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

Topochemical reduction of the n = 2 Ruddlesden-Popper oxide, LaSrCoRuO, yields LaSrCoRuO, a phase containing (Co/Ru)O squares which share corners to form 1D infinite double-chains. In contrast, fluorination of LaSrCoRuO yields the oxyfluoride LaSrCoRuOF, which can then be reduced to form LaSrCoRuOF. This reduced oxyfluoride is almost isoelectronic with LaSrCoRuO, but LaSrCoRuOF has a crystal structure in which the (Co/Ru)O 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 LaSrCoRuO and LaSrCoRuOF adopt glassy magnetic states at low temperature, but the magnetic interactions present in LaSrCoRuO appear to be significantly stronger than those in LaSrCoRuOF, attributable to the differing dimensionality of the transition-metal connectivity. The structural features of LaSrCoRuOF 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.