Cycloheptatrienyl-Pentadienyl Complexes of Zirconium (Half-Open

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Organometallics 2009, 28, 5866–5876 DOI: 10.1021/om900456s

Cycloheptatrienyl-Pentadienyl Complexes of Zirconium (Half-Open Trozircenes): Syntheses, Structures, Bonding, and Chemistry Andreas Gl€ ockner,†,‡ Thomas Bannenberg,‡ 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 May 28, 2009

The reactions of Zr(C7H7)(Cl)(tmeda) (tmeda = tetramethylethylenediamine) with pentadienyl anions lead to formally tetravalent Zr(C7H7)(Pdl) complexes, for Pdl = C5H7, 2,4-C7H11, 6,6-dmch, and c-C7H9 (C7H11 = dimethylpentadienyl, dmch = dimethylcyclohexadienyl, c-C7H9 = cycloheptadienyl). Structural characterizations of the first three have been carried out, revealing much shorter Zr-C distances for the C7H7 ligand and a pattern of Zr-C bond distances for the pentadienyl ligands that is consistent with a formally high (þ4) metal oxidation state, which is also supported by DFT calculations. As had been found for the analogous Cp complexes, these 16-electron species are susceptible to Lewis base coordination, and the 2,6-xylyl isocyanide adducts of the 2,4-C7H11 and 6,6-dmch complexes have been isolated and characterized by IR spectroscopy and single-crystal X-ray diffraction studies. The IR spectroscopic studies indicate that the pentadienyl ligands are serving as better net electron donors than Cp ligands, opposite what is typically found for related but lower valent species. At high temperatures the 16-electron Zr(C7H7)(C5H7) complex undergoes slow conversion to the corresponding Cp complex.

Introduction The organometallic chemistry of the group 4 transition metals titanium, zirconium, and hafnium is to a large extent dependent upon cyclic, aromatic ligands, such as cyclopentadienyl, benzene, cyclooctatetraene, and their many functionalized derivatives.1 Especially for the cyclopentadienyl systems, numerous synthetic and catalytic applications have been developed.2 In recent years it has become clear that some other delocalized π ligands can lead to interesting and potentially useful chemistry with these metals. One such example is the (η7-) cycloheptatrienyl (C7H7, tropylium) ligand,3 which has long been something of an enigma, having been proposed to coordinate formally as either aromatic monocationic4 or trianionic3,5 species, as well as antiaromatic (for at least η7 coordination) monoanions.6 Though *Corresponding authors. E-mail: [email protected] and [email protected]. (1) Comprehensive Organometallic Chemistry III; Crabtree, R. H.; Mingos, D. M. P., Eds.; Elsevier: Oxford, U.K., 2007; Vol. 4. (2) See, for example: (a) McKnight, A. L.; Waymouth, R. M. Chem. Rev. 1998, 98, 2587. (b) Alt, H. G.; K€oppl, A. Chem. Rev. 2000, 100, 1205. (c) Qian, Y.; Huang, J.; Bala, M. D.; Lian, B.; Zhang, H.; Zhang, H. Chem. Rev. 2003, 103, 2633. (3) Green, M. L. H.; Ng, D. K. P. Chem. Rev. 1995, 95, 439. (4) Dauben, H. J.; Honnen, L. R. J. Am. Chem. Soc. 1958, 80, 5570. (5) Tamm, M. Chem. Commun. 2008, 3089. (6) Cloke, F. G. N.; Green, M. L. H.; Lennon, P. J. J. Organomet. Chem. 1980, 188, C25. (7) (a) Timms, P. L.; Turney, T. W. J. Chem. Soc., Dalton Trans. 1976, 2021. (b) van Oven, H. O.; de Liefde Meijer, H. J. J. Organomet. Chem. 1971, 31, 71. (c) Basta, R.; Arif, A. M; Ernst, R. D. Organometallics 2003, 22, 812. pubs.acs.org/Organometallics

Published on Web 09/23/2009

M(C7H7)(c-C7H9) (M = Ti, Zr, Hf; c-C7H9 = cycloheptadienyl) complexes have long been known,6,7 relatively few related species have been reported. This could easily be a result of the lack of convenient anionic sources for the introduction of these ligands; indeed, most C7H7 complexes of these metals have been obtained via the initial introduction of cycloheptatriene or cycloheptadienyl ligands, which can subsequently undergo partial dehydrogenation to yield C7H7.3 Similarly, while pentadienyl complexes of titanium and zirconium had been reported in the 1970s,7,8 in-depth studies of group 4 transition metal-pentadienyl chemistry were not to come for years.9 Notably, when detailed studies were eventually pursued, it became clear that pentadienyl ligands not only could bond more strongly than cyclopentadienyl but also could be more reactive, particularly in coupling reactions. However, this stronger bonding had only been apparent for lower valent complexes. In the very rare higher valent species, such as Zr(C5H5)(6,6-dmch)X2 (dmch = dimethylcyclohexadienyl; X = Cl, Br, I),10 one observes the opposite favorability, at least on the basis of relative M-C distances. Thus, while divalent group 4 half-open metallocenes tend to have shorter M-C (pentadienyl) distances, by 0.06-0.20 A˚, just the reverse is observed for the tetravalent species. Furthermore, in the tetravalent species (8) Datta, S.; Wreford, S. S. J. Am. Chem. Soc. 1979, 101, 1053. (9) Stahl, L.; Ernst, R. D. Adv. Organomet. Chem. 2008, 55, 137. (10) Rajapakshe, A. J.; Gruhn, N. E.; Lichtenberger, D. L.; Basta, R.; Arif, A. M.; Ernst, R. D. J. Am. Chem. Soc. 2004, 126, 14105. r 2009 American Chemical Society

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Organometallics, Vol. 28, No. 20, 2009

one observes the onset of slippage of the electronically open dienyl ligand out of the metal coordination sphere, due to the difficulty in achieving decent overlap between the contracted metal orbitals and the relatively widespread π orbitals of 6,6-dmch. This feature thus renders pentadienyl ligands good “barometers” of metal oxidation states and can be put to good use in assessing the formal charge of a C7H7 ligand. As the M(C7H7)(C5H5) (M = Ti, Zr, Hf) complexes are all known,11,12 in addition to a variety of ansa-analogues,13 it became of interest to attempt the isolation of related pentadienyl species. For this purpose, the reported Zr(C7H7)(Cl)(tmeda)14 (tmeda = tetramethylethylenediamine) appeared ideal, as we have found to be the case for the incorporations of various cyclopentadienyl ligands.15 Notably, this promising species seems not to have been utilized previously as a starting material. As reported herein, this complex has indeed proven to be a viable precursor to Zr(C7H7)(Pdl) (Pdl = C5H7, 2,4-C7H11, 6,6-dmch, c-C7H9; C7H11 = dimethylpentadienyl) complexes, which appear ideally suited for further applications, such as for metal incorporations on supports or in materials or coupling chemistry.

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 either by distillation from sodium benzophenone ketyl or by passage through alumina columns. Pentadienyl anions were prepared by published procedures.16 Analytical data were obtained from the microanalytical facilities at the Technische Universit€at Carolo-Wilhelmina, Braunschweig, using an Elementar varioMicro instrument. NMR spectra were recorded on Varian VXL-300 spectrometers (the assignment of the H and C atoms refers to the numbering in the X-ray structures), whereas a Bruker Tresor 37 spectrometer was used for recording the IR spectra. Zr(C7H7)(Cl)(tmeda) (1). A modified literature procedure was used for the preparation of this complex.14 In a 250 mL Schlenk flask, 3.00 g (12.9 mmol) of ZrCl4 in 20 mL of toluene was treated with 3.86 mL (25.7 mmol) of tmeda. In a second 250 mL Schlenk flask, 2 equiv (591 mg, 25.7 mmol) of sodium amalgam (0.900%) was prepared. At -45 °C, the ZrCl4/tmeda suspension was then added via cannula transfer followed by 6.01 mL (57.9 mmol) of cycloheptatriene. The mixture was allowed to warm to room temperature, and stirring was continued for three days, resulting in a green suspension. Toluene (11) (a) van Oven, H. O.; Groenenboom, C. J.; de Liefde Meijer, H. J. J. Organomet. Chem. 1974, 81, 379. (b) van Oven, H. O.; de Liefde Meijer, H. J. J. Organomet. Chem. 1970, 23, 159. (c) Groenenboom, C. J.; de Liefde Meijer, H. J.; Jellinek, F.; Oskam, A. J. Organomet. Chem. 1975, 97, 73. (d) B€ uschel, S.; Bannenberg, T.; Hrib, C. G.; Gl€ockner, A.; Jones, P. G.; Tamm, M. J. Organomet. Chem. 2009, 694, 1244. (12) Tamm, M.; Kunst, A.; Bannenberg, T.; Herdtweck, E.; Schmid, R. Organometallics 2005, 24, 3163. (13) (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. (14) Diamond, G. M.; Green, M. L. H.; Mountford, P.; Walker, N. M. J. Chem. Soc., Dalton Trans. 1992, 2259. (15) Gl€ ockner, A.; Tamm, M.; Arif, A. M.; Ernst, R. D. Unpublished results. (16) (a) Yasuda, H.; Nishi, T.; Lee, K.; Nakamura, A. Organometallics 1983, 2, 21. (b) Wilson, D. R.; Stahl, L.; Ernst, R. D. Organomet. Synth. 1986, 3, 136. (c) DiMauro, P. T.; Wolczanski, P. T. Organometallics 1987, 6, 1947.

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was removed in vacuo, and the product was extracted with 40 mL, 2 30, and 20 mL of THF. The extracts were filtered through a pad of Celite, which resulted in a clear green solution. Evaporation to a volume of ∼8 mL led to a blue precipitate. The supernatant with impurities was removed and the solids were washed with 2 5 mL of toluene. After drying, a blue solid was obtained in yields ranging from 31 to 37% (1.34-1.58 g), the actual value probably depending in part on how thoroughly the extraction had been done. Single crystals were grown by dissolving the complex in hot toluene in a Schlenk tube and allowing the solution to cool to room temperature. 1H NMR (d6-benzene, ambient): δ 1.12-1.19 (m, 2 H, N-CH2), 1.63 (s, 6 H, CH3), 1.74-1.82 (m, 2 H, N-CH2), 2.22 (s, 6 H, CH3), 5.35 (s, 7 H, C7H7). Full characterization has been published.14 Zr(C7H7)(C5H7) (2). In a 250 mL Schlenk flask equipped with a dropping funnel 1.00 g (2.99 mmol) of Zr(C7H7)(Cl)(tmeda) was dissolved in 30 mL of THF. Then 0.324 g (3.05 mmol) of K(C5H7) in 10 mL of THF was added dropwise at -78 °C to the blue solution. The mixture was allowed to warm to room temperature over a period of 90 min. After a total time of 3.5 h, THF was removed in vacuo from the red solution, and the product was extracted with 2 30 mL, 2 20, and 10 mL of pentane followed by filtration through a pad of Celite. The resulting red solution was evaporated to dryness. After sublimation at 130 °C with a -78 °C coldfinger, 0.257 g (34%) of an orange-red solid was obtained. Fifteen milligrams of the product was sealed in a glass tube under vacuum, whose bottom was placed in a 110 °C oil bath. Crystals suitable for X-ray structure determination grew on the walls of the tube by slow sublimation overnight. 1H NMR (d6-benzene, ambient): δ 1.73 (dm, J = 15.8 Hz, 2 H, H8,12,endo), 3.72 (dm, J = 11.7 Hz, 2 H, H8,12,exo), 3.92 (tm, J = 8.9 Hz, 1 H, H10), 5.11 (s, 7 H, C7H7), 5.60 (m, 2 H, H9,11). 13C NMR (d6-benzene, ambient): δ 74.7 (C10), 81.5 (C7H7), 94.5 (C8,12), 117.1 (C9,11). Anal. Calcd for C12H14Zr: C, 57.78; H, 5.66. Found: C, 58.49; H, 5.90. Zr(C7H7)(2,4-C7H11) (3). In a 250 mL Schlenk flask equipped with a dropping funnel 1.00 g (2.99 mmol) of Zr(C7H7)(Cl)(tmeda) was dissolved in 30 mL of THF. Then 0.410 g (3.05 mmol) of K(2,4-C7H11) in 10 mL of THF was added dropwise at -78 °C to the blue solution. The mixture was allowed to warm to room temperature over a period of 90 min. After a total time of 3.5 h, THF was removed in vacuo from the red solution, and the product was extracted with 2 30 mL and 2 15 mL of pentane followed by filtration through a pad of Celite. The resulting red solution was evaporated to dryness. After sublimation at 130 °C with a -78 °C coldfinger, 0.436 g (53%) of a red solid was obtained. Single crystals were grown from a pentane solution at -20 °C. 1H NMR (d6-benzene, ambient): δ 1.82 (s, 8 H, CH3 and H7,endo), 3.74 (s, 2 H, H7,exo), 4.17 (m, 1 H, H5), 5.14 (s, 7 H, C7H7). 13C NMR (d6-benzene, ambient): δ 28.0 (CH3), 75.9 (C5), 82.6 (C7H7), 96.7 (C7), 132.9 (C6). Anal. Calcd for C14H18Zr: C, 60.59; H, 6.54. Found: C, 60.34; H, 6.55. Zr(C7H7)(6,6-dmch) (4). In a 250 mL Schlenk flask equipped with a dropping funnel 0.500 g (1.50 mmol) of Zr(C7H7)(Cl)(tmeda) was dissolved in 20 mL of THF. Then 0.223 g (1.53 mmol) of K(6,6-dmch) in 10 mL of THF was added dropwise at -78 °C to the blue solution. The mixture was allowed to warm to RT over a period of 80 min. After a total time of 3.5 h, THF was removed in vacuo from the red solution, and the product was extracted with 30 mL, 20 mL, and finally 10 mL of pentane followed by filtration through a pad of Celite. The resulting red solution was evaporated to dryness. After sublimation at 130 °C with a -78 °C coldfinger, 0.262 g (60%) of a dark red solid was obtained. Single crystals were obtained by cooling a toluene/ pentane solution to -20 °C. 1H NMR (d6-benzene, ambient): δ 0.31 (s, 3 H, exo-CH3), 1.03 (s, 3 H, endo-CH3), 3.51 (dd, 3J = 8.0 Hz, 4J = 1.9 Hz, 2 H, H8,12), 4.36 (tt, 3J = 6.5 Hz, 4J = 1.9 Hz, 1 H, H10), 5.23 (s, 7 H, C7H7), 5.26 (dd, 3J = 7.9 Hz, 3J = 6.6 Hz, 2 H, H9,11). 13C NMR (d6-benzene, ambient): δ 31.6 (exoCH3), 32.3 (endo-CH3), 32.4 (C13), 81.5 (C7H7), 82.5 (C10), 93.2

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(C8,12), 112.5 (C9,11). Anal. Calcd for C15H18Zr: C, 62.23; H, 6.27. Found: C, 62.21; H, 6.36. Zr(C7H7)(c-C7H9) (5). A 250 mL Schlenk flask was charged with 0.30 g (0.80 mmol) of Zr(C7H7)(Cl)(tmeda) and 15 mL of THF. Then 125 mg (0.943 mmol) of K(C7H9) in 10 mL of THF was added at -78 °C. During the slow warm-up, the color changed to red. After being stirred overnight, the mixture was evaporated to dryness, and the product was extracted with 2 20 mL and 4 10 mL of pentane. The extracts were filtered through a pad of Celite. After pentane was removed in vacuo, sublimation at 130 °C with a -78 °C coldfinger was performed, yielding 0.072 g (29%) of a red solid. The 1H NMR spectrum was analogous to that previously reported from a metal atom reaction with cycloheptatriene.6 Single crystals were grown by slow sublimation in a sealed glass tube. 1H NMR (d6-benzene, ambient): δ 1.60 (m, 2 H), 2.05 (m, 2 H), 4.03 (tt, 3J = 8 Hz, 4J = 1.8 Hz, 1 H, H10), 4.97 (m, 2 H, H8,12), 5.11 (s, 7 H, C7H7), 5.53 (dd, 3J = 11 Hz, 3J = 8 Hz, 2 H, H9,11). 13C NMR (d6-benzene): δ 36.9 (CH2), 81.9 (C7H7), 96.3 (C10), 102.2 (C8,12), 117.4 (C9,11). Zr(C7H7)(2,4-C7H11){CN[2,6-C6H3(CH3)2]} (6). Zr(C7H7)(2,4-C7H11) (0.10 g, 0.36 mmol) was dissolved in 10 mL of THF, and 0.048 g (0.37 mmol) of 2,6-dimethylphenyl isocyanide in 5 mL of THF was added via syringe. The color changed to a slightly darker red. After the mixture was stirred for 3 h, the solvent was removed in vacuo. Then 10 mL of pentane was added, and the mixture was allowed to settle overnight at -20 °C. The supernatant was syringed off, and the remaining red solid was dried at 0 °C. Yield: 0.119 mg (82%). Single crystals were obtained by cooling a THF/pentane mixture to -30 °C. 1H NMR (d6-benzene, ambient): δ 1.73 (s, 2 H, H8,12,endo), 1.95 (s, 6 H, CH3), 2.01 (s, 6 H, CH3), 4.10 (s, 2 H, H8,12,exo), 4.85 (s, 7 H, C7H7), 5.11 (s, 1 H, H10), 6.50 (d, orthoJ = 7.6 Hz, 2 H, H18,20), 6.66 (t, orthoJ = 7.6 Hz, 1 H, H19). 13C NMR (d6-benzene, ambient): δ 16.4 (CH3), 26.5 (CH3), 72.6 (C10), 80.2 (C7H7), 91.5 (C8,12), 119.8 (C9,11), 133.3 (C16, i-C6H3); the resonances of C17-C21 were hidden under the solvent peak and C15 was not observed. Anal. Calcd for C23H27NZr: C, 67.59; H, 6.66. Found: C, 66.16; H, 6.55. The low carbon values, found for several crystalline samples, are consistent with experiences with the C5H5 analogue12 and theoretical calculations indicating weak RNC bonding. IR (KBr): ν 2100 cm-1 (CN). Zr(C7H7)(6,6-dmch){CN[2,6-C6H3(CH3)2]} (7). Zr(C7H7)(6,6-dmch) (0.10 g, 0.35 mmol) was dissolved in 10 mL of THF, and 0.046 g (0.352 mmol) of 2,6-dimethylphenyl isocyanide in 5 mL of THF was added via syringe. The color changed to a slightly darker red. After the mixture was stirred for 6 h, the solvent was removed in vacuo. Then 10 mL of pentane was added, and the mixture was allowed to settle overnight at -20 °C. The supernatant was syringed off, and the remaining red solid was dried at 0 °C. Yield: 0.118 mg (81%). Single crystals were obtained by the slow diffusion of pentane vapors into a THF solution at room temperature. 1H NMR (d6benzene, ambient): δ 0.61 (s, 3 H, exo-CH3), 1.03 (s, 3 H, endo-CH3), 1.89 (s, 6 H, CH3), 4.16 (dd, 3J = 7.3 Hz, 4J = 1.6 Hz, 2 H, H8,12), 4.97 (s, 7 H, C7H7), 5.02 (tt, 3J = 5.8 Hz, 4 J = 1.8 Hz, 1 H, H10), 5.16 (dd, 3J = 7.2 Hz, 3J = 6.0 Hz, 2 H, H9,11), 6.48 (d, orthoJ = 7.6 Hz, 2 H, H19,21), 6.65 (t, orthoJ = 7.6 Hz, 1 H, H20). 13C NMR (d6-benzene, ambient): δ 18.2 (CH3), 28.3 (exo-CH3), 33.4 (endo-CH3), 33.7 (C13), 82.2 (C7H7), 84.0 (C10), 87.2 (C8,12), 106.3 (C9,11), 135.0 (C17, i-C6H3); the resonances of C18-C22 were hidden under the solvent peak and C16 was not observed. Anal. Calcd for C24H27NZr: C, 68.52; H, 6.47. Found: C, 66.12; H, 6.42. The low carbon values, found for several crystalline samples, are consistent with experiences with the C5H5 analogue12 and theoretical calculations indicating weak RNC bonding. IR (KBr): ν 2121 cm-1 (CN). 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

Gl€ ockner et al. SHELXL97 programs.17 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 Zr(C7H7)(2,4-C7H11){CN[2,6C6H3(CH3)2]} structure exhibited a twinning disorder, with the major/minor fragment ratio being 91:9. Pertinent crystallographic information is presented in Table 1, while bonding parameters are listed in Tables 2-4. Theoretical Calculations. The calculations were performed using the GAUSSIAN03 package.18 All structures were fully optimized on the density functional theory (DFT) level employing the B3LYP and the M05-2X hybrid functional.19,20 For all main-group elements (C, H, and N) the all-electron triple-ζ basis set (6-311G**) was used, whereas for zirconium a small-core relativistic ECP together with the corresponding triple-ζ valence basis set was employed (Stuttgart RSC 1997 ECP).21 The standard enthalpies for the formations of the isocyanide complexes were calculated by subtracting the enthalpies of the ground-state electronic structures of the sandwich complexes and the isocyanide ligand from those of the resulting complexes.

Results and Discussion Synthesis and Characterization of CycloheptatrienylPentadienyl Zirconium Complexes. As had been desired, Zr(C7H7)(Cl)(tmeda) does indeed react with a variety of pentadienyl anions (Pdl = C5H7, 2,4-C7H11, 6,6-dmch, c-C7H9) to provide reasonable to good yields of the expected Zr(C5H7)(Pdl) complexes (eq 1). The products are orange-red to red in color, fairly air-sensitive, and soluble in THF and hydrocarbon solvents, and possess generally high thermal

stabilities, being isolable after vacuum sublimation at ca. 130 °C. However, to date attempts to make the cyclooctadienyl analogue have been unsuccessful. Conceivably, this (17) (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. (18) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, Revision C.02; Gaussian, Inc.: Wallingford, CT, 2004. (19) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (b) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785. (20) Zhao, Y.; Schultz, N. E.; Truhlar, D. G. J. Chem. Theory Comput. 2006, 2, 364. (21) Dolg, M.; Stoll, H.; Preuss, H.; Pitzer, R. M. J. Phys. Chem. 1993, 97, 5852.

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Table 1. Crystallographic Parameters for Zr(C7H7)(Cl)(tmeda), Zr(C7H7)(Pdl) Complexes (Pdl = C5H7, 2,4-C7H11, 6,6-dmch, respectively), Zr(C7H7)(2,4-C7H11){CN[2,6-C6H3(CH3)2]}, and Zr(C7H7)(6,6-dmch){CN[2,6-C6H3(CH3)2]} formula

C13H23ClN2Zr

fw temperature (K) λ (A˚) cryst syst space group unit cell dimens a (A˚) b (A˚) c (A˚) β (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

C12H14Zr

C23H27NZr

C24H27NZr

249.45 150(1) 0.71073 monoclinic P21/c

277.50 150(1) 0.71073 orthorhombic Pnma

289.51 150(1) 0.71073 monoclinic P21/a

408.68 150(1) 0.71073 monoclinic C2/c

420.69 150(1) 0.71073 orthorhombic P212121

10.6477(2) 11.1863(2) 12.3060(2) 93.9268(11) 1462.31(4); 4 1.517 9.17 2.4 - 27.5 -13 e h e 13 -14 e k e 13 -15 e l e 15 6294 3307; 2 0.0302 0.0863 0.69/-0.47 98.9%

10.6566(13) 8.0036(11) 12.4387(12) 109.278(7) 1001.4(2); 4 1.655 10.45 3.0 - 27.6 -13 e h e 13 -10 e k e 10 -16 e l e 16 4239 2279; 2 0.0392 0.0889 1.19/-0.94 98.9%

10.5476(4) 13.5076(6) 8.6593(3) 90 1233.71(8); 4 1.494 8.57 2.7 - 27.5 -13 e h e 13 -17 e k e 17 -11 e l e 11 2609 1474; 2 0.0296 0.0617 0.86/-0.52 99.9%

11.1345(4) 9.4499(2) 12.9780(5) 112.1154(14) 1265.08(7); 4 1.520 8.39 2.7 - 27.5 -14 e h e 14 -10 e k e 12 -16 e l e 16 5029 2896; 2 0.0322 0.0611 0.47/-0.60 99.9%

27.7808(5) 10.3825(2) 13.8564(3) 103.9403(11) 3878.95(13); 8 1.400 5.71 2.4 - 27.5 -35 e h e 36 -13 e k e 13 -17 e l e 17 8509 4430; 2 0.0344 0.0794 0.58/-0.45 99.7%

6.7456(2) 10.9052(3) 26.8823(8) 90 1977.52(10); 4 1.413 5.63 2.0 - 27.5 -8 e h e 8 -14 e k e 14 -34 e l e 34 4398 4398 0.0426 0.1209 0.77/-0.83 97.6%

Bond Distances (A˚) 2.374(3) 2.359(3) 2.336(3) 2.358(3) 2.353(3)

C15H18Zr

334.00 150(1) 0.71073 monoclinic P21/c

Table 2. Pertinent Bonding Parameters for Zr(C7H7)(Cl)(tmeda)a

Zr-C1 Zr-C2 Zr-C3 Zr-C4 Zr-C5

C14H18Zr

Zr-C6 Zr-C7 Zr-Cl Zr-N1 Zr-N2

formation of weak Zr-metal bonds.12,22 In the present case, two representative 16-electron complexes react with 2,6-xylyl isocyanide {CN[2,6-C6H3(CH3)2]} to yield the 18-electron mono(adduct) complexes (eq 2). It is notable that coupling

2.361(3) 2.348(3) 2.5724(6) 2.477(2) 2.519(2)

Bond Angles (deg) Cl-Zr-N1 82.61(5) CNT-Zr-Cl 128.4 Cl-Zr-N2 85.92(5) CNT-Zr-N1 133.0 N1-Zr-N2 72.56(7) CNT-Zr-N2 133.9 a CNT represents the center of mass of the C7H7 ligand, defined by its carbon atoms.

could be due to the ease of the abstraction of hydrogen atoms from the C3H6 bridge.7c Red THF solutions were in fact isolated initially, but neither sublimation nor hydrocarbon extraction led to the isolation of the desired product.

These 16-electron complexes are diamagnetic, and their H NMR spectra are entirely consistent with expectations, displaying in each case a singlet for the C7H7 ligand and the additional resonances characteristic of a pentadienyl ligand having mirror plane symmetry (see Experimental Section). The Lewis-acidic metal center in related cycloheptatrienyl zirconium complexes allows the coordination of two electron donor ligands and, in appropriately substituted species, also the development of secondary interactions, e.g., the 1

(22) (a) Baker, R. J.; Bannenberg, T.; Kunst, A.; Randoll, S.; Tamm, M. Inorg. Chim. Acta 2006, 395, 4797. (b) B€uschel, S.; Jungton, A.-K.; Bannenberg, T.; Randoll, S.; Hrib, C. G.; Jones, P. G.; Tamm, M. Chem.; Eur. J. 2009, 15, 2176.

with the pentadienyl ligands was not realized, which appears nearly inevitable for low-valent analogues,23 as readily evidenced by the presence of well-defined CtN stretching vibrations (vide infra), although the 2,6-disubstitution on the phenyl ring may be expected to greatly hinder such reactions even for the lower valent species. These couplings for the lower valent species in each case led to the achievement of a þ4 oxidation state for the metal center, which would thus produce an extra driving force for their couplings. While the 1H and 13C NMR spectra of these adducts are not unusual, the IR spectroscopic data are of interest. In particular, the isocyanide CtN stretching vibrations provide good indications of the donor/acceptor tendencies of the metal fragment. Indeed, the IR spectra of Zr(C7H7)(C5H5){2,6-CN[C6H3(CH3)2]} and of the free isocyanide display respective C-N stretches of 2134 and 2122 cm-1, indicating (23) (a) Waldman, T. E.; Wilson, A. M.; Rheingold, A. L.; Melendez, E. M.; Ernst, R. D. Organometallics 1992, 11, 3201. (b) See also: Ernst, R. D.; Basta, R.; Arif, A. M. Z. Kristallogr. New Cryst. Struct. 2004, 219, 403.

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Table 3. Pertinent Bonding Parameters for the Zr(C7H7)(Pdl) Complexes Pdla

C5H7

2,4-C7H11

6,6-dmch

Table 4. Pertinent Bonding Parameters for the Zr(C7H7)(Pdl){CN[2,6-C6H3(CH3)2]} Complexes Pdl

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

2.333(3) 2.347(3) 2.364(3) 2.343(3) 2.378(3) 2.354(3) 2.335(3) 2.604(3) 2.508(3) 2.437(3) 2.480(3) 2.571(3) 1.366(4) 1.409(4) 1.414(5) 1.365(5)

2.337(3) 2.337(3) 2.345(2) 2.341(2)

2.606(2) 2.512(2) 2.431(3) 1.376(3) 1.427(2)

2,4-C7H11a

6,6-dmch

Bond Distances (A˚) 2.336(3) 2.357(3) 2.341(3) 2.370(3) 2.352(3) 2.338(3) 2.327(3) 2.577(2) 2.486(2) 2.459(2) 2.493(2) 2.595(2) 1.377(4) 1.420(3) 1.418(3) 1.379(3)

Bond Angles (deg) C8-C9-C10 128.6(3) 125.9(2) 120.2(2) C9-C10-C11 127.8(3) 130.3(3) 117.9(2) C10-C11-C12 129.0(3) 120.4(2) a For the 2,4-C7H11 complex, with crystallographic m symmetry, the pentadienyl parameters have been placed adjacent to the corresponding values for the other pentadienyl complexes. The carbon atom labels for these values are those given in parentheses.

an absence of any significant back-bonding, which leads to an increase in the C-N frequency on coordination as a result of the transfer of some isocyanide lone pair electron density to the metal center. This provided clear evidence in support of the presence of formally d0 Zr(IV), and thus a C7H7 trianion.12 The C-N stretches for the related 2,4-C7H11 and 6,6-dmch complexes, appearing at 2100 and 2121 cm-1, respectively, reveal that the pentadienyl ligands appear to be serving as better net electron donors than C5H5, exactly the opposite of what is generally observed for lower (þ2) valent early transition metal complexes.24 In each case, however, the 6,6-dmch ligand is observed to be intermediate between C5H5 and nonbridged pentadienyl ligands such as C5H7 or 2,4-C7H11. The key to explaining these observations may be the fact that the nonaromatic pentadienyl ligands are capable of serving as both better donors and acceptors than C5H5.24,25 While in the lower valent Zr(II) species, the pentadienyls function as particularly strong δ-acids, the Zr(IV) complexes should have minimal back-bonding, due to both the formal d0 configuration and the high metal oxidation state. In such species, the greater donor abilities of the pentadienyl ligands become crucial and lead to a reduction in the metal center’s acidity, and thereby also to a reduction in the σ donation from the isocyanide ligand. Additionally, there could still be a role played by a relatively weak back-bonding interaction, particularly considering that the CtN stretching frequencies of the pentadienyl complexes are slightly lower than that of the free isocyanide. (24) (a) Ernst, R. D. Acc. Chem. Res. 1985, 18, 56. (b) Ernst, R. D. Chem. Rev. 1988, 88, 1255. (25) (a) Wilson, D. R.; Liu, J.-Z.; Ernst, R. D. J. Am. Chem. Soc. 1982, 104, 1120. (b) Ernst, R. D. Struct. Bond. (Berlin) 1984, 57, 1. (26) (a) Pillet, S.; Wu, G.; Kulsomphob, V.; Harvey, B. G.; Ernst, R. D.; Coppens, P. J. Am. Chem. Soc. 2003, 125, 1937. (b) Scheins, S.; Messerschmidt, M.; Gembicky, M.; Pitak, M.; Volkov, A.; Coppens, P.; Harvey, B. G.; Turpin, G. C.; Arif, A. M.; Ernst, R. D. J. Am. Chem. Soc. 2009, 131, 6154.

Zr-C1 Zr-C2 Zr-C3 Zr-C4 Zr-C5 Zr-C6 Zr-C7 Zr-C8 Zr-C9 Zr-C10 Zr-C11 Zr-C12 Zr-C(15/16) C8-C9 C9-C10 C10-C11 C11-C12 N-C(15/16)

2.371(3) 2.409(5) 2.423(3) 2.410(4) 2.375(3) 2.428(4) 2.423(3) 2.602(3) 2.571(2) 2.509(2) 2.571(2) 2.597(3) 2.364(2) 1.379(4) 1.432(3) 1.421(3) 1.384(4) 1.157(3)

2.373(4) 2.400(4) 2.439(4) 2.456(4) 2.370(4) 2.383(4) 2.427(4) 2.646(4) 2.533(4) 2.509(4) 2.523(4) 2.639(4) 2.347(3) 1.382(6) 1.416(7) 1.416(7) 1.387(7) 1.173(4)

Bond Angles (deg) C8-C9-C10 124.6(3) 119.5(4) C9-C10-C11 131.1(2) 118.8(4) C10-C11-C12 125.1(3) 119.8(4) Zr-C(15/16)-N 176.9(2) 178.3(4) a For the 2,4-C7H11 complex the bonding parameters are given only for the major image.

Although this seems at odds with the d0 metal configuration, related formally Ti(IV) and Zr(IV) complexes have been shown to have nonzero d orbital electron populations,26 which could thus lead to some back-bonding. In such a case, the electron density transferred to the metal by the pentadienyl ligand donor interactions could then lead to an increase in back-bonding to the isocyanide ligand and to a decrease in the CtN stretching frequency. One final example of reaction chemistry was discovered on the sublimation of Zr(C7H7)(C5H7), undertaken in an attempt to obtain crystals of this complex. It was observed that some of the crystals had a coloration more like that of the C5H5 complex, and indeed the unit cell of these crystals was shown to be identical to that of the known Zr(C7H7)(C5H5).12 To eliminate the possibility that this complex may have arisen from the presence of cyclopentadiene contaminant in the 1,3-pentadiene, a sample of the compound in a sealed NMR tube was heated to 125 °C. The 1H and 13C NMR spectra both revealed that the heating led to the formation of Zr(C7H7)(C5H5), although even after two days only about 10% of the complex had undergone conversion. Other examples of pentadienyl-to-cyclopentadienyl conversions have been observed, particularly under high-energy conditions such as mass spectroscopic examination or thermolysis.26,27 Structural Studies. The structure of Zr(C7H7)(Cl)(tmeda) exhibits the expected piano stool geometry (Table 2, Figure 1). The Zr-C bonds are fairly uniform, averaging 2.356(4) A˚, corresponding to a 1.700 A˚ deviation of the metal center from the ligand plane. The C7H7 substituents tilt down out of this plane by an average of 6.6(6)°. In comparison, the (27) (a) Mann, B. E.; Manning, P. W.; Spencer, C. M. J. Organomet. Chem. 1986, 312, C64. (b) Kralik, M. S.; Rheingold, A. L.; Ernst, R. D. Organometallics 1987, 6, 2612. (c) Kirss, R. U.; Quazi, A.; Lake, C. H.; Churchill, M. R. Organometallics 1993, 12, 4145.

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Figure 2. Perspective view of Zr(C7H7)(C5H7) (2). Figure 1. Perspective view of Zr(C7H7)(Cl)(tmeda) (1).

Zr-Cl bond length is 2.5724(6) A˚, while the Zr-N(1,2) bond lengths differ slightly, at 2.477(2) and 2.519(2) A˚. The angles between the Zr-(Cl,N1,N2) vectors and the Zr-C7H7 centroid are 128.4°, 133.0°, and 133.9°, respectively. The larger angles for the nitrogen atoms can be attributed to steric interactions between the C7H7 ligand and the organic portion of the tmeda ligand. The structures of the Zr(C7H7)(Pdl) complexes (Pdl = C5H7, 2,4-C7H11, 6,6-dmch) appear in Figures 2-4, while selected bonding parameters may be found in Table 3. Preliminary unit cell data were also obtained for the cycloheptadienyl complex that had been prepared previously via a metal atom reaction involving cycloheptatriene. However, the complex was found to be isomorphous with its titanium and vanadium analogues, each of which was highly disordered.7c As a result, diffraction data were not collected for Zr(C7H7)(c-C7H9). A discussion of the relative ligand orientations in 2-4 is complicated by the acyclic nature of pentadienyl ligands, the resulting nonequivalence of the three types of carbon atom positions (C[1,5], C[2,4], C[3]), and the resulting nonequivalence of the corresponding Zr-C distances. In subsequent discussions, the use of these brackets rather than parentheses will be used when referring to these three types of pentadienyl carbon atoms. While the two ligand least-squares planes, defined only by the metal-bound carbon atoms, deviate from being parallel by 26.93(16)°, 15.74(18)°, and 14.36(12)°, if one instead makes these judgments based on ligand centroids, the values become 15.45°, 5.50°, and 10.98°, respectively (corresponding to centroid-Zr-centroid0 angles of 164.55°, 174.50°, and 169.02°). Although other measures of this deviation can be suggested, by any of these measures there is a greater deviation than observed for the analogous Zr(C7H7)(C5H5) (28) Rogers, R. D.; Teuben, J. H. J. Organomet. Chem. 1988, 354, 169.

Figure 3. Perspective view of Zr(C7H7)(2,4-C7H11) (3). The molecule possesses crystallographic m symmetry.

or Zr(C7H7)(C5Me5) complexes.12,28 The greater deviation can be attributed to the asymmetry in Zr-C bond distances, for which one observes a significant increase on going from the Zr-C[3] bond to the Zr-C[2,4] and Zr-C[1,5] bond types. Thus, with the shortest Zr-C bonds involving the region by the central carbon atom, one might expect this region to be positioned opposite the C7H7 ligand, as is in fact evident (e.g., Figure 4). It is notable that the C5H7 ligand remained in the U conformation, given that the free anion,29 unlike that of 2,4-C7H11,30 strongly disfavors that orientation. This demonstrates that there must be a significant (29) Bates, R. B.; Gosselink, D. W.; Kaczynski, J. A. Tetrahedron Lett. 1967, 3, 205. (30) Schlosser, M.; Rauchschwalbe, G. J. Am. Chem. Soc. 1978, 100, 3258.

5872


Figure 4. Perspective view of Zr(C7H7)(6,6-dmch) (4).

interaction between the metal center and the dienyl termini, despite their longer Zr-C distances, as otherwise an η3 bonding mode with either an S or a W conformation should be adopted. The asymmetry in the Zr-C (Pdl) bond lengths, and their magnitudes relative to the M-C (Cp) lengths in the analogous Cp complexes, can provide further information of interest, particularly regarding the formal charge on the C7H7 ligand. As previous considerations have led to the formulation of this ligand as a trianion in Zr(C7H7)(C5H5) and related species,3,5,12 this would lead to a þ4 oxidation state for the metal center. Oxidation states that high have significant negative impacts on metal-pentadienyl versus metal-cyclopentadienyl bonding and indeed relatively few higher valent zirconium pentadienyl complexes are known.10,26,31,32 These observations have been traced to a loss in overlap between the π molecular orbitals of the large girth pentadienyl ligands and the contracted Mþ4 orbitals, and also to a potential loss of δ back-bonding interactions, which can be significant in lower valent species.33 Thus, while low-valent half-open titanocenes and zirconocenes display significant shortening of their M-C distances for the pentadienyl versus cyclopentadienyl ligands,9 exactly the opposite (by ca. 0.1 A˚) is observed for the Zr(C5H5)(6,6-dmch)X2 (X = Cl, Br, I) complexes.10 This is accompanied by a pattern of increasing Zr-C bond distances as one goes sequentially from the central pentadienyl carbon atom (C[3]) to the terminal positions. This pattern has also been ascribed to the inability of the contracted Zr4þ orbitals to overlap effectively (31) (a) Kulsomphob, V.; Arif, A. M.; Ernst, R. D. Organometallics 2002, 21, 3182. (b) Kulsomphob, V.; Harvey, B. G.; Arif, A. M.; Ernst, R. D. Inorg. Chim. Acta 2002, 334, 17. (c) Basta, R.; Ernst, R. D.; Arif, A. M. J. Organomet. Chem. 2003, 683, 64. (d) Arif, A. M.; Basta, R.; Ernst, R. D. Polyhedron 2006, 25, 876. (e) Ernst, R. D.; Kulsomphob, V.; Arif, A. M. Z. Kristallogr.-New Cryst. Struct. 2006, 221, 293. (f) Harvey, B. G.; Arif, A. M.; Ernst, R. D. J. Mol. Struct. 2008, 890, 107. (g) Basta, R.; Arif, A. M.; Ernst, R. D. Organometallics 2005, 24, 3974. (h) See also: Harvey, B. G.; Basta, R.; Arif, A. M.; Ernst, R. D. J. Organomet. Chem. 2008, 693, 1420. (32) Rajapakshe, A.; Basta, R.; Arif, A. M.; Ernst, R. D.; Lichtenberger, D. L. Organometallics 2007, 26, 2867. (33) (a) B€ ohm, M. C.; Eckert-Maksı´ c, M.; Ernst, R. D.; Wilson, D. R.; Gleiter, R. J. Am. Chem. Soc. 1982, 104, 2699. (b) Ernst, R. D.; Liu, J.-Z.; Wilson, D. R. J. Organomet. Chem. 1983, 250, 257. (c) Ernst, R. D. Comm. Inorg. Chem. 1999, 21, 285.

Gl€ ockner et al.

with the π orbitals of the widespread pentadienyl ligands. As already noted above, one observes this same pattern in the Zr(C7H7)(Pdl) structures. Furthermore, the average Zr-C distances for the pentadienyl ligands in these complexes (2.53, 2.52, and 2.52 A˚, respectively) are indeed longer than their counterparts in the analogous Cp complexes (ca. 2.50 A˚, vide supra). Additionally, the hydrogen atom substituents on the central pentadienyl carbon atoms tilt out of the ligand plane away from the metal center, opposite the norm, and one observes a distinctive short-long-long-short pattern for the pentadienyl ligands’ delocalized C-C bonds. For the 16-electron complexes, the average short and long distances are 1.373(3) and 1.420(2) A˚, whereas for the 18-electron complexes they are 1.383(1) and 1.422(2) A˚. This can be attributed to the partial contribution of a resonance form like that below, for which partial sp3 hybridization is present on that central atom. Hence, these observations support the formulation of the C7H7 ligands as trianions. On the other hand, the increase in Zr-C (Pdl vs Cp) distances is not nearly so great as observed in the Zr(C5H5)(6,6-dmch)X2 complexes,10 which may reflect a significant degree of covalency in the Zr-C7H7 bonding, due to the relatively low electronegativity of the carbon atoms. This appears consistent with the results of the MO calculations, which reveal significant Zr/ C7H7 orbital mixings (vide infra).

There is an issue remaining that concerns the above argument, having to do with the validity of comparing average M-C bond lengths for the pentadienyl and cyclopentadienyl ligands. Given that there is a large variation in M-C distances for the pentadienyl ligands in these higher valent species, there is every reason to question whether the use of the average M-C distance will serve as a valid indicator of relative bonding favorability. In fact, in the Zr(C5H5)(6,6-dmch)Cl2 complex, even with significantly longer Zr-C (6,6-dmch) distances, Mayer bond order analyses indicated slightly stronger interactions for 6,6-dmch versus C5H5.32 In the present case, a comparison of the Zr-C (C7H7) distances in the Zr(C7H7)(C5H4R) [R = H, CH3, Si(CH3)3]12,15 and Zr(C7H7)(Pdl) (Pdl = C5H7, 2,4-C7H11, 6,6-dmch) complexes provide some interesting information. For the C5H4R species, the average Zr-C (C7H7) value is 2.33 A˚, whereas for the pentadienyl complexes, the value is 2.346 A˚, providing at least some indication that the Zr-Pdl bonding could be stronger than the Zr-Cp bonding, despite the shorter Zr-C bonds for the C5H4R versus the pentadienyl ligands. Such a circumstance has also been found in other high-valent systems.10 One also observes slightly longer (ca. 0.033 A˚) Zr-C (C7H7) distances for the 18-electron mono(ligand) Pdl complexes versus their Cp analogue (vide infra). The addition of the Lewis base to the metal coordination spheres leads to the expected geometric adjustments. The discussion of this point is again complicated by the asymmetry in the bonding of the pentadienyl ligands, except for the angle between the M-C7H7 centroid and the added M-L bond. These values for Zr(C7H7)(2,4-C7H11){CN[2,6-C6H3(CH3)2]} and Zr(C7H7)(6,6-dmch){CN[2,6-C6H3(CH3)2]} are 113.72° and 111.16°, respectively, which are

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Figure 5. Perspective view of Zr(C7H7)(2,4-C7H11){CN[2,6-C6H3(CH3)2]} (6).

quite similar to the values found in the analogous C5H5 complex, Zr(C7H7)(C5H5){[2,6-C6H3(CH3)2]}.15 This may reflect the dominant steric effect that the C7H7 ligand has on the geometry, as otherwise the greater steric demands of pentadienyl versus cyclopentadienyl ligands would be expected to lead to some differences. The respective angles between the M-centroid vectors are 150.59° and 151.87°, which closely match that of the analogous Cp complex (150.33°).34 One other parameter that will be of interest in subsequent MO discussions is the tilt of a given isocyanide’s arene ring relative to the aromatic ligand orientations. A twist angle can be defined as the angle between the arene plane, defined by its six carbon atoms, and the plane formed by Zr and the two aromatic ligand centroids. For the 2,4-C7H11 and 6,6-dmch complexes, these angles are 3.4° and 17.4°, respectively, compared to a value of 14.8° in the analogous C5H5 complex. The incorporations of the additional Lewis bases led to approximate increases in the average Zr-C bond lengths of ca. 0.06 and 0.05 A˚ for the C7H7 and Pdl ligands, respectively, as compared to the ca. 0.05 and 0.01 A˚ values for the Cp analogue.15 One other comparison involves the degree of tilting by the C7H7 substituents. Similar to their Cp analogues, these substituent tilts of the 18-electron complexes, e.g., 5.8° for Zr(C7H7)(2,4-C7H11){CN[2,6-C7H3(CH3)2]}, are reduced as compared to the values in the 16-electron Zr(C7H7)(Pdl) species. With the availability of these new formal Zr(IV) pentadienyl complexes, an examination of their structural systematics becomes possible. In Table 5 are presented the average Zr-C bond lengths for the central (C[3]) and the terminal (C[1,5]) carbon atoms as a function of the number (34) These coordination angles can also be approximated using the interplanar angles for the 2,4-C7H11, 6,6-dmch, and C5H5 complexes. The respective angles of 42.03(12)°, 31.21(19)°, and 31.15° lead to the corresponding estimates (from their supplements) of 137.97(12)°, 148.79(19)°, and 148.55°.

of heteroatoms bound to Zr. As can be seen, for the one complex with three heteroatoms, the difference in these two Zr-C bond types is 0.253 A˚, whereas with two, one, or zero heteroatoms, respective ranges of 0.135-0.336, 0.137-0.179, and 0.031-0.175 A˚ are observed. While there is some overlap of these ranges, that is mostly the result of formally “enediyl” and “yl-amide” complexes, for which a contribution of a diene or π-imine resonance form would lead to some Zr(II) character. Most of these species are in fact found at the bottom of their groupings. The only other anomaly would appear to be the relatively small Δ value for Zr(6,6-dmch)Br3(dmpe). In this case, the strongly donating diphosphine likely leads to a significant reduction in metal charge, and thus to a smaller Δ value. Nonetheless, even with the inclusion of these three, the average Δ values for the 3-0 heteroatom ranges are 0.253, 0.228, 0.158, and 0.126 A˚, respectively. This correlation reveals that as the number of electronegative heteroatoms is increased, along with the presumed positive charge on the Zr center, there is indeed a greater difference between the Zr-C[3] and Zr-C[1,5] distances, in accord with the general correlation of those structural differences with the presence of contracted valence orbitals of a higher valent metal center. One finds similar distortions in Ti(IV)35 and Ta(V)36 complexes, but alternative variations in Re(V)37 and W(VI)38 complexes. In the Re(V) complex, possessing two aryl ligands, a dianionic nitrogen center, and a cyclohexadienyl ligand, the short M-C interactions involve two carbon atoms on one side of the ligand. In contrast, for the cationic W(VI) complex, possessing three aryloxides, one aryl ligand, and one (35) Feng, S.; Klosin, J.; Kruper, W. J., Jr.; McAdon, M. H.; Neithamer, D. R.; Nickias, P. N.; Patton, J. T.; Wilson, D. R.; Abboud, K. A.; Stern, C. L. Organometallics 1999, 18, 1159. (36) Gavenonis, J.; Tilley, T. D. Organometallics 2002, 21, 5549. (37) Gutierrez, A.; Wilkinson, G.; Hussain-Bates, B.; Hursthouse, M. B. Polyhedron 1990, 9, 2081. (38) Lentz, M. R.; Fanwick, P. E.; Rothwell, I. P. Organometallics 2003, 22, 2259.

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Table 5. Comparison of Average Zr-C[3] and Zr-C[1,5] Distances for High-Valent Pentadienyl Complexes complex description

Zr-C[3]

Zr-C[1,5]

Δ

reference

3 Heteroatoms Zr(6,6-dmch)Br3(dmpe)

2.482

2.735

0.253

31d

2.775 2.722 2.732 2.705 2.703 2.688 2.690 2.666 2.684 2.660

0.336 0.265 0.249 0.239 0.231 0.219 0.216 0.202 0.188 0.135

31c 10 31 g 31 g 10 31 g 31 g 10 32 31e

2.674 2.608

0.179 0.137

31 g 26a

2.606 2.672 2.588 2.614 2.642 2.586 2.600 2.645

0.175 0.167 0.151 0.135 0.133 0.127 0.091 0.031

a

2 Heteroatoms Zr(6,6-dmch)(allyl)(amide)2 Zr(C5H5)(6,6-dmch)Br2 Zr(6,6-dmch)2Cl(OMe) Zr(6,6-dmch)2Cl2 Zr(C5H5)(6,6-dmch)Cl2 Zr(6,6-dmch)2I2 Zr(6,6-dmch)2Br2 Zr(C5H5)(6,6-dmch)I2 Zr(C5H5)(3-Me3Si-6,6-dmch)I2 Zr(2,4-C7H11)(amide)(π-imine)(CNCMe3)

2.439 2.458 2.483 2.466 2.472 2.469 2.474 2.464 2.496 2.525 1 Heteroatom

Zr(6,6-dmch)2(CH3)Br Zr(2,4-C7H11)(enediyl)(amide)

2.495 2.471

Zr(C7H7)(2,4-C7H11) Zr(C5H5)(6,6-dmch)(vinyl)2 Zr(C7H7)(C5H7) Zr(6,6-dmch)(allyl)(enediyl) Zr(C7H7)(6,6-dmch)[CN(2,6-xylyl)] Zr(C7H7)(6,6-dmch) Zr(C7H7)(2,4-C7H11)[CN(2,6-xylyl)] Zr(C5H5)(“2,4-C7H11”)(enediyl)

2.431 2.505 2.437 2.479 2.509 2.459 2.509 2.614

0 Heteroatoms

a

31b a

31f a a a

31a

This work.

cyclohexadienyl ligand, two short M-C distances involve the terminal carbon atoms, with the M-C[2,4] and M-C[3] distances becoming progressively longer. Possibly the presence of three π donor aryloxides led to a sufficient enough reduction in metal charge to allow for decent overlap between the metal center and the two terminal positions, despite their spatial separation. Theoretical Studies. To obtain more detailed insight into the bonding of half-open trozircenes, a series of DFT calculations was performed for Zr(C7H7)(2,4-C7H11) and Zr(C7H7)(6,6-dmch) and for their 2,6-xylyl isocyanide adducts. Initially, these calculations were conducted by employing the B3LYP hybrid functional19 to allow comparison with the previously published results obtained for the bonding in Zr(C7H7)(C5H5).5,12 As it was shown that the MO5-2X functional is superior in describing weak, noncovalent interactions,39,40 additional calculations were carried out, and these results are presented along with the others. The optimized structures are in good agreement with those determined by X-ray diffraction analysis; these structures and their Cartesian coordinates are presented in the Supporting Information. Further calculations were performed to address the question of formal oxidation state assignments in these complexes. The Eigenvalues together with the metal and ligand fragment contributions to the frontier molecular orbitals of Zr(C7H7)(2,4(39) Zhao, Y.; Truhlar, D. G. Acc. Chem. Res. 2008, 41, 157. (40) (a) Holschumacher, D.; Bannenberg, T.; Hrib, C. G.; Jones, P. G.; Tamm, M. Angew. Chem. 2008, 120, 7538. (b) Holschumacher, D.; Bannenberg, T.; Hrib, C. G.; Jones, P. G.; Tamm, M. Angew. Chem., Int. Ed. 2008, 47, 7428. (41) (a) O’Boyle, N. M. GaussSum 2.1, 2007; available at http:// gausssum.sf.net. (b) O'Boyle, N. M.; Tenderholt, A. L.; Langner, K. M. J. Comput. Chem. 2008, 29, 839.

C7H11) (3) and Zr(C7H7)(6,6-dmch) (4) are summarized in Table 6.41 For the 2,4-dimethylpentadienyl complex 3, contour plots of the relevant frontier molecular orbitals are shown in Figure 7, nicely illustrating the symmetry of the interactions between the metal and the Cht (Cht = C7H7) and Pdl ligands. Thus, the HOMO and the HOMO-1 exhibit δ interactions between the metal and the seven-membered ring. A great contribution of both the zirconium and the Cht orbitals can be identified, which implicates a strongly covalent interaction. Since the electrons in these two orbitals are significantly more ligand- than metal-localized, this bonding situation is in agreement with assigning a -3 formal charge to the C7H7 ligand. Consequently, the half-open trozircenes should be regarded as high oxidation state complexes, which is nicely in accord with our experimental results. The next two energy levels, HOMO-2 and HOMO-3, can be mainly attributed to the metalpentadienyl π interactions and are predominantly of ligand character. Therefore, this interaction is largely ionic. Finally, HOMO-4 and HOMO-5 correspond to the π interaction of the metal orbitals with the Cht orbitals. Remarkably, mainly dz2 character can be found for the LUMO. All these results are similar to what was previously established for M(C7H7)(C5H5) complexes (M = Ti, Zr, Hf);5,42 however, due to the lower symmetry of the pentadienyl complexes, no degeneracy is observed. The electronic structure calculations reveal that the 16electron complex 3 contains frontier orbitals that seem to be suitably oriented for σ and π interaction with additional two-electron-donor ligands such as isocyanides. The LUMO has the required symmetry for a σ donor interaction; (42) Menconi, G.; Kaltsoyannis, N. Organometallics 2005, 24, 1189.

Article


5875

Figure 6. Perspective view of Zr(C7H7)(6,6-dmch){CN[2,6-C6H3(CH3)2]} (7). Table 6. Comparison of Eigenvalues and Composition of Frontier Orbitals energy (eV) Pdl 6,6-dmch 2,4-C7H11 6,6-dmch 2,4-C7H11 6,6-dmch 2,4-C7H11 6,6-dmch 2,4-C7H11 6,6-dmch 2,4-C7H11 6,6-dmch 2,4-C7H11 6,6-dmch 2,4-C7H11

orbital LUMO HOMO HOMO-1 HOMO-2 HOMO-3 HOMO-4 HOMO-5

Zr (%)

Cht (%)

Pdl (%)

B3LYP

M05-2X

B3LYP

M05-2X

B3LYP

M05-2X

B3LYP

M05-2X

-1.51 -1.40 -4.86 -4.76 -4.89 -4.76 -5.58 -5.87 -7.57 -7.53 -8.30 -8.24 -8.37 -8.28

-0.27 -0.23 -6.05 -5.94 -6.07 -5.98 -6.85 -7.18 -9.16 -9.12 -9.98 -9.93 -10.05 -10.00

89 89 31 34 35 32 16 18 5 7 11 11 13 13

91 90 31 30 35 35 17 18 5 7 11 12 13 14

8 7 64 59 60 64 5 4 9 7 85 86 77 82

7 6 63 64 61 59 6 6 7 8 84 86 79 81

3 5 6 8 6 4 79 78 86 86 4 2 10 4

2 3 7 6 5 6 78 76 87 86 4 2 8 5

furthermore, the experimentally and theoretically observed vertical orientation of the 2,6-xylyl isocyanide ligand (vide supra) can be explained by the resulting proper alignment of the LUMO of the isocyanide (Figure 8) and the HOMO-1 orbital of complex 3 for efficient π interaction. Table 7 summarizes the CN stretching frequencies and the calculated enthalpies for the isocyanide adduct formations. The experimental trend in the frequencies is reproduced, thus giving Zr(C7H7)(2,4-C7H11){CN[2,6-C6H3(CH3)2]} the lowest and the analogous Cp complex the highest value. Notably, the B3LYP results are fairly close to what was experimentally observed by IR spectroscopy, whereas the MO5-2X values differ considerably by about 100 cm-1. The lowest stretching frequency should correspond to the strongest metalisocyanide bond and therefore to the highest enthalpy of formation. This is actually observed theoretically for the 2,4-dimethylpentadienyl complex 3; however, the related 6,6-dimethylhexadienyl and cyclopentadienyl complexes exhibit almost identical enthalpies. With values for ΔH° ranging from -9.1 to -11.4 kcal mol-1 (B3LYP) and -11.8 to -13.9 kcal mol-1 (M05-2X), respectively, the zirconium-isocyanide interaction can be considered as rather weak in comparison with conventional transition

metal isocyanide complexes,43 indicating that the cycloheptatrienyl-pentadienyl complexes have a relatively small propensity to efficiently interact with σ donor/π acceptor ligands. As previously described for Cht-Cp group 4 metal complexes,5 this behavior can be mainly attributed to the strong and appreciably covalent zirconiumcycloheptatrienyl interaction, leading to highly stabilized frontier orbitals and consequently to a diminishing π electron release capability.

Summary Zr(C7H7)(Cl)(tmeda) has been shown to be an effective starting material for the preparations of 16-electron Zr(C7H7)(Pdl) complexes and their weakly bound isocyanide adducts. Structural, IR spectroscopic, and theoretical data are all in accord with the formulation of these species as formal Zr(IV) complexes. Notably, Zr(C7H7)(Cl)(tmeda) also serves as a useful starting material for the preparations of several modified Cp complexes.15 The recent report of a (43) Wang, K.; Rosini, G. P.; Nolan, S. P.; Goldman, A. S. J. Am. Chem. Soc. 1995, 117, 5082.

5876


Gl€ ockner et al.

Table 7. Comparison of Stretching Frequencies and Enthalpies ν (cm-1)

CN[2,6-C6H3(CH3)2] Zr(C7H7)(2,4-C7H11){CN[2,6-C6H3(CH3)2]} Zr(C7H7)(6,6-dmch){CN[2,6-C6H3(CH3)2]} Zr(C7H7)(C5H5){CN[2,6-C6H3(CH3)2]}

ΔH° (kcal mol-1)

exptl

B3LYP

MO5-2X

B3LYP

MO5-2X

2122 2100 2121 2134

2184 2110 2128 2141

2242 2199 2218 2232

-11.4 -9.1 -9.1

-13.9 -11.7 -11.8

Figure 8. LUMO of the 2,6-xylyl isocyanide complex based on M05-2X.

ligands to protonation. While lower valent (Zr(II)) open and half-open zirconocenes could also be employed for this purpose, except for Zr[1,5-(Me3Si)2C5H5]2, they all require one or two additional two-electron donors, particularly phosphines, for their stabilization,9 and these ligands typically would be disadvantageous to subsequent catalytic applications. Additionally, it is already clear that the coupling chemistry of higher valent pentadienyl complexes will differ dramatically from the widely explored chemistry of lower valent species.9 Hence, these complexes offer substantial promise for further, unique reaction chemistry. Figure 7. Contour plots of selected frontier orbitals of Zr(C7H7)(2,4-C7H11) (3) based on the M05-2X values.

Ta(C7H7)(Cl)(PMe3)2 complex might allow for an extension of some of this chemistry to tantalum.44 The Zr(C7H7)(Pdl) species should serve as useful complexes for the incorporation of zirconium into catalytic supports,45 due to the high susceptibility of their pentadienyl

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. wishes to thank the German Academic Exchange Service and the Bayer Science & Education Foundation for financial support.

(44) Noh, W.; Girolami, G. S. Inorg. Chem. 2008, 47, 10682. (45) (a) Robert, G.; Carr, N. L. U.S. Patent 5,780,381, 1998. (b) Barton, D. G.; Soled, S. L.; Meitzner, G. D.; Fuentes, G. A.; Iglesia, E. J. Catal. 1999, 181, 57. (c) Kunkes, E. L.; Simonetti, D. A.; West, R. M.; Serrano-Ruiz, J. C.; G€artner, C. A.; Dumesic, J. A. Science 2008, 322, 417. (d) Zhong, Z.; Zhang, Y.; Tierney, J. W.; Wender, I. Fuel Process. Technol. 2003, 83, 67. (e) Dutta, P.; Seehra, M. S.; Zheng, Y.; Wender, I. J. Appl. Phys. 2008, 103, 07D104.

Supporting Information Available: Cif files for each of the crystal structures included; details of the electronic structure calculations together with presentations and coordinates in x, y, z format of all optimized structures. This material is available free of charge via the Internet at http:// pubs.acs.org.

Cycloheptatrienyl-Pentadienyl Complexes of Zirconium (Half-Open

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