In this communication we present a stereochemical NMR study of
paramagnetic labile Werner’s type complexes. The complexes possess a
cobalt(II) coordination centre around which two bidentate β-diketonato and
two monodentate substituted pyridine ligands or one bipyridine type ligand
are coordinated. The bidentate ligand contains only one CF3 fragment which
allows to distinguish, by 19F NMR spectroscopy, the various isomers existing in
solution. The paramagnetic cobalt(II) (4T1g) centre induces an important dipolar
contribution to the chemical shift, and leads to good signals separation and
resolution.

In spite of its apparent simplicity, this system embraces a real stereochemical
diversity hinging on the nature of β-diketonato moiety (R1) and bidentate or
monodentate pyridine type ligands. As a result, one (I) or two isomers (II &
III) are obtained using symmetric β-diketonato ligand and either bidentate or
monodentate pyridine type ligands, respectively. Three (IV-VI) or five isomers
(VII-XI) are obtained using asymmetric β-diketonato ligand, and either bidentate
or monodentate pyridine type ligands, respectively.

The structures for the all possible diastereoisomers and their simplified

nomenclature are presented in scheme 2.
The criteria used for full NMR assignment (180-265 K) include: comparison
of integral ratios, cis(N) and trans(N) differentiation in presence of the chiral
amine (S(-)-1-(4-pyridyl)ethanol), effect of solvent polarity on the relative
stabilities of the five isomers and observation of trans-influences in heteroligand
(either mono- or bidentates) mixed complexes.
Acknowledgments
This work has been supported by the Swiss National Science Foundation
(Gh.N.) and the SCOPES (Scientific Co-operation between Eastern Europe and
Switzerland) program.