Arrays of PO Dipoles As a Recurrent Structural Motif in Bis-Diphenylphosphine Oxides, Established from Powder X-ray Diffraction

Rationalization of the solid-state structural properties of odd members (<i>n</i> = 3, 5, 7) of the series Ph<sub>2</sub>P(O)(CH<sub>2</sub>)<sub><i>n</i></sub>P(O)Ph<sub>2</sub> leads to insights concerning the structural determinants of this class of material, particularly with regard to the formation of preferred arrangements of PO dipoles. The odd members of this series are recalcitrant to the formation of single crystals of suitable size and quality for single-crystal X-ray diffraction, and modern techniques for carrying out crystal structure determination directly from powder X-ray diffraction data were essential for determining the structural properties of these materials. In the present work, nonsolvate crystal phases of the materials with <i>n</i> = 3 and 5 were prepared by appropriate solid-state desolvation processes (starting from hydrate and toluene solvate phases, respectively), yielding microcrystalline powders of the nonsolvate phase in each case. Structure determination was carried out directly from powder X-ray diffraction data, employing the direct-space genetic algorithm technique for structure solution followed by Rietveld refinement.