DOI: 10.1002/ejic.201601475 Full Paper Metal–Nucleobase Composites Supramolecular Architectures Based on Metal–Cytosine Systems Jintha Thomas-Gipson, [a] Garikoitz Beobide, [a] Oscar Castillo,* [a] Antonio Luque, [a] Sonia Pérez-Yáñez, [a,b] and Pascual Román [a] Abstract: Five new compounds of cytosine with first-row tran- sition metals, (H 2 Cyt) 2 [CoCl 4 ]·2(HCyt) (1), [CoBr 2 (HCyt) 2 ] (2), [ZnCl 2 (HCyt) 2 ](3), [CuCl 2 (HCyt) 2 ](4), and [CuBr 2 (HCyt) 2 ](5) (in which HCyt = cytosine, H 2 Cyt = cytosinium cation), are re- ported. In 1, the protonation of one of the cytosine molecules facilitates base pairing with the neutral cytosine, instead of the expected coordination to the metal center. In all other com- pounds, neutral cytosine molecules are coordinated to the M II centers through N3 positions, forming monomeric entities. The supramolecular crystal structure of 1 is sustained by the base- pairing interactions between the 1H,3H-cytosinium cation and Introduction The interaction of metal ions with nucleobases has generated great interest in biochemistry during the last few decades. [1–3] Nucleobases are molecules with a vast spectrum of binding possibilities, and the reactivity of both purine and pyrimidine bases have been widely investigated. The reactivity of nucleo- bases towards metal ions varies, depending on the nature of the metal ions and the nucleobases selected. [4] Although there are many examples of purine-based systems, it still remains a challenge to provide similar compounds with pyrimidine nu- cleobases, since, as can be deduced from their lower number of heteroatoms, their coordination and hydrogen-bonding in- teractions with other species are reduced. [5] On the other hand, there exist interesting examples of cytosine derivatives linked to transition-metal complexes, but the coordination of unsubsti- tuted neutral cytosine to metal centers has been less stud- ied. [6,7] Neutral cytosine, although presenting different donor posi- tions, shows a relatively predictable coordination mode through N3. [7] The other possible binding modes are through N1, and simultaneously, through N3 and O2 atoms, while in some cases, coordination solely through O2 is reported. In a search of the CSD database [8] (version May 2016), the N3 coordination mode showed 28 hits. Less common coordination modes involve [a] Departamento de Química Inorgánica, Facultad de Ciencia y Tecnología, Universidad del País Vasco (UPV/EHU), Apartado 644, 48080 Bilbao, Spain E-mail: oscar.castillo@ehu.es [b] Departamento de Química Inorgánica, Facultad de Farmacia, Universidad del País Vasco (UPV/EHU), 01006 Vitoria-Gasteiz, Spain ORCID(s) from the author(s) for this article is/are available on the WWW under http://dx.doi.org/10.1002/ejic.201601475. Eur. J. Inorg. Chem. 2017, 1333–1340 © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 1333 the neutral 1H-cytosine molecule through their Watson–Crick and sugar edges, giving rise to zig-zag supramolecular ribbons, in which neutral and cationic cytosine molecules alternate. In the isostructural compounds 2 and 3, only one of the cytosine molecules establishes base pairing through sugar-edge-forming supramolecular dimers, and the 3D cohesiveness is achieved by means of noncomplementary hydrogen bonding, involving the sugar edge of the other cytosine with the amino group and the bromide anions of the adjacent dimer. Compounds 4 and 5 show similar supramolecular arrangements, with cytosine mol- ecules self-assembling along the sugar edge into 1D chains. bonding through the exocyclic O2, N1, or even, in two cases, simultaneously through N3 and O2, both as chelates or bridg- ing ligands. A recently reported case shows chelation through N3 and exocyclic O2, together with N1 coordination. [9] In this work, we have analyzed the role of coordinated cytosine molecules in the final crystal structure. It takes place, not only due to its coordination to the metal center, but also because of its capacity to recognize adjacent nucleobases through base-pairing interactions. There are some examples of discrete complex entities that yield porous structures sustained by this kind of supramolecular base-pairing interaction involv- ing metal–adenine entities. [10] An analysis of the hydrogen- bond donor and acceptor positions in cytosine and adenine nucleobases coordinated to the metal centers through their most common coordination modes is depicted in Figure 1. It Figure 1. Hydrogen-bond donor and acceptor positions in coordinated cytosine (HCyt), adenine (HAde), cytosinato (Cyt – ), and adeninato (Ade – ) li- gands in their most common coordination mode. Base-pairing-capable sides are also depicted.