2023 Volume 6 Issue 1

Open Access

INEOS OPEN, 2023, 6 (1), 5–9  Journal of Nesmeyanov Institute of Organoelement Compounds of the Russian Academy of Sciences Download PDF Electronic supplementary information DOI: 10.32931/io2301a

An Organometallic Approach to the Synthesis of
Cationic Yttrium Bis(Alkoxide) Complexes

Abstract

The synthesis and structure of a cationic bis(alkoxide) yttrium complex {[(C₆F₅)₂CHO]₂Y(THF)₄}⁺{BPh₄}^– that represents a separated ion pair with octahedral coordination environment of the Y³⁺ ion and angular mutual arrangement of the alkoxide ligands are reported. The formation of the related yttrium complexes containing {Ph(Me)(CF₃)CO}^– and {(C₆F₅)Me₂CO}^– ligands is evidenced by multinuclear NMR spectroscopy.

Key words: yttrium, alkoxide complexes, synthesis, structure.

 

Introduction

The molecular design of the coordination environment of rare-earth ions represents an important fundamental and applied problem in terms of fine-tuning the chemical and physical properties of the complexes [1, 2]. This is of particular importance for the elaboration of new materials for application in electronics. At a certain geometry of the ligand environment, the complexes of some lanthanide metals (Dy, Tb, Er) exhibit slow relaxation of magnetization, which is due to bistability at the molecular level [3, 4]. Such single-ion magnet (SIM) properties make lanthanide complexes promising candidates for the creation of innovative methods of information storage and development of quantum computers [5, 6].

A significant limitation that negatively affects the dynamics of magnetization relaxation in the zero magnetic field is quantum tunneling of magnetization, as well as Raman and direct relaxations [7, 8]. Recently, mononuclear cationic bis(alkoxide) Dy³⁺ complexes featuring an axial arrangement of ligands, which provides stabilization of an electron shell of the lanthanide ion and minimizes the effect of quantum tunneling, were shown to be the most promising high-performance SIMs [9–12]. However, such complexes are still characterized by blocking temperatures, which are inferior compared to those typical for metallocene type SIMs. Probably this may be related to the Raman relaxation pathway [10, 11]. A contribution of the Raman relaxation can be attenuated by reducing the ligand molecular C–H vibrations using fluorinated ligands [13, 14]. The application of fluorinated ligands can also have a positive impact on the magnetic properties due to non-valent Ln–F interactions, leading to a decrease in the number of equatorial ligands [15–17]. The elaboration of new synthetic approaches allowing for the design of low-coordinate Ln³⁺ cationic bis(alkoxide) complexes featuring axial symmetry remains a challenge for coordination chemistry. The diamagnetic Y³⁺ ion that has an ionic radius close to that of Dy³⁺ [18] seems to be an ideal candidate for the development of the synthetic methods as well for the investigation of the complex behavior in solution.

Herein, we report on the application of the organometallic approach to the cationic bis(alkoxy) Ln³⁺ complexes through alkane elimination reactions involving Y(CH₂SiMe₃)₃(THF)₂, [NEt₃H][BPh₄], and a fluorinated alcohol. This method allowed for the synthesis and structural characterization of complex [((C₆F₅)₂CHO)₂Y(THF)₄][BPh₄] (1) ((C₆F₅)₂CHOH = L¹OH). Additionally, the formation of the related complexes [RO₂Y(THF)₄][BPh₄] (RO = (C₆H₅)(CF₃)(Me)CO, (C₆F₅)Me₂CO)) was evidenced by the NMR scale reactions of 1,1,1-trifluoro-2-phenylpropan-2-ol (L²OH) and 2-(perfluorophenyl)propan-2-ol (L³OH).

Results and discussion

The alkane elimination reaction was used to develop a methodology for the synthesis of cationic bis(alkoxide) complexes. The interaction of Y(CH₂SiMe₃)₃(THF)₂ in THF-d₈ with one equivalent of [HNEt₃][BPh₄] followed by the treatment with two equivalents of L¹OH resulted in the formation of yttrium complex {[(C₆F₅)₂CHO]₂Y(THF)₄}⁺{BPh₄}^– (1) (Scheme 1). The ¹H NMR spectrum of the reaction mixture evidenced the formation of three equivalents of SiMe₄ and one equivalent of Et₃N (a quadruplet at 2.11 and a triplet at 1.05 ppm), while the signal characteristic of the hydroxyl group of L¹OH disappeared.

Scheme 1. Synthesis of bis(alkoxide) yttrium complexes 1–3.

In order to isolate complex 1, a preparative scale reaction of Y(CH₂SiMe₃)₃(THF)₂ with 1 eq. of [HNEt₃][BPh₄] in THF was carried out at room temperature (Scheme 1). The reaction mixture was stirred for 30 min, then a solution of 2 eq. of L¹OH in THF was added. An important condition for the successful isolation and crystallization of cationic complexes is the evaporation of the volatiles and drying of the solid residue for 40–50 min under vacuum to remove Et₃N. A similar procedure should be performed after the addition of the alcohol to the cationic bis(alkyl) complex to remove SiMe₄. The recrystallization of the solid residue from a THF/hexane mixture (3/1) afforded white crystals of complex 1 in 77% yield.

Figure 1. General view of the complex cation in 1 in representation of atoms via thermal ellipsoids at 30% probability level. Hydrogen atoms and the minor component of the disordered perfluorophenyl group are omitted for clarity. The labels are given only for the metal ion and oxygen atoms it coordinates. Selected bond lengths (Å) and angles (°): Y–O1 2.061(3), Y–O2 2.070(3), Y–O1S 2.345(3), Y–O2S 2.369(2), Y–O3S 2.344(3), Y–O4S 2.392(2), O1–Y–O2 105.44(11).

Bis(alkoxide) complex 1 is moisture- and air-sensitive. It is well soluble in THF and pyridine, but sparingly soluble in toluene and hexane. It can be stored under an inert atmosphere at room temperature for several months without any sign of decomposition.

The ¹H NMR spectrum of complex 1 shows a broadened singlet at 6.45 ppm which corresponds to the proton of the methanide group of the ligand. In the ¹³C{¹H} NMR spectrum, the carbon of methanide group gives rise to a singlet at 60.9 ppm. The signals at 7.28, 6.86, and 6.72 ppm in the ¹H NMR spectrum are attributed to the phenyl protons of the BPh₄^– anion. The ¹¹B{¹H} NMR spectrum shows a singlet at –4.7 ppm which corresponds to the boron atom of the BPh₄^– anion. Three characteristic signals are observed for the fluorine nuclei in the ¹⁹F spectrum: a doublet at –143.4 ppm (³J_FF = 16.0 Hz) can be assigned to the ortho-F nucleus, a triplet at –152.9 ppm (³J_FF = 23.0 Hz) corresponds to the meta-F nucleus, and a triplet at –160.9 ppm (³J_FF = 21.2 Hz) can be attributed to the para-F nucleus.

Single crystals suitable for X-ray diffraction analysis were obtained by slow concentration of a solution of 1 in a THF/hexane (3/1) mixture at room temperature. Complex 1 crystallizes in the monoclinic space group P2₁/c as a separated ion pair consisting of the complex cation, yttrium bis(alkoxide) {[(C₆F₅)₂CHO]₂Y(THF)₄}⁺, and the anion [BPh₄]^–. The coordination environment of the Y³⁺ ion is formed by two oxygen atoms of the two alkoxide ligands L¹O and four oxygen atoms of the THF molecules. The Y³⁺ ion has a coordination number of six and adopts a geometry of a slightly distorted octahedron, as quantified by "continuous shape measures" (the crystal data and structure refinement parameters are given in Table S1 in the Electronic Supplementary Information (ESI), the SHAPE analysis is given in Table S2) [19]. Surprisingly, in contrast to the previously reported related cationic Dy³⁺ complexes with fluorinated ligands featuring linear arrangement of the alkoxide ligands [14], the alkoxy ligands in 1 are in cis-positions with the O–Y–O angle of 105.44(11)°. The dihedral angles between the planes of the perfluorophenyl fragments in these ligands are 84.15(15)° and 77.6(2)° (80.7(3)° with the minor component of the disordered perfluorophenyl group). The bond distances between Y³⁺ and the oxygen atoms of the alkoxide ligands are nearly identical (2.070(3) and 2.061(3) Å), while the Y–O(THF) bond lengths range from 2.344(3) to 2.392(2) Å. The Y–O_RO bonds in 1 appeared to be shorter than in six-coordinate yttrium phenoxides but comparable to the Y–O^tBu bonds. For example, in the six-coordinate yttrium complex NH{C₂H₄N=PPh₂(2,4-^tBu₂-1-C₆H₄O)}₂Y(O^tBu) coordinated by an iminophosphorane "salen" ligand, the Y–OPh bonds are as long as 2.223(5) Å and 2.161(6) Å, but the Y-O^tBu bond length is only 2.069(6) Å [20]. In complex {(3,5-^tBu₂-1-OC₆H₄)CH=N(trans-1,2-cyclo-C₆H₁₀)N=C(Me) CH₂C(CF₃)₂O}Y(N(SiHMe₂)₂)(THF), the Y–OPh bond length is 2.1530(18) Å, while Y–O(Alkoxide) and Y–O(THF) bond lengths are 2.1358(17) and 2.4240(18) Å, respectively [21, 22]. The bond lengths of 2.094(10), 2.074(12), and 2.123(11) Å were found in the neutral complex Y{OCMe(CF₃)₂}₃(THF)₃ [23]. In this complex, the Y–O(THF) bonds (2.413(12)–2.453(10) Å) are noticeably longer than those in 1.

In compound 1, the O–Y–O angle between the axial ligands (the THF molecule and one of the alkoxide ligands) is 166.06(9)°. The same angles between the equatorial ligands range from 81.72(9) to 99.92(10)°. No close contacts between the yttrium center and the fluorine atoms were found; the shortest Y…F distance is 3.513(3) Å.

Similarly to the synthesis of 1, the reactions of Y(CH₂SiMe₃)₃(THF)₂ with one equivalent of [HNEt₃][BPh₄] and two equivalents of L²OH or L³OH were carried out in THF-d₈ and monitored by ¹H, ¹³C{¹H}, ¹¹B{¹H}, and ¹⁹F NMR spectroscopy. The formation of complexes 2 and 3 was detected in solution: the signals of the hydroxyl protons of alcohols L^2,3OH in the ¹H spectra disappeared (3.20 for L²OH and 2.61 for L³OH), and the signals typical of SiMe₄ and Et₃N (2.63, 0.99 ppm for 2; 2.48, 0.93 ppm for 3) appeared. No formation of C–F bond activation products was observed in any case. The ¹H NMR spectra of the complexes 2 and 3 contain three distinct low-field signals (7.27, 6.86, and 6.72 ppm for 2; 7.21, 6.88, and 6.74 ppm for 3) which correspond to the phenyl protons of the BPh₄^– anion. In the ¹H NMR spectrum of complex 2, the broadened multiplets observed at 7.62, 7.29, and 7.03 ppm can be attributed to the aromatic protons of the L²O ligand. Additionally, a multiplet at 1.58 ppm can be assigned to the protons of the methyl group (split, presumably, due to its proximity to the CF₃-group). In the ¹³C{¹H} NMR spectrum, the methyl group of the ligand is represented by a signal at 26.4 ppm, while the aromatic carbons of the ligands give rise to the singlets at 128.6 and 127.8 ppm. The ¹¹B{¹H} NMR spectrum displays a singlet at –6.6 ppm referring to the boron atom of the anion. Finally, a multiplet in the range from –80.2 to –80.5 ppm in the ¹⁹F spectrum can be attributed to the fluorine nuclei of the ligand.

For the product formed in the reaction of L³OH, the singlet observed at 1.60 ppm in the ¹H NMR spectrum corresponds to the methyl groups of the alkoxide ligand. In the ¹³C{¹H} NMR spectrum, this group gives rise to a singlet at 26.3 ppm. The ¹¹B{¹H} spectrum displays one singlet of the boron nucleus at –4.8 ppm. The ¹⁹F spectrum contains three signals attributed to the fluorine nuclei of the ligands: a doublet at –138.2 ppm (ortho-F) and two triplets at –159.1 (meta-F) and –163.3 (para-F) ppm.

In general, the signals of quarternary carbon nuclei are of low intensity; in the case of complexes 2 and 3, they were not observed.

Experimental section

General remarks

All the reactions and manipulations were carried out under an argon atmosphere using Schlenk techniques or in a nitrogen atmosphere glovebox. After drying over NaOH, THF was purified by distillation from sodium/benzophenone ketyl. Hexane was dried over sodium and then distilled. THF-d₈ was purified by distillation from sodium/benzophenone ketyl and stored in a glovebox. Fluorinated carbinols, bis(2,3,4,5,6-pentafluorophenyl)methanol (L¹OH) 1,1,1-trifluoro-2-phenylpropan-2-ol (L²OH), and 2-(perfluorophenyl)propan-2-ol (L³OH), were purchased from SIA P&M-Invest Ltd, kept in an inert atmosphere over CaH₂ for several days, condensed under vacuum, and stored in a glovebox. [Y(CH₂SiMe₃)₃THF₂ [24] and [HNEt₃][BPh₄] [25] were synthesized according to the published procedures. Rare-earth analysis was carried out by complexometric titration [26]. Elemental analysis was performed in the Laboratory of Microanalysis of INEOS RAS.

Syntheses

NMR reaction of Y(CH₂SiMe₃)₃THF₂ with [HNEt₃][BPh₄] and L¹OH (1). In a glovebox, a solution of [HNEt₃][BPh₄] (0.045 g, 0.107 mmol) in 0.2 mL of THF-d₈ was added to a solution of Y(CH₂SiMe₃)₃THF₂ (0.053 g, 0.107 mmol) in 0.2 mL of THF-d₈. The reaction mixture was stirred at ambient temperature for 20 min, then a solution of L¹OH (0.078 g, 0.215 mmol) in 0.3 of mL THF-d₈ was added. The resulting mixture was transferred to an NMR tube. ¹H NMR (400 MHz, THF-d₈, 293 K): 7.29 (m, 8H, o-CH, BPh₄), 6.87 (t, ³J_HH = 6.6 Hz, 8H, m-CH, BPh₄), 6.73 (t, ³J_HH = 6.7 Hz, 4H, p-CH, BPh₄), 6.46 (br. s, 2H, Ar₂CH), 3.57 (m, α-CH₂, THF, together with THF-d₈), 2.66 (q, ³J_HH = 7.5 Hz, 6H, CH₂, Et₃N), 1.72 (m, β-CH₂, THF, together with THF-d₈), 0.99 (br. t, ³J_HH = 7.6 Hz, 9H, CH₃, Et₃N), 0.07 (s, 36H, Si(CH₃)₄) ppm. ¹³C{¹H} NMR (100 MHz, THF-d₈, 293 K): 143.3 (s, ArC), 139.8 (s, ArC), 126.8 (s, ArC), 124.5 (s, ArC), 124.3 (s, ArC), 123.9 (s, ArC), 122.3 (s, ArC), 120.1 (s, ArC), 112.9 (s, ArC), 67.7 (m, α-CH₂, together with THF-d₈), 61.2 (s, Ar₂CH), 45.7 (s, CH₂, Et₃N), 25.8 (m, β-CH₂, together with THF-d₈), 12.3 (s, CH₃, Et₃N), 0.9 (s, Si(CH₃)₄) ppm. ¹¹B{¹H} NMR (128 MHz, THF-d₈, 293 K): –4.7 (s, BPh₄) ppm. ¹⁹F NMR (376 MHz, THF-d₈, 293 K): –144.1 (d, ³J_FF = 16.0 Hz, o-F), –152.3 (t, ³J_FF = 23.0 Hz, m-F), –161.1 (t, ³J_FF = 21.2 Hz, p-F) ppm.

NMR reaction of Y(CH₂SiMe₃)₃THF₂ with [HNEt₃][BPh₄] and L²OH (2). In a glovebox, a solution of [HNEt₃][BPh₄] (0.045 g, 0.107 mmol) in 0.2 mL of THF-d₈ was added to a solution of Y(CH₂SiMe₃)₃THF₂ (0.053 g, 0.107 mmol) in 0.2 mL of THF-d₈. The reaction mixture was stirred at ambient temperature for 20 min, then a solution of L²OH (0.041 g, 0.215 mmol) in 0.3 mL of THF-d₈ was added. The resulting mixture was transferred to an NMR tube. ¹H NMR (400 MHz, THF-d₈, 293 K): 7.62 (m, 4H, o-CH, Ar, L²O), 7.29 (m, 12H, o-CH, BPh₄, together with m-CH, Ar, L²O), 7.03 (m, 2H, p-CH, Ar, L₂O), 6.87 (t, ³J_HH = 6.7 Hz, 8H, m-CH, BPh₄), 6.73 (t, ³J_HH = 6.4 Hz, 4H, p-CH, BPh₄), 3.57 (m, α-CH₂, THF, together with THF-d₈), 2.63 (q, ³J_HH = 7.0 Hz, 6H, CH₂, Et₃N), 1.58 (m, CH_3, L²O, together with β-CH₂ of THF and THF-d₈), 0.99 (t, ³J_HH = 7.0 Hz, 9H, CH₃, Et₃N), 0.00 (s, 36H, Si(CH₃)₄) ppm. ¹³C{¹H} NMR (100 MHz, THF-d₈, 293 K): 136.1 (s, o-CH, BPh₄), 127.5 (m, Ar, L²O, together with CF₃), 124.8 (s, m-CH, BPh₄), 121.0 (s, p-CH, BPh₄), 66.5 (m, α-CH₂ THF, together with THF-d₈), 46.4 (s, CH₂, Et₃N), 25.3 (s, CH₃, L²O), 24.3 (m, β-CH₂, THF, together with THF-d₈), 10.4 (s, CH₃, Et₃N), –1.0 (s, Si(CH₃)₄) ppm. ¹¹B{¹H} NMR (128 MHz, THF-d₈, 293 K): –6.6 (s, BPh₄) ppm. ¹⁹F NMR (376 MHz, THF-d₈, 293 K): –80.5 – –80.2 (m, CF₃) ppm.

NMR reaction of Y(CH₂SiMe₃)₃THF₂ with [HNEt₃][BPh₄] and L³OH (3). In a glovebox, a solution of [HNEt₃][BPh₄] (0.045 g, 0.107 mmol) in 0.2 mL of THF-d₈ was added to a solution of Y(CH₂SiMe₃)₃THF₂ (0.053 g, 0.107 mmol) in 0.2 mL of THF-d₈. The reaction mixture was stirred at ambient temperature for 20 min, then a solution of L³OH (0.049 g, 0.215 mmol) in 0.3 mL of THF-d₈ was added. The resulting mixture was transferred to an NMR tube. ¹H NMR (400 MHz, THF-d₈, 293 K): 7.24 (m, 8H, o-CH, BPh₄), 6.83 (t, ³J_HH = 7.0 Hz, 8H, m-CH, BPh₄), 6.69 (t, ³J_HH = 7.1 Hz, 4H, p-CH, BPh₄), 3.55 (m, α-CH₂, THF, together with THF-d₈), 2.42 (q, ³J_HH = 7.1 Hz, 6H, CH₂, Et₃N), 1.69 (m, β-CH₂, THF, together with THF-d₈), 1.57 (s, 12H, CH₃, L³O), 0.93 (t, ³J_HH = 7.6 Hz, 9H, CH₃, Et₃N), –0.03 (s, 36H, Si(CH₃)₄) ppm. ¹³C{¹H} NMR (100 MHz, THF-d₈, 293 K): 137.2 (s, o-CH, BPh₄), 125.8 (s, m-CH, BPh₄), 122.0 (s, p-CH, BPh₄), 68.4 (s, α-CH₂), 47.4 (s, CH₂, Et₃N), 34.8 (s, CH₃, L³O), 26.4 (s, β-CH₂), 13.6 (s, CH₃, Et₃N), 0.1 (s, Si(CH₃)₄) ppm. ¹¹B{¹H} NMR (128 MHz, THF-d₈, 293 K): –4.8 (s, BPh₄) ppm. ¹⁹F NMR (376 MHz, THF-d₈, 293 K): –138.2 (d, ³J_FF = 26.4 Hz, o-F), –159.1 (t, ³J_FF = 22.9 Hz, m-F), –163.3 (t, ³J_FF = 21.9 Hz, p-F) ppm.

Synthesis of [Y(L¹O)₂THF₄][BPh₄] (1). A solution of [HNEt₃][BPh₄] (0.250 g, 0.593 mmol) in 5 mL of THF was added to a solution of Y(CH₂SiMe₃)₃THF₂ (0.294 g, 0.593 mmol) in 5 mL of THF. The reaction mixture was stirred at room temperature for 30 min. The volatiles were removed under vacuum, and the residue obtained was dissolved in 5 mL of THF. Then a solution of L¹OH (0.432 g, 1.186 mmol) in 4 mL of THF was added, and the reaction mixture was stirred at room temperature for 1 h. THF was removed under vacuum, and the solid residue obtained was dissolved in a THF/hexane mixture (3/1, 7 mL). Concentrating the resulting solution at room temperature resulted in the formation of white crystals of 1. The mother liquid was decanted, and the crystals were washed with cold hexane and dried under vacuum at ambient temperature for 10 min. Complex 1 was isolated in 77% yield (0.649 mg, 0.457 mmol). Anal. Calcd for C₆₆H₅₄BF₂₀O₆Y (1422.84 g·mol^–1): C, 55.71; H, 3.83; Y, 6.25. Found: C, 55.74; H, 3.82; Y, 6.31%.

X-ray crystallography

X-ray diffraction data for 1 were collected at 120 K with a Bruker APEX2 CCD diffractometer, using graphite monochromated Mo-Ka radiation (λ = 0.71073 Å, w-scans). The structure was solved using Intrinsic Phasing with the ShelXT [27] structure solution program in Olex2 [28] and then refined with the XL [29] refinement package using Least-Squares minimization against F² in the anisotropic approximation for non-hydrogen atoms. The positions of hydrogen atoms were calculated, and they were refined in the isotropic approximation within the riding model. The disordered lattice molecules of THF were treated as diffuse contributions to the overall scattering without specific atom positions using the Solvent Mask routine implemented in OLEX2. The crystal data and structure refinement parameters are given in Table S1 in the ESI. CCDC 2283275 contains the supplementary crystallographic information for this paper.

Conclusions

Hence, we demonstrated the applicability of the organometallic approach to the synthesis of cationic yttrium bis(alkoxide) species containing fluorinated alkoxide ligands. The stepwise protonolysis of the Y–C bonds in the parent complex Y(CH₂SiMe₃)₃(THF)₂ with one equivalent of [HNEt₃][BPh₄] and two equivalents of L^1–3OH allowed for the clear formation of the cationic bis(alkoxide) complexes in nearly quantitative yields. Complex 1 was isolated in the crystalline form in 77% yield. The X-ray diffraction study showed that 1 represents a separated ion pair. At present, the works on obtaining single-crystals of complexes 2 and 3 and establishing their structures are ongoing in our lab.

Acknowledgements

This work was supported by the Ministry of Science and Higher Education of the Russian Federation (agreement no. 075-00697-22-00).

X-ray diffraction and NMR data were collected using the equipment of the Center for Molecular Composition Studies of INEOS RAS.

Electronic supplementary information

The crystal data and structure refinement parameters for complex 1 (Table S1); the results of the SHAPE analysis for complex 1 (Table S2); the NMR spectra of the reaction of Y(CH₂SiMe₃)₃THF₂ with [HNEt₃][BPh₄] and L¹OH (Figs. S1–S4); the NMR spectra of complex 1 (Figs. S5–S8); the NMR spectra of the reaction of Y(CH₂SiMe₃)₃THF₂ with [HNEt₃][BPh₄] and L²OH or L³OH (Figs. S9–S16). For ESI, see DOI: 10.32931/io2301a.

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