Influence of fluoro and cyano substituents in the aromatic and antiaromatic characteristics of cyclooctatetraene

Abstract

An exhaustive and systematic study of the structural and electronic properties of cyclooctatetraene (COT) upon substitution of hydrogen atoms by fluoro and cyano groups has been carried out in order to analyse the influence of both substituents on the aromaticity. We found that C-C distances decrease with fluoro substitution while in cyano derivatives the opposite happens. All the compounds retain their original structural type, with the exception of the cyano derivatives; thus, compounds 25CN 6T , 27CN 6T and 30CN 8T show boat-like structure, whereas compounds 20CN 5T , 26CN 6T , and 29CN 7T present twisted structures. Regarding the relative energies of those compounds with the same number of substitutions, it was found that compounds where the X groups were more separated among them were the most stable ones. Inversion barriers (DE TS ) were found to increase with the number of substitutions; in the case of fluoro derivatives these barriers have a two-fold, increase compared to the parent compound while in the cyano ones a three-fold increase was observed. The aromatic character based on the NICS values, was found to increase in the ground singlet states and in the transition states of both fluoro and cyano derivatives. For triplet states, a decrease of the aromatic behaviour was found upon substitution. NICS profiles and 3D NICS isosurfaces confirm such findings. Finally, HOMA indexes corroborate the aromatic changes described by the NICS values, although, no good correlations between both quantities were found.

Regium Bonds between Silver(I) Pyrazolates Dinuclear Complexes and Lewis Bases (N2, OH2, NCH, SH2, NH3, PH3, CO and CNH)

“…Despite NICS isotropic values being widely used and well established, there is still a controversy about the reliability of NICS values for assessment of the aromaticity of certain molecules [86,87]. Nevertheless, and following our previous experience, the isotropic values have in several cases been shown to present an accurate description of the aromatic behaviour in poly-aromatic systems [88][89][90].…”

A Study of π–π Stacking Interactions and Aromaticity in Polycyclic Aromatic Hydrocarbon/Nucleobase Complexes

“…Moreover, by using ring currents, it has also been proven that NICS values do not provide an accurate view of PAHs, particularly in fused systems with contiguous rings . Nevertheless, in previous works it has been demonstrated that in several cases the NICS and NICS zz values show similar trends . In the present work, we have analysed both NICS and NICS zz values for all the systems studied.…”

Ring-whizzing in polyene-PtL2 complexes revisited

“…For COT itself this entails an energy cost of 10–13 kcal/mol [ 59 ]. We find that the conversion for C 8 F 8 is nearly triple this amount, namely 29.9 kcal/mol [ 60 ].…”

“…An X-ray of the COT–Ni complex [ 56 ] reveals the structure is analogous to that for 29 . An X-ray of another Ni complex [ 60 ] produces a bis-η 2 isomer, 35 . This is also true for C 8 F 8 –Ni complexes with certain ligand sets [ 57 – 58 ].…”