Ytterbium(III) Complexes Coordinated by Dianionic 1,4-Diazabutadiene Ligands

A series of new Yb(III) complexes XYb(DAD)2–(L) (X = C5Me5, C5Me4H, N(SiMe3)2, tBuO; DAD = 2,6-R″2C6H3N═C(R′)-C(R′)═NC6H3R″2-2,6, R′ = H, Me, R″ = Me, iPr; L = thf, dme) coordinated by redox-active diazabutadiene ligands in dianionic form were synthesized and characterized. The half-sandwich complexes Cp#Yb(DAD)2–(THF) (Cp# = C5Me5, C5Me4H) were synthesized by the reactions of the ytterbocenes Cp#2Yb(THF)2 with the corresponding DADs in a 1:1 molar ratio. These reactions are accompanied by oxidation of the Yb(II) to Yb(III), cleavage of one Cp#–Yb bond, oxidation of cyclopentadienyl anion, and reduction of the diazabutadiene to dianionic form. It was found that the substituents by the DAD nitrogens (2,6-iPr2C6H3 vs 2,6-Me2C6H3) and imino carbons (H vs Me) do not affect the reaction outcome and afford Cp#Yb(DAD)2–(thf). The amido and alkoxo derivatives XYb(DAD)2–(dme) (X = N(SiMe3)2, tBuO) were obtained by the salt metathesis reactions of the in situ generated species [XYbCl2(thf)n] and Na2(thf)n[2,6-iPr2C6H3NC(Me)C(Me)NC6H3iPr2-2,6]. If the reaction was carried out in the presence of Li ions, it afforded an ate-complex {Li(thf)3}{Yb[2,6-iPr2C6H3NC(Me)C(Me)NC6H3iPr2-2,6]2–[N(SiMe3)2](μ-Cl)}. The X-ray studies of complexes XYbIII(DAD)2–(L) revealed that they feature the 2σ:η2-type of coordination of dianionic DAD ligands. Introduction of Me-substituents by the imino carbons of DADs leads to some elongation of Yb–Cp# bonds compared to the NCHCHN-analogues. The Yb–CNCCN bonds and the dihedral YbNN–NCCN angles were found to be the most sensitive to replacing H by Me. Unlike the formerly reported complex Cp*Yb[2,6-iPr2C6H3NCHCHNC6H3iPr2-2,6]2–(thf), the variable-temperature magnetic measurements (1.8–300 K) of complexes 3–5 and 7–9 did not reveal thermally induced redox isomeric transformations for these compounds. However, for complex (C5Me4H)Yb[2,6-iPr2C6H3NC(Me)C(Me)NC6H3iPr2-2,6](thf) at 9 K, the structural phase transition accompanied by changes of the coordination behavior of the DAD ligand was detected, which might hint for an onset of a temperature-induced redox isomerism. These results clearly indicate high sensitivity of redox isomeric transformations of XYbIII(DAD)2–L to the smallest changes of the structural and electronic properties of the DAD ligands.