Synthesis and X-ray crystal structure of 3-cyclopentadienyl- 9-(acetomercuro)-3-cobalta-1,2-dicarba-closo -dodecaborane

Electrophilic mercuration of 3-η 5 -cyclopentadienyl-1,2-dicarba-3-cobalta-closo -dodecaborane 1 with (AcO) 2 Hg in CH 2 Cl 2 /AcOH (7:1) leads mainly to 9-acetomercuro-cobaltacarborane 2 . In addition, 8-mono-and 9,12-dimercurated products 3 and 4 were isolated in small yields. The X-Ray structural analysis of 2 reveals the Hg–B bond length of 2.10(1) Å.


Results and Discussion
The purpose of this work is the synthesis of a monomercurated cobaltacarborane by electrophilic mercuration, its isolation, and the structure proof by X-ray analysis.

Scheme 1
In order to avoid a formation of polymercurated and large amounts of dimercurated product, an excess (20%) of 1 was used.Compounds 2, 3 and 4 were separated by column chromatography on silica gel (yields 60, 14 and 9% respectively).The structure of 3-η 5 -cyclopentadienyl-9-(acetomercuro)-3-cobalta-1,2-dicarba-closo-dodecaborane 2 was confirmed by Xray diffraction study, 1 H and 11 B NMR spectra.A composition of 3 and 4 and the positions of substitution in them were determined by 11 B NMR spectra and elemental analysis.

Experimental Section
General Procedures.The starting compound 1 was prepared by the previously reported procedure. 15 1H NMR and 11 B NMR spectra were recorded on a Bruker WP 200 SY spectrometer (200 MHz for 1 H and 64.2 MHz for 11 B) using TMS and BF 3 •OEt 2 as the external standards.
X-ray analysis of 2. C 9 H 18 B 9 O 2 CoHg, M = 515.04,monoclinic, space group C2/c, at 293 K a = 14.144( 8), b = 13.889(9), c = 16.888(9)Å, β = 107.71(4)o,V = 3160(3) Å 3 , Z = 8, d calc .= 2.165 g/cm 3 .The X-ray diffraction experiment was carried out with a Siemens P3/PC diffractometer (T=293 K, graphite-monochromated MoKα radiation, λ = 0.71073 Å, θ/2θ scan technique, 2θ<56 o ).The total of 3188 reflections was collected.The absorption correction (µ(MoKα) = 107.45cm -1 ) was applied with using the psi-scan technique (T min/max = 0.232/0.997).The structure was solved by direct methods and refined by the SHELXTL PLUS 5 program. 16he H atoms of the carborane cage were located in the difference Fourier synthesis and included in the refinement in the isotropic approximation; the rest H atoms were placed in the geometrically calculated positions and refined in the riding model approximation.Final discrepancy factors were: R 1 = 0.0675 (on F for 2540 observed reflections with I>2σ(I)), wR 2 = 0.1941 (on F 2 for all 2986 independent reflections used in the refinement of 239 parameters).-Crystallographic data, final positional and thermal parameters for atoms and full list of bond lengths and angles for the structure reported in this paper have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication no.CCDC-178487.Copies of the data can be obtained free of charge on application to CCDC, 12 Union Road, Cambridge CB21EZ, UK; fax: +44-1223-336-033; e-mail: deposit@ccdc.cam.ac.uk.

3-Cyclopentadienyl-8-(acetomercuro)-3-cobalta-1,2-dicarba-closo-dodecaborane
(3).Yellow crystals (0.27 g, 14%), mp 197-198 °C. 11 1,2-C 2 B 9 H 11 ): 8 the Co-C carb distances are just barely shorter than Co-B, the C 2 B 5 "open face" is planar within 0.006 Å, and the two 5-membered rings are in a staggered conformation, the torsion angle B(7)X(1)X(2)C(14) involving the centroids of the C 2 B 3 and the Cp rings (X(1) and X(2)) being equal to -40.2 o .The C 2 B 3 /Cp dihedral angle is equal to 4.2 o .The crystal packing of 2 features a short intermolecular Hg•••Hg contact (3.491(2)Å) between the Hg atoms of two molecules related by the two-fold axis.However, the distances of this range are rather common for Hg(II) derivatives and may hardly indicate anything but the normal Van der Waals contact between the metal atoms, which are low-coordinate, and therefore, much more easily exposed to intermolecular contacts than most of the other usually high-coordinated metals.