Flexible Coordination Mode of a Donor-Functionalized Terphenyl

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Organometallics 2009, 28, 5277–5280 DOI: 10.1021/om900465c

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Flexible Coordination Mode of a Donor-Functionalized Terphenyl Ligand in Monomeric Cp-Based Lanthanocenes Gerd W. Rabe,*,† Florian A. Riederer,‡ Mei Zhang-Presse,‡ and Arnold L. Rheingold§ †

Department of Chemistry, Texas A&M University, College Station, Texas 77842, ‡Department f€ ur Chemie, Technische Universit€ at M€ unchen, Lichtenbergstrasse 4, 85747 Garching, Germany, and §Department of Chemistry & Biochemistry, University of California, San Diego, La Jolla, California 92093 Received June 1, 2009

Summary: The synthesis and structural characterization of monomeric Cp-based metallocene compounds of the rare-earth elements samarium, yttrium, and ytterbium of general composition Cp2LnDanip (Danip=2,6-di-o-anisylphenyl) is reported, featuring a donor-functionalized terphenyl moiety as a coligand. Depending on the size of the rare-earth element employed, different coordination modes of the terphenyl moiety are observed. In the case of the relatively large samarium cation both methoxy groups from the terphenyl ligand symmetrically coordinate to the metal cation. However, when smaller cations (yttrium and ytterbium) are employed, a flexible, asymmetric coordination mode of the terphenyl ligand is found.

Metallocenes of the f elements are known to be excellent catalysts for a variety of transformations, e.g., hydroamination, hydrosilylation, and hydrogenation reactions.1 A very often encountered problem in such compounds that are based on the unsubstituted Cp ligand is dimerization of the obtained complexes, which can be prevented by use of sterically more demanding ligands such as the Cp* ligand and Cp ligands bearing either tert-butyl or trimethylsilyl substituents or introduction of donor groups to the Cp moiety.2,3 Naturally, use of sterically more demanding or donor-functionalized Cp ligands results in a lower catalytic activity of the resulting complexes. Other concepts involve the use of ansa or constrained-geometry type ligands in order to prevent dimerization while still retaining desirable catalytic activity.4 Over the past two decades there has been much interest in the chemistry of terphenyl compounds of both main-group elements and transition metals.5-9 We previously reported a number of stable terphenyl-based compounds of the f elements employing the donor-functionalized terphenyl ligand

Danip (=2,6-di-o-anisylphenyl),10-12 which can easily be prepared from rather inexpensive starting materials.13 We are now interested in probing the accessibility of unsolvated monomeric metallocene compounds of the lanthanides based on the unsubstituted Cp ligand on one hand, in combination with a donor-functionalized terphenyl moiety vis- a-vis the metallocene framework, thereby attempting to kinetically stabilize a monomeric metallocene unit by coordinative saturation through its potentially reactive site. Previously Niemeyer et al. reported the synthesis of nonfunctionalized terphenyl-based lanthanocene compounds of the composition (CpMe)2LnDmp (Ln = Yb, Er)14,15 and Cp2SmDmp14 (Dmp = 2,6-dimesitylphenyl). These compounds were prepared from DmpLi and the corresponding tris(cyclopentadienyl) precursor compounds. Reaction of equimolar amounts of DanipLi with LnCl3 (Ln=Sm, Y, Yb) in THF at ambient temperature followed by addition of 2 equiv of NaCp at -25 °C produces monomeric unsolvated metallocene compounds of the general composition Cp2LnDanip (Ln = Sm (1), Y (2), Yb (3)) in 50-60% yield (Scheme 1). 1-3 are arene-soluble but insoluble in hexanes. We observed that the order of addition of reagents (DanipLi/NaCp) cannot be reversed for the synthesis of these particular metallocenes; otherwise, formation of tris(cyclopentadienyl) compounds is observed instead. Single crystals of 1 (yellow), 2 3 toluene (colorless), and 3 3 toluene (orange) were obtained by slow evaporation of toluene solutions at ambient temperature inside a glovebox (Table 1). 1-3 show no visible signs of decomposition up to 200 °C. We note that, as a minor side product, formation of a small amount of DanipMe16 is observed, which was isolated from the hexanes cut of the crude product of the reaction mixture and identified by 1H NMR spectroscopy as well as GC/MS. Formation of DanipMe was seen earlier in the analogous reaction using 1 or 2 equiv of KCp* instead of 2 equiv of NaCp.17

*To whom correspondence should be addressed. E-mail: rabe@ chem.tamu.edu. (1) Molander, G. A.; Romero, J. A. C. Chem. Rev. 2002, 102, 2161. (2) Hultzsch, K. C.; Spaniol, T. P.; Okuda, J. Organometallics 1997, 16, 4845. (3) Schumann, H.; Rosenthal, E. C. E.; Demtschuk, J.; Molander, G. A. Organometallics 1998, 17, 5324. (4) Stern, D.; Sabat, M.; Marks, T. J. J. Am. Chem. Soc. 1990, 112, 9558. (5) Wang, Y.; Robinson, G. H. Organometallics 2007, 26, 2. (6) Nguyen, T.; Sutton, A. D.; Brynda, M.; Fettinger, J. C.; Long, G. J.; Power, P. P. Science 2005, 310, 844. (7) Twamley, B.; Haubrich, S. T.; Power, P. P. Adv. Organomet. Chem. 1999, 44, 1. (8) Clyburne, J. A. C.; McMullen, N. Coord. Chem. Rev. 2000, 210, 73. (9) Robinson, G. H. Acc. Chem. Res. 1999, 32, 773. (10) Rabe, G. W.; Berube, C. D.; Yap, G. P. A. Inorg. Chem. 2001, 40, 4780. (11) Rabe, G. W.; Zhang-Presse, M.; Riederer, F. A.; Yap, G. P. A. Inorg. Chem. 2003, 42, 3527.

(12) Rabe, G. W.; Zhang-Presse, M.; Riederer, F. A.; Golen, J. A.; Incarvito, C. D.; Rheingold, A. L. Inorg. Chem. 2003, 42, 7587. (13) Ionkin, A. S.; Marshall, W. J. Heteroat. Chem. 2003, 14, 360. (14) Niemeyer, M.; Hauber, S.-O. Z. Anorg. Allg. Chem. 1999, 625, 137. (15) Niemeyer, M. Acta Crystallogr. 2007, E63, m2188. (16) Rabe, G. W.; Yap, G. P. A. Private communication, CCDC Dep. No. 153825. (17) Rabe, G. W.; Riederer, F. A.; Zhang-Presse, M.; Rheingold, A. L. Organometallics 2007, 26, 5724.

r 2009 American Chemical Society

Published on Web 08/12/2009

Scheme 1. Formation of Danip-Based Monomeric Lanthanocenes

pubs.acs.org/Organometallics

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Rabe et al.

Table 1. Crystallographic Data for Cp2LnDanip (1-3) and Danip[μ-Li(THF)]2I (4)a param formula fw space group a, A˚ b, A˚ c, A˚ β, deg V, A˚3 Z D(calcd), g cm-3 temp, K radiation (λ, A˚) μ(Mo KR), cm-1 GOF R1, % (obsd) wR2, % (all data)

1

2

3

4

C20H20O2Sm 442.71 P41212 9.4197(3) 9.4197(3) 26.592(2)

C30H27O2Y C30H27O2Yb C28H33ILi2O4 508.43 592.56 574.32 P21/n P21/n P21/n 11.5224(10) 11.5714(8) 10.6788(13) 13.4907(11) 13.3246(9) 18.338(2) 15.4089(13) 15.2776(10) 14.1827(18) 93.8110(10) 92.9300(10) 100.334(2) 2359.5(2) 2389.9(3) 2352.5(3) 2732.3(6) 4 4 4 4 1.246 1.413 1.673 1.396 100(2) 100(2) 213(2) 208(2) Mo KR (0.710 73) Mo KR (0.710 73) Mo KR (0.710 73) Mo KR (0.710 73) 24.93 24.65 40.00 12.02 1.090 1.036 1.032 1.039 1.79 3.40 1.85 3.67 4.53 9.04 4.83 10.42 P P P P a The quantity minimized was wR2 = [w(F2o - F2c )2]/ [(wF2o)2]1/2; R1 = Δ/ (Fo), Δ = |(Fo - Fc)|, w = 1/[σ2(F2o) þ (aP)2 þ bP], P = [2F2c þ Max(Fo, 0)]/3.

Figure 1. Molecular structure of 1.

However, it should be pointed out that in the reactions reported in this work formation of such ether-cleavage products clearly is a side reaction only, with the main products being the Danipbased metallocene compounds. The molecular structure of 1 (Figure 1) is comprised of two Cp rings (Cnt-Sm-Cnt=124.4(5)°, Sm-Cnt=2.477(5) A˚, Sm-C(ring) =2.75 A˚ (av)) and a terphenyl moiety coordinating symmetrically in a d,l-type fashion to the metal atom through both methoxy groups (Sm-C(ipso)-C= 122.75(14)°, C(ipso)-C(ortho)-C(ipso0 ) = 121.2(2)°). The dihedral angle between the central phenyl ring and the rings in the ortho positions is 43°. The Sm-C(ipso) distance of 2.520(3) A˚ and the Sm-O distance of 2.5622(15) A˚ are about in the range of those in previously reported DanipSm(THF)COT (Sm-C =2.543(3) A˚; Sm-O= 2.5436(19) and 2.576(2) A˚)12 but are somewhat longer than those in fivecoordinate DanipSm[N(SiHMe2)2]2 (Sm-C = 2.484(4) A˚; Sm-O=2.496(3) and 2.512(3) A˚)11 or in seven-coordinate [DanipSm(μ2-Cl)2(μ2-Cl)Li(THF)2]2 (Sm-C = 2.489(3) A˚; Sm-O=2.503(2) and 2.520(2) A˚).10 The O-Sm-O angle of 143.12(7)° in 1 can be compared, e.g., with the corresponding angles in the also d,l-type bonded DanipSm[N(SiHMe2)2]2 (144.51(9)°)11 and [DanipSm(μ 2 -Cl)2 (μ 2 -Cl)Li(THF)2 ]2 (137.71(8)°),10 as well as in meso-type bonded DanipSm(THF)COT (103.56(6)°).12 In the isomorphous molecular structures of compounds 2 and 3 (Figure 2) an asymmetric or flexible coordination

Figure 2. Molecular structure of 3.

mode of the terphenyl moiety is seen, presumably due to too much steric crowding around the somewhat smaller yttrium and ytterbium cations. In both 2 and 3 only one of the anisyl groups coordinates to the metal cation (M-O = 2.3899(16) A˚ (2) and 2.3602(15) A˚ (3)) with O-M-C(ipso) angles of 77.25(6)° (2) and 78.89(6)° (3), respectively. In both compounds this angle is noticeably larger than the corresponding angle of 71.56(4)° in the symmetrically coordinated samarium compound 1. Additionally, the Cnt-M-Cnt angles of 128.6(5)° (2) and 128.8(5)° (3) are slightly larger than the same angle in the samarium compound, as one would expect as a consequence of the presence of only one coordinating anisyl group. The average metal-ring carbon distances of both Cp rings differ only little in both 2 (2.64 and 2.66 A˚) and 3 (2.61 and 2.62 A˚), with the Cnt-M distances being 2.365(5) and 2.379(5) A˚ for 2 and 2.320(5) and 2.330(5) A˚ for 3. The M-C(ipso) distances (2, 2.4538(19) A˚; 3, 2.402(2) A˚) are shorter than the same distance in the samarium compound. The relevant bond distances in the ytterbium compound 3 can further be compared, for example, with those in DanipYb(Cp*)Cl (Yb-C = 2.415(7) A˚; Yb-O =2.335(5) and 2.340(6) A˚),17 featuring a meso-type bonded terphenyl moiety. As a consequence of the asymmetric bonding mode of the terphenyl ligand, this moiety is slightly tilted in both 2 and 3 with M-C(ipso)-C angles of 119.44(14) and 124.71(14)° in 2

Note

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Table 2. Comparison of Selected Interatomic Separations (A˚) and Angles (deg) of 1-3 complex 1 (M = Sm) 2 (M = Y)

3 (M = Yb)

M-C(ipso) M-O C(ipso)-M-O M-Cipso-Cortho

2.520(3) 2.5622(15) 71.56(4) 122.75(14)

Cipso-CorthoCipso0

121.2(2)

2.402(2) 2.3602(15) 78.89(6) 118.91(14) (C(21)) 125.41(14) (C(17)) 119.60(18) (C(17)) 121.40(18) (C(21))

2.4538(19) 2.3899(16) 77.25(6) 119.44(14) (C(21)) 124.71(14) (C(17)) 118.60(18) (C(17)) 121.60(18) (C(21))

and 118.91(14) and 125.41(14)° in 3. Further bond distances and angles can be derived from Table 2. We note that both the proton and the carbon-13 NMR spectra of diamagnetic 2 show only one signal for both methoxy groups (a very broad signal in the carbon-13 NMR spectrum with ν1/2=ca. 800 Hz). Our efforts to resolve the singlet in the proton NMR spectrum into two signals by lowering the temperature, even to -80 °C in d8-toluene, did not give the desired outcome, though. For reasons of comparison with the Danip-based metallocenes we also wanted to investigate the accessibility and stability of unsubstituted Cp-based metallocene compounds bearing the nonfunctionalized terphenyl ligand Dpp (=2,6diphenylphenyl). However, in our hands such systems of assumed composition Cp2Ln(THF)Dpp (Ln=Sm, Y, Yb), obtained from the reaction of equimolar amounts of DppLi and LnCl3 followed by addition of 2 equiv of NaCp at -25 °C, are found to be of very limited stability only. Our attempts to obtain single-crystal material yielded Cp3Ln(THF) instead, presumably as a result of a ligand redistribution reaction: i.e., ligand scrambling. Finally, we were also interested in extending the idea of employing a donor-functionalized terphenyl moiety vis- a-vis a Cp ligand to divalent lanthanide chemistry. Our attempts to obtain a divalent Danip-based terphenyl precursor compound by reacting equimolar amounts of SmI2(THF)2 and DanipLi in tetrahydrofuran yielded a brown powder. Our repeated efforts to obtain single-crystal material of the obtained crude product from toluene solutions reproducibly gave colorless crystals (in yields of less than 10%) of Danip[μ-Li(THF)]2I (4), which can be described as a double salt or as a tetrahydrofuran-solvated lithium iodide adduct of DanipLi. We attempted a direct synthesis of compound 4 using equimolar amounts of DanipLi and LiI in tetrahydrofuran solution. However, in our hands the reaction did not yield the expected product after evaporation of all volatiles, followed by extraction with toluene, centrifugation, and slow evaporation of the toluene solution at ambient temperature inside a glovebox. The molecular structure of 4 (Figure 3) features two fourcoordinate lithium atoms (Li 3 3 3 Li = 2.663(6) A˚) in distorted-tetrahedral coordination environments, each of them being complexed by one tetrahydrofuran molecule (Li-O= 1.930(4) and 1.901(5) A˚), and one methoxy group from the terphenyl ligand (Li-O=1.960(4) and 1.988(5) A˚). The two other positions of the coordination polyhedron are filled by the bridging ipso carbon atom from the central phenyl ring of the Danip ligand (Li-C=2.162(5) and 2.180(5) A˚; Li-C-Li =75.65(17)°) and a bridiging iodine atom (Li-I=2.786(4) and 2.793(4) A˚; Li-I-Li=57.01(12)°). Each anisyl group of the terphenyl ligand coordinates to a different lithium atom, thereby forming a six-membered chelate ring, resulting in an

Figure 3. Molecular structure of 4.

overall d,l-type coordination of the ligand, with dihedral angles between the central phenyl ring and the two rings in the ortho positions of 50° (C2-C7 ring) and 56° (C14-C19 ring). The molecular structure of 4 can best be compared with the solvent-modified dimeric double salt {Danip[μ-Li(THF)](μ-Li)I}2, featuring a four-step ladder-type structure in the solid state.18 Both structures differ mainly in the amount of THF per lithium atom: i.e., one (4) vs half a molecule in {Danip[μ-Li(THF)](μ-Li)I}2, respectively, thereby resulting in a monomeric vs a dimeric arrangement. Further comparisons of compound 4 can be made with another structurally characterized “halide rich” terphenyl lithium compound, ArDbp2[Li(OEt2)]2I, in which the ipso carbon atom of the central aryl ring is bound to two ethersolvated lithium atoms which are also iodine-bridged.19

Experimental Section The compounds described below were handled under nitrogen using Schlenk double-manifold, high-vacuum, and glovebox (MBraun, Labmaster 130) techniques. Solvents were dried, and physical measurements were obtained by following typical laboratory procedures. NaCp was purchased from Aldrich as a 2.0 M solution in tetrahydrofuran. Prior to use it was crystallized from the purple tetrahydrofuran solution (in a glovebox refrigerator at -25 °C), filtered, washed with hexanes, and dried under vacuum, yielding a colorless powder. DanipI13 was prepared according to the literature. DanipLi was prepared as reported earlier11,18 and was stored in a glovebox refrigerator. SmCl3, YCl3, and YbCl3 were purchased from Aldrich (packaged under argon in ampules) and were used as received. SmI2(THF)2 was purchased from Aldrich as a 0.1 M solution in tetrahydrofuran. Prior to use it was isolated by evaporation of the solvent, washing with hexanes, and drying under vacuum and was stored inside a glovebox refrigerator. NMR spectra were recorded on a JMN-GX 400 instrument. Cp2LnDanip (Ln=Sm, 1; Ln=Y, 2; Ln=Yb, 3). In a glovebox, a freshly prepared solution of DanipLi (296 mg, 1.0 mmol) dissolved in 5 mL of tetrahydrofuran was added to a suspension of LnCl3 (1.0 mmol; 1, 257 mg; 2, 195 mg; 3, 279 mg) in 5 mL of tetrahydrofuran at ambient temperature and stirred for 30 min, which resulted in a yellow (Sm), colorless (Y), or purple (Yb) (18) Rabe, G. W.; Zhang-Presse, M.; Yap, G. P. A. Acta Crystallogr. 2002, E58, m434. (19) Hino, S.; Olmstead, M. M.; Fettinger, J. C.; Power, P. P. J. Organomet. Chem. 2005, 690, 1638.

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solution, respectively.10 The solution was cooled to -25 °C (glovebox refrigerator). A freshly prepared solution of NaCp (176 mg, 2.0 mmol) dissolved in 5 mL of tetrahydrofuran was also cooled to -25 °C. Both solutions were then combined by adding the NaCp solution dropwise to the other solution, and the reaction mixture was then warmed to ambient temperature. Centrifugation and removal of all volatiles followed by washing of the residues several times with a few milliliters of hexanes yielded a solid (1, yellow; 2, colorless; 3, orange), which was extracted with toluene and then centrifuged. Storage of the obtained solutions at -25 °C over several days produced microcrystalline material of compounds 1-3 (1, 285 mg, 50%; 2, 280 mg, 55%; 3, 305 mg, 60%), after removal of the mother liquor, washing of the residues with hexanes, and drying under vacuum for several hours. Characterization Data. 1: 1H NMR (C6D6) δ 0.91 (br, 6H, OMe), 2.50 (br), 3.37 (v br), 5.59 (v br), 6.55 (v br), 8.20 (v br, 10H, Cp), 8.33 (d, aromatic H), 8.65 (t, aromatic H); we failed to detect any signals in the carbon-13 NMR spectra in both deuterated benzene and tetrahydrofuran, which is probably due to both the paramagnetic nature of compound 1 and fluctionality of the molecule in the solution state; IR (Nujol, cm-1) ν 1461 m, 1387 s, 1340 w, 1242 w, 1023 m, 928 w, 860 m, 788 s, 753 s, 666 m, 452 vs, 412 vs. Anal. Calcd for C30H27O2Sm: C, 63.23; H, 4.78. Found: C, 63.43; H, 4.95. 2: 1H NMR (C6D6) δ 3.29 (6H, s, OMe), 5.75 (10H, s, Cp), plus several multipletts in the aromatic region ranging from 6.68 to 7.58 ppm; 13C NMR (C6D6) δ 63.2 (v br, OMe, ν1/2 =ca. 800 Hz), 110.7 (Cp), 124.7, 126.0, 126.7, 131.7, 139.8, 146.1, 156.6, 185.1 (d, JY-C=51 Hz, ipso C); IR (Nujol, cm-1) ν 1291 s, 1232 s, 1175 m, 1158 m, 1107 m, 1046 m, 1008 s, 966 m, 796 m, 720 s, 415 vs. Anal. Calcd for C30H27O2Y: C, 70.87; H, 5.35. Found: C, 71.02; H, 5.52. 3: IR (Nujol, cm-1) ν 1479 m, 1431 m, 1370 s, 1294 m, 1178 m, 1161 s, 1119 m, 1107 s, 1048 m, 110 m, 970 s, 895 w, 849 m, 783 s, 728 s, 666 w, 630 w, 616 m, 584 s, 514 s, 460 s. Anal. Calcd for C30H27O2Yb: C, 60.81; H, 4.59. Found: C, 61.01; H, 4.70. Characterization Data for DanipMe. 1H NMR (C6D6): δ 2.19, 2.23 (s, 3H, Me, approximate ratio: 1:2), 3.20, 3.25 (s, 6H, OMe, approximate ratio: 2:1), plus several multiplets in the aromatic region ranging from 6.5 to 7.3 ppm (m, 11H). GC/MS (EI, 70 eV):

Rabe et al. m/z (%) 304 (100), 273 (16), 258 (7), 202 (6), 152 (4), 119 (6), 101 (6), 39 (2), 28 (6). Danip[μ-Li(THF)]2I (4). In a glovebox, a freshly prepared solution of DanipLi (296 mg, 1.0 mmol) dissolved in 5 mL of tetrahydrofuran was cooled to -25 °C (glovebox refrigerator) and then slowly added, upon stirring, to a blue solution of SmI2(THF)2 (1.0 mmol, 548 mg) in 10 mL of tetrahydrofuran at -25 °C. The reaction mixture was stirred for 30 min and warmed to ambient temperature, resulting in a brown solution. Removal of all volatiles, followed by washing of the residues several times with a few milliliters of hexanes, yielded a brown solid, which was extracted with toluene and then centrifuged. Slow evaporization of the obtained solution inside a glovebox at ambient temperature over a period of several days resulted in decolorization of the solution and formation of colorless crystals of 4 in yields of less than 10%. General Aspects of X-ray Data Collection, Structure Determination, and Refinement for Complexes 1-4. Crystal, data collection, and refinement parameters are given in Table 1. Data were collected on a Bruker Apex AXS CCD system. The systematic absences in the diffraction data are uniquely consistent for the reported space groups. All crystals were mounted on glass fibers with Paratone-N oil and cooled to indicated temperatures. The structures were solved using direct methods, completed by subsequent difference Fourier syntheses, and refined by full-matrix, least-squares procedures. SADABS absorption corrections were applied to all data sets. All nonhydrogen atoms, except those exhibiting disorder, were refined with anisotropic displacement coefficients. All hydrogen atoms were treated as idealized contributions. All software and sources of the scattering factors are contained in the SHELXTL (5.10) program library (G. M. Sheldrick, Siemens XRD, Madison, WI).

Acknowledgment. We thank the Deutsche Forschungsgemeinschaft for financial support. Supporting Information Available: CIF files giving X-ray crystallographic data for 1-4. This material is available free of charge via the Internet at http://pubs.acs.org.