A Combined “Electrochemical–Frustrated Lewis Pair” Approach to Hydrogen Activation: Surface Catalytic Effects at Platinum Electrodes

Abstract

Herein, we extend our “combined electrochemical–frustrated Lewis pair” approach to include Pt electrode surfaces for the first time. We found that the voltammetric response of an electrochemical–frustrated Lewis pair (FLP) system involving the B(C6F5)3/[HB(C6F5)3]− redox couple exhibits a strong surface electrocatalytic effect at Pt electrodes. Using a combination of kinetic competition studies in the presence of a H atom scavenger, 6-bromohexene, and by changing the steric bulk of the Lewis acid borane catalyst from B(C6F5)3 to B(C6Cl5)3, the mechanism of electrochemical–FLP reactions on Pt surfaces was shown to be dominated by hydrogen-atom transfer (HAT) between Pt, [Pt–H] adatoms and transient [HB(C6F5)3]⋅ electrooxidation intermediates. These findings provide further insight into this new area of combining electrochemical and FLP reactions, and proffers additional avenues for exploration beyond energy generation, such as in electrosynthesis.

Exploring structural and electronic effects in three isomers of tris{bis(trifluoromethyl)phenyl}borane: towards the combined electrochemical-frustrated Lewis pair activation of H2

“…Whilst electrochemical studies show that all three isomers are less electrophilic than the archetypal Lewis acid B(C 6 F 5 ) 3 the link between Lewis acidity, activity towards H 2 in an FLP, and electrophilicity, as shown in our previous studies 20,25,26 and those of other groups [33][34][35] is complex and requires further study. This lack of reactivity is due to a combination of, kinetic effects resulting from in part steric shielding of the boron centre; but also quenching of the boranes' electrophilicity through B⋯F bonding interactions, which are quantified by DFT calculations and electrochemical measurements.…”

“…22 FLPs are not limited to the main group, with zirconocene-phosphane complexes pioneered by Wass and co-workers shown to act as intramolecular FLPs that exhibit unprecedented reactivity towards small molecules; 23 similar chemistry has also been demonstrated with zirconocene-amines which act as hydrogenation catalysts with a wide range of substrates. 24 In 2014 we introduced the concept of "combined electrochemical-frustrated Lewis pairs", 20,25,26 that couple the heterolytic cleavage of H 2 by a conventional FLP with in situ electrochemical oxidation of the resultant borohydride and subsequent regeneration of the parent borane. The combined electrochemical-FLP systems were shown to be electrocatalytic for the oxidation of H 2 (to form two protons and two electrons a key reaction in many hydrogen-based energy technologies).…”

ON/OFF switching of silicon wafer electrochemistry by pH-responsive polymer brushes

“…For conventional electrochemical systems, curve crossing is interpreted as a consequence of a series of chemical reactions occurring within the electrochemical time scale of the voltammetry scan. [48][49][50][51][52] Due to the well-known stability and reliability of the hexamine ruthenium complex, however, this hypothesis would be hard to sustain in the present case. It is noteworthy to mention that until now there is only one example of this kind of behavior described for polymer brushes.…”

H2 activation using the first 1 : 1 : 1 hetero-tri(aryl)borane

“…In an effort to expand the range of Lewis acidic boranes for FLP reactions, we have previously reported studies of homotri(aryl)boranes {aryl ¼ C 6 F 5 , C 6 Cl 5 , 3,5-(CF 3 ) 2 C 6 H 3 , 2,4-(CF 3 ) 2 C 6 H 3 , 2,5-(CF 3 ) 2 C 6 H 3 } 24,25,27,28 and 2 : 1 hetero-tri(aryl) boranes {aryl ¼ C 6 F 5 /Ar F 5 , C 6 Cl 5 /Ar Cl 5 , 3,5-(CF 3 ) 2 C 6 H 3 /Ar F 6 }. 29 In these publications we also introduced the concept of using the standard reduction potential of tri(aryl)boranes as a proxy measure of their Lewis acidities; such a method is base independent, and measures a property of the trigonal-planar reactive species.…”