Polystyrene stabilized Pd-Au nanoalloy for efficient synthesis of bis(indolyl)methanes from aryl iodides using oxalic acid as CO and H2 source

•First example of heterogeneously catalyzed carbonylative synthesis of bis(indolyl)methanes from aryl iodides.•Air/moisture stable, easy to handle and recyclable Pd-Au@PS catalyst.•Oxalic acid as green, economic and readily available CO, H2 source as well as acid catalyst.•Green solvent (PEG-200) favored, phosphine ligand free carbonylative approach.•Very good yields (upto 91%), appreciable functional group tolerance.•Easy accessibility and affordability to starting materials.