Crystal structure of catena-poly[[[aquabis(dimethylformamide-κO)magnesium(II)]-μ3-(2,2′-bipyridine-5,5′-dicarboxylato-κ5 O 2:O 2′:N,N′:O 5)-[dichloridoplatinum(II)]] dimethylformamide monosolvate]

A new one-dimensional coordination polymer formed unexpectedly during the synthesis of a Pt-functionalized bipyridine linker for metal–organic frameworks. We report here the synthesis, structure determination and energy-dispersive X-ray spectroscopy analysis of this new coordination polymer.

The title compound, {[MgPtCl 2 (C 12 H 6 N 2 O 4 )(C 3 H 7 NO) 2 (H 2 O)]ÁC 3 H 7 NO} n , is a one-dimensional coordination polymer. The structure consists of Ptfunctionalized bipyridine ligands connected by Mg II cations, as well as coordinating and non-coordinating solvent molecules. The Pt II cation is coordinated by the two N atoms of the bipyridine moiety and two Cl atoms in a square-planar fashion. This coordination induces an in-plane bend along the bipyridine backbone of approximately 10 from the linear ideal of a conjugated -system. Likewise, the coordination to the Mg II cation induces a significant bowing of the plane of the bipyridine of about 12 , giving it a distinct curved appearance. The carboxylate groups of the bipyridine ligand exhibit moderate rotations relative to their parent pyridine rings. The Mg II cation has a fairly regular octahedral coordination polyhedron, in which three vertices are occupied by O atoms from the carboxylate groups of three different bipyridine ligands. The remaining three vertices are occupied by the O atoms of two dimethylformamide (DMF) molecules and one water molecule. The onedimensional chains are oriented in the [011] direction, and non-coordinating DMF molecules can be found in the space between the chains. The shortest intermolecular OÁ Á ÁH contacts are 2.844 (4) and 2.659 (4) Å , suggesting moderate hydrogen-bonding interactions. In addition, there is a short intermolecular PtÁ Á ÁPt contact of 3.491 (1) Å , indicating a Pt stacking interaction. Some structure-directing contribution from the hydrogen bonding and PtÁ Á ÁPt interaction is probable. However, the crystal packing seems to be directed primarily by van der Waals interactions.

Chemical context
Metal-organic frameworks (MOFs) are porous materials that have attracted significant attention over the last two decades. The materials are formed from inorganic and organic components, typically a cationic unit linked by an organic ligand commonly referred to as a linker. Incorporating a catalytically active site in the linker of a porous MOF has the potential to create a heterogenous catalyst with the same selectivity often associated with homogenous catalysts. To this end, there are two main strategies for incorporating the active species. One method is to add the active species to the MOF after the frameworks has been formed, so called post-synthetic modification. The other option is to functionalize the linker either before or during the MOF synthesis (Cohen, 2017). ISSN 2056-9890 The title compound is an unexpected byproduct from the synthesis of the functionalized linker (2,2 0 -bipyridine-5,5 0dicarboxcylic acid)tetrachloridoplatinum(IV). 2,2 0 -bipyridine-5,5 0 -dicarboxcylic acid is highly suitable for incorporation in the UiO-67 MOF, where it can partially substitute the biphenyl linker of the parent structure (Cavka et al., 2008). Furthermore, the N atoms of the bipyridine linker can be used to anchor and functionalize the linker with e.g. Pt or other noble metals. The Pt site of the target linker is interesting in a catalytic context. Pt has a rich redox chemistry and is know to readily switch between oxidation states Pt II and Pt IV , thus providing an active site for e.g. C-H activation. The target linker and its successful inclusion in the UiO-67 MOF has been reported in the literature (Øien et al., 2015).

Structural commentary
The asymmetric unit of the title compound comprises a Mg II cation coordinated by two dimethylformamide (DMF) molecules and one water molecule, as well as a bipyridine moiety with two Cl atoms and Pt II in a square-planar coordination. In addition, the asymmetric unit contains a DMF solvent molecule that does not coordinate to the rest of the structure (Fig. 1). The Mg II cation is octahedrally coordinated, with the vertices occupied by O atoms from two DMF molecules, one water molecule and three carboxylate groups from three different bipyridine moieties.
The carboxylate groups coordinate to the cation in a monodentate fashion, thus each bipyridine moiety coordinates to three different Mg II cations. The fourth O atom of the carboxylate groups (O4) is uncoordinating, and has a more pronounced displacement ellipsoid when compared to the coordinating O atoms O1, O2 and O3. Moderate torsion angles of 12.56 (29) and 12.29 (25) can be observed for the two carboxylate groups relative to their parent pyridine rings.
One Pt II and two Cl atoms are coordinated by the N atoms of the bipyridine ligand in a square-planar coordination. This type of coordination is commonly observed in complexes with Pt II and other transition metals with a d 8 electron configuration (Krogmann, 1969). The square plane itself is regular with an r.m.s. deviation from the flat plane of only 0.013 Å . Angles of 88.94 (5) and 80.50 (13) are observed for Cl1-Pt1-Cl2 and N1-Pt1-N2, respectively. Notably, the Pt-Cl bonds are slightly longer ($2.30 Å ) than the Pt-N bonds ($2.02 Å ). This indicates that there is a stronger trans effect from the bipyridine ligand than the Cl atoms. The bond lengths and angles (Table 1) are consistent with other similar structures (Hazell et al., 1986;Kato & Ikemori, 2003;Kato et al., 2006;Hazell, 2004;Maheshwari et al., 2007).
The bipyridine backbone exhibits a distinct bowing relative to the plane of the molecule (Figs. 2 and 3) as well as an inplane bend (Fig. 4). The bowing has been calculated to 12.74 (20) by comparing the angle between the least-squares planes of the pyridine rings. Deviations from the ideal 120 for the N1-C1-C7 and C1-C7-N2 angles give an estimation of the in-plane bending of about 10 . Such in-plane bending and bowing has been observed in several similar, albeit noncoordinating, bipyridine compounds (Hazell et al., 1986;Kato & Ikemori, 2003;Kato et al., 2006;Hazell, 2004;Maheshwari et al., 2007). However, it is likely that the distortion of the bipyridine is influenced by the coordination to Mg as well as the intermolecular PtÁ Á ÁPt interaction.

Supramolecular features
The title compound forms one-dimensional chains comprising two bipyridine linkers and two Mg II cations with associated coordinating solvent molecules as the repeating unit. These 972 Lundvall and Tilset [MgPtCl 2 (C 12 Table 1 Selected geometric parameters (Å , ).

Figure 1
The asymmetric unit of the title compound, with atom labels and 50% probability displacement ellipsoids. H atoms have been omitted for clarity, excluding the H atoms of the coordinating water molecule (H1WA/B).
chains are oriented in the [011] direction ( Fig. 1). DMF solvent molecules can be found between the chains, oriented side-on to the plane of the bipyridine linker. Hydrogen-bonding interactions ( Table 2) are found between the coordinating water molecule O2W and atoms O2C and O4 of neighboring DMF and bipyridine moieties. The donor-acceptor distances are 2.844 (4) and 2.659 (4) Å , indicating moderately strong bonds. There is also a short intermolecular PtÁ Á ÁPt contact of 3.491 (1) Å , indicating a Pt stacking interaction between pairs of bipyridine ligands in the chain. These types of stacking interactions are common in square-planar complexes of metals in a d 8 electronic configuration (Krogmann, 1969). The hydrogen bonding and PtÁ Á ÁPt stacking interaction are likely to contribute to the overall structure and crystal packing.

Synthesis and crystallization
2,2 0 -Bipyridine-5,5 0 -dicarboxylic acid, was synthesized according to literature methods (Szeto et al., 2008). Dimethylformamide (DMF) was supplied by Sigma-Aldrich and dried before use. K 2 PtCl 6 and 35% wt HCl were used as received from Sigma-Aldrich. The title compound was synthesized by dissolving 16.3 mg (0.067 mmol) 2,2 0 -bipyridine-5,5 0 -dicarboxylic acid, 65.3 mg (0.134 mmol) K 2 PtCl 6 and three drops of 35% HCl in 4 ml of DMF. The mixture was heated in a closed glass vial in a convection oven at 323 K for 48 h, followed by 24 h at 343 K and finally 48 h at 353 K. This procedure yielded clusters of yellow needle-shaped crystals suitable for single crystal X-ray diffraction, as well as a yet unidentified red compound.
Note that the synthesis procedure does not include a source of Mg, despite its inclusion as cation in the title compound. The initial structural solution included K + as the cation. However, the refinement of this initial model indicated several problems. First of all, a fully deprotonated organic ligand (L 2À ) and just one K + cation would imply a charge imbalance in the structure. Secondly, the model had unrealistic displacement ellipsoids for the metal species as well as an unusual weighting scheme. Lastly, the metal-to-oxygen bond lengths were significantly shorter than expected for K-O bonds in an Packing diagram of the title compound, viewed along the c axis. H atoms, non-coordinating solvent molecules and non-O atoms of coordinating solvent molecules have been omitted for clarity. Table 2 Hydrogen-bond geometry (Å , ). Symmetry codes: (iv) Àx þ 1; Ày; Àz þ 2; (v) Àx þ 1; Ày þ 1; Àz þ 1.

Figure 2
Packing diagram of the title compound, viewed along the a axis. H atoms have been omitted for clarity.

Figure 3
Detailed view of the title compound viewed along the a axis, with 50% probability displacement ellipsoids. H atoms, non-coordinating solvent molecules and non-O atoms of coordinating solvent molecules have been omitted for clarity. The PtÁ Á ÁPt interaction is indicated by a red dashed line. The second bipyridine moiety is generated by the symmetry operation (Àx + 2, Ày + 1, Àz + 1).
octahedral environment when applying the bond-valence method (Brown & Altermatt, 1985). Thus we hypothesized that the coordination polymer must contain a contamination from the synthesis. The correct cation would likely be a divalent metal that is commonly encountered in organic chemistry, often exhibits octahedral coordination, and most importantly has a short metal-to-oxygen bond. Based on these criteria, the cation of the initial model was replaced with Mg, which solved the aforementioned refinement issues. Subsequent energy-dispersive X-ray spectroscopy (EDX) confirmed the presence of Mg in the sample (Fig. 5). The source of the contamination is likely from a batch of DMF incorrectly dried over MgSO 4 .

Refinement
Crystal data, data collection and structure refinement details are summarized in Table 3. H atoms were positioned geometrically at distances of 0.87 (OH), 0.95 (CH) and 0.98 Å (CH 3 ) and refined using a riding model with U iso (H) = 1.2 U eq (CH) and U iso (H) = 1.5U eq (OH and CH 3 ).

Crystal structure of catena-poly[[[aquabis(dimethylformamide-κO)magnesium(II)]-µ
ChemBioDraw Ultra (Cambridge Soft, 2012); software used to prepare material for publication: publCIF (Westrip, 2010). where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.002 Δρ max = 1.92 e Å −3 Δρ min = −2.42 e Å −3 Special details Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds involving l.s. planes.