Phosphorus-halogen bond elongation and autoionization in a Cl,Br,I-series of N-heterocyclic phosphane halides with saturated backbone: diazaphosphorinane vs. diazaphosphorinenium salt

Abstract

Halide exchange reactions of PX3 (X = Br, I) and the known 1,3-di-tert-butyl-2-chloro-1,3,2-diazaphosphorinane (1) gave the bromide (2) and the iodide (3). The new compounds have been characterized by elemental analysis, IR and NMR spectroscopy. 1H, 13C and 31P NMR spectroscopic data give strong evidence for a non-ionic behaviour of 1, 2 and 3 in solution. Single crystal X-ray structure determinations of 2 and 3 for the first time allowed for a detailed comparative study of the solid state structures of a Cl,Br,I-series of NHPXs with aliphatic backbone. Like in 1 in 2 a partially flattened cyclohexane-like chair conformation with the nitrogen atom substituents in a diequatorial orientation and a substantially elongated covalent bond between the phosphorus atom and the axial oriented halogen atom is found. Neither a covalent P–I bond nor a preferred chair conformation is present in the solid autoionization product 3 that should be labelled phosphorinenium iodide instead of P-iodophosphorinane. Its solid state structural parameters are in line with the expectations for a [−N(tBu)]2>P+-fragment with high diazaphosphaallyl cationic character.