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

The reaction of the donor‐functionalised N , N ‐bis(2‐{pyrid‐2‐yl}ethyl)hydroxylamine and [LnCp 3 ] (Cp=cyclopentadiene) resulted in the formation of bis(cyclopentadienyl) hydroxylaminato rare‐earth metal complexes of the general constitution [Ln(C 5 H 5 ) 2 {ON(C 2 H 4 ‐ 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(C 5 H 5 ) 2 {η 2 ‐ON(C 2 H 4 ‐η 1 ‐ o ‐Py)(C 2 H 4 ‐ 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 Nd 3+ and Pr 3+ ions in 5 and 6 make the hydroxylaminato unit capable of dimerising through the oxygen atoms. This leads to the dimeric complexes [(Ln(C 5 H 5 ) 2 {μ‐η 1 :η 2 ‐ON(C 2 H 4 ‐ 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(C 5 H 5 ) 2 {ON(C 2 H 4 ‐ o ‐Py)}{μ‐η 1 :η 2 ‐ON(C 2 H 4 ‐η 1 ‐ 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.