Homo‐ and Heterobinuclear Cu and Pd Complexes with a Bridging Redox‐Active Bisguanidino‐Substituted Dioxolene Ligand: Electronic Structure and Metal–Ligand Electron‐Transfer

Abstract

A new redox-active 4,5-bisguanidino-substituted o-benzoquinone ligand L is synthesized, which allows rational access to heterobinuclear complexes through the sequential coordination of two metals. In the examples discussed in this work, mononuclear Cu and Pd complexes are prepared in a first coordination step, and these complexes are then used as precursors to homobinuclear [Cu -L -Cu ] and heterobinuclear [Pd -L -Cu ] complexes. In the heterobinuclear complex, the Pd is coordinated by the softer bisguanidine side of L and the Cu by the harder dioxolene side (in line with the HSAB concept). The heterobinuclear complex is in a temperature-dependent equilibrium with its dimer, with two unsymmetrical Cu-Cl-Cu bridges. The redox-chemistry of the [Cu -L-Cu ] and [Pd -L-Cu ] complexes is studied. One-electron oxidation of both complexes was found to be quasi-reversible in CV experiments, and chemical one-electron oxidation was achieved with NO (SbF ). In the case of the homobinuclear complex [L(CuCl ) ] , intramolecular ligand-metal electron-transfer, triggered by coordination of a CH CN solvent molecule, leads to a temperature-dependent equilibrium between the form [Cu -L -Cu ] at low temperatures (with CH CN coordinated to the Cu atom) and [Cu -L -Cu ] at higher temperatures (without CH CN).

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

“…[36,38,39]), to achievet he optimal balance between fast activation of dioxygen with an efficient phenol deprotonation and the ability to oxidizel ess electron-rich phenols, is currently under development. The expansion of this catalytic system to redox-activeg uanidine ligands with slightly higher redox-potential (see the bisguanidine ligands in refs.…”

“…The expansion of this catalytic system to redox-activeg uanidine ligands with slightly higher redox-potential (see the bisguanidine ligands in refs. [36,38,39]), to achievet he optimal balance between fast activation of dioxygen with an efficient phenol deprotonation and the ability to oxidizel ess electron-rich phenols, is currently under development.…”

“…The oxidant, dioxygen, is activatedb yc opperc omplexesb earing the redox-active guanidino-functionalized aromatics( GFA) 1,2,4,5tetrakis-(tetramethylguanidino)benzene (1)a sl igand (see Scheme4a). [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. 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] ).…”

“…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.…”

“…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. [41] The high electron-donor capability of the ligand warrants an efficient dioxygen activation or OÀOb ond cleavage reactiont og ive Cu 2 O 2 complexes.…”