Hydrogen Bonding Controls the Structural Evolution in Perovskite-Related Hybrid Platinum(IV) Iodides

We describe the solid-state structural evolution in four hybrid hexaiodoplatinate(IV) compounds, demonstrating the increasingly important role that extended hydrogen bonding plays in directing the structure across the series. The compounds are A(2)PtI(6), where A is one of the following amines: ammonium, NH4+ methylammonium, CH3NH3+; formamidinium, CH(NH2)(2)(+); guanidinium, C(NH2)(3)(+). These are closely related in structure and properties to the hybrid halide perovskites of lead(II) that have recently established their prowess in optoelectronics. The first three of these compounds crystallize in the vacancy-ordered double perovskite A(2)Pt square I-6 (square indicates a vacant site) structure in the K2PtCl6 archetype, despite the relatively large perovskite tolerance factors involved. The last compound, (GUA)(2)PtI6, crystallizes in a vacancy-ordered variant of the hexagonal CsNiCl3 structure: the K2MnF6 structure. A combination of solid-state Pt-195 and H-1 NMR spectroscopy and detailed density functional theory calculations helps to reveal structural trends and establish the hydrogen-bonding tendencies. The calculations and measured optical properties support the surprising observation in these iodosalt compounds that, for smaller A cations, the conduction bands are considerably disperse, despite lacking extended I-Pt-I connectivity.