& Ion Effects Effect of the Metal Ion on the Enantioselectivity and Linkage Isomerization of Thiosemicarbazone Helicates María J. Romero, [a] Vanesa Suµrez, [a] Sandra Fernµndez-FariÇa, [a] Marcelino Maneiro, [b] Emilio Martínez-NfflÇez, [c] Guillermo Zaragoza, [d] Ana M. Gonzµlez-Noya,* [a] and Rosa Pedrido* [a] Dedicated to Prof. Manuel R. Bermejo on the occasion of his honorary distinction as Emeritus Professor at the University of Santiago de Compostela Abstract: The effect of the metal ion and ligand design on the enantioselectivity and linkage isomerization of neutral cobalt and zinc bisthiosemicarbazone metallohelicates has been investigated in this work. The electrochemical synthesis has afforded the enantioselective formation of chirally pure cobalt helicates, and the LL isomer of a single enantiomer has been crystallized as only product for the cobalt methyl- substituted thiosemicarbazone helicate. Interestingly linkage isomers have been formed from zinc ethyl-substituted thio- semicarbazone helicate enantiomers for the first time. The co-existence of these isomers has been evaluated from the point of view of both experimental results and computation- al calculations. Introduction Among the wide variety of discrete supramolecular species re- ported to date, helicates are considered to be the simplest and most promising architectures [1] because of their involvement in different areas of nanotechnology related to medicine, [2] cataly- sis, [3] and magnetic materials. [4] The term helicate was intro- duced by Lehn in 1987 to describe chiral structures in which organic ligands wrap around two or more metal ions in a heli- cal array. [5] The intrinsic chirality of helicates arises from the homochiral mechanical coupling of D or L chiral metal cen- ters, that is, DD or LL configurations, respectively. Neverthe- less these compounds are usually formed as a racemic mixture. Isomerism is an important tool in the development of phar- macotherapeutics and molecular materials due to the different physical and optical properties that two isomers of the same molecule can exhibit. [6] In this context, the resolution of race- mic helicates in order to obtain stereochemically pure systems has been achieved for helicates in a small number of cases by either chromatographic methods [7] or spontaneous separation upon crystallization. [8] Other type of isomerism is the linkage isomerism, which is a structural event that can be observed in complexes with ambidentate ligands. [9] Linkage isomerization can be stimulat- ed by external factors such as redox changes in the metal ion, [10] photoirradiation, [11] temperature changes, [12] or solva- tion. [13] These processes open up new pathways for the appli- cation of linkage isomers in optical devices or for information storage at the molecular level. [14] Linkage isomerization has also been studied in an effort to develop solar cells [15] and anti- cancer drugs, [16] which makes the study of this structural phe- nomenon even more relevant. An exhaustive review of the literature shows that a large collection of helicate structures have been reported to date but none of them display linkage isomerism. Our research group has pioneered the use of bisbidentate thiosemicarbazone ligands that incorporate a short spacer to achieve helical or meso-helical arrays. [17] We have observed that linkage isomerization occurs in cobalt(II) and zinc(II) mesocates derived from an ethyl-substituted tetradentate bisthiosemicar- bazone ligand bearing a benzene as a spacer. [18] Interestingly such mesocates crystallize as a mixture of conformational and [a] Dr. M. J. Romero, Dr. V. Suµrez, S. Fernµndez-FariÇa, Dr. A. M. Gonzµlez-Noya, Dr. R. Pedrido Departamento de Química Inorgµnica, Facultade de Química Campus Vida, Universidade de Santiago de Compostela Santiago de Compostela, Galicia, 15782 (Spain) E-mail : ana.gonzalez.noya@usc.es rosa.pedrido@usc.es [b] Dr. M. Maneiro Departamento de Química Inorgµnica, Facultade de Ciencias Universidade de Santiago de Compostela, 27002 Lugo (Spain) [c] Dr. E. Martínez-NfflÇez Departamento de Química Física, Facultade de Química Campus Vida, Universidade de Santiago de Compostela Santiago de Compostela, Galicia, 15782 (Spain) [d] G. Zaragoza Unidade de Difracción de Raios X, Edificio CACTUS Universidade de Santiago de Compostela, Campus Sur, Santiago de Com- postela Galicia, 15782 (Spain) Supporting information and the ORCID identification number(s) for the au- thor(s) of this article can be found under: http ://dx.doi.org/10.1002/chem.201700195. Chem. Eur. J. 2017, 23, 4884 – 4892  2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 4884 Full Paper DOI: 10.1002/chem.201700195