Biquinoline Ligand Research Articles

Symmetry, aggregation and counterion role in tuning NIR absorbance of a Cu(I) coordination complex

A homoleptic Cu(I) coordination complex bearing N^N-bischelating functionalised biquinoline (BQCOOTEG) ligand and weakly coordinating counterions, BF4 or ClO4 respectively, was designed, synthesized and characterized. Hydrophilic tetraethyleneglycol chains functionalizing the ligands help water solubility of these compounds, that show aggregation in water with increasing concentration. Self-assembly behaviour in water was investigated by accurate 1H NMR and UV–Vis-NIR absorption studies, supported by computational studies, highlighting the influence of counterion-metal distance in tuning NIR absorbance of the complex.

Synthesis, crystal structure and Hirshfeld surface analysis of di-μ2-iodido-bis-[(2,2'-bi-quinoline-κ2 N,N')copper(I)

The mol-ecular and crystal structures of the title compound, [Cu2I2(C18H12N2)2], were examined by single-crystal X-ray diffraction and Hirshfeld surface analysis. The Cu atom is coordinated in a distorted tetra-hedral geometry by two N atoms from the 2,2'-bi-quinoline ligands and the two μ2-bridging iodide ligands. The mol-ecules are in contact via π-π-stacking inter-actions. Hirshfeld surface analysis showed that the most important contributions to the inter-molecular inter-actions are H⋯H (39.7%), H⋯I/I⋯H (17.8%), C⋯H/H⋯C (17.5%), C⋯C (16.5%), N⋯C/C⋯N (3.9%) and N⋯H/H⋯N (3.5%).

Mechanism-Guided Design of Robust Palladium Catalysts for Selective Aerobic Oxidation of Polyols

The palladium complex [(L1)Pd(μ-OAc)]2[OTf]2 (L1 = neocuproine) is a selective catalyst for the aerobic oxidation of vicinal polyols to α-hydroxyketones, but competitive oxidation of the ligand methyl groups limits the turnover number and necessitates high Pd loadings. Replacement of the neocuproine ligand with 2,2'-biquinoline ligands was investigated as a strategy to improve catalyst performance and explore the relationship between ligand structure and reactivity. Evaluation of [(L2)Pd(μ-OAc)]2[OTf]2 (L2 = 2,2'-biquinoline) as a catalyst for aerobic alcohol oxidation revealed a threefold enhancement in turnover number relative to the neocuproine congener, but a much slower rate. Mechanistic studies indicated that the slow rates observed with L2 were a consequence of precipitation of an insoluble trinuclear palladium species─(L2Pd)3(μ-O)22+─formed during catalysis and characterized by high-resolution electrospray ionization mass spectrometry. Density functional theory was used to predict that a sterically modified biquinoline ligand, L3 = 7,7'-di-tert-butyl-2,2'-biquinoline, would disfavor the formation of the trinuclear (LPd)3(μ-O)22+ species. This design strategy was validated as catalytic aerobic oxidation with [(L3)Pd(μ-OAc)]2[OTf]2 is both robust and rapid, marrying the kinetics of the parent L1-supported system with the high aerobic turnover numbers of the L2-supported system. Changes in ligand structure were also found to modulate regioselectivity in the oxidation of complex glycoside substrates, providing new insights into structure-selectivity relationships with this class of catalysts.

Metal-Organic Frameworks Based on a Janus-Head Biquinoline Ligand as Catalysts in the Transformation of Carbonyl Compounds into Cyanohydrins and Alcohols.

A new family of metal-organic frameworks (MOFs) named GR-MOFs with the chemical formula {[M x (BCA) y ](H2O) z (DMF) w } (x,y,z,w: 1,1,2,0; 1,1.5,0,1; 1,2,2,1; and 1,1,0,2 for GR-MOF-11 to 14, respectively) based on s-block [M: Sr (GR-MOF-11), Ba (GR-MOF-14)] and d-block [M: Y (GR-MOF-12) and Cd (GR-MOF-13)] metals together with the biquinoline ligand 2,2'-bicinchoninic acid (H2BCA) has been synthetized by a solvothermal route and fully characterized by elemental and thermogravimetric analysis, Fourier transform infrared spectroscopy, photoluminescence, particle size distribution through optical microscopy, electrophoretic mobility, and finally, X-ray single-crystal and powder diffraction. The structural characterization reveals that these 2D and 3D MOFs possess a rich variety of coordination modes that maintained the Janus-head topology on the ligand in most of the cases. The new MOFs were studied in the catalyzed cyanosilylation and hydroboration of an extensive group of aldehydes and ketones, wherein the s-block metal-based MOFs GR-MOF-11 and GR-MOF-14 provided the highest efficiency ever reported in the MOF-catalyzed cyanosilylation of carbonyl compounds by using only 0.5 mol % of catalyst loading, room temperature, and solvent-free conditions. Furthermore, the hydroboration of ketones has been reported for the first time with this type of s-block metal catalysts obtaining from moderate to good conversions.

NO Photorelease from a Ruthenium Complex Assisted by Formation of a Supramolecular Dimer

This work presents the NO release from compound [Ru(biq)2(H2O)(NO)](PF6)3 (biq = 2,2’-biquinoline) with visible light irradiation (λirrad = 660 nm), assisted by the low-absorbing photosensitizer [Ru(biq)2Cl2]. The structure of both compounds were characterized by means of ESI-MS (electrospray ionization mass spectrometry). The NO+ stretching, ν(NO) = 1995 cm-1, is atypically shifted to higher energy. This observation, along with the E1/2 = 0.49 V (vs. Ag/AgCl) assigned to the Ru2+/3+ redox pair observed for compound [Ru(biq)2Cl2] and its photoreactivity in solution suggest that the RuII ion, when coordinated to two biquinolines, behaves as a hard Pearson acid. Molecular modelling results confirmed the typical geometry distortion of ruthenium-polypyridine complexes bearing sterically hindered ligands. They also suggest the formation of a supramolecular dimer, assembled by weak interaction between biquinoline ligands from each compound, that is claimed to be responsible for the high efficiency of the NO photorelease bimolecular sensitization.

Cu(I) Coordination Complex Precursor for Randomized CuOx Microarray Loaded on Carbon Nanofiber with Excellent Electrocatalytic Performance for Electrochemical Glucose Detection.

A homoleptic ionic Cu(I) coordination complex that was based on 2,2′-biquinoline ligand functionalized with long alkyl chains (Cu(I)–C18) was used as a precursor to modify a carbon nanofiber paste electrode (Cu–C18/CNF). Randomized copper oxide microelectrode arrays dispersed within carbon nanofiber paste (CuOx/CNF) were obtained by electrochemical treatment of Cu–C18/CNF while using cyclic voltammetry (CV). The CuOx/CNF exhibited high electrocatalytic activity towards glucose oxidation at +0.6 V and +1.2 V vs. Ag/AgCl. Infrared Spectroscopy (FTIR) and scanning electron microscopy (SEM) characterized the electrodes composition. Cyclic voltammetry (CV), square wave-voltammetry (SWV), and multiple-pulsed amperometry (MPA) techniques provided optimized conditions for glucose oxidation and detection. A preconcentration step that involved 10 minutes accumulation at open circuit potential before SWV running led to the lowest limit of detection and the highest sensitivity for glucose detection (5419.77 µA·mM−1·cm−2 at + 1.1 V vs. Ag/AgCl) vs. Cu-based electrodes reported to date in literature.

Efficient and Saturated Red Light‐Emitting Electrochemical Cells Based on Cationic Iridium(III) Complexes with EQE up to 9.4 %

Solid-state light-emitting electrochemical cells (LECs) have several advantages, such as low-voltage operation, compatibility with inert metal electrodes, large-area flexible substrates, and simple solution-processable device architectures. However, most of the studies on saturated red LECs show low or moderate device efficiencies (external quantum efficiency (EQE) <3.3 %). In this work, we demonstrate a series of five red-emitting cationic iridium complexes (RED1--RED5) with 2,2'-biquinoline ligands and test their electroluminescence (EL) characteristics in LECs. The Commission Internationale de l'Eclairage (CIE) 1931 coordinates for the LECs based on these complexes are all beyond the National Television System Committee (NTSC) red standard point (0.67, 0.33). The maximal EQE of the neat-film RED1-based LECs reaches 7.4 %. The reddest complex, RED3, is doped in the blue-emitting host complex, BG, to fabricate host-guest LECs. The maximal EQE of the host-guest LECs (1 wt % complex RED3) reaches 9.4 %, which is among the highest reported for the saturated red LECs.

Tuning the Photophysics and Reverse Saturable Absorption of Heteroleptic Cationic Iridium(III) Complexes via Substituents on the 6,6'-Bis(fluoren-2-yl)-2,2'-biquinoline Ligand.

To understand the effects of the terminal substituent at the diimine ligand on the photophysics of heteroleptic cationic Ir(III) complexes and to obtain Ir(III) complexes with extended ground-state absorption to the near-IR region while retaining the long-lived and broadly absorbing triplet excited state, we synthesized three heteroleptic cationic iridium(III) complexes bearing cyclometalating 1-phenylisoquinoline (C^N) ligands and substituted 6,6'-bis(7-R-fluoren-2-yl)-2,2'-biquinoline (N^N) ligand (R = H, NO2, or NPh2). The photophysics of these complexes was systematically investigated via spectroscopic methods and time-dependent density functional theory. All complexes possess strong ligand-localized 1π,π* transitions mixed with ligand-to-ligand charge transfer (1LLCT)/metal-to-ligand charge transfer (1MLCT) transitions below 400 nm, and a broad and featureless absorption band above 400 nm that arises from the N^N ligand-localized 1π,π*/1ILCT (intraligand charge transfer) transitions as well as the very weak 1,3LLCT/1,3MLCT transitions at longer wavelengths. The electron-withdrawing NO2 substituent on the N^N ligand leads to a blue-shift of the 1π,π*/1ILCT absorption band, while the electron-donating NPh2 substituent causes a pronounced red-shift of this band. The unsubstituted and NO2-substituted complexes (complexes 1 and 2, respectively) are moderately emissive at room temperature (RT) in solution as well as at 77 K in the glassy matrix, while the NPh2-substituted complex (3) is weakly emissive at RT, but the emission becomes much brighter at 77 K. Complexes 1 and 2 show very broad and strong triplet excited-state absorption from 460 to 800 nm with moderately long lifetimes, while complex 3 exhibits weak but broad absorption bands from 384 to 800 nm with a longer lifetime than those of 1 and 2. The nonlinear transmission experiment manifests that complexes 1 and 2 are strong reverse saturable absorbers (RSA) at 532 nm, while 3 shows weaker RSA at this wavelength. These results clearly demonstrate that it is feasible to tune the ground-state and excited-state properties of the Ir(III) complexes via the terminal substituents at the diimine ligand. By introducing the fluoren-2-yl groups to the 2,2'-biquinoline ligand to extend the diimine ligand π-conjugation, we can obtain Ir(III) complexes with reasonably long-lived and strongly absorbing triplet excited state while red-shifting their 1,3LLCT/1,3MLCT absorption band into the near-IR region. These features are critical in developing visible to near-IR broadband reverse saturable absorbers.

Synthesis, characterization, crystal structure, solvatochromism, fluorescence and electrochemical studies of new organometallic platinum complexes, kinetic investigation of oxidative addition reaction

A new organoplatinum(II) complex containing 2,2′-biquinoline ligand (biq) was synthesized by the reaction of [Pt(p-MeC6H4)2(SMe2)2] with 2,2′-biquinoline in a 1:1 molar ratio. In this complex the ligand was coordinated to metal via the chelating nitrogen donor atoms. Also, platinum(IV) complex was obtained by the oxidative addition reaction of the methyl iodide with the platinum(II) complex in acetone. The platinum complexes have been found to possess 1:1 metal to ligand stoichiometry and the molar conductance data revealed that the metal complexes were non-electrolytes. The platinum(II) and platinum(IV) complexes exhibited square planar and octahedral coordination geometry, respectively. The emission spectra of the platinum(II) and platinum(IV) complexes were studied in dichloromethane. Furthermore, electrochemical properties of the metal complexes were investigated in dimethylformamide (DMF) solvent at 150 mV s−1 scan rate. The ligand and metal complexes showed both reversible and irreversible processes at this scan rate. The complexes have been characterized by IR, 1H NMR, UV/Vis, elemental analysis and conductometry. The crystal structure of the platinum(II) complex containing 2,2′-biquinoline has been determined by single crystal X-ray diffraction. Moreover, kinetic studies of the oxidative addition reaction of methyl iodide with the platinum(II) complex in different temperatures were investigated. It was indicated that the reaction occurred by the SN2 mechanism. The rate of reaction in two different solvents was compared and the activation parameters were determined.

Comparative study of copper complexes with different anchoring groups by molecular modeling and its application to dye-sensitized solar cells

Copper complexes with different anchoring groups (based on biquinoline ligands with acetylacetone, catechol, hydroxamic acid, phosphonic acid and thiocarboxylic acid groups) have been studied by Density Functional Theory and Time-Dependent Density Functional Theory, also the integral equation formalism of the polarizable continuum model has been extended to the calculation of solvent effects. Maximum absorption wavelength reaches the highest value for the complex with hydroxamic acid as anchoring group but a low oscillator strength; the rest of the complexes fulfill both properties. In general, the molecular orbitals energy levels and the schematic representation of them suppose an appropriate electron transfer between the sensitizer and the semiconductor (TiO2), emphasizing more clearly for the copper complex with the thiocarboxylic acid. Chemical reactivity parameters like chemical hardness shows similitude for all molecular systems; but not so in the case of the electrophilicity index and electroaccepting power.

Influence of Auxiliary Ligands on the Photophysical Characteristics of a Series of Ruthenium(II)–Polypyridyl Complexes

A series of ruthenium-polypyridyl complexes were studied using UV/vis absorption and luminescence spectroscopy as well as luminescence lifetime determination by time-correlated single photon counting (TCSPC). The complexes were characterized with regard to the variation in the electronic band gap as a result of the sequential variation of the auxiliary ligand 2,2'-bipyridine (bpy), 1,10-phenanthroline (phen), and 2,2'-biquinoline (biq) ligands while the main ligand remained constant for three different main ligand types. Luminescence yields were calculated and correlated with structural and electronic variation. It was found that both the absorption and emission characteristics could be tailored through the systematic variation of the reduction potential of the individual auxiliary ligand. This was shown to be the case regardless of the functional group at the end of the main ligand. Stokes shift and Raman spectroscopy were employed as a means to gauge the effect of ligand change on the conjugation and vibrational characteristics of the complexes. Luminescence yield and lifetimes were also shown to be well-defined with regards to systematic structure variations. The well-defined trends established elucidate the effect which variation of auxiliary ligands has on the electronic and excited state characteristics of the ruthenium-polypyridyl systems. These well-defined relationships can potentially be extended to optimize luminescence yield and lifetimes and therefore suitability of such compounds for the application in for example photodynamic therapy.

(2,2′-Biquinoline-κ2N,N′)dibromidopalladium(II)

The PdII ion in the title complex, [PdBr2(C18H12N2)], is four-coordinated in a distorted square-planar environment by the two N atoms from the chelating 2,2′-biquinoline (Biqu) ligand and two mutually cis Br− anions. The Biqu ligand is not planar, the dihedral angle between the quinoline systems being 17.2 (2)°. In the crystal, the complex mol­ecules are connected by C—H⋯Br hydrogen bonds, forming chains along the c axis. When viewed down the b axis, successive chains are stacked in opposite directions. Intra­molecular C—H⋯Br hydrogen bonds are also observed.

Light-activated ruthenium complexes photobind DNA and are cytotoxic in the photodynamic therapy window

Incorporation of biquinoline ligands into Ru(II) polypyridyl complexes produces light-activated systems that eject a ligand and photobind DNA after irradiation with visible and near-IR light. Structural analysis shows that distortion facilitates the photochemistry, and gel shift and cytotoxicity studies prove the compounds act as anti-cancer photodynamic therapy (PDT) agents in the tissue penetrant region.

Long-Wavelength Sensitization of TiO2 by Ruthenium Diimine Compounds with Low-Lying π* Orbitals

The role of low-lying π* orbitals in dye-sensitized solar cells based on mesoporous thin films of anatase TiO(2) nanocrystallites remains unknown. Herein we report three ruthenium compounds, cis-Ru(dcbq)(2)(NCS)(2), cis-Ru(dcbq)(bpy)(NCS)(2), and cis-Ru(dcb)(bq)(NCS)(2), where bpy is 2,2'-bipyridine, dcb is 4,4'-(CO(2)H)(2)-2,2'-bipyridine, bq is 2,2'-biquinoline, and dcbq is 4,4'-(CO(2)H)(2)-2,2'-biquinoline, that were synthesized, characterized, and contrasted with the well-known N3 compound (i.e., cis-Ru(dcb)(2)(NCS)(2)) in dye-sensitized solar cells. These compounds maintain the same cis-Ru(NCS)(2) core with a systematic variation in the energy of the π* orbitals of the diimine ligand: bpy > dcb > bq > dcbq. The lowered π* orbitals resulted in enhanced red absorption relative to N3. With HCl pretreated TiO(2) in regenerative solar cells, sensitization from 400 to 900 nm was realized with cis-Ru(dcb)(bq)(NCS)(2) and global power conversion efficiencies as high as 6.5% were achieved under 1 sun of AM 1.5 irradiation. The energy conversion efficiency was found to be acutely sensitive to the presence of p-tert-butylpyridine (TBP) in a 0.5 M LiI/0.05 M I(2) acetonitrile electrolyte. Nanosecond transient absorption studies revealed that the addition of TBP decreased the excited-state injection yield for the compounds with biquinoline ligands. Spectro-electrochemical studies showed that the HCl pretreatment lowered the effective density of TiO(2) acceptor states and confirmed that the presence of TBP raised them toward the vacuum level. There was no spectroscopic data to support the hypothesis that the π* levels of the diimine ligand mediate back-electron transfer to the oxidized dye or the redox mediator was found.

A Porphyrin with Two Coordination Sites: The Biquinoline Ligand as a New Potential External Chelate

AbstractA highly symmetrical doubly fused porphyrin was obtained after two successive intramolecular cyclizations.1 The first supplementary ring was closed by using the Cadogan reaction, involving the generation of an intermediate nitrene from the starting nitro function. By trying to use the same methodology for the second ring closure, we found that the Cadogan reaction could be used but that cyclization yields remained low. We showed, by taking into account unusual mass spectroscopy results, that the cyclization of the nitro functionality with the neighboring meso‐phenyl took place in the absence of the oxygen abstracting phosphite used by Cadogan. The bis‐biquinolino‐fused porphyrin, bearing an internal and an external coordination site, was obtained in very good yield and showed strong absorbance between 700 and 800 nm.

Synthesis, Characterization, and Crystal Structure of Tetrahedral Cobalt(II) Complex with 2,2′-Biquinoline Ligand

The complex [Co(biq)Cl2] (biq is 2,2′-biquinoline) was prepared from the reaction of CoCl2.6H2O with 2,2′-biquinoline in a 1:1 mixture of methanol/chloroform. This complex was characterized by IR, UV-vis, CV, and the single crystal X-ray diffraction method. The crystal data for [Co(biq)Cl2] at 25°C are as follows: monoclinic, space group P21/n with a = 7.9925(10)Å, b = 12.2108(8)Å, c = 16.8030(19)Å, β = 102.617(9)° and Z = 4.

(2,2′-Biquinoline-κ2N,N′)dibromidozinc(II)

In the title compound, [ZnBr2(C18H12N2)], the ZnII atom is four-coordinated in a distorted tetra­hedral configuration by two N atoms from the 2,2′-biquinoline ligand and two terminal Br atoms. The crystal packing is stabilized by weak inter­molecular C—H⋯Br hydrogen bonds and extensive inter­molecular π–π contacts between the pyridine and benzene rings [centroid–centroid distances = 3.775 (4), 3.748 (4), 3.735 (4), 3.538 (4), 3.678 (4) and 3.513 (4) Å].

Bis(benzenethiolato)(2,2′-biquinoline)zinc(II)

The title compound, [Zn(C6H5S)2(C18H12N2)], was prepared as a model for future complexes that will be incorporated into light-harvesting arrays. The ZnII atom lies on a twofold rotation axis and the ligands are arranged tetra­hedrally around this atom. The benzene­thiol­ate ligand and the biquinoline ligand are nearly perpendicular to one another, making a dihedral angle of 84.09 (5)°. The biquinoline ligand is nearly planar, with a maximum deviation of 0.055 (3) Å from the mean plane of the ring system. In the crystal, the mol­ecules pack in a manner such that the biquinoline ligands are parallel to one another, with a π–π inter­action [interplanar distance = 3.38 (1) Å] with the neighboring biquinoline ligand.

(2,2′-Biquinoline-κ2N,N′)dichloridoiron(II)

In the title compound, [FeCl2(C18H12N2)], the FeII atom is four-coordinated in a distorted tetra­hedral arrangement by an N,N′-bidentate 2,2′-biquinoline ligand and two chloride ions. In the crystal, there are extensive π–π contacts between the pyridine rings [centroid–centroid distances = 3.7611 (3), 3.7603 (4), 3.5292 (4), 3.5336 (5) and 3.6656 (4) Å].

Catena-Poly[[(2,2′-biquinoline-κ2 N,N′)copper(I)]-μ-isothiocyanato-κ2 N:S

The isothiocyanate ligand in the title compound, [Cu(NCS)(C18H12N2)] n , links adjacent tetrahedral CuI atoms into a zigzag chain; the Cu lies on a mirror plane that bisects the chelating biquinoline ligand.