The structure of R3PBr2 compounds in the solid state and in solution; geometrical dependence on R, the crystal structures of tetrahedral ionic Et3PBr2 and molecular trigonal bipyramidal (C6F5)3PBr2

Abstract

Twenty-one triorganophosphorus dibromide compounds, R₃PBr₂ (R₃ = substituted aryl, mixed aryl–alkyl, triaryl or trialkyl) have been synthesized from diethyl ether solution and characterised by analytical and ³¹P-{H} NMR data in CDCl₃ solution, the vast majority being reported for the first time. All but one of the compounds are ionic, [R₃PBr]Br in CDCl₃ solution, in keeping with analogous species containing iodine or chlorine, [R₃PX]X (X = I or Cl) according to ³¹P-{H} NMR studies. In contrast, (C₆F₅)₃PBr₂ has a molecular five-co-ordinate trigonal bipyramidal structure both in CDCl₃ solution and in the solid state. The single crystal structure of this compound has been determined and it represents the only reported R₃PBr₂ species which contains a five-co-ordinate phosphorus atom. It has D₃ symmetry and perfect trigonal bipyramidal geometry. Why (C₆F₅)₃PBr₂ is the only R₃PBr₂ compound which adopts trigonal bipyramidal geometry is reasoned to be due to the very low basicity of the parent tertiary phosphine, which favours this geometry for the dihalogen adducts, a phenomenon previously observed for dichlorine adducts of tertiary phosphines and dihalogen adducts of tertiary arsines. The crystal structure of the first non-solvated trialkylphosphine dibromide compound, Et₃PBr₂, has also been determined and contains a tertrahedral phosphorus atom, exhibiting a long Br· · ·Br contact, 3.303(2) Å, and is probably better described as ionic, [Et₃PBr]Br, with significant cation–anion interaction.