posted on 2013-11-18, 00:00authored byFemke
F. B. J. Janssen, Laurens
C. J. M. Peters, Paul P.
J. Schlebos, Jan M. M. Smits, René de
Gelder, Alan E. Rowan
Tetranuclear carboxylate
clusters with the general structural formula
[M4(L)2(O2CR)4] (M = Cd,
Zn; LH2 = 2,6-bis(1-(2-hydroxyphenyl)-iminoethyl)pyridine;
R = CH3, C6H5) were studied by variable-temperature
(VT) 1H NMR spectroscopy. The dynamics of these clusters
in solution can be described by two uncorrelated dynamical processes.
The first dynamical process is the interconversion, both inter- as
well as intramolecular, between syn–syn bridging and chelating carboxylate ligands. It is shown
that this carboxylate interconversion mechanism is predominantly intramolecular
for [Cd4(L)2(O2CCH3)4] (1a), whereas for [Zn4(L)2(O2CCH3)4] (2a) it
is predominantly intermolecular. Two models for the second dynamic
process, which involves the diiminepyridine ligand, are described.
The first model comprises a nondissociative rotation around an internal
axis, which changes the chirality of the cluster. The second model
is based on the dissociation of the tetranuclear cluster into two
dimeric species, which recombine again. This last model is supported
by scrambling experiments between [Zn4(L)2(O2CCH3)4] (2a) and [Zn4(L3)2(O2CCH3)4] (5) (L3H2 = 2,6-bis(1-(2-hydroxyphenyl)-iminoethyl)4-chloropyridine).