Types of Six-Membered N-Heterocyclic Germanium Radicals: A Combined Computational and Experimental Study

A new stable six-membered cyclic germylene radical C (^•L³Ge:; ^•L³ = ^•[CH₃C(PhCN-Dip)₂]^2-, where Dip = 2,6- ⁱPr₂C₆H₃) has been synthesized and structurally characterized. Unlike the germanium-centered radical A (L^1•Ge:, L¹ = [HC( ^tBuCN-Dip)₂]^-), C is a π-type radical with spin density mainly distributed on the NC₃N backbone, similar to that in the germylene radical B (^•L²Ge:; ^•L² = ^•[PhC(PhCN-Dip)₂]^2-). The electronic effects in six-membered N-heterocyclic germanium radicals were systematically investigated using density functional theory calculations. The type of radical, which basically depends on the strong inductive effect of substituents on the side C atoms of the NC₃N backbone, is confirmed by monitoring the change in the ordering of the frontier molecular orbitals during radical formation. However, the radical with moderately electronegative substituents or two substituents with comparable electron pushing and pulling abilities could not be isolated in experiments, probably because of the kinetic instability during the reduction process from the germylene chloride precursor to radical.