Synthesis and reactivity of cyclo-tetra(stibinophosphonium) tetracations: redox and coordination chemistry of phosphine–antimony complexes†
Reductive elimination of [R3PPR3]2+, [11(R)]2+, from the highly electrophilic SbIII centres in [(R3P)3Sb]3+, [8(R)]3+, gives SbI containing cations [(R3P)Sb]1+, [9(R)]1+, which assemble into frameworks identified as cyclo-tetra(stibinophosphonium) tetracations, [(R3P)4Sb4]4+, [10(R)]4+. A phosphine catalyzed mechanism is proposed for conversion of fluoroantimony complexes [(R3P)2SbF]2+, [7(R)]2+, to [10(R)]4+, and the characterization of key intermediates is presented. The results constitute evidence of a novel ligand activation pathway for phosphines in the coordination sphere of hard, electron deficient acceptors. Characterization of the associated reactants and products supports earlier, albeit less definitive, detection of analogous phosphine ligand activation in CuIII and TlIII complexes, demonstrating that these prototypical ligands can behave simultaneously as reducing agents and σ donors towards a variety of hard acceptors. The reactivity of the parent cyclo-tetra(stibinophosphonium) tetracation, [10(Me)]4+, is directed by high charge concentration and strong polarization of the P–Sb bonds. The former explains the observed facility for reductive elimination to yield elemental antimony and the latter enabled activation of P–Cl and P–H bonds to give phosphinophosphonium cations, [Me3PPR′2]1+, including the first example of an H-phosphinophosphonium, [(Me3P)P(H)R′]1+, and 2-phosphino-1,3-diphosphonium cations, [(Me3P)2PR′]2+. Exchange of a phosphine ligand in [10(Me)]4+ with [nacnac]1− gives [(Me3P)3Sb4(nacnac)]3+, [15(Me)]3+, and with dmap gives [(Me3P)3Sb4(dmap)]4+, 4+. The lability of P–Sb or Sb–Sb interactions in [10(Me)]4+ has also been illustrated by characterization of heteroleptically substituted derivatives featuring PMe3 and PEt3 ligands.