2,6-Di­iodo­pyridine

The title compound, C₅H₃I₂N, crystallizes in the polar space group Fmm2, with crystallographic mm2 symmetry imposed on the mol­ecule. Mol­ecules are linked through C—H...N hydrogen bonding to form chains which are, in turn, joined through weak I...I halogen-bonding interactions to form layers. The pyridine ring lies parallel to the polar z axis and has the N atom pointing in the +z direction. The layers stack in a polar fashion normal to the a axis and the absolute structure has been determined.

S1. Comment

Haloaromatic compounds are important for synthetic purposes, especially cross-coupling reactions with organometallic reagents (Li & Gribble, 2000; Finet, 1998). Iodoaromatics typically display increased reactivity compared with their bromo and chloro counterparts. Disubstituted iodoheteroaromatic compounds are useful for the preparation of multifunctional ligands, but few are available due to the difficulty of their formation (Smith & Ho, 1990; Yang et al., 1986).

This paper reports the preparation and crystal structure of 2,6-diiodopyridine, (I), which will be used as a more reactive base unit in an alternate synthesis of acetylene-expanded tridentate ligands (Holmes et al., 2002). This appears to be only the second reported structure analysis of an iodopyridine, and the first of a diiodo derivative. Similar to the first example, viz. 4-iodopyridine (Ahrens & Jones, 1999), (I) crystallizes in polar space group Fmm2 (cf. Fdd2 for 4-iodopyridine).

The molecule of (I) lies on a crystallographic mm2 site (Fig. 1), with the molecular plane coincident with one mirror and the molecule bisected by the other. Atoms N1 and C3 lie on the twofold rotational axis at the intersection of the mirrors.

The geometric parameters for (I) are somewhat distorted relative to those found in both pyridine (Mootz & Wussow, 1981) and 4-iodopyridine, with expansion of the ring angle at atoms C1 and C3 and contraction at atoms N1 and C2. This could be due to steric and electronic factors associated with the large electron-dense substituents, but may also be the result of low precision in the positional parameters of the light atoms (which comprise only 27% of the scattering power of the unit cell).

The molecules of (I) form linear chains along the c axis through C—H···N hydrogen-bonding interactions (Table 2). These chains are linked through weak halogen bonding between I atoms (Walsh et al., 2001; Bailey et al., 2000; Bosch & Barnes, 2002) to form planar layers normal to the a axis (Fig. 2). The I···I distance (Table 1) is at the limit of the van der Waals contact distance, which has been estimated to be 4.30 Å by Pauling (1960), 3.96 Å by Bondi (1964) and 4.00 Å by Rowland & Taylor (1996), but the angular orientation of the interactions fits the standard description of a type II interaction quite well (Desiraju & Parthasarathy, 1989). Each I atom undergoes two close contacts with neighboring I atoms within the layer, with one being nearly linear with respect to the C1—I1 bond and the other being closer to a tetrahedral angle (Table 1). The layers are parallel to each other, with all of the N atoms aligned in the same absolute direction. Alternating layers are staggered by one half translation along the c axis. The absolute direction of the polar axis has been determined successfully.

S2. Experimental

2,6-Diiodopyridine was synthesized by reacting commercially available 2,6-dibromopyridine with six equivalents of ethylmagnesium bromide in tetrahydrofuran. The resulting pyridyl dimagnesium bromide was quenched with iodine to afford the desired white solid in 42% yield following purification by chromatography on silica (hexane/dichloromethane) and recrystallization (hexane/chloroform). ¹H NMR (300 MHz, δ, CDCl₃, p.p.m.): 7.67 (d, J = 7.88 Hz, 2H), 6.93 (t, J = 7.75 Hz, 1H); ¹³C NMR (75 MHz, δ, CDCl₃, p.p.m.): 138.36 (CH), 134.19 (CH), 116.21. Diffraction-quality crystals of 2,6-diiodopyridine were obtained by slow evaporation of a xylene solution at room temperature.

Computing details

Data collection: CrystalClear (Rigaku/MSC, 2001); cell refinement: CrystalClear; data reduction: CrystalClear; program(s) used to solve structure: SHELXTL-Plus (Sheldrick, 2000); program(s) used to refine structure: SHELXTL-Plus; molecular graphics: SHELXTL-Plus; software used to prepare material for publication: SHELXTL-Plus.

Figures

	Fig. 1. The molecular structure of (I). Displacement ellipsoids are shown with 50% probability and H atoms are of arbitrary radii. [Symmetry code: (i) -x, 1 - y, z.]
	Fig. 2. The molecular packing, viewed down the a axis. Atoms are shown with arbitrary radii. Filled dashed lines represent C—H···N hydrogen bonds, while single dashed lines represent I···I halogen-bonding contacts.

2,6-diiodopyridine

Crystal data

C5H3I2N	Dx = 3.065 Mg m^−3
Mr = 330.88	Melting point: 186 K
Orthorhombic, Fmm2	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: F 2 -2	Cell parameters from 1322 reflections
a = 6.8039 (14) Å	θ = 4.6–26.4°
b = 17.011 (3) Å	µ = 8.67 mm^−1
c = 6.1959 (12) Å	T = 148 K
V = 717.1 (2) Å^3	Prism, colorless
Z = 4	0.17 × 0.05 × 0.05 mm
F(000) = 584

Data collection

Mercury AFC-8S diffractometer	406 independent reflections
Radiation source: fine-focus sealed tube	405 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.032
ω scans	θmax = 26.3°, θmin = 4.6°
Absorption correction: multi-scan (REQAB; Jacobson, 1998)	h = −8→8
Tmin = 0.300, Tmax = 0.660	k = −20→20
1721 measured reflections	l = −7→7

Refinement

Refinement on F^2	Secondary atom site location: difference Fourier map
Least-squares matrix: full	Hydrogen site location: inferred from neighbouring sites
R[F^2 > 2σ(F^2)] = 0.020	H-atom parameters constrained
wR(F^2) = 0.050	w = 1/[σ^2(Fo^2) + (0.0341P)^2] where P = (Fo^2 + 2Fc^2)/3
S = 1.11	(Δ/σ)max < 0.001
406 reflections	Δρmax = 0.67 e Å^−3
27 parameters	Δρmin = −0.58 e Å^−3
1 restraint	Absolute structure: Flack (1983), 180 Friedel pairs
Primary atom site location: structure-invariant direct methods	Absolute structure parameter: 0.00 (8)

Special details

Experimental. REQAB (Jacobson, 1998)

Geometry. All e.s.d.'s (except the e.s.d. in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell e.s.d.'s are taken into account individually in the estimation of e.s.d.'s in distances, angles and torsion angles; correlations between e.s.d.'s in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell e.s.d.'s is used for estimating e.s.d.'s involving l.s. planes.

Fractional atomic coordinates and isotropic or equivalent isotropic displacement parameters (Ų)

	x	y	z	Uiso*/Ueq
I1	0.0000	0.329108 (17)	0.3552	0.03003 (16)
N1	0.0000	0.5000	0.2865 (12)	0.0204 (13)
C1	0.0000	0.4342 (3)	0.1738 (11)	0.0226 (11)
C2	0.0000	0.4290 (4)	−0.0497 (11)	0.0269 (11)
H2A	0.0000	0.3810	−0.1215	0.032*
C3	0.0000	0.5000	−0.158 (6)	0.017 (3)
H3A	0.0000	0.5000	−0.3086	0.021*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
I1	0.0407 (2)	0.0160 (2)	0.0334 (2)	0.000	0.000	0.0018 (4)
N1	0.024 (3)	0.015 (3)	0.022 (3)	0.000	0.000	0.000
C1	0.023 (2)	0.018 (3)	0.027 (3)	0.000	0.000	0.005 (2)
C2	0.034 (3)	0.023 (3)	0.024 (2)	0.000	0.000	−0.004 (2)
C3	0.014 (2)	0.023 (3)	0.014 (7)	0.000	0.000	0.000

Geometric parameters (Å, º)

I1—C1	2.112 (5)	C2—C3	1.383 (19)
I1—I1^i	4.1038 (7)	C2—H2A	0.930
N1—C1	1.319 (7)	C3—H3A	0.933
C1—C2	1.388 (8)
C1—I1—I1^i	98.84 (17)	C3—C2—C1	115.5 (14)
C1—I1—I1^ii	163.13 (18)	C2^iii—C3—C2	122 (3)
I1^i—I1—I1^ii	98.03 (2)	C1—C2—H2A	122.24
C1—N1—C1^iii	116.1 (7)	C3—C2—H2A	122.36
N1—C1—C2	125.7 (5)	C2—C3—H3A	119.06
N1—C1—I1	115.9 (4)	C2—C3—H3A	119.06
C2—C1—I1	118.5 (4)

Symmetry codes: (i) −x, −y+1/2, z−1/2; (ii) −x, −y+1/2, z+1/2; (iii) −x, −y+1, z.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
C3—H3A···N1^iv	0.93	2.51	3.44 (4)	180

Symmetry code: (iv) x, y, z−1.