Easy Access to Silicon(0) and Silicon(II) Compounds

Two different synthetic methodologies
of silicon dihalide bridged biradicals of the general formula (Ln•)2SiX2 (n = 1, 2) have been developed. First, the metathesis reaction
between NHC:SiX2 and Ln: (Ln: = cyclic akyl­(amino) carbene in a 1:3 molar
ratio leads to the products 2 (n = 1,
X = Cl), 4 (n = 2, X = Cl), 6 (n = 1, X = Br), and 7 (n = 2, X = Br). These reactions also produce coupled NHCs (3, 5) under C–C bond formation. The formation
of the coupled NHCs (Lm = cyclic alkyl­(amino)
carbene substituted N-heterocyclic carbene; m = 3, n = 1 (3) and m = 4, n =2 (5)) is faster during the metathesis reaction
between NHC:SiBr2 and Ln: when
compared with that of NHC:SiCl2. Second, the reaction of
L1:SiCl4 (8) (L1: =:C­(CH2)­(CMe2)2N-2,6-iPr2C6H3) with a non-nucleophilic base LiN­(iPr)2 in a 1:1 molar ratio shows an unprecedented
methodology for the synthesis of the biradical (L1•)2SiCl2 (2). The blue blocks of silicon
dichloride bridged biradicals (2, 4) are
stable for more than six months under an inert atmosphere and in air
for one week. Compounds 2 and 4 melt in
the temperature range of 185 to 195 °C. The dibromide (6, 7) analogue is more prone to decomposition
in the solution but comparatively more stable in the solid state than
in the solution. Decomposition of the products has been observed in
the UV–vis spectra. Moreover, compounds 2 and 4 were further converted to stable singlet biradicaloid dicarbene-coordinated
(Ln:)2Si­(0) (n = 1 (9), 2 (10)) under KC8 reduction.
Compounds 2 and 4 were also reduced to dehalogenated
products 9 and 10, respectively when treated
with RLi (R = Ph, Me, tBu). Cyclic voltametry measurements
show that 10 can irreversibly undergo both one electron
oxidation and reduction.