2,5-Bis{N-(2,6-diisopropylphenyl)iminomethyl}pyrrolyl borohydride complexes of the divalent lanthanides – Synthesis, structures and ring-opening polymerization of ε-caprolactone

The borohydride complexes [(DIP2pyr)Ln(BH4)(THF)3] (Ln = Eu (1), Yb (2); (DIP2pyr)− = 2,5-bis{N-(2,6-diisopropylphenyl)iminomethyl}pyrrolyl) were synthesized by reaction of [Ln(BH4)2(THF)2] (Ln = Eu, Yb) with [(DIP2pyr)K] in THF. The X-ray solid-state structures of 1 and 2 show that the (DIP2pyr)− ligand coordinates in a κ3-fashion to the europium atom, while the smaller ytterbium atom is only κ2-coordinated by the (DIP2pyr)− ligand with one Schiff base nitrogen atom pointing away from the metal center. Whereas compound 1 can be stored under nitrogen at room temperature for several months, compound 2 is less stable, decomposing under these same conditions. The corresponding two-fold ligated compound [(DIP2pyr)2Yb(THF)] was isolated from the reaction of [(DIP2pyr)K] with anhydrous ytterbium diiodide in THF. The X-ray solid state structure suggests one κ3-coordinated (DIP2pyr)− ligand while the second ligand is κ2-coordinated to the metal center. Compound 1 was found active in promoting the ring-opening polymerization (ROP) of ε-caprolactone under mild conditions.