The Si2H radical supported by two N-heterocyclic carbenes

Abstract

Cyclic voltammetric studies of the hydridodisilicon(0,II) borate [(Idipp)(H)Si^IISi⁰(Idipp)][B(Ar^F)₄] (1H[B(Ar^F)₄], Idipp = C[N(C₆H₃-2,6-iPr₂)CH]₂, Ar^F = C₆H₃-3,5-(CF₃)₂) reveal a reversible one-electron reduction at a low redox potential (E_1/2 = −2.15 V vs. Fc⁺/Fc). Chemical reduction of 1H[B(Ar^F)₄] with KC₈ affords selectively the green, room-temperature stable mixed-valent disilicon(0,I) hydride Si₂(H)(Idipp)₂ (1H), in which the highly reactive Si₂H molecule is trapped between two N-heterocyclic carbenes (NHCs). The molecular and electronic structure of 1H was investigated by a combination of experimental and theoretical methods and reveals the presence of a π-type radical featuring a terminal bonded H atom at a flattened trigonal pyramidal coordinated Si center, that is connected via a Si–Si bond to a bent two-coordinated Si center carrying a lone pair of electrons. The unpaired electron occupies the SiSi π* orbital leading to a formal Si–Si bond order of 1.5. Extensive delocalization of the spin density occurs via conjugation with the coplanar arranged NHC rings with the higher spin density lying on the site of the two-coordinated silicon atom.