4,4′-Bipyridine–pyroglutamic acid (1/2)

In the title co-crystal, C10H8N2·2C5H7NO3, the 4,4′-bipyridine molecule [dihedral angle between the pyridine rings = 36.33 (11)°] accepts O—H⋯N hydrogen bonds from the two pyroglutamic (pga) acid molecules. The pga molecules at each end of the trimeric aggregate self-associate via centrosymmetric eight-membered amide {⋯HNCO}2 synthons, so that the crystal structure comprises one-dimensional supramolecular chains propagating in [13]. C—H⋯O and π–π stacking interactions [centroid–centroid separation = 3.590 (2) Å] consolidate the structure.

Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: HB5194).

Comment
The co-crystallization of active pharmaceutical ingredients is an active area of contemporary crystal engineering (Shan & Zaworotko, 2008); see Zukerman-Schpector & Tiekink (2008) for a discussion of terminology. As a continuation of studies into the phenomenon of co-crystallization (Broker & Tiekink, 2007;Broker et al., 2008;Ellis et al., 2009), the co-crystallization of DL-pyroglutamic acid with 4,4'-bipyridine was investigated.
The trimeric aggregates associate into a supramolecular chain via eight-membered amide {···HNCO} 2 synthons. The most convenient description of the chain is given in the following terms. Centrosymmetrically related pyroglutamic acid molecules are connected by the {···HNCO} 2 synthons and these are bridged by the 4,4'-bipyridine molecules, Table 1 and The chains are consolidated into the 3-D crystal structure by a large number of C-H···O contacts, the shortest two are listed in Table 1, as well as π···π interactions involving both pyridyl rings [the closest Cg···Cg i = 3.590 (2) Å where Cg is the ring centroid of N2, C16-C20 for i: 2 -x, -y, 1 -z].

Refinement
The H-atoms were placed in calculated positions (O-H = 0.84 Å, N-H = 0.88 Å and C-H 0.95-1.00 Å) and were included in the refinement in the riding model approximation with U iso (H) set to 1.2-1.5U eq (carrier atom).

Special details
Geometry. All s.u.'s (except the s.u. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell s.u.'s are taken into account individually in the estimation of s.u.'s in distances, angles and torsion angles; correlations between s.u.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell s.u.'s is used for estimating s.u.'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 > 2σ(F 2 ) is used only for calculating Rfactors(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.