Reduction chemistry yields stable and soluble divalent lanthanide tris(pyrazolyl)borate complexes

Reduction of the heteroleptic Ln(iii) precursors [Ln(Tp)(2)(OTf)] (Tp = hydrotris(1-pyrazolyl)borate; OTf = triflate) with either an aluminyl(i) anion or KC8 yielded the adduct-free homoleptic Ln(ii) complexes dimeric 1-Eu [{Eu(Tp)(mu-kappa(1):eta(5)-Tp)}(2)] and monomeric 1-Yb [Yb(Tp)(2)]. Complexes 1-Ln have good solubility and stability in both non-coordinating and coordinating solvents. Reaction of 1-Ln with 2 Ph3PO yielded 1-Ln(OPPh3)(2). All complexes are intensely coloured and 1-Eu is photoluminescent. The electronic absorption data show the 4f-5d electronic transitions in Ln(ii). Single-crystal X-ray diffraction data reveal first mu-kappa(1):eta(5)-coordination mode of the unsubstituted Tp ligand to lanthanides in 1-Eu.