Lanthanoid Semiquinonate/Tropolonate Complexes: Redox Chemistry and Luminescence

Heteroleptic lanthanoid (Ln = Eu, Gd, Tb, Dy, Ho and Yb) complexes featuring hydro‐tris(1‐pyrazolyl)borate (Tp−) ligands combined with the bidentate O‐donor ligands tropolonate (trop−) or 3,5‐di‐tertbutylseminquinonate (dbsq⋅−) have been synthesized and investigated. Despite the similarity of the bidentate O‐donor trop− and dbsq⋅− ligands, and the lanthanoid coordination geometries, the complexes [LnTp2dbsq] (Ln‐dbsq) and [LnTp2trop] (Ln‐trop) exhibit markedly different redox and photophysical properties. The electrochemistry of Ln‐dbsq is dominated by processes associated with the readily redox‐active dbsq⋅− moiety. In contrast, the relative redox inactivity of Ln‐trop allows for the observation of the Eu(III) to Eu(II) reduction process. Photophysical measurements reveal no sensitized emission for the Ln‐dbsq family, while the trop− antenna ligand is able to sensitize emission in the visible range for Eu(III) and Ho(III), and in the near‐infrared for Ho(III) and Yb(III). Despite their structural similarity, the heteroleptic lanthanoid complexes [LnTp2dbsq] and [LnTp2trop] (Ln=Eu, Gd, Tb, Dy, Ho and Yb; Tp−=hydro‐tris(1‐pyrazolyl)borate; dbsq•−=3,5‐di‐tertbutylseminquinonate; trop−=tropolonate) exhibit very different redox and photophysical properties.