Understanding M-ligand bonding and mer-/fac-isomerism in tris(8-hydroxyquinolinate) metallic complexes

Aluminium 8-Hydroxyquinolinate N-Oxide as a Precursor to Heterometallic Aluminium-Lanthanide Complexes

A reaction in anhydrous toluene between the formally unsaturated fragment [Ln(hfac)3] (Ln3+ = Eu3+, Gd3+ and Er3+; Hhfac = hexafluoroacetylacetone) and [Al(qNO)3] (HqNO = 8-hydroxyquinoline N-oxide), here prepared for the first time from [Al(OtBu)3] and HqNO, affords the dinuclear heterometallic compounds [Ln(hfac)3Al(qNO)3] (Ln3+ = Eu3+, Gd3+ and Er3+) in high yields. The molecular structures of these new compounds revealed a dinuclear species with three phenolic oxygen atoms bridging the two metal atoms. While the europium and gadolinium complexes show the coordination number (CN) 9 for the lanthanide centre, in the complex featuring the smaller erbium ion, only two oxygens bridge the two metal atoms for a resulting CN of 8. The reaction of [Eu(hfac)3] with [Alq3] (Hq = 8-hydroxyquinoline) in the same conditions yields a heterometallic product of composition [Eu(hfac)3Alq3]. A recrystallization attempt from hot heptane in air produced single crystals of two different morphologies and compositions: [Eu2(hfac)6Al2q4(OH)2] and [Eu2(hfac)6(µ-Hq)2]. The latter compound can be directly prepared from [Eu(hfac)3] and Hq at room temperature. Quantum mechanical calculations confirm (i) the higher stability of [Eu(hfac)3Al(qNO)3] vs. the corresponding [Eu(hfac)3Alq3] and (ii) the preference of the Er complexes for the CN 8, justifying the different behaviour in terms of the Lewis acidity of the metal centre.

Tris(β-ketoiminate) Aluminium(III) Compounds as Aluminium Oxide Precursors

Precursor design is the crucial step in tailoring the deposition profile towards a multitude of functional materials. Most commercially available aluminium oxide precursors require high processing temperatures (>500 °C). Herein, we report the tuning of the decomposition profile (200-350 °C) of a range of octahedrally coordinated tris(β-ketoiminate) aluminium complexes of the type [Al(MeCN(R)CHC=OMe)₃ ], by varying the R substituents in the ligands. The complexes are derived from the reaction of trimethylamine alane (TMAA) and a series of N-substituted β-ketoiminate ligands (R-acnacH, R=Me, Et, ⁱ Pr, Ph) with varying R-substituents sizes. When the more sterically encumbered ligand (R=Mes) was used, the Al atom became five-coordinate, therefore representing the threshold to octahedral coordination around the metal in these type of compounds, which, consequently, lead to a change of decomposition profile. The resulting compounds have been characterised by NMR spectroscopy, mass spectrometry, elemental analysis and single crystal X-ray diffraction. [Al(MeCN(Me)CHC=OMe)₃ ] has been used as a single source precursor for the deposition of Al₂ O₃ . Thin films were deposited via aerosol assisted chemical vapour deposition (AACVD), with toluene as the solvent, and were analysed using SEM, EDX and XPS.

Complexes of In(III) with 8-hydroxyquinoline-5-sulfonate in solution: structural studies and the effect of cationic surfactants on the photophysical behaviour

Following previous studies on the complexation in aqueous solutions of 8-hydroxyquinoline-5-sulfonate (8-HQS) with the trivalent metal ions, Al(III) and Ga(III) and various other metal ions, using multinuclear NMR, DFT calculations, UV-vis absorption and luminescence techniques, we have extended our studies on 8-HQS complexation to the trivalent metal ion In(III). The study combines the high sensitivity of luminescence techniques and the selectivity of multinuclear NMR spectroscopy with the structural details accessible through DFT calculations, and aims to obtain a complete understanding of the complexation between the In³⁺ metal ion and 8-HQS, and how this influences the luminescence behaviour. A full speciation study has been performed and, as has been reported for the complexes of 8-hydroxyquinoline (8-HQ), the dominant complexes of 8-HQS with In(III) show marked differences in the complexation behaviour when compared with the equivalent complexes with the other group 13 cations Al(III) and Ga(III). While all three complexes have a 1 : 3 (metal : ligand) stoichiometry, those with Al(III) and Ga(III) show a mer-geometry of the ligands around the metal centre, whereas the fac-geometry is observed for the complexes with In(III). On binding to metal ions, 8-HQS shows a marked increase in the intensity of the fluorescence emission band compared to that of the virtually non-luminescent free ligand. However, the increase for In(III) is less pronounced than with Al(III) or Ga(III). These observations have important implications for the application of the complexes in sensing, light emitting devices (e.g. OLEDs), or as electron transport layers in photovoltaics for solar energy conversion. Furthermore, surfactant complexation is known to improve the fluorescence intensity in metal complexes with 8-HQS, by inhibiting the ligand exchange, as we have reported for complexes of HQS with Al(III) and Ga(III). Accordingly, in view of the development of applications in either sensing or optoelectronics, our interest also includes the study of HQS complexes of In(III) in the presence of cationic surfactants, in comparison with previous results with Al(III) and Ga(III).

Tuning of bandgaps and emission properties of light-emitting diode materials through homogeneous alloying in molecular crystals

Alloy formation is ubiquitous in inorganic materials science, and it strongly depends on the similarity between the alloyed atoms. Since molecules have widely different shapes, sizes and bonding properties, it is highly challenging to make alloyed molecular crystals. Here we report the generation of homogenous molecular alloys of organic light emitting diode materials that leads to tuning in their bandgaps and fluorescence emission. Tris(8-hydroxyquinolinato)aluminium (Alq₃) and its Ga, In and Cr analogues (Gaq₃, Inq₃, and Crq₃) form homogeneous mixed crystal phases thereby resulting in binary, ternary and even quaternary molecular alloys. The M_xM′_(1−x)q₃ alloy crystals are investigated using X-ray diffraction, energy dispersive X-ray spectroscopy and Raman spectroscopy on single crystal samples, and photoluminescence properties are measured on the exact same single crystal specimens. The different series of alloys exhibit distinct trends in their optical bandgaps compared with their parent crystals. In the Al_xGa_(1−x)q₃ alloys the emission wavelengths lie in between those of the parent crystals, while the Al_xIn_(1−x)q₃ and Ga_xIn_(1−x)q₃ alloys have red shifts. Intriguingly, efficient fluorescence quenching is observed for the M_xCr_(1−x)q₃ alloys (M = Al, Ga) revealing the effect of paramagnetic molecular doping, and corroborating the molecular scale phase homogeneity.

Multicomponent molecular alloy crystals exhibit intriguing effects of tuning and quenching in their photoluminescence, suggesting ‘alloy-crystal engineering’ as a useful design strategy for molecular functional materials.

Comparison of a dimeric and a monomeric indium-quinolinato complex: synthesis, structure and photoluminescence

Two indium(iii) complexes, mer-[In(Ox)₃]·2H₂O (1) and [In₂(Ox)₂Cl₂-μ-[κ²-O,O′-(Ox)₂]]·C₇H₈ (2) (Ox = 8-hydroxyquinolinate), were synthesized and characterised for comparison by NMR, X-ray diffraction and photoluminescence.

Unveiling the Potential of an Fe Bis(terpyridine) Complex for Precise Development of an Fe-N-C Electrocatalyst to Promote the Oxygen Reduction Reaction

Improvements in highly efficient precious-metal-free electrocatalysts for the oxygen reduction reaction (ORR) are extremely important but still a significant challenge. Herein, we report a novel catalyst design strategy integrating a bis(terpyridine) (hexadentate chelating ligand) with Fe which acts as nitrogen, a self-supporting carbon source, and a potent metal-ligand active site binding structure (Fe-btpy) and promotes the formation of Fe-N_x/C active sites, bypassing the complications induced during Fe-N-C catalyst synthesis. The resulting Fe-N/C_(H,P) electrocatalyst shows a very high ORR onset (E_onset) and half-wave potential (E_1/2) of 1.05 and 0.89 V (vs reversible hydrogen electrode), respectively, outperforming the commercial Pt/C catalyst in alkaline medium. Most importantly, the Fe-N/C_(H,P) catalyst displays decent stability and remarkable methanol tolerance in comparison to the Pt/C catalyst. A fabricated rechargeable zinc-air battery with an Fe-N/C_(H,P) cathode catalyst demonstrated an excellent peak power density of 225 mW cm^-2 at a current density of 240 mA cm^-2, in comparison to the Pt/C cathode catalyst. This work illuminates blueprints utilizing a new long-chain one-dimensional macromolecule that could be viable to produce Fe-N/C-based carbon electrocatalysts toward energy conversion applications.

Absorption and emission properties of 5-phenyl tris(8-hydroxyquinolinato) M(III) complexes (M = Al, Ga, In) and correlations with molecular electrostatic potential

Substituent effect for a series of 5-phenyl tris(8-hydroxyquinolinato) M(III) complexes (Mq3) of aluminum, gallium, and indium are investigated using density functional theory (DFT) for the ground state properties and the time-dependent version of DFT (TDDFT) for their absorption and emission properties. A comparison between the ground state energy of mer and fac isomers of all the complexes revealed that the mer configuration is always more stable than fac. The substituent effect is significantly reflected at the fluorescence maximum (λ_F ) values whereas the effect is moderate at the absorption maximum (λ_abs ) values. The molecular electrostatic potential (MESP) at the metal center (V_M ) and the most electron rich region indicated by MESP minimum (V_min ), located at the oxygen of phenoxide ring exhibit excellent correlations with the λ_F and Stokes shift (λ_F -λ_abs ) values. The study suggests the use of Stokes shift as an experimental quantity to measure the excited state substituent effect while the V_min or V_M emerge as theoretical quantities to measure the same.

8-Hydroxyquinoline complexes (Alq3) on Al(111): atomic scale structure, energetics and charge distribution

8-Hydroxyquinoline (8Hq) is known to efficiently inhibit the corrosion of aluminium by forming metal–organic layers (8Hq forms complexes with aluminium atoms).

Elucidating the Solution-Phase Structure and Behavior of 8-Hydroxyquinoline Zinc in DMSO

The solution-phase structure and electronic relaxation dynamics of zinc bis-8-hydroxyquinoline [Zn(8HQ)₂] in dimethyl sulfoxide (DMSO) were examined using a broad array of spectroscopic techniques, complimented by ab initio calculations of molecular structure. The ground-state structure was determined using extended X-ray absorption fine structure (EXAFS) data collected on the Zn K-edge and diffusion ordered spectroscopy (DOSY) NMR. The complex was found to be monomeric and octahedral, with two bidentate 8-hydroxyquinolate ligands and two DMSO molecules coordinated to the zinc through oxygen atoms. Electronic relaxation dynamics were examined with ultrafast transient absorption spectroscopy and complementary density functional calculations. Electronic relaxation was observed to proceed through both singlet and triplet pathways. This solution-phase data provides a deeper physical understanding of the behavior of this molecule, which has a variety of uses such as sensing, OLEDs, and biological applications.

Heteroassembly Ability of Dicationic Ionic Liquids and Neutral Active Pharmaceutical Ingredients

Extensive investigation of interactions and aggregation properties of IL + API systems is necessary to apply ionic liquids (ILs) with different hydrophobic characteristics to drug delivery or in active pharmaceutical ingredient (API) formulations. Therefore, this study aims to investigate the heteroassembly between dicationic ILs ([BisOct(MIM)₂][2X], in which X is Br or BF₄, and [BisOct(BnIM)₂][2Br]), both in the absence and the presence of neutral APIs (salicylic acid, ibuprofen, and paracetamol) with different functional groups. Isothermal titration calorimetry results demonstrate that IL-API associations occur at very low concentrations of IL. These results were reinforced by electrospray ionization mass spectrometry with variable collision-induced dissociation, in which the IL dication interactions with APIs were detected. The strength of the dication-API interaction was determined from E _cm,1/2 data. The aggregation parameters (cac, ΔG _agg ^°, and K) between ILs and APIs were evaluated by conductivity. The ¹H NMR data showed that differences in chemical shifts provided relevant insights about interaction sites in both components.

Two 8-Hydroxyquinolinate Based Supramolecular Coordination Compounds: Synthesis, Structures and Spectral Properties

Two new Cr(III) complexes based on 2-substituted 8-hydroxyquinoline ligands, namely [Cr(L₁)₃] (1), (HL₁=(E)-2-[2-(4-nitro-phenyl)-vinyl]-8-hydroxy-quinoline) and [Cr(L₂)₃] (2), (HL₂=(E)-2-[2-(4-chloro-phenyl)vinyl]-8-hydroxy-quinoline), were prepared by a facile hydrothermal method and characterized thoroughly by single crystal X-ray diffraction, powder X-ray diffraction, FTIR, TGA, ESI-MS, UV-Visible absorption spectra and fluorescence emission spectra. Single crystal X-ray diffraction analyses showed that the two compounds featured 3D supramolecular architectures constructed from noncovalent interactions, such as π···π stacking, C-H···π, C-H···O, C-Cl···π, C-H···Cl interactions. The thermogravimetric analysis and ESI-MS study of compounds 1 and 2 suggested that the Cr(III) complexes possessed good stability both in solid and solution. In addition, the ultraviolet and fluorescence response of the HL₁ and HL₂ shown marked changes upon their complexation with Cr(III) ion, which indicated that the two 8-hydroxyquinolinate based ligand are promising heavy metal chelating agent for Cr³⁺.