Aquabromo(6-carboxypyridine-2-carboxylato-O,N,O′)mercury(II)

The title compound, [HgBr(C₇H₄NO₄)(H₂O)], was obtained by the reaction of an aqueous solution of mercury(II) bromide and pyridine-2,6-di­carboxylic acid (picolinic acid, dipicH₂). The shortest bond distances to Hg are Hg—Br 2.412 (1) Å and Hg—N 2.208 (5) Å; the corresponding N—Hg—Br angle of 169.6 (1)° corresponds to a slightly distorted linear coordination. There are also four longer Hg—O interactions, three from dipicH⁻ [2.425 (4) and 2.599 (4) Å within the asymmetric unit, and 2.837 (4) Å from a symmetry-related mol­ecule] and one from the bonded water mol­ecule [2.634 (4) Å]. The effective coordination of Hg can thus be described as 2+4. The mol­ecules are connected to form double-layer chains parallel to the y axis by strong O—H...O hydrogen bonds between carboxylic acid groups of neighbouring mol­ecules, and by weaker hydrogen bonds involving both H atoms of the water mol­ecule and the O atoms of the carboxylic acid groups.

S1. Comment

The first discovery of dipicolinic acid in a biological system was reported by Udo (1936), who found dipicH₂ in the viscous matter of natto, a Japanese food made of steamed soybeans fermented with Bacillus natto. DipicH₂ is present in large amounts in bacterial spores of the Bacillus group (Powell & Strange, 1953). The crystal structure of dipicH₂, as the monohydrate, has been known for many years (Takusagawa et al., 1973). DipicH₂ exhibits biological activity such as inhibition of the zinc enzyme bovine carbonic anhydrase (Pocker & Fong, 1980) or of E. coli dihydropicolinate reductase (Scapin et al., 1997). To date, no crystal structures of Hg complexes with dipicH₂ have been published. There are two structures of Zn complexes known, one with two deprotonated dipic^2- ligands (Hakansson et al., 1993), and the other with two monodeprotonated dipicH^- ligands bonded to the Zn atom (Hakansson et al., 1993; Okabe & Oya, 2000). In an Fe^III complex with dipic^2-, one Cl ligand and two water molecules are also within the coordination sphere and form an octahedral complex (Lainé et al., 1995). The ligand is tridentately bound in all of these structures and forms typical chelating complexes, with M—O and M—N bonds of similar length.

The Hg^II ion, as a soft Lewis acid, forms covalent complexes with various soft Lewis bases, mostly by binding to S-donor groups, or, if these are not available, to N– or O-donor groups. As part of our wider research programme, we are interested in the competition of halide or pseudohalide ligands with N– and O-ligands towards Hg, and the structural characterization of Hg^II complexes with such ligands (Popović et al., 1999; Matković-Čalogović, Picek et al., 2001; Matković-Čalogović, Pavlovic et al., 2001). We present here the crystal structure of the first complex of Hg with dipicH₂, the title compound, (I). \sch

In (I), Hg is coordinated by a tridentate monodeprotonated dipicH^- ligand, a Br atom and a weakly bonded water molecule. Hg has a strong tendency to preserve linear coordination, as can be seen from the two shortest bonds, Hg—N 2.208 (5) and Hg—Br1 2.412 (1) Å, and from the N—Hg—Br1 angle of 169.6 (1)°. The Hg—Br distance is close to the sum of the covalent radii of Hg(linear) (Grdenić, 1965, 1981) and Br, while that of Hg—N is longer than the corresponding sum. This elongation, together with the deviation from linearity, may be attributed to additional contacts with the O atoms, two from the monoanion [Hg—O1 2.425 (4) and Hg—O4 2.599 (4) Å], the third from the water molecule [Hg—O5 2.635 (4) Å] and the fourth, the weakest, from the monoanion of a neighbouring complex molecule [Hg···O2(-1/2 - x, y - 1/2, 1/2 - z) 2.837 (4) Å]. These four O atoms are at distances longer then the sum of the covalent radii but shorter than the sum of van der Waals radii, so the effective coordination can be described as 2 + 4. The Hg—N distance is comparable with that in ethylenediaminemercury(II) dibromide, where two Hg—N bonds are 2.19 (2) Å, yet in this structure four Br atoms are weakly bound at 3.012 (2) Å (2 + 4 coordination; Matković-Čalogović & Sikirica, 1990). The weak bonding of the water molecule to Hg in (I) can be recognized in comparison with the much stronger Hg—OH₂ bond in [Hg(H₂OHg)(NO₃Hg)CCOO]NO₃, regarded as a monomercurated oxonium ion (Grdenić et al., 1986), where the bond length amounts to 2.17 (3) Å.

The molecules in (I) are interconnected by O—H···O intermolecular hydrogen bonds (Table 2). The shortest is between the protonated and deprotonated carboxylic acid groups and joins the molecules into chains. The two longer hydrogen bonds join H atoms from the bonded water molecule to the carboxylic acid groups of two neighbouring molecules from the parallel chain. In this way, a double-layer chain is formed parallel to the y axis (Fig. 2).

S2. Experimental

Crystals of (I) were obtained by slow evaporation from aqueous solution of a mixture containing pyridine-2,6-dicarboxylic acid (0.14 g, 83.8 mmol in 10 ml H₂O) and mercury(II) bromide (0.3 g, 83.3 mmol in 25 ml H₂O) at room temperature.

S3. Refinement

The data are 94% complete to 55°. The H atoms belonging to the water molecule and one carboxylic acid group were located in the difference Fourier map and isotropically refined with restrained bond lengths. H atoms belonging to the pyridine ring were generated geometrically and refined using a riding model.

Computing details

Data collection: STADI4 (Stoe & Cie, 1995); cell refinement: STADI4; data reduction: X-RED (Stoe & Cie, 1995); program(s) used to solve structure: SHELXS86 (Sheldrick, 1990); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: PLATON98 (Spek, 1990); software used to prepare material for publication: SHELXL97.

Figures

	Fig. 1. A view of the molecular structure of (I) with the atom-numbering scheme. Displacement ellipsoids are at the 50% probability level and H atoms are shown as spheres of arbitrary radii.
	Fig. 2. One double-layer chain formed by hydrogen bonding within the unit cell of (I). The hydrogen bonds are indicated by dashed lines.

Aquabromo(6-carboxypyridine-2-carboxylato-O,N,O')mercury(II)

Crystal data

[HgBr(C7H4NO4)(H2O)]	F(000) = 840
Mr = 464.63	Dx = 3.052 Mg m^−3
Monoclinic, P21/n	Mo Kα radiation, λ = 0.71073 Å
a = 6.967 (3) Å	Cell parameters from 56 reflections
b = 9.068 (3) Å	θ = 10–19°
c = 16.007 (5) Å	µ = 19.17 mm^−1
β = 91.05 (3)°	T = 293 K
V = 1011.1 (6) Å^3	Parallelepiped, colourless
Z = 4	0.28 × 0.28 × 0.22 mm

Data collection

Philips PW1100 updated by Stoe diffractometer	1907 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.030
Graphite monochromator	θmax = 30.0°, θmin = 3.2°
ω scans	h = −9→9
Absorption correction: integration (X-RED; Stoe & Cie, 1995)	k = −5→12
Tmin = 0.031, Tmax = 0.091	l = 0→17
4339 measured reflections	5 standard reflections every 90 min
2658 independent reflections	intensity decay: 2.2%

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)] = 0.028	H atoms treated by a mixture of independent and constrained refinement
wR(F^2) = 0.066	w = 1/[σ^2(Fo^2) + (0.0312P)^2 + 0.6877P] where P = (Fo^2 + 2Fc^2)/3
S = 0.99	(Δ/σ)max = 0.001
2658 reflections	Δρmax = 0.98 e Å^−3
149 parameters	Δρmin = −0.97 e Å^−3
3 restraints	Extinction correction: SHELXL97 (Sheldrick, 1997), Fc^*=kFc[1+0.001xFc^2λ^3/sin(2θ)]^-1/4
Primary atom site location: structure-invariant direct methods	Extinction coefficient: 0.00157 (14)

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.

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

	x	y	z	Uiso*/Ueq
Hg	0.04957 (3)	0.02939 (2)	0.278042 (14)	0.03554 (9)
Br1	0.19617 (10)	−0.04826 (8)	0.40844 (4)	0.04991 (17)
N	−0.1059 (6)	0.0594 (5)	0.1578 (3)	0.0297 (9)
O1	−0.0253 (6)	0.2893 (5)	0.2626 (3)	0.0451 (10)
O2	−0.1743 (6)	0.4440 (4)	0.1759 (3)	0.0486 (11)
O3	−0.1577 (7)	−0.3122 (5)	0.0919 (3)	0.0517 (11)
H31	−0.134 (13)	−0.399 (4)	0.109 (6)	0.10 (3)*
O4	−0.0080 (6)	−0.2228 (5)	0.2047 (3)	0.0458 (10)
O5	0.3513 (7)	0.0505 (5)	0.1820 (3)	0.0464 (10)
H51	0.422 (10)	0.117 (7)	0.204 (5)	0.09 (3)*
H52	0.423 (11)	−0.025 (6)	0.188 (7)	0.10 (4)*
C1	−0.1473 (7)	−0.0565 (5)	0.1067 (3)	0.0283 (11)
C2	−0.2330 (8)	−0.0351 (7)	0.0290 (4)	0.0400 (13)
H2	−0.2595	−0.1146	−0.0060	0.048*
C3	−0.2782 (8)	0.1075 (7)	0.0047 (4)	0.0418 (14)
H3	−0.3345	0.1242	−0.0476	0.050*
C4	−0.2414 (8)	0.2228 (7)	0.0565 (4)	0.0422 (14)
H4	−0.2740	0.3184	0.0406	0.051*
C5	−0.1534 (7)	0.1955 (6)	0.1340 (4)	0.0319 (11)
C6	−0.1130 (8)	0.3199 (6)	0.1953 (4)	0.0385 (13)
C7	−0.0960 (8)	−0.2054 (6)	0.1392 (4)	0.0349 (12)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Hg	0.04376 (12)	0.03023 (11)	0.03230 (14)	0.00064 (10)	−0.00815 (8)	0.00045 (10)
Br1	0.0570 (4)	0.0622 (5)	0.0302 (3)	0.0011 (3)	−0.0106 (3)	0.0019 (3)
N	0.0314 (19)	0.025 (2)	0.033 (3)	−0.0021 (16)	−0.0015 (17)	0.0032 (18)
O1	0.064 (3)	0.027 (2)	0.043 (3)	0.0002 (19)	−0.010 (2)	−0.0040 (18)
O2	0.054 (2)	0.025 (2)	0.068 (3)	0.0013 (17)	0.008 (2)	0.006 (2)
O3	0.086 (3)	0.026 (2)	0.043 (3)	−0.003 (2)	−0.017 (2)	−0.0049 (19)
O4	0.064 (3)	0.029 (2)	0.043 (3)	0.0034 (19)	−0.020 (2)	0.0011 (18)
O5	0.051 (2)	0.039 (3)	0.049 (3)	0.001 (2)	−0.004 (2)	0.001 (2)
C1	0.032 (2)	0.025 (3)	0.028 (3)	−0.0027 (18)	−0.001 (2)	0.0016 (19)
C2	0.042 (3)	0.040 (3)	0.038 (3)	−0.005 (3)	−0.004 (2)	0.002 (3)
C3	0.044 (3)	0.045 (4)	0.036 (3)	0.002 (3)	−0.009 (2)	0.015 (3)
C4	0.045 (3)	0.038 (3)	0.043 (4)	0.005 (2)	0.000 (3)	0.015 (3)
C5	0.032 (2)	0.028 (3)	0.035 (3)	−0.001 (2)	0.002 (2)	0.002 (2)
C6	0.038 (3)	0.031 (3)	0.048 (4)	0.003 (2)	0.011 (3)	0.006 (3)
C7	0.040 (3)	0.029 (3)	0.036 (3)	−0.003 (2)	0.000 (2)	−0.004 (2)

Geometric parameters (Å, º)

Hg—N	2.208 (5)	O5—H51	0.85 (7)
Hg—Br1	2.4120 (10)	O5—H52	0.85 (7)
Hg—O1	2.425 (4)	C1—C2	1.383 (8)
Hg—O4	2.599 (4)	C1—C7	1.488 (8)
Hg—O5	2.634 (5)	C2—C3	1.385 (8)
N—C5	1.332 (7)	C2—H2	0.9300
N—C1	1.358 (7)	C3—C4	1.356 (9)
O1—C6	1.260 (8)	C3—H3	0.9300
O2—C6	1.241 (7)	C4—C5	1.396 (8)
O3—C7	1.298 (7)	C4—H4	0.9300
O3—H31	0.85 (5)	C5—C6	1.518 (8)
O4—C7	1.215 (6)
N—Hg—Br1	169.57 (11)	N—C1—C7	116.4 (5)
N—Hg—O1	71.98 (15)	C2—C1—C7	122.6 (5)
Br1—Hg—O1	117.29 (11)	C1—C2—C3	118.4 (6)
N—Hg—O4	69.22 (14)	C1—C2—H2	120.8
Br1—Hg—O4	101.16 (9)	C3—C2—H2	120.8
O1—Hg—O4	141.10 (13)	C4—C3—C2	120.6 (5)
N—Hg—O5	82.31 (16)	C4—C3—H3	119.7
Br1—Hg—O5	101.33 (11)	C2—C3—H3	119.7
O1—Hg—O5	92.44 (14)	C3—C4—C5	118.8 (5)
O4—Hg—O5	85.32 (15)	C3—C4—H4	120.6
C5—N—C1	119.7 (5)	C5—C4—H4	120.6
C5—N—Hg	118.6 (4)	N—C5—C4	121.4 (5)
C1—N—Hg	121.6 (3)	N—C5—C6	117.5 (5)
C6—O1—Hg	113.6 (4)	C4—C5—C6	121.1 (5)
C7—O3—H31	117 (7)	O2—C6—O1	124.9 (6)
C7—O4—Hg	110.3 (4)	O2—C6—C5	116.9 (6)
Hg—O5—H51	106 (6)	O1—C6—C5	118.1 (5)
Hg—O5—H52	110 (6)	O4—C7—O3	124.2 (6)
H51—O5—H52	101 (9)	O4—C7—C1	122.3 (5)
N—C1—C2	121.1 (5)	O3—C7—C1	113.5 (5)

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O3—H31···O2^i	0.85 (5)	1.81 (7)	2.591 (6)	153 (9)
O5—H51···O4^ii	0.85 (7)	2.14 (7)	2.938 (7)	157 (7)
O5—H52···O1^iii	0.86 (7)	1.99 (7)	2.798 (7)	159 (8)

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