Revisited crystal symmetry of the high-spin form of the iron(II) spin-crossover complex di­cyano­[2,13-di­methyl-6,9-dioxa-3,12,18-tri­aza­bi­cyclo­[12.3.1]­octadeca-1(18),2,12,14,16-pentaene]­iron(II) monohydrate

The title iron(II) complex, [Fe(CN)₂(C₁₅H₂₃N₃O₂)]·H₂O, is of interest to the spin-crossover community because of its unusual temperature-dependent magnetic behaviour as well as its relatively high relaxation temperature for the light-induced spin-crossover phenomenon. Structural modifications are strongly suspected to cause the unusual thermal spin-crossover features. Recently, the high-spin crystal structure has been reported but with an inadequate space group. In the present paper, the crystal structure is corrected by a new investigation, and some consequences for the structure-property relationships of this complex are discussed. The Fe^II ion is seven-coordinate and lies on a twofold axis.

S1. Comment

Since its first description (Nelson et al., 1986), the variable-temperature magnetic behaviour of the title spin-crossover compound has proved to be very unusual (König et al., 1987; Hayami et al., 2001) and is still debated. These properties will be detailed in a forthcoming paper (Sánchez-costa et al., 2004). When cooling for the first time, this complex undergoes a sharp spin-crossover from a high-spin state (HS) to a low-spin state (LS) at 160 K. Subsequent temperature changes strongly affect the features of the spin-crossover, such as the critical temperature, the hysteresis width and the relaxation process for the light-induced spin-crossover phenomenon. The commonly admitted justification to date for this difference between the first and a subsequent cooling is that a major change of the molecular structure occurs at low temperature. It has been suggested that the Fe atom could undergo a modification from a six- to a seven- coordination sphere (König et al. 1987). However, so far there is no experimental proof of this transition, which would be an unusual feature in the solid state.

The crystal structure of this complex in its high-spin form has been determined recently at 250 K (Hayami et al., 2001). The suggested unit-cell symmetry was monoclinic, Cc, and the determination revealed the presence of one solvent water molecule per iron complex. The water molecules and the complexes form infinite pseudo-chains along the a axis. The Fe^II ion is in a pentagonal bipyramidal environment. Surprisingly, one of the two Fe—O bond lengths appears to be very long [2.405 (6) Å], while the other is in the expected range [2.277 (6) Å]. In addition, the structural data exhibit anomalies, such as, for instance, the high values of the shift/s.u. ratio after the final refinement cycle (0.386) and the unexplained value of the Flack (1983) parameter (−0.57). These anomalies could be due to a real physical phenomenon, such as, for example, a fraction of iron ions remaining in a LS state as a result of the relatively low temperature of the structural determination, or could simply reflect an error in the structural determination. On the basis of the available data, it was impossible to be sure of the nature of the problem raised by this structural report. In order to answer the question, we haved recrystallized this complex and recollected the data at room temperature (293 K). The structure was solved and refined in the monoclinic C2/c space group without encountering any of the above problem. It is interesting to note that if the crystal structure is solved from our data set using the Cc space group then exactly the same anomalies as were present in the previous determination are obtained. Consequently, we assume that the true space group for the title complex is C2/c.

The general description of the crystal packing is unchanged. However, the present space group correction is of importance because it affects the iron environment. First, the iron coordination sphere angles (Table 1) are slighlty different from the previously reported values; this difference could be crucial in the future when analysing the magnetic data. Indeed, the iron environment distortion is known to strongly influence the spin-crossover features (Guionneau et al., 2004). Furthermore, in the C2/c unit cell, the iron ion lies on a twofold axis (Fig. 1), and so the two Fe—O bonds are crystallographically equivalent and of identical length (Table 1). The iron ion is thus unambiguously seven-coordinate at 293 K in the HS state. One consequence of importance is that, if there is a transition to a six-coordinate iron ion at low temperature, there must necessarily be a change in the crystal symmetry. Indeed, on going from an FeN₃C₂O₂ to an FeN₃C₂O coordination polyhedron, as expected, the two Fe—O bonds cannot remain symmetrically equivalent and the iron ion cannot lie on a twofold axis.

A large structural change at low temperature can also be predicted by considerations of the volume of the iron coordination sphere. The volume of the iron polyhedron has been widely studied for six-coordinate spin-crossover complexes (Guionneau et al., 2002; Marchivie et al., 2002; Thompson et al., 2004). The calculation of this volume was performed from the atomic coordinates using IVTON (Balic Zunic & Vickovic, 1996). It has been shown, on the basis of a large number of six-coordinate complexes, that the spin-crossover corresponds to a decrease of the polyhedron volume from 13.0 (5) (HS) to 10.0 (5) ų (LS), i.e. a decrease of 25% (Guionneau et al., 2004). Seven-coordinate spin-crossover iron complexes are very rare and such a statistical approach is not yet possible. For the title complex, the calculated volume of the iron polyhedron is 17.0 ų in the HS state. Consequently, the expected transition from a seven-coordinate iron ion at room temperature (HS) to a six-coordinate iron ion at low temperature (LS) would correspond to a change of the iron polyhedron volume from 17.0 to 10.0 (5) ų. This large modification, almost 60% of the initial value, should be propagated to the whole molecular geometry and probably to the crystal packing.

The above remarks are consistent with the low-temperature single-crystal X-ray diffraction experiments that we have performed. Indeed, the single crystals are damaged when cooling, whatever the cooling rate. A large structural rearrangement is certainly the cause of this behaviour. Low-temperature single-crystal X-ray diffraction investigation being unsuccessful so far, a powder investigation should be attempted. We assume that the present results will then be useful in the analysis of the diffractograms.

S2. Experimental

The complex was synthetized as described previously (Nelson et al., 1986), except that the volume of the [FeL(CN)₂]Cl₂ solution was not reduced after the addition of NaCN. The entire solution was kept at a constant temperature of 293 K for a period of one week. This yielded small prismatic dark-violet single crystals.

S3. Refinement

All H atoms attached to C atoms were placed in calculated positions (C—H = 0.93–0.97 Å) and treated as riding. The H atoms of the water molecule were located in a difference map and refined with O—H and H···H distance restraints of 0.85 (1) and 1.39 (1) Å, respectively, and with U_iso(H) = 1.2U_eq(O).

Computing details

Data collection: COLLECT (Nonius, 2003); cell refinement: HKL SCALEPACK (Otwinowski & Minor, 1997); data reduction: HKL DENZO (Otwinowski & Minor, 1997) and SCALEPACK; program(s) used to solve structure: SHELXS97 (Sheldrick, 1997); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997) and WinGX (Farrugia, 1999); molecular graphics: ORTEP-3 for Windows (Farrugia, 1997); software used to prepare material for publication: WinGX (Farrugia, 1999).

Figures

Fig. 1. : An ORTEP-3 (Farrugia, 1997) view of the title complex, showing 30% probability displacement ellipsoids and the atom-numbering scheme. The water molecules have been omitted for clarity.

dicyano[2,13-dimethyl-6,9-dioxa-3,12,18-triazabicyclo[12.3.1]octadeca- 1(18),2,12,14,16-pentaene]iron(II)

Crystal data

[Fe(C15H23N3O2)(CN)2]·H20	F(000) = 840
Mr = 401.25	Dx = 1.406 Mg m^−3
Monoclinic, C2/c	Mo Kα radiation, λ = 0.71069 Å
Hall symbol: -C2yc	Cell parameters from 6605 reflections
a = 17.326 (5) Å	θ = 1.0–27.5°
b = 12.054 (5) Å	µ = 0.82 mm^−1
c = 10.125 (5) Å	T = 293 K
β = 116.27 (1)°	Prism, dark violet
V = 1896.2 (13) Å^3	0.05 × 0.05 × 0.03 mm
Z = 4

Data collection

Nonius KappaCCD diffractometer	1590 reflections with I > 2σ(I)
Radiation source: fine-focus sealed tube	Rint = 0.045
Graphite monochromator	θmax = 26.4°, θmin = 4.1°
ω scans with κ offsets	h = −21→21
6183 measured reflections	k = −15→15
1933 independent reflections	l = −12→12

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)] = 0.038	Hydrogen site location: inferred from neighbouring sites
wR(F^2) = 0.094	H atoms treated by a mixture of independent and constrained refinement
S = 1.09	w = 1/[σ^2(Fo^2) + (0.0364P)^2 + 1.1283P] where P = (Fo^2 + 2Fc^2)/3
1933 reflections	(Δ/σ)max = 0.003
131 parameters	Δρmax = 0.27 e Å^−3
3 restraints	Δρmin = −0.29 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	Occ. (<1)
Fe1	1.0000	0.26200 (3)	0.2500	0.03899 (17)
O1	0.94232 (10)	0.41946 (12)	0.1054 (2)	0.0549 (4)
N1	1.0000	0.08722 (19)	0.2500	0.0379 (6)
N2	0.90301 (12)	0.20780 (15)	0.0315 (2)	0.0443 (4)
C1	0.94694 (13)	0.03144 (17)	0.1281 (3)	0.0420 (5)
C2	0.89177 (14)	0.10427 (18)	0.0030 (3)	0.0436 (5)
N3	0.85158 (15)	0.2864 (2)	0.3687 (3)	0.0689 (7)
C4	0.94602 (16)	−0.08329 (19)	0.1245 (3)	0.0558 (7)
H4	0.9099	−0.1212	0.0395	0.067*
C3	0.83077 (17)	0.0519 (2)	−0.1393 (3)	0.0613 (7)
H3A	0.8044	0.1085	−0.2122	0.092*
H3B	0.7871	0.0121	−0.1249	0.092*
H3C	0.8617	0.0015	−0.1718	0.092*
C6	0.85577 (17)	0.2933 (2)	−0.0786 (3)	0.0567 (6)
H6A	0.7968	0.2696	−0.1375	0.068*
H6B	0.8826	0.3046	−0.1437	0.068*
C9	0.90158 (15)	0.27860 (18)	0.3253 (3)	0.0477 (5)
C7	0.85683 (17)	0.3992 (2)	−0.0008 (3)	0.0648 (7)
H7A	0.8367	0.4599	−0.0708	0.078*
H7B	0.8192	0.3931	0.0468	0.078*
C5	1.0000	−0.1404 (3)	0.2500	0.0639 (10)
H5	1.0000	−0.2175	0.2500	0.077*
C8	0.9554 (2)	0.5164 (2)	0.1911 (4)	0.0773 (9)
H8A	0.9152	0.5182	0.2339	0.093*
H8B	0.9457	0.5816	0.1295	0.093*
O2	0.7731 (4)	0.6722 (5)	0.0580 (7)	0.1060 (18)	0.50
H2O	0.731 (3)	0.693 (7)	−0.024 (6)	0.127*	0.50
H1O	0.820 (2)	0.697 (7)	0.058 (9)	0.127*	0.50

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
Fe1	0.0392 (3)	0.0363 (2)	0.0409 (3)	0.000	0.0171 (2)	0.000
O1	0.0566 (10)	0.0404 (8)	0.0594 (12)	0.0029 (7)	0.0180 (9)	0.0022 (7)
N1	0.0386 (12)	0.0377 (12)	0.0381 (16)	0.000	0.0177 (12)	0.000
N2	0.0432 (10)	0.0457 (10)	0.0402 (12)	−0.0006 (8)	0.0148 (9)	0.0031 (8)
C1	0.0439 (11)	0.0399 (11)	0.0445 (14)	−0.0051 (9)	0.0216 (11)	−0.0059 (9)
C2	0.0425 (11)	0.0491 (12)	0.0406 (14)	−0.0063 (9)	0.0195 (11)	−0.0055 (10)
N3	0.0590 (13)	0.0831 (16)	0.0759 (18)	0.0123 (11)	0.0401 (14)	0.0203 (13)
C4	0.0638 (15)	0.0422 (12)	0.0639 (19)	−0.0088 (11)	0.0304 (14)	−0.0110 (11)
C3	0.0566 (15)	0.0675 (15)	0.0494 (17)	−0.0073 (12)	0.0141 (13)	−0.0109 (13)
C6	0.0573 (14)	0.0575 (14)	0.0399 (16)	0.0027 (11)	0.0077 (12)	0.0085 (11)
C9	0.0462 (12)	0.0456 (12)	0.0491 (15)	0.0018 (9)	0.0191 (12)	0.0080 (10)
C7	0.0609 (15)	0.0573 (14)	0.0629 (19)	0.0132 (12)	0.0153 (15)	0.0141 (13)
C5	0.082 (3)	0.0326 (16)	0.078 (3)	0.000	0.037 (2)	0.000
C8	0.096 (2)	0.0357 (12)	0.080 (2)	0.0096 (13)	0.0204 (17)	0.0001 (13)
O2	0.117 (4)	0.118 (4)	0.120 (5)	0.039 (4)	0.086 (4)	0.037 (4)

Geometric parameters (Å, º)

Fe1—N1	2.107 (2)	C4—C5	1.382 (3)
Fe1—C9	2.163 (3)	C4—H4	0.9300
Fe1—C9^i	2.163 (3)	C3—H3A	0.9600
Fe1—N2	2.203 (2)	C3—H3B	0.9600
Fe1—N2^i	2.203 (2)	C3—H3C	0.9600
Fe1—O1^i	2.3342 (18)	C6—C7	1.496 (4)
Fe1—O1	2.3342 (18)	C6—H6A	0.9700
O1—C8	1.413 (3)	C6—H6B	0.9700
O1—C7	1.414 (3)	C7—H7A	0.9700
N1—C1	1.348 (3)	C7—H7B	0.9700
N1—C1^i	1.348 (3)	C5—C4^i	1.382 (3)
N2—C2	1.276 (3)	C5—H5	0.9300
N2—C6	1.472 (3)	C8—C8^i	1.477 (6)
C1—C4	1.383 (3)	C8—H8A	0.9700
C1—C2	1.489 (3)	C8—H8B	0.9700
C2—C3	1.498 (3)	O2—H2O	0.86 (2)
N3—C9	1.135 (3)	O2—H1O	0.86 (2)
N1—Fe1—C9	95.31 (6)	N2—C2—C3	127.0 (2)
N1—Fe1—C9^i	95.31 (6)	C1—C2—C3	118.9 (2)
C9—Fe1—C9^i	169.39 (12)	C5—C4—C1	118.5 (3)
N1—Fe1—N2	72.75 (5)	C5—C4—H4	120.7
C9—Fe1—N2	90.66 (9)	C1—C4—H4	120.7
C9^i—Fe1—N2	92.49 (9)	C2—C3—H3A	109.5
N1—Fe1—N2^i	72.75 (5)	C2—C3—H3B	109.5
C9—Fe1—N2^i	92.49 (9)	H3A—C3—H3B	109.5
C9^i—Fe1—N2^i	90.66 (9)	C2—C3—H3C	109.5
N2—Fe1—N2^i	145.50 (10)	H3A—C3—H3C	109.5
N1—Fe1—O1^i	144.40 (5)	H3B—C3—H3C	109.5
C9—Fe1—O1^i	83.65 (8)	N2—C6—C7	109.0 (2)
C9^i—Fe1—O1^i	87.72 (8)	N2—C6—H6A	109.9
N2—Fe1—O1^i	142.69 (7)	C7—C6—H6A	109.9
N2^i—Fe1—O1^i	71.76 (7)	N2—C6—H6B	109.9
N1—Fe1—O1	144.40 (5)	C7—C6—H6B	109.9
C9—Fe1—O1	87.72 (8)	H6A—C6—H6B	108.3
C9^i—Fe1—O1	83.65 (8)	N3—C9—Fe1	178.1 (2)
N2—Fe1—O1	71.76 (7)	O1—C7—C6	108.2 (2)
N2^i—Fe1—O1	142.69 (7)	O1—C7—H7A	110.1
O1^i—Fe1—O1	71.20 (9)	C6—C7—H7A	110.1
C8—O1—C7	116.0 (2)	O1—C7—H7B	110.1
C8—O1—Fe1	112.35 (17)	C6—C7—H7B	110.1
C7—O1—Fe1	110.50 (14)	H7A—C7—H7B	108.4
C1—N1—C1^i	120.1 (3)	C4^i—C5—C4	120.3 (3)
C1—N1—Fe1	119.93 (13)	C4^i—C5—H5	119.8
C1^i—N1—Fe1	119.93 (13)	C4—C5—H5	119.8
C2—N2—C6	122.4 (2)	O1—C8—C8^i	108.8 (2)
C2—N2—Fe1	119.29 (16)	O1—C8—H8A	109.9
C6—N2—Fe1	118.30 (16)	C8^i—C8—H8A	109.9
N1—C1—C4	121.2 (2)	O1—C8—H8B	109.9
N1—C1—C2	113.94 (19)	C8^i—C8—H8B	109.9
C4—C1—C2	124.8 (2)	H8A—C8—H8B	108.3
N2—C2—C1	114.1 (2)	H2O—O2—H1O	106 (3)

Symmetry code: (i) −x+2, y, −z+1/2.