catena-Poly[[[di-μ2-hydroxido-bis[(di-2-pyridylamine)nickel(II)]]-μ-fumarato] dihydrate]

The NiII ion in the one-dimensional title complex, {[Ni2(C4H2O4)(OH)2(C10H9N3)2]·2H2O}n, has a distorted square-pyramidal coordination environment formed by three O atoms from two bridging hydroxide groups and one carboxylate group of the fumarate ligand and two pyridine N atoms from a di-2-pyridylamine (dpa) ligand. Two hydroxide groups link adjacent metal centers, forming a centrosymmetric four-membered [Ni2(OH)2] ring. In the crystal structure, the H atoms of the bridging hydroxide groups form intermolecular hydrogen bonds to both water molecules. These are further linked to the uncoordinated O atoms of the carboxylate groups and the NH group of a dpa ligand to generate a three-dimensional network from the chains of the coordination polymer.


Related literature
For applications of transition metal complexes with polypyridylamine ligands, see: Cotton et al. (1998). For details of complexes of bispyridine ligands, see: Liu et al. (2008). For the role of carboxylate substituents in building coordination networks, see: Nathan & Traina (2003).

S1. Comment
Transition metal complexes with polypyridylamine ligands have diverse structures and special optical and electromagnetic properties (Cotton et al., 1998) and have aroused great interest among researchers. Multidentate amine ligands usually exhibit donor as well as acceptor properties and can be used as popular chelating ligands (Nathan & Traina, 2003). On the other hand, carboxylates are attractive as metal-binding units in coordination networks because the negative charge significantly enhances their ability to bind strongly to metal centers, a feature which undoubtedly contributes to the robust nature of the resulting materials (Liu et al., 2008). In this paper, we report the synthesis and crystal structure of the title compound (I), Figure 1.
The Ni1 atom in the title complex has a distorted square pyramidal coordination environment formed by one bidentate dpa ligand, two hydroxyl groups and one carboxylate group. The two peripheral pyridine N atoms from the dpa ligand  Table 1. In addition, the inversion related hydroxyl groups link Ni(II) ions into a centrosymmetric [Ni 2 (OH) 2 ] four-membered ring. Furthermore, the fum 2ligands bridge to an adjacent Ni II linking the four-membered rings into a zigzag chain ( Figure 2).
In the crystal structure, the H atoms of both water molecules and hydroxyl groups are involved in intermolecular hydrogen bonds with the O atoms of uncoordinated carboxylate groups and -NH group from dpa ligand which link the one-dimensional chains to form a three-dimensional network (Table 2).

S3. Refinement
All H atoms attached to C and N atoms were fixed geometrically and treated as riding with C-H = 0.93 Å (ring) or N-H-0.86 Å with U iso (H) = 1.2U eq . H atoms of water molecule and hydroxyl group were located in difference Fourier maps and included in the subsequent refinement using restraints (O-H= 0.83 (1) Å) with U iso (H) = 1.5U eq (O).  Displacement ellipsoids are shown at the 30% probability level.

Figure 2
Partial packing diagram showing the formation of the one-dimensional zigzag chain.

Special details
Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes. Refinement. Refinement of F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 are statistically about twice as large as those based on F, and R-factors based on ALL data will be even larger.