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)

Abstract

Tris-(dimethylamino)phosphine reacts with I₂ to form (Me₂N)₃PI₂, which when recrystallised from acetonitrile displays a structure of overall stoichiometry [{(Me₂N)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 (Me₂N)₃P and ICl. The reaction of (Me₂N)₃P with IBr forms [(Me₂N)₃PBr]I, 3, which when recrystallised from chlorinated solvents forms [(Me₂N)₃PCl_0.5Br_0.5]I, 3b. The analogous [(Me₂N)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 C_s 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.