XPS determination of the binding energies of phosphorus and nitrogen in phosphazenes

Abstract

Phosphazenes are compounds in which repeating units -N = P(L1, L2)— are arranged in cyclic or linear structures including polymeric ones. Representatives are numerous due to the possibility of varying the ligands L1 and L2. This determines the wide spectrum of useful properties of the phosphazenes such as elasticity, thermal stability, biological compatibility etc. and, a consequence there is enhanced interest in these compounds [1–3]. Although the literature on phosphazenes is impressive [4–6], it seems that the nitrogenphosphorus bond and the effect of substituents on this bond are not yet sufficiently investigated [7]. Dake et al. [8] have used XPS and Raman spectroscopy to investigate the effect of electron-accepting substituents on the distribution of the electron charge in the nitrogen-phosphorus chains, comparing cyclic trimers and linear polymers when substitution is homogeneous (L1 = L2). The results of these authors confirm the assumptions based on theoretical considerations, according to which the N P bond is strongly polarized, this effect being more pronounced with polymers rather than with cyclic trimers. As to the influence of substituents on the binding energies of nitrogen and phosphorus, it is found to depend not only on the electromeric effect of the substituent but also on the conformation of the phosphazene molecule. The present paper is aimed at determining the binding energies of nitrogen and phosphorus in some cyclic phosphazene molecules in the cases of identical (L1 = L2) or different (L1 = L2) substituents as well as in linear polymers having electron-donating or electronaccepting ligands. The following 7 phosphazenes were investigated: Cyclic trimers (No1 hexachlorocyclotriphosphazene (NPCl2)3, No2 cis-bis-(dimethylamino)-tetrachlorocyclotriphosphazene N3P3[N(CH3)2]Cl4, No3 bis(dimethylamino)-bis-(trifluorethoxy)-dichlorocyclotriphosphazene and N3P3[N(CH3)2]2(OCH2CF3)2Cl2), Cyclic tetramert (No4 octabutylaminocyclotetraphosphazene N4P4[NH(C4H9)]8), Cyclomatrix polymer (No5 phospham [(NPNH)3]n) and Linear polymers (No6 poly-bis-(butylamino)phosphazene {NP[NH(C4H9)]2}n, No7 poly-bis-(trifluoroethoxy)phosphazene [NP(OCH2CF3)2]n). The compounds were prepared according to usual methods [9, 10]. Elemental analysis, IR and 31P-NMR spectra, X-ray phase analysis and measurement of the