New Shapes of PC(sp3)P Pincer Complexes - Organometallics (ACS

Oct 22, 2009 - Trans-chelating bis(diisopropylphosphino)triptycene (1) was employed as a platform for the construction of a new class of C(sp3)-metala...
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Organometallics 2009, 28, 6578–6584 DOI: 10.1021/om900723s

New Shapes of PC(sp3)P Pincer Complexes Clarite Azerraf and Dmitri Gelman* Institute of Chemistry, The Hebrew University of Jerusalem, Jerusalem, 91904 Israel Received August 18, 2009

Trans-chelating bis(diisopropylphosphino)triptycene (1) was employed as a platform for the construction of a new class of C(sp3)-metalated pincer complexes via C-H activation. We synthesized and fully characterized platinum(II), ruthenium(II), and iridium(III) compounds bearing this tridentate ligand. In contrast to the known all-aliphatic C(sp3)-metalated compounds, the new complexes exhibit excellent thermal and conformational stability even under very harsh and noninert conditions, likely due to the lack of labile R- or β-hydrogens. In addition, the three-dimensional barrelene-based scaffold represents a unique structural motif for the design of PC(sp3)P-based compounds with unexplored steric and electronic features.

Introduction Carbometalated pincer-type complexes were pioneered more than 40 years ago.1-3 Since then, these compounds have been studied extensively and became very popular in many areas of chemistry. For instance, they are widely used as homogeneous catalysts and stoichiometric promoters in a variety of chemical transformations,4 as building blocks for the construction of advanced functional materials,5 and as components in supramolecular systems.5,6 Furthermore, carbometalated pincer complexes were found relevant to biological applications.7 The popularity of pincer complexes is likely due to their unique stability/reactivity balance; unlike many other classes *To whom correspondence should be addressed. E-mail: dgelman@ chem.ch.huji.ac.il. Fax: þ972-2-6585279. (1) (a) Creaser, C. S.; Kaska, W. C. Inorg. Chim. Acta 1978, 30, L325. (b) Van Koten, G.; Timmer, K.; Noltes, J. G.; Spek, A. L. J. Chem. Soc., Chem. Commun. 1978, 250. (2) Empsall, H. D.; Hyde, E. M.; Markham, R.; McDonald, W. S.; Norton, M. C.; Shaw, B. L.; Weeks, B. J. Chem. Soc., Chem. Commun. 1977, 589. (3) Moulton, C. J.; Shaw, B. L. J. Chem. Soc., Dalton Trans. 1976, 1020. (4) (a) Benito-Garagorri, D.; Kirchner, K. Acc. Chem. Res. 2008, 41, 201. (b) Ghosh, R.; Zhang, X.; Achord, P.; Emge, T. J.; Krogh-Jespersen, K.; Goldman, A. S. J. Am. Chem. Soc. 2007, 129, 853. (c) Goldman, A. S.; Roy, A. H.; Huang, Z.; Ahuja, R.; Schinski, W.; Brookhart, M. Science 2006, 312, 257. (d) Jensen, C. M. Chem. Commun. 1999, 2443. (e) Morales-Morales, D. Mini-Rev. Org. Chem. 2008, 5, 141. (f) van der Boom, M. E.; Milstein, D. Chem. Rev. 2003, 103, 1759. (g) Zhao, J.; Goldman, A. S.; Hartwig, J. F. Science 2005, 307, 1080. (5) (a) Albrecht, M.; van Koten, G. Angew. Chem., Int. Ed. 2001, 40, 3750. (b) Lang, H.; Packheiser, R.; Walfort, B. Organometallics 2006, 25, 1836. (6) (a) Gerhardt, W. W.; Weck, M. J. Org. Chem. 2006, 71, 6333. (b) Gerhardt, W. W.; Zucchero, A. J.; Wilson, J. N.; South, C. R.; Bunz, U. H. F.; Weck, M. Chem. Commun. 2006, 2141. (c) Stiriba, S.-E.; Slagt, M. Q.; Kautz, H.; Gebbink, R. J. M. K.; Thomann, R.; Frey, H.; Van Koten, G. Chem.—Eur. J. 2004, 10, 1267. (d) Wieczorek, B.; Dijkstra, H. P.; Egmond, M. R.; Gebbink, R. J. M. K.; van Koten, G. J. Organomet. Chem. 2009, 694, 812. (7) (a) Huang, Y.; Zhong, A.; Rong, C.; Xiao, X.; Liu, S. J. Phys. Chem. A 2008, 112, 305. (b) Melaiye, A.; Simons, R. S.; Milsted, A.; Pingitore, F.; Wesdemiotis, C.; Tessier, C. A.; Youngs, W. J. J. Med. Chem. 2004, 47, 973. pubs.acs.org/Organometallics

Published on Web 10/22/2009

of organometallics, the carbon-metal bonds of the pincertype compounds are atypically strong due to the presence of two donor groups that form two thermodynamically favored ipso-C-M bond-sharing five-membered rings (Figure 1).8,9 On the other hand, their steric and electronic properties can be easily tuned by modifying metal, substituents, or donor atoms to achieve the desired level of reactivity. Arguably, the most extreme change in the electronic properties of such compounds can be achieved upon changing hybridization of the metalated carbon from sp2 to sp3, because the stronger σ-donating nature of the last significantly increases electron density at the metal center. However, despite the great fundamental interest in the electronrich complexes, the majority of studies performed so far have focused on sp2- rather than sp3-carbometalated transition metal ones (Figure 1, top), possibly because excessive flexibility of all-aliphatic ligands complicates the selective synthesis of structurally well-defined species. Additionally, the presence of easily abstractable R- and β-hydrogens makes them less thermally stable2 and more difficult to manipulate (Figure 1, bottom).10 Of course, C(sp3)-cyclometalated pincer compounds lacking labile β-hydrogens have been previously described. To illustrate, Milstein and co-workers synthesized a series of complexes possessing a benzylic C(sp3)-metal bond.11 Another approach was suggested by Kaska and Mayer, who (8) (a) Shaw, B. L. J. Organomet. Chem. 1980, 200, 307. (b) Van Koten, G. Pure Appl. Chem. 1989, 61, 1681. (9) However, other coordination modes in pincer-type compounds are also known. See for example: (a) Minakawa, M.; Takenaka, K.; Uozumi, Y. Eur. J. Inorg. Chem. 2007, 1629 (monodenate); (b) Carre, F.; Chuit, C.; Corriu, J. P.; Fanta, A.; Mehdi, A.; Reye, C. Organometallics 1995, 14, 194 (bidentate); (c) Albrecht, M.; James, S. L.; Veldman, N.; Spek, A. L.; van Koten, G. Can. J. Chem. 2001, 79, 709 (bridging); (d) Abbenhuis, H. C. L.; Feiken, N.; Grove, D. M.; Jastrzebski, J. T. B. H.; Kooijman, H.; Van der Sluis, P.; Smeets, W. J. J.; Spek, A. L.; Van Koten, G. J. Am. Chem. Soc. 1992, 114, 9773 (fac-terdentate). (10) (a) Al-Salem, N. A.; Empsall, H. D.; Markham, R.; Shaw, B. L.; Weeks, B. J. Chem. Soc., Dalton Trans. 1979, 1972. (b) Al-Salem, N. A.; McDonald, W. S.; Markham, R.; Norton, M. C.; Shaw, B. L. J. Chem. Soc., Dalton Trans. 1980, 59. (c) Crocker, C.; Errington, R. J.; Markham, R.; Moulton, C. J.; Odell, K. J.; Shaw, B. L. J. Am. Chem. Soc. 1980, 102, 4373. r 2009 American Chemical Society

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took advantage of a cycloheptatriene scaffold lacking β-hydrogens to design a new type of pincer diphosphine ligand.12 However, the number of such examples is very limited and the design of new modular platforms for the construction of reasonably stable sp3-carbometalated transition metal complexes is much desired in order to stimulate their utilization in different areas of chemistry. Recently, we demonstrated the utilization of the barrelenetype scaffold, namely, 1,8-bis(diisopropylphosphino)triptycene (1), as a platform for the construction of the first representative of a new class of sp3-carbometalated complexes.13 The complex Ir-1 was synthesized using the traditional metal-induced C-H activation strategy according to eq 1. Unfortunately, the method was not very versatile, and other metal precursors failed to displace the methine hydrogen upon coordination to 1.13,14

However, in contrast to the known all-aliphatic C(sp3)metalated compounds, the novel Ir-1 exhibited excellent thermal and conformational stability even under very harsh and noninert conditions, likely due to the lack of labile R- or β-hydrogens. Moreover, it proved itself as a highly active catalyst for the transfer hydrogenation of ketones. This justified our further attempts to expand the scope of the method. In this article, we wish to report on the synthesis, characterization, and properties of new complexes that now constitute an entire series of robust C(sp3)-metalated pincer compounds (PC(sp3)Ps).

Results and Discussion Synthesis of Platinum and Ruthenium Complexes via C-H Activation. It would be reasonable to expect that the coordination of the trans-chelating 1,8-bis(diisopropylphosphino)triptycene (1) to a metal precursor15-17 will bring about a (11) (a) Gozin, M.; Weisman, A.; Ben-David, Y.; Milstein, D. Nature 1993, 364, 699. (b) Ohff, M.; Ohff, A.; van der Boom, M. E.; Milstein, D. J. Am. Chem. Soc. 1997, 119, 11687. (c) Rybtchinski, B.; Vigalok, A.; BenDavid, Y.; Milstein, D. J. Am. Chem. Soc. 1996, 118, 12406. (d) van der Boom, M. E.; Liou, S.-Y.; Shimon, L. J. W.; Ben-David, Y.; Milstein, D. Inorg. Chim. Acta 2004, 357, 4015. (f) Vigalok, A.; Rybtchinski, B.; Shimon, L. J. W.; Ben-David, Y.; Milstein, D. Organometallics 1999, 18, 895. (12) (a) Nemeh, S.; Flesher, R. J.; Gierling, K.; Maichle-Moessmer, C.; Mayer, H. A.; Kaska, W. C. Organometallics 1998, 17, 2003. (b) Winter, A. M.; Eichele, K.; Mack, H.-G.; Kaska, W. C.; Mayer, H. A. Organometallics 2005, 24, 1837. (c) Winter, A. M.; Eichele, K.; Mack, H.-G.; Kaska, W. C.; Mayer, H. A. Dalton Trans. 2008, 527. (13) Azerraf, C.; Gelman, D. Chem.;Eur. J. 2008, 14, 10364. (14) Azerraf, C.; Shpruhman, A.; Gelman, D. Chem. Commun. 2009, 466. (15) Azerraf, C.; Grossman, O.; Gelman, D. J. Organomet. Chem. 2007, 692, 761. (16) (a) Bini, L.; Mueller, C.; Wilting, J.; Von Chrzanowski, L.; Spek, A. L.; Vogt, D. J. Am. Chem. Soc. 2007, 129, 12622. (b) Schnetz, T.; Roeder, M.; Rominger, F.; Hofmann, P. Dalton Trans. 2008, 2238. (17) Grossman, O.; Azerraf, C.; Gelman, D. Organometallics 2006, 25, 375.

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Scheme 1

spontaneous abstraction of the only available methine hydrogen as for other trans-chelating ligands possessing similar coordinating properties (Scheme 1).18,19 Thus, the most likely reason for the inertness of the methine hydrogen in 1 is an insufficient acidity of the bridgehead hydrogens in the rigid tricyclic systems,20 and the use of larger and more electrophilic metal precursors could favor metalation in this case. Initially we focused on the reaction between 1 and different platinum precursors. Unfortunately, we observed very little (if any) formation of C-metalated species upon heating the DMF or diethyleneglycol monomethyl ether solution of the ligand with trans-PtCl2(CH3CN)2, K2PtCl4, or PtCl2(COD), and only the known nonmetalated [PtCl2(1)] complex resulted from the reaction.15 However, when the preformed [PtCl2(1)] complex was treated with AgBF4 in diethyleneglycol monomethyl ether and heated to 120 °C for 20 h under nitrogen in the presence of diisopropylethylamine as a base (eq 2), the desired compound Pt-1 was observed and isolated in 50% yield.

The existence of the C(sp3)-Pt bond in Pt-1 was initially implied by NMR spectroscopic analysis. The 1H NMR spectrum of the nonmetalated [PtCl2(1)] complex shows a very characteristic low-field resonance at 8.95 ppm for the central methine hydrogen.15 This signal does not appear in the spectrum of Pt-1. 31P{1H} NMR measurements of Pt-1 indicated a signal centered at 59.7 ppm flanked by 195Pt satellites with a 1JPt-P coupling constant of 1518 Hz. The magnitude of the coupling constant implies the trans-located phosphine donors.21 The trans disposition of the phosphorus atoms in Pt-1 was also supported by the presence of virtual triplets in the 13C{1H} NMR for P-neighboring R- and β- carbons (18) (a) Haenel, M. W.; Jakubik, D.; Krueger, C.; Betz, P. Chem. Ber. 1991, 124, 333. (b) Haenel, M. W.; Oevers, S.; Angermund, K.; Kaska, W. C.; Fan, H.-J.; Hall, M. B. Angew. Chem., Int. Ed. 2001, 40, 3596. (c) McLoughlin, M. A.; Flesher, R. J.; Kaska, W. C.; Mayer, H. A. Organometallics 1994, 13, 3816. (d) Neo, K. E.; Huynh, H. V.; Koh, L. L.; Henderson, W.; Hor, T. S. A. Dalton Trans. 2007, 5701. (e) Sundermann, A.; Uzan, O.; Milstein, D.; Martin, J. M. L. J. Am. Chem. Soc. 2000, 122, 7095. (19) Olsson, D.; Arunachalampillai, A.; Wendt, O. F. Dalton Trans. 2007, 5427. (20) Schwartz, L. H. J. Org. Chem. 1968, 33, 3977. (21) (a) Dixon, K. R.; Fakley, M.; Pidcock, A. Can. J. Chem. 1976, 54, 2733. (b) Hitchcock, P. B.; Jacobson, B.; Pidcock, A. J. Chem. Soc., Dalton Trans. 1977, 2038.

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Figure 1

of the complex. This splitting of the phosphorus-coupled carbon signals into 1:2:1 triplets is characteristic of compounds forming AXX0 spin systems where two phosphorus nuclei couple with a large coupling constant (e.g. trans diphosphine complexes).22 It is interesting to note, however, that the signal assigned to the metalated carbon (δ 55.8 ppm) appears as a singlet. Although the absence of the splitting was observed previously by others,19 this was unexpected for us because it was visible in a parent nonmetalated [PtCl2(1)].15 X-ray crystallographic analysis unequivocally proved the structural arrangement of Pt-1 (Figure 2; the crystal data and collection details are listed in Table 1). As one would expect for the complexes bearing strongly bent triptycene-based ligands, the platinum center in Pt-1 is distorted from the ideal square-planar geometry toward a butterfly-like environment. For example, the observed interplanar angle between the C(1)-Pt(1)-P(1) and C(1)-Pt(1)-P(2) planes is 157.9°, while the C(1)-Pt(1)-Cl(1) angle is 179.34(2)o. Interestingly, despite a suggested stronger σ-donating character of the sp3-hybridized carbon, the bond lengths are within the normal range. Encouraged by this success, we decided to employ this protocol to the synthesis of a ruthenium analogue. Treatment of RuCl3 and 1 in anhydrous DMF even in the absence of AgBF4 leads to the formation of two new substances in ca. 10:1 ratio according to the 31P{1H} NMR analysis. The major product (Ru-1) was isolated in 30% yield and first characterized using NMR, which was very helpful in predicting the structure of the complex, as follows. The absence of a low-field-shifted signal characteristic of trans-chelated complexes of this type gave the first indication of the metalated nature of the product. The equivalence of the phosphorus nuclei was confirmed by the presence of one singlet resonance (70.1 ppm) in the room-temperature 31 P{1H} NMR spectrum. Here again, the triplet splitting of the signals assigned to R- and β-carbons of the complex in the 13 C{1H} NMR indicated the trans location of the phosphorus donors. However, unlike Pt-1, the metalated carbon in Ru-1 appeared as a virtual triplet centered at 69.9 ppm (JP-C =4 Hz). Unexpectedly, the 13C{1H} NMR spectrum showed the presence of two triplet signals centered at 202.1 (JP-C=120 Hz) and 196.2 (JP-C = 80 Hz) ppm, which could point to the presence of two different CO ligands. The assumption was

also confirmed by FT-IR, which displayed two νCO bands at 2027 and 1941 cm-1, typical of ruthenium carbonyl bonds.23 Considering this and that ruthenium is at most hexacoordinate, we could think only of the formation of a bis-carbonylated Ru(II) pincer complex. The solid state structure of Ru-1 was established by X-ray analysis of its single crystals. The ORTEP view is shown in Figure 3. Indeed, the ruthenium(II) center adopted a slightly distorted octahedral coordination environment (the angles C1-Ru1-CO1, Cl1-Ru1-CO2, and P1-Ru1-P2 are 177.48°, 178.34°, and 159.32°, respectively) with two nonequivalent CO ligands located in trans and cis positions with respect to the metalated carbon and to the single chlorine atom. An interesting common structural feature of the complexes Pt-1 and Ru-1 is a strong distortion of the metalated carbon from the ideal tetrahedral geometry. For example, the observed C15-C1-Pt1 angle in Pt-1 was found to be 121° instead of the expected 109°. The same parameter for Ru-1 was even more illustrative, 129°! However, this extreme distortion does not affect the stability of the complexes, and they were perfectly stable in both the solid state (decomposition after 220 °C) and solution (nearly boiling DMSO-d6) under a not inert atmosphere. DMF is a convenient source of the carbonyl ligands,24 and therefore the following scenario for the formation of Ru-1 looks reasonable: dissolution of RuCl3 in DMF produces cationic DMF-containing species that upon heating transform into cationic carbonyl complexes in the same oxidation state as a result of DMF decomposition. Further, as was reported earlier,23 traces of water accelerate metal reduction to form dicarbonyl a Ru(II) complex that upon coordination to 1 leads to Ru-1 (Scheme 2).

(22) (a) Fild, M.; Althoff, W. J. Chem. Soc., Chem. Commun. 1973, 933. (b) Redfield, D. A.; Cary, L. W.; Nelson, J. H. Inorg. Chem. 1975, 14, 50. (23) Serp, P.; Hernandez, M.; Richard, B.; Kalck, P. Eur. J. Inorg. Chem. 2001, 2327.

(24) (a) Bondarenko, I. B.; Buzina, N. A.; Varshavskii, Y. S.; Gel’fman, M. I.; Razumovskii, V. V.; Cherkasova, T. G. Zh. Neorg. Khim. 1971, 16, 3071. (b) Ciuhandu, G.; Tirnaveanu, A. Rev. Roum. Chim. 1971, 16, 1231. (c) Serp, P.; Hernandez, M.; Kalck, P.; Hoyne, J. H.; Shapley, J. R. Inorg. Synth. 2004, 34, 118.

Figure 2. ORTEP drawing (50% probability ellipsoids) of the structure of Pt-1. Hydrogen atoms and solvent molecules are omitted for clarity. Selected bond lengths (A˚) and angles (deg): Pt1-Cl1 (2.3925(9)), Pt1-C1 (2.068(3)), Pt1-P1 (2.2847(10)), Pt1-P2 (2.2832(11)). C15-C1Pt1 (121.1(2)), C1-Pt1Cl1 (179.34(10)), P1-Pt1-P2 (156.62(4)).

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Table 1. Crystal Data and Structure Refinement Details for Pt-1, Ru-1, Ir-2, and Ir-3 Pt-1 empirical formula fw temp (K) wavelength (A˚) cryst syst space group unit cell dimens a (A˚) b (A˚) c (A˚) R (deg) β (deg) γ (deg) V (A˚3) Z calc density (mg/m3) abs coeff (mm-1) F(000) cryst size (mm) θ range, data collecn (deg) limiting indices no. of reflns collected no. of indep reflns refinement method no. of data/restraints/ params goodness of fit on F2 final R indices (I > 2σ(I)) R1 wR1 R indices (all data) R1 wR1

C36H45ClN2P2Pt 798.22 295(1) 0.71073 monoclinic P2(1)/n 14.6217(9) 15.048(1) 15.642(1) 90 96.244(1) 90 3421.4(4) 4 1.550 4.300 1600 0.20  0.19  0.09 1.88 to 27.00 -18 < h < 18 -19 < k < 19 -19 < l < 19 37 197 7445 7445/0/389 1.048 0.0346 0.0779 0.0489 0.0828

Ru-1

Ir-2

C34H39ClO2P2Ru C34H41Cl4IrOP2 678.11 861.61 295(1) 173(1) 0.71073 0.71073 monoclinic orthorhombic P2(1)/n Pnma 10.4228(9) 19.2541(18) 11.791(1) 15.9466(15) 25.298(2) 11.2940(10) 90 90 91.614(1) 90 90 90 3107.8(5) 3467.7(6) 4 4 1.449 1.650 0.723 4.277 1400 1712 0.24  0.22  0.16 0.40  0.33  0.30 2.13 to 27.00 2.09 to 28.00 -13 < h < 13 -25 < h < 25 -15 < k < 15 -20 < k < 20 -32 < l < 32 -14 < l < 14 33 045 38 432 6736 4324 full-matrix least-squares on F2 6736/ 0/369 4324/0/219 1.273 1.126 0.0562 0.0192 0.1282 0.0519 0.0597 0.0199 0.1298 0.0522

Indeed, we found that the presence of water is crucial, and treating RuCl3 3 3H2O in wet DMF improves the isolated yield of the product from 30% to 62%. Synthesis of New Iridium Complexes. The latter findings were especially interesting regarding the background of the synthesis of the prototype of these compounds, Ir-1 (eq 1), that forms without changing the oxidation state of the metal under anhydrous conditions.13 We assumed that the use of wet DMF in the reaction of the ligand 1 and IrCl3 3 3H2O will result in the formation of intermediates at higher oxidation state and, in due course, will deliver a different product. Indeed, heating the reactants for 20 h under nitrogen in wet solvent yielded one isolable product,25 which was identified as a carbometalated Ir(III) hydride complex (Ir-2), as demonstrated in eq 3.

The structure assignment was made as follows: the P NMR spectrum indicated a hydride-coupled signal at 47.4 ppm indicative of the equivalent P nuclei in Ir-2. Additionally, the 1H NMR spectrum revealed a triplet of hydride resonances at -19.86 ppm with a P-H coupling constant of 14 Hz. The relatively low-field-shifted signal may indicate the location of the hydride ligand trans to an occupied orbital and, therefore, most likely, the octahedral 31

(25) The crude P NMR spectrum does indicate the presence of several less intense signals along with the reported one. (26) Renkema, K. B.; Bosque, R.; Streib, W. E.; Maseras, F.; Eisenstein, O.; Caulton, K. G. J. Am. Chem. Soc. 1999, 121, 10895.

Ir-3 C33H40ClIrOP2 742.24 173(1) 0.71073 monoclinic C2/c 37.475(2) 11.4470(6) 14.6347(8) 90 108.092(1) 90 5967.6(6) 8 1.652 4.697 2960 0.23  0.15  0.14 12.47 to 28.02 -49 < h < 49 -14 < k < 15 -19 < l < 18 33 484 7092 7091/0/355 1.060 0.0207 0.0488 0.0224 0.0496

geometry around iridium.3,26 The presence of the carbon monoxide ligand was confirmed by the presence of the phosphorus-coupled carbon signal at 176.4 ppm (J=11 Hz). Although the spectroscopic data were helpful for establishing the structure of Ir-2, the exact order of the ligands around the metal center could be established only using X-ray analysis. So we found the expectedly distorted octahedral iridium center with the carbon monoxide ligand located in a trans position and the chlorine and hydrogen atoms located in cis positions to the metalated C1, as depicted in the ORTEP view in Figure 3 (left). The P1-Ir1-P2, C1-Ir1-CO, and Cl1-Ir1-H angles are equal to 155.9°, 176.3°, and 176.1°, respectively, and these parameters very much resemble those observed for the complexes Ir-1,13 Ru-1, and Pt-1. The bond lengths in Ir-2 are within the reported range. The formation of Ir-2 is not surprising and apparently starts from the aqueous DMF-mediated reduction of Ir(III) chloride into the Vaska-type trans-[IrCl(CO)(1)] intermediate (eq 3), which oxidatively adds across the proximate C-H bond. If so, the same complex can be prepared more easily by a simple ligand exchange reaction between the classical Vaska’s complex and the chelate 1. Indeed, according to the NMR monitoring, the reaction proceeds at room temperature, causing a gradual disappearance of the signals that correspond to the starting materials and appearance of a new spectral pattern that is similar but not identical to the expected Ir-2. For example, the hydridecoupled doublet at 50.4 ppm (versus 47.9 ppm for Ir-2) was observed by 31P NMR. The signal assigned to the hydride ligand appeared at 17.7 ppm as a triplet with JP-H =11 Hz. To fully characterize the apparently isomeric product Ir-3, we repeated the reaction on a larger scale and isolated the product in 80% yield (eq 4). X-ray analysis of the colorless crystals of Ir-3 revealed that only the mutually

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solution.27 The compounds were not amenable to oxidative decarbonylation with triethylamine N-oxide.28 On the other hand, even a brief experimentation revealed that they are catalytically active and promote transfer hydrogenation of ketones in 2-propanol with good TOFs. For example, Ir-2- or Ir-3-catalyzed transfer hydrogenation of 20 -chloroacetophenone in 2-propanol in the presence of NaOt-Bu (cat:sub:base = 1:1000:50) results in its complete conversion into the corresponding secondary alcohol in only 15 min (eq 55), which corresponds to a TOF of 6  103 h-1. More detailed studies will follow.

Figure 3. ORTEP drawing (50% probability ellipsoids) of the structure of Ru-1. Hydrogen atoms and solvent molecules are omitted for clarity. Selected bond lengths (A˚) and angles (deg): Ru1-Cl1 (2.4514(11)), Ru1-C1 (1.931(5)), Ru1-CO1 (1.851(5)), Ru1-CO2 (1.931(5)), Ru1-P1 (2.3740(11)), Ru1-P2 (2.3804(11)). C15-C1-Ru1(129.2(3)), C1-Ru1-CO1 (177.48(19)), Cl1-Ru1-CO2 (178.34(15)), P1-Ru1-P2 (159.32(4)). Scheme 2

interconverted location of the hydride and chloride ligands differs between the isomers, whereas other structural parameters including bond lengths and angles are essentially identical (Figure 4). It is also worth noting that eq 4 represents a more reliable route to the iridium(III) hydride complexes of this type, as it leads to the exclusive formation of one single isomer in high yield.

We also attempted to convert Ir-2 and Ir-3 into coordinatively unsaturated species by vacuum- and photoinduced decarbonylation. Unfortunately, the complexes remained unchanged under prolonged heating under vacuum, and only isomerization of Ir-3 into Ir-2 and into an unidentified isomer was observed after photolysis of the its chloroform (27) (a) Doetz, K. H.; Rau, A.; Harms, K. J. Organomet. Chem. 1992, 439, 263. (b) Ishii, K.; Hoshino, S.; Kobayashi, N. Inorg. Chem. 2004, 43, 7969. (c) Jin, G.-X.; Herberhold, M. Transition Met. Chem. 2001, 26, 445. (28) (a) Blumer, D. J.; Barnett, K. W.; Brown, T. L. J. Organomet. Chem. 1979, 173, 71. (b) Shvo, Y.; Hazum, E. J. Chem. Soc., Chem. Commun. 1974, 336.

To conclude, we described here a new platform for the synthesis of C(sp3)-metalated pincers via C-H activation. We exemplified this approach by the preparation and full structural characterization of some new complexes of platinum, ruthenium, and iridium, which together with those previously published13,14 represent a novel structural motif for the design of pincer-type compounds. With some exceptions of C2-symmetrically twisted molecules,19,29 ipso-C-M bond-sharing five-membered rings characteristic of pincer complexes are nearly planar. Unlike those, barrelene-based complexes have an unusual three-dimensional steric environment produced by the roof-shaped ligand (Figure 5).30 This three-dimensionality might be very useful in catalysis, for example, for designing rare “chiral-at-frame”31 pincer complexes lacking C2-symmetry. Yet, the bulkiness of the three-dimensional frame can be beneficial for material applications.32 Studies focusing on chiral ligands as well as of coordinatively unsaturated complexes and their catalytic application are under way.

Experimental Section All manipulations were performed using standard Schlenk techniques under an atmosphere of dry N2. Anhydrous solvents and metal precursors were purchased from the usual sources and used without further purification. 1,8-Bis(diisopropylphosphino)triptycene17,33 and PtCl2(1,8-bis(diisopropylphosphino)triptycene15 were prepared following published procedures. NMR spectra were recorded on a Bruker instrument operating at 400 MHz for protons, 100 MHz for carbons, and 121 MHz for (29) (a) Danopoulos, A. A.; Tulloch, A. A. D.; Winston, S.; Eastham, G.; Hursthouse, M. B. Dalton Trans. 2003, 1009. (b) Diez-Barra, E.; Guerra, J.; Lopez-Solera, I.; Merino, S.; Rodriguez-Lopez, J.; SanchezVerdu, P.; Tejeda, J. Organometallics 2003, 22, 541. (c) Lee, H. M.; Zeng, J. Y.; Hu, C.-H.; Lee, M.-T. Inorg. Chem. 2004, 43, 6822. (d) Ma, L.; Woloszynek, R. A.; Chen, W.; Ren, T.; Protasiewicz, J. D. Organometallics 2006, 25, 3301. (e) Nielsen, D. J.; Cavell, K. J.; Skelton, B. W.; White, A. H. Inorg. Chim. Acta 2002, 327, 116. (f) Nilsson, P.; Wendt, O. F. J. Organomet. Chem. 2005, 690, 4197. (g) Tulloch, A. A. D.; Danopoulos, A. A.; Tizzard, G. J.; Coles, S. J.; Hursthouse, M. B.; Hay-Motherwell, R. S.; Motherwell, W. B. Chem. Commun. 2001, 1270. (30) The bent shape is rarely seen in pincer complexes. See, for example, ref 9d. (31) (a) Albrecht, M.; Kocks, B. M.; Spek, A. L.; van Koten, G. J. Organomet. Chem. 2001, 624, 271. (b) Dani, P.; Albrecht, M.; van Klink, G. P. M.; van Koten, G. Organometallics 2000, 19, 4468. (c) Gorla, F.; Togni, A.; Venanzi, L. M.; Albinati, A.; Lianza, F. Organometallics 1994, 13, 1607. (32) For example, being incorporated into functional polymeric materials they will act as spacers, separating individual chains in bulk materials and preventing their aggregation. See for example: Swager, T. M. Acc. Chem. Res. 2008, 41, 1181. (33) Grossman, O.; Gelman, D. Org. Lett. 2006, 8, 1189.

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Figure 4. ORTEP drawing (50% probability ellipsoids) of the structures of Ir-2 and Ir-3. Hydrogen atoms and solvent molecules are omitted for clarity. Selected bond lengths (A˚) and angles (deg) for Ir-2: Ir1-H (1.41(5)), Ir1-Cl1 (2.5169(7)), Ir1-C1 (2.193(3)), Ir11CO (1.898(3)), Ir1-P1 (2.3166(6)), Ir1-P2 (2.3166(6)). C9-C1-Ir1 (128.05(18)), H-Ir1-Cl1 (176.1(2)), C1-Ir1-CO (176.32(12)), P1-Ir1-P2 (155.98(3)). Selected bond lengths (A˚) and angles (deg) for Ir-3: Ir1-H (1.39(3)), Ir1-Cl1 (2.4794(5)), Ir1-C1 (2.163(2)), Ir11-CO (1.895(2)), Ir1-P1 (2.3372(6)), Ir1-P2 (2.3147(6)). C15-C1-Ir1 (125.42(15)), H-Ir1-Cl1 (173.5(13)), C1-Ir1-CO (172.14(9)), P1-Ir1-P2 (160.94(2)).

Figure 5 phosphorus. Diffraction data were collected with a Bruker APEX CCD instrument (Mo KR radiation (λ = 0.71073 A˚)). Crystals were mounted onto glass fibers using epoxy. Singlecrystal reflection data were collected on a Bruker APEX CCD X-ray diffraction system controlled by a Pentium-based PC running the SMART software package.34 The integration of data frames and refinement of cell structure were done by the SAINTþ program package.35 Refinement of the structure on F2 was carried out by the SHELXTL software package.36 Further information may be found within CIF files provided as Supporting Information. Synthesis Pt-1. PtCl2(1,8-bis(diisopropylphosphino)triptycene (100 mg, 0.133 mmol) in diethyleneglycol monomethyl ether (10 mL), diisopropylethylamine (6 equiv, 130 μL, 0.8 mmol), and AgBF4 (1 equiv, 26 mg, 0.133 mmol) were added. The solution was refluxed (120 °C) for 20 h under N2. The solvent evaporated to dryness, and the product was washed with methanol, redissolved in chloroform, and filtered through Celite. Evaporation afforded white crystals (48 mg, 50% yield). Crystals suitable for X-ray analysis were obtained by slow evaporation of its saturated acetonitrile solution. 1H NMR, 400 MHz (CDCl3), δ (ppm): 0.76 (6H, dd, J=7 Hz, J=16 Hz); 1.34 (6H, dd, J=7.6, J=16 Hz); 1.42 (6H, dd, J=7.2, J=14 Hz); 1.48 (6H, dd, J=7 Hz, J=16 Hz); 2.43 (2H, m); 3.83 (2H, m); 5.39 (1H, s); 6.73 (1H, t, J = 7 Hz); 6.85 (1H, t, J = 7 Hz); 7.05-7.18 (5H, m); 7.42 (2H, d, J=7 Hz); 7.80 (1H, d, J=7.2 Hz). 13 C NMR, 100 (CDCl3), δ (ppm): 18.6, 18.7, 18.8 (t, J=2 Hz), 19.3 (t, J=2 Hz), 24.3 (t, J=16 Hz), 27.6, 54.1, 55.8, 122.5, 123.4, (34) SMART-NT, V. 5.6; Bruker AXS GMBH: Karlsruhe, Germany, 2002. (35) SAINT-NT, V. 5.0; Bruker AXS GMBH: Karlsruhe, Germany, 2002. (36) SHELXTL-NT, V. 6.1; Bruker AXS GMBH: Karlsruhe, Germany, 2002.

124.0, 125.6, 125.9, 126.7, 128.9, 131.0 (t, J=19 Hz), 145.3 (t, J= 8 Hz), 147.3, 153.7, 164.9 (t, J=15 Hz). 31P{1H} NMR, 161 MHz (CDCl3), δ: 59.7 (t, J=1518 Hz). Anal. Calcd for C32H39ClP2Pt: C, 53.67; H, 5.49. Found: C, 53.60; H, 5.35. Synthesis of Ru-1. A solution of RuCl3 3 3H2O (0.2 g, 0.76 mmol) and 1,8-bis(diisopropylphosphino)triptycene (1) (0.56 g, 1.15 mmol) in commercial N,N-dimethylformamide (DMF, 10 mL) was heated at reflux for 48 h under nitrogen. DMF was removed from the reaction mixture by distillation under reduced pressure, and the offwhite residue was recrystallized three times from methanol, affording 0.32 g (62%). 1H NMR, 400 MHz (CDCl3), δ (ppm): 1.09 (12H, dd, J=7 Hz, J=14 Hz); 1.60 (6H, dd, J=6.8, J=14.0 Hz); 1.69 (6H, dd, J=7 Hz, J=14 Hz); 2.29 (2H, m); 3.14 (2H, m); 5.32 (1H, s); 6.79 (2H, m); 6.85 (2H, t, J=7 Hz); 7.20 (3H, m); 7.41 (2H, d, J=7 Hz); 7.69 (1H, d, J=7 Hz). 13C{1H} NMR, 100 (CDCl3), δ, ppm: 18.9, 19.8, 20.7, 22.6, 25.7 (t, J=13 Hz), 29.7, 30.3 (t, J=12 Hz), 55.0, 69.9 (t, J=5 Hz), 122.8, 123.1, 124.1, 124.4 (t, J=3 Hz), 125.0, 126.7 (d, J=6 Hz), 133.4 (t, J=22 Hz), 146.3 (t, J=7 Hz), 150.6, 155.1, 163.7 (t, J=15 Hz), 196.2 (t, J=8 Hz), 202.1 (t, J=11 Hz). 31P NMR, 161 MHz (CDCl3), δ (ppm): 71.3. IR (KBr): 2027, 1941 cm-1 (CdO). Anal. Calcd for C34H41ClO2P2Ru: C, 60.04; H, 6.08. Found: C, 59.95; H, 6.02. Synthesis of Ir-2. This compound was prepared from IrCl3 3 3H2O (0.2 g, 0.67 mmol) and 1,8-bis(diisopropylphosphino)triptycene (1) (0.65 g, 1.34 mmol) in DMF (10 mL) and 2 mL of water. The solution was refluxed for 2 days under N2. The solvent was distilled off under reduced pressure from the reaction mixture, and the off-white product was recrystallized from methanol, affording 0.30 g of Ir-2 (61%). 1H NMR, 400 MHz (CDCl3), δ (ppm): -19.86 (1H, t, J=14 Hz); 1.06 (12H, dd, J= 6.4 Hz, J=14 Hz); 1.18 (6H, dd, J=7 Hz, J=14 Hz); 1.44 (6H, dd, J=7, J=15 Hz); 2.79 (2H, m); 2.94 (2H, m); 5.34 (1H, s); 6.75 (1H, t, J=7 Hz); 6.82 (1H, t, J=7 Hz); 7.02-7.09 (5H, m); 7.40 (2H, d, J=6 Hz); 9.13 (1H, d, J=7 Hz). 13C{1H} NMR, 100 (CDCl3), δ (ppm): 14.1, 17.7 (t, J=22 Hz), 18.6, 19.9, 22.7, 24.2 (t, J = 17 Hz), 27.4 (t, J = 13 Hz), 29.3, 29.7, 31.9, 54.5, 60.0, 121.6, 122.9, 124.2, 124.9 (t, J = 4 Hz), 125.2, 125.5, 129.4, 132.1 (t, J=26 Hz), 146.5 (t, J=7 Hz), 147.2, 155.8, 167.2 (t, J=16 Hz). 31 P NMR, 161 MHz (CDCl3), δ (ppm): 47.5 (d, JP-H=14.0 Hz). IR (KBr): 2017 cm-1 (CO). Anal. Calcd for C33H41ClIrOP2: C, 53.32; H, 5.56. Found: C, 52.89; H, 5.38. Synthesis of Ir-3. This compound was prepared from the commercially available IrClCO(PPh3)2 (Vask as complex) (0.1 g, 0.26 mmol) and 1,8-bis(diisopropylphosphino)triptycene (1) (1 equiv, 0.12 g, 0.26 mmol) in chloroform (10 mL) at room

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temperature for 4 h. White crystals were obtained from methanol under cooling (-10 °C), affording 0.15 g (80% yield). Crystals suitable for X-ray analysis were obtained by slow evaporation of the product solution in diethyl ether. 1H NMR, 400 MHz (CDCl3), δ: -17.72 (1H, t, J = 11 Hz); 1.05 (6H, dd, J=8.0 J=16 Hz); 1.15 (6H, dd, J=7.2, J=14 Hz); 1.66 (12H, m); 1.86 (2H, m); 3.27 (2H, m); 5.34 (1H, s); 6.71-6.78 (2H, m); 7.06-7,2 (3Hm); 7.39-7.47 (4H, m); 7.76 (1H, s). 13C NMR, 100 (CDCl3), δ: 18.1, 20.4, 20.6, 22.6, 24.5 (t, J=16 Hz), 29.1 ((t, J=15 Hz), 54.6, 61.0, 122.9, 123.2, 123.6, 125.0, 125.4, 126.5, 127.9, 133.8 (t, J=26 Hz), 146.2 (t, J=7 Hz), 150.1, 153.4, 163.8 (t, J=15 Hz), 176.4 (t, J=11 Hz). 31P NMR, 161 MHz (CDCl3), δ: 50.5 ppm (d, JP-H =11 Hz). IR (KBr): 2017 cm-1 (s, CO). Anal. Calcd for C33H41ClIrOP2: C, 53.32; H, 5.56. Found: C, 52.73; H, 5.26. Attempted Photodecarbonylation of Ir-3. Photolysis of Ir-3 was performed in chloroform for 15 h using a xenon lamp (125-150 W, model E7536, Hamamatsu Photonics), resulting in a mixture of three Ir-H-CO isomers as detected by 31P{1H} NMR analysis: 50.4 ppm (Ir-3), 47.5 (Ir-2) ppm, and an additional unidentified 45.3 ppm. Procedure for Transfer Hydrogenation of 20 -Chloroacetophenone. A 10 mL Schlenk tube was charged with the precatalyst

Azerraf and Gelman Ir-2 (5 mg, 6.7  10-3 mmol) and t-BuONa (32.3 mg, 0.33 mmol) in i-PrOH (1 mL) and was preheated to 82 °C. A solution of 20 chloroacetophenone (1.35 mL, 6.7 mmol) and i-PrOH (4 mL) was then injected into the reaction mixture. After attainment of equilibrium (15 min), the reaction was quenched with water, extracted with ethyl acetate, and analyzed by GC using dodecane as the internal standard. The product was later isolated by column chromatography on silica, giving 1-(2-chlorophenyl)ethanol (CAS Registry No.: 13524-04-4). 1H NMR, 400 MHz (CDCl3), δ: 1.49 (3H, d, J=6.5 Hz); 1.94 (1H, br); 5.29 (1H, q, J=6.5 Hz); 7.19 (1H, td, 2J=1.5, 3J=7.5 Hz); 7.27-7.33 (2H, m); 7.19 (1H, dd, 2J = 1.7, 3J = 7.8 Hz). 13C NMR, 400 MHz (CDCl3), δ: 23.5, 66.8, 126.4, 127.2, 128.3, 129.3, 131.6, 143.1.

Acknowledgment. We thank the Israel Science Foundation (Grant No. 866/06) for the financial support and Dr. Shmuel Cohen for solving X-ray structures. Supporting Information Available: X-ray crystallographic files in CIF format for all structures. Spectroscopic data for compounds. This material is available free of charge via the Internet at http://pubs.acs.org.