Crystal structures of three 4-substituted-2,2′-bipyridines synthesized by Sonogashira and Suzuki–Miyaura cross-coupling reactions

In the crystal structures of three 4-substituted-2,2′-bipyridines prepared using facile synthetic procedures, two novel 4-alkynyl-2,2-bipyridines via the Sonogashira cross-coupling reaction and one 4-aryl-2,2′-bipyridine via the Suzuki–Miyaura cross-coupling reaction, the planar 4-alkynyl-substituted derivatives are in contrast to the non-planar 4-aryl derivative.

In conclusion, we have described facile synthetic procedures for 4-alkynylated and 4-arylated 2,2 0 -bipyridines by means of the Sonogashira and Suzuki-Miyaura cross-coupling reactions of 4-bromo-2,2 0 -bipyridine. Based on this strategy, two novel 4-alkynylbipyridines and one 4-aryl-2,2 0 -bipyridine were synthesized whose structures were partially elucidated by NMR spectroscopic methods. In addition, the X-ray structural analysis revealed the planarity of the 4-alkynylbipyridines as the triple-bond linker separates the bipyridine and the introduced aromatic parts. This provides a hint for fine-tuning the electronic properties of this ligand by introducing suitable substituents. On the other hand, the introduced heterocyclic ring in compound (III), formed via Suzuki-Miyaura cross-coupling is twisted from the 2,2 0 -bipyridine ring due to the van der Waals repulsive force of the hydrogen atoms in close proximity.

Database survey
An extension of the -conjugated system of 2,2 0 -bipyridine can be obtained by the introduction of an aromatic substituent. A search in the Cambridge Structural Database (CSD, Version 5.38, last update February 2017; Groom et al., 2016) for crystal structures of 2,2 0 -bipyridine derivatives substituted at the 4-position with an aromatic substituent resulted in 13 unique hits (excluding organometallic compounds) with substituents ranging from smaller phenyl and triazine rings to bipyridine, naphthalene, anthracene and phenanthrene to a larger pyrene ring (Table 4). However, it is evident from the dihedral angle between the best planes through pyridine and its aromatic 4-substituent (varying from 0.0 to 73.8 ) that the degree of extension of the -conjugated system depends on the steric hindrance of the substituent and theinteractions in the crystal packing.
The dihedral angle py-py is defined as the angle between the best planes through both pyridine rings and the dihedral angle py-Ar is defined as the angle between the best planes through the 4-substituted pyridine and the aromatic substituent.

Structure solution and refinement
Crystal data, data collection and structure refinement details are summarized in Table 5. The structures of (I) and (III) were solved using SHELXS97 (Sheldrick, 2008) and for (II) by charge flipping using Olex2.solve (Bourhis et al., 2015). All hydrogen atoms were placed in idealized positions and refined in a riding mode with U iso (H) = 1.2 times those of their parent atoms (1.5 times for methyl groups), with C-H distances of 0.95 Å (aromatic) and 0.98 Å (CH 3 ) and N-H distances of 0.88 Å . For (II) a region of electron density amounting to the scattering from approximately 10.7 carbon atoms, apparently disordered in channels between columns of stacking molecules, was removed with the SQUEEZE routine of PLATON (Spek, 2015) after it proved impossible to identify it with any reasonable solvent molecule. A suggestion of possible twinning generated by PLATON (Spek, 2009) was further checked but subsequent refinement did not improve and was neglected. For all compounds, data collection: CrysAlis PRO (Rigaku OD, 2015); cell refinement: CrysAlis PRO (Rigaku OD, 2015); data reduction: CrysAlis PRO (Rigaku OD, 2015). Program(s) used to solve structure: SHELXS97 (Sheldrick, 2008) for (I), (III); Olex2.solve (Bourhis et al., 2015) for (II). Program(s) used to refine structure: SHELXL2014 (Sheldrick, 2015) for (I), (III); SHELXL (Sheldrick, 2015) for (II). For all compounds, molecular graphics: OLEX2 (Dolomanov et al., 2009); software used to prepare material for publication: OLEX2 (Dolomanov et al., 2009). T min = 0.552, T max = 1.000 12597 measured reflections 5747 independent reflections 3728 reflections with I > 2σ(I) Hydrogen site location: inferred from neighbouring sites H-atom parameters constrained

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.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Å 2 )
x y z U iso */U eq N1 0.2656 (9) 0.68966 (10 where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max < 0.001 Δρ max = 0.21 e Å −3 Δρ min = −0.23 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. 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 > 2sigma(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.