Valuable insights into the structural, electronic, and aromaticity aspects of azulene and its monophospha-derivatives

Abstract

Azulene-based conjugated systems have garnered considerable interest owing to their atypical structures. The incorporation of a phosphorous atom into the azulene framework offers an enticing prospect for the creation of unique organophosphorous π-conjugated compounds. No such phosphaazulene skeleton however has been reported to date. Here, density functional theory calculations were employed to examine the physicochemical properties and aromaticity of azulene and its monophospha-derivatives. The electronic, magnetic, and structural properties were taken into consideration. The reactivity of azulene and its monophospha-analogs including 1-phospha (1p), 2-phospha (2p), 4-phospha (3p), 5-phospha (4p), and 6-phosphaazulene (5p), was examined through local electrophilicity and local nucleophilicity indices. The stability and aromaticity of these compounds were determined using DFT calculations with the B3LYP/6–311 + G(d,p) method. The stability of monophosphaazulene derivatives is significantly influenced by the position of the phosphorus atom, as indicated by the energy calculations. The order of stability was found to be 1p > 2p > 5p > 3p > 4p. Aromaticity studies based on HOMA, ΔBl, FLUπ, SA, and NICS indices indicated that the incorporation of a phosphorus atom into the 5-membered ring decreased its aromaticity while simultaneously increasing the aromaticity of the 7-membered ring. However, replacing the carbon atom with phosphorus atom in the 7-membered ring reduced the aromaticity of both rings. Additionally, the ability of π-stacking, as determined by the LOLIPOP criterion, was reduced in the monophosphaazulene derivatives compared to azulene, except for compound 5p. The order of π-stacking ability was 5p > azulene > 2p > 1p > 4p > 3p. Overall, there was a good agreement between the various aromaticity indices, providing consistent results for both rings in azulene and monophosphaazulenes.