Tricoordinate Beryllium Radicals and Their Reactivity.

The one-electron reduction of [(CAAC)Be(Dur)Br] (CAAC = cyclic alkyl(amino)carbene, Dur = 2,3,5,6-tetramethylphenyl = duryl) with lithium sand in diethyl ether yields the first neutral, tricoordinate, and moderately stable beryllium radical, [(CAAC)(Et 2 O)BeDur] • ( 2-Et 2 O ), which undergoes a facile second one-electron reduction concomitant with the insertion of the beryllium center into the endocyclic C-N CAAC bond and a cyclopropane-forming C-H bond activation of an adjacent methyl group. In situ generation of 2-Et 2 O and addition of PMe 3 yield the stable analogue, [(CAAC)(Me 3 P)BeDur] • ( 2-PMe 3 ), which serves as a platform for PMe 3 -ligand exchange with stronger donors, generating the radicals [(CAAC)LBeDur] • ( 2-L , L = isocyanides, pyridines, and N -heterocyclic carbenes). X-ray structural analyses show trigonal-planar beryllium centers and strong π backbonding from the metal to the CAAC ligand. The EPR signals of all six isolated [(CAAC)LBeDur] • radicals display significant, albeit small, hyperfine coupling to the 9 Be nucleus. DFT calculations show that the spin density is mostly delocalized over the CAAC π framework and, where present, the isocyanide CN moiety, with only a small proportion (3-6%) on the beryllium center. 2-PMe 3 proved thermally unstable at 80 °C, first undergoing radical hydrogen abstraction with the solvent, followed by insertion of beryllium into the endocyclic C-N CAAC bond and PMe 3 transfer to the former carbene carbon atom. The reactions with diphenyl disulfide and phenyl azide occur at the beryllium center and yield the corresponding Be(II) phenyl sulfide and amino complexes, respectively, the latter concomitant with radical transfer and hydrogen abstraction by the beryllium-bound nitrogen center.