Dedicated to Professor Karl Otto Christe on the occasion of his 70th birthday
Compounds of the elements phosphorus and nitrogen can exist either as molecular species or as three-dimensional polymeric solids. Examples of known solid-state compounds include the structurally well-characterized phases of the binary compound P 3 N 5 , which were reported by Schnick et al. [1] In contrast, none of the four binary P-N molecules described in the literature, namely P 4 N 4 , P(N 3 ) 3 , P(N 3 ) 5 , P 3 N 21 , [5] and the ionic compound (N 5 )P(N 3 ) 6 , have been structurally characterized. As shown by Christe et al. for (N 5 )P(N 3 ) 6 , the difficulties in the isolation and handling of these compounds arise from their highly endothermic character and their extremely low energy barriers, which lead to an often uncontrollable, explosive decomposition. [7] Herein, we report the single-crystal X-ray structure of P 3 N 21 (1) and, thereby, the first structural characterization of a binary P-N molecule. Although the synthesis of this compound was first reported over 50 years ago through the reaction of hexachlorophosphazene with sodium azide, [5a] compound 1 has only been characterized by elemental analysis, [5a] and vibrational [5b,c] and NMR spectroscopy. [5c] The experimental difficulties involved in the structural characterization of P 3 N 21 are a consequence of its high energy content; our calculated gas-phase enthalpy of formation is + 341.4 kcal mol À1 .
To obtain as pure a product as possible, a new synthetic strategy for P 3 N 21 , in which the azide (N 3 ) group is introduced using trimethylsilylazide, was chosen [Eq. ( )]. The trime-[