Dinuclear Manganese Carbonyl Complexes: Electrocatalytic Reduction of Protons to Dihydrogen

Abstract

Studies on Mn complexes as electrocatalysts for proton reduction are limited. Mostly Mn complexes have been investigated for CO 2 activation. Hence, hydrogen production by three Mn complexes [(Mn(CO) 3 (μ-SN 2 C 7 H 5 )) 2 ] 1, [(Mn(CO) 3 (μ-S 2 NC 7 H 4 )) 2 ] 3 and [Mn 2 (CO) 7 (μ-S 2 NC 7 H 4 ) 2 ] 4 is reported in the paper. The complexes 1, 3 and 4 effectively catalyzed electro-chemical proton reduction in CH 3 CN with trifluoroacetic acid (TFA) as the proton source. Complexes 1, 3 and 4 displayed turnover frequency (TOF / s À 1 ) (overpotential, η / V) of 358 (0.83), 111 (0.79) and 102 (0.77), respectively for the catalytic process.

Manganese Complexes: Hydrogen Generation and Oxidation

“…Recently, three dinuclear Mn complexes [{Mn(CO) 3 (µ‐SN 2 C 7 H 5 )} 2 ] 15 , [{Mn(CO) 3 (µ‐S 2 NC 7 H 4 )} 2 ] 16 and [Mn 2 (CO) 7 (µ‐S 2 NC 7 H 4 ) 2 ] 17 (Figure ) were reported by our group . The three complexes 15 – 17 showed catalytic reduction peaks at –1.72, –1.68 and –1.66 V, respectively on addition of TFA in CH 3 CN.…”

“…The dinuclear complexes reported include [Mn(CO) 4 (µ‐SePh)] 2 10 , [Mn(CO) 4 (µ‐SePh) 2 Mn(CO) 3 (Py)] (Py=pyridine) 11 , [Mn 2 (CO) 4 (µ‐CO)(µ‐SePh) 2 (PBu 3 ) 2 ] (PBu 3 =Tri‐ n ‐butylphosphine) 12 , [Mn(bpy) 3 ] + [(CO) 3 Mn(µ‐SPh) 3 Mn(CO) 3 ] – (bpy = 2,2′‐bipyridine) 13 , [{Mn(CO) 3 } 2 (µ‐SC 6 H 4 ‐ o ‐S–S‐C 6 H 4 ‐ o ‐µ‐S‐)] 14 , [{Mn(CO) 3 (µ‐SN 2 C 7 H 5 )} 2 ] 15 , [{Mn(CO) 3 (µ‐S 2 NC 7 H 4 )} 2 ] 16 , and [Mn 2 (CO) 7 (µ‐S 2 NC 7 H 4 ) 2 ] 17 . (Figure ).…”

“…The complexes that have so far been studied for the hydrogen evolution reaction (HER) are classified into {Mn}, {MnMn} and {MnM′} (M′ = any other metal) type of complexes . The sections given below describe the characteristics and properties of these complexes.…”

“…Manganese inspite of being one of the most earth‐abundant element after Fe and Ti has been less explored. Though the protonation reactions of Mn‐oxo complexes have been reported since the early 1990s,, it is only recently that Mn catalysts which exploit an amplitude of ligand environments have been investigated for electrochemical hydrogen production and hydrogen oxidation . Recent developments on Mn‐catalyzed hydrogenation are also a source of inspiration in this regard …”

“…Moreover, though there are numerous reviews on Fe, Ni and Co catalysts, there is a lack of reviews that specifically cover the Mn catalysts , . Herein, we have provided a comprehensive overview of the applications of manganese complexes in electrochemical hydrogen generation and oxidation . This is an exhaustive review, covering literature up to August 2019, in comparison to previous reviews that discuss mostly general manganese catalysis and CO 2 reduction catalysts , …”