Mechanistic studies on single-electron transfer in frustrated Lewis pairs and its application to main-group chemistry

Advances in the field of frustrated Lewis pair (FLP) chemistry have led to the discovery of radical pairs, obtained by a single-electron transfer (SET) from the Lewis base to the Lewis acid. Radical pairs are intriguing for their potential to enable cooperative activation of challenging substrates (e.g., CH4, N2) in a homolytic fashion, as well as the exploration of novel radical reactions. In this review, we will cover the two known mechanisms of SET in FLPs—thermal and photoinduced—along with methods (i.e., CV, DFT, UV-vis) to predict the mechanism and to characterise the involved electron donors and acceptors. Furthermore, the available techniques (i.e., EPR, UV-vis, transient absorption spectroscopy) for studying the corresponding radical pairs will be discussed. Initially, two model systems (PMes3/CPh3+ and PMes3/B(C6F5)3) will be reviewed to highlight the difference between a thermal and a photoinduced SET mechanism. Additionally, three cases are analysed to provide further tools and insights into characterizing electron donors and acceptors, and the associated radical pairs. Firstly, a thermal SET process between LiHMDS and [TEMPO][BF4] is discussed. Next, the influence of Lewis acid complexation on the electron acceptor will be highlighted to facilitate a SET between (pBrPh)3N and TCNQ. Finally, an analysis of sulfonium salts as electron acceptors will demonstrate how to manage systems with rapidly decomposing radical species. This framework equips the reader with an expanded array of tools for both predicting and characterizing SET events within FLP chemistry, thereby enabling its extension and application to the broader domain of main-group (photo)redox chemistry.


Table of Contents
All experimental and computational data presented in the tutorial review and accompanying supporting information were generated by the authors.This effort aimed to replicate previously published work and offer readers a comprehensive summary of all essential data.Additionally, we created the images ourselves to enhance this resource.

II. Electrochemistry
General procedures.Cyclic voltammetry measurements were done using a standard three electrode set-up in a commercially available, one compartment Dr. Bob cell using an Ivium Vertex 5A potentiostat.Prior to use, the cell and bubbler were rinsed with water, ethanol and acetone or, when needed, cleaned using a potassium permanganate (1 mg/mL) and sulfuric acid (0.5 M) solution in water, followed by a hydrochloric acid (0.01 M) and peroxide (10%) solution in water.It was then dried in an oven at 130 o C for 4 hours, assembled while hot and purged with nitrogen for at least 30 minutes.Prior to use, the working electrodes were polished using a 0.05 µm alumina slurry (BASi PK4-polishing kit), washed with demineralized water, sonicated in demineralized water for several minutes and dried using compressed air.If insufficient for a clean polish, diamond polish was used as needed (15 µm, 3 µm and 1 µm), which was washed off using methanol and dried using compressed air. 4 It was then polished using alumina.To minimize the Ohmic resistance of the cell, the electrodes were placed as closely together as possible without touching.Dry solvents were syringe filtered prior to use to remove mol sieve dust.Measurements were generally performed at RT, starting at 0 V, going to negative potentials first at a scan rate of 100 mV/s and a step size of 1 mV.Three scans were taken per measurement and the second scan was used.Between each measurement the diffusion layer was refreshed by gently shaking the electrochemical cell.After the final measurement, internal standard was added to the cell, nitrogen was bubbled through for 15 minutes and a final scan was taken to calibrate the reference electrode.CVs were plotted using MATLAB in the IUPAC style.

Determination of Potentials
Half wave potentials were determined by adding the anodic and cathodic peak potential for reversible and quasi-reversible processes and dividing by two.Half peak potentials were determined by first determining the baseline corrected peak anodic or cathodic current according to literature procedures. 5In short, this is done by taking the tangent to the baseline and subtracting this from the measured peak current, as shown in Figure S1.The half peak current is then taken as half of this value, after which the corresponding half peak potential can be determined.

Cyclic Voltammetry: B(C 6 F 5 ) 3
The procedure was adapted from literature. 3 In a glovebox, tris(pentafluorophenyl)borate (12.8 mg) and tetra-n-butyl ammonium tetrakis (pentafluorophenyl)borate [nBu 4 N][B(C 6 F 5 ) 4 ] (234 mg) were dissolved in dry and degassed DCM (5 mL).This was added to the dr.Bob cell containing a Pt working electrode (Gamry, 3 mm diameter), Pt wire counter electrode (Thermo Scientific, 99.9% Pt, 0.25 mm diameter) and Ag wire reference electrode (STREM chemicals, 99.9%).The electrodes were left in solution for an hour prior to measuring to calibrate the reference electrode.After the measurements, ferrocene was added as internal standard.

Cyclic voltammetry: TEMPO
The procedure was adapted from literature.9ref In a glovebox TEMPO (15.6 mg) was dissolved in dry and degassed ortho-difluorobenzene (5 mL), diluted 1:10 and added to a dried dr.Bob

Cyclic voltammetry: LiHMDS
The procedure was adapted from literature. 9 In a glovebox LiHMDS (16.8 mg) was dissolved in dry and degassed ortho-difluorobenzene (5 mL), diluted 1:10 and added to a dried dr.Bob

Cyclic voltammetry: Ferrocene
Under a nitrogen atmosphere [nBu 4 N][PF 6 ] (386 mg) was dissolved in dry and degassed orthodifluorobenzene (5 mL).This was added to the dr.Bob cell, containing a few granules of

Cyclic voltammetry: N(pBrPh) 3
The procedure was adapted from literature. 8In a glovebox tris(4-bromo-phenyl)amine (12.1 mg) was dissolved in dry and degassed DCM (5 mL), diluted 1:5 and added to a dried dr.Bob cell containing [nBu 4 N][PF 6 ] (960 mg).A Pt working electrode (Metrohm, 3 mm diameter), Pt wire counter electrode (Thermo Scientific, 99.9% Pt, 0.25 mm diameter) and Pt reference electrode was used.Prior to measuring the Pt reference electrode was allowed to equilibrate for an hour to prevent excessive drifting.After the measurements ferrocene was added as an internal standard and an additional measurement was taken.

Cyclic voltammetry: TCNQ-{B(C 6 F 5 ) 3 } 4
The procedure was adapted from literature. 11In a glovebox TCNQ-{B(C 6 F 5 ) 3 } 4 was synthesized according to literature procedure.This was added directly to the Dr. Bob cell for CV without further purification and without addition of electrolyte.A Pt working electrode (Metrohm, 3 mm diameter), Pt wire counter electrode (Thermo Scientific, 99.9% Pt, 0.25 mm diameter) and Pt reference electrode was used.Prior to measuring the Pt reference electrode was allowed to equilibrate for an hour to prevent excessive drifting.After the measurements ferrocene was added as an internal standard and an additional measurement was taken.

Cyclic Voltammetry: NMM
The procedure was adapted from literature. 12, 13 In a glovebox, N-methylmorpholine (53 mg) was dissolved in dry and degassed MeCN, diluted 1:10 and added to the dr.Bob cell containing  in MeCN) was used.After the measurements, ferrocene was added as internal standard.Experimental results.NMM / UR UV-vis Spectroscopy

V. Computational Details
General procedures.All geometry optimizations were calculated using the (U)ωB97X-D density functional 19 and the 6-31G(d) 20,21 basis set as implemented in Gaussian16 (Revision C.01) 22 without symmetry constraints.The obtained geometries were characterized as true minima having no imaginary frequency.Single-point calculations on the optimized structures were performed using (U)ωB97X-D and the 6-311+G(d,p)

Figure S1
Figure S1 Determination of the baseline corrected peak current for NMM.

Figure S2
Figure S2 CV of PMes 3 (10 mM) in dry and degassed DCM with [nBu 4 N][PF 6 ] (0.5 M) as electrolyte.A Pt working electrode, Pt wire counter electrode and Ag wire reference electrode was used.

Figure S3
Figure S3 CV of PMes 3 (5 mM) in dry and degassed DCM with [nBu 4 N][PF 6 ] (0.5 M) as electrolyte.A Pt working electrode, Pt wire counter electrode and Ag wire reference electrode was used.

Figure S6
Figure S6 CV of TEMPO (2 mM) in dry and degassed ortho-difluorobenzene with [nBu 4 N][PF 6 ] (0.2 M) as electrolyte.A GC working electrode, Pt wire counter electrode and Ag/AgNO 3 reference electrode was used.

Figure S7
Figure S7 CV of LiHMDS (2 mM) in dry and degassed ortho-difluorobenzene with [nBu 4 N][PF 6 ] (0.2 M) as electrolyte.A GC working electrode, Pt wire counter electrode and Ag/AgNO 3 reference electrode was used.Scan 2 and scan 3 do not fully overlap due to a change in current between scans.

Figure S8
Figure S8 CV of ferrocene in dry and degassed ortho-difluorobenzene with [nBu 4 N][PF 6 ] (0.2 M) as electrolyte.A GC working electrode, Pt wire counter electrode and Ag/AgNO 3 reference electrode was used.A scan rate of 10 mV/s was used.

Figure S9
Figure S9 CV of N(pBrPh) 3 (10 mM) in dry and degassed DCM with [nBu 4 N][PF 6 ] (0.5 M) as electrolyte.A Pt working electrode, Pt wire counter electrode and Pt wire reference electrode was used.Scan 2 and scan 3 do not fully overlap due to a change in current between scans.
1 mg) was dissolved in dry and degassed DCM (10 mL) and added to a dried dr.Bob cell containing [nBu 4 N][PF 6 ] (387 mg).A Pt working electrode (Metrohm, 3 mm diameter), Pt wire counter electrode (Thermo Scientific, 99.9% Pt, 0.25 mm diameter) and Pt reference electrode was used.Prior to measuring the Pt reference electrode was allowed to equilibrate for an hour to prevent excessive drifting.After the measurements ferrocene was added as an internal standard and an additional measurement was taken.

Figure S10
Figure S10 CV of TCNQ (5 mM) in dry and degassed DCM with [nBu 4 N][PF 6 ] (0.1 M) as electrolyte.A Pt working electrode, Pt wire counter electrode and Pt wire reference electrode was used.

Figure S11
Figure S11 CV of TCNQ-{B(C 6 F 5 ) 3 } 4 (10 mM) in dry and degassed DCM without electrolyte.A Pt working electrode, Pt wire counter electrode and Pt wire reference electrode was used.

Figure S12
Figure S12 CV of N-methylmorpholine (10 mM) in dry and degassed MeCN with [nBu 4 N][BF 4 ] (0.5 M) as electrolyte.A Pt working electrode, Pt wire counter electrode and AgNO 3 reference electrode was used.

Figure S16 :
Figure S16: UV-vis spectra of acetonitrile solutions of NMM (11.25 mM) and UR (11.25 mM) and the combination thereof.17