Elliot J. Lawrence

An electrochemical study of frustrated Lewis pairs: a metal-free route to hydrogen oxidation.

Frustrated Lewis pairs have found many applications in the heterolytic activation of H2 and subsequent hydrogenation of small molecules through delivery of the resulting proton and hydride equivalents. Herein, we describe how H2 can be preactivated using classical frustrated Lewis pair chemistry and combined with in situ nonaqueous electrochemical oxidation of the resulting borohydride. Our approach allows hydrogen to be cleanly converted into two protons and two electrons in situ, and reduces the potential (the required energetic driving force) for nonaqueous H2 oxidation by 610 mV (117.7 kJ mol–1). This significant energy reduction opens routes to the development of nonaqueous hydrogen energy technology.

Metal‐Free Dihydrogen Oxidation by a Borenium Cation: A Combined Electrochemical/Frustrated Lewis Pair Approach

In order to use H2 as a clean source of electricity, prohibitively rare and expensive precious metal electrocatalysts, such as Pt, are often used to overcome the large oxidative voltage required to convert H2 into 2 H+ and 2 e−. Herein, we report a metal-free approach to catalyze the oxidation of H2 by combining the ability of frustrated Lewis pairs (FLPs) to heterolytically cleave H2 with the in situ electrochemical oxidation of the resulting borohydride. The use of the NHC-stabilized borenium cation [(IiPr2)(BC8H14)]+ (IiPr2=C3H2(NiPr)2, NHC=N-heterocyclic carbene) as the Lewis acidic component of the FLP is shown to decrease the voltage required for H2 oxidation by 910 mV at inexpensive carbon electrodes, a significant energy saving equivalent to 175.6 kJ mol−1. The NHC–borenium Lewis acid also offers improved catalyst recyclability and chemical stability compared to B(C6F5)3, the paradigm Lewis acid originally used to pioneer our combined electrochemical/frustrated Lewis pair approach.

Synthesis, Photochemical, and Redox Properties of Gold(I) and Gold(III) Pincer Complexes Incorporating a 2,2′:6′,2″-Terpyridine Ligand Framework

Reaction of [Au(C6F5)(tht)] (tht = tetrahydrothiophene) with 2,2′:6′,2″-terpyridine (terpy) leads to complex [Au(C6F5)(η1-terpy)] (1). The chemical oxidation of complex (1) with 2 equiv of [N(C6H4Br-4)3](PF6) or using electrosynthetic techniques affords the Au(III) complex [Au(C6F5)(η3-terpy)](PF6)2 (2). The X-ray diffraction study of complex 2 reveals that the terpyridine acts as tridentate chelate ligand, which leads to a slightly distorted square-planar geometry. Complex 1 displays fluorescence in the solid state at 77 K due to a metal (gold) to ligand (terpy) charge transfer transition, whereas complex 2 displays fluorescence in acetonitrile due to excimer or exciplex formation. Time-dependent density functional theory calculations match the experimental absorption spectra of the synthesized complexes. In order to further probe the frontier orbitals of both complexes and study their redox behavior, each compound was separately characterized using cyclic voltammetry. The bulk electrolysis of a solution of complex 1 was analyzed by spectroscopic methods confirming the electrochemical synthesis of complex 2.

A combined "electrochemical-frustrated Lewis pair" approach to hydrogen activation: surface catalytic effects at platinum electrodes

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.

“Janus” Calixarenes: Double-Sided Molecular Linkers for Facile, Multianchor Point, Multifunctional, Surface Modification

We herein report the synthesis of novel “Janus” calix[4]arenes bearing four “molecular tethering” functional groups on either the upper or lower rims of the calixarene. These enable facile multipoint covalent attachment to electrode surfaces with monolayer coverage. The other rim of the calixarenes bear either four azide or four ethynyl functional groups, which are easily modified by the copper(I)-catalyzed azide–alkyne cycloaddition reaction (CuAAC), either pre- or postsurface modification, enabling these conical, nanocavity reactor sites to be decorated with a wide range of substrates to impart desired chemical properties. Redox active species decorating the peripheral rim are shown to be electrically connected by the calixarene to the electrode surface in either “up” or “down” orientations of the calixarene.