Organometallics 1995, 14, 3724-3731
3724
Isolation and Complete NMR and X-ray Crystallographic Characterization of an Unusual Pentadienyllutetium Complex, (q5-Me2C5Hs)Lu( q5:q3-MeC5H5CH2CH2CHMeC3H3Me) Matthew B. Zielinski,*32aDonald K. Drummond,2bPradeep S. Iyer,2a John T. Leman,2band William J. Evans*Izb Unocal Corporation, P.O. Box 76, Brea, California 92621, and Department of Chemistry, University of California, Irvine, Irvine, California 92717 Received December 6, 1994@ From the metathesis reaction of (2,4-dimethylpentadienyl)potassium with lutetium trichloride (3:1 reaction stoichiometry) has been isolated a novel pentadienyllutetium (11,which contains complex, (115-(CH3)2C5H5)L~(1;15:113-(CH3)C5H5CH2CH2CH(CH3)C3H3(CH3)) a dimeric chelate ligand derived from the end-to-end fusion of two 2,4-dimethylpentadienyl groups. The solid-state structure of 1 was established through single-crystal X-ray diffraction analysis. Resonances in the IH and 13CNMR spectra were completely and unequivocally assigned by 2D NMR techniques and show the complex to be in a n asymmetric conformation in solution.
Introduction Significant advances in the field of metal pentadienyl chemistry have been made within the past 10 years.3 Much of this work has been stimulated by speculation that pentadienyl complexes may show unusual chemical reactivity by virtue of ready interconversion among any of three ligand-metal bonding modes, i.e., y', y3, and q5. Investigations have now led to the synthesis of a variety of transition-metal pentadienyl complexes which display the entire range of metal-ligand bonding interactions. In contrast, few pentadienyl-f-element complexes have been r e p ~ r t e dthose ; ~ which have been crystallographically characterized predominantly display y5-pentadienyl-metal b ~ n d i n g . ~ *Our ~ ~ ~interest ~,g in the catalytic chemistry of organolanthanides with unusual hydrocarbyl ligation, as well as our desire to investigate the extent t o which metal ion size will influence metal-pentadienyl bonding interactions, has led us to a further investigation of this area. With the realization that coordination sphere oversaturation in trivalent organolanthanide chemistry can force complexes into higher reactivity manifold^,^ our Abstract published in Advance ACS Abstracts, June 15, 1995. (1)Reported in part at the California Catalysis Society Fall Meeting, Lake Arrowhead, CA, Oct 20-21, 1988. (2) (a) Unocal Corporation. (b) University of California, Irvine. (3)General reviews of this chemistry include: (a)Emst, R. D. Chem. Rev. 1988,88, 1255-1291. (b) Powell, P. In Advances in Organometallic Chemistry; West, R., Stone, F. G. A,, Eds.; Academic Press: New York, 1986; Vol. 26, pp 125-164. (c) Ernst, R. D. Acc. Chem. Res. 1985, 18, 56-62. (d)Yasuda, H.; Nakamura, A. J . Organomet. Chem. 1985,285, 15-29. (e) Ernst, R. D. Struct. Bonding (Berlin) 1984,57, 1-53. Also see: (D Bleeke, J. R.; Wittenbrink, R. J.; Clayton, T. W., Jr.; Chiang, M. Y. J . Am. Chem. Soc. 1990, 112, 6539-6545 and references cited therein. (4) (a) Baudry, D.; Bulot, E.; Charpin, P.; Ephritikhine, M.; Lance, M.; Nierlich, M.; Vigner, J. J. Organomet. Chem. 1989,371,163-174. (b) Sieler, J.; Simon, A.; Peters, K.; Taube, R.; Geitner, M. J . Organomet. Chem. 1989, 362, 297-303. (c) Baudry, D.; Bulot, E.; Ephritikhine, M. J . Chem. Soc., Chem. Commun. 1989, 1316-1317. (d) Baudry, D.; Bulot, E.; Ephritikhine, M. J . Chem. Soc., Chem. Commun. 1988, 1369-1370. (e) Cymbaluk, T. H.; Liu, J.-Z.;Ernst, R. D. J . Organomet. Chem. 1983, 255, 311-315. (0 Emst, R. D.; Cymbaluk, T. H. Organometallics 1982, 1 , 708-713. (g) Schumann, H.; Dietrich, A. J. Organomet. Chem. 1991, 401, C33-C36.
immediate goal was to maximize steric crowding within a pentadienyllanthanide complex. Because of the considerable steric congestion already present within the only known homoleptic pentadienyllanthanide complex, tri~(y~-2,4-dimethyIpentadienyl)neodymium,~~ we envisioned that substitution of a significantly smaller ionic radius lanthanide ion into a related synthetic sequence might effect pivotal changes in structure and/or reactivity. We chose to work with Lu3+ because it is the smallest ionic radius lanthanide ion and because its diamagnetic character contributes to a simplification of NMR analysis. During the course of our study on the metathesis reaction of (2,4-dimethylpentadienyl)potassium with lutetium trichloride, an independent report on this system has appeared4gwhich describes the main product of this reaction, (115-(CH3)2C5H5)2L~(y3-(CH3)2C5H5). We report here the isolation of an unusual minor reaction product derived from the coupling of two pentadienyl ligands. Both X-ray crystallography and 2D NMR were used to characterize the complex. The power of the latter method for defining the organolanthanide structure in solution is clearly demonstrated.
Experimental Section The acute air and moisture sensitivity of each organometallic complex required t h a t all synthetic procedures and subsequent manipulations be performed using vacuum line, Schlenk, or glovebox (VacuudAtmospheres HE-43 Dri-Lab with M040 Dri-Train) techniques. Atmospheres of high-purity nitrogen or ultrahigh-purity argon were used in both Schlenk and glovebox operations. Materials. Hexane and tetrahydrofuran (THF) were distilled from potassium benzophenone ketyl. Benzene-& was vacuum-transferred from potassium benzophenone ketyl. Anhydrous lutetium trichloride (Cerac) was used as received. (2,4Dimethylpentadieny1)potassiumwas prepared according t o the literature6 from dispersed potassium and 2,4-dimethyl-1,3pentadiene (Aldrich). (5) Evans, W. J. Polyhedron 1987, 6, 803-835. (6)Yasuda, H.; Ohnuma, Y.; Yamauchi, M.; Tani, H.; Nakamura, A. Bull. Chem. SOC. Jpn. 1979,52, 2036-2045.
0276-733319512314-3724$09.00/0 0 1995 American Chemical Society
An Unusual Pentadienyllutetium Complex Physical Measurements. Infrared spectra were recorded on a Perkin-Elmer Model 1430 spectrophotometer. Samples were prepared as Nujol mulls within the glovebox. lH and 13C NMR spectra were obtained as described below. CH analyses were performed on a Carlo Erba EA 1108 elemental analyzer, and complexometric analysis for Lu was obtained as previously d e ~ c r i b e d . ~
Organometallics, Vol. 14, No. 8,1995 3725 Table 1. Crystallographic Data for Complex 1
(?5-(CIIQig)zC5)4)L~(~5:~3-(CIIQ)CsIIsCH2CH2CH(C~)C~-
(CH3))(1). Into a lOO-mL, three-necked, round-bottomed flask equipped with a spinbar, rubber septum, glass stopper, and gas inlet was placed 1.50 g (5.33 mmol) of anhydrous lutetium trichloride and 20 mL of THF. The assembly was attached t o the Schlenk line, and the solution was stirred overnight t o disperse the undissolved salt. A solution containing 2.15 g (16.0 mmol) of (2,4-dimethylpentadienyl)potassiumin 30 mL of THF was slowly syringed into the rapidly stirred slurry of lutetium trichloride, previously cooled to -78 "C. Upon dropwise addition of the light amber potassium salt solution, a localized yellow color appeared momentarily and then dissipated. This occurred until approximately 1mL of solution had been added. The yellow color then remained as the balance of the potassium salt was added. After complete addition, the solution was stirred for a n additional 1.5 h. The cooling bath was then removed and the solution was warmed slowly to room temperature. During this period the solution gradually turned dark brown. After the mixture was stirred overnight, the solvent was vacuum-evaporated. The residue was extracted with hexane (4 x 20 mL), and the resulting extract was concentrated to a volume of ca. 30 mL. The solution was then cooled to -78 "C for 8 h, which resulted in the formation of olive-colored crystals. Using a double-ended filter, the crystals were separated from the mother liquor, which itself yielded a noncrystallizable, as yet uncharacterized, brown-black oil. The crystals were redissolved in a minimum amount of THFhexane, and the solution was cooled to ca. -30 "C overnight. This resulted in the formation of pale orangeyellow crystals suitable for X-ray diffraction analysis; yield 0.13 g (5%). IR (Nujol mull): 3110 (vw), 3090 (w), 3080 (w), 3025 (w), 1525 (s, br), 1425 (sh), 1350 (w), 1340 (vw), 1320 (vw), 1290 (w), 1270 (w), 1250 (w-m), 1230 (w), 1210 (vw), 1180 (w), 1155 (vw), 1090 (w-m), 1060 (m), 1030 (m), 1015 (w), 990 (sh), 980 (w), 945 (w), 925 (w), 890 (w), 875 (w, br), 850 (w-m), 835 (w-m), 810 (s), 800 (w), 795 (w), 770 (s, br), 700 (sh), 690 (sh), 630 (m), 600 (w), and 565 (sh) cm-l. Anal. Calcd for LuCz1H33: C, 54.78; H, 7.22; Lu, 38.00. Found: C, 54.13; H, 7.28; Lu, 37.8.
X-ray Crystallography of (s5-(CIIQ)zCsYs)L~(;5:?3-(C~)C~H&H~CH&H(CHS)C~H~(CH~)) ( 1 ) . A single crystal of approximate dimensions 0.20 x 0.30 x 0.40 mm was sealed into a thin-walled glass capillary under an inert atmosphere (Nz)and mounted on a Syntex P21 diffractometer. Subsequent setup operations (determination of accurate unit cell dimensions and orientation matrix) and collection of room temperature (296 K) intensity data were carried out using standard techniques similar t o those of ChurchilL8 Details appear in Table 1. All 4439 data were corrected for the effects of absorption and for Lorentz and polarization effects and placed on a n approximately absolute scale by means of a Wilson plot. Any reflection with Z(net) < 0 was assigned the value lFol = 0. A careful examination of a preliminary data set revealed no systematic extinctions or any diffraction symmetry other than the Friedel condition. The centrosymmetric triclinic space group Pi (Ctl; No. 2) was chosen and later determined to be correct by successful solution of the structure. All crystallographic calculations were carried out using either a modified version of the UCLA Crystallographic Computing Packageg or the SHELXTL PLUS program set.1°
formula: C21H33Lu fw: 460.4 crystal syst: trkclinic space group: P1 a = 7.382(4)A b = 8.703(2) A c = 16.443(6)A a = 78.54(2)" p = 84.74(4)" y = 68.11(3)" V = 960.5(6) A3 2=2 Deaicd= 1.592 Mg/m3 diffractometer: SpJex P21 radiation: Mo Ka (A = 0.710 730 A) monochromator: highly oriented graphite data collected: + h , f k , f l scan type: 8-28 scan width: 1.2" scan speed: 2.0" min-1 (in o) 28,,, = 55.0" p(Mo Ka)= 5.144 mm-' abs cor: semiempirical (y-scan method) midmax transmissn: 0.4002/0.9352 no. of rflns collected: 4439 no. of rflns with lFol > 0: 4380 no. of variables: 200 RF= 3.3% R,,.F = 4.8% goodness of fit: 1.23
The analytical scattering factors for neutral atoms were used throughout the analysis;lIa both the real ( A f ' ) and imaginary (iAf'') components of anomalous dispersionlIb were included. The quantity minimized during least-squares analysis was Zw(lFol - IF,1)2, where w - I = u2(IFo0+ 0.0007(IF,1)2. The structure was solved by direct methods (MITHRIL)12 and refined by full-matrix least-squares techniques (SHELXTL). Hydrogen atom contributions were included using a riding model with d(C-H) = 0.96 and U(iso) = 0.08 A2. Refinement of positional and anisotropic thermal parameters led to convergence with RF = 3.3%, R w =~ 4.8%, and GOF = 1.23 for 200 variables refined against all 4380 unique data (RF= 3.1%; R\*.F= 4.6% for those 4185 data with lFol > 6 . 0 ~ (lFol)).A final difference-Four@ map was devoid of significant features; e(max) = 1.37 e A-3. Atomic coordinates and equivalent isotropic displacement coefficients are presented in Table 2.
NMR Spectra of (qS-(CH3)2CaH5)Lu(95:?3-(CH3)C5H5CH~CH~CH(CH~)C~HS(CH~)) (I). IH and 13C NMR spectra were acquired at ambient temperature with a n IBM AF-270 FT NMR narrow-bore spectrometer. All data processing was done on a n Aspect-3000 computer using DISNMR standard software. A 5-mm dual-tuned probe was used to observe 'H and 13Cnuclei at 270 and 68 MHz, respectively. The 90" pulse widths for IH and 13C were 8.6 and 4.6 ys, respectively, while the decoupler coil pulse length was measured t o be 14.2 p s . The lutetium complex was dissolved in benzene-& solvent in a 5-mm Wilmad glass NMR tube, and the sample was sealed under vacuum. Chemical shifts are reported in ppm from TMS by setting the residual proton signal of the solvent a t 7.15 ppm and the corresponding 13C solvent resonance a t 128.0 ppm (Table 3). Carbon signal multiplicities were determined using the J-modulated spin echo pulse sequence. A 2D 'H COSY spectrum was acquired using Jeener's two-pulse sequence, 9O0-tl-45"-ACQ(t2), minimizing the diagonal peak intensi(9) UCLA Crystallographic Computing Package, University of California, Los Angeles, 1981. Strouse, C. Personal communication. (10)Nicolet Instrument Corp., Madison, WI, 1988. (11)(a) International Tables for X-Ray Crystallography; Kynoch Press: Birmingham, England, 1974; pp 99-101. (b) Ibid., pp 149-
150.
(12) Gilmore, C. J. J . Appl. Crystallogr. 1984, 17, 42-46.
Zielinski et al.
3726 Organometallics, Vol. 14, No. 8,1995
Table 2. Atomic Coordinates ( x 104) and Equivalent Isotropic Displacement Coefficients (A2 x lo4)for Complex la X
Lu(1) C(1) C(2) C(3) C(4) C(5) C(6) C(7) C(8) C(9) C(10) C(11) C(12) C(13) C(14) C(15) C(16) C(17) C(18) C(19) C(20) C(21)
-2242.5(0.2) 361(9) -164(7) -167(7) 481(7) 1181(8) -974(10) 256(10) -1403(7) -2849(7) -2798(7) -4231(10) -5561(10) -6885(8) -5909(6) -5981(6) -5152(6) -5375(9) -4531(9) -3045(14) -4972(7) -4058(7)
Y
z
347.4(0.2) -1464(7) -2687(6) -2961(6) -2203(6) -921(8) -3737(7) -2858(8) 2854(6) 3468(5) 2535(6) 3020(8) 1993(9) 2158(8) 1106(6) 2044(5) 1483(6) 2847(7) 5100(6) 2756(12) -585(6) -142(6)
2205.9(0.1) 3336(3) 3091(3) 2291(3) 1517(3) 1419(4) 3761(4) 771(4) 1785(3) 2387(3) 3187(3) 3887(3) 4090(3) 3381(3) 2721(3) 1896(3) 1143(3) 391(3) 2112(3) 4668(4) 2951(3) 1046(3)
U(ed 273.5(0.9) 549(21) 469(17) 406(15) 448(16) 603(22) 741(24) 657(24) 454(17) 376(15) 434(17) 558(23) 607(25) 553(21) 402(16) 374(14) 385(15) 557(21) 509(19) 951(47) 444(17) 450(17)
Equivalent isotropic U,defined as one-third of the trace of the orthogonalized U, tensor. 0
Table 3. 68 MHz lsC(lH) and 270 MHz 'H NMR Data for Complex 1" chem shift (6, ppm)b
carbon no.csd 2 3 4 5 6 7 8
type' CHz C CH C CHz CH3 CH3 CHz
13C{1H1 81.1 147.3 90.0 145.2 82.2 30.0 29.9 59.5
9 10 11 12 13 14 15 16 17 18 19 20 21
C CH CH CHz CHz C CH C CH3 CH3 CH3 CHz CHz
151.3 80.9 33.8 44.3 41.5 155.0 98.0 149.0 27.6 24.2 24.0 73.0 76.0
1
'Hf 3.68 (s, lH), 2.68 (s, 1H) 4.73 (5, 1H) 4.35 (s, lH), 3.37 (s, 1H) 1.86 (s, 3H) 1.98 (s, 3H) 1.95 (m, 1H) 1.51 (d, lH, J = 4.6 Hz) 3.58 (d, lH, J = 7.6 Hz) 2.46 (m, 1H) 1.75 (m, lH), 1.25 (m, 1H) 2.73 (m, lH), 1.93 (m, 1H)
A 2D heteronuclear correlation spectrum (XHCORR)15was recorded using the pulse sequence 90"(H)-1/~tl-1800(C)1/ztl-D3-900(C)900(H)-d4-ACQ(t2) (under proton decoupling). The acquisition involved 128 scans for each of 128 t l increments using a 3 s recycle delay. Delays D3 and D4 were optimized for J = 160 Hz (Le., set to 3.125 and 1.563 ms, respectively). The spectral widths used in the F1 and F2 domains were 2702 and 13 514 Hz, respectively. The t2 data were exponentially weighted using a line-broadening factor of 5 Hz and Fourier-transformed over 2048 data points. The t l interferograms were modified with a shifted ( d 4 ) sine squared bell function before Fourier transformation over 256W data points as a magnitude spectrum. Finally, the pulse sequence that worked best for obtaining a long-range heteronuclear correlation spectrum of this complex was the modified version of XHCORR suggested by Krishnamurthy and Nunlist.16 The sequence involves the elimination of the refocusing D4 delay and BB decoupling during acquisition. The D3 delay was optimized for long-range couplings of 8 Hz magnitude and set to be 62.5 ms. The number of scans was increased to 512 for each increment of tl.
Results and Discussion
A metathesis reaction involving the addition of 3 equivalents of (2,4-dimethylpentadienyl)potassiumto a THF solution of lutetium trichloride a t -78 "C led to a mixture of products. While removal of solvent from the reaction mixture and extraction with toluene has been reported4g to give (175-(CH3)2C5H5)2L~(173-(CH3)2C5H5)) (2) in 46% yield, initial extraction with hexanes, a affords a solvent in which 2 has limited ~olubility,~g second product, (175-(CH3)2C5H5)L~(r5:r3-(CH3)CgH5CH2CH2CH(CH3)C3H3(CH3))(1). The solid-state structure of 1 was unequivocally determined through singlecrystal X-ray diffraction analysis, which revealed the presence of a solitary 2,4-dimethylpentadienyl ligand and one chelate ligand resulting from the coupling of two 2,4-dimethylpentadienyl units. The complex displays one v3-allyl and two $-pentadienyl bonding interactions, as are found in 2.4g The presence of preformed coupling product in the (2,4-dimethylpentadieny1)potassiumstarting material was ruled out through lH NMR and IR analysis.
4.63 (s, 1H) 1.82 (s, 3H) 2.23 (s, 3H) 1.07 (d, 3H, J = 6.6 Hz) 3.12 (s, lH), 3.01 (s, 1H) 2.95 (s, lH), 2.61 (s, 1H)
In benzene-& solution under ambient conditions. * Chemical shifts (6) are reported in ppm with respect to TMS. Refer to carbon numbering scheme in Figure 1. Signal assignments for atoms 1-7 were inferred from a combination of 2D NMR experiments and X-ray data. e Discerned from the J-modulated spin echo 13C experiment. f Multiplicities, integrations, and coupling constants are indicated in parentheses.
&-
(1
ties.13 Thirty-two scans were collected over a spectral width of 2703 Hz for each of 256 time increments to give a matrix of 1024 x 1024 data points. The recycle delay used was 2 s. The long-range COSY experiment used the pulse sequence of Bax and Freeman,'* 9O0-tl-A-45"-A-ACQ(t2), under the same conditions but with A set to 80 ms to observe weak cross-peaks from long-range couplings. The free induction decays were multiplied by a n unshifted sine squared bell function, and symmetrization was applied to the final spectrum. (13)Nagayama, K.; Kumar, A.; Wuthrich, K.; Ernst, R. R. J . Mugn. Reson. 1980,40, 321-334. (14)Bax, A.; Freeman, R. J . Mugn. Reson. 1981,4 4 , 542-561.
1
a 2
Solid-state Structure of 1. An ORTEP diagram of complex 1 is shown in Figure 1 with interatomic distances and angles presented in Table 4. Both the IR spectrum and X-ray crystallographic data are consistent with v3-allyland +pentadienyl bonding modes. The strong IR absorption a t 1525 cm-I clearly indicates delocalized C-C bond stretching within the unsaturated carbon frameworks of the complex. No localized C=C or C=C-C=C bond stretching absorptions are evident. Furthermore, least-squares analysis of the (15)Bax, A.; Morris, G . A. J.Magn. Reson. 1981,42, 501-505. (16)Krishnamurthy, V. V.; Nunlist, R. J . Mugn. Reson. 1988,80,
280-295.
An Unusual Pentadienyllutetium Complex
Organometallics, Vol. 14, No. 8, 1995 3727
C13 f ? 3
Figure 1. ORTEP diagram of complex 1. Thermal ellipsoids are drawn at the 30% probability level. Table 4. Selected Interatomic Distances (A)and Angles (deg) for Complex 1" Lu(1)- C(1) Lu(1)-C(3) LU(l)-C(5) Lu( 1)-C(9) Lu( 1)-c(14) Lu(l)-C( 16) Lu(1)-C(21) Lu( l)-Cent(2) C(l)-C(2) C(2)-C(6) C(4)-C(5) C(8)-C(9) C(9)-C( 18) C(ll)-C(12) C(12)-C(13) C(14)-C( 15) C(15)-C(16) C(16)-C(21)
C(9)-C(lO)-C(ll) C(lO>-C(ll)-C(l9) C(11)-C(12)-C(13) C(13)-C(14)-C(15) C(15)-C(14)-C(20) C(15)-C(16)-C(17) C(17)-C(16)-C(21) Cent(l)-Lu(l)Cent(2) Cent(B)-Lu( 1)Cent(3)
Interatomic 2.620(6) 2.693(4) 2.677(6) 2.656(5) 2.636(5) 2.658(5) 2.614(6) 2.353 1.398(10) 1.514(9) 1.371(10) 1.412(6) 1.515(6) 1.533(12) 1.541(9) 1.432(6) 1.423(7) 1.376(6)
Distances Lu(l)-C(2) Lu(l)-C(4) Lu( 1)-C(8) Lu(l)-C( 10) Lu( 1)-C( 15) Lu( 1)-C(20) Lu(1)-Cent(1) Lu(l)-Cent(3) C(2)-C(3) C(3)-C(4) C(4)-C(7) C(S)-C(lO) C(lO)-C(ll) C(ll)-C(19) C(13)-C( 14) C(14)-C(20) C(16)-C( 17)
Interatomic Angles 127.1(5) C(l)-C( 2)-C( 6) 116.0(6) C(2)-C(3)-C(4) 125.4(5) C(3)-C(4)-C(7) 119.6(5) C(8)-C(9)-C( 10) 117.5(4) C(lO)-C(9)-C(lS) 126.8(4) C(lO)-C(ll)-C(l2) 107.6(6) C(l2)-C(ll)-C(l9) 115.2(5) C(12)-C(13)-C(14) 114.8(4) C(13)-C(14)-C(20) 125.8(5) C(14)-C(15)-C(16) 115.7(4) C(15)-C(16)-C(21) 117.3(4) 126.7 Cent(1)-Lu(1)Cent(3) 104.2
2.703(4) 2.740(5) 2.440(6) 2.629(5) 2.642(4) 2.567(5) 2.227 2.149 1.383(8) 1.439(7) 1.497(9) 1.398(6) 1.504(7) 1.551(11) 1.515(8) 1.362(7) 1.508(7)
116.7(5) 130.8(5) 115.0(6) 120.9(4) 121.5(4) 113.8(6) 108.8(5) 115.5(4) 119.2(5) 129.8(4) 126.9(4) 128.9
a Cent(1) is the centroid of the unit defined by C(l)-C(2)-C(3)C(4)-C(5), Cent(2) is the centroid of the unit defined by C(8)C(9)-C(lO), and Cent(3) is the centroid of the unit defined by C(20)-C( 14)-C(15)-C( 16)-C(21).
pentadienyl planes, defined by the five unsaturated carbon atoms of each pentadienyl array, indicates a high degree of planarity (maximum carbon atom deviation: 0.02 A). The pentadienyl units in complex 1 are oriented in a head-to-tail fashion,4fanalogous to the arrangement of adjacent +pentadienyl ligands in 2 and tris(y5-2,4-
dimethylpentadieny1)neodymium (3). The allyl group, however, with its syn alkyl bridge connection at C(lo), is oriented head-to-head with respect to the chelate pentadienyl unit. This is in contrast to 2, where a headto-tail allyl-to-pentadienylarrangement is f o ~ d . ~Viewg ing complex 1 in perspective (Figure 1) shows this arrangement to minimize steric interaction among the C(17), C(l8), and C(19) methyl groups and the alkyl bridge. Complex 1 displays pseudotrigonal coordination as indicated by the incongruent (ligand centroid)-Lu(ligand centroid) angles. Thus, the (allyl centroid)-Lu(pentadienyl (chelate) centroid) angle (104.2') is considerably narrower than either of the angles involving the centroid of the solitary pentadienyl ligand ((allyl centroid)-Lu-(pentadienyl centroid), 126.7'; (pentadienyl centroid)-Lu-(pentadienyl (chelate) centroid), 128.9'). The narrowness of the first angle clearly results from the restricted three-carbon bridge connecting the terminal n systems of the chelate ligand. The two remaining angles are necessarily much larger because of the placement of the solitary 2,4-dimethylpentadienyl ligand in a less congested area of the coordination sphere. The solitary pentadienyl ligand of complex 1 displays the shortest metal-carbon bonds to the terminal carbon atoms C(1) (2.620(6) A) and C(5) (2.677(6) A). These distances are equivalent to the analogous bonds in 2 (2.63(1)-2.66(1) A)$ The Lu-C(3) and Lu-C(2) bonds are intermediate in length (2.693(4) and 2.703(4) A, respectively),while the Lu-C(4) bond is longest (2.740(5) A). This trend is somewhat similar to that found in both the lutetium and neodymium complexes 2 and 3, where the shortest bonds are to the anionic carbon atoms in the 1-, 3-, and 5-positions of each pentadienyl ligand. Carbons at these positions have formal electron charge densities derived from the three major resonance forms which describe the overall pentadienyl anion hybrid. Bond orders derived from these resonance contributors, however, would predict shorter external and longer internal carbon-carbon bond distances for the solitary pentadienyl ligand. Instead, the external C(l)-C(2) and C(4)-C(5) bonds (1.398(10) and 1.371(10) A,respectively) as well as the internal C(2)-C(3) bond (1.383(8)A) have similar lengths while the C(3)C(4) bond (1.439(7)A>is longer. These bond distances are, nevertheless, similar enough to confirm an v5 bonding mode. Failure to observe more systematic variations in metal-carbon and carbon-carbon bond distances, as found for the symmetrical complex 2, is largely attributed to the unsymmetrical nature of 1. In the chelate pentadienyl ligand, metal-carbon bond lengths are again shortest for the terminal carbon atoms (Lu-C(20), 2.567(5) A; Lu-C(21), 2.614(6) A). The central Lu-C(15) bond (2.642(4)A) is intermediate in length, as is the Lu-C(14) bond (2.636(5)A). The LuC(14)bond is shorter than expected and may result from ligand distortion a t C(14) brought about by attachment of the bridging unit;this presumably forces C(14)toward the metal as the allyl unit a t the opposite end of the bridge is arranged for maximum coordination. Longest is the Lu-C(16) bond (2.658(5) A). Carbon-carbon bond distances in this ligand clearly reflect bond orders derived from the major resonance forms contributing to the pentadienyl anion hybrid. Thus, the external
3728 Organometallics, Vol. 14, No. 8, 1995
Zielinski et al. benzene-d,
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Figure 2. 68 MHz 13C{lH) NMR spectrum of complex 1 in benzene-ds (ambient temperature). Refer to the numbering scheme in Figure 1. pentadienyl carbon-carbon bonds (C(14)-C(20) and C(16)-C(21)) average 1.369(7)A in length, which compares well with the 1.373(12) A average length observed for the corresponding bonds in neodymium complex 2. Similarly, the internal bonds C(14)-C(15) and C(15)C(16) average 1.428(5) A in length, compared with 1.421(12) A for the internal bonds in 2. Within the allyl unit, carbon atom C(8) is bound more closely to lutetium (2.440(6)A>than carbon atoms C(9) and C(10) (2.656(5) and 2.629(5) A, respectively). In com arison, the average Lu-allyl distance in 2 is 2.59(1) .f.4g The exceedingly short Lu-C(8) bond length is not the result of an y1 orientation of the allyl ligand, since the C(8)-C(9) and C(9)-C(lO) bond distances (1.412(6) and 1.398(6) A, respectively) are statistically equivalent. Rather, it is likely the result of steric constraints imposed by coordination of the chelating ligand. These results attest t o an y3 bonding mode where greater formal charge density resides on the terminal allyl carbon atoms. The equivalent carboncarbon bond distances further reflect the equivalence of the two major resonance structures describing the allyl anion hybrid. Related f-element allyl complexes 2, (C~,Mes)U(y~-2-methylally1)3,~~ and (C5Me5)zSm(q3ally1)18display similar trends. The larger size of the pentadienyl ligand compared with the cyclopentadienyl ligand manifests itself in a longer average metal-carbon bond distance and shorter metal-(ligand centroid) distances. Thus, the average metal-carbon bond distance within the Lu-(y5-pentadienyl) framework of the solitary pentadienyl ligand in 1 is 2.69(4)A compared with the mean Lu-(y5-cyclopentadienyl carbon) bond distance of 2.60 A in the eight(17)Cymbaluk, T. H.;Ernst, R. D.; Day, V. W. Organometallics
coordinate tris(cyclopentadieny1)lutetium system.lg Metal-(y5-pentadienyl centroid) distances in 1 (pentadienyl (chelate) centroid, 2.149 A; solitary pentadienyl centroid, 2.227 A), on the other hand, are shorter than the corresponding metal-(q5-cyclopentadienyl centroid) distances (2.285 and 2.321 A). Ligand size differences have effected similar results in the actinide complexes (y5-2,4-dimethylpentadienyl)U(BJ&)3and (y5-cyclopentadien~l)U(BH&..~* Interestingly, internal C-C-C bond angles within the pentadienyl units vary systematically. Those angles in which the central carbon atom bears an alkyl substituent are narrower than those angles which do not have a central carbon atom substituent. The same effect has been observed for 3. The internal C-C-C bond angle of the methylallyl unit in 1 (120.9(4)")is comparable to the average internal C-C-C bond angle for the methylallyl groups in (C5Me5)U(y3-2-methylally1)317 (120.3(7)"). This is in contrast to an internal angle of 125.6(20)"observed for the unsubstituted allyl unit in (CsMe5)2Sm(y3-allyl).ls Solution Structure of 1. It was of interest to determine if a complete NMR analysis of such a complicated ligand set could be accomplished with a lanthanide complex given the fluxionality and ligand redistribution equilibria common for these metals. The solution structure of 1 has been shown by NMR to correlate well with the solid-state structure. The protondecoupled 13C and lH NMR spectra of the complex, dissolved in benzene-ds solvent, are shown in Figures 2 and 3, respectively. The 13C spectrum shows 21 resonances, consistent with the presence of 21 nonequivalent carbon nuclei making up the ligands about the lutetium metal center. Using the J-modulated spin echo 13Cexperiment, these
1983,2, 963-969.
(18)Evans, W. J.; Ulibarri, T.A.; Ziller, J. W. J.Am. Chem. SOC. 1990, 112, 2314-2324.
(19)Eggers, S. H.;Schultze, H.; Kopf, J.; Fischer, R. D. Angew. Chem., Int. Ed. Engl. 1986,25, 656-657.
An Unusual Pentadienyllutetium Complex
Organometallics, Vol. 14, No. 8, 1995 3729
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Figure 3. 270 MHz 'H NMR spectrum of complex 1 in benzene-de (ambient temperature). Refer to the numbering scheme in Figure 1. resonances were differentiated into sets of five methyl, seven methylene, four methine, and five quaternary carbon nuclei. Evidence for the allylkhelate-bridge substructure is readily apparent from the 'H NMR signal multiplicities observed for H(10) (doublet, lH, 3.58 ppm), H(11) (multiplet, lH, 2.46 ppm), and H(19) (doublet, 3H, 1.07 ppm) (Figure 3). These resonances provide the basis from which the remainder of the chelate ligand structure is established. Other proton resonances within the complex include four methyl group singlets (3H each) as well as ten other singlets which are integrated nominally as single protons. Six remaining singleproton resonances appear as multiplets which arise from methylenes H(8), H(12), and H(13). Both lH and 13C NMR spectra clearly reflect the asymmetric conformation of 1 in solution. No direct evidence for fluxional behavior involving the solitary pentadienyl ligand (e.g., 1,3-or 1,5-metal migrations) can be inferred under these conditions. This is in contrast to the room-temperature fluxional behavior noted for 2.4g Nonfluxionality in 1 also is in contrast with the rapid room-temperature ligand oscillation reported for bis(2,4-dimethylpentadienyl)complexes of magnesium,20 beryllium,21 zinc,21 and ruthenium3e but is more in line with the less fluxional behavior of bi~(2,4-dimethylpentadienyl)titanium.~~ The complete assignment of all lH and 13Cresonances required further analyses by a combination of 2D NMR experiments. Using resonances H(10), H(11), and H(19) as unequivocal starting points, the proton coupling network was traced from the corresponding cross-peaks observed in COSY-45 and long-range COSY-45 spectra. The latter is shown in Figure 4. The long-range correlation proved particularly useful in assigning the four methyls, H(6), H(71, H(17), and H(181, that are (20)Yasuda, H.; Yamauchi, M.; Nakamura, A,; Sei, T.; Kai, Y.; Yasuoka, N.; Kasai, N. Bull. Chem. SOC.Jpn. 1980, 53, 1089-1100. (21) Yasuda, H.; Ohnuma, Y.; Nakamura, A,; Kai, Y.; Yasuoka, N.; Kasai, N. BuZ1. Chem. SOC.Jpn. 1980, 53,1101-1111. (22) Lehmkuhl, H.; Naydowski, C. J . Organomet. Chem. 1982,240, C30-C32.
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Figure 4. Long-range IH COSY-45 spectrum of complex 1 in benzene-d6. attached to quaternary carbons. The corresponding 13C resonances were assigned through a 2D heteronuclear correlation (XHCORR) study, shown in Figure 5. The spread of carbon frequencies in the f 2 domain further helped corroborate and confirm the assignment of the nonequivalent methylene proton resonances, despite the fact that some were obscured by signal overlap in the 'H spectrum (e.g., H(8) and H(13)). Finally, the longrange analog of this experiment (not shown) helped in the assignment of the quaternary carbon signals and also provided further confirmation of all lH and 13C signal assignments. The final outcome from these studies has been summarized in Table 3. On the basis of the chemical shifts observed in both 13C and lH NMR spectra, it is clear that the metal-
Zielinski et al.
3730 Organometallics, Vol. 14, No. 8, 1995
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Figure 5. 2D 13C-lH heteronuclear correlation (XHCORR) spectrum of complex 1 in benzene-&. carbon bonding hapticities observed in the solid state are also retained in the solution state. Establishing that the +bound pentadienyl moieties are present in the “ U form, however, was less straightforward. The presence of alkyl substituents in the 2- and 4-positions precluded the observation of spin-spin coupling constants which have previously proven successful in establishing conformations of allyl and pentadienyl metal complexes in solution. Considering the large effective van der Waals radius of the methyl group (ca. 2.0 A), it is nevertheless unlikely that the ligand would prefer either a “ W or “S” form in solution while adopting a “ U form in the solid state (vide supra). Finally, the I3C chemical shifts of the anionic terminal methylene carbons show an interesting trend that can, at least in part, be related to their corresponding metalcarbon bond distances. For example, the C(1) and C(5) atoms of the solitary pentadienyl unit are 2.620(6) and 2.677(6) A away from the lutetium metal center and show 13C resonances at 81.1 and 82.2 ppm, respectively. Carbons C(20) and C(21) are situated closer at 2.567(5) and 2.614(6) A, respectively, and show corresponding 13C signals at 73.0 and 76.0 ppm that are relatively shielded. By far the shortest bond distance measured is that between lutetium and C(8) (2.440(6) A), which also exhibits the most shielded resonance (59.5 ppm). Though bond distances measured in the solid state may be expected to change in solution, the observed trend is consistent with the expected relative charge densities at these carbon atoms. Similar effects arising on account of the relative importance of the resonance contributing structures are also evident when comparing the chemical shifts of the quaternary carbons (uncharged) with those of their neighbors which carry some negative charge. These observations are especially interesting when comparing metal-carbon bond distances in transition-
metal pentadienyl complexes. Just as the shortest pentadienyl metal-carbon bonds in 1 and 2 are primarily to those carbons bearing formal electron charge density, the terminal pentadienyl metal-carbon bonds of the early-transition-metal complexes bis(y5-2,4-dimethy1pentadienyl)~anadium~~ and bis(v5-2,4-dimethy1pentadienyl)chromi~m~~ are also observed to be the shortest. Such bonding effects are attributable to the ionic character of these complexes. However, bis(q5-2,4dimethy1pentadien~l)iron~~ displays an opposite trend in which the shortest metal-carbon bonds are to the 2and 4-carbon atoms. This behavior has been explained by the growing importance of back-bonding in later transition metal c ~ m p l e x e s ,Back-bonding, ~~ as such, is inconsequential in f-block chemistry and does not play a role in describing the structures of 1 and 2. For transition-metal allyl complexes, it is more common that the central metal-carbon bond distance is shorter than the terminal allyl metal-carbon bond distances.26 This is clearly opposite to that situation observed within the allyl framework of 1,in which the shorter metal-carbon bonds are again to those carbons bearing formal electron charge density. These observations thus show that, steric constraints notwithstanding, charge plays a critical role in the architecture of lanthanide complex 1. Factors Leading to the Formation of 1. The dimeric chelate ligand in 1 clearly derives from the endto-end fusion of two 2,4-dimethylpentadienyl groups. Although pentadienyl dimerizations have previously been observed in transition-metal ~hemistry,~” this is the first time that pentadienyl dimerization has been reported in the lanthanide series. Dimerizations in transition-metal pentadienyl chemistry are believed to result from reorganizations involving unstable, non-Welectron intermediates. Since stabilities in organolanthanide chemistry are not formally governed by the inert-gas rule but, rather, are a function of electrostatic factors and coordination sphere saturation, it is likely that 1 originates, at least in part, from the steric overcrowding which results when three 2,4-dimethylpentadienyl ligands surround a lutetium center. This would appear reasonable, given that Lu3+ is significantly smaller in ionic radius compared with Nd3+,whose coordination sphere is already fully saturated when v5-complexed to three 2,4-dimethylpentadienyl ligands (vide supra). As reported earlier,4g one way to avoid the overcrowding of three y5-pentadienyl ligands is to form the (r15-(CH3)2C5H5)2L~(r13_(CH3)2C5H5) complex, 2. A parallel alternative route to a less (23)Campana, C. F.;Ernst, R. D.; Wilson, D. R.; Liu, J.-Z. Inorg. Chem. 1984,23,2732-2734. (24)Newbound, T.D.;Freeman, J. W.; Wilson, D. R.; Kralik, M. S.; Patton, A. T.; Campana, C. F.; Ernst, R. D. Organometallics 1987,6, 2432-2437. (25)Wilson, D.R.;Ernst, R. D.; Cymbaluk, T. H. Organometallics 1983,2,1220-1228. (26)(a) Jolly, P. W. In Comprehensive organometallic Chemistry; Wilkinson, G., Stone, F. G. A,, Abel, E. W., Eds.; Pergamon Press: Oxford, England, 1982;Vol. 6,Chapter 37.(b)Maitlis, P. M.; Espinet, P.; Russell, M. J. H. In Comprehensive Organometallic Chemistry; Wilkinson, G., Stone, F. G. A., Abel, E. W., Eds.; Pergamon Press: Oxford, England, 1982; Vol. 6, Chapter 38. (c) Hartley, F. R. In Comprehensiue Organometallic Chemistry; Wilkinson, G., Stone, F. G. A,, Abel, E. W., Eds.; Pergamon Press: Oxford, England, 1982;Vol. 6, ChaDter 39. (d) Putnik. C. F.: Welter, J. J.: Stuckv. G. D.: D’Aniello. M. i,Jr.;Sosinsky, B. A.; Kiker, J. F.; Muetterties,-E. L. J.Am. Chem. SOC.1978,100,4107-4116.(e) Kaduk, J. A.; Poulos, A. T.; Ibers, J. A. J. Organomet. Chem. 1977, 127, 245-260. (D Clarke, H. L. J . Organomet. Chem. 1974,80,155-173.
An Unusual Pentadienyllutetium Complex
Organometallics, Vol. 14, No. 8, 1995 3731
Scheme 1
\b
would then afford the final complex. Such transformations have precedent or have been proposed to occur in organolutetium chemistry: alkene insertion into Lu-C and Lu-H bonds,28,%hydrogen elimination,28and interligand proton abstraction.28e
Conclusion
0 LU -
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crowded complex which is consistent with the propensity for Lu3+t o activate carbon-hydrogen bonds27involves a metal-assisted ligand reorganization leading to 1. Details of this process have not been investigated, but a route involving an vl-complexed pentadienyl unit can be envisioned (see Scheme 1). Interligand proton abstraction by a terminally bound pentadienyl group could lead to a metallacyclic intermediate which upon diene reinsertion would generate a ligand-dimerized product. /?-Hydrogen elimination followed by hydride addition (27)Watson, P.L.J.Am. Chem. SOC.1983,105,6491-6493.
In contrast to the reaction of neodymium trichloride with the 2,4-dimethylpentadienyl anion which affords a simple tris(v5-2,4-dimethylpentadienyl)complex, the analogous reaction with lutetium trichloride leads to bis($-pentadienyl) y3-allyl complexes. The predominant product of this reaction, (v5-(CH3)2C5H5)2Lu(v3-(CH3)~C ~ H S )is, ~accompanied ~ by a complex resulting from coupling of 2,4-dimethylpentadienyl ligands to form a C14 unit. The solution and solid-state structures, as inferred from 2D NMR and X-ray analysis, have been shown to be in congruence.
Acknowledgment. We thank the Division of Chemical Sciences of the Office of Basic Energy Sciences of the Department of Energy for partial support of this work. Supporting Information Available: Tables of anisotropic displacement coefficients, H-atom coordinates and isotropic displacement coefficients, complete interatomic distances and angles, and least-squares planes for 1 (8 pages). Ordering information is given on any current masthead page. 0M94092 6D (28)(a)Jeske, G.;Lauke, H.; Mauermann, H.; Schumann, H.; Marks, T. J. J. Am. Chem. SOC.1985,107,8111-8118. (b) Jeske, G.;Schock, L. E.; Swepston,P. N.; Schumann, H.; Marks, T. J. J . Am. Chem. SOC. 1985, 107, 8103-8110. ( c ) Jeske, G.;Lauke, H.; Mauermann, H.; Swepston,P. N.; Schumann, H.; Marks, T. J. J . Am. Chem. SOC.1985, 107,8091-8103.(d) Watson, P.L. J . Am. Chem. SOC.1982,104,337339.(e)Watson, P.L.; Roe, D. C. J . Am. Chem. SOC.1982,104,64716473.
