Structural Diversity of Lanthanide 3-Nitrotrispyrazolylborates: Tunable Nuclearity and Intra-Ligand Charge Transfer Sensitization of Visible and NIR Ln3+ Emission

Reported are the synthesis, crystal structures and photophysical properties of twenty-eight, novel lanthanide compounds across five structural types, [Ln(3-NO2Tp)2(NO3)] (1-Ln, Ln = La – Tm, except Pm), [Bu4N][Ln(3-NO2Tp)(NO3)3] (2-Ln, Ln = Yb, Lu), [Eu(3-NO2Tp)2Cl(H2O)]·2iPrOH (3-Eu), [{Ln(3-NO2Tp)2}2(μ2-CO3)] (4-Ln, Ln = La – Gd, except Pm) and [{Ln(3-NO2Tp)}4(μ2-OMe)6(μ4-O)] (5-Ln, Ln = Pr – Tb, except Pm) with the 3-nitrotrispyrazolylborate (3-NO2Tp–) ligand. Reaction of methanol or isopropanol solutions of LnX3 (X = Cl, NO3) with the tetrabutyl ammonium salt of the flexidentate 3-NO2Tp–, ([Bu4N][3-NO2Tp]) yields Ln(3-NO2Tp)x complexes of various nuclearities as either monomers (1-Ln, 2-Ln, 3-Eu), dimers (4-Ln) or tetramers (5-Ln) owing to the efficient conversion of atmospheric CO2 to CO32– (dimers) or ligand controlled solvolysis of lanthanide ions (tetramers). 3-NO2Tp– is an efficient sensitizer for both the visible and near-IR (NIR) emissions of most of the lanthanide series, except thulium. Optical measurements, supported by density functional theory calculations, indicate that the dual visible and NIR Ln3+ emission arises from two intra-ligand charge transfer (ILCT) transitions of 3-NO2Tp–. This is the first report of lanthanide complexes with a nitro-functionalized pyrazolylborate ligand. The derivatization of the known Tp– ligand results in new coordination chemistry governed by the increased denticity of 3-NO2Tp–, imparting remarkable structural diversity and charge transfer properties to resultant lanthanide complexes.