4,4′-Di-tert-butyl-2,2′-bipyridine

In the title compound, C18H24N2, the molecular unit adopts a trans conformation around the central C—C bond [N—C—C—N torsion angle of 179.2 (3)°], with the two aromatic rings almost coplanar [dihedral angle of only 0.70 (4)°]. The crystal packing is driven by co-operative contacts involving weak C—H⋯N and C—H⋯π interactions, and also the need to fill effectively the available space.

In the title compound, C 18 H 24 N 2 , the molecular unit adopts a trans conformation around the central C-C bond [N-C-C-N torsion angle of 179.2 (3) ], with the two aromatic rings almost coplanar [dihedral angle of only 0.70 (4) ]. The crystal packing is driven by co-operative contacts involving weak C-HÁ Á ÁN and C-HÁ Á Á interactions, and also the need to fill effectively the available space.   Table 1 Hydrogen-bond geometry (Å , ).
The asymmetric unit is composed of an entire molecular unit as depicted in Fig. 1. The molecule adopts in the crystal structure a trans conformation around the central C-C bond, a feature also reported by Batsanov et al. for the co-crystal with hexafluorobenzene. This conformation seems to minimize steric repulsion between the substituent tert-butyl groups and the heteroatoms from the aromatic rings. While in the structure of Batsanov et al. the 4,4'-di-tert-butyl-2,2'-dipyridyl residue is structurally located on a mirror plane, which ensures coplanarity for the two aromatic rings, in the standalone crystal here reported the atoms are located on generic positions. Nevertheless, the average planes containing the two aromatic rings subtend a dihedral angle of only ca 0.70°, with the corresponding <(N1-C5-C6-N2) torsion angle around the central bond being of 179.2 (3)°.
Individual molecules close pack in the solid state forming layers placed in the (001) plane of the unit cell (Fig. 2). The presence of the large tert-butyl groups seems to prevent the presence of π-π stacking interactions as it can be easily observed by manipulating Enhanced Fig. 4. We note the existence of a terminal -CH 3 group engaged in a C-H···N hydrogen bonding interaction: even though this contact is considered as weak (d D···A being ca 3.64 Å) it is directional with <(DHA) being above 150° (Table 1). In addition, the same -CH 3 group is involved in a C-H···π contact with the aromatic ring of an adjacent molecular unit [not shown; d C···π = ca 3.78 Å; <(C12-H12A···π) = ca 140°]. A similar contact connects two adjacent aromatic rings [not shown; d C1···π = ca 3.40 Å; <(C1-H1···π) = ca 137°]. Besides these weak cooperative interactions, close packing in (I) is further mediated by van der Waals interactions so to promote an effective filling of the available space. Noteworthy, in the C 18 H 24 N 2 . C 6 F 6 organic crystal π-π contacts mediate the close packing because the auxiliary C 6 F 6 molecule is small and can easily be accommodated on top of the 2,2'-dipyridyl residue.
Experimental 4,4'-Di-tert-butyl-2,2'-dipyridyl was purchased from Sigma-Aldrich (98% purity) and used as received without further purification. Single crystals were isolated from the slow evaporation (at ambient temperature) over the period of one month from a solution of the title compound in toluene (Sigma-Aldrich, ACS reagent, >99.5%).

supplementary materials sup-2 Refinement
Hydrogen atoms bound to carbon were located at their idealized positions and were included in the final structural model in riding-motion approximation with C-H = 0.93 (aromatic C-H) or 0.96 Å (for the -CH 3 moieties). The isotropic thermal displacement parameters for these atoms were fixed at 1.2 or 1.5 for the aromatic C-H or the -CH 3 moieties, respectively, times U eq (C). Fig. 1. Schematic representation of the molecular unit of the title compound, with non-hydrogen atoms being represented as thermal displacement ellipsoids drawn at the 50% probability level. The atomic labeling is provided for all non-hydrogen atoms. Bond lengths and angles are provided as supplementary material.