Trimethylammonium Sn(IV) and Pb(IV) Chlorometalate Complexes with Incorporated Dichlorine

Abstract

Supramolecular dichloro-chlorostannate(IV) and -plumbate(IV) complexes (Me₃NH)₂{[MCl₆]Cl₂} (M = Sn (1), Pb (2)) feature dichlorine units incorporated into a halometalate framework. Both compounds were characterized by X-ray diffractometry and Raman spectroscopy.

1. Introduction

Anionic halide complexes (halometalates) [1,2,3,4,5,6,7,8,9] are being intensively investigated for years. Refs. [10,11,12,13,14,15,16,17] In the last decades, this research is strongly promoted by materials science, especially by photovoltaics where iodometalates, especially iodoplumbates(II), are widely used as light absorbers. Refs. [18,19,20,21,22,23,24] On the other hand, there are many works focusing on ability of halometalates to build supramolecular associates with di- or polyhalogens due to halogen bonding (XB), and a specific type of non-covalent interactions [25,26,27,28,29]. Although this feature was known for decades, [30,31,32,33,34] its systematic studies began rather recently; Refs. [35,36] in our reports, [37] we demonstrated that such behavior is rather common for Bi(III), Te(IV) and Sb(V) halide complexes. Simultaneously, the works by Shevelkov et al. demonstrated [38,39,40,41] that polyiodide-containing iodobismuthates commonly reveal narrow optical band gaps and, sometimes, rather high thermal stability, making such hybrids promising candidates for photovoltaic applications.

For dichlorine-containing halometalates, the very first report was published over 30 years ago. Ref. [42] It was shown that tetramethylammonium chloropalladate(IV) and –stannate(IV) readily form complexes of the general formula (Me₄N)₂{[MCl₆](Cl₂)_x}, where X ≤ 1. Surprisingly, this work remained overlooked for years. Only very recently, we demonstrated that such complexes can be formed: a) by other elements, including Te and Pb, and b) in presence of other cations. Refs. [43,44] Continuing this work, we hereby present two new dichlorine-chlorometalates—(Me₃NH)₂{[MCl₆]Cl₂} (M = Sn (1), Pb (2)).

2. Materials and Methods

All reagents were used as purchased. Caution: All experiments with Cl₂ require obligatory safety precautions—sufficient exhaust ventilation (fume hood must be used), and obligatory eye (goggles) and skin (gloves) protection. Soluble Pb(II) salts are toxic.

2.1. Preparation of 1

50 mg (0.22 mmol) of SnCl₂·2H₂O and 42 mg (0.44 mmol) of Me₃NHCl were dissolved in 4 mL of concentrated HCl at 60 °C. Then gaseous Cl₂ was bubbled through the solution at the same temperature for 10 min. After that, the flask was closed and slowly cooled to room temperature, resulting in the formation of transparent crystals of 1 within several hours. The yield was 69%. The element analysis for C₆H₂₀N₂SnCl₆ is (see Discussion): calculated, %: C, 16.00; H, 4.48; N, 6.23; found C, 15.94; H, 4.52; N, 6.29.

2.2. Preparation of 2

111 mg (0.22 mmol) of PbO and 96 mg (1 mmol) of Me₃NHCl were dissolved in 5 mL of concentrated HCl at 60 °C. Then gaseous Cl₂ was bubbled through the solution at the same temperature for 10 min. After that, the flask was closed and slowly cooled to room temperature, resulting in formation of transparent crystals of 1 within several hours. The yield was 71%. The element analysis for C₆H₂₀N₂PbCl₆ is (see Discussion): calculated, %: C, 13.38; H, 3.74; N, 5.20; found C, 13.33; H, 3.77; N, 5.27.

2.3. X-Ray Diffractometry

X-ray diffraction data for oligocrystalline samples of (Me₃NH)₂{[MCl₆]Cl₂} (M = Sn (1), Pb (2)) were collected on a Bruker D8 Venture diffractometer (PHOTON III CMOS detector, Mo IµS3.0 X-ray source, Montel mirror focused MoKα radiation λ = 0.71073 Å, N_2-flow cryostat) via 0.5° ω- and φ-scan techniques. The experimental data reductions were performed using the APEX3 suite (Bruker APEX3 Software Suite (APEX3 v.2019.1-0, SADABS v.2016/2, TWINABS v.2012/2, SAINT v.8.40a), Bruker Nonius (2003–2004), Bruker AXS (2005–2018), Bruker Nano (2019): Madison, WI, USA). The only one major crystal domain of 1 and both major domains of 2 were used for the intensity integration via SAINT. Scaling and absorption corrections of the experimental intensities were performed empirically in the medium absorber (3 odd/6 even orders for spherical harmonics, spherical correction µ·r = 0.2) and strong absorber models (7 odd/8 even OSH, µ·r = 1.2) using SADABS and TWINABS programs for 1 and 2, respectively. The structures were solved by SHELXT [45] and refined using the full-matrix least-squares by SHELXL [46] assisted with Olex2 GUI [47].

Non-H atoms for all structures were located from the electron density map and refined in the anisotropic approximation. H atoms were located from the electron difference maps and refined in a riding model with the constrained U_iso. Site occupation factors of Cl atoms of guest Cl₂ molecules, located around special positions (Wyckoff positions 6a, 32 point symmetry) were fixed as 1/3 (i.e., the guest positions are singly occupied by Cl₂ molecules). The crystallographic characteristics, experimental data, and structure refinement indicators are shown in

Table 1. Details of the XRD experiments for 1 and 2.

	1	2
Empirical formula	C6H20Cl8N2Sn	C6H20Cl8N2Pb
Formula weight	522.53	611.03
Temperature, K	150(2)	250(2)
Crystal system	Trigonal	trigonal
Space group	R–3c	R–3c
a, Å/α, °	9.4097(6)/90	9.5183(2)/90
b, Å/β, °	9.4097(6)/90	9.5183(2)/90
c, Å/γ, °	36.738(3)/120	37.3034(8)/120
Volume, Å3	2817.1(4)	2926.83(14)
Z	6	6
ρcalc, g/cm3	1.848	2.080
μ, mm−1	2.482	9.726
F(000)	1536.0	1728.0
Crystal size, mm3	0.13 × 0.08 × 0.05	0.15 × 0.15 × 0.15
Radiation	MoKα (λ = 0.71073)	MoKα (λ = 0.71073)
2θ range for data collection, °	5.47/62.95	6.55/63.03
Index ranges	–13 ≤ h ≤ 13, –12 ≤ k ≤ 13, –53 ≤ l ≤ 53	–12 ≤ h ≤ 0, 0 ≤ k ≤ 13, 0 ≤ l ≤ 54 *
Reflections collected/independent	12026/1019	25228 **/1082
Rint/Rσ	0.0438/0.0196	0.0371/0.0106
Data/restraints/parameters	1019/0/34	1082/0/34
Goodness-of-fit on F2	1.116	1.112
R1/wR2 for I ≥ 2σ(I)	0.0179/0.0388	0.0206/0.0376
for all data	0.0197/0.0394	0.0282/0.0401
Largest diff. peak/hole/e Å−3	0.23/–0.36	0.41/–0.47

. The crystallographic data and experimental details were deposited in the Cambridge Crystallographic Data Centre under the deposition codes CCDC 2154812 (1) and 2167558 (2) and can be obtained at https://www.ccdc.cam.ac.uk/structures (accessed on 10 November 2022).

2.4. Raman Spectroscopy

Raman spectra were collected using a LabRAM HR Evolution (Horiba) spectrometer with the excitation by the 633 nm line of the He-Ne laser. The spectra at room temperatures were obtained in the backscattering geometry with a Raman microscope. The laser beam was focused to a diameter of 2 μm using a LMPlan FL 50×/0.50 Olympus objective. The spectral resolution was 0.7 cm⁻¹. The laser power on the sample surface was about 0.03 mW.

3. Results and Discussion

Both complexes were prepared via bubbling of Cl₂ through HCl solution of corresponding chlorometalate(IV) (in the case of Pb, it is generated in situ during dissolution of oxide in HCl) with trimethylammonium chloride, resulting in crystals suitable for XRD. Both compounds are isostructural. There are mononuclear [MCl₆]²⁻ anions (M-Cl = 2.425–2.427 and 2.504–2.507 Å for Sn and Pb, respectively). Similar to (Me₄N)₂{[MCl₆](Cl₂)} (M = Sn, Pb) described earlier [43], the dichlorine units (the Cl-Cl bond lengths are 1.994 in 1 and 1.996 in 2, respectively) are disordered over three positions with equal occupancies so the system of Cl···Cl non-covalent interactions (Figure 1) is three-dimensional (Cl···Cl = 2.900 and 2.892 Å, M-Cl-Cl = 159.1 and 160.0°, respectively). The proximity of the measured intramolecular Cl-Cl distances to “canonical” values as well as the low anisotropy of the atomic displacements indicates the absence of significant librations of the guest molecules. The crystal packing in 1 and 2 are shown on Figure 2. Details of cation···anion interactions (figures demonstrating minor differences in NH···Cl distances) are given in Supplementary Materials.

Both complexes demonstrate poor stability while kept outside the Cl₂-containing mother liquor, and lose Cl₂, transforming into (Me₃NH)₂[MCl₆], as follows from element analysis of residues (see Experimental part). The PXRD data (Figure 3 and Figure 4) confirm that after 1 h the samples of 1 and 2 contain up to 33% of “dichlorine-free” salts (for comparison of the structural data, we used the XRD information for (Me₃NH)₂[SnCl₆] which was described earlier [48]; the (Me₃NH)₂[PbCl₆] salt was found to be isostructural).

It is worth mentioning that supramolecular complexes with halide···dichlorine non-covalent interactions are yet very rare. Apart from polychlorides extensively studied in last decade by Riedel et al. [49,50,51], the number of such examples is very limited.

Hirshfeld surface analysis of the structures of 1 and 2 is given in Supplementary Materials. The stability of compounds did not allow performance of TGA experiments; however, we succeeded in recording of Raman spectra (Figure 5 and Figure 6). The bands corresponding to the {Cl₂} unit vibrations (518–531 and 508–520 cm⁻¹, respectively) are shifted to the lower wavelengths; this is a common feature for the compound of this family [43] (for gaseous Cl₂, the bands were detected at 539, 547 and 554 cm⁻¹ [52]). There are also bands at 312, 242, 165 cm⁻¹ for 1 and 278, 218, 143 cm⁻¹ for 2 corresponding to ν₁, ν₂ (stretching) and ν₅ (deformation) vibrations in {MCl₆} octahedral units [53].

4. Conclusions

To conclude, the series of Sn and Pb dichlorine-contanining supramolecular compounds featuring Type I Cl···Cl interactions (according to the classification proposed by Metrangolo et al. [54]) was expanded by adding two new complexes. It is noteworthy that, unlike tetramethylammonium-containing relatives, 1 and 2 demonstrate poor stability. This observation confirms the crucial role of multiple cation···anion hydrogen bonds in overall stabilization of the compounds of this type. While all complexes of this family reported contained dichlorine units, we, as stated previously [43], cannot exclude the existence of compounds where other, more sophisticated polychlorine fragments would be stabilized (the overall progress in research of polychlorides [49,50,55] encourages this hypothesis). The corresponding experiments are underway in our group.