Issue 1, 2014

1,4-Disilacyclohexa-2,5-diene: a molecular building block that allows for remarkably strong neutral cyclic cross-hyperconjugation

Abstract

2,3,5,6-Tetraethyl-1,4-disilacyclohexa-2,5-dienes with either four chloro (1a), methyl (1b), or trimethylsilyl (TMS) (1c) substituents at the two silicon atoms were examined in an effort to design rigid compounds with strong neutral cross-hyperconjugation between π- and σ-bonded molecular segments arranged into a cycle. Remarkable variations in the lowest electronic excitation energies, lowest ionization energies, and the first oxidation potentials were observed upon change of substituents, as determined by gas phase ultraviolet (UV) absorption spectroscopy, ultraviolet photoelectron spectroscopy (UPS), and cyclic voltammetry. A particularly strong neutral cyclic cross-hyperconjugation was observed in 1c. Its lowest electron binding energy (7.1 eV) is distinctly different from that of 1b (8.5 eV). Molecular orbital analysis reveals a stronger interaction between filled π(C[double bond, length as m-dash]C) and π(SiR2) group orbitals in 1c than in 1a and 1b. The energy shift in the highest occupied molecular orbital is also reflected in the first oxidation potentials as observed in the cyclic voltammograms of the respective compounds (1.47, 0.88, and 0.46 V for 1a, 1b and 1c, respectively). Furthermore, 1,4-disilacyclohexadiene 1c absorbs strongly at 273 nm (4.55 eV), whereas 1a and 1b have no symmetry allowed excitations above 215 nm (below 5.77 eV). Thus, suitably substituted 1,4-disilacyclohexa-2,5-dienes could represent novel building blocks for the design of larger cross-hyperconjugated molecules as alternatives to traditional purely cross-π-conjugated analogues, and could allow for design of molecules with properties that are not accessible to those that are exclusively π-conjugated.

Graphical abstract: 1,4-Disilacyclohexa-2,5-diene: a molecular building block that allows for remarkably strong neutral cyclic cross-hyperconjugation

Supplementary files

Article information

Article type
Edge Article
Submitted
26 Aug 2013
Accepted
23 Sep 2013
First published
26 Sep 2013

Chem. Sci., 2014,5, 360-371

1,4-Disilacyclohexa-2,5-diene: a molecular building block that allows for remarkably strong neutral cyclic cross-hyperconjugation

J. Tibbelin, A. Wallner, R. Emanuelsson, F. Heijkenskjöld, M. Rosenberg, K. Yamazaki, D. Nauroozi, L. Karlsson, R. Feifel, R. Pettersson, J. Baumgartner, S. Ott and H. Ottosson, Chem. Sci., 2014, 5, 360 DOI: 10.1039/C3SC52389F

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