(Ferrocenylmethyl)dimethylammonium bromide

The title compound, [Fe(C5H5)(C8H13N)]Br, is isotypic with the analogous chloride compound. The Fe—C bond lengths are in the range 2.020 (6)–2.048 (7) Å. In the crystal, the cations and bromide anions are connected by N+—H⋯Br− hydrogen bonds.

The Fe atom is bonded to the two five-membered carbon rings with Fe-C bond lengths in the range 2.020 (6)-2.048 (7) Å, with mean values of 2.036 and 2.025Å for the unsubstituted and substituted Cp rings, respectively. The Fe···Cp plane distances are 1.638 and 1.651 Å for Cp1 and Cp2, respectively, and the Cp1-Fe-Cp2 angle is 178.58°. These suggest that an interaction may exist between the methyldimethylamine group and the Fe atom, drawing the less electron-rich substituted Cp ligand marginally closer to the metal centre. The two rings, Cp1(C1-C5) and Cp2 (C6-C10), are nearly parallel with a dihedral angle between their mean planes of 1.7°. They exhibit a nearly eclipsed conformation, as is usually found in other ferrocene derivatives (for example, the structures listed above).
In the crystal structure, the cations and bromide ions are connected by N + -H···Brhydrogen bonds (Fig. 2).

Experimental
(Ferrocenylmethyl)dimethylamine (0.607 g, 2.5 mmol) was dissolved in ethanol (15 ml) and a yellow solid was obtained after adding HBr (0.5 g, 40%). The precipitate was dissolved by adding DMF and single crystals of the title compound suitable for X-ray analysis were obtained on slow evaporation of the solvents over a period of 7 days.
The dielectric constant of the compound as a function of temperature indicates that the permittivity is basically temperature-independent (ε = C/(T-T 0 )), suggesting that this compound is not ferroelectric or there may be no distinct phase transition occurring within the measured temperature range. Similarly, below the melting point of the compound (184°C), the dielectric constant as a function of temperature goes smoothly, and no dielectric anomaly is observed.

Refinement
H atoms bound to C atoms were positioned geometrically, with C-H = 0.98, 0.97 and 0.96 Å for those on cyclopentadienyl, methylene and methyl C atoms, respectively, and refined as riding with U iso (H) = 1.2U eq (C) or 1.5U eq (methyl C). The methyl groups were allowed to rotate about their local threefold axes. Atom H1A was positioned geometrically and allowed to ride on N1, with N-H = 0.91 Å and U iso (H) = 1.2U eq (N).

Special details
Geometry. All esds (except the esd in the dihedral angle between two l.s. planes) are estimated using the full covariance matrix. The cell esds are taken into account individually in the estimation of esds in distances, angles and torsion angles; correlations between esds in cell parameters are only used when they are defined by crystal symmetry. An approximate (isotropic) treatment of cell esds is used for estimating esds 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 > 2sigma(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.