Comment

2-Aminopyridine is used in the manufacture of pharmaceuticals, especially antihistaminic drugs (Windholz, 1976). As part of our investigation of the reactions of 2-aminopyridine with metals, we report here the crystal structure of the title compound, (I).

The asymmetric unit of (I) contains a 2-aminopyridinium cation and half of a [CoCl₄]^2- anion. The Co atom lies on a crystallographic twofold rotation axis. Protonation of atom N1 of the 2-aminopyridine results in the widening of the C2-N1-C6 angle to 122.7 (2)°. This compares with 117.7 (1)° in neutral 2-aminopyridine (Chao et al., 1975). The bond lengths and angles in (I) are comparable to those in other 2-aminopyridinium complexes (Bis & Zaworotko, 2005; Smith et al., 2000; Jebas & Balasubramanian, 2006). The pyridinium ring deviates somewhat from planarity, with a maximum deviation from the mean plane of 0.026 (2) Å for atom C6.

The anion exhibits tetrahedral geometry, with the Co^II ion surrounded by four Cl atoms, with Cl-Co-Cl angles ranging from 109.85 (4) to 115.98 (3)°. The mean Co-Cl bond length, 2.27 (7) Å, is close to those observed in similar complexes (Zhang et al., 2005).

There are N-HCl hydrogen-bonding interactions between the cations and the anions (Table 2).

Figure 1 The structure of (I), showing the atom-numbering scheme, with 50% probability displacement ellipsoids. The suffix a indicates the symmetry position (-x, y, - z).

Experimental

Solutions of 2-aminopyridine and CoCl₂·2H₂O in water were mixed in a 1:1 molar ratio and heated at 363 K for 2 h. Blue crystals of (I) were obtained by slow evaporation over a period of one week.

Crystal data

(C5H7N2)2[CoCl4] Mr = 390.98 Monoclinic, C 2/c a = 8.2152 (3) Å b = 14.0713 (5) Å c = 13.5731 (5) Å = 95.190 (2)° V = 1562.52 (10) Å3 Z = 4 Dx = 1.662 Mg m-3 Mo K radiation = 1.77 mm-1 T = 120 (2) K Block, blue 0.4 × 0.25 × 0.2 mm

Data collection

Bruker-Nonius FR591 rotating-anode diffractometer and scans Absorption correction: multi-scan SADABS (Sheldrick, 2003) Tmin = 0.595, Tmax = 0.701 8864 measured reflections 1801 independent reflections 1488 reflections with I > 2(I) Rint = 0.032 max = 27.5° 3 standard reflections every 60 reflections intensity decay: none

Refinement

Refinement on F2 R[F2 > 2(F2)] = 0.028 wR(F2) = 0.119 S = 1.26 1801 reflections 87 parameters H-atom parameters constrained w = 1/[2(Fo2) + (0.0653P)2 + 0.2962P] where P = (Fo2 + 2Fc2)/3 (/)max < 0.001 max = 0.57 e Å-3 min = -0.67 e Å-3

Table 1 Selected geometric parameters (Å, °) Co-Cl2 2.2724 (7) Co-Cl1 2.2755 (7) C2-N1-C6 122.7 (2) Cl1-Co-Cl1i 109.37 (4) Cl2-Co-Cl2i 109.85 (4) Cl2-Co-Cl1 115.98 (3) Symmetry code: (i) .

Table 2 Hydrogen-bond geometry (Å, °) D-HA D-H HA DA D-HA N7-H2ACl2ii 0.86 2.42 3.258 (2) 165 N7-H2BCl1iii 0.86 2.44 3.286 (2) 169 N1-H1Cl1iv 0.86 2.58 3.275 (2) 139 Symmetry codes: (ii) ; (iii) ; (iv) .

H atoms were placed in calculated positions, with C-H = 0.93 Å and N-H = 0.86 Å, and refined using a riding model, with U_iso(H) = 1.2U_eq(C,N).

Data collection: COLLECT (Nonius, 1998); cell refinement: DENZO (Otwinowski & Minor, 1997) and COLLECT; data reduction: DENZO and COLLECT; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: PLATON (Spek, 2003); software used to prepare material for publication: SHELXL97 (Sheldrick, 1997).