Unraveling the influence mechanisms of different substituents on the chemical activity of N-heterocyclic phosphines via theoretical calculations

Context

N-Heterocyclic phosphines (NHP-H) represent a distinctive class of phosphorus-containing heterocycles characterized by “polarity-inverted” P–H bonds. These unique bonds facilitate a wide array of P–H reactions, rendering NHP-H compounds promising candidates for applications in organocatalysis. Although significant advancements have been made in NHP-H research, the experimental quantification of their reactivity parameters poses considerable challenges due to their high reactivity. Furthermore, the influence of various substituents on the chemical activity of NHP-H compounds remains insufficiently understood. This study examines eight NHP-H compounds with varying substituents. The findings indicate that electron-donating substituents decrease the P–H bond order, increase the negative charge on the phosphorus atom, and enhance nucleophilicity. Conversely, electron-withdrawing substituents exhibit opposite effects. Furthermore, substituents influence the local electron attachment energy of the phosphorus atom, thereby affecting reactivity in proton-transfer reactions. According to conceptual density functional theory, electron-donating substituents are associated with lower electrophilicity and higher nucleophilicity indices, whereas electron-withdrawing substituents demonstrate the opposite trend. Charge-transfer spectra suggest that electron-donating substituents reduce the excitation energy of NHP-H, thereby increasing its reactivity. Additionally, IRI analysis indicates that electron-donating substituents weaken the P–H bond, while electron-withdrawing substituents strengthen it, along with alterations in other intramolecular interactions.

Methods

The study utilized the M06-2X functional in conjunction with the def2-TZVP basis set within the SMD model, employing acetonitrile as the solvent, to perform structural optimization and frequency analysis of NHP-H compounds. Computational analyses were conducted using Gaussian 09 software, with 30 excited states calculated for each compound. Multiwfn software facilitated the determination of atomic dipole moment-corrected Hirshfeld population, local electron attachment energy, the Interaction Region Indicator, and charge-transfer spectrum, which were subsequently visualized using VMD 1.9.3. Additionally, GaussView 6.0.16 software was employed to generate three-dimensional molecular configurations and prepare input files.