Understanding the structural properties of a homologous series of bis-diphenylphosphine oxides

A homologous series of bis-diphenylphosphine oxides (CH2)(2)PO(CH2)(n)PO(C6 H5)(2) (with n = 2-8; denoted 2-8] have been investigated to explore the ef fects of a range of competing and cooperative intermolecular and intramolec ular interactions on the structural properties in the solid state. The impo rtant factors influencing the structural properties include intramolecular aspects such as the conformation of the aliphatic chain and the intramolecu lar interaction between the two P=O dipoles in the molecule, and intermolec ular aspects such as long-range electrostatic interactions (dominated by th e arrangement of the P=O dipoles), C-H ... O interactions, C-H ...pi intera ctions and pi ...pi interactions. Compounds 3 and 5 could be crystallized o nly as solvate co-crystals (3.water and 5.(toluene)(2)], whereas the crysta l structures of all the other compounds contain only the bis-diphenylphospk ine oxide molecule. The crystal structures have been determined from single -crystal X-ray diffraction data, with the exception of 7 (which has been de termined here from powder X-ray diffraction data) and 4 (which was known pr eviously), The compounds with even n represent a systematic structural seri es, exhibiting characteristic, essentially linear P=O ... P=O ... P=O dipol ar arrays, together with C-H ... O and C-H ...pi interactions. For the comp ounds with odd n, on the other hand, uniform structural behaviour is not ob served across the series, although certain aspects of these crystal structu res contribute in a general sense to our understanding of the structural pr operties of bis-diphenylphosphine oxides. Importantly, for the compounds wi th odd n, there is "frustration" with regard to the molecular conformation, as the preferred all-anti conformation of the aliphatic chain gives rise t o an unfavourable parallel alignment of the two P=O dipoles within the mole cule. Clearly the importance of avoiding a parallel alignment of the P=O di poles becomes greater as n decreases. Local structural aspects (investigate d by high-resolution solid-state P-31 NMR spectroscopy) and thermal propert ies of the bis-diphenylphosphine oxide materials are also reported.