Synthesis, structures and luminescence properties of two gallium(III) complexes with 5,7-dimethyl-8-hydroxyquinoline

New insights into bioactive Ga(III) hydroxyquinolinate complexes from UV-vis, fluorescence and multinuclear high-field NMR studies

There is current interest in the anticancer and antimicrobial activities of Ga(III) tris-hydroxyquinolinate complexes, and hence their solution and solid-state chemistry. Here, we have studied the formation, stability and structure of a novel tris-5,7-dibromo-8-hydroxyquinolinate Ga(III) complex [Ga(Br2-HQ)3]. Reactions of 5,7-dibromo-8-hydroxyquinoline with Ga(NO)3 in DMSO were followed using electronic absorption and emission spectroscopy, and revealed the slow but concerted coordination of three chelated ligands, with ligand deprotonation being the apparent rate-limiting step, facilitated by basic Ga(III) hydroxido species. The emissive excited state of [Ga(Br2-HQ)3] in DMSO had a short half-life of 1.2 ns, and the fluorescence (550 nm, λex = 400 nm) was characterized by TDDFT calculations as arising from a ligand-centred singlet S1 state. We compared the structures of [Ga(Br2-HQ)3] and the clinical tris-hydroxyquinolinate complex [Ga(HQ)3] using high-field magic-angle-spinning solid-state 1D and 2D 850 MHz and 1 GHz 1H, 13C and 71Ga NMR spectroscopy. The similarity of their coordination spheres was confirmed by their 71Ga chemical shifts of 101 and 98 ppm, respectively, and quadrupolar coupling constants of 9.265 MHz and 9.282 MHz. 1H-1H 2D NOESY experiments revealed second coordination sphere interactions between an acetic acid solvent molecule and the bound hydroxyquinolinate ligands of [Ga(HQ)3]·0.5CH3CO2H. This finding suggests that carboxylic acids could play a role in modifying the formulation properties of this drug for clinical use.

Insights of metal 8-hydroxylquinolinol complexes as the potential anticancer drugs

8-Hydroxyquinoline and its derivatives, which belong to a well-known class of quinoline based drugs with varied biological activities, have been extensively explored for the treatments of cancer, Alzheimer's disease, neurodegenerative diseases and other life-threatening diseases. In virtue of the existence of bicyclic heterocyclic scaffold, their bidentate chelators can further bind to metal ions via O- and N-donors from 8-hydroxylquinolinol skeletons to yield a variety of metal 8-hydroxylquinolinol complexes appealing as the anticancer drugs with low toxicity, due to their better biological effects and higher anticancer activities than free 8-hydroxylquinolinol ligands and cis-diammine-dichloro-platinum. The present review summarizes the recent developments in the syntheses, crystal structures, and anticancer activities of metal 8-hydroxylquinolinol complexes, attempting to discover a correlation between their structures and anticancer activities, and to provide an evidence for their potential application perspectives. It means to offer the helpful and meaningful guidance for the researchers in the future syntheses of new and highly efficient anticancer metal 8-hydroxylquinolinol complexes based drugs.

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.

Low-dimensional compounds containing bioactive ligands. Part XIV: High selective antiproliferative activity of tris(5-chloro-8-quinolinolato)gallium(III) complex against human cancer cell lines

Four gallium(III) complexes, [Ga(ClQ)₃]⋅MeOH (1 - MeOH), [Ga(ClQ)₃] (1), [Ga(BrQ)₃] (2), [Ga(dIQ)₃] (3) and [Ga(CQ)₃] (4), were prepared (H-ClQ = 5-chloro-8-quinolinol, H-BrQ = 7-bromo-8-quinolinol, H-dIQ = 5,7-diiodo-8-quinolinol, H-CQ = 5-chloro-7-iodo-8-quinolinol) and characterised by elemental analysis, IR and NMR spectroscopy. Single crystal structure analysis of 1 - MeOH confirmed that the complex has a molecular structure with gallium(III) metal ion coordinated in mer-fashion by N- and O-donor atoms of three ClQ ligands. Stability of all complexes in DMSO was proved by ¹H NMR spectroscopy. The in vitro antiproliferative activity of 1 was evaluated against the A2780, MBA-MB-231 and HCT116 cell lines. Complex 1 displays higher antiproliferative activity (IC₅₀ values in the range 2.1-6 μm) compared to the ClQ ligand and cisplatin; and a significant selective antiproliferative potency (IC₅₀ = 136 μm, for normal MRC5pd30 cell line). Radical scavenging experiments revealed that complex 1 exhibits the highest antioxidant activity of the prepared complexes as well as the ligands.

Symmetry correlations between crystallographic and photoluminescence study of ternary β-diketone europium(iii) based complexes using 1,10-phenanthroline as the ancillary ligand

This work entails deep red emitting Eu^III based complexes with a variety of ternary β-diketonate ligands and 1,10-phenanthroline as the ancillary ligand in the system. The solid state structure and spectroscopic analysis has been outlaid in terms of photoluminescence and crystallography. A luminescence quantum efficiency of 50% was obtained for the [tris-(4,4,4-trifluoro-1-chlorophenyl-butanedione)mono-(1,10-phenanthroline)europium(iii)] complexes. Moreover, complexes [tris-(2,2,6,6-tetramethyl-heptanedione)mono-(1,10-phenanthroline) europium(iii)] and {[hexa-(benzyl carboxylic acid) bis-(1,10-phenanthroline)di-europium(iii)]-μ-[κ²-O,O'-(benzyl carboxylic acid)]₂} were found to also have quantum yields of 9% and 28% with respective sensitization efficiencies of 85%, 15% and 58%. These results were articulated with crystallographic details pertaining to the nature of coordination and the effect of steric and electronic properties thereof which somewhat impacts the Eu-N bond distances. A symmetry correlation was drawn between the crystallographic data and the photoluminescence data.

Synthesis, crystal structures, photoluminescence, electrochemistry and DFT study of aluminium(III) and gallium(III) complexes containing a novel tetradentate Schiff base ligand

Single crystals of the aluminium and gallium complexes of 6,6'-{(1E,1'E)-[1,2-phenylenebis(azanylylidene)]bis(methanylylidene)}bis(2-methoxyphenol), namely diaqua(6,6'-{(1E,1'E)-[1,2-phenylenebis(azanylylidene)]bis(methanylylidene)}bis(2-methoxyphenolato)-κ⁴O¹,N,N',O^1')aluminium(III) nitrate ethanol monosolvate, [Al(C₂₂H₁₈N₂O₄)(H₂O)₂]NO₃·C₂H₅OH, 1, and diaqua(6,6'-{(1E,1'E)-[1,2-phenylenebis(azanylylidene)]bis(methanylylidene)}bis(2-methoxyphenolato)-κ⁴O¹,N,N',O^1')gallium(III) nitrate ethanol monosolvate, [Ga(C₂₂H₁₈N₂O₄)(H₂O)₂]NO₃·C₂H₅OH, 2, were obtained after successful synthesis in ethanol. Both complexes crystallized in the triclinic space group P-1, with two molecules in the asymmetric unit. In both structures, in one of the independent molecules the tetradentate ligand is almost planar while in the other independent molecule the ligand shows significant distortions from planarity, as illustrated by the largest distance from the plane constructed through the central metal atom and the O,N,N',O'-coordinating atoms of the ligand in 1 of 1.155 (3) Å and a distance of 1.1707 (3) Å in 2. The possible reason for this is that there are various strong π-interactions in the structures. This was confirmed by density functional theory (DFT) calculations, as were the other crystallographic data. DFT was also used to predict the outcome of cyclic voltammetry experiments. Ligand oxidation is more stabilized in the gallium complex. Solid-state photoluminescence gave an 80 nm red-shifted spectrum for the gallium complex, whereas the aluminium complex maintains the ligand curve with a smaller red shift of 40 nm.