**2. Results and Discussion**

### *2.1. Synthesis of the Complexes*

The study of the reactivity in the CuCl2/BMTTM or BMTTE system was performed using different stoichiometric ratios and four synthetic methods: stirring at room temperature, diffusion, reflux under microwave irradiation and hydro/solvothermal techniques. Conventional synthesis at room temperature afforded the compounds in good yields. However, hydrothermal and microwave methods, providing the same compounds in lower yields, were used to synthesized compounds as pure materials and as single crystals. Acetonitrile was used as the solvent for the reactions due to the better solubility of the ligands in this solvent. The synthetic conditions allowed the preparation of the crystalline complexes **1**, **1**·**solv**, **2** and **3** (Scheme 1).

In the reactions of copper(II) chloride with BMTTM, compound **1** was obtained in good yield with a 1:1 metal/ligand ratio by diffusion and by stirring at room temperature. However, when the reaction was performed under microwave irradiation, in addition to **1**, some crystals of the pseudopolymorph **1**·**solv** (4CH3CN) were also obtained. The hydro/solvothermal methods tested at different metal/BMTTM ligand ratios at different temperatures between 70 and 160 ◦C in all cases yielded the copper(I) 2D

coordination polymer **Cu(mtS)** (mtS = 1-methyl-1*H*-tetrazole-5-thiolato) [15], which indicates that cleavage of the BMTTM ligand had taken place under these synthetic conditions.

Reactions with BMTTE were conditioned by its low solubility in all of the solvents tested and, as a consequence, a copper complex could not be isolated by diffusion due to the rapid crystallization of BMTTE [16,17]. Copper(II) compound **3** was obtained as an orange crystalline powder upon stirring at room temperature using a 1:1 molar ratio, and as orange single crystals by reaction under microwave irradiation using a 4:1 metal/ligand molar ratio. The copper(I) complex **2** was obtained under hydrothermal conditions at 160 ◦C with a 3:1 metal/ligand ratio. This hydrothermal reduction of copper(II) to copper(I) was previously observed in the preparation of copper polyoxometalates of BMTTM and BMTTE from copper(II) acetate, although the authors indicated that metal/ligand ratios greater than 10:1 are required for this reduction to occur [11–14].

**Scheme 1.** Summary of the synthetic routes for the complexes and the Cu(II):L stoichiometric ratios, temperature and solvent used where relevant (RT: room temperature; MW: microwave).

### *2.2. Structural Studies: General Features*

All of the compounds described here were isolated as single crystals and their structures were elucidated by X-ray diffraction. The combination of the flexible polydentate ligands with different chlorocuprate clusters resulted in the formation of 1D coordination polymer chains (**1** and **1**·**solv**) or a 2D coordination layer (**3**). Moreover, the reduction of copper(II) to copper(I) produced a stable discrete tetrameric Cu(I) coordination compound (**2**). The main structural features for each compound are provided in Table 1. The structures can be deconstructed into two components: trinuclear units {Cu3Cl6N6O2} in **1** and **1**·**solv**, tetranuclear units {Cu4Cl4N6} in **2**, and dinuclear units {Cu2Cl4N} in **3** as inorganic chlorocuprate building clusters, and the corresponding flexible BMTTM and BMTTE tetrazole bridging ligands.

The two methyl tetrazole groups in BMTTM and BMTTE are separated by a flexible organosulfur spacer that allows rotation around the C–S and C–C bonds to adjust the direction of the coordination nitrogen atoms. It is therefore apparent that the rigid and geometrically well-defined structures of the inorganic units with the flexibility and the potential N-hexacoordination ligands are essential to achieve the structural diversity observed in these systems.

Compounds **1**, **1**·**solv** and **3** are polymeric coordination compounds. The layers of **3** are formed by chloride-bridged Cu(μ-Cl)2 chains and bridging N-donor ligands in a second dimension that act as cross-linking ligands. The copper-chloride chain is based on the repetition of the dinuclear Cu2Cl2 unit, as shown in Table 1. The polymeric chains of **1** and **1**·**solv** incorporate trinuclear Cu3Cl6 units and organic subunits in an alternate manner.

**Table 1.** Main structural features.

Compound **2** is a discrete tetrameric compound based on a stair-step Cu4Cl4 cluster coordinated by two BMTTE ligands in an octahedral {Cu4Cl4N6} motif, which has been less widely studied than other copper halide clusters such as cubane organizations. In this octahedral motif, a tetranuclear copper core defines the basal plane of an octahedron with two capping μ3-chloride atoms in the apical positions and bridging μ2-chloride and nitrogen BMTTE atoms along the meridian positions [18].

### *2.3. Crystal Structures of* **1** *and* **1**·**solv**

Complex **1** and the solvate **1**·**solv** crystallize in the monoclinic *P21*/*n* and triclinic *P-1* space groups, respectively, and they could be considered pseudopolymorphs [19] since they have the same crystalline form, although **1**·**solv** crystallizes with acetonitrile molecules trapped within the crystal network. Significant structural parameters for compounds **1**–**1.solv** are listed in Table 2 and crystal structure and refinement data are listed in Table S1. Both structures are 1D chains based on a {Cu3Cl6} cluster and the BMTTM ligand, as shown in Figure 1. The three copper atoms in the {Cu3Cl6} unit are aligned with a Cu1–Cu2–Cu1 angle of 180◦ [20]. The conformation of the outer two coppers is identical. The Cu1··· Cu2 distance is 3.903 Å in **1** and slightly shorter (3.842 Å) in **1**·**solv**.


**Table 2.** Selected bond lengths/Å and angles/ ◦.

Symmetry code: (**1**) #1 −x + 2, −y + 1, −z + 1; #2 −x + 1, −y + 2, −z + 1; (**1**·**solv**) #1 −x + 1, −y + 1, −z + 2; #2 −x + 1, −y + 2, −z + 2.

The two bridges of chloride and N-N tetrazole atoms between the Cu1 and Cu2 lead to the formation of an unusual planar {Cu3Cl6N6O2} cluster. These units are connected by two BMTTM ligands to form infinite tapes, as shown in Figure 1.

**Figure 1.** Coordination environments and polymeric chains of **1** and **1**·**solv.**

The Cu1 center is pentacoordinated and the value of the Addison parameter [21] τ of 0.25 (0.28 in **1**·**solv**) indicates a square pyramidal coordination geometry around this copper atom, with the two nitrogen atoms and the two terminal chloride atoms in the *trans* positions of the pyramid base and a chloride atom acting as a bridge between Cu1 and Cu2 at the apex position. The Cu–Cl bond lengths with the chloride terminal groups are in the range 2.27–2.29 Å and the bridging chloride atom in the apical position is located at a longer distance, with Cu–Cl distances of 2.5477(12) Å in **1** and 2.5119(4) Å in **1**·**solv**.

The Cu2 atom is in an elongated octahedral environment and it exhibits the expected Jahn–Teller distortion, with four short metal–ligand bonds (Cu–Ow and Cu–Cl) and two long bonds with nitrogen atoms of BMTTM bridging ligands [Cu2–N distances of 2.561(4) and 2.4137(11) Å in **1** and **1**·**solv**, respectively]. Here, it is worth highlighting the equatorial coordination position of the water molecule, which contrasts with the typical axial position in similar systems.

The BMTTM ligand uses three nitrogen atoms of the two tetrazole rings to coordinate three metal centers, as shown in Table 2, with a bridging role that results in the formation of a tape, as shown in Figure 1, with 16-membered macrocycles (Cu2N4S4C6). This macrocyclic motif has previously been observed in complexes with other thioether N-donor ligands [22]. This coordination mode (μ3-1κN3:2κN4:3κN4) of BMTTM was not observed in the copper-POM complexes, in which the ligand had a bidentate chelating coordination mode with one, two or three copper cations [12,14]. The intermetallic distances through the BMTTM-bridge are 6.97 Å for compound **1** and 6.69 Å for **1**·**solv**. Moreover, the intramolecular N··· N distance is shorter in **1**·**solv** (6.48 Å) than in **1** (6.70 Å). On comparing the two structures, it is evident that BMTTM acts as a semirigid ligand, which shrinks due to the presence of the gues<sup>t</sup> molecules in **1**·**solv** but retains the same connectivity and geometric environment in both cases. In accordance with this situation, the value of the C–S–C–S torsion angles are 81.78◦ and 83.20◦ in **1** and 77.14◦ and 84.45◦ in **1**·**solv**. The chains are achiral and incorporate molecules of BMTTM with opposite helicity in each macrocycle unit.

### *2.4. Crystal Structure of* **2**

The Cu(I) compound **2** crystallizes in the *P21*/*n* space group with an inversion center located between the Cu2 central atoms. Significant structural parameters are listed in Table 3, and crystal structure and refinement data are listed in Table S1. Four Cu(I) atoms and four chloride atoms form a stair-like [Cu4Cl4] cluster core with each of the two BMTTE ligands anchored at each end of the stair through two Cu(I) centers, as shown in Figure 2 [23,24]. In the cluster, the Cu2··· Cu2 distance of 2.7651(5) Å is slightly shorter than the sum of the van der Waals radii of two Cu atoms (2.80 Å), thus indicating attractive Cu··· Cu interactions, but the Cu1··· Cu2 distance of 2.9556(3) Å suggests that copper–copper interactions are not present [25].


**Table 3.** Selected bond lengths/Å and angles/ ◦

.

Symmetry code: (**2**) #1 −x + 1, −y + 2, −z + 1; (**3**) #1 −x + 1/2, y + 1/2, −z + 3/2; #2 −x + 1/2, y − 1/2, −z + 3/2.

**Figure 2.** Molecular structure of **2** showing the hydrophobic contour of the tetrameric unit.

The chloride ligands display μ3-Cl1 and μ-Cl2 bridging modes. Each Cu(I) center is tetrahedrally coordinated—Cu1 by two BMTTE nitrogen atoms and Cl1 and Cl2 atoms, and Cu2 by one BMTTE nitrogen atom, two μ3-Cl1 and one μ-Cl2 atoms, so the values of the τ4 parameters [26] of 0.80 for Cu1 and 0.77 for Cu2 indicate a fairly regular tetrahedral geometry. Cl2, which is bonded to adjacent copper atoms with Cu–Cl distances of 2.4794(4) Å and 2.2330(4) Å, resides 0.813 Å above and below the planar copper core. The triply bonded chloride ligand connects Cu1 and Cu2 atoms with variable distances [2.4039(4) Å, 2.3556(5) Å and 2.6728(5) Å].

In compound **2**, each BMTTE molecule provides three N atoms to coordinate two copper ions, as shown in Figure 2. Thus, BMTTE acts as a bis-monodentate bridging ligand to link Cu1 and Cu2 through the tetrazole fragment and also as a bidentate chelating ligand with Cu1 to give a nine-membered ring (CuN2S2C4). This coordination mode of BMTTE has also been observed in a copper-POM 1D coordination polymer [13]. The S–C–C–S torsion angle is 168.93◦ and the two ligands in the tetramer have opposite axial chirality.

### *2.5. Crystal Structure of* **3**

The Cu(II) polymeric compound **3** crystallizes in the *C2*/*c* space group with one crystallographically independent copper atom. Significant structural parameters are listed in Table 3, and crystal structure and refinement data are listed in Table S1. A dimeric Cu2Cl4 motif with two metal cations linked by two bridging chloride atoms leads to the formation of Cu2Cl4-based inorganic chains along the *b* axis, as shown in Figure 3. The nonbonding Cu··· Cu distance through the halide bridge is 3.276 Å. These chains are bridged by bis-monodentate BMTTE ligands to yield the final 2D coordination compound. The 2D structure contains 32-membered macrometallocycles formed by (Cu3Cl4) inorganic units and two BMTTE ligands, as shown in Figure 3, with the methyl groups of each ligand molecule orientated to opposite sides.

The copper(II) cation is pentacoordinated, as shown in Figure 3, and the value of the Addison parameter τ [21] of 0.04 indicates that the coordination sphere is almost an ideal square pyramid, with four chloride atoms in basal positions (Cu—Cl distances between 2.313 and 2.298 Å) and a nitrogen atom belonging to a BMTTE molecule at the apex (Cu—N2 = 2.3981(13) Å). At a longer distance (2.854 Å) in the sixth coordination position is a nitrogen atom (N3) of an opposite ligand molecule, so that each tetrazole group coordinates a metal center and establishes a weak contact with a neighboring metal center.

**Figure 3.** Top; 2D structure of **3** showing in detail the macrometallocycle. Bottom; supramolecular arrangemen<sup>t</sup> of the layers.

The BMTTE acts as a bis-monodentate bridging (μ-<sup>κ</sup>N2) ligand. This coordination mode is different to the bis-monodentate bridging mode observed in some copper-POM complexes, where the nitrogen atom involved in coordination is N3 [11,12] and not N2. The S–C–C–S torsion angle is 180◦ and each metallocycle contains two ligands of opposite chirality to yield an achiral layer. The distance between two neighboring copper(II) ions through the BMTTE ligand is 14.585 Å and the intramolecular N··· N distance is 9.794 Å.
