Three new amidophosphoric acid esters with a P(O)[OCH2C(CH3)2CH2O] segment: X-ray diffraction, DFT, AIM and Hirshfeld surface investigations of bi- and tri-furcated (three and four-center) hydrogen bond interactions

Abstract

AbstractStructural and packing features of three new amidophosphoric acid esters having a common partXP(O)[OCH2C(CH3)2CH2O], withX = [(CH3)3CNH] (1), [(CH3)2HCNH] (2) and [C6H11(CH3)N] (3), are investigated by single crystal X-ray diffraction. The results illustrate that the compounds1and3crystallize with one independent molecule in the asymmetric unit; whereas, for2, the compound crystallizes with three independent molecules in the asymmetric unit. The crystal structures are mostly stabilizedviatri-furcated hydrogen bond interactions (C–H · · ·)2(N–H · · ·)O=P in1and (C–H · · ·)3O=P in3, while the stability is given by bi-furcated hydrogen bond interactions (C–H · · ·)(N–H · · ·)O=P in2. For a better understanding of the nature, strength and energetics associated with the formation of the quoted multi-center hydrogen bond interactions, the Natural Bond Order (NBO) method from Density Functional Theory (DFT) and a topological analysis by means of Atoms In Molecules (AIM) and Hirshfeld surface procedures were performed. These studies reveal that the studied multi-center hydrogen bond interactions of the type O · · · H are favoured in the crystal packing displaying enrichment ratios larger than unity. The detailed nature of the different interactions in these multi-center interactions is studied for the first time in such compounds. It is shown that the N–H · · · O interaction is rather non-covalent closed-shell whereas the C–H · · · O interaction is more van der Waals closed-shell. Stronger hydrogen bond interactions are observed for a lower multiple H-atom acceptor oxygen in three-center hydrogen bond interactions (C–H · · ·)(N–H · · ·)O=P of2than for four-center interactions in1[i.e. (C–H · · ·)2(N–H · · ·)O=P] and3[i.e. (C–H · · ·)3O=P]. The better H-atom acceptability of the O atom of P=O compared with the esteric O atom is explained by the richers-character of the hybrid orbital of the O atom acceptor of P=O coupled with enhance of the polarization and charge. The obtained results are also confirmed by Molecular Electrostatic Potential (MEP).