Organometallics 2009, 28, 4649–4651 DOI: 10.1021/om900541r
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Ethylene Coordination and Acetylene Dimerization at Tp0 IrIII Centers Patricia Lara, Joaquı´ n L opez-Serrano, Celia Maya, Margarita Paneque, Manuel L. Poveda,* Luis J. S anchez, Jose E. V. Valpuesta, and Ernesto Carmona* Instituto de Investigaciones Quı´micas, Departamento de Quı´mica Inorg anica, Consejo Superior de Investigaciones Cientı´ficas (CSIC) and Universidad de Sevilla, Avenida Am erico Vespucio 49, Isla de la Cartuja, 41092 Sevilla, Spain Received June 22, 2009 00
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Summary: The compound TpMs Ir(N2) (1 3 N2; TpMs =doubly metalated hydrotris(3-mesitylpyrazol-1-yl)borate ligand) re00 acts with ethylene with formation of the adduct TpMs Ir(C2H4) 00 (2), while with acetylene it gives TpMs Ir(H2CdCHCtCH) (3; the enyne coordinated through the CdC double bond), as the result of the dimerization of the alkyne. Experimental and theoretical studies have been carried out in order to address the 00 kinetic reactivity of the fragment [TpMs Ir] (1) toward C2H4 and C2H2 and the thermodynamic stabilities of the resulting adducts. Alkenes and alkynes are versatile unsaturated hydrocarbons that take part in many important transformations induced by transition-metal compounds.1,2 The bonding between transition metals and alkenes or alkynes is usually discussed with the aid of the Dewar, Chatt, and Duncanson model, which assumes σ donation from a π bond of the hydrocarbon to an empty metal orbital, complemented by π back-bonding from a filled metal d orbital to an antibonding *To whom correspondence should be addressed. E-mail: guzman@ us.es. (1) (a) Elschenbroich, C. Organometallics, 3rd ed.; Wiley-VCH: Weinheim, Germany, 2006. (b) Comprehensive Organometallic Chemistry III; Mingos, D. M. P., Crabtree, R. H., Parkin, G., Eds.; Elsevier: New York, 2007; Vols. 10 and 11. (2) (a) F€ urstner, A.; Davies, P. W. Angew. Chem., Int. Ed. 2007, 46, 3410. (b) Jimenez-N u~ nez, E.; Echavarren, A. M. Chem. Rev. 2008, 108, 3326. (c) Defieber, C.; Gr€utzmacher, H.; Carreira, E. M. Angew. Chem., Int. Ed. 2008, 47, 4482. (d) Johnson, J. B.; Rovis, T. Angew. Chem., Int. Ed. 2008, 47, 840. (3) (a) Dewar, M. J. S. Bull. Soc. Chim. Fr. 1951, 18, C71–C79. (b) Chatt, J.; Duncanson, L. A. J. Chem. Soc. 1953, 2939. (4) See for example: (a) Peng, T. S.; Wang, Y.; Arif, A. M.; Gladysz, J. A. Organometallics 1993, 12, 4535. (b) Casey, C. P.; Chung, S. Inorg. Chim. Acta 2002, 334, 283. (c) Yu, Y.; Smith, J. M.; Flaschenriem, C. J.; Holland, P. L. Inorg. Chem. 2006, 45, 5742. (d) Marchenko, A. V.; Gerard, H.; Eisenstein, O.; Caulton, K. G. New J. Chem. 2001, 25, 1382. (5) (a) Frenking, G.; Frohlich, N. Chem. Rev. 2000, 100, 717. (b) Massera, C.; Frenking, G. Organometallics 2003, 22, 2758. (c) Nechaev, M. S; Ray on, V. M; Frenking, G. J. Phys. Chem. A 2004, 108, 3134. (6) (a) Burgess, B. K.; Lowe, D. J. Chem. Rev. 1996, 96, 2983. (b) Newton, W. E.; Otsuka, S. Molybdenum Chemistry of Biological Significance; Plenum Press: New York, 1980. (7) (a) Parshall, G. W.; Ittel, S. D. Homogeneous Catalysis, 2nd ed.; Wiley: New York, 1992; Chapter 8.2. (b) Bustelo, E.; Dixneuf, P. H. In Handbook of C;H Bond Transformations; Dycker, G., Ed.; Wiley-VCH: Weinheim, Germany, 2005; Vol. 1, Chapter 2. (c) Tachiyama, T.; Yoshida, M.; Aoyagi, T.; Fukuzumi, S. Appl. Organomet. Chem. 2008, 22, 205. (8) (a) Katayama, H.; Ozawa, F. Coord. Chem. Rev. 2004, 248, 1703. (b) Werner, H. Coord. Chem. Rev. 2004, 248, 1693. (c) Schafer, M.; Wolf, J.; Werner, H. Organometallics 2004, 23, 5713. (d) Bianchini, C.; Frediani, P.; Masi, D.; Peruzzini, M.; Zanobini, F. Organometallics 1994, 13, 4616. (e) Slugovc, C.; Mereiter, K.; Zobetz, E.; Schmid, R.; Kirchner, K. Organometallics 1996, 15, 5275. (f) Yi, C. S.; Liu, N.; Rheingold, A. L.; Liable-Sands, L. M. Organometallics 1997, 16, 3910. (g) Boese, W. T.; Goldman, A. S. Organometallics 1991, 10, 782. (h) Le Berre, N.; Kergoat, R.; Kubicki, M. M.; Guerchais, J. E.; L'Haridon, P. J. Organomet. Chem. 1990, 389, 61. r 2009 American Chemical Society
π* orbital of the unsaturated hydrocarbon.3 Several experimental4 and theoretical5 studies have addressed the kinetic or thermodynamic preferences of alkene versus alkyne coordination, a relevant problem in various transformations as, for example, the selectivity of nitrogenase enzymes to reduce acetylene or ethylene,6 the catalytic or stoichiometric dimerization of terminal alkynes to enynes,7-9 the gold, platinum, or other transition metal catalyzed cycloisomerization of enynes, as well as many carbophilic activations.2 In this contribution we wish to report that iridium fragment 1, which contains two Ir-CH2 bonds due to metalation of two o-methyl groups of mesityl substituents of the hydrotris(3-mesitylpyrazol-1-yl)borate ligand, exhibits comparable kinetic selectivity but distinct thermodynamic preference for alkene vs alkyne coordination.
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Reaction 0of the dinitrogen complex10 TpMs Ir(N2) (1 3 N2), Ms 0 represents the doubly metalated TpMs ligand in where Tp fragment 1, with C2H4 (2 bar, 60 C, 3 h) proceeds quantitatively (1H NMR) to yield the ethylene complex 2 (Scheme 1a). Characteristic spectroscopic features in the 1 H NMR spectrum of 2 are four doublets between δ 4.04 and 1.39, due to the two Ir-CH2 units, and an AA0 XX0 pattern centered at 2.85 and 2.42 ppm, attributed to the coordinated molecule of ethylene. The latter observation indicates free olefin rotation around the Ir-C2H4 bond, which is also noticeable in the appearance of a single olefinic 13 C{1H} NMR resonance at 56.2 ppm. The structure of 2 has been confirmed by X-ray diffraction studies (Figure 1). The alkene ligand occupies a trans position with respect to the nitrogen atom of one of the pyrazolyl rings metalated at (9) (a) Ciclosi, M.; Estevan, F.; Lahuerta, P.; Passarelli, V.; Perez-Prieto, J.; Sana u, M. Adv. Synth. Catal. 2008, 350, 234. (b) West, N. M.; White, P. S.; Templeton, J. L. Organometallics 2008, 27, 5252. (c) Jimenez-Tenorio, M.; Puerta, M. C.; Valerga, P. Organometallics 2009, 28, 2787. (10) L opez, J. A.; Mereiter, K.; Paneque, M.; Poveda, M. L.; Serrano, O.; Trofimenko, S.; Carmona, E. Chem. Commun. 2006, 3921. Published on Web 07/30/2009
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Figure 1. ORTEP view of complex 2. Scheme 1. Reactions of Complex 1 3 N2 with C2H4 and C2H2
the mesityl substituent (N3) and eclipses, approximately, the C24;Ir1;N1 vector in the equatorial plane (dihedral angle 12.1). The Ir-C distances are identical within experimental error (2.143(3) and 2.154(3) A˚), and the coordinated CdC bond has a length of 1.402(5) A˚. NOESY experiments uncover that this is the predominant solution rotamer. At variance with this result, the reaction of 1 3 N2 and C2H2 (Scheme 1b) does not permit detection of the expected πC2H2 complex analogous to 2. Vinylacetylene derivative 3, which results from acetylene dimerization and contains the enyne coordinated to iridium through the CdC double bond, is the only detectable product. NMR data, in particular the similarity of the chemical shifts of the sp2 carbon atoms in 2 and 3 and the comparability of NMR data for the -CtCH terminus in 3 and the free enyne,4a suggest coordination of the polyunsaturated ligand of 3 through its carbon-carbon double bond. Only one of the two possible stereoisomers (notice that the CH2dCHR ligand has prochiral faces) is formed, and unequivocal proof of its structure has been obtained by NOESY experiments and X-ray
Lara et al.
Figure 2. ORTEP view of complex 3.
crystallography (Figure 2). The alkene moiety of 3 adopts an orientation similar to that found in 2 (dihedral angle 6.6), but its coordination to iridium is somewhat less symmetrical, as the Ir-C distance to the alkenyl-substituted carbon, C38 (2.171(3) A˚), is slightly longer than that to the terminal carbon C37 (2.142(3) A˚). The coordinated carbon-carbon bond has a length of 1.393(4) A˚, whereas the C39;C40 distance of 1.183(4) A˚ is typical of a CtC triple bond. It seems most probable that compound 3 forms by a mechanism involving coordination of acetylene to fragment 1, followed by a rearrangement that entails C-H bond activation and C-C coupling to a second molecule of C2H2. However, no Ir-C2H2 complex intermediate, or isomeric vinylidene structure, IrdCdCH2, have been detected, which concurs with the scarcity of literature information on iridium species of these types. Thus, only one IrIIIdCdCR211 compound has been characterized by X-ray crystallography (R=SiMe3) and no mononuclear IrIII-π-C2R2 complex has been structurally authenticated. A binuclear complex in which one Ir(III) atom is coordinated to the CtC bond of a Ir-CtCPh unit has been reported recently.12 When the reaction of Scheme 1b is repeated using C2D2, the resulting 3-d4 has an odd deuterium distribution, with only three deuterium atoms in the enyne ligand; one of these is scrambled between the terminal alkene sites. The fourth deuterium distributes among the two methylene sites of one of the Ir-CH2 bonds and some of the o-Me groups of mesityl substituents. Even if the complexity of this experiment prevents drawing definite mechanistic conclusions, it unmistakably shows that, similar to other systems we have previously investigated,13 the C-H bond activation is assisted by one of the Ir-CH2 bonds of fragment 1. To determine the kinetic preference of iridium fragment 1 toward C2H4 and C2H2, we have reacted complex 1 3 N2 with a ca. 1:1 mixture of the two hydrocarbons, at 60 C (2-3 bar total pressure of C2H2 plus C2H4, composition checked by (11) Ilg, K.; Paneque, M.; Poveda, M. L.; Rend on, N.; Santos, L. L.; Carmona, E.; Mereiter, K. Organometallics 2006, 25, 2230. (12) Sau, Y.-K.; Lee, H.-K.; Williams, I. D.; Leung, W.-H. Chem. Eur. J. 2006, 12, 9323. (13) Alvarez, E.; Conejero, S.; Lara, P.; L opez, J. A.; Paneque, M.; Petronilho, A.; Poveda, M. L.; del Rı´ o, D.; Serrano, O.; Carmona, E. J. Am. Chem. Soc. 2007, 129, 14130.
Communication Scheme 2. Reaction of Complex 3 with CO
GC prior to the experiment). A kinetic mixture of 2 and 3 in a ca. 2.5:1 ratio results, but this is accompanied by minor amounts of two so far unidentified products (ca. 25% of the total mixture). Accordingly, and irrespective of the nature of the latter products, C2H4 and C2H2 are trapped by 1 with comparable rates. Vinylacetylene complex 3 is thermally stable and can be heated in solution at 90 C, for several days, without noticeable decomposition. If the heating is carried out under 2 bar of CO for 3 h (Scheme 2), liberation of HCtCCHdCH2 (NMR evidence) and concomitant formation of the carbonyl derivative 4 is observed. The carbonyl adduct 4, which is also obtained from 1 3 N2 and CO at 60 C, features an IR absorption at 2020 cm-1 and a 13C{1H} NMR resonance at δ 169.7 ppm, due to the Ir-CO linkage. We conclude, therefore, that coordination of the enyne through the CdC terminus is thermodynamically more favorable than through the alkyne unit and infer that the Ir-C2H4 complex 2 is thermodynamically more stable than the alleged and undetected Ir-C2H2 intermediate in the route leading to 3. For further confirmation, the reactions of 2 and 3 with C2H2 and C2H4, respectively, have been investigated. Heating ethylene complex 2 with C2H2 (2 bar) at 90 C overnight results in no observable reaction, whereas under identical conditions enyne 3 reacts with C2H4 to yield quantitatively compound 2. Unfortunately, compound 3 decomposes to a complex mixture of products in the presence of acetylene at 90 C, so that the dimerization of C2H2 cannot be performed catalytically. Theoretical support for these findings has also been obtained. DFT calculations at the BHandH level located two minima in the potential energy surface for the ethylene complex 2C (where the subscript C denotes calculated species) and the related π-C2H2 derivative 5C. Complex 2C is 14.4 kJ mol-1 more stable than its undetected acetylene π-C2H2 counterpart. The calculations also reveal that the
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formation of the enyne complex 3C from 5C and acetylene is very exothermic, with an energy return of ca. 267 kJ mol-1. A minimum corresponding to the η2-alkyne-coordinated isomer of 3C (3bC) has been located in the potential energy surface, which lies 18.0 kJ mol-1 higher in energy than the η2-alkene-coordinated species 3C. Replacement of the enyne ligand in 3C by ethylene to yield 2C is exothermic by 11.0 kJ mol-1. However, when the substitution of the enyne in 3C by acetylene to recover intermediate 5C is evaluated, the calculations indicate that this transformation is endothermic by just 3.4 kJ mol-1. An NBO analysis of species 2C and 5C was done in order to rationalize the thermodynamic preference for alkene coordination. Analysis of the second-order perturbation energies reveals that σ donation from the alkene and alkyne ligands is more important than back-donation in both cases. However, in the case of the η2-alkyne complex 5C, the occupied π^ orbital that contributes to the CtC triple bond overlaps with occupied d orbitals localized on the Ir center, causing a repulsive interaction. Such an interaction, which is absent in the alkene complex 2C, may account for the higher stability of the latter type of complexes. In summary, the iridium(III) fragment 1 exhibits comparable kinetic selectivity toward C2H4 and C2H2 but clear thermodynamic preference for ethylene coordination. Since C2H4 is a better donor than C2H2 but a less efficient π-acceptor, it seems that in these complexes the σ component of the bond is comparatively more important than π backdonation.
Acknowledgment. This work was supported by the Spanish Ministry of Science and Innovation (MICINN; grants CTQ2007-62814 and Consolider Ingenio 2010 CSD 2007-0006) and the Junta de Andalucı´ a (group FQM119) (FEDER support). A. Lled os and O. Eisenstein are gratefully acknowledged for useful discussion. The use of computational facilities of the Centro Informatico Cientı´ fico de Andalucı´ a (CICA) is also appreciated. P.L. and J.E.V.V. thank the MICINN for research grants. Supporting Information Available: Text, figures, and tables giving detailed procedures for the synthesis and characterization of complexes 2-4 and computational and crystallographic details and a CIF file giving crystallographic data of 2 and 3. This material is available free of charge via the Internet at http:// pubs.acs.org.
