Disorder at the Chiral Cα Center and Room-Temperature Solid-State cis–trans Isomerization; Synthesis and Structural Characterization of Copper(II) Complexes with d-allo,l-Isoleucine

Four new complexes of copper(II) with D-allo,L-isoleucine were prepared by solution and mechanochemical synthesis: three complexes of aqua cis isomers, cis-[Cu(D-allo-Ile)(2)(H2O)] (2), cis-[Cu(D-allo-Ile)(L-Ile)(H2O)](0.7)[Cu(L-Ile)(2)(H2O)](0.3 )(3), cis-[Cu(D-allo-Ile)(2)(H2O)](0.75)[Cu(D-allo-Ile)(L-Ile)(H2O)](0.25) (4), and anhydrous trans-[Cu(D-allo-Ile)(L-Ile)] (5). The previously characterized orthorhombic cis-[Cu(L-Ile)(2)(H2O)] (1a) cocrystallized with 3 and either 2 or 4, depending on the synthetic conditions. In 2, a disorder of the side chain was found, while 3 and 4 had a rare positional disorder of two diastereomers, namely, D-allo-Ile and L-Ile, resulting in two positions of the C(alpha)chiral center in their crystal structures. The complexes were analyzed by X-ray diffraction methods, solid-state NMR spectroscopy, and density functional theory calculated( 13)C and H-1 Fermi contact shifts. Room- temperature solid-state cis-to-trans isomerization of Cu(D-al/o-Ile)(L-Ile) occurred by liquid-assisted grinding (LAG) of 3 with methanol, by aging in methanol vapor, and by soaking 3 in methanol. The latter was a fast process. The isomerization also happened in the 140-160 degrees C temperature interval. Reverse trans-to-cis isomerization occurred by LAG of 5 with water and very slowly by aging in water vapor. Low activation energy for the Cu(D-allo-Ile)(L-Ile) cis-trans isomerization in the solid state could explain why subtle changes in the experimental conditions affected the final product.