Uncorrelated Dynamical Processes in Tetranuclear Carboxylate Clusters Studied by Variable-Temperature 1H NMR Spectroscopy.

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).