A New Versatile Approach to Substituted Cyclopentadienyl

Nov 30, 2009 - From a Cycloheptatrienylzirconium Allyl Complex to a Cycloheptatrienylzirconium Imidazolin-2-iminato “Pogo Stick” Complex with Imid...
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Organometallics 2009, 28, 7041–7046 DOI: 10.1021/om900847y

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A New Versatile Approach to Substituted Cyclopentadienyl-Cycloheptatrienyl Complexes of Zirconium (Trozircenes) Andreas Gl€ ockner,†,‡ Matthias Tamm,*,‡ Atta M. Arif,† and Richard D. Ernst*,† †

Department of Chemistry, University of Utah, Salt Lake City, Utah 84112 and ‡Institut f€ ur Anorganische und Analytische Chemie, Technische Universit€ at Braunschweig, Hagenring 30, D-38106 Braunschweig, Germany Received September 30, 2009

The reactions of Zr(C7H7)(Cl)(tmeda) (tmeda = N,N,N0 ,N0 -tetramethylethylene-1,2-diamine) with cyclopentadienyl and substituted cyclopentadienyl anions have led to the expected Zr(C7H7)(C5H4R) and Zr(C7H7)(C9H7) complexes (R=H, CH3, SiMe3, C3H5 (allyl), PPh2; C9H7=indenyl). The R=H and PPh2 complexes had previously been reported, but their preparations were accompanied by lower yields, among other drawbacks. The approach reported herein thus appears to offer a fairly general, effective, and convenient method for the preparation of zirconium complexes containing the C7H7 and a variety of monoanionic ligands. Structural data have been obtained for the R=CH3, SiMe3, and allyl species, as well as the indenyl complex. These data are consistent with previous conclusions that the zirconium center is present in a formal þ4 oxidation state. Despite the 16-electron configurations for these species, the R=allyl complex showed no coordination of this substituent. As an apparent result of the hard nature of Zr(IV), weak THF coordination to the R=SiMe3 complex was observed to take place, via a structural study. Structural data were also obtained for the previously characterized Zr(C7H7)(C5H5){CN[2,6-C6H3(CH3)2]} complex.

Introduction Although the discovery of ferrocene occurred more than 50 years ago, sandwich complexes still represent an especially prominent group of organometallic compounds.1 In particular, bis(benzene) complexes, such as the classic bis(η6benzene)chromium,2 and many metallocenes as well as their derivatives play very important roles. However, a comparable extrapolation to bis(cycloheptatrienyl) transition metal complexes has not been accomplished synthetically to date and has been addressed only in a recent theoretical study.3 Nevertheless, mixed cycloheptatrienyl-cyclopentadienyl (Cht-Cp) complexes of the type M(C7H7)(Cp) have been known for some time and have been characterized in detail (Cp=C5H5 as well as modified analogues).4 In recent years, *Corresponding authors. E-mail: [email protected]; m.tamm@ tu-bs.de. (1) (a) Special issue: 50th anniversary of the discovery of ferrocene; Adams, R. D., Ed. J. Organomet. Chem. 2001, 637-639. (b) Comprehensive Organometallic Chemistry III; Crabtree, R. H.; Mingos, D. M. P., Eds.; Elsevier: Oxford, U.K., 2007; Vol. 4. (2) Fischer, E. O.; Hafner, W. Z. Naturforsch. 1955, 10b, 665. (3) Wang, H.; Xie, Y.; King, R. B.; Schaefer, H. F. III. Eur. J. Inorg. Chem. 2008, 3698. (4) (a) Green, M. L. H.; Ng, D. K. P. Chem. Rev. 1995, 95, 439. For more recent examples see: (b) Basta, R.; Arif, A. M.; Ernst, R. D. Organometallics 2003, 22, 812. (c) Braunschweig, H.; Lutz, M.; Radacki, K.; Schauml€ offel, A.; Seeler, F.; Unkelbach, C. Organometallics 2006, 25, 4434. (d) Braunschweig, H.; Kupfer, T.; Lutz, M.; Radacki, K. J. Am. Chem. Soc. 2007, 129, 8893. (e) Noh, W.; Girolami, G. S. Inorg. Chem. 2008, 47, 10682. (f) B€ uschel, S.; Bannenberg, T.; Hrib, C. G.; Gl€ockner, A.; Jones, P. G.; Tamm, M. J. Organomet. Chem. 2009, 694, 1244. r 2009 American Chemical Society

we have become particularly interested in the bonding aspects as well as possible chemical functionalization of group 4 Cht-Cp complexes.5 In this regard, experimental and theoretical investigations have suggested that cycloheptatrienyl, also referred to as tropylium, is better formulated as a “-3” ligand instead of a “þ1” ligand in these complexes, in accord with a formal, and typically favored, þ4 oxidation state of the Lewis acid metal centers.6,7 The incorporation of a Lewis basic phosphine substituent on the five-membered ring in M(C7H7)(C5H4PR2) (M=Zr, Hf) led to the formation of dimers in the solid state and to interesting secondary interactions, e.g., weak Zr-metal contacts.8 The syntheses of M(C7H7)(C5H4PR2) complexes by the reduction of M(C5H4PR2)2Cl2 with magnesium in the presence of cycloheptatriene, however, suffered from low yields, especially considering that one of the valuable C5H4PR2 ligands had to be sacrificed.8 In the case of zirconium, other Zr(C7H7)(Cp) complexes have been reported for Cp=C5H5, C5Me5, and C9H7 (indenyl), utilizing as starting materials Zr(C5H5)Cl3, Zr(C5Me5)Cl3, and Zr(c-C7H8)(Cl2)(PEt3)2, respectively.6,9 While one might expect the latter cycloheptatriene complex to (5) Tamm, M. Chem. Commun. 2008, 3089. (6) Tamm, M.; Kunst, A.; Bannenberg, T.; Herdtweck, E.; Schmid, R. Organometallics 2005, 24, 3163. (7) Menconi, G.; Kaltsoyannis, N. Organometallics 2005, 24, 1189. (8) B€ uschel, S.; Jungton, A.-K.; Bannenberg, T.; Randoll, S.; Hrib, C. G.; Jones, P. G.; Tamm, M. Chem.-Eur. J. 2009, 15, 2176. (9) (a) Van Oven, H. O.; Groenenboom, C. J.; De Liefde Meijer, H. J. J. Organomet. Chem. 1974, 81, 379. (b) Blenkers, J.; Bruin, P.; Teuben, J. H. J. Organomet. Chem. 1985, 297, 61. (c) Diamond, G. M.; Green, M. L. H.; Mountford, P.; Walker, N. M.; Howard, J. A. K. J. Chem. Soc., Dalton Trans. 1992, 417. Published on Web 11/30/2009

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Table 1. Crystallographic Parameters for Zr(C7H7)(C5H4CH3), Zr(C7H7)(C5H4SiMe3), Zr(C7H7)(C5H4CH2CHdCH2), Zr(C7H7)(C9H7), Zr(C7H7)(C5H5){CN[2,6-C6H3(CH3)2]}, and Zr(C7H7)(C5H4SiMe3)(THF) C13H14Zr fw temperature (K) λ (A˚) cryst syst space group unit cell dimens a (A˚) b (A˚) c (A˚) R (deg) β (deg) γ (deg) volume (A˚3); Z Dcalc absorp coeff (cm-1) θ range (deg) limiting indices no. of reflns collected no. of indep reflns; nI > nσ(I) R (F) Rw (F2) max./min. diff Fourier peak (e A˚-3) data completeness

C15H20SiZr

C15H16Zr

C16H14Zr

C21H21NZr

C19H28OSiZr

261.46 150(1) 0.71073 monoclinic P21/a

319.62 150(1) 0.71073 monoclinic P21/c

287.50 150(1) 0.71073 monoclinic P21/a

297.49 150(2) 0.71073 monoclinic P21/n

378.61 150(1) 0.71073 monoclinic P21

391.72 150(1) 0.71073 triclinic P1

11.5011(4) 8.2266(3) 12.4493(5) 90 113.647(2) 90 1078.99(7); 4 1.610 9.74 3.0-27.5 -14 e h e 14 -10 e k e 10 -16 e l e 16 4485 2461; 2 0.0283 0.0670 1.35/-0.78 99.6%

11.5132(3) 11.1425(3) 12.5058(3) 90 114.7267(13) 90 1457.22(6); 4 1.457 8.14 2.5-27.5 -14 e h e 14 -14 e k e 13 -16 e l e 16 6275 3335; 2 0.0416 0.0989 0.93/-0.72 99.9%

11.2763(3) 8.2112(3) 13.6876(5) 90 100.433(2) 90 1246.4(1); 4 1.532 8.51 2.9-27.5 -14 e h e 14 -10 e k e 10 -17 e l e 17 4682 2845; 2 0.0303 0.0688 0.63/-0.51 99.5%

6.9203(2) 8.3288(3) 11.0045(3) 90 105.1948(18) 90 612.10(3); 2 1.614 8.70 3.8-27.5 -8 e h e 8 -9 e k e 10 -14 e l e 14 2383 1396 0.0272 0.0612 0.19/-0.29 99.8%

10.9094(3) 6.81180(10) 11.5925(3) 90 101.7549(12) 90 843.40(3); 2 1.491 6.51 1.8-27.5 -14 e h e 14 -8 e k e 7 -15 e l e 14 3365 3365 0.0266 0.0630 0.62/-0.42 99.2%

7.6814(2) 8.24370(10) 15.3305(3) 94.3128(13) 104.4572(9) 100.0571(12) 918.38(3); 2 1.417 6.64 2.5-27.5 -9 e h e 9 -10 e k e 10 -19 e l e 19 7884 4178 0.0269 0.0584 0.42/-0.41 99.3%

be capable of yielding all of the above trozircene complexes, it apparently does not lead to the unsubstituted C5H5 complex to any noticeable extent.9c Therefore, our recent success in employing the previously reported Zr(C7H7)(Cl)(tmeda)10 (tmeda = N,N,N0 ,N0 -tetramethylethylene-1,2-diamine) for the synthesis of half-open trozircenes of the type Zr(C7H7)(Pdl)11 (Pdl=pentadienyl or substituted pentadienyl ligand) encouraged us to attempt the use of this complex also for the incorporation of various cyclopentadienyl ligands, which could then lead to general and straightforward access to Zr(C7H7)(Cp) complexes.

Experimental Section All synthetic and spectroscopic manipulations were carried out under an atmosphere of prepurified nitrogen, either in Schlenk apparatus or in a glovebox. Solvents were dried and deoxygenated under a nitrogen atmosphere either by distillation from sodium benzophenone ketyl or by passage through alumina columns. Zr(C7H7)(Cl)(tmeda) (1) and cyclopentadienyl anions were prepared by modifications of published procedures.11,12 Zr(C7H7)(C5H5){CN[2,6-C6H3(CH3)2]} was prepared according to the literature,6 and single crystals suitable for an X-ray diffraction study were obtained by cooling a THF/ pentane solution to -20 C. Analytical data were obtained from the microanalytical facilities at the Technische Universit€ at Carolo-Wilhelmina, Braunschweig, using an Elementar VarioMicro instrument, whereas NMR spectra were recorded on Varian VXL-300 spectrometers (the assignments of the H and (10) Diamond, G. M.; Green, M. L. H.; Mountford, P.; Walker, N. M. J. Chem. Soc., Dalton Trans. 1992, 2259. (11) Gl€ ockner, A.; Bannenberg, T.; Tamm, M.; Arif, A. M.; Ernst, R. D. Organometallics 2009, 28, 5866. (12) (a) Panda, T. K.; Gamer, M. T.; Roesky, P. W. Organometallics 2003, 22, 877. (b) Cornellissen, C.; Erker, G.; Kehr, G; Fr€ohlich, R. Organometallics 2005, 24, 214. (c) Beachley, O. T. Jr.; Pazik, J. C.; Glassman, T. E.; Churchill, M. R.; Fettinger, J. C.; Blom, R. Organometallics 1988, 7, 1051. (d) Blais, M. S.; Rausch, M. D. Organometallics 1994, 13, 3557. (e) Erker, G.; Aul, R. Chem. Ber. 1991, 124, 1301. (f) Dinnebier, R. E.; Neander, S.; Behrens, U.; Olbrich, F. Organometallics 1999, 18, 2915.

C resonances are taken from the numbering in the X-ray structures). Zr(C7H7)(C5H5) (2). In a 250 mL Schlenk flask, 0.410 g (1.23 mmol) of Zr(C7H7)(Cl)(tmeda) was dissolved in 20 mL of THF. After this mixture had been cooled to -78 C, 0.110 mg (1.25 mmol) of NaC5H512a in 10 mL of THF was added with a syringe. During the subsequent warmup (80 min), the color changed to orange-red. After 3 h of stirring, the solvent was removed in vacuo. Sublimation at 130 C afforded a purple solid on the -78 C coldfinger in an unoptimized yield of 53% (162 mg). Spectroscopic data match those previously published.9a 1 H NMR (d6-benzene, ambient): δ 5.21 (s, 12 H, C5H5 and C7H7). 13 C NMR (d6-benzene, ambient): δ 80.4 (C7H7), 101.0 (C5H5). Zr(C7H7)(C5H4PPh2) (3). A 0.50 g (1.50 mmol) sample of Zr(C7H7)(Cl)(tmeda) was dissolved in 20 mL of THF in a 250 mL Schlenk flask. Then 0.391 g (1.53 mmol) of Li(C5H4PPh2) (obtained from NaCp and ClPPh2 in hexane, followed by subsequent deprotonation)12b in 10 mL of THF was slowly added with a syringe at -78 C, resulting in a red solution after the warmup to room temperature. After the mixture had been stirred for 2.5 h and the solvent subsequently removed in vacuo, sublimation at 175 C afforded 0.351 g (54%) of an orange solid on a -78 C coldfinger. Spectroscopic data match those previously published.8 1 H NMR (d8-THF, ambient): δ 4.61 (s, 7 H, C7H7), 5.17 (m, 2 H, C5H4), 5.73 (br t, 2 H, C5H4), 7.27-7.35 (m, 10 H, C6H5) Zr(C7H7)(C5H4CH3) (4). A 0.500 g (1.50 mmol) amount of Zr(C7H7)(Cl)(tmeda) was dissolved in 20 mL of THF in a 250 mL Schlenk flask. Then 0.133 g (1.55 mmol) of Li(C5H4CH3)12c was added dropwise at -78 C, resulting in an orange solution, which changed to orange-red during the slow warmup to room temperature. After stirring for 3.5 h, the solvent was removed in vacuo and the oily residue was dried at 50 C. Finally, sublimation at 120 C led to 0.289 g (78%) of a purple solid on a -78 C coldfinger. Single crystals were obtained by slow sublimation in a sealed glass tube. 1 H NMR (d6-benzene, ambient): δ 1.77 (s, 3 H, CH3), 5.10 (m, 2 H, C5H4), 5.19 (m, 2 H, C5H4), 5.20 (s, 7 H, C7H7). 13C NMR (d6-benzene, ambient): δ 14.1 (CH3), 80.9 (C7H7), 100.4 (C5H4), 102.9 (C5H4), 114.1 (ipso-C5H4). Anal. Calcd for C13H14Zr: C, 59.71; H, 5.40. Found: C, 59.35; H, 5.51.

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Organometallics, Vol. 28, No. 24, 2009

Table 2. Pertinent Bonding Parameters for the Zr(C7H7)(Cp) Complexes Cp

C5H4CH3

C5H4SiMe3

C5H4CH2CHdCH2

indenyl

Table 3. Pertinent Bonding Parameters for the Zr(C7H7)(Cp)(L) Complexes Cp/L

Bond Distances (A˚) Zr-C1 Zr-C2 Zr-C3 Zr-C4 Zr-C5 Zr-C6 Zr-C7 Zr-C(av) Zr-C8 Zr-C9 Zr-C10 Zr-C11 Zr-C12 Zr-C(av)

2.316(3) 2.316(2) 2.324(3) 2.340(2) 2.338(2) 2.336(2) 2.326(2) 2.328(4) 2.499(2) 2.480(2) 2.477(2) 2.498(2) 2.523(2) 2.495(8)

2.337(4) 2.323(4) 2.316(4) 2.340(4) 2.339(4) 2.347(4) 2.330(4) 2.333(4) 2.497(3) 2.516(3) 2.509(3) 2.485(3) 2.500(3) 2.501(5)

1.658(1) 2.187(1) 4.1(1)

1.667(1) 2.193(2) 4.6(2)

C5H5/CN[2,6-C6H3(CH3)2]a

C5H4SiMe3/THF

Bond Distances (A˚) 2.322(3) 2.326(3) 2.330(3) 2.330(3) 2.332(3) 2.330(3) 2.329(3) 2.328(2) 2.488(2) 2.487(3) 2.495(2) 2.506(2) 2.517(2) 2.499(6)

2.302(5) 2.315(9) 2.331(6) 2.335(5) 2.323(6) 2.306(7) 2.321(4) 2.319(5) 2.554(3) 2.503(6) 2.488(4) 2.493(4) 2.544(3) 2.516(14)

Ligand Plane Parametersa Zr-CNT(C7) (A˚) Zr-CNT(Cp) (A˚) — C7/Cp (deg)

7043

1.663(1) 2.193(1) 2.2(2)

1.648(2) 2.204(2) 3.2(3)

a CNT represents the center of mass of the cyclic ligand, defined by its carbon atoms.

Zr(C7H7)(C5H4SiMe3) (5). In a 250 mL Schlenk flask, 0.500 g (1.50 mmol) of Zr(C7H7)(Cl)(tmeda) was dissolved in 20 mL of THF. A pale yellow solution of 0.220 g (1.53 mmol) of Li(C5H4SiMe3)12d in 10 mL of THF was added dropwise to the blue solution at -78 C. While the mixture was allowed to warm slowly to room temperature over a period of ∼60 min, the color changed to red. After 4 h of stirring, the solvent was removed in vacuo, and sublimation at 120 C in vacuo led to 334 mg (70%) of a purple solid on a -78 C coldfinger. Single crystals were obtained by slow sublimation in a sealed glass tube. In addition, cooling 5 in a THF/hexane mixture to -60 C overnight gave crystals of the respective THF adduct. 1 H NMR (d6-benzene, ambient): δ 0.08 (s, 9 H, SiMe3), 5.19 (s, 7 H, C7H7), 5.40 (m, 2 H, C5H4), 5.48 (m, 2 H, C5H4). 13C NMR (d6-benzene, ambient): δ 0.01 (s, SiMe3), 80.3 (d, J=166 Hz, C7H7), 104.3 (d, J=170 Hz, C5H4), 106.8 (d, J=170 Hz, C5H4), 114.2 (s, ipso-C5H4). Anal. Calcd for C15H20SiZr: C, 56.37; H, 6.31. Found: C, 55.66; H, 6.41. Zr(C7H7)(C5H4CH2CHdCH2) (6). A 250 mL Schlenk flask was charged with 1.00 g (2.99 mmol) of Zr(C7H7)(Cl)(tmeda) in 30 mL of THF. Then 0.342 g (3.05 mmol) of Li(C5H4CH2CHdCH2)12e in 10 mL of THF was added dropwise to the blue solution at -78 C. After 3.5 h of stirring, during which time the solution was allowed to warm to room temperature, the solvent was removed in vacuo. The product was extracted from the red oily residue with 2  20 and 10 mL of pentane, and these extracts were then filtered through a pad of Celite. After the solvent was removed, the resulting purple solid was further purified by sublimation at 130 C in vacuo to give 0.581 g (68%) of a highly air-sensitive purple solid on a -78 C coldfinger. The product has a melting point of ∼60 C (oil bath temperature). Single crystals were obtained from a pentane solution at 0 C. 1 H NMR (d6-benzene, ambient): δ 2.82 (br d, 3J = 6.6 Hz, 2 H, CH2), 4.84-4.91 (m, 2 H, CHdCH2), 5.15 (m, 2 H, C5H4), 5.18 (m, 2 H, C5H4), 5.19 (s, 7 H, C7H7), 5.64-5.77 (m, 1 H, CHdCH2). 13C NMR (d6-benzene, ambient): δ 33.3 (CH2), 80.8 (C7H7), 100.5 (C5H4), 101.5 (C5H4), 115.4 (CHdCH2), 120.1 (ipso-C5H4), 136.4 (CHdCH2). Anal. Calcd for C15H16Zr: C, 62.66; H, 5.61. Found: C, 62.43; H, 5.74 Zr(C7H7)(C9H7) (7). A 0.50 g (1.50 mmol) portion of Zr(C7H7)(Cl)(tmeda) was dissolved in 20 mL of THF in a 250 mL

Zr-C1 Zr-C2 Zr-C3 Zr-C4 Zr-C5 Zr-C6 Zr-C7 Zr-C(av) Zr-C8 Zr-C9 Zr-C10 Zr-C11 Zr-C12 Zr-C(av) Zr-X(C/O)

2.420(7) 2.362(6) 2.371(6) 2.446(5) 2.423(7) 2.352(6) 2.349(5) 2.389(15) 2.520(3) 2.509(4) 2.510(3) 2.503(4) 2.521(4) 2.513(3) 2.367(2)

2.390(2) 2.337(2) 2.355(2) 2.428(2) 2.429(2) 2.363(2) 2.344(2) 2.378(15) 2.522(2) 2.531(2) 2.525(2) 2.525(2) 2.548(2) 2.530(5) 2.4059(13)

Ligand Plane Parametersb Zr-CNT(C7) (A˚) Zr-CNT(Cp) (A˚) — C7/Cp (deg) — X-Zr-CNT(C7) (deg) — X-Zr-CNT(Cp) (deg)

1.754(4) 2.219(2) 31 112.7

1.745(1) 2.227(1) 33 113.9

98.0

97.1

a For this complex, when values are given that involve the C7H7 ligand, an average over the two images is presented. b CNT represents the center of mass of the cyclic ligand, defined by its carbon atoms.

Schlenk flask. Then 0.186 g (1.53 mmol) of Li(indenide)12f in 10 mL of THF was slowly added with a syringe at -78 C, resulting in a red solution, which was allowed to warm to room temperature. After stirring for 3.5 h, the solvent was removed and the oily residue was dried at 70 C for 1 h. Finally, sublimation at 150 C led to 0.271 g (61%) of a dark purple solid on a -78 C coldfinger. Single crystals were obtained by slow sublimation in a sealed glass tube at 140 C. Spectroscopic data match those previously published.9c 1 H NMR (d6-benzene, ambient): δ 4.98 (s, 7 H, C7H7), 5.35 3 (t, J=3.5 Hz, 1 H, C10), 5.59 (d, 2 H, 3J = 3.5 Hz, H9,11), 6.68-6.72 (m, 2 H, H13,16 or H14,15), 7.12-7.16 (m, 2 H, H13,16 or H14,15). 13C NMR (d6-benzene, ambient): δ 83.2 (C7H7), 92.3 (C5H3), 106.0 (C5H3), 121.7 (ipso-C5H3), 122.5 (phenyl), 122.7 (phenyl). X-ray Diffraction Studies. Single crystals of each compound were examined under Paratone oil and transferred to an EnrafNonius Kappa CD diffractometer for unit cell determination and data collection. The data were analyzed using the SIR97 and SHELXL97 programs.13 In all cases the non-hydrogen atoms could be refined anisotropically. Hydrogen atoms were either located and refined isotropically or allowed to ride on their attached carbon atoms. The terminal CH2 group of the allyl substituent in Zr(C7H7)(C5H4CH2CHdCH2) was found to be disordered over two positions in a 65:35 ratio, but their refinements could still be carried out anisotropically. The indenyl complex was found to lie on a crystallographic mirror plane, leading to the unit cell (space group P21/n) containing only two molecules. However, the mirror plane was positioned between the two π ligands. Even so, their partially overlapping images could still be refined adequately. The Zr(C7H7)(C5H5)[CN(2,6-C6H3(CH3)2] structure contained two images of the (13) (a) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G. L.; Giacovazzo, C.; Guagliardi, A.; Moliterni, A. G. G.; Polidori, G.; Spagna, R. J. Appl. Crystallogr. 1999, 32, 115–119. (b) Sheldrick, G. M. SHELXL-97, Program for the Refinement of Crystal Structures; University of G€ottingen, Germany 1997.

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C7H7 ligand, in a nearly 1:1 ratio. Both could be refined satisfactorily, but isotropically, in space group P21. The choice of this space group was supported by statistical analyses, and attempts to refine the structure in centrosymmetric space groups led to increased disorder and higher R factors. Pertinent crystallographic information is provided in Table 1, while selected bonding parameters are presented in Tables 2 and 3.

Results and Discussion As had been desired, Zr(C7H7)(Cl)(tmeda) (1) has been proven to be an ideal candidate for the successful incorporation of a variety of Cp ligands (eq 1), including C5H5, C5H4PPh2, C5H4CH3, C5H4SiMe3, C5H4CH2CHdCH2, and

Figure 1. Perspective view of Zr(C7H7)(C5H4CH3).

C9H7 (indenyl), and each complex could be isolated in good yield after vacuum sublimation. The C5H4CH3, C5H4SiMe3, and C5H4(CH2CHdCH2) complexes have not been previously reported, whereas the other three were synthesized recently, applying different procedures, which led to yields that were at the most half as high as presented herein.6,8,9c Thus far, however, this approach has allowed for the isolation of neither the hypothetical C13H9 (fluorenyl) complex nor the zirconium analogue of [5,5]bitrovacene,14 utilizing an in situ-prepared fulvalene dianion.15 The 1H NMR spectrum of each complex showed a sharp singlet for the protons of the seven-membered ring and for the substituted C5H4R complexes a more complicated pattern for the protons of the given substituted cyclopentadienyl ligand, thus confirming the substitution at the five-membered ring. The chemical shifts of the allyl substituent reveal the presence of an ordinary CdC bond (e.g., 13C NMR: 115.4 and 136.4 ppm). Similar to Zr(C7H7)(C5H4PR2) complexes, Zr(C7H7)(C5H4CH2CHdCH2) can be considered as a bifunctional complex with a Lewis acidic center and a substituent capable of donating two electrons. Therefore, one must allow for the possibility of weak alkene coordination, perhaps especially in the solid state, which might result in the formation of dimers or oligomers; alternatively, favored by the chelate effect, intramolecular coordination might occur. However, in accord with the NMR data, an X-ray diffraction study showed that the alkene remained as a simple substituent without an interaction with any metal center (vide infra). The structures of Zr(C7H7)(C5H4CH3), Zr(C7H7)(C5H4SiMe3), Zr(C7H7)(C5H4CH2CHdCH2), and Zr(C7H7)(C9H7) are presented in Figures 1-4, while pertinent bonding parameters are included in Table 2. The 16-electron C5H4CH3 complex was found to be isomorphous with its tantalum16 and molybdenum17 (14) Elschenbroich, C.; Schiemann, O; Burghaus, O.; Harms, K.; Pebler, J. Organometallics 1999, 18, 3273. (15) Smart, J. C.; Pinsky, B. L. J. Am. Chem. Soc. 1980, 102, 1009. (16) Noh, W.; Girolami, G. S. Inorg. Chem. 2008, 47, 535. (17) Green, M. L. H.; Ng, D. N. K.; Tovey, R. C.; Chernega, A. N. J. Chem. Soc., Dalton Trans. 1993, 3203.

Figure 2. Perspective view of Zr(C7H7)(C5H4SiMe3).

Figure 3. Perspective view of Zr(C7H7)(C5H4C3H5).

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Figure 5. Perspective view of Zr(C7H7)(C5H4SiMe3)(THF). Figure 4. Perspective view of Zr(C7H7)(C9H7).

analogues, and the decrease of the M-CpCentroid distances from 2.187 A˚ to 2.050 A˚ and to 1.982 A˚ reflects the decreasing metal radii on going from zirconium to tantalum to molybdenum. In each of the zirconium compounds, the two aromatic ligands lie in nearly parallel planes, the deviations being 4.05(14), 4.56(21), 2.16(16), and 3.16(32), respectively, as compared to 5.33 for the C5H5 complex.6 Although the carbon atoms of a given ligand may be nearly planar, some of their substituents deviate noticeably from these planes. While a disorder in the indenyl structure prevented a decent analysis, the data for the other three complexes were sufficiently accurate and consistent. Thus, for the Cp ligands, the hydrogen substituents tilt slightly toward the metal, by 1.2, 0.8, and 1.9, respectively, while the CH3, SiMe3, and CH2CHdCH2 groups tilt away from the metal, by 2.1, 1.7, and 5.0. Significantly larger tilts of 7.2, 9.1, and 6.5, toward the metal, are observed for the C7H7 ligands. This is consistent with the larger girth of this ligand, for which a downward tilting of the substituents would allow for improved metal-ligand orbital overlap, as below.18 The respective tilts for the C5H5 and C7H7 ligands in the C5H5 complex were found to be similar to the above (1.6 and 6.4).6

As was found for the C5H5 analogue, the Zr-C bond distances for the C7H7 ligands in the substituted complexes are nearly 0.2 A˚ on average shorter than those for the Cp ligand, leading to dramatically differing distances from the metal center to the ligand planes, e.g., 1.658 and 2.187 A˚, respectively, for the C7H7 and C5H4CH3 ligands (cf., 1.664 and 2.195 A˚ for the C5H5 complex). The shorter Zr-C (C7H7) bonds clearly reveal a significantly stronger interaction, which has been explained theoretically.19 Such an (18) (a) Kettle, S. F. A. Inorg. Chim. Acta 1967, 1, 303. (b) Elian, M.; Chen, M. M. L.; Mingos, D. M. P.; Hoffmann, R. Inorg. Chem. 1976, 15, 1148. (c) Haaland, A. Acc. Chem. Res. 1979, 12, 415. (d) Ernst, R. D. Struct. Bond. (Berlin) 1984, 57, 1. (e) Ernst, R. D. Comments Inorg. Chem. 1999, 21, 285. (19) (a) Clack, D. W.; Warren, K. D. Theoret. Chim. Acta 1977, 46, 313. (b) Zeinstra, J. D.; Nieuwpoort, W. C. Inorg. Chim. Acta 1978, 30, 103.

observation has already been made for related complexes of titanium, zirconium, and hafnium.5 This strongly implicates the presence of the C7H73- ligand, as simple þ1 or -1 charges could not be expected to lead to such an enhancement of metal-ligand bonding vs. the C5H5 monoanion. Notably, the difference in M-C distances for the two ligands drops on going from M=Ti (ca. 0.21 A˚) to M=V (ca. 0.07 A˚) and M = Cr (ca. 0.03 A˚), suggesting progressively lesser contributions of a C7H73- resonance form, which is in agreement with theoretical studies.7 The coordination of THF to Zr(C7H7)(C5H4SiMe3) was unexpectedly found during a crystallization attempt from a THF solution at -60 C. Even though the coordination once more reflects the Lewis acidity of the metal center, it is important to note that the THF coordination of vacuum-dried material could not be established by NMR techniques, so that the binding can be concluded to be extremely weak. In fact, the Zr-O bond length of 2.406(1) A˚ reflects the weakness via a comparison with the isoelectronic, but cationic, complex [Nb(C7H7)(C5H4CH3)(THF)]þ (Nb-O: 2.308(4) A˚; 2.316 A˚ for Nb-C7 ligand atoms; 2.409 A˚ for Nb-C5 ligand atoms).20 The 16-electron complexes Zr(C7H7)I(THF)2 and [Ti(C7H7)(THF)μ-Cl]2 also exhibit significantly shorter M-O bonds, 2.321(3) and 2.187(1) A˚, respectively.21,22 Coordination of an additional ligand to the 16-electron Zr(C7H7)(C5H4R) fragment does lead to distortions in line with those already reported for complexes having PMe3, (t-C4H9)NC, or N-heterocyclic ligands.4f,6,23 Notably, coordination in the related titanium complexes is only possible if both aromatic ligands are tilted by a bridge in order to create a larger gap (20) (a) Green, M. L. H.; Mountford, P.; Mtetwa, V. S. B.; Scott, P.; Simpson, S. J. J. Chem. Soc., Chem. Commun. 1992, 314. (b) Green, J. C.; Green, M. L. H.; Kaltsoyannis, N.; Mountford, P.; Scott, P.; Simpson, S. J. Organometallics 1992, 11, 3353. (21) Green, M. L. H.; Mountford, P.; Walker, N. M. J. Chem. Soc., Chem. Commun. 1989, 908. (22) Davies, C. E.; Gardiner, I. M.; Green, J. C.; Green, M. L. H.; Hazel, N. J.; Grebenik, P.; Mtewa, V. S. B.; Prout, K. J. Chem. Soc., Dalton Trans. 1985, 669. (23) Baker, R. J.; Bannenberg, T.; Kunst, A.; Randoll, S.; Tamm, M. Inorg. Chim. Acta 2006, 395, 4797. (24) (a) Tamm, M.; Kunst, A.; Bannenberg, T.; Herdtweck, E.; Sirsch, P.; Elsevier, C. J.; Ernsting, J. M. Angew. Chem., Int. Ed. 2004, 43, 5530. (b) Tamm, M.; Kunst, A.; Herdtweck, E. Chem. Commun. 2005, 1729. (c) Tamm, M.; Kunst, A.; Bannenberg, T.; Randoll, S.; Jones, P. G. Organometallics 2007, 26, 761.

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C7H7 ligand to back away somewhat from the metal in order to free up sufficient space.

Summary

Figure 6. Perspective view of Zr(C7H7)(C5H5){CN[2,6-C6H3(CH3)2]}.

between the ligands.24 Although a recent structure analysis of the 2,6-xylyl isocyanide adduct of Zr(C7H7)(C5H5) had been hampered by an unresolvable disorder,6 a repeat of these studies resulted in reasonable success. The deviations of the aromatic ligand planes from parallel orientations increase on Lewis base coordination, to 33.0 for Zr(C7H7)(C5H4SiMe3)(THF) and 31 for Zr(C7H7)(C5H5){CN[2,6-C6H3(CH3)2]}. The closely related angles between the M-centroid vectors are 148.9 and 149.3, respectively, which seem somewhat large relative to complexes such as V(C5H5)2Cl (139.5),25 which may be attributed to the previously discussed closeness of the C7H7 ligand to the metal. This close approach is also likely responsible for greater angles between the M-C7H7 centroids and the THF or isocyanide ligands (113.9 and 112.7), as compared to the corresponding angles involving the Cp ligand (97.1 and 98.0). As compared to the unligated species, there is a slight tendency for the ring substituents to be positioned further from the metal; as for the more ordered THF complex, the C7H7 substituent tilts average 5.2 toward the metal, whereas those for the C5H4SiMe3 ligand average 0.5 toward the metal for hydrogen but 5.9 away from the metal for SiMe3. The incorporation of the additional ligands also leads to increased Zr-C bond lengths for both aromatic ligands. Notably, an increase of ca. 0.05 A˚ is observed for the C7H7 ligands, but only ca. 0.02-0.03 A˚ for the C5H4R ligands. Although one might have expected the added Lewis base to compete more effectively with the relatively weakly bound C5H4R ligands than with C7H7, it may be that the incorporation of the Lewis base required the sterically demanding (25) Fieselmann, B. F.; Stucky, G. D. J. Organomet. Chem. 1977, 137, 43.

With this contribution, we have extended our previous studies involving the incorporation of pentadienyl ligands into the Zr(C7H7) coordination sphere, through the successful syntheses of a variety of substituted trozircenes, in all cases using Zr(C7H7)(Cl)(tmeda) as the starting material.11 In comparison to other methods, which usually incorporate the seven-membered ring under reductive conditions in the final step,4a,5 this approach allows more convenient access to the class of Zr(C7H7)(Cp) sandwich complexes, as the same precursor is used in all cases. Just as the readily available Zr(C5H5)Cl3 is a valuable material for the synthesis of many types of zirconium-cyclopentadienyl complexes,26 Zr(C7H7)(Cl)(tmeda) could develop into a counterpart for cycloheptatrienyl chemistry. The incorporation of even more sophisticated monoanionic ligands is an ongoing subject of study in our laboratories. Furthermore, it has been shown in the past that the ancillary allyl substituent on a cyclopentadienyl ring is susceptible to hydroboration and also can allow for the formation of homo- and heteronuclear bimetallic complexes by olefin metathesis.27,28 The latter reaction in particular should be interesting for Zr(C7H7)(C5H4CHdCH2), as it might lead to the formation of a bitrozircene. Supporting Information Available: Cif files for each of the crystal structures included. This material is available free of charge via the Internet at http://pubs.acs.org.

Acknowledgment. R.D.E. is grateful to the University of Utah and the U.S. Department of Energy, Office of Fossil Fuel Energy, under contract number DE-FC2605NT42456 for partial support of this research. A.G. gratefully acknowledges the German Academic Exchange Service, the Bayer Science & Education Foundation, and the Fonds der Chemischen Industrie for financial support. (26) Selected examples: (a) Poli, R. Chem. Rev. 1991, 91, 509. (b) Kravchenko, R.; Masood, A.; Waymouth, R. M.; Myers, C. L. J. Am. Chem. Soc. 1998, 120, 2039. (c) H€uerl€ander, D.; Fr€ohlich, R.; Erker, G. J. Chem. Soc., Dalton Trans. 2002, 1513. (d) Buccella, D.; Shultz, A.; Melnick, J. G.; Konopka, F.; Parkin, G. Organometallics 2006, 25, 5496. (e) Zhang, J.; Lin, Y.-J.; Jin, G.-X. Organometallics 2007, 26, 4042. (f) Panhans, J.; Heinemann, F. W.; Zenneck, U. J. Organomet. Chem. 2009, 694, 1223. (27) Spence, R. E. v. H.; Piers, W. E. Organometallics 1995, 14, 4617. (28) Kuwabara, J.; Takeuchi, D.; Osakada, K. Organometallics 2005, 24, 2705.