Electronic communication in phosphine substituted bridged dirhenium complexes – clarifying ambiguities raised by the redox non-innocence of the C4H2- and C4-bridges

Abstract

The mononuclear rhenium carbyne complex trans-[Re(CCSiMe₃)(C–Me)(PMe₃)₄][PF₆] (2) was prepared in 90% yield by heating a mixture of the dinitrogen complex trans-[ReCl(N₂)(PMe₃)₄] (1), TlPF₆, and an excess of HCCSiMe₃. 2 could be deprotonated with KOtBu to the vinylidene complex trans-[Re(CCSiMe₃)(CCH₂)(PMe₃)₄] (3) in 98% yield. Oxidation of 3 with 1.2 equiv. of [Cp₂Fe][PF₆] at −78 °C gave the C_β–C′_β coupled dinuclear rhenium biscarbyne complex trans-[(Me₃SiCC)(PMe₃)₄ReC–CH₂–CH₂–CRe(PMe₃)₄(CCSiMe₃)][PF₆]₂ (5) in 92% yield. Deprotonation of 5 with an excess of KOtBu in THF produced the diamagnetic trans-[(Me₃SiCC)(PMe₃)₄ReCCH–CHCRe(PMe₃)₄(CCSiMe₃)] complex (E-6(S)) in 87% yield with an E-butadienediylidene bridge. Density functional theory (DFT) calculations of E-6(S) confirmed its singlet ground state. The Z-form of 6 (Z-6(S)) could not be observed, which is in accord with its DFT calculated 17.8 kJ mol⁻¹ higher energy. Oxidation of E-6 with 2 equiv. of [Cp₂Fe][PF₆] resulted in the stable diamagnetic dicationic trans-[(Me₃SiCC)(PMe₃)₄ReC–CHCH–CRe(PMe₃)₄(CCSiMe₃)][PF₆]₂ complex (E-6[PF₆]₂) with an ethylenylidene dicarbyne structure of the bridge. The paramagnetic mixed-valence (MV) complex E-6[PF₆] was obtained by comproportionation of E-6(S) and E-6[PF₆]₂ or by oxidation of E-6(S) with 1 equiv. of [Cp₂Fe][PF₆]. The dicationic trans-[(Me₃SiCC)(PMe₃)₄ReC–CC–CRe(PMe₃)₄(CCSiMe₃)][PF₆]₂ (7[PF₆]₂) complex, attributed a butynedi(triyl) bridge structure, was obtained by deprotonation of E-6[PF₆]₂ with KOtBu followed by oxidation with 2 equiv. of [Cp₂Fe][PF₆]. The neutral complex 7 could be accessed best by reduction of 7[PF₆]₂ with KH in the presence of 18-crown-6. According to DFT calculations 7 possesses two equilibrating electronic states: diamagnetic 7(S) and triplet 7(F) with ferromagnetically coupled spins. The latter is calculated to be 5.2 kcal mol⁻¹ lower in energy than 7(S). There is experimental evidence that 7(S) prevails in solution. 7 could not be isolated in the crystalline state and is unstable transforming mainly by H-abstraction to give E-6(S). UV-Vis-NIR spectroscopy for the dinuclear rhenium complexes E-6(S), E-6[PF₆] and E-6[PF₆]₂, as well as EPR spectroscopic and variable-temperature magnetization measurements for the MV complex E-6[PF₆] were also conducted. Spectro-electrochemical reduction studies on 7[PF₆]₂ allowed the characterization of the mono- and direduced forms of 7⁺ and 7 by means of IR- and UV-Vis-NIR-spectroscopy and revealed the chemical fate of the higher reduced form.