2,3,5,6-Tetrasilyl- and 2,3,5,6-Tetragermyl-1,4-benzoquinones: X-ray Crystallographic Analysis, Cyclic Voltammetry, and DFT Calculations

By X-ray crystallographic analysis, it was proved that 2,3,5,6-tetrakis(trimethylgermyl)-1,4-benzoquinone, 2,3,5,6-tetrakis(dimethylvinylsilyl)-1,4-benzoquinone, and 2,3,5-tris(trimethylsilyl)-1,4-benzoquinone took a chair conformation, while 4,4,6,6,10,10,12,12-octamethyl-4,6,10,12-tetrasilatricyclo[7.3.0.03,7]dodeca-1(9),3(7)-diene-2,8-dione had a planar structure.
Cyclic voltammograms of 2,5-bis(trimethylsilyl)-1,4-benzoquinone (5a) and 1,4-benzoquinone
(1f) showed two sets of reversible redox peaks. On the other hand, the voltammogram of
2,3,5,6-tetrakis(trimethylsilyl)-1,4-benzoquinone (1a) showed that the first reduction step
was reversible and the second step was irreversible. The first reduction potential of 1a (E1/2I
= −1.12 V vs Ag/Ag+) was higher by more than 0.2 V compared to those of 5a (E1/2I = −0.88
V) and 1f (E1/2I = −0.81 V). DFT calculations for trimethylsilyl-substituted 1,4-benzoquinones
revealed that the LUMOs shifted to higher energies, whereas the calculated vibrational
frequencies shifted to lower frequencies as the number of silyl groups increased. These results
demonstrated that the trimethylsilyl group acted as an electron-donating group to the 1,4-benzoquinone unit.