Interplay and Competition Between Two Different Types of Redox‐Active Ligands in Cobalt Complexes: How to Allocate the Electrons?

Abstract

The field of molecular transition metal complexes with redox-active ligands is dominated by compounds with one or two units of the same redox-active ligand; complexes in which different redox-active ligands are bound to the same metal are uncommon. This work reports the first molecular coordination compounds in which redox-active bisguanidine or urea azine (biguanidine) ligands as well as oxolene ligands are bound to the same cobalt atom. The combination of two different redox-active ligands leads to mono-as well as unprecedented dinuclear cobalt complexes, being multiple (four or six) center redox systems with intriguing electronic structures, all exhibiting radical ligands. By changing the redox potential of the ligands through derivatisation, the electronic structure of the complexes could be altered in a rational way.

Chemical and Redox Noninnocence of Pentane-2,4-dione Bis(S-methylisothiosemicarbazone) in Cobalt Complexes and Their Application in Wacker-Type Oxidation

“…Metal complexes with redox-active and redox noninnocent ligands − continue to spark the interest of the chemical community − as they not only boast intriguing electronic structures, − as well as spectroscopic − and magnetochemical properties, − but also promise new opportunities for catalysis. − A particular point of interest of redox-active ligands is their ability to act as electron reservoirs, enabling substrate activation at a metal center via intramolecular ligand–metal electron transfer. , The propensity of redox-active ligands to bring about new reactivity at a bound transition metal and to promote challenging multielectron catalytic transformations by avoiding pathways with high-energy intermediates also motivates their use in catalysis. , The recent replacement of noble metals (classically undergoing 2e – transformations) by a combination of earth-abundant first-row transition metals and redox-active ligands (both enabling 1e – redox events under mild conditions) in catalytic transformations such as water oxidation is just one prominent example (Scheme a). , …”

“…Metal complexes with redox-active and redox noninnocent ligands 1 − 7 continue to spark the interest of the chemical community 8 − 11 as they not only boast intriguing electronic structures, 12 − 14 as well as spectroscopic 15 − 17 and magnetochemical properties, 18 − 21 but also promise new opportunities for catalysis. 22 − 28 A particular point of interest of redox-active ligands is their ability to act as electron reservoirs, enabling substrate activation at a metal center via intramolecular ligand–metal electron transfer.…”