Ag^II‐Mediated Electrocatalytic Ambient CH₄ Functionalization Inspired by HSAB Theory

Abstract

Although most class (b) transition metals have been studied in regardt oC H 4 activation, divalent silver (Ag II ), possibly owing to its reactive nature,isthe only class (b) highvalent transition metal center that is not yet reported to exhibit reactivities towards CH 4 activation. We now report that electrochemically generated Ag II metalloradical readily functionalizes CH 4 into methyl bisulfate (CH 3 OSO 3 H) at ambient conditions in 98 %H 2 SO 4 .M echanistic investigation experimentally unveils al ow activation energy of 13.1 kcal mol À1 , ahigh pseudo-first-order rate constant of CH 4 activation up to 2.8 10 3 h À1 at room temperature and aCH 4 pressure of 85 psi, and two competing reaction pathwayspreferable towards CH 4 activation over solvent oxidation. Reaction kinetic data suggest aF aradaic efficiency exceeding 99 %b eyond 180 psi CH 4 at room temperature for potential chemical production from widely distributed natural gas resources with minimal infrastructure reliance.electrophilicity may be reactive towards CH 4 via either electrophilic activation [6] or ar adical-based mechanism. [7] Consistently,many high-valent class (b) metals in the d-block of periodic table,i ncluding Rh I,II , [8] Pd II,III , [9] Ir III , [10] Pt II,IV , [11] Au I,III , [12] Hg II , [6b] have been reported for CH 4 activation (Figure 1a). Some of the borderline metals with intermediate chemical softness,i ncluding Mn III , [13] Co III , [13] Ru IV,VIII , [14] Os IV,VIII , [15] Tl III , [6c] and Pb IV , [6c, 13] are reactive towards CH 4 , too.Yet there is one exception, silver (Ag). While monovalent Ag I as am ild oxidant (h = 6.96; E8 8 = 0.80 Vv s. normal hydrogen electrode (NHE) for Ag + (aq.)/Ag (s)) [5,16] may not be oxidative enough to break the CÀHb ond in CH 4 (E8 8 = 0.59 Vvs. NHE for CH 3 OH (l)/CH 4 (g)), [2b, 16] divalent Ag II is similarly soft (h = 6.7) [5] and possesses aA g II /Ag I redox potential (E8 8 % 2.5 Vv s. NHE for Ag II /Ag I in 98 % H 2 SO 4 ) [17] comparable to other reported CH 4 -activation catalysts. [2a,b,9b] Therefore,i ti si ntriguing that divalent Ag II has not been known for CH 4 activation despite the reported Ag II -based reactivities on much weaker C À Hbonds in organic synthesis. [18] Thed 9 electronic configuration of Ag II not only renders it am etalloradical, but also introduces the Jahn-Te ller effect in an O h ligand field that elongates the ligand bond in the axial position (Figure 1b). [19] Such weakly bound axial ligands and Ag II sl ikely radical nature may offer an opportunity for substrate binding and CH 4 activation in aradical-based activation pathway with low reaction barrier, leading to our hypothesis that Ag II ,o nce continuously generated, may serve as the active species towards ambient CH 4 functionalization catalytically.Our strategy of investigating Ag II as ap otential active species towards CH 4 activation includes continuous electrogeneration of reactive Ag II species in an inert solvent environment (Figure 1c). Electrochemistry offers av iable and clean ...

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