Syntheses and structures of cyclopentadienyl arsenic compounds .1. Pentamethylcylopentadienyl arsenic dihalides (Cp(*)AsX(2), X=F, Cl, Br, I)

Treatment of 1,2,3,4,5-pentamethylcyclopenta-2,4-dien-1-yl arsenic(III) dichloride (II) with two equivalents of cobaltocenium fluoride or potassium iodide affords the analogous compounds Cp*AsF2 (I) and Cp*AsI2 (IV) in 85% and 98% yield respectively. Cp*AsBr2 (III) has been synthesized by direct metathesis reaction between AsBr3 and Cp*Li in 70% yield. The new compounds have been characterized by spectroscopic methods (H-1, C-13, F-19 NMR, EI-MS) and elemental analyses, The crystal structures of I-IV were determined by X-ray diffraction methods. The AsX(2)-moiety in I-IV is sigma-bound to the cyclopentadienyl ligand, indicating remarkable pi-interaction with the diene fragment of the cyclopentadienyl ring. In addition, weak intermolecular contacts are observed in I and IV, completing the coordination sphere around the arsenic atom to pseudo-tetrahedral.