Structure and Conformation of the Medium-Sized Chlorophosphazene
Rings
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Abstract
Medium-sized
cyclic oligomeric phosphazenes [PCl<sub>2</sub>N]<sub><i>m</i></sub> (where <i>m</i> = 5–9) that were prepared
from the reaction of PCl<sub>5</sub> and NH<sub>4</sub>Cl in refluxing
chlorobenzene have been isolated by a combination of sublimation/extraction
and column chromatography from the predominant products [PCl<sub>2</sub>N]<sub>3</sub> and [PCl<sub>2</sub>N]<sub>4</sub>. The medium-sized
rings [PCl<sub>2</sub>N]<sub><i>m</i></sub> have been characterized
by electrospray ionization–mass spectroscopy (ESI-MS), their <sup>31</sup>P chemical shifts have been reassigned, and their T<sub>1</sub> relaxation times have been obtained. Crystallographic data has been
recollected for [PCl<sub>2</sub>N]<sub>5</sub>, and the crystal structures
of [PCl<sub>2</sub>N]<sub>6</sub>, and [PCl<sub>2</sub>N]<sub>8</sub> are reported. Halogen-bonding interactions were observed in all
the crystal structures of cyclic [PCl<sub>2</sub>N]<sub><i>m</i></sub> (<i>m</i> = 3–5, 6, 8). The crystal structures
of [P(OPh)<sub>2</sub>N]<sub>7</sub> and [P(OPh)<sub>2</sub>N]<sub>8</sub>, which are derivatives of the respective [PCl<sub>2</sub>N]<sub><i>m</i></sub>, are also reported. Comparisons of
the intermolecular forces and torsion angles of [PCl<sub>2</sub>N]<sub>8</sub> and [P(OPh)<sub>2</sub>N]<sub>8</sub> with those of three
other octameric rings are described. The comparisons show that chlorophosphazenes
should not be considered prototypical, in terms of solid-state structure,
because of the strong influence of halogen bonding
