On bonding in cyclic triphosphazenes

Formation Mechanism of the Unsubstituted Chlorophosphazene Cl3P═NH: A Theoretical Study via Quantum Mechanical Calculations

Although the synthesis of chlorophosphazene polymers has been explored for more than 100 years, the shortest yet most illusive monomer, Cl3P═NH, has never been isolated and fully characterized. Here we investigate the formation of Cl3P═NH from PCl5 and NH3 in chlorobenzene through quantum mechanical calculations. The potential energy surface was mapped using the MP2 Hamiltonian in conjunction with Dunning's correlation-consistent basis sets (aug-cc-pVXZ, where X = D and T). Along with HOMO/LUMO frontier molecular orbitals and natural bond orbital analyses, we found that instead of following the SN1 path proposed in the literature, the reaction proceeds via an addition-elimination mechanism. Our results also indicate that due to the low-lying stable intermediates (IM), most steps are exothermic such that the production of Cl3P═NH·2HCl can be completed once the energy barrier for the formation of [PCl4-NH3]+Cl- is overcome. Therefore, our theoretical work might explain the challenges in isolating any of the IMs in a typical chlorophosphazene reaction in chlorobenzene.

Modulation of tris(o-phenylenedioxy)cyclotrisphosphazene (TPP) properties for zeolite use: effect of pi-conjugation length and CH/N heterosubstitution

Highly accurate quantum chemical calculations were used in a comparative study of the pi-conjugation length and CH/N heterosubstitution effects on the structure and a number of properties of tris(o-phenylenedioxy)cyclotrisphosphazene (TPP) derivatives to identify the role of each of them in the modulation of the host...guest complex stability and available space for the adsorbate within TPP-like organic zeolite (OZ). From the BH&HLYP/6-31G(d,p) structures it was found that extending the side fragment with a phenyl ring and substituting CH/N preserve the "paddle wheel" molecular shape, a key factor in the tunnel formation on which the organic zeolite use of TPP is based on. Compared to the unsubstituted molecules, the side fragment is shortened for CH/N heterosubstitution, which may decrease the diameter of the tunnel. In addition, through accurate ionization potential (IP) calculations (at the HF/6-311+G(d) level) the electron-donor (E-D) capacity was found to be more significantly enhanced by a lateral than a linear extension with phenyl ring, while it decreased upon CH/N heterosubstitution, which can affect the stability of some related host...guest complexes in the same order. Finally, as recently reported for TPP, the current results based on PBE0/6-31G(d,p) show that upon release of an electron the structural stability may not be altered by a CH/N mono- or disubstitution. From these features it was concluded that if the crystal requirement can be attained clathrates of good tolerance toward guest molecules in a wide range of Lewis acidity with variable E-D strength and available space for adsorbates may be reasonably awaited by exploiting these two approaches.

Influence of the substituted side group on the molecular structure and electronic properties of TPP and related implications on organic zeolites use

Tris(o-phenylenedioxy)cyclotriphosphazene (TPP) became a compound of choice to investigate the structural features of organic zeolite and their potential applications. Different TPP-like materials are studied in this Letter from the electron-donor (E-D) capacity viewpoint, since this was reported as a stabilizing parameter of the TPP-Lewis acid inclusion compound up to high temperatures. On the basis of DFT-PBE0/6-31G(d,p) calculations, the results reported herein show a tight dependence of the E-D of the entire molecule on that of the side group. It was shown that both the O/NH substitution and the extension of the phenylenedioxyl group with an aromatic ring significantly enhance the E-D. As a result, the corresponding clathrates, including some reported ones, may also be exploited for the same issue, with an even wider range of operating temperatures when trapping compounds of Lewis acidity character comparable to that of I2. Furthermore, it was concluded that these two strategies may significantly enhance the E-D capacity without altering the tolerance of TPP-like host materials to the guest molecules.

Design of TTF-Based Phosphazenes Combining a Good Electron-Donor Capacity and Possible Inclusion Adduct Formation

The physical properties of porous material can be modulated by intercalation of small molecules, whose size, in the case of tris-o-phenylenedioxy)cyclotriphosphazene (TPP)-based materials, vary with the different side groups. Starting from the TPP structure, a series of new derivatives were constructed through the core ring [(NP)(3)] substitution by [(CNH)(3)], [(CO)(3)], and [(CS)(3)] or/and the side group substitution by tetrathiafulvalene and a series of related fragments including bis(ethylenedithio)tetrathiafulvalene, 2-methylene-1, 3-dithiole, and 2-methylene-5,6-dihydro[1,3]dithiolo[4,5-b][1,4]dithiine. In the side fragment, such a substitution corresponds to the replacement of a ring heteroatom, an addition of substituents, or both. With use of theoretical methodologies based on DFT-B3LYP/6-31G* and HF/6-31G*//B3LYP/6-31G* approaches, molecular geometries and electronic properties including the LUMO and HOMO energies, the HOMO-LUMO gap, as well as the ionization potential (IP) were calculated. In comparison with the commonly used organic superconductors, most of the molecules investigated were predicted to show comparable or better electron-donor strength. Interestingly, a number of cyclophosphazene [(NP)(3)]-containing compounds were predicted to show the "paddle wheel" shape responsible for inclusion adducts formation, making these compounds to be potential candidates for organic superconductors with the ease of modulating their conducting properties by intercalation of suitable acceptors.

Revisiting the Electronic Structure of Phosphazenes

Natural bond orbital (NBO) and topological electron density analyses have been used to investigate the electronic structure of phosphazenes [N3P3R6] (R = H, F, Cl, Br, CH3, CF3, N(C2H4); 2R = O2C6H4), [N4P4Cl8], and H[NPCl2]4H. Using the former, the two most likely phosphazene bonding alternatives, negative hyperconjugation and ionic bonding have been critically evaluated. Ionic bonding, as suggested by topological analysis, was found to be the dominant bonding feature, although contributions from negative hyperconjugation are necessary for a more complete bonding description. Substituent effects on the P-N bond have been assessed and cases of bond length alternation have been rationalized using this combined bonding model, which supersedes previous models involving d-orbital participation, leading to an explanation for the observed bond length alternation found in some linear polyphosphazenes. In addition, common aromaticity indicators, nucleus independent chemical shifts (NICS) and para-delocalization indices (PDI), have been determined for the cyclophosphazenes.