A Bioinspired Disulfide/Dithiol Redox Switch in a Rhenium Complex as Proton, H Atom, and Hydride Transfer Reagent.

Abstract

The transfer of multiple electrons and protons is of crucial importance in many reactions relevant in biology and chemistry. Natural redox-active cofactors are capable of storing and releasing electrons and protons under relatively mild conditions and thus serve as blueprints for synthetic proton-coupled electron transfer (PCET) reagents. Inspired by the prominence of the 2e-/2H+ disulfide/dithiol couple in biology, we investigate herein the diverse PCET reactivity of a Re complex equipped with a bipyridine ligand featuring a unique SH···-S moiety in the backbone. The disulfide bond in fac-[Re(S-Sbpy)(CO)3Cl] (1, S-Sbpy = [1,2]dithiino[4,3-b:5,6-b']dipyridine) undergoes two successive reductions at equal potentials of -1.16 V vs Fc+|0 at room temperature forming [Re(S2bpy)(CO)3Cl]2- (12-, S2bpy = [2,2'-bipyridine]-3,3'-bis(thiolate)). 12- has two adjacent thiolate functions at the bpy periphery, which can be protonated forming the S-H···-S unit, 1H-. The disulfide/dithiol switch exhibits a rich PCET reactivity and can release a proton (ΔG°H+ = 34 kcal mol-1, pKa = 24.7), an H atom (ΔG°H• = 59 kcal mol-1), or a hydride ion (ΔG°H- = 60 kcal mol-1) as demonstrated in the reactivity with various organic test substrates.