Preference for a C3 conformation in tris- dimethylaminophosphonium cations: A comparison of the reactions of (Me 2N)3P with I-X (X = Cl, Br, I, CN)

Tris-(dimethylamino)phosphine reacts with I2 to form (Me 2N)3PI2, which when recrystallised from acetonitrile displays a structure of overall stoichiometry [{(Me 2N)3PI}I]6·CH3CN 1. The asymmetric unit of 1 consists of four different [(Me2N) 3PI]+ cations, one of these exhibits a cation-anion interaction to an iodide ion, with an I-I contact distance of 3.6378(14) Å, 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) 3PI2 is recrystallised from dichloromethane the molecule abstracts chlorine from the solvent to form [(Me2N)3PCl]I 2; this latter compound can also be synthesised directly from (Me 2N)3P and ICl. The reaction of (Me2N) 3P with IBr forms [(Me2N)3PBr]I, 3, which when recrystallised from chlorinated solvents forms [(Me2N) 3PCl0.5Br0.5]I, 3b. The analogous [(Me 2N)3PCN]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)3P groups in the cations in 1, 2, 3b and 4 exhibit a C3 conformation, in contrast to the majority of (R2N)3P systems where a Cs conformation is usually preferred. This C3 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 π-bonding, and as a result the P-N bonds are shortened, varying between 1.566(10) and 1.624(10) Å in these compounds. © The Royal Society of Chemistry.