One-Electron Oxidation of a Disilicon(0) Compound: An Experimental and Theoretical Study of [Si-2](+) Trapped by N-Heterocyclic Carbenes

摘要

One-electron oxidation of the disilicon(0) compound Si-2(Idipp)(2) (1, Idipp=1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene) with [Fe(C5Me5)(2)][B(Ar-F)(4)] (Ar-F = C6H3-3,5(CF3)(2)) affords selectively the green radical salt [Si-2(Idipp)(2)] [B(Ar-F)(4)] (1-[B(Ar-F)(4)). Oxidation of the centrosymmetric 1 occurs reversibly at a low redox potential (E-1/2=-1.250 V vs. Fc(+)/Fc), and is accompanied by considerable structural changes as shown by single-crystal X-ray structural analysis of 1-B(Ar-F)(4). These include a shortening of the Si-Si bond, a widening of the Si-Si-C-NHC angles, and a lowering of the symmetry, leading to a quite different conformation of the NHC substituents at the two inequivalent Si sites in 1(+). Comparative quantum chemical calculations of 1 and 1(+) indicate that electron ejection occurs from the symmetric (n(+)) combination of the Si lone pairs (HOMO). EPR studies of 1-B(Ar-F)(4) in frozen solution verified the inequivalency of the two Si sites observed in the solid-state, and point in agreement with the theoretical results to an almost equal distribution of the spin density over the two Si atoms, leading to quite similar Si-29 hyperfine coupling tensors in 1(+). EPR studies of 1-B(Ar-F)(4) in liquid solution unraveled a topomerization with a low activation barrier that interconverts the two Si sites in 1(+).