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Article

Weakly Bound Dimer of a Diaryloxygermylene Derived from a tBuPh2Si-Substituted 2,2′-Methylenediphenol

by
Ryo Yamazaki
1,
Ryunosuke Kuriki
1,
Asuka Sugihara
1,
Youichi Ishii
1 and
Takuya Kuwabara
1,2,*
1
Department of Applied Chemistry, Faculty of Science and Engineering, Chuo University, 1-13-27, Kasuga, Bunkyo-ku, Tokyo 112-8551, Japan
2
Department of Chemistry and Biochemistry, Faculty of Humanities and Sciences, Ochanomizu University, 2-1-1, Otsuka, Bunkyo-ku, Tokyo 112-8610, Japan
*
Author to whom correspondence should be addressed.
Crystals 2022, 12(5), 605; https://doi.org/10.3390/cryst12050605
Submission received: 23 March 2022 / Revised: 15 April 2022 / Accepted: 22 April 2022 / Published: 25 April 2022
(This article belongs to the Section Crystal Engineering)
Browse Figures
Versions Notes

Abstract

:
Novel diaryloxygermylenes have been prepared by the reaction of Lappert’s germylene, Ge[N(SiMe3)2]2, with 2,2′-methylenediphenols bearing different substituents. The bulkiness of the substituents on the ortho positions of the phenolic oxygen (6 and 6′ positions) affects the structure of the products both in the solid-state and in solution. When the ortho substituents are SitBuPh2, the diaryloxygemylene crystalizes as a weakly bound dimer with intermolecular Ge…O distances of ca. 3.0 Å and exists as a monomer in solution. In contrast, the germylene with SiMePh2 groups as the ortho substituents form a tightly bound dimer featuring a Ge2O2 rhombus with cis-oriented terminal aryloxy groups in the crystalline state, which is confirmed to be maintained in solution through the VT (variable-temperature)-1H NMR studies. To the best of our knowledge, the former dimeric structure is unprecedented in the family of dioxytetrylenes.

1. Introduction

Divalent heavier group 14 element species (:ER2, E = Si, Ge, Sn, Pb), so-called tetrylenes or metallylenes, have gained increasing attention due to their potential for mimicking the reactivities of transition metals [1,2,3,4]. Since tetrylenes are highly reactive, judicious choice of the substituents is essential to isolate such species. After the seminal work of Lappert utilizing bulky -N(SiMe3)2 and -C(SiMe3)3 groups as the substituents for Ge(II), Sn(II) and Pb(II) species [5,6], various types of substituents that enable isolation of tetrylenes have been explored, for instance, m-terphenyl [7,8], Rind (1,1,3,3,5,5,7,7-octa-R-substituted s-hydrindacenyl) [9], Trp* (extended 9-triptysyl) [10], and boryl groups [11,12], just to name a few. In contrast with these B-, C- and N-based substituents that can bring two or three pendant substituents, O-based substituents offer only one pendant substituent. Thus, most of O-disubstituted tetrylenes (dioxytetrylenes; :E(OR)2) easily form a dimer [13], trimer [14,15,16], tetramer [16] or polymer [14,17], due to the lack of steric protection. However, isolation of two-coordinate dioxytetrylenes has been achieved by utilizing bulky OR groups, where R = m-terphenyl [13,18,19,20,21], 2,6-dialkylphenyl [22,23], binaphthyl [24], boryl [25,26] substituents, and so on.
Recently, our group has demonstrated that a tetrathiacalix[4]arene-supported stannylene and plumbylene have a two-coordinate metal center with an E(OAr)2 substructure (E = Sn, Pb) [27], which is in stark contrast to the tetra-coordinated Ge(II) and Sn(II) centers found in the related 1,3-diether of calix[4]arene–tetrylenes reported by Parkin and his co-workers [28,29]. It has also been reported that germylenes and stannylenes incorporated in calix[n]arene scaffolds, where n = 4, 5, 8, feature a Ge2O2 and Sn2O2 rhombi in the solid-state [30,31,32]. Based on these studies on calixarene–tetrylene compounds, we next became interested in the structure of diaryloxytetrylenes derived from 2,2′-methylenediphenol derivatives that can be viewed as a partial structure of calixarenes. Although transition metal complexes bearing fragment structure of calixarenes have been well-investigated [33,34,35,36,37,38], their tetrylene counterparts have not been reported so far. In this contribution, we report the synthesis and solid-state structures of two novel diaryloxygermylenes derived from 2,2′-methylenediphenols with bulky silyl substituents on the ortho positions of the phenolic oxygen atoms. One of the products has a weakly bound dimeric structure in the solid state, which is, as far as we know, unprecedented in the solid-state structure of related dioxytetrylenes. The other one crystalizes as an O-bridged dimer where the terminal OAr groups are oriented in a cis-fashion. Solution behavior of the products has also been investigated by VT-1H NMR studies.

2. Materials and Methods

2.1. General Procedures

All manipulations were performed under an argon atmosphere by using standard Schlenk techniques or a conventional glovebox. Toluene, hexane, and toluene-d8 were dried with a potassium mirror before use. Dichloromethane (CH2Cl2) and chloroform-d1 were dried over P4O10, distilled, degassed, and stored under argon with molecular sieves. Unless otherwise specified, commercially available compounds were used as received. Ge[N(SiMe3)2]2 was synthesized by the literature method [6]. 1H (500 MHz), 13C{1H} (126 MHz), and 29Si{1H} (98 MHz) NMR spectra were recorded on a JEOL ECZ-500 spectrometer at 20 °C unless otherwise stated. Chemical shifts are reported in δ and referenced to residual 1H and 13C signals of the deuterated solvents as internal standards or to the 29Si NMR signal of SiMe4 in CDCl3 (δ 0). Elemental analyses were performed on a Perkin Elmer 2400 series II CHN analyzer.

2.2. Synthesis of 2-(tert-Butyldiphenylsilyl)-4-methylphenol (1a)

Imidazole (1.3607 g, 19.98 mmol) and tBuPh2SiCl (1.38 mL, 5.00 mmol) were added to a CH2Cl2 solution (15 mL) of 2-bromo-4-methylphenol (0.62 mL, 5.14 mmol). After stirring at r.t. for 24 h, the resulting mixture was poured into a saturated aqueous solution of NH4Cl and extracted with CH2Cl2. The organic layer was dried over MgSO4 and concentrated under reduced pressure. Rough purification by silica gel column chromatography (eluent = hexane) followed by removal of the solvent provided a colorless oil (1.9 g) that was used directly for retro-Brook rearrangement. The oil (1.9 g) was dissolved in THF (7 mL), and the solution was cooled to −78 °C. nBuLi (1.57 M in hexane; 3.2 mL, 4.9 mmol) was added dropwise to the solution, and the mixture was stirred for 30 min at this temperature and then 24 h at room temperature. After removal of the volatiles under reduced pressure, the residue was extracted by CH2Cl2/NH4Cl(aq). The organic layer was dried over MgSO4, and the solvent was removed in vacuo. The crude mixture was washed with hexane to give analytically pure 1a as a white powder (1.700 g, 4.90 mmol, 95% over two steps). 1H NMR (CDCl3): δ 7.65 (dd, 3J = 8.5 Hz, 4J = 1.5 Hz, 4H, Si−Ph(o)), 7.44 (tt, 3J = 7.5 Hz, 4J = 1.5 Hz, 2H, Si−Ph(p)), 7.39 (t, 3J = 7.5 Hz, 4H, Si−Ph(m)), 7.30 (d, 4J = 2.0 Hz, 1H, C(3)−H), 7.16 (dd, 3J = 8.0 Hz, 4J = 2.0 Hz, 1H, C(5)−H), 6.72 (d, 3J = 8.0 Hz, 1H, C(6)−H), 4.77 (s, 1H, OH), 2.27 (s, 3H, ArMe), 1.23 (s, 9H, tBu); 13C{1H} NMR (CDCl3): δ 159.1 (s, 4°, C(1)−OH), 138.1 (s, 3°, C(3)), 136.4 (s, 3°, C(o) of SiPh), 134.5 (s, 4°, C(ipso) of SiPh), 132.5 (s, 3°, C(5)), 129.8 (s, 3°, C(p) of SiPh), 129.3 (s, 4°, C(4)), 128.3 (s, 3°, C(m) of SiPh), 118.9 (s, 4°, C(2)), 116.1 (s, 3°, C(6)), 29.2 (s, 1°, tBu), 20.9 (s, 1°, ArMe), 18.9 (s, 4°, tBu); 29Si NMR (CDCl3): δ −7.2 (s). Anal. Calcd for C23H26OSi (1a): C, 78.72, H, 7.56. Found: C, 79.13, H, 7.86.

2.3. Synthesis of 2-(Methyldiphenylsilyl)-4-methylphenol (1b)

Imidazole (1.3615 g, 19.99 mmol) and MePh2SiCl (1.05 mL, 5.00 mmol) were added to a CH2Cl2 solution (15 mL) of 2-bromo-4-methylphenol (0.60 mL, 4.97 mmol). After heating at 40 °C for 24 h, the resulting mixture was poured into a saturated aqueous solution of NH4Cl and extracted with CH2Cl2. The organic layer was dried over MgSO4, and the solvent was removed in vacuo. Rough purification by silica gel column chromatography (eluent = hexane:EtOAc = 15:1) provided a colorless oil (687.1 mg) that was directly used for retro-Brook rearrangement. The oil was dissolved in THF (7 mL), and the solution was cooled to −78 °C. nBuLi (1.57 M in hexane; 1.37 mL, 2.15 mmol) was added dropwise to the solution, and the mixture was stirred for 30 min at this temperature and then 24 h at room temperature. After removal of the volatiles under reduced pressure, the residue was extracted by CH2Cl2/NH4Cl(aq). The organic layer was dried over MgSO4, and the solvent was removed in vacuo. The crude mixture was washed with hexane to give analytically pure 1b as a white powder (303.7 mg, 0.998 mmol, 20% over two steps). 1H NMR (CDCl3): δ 7.58 (d, 3J = 7.5 Hz, 4H, Si−Ph(o)), 7.45–7.37 (m, 6H, Si−Ph(p, m)), 7.13 (dd, 3J = 8.5 Hz, 4J = 2.5 Hz, 1H, C(5)−H), 7.06 (d, 4J = 2.5 Hz, 1H, C(3)−H), 6.68 (d, 3J = 8.5 Hz, 1H, C(6)−H), 4.88 (s, 1H, OH), 2.24 (s, 3H, ArMe), 0.88 (s, 3H, SiMe); 13C{1H} NMR (CDCl3): δ 158.8 (s, 4°, C(1)−OH), 137.3 (s, 3°, C(3)), 135.9 (s, 4°, C(ipso) of SiPh), 135.3 (s, 3°, C(o) of SiPh), 132.6 (s, 3°, C(5)), 129.83 (s, 3°, C(p) of SiPh), 129.78 (s, 4°, C(4)), 128.3 (s, 3°, C(m) of SiPh), 120.7 (s, 4°, C(2)), 115.6 (s, 3°, C(6)), 20.7 (s, 1°, ArMe), −2.94 (s, 1°, SiMe), 29Si NMR (CDCl3): δ −13.1 (s). Anal. Calcd for C20H20OSi (1b): C, 78.90, H, 6.62. Found: C, 78.81, H, 6.61.

2.4. Synthesis of 2,2′-Methylenebis{6-(tert-butyldiphenylsilyl)-4-methylphenol} (2a)

A solution of 1a (1.2003 g, 3.464 mmol) in Et2O (18 mL) was cooled to 0 °C. To this solution, MeMgBr (1.0 M in THF; 3.09 mL, 3.09 mmol) was added dropwise, and the mixture was warmed up to r.t. and stirred for 30 min. Volatiles were removed under reduced pressure, and then toluene (20 mL) and paraformaldehyde (46.7 mg, 1.55 mmol) were added. This solution was stirred for 14 h at 80 °C, and the resultant mixture was extracted with Et2O. The organic layer was washed with NH4Cl(aq), dried over MgSO4, and evaporated to give a crude product, which was further purified by silica gel column chromatography (eluent = hexane:CH2Cl2 = 3:1) to afford 2a as a white powder (842.4 mg, 1.19 mmol, 69%). 1H NMR (CDCl3): δ 7.53 (dd, 3J = 8.5, 4J = 1.4 Hz, 8H, Si−Ph(o)), 7.37 (tt, 3J = 7.5, 4J = 1.4 Hz, 4H, Si−Ph(p)), 7.29 (t, 3J = 7.5 Hz, 8H, Si−Ph(m)), 7.16 (d, 4J = 2.2 Hz, 2H, C(3)−H), 7.02 (d, 4J = 2.2 Hz, 2H, C(5)−H), 6.14 (s, 2H, OH), 3.77 (s, 2H, CH2), 2.21 (s, 6H, ArMe), 1.15 (s, 18H, tBu); 13C{1H} NMR (CDCl3): δ 156.1 (s, 4°, C(1)−OH), 137.0 (s, 3°, C(5)), 136.4 (s, 3°, C(o) of SiPh), 134.8 (s, 4°, C(ipso) of SiPh), 133.6 (s, 3°, C(3)), 129.68 (s, 4°, C(4)), 129.63 (s, 3°, C(p) of SiPh), 128.2 (s, 3°, C(m) of SiPh), 126.8 (s, 4°, C(2)), 119.9 (s, 4°, C(6)), 31.5 (s, 2°, CH2), 29.5 (s, 1°, tBu), 20.9 (s, 1°, ArMe), 18.9 (s, 4°, tBu); 29Si NMR (CDCl3): δ −7.2 (s). Anal. Calcd for C47H52O2Si2 (2a): C, 80.06, H, 7.43. Found: C, 79.96, H, 7.58.

2.5. Synthesis of 2,2′-Methylenebis{6-(methyldiphenylsilyl)-4-methylphenol} (2b)

A solution of 1b (502.7 g, 1.651 mmol) in Et2O (9 mL) was cooled to 0 °C. To this solution, MeMgBr (1.0 M in THF; 1.65 mL, 1.65 mmol) was added dropwise, and the mixture was warmed up to r.t. and stirred for 30 min. Volatiles were removed under reduced pressure, and then toluene (10 mL) and paraformaldehyde (25.1 mg, 0.825 mmol) were added. This solution was stirred for 24 h at 80 °C, and the resultant mixture was extracted with Et2O. The organic layer was washed with NH4Cl(aq), dried over MgSO4, and evaporated to give a crude product, which was further purified by silica gel column chromatography (eluent = hexane:CH2Cl2 = 1:1) to afford 2b as a white powder (203.8 mg, 0.328 mmol, 40%).1H NMR (CDCl3): δ 7.50 (dd, 3J = 8.0, 4J = 1.5 Hz, 8H, Si−Ph(o)), 7.40 (tt, 3J = 7.5, 4J = 1.4 Hz, 4H, Si−Ph(p)), 7.33 (t, 3J = 8.0 Hz, 8H, Si−Ph(m)), 7.15 (d, 4J = 2.0 Hz, 2H, C(3)−H), 6.87 (d, 4J = 2.0 Hz, 2H, C(5)−H), 6.08 (s, 2H, OH), 3.77 (s, 2H, CH2), 2.20 (s, 6H, ArMe), 0.82 (s, 6H, SiMe); 13C{1H} NMR (CDCl3): δ 156.0 (s, 4°, C(1)−OH), 136.0 (s, 3°, C(5)), 135.9 (s, 4°, C(ipso) of SiPh), 135.3 (s, 3°, C(o) of SiPh), 133.8 (s, 3°, C(3)), 130.3 (s, 4°, C(4)), 129.8 (s, 3°, C(p) of SiPh), 128.3 (s, 3°, C(m) of SiPh), 126.7 (s, 4°, C(2)), 121.4 (s, 4°, C(6)), 31.3 (s, 2°, CH2), 20.7 (s, 1°, ArMe), −2.9 (s, 1°, SiMe); 29Si NMR (CDCl3): δ −11.8 (s). Anal. Calcd for C41H40O2Si2 (2b): C, 79.31, H, 6.49. Found: C, 78.92, H, 6.59.

2.6. Synthesis of Diaryloxygermylene (3a)

In a J. Young tube, compound 2a (49.8 mg, 0.071 mmol) was dissolved in hexane (2 mL). Then a solution of Ge[N(SiMe3)2]2 (34.1 mg, 0.087 mmol) in hexane (2 mL) was added, and the mixture was heated at 50 °C for 48 h, during which period a white crystalline powder precipitated out. The solvent was removed by a syringe, and the remaining powder was dried under reduced pressure to give 3a as a white crystalline powder (28.4 mg, 0.037 mmol, 52%). The sample of 3a thus obtained was spectroscopically pure, although elemental analysis could not be performed because of its high sensitivity toward oxygen and moisture. 1H NMR (r.t., toluene-d8): δ 7.61–7.53 (m, 10 H, Si−Ph(o) + C(5)−H), 7.10 (d, 4J = 2.0 Hz, 2H, C(3)−H), 7.05 (t, 3J = 7.5 Hz, 4H, Si−Ph(p)), 6.92 (t, 3J = 7.5 Hz, 8H, Si−Ph(m)), 3.36 (brs, 2H, CH2), 2.18 (s, 6H, ArMe), 1.24 (s, 18H, tBu); 1H NMR (−60 °C, toluene-d8): δ 7.68 (s, 2H, C(5)−H), 7.63 (d, 3J = 7.5 Hz, 4H, Si−Ph(o)), 7.53 (d, 3J = 7.6 Hz, 4H, Si−Ph(o)), 7.14 (s, 2H, C(3)−H), 7.07–7.03 (m, 4H, Si−Ph(p)), 6.90 (t, 3J = 7.5 Hz, 4H, Si−Ph(m)), 6.84 (t, 3J = 7.6 Hz, 4H, Si−Ph(m)), 3.46 (d, 2J = 14.2 Hz, 1H, CH2), 3.20 (d, 2J = 14.2 Hz, 1H, CH2), 2.20 (s, 6H, ArMe), 1.29 (s, 18H, tBu);13C{1H} NMR (r.t., toluene-d8): δ 161.0 (s, 4°, C(1)−OH), 137.2 (brs, s, 4°, C(ipso) of SiPh), 136.4 (s, 3°, C(5)), 136.0 (brs, 3°, C(o) of SiPh), 133.3 (s, 3°, C(3)), 129.65 (s, 4°, C(2) or C(4)), 129.60 (s, 4°, C(2) or C(4)), 129.2 (brs, 3°, C(p) of SiPh), 128.4 (s, 3°, C(m) of SiPh), 122.5 (s, 4°, C(6)), 35.9 (s, 2°, CH2), 29.2 (s, 1°, tBu), 20.9 (s, 1°, ArMe), 19.1 (s, 4°, tBu) 29Si NMR (toluene-d8): δ −7.1 (s).

2.7. Synthesis of Diaryloxygermylene Dimer (3b)

In a J. Young tube, compound 2b (50.5 mg, 0.081 mmol) was dissolved in hexane (2 mL). Then a solution of Ge[N(SiMe3)2]2 (39.3 mg, 0.100 mmol) in hexane (2 mL) was added, and the mixture was heated at 50 °C for 24 h, during which period a white crystalline powder precipitated out. The solvent was removed by a syringe, and the remaining powder was dried under reduced pressure to give dimer 3b as a white crystalline powder (30.5 mg, 0.022 mmol, 54%). The sample of 3b thus obtained was spectroscopically pure, although elemental analysis could not be performed because of its high sensitivity toward oxygen and moisture. 1H NMR (r.t., toluene-d8): δ 7.41 (d, 3J = 6.5 Hz, 16H, Si−Ph(o)), 7.17–7.12 (m, 12H, Si−Ph(p) + C(3)−H), 7.10–7.03 (m, 20H, Si−Ph(m) + C(5)−H), 4.94 (brs, 2H, CH2), 3.28 (brs, 2H, CH2), 2.03 (s, 12H, ArMe), 0.72 (s, 12H, SiMe); 1H NMR (−60 °C, toluene-d8): δ 7.53 (d, 3J = 6.6 Hz, 8H, Si−Ph(o)), 7.44 (d, 3J = 7.0 Hz, 4H, Si−Ph(o)), 7.31 (s, 2H, C(3)−H), 7.26–7.06 (m, 30H, Si−Ph + C(3)−H + C(5)−H), 6.89 (t, 3J = 7.2 Hz, 4H, Si−Ph(m)), 6.36 (brd, 3J = 12 Hz, 2H, CH2), 3.34 (brd, 3J = 12 Hz, 2H, CH2), 2.07 (s, 6H, ArMe), 1.89 (s, 6H, ArMe), 0.80 (s, 6H, SiMe), 0.46 (s, 6H, SiMe); 13C{1H} NMR (r.t., toluene-d8): δ 157.6 (s, 4°, C(1)−OH), 138.0 (brs, 4°, C(ipso) of SiPh), 136.4 (s, 3°, C(5)), 135.5 (s, 3°, C(o) of SiPh), 133.9 (s, 3°, C(3)), 133.2 (brs, 4°, C(2)), 131.2 (brs, 4°, C(4)), 129.4 (s, 3°, C(p) of SiPh), 128.4 (s, 3°, C(m) of SiPh), 126.3 (brs, 4°, C(6)), 35.0 (s, 2°, CH2), 20.7 (s, 1°, ArMe), −1.2 (s, 1°, SiMe); 29Si NMR (toluene-d8): δ −5.5 (s).

2.8. Single-Crystal XRD Analysis

Diffraction data for 3a–b were collected on a VariMax Saturn CCD diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.71075 Å) at −180 °C. Intensity data were corrected for Lorenz-polarization effects and for empirical absorption (REQAB) [39]. Calculations were performed using the CrystalStructure [40] and OLEX2 crystallographic [41,42] software packages except for refinements, which were performed using SHELXL-2018/3 [43]. All non-hydrogen atoms were refined on Fo2 anisotropically using full-matrix least-square techniques. All hydrogen atoms were placed at the calculated positions with fixed isotropic parameters.
Crystal Data for C47H50GeO2Si2 (3a) (M = 775.703 g/mol): triclinic, space group P-1 (no. 2), a = 10.745(2) Å, b = 13.378(3) Å, c = 14.756(3) Å, α = 79.564(11)°, β = 76.944(10)°, γ = 78.66(1)°, V = 2005.0(7) Å3, Z = 2, T = 93(2) K, μ(MoKα) = 0.71075 mm−1, Dcalc = 1.285 g/cm3, 16,588 reflections measured (7.1° ≤ 2Θ ≤ 54.9°), 8827 unique (Rint = 0.046) which were used in all calculations. The final R1 was 0.0657 (I > 2σ(I)) and wR2 was 0.1691 (all data).
Crystal Data for C82H76Ge2O4Si4 (3b) (M = 1383.01 g/mol): orthorhombic, space group Pca21 (no. 29), a = 27.190(6) Å, b = 10.177(2) Å, c = 25.324(6) Å, V = 7008(3) Å3, Z = 4, T = 93(2) K, μ(MoKα) = 0.71075 mm−1, Dcalc = 1.311 g/cm3, 54,569 reflections measured (6.0° ≤ 2Θ ≤ 55.0°), 15,982 unique (Rint = 0. 1268) which were used in all calculations. The final R1 was 0.1074 (I > 2σ(I)) and wR2 was 0.2835 (all data).

3. Results and Discussion

3.1. Synthesis

Scheme 1 illustrates the synthetic route to novel diaryloxygermylenes from commercially available 2-bromo-4-methylphenol. The O-silylation of the cresol followed by retro-Brook rearrangement provided 2-silyl-4-methylphenols 1a–b [44,45], which are transformed into the corresponding 2,2′-methylenediphenols 2a–b in moderate yields [33,46]. Treatment of Ge[N(SiMe3)2]2 and 2a–b in hexane at 50 °C resulted in the formation of diaryloxygermylene 3a and dimer 3b, respectively, as a white powder. Although the isolated yields of 3a and 3b are not high, nearly quantitative formation of these products has been confirmed by the 1H NMR spectra of the crude products.

3.2. Solid-State Structures

Slow cooling of hot hexane solution of 3a provided colorless single-crystals, whereas slow diffusion of pentane into a toluene solution of 3b deposited colorless crystals. X-ray diffraction analysis of these crystals revealed their solid-state structures as shown in Figure 1 and Figure 2, although the data of 3b should be regarded as preliminary results because of the high wR2 value.
In 3a, the Ge−O1/2 bond lengths are nearly identical (1.820(3)/1.829(3) Å), and the O1−Ge−O2 angle is 99.89(11)°. This angle is larger than the O−Ge−O angles found in monomeric GeII(OR)2 type germylenes (85–92°) [13,23,24,47,48], although smaller than the angle in a related compound with a Ge(IV) center (107.5°) [49]. Notably, the closest Ge…O2 distance in the packing structure is 3.0292(3) Å, significantly longer than those in well-known dimers of dioxygermylenes (ca. 1.98 Å) [13,48]. However, considering that the sum of the van der Waals radii of Ge and O atoms is 3.63 Å [50], the solid-state structure of 3a can be best described as a weakly bound dimer. To the best of our knowledge, such dimeric structure with relatively large Ge…O separation, yet within the sum of the van der Waals radii, is unprecedented in the family of dioxytetrylenes reported so far. In fact, most dioxytetrylenes exist as monomers or tightly bound dimers in the crystalline state, and weakly bound dimeric structures of dioxytetrylenes have never been reported. It should also be mentioned that the weakly bound dimer possesses a crystallographic center of symmetry, and therefore two 2,2′-methylenebisphenol moieties are located mutually trans with respect to the Ge2O2 tetragon.
In contrast to the loosely bound dimer in 3a, compound 3b forms a tightly bound dimer as illustrated in Figure 2. The average Ge−Oterminal bond length (Ge1−O2 and Ge2−O4; 1.814(6) Å) is shorter than the average Ge−Obridging bond (Ge1−O1, Ge1−O3, Ge2−O1, and Ge2−O3; 2.010(6) Å) as was found in the related diaryloxygermylene dimers [13,48]. The O2−Ge1−O3 and O1−Ge2−O4 angles are 95.6(3) and 97.6(3)°, slightly smaller than that of 3a (99.89(11)°). The terminal aryloxy groups are arranged in a cis fashion, which is also an uncommon feature in the family of dioxytetrylenes; most dimers of dioxytetrylenes have trans-orientated terminal O-substituents, and only a few examples have been reported to possess crystal structure of cis-oriented dioxytetrylene dimers [48,51,52]. Because of the cis-orientation, one of the methylene protons in a dioxygermylene unit is spatially close to the oxygen atom of the other, which enables CH…O hydrogen bonding. Indeed, the distances between the methylene CH…Oterminal are 2.25(1) and 2.46(1) Å, and the C−H−O angles are 142(1) and 151.8(9)° (Supplementary Materials Figure S1), within the normal range for CH…O hydrogen bonding [53,54,55].

3.3. NMR Studies

To investigate the solution behavior of 3a and 3b, VT-1H NMR spectra were recorded in toluene-d8 (Figures S2–S4). In the 1H NMR spectra of 3a at 20 °C, only one broad methylene signal with a half width of ca. 50 Hz is observed at δ 3.36, indicating that the inversion of the central eight-membered C5O2Ge ring occurs slowly at this temperature (Figure S5). Upon cooling to 0 °C, the methylene signal splits into two broad signals (δ 3.46, 3.20), which are observed as two sharp doublets (δ 3.46, 3.21) with a coupling constant of ca. 14 Hz below −40 °C (Figure S2). Thus, the ring inversion is frozen at lower temperatures. Accordingly, two sets of SiPh signals were observed below −40 °C because of their diastereotopic nature. It should be noted that only one set of 1H NMR signals originating from the two aryloxy groups is observed over the temperature range −80 to 20 °C, which is not the case for 3b as mentioned below. These observations clearly indicate that 3a exists substantially as a monomeric germylene in the solution state.
The 1H NMR spectrum of 3b at 20 °C exhibits one set of aryloxy signals, e.g., δ 0.72 (SiMe) and 2.03 (ArMe), indicating a fluxional nature of the dimer structure (Figure S5), and two broadened methylene signals (δ 4.94 and 3.28) as shown in Figures S3 and S4. The latter methylene signals coalesce at 40 °C and change to a broad singlet at δ 3.65 at 60 °C, revealing that the ring inversion in 3b requires higher energy than that in 3a. This difference implies that the solution behavior of 3a and 3b is different; it is highly likely that dimeric structure of 3b is maintained in solution, while it undergoes partial dissociation and ring inversion at or above r.t., which explains the fluxionality. In fact, in the 1H NMR spectrum of 3b at −80 °C, two sets of aryloxy signals are observed, for instance, δ 0.84 and 0.41 for the SiMe groups, with an integration ratio of 1:1. This nonequivalence of the two aryloxy groups can be reasonably explained by assuming that the Ge2O2 dimeric core is rigid and no exchange takes place between the terminal and bridging aryloxy groups at −80 °C. To our surprise, one of the methylene signals exhibits a significant downfield shift upon cooling (δ 4.94 (20 °C), 5.47 (0 °C), 5.95 (−20 °C), 6.30 (−40 °C), 6.36 (−60 °C), 6.43 (−80 °C)), which is in contrast to the normal chemical shifts for the methylene in 3a (δ 3.46, 3.20 at −60 °C). We infer that the origin of the downfield shifts is the CH…O hydrogen bonding in the cis-oriented dimeric structure [56,57,58].

4. Conclusions

Novel two aryloxygermylenes 3a–b derived from 2,2′-methylenediphenols have been synthesized and structurally characterized. Compound 3a crystalizes as a weakly bound dimer that is unprecedented in the structures of dioxytetrylenes reported so far, whereas 3b has a tightly bound rhombic Ge2O2 ring with cis-oriented terminal aryloxy groups. The VT-NMR studies unveiled that 3a and 3b exist as a monomer and a dimer, respectively, in solution. These structural differences stem from the different bulkiness between SitBuPh2 and SiMePh2 at the 6 and 6′-positions of the 2,2′-methylenediphenoxide ligand.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/cryst12050605/s1, Figure S1: Hydrogen bonds in 3b; Figures S2–S4: VT-NMR spectra; Figure S5: dynamic behaviors of 3a,b; Table S1: Crystal data for 3a,b; Figures S6–S20: NMR spectra of new compounds.

Author Contributions

Conceptualization, T.K.; investigation, R.Y., R.K. and A.S.; validation, T.K. and Y.I.; writing—original draft preparation, T.K.; writing—review and editing, Y.I.; project administration, T.K. and Y.I.; funding acquisition, T.K. and Y.I. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by JSPS KAKENHI, grant number 20K15265.

Institutional Review Board Statement

Not applicable.

Data Availability Statement

CCDC 2159874 (3a), 2159875 (3b) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif (accessed on 24 April 2022), or by emailing data_request@ccdc.cam.ac.uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, U.K.; Fax: +44 1223 336033.

Conflicts of Interest

The authors declare no conflict of interest.

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Crystals 12 00605 sch001 550
Scheme 1. Synthetic route to diaryloxygermylenes 3a–b from 2-bromo-4-methylphenol. (a) imidazole (4 equiv), tBuPh2SiCl or MePh2SiCl (1.1 equiv), CH2Cl2, r.t. or 40 °C, 24 h. (b) nBuLi (1.1 equiv), THF, −78 °C, 30 min., then r.t. for 24 h. (c) MeMgBr (1 equiv), Et2O, 0 °C to r.t., 30 min., then (CH2O)n (0.5 equiv), toluene, 80 °C, 24 h. (d) Ge[N(SiMe3)2]2, hexane, 50 °C.
Scheme 1. Synthetic route to diaryloxygermylenes 3a–b from 2-bromo-4-methylphenol. (a) imidazole (4 equiv), tBuPh2SiCl or MePh2SiCl (1.1 equiv), CH2Cl2, r.t. or 40 °C, 24 h. (b) nBuLi (1.1 equiv), THF, −78 °C, 30 min., then r.t. for 24 h. (c) MeMgBr (1 equiv), Et2O, 0 °C to r.t., 30 min., then (CH2O)n (0.5 equiv), toluene, 80 °C, 24 h. (d) Ge[N(SiMe3)2]2, hexane, 50 °C.
Crystals 12 00605 sch001
Crystals 12 00605 g001 550
Figure 1. Solid-state structures of 3a with thermal ellipsoid plots at 50% probability. All hydrogen atoms except for those of the methylene in the left figure are omitted for clarity. Left: monomeric structure. Right: two closest germylene units in the packing structure. The SitBuPh2 and Me groups are shown in wireframe.
Figure 1. Solid-state structures of 3a with thermal ellipsoid plots at 50% probability. All hydrogen atoms except for those of the methylene in the left figure are omitted for clarity. Left: monomeric structure. Right: two closest germylene units in the packing structure. The SitBuPh2 and Me groups are shown in wireframe.
Crystals 12 00605 g001
Crystals 12 00605 g002 550
Figure 2. Solid-state structure of 3b with thermal ellipsoid plots at 50% probability. All hydrogen atoms are omitted for clarity. The SiMePh2 and Me groups are shown in wireframe.
Figure 2. Solid-state structure of 3b with thermal ellipsoid plots at 50% probability. All hydrogen atoms are omitted for clarity. The SiMePh2 and Me groups are shown in wireframe.
Crystals 12 00605 g002
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Yamazaki, R.; Kuriki, R.; Sugihara, A.; Ishii, Y.; Kuwabara, T. Weakly Bound Dimer of a Diaryloxygermylene Derived from a tBuPh2Si-Substituted 2,2′-Methylenediphenol. Crystals 2022, 12, 605. https://doi.org/10.3390/cryst12050605

AMA Style

Yamazaki R, Kuriki R, Sugihara A, Ishii Y, Kuwabara T. Weakly Bound Dimer of a Diaryloxygermylene Derived from a tBuPh2Si-Substituted 2,2′-Methylenediphenol. Crystals. 2022; 12(5):605. https://doi.org/10.3390/cryst12050605

Chicago/Turabian Style

Yamazaki, Ryo, Ryunosuke Kuriki, Asuka Sugihara, Youichi Ishii, and Takuya Kuwabara. 2022. "Weakly Bound Dimer of a Diaryloxygermylene Derived from a tBuPh2Si-Substituted 2,2′-Methylenediphenol" Crystals 12, no. 5: 605. https://doi.org/10.3390/cryst12050605

APA Style

Yamazaki, R., Kuriki, R., Sugihara, A., Ishii, Y., & Kuwabara, T. (2022). Weakly Bound Dimer of a Diaryloxygermylene Derived from a tBuPh2Si-Substituted 2,2′-Methylenediphenol. Crystals, 12(5), 605. https://doi.org/10.3390/cryst12050605

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