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.