Probing antiaromaticity: resonance Raman investigation of a series of differently substituted boroles

Abstract

We report an investigation of four differently substituted boroles by resonance Raman (RR) spectroscopy with the aim of gaining insight into the structural changes that occur upon electronic π -π * excitation in the five-membered BC 4 ring. Such boroles are prototypes for the theory of antiaromaticity, because the empty p z orbital at the boron interacts with the π system of the carbon backbone to delocalize the four π electrons. The magnitude of this interaction depends on the substituents at the boron atom. In all compounds, an intense band appears around 1600 cm −1 , which can be assigned to a C C stretching vibration. The selective amplification of this mode indicates an expansion of the borole ring upon electronic excitation. The resonant enhancement of another mode at around 1300 cm −1 , which can be assigned to a vibration between the boron and the substituent, gives evidence that the degree of antiaromaticity in the borole depends strongly on the interaction between the substituent at the boron atom with the empty p z orbital of the boron. When the boron is connected to a ferrocene, this band appears with high intensity, indicating a strong interaction between B and Fe. Furthermore, we studied the dependence of the excitation wavelength on the Raman intensities. In addition, we used DFT calculations to determine the vibrational wavenumbers.

A theoretical study of the aromaticity in neutral and anionic borole compounds

“…This is further reflected in the bending of the borolyl unit toward the metal center in the molecular structures. [46][47][48] While the gasphase optimized geometry matches the experimentally observed structure well (the dip angle α*, defined between the cyclopentadienyl ring plane and the borolyl unit, is 29.4° (exp.) [5] vs. 19.2° (calc.…”

“…It is worth mentioning, however, that on the basis of a resonance Raman investigation this π-interaction of the aryl moiety with the borole unit is rather weak. [46] More conclusive evidence for π-conjugation of an exocyclic substituent with the borole ring was found in 1-heteroaromatic-substituted tetraphenylboroles, in which the electron-rich heterocycles (furan, thiophene or pyrrole) are essentially coplanar with the borole ring plane. This interaction is further manifested in characteristic hypsochromic shifts in their UV-vis absorption maxima compared to borole 1, which correspond to a reduced antiaromaticity, as well as in the calculated NICS values for the contrasting ring systems.…”

High‐Yield Synthesis of a Hybrid 2,3,4,5‐Tetracarba‐1,6‐nido‐hexaborane(6) Cluster with an exo‐Polyhedral Boracycle

“…As part of our ongoing studies on boroles and their derivatives, [12][13][14] we recently described the unusual dimerization/ rearrangement of 1-chloro-2,3,4,5-tetraphenylborole at slightly elevated temperatures (40 8C) to afford the [4.5]decatriene derivative 1.…”

“…[18,19] The 11 B NMR chemical shifts of 2 proved temperature independent in the inspected range between 75 and À80 8C. The number of signals determined by 13 C NMR spectroscopy indicates rotational hindrance of several phenyl groups around the cluster framework, which results in chemical inequivalence of the respective phenyl carbon atoms and concomitant line broadening of some resonances.…”

Reduktion von Ferrocenylborol

“…Das hervorstechendste Strukturmerkmal von 1-Ferrocenyl-2,3,4,5-tetraphenylborol (1) ist der relativ große "dip"-Winkel von 29.48, [12] der eindrucksvoll die Stärke der Fe-BWechselwirkung, bedingt durch die antiaromatische Natur der Boroleinheit, [13] [14] Den Untersuchungen an 9-Ferrocenylborafluoren zufolge ist die Einelektronenoxidation mit der Bildung des entsprechenden Ferroceniumkations und einer Verkleinerung des "dip"-Winkels von 25.58 auf 6.38 verbunden.…”