**4. Conclusions**

In summary, eight Cu(I)Br coordination polymers have been synthesized, and their single-crystal structures characterized by using X-ray diffraction analysis. Three different types of (CuBr)n arrangements with n ≥ 2 were realized. Six of the eight complexes form characteristic catenated μ2-bromo staircase polymers that are reminiscent of those reported in related copper(I) halide complexes. Our findings indicate that a combination of cyclic and non-cyclic hydrogen bonds (HBs), between the C2-amino group and Cu(I) bound bromides (N–H···Br–Cu) and halogen bonding (C–Cl···Br–Cu) interactions, underpins the formation of catenated μ2- and μ3-bromo (CuBr)n polymeric structures.

It was found that the mode of coordination by the dichloro-substituted aminopyrazines **4**–**6** to the Cu(I) could be readily predicted based on simple electronic and steric arguments. In the mono chloro-substituted pyrazines **1**–**3**, these effects were less obvious, as evidenced by the realization of both monodentate (**1**·CuBr and **2a**·CuBr), and polymeric structures (**2b**·CuBr, **2c**·CuBr and **3**·CuBr). The polymeric **3**·CuBr suggests that there is little steric hindrance afforded by the combination of a C2-amino and C6-chloro substituents vicinal to the N1–Cu coordination site, and may indicate that this is not a major factor in determining N1-Cu coordination passiveness in **4**·CuBr and **5**·CuBr. This could also account for the unique polymeric (μ2-bromo)-Cu<sup>I</sup> chain in **3**·CuBr, which is able to accommodate the more sterically demanding N1–Cu coordination, and allows stabilizing cyclic N–H···Br HBs.

Three polymorphs were isolated and structurally characterized based on the simple 2-amino-5- chloropyrazine ligand, **2**. We demonstrate that concomitant polymorphism leads to both 1D and 2D polymeric structures based on theμ3-bromo staircase polymer motif in **2a**·CuBr and **2c**·CuBr, respectively. The third polymorph, **2b**·CuBr, displays a unique honeycomb network composed of discrete Cu2Br2 rhomboid (μ2-bromo)-Cu<sup>I</sup> dimers and μ2-*N*,*N'* bridging ligands. These three polymorphs demonstrate the role of the C2-amino group in forming three types of hydrogen bonding patterns recognized in all eight of these complexes. The self-complementary, symmetry-related N–H···Npz pyrazine dimers

described by the graph set notation R2 2(8) with distances varying from ca. 2.138(5)–2.308(12) Å [∠Br···H–N = 160.1(3)◦–177.7(8)◦] were found in N4–Cu coordination structures **4**·CuBr and **5**·CuBr, and resulted in the formation of laminar 2D sheet structures. Based on the competitive binding sites in the simple mono-substituted aminopyrazine ligands, and the role of the amino-group to influence structural features through hydrogen bonding, we are currently investigating the formation of dihalopyrazine-copper complexes.

The Cu(I) halide complexes have attracted considerable attention because of their physical properties and functional applications in future technologies, such as solar energy conversion, light emitting devices, and possible sensing applications. As a result, there has been considerable focus on exploring and developing structural relationships that will provide insights for future materials design and synthesis. The interesting 2D-honeycomb network found in **2b**·CuBr is a particularly appealing architecture that could have applications in gas storage or separation, if the porosity can be tuned through ligand design. While systematic structural studies on Cu(I) halide and pyrazine derivatives have not received much attention to date, this work demonstrates a level of structural predictability not typically observed in Cu(I) halide complexes. We believe future investigations of halo-substituted aminopyrazines and related pyrazine derivatives will provide the basis for an empirical structural database that will aid future material designs.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/2624-8549/2/3/45/s1, Full experimental, the single-crystal experimental and CCDC number details for **1**·CuBr–**6**·CuBr (Tables S1–S3), structure refinements for **1**·CuBr–**6**·CuBr, crystal habits of **2(a,b,c)**·CuBr (Figure S1), powder X-ray di ffraction analysis (Tables S4–S6, Figures S5–S9), FT-IR spectroscopic details (Figures S10–S16), and X-ray crystal figures (Figures S2–S4).

**Author Contributions:** The manuscript was written and edited through the contributions of all authors. R.P. and A.M. were responsible for the original concept, methodology development and SCXRD analysis. The ligand synthesis was carried out by N.S., while the PXRD data collection, analysis and Pawley fittings were performed by M.L. K.R. and A.M. were responsible for proof reading and the final manuscript version. All authors have read and agreed to the published version of the manuscript.

**Funding:** This work was financially supported by the Academy of Finland (projects 298817 and 289172), the University of Jyväskylä, and in part by the European Union's H2020 program, under the Marie Skłodowska-Curie gran<sup>t</sup> agreemen<sup>t</sup> 659123 for A.M.

**Acknowledgments:** The authors gratefully acknowledge the University of Jyväskylä for providing laboratory and SCXRD resources.

**Conflicts of Interest:** The authors declare no conflict of interest.
