Solvated copper(I) hexafluorosilicate π-complexes based on [Cu2(amtd)2]2+ (amtd = 2-allylamino-5-methyl-1,3,4-thiadiazole) dimer

[Cu2(amdt)2]SiF6·C6H6 and [Cu2(amdt)2(H2O)2]SiF6·CH3CN·2H2O (amdt = 2-allylamino-5methyl-1,3,4-thiadiazole) were obtained by alternat ing-current electrochemical synthesis, starting from water–acetonitrile–benzene mixtures c ontaining 2-allylamino-5-methyl-1,3,4thiadiazole and CuSiF 6·4H2O. The electrochemical reduction of the saturated c opper hexafluorosilicate water solution beneath the neatl y poured layer of acetonitrile-benzene amdt solution resulted in the formation of crystall ine [Cu2(amdt)2]SiF6·C6H6. The initial stirring of the same mixture before subjecting it t o he electrochemical reduction resulted in the formation of [Cu2(amdt)2(H2O)2]SiF6·CH3CN·2H2O. A sluggish hydrolysis of the acetonitrile over 2 years in a closed test tube wit h [Cu2(amdt)2]SiF6·C6H6 crystals in a mother liquor resulted in the formation of [Cu 2L2(H2O)2]SiF6·CH3CONH2·2H2O. All the compounds were studied using X-ray single-crystal d iffraction and Raman spectroscopy. The molecular structures and the Raman spectra of t he compounds were discussed on the basis of computational modeling with the DFT/B3LYP/ cc-pVDZ method.

The 1,3,4-thiadiazoles are well known as efficient building blocks for the crystal engineering of metal-organic complexes [1][2][3]. Soft Lewis acids such as copper(I) have a good affinity for the thiadiazole core and are able to engage in an effective interaction with the C=C bond; therefore, compounds containing both a 1,3,4-thiadiazole ring and a flexible allyl group appear to be suitable for crystal engineering. Despite the fact that allyl derivatives of such a thiadiazole have been known since the 19th Century, crystallographic data on its π-coordination behavior towards transition-metal ions has appeared only recently.
Generally, π-complexes based on the Cu 2 SiF 6 salt appear only rarely. Only eight entries corresponding to such compounds were found in the Cambridge Structural Database [5][6][7][8][9][10]. The fact that just a single compound with the direct Cu + -SiF 6 2bond is known [9], indicates that the SiF 6 2moiety behaves as a weakly bonded anion with respect to the Cu + ion, making the univalent copper cation easily accessible to ligands. The strong affinity of the hexafluorosilicate anion for water frequently leads to the presence of water or other solvent molecules in the crystal structure. For example, in the structure of [Cu 2 (amdt) 2 (H 2 O) 2 ]SiF 6 ·2.5H 2 O there are two types of water molecules, i.e., bound to the metal center or attached to the SiF 6 2anions by strong hydrogen bonds [10].
In the present work we report on three new π-coordination compounds containing six-membered Cu 2 N 4 cycles formed by two copper(I) ions and two amdt ligands, of reflns and N p = no. of refined parameters.
The crystal data and refinement results are summarized in Table 1 [10] and in the structure of copper(I) π-complexes with 5-(S-allyl)-1H-tetrazole derivatives, where the copper(I) centers connect two of the most nucleophilic N(3) and N(4) atoms in the adjacent tetrazole rings [11,12]. The copper(I) ion has a trigonal pyramidal environment. Its equatorial plane is arranged by the N atom and the C=C bond from the N-allyl group from one ligand moiety and another N atom from the adjacent thiadiazole core. The next feature of the structure discussed is the presence of a fluorine atom from the SiF 6 2- , is preferentially bonded with Lewis acids stronger than the Cu + , i.e., hydrogen atoms.
[Cu 2 (amdt) 2 ]SiF 6 ·C 6 H 6 appears to be the second reported exception to the general tendency.
The first example of a direct Cu + -F(SiF 6 2-) bond was reported only few years ago [9]. Here we should note that even a single fluoride anion forms a direct bond with the Cu + center, as was observed in the crystal structure of tris(triphenylphosphane) fluorido copper(I) [14,15].
Probably the reason for the formation of such a bond in [Cu 2 (amdt) 2 ]SiF 6 ·C 6 H 6 is the absence of water in the benzene layer, wherein apparently the reaction occurred. Each SiF 6 2anion acts as a trans-bridge, being bound to two metal centers from different dimers, resulting in the formation of infinite chains. The third, and possibly the most intriguing, peculiarity of this structure is the presence of benzene molecules, included between the above-mentioned chains ( Fig. 2; for bond lengths see Table 2). Each benzene ring is located between the two Cu 2 N 4 cycles with rather short plane-plane distances of 3.6 Å. The centroid of the benzene ring is located roughly against the N4 atom from the Cu 2 N 4 ring (N4…Cg 3.507 Å), and the plane of the benzene ring is very slightly (3.9 o ) tilted with respect to the metal-organic planes. The value of 3.757(4) Å for the π-π stacking interactions between the benzene and imidazothiadiazole ring systems was reported recently [16]. To the best of our knowledge, there are only two known copper(I) compounds involving the Cu-F (PF 6 -) bond and solvated by benzene [17], and only one reported structure of a solvated-by-benzene copper(I) π-complex with a fluorine-containing anion [18].
From the Dewar-Chatt-Duncanson concept [19] we know that the Cu-(C=C) bond The main difference between [Cu 2 (amdt) 2  Only one water-molecule orientation is shown (see text below). Table  4.  (Fig. 6). The SiF 6 2anion forms, besides O-H···F, a pair of weak N-H···F bonds. The most interesting peculiarity of the discussed structure is the presence of an uncoordinated acetonitrile molecule, disordered in an unusual manner: two half-occupied nitrile groups are nearly perpendicularly oriented and bound to the common methyl group (Fig. 7). The acetonitrile molecules are located between two dimers, close to the "empty" side of each dimer. The appearance of the non-bonded acetonitrile in the crystal structure of the copper(I) complexes containing a fluorinated anion was described earlier [24].  Table 5.

Geometry optimization
Computational modeling was performed on the [Cu 2 (amdt) 2 ] 2+ cation, since it is the element that is present in all the structures and the residual fragments, i.e., the SiF

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18  (2) The equilibrium geometries obtained through the optimization appear to be close to giving the cation a saddle-shaped conformation. For α-Cu 2 L 2 no such changes were observed.

Raman spectroscopy
The Raman spectra, calculated from the optimized geometry, had no imaginary frequencies, indicating that the equilibrium geometry was determined correctly. Simulated spectra were used for the detailed vibrational band assignments of the experimental spectra.
In our previous experience with Cu + coordination compounds [25] it was established that a Raman spectra could be measured directly in the air without decomposition; however, the laser power has to be reduced to 1.7 mW using a density filter. The recorded Raman spectra for all of the compounds are shown at Fig. 9; the part 1750-2700 cm -1 is omitted for convenience. Among all the spectra, only the C-N stretching band from the acetonitrile, which is present in the compound [Cu 2 (amdt) 2 (H 2 O) 2 ]SiF 6 ·CH 3 CN ·2H 2 O, emerged at 2245 cm -1 (see Fig. S1, Supplementary materials). The spectra are relatively complicated due to the number of bands appearing from the organic ligand. For a clearer representation, the bands with relative intensities and assignments that are based on a simulated spectrum are listed in Tables 6 and 7. The computed bands are omitted in Table 7 [26], acetamide, in the structure of the compound II [27], and acetonitrile, for compound III [26], we referred to the literature data. For the assignments of the bands resulting from the SiF 6 2anion, we referred to the article by A. Ouasri et al. [28].

Conclusions
We report on synthesis and characterization of three novel copper(I) coordination compounds, i.e., [Cu 2 (amdt) 2 ]SiF 6 ·C 6 H 6 , [Cu 2 (amdt) 2  Furthermore, even crystallization in the presence of acetonitrile, which is known to have a strong affinity for Cu + ions, retains these dimers and leads to a structure with non-bonded CH 3 CN molecules embedded into the lattice.
Ab-initio quantum-chemical calculations using the DFT/B3LYP/cc-pVDZ approach were used to obtain the equilibrium geometries of two types of [Cu 2 (amdt) 2 ] 2+ cations, which were marked as the α-form and β-form for convenience. The difference between these two forms is in the positioning of the allyl groups (see the section Results and Discussion for more details).
Optimized geometries were applied as the benchmarks for the Raman spectra calculations. The band assignments of the recorded Raman spectra were made on the basis of the calculated spectra.

Electrochemical syntheses
The syntheses were performed in small (~10-mm i.d., 5-ml volume) test tubes. A copper wire was wrapped into a spiral of 1-cm diameter, and a straight copper wire was placed inside the spiral.
These copper electrodes were inserted into cork and immersed in solutions containing copper(II) hexafluorosilicate and the ligand. Syntheses were performed using an alternating current of 50 Hz applied to both wire electrodes.
The main advantage of such a synthesis route is the one-step growth of high-quality single crystals suitable for X-ray structure experiments. Also, this technique allows the use as starting materials easily accessible and stable Cu 2+ salts instead of the sensitive-to-oxidation (or even unknown) Cu + derivatives. The electrochemical process does not require additional reducing agents, making the reaction products free from undesirable contaminants.
The main disadvantage of this method is the relatively low yield caused by an abrupt decrease of the electrical conductivity of the electrode(s) due to the appearance of crystalline products at the electrode surface.

Synthesis of [Cu 2 (amdt) 2 ]SiF 6 ·C 6 H 6
A solution of 2-(allyl)-amino-5-methyl-1,3,4-thiadiazole (1.3 mmol, 0.20 g) in a mixture of 3.1 mL of acetonitrile and 0.8 mL of benzene was carefully layered over a 0.8-mL water-saturated solution of CuSiF 6 ·4H 2 O (into 5 mL test-tube). The upper layer was transparently yellowish, the lower one was cyan colored. Darkening of the lower part of the acetonitrile-benzene layer, possibly because of diffusion from the water layer, was observed, but it disappeared after the application of the tension. Then copper-wire electrodes in the cork were inserted and colorless crystals of the compound appeared directly on the electrodes under the alternating-current tension [30] (frequency 50 Hz) of 0.60 V after 3 days.

Raman spectroscopy
The Raman spectra were measured on crystals of the coordination compounds and ligand powder with a Horiba Jobin-Yvon LabRAM HR spectrometer using the 632.81-nm excitation line of a He-Ne laser with a power of 17 mW. To avoid decomposition of the sample, a density filter was applied to reduce the power of the laser to 1.7 mW. An Olympus ×50 long-distance lens was used.
The spectra were obtained by accumulating 50 scans with an integration time of 5 seconds directly in the air at room temperature. Prior to recording, the spectrometer was calibrated using a Si polycrystalline plate as a standard with a characteristic band at 520.6 cm -1 .

Computational details
For the interpretation of the measured vibrational spectra, quantum-mechanical methods were applied. All the calculations were performed using the DFT with the B3LYP functional [41,42] and cc-PVDZ basis set using the GAMESS(US) program package [43]. All the calculations were carried out using the C1 symmetry group. Effective core potentials (ECPs) were additionally applied for the copper atoms. In order to reduce the complexity of the computation and avoid free shifts of the weakly bonded ligands, which will no doubt contribute to the equilibrium geometry and spectrum, only the [Cu 2 (amdt) 2 ] 2+ cation was modeled. Initial geometries of the dimeric cations, obtained by an X-ray crystal structure determination of the compounds  Highlights: • Three novel copper(I) hexafluorosilicate π-complexes with amdt ligand are prepared • Single crystal X-ray diffraction revealed two geometries of [Cu 2 (amdt) 2 ] 2+ • Chelate-bridging role of the ligand leads to extreme stability of [Cu 2 (amdt) 2 ] 2+ • Raman spectra were analyzed on the basis of quantum-chemical calculations