N,N′-(4,5-Dimethyl-1,2-phenylene)bis(pyridine-2-carboxamide)

In the title compound, C20H18N4O2, the dihedral angles between the central benzene ring and the pyridine rings are 57.55 (6) and 22.05 (8)°. The molecular conformation is stabilized by intramolecular N—H⋯N interactions and in the crystal structure an intermolecular asymmetric cyclic hydrogen-bonding association involving both amide N—H donors and a common amide O-atom acceptor gives a chain extending along the c axis.


Comment
The title compound C 20 H 18 N 4 O 2 was synthesized as a ligand for potential use in medical and radiopharmaceutical applications. In this compound, which has one molecule in the asymmetric unit ( Fig. 1), the dihedral angles between the central benzene ring and the pyridine rings are 57.55 (6) and 22.05 (8)°. The molecular conformation is stabilized by intramolecular N-H···N interactions and in the crystal structure an intermolecular asymmetric cyclic hydrogen-bonding association involving both amide N-H donors and a common amide O-atom acceptor (O2 i ) (Table 1), give a onedimensional chain extending along c. The related structures from Roodt et al. (2011) and Schutte et al. (2011) also contribute to our studies in radiopharmaceutical design and reactivity.

Experimental
Under oxygen atmosphere, picolinic acid (5.73 g, 0.0465 mol) was added as a solid in one portion to a suspension of 4,5dimethyl-1,2-phenylenediamine (3.00 g, 0.0220 mol) in pyridine (20 ml) and the mixture was stirred at 40 °C for 40 min.
Triphenylphosphite (30 ml) was added dropwise over 10 minutes after which the temperature was increased to 90-100 °C and stirred for a further 24 h. On cooling the precipitate was filtered, washed with H 2 O (50 ml) and then MeOH (50 ml).
The precipitate was recrystallized in chloroform to giving colourless crystals after five days

Refinement
The amides, aromatic and methyl hydrogen atoms were positioned geometrically and allowed to ride on their parent atoms, N-H = 0.86 Å and U iso (H) = 1.2U eq , C-H (aromatic C) = 0.95 Å and U iso (H) = 1.2U eq and C-H (methyl C) = 0.98 Å and U iso (H) = 1.5U eq respectively. The methyl groups were allowed to rotate, giving six half-H sites.

Special details
Experimental. The intensity data was collected on a Bruker X8 ApexII 4 K Kappa CCD diffractometer using an exposure time of 30 s/frame. A total of 1895 frames was collected with a frame width of 0.5° covering up to θ = 28.29° with 99.9% completeness accomplished. 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 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.