Regioselective Synthesis of 1, 2-and 1, 3-Di (silylamido

Dec 1, 2009 - Luis M. Frutos,‡ and Obis Casta˜no‡. ^. Departamento de Quımica Inorg´anica, Universidad de Alcal´a, Campus Universitario, E-288...
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Organometallics 2010, 29, 263–268 DOI: 10.1021/om9009087

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Regioselective Synthesis of 1,2- and 1,3-Di(silylamido)cyclopentadienyl Zirconium Complexes Marı´ a Sudupe,^ Jes us Cano,*,^ Pascual Royo,*,^ Marta E. G. Mosquera,^,† Luis M. Frutos,‡ and Obis Casta~ no‡ ^

Departamento de Quı´mica Inorg anica, Universidad de Alcal a, Campus Universitario, E-28871 Alcal a de Henares, Spain and ‡Departamento de Quı´mica Fı´sica, Universidad de Alcal a, Campus Universitario, E-28871 Alcal a de Henares, Spain. †X-ray diffraction studies. Received October 15, 2009

Regioselective methods to synthesize [Zr{η5-C5H3-1,3-(SiMe2-η-NtBu)2}R] and [Zr{η5-C5H31,2-(SiMe2-η-NtBu)2}R] have been investigated. Selective formation of 1,3- and 1,2-di(silylamido)cyclopentadienyl zirconium complexes is controlled by the generation of the intermediate lithium salts Li[C5H3-1,3-(SiMe2NHtBu)2] (2) and Li2[C5H4-1,1-(SiMe2NtBu)2] (3). Experimental data and ab initio calculations justify the formation of 2 and 3 starting from C5H4(SiMe2NHtBu)2 (1). The structural characterization of the lithium salt 3 shows the cyclopentadiene ring unaltered, while the two deprotonated 1,1-di(silylamido) groups are bridging both lithium atoms with one additional coordinated THF molecule. The X-ray diffraction studies of 1,2- and 1,3-di(silylamido)cyclopentadienyl zirconium complexes show a remarkably different coordination site in these molecules.

Introduction Metal complexes with chelating linked η-amido-η5-cyclopentadienyl ligands are fascinating types of compounds that have become an important class of homogeneous Ziegler-Natta constrained geometry catalysts (CGC). Many efforts have been made to modify the active center in order to influence the electronic and steric properties and the coordination environment of the metal.1 The special reactivity of these complexes compared to the ansa-dicyclopentadienyl compounds2 derives from their more open coordination site and the higher acidity of the metal center. We reported previously a new class of metal complexes derived from a tridentate chelating di(silyl-η-amido)-η5cyclopentadienyl ligand that was isolated by deprotonation of the precursor di(silylamino)cyclopentadiene C5H4(SiMe2NHtBu)2 (1) with the basic group 4 metal compounds MX4 (M=Ti, Zr; X=CH2Ph, NMe2) (A). It is remarkable that this synthetic approach always gave complexes containing

the 1,3-di(silyl-η-amido)-η5-cyclopentadienyl ring3 and that the monobenzyl zirconium complex exhibited ethylene polymerization activity, in spite of generating alkyl-free cations,3a similar to related singly silyl-η-amido zirconium dicyclopentadienyl compounds (B, C)4 and to cationic Co(I)5,6(D) species, which also show ethene polymerization activity (Chart 1). The reactivity of these 1,3-di(silylamido)cyclopentadienyl complexes3 prompted us to explore selective synthetic methods to isolate the 1,2-di(silyl-η-amido)-η5-cyclopentadienyl derivatives, for which a more open coordination site7 and therefore higher reactivity should be expected. Here we report selective methods to isolate 1,2- and 1, 3-di(silyl-η-amido)-η5-cyclopentadienyl group 4 metal complexes and the ab initio calculations to support the pathways of their formation reactions.

Results and Discussion The precursor cyclopentadiene C5H4(SiMe2NHtBu)2 (1) may exist as a mixture of seven isomers, three of which

*Corresponding authors. Tel: 34918854765. Fax: 34 918854683. E-mail: [email protected]. (1) (a) Shapiro, P. J.; Bunel, E. E.; Piers, W. E.; Bercaw, J. E. Synlett 1990, 2, 74. (b) Shapiro, P. J.; Bunel, E.; Schaefer, W. P.; Bercaw, J. E. Organometallics 1990, 9, 867. (c) Okuda, J. Chem. Ber. 1990, 123, 1649. (d) Shapiro, P. J.; Cotter, W. E.; Schaefer, W. P.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc. 1994, 116, 4623. (e) McKnight, A. L.; Waymouth, R. M. Chem. Rev. 1998, 98, 2587. (f) Braunschweig, H.; Breitling, F. M. Coord. Chem. Rev. 2006, 250, 2691. (g) Cano, J.; Kunz, K. J. Organomet. Chem. 2007, 4411. (2) (a) M€ ohring, P. C.; Coville, N. J. J. Organomet. Chem. 1994, 479, 1. (b) Brintzinger, H.-H.; Fischer, D.; M€ulhaupt, R.; Rieger, B.; Waymouth, R. M. Angew. Chem. 1995, 107, 1255. Angew. Chem., Int. Ed. Engl. 1995, 34, 1143. (Corrigendum: Brintzinger, H.-H.; Fischer, D.; M€ulhaupt, R.; Rieger, B.; Waymouth, R. M. Angew. Chem. 1995, 107, 1652. Angew. Chem., Int. Ed. Engl. 1995, 34, 1368) (c) Kaminsky, W. J. Chem. Soc., Dalton Trans. 1998, 1413. (d) Hlatky, G. G. Coord. Chem. Rev. 1999, 181, 256.

(3) (a) Cano, J.; Royo, P.; Lanfranchi, M.; Pellinghelli, M. A.; Tiripicchio, A. Angew. Chem., Int. Ed. 2001, 40, 2495. (b) Cano, J.; Sudupe, M.; Royo, P.; Mosquera, M. E. G. Organometallics 2005, 24, 2424. (c) Cano, J.; Sudupe, M.; Royo, P.; Mosquera, M. E. G. Angew. Chem., Int. Ed. 2006, 45, 7572. (4) Jin, J. Z.; Wilson, D. R.; Chen, E. Y. X. Chem. Commun. 2002, 708. (b) Wang, C.; Luo, H.-K.; van Meurs, M.; Stubbs, L. P.; Wong, P.-K. Organometallics 2008, 2908. (5) Steffen, W.; Bl€ omker, T.; Kleigrewe, N.; Kehr, G.; Fr€ ohlich, R.; Erker, G. Chem. Commun. 2004, 1188. (b) Kleigrewe, N.; Steffen, W.; Bl€omker, T.; Kehr, G.; Fr€ohlich, R.; Wibbeling, B.; Erker, G.; Wasilke, J. C.; Wu, G.; Bazan, G. C. J. Am. Chem. Soc. 2005, 127, 13955. (6) Humphries, M. J.; Tellmann, K. P.; Gibson, V. C.; White, A. J. P.; Williams, D. J. Organometallics 2005, 24, 2039. (7) Mengele, W.; Diebold, J.; Troll, C.; Roll, W.; Brintzinger, H.-H. Organometallics 1993, 12, 1931.

r 2009 American Chemical Society

Published on Web 12/01/2009

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Sudupe et al. Chart 1

Chart 2. Isomers of C5H4(SiR3)2

contain at least one C(sp3)-bound silyl group and are 1,1- 1a, 1,2- 1b, and 1,3-ring 1c disubstituted isomers8 that can be interconverted through degenerate successive silatropic 1, 2-shifts of the allylic silyl group. The 1H and 13C NMR spectra of CD2Cl2 solutions of 1 recorded at -70 to 80 C indicate the presence of variable molar ratios of isomers 1a and 1c with a negligible proportion of isomer 1b, considered to be the higher energy intermediate in the interconversion of 1a and 1c (Chart 2). An attractive strategy to isolate metal complexes containing the deprotonated 1,2-di(silylamido) ligand could be based on getting a silatropic exchange slow enough to produce the deprotonation of the unstable isomer 1b; otherwise the 1c/1a molar ratio has been observed to be 1:1 at -70 C, increasing to 2:1 at 20 C and to 10:1 at 80 C (Chart 2). The steric hindrance caused by the bulkier (R = NHtBu) substituents makes the isomer 1c the major component at room temperature,8 in contrast to other disilyl-substituted compounds C5H4(SiR3)2 reported9 previously, for which the isomer 1a (R =Me, Cl) is by far the major component; other isomers resulting from nondegenerate prototropic 1,2-shifts were not observed. The reported8 reactions of M(NMe2)4 (M=Ti, Zr) with 1 to give the triamido complexes [M{η5-C5H3-1,3-[SiMe2(NHtBu)]2}(NMe2)3] (I) indicate that these basic metal amides produce a selective deprotonation of the more acidic cyclopentadiene allylic H of isomer 1c, generating the aromatic cyclopentadienyl ligand with irremovable 1,3-disilyl substituents (Scheme 1). Successive thermal deprotonation at increasing temperatures of one less basic amine NHtBu to give the mono(amidosilyl)-bridged derivatives followed by deprotonation of the second NHtBu group finally afforded the doubly silylamido-bridged compounds [M{η5-C5H3-1,3-[SiMe2(η1NtBu)]2}(NMe2)] (II). Similar but more favorable reactions occurred with the more basic M(CH2Ph)4 (M = Ti, Zr) reagents, affording [M{η5-C5H3-1,3-[SiMe2(η1-NtBu)]2}(CH2Ph)] (II). We also reported10 that metalation of 1 with 1 equiv of nBuLi or MeLi gave selectively the lithium salt Li[C5H3-1,3-(SiMe2NHtBu)2] (2) after warming the hexane solution, initially cooled at -78 C, to room temperature (Scheme 1). This result is also consistent with the expected deprotonation of isomer 1c leaving both 1,3-disilylamino bridges unaltered, although it could also be justified considering isomer 1a as the reactive species (see below). (8) Cano, J.; Royo, P.; Jacobsen, H.; Blacque, O.; Berke, H.; Herdtweck, E. Eur. J. Inorg. Chem. 2003, 2463. (9) Jutzi, P. Chem. Rev. 1986, 86, 983. (10) Sudupe, M.; Cano, J.; Royo, P.; Herdtweck, E. Eur. J. Inorg. Chem. 2004, 3074.

Scheme 1. 1,3-Disilyl Cyclopentadienyl Derivatives

Scheme 2. Synthesis of the Lithium Salts Li[C5H3-1,3(SiMe2NHtBu)2] (2) and Li2[C5H4-1,1-(SiMe2NtBu)2] (3)

When 2 equiv of nBuLi was slowly added at -78 C to a hexane solution of 1 (Scheme 2), the dilithium salt identified as Li2[C5H4-1,1-(SiMe2NtBu)2] (3) was isolated, after warming to room temperature, in an almost quantitative yield. The X-ray diffraction study of the dilithium salt 3 (Figure 1) showed that the cyclopentadiene ring remained unaltered with the two deprotonated 1,1-di(silylamido) groups bridging both lithium atoms with one additional coordinated THF molecule. Formation of 3 cannot occur following the selective progressive deprotonation of isomer 1c discussed above, as it would give the lithium salt Li[C5H3-1,3-(SiMe2NHtBu)2] (2) in the first step, keeping the irremovable 1,3-disilyl substituents. Rather, formation of 3 could be justified assuming that at low temperatures (-78 C) addition of 2 equiv of base produces deprotonation of both amine protons of 1, implying that the kinetic acidity of N-H is greater than that of Cp-H. However, in the presence of 1 equiv of base at low temperature (heating from -78 to 25 C) 1 undergoes

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deprotonation of one of the NHtBu groups to give the intermediate (silylamino)(silylamido)cyclopentadiene compounds 1,1-, 1,2-, and 1,3- Li[C5H4{SiMe2(NtBu)}{SiMe2(NHtBu)}] (Scheme 2), which at 25 C are further transformed into the 1,3-disilyl cyclopentadiene Li[C5H4-1-(SiMe2NtBu)-3-(SiMe2NHtBu)] by a silatropic exchange, which is finally followed by an intramolecular proton transfer from the cyclopentadienyl ring to the amido moiety (see below) to give the reaction product as the monolithium salt 2. Nevertheless, when the intermediates 1,1-, 1,2-, and 1,3-Li[C5H4{SiMe2(NtBu)}{SiMe2(NHtBu)}] are kept at -78 C and

Figure 1. ORTEP view of Li2[C5H4-1,1-(SiMe2NtBu)2] (3) with probability ellipsoids set at 30%.

Figure 2. Transition-state structure for the intramolecular proton transfer from the Cp ring to the amido -tBuN- Liþ moiety. The transition vector indicating the movement of the proton from the transition state is shown together with the NH and CpH distances.

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treated with a second equivalent of MeLi, the 1,1- compound Li2[C5H4-1,1-(SiMe2NtBu)2] (3) is obtained selectively, implying that the kinetic acidity of N-H is greater than that of Cp-H and showing that there is no reaction between Cp-H and MeLi at -78 C. Ab initio calculations (see Supporting Information for details concerning the theoretical methods and computational results) have been carried out in order to study the possible intramolecular proton transfer from the cyclopentadienyl ring to the amido -tBuN- Liþ moiety, the former acting as the base in the reaction. We have determined the structure of the transition state for this reaction (Figure 2), which shows the transferring proton being coplanar with the C atom of the Cp, and the Si and N atoms, forming a fourcenter transition state with large N-H and Cp-H bond distances (1.53 and 1.36 A˚, respectively). Synthesis of a Mixture of 1,3- and 1,2-Di(silylamido)cyclopentadienyl Zirconium Complexes. The reaction of 1 with 3 equiv of MeLi in refluxing diethyl ether (Scheme 3) gave a mixture of different lithium salts that could not be identified by NMR spectroscopy. Evidence of their formulation as a mixture containing a ca. 1:1 molar ratio of the lithium salts Li3[C5H3-1,3-(SiMe2NtBu)2] (4a) and Li3[C5H3-1,2-(SiMe2NtBu)2] (4b) was obtained following reaction of the mixture with 1 equiv of ZrCl4 in toluene. The resulting products were fully characterized as the chloro derivatives [Zr{η5-C5H3-1,3-(SiMe2-η-NtBu)2}Cl] (5) (42%) and [Zr{η5-C5H3-1,2-(SiMe2-η-NtBu)2}Cl] (6) (40%). The same result was obtained when the isolated dilithium salt 3 was reacted with an extra equivalent of MeLi in refluxing diethyl ether (Scheme 3), indicating that 3 is formed as an intermediate in the reaction of 1 with 3 equiv of MeLi. Selective Synthesis of 1,3- and 1,2-Di(silylamido)cyclopentadienyl Zirconium Complexes. Alternative synthetic routes were designed to synthesize both types of complexes selectively. Despite giving mixtures of 1,2- and 1,3-disilylcyclopentadienyl complexes, the intermediate dilithium salt 3 provides an excellent solution to the problem of controlling the selective formation of 1,2-complexes. The 1,1-disilylcyclopentadiene ring of 3 exhibits an extremely slow silatropic migration due to the rigidity of the stable cyclic system, increasing the lifetime of isomer 4b formed after the first 1, 2-shift. Simultaneously the rate of deprotonation of the allylic H of this isomer can be increased by addition of excess base (Scheme 4). Under these conditions the resulting isomer can be trapped as an aromatic cyclopentadienyl ligand with irremovable 1,2-disilylamido substituents in a highly selective reaction.

Scheme 3. Synthesis of the Mixture of 5 and 6

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Figure 3. ORTEP view of [Zr{η5-C5H3-1,2-(SiMe2-η-NtBu)2}Me] (6) with probability ellipsoids set at 30%. The hydrogen atoms are omitted for clarity. Scheme 4. Deprotonation of Li2[C5H4-1,1-(SiMe2NtBu)2] (3)

Figure 4. ORTEP view of [Zr{η5-C5H3-1,2-(SiMe2-η-NtBu)2}Me] (8) with probability ellipsoids set at 30%. The hydrogen atoms are omitted for clarity. Table 1. Selected Bond Distances (A˚) and Angles (deg) in [1,2]and [1,3]-Complexesa [1,3]-ZrMe (7) M-Cg M-N(1) M-N(2) M-L

Scheme 5. Selective Synthesis of 1,2- and 1,3-Complexes

When 5 equiv of MeLi and 1 equiv of ZrCl4 were added to a solution of 1, the methyl complex [Zr{η5-C5H3-1,2-(SiMe2η-NtBu)2}Me] (8) was isolated as the unique product of the reaction (Scheme 5), occurring through the intermediate formation of the dilithium salt 3 and its further metathesis with ZrCl4 to give complex 6, which is immediately alkylated with the excess MeLi to afford complex 8. The selective synthesis of 8 starting from 3 shows that the key intermediate in the formation of the [1,2]-MX complexes is compound Li2[C5H4-1,1-(SiMe2NtBu)2], 3. In addition, 1,3-complexes can be synthesized selectively when the isolated monolithium salt 2 is treated with two additional equivalents of MeLi and 1 equiv of ZrCl4. Complex [Zr{η5-C5H3-1,3-(SiMe2-η-NtBu)2}Cl] (5) is then obtained selectively in high yield (72%), and its methylation with 1 equiv of MeLi gave the methyl complex [Zr{η5-C5H31,3-(SiMe2-η-NtBu)2}Me] (7) (Scheme 5). The 1H and 13C NMR spectra of complexes 5-8 showed the behavior expected for Cs symmetric molecules. Recrystallization of 6 and 8 gave appropriate crystals for an X-ray study showing different coordination geometries. While in 8

2.1457 2.123(2) 2.112(2) 2.318(3) (C10)

[1,2]-ZrCl (6)

2.191 2.1041(19) 2.1186(18) 2.6117(6) Cl(1) 2.6908(6) Cl(1)#1 N(1)-Si(1) 1.738(2) 1.719(2) N(2)-Si(2) 1.735(3) 1.737(2) Zr(1)-C(1) 2.451(3) 2.432(2) Zr(1)-C(2) 2.494(3) 2.364(2) Zr(1)-C(3) 2.502(3) 2.496(2) Zr(1)-C(4) 2.457(3) 2.631(2) Zr(1)-C(5) 2.407(3) 2.578(2) C(1)-C(2) 1.423(4) 1.465(3) C(2)-C(3) 1.402(5) 1.433(4) C(3)-C(4) 1.423(4) 1.402(4) C(4)-C(5) 1.428(4) 1.389(4) C(1)-C(5) 1.429(4) 1.420(3) Cg-M-N(1) 101.23 100.12 Cg-M-N(2) 100.52 101.89 Cg-M-L 110.90 (C10) 112.07 Cl(1) 101.96 Cl(1)#1 M-N(1)-Si(1) 104.26(12) 106.44(10) M-N(2)-Si(2) 104.92(11) 101.24(9) N(1)-Si(1)-C(1) 95.52(12) 93.41(10) N(2)-Si(2)-C(2) 94.84(12) (C(4)) 95.19(9) N(1)-M-N(2) 131.32(10) 99.72(7) 106.16(10) 144.63(5) N(1)-M-L N(2)-M-L 105.77(10) 87.95(5)

[1,2]-ZrMe (8) 2.194 2.1040(19) 2.0993(19) 2.274(2) 1.729(2) 1.724(2) 2.389(2) 2.391(2) 2.551(2) 2.639(3) 2.543(2) 1.447(4) 1.427(3) 1.400(4) 1.398(4) 1.428(4) 101.33 100.69 111.61 103.46(10) 103.72(9) 94.69(10) 94.34(10) 108.41(7) 115.14(9) 117.53(9)

a Cg denotes the centroid of the ring. Symmetry transformations used to generate equivalent atoms: #1 -xþ2, -yþ1, -z.

the metal is at the center of tetrahedral coordination, complex 6 is a dinuclear species showing the Zr atoms doubly bridged by two chloride ligands in a square-pyramidal coordination. The most remarkable difference between the X-ray structures of the 1,3-complex 73b and the 1,2-complexes 6 and 8 is related to the coordination of the cyclopentadienyl ring (Figure 3). While the Zr-C(Cp) and C(Cp)-C(Cp) distances for the 1,3-isomer 7 are in agreement with a η5-coordination of the Cp ring (the maximum difference between these bonds is 0.095 A˚ for [Zr-C(5)/Zr-C(1)] and 0.027 A˚ for [C(1)-C(5)/C(2)-C(3)]), the [1,2]-isomers 6 and 8 exhibit a profoundly distorted Cp ring with the maximum difference between the above-mentioned Zr-C and C-C distances of 0.267 and

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0.076 A˚ for 6 and 0.250 and 0.049 A˚ for 8, respectively (Table 1). In addition, the coordination sphere is clearly more open for the zirconium 1,2-isomers 6 and 8, which show N-M-N angles around 30 larger than those observed for the 1,3isomers of titanium-benzyl8 and zirconium-methyl 7 complexes. Considering that in ansa-metallocenes small variations produce important differences in reactivity,11 these structural features should enhance the reactivity of 1,2complexes due to openness of the coordination site.

Conclusions We have developed two methods to synthesize selectively 1,3- and 1,2-di(silylamido)cyclopentadienyl zirconium complexes that are controlled by the intermediate lithium salts 2 and 3. Experimental data and ab initio calculations provide an explanation for the formation of the lithium salts Li[C5H3-1,3-(SiMe2NHtBu)2] (2) and Li2[C5H4-1,1-(SiMe2NtBu)2] (3) starting from C5H4(SiMe2NHtBu)2 (1). The isolation of the lithium salt Li2[C5H4-1,1-(SiMe2NtBu)2] (3) shows that the cyclopentadiene ring is unaltered, while the two silylamino groups are deprotonated, implying that the kinetic acidity of N-H is greater than that of Cp-H, with complex 3 as the key intermediate to obtain one or two di(silylamido)cyclopentadienyl metal compounds. The X-ray diffraction studies of 1,2- and 1,3-di(silylamido)cyclopentadienyl zirconium complexes show that the coordination site of both molecules is remarkably different. Further experimental and theoretical studies will be carried out to gain insight into the structural differences observed for these complexes and their relative reactivity.

Experimental Section General Procedures. All manipulations involving syntheses of metal complexes were performed at an argon/vacuum manifold by using standard Schlenk-line techniques under an argon atmosphere or in a glovebox MBraun MOD system. Solvents were dried by conventional procedures and freshly distilled prior to use. Commercially available reagents were used without further purification. NMR spectra were recorded with a Bruker 400 Ultrashield. 1H and 13C chemical shifts are reported relative to tetramethylsilane. Coupling constants J are given in hertz. Elemental analysis was performed in our laboratories (UAH) with a Perkin-Elmer 2400 CHNS/O analyzer, Series II. Preparation of Li2[C5H4-1,1-(SiMe2NtBu)2] (3). nBuLi (8 mL, 16 mmol), 1.6 M in hexane, was slowly added to a solution of C5H4(SiMe2NHtBu)2 (2.60 g, 8 mmol) in a mixture of hexane/ THF (75:25) at -78 C. The reaction mixture was warmed to room temperature and stirred for 12 h. The solvent was removed under vacuum, and the residue was extracted into hexane. After filtration, partial removal of the solvent and cooling to -40 C gave compound 3 as a crystalline white solid (3.61 g, 7.52 mmol, 94%). 1H NMR (300 MHz, C6D6, 20 C, TMS): 0.30 (bs, 12H, SiMe), 1.27 (m, 4H, THF), 1.39 (s, 18H, NtBu), 3.67 (m, 4H, THF), 6.81 (m, 2H, C5H4), 6.98 (m, 2H, C5H4). 13C NMR (300 MHz, C6D6, 20 C, TMS): 25.3 (SiMe), 34.0 (THF), 37.6 (NtBu), 37.8 (THF), 52.5 (NtBu ipso), 68.6 (C5H4ipso), 127.6 (C5H4), 138.8 (C5H4). Anal. Found: C 62.59, H 10.46, N 6.11. Calcd: C 62.46, H 10.90, N 5.83. Preparation of Li3[C5H3-1,3-(SiMe2NtBu)2] (4a) and Li3[C5H3-1,2-(SiMe2NtBu)2] (4b). MeLi (23.9 mL, 3.8 mmol), 1.6 M in Et2O, was slowly added to a solution of C5H4(11) (a) Shapiro, P. J. Coord. Chem. Rev. 2002, 231, 67. (b) Wang, B. Q. Coord. Chem. Rev. 2006, 250, 242.

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(SiMe2NHtBu)2 (4.14 g, 12.7 mmol) in Et2O (150 mL) at -78 C. The reaction mixture was then warmed to room temperature and stirred under reflux for 12 h. The product obtained after removal of the solvent was washed with pentane (2  25 mL), providing a white solid (3.7 g, 10.8 mmol, 86%), which was identified as the mixture of isomers 4a and 4b in the subsequent transfer of the ligand to the metal center. The same product was obtained when MeLi (0.63 mL, 1.04 mmol), 1.6 M in Et2O, was slowly added to a solution of compound 3 (0.5 g, 1.04 mmol) in Et2O (25 mL) (0.31 g, 0.92 mmol, 89%). Anal. Found: C 61.12, H 10.50, N 7.35. Calcd: C 59.62, H 9.71, N 7.35. Preparation of Li3[C5H3-1,3-(SiMe2NtBu)2] (4a). MeLi (8 mL, 12.8 mmol), 1.6 M in Et2O, was slowly added to a solution of Li[C5H3-1,3-(SiMe2NHtBu)2] (2) (2.12 g, 6.4 mmol) in Et2O (75 mL) at -78 C. The reaction mixture was warmed to room temperature and stirred under reflux for 12 h. The initial light yellow solution turned into a white suspension. After removal of the solvent the residue was washed with pentane (2  25 mL) and dried under vacuum, providing a white solid (1.5 g, 4.9 mmol, 77%), which was identified as the unique isomer 4a in the subsequent transfer of the ligand to the metal center. Anal. Found: C 59.37, H 9.69, N 8.16. Calcd: C 59.62, H 9.71, N 8.18. Preparation of [Zr{η5-C5H3-1,3-(SiMe2-η-NtBu)2}Cl] (5). Toluene (50 mL) was added to a cooled mixture (-78 C) of solid compounds Li3[C5H3-1,3-(SiMe2NtBu)2] (4a) (2 g, 5.8 mmol) and ZrCl4 (1.36 g, 5.8 mmol). The reaction mixture was warmed to room temperature and stirred for 12 h. The solvent was then removed under vacuum, and the residue was extracted into hexane (2  25 mL). After filtration, partial removal of the solvent, and cooling the saturated solution to -40 C, compound 5 was isolated as a colorless crystalline solid (1.8 g, 3.9 mmol, 72%). 1H NMR (300 MHz, C6D6, 20 C, TMS): 0.42 (s, 6H, SiMe), 0.44 (s, 6H, SiMe), 1.30 (s, 18H, NtBu), 6.62 (d, 2H, C5H3), 6.67 (t, 1H, C5H3). 13C NMR (300 MHz, C6D6, 20 C, TMS): 2.4 (SiMe), 2.6 (SiMe), 35.3 (NtBu), 55.9 (NtBuipso), 118.9 (C5H3ipso), 124.2 (C5H3), 134.5 (C5H3). Anal. Found: C 45.54, H 7.77, N 6.08. Calcd: C 45.55, H 7.42, N 6.25. Preparation of [Zr{η5-C5H3-1,2-(SiMe2-η-NtBu)2}Cl] (6). Toluene (50 mL) was added to a cooled (-78 C) mixture of the trilithium salts Li3[C5H3-1,3-(SiMe2NtBu)2] (4a), Li3[C5H31,2-(SiMe2NtBu)2] (4b) (1.48 g, 4.3 mmol), and ZrCl4 (1 g, 4.3 mmol). The reaction mixture was warmed to room temperature and stirred for 12 h. The solvent was removed under vacuum and the residue extracted into hexane (2  25 mL). After filtration, partial removal of the solvent, and cooling the saturated solution to -40 C, compound 6 was isolated as a colorless crystalline solid (0.77 g, 1.7 mmol, 40%). 1H NMR (300 MHz, C6D6, 20 C, TMS): 0.46 (s, 12H, SiMe), 1.30 (s, 18H, NtBu), 6.79 (d, 2H, C5H3), 7.06 (t, 1H, C5H3). 13C NMR (300 MHz, C6D6, 20 C, TMS): 3.1 (SiMe), 5.3 (SiMe), 34.9 (NtBu), 55.2 (NtBu ipso), 117.5 (C5H3ipso), 128.6 (C5H3), 130.6 (C5H3). Anal. Found: C 45.43, H 7.48, N 6.17. Calcd: C 45.55, H 7.42, N 6.25. Preparation of [Zr{η5-C5H3-1,2-(SiMe2-η-NtBu)2}Me] (8). Method A. MeLi (19.1 mL, 30.5 mmol), 1.6 M in Et2O, was slowly added to a solution of C5H4(SiMe2NHtBu)2 (2.36 g, 7.27 mmol) in toluene (50 mL) at -78 C. The reaction mixture was warmed to room temperature and subsequently stirred for 12 h at 60 C. Then, after cooling the reaction mixture to -78 C, a suspension of ZrCl4 (1.69 g, 7.26 mmol) in toluene (25 mL) was added and stirred for 12 h at room temperature. The solvent was removed under vacuum and the residue extracted into hexane (2  50 mL). After filtration, partial removal of the solvent, and cooling the saturated solution to -40 C, compound 8 was isolated as a crystalline light brown solid (2.2 g, 5.1 mmol, 70%). Method B. MeMgCl (3 M in THF, 0.22 mL, 0.67 mmol) was added to a solution of complex 6 (0.25 g, 0.56 mmol) at -78 C. The reaction mixture was warmed to room temperature and stirred for 12 h. The solvent was removed under vacuum, and the

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residue was extracted into hexane (2  25 mL). After filtration, partial removal of the solvent, and cooling the saturated solution to -40 C, compound 8 was isolated as a light yellow solid (0.52 g, 1.22 mmol, 94%). 1H NMR (300 MHz, C6D6, 20 C, TMS): -0.01 (s, 3H, ZrMe), 0.50 (s, 12H, SiMe), 1.31 (s, 18H, NtBu), 6.75 (d, 2H, C5H3), 7.17 (t, 1H, C5H3). 13C NMR (300 MHz, C6D6, 20 C, TMS): 3.4 (SiMe), 5.5 (SiMe), 31.2 (ZrMe), 35.3 (NtBu), 54.9 (NtBu ipso), 115.7 (C5H3ipso), 126.4 (C5H3), 130.4 (C5H3). Anal. Found: C 50.25, H 8.59, N 6.52. Calcd: C 50.53, H 8.48, N 6.55. Crystal Structure Determination. Suitable single crystals of 3, 6, and 8 for X-ray diffraction studies were selected. Crystals were mounted on the top of a glass fiber using perfluoropolyether oil and cooled to 150 K (6) and 200(2) K (3 and 8). Data collection was performed on a Bruker-Nonius Kappa CCD single-crystal diffractometer. The structures were solved, using the WINGX package,12 by direct methods (SHELXS-97) and refined by using full-matrix least-squares against F2 (SHELXL97).13 All non-hydrogen atoms were anisotropically refined expect for C20, C21 C22, and C23 in 3, which were disordered in two positions and left isotropic. In 3, the Flack parameter (12) Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 837–938. (13) Sheldrick, G. M., SHELXL-97; Universit€at G€ottingen: G€ottingen (Germany), 1998.

Sudupe et al. could not be reliably determined because of insufficient anomalous scattering effects. Hydrogen atoms were geometrically placed and left riding on their parent atoms. Crystallographic data and details of refinement of 3, 6, and 8 are summarized in the Supporting Information. CCDC-686750 (3), CCDC-686751 (6), CCDC-686752 (8), and CCDC-686753 (9) contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif

Acknowledgment. The authors acknowledge the MCyT (project MAT2007-60997 and CTQ2006-07643) and the CAM (projects S-0505-PPQ/0328 and CGO8UAH/PPQ-3790) for financial support. M.S. and L.M.F. acknowledge the MEC for a fellowship and for a RyC2007-0095 “Ram on y Cajal” contract, respectively. Supporting Information Available: Tables of crystal data, data collection parameters, atomic coordinates, bond lengths, bond angles, and thermal displacement parameters for complex 3, 6, and 8 in CIF format. This material and computational details are available free of charge via the Internet at http://pubs.acs. org.