A DFT quest for effects of fused rings on the stability of remote N -heterocyclic carbenes
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ORIGINAL RESEARCH
A DFT quest for effects of fused rings on the stability of remote N-heterocyclic carbenes Parvaneh Delir Kheirollahi Nezhad 1 & Leila Youseftabar-Miri 2 & Sheida Ahmadi 1 & Saeideh Ebrahimiasl 3,4 & Esmail Vessally 1 Received: 2 June 2020 / Accepted: 22 September 2020 # Springer Science+Business Media, LLC, part of Springer Nature 2020
Abstract Assuming aromaticity (cyclic continuous conjugation, planarity, and obeying the Hückel 4n + 2 rule), effects of one and two fused six-membered heterocyclic rings are investigated on the energy lowering (stabilization) of 22 novel singlet (s) and triplet (t) carbenes, at B3LYP/AUG-cc-pVTZ and M06-2X/AUG-cc-pVTZ. Results display that (1) exclusive of triplet pyridine-4-ylidene, and s and t states, other species appear as ground state, so every s Hammick carbene exhibits more stability than its corresponding t state; (2) the highest stability is demonstrated by unsubstituted pyridine-4-ylidene as reference carbene, and the lowest stability is shown by carbene situated between two nitrogen heteroatoms of two fused rings, in a “W” arrangement; (3) regarding the relationship between carbenic center (CC) and substituted heteroatom, the order of stabilization for fused rings is meta > para > ortho; (4) regardless of how organized, fusion of one six-membered ring, in a given arrangement, has more stabilizing effect than two six-membered rings; (5) contrary to our expectation, t Hammick carbenes show higher band gap (ΔΕHOMO-LUMO) than their corresponding s species; (6) based on the NICS (nuclear independent chemical shift) results, the least stable carbene has the most aromaticity in its pyridine ring; and (7) according to proposed homomolecular isodesmotic reactions, all s states are stabilized via π-donor/σ-acceptor substitution more than the t states. Keywords Hammick carbene . Stability . Nucleophilicity . Aromaticity
Introduction Carbenes as reactivate intermediates are of great current interest, because of their individual structural properties, their catalytic reactions in transition metal complexes or metal-free Electronic supplementary material The online version of this article (https://doi.org/10.1007/s11224-020-01650-5) contains supplementary material, which is available to authorized users. * Sheida Ahmadi [email protected] 1
Department of Chemistry, Payame Noor University, Tehran, Iran
2
Department of Organic Chemistry, Faculty of Pharmaceutical Chemistry, Tehran Medical Sciences, Islamic Azad University, Tehran, Iran
3
Department of Chemistry, Ahar Branch, Islamic Azad University, Ahar, Iran
4
Industrial Nanotechnology Research Center, Tabriz Branch, Islamic Azad University, Tabriz, Iran
organocatalysts, their metallopharmaceuticals, and their coordination to p-block elements in many fields of applied chemistry [1, 2]. The size and substitution pattern can have a large effect on the properties of N-heterocyclic carbenes (NHCs) [3]. Initially, Buchner and Curtius discovered carbenes that seemed impossible to isolate [4]. Nevertheless, Be
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