Anew layered Cu-formate hydrate of Cu(HCOO)
2·1/3H
2O is unique in its water content and the strongly waved (4,4) Cu-formate layers held by interlayer weak axial Cu-O
formate bonds and O-H
water···O
formate hydrogen bonds. The crystal is in orthorhombic space group Pbcn, with cell parameters at 80 K:a=7.9777(2) Å, b=7.3656(2) Å, c=21.0317(5) Å(1 Å=10
−1 nm), and V=1235.83(5) Å
3. The Cu
2+ ions are in the environments of a square pyramid and elongated octahedron, in a ratio of 1/2 within the structure. In the layer, Cu
2+ ions are connected by anti-anti formates via short basal Cu-O bonds. The structure remains unchanged until the dehydration that produces the layered anhydrous β-Cu(HCOO)
2, and the possible transformation mechanism, supported by diffraction evidence, is the reorganization of the Cu-O
formate bonds across the parent layers after dehydration. The two phases exhibit anisotropic thermal expansion behaviors closely relevant to the transverse thermal vibrations of the constituents. Cu(HCOO)
2·1/3H
2O is a 2-dimensional Heisenberg antiferromagnet, and exhibits a global spin-canted antiferromagnetism with the Néel temperature of 32.1 K. This is not only higher than that of the magnetically denser β-Cu(HCOO)
2, but also the highest among the copper formate frameworks.
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