Controlling the Regioselectivity of Topochemical Reduction Reactions Through Sequential Anion Insertion and Extraction

Topochemical reduction of the n = 2 Ruddlesden-Popper oxide, LaSr₂CoRuO₇, yields LaSr₂CoRuO_5.3, a phase containing (Co/Ru)O₄ squares which share corners to form 1D infinite double-chains. In contrast, fluorination of LaSr₂CoRuO₇ yields the oxyfluoride LaSr₂CoRuO_5.5F_3.5, which can then be reduced to form LaSr₂CoRuO_4.5F_1.5. This reduced oxyfluoride is almost isoelectronic with LaSr₂CoRuO_5.3, but LaSr₂CoRuO_4.5F_1.5 has a crystal structure in which the (Co/Ru)O₄ squares are connected into 2D infinite sheets. Thus, by following a fluorinate-then-reduce reaction scheme, the regiochemistry of topochemical reduction reactions can be modified, and compounds with different transition-metal-centre interconnectivity can be prepared. Both LaSr₂CoRuO_5.3 and LaSr₂CoRuO_4.5F_1.5 adopt glassy magnetic states at low temperature, but the magnetic interactions present in LaSr₂CoRuO_5.3 appear to be significantly stronger than those in LaSr₂CoRuO_4.5F_1.5, attributable to the differing dimensionality of the transition-metal connectivity. The structural features of LaSr₂CoRuO_5.5F_3.5 that modify the regioselectivity of the topochemical reduction reaction appear to be common to many fluorinated Ruddlesden-Popper oxides, suggesting this fluorinate-then-reduce strategy could be used to prepare a range of "infinite-layer" reduced phases which cannot be made by direct reduction of Ruddlesden-Popper oxide precursors.