Exploring Ligand-Centered Hydride and H-Atom Transfer

Metal-Ligand Role Reversal: Hydride-Transfer Catalysis by a Functional Phosphorus Ligand with a Spectator Metal

Hydride transfer catalysis is shown to be enabled by the nonspectator reactivity of a transition metal-bound low-symmetry tricoordinate phosphorus ligand. Complex 1·[Ru]+, comprising a nontrigonal phosphorus chelate (1, P(N(o-N(2-pyridyl)C6H4)2) and an inert metal fragment ([Ru] = (Me5C5)Ru), reacts with NaBH4 to give a metallohydridophosphorane (1H·[Ru]) by P-H bond formation. Complex 1H·[Ru] is revealed to be a potent hydride donor (ΔG°H-,exp < 41 kcal/mol, ΔG°H-,calc = 38 ± 2 kcal/mol in MeCN). Taken together, the reactivity of the 1·[Ru]+/1H·[Ru] pair comprises a catalytic couple, enabling catalytic hydrodechlorination in which phosphorus is the sole reactive site of hydride transfer.

Metal-Ligand Cooperative Transfer of Protons and Electrons

Realizing cooperativity between ligands and metal centers in the transfer of proton and electron equivalents has the potential to facilitate faster, selective, and novel transformations. Recent advances in the synthesis and application of ligands with these design features illustrate the value of this biomimetic strategy in synthetic chemistry.

Rhenium-Mediated Conversion of Dinitrogen and Nitric Oxide to Nitrous Oxide

Reductive splitting of N₂ is an attractive strategy towards nitrogen fixation beyond ammonia at ambient conditions. However, the resulting nitride complexes often suffer from thermodynamic overstabilization hampering functionalization. Furthermore, oxidative nitrogen atom transfer of N₂ derived nitrides remains unknown. We here report a Re^IV pincer platform that mediates N₂ splitting upon chemical reduction or electrolysis with unprecedented yield. The N₂ derived Re^V nitrides undergo facile nitrogen atom transfer to nitric oxide, giving nitrous oxide nearly quantitatively. Experimental and computational results indicate that outer‐sphere ReN/NO radical coupling is facilitated by the activation of the nitride via initial coordination of NO.

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

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- (1^2-, ^S2bpy = [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.