Flexible Square Supramolecular Rings with Hydrogen-Bonded Bushing in Solid-State Oxalurate Complexes: Versatility of the Oxalurate Ligand in Covalent and Noncovalent Binding

Isotypic pseudooctahedral complexes of Co, Ni, and Cu with two chelating oxalurate ligands and two water molecules,
trans-[M(oxalurate)2(H2O)2], have been synthesized and isolated by a novel progressive crystallization technique.
Diffraction analyses reveal that the three complexes form isotypic solid-state structures in which the molecular
connectivity and complex network of noncovalent interactions are qualitatively identical throughout the series. The
oxalurate groups form unbounded chains through two different self-recognition patternsa typical DA−AD motif
and an unusual DDA‘−A‘DD form (D = hydrogen bond donor, A‘ = double acceptor). The unsymmetrical oxalurate
group possesses the topological properties necessary to form aggregates of higher symmetry, and the “M(oxalurate)2”
fragments form a rhombic 2-D motif with hydrogen-bonded corners and with hydrogen-bond acceptors directed to
the inside of the cyclic aggregate. The 2-D net is stacked to form a channeled 3-D structure, in which the coordinated
aqua ligands form the principal interlayer interactions. The slanted channels are occupied by the axial waters and
by waters of crystallization, which are hydrogen bonded to the channel walls to form an ordered bushing. The
extensive 3-D hydrogen-bonded superstructure is flexible enough to accommodate the distortion produced by the
Jahn−Teller effect in the copper compound without requiring a qualitative structural change. The bonds affected by
Jahn−Teller distortion in the Cu complex [Cu−O = 2.3788(15) Å] are significantly longer than their analogues in
the Co and Ni complexes [Co−O = 2.175(2), Ni−O = 2.094(9) Å].