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

The air-stable, free-radical, low-spin Co(III) complex, (Bu(4)N)(2) [3,5-Co(DBSQ)(CN)(4)].(1)/(2)H(2)O.(1)/(4)CH(2)Cl(2) (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 Bu(4)NCN 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, (Bu(4)N)(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 &mgr;(B) and a typical semiquinone, S = (1)/(2), free-radical EPR signature (CH(2)Cl(2) solution, 293K) with g = 2.002, a(Co) = 8.8G, and a(H) = 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-DBCat(2)(-) solution.