1-(Bromomercurio)ferrocene

The asymmetric unit of the title compound, [Fe(C5H5)(C5H4BrHg)], contains two independent molecules, A and B, in which the Hg—C bond lengths are 2.045 (6) and 2.046 (6) Å, the Hg—Br bond lengths are 2.4511 (9) and 2.4562 (7) Å, and the C—Hg—Br angles are 176.42 (17) and 177.32 (17)°. The two cyclopentadienyl rings of molecule A are eclipsed, while those of molecule B are almost staggered. The HgBr groups are connected by intermolecular Hg⋯Br contacts of 3.3142 (9)–3.4895 (11) Å, forming layers parallel to (001). These layers contain both four-membered (HgBr)2 and eight-membered (HgBr)4 rings. Ferrocene–ferrocene C—H⋯π contacts connect the molecular layers along the c-axis direction.

Supplementary data and figures for this paper are available from the IUCr electronic archives (Reference: NC2322).

Comment
Our group has recently reported on the chemical and structural features of compounds containing the group 12 elements Zn, Cd and Hg (Meyer-Wegner et al., 2012;Hayashi et al., 2011;Franz et al., 2011;Lerner, 2005;Wiberg et al., 1997;Wiberg et al., 2001;Margraf et al., 2004). To extend our long-standing investigations in this field, we are currently studying the title compound 1-(bromomercurio)ferrocene. A protocol for the synthesis of 1-(chloromercurio)ferrocene has been reported by Fish & Rosenblum (1965). However, we were not able to prepare the title compound using that method. Therefore we decided to synthesize the compound via a new route by the reaction of 1-(tri-n-butylstannyl)ferrocene with HgBr 2 (see experimental section). Here we report the crystal structure of the resulting compound.
Each HgBr group is connected by four long Hg···Br contacts to three neighboring HgBr groups as shown in Fig. 3.
These additional Hg···Br contacts result in layers of molecules parallel to the (001) plane. Molecule A is connected by two Hg···Br contacts to a symmetry-related molecule A to form a four-membered (HgBr) 2 ring. A similar four-membered ring is formed by two symmetry-related molecules B. Two molecules A and two molecules B are connected by four additional Hg···Br contacts to form eight-membered (HgBr) 4 rings. No intermolecular Hg···Br contacts occur in the crystal structure of 1-(bromomercurio)-2,4,6-trimethylbenzene (Meyer-Wegner et al., 2012). There an undistorted HgBr single bond length of 2.4307 (8) Å was found which is about 0.02 Å shorter than the values of 2.4511 (9) and 2.4562 (7) Å observed for molecules A and B of the title compound. In the crystal structure of 1-(bromomercurio)-4-methoxybenzene (Singh et al., 2005) each HgBr bond is connected to the HgBr bonds of four neighboring molecules via intermolecular Hg···Br contacts of 3.4041 (7) to 3.5461 (7) Å and the HgBr single bond length is elongated to 2.4700 (7) Å. Although the long intermolecular Hg···Br contact distances are near the value of 3.40 Å, generally taken as the nonbonding distance between Hg and Br (Bondi, 1964), they do result in a small lengthening of the HgBr single bond.
The layers are connected along the c-axis direction by ferrocene···ferrocene contacts (Fig. 4). There is a weak intermolecular C ferrocene -H···π ferrocene interaction between the C3-H3A bond and the cyclopentadienyl ring labelled C16-C20 from an adjacent layer (first entry in Table 1). An additional C ferrocene -H···π ferrocene interaction connects the ferrocene supplementary materials groups within a single layer (second entry in Table 2).

Experimental
The starting material 1-(tri-n-butylstannyl)ferrocene was prepared as described by Guillaneux & Kagan (1995). The starting material (5.583 g, 11.75 mmol) was dissolved in CH 2 Cl 2 (20 ml) and was added to HgBr 2 (4.248 g, 11.79 mmol, 1 equivalent) in CH 2 Cl 2 (80 ml). The reaction mixture was stirred for 30 minutes at room temperature and then filtered. All volatiles were removed in vacuo and the resulting solid residue was washed with n-hexane (250 ml). Drying to constant mass yielded the title compound 1-(bromomercurio)ferrocene (3.714 g, 7.978 mmol, 68%). The compound was recrystallized from CH 2 Cl 2 /n-pentane (1:1), resulting in brown blades. 1 H and 13 C{ 1 H} NMR spectra were recorded at 298 K on a Bruker Avance 300 spectrometer using D 6 -benzene as solvent. Chemical shifts are reported in p.p.m. relative to

Refinement
H atoms were positioned geometrically and treated as riding: C-H=0.95 Å and U iso (H)=1.2U eq (C). The minimum and maximum residual peaks of -1.93 and +2.78 e.Å -3 in the difference Fourier synthesis are found at about 0.9 Å from the Hg atoms.

Computing details
Data collection: SMART (Siemens, 1995); cell refinement: SMART (Siemens, 1995); data reduction: SAINT (Siemens, 1995); program(s) used to solve structure: SHELXS97 (Sheldrick, 2008); program(s) used to refine structure: SHELXL97 (Sheldrick, 2008); molecular graphics: SHELXTL (Sheldrick, 2008); software used to prepare material for publication: SHELXL97 (Sheldrick, 2008).     where P = (F o 2 + 2F c 2 )/3 (Δ/σ) max = 0.001 Δρ max = 2.78 e Å −3 Δρ min = −1.93 e Å −3 Extinction correction: SHELXL97 (Sheldrick, 2008), Fc * =kFc[1+0.001xFc 2 λ 3 /sin(2θ)] -1/4 Extinction coefficient: 0.0014 (2) 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 F 2 against ALL reflections. The weighted R-factor wR and goodness of fit S are based on F 2 , conventional R-factors R are based on F, with F set to zero for negative F 2 . The threshold expression of F 2 > σ(F 2 ) is used only for calculating R-factors(gt) etc. and is not relevant to the choice of reflections for refinement. R-factors based on F 2 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 (Å 2 )
x y z U iso */U eq