Communication pubs.acs.org/Organometallics
Remarkably Robust Mono‑n‑butyl Group IV Dicyclohexylamido Complexes {(Cy)2N}3M(n‑butyl) (Cy: cyclohexyl [C6H11], M: Ti, Zr) Christian Adler, Gabriele Tomaschun, 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 syntheses of alkyl titanium and zirconium dicyclohexylamido complexes from (Cy2N)3MCl (Cy: cyclohexyl, M: Ti 1a, M: Zr 1b) are presented. Particularly the β-Hcontaining n-butyl derivatives (Cy2N)3MnBu (M: Ti 3a, M: Zr 3b) become available as isolable, thermally stable complexes by reactions of 1 with n-BuLi. The solid-state structures of 3a and 3b are presented. No hints for C−H agostic interactions are found. The behavior of 3 is discussed in comparison to the corresponding methyl complexes (Cy2N)3MMe (M: Ti 2a, M: Zr 2b) in the solid state, as well as in solution. arly transition metal alkyl complexes containing βhydrogen atoms are of general interest, particularly as models for the growing alkyl chain in olefin polymerization processes.1 Generally, dominant β-H elimination processes are found for titanium as well as zirconium σ-alkyls (ethyl and homologues), even at low temperatures.2 Up to now only a small number of n-butyl complexes of titanium3−5 or zirconium6−14 characterized by X-ray diffraction analysis can be found in the CCDC database. The early transition metal chemistry with cyclopentadienyltype ancillary ligands is widely developed. In the last decades the reactivity of early transition metal complexes with amido ligands is an area of growing interest.15,16 Amido ligands are generally able to stabilize electron-deficient transition metal complexes through N(pπ)−M(dπ) interactions.17 The diverse possibilities of organic substituents attached to the nitrogen atom allow an adjustment of electronic features and steric hindrance around the transition metal center.18 These properties make amido ligands as versatile as Cp-type ligands, and their complexes are alternatives for various catalytic processes.19−21 The utilization of dicyclohexylamine as a ligand in particular has many advantages.22,23 The bulky substituents result in a steric demand similar to cyclopentadienyl ligands. Transition metal complexes with this ligand have excellent crystallization properties. Solid-state structures of dicyclohexylamido titanium complexes reveal a close intramolecular contact between the metal center and one ipso carbon of the ligand.24,25 We recently reported on the chemistry of titanium dicyclohexylamido complexes, which are able to activate C−H bonds under mild conditions, forming titanaaziridines or azatitanacyclobutanes.24 Generally, α-, β-, or γ-C−H bond activation processes are of fundamental significance for the understanding of organometallic reactions.26−30 Due to these results, we were interested in transferring this ligand system to zirconium. By the reaction of zirconium(IV) chloride with LiNCy2 it was not possible to synthesize (Cy2N)ZrCl3 or
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(Cy2N)2ZrCl2. Regardless of the stoichiometry, (Cy2N)3ZrCl (1b) was formed, analogous to a known process.25 The use of ZrCl4 and n-hexane as solvent results in an improved yield. We suspect that the usage of THF leads to the formation of octahedral-coordinated anionic zirconium complexes as a byproduct. A similar phenomenon was reported by Petersen et al. for the synthesis of Zr(NMe2)4 from ZrCl4 and LiNMe2.31 In contrast to Verkade et al. we were able to obtain single crystals suitable for X-ray diffraction analysis of 1b from a saturated n-hexane solution at −30 °C.25 The molecular structure of 1b is shown in Figure 1. The tetrahedralcoordinated complex crystallizes in the monoclinic space group Cc. The dicyclohexylamide ligands are coordinated trigonal planar (sum of all angles: N1: 359.5°, N2: 359.6°, N3: 359.4°). All Zr−N bonds are of comparable length (Zr1−N1 2.041(2) Å, Zr1−N2 2.042(3) Å, Zr1−N3 2.048(3) Å). The Zr−N bonds are shorter than a typical single bond, which indicates an attractive N(pπ)−Zr(dπ) interaction.17,32 However, these bond lengths correspond well to known zirconium complexes with secondary amido ligands.16,17,32,33 The Zr1− Cl1 bond is within the expected range (2.4375(8) Å) for similar compounds.16,17,32 The crystallographic properties of complex 1b are similar to the analogous titanium complex (Cy2N)3TiCl (1a).24 The remaining chlorine of complex 1b can easily be substituted with salt metathesis reactions, forming alkylated compounds (Scheme 1). The reaction of 1b with one equivalent of methyllithium in n-hexane yields the methylated complex (Cy2N)3ZrMe (2b). After subsequent purification, single crystals, suitable for X-ray diffraction analysis, could be obtained from a saturated n-hexane solution at −30 °C. Complex 2b crystallizes in the monoclinic space group Cc. The ORTEP plot is given in Figure 2. All nitrogen atoms are Received: August 7, 2014 Published: December 8, 2014 7011
dx.doi.org/10.1021/om5008105 | Organometallics 2014, 33, 7011−7014
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Communication
Figure 1. Molecular structure of complex 1b. 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]: Zr1−N1 2.041(2), Zr1−N2 2.042(3), Zr1−N3 2.048(3), Zr1−Cl1 2.4375(8), N1−C1 1.485(4), N1−C7 1.472(4), N2−C13 1.482(4), N2−C19 1.470(4), N3−C25A 1.574(8), N1−C31 1.475(7), N1−Zr1−N2 111.73(10), N1−Zr1−N3 110.33(10), N1−Zr1−Cl1 107.03(7), N2−Zr1−N3 112.33(11), Zr1−N1−C1 105.90(17), Zr1−N1−C7 138.5(2), C7−N1−C1 115.1(2), Zr1−N2−C13 105.33(18), Zr1−N2−C19 139.4(2), C13− N2−C19 114.90(17), Zr1−N3−C25A 104.4(3), Zr1−N3−C31 136.2(2), C25A−N1−C31 118.8(4).
Figure 2. Molecular structure of complex 2b. 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]: Zr1−N1 2.048(2), Zr1−N2 2.054(2), Zr1−N3 2.064(3), Zr1−C73 2.286(3), N1−C1 1.482(4), N1−C7 1.472(4), N2−C13 1.480(4), N2−C19 1.467(4), N3−C25A 1.567(8), N1−C31 1.465(4), N1−Zr1−N2 113.16(10), N1−Zr1−N3 111.89(10), N1−Zr1−C73 105.92(11), N2−Zr1−N3 113.52(10), Zr1−N1−C1 106.54(17), Zr1−N1−C7 137.86(19), C7−N1−C1 115.3(2), Zr1−N2−C13 105.76(17), Zr1−N2−C19 139.0(2), C13− N2−C19 114.9(2), Zr1−N3−C25A 104.2(3), Zr1−N3−C31 135.9(2), C25A−N3−C31 119.5(3).
Scheme 1. Preparation of (Cy2N)3MCH3 and (Cy2N)3MnBu
Scheme 2. Subsequent Reactions of Alkylated Titanium and Zirconium Dicyclohexylamido Complexes
coordinated trigonal planar (sum of all angles: N1: 359.7°, N2: 359.7°, N3: 359.6°). The Zr−N bonds are of comparable length (Zr1−N1 2.048(2) Å, Zr1−N2 2.054(2) Å, Zr1−N3 2.064(3) Å) and in agreement with comparable known complexes, as well as the Zr1−C73 bond (2.286(3) Å).18,34−37 In general, 4d- and 5d-transition metal alkyl complexes are more stable than the corresponding 3d-complexes due to a more efficient orbital overlap between the ligand and the metal center.38−41 The analogous titanium complex (Cy2N)3TiMe (2a) is not stable in solution at ambient temperature. A C−H bond elimination occurs in α-position to the nitrogen atom, methane and cyclohexene are released, and a binuclear titanium imido complex is formed (Scheme 2), as a subsequent product of an imine intermediate.24 In contrast, the formation of degradation products of the analogous zirconium complex (2b) could be observed above 90 °C using temperature-dependent 1 H NMR spectroscopic experiments. After 16 h at 100 °C the characteristic signals of 2b completely disappeared. However, only the signals of dissolved methane and cyclohexene are detected, whereas the nature of the subsequent zirconiumcontaining product could not be solved. Treatment of an n-hexane solution of 1b with one equivalent of n-butyllithium leads to butylated complex (Cy2N)3ZrnBu (3b) in excellent yields. Crystals suitable for X-ray diffraction analysis could be obtained in the form of colorless needles from
a saturated n-hexane solution at −30 °C. Complex 3b crystallizes in the monoclinic space group P21/n and has a slightly distorted tetrahedral coordination geometry. The molecular structure of 3b is shown in Figure 3. Analogous to 1b and 2b all Zr−N bonds are of comparable length (Zr1−N1 2.0651(8) Å, Zr1−N2 2.0612(8) Å, Zr1−N3 2.0616(8) Å), and all nitrogen atoms are coordinated trigonal planar (sum of all angles: N1: 359.8°, N2: 359.9°, N3: 359.9°). The Zr1−C37 bond (2.2938(10) Å) is slightly elongated compared to that in complex 2b, but corresponds well to other known Zr−nBu bond lengths.8,11,14 The molecular structures of complexes 1b−3b exhibit no great differences except for the Zr−N bond lengths, which increase from compound 1b to 3b. This effect can be attributed to the exchange of the halide to alkyl ligands. All complexes are slightly distorted tetrahedral coordinated, and compounds 1b and 2b are similar to the 7012
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Communication
reaction of (Cy2N)3TiCl with n-butyllithium in n-hexane at −30 °C produces the butylated complex (Cy2N)3TinBu (3a) in moderate yield. Single crystals of 3a suitable for X-ray diffraction analysis could be obtained from a saturated nhexane solution at 0 °C. Complex 3a crystallizes in the triclinic space group P1̅. The ORTEP plot is given in Figure 4. Analogous to the zirconium
Figure 3. Molecular structure of complex 3b. Thermal ellipsoids are drawn at the 50% probability level. Hydrogen atoms are omitted for clarity. Selected bond lengths [Å] and angles [deg]: Zr1−N1 2.0651(8), Zr1−N2 2.0612(8), Zr1−N3 2.0616(8), Zr1−C37 2.2938(10), N1−C1 1.4790(12), N1−C7 1.4709(12), N2−C13 1.4791(12), N2−C19 1.4684(12), N3−C25 1.4784(12), N1−C31 1.4683(12), N1−Zr1−N2 112.10(3), N1−Zr1−N3 113.63(3), N1− Zr1−C37 105.55(3), N2−Zr1−N3 111.48(3), Zr1−N1−C1 108.27(5), Zr1−N1−C7 136.59(6), C7−N1−C1 114.95(7), Zr1− N2−C13 104.88(5), Zr1−N2−C19 137.91(6), C13−N2−C19 117.13(7), Zr1−N3−C25 106.72(5), Zr1−N3−C31 136.98(6), C25−N3−C31 116.23(7).
Figure 4. Molecular structure of complex 3a. Thermal ellipsoids are drawn at the 50% probability level. Hydrogen atoms and the second position of the disordered part of the molecule are omitted for clarity. Selected bond lengths [Å] and angles [deg]: Ti1−N1 1.9150(10), Ti1−N2 1.9184(10), Ti1−N3 1.9101(8), Ti1−C37 2.1464(10), N1− C1 1.4795(3), N1−C7A 1.503(2), N2−C13 1.4759(13), N2−C19A 1.5162(19), N3−C25 1.4803(12), N1−C31 1.4693(12), N1−Ti1−N2 111.27(4), N1−Ti1−N3 109.58(4), N1−Ti1−C37 111.16(4), N2− Ti1−N3 113.22(4), Ti1−N1−C1 113.58(6), Ti1−N1−C7A 117.79(11), C7A−N1−C1 128.63(12), Ti1−N2−C13 108.85(6), Ti1−N2−C19A 139.95(10), C13−N2−C19A 111.18(11), Ti1−N3− C25 114.37(6), Ti1−N3−C31 124.48(6), C25−N3−C31 121.14(7).
corresponding titanium complexes (Cy2N)3TiCl (1a) and (Cy2N)3TiMe (2a) as well.24 Another interesting feature of the solid-state structures is a short intramolecular contact between the metal center and one ipso carbon of each dicyclohexylamine ligand (see Supporting Information, Table S5). They are significantly shorter (on average 0.85 Å) than the sum of the van der Waals radii of zirconium and carbon. These close contacts are typical for complexes with this bulky secondary amine ligand.24,25 Similar to the methylated complex 2b, temperature-dependent 1H NMR spectroscopic experiments demonstrate that the monobutyl complex 3b is rather stable. First indications for a degradation process can be noticed at 80 °C. After 16 h at 100 °C the characteristic signal of the ipso protons has completely disappeared, but cyclohexene is released. The remaining decomposition products could not be identified yet. Especially dialkyl complexes of zirconium are known for β-H elimination reactions at low temperatures. The Negishi system Cp2ZrCl2/n-butyllithium has to be prepared at −78 °C and eliminates 1-butene and butane to yield a zirconocene intermediate, which can undergo various reactions.42−45 To our knowledge there are only two comparable monobutylated complexes, reported by Jia, Brennessel, et al.11 These compounds exhibit an n-butyl group and three nitrogencontaining ligands like complex 3b. Solid-state structures reveal indications for weak β-agostic interactions. The Zr−Cα−Cβ angle of 95.7° and 97.1°, respectively, is much smaller than the corresponding angle in nonagostic compounds (108−126°), but larger than angles in known β-agostic complexes (about 85°).46−48 The Zr−Cα−Cβ angle in complex 3b is 125.26(7)°. We assume that the electron deficiency of the metal center is compensated due to the good N(pπ)−Zr(dπ) interactions between the secondary amine ligands and the metal. Furthermore, the solid structure of 3b indicates that there is no space for an interaction between the metal center and the butyl group because of the high steric hindrance achieved by the bulky cyclohexyl groups. Due to the unexpected stability of complex 3b, we transferred the synthetic procedure to titanium. The equimolar
compound (3b) the central titanium atom is slightly distorted tetrahedral coordinated. The titanium−nitrogen distances are of comparable length (Ti1−N1 1.9150(10) Å, Ti1−N2 1.9184(10) Å, Ti1−N3 1.9101(8) Å). The Ti−N bonds and the Ti1−C37 bond length of 2.1464(10) Å are within the range for comparable complexes.5,24 All nitrogen atoms are trigonal planar coordinated (sum of all angles: N1: 360°, N2: 360°, N3: 360°).The solid-state structure gives no evidence for a β-agostic interaction. The angle Ti1−C37−C38 is 123.34(7)°, which is a typical value for known nonagostic compounds.46−48 As explained earlier for the corresponding zirconium complex, we suspect that the sterical hindrance of the amido ligands and the reduced Lewis acidity because of the N(pπ)−Ti(dπ) interactions are responsible for this effect. However, the titanium compound 3a is considerably less stable compared to the analogous zirconium complex 3b. 1H NMR spectroscopic experiments show a complete degradation in solution at ambient temperature within 1 day, while cyclohexene is released. Furthermore, the formation of the binuclear imido-bridged titanium complex could be found after a certain time. This indicates a similar degradation process for the methylated (2a) and the butylated complexes (3a) (Scheme 2).24 There are no hints for a butyl-specific β-C−H elimination reaction at the alkyl chain. In summary, we demonstrated the preparation of monoalkylated dicyclohexylamido titanium and zirconium complexes from the corresponding chlorines by the use of methyl- and butyllithium, respectively. The molecular structures of the chlorine 1b and the alkylated complexes 2b, 3a, and 3b could 7013
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(18) Kasani, A.; Gambarotta, S.; Bensimon, C. Can. J. Chem. 1997, 75, 1494−1499. (19) Scollard, J. D.; McConville, D. H.; Payne, N. C.; Vittal, J. J. Macromolecules 1996, 29, 5241−5243. (20) Britovsek, G. J. P.; Gibson, V. C.; Wass, D. F. Angew. Chem., Int. Ed. 1999, 38, 428−447; Angew. Chem. 1999, 111, 448−468. (21) Horton, A. D.; de With, J.; van der Linden, A. J.; van de Weg, H. Organometallics 1996, 15, 2672−2674. (22) Berno, P.; Gambarotta, S. Organometallics 1995, 14, 2159−2161. (23) Scoles, L.; Minhas, R.; Duchateau, R.; Jubb, J.; Gambarotta, S. Organometallics 1994, 13, 4978−4983. (24) Adler, C.; Bekurdts, A.; Haase, D.; Saak, W.; Schmidtmann, M.; Beckhaus, R. Eur. J. Inorg. Chem. 2014, 1289−1302. (25) Duan, Z.; Thomas, L. M.; Verkade, J. G. Polyhedron 1997, 16, 635−641. (26) Scherer, W.; McGrady, G. S. Angew. Chem., Int. Ed. 2004, 43, 1782−1806; Angew. Chem. 2004, 116, 1816−1842. (27) Scherer, W.; Wolstenholme, D. J.; Herz, V.; Eickerling, G.; Brück, A.; Benndorf, P.; Roesky, P. W. Angew. Chem., Int. Ed. 2010, 49, 2242−2246; Angew. Chem. 2010, 122 (12), 2291−2295. (28) Pudasaini, B.; Janesko, B. G. Organometallics 2014, 33, 84−93. (29) Matas, I.; Campora, J.; Palma, P.; Alvarez, E. Organometallics 2009, 28, 6515−6523. (30) Dunlop-Briere, A. F.; Budzelaar, P. H. M.; Baird, M. C. Organometallics 2012, 31, 1591−1594. (31) Diamond, G. M.; Jordan, R. F.; Petersen, J. L. J. Am. Chem. Soc. 1996, 118, 8024−8033. (32) Airoldi, C.; Bradley, D. C.; Chudzynska, H.; Hursthouse, M. B.; Malik, K. M. A.; Raithby, P. R. J. Chem. Soc., Dalton Trans. 1980, 2010−2015. (33) Orpen, A. G.; Brammer, L.; Allen, F. H.; Kennard, O.; Watson, D. G.; Taylor, R. J. Chem. Soc., Dalton Trans. 1989, S1−S83. (34) Gade, L. H.; Renner, P.; Memmler, H.; Fecher, F.; Galka, C. H.; Laubender, M.; Radojevic, S.; McPartlin, M.; Lauher, J. W. Chem.−Eur. J. 2001, 7, 2563−2580. (35) Cummins, C. C.; Van, D. G. D.; Schaller, C. P.; Wolczanski, P. T. Organometallics 1991, 10, 164−170. (36) Schrock, R. R.; Casado, A. L.; Goodman, J. T.; Liang, L.-C.; Bonitatebus, P. J., Jr.; Davis, W. M. Organometallics 2000, 19, 5325− 5341. (37) Bradley, D. C.; Chudzynska, H.; Backer-Dirks, J. D. J.; Hursthouse, M. B.; Ibrahim, A. A.; Motevalli, M.; Sullivan, A. C. Polyhedron 1990, 9, 1423−1427. (38) Li, J.; Schreckenbach, G.; Ziegler, T. Inorg. Chem. 1995, 34, 3245−3252. (39) Li, J.; Ziegler, T. Organometallics 1996, 15, 3844−3849. (40) Simoes, J. A. M.; Beauchamp, J. L. Chem. Rev. 1990, 90, 629− 688. (41) Ziegler, T.; Tschinke, V. Periodic Trends in the Bonding Energies of Transition Metal Complexes - Density Functional Theory. In Bonding Energetics in Organometallic Compounds; ACS Symposium Series 428; Marks, T. J., Ed.; American Chemical Society: Washington DC, 1990; Chapter 19, pp 279−292 (42) Negishi, E.-i.; Takahashi, T. Bull. Chem. Soc. Jpn. 1998, 71, 755− 769. (43) Derat, E.; Bouquant, J.; Bertus, P.; Szymoniak, J.; Humbel, S. Int. J. Quantum Chem. 2006, 106, 704−711. (44) Derat, E.; Bouquant, J.; Bertus, P.; Szymoniak, J.; Humbel, S. J. Organomet. Chem. 2002, 664, 268−276. (45) Dioumaev, V. K.; Harrod, J. F. Organometallics 1997, 16, 1452− 1464. (46) Brookhart, M.; Green, M. L. H.; Wong, L. L. Prog. Inorg. Chem. 1988, 36, 1−124. (47) Dawoodi, Z.; Green, M. L. H.; Mtetwa, V. S. B.; Prout, K.; Schultz, A. J.; Williams, J. M.; Koetzle, T. F. J. Chem. Soc., Dalton Trans. 1986, 1629−1637. (48) Jordan, R. F.; Bradley, P. K.; Baenziger, N. C.; LaPointe, R. E. J. Am. Chem. Soc. 1990, 112, 1289−1291.
be determined by single-crystal diffraction analysis. The solid structures of these compounds are very similar. There are no indications for agostic interactions between the metal center and the alkyl group in the solid state. Complexes 2b, 3a, and 3b were found to be remarkably robust. Temperature-dependent 1 H NMR spectroscopic experiments prove that decomposition processes in solution begin from temperatures above 80 °C for the zirconium complexes but at room temperature for the titanium complex. The amido ligands are able to stabilize the alkyl function at the metal center.
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ASSOCIATED CONTENT
S Supporting Information *
Experimental details, relevant NMR spectra, X-ray crystallographic information, and CIF files for compounds 1b, 2b, 3a, and 3b. 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 This work was supported by the Deutsche Forschungsgemeinschaft (BE 1400/7-1). REFERENCES
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