Structural relationship of various squarates

(T)his work illustrates the structural relationship between three types of metal squarates as well as the ligand in its acid form and in its monoanion salt. Squaric acid, H2C4O4, is known to have a polymeric layer structure with planar molecules connected through intermolecular hydrogen bonds. The interlayer distance is only 2.649 Angstrom. The crystal of H(2)NMe(2)[H-3(C4O4)(2)] is found to contain columns of [H-3(C4O4)(2)(-)] repeating units, again connected by intermolecular hydrogen bonds. Within the repeated unit, there is a symmetric hydrogen bond connected to two HC4O4 moieties. A new type of metal squarate with M(HC4O4)(2)(H2O)(4) [M=Mn-II, Fe-II both belong to space group <P(1)over bar>, Z=1, a=5.194(3), b=7.454(2), c=8.901(2) Angstrom, alpha=67.07(2), beta=77.26(3), gamma=74.46(4)degrees, for Mn-II] is shown to have a layer-type structure, where all [HC4O4] units are bonded into infinite chains via symmetric hydrogen bonds, each (HC4O4)(2)(2-) ligand bridging two metal ions (mu-2) in a trans fashion. The structurally most well understood metal squarate M(C4O4)(H2O)(4) (M=Mn-II, Fe-II, Co-II, Ni-II and Zn-II, space group C2/c, Z=4) is again a polymeric chain with C4O42- serving as a bridging ligand between two metal ions (mu-2) in trans positions. A three-dimensional polymeric structure is found to have the formula M(C4O4)(H2O)(2), where C4O42- is a bridging ligand between four metal ions (mu-4). Due to the slight difference in packing, there are two structure types in this category: one is in space group <R(3)over bar> [M=Fe-II, a=11.440 (2), c=14.504 (3) Angstrom, Z=9], the other is in <Pn(3)over bar n> [M=Co-II, a=16.255(3) Angstrom, Z=24]. The structural relationship between all these structures relies heavily on the understanding of intra- and intermolecular hydrogen bonds. The interesting building blocks of each compound will be illustrated. There are tunnels of various sizes in all these structures.