Phosphane‐borane chemistry. Open‐chain and cyclic phosphane‐boranes based on tetramethyldiphosphane

Abstract

Abstract1:2 addition compounds of tetramethyldiphosphane with BH3, BH2Br, BHBr2, and BBr3 (1a–d) have been prepared from the parent diphosphane and the dimethyl sulfide complexes of the boranes in the appropriate stoichiometric ratio. Open‐chain and cyclic phosphane‐borane cations were obtained from Me4P2 and H2BrB · SMe2 in the molar ratio 2:1 or 1:1, respectively: [Me2PPMe2 · BH2 · PMe2  PMe2]+Br− (2) or [H2B(Me2PPMe2)2‐ BH2]2+2Br− (3). Treatment of 1b with trimethylphosphane yields the salt [H2BrB·Me2PPMe2 · BH2 · PMe3]+Br− (4). – The crystal structures of 1b and 3 · 2MeOH have been determined by single‐crystal X‐ray diffraction methods. Molecules 1b have crystalographic C2h symmetry corresponding to the anti‐configuration of the diphosphane and staggered conformation of the BH2Br moieties. The monomers are arranged in layers with Br …︁ Br contacts of 3.66 Å. – Crystals of 3 · 2 MeOH have an ionic structure built from bromide ions, interstitial methanol molecules, and crystallographically centrosymmetric dications [H2B(Me2PPMe2)2‐BH2]2+. These dications are in a chair configuration approaching (non‐crystallographical) mirror symmetry. The intra‐ring valence angles at boron [116.2(2)°] are significantly larger than those at phosphorus (108.4°, average). The BP distances are shorter than in standard reference compounds (e.g. 1b), indicating extra stabilisation in the dicationic phosphane‐borane ring system. In the unit cell there are no sub‐van der Waals contacts between the individual components.