Removing the sting from the tail: Reversible protonation of scorpionate ligands in cobalt(II) tris(carbene) borate complexes

Low-temperature deprotonation of the phenylborane dications, PhB(RIm)(3)OTf2 (R = Bu-t, Mes), followed by in situ reaction with CoCI2(thf)(1.5), results in the formation of the four-coordinate complexes, k(3)-PhB(RIm)(3)CoCl, in which the metal is supported by tripodal N-heterocyclic carbene-based ligands. The chloride complexes are exceptionally sensitive to acid and can be reversibly protonated to form the zwitterions k(2)-{PhB(RIm)(2)(RIm center dot H)} CoCl2. This unexpected reactivity is attributed to the highly basic nature of the tris( carbene) borate ligands. Reaction of the chloride complexes with methylating reagents results in products that depend on the N-heterocyclic carbene substituent. For R = Bu-t, the four-coordinate high-spin complex, k(3)-PhB((t)BuIm)(3)CoMe, is formed, while for R = Mes, reduction to a multitude of complexes occurs.