Conversion of hydrogen-bonded manganese(II) and zinc(II) squarate (C4O42–) molecules, chains and sheets to three-dimensional cage networks

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Abstract
Very strong hydrogen-bonding interactions can be utilized to guide the assembly of the molecular building block [Mn(HC4O4)2(OH2)4](H2C4O4= squaric acid, 3,4-dihydroxycyclobut-3-ene-1,2-dione) in the crystal. X-Ray structural analysis of these crystals [triclinic, P, a= 5.1910(4), b= 7.4605(7), c= 8.9088(11)Å, α= 67.07(1), β= 77.23(1), γ= 74.49(1)°, Z= 1] revealed the presence of two very short hydrogen-bonding contacts, O–H ⋯ O = 2.464(1) and 2.481(1)Å, which are responsible for the formation of chains and sheets of manganese squarates(1–) in the crystal. Additional hydrogen-bonding contacts that are weaker but numerous are present and they lead to the formation of a strongly associated three-dimensional manganese squarate(1–) network. The complexity of this hydrogen-bonding pattern was simplified by substituting, in the zinc analogue, two water molecules with dmso (dmso = dimethyl sulfoxide) to give [Zn(C4O4)(OH2)2(dmso)2], which was characterized by single-crystal X-ray diffraction [monoclinic, P21/c, a= 15.697(6), b= 8.116(4), c= 12.211(5)Å, β= 115.83(3)°, Z= 4]. Here, extended chains of zinc squarate are present and found to be linked together by hydrogen bonds to form a layered solid. Further growth of this structure to produce a three-dimensional framework was prevented by virtue of the dmso ligands occupying axial positions at the zinc centre. The ability of the hydrogen bonds to act collectively over a large surface area of these solids adds to their overall crystal stability, as these compounds are insoluble at room temperature in aqueous and non-aqueous media. However, in aqueous media at 25–100°C they undergo conversion reactions leading to the known extended three-dimensional cage network M(C4O4)(OH2)2(M = Mn or Zn).