Peri-substituted phosphorus-selenium and -tin acenaphthenes : syntheses, reactivities and radical species

Abstract

The investigation of 𝘱𝘦𝘳𝘪-substitution has yielded fascinating approaches to unusual chemical bonds and interactions involving two or more atoms forced to close proximity. In this thesis, we discuss the syntheses and reactivities of a series of 𝘱𝘦𝘳𝘪-substituted P-Se and P-Sn species. Several dialkylphosphino-arylselanyl acenaphthenes Acenap (P𝘪Pr₂)(SeAr) (Ar = Mes, TRIP, Mes*), along with their transition metal complexes [M(Acenap(P𝘪Pr₂)(SeAr))ₙ] (M = Mo, Pd, Hg, Ag), have been prepared. Crystal structures and NMR properties (e.g. 𝘑MP and 𝘑MSe couplings) of these compounds have also been examined. We aimed to develop stable 𝘱𝘦𝘳𝘪-substituted systems that contain a captodative P-Se hemibond (2c-3e bond). We used P-Se acenaphthenes as radical candidates, investigating potential single-electron oxidation reactions with nitrosonium and silver (I) salts, expecting the formation of respective radical cations. The -P𝘪Pr₂ substituent is strongly electron-donating, and the bulky -SeAr groups are relatively electron-withdrawing. Therefore, the two captodative motifs were expected to form a stable P-Se hemibond. The stability of the radical centre was expected to be increased by steric shielding from the vicinal bulky arylselanyl groups and the presence of a large AI(ORꟳ)₄ weakly coordinated anion. The redox properties of the phosphine-selanes have been tested via electrochemical methods (e.g., cyclic voltammetry). The products of the potential single-electron oxidation reactions have been characterised by EPR spectroscopy. Despite all our efforts, the isolation of the desired cation radicals was not successful. In a separate project, we have investigated P-Sn acenaphthenes Acenap (P𝘪Pr₂)(SnHR₂) (R=Me, Ph) to get insight into their potential use as C-H coupling precursors. We have shown that the thermally induced reaction of these generates a phosphine-stabilised stannylene via elimination of benzene. Our observations have been supported by NMR spectroscopy; also, a gas trapping experiment indicates that benzene is formed as one of the products from fermal decomposition.