Palladium(ii) and platinum(ii) complexes of disubstituted imidazo[1,5-a]pyridine and imidazolylpyridine: coordination chemistry, versatile catalysis, and biophysical study

Abstract

Pincer-type mono- and poly-nuclear Pd(II) and Pt(II) complexes bearing imidazo[1,5-a]pyridine and imidazolylpyridine moieties were synthesized and characterized using several spectroscopic methods. Determination of molecular structures of these complexes using single crystal X-ray diffraction studies revealed a distorted square planar geometry around the bivalent palladium and platinum in all the complexes. These Pd(II) complexes displayed high catalytic activity in various reactions, such as the Suzuki–Miyaura cross-coupling reaction, transfer hydrogenation reaction, and alkyne homocoupling. The experimental results matched well with the theoretical data of all catalysts. Substantial deviations in the catalytic activity were observed by changing the co-ligand, binding mode of the ligand and the number of metal centres. Under optimal conditions, the Suzuki cross-coupling and transfer hydrogenation reactions were successfully accomplished with a wide range of functional groups by taking only 0.1 mol% of tetranuclear Pd(II) complex (5) as the catalyst. Intermediates in the Suzuki coupling reaction were also detected using mass spectroscopy. Among the studied complexes, the tetranuclear palladium complex exhibited the highest catalytic activity. Further, Pd(II) complexes were tested in a model reaction of the homocoupling of phenylacetylene, and diphenylbutadiyne was produced in excellent yield. Additionally, the interactions of all the complexes with calf thymus DNA (CT-DNA) and bovine serum albumin (BSA) were investigated using electronic spectroscopy. Absorption study showed minor groove binding of DNA with these complexes, while intercalative binding through displacement of ethidium bromide (EB) in EB-DNA was observed in all the complexes, quenching the fluorescence intensity. The complexes also displayed high binding affinity toward BSA, as confirmed by emission, synchronous fluorescence, and steady-state fluorescence anisotropy measurements. Moreover, the pharmacokinetic properties of two bioactive compounds (3s and 3t) obtained from the Suzuki coupling reaction were calculated, and to evaluate their activity as leukotriene A4 hydrolase (LTA₄H) inhibitor, these molecules were docked with human LTA₄H enzyme.