catena-Poly[[diaqua­diisothio­cyanatoiron(II)]-μ-4,4′-bi-1,2,4-tri­azole]: a one-dimensional coordination polymer

In the title complex, [Fe(NCS)₂(C₄H₂N₆)₂(H₂O)₂]_n, the Fe^II atom is on an inversion centre and the 4,4′-bi-1,2,4-triazole (btr) group is bisected by a twofold axis through the central N—N bond. The coordination geometry of the Fe^II atom is elongated distorted FeN₄O₂ octa­hedral, where the cation is coordinated by two N atoms from the triazole rings of two btr groups, two N atoms from NCS⁻ ligands and two water mol­ecules. Btr is a bidentate ligand, coordinating one Fe^II atom through a peripheral N atom of each triazole ring, leading to a one-dimensional polymeric (chain) structure extending along [101]. The chains are further connected through a network of O—H...N and C—H...S hydrogen bonds.

S1. Comment

Recently, considerable interest has been attached to iron coordination complexes which exhibit a large variety of magnetic behaviour ranging from purely high spin (S = 2) to low spin (S = 0) and solid-state spin-crossover. In general, the difference in electron configuration of the central Fe atom parallels large structural modifications of the coordination geometry, especially the Fe–ligand bond lengths. In the case of spin-crossover materials, the characteristics of the spin transition (e.g. transition temperature, transition abruptness or hysteresis) are closely related to the structural features, such as Fe^II coordination geometry, crystal packing, or the presence of solvent molecules or counterions in the voids of the structure (Gütlich et al., 1994). Cooperative interactions in the solid are a prerequisite for practical applications of these materials in nanotechnology and molecular electronics, since cooperativity may govern hysteretic spin transitions. One strategy for designing highly cooperative systems relies on the use of polydentate bridging ligands with the hope of creating a polymeric structural organization in the solid state. Numerous Fe spin-crossover materials exhibit an extended polymeric structure, and among these infinite chains, planar and three-dimensional networks built up from 4-substituted 1,2,4-triazole moieties have been described. We report here the crystal structure analysis of the title new polymeric Fe^II coordination complex, [Fe(btr)₂(NCS)₂(H₂O)₂], (I), with a one-dimensional topology.

The Fe atom in (I) lies on an inversion centre and the btr group is bisected by a twofold axis through the central N—N bond. The coordination geometry of the cation is an elongated distorted FeN₄O₂ octahedron involving two N atoms from two triazole rings, two N atoms from thiocyanate ligands and two water molecules (Fig. 1). The coordination polyhedron is highly elongated in the triazole directions with similar Fe—N1 and Fe—O1 bond lengths in the basal plane, which are typical for [Fe(L)₂(NCS)₂(H₂O)₂] complexes in the high-spin electron configuration (References?). In contrast, the Fe—N2 bond of 2.2319 (5) Å is the longest observed for any 4-substituted-1,2,4-triazole ligand. For comparison, values from 2.180 (3) to 2.223 (3) Å have been reported for the layer systems [Fe(btre)₂(NCS)₂] (Garcia et al., 2005) and [Fe(btr)₂(NCS)₂]·H₂O (Vreugdenhil et al., 1990), and from 2.151 (3) to 2.164 (3) Å for the three-dimensional complex [Fe(btr)₃]·(ClO₄)₂ (Garcia et al., 1999). Considerably shorter Fe—N bonds, in the range 1.950 (2)–1.994 (5) Å, are observed for the corresponding low-spin states (Pillet et al., 2004; Garcia et al., 1999).

According to the coordination geometry in (I), it is expected that the electron configuration of the central Fe atom is high spin, with an attempted assignment of the quantum axis as x and y along N1 and O1 and z along N2, as a result of the pseudo D4h distortion. The high-spin electron configuration would also be in agreement with the colourless aspect of the crystals at room temperature and even at 120 K; the lowest energy d–d spin-allowed transition (⁵T_{2 g} → ⁵E_g) occurs generally in the near-IR for this class of compound. In addition, several other complexes for which the central Fe^II ion is FeN₄O₂-coordinated by two water molecules and four btr ligands have been reported and their high-spin state confirmed by magnetic and Mössbauer measurements (Garcia et al., 2001).

The octahedral angular distortion in (I), defined as the sum of the deviations from 90° of the 12 X—Fe—Y angles (Σ = 13°), is at the lower limit of all the other bis-triazole (btr and btre) complexes listed above (14.8–21.2°) and is small compared with typical high-spin Fe^II complexes (Guionneau et al., 2004).

The NCS group of (I) adopts a quasi-linear geometry with respect to the Fe atom, with an Fe—N—C angle of 175.48 (6)°. The triazole rings are oriented in an almost planar configuration with respect to the Fe atom and NCS groups, induced by a weak C2—H2···N1ⁱⁱ hydrogen bond [symmetry code: (ii) -x + 1/2, -y + 3/2, -z + 1]; the r.m.s. deviation of all the atoms concerned from the mean plane is 0.04 Å. As for the btr group, the C—N and N—N bond lengths, and especially the differences between the longest (C2—N4 and C3—N4) and the shortest (C2—N2 and C3—N3) bonds, are consistent with those reported in the literature (References?) and indicative of the main mesomeric form depicted in the chemical scheme and discussed recently in the light of electron-density results from [Fe(btr)₂(NCS)₂]·H₂O (Legrand et al., 2006). On the other hand, the dihedral angle between the two symmetry-related (-x, y, -z + 1/2) triazole rings of one btr fragment [72.58 (2)°] is clearly lower than in all other btr-containing materials, with dihedral angles ranging from 90° in crystalline btr (Domiano, 1977) to 77.35° in [Fe(btr)₃]·(ClO₄)₂ (Garcia et al., 1999). Free rotation of the two triazole rings around the central N4—N4ⁱ bond [symmetry code: (i) -x, y, -z + 1/2] is hindered by the potential H···H close contact that would arise if the dihedral angle were too low.

The crystal packing of compound (I) consists of Fe atoms bridged by btr ligands to form infinite parallel chains running along the [101] crystallographic direction. The corresponding Fe···Fe separation along the chains [9.1937 (3) Å] is in the same range as the separation in the high-spin crystal structure of [Fe(btr)₂(NCS)₂]·H₂O at room temperature [9.30 (2) Å] and is much longer than that in the high-spin structure of [Fe(btr)₃]·(ClO₄)₂ at 260 K (8.67 Å). The chains are connected through a network of O—H···N and C—H···S hydrogen bonds in the (010) plane and in the perpendicular [010] direction, leading to a three-dimensional network with corresponding shortest inter-chain Fe···Fe separations of 6.6559 (1) and 5.8299 (1) Å, respectively (Fig. 2). The shortest hydrogen bond, O1—H5···N3ⁱⁱⁱ, occurs in the [010] direction [symmetry code: (iii) x, y - 1, z]. Even though the C2—H2···S1^v [symmetry code: (v) x - 1/2, y - 1/2, z] hydrogen bond might seem rather long at first sight, we suspect it is nevertheless involved in the stabilization of the chain packing, as can be judged in Fig. 2(b).

Related literature

For related literature, see: Domiano (1977); Gütlich et al. (1994); Garcia et al. (1999, 2001, 2005); Guionneau et al. (2004); Haasnoot & Groeneveld (1979); Legrand et al. (2006); Pillet et al. (2004); Vreugdenhil et al. (1990).

S2. Experimental

The ligand 4–4'-1,2,4-bis-triazole (btr) was synthesized as described by Haasnoot & Groeneveld (1979). A solution containing btr (0.40 g, 2.94 mmol) dissolved in a mixture of water (5 ml) and methanol (5 ml) was warmed to 333 K and added to a solution containing [Fe(H₂O)₆](ClO₄)₂ (0.35 g, 0.96 mmol) and ascorbic acid (1 mg) dissolved in water (10 ml) warmed to 333 K. The resulting solution was layered on top of dichloromethane (10 ml) in a closed test tube. Large colourless single crystals of (I) grew at the (water–methanol)/(dichloromethane) interface over several weeks. The crystals are air sensitive.

S3. Refinement

H atoms were located in a difference Fourier synthesis and refined freely, with (different) global isotropic displacement parameters for C– and O-bound H (Ranges of refined C—H and O—H distances?). The highest positive residual electron-density peak is located at the Fe site, on the centre of inversion.

Computing details

Data collection: CrysAlis CCD (Oxford Diffraction, 2003); cell refinement: CrysAlis RED (Oxford Diffraction, 2003); data reduction: CrysAlis RED; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: ORTEPII (Johnson, 1976) and PLATON (Spek, 2003); software used to prepare material for publication: WinGX (Farrugia, 1999) and PLATON.

Figures

	Fig. 1. The coordination environment of atom Fe1 in (I), showing 50% probability displacement ellipsoids and the atom-numbering scheme. [Symmetry code: (ii) -x + 1/2, -y + 3/2, -z + 1].
	Fig. 2. Two views of the chain packing, at right angles to each other around the [101] chain direction and showing hydrogen-bond contacts. (a) A view normal to the b axis. (b) A view along the b axis. [Symmetry codes: (iii) x, y - 1, z; (iv) -x + 1/2, y - 1/2, -z + 1/2; (vi) x + 1/2, y - 1/2, z.]

catena-Poly[[Diaquabis(isothiocyanato)iron(II)]-µ-4,4'-bi-1,2,4-triazole]

Crystal data

[Fe(NCS)2(C4H2N6)2(H2O)2]	F(000) = 696
Mr = 343.85	Dx = 1.832 Mg m^−3
Monoclinic, C2/c	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -C2yc	Cell parameters from 13863 reflections
a = 19.2178 (4) Å	θ = 3.5–41.3°
b = 5.8299 (1) Å	µ = 1.56 mm^−1
c = 11.9673 (2) Å	T = 120 K
β = 112.287 (2)°	Block, colourless
V = 1240.63 (4) Å^3	0.19 × 0.14 × 0.09 mm
Z = 4

Data collection

Oxford Xcalibur CCD area-detector diffractometer	3829 independent reflections
Radiation source: fine-focus sealed tube	3300 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.015
ω scans	θmax = 40.3°, θmin = 3.5°
Absorption correction: gaussian (CrysAlis; Oxford Diffraction, 2003)	h = −24→34
Tmin = 0.69, Tmax = 0.83	k = −10→10
13719 measured reflections	l = −21→21

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)] = 0.028	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.073	H atoms treated by a mixture of independent and constrained refinement
S = 1.09	w = 1/[σ^2(Fo^2) + (0.0433P)^2 + 0.3921P] where P = (Fo^2 + 2Fc^2)/3
3829 reflections	(Δ/σ)max = 0.001
102 parameters	Δρmax = 1.39 e Å^−3
0 restraints	Δρmin = −0.30 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
Fe1	0.2500	0.7500	0.5000	0.00983 (4)
S1	0.434595 (13)	1.25814 (3)	0.438911 (19)	0.01977 (5)
N1	0.32735 (3)	0.96206 (11)	0.46525 (6)	0.01623 (10)
C1	0.37188 (3)	1.08578 (11)	0.45364 (6)	0.01222 (9)
C2	0.08366 (3)	0.90225 (11)	0.35208 (5)	0.01169 (9)
H2	0.0696 (8)	0.783 (2)	0.3896 (13)	0.019 (2)*
C3	0.08482 (4)	1.20345 (13)	0.24436 (6)	0.01504 (11)
H3	0.0690 (7)	1.321 (2)	0.1908 (12)	0.019 (2)*
N2	0.15352 (3)	0.94975 (10)	0.37225 (5)	0.01227 (9)
N3	0.15457 (3)	1.14245 (11)	0.30453 (6)	0.01646 (10)
N4	0.03860 (3)	1.05711 (9)	0.27258 (5)	0.01013 (8)
O1	0.24956 (3)	0.53466 (10)	0.35707 (5)	0.01591 (9)
H4	0.2846 (10)	0.536 (3)	0.3317 (17)	0.050 (3)*
H5	0.2273 (10)	0.408 (3)	0.3445 (17)	0.050 (3)*

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Fe1	0.00715 (6)	0.01028 (6)	0.01155 (6)	−0.00106 (3)	0.00297 (4)	0.00105 (3)
S1	0.02195 (9)	0.02119 (9)	0.02021 (9)	−0.01122 (6)	0.01255 (7)	−0.00246 (6)
N1	0.0122 (2)	0.0158 (2)	0.0206 (2)	−0.00214 (17)	0.00615 (18)	0.00337 (19)
C1	0.0117 (2)	0.0123 (2)	0.0127 (2)	−0.00078 (17)	0.00471 (17)	0.00110 (17)
C2	0.0084 (2)	0.0135 (2)	0.0126 (2)	0.00028 (16)	0.00322 (16)	0.00242 (17)
C3	0.0103 (2)	0.0144 (2)	0.0183 (3)	−0.00193 (19)	0.00304 (19)	0.0044 (2)
N2	0.00840 (18)	0.0133 (2)	0.0140 (2)	−0.00039 (15)	0.00304 (15)	0.00206 (16)
N3	0.0093 (2)	0.0162 (2)	0.0214 (2)	−0.00197 (17)	0.00300 (17)	0.0054 (2)
N4	0.00646 (16)	0.01190 (19)	0.01097 (17)	−0.00025 (14)	0.00211 (13)	0.00094 (15)
O1	0.0156 (2)	0.0152 (2)	0.0191 (2)	−0.00418 (16)	0.00903 (17)	−0.00388 (16)

Geometric parameters (Å, º)

Fe1—N1^i	2.0928 (6)	C2—N4	1.3584 (8)
Fe1—N1	2.0928 (6)	C2—H2	0.923 (14)
Fe1—N2^i	2.2319 (5)	C3—N4	1.3633 (9)
Fe1—N2	2.2319 (5)	N4—N4^ii	1.3728 (10)
Fe1—O1	2.1192 (5)	N2—N3	1.3898 (8)
Fe1—O1^i	2.1192 (5)	N4—N4^ii	1.3728 (10)
N1—C1	1.1671 (8)	C3—H3	0.908 (14)
C1—S1	1.6293 (7)	O1—H4	0.837 (19)
C2—N2	1.3006 (8)	O1—H5	0.835 (19)
C3—N3	1.3074 (8)
N1^i—Fe1—N1	180.00 (2)	N2—C2—N4	109.11 (5)
N1^i—Fe1—O1	91.42 (2)	N2—C2—H2	122.8 (8)
N1—Fe1—N2	91.36 (2)	N4—C2—H2	128.0 (8)
N1—Fe1—O1	88.58 (2)	C2—N4—C3	106.69 (5)
N1^i—Fe1—O1^i	88.58 (2)	C2—N4—N4^ii	125.62 (5)
N1—Fe1—O1^i	91.42 (2)	C3—N4—N4^ii	127.66 (5)
O1—Fe1—O1^i	180.0	C2—N2—N3	107.83 (5)
N1^i—Fe1—N2^i	91.36 (2)	C2—N2—Fe1	123.16 (4)
N1—Fe1—N2^i	88.64 (2)	N3—N2—Fe1	128.99 (4)
O1—Fe1—N2^i	89.53 (2)	C3—N3—N2	107.60 (5)
O1^i—Fe1—N2^i	90.47 (2)	N3—C3—N4	108.76 (6)
N1^i—Fe1—N2	88.64 (2)	N3—C3—H3	126.3 (8)
O1—Fe1—N2	90.47 (2)	N4—C3—H3	124.9 (8)
O1^i—Fe1—N2	89.53 (2)	Fe1—O1—H4	122.0 (13)
N2^i—Fe1—N2	180.0	Fe1—O1—H5	120.8 (14)
Fe1—N1—C1	175.48 (6)	H4—O1—H5	111.6 (18)
N1—C1—S1	179.40 (6)

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

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
C2—H2···N1^i	0.923 (14)	2.523 (13)	3.0625 (9)	117.7 (10)
O1—H5···N3^iii	0.835 (19)	2.021 (19)	2.8435 (8)	168.4 (17)
O1—H4···N3^iv	0.837 (19)	2.42 (2)	3.1989 (9)	155.1 (16)
C2—H2···S1^v	0.923 (14)	2.875 (14)	3.5000 (6)	126.2 (10)

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