A Cationic Bis(phosphiniminomethanide) Europium(II) Complex

The ionic Eu(II) complex [CH(PPh2NSiMe3)2}Eu(THF)3]BPh4 was ..... Subsequent computations were carried out on a Intel Pentium IV PC or an Intel Core2 ...
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Organometallics 2009, 28, 1266–1269

A Cationic Bis(phosphiniminomethanide) Europium(II) Complex Michal Wiecko† and Peter W. Roesky*,‡ Institut fu¨r Chemie and Biochemie, Freie UniVersita¨t Berlin, Fabeckstrasse 34-36, 14195 Berlin, Germany, and Institut fu¨r Anorganische Chemie, UniVersita¨t Karlsruhe, Engesserstrasse 15, 76128 Karlsruhe, Germany ReceiVed October 14, 2008 Summary: The ionic Eu(II) complex [CH(PPh2NSiMe3)2}Eu(THF)3]BPh4 was synthesized Via the salt elimination reaction of [{CH(PPh2NSiMe3)2}Eu(µ-I)(THF)]2 and NaBPh4 in CH2Cl2. Subsequent reactions led to the Eu(II) compounds [{CH(PPh2NSiMe3)2}2Eu] and [{CH(PPh2NSiMe3)2}Eu(THF){N(PPh2)2}]. In recent years the coordination chemistry of organometallic compounds of the rare earth elements containing a metalcentered cation has attracted much attention.1,2 This interest has been mostly fueled by the synthetic application of isolated or intermediate cationic rare earth compounds in various polymerization reactions of organic monomers.2,3 In this context weakly coordinating anions, in particular the borates BPh4- and B(C6F5)4- or their derivatives, were used to induce charge separation, increase the cationic character of the metal center, and/or predetermine easily accessible coordination sites. While the number of known cationic Ln(III) organyls is therefore rapidly increasing, cationic complexes of the divalent lanthanides remain rare. As first reported by Evans, monocationic complexes of Sm(II) and Yb(II) with the BPh4- counteranion tend to undergo ligand redistribution reactions, forming dicationic species.4 Therefore, protonation of the amide ligand in [(η5C5Me5)Ln{N(SiMe3)2}(THF)2] (Ln ) Sm, Yb) with HNEt2BPh4 resulted in the formation of the fully solvated dications [Ln(THF)n][BPh4]2 (Ln ) Yb, n ) 6; Ln ) Sm, n ) 7) and the corresponding permethylmetallocenes [(η5-C5Me5)2Ln(THF)1,2]. Deacon and co-workers successfully prepared the perfluoroaryllanthanide(II) monocations [(C6F5)Ln(THF)n]+ (Ln ) Yb, * To whom correspondence should be addressed. E-mail: roesky@ chemie.uni-karlsruhe.de. † Freie Universita¨t Berlin. ‡ Universita¨t Karlsruhe. (1) Reviews: (a) Arndt, S.; Okuda, J. AdV. Synth. Catal. 2005, 347, 339. (b) Zeimentz, P. M.; Arndt, S.; Elvidge, B. R.; Okuda, J. Chem. ReV. 2006, 106, 2404. (c) Deacon, G. B.; Evans, D. J.; Forsyth, C. M.; Junk, P. C. Coord. Chem. ReV. 2007, 251, 1699. (2) Recent examples: (a) Robert, D.; Spaniol, T. P.; Okuda, J. Eur. J. Inorg. Chem. 2008, 2801. (b) Tredget, C. S.; Clot, E.; Mountford, P. Organometallics 2008, 27, 3458. (c) Robert, D.; Kondracka, M.; Okuda, J. Dalton Trans. 2008, 2667. (d) Nishiura, M.; Mashiko, T.; Hou, Z. Chem. Commun. 2008, 2019. (e) Zimmermann, M.; To¨rnroos, K. W.; Waymouth, R. M.; Anwander, R. Organometallics 2008, 27, 4310. (f) Kramer, M. U.; Robert, D.; Nakajima, Y.; Englert, U.; Spaniol, T. P.; Okuda, J. Eur. J. Inorg. Chem. 2007, 665. (g) Hayes, P. G.; Piers, W. E.; Parvez, M. Chem. Eur. J. 2007, 13, 2632. (3) For examples, see: (a) Li, X.; Nishiura, M.; Mori, K.; Mashiko, T.; Hou, Z. Chem. Commun. 2007, 4137. (b) Li, S.; Miao, W.; Tang, T.; Cui, D.; Chen, X.; Jing, X. J. Organomet. Chem. 2007, 692, 4943. (c) Luo, Y.; Hou, Z. Organometallics 2006, 25, 6162. (d) Li, X.; Baldamus, J.; Nishiura, M.; Tardif, O.; Hou, Z. Angew. Chem., Int. Ed. 2006, 45, 8184. (e) Bambirra, S.; van Leusen, D.; Tazelaar, C. G. J.; Meetsma, A.; Hessen, B. Organometallics 2007, 26, 1014. (f) Zhang, L.; Suzuki, T.; Luo, Y.; Nishiura, M.; Hou, Z. Angew. Chem., Int. Ed. 2007, 46, 1909. (4) Evans, W. J.; Johnston, M. A.; Greci, M. A.; Gummersheimer, T. S.; Ziller, J. W. Polyhedron 2003, 22, 119.

n ) 5; Ln ) Eu, n ) 6);5 however, as recently reviewed by Junk,1c the attempted ligand exchange reactions of [(C6F5)Yb(THF)5]BPh4 with protic reagents in coordinating solvents always resulted in the formation of the solvated dications [Yb(THF)6][BPh4]2, [Yb([18]-crown-6)(pyridine)2][BPh4]2, and [Yb([18]-crown-6)(NCMe)3][BPh4]2 respectively. In contrast, in “noncoordinating” solvents such as toluene and benzene unsolvated complexes such as [(C5Me5)Ln(µ-η6-Ph)2BPh2]6 (Ln ) Sm, Yb), [{(SiMe3)2N}Yb(THF)(µ-η6-Ph)(µ-η2-Ph)BPh2],7a and [{(SiMe3)2N}Yb(µ-η6-Ph)2BPh2]7b are accessible via protolysis of the corresponding homoleptic compounds. For several years now our group has been exploring monoanionic N-donor ligands as an alternative to cyclopentadiene-based systems. We used the bis(phosphiniminomethanide) {CH(PPh2NSiMe3)2}- as an ancillary ligand in rare earth8 and alkaline earth metal chemistry, and several Ln(II) (Ln ) Eu, Sm, Yb) compounds have already been reported by our group.9,10 Now we are interested in using this ligand to stabilize monocationic Ln(II) compounds in donor solvents. Herein we report on the facile preparation of a rare monocationic Eu(II) complex and its use in subsequent synthesis.

Results and Discussion The ionic compound [{CH(PPh2NSiMe3)2}Eu(THF)3]BPh4 (1) was synthesized by a salt metathesis reaction of [{CH(PPh2NSiMe3)2}Eu(THF)(µ-I)]2 with NaBPh4, following the route shown in Scheme 1. The reaction in CH2Cl2 resulted after 12 h in the formation of a yellow powder, which gave compound 1 after dissolution in THF. The density of NaBPh411 is smaller than that of CH2Cl2 and NaI;12 therefore, the progress of the reaction can be easily monitored by its consumption. However, analogous reactions with the metal centers Yb(II) and Sm(II) failed, presumably due (5) Deacon, G. B.; Forsyth, C. M. Chem. Eur. J. 2004, 10, 1798. (6) Evans, W. J.; Champagne, T. M.; Ziller, J. W. Organometallics 2007, 26, 1204. (7) (a) Deacon, G. B.; Forsyth, C. M. Chem. Commun. 2002, 2522. (b) Deacon, G. B.; Forsyth, C. M.; Junk, P. C. Eur. J. Inorg. Chem. 2005, 817. (8) (a) Gamer, M. T.; Dehnen, S.; Roesky, P. W. Organometallics 2001, 20, 4230. (b) Zulys, A.; Panda, T. K.; Gamer, M. T.; Roesky, P. W. Chem. Commun. 2004, 2584. (c) Zulys, A.; Panda, T. K.; Gamer, M. T.; Roesky, P. W. Organometallics 2005, 24, 2197. (d) Gamer, M. T.; Roesky, P. W.; Rasta¨tter, M.; Steffens, A.; Glanz, M. Chem. Eur. J. 2005, 11, 3165. (e) Rasta¨tter, M.; Zulys, A.; Roesky, P. W. Chem. Commun. 2006, 874. (f) Rasta¨tter, M.; Zulys, A.; Roesky, P. W. Chem. Eur. J. 2007, 13, 3606. (g) Gamer, M. T.; Roesky, P. W.; Palard, I.; Le Hellaye, M.; Guillaume, S. M. Organometallics 2007, 26, 651. (9) Panda, T. K.; Zulys, A.; Gamer, M. T.; Roesky, P. W. J. Organomet. Chem. 2005, 690, 5078. (10) Wiecko, M.; Roesky, P. W.; Burlakov, V. V.; Spannenberg, A. Eur. J. Inorg. Chem. 2007, 876. (11) Arnott, S.; Abrahams, S. C. Acta Crystallogr. 1958, 11, 449. (12) Sathe, N. V.; Phalnikar, N. L.; Bhide, B. V. J. Indian Chem. Soc. 1945, 22, 29.

10.1021/om800989x CCC: $40.75  2009 American Chemical Society Publication on Web 01/20/2009

Notes

Organometallics, Vol. 28, No. 4, 2009 1267 Scheme 1

Table 1. Crystallographic Details of 1 and 3 1 · THF formula formula wt space group a (Å) b (Å) c (Å) R (deg) β (deg) γ (deg) V (Å3) Z Fcalcd (g/cm3) radiation µ (mm-1) T (K) abs cor no. of collected rflns no. of unique rflns no. of obsd rflns no. of data, params R1 wR2 GOF

2 3 · (toluene)

C71H91BEuN2O4P2Si2 C131H164Eu2N8P8Si8 1317.35 2627.10 P1j (No. 2) P1j (No. 2) 11.5552(7) 11.819(2) 16.4704(11) 15.421(3) 17.8916(11) 19.688(4) 89.858(5) 75.96(3) 94.073(5) 82.43(3) 95.634(5) 79.30(3) 3380.1(4) 3406.5(11) 2 1 1.294 1.281 Mo KR (λ ) 0.710 73 Å) 1.059 1.125 200(2) 150(2) numerical 26 239 19 733 11 853 (Rint ) 0.0528) 11 542 (Rint ) 0.0670) 9442 9622 11 853, 743 11 542, 688 0.0370 0.0509 0.0718 0.1304 0.963 1.005

to their higher redox potential and decomposition in CH2Cl2 via redox processes.13 In all cases, in THF, toluene, or their mixtures no reaction was observed. Compound 1 was characterized by standard analytical and spectroscopic techniques. As a result of the strongly paramagnetic center metal, NMR spectroscopy is not very useful for a clear characterization of bis(phosphinimino)methanide complexes of the divalent lanthanide europium. Single crystals of 1 suitable for X-ray structure analysis were obtained by layering a THF solution of the compound with n-pentane. Data collection parameters and selected bond lengths and angles are given in Table 1 and the caption of Figure 1, respectively. Complex 1 crystallizes in the triclinic space group P1j with an additional THF molecule per ion pair remaining in the crystal (Figure 1). In the solid state no close contacts between cation and anion were observed, with the shortest distance Eu · · · CBPh4 being approximately 6.70 Å. The six-membered ring formed by the {CH(PPh2NSiMe3)2}ligand and the Eu(II) center adopts a twist-boat conformation, and the methine carbon and europium atoms are displaced from the P2N2 plane. The observed coordination mode, having a long C1-metal interaction which was discussed earlier, is very common for bis(phosphiniminomethanide) complexes of the rare earth metals.8 In contrast to other cationic rare earth complexes, in which the bond distances are shorter than in their neutral counterparts (e.g., in [(C6F5)Eu(THF)6]+ Eu-C ) 2.735(2) Å;5 in [(C6F5)2Eu(THF)5] Eu-C ) 2.822(2) Å14), we do not see such an effect in compound 1 in comparison to neutral 3+

2+

(13) E°(Ln /Ln ) in H2O: Eu,-0.31 V; Yb,-1.05 V; Sm,-1.55 V: Hollemann, A. F.; Wiberg, E. Lehrbuch der Anorganischen Chemie, 101st ed.; De Gruyter: Berlin, 1995. (14) Forsyth, C. M.; Deacon, G. B. Organometallics 2000, 19, 1205.

bis(phosphiniminomethanide) complexes of europium. The Eu-N and Eu-C distances in compound 1 (Eu-N1 ) 2.579(3) and Eu-N2 ) 2.592(3) Å, Eu-C1 ) 2.943(3) Å) are in range similar to that for other 6-fold-coordinated bis(phosphiniminomethanide) complexes of europium (e.g., in [{CH(PPh2NSiMe3)2}Eu(THF)(µ-I)]2 Eu-N1 ) 2.596(2) Å, Eu-N2 ) 2.563(2)Å,andEu-C1)2.945(2)Åandin[{CH(PPh2NSiMe3)2}Eu{η2-N(PPh2)2)(THF)] Eu-N1 ) 2.539(4) Å, Eu-N2 ) 2.579(4) Å, and Eu-C1 ) 2.878(4) Å).9 On the other hand, the Eu-O distances to the coordinating THF molecules in compound 1 (Eu-O1 ) 2.569(2), Eu-O2 ) 2.602(3), Eu-O3 ) 2.522(3) Å; average Eu-O ) 2.565 Å) are even longer than those in [{CH(PPh2NSiMe3)2}Eu(THF)(µ-I)]2 (Eu-O ) 2.479(2) Å). We suggest that these observations are a result of steric effects. To investigate the synthetic utility of compound 1, we performed two model reactions. The known complex [CH(PPh2NSiMe3)2}Eu{η2-N(PPh2)2)(THF)] (2)9 was obtained by the reaction of compound 1 with K(THF){N(PPh2)2}15 in toluene (Scheme 2). Moreover, reacting compound 1 with 1 equiv of the potassium salt K{CH(PPh2NSiMe3)2}16 led to the homoleptic, THF-free compound [CH(PPh2NSiMe3)2}2Eu] (3) in moderate yields (63%). Complex 3 was also accessible in a higher yield by treatment of EuI2(THF)2 with 2 equiv of K{CH(PPh2NSiMe3)2} in THF (Scheme 2). Compound 3 has been characterized by standard analytical/ spectroscopic techniques. The spectral data of compound 3 are more conclusive than for the ionic compound 1. In the 1H NMR spectrum of compound 3 the hydrogen atom of the methine group can be observed at δ 2.11 ppm. The 31P{1H} NMR shows one singlet for the four equivalent phosphorus atoms at δ 532 ppm. Additionally we recorded high-resolution mass spectra to confirm the composition of compound 3. The solid-state structure of compound 3 was established by X-ray techniques (Figure 2). As expected, due to the very similar ionic radii of Eu2+ and Sm2+ (Eu2+ ) 1.20 Å and Sm2+ ) 1.22 Å for coordination number 7),17 the structure of compound 3 is related to those of the known samarium complexes [{CH(PPh2NSiMe3)2}2Sm] and [{CH(PPh2NMes)2}2Sm] (Mes ) 2,4,6-Me3C6H3).18 Thus, the two {CH(PPh2NSiMe3)2}ligands coordinate asymmetrically via the nitrogen atoms in a chelating fashion with long Eu-N (Eu-N1 ) 2.778(4) and Eu-N4 ) 2.765(4) Å) and short Eu-N (Eu-N2 2.605(4) and Eu-N3 2.602(4) Å) bond lengths for each ligand. Additionally an overall 6-fold coordination of the metal center is achieved by coordination of the methine carbon atoms. The Eu-Cmethine distances in compound 3 (Eu-C1 ) 2.865(5), Eu-C32 ) 2.880(5) Å) are significantly shorter than in the cation of (15) Roesky, P. W.; Gamer, M. T.; Puchner, M.; Greiner, A. Chem. Eur. J. 2002, 8, 5265. (16) Gamer, M. T.; Roesky, P. W. Z. Anorg. Allg. Chem. 2001, 627, 877. (17) Shannon, R. D. Acta Cryst. 1976, A32, 751-760. (18) Hill, M. S.; Hitchcock, P. B. Dalton Trans. 2003, 4570.

1268 Organometallics, Vol. 28, No. 4, 2009

Notes

Figure 1. Molecular structure of 1. Hydrogen atoms are omitted for clarity. Ellipsoids are drawn to encompass 30% probability. Selected bond lengths (Å) and angles (deg): Eu-N1 ) 2.579(3), Eu-N2 ) 2.592(3), Eu-O1 ) 2.569(2), Eu-O2 ) 2.602(3), Eu-O3 ) 2.522(3), Eu-C1 ) 2.943(3), N1-P1 ) 1.577(3), N1-Si1 ) 1.693(3), N2-P2 ) 1.595(3), N2-Si2 ) 1.713(3), C1-P1 ) 1.729(3), C1-P2 ) 1.734(3); N1-P1-C1 ) 111.50(15), N2-P2-C1 ) 122.3(2), N1-Eu-N2 ) 85.49(8), P1-C1-P2 ) 129.9(2), O3-Eu-O1 ) 97.40(9), O3-Eu-O2 ) 80.24(11), O1-Eu-O2 ) 76.07(9). Scheme 2

compound 1 and also shorter than in the related Sm(II) complexes (Sm-Cmethine ) 2.8991(15) and 2.8984(14) Å in [{CH(PPh2NSiMe3)2}2Sm];10 Sm-Cmethine ) 2.900(5) and 2.877(5) Å in [{CH(PPh2NMes)2}2Sm]).18

Summary In summary, we have prepared the bis(phosphiniminomethanide) Eu(II) monocation [{CH(PPh2NSiMe3)2}Eu(THF)3]+. In the solid state the title compound showed no close contacts between cation and anion. Further reaction of [{CH(PPh2NSiMe3)2}Eu(THF)3]BPh4 with potassium reagents resulted in the neutral Eu(II) compounds [{CH(PPh2NSiMe3)2}2Eu] and [{CH(PPh2NSiMe3)2}Eu(THF){N(PPh2)2}], while KBPh4 is formed as a byproduct.

Experimental Section General Considerations. All manipulations of air-sensitive materials were performed with the rigorous exclusion of oxygen

and moisture in flame-dried Schlenk-type glassware either on a dualmanifold Schlenk line, interfaced to a high-vacuum (10-3 Torr) line, or in an argon-filled MBraun glovebox. THF was predried over potassium and distilled under nitrogen from Na/K alloy benzophenone ketyl prior to use. Hydrocarbon solvents (toluene and n-pentane) were distilled under nitrogen from LiAlH4. All solvents for vacuum-line manipulations were stored in vacuo over LiAlH4 in resealable flasks. Deuterated solvents were obtained from Chemotrade or Euriso-Top GmbH (99 atom % D). NMR spectra were recorded on a Jeol JNM-LA 400 FT-NMR spectrometer. Chemical shifts are referenced to internal solvent resonances and are reported relative to tetramethylsilane. Raman spectra were performed on a Bruker RFS 100. Elemental analyses were carried out with an Elementar Vario EL III. Mass spectra were acquired with a Finnigan MAT 711 mass spectrometer at an ionization potential of 70 eV and were reported as mass/charge (m/e) with percent relative abundance. [{CH(PPh2NSiMe3)2}Eu(THF)(µ-I)]2 was prepared following a literature procedure.9 [{CH(PPh2NSiMe3)2}Eu(THF)3]BPh4 (1). To a mixture of [{CH(PPh2NSiMe3)2}Eu(THF)(µ-I)]2 (227 mg, 0.12 mmol) and

Notes

Figure 2. Molecular structure of 3. Hydrogen atoms are omitted for clarity. Ellipsoids are drawn to encompass 30% probability. Selected bond lengths (Å) and angles (deg): Eu-N1 ) 2.778(4), Eu-N2 ) 2.605(4), Eu-N3 ) 2.602(4), Eu-N4 ) 2.765(4), Eu-C1 ) 2.865(5), Eu-C32 ) 2.880(5), N1-P1 ) 1.592(4), N2-P2 ) 1.577(5), N3-P3 ) 1.595(4), N4-P4 ) 1.576(5); P1-C1-P2 ) 124.7(3), P3-C32-P4 ) 125.5(3), N2-Eu-N3 ) 106.52(13), N3-Eu-N4 ) 85.70(12), N2-Eu-N4 ) 135.83(14), N3-Eu-N1 ) 135.08(13), N2-Eu-N1 ) 85.49(13), N4-Eu-N1 ) 115.77(12), C1-Eu-C32 ) 132.38(13). NaBPh4 (83 mg, 0.26 mmol) was added CH2Cl2 (10 mL). The mixture was stirred for 12 h at room temperature and filtered through a glass frit. The solvent was removed in vacuo, and the obtained yellow powder was dissolved in 3-5 mL of THF. n-Pentane was layered on top of the solution, yielding 87 mg (28%) of 1 as pale yellow crystals. 1H NMR (d8-THF, 400 MHz, 20 °C): δ 0.08 (s, 18 H, SiMe3), 6.50-8.00 (m, 40 H, Ph). Raman (solid): 609 (m), 667 (w), 923 (w), 998 (s), 1029 (m), 1103 (w), 1581 (w), 2893 (br), 2950 (br), 3047 (s) cm-1. Anal. Calcd for C67H83BEuN2O3P2Si2 (Mr ) 1245.28): C, 64.62; H, 6.72. N, 2.25. Found: C, 63.84; H, 6.51; N, 2.02. [CH(PPh2NSiMe3)2}Eu{η2-N(PPh2)2)(THF)] (2). The synthesis was performed following the route A to 3. Recrystallization of the crude product from toluene yielded 160 mg (55%) of 2 as single crystals. The data obtained from a single-crystal X-ray structure analysis were consistent with the structural data of compound 2 reported earlier.9 [{CH(PPh2NSiMe3)2}2Eu] (3). Route A. To a mixture of K{CH(PPh2NSiMe3)2} (149 mg, 0.25 mmol) and 1 (311 mg, 0.25 mmol) was added toluene (15 mL). The mixture was stirred overnight and filtered through a glass frit. The solution was concentrated to 5 mL, and the precipitate that formed recrystallized from the hot solvent, yielding 200 mg (63%) of crystalline 3 as large orange blocks. Route B. EuI2(THF)2 (275 mg, 0.5 mmol) and K{CH(PPh2NSiMe3)2} (597 mg, 1.0 mmol) were dissolved in THF (15 mL) and stirred for 16 h. The solvent was removed and the

Organometallics, Vol. 28, No. 4, 2009 1269 residue suspended in toluene (15 mL). Filtration and removal of the toluene yielded 445 mg of (86%) 3. 1 H NMR (C6D6, 400 MHz, 21 °C): δ 0.26 (s, 36 H, SiMe3), 2.11 (br, 2 H, CH), 6.90-7.15 (m, 16 H, Ph), 7.65-7.75 (m, 24 H, Ph). 31P{1H} NMR (C6D6, 160.7 MHz, 21 °C): δ 532 ppm. MS (EI, 240 °C): 1267 (M+, 4), 709 (M+ - CH(PPh2NSiMe3)2, 13), 543 (CH2(PPh2NSiMe3)2+ - CH3, 100), 454 (14). Raman (solid): 613 (m), 784 (w), 999 (s), 1028 (m), 1103 (w), 1157 (w), 1572 (w), 1588 (m), 2895 (br), 2951 (br), 3057 (s) cm-1. Anal. Calcd for C62H78N4P4Si4Eu (Mr ) 1267.51): C, 58.75; H, 6.20; N, 4.42. Found: C, 58.75; H 5.76; N, 4.07. X-ray Crystallographic Studies of 1 and 3. Crystals of 1 were obtained from THF/n-pentane, and crystals of 3 were obtained from toluene. A suitable crystal was covered in mineral oil (Aldrich) and mounted onto a glass fiber. The crystal was transferred directly to a cold N2 stream of a STOE IPDS 2T diffractometer. Subsequent computations were carried out on a Intel Pentium IV PC or an Intel Core2 Duo PC. All structures were solved by the Patterson method (SHELXS-9719). The remaining non-hydrogen atoms were located from successive difference Fourier map calculations. The refinements were carried out by using full-matrix least-squares techniques on F, minimizing the function (Fo - Fc),2 where the weight is defined as 4Fo2/2(Fo2) and Fo and Fc are the observed and calculated structure factor amplitudes using the program SHELXL-97,20 respectively. Carbon-bound hydrogen atom positions were calculated. The hydrogen atom contributions were calculated but not refined. The final values of refinement parameters are given in Table 1. The locations of the largest peaks in the final difference Fourier map calculation as well as the magnitude of the residual electron densities in each case were of no chemical significance. Positional parameters, hydrogen atom parameters, thermal parameters, and bond distances and angles have been deposited as Supporting Information. Crystallographic data (excluding structure factors) for the structures reported in this paper have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication nos. CCDC-710924 (1) and CCDC-710925 (3). Copies of the data can be obtained free of charge on application to the CCDC, 12 Union Road, Cambridge CB2 1EZ, U.K. (fax (+(44)1223-336033; e-mail [email protected]).

Acknowledgment. This work was supported by the Deutsche Forschungsgemeinschaft and the Fonds der Chemischen Industrie. Supporting Information Available: CIF files giving X-ray crystallographic data for the structure determinations of 1 and 3. This material is available free of charge via the Internet at http://pubs.acs.org. OM800989X (19) Sheldrick, G. M. SHELXS-97: Program for Crystal Structure Solution; University of Go¨ttingen, Go¨ttingen, Germany, 1997. (20) Sheldrick, G. M. SHELXL-97: Program for Crystal Structure Refinement; University of Go¨ttingen, Go¨ttingen, Germany, 1997.