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

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 pi - pi* excitation in the five-membered BC4 ring. Such boroles are prototypes for the theory of antiaromaticity, because the empty p(z) orbital at the boron interacts with the pi system of the carbon backbone to delocalize the four pi 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 OFT calculations to determine the vibrational wavenumbers. Copyright (C) 2009 John Wiley & Sons, Ltd.