Synthesis of gold(III) ← gold(I)-NHC through disproportionation: the role of gold(I)-NHC in the induction of apoptosis in HepG2 cells

Starting from the proligand 2-[(6-methylpyridin-2-yl)] imidazo[1,5-a] pyridin-4-ylium hexafluorophosphate(1 center dot PF6), three new complexes, viz. [Au(1)(2)] [PF6] (2), [1/2AuCl(2), 1/2AuCl(4)](-) [(1H)](+) (3), and [Au(1)Cl-3] (4), have been synthesized and characterized employing different spectroscopic methods. [Au(1)Cl-3] (4) has been synthesized by the disproportionation process. During the transformation of 2 to 4, the annelated proligand stabilizes both Au(I) and Au(III), and the isolation of the intermediate (3) confirms the conversion of Au(I) - Au(III) through the disproportionation pathway. The solid state structures of 2, 3 and 4 have been determined. Linear geometry was observed in 2, whereas complex 4 adopted square-planar geometry. Gold complexes 2 and 4 have been subjected to growth inhibitory studies. Complex 2 induced apoptosis in HepG2 cells along with increased expression of proteins involved in the mitochondrial death pathway, suggesting that apoptosis may occur via the mitochondrial death pathway.