Synthesis and structural characterisation of R3AsX2 compounds (R = Me, Ph, p-FC6H4 or p-MeOC6H4; X2= Br2, I2 or IBr); dependency of structure on R, X and the solvent of preparation

Abstract

Compounds of stoichiometry R₃AsX₂(R =p-MeOC₆H₄, Ph or Me; X = Br or I; R =p-FC₆H₄, X = I), R₃AsI₄(R =p-FC₆H₄ or Ph) and Ph₃AsIBr have been synthesised in anhydrous diethyl ether and characterised by elemental analysis and Raman spectroscopy. Single-crystal X-ray diffraction studies of Ph₃AsBr₂ showed it to possess a molecular trigonal-bipyramidal structure, in contrast to Ph₃AsI₂ which adopts the molecular four-co-ordinate structure Ph₃As-I-I. Both Me₃AsI₂ and Me₃AsBr₂ have the molecular four-co-ordinate structure, Me₃As-X-X, thus illustrating that the structure of R₃AsX₂ compounds isolated from diethyl ether is dependent on R and X. The structure of Ph₃AsIBr also revealed four-co-ordinate molecular geometry in contrast to Ph₃AsBr₂. The compound Ph₃AsIBr showed no dual occupancy of the halogen sites, and the heavier halogen is bound directly to the arsenic. The X-ray powder diffraction patterns of Ph₃AsX₂(X₂= Br₂, I₂ or IBr) have been recorded and are compared. Solution studies on R₃AsI₂(R =p-FC₆H₄, Ph or p-MeOC₆H₄) again showed the sensitivity of these molecules to the nature of R. For example, (p-FC₆H₄)₃AsI₂ was not formed in any solvent employed; instead, equimolar quantities of (p-FC₆H₄)₃AsI₄ and (p-FC₆H₄)₃As were isolated; Ph₃AsI₂ was only formed in non-polar solvents upon dropwise addition of a light petroleum solution of I₂ to a saturated solution of Ph₃As in the same solvent. The compound (p-MeOC₆H₄)₃AsI₂ was the quantitative product from the direct reaction of the tertiary arsine with I₂ in diethyl ether. Its stability constant in CCl₄ is approximately 2.5 times greater than that calculated for Ph₃AsI₂.