Singlet and triplet cyclonona‐3,5,7‐trienylidenes and their α, ά‐halogenated derivatives at DFT

Abstract

AbstractSinglet and triplet boat‐shaped cyclonona‐3,5,7‐trienylidene (1H‐S and 1H‐T) and their corresponding α, ά‐tetrahalo derivatives (1F‐S, 1F‐T, 1Cl‐S, 1Cl‐T, 1Br‐S, 1Br‐T, 1I‐S, and 1I‐T) are compared at M06‐2X/AUG‐cc‐pVDZ, B3LYP/AUG‐cc‐pVTZ, and B3LYP/6‐311++G** levels of theory. Apart from 1H‐S which turns out as a transition state for having a negative force constant, the other structures appear as minima on their potential energy surfaces. 1H, 1Cl, and 1Br carbenes have triplet ground states, while 1F and 1I which initially take on a singlet multiplicity and eventually transform to a more stable intramolecular [1 + 2] cycloaddition product, and trans alkene, respectively. Except for 1H‐T and 1F‐T carbenes, the other triplet states emerge as allene isomer after optimization. We conclude that halogen substitution increases stability based on absolute value of singlet‐triplet energy separation (|ΔES‐T|), in the following order 1Br < 1H ≤ 1Cl < 1I < 1F at B3LYP/AUG‐cc‐pVTZ, and 1Br < 1Cl < 1H < 1I < 1F at M06‐2X/AUG‐cc‐pVDZ. Nucleophilicity of the carbenic center is decreased due to higher electronegativity of substituted halogens. The scrutinized carbenes 1Cl‐S, 1Br‐S, and 1I‐S show higher nucleophilicity and higher electrophilicity than the synthesized common NHCs (2‐6). Hence, compared with 2‐6 carbenes, these halogenated carbenes exhibit ambiphilic character owing to the presence of a σ2‐orbital and vacant 2pπ‐orbital that makes them theoretically more susceptible for attacking an electrophile or a nucleophile.