Solvent‐Dependent Structure of Iridium Dihydride Complexes: Different Geometries at Low and High Dielectricity of the Medium

The hydride iridium pincer complex [(PCyP)IrH₂] (PCyP=cis‐1,3‐bis[(di‐tert‐butylphosphino)methyl]cyclohexane, 1 ) reveals remarkably solvent‐dependent hydride chemical shifts, isotope chemical shifts, J_HD and T₁(min), with r_HH increasing upon moving to more polar medium. The only known example of such behaviour (complex [(POCOP)IrH₂], POCOP=2,6‐(tBu₂PO)₂C₆H₃) was explained by the coordination of a polar solvent molecule to the iridium (J. Am. Chem. Soc. 2006 , 128, 17114). Based on the existence of an agostic bond between α‐C?H and iridium in 1 in all solvents, we argue that the coordination of solvent can be rejected. DFT calculations revealed that the structures of 1 and [(POCOP)IrH₂] depend on the dielectric permittivity of the medium and these compounds adopt trigonal‐bipyramidal geometries in non‐polar media and square‐pyramidal geometries in polar media.     

Photophysical Properties of Heteroleptic Iridium Complexes Containing Carbazole‐Functionalized β‐Diketonates

Twelve iridium complexes with general formula of Ir(C^N)₂(LX) [C^N represents the cyclometalated ligand, i.e. 2‐(2,4‐difluorophenyl) pyridine (dfppy), 2‐phenylpyridine (ppy), dibenzo{f, h}quinoxaline (DBQ); LX stands for β‐diketonate, i.e. acetyl acetonate (acac), 1‐(carbazol‐9‐yl)‐5,5‐dimethylhexane‐2,4‐diketonate (CBDK), 1‐(carbazol‐9‐yl)‐5,5,6,6,7,7,7‐heptafluoroheptane‐2,4‐diketonate (CHFDK), 1‐(N‐ethyl‐carbazol‐3‐yl)‐4,4,5,5,6,6,6‐heptafluorohexane‐1,3‐diketonate (ECHFDK)] are synthesized, characterized and their photophysical properties are systemically studied. In addition, crystals of Ir(DBQ)₂(CHFDK) and Ir(DBQ)₂(acac) are obtained and characterized by single crystal X‐ray diffraction. The choice of these iridium complexes provides an opportunity for tracing the effect of the triplet energy level of ancillary ligands on the photophysical and electrochemical behaviors. Data show that if the triplet energy level of the β‐diketonate is higher than that of the Ir(C^N)₂ fragment and there is no superposition on the state density map, strong ³LC or ³MLCT‐based phosphorescence can be obtained. Alternatively, if the state density map of the two parts are in superposition, the ³LC or ³MLCT‐based transition will be quenched at room temperature. Density functional theory calculations show that these complexes can be divided into two categories. The lowest excited state is mainly determined by C^N but not β‐diketonate when the difference between the triplet energy levels of the two parts is large. However, when this difference is very small, the lowest excited state will be determined by both sides. This provides a satisfactory explanation for the experimental observations.     

Cationic Heteroleptic Cyclometalated Iridium Complexes with 1‐Pyridylimidazo[1,5‐α]pyridine Ligands: Exploitation of an Efficient Intersystem Crossing

Luminescent ligands in Ir^III cyclometalated complexes. The photophysical and photochemical properties of Ir‐cyclometalated complexes containing luminescent ligands are evaluated (see figure). Significant admixture between Ir and ligand orbitals induces an efficient intersystem crossing. Photochemical reactions performed in the presence of oxygen lead to new Ir‐cyclometalated complexes containing N(amido) groups directly bound to Ir.

     

Understanding Electrogenerated Chemiluminescence Efficiency in Blue‐Shifted Iridium(III)‐Complexes: An Experimental and Theoretical Study

Compared to tris(2‐phenylpyridine)iridium(III) ([Ir(ppy)₃]), iridium(III) complexes containing difluorophenylpyridine (df‐ppy) and/or an ancillary triazolylpyridine ligand [3‐phenyl‐1,2,4‐triazol‐5‐ylpyridinato (ptp) or 1‐benzyl‐1,2,3‐triazol‐4‐ylpyridine (ptb)] exhibit considerable hypsochromic shifts (ca. 25–60 nm), due to the significant stabilising effect of these ligands on the HOMO energy, whilst having relatively little effect on the LUMO. Despite their lower photoluminescence quantum yields compared with [Ir(ppy)₃] and [Ir(df‐ppy)₃], the iridium(III) complexes containing triazolylpyridine ligands gave greater electrogenerated chemiluminescence (ECL) intensities (using tri‐n‐propylamine (TPA) as a co‐reactant), which can in part be ascribed to the more energetically favourable reactions of the oxidised complex (M⁺) with both TPA and its neutral radical oxidation product. The calculated iridium(III) complex LUMO energies were shown to be a good predictor of the corresponding M⁺ LUMO energies, and both HOMO and LUMO levels are related to ECL efficiency. The theoretical and experimental data together show that the best strategy for the design of efficient new blue‐shifted electrochemiluminophores is to aim to stabilise the HOMO, while only moderately stabilising the LUMO, thereby increasing the energy gap but ensuring favourable thermodynamics and kinetics for the ECL reaction. Of the iridium(III) complexes examined, [Ir(df‐ppy)₂(ptb)]⁺ was most attractive as a blue‐emitter for ECL detection, featuring a large hypsochromic shift (λ_max=454 and 484 nm), superior co‐reactant ECL intensity than the archetypal homoleptic green and blue emitters: [Ir(ppy)₃] and [Ir(df‐ppy)₃] (by over 16‐fold and threefold, respectively), and greater solubility in polar solvents.     

Near‐IR Emitting Iridium(III) Complexes with Heteroaromatic β‐Diketonate Ancillary Ligands for Efficient Solution‐Processed OLEDs: Structure–Property Correlations

Three NIR‐emitting neutral Ir^III complexes [Ir(iqbt)₂(dpm)] ( 1 ), [Ir(iqbt)₂(tta)] ( 2 ), and [Ir(iqbt)₂(dtdk)] ( 3 ) based on the 1‐(benzo[b]thiophen‐2‐yl)‐isoquinolinate (iqtb) were synthesized and characterized (dpm=2,2,6,6‐tetramethyl‐3,5‐heptanedionate; tta=2‐thienoyltrifluoroacetonate; dtdk=1,3‐di(thiophen‐2‐yl)propane‐1,3‐dionate). The compounds emit between λ=680 and 850 nm with high luminescence quantum yields (up to 16 %). By combining electrochemistry, photophysical measurements, and computational modelling, the relationship between the structure, energy levels, and properties were investigated. NIR‐emitting, solution‐processed phosphorescent organic light‐emitting devices (PHOLEDs) were fabricated using the complexes. The devices show remarkable external quantum efficiencies (above 3 % with 1 ) with negligible efficiency roll‐off values, exceeding the highest reported values for solution‐processible NIR emitters.     

Factors Controlling β‐Elimination Reactions in Group 10 Metal Complexes

Trends in reactivity of β‐chloride and β‐hydride elimination reactions involving Group 10 transition‐metal complexes have been computationally explored and analyzed in detail by DFT. These reactions do not require the initial formation of a vacant coordination site; they proceed concertedly without a prior ligand‐dissociation step. Whereas β‐chloride elimination is associated with relatively moderate activation barriers, the high barriers calculated for analogous β‐hydride eliminations suggest that the latter process is unfeasible for this type of compounds. This differential behavior is analyzed within the activation strain model, which provides quantitative insight into the physical factors controlling these β‐elimination reactions. The effects of the nature of the Group 10 transition metal (Ni, Pd, Pt), as well as the substituents attached to the β‐eliminating fragment (R₂C?CR₂X; R, X=H, Cl) on the transformation have also been considered and are rationalized herein.     

Mechanism of the Asymmetric Hydrogenation of Exocyclic α,β‐Unsaturated Carbonyl Compounds with an Iridium/BiphPhox Catalyst: NMR and DFT Studies

The mechanism of the asymmetric hydrogenation of exocyclic α,β‐unsaturated carbonyl compounds with the (aS)‐Ir/iPr‐BiphPhox catalyst was studied by NMR experiments and DFT computational analyses. Computed optical yields of the asymmetric hydrogenation proceeding by an iridium(I)/iridium(III) mechanism involving a transition state stabilized through two intramolecular hydrogen bonds are in good accordance with the experimental ee values.     

Walking Metals in d8⋅⋅⋅d8 Hetero‐bimetallic Complexes: An Original Dynamic Phenomenon

In the course of our investigations on polymetallic complexes derived from 1,3‐bis(thiophosphinoyl)indene (Ind(Ph₂P?S)₂), we observed original fluxional behavior and report herein a joint experimental/computational study of this dynamic process. Starting from the indenylidene chloropalladate species [Pd{Ind(Ph₂P?S)₂}Cl]^? ( 1 ), the new Pd^II???Rh^I hetero‐bimetallic pincer complex [PdCl{Ind(Ph₂P?S)₂}Rh(nbd)] ( 2 ; nbd=2,5‐norbornadiene) was prepared. X‐ray crystallography and DFT calculations substantiate the presence of a d⁸???d⁸ interaction. According to multinuclear variable‐temperature NMR spectroscopic experiments, the pendant {Rh(nbd)} fragment of 2 readily shifts in solution at room temperature between the two edges of the SCS tridentate ligand. To assess the role of the pincer‐based polymetallic structure on this fluxional behavior, the related monometallic Rh complex [Rh{IndH(Ph₂P?S)₂}(nbd)] ( 3 ) was prepared. No evidence for a metal shift was observed in that case, even at high temperature, thus indicating that inplane pincer coordination to the Pd center plays a crucial role. The previously described Pd^II???Ir^I bimetallic complex 4 exhibited fluxional behavior in solution, but with a significantly higher activation barrier than 2 . This finding demonstrates the generality of this metal‐shift process and the strong influence of the involved metal centers on the associated activation barrier. DFT calculations were performed to shed light onto the mechanism of such metal‐shift processes and to identify the factors that influence the associated activation barriers. Significantly different pathways were found for bimetallic complexes 2 and 4 on one hand and the monometallic complex 3 on the other hand. The corresponding activation barriers predicted computationally are in very good agreement with the experimental observations.     

β‐Hydrogen Elimination Reactions of Nickel and Palladium Methoxides Stabilised by PCP Pincer Ligands

Nickel and palladium methoxides [(^iPrPCP)M‐OMe], which contain the ^iPrPCP pincer ligand, decompose upon heating to give products of different kinds. The palladium derivative cleanly gives the dimeric Pd⁰ complex [Pd(μ‐^iPrPCHP)]₂ (^iPrPCHP=2,6‐bis(diisopropylphosphinomethyl)phenyl) and formaldehyde. In contrast, decomposition of [(^iPrPCP)Ni‐OMe] affords polynuclear carbonyl phosphine complexes. Both decomposition processes are initiated by β‐hydrogen elimination (BHE), but the resulting [(^iPrPCP)M‐H] hydrides undergo divergent reaction sequences that ultimately lead to the irreversible breakdown of the pincer units. Whereas the Pd hydride spontaneously experiences reductive C?H coupling, the decay of its Ni analogue is brought about by its reaction with formaldehyde released in the BHE step. Kinetic measurements showed that the BHE reaction is reversible and less favourable for Ni than for Pd for both kinetic and thermodynamic reasons. DFT calculations confirmed the main conclusions of the kinetic studies and provided further insight into the mechanisms of the decomposition reactions.     

Unusual NHC–Iridium(I) Complexes and Their Use in the Intramolecular Hydroamination of Unactivated Aminoalkenes

N‐heterocyclic carbene (NHC) ligands with naphthyl side chains were employed for the synthesis of unsaturated, yet isolable [(NHC)Ir(cod)]⁺ (cod=1,5‐cyclooctadiene) complexes. These compounds are stabilised by an interaction of the aromatic wingtip that leads to a sideways tilt of the NHC?Ir bond. Detailed studies show how the tilting of such N‐heterocyclic carbenes affects the electronic shielding properties of the carbene carbon atom and how this is reflected by significant upfield shifts in the ¹³C NMR signals. When employed in the intramolecular hydroamination, these [(NHC)Ir(cod)]⁺ species show very high catalytic activity under mild reaction conditions. An enantiopure version of the catalyst system produces pyrrolidines with excellent enantioselectivities.     

Theoretical investigation on the photophysical properties of N‐heterocyclic carbene iridium (III) complexes (fpmb)xIr(bptz)3‐x (x = 1−2)

In the search for efficiently phosphorescent materials, this article presents a rational design and theoretical comparative study of some photophysical properties in the (fpmb)_xIr(bptz)_3‐x (x = 1–2), which involve the usage of two 2‐pyridyl triazolate ( bptz ) chromophores and a strong‐field ligand fpmb ( fpmb = 1‐(4‐difluorobenzyl)‐3‐methylbenzimidazolium). The first principle theoretical analysis under the framework of the time‐dependent density functional theory approach is implemented in this article to investigate the electronic structures, absorption and phosphorescence spectra. It is intriguing to note that 1 and 2 exhibit theirs blue phosphorescent emissions with maxima at 504 and 516 nm, respectively. Furthermore, to obtain the mechanism of low phosphorescence yield in 1 and estimate the radiative rate constant k_r for 2 , we approximately measure the radiative rate constant k_r, the spin‐orbital coupling (SOC) value, ΔE (S ? T), and the square of the SOC matrix element (<Ψ_S1.H_SO.Ψ_T1>²) for 1 and 2 . Finally, we tentatively come to conclusion that the switch of the cyclometalated ligand from the main to ancillary chelate seems to lower the splitting ΔE (S ? T) in the current system. © 2012 Wiley Periodicals, Inc. J Comput Chem, 2012     

A Phosphorus Analogue of Bis(η4‐cyclobutadiene)iron(0)

P makes it possible : The convenient oxidative synthesis of the 16‐electron organophosphorus iron sandwich complex [Fe(η⁴‐P₂C₂tBu₂)₂] (see structure) suggests that the elusive all‐carbon complex [Fe(η⁴‐C₄H₄)₂] is a viable synthetic target.

     

Highly Stereocontrolled Ring‐Opening Polymerization of Racemic Alkyl β‐Malolactonates Mediated by Yttrium [Amino‐alkoxy‐bis(phenolate)] Complexes

Yttrium [amino‐alkoxy‐bis(phenolate)]amido complexes have been used for the ring‐opening polymerization (ROP) of racemic alkyl β‐malolactonates (4‐alkoxycarbonyl‐2‐oxetanones, rac‐MLA^Rs) bearing an allyl (All), benzyl (Bz) or methyl (Me) lateral ester function. The nature of the ortho‐substituent on the phenolate rings in the metal ancillary dictated the stereocontrol of the ROP, and consequently the syndiotactic enrichment of the resulting polyesters. ROP promoted by catalysts with halogen (Cl, Br)‐disubstituted ligands allowed the first reported synthesis of highly syndiotactic PMLA^Rs (P_r ≥ 0.95); conversely, catalysts bearing bulky alkyl and aryl ortho‐substituted ligands proved largely ineffective. All polymers have been characterized by ¹H and ¹³C{¹H} NMR spectroscopy, MALDI‐ToF mass spectrometry and DSC analyses. Statistical and thermal analyses enabled the rationalization of the chain‐end control mechanism. Whereas the stereocontrol of the polymerization obeyed a Markov first‐order (Mk1) model for the ROP of rac‐MLA^Bz and rac‐MLA^All, the ROP of rac‐MLA^Me led to a chain end‐control of Markov second‐order type (Mk2). DFT computations suggest that the high stereocontrol ability featured by catalysts bearing Cl‐ and Br‐substituted ligands does not likely originate from halogen bonding between the halogen substituent and the growing polyester chain.     

Efficient Green‐Light‐Emitting Electrochemical Cells Based on Ionic Iridium Complexes with Sulfone‐Containing Cyclometalating Ligands

A new approach to obtain green‐emitting iridium(III) complexes is described. The synthetic approach consists of introducing a methylsulfone electron‐withdrawing substituent into a 4‐phenylpyrazole cyclometalating ligand in order to stabilize the highest‐occupied molecular orbital (HOMO). Six new complexes have been synthesized incorporating the conjugate base of 1‐(4‐(methylsulfonyl)phenyl)‐1 H‐pyrazole as the cyclometalating ligand. The complexes show green emission and very high photoluminescence quantum yields in both diluted and concentrated films. When used as the main active component in light‐emitting electrochemical cells (LECs), green electroluminance is observed. High efficiencies and luminances are obtained at low driving voltages. This approach for green emitters is an alternative to the widely used fluorine‐based substituents in the cyclometalating ligands and opens new design possibilities for the synthesis of green emitters for LECs.     

Heteroatom‐Guided Torquoselective Olefination of α‐Oxy and α‐Amino Ketones via Ynolates

Ynolates were found to react with α‐alkoxy‐, α‐siloxy‐, and α‐aryloxyketones at room temperature to afford tetrasubstituted olefins with high Z selectivity. Since the geometrical selectivity was determined in the ring opening of the β‐lactone enolate intermediates, the torquoselectivity was controlled by the ethereal oxygen atoms. From experimental and theoretical studies, the high Z selectivity is induced by orbital and steric interactions rather than by chelation. In a similar manner, α‐dialkylamino ketones provided olefins with excellent Z selectivity. These products can be easily converted into multisubstituted butenolides and γ‐butyrolactams in good yield.