A novel silver(I) di-iso-butyldithiocarbamate: Unusually complicated 1-D polymeric structure, multiple ligand-supported Ag–Ag interactions and its capability to bind gold(III). Preparation, structural organisation and (13C, 15N) CP-MAS NMR of [Ag6(S2CNiBu2)6] and [Au(S2CNiBu2)2][AgCl2]