**3. Results and Discussion**

This new class of chloro-substituted pyrazin-2-amine ligands given in Chart 1 were made to react with cupric bromide (CuBr2), either by refluxing in a 1:1:1 (*v*/*v*/*v*) mixture of acetone (Ace), ethanol (EtOH) and acetonitrile (MeCN) or 1:5 (*v*/*v*) mixture of Ace:EtOH to afford the desired Cu(I)Br complexes, via an in situ redox process, according to Scheme 1. An excess of pyrazine ligand was used to act as ligand and auxiliary base for the HBr liberated during the reduction of CuBr2 to CuBr by acetone [70]. The reduction was accompanied by a color change from a deep blue-green solution characteristic of Cu(II) ions, to a clear, yellow solution upon refluxing for several minutes. In total, eight different Cu(I)Br coordination complexes obtained from the combination of these six ligands will be discussed.

**Chart 1.** List of chloro-substituted pyrazin-2-amines and numbering scheme used throughout: 2-amino-3-chloropyrazine (**1**), 2-amino-5-chloropyrazine (**2**), 2-amino-6-chloropyrazine (**3**), 2-amino-5,6- dichloropyrazine (**4**), 2-amino-3,6-dichloropyrazine (**5**), and 2-amino-3,5-dichloropyrazine (**6**).


**Scheme 1.** General synthetic route to Cu(I)-complexes of ligands **1**–**6** via an in situ redox process from CuBr2, and the nomenclature used.

X-ray quality crystals of the complexes were obtained by slowly concentrating the reaction mixtures by controlled evaporation. Interestingly, **2c**·CuBr was only obtained from a 1:5 (*v*/*v*) Ace and EtOH reaction mixture, together with **2b**·CuBr. The concomitant polymorphism did not allow the isolation of **2c**·CuBr as a phase pure material; however, **2b**·CuBr could be prepared independently by alternative methodologies in moderate yield. Moreover, attempts to recrystallize the mixture of **2c**·CuBr and **2b**·CuBr from MeCN afforded only complexes **2a**·CuBr and **2b**·CuBr. This suggests the **2c**·CuBr is only somewhat stable when prepared from a 1:5 (*v*/*v*) Ace:EtOH mixture and readily dissociates in the polar aprotic solvent MeCN. Optical microscopy could be routinely used to distinguish the different color and crystal habits of the three different polymorphs of (**2a**–**<sup>c</sup>**) CuBr (See Figure S1). Powder X-ray diffraction methods were used to demonstrate the solid-state structures and estimate the phase purity of the bulk material by using Pawley full pattern fittings. The FT-IR spectra of the isolated complexes were compared to the respective ligands, and display two specific regions from 3500–2800 cm<sup>−</sup><sup>1</sup> and 1200–400 cm<sup>−</sup><sup>1</sup> for the hydrogen-bonded N–H stretching [71] and fingerprint regions, respectively, which are distinctive for each ligand and complex (see Figures S10–S16).

In general, reactions between donor ligands (L) and CuX (X= Cl, Br, I) yield complexes with formula Cu*n*X*n*L*<sup>m</sup>*, that display diverse structural topologies, containing rhomboid dimer, zigzag polymer, staircase polymer, closed cubane, and hexagon clusters, have been reported in the literature [72]. The structural diversification stems from the very nature of the ligand coordination modes and the tendency of copper halides to form clusters via μ2- and μ3-halide bridges [73]. In our eight complexes, we obtained three topologies exclusively, namely staircase polymer (**1**·CuBr, **2a**·CuBr, **2c**·CuBr, **4**·CuBr, **5**·CuBr, and **6**·CuBr), rhomboid dimer (**2b**·CuBr), and zigzag polymer (**3**·CuBr), as depicted in Figure 1. The staircase and rhomboid structures feature well-known Cu···Cu distances (Cuprophilic interactions) [74], ranging from 2.7547(13) to 3.086(4) Å, but will not be discussed further in the text.

The complexes **1**·CuBr and **2a**·CuBr both crystallize as clear, pale-yellow or colorless needles in an orthorhombic space group, *P*na21 and *P*212121, respectively. Both contain discrete 1D polymeric chains of the catenated (μ3-bromo)-Cu<sup>I</sup> 'staircase' that run parallel to the *c*-axis and *a*-axis in **1**·CuBr and **2a**·CuBr, respectively. In these chains, the Cu(I) ions have tetrahedral geometry, coordinated by three bromines and the pyrazine N1-atom. Since the pyrazine N4-atom is not involved in the N–Cu bond formation, it plays a central role in the HB formation involving a C2-amino group of an adjacent chain. The orthogonal arrangemen<sup>t</sup> of the discrete 1-D polymeric chains leads to a herringbone packing structure in both **1**·CuBr and **2a**·CuBr, as illustrated in Figure 2. The N4···H–N HBs in **1**·CuBr of 2.245(11) Å, [∠N4···H–N = 155.6(9)◦] are shorter than in **2a**·CuBr [2.349(7) Å, ∠N4···H–N = 133.4(5)◦]. The XBs in **1**·CuBr and **2a**·CuBr complexes are, however, more comparable at 3.465(3) Å [∠Br···Cl–C = 165.4(3)◦] and 3.509(3) Å [∠Br···Cl–C = 159.5(3)◦], respectively. This network of HBs and XBs does lead to a complex molecular packing that extends in three dimensions.

**Figure 1.** The three structural topologies realized in the Cu(I)-complexes of **1**–**6**, (**a**) catenated (μ3-bromo)-Cu<sup>I</sup> 'staircase' polymer, (**b**) Cu2Br2 rhomboid (μ2-bromo)-Cu<sup>I</sup> dimers, and (**c**) zigzag polymer (μ2-bromo)-Cu<sup>I</sup> chains.

The complex **6**·CuBr crystallizes in a monoclinic *P*21/*c* space group, and is composed of 1D polymeric chains of the catenated (μ3-bromo)-Cu<sup>I</sup> 'staircase' that run along the shortest unit cell *a*-axis. The asymmetric unit contains one bromide anion and a one Cu(I) cation coordinated by the sterically less hindered N1-atom of **6**. Not surprisingly, the C3 and C5 chloro-substituents vicinal to the N4-atom render this ring nitrogen Cu-coordination passive. The orthogonal arrangemen<sup>t</sup> of the discrete 1D polymeric chains leads to a herringbone packing structure that is similar to **1**·CuBr and **2a**·CuBr (for **6**·CuBr, see Figure S2). The 1D chains are aligned by a more extensive network of XBs, afforded by the C3- and C5-Cl substituents (C3–Cl···Br–Cu [3.4794(14) Å, ∠Br···Cl–C = 162.44(19)◦], C5–Cl···Br–Cu [3.4178(14) Å, ∠Br···Cl–C = 175.34(19)◦]) and longer N4···H–N of 2.511(4) Å, [∠N4···H–N = 135.8(3)◦] HBs, compared to **1**·CuBr and **2a**·CuBr.

The N1–Cu mode of coordination realized in **1**·CuBr, **2a**·CuBr, and **6**·CuBr allows the vicinal C2-amino group hydrogen and a Cu(I) bound bromide to form cyclic N–H···Br–Cu HBs, with distances varying from ca. 2.609(7)–2.774(2) Å [∠Br···H–N = 146(1)◦–168.9(4)◦], within the discrete polymeric (μ3-bromo)-Cu<sup>I</sup> 'staircase' chain (See Figure S3). These cyclic intra-chain N–H···Br HBs afford a pseudo-helical type arrangemen<sup>t</sup> of the pyrazine ligands along the polymeric (μ3-bromo)-Cu<sup>I</sup> 'staircase' backbone. The HBs, in combination with the C–Cl···Br–Cu XBs from neighboring chains, aids the stabilization of the catenated (μ3-bromo)-Cu<sup>I</sup> 'staircase' polymeric chains in these complexes.

The complexes **4**·CuBr and **5**·CuBr crystallize in a triclinic *P*-1 and monoclinic *P*21/n space groups, respectively. In both complexes, the catenated (μ3-bromo)-Cu<sup>I</sup> 'staircase' motif is preserved, however, the tetrahedral Cu(I) centers are coordinated by three bromides and a pyrazine N4-atom. In the case of **4**·CuBr, it crystallizes with an additional pyrazine molecule in the asymmetric unit that is not involved in coordination with the Cu(I) centers. The N4–Cu coordination mode found in both complexes allows the C2-amino group to participate in hydrogen-bonding with a neighboring pyrazine in a self-complimentary N–H···Npz pyrazine dimer. The resultant hydrogen-bonded dimers in **4**·CuBr and **5**·CuBr are described by the graph set notation R22(8), with HB parameters of ca. 2.308(12) Å, [166.4(7)◦] and 2.138(5) Å [160.1(3)◦], respectively (Figure 3). These Watson–Crick-like base pairing structures are characteristic of aminopyrazine derivatives [75,76]. The R22(8) hydrogen bonding in **4**·CuBr and **5**·CuBr leads to essentially linear chains that pack into laminar 2D polymeric sheets held together by the polymeric (μ3-bromo)-Cu<sup>I</sup> 'staircase' motif. The non-coordinating pyrazine in **4**·CuBr forms an additional set of R22(8) hydrogen-bonded dimers that cross-link the 2D sheets via non-cyclic

N–H···N-ring hydrogen bonds [2.219(11) Å <sup>∠</sup>N–H···N4, 177.7(8)◦], into a 3D structure shown in Figure 3a. Although **4**·CuBr and **5**·CuBr lack the intra-chain cyclic N–H···Br–Cu HBs (see Figure S4) found in **1**·CuBr, **2a**·CuBr, and **6**·CuBr, a network of non-cyclic inter-chain N–H···Br–Cu and C–H···Br–Cu HBs are established between neighboring chains in **5**·CuBr or the non-complexed pyrazine in **4**·CuBr. Overall, the N–H···Br–Cu and C–H···Br–Cu HBs together with the C–Cl···Br–Cu, and C–Cl···Cl–C XBs stabilize the polymeric μ3-bromo 'staircase', and further cross-link the laminar 2D polymeric sheets into complex 3D structures. Notwithstanding the structural similarities between **4**·CuBr and **5**·CuBr, as determined by single-crystal X-ray diffraction, the solid-state structure of **4**·CuBr could not be demonstrated in the bulk material, as determined by powder X-ray diffraction. Full pattern Pawley analysis of **4**·CuBr suggests that the bulk material is effectively isomorphous with **6**·CuBr (See Table S6 and Figure S8). This suggests that additional polymorphs may exist where the Cu(I) center is coordinated by the sterically more congested N1 in **4**. This possibility has also been observed in **3**·CuBr, and polymorphism has been demonstrated in **2**(**<sup>a</sup>**–**<sup>c</sup>**)·CuBr, as described below. Despite repeated attempts, under different conditions, alternative single-crystal structures of Cu(I) complexes of **4** have still not been realized.

**Figure 2.** The herringbone packing arrangemen<sup>t</sup> of the discrete 1D polymeric chains of (**a**) **1**·CuBr, (**b**) and **2a**·CuBr. The red- and black- dotted lines represent hydrogen and halogen bonds, respectively.

**Figure 3.** The R22(8) hydrogen-bonded ring motif realized in the crystal packing of (**a**) **4**·CuBr, and (**b**) **5**·CuBr. Red dotted lines are N–H···N R22(8) and N–H···Br–Cu hydrogen bonds, and black are C–Cl···Cl–Cu halogen bonds.

The three polymeric structures **2b**, **2c**·and **3**·CuBr all contain bridging μ2-*N*,*N'* pyrazine ligands, and display the three (CuBr)n structural motifs outlined in Figure 1. Complex **2b**·CuBr crystallizes in the orthorhombic space group *Pbcn* as amber, prismatic blocks (See Figure S1a). In this complex, the pyrazine ligands are μ2-*N*,*N'* bridging between Cu2Br2 rhomboid dimer units to afford a distorted tetrahedral coordination geometry at the Cu(I) ion formed by two symmetry-equivalent bromines, and the N1- and N4-atoms of two different pyrazine ligands. This leads to a 2D dimensional honey-comb sheet that propagates in the *ab* plane, as illustrated in Figure 4a. The honey comb structure is similar to the [CuCl(μ-2,5-dimethylpyrazine-*<sup>N</sup>*,*N*)]n structure reported in the literature [77]. Adjacent layers are held together by C5–Cl···Br–Cu [3.5105(7) Å, ∠Br···Cl–C = 161.83(8)◦] XBs and N–H···Br–Cu [2.6402(2) Å, ∠Br···H–N = 159.28(14)◦] HBs, as given in Figure 4b. Moreover, these HBs and XBs, together with steric effects, prevent the formation of a catenated μ-bromo-Cu<sup>I</sup> motif, as in the staircase polymers previously mentioned.

**Figure 4.** (**a**) 2-D Honey comb motif of **2b**·CuBr, and (**b**) 3-D packing unit displaying hydrogen (red dotted lines) and halogen bonds (black dotted lines) between 2-D sheets.

The bright yellow, needles of **2c**·CuBr crystallize in the monoclinic space group *P*21/*<sup>n</sup>*. The asymmetric unit consists of a Cu(I) ion; one bromide and one half of a pyrazine ligand. The C5-chlorine and C2-amine substituents exhibit a positional disorder with 50:50 occupancies. The distorted tetrahedral coordination environment of Cu(I) consists of one pyrazine nitrogen and three symmetry equivalent bromide anions. Ligand **2** is μ2-*N*,*N'* bridging to yield a 2D polymeric sheet containing the catenated (μ3-bromo)-Cu<sup>I</sup> 'staircase' chain structure, as shown in Figure 5. The polymeric structure of **2c**·CuBr is similar to that found in the 2D coordination polymer of 2-aminopyrazine with CuI (i.e., [Cu2I2(2-aminopyrazine)]*n*) [78] and [2(μ-2,5-dimethylpyrazine-*<sup>N</sup>*,*N*)Cu2]n where X = Br, I [77]. The adjacent sheets are linked by N–H···Br–Cu [2.6054(17) Å, ∠Br···H–N = 175(4)◦] interactions between the C2-amino group and a Cu(I)-bound bromide. As a result of positional disorder, the C–Cl bond length associated with the 50% occupancy is close to the standard C–Cl bond distance, and results in a weak C5–Cl···Br–Cu [3.582(15) Å, ∠Cl···Br–Cu = 103.9(9)◦] 'contact'. These weak interactions are likely triggered by the N–H···Br–Cu interactions between the neighboring molecules in this laminar structure. Due to the disorder, there are possible HB and XB interactions that could alternate along a plane that passes through the amino- and chloro-substituents of the pyrazines of adjacent sheets, thus forming moderately short Cl···Cl halogen contacts with RXB = 0.88. These HBs and XB interactions

further supports the catenated (μ3-bromo)-Cu<sup>I</sup> 'staircase' motif as a common structural feature in these complexes.

**Figure 5.** (**a**) Here, 2D polymeric sheet structure of **2c**·CuBr, and (**b**) the hydrogen and halogen bonds (black dotted lines) between sheets in the laminar structure. Note: the disordered atoms were not omitted to reflect the discussion.

The complex **3**·CuBr, crystallized in the monoclinic space group *P*21/*<sup>n</sup>*. The distorted tetrahedral coordination environment of the Cu(I) is composed of two symmetry-related bromides and two crystallographically different N-atoms from two distinct pyrazine ligands. The Cu(I) ions are bridged by μ2-*N*,*N'* pyrazine ligands, forming 1D linear chains that run along the *c*-axis as illustrated in Figure 6a. These chains are further linked into a 2D sheet structure by polymeric (μ2-bromo)-CuI-Br chains along the *a*-axis. The 2D layers are not flat, but instead adopt a slightly corrugated pattern because of the ∠N–Cu–N = 136.6(4)◦ at the distorted tetrahedron at the Cu(I) ion, as shown in Figure 6b. Two types of interactions assemble the layers into a 3D structure. First is the hydrogen bonding between a C2-amino group and Cu(I)-bound bromide from an adjacent layer [2.3615(10) Å, ∠Br···H–N = 164.2 (7)◦; 2.4846(12) Å, ∠Br···H–N = 158.9(6)◦], to afford a R22(8) ring motif from a set of two N–H···Br HBs. The second interaction is C6–Cl···Br–Cu halogen bonds [3.527(3) Å, ∠Br···Cl–C = 158.8(4)◦] between a chlorine on the pyrazine ring and bromide coordinated to Cu(I) on an adjacent layers, as shown in Figure 6b. Similar to the (μ3-bromo)-Cu<sup>I</sup> 'staircase' motif, the μ2-bromo polymeric chains found in **3**·CuBr is fully stabilized by the network of HB and XBs, and is likely adopted because of the steric demands of the N1 coordination, compared to the more accessible N4-coordination mode in ligand **3**.

**Figure 6.** (**a**) Here, 2D Layered structure of **3**·CuBr, and (**b**) 2-D layers stack viewed along the a-axis. Red dotted lines are N–H···Br–Cu hydrogen bonds, and black are C6–Cl···Br–Cu halogen bonds.
