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Anisotropic and amphoteric characteristics of diverse carbenes

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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, π conjugation, singlet–triplet energy gaps, anisotropic hard wall radii, anisotropic electrostatic potentials, and amphotericities of carbenes, which are vital to carbene chemistry. The (σ2, π2 or σπ) 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 σ2 singlet carbenes tend to show negative and positive EPs, favoring nucleophiles and electrophiles, respectively; meanwhile, those of π2 singlet carbenes, such as very highly π-conjugated 5-membered cyclic C(NCH)2, tend to show the opposite behavior. Open-shell σπ singlet (such as highly π-conjugated 5-membered cyclic C(CHCH)2) and triplet carbenes show less anisotropic and amphoteric behaviors.

Graphical abstract: Anisotropic and amphoteric characteristics of diverse carbenes

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Publication details

The article was received on 21 Jan 2018, accepted on 08 Mar 2018 and first published on 15 Mar 2018


Article type: Paper
DOI: 10.1039/C8CP00457A
Citation: Phys. Chem. Chem. Phys., 2018, Advance Article
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    Anisotropic and amphoteric characteristics of diverse carbenes

    D. Y. Kim, D. C. Yang, J. M. L. Madridejos, A. Hajibabaei, C. Baig and K. S. Kim, Phys. Chem. Chem. Phys., 2018, Advance Article , DOI: 10.1039/C8CP00457A

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