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 A₂PtI₆, 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 A₂Pt□I₆ (□ indicates a vacant site) structure in the K₂PtCl₆ archetype, despite the relatively large perovskite tolerance factors involved. The last compound, (GUA)₂PtI₆, crystallizes in a vacancy-ordered variant of the hexagonal CsNiCl₃ structure: the K₂MnF₆ structure. A combination of solid-state ¹⁹⁵Pt and ¹H 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.