Single-Electron Transfer in Frustrated Lewis Pair Chemistry

Abstract

Frustrated Lewis pairs (FLPs) are well known for their ability to activate small molecules. Recent reports of radical formation within such systems indicate single-electron transfer (SET) could play an important role in their chemistry. Herein, we investigate radical formation upon reacting FLP systems with dihydrogen, triphenyltin hydride, or tetrachloro-1,4-benzoquinone (TCQ) both experimentally and computationally to determine the nature of the single-electron transfer (SET) events; that is, being direct SET to B(C₆ F₅ )₃ or not. The reactions of H₂ and Ph₃ SnH with archetypal P/B FLP systems do not proceed via a radical mechanism. In contrast, reaction with TCQ proceeds via SET, which is only feasible by Lewis acid coordination to the substrate. Furthermore, SET from the Lewis base to the Lewis acid-substrate adduct may be prevalent in other reported examples of radical FLP chemistry, which provides important design principles for radical main-group chemistry.

Keywords: frustrated Lewis pairs; radicals; reactivity; single-electron transfer; substrate coordination.

Scheme 1

a) Different pathways proposed by Stephan et al. for reactions of FLPs with Ph₃SnH. b) Reactivity observed by Stephan et al. for Mes₃P/B(C₆F₅)₃ with tetrachloro‐1,4‐quinone (TCQ). c) Reactivity observed by Melen et al. (Ar^F=Ph, p‐F‐Ph or fluorene; Ar=variety of aryl groups, see Ref. [6]. d) Reactivity observed by Ooi et al. utilizing catalytic B(C₆F₅)₃ (10 mol %) (R=Me or Br); e) Light dependence for radical ion pair generation from archetypal FLP systems observed by Slootweg et al. (For P: R=Mes or tBu, for N: R=Ph or p‐Me‐Ph).

Scheme 2

Reactivity of PMes₃/B(C₆F₅)₃ with H₂ for which no light dependence was observed.

Scheme 3

Reactivity of PtBu₃/B(C₆F₅)₃ with Ph₃SnH.

Figure 1

Computed structure for the adducts of Ph₃SnH with B(C₆F₅)₃ (left) and Ph₃Sn⁺ (right) featuring a bridging hydride (DFT: ωB97X‐D/def2‐TZVP). Selected bond lengths and angles: Ph₃Sn−H−B(C₆F₅)₃: Sn−H 1.83 Å, B−H 1.37 Å; Sn‐H‐B 180°. [Ph₃Sn−H−SnPh₃]⁺: Both Sn−H 1.87 Å; Sn‐H‐B 147°.

Figure 2

Proposed reaction mechanism based on DFT calculations at the ωB97X‐D/def2‐TZVP level of theory. R=tBu (blue, dashed) or Mes (green, dotted). [HB(C₆F₅)₃]⁻ anion has been omitted for clarity. Energies in kcal mol⁻¹.

Scheme 4

Hydride abstraction from Ph₃SnH using [Ph₃C][B(C₆F₅)₄] and subsequent reaction with PMes₃.

Figure 3

Experimental EPR spectrum (bottom) for reaction of PMes₃, B(C₆F₅)₃ and TCQ (2:2:1) and simulated spectra for PMes₃ ^.+, TCQ‐B(C₆F₅)₃ ^.− and the third smaller signal. See the Supporting Information for experimental and simulation parameters. HFI=hyperfine interaction.

Scheme 5

a) Orbitals involved in the SET between PMes₃ and the TCQ‐B(C₆F₅)₃ adduct. b) Reactivity, featuring all possible pathways for the reaction of TCQ, B(C₆F₅)₃, and PMes₃.

Scheme 6

Lewis acid coordination to a carbonyl moiety facilitating SET. LB=Lewis base.

Scheme 7

Change in electron affinity when B(C₆F₅)₃ coordinates and the resulting LUMO for two different B(C₆F₅)₃‐coordinated substrates.