Stabilization of Three-Coordinated Germanium(II) and Tin(II) Cations

Article pubs.acs.org/Organometallics

Stabilization of Three-Coordinated Germanium(II) and Tin(II) Cations by a Neutral Chelating Ligand Marek Bouška, Libor Dostál, Aleš Růzǐ čka, and Roman Jambor* Department of General and Inorganic Chemistry, Faculty of Chemical Technology, University of Pardubice, Studentská 95, CZ-532 10, Pardubice, Czech Republic S Supporting Information *

ABSTRACT: Treatment of the neutral 2-[C(CH3)N(C6H3-2,6-iPr2)]-6-(CH3O)C6H3N ligand (hereafter assigned as L) with SnCl2 and GeCl2 provided the ionic germanium(II) and tin(II) complexes [LGeIICl]+[GeIICl3]− (1) and [LSnIICl]+[SnIICl3]− (2), respectively, as the result of spontaneous dissociation of ECl2 (E = Ge, Sn). The cationic parts [LEIICl]+ of 1 and 2 contain three-coordinated germanium(II) and tin(II) atoms. In comparison, treatment of the ligand L with GeCl4 and SnBr4 yielded the germanium(IV) and tin(IV) complexes LGeCl4 (3) and LSnBr4 (4), respectively, and no dissociation process was observed. Compounds 1−4 were characterized by means of elemental analyses, 1H, 13C, and 119Sn NMR spectroscopies, and single-crystal X-ray diffraction analysis in (compounds 1 and 4).

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INTRODUCTION The field of group 14 element(II) monocation RE:+ chemistry remains a challenge for the inorganic chemists.1 The positive charge of the central atom suggests that these species should be electrophilic, but at the same time, they hold potential to have nucleophilic character through their lone pair. The pioneering works showed the possible stabilization of RSn:+ or RGe:+ cations by cyclopentadienyl, N-isopropyl-2-(isopropylamino)troponimine, or cyclophane groups.2 The area of divalent cations RE:+ was further explored by Jutzi, Power, Driess, and Müller, who reported the synthesis of two-coordinated RSi:+ or RGe:+ cations stabilized by sterically encumbered β-diketiminate ligands.3 The use of the bulky terphenyl ligand provided stable lead cation RPb:+ that is weakly coordinated by a molecule of toluene.4 The recent work of Krossing and Jones reported on the synthesis of one coordinated RGe:+ and RSn:+ cations stabilized by an extremely hindered amide ligand.5 All of these strategies involved kinetic stabilization of corresponding monocations RE:+ (Chart 1A). Another approach to stabilize extremely electrophilic monocations RE:+ is the electronic stabilization. This can be achieved by the coordination of divalent cations of group 14 elements to the transition metal, successfully applied for the synthesis of trans-[(MeCN)(Ph2PCH2CH2PPh2)2WGe(η1-Cp*)][B(C6F5)4],6 [{NPhC(Me)CHC(Me)NPh}(OTf)Ge]W(CO)5,7 and [(Ph2PCH2CH2PPh2)2WSnC6H3-2,6-Mes2]PF6.8 The groups of Reid and Baines used bi-, or multidentate ligands for autoionization of GeX2 (X = Cl, Br) to stabilize monocation [RXGe:]+[GeCl3]− or even dication [(crown-ether)Ge:]2+2[GeCl3]−.9 The electronic stabilization was also applied in the synthesis of Y → Sn coordinated organotin(II) cations (Chart 1B).10 The literature search revealed that neutral N,N,N-ligand (Diimp), © 2013 American Chemical Society

Chart 1

structurally and electronically resembling Y,C,Y−chelating ligands, has been applied in main group chemistry.11 Recently, Roesky et al. reported on the spontaneous dissociation of ECl2 in the presence of the Diimp ligand, yielding cation [(Diimp)EIICl]+ containing four-coordinated metal centers E (E = Ge, Sn) (Chart 1C).12 As part of a comprehensive study on the chemistry of coordinated monocation RE:+ of group 14 element(II), we report here the synthesis of three-coordinated tin and germanium monocations by the use of neutral chelating ligand 2-[C(CH3)N(C6H3-2,6-iPr2)]-6-(CH3O)C6H3N (hereafter, assigned as L). Treatment of ligand L with SnCl2 and GeCl2 provided complexes [LGeIICl]+[GeIICl3]− (1) and [LSnIICl]+[SnIICl3]− (2), respectively, as the result of spontaneously Received: January 30, 2013 Published: March 5, 2013 1995

dx.doi.org/10.1021/om400082t | Organometallics 2013, 32, 1995−1999

Organometallics

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1; the crystallographic data are given in Table S1 (see the Supporting Information).

dissociation of ECl2 (E = Ge, Sn). In comparison, when more Lewis acidic halides GeCl4 and SnBr4 were treated with ligand L, no autoionization had occurred and the complexes LGeCl4 (3) and LSnBr4 (4) were isolated, respectively.

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RESULTS AND DISCUSSION Chelating ligand 2-[C(CH3)N(C6H3-2,6-iPr2)]-6-(CH3O)C6H3N was synthesized by treatment of the 2-acetyl-6methoxypyridine with diisopropylaniline to give ligand L in quantitative yield as a yellowish powder soluble in CH2Cl2 and toluene. Treatment of L with 2 equiv of ECl2 provided the ionic complexes [LGeIICl]+[GeIICl3]− (1) and [LSnIICl]+[SnIICl3]− (2), respectively (Scheme 1). Scheme 1. Synthesis of Compounds 1 and 2

Figure 1. Molecular structure of 1 together with selected bond lengths (Å) and angles (deg): Ge1−N1 2.045(4), Ge1−N2 2.070(4), Ge1− Cl1 2.215(2), N2−C6 1.289(6); N1−Ge1−Cl1 94.12(1), N2−Ge1− Cl1 94.92(1), N1−Ge1−N2 76.50(2).

Compounds 1 and 2 are highly soluble in chlorinated solvents. Both compounds have been characterized by elemental analysis and NMR spectroscopy and compound 1 by single-crystal X-ray structural analysis. In the 1H NMR spectrum of both compounds 1 and 2, the aromatic protons are shifted downfield (δ 7.63, 8.25, and 8.67 ppm for 1, δ 7.48, 8.12, and 8.37 ppm for 2) when compared with the free ligand (δ 6.75, 7.68, and 7.95 ppm). Similarly, the signal of the CH3 protons of the (CH3)CN group are shifted downfield (δ 2.66 ppm for 1, δ 2.87 ppm for 2) in comparison to ligand L (δ 2.10 ppm). On the other hand, protons of the OMe group revealed similar chemical shifts in the 1H NMR spectrum of 1 and 2 (δ 4.32 for 1 and 4.05 ppm for 2) in comparison to ligand L (δ 3.99 ppm). The CH3 protons of iPr groups showed two different resonances (δ 1.13, 1.21 ppm for 1 and δ 1.17, 1.29 ppm for 2) in comparison to the free ligand L, where CH3 protons of iPr groups resonate at 1.14 ppm. The 119Sn NMR spectrum of 2 revealed two resonances at −73.2 and −330.4 ppm. While the signal at −73.2 ppm corresponds to the SnCl3− anion,13 a second resonance at −330.4 ppm was assigned to the [LSnCl]+ cation. This value is, however, shifted downfield when compared to the related cationic complex [(Diimp)SnIICl]+ (−435.0 ppm), where a four-coordinated tin center was found.12 The NMR spectra of 1 and 2 thus indicate that the ligand L strongly coordinates metal atoms E (E = Ge, Sn) by N → E coordination of the (CH3)CN moiety and suggest the absence of O → E coordination of the OMe group in solution. This coordination behavior of L provided three-coordinated metal centers, as suggested by the downfield shift in the 119Sn NMR spectrum. The existence of ionic structures of [L*GeIICl]+[GeIICl3]− (1), as predicted by NMR spectroscopy, was unambiguously established by the single-crystal X-ray diffraction analysis of 1. Suitable single crystals of 1 were obtained from saturated toluene solutions at room temperature. The molecular structure of 1 and selected bond lengths and angles are shown in Figure

The molecular structure of 1 reveals the presence of an ionic pair consisting of the cation [L*GeIICl]+ and anion [GeIICl3]−. The germanium center is three-coordinated in the cation [L*GeIICl]+, as the result of two strong N → Ge interactions from ligand L and one chlorine atom. The Ge1−O1 bond distance (2.984(4) Å) suggests the absence of a donor O → Ge interaction (ΣcovGe,O = 1.84 Å),14 leaving the oxygen atom of ligand L uncoordinated to the Ge atom. The nitrogen atoms N1 and N2 of the ligand L and chlorine atom Cl1 form a basal plane and define a distorted trigonal-pyramidal geometry of the Ge1 atom when the electron lone pair of Ge1 is considered. The Ge center is twisted by 0.335 Å from the N1−N2−C6−C1 plane defined by the ligand L. Both Ge−N bond lengths are similar (Ge1−N1 is 2.045(4) Å and Ge1−N2 is 2.070(4) Å) and define the presence of strong N → Ge interactions that is practically unknown for structurally characterized Ge(II) cations with N-donor ligands, where the range of 2.071(2)− 2.4286(5) Å was established for Ge−N bonds.9c,12,15 The recently published chlorogermyliumylidene complex {[1,8bis(tri-n-butylphosphanzenyl)naphthalene]GeCl}+Cl−, with Ge−N bond lengths being 1.981(1) and 1.960(3), is the only example with a stronger Ge−N interaction reported to date.16 The coordination of two nitrogen atoms to the Ge atom provided one five-membered ring N1−Ge1−N2−C1−C6 with an acute angle N1−Ge1−N2 being 76.50(2)°. To see whether similar autoionization will take place with more Lewis acidic halides of 14 group elements, ligand L was further treated with GeCl4 and SnBr4. As no autoionization occurred, complexes LGeCl4 (3) and LSnBr4 (4) were isolated (Scheme 2). Whereas compound 3 is soluble in nonpolar solvents, compound 4 is soluble in polar solvents, such as CH2Cl2 or THF only. Both compounds have been characterized by the 1996

dx.doi.org/10.1021/om400082t | Organometallics 2013, 32, 1995−1999

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is 2.320(12) Å) and indicate the existence of strong Sn ← N interactions. While both nitrogen atoms N1 and N2 of ligand L are coordinated to the tin atom, the Sn1−O1 bond distance (3.205(13) Å) suggests the absence of Sn−O interaction, leaving the oxygen atom of ligand L uncoordinated to Sn atom. In conclusion, we have prepared neutral chelating ligand L and showed that this ligand is able to stabilize cationic complexes [LGeIICl]+[GeIICl3]− (1) and [LSnIICl]+[SnIICl3]− (2), as the result of spontaneous dissociation of ECl2. In contrast, similar reaction of L with EX4 provided the neutral complexes LGeCl4 (3) and LSnBr4 (4), respectively. Molecular structures of complexes 1 and 4 revealed that the neutral ligand L behaved as a 4e− N,N-chelating ligand and no E ← O donation was established. This coordination behavior of the ligand L allowed the stabilization of three-coordinated EII cations (E = Ge, Sn).

Scheme 2. Synthesis of Compounds 3 and 4

elemental analysis and NMR spectroscopy and compound 4 by single-crystal X-ray diffraction analysis. The 1H NMR spectra of compounds 3 and 4 revealed one set of signals of ligand L, indicating a highly symmetrical arrangement around central E atoms. In addition, the aromatic protons and the signals of the CH3 protons of the CH3CN group did not showed reasonable downfield shifts when compared with the free ligand L, which hints that no ionization occurred in 3 and 4. All attempts to get a 119Sn NMR signal of 4 were unsuccessful, although long experiments were measured even at 220 K. These suggestions were proved by the help of single-crystal Xray diffraction analysis of 4. Suitable single crystals of 4 were obtained from saturated CHCl3 solution at room temperature. The molecular structure of 4 and selected bond lengths and angles are shown in Figure 2; the crystallographic data are given

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EXPERIMENTAL SECTION

General Methods. The starting compounds 2-acetyl-6-methoxypyridine, diisopropylaniline, SnCl2, GeCl2*dioxane, SnBr4, and GeCl4 were purchased by Sigma Aldrich. All reactions were carried out under argon, using standard Schlenk techniques. Solvents were dried by standard methods and distilled prior to use. The 1H, 13C, and 119Sn NMR spectra were recorded on a Bruker Avance500 spectrometer at 300 K in CDCl3 or THF-d8. The 1H, 13C, and 119Sn NMR chemical shifts δ are given in parts per million and referenced to external Me4Sn (119Sn) and Me4Si (13C, 1H). Elemental analyses were performed on an LECO-CHNS-932 analyzer. Synthesis of Ligand L. To a solution of 2-acetyl-6-methoxypyridine (5 g, 33.1 mmol) in methanol (30 mL) was added 2,6diisopropylaniline (6.24 mL, 33.1 mmol) and 10 drops of formic acid. The reaction mixture was heated for 24 h at 70 °C, and yellow powder was precipitated. The yellow powder was filtered off and washed with hexane. Yield: 8.9 g (87%). mp: 155.8−156.1 °C. Anal. Calcd for C20H26N2O (310.4 g/mol): C, 77.38; H, 8.44. Found: C, 77.35; H, 8.42. 1H NMR (CDCl3, 500.18 MHz): δ = 1.14 (d, 12H, CH(CH3)2), 2.10 (s, 3H, (CH3)CN), 2.66 (h, 2H, CH(CH3)2), 3.99 (s, 3H, OCH3), 6.75 (d, 1H, ArH) 7.01−7.20 (m, 3H, ArH), 7.68 ppm (t, 1H, ArH), 7.95 ppm (d, 1H, ArH). 13C NMR (CDCl3, 125.77 MHz): δ = 17.2 CH(CH3)2, 22.9 CH(CH3)2, 23.1 CH(CH3)2, 28.2(CH3)CN, 53.2 OCH3, 111.6, 113.9, 122.9, 123.4, 135.8, 138.9, 146.6 (CH3)C N, 153.9, 163.2, 166.8. Synthesis of [LGeIICl]+[GeIICl3]− (1). GeCl2·dioxane powder (0.37 g, 1.6 mmol) was added with stirring to the solution of L (0.25 g, 0.8 mmol) in CH2Cl2 (20 mL) at room temperature. The reaction mixture was stirred for an additional 24 h. The solution was filtered, and the filtrate was concentrated to a volume of approximately 10 mL. Storage overnight at room temperature gave yellow crystals. Yield: 0.40 g (83%). mp: 197.0−200.3 °C. Anal. Calcd for C20H26N2OGe2Cl4 (601.4 g/mol): C, 39.94; H, 5.03. Found: C, 39.99; H, 5.01. 1H NMR (CDCl3, 500.18 MHz): δ = 1.13 (d, 6H, CH(CH3)2), 1.21 (d, 6H, CH(CH3)2), 2.66 (s, 3H, (CH3)CN), 2.75 (h, 2H, CH(CH3)2), 4.32 (s, 3H, OCH3), 7.32−7.42 (m, 3H, ArH), 7.63 (d, 1H, ArH), 8.25 ppm (d, 1H, ArH), 8.67 ppm (t, 1H, ArH). 13C NMR (CDCl3, 125.77 MHz): δ = 19.1 CH(CH3)2, 25.0 CH(CH3)2, 29.3(CH3)CN, 59.0 OCH3, 114.6, 123.5, 130.1, 132.9, 137.8, 141.7, 145.6 (CH3)CN, 149.2, 163.7, 176.9. Synthesis of [LSnIICl]+[SnIICl3]− (2). SnCl2 powder (0.35 g, 1.84 mmol) was added with stirring to the solution of L (0.29 g, 0.92 mmol) in CH2Cl2 (20 mL) at room temperature. The reaction mixture was stirred for an additional 24 h, the CH2Cl2 suspension was filtered, and the filtrate was concentrated to a volume of approximately 10 mL. Storage overnight at 5 °C gave a yellow solid. Yield: 0.46 g (71%). mp: 184.2−187.4 °C. Anal. Calcd for C20H26N2OSn2Cl4 (MW = 689.6): C, 34.83; H, 3.80. Found: 34.90 C, %; 3.86 H, %. 1H NMR (CDCl3, 500.18 MHz): δ = 1.17 (d, 6H, CH(CH3)2), 1.29 (d, 6H, CH(CH3)2), 2.78 (bs, 2H, CH(CH3)2), 2.87 (s, 3H, (CH3)CN), 4.05 (s, 3H, OCH3), 7.21−7.33 (m, 3H, ArH), 7.48 (bs, 1H, ArH), 8.12 ppm (bs,

Figure 2. Molecular structure of 4 together with selected bond lengths (Å) and angles (deg): Sn1−N1 2.304(14), Sn1−N2 2.320(12), Sn1− O1 3.205(13); N1−Sn1−N2 71.7(5), N1−Sn1−Br4 158.6(4), N2− Sn1−Br1 177.3(3).

in Table S1 (Supporting Information). Although the periphery carbon atoms of the phenyl rings are disordered, the primary coordination sphere of the tin atom can be described. The molecular structure of 4 reveals the presence of a hexacoordinated tin atom. A slightly distorted octahedral geometry is formed by two N atoms from ligand L and four bromine atoms, showing the symmetrical arrangement of the tin atom, as suggested by NMR spectroscopy. Both Sn−N bond lengths are similar (Sn1−N1 is 2.304(14) Å and Sn1−N2 1997

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1H, ArH), 8.37 ppm (bs, 1H, ArH). 13C NMR (CDCl3, 125.77 MHz): δ = 16.9 CH(CH3)2, 21.3 CH(CH3)2, 27.4 (CH3)CN, 53.3 OCH3, 112.4, 119.8, 122.8, 127.8, 130.0, 139.8, 141.6 (CH3)CN, 145.2, 162.2, 180.8. 119Sn NMR (CDCl3, 186.49 MHz): δ = −73.2, −330.4 ppm Synthesis of LGeCl4 (3). GeCl4 (0.22 mL, 1.9 mmol) was added with stirring to a solution of L (0.58 g, 1.9 mmol) in CH2Cl2 (20 mL), at room temperature. The reaction mixture was stirred for an additional 24 h. The suspension was filtered, and the filtrate was concentrated to a volume of approximately 5 mL. Storage overnight at 5 °C gave a yellow solid. Yield: 0.90 g (92%). mp: 149.3−151.8 °C. Anal. Calcd for C20H26N2OGeCl4 (MW = 534.8): C, 45.77; H, 4.99. Found: 45.85 C, %; 5.05 H, %. 1H NMR (CDCl3, 500.13 MHz): δ 1.26 (d, 6H, CH(CH3)2, 1.30 (d, 6H, CH(CH3)2, 2.34 (s, 3H, CH3C N), 2.88 (septet, 2H, CH(CH3)2), 4.10 (s, 3H, OCH3), 6.97 ppm (d, 2H, ArH), 7.23−7.29 (m, 3H, ArH), 7.80 ppm (t, 1H, ArH), 8.20 ppm (d, 2H, ArH). 13C NMR (CDCl3, 125.77 MHz): δ = 17.5 CH(CH3)2, 22.9 CH(CH3)2, 23.3 CH(CH3)2, 28.3 (CH3)CN, 53.3 OCH3, 112.6, 114.8, 122.8, 124.6, 136.6, 139.1, 145.4 (CH3)CN, 153.1, 163.1, 168.6. Synthesis of LSnBr4 (4). SnBr4 powder (1.3 g, 2.9 mmol) was added with stirring to a solution of 1 (0.90 g, 2.9 mmol) in CH2Cl2 (20 mL), at room temperature. The reaction mixture was stirred for an additional 24 h. The suspension was filtered, and the filtrate was concentrated to a volume of approximately 5 mL. Storage overnight at room temperature gave yellow crystals. Yield 1.95 g (90%). mp: 162 °C-decomp. Anal. Calcd for C20H26N2OSnBr4 (MW = 748.7): C, 32.08; H, 3.50. Found: 32.13 C, %; 3.44 H, %. 1H NMR (THF-d8, 500.13 MHz): δ 1.10 (d, 6H, CH(CH3)2, 1.14 (d, 6H, CH(CH3)2, 1.73 (s, 3H, CH3CN), 2.74 (septet, 2H, CH(CH3)2), 3.58 (s, 3H, OCH3), 6.85 ppm (d, 2H, ArH), 6.88−7.13 (m, 3H, ArH), 7.73 ppm (t, 1H, ArH), 7.95 ppm (d, 2H, ArH). 13C NMR (THF-d8, 125.77 MHz): δ 17.8 CH(CH3)2, 23.6 CH(CH3)2, 24.1 CH(CH3)2, 29.7(CH3)CN, 53.9 OCH3, 113.4, 115.1, 124.1, 124.8, 136.8, 140.3, 148.1 (CH3)CN, 155.2, 164.9, 167.8. 119Sn NMR (THF-d8, 186.49 MHz): δ not found. Crystallography. Compounds 1 and 4 were dissolved in CHCl3, and slow diffusion of solvent produced material that was suitable for Xray analysis and characterized as compounds 1 and 4·CHCl3. The X-ray data (Table S1, Supporting Information) for colorless crystals of 1 and 4·CHCl3 were obtained at 150 K using an Oxford Cryostream low-temperature device on a Nonius KappaCCD diffractometer with Mo Kα radiation (λ = 0.71073 Å), a graphite monochromator, and the ϕ and χ scan mode. Data reductions were performed with DENZO-SMN.17 The absorption was corrected by integration methods.18 Structures were solved by direct methods (Sir92)19 and refined by full-matrix least-squares based on F2 (SHELXL97).20 Hydrogen atoms were mostly localized on a difference Fourier map; however, to ensure uniformity of the treatment of the crystal, all hydrogen atoms were recalculated into idealized positions (riding model) and assigned temperature factors Hiso(H) = 1.2 Ueq(pivot atom) or of 1.5Ueq for the methyl moiety with C−H = 0.96, 0.97, and 0.93 Å for methyl, methylene, and hydrogen atoms in aromatic rings, respectively. Only bad quality crystals of 4 were measured, giving positional disorder of the phenyl ring with low precision of C−C bond distances and about 200 Å large solvent accessible voids. We tried to treat these disorders and voids by standard methods implemented in SHELXL or Platon software packages, but these tries were unsuccessful. Crystallographic data for structural analysis have been deposited with the Cambridge Crystallographic Data Centre. CCDC nos. 919630 and 919631 for 1 and 4·CHCl3, respectively. Copies of this information may be obtained free of charge from The Director, CCDC, 12 Union Road, Cambridge CB2 1EY, U.K. (Fax: +44-1223-336033; E-mail: [email protected]. uk or www: http://www.ccdc.cam.ac.uk).

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ASSOCIATED CONTENT

S Supporting Information *

Further details of the structure determination of compounds 1 and 4, including atomic coordinates, anisotropic displacement parameters, and geometric data. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors wish to thank the Ministry of Education of the Czech Republic for financial support.

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REFERENCES

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dx.doi.org/10.1021/om400082t | Organometallics 2013, 32, 1995−1999