Crystal Structures and Solution Behavior of Paramagnetic, Trinuclear, Mixed‐Valent Cobalt Complexes with Salen‐Type Ligands

New trinuclear mixed-valent CoIII-CoII-CoIII complexes with salen-type ligands have been synthesized and characterized both in the solid state and in solution. These complexescomprise 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 CoII ion completes its octahedral coordination through the salen oxygen donors. Complex 1 (R = CH2Cl), which has cis Cl ligands at the central CoII ion, has approximate C2 symmetry, whereas the centrosymmetric complex 2 (R = CF3CH2) contains trans Cl ligands (C2h symmetry). Complexes 1 and 2 are paramagnetic with magnetic moments of 4.7 and 4.4 μB, respectively, which indicate the presence of a high-spin CoII center in the molecule. The 1H 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 1H NMR signals have been fully assigned by comparison with those of appropriately substituted complexes and by T1 measurements and 2D exchange experiments (EXSY). The K constant for the trans [rlarr2] cis equilibrium, as evaluated from the 1H NMR signal intensity ratio at various temperatures, has allowed us to estimate, at least qualitatively, the thermodynamic parameters ΔH° and ΔS°.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)