Polymorphic layered copper phosphonates: exfoliation and proton conductivity studies

Abstract

Three polymorphic copper carboxylate-phosphonates are reported, namely, α-Cu(4-cnappH)(H₂O) (α-Cu-1), α-Cu(4-cnappH)(H₂O)·0.5H₂O (α-Cu-2) and β-Cu(4-cnappH)(H₂O) (β-Cu), where 4-cnappH₃ is (4-carboxynaphthalen-1-yl)phosphonic acid. All show layer structures but with different layer topologies. The layer topologies of the α-phases are similar, where the distorted {CuO₄} planes and {PO₃C} tetrahedra are corner-shared forming an inorganic layer containing 8- and 16-member rings. In β-Cu, double chains made up of corner-sharing {CuO₅} and {PO₃C} are crossed-linked through edge-sharing of the {CuO₅} square pyramids to form an inorganic layer containing 4- and 8-member rings. The uncoordinated carboxylate groups are protonated and are involved in the interlayer hydrogen bonds in all cases. Antiferromagnetic interactions are dominant in all cases. Compounds α-Cu-1, α-Cu-2 and β-Cu can be exfoliated in THF resulting in 2D nanosheets of mainly two-layer thickness. Proton conductivity measurements reveal that the proton conduction is more favorable in α-Cu-2 than in α-Cu-1, but exfoliation into nanosheets decreases the conductivity.