Understanding the Structural Properties of a Homologous Series of Bis-diphenylphosphine Oxides

摘要

A homologous series of bis-diphenylphosphine oxides (C_6H_5)_2 POO-(CH_2)_nPO(C_6H_5)_2 (with n=2-8; denoted 2-8] have been investigated to explore the effects of a range of competing and cooperative intermolecular and intramolecular interactions on the structural properties in the solid state. The imortant factors influencing the structural properties include intramolecular aspects such as the conformation of the aliphatic chain and the intramoelcular interaction between the two P=O dipoles in the molecule, and intermolecular aspects such as long-range electrostatic interactions (dominated by the arrangement of the P=O dipoles), C-H...O interactions, C-H...#pi# interactions and #pi#...#pi# interactions. Compounds 3 and 5 could be crystalized only as solvate co-crystals (3 centre dot water and 5 centre dot (tolene)_2], whereas the crystal structures of the all the other compounds contain only the bis-diphenylphosphine oxide molecule. The crystal structures have been determined from singel-crystal X-ray diffraction data, with the exception of 7 (which has been determined here from powder X-ray diffraction data) and 4 (which was known previously). The compounds with even n represent a systematic structural series, exhibiting characterisitic, essentially linear P=O... P=O... P=O dipolar arrays, together with C-H...O and C-H...#pi# interactions. For the compounds with odd n, on the other hand,uniform structural behaviour is not observed across the series, although certain aspects of these crystal structures contribute in a general sense of our under-standing of the structural properties of bis-diphenylphosphine oxides. Importantly, for the compounds with odd n, there is "frustration" with regard to the molecular conformation, as the preferred all-anti conformation of the aliphatic chain gives rise to an unfavourable parallel alignment of the two P=O dipoles within the molecule. Clearly the importance of avoiding a parallel alignment of the P=O dipoles becomes greater as n decreases. Local structural aspects (investigated by high-resolution solid-state ~31P NMR spectroscopy) and thermal properties of the bis-diphenylphospine oxide materials are also reported.