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

Reported are the syntheses, crystal structures, and photophysical properties of 28, novel lanthanide compounds across five structural types, [Ln(3-NOTp)(NO)] (-Ln, Ln = La-Tm, except Pm), [BuN][Ln(3-NOTp)(NO)] (-Ln, Ln = Yb, Lu), [Eu(3-NOTp)Cl(HO)]·2PrOH (-Eu), [{Ln(3-NOTp)}(μ-CO)]·MeOH (-Ln, Ln = La-Gd, except Pm), and [{Ln(3-NOTp)}(μ-OMe)(μ-O)] (-Ln, Ln = Pr-Tb, except Pm) with the 3-nitrotrispyrazolylborate (3-NOTp) ligand. The reaction of methanol or isopropanol solutions of LnX (X = Cl, NO) with the tetrabutyl ammonium salt of the flexidentate 3-NOTp ([BuN][3-NOTp]) yields Ln(3-NOTp) complexes of various nuclearities as either monomers (-Ln, -Ln, -Eu), dimers (-Ln), or tetramers (-Ln) owing to the efficient conversion of atmospheric CO to CO (dimers) or ligand controlled solvolysis of lanthanide ions (tetramers). 3-NOTp 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 Ln emission arises from two intraligand charge transfer (ILCT) transitions of 3-NOTp. 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-NOTp, imparting remarkable structural diversity and charge transfer properties to resultant lanthanide complexes.