The syntheses, crystal structures, and spectroscopic, chiral, thermal and free radical scavenging properties of fully and partially substituted indolyl-phosphazenes

Abstract

This study covers the syntheses, crystal structures, and spectral, stereogenic, thermal and free radical scavenging (FRS) properties of partially and fully substituted (indol-3-yl)spiro(N/N)cyclotri-phosphazenes. Primarily, N¹-[(1H-indol-3-yl)methyl]-N²-methyl-1,2-diaminoethane, synthesized from the 1 : 1 reaction of indol-3-carboxaldehyde with N-methyl-1,2-diaminoethane, was made to react with equimolar hexachlorocyclotriphosphazene (N₃P₃Cl₃, HCCP) to obtain tetrachloro-N¹-[(1H-indol-3-yl)methyl]-N²-methylspiro(N/N)cyclotriphosphazene. Cl substitution reactions of n-propylamine, iso-propylamine and n-butylamine separately with varying stoichiometric equimolar ratios (1 : 2 and 1 : 4) resulted in the formation of geminal-bis and tetrakis-amino cyclotriphosphazenes. Geminal-phosphazenes possess “chiral P atoms”. The ³¹P{¹H}NMR spectra of geminal-bis-iso-propylamino and geminal-bis-n-butylaminocyclotriphosphazenes with the addition of the “chiral solvating agent” (CSA) proved that these compounds exist as “racemates”. The structures of all compounds were characterized using spectral and elemental analyses data. Single crystal X-ray crystallography revealed the molecular and crystal structures of geminal-bis-n-propylamino and tetrakis-n-propylamino cyclotriphosphazenes, complemented by Hirshfeld surface (HS) analysis to evaluate intermolecular interactions. Furthermore, the highest free radical scavenging activities (FRSAs, 30.6 ± 0.2%) were observed for tetrakis-n-butylaminocyclotriphosphazenes. The thermal properties of phosphazenes were investigated using thermogravimetric analysis (TGA) and differential thermogravimetric analysis (DTG). The limited oxygen index (LOI) values, used as an indicator of flame retardancy, were calculated from TGA data. The calculated LOI values, ranging from 31.2 to 39.0, classify these compounds as self-extinguishing materials.