Cyclic Alkyl(amino) Carbene-Stabilized Monoradicals of Organosilicon(IV) Compounds with Small Substituents

Radicals and biradicals can easily be obtained when silicon(IV) compounds are reduced and stabilized by the Lewis base cyclic(alkyl)(amino)carbene (cAAC = C(CH₂)(CMe₂)₂N-Dipp, Dipp = 2,6-i-Pr₂C₆H₃) featuring strong σ-donor and π-acceptor properties. To obtain the cAAC-stabilized monoradicals with halogen and alkyl substituents at the silicon(IV) atoms, the Ar substituent (Ar = o-C₆H₄NMe₂) was employed to act as a side-arm nitrogen donor ligand. The resulting monoradicals ArSiCl₂(cAAC) (4) and ArSiRCl(cAAC) (R = Me 5; Et 6) were isolated by the reduction of ArSiCl₂ (1) and ArSiRCl₂ (R = Me 2; Et 3) with KC₈ in the presence of cAAC with a strict ratio of 1:1:1. The three room-temperature stable radicals were kept for at least 3 months in the solid state under dry N₂ atmosphere without decomposition. All the compounds were fully characterized by liquid injection field desorption/ionization-mass spectrometry, electron paramagnetic resonance spectroscopy, UV/vis spectroscopy, elemental analysis, and single-crystal X-ray crystallographic studies.