PARASHIFT Probes: Solution NMR and X‑ray Structural Studies of Macrocyclic Ytterbium and Yttrium Complexes

Ytterbium and yttrium complexes of
octadentate ligands based on 1,4,7,10-tetraazacyclododecane with a
coordinated pyridyl group and either tricarboxylate (L1) or triphosphinate (L2) donors form twisted-square-antiprismatic
structures. The former crystallizes in the centrosymmetric group P21/c, with the two molecules
related by an inversion center, whereas the latter was found as an
unusual kryptoracemate in the chiral space group P21. Pure shift NMR and EXSY spectroscopy allowed the dynamic
exchange between the (RRR)-Δ-(δδδδ)
and (RRR)-Λ-(λλλλ)
TSAP diastereomers of the [Y.L2] complex to be detected.
The rate-limiting step in the exchange between Δ and Λ
isomers involves cooperative ligand arm rotation, which is much faster
for [Ln.L1] than for [Ln.L2]. Detailed analysis
of NOESY, COSY, HSQC, and HMBC spectra confirms that the major conformer
in solution is (RRR)-Λ-(λλλλ),
consistent with crystal structure analysis and DFT calculations. The
magnetic susceptibility tensors for [Yb.L1] and [Yb.L2], obtained from a full pseudocontact chemical shift analysis,
are very different, in agreement with a CASSCF calculation. The remarkably
different pseudocontact shift behavior is explained by the change
in the orientation of the pseudocontact shift field, as defined by
the Euler angles of the susceptibility tensor.