New pathways of stability for NHCs derived from azole, di -azole, n -tetrazole, and ab -tetrazole, by DFT
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ORIGINAL PAPER
New pathways of stability for NHCs derived from azole, di-azole, n-tetrazole, and ab-tetrazole, by DFT Aliakbar Ahmadi 1 & Mohamad Z. Kassaee 1
&
Mojgan Ayoubi-Chianeh 1
&
Alireza Fattahi 2
Received: 18 August 2020 / Accepted: 15 October 2020 # Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract We have investigated the pathways of stability for NHCs derived from azole, di-azole, n-tetrazole, and ab-tetrazole (1a, 2a, 3a, and 4a, respectively), at the M06/6-311++G** level of theory. Optimization and vibrational frequency calculations of ground states (GS) and transition states (TS) are performed to identify Gibbs free energies and nature of stationary points, respectively. Two possible pathways of stability for 1a-4a are compared and contrasted which entail dimerization through hydrogen bonding (HB) and covalent bonding (CB). The CB pathway comprises head to head (HH) and head to tail (HT) dimerizations. Plausible reaction profiles are illustrated for 1a-4a along with the mechanism of each dimerization. Structures 1a-3a show one possibility for HB while 4a represents two possibilities. Structures 1a and 4a display two HH dimers while 2a and 3a show one. Structures 1a-4a undergo HT dimerizations to yield three possible dimers which include trans, cis, and [2+3] isomers. Interestingly, for all 1a-4a, HB dimerization turns out as the most favorable stability pathway for showing no barrier of reaction. Structures 4b and 4c indicate the highest stability with respect to their initial 4a compared to remaining HB dimers 1b-3b. In addition, the 1,2-H shift appears as a possible rearrangement for 1a-4a to yield their corresponding tautomers (1i, 2h, 3h, and 4k, respectively). The reaction profile of this rearrangement indicates that 1a-4a favor HB dimerization pathway more than 1,2-H shift, in terms of kinetic and thermodynamic. Keywords NHC . Stability . Dimerization . Azole . di-Azole . n-Tetrazole . ab-Tetrazole . DFT
Introduction The N-heterocyclic carbenes (NHCs) have been known as stable carbenes because of their electronic [1] and steric [2, 3] features. Due to the electron-rich nature and σ-donation properties of NHCs, they have been remarkably considered as ligands for many years [4–9]. Many routes have been made on the synthesis and characterization of NHCs [10–12]. The first stable and persistent NHC was synthesized by Arduengo [13]. The kinetic and thermodynamic stabilities of the NHC play an important role in its reactivity [14]. The singlet–triplet energy gap (ΔEs-t) appears as the most effective thermodynamic parameter on the relative stability of NHCs.
* Mohamad Z. Kassaee [email protected] 1
Department of Chemistry, Tarbiat Modares University, Tehran, Iran
2
Department of Chemistry, Sharif University of Technology, Tehran, Iran
Heinemann studied the stability of diaminocarbenes and revealed the higher stability of imidazol-2-ylidene with respect to its corresponding saturated structure, imidazolin-2-ylidene [15]. This type of stabilization depends on the σ-wi
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