Three isomorphous 2,6-dibromo­pyridinium tetra­bromidometallates: (C₅H₄Br₂N)₂[MBr₄]·2H₂O (M = Cu, Cd and Hg)

The structures of three isomorphous compounds, namely bis­(2,6-dibromo­pyridinium) tetra­bromidocuprate(II) dihydrate, (C₅H₄Br₂N)₂[CuBr₄]·2H₂O, bis­(2,6-dibromo­pyridinium) tetra­bromidocadmate(II) dihydrate, (C₅H₄Br₂N)₂[CdBr₄]·2H₂O, and bis­(2,6-dibromo­pyridinium) tetra­bromidomercurate(II) dihydrate, (C₅H₄Br₂N)₂[HgBr₄]·2H₂O, show a crystal supra­molecularity represented by M—Br...H—O—H...Br—M inter­molecular inter­actions along with (π)N—H...OH₂ hydrogen-bonding inter­actions forming layers connected via ar­yl–aryl face-to-face stacking of cations, leading to a three-dimensional network. The anions have significantly distorted tetra­hedral geometry and crystallographic C₂ symmetry. The stability of this crystal lattice is evidenced by the crystallization of a whole series of isomorphous compounds.

S1. Comment

Non-covalent interactions play an important role in the organization of structural units in both natural and artificial systems (Desiraju, 1997). The consequences of such interactions may affect the properties of many materials found and utilized in areas such as biology (Hunter, 1994; Desiraju & Steiner, 1999), crystal engineering (Allen et al., 1997; Dolling et al., 2001) and materials science (Panunto et al., 1987; Robinson et al., 2000). The interactions governing the crystal organization are expected to affect the packing and the specific properties of solids.

Organic–inorganic hybrid compounds are of great interest ro researchers because of their special magnetic (Cui et al., 2000), electronic (Lacroix et al., 1994) and optoelectronic properties (Chakravarthy & Guloy, 1997). The influence of the features of the organic cations on the packing interactions that govern the crystal organization is expected to affect the packing and the specific properties of solids. On the other hand, the results of a series of structure analyses and theoretical calculations (Awwadi et al., 2007, and references therein) show the significance of linear C—Br···Br synthons in influencing the structures of crystalline materials, suggesting their use as potential building blocks in crystal engineering via supramolecular synthesis. This inspired our interest in the role of the C—Br···Br—M synthon in the control of the packing of different metal halide anions such as MBr₄²⁻ in crystalline lattices. In continuation of our work (Luque et al., 2001; Haddad et al., 2006; Al-Far & Ali, 2007a,b; Ali & Al-Far, 2007) on complexes containing cationic pyridine derivatives with bromo-metal anions, herein we describe the crystallization of three isomorphous compounds containing the 2,6-dibromopyridinium cation (denoted 2,6-dbpH), namely bis(2,6-dibromopyridinium) tetrabromidocuprate(II) dihydrate, (I), bis(2,6-dibromopyridinium) tetrabromidocadmate(II) dihydrate, (II), and bis(2,6-dibromopyridinium) tetrabromidomercurate(II) dihydrate, (III), along with their crystal packing and crystal supramolecularity analysis. Comparison of packing forces as related to metal halide distortions, [MBr₄]²⁻ (M = Cu^II, Cd^II and Hg^II), with different electronic configurations, is of interest to us. Attempts to make other isomorphous salts of the above failed. The reaction of two equivalents of 2,6-dibromopyridine with one equivalent of the corresponding M^II salt in the presence of excess aqueous HBr gave compounds (I), (II) and (III) in 90%, 84% and 82% yield, respectively. The introduction of bromo groups at the 2- and 6- positions increases the basicity at the ring N atom (Al-Far & Ali, 2007a). Therefore, the resulting protonated 2,6-dibromopyridine was expected to create many important centres of interaction with the bromo-metal anions, e.g. N—H···Br, (π)C—H···Br and possibly aryl···aryl stacking.

The title compounds are isomorphous and crystallize in the orthorhombic space group Pccn. The asymmetric unit consists of one cation, one half anion, which lies across a crystallographic C₂ axis, and one water molecule (Fig. 1). The anions all have a significantly distorted tetrahedral geometry. The unique M—Br distances are 2.3770 (8) and 2.3797 (8), 2.5803 (5) and 2.5947 (5), and 2.5928 (7) and 2.6215 (7) Å for (I), (II) and (III), respectively. The Br—Cu—Br angles are in the ranges 99.24 (2)–128.09 (2), 102.90 (3)–117.970 (16) and 102.03 (3)–118.19 (2)°, [for (I), (II) and (III)?], respectively. These distances and angles are in accordance with previously reported values for corresponding [MBr₄]²⁻-containing complexes (Coffey et al., 2000; Al-Far & Ali, 2008; Ali et al., 2006). The bond distances and angles in the planar cations in each structure are in the normal range (Allen et al., 1987).

The MBr₄^2– anions and water molecules, that act as bridging units between the anions, form cooperative infinite chains parallel to the crystallographic c axis (Fig. 2). The anions···water chains are held up through M—Br···H—O—H···Br—M intermolecular interactions (Fig. 2, Tables 1, 2 and 3). These chains are further connected to the cations, leading to 'ribbons' (Fig. 2), the connecting unit also being H₂O molecules via short (π)N—H···OH₂ intermolecular hydrogen bonds. Within the ribbons, Br···Br halogen bonding plays a significant and complementary role in bringing all these interacting moieties together (Fig. 2). This type of interaction results from C—Br···Br—M contacts in which the Br···Br distances (Table 4) are in the range 3.3928 (7)–3.5744 (9) Å which is significantly less than the sum of the van der Waals radii (3.7 Å). It is worth mentioning that halide···halide interactions of the type M—Br···Br—M are absent since the shortest contact in the series [4.3394 (10) Å in (III)] is much larger than the sum of the commonly accepted van der Waals radii.

The discussed ribbons, in turn, interact with neighbouring ones via aryl···aryl face-to-face interactions (π···π stacking) between almost parallel oppositely oriented pyridinium cations, giving rise to layers parallel to the ac plane (Fig. 3a). The distances between the centroids (Cg) of adjacent rings are 3.53 (3), 3.601 (3) and 3.59 (8) Å for Cg (x, y, z)···Cg (1/2 − x, 1/2 − y, z) in (I), (II) and (III), respectively. The angles between the centroid ···centroid line and the perpendicular distance line between planes are calculated to be 2.3, 7.7 and 7.6° in (I), (II) and (III), respectively.

Intermolecular interactions result in two distinguishable regions in the lattice (Fig.3b). One is hydrophobic, which represents the cation layers that interact via offset face-to-face π···π stacking interactions. The other is the hydrophilic region, which represents the zone where C—Br···Br—M, H—O—H···Br—M and N—H···OH₂ interactions are assembled.

S2. Experimental

All compounds were synthesized by dissolving 2,6-dibromopyridine (2 mmol) in 95% EtOH (10 ml, with warming) and an additional 2 ml of HBr (60%). The solution was added slowly with constant stirring to a warm solution of M^II salts, CuBr₂,CdBr₂ and HgCl₂ (1 mmol), dissolved in EtOH (10 ml). The resulting mixture was refluxed for 2 h, cooled undisturbed at room temperature and allowed to evaporate slowly until crystals appeared (generally in a few days). The products were as follows: (2,6-dbpH)₂[CuBr₄]·2H₂O, (I), brown crystals, yield 90%; (2,6-dbpH)₂[CdBr₄]·2H₂O, (II), colourless parallelepiped crystals, yield 84%; (2,6-dbpH)₂[HgBr₄]·2H₂O,(III), colourless crystals, yield 82%.

S3. Refinement

H atoms were positioned geometrically, with NH = 0.86 Å and CH = 0.93 Å, and constrained to ride on their parent atoms, with U_iso(H) = 1.2[U?]_eq(C, N). The water H atoms were located first in a difference Fourier map, then refined with restraints [O—H: 0.88 (1) Å] and finally allowed to ride. There is some disorder in the water molecules in all three structures, reflected in the thermal ellipsoids for the water O atoms being quite elongated in a direction perpendicular to the molecular plane, particularly in (I). However, refinement of the water O atoms over several locations with partial occupancy did not improve the picture for what [and?] a single site model with a large displacement factor was preferred.

Computing details

For all compounds, data collection: SMART (Bruker, 2001); cell refinement: SAINTPlus (Bruker, 2001); data reduction: SAINTPlus (Bruker, 2001); program(s) used to solve structure: XS, SHELXTL (Sheldrick, 2008); program(s) used to refine structure: XL, SHELXTL (Sheldrick, 2008); molecular graphics: XP, SHELXTL (Sheldrick, 2008); software used to prepare material for publication: XCIF, SHELXTL (Sheldrick, 2008).

Figures

	Fig. 1. The molecular structure of (I), with the atom-labelling scheme. Thermal ellipsoids are drawn at the 50% probability level. The same view and atom-numbering scheme were applied to the other compounds. Symmetry code: (i) 3/2 − x, 1/2 − y, z.
	Fig. 2. A packing view of (I), projected down b, showing the ribbons parallel to c resulting from the hydrogen-bonding interactions (dotted lines) within the chains of anions and water molecules (in black) and with the cations (in grey). The Br···Br interactions between anions and cations are also shown by dashed lines.
	Fig. 3. (a) The overall packing diagram of (I), showing a layered arrangement of cations and anions (cations in grey). (b) Two layers are shown, where the cations from each are further connected to each other parallel to the a axis via an aryl···aryl face-to-face motif (π···π stacking) (viewed down the crystallographic c axis). One layer is shown in black and the other in grey. Intermolecular interactions are shown as dotted solid lines.

(I) bis(2,6-dibromopyridinium) tetrabromidocuprate(II) dihydrate

Crystal data

(C5H4Br2N)2[CuBr4]·2H2O	F(000) = 1644
Mr = 894.97	Dx = 2.729 Mg m^−3
Orthorhombic, Pccn	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ab 2ac	Cell parameters from 4142 reflections
a = 10.2861 (7) Å	θ = 2.5–26.9°
b = 13.4443 (9) Å	µ = 15.68 mm^−1
c = 15.7523 (11) Å	T = 296 K
V = 2178.4 (3) Å^3	Fragment, brown
Z = 4	0.22 × 0.16 × 0.12 mm

Data collection

Bruker/Siemens SMART APEX diffractometer	1921 independent reflections
Radiation source: fine-focus sealed tube	1426 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.067
Detector resolution: 8.3 pixels mm^-1	θmax = 25.0°, θmin = 2.6°
ω scans	h = −12→12
Absorption correction: multi-scan (SADABS; Bruker, 2001)	k = −15→15
Tmin = 0.130, Tmax = 0.255	l = −18→18
19097 measured reflections

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)] = 0.033	H-atom parameters constrained
wR(F^2) = 0.088	w = 1/[σ^2(Fo^2) + (0.0342P)^2 + 3.8947P] where P = (Fo^2 + 2Fc^2)/3
S = 1.03	(Δ/σ)max < 0.001
1921 reflections	Δρmax = 1.32 e Å^−3
106 parameters	Δρmin = −0.49 e Å^−3
0 restraints	Extinction correction: SHELXL, Fc^*^=kFc[1+0.001xFc^2^λ^3^/sin(2θ)]^-1/4^
Primary atom site location: structure-invariant direct methods	Extinction coefficient: 0.00064 (8)

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
N1	0.3876 (4)	0.1195 (3)	0.1609 (3)	0.0339 (11)
H1	0.4712	0.1205	0.1603	0.041*
Br1	0.64522 (6)	0.36123 (5)	0.31699 (4)	0.0454 (2)
Cu1	0.7500	0.2500	0.41253 (6)	0.0347 (3)
O1	0.6405 (4)	0.1167 (5)	0.1624 (3)	0.0748 (18)
H1'	0.6956	0.1393	0.2009	0.112*
H2'	0.6796	0.0836	0.1210	0.112*
C2	0.3226 (6)	0.1206 (4)	0.0875 (4)	0.0347 (13)
Br2	0.84739 (6)	0.36789 (5)	0.50528 (4)	0.0487 (2)
C3	0.1898 (6)	0.1195 (4)	0.0853 (4)	0.0431 (15)
H3	0.1457	0.1187	0.0338	0.052*
Br3	0.42358 (7)	0.12737 (5)	−0.01139 (4)	0.0525 (2)
C4	0.1206 (7)	0.1196 (4)	0.1632 (4)	0.0478 (16)
H4	0.0302	0.1219	0.1643	0.057*
Br4	0.42840 (7)	0.11425 (5)	0.33258 (4)	0.0520 (2)
C5	0.1922 (6)	0.1161 (4)	0.2373 (4)	0.0431 (15)
H5	0.1496	0.1132	0.2894	0.052*
C6	0.3256 (6)	0.1169 (4)	0.2349 (4)	0.0390 (14)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
N1	0.031 (2)	0.034 (3)	0.036 (3)	0.0022 (19)	−0.001 (2)	0.002 (2)
Br1	0.0400 (4)	0.0507 (4)	0.0455 (4)	0.0069 (3)	−0.0023 (3)	0.0129 (3)
Cu1	0.0364 (5)	0.0396 (5)	0.0280 (5)	0.0025 (4)	0.000	0.000
O1	0.026 (2)	0.168 (6)	0.030 (3)	−0.007 (3)	−0.0011 (19)	−0.006 (3)
C2	0.040 (3)	0.029 (3)	0.035 (3)	−0.004 (3)	−0.001 (3)	0.000 (2)
Br2	0.0420 (4)	0.0609 (4)	0.0433 (4)	−0.0062 (3)	−0.0016 (3)	−0.0151 (3)
C3	0.036 (3)	0.046 (4)	0.047 (4)	−0.002 (3)	−0.008 (3)	0.003 (3)
Br3	0.0519 (4)	0.0710 (5)	0.0346 (4)	−0.0048 (3)	0.0024 (3)	0.0034 (3)
C4	0.046 (4)	0.041 (4)	0.056 (5)	−0.003 (3)	−0.013 (3)	0.003 (3)
Br4	0.0515 (4)	0.0713 (5)	0.0332 (4)	−0.0048 (3)	−0.0035 (3)	0.0001 (3)
C5	0.040 (3)	0.051 (4)	0.039 (4)	−0.003 (3)	0.012 (3)	−0.001 (3)
C6	0.042 (4)	0.035 (3)	0.040 (4)	−0.004 (3)	0.002 (3)	−0.003 (3)

Geometric parameters (Å, º)

N1—C6	1.329 (7)	C2—Br3	1.875 (6)
N1—C2	1.336 (7)	C3—C4	1.419 (9)
N1—H1	0.8600	C3—H3	0.9300
Cu1—Br1	2.3797 (8)	C4—C5	1.382 (8)
Cu1—Br2	2.3770 (8)	C4—H4	0.9300
O1—H1'	0.8837	Br4—C6	1.867 (6)
O1—H2'	0.8866	C5—C6	1.372 (8)
C2—C3	1.366 (9)	C5—H5	0.9300
C6—N1—C2	121.2 (5)	C2—C3—C4	118.7 (6)
C6—N1—H1	119.4	C2—C3—H3	120.7
C2—N1—H1	119.4	C4—C3—H3	120.7
Br2—Cu1—Br2^i	104.15 (5)	C5—C4—C3	117.6 (6)
Br2—Cu1—Br1^i	128.09 (2)	C5—C4—H4	121.2
Br2^i—Cu1—Br1^i	99.24 (2)	C3—C4—H4	121.2
Br2—Cu1—Br1	99.24 (2)	C6—C5—C4	120.6 (6)
Br2^i—Cu1—Br1	128.09 (2)	C6—C5—H5	119.7
Br1^i—Cu1—Br1	101.54 (5)	C4—C5—H5	119.7
H1'—O1—H2'	112.8	N1—C6—C5	120.3 (6)
N1—C2—C3	121.5 (6)	N1—C6—Br4	116.8 (4)
N1—C2—Br3	116.3 (4)	C5—C6—Br4	122.9 (5)
C3—C2—Br3	122.2 (5)
C6—N1—C2—C3	0.3 (8)	C3—C4—C5—C6	2.7 (9)
C6—N1—C2—Br3	178.6 (4)	C2—N1—C6—C5	−0.6 (8)
N1—C2—C3—C4	1.5 (9)	C2—N1—C6—Br4	179.4 (4)
Br3—C2—C3—C4	−176.7 (4)	C4—C5—C6—N1	−1.0 (9)
C2—C3—C4—C5	−3.0 (8)	C4—C5—C6—Br4	179.1 (4)

Symmetry code: (i) −x+3/2, −y+1/2, z.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
N1—H1···O1	0.86	1.74	2.602 (6)	177
O1—H1′···Br1^i	0.88	2.45	3.298 (4)	160
O1—H2′···Br2^ii	0.89	2.59	3.271 (4)	134
C4—H4···Br1^iii	0.93	3.02	3.662 (7)	128
C5—H5···Br1^iii	0.93	3.08	3.703 (6)	126

Symmetry codes: (i) −x+3/2, −y+1/2, z; (ii) x, −y+1/2, z−1/2; (iii) −x+1/2, −y+1/2, z.

(II) bis(2,6-dibromopyridinium) tetrabromidocadmate(II) dihydrate

Crystal data

(C5H4Br2N)2[CdBr4]·2H2O	F(000) = 1720
Mr = 943.83	Dx = 2.788 Mg m^−3
Orthorhombic, Pccn	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ab 2ac	Cell parameters from 5378 reflections
a = 10.6168 (7) Å	θ = 2.4–28.5°
b = 13.5358 (9) Å	µ = 15.19 mm^−1
c = 15.6473 (11) Å	T = 296 K
V = 2248.6 (3) Å^3	Parallepiped, colourless
Z = 4	0.16 × 0.16 × 0.04 mm

Data collection

Bruker/Siemens SMART APEX diffractometer	2034 independent reflections
Radiation source: fine-focus sealed tube	1607 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.068
Detector resolution: 8.3 pixels mm^-1	θmax = 25.2°, θmin = 2.8°
ω scan	h = −12→12
Absorption correction: multi-scan (SADABS; Bruker, 2001)	k = −16→16
Tmin = 0.111, Tmax = 0.551	l = −18→18
20158 measured reflections

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)] = 0.027	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.064	H-atom parameters constrained
S = 1.04	w = 1/[s^2(Fo^2) + (0.0268P)^2] where P = (Fo^2 + 2Fc^2)/3
2034 reflections	(Δ/σ)max = 0.001
105 parameters	Δρmax = 0.43 e Å^−3
0 restraints	Δρmin = −0.39 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
N1	0.3993 (3)	0.1188 (3)	0.1583 (2)	0.0353 (9)
H1	0.4803	0.1203	0.1579	0.042*
Br1	0.61989 (4)	0.35982 (4)	0.30928 (3)	0.04893 (16)
Cd1	0.7500	0.2500	0.41263 (3)	0.03762 (15)
O1	0.6475 (3)	0.1097 (3)	0.1599 (2)	0.0723 (13)
H1'	0.6956	0.1393	0.2009	0.108*
H2'	0.6796	0.0836	0.1210	0.108*
C2	0.3362 (4)	0.1209 (3)	0.0842 (3)	0.0390 (11)
Br2	0.87009 (4)	0.37414 (4)	0.50762 (3)	0.05094 (16)
C3	0.2070 (4)	0.1206 (4)	0.0826 (3)	0.0488 (12)
H3	0.1632	0.1218	0.0311	0.059*
Br3	0.43117 (5)	0.12726 (4)	−0.01609 (3)	0.05361 (16)
C4	0.1440 (4)	0.1183 (4)	0.1602 (3)	0.0503 (13)
H4	0.0564	0.1187	0.1608	0.060*
Br4	0.43658 (5)	0.10859 (4)	0.33169 (3)	0.05349 (16)
C5	0.2083 (4)	0.1155 (4)	0.2357 (3)	0.0467 (12)
H5	0.1655	0.1143	0.2875	0.056*
C6	0.3378 (4)	0.1145 (3)	0.2335 (3)	0.0402 (11)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
N1	0.0289 (18)	0.039 (2)	0.038 (2)	−0.0010 (15)	−0.0020 (15)	0.0024 (16)
Br1	0.0422 (3)	0.0584 (4)	0.0462 (3)	0.0062 (2)	−0.0018 (2)	0.0138 (2)
Cd1	0.0362 (2)	0.0434 (3)	0.0333 (3)	0.0002 (2)	0.000	0.000
O1	0.036 (2)	0.133 (4)	0.049 (2)	−0.0082 (19)	0.0015 (14)	−0.009 (2)
C2	0.041 (3)	0.031 (3)	0.045 (3)	−0.0023 (18)	0.005 (2)	0.006 (2)
Br2	0.0430 (3)	0.0635 (4)	0.0464 (3)	−0.0091 (2)	−0.0004 (2)	−0.0144 (2)
C3	0.043 (3)	0.051 (3)	0.052 (3)	−0.001 (2)	−0.011 (2)	0.003 (2)
Br3	0.0529 (3)	0.0697 (4)	0.0382 (3)	−0.0056 (2)	0.0043 (2)	0.0041 (2)
C4	0.035 (3)	0.060 (4)	0.056 (3)	−0.003 (2)	0.002 (2)	0.003 (3)
Br4	0.0546 (3)	0.0683 (4)	0.0377 (3)	−0.0061 (2)	−0.0046 (2)	0.0006 (2)
C5	0.038 (2)	0.054 (3)	0.049 (3)	0.000 (2)	0.007 (2)	0.001 (2)
C6	0.044 (3)	0.039 (3)	0.038 (3)	0.000 (2)	−0.001 (2)	−0.002 (2)

Geometric parameters (Å, º)

N1—C2	1.339 (5)	C2—C3	1.372 (6)
N1—C6	1.345 (5)	C2—Br3	1.868 (4)
N1—H1	0.8600	C3—C4	1.386 (6)
Br1—Cd1	2.5947 (5)	C3—H3	0.9300
Cd1—Br2	2.5803 (5)	C4—C5	1.365 (6)
Cd1—Br2^i	2.5803 (5)	C4—H4	0.9300
Cd1—Br1^i	2.5947 (5)	Br4—C6	1.862 (4)
O1—H1'	0.9122	C5—C6	1.376 (6)
O1—H2'	0.7827	C5—H5	0.9300
C2—N1—C6	121.0 (4)	C2—C3—C4	117.8 (5)
C2—N1—H1	119.5	C2—C3—H3	121.1
C6—N1—H1	119.5	C4—C3—H3	121.1
Br2—Cd1—Br2^i	109.65 (3)	C5—C4—C3	121.2 (4)
Br2—Cd1—Br1^i	117.970 (16)	C5—C4—H4	119.4
Br2^i—Cd1—Br1^i	104.417 (19)	C3—C4—H4	119.4
Br2—Cd1—Br1	104.417 (19)	C4—C5—C6	118.6 (4)
Br2^i—Cd1—Br1	117.970 (16)	C4—C5—H5	120.7
Br1^i—Cd1—Br1	102.90 (3)	C6—C5—H5	120.7
H1'—O1—H2'	120.1	N1—C6—C5	120.4 (4)
N1—C2—C3	121.1 (4)	N1—C6—Br4	116.7 (3)
N1—C2—Br3	117.3 (3)	C5—C6—Br4	122.9 (3)
C3—C2—Br3	121.6 (4)
C6—N1—C2—C3	1.3 (6)	C3—C4—C5—C6	−0.3 (8)
C6—N1—C2—Br3	179.8 (3)	C2—N1—C6—C5	−2.2 (6)
N1—C2—C3—C4	0.2 (6)	C2—N1—C6—Br4	178.4 (3)
Br3—C2—C3—C4	−178.4 (4)	C4—C5—C6—N1	1.7 (6)
C2—C3—C4—C5	−0.6 (8)	C4—C5—C6—Br4	−178.9 (4)

Symmetry code: (i) −x+3/2, −y+1/2, z.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
N1—H1···O1	0.86	1.78	2.639 (4)	174
O1—H1′···Br1^i	0.91	2.59	3.425 (3)	152
O1—H2′···Br2^ii	0.78	2.75	3.363 (3)	137
C4—H4···Br1^iii	0.93	3.00	3.658 (5)	129
C5—H5···Br1^iii	0.93	3.07	3.685 (4)	125

Symmetry codes: (i) −x+3/2, −y+1/2, z; (ii) x, −y+1/2, z−1/2; (iii) −x+1/2, −y+1/2, z.

(III) bis(2,6-dibromopyridinium) tetrabromidomercurate(II) dihydrate

Crystal data

(C5H4Br2N)2[HgBr4]·2H2O	F(000) = 1848
Mr = 1032.01	Dx = 3.055 Mg m^−3
Orthorhombic, Pccn	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ab 2ac	Cell parameters from 3699 reflections
a = 10.6328 (7) Å	θ = 2.4–28.5°
b = 13.5144 (9) Å	µ = 21.11 mm^−1
c = 15.6141 (11) Å	T = 296 K
V = 2243.7 (3) Å^3	Chunk, colourless
Z = 4	0.22 × 0.20 × 0.14 mm

Data collection

Bruker/Siemens SMART APEX diffractometer	2024 independent reflections
Radiation source: fine-focus sealed tube	1535 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.061
Detector resolution: 8.3 pixels mm^-1	θmax = 25.3°, θmin = 2.4°
ω scans	h = −12→9
Absorption correction: numerical (SADABS; Bruker, 2001)	k = −15→16
Tmin = 0.019, Tmax = 0.051	l = −18→18
13744 measured reflections

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)] = 0.031	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.068	H-atom parameters constrained
S = 1.03	w = 1/[s^2(Fo^2) + (0.0297P)^2] where P = (Fo^2 + 2Fc^2)/3
2024 reflections	(Δ/σ)max < 0.001
105 parameters	Δρmax = 0.94 e Å^−3
0 restraints	Δρmin = −0.68 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
N1	0.3992 (5)	0.1193 (3)	0.1580 (3)	0.0325 (12)
H1	0.4800	0.1206	0.1574	0.039*
Br1	0.61984 (7)	0.36068 (5)	0.30855 (5)	0.0472 (2)
Hg1	0.7500	0.2500	0.41417 (2)	0.04000 (13)
O1	0.6467 (5)	0.1124 (5)	0.1605 (3)	0.0745 (18)
H1'	0.6956	0.1393	0.2009	0.112*
H2'	0.6796	0.0836	0.1210	0.112*
C2	0.3365 (6)	0.1217 (5)	0.0842 (4)	0.0387 (16)
Br2	0.87038 (7)	0.37639 (5)	0.50844 (4)	0.0495 (2)
C3	0.2084 (6)	0.1205 (5)	0.0828 (5)	0.0444 (17)
H3	0.1649	0.1217	0.0311	0.053*
Br3	0.43205 (7)	0.12828 (5)	−0.01581 (4)	0.0520 (2)
C4	0.1448 (7)	0.1175 (5)	0.1594 (5)	0.0510 (19)
H4	0.0573	0.1172	0.1598	0.061*
Br4	0.43698 (7)	0.10954 (6)	0.33229 (4)	0.0515 (2)
C5	0.2098 (6)	0.1150 (5)	0.2359 (5)	0.0455 (17)
H5	0.1670	0.1136	0.2878	0.055*
C6	0.3380 (6)	0.1146 (5)	0.2337 (4)	0.0375 (16)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
N1	0.032 (3)	0.033 (3)	0.033 (3)	−0.002 (2)	−0.002 (2)	−0.001 (2)
Br1	0.0459 (4)	0.0526 (4)	0.0431 (4)	0.0068 (3)	−0.0010 (3)	0.0139 (3)
Hg1	0.0444 (2)	0.0421 (2)	0.0335 (2)	−0.00014 (18)	0.000	0.000
O1	0.040 (3)	0.139 (6)	0.044 (3)	−0.007 (3)	0.000 (2)	−0.001 (3)
C2	0.046 (4)	0.037 (4)	0.034 (4)	−0.008 (3)	0.001 (3)	0.002 (3)
Br2	0.0470 (4)	0.0585 (5)	0.0431 (4)	−0.0088 (3)	−0.0006 (3)	−0.0142 (3)
C3	0.042 (4)	0.048 (4)	0.043 (4)	−0.007 (3)	−0.009 (3)	0.005 (3)
Br3	0.0569 (4)	0.0650 (5)	0.0340 (4)	−0.0051 (4)	0.0045 (3)	0.0041 (3)
C4	0.041 (4)	0.058 (5)	0.055 (5)	0.000 (3)	−0.001 (4)	−0.005 (4)
Br4	0.0573 (5)	0.0633 (5)	0.0338 (4)	−0.0062 (4)	−0.0048 (3)	0.0008 (3)
C5	0.048 (4)	0.045 (4)	0.043 (4)	−0.003 (3)	0.006 (3)	−0.003 (3)
C6	0.048 (4)	0.034 (4)	0.030 (4)	−0.006 (3)	−0.006 (3)	0.001 (3)

Geometric parameters (Å, º)

N1—C6	1.351 (7)	C2—C3	1.363 (10)
N1—C2	1.331 (7)	C2—Br3	1.865 (6)
N1—H1	0.8600	C3—C4	1.375 (9)
Br1—Hg1	2.6215 (7)	C3—H3	0.9300
Hg1—Br2	2.5928 (7)	C4—C5	1.379 (9)
Hg1—Br2^i	2.5928 (7)	C4—H4	0.9300
Hg1—Br1^i	2.6215 (7)	Br4—C6	1.866 (6)
O1—H1'	0.8951	C5—C6	1.364 (9)
O1—H2'	0.8093	C5—H5	0.9300
C6—N1—C2	121.2 (5)	C4—C3—C2	118.6 (6)
C6—N1—H1	119.4	C4—C3—H3	120.7
C2—N1—H1	119.4	C2—C3—H3	120.7
Br2—Hg1—Br2^i	110.82 (3)	C5—C4—C3	120.5 (7)
Br2—Hg1—Br1^i	118.19 (2)	C5—C4—H4	119.8
Br2^i—Hg1—Br1^i	104.00 (3)	C3—C4—H4	119.8
Br2—Hg1—Br1	104.00 (3)	C4—C5—C6	118.7 (6)
Br2^i—Hg1—Br1	118.19 (2)	C4—C5—H5	120.7
Br1^i—Hg1—Br1	102.03 (3)	C6—C5—H5	120.7
H1'—O1—H2'	118.8	N1—C6—C5	120.1 (6)
N1—C2—C3	120.9 (6)	N1—C6—Br4	116.9 (5)
N1—C2—Br3	117.0 (5)	C5—C6—Br4	123.0 (5)
C3—C2—Br3	122.1 (5)
C6—N1—C2—C3	0.6 (9)	C3—C4—C5—C6	−0.6 (11)
C6—N1—C2—Br3	179.9 (4)	C2—N1—C6—C5	−1.7 (9)
N1—C2—C3—C4	0.5 (9)	C2—N1—C6—Br4	179.0 (5)
Br3—C2—C3—C4	−178.8 (5)	C4—C5—C6—N1	1.7 (9)
C2—C3—C4—C5	−0.5 (10)	C4—C5—C6—Br4	−179.1 (5)

Symmetry code: (i) −x+3/2, −y+1/2, z.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
N1—H1···O1	0.86	1.78	2.634 (7)	175
O1—H1′···Br1^i	0.90	2.58	3.412 (5)	154
O1—H2′···Br2^ii	0.81	2.74	3.364 (5)	136
C4—H4···Br1^iii	0.93	3.01	3.664 (7)	129
C5—H5···Br1^iii	0.93	3.09	3.699 (7)	125

Symmetry codes: (i) −x+3/2, −y+1/2, z; (ii) x, −y+1/2, z−1/2; (iii) −x+1/2, −y+1/2, z.