Preference for a C3 conformation in tris-dimethylaminophosphonium cations:: A comparison of the reactions of (Me2N)3P with I-X (X = Cl, Br, I, CN)

Tris-(dimethylamino)phosphine reacts with I-2 to form (Me2N)(3)PI2 which when recrystallised from acetonitrile displays a structure of overall stoichiometry [{(Me2N)(3)PI}I](6)center dot CH3CN 1. The asymmetric unit of 1 consists of four different [(Me2N)(3)PI](+) cations, one of these exhibits a cation-anion interaction to an iodide ion, with an I-I contact distance of 3.6378(14) angstrom, the longest yet observed for an R3PI2 compound. Two of the other three cations display no interactions, whilst a cation-solvent interaction is observed for the fourth. When (Me2N)(3)PI2 is recrystallised from dichloromethane the molecule abstracts chlorine from the solvent to form [(Me2N)(3)PCl]I 2; this latter compound can also be synthesised directly from (Me2N)(3)P and ICI. The reaction of (Me2N)(3)P with IBr forms [(Me2N)(3)PBr]I, 3, which when recrystallised from chlorinated solvents forms [(Me2N)(3)PCl0.5Br0.5]I, 3b. The analogous [(Me2N)(3)PCN]I, 4 does not display CN-Cl exchange and can be recrystallised from dichloromethane. The structures of 1, 2, 3b and 4 have all been determined by X-ray diffraction. All of the (Me2N)(3)p groups in the cations in 1, 2, 3b and 4 exhibit a C-s conformation, in contrast to the majority of (R2N)(3)P systems where a C-s conformation is usually preferred. This C-3 conformation appears to be favoured where there is increased positive charge on phosphorus, as is the case in the phosphorus(v) ionic species described herein. This conformation allows greater P-N pi-bonding, and as a result the P- N bonds are shortened, varying between 1.566(10) and 1.624(10) angstrom in these compounds.