Abstract
New trinuclear mixed-valent Co-III-Co-II-Co-III. complexes with salen-type ligands have been synthesized and characterized both in the solid state and in solution. These complexes comprise a central CoCl2 unit connecting two [RCo(4,4',7,7'tmsalen)] fragments with chlorides acting as bridging ligands between the metals, while the central Co-II ion completes its octahedral coordination through the salen oxygen donors. Complex 1 (R = CH2Cl), which has cis Cl ligands at the central Co-II ion, has approximate C-2 symmetry, whereas the centrosymmetric complex 2 (R = CF3CH2) contains trans Cl ligands (C-2h symmetry). Complexes 1 and 2 are paramagnetic with magnetic moments of 4.7 and 4.4 mu(B), respectively, which indicate the presence of a high-spin Co-II center in the molecule. The H-1 NMR spectra, which spread over a range of nearly 190 ppm, are consistent with the presence of both cis and trans isomers in solution (with cis being the prevailing species), and indicate that exchange is slow on the NMR time-scale. Isotropically shifted H-1 NMR signals have been fully assigned by comparison with those of appropriately Substituted complexes and by T-1 measurements and 2D exchange experiments (EXSY). The K constant for the trans reversible arrow cis equilibrium, as evaluated from the H-1 NMR signal intensity ratio at various temperatures, has allowed us to estimate, at least qualitatively, the thermodynamic parameters Delta H degrees and Delta S degrees. ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)