**1. Introduction**

Construction of supramolecular structures from small molecules that self-assemble using hydrogen bonds (HBs) and other non-covalent interactions is the ultimate goal of the crystal engineering discipline [1]. Hydrogen bonding, due to the smaller size and easy polarizable nature of H-atom, has become a reliable tool to fabricate highly symmetric and exotic solid-state networks with tunable properties for applications in biology [2], and materials sciences [3]. The H-bonding knowledge gleaned from organic co-crystals have been applied to self-assemble metal-organic/coordination networks [4–7]. In fact, under the "crystal engineering umbrella", the design and synthesis of coordination compounds have received wide attention, due to their intriguing structural topologies [8–10]. Unlike organic co-crystals, engineering inorganic compounds is dependent on two principals; primary coordination sphere (metal-ligand interactions) and secondary coordination sphere (non-covalent interactions) [11].

Despite several factors (e.g., pH, temperature) [12,13] could influence these two "parameters" for structurally diverse outcomes, modulation of networks based on the metal-ions geometry, organic ligands and their functional groups stand out in coordination chemistry research [14–21]. This fact is due to reproducible outcomes, that reflect the strong metal-ligand coordination bonds, and can be achieved with the judicious choice of organic ligands and metal-ions [14–21]. In this context, for example, O- and N-atoms are typical donors for the coordination bond formation; the former are derived from functional groups such as –COOH, –SO3H and phosphonates [22–24], and the latter primarily from N-heterocycles [25]. The combined use of these two groups render ligands a strong coordinating ability, and are well-known for the preparation of homometallic and heterometallic coordination compounds [25].

In this regard, aminopyrazine carboxylic acids (L), of the forms L, and L+, for the construction of hydrogen-bonded organic co-crystals [26–31], and L− functioning as a polydentate ligand in the preparation of coordination compounds [32–39] are reported in the literature. The presence of aromatic N-atoms and carboxylic acid/carboxylate groups within the same ligand may enhance the N–M bond strengths, often assisted by the polydentate bonding nature of O-atoms [26]. However, if the –COOH group is replaced by an aprotic donor substituent such as chlorine, and in combination with copper halides, what will happen to N-atom coordination nature? Will the chlorine substituents and metal-bound halides establish halogen bonds (XBs) [40–44], and halogen···halogen interactions [45–48]? This knowledge of XBs in metal complexes is derived from our previous experiences in halopyridine-Cu(I)/Cu(II) compounds [49–52]. When the bulky chlorines are installed close to an N-atom, can this affect the N–M coordination? How do the substituents mediate a hybrid topology containing –HN–H···Npz (pz = pyrazine) hydrogen bonds and C–Cl···Br halogen bonds? To test our hypothesis, we synthesized six chloro-substituted pyrazin-2-amine ligands (**1–6**) using procedures reported in the literature [53–57], and each ligand was combined with CuBr2 in a 1:2 metal:ligand ratio. The CuBr complexes of these ligands were obtained by exploiting the known redox activity of Cu(II) halides in the presence of organic carbonyl compounds [31]. This work represents the first systematic study of the metal-ligand, HB, and XB interactions of chloro-substituted pyrazin-2-amines with copper halides and recent results will be discussed.
