Organometallics 2000, 19, 2809-2812
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Crystal Packing Forces Dictate η1- versus η2-Coordination of Benzyl Groups in [Guanidinate]Zr(CH2Ph)3 Garth R. Giesbrecht, Glenn D. Whitener, and John Arnold* Department of Chemistry, University of California, Berkeley, and the Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720-1460 Received February 9, 2000 Summary: An X-ray structure determination of {CyNC[N(SiMe3)2]NCy}Zr(CH2Ph)3, crystallized from pentane, differs from the solid-state structure previously reported for the same compound crystallized from toluene. In contrast to the earlier report, we find no evidence for the presence of an η2 interaction in the solid state by X-ray crystallography or in solution by means of 1H or 13C NMR spectroscopy. Introduction Benzyl ligands (Bz) are well-established in organometallic chemistry and are distinct from other alkyls by virtue of their ability to exhibit a range of hapticities when binding to metals. In addition to the simple η1 case, bonding to the metal through the π-orbitals of the aromatic ring may also occur, resulting in η2, η3, or even η7 binding modes.1-4 In cases where two or more Bz ligands are coordinated to a single metal, differing modes have been seen2,4-13 and many studies of the fluxional behavior of such species in solution have been reported.
electrophilicity of the metal center and, by the same argument, the electron-donating ability of the ancillary ligands. Nevertheless, in the absence of confirming evidence by, for example, 1H or 13C NMR spectroscopy, the difference in energy between η1 and η2 bonding modes is uncertain.14,15 Here we compare two independent crystallographic determinations of the same zirconium tribenzyl complex: in one, an η2 interaction is present; in the other there is no evidence for such behavior. These data confirm that simple crystal packing forces may be sufficient to change hapticitites in metal benzyl complexes; they further serve to highlight the difficulties associated with using crystal structure data to reach conclusions regarding chemical bonding, especially where the energy differences involved are small. Results and Discussion Richeson and co-workers recently described the preparation of {CyNC[N(SiMe3)2]NCy}Zr(CH2Ph)3,16 according to eq 1.
[{CyNC[N(SiMe3)2]NCy}ZrCl4][Li(thf)2](ether) + toluene, RT
3PhCH2MgCl 9 8 -LiCl, -3MgCl 2
{CyNC[N(SiMe3)2]NCy}Zr(η -CH2Ph)(CH2Ph)2 (1) A 2
It is generally believed that observation of η2 Bz groups in the solid state is a good indicator of the (1) Scholz, J.; Rehbaum, F.; Thiele, K. H.; Goddard, R.; Betz, P.; Kruger, C. J. Organomet. Chem. 1993, 443, 93. (2) Legzdins, P.; Jones, R. H.; Phillips, E. C.; Yee, V. C.; Trotter, J.; Einstein, F. W. B. Organometallics 1991, 10, 986. (3) Mintz, E. A.; Moloy, K. G.; Marks, T. J. J. Am. Chem. Soc. 1982, 104, 4692. (4) Pellecchia, C.; Immirzi, A.; Pappalardo, D.; Peluso, A. Organometallics 1994, 13, 3773. (5) Ciruelo, G.; Cuenca, T.; Gomez, R.; Gomez-Sal, P.; Martin, A.; Rodriguez, G.; Royo, P. J. Organomet. Chem. 1997, 547, 287. (6) Pellecchia, C.; Grassi, A.; Immirzi, A. J. Am. Chem. Soc. 1993, 115, 1160. (7) Girolami, G. S.; Wilkinson, G.; Thornton-Pett, M.; Hursthouse, M. B. J. Chem. Soc., Dalton Trans. 1984, 2789. (8) Bei, X.; Swenson, D. C.; Jordan, R. F. Organometallics 1997, 16, 3282. (9) Tedesco, C.; Immirzi, A.; Proto, A. Acta Crystallogr. 1998, B54, 431. (10) Tsukahara, T.; Swenson, D. C.; Jordan, R. F. Organometallics 1997, 16, 3303. (11) Latesky, S. L.; McMullen, A. K.; Niccolai, G. P.; Rothwell, I. P.; Huffman, J. C. Organometallics 1985, 4, 902. (12) Edwards, P. G.; Andersen, R. A.; Zalkin, A. Organometallics 1984, 3, 293. (13) Mena, M.; Pellinghelli, M. A.; Royo, P.; Serrano, R.; Tiripicchio, A. J. Chem. Soc., Chem. Commun. 1986, 1118.
Orange crystals were obtained from concentrated toluene at -34 °C in good yield. An X-ray crystal structure revealed an acute Zr-C-Cipso angle for one of the benzyl groups, implying a single benzyl group bound to the metal center in an η2 fashion and two η1 benzyl groups. We had independently synthesized the same compound via an alkane elimination reaction between the guanidine {CyNC[N(SiMe3)2]NCy}H and Zr(CH2Ph)4 (14) Jordan et al. have estimated the η2-η1 isomerization barrier to be 14.0 kcal/mol for a cationic zirconium species; a lower value could be expected for a neutral complex (see ref 8). Legzdins et al. have determined a barrier of ∼10 kcal/mol for a series of molybdenum and tungsten bis(benzyl) nitrosyl complexes (see ref 2); similarly, the tribenzyl species Cp*M(CH2Ph)3 (M ) Th, U) exhibit barriers of 6.8 ppm).11 In addition, the 1JCH of the -CH2 groups increases (>130 Hz) as the Zr-C-Ph angle decreases, mirroring the increase in the amount of sp2 character of the R-carbon. Solid-state structures of related polybenzyl complexes frequently contain benzyl ligands that are distorted to varying degrees; still, these are equivalent in solution, where time-averaged NMR properties are observed.17 For example, the solid-state structure of Cp*Ti(CH2Ph)3 shows a single distorted -CH2Ph ligand13,18 and exhibits a high-field ortho hydrogen resonance (δ 6.57) in solu(17) Crowther, D. J.; Jordan, R. F.; Baenziger, N. C.; Verma, A. Organometallics 1990, 9, 2574. (18) Mena, M.; Royo, P.; Serrano, R.; Pellinghelli, M. A.; Tiripicchio, A. Organometallics 1989, 8, 476.
Notes
Organometallics, Vol. 19, No. 14, 2000 2811
Table 1. Crystallographic Data for A and B temp, K λ, Å space group a, Å b, Å c, Å β, deg volume, Å3 Z dcalcd, g/cm3 abs coeff, mm-1 R Rw no. of reflns no. of params
A
B
203(2) 0.71073 P21/n 10.483(1) 18.889(2) 20.459(3) 102.431(2) 3956.0(9) 4 1.228 0.369 0.0473 0.0918 4145 415
153(2) 0.71073 P21/n 10.810(1) 23.778(1) 15.755(1) 97.144(1) 4018.1(2) 4 1.19 0.45 0.026 0.031 5213 659
tion, but a normal sp3 1JCH value of 122 Hz. In our case, the chemical shift of the ortho protons in the 1H NMR spectrum is unexceptional (δ 6.94), as is the 1JCH (123 Hz). These data show that an η2 interaction is not maintained in solution. Since there is no spectroscopic evidence for an η2coordinated benzyl group in solution for the present case, what is the source of this interaction in the solidstate structure of A? A contribution from a zwitterionic resonance structure (II), which would render the metal center less electrophilic and less prone to engage in additional interactions with benzyl groups, may be ruled out due to the similar dihedral angles between N(SiMe3)2 and NCN planes in A and B. It appears that the presence of an η2-coordinated benzyl group in the solidstate structure of A is simply due to crystal packing effects resulting from the slight difference in crystallization conditions as compared to B. Thus, in the absence of corroborating evidence from solution spectroscopy, conclusions based on X-ray crystallography regarding electronic effects, and their influence on chemical bonding, must be viewed with caution. Experimental Section General Considerations. Standard Schlenk-line and glovebox techniques were used unless stated otherwise. Diethyl ether and pentane were purified by passage through a column of activated alumina and degassed with argon.19 C6D6 was vacuum transferred from sodium/benzophenone. 1,3-Dicyclohexylcarbodiimide was purchased from Sigma and distilled prior to use. LiN(SiMe3)2 and Zr(CH2Ph)4 were prepared according to literature procedures.20,21 Melting points were determined in sealed capillary tubes under nitrogen and are uncorrected. 1H and 13C{1H} NMR spectra were recorded at ambient temperature on a Bruker AM-300 spectrometer. 1H NMR chemical shifts are given relative to C6D5H (7.15 ppm). 13C{1H} NMR spectra are relative to C D (128.39 ppm). IR 6 6 samples were prepared as Nujol mulls and taken between KBr plates. Elemental analyses were determined at the College of Chemistry, University of California, Berkeley. Single-crystal X-ray structure determination was performed at CHEXRAY, University of California, Berkeley. {CyNC[N(SiMe3)2]NCy}H. 1,3-Dicyclohexylcarbodiimide (5.52 g, 26.8 mmol) and LiN(SiMe3)2 (4.48 g, 26.8 mmol) were combined in a 500 mL round-bottomed flask, and 300 mL of diethyl ether was added. The resultant pale yellow solution (19) Pangborn, A. B.; Giardello, M. A.; Grubbs, R. H.; Rosen, R. K.; Timmers, F. J. Organometallics 1996, 15, 1518. (20) Kruger, C. R.; Niederprum, H. Inorganic Syntheses; McGrawHill: New York, 1966; Vol. 8. (21) Zucchini, U.; Giannini, U.; Albizzati, E. J. Organomet. Chem. 1971, 26, 357.
Table 2. Bond Lengths (Å) for A and B Zr1-N1 Zr1-N2 Zr1-C20 Zr1-C27 Zr1-C34 N1-C6 N2-C12 N1-C13 N2-C13 N3-C13 N3-Si1 N3-Si2 C20-C26 C27-C33 C34-C40 Zr1-C26 Zr1-C33 Zr1-C40
A
B
2.241(4) 2.250(5) 2.268(5) 2.263(7) 2.288(6) 1.465(6) 1.461(6) 1.339(7) 1.344(7) 1.421(7) 1.756(5) 1.775(4) 1.466(8) 1.463(9) 1.491(8) 2.672(6) 3.187 3.345
2.249(2) 2.203(2) 2.289(3) 2.255(2) 2.266(3) 1.468(3) 1.471(3) 1.340(3) 1.337(3) 1.402(3) 1.776(2) 1.772(2) 1.491(4) 1.488(3) 1.483(4) 3.221(2) 3.198(2) 3.006(2)
Table 3. Bond Angles (deg) for A and B N1-Zr1-N2 N1-Zr1-C20 N1-Zr1-C27 N1-Zr1-C34 N2-Zr1-C20 N2-Zr1-C27 N1-Zr1-C34 C20-Zr1-C27 C20-Zr1-C34 C27-Zr1-C34 Zr1-C20-C26 Zr1-C27-C33 Zr1-C34-C40 N1-C13-N2 N1-C13-N3 N2-C13-N3 C13-N3-Si1 C13-N3-Si2 Si1-N3-Si2
A
B
59.77(17) 95.6(2) 111.7(3) 138.9(2) 126.4(2) 113.8(2) 84.0(2) 119.6(3) 90.7(2) 99.9(3) 88.7(4) 116.0(5) 123.2(4) 113.1(5) 123.6(5) 123.3(6) 117.1(3) 119.2(4) 123.7(3)
59.38(6) 86.09(8) 108.46(9) 130.64(9) 139.54(8) 98.72(8) 94.25(8) 112.8(1) 93.4(1) 116.9(1) 115.2(2) 115.9(2) 104.6(2) 110.9(2) 125.0(2) 124.1(2) 117.9(1) 119.5(1) 118.1(1)
was stirred at room temperature overnight. To this was added water (0.50 g, 27.8 mmol) via syringe, causing the solution to gradually turn cloudy. This was then stirred for 24 h. The mixture was filtered through a frit lined with Celite to remove LiCl to yield a pale yellow solution. Removal of the solvent under vacuum gave a golden oil, which yielded colorless plates upon mechanical agitation (4.92 g, 50%). Mp: 58-60 °C. 1H NMR (C6D6): δ 3.84 (m, 1H, unique CyH), 3.45 (m, 1H, unique CyH), 3.09 (d, 1H, NH, 3JH-H ) 7.0 Hz), 2.11-0.90 (m, 20H, C6H10), 0.21 (s, 18H, SiMe3). 13C{1H} NMR (C6D6): δ 147.22 (NCN), 56.95, 50.00, 36.21, 34.89, 33.92, 26.91, 26.76 and 25.73 (C6H11), 2.32 (SiMe3). IR (cm-1): 3441 (NH, m), 1636 (CdN, s), 1346 (w), 1296 (m), 1251 (s), 1220 (s), 1154 (w), 1118 (m), 1009 (m), 956 (s), 938 (s), 889 (sh), 868 (sh), 840 (s), 757 (m), 684 (m), 639 (w). Anal. Calcd for C19H41N3Si2: C, 62.06; H, 11.24; N, 11.43. Found: C, 62.44; H, 11.13; N, 11.64. {CyNC[N(SiMe3)2]NCy}Zr(CH2Ph)3. To an ethereal solution (30 mL) of Zr(CH2Ph)4 (2.01 g, 4.41 mmol) maintained at -78 °C was added {[CyNC[N(SiMe3)2]NCy}H (1.62 g, 4.41 mmol) in diethyl ether (20 mL) via cannula. The reaction mixture was left to stir and warm to room temperature overnight. The solvent was then removed under vacuum and the resultant yellow solid extracted with pentane. Cooling to -30 °C yielded large yellow blocks (3.09 g, 96%). Mp: 141143 °C. 1H NMR (C6D6): δ 7.22-6.91 (m, 15H, C6H5), 3.50 (m, 2H, unique CyH), 2.50 (s, 6H, CH2), 1.70-0.90 (m, 20H, C6H10), 0.18 (s, 18H, SiMe3). 13C{1H} NMR (C6D6): δ 174.55 (NCN), 145.13, 129.80, 128.58 and 123.20 (C6H5), 77.77 (CH2), 56.75, 35.43, 26.81 and 26.04 (C6H11), 2.72 (SiMe3). IR (cm-1): 1589 (CdN, s), 1309 (m), 1253 (s), 1195 (s), 1139 (m), 1062 (w), 1025 (sh), 1014 (m), 971 (m), 929 (s), 860 (m), 840 (s), 825 (s), 798
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(sh), 759 (w), 742 (s), 694 (s), 671 (sh), 642 (s). Anal. Calcd. for C40H61N3Si2Zr: C, 65.69; H, 8.41; N, 5.75. Found: C, 64.78; H, 8.69; N, 5.87.
Acknowledgment. We thank the NSF for the award of a predoctoral fellowship to G.D.W. and the DOE for support of this work.
Notes Supporting Information Available: Full details of the single-crystal X-ray analysis of {CyNC[N(SiMe3)2]NCy}Zr(CH2Ph)3. This material is available free of charge via the Internet at http://pubs.acs.org.
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