Modulating the frontier orbitals of L(X)Ga-substituted diphosphenes [L(X)GaP]2 (X = Cl, Br) and their facile oxidation to radical cations

Abstract

Modulating the electronic structures of main group element compounds is crucial to control their chemical reactivity. Herein we report on the synthesis, frontier orbital modulation, and one-electron oxidation of two L(X)Ga-substituted diphosphenes [L(X)GaP]₂ (X = Cl 2a, Br 2b; L = HC[C(Me)N(Ar)]₂, Ar = 2,6-i-Pr₂C₆H₃). Photolysis of L(Cl)GaPCO 1 gave [L(Cl)GaP]₂2a, which reacted with Me₃SiBr with halide exchange to [L(Br)GaP]₂2b. Reactions with ^MeNHC (^MeNHC = 1,3,4,5-tetramethylimidazol-2-ylidene) gave the corresponding carbene-coordinated complexes L(X)GaPP(^MeNHC)Ga(X)L (X = Cl 3a, Br 3b). DFT calculations revealed that the carbene coordination modulates the frontier orbitals (i.e. HOMO/LUMO) of diphosphenes 2a and 2b, thereby affecting the reactivity of 3a and 3b. In marked contrast to diphosphenes 2a and 2b, the cyclic voltammograms (CVs) of the carbene-coordinated complexes each show one reversible redox event at E_1/2 = −0.65 V (3a) and −0.36 V (3b), indicating their one-electron oxidation to the corresponding radical cations as was confirmed by reactions of 3a and 3b with the [FeCp₂][B(C₆F₅)₄], yielding the radical cations [L(X)GaPP(^MeNHC)Ga(X)L]B(C₆F₅)₄ (X = Cl 4a, Br 4b). The unpaired spin in 4a (79%) and 4b (80%) is mainly located at the carbene-uncoordinated phosphorus atoms as was revealed by DFT calculations and furthermore experimentally proven in reactions with ⁿBu₃SnH, yielding the diphosphane cations [L(X)GaPHP(^MeNHC)Ga(X)L]B(C₆F₅)₄ (X = Cl 5a, Br 5b). Compounds 2–5 were fully characterized by NMR and IR spectroscopy as well as by single crystal X-ray diffraction (sc-XRD), and compounds 4a and 4b were further studied by EPR spectroscopy, while their bonding nature was investigated by DFT calculations.