Geometrical Isomerism and Redox Behaviour in Zirconium-Schiff Base Complexes: the Formation of C-C Bonds Functioning as Two-Electron Reservoirs

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 [Zr2LL'(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 PhlCl(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).