Solvothermal syntheses of lanthanide thiogermanates displaying three new structural moieties

Abstract

A series of lanthanide thiogermanates [Ln(dien)₃]₂[Ge₂S₆]Cl₂ [Ln = Pr (Ia), Sm (Ib), Gd (Ic), Dy (Id); dien = diethylenetriamine], [Er₂(dien)₄(μ-OH)₂][Ge₂S₆] (II) and [Ho(trien)(en)GeS₃(SH)] (III, trien = triethylenetetramine, en = ethylenediamine) have been hydrothermally synthesized and structurally characterized. The structures of Ia–d consist of isolated [Ln(dien)₃]³⁺ cations, [Ge₂S₆]⁴⁻ anions built up from the connection of two [GeS₄] tetrahedra sharing a common edge and Cl⁻ ions. II contains binuclear [Er₂(dien)₄(μ-OH)₂]⁴⁺ cations constructed by the linkage of [Er(dien)₂]³⁺ ions and –OH bridging groups, and [Ge₂S₆]⁴⁻ anions. III contains neutral holmium-centred complexes, where the unusual protonated tetrahedral anion [GeS₃(SH)]³⁻ acts as a chelating ligand to complex the [Ho(en)(trien)]³⁺ cation. A systematic investigation of six lanthanide thiogermanates and four reported compounds revealed that both the well-known lanthanide contraction and different chelating organic amines have a significant influence on the formation of lanthanide thiogermanates under solvothermal conditions. Density functional theory calculation for III has also been performed and the absorption edges of all compounds have been investigated by UV-vis spectroscopy.

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Introduction

Solvothermal synthesis of new chalcogenidogermanates has received increasing attention since 1989,¹ due to their fascinating architectures and potential applications in gas separation, ion conductivity, and photocatalysis.² Among these compounds, the thiogermanates have been well studied, fundamental [GeS₄] tetrahedral units or combination of [GeS₄] and [MS₄] (M = Ga, In) tetrahedral units can condense into the [M_xGe_4−xS₁₀] (x = 0–3) supertetrahedral units (T₂) as secondary building units, which further self-assemble into a variety of open framework thiogermanates.^2d The T₂ unit and other [MS_x] polyhedra (M = transition-metal ions) are interconnected to form the various 3-D network structures with pores or cavities.³ Protonated nonchelating amines or tetraalkyl-ammonium ions as structure-directing agents are commonly retained within pore or cavity spaces. The [GeS₄] unit can also combine with acentric [SbS_n] (n = 3, 4) polyhedra via corner sharing of vertex S atoms to produce 1-D, 2-D, and 3-D thiogermanate-thioantimonates.⁴ When transition metal complexes substitute for nonchelating amines or tetraalkyl-ammonium ions, thiogermanate anions are strongly dominated by [Ge₂S₆]⁴⁻ or [Ge₄S₁₀]⁴⁻ moieties.⁵ Notably, the noncondensed [GeS₄]⁴⁻ mode in organic hybrid thiogermanates is very scarce, and appear to be limited to that of [VO(dien)]₂GeS₄,⁶ [Ln(dien)₂(GeS₃(SH))]_n (Ln = La, Nd),⁷ and [Na(H₂O)₃]-[Cr(en)₃]₂[GeS₃(OH)]₂[Cr(en)₂(GeS₄)],⁸ where the [GeS₄]⁴⁻ ion is stabilized by coordinating to metal complex. More interestingly, the cations of these thiogermanates are either protonated amine or transition metal complex cations,^3,5,6,8 but the thiogermanates with lanthanide-containing counter cation prepared under mild solvothermal conditions have been still a less explored area.

The integration of lanthanide (Ln) metals in chalcogenide frameworks can lead to new materials with interesting semiconducting, magnetic, luminescent and infrared nonlinear optical properties.⁹ These materials have traditionally been prepared by flux methods at high temperature, as exemplified by La₂Ga₂GeS₈,¹⁰ Ba₃AGa₅Se₁₀Cl₂ (A = Cs, Rb, K),¹¹ and Ba₂₃Ga₈Sb₂S₃₈.¹² However, the synthesis of lanthanide chalcogenides modified with organic components are hampered by the heat, water, oxygen and light sensitivity of these materials, and the difficulty of soft Lewis basic Q²⁻ ligands (Q = S, Se, Te) coordinating to the hard Lewis acidic Ln³⁺ ions via Ln–Q bonds in the solvents.¹³ Recently, we have attempted the preparation of lanthanide thiogermanates in chelating amine solutions by the solvothermal methods allowing access to thermodynamically metastable phases, and prepared four organic hybrid lanthanide thiogermanates, [Eu(dien)₃]₂[Ge₂S₆]Cl,¹⁴ [Y₂(tepa)₂(μ-OH)₂(μ-Ge₂S₆)](tepa)_0.5·H₂O (tepa = tetraethylenepentamine),¹⁴ and [Ln(dien)₂(GeS₃(SH))]_n (Ln = La, Nd),⁷ where the soft Lewis basic [Ge₂S₆]⁴⁻ and [GeS₃(SH)]³⁻ units act as ligands to the hard Lewis acidic Ln³⁺ ions. To investigate systematically influence of the well-known lanthanide contraction and different chelating organic amines on the formation of lanthanide thiogermanates, we started to explore the system LnCl₃/GeO₂/S/amine under solvothermal conditions, and successfully synthesized a series of organic hybrid lanthanide thiogermanates, [Ln(dien)₃]₂[Ge₂S₆]Cl₂ [Ln = Pr (Ia), Sm (Ib), Gd (Ic), Dy (Id)], [Er₂(dien)₄(μ-OH)₂][Ge₂S₆] (II) and [Ho(trien)(en)GeS₃(SH)] (III). I and II contain isolated [Ge₂S₆]⁴⁻ anions with lanthanide complexes as counterions, while III consists of neutral holmium-centred complex with noncondensed tetrahedral anion [GeS₃(SH)]³⁻ as a chelating ligand to complex [Ho(en)(trien)]³⁺ cation.

Experimental section

General remarks

All analytical grade chemicals were obtained commercially and used without further purification. Elemental analyses (C, H, and N) were performed using a PE2400 II elemental analyzer. The UV/Vis spectra were recorded at room temperature using a computer-controlled PE Lambda 900 UV/Vis spectrometer equipped with an integrating sphere in the wavelength range of 200–1250 nm. FT-IR spectra were recorded with a Nicolet Magna-IR 550 spectrometer in dry KBr disks in the 4000–400 cm⁻¹ range. Powder XRD patterns were collected on a D/MAX-3C diffractometer using graphite-mono-chromatized CuKa radiation (λ = 1.5406 Å).

Synthesis of [Pr(dien)₃]₂[Ge₂S₆]Cl₂ (Ia). The reagents of PrCl₃ (0.0247 g, 0.10 mmol), S (0.0256 g, 0.80 mmol), GeO₂ (0.0104 g, 0.10 mmol) and dien (2.0 mL) were placed in a thick Pyrex tube (ca. 20 cm long). The sealed tube was heated at 170 °C for 8 days to yield green crystals. The crystals were washed with ethanol, dried and stored under vacuum (46% yield based on GeO₂). Anal. calcd. for Ia, C₁₂H₃₉ClGeN₉PrS₃, C 22.02%, H 6.00%, N 19.26%, found: C 21.88%, H 5.91%, N 19.13%. IR (cm⁻¹): 3244(s), 3121(s), 2909(s), 2852(s), 1577(m), 1461(m), 1337(m), 1134(w), 1090(m), 1017(m), 974(m), 944(m), 893(m), 814(w), 748(vw), 675(w), 588(m), 494(w), 428(s).

Synthesis of [Sm(dien)₃]₂[Ge₂S₆]Cl₂ (Ib). The light yellow crystals of Ib were prepared similarly from SmCl₃ (yield 65% based on GeO₂). Anal. calcd. for Ib, C₁₂H₃₉ClGeN₉S₃Sm, C 21.70%, H 5.92%, N 18.98%, found: C 21.85%, H 6.11%, N 19.12%. IR (cm⁻¹): 3223(s), 3143(s), 2917(s), 2859(s), 1571(s), 1491(s), 1381(vw), 1323(m), 1156(w), 1098(m), 1068(m), 1025(w), 966(m), 901(m), 850(w), 807(w), 756(w), 602(m), 494(w), 435(s).

Synthesis of [Gd(dien)₃]₂[Ge₂S₆]Cl₂ (Ic). The colorless crystals of Ic were prepared similarly from GdCl₃ (yield 51% based on GeO₂). Anal. calcd. for Ic, C₁₂H₃₉ClGdGeN₉S₃, C 21.48%, H 5.86%, N 18.79%, found: C 21.36%, H 5.75%, N 18.68%. IR (cm⁻¹): 3222(s), 3127(s), 2909(s), 2844(s), 1571(s), 1461(m), 1396(w), 1331(m), 1280(w), 1257(w), 1163(m), 1105(s), 1076(m), 1025(m), 966(s), 893(s), 828(w), 610(m), 500(m), 435(s).

Synthesis of [Dy(dien)₃]₂[Ge₂S₆]Cl₂ (Id). The light yellow crystals of Id were prepared similarly from DyCl₃ (yield 64% based on GeO₂). Anal. calcd. for Id, C₁₂H₃₉ClDyGeN₉S₃, C 21.31%, H 5.81%, N 18.64%, found: C 21.18%, H 5.78%, N 21.15%. IR (cm⁻¹): 3207(s), 3134(s), 2939(s), 2860(s), 1566(s), 1494(s), 1335(m), 1104(m), 1067(w), 974(m), 894(w), 757(m), 670(m), 597(m), 504(w), 446(s).

Synthesis of [Er₂(dien)₄(μ-OH)₂][Ge₂S₆] (II). The pink crystals of II were prepared similarly from ErCl₃ (yield 56% based on GeO₂). Anal. calcd. for II, C₁₆H₅₄Er₂Ge₂N₁₂O₂S₆, C 17.18%, H 4.87%, N 15.02%, found: C 17.32%, H 5.03%, N 15.21%. IR (cm⁻¹): 3346(s), 3222(s), 3095(s), 2938(s), 2858(s), 1571(vs), 1505(s), 1454(m), 1331(m), 1280(w), 1156(w), 1076(m), 1025(w), 995(w), 952(s), 901(s), 850(w), 763(w), 712(w), 617(m), 515(w), 472(s), 428(s).

Synthesis of [Ho(trien)(en)GeS₃(SH)] (III). The reagents of HoCl₃ (0.0271 g, 0.10 mmol), S (0.0264 g, 0.82 mmol), GeO₂ (0.0168 g, 0.16 mmol), teta (1.0 mL) and en (1.0 mL) were placed in a thick Pyrex tube (ca. 20 cm long). The sealed tube was heated at 170 °C for 10 days to yield pink crystals. The crystals were washed with ethanol, dried and stored under vacuum (yield 63% based on GeO₂). Anal. calcd. for III, C₈H₂₇GeHoN₆S₄, C 16.77%, H 4.75%, N 14.66%, found: C 16.55%, H 5.03%, N 14.85%. IR (cm⁻¹): 3251(s), 3127(s), 2931(s), 2866(s), 2575(vw), 1577(s), 1439(s), 1323(m), 1280(m), 1235(w), 1163(m), 1097(s), 1057(s), 1003(s), 915(s), 821(m), 602(m), 573(m), 545(w), 443(s).

Crystal structure determination

Single-crystal X-ray diffraction data for all compounds were recorded on a Rigaku Mercury CCD diffractometer using a ω-scan method with graphite monochromated Mo Kα radiation (λ = 0.71073 Å) at 296(2) K to a maximum 2θ value (52.00°). Absorption corrections were applied using multi-scan technique. The structures of all compounds were solved by Direct Methods (SHELXS-97)¹⁵ and refined by full-matrix least-squares techniques using the SHELXL-97 program.¹⁶ Non-hydrogen atoms were refined with anisotropic displacement parameters. The H atoms bonded to C and N atoms were positioned with idealized geometry and a riding model was used. H atom associated with –SH group in III was located from the difference Fourier map. Relevant crystal and collection data parameters and refinement results can be found in Table 1.

Table 1 Crystallographic data for all compounds

	Ia	Ib	Ic	Id	II	III
Formula	C12H39ClGeN9PrS3	C12H39ClGeN9S3Sm	C12H39ClGdGeN9S3	C12H39ClDyGeN9S3	C16H54Er2Ge2N12O2S6	C8H27GeHoN6S4
Fw	654.70	664.15	671.04	676.29	1118.87	573.18
Crystal system	Monoclinic	Monoclinic	Monoclinic	Monoclinic	Monoclinic	Monoclinic
Space group	P21/n	P21/n	P21/n	P21/n	P21/n	P21/c
a, Å	11.637(3)	11.532(3)	11.5484(14)	11.5035(11)	11.710(2)	9.7755(15)
b, Å	14.143(3)	14.423(4)	14.6767(17)	14.6453(14)	11.318(2)	13.4516(19)
c, Å	15.120(3)	14.573(4)	14.4271(17)	14.3396(13)	13.548(2)	14.388(2)
β, deg	98.149(5)	97.105(5)	96.332(2)	96.178(2)	97.635(3)	99.264(2)
V, Å^3	2463.5(10)	2405.3(10)	2430.4(5)	2401.8(4)	1779.6(5)	1867.3(5)
Z	4	4	4	4	2	4
T, K	296(2)	296(2)	296(2)	296(2)	296(2)	296(2)
Calcd density, Mg m^−3	1.765	1.834	1.834	1.870	2.088	2.039
Abs.coeff., mm^−1	3.555	4.056	4.327	4.729	6.728	6.262
F(000)	1320	1332	1340	1348	1092	1120
2θ (max), deg	50.20	50.20	46.78	50.20	50.20	50.18
Total reflns collected	17095	16607	20881	16686	12194	13209
Unique reflns	4249	4206	5982	4237	3110	3310
No. of param.	244	244	244	244	185	184
R1[I > 2σ(I)]	0.0235	0.0245	0.0213	0.0573	0.0171	0.0513
wR2 (all data)	0.0570	0.0667	0.0640	0.1568	0.0418	0.1331

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Results and discussion

Synthetic aspects

Solvothermal techniques have proven to be an extremely effective method for the preparation of new chalcogenometalates, which overcome the difficulties associated with differential solubilities of organic and inorganic starting materials.¹⁷ The crystals of Ia–d, [Eu(dien)₃]₂[Ge₂S₆]Cl₂,¹⁴ and II were obtained in the presence of dien. Dien preferably coordinate to Ln³⁺ ions to form isolated complex cations [Ln(dien)₃]³⁺ or [Ln₂(dien)₄(μ-OH)₂]⁴⁺, which prevent the lanthanide complexes from bonding with S atom of thiogermanate anion. Penta-dentate chelating amine coordinate to Ln³⁺ ion to form unsaturated lanthanide complex that can effectively incorporate into chalcogenido-germanate frameworks to product novel organic hybrid lanthanide chalcogenido-germanates with diverse structures.^14,18 Mixed chelating ammines can also lead to the formation of unsaturated lanthanide complex, as exemplified by [Ho(trien)(en)GeS₃(SH)] (III). These results demonstrated that different chelating organic amines as structure-directing agents have a significant influence on the formation of lanthanide chalcogenidogermanate under solvothermal conditions (Scheme 1).

Scheme 1 View of the structural moieties of lanthanide thiogermanates with different chelating organic amine under solvothermal conditions.

Crystal structure

Description of the structures of Ia–d. Ia, Ib, Ic and Id are isomorphic, so we only discussed the structure of Ia. Ia crystallizes in monoclinic centrosymmetric space group of P2₁/n and contains one half [Ge₂S₆]⁴⁻ anion, one complete [Pr(dien)₃]³⁺ and one Cl⁻ ion in the asymmetric unit (Fig. 1a). The [Pr(dien)₃]³⁺ cation needs additional one Cl⁻ ion to compensate its positive charge. The dimeric [Ge₂S₆]⁴⁻ anion exhibits crystallographic C_i symmetry, which is formed by two edge-sharing [GeS₄] tetrahedra with four terminal S atoms. The Ge–S_b (b = bridging) bond distances (2.2596(10)-2.2803(10) Å) are slightly longer than the Ge–S_t (t = terminal) bond distances (2.1518(10)–2.1627(10) Å). The distorted tetrahedral [GeS₄] geometry can be seen from the S–Ge–S bond angles (92.09(4)–113.78(5)°), which deviate significantly from the ideal value of 109.5°. The Pr³⁺ ion is coordinated by 9 N atoms from three dien ligands. The [EuN₉] polyhedron can be described as a distorted tri-capped trigonal prism (Fig. 1b). The Ln–N bond lengths are 2.651(3)–2.733(3) Å for Pr–N, 2.599(4)–2.668(4) Å for Sm–N, 2.577(3)–2.661(3) Å for Gd–N and 2.544(6)–2.629(6) Å for Dy–N, and are consistent with those in other amino Ln³⁺ complexes.¹⁹

Fig. 1 (a) Molecular structure of Ia [all H atoms are omitted for clarity]. (b) The coordination environments of Pr³⁺ ion. (c) The stack of [Pr(dien)₃]³⁺ and Cl⁻ ions in Ia, showing 1-D tunnel-like features. (d) The 3-D supermolecular network in Ia. H atoms bonded to C atoms are omitted for clarity.

The [Pr(dien)₃]³⁺ cations adopt the NaCl-type stacking mode, (Fig. 1c). The shortest Pr⋯Pr distance is 9.44 Å. Cl⁻ ions are involved in intermolecular N–H⋯Cl hydrogen bonding with the –NH₂ groups of adjacent [Pr(dien)₃]³⁺ ions resulting in a 2-D layer (Fig. S1†). These layers are arranged in parallel with an interlayer distance of about 11.64 Å, leading to the formation of 1-D tunnel-like features that are filled by the [Ge₂S₆]⁴⁻ anions (Fig. 1d). There are a lot of short intermolecular N–H⋯S interactions that play an important role in stabilizing 1 in the solid state.

Description of the structure of II. II crystallizes in monoclinic centrosymmetric space group of P2₁/n with two formula units in the unit cell. II consists of a discrete dimeric [Ge₂S₆]⁴⁻ anion and a charge compensating binuclear [Er₂(dien)₄(μ-OH)₂]⁴⁺ complex cation. Two [Er(dien)₂]³⁺ groups are joined by two [μ₂-OH] bridging groups to form a centrosymmetric dinuclear complex cation with a [Er₂O₂] rhomboidal core. The Er⋯Er separation is 3.7231(5) Å, which is similar to that observed in other polynuclear erbium complexes.²⁰ The Er³⁺ center is in a 8-fold coordination of six N atoms of two dien ligands and two O atoms of two [μ₂-OH] groups. The [ErN₆O₂] polyhedron can be described as a distorted bicapped trigonal prism. The Er–O (2.216(2)–2.251(2) Å) and Er–N (2.466(3)–2.563(3) Å) bond lengths are compared with those reported for [Er₂(en)₆(μ₂-OH)₂][Sn₂S₆].²⁰ The dien ligands in [Er₂(dien)₄(μ-OH)₂]⁴⁺ ion display two different conformation, namely facial and U-shape, while the dien ligands of [Ln(dien)₃]³⁺ ions in Ia–d show only one type of facial conformation. The interatomic distances and angles in the [Ge₂S₆]⁴⁻ anion are similar to those in Ia–d and other discrete [Ge₂S₆]⁴⁻ anions.⁵ There are many important N–H⋯S and O–H⋯S hydrogen bonds between the [Ge₂S₆]⁴⁻ anions and [Er₂(dien)₄(μ-OH)₂]⁴⁺ cations. By these hydrogen-bond interactions, the [Ge₂S₆]⁴⁻ anions and Er₂(dien)₄(μ-OH)₂]⁴⁺ cations are assembled into a 3-D hydrogen-bond network structure (Fig. 2b).

Fig. 2 (a) Molecular structure of II [H atoms bonded to C/N atoms are omitted for clarity]. (b) 3-D hydrogen bonding network structure [H atoms bonded to C atoms are omitted for clarity].

Description of the structure of III. III crystallizes in monoclinic centrosymmetric space group of P2₁/c with four formula units in the unit cell. The Ho³⁺ ion is coordinated by one teta ligand and one bidentate [η²-GeS₃(SH)] chelating anion to give a neutral molecule [Ho(trien)(en)GeS₃(SH)] with an Ho⋯Ge distance of 3.521(1) Å (Fig. 3a). The polyhedron [HoN₆S₂] can be described as a distorted bicapped trigonal prism (Fig. 3b). The Ho–N (2.474(7)–2.541(6) Å) and Ho–S (2.7927(19)–2.833(2) Å) bonds are comparable to those in [Ho(1,10-phen)L₃] (L = N,N-diethylithiocarbato).²¹ The [GeS₃(SH)]³⁻ anion show a distorted tetrahedral geometry. The Ge1–S4 bond (2.317(3) Å) is significantly longer than other Ge–S bonds (2.163(2)–2.201(2) Å), showing that S4 is protonated. A very weak band at 2575 cm⁻¹, assigned to the S–H stretching vibration, confirms the presence of an –SH group. The –SH group has also been reported in the other sulfides [Ni(tepa)]₂[In₄S₇(SH)₂]·H₂O²² and [Ln(dien)₂(μ-η¹,η²-GeS₃(SH))]_n (Ln = La, Nd).⁷

Fig. 3 (a) Molecular structure of III [all H atoms are omitted for clarity]. (b) The coordination environments of Ho³⁺ ion. (c) Part of the crystal structure of III, showing the formation of a (100) sheet constructed from N–H⋯S hydrogen bonds. (d) 3-D hydrogen bond network structure. H atoms bonded to C atoms are omitted for clarity.

There are a lot of weak N–H⋯S hydrogen bonds between –NH groups and S atoms of [GeS₃(SH)]³⁻ anions. The [Ho(trien)(en)-GeS₄H] molecules are connected via N–H⋯S hydrogen bonds to form lays parallel to the (100) plane (Fig. 3c) with an interlayer distance of about 9.78 Å. Further interactions via N–H⋯S H-bonds gives a 3-D hydrogen bonding network (Fig. 3d).

The [GeS₄] tetrahedra under solvothermal conditions exhibit a characteristic tendency to condense via corner- or edge-bridging forming dimeric [Ge₂S₆]⁴⁻ or [Ge₄S₁₀]⁴⁻ moieties with transition metal complexes as counterions.⁵ But simply noncondensed [GeS₄]⁴⁻ or [GeS₃(SH)]³⁻ anion combined with lanthanide complexes is very scarce, and appear to be limited to that of [Ln(dien)₂(GeS₃(SH))]_n (Ln = La, Nd),⁷ where the [GeS₃(SH)]³⁻ anion acts as a [μ-η¹,η²-GeS₃(SH)] bridging ligand to [Ln(dien)₂]³⁺ group to form a 1-D coordination polymer. The [GeS₃(SH)]³⁻ anion in III acting as bidentate chelating ligand is the second example of noncondensed [GeS₄]⁴⁻ or [GeS₃(SH)]³⁻ species with lanthanide complexes under solvothermal conditions. Moreover, these organic hybrid lanthanide thiogermanates made by using solvothermal techniques usually contain only one type of organic amine, but different organic amines as chelating ligands and structure-directing agents have not been documented to date. Hence, III is the only known lanthanide thiogermanate combined with two different organic ligands.

Lanthanide contraction effect on the structures of lanthanide thiogermanates

Lanthanide contraction is a term used in chemistry to describe the steady decrease in the size of the atoms and ions of the lanthanide elements with increasing atomic number, as shown in compounds with coordination numbers nine for the earlier lanthanide(III) ions and eight for the later ones in aqueous solution.²³ The dien ligand prefers to coordinate to lighter lanthanide ions to form [Ln(dien)₃]³⁺ ions [Ln = Pr (Ia), Sm (Ib), Gd (Ic), Dy (Id) and Eu¹⁴], and to coordinate to heavier ones to give saturated binuclear [Ln₂(dien)₄(μ-OH)₂]⁴⁺ (Ln = Er(II)). The saturated [Ln(dien)₃]³⁺ or [Ln₂(dien)₄(μ-OH)₂]⁴⁺ ions only act as [Ge₂S₆]⁴⁻ anionic counterion. Evidently, two types of lanthanide complex cations are related to the Ln³⁺ ion size. To complete a coordination number of nine for both La³⁺ and Nd³⁺ ions, and eight for Ho³⁺ ion, the [GeS₃(SH)]³⁻ anion coordinates to unsaturated [Ln(dien)₂]³⁺ or [Ho(trien)(en)]³⁺ ions forming 1-D neutral chains [Ln(dien)₂(GeS₃(SH))]_n (Ln = La, Nd)⁷ and molecular [Ho(trien)(en)GeS₃(SH)] (III). These results demonstrated that lanthanide contraction has a significant influence on the formation of lanthanide thiogermanates under solvothermal conditions.

Optical properties

The solid-state UV/Vis absorption spectra of all compounds measured at room temperature are shown in Fig. 4 and S2.† The absorption edges are 2.35 eV for Ia, 2.36 eV for Ib, 2.36 eV for Ic, 3.59 eV for Id, 3.23 eV for II, and 3.54 eV for III, which are compared with values of other thiogermanates (2.10–3.53 eV), such as [Mn(en)₃]GeSb₂S₆ (2.10 eV, en = ethylenediamine),^4c [Co(dien)₂]₂GeSb₄S₁₀ (2.36 eV),^4c [Eu(dien)₃]₂[Ge₂S₆]Cl₂ (2.27 eV),¹⁴ [Y₂(tepa)₂(μ-OH)₂(μ-Ge₂S₆)](tepa)_0.5·H₂O (2.82 eV, tepa = tetraethylenepentamine),¹⁴ [M(dap)₃]₄Ge₄S₁₀Cl₄ [M = Co (3.21 eV) and Ni (3.31 eV), dap = diaminopropane],^5b [Ge₃S₆Zn(H₂O)S₃Zn(H₂O)][(Zn(tren)(H₂O)] (3.40 eV, tren = tris(2-aminoethyl)amine)²⁴ and (trenH₂)₂[Ge₂S₆] (3.53 eV).¹⁴ Moreover, there are several absorptions characteristic of the f–f transitions of Pr³⁺ {1.22 eV (³H₄ → ¹G₄) and 2.07 eV (³H₄ → ¹D₂)}, Sm³⁺ {1.00 eV (⁶H_5/2 → ⁶F_7/2), 1.14 eV (⁶H_5/2 → ⁶F_9/2), 1.29 eV (⁶H_5/2 → ⁶F_11/2)}, Dy³⁺ {1.13 eV (⁶H_15/2 → ⁶H_7/2 + ⁶F_9/2), 1.37 Ev (⁶H_15/2 → ⁶F_7/2), 1.53 eV (⁶H_15/2 → ⁶F_5/2)}, Ho³⁺ {1.27 eV (⁵I₈ → ⁵I₆), 1.89 eV (⁵I₈ → ⁵I₅), 2.38 eV (⁵I₈ → ⁵S₂ + ⁵F₄), 2.55 eV (⁵I₈ → ⁵F₃), 2.74 eV (⁵I₈ → ⁵G₆), 3.05 eV (⁵I₈ → ⁵G₅ + ³G₅)} and Er³⁺ {1.07 eV (⁴I_15/2 → ⁴F_11/2), 1.93 eV (⁴I_15/2 → ⁴F_9/2), 2.30 eV (⁴I_15/2 → ⁴S_3/2), 2.54 eV (⁴I_15/2 → ⁴F_7/2), 2.73 eV (⁴I_15/2 → ⁴F_5/2), 2.98 eV (⁴I_15/2 → ⁴F_3/2), 3.43 eV (⁴I_15/2 → ²H_9/2)} observed, which are agreement with the complicated UV-vis spectra of previously reported molecular lanthanide complexes.²⁵ No such band could be observed for Id.

Fig. 4 The solid-state UV-vis absorption spectra of Ia, II and III.

Theoretical studies

The band structure and density of states (DOS) of III were theoretically calculated using the computer code CASTEP. The valence bands (VBs) are dominated by the S 3p block, while the conduction bands (CBs) are mainly derived from Ho 4f states with a small contribution from Ge 4s orbitals (Fig. 5). Notably, the contributions from N and C atoms near the Fermi level are almost neglectable. The electronic absorption responsible for the optical gap is likely an electronic transfer excitation of S 3p to Ho 4f orbital electrons. The state energies of the lowest CBs and the highest VBs are located in the same Gamma point (Fig. S3†), and computational band gap of III is about 1.55 eV. Such value is smaller than the measured optical band gap, which may be related to the underestimation of the band gap by the DFT method.²⁶

Fig. 5 The total density-of-states and partial density-of-states for III. The Fermi level is set at 0 eV (dotted line).

Conclusions

Three structural types of lanthanide thiogermanates were obtained from LnCl₃/GeO₂/S/amine system by mild solvothermal reaction. Although some thiogermanates combinated with metal complexes have been reported, their anions are strongly dominated by [Ge₂S₆]⁴⁻ or [Ge₄S₁₀]⁴⁻ moieties. III contains simple tetrahedral [GeS₃(SH)]³⁻ acting as a chelating ligand to complex [Ho(en)(trien)]³⁺ cation, which is the rare example of noncondensed [GeS₄]⁴⁻ or [GeS₃(SH)]³⁻ species under solvothermal conditions. The above observations on the organic hybrid lanthanide thiogermanates could be helpful for a full complete comprehension of the coordination chemistry of Ln³⁺ ions.

Acknowledgements

This work was supported by the NNSF of china (no. 21163022), the NSF of Chongqing municipality (no. cstc2014jcyjA50002), the Key Project of Chinese Ministry of Education (no. 212132), and Program for Excellent Talents in Chongqing Higher Education Institutions. The authors are also grateful to Chongqing Normal University for financial support (13XLZ07).

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Footnote

† Electronic supplementary information (ESI) available: Crystal data in CIF format, additional figure, XRD data, and theoretical calculation. CCDC 1005128–1005132. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c4ra07812h

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