Isomerization Reactions in Anionic Mesoionic Carbene-Borates and Control of Properties and Reactivities in the Resulting Co Complexes through Agostic Interactions.

We present herein anionic borate-based bi-mesoionic carbene compounds of the 1,2,3-triazol-4-ylidene type that undergo C-N isomerization reactions. The isomerized compounds are excellent ligands for Co  centers. Strong agostic interactions with the "C-H"-groups of the cyclohexyl substituents result in an unusual low-spin square planar Co  complex, which is unreactive towards external substrates. Such agostic interactions are absent in the complex with phenyl substituents on the borate backbone. This complex displays a high-spin tetrahedral Co  center, which is reactive towards external substrates including dioxygen. To the best of our knowledge, this is also the first investigation of agostic interactions through single-crystal EPR spectroscopy. We conclusively show here that the structure and properties of these Co complexes can be strongly influenced through interactions in the secondary coordination sphere. Additionally, we unravel a unique ligand rearrangement for these classes of anionic mesoionic carbene-based ligands.