Lanthanide(III) complexes with μ-SnSe₄ and μ-Sn₂Se₆ linkers: solvothermal syntheses and properties of new Ln(III) selenidostannates decorated with linear polyamine

2.1 Crystal structure of 1

Compound 1 crystallizes in the triclinic space group P1̅ with two formula units in the unit cell. Both La(1)³⁺ and La(2)³⁺ions are coordinated by a hexadentate peha molecule, and are further bound to a Cl⁻ and NO₃⁻ ions, forming [La(peha)(Cl)]²⁺ and [La(peha)(NO₃)]²⁺ units, respectively. The NO₃⁻ ion chelates La(2)³⁺in a μ-NO₃ mode. The Sn⁴⁺ion binds four Se²⁻ anions at distances of 2.4973(10)−2.5145(10) Å, forming a tetrahedral [SnSe₄]⁴⁻unit with Se−Sn−Se angles in the range of 103.52(4)−114.62(3)°. The bond lengths and angles are in accordance with those observed in other compounds containing SnSe₄ structural units [33]. Acting as a tetradentate chelating and bridging ligand of the μ-1κ²:2κ² connectivity, the [SnSe₄]⁴⁻ unit joins {La(peha)(Cl)}²⁺ and {La(peha)(NO₃)}²⁺ cations generating the neutral binuclear coordination compound [{La(peha)(Cl)}{La(peha)(NO₃)}(μ-1κ²:2κ²-SnSe₄)] (1) (Fig. 1a). As a result, two four-membered heterometallic SnLaSe₂ rings are formed. The La(1)³⁺ ion is in 9-fold coordination involving six N atoms from a peha molecule, two Se atoms from a SnSe₄ unit and a Cl⁻, forming a coordination polyhedron LaN₆Se₂Cl in a distorted tricapped trigonal prismatic geometry (Fig. 1b). The La(2)³⁺ ion is 10-fold coordinated by six N atoms from a peha molecule, two Se atoms from a SnSe₄ unit and two O atoms from a NO₃⁻ ion, forming a bicapped square antiprism LaN₆Se₂O₂ (Fig. 1c). Except for the smaller angle O−La(2)−O [47.46(15)°] formed by the chelating μ-NO₃ ion, the angles around La(1) [60.93(15)−151.06(15)°] and La(2) [61.23(16)−151.28(12)°] exhibit no distinct differences. The La−Se, La−N and La−O bond lengths are 3.0998(13)–3.2376(12) Å, 2.678(8)−2.791(5) Å and 2.646(4)−2.694(5) Å (Table 1), respectively, which are in the typical ranges of the corresponding bond lengths reported in the literature [27], [28], [36], [39]. Additional bond lengths and angles are summarized in Table S1 (Supplementary Information).

Fig. 1:

(a) Molecular structure of 1 in the crystal with atom labeling scheme adopted (displacement ellipsoids are drawn at the 50% probability level; hydrogen atoms are omitted for clarity). (b) Distorted tricapped trigonal prism of La(1)N₆Se₂Cl in 1. (c) Distorted bicapped square antiprism of La(2)N₆Se₂O₂ in 1.

In 1, the individual [{La(peha)(Cl)}{La(peha)(NO₃)}(μ-SnSe₄)] molecules are arranged end to end via N−H···Cl hydrogen bonding [N(10)···Cl(1): 3.426(5) Å; N(10)–H(10)···Cl(1): 153.4°] (Table S5, Supplementary Information), resulting in parallel chains (Fig. 2a; Fig. S1, Supplementary Information). The parallel chains contact each other via N−H···Se hydrogen bonds, forming layers perpendicular to the c axis of the unit cell (Fig. 2a). The layers are connected into a three-dimensional supramolecular network via interlayer N−H···Se hydrogen bonds (Fig. 2b). The Se(1) and Se(4) atoms are involved in the hydrogen bonding formation. The parameters of N−H···Se hydrogen bonds [N···Se: 3.524(5)–3.591(5) Å; N–H···Se: 138.5–172.8°] (Table S5, Supplementary Information) are consistent with those observed in the literature [36].

Fig. 2:

(a) Sectional layered structure assembled by N−H···Cl and N−H···Se interactions in 1. (b) Packing diagram of 1. Hydrogen atoms of C−H are omitted for clarity. The SnSe₄ units are shown as yellow tetrahedra.

Several TM complexes with SnSe₄ ligands have been prepared. A well-known example is the pentasupertetrahedral family of anionic clusters [TM₄Sn₄Se₁₇]¹⁰⁻ (TM=Mn, Fe, Co, Zn, Hg) [40], [41], [42], [43], [44]. In the ternary cluster, the tetrahedral SnSe₄ unit coordinates with the TM(II) centers as a tridentate μ-1κ:2κ :3κ-SnSe₄ bridging ligand. The same ligand is also observed in the [Zn₄Sn₃Se₁₆]⁴⁻ cluster [45]. In the organic hybrid complex [K₆(MeOH)₉][Sn₂Se₆][Cr(en)₂(SnSe₄)]₂, the SnSe₄ unit ligates Cr(III) centers in a bidentate 1κ²-SnSe₄ chelating mode [46]. However, SnSe₄ coordination with lanthanide(III) centers is rare. The only example of a lanthanide complex ligated with a SnSe₄ unit is [Hdien][Ln(dien)₂(SnSe₄)] (Ln=Sm, Eu), in which a bidentate 1κ²-SnSe₄ chelating mode is obtained [33]. The tetradentate μ-1κ²:2κ² ligand with four Se atoms in compound 1 represents a new coordination fashion for the SnSe₄ unit.

2.2 Crystal structure of 2

Compound 2 crystallizes in the monoclinic space group P2₁/c with two formula units in the unit cell. It consists of [H₂trien]²⁺, [{La(trien)₂}₂(μ-1κ:2κ-Sn₂Se₆)]²⁺ and [Sn₂Se₆]⁴⁻ ions, and a water molecule. The La³⁺ion is coordinated by a tetradentate trien ligand, forming a [La(trien)]³⁺ unit. There are two crystallographically independent Sn⁴⁺ions. Both Sn(1)⁴⁺ and Sn(2)⁴⁺ are tetrahedrally bound with four Se²⁻ anions forming [SnSe₄]⁴⁻ units. A dimeric [Sn₂Se₆]⁴⁻ anion is constructed by two [Sn(1)Se₄]⁴⁻units via edge-sharing. It connects two [La(trien)]³⁺ units with its two trans-terminal Se atoms in a μ-1κ:2κ bridging mode to form a binuclear [{La(trien)₂}₂(μ-1κ:2κ-Sn₂Se₆)]²⁺ complex cation (Fig. 3). The same connectivity of two [Sn(2)Se₄]⁴⁻units gives another dimeric [Sn₂Se₆]⁴⁻ion, which acts as the counter anion in compound 2. The terminal Sn−Se_t bond lengths of Sn₂Se₆ units are comparable to those of the SnSe₄ unit in 1. As expected, the bridging Sn−Se_b bonds are longer than the terminal ones (Table 2; additional bond lengths and angles are summarized in Table S2 of the Supplementary Information). The La³⁺ ion is coordinated by eight N atoms from two trien molecules, and a Se atom from a Sn₂Se₆ unit, forming a monocapped square antiprism LaN₈Se (Fig. S2, Supplementary Information). The Sn−Se−La angle (112.29(6)°) is much larger than the corresponding angles in 1 [Sn−Se−La: 86.98(4)−88.91(4)°] (Tables 1 and 2) due to the formation of SnLaSe₂ rings in 1. The La−Se and La−N bond lengths are in the range of the corresponding bond lengths observed in 1. It is worthy of note that similar syntheses using reactants LnCl₃ (Ln=Ce, Nd), Sn and Se in trien produce compounds [Htrien]₂[{Ln(trien)(tren)}₂(μ-1κ:2κ-Sn₂Se₆)][Sn₂Se₆] (triclinic, P1̅) [34]. The trien molecule undergoes rearrangement during solvothermal reactions to give the isomer tren. The Ln³⁺ ions are coordinated by a trien and a tren molecules to form the mixed-coordinate [Ln(trien)(tren)]³⁺fragment in these compounds.

Fig. 3:

Molecular structure of the binuclear [{La(trien)₂}₂(μ-1κ:2κ-Sn₂Se₆)]²⁺ cation in crystals of 2 with the atom labeling scheme (displacement ellipsoids are drawn at the 50% probability level; hydrogen atoms are omitted for clarity).

In 2, each [{La(trien)₂}₂(μ-1κ:2κ-Sn₂Se₆)]²⁺ moiety contacts four neighbors via N−H···Se interactions with the terminal Se(1) and Se(3) atoms (Fig. 4a; Table S5, Supplementary Information). As a result, a cationic layer perpendicular to the a axis is formed. The [H₂trien]²⁺ and [Sn₂Se₆]⁴⁻ ions are connected into a [H₂trien]²⁺/[Sn₂Se₆]⁴⁻anionic layer through N···Se interactions (Fig. 4c). The cationic and anionic layers run parallel and stack alternatively along the a axis (Fig. 4b).

Fig. 4:

(a) Sectional layered structure of [{La(trien)₂}₂(μ-1κ:2κ-Sn₂Se₆)]²⁺ moieties assembled by N−H···Se interactions in 2. (b) Packing diagram of 2. (c) Sectional layered structure of [H₂trien]²⁺/[Sn₂Se₆]⁴⁻ moieties assembled by interaction N−H···Se in 2. Hydrogen atoms of C−H are omitted for clarity. Yellow tetrahedra: SnSe₄ of [{La(trien)₂}₂(μ-1κ:2κ-Sn₂Se₆)]²⁺. Blue tetrahedra: SnSe₄ of free [Sn₂Se₆]⁴⁻.

2.3 Crystal structures of 3 and 4

Compounds 3 and 4 are isostructural. They crystallize in the monoclinic space group C2/c with four formula units in the unit cell. They consist of coordination polymers [{Ln(tepa)(μ-OH)}₂(μ-1κ:2κ-Sn₂Se₆)]_n (Ln=Sm(3), Eu(4)) and a H₂O molecule. The polymer is constructed by [{Ln(tepa)(μ-OH)}₂]²⁺ and [Sn₂Se₆]⁴⁻ fragments. The crystal structure of 3 is shown in Figs. 5 and 6. The Sm³⁺ion is coordinated by a pentadentate tepa ligand, forming a [Sm(tepa)]³⁺ unit. Two μ-OH bridging ligands join two [Sm(tepa)]³⁺ units to form a binuclear [{Sm(tepa)(μ-OH)}₂]²⁺ complex fragment (Fig. 5). The [Sn₂Se₆]⁴⁻anion interconnects [{Sm(tepa)(μ-OH)}₂]²⁺ fragments using two trans-terminal Se atoms. As a result, a 1D coordination polymer [{Sm(tepa)(μ-OH)}₂(μ-1κ:2κ-Sn₂Se₆)]_n is formed. The angle of Se(1) (111.92(6)°) joining the Sn⁴⁺ and Sm³⁺ ions is similar to the corresponding angle of 2 (112.29(6)°). The [Sn₂Se₆]⁴⁻ligand adopts the same coordination mode of a μ-1κ:2κ-Sn₂Se₆ ligand as in 2 and exhibits similar structural parameters (Tables 2 and 3). Unlike the La³⁺ ions which are in 10- or 9-fold coordination environments in 1 and 2, the Sm³⁺and Eu³⁺ions form eight-coordinated polyhedra involving five N, two O and one Se atoms. The polyhedron LnN₅O₂Se (Ln=Sm, Eu) can be viewed as a distorted bicapped trigonal prism (Fig. S3, Supplementary Information). The Ln–Se, Ln–N and Ln–O bond lengths (Table 3) are comparable to the corresponding bonds observed in other Sm(III) and Eu(III) complexes [29], [33], [47]. Tables S3 and S4 (Supplementary Information) contain additional bond lengths and angles.

Fig. 5:

Molecular structure of 3 in the crystal with the atom labeling scheme (displacement ellipsoids are drawn at the 50% probability level; hydrogen atoms and water molecule are omitted for clarity).

Fig. 6:

Packing diagram of 3. Hydrogen atoms and CH₂ groups are omitted for clarity. Yellow tetrahedra: SnSe₄. Blue polyhedra: SmN₅O₂Se.

In 3, [{Sm(tepa)(μ-OH)}₂(μ-1κ:2κ-Sn₂Se₆)]_n chains run parallel generating a layered structure. The chains in different layers are cross-stacked. The cross-stacked chains generate 1D channels, in which the water molecules are located (Fig. 6; Fig. S4, Supplementary Information). The Se(1) and Se(2) atoms contact with the NH₂ and NH groups of the cross-stacked chains with N···Se distances varying between 3.472(10) and 3.707(12) Å, with N−H···Se angles in the range 143.8−177.4°, indicating N−H···Se hydrogen bonds.

Summarizing the results of the solvothermal reactions of the Ln/Sn/Se (Ln=La, Ce, Nd, Sm, Eu, Gd, Yb) systems in tepa solvent, three different types of Ln(III) complexes were obtained. La³⁺ and Ce³⁺ions with largest ionic radii form compounds (H₃O)_n[Ln(tepa)(μ-1κ²:2κ²-Sn₂Se₆)]_n (Ln=La, Ce), containing [Ln(tepa)(μ-1κ²:2κ²-Sn₂Se₆)⁻]_n anionic chains with a tetradentate μ-1κ²:2κ²-Sn₂Se₆ ligand [34], [35]. Nd³⁺ion afforded a neutral coordination polymer [{Nd(tepa)(μ-OH)}₂(μ-1κ:2κ-Sn₂Se₆)]_n (orthorhombic, Pbca), containing a bidentate μ-1κ:2κ-Sn₂Se₆ ligand [35]. From Sm³⁺, Eu³⁺, Gd³⁺, Yb³⁺ ions, coordination polymers [{Ln(tepa)(μ-OH)}₂(μ-1κ:2κ-Sn₂Se₆)]_n·nH₂O (Ln=Sm, Eu, Gd, Yb) crystallizing in the monoclinic space group C2/c were formed [34].

2.4 Influence of ethylene polyamines on the La/Sn/Se system

Polyethylene polyamines play an important role in the solvothermal syntheses of lanthanide selenidostannate complexes, as is illustrated by the La/Sn/Se system. In our previous investigations on the solvothermal system La/Sn/Se in different ethylene polyamines, we had prepared La(III) complexes [La₂(en)₈(μ-Se₂)]Sn₂Se₆ (5), [{La(dien)₂}₄(μ₄-Sn₂Se₉)(μ-Sn₂Se₆)]_n (6) and (H₃O)_n[La(tepa)(μ-1κ²:2κ²-Sn₂Se₆)]_n (7) by the reactions of La₂O₃, Sn and Se using en, dien and tepa as reaction media, respectively [35], [36]. In the en, trien and tepa solvents, the dinuclear Sn₂Se₆ unit is formed. It does not bind a La(III) center, and just acts as counter anion in compound 5. It joins two [La(trien)]³⁺ units with two trans-terminal Se atoms in a μ-1κ:2κ bridging mode, to form a binuclear [{La(trien)₂}₂(μ-1κ:2κ-Sn₂Se₆)]²⁺ complex cation in compound 2. But it acts as a tetradentate ligand with four terminal Se atoms, and interlinks the [La(tepa)]³⁺ cations into anionic chains [La(tepa)(μ-1κ²:2κ²-Sn₂Se₆)]_nⁿ⁻ in compound 7. In dien, polyselenidostannate Sn₂Se₉ and selenidostannate Sn₂Se₆ units are simultaneously formed. They act as nonadentate and bidentate bridging ligands, respectively, to interconnect [La(dien)₂]³⁺ cations into the neutral coordination polymer of 6. In peha, the selenidostannate SnSe₄ unit is obtained. It joins two La(III) complex cations as a tetradentate chelating and bridging ligand with four Se atoms in the μ-1κ²:2κ² connection mode, forming the neutral coordination compound 1. All N atoms of the polyethylene polyamines take part in coordination to the La(III) ions in the La(III) compounds 1, 2, 5–7, and different numbers of coordination sites are left at the La(III) centers. As a result, different coordination modes of Sn-Se units are formed.

2.5 Optical properties

The UV/Vis reflectance spectra of the title compounds were measured on powder samples at room temperature. The absorption data from the reflectance spectroscopy by the Kubelka–Munk function [48] demonstrate that compounds 1–4 show well-defined abrupt absorption edges from which the band gaps (E_g) can be estimated at 2.33, 2.21, 2.37 and 2.42 eV, respectively (Figs. 7 and 8). The La-selenidostannate complexes show a distinct red shift of the band gaps from 1 and 2 to [{La(dien)₂}₄(μ₄-Sn₂Se₉)(μ-Sn₂Se₆)]_n (E_g=2.09 eV) and (H₃O)_n[La(tepa)(μ-1κ²:2κ²-Sn₂Se₆)]_n (E_g=1.95 eV). The gaps are larger than that of the La(III) complex [La₂(en)₈(μ-Se₂)]Sn₂Se₆ containing a free selenidostannate unit (E_g=2.47 eV) [35], [36].

Fig. 7:

Optical absorption spectra of compounds 1 (blue) and 2 (black).

Fig. 8:

Optical absorption spectra of compounds 3 (black) and 4 (red).

2.6 Thermal properties

The thermal stabilities of compounds 1–4 were investigated by thermogravimetric analysis (TGA) under N₂ atmosphere (Fig. 9). The TG curve shows that compound 1 decomposes in one step with a weight loss of a 35.2% between 280 and 380°C. The weight loss is in accordance with the complete removal of two peha ligands (theoretical value: 36.5%). Compound 2 decomposes in three steps with a total weight loss of 33.6% between 90 and 350°C. The weight loss of 1.1% in the first step corresponds to the removal of one water molecule per formula unit in 2. The decomposition is followed by a weight loss of 8.7%, which is attributed to removal of a trien and a H₂Se molecules. The weight loss of 23.8% in the third step corresponds to the removal of four trien ligands. Compound 3 exhibits a weight loss of 1.0% at about 105°C due to the removal of lattice water molecules (Fig. 9). The weight loss of 26.8% between 290 and 380°C corresponds to the removal of tepa ligands (theoretical value: 26.2%). Compound 4 exhibits a similar thermal decomposing behavior as compound 3 (Fig. 9).

Fig. 9:

TG curves of compounds 1 (black), 2 (blue), 3 (red), and 4 (cyan).

4.1 Materials and general methods

All starting chemicals were analytical grade and used as received. Elemental analyses were conducted on an EA1110-CHNS-O elemental analyzer. FT-IR spectra were recorded with a Nicolet Magna-IR 550 spectrometer in dry KBr disks in the 4000–400 cm⁻¹ range. TGA was conducted on a TG 6300 microanalyzer. The samples were heated at a rate of 5°C min⁻¹ under a nitrogen stream of 100 mL min⁻¹. Powder X-ray diffraction (PXRD) patterns were collected on a D/MAX-3C diffractometer using graphite monochromatized CuK_α radiation (λ=1.5406 Å). Room-temperature optical diffuse reflectance spectra of the powdered samples were obtained with a Shimadzu UV-3150 spectrometer. The absorption (α/S) data were calculated from the reflectance using the Kubelka–Munk function α/S=(1−R)²/2R, where R is the reflectance at a given energy, α is the absorption and S is the scattering coefficient [48].

4.2 Synthesis of {La(peha)(Cl)}{La(peha)(NO₃)}(μ-1κ²:2κ²-SnSe₄)] (1)

La(NO₃)₃ (0.162 g, 0.50 mmol), SnCl₄·5H₂O (0.175 g, 0.50 mmol) and Se (0.158 g, 2.0 mmol) were dispersed in 5 mL of peha by stirring, and the dispersion was loaded into a polytetrafluoroethylene (PTFE)-lined stainless steel autoclave of volume 10 mL. The sealed autoclave was heated to 160°C for 6 days. After cooling to ambient temperature, the resulting light yellow block crystals of 1 were filtered off, washed with ethanol and dried in vacuo. Yield 0.39 g (61% based on La(NO₃)₃). – Analysis for C₂₀H₅₆N₁₃O₃ClSe₄SnLa₂ (1): calcd. C 18.85, H 4.43, N 14.29; found C 18.74, H 4.38, N 14.18%. – IR (KBr disk): ῡ=3730 (w), 3664 (w), 3264 (w), 3125 (w), 2918 (w) ,2869 (w), 2351 (s), 2013 (w), 1648 (w), 1573 (m), 1455 (w), 1332 (m), 1270 (w), 1156 (w), 993 (s), 853 (w), 765 (w), 624 (w), 523 (w), 453 (w), 409 (w) cm⁻¹.

4.3 Synthesis of [H₂trien][{La(trien)₂}₂(μ-1κ:2κ-Sn₂Se₆)][Sn₂Se₆]·H₂O (2)

Compound 2 was prepared with a procedure similar to that for the synthesis of 1, except that peha was replaced by trien. Orange block crystals of 2 were obtained in 54% yield (0.17 g) based on Sn. – Analysis for C₃₀H₉₄N₂₀OSe₁₂Sn₄La₂ (2): calcd. C 14.81, H 3.81, N 11.51; found C 14.65, H 3.68, N 11.39%. – IR (KBr disk): ῡ=3747 (w), 3411 (m), 3253 (w), 2913 (w), 2364 (w), 1995 (w), 1556 (s), 1516 (s), 1380 (m), 1332 (m), 1143 (w), 1050 (w), 989 (w), 945 (w), 814 (w), 765 (w), 581 (w), 510 (w), 414 (w) cm⁻¹.

4.4 Synthesis of [{Sm(tepa)(μ-OH)}₂ (μ-1κ:2κ-Sn₂Se₆)]_n·nH₂O (3)

Sm(NO₃)₃ (0.168 g, 0.50 mmol), SnCl₄·5H₂O (0.175 g, 0.50 mmol) and Se (0.158 g, 2.0 mmol) were dispersed in 5 mL of tepa by stirring, and the dispersion was loaded into a PTFE-lined stainless steel autoclave of volume 10 mL. The sealed autoclave was heated to 160°C for 6 days. After cooling to ambient temperature, the resulting yellow block crystals of 3 were filtered off, washed with ethanol and dried in vacuo. Yield 0.24 g (67% based on Sn). – Analysis for C₁₆H₅₀N₁₀O₃Se₆Sn₂Sm₂ (3): calcd. C 13.32, H 3.48, N 9.71; found C 13.25, H 3.41, N 9.58%. – IR (KBr disk): ῡ=3626 (s), 3464 (w), 3296 (s), 3258 (s), 3164 (w), 3059 (m), 2911 (w), 2864 (w), 1567 (s), 1473 (m), 1379 (w), 1314 (w), 1126 (m), 1075 (s), 1006 (s), 882 (m), 745 (w), 664 (w), 617 (w), 518 (m), 418 (m) cm⁻¹.

4.5 Synthesis of [{Eu(tepa)(μ-OH)}₂ (μ-1κ:2κ-Sn₂Se₆)]_n·nH₂O (4)

Compound 4 was prepared with a procedure similar to that for the synthesis of 3, except that Sm(NO₃)₃ was replaced by Eu(NO₃)₃. Yellow block crystals of 4 were obtained in 58% yield (0.21 g) based on Sn. – Analysis for C₁₆H₅₀N₁₀O₃ Se₆Sn₂Eu₂ (4): calcd. C 13.29, H 3.49, N 9.69; found C 13.11, H 3.37, N 9.52%. – IR (KBr disk): ῡ=3398 (m), 2920 (w), 1576 (s), 1493 (s), 1381 (s), 1322 (m), 1110 (w), 1051 (w), 803 (w), 679 (m), 585 (m), 502 (m), 421 (w) cm⁻¹.

4.6 Single-crystal X-ray crystallography

Data were collected on a Rigaku Saturn CCD diffractometer at 223(2) K using graphite-monochromated MoK_α radiation with a ω scan method to a maximum 2θ value of 50.70° for 1, 3 and 4, and 51.00° for 2. An absorption correction was applied for all the compounds using multi-scan. The structures were solved with Direct Methods using the program SHELXS-97 [49], [50], and the refinement was performed against F² using SHELXL-97 [51], [52]. All non-hydrogen atoms were refined anisotropically. Atom C(11) in 1 was disordered, and the occupancies of the disordered atom were refined as 70% and 30% for C/C′. The disordered C(8) atoms in 3 were refined as 60% and 40%, whereas the same disordered C(8) atoms in 4 were refined as 70% and 30%. The hydrogen atoms were added geometrically and refined using a riding model. Technical details of data acquisition and selected refinement results are summarized in Table 4. Figures S5 and S6 (Supplementary Information) contain comparisons of the measured and calculated powder diffraction patterns of compounds 1 and 2.

CCDC 1475477 (1), 1475478 (2), 1503349 (3) and 1503350 (4) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif.