Lanthanide(II) Complexes Supported by N,O-Donor Tripodal Ligands: Synthesis, Structure, and Ligand-Dependent Redox Behavior

Abstract : The preparation and characterization of a series of complexes of the Yb and Eu cations in the oxidation state II and III with the tetradentate N,O-donor tripodal ligands (tris(2-pyridylmethyl)amine (TPA), BPA(-) (HBPA=bis(2-pyridylmethyl)(2-hydroxybenzyl)amine), BPPA(-) (HBPPA=bis(2-pyridylmethyl)(3.5-di-tert-butyl-2-hydroxybenzyl)amine), and MPA(2-) (H(2)MPA=(2-pyridylmethyl)bis(3.5-di-tert-butyl-2-hydroxybenzyl)amine) is reported. The X-ray crystal structures of the heteroleptic Ln(2+) complexes [Ln(TPA)I-2] (Ln=Eu, Yb) and [Yb(BPA)I(CH3CN)](2), of the Ln(2+) homoleptic [Ln(TPA)(2)]I-2 (Ln=Sm, Eu, Yb) and [Eu(BPA)(2)] complexes, and of the Ln(3+) [Eu(BPPA)(2)]OTf and [Yb(MPA)(2)K(dme)(2)] (dme=dimethoxyethane) complexes have been determined. Cyclic voltammetry studies carried out on the bis-ligand complexes of Eu3+ and Yb3+ show that the metal center reduction occurs at significantly lower potentials for the BPA(-) ligand as compared with the TPA ligand. This suggests that the more electron-rich character of the BPA(-) ligand results in a higher reducing character of the lanthanide complexes of BPA(-) compared with those of TPA. The important differences in the stability and reactivity of the investigated complexes are probably due to the observed difference in redox potential. Preliminary reactivity studies show that whereas the bis-TPA complexes of Eu2+ and Yb2+ do not show any reactivity with heteroallenes, the [Eu(BPA)(2)] complex reduces CS2 to afford the first example of a lanthanide trithiocarbonate complex.