Ir-H Bond Research Articles

A Theoretical Investigation of Chemical Bonding of a Heterometallic Trinuclear Cluster Containing Iridium and Ruthenium: [(Cp*Ir) (CpRu)2 (μ3-H) (μ-H)3] by QTAIM Approach

Numerous integral and local electron density’s topological parameters of significant metal-metal and metal-ligand bonding interactions in a trinuclear tetrahydrido cluster [(Cp* Ir) (Cp Ru)2 (μ3-H) (μ-H)3]1 (Cp = η5 -C5Me5), (Cp* = η5 -C5Me4Et) were calculated and interpreted by using the quantum theory of atoms in molecules (QTAIM). The properties of bond critical points such as the delocalization indices δ (A, B), the electron density ρ(r), the local kinetic energy density G(r), the Laplacian of the electron density ∇2ρ(r), the local energy density H(r), the local potential energy density V(r) and ellipticity ε(r) are compared with data from earlier organometallic system studies. A comparison of the topological processes of different atom-atom interactions has become possible thanks to these results. In the core of the heterometallic tetrahydrido cluster, the Ru2IrH4 part, the calculations showed that there are no bond critical points (BCPs) or identical bond paths (BPs) between Ru-Ru and Ru-Ir. The distribution of electron densities is determined by the position of bridging hydride atoms coordinated to Ru-Ru and Ru-Ir, which significantly affects the bonds between these transition metal atoms. On the other hand, the results confirm that the cluster under study contains a 7c–11e bonding interaction delocalized over M3H4, as shown by the non-negligible delocalization index calculations. The small values for ρ(b) above zero, together with the small values, again above zero, for Laplacian ∇2ρ(b) and the small positive values for total energy density H(b), are shown by the Ru-H and Ir-H bonds in this cluster is typical for open-shell interactions. Also, the topological data for the bond interactions between Ir and Ru metal atoms with the C atoms of the cyclopentadienyl Cp ring ligands are similar. They show properties very identical to open-shell interactions in the QTAIM classification.

CH bond activation in aromatic ketones mediated by iridium-tris(pyrazolyl)borate complexes.

Reaction of complex [TpMe2Ir(η4-CH2C(Me)C(Me)C2)] (1) with a series of aromatic ketones at 130 °C renders, by means of a selective ortho-CH activation, Ir(III)-metallacycles 2-5, which display an Ir-H bond. When [TpMe2Ir(C6H5)2N2] (6) is treated with 2-(trifluoromethyl)acetophenone and 2-fluoroacetophenone at 80 °C, the formation of dimeric (7) and trimeric architectures (8) is achieved through the meta- and para-CH activation of the aromatic ketone, respectively. The generation of complexes 2-5 is proposed to occur by the initial formation of Ir(III) η1-ketone adducts as key intermediates, followed by aromatic CH activations and the release of a butadiene ligand. The formation of complexes 7 and 8 involves an assisted process in which a metal center activation of the less sterically hindered C-H bond of the aromatic ketone takes place (releasing a benzene molecule), followed by the coordination of the carbonyl group, which generates the respective dimeric and trimeric structures. Complexes 7 and 8 are efficient catalysts for the transfer hydrogenation of ketones and aldehydes using isopropanol as the hydrogen source. All complexes have been fully characterized by NMR spectroscopy, FT-IR, elemental analysis and, in the cases of 7 and 8, X-ray crystallography. Details of the reaction conditions, isolation of the products, and proposals for the pathways of formation of complexes 2-5 and 7-8 are discussed.

URVA and Local Mode Analysis of an Iridium Pincer Complex Efficiently Catalyzing the Hydrogenation of Carbon Dioxide

The catalytic effects of iridium pincer complexes for the hydrogenation of carbon dioxide were investigated with the Unified Reaction Valley Approach (URVA), exploring the reaction mechanism along the reaction path traced out by the reacting species on the potential energy surface. Further details were obtained with the Local Mode Analysis performed at all stationary points, complemented by the Natural Bond Orbital and Bader’s Quantum Atoms in Molecules analyses. Each of the five reaction paths forming the catalytic cycle were calculated at the DFT level complemented with DLPNO-CCSD(T) single point calculations at the stationary points. For comparison, the non-catalytic reaction was also investigated. URVA curvature profiles identified all important chemical events taking place in the non-catalyzed reaction and in the five reactions forming the catalytic cycle, and their contribution to the activation energy was disclosed. The non-catalytic reaction has a large unfavorable activation energy of 76.3 kcal/mol, predominately caused by HH bond cleave in the H2 reactant. As shown by our study, the main function of the iridium pincer catalyst is to split up the one–step non-catalytic reaction into an energy efficient multistep cycle, where HH bond cleavage is replaced by the cleavage of a weaker IrH bond with a small contribution to the activation energy. The dissociation of the final product from the catalyst requires the cleavage of an IrO bond, which is also weak, and contributes only to a minor extent to the activation energy. This, in summary, leads to the substantial lowering of the overall activation barrier by about 50 kcal/mol for the catalyzed reaction. We hope that this study inspires the community to add URVA to their repertoire for the investigation of catalysis reactions.

Oxidative Addition of Water to Ir(I) Complexes Bearing a Pincer-Type Silyl Ligand.

Reaction of water with in situ generated [(PSiP-R)IrI] (PSiP-R = [{2-(R2P)C6H4}2MeSi]−; R = cyclohexyl, tBu or iPr) from [(PSiP-R)Ir(H)4] and tert-butylethylene (tbe) showed high ligand dependency. Oxidative addition of water cleanly proceeded in the reaction of [(PSiP-tBu)IrI] in THF at room temperature to selectively afford a 16-electron hydrido–hydroxo complex [(PSiP-tBu)Ir(H)(OH)] almost quantitatively. In contrast, the reaction of cyclohexyl and iPr derivatives was unselective and formed various products containing Ir–H bonds. In the case of iPr-derivative, a small amount of 18-electron hydrido–hydroxo aqua complex [(PSiP-iPr)Ir(H)(OH)(H2O)] was isolated and structurally characterized by X-ray crystallography.

Electron-Rich Phenoxyl Mediators Improve Thermodynamic Performance of Electrocatalytic Alcohol Oxidation with an Iridium Pincer Complex.

Electron-rich phenols, including α-rac-tocopherol Ar1OH, 2,4,6,-tri-tert-butylphenol Ar3OH, and butylated hydroxy-toluene Ar4OH, are effective electrochemical mediators for the electrocatalytic oxidation of alcohols by an iridium amido dihyride complex (PNP)Ir(H)2 (IrN 1, PNP = bis[2-diisopropylphosphino)ethyl]amide). Addition of phenol mediators leads to a decrease in the onset potential of catalysis from -0.65 V vs Fc+/0 under unmediated conditions to -1.07 V vs Fc+/0 in the presence of phenols. Mechanistic analysis suggests that oxidative turnover of the iridium amino trihydride (PNHP)Ir(H)3 (IrH 2, PNHP = bis[2-diisopropylphosphino)ethyl]amine) to IrN 1 proceeds through two successive hydrogen atom transfers (HAT) to 2 equiv of phenoxyl that are generated transiently at the anode. Isotope studies and comparison to known systems are consistent with initial homolysis of an Ir-H bond being rate-determining. Turnover frequencies up to 14.6 s-1 and an average Faradaic efficiency of 93% are observed. The mediated system shows excellent chemoselectivity in bulk oxidations of 2-propanol and 1,2-benzenedimethanol in THF and is also viable in neat 2-propanol.

H2 Addition to Pincer Iridium Complexes Yielding trans-Dihydride Products: Unexpected Correlations of Bond Strength with Bond Length and Vibrational Frequencies.

R4PONOP-Ir-Me (R1) and R4POCOP-Ir-CO (R2), R = tBu or iPr, are known to undergo acid-catalyzed oxidative addition of H2 that yields octahedral products with two hydrides in a trans-configuration. We use density functional theory to study the free energies (Δ Gtrans) and equilibrium isotope effects (EIEtrans) for H2/D2 addition to R1, R2, and related complexes for R = tBu, iPr, and Me. For a given R, reaction of R1 is ∼5 kcal/mol more exergonic than R2. For a given subclass of complexes, Δ Gtrans is more exergonic for the smaller R. The computed values of Δ Gtrans vary between +5.1 and -17.4 kcal/mol. EIEtrans varies between 0.78 and 1.22. Counterintuitively, it is the less-exergonic reactions that afford products with shorter Ir-H bonds, greater symmetric and asymmetric trans-Ir-(H)2 stretching vibrational frequencies, and more inverse EIEtrans. This disparity is amplified in Me4PONOP-Os-CO, where Δ Gtrans is -35.2 kcal/mol, yet the Os-H bonds are long, and the Os-H vibrational frequencies are low as compared with the Ir-H bonds, and EIEtrans is high (1.20). Attempts are made to account for the inverted bond strength-bond length correlation based on the hydricity of the products and the total negative charge on the trans-Ir(H)2 unit as computed using the Quantum Theory of Atoms in Molecules.

Cyclic cooperative intramolecular hydrogen bond in p-tert-butylcalix[6]arene according to FTIR spectroscopy and DFT studies

This article describes a comparative research of IR spectra and H-bonds in the p-tert-butylcalix[6]arene (TB6) and calix[6]arene (C6). IR spectra were computed for compressed cone conformation by DFT method. The assignment of the bands in the IR spectra of the TB6 and C6 was made. The effect of the bulky tert-butyl substituents on the structure and H-bonding in TB6 was established. Our research has shown that in TB6 and C6 the cyclic H-bond is realized, which ensures the existence of a compressed cone conformation. Introduction of tert-butyl substituents in TB6 leads to hardening of H-bonds. Examination of IR spectra showed that when heated TB6 remains in a compressed cone conformation. In a molecule of TB6 and C6 oxygen atoms are in a “boat” conformation.

Mechanism Studies of Ir-Catalyzed Asymmetric Hydrogenation of Unsaturated Carboxylic Acids.

The Ir-catalyzed asymmetric hydrogenation of olefins is widely used for production of value-added bulk and fine chemicals. The iridium catalysts with chiral spiro phosphine-oxazoline ligands developed in our group show high activity and high enantioselectivity in the hydrogenation of olefins bearing a coordinative carboxyl group, such as α,β-unsaturated carboxylic acids, β,γ-unsaturated carboxylic acids, and γ,δ-unsaturated carboxylic acids. Here we conducted detailed mechanistic studies on these Ir-catalyzed asymmetric hydrogenation reactions by using (E)-2-methyl-3-phenylacrylic acid as a model substrate. We isolated and characterized several key intermediates having Ir-H bonds under the real hydrogenation conditions. Particularly, an Ir(III) migratory insertion intermediate was first isolated in an asymmetric hydrogenation reaction promoted by chiral Ir catalysts. That this intermediate cannot undergo reductive elimination in the absence of hydrogen strongly supports the involvement of an Ir(III)/Ir(V) cycle in the hydrogenation. On the basis of the structure of the Ir(III) intermediate, variable-temperature NMR spectroscopy, and density functional theory calculations, we elucidated the mechanistic details of the Ir-catalyzed hydrogenation of unsaturated carboxylic acids and explained the enantioselectivity of the reactions. These findings experimentally and computationally elucidate the mechanism of Ir-catalyzed asymmetric hydrogenation of olefins with a strong coordinative carboxyl group and will likely inspire further catalyst design.

Mild activation of Ir[sbnd]Cl bond upon the interaction of pincer iridium hydride (tBuPCP)IrH(Cl) with acids and bases

The coordination of nitrogen bases (CH3CN, PhCN and 2-hydroxymetylpyridine (HMPy)) to (tBuPCP)IrH(Cl) (1) was investigated by variable temperature NMR, IR, UV–vis spectroscopies combined with DFT/M06 calculations. Hydrogen bonding of 1 and its (tBuPCP)IrH(Cl)(CO) with alcohols was also studied. At room temperature the formation constants Kf of complex (tBuPCP)IrH(Cl)(PhCN) (2) is ca. 3.5 times higher than for complex (tBuPCP)IrH(Cl)(CH3CN) (3) (6.7 × 102 and 1.9 × 102 L mol−1, respectively). At that quantitative analysis of variable temperature UV–vis spectra gave ΔH° = −7.0 kcal mol−1 and ΔS° = −13.7 cal mol−1 K−1 for 3 and ΔH° = −5.8 kcal mol−1 and ΔS° = −7.1 cal mol−1 K−1 for 2. Relatively weak binding of 2-hydroxymethylpyridine to complex 1 yields two isomers, with HMPy ligand trans to hydride (4a) and trans to arene ring (4b) with formation constants Kf(4a) and Kf(4b) of only 28 and 18 L mol−1 at room temperature. Variable temperature measurements give ΔH°(4a) = −12.3 kcal mol−1, ΔS°(4a) = −35.5 cal mol−1 K−1 and ΔH°(4b) = −11.7 kcal mol−1, ΔS°(4b) = −34.5 cal mol−1 K−1. DFT calculations on (tBuPCP)IrH(Cl)(L) confirm preferential coordination of L = RCN, pyridines trans to the hydride ligand but show no alteration of IrH bond properties. Instead, the elongation of IrCl bond is revealed that correlates with the strength of the Ir-L binding. The second isomers with L coordinated trans to arene ring of the pincer ligand are slightly less stable (≤2.8 kcal mol−1 according to DFT calculations) and are observed only for pyridines. In 1 and its (tBuPCP)IrH(Cl)(L) derivatives, chloride ligand is able to serve as a proton accepting site forming Cl⋯HX hydrogen bonds in solution or in solid state. These interactions further activate the IrCl bond.

Transition-Metal-Mediated Cleavage of Fluoro-Silanes under Mild Conditions.

Si-F bond cleavage of fluoro-silanes was achieved by transition-metal complexes under mild and neutral conditions. The Iridium-hydride complex [Ir(H)(CO)(PPh3 )3 ] was found to readily break the Si-F bond of the diphosphine- difluorosilane {(o-Ph2 P)C6 H4 }2 Si(F)2 to afford a silyl complex [{[o-(iPh2 P)C6 H4 ]2 (F)Si}Ir(CO)(PPh3 )] and HF. Density functional theory calculations disclose a reaction mechanism in which a hypervalent silicon species with a dative Ir→Si interaction plays a crucial role. The Ir→Si interaction changes the character of the H on the Ir from hydridic to protic, and makes the F on Si more anionic, leading to the formation of H(δ+) ⋅⋅⋅F(δ-) interaction. Then the Si-F and Ir-H bonds are readily broken to afford the silyl complex and HF through σ-bond metathesis. Furthermore, the analogous rhodium complex [Rh(H)(CO)(PPh3 )3 ] was found to promote the cleavage of the Si-F bond of the triphosphine-monofluorosilane {(o-Ph2 P)C6 H4 }3 Si(F) even at ambient temperature.

Dehydrogenation of n-Alkanes by Solid-Phase Molecular Pincer-Iridium Catalysts. High Yields of α-Olefin Product.

We report the transfer-dehydrogenation of gas-phase alkanes catalyzed by solid-phase, molecular, pincer-ligated iridium catalysts, using ethylene or propene as hydrogen acceptor. Iridium complexes of sterically unhindered pincer ligands such as (iPr4)PCP, in the solid phase, are found to give extremely high rates and turnover numbers for n-alkane dehydrogenation, and yields of terminal dehydrogenation product (α-olefin) that are much higher than those previously reported for solution-phase experiments. These results are explained by mechanistic studies and DFT calculations which jointly lead to the conclusion that olefin isomerization, which limits yields of α-olefin from pincer-Ir catalyzed alkane dehydrogenation, proceeds via two mechanistically distinct pathways in the case of ((iPr4)PCP)Ir. The more conventional pathway involves 2,1-insertion of the α-olefin into an Ir-H bond of ((iPr4)PCP)IrH2, followed by 3,2-β-H elimination. The use of ethylene as hydrogen acceptor, or high pressures of propene, precludes this pathway by rapid hydrogenation of these small olefins by the dihydride. The second isomerization pathway proceeds via α-olefin C-H addition to (pincer)Ir to give an allyl intermediate as was previously reported for ((tBu4)PCP)Ir. The improved understanding of the factors controlling rates and selectivity has led to solution-phase systems that afford improved yields of α-olefin, and provides a framework required for the future development of more active and selective catalytic systems.

Iridium Compounds with κ-P,P,Si(biPSi) Pincer Ligands: Favoring Reactive Structures in Unsaturated Complexes

The structure, coordination properties, insertion processes, and dynamic behavior in solution of the five-coordinate complexes [IrXH(biPSi)] (biPSi = kappa-P,P,Si-Si(Me){(CH(2))(3)PPh(2)}(2); X = Cl (1), Br (2), or I (3)) have been investigated. The compounds are formed as mixtures of two isomers, anti and syn, in slow equilibrium in solution. The equilibrium position depends on the halogen and the solvent. Both isomers display distorted square-based pyramidal structures in which the vacant position sits trans to silicon. The equatorial plane of the syn isomer is closer to the T structure due to distortions of steric origin. The small structural differences between the isomers trigger remarkable differences in reactivity. The syn isomers form six-coordinate adducts with chlorinated solvents, CO, P(OMe)(3), or NCMe, always after ligand coordination trans to silicon. The anti isomers do not form detectable adducts with chlorinated solvents and coordinate CO or P(OMe)(3) either trans to silicon (kinetic) or trans to hydride (thermodynamic). NCMe coordinates the anti isomers exclusively at the position trans to hydride. Qualitative and quantitative details (equilibrium constants, enthalpies, entropies, etc.) on these coordination processes are given and discussed. As a result of the different coordination properties, insertion reagents such as acetylene, diphenylacetylene, or the alkylidene resulting from the decomposition of ethyl diazoacetate selectively insert into the Ir-H bond of 1-syn, not into that of 1-anti. These reactions give five-coordinate syn alkenyl or alkyl compounds in which the vacancy also sits trans to silicon. Acetylene is polymerized in the coordination sphere of 1. The nonreactive isomer 1-anti also evolves into the syn insertion products via anti<-->syn isomerizations, the rates of which are notably dependent on the nature of the insertion reactants. H(2) renders anti<-->syn isomerization rates of the same order as the NMR time scale. The reactions are second order (k(obs) = k(anti<-->syn)[H(2)]) and do not involve H(2)/IrH hydrogen atom scrambling. A possible isomerization mechanism, supported by MP2 calculations and compatible with the various experimental observations, is described. It involves Ir(V) intermediates and a key sigma Ir-(eta(2)-SiH) agostic transition state. A similar transition state could also explain the anti<-->syn isomerizations in the absence of oxidative addition reactants, although at the expense of high kinetic barriers strongly dependent on the presence of potential ligands and their nature.

Mechanistic Investigation of the Hydrogenation of O2 by a Transfer Hydrogenation Catalyst

The mechanistic details of the hydrogenation of molecular oxygen by the 18e amino-hydride Cp*IrH(TsDPEN) (1H(H)) complex to give Cp*Ir(TsDPEN-H) (1) and 1 equiv of H(2)O were investigated by means of hybrid density functional calculations (B3LYP). To comprehensively describe the overall catalytic cycle of the hydrogenation of dioxygen using H(2) catalyzed by the Ir complex 1, the potential energy surfaces for the hydrogenation process of both the catalyst 1 and the corresponding unsaturated iridium(III) amine cation ([1H](+)) were explored at the same level of theory. The results of our computations, in agreement with experimental findings, confirm that the addition of H(2) to the 16e diamido complexes 1 is favorable but is slow and is accelerated by the presence of Bronsted acids, such as HOTf, which convert 1 into the corresponding amine cation [1H](+). By deprotonation of the subsequently hydrogenated [1H(H(2))](+) complex the amine hydride catalyst 1H(H) is generated, which is able to reduce molecular oxygen. Calculations corroborate that the O(2) reduction goes through formation of an intermediate iridium hydroperoxo complex that reacts with 1H(H) to eliminate water, restore 1, and restart the catalytic cycle. From the outcomes of our computational analysis it results that the slow step of the overall O(2) hydrogenation process is the O(2) insertion into the Ir-H bond, and the highest calculated barrier along this pathway to give the hydroperoxo product shows a good agreement with the experimentally estimated value. As a consequence, unreacted 1H(H) approaches 1H(OOH) to give 1H(OH) and water according to the experimentally observed second-order kinetics with respect to [1H(H)]. Calculations were carried out to explore the possibility that H(2)O(2) is released from the hydroperoxo intermediate together with catalyst 1, and the subsequent water elimination reaction occurs by reduction of produced H(2)O(2) with 1H(H) to regenerate catalyst 1. Preliminary results concerning the O(2) reduction in acidic conditions show that the reaction proceeds by intermediate production of H(2)O(2), which reacts with 1H(H) to eliminate water, restore [1H](+), and restart the catalytic cycle. The energetics of the process appear to be definitely more favorable with respect the analogous pathways in neutral conditions.

Dynamics of a cis-Dihydrogen/Hydride Complex of Iridium

Insertion of CS2 into one of the Ir-H bonds of [Ir(H)5(PCy3)2] takes place to afford the dihydrido dithioformate complex cis-[Ir(H)2(eta2-S2CH)(PCy3)2] accompanied by the elimination of H2. Protonation of the dithioformate complex using HBF4.Et2O gives cis-[Ir(H)(eta2-H2)(eta2-S2CH)(PCy3)2][BF4] wherein the H atom undergoes site exchange between the dihydrogen and the hydride ligands. The dynamics was found to be so extremely rapid with respect to the NMR time scale that the barrier to exchange could not be measured. Partial deuteration of the hydride ligands resulted in a J(H,D) of 6.5 and 7.7 Hz for the H2D and the HD2 isotopomers of cis-[Ir(H)(eta2-H2)(eta2-S2CH)(PCy3)2][BF4], respectively. The H-H distance (d(HH)) for this complex has been calculated to be 1.05 A, which can be categorized under the class of elongated dihydrogen complexes. The cis-[Ir(H)(eta2-H2)(eta2-S2CH)(PCy3)2][BF4] complex undergoes substitution of the bound H2 moiety with CH(3)CN and CO resulting in new hydride derivatives, cis-[Ir(H)(L)(eta2-S2CH)(PCy3)2][BF4] (L = CH3CN, CO). Reaction of cis-[Ir(H)2(eta2-S2CH)(PCy3)2] with electrophilic reagents such as MeOTf and Me3SiOTf afforded a new hydride aquo complex cis-[Ir(H)(H2O)(eta2-S2CH)(PCy3)2][OTf] via the elimination of CH4 and Me3SiH, respectively, followed by the binding of a water molecule (present in trace quantities in the solvent) to the iridium center. The X-ray crystal structures of cis-[Ir(H)2(eta2-S2CH)(PCy3)2] and cis-[Ir(H)(H2O)(eta2-S2CH)(PCy3)2][OTf] have been determined.

Double geminal C-H activation and reversible alpha-elimination in 2-aminopyridine iridium(III) complexes: the role of hydrides and solvent in flattening the free energy surface.

[H2Ir(OCMe2)2L2]BF4 (1) (L = PPh3), a preferred catalyst for tritiation of pharmaceuticals, reacts with model substrate 2-(dimethylamino)pyridine (py-NMe2; py = 2-pyridyl) to give chelate carbene [H2Ir(py-N(Me)CH=)L2]BF4 (2a) via cyclometalation, H2 loss, and reversible alpha-elimination. Agostic intermediate [H2Ir(py-N(Me)CH2-H)L2]BF4) (4a), seen by NMR, is predicted (DFT(B3PW91) computations) to give C-H oxidative addition to form the alkyl intermediate [(H)(eta2-H2)Ir(py-N(Me)CH2-)L2]BF4. Loss of H2 leads to the fully characterized alkyl [HIr(OCMe2)(py-N(Me)CH2-)L2]BF4 (3a(Me2CO)), which loses acetone to give alkylidene hydride 2a by rapid reversible alpha-elimination. 2a rapidly reacts with excess H2 in d6-acetone to generate [H2Ir(OC(CD3)2)2L2]BF4 (1-d12), 3a((CD3)2CO), and py-NMe2 in a 1:1:1 ratio, showing reversibility and accounting for the selective isotope exchange catalyzed by 1. Reaction of 1 with py-N(CH2)4 gives the fully characterized carbene 2c. A cis-L(2) carbene intermediate, cis-2c, observed by NMR, reacts with CO via retro alpha-elimination to give the alkyl 3cCO, while the trans isomer, 2c, does not react; retro alpha-elimination thus requires the Ir-H bond to be orthogonal to the carbene plane. Consistent with experiment, computational studies show a particularly flat PE surface with activation of the agostic C-H bond giving a less stable H2 complex, then formation of a kinetic carbene complex with cis-L, only seen experimentally for py-N(CH2)4. Hydrides at key positions, together with gain or loss of solvent and H2, flatten the PE (DeltaG) surfaces to allow fast catalysis.

Mechanism of alkane transfer-dehydrogenation catalyzed by a pincer-ligated iridium complex.

The mechanism of (PCP)Ir-catalyzed transfer-dehydrogenation has been elucidated for the prototypical substrate/acceptor couple, COA/TBE, at 55 degrees C (COA = cyclooctane; TBE = tert-butylethylene). The catalytic cycle may be viewed as the sum of two reactions: (i) hydrogenation of TBE by (PCP)IrH2 and C-H addition of a second mole of TBE to give (PCP)IrH(tert-butylvinyl), and (ii) dehydrogenation of COA by (PCP)IrH(tert-butylvinyl) to give (PCP)IrH2, COE, and TBE. These two stoichiometric reactions have been observed independently and their kinetics determined. The overall catalysis has also been monitored in situ, and (PCP)IrH2 and (PCP)IrH(tert-butylvinyl) have been observed as the resting states; the ratio of these two complexes is found to be proportional to [TBE]2. Based upon the proportionality constant thus obtained and the catalytic rate as a function of [TBE] (which reaches a maximum at ca. 0.3 M), the respective rate constants for the hydrogenation and dehydrogenation segments can be obtained. Good agreement is found between the rates independently obtained from stoichiometric and catalytic runs. Within the overall TBE-hydrogenation reaction, labeling experiments indicate that the rate-determining step is the reductive elimination of TBA (2,2-dimethylbutane) from (PCP)IrH(tert-butylethyl) (which is formed via insertion of TBE into an Ir-H bond of (PCP)IrH2). Based upon considerations of microscopic reversibility, it can be further inferred that the rate-determining step for the alkane dehydrogenations is C-H addition (and not beta-H elimination).

Minimum energy structure of hydridotris(pyrazolyl)borato iridium(V) tetrahydride is not a C(3upsilon) capped octahedron.

Recent synthesis and NMR spectroscopy of neutral Ir(V) complexes hydridotris(3,5-dimethylpyrazol-1-yl)borato tetrahydride (Tp*IrH(4)) and hydridotris(pyrazol-1-yl)borato tetrahydride (TpIrH(4)) have been interpreted as supporting face-capped octahedral structures (C(3upsilon)) with each of three Ir-H bonds trans to an Ir-N bond and the fourth hydride capping the IrH(3) face. Here, density functional geometry optimizations and coupled cluster calculations on hydridotris(pyrazol-1-yl)borato iridium tetrahydrogen find that a C(s) edge-bridged octahedral tetrahydride structure and a C(1) eta(2)-dihydrogen, dihydride structure are local minima and find that the C(3upsilon) structure is a local maximum (second-order saddle point). Several low energy transition states connecting the local minima have been located, and these minima can be used to simulate the experimental NMR spectra. A comparison of the experimental infrared spectrum of Tp*IrH(4) and the harmonic frequency calculations on the C(s), C(1), and C(3upsilon) structures also supports the assignment of the C(s)and C(1) structures as the observed ones.

Structural studies of the hydrido-bridged iridium gold complexes ‘IrHm{CH3C(CH2PPh2)3}{Au(PR3)}n’

The X-ray crystal structure of [{(triphos)H(3−x)Ir}(μ-H)x{Au(PR3)}][PF6] (triphos=CH3C(CH2PPh2)3, x=2) shows that the gold atom builds two almost equal Ir-H-Au bridges with the he ‘IrH3(triphos)’ building block. The Ir-H-Au bridging parameters are typical of three-center-two-electron interactions. The X-ray crystal structure of [{(triphos)H(3−y)Ir}(μ-H)y{Au(PR3)}2][PF6]2 shows that each gold atom builds two Ir(μ2-H)Au bridges with the three hydrides of the ‘IrH3(triphos)’ building block; one Ir(μ3-H)Au2 bridge is also present (y=3). The relative positions of the Ir, H, Au and P atoms show that typical three-center-two-electron interactions predominate in this compound, in which there is no direct Au-Au bonding. The neutron diffraction structure of [{(triphos)Ir}(μ-H)2{Au(PPh3)}3][PF6]2 confirms the earlier hypothesis that only two of the three Ir-Au edges are associated with a hydride with formation of Ir(μ2-H)Au bridges. The presence or absence of the latter ligand changes the Ir-Au distance only marginally, in contrast to the general trend in hydride clusters. It is shown that the formation of a ‘classical’ cluster in this set of compounds requires a quadrimetallic unit and the two additional electrons generated by loss of a proton from an Ir-H bond in the trication [{(triphos)Ir (μ2-H)3{Au(PR3)}3}3+.

Ba 3Ir 2H 12, a new ternary hydride containing octahedral [IrH 6] 3 - complex anions

A new ternary hydride, Ba 3Ir 2H 12, has been synthesized by reacting a mixture of BaH 2 and iridium powders in a hydrogen atmosphere. The crystal structure was determined by combining Guinier-Hägg X-ray powder diffraction and neutron diffraction data from a deuterated compound. The structure was refined (space group, P-3m1; a = 5.4865(3) A ̊ and c = 8.8459(10) A ̊ ; Z = 1 ). The unit cell contains two slightly distorted [IrH 6] 3− octahedra which form a La 2O 3-type structure with the three Ba 2+ counterions. Two different Ir-H bond lengths were refined with neutron diffraction data from Ba 3Ir 2D 12: 1.60(1) Å and 1.77(2) Å.

Mechanism and Energetics for Dehydrogenation of Methane by Gaseous Iridium Ions

Ir^+ has been observed as the most efficient transition-metal ion for dehydrogenation of CH_4 in the gas phase. We carried out theoretical studies of the complete reaction profile for this process and find three salient factors responsible for the high reactivity: (i) the ability of Ir^+ to change spin easily, (ii) the strength of the Ir-C and Ir-H bonds, and (iii) the ability of Ir^+ to form up to four covalent bonds. We show that among transition-metal ions Ir^+ is unique in best possessing all three characteristics. The combination of these factors leads to a global minimum for the singlet Ir(H)_2(CH_2)^+ structure, which plays an important role in the activation. On the basis of these results, we suggest solution-phase analogues that may also activate CH_4.