Variable coordination of carbazolyl-bis(tetrazole) ligands in lanthanide chemistry

Abstract

The synthesis of the protioligand 1,8-bis(2′-isopropyltetrazo-5′-yl)-3,6-di-tert-butylcarbazole, HCzT^iPr2, is described. The acid–base reaction of 2 with various Ln[N(SiMe₃)₂]₃ (Ln = Ce, Sm, Er, Yb, Y) precursors leads to products displaying a variety of different bonding modes for the CzT^iPr− ligand as revealed by X-ray crystallographic studies. The smaller lanthanides form Ln(CzT^iPr)[N(SiMe₃)₂]₂ complexes (Ln = Er (3), Yb(4)) with a near planar, tridentate coordination mode for the CzT^iPr− ligand. In contrast, the larger lanthanides form Ln(CzT^iPr)₂[N(SiMe₃)₂] complexes (Ln = Ce (5), Sm (6)) at room temperature that feature a sandwich structure with a highly distorted tridentate mode where tetrazole canting allows the metal to sit well out of the carbazole plane. Attempts to force additional CzT^iPr− ligands onto Er or Yb at elevated temperature results in isolation of trace siloxide complexes Ln(CzT^iPr)₂(OSiMe₃) (Ln = Er (3a), Yb (4a)) that adopt a flat tridentate ligand bonding mode similar to 3 and 4. Surprisingly, the high temperature reaction of 2 with Y[N(SiMe₃)₂]₃ affords Y(CzT^iPr)₃, 7, featuring two tridentate and one bidentate CzT^iPr ligands. Partial hydrolysis of 6 occurs over time to produce a hydroxo-bridged dimer that also features one bridging and one bidentate CzT^iPr ligand, Sm(κ³-CzT^iPr)(κ³-μ-CzT^iPr)(μ-OH)₂Sm(κ³-CzT^iPr)(κ²-CzT^iPr), 6a, further illustrating the remarkable coordinative flexibility of the CzT^iPr ligand system.