A linear Fe–O–Fe unit in bis­(dibenzyl­dimethyl­ammonium) μ-oxo-bis­[tribromo­ferrate(III)]

The title compound, (C₁₆H₂₀N)₂[Fe₂Br₆O], crystallizes with one dibenzyl­dimethylammonium cation and one half of a μ-oxo-bis­[tribromo­ferrate(III)] anion in the asymmetric unit. The bridging oxo group is situated on an inversion centre, resulting in a linear conformation for the Fe—O—Fe unit. The iron(III) cations have tetra­hedral geometry, with bond angles in the range 106.8 (1)–112.2 (1)°. The ion pairs are held together by Coulombic forces and C—H...Br hydrogen bonds. Each Br⁻ anion forms one hydrogen bond. No C—H...O hydrogen bonds are found between the O atom in the Fe—O—Fe unit and surrounding counter-cations, consistent with the linear configuration of the Fe—O—Fe unit.

S1. Comment

The Fe—O—Fe unit has been of interest in inorganic chemistry because of its magnetic properties, its stability (diferric form) and its occurrence at the active sites of proteins (Kurtz, 1990; Gorun & Lippard, 1991; Davydov et al., 1997; Scarrow et al., 1986; Stenkamp et al., 1984; Klotz & Kurtz, 1984; Nordlund et al., 1990; Reichard & Ehrenberg, 1983; Lynch et al., 1989; Gatteschi et al., 2000). The synthesis, crystal structure and magnetic properties of [Hpy]₂[Fe₂Cl₆O] were reported by Drew et al. (1978). Since then, dozens of similar complexes incorporating the [Fe₂Cl₆O]²⁻ unit have been synthesized and characterized by various analytical methods and by X-ray crystallography (Petridis & Terzis, 1986; Healy et al., 1983; Vasilevsky et al., 1988; Haselhorst et al., 1993; Solbrig et al., 1982; Bullen et al., 1986; Armstrong & Lippard, 1985; Do et al., 1983; Adler et al., 1988; Molins et al., 1998; Senda et al., 2000; Lledós et al., 2003; Wei et al., 2004). In some cases, crystallographic problems have been associated with crystallographically imposed site symmetry C_s, C_i or C₂, which masks an orientational disorder of the anion (Haselhorst et al., 1993).

At present, approximately 50 structures containing the dinuclear [Fe₂Cl₆O]²⁻ unit have been published. About one-third of them have nearly linear Fe—O—Fe cores, but the real number is difficult to estimate due to the orientational disorder. The Fe—O—Fe angle in the solid state varies from 180° to about 140°, depending on the countercation. The structure has been considered as bent if the Fe—O—Fe angle is between 146 and 171° (Lledós et al., 2003). Lledós et al. (2003) analyzed the diversity of Fe—O—Fe angles varying from 140 to 180° in the X-ray crystal structures of the [Fe₂Cl₆O]²⁻ dianion. Only the linear isomer was found as a minimum on the potential energy surface by theoretical calculations. Detailed studies of the crystal packing showed that the angular form occurs when attractive intermolecular interactions (C—H···O contacts) are involved. If the interactions are strong and cooperative, the O atom is displaced from its central position and the bent form is present. If no interactions are found or if they are opposed, the configuration remains linear at the potential minimum.

In the only report to date of a structure with the dinuclear entity [Fe₂Br₆O]²⁻, Evans et al. (1992) described the crystal structure of (Fc⁺)₂[(FeBr₃)₂O]²⁻, which was prepared from FeBr₃ and ferrocene (Fc). The anion consists of two corner-sharing FeBr₃O tetrahedra, and the cations are ferroceniums with eclipsed cyclopentadienyl rings. A bent Fe—O—Fe angle of 159.84 (13)° was observed.

Our aim has been to synthesize different types of tetrahalometallate complexes and investigate the structural properties of the MX₄²⁻ anions in the solid state, by varying the first-row transition metal cations (M^II) and the halides (X = Br⁻ and/or Cl⁻) in these anions. The same counter-cation, dibenzyldimethylammonium (Busi et al., 2004; Ropponen et al., 2004), was used in all experiments. A new and interesting compound, the title complex, (I), was crystallized as part of these studies.

The molecular structure of (I) consists of an [Fe₂Br₆O]²⁻ anion with a linear Fe—O—Fe core and two dibenzyldimethylammonium cations. The asymmetric unit contains one cation and one half of the µ-oxo-di[trihaloferrate(III)] anion, which sits on an inversion centre. The structure and labelling scheme of (I) are presented in Fig. 1. Selected bond lengths and angles are presented in Table 1, which gives the geometries around the Fe^III cation and N atom. The linearity of the Fe1—O1—Fe1ⁱ fragment is required by the centre of symmetry [symmetry code: (i) 1 − x, −y, 1 − z]. The Fe^III cation is four-coordinated by three Br⁻ ions and the bridging oxo group. The bond angles around Fe^III vary between 106.82 (4) and 112.21 (4)°, which are typical values for [Fe₂X₆O]²⁻ anions (X = Br⁻ or Cl⁻). The configuration around the N atoms in the cation is also tetrahedral, with angles in the range 107.4 (4)–111.5 (4)°, typical for N atoms in quaternary ammonium cations (Busi et al., 2004, 2005, 2006).

As already mentioned, the linear shape of the Fe—O—Fe unit is obliged by the crystallographic centre of symmetry. The displacement ellipsoid of the O atom has a slightly elongated shape, which could indicate a dynamic or static disorder between two rotational conformers. However, we note that distortions away from tetrahedral geometry are unusual for the Fe^III ions in [Fe₂X₆O]²⁻ anions with bent Fe—O—Fe moieties. During the refinement of a bent model for (I), the coordination sphere around the Fe^III cations is distorted away from tetrahedral geometry, giving bond angles in the range 104.3 (7)–119.5 (7)°. In addition, the work of Lledós et al. (2003) points out that the linear configuration of the [Fe₂Br₆O]²⁻ dianion is at the potential minimum and that, generally, the bent Fe—O—Fe moiety demands C—H···O hydrogen bonds from cations to bridging O atoms, which are not found in this case. Thus, two angular conformers are very unlikely. This is then the first compound including the [Fe₂Br₆O]²⁻ anion with a linear Fe—O—Fe moiety to have been crystallized.

In the extended structure of (I), the [Fe₂Br₆O]²⁻ anions are located at the corners of the cell and in the middle of each cell face. Each anion is surrounded by six cations. The cations are not in the W conformation, as was the case for dibenzyldimethylammonium bromide (Busi et al., 2004). In (I), the cations have a somewhat twisted conformation. In addition to Coulombic forces between the cations and anions, the packing in (I) is influenced by C—H···Br hydrogen bonds (Table 2) [H···Br <3.0 Å; a mean value for H···Br of 2.96 (1) Å was given by Desiraju & Steiner (1999)], which are shown in Fig. 2. One [Fe₂Br₆O]²⁻ anion acts as acceptor for six hydrogen bonds (<3.0 Å) from the dibenzyldimethylammonium cations, related in pairs by the centre of symmetry in the middle of the anion.

S2. Experimental

Single crystals of the title compound were obtained from an acetonitrile solution containing stoichiometric amounts (2:1) of the dibenzyldimethylammonium halide salt (Busi et al., 2004) and anhydrous FeBr₂ salt. The crystals were obtained at room temperature by slow evaporation of the solvent.

S3. Refinement

H atoms were positioned geometrically and treated as riding, with C—H = 0.95–0.99 Å and with U_iso(H) = 1.2U_eq(C), or 1.5U_eq(C) for methyl atoms. [Please check added text]

Computing details

Data collection: COLLECT (Nonius, 1998); cell refinement: COLLECT; data reduction: DENZO-SMN (Otwinowski & Minor, 1997); program(s) used to solve structure: SIR97 (Altomare et al., 1999); program(s) used to refine structure: SHELXL97 (Sheldrick, 1997); molecular graphics: DIAMOND (Brandenburg, 2001); software used to prepare material for publication: WinGX (Farrugia, 1999).

Figures

	Fig. 1. The molecular structure of (I), with the atom-numbering scheme. Displacement ellipsoids are drawn at the 50% probability level and H atoms are shown as small spheres of arbitrary radii. Two of six hydrogen bonds (Br···H—C < 3.0 Å) around the anion are shown as dashed lines. [Symmetry code: (i) 1 − x, −y, 1 − z.]
	Fig. 2. The packing in (I), showing C—H···Br hydrogen bonding around the [Fe2Br6O]^2− anion. [Symmetry code: (i) 1 − x, −y, 1 − z.]

bis(dibenzyldimethylammonium) µ-oxo-bis[tribromoferrate(III)]

Crystal data

(C16H20N)2[Fe2Br6O]	F(000) = 2064
Mr = 1059.82	Dx = 1.862 Mg m^−3
Orthorhombic, Pbca	Mo Kα radiation, λ = 0.71073 Å
Hall symbol: -P 2ac 2ab	Cell parameters from 4645 reflections
a = 13.800 (3) Å	θ = 0.4–27.1°
b = 14.984 (3) Å	µ = 7.14 mm^−1
c = 18.283 (4) Å	T = 173 K
V = 3780.5 (13) Å^3	Block, orange
Z = 4	0.25 × 0.15 × 0.05 mm

Data collection

Bruker Nonius Kappa APEXII CCD area-detector diffractometer	4618 independent reflections
Radiation source: fine-focus sealed tube	3215 reflections with I > 2σ(I)
Graphite monochromator	Rint = 0.056
ω and ϕ scans	θmax = 28.1°, θmin = 2.3°
Absorption correction: multi-scan (DENZO-SMN; Otwinowski & Minor, 1997)	h = −18→18
Tmin = 0.269, Tmax = 0.717	k = −19→19
8735 measured reflections	l = −24→24

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)] = 0.049	H-atom parameters constrained
wR(F^2) = 0.110	w = 1/[σ^2(Fo^2) + (0.0304P)^2 + 14.2938P] where P = (Fo^2 + 2Fc^2)/3
S = 1.06	(Δ/σ)max = 0.001
4618 reflections	Δρmax = 0.96 e Å^−3
199 parameters	Δρmin = −0.73 e Å^−3
0 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: 0.00036 (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.

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

	x	y	z	Uiso*/Ueq
C11	0.0121 (4)	−0.0478 (4)	0.6217 (3)	0.0321 (11)
H11A	0.0523	−0.0837	0.5879	0.038*
H11B	−0.0540	−0.0736	0.6215	0.038*
C12	0.0069 (4)	0.0462 (3)	0.5934 (3)	0.0299 (11)
C13	−0.0766 (4)	0.0972 (3)	0.6018 (3)	0.0349 (12)
H13A	−0.1314	0.0724	0.6258	0.042*
C14	−0.0806 (4)	0.1842 (4)	0.5756 (3)	0.0382 (13)
H14A	−0.1376	0.2189	0.5817	0.046*
C15	−0.0013 (4)	0.2193 (4)	0.5406 (3)	0.0374 (12)
H15A	−0.0031	0.2793	0.5237	0.045*
C16	0.0811 (4)	0.1686 (4)	0.5298 (3)	0.0346 (12)
H16A	0.1351	0.1933	0.5047	0.041*
C17	0.0846 (4)	0.0825 (3)	0.5554 (3)	0.0300 (11)
H17A	0.1409	0.0473	0.5470	0.036*
C21	0.0598 (4)	−0.1552 (3)	0.7195 (3)	0.0329 (11)
H21A	0.1016	−0.1859	0.6834	0.039*
H21B	0.0917	−0.1601	0.7678	0.039*
C22	−0.0352 (3)	−0.2030 (3)	0.7230 (3)	0.0309 (11)
C23	−0.0883 (4)	−0.2044 (4)	0.7874 (3)	0.0368 (12)
H23A	−0.0641	−0.1747	0.8295	0.044*
C24	−0.1765 (4)	−0.2489 (4)	0.7909 (3)	0.0388 (12)
H24A	−0.2133	−0.2484	0.8348	0.047*
C25	−0.2101 (4)	−0.2936 (4)	0.7303 (3)	0.0401 (13)
H25A	−0.2705	−0.3239	0.7325	0.048*
C26	−0.1571 (4)	−0.2950 (3)	0.6664 (3)	0.0361 (12)
H26A	−0.1805	−0.3269	0.6251	0.043*
C27	−0.0702 (4)	−0.2502 (3)	0.6626 (3)	0.0320 (11)
H27A	−0.0337	−0.2514	0.6186	0.038*
C31	−0.0048 (4)	−0.0051 (4)	0.7530 (3)	0.0385 (12)
H31A	0.0235	−0.0122	0.8018	0.058*
H31B	−0.0713	−0.0279	0.7532	0.058*
H31C	−0.0051	0.0583	0.7397	0.058*
C41	0.1557 (4)	−0.0196 (4)	0.7004 (3)	0.0355 (12)
H41A	0.1546	0.0440	0.6877	0.053*
H41B	0.1958	−0.0520	0.6651	0.053*
H41C	0.1828	−0.0270	0.7496	0.053*
N1	0.0540 (3)	−0.0561 (3)	0.6988 (2)	0.0298 (9)
Fe1	0.37569 (5)	0.01945 (5)	0.51130 (4)	0.03305 (19)
Br1	0.29221 (4)	−0.11490 (4)	0.53578 (3)	0.04031 (16)
Br2	0.35522 (4)	0.11681 (4)	0.61160 (3)	0.04042 (16)
Br3	0.30371 (5)	0.08244 (4)	0.40481 (3)	0.04824 (18)
O1	0.5000	0.0000	0.5000	0.068 (2)

Atomic displacement parameters (Ų)

	U^11	U^22	U^33	U^12	U^13	U^23
C11	0.030 (2)	0.037 (3)	0.029 (3)	−0.003 (2)	−0.005 (2)	−0.001 (2)
C12	0.036 (3)	0.027 (2)	0.027 (3)	−0.002 (2)	−0.001 (2)	−0.003 (2)
C13	0.028 (2)	0.036 (3)	0.040 (3)	−0.002 (2)	0.004 (2)	0.001 (2)
C14	0.030 (3)	0.034 (3)	0.051 (3)	0.005 (2)	−0.004 (2)	−0.002 (3)
C15	0.047 (3)	0.028 (3)	0.036 (3)	−0.004 (2)	−0.006 (2)	0.002 (2)
C16	0.035 (3)	0.039 (3)	0.030 (3)	−0.006 (2)	0.001 (2)	0.001 (2)
C17	0.031 (2)	0.035 (3)	0.024 (2)	−0.001 (2)	−0.0009 (19)	−0.002 (2)
C21	0.032 (3)	0.034 (3)	0.032 (3)	0.003 (2)	−0.002 (2)	0.003 (2)
C22	0.032 (2)	0.029 (3)	0.032 (3)	0.001 (2)	0.003 (2)	0.004 (2)
C23	0.042 (3)	0.044 (3)	0.024 (3)	−0.003 (3)	0.001 (2)	0.001 (2)
C24	0.043 (3)	0.044 (3)	0.029 (3)	−0.008 (3)	0.008 (2)	0.003 (2)
C25	0.039 (3)	0.041 (3)	0.041 (3)	−0.012 (3)	−0.002 (2)	0.009 (2)
C26	0.050 (3)	0.031 (3)	0.027 (3)	−0.002 (2)	−0.009 (2)	0.001 (2)
C27	0.041 (3)	0.032 (3)	0.023 (2)	0.004 (2)	0.002 (2)	0.001 (2)
C31	0.042 (3)	0.037 (3)	0.036 (3)	0.007 (2)	0.006 (2)	0.002 (2)
C41	0.032 (3)	0.047 (3)	0.027 (3)	−0.009 (2)	−0.004 (2)	0.003 (2)
N1	0.029 (2)	0.030 (2)	0.030 (2)	−0.0021 (18)	0.0007 (17)	−0.0013 (18)
Fe1	0.0253 (3)	0.0392 (4)	0.0346 (4)	0.0030 (3)	0.0062 (3)	−0.0074 (3)
Br1	0.0523 (3)	0.0313 (3)	0.0373 (3)	0.0034 (2)	0.0004 (2)	0.0005 (2)
Br2	0.0360 (3)	0.0458 (3)	0.0394 (3)	−0.0018 (2)	0.0033 (2)	−0.0143 (2)
Br3	0.0683 (4)	0.0455 (3)	0.0309 (3)	−0.0070 (3)	0.0039 (3)	0.0049 (2)
O1	0.024 (3)	0.077 (5)	0.102 (6)	0.009 (3)	0.010 (3)	−0.022 (4)

Geometric parameters (Å, º)

C11—C12	1.503 (7)	C23—C24	1.389 (7)
C11—N1	1.528 (6)	C23—H23A	0.9500
C11—H11A	0.9900	C24—C25	1.375 (8)
C11—H11B	0.9900	C24—H24A	0.9500
C12—C17	1.389 (7)	C25—C26	1.379 (8)
C12—C13	1.391 (7)	C25—H25A	0.9500
C13—C14	1.389 (7)	C26—C27	1.376 (7)
C13—H13A	0.9500	C26—H26A	0.9500
C14—C15	1.371 (8)	C27—H27A	0.9500
C14—H14A	0.9500	C31—N1	1.492 (6)
C15—C16	1.382 (8)	C31—H31A	0.9800
C15—H15A	0.9500	C31—H31B	0.9800
C16—C17	1.374 (7)	C31—H31C	0.9800
C16—H16A	0.9500	C41—N1	1.506 (6)
C17—H17A	0.9500	C41—H41A	0.9800
C21—C22	1.495 (7)	C41—H41B	0.9800
C21—N1	1.535 (6)	C41—H41C	0.9800
C21—H21A	0.9900	Fe1—O1	1.7523 (7)
C21—H21B	0.9900	Fe1—Br2	2.3601 (10)
C22—C23	1.388 (7)	Fe1—Br1	2.3622 (10)
C22—C27	1.397 (7)	Fe1—Br3	2.3808 (11)
C12—C11—N1	114.3 (4)	C25—C24—H24A	120.3
C12—C11—H11A	108.7	C23—C24—H24A	120.3
N1—C11—H11A	108.7	C24—C25—C26	120.8 (5)
C12—C11—H11B	108.7	C24—C25—H25A	119.6
N1—C11—H11B	108.7	C26—C25—H25A	119.6
H11A—C11—H11B	107.6	C27—C26—C25	119.8 (5)
C17—C12—C13	118.7 (5)	C27—C26—H26A	120.1
C17—C12—C11	120.1 (5)	C25—C26—H26A	120.1
C13—C12—C11	121.1 (5)	C26—C27—C22	120.6 (5)
C14—C13—C12	120.7 (5)	C26—C27—H27A	119.7
C14—C13—H13A	119.7	C22—C27—H27A	119.7
C12—C13—H13A	119.7	N1—C31—H31A	109.5
C15—C14—C13	119.3 (5)	N1—C31—H31B	109.5
C15—C14—H14A	120.3	H31A—C31—H31B	109.5
C13—C14—H14A	120.3	N1—C31—H31C	109.5
C14—C15—C16	120.8 (5)	H31A—C31—H31C	109.5
C14—C15—H15A	119.6	H31B—C31—H31C	109.5
C16—C15—H15A	119.6	N1—C41—H41A	109.5
C17—C16—C15	119.8 (5)	N1—C41—H41B	109.5
C17—C16—H16A	120.1	H41A—C41—H41B	109.5
C15—C16—H16A	120.1	N1—C41—H41C	109.5
C16—C17—C12	120.7 (5)	H41A—C41—H41C	109.5
C16—C17—H17A	119.7	H41B—C41—H41C	109.5
C12—C17—H17A	119.7	C31—N1—C41	107.9 (4)
C22—C21—N1	115.4 (4)	C31—N1—C11	111.5 (4)
C22—C21—H21A	108.4	C41—N1—C11	110.0 (4)
N1—C21—H21A	108.4	C31—N1—C21	111.0 (4)
C22—C21—H21B	108.4	C41—N1—C21	107.4 (4)
N1—C21—H21B	108.4	C11—N1—C21	109.0 (4)
H21A—C21—H21B	107.5	O1—Fe1—Br2	108.16 (4)
C23—C22—C27	118.7 (5)	O1—Fe1—Br1	110.98 (4)
C23—C22—C21	120.4 (5)	Br2—Fe1—Br1	108.73 (4)
C27—C22—C21	120.8 (5)	O1—Fe1—Br3	112.21 (4)
C22—C23—C24	120.6 (5)	Br2—Fe1—Br3	109.91 (4)
C22—C23—H23A	119.7	Br1—Fe1—Br3	106.82 (4)
C24—C23—H23A	119.7	Fe1^i—O1—Fe1	180.0
C25—C24—C23	119.5 (5)
N1—C11—C12—C17	89.5 (6)	C21—C22—C23—C24	−179.9 (5)
N1—C11—C12—C13	−93.1 (6)	C22—C23—C24—C25	1.6 (9)
C17—C12—C13—C14	−2.8 (8)	C23—C24—C25—C26	0.2 (9)
C11—C12—C13—C14	179.9 (5)	C24—C25—C26—C27	−0.9 (8)
C12—C13—C14—C15	0.4 (8)	C25—C26—C27—C22	−0.2 (8)
C13—C14—C15—C16	1.6 (8)	C23—C22—C27—C26	1.9 (8)
C14—C15—C16—C17	−1.2 (8)	C21—C22—C27—C26	179.2 (5)
C15—C16—C17—C12	−1.3 (8)	C12—C11—N1—C31	59.8 (5)
C13—C12—C17—C16	3.2 (7)	C12—C11—N1—C41	−59.9 (5)
C11—C12—C17—C16	−179.4 (5)	C12—C11—N1—C21	−177.4 (4)
N1—C21—C22—C23	−88.6 (6)	C22—C21—N1—C31	60.6 (6)
N1—C21—C22—C27	94.3 (6)	C22—C21—N1—C41	178.3 (4)
C27—C22—C23—C24	−2.6 (8)	C22—C21—N1—C11	−62.5 (5)

Symmetry code: (i) −x+1, −y, −z+1.

Hydrogen-bond geometry (Å, º)

D—H···A	D—H	H···A	D···A	D—H···A
C41—H41B···Br1	0.98	2.87	3.827 (6)	165
C24—H24A···Br2^ii	0.95	2.98	3.648 (6)	128
C41—H41C···Br3^iii	0.98	2.96	3.895 (6)	159

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