Structural Variations and Molecular Dynamics of Rare‐Earth Metal Complexes with the N,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.

AlMe3, GaMe3 and InMe3 Adducts of N,N‐Bis(2‐{pyrid‐2‐yl}ethyl)hydroxylaminato Rare‐Earth Metal Complexes and Their Molecular Dynamics

“…The singlet of the protons of GaMe 3 does not split into different signals, indicating that the gallium atom remains bonded to the hydroxylaminato oxygen atom during the exchange process. Compared to the spectra of the EMe 3 -free [YCp 2 ON(C 2 H 4 -oPy) 2 ] [14] (Type A) the coordination of the EMe 3 -units to the ligand oxygen atom effects a simplification of the dynamic, because in this case solely the pyridyl exchange process and no further splitting is observed. By means of the Eyring equation it is possible to estimate the activation energy of this exchange process for 2 to be 44 kJ mol -1 .…”

“…[14] These processes lead to very complex VT-NMR spectra and we found that besides an exchange of the pyridyl groups, further dynamic processes take place.…”

“…In contrast to complexes 1-5 these bonds are shorter than those in the . [14] The N( (2) and 2.737(2) Å, respectively. Taking the differences of the ionic radii into account, these interactions appear to be weaker than in the group of compounds 1-5.…”

“…[14] With this ligand system the coordination of one pyridine donor atom effects that a dimerisation via an Ln 2 O 2 linkage is disfavoured. Also the molecular structures, ligand coordination modes and aggregation types of these complexes depend strongly on the ionic radii of the involved rare-earth metal ions.…”

“…In order to investigate the hemilability we tried to occupy the free pyridine donor-site with Lewis acids to block such exchange processes. Here we present the formation of complexes resulting from the interactions of the Lewis-acidic metal trialkyls EMe 3 2 in thf, [14] complexes 1-17 were synthesised by reacting them with pure AlMe 3 , GaMe 3 and InMe 3 . This resulted in a variety of ligand binding and aggregation modes.…”