Synthesis of an Amino-Functionalized ansa-Zirconocene by

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Organometallics 2009, 28, 4513–4518 DOI: 10.1021/om9003576

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Synthesis of an Amino-Functionalized ansa-Zirconocene by Intramolecular Photochemical Enamine [2þ2]Cycloaddition T€ ulay Aslı Tumay, Gerald Kehr, Roland Fr€ ohlich, and Gerhard Erker* Organisch-Chemisches Institut der Universit€ at M€ unster, Corrensstrasse 40, 48149 M€ unster, Germany Received May 6, 2009

Deprotonation of 6-dimethylamino-6-methylpentafulvene (3a) with methyl lithium yielded the (1-dimethylaminoethenyl)Cp lithium reagent 4a. Its treatment with zirconium tetrachloride under carefully controlled reaction conditions gave the corresponding bis(enamino-Cp)ZrCl2 complex 5a. The product 5a is very sensitive to acid-catalyzed Mannich condensation. Photolysis of 5a (HPK 125, Pyrex filter) resulted in a rapid intramolecular [2þ2]cycloaddition reaction to yield the cis-bis(dimethylamino)cyclobutylene-bridged ansa-zirconocene complex 7a in a photostationary equilibrium (5a:7a ≈ 30:70). After acid-catalyzed Mannich conversion of the minor component (5a to 6a) the new photoproduct 7a was isolated pure by fractional crystallization. The complexes 5a and 7a were both characterized by X-ray diffraction.

Introduction ansa-Group 4 metallocene complexes have been of enormous importance for the development of homogeneous Ziegler-Natta olefin polymerization chemistry.1 Mostly, the ansa-bridge is introduced at an early stage of the synthesis before the transmetalation step to the group 4 metal because the development of a suitable organic functional group chemistry at the metallocene framework has been difficult due to the sensitivity of these early metal complexes. However, there is some development in this synthetic organometallic chemistry lately.2 Some ansa-metallocenes were recently prepared by functional group interconversion at the bent metallocene framework. Notable examples include ansa-metallocene synthesis by ring-closing olefin metathe*Corresponding author. E-mail: [email protected]. (1) (a) Marks, T. J. Acc. Chem. Res. 1992, 25, 57–65. (b) Brintzinger, H.-H.; Fischer, D.; M€ulhaupt, R.; Rieger, B.; Waymouth, R. M. Angew. Chem. 1995, 107, 1255–1283; Angew. Chem., Int. Ed. Engl. 1995, 34, 1143-1170. (c) Kaminsky, W. J. Chem. Soc., Dalton Trans. 1998, 1413– 1418. (d) Alt, H. G.; Samuel, E. Chem. Soc. Rev. 1998, 27, 323–329. (e) Alt, H. G.; K€ oppl, A. Chem. Rev. 2000, 100, 1205–1221. (f ) Erker, G. Acc. Chem. Res. 2001, 34, 309–317. (g) Erker, G. Chem. Commun. 2003, 1469– 1476. (2) (a) Erker, G.; Kehr, G.; Fr€ ohlich, R. J. Organomet. Chem. 2004, 689, 1402–1412. (b) Erker, G. Polyhedron 2005, 24, 1289–1297. (c) Erker, G.; Kehr, G.; Fr€ ohlich, R. Coord. Chem. Rev. 2006, 250, 36–46. (d) Erker, G. Macromol. Symp. 2006, 236, 1–13. ander, D.; Kleigrewe, N.; Kehr, G.; Erker, G.; Fr€ ohlich, (3) (a) H€ uerl€ R. Eur. J. Inorg. Chem. 2002, 2633–2642. (b) Ogasawara, M.; Nagano, T.; Hayashi, T. J. Am. Chem. Soc. 2002, 124, 9068–9069. (c) Sierra, J. C.; H€ uerl€ander, D.; Hill, M.; Kehr, G.; Erker, G.; Fr€ohlich, R. Chem. Eur. J. 2003, 9, 3618–3622. (d) Kuwabara, J.; Takeuchi, D.; Osakada, K. Organometallics 2005, 24, 2705–2712. (4) (a) Kn€ uppel, S.; Erker, G.; Fr€ ohlich, R. Angew. Chem. 1999, 111, 2048–2051; Angew. Chem., Int. Ed. 1999, 38, 1923-1926. (b) Kn€uppel, S.; Wang, C.; Kehr, G.; Fr€ ohlich, R.; Erker, G. J. Organomet. Chem. 2005, 690, 14–32. (5) Venne-Dunker, S.; Kehr, G.; Fr€ ohlich, R.; Erker, G. Organometallics 2003, 22, 948–958. (6) Bai, S.-D.; Wei, X.-H.; Guo, J.-P.; Liu, D.-S.; Zhou, Z.-Y. Angew. Chem. 1999, 111, 2051–2054; Angew. Chem., Int. Ed. 1999, 38, 19261928. r 2009 American Chemical Society

sis,3 by a variant of the Mannich-reaction,4-7 and by photochemical [2þ2]cycloaddition of alkenyl groups conjugated with the Cp-ligand systems (see Scheme 1). Scheme 1

Intramolecular [2þ2]cycloaddition of bis(alkenyl-Cp)ZrCl2 complexes had first been achieved employing alkenyl groups bearing rather large substituents at their R-positions.8 These systems underwent rapid intramolecular [2þ2]cycloaddition reactions to generate the respective substituted cyclobutylene-bridged ansa-metallocenes, but many of these early systems suffered from unfavorable photostationary equilibria between the open and ring-closed isomers under the practical photolytic conditions. A breakthrough in this synthetic methodology of ansa-metallocene formation was achieved when we realized that the attachment of small methyl groups (or even hydrogen) at the alkenyl R-positions led to practically complete photochemical conversion to the ansa-metallocenes.9 The application of this observation has made photochemical [2þ2]cycloaddition of directly bonded 2-propenyl (or even vinyl)10 substituents a valuable tool for the organometallic (7) See for comparison: (a) Kn€ uppel, S.; Fr€ ohlich, R.; Erker, G. J. Organomet. Chem. 1999, 586, 218–222. (b) Kn€uppel, S.; Fr€ohlich, R.; Erker, G. J. Organomet. Chem. 2000, 595, 308–312. (c) Liptau, P.; Kn€uppel, S.; Kehr, G.; Kataeva, O.; Fr€ohlich, R.; Erker, G. J. Organomet. Chem. 2001, 637-639, 621–630. (8) (a) Erker, G.; Wilker, S.; Kr€ uger, C.; Goddard, R. J. Am. Chem. Soc. 1992, 114, 10983–10984. (b) Erker, G.; Wilker, S.; Kr€uger, C.; Nolte, M. Organometallics 1993, 12, 2140–2151. (9) Paradies, J.; Kehr, G.; Fr€ ohlich, R.; Erker, G. Proc. Natl. Acad. Sci. 2006, 103, 15333–15337. (10) Erker, G.; Greger, I. Unpublished results. Published on Web 07/14/2009

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synthesis of ansa-metallocences by functional group interconversion at the actual group 4 bent metallocene stage.11 We have now been able to extend this reaction to the [2þ2] coupling of enamine units at the group 4 bent metallocene stage. In this account we will describe first examples and some of the specific characteristics of the enamine cycloaddition,12 such as its strong dependency of the substituents at the amine units. To the best of our knowledge, this reaction represents a novel way of synthesizing ansa-metallocenes with amino-functional groups at the bridge.

Results and Discussion We had previously shown that 6-dialkylaminofulvenes 313 are readily deprotonated to yield the enamino-substituted Cp-anion equivalents 4.4,5,14 Treatment of the examples 4b (diethylamino) and 4c (piperidino) with zirconium tetrachloride under carefully controlled reaction conditions gave the bis(enamino-Cp)ZrCl2 products 5b and 5c, respectively. Their treatment with a Lewis acid (ZrCl4) or a Broensted acid (R3NHþ) catalyst resulted in a rapid intramolecular Mannich-type condensation reaction to yield the unsaturated C3-bridged ansa-metallocenes 6b and 6c, respectively (see Scheme 2). The dimethylamino-substituted “parent” compound principally reacted in the same way. However, previous to this study we had not been able to isolate or even spectroscopically observe the respective bis(dimethylaminoethenyl-Cp)ZrCl2 intermediate 5a. It was so reactive that it always reacted further under the previously applied reaction conditions to directly yield the Mannich condensation product 6a instead.4-6 We were now for the first time able to observe and then consequently isolate the very sensitive complex 5a. This allowed us to study its structural, Scheme 2

Figure 1. View of the molecular geometry of the (1-dimethylaminoethenyl-Cp)2ZrCl2 complex 5a. Table 1. Selected Structural Parameters of the (Enamino-Cp)2ZrCl2 Complexes 5a and 5ca Cl-C6 C11-C16 C6-C7 C16-C17 C6-N8 C16-N18 Zr-Cl1 Zr-Cl2 Cl1-Zr-Cl2 C1-C6-C7 C1-C6-N8 C11-C16-C17 C11-C16-N18 C5-C1-C6-C7 C5-C1-C6-N8 C15-C11-C16-C17 C15-C11-C16-N18 a

(11) For related examples see: (a) Nie, W.-L.; Erker, G.; Kehr, G.; Fr€ ohlich, R. Angew. Chem. 2004, 116, 313–317; Angew. Chem., Int. Ed. 2004, 43, 310-313. (b) Chen, L.; Nie, W.-L.; Paradies, J.; Kehr, G.; Fr€ohlich, R.; Wedeking, K.; Erker, G. Organometallics 2006, 25, 5333–5344. (c) Paradies, J.; Erker, G.; Fr€ohlich, R. Angew. Chem. 2006, 118, 3150– 3153; Angew. Chem., Int. Ed. 2006, 45, 3079-3082. (d) Paradies, J.; Greger, I.; Kehr, G.; Erker, G.; Bergander, K.; Fr€ohlich, R. Angew. Chem. 2006, 118, 7792–7795; Angew. Chem., Int. Ed. 2006, 45, 7630-7633. (e) Paradies, J.; Fr€ ohlich, R.; Kehr, G.; Erker, G. Organometallics 2006, 25, 3920–3925. (12) For organic examples of photochemical enamine cycloaddition reactions see for example: (a) Winkler, J. D.; Muller, C. L.; Scott, R. D. J. Am. Chem. Soc. 1988, 110, 4831–4832. (b) Amougay, A.; Pete, J.-P.; Piva, O. Tetrahedron Lett. 1992, 33, 7347–7350. (c) Johnson, B. L.; Kitahara, Y.; Weakley, T. J. R.; Keana, J. F. W. Tetrahedron Lett. 1993, 34, 5555–5558. (d) Oldroyd, D. L.; Payne, N. C.; Vittal, J. J.; Weedon, A. C.; Zhang, B. Tetrahedron Lett. 1993, 34, 1087–1090. (e) Carell, T.; Epple, R.; Gramlich, V. Helv. Chim. Acta 1997, 80, 2191–2203. (f ) Itoh, K.; Fujimoto, M.; Hashimoto, M. New J. Chem. 2002, 26, 1070–1075. (g) White, J. D.; Ihle, D. C. Org. Lett. 2006, 8, 1081–1084.

5a

5c

1.487(5) 1.478(5) 1.337(6) 1.325(5) 1.397(5) 1.404(5) 2.439(1) 2.444(1) 97.2(1) 120.1(4) 115.1(3) 121.3(4) 115.1(3) 150.6(4) -23.3(6) 158.1(4) -16.1(6)

1.490(7) 1.483(6) 1.327(7) 1.332(7) 1.409(6) 1.402(6) 2.428(1) 2.425(1) 96.7(1) 121.3(5) 113.4(4) 121.6(5) 112.9(4) -151.7(6) 25.2(7) -145.1(5) 32.1(6)

Bond lengths in A˚, angles and dihedral angles in deg.

spectroscopic, and chemical properties, most notably its ability to undergo a photochemical [2þ2]cycloaddition reaction. Synthesis and Characterization of the (Enamino-Cp)2ZrCl2 Complexes. We now treated ZrCl4 in toluene at room temperature with the reagent [H2CdC(NMe2)-Cp)]Li 4a. In order to diminish the presence of the ZrCl4 Lewis acid, we used a slight excess (2.5 equiv) of 4a. The reaction mixture was stirred for a minimum amount of time (ca. 30 min) to ensure the reaction to go to completion and then worked up (involving filtration) as quickly as possible to prevent the acid-catalyzed subsequent Mannich reaction from taking place. Following this procedure allowed us to isolate the compound 5a practically free from the Mannich product 6a. Recrystallization of 5a from pentane at -30 °C gave single crystals suited for the X-ray crystal structure analysis (see above). However, the H2CdC(NMe2)-substituted bent metallocene system 5a is a very sensitive compound. (13) (a) Hafner, K.; V€ opel, K. H.; Ploss, G.; K€ onig, C. Liebigs Ann. Chem. 1963, 661, 52–75. (b) Hafner, K.; Schulz, G.; Wagner, K. Liebigs Ann. Chem. 1964, 678, 39–53. (c) Hafner, K.; V€opel, K. H.; Ploss, G.; K€onig, C. Org. Synth. 1967, 47, 52–54. (14) Kunz, K.; Pflug, J.; Bertuleit, A.; Fr€ ohlich, R.; Wegelius, E.; Erker, G.; W€ urthwein, E.-U. Organometallics 2000, 19, 4208–4216.

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Scheme 3

Figure 2. Molecular structure of complex 5c.

It is relatively resistant to acid-catalyzed Mannich coupling at ambient conditions in pentane. Addition of solvents such as dichloromethane and the addition of traces of Lewis or Broensted acids resulted in instantaneous Mannich condensation to yield 6a. Mannich coupling also took place in THF, and the sensitive complex 5a was slowly converted to 6a in the solid state upon storage at room temperature. Complex 5a features typical 1H NMR signals of the enamino substituents at δ 4.82/4.17 (dCH2) and δ 2.25 (NMe2) with corresponding 13C NMR resonances at δ 150.8/94.1 (CdCH2) and δ 41.5 (NMe2). The 1H NMR signals of the η5-C5H4 units appear at δ 6.60 and 6.07 (in d6-benzene). Complex 5a was characterized by X-ray diffraction (see Figure 1 and Table 1). It features a typical bent metallocene structure. The Zr-C(Cp) bond lengths are in a range between 2.481(4) and 2.549(4) A˚. Each Cp ring has an enamino unit attached to it. The enamino π-system is in conjugation with the Cp ring, which leads to an overall close to in-plane arrangement of the subgroups at each enamino-Cp ligand system. The enamino substituents are oriented close to C2-symmetric toward the narrow hind side of the bent metallocene wedge. In this arrangement the dCH2 groups of the coplanar enamino units are oriented toward opposite lateral sectors of the bent metallocene framework, whereas the -NMe2 groups are pointing to the bent metallocene backside. We have also investigated the molecular structure of the rather stable previously described analogue 5c.4c,5 It features an overall very similar structure (for details see Figure 2 and Table 1). Photolytic Reactions. Photolysis of a solution of complex 5a in d6-benzene with Pyrex-filtered UV light (HPK 125) at room temperature resulted in intramolecular [2þ2]cycloaddition of the pair of H2CdC(NMe2) substituents with formation of the new ansa-zirconocene complex 7a (see Scheme 3). Under typical reaction conditions (3 h irradiation time) a photostationary equilibrium of 5a and 7a in a 30:70 ratio is reached (see Figure 3). The photostationary equilibrium 5a:7a is wavelength dependent, as expected. Quartzfiltered HPK 125 light results in a ca. 40:60 mixture of the open and closed isomers, whereas irradiation of either the pure 5a starting material or the 30:70 5a:7a mixture with 350 nm light resulted in a final product mixture of 5a:7a in a 50:50 ratio.

In addition, we have carried out the following experiment: the 30:70 photostationary 5a/7a product mixture was exposed to a catalytic quantity of ZrCl4. This resulted in a rapid conversion of the minor enamino-Cp component 5a to 6a by Lewis acid-catalyzed intramolecular Mannich condensation (see above and Scheme 4). The resulting ca. 30:70 mixture of 6a/7a was then photolyzed, which regenerated the original 30:70 mixture of 5a and 7a from the pure 7a component, to independently prove the photostationary situation encountered here. Scheme 4

The removal of the reactive minor 5a component from the photostationary 30:70 5a/7a mixture by the acid-catalyzed Mannich reaction eventually led to the isolation of the pure ansa-metallocene photoproduct 7a. It was obtained by fractional crystallization of the resulting 6a/7a mixture from toluene/pentane. Diffusion of pentane vapor into a solution of pure 7a in benzene eventually furnished single crystals of this new ansa-metallocene system for the X-ray crystal structure analysis of 7a (see below). The ansa-zirconocene 7a exhibits a very characteristic set of NMR data. It shows a 1H NMR -NMe2 singlet of 12H relative intensity at δ 2.17 and an ABCD pattern of the η5-C5H4 protons at δ 6.65, 6.32, 5.95, 5.77, each of 2H intensity [corresponding 13C NMR signals at δ 127.1, 117.2, 107.3, 113.8, plus δ 136.8 (ipso-C)]. The saturated -CH2CH2- unit of the newly formed cyclobutane ring features a characteristic AA0 BB0 1H NMR pattern centered at δ 2.02 and 1.78. The bis(dimethylamino)cyclobutylene moiety shows a pair of 13C NMR resonances of its core at δ 73.5 and 24.9. The ansa-zirconocene complex 7a was characterized by X-ray diffraction (see Figure 4). It features a C2-bridged ansa-metallocene core. The Zr-C(Cp) bond lengths are found in a range between 2.466(2) and 2.524(3) A˚. The connecting ansa-C-C bond (C6-C16) amounts to

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Figure 3. 1H NMR (d6-benzene(*)) spectra of pure 5a (bottom, 500 MHz) and the photostationary mixture of 5a and 7a (top, 600 MHz) after irradiation [Pyrex-filtered UV light (HPK 125)].

(room temperature, benzene, HPK 125, Pyrex filter) to form the new amino-substituted cyclobutylene-bridged ansametallocene 7b. In contrast to the 5a a 7a case here the photostationary equilibrium lies markedly on the side of the open metallocene starting material. We have here reached an equilibrium state of 5b:7b ≈ 75:25. The ansa-metallocene isomer 7b was not isolated pure but spectroscopically characterized from the photostationary product equilibrium mixture [7b, 1H NMR: δ 2.21/1.90 (-CH2-CH2-), δ 2.73, 2.66/ 0.84 (-NEt2), δ 6.66/6.31/5.92/5.85 (η5-C5H4)]. Scheme 5

Figure 4. Projection of the molecular structure of the ansazirconocene complex 7a. Selected bond lengths (A˚) and angles (deg): C1-C6 1.534(3), C11-C16 1.508(3), C6-C7 1.552(3), C16-C17 1.564(3), C6-C16 1.603(3), C7-C17 1.531(4), C6N8 1.451(3), C16-N18 1.479(3); C1-C6-C7 110.2(2), C11C16-C17 115.5(2), C1-C6-N8 114.6(2), C11-C16-N18 107.4(2), C1-C6-C16 110.3(2), C11-C16-C6 113.5(2), C6C7-C17 90.6(2), C16-C17-C7 89.8(2), C6-C16-C17 87.6(2), C16-C6-C7 87.7(2).

1.603(3) A˚. The newly formed cyclobutane ring is puckered.15 The “puckering angle” between the C6-C16-C17 and C6-C7-C17 planes amounts to 21.9°. The dimethylamino substituents at the cyclobutane ring are found in a cisposition to each other. The puckering of the cyclobutane framework brings them into slightly different orientations to the Cp core. The diethylaminoethenyl-substituted zirconocene 5b also undergoes intramolecular [2þ2]cycloaddition upon photolysis (15) See for a comparison: Eliel, E. L.; Wilen, S. H.; Doyle, M. P. Basic Organic Stereochemistry; Wiley-Interscience: New York, 2001.

We also photolyzed the piperidino-substituted enaminometallocene system 5c. Under similar conditions we could, however, not observe any significant ansa-metallocene formation in this case.

Conclusions Our study has shown that conjugated enamino substituents at the Cp rings of the bent zirconocene framework can be used for ansa-metallocene formation by means of photochemical [2þ2]cycloaddition. This observation increases the repertoire of organic functional group chemistry at these sensitive early metal bent metallocene systems. We have now an increasing number of reactions available to perform functional group interconversions at the group 4 metallocenes. The characteristics of our new organometallic enamine cyclization are similar to the alkenyl [2þ2]cycloaddition reactions at the zirconocene framework that we had recently reported. The new reaction also leads to photostationary equilibria that are strongly dependent on the size of the substituent at the R-carbon atom of the alkenyl substituents. The presence of the smaller -NMe2 groups leads to a better

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ansa-metallocene formation than the more bulky -NEt2 systems. This congruent substituent dependency of the photostationary equilibrium in both the alkenyl- and the enamino-substituted (RCp)2ZrCl2 systems is not so easy to rationalize; it may perhaps indicate that a metallocene local minimum conformer16 needs to be populated in a preequilibrium situation that brings the two -C(R)dCH2 double bonds in a favorable orientation relative to each other to allow the intramolecular [2þ2]photocycloaddition reaction to take place. This would in fact place the here observed reaction type within the family of the topochemical reactions17 and might justify describing the 5 to 7 interconversion as an example of a reaction proceeding under “dynamic topochemical control”, just as we had proposed previously for the related 1 to 2 interconversions.8-11 With these new examples the scope of useful photochemically induced carbon-carbon coupling reactions at the sensitive group 4 metallocenes and related organometallic systems is progressively increasing. The properties of the newly available ansa-metallocene systems such as the aminofunctionalized complexes 7, e.g., for Ziegler-Natta catalysis, will be explored.

Experimental Section General Information. Reactions with air- and moisture-sensitive compounds were carried out under an argon atmosphere using Schlenk-type glassware or in a glovebox. Solvents (including deuterated solvents used for NMR spectroscopy) were dried and distilled from appropriate drying agents under argon prior to use. The following instruments were used for physical characterization of the compounds. Elemental analyses: Foss-Heraeus CHN-O-Rapid. NMR: Varian Inova 500 (1H, 500 MHz; 13C, 126 MHz), Varian UnityPlus 600 (1H, 600 MHz; 13C, 151 MHz). Assignments of the resonances were supported by 2D experiments. X-ray diffraction: Data sets were collected with a Nonius KappaCCD diffractometer, equipped with a rotating anode generator. Programs used: data collection COLLECT (Nonius B.V., 1998), data reduction Denzo-SMN (Otwinowski, Z.; Minor, W. Methods Enzymol. 1997, 276, 307326), absorption correction SORTAV (Blessing, R. H. Acta Crystallogr. 1995, A51, 33-38; Blessing, R. H. J. Appl. Crystallogr. 1997, 30, 421-426) and Denzo (Otwinowski, Z.; Borek, D.; Majewski, W.; Minor, W. Acta Crystallogr. 2003, A59, 228234), structure solution SHELXS-97 (Sheldrick, G. M. Acta Crystallogr. 1990, A46, 467-473), structure refinement SHELXL-97 (Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112-122), graphics XP (BrukerAXS, 2000). 6-Dimethylamino-6-methylfulvene (3a),13 6-diethylamino6-methylfulvene (3b),5 6-piperidino-6-methylfulvene (3c),4 [(1-dimethylaminoethenyl)cyclopentadienyl]lithium (4a),4 [(1diethylaminoethenyl)cyclopentadienyl]lithium (4b),5 [(1-piperidinoethenyl)cyclopentadienyl]lithium (4c),4 bis[(1-diethylaminoethenyl)cyclopentadienyl]zirconium dichloride (5b),5 and bis[(1-piperidinoethenyl)cyclopentadienyl]zirconium dichloride (5c)4 were prepared according to modified literature procedures. (16) (a) Erker, G.; M€ uhlenbernd, T.; Benn, R.; Rufinska, A.; Tsay, Y.-H.; Kr€ uger, C. Angew. Chem. 1985, 97, 336–337; Angew. Chem., Int. Ed. Engl. 1985, 24, 321-323. (b) Erker, G.; Nolte, R.; Tainturier, G.; Rheingold, A. Organometallics 1989, 8, 454–460. (c) Erker, G.; Nolte, R.; Kr€uger, C.; Schlund, R.; Benn, R.; Grondey, H.; Mynott, R. J. Organomet. Chem. 1989, 364, 119–132. (d) Erker, G.; Aulbach, M.; Knickmeier, M.; Wingberm€ uhle, D.; Kr€uger, C.; Nolte, M.; Werner, S. J. Am. Chem. Soc. 1993, 115, 4590–4601. (e) Knickmeier, M.; Erker, G.; Fox, T. J. Am. Chem. Soc. 1996, 118, 9623–9630. (f ) J€odicke, T.; Menges, F.; Kehr, G.; Erker, G.; H€ oweler, U.; Fr€ ohlich, R. Eur. J. Inorg. Chem. 2001, 2097–2106. (17) (a) Cohen, M. D.; Schmidt, G. M. J. J. Chem. Soc. 1964, 1996– 2000. (b) Schmidt, G. M. J. J. Chem. Soc. 1964, 2014–2021. (c) Bregman, J.; Osaki, K.; Schmidt, G. M. J.; Sonntag, F. I. J. Chem. Soc. 1964, 2021–2030.

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Preparation of Bis[(1-dimethylaminoethenyl)cyclopentadienyl]zirconium Dichloride (5a). Toluene (4 mL) was added to a mixture of [(1-dimethylaminoethenyl)cyclopentadienyl]lithium (4a) (215 mg, 1.523 mmol) and zirconium tetrachloride (142 mg, 0.609 mmol). The resulting suspension was stirred at room temperature for 30 min. Filtration of the brown mixture gave an orange solution. Then all the volatiles were removed in vacuo, and the remaining crude product was washed with pentane (2  3 mL). The compound was dried in vacuo to afford the product as a yellow powder (170 mg, 0.395 mmol, 65%). Crystallization from pentane solution at -30 °C yielded yellow crystals suitable for X-ray diffraction. 1H NMR (C6D6, 500 MHz, 298 K): δ 6.60 (m, 4H, R-C5H4), 6.07 (m, 4H, β-C5H4), 4.82 (d, 2JH,H =0.9 Hz, 2H, dCH2(E)), 4.17 (d, 2JH,H =0.9 Hz, 2H, dCH2(Z)), 2.25 (s, 12H, CH3). 13C{1H} NMR (C6D6, 126 MHz, 298 K): δ 150.8 (dC), 124.0 (ipso-C of C5H4), 119.0 (R-C5H4), 115.9 (β-C5H4), 94.1 (dCH2), 41.5 (CH3). X-ray Crystal Structure Analysis of Complex 5a: formula C18H24Cl2N2Zr, M = 430.51, yellow-orange crystal 0.25  0.15  0.10 mm, a = 9.6081(4) A˚, b = 9.6579(5) A˚, c = 11.8586(7) A˚, R = 84.225(3)°, β = 82.718(3)°, γ = 66.429(2)°, V = 998.95(9) A˚3, Fcalc = 1.431 g cm-3, μ = 0.818 mm-1, empirical absorption correction (0.822 e T e 0.923), Z=2, triclinic, space group P1 bar (no. 2), λ = 0.71073 A˚, T = 223(2) K, ω and j scans, 10 477 reflections collected (( h, ( k, ( l), [(sin θ)/λ]=0.66 A˚-1, 4555 independent (Rint=0.058) and 3134 observed reflections [I g 2 σ(I)], 212 refined parameters, R = 0.053, wR2 = 0.139, max. (min.) residual electron density 0.48 (-0.93) e A˚-3, hydrogen atoms calculated and refined as riding atoms. Photolysis of Bis[(1-dimethylaminoethenyl)cyclopentadienyl]zirconium Dichloride (5a). Bis[(1-dimethylaminoethenyl)cyclopentadienyl]zirconium dichloride (5a) (ca. 30 mg) was dissolved in d6-benzene (1 mL) and exposed to UV light in a 5 mm NMR tube with a Philips HPK 125 lamp (Pyrex filter) at room temperature. After 3 h a photostationary equilibrium was reached with 30:70 ratio between complex 5a and 7a, respectively. Complex 5a: 1H NMR (C6D6, 600 MHz, 298 K): δ 6.60 (m, 4H, R-C5H4), 6.07 (m, 4H, β-C5H4), 4.81 (d, 2JH,H=0.9 Hz, 2H, dCH2(E)), 4.17 (d, 2JHH = 0.9 Hz, 2H, dCH2(Z)), 2.25 (s, 12H, CH3). 13C{1H} NMR (C6D6, 151 MHz, 298 K): δ 150.8 (dC), 124.0 (ipso-C5H4), 119.0 (R-C5H4), 115.9 ( β-C5H4), 94.1 (dCH2), 41.5 (CH3). Complex 7a: 1H NMR (C6D6, 600 MHz, 298 K): δ 6.65 (m, 2H, β-C5H4), 6.32 (m, 2H, β0 -C5H4), 5.95 (m, 2H, R-C5H4), 5.77 (m, 2H, R0 -C5H4), 2.18 (s, 12H, CH3), 2.03 (m, 2H, CH2-cis), 1.79 (m, 2H, CH2-trans). 13C{1H} NMR (C6D6, 151 MHz, 298 K): δ 136.8 (ipso-C5H4), 127.1 ( β-C5H4), 117.2 ( β0 -C5H4), 113.8 (R0 -C5H4), 107.3 (R-C5H4), 73.5 (ipso-cyclobutane), 39.5 (CH3), 24.9 (CH2). Preparation of [1,2-Bis(dimethylamino)cyclobutylenebis(cyclopentadienyl)]zirconium Dichloride (7a). A solution of bis[(1-dimethylaminoethenyl)cyclopentadienyl]zirconium dichloride (5a) (250 mg, 0.581 mmol) in d6-benzene (1.8 mL) was irradiated in a 5 mm NMR tube with a Philips HPK 125 lamp (Pyrex filter) at room temperature for 6 h with periodical shaking every 30 min. Then the reaction mixture was transferred into a Schlenk flask, and catalytical amount of zirconium tetrachloride (20 mg, 0.086 mmol) and dichloromethane (5 mL) were added to the flask. The resulting mixture was stirred at room temperature for 20 min. After removing all the volatiles in vacuo, the residue was dissolved in toluene (10 mL), and pentane (4 mL) was layered on the top of the toluene phase. The reaction setup was kept overnight at -30 °C, and then the liquid phase was decanted from the precipitated solid and evaporated in vacuo to give the product as a yellow-brown solid (110 mg, 0.255 mmol, 44%). Anal. Calcd for C18H24N2Cl2Zr: C, 50.22; H, 5.62; N, 6.51. Found: C, 49.85; H, 5.74; N, 5.72. 1H NMR (C6D6, 600 MHz, 298 K): δ 6.65 (m, 2H, β-C5H4), 6.32 (m, 2H, β0 -C5H4), 5.95 (m, 2H, R-C5H4), 5.77 (m, 2H, R0 -C5H4), 2.17 (s, 12H, CH3), 2.02 (m, 2H, CH2-cis), 1.78 (m, 2H, CH2-trans).

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C{1H} NMR (C6D6, 151 MHz, 298 K): δ 136.8 (ipso-C5H4), 127.1 ( β-C5H4), 117.2 ( β0 -C5H4), 113.8 (R0 -C5H4), 107.3 (R-C5H4), 73.5 (ipso-cyclobutane), 39.5 (CH3), 24.9 (CH2). X-ray Crystal Structure Analysis of Complex 7a: formula C18H24Cl2N2Zr, M=430.51, light yellow crystal 0.35  0.20  0.06 mm, a = 13.6056(2) A˚, b = 15.4198(2) A˚, c = 17.3531(3) A˚, V = 3640.6(1) A˚3, Fcalc = 1.571 g cm-3, μ = 0.898 mm-1, empirical absorption correction (0.744 e T e 0.948), Z = 8, orthorhombic, space group Pbca (No. 61), λ=0.71073 A˚, T= 223(2) K, ω and j scans, 25 980 reflections collected (( h, ( k, ( l), [(sin θ)/λ]=0.66 A˚-1, 4315 independent (Rint=0.063) and 3253 observed reflections [Ig2σ(I)], 212 refined parameters, R=0.034, wR2=0.082, max. (min.) residual electron density 0.47 (-0.51) e A˚-3, hydrogen atoms calculated and refined as riding atoms. Photolysis of Bis[(1-diethylaminoethenyl)cyclopentadienyl]zirconium Dichloride (5b). A solution of bis[(1-diethylaminoethenyl)cyclopentadienyl]zirconium dichloride (5b) (ca. 30 mg) in d6-benzene (1 mL) was irradiated in a 5 mm NMR tube for 6 h at room temperature with a Philips HPK 125 lamp (Pyrex filter). The compound produces a photostationary equilibrium at 5b/7b ≈ 75:25. Complex 5b: 1H NMR (C6D6, 500 MHz, 298 K): δ 6.69 (m, 4H, R-C5H4), 6.09 (m, 4H, βC5H4), 4.93 (d, 2JH,H =0.8 Hz, 2H, dCH2(E)), 4.19 (d, 2JH,H = 0.8 Hz, 2H, dCH2(Z)), 2.73 (q, 3JH,H =7.1 Hz, 8H, CHEt 2 ), 0.85 13 1 (t, 3JH,H=7.1 Hz, 12H, CHEt 3 ). C{ H} NMR (C6D6, 126 MHz, 298 K): δ 148.0 (dC), 123.8 (ipso-C5H4), 119.0 (R-C5H4), 116.3 Et (β-C5H4), 96.3 (dCH2), 43.6 (CHEt 2 ), 11.6 (CH3 ). Complex 7b: 1H NMR (C6D6, 500 MHz, 298 K): δ 6.66 (m, 2H, β-C5H4), 6.31 (m, 2H, β0 -C5H4), 5.92 (m, 2H, R-C5H4), 5.85 (m, 2H, R0 -C5H4), 2.73, 2.66 (each m, each 4H, CHEt 2 ), 2.21 (m, 2H, CH2-cis), 1.90 (m, 2H, CH2-trans), 0.84 (t, 3JH,H=7.1 Hz, 13

Tumay et al. 13 12H, CHEt C{1H} NMR (C6D6, 126 MHz, 298 K): δ 137.9 3 ). (ipso-C5H4), 127.2 ( β-C5H4), 116.7 ( β0 -C5H4), 114.2 (R0 -C5H4), 107.9 (R-C5H4), 75.4 (ipso-cyclobutane), 43.7 (CHEt 2 ), 28.7 (CH2), 15.0 (CHEt 3 ). Preparation of Bis[(1-piperidinoethenyl)cyclopentadienyl]zirconium Dichloride (5c). The complex was synthesized according to modified literature procedures.4 Single crystals suitable for X-ray diffraction were obtained from a concentrated toluene solution at room temperature. X-ray Crystal Structure Analysis of Complex 5c: formula C24H32Cl2N2Zr, M=510.64, yellow crystal 0.50  0.15  0.10 mm, a=10.5561(2) A˚, b=10.9379(2) A˚, c=12.3165(3) A˚, R= 86.038(1)°, β = 68.639(1)°, γ = 63.421(1)°, V = 1176.88(4) A˚3, Fcalc = 1.441 g cm-3, μ = 0.707 mm-1, empirical absorption correction (0.719 e T e 0.933), Z=2, triclinic, space group P1 (No. 2), λ = 0.71073 A˚, T = 223(2) K, ω and j scans, 11 489 reflections collected (( h, ( k, ( l), [(sin θ)/λ]=0.66 A˚-1, 5363 independent (Rint = 0.064) and 3868 observed reflections [I g 2σ(I)], 263 refined parameters, R = 0.049, wR2 = 0.149, max. (min.) residual electron density 1.02 (-1.16) e A˚-3, hydrogen atoms calculated and refined as riding atoms.

Acknowledgment. Financial support from the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie is gratefully acknowledged. We thank the BASF for a gift of solvents. Supporting Information Available: Experimental details and information about the X-ray crystal structure analyses. This material is available free of charge via the Internet at http:// pubs.acs.org.