Synthesis and structural, redox and photophysical properties of tris-(2,5-di(2-pyridyl)pyrrolide) lanthanide complexes

Abstract

A first series of lanthanide complexes of tris(dipyridyl)pyrrolide ligands has been prepared. The [Ln(dpp^R1,R2)₃] complexes (Ln = La(III), Sm(III), Eu(III), Gd(III) and Yb(III); and dpp^R1,R2 = 2,5-di(2-pyridyl-3-(R¹)-4-(R²)) pyrrolide) have been isolated and their structures and photophysical and redox properties characterised, both in the solid-state and in solution. In the complexes, the three dpp⁻ ligands form a distorted tricapped trigonal prismatic coordination geometry about the lanthanide ions, with the antiparallel isomer observed in the solid state for non-symmetric (dpp^CO₂Me,Me)⁻. However, ¹H NMR spectroscopy of the diamagnetic and paramagnetic [Ln(dpp^R1,R2)₃] complexes in d₆-benzene solution reveal evidence for a statistical distribution of all possible isomers. Time-resolved luminescence studies suggest that the dpp⁻ ligand (with triplet excited state T₁ energy at 18 622 cm⁻¹) sensitises red emission from [Eu(dpp^CO₂Me,Me)₃] and near-infrared emission from [Yb(dpp^CO₂Me,Me)₃] through the antenna effect. Cyclic voltammetry reveals three consecutive, reversible, one-electron oxidation processes for each [Ln(dpp^R1,R2)₃] complex, corresponding to oxidations of each dpp⁻ ligand between 0.3–0.8 V vs. E_1/2 (Fc^+/0), and for [Eu(dpp^CO₂Me,Me)₃] the Eu^III/II couple was −2.099 V vs. E_1/2 (Fc^+/0).