One-Electron Oxidation of a Disilicon(0) Compound: An Experimental and Theoretical Study of [Si₂]⁺ Trapped by N-Heterocyclic Carbenes

One-electron oxidation of the disilicon(0) compound Si₂(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 [Si₂(Idipp)₂][B(Ar^F)₄] (1-[B(Ar^F)₄). Oxidation of the centrosymmetric1 occurs reversibly at a low redox potential (E_1/2=-1.250V 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 S-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 ²⁹Si 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⁺. The radical salt [Si₂(Idipp)₂][B(Ar^F)₄] was obtained upon one-electron oxidation of the disilicon(0) compound Si₂(Idipp)₂. The open-shell compound was characterized by experimental and quantum-chemical methods, providing a detailed insight into the unusual electronic structure and bonding situation of this σ-type radical.