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

One-electron oxidation of the disilicon(0) compound Si sub(2)(Idipp) sub(2) (1, Idipp=1,3-bis(2,6-diisopropylphenyl)imidazolin-2-ylidene) with [Fe(C sub(5)Me sub(5)) sub( 2)][B(Ar super(F)) sub(4)] (Ar super(F)=C sub(6)H sub(3)-3,5-(CF sub(3)) sub(2)) affords selectively the green radical salt [Si sub(2)(Idipp) sub(2)][B(Ar super(F)) sub(4)] (1-[B(Ar super(F)) sub(4)). Oxidation of the centrosymmetric1 occurs reversibly at a low redox potential (E sub(1/2)=-1.250V vs. Fc super(+)/Fc), and is accompanied by considerable structural changes as shown by single-crystal X-ray structural analysis of 1-B(Ar super(F)) sub(4). These include a shortening of the Si--Si bond, a widening of the Si-Si-C sub(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 super(+). Comparative quantum chemical calculations of 1 and 1 super(+) indicate that electron ejection occurs from the symmetric (n sub(+)) combination of the Si lone pairs (HOMO). EPR studies of 1-B(Ar super(F)) sub(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 super(29)Si hyperfine coupling tensors in 1 super(+). EPR studies of 1-B(Ar super(F)) sub(4) in liquid solution unraveled a topomerization with a low activation barrier that interconverts the two Si sites in 1 super(+). The radical salt [Si sub(2)(Idipp) sub(2)][B(Ar super(F)) sub(4)] was obtained upon one-electron oxidation of the disilicon(0) compound Si sub(2)(Idipp) sub(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 sigma -type radical.