Zincocenes as Mild Cyclopentadienyl Transfer Reagents toward

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Zincocenes as Mild Cyclopentadienyl Transfer Reagents toward Rhodium(I) Olefin Precursors. Facile Synthesis of (η5-Cp′)Rh(olefin)2 Compounds Ana Cristina Esqueda, Salvador Conejero, Celia Maya, and Ernesto Carmona* Instituto de InVestigaciones Quı´micas, Departamento de Quı´mica Inorga´nica, CSIC, and UniVersidad de SeVilla, AVda. Ame´rico Vespucio 49, 41092 SeVilla, Spain ReceiVed October 30, 2008 Summary: The Rh(I) cyclopentadienyl olefin compounds [(η5C5Me4R)Rh(olefin)2] (R ) CMe3, SiMe3, SiMe2CMe3; olefin ) C2H4, C2H3SiMe3) are readily prepared from the corresponding [RhCl(olefin)2]2 and zincocene, [Zn(C5Me4R)2], precursors. The new compounds undergo hydrogen/deuterium exchange at the olefinic sites in deuterated benzene, at a rate dependent on the cyclopentadienyl substituents. Cyclopentadienyl complexes of the transition metals are among the most widely used compounds in organometallic chemistry and catalysis,1 the cyclopentadienyl fragment being a reliable ligand that has provided very important results in these fields of research. Both the parent cyclopentadienyl (Cp) and substituted cyclopentadienyls (Cp′), in particular pentamethylcyclopentadienyl, are often used, but recently, substituted Cp′ ligands with bulky substituents are gaining importance, as they permit kinetic stabilization of otherwise unstable species, enhance the regioselectivity of some catalytic reactions, and can even act as suitable ligands for atomic layer deposition.2 Generally their synthesis requires the use of cyclopentadienyl derivatives of the s elements, although various other reagents may be used and, for instance, Cp transfer from ZrCp2(NBut)(thf) to iridium has been reported.3 Rhodium cyclopentadienyls of the type (η5-Cp′)Rh(olefin)24 and related compounds have a distinguished history as key precursors in important organometallic transformations such as hydrosilylation,5 C-H bond activation,6 and alkane borylation.7 Along with their iridium counterparts, they are often generated in a two-step process,8 which involves formation of [(η5Cp′)RhCl2]2 dimers,9 followed by reduction to the desired M(I) derivatives.8 Even though the synthesis of the M(III) dimers9 is a general, high-yield procedure, utilization of commercial M(III) chlorides, e.g. RhCl3 · 3H2O, and the generation of HCl as a byproduct thwart its use for Cp′ ligands with substituents * To whom correspondence should be addressed. E-mail: [email protected]. (1) (a) Deck, P. A. Coord. Chem. ReV. 2006, 250, 1032, and references therein. (b) Scheirs, J.; Kaminsky, W. Metallocene-Based Polyolefins; Wiley: Chichester, U.K., 2000; Vols. 1 and 2. (c) Togni, A.; Hayashi, T. Ferrocenes; Verlach Chemie: Weinheim, Germany, 1995. (2) (a) Choukroun, R.; Wolff, F.; Lorber, C.; Donnadieu, B. Organometallics 2003, 22, 2245. (b) Yu, Y.; Bond, A. D.; Leonard, P. W.; Vollhardt, K. P. C.; Whitener, G. D. Angew. Chem., Int. Ed. 2006, 45, 1794. (c) Hatanpa¨a¨, T.; Ritala, M.; Leskela¨, M. J. Organomet. Chem. 2007, 692, 5256. (d) Dutta, B.; Solari, E.; Gauthier, S.; Scopelleti, R.; Severin, K. Organometallics 2007, 26, 4791. (e) Zhang, H.-J; Demerseman, B.; Toupet, L.; Xi, Z.; Bruneau, C. AdV. Synth. Catal. 2008, 350, 1601. (f) Janiak, C.; Schumann, H. AdV. Organomet. Chem. 1991, 23, 291. (g) Ruspic, C.; Moss, J. R.; Schu¨rmann, M.; Harder, S. Angew. Chem., Int. Ed. 2008, 47, 2121. (3) Holland, P. L.; Andersen, R. A.; Bergman, R. G. Organometallics 1998, 17, 433. (4) See for example: (a) King, R. B. Inorg. Chem. 1963, 2, 528. (b) Cramer, R. J. Am. Chem. Soc. 1964, 86, 217. (c) Moseley, K.; Kang, J. W.; Maitlis, P. M. J. Chem. Soc. A 1970, 2875.

prone to hydrolytic cleavage: e.g., the widely employed silylsubstituted cyclopentadienyls C5H4SiMe3, C5Me4SiMe3, C5Me4SiMe2But, and others.10 Indeed, as part of this work, attempts to obtain [(η5-C5Me4SiMe3)RhCl2]2 have yielded instead the C5Me4H derivative [(η5-C5Me4H)RhCl2]2. Since zinc organometallics ZnR2 (R ) alkyl, aryl) are mild hydrocarbyl transfer reagents,11 we have considered the possibility of using zincocenes, ZnCp′2, as also mild, Cp′ transfer reagents. A variety of zincocenes may be generated from ZnCl2 and the appropriate MCp′ reagent (M ) alkali metal).12 Hence, the synthetic methodology herein reported appears to enjoy wide applicability using common [RhCl(olefin)2]2 precursors. Moreover, ZnCp′2 reagents need not be isolated. Reaction of the Rh(I) complex with in situ formed ZnCp′2 (from ZnCl2 and KCp′, 5 h, 20 °C) provides the desired compounds in good isolated yields. To test our hypothesis, we first mixed Zn(C5Me5)212a and [RhCl(C2H4)2]213 in THF, at 0 °C, and allowed them to react at room temperature for 12 h, to obtain (η5-C5Me5)Rh(C2H4)24c in good yields (>85%). Next, the new compound (η5C5Me4But)Rh(C2H4)2 (1) was prepared by a similar procedure using Zn(C5Me4But)212c,d (Scheme 1) and was isolated in ca. 80% yield, following extraction with pentane of the crude (5) (a) Duckett, S. B.; Haddleton, D. M.; Jackson, S. A.; Perutz, R. N.; Poliakoff, M.; Upmacis, R. K. Organometallics 1988, 7, 1526. (b) Duckett, S. B.; Perutz, R. N. J. Chem. Soc., Chem. Commun. 1991, 28. (c) Perutz, R. N.; Haddleton, D. M.; Duckett, S. B.; Belt, S. T. Organometallics 1989, 8, 748. (d) Fernandez, M. J.; Bailey, P. M.; Bentz, P. O.; Ricci, J. S.; Koetzle, T. F.; Maitlis, P. M. J. Am. Chem. Soc. 1984, 106, 5458. (e) Duckett, S. B.; Perutz, R. N. Organometallics 1992, 11, 90. (f) Nikonov, G. J. AdV. Organomet. Chem. 2005, 53, 217. (g) Vyboishchikov, S. F.; Nikonov, G. I. Organometallics 2007, 26, 4160. (6) See for example: (a) Jones, W. D.; Feher, F. J. J. Am. Chem. Soc. 1984, 106, 1650. (b) Jones, W. D. Inorg. Chem. 2005, 44, 4475. (c) Periana, R. A.; Bergman, R. G. J. Am. Chem. Soc. 1986, 108, 7332. (d) Periana, R. A.; Bergman, R. G. J. Am. Chem. Soc. 1986, 108, 7336. (e) Arndtsen, B. A.; Bergman, R. G.; Mobley, T. A.; Peterson, T. H. Acc. Chem. Res. 1995, 28, 154. (7) (a) Chen, H.; Schlecht, S.; Semple, T. C.; Hartwig, J. F. Science 2000, 287, 1995. (b) Hartwig, J. F.; Cook, K. S.; Hapke, M.; Incarvito, C. D; Fan, Y.; Webster, C. E.; Hall, M. B. J. Am. Chem. Soc. 2005, 127, 2538. (c) Hartwig, J. F. In ActiVation and Functionalization of C-H Bonds; Goldberg, K. I., Goldman, A. S., Eds.; American Chemical Society: Washington, DC, 2004; ACS Symposium Series 885. (8) (a) Lenges, C. P.; White, P. S.; Brookhart, M. J. Am. Chem. Soc. 1999, 121, 4385. (b) Lenges, C. P.; Brookhart, M. Angew. Chem., Int. Ed. 1999, 38, 3533. (c) Daugulis, O.; Brookhart, M. Organometallics 2004, 23, 527. (9) White, C.; Yates, A.; Maitlis, P. M.; Heinekey, D. M. Inorg. Synth. 1992, 29, 228. (10) (a) Horacek, H.; Gyepes, R.; Cisarova, I.; Polasek, M.; Varga, V.; Mach, K. Collect. Czech. Chem. Commun. 1996, 61, 1307. (b) Evans, W. J.; Davis, B. L.; Ziller, J. W. Inorg. Chem. 2001, 40, 6341. (c) Hitchcock, P. B.; Kerton, F. M.; Lawless, G. A. J. Am. Chem. Soc. 1998, 120, 10264. (11) Elschenbroich, C. Organometallics, 3rd ed.; Wiley-VCH: Weinheim, Germany, 2006.

10.1021/om801046d CCC: $40.75  2009 American Chemical Society Publication on Web 12/16/2008

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Scheme 1. Synthesis of (η5-Cp′)Rh(olefin)2 Compounds 1-6 using Zincocenes as Cyclopentadienyl Transfer Reagents

reaction product. 1H NMR signals for the Cp′ ring are found with δ 1.38 (But), 1.46 (R-Me), and 1.67 (β-Me), while the C2H4 ligands give rise to broad resonances at 1.52 and 1.98 ppm. In the 13C{1H} NMR spectrum the ethylene carbon nuclei resonate at δ 44.3 and couple with 103Rh (J ) 14 Hz), whereas the quaternary ring carbon nuclei appear at δ 110.2 (CBut) and 99.3 and 93.8 (β-CMe and R-CMe) and exhibit coupling constants to rhodium of ca. 4-5 Hz. The structure of 1 has been determined by X-ray crystallography and will be reported elsewhere. The procedure employed for the generation of 1 is general and can be applied to the synthesis of related complexes with 1,3-dienes, e.g. CH2dC(Me)C(R))CH2 (R ) H, Me), or the alkenylsilane CH2dCHSiMe3, as well as to that of the analogous derivatives of the silyl-substituted cyclopentadienyls C5Me4SiMe3 and C5Me4SiMe2But. As briefly noted earlier, attempts to obtain (η5-C5Me4SiMe3)Rh(C2H4)2 by reaction of RhCl3 · 3H2O with C5Me4(SiMe3)H, followed by reduction with zinc powder,8 were thwarted by the generation in the first step of [(η5-C5Me4H)RhCl2]2, as a result of hydrolytic cleavage of the C-SiMe3 bond of the Cp′ ring. All attempted reactions behave well and give corresponding products in high yields. For comparison with recent work,8 we concentrate here on the C2H4 and C2H3SiMe3 derivatives of the above Cp′ ligands. Accordingly, the vinylsilane complex (η5-C5Me4But)Rh(CH2dCHSiMe3)2 (2) can be obtained as indicated in Scheme 1 but using [RhCl(C2H3SiMe3)2]2 as the starting material, while the related (η5-C5Me4SiMe2R)Rh(olefin)2 derivatives 3-6 are formed from the appropriate Rh(I) olefin and ZnCp′212c precursors (R ) SiMe3, R′ ) H (3), SiMe3 (4); R ) SiMe2But, R′ ) H (5), SiMe3 (6)). The bis(ethene) compounds 3 and 5 feature spectroscopic data similar to those of 1, suggesting a similar structure, subsequently confirmed for 3 by X-ray crystallography (Figure 1). Despite the presence of Me and SiMe3 ring substituents of different inductive effects, coordination of the Cp′ ring is symmetrical, with Rh-C distances in the range 2.26-2.29 Å. Rh-C distances to the C2H4 ligands of ca. 2.12 Å are comparable to literature values for Rh-C2H4 compounds,8c,14 as are the CdC bond lengths of the coordinated ethene molecules (ca. 1.41 Å). The bis(vinylsilane) derivatives 2 and 4 are produced as a major isomer (>95%), and the same applies to (η5C5Me4SiMe2But)Rh(CH2dCHSiMe3)2 (6). This major isomer (12) (a) Blom, R.; Boersma, J.; Budzelaar, P. H. M.; Fischer, B.; Haalan, A.; Volden, H. V.; Weidlein, J. Acta Chem. Scand. 1986, A40, 113. (b) Burkey, D. J.; Hanusa, T. P. J. Organomet. Chem. 1996, 512, 165. (c) Ferna´ndez, R.; Resa, I.; del Rı´o, D.; Carmona, E. Organometallics 2003, 22, 381. (d) Ferna´ndez, R.; Grirrane, A.; Resa, I.; Rodrı´guez, A.; Carmona, ´ lvarez, E.; Gutie´rrez-Puebla, E.; Monge, A.; Lo´pez del Amo, J. M.; E.; A Limbach, H.-H.; Lledo´s, A.; Masseras, F.; del Rı´o, D. , Submitted for publication. (13) Cramer, R. Inorg. Synth. 1974, 15, 14. (14) (a) Nicasio, M. C.; Paneque, M.; Pe´rez, P. J.; Pizzano, A.; Poveda, M. L.; Rey, L.; Sirol, S.; Taboada, S.; Trujillo, M.; Monge, A.; Ruiz, C.; Carmona, E. Inorg. Chem. 2000, 39, 180. (b) Day, V. M.; Stults, B. R.; Reimer, K. J.; Shaver, A. J. Am. Chem. Soc. 1974, 96, 1227.

Figure 1. ORTEP view of complex 3 (50% thermal ellipsoids).

contains equivalent olefins but inequivalent ring R and β sites. Thus, for 2 the olefin ligands yield two complex multiplets at 2.34 (2 H, Hβ) and 1.19 ppm (4 H, HR and Hβ), together with a shielded singlet at 0.08 ppm (18H) due to the SiMe3 substituent. The methyl substituents of the C5Me4But ring appear at 1.98, 1.72, 1.59, and 1.57 ppm. Thus, while the two olefin ligands of these compounds are equivalent by NMR spectroscopy, the two R-Me and the two β-Me rings are not equivalent. These data rule out structures such as A, which possess a plane of symmetry, and are consistent with structures B and C (that would actually interconvert by internal rotation around the Rh-olefin bonds). Despite the lack of rotational symmetry of the Cp′ ring caused by the R substituents, fast rotation of the ring creates an effective C2 axis responsible for the equivalency by NMR of the olefin ligands of these molecules.8a,15 Steric repulsions between olefin and cyclopentadienyl substituents probably favor B, in which the two bulky SiMe3 groups point away from the Cp′ ring.

To complete the characterization of new compounds by chemical methods, incorporation of deuterium from C6D6 into the olefinic sites of vinylsilane derivatives 2, 4, and 6 has been investigated. Hydrogen/deuterium exchange reactions between different carbon centers are of much current importance, since they may provide relevant mechanistic information and allow the preparation of deuterium-labeled organic molecules.16 Hydrogen/deuterium exchange reactions at the olefinic sites of (15) (a) Cramer, R.; Reddy, G. S. Inorg. Chem. 1973, 12, 346. (b) Hauptman, E.; Sabo-Etienne, S.; White, P. S.; Brookhart, M.; Garner, J. M.; Fagan, P. J.; Calabrese, J. C. J. Am. Chem. Soc. 1994, 116, 8038. (16) For recent examples see: (a) Santos, L. L.; Mereiter, K.; Paneque, M.; Slugovc, C.; Carmona, E. New J. Chem. 2003, 27, 107. (b) Yung, C. M.; Skaddan, M. B.; Bergman, R. G. J. Am. Chem. Soc. 2004, 126, 13033. (c) Feng, Y.; Lail, M.; Barakat, K. A.; Cundari, T. R.; Gunnoe, T. B.; Petersen, J. L. J. Am. Chem. Soc. 2005, 127, 14174. (d) Tenn, W. J., III.; Young, K. J. H.; Bhalla, G.; Oxgaard, J.; Goddard, W. A., III.; Periana, R. A. J. Am. Chem. Soc. 2005, 127, 14172. (e) Corbera´n, R.; Sanau´, M.; Peris, E. J. Am. Chem. Soc. 2006, 128, 3974. (f) Hanson, S. K.; Heinekey, D. M.; Goldberg, K. I. Organometallics 2008, 27, 1454. (g) Atzrdot, J.; Derdan, V.; Fey, T.; Zimmermann, J. Angew. Chem., Int. Ed. 2007, 46, 7744.

Communications

(η5-Cp′)Rh(olefin)2 compounds have long been known17 and have been studied recently by Brookhart and co-workers for (η5-C5Me5)Rh(CH2dCHSiMe3)2.8a In C6D6 the 1H NMR signal of the R olefinic protons of the latter compound undergoes a 50% decrease in intensity in ca. 115 min at 50 °C, while the β-hydrogens require 250 min for their intensity to become half of the original values. The exchange is faster than for the related (η5-C5Me5)Rh(C2H4)2, investigated by Jones and co-workers,17b indicating more facile access to the unsaturated, 16-electron [(η5C5Me5)Rh(olefin)] species that permits H/D exchange by activation of C6D6, followed by successive, reversible insertion and reductive elimination steps (for mechanistic details and reaction intermediates, see eq 3 in ref 8a). Replacing C5Me5 by C5Me4But increases the electrondonating properties of the Cp′ ligand, whereas the opposite is expected for C5Me4SiMe3, according to average ν(CO) values for a large series of (CpR)2Zr(CO)2 complexes.18 As judged by corresponding ∆ν values, the change is not very large and to a first approximation can be neglected. Larger differences in the steric effects of the cyclopentadienyl ligands are, however, to be expected, and they may be understood by consideration of their respective solid angles.19a The value reported for C5Me5 is 187°, and since substituent effects are additive,19a solid angles of 197 and 194° can be estimated for C5Me4But and C5Me4SiMe3, respectively. Hence, H/D substitution at the vinylic sites of 2 and 4 should be faster than for the C5Me5 complex analogue. Accordingly, the half-lives for deuterium incorporation into the R sites of 2 and 4 are ca. 10 min (150 min for the C5Me5 derivative8a) but, interestingly, while deuterium incorporation into the β sites of 2 is also faster (50 vs 250 min), it is slower for the SiMe3-substituted complex 4 (ca. 550 min for 50% incorporation of deuterium). This implies that the secondary alkyl intermediate D, which permits H/D exchange at β-olefinic positions, is sterically more hindered for the C5Me4SiMe3 complex 4 than for the C5Me4But analogue 2. This could be due to C-SiMe3 bonds being longer than C-CMe3 bonds (covalent radius values for Csp2, Csp3, and Si are 0.73, 0.76, and 1.11,Å, respectively20), which may force the ring and alkyl SiMe3 substituents in D to occupy the same region of space. In (17) (a) Seiwell, L. P. J. Am. Chem. Soc. 1974, 96, 7134. (b) Jones, W. D.; Duttweiler, R. P. J.; Feher, F. J.; Hessell, E. T. New J. Chem. 1989, 13, 725. (18) Zachmanoglou, C. E.; Docrat, A.; Bridgewater, B. M.; Parkin, G.; Brandow, C. G.; Bercaw, J. E.; Jardine, C. N.; Lyall, M.; Green, J. C.; Keister, J. B. J. Am. Chem. Soc. 2002, 124, 9525.

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fact, in a study of (η5-C5H3R2)Fe(CO)(PR′3)I complexes, for R ) But, SiMe3,19b it has been found that bulk solid angles are a poor measure of the steric demands of the ligands at certain points in space. In accord with this assumption, the half-life for the exchange of the R and β hydrogens of (η5C5Me4SiMe2But)Rh(CH2dCHSiMe3)2 (6) is comparable to that of 4 (15 min for the R proton and 550 min for the β proton at 50 °C).

In summary, a convenient, facile synthesis of (η5Cp′)Rh(olefin)2 compounds has been developed using corresponding zincocenes, ZnCp′2, as the cyclopentadienyl transfer reagents. This avoids low-yield procedures involving insoluble alkali-metal MCp′ derivatives4a or undesirable TlCp′ salts,5a-c and allows the preparation of compounds with silyl-substituted rings that cannot be obtained by other established methods.8 The new compounds described exhibit the expected reactivity in C6D6-assisted H/D exchange processes.

Acknowledgment. Financial support from the Spanish Ministerio de Educacio´n y Ciencia (MEC) (Project No. CTQ2007-62814)andConsolider-Ingenio2010(No.CSD200700006) (FEDER support), the Junta de Andalucı´a (Project Nos. FQM-3151 and FQM-672) is gratefully acknowledged. A.C.E. thanks the CONACYT for a research grant (Ref. No. 229340). Supporting Information Available: Text and figures giving detailed procedures for the synthesis and characterization of complexes 1-6 and a CIF file giving crystallographic data of 3. This material is available free of charge via the Internet at http://pubs.acs.org. OM801046D (19) (a) White, D.; Coville, N. J. AdV. Organomet. Chem. 1994, 36, 95. (b) White, D.; Carlton, L.; Coville, N. J. J. Organomet. Chem. 1992, 440, 15. (20) Cordero, B.; Gomez, V.; Platero-Plats, A. E.; Reves, M.; Echeverria, J.; Cremades, E.; Barragan, F.; Alvarez, S. Dalton Trans. 2008, 2832.