Exploring Ligand-Centered Hydride and H-Atom Transfer

Two-fold proton coupled electron transfer of a Ta(V) aniline complex mediated by a redox active NNN pincer ligand

We report the proton-coupled electron transfer (PCET) reactivity of an octahedral Ta(V) aniline complex supported by an acridane-derived redox active NNN pincer ligand. The reversible binding of aniline to a Ta(V) dichloride induces significant coordination-induced bond weakening (CIBW) of the aniline N-H bonds. This enables a rare two-fold hydrogen atom abstraction, resulting in a terminal imido complex and a two-electron oxidation of the NNN pincer ligand, all while maintaining the metal's oxidation state. The bond dissociation free energies (BDFEs) of the aniline and a transient radical amido complex are estimated through stoichiometric reactions with different hydrogen atom abstractors and donors, further supported by density functional theory calculations.

Leveraging ligand-based proton and electron transfer for aerobic reactivity and catalysis

While O2 is an abundant, benign, and thermodynamically potent oxidant, it is also kinetically inert. This frequently limits its use in synthetic transformations. Correspondingly, direct aerobic reactivity with O2 often requires comparatively harsh or forcing conditions to overcome this kinetic barrier. Forcing conditions limit product selectivity and can lead to over oxidation. Alternatively, O2 can be activated by a catalyst to facilitate oxidative reactivity, and there are a variety of sophisticated examples where transition metal catalysts facilitate aerobic reactivity. Many efforts have focused on using metal-ligand cooperativity to facilitate the movement of protons and electrons for O2 activation. This approach is inspired by enzyme active sites, which frequently use the secondary sphere to facilitate both the activation of O2 and the oxidation of substrates. However, there has only recently been a focus on harnessing metal-ligand cooperativity for aerobic reactivity and, especially, catalysis. This perspective will discuss recent efforts to channel metal-ligand cooperativity for the activation of O2, the generation and stabilization of reactive metal-oxygen intermediates, and oxidative reactivity and catalysis. While significant progress has been made in this area, there are still challenges to overcome and opportunities for the development of efficient catalysts which leverage this biomimetic strategy.

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.