Insight into systematic formation of hexafluorosilicate during crystallization via self-assembly in a glass vessel

Formation of the unexpected hexafluorosilicate (SiF62−) anion during crystallization via self-assembly in glassware is scrutinized. Self-assembly of M(BF4)2 (M2+ = Cu2+ and Zn2+) with tridentate N-donors (L) in a mixture solvent including methanol in a glass vessel gives rise to an SiF62−-encapsulated Cu3L4 double-decker cage and a Zn2L4 cage, respectively. Induced reaction of CuX2 (X− = PF6− and SbF6−) instead of Cu(BF4)2, with the tridentate ligands, produces the same species. The formation time of SiF62− is in the order of anions BF4− < PF6− < SbF6− under the given reaction conditions. The SiF62− anion, acting as a cage template or cage-to-cage bridge, seems to be formed from the reaction of polyatomic anions containing fluoride with the SiO2 of the surface of the glass vessel.

Method 2. A methanol solution (1.0 mL) of silver(I) hexa-uorophosphate (10.1 mg, 0.04 mmol) was added to copper(II) chloride (2.7 mg, 0.02 mmol) dispersed in methanol at room temperature. The reaction mixture was stirred for 30 min, aer which, the precipitated silver chloride was ltered off. The methanol solution of Cu(PF 6 ) 2 was carefully layered onto a dichloromethane solution (2.0 mL) of L 1 (18.4 mg, 0.02 mmol). Aer 4 weeks, blue crystals suitable for single crystal Xray structure determination were obtained in a 78% yield.
Method 3. A methanol solution (1.0 mL) of silver(I) hexa-uoroantimonate (13.7 mg, 0.04 mmol) was added to a suspension of copper(II) chloride (2.7 mg, 0.02 mmol) dispersed in methanol at room temperature. The reaction mixture was stirred for 30 min, aer which, the precipitated silver chloride was ltered off. The methanol solution of Cu(SbF 6 ) 2 was carefully layered onto a dichloromethane solution (2.0 mL) of L 1 (18.4 mg, 0.02 mmol). Aer 6 weeks, blue crystals suitable for single crystal X-ray structure determination were obtained in a 66% yield. [(SiF 6 )@Zn 2 L 2 4 ](SiF 6 )$2C 4 H 8 O$4CH 2 Cl 2 A mixture of tetrahydrofuran with a methanol solution (3.0 mL/ 1.0 mL) of zinc(II) tetrauoroborate (9.6 mg, 0.04 mmol) and a dichloromethane solution (3.0 mL) of L 2 (14.5 mg, 0.04 mmol) were le at room temperature, and aer 2 weeks, blue crystals suitable for single crystal X-ray structure determination were obtained in a 67% yield. m. p. 260 C (dec). Anal. Calcd for: C, Single crystal X-ray structure determination The X-ray diffraction data for [(SiF 6 ) 2 @Cu 3 L 1 4 ](SiF 6 )$16CH 3 OH were measured at 100 K with synchrotron radiation (l ¼ 0.76000Å ) on a Rayonix MX225HS detector at 2D SMC with a silicon (111) double-crystal monochromator (DCM) at the Pohang Accelerator Laboratory (PAL), Korea. The PAL BL2D-SMDC program 50 was used for data collection (detector distance: 66 mm, omega scan: Du ¼ 3 , exposure time: 1 s per frame), and HKL3000sm (ver. 703r) 51 was used for cell renement, reduction, and absorption correction. X-ray data on [(SiF 6 )@Zn 2 -L 2 4 ](SiF 6 )$2C 4 H 8 O$4CH 2 Cl 2 were collected on a Bruker SMART automatic diffractometer with graphite-monochromated Mo Ka radiation (l ¼ 0.71073Å). Thirty-six (36) frames of 2D diffraction images were collected and processed to obtain the cell parameters and orientation matrix. The data were corrected for Lorentz and polarization effects. The absorption effects were corrected using the multi-scan method (SADABS). 52 The structures were resolved using the direct method (SHELXS) and rened by full-matrix least squares techniques (SHELXL 2018/ 3). 53 The non-hydrogen atoms were rened anisotropically, and the hydrogen atoms were placed in calculated positions and rened only for the isotropic thermal factors. The crystal parameters and procedural information corresponding to the data collection and structure renement are listed in Table 1.

Synthesis aspects
Self-assembly of Cu(BF 4 ) 2 with L 1 in a mixture of methanol and dichloromethane produced crystals consisting of [(SiF 6 ) 2 @-Cu 3 L 1 4 ](SiF 6 )$16CH 3 OH suitable for single crystal X-ray structure determination aer 2 weeks, as shown in Scheme 1. Moreover, the reactions with Cu(PF 6 ) 2 and Cu(SbF 6 ) 2 instead of Cu(BF 4 ) 2 gave rise to the same product aer signicant durations, specically 4 and 6 weeks, respectively. The reaction time may be owed to the stability of the BF 4 À < PF 6 À < SbF 6 À anions under the given reaction conditions. As expected, self-assembly of Cu(BF 4 ) 2 with L 1 in the presence of (NH 4 ) 2 SiF 6 produced blue crystals within 1 day. In order to investigate the metal effects, self-assembly of Zn(BF 4 ) 2 instead of Cu(BF 4 ) 2 with L 2 was performed, which generated crystals of [(SiF 6 )@Zn 2 L 2 4 ](SiF 6 )$2C 4 -H 8 O$4CH 2 Cl 2 aer 2 weeks, indicating that the central metal cation is not a signicant factor in anion transformation in a glass vessel. For all of the reactions, the SiF 6 2À anion acted  (2) Zn (1) 180.0 a x À 1, y + 1, z, x + 1, y À 1, z. b x + 1, y, z À 1. c Àx + 1/2, Ày + 1/2, z either as a template by which to form the cage products or as a cage-to-cage bridge. Both crystalline products are stable under anaerobic condition at room temperature. The crystals are insoluble in common organic solvents such as acetone, chloroform, toluene, benzene and tetrahydrofuran, but are partially dissociated in dimethyl sulfoxide and N,N-dimethylformamide. Their compositions and structures were conrmed by elemental analyses, IR-spectral analysis ( Fig. S1 and S2 †), thermal analysis (Fig. S3 †), 1 H NMR (Fig. S4, as dissoociated in Me 2 SO-d 6 †), and single-crystal X-ray crystallography. The characteristic IR bands of SiF 6 2À were easily assigned. The thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC) showed [(SiF 6 ) 2 @-Cu 3 L 1 4 ](SiF 6 )$16CH 3 OH and [(SiF 6 )@Zn 2 L 2 4 ](SiF 6 )$2C 4 H 8 -O$4CH 2 Cl 2 to be stable up to 277 and 257 C, respectively. Evaporation of the solvate molecules of the two kinds of crystals occurred in the 10-60 and 20-135 C temperature ranges, respectively (Fig. S3 †).

Discussion
Formation of SiF 6 2À during self-assembly of more than 2 weeks' duration requires some explanation, because the anion was not added to the self-assembly solution in a glass vessel. It should Scheme 1 Construction of cages (a) via SiF 6 2À formation (b) during self-assembly in a glass vessel. be noted that the SiF 6 2À anion replaces the starting BF 4 À anion in the self-assembly reaction and that it is formed aer extracting Si 4+ from the glass vessel in methanol solvent-a rare but well-known phenomenon. 55 The generation of SiF 6 2À by slow release of Si 4+ from the glass vessel in the methanol solution was clearly conrmed, and the resultant anions were separated as counteranions. The SiF 6 2À anions appear to play signicant roles as both encapsulator and bridge in the construction of the cage skeleton. The stable anion, by contrast, forms as a consequence of the hydrolysis of the BF 4 À anion and subsequent reaction of F À with the Si-O moieties of the glass vessel 56,57 in which a methanol solution of M(BF 4 ) 2 is kept for self-assembly or crystallization. The formation of SiF 6 2À seems to be promoted by the presence of a small quantity of H 2 O in addition to a mixture solvent including methanol. Formation of SiF 6 2À from BF 4 À and PF 6 À in a glass vessel in water has been observed by other groups, [30][31][32]56 but that of SiF 6 2À from SbF 6 À heretofore has not been recorded. The present self-assembly of MX 2 (M 2+ ¼ Cu 2+ , Zn 2+ ; X À ¼ BF 4 À , PF 6 À , SbF 6 À ) with each tridentate ligand in each glass vessel systematically transformed BF 4 À , PF 6 À or SbF 6 À into the SiF 6 2À anion according to the following eqn (1): Of course, formation of SiF 6 2À from CF 3 SO 3 À (C-F compound) could not be carried out in a similar reaction performed in a polypropylene vessel instead of a glass one; that is, the reaction did not yield crystals containing the SiF 6 2À anion.
The presence of SiF 6 2À in Me 2 SO-d 6 was conrmed by 19 F NMR spectra (Fig. S6 †). The SiF 6 2À occurred as a singlet at d ¼ À134 ppm. Spectra taken of the sample preparation one month later indicated that almost all of the original BF 4 À anions had been converted to SiF 6 2À anions. As regards the formation of SiF 6 2À from PF 6 À and SbF 6 À anions, PF 6 À (À70.8 ppm) and SbF 6 À chemical shis (À119.9 ppm) disappeared aer 4 and 6 weeks, respectively, and appeared at the position of the SiF 6 2À anion. As for construction of the coordination architecture, insight into the stability and geometry of a specic anion as a template is useful to the tuning of the shape and topology of coordination species. In the present study, the encapsulated and bridged SiF 6 2À anions helped to stabilize the crystal structure of the cages. Certainly, transformation of polyatomic anions and their role is a topic worthy of further investigation.

Conclusions
In summary, transformation of polyatomic anions BF 4 À , PF 6 À , and SbF 6 À into SiF 6 2À during self-assembly or recrystallization in a glass vessel was systematically conrmed. This shows that the surface of regular laboratory glassware should be given serious consideration for long-duration reactions or selfassembly with F À species. Further studies on other means of anion-bridging or -encapsulation of cage units will contribute to the development of task-specic polyatomic anions for recognition as well as environmental molecular materials such as adsorbents and sensing materials.

Conflicts of interest
There are no conicts to declare.