Comment

Functionalized 4,4'-bipyridine building blocks are of great interest in designing coordination polymers (Biradha et al., 2006) and are also useful intermediates in viologen chemistry (Benniston et al., 2007). However, the known methods for their preparation often require long reaction sequences (Swahn et al., 2006). Recently, a new one-step procedure was described for the preparation of a series of 4,4'-bipyridines bearing four halogen atoms in their structure, which can serve for further functionalization using cross-coupling reactions (Abboud, Mamane et al., 2010). Therefore, the nonsymmetrical bipyridine 4-(5,5'-dibromo-2'-chloro-4,4'-bipyridin-2-yl)benzaldehyde could be prepared by a selective Suzuki reaction on a single halogen atom (Abboud, Kadimi et al., 2010). Here, we report on the structural analysis of a symmetrical derivative, namely 5,5'-dibromo-2,2'-bis[4-(methylsulfanyl)phenyl]-4,4'-bipyridine, (I). Its preparation involves a selective Suzuki reaction on the C2 and C2' positions of 2,2',5,5'-tetrabromo-4,4'-bipyridine, due to the increased reactivity of Br atoms in proximity to N atoms (Abboud et al., 2012). Crystals of (I) suitable for X-ray analysis were obtained by evaporation from dichloromethane at room temperature.

Compound (I) crystallizes in the C2/c space group, with half the molecule in the asymmetric unit (Fig. 1); the molecule lies on the crystallographic twofold symmetry axis. The dihedral angle between the planes of the pyridine rings is 85.43 (4)°, leading to an intramolecular Br1Br1(-x + 1, y, -z + ) separation of 4.2745 (4) Å, shorter than the corresponding distance found in a previously reported partially functionalized bipyridine [4.29549 (3) Å; Abboud, Kadimi et al., 2010] but still significantly longer than twice the van der Waals radius of bromine (1.85 Å; Bondi, 1964). The methylsulfanyl group is essentially coplanar with the benzene ring [C11-C10-S13-C14 = -0.12 (15)°], but the latter makes a dihedral angle of 19.88 (9)° with the plane of the attached pyridine ring.

The intermolecular interactions in (I) were analysed using the PIXEL method (OPiX; Gavezzotti, 2003a,b), which allows the separation of the intermolecular interaction energies between pairs of molecules into the individual Coulombic, polarization, dispersion and repulsion contributions. In this PIXEL method, all symmetry-related molecules within 22 Å of a reference molecule were considered. Atomic coordinates derived from the X-ray diffraction experiment were used, except for H atoms, which were repositioned at standard average distances derived from neutron diffraction (Allen et al., 1995). Molecular charge densities from a quantum chemical calculation (MP2 level with the 6-31G** basis set using GAUSSIAN03; Frisch et al., 2003) and tabulated atomic polarizabilities were used to compute the Coulombic, polarization, dispersion and repulsion terms. Table 1 reports the intermolecular interaction energies for all molecular pairs with an energy below -1.0 kJ mol^-1.

It can be seen (Fig. 2) that (I) forms infinite molecular columns parallel to [010]. Indeed, the molecules are tightly bonded (entry 1; intermolecular interaction energy = 60.0 kJ mol^-1) through - stacking (Table 2) between the benzene and pyridine rings. The cohesion of this molecular arrangement is also effected through a C3-H3Br1^iv hydrogen bond (Table 3) and methyl contacts [C14-H14B(C9ⁱ), with HA = 2.70 Å and D-HA = 146°; symmetry code: (i) x, y - 1, z]. This particular molecular interaction is characterized by a very large dispersion contribution, which may also result in part from the proximity between the pyridine ring -cloud and the polarizable atom Br1ⁱ [C3Br1ⁱ = 4.256 (2) Å and C3Br1ⁱ-C5ⁱ = 85.96 (6)°].

These neighbouring columns interact through slightly weaker - stacking between the benzene rings, C14-H14C(C7ⁱⁱ) [HA = 2.76 Å and D-HA = 150°; symmetry code: (ii) -x + , -y - , -z + 1] and C14-H14ABr1^vii [HA = 3.00 Å and D-HA = 172°; symmetry code: (vii) x - , -y - , z - ] contacts (Table 1, entry 2), forming infinite (01) molecular planes (Fig. 2). This lower interaction energy compared with the first molecular pair, where two - interactions are present (Table 1, entry 1), may be related to the fact that in this second molecular pair only one such interaction is present.

Of the less stabilizing interactions listed in Table 1, entries 5 and 7 also participate in the internal cohesion of these molecular planes. The former is characterized by two crystallographically equivalent C5-Br1S13^viii contacts [symmetry code: (viii) x + , -y + , z + ; Fig. 3]. Although the BrS distance [3.8173 (6) Å] is slightly larger than the sum of the corresponding van der Waals radii [r_vdW(Br) = 1.85 Å and r_vdW(S) = 1.80 Å; Bondi, 1964], these atoms show a favourable mutual orientation, with the positive hole of the halogen atom pointing towards one of the S-atom lone pairs [C5-Br1S13^viii = 170.72 (5)°]. Interestingly, a search of the Cambridge Structural Database (CSD, Version 5.33 of November 2011, plus three updates; Allen, 2002) for intermolecular BrS contacts reveals a sharp peak centred at 3.9 Å in the normalized contact distribution function (Gavezzotti, 2010) calculated from these data (Fig. 4) (the search was for C-BrSXY contacts < r_vdW + 3 Å, with three-dimensional coordinates determined, with no errors and no disorder, yielding 678 structures and 4454 contacts). Thus, although this distance is slightly longer than the sum of the van der Waals radii of Br and S atoms, it corresponds to a preferential statistical geometric relationship between these two atoms. Scatter plots of the C-BrS and BrS-Cg angles versus BrS distance (where Cg is the centroid of the S, X and Y atoms of the SXY fragment) show that, as the BrS distance increases, the observed ranges for these angles widen (Figs. 5 and 6). More precisely, for the C-BrS angle there is first a rapid widening for BrS ranging from 3.3 to 3.6 Å, and then a smooth enlarging of the observed contact angles. At the observed contact distance (nearly corresponding to the peak in the contact distribution function, i.e. 3.9 Å), the observed range is about 60-180°. Thus, this type of contact is not strongly directional at this interatomic distance; the positive hole of the Br atom is not systematically oriented towards the S atom. For the BrS-Cg angle, the observed range is about 80-180° for the shortest distances, and about 0-180° for BrS distances above 5.6 Å. Thus, from this CSD survey it appears that the observed orientation of the C-BrS contact in (I), with the Br hole pointing towards the S-atom lone pair, results more from a coincidence induced by the packing than from the anisotropy of the electron density around the Br and S atoms. Moreover, the situation within the structure of (I) is additionally complicated: the two molecules involved in this BrS contact interact with two other molecules at (-x + 1, y + 1, -z + ) and (x + , -y - , z + ) through the two strongest interactions (entries 1 and 2 of Table 1). The interaction listed in entry 7 is characterized by a cyclic methylaromatic H atom C14H11(-x + , -y - , -z + 1) contact. The dispersion term is the leading contribution, arising from the proximity of the benzene rings and the S atoms.

All the other interactions listed in Table 1 ensure cohesion between adjacent (01) molecular planes. The strongest (Table 1, entry 3) corresponds to - stacking involving half the molecule through the pyridine and benzene rings (Table 2). In this molecular pair, a C-BrS contact is observed with a significantly longer BrS distance than the previous one [C5-Br1S13(-x + 1, -y, -z + 1), with BrS = 4.1194 (5) Å and C-BrS = 104.90 (4)°], falling on the right-hand side of the peak in the BrS distance distribution (Fig. 4). In this latter case, the mutual orientation of the Br and S atoms is not favourable according to the -hole model, the lone pairs of the two atoms being oriented approximately towards each other. Contrary to the first two - interactions mentioned above, this particular contact has relatively small Coulombic and polarization contributions besides the large dispersion term; in the first two cases, C-H/Br interactions also contribute to the total energy. In comparison, the cyclic R₂²(6) (Bernstein et al., 1995) C6-H6N1(-x + 1, -y + 1, -z + 1) hydrogen bonds (Table 1, entry 4) have a Coulombic plus polarization term almost twice as large; the dispersion contribution is smaller but is still the leading term, resulting from the close proximity of the parallel pyridine rings (distance between ring centroids = 5.591 Å) (Fig. 7). Aromatic atom H9 is involved in hydrogen bonding with atom S13, viz. C9-H9S13^vi (Table 3), giving four such contacts per molecule (Table 1, entry 6). The last two interactions listed in Table 1 (entries 8 and 9) involve distant molecules belonging to the second interaction shell.

The lattice energy of (I) was computed using the PIXEL method as the sum of pair contributions to a central molecule embedded in a crystal cluster of radius 22 Å. This cohesion energy reached -198.1 kJ mol^-1, which is significantly larger than the corresponding value obtained in a similar calculation performed on 4-(5,5'-dibromo-2'-chloro-4,4'-bipyridin-2-yl)benzaldehyde (-135.0 kJ mol^-1; Abboud, Kadimi et al., 2010). This is in line with the higher melting point measured for (I) (488-489 K) compared with 4-(5,5'-dibromo-2'-chloro-4,4'- bipyridin-2-yl)benzaldehyde (443-444 K).

Experimental

To a degassed toluene solution (6 ml) containing Pd(PPh₃)₄ (87 mg, 0.075 mmol) and 2,2',5,5'-tetrabromo-4,4'-bipyridine (575 mg, 1.5 mmol) were successively added degassed solutions of 4-(methylsulfanyl)phenylboronic acid (450 mg, 3 mmol) in methanol (3 ml) and Na₂CO₃ (636 mg, 6 mmol) in water (3 ml). After heating for 15 h at 373 K, the reaction mixture was cooled to room temperature, extracted with ethyl acetate and dried over MgSO₄. After concentration using a rotary evaporator, the residue was purified by chromatography on silica gel (hexanes-ethyl acetate, 9:1 v/v) to give compound (I) as a yellow powder (yield 136 mg, 45%). Crystals of (I) were obtained by slow evaporation from dichloromethane at room temperature and in air (m.p. 488-489 K).

H atoms were located from difference Fourier maps. The final structure was constructed using riding models for C-H bonds, with C-H = 0.95 Å and U_iso(H) = 1.2U_eq(C) for all aromatic H atoms, and with C-H = 0.98 Å and 1.5U_eq(C) for all methyl H atoms.

Data collection: CrysAlis PRO (Oxford Diffraction, 2009); cell refinement: CrysAlis PRO; data reduction: CrysAlis PRO; program(s) used to solve structure: SIR92 (Altomare et al., 1994); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: Mercury (Macrae et al., 2008); software used to prepare material for publication: publCIF (Westrip, 2010).

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Supplementary data for this paper are available from the IUCr electronic archives (Reference: FG3278 ). Services for accessing these data are described at the back of the journal.

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