Douglas J. Raber

Analysis of multinuclear lanthanide-induced shifts. 1. Investigations of some approximations in the procedure for separation of diamagnetic, contact, and pseudocontact shifts

For the 1:1 adducts of La(fod)3, Pr(fod)3, Eu(fod)3, Gd(fod)3, Dy(fod)3, and Yb(fod)3 and adamantanone the 17O, 133C, and 1H bound shifts were determined. With the data set obtained it is demonstrated that a separation of diamagnetic, contact, and pseudocontact shifts gives reliable values for all shift contributions only when a modified procedure is used in the separation. Our data show that diamagnetic and contact shifts are very large for the carbonyl 17O and 13C nuclei. Diamagnetic and contact shifts are also substantial for the remaining 13C nuclei and for 1Hα in the Eu(fod)3 adduct. For all other nuclei the pseudocontact shift is dominant, and a direct comparison of experimental bound shifts with calculated pseudocontact shifts is possible.

Reduction of aldehydes and ketones with tetraalkylammonium borohydrides

Abstract Misinterpretations regarding the selectivity of tetraalkylaminonium borohydride reductions in dichloromethane are resolved. Tetrabutyl ammonium borohydride offers several advantages, but both it and tetraethyl ammoniumborohydride are highly useful synthetic reagents.

Structures of lanthanide shift reagent complexes by molecular mechanics computations

Abstract Molecular mechanics calculations have been applied to the structure determination of 7-coordinate lanthanide complexes. To circumvent problems in defining oxygen—lanthanide—oxygen bond angles, the energy of angle deformations at the metal center are not evaluated explicitly. Instead the standard approach to molecular mechanics calculations is modified by including 1,3-nonbonded interactions between atoms that are both bonded to the metal center. Geometry optimization for two known lanthanide complexes afforded structures that are in reasonable agreement with X-ray crystal structures, and small discrepancies are attributed to cyrstal packing forces.

Structure elucidation with lanthanide induced shifts. 9. bicyclo[3.3.1]nonan-9-one

Abstract Lanthanide shift reagents have been used for the conformational analysis of bicyclo[3.3.1]-nonan-9-one. Comparison of predicted shifts and experimental values obtained with Eu(fod)3 indicate that the boat-chair conformation constitutes 22% of the mixture of conformers.

Analysis of multinuclear lanthanide-induced shifts. Part 2. The geometry of ketone binding to lanthanides

Multinuclear lanthanide-induced shifts for a variety of Ln(fod)3 shift reagents have been combined with Gd(fod)3-induced longitudinal relaxation-rate enhancements in geometry calculations for lanthanide adducts of adamantanone. The experimental results are most consistent with an Ln–O–C angle of about 180° and an Ln–O distance of approximately 2.5 A. The fit between calculated and experimental data is very soft, and simulated data indicate that this is characteristic of the linear Ln–O–C array. When induced shifts and relaxation rate enhancements are simulated for nonlinear co-ordination geometries, the results are inconsistent with the experimental data.

Structure elucidation with lanthanide-induced shifts 5. Evaluation of the binding ability of various functional groups

The equilibrium binding constants have been evaluated for the association in CCl4 of the lanthanide shift reagent. Eu(fod)3, with a series of adamantane derivatives containing a variety of different functional groups. It is shown that if steric effects are held constant the binding abilities of the adamantane derivatives exhibit a good correlation with the proton affinities of the methyl analogs containing the same functional groups. The results permit the prediction of binding strength of other functional groups with Eu(dpm)3 as well as with Eu(fod)3 and thus can be of great assistance in planning and interpreting the results of experiments with lanthanide shift reagents.ZusammenfassungEs wurden die Gleichgewichts-Komplexbildungskonstanten für eine Reihe von Adamantanderivaten mit verschiedenen funktionellen Gruppen mit Eu(fod)3 in CCl4 bestimmt. Es wird gezeigt, daß bei Konstanthaltung sterischer Effekte die Komplex-Bindungsenergien eine gute Korrelation mit den Protonenaktivitäten der entsprechenden Methylhomologen ergibt. Die gewonnenen Resultate geben (auch für andere funktionelle Gruppen als die unmittelbar untersuchten) eine gute Möglichkeit zur Abschätzung zu erwartender Komplexbildungsstärken.

Analysis of multinuclear lanthanide induced shifts. Part 5. The co-ordination polyhedron of 1 : 3 lanthanide(III)–glycolate complexes in aqueous solution

Experimental 17O, 13C, and 1H bound shifts have been determined for the complexes of lanthanide(III) cations with glycolate (L) in D2O at pD 4.5. These shifts were separated into diamagnetic, contact, and pseudocontact contributions. The contact shifts show that the stoicheiometry of these complexes is LnL3(D2O)3. Glycolate is co-ordinated in a bidentate fashion via one of the carboxylate oxygens and the hydroxylic oxygen. From the pseudocontact shifts it is concluded that lanthanide(III) cations are surrounded by nine oxygens constituting a distorted tricapped trigonal prism, in which the three capping positions are occupied by carboxylate oxygens.

Eu(FOD)3 induced shifts of alkyl substituted adamantanones and molecular geometries of the adducts

Abstract The geometries of adducts of europium tris(fod) and a series of alkyl substituted adamantanones have been determined with the use of lanthanide induced 1H shift measurements. The position of the Eu(III) cation relative to the substrate ligand appears to be strongly dependent upon steric interactions between the fod ligands and the alkyl groups in the substrate. The alkyl substitution causes distortions from the otherwise linear C-O-Eu array. In general the Eu(III) cation moves away from the alkyl substituents.

Structure elucidation with lanthanide-induced shifts. 11. Analysis of alkyl-substituted benzonitriles

Abstract The 1H NMR spectra for a series of alkyl-substituted benzonitriles were investigated in the presence of Eu(fod)3. The experimental lanthanide-induced shifts (LIS) were determined for the 1:1 complexes, and the structures of the complexes were evaluated by comparison of the experimental LIS with values predicted with the pseudocontact equation. The contribution to the observed LIS from contact shifts and complex formation shifts is shown to be small, and excellent structure fits are obtained. The correct structure of a compound can be reliably selected from several possibilities, even if only one of the structural alternatives is available for experimental study.

Structure Elucidation with Lanthanide Induced Shifts. 14. Structural Effects on Equilibria between Nitriles and EU(fod)3

Abstract In recent years lanthanide shift reagents have become increasingly valuable as aids in the analysis of organic structures by nmr spectroscopy.2. They can be used qualitatively in order to simplify nmr spectra, and the lanthanide induced shifts can also be used quantitatively for the mathamatical analysis of a proposed structure.3–6 Whatever the particular case may demand, these reagents can be utilized most effectively when the equilibria between shift reagent and substrate are well understood. We have previously reported7 the binding abilities of a variety of functional groups with Eu(fod)3, and we have also discussed8 the effects of structural variation on the corresponding equilibria of ketones. We now consider structural effects on the association equilibria for a variety of nitriles with Eu(fod)3 in CCl4.