Article pubs.acs.org/Organometallics
Zwitterionic d0 Metal Complexes [(Cy2N)3M]+[(μ-Me)B(C6F5)3]− (M = Ti, Zr, Hf) Derived from Tris(dicyclohexylamido)methyl Metal Precursors Christian Adler, Nils Frerichs, Marc Schmidtmann, and Rüdiger Beckhaus* Institut für Chemie, Carl von Ossietzky Universität Oldenburg, Carl-von-Ossietzky-Straße 9-11, D-26129 Oldenburg, Federal Republic of Germany S Supporting Information *
ABSTRACT: The synthesis and characterization of tris(dicyclohexyl)amido hafnium complexes ClHf(NCy2)3 (1-Hf) and MeHf(NCy2)3 (2Hf) are presented. The reactions of the group 4 derivatives of this compound class (MeM(NCy2)3; M = Ti (2-Ti), Zr (2-Zr), Hf (2-Hf)) with B(C6F5)3 are investigated. Their reactions with strong Lewis acids lead to the first examples of zwitterionic group 4 complexes employing three amido ligands at the electrophilic metal center ([(Cy2N)3M]+[(μMe)B(C6F5)3]−). The solid-state structures of all of the betaines (3-Ti, 3Zr, 3-Hf) are presented and compared. In all homologues the methyl group is abstracted by the Lewis acid but remains in interaction with the electron-deficient metal center, resulting in a linearly bridging methyl group. The M···C distances of 3-M are elongated by 0.25 Å (av) in comparison to 2-M.
T
known and characterized d0 cations of group 4 metals have Cptype ancillary ligands due to the great industrial interest in this compound class as precatalysts.1 Only a few examples of cationic metal tris-amides have been reported. Green and coworkers synthesized the cations [M(N(SiMe 3 ) 2 ) 3 ] + [MeB(C6F5)3]− by reacting the strong Lewis acid B(C6F5)3 with MeM(N(SiMe3)2)3 (M = Zr, Hf).18 Recently, Cummins and co-workers presented the synthesis of a cationic metal trisamide. The oxidation of their well-established complex Ti(N[tBu]Ar)3 with [FeCp2][B(C6F5)4] resulted in the formation of the corresponding salt [Ti(N[ t Bu]Ar) 3 ] [B(C 6 F 5 ) 4 ] exhibiting the cationic titanium tris-amide moiety.19,20 We recently reported a facile synthesis for stable alkylated tris(dicyclohexylamido) complexes of titanium and zirconium.21,22 The methyl derivatives are promising precursors for the reaction with strong Lewis acids. Herein, we first report the synthesis and solid-state structures of the analogous hafnium complexes ClHf(NCy2)3 (1-Hf) and MeHf(NCy2)3 (2-Hf). We present the reaction of the methyl tris(dicyclohexylamido) complexes MeM(NCy2)3 (2-Ti, 2-Zr, 2Hf) with B(C6F5)3 in order to produce cationic metal trisamide complexes. We successfully applied the synthesis of tris(dicyclohexylamido)metal chlorides to hafnium. The reaction of 3 equiv of lithium dicyclohexylamide with HfCl4 in n-hexane produces ClHf(NCy2)3 in good yield according to the procedure known for the analogous zirconium complex.21,23 Single crystals
he chemistry of highly electrophilic d0 cations of group 4 metals is of great academic and industrial interest.1 In particular, the so-called Jordan cation [Cp2ZrR]+ is considered to be the active species in the well-defined Ziegler catalysis polyolefin chemistry.2 A wide range of substitution patterns for the ancillary ligands of these cations were investigated which influence the reactivity of the active species in polymerization processes. During these developments, there are some overlaps with the chemistry of frustrated Lewis pairs (FLP), in particular when sterically encumbered Lewis acids were used as activation reagents.3−5 The frustrated Lewis pair chemistry is known for its diverse bond activation reactions, including C−C and C−H bond formation reactions.6−9 In general, the abstraction of methyl groups by strong Lewis acids such as boranes or aluminum derivatives, including methylalumoxane (MAO), is the first step in the activation of the precatalysts in Ziegler-type polymerization chemistry and leads to highly reactive, electron-deficient metal centers (Scheme 1).10,11 The reaction of B(C6F5)3 with group 4 metallocene alkyl compounds results in the abstraction of the alkyl moiety and the formation of electrophilic metal centers.1,7,12−14 This reaction pattern can also be applied to amido and imido complexes of early transition metals.3,15−17 The majority of Scheme 1. Abstraction of a Methyl Group by a Strong Lewis Acid
Received: August 30, 2016
© XXXX American Chemical Society
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DOI: 10.1021/acs.organomet.6b00688 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics suitable for X-ray diffraction analysis were obtained from a saturated n-hexane solution at −30 °C. The molecular structure of 1-Hf is shown in Figure 1.
Figure 1. Molecular structure of the complex ClHf(NCy2)3 (1-Hf). Thermal ellipsoids are drawn at the 50% probability level. Hydrogen atoms and the second molecule of the asymmetric unit are omitted for clarity. Selected bond lengths (Å) and angles (deg): Hf1−N1 2.042(3), Hf1−N2 2.022(3), Hf1−N3 2.023(3), Hf1−Cl1 2.4016(8), N1−C1 1.473(4), N1−C7 1.496(4), N2−C13 1.477(4), N2−C19 1.488(4), N3−C25 1.476(4), N1−C31 1.484(4); N1−Hf1− N2 110.32(11), N1−Hf1−N3 113.08(11), N1−Hf1−Cl1 109.34(8), N2−Hf1−N3 111.42(11), Hf1−N1−C1 135.1(2), Hf1−N1−C7 108.0(2), C7−N1−C1 116.8(3), Hf1−N2−C13 138.4(3), Hf1− N2−C19 107.25(19), C13−N2−C19 114.0(3), Hf1−N3−C25 138.4(2), Hf1−N3−C31 106.72(19), C25−N3−C31 114.0(4).
Figure 2. Molecular structure of the complex MeHf(NCy2)3 (2-Hf). Thermal ellipsoids are drawn at the 50% probability level. Hydrogen atoms, the second position of the disordered part of the molecule, and the second molecule of the asymmetric unit are omitted for clarity. Selected bond lengths (Å) and angles (deg): Hf1−N1 2.039(6), Hf1− N2 2.044(6), Hf1−N3 2.051(6), Hf1−C73 2.248(8), N1−C1 1.484(9), N1−C7 1.467(8), N2−C13 1.483(9), N2−C19 1.477(9), N3−C25A 1.548(15), N3−C31 1.466(10); N1−Hf1−N2 112.9(2), N1−Hf1−N3 111.8(2), N1−Hf1−C73 106.0(3), N2−Hf1−N3 114.2(2), Hf1−N1−C1 107.5(4), Hf1−N1−C7 136.8(4), C7−N1− C1 115.2(5), Hf1−N2−C13 106.6(4), Hf1−N2−C19 139.0(5), C13− N2−C19 114.1(5), Hf1−N3−C25A 105.6(6), Hf1−N3−C31 134.5(5), C25A−N3−C31 119.2(7).
Complex 1-Hf crystallizes in the triclinic space group P1̅. The metal center shows a tetrahedral coordination geometry, and the three Hf−N bond lengths are very similar (Hf1−N1 2.042(3) Å, Hf1−N2 2.022(3) Å, Hf1−N3 2.023(3) Å). They are shortened in comparison to a typical single bond, which can be explained by attractive Hf(dπ)−N(pπ) interactions.24−26 All of the nitrogen atoms of the amido ligands are in trigonalplanar coodination (sum of all angles: N1, 359.9°; N2, 359.7°; N3, 359.4°). The Hf1−Cl1 bond length (2.4016(8) Å) is within the expected range for terminal chlorides.24,25 Reaction of tris(dicyclohexylamido)hafnium chloride (ClHf(NCy2)3; 1-Hf) with 1 equiv of methyllithium solution (1.6 M in Et2O) in n-hexane yields the corresponding methyl complex MeHf(NCy2)3 (2-Hf) after subsequent purification steps (Scheme 2).
are also shortened in comparison to a single bond, as discussed for complex 1-Hf.24−26 The interaction of the nitrogen lone pairs with the electron-deficient metal is supported by the trigonal-planar coordination spheres, as indicated by the sp2 hybridization of the nitrogen atoms (sum of all angles: N1, 359.5°; N2, 359.7°; N3, 359.3°). The hafnium−carbon bond length to the terminal methyl group is within the expected range for such bonds (2.248(8) Å).24,27 In summary, the hafnium derivatives 1-Hf and 2-Hf could be synthesized according to procedures reported for the titanium and zirconium complexes and they are very similar with regard to their reactivities and solid-state structures.21,22 Reaction of the tris(dicyclohexylamido)methyl complexes (2M) with the Lewis acid B(C6F5)3 in n-hexane at room temperature results in an immediate precipitation of the product. After the reaction mixture was stirred for 16 h, the solids were separated, washed with n-hexane, and dried under vacuum. The solids proved to be the product of the methyl abstraction from the metal tris-amides [(Cy2N)3M]+[(μMe)B(C6F5)]− (3-M) by the Lewis acid (Scheme 3). Characterization of the products with 11B NMR spectroscopy clearly indicates the abstraction of the methyl moiety and the formation of the corresponding borates (δ −16.7 ppm (3-Ti), −14.1 ppm (3-Zr), −15.0 ppm (3-Hf)). Furthermore, these chemical shifts do not indicate an interaction between the cationic metal complex and the borate in solution.18 Horton et al. introduced the difference of the chemical shifts of m- and pfluorine atoms of the borate anion in 19F NMR as a good probe
Scheme 2. Synthesis of Methyl Tris(dicyclohexylamido) Complexes 2-M (M = Ti, Zr, Hf)
The molecular structure of 2-Hf was determined as well. Single crystals were obtained from a saturated n-hexane solution at −30 °C. The ORTEP plot is shown in Figure 2. The methylated complex 2-Hf shows a tetrahedral coordination geometry and crystallizes in the monoclinic space group Cc. The Hf−N bond lengths are similar (Hf1− N1 2.039(6) Å, Hf1−N2 2.044(6) Å, Hf1−N3 2.051(6) Å) and B
DOI: 10.1021/acs.organomet.6b00688 Organometallics XXXX, XXX, XXX−XXX
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Organometallics Scheme 3. Formation of Zwitterionic Group 4 Metal Complexes (3-M) by Methyl Abstraction Reactions
of the anion coordination to the cationic d0 metal centers.28 According to their work coordinative interactions are assumed for Δδ(m,p-F) values greater than 3.5 ppm, whereas for values less than 3.0 ppm noncoordination modes are expected.29 It was found that the differences in the chemical shifts depend on the solvent used for the measurements. Following Horton’s analysis, the titanium complex 3-Ti shows no interaction of cation and anion in solution (Δδ(m,p-F) = 2.1 ppm (THF-d8), 2.9 ppm (CDCl3), 2.9 ppm (C6D6), 3.1 ppm (C6D5Br)). However, the lowest Δ value is found for the wellcoordinating solvent, indicating an interaction with THF molecules in solution. The Δ values found for the analogous hafnium complex 3-Hf are also typical of noncoordinative modes (Δδ(m,p-F) = 2.7 ppm (CDCl3), 2.6 ppm (C6D6)). In the case of 3-Zr a stronger dependence on the solvent was found. Whereas the Δδ(m,p-F) value in C6D6 (4.8 ppm) clearly indicates coordinative interactions of the ions, a lower value of 3.0 ppm was determined in CDCl3. This value is similar to those for the corresponding titanium and hafnium compounds and rather tends to indicate a noncoordinative mode. The molecular structures of all compounds were determined by single-crystal X-ray diffraction. Crystals were obtained from the mother liquor of the reaction mixtures at −30 °C (3-Zr, 3Hf) or from a saturated toluene solution at −30 °C (3-Ti). The molecular structures are shown in Figures 3−5. The crystallographic features of all complexes are very similar. In all cases, the strong Lewis acid B(C6F5)3 abstracted the methyl group from the metal complexes. In contrast to liquid-phase NMR studies, for all derivatives in the solid state an interaction between the alkyl group of the borate and the metal center is observed. The titanium and the hafnium complexes crystallize in the trigonal space group R3̅, but they are not isotypes due to different cocrystallizing solvent molecules. Both complexes have a high symmetry and are situated on a 3-fold rotational axis (M1···C19−B1). The distance Ti1···C19 (2.379 Å) is significantly elongated in comparison to the titanium−carbon single bond (2.147 Å) in the methyl titanium complex (2-Ti).22 The newly formed bond B1−C19 (1.677 Å) lies within the range for a carbon−boron single bond.24,27,30 The methyl group points directly to the electron-deficient metal and, due to crystallographic symmetry, results in a 180° Ti1···C19−B1 angle. The titanium and the boron atoms are tetrahedrally coordinated (angles between 108 and 110°). The abstraction of the methyl group by the borane increases the Lewis acidity of the metal center, resulting in contracted Ti1−N1 bonds (1.885 Å) (2-Ti: Ti−N 1.917− 1.930 Å). The molecular structure of the analogous hafnium compound 3-Hf has very similar features. The methyl group (2-Hf: Hf−C 2.248 Å) is abstracted but remains in interaction, if weakened, with the hafnium center (Hf1···C19 2.502 Å, Hf1···C19−B1 180°, B1−C19 1.702 Å). Both the hafnium and boron atoms have a near-perfect tetrahedral coordination geometry.
Figure 3. Molecular structure of the complex [(Cy2N)3Ti]+[(μMe)B(C6F5)3]− (3-Ti). Thermal ellipsoids are drawn at the 50% probability level. Hydrogen atoms (except for H19) and solvent molecules are omitted for clarity. Selected bond lengths (Å) and angles (deg): Ti1−N1 1.8850(14), Ti1···C19 2.3786(25), Ti1···H19 2.260(19), N1−C1 1.482(2), N1−C7 1.478(2), B1−C19 1.677(4), C19−H19 0.972(19), B1−C13 1.6496(16); Ti1···C19−B1 180, N1− Ti1···C19 110.339(37), C19−B1−C13 108.52(10), B1−C19−H19 108.8(12), Ti1−N1−C1 112.63(10), Ti−N1−C7 125.01(11).
Figure 4. Molecular structure of the complex [(Cy2N)3Hf]+[(μMe)B(C6F5)3]− (3-Hf). Thermal ellipsoids are drawn at the 50% probability level. Hydrogen atoms (except for H19) and solvent molecules are omitted for clarity. Selected bond lengths (Å) and angles (deg): Hf1−N1 2.0114(18), Hf1···C19 2.502(3), Hf1···H19 2.393(32), N1−C1 1.485(3), N1−C7 1.479(3), B1−C19 1.702(5), C19−H19 0.94(3), B1−C13 1.648(2); Hf1···C19−B1 180, N1−Hf1··· C19 107.86(5), C19−B1−C13 107.33(14), B1−C19−H19 107.5(18), Hf1−N1−C1 107.86(5), Hf−N1−C7 138.37(17).
In comparison to the methyl complex (2-Hf: Hf−N 2.038− 2.051 Å) the metal−nitrogen bonds are slightly shortened (Hf1−N1 2.012 Å), indicating a stronger Hf(dπ)−N(pπ) C
DOI: 10.1021/acs.organomet.6b00688 Organometallics XXXX, XXX, XXX−XXX
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Organometallics
Table 1. Selected Bond Lengths and Angles of 2-M and 3-M in Comparison with Those in Selected Refs 4−6
Figure 5. Molecular structure of the complex [(Cy2N)3Zr]+[(μMe)B(C6F5)3]− (3-Zr). Thermal ellipsoids are drawn at the 50% probability level. Hydrogen atoms (except for H55) are omitted for clarity. Selected bond lengths (Å) and angles (deg): Zr1−N1 2.038(3), Zr1−N2 2.024(4), Zr1−N3 2.025(3), Zr1···C55 2.569(5), Zr1···H55A 2.53(5), Zr1···H55B 2.36(4), Zr1···H55C 2.36(5), C55− H55A 0.97(5), C55−H55B 0.98(5), C55−H55C 0.88(5), B1−C55 1.681(7), B1−C37 1.657(6), B1−C43 1.650(7), B1−C49 1.639(7); Zr1···C55−B1 172.6(3), C55···Zr1−N1 102.09(15), C55···Zr1−N2 104.97(16), C55···Zr1−N3 114.84(15), C55−B1−C37 105.6(4), C55−B1−C43 105.9(4), C55−B1−C49 109.8(4), B1−C55−H55A 111(3), B1−C55−H55B 108(3), B1−C55−H55C 111(3).
interaction due to the increased Lewis acidity of the cationic hafnium center. The zirconium compound 2-Zr crystallizes in the orthorhombic space group Pna21. Again, the molecular structure shows the abstraction of the methyl group by the borane (Zr1··· C55 2.569 Å, C55−B1 1.681 Å) but there is still an interaction between the carbon atom and the metal center. In contrast to the titanium and hafnium complexes, the bond angle Zr1··· C55−B1 of 172.6° is not perfectly linear. The [(μ-Me)B(C6F5)3]− moiety is tilted toward the dicyclohexylamido ligands containing N1 and N2, respectively, resulting in reduced C55··· Zr1−N1 (102.1°) and C55···Zr1−N2 bond angles (105.0°). The zirconium−nitrogen bond lengths in [(Cy2N)3Zr]+[(μMe)B(C6F5)3]− (Zr−N 2.024−2.038 Å) are shortened in comparison to those in 2-Zr (Zr−N 2.048−2.064 Å), as observed for the titanium and hafnium complexes.21 Table 1 shows representative crystallographic bond lengths and angles of 3-Ti, 3-Zr, and 3-Hf in comparison to selected reference data. The cationic group 4 tris-amide complexes presented here can all be obtained in good yields (about 80−90%) by facile and reproducible reaction procedures. They are perfectly stable in the solid state and can be stored under inert conditions for months. However, the zwitterionic compounds are extremely sensitive toward moisture and air. The titanium compound is very poorly soluble in aliphatic and aromatic solvents. The zirconium and hafnium complexes are slightly soluble in aromatic solvents. All compounds are readily soluble in tetrahydrofuran but induce the polymerization of THF after some time. In contrast to the findings of Green and co-workers, the abstraction of the methyl group by the strong Lewis acid
B(C6F5)3 does not lead to separated ion pairs. Instead, an interaction between the electrophilic metal center and the borate anion is observed. A similar formation of zwitterionic compounds has been observed by Stephan and co-workers by reacting dimethyltitanium complexes featuring phosphanimide ligands with B(C6F5)3.3,17 To our knowledge the compounds [(Cy2N)3M]+[((μ-Me)B(C6F5)3]− (3-Ti, 3-Zr, 3-Hf) are the first examples of zwitterionic group 4 tris-amide complexes featuring a bridging methyl group. In conclusion, we successfully extended the reaction procedure for the synthesis of group 4 tris(dicyclohexyl)amido complexes to hafnium. We synthesized and characterized the derivatives ClHf(NCy2)3 (1-Hf) and MeHf(NCy2)3 (2-Hf). In particular, the alkylated complexes are well suited for the investigation of the reactivity toward strong Lewis acids. The reported reactions of all group 4 tris(dicyclohexyl)amido methyl complexes with B(C6F5)3 lead to the zwitterionic species [(Cy2N)3M]+[((μ-Me)B(C6F5)3]− (3-Ti, 3-Zr, 3-Hf). The methyl group is abstracted selectively by the Lewis acid. No reactions of the Lewis acid with the lone pairs of the amido ligands were observed. In the solid-state structures of all compounds, an interaction between the electrophilic metal center and the methyl group of the borate is observed. This results in the first structurally characterized examples for betaines of all group 4 metals employing three amido ligands at the metal center. However, liquid-phase NMR spectroscopy indicates separated ions in solution depending on the solvent. The capability of 3-Ti, 3-Zr, and 3-Hf of polymerizing THF is D
DOI: 10.1021/acs.organomet.6b00688 Organometallics XXXX, XXX, XXX−XXX
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Organometallics
[(Cy2N)3Ti]+[(μ-Me)B(C6F5)3]− (3-Ti). A mixture of MeTi(NCy2)3 (500 mg, 0.828 mmol) and B(C6F5)3 (423.9 mg, 0.828 mmol) was stirred in n-hexane (20 mL). After a few minutes, a yellow solid began to precipitate. The reaction mixture was stirred for 16 h. The solvent was decanted, and the yellow solid was washed with n-hexane (3 × 5 mL). The yellow product was dried under vacuum. Single crystals suitable for X-ray structure determination were obtained from a saturated toluene solution at −30 °C. Yield: 832 mg (90%). 1H NMR (499.9 MHz, THF-d8): δ 4.18−4.13 (m, 6 H, Cy-CH), 1.90−1.87 (m, 24 H, Cy-CH2), 1.75−1.67 (m, 18 H, Cy-CH2), 1.42−1.38 (m, 12 H, Cy-CH2), 1.26−1.20 (m, 6 H, Cy-CH2), 0.51 (s (br), 3 H, (C6F5)3BCH3) ppm. 13C NMR (125.7 MHz, THF-d8): δ 150.2, 138.0, 136.3 (Aryl-CF), 130.8 (C6F5)3BCH3), 101.0 (Aryl-CF), 64.8 (Cy-CH), 37.0 (Cy-CH2), 27.5 (Cy-CH2), 26.3 (Cy-CH2), 9.8 (C6F5)3BCH3) ppm. 11B NMR (160.4 MHz, THF-d8): δ −16.7 ppm. 19F NMR (470.3 MHz, THF-d8): δ −134.6 (d, J = 17.6 Hz), −168.9 (t, J = 20.2 Hz), −171.0 (t, J = 24.2 Hz) ppm. IR (ATR, 16 scans): ν̃ 2931 (m), 2856 (m), 1643 (w), 1510 (m), 1449 (s), 1391 (w), 1349 (w), 1267 (m), 1087 (s), 1018 (w), 965 (s), 953 (s), 891 (w), 868 (w), 844 (m), 801 (w), 783 (w), 766 (w), 736 (m), 688 (w), 661 (w), 610 (w), 594 (w), 572 (w), 520 (w), 483 (w), 461 (w), 447 (w) cm−1. Mp: 95 °C dec. Anal. Calcd for C55H69BF15N3Ti: C, 59.20; H, 6.23; N, 3.77. Found: C, 57.54; H, 6.64; N, 3.66. The carbon value is lowered by carbide formation. [(Cy2N)3Zr]+[(μ-Me)B(C6F5)3]− (3-Zr). A mixture of MeZr(NCy2)3 (500 mg, 0.773 mmol) and B(C6F5)3 (395.5 mg, 0.773 mmol) was stirred in n-hexane (20 mL). After a few minutes, a pale yellow solid began to precipitate. The reaction mixture was stirred for 16 h. The solvent was decanted, and the pale yellow solid was washed with nhexane (3 × 5 mL). The product was dried under vacuum. Single crystals suitable for X-ray structure determination were obtained from the mother liquor at −30 °C. Yield: 706 mg (79%). 1H NMR (499.9 MHz, C6D6): δ 3.32−3.27 (m, 6 H, Cy-CH), 1.66−1.60 (m, 24 H, CyCH2), 1.46−1.42 (m, 18 H, Cy-CH2), 1.20−1.12 (m, 12 H, Cy-CH2), 0.94−0.89 (m, 6 H, Cy-CH2), ppm. (The (C6F5)3BCH3) resonance was not found.) 13C NMR (125.7 MHz, C6D6): δ 150.2, 138.0, 136.3 (Aryl-CF), 130.8 (Aryl-ipso-C), 101.0 (Aryl-CF), 58.7 (Cy-CH), 37.3 (Cy-CH2), 26.4 (Cy-CH2), 25.5 (Cy-CH2) ppm. 11B NMR (160.4 MHz, C6D6): δ −14.1 ppm. 19F NMR (470.3 MHz, C6D6): δ −134.6 (d, J = 15.5 Hz), −168.9 (t, J = 20.5 Hz), −171.0 (t, J = 23.2 Hz) ppm. 1 H NMR (499.9 MHz, CDCl3): δ 3.09−3.03 (m, 6 H, Cy-CH), 1.70− 1.59 (m, 24 H, Cy-CH2), 1.53−1.37 (m, 18 H, Cy-CH2), 1.21−1.13 (m, 12 H, Cy-CH2), 1.08−0.98 (m, 6 H, Cy-CH2), 0.40 (s (br), 3 H, (C6F5)3BCH3) ppm. 13C NMR (125.7 MHz, CDCl3): δ 149.5, 147.6 137.7, 135.7 (Aryl-CF), 128.1 (Aryl-ipso-C), 54.1 (Cy-CH), 37.8 (CyCH2), 25.9 (Cy-CH2), 25.1 (Cy-CH2), 14.2 ((C6F5)3BCH3) ppm. 11B NMR (160.4 MHz, CDCl3): δ −15.0 ppm. 19F NMR (470.3 MHz, CDCl3): δ −133.6 (d, J = 18.1 Hz), −163.5 (t, J = 20.9 Hz), −166.5 (t, J = 20.1 Hz) ppm. IR (ATR, 16 scans): ν̃ 2931 (m), 2857 (m), 1641 (w), 1606 (w), 1510 (m), 1449 (s), 1382 (w), 1350 (w), 1266 (m), 1083 (s), 1029 (w), 965 (s), 951 (s), 890 (m), 874 (w), 843 (m), 802 (m), 768 (w), 736 (w), 696 (w), 682 (w), 662 (m), 608 (w), 570 (w), 520 (m), 508 (m), 494 (m), 475 (m) cm−1. Mp: 119 °C dec. Anal. Calcd for C55H69BF15N3Zr: C, 56.99; H, 6.00, N, 3.63. Found: C, 57.10; H, 6.33; N, 3.57. [(Cy2N)3Hf]+[(μ-Me)B(C6F5)3]− (3-Hf). A mixture of MeHf(NCy2)3 (500 mg, 0.681 mmol) and B(C6F5)3 (348.5 mg, 0.681 mmol) was stirred in n-hexane (20 mL). After a few minutes, a pale yellow solid began to precipitate. The reaction mixture was stirred for 16 h. The solvent was decanted, and the product was dried under vacuum. Single crystals suitable for X-ray structure determination were obtained from the mother liquor at −30 °C. Yield: 661 mg (78%). 1H NMR (499.9 MHz, CDCl3): δ 3.18−3.14 (m, 6 H, Cy-CH), 1.82−1.76 (m, 24 H, Cy-CH2), 1.54−1.51 (m, 18 H, Cy-CH2), 1.29−1.26 (m, 12 H, CyCH2), 1.15−1.00 (m, 6 H, Cy-CH2), 0.51 (s (br), 3 H, (C6F5)3BCH3) ppm. 13C NMR (125.7 MHz, CDCl3): δ 149.3, 147.6 (Aryl-CF), 129.5 (Aryl-ipso-C), 110.1 (Aryl-CF), 56.3 (Cy-CH), 37.2 (Cy-CH2), 27.1 (Cy-CH2), 25.9 (Cy-CH2), 10.1 ((C6F5)3BCH3) ppm. 11B NMR (160.4 MHz, CDCl3): δ −15.0 ppm. 19F NMR (470.3 MHz, CDCl3): δ −132.8 (d, J = 18.2 Hz), −164.1 (t, J = 20.1 Hz), −166.9 (t, J = 19.4
also an evidence for free d0 cations in solution. The synthesis and characterization of the complete homologous series allows a comparison of the group 4 metal derivatives toward their chemical properties and their reactivity.
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EXPERIMENTAL SECTION
General Considerations. All reactions were carried out under an inert atmosphere of argon or nitrogen with rigorous exclusion of oxygen and moisture using standard Schlenk techniques and gloveboxes. Solvents were dried according to standard procedures over Na/K alloy with benzophenone as indicator and distilled under a nitrogen atmosphere. NMR spectra were recorded on a Bruker AVANCE III 500 spectrometer (1H, 499.9 MHz; 11B, 160.4 MHz; 13C, 125.7 MHz; 19F, 470.3 MHz). 1H NMR spectra were calibrated using residual protio signals of the solvents. The signals at δ 0.29 ppm (C6D6), 0.11 ppm (THF-d8), and 0.07 ppm (CDCl3) can be attributed to silcon grease. Due to the generally low solubility of the B(C6F5)3containing substances, the silicon grease signals are extraordinarily high. 13C NMR spectra were calibrated using the solvent signals.33 11B NMR spectra were calibrated using external BF3·OEt2 (δ11B(BF3·OEt2) 0.0). 19 F NMR spectra were calibrated using external CFCl3 (δ19F(CFCl3) 0.0). The expected 1JCF coupling constants of 3-M are not observed due to the low solubility. Group 4 tris-amide compounds and the borane B(C6F5)3 were synthesized according to known procedures.21,22,34 Melting points were determined using a Mel-Temp apparatus by Laboratory Devices, Cambridge, MA. IR spectra were recorded with a Bruker Tensor 27 spectrometer using an attenuated total reflection (ATR) method. Elemental analyses were carried out on an EuroEA 3000 Elemental Analyzer by EuroVector. ClHf(NCy2)3 (1-Hf). A suspension of 1000 mg of HfCl4 (3.122 mmol) and 1754 mg of lithium dicyclohexylamide (9.366 mmol) was stirred in 100 mL of n-hexane for 16 h. The reaction mixture was filtered hot (P4-frit). The residue was washed with n-hexane (3 × 10 mL). The combined solution was concentrated to 20 mL. Colorless cystals of 1-Hf were obtained upon cooling the solution overnight at −30 °C. Yield: 1833 mg (78%). 1H NMR (499.9 MHz, C6D6): δ 3.37−3.32 (m, 6 H, Cy-CH), 2.03−2.00 (m, 12 H, Cy-CH2), 1.83− 1.80 (m, 12 H, Cy-CH2), 1.72−1.64 (m, 12 H, Cy-CH2), 1.60−1.58 (m, 6 H, Cy-CH2), 1.40−1.32 (m, 12 H, Cy-CH2), 1.19−1.10 (m, 6 H, Cy-CH2). 13C NMR (125.7 MHz, C6D6): δ 56.6 (Cy-CH), 37.6 (CyCH2), 27.3 (Cy-CH2), 26.1 (Cy-CH2). IR (ATR, 16 scans): ν̃ 2922 (s), 2850 (s), 1466 (m), 1446 (s), 1410 (w), 1369 (w), 1346 (w), 1316 (w), 1260 (m), 1177 (w), 1160 (w), 1144 (w), 1113 (s), 1068 (w), 1027 (s), 982 (w), 948 (s), 917 (w), 891 (m), 841 (m), 801 (m), 781 (m), 680 (m), 587 (m) cm−1. Mp: 149 °C dec. Anal. Calcd for C36H66ClHfN3: C, 57.28; H, 8.81; N, 5.57. Found: C, 58.04; H, 9.17; N, 5.63. MeHf(NCy2)3 (2-Hf). To a suspension of 1400 mg of ClHf(NCy2)3 (2.243 mmol) in 50 mL of n-hexane was added 1.4 mL of a methyllithium solution (1.6 M in Et2O). The rection mixture was stirred at ambient temperature for 16 h. The solvent was completely removed, and the residue was dissolved in 50 mL of n-hexane. The solution was filtered (P4-frit), and the residue was washed with nhexane (4 × 5 mL). The solution was concentrated to 15 mL and stored at −30 °C overnight. The product was obtained as colorless cyrstals. Yield: 1021 mg (75%). 1H NMR (499.9 MHz, C6D6): δ 3.31−3.25 (m, 6 H, Cy-CH), 1.99−1.95 (m, 12 H, Cy-CH2), 1.86− 1.82 (m, 12 H, Cy-CH2), 1.70−1.60 (m, 18 H, Cy-CH2), 1.42−1.33 (m, 12 H, Cy-CH2), 1.20−1.11 (m, 6 H, Cy-CH2).0.35 (s, 3 H, HfCH3). 13C NMR (125.7 MHz, C6D6): δ 54.9 (Cy-CH), 41.2 (HfCH3), 38.0 (Cy-CH2), 27.5 (Cy-CH2), 26.3 (Cy-CH2). IR (ATR, 16 scans): ν̃ 2922 (s), 2850 (m), 1464 (w), 1448 (s), 1403 (w), 1367 (w), 1345 (w), 1311 (w), 1255 (m), 1144 (m), 1116 (m), 1027 (m), 983 (w), 950 (m), 899 (w), 842 (w), 799 (m), 781 (m), 678 (m), 584 (m) cm−1. Mp: 125 °C dec. Anal. Calcd for C37H69HfN3: C, 60.51; H, 9.47; N, 5.72. Found: C, 57.52; H, 9.32; N, 5.43. The carbon value is lowered by carbide formation. E
DOI: 10.1021/acs.organomet.6b00688 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics
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Hz) ppm. Additional signals are attributed to a slow decomposition in CDCl3 solution. IR (ATR, 16 scans): ν̃ 2930 (m), 2856 (m), 1643 (w), 1607 (w), 1510 (m), 1449 (s), 1367 (w), 1268 (m), 1081 (s), 1029 (w), 965 (s), 951 (s), 890 (w), 866 (w), 842 (m), 801 (w), 779 (w), 768 (w), 735 (w), 724 (w), 680 (w), 665 (w), 612 (w), 571 (w), 510 (w), 474 (w), 452 (w), 425 (w) cm−1. Mp: 105 °C dec. Anal. Calcd for C55H69BF15HfN3: C, 53.00; H, 5.58; N, 3.37. Found: C, 52.67; H, 6.10; N, 3.23. X-ray Crystal Structure Determinations. Single-crystal X-ray data were measured on a Bruker AXS Apex II diffractometer (Mo Kα radiation, λ = 0.71073 Å, Kappa four-circle goniometer, Bruker Apex II detector). Absorption correction based on symmetry-related measurements (multiscan) was performed for 3-Ti, 3-Zr, and 3-Hf. Numerical absorption corrections were performed for 1-Hf and 2-Hf with the program SADABS.35 The structures were solved with the program SHELXS and refined with SHELXL.36 Non-H atoms were refined anisotropically; non-H atoms involved in disorder were partially refined isotropically. H atoms bonded to C were located from the difference Fourier maps but subsequently fixed to geometric positions using appropriate riding models; H atoms bonded to C in bridging methyl groups were refined freely.
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ASSOCIATED CONTENT
S Supporting Information *
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.6b00688. Crystallographic parameters and close intramolecular contacts for compounds 1-Hf, 2-Hf, 3-Ti, 3-Zr, and 3Hf, 1H and 13C NMR spectra of compounds 1-Hf and 2Hf, and 1H, 13C, 11B, and 19F NMR spectra of compounds 3-Ti, 3-Zr, and 3-Hf (PDF) Crystallographic data of compounds 1-Hf, 2-Hf, 3-Ti, 3Zr, and 3-Hf (CIF)
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AUTHOR INFORMATION
Corresponding Author
*E-mail for R.B.:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We kindly thank Friederike Kirschner for the Table of Contents drawing. REFERENCES
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DOI: 10.1021/acs.organomet.6b00688 Organometallics XXXX, XXX, XXX−XXX
