IrIII and RuII Complexes Containing Triazole‐Pyridine Ligands: Luminescence Enhancement upon Substitution with β‐Cyclodextrin

Novel 2‐(1‐substituted‐1H‐1,2,3‐triazol‐4‐yl)pyridine (pytl) ligands have been prepared by “click chemistry” and used in the preparation of heteroleptic complexes of Ru and Ir with bipyridine (bpy) and phenylpyridine (ppy) ligands, respectively, resulting in [Ru(bpy)₂(pytl‐R)]Cl₂ and [Ir(ppy)₂(pytl‐R)]Cl (R=methyl, adamantane (ada), β‐cyclodextrin (βCD)). The two diastereoisomers of the Ir complex with the appended β‐cyclodextrin, [Ir(ppy)₂(pytl‐βCD)]Cl, were separated. The [Ru(bpy)₂(pytl‐R)]Cl₂ (R=Me, ada or βCD) complexes have lower lifetimes and quantum yields than other polypyridine complexes. In contrast, the cyclometalated Ir complexes display rather long lifetimes and very high emission quantum yields. The emission quantum yield and lifetime (Φ=0.23, τ=1000 ns) of [Ir(ppy)₂(pytl‐ada)]Cl are surprisingly enhanced in [Ir(ppy)₂(pytl‐βCD)]Cl (Φ=0.54, τ=2800 ns). This behavior is unprecedented for a metal complex and is most likely due to its increased rigidity and protection from water molecules as well as from dioxygen quenching, because of the hydrophobic cavity of the βCD covalently attached to pytl. The emissive excited state is localized on these cyclometalating ligands, as underlined by the shift to the blue (450 nm) upon substitution with two electron‐withdrawing fluorine substituents on the phenyl unit. The significant differences between the quantum yields of the two separate diastereoisomers of [Ir(ppy)₂(pytl‐βCD)]Cl (0.49 vs. 0.70) are attributed to different interactions of the chiral cyclodextrin substituent with the Δ and Λ isomers of the metal complex.     

Easy Separation of Δ and Λ Isomers of Highly Luminescent [IrIII]‐Cyclometalated Complexes Based on Chiral Phenol‐Oxazoline Ancillary Ligands

A new class of neutral cyclometalated iridium(III) complexes with enantiomerically pure C₁‐symmetric phenol‐oxazolines ( 3 a,b ) have been synthetized in high yields and fully characterized. Resolution of the corresponding Δ_R and Λ_R or Δ_S and Λ_S isomers was easily achieved by conventional flash chromatography. The corresponding Δ and Λ helicities have been confirmed by CD spectroscopy and X‐ray crystallography. Regarding the absorption and luminescence properties with unpolarized light, no significant difference between Δ and Λ isomers has been observed. A strong blue luminescence is observed for deaerated solutions of complexes 5 a and 5 b in CH₃CN.     

Sublimable Cationic Iridium(III) Complexes with 1,10‐Phenanthroline Derivatives as Ancillary Ligands for Highly Efficient and Polychromic Electroluminescence

A novel series of four sublimable cationic iridium(III) complexes have been prepared with 1,10‐phenanthroline derivatives as ancillary ligands and the same negative counter‐ion, tetrakis[3,5‐bis(trifluoromethyl)phenyl]borate, which has a large steric hindrance and widely dispersed charges, thereby increasing the ionic radii, reducing the electrostatic interaction, and thus improving the volatility. Their structural, photophysical, electrochemical, and thermal properties have been fully characterized. Upon excitation, these compounds show polychromic emission varying from green to orange in solution, which are blue‐shifted in the solid state to different extents due to π–π conjugate effects in the ancillary ligands and the resulting molecular aggregation. OLEDs fabricated by vacuum evaporation deposition demonstrated desirable device performance with high efficiency and brightness, exhibiting various electroluminescent colors dependent upon doping concentration.     

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.     

Emissive Iridium(III) Complexes with Phosphorous‐Containing Ancillary

Ir(III) metal complexes and related emitters bearing all kind of cyclometalated chromophoric chelates and non‐chromophoric ancillary are extensively studied during the past three decades. Many of them have been found to display bright room temperature phosphorescence from triplet excited states in both solution and solid states, offering a possible application in contemporary optoelectronic technologies, including organic light emitting diodes, electrochemiluminescence, biological imaging and chemical sensing. Among reported materials, there are Ir(III) complexes with at least one phosphorus (P)‐containing ligand and/or ancillary chelate, together with cyclometalates or equivalents that are in control of the actual emission energy. Particularly, possession of P‐based donor can lead to divergent structural and photophysical properties compared to the traditional designs. This review aims to provide a literature overview as well as the authors’ personal account to the development of relevant Ir(III) based phosphors bearing these P‐donors. To the readers’ convenience, the contents are subdivided into six sessions, according to whether or not they are charge natural, or with mono‐ or dianionic electronic character, and in accordance to their divergent bonding modes, i. e. monodentate, bidentate and tripodal coordination. In many cases, the P‐based ancillaries offer an easy accessible route to the formation of efficient sky‐blue and true‐blue emitters due to their π‐accepting property, together with enlarged emission energy gap and destabilized upper lying quenching state.     

Stepwise Strategy to Cyclometallated PtII Complexes with N‐Heterocyclic Carbene Ligands: A Luminescence Study on New β‐Diketonate Complexes

The imidazolium salt 3‐methyl‐1‐(naphthalen‐2‐yl)‐1H‐imidazolium iodide ( 2 ) has been treated with silver(I) oxide and [{Pt(μ‐Cl)(η³‐2‐Me‐C₃H₄)}₂] (η³‐2‐Me‐C₃H₄=η³‐2‐methylallyl) to give the intermediate N‐heterocyclic carbene complex [PtCl(η³‐2‐Me‐C₃H₄)(H$\widehat{CC}$ *‐κC*)] ( 3 ) (H$\widehat{CC}$ *‐κC*=3‐methyl‐1‐(naphthalen‐2‐yl)‐1H‐imidazol‐2‐ylidene). Compound 3 undergoes regiospecific cyclometallation at the naphthyl ring of the NHC ligand to give the five‐membered platinacycle compound [{Pt(μ‐Cl)($\widehat{CC}$ *)}₂] ( 4 ). Chlorine abstraction from 4 with β‐diketonate Tl derivatives rendered the corresponding neutral compounds [Pt($\widehat{CC}$ *)(L‐O,O′)] {L=acac (HL=acetylacetone) 5 , phacac (HL=1,3‐diphenyl‐1,3‐propanedione) 6 , hfacac (HL=hexafluoroacetylacetone) 7 }. All of the compounds ( 3 – 7 ) were fully characterized by standard spectroscopic and analytical methods. X‐ray diffraction studies were performed on 5 – 7 , revealing short Pt?Pt and π–π interactions in the solid‐state structure. The influence of the R‐substituents of the β‐diketonate ligand on the photophysical properties and the use of the most efficient emitter, 5 , as phosphor converter has also been studied.     

Development of Chiral Spiro P‐N‐S Ligands for Iridium‐Catalyzed Asymmetric Hydrogenation of β‐Alkyl‐β‐Ketoesters

The chiral tridentate spiro P‐N‐S ligands (SpiroSAP) were developed, and their iridium complexes were prepared. Introduction of a 1,3‐dithiane moiety into the ligand resulted in a highly efficient chiral iridium catalyst for asymmetric hydrogenation of β‐alkyl‐β‐ketoesters, producing chiral β‐alkyl‐β‐hydroxyesters with excellent enantioselectivities (95–99.9 % ee) and turnover numbers of up to 355 000.     

Phosphorescent Cyclometalated Iridium(III) Complexes That Contain Substituted 2‐Acetylbenzo[b]thiophen‐3‐olate Ligand for Red Organic Light‐Emitting Devices

We report the synthesis of a new class of thermally stable and strongly luminescent cyclometalated iridium(III) complexes 1 – 6 , which contain the 2‐acetylbenzo[b]thiophene‐3‐olate (bt) ligand, and their application in organic light‐emitting diodes (OLEDs). These heteroleptic iridium(III) complexes with bt as the ancillary ligand have a decomposition temperature that is 10–20 % higher and lower emission self‐quenching constants than those of their corresponding complexes with acetylacetonate (acac). The luminescent color of these iridium(III) complexes could be fine‐tuned from orange (e.g., 2‐phenyl‐6‐(trifluoromethyl)benzo[d]thiazole (cf₃bta) for 4 ) to pure red (e.g., lpt (Hlpt=4‐methyl‐2‐(thiophen‐2‐yl)quinolone) for 6 ) by varying the cyclometalating ligands (C‐deprotonated C^N). In particular, highly efficient OLEDs based on 6 as dopant (emitter) and 1,3‐bis(carbazol‐9‐yl)benzene (mCP) as host that exhibit stable red emission over a wide range of brightness with CIE chromaticity coordinates of (0.67, 0.33) well matched to the National Television System Committee (NTSC) standard have been fabricated along with an external quantum efficiency (EQE) and current efficiency of 9 % and 10 cd A^?1, respectively. A further 50 % increase in EQE (>13 %) by replacing mCP with bis[4‐(6H‐indolo[2,3‐b]quinoxalin‐6‐yl)phenyl]diphenylsilane (BIQS) as host for 6 in the red OLED is demonstrated. The performance of OLEDs fabricated with 6 (i.e., [(lpt)₂Ir(bt)]) was comparable to that of the analogous iridium(III) complex that bore acac (i.e., [(lpt)₂Ir(acac)]; 6 a in this work) [Adv. Mater.­ 2011 , 23, 2981] fabricated under similar conditions. By using ntt (Hnnt=3‐hydroxynaphtho[2,3‐b]thiophen‐2‐yl)(thiophen‐2‐yl)methanone) ligand, a substituted derivative of bt, the [(cf₃bta)₂Ir(ntt)] was prepared and found to display deep red emission at around 700 nm with a quantum yield of 12 % in mCP thin film.     

Oxidation and β‐Alkylation of Alcohols Catalysed by Iridium(I) Complexes with Functionalised N‐Heterocyclic Carbene Ligands

The borrowing hydrogen methodology allows for the use of alcohols as alkylating agents for C?C bond forming processes offering significant environmental benefits over traditional approaches. Iridium(I)‐cyclooctadiene complexes having a NHC ligand with a O‐ or N‐functionalised wingtip efficiently catalysed the oxidation and β‐alkylation of secondary alcohols with primary alcohols in the presence of a base. The cationic complex [Ir(NCCH₃)(cod)(MeIm(2‐ methoxybenzyl))][BF₄] (cod=1,5‐cyclooctadiene, MeIm=1‐methylimidazolyl) having a rigid O‐functionalised wingtip, shows the best catalyst performance in the dehydrogenation of benzyl alcohol in acetone, with an initial turnover frequency (TOF₀) of 1283 h^?1, and also in the β‐alkylation of 2‐propanol with butan‐1‐ol, which gives a conversion of 94 % in 10 h with a selectivity of 99 % for heptan‐2‐ol. We have investigated the full reaction mechanism including the dehydrogenation, the cross‐aldol condensation and the hydrogenation step by DFT calculations. Interestingly, these studies revealed the participation of the iridium catalyst in the key step leading to the formation of the new C?C bond that involves the reaction of an O‐bound enolate generated in the basic medium with the electrophilic aldehyde.     

Catalytic Enantioselective α‐Fluorination of 2‐Acyl Imidazoles via Iridium Complexes

The first highly enantioselective α‐fluorination of 2‐acyl imidazoles utilizing iridium catalysis has been accomplished. This transformation features mild conditions and a remarkably broad substrate scope, providing an efficient and highly enantioselective approach to obtain a wide range of fluorine‐containing 2‐acyl imidazoles which are found in a variety of bioactive compounds and prodrugs. A large scale synthesis has also been tested to demonstrate the potential utility of this fluorination method.     

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.     

Asymmetric Hydrogenation of α,β‐Unsaturated Nitriles with Base‐Activated Iridium N,P Ligand Complexes

Although many chiral catalysts are known that allow highly enantioselective hydrogenation of a wide range of olefins, no suitable catalysts for the asymmetric hydrogenation of α,β‐unsaturated nitriles have been reported so far. We have found that Ir N,P ligand complexes, which under normal conditions do not show any reactivity towards α,β‐unsaturated nitriles, become highly active catalysts upon addition of N,N‐diisopropylethylamine. The base‐activated catalysts enable conjugate reduction of α,β‐unsaturated nitriles with H₂ at low catalyst loadings, affording the corresponding saturated nitriles with high conversion and excellent enantioselectivity. In contrast, alkenes lacking a conjugated cyano group do not react under these conditions, making it possible to selectively reduce the conjugated C?C bond of an α,β‐unsaturated nitrile, while leaving other types of C?C bonds in the molecule intact.     

Cyclometalated Mono‐ and Dinuclear IrIII Complexes with “Click”‐Derived Triazoles and Mesoionic Carbenes

Orthometalation at Ir^III centers is usually facile, and such orthometalated complexes often display intriguing electronic and catalytic properties. By using a central phenyl ring as C?H activation sites, we present here mono‐ and dinuclear Ir^III complexes with “click”‐derived 1,2,3‐triazole and 1,2,3‐triazol‐5‐ylidene ligands, in which the wingtip phenyl groups in the aforementioned ligands are additionally orthometalated and bind as carbanionic donors to the Ir^III centers. Structural characterization of the complexes reveal a piano stool‐type of coordination around the metal centers with the “click”‐derived ligands bound either with C^N or C^C donor sets to the Ir^III centers. Furthermore, whereas bond localization is observed within the 1,2,3‐triazole ligands, a more delocalized situation is found in their 1,2,3‐triazol‐5‐ylidene counterparts. All complexes were subjected to catalytic tests for the transfer hydrogenation of benzaldehyde and acetophenone. The dinuclear complexes turned out to be more active than their mononuclear counterparts. We present here the first examples of stable, isomer‐pure, dinuclear cyclometalated Ir^III complexes with poly‐mesoionic‐carbene ligands.     

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.

     

Sensing of 2,4,6‐Trinitrotoluene (TNT) and 2,4‐Dinitrotoluene (2,4‐DNT) in the Solid State with Photoluminescent RuII and IrIII Complexes

A series of metal–organic chromophores containing Ru^II or Ir^III were studied for the luminometric detection of nitroaromatic compounds, including trinitrotoluene (TNT). These complexes display long‐lived, intense photoluminescence in the visible region and are demonstrated to serve as luminescent sensors for nitroaromatics. The solution‐based behavior of these photoluminescent molecules has been studied in detail in order to identify the mechanism responsible for metal‐to‐ligand charge‐transfer (MLCT) excited state quenching upon addition of TNT and 2,4‐dinitrotoluene (2,4‐DNT). A combination of static and dynamic spectroscopic measurements unequivocally confirmed that the quenching was due to a photoinduced electron transfer (PET) process. Ultrafast transient absorption experiments confirmed the formation of the TNT radical anion product following excited state electron transfer from these metal complexes. Reported for the first time, photoluminescence quenching realized through ink‐jet printing and solid‐state titrations was used for the solid‐state detection of TNT; achieving a limit‐of‐quantitation (LOQ) as low as 5.6 ng cm^?2. The combined effect of a long‐lived excited state and an energetically favorable driving force for the PET process makes the Ru^II and Ir^III MLCT complexes discussed here particularly appealing for the detection of nitroaromatic volatiles and related high‐energy compounds.     

Iron and Nickel Complexes Containing β‐Diketiminato Ligands with Thioether Tethers

A novel β‐diketiminato ligand precursor, LH ( II ), containing thioether tethers was synthesized by the reaction of acetylacetone and 2‐methylthioaniline. II was deprotonated and used in the synthesis of two iron(II) complexes, [LFeCl] ( 1 ), and [LFeOTf] ( 2 ), and one nickel(II) complex, [LNiBr] ( 3 ). All three compounds were characterized by means of single crystal X‐ray diffraction and their structures are discussed.     

Asymmetric Hydrogenation of α,β‐Unsaturated Carboxylic Esters with Chiral Iridium N,P Ligand Complexes

Enantioselective conjugate reduction of a wide range of α,β‐unsaturated carboxylic esters was achieved using chiral Ir N,P complexes as hydrogenation catalysts. Depending on the substitution pattern of the substrate, different ligands perform best. α,β‐Unsaturated carboxylic esters substituted at the α position are less problematic substrates than originally anticipated and in some cases α‐substituted substrates actually reacted with higher enantioselectivity than their β‐substituted analogues. The resulting saturated esters with a stereogenic center in the α or β position were obtained in high enantiomeric purity.     

Magnesium Complexes Bearing η2-Pyrazolato Ligands

Despite the small size of the magnesium ion , η²-bound pyrazolato ligands are found in complexes 1 – 3 . These complexes provide new insight into the design of volatile Group 2 metal complexes for use in chemical vapor deposition processes.     

Reversible α‐Hydrogen and α‐Alkyl Elimination in PC(sp3)P Pincer Complexes of Iridium

Despite significant progress in recent years, the cleavage of unstrained C(sp³)? C(sp³) bonds remains challenging. A C? C coupling and cleavage reaction in a PC(sp³)P iridium pincer complex is mechanistically studied; the reaction proceeds via the formation of a carbene intermediate and can be described as a competition between α‐hydrogen and α‐alkyl elimination; the latter process was observed experimentally and is an unusual way of C(sp³)? C(sp³) bond scission, which has previously not been studied in detail. Mechanistic details that are based upon kinetic studies, activation parameters, and DFT calculations are also discussed. A full characterization of a C? C agostic intermediate is presented.     

Stereodivergent Dual Catalytic α‐Allylation of Protected α‐Amino‐ and α‐Hydroxyacetaldehydes

Fully stereodivergent dual‐catalytic α‐allylation of protected α‐amino‐ and α‐hydroxyacetaldehydes is achieved through iridium‐ and amine‐catalyzed substitution of racemic allylic alcohols with chiral enamines generated in situ. The operationally simple method furnishes useful aldehyde building blocks in good yields, more than 99 % ee, and with d.r. values greater than 20:1 in some cases. Additionally, the γ,δ‐unsaturated products can be further functionalized in a stereodivergent fashion with high selectivity and with preservation of stereochemical integrity at the C_α position.