Broadening the Scope of Cobalt Complexes Supported by Macrocyclic Tetra-NHCs

A series of cobalt(II) complexes ligated by 18-atom macrocyclic tetra-NHCs (N-heterocyclic carbenes) were synthesized via two distinct strategies. The complexes [( Me,Et TC Ph )Co(NCCH 3 )](PF 6 ) 2, 1, and [( ( S, S )-1,2-Ph,Me TC H )Co(NCPh)](PF 6 ) 2, 2, were prepared with neutral macrocycles through transmetalation with silver(I) dimer precursors. In contrast, neutral cobalt(II) species, ( BMe 2,Et TC H )Co, 3, and ( ( S, S )-1,2-Cy,BMe 2 TC H )Co, 4, were synthesized from anionic macrocyclic precursors by deprotonation with n BuLi prior to ligation. Complexes 2 and 4 are the first examples of chiral macrocyclic NHCs on cobalt. Each complex was characterized by multinuclear NMR spectroscopy, UV–vis, IR, ESI-MS, Evans method measurements, and single-crystal X-ray diffraction (SCXRD). Despite having different coordination geometries, each of the cobalt(II) complexes is low spin, S = 1/2. Finally, a stable diamagnetic cobalt(III) complex, [( Me,Et TC Ph )Co(NCCH 3 ) 2 ](PF 6 ) 3, 5, was prepared through a one electron oxidation of 1 .