Bis(2-amino-6-methyl­pyridinium) tetra­chloro­zincate(II)

The title compound, (C₆H₉N₂)₂[Zn^IICl₄], consists of two 2-amino-6-methyl­pyridinium (AMP) cations and one [ZnCl₄]²⁻ anion, which are held together by N—H...Cl hydrogen bonds. Bond lengths within the AMP cation indicate that the imine tautomer makes a significant contribution to the structure. The mol­ecules are associated by two different π–π interactions between identical antiparallel AMP cations, with face-to-face distances of 3.627 (4) and 3.342 (3) Å, to form a one-dimensional chain.

S1. Comment

Protonated 2-aminopyridine alway undergoes aminium–iminium tautomerism (see first scheme below) (Inuzuka & Fujimoto, 1986; Inuzuka & Fujimoto, 1990, Ishikawa et al., 2002). Previously, the tautomerism has been evidenced by X-ray diffraction in some 2-amino-X-methylpyridine (2AXMP; X indicates the methyl position) adducts, such as 2 A3MP-maleic acid (2:3; Jin et al., 2002), 2 A6MP-neoabietic acid (1:1; Jin et al., 2000) and 2 A3MP-phthalic acid (2:1; Jin et al., 2001). As a continuation of our research, the title compound, (I), has been synthesized. The structure is discussed here.

There are two crystallographically independent 2-amino-6-methylpyridinium (AMP) cations and a [ZnCl₄]²⁻ anion in the formula unit (Fig. 1). The [ZnCl₄]²⁻ is linked to cation AMP(A) (N1/N2/C1–C6) by N1—H1···Cl2 and N2—H21···Cl2 hydrogen bonds, and to cation AMP(B)(N3/N4/C7–C12) by N3—HN···Cl4 and N4—H41···Cl4 hydrogen bonds (Fig. 1 and Table 2). The hydrogen-bonded rings shown in Fig. 1 could be described by the graph-set motif (Etter, 1990; Grell et al., 2000) R₂¹(6). Atom Cl4 is perfectly coplanar with the AMP(B) ring, with a deviation of 0.007 (5) Å from the best plane, whereas atom Cl2 lies 0.337 (5) Å from the plane of the AMP(A) ring. In the formula unit, the dihedral angle between the planes of the two AMP cations is 95.53 (8)°.

The geometric features of both ammonium and iminium tautomers are obvious in a compound in which coordinated and protonated 2-aminopyridine coexists (Luque et al., 1997). In contrast, features of the iminium tautomer are clearly observed in (I), suggesting that the imime tautomer makes a greater contribution to the structure. In cation AMP(A), the N2—C1 bond [1.326 (4) Å] is slightly but significantly shorter than the N1—C1 [1.351 (4) Å] and N1—C5 [1.359 (4) Å] bonds, consistent with the iminium tautomer. Moreover, the existence of the iminium tautomer is supported by the fact that the C1—C2 [1.403 (4) Å] and C3—C4 [1.387 (4) Å] bonds are longer than the C2—C3 [1.347 (4) Å] and C4—C5 [1.357 (4) Å] bonds. Similar features are also observed in cation AMP(B).

Generally, the Zn—Cl bond lengths and Cl—Zn—Cl bond angles in a [ZnCl₄]²⁻ anion are not equal to one another (Ferbinteanu et al., 1998; Kubicki & Szafranski, 1998; Wickleder, 2001; Albrecht, Landee & Turnbull, 2003) but vary with the environment around the Cl atoms. In (I), as atoms Cl2 and Cl4 are involved in stronger and more numerous N—H···Cl hydrogen bonds than atoms Cl1 and Cl3, the Zn—Cl2 and Zn—Cl4 bonds are obviously longer than the Zn—Cl1 and Zn—Cl3 bonds. The mean value of the Zn—Cl bond lengths is 2.2579 (9)%A. The Cl—Zn—Cl angles vary from 106.87 (4) to 112.37 (4)°. Owing to the obvious differences of the Zn—Cl distances and the Cl—Zn—Cl angles, the coordination geometry of the Zn atom could be regarded as a distorted tetrahedron.

It is expected that two AMP cations will repell each other due to the electrostatic force between positive charges. However, in the solid state of (I), there are antiparallel pairs of cations AMP(A) and AMP(A) at (1 − x, 1 − y, 1 − z), as well as pairs of cations AMP(B) and AMP(B) at (1 − x, −y, −z), these pairs being governed by π–π interactions (Sharma et al., 1993; Pedireddi et al., 1996) with face-to-face distances of 3.627 (4) and 3.748 (3) Å, respectively. The π–π interactions can be interpreted as the electrostatic attraction between uneven distributed charge across the π system of the antiparallel AMP cations (Muehldorf et al., 1988).

As shown in Fig. 2, (I) is connected by π–π interactions between antiparallel pairs of AMP cations to form a one-dimensional chain along the [111] direction. Neighboring chains are associated with one another principally by N2—H22···Cl1ⁱ and N4—H42···Cl3ⁱⁱ hydrogen bonding and are interrelated by translation, thus resulting in the building up of the whole crystal structure. In general, the structure is characterized of organic and inorganic layers (Fig. 3). There is a C6—H6B···Cl1 hydrogen bond (Braga et al., 1999; Janiak & Scharmann, 2003) playing a subordinative role in stabilizing the structure (Table 2).

S2. Experimental

ZnCl₂, aqueous HCl and 2-amino-6-methylpyridine in a molar ratio of 1:1:2 were mixed and dissolved in sufficient ethanol by heating to a temperature at which a clear solution resulted. Crystals of (I) were formed by gradual evaporation of ethanol over a period of one week at 293 K, with a yield of 68% based on the pyridine. IR (KBr, cm⁻¹): 3411, 3296, 3192, 3094, 2972, 1656 (versus), 1633 (sh), 1391, 1306, 1171, 790, 726, 566.

S3. Refinement

All H atoms could be found in difference Fourier maps, but they were introduced in calculated positions and allowed to ride on their parent atoms at distances of 0.86 (N—H), 0.93 (C—H aromatic) and 0.96 Å (methyl), with U_iso(H) values of 1.2–1.5 times U_eq of the parent atoms.

Computing details

Data collection: SMART (Bruker, 2000); cell refinement: SMART; data reduction: SAINT (Bruker, 2000); program(s) used to solve structure: SHELXTL (Bruker, 2000); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: ORTEPIII for Windows (Farrugia, 1997) and SHELXTL; software used to prepare material for publication: SHELXTL.

Figures

	Fig. 1. The AMP and [ZnCl4]^2− units with atom labels, showing 30% probability displacement ellipsoids. Hydrogen bonds are illustrated as dashed lines.
	Fig. 2. Part of the crystal structure, showing the infinite chain of π–π interaction along the [111] direction. Hydrogen bonds are indicated by dashed lines.
	Fig. 3. A packing diagram, viewed down along the a axis. Hydrogen bonds are indicated by dashed lines.

Bis(2-amino-6-methylpyridinium) tetrachlorozincate(II)

Crystal data

(C6H9N2)2[ZnCl4]	Z = 2
Mr = 425.49	F(000) = 432.0
Triclinic, P1	Dx = 1.531 Mg m^−3
Hall symbol: -P 1	Mo Kα radiation, λ = 0.71073 Å
a = 7.6642 (8) Å	Cell parameters from 39 reflections
b = 7.9235 (8) Å	θ = 2.8–14.9°
c = 15.6651 (16) Å	µ = 1.91 mm^−1
α = 81.177 (2)°	T = 273 K
β = 79.128 (2)°	Block, colorless
γ = 89.983 (2)°	0.37 × 0.19 × 0.14 mm
V = 922.77 (16) Å^3

Data collection

Bruker Smart Apex CCD area detector diffractometer	3335 independent reflections
Radiation source: fine-focus sealed tube	2791 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.012
ϕ and ω scans	θmax = 25.2°, θmin = 2.6°
Absorption correction: multi-scan (SADABS; Bruker, 2000)	h = −9→8
Tmin = 0.654, Tmax = 0.776	k = −9→9
4948 measured reflections	l = −14→18

Refinement

Refinement on F^2	Primary atom site location: structure-invariant direct methods
Least-squares matrix: full	Secondary atom site location: difference Fourier map
R[F^2 > 2σ(F^2)] = 0.033	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.087	H-atom parameters constrained
S = 1.06	w = 1/[σ^2(Fo^2) + (0.0457P)^2 + 0.1635P] where P = (Fo^2 + 2Fc^2)/3
3272 reflections	(Δ/σ)max = 0.001
192 parameters	Δρmax = 0.40 e Å^−3
0 restraints	Δρmin = −0.29 e Å^−3

Special details

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.

Refinement. Refinement of F² against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F², conventional R-factors R are based on F, with F set to zero for negative F². The threshold expression of F² > σ(F²) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F² are statistically about twice as large as those based on F, and R- factors based on ALL data will be even larger.

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

	x	y	z	Uiso*/Ueq
Cl1	0.62077 (14)	0.03528 (12)	0.34772 (7)	0.0989 (4)
Cl2	1.04628 (10)	0.29249 (13)	0.29792 (6)	0.0781 (3)
Cl3	0.93550 (11)	0.04961 (12)	0.13449 (6)	0.0789 (3)
Cl4	0.67477 (11)	0.42709 (9)	0.18962 (5)	0.0620 (2)
Zn	0.81829 (4)	0.19389 (4)	0.24287 (2)	0.04909 (13)
N1	0.8096 (3)	0.4174 (3)	0.46278 (16)	0.0539 (6)
H1	0.8664	0.3668	0.4216	0.065*
N2	0.8056 (4)	0.6540 (3)	0.35723 (17)	0.0749 (8)
H21	0.8616	0.5979	0.3182	0.090*
H22	0.7771	0.7578	0.3421	0.090*
C1	0.7634 (4)	0.5805 (4)	0.44064 (19)	0.0526 (7)
C2	0.6738 (4)	0.6628 (4)	0.5087 (2)	0.0565 (7)
H2	0.6400	0.7755	0.4968	0.068*
C3	0.6375 (4)	0.5766 (4)	0.5913 (2)	0.0655 (8)
H3A	0.5796	0.6316	0.6365	0.079*
C4	0.6843 (4)	0.4080 (4)	0.6109 (2)	0.0672 (8)
H4	0.6554	0.3502	0.6683	0.081*
C5	0.7724 (4)	0.3280 (4)	0.5456 (2)	0.0606 (8)
C6	0.8322 (5)	0.1475 (4)	0.5559 (3)	0.0832 (10)
H6A	0.7794	0.0853	0.5188	0.125*
H6B	0.7962	0.0948	0.6162	0.125*
H6C	0.9593	0.1466	0.5394	0.125*
N3	0.6018 (3)	0.2936 (3)	0.01523 (16)	0.0529 (6)
H3	0.6432	0.3348	0.0554	0.064*
N4	0.3409 (4)	0.2346 (4)	0.1155 (2)	0.0832 (9)
H41	0.3897	0.2751	0.1535	0.100*
H42	0.2327	0.1960	0.1297	0.100*
C7	0.4326 (4)	0.2317 (3)	0.0348 (2)	0.0552 (7)
C8	0.3651 (4)	0.1688 (4)	−0.0312 (2)	0.0620 (8)
H8	0.2486	0.1264	−0.0206	0.074*
C9	0.4715 (5)	0.1702 (4)	−0.1108 (2)	0.0696 (9)
H9	0.4267	0.1288	−0.1551	0.084*
C10	0.6461 (5)	0.2323 (4)	−0.1277 (2)	0.0683 (8)
H10	0.7177	0.2307	−0.1825	0.082*
C11	0.7114 (4)	0.2948 (4)	−0.0644 (2)	0.0556 (7)
C12	0.8956 (4)	0.3660 (5)	−0.0732 (2)	0.0762 (9)
H12A	0.9646	0.3512	−0.1293	0.114*
H12B	0.9498	0.3070	−0.0271	0.114*
H12C	0.8908	0.4855	−0.0687	0.114*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Cl1	0.0849 (7)	0.0693 (6)	0.1105 (8)	0.0064 (5)	0.0310 (5)	0.0249 (5)
Cl2	0.0510 (4)	0.1244 (7)	0.0711 (5)	0.0065 (4)	−0.0133 (4)	−0.0512 (5)
Cl3	0.0546 (5)	0.0959 (6)	0.0971 (6)	0.0014 (4)	−0.0042 (4)	−0.0614 (5)
Cl4	0.0708 (5)	0.0528 (4)	0.0609 (5)	0.0095 (3)	−0.0109 (4)	−0.0068 (3)
Zn	0.0445 (2)	0.0515 (2)	0.0500 (2)	0.00078 (14)	−0.00280 (14)	−0.01168 (15)
N1	0.0515 (14)	0.0550 (14)	0.0563 (15)	0.0033 (11)	−0.0031 (11)	−0.0208 (12)
N2	0.096 (2)	0.0630 (16)	0.0596 (17)	0.0064 (15)	0.0002 (14)	−0.0100 (13)
C1	0.0506 (16)	0.0547 (16)	0.0544 (17)	−0.0010 (13)	−0.0083 (13)	−0.0170 (14)
C2	0.0553 (17)	0.0560 (16)	0.0625 (19)	0.0052 (13)	−0.0112 (14)	−0.0228 (15)
C3	0.0567 (18)	0.085 (2)	0.061 (2)	0.0088 (16)	−0.0092 (15)	−0.0354 (18)
C4	0.067 (2)	0.083 (2)	0.0510 (18)	0.0000 (17)	−0.0092 (15)	−0.0112 (17)
C5	0.0512 (17)	0.0638 (18)	0.068 (2)	−0.0031 (14)	−0.0151 (15)	−0.0087 (16)
C6	0.080 (2)	0.063 (2)	0.102 (3)	0.0045 (18)	−0.015 (2)	−0.002 (2)
N3	0.0471 (14)	0.0543 (13)	0.0586 (15)	−0.0004 (11)	−0.0104 (11)	−0.0122 (11)
N4	0.0521 (16)	0.100 (2)	0.095 (2)	−0.0093 (15)	0.0129 (15)	−0.0417 (18)
C7	0.0442 (16)	0.0484 (15)	0.072 (2)	0.0054 (12)	−0.0073 (14)	−0.0109 (14)
C8	0.0484 (17)	0.0544 (17)	0.087 (2)	0.0055 (14)	−0.0254 (17)	−0.0074 (16)
C9	0.079 (2)	0.071 (2)	0.066 (2)	0.0025 (17)	−0.0382 (18)	−0.0024 (17)
C10	0.072 (2)	0.083 (2)	0.0477 (17)	0.0007 (17)	−0.0140 (15)	−0.0002 (16)
C11	0.0489 (16)	0.0550 (16)	0.0588 (18)	0.0021 (13)	−0.0084 (14)	0.0014 (14)
C12	0.057 (2)	0.088 (2)	0.076 (2)	−0.0113 (17)	−0.0034 (16)	−0.0011 (19)

Geometric parameters (Å, º)

Cl1—Zn	2.2358 (9)	C6—H6B	0.9600
Cl2—Zn	2.2767 (8)	C6—H6C	0.9600
Cl3—Zn	2.2396 (8)	N3—C7	1.350 (4)
Cl4—Zn	2.2786 (8)	N3—C11	1.365 (4)
N1—C1	1.351 (4)	N3—H3	0.8600
N1—C5	1.359 (4)	N4—C7	1.329 (4)
N1—H1	0.8600	N4—H41	0.8600
N2—C1	1.326 (4)	N4—H42	0.8600
N2—H21	0.8600	C7—C8	1.394 (4)
N2—H22	0.8600	C8—C9	1.354 (5)
C1—C2	1.403 (4)	C8—H8	0.9300
C2—C3	1.347 (4)	C9—C10	1.389 (5)
C2—H2	0.9300	C9—H9	0.9300
C3—C4	1.387 (4)	C10—C11	1.348 (4)
C3—H3A	0.9300	C10—H10	0.9300
C4—C5	1.357 (4)	C11—C12	1.495 (4)
C4—H4	0.9300	C12—H12A	0.9600
C5—C6	1.496 (4)	C12—H12B	0.9600
C6—H6A	0.9600	C12—H12C	0.9600
Cl1—Zn—Cl3	112.41 (4)	C5—C6—H6C	109.5
Cl1—Zn—Cl2	111.75 (4)	H6A—C6—H6C	109.5
Cl3—Zn—Cl2	107.77 (3)	H6B—C6—H6C	109.5
Cl1—Zn—Cl4	107.17 (3)	C7—N3—C11	123.9 (3)
Cl3—Zn—Cl4	110.74 (4)	C7—N3—H3	118.0
Cl2—Zn—Cl4	106.88 (3)	C11—N3—H3	118.0
C1—N1—C5	124.4 (2)	C7—N4—H41	120.0
C1—N1—H1	117.8	C7—N4—H42	120.0
C5—N1—H1	117.8	H41—N4—H42	120.0
C1—N2—H21	120.0	N4—C7—N3	118.1 (3)
C1—N2—H22	120.0	N4—C7—C8	124.2 (3)
H21—N2—H22	120.0	N3—C7—C8	117.8 (3)
N2—C1—N1	118.9 (3)	C9—C8—C7	119.1 (3)
N2—C1—C2	123.9 (3)	C9—C8—H8	120.4
N1—C1—C2	117.2 (3)	C7—C8—H8	120.4
C3—C2—C1	119.1 (3)	C8—C9—C10	121.3 (3)
C3—C2—H2	120.4	C8—C9—H9	119.4
C1—C2—H2	120.4	C10—C9—H9	119.4
C2—C3—C4	121.8 (3)	C11—C10—C9	119.8 (3)
C2—C3—H3A	119.1	C11—C10—H10	120.1
C4—C3—H3A	119.1	C9—C10—H10	120.1
C5—C4—C3	119.4 (3)	C10—C11—N3	118.1 (3)
C5—C4—H4	120.3	C10—C11—C12	125.8 (3)
C3—C4—H4	120.3	N3—C11—C12	116.1 (3)
C4—C5—N1	118.0 (3)	C11—C12—H12A	109.5
C4—C5—C6	125.9 (3)	C11—C12—H12B	109.5
N1—C5—C6	116.1 (3)	H12A—C12—H12B	109.5
C5—C6—H6A	109.5	C11—C12—H12C	109.5
C5—C6—H6B	109.5	H12A—C12—H12C	109.5
H6A—C6—H6B	109.5	H12B—C12—H12C	109.5

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
N1—H1···Cl2	0.86	2.31	3.155 (2)	167
N2—H21···Cl2	0.86	2.83	3.555 (3)	143
N2—H22···Cl1^i	0.86	2.51	3.331 (3)	160
N3—H3···Cl4	0.86	2.38	3.218 (2)	164
N4—H41···Cl4	0.86	2.69	3.448 (3)	148
N4—H42···Cl3^ii	0.86	2.54	3.378 (3)	165
N4—H42···Cl2^ii	0.86	2.96	3.382 (3)	112
C2—H2···Cl1^i	0.93	2.89	3.651 (3)	140
C6—H6B···Cl1^iii	0.96	3.28	3.713 (4)	110
C12—H12B···Cl3	0.96	2.98	3.851 (4)	151
C8—H8···Cl3^iv	0.93	2.97	3.632 (3)	129

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