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. 2023 Jan 30;62(6):2913–2923. doi: 10.1021/acs.inorgchem.2c04371

Synthesis, Crystal Structures, and Optical and Magnetic Properties of Samarium, Terbium, and Erbium Coordination Entities Containing Mono-Substituted Imine Silsesquioxane Ligands

Patrycja Wytrych †, Józef Utko †, Mariusz Stefanski ‡, Julia Kłak †, Tadeusz Lis †, Łukasz John †,*
PMCID: PMC9930112  PMID: 36716237

Abstract

graphic file with name ic2c04371_0009.jpg

Mono-substituted cage-like silsesquioxanes of the T8-type can play the role of potential ligands in the coordination chemistry. In this paper, we report on imine derivatives as ligands for samarium, terbium, and erbium cations and discuss their efficient synthesis, crystal structures, and magnetic and optical properties. X-ray analysis of the lanthanide coordination entities [MCl3(POSS)3]·2THF [M = Er3+ (3), Tb3+ (4), Sm3+ (5)] showed that all three compounds crystallize in the same space group with similar lattice parameters. All compounds contain an octahedrally coordinated metal atom, and additionally, 3 and 5 structures are strictly isomorphous. However, surprisingly, there are two different molecules in the crystal structure of the terbium coordination entity 4, monomer (sof 65%) and dimer (sof 35%), with one and two metal centers. Absorption measurements of the investigated materials recorded at 300 K showed that regardless of the lanthanide involved, their energy band gap equals 2.7 eV. Moreover, the analogues containing Tb3+ and Sm3+ exhibit luminescence typical of these rare earth ions in the visible and infrared spectral range, while the compound with Er3+ does not generate any emission. Direct current variable-temperature magnetic susceptibility measurements on polycrystalline samples of 3–5 were performed between 1.8 and 300 K. The magnetic properties of 3 and 4 are dominated by the crystal field effect on the Er3+ and Tb3+ ions, respectively, hiding the magnetic influence between the magnetic cations of adjacent molecules. Complex 5 exhibits a nature typical for the paramagnetism of the samarium(III) cation.

Short abstract

In this paper, we report on imine-polyhedral oligomeric silsesquioxane derivatives as ligands for samarium, terbium, and erbium cations and discuss their efficient synthesis, crystal structures, and optical and magnetic properties.

Introduction

The variety of functionalization and further modification of the organic arms anchored to the silicon atoms in the polyhedral oligomeric silsesquioxane (POSS) cages makes these compounds unique systems with a wide range of applications. One of the most exciting futures is using mono-substituted or fully substituted cage-like architectures as a new class of potential ligands in coordination chemistry. For instance, in the case of POSS-containing complexes, the most investigated are metalated T7 moieties of formula R7Si7O12M (where R = a cyclopentyl or isobutyl group; M = a d-block cation) with additional ligands coordinated to the metal-containing corner.1−5 Zinc(II) alkyl silsesquioxanes were reported to co-polymerize carbon dioxide and cyclohexene oxide.6 Tin(II), aluminum(III), gallium(III), and zirconium(IV) POSS-based species were also investigated and used, among others, as models for alkene polymerization.7−9 Also, silsesquioxane-like complexes containing iron,10 titanium,11 and zirconium12 cation connections were reported as molecular models for heterogeneous catalysis. Other exciting applications of POSS-containing coordination entities and their hybrids were found, among other things, in the area of flexible displays, light-harvesting systems, up-converters for photovoltaic materials, chemical sensors, and so forth.13−16

More recently, POSS-based octahedral systems were also intensively explored. Cubic metal-containing silsesquioxanes can be divided into two groups. The first includes species with a metal ion directly anchored to the inorganic core, mainly via silicon or oxygen atoms. The second group comprises seven non-reactive organic arms and one metalated corner. Such compounds possess various features. As mentioned above, the metal sites attached to cubic inorganics can constitute efficient catalytic centers and play the role of soluble models for, for example, tethered Os4+ and Rh2+ species.17,18 An organopalladium(II) cage-like silsesquioxane was also studied as a catalyst for the Suzuki–Miyaura coupling.19 Also, our group described the molecular structures of the first Pd(II) coordination entities containing mono-functionalized amine-POSS ligands examined in the C–C coupling.20

Furthermore, a ruthenium-containing POSS was examined in silylative couplings.21 There are also known Al3+ and Zn2+ POSS species active in the ROP of rac-lactide.22,23 Also, it was shown that a reaction between ZnR2 (R = OAc, Et) and fully substituted imine-POSS leads to a tetrazinc(II) compound. The resulting coordination entity was studied as a catalyst for converting CO2 and epoxides into cyclic carbonates.24 In turn, the Hoveyda–Grubbs olefin metathesis can be performed using a ruthenium cage-like silsesquioxane.25 Other metal-based silsesquioxanes active in catalytic transformations include zirconium(IV) species active in SiO2-supported olefin polymerization.26 The literature reports are relatively sparse in the case of copper-containing silsesquioxanes. For example, Shul’pin et al. reported on isomeric copper(II)-sodium silsesquioxanes as efficient catalysts in alkane oxidation with peroxides.27 The same group also described the synthesis of a series of copper(II) silsesquioxanes with 2,9-dimethyl-1,10-phenanthroline28 and hexanuclear and “peanut cage” Cu9-cluster-containing phenylsilsesquioxane moieties.29,30 Such systems can be used in the oxidation of alcohols and alkanes with peroxides, and alcohols to ketones with tert-butyl hydroperoxide and in hyperperoxidation of alkanes with H2O2. Crucial findings concerning the reactivity with azide ions and voltammetric application of copper-containing octakis(3-aminopropyl)octasilsesquioxane were also conveyed.31 Another example is mono-substituted T8-type cage-like silsesquioxanes bound by trifunctional acyl chloride as N,O-donor ligand for Cu2+ cations.32

Lanthanide coordination entities are attractive because of their interesting physicochemical features and numerous applications as so-called new materials.33−35 Incredibly, the luminescence36 and magnetism37 of 4f element coordination entities have stimulated groundbreaking discoveries in these particular areas of interest. In the case of magnetism, various lanthanide cations are perfect candidates for magnetic materials manifesting high spins and introducing anisotropy derived from the nature of the f-electron shell.38,39 Several complexes possessing f-block cations have been available. However, it should be emphasized that little is known about the nature of the exchange interactions between rare earth cations and other magnetic centers.40 Furthermore, the detailed description of the magnitude and nature of the mentioned-above interactions is complex due to the significant orbital contributions dealing with lanthanide ions and the specific effects of the crystal field.41

We herein report on less-explored imine derivatives of mono-substituted POSSs applied as ligands for samarium, terbium, and erbium cations and discuss their efficient synthesis, crystal structures, and magnetic and optical properties.

Experimental Section

Materials

Trisilanolisobutyl-POSS (Hybrid Plastic, USA), tetramethylammonium hydroxide (25% in methanol, Alfa Aesar), and (3-aminopropyl)triethoxysilane (98%, Alfa Aesar) were purchased. Salicylaldehyde (98%), anhydrous erbium chloride (99.9%), anhydrous terbium chloride (99.9%), and anhydrous samarium chloride (99.9%) were purchased from Sigma-Aldrich (Darmstadt, Germany) and used without further purification. Ethanol (HPLC grade, J. T. Baker, Gdańsk, Poland), methanol (HPLC grade, J. T. Baker, Gdańsk, Poland), acetonitrile (HPLC grade, J. T. Baker, Gdańsk, Poland), toluene (HPLC grade, J. T. Baker, Gdańsk, Poland), and tetrahydrofuran (Chempur) were purchased. All solvents, except methanol and acetonitrile, were used with further purification. Tetrahydrofuran and toluene were purified by distillation with sodium wires, and ethanol was purified with metallic magnesium.

Methods

1H, 13C, and 29Si NMR spectra were recorded using a Bruker Avance 500 or equipped with broadband inverse gradient probe heads. All spectra were collected at 500 MHz using a relaxation delay of 1.0 s and a pulse width of 7°. Spectra were referred to the residual solvent signal (CDCl3 for 1H NMR and 13C NMR) or TMS (0.00 ppm for 29Si NMR) as an internal reference. FT-IR spectra of all compounds were measured using a Bruker Vertex 70 FTIR spectrometer. Samples spectra were recorded as KBr pellets. The FT-IR sample chamber was flushed continuously with N2 prior to data acquisition in the range 4000–400 cm–1 with a precision of ±1 cm–1. Elemental analyses (C, H, N, Cl) were performed using a Vario EL III element analyzer (Hanau, Germany). Quantitative analyses of Si, Er, Tb, and Sm were performed using an emission spectrometer iCAP 7400 DUO icp–Thermo Fisher Scientific (Waltham, MA, USA). Diffraction data for all resulting coordination entities were collected at 100 K on monocrystalline diffractometer with a XtaLAB Synergy R, DW system HyPix-Arc 150 κ-axis four-circle diffractometer with mirror-monochromated Mo Kα radiation. The CrysAlisPro software from Oxford Diffraction was used to determine cell parameters, data reduction, and absorption correction for all crystals. The structures were solved by direct methods (SHELXS97) and refined using the least-squares technique (SHELXL2013), CCDC nos.: 2226038 (for 3), 2226039 (for 4), and 2226040 (for 5). The absorption measurement was carried out in the backscattering mode using an Agilent Cary 5000 spectrophotometer. The emission spectra were collected using an FLS980 Fluorescence Spectrometer from Edinburgh Instruments. The laser diodes operating under 375 and 405 nm were used as excitation sources. The luminescence decay profiles were recorded using a femtosecond laser (Coherent Model Libra). Magnetic susceptibility and magnetization measurements were performed on a Quantum Design SQUID magnetometer (MPMS-3). Direct current (dc) magnetic measurements were carried out on polycrystalline samples of 3–5 in the 1.8–300 K temperature range with an applied external magnetic field of 0.5 T. Corrections are based on subtracting the sample-holder signal and contribution χD estimated from Pascal’s constants.42 The samples were restrained by adding a small amount of paraffin oil to prevent torquing.

Syntheses

All reactions were carried out under a dinitrogen atmosphere.

Synthesis of 1

1 was prepared following a modified procedure described by X. Zhou et al.43 Trisilanolisobutyl-POSS (5 g, 6.32 mmol) was dissolved in an ethanol/methanol mixture (30/10 mL). When stirring, to the solution was added tetramethylammonium hydroxide (14 μL, 0.13 mmol, 0.021 equiv), followed by the dropwise addition of (3-aminopropyl)triethoxysilane (1.47 mL, 6.32 mmol, 1 equiv). The mixture was stirred at RT for 7 days. After this time, the reaction mixture was concentrated on a rotary evaporator to give a white suspension, which was precipitated with acetonitrile. Then, the resulting white solid was filtered and dried under reduced pressure to a white powder (5.36 g, 97%). 1H NMR (500 MHz, CDCl3, 300 K, ppm): δ 2.64 (t, 2H, 3JHH = 7,14 Hz, CH2–N), 1.83 (quint, 7H, 3JHH = 6,5 Hz, CH, iBu), 1.50 (m, 2H, CH2), 0.93 (dd, 42H, 3JHH = 6,71 Hz, CH3, iBu), 0.58 (m, 16H, two signals 14H and 2H overlapped, SiCH2+ SiCH2, iBu). 13C NMR (500 MHz, CDCl3, 300 K): δ 44.92 (CH2–N), 27.31 (CH2), 25.84 (CH3, iBu), 24.00 (CH, iBu), 22.66 (CH2, iBu), 9.41 (SiCH2). 29Si NMR (500 MHz, CDCl3, 300 K): δ −67.28 (SiCH2CH2CH2NH2), −67.70 (3 Si closed to amine arm), −67.89 (remaining 4 Si). FT-IR (cm–1, KBr): νNH 3436 (s), νCH 2955 (m), νNH 1623 (m), νCH 1466 (s), δCH3 1367 (s), νC–N 1333 (s), νSi–CH 1231 (s), νSi–O–Si 1114 (s), δCH 838 (s), δCH 746 (m), δO–Si–C 483 (s). Elemental analysis (%) for C31H71NO12Si8: calcd C 42.57, H 8.18, N 1.60, Si 25.69; found, C 42.48, H 8.14, N 1.53, Si 25.64.

Synthesis of 2

Imine-functionalized POSS 2 was prepared following a modified procedure described by Jones and Mahon et al.231 (2.00 g, 2.29 mmol) was dissolved in toluene (30 mL), and salicylaldehyde (0.30 mL, 2.86 mmol, 1.25 equiv) was added. The reaction was performed at RT for 24 h. The resulting yellow suspension was then filtered and concentrated under vacuum. A yellow oil was obtained, which was recrystallized from EtOH, leading to precipitation. The precipitate was filtered and dried in vacuo to give a yellow powder of 2 (1.96 g, 87.5% yield). 1H NMR (500 MHz, CDCl3, 300 K): δ 8.32 (s, 1H, N=CH), 7.30 (td, 1H, 3JHH = 8.68 Hz, Ar), 7.23 (dd, 1H, 3JHH = 7.71 Hz, Ar), 6.97 (d, 1H, 3JHH = 8.16 Hz, Ar), 6.87 (td, 1H, 3JHH = 7.35 Hz, Ar), 3.58 (t, 2H, 3JHH = 6.77 Hz, CH2), 1.85 (m, 9H, CH, iBu), 0.95 (m, 42H, CH3, iBu), 0.67 (m, 2H, Si–CH2), 0.61 (m, 14H, Si–CH2). 13C NMR (500 MHz, CDCl3, 300 K): δ 164.84 (CH, C=N), 161.65 (C, Ar–OH), 132.19 (Ar), 131.32 (Ar), 118.99 (Ar), 118.52 (Ar), 117.20 (Ar), 62.21 (CH2–N), 25.84 (CH3, iBu), 24.54 (CH2), 24.04 (CH, iBu), 22.66 (CH2, iBu), 9.80 (Si–CH2). 29Si NMR (500 MHz, CDCl3, 300 K): δ −67.28 (SiCH2CH2CH2NH=C), −67.70 (3 Si close to functionalized arm), −67.89 (remaining 4 Si). FT-IR (cm–1, KBr): νOH 3435 (s), νCH 2955 (m), νC=N 1635 (s), δN–H 1585 (m), νCH 1466 (s), δCH3 1367 (s), νC–N 1334 (s), νC–O 1282 (s) νSi–CH 1231 (s), νSi–O–Si 1112 (s), δCH 838 (s), δCH 746 (m), δO–Si–C 483 (s). Elemental analysis (%) for C38H75NO13Si8: calcd C 46.63, H 7.72, N 1.43, Si 22.96; found, C 46.58, H 7.62, N 1.37, Si 22.91.

Synthesis of 3

Anhydrous erbium chloride (0.12 g, 0.45 mmol) and 2 (1.31 g, 1.34 mmol, 3 equiv) were dissolved in THF (30 mL). The reaction mixture was stirred at RT for 24 h. The resulting yellow suspension was then filtered and concentrated to ca. 10 mL. The solution was stored in a refrigerator. After about 10 days, colorless crystals of 3 were obtained (1.31 g, 91%). FT-IR (cm–1, KBr): νOH 3435 (s), νCH 2955 (m), νC=N 1659 (s), δN–H 1610 (s), δC=C 1540 (s), νCH 1467 (s), δCH3 1367 (s), νC–N 1334 (s), νC–O 1292 (s) νSi–CH 1231 (s), νSi–O–Si 1109 (s), δCH 838 (s), δCH 741 (s), δO–Si–C 481 (s). Elemental analyses (%) for ErCl3(C38H75NO13Si8)3(C4H8O): calcd C 43.19, H 7.16, N 1.28, Cl 3.24, Er 5.10, Si 20.54; found, C 43.01, H 7.02, N 1.19, Cl 3.21, Er 5.02, Si 20.49.

Synthesis of 4

Anhydrous terbium chloride (0.083 g, 0.31 mmol) and 2 (0.91 g, 0.93 mmol, 3 equiv) were dissolved in THF (30 mL). The reaction mixture was stirred at RT for 24 h. The resulting yellow suspension was then filtered and concentrated to ca. 10 mL. The solution was stored in the refrigerator. After about 10 days, colorless crystals of 4 were obtained (0.90 g, 88%). FT-IR (cm–1, KBr): νOH 3435 (s), νCH 2955 (m), νC=N 1657 (s), δN–H 1610 (s), δC=C 1540 (s), νCH 1467 (s), δCH3 1367 (s), νC–N 1333 (s), νC–O 1291 (s) νSi–CH 1231 (s), νSi–O–Si 1107 (s), δCH 838 (s), δCH 741 (s), δO–Si–C 482 (s). Elemental analyses (%) for TbCl3(C38H75NO13Si8)3(C4H8O): calcd C 43.30, H 7.17, N 1.28, Cl 3.25, Tb 4.86, Si 20.59; found, C 43.49, H 7.01, N 1.21, Cl 3.18, Tb 4.76, Si 20.53.

Synthesis of 5

Anhydrous samarium chloride (0.073 g, 0.28 mmol) and 2 (0.83 g, 0.85 mmol, 3 equiv) were dissolved in THF (30 mL). The reaction mixture was stirred at RT for 24 h. The resulting yellow suspension was then filtered and concentrated to ca. 10 mL. The solution was stored in the refrigerator. After about 10 days, colorless crystals of 5 were obtained (0.85 g, 92%). FT-IR (cm–1, KBr): νOH 3430 (s), νCH 2955 (m), νC=N 1656 (s), δN–H 1610 (s), δC=C 1540 (s), νCH 1466 (s), δCH3 1367 (s), νC–N 1333 (s), νC–O 1290 (s) νSi–CH 1231 (s), νSi–O–Si 1108 (s), δCH 838 (s), δCH 741 (s), δO–Si–C 482 (s). Elemental analyses (%) for SmCl3(C38H75NO13Si8)3(C4H8O): calcd C 43.41, H 7.19, N 1.29, Cl 3.26, Sm 4.61, Si 20.64; found, C 43.32, H 7.08, N 1.22, Cl 3.21, Sm 4.55, Si 20.61.

Results and Discussion

Synthesis and Crystal Structures

In this study, we focused on imine-POSS-based coordination entities containing Er3+, Tb3+, and Sm3+ cations with well-defined crystal structures. In the first step, an imine silsesquioxane ligand 2 was obtained in the reaction between 3-aminopropylheptaisobutyl-POSS 1 with salicylaldehyde (Scheme 1).

Scheme 1. Synthesis of Imine-POSS Ligand 2.

Scheme 1

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In the next step, we reacted 2 and anhydrous lanthanide chlorides LnCl3 (Ln = Er3+, Tb3+, Sm3+) in a 3:1 molar ratio. Reactions were performed in tetrahydrofuran at room temperature for 24 h. All reactions led to the isolation of three lanthanide complexes of the formula [MCl3(POSS)3]·2THF (M = Er (3), Tb (4), and Sm (5)) in a crystalline form with 91, 88, and 92% yields for 3, 4 and 5, respectively.

The structure of 2 was unambiguously confirmed by multinuclear NMR analyses (1H, 13C, and 29Si), infrared spectroscopy, and elemental analysis. The chemical shifts of 29Si NMR signals within the expected region for mono-alkyl-substituted T8 cage-like silsesquioxanes at δ = −67.28, −67.70, and −67.89 ppm for one SinPrNH=C functionalized arm, three Si atoms which are close to reactive organic arm, and four remaining silicon nuclei, respectively, confirmed the presence of three kinds of Si atoms forming an inorganic silsesquioxane core. Moreover, the characteristic peak at δ = 8.32 ppm in the 1H NMR spectra indicated the presence of an N=CH fragment belonging to one imine arm attached to a cage-like core. Also, the presence of the N=C bond was confirmed by 13C NMR (chemical shift at 164.84 ppm). Moreover, the ν(O–H) vibration from the salicylimine fragment was observed at 3435 cm–1.

The structures of 3–5 coordination entities were confirmed by FT-IR, elemental analysis, and X-ray studies. In the FT-IR spectra of lanthanide complexes, the Si–O–Si stretching gave strong absorptions around 1109, 1107, and 1108 cm–1 for 3, 4, and 5, respectively. Moreover, the absorption of ν(C=N) and ν(O–H) groups was observed at 1659 and 3435, 1657 and 3435 cm–1, and 1656 and 3430 cm–1 for 3, 4, and 5, respectively.

In turn, the X-ray analysis of single crystals showed that compounds [ErCl3(POSS)3]·2(C4H8O) (3), [TbCl3(POSS)3]·2(C4H8O) (4), and [SmCl3(POSS)3]·2(C4H8O) (5) crystallize in a triclinic system in the P1̅ space group. Crystallographic data for 3, 4, and 5 are presented in Table 1.

Table 1. Crystallographic Data for 3, 4, and 5a.

  3 4 5
Crystal Data
chemical formula C114H225Cl3N3O39Si24Er·2(C4H8O) C114H225Cl3N3O39Si24Tb·2(C4H8O) C114H225Cl3N3O39Si24Sm·2(C4H8O)
Mr 3353.93 3345.59 3337.02
crystal system, space group triclinic, P1̅ triclinic, P1̅ triclinic, P1̅
temperature (K) 100 100 100
a, b, c (Å) 16.230 (2), 22.743 (3), 27.092 (3) 16.258 (2), 22.790 (3), 27.159 (3) 16.241 (2), 22.783 (3), 27.202 (3)
α, β, γ (°) 110.97 (2), 91.51 (2), 108.79 (2) 111.10 (2), 91.42 (2), 108.75 (2) 111.19 (2), 91.62 (2), 108.66 (2)
V (Å3) 8726 (2) 8777 (2) 8773 (2)
Z 2 2 2
radiation type Mo Kα Mo Kα Mo Kα
μ (mm–1) 0.76 0.68 0.61
crystal size (mm) 0.24 × 0.16 × 0.09 0.21 × 0.11 × 0.10 0.17 × 0.09 × 0.09
Data Collection
diffractometer XtaLABSynergy R, DW system, HyPix-Arc 150 XtaLABSynergy R, DW system, HyPix-Arc 150 XtaLABSynergy R, DW system, HyPix-Arc 150
absorption correction Gaussian multi-scan Gaussian
Tmin, Tmax 0.651, 1.000 0.898, 1.000 0.739, 1.000
no. of measured, independent, and observed [I > 2σ(I)] reflections 227894, 34280, 31771 212386, 32675, 28988 236299, 44455, 39035
Rint 0.037 0.076 0.060
(sin θ/λ)max (Å–1) 0.617 0.606 0.671
Refinement
R[F2> 2σ(F2)], wR(F2), S 0.060, 0.157, 1.10 0.072, 0.188, 1.10 0.058, 0.157, 1.05
no. of reflections 34280 32675 44455
no. of parameters 1741 1773 1746
no. of restraints 34 1 36
H-atom treatment H-atom parameters constrained H-atom parameters constrained H-atom parameters constrained
  w = 1/[σ2(Fo2) + (0.0636P)2 + 40.2234P] where P = (Fo2 + 2Fc2)/3 w = 1/[σ2(Fo2) + (0.0765P)2 + 47.4307P] where P = (Fo2 + 2Fc2)/3 w = 1/[σ2(Fo2) + (0.0728P)2 + 22.8589P] where P = (Fo2 + 2Fc2)/3
Δ⟩max, Δ⟩min(e Å–3) 2.26, −1.43 2.42, −1.36 2.44, −1.06
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a

Computer programs: CrysAlis PRO 1.171.41.112a (Rigaku OD, 2021), SHELXS97 (Sheldrick, 2008), and SHELXL2018/1 (Sheldrick, 2018).

The general formula of all three compounds is [MCl3(POSS)3]·2THF (M = Er3+, Tb3+, Sm3+), and they all contain an octahedrally coordinated metal cation (Er3+, Tb3+, and Sm3+ for 3, 4 and 5, respectively). Erbium (3) and samarium (5) structures are strictly isomorphous because they differ only in the metal atom in the molecule’s center.

Compounds 3 and 5 consist of a six-coordinated metal cation bound to three oxygen atoms and three chlorine anions (Figure 1). The oxygen atoms belong to the three phenolate groups of the three POSS ligands, while the chlorine atoms are derived from the lanthanide(III) chloride salts used in the synthesis. Those molecules have a distorted octahedral geometry and are meridional (mer) isomers.

Figure 1.

Figure 1

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Molecular structures of 3, 4, and 5. Hydrogen atoms are omitted for clarity. For the sake of clarity, the image is presented in stick mode, omitting the thermal ellipsoids.

However, surprisingly, the terbium compound is only 65% identical to compounds 3 and 5. In the crystal structure of 4, there are two different molecules. One of them is a monomer with the same structure as the erbium and samarium species. On the other hand, 35% is a dimer, in which there are two metallic centers. In this case, the Tb3+ cation is bonded to only two oxygen atoms of POSS ligands. The imine arm of the third cage is bent in the opposite direction around the Si10C–C10C bond. As a result, the torsion angle between O13C–Si1C–C10C–C9C/C9E changes from 81.21(2) to −75.73(2)o, and the oxygen atom of the third POSS cage is coordinated to the second lanthanide cation so that both metal centers have octahedral geometry (Figure 2). The dimer molecule of the terbium compound was created as a result of a different arrangement in the space of one of the imine arms of the POSS and is probably also energetically beneficial.

Figure 2.

Figure 2

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Asymmetric unit (a) and dimer molecule (b) of 4. Hydrogen atoms are omitted for clarity. For the sake of clarity, the image is presented in stick mode, omitting the thermal ellipsoids.

In the crystal structures of 3–5, apart from the lanthanide complexes, there are also two non-coordinated molecules of tetrahydrofuran, which was used as a solvent during synthesis. Selected bond lengths and angles for 3, 4, and 5 are presented in Table 2.

Table 2. Selected Bond Lengths (Å) and Angles (°) for 3, 4, and 5a.

3 Er–O2A 2.230 (3) O2C–Er–O2B 171.17 (14) O2B–Er–Cl3 88.99 (9)
  Er–O2B 2.239 (3) O2A–Er–O2B 96.82 (11) Cl1–Er–Cl3 95.78 (10)
  Er–O2C 2.161 (3) O2C–Er–Cl1 85.48 (12) O2C–Er–Cl2 96.81 (10)
  Er–Cl1 2.6155 (14) O2A–Er–Cl1 175.71 (12) O2A–Er–Cl2 81.64 (8)
  Er–Cl2 2.6490 (13) O2B–Er–Cl1 85.93 (10) O2B–Er–Cl2 81.96 (9)
  Er–Cl3 2.6163 (16) O2C–Er–Cl3 93.93 (11) Cl1–Er–Cl2 95.52 (7)
  O2C–Er–O2A 91.64 (14) O2A–Er–Cl3 87.58 (11) Cl3–Er–Cl2 164.97 (7)
4 Tb–O2A 2.266 (4) O2Ei–Tb–O2B 167.5 (3) O2A–Tb–Cl3 84.4 (3)
  Tb–O2B 2.271 (4) O2C–Tb–O2B 177.3 (3) O2B–Tb–Cl3 88.49 (16)
  Tb–O2C 2.253 (11) O2A–Tb–O2B 97.99 (14) Cl1–Tb–Cl3 100.4 (2)
  Tb–O2Ei 2.156 (17) O2Ei–Tb–Cl1 81.6 (3) O2Ei–Tb–Cl2 96.2 (4)
  Tb–Cl1 2.6457 (16) O2C–Tb–Cl1 92.2 (4) O2C–Tb–Cl2 100.8 (3)
  Tb–Cl2 2.6930 (15) O2A–Tb–Cl1 173.71 (12) O2A–Tb–Cl2 81.18 (10)
  Tb–Cl3 2.678 (4) O2B–Tb–Cl1 86.30 (10) O2B–Tb–Cl2 81.50 (10)
  O2Ei–Tb–O2A 93.8 (4) O2Ei–Tb–Cl3 96.9 (4) Cl1–Tb–Cl2 94.99 (5)
  O2C–Tb–O2A 83.7 (4) O2C–Tb–Cl3 89.6 (3) Cl3–Tb–Cl2 161.04 (15)
5 Sm–O2A 2.309 (2) O2C–Sm–O2B 169.93 (10) O2B–Sm–Cl3 87.61 (7)
  Sm–O2B 2.321 (2) O2A–Sm–O2B 99.86 (9) Cl1–Sm–Cl3 98.47 (6)
  Sm–O2C 2.245 (3) O2C–Sm–Cl1 84.47 (9) O2C–Sm–Cl2 99.06 (7)
  Sm–Cl1 2.6806 (11) O2A–Sm–Cl1 173.54 (8) O2A–Sm–Cl2 80.50 (6)
  Sm–Cl2 2.7283 (11) O2B–Sm–Cl1 85.59 (7) O2B–Sm–Cl2 80.54 (7)
  Sm–Cl3 2.7042 (12) O2C–Sm–Cl3 95.47 (8) Cl1–Sm–Cl2 97.10 (5)
  O2C–Sm–O2A 89.97 (10) O2A–Sm–Cl3 85.31 (7) Cl3–Sm–Cl2 159.61 (3)
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a

Symmetry code (s): (i) −x + 1, −y + 1, −z + 1.

The obtained compounds can be compared with molecules containing similar fragments, in which the lanthanide cation is coordinated with the chlorine atoms and the oxygen atoms of the phenolate group. The lengths of M–O bonds with the oxygen atoms of the phenolate groups of POSS ligands in compounds 3, 4, and 5 are similar. Their values are given in the following ranges: 2.161 (3)–2.239 (3) Å for 3, 2.253 (11)–2.271 (4) Å for 4, 2.245 (3)–2.321 (2) Å for 5. Slight differences in the lengths of the M–O bonds in compounds 3, 4, and 5 result from the different lengths of the ionic radii of the lanthanide cations. These values correspond to the length of the M–O bonds found in the compounds Eu, Dy, and Er described by Huang et al.44 and Tb, Dy, Ho, and Er reported by Li et al.,45 which contain M–O–Ph fragments.

A similar relation occurs for the length of M–Cl bonds in compounds 3, 4, and 5, the values of which are in the following ranges: 3, 2.6155 (14)–2.6490 (13) Å; 4, 2.6457 (16)–2.6930 (15) Å; and 5, 2.6806 (11)–2.7283 (11) Å. These values also correspond to the literature data. In this case, erbium, terbium, and samarium compounds were compared with octahedral lanthanide compounds described by Li et al.,45 in which three chlorine anions coordinate to the metal cation.

Intramolecular hydrogen bonds additionally stabilize the structures of 3–5 coordination entities. These bonds occur in all silsesquioxane ligands between the oxygen atoms of the phenolate groups and the NH+ moiety (Figure 3). Due to the tautomerism that occurs on the imine arms of the POSS, these moieties are zwitterions. As a result, the hydroxyl groups of the salicylic fragment of the molecule are deprotonated, while the positive charge is on the nitrogen atom.

Figure 3.

Figure 3

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Fragment of the molecular structure of 3 showing the POSS ligand with marked hydrogen bond (a); the structural formula of the POSS ligand (b).

Optical Properties of 3–5

The diffuse reflectance spectra of 3–5 compounds measured at 300 K are presented in Figure 4.

Figure 4.

Figure 4

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Diffuse reflectance spectra of 3 (a), 4 (b), and 5 (c) samples recorded at room temperature.

It can be seen that all samples reveal a broad band in the range of 200–490 nm, which can be attributed to material absorption. The mentioned band is so wide and intense that it covers the visible range in which the absorption transitions of embedded lanthanides usually occur. Because of its electronic structure, only narrow bands from Er3+(3) ions can be observed there.46 In the case of the NIR range, the absorption transitions characteristic of Er3+(3) and Sm3+(5) ions in the respective analogues have been identified (see Figure 4, insets).46,47

The absorption spectra were recalculated to determine the energy band gap (Eg) of the 3–5 samples using Kubelka–Munk formula48

graphic file with name ic2c04371_m001.jpg 1

where R means reflectance. The Eg of analyzed materials was calculated to be 2.7 eV regardless of the lanthanide contained in the crystal structure (see Figure S12).

In order to characterize the optical properties of the investigated materials, their emission spectra were recorded. The measurements were performed using the excitation lines of 375 and 405 nm for the sample containing Tb3+(4) and Sm3+(5) ions, respectively, for the direct population of higher levels of these lanthanides (Figure 5).

Figure 5.

Figure 5

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Emission spectra of 4 (a) and 5 (b,c) samples recorded at room temperature and their chromatic coordinates (d).

During the research, it was found that 3 and 4 excited by the 405 nm line corresponding to the most intense absorption band did not generate any spectral response. This proves the lack of energy transfer between the lanthanide ions and the ligand in these species. It is worth mentioning that none of the excitation lines typical of the Er3+ (3) produced no luminescence from this rare earth ion, in neither the visible (VIS) range nor the infrared (NIR) range, probably due to strong non-radiative processes.49 However, further detailed analysis is needed to investigate the nature of this luminescence quenching thoroughly. Figure 5a demonstrates the emission spectrum of 4 recorded under 375 nm excitation at 300 K. It can be seen that the sample shows four narrow spectral lines in the range of 480–630 nm, characteristic for Tb3+ ions. Bands with a maximum at 488, 542, 584, and 620 nm can be assigned to transitions from the 5D4 emission level to 7F6, 7F5, 7F4, and 7F3 states, respectively.50,51Figure 5b,c shows the emission spectra of 5 compound measured upon 405 nm excitation at 300 K. The sample exhibits typical Sm3+ transitions in both the visible and infrared spectral ranges. Bands centered at 564, 600, 647, and 711 nm in VIS and 909, 953, 1037, and 1175 nm in NIR were attributed to transitions from the 4G5/2 emission level to 6H5/2, 6H7/2, 6H9/2, and 6H11/2 and 6F3/2, 6F5/2, 6F7/2, and 6F9/2 states, respectively.52,53 The luminescence decay profiles for the dominant Tb3+ (4) (5D4 → 7F5) and Sm3+ (5) (4G5/2 → 6H9/2) bands were recorded and are plotted in Figure S13. The curves were well fitted by a biexponential function indicating the presence of some non-radiative processes. The average lifetimes for the Tb3+ and Sm3+ ions calculated using eq 2 were equal to 0.68 ms and 13.7 μs, respectively

graphic file with name ic2c04371_m002.jpg 2

Here, A1 and A2 are the fitting constants, and τi (i = 1, 2) means the decay time of the i component.

In order to visualize the color of the registered luminescence of 4 and 5 coordination entities, their chromatic coordinates were determined and are shown in Figure 5d. It turned out that their emission color is located in the yellowish-green and reddish-orange regions, respectively.

Magnetic Properties of 3–5

Because of their large and anisotropic magnetic moment, lanthanide cations and their coordination entities are of constant interest in molecular magnetism.54−56 The anisotropy of magnetic susceptibility at low temperatures is primarily determined by the anisotropy of the g-tensor of the ground electronic level and a few low-lying crystal field levels of the lanthanide ions. With increasing temperature, the crystal field levels at higher energy are progressively occupied, and magnetic anisotropy decreases rapidly, although still very noticeable at room temperature and above. However, interpreting the magnetic data of Ln3+ compounds is continuously challenging because of the large orbital contribution of these ions. Another issue for these, usually of low symmetry, species is the disentanglement of the single-ion magnetic anisotropy from the exchange contributions in the analysis of the temperature dependence of the magnetic susceptibility.

Direct current (dc) magnetic susceptibility measurements were carried out on a polycrystalline sample of 3–5 lanthanide complexes between 1.8 and 300 K with an applied magnetic field of 0.5 Tesla. Plots of χmT product versus T (χm is the molar magnetic susceptibility per Ln3+ ion) for 3–5 are given in Figure 6.

Figure 6.

Figure 6

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Temperature dependence of experimental χmT for 3–5 (χm per Ln3+ ion).

The experimentally determined values of χmT of investigated coordination entities at room temperature were compared with the theoretical values of χmT calculated by the equation Inline graphic, where N is the Avogadro constant, β is the Bohr magneton, k is Boltzman’s constant, and gLn is the g factor of the ground J terms of Ln3+ expressed as gLn = 3/2 + S(S + 1) – L(L + 1)/2J(J + 1).57 At 300 K, the χmT products of 10.62 cm3 K mol–1 (3) and 12.06 cm3 K mol–1 (4) are close to the expected values for single non-interacting Er3+ (11.48 cm3 K mol–1, 4I15/2, S = 3/2, L = 6, J = 15/2, g = 6/5) and Tb3+ (11.80 cm3 K mol–1: 7F6, S = 3, L = 3, J = 6, g = 3/2) ions, respectively. The values are within the range previously found for mononuclear Er3+ and Tb3+ compounds.58−61 Although there are two different molecules in the crystal structure of the terbium compound, monomer and dimer, magnetic studies correspond to only one metal center. Upon cooling, the χmT for both compounds remains relatively constant down to 100 K, below which it begins a steeper reduction to 4.27 cm3 mol–1 K (3) and 7.08 cm3 mol–1 K (4) at 1.8 K. The gradual decrease of χmT upon cooling can be explained by a progressive depopulation of excited Stark sublevels because of the ligand field effects. This suggests the presence of significant magnetic anisotropy characteristics for lanthanide(III)-containing coordination entities.61−66 Such behavior of 3 and 4 can also be attributed to weak intermolecular interactions. The plot of χm–1 versus T obeys the Curie–Weiss law in the whole temperature range with a negative Weiss constant θ = −3.52 K (3) and −1.47 K (4). The negative values of the Weiss temperatures, in combination with the changing tendency of the temperature dependence of χmT for both complexes, lead to a conclusion that the magnetic interaction is weak antiferromagnetic. However, the great intermolecular metal–metal separation in these mononuclear coordination entities could not favor the exchange coupling. Unfortunately, the quantitative description of the magnetic properties of lanthanide(III) species is cumbersome due to the ligand-field effect and spin–orbit coupling of the Ln3+ cation.67 Magnetization studies verified the nature of the ground state of 3 and 4 at 2–8 K in the field range of 0–7 T (Figures S14 and S15, respectively). The M versus H curves for both complexes show a sharp increase in magnetization at a low field limit with a linear response to the magnetic moment upon increasing the magnetic field without any saturation. The values of the magnetization at 2 K and 7 T are 5.63 μB (3) and 5.53 μB (4), respectively. They are well below the expected value for the saturation of the magnetization of free, non-interacting Er3+ and Tb3+ ions, which is 9 μB (considering J = 15/2 and J = 6 ground state, respectively). Non-saturation of magnetization at a high field limit (at 2 K) might arise from the intermolecular interactions, the magnetic anisotropy, and significant crystal field effects associated with both complexes. As discussed above, taking into account the quite long intermolecular distance between the closest Ln···Ln centers, it seems unlikely that the intermolecular interactions will have a substantial impact on the magnetism. The presence of single-ion magnetic anisotropy is also confirmed by the non-superimposable nature of the magnetization curves at higher temperatures (Figures S14 and S15, respectively). Due to the large anisotropy,60,61 some terbium(III) or erbium(III) complexes show SMM behavior. Upon application of a static dc field of 0.1 T, both in-phase (χ′) and out-of-phase (χ″) susceptibilities show no frequency dependence peaks in the 1.8–25 K temperature range for an oscillating field range of 1 to 900 Hz for complexes 3 and 4, which exclude the presence of magnetic ordering or slow paramagnetic relaxation. This suggests that the anisotropy around lanthanide(III) ions is not large enough in both complexes for the requirement of SMM, which may probably be quenched by the very weak intermolecular interaction. The other important reason is the poorly axial symmetry of the coordination geometry of the Tb3+ and Er3+ ions.

The 6H ground term for the free Sm3+ ion is split into six levels by spin–orbit coupling, and the spin–orbit coupling parameter is 200 cm–1. This means that the crystal-field effect and the possible thermal population of the higher states should be assessed for the samarium(III) coordination entity. As shown in Figure 6, the plot of χmT versus T for 5 is nearly linear over the whole temperature range and similar to that reported for mononuclear samarium(III) compounds.68−72 Therefore, the relationship of 1/χm versus T does not obey Curie law or Curie–Weiss law. The value of χmT is equal to 0.279 cm3 K mol–1 at room temperature and decreases rapidly with the temperature to a value of 0.03 cm3 K mol–1 at 1.8 K, which is lower than the value of 0.089 cm3 K mol–1 predicted by theory. This difference may be because the 6H5/2 ground state of Sm3+ is split into three Kramers doublets.57

Conclusions

In this publication, we presented the use of the mono-substituted imine-POSS 2, which was obtained in the reaction of 3-aminopropylheptaisobutyl-POSS, as a ligand for selected lanthanide ions, such as erbium (3), terbium (4), and samarium (5) of the formula [MCl3(POSS)3]·2THF (M = Er3+, Tb3+, Sm3+). The structure of the ligand and the coordination compounds was fully confirmed in solution and solid states. The X-ray analysis of single crystals showed that the erbium and samarium complexes are isomorphous because they differ only in the metal cation in the molecule’s center. However, surprisingly, the terbium compound is only 65% identical to compounds 3 and 5. In the crystal structure of 4, there are two different molecules. One of them is a monomer with the same structure as the erbium and samarium species. On the other hand, 35% is a dimer, in which there are two metallic centers. Measurements of the optical properties of 3–5 coordination entities have shown that apart from the narrow absorption transitions characteristic of lanthanides, a wide broad material band corresponding to the matrix’s absorption, covering the spectral range up to 490 nm, is also observed. These measurements made it possible to determine the energy band gap of the studied compounds equal to 2.7 eV, regardless of the lanthanide contained in their crystal structure. Direct excitation of rare earth ions led to the appearance of luminescence with spectral characteristics typical for the investigated lanthanides. The exception was the analogue containing Er3+ ions, whose luminescence was quenched through non-radiative processes. In turn, 5 shows behavior characteristics for the well-isolated mononuclear system, whereas the magnetic properties of 3 and 4 are dominated by the significant orbital contributions and the crystal field effect on the lanthanide(III) site, masking the magnetic interaction between the paramagnetic centers.

Acknowledgments

This work was financially supported by the National Science Centre, Poland (grant no. 2020/39/B/ST4/00910).

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.inorgchem.2c04371.

  • NMR (1H, 13C, 29Si), FT-IR spectra, additional optical and magnetic properties, and crystallographic refinement details (PDF)

Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

National Science Centre, Poland (grant no. 2020/39/B/ST4/00910).

The authors declare no competing financial interest.

Supplementary Material

ic2c04371_si_001.pdf (999.1KB, pdf)

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