Synthesis and Structure of an Air-Stable, Free-Radical Cobalt(III) Semiquinone Complex

The air-stable, free-radical, low-spin Co(III) complex, (Bu4N)2 [3,5-Co(DBSQ)(CN)4]·1/2H2O·1/4CH2Cl2 (1), where
3,5-DBSQ is the semiquinone anion derived from the one-electron reduction of 3,5-di-tert-butyl-1,2-benzoquinone,
has been synthesized by the reaction of the cobalt(II) tetramer [Co(3,5-DBSQ)2]4 with Bu4NCN in THF. This is
a cyanide-induced redox reaction resulting in the formation of cobalt (II) and cobalt(III) products as follows,
where 3,5-DBCat is the respective catecholate dianion:  [Co(3,5-DBSQ)2]4 + 8CN- → 2[Co(3,5-DBSQ)(CN)4]2-
+ 2[Co(3,5-DBSQ)2(3,5-DBCat)]2-. The Co(III) product, (Bu4N)2 [Co(3,5-DBSQ)(CN)4], is insoluble in THF
while the Co(II) product remains in solution. Single-crystal X-ray diffraction of (1) reveals octahedrally coordinated
cobalt(III) and C−O and C−C bond lengths indicative of semiquinone. The cyanide ligands occupy the remaining
four sites with essentially linear Co−CN bond angles and average Co−C and Co−O bond distances of 1.89(2)
Å and 1.97(2) Å, respectively. The complex has a magnetic moment of 1.80 μB and a typical semiquinone, S =
1/2, free-radical EPR signature (CH2Cl2 solution, 293K) with g = 2.002, aCo = 8.8G, and aH = 2.6G. The identity
of [Co(3,5-DBSQ)2(3,5-DBCat)]2- (2) in the above reaction was confirmed by independent in situ generation of
this anion from the reaction of [Co(3,5-DBSQ)2]4 with 3,5-DBCat2- solution.