Synthesis, Characterization, and Theoretical Studies of Group 4 Amido

The structures of 1, 2, and 4·CH2Cl2 were determined by X-ray diffraction and show octahedral coordination geometry around the metal centers. Density...
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Inorg. Chem. 2003, 42, 3008−3015

Synthesis, Characterization, and Theoretical Studies of Group 4 Amido Hydrotris(pyrazolyl)borate Complexes Hu Cai,† Wai Han Lam,‡ Xianghua Yu,† Xiaozhan Liu,† Zhong-Zhi Wu,† Tianniu Chen,† Zhenyang Lin,*,‡ Xue-Tai Chen,§ Xiao-Zeng You,§ and Ziling Xue*,† Department of Chemistry, The UniVersity of Tennessee, KnoxVille, Tennessee 37996, Department of Chemistry, The Hong Kong UniVersity of Science and Technology, Clearwater Bay, Kowloon, Hong Kong, P. R. China, and State Key Laboratory of Coordination Chemistry, Nanjing UniVersity, Nanjing, 210093, P. R. China Received September 27, 2002

Titanium and zirconium amido complexes containing a hydrotris(pyrazolyl)borate (Tp) or hydrotris(3,5-dimethylpyrazolyl)borate (Tp*) ligand TpM(NMe2)3 (M ) Ti, 1; M ) Zr, 2) and Tp*M(NMe2)3 (M ) Ti, 3; M ) Zr, 4) were prepared by the reactions of M(NMe2)3Cl (M ) Ti, Zr) with sodium hydridotris(pyrazol-1-yl)borate and potassium hydridotris(3,5-dimethylpyrazol-1-yl)borate, respectively. The structures of 1, 2, and 4‚CH2Cl2 were determined by X-ray diffraction and show octahedral coordination geometry around the metal centers. Density functional theory calculations at the B3PW91 level were performed to understand the orientations and the rotational behavior of amido ligands in these metal complexes.

Group 4 complexes containing amido ligands have been studied1 because of their unique chemistry and their applications as catalysts for alkene polymerization2 and precursors for the chemical vapor deposition of metal nitride thin films.3 Many complexes containing NMe2 and cyclopentadienyl (Cp) or other ancillary ligands have been prepared and structurally characterized.4 Hydrotris(pyrazolyl)borate (Tp) and hydrotris(3,5-dimethylpyrazolyl)borate (Tp*) are two tridentate, anionic six-e donor ligands, and they are well-known Cp analogues. Compared to Cp ligands, these tris(pyrazolyl)borate ligands

are stronger electron donors and are more sterically hindered. In addition, they have more possible substitution positions for modifying the electron-donating and steric properties.5 Despite the disadvantages of metathesis reactions involving alkali-metal Tp compounds, which often result in inseparable byproducts,5,6 group 4 tris(pyrazolyl)borate complexes have been actively studied. Known group 4 tris(pyrazolyl)borate complexes include halide,7 alkyl,8 alkoxide,7d-g,8,9 CO,10 acetoacetate,11 Cp,9b,c cyclooctatetraenyl (COT),12 hydrazido,13 Tp2Ti,14 and imido derivatives.15 Recently, [tris(pyrazolyl)borate] halide and alkoxide complexes have been studied as catalysts for ethylene polymerization.7g,16 Examples

* Authors to whom correspondence should be addressed. E-mail: xue@ novell.chem.utk.edu; [email protected]. † University of Tennessee. ‡ Hong Kong University of Science and Technology. § Nanjing University. (1) (a) Bradley, D. C.; Chisholm, M. H. Acc. Chem. Res. 1976, 9, 273280. (b) Lappert, M. F.; Power, P. P.; Sanger, A. R.; Srivastava, R. C. Metal and Metalloid Amides: Syntheses, Structures, and Physical and Chemical Properties; Ellis Horwood Limited: Chichester, U.K., 1980. (c) Cotton, S. A. Annu. Rep. Prog. Chem., Sect. A: Inorg. Chem. 1992, 89, 107-116. (d) Kempe, R. Angew. Chem., Int. Ed. 2000, 39, 468-493. (e) Gade, L. H.; Mountford, P. Coord. Chem. ReV. 2001, 216-217, 65-97. (2) (a) McKnight, A. L.; Waymouth, R. M. Chem. ReV. 1998, 98, 25872598. (b) Britovsek, G. J. P.; Gibson, V. C.; Wass, D. F. Angew. Chem., Int. Ed. 1999, 38, 428-447. (3) (a) Fix, R. M.; Gordon, R. G.; Hoffman, D. M. Chem. Mater. 1990, 2, 235-241. (b) Chiu, H.-T.; Huang, C.-C. Mater. Lett. 1993, 16, 194199.

(4) (a) Nugent, V. A.; Harlow, R. L. Inorg. Chem. 1979, 18, 2030-2032. (b) Chisholm, M. H.; Hammond, C. E.; Huffman, J. C. Polyhedron 1988, 7, 2515-2520. (c) Diamond, G. M.; Jordan, R. F.; Petersen, J. L. J. Am. Chem. Soc. 1996, 118, 8024-8033. (d) Wu, Z.; Diminnie, J. B.; Xue, Z. Inorg. Chem. 1998, 37, 6366-6372. (e) Kunz, K.; Erker, G.; Do¨ring, S.; Bredeau, S.; Kehr, G.; Fro¨hlich, R. Organometallics 2002, 21, 1031-1041. (5) (a) Trofimenko, S. Chem. ReV. 1972, 72, 497-509. (b) Trofimenko, S. Prog. Inorg. Chem. 1986, 34, 115-210. (c) Niedenzu, K.; Trofimenko, S. Top. Curr. Chem. 1986, 131, 1-37. (d) Shaver, A. In ComprehensiVe Coordination Chemistry; Wilkinson, G., Gillard, R. D., McCleverty, J. A., Eds.; Pergamon: Oxford, 1987; Vol. 2, pp 245259. (e) Trofimenko, S. Chem. ReV. 1993, 93, 943-980. (f) Kitajima, N.; Tolman, W. B. Prog. Inorg. Chem. 1995, 43, 419-531. (g) Trofimenko, S. Scorpionates;The Coordination Chemistry of Polypyrazolylborate Ligands; Imperial College Press: London, 1999. (6) (a) Mashima, K.; Oshiki, T.; Tani, K. Organometallics 1997, 16, 2760-2762. (b) Oshiki, T.; Mashima, K.; Kawamura, S.; Tani, K.; Kitaura, K. Bull. Chem. Soc. Jpn. 2000, 73, 1735-1748.

Introduction

3008 Inorganic Chemistry, Vol. 42, No. 9, 2003

10.1021/ic026063v CCC: $25.00

© 2003 American Chemical Society Published on Web 04/05/2003

Group 4 Amino Hydrotris(pyrazolyl)borate Complexes

of group 4 tris(pyrazolyl)borate amido complexes are relatively rare.13 We have studied [tris(pyrazolyl)borate]M(NMe2)3 (M ) Ti, Zr) complexes, in part to see whether they would be precursors to other tris(pyrazolyl)borate complexes. In the structures of TpM(NMe2)3 and Tp*M(NMe2)3 with six M-N bonds, three M-N(amido) bonds are σ bonds with significant d-p π interaction, and three Ti r N(pz) bonds are dative bonds confined by the chelating tris(pyrazolyl)borate ligands. The structures of these Tp and Tp* triamido complexes were thus studied and compared, to understand, from a theoretical point of view, the orientations and the rotational behavior of the amido ligands in these complexes. In addition, we found that the reactions of TpMCl3 and Tp*MCl3 (M ) Ti, Zr) with LiNMe2 did not give TpM(NMe2)3 (M ) Ti, 1; Zr, 2) and Tp*M(NMe2)3 (M ) Ti, 3; Zr, 4) complexes in our hands. These TpM(NMe2)3 and Tp*M(NMe2)3 were prepared instead from the reactions of Cl-M(NMe2)3 with sodium tris(1-pyrazyolyl)borohydride (NaTp) and potassium hydridotris(3,5-dimethylpyrazol-1-yl)borate (KTp*), respectively. We report here the preparation and structures of these titanium and zirconium amido tris(pyrazolyl)borate complexes and theoretical studies of the differences in the relative orientations of the three amido ligands in these complexes. (7) (a) Kouba, J. K.; Wreford, S. S. Inorg. Chem. 1976, 15, 2313-2314. (b) Reger, D. L.; Tarquini, M. E. Inorg. Chem. 1982, 21, 840-842. (c) LeCloux, D. D.; Keyes, M. C.; Osawa, M.; Reynolds, V.; Tolman, W. B. Inorg. Chem. 1994, 33, 6361-6368. (d) Reger, D. L.; Tarquini, M. E. Inorg. Chem. 1983, 22, 1064-1068. (e) Ipaktschi, J.; Sulzbach, W. J. Organomet. Chem. 1992, 434, 287-302. (f) Antin˜olo, A.; Carrillo-Hermosilla, F.; Corrochano, A. E.; Ferna´ndez-Baeza, J.; Lanfranchi, M.; Otero, A.; Pellinghelli, M. A. J. Organomet. Chem. 1999, 577, 174-180. (g) Murtuza, S.; Casagrande, O. L., Jr.; Jordan, R. F. Organometallics 2002, 21, 1882-1890. (8) (a) Reger, D. L.; Tarquini, M. E.; Lebioda, L. Organometallics 1983, 2, 1763-1769. (b) Ipaktschi, J.; Sulzbach, W. J. Organomet. Chem. 1992, 426, 59-70. (9) (a) Kresinski, R. A.; Hamor, T. A.; Isam, L.; Jones, C. J.; McCleverty, J. A. Polyhedron 1989, 8, 845-847. (b) Kresinski, R. A.; Jones, C. J.; McCleverty, J. A. Polyhedron 1990, 9, 2185-2187. (c) Kresinski, R. A.; Hamor, T. A.; Jones, C. J.; McCleverty, J. A. J. Chem. Soc., Dalton Trans. 1991, 603-607. (d) Kresinski, R. A.; Isam, L.; Hamor, T. A.; Jones, C. J.; McCleverty, J. A. J. Chem. Soc., Dalton Trans. 1991, 1835-1842. (e) Ku¨hl, O.; Blaurock, S.; Sieler, J.; Hey-Hawkins, E. Polyhedron 2001, 20, 2171-2177. (10) Chi, K. M.; Frerichs, S. R.; Stein, B. K.; Blackburn, D. W.; Ellis, J. E. J. Am. Chem. Soc. 1988, 110, 163-171. (11) Satish, C. D.; Gururaj, M. V.; Bajgur, C. S. Indian J. Chem. Sect. A: Inorg., Bio-inorg., Phys., Theor., Anal. Chem. 2000, 39, 446-449. (12) Kilimann, U.; Noltemeyer, M.; Scha¨fer, M.; Herbst-Irmer, R.; Schmidt, H.-G.; Edelmann, F. T. J. Organomet. Chem. 1994, 469, C27-C30. (13) (a) Hughes, D. L.; Leigh, G. J.; Walker, D. G. J. Chem. Soc., Dalton Trans. 1988, 1153-1157. (b) Hughes, D. L.; Leigh, G. J.; Walker, D. G. J. Chem. Soc., Dalton Trans. 1989, 1413-1416. (14) Kayal, A.; Kuncheria, J.; Lee, S. C. Chem. Commun. 2001, 24822483. (15) (a) Dunn, S. C.; Batsanov, A. S.; Mountford, P. Chem. Commun. 1994, 2007-2008. (b) Dunn, S. C.; Mountford, P.; Shishkin, O. V. Inorg. Chem. 1996, 35, 1006-1012. (16) (a) Nakazawa, H.; Ikai, S.; Imaoka, K.; Kai, Y.; Yano, T. J. Mol. Catal. A 1998, 132, 33-41. (b) Furlan, L. G.; Gil, M. P.; Casagrande, O. L., Jr. Macromol. Rapid Commun. 2000, 21, 1054-1057. (c) Janiak, C.; Lange, K. C. H.; Scharmann, T. G. Appl. Organomet. Chem. 2000, 14, 316-324. (d) Karam, A.; Jimeno, M.; Lezama, J.; Catarı´, E.; Figueroa, A.; de Gascue, B. R. J. Mol. Catal. A 2001, 176, 65-72. (e) Otero, A.; Ferna´ndez-Baeza, J.; Antin˜olo, A.; Carrillo-Hermosilla, F.; Tejeda, J.; Dı´ez-Barra, E.; Lara-Sanchez, A.; Sanchez-Barba, L.; Lo´pez-Solera, I. Organometallics 2001, 20, 2428-2430. (f) Lo´pezLinares, F.; Barrios, A. D.; Ortega, H.; Karam, A.; Agrifoglio, G.; Gonza´lez, E. J. Mol. Catal. A 2002, 179, 87-92.

Experimental Section All manipulations were performed under a dry N2 atmosphere with the use of either a drybox or standard Schlenk techniques. All solvents except CH2Cl2 were purified by distillation from potassium/benzophenone ketyl; CH2Cl2 was purified by distillation from calcium hydride. Benzene-d6 and toluene-d8 were dried over activated molecular sieves and stored under N2. ZrCl4 (Strem) was freshly sublimed under vacuum. TiCl4(THF)217a and Li(THF)SiButPh217b were prepared according to the literature procedures. Potassium tris(3,5-dimethyl-1-pyrazyolyl)borohydride (KTp*) (Aldrich), sodium tris(1-pyrazyolyl)borohydride (NaTp) (Aldrich), and H3SiPh (Acros) were used as received. 1H and 13C{1H} NMR spectra were recorded on a Bruker AC-250 or AMX-400 spectrometer and referenced to the solvent (residual protons in the 1H spectra). Elemental analyses were performed by Complete Analysis Laboratories, Inc. (Parsippany, NJ). Preparation of TpTi(NMe2)3 (1). To a yellow slurry of TiCl4(THF)2 (1.67 g, 5.0 mmol) in THF (30 mL), 3 equiv of LiNMe2 (0.77 g, 15.0 mmol) in THF (20 mL) was added dropwise with stirring at -30 °C. After the reaction mixture was stirred overnight at room temperature, it was cooled to -30 °C and 1 equiv of NaTp (1.18 g, 5.0 mmol) in THF (30 mL) was added over 20 min. The red-brown solution was then slowly warmed to room temperature and stirred overnight. A red-brown solid was afforded by removing the volatiles under reduced pressure. Extraction of the solid with hexanes, followed by filtration and removal of the solvents and crystallization in CH2Cl2 at -32 °C, yielded red-brown crystals of 1 (1.37 g, 69% yield). 1H NMR (benzene-d6, 250.1 MHz, 23 °C): δ 7.49, 7.41 [d, d, 3H, 3H, 3-H, 5-H (pz)], 5.91 [t, 3H, 4-H (pz)], 3.27 (s, 18H, NMe2). 13C NMR (benzene-d6, 62.9 MHz, 23 °C): δ 141.88, 133.82 [3-C, 5-C (pz)], 104.70 [4-C (pz)], 49.74 (NMe2). Anal. Calcd for C15H28BN9Ti: C, 45.83; H, 7.18. Found: C, 45.63; H, 7.26. Preparation of TpZr(NMe2)3 (2). To a white slurry of ZrCl4 (1.17 g, 5.0 mmol) in THF (30 mL), 3 equiv of LiNMe2 (0.77 g, 15.0 mmol) in THF (20 mL) was added dropwise with stirring at -30 °C. After the reaction mixture was stirred overnight at room temperature, it was cooled to -30 °C and 1 equiv of NaTp (1.18 g, 5.0 mmol) in THF (30 mL) was added over 20 min. The mixture was then warmed gradually to room temperature and stirred overnight. A pale yellow solid was afforded by removing of the volatiles under reduced pressure. Extraction of the solid with hexanes, followed by filtration and removal of solvents and crystallization in CH2Cl2 at -32 °C, yielded colorless crystals of 2 (1.39 g, 64% yield). 1H NMR (benzene-d6, 250.1 MHz, 23 °C): δ 7.62, 7.36 [d, d, 3H, 3H, 3-H, 5-H (pz)], 5.86 [t, 3H, 4-H (pz)], 3.13 (s, 18H, NMe2). 1H NMR (toluene-d8, 400.1 MHz, -90 °C): δ 7.56, 7.26 [d, d, 3H, 3H, 3-H, 5-H (pz)], 5.75 [t, 3H, 4-H (pz)], 3.25 (s, 18H, NMe2). 13C NMR (benzene-d6, 62.9 MHz, 23 °C): δ 142.34, 134.99 [3-C, 5-C (pz)], 105.00 [4-C (pz)], 44.95 (NMe2). Anal. Calcd for C15H28BN9Zr: C, 41.28; H, 6.47. Found: C, 41.17; H, 6.51. Preparation of Tp*Ti(NMe2)3 (3). To a yellow slurry of TiCl4(THF)2 (1.67 g, 5.0 mmol) in THF (30 mL), 3 equiv of LiNMe2 (0.77 g, 15.0 mmol) in THF (20 mL) was added dropwise with stirring at -30 °C. After the reaction mixture was stirred overnight at room temperature, it was cooled to -30 °C and 1 equiv of KTp* (17) (a) Manzer, L. E. Inorg. Synth. 1982, 21, 135-140. (b) Tilley and co-workers reported the preparation of Li(THF)3SiButPh2 (Campion, B. K.; Heyn, R. H.; Tilley, T. D. Organometallics 1993, 12, 25842590). In the current studies, Li(THF)SiButPh2 was prepared by a similar procedure.

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Cai et al. (1.68 g, 5.0 mmol) in THF (30 mL) was added over 20 min. The red-brown solution was then slowly warmed to room temperature and stirred overnight. A red-brown solid was afforded by removing the volatiles under reduced pressure. Extraction of the solid with hexanes, followed by filtration and removal of solvents and crystallization in CH2Cl2 at -32 °C, yielded red-brown crystals of 3 (1.66 g, 70% yield). 1H NMR (benzene-d6, 250.1 MHz, 23 °C): δ 5.69 [s, 3H, 4-H (pz)], 3.23 (s, 18H, NMe2), 2.26, 2.17 (s, s, 9H, 9H, 3,5-Me2pz). 13C NMR (benzene-d6, 62.9 MHz, 23 °C): δ 151.37, 142.91 [3-C, 5-C (pz)], 106.33 [4-C (pz)], 49.37 (NMe2), 13.95, 12.77 (3,5-Me2pz). Anal. Calcd for C21H40BN9Ti: C, 52.84; H, 8.45. Found: C, 52.76; H, 8.56. Preparation of Tp*Zr(NMe2)3 (4). To a white slurry of ZrCl4 (1.17 g, 5.0 mmol) in THF (30 mL), 3 equiv of LiNMe2 (0.77 g, 15.0 mmol) in THF (20 mL) was added dropwise with stirring at -30 °C. After the reaction mixture was stirred overnight at room temperature, it was cooled to -30 °C and 1 equiv of KTp* (1.68 g, 5.0 mmol) in THF (30 mL) was added over 20 min. The mixture was then warmed gradually to room temperature and stirred overnight. A yellow solid was afforded by removing the volatiles under reduced pressure. Extraction of the solid with hexanes, followed by filtration and removal of solvents and crystallization in CH2Cl2 at -32 °C, yielded pale-yellow crystals of 4‚CH2Cl2 (1.78 g, 68% yield). The CH2Cl2 in the crystals of 4‚CH2Cl2 could be removed under vacuum (10-3 Torr). 1H NMR (benzene-d6, 250.1 MHz, 23 °C) of 4: δ 5.65 [s, 3H, 4-H (pz)], 3.03 (s, 18H, NMe2), 2.35, 2.10 (s, s, 9H, 9H, 3,5-Me2pz). 1H NMR (toluene-d8, 400.1 MHz, -90 °C) of 4: δ 5.67 [s, 3H, 4-H (pz)], 3.16 (s, 18H, NMe2), 2.44, 1.99 (s, s, 9H, 9H, 3,5-Me2pz). 13C NMR (benzene-d6, 62.9 MHz, 23 °C) of 4: δ 151.92, 144.23 [3-C, 5-C (pz)], 106.34 [4-C (pz)], 44.66 (NMe2), 12.75 (3,5-Me2pz). 13C NMR (toluene-d8, 100.6 MHz, -90 °C) of 4: δ 151.41, 144.12 [3-C, 5-C (pz)], 105.91 [4-C (pz)], 44.85 (NMe2), 12.80 (3,5-Me2pz). Anal. Calcd for the CH2Cl2-free solid of 4 (C21H40BN9Zr): C, 48.45; H, 7.74. Found: C, 48.37; H, 7.66. Studies of Reactivities of TpM(NMe2)3 (M ) Ti, 1; Zr, 2). In the studies of the reactions of 1 and 2 with O2, a benzene-d6 solution containing either 1 (44.6 mg, 0.113 mmol) or 2 (59.8 mg, 0.137 mmol) in a J. Young NMR tube was frozen by liquid nitrogen, and the gases in the NMR tube were removed under vacuum. O2 (ca. 1 atm at 296 K, 0.08 mmol) was then introduced, and the solution was monitored by 1H and 13C NMR spectroscopy. In the studies of reactivities of 2 toward H3SiPh conducted in a benzene-d6 solution, the solution containing 2 (58.0 mg, 0.115 mmol) was added to H3SiPh (47.6 mg, 0.149 mmol). The solution was heated to 40 °C and monitored by 1H and 13C NMR spectroscopy. In the studies of reactivities of 2 toward H3SiPh conducted in liquid H3SiPh, 2 (58.0 mg, 0.115 mmol) was added to excess H3SiPh at 23 °C. Part of this solution was removed and dissolved in benzene-d6 for 1H and 13C NMR spectroscopy. In the studies of reactivities of 2 toward Li(THF)SiButPh2, a toluene-d8 solution of 2 (68.4 mg, 0.157 mmol) was added to Li(THF)SiButPh2 (49.9 mg, 0.157 mmol). The solution was monitored by 1H NMR spectroscopy. X-ray Crystal Structure Determination of 1, 2, and 4‚CH2Cl2. Crystal data and a summary of intensity data collection parameters for 1, 2, and 4‚CH2Cl2 are given in Table 1. All crystal structures were determined on a Bruker model AXS Smart 1000 X-ray diffractometer equipped with a CCD area detector and a graphitemonochromated Mo source (KR radiation, wavelength of 0.71073 Å) and fitted with an upgraded Nicolet model LT-2 low-temperature device. Suitable crystals were coated with paratone oil (Exxon) and

3010 Inorganic Chemistry, Vol. 42, No. 9, 2003

Table 1. Crystal Data for 1, 2, and 4‚CH2Cl2 1 formula fw color cryst size, mm

C15H28BN9Ti 393.17 red 0.41 × 0.22 × 0.20 T, K 173 ( 2 λ (Mo KR), Å 0.71073 cryst syst monoclinic space group P21/c a, Å 15.6637(8) b, Å 23.6788(12) c, Å 16.8363(8) R, deg 90 β, deg 100.7490(10) γ, deg 90 V, Å3 6135.0(5) Z 12 3 Dcalc, g/cm 1.277 total no. of reflns 59 427 independent reflns 12 571 R1 (wR2)a 0.0540 (0.1546) GOF 1.025 a

2

4‚CH2Cl2

C15H28BN9Zr 436.49 colorless 0.49 × 0.35 × 0.23 173 ( 2 0.71073 orthorhombic Pnma 18.7451(8) 14.6284(6) 7.6923(3) 90 90 90 2109.31(15) 4 1.375 21 507 2658 0.0263 (0.0747) 1.091

C22H42BCl2N9Zr 605.58 pale yellow 0.50 × 0.48 × 0.25 173 ( 2 0.71073 orthorhombic Pbca 11.3015(8) 19.7543(14) 26.9199(19) 90 90 90 6010.0(7) 8 1.339 62 226 7417 0.0434 (0.1079) 1.043

R1 ) ∑||Fo| - |Fc||/∑|Fo|; wR2 ) (∑[w(Fo2 - Fc2)2]/∑[w(Fo2)2])1/2.

mounted on a glass fiber under a stream of nitrogen at -100 ( 2 °C. All three structures were solved by direct methods. Nonhydrogen atoms were anisotropically refined. All H atoms in the X-ray structures were treated as idealized contributions. Empirical absorption correction used the SADABS program.18a In addition, the global refinements for the unit cells and data reductions in the X-ray structures were performed under the SAINT program (Version 6.02), and all calculations were performed using the SHELXTL (Version 5.1) proprietary software package.18b Computation Details. Theoretical calculations at the B3PW91 level of the density functional theory (DFT) on model complexes TpTi(NMe2)3 and TpZr(NMe2)3 were performed. In these calculations,19 the effective core potentials of Hay and Wadt with a double-ζ valence basis set (LanL2DZ)20 were used to describe Ti and Zr, whereas the standard 6-31G basis set was used for all the other atoms. Polarization functions were added for Zr (ζf ) 0.875), Ti (ζf ) 1.506), and N (ζd ) 0.800) on the amido ligands.21

Results and Discussion Preparation of 1-4 and Reactivities of 1 and 2. Complexes 1-4 were prepared in 64%-70% yields by the (18) (a) Sheldrick, G. M. SADABS, A Program for Empirical Absorption Correction of Area Detector Data; University of Go¨ttingen, Go¨ttingen, Germany, 1996. (b) Sheldrick, G. M. SHELXL-97, A Program for the Refinement of Crystal Structures; University of Go¨ttingen: Go¨ttingen, Germany, 1997. (19) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Zakrzewski, V. G.; Montgomery, J. A., Jr.; Stratmann, R. E.; Burant, J. C.; Dapprich, S.; Millam, J. M.; Daniels, A. D.; Kudin, K. N.; Strain, M. C.; Farkas, O.; Tomasi, J.; Barone, V.; Cossi, M.; Cammi, R.; Mennucci, B.; Pomelli, C.; Adamo, C.; Clifford, S.; Ochterski, J.; Petersson, G. A.; Ayala, P. Y.; Cui, Q.; Morokuma, K.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Cioslowski, J.; Ortiz, J. V.; Baboul, A. G.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Gomperts, R.; 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.; Andres, J. L.; Gonzalez, C.; Head-Gordon, M.; Replogle, E. S.; Pople, J. A. Gaussian 98 (ReVision A.9); Gaussian, Inc.: Pittsburgh, PA, 1998. (20) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299-310. (21) Ehlers, A. W.; Bo¨hme, M.; Dapprich, S.; Gobbi, A.; Ho¨llwarth, A.; Jonas, V.; Ko¨hler, K. F.; Stegmann, R.; Veldkamp, A.; Frenking, G. Chem. Phys. Lett. 1993, 208, 111-114.

Group 4 Amino Hydrotris(pyrazolyl)borate Complexes Scheme 1

metathesis reactions of NaTp and KTp* with (Me2N)3MCl (M ) Ti, Zr) (Scheme 1), respectively. (Me2N)3MCl was prepared in situ by reactions of 3 equiv of LiNMe2 with TiCl4(THF)2 or ZrCl4, respectively. Although TpMCl3 and Tp*MCl3 (M ) Ti, Zr) are convenient starting materials for a variety of Tp and Tp* complexes,5e,g our attempts to prepare 1-4 by the reactions of TpMCl3 and Tp*MCl3 with 3 equiv of LiNMe2 failed to yield isolable products (Scheme 1). When TpMCl3 or Tp*MCl3 was treated with 3 equiv of LiNMe2 at 23 °C, instantaneous reactions occurred to give unidentified dark-brown materials. It is not clear why the reactions of TpMCl3 or Tp*MCl3 with LiNMe2 failed to give 1-4. Perhaps the NMe2- anions attacked the B atoms in TpMCl3 or Tp*MCl3, leading to the decomposition of the Tp and Tp* ligands. Complexes 1-4 are thermally stable both in the solid state and in solution. No signs of decomposition of their solutions were observed after 1 week at room temperature. The strong donor nature of the Tp and Tp* ligands and the p-d π bonding from the N atoms in amido ligands to the electron-deficient metal centers perhaps help stabilize these complexes.4b The 1H and 13C{1H} NMR spectra of the complexes at 23 °C are consistent with the appropriate C3 symmetry in these complexes, in which the three pyrazolyl groups in the Tp or Tp* and three NMe2 groups are equivalent. These spectra, at 23 °C, contain one set of pyrazolyl and NMe2 resonances, respectively. The 1H and 13C NMR resonances of the Tp and Tp* groups in complexes 1-4 are similar to those observed in other group 4 Tp complexes.7-9 The 1H NMR resonances of NMe2 ligands in TpM(NMe2)3 (M ) Ti, 1; Zr, 2), at 3.27 and 3.13 ppm, respectively, and those of the NMe2 ligands in Tp*M(NMe2)3 (M ) Ti, 3; Zr, 4), at 3.23 and 3.03 ppm, respectively, are shifted downfield from those in Ti(NMe2)4 and Zr(NMe2)4.22 Noteworthy is the fact that the NMe2 resonances of Tp*M(NMe2)3 are shifted slightly upfield from those of TpM(NMe2)3.

Scheme 2

The 1H NMR spectra of TpZr(NMe2)3 (2) and Tp*Zr(NMe2)3 (4) at -90 °C were similar to those at 23 °C, and no splitting of the NMe2 resonances was observed. In addition, the 13C{1H} spectrum of Tp* complex 4 at -90 °C did not show either splitting of the NMe2 resonance at 44.85 ppm. These results suggest that the amido ligands in 2 and 4 are very fluxional and their rotational barriers are extremely small, as suggested by the DFT calculations that are discussed below. The solutions of TpM(NMe2)3 (M ) Ti, 1; Zr, 2) were found to be fairly stable against O2. Under 1 atm of O2 at room temperature for 24 h,