Imidazolin-2-imine and Imidazolin-2-methylidene Substitutions to Benzene, Pyridine, Phosphine, and N‑Heterocyclic Carbene Predict Highly Electron-rich Ligands

Majority of the substituents in organic and organometallic chemistry are electron-withdrawing in nature toward substrates/ligands. Even for the most electron-donating neutral substituents such as N,N-dialkyl amino groups, the magnitude of electron donation is only nearly half of the magnitude of electron withdrawal by some of the most withdrawing groups. Therefore, development of highly electron-donating substituents promises the discovery of new chemistry. Density functional theory (DFT) study at the M06L/6-311++G(d,p) level in conjunction with molecular electrostatic potential (MESP) topology analysis unraveled the high electron donating nature of imidazolin-2-imine (X and X′) and imidazolin-2-methylidene (Y and Y′) types of moieties as substituents to benzene, pyridine, phosphine (PR₃), and N-heterocyclic carbene (NHC). The MESP minimum (V_min) is derived for aromatic π-regions and lone-pair regions of the molecules, and the X-, X′-, Y-, and Y′-substituted systems showed a substantial increase in the negative character of V_min compared to that of the corresponding unsubstituted systems. Multiple substitutions led to a further increase in the magnitude of V_min, by 195% for benzene with tri-Y′ substitution, 541% for pyridine with tri-Y substitution, 162% for PH₃ with tri-X′ substitution, and 23% for NHC with di-X′ substitution, which suggests the amazing electron donating ability of these substituents. The substituted benzene, pyridine, phosphine, and NHC behaved as very strongly coordinating ligands toward Li⁺, CuCl, and Cr(CO)₃. The coordination energy is proportional to the electron richness of the ligands measured in terms of V_min. The V_min serves as an electronic parameter and leads to a priori prediction of the coordination reactivity of the ligands. The recent discovery of the complex of PMeX″₂ (X″ = imidazolin-2-imine group) with C₂ is also rationalized in terms of the high electron donating character of X″. The substitution with imidazolin-2-imine and imidazolin-2-methylidene moieties suggests a powerful molecular design strategy toward the development of highly electron-rich substrates and ligands.