Synthesis and structure of heteroleptic ytterbium (II) tetrahydroborate complexes

The synthesis and characterization of (TptBu,Me)Yb(BH4)(THF)n (n = 0, 3; n = 1, 4) complexes are reported. The compounds represent rare examples of lanthanide (II) tetrahydroborate complexes. The X-ray crystal structure of complex 4 has been determined and it shows a monomeric, formally seven coordinate ytterbium center, bearing one κ3 bonded TptBu,Me ligand, a tetrahydroborate ligand and a coordinated THF molecule. The tetrahydroborate ligand binds in a κ3 fashion, via three bridging hydrogen atoms. IR spectroscopy data are consistent with the solid-state structure and the corresponding BD4 analog of 4 shows the expected IR isotope shifts. The 1H NMR spectra of 3 and 4 shows one set of resonances each for the BH4 and the pyrazolylborate ligands indicating dynamic solution behavior. For complex 3, although X-ray quality crystals could not be obtained, the IR and NMR data are consistent with its formulation as the solvent-free analog of complex 4 with κ3-bonded BH4 ligand.

Graphical abstractMonomeric ytterbium (II) tetrahydroborate complexes supported by tris(pyrazolylborate) ligands, (TptBu,Me)Yb(BH4)(THF)0/1 were synthesized and characterized. The synthesis of (TptBu,Me)Yb(BH4)(THF) was found to depend on both solvent and alkali metal borohydride used. The synthesis proceeded optimally using NaBH4 in acetonitrile. The structure of (TptBu,Me)Yb(BH4)(THF) revealed a monomeric ytterbium (II) center in which the tetrahydroborate ligand is κ3 coordinated to the ytterbium center. A similar structure is proposed for (TptBu,Me)Yb(BH4) based on spectroscopic evidence.Figure optionsDownload full-size imageDownload as PowerPoint slide