ANALYZING LANTHANIDE-INDUCED SHIFTS IN THE NMR-SPECTRA OF LANTHANIDE(III) COMPLEXES DERIVED FROM -DIETHYLACELTAMIDO)-1,4,7,10-TETRAAZACYCLODODECANE

Variable-temperature H-1 and C-13 NMR spectra have been obtained for s olutions of lanthanide(III) complexes derived from N-diethylacetamido) -1,4,7,10-tetraazacyclododecane in deuterated acetonitrile. The lantha nide-induced shifts (LIS) observed in the spectra of the paramagnetic complexes (Ln = Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er, Tm, and Yb) are analyz ed using a linear least-squares procedure involving the five componene nts of the traceless part of the magnetic susceptibility tenser as fit ting parameters (C-13 data is limited to Ln = Pr, Nd, Sm, and Eu). No assumptions are made regarding the orientation of the molecular coordi nate system and principal magnetic axis system in this method of analy sis. Furthermore, by using a linear least-squares fitting of the data, we establish that it is possible to allow a computer program to permu te LIS values over any number of nuclei and determine which particular assignment gives the best fit of the LIS data. A new method of evalua ting the contact component of the LIS in the proton spectra is introdu ced. Molecular mechanics calculations are combined with analyses of LI S data to determine structures of the complexes in solution. Through s uch an approach, we establish that the eight-coordinate lanthanide ion s are encapsulated by the octadentate macrocyclic ligand, with the fou r nitrogen atoms and four oxygen atoms situated at the vertices of a d istorted square antiprism having C-4 symmetry (the rotational angle be tween the two square pyramids is 47 degrees). The lanthanide ions lie above the plane of nitrogen atoms of the macrocycle, with the distance decreasing from 1.74 to 1.41 Angstrom across the lanthanide series. T he distance to the mean plane of donor oxygen atoms increases from 0.6 3 to 0.83 Angstrom across the series. The Ln(3+)-N and Ln(3+)-O bond d istances range from 2.82 to 2.54 Angstrom and 2.47 to 2.21 Angstrom, r espectively. The NMR data establish that each complex exists in soluti on as a single pair of enantiomers. Analysis of the LIS data establish es that the isomer with each ethylenediamine ring of the macrocycle in the lambda conformation with clockwise rotation of the pendant arms, or its enantiomer, is favored in solution.