Improving the performance of Pt(II) complexes for blue light emission by enhancing the molecular rigidity

This study highlights the potential benefits of using terdentate over bidentate ligands in the construction of organometallic complexes as organic light-emitting diode (OLED) emitters offering better color purity, and explores in detail the molecular origins of the differences between the two. A pair of closely related platinum(II) complexes has been selected, incorporating a bidentate and a terdentate cyclometallating ligand, respectively, namely, Pt(4,6-dFppy)(acac) (1) {4,6-dFppy = 2-(4,6-difluorophenyl)pyridine metalated at C(2) of the phenyl ring} and Pt(4,6-dFdpyb)Cl (2) {4,6-dFdpyb = 4,6-difluoro-1,3-di(2-pyridyl)benzene, metalated at C(2) of the phenyl ring}. The emission properties over the range of temperatures from 1.2 to 300 K have been investigated, including optical high-resolution studies. The results reveal a detailed insight into the electronic and vibronic structures of the two compounds. In particular, the Huang-Rhys parameter S that serves to quantify the degree of molecular distortion in the excited state with respect to the ground state, though small in both cases, is smaller by a factor of 2 for the terdentate than the bidentate complex (S ≈ 0.1 and ≈0.2, respectively). The smaller value for the former reflects the greater degree of rigidity induced by the terdentate ligand, leading to a lesser contribution of intraligand Franck-Condon vibrational modes in the green spectral range of the emission spectra. Consequently, an enhanced color purity with respect to blue light emission results. The high rigidity and the short Pt-C bond in Pt(4,6-dFdpyb)Cl also serve to disfavor nonradiative decay pathways, including those involving higher-lying dd* states. These effects account for the greatly superior luminescence quantum yield of the terdentate complex in fluid solution, amounting to φ(PL) = 80% versus only 2% found for the bidentate complex.     

Near-UV to red-emitting charged bis-cyclometallated iridium(III) complexes for light-emitting electrochemical cells

Herein we report a series of charged iridium complexes emitting from near-UV to red using carbene-based N^C: ancillary ligands. Synthesis, photophysical and electrochemical properties of this series are described in detail together with X-ray crystal structures. Density Functional Theory calculations show that the emission originates from the cyclometallated main ligand, in contrast to commonly designed charged complexes using bidentate N^N ancillary ligands, where the emission originates from the ancillary N^N ligand. The radiative process of this series of compounds is characterized by relatively low photoluminescence quantum yields in solution that is ascribed to non-radiative deactivation of the excited state by thermally accessible metal-centered states. Despite the poor photophysical properties of this series of complexes in solution, electroluminescent emission from the bluish-green to orange region of the visible spectrum is obtained when they are used as active compounds in light-emitting electrochemical cells.     

Synthesis,Spectral and Magnetic Studies of Mononuclear and Binuclear Mn(Ii), Co(Ii), Ni(Ii) and Cu(Ii) Complexes with Semicarbazone Ligands Derived from Sulfonamide

Mononuclear and binuclear Mn(II), Co(II), Ni(II) and Cu(II) complexes of new semicarbazone ligands derived from sulfonamide were synthesized and characterized by elemental analysis and IR spectra. In mononuclear complexes, the semicarbazone behaves as a monoanionic terdentate or neutral terdentate ligand towards the metal ion. However, in binuclear complexes, it behaves as a monoanionic terdentate towards one of the bivalent metal ions and monoanionic bidentate ligand towards the other metal ion in the same complex. Electronic spectra and magnetic susceptibility measurements of the solid complexes indicated octahedral geometry around Mn(II), Co(II) and Ni(II) and square planar around the Cu(II) ion. These geometries were confirmed by the results obtained from thermal analyses. The antifungus properties of the ligands and their complexes were investigated.     

Zinc(II)-templated synthesis of a [2]-catenane consisting of a 2,2',6',2' '-terpyridine-incorporating cycle and a 1,10-phenanthroline-containing ring

A nonsymmetrical [2]-catenane has been synthesized, with a 5-coordinated metal center (Zn(2+)) as template. One of the two rings contains a terdentate ligand (2,2',6',2' '-terpyridine) and the other one incorporates a bidentate chelate (1,10-phenanthroline). The first ring was prepared separately and, subsequently, Zn(2+) was used as the gathering and threading element to pass the stringlike component through the ring. This open-chain species bears two terminal olefins, which were reacted with Grubbs first-generation catalyst (ring-closing metathesis) to afford the desired catenane. Hydrogenation of the double bond and removal of the zinc(II) template afforded the final free [2]-catenane in 40% yield from the terdentate ligand-containing cycle and the diolefinic compound. Complexation studies on this new pentacoordinating catenane were carried out with Fe(II) or Cu(II). The most interesting observation is that the 5-coordinated complexes obtained are strongly stabilized. Their electrochemical reduction occurs at negative potentials.     

Complexes with a Pincers. 2,6-Diphenylpyridine as Twofold-Deprotonated (C ∧ N ∧ C) terdentate ligand in C,C-trans-, and as mono-deprotonated (C ∧ N) chelate ligand in chiral C,C-cis-complexes of platinum(II) and palladium(II). Preliminary communication

2,6-Diphenylpyridine forms, as twofold-deprotonated, terdentate ligand, complexes with Pt(II) and Pd(II), having two adjacent five-membered metallocycles. As mono-deprotonated, bidentate ligand, it forms cis-bis-complexes having a chirality axis. Pt(II) complexes undergo thermal and photochemical oxidative addition reactions, yielding stable Pt(IV) compounds. Pd(II) complexes yield substitued 2,6-diphenylpyridine in photochemical reactions.     

New chiral P-N ligands for the regio- and stereoselective Pd-catalyzed dimerization of styrene

Two new chiral, enantiomerically pure, hybrid P-N ligands, namely (2R,5S)-2-phenyl-3-(2-pyridyl)-1,3-diaza-2-phosphanicyclo[3,3,0]octan-4-one (1) and (2R,5S)-2-phenyl-3-(2-pyridyl)-1,3-diaza-2-phosphanicyclo[3,3,0]octane (2), have been synthesized starting from L-proline. The two ligands differ in the presence or not of a carbonyl group in the diazaphosphane ring. Their coordination chemistry towards Pd(II) was studied by reacting them with [Pd(CH?)Cl(cod)]. A different behaviour was observed: ligand 2 shows the expected bidentate chelating behaviour leading to the mononuclear Pd-complex, while ligand 1 acts as a terdentate ligand giving a dinuclear species. The corresponding cationic derivatives were obtained from the palladium neutral complexes, both as mono- and dinuclear derivatives, and tested as precatalysts for styrene dimerization, yielding E-1,3-diphenyl-1-butene regio- and stereoselectively as the sole product. A detailed analysis of the catalytic behaviour is reported.     

Potentiometric studies of ternary complex formation Cu(II), Ni, Zn or Cd iminodiacetic acid/amino-acid complexes

The formation constants, log K(mab), for the reactions MA + B right harpoon over left harpoon MAB [where M = Cu(II), Ni, Zn or Cd, A = terdentate ligand and B = bidentate or terdentate ligand] have been determined. Potentiometric evidence is presented for the stepwise addition of the secondary ligand B to the 1:1 metal iminodiacetate (MA). The formation constants and the free energies of formation (DeltaG) have been calculated at 25 +/- 1 degrees and mu = 0.10. The order in terms of secondary ligands has been found to be ASPA > Gly > Aln and Gly > Aln > ASPA with iminodiacetic and nitrilotriacetic acid as primary ligands respectively (ASPA = aspartic acid, Gly = glycine, Aln = dl-alanine). The plot of log K(mab) against log k(mb)(2) shows a linear relationship between the formation constants of the ternary and 1:2 M(II)secondary ligand complexes.     

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.     

The effect of phenyl group on the electronic and phosphorescent properties of cyclometalated analogues of platinum(II) terpyridine complexes: a theoretical study

In this work, we theoretically investigate the effect of phenyl group on the electronic and phosphorescent properties of cyclometalated platinum(II) complexes, thereby designing an efficient blue emitting material. Three platinum(II) complexes Pt(N^N^N)Cl (N^N^N = terpyridine), Pt(N^C^N)Cl (N^C^N = 1,3-di(2-pyridyl)-benzene) and Pt(N^N^C)Cl (N^N^C = 6-phenyl-2,2′-bipyridines) are chosen as the models. Their electronic and phosphorescent properties are investigated utilizing quantum theoretical calculations. The results reveal that the phenyl group significantly affects the molecular and electronic structures, charge distribution and phosphorescent properties. The coordination bond length trans to phenyl group is the longest among the same type of bonds owing to the trans influence of phenyl group. Moreover, the phenyl group largely restricts the geometry relaxation of cyclometalated ligand. The strong σ-donor ability of Pt–C bond makes more electrons center at Pt atom and the fragments trans to phenyl group. In comparison with Pt(N^N^N)Cl and Pt(N^N^C)Cl, the complex Pt(N^C^N)Cl has the smallest excited-state geometry relaxation and the biggest emission energy and spatial overlap between the transition orbitals in the emission process. As a result, Pt(N^C^N)Cl has the largest emission efficiency, which well agrees with the experimental observation. Based on these calculation results, a potentially efficient blue-emitting material is designed via replacing pyridine groups in Pt(N^C^N)Cl by 3-methylimidazolin-2-ylidene.     

pH-Responsive N^C-Cyclometalated Iridium(III) Complexes: Synthesis,Photophysical Properties,Computational Results,and Bioimaging Application

Herein we report four [Ir(N^C)₂(L^L)]ⁿ⁺, n = 0,1 complexes (1–4) containing cyclometallated N^C ligand (N^CH = 1-phenyl-2-(4-(pyridin-2-yl)phenyl)-1H-phenanthro[9,10-d]imidazole) and various bidentate L^L ligands (picolinic acid (1), 2,2′-bipyridine (2), [2,2′-bipyridine]-4,4′-dicarboxylic acid (3), and sodium 4,4′,4″,4‴-(1,2-phenylenebis(phosphanetriyl))tetrabenzenesulfonate (4). The N^CH ligand precursor and iridium complexes 1–4 were synthesized in good yield and characterized using chemical analysis, ESI mass spectrometry, and NMR spectroscopy. The solid-state structure of 2 was also determined by XRD analysis. The complexes display moderate to strong phosphorescence in the 550–670 nm range with the quantum yields up to 30% and lifetimes of the excited state up to 60 µs in deoxygenated solution. Emission properties of 1–4 and N^CH are strongly pH-dependent to give considerable variations in excitation and emission profiles accompanied by changes in emission efficiency and dynamics of the excited state. Density functional theory (DFT) and time-dependent density functional theory (TD DFT) calculations made it possible to assign the nature of emissive excited states in both deprotonated and protonated forms of these molecules. The complexes 3 and 4 internalize into living CHO-K1 cells, localize in cytoplasmic vesicles, primarily in lysosomes and acidified endosomes, and demonstrate relatively low toxicity, showing more than 80% cells viability up to the concentration of 10 µM after 24 h incubation. Phosphorescence lifetime imaging microscopy (PLIM) experiments in these cells display lifetime distribution, the conversion of which into pH values using calibration curves gives the magnitudes of this parameter compatible with the physiologically relevant interval of the cell compartments pH.     

Phosphorescent Pt(II) Emitters for OLEDs: From Triarylboron‐Functionalized Bidentate Complexes to Compounds with Macrocyclic Chelating Ligands

The development of organic light‐emitting diodes (OLEDs) has attracted enormous research efforts from both academia and industry in the past decades and tremendous progress has been made. However, the low operation lifetime of the blue phosphorescent OLEDs remains as one of the greatest bottlenecks limiting further applications of OLEDs. To address this problem, design and synthesis of triplet emitters with high phosphorescence quantum yield (Φ_P) and adequate thermal, chemical, electrical and ultraviolet (UV) stabilities are vital. This review summarizes the progress we made on the development of efficient and robust phosphorescent emitters based on cyclometalated Pt(II) compounds, particularly the ones with blue emission, starting from complexes with triarylboron‐functionalized bidentate ligand to molecules incorporating tetradentate and macrocyclic ligands, with emphasis on their structure‐property relationships.     

Synthesis and Characterization of Luminescent Cyclometalated Platinum(II) Complexes of 1,3‐Bis‐Hetero‐Azolylbenzenes with Tunable Color for Applications in Organic Light‐Emitting Devices through Extension of π Conjugation by Variation of the Heteroatom

A series of luminescent cyclometalated platinum(II) complexes of N^C^N ligands [N^C^N=2,6‐bis(benzoxazol‐2′‐yl)benzene (bzoxb), 2,6‐bis(benzothiazol‐2′‐yl)benzene (bzthb), and 2,6‐bis(N‐alkylnaphthoimidazol‐2′‐yl)benzene (naphimb)] has been synthesized and characterized. Two of the platinum(II) complexes have been structurally characterized by X‐ray crystallography. Their electrochemical, electronic absorption, and luminescence properties have been investigated. In dichloromethane solution at room temperature, the cyclometalated N^C^N platinum(II) complexes exhibited rich luminescence with well‐resolved vibronic‐structured emission bands. The emission energies of the complexes are found to be closely related to the electronic properties of the N^C^N ligands. By varying the electronic properties of the cyclometalated ligands, a fine‐tuning of the emission energies can be achieved, as supported by computational studies. Multilayer organic light‐emitting devices have been prepared by utilizing two of these platinum(II) complexes as phosphorescent dopants, in which a saturated yellow emission with Commission International de I′Eclairage coordinates of (0.50, 0.49) was achieved.     

Stepwise formation of quasi-octahedral macrocyclic complexes of rhodium(III) and iridium(III) bearing a pentamethylcyclopentadienyl group

Reactions of [[MCl2(Cp*)]2] (1: M=Ir, 2: M=Rh) with bidentate ligands (L) such as 1,4-diisocyano-2,5-dimethylbenzene (a), 1,4-diisocyano-2,3,5,6-tetramethylbenzene (b), pyrazine (c) or 4,4'-dipyridyl (d) gave the corresponding dinuclear complexes [[MCl2(Cp*)]2(L)] (M=Ir: 3a, 3b, 5c, 5d; M=Rh: 4b, 6c, 6d), which were converted into tetranuclear complexes [[M2(mu-Cl)2(Cp*)2]2(L)2](OTf)4 (M=Ir: 7c, 7d, 9a, 9b; M=Rh: 8e, 8d, 10b) on treatment with Ag(OTf). X-ray analyses of 8c and 8d revealed that each of four pentamethylcyclopentadienyl metal moieties was connected by two mu-Cl-bridged atoms and a bidentate ligand to construct a rectangular cavity with the dimensions of 3.7 x 7.0 A for 8c and 3.7 x 11.5 A for 8d. Both the Rh2Cl2 and pyrazine (or 4,4'dipyridyl) ring planes are perpendicular to the Rh4 plane. Treatment of Cl-bridged complexes (7c, 7d, 8e, 8d, 9b, and 10b) with a different ligand (L') resulted in cleavage of the Cl bridges to produce two-dimensional complexes [[MCl(Cp*)]4[(L)-(L')]2](OTf)4 (11ac, 11bc, 11bd, 12bc, and 12bd) with two different ligand "edges". Complex 10b reacted readily with 1,4-diisocyano-2,3,5,6-tetramethylbenzene (b) to give a tetranuclear rhodium(III) complex 12bb. The structure of tetranuclear complexes was confirmed by X-ray analysis of 11bc. Each [MCp*] moiety is surrounded by a Cl atom, isocyanide, and pyrazine (or 4,4'-dipyridyl) and the dimensions of its cavity are 7.0 x 11.6 A.     

Increased antenna effect of the lanthanide complexes by control of a number of terdentate N-donor pyridine ligands

Three europium complexes with the terdentate N-donor ligand 2,6-bis(1H-pyrazol-3-yl)pyridine (L) have been synthesized, and their crystal structures have been determined. The ligand/metal ratios in these complexes are 3, 2, and 1. The photophysical properties of the complexes indicate more efficient ligand sensitization of europium emission for the homoleptic complex.     

Kinetico-mechanistic studies of C-H bond activation on new Pd complexes containing N,N'-chelating ligands

The hybrid imine/amine palladium(II) coordination complexes [PdX2(kappa2-N(imino),N(amino))](X = Cl, AcO; kappa2-N(imino),N(amino)= 4ClC6H4CHNCH2(CH2)nN(CH3)2, n= 1, 2) have been prepared in different isomeric forms which include E/Z arrangement around the C[double bond]N bond of the hybrid ligand and {Pd(kappa(2)-N(imino),N(amino))} ring conformation. The crystal structures of four of them, E-1AcO, Z-1AcO, E-2AcO and E-2Cl, have been determined and the solution behaviour in acetic acid, the common cyclometallating solvent, for all these systems studied. The complexes in acetic acid solution are shown to maintain the structure determined by X-ray crystallography, as they do in deuterated chloroform. Nevertheless, a partial opening equilibrium of the {Pd(kappa2-N(imino),N(amino))} ring is observed by NMR experiments. When the complexes are held in solution for longer periods the corresponding cyclometallated derivatives, 1AcO-CM, 2AcO-CM, 1Cl-CM and 2Cl-CM, containing the {Pd(kappa2-C,N(imino))} palladacycle are obtained, as characterized by 1H NMR spectroscopy. In these compounds the total opening of the N(amino) moiety of the ligand has occurred. The C-H bond activation process has been studied kinetico-mechanistically at different temperatures, pressures and acid concentrations; the results agree with the need of an opening of the chelate ring in [PdX2(kappa2-N(imino),N(amino))] prior to the proper cyclometallation reaction. The values of the enthalpies of activation are higher than those observed for known N-monodentated cyclometallating ligands, as should correspond to the contribution of a ligand dechelation pre-equilibrium. The entropies and volumes of activation are also indicative of this predissociation that include an important amount of contractive ordering. The presence of small amounts of triflic acid in the reaction medium accelerates the reaction to the value observed for N(imino)-monodentate systems, indicating that the full opening of the chelate ring has taken place. For the badly oriented isomeric forms of the ligand in the chelated complex (Z), the cyclometallation process is even more slow and corresponds directly to the reorganization of the ligand to its cyclopalladation-active (E) conformation.     

N-heterocyclic carbenes as π*-acceptors in luminescent Re(I) triscarbonyl complexes

Two rhenium(I) carbonyl complexes of the type fac-[Re(CO)(3)(N^C)X] where N^C is an N-heterocyclic carbene [3-butyl-1-(2'-pyridyl)benzimidazolin-2-ylidene] and X is either Cl or Br have been synthesised via an in situ method from [Re(CO)(5)X] and a respective benzimidazolium salt. The complexes have been characterised by (1)H and (13)C NMR, infra-red spectroscopy and in the case of the bromo-complex by a single-crystal X-ray diffraction study. The photophysical properties of the complexes have been investigated, revealing similar phosphorescent emission which was attributed to radiative decay from a (3)MLCT state partially mixed with a (3)LLCT state. However, the analysis of excited state lifetime and quantum yield values revealed distinct photophysical behaviour for the two complexes, which was attributed to the more labile nature of the bromo ligand with respect to the chloro one. The explanation was supported by the time-dependent emission profile change in diluted acetonitrile solutions.     

An alternative route to highly luminescent platinum(II) complexes: cyclometalation with N=C=N-coordinating dipyridylbenzene ligands

The remarkable luminescence properties of the platinum(II) complex of 1,3-di(2-pyridyl)benzene, acting as a terdentate N=C=N-coordinating ligand cyclometalated at C2 of the benzene ring ([PtL(1)Cl]), have been investigated, together with those of two new 5-substituted analogues [PtL(2)Cl] and [PtL(3)Cl] [HL(2) = methyl-3,5-di(2-pyridyl)benzoate; HL(3) = 3,5-di(2-pyridyl)toluene]. All three complexes are intense emitters in degassed solution at 298 K (lambda(max) 480-580 nm; phi(lum) = 0.60, 0.58, and 0.68 in CH(2)Cl(2)), displaying highly structured emission spectra in dilute solution, with lifetimes in the microsecond range (7.2, 8.0, and 7.8 micros). On the basis of the very small Stokes shift, the highly structured profiles, and the relatively long lifetimes, the emission is attributed to an excited state of primarily (3)pi-pi character. At concentrations >1 x 10(-)(5) M, structureless excimer emission centered at ca. 700 nm is observed. The X-ray crystal structure of [PtL(2)Cl] is also reported.     

Synthesis, characterization, and reactivity of mononuclear O,N-chelated vanadium(IV) and -(III) complexes of methyl 2-Aminocyclopent-1-ene-1-dithiocarboxylate based ligand: reporting an example of conformational isomerism in the solid state

Vanadium(IV) and -(III) complexes of a tetradentate N(2)OS Schiff base ligand H(2)L [derived from methyl 2-((beta-aminoethyl)amino)cyclopent-1-ene-1-dithiocarboxylate and salicylaldehyde] are reported. In all the complexes, the ligand acts in a bidentate (N,O) fashion leaving a part containing the N,S donor set uncoordinated. The oxovanadium(IV) complex [VO(HL)(2)] (1) is obtained by the reaction between [VO(acac)(2)] and H(2)L. In the solid state, compound 1 has two conformational isomers 1a and 1b; both have been characterized by X-ray crystallography. Compound 1a has the syn conformation that enforces the donor atoms around the metal center to adopt a distorted tbp structure (tau = 0.55). Isomer 1b on the other hand has an anti conformation with almost a regular square pyramidal geometry (tau = 0.06) around vanadium. In solution, however, 1 prefers to be in the square pyramidal form. A second variety of vanadyl complex [VO(L(cyclic))(2)](I(3))(2) (2) with a new bidentate O,N donor ligand involving isothiazolium moiety has been obtained by a ligand-based oxidation of the precursor complex 1 with iodine. Preliminary X-ray and FAB mass spectroscopic data of 2 have supported the formation of a heterocyclic moiety by a ring closure reaction involving a N-S bond. Vanadium(III) complex [V(acac)(HL)(2)] (3) has been obtained through partial ligand displacement of [V(acac)(3)] with H(2)L. Compound 3 has almost a regular octahedral structure completed by two bidentate HL ligands along with an acetylacetonate molecule. Electronic spectra, magnetism, EPR, and redox properties of these compounds are reported.     

A new class of iridium complexes suitable for stepwise incorporation into linear assemblies: synthesis, electrochemistry, and luminescence

A new family of cationic iridium(III) complexes is reported that contain two cyclometalating terdentate ligands. The complex [Ir(N--C--N-dpyx)(N--N--C-phbpy)]+ (1) contains one N--C--N-coordinating ligand, cyclometalating through the central phenyl ring, and one N--N--C-coordinated ligand, cyclometalated at the peripheral phenyl ring [dpyxH = 1,3-di(2-pyridyl)-4,6-dimethylbenzene; phbpyH = 6-phenyl-2,2'-bipyridine]. This binding mode dictates a mutually cis arrangement of the cyclometalated carbon atoms: the complexes are thus bis-terdentate analogues of the well-known [Ir(N--C-ppy)2(N--N-bpy)]+ family of complexes, which similarly contain a cis-C2N4 coordination environment. The dpyx ligand can be brominated regioselectively at the carbon atom para to the metal under mild conditions. Starting from a modified complex, [Ir(N--C--N-dpyx)(N--N--C-mtbpy-phi-Br)]+ (2), which incorporates a pendent bromophenyl group, a sequential cross-coupling-bromination-cross-coupling strategy can be applied for the stepwise introduction of aryl groups into the ligands, using in situ palladium-catalyzed Suzuki reactions with arylboronic acids [mtbpyH-phi-Br = 4-(p-bromophenyl)-6-(m-tolyl)bipyridine]. Dimetallic complexes 6 and 7 have similarly been prepared by a palladium-catalyzed reaction of complex 2 with 1,4-benzenediboronic acid and 4,4'-biphenyldiboronic acid, respectively. All five monometallic complexes and both dimetallic systems are luminescent in solution, emitting around 630 nm in MeCN at 298 K, with quantum yields in the range of 0.02-0.06, superior to [Ir(ppy)2(bpy)]+. The luminescence, electrochemistry, and singlet-oxygen-sensitizing abilities of the new family of complexes are discussed in the context of the tris-bidentate analogues and related bis-terdentate compounds that contain a trans arrangement of cyclometalated carbon atoms.     

Mixed ligand complexes of palladium(II) and platinum(II) with methionine and 2,4-disubstituted pyrimidines

Summary Mixed ligand complexes ofcis-[M(MetH)Cl₂] (M=Pd²⁺ and Pt²⁺; MetH=methionine) with 2,4-disubstituted pyrimidines were prepared and characterised. Thecis-[Pd(MetH)Cl₂] complex reacted with cytosine (2-hydroxy-4-aminopyrimidine), isocytosine (2-amino-4-hydroxypyrimidine) and thiocytosine (2-thio-4-amino-pyrimidine) to form ternary complexes.cis-[Pt(MetH)Cl₂] however reacted with cytosine, uracil (2,4-pyrimidine dione or 2,4-dihydroxypyrimidine) to yield the corresponding mixed ligand complexes. The primary ligand, methionine, binds to the metal ion through sulphur and amino nitrogenvia a six membered chelate ring. The secondary ligands (substituted pyrimidines) bind to the Pd²⁺ or Pt²⁺ metal ion through the ring nitrogen (N₃), as monodentate ligand. Thiocytosine however acts as a bidentate ligand, coordinating to the metal ion through-SH and ring nitrogen (N₃). All complexes are 11 electrolytes, except the thiocytosine complex, which is a 12 electrolyte.