Triphenylphosphane Oxide Complexes of Lanthanide Nitrates: Polymorphs and Photophysics

A series of mer‐[Ln(NO3)3(Ph3PO)3] complexes were prepared from Ln(NO3)3·xH2O and Ph3PO in chloroform (Ln = La, Nd, Sm, Eu, Gd, Tb, Dy, and Er). The La and Nd complexes were 0.25 CHCl3 solvates, whereas the others were solvent‐free. The identical reaction using Yb(NO3)3·xH2O produced the unique salt trans‐[Yb(NO3)2(Ph3PO)4][Yb(NO3)4(Ph3PO)]·Et2O. All nitrate ions in all complexes are η2‐chelating. A comparison of the various [Ln(NO3)3(Ph3PO)3] structures, including those in the literature, reveals at least four common polymorphs, each of which is represented by isomorphic structures of multiple Ln ions. Luminescence of mer‐[Ln(NO3)3(Ph3PO)3] (Ln = Y, La, Nd, Sm, Eu, Gd, Tb, and Dy), trans‐[Yb(NO3)2(Ph3PO)4][Yb(NO3)4(Ph3PO)] and Ph3PO assignments are reported. Latva's empirical rule allows for the antenna effect, in which energy is transferred from the triplet state of the Ph3PO ligand, to occur only for Tb3+. Excitation via Ph3PO results in strong green luminescence for Tb3+ having twice the intensity as that which results from direct excitation of the f‐f transitions.