First Example of Low-Valence Ion Substitution in Ln₅O(OPrⁱ)₁₃: Mixed-Valence Europium Oxoalkoxide [Eu^III₄Eu^IIO(OPrⁱ)₁₂(HOPrⁱ)]*HOPrⁱ

The novel mixed-valence alkoxide [Eu3+4Eu2+O(OPri)12(HOPri)]*HOPri (1) has been prepared and structurally and spectroscopically characterized. The three synthesis routes (i) metathesis of 4EuCl3, EuI2, and 14KOPri combined with hydrolysis with 1H2O, (ii) oxidation of 5[Eu4(OPri)10(HOPri)3]*2HOPri with 1.5O2, and (iii) reduction of Eu5O(OPri)13 with 0.8[Eu4(OPri)10(HOPri)3]*2HOPri all yielded pure 1, whereas (iv) reduction of Eu5O(OPri)13 with 0.36−0.5 mol of europium metal produced impure 1. The compound, having the average Eu oxidation number +2.8, is very sensitive toward further oxidation to Eu5O(OPri)13 and is part of a redox series of europium 2-propoxides with average oxidation states +2.5, +2.8, and +3. The square pyramidal molecular structure, containing an oxo-oxygen atom in the basal plane, is similar to that of the well-known Ln5O(OPri)13; the main difference is the substitution of an Eu3+-OPri pair for an Eu2+-HOPri pair in the basal plane. Fourier transform infrared (FT-IR) and UV−visible spectroscopy showed that the solid-state structure was retained on dissolution in hexane and toluene−HOPri. The compound was further characterized by differential scanning calorimetry and solubility studies.