Bis(Indazole-imine) ligands for gold(III): towards new scaffolds for metal-based topoisomerase inhibitor

Abstract

The purpose of this work was to synthesize and characterize several novel bis(indazole-imine) ligands suitable for chelating gold(III), thereby contributing to the development of a new library of square-planar Au(III) complexes with the potential ability to target DNA and consequently potential application as anticancer agents. Four bis(indazole-imine) ligands were synthesized and characterized by proton and carbon-13 NMR, FTIR, UV visible, and mass spectrometry. The structures of the ligands possess two indazole rings connected by diamine bridges. The ligands, however, are varied structurally through changing the length of the diamine bridge and the substituent on the diamine bridge. The length of the diamine bridge was varied such that the first ligand had a propyl chain with a short notation: H2PrMeIndz, the second ligand had a dimethyl group on the propyl chain and was notated H2PrMe2Indz, the third ligand had a hydroxy group (H2PrOHIndz), while the fourth had a butyl chain (H2BuIndz). Among these ligands, only the dimethyl and the hydroxy-bridged ligands (H2PrMe2Indz and H2PrOHIndz) chelated successfully to the Au(III) metal ion forming square-planar Au(III) chelates: [Au(PrMe2Indz)]PF6 and [Au(PrOHIndz)]PF6. These salts were also fully characterized by 1H and 13C NMR, UV-vis, FTIR and MS. During metal coordination, the indazole NH acts as a proton donor and the imino nitrogen acts as a proton acceptor which results in deprotonation of two NH protons forming +1 charged chelates of the general type [Au(L)]+. The two successfully synthsised Au(III) chelates ([Au(PrMe2Indz)]PF6 and [Au(PrOHIndz)]PF6) were studied by X-ray crystallography. Both chelates had a d8 electronic configuration and a square-planar geometry. The average Au Nindazole and Au-Nimine bond lengths were 1.99 Å and 2.01 Å, respectively and the average Nindazole-Au-Nindazole and Nimine-Au-Nimine angles measured 90.01° and 89.73°, respectively. Density functional theory (DFT) simulations were performed. The simulations were carried out for geometry optimizations, electronic transitions, NMR spectral data, as well as vibrational spectra. Overall, the DFT simulations produced accurate results that correlated well with experimental results. Geometry optimization data revealed a good correlation between the DFT-calculated and the X-ray structure with low RMSD values (when compared with the X ray structures) indicating that the DFT calculations were accurate. - 17 - A comparison between the experimental and the calculated UV-vis data showed that DFT calculations consistently overestimated the band energies. However, there was still a good correlation between the intensities and the frequencies of the simulated and experimental spectra. The proton and carbon-13 NMR shielding tensors were calculated and the chemical shifts were compared to the experimental chemical shifts. In most cases, the calculated chemical shifts were overestimated with slight percentage differences for both proton and carbon-13 NMR. Three DNA binding studies, namely gel electrophoresis, viscometry, and UV-vis absorption, were conducted to determine the binding mode of the Au(III) chelates. Gel electrophoresis and UV-vis absorption studies were consistent with each other as they both suggested a possibility of two binding modes which are intercalation and groove binding. Viscosity measurements, however, suggested intercalation as the main binding mode. The Au(III) chelates were screened against two human breast cancer cell lines, MDA-MB-231 and MCF-7, and one normal human cell line, HEK-293. [Au(PrMe2Indz)]PF6 proved to be more selective towards the two breast cancer cell lines compared with [Au(PrOHIndz)]PF6. [Au(PrOHIndz)]PF6, however, displayed greater cytotoxicity than cisplatin against both breast cancer cell lines.