Estimating the stability and reactivity of cyclic tetrahalo substituted germylenes: A density functional theory investigation

Abstract

AbstractThe largest unsaturated carbene, cyclonona‐3,5,7‐trienylidene (1H), emerges as a boat‐shaped transition state (TS) for having a negative force constant (FC). Following the quest for stable germylenes (2X, where X = H, F, Cl, Br, and I) with C8H6X4Ge: formula, the density functional theory (DFT) calculations are carried out to assay the substituent effect on their thermodynamic and kinetic possibilities. These singlet and triplet germylenes (2X‐S and 2X‐T, respectively) appear as boat‐shaped minima without imaginary frequencies and the order of their stability estimated by singlet (S)–triplet (T) energy difference (ΔES‐T = ET − ES) is 1H < 2H < 2F < 2Cl < 2I < 2Br. The purpose of the present work is to swiftly switch from T carbene to S germylenes through alteration of the substituents and assess the influence of different halogens on the stability and other properties. The 2Br‐S seems as the most stable species. From kinetic point of view, 2Cl, 2Br, and 2I are more stable than some of synthesized germylenes. The stabilization of the S state is essential to stabilize germylenes. This can be achieved by σ‐electron withdrawing and π‐electron donating substituents, of which stabilizing effect can be further increased in the order of steric effect < positive hyperconjugation < negative hyperconjugation < mesomeric effect. Applying suitable isodesmic reactions, we show the π‐donor/σ‐acceptor halogen substituents stabilize not only the S but also the T states, and these groups stabilize 2X‐S more than 2X‐T.