Poly­[iron(II)-di-[mu]-imidazole-4,5-di­carboxyl­ato-[kappa]³N³,O⁴:O⁵]

In the title compound, [Fe(C₅H₃N₂O₄)₂]_n, each Fe atom lies on a centre of symmetry, in an octahedral coordination environment consisting of two chelate rings [Fe-N = 2.154 (3) Å and Fe-O = 2.180 (3) Å] and two carboxyl­ate O atoms [Fe-O = 2.111 (2) Å] from imidazole-4,5-di­carboxyl­ate ligands. Extensive hydrogen-bonding interactions exist between layers constructed of Fe₄ squares, forming tunnels along the a axis with large voids.

S1. Comment

The structures and properties of metal complexes with biologically relevant ligands are currently attractive, on account of their promising contribution to understanding the active mechanism of metalloproteins by means of modelling their metal binding site (Chauvin et al., 2003; Mukherjee, 2000). However, as a derivative of imidazole which is ubiquitous at the active site of many metalloproteins, 4,5-imidazoledicarboxylic acid (H₃IMDC) has been less well studied (Wang et al., 2004; Rajendiran et al., 2003; Huang et al., 2001; Sengupta et al., 2001; Caudle et al., 1997). Here, we report the synthesis and crystal structure of the title compound, [Fe(H₂IMDC)₂]_n, (I). \sch

In compound (I), the Fe atom lies on centre of symmetry and the coordination enviroment is octahedral (Fig. 1 and Table 1). Each Fe atom is hexacoordinated by N₂O₄, with two chelating rings from H₂IMDC⁻ ligands [Fe—N1 2.154 (3) and Fe—O3 2.180 (3) Å] arranged symmetrically in the equatorial plane and two O atoms from H₂IMDC⁻ ligands [Fe—O1(1/2 − x, y − 1/2, 1/2 − z) 2.111 (2) Å] completing the apical sites.

Complex (1) displays an extended two-dimensional layer structure constructed of quasi-squares, with four Fe atoms at the corners and H₂IMDC⁻ as linkers (Fig. 2). Each H₂IMDC⁻ anion is nearly planar and connects two Fe^II ions in an individual mono- and bidentate mode. The lateral Fe···Fe lengths of the Fe₄ square are all 8.572 (1) Å, the diagonal Fe···Fe distances are 11.719 (2) and 12.512 (2) Å, and the vertex angles are 86.25 (1) and 93.75 (1)°, indicating only slight deviations from the ideal square geometry.

Recently, a cobalt complex containing Co₄ molecular squares with 4,5-imidazoledicarboxylate bridges has been reported (Wang et al., 2004), in which IMDC³⁻ links two cobalt ions in a bis(bidentate) mode and the two chelating rings of two IMDC³⁻ ligands coordinated to one metal atom are almost perpendicular to each other. Furthermore, 2,2'-bipyridine exists as a terminal ligand, chelating the central atom to prevent the structure expanding in two dimensions. However, in (I), the Fe₄ squares are extended by the H₂IMDC⁻ ligand and form an infinite layer structure without terminal ligands to end it. It is noted that the H₂IMDC⁻ ligand in (I) is less geometrically symmetrical than the IMDC³⁻ in the reported molecular squares, but it provides much smaller deviations from the ideal square geometry.

The layers of complex (I) are connected by N—H···O hydrogen bonds, leading to the formation of a three-dimensional network structure with tunnels along the a axis (Fig. 3). The packing diagram of (I) along the b axis is shown in Fig. 4. It is very interesting that this crystal structure of hydrogen-bonded networks survives with solvent-accessible voids of 180 ų. The maximum peak in the final difference Fourier map is 0.82 e Å⁻³ at the Fe atom, and the minimum peak is −0.37 e Å⁻³, 0.69 Å away from atom H2A.

According to charge-balance requirements, the oxidation state of Fe in (I) is tentatively assigned as +2, since the reaction of aromatic N ligands such as imidazole or pyridine derivatives with transition metal salts can result in spontaneous metal ion reduction (Stupka et al., 2004; Wang et al., 2004; Lu & Babb, 2002; Sugiyama et al., 2002). In addition, we have also recently obtained a type of Fe^II coordination complex from the reaction of FeCl₃ and 2,5-pyridinedicarboxylic acid via the solvothermal method in N,N-dimethylformamide. The mechanism of the redox reactions involving the usual aromatic ligands with N or N₂ donors under solvo(hydro)thermal conditions are still under study.

S2. Experimental

The title compound, (I), was prepared by the solvothermal reaction of iron(III) chloride (0.008 g, 0.05 mmol) and 4,5-imidazoledicarboxylic acid (0.016 g, 0.1 mmol) in 99.5% ethanol (10 ml). The reaction was performed in a 23 ml Teflon-lined stainless steel Parr bomb under autogeneous pressure. After heating at 433 K for 2 d and cooling to room temperature at 13 K h⁻¹, light-yellow crystals of (I) were obtained. The title compound was the only product (yield 0.4 mg, 2%).

S3. Refinement

The H atoms were positioned geometrically, with O—H 0.82, N—H 0.86 and C—H 0.93 Å. They were constrained to ride on their parent atoms, with U_iso(H) = 1.5U_eq(O) or 1.2U_eq(C,N).

Computing details

Data collection: CrystalClear (Rigaku, 2000); cell refinement: CrystalClear; data reduction: CrystalClear; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997a); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997a); molecular graphics: SHELXL97; software used to prepare material for publication: SHELXTL (Sheldrick, 1997b).

Figures

	Fig. 1. The coordination environment of the Fe atom in (I). Displacement ellipsoids are drawn at the 50% probability level and H atoms are shown as small spheres of arbitrary radii. [Symmetry codes: (A) 1/2 − x, y − 1/2, 1/2 − z; (B) −x, −y, −z; (AA) x − 1/2, 1/2 − y, z − 1/2.]
	Fig. 2. The extended lamellar structure of (I).
	Fig. 3. A packing diagram for (I), viewed along the a axis.
	Fig. 4. A packing diagram for (I), viewed along the b axis. Dashed lines indicate hydrogen bonding.

Poly[iron(II)-di-µ-imidazole-4,5-dicarboxylato-κ³N³,O⁴:O⁵]

Crystal data

[Fe(C5H3N2O4)2]	F(000) = 368
Mr = 366.04	Dx = 1.328 Mg m^−3
Monoclinic, P21/n	Mo Kα radiation, λ = 0.71073 Å
a = 6.9909 (14) Å	Cell parameters from 2242 reflections
b = 11.719 (2) Å	θ = 3.3–25.0°
c = 11.2519 (16) Å	µ = 0.86 mm^−1
β = 96.909 (9)°	T = 293 K
V = 915.2 (3) Å^3	Dipyramid, light yellow
Z = 2	0.30 × 0.18 × 0.10 mm

Data collection

MAKE Mercury CCD diffractometer	1614 independent reflections
Radiation source: fine-focus sealed tube	1455 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.031
ω scans	θmax = 25.0°, θmin = 3.3°
Absorption correction: multi-scan (CrystalClear; Rigaku, 2000)	h = −5→8
Tmin = 0.599, Tmax = 0.917	k = −13→13
5462 measured reflections	l = −12→13

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)] = 0.049	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.178	H-atom parameters constrained
S = 1.18	w = 1/[σ^2(Fo^2) + (0.113P)^2 + 0.6498P] where P = (Fo^2 + 2Fc^2)/3
1614 reflections	(Δ/σ)max < 0.001
106 parameters	Δρmax = 0.83 e Å^−3
0 restraints	Δρmin = −0.37 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
Fe	0.0000	0.0000	0.0000	0.0197 (3)
O1	0.4420 (4)	0.3767 (2)	0.3637 (3)	0.0347 (7)
O2	0.1243 (4)	0.3603 (3)	0.3564 (3)	0.0435 (8)
H2B	0.0391	0.3216	0.3195	0.065*
O3	−0.1605 (3)	0.1017 (2)	0.1165 (3)	0.0328 (7)
O4	−0.1370 (4)	0.2451 (3)	0.2468 (3)	0.0462 (8)
N1	0.2187 (4)	0.1101 (2)	0.0891 (3)	0.0255 (7)
N2	0.4527 (4)	0.2107 (3)	0.1854 (3)	0.0302 (8)
H2A	0.5669	0.2361	0.2073	0.036*
C1	0.4055 (5)	0.1304 (3)	0.1027 (4)	0.0295 (9)
H1A	0.4934	0.0934	0.0601	0.035*
C2	0.2888 (5)	0.2454 (3)	0.2291 (3)	0.0274 (8)
C3	0.1439 (5)	0.1812 (3)	0.1687 (3)	0.0250 (8)
C4	0.2876 (6)	0.3334 (3)	0.3228 (3)	0.0305 (9)
C5	−0.0645 (5)	0.1755 (3)	0.1777 (3)	0.0293 (9)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Fe	0.0167 (5)	0.0194 (5)	0.0217 (5)	−0.0003 (2)	−0.0022 (3)	−0.0006 (2)
O1	0.0243 (15)	0.0381 (15)	0.0410 (16)	−0.0069 (11)	0.0006 (12)	−0.0189 (12)
O2	0.0236 (15)	0.0526 (18)	0.0544 (19)	−0.0081 (13)	0.0050 (13)	−0.0324 (15)
O3	0.0166 (13)	0.0392 (15)	0.0426 (16)	−0.0048 (11)	0.0037 (11)	−0.0148 (12)
O4	0.0192 (14)	0.0575 (19)	0.063 (2)	−0.0050 (13)	0.0110 (13)	−0.0361 (16)
N1	0.0160 (15)	0.0287 (15)	0.0317 (17)	−0.0017 (12)	0.0020 (13)	−0.0085 (12)
N2	0.0149 (15)	0.0374 (18)	0.0379 (18)	−0.0057 (13)	0.0011 (13)	−0.0133 (14)
C1	0.0157 (18)	0.037 (2)	0.036 (2)	−0.0017 (15)	0.0044 (15)	−0.0109 (17)
C2	0.0171 (17)	0.032 (2)	0.0323 (19)	−0.0028 (15)	0.0016 (15)	−0.0085 (16)
C3	0.0174 (18)	0.0287 (18)	0.0287 (19)	−0.0027 (14)	0.0019 (14)	−0.0079 (15)
C4	0.024 (2)	0.033 (2)	0.034 (2)	−0.0027 (16)	0.0005 (16)	−0.0109 (16)
C5	0.0186 (19)	0.036 (2)	0.034 (2)	−0.0003 (16)	0.0027 (15)	−0.0097 (16)

Geometric parameters (Å, º)

Fe—O1^i	2.111 (2)	O4—C5	1.273 (5)
Fe—O1^ii	2.111 (2)	N1—C1	1.318 (5)
Fe—N1	2.154 (3)	N1—C3	1.372 (5)
Fe—N1^iii	2.154 (3)	N2—C1	1.337 (5)
Fe—O3	2.180 (3)	N2—C2	1.362 (5)
Fe—O3^iii	2.180 (3)	N2—H2A	0.8600
O1—C4	1.231 (5)	C1—H1A	0.9300
O1—Fe^iv	2.111 (2)	C2—C3	1.374 (5)
O2—C4	1.285 (5)	C2—C4	1.476 (5)
O2—H2B	0.8200	C3—C5	1.474 (5)
O3—C5	1.250 (5)
O1^i—Fe—O1^ii	180.00 (14)	C3—N1—Fe	111.3 (2)
O1^i—Fe—N1	90.50 (11)	C1—N2—C2	108.3 (3)
O1^ii—Fe—N1	89.50 (11)	C1—N2—H2A	125.9
O1^i—Fe—N1^iii	89.50 (11)	C2—N2—H2A	125.9
O1^ii—Fe—N1^iii	90.50 (11)	N1—C1—N2	111.4 (3)
N1—Fe—N1^iii	180.00 (18)	N1—C1—H1A	124.3
O1^i—Fe—O3	90.30 (11)	N2—C1—H1A	124.3
O1^ii—Fe—O3	89.70 (11)	N2—C2—C3	105.1 (3)
N1—Fe—O3	77.32 (10)	N2—C2—C4	122.9 (3)
N1^iii—Fe—O3	102.68 (10)	C3—C2—C4	132.0 (4)
O1^i—Fe—O3^iii	89.70 (11)	N1—C3—C2	109.8 (3)
O1^ii—Fe—O3^iii	90.30 (11)	N1—C3—C5	118.4 (3)
N1—Fe—O3^iii	102.68 (10)	C2—C3—C5	131.8 (3)
N1^iii—Fe—O3^iii	77.32 (10)	O1—C4—O2	124.0 (4)
O3—Fe—O3^iii	180.00 (18)	O1—C4—C2	118.4 (3)
C4—O1—Fe^iv	129.6 (3)	O2—C4—C2	117.6 (3)
C4—O2—H2B	109.5	O3—C5—O4	123.6 (3)
C5—O3—Fe	115.4 (2)	O3—C5—C3	117.6 (3)
C1—N1—C3	105.4 (3)	O4—C5—C3	118.8 (3)
C1—N1—Fe	143.1 (2)

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

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
O2—H2B···O4	0.82	1.66	2.477 (4)	179
N2—H2A···O4^v	0.86	2.07	2.897 (4)	162

Symmetry code: (v) x+1, y, z.