Synthesis, Spectroscopy, X-ray Crystallography, and DFT Computations of Nanosized Phosphazenes

Nanoparticles of nine phosphazenes with general formula 4‐CH3C6H4S(O)2N=PX3 [X = Cl (A), NC4H8 (1), NC6H12 (2), NC4H8N–C(O)OC2H5 (3), NC4H8N–C(O)OC6H5 (4), NC4H8O (5), NHCH2–C4H7O (6), N(CH3)(C6H11) (7), NHCH2–C6H5 (8), and 2‐NH‐NC5H4 (9)] were synthesized using ultrasonic method and characterized by 1H, 13C, 31P NMR, FT‐IR, fluorescence, as well as UV/Vis spectroscopy and additionally with XRD, FE‐SEM, N2 sorption, and elemental analysis. The 31P NMR spectra of compounds 1–9 reveal the most up field shift δ(31P) for 9 at –11.45 ppm reflecting the most electron donation of 2‐aminopyridinyl rings through resonance to the phosphorus atom. The 1H, 13C NMR spectra of 7 exhibit two sets of signals for the hydrogen and carbon atoms of its two isomers present in the solution state in 1:4 ratio. The FE‐SEM micrographs illustrate that the nanoparticles of compounds 1–9 have spherical morphology and a size of 27–42 nm. From the XRD patterns, the crystal sizes were estimated to about 24–86 nm. The highest bandgap was measured for 3 (3.81 eV) whereas the smallest was measured for 8 (3.50 eV). The structures of two polymorphs of compound 5 (5, 5′) were determined by X‐ray crystallography at 120 K. Both of these polymorphs are triclinic with P1 space group but 5 has a doubled unit cell volume and two symmetrically independent molecules (5a and 5b). In structures 5a and 5′, the phosphorus and all endocyclic atoms of two morpholinyl rings display disorder, whereas the molecule 5b does not show disorder. The strong intermolecular O–H···O hydrogen bonds plus weak intermolecular C–H···O and C–H···N interactions create three‐dimensional polymers in the crystalline networks of 5 and 5′. The DFT computations illustrate that molecule 5b is more stable than 5a by –1.1062 and –0.9779 kcal·mol–1 at B3LYP and B3PW91 levels, respectively. The NBO calculations presented sp3d hybridization for phosphorus and sulfur atoms and sp2, sp3 hybrids for the nitrogen and oxygen atoms.