Role of donor‐acceptor functional groups in N₃P₃ cyclic‐triphosphazene backbone. Unraveling bonding characteristics from natural orbitals within an extended transition state‐natural orbital for the chemical valence scheme

Abstract

The formation of cyclophosphazenes containing several ligands or substituent groups gives rise to an attractive derivative set, for development of novel applications, with variable properties. Here, it is possible to unravel the role of different functional groups attached to the N 3 P 3 backbone, to reach a better understanding of the bonding character in the cyclic [─P─N─] skeleton. We employed the extended transition state-natural orbital for the chemical valence scheme to unravel the σ and π orbital kernels that are involved in the assembling of such structures, by varying the acceptor-donor characteristics of the ─CF 3 , ─NO 2 , ─COOH, ─CN, ─NH 2 , ─OH, and ─OCH 3 groups, where ─NO 2 behaves as a stronger electron-withdrawing substituent rather than ─CF 3 , ─COOH, and ─CN, denoting that the nature of the ligandphosphazene interaction contributes to some degree to the bond strength of the cyclic [─P─N─] backbone. Our results reveal that the electron-withdrawing ─NO 2 group leads to higher σ and π [─P─N─] orbital-energy contributions, which is reflected in a shortening of the [─P─N─] distance, contrasting with the case of electron-donating groups such as ─NH 2 , ─OH, and ─OCH 3 within the phosphazene set. These insights allow further variation and modulation of the bonding in the [─P─N─] ring.

Cyclo- and Polyphosphazenes for Biomedical Applications

“…On these grounds, the quest concerning the nature of the P-N bond in phosphazenes has been focused on specific topics, i.e.,: (i) if dπ of the P are involved in the π electrons delocalization and (ii) if there is ring aromaticity when 4n + 2 π electrons are involved in the delocalization. In this context, it has been shown by charge density, Natural Bond Orbital (NBO), and Energy Decomposition Analysis–Natural Orbitals for Chemical Valence (EDA-NOCV) approaches that, both for cyclic- and polyphosphazenes, the PN bond is highly polar, with a remarkable ionic character [ 59 , 60 , 61 , 62 ] along with the presence of a negative hyperconjugation involving the N lone pair with the σ* PX (X = ligand at the P atom) and, to a lesser extent, the σ* PN orbitals [ 59 , 60 , 61 , 62 , 63 ]. These outcomes discarded the hypervalent character of the P due to the participation of P dπ orbitals in the delocalization of π electrons.…”

“…The role of the substituents on the properties of the P-N bond has also been investigated by several computational methods, e.g., electron density, NBO, and EDA-NOCV analysis [ 59 , 60 , 62 , 65 ]. The donor/acceptor as well as the electron-withdrawing behavior of the ligands bonded to the P modulates the electron density at the P center without affecting the electron density on N. Thus, a withdrawing group causes an electron density depletion at the P and then an increasing P-N polarization, leading to a shortening of the P-N bond and vice versa ( Figure 7 ).…”