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−S bpy)(CO) 3 Cl] (1, S−S bpy = [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( S2 bpy)(CO) 3 Cl] 2− (1 2− , S2 bpy = [2,2′-bipyridine]-3,3′-bis(thiolate)). 1 2− 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 , pK a = 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.

Luminescent Iridium Complexes with a Sulfurated Bipyridine Ligand: PCET Thermochemistry of the Disulfide Unit and Photophysical Properties

“…The N–C–C–N torsion angle in [1] + is reduced to around θ = 18° compared to 33° for free S–S bpy, thus maximizing orbital overlap between the donor N-atoms and the metal ion. However, this θ value is the largest among the series of S–S bpy complexes reported so far, , suggesting that [1] PF 6 has a more strained S–S bpy ligand compared to A and B . The C–S–S–C torsion angles of around 56° are virtually the same in all three complexes, and also, the S–S bond lengths are very similar in the narrow range 2.042–2.048 Å.…”

“…Concept of the ligand S–S bpy as a multi-PCET unit (top) and previously reported and new complexes of S–S bpy (bottom). , …”

“…The first reported complex of S–S bpy, [Ru­(bpy) 2 ( S–S bpy)] 2+ ( A in Figure ), was shown to undergo 2e – reduction centered on the disulfide moiety . While the free ligand exhibits potential inversion only in the presence of protons, it does so when N , N ′-chelating a metal ion in A , and the two potentials were found to be equal in rhenium­(I) complex B . After injection of the second electron, the disulfide bond breaks to form the corresponding dithiolate; however, in the case of A , it was observed that reduction triggers sulfide extrusion from S–S bpy to give N , N ′-chelating thieno­[3,2- b :4,5- b ′]­bipyridine.…”

“…To better understand the ground-state PCET thermodynamics of S–S bpy coordinated to a metal ion, specifically the reduction potentials and the protonation behavior, we used complex B that does not exhibit the drawback of rapid decomposition of the reduced species at room temperature (rt). It was found that upon two-electron reduction at moderate potentials ( E 1 = E 2 = −1.16 V versus Fc +/0 ) in acetonitrile (MeCN) solution, the resulting dithiolate can be protonated once with a p K a of 24.3 . The resulting doubly reduced and singly protonated S–H···S – complex can act as a formal hydride donor with a hydricity of 60 kcal mol –1 , similar to synthetic analogues of biological hydride transfer reagents such as nicotinamide adenine dinucleotide (NAD + |NADH). , In addition, H atom transfer of the S–H···S – complex is also possible [bond dissociatioon free energy, BDFE­(S–H) = 59 kcal mol –1 ], demonstrating the rich and versatile PCET chemistry, which can originate from the peripheral disulfide functional group.…”

Electrochemical CO2 Reduction Catalyzed by Binuclear LRe2(CO)6Cl2 and LMn2(CO)6Br2 Complexes with an Internal Proton Source

“…22a 13 operates at a potential of −2.05 V in MeCN with TFE as an acid, but the current enhancement i c /i p of 4.7 is rather moderate (Figure 3). 24 These selected examples show that the binuclear nature in combination with an anionic ligand could increase the catalytic rate, though, catalysis requires rather harsh reductive conditions, which could limit the FE and stability.…”