Histidinium-Driven Chirality Control of Self-Assembled Hybrid Molybdenum Oxyfluorides.

Exploring macroscopic chiral materials with extended structures has become an important and fundamental topic in chemistry. To systematically control the chirality of novel organic-inorganic frameworks, histidinium-based cationic structure-directing agents containing specific chiral information are introduced. In this way, two chiral compounds, [(l-hisH2 )MoO2 F4 ]3 ⋅H2 O (L) and [(d-hisH2 )MoO2 F4 ]3 ⋅H2 O (D), and an achiral oxyfluoride, (l/d-hisH2 )MoO2 F4 (LD) (his=histidine, C6 H9 N3 O2 ) have been successfully self-assembled by a slow evaporation method. The structures of these compounds are composed of histidinium cations and distorted [MoO2 F4 ]2- octahedra. Surprisingly, the histidinium cations not only control macroscopic chirality, but also induce O/F ordering in MoO2 F4 octahedra through hydrogen-bonding interactions. Compounds L and D crystallize in the extremely rare polar space group P1, and exhibit positive second harmonic generation (SHG) signals attributable to a net moment originating from the MoO2 F4 groups. Solid-state circular dichroism (CD) spectra indicate that the MoO2 F4 units templated by histidinium cations are chirally aligned through ionic interactions. Crystallization processes influenced by the chirality of the reported materials are also discussed herein.