Peri-substituted phosphorus–selenium and –tin cenaphthenes

: 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., J_MP and J_MSe 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 potentialsingle-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 Al(OR_F)₄^- weakly coordinating 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 thermal decomposition.

Date of Award	30 Jun 2021
Original language	English
Awarding Institution	University of St Andrews
Supervisor	Petr Kilian (Supervisor) & J Derek Woollins (Supervisor)