Crystal structures of 1,1′-bis(carboxymethyl)-4,4′-bipyridinium derivatives

The crystal structures of 2-[1′-(carboxymethyl)-4,4′-bipyridine-1,1′-diium-1-yl]acetate tetrafluoroborate, C14H13N2O4+·BF4− or (Hbcbpy)(BF4), and neutral 1,1′-bis(carboxylatomethyl)-4,4′-bipyridine-1,1′-diium (bcbpy), C14H20N2O8, are reported.


Chemical context
Viologen derivatives (N,N 0 -disubstituted bipyridinium salts) have been widely researched because of their reversible electroactivity, good photochromic properties, and high biochemical activity.Because of these interesting physical and chemical properties, the synthesis of organic polymers or metal-organic frameworks by assembling viologen derivatives using covalent bonds or coordination bonds is attracting attention (Jouhara et al., 2019;Madasamy et al., 2019;Sun & Zhang, 2015).Non-covalent interactions such as hydrogenbonding and electrostatic interactions have been used to assemble functional molecules in the field of crystal engineering (Desiraju, 2001;Horiuchi et al., 2007;Lehn, 1995).We have been working on hydrogen-bonded assemblies to synthesize functional materials and have previously reported hydrogen-bonded assemblies in which functional molecules such as redox-active ferrocene derivatives or tetrathiafulvalene are used (Kitagawa & Kawata, 2002;Nagayoshi et al., 2003).In the present study, we focus on hydrogen-bonded assemblies of N,N 0 -bis(2-carboxyethyl)-4,4 0 -bipyridinium derivatives.

Structural commentary
The asymmetric unit of (Hbcbpy)(BF 4 ) consists of a Hbcbpy + monocation, a BF 4 À anion, and one-half of a water molecule (Fig. 1).The BF 4 À anion is disordered.The key feature of the structure is a hydrogen-bonded one-dimensional chain structure in which the chains are connected by intermolecular -COO À � � �HOOC-hydrogen-bonding interactions (Table 1, Fig. 2).Because the water molecules of (Hbcbpy)(BF 4 ) are not involved in the hydrogen-bonding interactions, water molecules are easily lost from the crystal to give partial occupancy.Two pyridinium groups of the Hbcbpy + monocation are twisted at a C4-C5-C8-C9 torsion angle of 30.3 (2) � [dihedral angle between the rings = 30.18(8) � ] to each other.The carboxymethyl groups bonded to the pyridinium groups exhibit a bent structure and are nearly perpendicular to the pyridinium groups.The Hbcbpy + monocation contains a carboxylic acid group, -COOH, and a deprotonated negatively charged carboxylate group, -COO À , at each end of the monocation.The charge is compensated by a BF 4 À anion.The C-O and C O bond lengths in the carboxylic acid group, C1-O1 and C1-O2, are 1.294 (2) and 1.223 (2) A ˚, respectively, with a difference of 0.071 (2) A ˚.Although the carboxylate group is deprotonated, C14-O3 and C14-O4 also show two different bond lengths of 1.235 (2) and 1.287 (2) A ˚, respectively, where the difference is 0.052 (2) A ˚.
The carboxylic acid group acts as a hydrogen-bond donor, and O4 of the deprotonated carboxylate groups acts as a hydrogen-bond acceptor; the C14-O4 bond is longer than the C14-O3 bond.No hydrogen-bonding interactions are found for O3.The corresponding ClO 4 À salt also exhibits two different C-O bond lengths in the deprotonated carboxylate group and similar hydrogen-bonding interactions to give a zigzag chain structure (Gutov et al., 2008).While the measurements of the (Hbcbpy)(BF 4 ) were conducted at 100 K, the hydrogen-bonding distances of O(carboxylic acid)� � �O(carboxylate) is very similar to that of the ClO 4 salt measured at room temperature, indicative of a small influence of thermal libration (Gutov et al., 2008).We assume that the different C-O bond lengths in the carboxylate group arise from intermolecular hydrogen-bonding interactions.
The asymmetric unit of bcbpy consists of one-half of the neutral bcbpy molecule and two solvent water molecules (Fig. 3).The key feature of the structure is a hydrogen-bonded three-dimensional network in which the molecules are connected by intermolecular hydrogen bonding interactions Structure of (Hbcbpy)(BF 4 ) with labeling scheme and 50% probability displacement ellipsoids.
between bcbpy and water molecules (

Supramolecular features
The Hbcbpy + monocation contains a carboxylic acid and a deprotonated carboxylate group at each end of the monocation.Intermolecular hydrogen-bonding interactions occur between the carboxylic acid of one cation and the negatively charged carboxylate groups of another monocation to give one-dimensional chains (Table 1, Fig. 2).The chains zigzag because of the bent structure of the carboxymethyl groups attached to the pyridinium groups.Within the chains, the pyridinium rings are not coplanar, exhibiting no �-� stacking interactions.
The bcbpy molecule contains two deprotonated carboxylate groups at its ends.The negatively charged carboxylate groups undergo intermolecular hydrogen-bonding interactions between water molecules (Table 2).The negatively charged carboxylate groups act as hydrogen-bond acceptors, and water molecules act as hydrogen-bond donors.The water molecule bridges two bcbpy molecules by hydrogen-bonding interactions, forming a three-dimensional hydrogen bonding network.Although the two pyridinium groups are coplanar, no �-� stacking interactions are observed.
Both compounds lack �-� stacking interactions.A possible explanation for this is that the carboxymethyl groups bonded to the pyridinium groups, which are bent and nearly perpendicular to the pyridinium groups, prevent stacking interactions.Thus, the supramolecular structures of the Hbcbpy + monocation and bcbpy molecule are primarily stabilized by the hydrogen-bonding interactions between negatively charged carboxylate groups and carboxylic acids or water molecules.
dihydrate (MUPCEI) are ethylenic derivatives in which two pyridinium groups are linked by an ethylene group (Jouhara et al., 2019).MUPBUX and MUPCAE are monocations similar to the Hbcbpy + monocation.

Synthesis and crystallization
The dibromo salt H 2 bcbpy(Br) 2 was synthesized using a modified version of a reported procedure (Fajardo & Lewis, 1997).The route is presented in the supporting information (scheme S1).(Hbcbpy)(BF 4 ) and bcbpy were obtained as follows.An aqueous solution (50 mL) of Li(BF 4 ) (38.6 g, 320 mmol) was added to an aqueous solution (50 mL) of H 2 bcbpy(Br) 2 (20.0 g, 40 mmol).The mixture was stirred, the resultant white precipitate was collected by filtration, and the obtained solution was slowly evaporated to yield colorless crystals of (Hbcbpy)(BF 4 ).The crystals (4.9 g, 11 mmol) were dissolved in 30 mL of distilled water, and an aqueous solution (20 mL) of LiOH (0.42 g, 18 mmol) was added.The resultant solution was evaporated, and the obtained white precipitate was filtered.Colorless crystals began to form from the obtained solution at ambient temperature.One of these crystals was used for X-ray crystallographic analysis.

Refinement
The crystal data, data collection, and structure refinement details are summarized in Table 3.The hydrogen atoms of the carboxylic acid and water molecules, which are involved in hydrogen-bonding interactions were located in difference-Fourier maps and refined isotropically.Other hydrogen atoms were placed in idealized positions and refined using a riding model.The occupancy of the water molecule in (Hbcbpy)(BF 4 ) was refined.

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.Refinement.Refinement was performed using all reflections.The weighted R-factor (wR) and goodness of fit (S) are based on F 2 .R-factor (gt) are based on F. The threshold expression of F 2 > 2.0 sigma(F 2 ) is used only for calculating Rfactor (gt).

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.

Table 2
actions with three water molecules, and an interaction with one water molecule.The two C-O bond lengths in the carboxylate group are 1.235 (3) and 1.259 (3) A ˚; thus, the difference between the C-O bond lengths [0.024 (3) A ˚] is slightly larger than the corresponding difference for the first carboxylate group.These results indicate that the difference between the two C-O bond lengths in the carboxylate group is influenced by the type of hydrogen-bonding interactions in the bcbpy system.