Peculiarities of crystal structures and photophysical properties of GaIII/LnIII metallacrowns with a non-planar [12-MC-4] core

A new series of gallium(iii)/lanthanide(iii) metallacrown (MC) complexes (Ln-1) was synthesized by the direct reaction of salicylhydroxamic acid (H3shi) with GaIII and LnIII nitrates in a CH3OH/pyridine mixture. X-ray single crystal analysis revealed two types of structures depending on whether the nitrate counterion coordinate or not to the LnIII: [LnGa4(shi)4(H2shi)2(py)4(NO3)](py)2 (Ln = GdIII, TbIII, DyIII, HoIII) and [LnGa4(shi)4(H2shi)2(py)5](NO3)(py) (Ln = ErIII, TmIII, YbIII). The representative Tb-1 and Yb-1 MCs consist of a Tb/YbGa4 core with four [GaIII–N–O] repeating units forming a non-planar ring that coordinates the central LnIII through the oxygen atoms of the four shi3− groups. Two H2shi− groups bridge the LnIII to the GaIII ring ions. The YbIII in Yb-1 is eight-coordinated while the ligation of the nine-coordinated TbIII in Tb-1 is completed by one chelating nitrate ion. Ln-1 complexes in the solid state showed characteristic sharp f–f transitions in the visible (Tb, Dy) and near-infrared (Dy, Ho, Er, Yb) spectral ranges upon excitation into the ligand-centered electronic levels at 350 nm. Observed luminescence lifetimes and absolute quantum yields were collected and discussed. For Yb-1, luminescence data were also acquired in CH3OH and CD3OD solutions and a more extensive analysis of photophysical properties was performed. This work demonstrates that while obtaining highly luminescent lanthanide(iii) MCs via a direct synthesis is feasible, many factors such as molar absorptivities, triplet state energies, non-radiative deactivations through vibronic coupling with overtones of O–H, N–H, and C–H oscillators and crystal packing will strongly contribute to the luminescent properties and should be carefully considered.