Intramolecular metal–ligand electron transfer triggered by co-ligand substitution

Abstract

The possibility of directed stimulation of intramolecular electron transfer between a metal and a redox-active ligand in a molecular coordination compound is the key to its application in molecular catalysis and other research themes. Although the stimulation by a substitution reaction of the co-ligands is often postulated as key step in catalytic cycles using redox-active ligands as electron reservoirs, there are only a few explicit examples for such reactions. Herein we report the synthesis of the first dicationic and dinuclear CuI complexes featuring the oxidized form of a redox-active tetrakisguanidine ligand (1,2,4,5-tetrakis(tetramethylguanidino)benzene 1) as a bridging ligand and two neutral co-ligands L (acetonitrile or pyridine), [1{Cu(Cl)L}2]2+. An intramolecular electron transfer between the copper atom and the tetrakisguanidine ligand 1, leading to a dinuclear CuII complex with the reduced form of the tetrakisguanidine ligand 1, is triggered by substitution of the neutral co-ligands L.

Copper catalysis with redox-active ligands

“…While such an behavior is a cornerstone in enzymatic reactions, the wide opportunities offered by these bioinspired approaches are only emerging and should blossom in the near future. In order to assert this field as a true game-changer in copper catalysis, further work should aim at rationalizing the "redox dialogue" between the metal and the ligand, and provide a deeper understanding of the valence tautomerism at work in such systems in order to develop predictive tools for reactivity control [41]. Such knowledge is most likely to result in new advances in the fast-expanding field of redox catalysis.…”

Catalytic Aerobic Phenol Homo‐ and Cross‐Coupling Reactions with Copper Complexes Bearing Redox‐Active Guanidine Ligands

“…Based on these results, the oxidative cross-coupling reactiono fp henolsw ith a non-complementary relationship is studied in detail. Hence it was possible to shift electrons between the redox-active guanidine ligand and the coppera tom in ac omplex by changing the counterions, [35] the solvent [36][37][38] or the temperature. This ligand, which is oxidized reversibly at low potential( E 1/2 = À0.7 Vv s. Fc + /Fc in CH 2 Cl 2 solution for the redox couple 1 2 + + /1), [34] supplies the metal atom with extra electrondensity,u pt ot he point of full transfer of two electrons from the ligand to the two coppera toms.…”

“…This ligand, which is oxidized reversibly at low potential( E 1/2 = À0.7 Vv s. Fc + /Fc in CH 2 Cl 2 solution for the redox couple 1 2 + + /1), [34] supplies the metal atom with extra electrondensity,u pt ot he point of full transfer of two electrons from the ligand to the two coppera toms. An umber of studies [35][36][37][38][39][40][41] have shown that the particularly low barrierf or intramolecular ligand-metal electron transfer in guanidine-copper complexes is due to the structural harmonization betweenC u II and Cu I complexes causedb yt he p-donor properties of the guanidine ligands( see also work by Herres-Pawlis et al on the entatic state concept in this context [42][43][44][45][46] ). Hence it was possible to shift electrons between the redox-active guanidine ligand and the coppera tom in ac omplex by changing the counterions, [35] the solvent [36][37][38] or the temperature.…”

“…An umber of studies [35][36][37][38][39][40][41] have shown that the particularly low barrierf or intramolecular ligand-metal electron transfer in guanidine-copper complexes is due to the structural harmonization betweenC u II and Cu I complexes causedb yt he p-donor properties of the guanidine ligands( see also work by Herres-Pawlis et al on the entatic state concept in this context [42][43][44][45][46] ). Hence it was possible to shift electrons between the redox-active guanidine ligand and the coppera tom in ac omplex by changing the counterions, [35] the solvent [36][37][38] or the temperature. [37,[39][40][41] An example for at hermally induced intramolecular electron transfer in ab inuclear copper complex with ar elated bridging redox-active tetrakis-guanidine ligand is given in Scheme 4b.…”

Redox‐Active Guanidines in Proton‐Coupled Electron‐Transfer Reactions: Real Alternatives to Benzoquinones?

“…25.3 for (1 + H) + in CH 3 CN sharplyd ecreasesu pon oxidation. Interestingly,g reen 1 2 + stilli saLewis [35] and Brønsted base, and is protonated with HBF 4 ·Et 2 Ot ob lue (1 + H) 3 + and orange (1 + 2H) 4 + (with ap K a value of ca. 13 in CH 3 CN close to CF 3 COOH,S cheme 1).…”