Structural Variations and Molecular Dynamics of Rare-Earth Metal Complexes with theN,N-Bis(2-{pyrid-2-yl}ethyl)hydroxylaminato Ligand

Abstract

The reaction of the donor-functionalised N,N-bis(2-{pyrid-2-yl}ethyl)hydroxylamine and [LnCp3] (Cp=cyclopentadiene) resulted in the formation of bis(cyclopentadienyl) hydroxylaminato rare-earth metal complexes of the general constitution [Ln(C5H5)2{ON(C2H4-o-Py)2}] (Py=pyridyl) with Ln=Lu (1), Y (2), Ho (3), Sm (4), Nd (5), Pr (6), La (7). These compounds were characterised by elemental analysis, mass spectrometry, NMR spectroscopy (for compounds 1, 2, 4 and 7) and single-crystal X-ray diffraction experiments. The complexes exhibit three different aggregation modes and binding motifs in the solid state. The late rare-earth metal atoms (Lu, Y, Ho and Sm) form monomeric complexes of the formula [Ln(C5H5)2{eta2-ON(C2H4-eta1-o-Py)(C2H4-o-Py)}] (1-4, respectively), in which one of the pyridyl nitrogen donor atoms is bonded to the metal atom in addition to the side-on coordinating hydroxylaminato unit. The larger Nd3+ and Pr3+ ions in 5 and 6 make the hydroxylaminato unit capable of dimerising through the oxygen atoms. This leads to the dimeric complexes [(Ln(C5H5)2{mu-eta1:eta2-ON(C2H4-o-Py)2})2] without metal-pyridine bonds. Compound 7 exhibits a dimeric coordination mode similar to the complexes 5 and 6, but, in addition, two pyridyl functions coordinate to the lanthanum atoms leading to the [(La(C5H5)2{ON(C2H4-o-Py)}{mu-eta1:eta2-ON(C2H4-eta1-o-Py)})2] complex. The aggregation trend is directly related to the size of the metal ions. The complexes with coordinative pyridine-metal bonds show highly dynamic behaviour in solution. The two pyridine nitrogen atoms rapidly change their coordination to the metal atom at ambient temperature. Variable-temperature (VT) NMR experiments showed that this dynamic exchange can be frozen on the NMR timescale.