A series of iridium complexes equipped with inert shields: Highly efficient bluish green emitters with reduced self-quenching effect in solid state

In this paper, we synthesize a series of cyclometalated ligands and their corresponding Ir(III) complexes using pentane-2,4-dione as the auxiliary ligand. We discuss the photophysical properties of these Ir(III) complexes in detail, including their UV-Vis absorption spectra, photoluminescence spectra in solid and liquid states, luminescence decay lifetimes, and luminescence quantum yields. The correlation between self-quenching effect and molecular structure is also investigated. It is found that these Ir(III) complexes are solid-emitting ones due to their reduced self-quenching in solid state. Theoretical calculation and experimental data reveal that the following two reasons should be responsible for the reduced self-quenching in solid state: (1) pentane-2,4-dione, phenyl, and triphenylamine moieties serve as inert shields for the excited state Ir(III) complexes; (2) the radiative decay process in these Ir(III) complexes is accelerated by the introduction of electron-donors, and thus partly immune from self-quenching caused by intermolecular action.     

Tuning iridium(III) complexes containing 2-benzo[b]thiophen-2-yl-pyridine based ligands in the red region

Synthesis and characterization of four iridium(III) complexes containing 2-benzo[b]thiophen-2-yl-pyridine based ligands are reported. The absorption, emission, electrochemistry, and thermostability of the complexes were systematically investigated. The (btmp)₂Ir(acac) (btmp = 2-benzo[b]thiophen-2-yl-4-methyl-pyridyl, acac = acetyl acetone) was characterized using X-ray crystallography. Calculation on the electronic ground state for (btmp)₂Ir(acac) was carried out using B3LYP density functional theory, HOMO levels are a mixture of Ir and btmp ligand orbitals, while the LUMO is predominantly btmp ligand based. Introduction of substituents (CH₃, CF₃) into pyridyl ring in a typical red emitter (btp)₂Ir(acac) leads to a marked decrease in the sublimation temperature, which is more suitable for OLEDs process. Electrochemical studies showed that (btmp)₂Ir(acac) has a slightly lower oxidation potential, but (btfmp)₂Ir(acac), (btfmp)₂Ir(dbm), and (btfmp)₂Ir(pic) (btfmp = 2-benzo[b]thiophen-2-yl-5-trifluoromethyl-pyridine, dbm = dibenzoylmethane, pic = 2-picolinic acid) containing CF₃ group are much difficult to oxidate than (btp)₂Ir(acac). The emission characteristics of these complexes can be tuned by either changing the substituents and their position on 2-benzo[b]thiophen-2-yl-pyridine or using different monoanionic ligands, showing emission λ_max values from 604 to 638 nm in CH₂Cl₂ solution at room temperature.     

Luminescence and structural studies of yttrium and heavier lanthanide-picrate complexes with pentaethylene glycol

The monoclinic structural isomers with molecular formula of [M(Pic)₂(EO5)](Pic) where EO5 = pentaethylene glycol, Pic = picrate anion, and M = Gd, Tb, Er, Tm, Yb, and Y have been synthesized and characterized. The current study was conducted in the solid state to evaluate the coordination pattern of the central metal ion with the EO5 ligand in the presence of Pic anion. The photoluminescence (PL) spectra of the Tb and Yb complexes have the typical 4f-4f transition emissions of Tb(III) and Yb(III) ions. The Gd, Er, Tm, and Y complexes had broad bands resulting from the ligands. The counteranion factor influenced the emission intensity in the Tb-Pic-EO5 and Tb-NO₃-EO5 complexes in several solvents also were studied. The Pic anion acts as a quencher in the [Tb(Pic)₂(EO5)](Pic) complex due to the nitro withdrawing groups was clearly observed both in solution and the solid state.     

Syntheses and structural characterizations of novel mono- and dinuclear iridium hydrido complexes with polydentate nitrogen donor ligands

New five mono- and dinuclear Ir hydrido complexes with polydentate nitrogen ligands, [Ir(H)₂(PPh₃)₂(tptz)]PF₆ (1), [Ir₂(H)₄(PPh₃)₄(tptz)](PF₆)₂ · 2H₂O (2 · 2H₂O), [Ir(H)₂(PPh₃)₂(tppz)]BF₄ (3), [Ir₂(H)₄(PPh₃)₄(tppz)](BF₄)₂ (4) and [Ir₂(H)₄(PPh₃)₄(bted)](BF₄)₂ · 6CHCl₃ (5 · 6CHCl₃), were systematically prepared by the reactions of the precursor Ir hydrido complex [Ir(H)₂(PPh₃)₂(Me₂CO)₂]X (X=PF₆ and BF₄) with 2,4,6-tris(2-pyridyl)-1,3,5-triazine (tptz), 2,3,5,6-tetrakis(2-pyridyl)pyrazine (tppz) and 1,4-bis(2,2^′:6^′,2″-terpyridine-4^′-yl)benzene (bted), and their structures and properties were characterized in the solid state and in solution. Each of the Ir hydrido complexes with polydentate nitrogen ligands crystallographically described a unique coordination mode. Their ¹H NMR spectra demonstrated unusual ¹H NMR chemical shifts of pyridyl rings that are likely induced by the ring current effect of neighboring ligands.     

Multinuclear nuclear magnetic resonance and density functional theoretical studies on the structure of bisperoxovanadium complexes with bidentate donors

Solid state and solution ⁵¹V and ¹³C NMR studies on four fundamental bisperoxovanadium complexes containing bidendate donor ligands were reported, together with DFT calculations of structural and NMR parameters. The ⁵¹V solid-state NMR characterization of the four complexes with [VO(O₂)₂L]ⁿ⁻ anion {abbr. bpVL, where L = oxalic acid dianion (ox), pyridine-2-carboxylic acid (pic), bipyridine (bipy), and 1,10-phenanthroline (phen)} show that the ligands have a significant effect on the electric-field gradient tensor, with the quadrupolar coupling constant ranging from 4.0 to 5.8 MHz. The experimental and theoretical results suggest that the vanadium center of bpVpic, bpVphen and bpVbipy in solid state and aqueous solution are all seven-coordinated except that bpVox is six-coordinated in aqueous solution. The steric space hindrance of the organic ligands and the bonding between vanadium with the coordination influences the activity of bpVL complexes.     

Structural comparison of (pentamethylcyclopentadienyl)iridium(III) azido complexes containing potentially N,S-bidentate N-heterocyclic thiolates: 2- or 8-quinolinethiolate and benzimidazole-2-thiolate

The crystal structures of mononuclear (azido)(pentamethylcyclopentadienyl)iridium(III) complexes bearing 2- or 8-quinolinethiolate (n-Sqn⁻), [Cp^∗Ir(N₃)(n-Sqn)] {n = 2 (1) or 8 (2); Cp^∗ = η⁵-C₅Me₅} have been determined by X-ray analysis. The 2-Sqn complex, 1, acquires severe steric strains in the four-membered κ²N,S chelate ring, while the 8-Sqn isomer, 2, forms a strain-free five-membered planar κ²N,S chelate ring. It has also been revealed that the corresponding benzimidazole-2-thiolate (Hbimt⁻) complex, which was obtained similarly to the above n-Sqn complexes from [Cp^∗Ir(N₃)₂]₂ and Na(Hbimt), takes an unsymmetrical dinuclear structure bridged by two Hbimt⁻ ligands with different bonding modes, [Cp^∗Ir(N₃){μ(S:N¹)-Hbimt}{μ(S:S)-Hbimt}Ir(N₃)Cp^∗] · MeOH (3).     

Synthesis, structures, and characterization of benzildiimine complexes of rhodium(III) and iridium(I)

The reactions of trans-[(PPh₃)₂M(CO)Cl] (M = Rh and Ir) with benzildiimine (H₂BDI = 2) derived from benzil-bis(trimethylsilyl)diimine (Si₂BDI) (1) in a 1:2 and 1:1 molar ratio afforded the cationic bis-benzildiiminato complexes [Rh(PPh₃)₂(HBDI)₂]Cl (3) and the mono-benzildiimine complex [Ir(PPh₃)₂(CO)(H₂BDI)]Cl (4), respectively. Both complexes are fully characterized using IR, FAB-MS, NMR spectroscopy and elemental analysis. The single crystal X-ray structure analysis reveals a distorted octahedral coordination geometry for the Rh(III) in 3 and a highly distorted square pyramidal geometry for Ir(I) in 4. In addition, the solid-state structure of Si₂BDI is reported here for the first time showing the substituents highly twisted because of steric reasons.     

Synthesis, structures and properties of new cyclometalated iridium(III) complexes of 3-methoxy-6-methyl-2-(naphthalen-2-yl)pyridine

The reaction between 3-methoxy-6-methyl-2-(naphthalen-2-yl)pyridine 1 and IrCl₃ was performed in an attempt to synthesize a cyclometalated Ir(III) Cl-bridged dimer 2. An unexpected Ir(III) complex 3 was isolated, which was a five-coordinate bis-cyclometalated Ir(III) complex. The complexes 2 and 3 were converted to the same mononuclear complex 4 upon reacting with acetylacetonate (acac), respectively. All of the new compounds have been fully characterized by elemental analysis, IR, ¹H, ¹³C{¹H} NMR and ESI-MS. Additionally, the crystal structures and properties of these Ir(III) complexes are investigated. The most striking common features of the structures of 2 and 3 is intramolecular C-H···Cl hydrogen bonds. The complex 4 shows yellow phosphorescence with structureless emission peaks at about 556 nm.     

Synthesis, crystal structure and photophysical properties of europium(III) and terbium(III) complexes with pyridine-2,6-dicarboxamide

The reactions of pyridine-2,6-dicarboxamide with europium(III) and terbium(III) triflates led to the formation of mononuclear complexes of formula [Ln(pcam)₃](CF₃SO₃)₃ (Ln = Eu 1, Tb 2; pcam stands for pyridine-2,6-dicarboxamide). From single-crystal X-ray diffraction analysis, the complexes were found to be isomorphous and isostructural. The [Ln(pcam)₃]³⁺ cations and triflate counterions are connected by intermolecular hydrogen bonds, resulting in a 3D network structure. Both the europium(III) and terbium(III) complexes exhibit efficient ligand sensitized luminescence in the visible region with lifetimes of 1.9 ms and 2.2 ms, respectively, in the solid state.     

Directing iridium-catalyzed C-C bond formation by selection of the ancillary ligands: Polymerization and cyclotrimerization of alkynes

The ability of organoiridium derivatives of catalyzing oligomerization and polymerization of terminal alkynes is markedly influenced by the nature of non-participative ligands coordinated to the metal. The dimeric species [Ir(cod)Cl]₂ and [Ir(cod)(OMe)]₂ (cod = 1,5-cyclooctadiene) as well as the phosphine complexes HIr(cod)(PR₃)₂ (PR₃ = PPh₃, P(p-MeOC₆H₄)₃, P(o-MeOC₆H₄)Ph₂, PCyPh₂) catalyze the polymerization reaction, whereas the diphosphine derivatives HIr(cod)(P-P) (P-P = Ph₂P(CH₂)_nPPh₂ (n = 1-4), o-C₆H₄(PPh₂)₂) promote the regioselective formation of 1,2,4-trisubstituted benzenes. On the other hand, the iridium complexes with nitrogen chelating ligands Ir(cod)(N-N)X and Ir(hd)(N-N)X (hd = 1,5-hexadiene; N-N = 1,10-phenanthroline and substituted derivatives; X = halogen) catalyze alkynes polymerization. In most cases one catalytic reaction predominates over the other possible routes, so that polymerization often takes place in the absence of oligomerization side reactions, and conversely cyclotrimerization is rarely accompanied by formation of either polyene or dimers.     

Syntheses and crystal structures of the first iridium complexes with m- and p-terphenyl (tp). {[Ir2(p-tp)(cod)2](BF4)2 · 2CH2Cl2}3 and [Ir(m-tp)(η-C5Me5)](BF4)2

Novel two iridium terphenyl complexes were prepared and their structures were characterized crystallographically. The reaction of [Ir(cod)₂]BF₄ with p-terphenyl (p-tp) in CH₂Cl₂ was carried out to afford dinuclear Ir(I) complex {[Ir₂(p-tp)(cod)₂](BF₄)₂ · 2CH₂Cl₂}₃ (cod=1,5-cyclooctadiene) (1 · 2CH₂Cl₂), whereas the reaction of the intermediate [Ir(η⁵-C₅Me₅)(Me₂CO)₃]³⁺ in Me₂CO with m-terphenyl (m-tp) was done to provide mononuclear Ir(III) complex [Ir(m-tp)(η⁵-C₅Me₅)](BF₄)₂ (2). In complex 1 · 2CH₂Cl₂, two Ir atoms are η⁶-coordinated to both sides of terminal benzene rings from the upper and lower sides in the p-tp ligand, while one Ir atom is η⁶-coordinated to one side of the terminal benzene ring in the m-tp ligand in complex 2. Each crystal structure describes the first coordination mode found in metal complexes with the m- and p-tp ligands.     

Synthesis and pH-sensitive luminescence of bis-terpyridyl iridium(III) complexes incorporating pendent pyridyl groups

A series of iridium(III) bis-terpyridine complexes have been prepared which incorporate pendent pyridyl groups at the 4′-positions of one or both of the terpyridine (tpy) ligands. These include: three mutually isomeric homoleptic complexes, in which the nitrogen atom of the pendent pyridyl is para, meta or ortho to the C-C bond to the terpyridine; their heteroleptic analogues in which the second ligand is 4′-tolyl-terpyridine (ttpy); analogous complexes of the new ligand, 4′-(2,6-dimethylpyrid-4-yl)-terpyridine; and related complexes incorporating an additional phenyl ring interposed between the terpyridine and the pendent pyridyl group. All of the complexes are luminescent in air-equilibrated aqueous solution at room temperature. The homoleptic complexes display structured emission resembling that of unsubstituted [Ir(tpy)₂]³⁺, with luminescence lifetimes of around 1 μs under these conditions. The heteroleptic analogues give broader, red-shifted emission spectra, similar to that of [Ir(ttpy)₂]³⁺, indicating that emission in these complexes arises primarily from a lower-energy excited state associated with the 4′-tolyl-terpyridine ligand. A further red-shift for the complexes incorporating the additional phenyl ring suggests that the emissive state involves the more conjugated phenylpyridyl-appended ligand in these cases. The luminescence of all of the heteroleptic complexes investigated, except the meta-substituted system, is sensitive to the protonation state of the pendent pyridyl group, and the structure of the ligand can have a significant influence on both the magnitude of the response and the pH region over which it occurs.     

Synthesis, structure, and photophysical properties of a bis-cyclometalated heteroleptic iridium(III) complex containing 2,2′-dipyridylamine

The synthesis, X-ray crystal structure and luminescence properties of a new bis-cyclometalated heteroleptic complex, [Ir(ppy)₂(HDPA)](PF₆) (1, ppy = 2-phenylpyridine, HDPA = 2,2′-dipyridylamine), are reported. Both at room temperature and 77 K, complex 1 exhibits intense blue emission, assigned as ³MLCT [dπ(Ir)-π^∗(HDPA)] phosphorescence. Analysis of voltammetric data also provides evidence in support of this assignment.     

Dicarbonyliridium(I) complexes of pyridine ester ligands and their reactivity towards various electrophiles

The reaction of dimeric precursor [Ir(CO)₂Cl]₂ with two molar equivalent of the pyridine-ester ligands (L) like methyl picolinate (a), ethyl picolinate (b), methyl nicotinate (c), ethyl nicotinate (d), methyl isonicotinate (e) and ethyl isonicotinate (f) affords the tetra coordinated neutral complexes of the type [Ir(CO)₂ClL] (1a-f). The single crystal X-ray structure of 1d reveals that the Ir atom occupies the centre of an approximately square planar geometry with two CO groups cis- to each other. Intermolecular C-H?O and Ir?C interactions greatly stabilize the supramolecular structure of 1d in the solid state. The oxidative addition (OA) reactions of 1a-f with different electrophiles such as CH₃I, C₂H₅I and I₂ undergo decarbonylation of one CO group to generate the oxidized products of the type [Ir(CO)RClIL] where R = -CH₃ (2a-f); -C₂H₅ (3a-f) and [Ir(CO)ClI₂L] (4a-f). Kinetic study of the reaction of 1c-f with CH₃I indicates a first order reaction which follow the order 1d > 1c > 1f > 1e. All the synthesized complexes were characterized by elemental analyses, IR, and multinuclear NMR spectroscopy.     

On the complex formation of iron(III) bromide in the space-demanding solvent N,N′-dimethylpropyleneurea and the structure of the trisbromoiron(III) complex in solution and crystalline state

The complex formation between iron(III) and bromide has been studied calorimetrically in N,N′-dimethylpropyleneurea (DMPU), and the structure of the DMPU solvated tribromoiron(III) complex has been studied in solution by extended X-ray absorption fine structure (EXAFS) and large angle X-ray scattering (LAXS), and in solid state by EXAFS and single crystal X-ray diffraction. The calorimetric study showed that iron(III) forms three medium strong bromide complexes in DMPU, and the thermodynamic pattern strongly indicates that all complexes are formed in entropy driven substitution reactions. In DMPU solution, the tribromoiron(III) complex has a regular trigonal planar configuration with a mean Fe-Br bond distance of 2.36 Å, and without any solvent molecules strongly bound to iron(III). In the solid state, however, the structure is a slightly distorted trigonal bipyramid, with one short and two slightly longer Fe-Br bonds, 2.37 and 2.44 Å, respectively, in a somewhat distorted trigonal plane, and two DMPU solvent molecules (mean Fe-O bond distance 1.98 Å) in the apical positions. The DMPU solution of iron(III) bromide and the [FeBr₃(dmpu)₂] crystals are both blackish red.     

Design of novel iron compounds as potential therapeutic agents against tuberculosis

In the search for new therapeutic tools against tuberculosis two novel iron complexes, [Fe(L-H)₃], with 3-aminoquinoxaline-2-carbonitrile N¹,N⁴-dioxide derivatives (L) as ligands, were synthesized, characterized by a combination of techniques, and in vitro evaluated. Results were compared with those previously reported for two analogous iron complexes of other ligands of the same family of quinoxaline derivatives. In addition, the complexes were studied by cyclic voltammetry and EPR spectroscopy. Cyclic voltammograms of the iron compounds showed several cathodic processes which were attributed to the reduction of the metal center (Fe(III)/Fe(II)) and the coordinated ligand. EPR signals were characteristic of magnetically isolated high-spin Fe(III) in a rhombic environment and arise from transitions between m_S = ± 1/2 (g_eff ~ 9) or m_S = ± 3/2 (g_eff ~ 4.3) states. Mössbauer experiments showed hyperfine parameters that are typical of high-spin Fe(III) ions in a not too distorted environment. The novel complexes showed in vitro growth inhibitory activity on Mycobacterium tuberculosis H₃₇Rv (ATCC 27294), together with very low unspecific cytotoxicity on eukaryotic cells (cultured murine cell line J774). Both complexes showed higher inhibitory effects on M. tuberculosis than the “second-line” therapeutic drugs.     

Synthesis and crystal structures of mono- and dinuclear silver(I) complexes bearing 1,8-naphthyridine ligand

Mononuclear and dinuclear silver(I) complexes bearing 1,8-naphthyridine (napy) were prepared. The crystal structures of [Ag(napy-κN)₂](PF₆) (1) and [Ag₂(μ-napy)₂](PF₆)₂ · 3CH₃CN (2 · 3CH₃CN) were determined by X-ray diffraction studies. In complex 1, intermolecular π-π interaction of napy ligands between neighboring molecules forms left-handed hexagonal columns in the solid state. On the other hand, two napy ligands bridging two Ag ions in the dinuclear complex 2 shape a face-to-face π-π stacking with those of the neighboring molecule to form the dimeric unit. Besides, two of four napy ligands, which are located in a diagonal position in the dimeric unit, build intermolecular back-to-back π-π stackings with those of the adjacent dimeric unit, and a ladder-like stairway structure is generated in the solid state. Irrespective of such characteristic structures of 1 and 2 in the solid state, both complexes show very rapid dynamic behavior in solutions. No conversion between 1 and 2 took place even in the presence of excess amounts of Ag⁺ or napy in solutions.     

[Ir(acac)(η-C8H14)2]: A precursor in the synthesis of cyclometalated iridium(III) complexes

A new convenient synthesis and the crystallographic characterization of [Ir(acac)(coe)₂] (2, acac = acetylacetonato; coe = cis-cyclooctene) are described. The title compound crystallized from THF/ethanol in two modifications (monoclinic P2₁/c, 2a, and triclinic , 2b). Complex 2 represents an efficient starting material in the synthesis of mononuclear iridium(III) complexes containing cyclometalated 2-phenylpyridinato ligands using oxidative addition reactions of the corresponding ligands towards 2. Thus [Ir(acac)(ppy)₂] (3, ppy = 2-phenylpyridinato) and [Ir(ppy)₃] (4) (mer, 4a; fac, 4b) were prepared in excellent yields and short reaction times in a kind of one-pot procedure starting from [{Ir(μ-Cl)(coe)₂}₂] (1). Furthermore a convenient synthesis of [{Ir(μ-Cl)(ppy)₂}₂] (5) from 1 and Hppy is described.     

New complexes of La(III), Ce(III), Pr(III), Nd(III), Sm(III), Eu(III) and Gd(III) ions with morin

New solid complex compounds of La(III), Ce(III), Pr(III), Nd(III), Sm(III), Eu(III) and Gd(III) ions with morin were synthesized. The molecular formula of the complexes is Ln(C₁₅H₉O₇)₃ · nH₂O, where Ln is the cation of lanthanide and n = 6 for La(III), Sm(III), Gd(III) or n = 8 for Ce(III), Pr(III), Nd(III) and Eu(III). Thermogravimetric studies and the values of dehydration enthalpy indicate that water occurring in the compounds is not present in the inner coordination sphere of the complex. The structure of the complexes was determined on the basis of UV-visible, IR, MS, ¹H NMR and ¹³C NMR analyses. It was found that in binding the lanthanide ions the following groups of morin take part: 3OH and 4CO in the case of complexes of La, Pr, Nd, Sm and Eu, or 5OH and 4CO in the case of complexes of Ce and Gd. The complexes are five- and six-membered chelate compounds.     

Mononuclear rhodium and iridium compounds with pyridyl-pyrazole ligands

Neutral [MCl(L₂)(Hpzpy)], [M(L₂)(pzpy)] and cationic [M(L₂)(Hpzpy)]CF₃SO₃ rhodium(I) or iridium(I) complexes [M = Rh or Ir; L₂ = diolefin or (CO)₂; pzpy = 3-(2-pyridyl)pyrazolate] have been prepared; the pzpy and Hpzpy ligands coordinate to the metal as bidentate chelate groups through one pyrazole nitrogen and the pyridine nitrogen atom. The reactivity of these complexes towards oxidative addition reactions of halogens, methyl iodide or triflic acid and towards displacement reactions has been studied. The neutral and cationic iridium(I) complexes are modest catalysts for the hydrosilylation of phenylacetylene with triethylsilane at 60 °C. The complexes have been characterised by analytical and spectroscopic data; their configuration has been confirmed by COSY and NOESY experiments and the molecular structure of [Rh(COD)(Mepzpy)(PPh₃)]CF₃SO₃ has been established by an X-ray diffraction study.