Stoichiometric and Catalytic Deuteration of Pyridine and

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Organometallics 2009, 28, 3700–3709 DOI: 10.1021/om900335b

Stoichiometric and Catalytic Deuteration of Pyridine and Methylpyridines by H/D Exchange with Benzene-d6 Promoted by an Unsaturated Osmium Tetrahydride Species Beatriz Eguillor, Miguel A. Esteruelas,* Jorge Garcıa-Raboso, Montserrat Olivan, and Enrique O~ nate Departamento de Quımica Inorg anica-Instituto de Ciencia de Materiales de Arag on, Universidad de Zaragoza-CSIC, 50009 Zaragoza, Spain Received April 29, 2009

Treatment of toluene solutions of the hexahydride complex OsH6(PiPr3)2 (1) with pyridine, 3-methylpyridine, and 4-methylpyridine affords the tetrahydride derivatives OsH4(Rpy)(PiPr3)2 (Rpy = py (3), 3-methylpyridine (4), and 4-methylpyridine (5)). In benzene-d6, these compounds release the heterocycles, and the resulting unsaturated tetrahydride OsH4(PiPr3)2 (2) promotes the stoichiometric and catalytic deuteration of pyridine, 3-methylpyridine, and 4-methylpyridine by means of H/D exchanges between the heterocycles and the solvent. The deuteration rates of the pyridinic C-H bonds depend upon their positions in the heterocycles. For pyridine, they increase as the C-H bonds are separated from the heteroatom. A methyl substituent has a marked negative effect on the deuteration of its adjacent C-H bonds. The kinetic analysis of the deuteration reveals that the rate-determining step for the H/D exchanges is the C-H activation of the bond that is deuterated. DFT calculations show that this step is formed by two elemental stages: the direct coordination of the C-H bond and its subsequent rupture.

Introduction Pyridines have a significant presence in the pharmaceutical industry,1 where labeled compounds are used for drug development.2 Several of these heterocycles incorporate deuterium atoms in D2O at 200-400 °C.3 In DCl at about 220 °C, pyridine undergoes exchange at measurable rates at the 2,6-positions, while in NaOD the protons at all positions exchange.4 Deuterium gas heterogeneous metal catalysis selectively affords 2,6-di(deuterated) compounds.5 *Corresponding author. E-mail: [email protected]. (1) Carey, J. S.; Laffan, D.; Thomson, C.; Williams, M. T. Org. Biomol. Chem. 2006, 4, 2337. (2) (a) Junk, T.; Catallo, W. J. Chem. Soc. Rev. 1997, 26, 401. (b) Elander, N.; Jones, J. R.; Lu, S. Y.; Stone-Elander, S. Chem. Soc. Rev. 2000, 29, 239. (c) Siskin, M.; Katritzky, A. R. Chem. Rev. 2001, 101, 825. (d) Atzrodt, J.; Derdau, V.; Fey, T.; Zimmermann, J. Angew. Chem., Int. Ed. 2007, 46, 7744. (3) (a) Werstuik, N. H.; Ju, C. Can. J. Chem. 1989, 67, 5. (b) Yao, J.; Evilia, R. F. J. Am. Chem. Soc. 1994, 116, 11229. (4) Zoltewicz, J. A.; Smith, C. L. J. Am. Chem. Soc. 1967, 89, 3358. (5) (a) Calf, G. E.; Garnett, J. L.; Pickles, V. A. Aust. J. Chem. 1968, 21, 961. (b) Rubottom, G. M.; Evain, E. J. Tetrahedron 1990, 46, 5055. (c) Alexakis, E.; Jones, J. R.; Lockley, W. J. S. Tetrahedron Lett. 2006, 47, 5025. (6) (a) Garnett, J. L.; Hodges, R. J. J. Chem. Soc., Chem. Commun. 1967, 1001. (b) Garnett, J. L.; Hodges, R. J. J. Am. Chem. Soc. 1967, 89, 4546. (c) Garnett, J. L.; Long, M. A.; McLaren, A. B.; Peterson, K. B. J. Chem. Soc., Chem. Commun. 1973, 749. (d) Garnett, J. L.; Kenyon, R. S. Aust. J. Chem. 1974, 27, 1023. (e) Kenyon, R. S.; Garnett, J. L. Aust. J. Chem. 1974, 27, 1033. (f ) Blake, M. R.; Garnett, J. L.; Gregor, I. K.; Hannan, W.; Hoa, K.; Long, M. A. J. Chem. Soc., Chem. Commun. 1975, 930. (g) Davis, K. P.; Garnett, J. L. Aust. J. Chem. 1975, 28, 1699. (h) Davis, K. P.; Garnett, J. L. Aust. J. Chem. 1975, 28, 1713. (i) Garnett, J. L.; O’Keefe, J. H. J. Labelled Compd. 1975, 11, 201. pubs.acs.org/Organometallics

H/D exchange by homogeneous metal catalysis offers many advantages over other methodologies,2d for instance, comparably mild reaction conditions. Moreover, very efficient deuterium incorporation with concomitant high regioselectivity can often be achieved in this way. Since the pioneering work of the research groups of Garnett6 and Shilov,7 iridium-mediated H/D exchange reactions make up by far the greatest number of examples in this area.8 Ru,9 Rh,10 and Pt11 complexes have also been used. H/D exchange by homogeneous metal catalysts involves activation of C-H bonds in the molecules and subsequent exchange for the isotopic label. Activation of C-H bonds has been reported with both early and late transition metal (7) (a) Gol’dshleger, N. F.; Tyabin, M. B.; Shilov, A. E.; Shteinman, A. A. Zh. Fiz. Khim. 1969, 43, 1222. (b) Gol’dshleger, N. F.; Es’kova, V. V.; Shilov, A. E.; Shteinman, A. A. Zh. Fiz. Khim. 1972, 46, 785. (8) For leading recent references see: (a) Golden, J. T.; Andersen, R. A.; Bergman, R. G. J. Am. Chem. Soc. 2001, 123, 5837. (b) Klei, S. R.; Golden, J. T.; Tilley, T. D.; Bergman, R. G. J. Am. Chem. Soc. 2002, 124, 2092. (c) Cross, P. W. C.; Ellames, G. J.; Gibson, J. S.; Herbert, J. M.; Kerr, W. J.; McNeill, A. H.; Mathers, T. W. Tetrahedron 2003, 59, 3349. (d) Wong-Foy, A. G.; Bhalla, G.; Liu, X. Y.; Periana, R. A. J. Am. Chem. Soc. 2003, 125, 14292. (e) Santos, L. L.; Mereiter, K.; Paneque, M.; Slugovc, C.; Carmona, E. New J. Chem. 2003, 27, 107. (f) Skaddan, M. B.; Yung, C. M.; Bergman, R. G. Org. Lett. 2004, 6, 11. (g) Yung, C. M.; Skaddan, M. B.; Bergman, R. G. J. Am. Chem. Soc. 2004, 126, 13033. (h) Ellames, G. J.; Gibson, J. S.; Herbert, J. M.; Kerr, W. J.; McNeill, A. H. J. Labelled Compd. Radiopharm. 2004, 47, 1. (i) Garman, R. N.; Hickey, M. J.; Kingston, L. P.; McAuley, B.; Jones, J. R; Lockley, W. J. S.; Mather, A. N.; Wilkinson, D. J. J. Labelled Compd. Radiopharm. 2005, 48, 75. (j) Corberan, R.; Sana u, M.; Peris, E. J. Am. Chem. Soc. 2006, 128, 3974. (k) Brown, J. A.; Irvine, S.; Kennedy, A. R.; Kerr, W. J.; Andersson, S.; Nilsson, G. N. Chem. Commun. 2008, 1115.

Published on Web 05/26/2009

r 2009 American Chemical Society

Article

Organometallics, Vol. 28, No. 13, 2009

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complexes and has been well studied by many research groups.12 Pyridine C-H bond activation has however received comparatively less attention than that of other aromatic compounds.13 The C-H activation reactions are promoted by low-valent metal complexes. The use of high-valent compounds is rare, in particular hydride derivatives. In spite of this, our group has reported that the saturated osmium(VI) hydride complex OsH6(PiPr3)2 is thermally activated to afford the nondetected unsaturated osmium(IV) species OsH4(PiPr3)2, which activates C-H bonds of different organic substrates.14 Now, we have trapped this tetrahydride with pyridine, 3-methylpyridine, and 4-methylpyridine and observed that it is an active catalyst for the H/D exchange between these heterocycles and benzene-d6. This paper reports (i) the preparation and spectroscopic and structural characterization of the new tetrahydride compounds, (ii) the stoichiometric and catalytic deuteration of pyridine, 3-methylpyridine, and 4-methylpyridine, (iii) the influence of the methyl group of the substituted heterocycles on the deuteration, and

Scheme 1

(9) See for example: (a) Moreno, B.; Sabo-Etienne, S.; Chaudret, B.; Rodriguez, A.; Jal on, F.; Trofimenko, S. J. Am. Chem. Soc. 1995, 117, 7441. (b) Collman, J. P.; Fish, H. T.; Wagenknecht, P. S.; Tyvoll, D. A.; Chng, L.-L.; Eberspacher, T. A.; Brouman, J. I.; Bacon, J. W.; Pignolet, L. H. Inorg. Chem. 1996, 35, 6746. (c) Giunta, D.; Ho¨lscher, M.; Lehmann, C. W.; Mynott, R.; Wirtz, C.; Leitner, W. Adv. Synth. Catal. 2003, 345, 1139. (d) Prechtl, M. H. G.; Ho¨lscher, M.; Ben-David, Y.; Theyssen, N.; Loschen, R.; Milstein, D.; Leitner, W. Angew. Chem., Int. Ed. 2007, 46, 2269. (e) Prechtl, M. H.G.; Ho¨lscher, M.; Ben-David, Y.; Theyssen, N.; Milstein, D.; Leitner, W. Eur. J. Inorg. Chem. 2008, 3493. (10) See for example: (a) Lenges, C. P.; White, P. S.; Brookhart, M. J. Am. Chem. Soc. 1999, 121, 4385. (b) Klei, S. R.; Tilley, T. D.; Bergman, R. G. Organometallics 2002, 21, 4905. (c) Rybtchinski, B.; Cohen, R.; Ben-David, Y.; Martin, J. M. L.; Milstein, D. J. Am. Chem. Soc. 2003, 125, 11041. (d) Hanson, S. K.; Heinekey, D. M.; Goldberg, K. I. Organometallics 2008, 27, 1454. (11) See for example: (a) Barthez, J. M.; Filikov, A. V.; Frederiksen, L. B.; Huguet, M.-L.; Jones, J. R.; Lu, S.-Y. Can. J. Chem. 1998, 76, 726. (b) Clement, O.; Roszak, A. W.; Buncel, E. J. Am. Chem. Soc. 1996, 118, 612. (12) (a) Shilov, A. E.; Shteinman, A. A. Coord. Chem. Rev. 1977, 24, 97. (b) Crabtree, R. H. Chem. Rev. 1985, 85, 245. (c) Jones, W. D.; Feher, F. J. Acc. Chem. Res. 1989, 22, 91. (d) Ryabov, A. D. Chem. Rev. 1990, 90, 403. (e) Shilov, A. E.; Shul’pin, G. B. Chem. Rev. 1997, 97, 2879. (f ) Crabtree, R. H. J. Chem. Soc., Dalton Trans. 2001, 2437. (g) Labinger, J. A.; Bercaw, J. E. Nature 2002, 417, 507. (h) Crabtree, R. H. J. Organomet. Chem. 2004, 689, 4083. (i) Carmona, E.; Paneque, M.; Santos, L. L.; Salazar, V. Coord. Chem. Rev. 2005, 249, 1729. (j) Lersch, M.; Tilset, M. Chem. Rev. 2005, 105, 2471. (k) Jones, W. D. Inorg. Chem. 2005, 44, 4475. (l) Godula, K.; Sames, D. Science 2006, 312, 67. (13) For some leading references see: (a) Jones, W. D.; Dong, L.; Myers, A. W. Organometallics 1995, 14, 855. (b) Ozerov, O. V.; Pink, M.; Watson, L. A.; Caulton, K. G. J. Am. Chem. Soc. 2004, 126, 2105. (c) Esteruelas, M. A.; Fernandez-Alvarez, F. J.; O~ nate, E. J. Am. Chem. Soc. 2006, 128, 13044. (d) Alvarez, E.; Conejero, S.; Paneque, M.; Petronilho, A.; Poveda, M. L.; Serrano, O.; Carmona, E. J. Am. Chem. Soc. 2006, 128, 13060. (e) Arndt, S.; Elvidge, B. R.; Zeimentz, P. M.; Spaniol, T. P.; Okuda, J. Organometallics 2006, 25, 793. (f ) Buil, M. L.; Esteruelas, M. A.; Garces, K.; Olivan, M.; O~ nate, E. J. Am. Chem. Soc. 2007, 129, 10998. (g) Alvarez, E.; Conejero, S.; Lara, P.; L opez, J. A.; Paneque, M.; Petronilho, A.; Poveda, M. L.; del Rio, D.; Serrano, O.; Carmona, E. J. Am. Chem. Soc. 2007, 129, 14130. (h) Delafuente, D. A.; Kosturko, G. W.; Graham, P. M.; Harman, W. H.; Myers, W. H.; Surendranath, Y.; Klet, R. C.; Welch, K. D.; Trindle, C. O.; Sabat, M.; Harman, W. D. J. Am. Chem. Soc. 2007, 129, 406. (i) Esteruelas, M. A.; Fern andez-Alvarez, F. J.; O~ nate, E. Organometallics 2007, 26, 5239. (j) Yang, P.; Warnke, I.; Martin, R. L.; Hay, P. J. Organometallics 2008, 27, 1384. (k) Conejero, S.; Lara, P.; Paneque, M.; Petronilho, A.; Poveda, M. L.; Serrano, O.; Vattier, F.; Alvarez, E.; Maya, C.; Salazar, V.; Carmona, E. Angew. Chem., Int. Ed. 2008, 47, 4380. (l) Buil, M. L.; Esteruelas, M. A.; Garces, K.; Olivan, M.; O~ nate, E. Organometallics 2008, 27, 4680. (m) Ohki, Y.; Hatanaka, T.; Tatsumi, K. J. Am. Chem. Soc. 2008, 130, 17174. (n) Esteruelas, M. A.; Fernandez-Alvarez, F. J.; O~ nate, E. Organometallics 2008, 27, 6236.

(iv) a theoretical study on the rate-determining step of the H/D exchanges.

Results and Discussion 1. Capture of the Tetrahydride Species OsH4(PiPr3)2: Formation and Characterization of OsH4(Rpy)(PiPr3)2. The heating between 1 and 3 h under reflux of toluene solutions of the hexahydride complex OsH6(PiPr3)2 (1) generates the unsaturated tetrahydride species OsH4(PiPr3)2 (2), which can be trapped with pyridine, 3-methylpyridine, and 4-methylpyridine to afford the saturated tetrahydride derivatives OsH4(Rpy)(PiPr3)2 (Rpy = py (3), 3-Mepy (4), 4-Mepy (5)), which are isolated as yellow solids in 60%, 90%, and 96% yield, respectively, according to Scheme 1.15 The structure of complex 3 proves the coordination of the heterocycles to the metal center of 2 (Figure 1). The geometry around the osmium atom can be rationalized as a distorted pentagonal bipyramid with axial phosphines (P(1)-Os-P(2) = 170.94(6)°). The metal coordination sphere is completed by the hydride ligands (H-H > 1.52(6) A˚) and the pyridine, which lies between H(01) and H(04). The tetrahydride nature of 3 was confirmed16 by means of the optimization of the structure of the (14) (a) Barea, G.; Esteruelas, M. A.; Lled os, A.; L opez, A. M.; O~ nate, E.; Tolosa, J. I. Organometallics 1998, 17, 4065. (b) Barrio, P.; Castarlenas, R.; Esteruelas, M. A.; Lled os, A.; Maseras, F.; O~ nate, E.; Tomas, J. Organometallics 2001, 20, 442. (c) Barrio, P.; Castarlenas, R.; Esteruelas, M. A.; O~ nate, E. Organometallics 2001, 20, 2635. (d) Barrio, P.; Esteruelas, M. A.; O~ nate, E. Organometallics 2004, 23, 1340. (e) Barrio, P.; Esteruelas, M. A.; O~ nate, E. Organometallics 2004, 23, 3627. (f ) Baya, M.; Eguillor, B.; Esteruelas, M. A.; Lled os, A.; Olivan, M.; O~ nate, E. Organometallics 2007, 26, 5140. (g) Baya, M.; Eguillor, B.; Esteruelas, M. A.; Olivan, M.; O~ nate, E. Organometallics 2007, 26, 6556. (h) Eguillor, B.; Esteruelas, M. A.; Olivan, M.; Puerta, M. Organometallics 2008, 27, 445. (i) Esteruelas, M. A.; Masamunt, A. B.; Olivan, M.; O~ nate, E.; Valencia, M. J. Am. Chem. Soc. 2008, 130, 11612. (j) Esteruelas, M. A.; Forcen, E.; Olivan, M.; O~ nate, E. Organometallics 2008, 27, 6188. (15) Treatment of toluene solutions of 1 with 2-methylpyridine under reflux leads to the η2(C,N)-pyridyl derivative OsH3{η2-C,N[NC5H3Me]}(PiPr3)2. See ref 14j. (16) The hydride positions obtained from X-ray diffraction data are, in general, imprecise. However, DFT calculations have been shown to provide useful accurate data for the hydrogen positions in both classical polyhydride and dihydrogen complexes. See for example: Barrio, P.; Esteruelas, M. A.; Lled os, A.; O~ nate, E.; Tomas, J. Organometallics 2004, 23, 3008.

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Figure 1. Molecular diagram of complex 3. Selected bond lengths (A˚) and angles (deg): Os-P(1) = 2.3186(19), OsP(2) = 2.3213(18), Os-N = 2.162(6), H(01)-H(02) = 1.70(6), H(02)-H(03) = 1.52(6); H(03)-H(04) 1.85(6); P(1)-Os-P(2) = 170.94(6), P(1)-Os-N = 94.84(15), P(2)-Os-N = 94.19(15).

Eguillor et al.

heterocycle and parallel to the P-Os-P direction. The H(01)-H(02) separation is 0.959 A˚. The 31P{1H} NMR spectra of 4 and 5 are consistent with that of 3. Like the latter, between 303 and 183 K, they show a singlet at about 42 ppm. The 1H NMR spectrum of 4 reveals the asymmetry of the heterocycle. Thus, at 183 K, it contains four hydride resonances at -4.38, -4.63, -15.48, and -15.55 ppm. At this temperature, the 1H NMR spectrum of 5, like that of 3, shows two hydride resonances centered at -4.5 and -15.6 ppm. 2. Stoichiometric and Catalytic Deuteration of Pyridine. In benzene-d6 at 25 °C, complex 3 exchanges the hydrogen atoms of the heterocycle and the hydride ligands with deuterium atoms of the solvent. As a result of this, the intensity of the pyridine and hydride resonances in the 1 H NMR spectrum diminishes with time, while the intensity of the C6D6-xHx signal increases. The decreases of the pyridine resonances are exponential functions of the time (Figure 2a), in agreement with firstorder processes. Thus, first-order rate constants at 25 °C can be calculated by means of the graphical representation (Figure 2b) of the expression

ln Chart 1

model compound OsH4(py)(PMe3)2 (3t) at the DFT/B3PW91 level (Chart 1), which yields H-H separations longer than 1.696 A˚. The 31P{1H} and 1H NMR spectra of 3 in dicloromethane-d2 are consistent with the structure shown in Figure 1. In agreement with equivalent phosphine ligands, the 31P{1H} NMR spectrum contains a singlet at 42.0 ppm, which is temperature invariant between 303 and 183 K. In contrast to the 31P{1H} NMR spectrum, the 1H NMR spectrum is temperature dependent. As expected for two inequivalent hydride positions, two broad resonances centered -4.4 and -15.5 ppm are observed in the high-field region of the spectrum at 183 K. At about 238 K, coalescence between them takes place. This behavior indicates that in dicloromethane-d2 the hydride ligands H(01) and H(02) (or H(04) and H(03)) undergo a thermally activated position exchange. A ΔGq238 value of 9.6 kcal 3 mol-1 can be estimated for the process. The change in free energy for the position exchange between the H(01) and H(02) hydride ligands of 3t has been computed at 238 K and P = 1 atm. The process takes place through the transition state TS1 (Chart 1), which in agreement with the ΔGq238 value, lies 9.2 kcal 3 mol-1 above 3t. It is a dihydride-dihydrogen species with the hydrogen molecule H(01)-H(02) disposed cis to the nitrogen atom of the

I0 ¼ kt I

ð1Þ

where I0 is the initial intensity for each resonance and I its intensity at time t. The obtained values are collected in Table 1. The deuteration rate of the pyridine C-H bonds increases as they are separated from the heteroatom. So, the rate constants show a marked dependence on the position, decreasing in the sequence k4 > k3,5 > k2,6. The deuteration of the hydride positions merits additional comments. The rate decreases as the deuterium atoms are added to the metal center (Figure 3). Thus, in order to reach a right analysis of the overall process, the deuteration of the hydride positions has been divided into four stages corresponding to the gradual formation of OsDH3, OsD2H2, OsD3H, and OsD4 species. The formation of an OsDH3 species is very fast, as much as the deuteration of position 4 of pyridine. The rate constants for both processes are higher than 0.2 min-1. The estimated first-order rate constants for the subsequent formation of OsD2H2, OsD3H, and OsD4 species decrease in the sequence (7.0 ( 0.7) 10-2 min-1 > (5.0 ( 0.5) 10-2 min-1 > (2.0 ( 0.2) 10-2 min-1. The latter of these values is similar to the rate constant for the deuteration of positions 3 and 5 of the pyridine ((2.4 ( 0.2) 10-2 min-1) and an order of magnitude higher than that for the deuteration of positions 2 and 6 ((2.7 ( 0.3) 10-3 min-1). Scheme 2 shows a mechanism proposal for the H/D exchange between the pyridine ligand of 3 and benzene-d6, which is consistent with the experimental observations previously mentioned and should also allow the catalytic sequential deuteration of pyridine. Because the deuteration of at least a hydride position occurs before the deuteration of any pyridine positions and complex 3 is a saturated species, the dissociation of the heterocycle from the metal center, to afford the tetrahydride 2 (Scheme 1), should be the initial step of the process. In agreement with this, we have observed that the addition at 25 °C of 1.0 equiv of 4-methylpyridine to a dichloromethane-d2 solution of 3 yields a 1:1.2 equilibrium

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Figure 2. (a) Intensity (I) of the pyridine resonances in the 1H NMR spectrum of OsH4(py)(PiPr3)2 vs time. (b) ln(I0/I) vs time for the stoichiometric deuteration of the pyridine ring of OsH4(py)(PiPr3)2. Position 4 (2); positions 3 and 5 (9); positions 2 and 6 (O). Table 1. Stoichiometric Rate Constants at 25 °C

Figure 3. Intensity (I) vs time of the hydride resonance in the 1H NMR spectrum of OsH4(py)(PiPr3)2.

mixture of 3 and 5 (eq 2).

The unsaturated tetrahydride 2 not only is the responsible species for the stoichiometric deuteration of the heterocycle but should also be an active catalyst. During both processes, it should activate a C-D bond of benzened6 and eliminate benzene-d5 to generate OsDH3(PiPr3)2

Scheme 2

(2-d1), which could undergo a D/H exchange with pyridine to give 4-deuteriumpyridine (4-Dpy). Since the formation of OsDxH4-x species (x = 2, 3, 4) is faster than the deuteration of the heterocycle at positions 3,5 and 2,6, once pyridine is deuterated at position 4, intermediate 2-d1 should evolve into OsDxH4-x(PiPr3)2 (2-dx) by means of the C-D bond activation of new benzene-d6 molecules and elimination of benzene-d5. Species 2-dx should promote the subsequent deuteration at positions 3,5 and 2,6 of 4-deuteriumpyridine. The formation of 2-dx after the deuteration at position 4 but before the deuteration at positions 3,5 and 2,6 indicates that the elimination of (deuterium)xpyridine from deuteride-osmium-pyridyl intermediates is fast and that the C-H bond activation of the position to be deuterated is the rate-determining step for each H/D exchange. As expected according to Scheme 2, in benzene-d6 and in the presence of inert 1,4-dioxane as internal standard, complex 3 acts as a catalyst precursor for H/D exchange between pyridine and the solvent. The decrease of the pyridine resonances in the 1H NMR spectrum are exponential functions of the time (Figure 4a), in agreement with pseudo-first-order processes. Thus, at constant catalyst concentrations, rate constants kobs between 45 and 75 °C were calculated for the deuteration of each position by means of the graphical representation (Figure 4b) of the expression

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Eguillor et al.

Figure 4. (a) Disappearance of the pyridine protons (I) vs time for the catalytic deuteration of pyridine. (b) ln(I0/I) vs time for the catalytic deuteration of pyridine. T = 75 °C, [cat] = 1.6 10-2 M, [py]initial = 0.54 M. Position 4 (2); positions 3 and 5 (9); positions 2 and 6 (O). Table 2. Rate Constants for the Catalytic Deuteration of Pyridinea

Figure 5. kobs versus catalyst concentration for the catalytic deuteration of pyridine. T = 75 °C, [py]initial = 0.54 M. Position 4 (2); positions 3 and 5 (9); positions 2 and 6 (O).

I0 ¼ kobs t I

ð3Þ

kobs ¼ k½Os

ð4Þ

ln

T (°C)

[Os ] (M)

positions

kobs (min-1)

75

8.1 10-3

2,6 3,5 4

(3.9 ( 0.4) 10-3 (7.9 ( 0.8) 10-3 (8.4 ( 0.8) 10-3

0.48 ( 0.05 0.97 ( 0.09 1.03 ( 0.10

75

1.6 10-2

2,6 3,5 4

(5.6 ( 0.6) 10-3 (1.3 ( 0.1) 10-2 (1.6 ( 0.2) 10-2

0.35 ( 0.03 0.81 ( 0.08 1.01 ( 0.10

75

3.2 10-2

2,6 3,5 4

(1.2 ( 0.1) 10-2 (2.5 ( 0.2) 10-2 (3.2 ( 0.3) 10-2

0.38 ( 0.04 0.79 ( 0.08 1.00 ( 0.10

75

4.4 10-2

2,6 3,5 4

(1.6 ( 0.2) 10-2 (3.4 ( 0.3) 10-2 (4.8 ( 0.5) 10-2

0.36 ( 0.04 0.77 ( 0.07 1.09 ( 0.10

65

1.6 10-2

2,6 3,5 4

(2.6 ( 0.3) 10-3 (6.0 ( 0.6) 10-3 (6.9 ( 0.7) 10-3

0.16 ( 0.02 0.38 ( 0.04 0.43 ( 0.04

55

1.6 10-2

2,6 3,5 4

(5.0 ( 0.5) 10-4 (1.3 ( 0.1) 10-3 (1.5 ( 0.1) 10-3

(3.2 ( 0.3) 10-2 (8.2 ( 0.8) 10-2 (9.4 ( 0.9) 10-2

45

1.6 10-2

2,6 3,5 4

(2.0 ( 0.2) 10-4 (3.0 ( 0.3) 10-4 (5.0 ( 0.5) 10-4

(1.3 ( 0.1) 10-2 (1.9 ( 0.2) 10-2 (3.2 ( 0.3) 10-2

where

Plots of kobs versus catalyst concentration yield k values for the deuteration of each position (Figure 5). Table 2 collects the obtained values of kobs and k. The catalytic deuteration of the heterocycle is sequential and completely consistent with the stoichiometric process. As in the latter, the deuteration rate of the C-H bonds increases as they are separated from the heteroatom. Thus, the k values decrease in the sequence k4 (1.09 ( 0.10 min-1 3 mol-1 3 L) > k3,5 (0.73 ( 0.07 min-1 3 mol-1 3 L) > k2,6 (0.35 ( 0.03 min-1 3 mol-1 3 L). The selective and sequential deuteration of pyridine is notable. In this context it should be noted that in contrast to this homogeneous procedure deuterium gas heterogeneous metal catalyst affords the 2,6-dideuterated product.5 3. Stoichiometric and Catalytic Deuteration of 3-Methylpyridine and 4-Methylpyridine: Influence of the Substituent. In benzene-d6 at room temperature, the heterocycles of 4 and 5 also undergo deuteration. A kinetic analysis of the intensities of the C(sp2)-H resonances of the heterocycles in the 1 H NMR spectra similar to that previously mentioned for 3 leads to the k values collected in Table 1.

a

k (min-1 3 mol-1 3 L)

[py]initial = 0.54 M.

The deuterations of the aromatic positions of the heterocycles of 4 and 5 are slower than those of pyridine. This is a consequence of two effects of the substituent: (i) the methyl group increases the coordinating power of the heterocyles,17 which leads to a decrease of the concentration of the active tetrahydride 2 in the solution, and (ii) the methyl group has a marked negative influence on the deuteration of its adjacent positions. The higher coordinating power of 3-methylpyridine and 4-methylpyridine with regard to pyridine is evident from the comparison of the obtained yields in the preparations of 3, 4, and 5 (60% (py) versus 90% (3-Mepy) and 96% (4-Mepy)) and from the 3:5 molar ratio (1:1.2) obtained for the addition of 1.0 equiv of 4-methylpyridine to a dichloromethane solution of 3. The effect of the substituent on the deuteration of its adjacent positions is particularly evident in complex 5, with (17) Berthelot, M.; Laurance, C.; Safar, M.; Besseau, F. J. Chem. Soc., Perkin Trans. 2 1998, 283.

Article Table 3. Rate Constants for the Catalytic Deuteration of 3-Methylpyridine and 4-Methylpyridinea

a


3705

Chart 2

T = 75 °C, [cat] = 2.1 10-2 M, [heterocycle]initial = 0.54 M.

a methyl group at position 4 of the heterocycle. While the rate constant value for the deuteration of positions 2 and 6 ((2.4 ( 0.2) 10-4 min-1) is one magnitude order lower than that of 3 ((2.7 ( 0.3) 10-3 min-1), the rate constant value for the deuteration of positions 3 and 5 is three magnitude orders lower ((7.0 ( 0.7) 10-5 min-1 versus (2.4 ( 0.2) 10-2 min-1). As a consequence of this, in contrast to 3, the deuteration of positions 2 and 6 of the heterocycle of 5 is faster than the deuteration of positions 3 and 5. Complexes 4 and 5, in agreement with 3, also act as catalyst precursors for the catalytic deuteration of 3-methylpyridine and 4-methylpyridine, respectively, in benzene-d6 at 75 °C and in the presence of 1,4-dioxane as internal standard. The k values obtained according to eqs 3 and 4 are given in Table 3. Like for pyridine, the catalytic deuterations of these substituted heterocycles are completely consistent with the stoichiometric processes. The methyl group prevents the deuteration of its adjacent positions. The effect is particularly marked for 4-methylpyridine, which, in a similar manner to 5, shows a k3,5 value two magnitude orders lower than that of pyridine ((9.5 ( 0.9) 10-3 min-1 3 mol-1 3 L versus 0.73 ( 0.07 min-1 3 mol-1 3 L). As expected, in agreement with the stoichiometric process, the deuteration of positions 2 and 6 of 4-methylpyridine is faster than the deuteration of positions 3 and 5. The reduction of the deuteration rate of the positions adjacent to the methyl group, in the substituted heterocycles with regard to pyridine, is additional evidence in favor of the C-H bond activation as rate-determining step for the deuteration, since the steric hindrance generated by the substituent should deter the approach of the C-H bond to the metal center. 4. Theoretical Calculations on the Rate-Determining Step. In an effort to obtain information about the reasons for the different deuteration rates of the pyridine positions, we have carried out DFT calculations (B3PW91) on the processes of C-H bond activation of pyridine and 4-methylpyridine. The changes in free energy (ΔG) have been computed at 348.15 K and P = 1 atm. (18) See for example: (a) Belt, S. T.; Duckett, S. B.; Helliwell, M.; Perutz, R. N. J. Chem. Soc., Chem. Commun. 1989, 928. (b) Belt, S. T.; Dong, L.; Duckett, S. B.; Jones, W. D.; Partridge, M. G.; Perutz, R. N. J. Chem. Soc., Chem. Commun. 1991, 266. (c) Chin, R. M.; Dong, L.; Duckett, S. B.; Partridge, M. G.; Jones, W. D.; Perutz, R. N. J. Am. Chem. Soc. 1993, 115, 7685. (d) Johansson, L.; Tilset, M.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc. 2000, 122, 10846.

The kinetic advantage of the aromatic C-H bond activation with regard to the aliphatic one appears to be due to a prior η2-C,C arene coordination.12c These intermediates have been observed in solution and shown to be at equilibrium with aryl hydride compounds.18 In the last years some of them have been isolated and characterized even by X-ray diffraction analysis.19 The slippage of the metal center from the C-C double bond to the C-H one affords η2-C,H species.13n The importance of the latter coordination has been demonstrated, and there is increasing evidence showing that aromatic C-H bond activation can also proceed through such intermediates without the direct involvement of η2-C,C coordination.10c,20 In order to know the coordination mode of pyridine in the C-H bond activation steps, we have optimized the structures of the possible OsH4(pyridine)(PMe3)2 adducts. In addition to 3t, seven minima were found. Four of them showed η2-C,C coordination,21 and three one have η2-C,H coordination. A minimum containing a coordinated η2-C,N heterocycle has not been found. Chart 2 shows the structures of the η2-C,C adducts. They are tetrahydride species, with H-H separations longer than 1.735 A˚, which lie between 19.9 and 22.6 kcal 3 mol-1 above 3t. The coordinated C-C bond is disposed parallel or perpendicular to the P-Os-P direction. The parallel coordination of the C(2)-C(3) bond (6ta) is 1.6 kcal 3 mol-1 more stable than that of the C(3)-C(4) bond (6tb), while the perpendicular coordination of the C(2)-C(3) bond (6ta1) is 0.9 kcal 3 mol-1 less stable than that of the C(3)-C(4) bond (6tb1). For both bonds the parallel coordination is slightly more stable than the perpendicular one. (19) Reinartz, S.; White, P. S.; Brookhart, M.; Templeton, J. L. J. Am. Chem. Soc. 2001, 123, 12724. (20) See for example: (a) Lavin, M.; Holt, E. M.; Crabtree, R. H. Organometallics 1989, 8, 99. (b) Toner, A. J.; Grundemann, S.; Clot, E.; Limbach, H.-H.; Donnadieu, B.; Sabo-Etienne, S.; Chaudret, B. J. Am. Chem. Soc. 2000, 122, 6777. (c) Peterson, T. H.; Golden, J. T.; Bergman, R. G. J. Am. Chem. Soc. 2001, 123, 455. (d) Asplund, M. C.; Snee, P. T.; Yeston, J. S.; Wilkens, M. J.; Payne, C. K.; Yang, H.; Kotz, K. T.; Frei, H.; Bergman, R. G.; Harris, C. B. J. Am. Chem. Soc. 2002, 124, 10605. (21) Osmium compounds containing a diverse array of η2-bond aromatic ligands have been reported. See for example: Harman, W. D. Chem. Rev. 1997, 97, 1953, and references therein.

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Eguillor et al. Chart 3

Figure 6. Energy profile (ΔG in kcal 3 mol-1) for the C(sp2)-H activation of the different positions of pyridine (green positions 3 and 5; purple position 4, and blue positions 2 and 6).

The η2-C,H adducts are between 0.9 and 4.1 kcal 3 mol-1 more stable than the η2-C,C ones, suggesting that the C-H activation of the C(2)-H (C(6)-H), C(3)-H (C(5)-H), and C(4)-H bonds of pyridine takes place via η2-C,H intermediates (7t2,6, 7t3,5, and 7t4, respectively) without direct involvement of η2-C,C species. Figure 6 shows the energy profiles, starting from the η2-C,H adducts, whereas Chart 3 collects the optimized structures and selected structural parameters. The η2-C,H intermediates are also tetrahydride species, which show H-H separations longer than 1.753 A˚. The bond lengths within the H-C-Os units are between 2.544 and 2.635 A˚ (Os-C), 1.899 and 1.957 A˚ (Os-H), and 1.138 and 1.146 A˚ (C-H). The rupture of the C-H bonds takes place through the transition states TS2,6 (C(2)-H and C(6)-H), TS3,5 (C(3)-H and C(5)-H) and TS4 (C(4)-H), where the C-H bonds are extended about 0.5 A˚, while the Os-C and Os-H separations are shortened about 0.3 A˚, respectively. The products of the C-H addition are the 2-pyridyl-, 3-pyridyl-, and 4-pyridylpentahydride derivatives 8t2, 8t3, and 8t4, respectively. These compounds have the typical dodecahedral structure of the eight-coordinate osmium complexes OsH5LP2,14h,22 which is defined by two intersecting orthogonal trapezoidal planes. One of them contains the phosphorus atoms of the phosphines, the metalated carbon atom of the pyridyl group, and the hydride resulting from the C-H addition, while the remaining hydride ligands are situated in the other one. The relationship between the energy barriers for the activation of the different positions is determined by the relative stability of the η2-C,H intermediates and the activation energies for the ruptures of the coordinated C-H bonds. The η2-C,H intermediates lie between 18.5 and 20.8 kcal 3 mol-1 above 3t. The most stable of them is 7t3,5, resulting from the coordination of the C(3)-H or C(5)-H bonds, while 7t2,6, containing a heterocycle coordinated by position 2 or 6, is less stable. The coordination of C(4)-H gives an intermediate, 7t4, which lies between 7t3,5 and 7t2,6 (19.0 kcal 3 mol-1). However, in accordance with what is expected for a nucleophilic aromatic substitution of pyridine, the rupture of the C(4)-H bond (5.9 kcal 3 mol-1) is the process requiring the lowest activation energy from the possible activations, while the rupture of the C(3)-H or C(5)-H bonds needs the highest (6.9 kcal 3 mol-1). The rupture of the (22) See for example: (a) Esteruelas, M. A.; Lled os, A.; Martın, M.; Maseras, F.; Oses, R.; Ruiz, N.; Tomas, J. Organometallics 2001, 20, 5297. (b) Esteruelas, M. A.; Lled os, A.; Maseras, F.; Olivan, M.; O~ nate, E.; Tajada, M. A.; Tomas, J. Organometallics 2003, 22, 2087.

C(2)-H or C(6)-H bonds requires an intermediate energy (6.4 kcal 3 mol-1) between those of the ruptures of C(4)-H and C(3)-H or C(5)-H. Thus, in agreement with the experimental stoichiometric and catalytic results, the energies of the transition states for the C-H bond activation of the positions to be deuterated increase in the sequence TS4 (24.9 kcal 3 mol-1) < TS3,5 (25.4 kcal 3 mol-1) < TS2,6 (27.2 kcal 3 mol-1). Once the C-H addition to the metal center has taken place, position exchanges in the osmium coordination sphere23 should dispose the pyridyl group coplanar to some deuterium atom. This should afford a rapid elimination of deuterated pyridine, which is favored by a marked kinetic preference of the deuterium by the heterocycle. The coordination of the C(2)-H or C(6)-H bonds of 4-methylpyridine to 2t leads to η2-C,H intermediate 9t2,6, whereas the coordination of the C(3)-H or C(5)-H bonds affords 9t3,5. Figure 7 shows the energy profiles for the C(sp2)-H bond activations of 4-methylpyridine starting from these intermediates. Chart 4 collects the optimized structures and selected structural parameters. (23) (a) Maseras, F.; Koga, N.; Morokuma, K. J. Am. Chem. Soc. 1993, 115, 8313. (b) Bosque, R.; Maseras, F.; Eisenstein, O.; Patel, B. P; Yao, W.; Crabtree, R. H. Inorg. Chem. 1997, 36, 5505.

Article


Figure 7. Energy profile (ΔG in kcal 3 mol-1) for the C(sp2)-H activation of the different positions of 4-methylpyridine (green positions 3 and 5 and blue positions 2 and 6). Chart 4

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pyridine case, the C-H bonds are extended about 0.5 A˚ and the Os-C and Os-H separations are shortened about 0.4 and 0.3 A˚, respectively. The presence of a donating group at position 4 has no significant influence on the rupture of the C(2)-H or C(6)-H bonds, which requires a similar energy to the rupture in the pyridine (6.6 kcal 3 mol-1). On the other hand, it increases the necessary energy for the rupture of the coordinated C(3)-H or C(5)-H bonds with regard to that in the pyridine (9.2 versus 6.9 kcal 3 mol-1). As a result of the combination of both parameters, the coordination and the rupture, the transition state TSp2,6 is 3.4 kcal 3 mol-1 lower in energy than the transition state TSp3,5 (27.2 versus 30.6 kcal 3 mol-1), in agreement with the experimental stoichiometric and catalytic results. The addition of the C-H bonds to the metal center leads to the pyridyl pentahydride derivatives 10t2 and 10t3, which have the same dodecahedral structure as 8t2, 8t3, and 8t4.

Concluding Remarks

Intermediates 9t2,6 and 9t3,5 are tetrahydride derivatives with H-H separations longer than 1.754 A˚. The methyl substituent hinders the coordination to the metal center of both types of C-H bonds. The methyl influence is particularly marked in 9t3,5. While the Os-C and Os-H bond lengths (2.611 and 1.978 A˚, respectively) are about 0.07 and 0.05 A˚ longer than those of 7t3,5, the C-H distance is 0.007 A˚ shorter. A weaker coordination gives rise to a lower stability. Intermediate 9t3,5, which lies 21.4 kcal 3 mol-1 above 5t, is 0.8 kcal 3 mol-1 less stable than 9t2,6. The rupture of the coordinated C-H bonds takes place through the transition states TSp2,6 and TSp3,5. Like in the

This study has revealed that the aromatic positions of pyridine, 3-methylpyridine, and 4-methylpyridine can be sequentially deuterated in the presence of the unsaturated tetrahydride osmium(IV) complex OsH4(PiPr3)2, by means of stoichiometric and catalytic H/D exchanges with benzene-d6. The hexahydride osmium(IV) compound OsH6(PiPr3)2 loses the hydrogen molecule in toluene under reflux to afford OsH4(PiPr3)2, which is trapped with pyridinic nitrogen donor ligands. The coordination of the heterocycles stabilizes the unsaturated tetrahydride and allows its manipulation as OsH4(Rpy)(PiPr3)2 derivatives (Rpy = py, 3-methylpyridine, 4-methylpyridine). In benzene-d6, these compounds release the heterocycles and the unsaturated tetrahydride undergoes H/D exchanges with the solvent to yield OsDxH4-x(PiPr3)2 species (x = 1-4), which exchange the deuteride ligands with hydrogen atoms of pyridine, 3-methylpyridine, and 4-methylpyridine. The deuteration rates of the pyridinic C-H bonds depend upon their positions in the heterocycles. This fact is a consequence of the rate-determining step to the overall H/D exchange between the heterocycles and benzene-d6 being the C-H activation of the bond that is deuterated. This step is formed by two elemental stages: (i) the direct coordination of the C-H bond to the metal center and (ii) its subsequent rupture. Thus, the relationship between the deuteration rates of the different positions is determined by the relative stability of the η2-C,H intermediates and the activation energies for the ruptures of the coordinated C-H bonds. For pyridine, the stability of the η2-C,H intermediates increases in the sequence positions 2,6 < position 4 < positions 3,5, while the necessary energy for the rupture of the C-H bond disminishes in the sequence positions 3,5 > positions 2,6 > position 4. As a result of the combination of both effects, the deuteration rate increases as the C-H bond is separated from the heteroatom, i.e., positions 2,6 < positions 3,5 < position 4. A methyl susbtituent hinders the coordination of its adjacent C-H bonds. According to this, a marked negative effect of the methyl susbstituent on the deuteration of its adjacent positions is also observed. In conclusion, we have discovered a catalytic procedure for the sequential deuteration of pyridine and methylpyridines

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using benzene-d6 as a deuterium source. Furthermore, we have analyzed the elemental steps of the deuteration from a kinetic point of view and by means of DFT calculations, obtaining the reasons for the observed behavior.

Experimental Section All reactions were carried out under an argon atmosphere using Schlenk tube techniques. Toluene was obtained oxygenand water-free from an MBraun solvent purification apparatus, while methanol was dried and distilled under argon prior to use. Infrared spectra were recorded on a Perkin-Elmer Spectrum 100 spectrometer as neat solids. 1H, 13C{1H}, and 31P{1H} NMR spectra were recorded on either a Varian Gemini 2000, a Bruker ARX 300, a Bruker Avance 400, and a Bruker Avance 500 instrument. Chemical shifts are referenced to residual solvent peaks (1H and 13C{1H}) and external H3PO4 (31P{1H}). Coupling constants J and N (N = JP-H + JP0 -H for 1H; N = JP-C + JP0 -C for 13C{1H}) are given in hertz. C, H, and N analyses were measured on a Perkin-Elmer 2400 CHNS/O analyzer. The complex OsH6(PiPr3)2 (1)24 was prepared as previously described. Reaction of 1 with Pyridine: Preparation of OsH4(py)(PiPr3)2 (3). A solution of 1 (100 mg, 0.193 mmol) in toluene (15 mL) was treated with a stoichiometric amount of pyridine (15.6 μL, 0.193 mmol). The resulting solution was heated at 110 °C for 1 h.25 During this time the color changed from colorless to lemon yellow. The solution was filtered through Celite and was taken to dryness. The yellow residue was dissolved in methanol, and the resulting solution was cooled at -30 °C overnight. The yellow solid formed was washed with cold methanol and dried in vacuo. Yield: 70 mg (61%). Anal. Calcd for C23H51NOsP2: C, 46.52; H, 8.66; N, 2.36. Found: C, 46.49; H, 8.73; N, 2.29. IR (cm-1): ν(OsH) 2114 (m), ν(NdC) 1594 (w), 1568 (w). 1H NMR (300 MHz, CD2Cl2, 293 K): δ 9.47 (d, JH-H = 6.2, 2H, Hpy), 7.38 (t, JH-H = 6.2, 1H, Hpy), 6.78 (t, JH-H = 6.2, 2H, Hpy), 1.71 (m, 6H, PCH(CH3)2), 1.11 (dvt, JH-H = 6.3, N = 12, 36H, PCH(CH3)2), -9.71 (t, JH-P = 13.5, 4H, OsH). 1H NMR (300 MHz, CD2Cl2, 178 K, high-field region): δ -4.44 (br, 2H, OsH), -15.49 (br, 2H, OsH). 31P{1H} NMR (121.42 MHz, CD2Cl2, 293K): δ 42.0 (s). 13C{1H} NMR (75.42 MHz, CD2Cl2, 293 K, plus apt): δ 161.5 (s, Cpy), 132.2 (s, Cpy), 124.6 (s, Cpy), 27.4 (vt, N = 22.8, PCH(CH3)2), 20.2 (s, PCH(CH3)2). Reaction of 1 with 3-Methylpyridine: Preparation of OsH4(3-Mepy)(PiPr3)2 (4). A colorless solution of 1 (100 mg, 0.19 mmol) in 15 mL of toluene was treated with 2 equiv of 3-methylpyridine (37.5 μL, 0.39 mmol) and heated under reflux for 2 h. The resulting solution was dried in vacuo. Methanol was added to afford a yellow solid, which was washed with further portions of methanol and dried in vacuo. Yield: 106 mg (90%). Anal. Calcd for C24H53NOsP2: C, 47.42; H, 8.79; N, 2.30. Found: C, 47.23; H, 8.66; N, 2.28. IR (cm-1): ν(OsH) 2107 (s); ν(CdN) 1574 (m). 1H NMR (300 MHz, CD2Cl2, 293 K): δ 9.38 (s, 1H, 3-Mepy), 9.25 (d, JH-H = 5.9, 1H, 3-Mepy), 7.18 (d, JH-H = 7.5, 1H, 3-Mepy), 6.67 (dd, JH-H = 5.9, JH-H = 7.5, 1H, 3-Mepy), 2.12 (s, 3H, -CH3), 1.69 (m, 6H, PCH (CH3)2), 1.11 (dvt, JH-H = 6.7, N = 12, 36H, PCH(CH3)2), (24) Aracama, M.; Esteruelas, M. A.; Lahoz, F. J.; L opez, J. A.; Meyer, U.; Oro, L. A.; Werner, H. Inorg. Chem. 1991, 30, 288. (25) Heating during 5 h leads to a mixture of OsH4(py)(PiPr3)2 (3) and OsH3{η2-C,N-[NC5H4]}(PiPr3)2 in a ratio of 60:40. Spectroscopic data for OsH3{η2-C,N-[NC5H4]}(PiPr3)2: 1H NMR (300 MHz, benzene-d6, 293 K): δ 7.81 (1H, NC5H4), 6.98 (1H, NC5H4), 6.09 (1H, NC5H4), 5.87 (1H, NC5H4), 1.81 (m, 6H, PCH(CH3)2), 1.25 (dvt, JH-H = 6.3, N = 12.3, 36H, PCH(CH3)2), -5.07 (br, 1H, OsH), -11.43 (br, 1H, OsH), -14.11 (t, JH-P = 13.5, 1H, OsH). 31P{1H} NMR (121.42 MHz, benzene-d6, 293 K): δ 37.5 (s). These spectroscopic data agree well with those previously reported for the related compound OsH3{η2-C,N[NC5H4]}(PiPr3)2. See reference 14j.

Eguillor et al. -9.73 (t, JH-P = 13.5, 4H, OsH). 1H NMR (400 MHz, CD2Cl2, 193 K, high-field region): δ -4.38 (br, 1H, OsH), -4.63 (br, 1H, OsH), -15.48 (br, 1H, OsH), -15.55 (br, 1H, OsH). 13C{1H} NMR (100.56 MHz, CD2Cl2, 293 K, plus APT): δ 162.4, 159.0, 134.1, 133.0, 124.0 (all s, 3-Mepy), 27.6 (vt, N = 22.9, PCH (CH3)2), 20.3 (s, PCH(CH3)2), 19.8 (s, CH3). 31P{1H} NMR (161.9 MHz, C6D6, 293 K): δ 43.2 (s). Reaction of 1 with 4-Methylpyridine: Preparation of OsH4(4-Mepy)(PiPr3)2 (5). A colorless solution of 1 (100 mg, 0.19 mmol) in 15 mL of toluene was treated with 2 equiv of 4-methylpyridine (37.5 μL, 0.39 mmol) and heated under reflux for 3 h. The resulting solution was dried in vacuo. Methanol was added to afford a yellow solid, which was washed with further portions of methanol at 195 K and dried in vacuo. Yield: 113 mg (96%). Anal. Calcd for C24H53NOsP2: C, 47.42; H, 8.79; N, 2.30. Found: C, 47.31; H, 8.66; N, 2.35. IR (cm-1): ν(OsH) 2120 (m); ν(CdN) 1614 (w). 1H NMR (400 MHz, CD2Cl2, 293 K): δ 9.24 (d, JH-H = 5.1, 2H, 4-Mepy), 6.60 (d, JH-H = 5.1, 2H, 4-Mepy), 2.17 (s, 3H, -CH3), 1.67 (m, 6H, PCH(CH3)2), 1.09 (dvt, JH-H = 5.5, N = 11, 36H, PCH(CH3)2), -9.80 (t, JP-H = 13.1, 4H, OsH). 1H NMR (400 MHz, CD2Cl2, 183 K, high-field region): δ -4.54 (br, 2H, OsH), -15.57 (br, 2H, OsH). 13C{1H} NMR (100.56 MHz, CD2Cl2, 293 K, plus APT): δ 161.4, 143.9, 125.7 (all s, 4-Mepy), 27.5 (vt, N = 22.9, PCH(CH3)2), 20.4 (s, CH3), 20.2 (s, PCH(CH3)2). 31P{1H} NMR (161.9 MHz, CD2Cl2, 293 K): δ 42.3 (s). General Procedure for the Stoichiometric H/D Exchange Reactions. In an NMR tube the osmium complex (5 10-2 M) was dissolved in benzene-d6 (0.5 mL). The tube was then introduced in an NMR probe at 25 °C, and the reaction was monitored by 1H NMR spectroscopy at intervals over the course of 440 min. Integration relative to the CH multiplet of the PiPr3 ligands revealed that the intensity of the pyridine and hydride peaks decreased in intensity, while the intensity of the residual solvent peak increased. Reaction of OsH4(py)(PiPr3)2 with 4-Methylpyridine. In an NMR tube, a solution of OsH4(py)(PiPr3)2 (3, 19 mg, 3.2 10-2 mmol) in dichloromethane-d2 (0.5 mL) was treated with a stoichiometric amount of 4-methylpyridine (3.1 μL, 3.2 10-2 mmol). The reaction was periodically checked by 31P{1H} NMR spectroscopy, and after 7 h at room temperature an equilibrium 1:1.2 between OsH4(py)(PiPr3)2 and OsH4(4-Mepy) (PiPr3)2 was reached. Reaction of OsH4(4-Mepy)(PiPr3)2 with Pyridine. In an NMR tube, a solution of OsH4(4-Me-py)(PiPr3)2 (5, 15 mg, 2.46 10-3 mmol) in dichloromethane-d2 (0.5 mL) was treated with a stoichiometric amount of pyridine (2 μL, 2.46 10-2 mmol). The reaction was periodically checked by 31P{1H} NMR spectroscopy, and after 7 h at room temperature an equilibrium 1:0.8 between OsH4(4-Mepy)(PiPr3)2 and OsH4(py)(PiPr3)2 was reached. General Procedure for the Catalytic H-D Exchange Reactions. The experimental procedure is described for a particular case, but the same method was used in all experiments. In an NMR tube the osmium complex (2.1 10-2 M) was dissolved in benzene-d6 (0.5 mL). 1,4-Dioxane (4.5 μL, 5.3 10-2 mmol) was added as an internal standard, and the heterocycle (0.54 M) was added. The tube was then introduced in an NMR probe preheated at the desired temperature, and the reaction was monitored by 1 H NMR at intervals of time, with the extent of exchange being determined by integration of the residual pyridine signals against the internal standard. In order ensure accurate integration of the signals, the spectra were recorded using one scan and a 5 s delay. Structural Analysis of Complex 3. Crystals suitable for the X-ray diffraction study were obtained by cooling at -30 °C a solution of 3 in methanol. X-ray data were collected on a Bruker Smart APEX CCD diffractometer equipped with a normal focus, 2.4 kW sealed tube source (Mo radiation, λ = 0.71073 A˚) operating at 50 kV and 30 mA. Data were

Article collected over the complete sphere by a combination of four sets. Each frame exposure time was 10 s covering 0.3° in ω. Data were corrected for absorption by using a multiscan method applied with the SADABS program.26 The structure was solved by the Patterson method. Refinement, by full-matrix least-squares on F2 with SHELXL97,27 included isotropic and subsequently anisotropic displacement parameters. The hydrogen atoms were observed or calculated and refined freely or using a restricted riding model. Hydride ligands were observed in the difference Fourier maps and refined with restrained Os-H bond lengths (1.59(1) A˚, CSD). The highest electronic residuals were observed in close proximity to the Os center and make no chemical sense. Crystal data for 3: C23H51NOsP2, Mw 593.79, yellow, irregular block (0.10 0.08 0.07), monoclinic, space group P2(1)/c, a = 7.9417(14) A˚, b = 18.470(3) A˚, c = 18.331(3) A˚, β = 93.388 (3)o, V = 2684.2(8) A˚3, Z = 4, Dcalc = 1.469 g cm-3, F(000) = 1208, T = 100(2) K, μ = 4.878 mm-1; 28 194 measured reflections (2θ: 3-58°, ω scans 0.3°), 6672 unique (Rint = 0.0958); min./max. transmn factors 0.516/0.751. Final agreement factors were R1 = 0.0462 (3929 observed reflections, I > 2σ(I)) and wR2 = 0.0811; data/restraints/parameters 6672/4/270; GoF = 0.746. Largest peak and hole 1.924 and -1.246 e/A˚3. (26) Blessing, R. H. Acta Crystallogr. 1995, A51, 33. SADABS: Area-detector absorption correction; Bruker-AXS: Madison, WI, 1996. (27) SHELXTL Package v. 6.10; Bruker-AXS: Madison, WI, 2000, Sheldrick, G. M. Acta Crystallogr. 2008, A64, 112.


3709

Computational Details. The calculations have been carried out using the Gaussian 03 computational package.28 All structures have been optimized using DFT and the B3PW91functional. The 6-31G** basis set has been used for all the atoms except the Os, where the LANL2DZ basis and pseudopotential has been used instead, and the CH3 groups of the phosphine ligands (6-31d). The transition states found have been confirmed by frequency calculations, and the connection between the starting and final reactants has been checked by slightly perturbing the TS geometry toward the minima geometries and reoptimizing.

Acknowledgment. Financial support from the MICINN (Projects CTQ2008-00810 and Consolider Ingenio 2010 CSD2007-00006) and Diputaci on General de Arag on (E35) is acknowledged. Note Added after ASAP Publication. In the version of this paper published on the Web May 26, 2009, an incorrect value was given in Table 3. All the values which appear in this table as of June 1, 2009, are correct. Supporting Information Available: X-ray analysis and crystal structure determination, including bond lengths and angles of compound 3, complete ref 28, orthogonal coordinates, and absolute energies of the optimized theoretical structures. This material is available free of charge via the Internet at http://pubs.acs.org. (28) Pople, J. A.; et al. et al. Gaussian 03, Revision C.03; Gaussian, Inc.: Wallingford, CT, 2004,

Stoichiometric and Catalytic Deuteration of Pyridine and

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