Photoinduced and Thermal Single-Electron Transfer to Generate Radicals from Frustrated Lewis Pairs

Abstract

Archetypal phosphine/borane frustrated Lewis pairs (FLPs) are famed for their ability to activate small molecules. The mechanism is generally believed to involve two-electron processes. However, the detection of radical intermediates indicates that single-electron transfer (SET) generating frustrated radical pairs could also play an important role. These highly reactive radical species typically have significantly higher energy than the FLP, which prompted this investigation into their formation. Herein, we provide evidence that the classical phosphine/borane combinations PMes₃ /B(C₆ F₅ )₃ and PtBu₃ /B(C₆ F₅ )₃ both form an electron donor-acceptor (charge-transfer) complex that undergoes visible-light-induced SET to form the corresponding highly reactive radical-ion pairs. Subsequently, we show that by tuning the properties of the Lewis acid/base pair, the energy required for SET can be reduced to become thermally accessible.

Keywords: Lewis pairs; charge transfer; electron transfer; photochemistry; radicals.

Scheme 1

Literature examples of detected radicals (highlighted in boxes) in cooperative main‐group Lewis acid/base chemistry.2c, 4b, 7 R=Mes, Tipp, C₆F₅, C₆Me₅, tBu (only in the case of CPh₃ ⁺); SiR′₃ ⁺=SiiPr₃ ⁺, Si(C₆Me₅)₃ ⁺, SitBuMe₂ ⁺, SiEt₃ ⁺.

Scheme 2

Electron donor–acceptor complex formation and subsequent SET to generate the corresponding radical‐ion pair.

Figure 1

DFT‐calculated energy needed to access the radical‐ion pairs from the FLP systems PtBu₃/PMes₃ and BCF.7

Figure 2

Frontier molecular orbitals of the PMes₃/BCF encounter complex, calculated at the ωB97X‐D/6‐311G(d,p) level of theory. Isovalue=0.05.

Figure 3

a) UV/Vis spectrum of PMes₃/BCF (both components: 1.5×10⁻²  m in toluene) compared with the spectra of the separate components (1.5×10⁻²  m). b) Experimental EPR spectrum of PMes₃/BCF in toluene measured at 30 K during irradiation with visible light (390–500 nm) and simulated EPR spectra of PMes₃ ^⋅+ and BCF^⋅−. c) Transient absorption spectra measured after pulsed excitation of PMes₃/BCF with 530 nm light.

Figure 4

a) UV/Vis spectrum of PtBu₃/BCF (both components: 1.5×10⁻²  m in toluene) compared with the spectra of the separate components (1.5×10⁻²  m). Inset shows the colour of the solution. b) Experimental EPR spectrum of PtBu₃/BCF in toluene measured at 30 K during irradiation with visible light (390–500 nm) and simulated EPR spectra of PtBu₃ ^⋅+ and BCF^⋅−. c) Transient absorption spectra measured after pulsed excitation of PtBu₃/BCF with 530 nm light.

Scheme 3

SET to afford the high‐energy radical‐ion pair, which, by decomposition of either the radical cation or radical anion, forms the stable complementary radical ion selectively.

Figure 5

Ionisation energies (IE_D, top) and electron affinities (EA_A, bottom) of donors and acceptors typical in FLP chemistry calculated at the SCRF/ωB97X‐D/6‐311+G(d,p) level of theory (solvent=toluene).

Figure 6

Energy diagram showing the thermal SET equilibrium for the NpTol₃/⁺CPh₃ EDA pair. For computational details, see Figure 5.