Geometrical isomerism and redox behaviour in zirconium–Schiff base complexes: the formation of C–C bonds functioning as two-electron reservoirs

Abstract

Two model compounds have been used for exploring the ligand-based redox chemistry of zirconium–Schiff base complexes, namely [Zr(salophen)Cl ₂ (thf)] 1 and [Zr(salophen) ₂ ] 2 [salophen = N,N′-bis (salicylidene)-o-phenylenediamine dianion, thf = tetrahydrofuran]. The latter occurs in two thermally non-interconvertible isomeric forms, 2A and 2B. Both contain eight-co-ordinate zirconium in a dodecahedral, 2A, and in a square-antiprismatic environment, 2B. The reduction of 1 with either Na or Mg led to the isolation of [Zr ₂ LL′ ₂ ] (L′ = thf 3 or pyridine 4). The centrosymmetric dinucleating octaanionic ligand L, which contains two-fold coupled salophen units joined by two C–C bonds, arises from the intermolecular reductive coupling of four imino groups belonging to two Zr(salophen) units. In an attempt to perform intramolecularly the same transformation, complexes 2A and 2B were reduced with sodium. Both isomers gave complex [ZrL″Na ₄ (dme) ₄ ] 5 (dme = 1,2-dimethoxyethane) containing a novel form of an hydrogenated coupled salophen ligand, L″, where the two salophen units are joined by a single C–C bond. The C–C bonds in the ligand L have been exploited as two-electron reservoirs in the reaction of 3 with PhICl ₂ and 9,10-phenanthrenequinone. In this reaction the original salophen moiety is restored in its original form and the Zr(salophen) moiety present in 3 behaves as a source of masked zirconium(II).