P, tubular: CNF‐T, herringbone: CNF‐H) with Ir₄(CO)₁₂ in mesitylene at 165 °C provided the corresponding CNF‐supported iridium nanoparticles, Ir/CNFs (Ir content=2.3–2.6 wt. %). Transmission electron microscopy (TEM) studies of these Ir/CNF samples revealed that size‐controlled Ir nanoparticles (average particle size of 1.1–1.5 nm) existed on the CNFs. Among the three Ir/CNF samples, Ir/CNF‐T showed an excellent catalytic activity and chemoselectivity towards hydrogenation of functionalized nitroarenes and imines; the corresponding aniline derivatives were obtained with high turnover numbers at ambient temperature under 10 atm of H₂, and the catalyst is reusable. Ir/CNF‐T was also effective for the reductive N‐alkylation of anilines with carbonyl compounds.     

Iridium(I)/N‐Heterocyclic Carbene Hybrid Materials: Surface Stabilization of Low‐Valent Iridium Species for High Catalytic Hydrogenation Performance

An Ir^I(NHC)‐based hybrid material was prepared using a methodology which allowed the precise positioning and isolation of the Ir centers along the pore channels of a silica framework. The full characterization of the material by solid‐state NMR spectroscopy showed that the supported Ir sites were stabilized by the silica surface, as low‐coordinated single‐site complexes. The material is extremely efficient for the hydrogenation of functional alkenes. The catalytic performance (TOF and TON) is one to two orders of magnitude higher than those of their molecular Ir analogues, and could be related to the prevention of the bimolecular deactivation of Ir complexes observed under homogeneous conditions.     

Silicon Carbide Supported Palladium‐Iridium Bimetallic Catalysts for Efficient Selective Hydrogenation of Cinnamaldehyde

Selective hydrogenation of α,β‐unsaturated carbonyls into saturated carbonyls is important to obtain remunerative products. However, it is still a challenge to achieve high activity and selectivity under mild conditions. Herein, Pd, Ir and bimetallic Pd‐Ir nanoparticles were uniformly deposited with high dispersity on the surface of SiC by a facile impregnation method, respectively. The as‐prepared Pd/SiC catalysts efficiently hydrogenate cinnamaldehyde to hydrocinnamaldehyde at room temperature and atmospheric pressure, and the activity of Pd/SiC is observed further enhanced by adding Ir component (conversion of 100%). In addition, the dependence of Pd‐Ir catalyst activity on Pd/Ir molar ratio confirms a synergistic effect between Ir and Pd, which originates from the electron transfer between Pd and Ir.     

Mononuclear Iridium Dinitrogen Complexes Bonded to Zeolite HY

The adsorption of N₂ on structurally well‐defined dealuminated HY zeolite‐supported iridium diethylene complexes was investigated. Iridium dinitrogen complexes formed when the sample was exposed to N₂ in H₂ at 298 K, as shown by infrared spectra recorded with isotopically labeled N₂. Four supported species formed in various flowing gases: Ir(N₂), Ir(N₂)(N₂), Ir(C₂H₅)(N₂), and Ir(H)(N₂). Their interconversions are summarized in a reaction network, showing, for example, that, in the presence of N₂, Ir(N₂) was the predominant dinitrogen species at temperatures of 273–373 K. Ir(CO)(N₂) formed transiently in flowing CO, and in the presence of H₂, rather stable iridium hydride complexes formed. Four structural models of each iridium complex bonded at the acidic sites of the zeolite were employed in a computational investigation, showing that the calculated vibrational frequencies agree well with experiment when full calculations are done at the level of density functional theory, independent of the size of the model of the zeolite.     

Catalytic Water Splitting with an Iridium Carbene Complex: A Theoretical Study

Catalytic water oxidation at Ir (OH)⁺ ( Ir =IrCp*(Me₂NHC), where Cp*=pentamethylcyclopentadienyl and Me₂NHC=N,N′‐dimethylimidazolin‐2‐ylidene) can occur through various competing channels. A potential‐energy surface showing these various multichannel reaction pathways provides a picture of how their importance can be influenced by changes in the oxidant potential. In the most favourable calculated mechanism, water oxidation occurs via a pathway that includes four sequential oxidation steps, prior to formation of the O?O bond. The first three oxidation steps are exothermic upon treatment with cerium ammonium nitrate and lead to formation of Ir ^V(?O)(O ^. )⁺, which is calculated to be the most stabile species under these conditions, whereas the fourth oxidation step is the potential‐energy‐determining step. O?O bond formation takes place by coupling of the two oxo ligands along a direct pathway in the rate‐limiting step. Dissociation of dioxygen occurs in two sequential steps, regenerating the starting material Ir (OH)⁺. The calculated mechanism fits well with the experimentally observed rate law: v=k_obs[ Ir ][oxidant]. The calculated effective barrier of 24.6 kcal mol^?1 fits well with the observed turnover frequency of 0.88 s^?1. Under strongly oxidative conditions, O?O bond formation after four sequential oxidation steps is the preferred pathway, whereas under milder conditions O?O bond formation after three sequential oxidation steps becomes competitive.     

Water Oxidation with Molecularly Defined Iridium Complexes: Insights into Homogeneous versus Heterogeneous Catalysis

Molecularly defined Ir complexes and different samples of supported IrO₂ nanoparticles have been tested and compared in the catalytic water oxidation with cerium ammonium nitrate (CAN) as the oxidant. By comparing the activity of nano‐scaled supported IrO₂ particles to the one of organometallic complexes it is shown that the overall activity of the homogeneous Ir precursors is defined by both the formation of the homogeneous active species and its conversion to Ir^IV ‐ oxo nanoparticles. In the first phase of the reaction the activity is dominated by the homogeneous active species. With increasing reaction time, the influence of nano‐sized Ir ‐ oxo particles becomes more evident. Notably, the different conversion rates of the homogeneous precursor into the active species as well as the conversion into Ir‐oxo nanoparticles and the different particle sizes have a significant influence on the overall activity. In addition to the homogeneous systems, IrO₂@MCM‐41 has also been synthesized, which contains stabilized nanoparticles of between 1 and 3 nm in size. This latter system shows a similar activity to IrCl₃ ? xH₂O and complexes 4 and 5 . Mechanistic insights were obtained by in situ X‐ray absorption spectroscopy and scanning transmission electron microscopy.     

Iridium Complexes of 2,2′‐Bidipyrrins

The reaction of [{Ir(cod)(μ‐Cl)}₂] and K₂CO₃ or of [{Ir(cod)(μ‐OMe)}₂] alone with the non‐natural tetrapyrrole 2,2′‐bidipyrrin (H₂BDP) yields, depending on the stoichiometry, the mononuclear complex [Ir(cod)(HBDP)] or the homodinuclear complex [{Ir(cod)}₂(BDP)]. Both complexes react readily with carbon monoxide to yield the species [Ir(CO)₂(HBDP)] and [{Ir(CO)₂}₂(BDP)], respectively. The results from NMR spectroscopy and X‐ray diffraction reveal different conformations for the tetrapyrrolic ligand in both complexes. The reaction of [{Ir(coe)₂(μ‐Cl)}₂] with H₂BDP proceeds differently and yields the macrocyclic [4e^?,2H⁺]‐oxidized product [IrCl₂(9‐Meic)] (9‐Meic = monoanion of 9‐methyl‐9,10‐isocorrole), which can be addressed as an iridium analog of cobalamin.     

A Single‐Atom Iridium Heterogeneous Catalyst in Oxygen Reduction Reaction

Combining the advantages of homogeneous and heterogeneous catalysts, single‐atom catalysts (SACs) are bringing new opportunities to revolutionize ORR catalysis in terms of cost, activity and durability. However, the lack of high‐performance SACs as well as the fundamental understanding of their unique catalytic mechanisms call for serious advances in this field. Herein, for the first time, we develop an Ir‐N‐C single‐atom catalyst (Ir‐SAC) which mimics homogeneous iridium porphyrins for high‐efficiency ORR catalysis. In accordance with theoretical predictions, the as‐developed Ir‐SAC exhibits orders of magnitude higher ORR activity than iridium nanoparticles with a record‐high turnover frequency (TOF) of 24.3 e^? site^?1 s^?1 at 0.85 V vs. RHE) and an impressive mass activity of 12.2 A mg^?1_Ir, which far outperforms the previously reported SACs and commercial Pt/C. Atomic structural characterizations and density functional theory calculations reveal that the high activity of Ir‐SAC is attributed to the moderate adsorption energy of reaction intermediates on the mononuclear iridium ion coordinated with four nitrogen atom sites.     

Synthesis,One‐ and Two‐Photon Photophysical and Excited‐State Properties,and Sensing Application of a New Phosphorescent Dinuclear Cationic Iridium(III) Complex

A new phosphorescent dinuclear cationic iridium(III) complex ( Ir1 ) with a donor–acceptor–π‐bridge–acceptor–donor (D? A? π? A? D)‐conjugated oligomer ( L1 ) as a N^N ligand and a triarylboron compound as a C^N ligand has been synthesized. The photophysical and excited‐state properties of Ir1 and L1 were investigated by UV/Vis absorption spectroscopy, photoluminescence spectroscopy, and molecular‐orbital calculations, and they were compared with those of the mononuclear iridium(III) complex [Ir(Bpq)₂(bpy)]⁺PF₆^? ( Ir0 ). Compared with Ir0 , complex Ir1 shows a more‐intense optical‐absorption capability, especially in the visible‐light region. For example, complex Ir1 shows an intense absorption band that is centered at λ=448 nm with a molar extinction coefficient (ε) of about 10⁴, which is rarely observed for iridium(III) complexes. Complex Ir1 displays highly efficient orange–red phosphorescent emission with an emission wavelength of 606 nm and a quantum efficiency of 0.13 at room temperature. We also investigated the two‐photon‐absorption properties of complexes Ir0 , Ir1 , and L1 . The free ligand ( L1 ) has a relatively small two‐photon absorption cross‐section (δ_max=195 GM), but, when complexed with iridium(III) to afford dinuclear complex Ir1 , it exhibits a higher two‐photon‐absorption cross‐section than ligand L1 in the near‐infrared region and an intense two‐photon‐excited phosphorescent emission. The maximum two‐photon‐absorption cross‐section of Ir1 is 481 GM, which is also significantly larger than that of Ir0 . In addition, because the strong B? F interaction between the dimesitylboryl groups and F^? ions interrupts the extended π‐conjugation, complex Ir1 can be used as an excellent one‐ and two‐photon‐excited “ON–OFF” phosphorescent probe for F^? ions.     

EQCM Study of Iridium Anodic Oxidation in H2SO4 and KOH Solutions

In the present study anodic oxidation of iridium layer formed thermally on a gold‐sputtered quartz crystal electrode has been investigated by electrochemical quartz crystal microgravimetry (EQCM) in the solutions of 0.5 M H₂SO₄ and 0.1 M KOH. The emphasis here has been put on the microgravimetric behavior of iridium as a metal, because a few previous EQCM studies reported in literature have been devoted to iridium oxide films (IROFs). The objective pursued here has been to elucidate the nature of the main voltammetric peaks, which occur at different ranges of potential in the solutions investigated. It has been found that anodic oxidation of iridium electrode in 0.5 M H₂SO₄ and 0.1 M KOH solutions is accompanied by irregular fluctuations of the electrode mass at 0.4 V<E<0.8 V followed by regular increase in mass at 0.8 V<E<1.2 V. The cathodic process initially, at 1.2 V>E>0.9 V, proceeds without any or with slight increase in electrode mass, whereas at E<0.8 V a regular decrease in mass is observed. It has been found that mass to charge ratio characterizing the processes of interest is 2 to 3 g F^?1in acidic medium, whereas in the case of alkaline one it is 4 to 6 g F^?1. The main pair of peaks seen in the voltammograms of Ir electrode in alkaline medium at E<0.8 V is attributable to redox transition Ir(0)→Ir(III), whereas those observed in the case of acidic medium at E>0.8 V should be related to the redox process Ir(0)→Ir(IV) going via intermediate stage of Ir(III) formation. As a consequence of these redox transitions, the gel‐like surface layer consisting of Ir(III) or Ir(IV) hydrous oxides forms on the electrode surface.     

Tuning the Selectivity of Catalytic Carbon Dioxide Hydrogenation over Iridium/Cerium Oxide Catalysts with a Strong Metal–Support Interaction

A one‐step ligand‐free method based on an adsorption–precipitation process was developed to fabricate iridium/cerium oxide (Ir/CeO₂) nanocatalysts. Ir species demonstrated a strong metal–support interaction (SMSI) with the CeO₂ substrate. The chemical state of Ir could be finely tuned by altering the loading of the metal. In the carbon dioxide (CO₂) hydrogenation reaction it was shown that the chemical state of Ir species—induced by a SMSI—has a major impact on the reaction selectivity. Direct evidence is provided indicating that a single‐site catalyst is not a prerequisite for inhibition of methanation and sole production of carbon monoxide (CO) in CO₂ hydrogenation. Instead, modulation of the chemical state of metal species by a strong metal–support interaction is more important for regulation of the observed selectivity (metallic Ir particles select for methane while partially oxidized Ir species select for CO production). The study provides insight into heterogeneous catalysts at nano, sub‐nano, and atomic scales.     

The Common Intermediates of Oxygen Evolution and Dissolution Reactions during Water Electrolysis on Iridium

Understanding the pathways of catalyst degradation during the oxygen evolution reaction is a cornerstone in the development of efficient and stable electrolyzers, since even for the most promising Ir based anodes the harsh reaction conditions are detrimental. The dissolution mechanism is complex and the correlation to the oxygen evolution reaction itself is still poorly understood. Here, by coupling a scanning flow cell with inductively coupled plasma and online electrochemical mass spectrometers, we monitor the oxygen evolution and degradation products of Ir and Ir oxides in situ. It is shown that at high anodic potentials several dissolution routes become possible, including formation of gaseous IrO₃. On the basis of experimental data, possible pathways are proposed for the oxygen‐evolution‐triggered dissolution of Ir and the role of common intermediates for these reactions is discussed.     

Oxygen Reduction with a Bifunctional Iridium Dihydride Complex

The iridium dihydride [Ir(H)₂(HPNP)]⁺ (PNP=N(CH₂CH₂PtBu₂)₂) reacts with O₂ to give the unusual, square‐planar iridium(III) hydroxide [Ir(OH)(PNP)]⁺ and water. Regeneration of the dihydride with H₂ closes a quasi‐catalytic synthetic oxygen‐reduction reaction (ORR) cycle that can be run several times. Experimental and computational examinations are in agreement with an oxygenation mechanism via rate‐limiting O₂ coordination followed by H‐transfer at a single metal site, facilitated by the cooperating pincer ligand. Hence, the four electrons required for the ORR are stored within the two covalent M? H bonds of a mononuclear metal complex.     

Structural,Electronic and Optical Properties of Multifunctional Iridium(III) and Platinum(II) Metallophosphors for Organic Light‐Emitting Diodes

An elaborated theoretical investigation on the optical and electronic properties of three fluorene‐based platinum(II) and iridium(III) cyclometalated complexes Pt‐a , Ir‐a and Ir‐b is reported. The geometric and electronic structures of the complexes in the ground state are studied with density functional theory and Hartree Fock approaches, while the lowest triplet excited states are optimized by singles configuration interaction (CIS) methods. At the time‐dependent density functional theory (TD‐DFT) level, molecular absorption and emission properties were calculated on the basis of optimized ground‐ and excited‐state geometries, respectively. The computational results show that the appearance of triphenylamino (TPA) moiety at the 9‐position of fluorene ring favors the hole‐creation and leads to red‐shifts of absorption and emission spectra. Moreover, Pt‐a and Ir‐b are nice hole‐transporting materials whereas Ir‐a has good charge‐transfer balance, which render them useful for the realization of efficient OLEDs (Organic Light‐Emitting Diodes).     

On the Extreme Oxidation States of Iridium

It has recently been suggested that the oxidation states of Ir run from the putative ?III in the synthesized solid Na₃[Ir(CO)₃] to the well‐documented +IX in the species IrO₄⁺. Furthermore, [Ir(CO)₃]^3? was identified as an 18‐electron species. A closer DFT study now finds support for this picture: The orbitals spanned by the 6s,6p,5d orbitals of the iridium are all occupied. Although some have considerable ligand character, the deviations from 18 e leave the orbital symmetries unchanged. The isoelectronic systems from Os^?IV to Au^?I behave similarly, suggesting further possible species. To paraphrase Richard P. Feynmann “there is plenty of room at the bottom”.     

Enhanced Iridium Complex Electrochemiluminescence Cytosensing and Dynamic Evaluation of Cell‐Surface Carbohydrate Expression

A newly prepared [(ppy)₂Ir(dcbpy)]⁺?PF₆^? (ppy: 2‐phenylpyridyl; dcbpy: 4,4′‐dicarboxy‐2,2′‐bipyridyl) and gold nanoparticle functionalized mesoporous silica nanoparticle (Au/Ir‐MSN) is reported. Based on the binding between concanavalin A (Con A) and mannose, the novel nanoparticle was applied to an ultrasensitive electrochemiluminescence (ECL) in situ cytosensing strategy and the dynamic evaluation of cell‐surface carbohydrate expression. The ECL activity of the presented Con A@Au/Ir‐MSN nanoprobe was greatly enhanced by employing a functionalized nanoparticle and graphene nanomaterial with an increased surface area and simultaneously improved electron‐transfer efficiency at the electrode interface. Under optimal conditions, the sandwich‐type ECL cytosensor showed a linear response to K562 cells at concentrations ranging from 1.0×10² to 1.0×10⁶ cells mL^?1 and realized a low detection limit of a single cell. The proposed method could also be successfully used for monitoring the dynamic variation of carbohydrate expression in cancer cells in response to external stimulation by an inhibitor.     

Iridium and Rhodium Complexes within a Macroreticular Acidic Resin: A Heterogeneous Photocatalyst for Visible‐light Driven H2 Production without an Electron Mediator

Direct ion exchange of cyclometalated iridium(III) and tris‐2,2′‐bipyridyl rhodium(III) complexes, of which the former acts as a photosensitizer and the latter as a proton reduction catalyst, within a macroreticular acidic resin has been accomplished with the aim of developing a photocatalyst for H₂ production under visible‐light irradiation. Ir L_III‐edge and Rh K‐edge X‐ray absorption fine structure (XAFS) measurements suggest that the Ir and Rh complexes are easily accommodated in the macroreticular space without considerable structural changes. The photoluminescence emission of the exchanged Ir complex due to a triplet ligand charge‐transfer (³LC) and metal‐to‐ligand charge‐transfer (³MLCT) transition near 550 nm decreases with increasing the amount of the Rh complex, thus suggesting the occurrence of an electron transfer from Ir to Rh. The Ir‐Rh/resin catalyst behaves as a heterogeneous photocatalyst capable of both visible‐light sensitization and H₂ production in an aqueous medium in the absence of an electron mediator. The photocatalytic activitity is strongly dependent on the amount of the components and reaches a maximum at a molar ratio of 2:1 of Ir/Rh complexes. Moreover, leaching and agglomeration of the active metal complexes are not observed, and the recovered photocatalyst can be recycled without loss in catalytic activity.     

Protonation and Hydrogenation Experiments with Iridium(0) and Iridium(−1) tropp Complexes: Formation of Hydrides

The reactions of three different tetracoordinated Ir complexes, [Ir(tropp^ph)₂]ⁿ (n=+1, 0, −1), which differ in the formal oxidation state of the metal from +1 to −1, with proton sources and dihydrogen were investigated (tropp=5‐(diphenylphosphanyl)dibenzo[a,d]cycloheptene). It was found that the cationic 16‐electron complex [Ir(tropp^ph)₂]⁺ ( 2 ) cannot be protonated but reacts with NaBH₄ to the very stable 18‐electron Ir^I hydride [IrH(tropp^ph)₂] ( 5 ), which is further protonated with medium strong acids to give the 18‐electron Ir^III dihydride [IrH₂(tropp^ph)₂]⁺ ( 6 ; pK_s in CH₂Cl₂/THF/H₂O 1 : 1 : 2 ca. 2.2). Both, the neutral 17‐electron Ir⁰ complex [Ir(tropp^ph)₂] ( 3 ) and the anionic 18‐electron complex [Ir(tropp^ph)₂]⁻ ( 4 ) react rapidly with H₂O to give the monohydride 5 . In reactions of 3 with H₂O, the terminal Ir^I hydroxide [Ir(OH)(tropp^ph)₂] ( 8 ) is formed in equal amounts. All these complexes, apart from 5 , which is inert, do react rapidly with dihydrogen. The complex 2 gives the dihydride 6 in an oxidative addition reaction, while 3 , 4 , and 8 give the monohydride 5 . Interestingly, a salt‐type hydride (i.e., LiH) is formed as further product in the unexpected reaction with [Li(thf)_x]⁺[Ir(tropp^ph)₂]⁻ ( 4 ). Because 3 undergoes disproportionation into 2 and 4 according to 2 3 ⇄ 2 + 4 (K_disp=2.7⋅10⁻⁵), it is likely that actually the diamagnetic species and not the odd‐electron complex 3 is involved in the reactions studied here, and possible mechanisms for these are discussed.     

Enhanced Electrochemiluminescence from a Stoichiometric Ruthenium(II)–Iridium(III) Complex Soft Salt

Electrochemiluminescence (ECL) and electrochemistry are reported for a heterometallic soft salt, [Ru(dtbubpy)₃][Ir(ppy)₂(CN)₂]₂ ( [Ir][Ru][Ir] ), consisting of a 2:1 ratio of complementary charged Ru and Ir complexes possessing two different emission colors. The [Ru]²⁺ and [Ir]^? moieties in the [Ir][Ru][Ir] greatly reduce the energy required to produce ECL. Though ECL intensity in the annihilation path was enhanced 18× relative to that of [Ru(bpy)₃]²⁺, ECL in the co‐reactant path with tri‐n‐propylamine was enhanced a further 4×. Spooling spectroscopy gives insight into ECL mechanisms: the unique light emission at 634 nm is due to the [Ru]²⁺* excited state and no [Ir]^?* was generated in either route. Overall, the soft salt system is anticipated to be attractive and suitable for the development of efficient and low‐energy‐cost ECL detection systems.