Anisotropic and amphoteric characteristics of diverse carbenes

Abstract

Despite its key importance in carbene chemistry, the amphoteric (i. e., both nucleophilic and electrophilic) behavior of the divalent carbon atom (: C) in carbenes is not well understood. The electrostatic potential (EP) around : C is often incorrectly described by simple isotropic atomic charges (particularly, as in singlet CF2); therefore, it should be described by the multipole model, which can illustrate both negative and positive EPs, favoring the positively and negatively charged species that are often present around : C. This amphotericity is much stronger in the singlet state, which has more conspicuous anisotropic charge distribution than the triplet state; this is validated by the complexation structures of carbenes interacting with Na+, Cl-, H2O, and Ag+. From the study of diverse carbenes [including CH2, CLi2/CNa2, CBe2/CMg2, CF2/CCl2, C(BH2) 2/C(AlH2) 2, C(CH3) 2/C(SiH3) 2, C(NH2) 2/C(PH2) 2, cyclic systems of C(CH2) 2/C(CH) 2, C(BHCH) 2, C(CH2CH) 2/C(CHCH) 2, and C(NHCH) 2/C(NCH) 2], we elucidate the relationships between the electron configurations, electron accepting/donating strengths of atoms attached to : C, p conjugation, singlet-triplet energy gaps, anisotropic hard wall radii, anisotropic electrostatic potentials, and amphotericities of carbenes, which are vital to carbene chemistry. The (s 2, p 2 or sp) electronic configuration associated with : C on the : CA2 plane (where A is an adjacent atom) in singlet and triplet carbenes largely governs the amphoteric behaviors along the : C tip and : C face-on directions. The : C tip and : C face-on sites of s 2 singlet carbenes tend to show negative and positive EPs, favoring nucleophiles and electrophiles, respectively; meanwhile, those of p 2 singlet carbenes, such as very highly p-conjugated 5-membered cyclic C(NCH) 2, tend to show the opposite behavior. Open-shell sp singlet (such as highly p-conjugated 5-membered cyclic C(CHCH) 2) and triplet carbenes show less anisotropic and amphoteric behaviors.