Syntheses of novel di- and trinucleating ligands having a triethylbenzene core with N,N-bidentate tethers: their complexation toward Pd and Rh organometallic fragments

Abstract

A series of di- and trinucleating ligands with a 1,3,5-triethylbenzene core connected to N,N-bidentate tethers was synthesized. The ligands readily reacted with monuclear Rh and Pd precursors to give the corresponding di- and trinuclear complexes, which were characterized by using NMR and ESI mass spectroscopy. In the solid state, the trinuclear complexes with ligands having pyridylpyrazolyl tethers adopt the most stable ababab configuration, in which the organometallic fragments are on the same side of the benzene plane. On the other hand, in solution, the linker moieties between the benzene core and the metals are flexible enough to interconvert between other configurations, that is, they exhibit dynamic behavior, and the rotational barrier was dependent on the length of the linkers. From variable temperature (VT) ¹H NMR measurements, the rotational barrier for a trinuclear Rh–CO complex with a ligand having methylene linkers was estimated to be ∼12.6 kcal mol⁻¹. However, no spectral changes were observed for the ethylene derivative in the temperature range of −60 °C to 50 °C, indicating that the rotation was not frozen out on the ¹H NMR timescale, even at −60 °C.