X-ray crystal structure of trans-bis­­(pyridin-3-yl)ethyl­ene: comparing the supra­molecular structural features among the symmetrical bis­­(n-pyrid­yl)ethyl­enes (n = 2, 3, or 4) constitutional isomers

The mol­ecular structure of trans-bis­(pyridin-3-yl)ethyl­ene (3,3′-bpe), C₁₂H₁₀N₂, as determined by single-crystal X-ray diffraction is reported. The mol­ecule self-assembles into two dimensional arrays by a combination of C—H⋯N hydrogen bonds and edge-to-face C—H⋯π inter­actions that stack in a herringbone arrangement perpendicular to the crystallographic c-axis. The supra­molecular forces that direct the packing of 3,3′-bpe as well as its packing assembly within the crystal are also compared to those observed within the structures of the other symmetrical isomers trans-1,2-bis­(n-pyrid­yl)ethyl­ene (n,n′-bpe, where n = n′ = 2 or 4).

1. Chemical context

Bis(pyrid­yl)ethyl­enes have arisen as somewhat of a natural extension of cinnamic acid as a series of mol­ecules capable of undergoing [2+2] photodimerization in the solid state to generate cyclo­butanes. Foundational work by Schmidt and coworkers on trans-cinnamic acids led to the formation of the `Topochemical Postulate', which dictated that olefins within 4.2 Å of one another are capable of undergoing the photodimerization process. Unlike cinnamic acid, which crystallizes in such a way that the olefins are rendered photoactive (olefins within 4.2 Å of one another), the native crystalline forms of bis­(pyrid­yl)ethyl­enes are photostable (olefins separated by distances > 4.2 Å in the crystal). To achieve photoreactivity of these olefins, it often becomes necessary to use a `mol­ecular template' that can inter­act with the olefin-containing bi­pyridine via supra­molecular inter­actions such as hydrogen bonding, halogen bonding, argento- and aurophilic inter­actions, and dative N→B inter­actions. Analyses of the crystal structures of symmetric bis­(pyrid­yl)ethyl­enes derivatives such as the trans-bis­(n-pyrid­yl)ethyl­enes series of isomers (n = 2, 3 or 4) is necessary to understand the forces that govern their crystallization, why they are photostable, and why use templates to achieve photoreactivity (Campillo-Alvarado et al., 2019; Chanthapally et al., 2014; MacGillivray et al., 2008; Pahari et al., 2019; Sezer et al., 2017; Volodin et al., 2018).

2. Structural commentary

The alkene 3,3′-bpe crystallizes in the centrosymmetric monoclinic space group P2₁/n (Fig. 1). The asymmetric unit consists of one-half mol­ecule of 3,3′-bpe with the C=C bond sitting on a crystallographic center of inversion. The pyridyl rings adopt an anti-conformation with respect to each other (Fig. 1).

Figure 1 Single crystal structure for trans-bis­(pyridin-3-yl)ethyl­ene (3,3′-bpe) with anisotropic displacement ellipsoids at 50% probability.

3. Supra­molecular features

Adjacent 3,3′-bpe mol­ecules inter­act primarily via edge-to-face C—H⋯π[d(C6⋯pyr) 3.58 Å; Θ(C6—H6⋯pyr) 131.8°] forces between pyridyl rings (Fig. 2). Those rings also participate in C—H⋯N [d(C4⋯N1) 3.59 Å; Θ(C4—H4⋯N1) 139.5°] hydrogen bonds (Fig. 2). The forces generate nearly planar sheets (Fig. 3), which aggregate into a herringbone arrangement of adjacent sheets (Fig. 4). Nearest-neighbor alkene C=C bonds of 3,3′-bpe between adjacent sheets reveals a parallel, but offset orientation of the neighboring alkenes relative to one another at a distance of 5.50 Å. The distance exceeds the inter-alkene separation of Schmidt for photodimerizarion and suggests that 3,3′-bpe is photostable (Schmidt, 1971).

Figure 2 C—H⋯N and edge-to-face C—H⋯π inter­molecular inter­actions (both yellow dotted lines) highlighting nearest-neighbor alkene separations (red dashed arrow) (view along a).

Figure 3 Edge-on view of sheets encompassing neighboring mol­ecules of 3,3′-bpe supported by C—H⋯N and C—H⋯π inter­molecular inter­actions.

Figure 4 Herringbone arrangement of neighboring sheets of 3,3′-bpe mol­ecules.

4. Database survey

For the n,n′-bpe (where: n = n′ = 2, 3, or 4) series of symmetric alkenes, all three adopt nearly planar conformations (Table 1), with the pyridyl rings of 3,3′-bpe and 2,2′-bpe adopting anti-conformations with respect to each other. The packings of the symmetric alkenes are defined by combinations of C—H⋯π and/or C—H⋯N hydrogen bonds (Table 1) to form either one-dimensional chain (2,2′-bpe, Fig. 5) or two-dimensional sheet (3,3′-bpe and 4,4′-bpe) structures (Fig. 6). Similar to 3,3′-bpe, the alkene C=C bonds of 2,2′-bpe (6.09 Å; Vansant et al., 1980) and 4,4′-bpe (5.72 Å; Tinnemans et al., 2018) (Table 1) are beyond the separation distance of Schmidt (1971).

Table 1
Structural features of the n,n′-bpe series of constitutional isomers

The twist angle is defined as the angle between the plane defined by the four alkene atoms and the plane defined by either pyridine ring.

Compound	2,2′-bpe	3,3′-bpe	4,4′-bpe
Twist angle φ (°)	7.43	5.17	9.14
Solid-state packing assembly	corrugated chains	approximately planar sheets	planar sheets
Assembly forces	edge-to-face C—H⋯π	edge-to-face C—H⋯π, C—H⋯N	C—H⋯N, face-to-face π–π
Nearest-neighbor alkene separation (Å)	6.09	5.50	5.72

Figure 5 Corrugated, one-dimensional chains of 2,2′-bpe.

Figure 6 Planar, two-dimensional sheets of 4,4′-bpe.

5. Synthesis and crystallization

The alkene 3,3′-bpe was prepared as described (Quentin et al., 2020; Gordillo et al., 2007, 2013) via a one-pot, aqueous Pd-catalyzed Hiyama-Heck cross-coupling between 3-bromo­pyridine and tri­eth­oxy­vinyl­silane (2:1 molar ratio) (Fig. 7). Flash chromatography (SiO₂, 10% MeOH/CH₂Cl₂) furnished 3,3′-bpe as yellow crystals: 222.3 mg (23%). A portion of 3,3′-bpe was dissolved in CHCl₃ and allowed to slowly evaporate at room temperature. Single crystals in the form of colorless plates suitable for single crystal X-ray diffraction formed within seven days.

Figure 7 Synthesis of 3,3′-bpe via Pd-catalyzed Hiyama–Heck cross-coupling.

6. Refinement

Crystal data, data collection and structure refinement details for 3,3′-bpe are summarized in Table 2. All non-hydrogen atoms were refined anisotropically. Hydrogen atoms were located in the difference-Fourier map and freely refined with 0.93 < C—H < 0.99 Å. Refinement of the hydrogen atoms led to a data-to-parameter ratio of ∼10. The single-crystal data were collected at room temperature to best reflect conditions under which photochemical reactions are typically conducted. Room-temperature data can also lead to fewer reflections and/or scaling anomalies.

Table 2
Experimental details

Crystal data
Chemical formula	C12H10N2
Mr	182.22
Crystal system, space group	Monoclinic, P21/n
Temperature (K)	296
a, b, c (Å)	7.4591 (7), 5.5045 (6), 11.7803 (12)
β (°)	99.638 (5)
V (Å^3)	476.86 (8)
Z	2
Radiation type	Mo Kα
μ (mm^−1)	0.08
Crystal size (mm)	0.18 × 0.12 × 0.06
Data collection
Diffractometer	Bruker Nonius KappaCCD
Absorption correction	Multi-scan (SADABS; Krause et al., 2015)
Tmin, Tmax	0.989, 0.995
No. of measured, independent and observed [I > 2σ(I)] reflections	2410, 836, 587
Rint	0.034
Refinement
R[F^2 > 2σ(F^2)], wR(F^2), S	0.050, 0.137, 1.07
No. of reflections	836
No. of parameters	84
H-atom treatment	All H-atom parameters refined
Δρmax, Δρmin (e Å^−3)	0.13, −0.16
Computer programs: COLLECT (Nonius, 1988), HKL DENZO and SCALEPACK (Otwinowski & Minor, 1997), SHELXT (Sheldrick, 2015a), SHELXL (Sheldrick, 2015b) and OLEX2 (Dolomanov et al., 2009).

Supporting information

Computing details

Data collection: HKL SCALEPACK (Otwinowski & Minor, 1997); cell refinement: COLLECT (Nonius, 1998); data reduction: HKL DENZO and SCALEPACK (Otwinowski & Minor, 1997); program(s) used to solve structure: ShelXT (Sheldrick, 2015a); program(s) used to refine structure: SHELXL (Sheldrick, 2015b); molecular graphics: OLEX2 (Dolomanov et al., 2009); software used to prepare material for publication: OLEX2 (Dolomanov et al., 2009).

trans-1,2-Bis(pyridin-3-yl)ethene

Crystal data

C12H10N2	F(000) = 192
Mr = 182.22	Dx = 1.269 Mg m^−3
Monoclinic, P21/n	Mo Kα radiation, λ = 0.71073 Å
a = 7.4591 (7) Å	Cell parameters from 1169 reflections
b = 5.5045 (6) Å	θ = 1.0–26.7°
c = 11.7803 (12) Å	µ = 0.08 mm^−1
β = 99.638 (5)°	T = 296 K
V = 476.86 (8) Å^3	Plate, colourless
Z = 2	0.18 × 0.12 × 0.06 mm

Data collection

Bruker Nonius KappaCCD diffractometer	587 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.034
CCD phi and ω scans	θmax = 25.0°, θmin = 3.0°
Absorption correction: multi-scan (SADABS; Krause et al., 2015)	h = −8→8
Tmin = 0.989, Tmax = 0.995	k = −6→6
2410 measured reflections	l = −13→13
836 independent reflections

Refinement

Refinement on F^2	Primary atom site location: dual
Least-squares matrix: full	Hydrogen site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.050	All H-atom parameters refined
wR(F^2) = 0.137	w = 1/[σ^2(Fo^2) + (0.0703P)^2 + 0.056P] where P = (Fo^2 + 2Fc^2)/3
S = 1.07	(Δ/σ)max < 0.001
836 reflections	Δρmax = 0.13 e Å^−3
84 parameters	Δρmin = −0.16 e Å^−3
0 restraints

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 (Ų)

	x	y	z	Uiso*/Ueq
N1	0.5400 (2)	0.7577 (3)	0.63093 (15)	0.0609 (6)
C2	0.2479 (2)	0.5639 (3)	0.57302 (15)	0.0459 (5)
C3	0.3752 (3)	0.7464 (4)	0.56601 (17)	0.0537 (6)
C6	0.2998 (3)	0.3821 (4)	0.65272 (17)	0.0529 (6)
C4	0.5835 (3)	0.5788 (4)	0.70678 (19)	0.0564 (6)
C1	0.0695 (3)	0.5737 (4)	0.49890 (16)	0.0509 (6)
C5	0.4688 (3)	0.3894 (4)	0.71993 (19)	0.0556 (6)
H4	0.705 (3)	0.590 (3)	0.7528 (19)	0.062 (6)*
H3	0.345 (3)	0.875 (4)	0.507 (2)	0.068 (6)*
H5	0.504 (3)	0.265 (4)	0.7803 (18)	0.063 (6)*
H6	0.215 (3)	0.250 (4)	0.6607 (17)	0.066 (6)*
H1	0.051 (3)	0.706 (4)	0.4498 (19)	0.071 (7)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
N1	0.0566 (11)	0.0569 (11)	0.0674 (11)	−0.0077 (8)	0.0048 (9)	0.0012 (9)
C2	0.0493 (11)	0.0476 (11)	0.0416 (10)	−0.0010 (9)	0.0103 (8)	−0.0024 (8)
C3	0.0562 (13)	0.0522 (13)	0.0519 (12)	−0.0045 (9)	0.0068 (10)	0.0027 (10)
C6	0.0491 (12)	0.0522 (13)	0.0585 (13)	−0.0029 (9)	0.0120 (10)	0.0048 (10)
C4	0.0465 (12)	0.0671 (14)	0.0551 (12)	0.0010 (10)	0.0069 (10)	−0.0019 (11)
C1	0.0553 (12)	0.0526 (12)	0.0448 (11)	−0.0034 (8)	0.0085 (9)	0.0020 (10)
C5	0.0517 (12)	0.0591 (13)	0.0570 (12)	0.0067 (9)	0.0117 (10)	0.0095 (10)

Geometric parameters (Å, º)

N1—C3	1.336 (3)	C6—H6	0.98 (2)
N1—C4	1.333 (3)	C4—C5	1.374 (3)
C2—C3	1.395 (3)	C4—H4	0.98 (2)
C2—C6	1.382 (3)	C1—C1^i	1.320 (4)
C2—C1	1.465 (3)	C1—H1	0.93 (2)
C3—H3	0.99 (2)	C5—H5	0.99 (2)
C6—C5	1.372 (3)
C4—N1—C3	116.54 (18)	N1—C4—C5	123.4 (2)
C3—C2—C1	119.85 (19)	N1—C4—H4	115.2 (11)
C6—C2—C3	116.44 (19)	C5—C4—H4	121.4 (11)
C6—C2—C1	123.71 (18)	C2—C1—H1	115.3 (13)
N1—C3—C2	124.8 (2)	C1^i—C1—C2	127.1 (3)
N1—C3—H3	116.6 (12)	C1^i—C1—H1	117.4 (13)
C2—C3—H3	118.6 (12)	C6—C5—C4	119.1 (2)
C2—C6—H6	119.4 (12)	C6—C5—H5	120.0 (11)
C5—C6—C2	119.80 (19)	C4—C5—H5	120.8 (11)
C5—C6—H6	120.8 (12)
N1—C4—C5—C6	−0.5 (3)	C6—C2—C3—N1	−0.7 (3)
C2—C6—C5—C4	0.2 (3)	C6—C2—C1—C1^i	4.7 (4)
C3—N1—C4—C5	0.1 (3)	C4—N1—C3—C2	0.5 (3)
C3—C2—C6—C5	0.3 (3)	C1—C2—C3—N1	178.62 (17)
C3—C2—C1—C1^i	−174.6 (2)	C1—C2—C6—C5	−178.96 (17)

Symmetry code: (i) −x, −y+1, −z+1.