Published on Web 08/14/2009
High Oxidation State Rhodium and Iridium Bis(silyl)dihydride Complexes Supported by a Chelating Pyridyl-Pyrrolide Ligand Jennifer L. McBee, Jose Escalada, and T. Don Tilley* Department of Chemistry, UniVersity of California, Berkeley, Berkeley, California 94720 Received April 30, 2009; E-mail:
[email protected] Abstract: New rhodium and iridium complexes containing the bidentate ligand 3,5-diphenyl-2-(2-pyridyl)pyrrolide (PyPyr) were prepared. The bis(ethylene) complex (PyPyr)Rh(C2H4)2 (3) reacted with HSiEt3, HSiPh3, and HSitBuPh2 to produce the 16-electron Rh(V) bis(silyl)dihydrides (PyPyr)Rh(H)2(SiEt3)2 (8), (PyPyr)Rh(H)2(SiPh3)2 (9), and (PyPyr)Rh(H)2(SitBuPh2)2 (10), respectively. The analogous Ir(V) bis(silyl)dihyride (PyPyr)Ir(H)2(SiPh3)2 (11) has also been synthesized. X-ray crystallography reveals that 9-11 adopt a coordination geometry best described as a bicapped tetrahedron. Silane elimination from 9 and 10 occurred in the presence of either HSiEt3 or PPh3. Mechanistic studies of the silane exchange process involving 10 and free HSiEt3 (to give 8) indicate that this process occurs by rate-limiting reductive elimination of HSitBuPh2 from 10 to generate a 14-electron Rh(III) intermediate of the type (PyPyr)Rh(H)(SitBuPh2).
Introduction
The activation of Si-H bonds by late transition metal complexes represents an important step in numerous catalytic reactions,1-10 including silane redistribution,11-15 reductions of haloarenes by silanes,16-18 and the dehydrogenative Si-C coupling of silanes with arenes.3,19-24 Complexes of Rh and Ir feature prominently in known Si-H bond activation chemistry, (1) Corey, J. Y.; Braddock-Wilking, J. Chem. ReV. 1999, 99, 175–292. (2) Tilley, T. D. In The Chemistry of Organic Silicon Compounds; Patai, S., Rappoport, Z., Eds.; Wiley: New York, 1989. (3) Tilley, T. D. Comments Inorg. Chem. 1990, 10, 37–46. (4) Tilley, T. D. In The Silicon-Heteroatom Bond; Patai, S., Rappoport, Z., Eds.; Wiley: New York, 1991. (5) Catalyzed Direct Reactions of Silicon; Lewis, K. M.; Rethwisch, D. G., Eds.; Elsevier: Amsterdam, The Netherlands, 1993. (6) Waterman, R.; Hayes, P. G.; Tilley, T. D. Acc. Chem. Res. 2007, 40, 712–719. (7) Eisen, M. S. In The Chemistry of Organosilicon Compounds; Rappoport, Z., Apeloig, Y., Eds.; Wiley: New York, 1998. (8) ComprehensiVe Handbook of Hydrosilation; Marciniec, B., Gulinski, J., Urbaniak, W., Kornetka, Z. W., Eds.; Pergamon: Oxford, 1992. (9) Ong, C. M.; Burchell, T. J.; Puddephatt, R. J. Organometallics 2004, 23, 1493–1495. (10) Bosnich, B. Acc. Chem. Res. 1998, 31, 667–674. (11) Gavenonis, J.; Tilley, T. D. Organometallics 2004, 23, 31–43. (12) Hashimoto, H.; Tobita, H.; Ogino, H. J. Organomet. Chem. 1995, 499, 205–211. (13) Radu, N. S.; Hollander, F. J.; Tilley, T. D. Chem. Comm. 1996, 21, 2459–2460. (14) Curtis, M. D.; Epstein, P. S. AdV. Organomet. Chem. 1981, 19, 213– 232. (15) Sharma, H. K.; Pannell, K. H. Chem. ReV. 1995, 95, 1351–1374. (16) Alonso, F.; Beletskaya, I. P.; Yus, M. Chem. ReV. 2002, 102, 4009– 4092. (17) Boukherroub, R.; Chatgiliaoglu, C.; Manuel, G. Organometallics 1996, 15, 1508–1510. (18) Esteruelas, M. A.; Herrero, J.; Lopez, F. M.; Martin, M.; Oro, L. A. Organometallics 1999, 18, 1110–1112. (19) Corey, J. Y. AdV. Organomet. Chem. 2004, 51, 1–52. (20) Gauvin, F.; Harrod, J. F.; Woo, H. G. AdV. Organomet. Chem. 1988, 42, 363–405. (21) Ezbiansky, K.; Djurovich, P. I.; LaForest, M.; Sinning, D.; Zayes, R.; Berry, D. H. Organometallics 1998, 17, 1455–1457. (22) Tsukada, N.; Hartwig, J. F. J. Am. Chem. Soc. 2005, 127, 5022–5023. 10.1021/ja9035169 CCC: $40.75 2009 American Chemical Society
and this may be at least partly due to the ability of metals to support silyl complexes in high oxidation states.25-36 In addition, these metals may activate Si-H bonds by way of formation of η2-silane complexes.37-41 Thus, it is of interest to explore new Si-H bond activations and related catalytic transformations for group 9 metal complexes, and in this context it is worth noting that most of the previous investigations have featured soft ancillary donor ligands (e.g., cyclopentadienyl and phosphine derivatives). Fewer investigations have involved silane conver-
(23) Murata, M.; Ishikura, M.; Nagata, M.; Watanabe, S.; Masuda, Y. Org. Lett. 2002, 4, 1843–1845. (24) Manoso, A. S.; DeShong, P. J. Org. Chem. 2001, 66, 7449–7455. (25) Cook, K. S.; Incarvito, C. D.; Webster, C. E.; Fan, Y. B.; Hall, M. B.; Hartwig, J. F. Angew. Chem., Int. Ed. 2004, 43, 5474–5477. (26) Duckett, S. B.; Haddleton, D. M.; Jackson, S. A.; Perutz, R. N.; Poliakoff, M.; Upmacis, R. K. Organometallics 1988, 7, 1526–1532. (27) Duckett, S. B.; Perutz, R. N. Chem. Comm. 1991, 1, 28–31. (28) Fernandez, M. J.; Bailey, P. M.; Bentz, P. O.; Ricci, J. S.; Koetzle, T. F.; Maitlis, P. M. J. Am. Chem. Soc. 1984, 106, 5458–5463. (29) Fernandez, M. J.; Maitlis, P. M. Dalton Trans. 1984, 9, 2063–2066. (30) Feldman, J. D.; Peters, J. C.; Tilley, T. D. Organometallics 2002, 21, 4065–4075. (31) Fernandez, M. J.; Maitlis, P. M. Organometallics 1983, 2, 164–165. (32) Gutierrez-Puebla, E.; Monge, A.; Paneque, M.; Poveda, M. L.; Taboada, S.; Trujillo, M.; Carmona, E. J. Am. Chem. Soc. 1999, 121, 346–354. (33) Klei, S. R.; Tilley, T. D.; Bergman, R. G. Organometallics 2002, 21, 4648–4661. (34) Loza, M. L.; de Gala, S. R.; Crabtree, R. H. Inorg. Chem. 1994, 33, 5073–5078. (35) Tanke, R. S.; Crabtree, R. H. Organometallics 1991, 10, 415–418. (36) Turculet, L.; Feldman, J. D.; Tilley, T. D. Organometallics 2004, 23, 2488–2502. (37) Bart, S. C.; Lobkovsky, E.; Chirik, P. J. J. Am. Chem. Soc. 2004, 126, 13794–13807. (38) Delpech, F.; Sabo-Etienne, S.; Chaudret, B.; Daran, J. C. J. Am. Chem. Soc. 1997, 119, 3167–3168. (39) Lin, Z. Chem. Soc. ReV. 2002, 31, 239–245. (40) Schneider, J. J. Angew. Chem., Int. Ed. 1996, 35, 1068–1075. (41) Taw, F. L.; Bergman, R. G.; Brookhart, M. Organometallics 2004, 23, 886–890. J. AM. CHEM. SOC. 2009, 131, 12703–12713
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Scheme 1
Scheme 2
sions at group 9 metal centers supported by hard ancillary ligands with oxygen or nitrogen donos.42 Recent investigations in these laboratories have explored use of the monoanionic, chelating 2-(2-pyridyl)indolide (PyInd) ligand with late transition metals.42,43 Interestingly, this ligand stabilizes coordinatively unsaturated Ir(V) and Rh(V) bis(silyl)dihydride complexes. The solid-state structure of the Ir(V) complex, (PyInd)Ir(H)2(SiPh2Me)2, revealed a highly distorted octahedral geometry. Furthermore, the Rh(V) bis(silyl)dihydride (PyInd)Rh(H)2(SiEt3)2 mediates the catalytic dehydrochlorinative coupling of chlorobenzene with triethylsilane. Given the results observed for (PyInd)Rh and (PyInd)Ir complexes, it was of interest to examine related systems with nitrogen-donor ligands that possess different steric and/or electronic properties. In this regard, the ligand 3,5-diphenyl-2(2-pyridyl)pyrrolide (PyPyr), reported by McNeill and coworkers,44 seemed of interest as it is more sterically demanding than PyInd. The PyPyr ligand has been found to support latemetal complexes in relatively high oxidation states, such as (PyPyr)AuMe245 and (PyPyr)PtMe3.46 This report describes the synthesis, structure and reactivity of Rh and Ir complexes containing the PyPyr ligand. This ligand, like PyInd, supports Rh(V) and Ir(V) bis(silyl)dihydrides which also exhibit geometries that are highly distorted from an ideal octahedron. Interestingly, the rhodium complexes have been found to exchange silyl hydride ligands with added tertiary silanes, and these reactions appear to proceed via 14 electron (PyPyr)Rh(H)(SiR3) intermediates. Results and Discussion Synthesis of (PyPyr)M(alkene)2 Complexes. The Rh(I) complexes were prepared using both (PyPyr)H and Li(PyPyr) (1) as reagents (Scheme 1). Treatment of (PyPyr)H with LiN(SiMe3)2 or nBuLi in benzene afforded Li(PyPyr) (1) as a yellow powder in good yields (75-91%) after removal of solvent in vacuo. Excess LiN(SiMe3)2 or nBuLi was removed by washing the isolated solid with hexane. The 1H NMR spectrum of 1 in benzene-d6 contains only broad aromatic peaks and confirms (42) Karshtedt, D.; Bell, A. T.; Tilley, T. D. Organometallics 2006, 25, 4471–4482. (43) Karshtedt, D.; McBee, J. L.; Bell, A. T.; Tilley, T. D. Organometallics 2006, 25, 1801–1811. (44) Klappa, J. J.; Rich, A. E.; McNeill, K. Org. Lett. 2002, 4, 435–437. (45) Schouteeten, S.; Allen, O. R.; Haley, A. D.; Ong, G. L.; Jones, G. D.; Vicic, D. A. J. Organomet. Chem. 2006, 691, 4975–4981. (46) Luedtke, A. T.; Goldberg, K. I. Inorg. Chem. 2007, 46, 8496–8498. 12704
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the complete consumption of starting material. In subsequent procedures, this solid was used without further purification. Addition of (PyPyr)H to 0.5 equiv of [RhCl(C2H4)2]247 in toluene proceeded partially (in 52% yield) to (PyPyrH)RhCl(C2H4)2 (2), characterized by 1H NMR spectroscopy as part of an equilibrium mixture with (PyPyr)H and [RhCl(C2H4)2]2 (Scheme 1). Subsequent treatment of this mixture with LiN(SiMe3)2 in benzene generated (PyPyr)Rh(C2H4)2 (3), which was isolated in 64% yield. A more direct route to 3 was accomplished via addition of 1 to [RhCl(C2H4)2]2 in benzene. After filtration and removal of the solvent in vacuo, complex 3 was isolated in 97% yield as an analytically pure solid. At 25 °C, the 1H NMR spectrum of 3 in benzene-d6 exhibits three broad vinylic resonances at 2.06 (2H), 2.86 (2H), and 3.07 (4H) ppm associated with the two ethylene ligands. Investigation of the 1H NMR spectrum of 3 by low temperature 1H NMR spectroscopy revealed that the ethylene resonance at 3.07 ppm decoalesces into two resonances at 4.08 (2H) and 2.09 (2H) ppm at -60 °C. The assignments of the 1H NMR resonances of the ethylene ligands in 3 are based on 1D NOESY at -60 °C. The 1H NMR resonance at 3.07 ppm corresponds to the four protons of the ethylene ligand cis to the pyrrolide donor and trans to the pyridyl ligand donor. The free energy of activation for the rotation of this ethylene ligand was determined to be 9.1 ( 2 kcal/mol at -60 °C. The Rh(I) and Ir(I) complexes with 1,5-cyclooctadiene (COD) as the ancillary ligand were obtained by a method similar to that used for 3, via a salt metathesis reaction (Scheme 2). Addition of a slurry of 1 in benzene to a solution of [(COD)RhCl]248 in benzene gave the brown solid (PyPyr)Rh(COD) (4) in 51% yield after recrystallization by vapor diffusion of pentane into a benzene solution of 4 at 25 °C. The Ir analogue was synthesized in a similar manner, from 1 and [(COD)IrCl]2,49 to give analytically pure orange platelets of (PyPyr)Ir(COD) (5) in 34%. The 1H NMR spectrum of 4 in benzene-d6 is similar to that of 3 in that the COD ligand occupies two inequivalent coordination sites, as indicated by two vinylic resonances (3.63 and 4.63 ppm) in the 1H NMR spectrum. Similarly for 5, two vinylic proton resonances for the COD ligand are observed at 3.39 and 4.60 ppm. Compounds 4 and 5 crystallized with isostructural solid-state structures in the space group C2/c (Figures 1 and 2). The pyridyl and pyrrolide ligand fragments are coplanar, while the phenyl substituents twist out of the pyrrolide plane. This aspect of the molecular structure may be described by the torsion angles between the phenyl planes and the pyridyl plane, which are 74° and 46° for 4 and 74° and 47° for 5. These structures are similar to that previously observed for [(bipy)Rh(COD)]PF6,50 as indicated by comparison of N-M-N angles (ca. 79°) and N-M bond distances (∼2.09 Å, Table 1). (47) (48) (49) (50)
Cramer, R. Inorg. Syn. 1990, 28, 86. Giordano, G.; Crabtree, R. H. Inorg. Syn. 1990, 28, 88. Herde, J. L.; Lambert, J. C.; Senoff, C. V. Inorg. Syn. 1974, 15, 18. Alejandro, F.; Guadalupe, A. R.; Huang, S. D. Z. Kristallogr-New Crystal Structures 1999, 214, 463.
Rhodium and Iridium Bis(silyl)dihydride Complexes
ARTICLES Scheme 3
Figure 1. ORTEP diagram of the X-ray crystal structure of 4. Hydrogen
atoms are omitted for clarity. Bond lengths (Å) and angles (deg): Rh1-N1 ) 2.079(7), Rh1-N2 ) 2.094(7), ∠N1-Rh1-N2 ) 79.1(3).
Figure 2. ORTEP diagram of the X-ray crystal structure of 5. Hydrogen atoms are omitted for clarity. Bond lengths (Å) and angles (deg): Ir1-N1 ) 2.087(5), Ir1-N2 ) 2.055(5), ∠N1-Ir1-N2 ) 79.2(2). Table 1. Selected Distances (Å) and Angles (deg) for Related Complexes complex
d(N-M), Å
∠N-M-N, deg
[(bipy)Rh(COD)]PF6 4 5
2.091(4), 2.098(4) 2.080(7), 2.087(7) 2.058(7), 2.082(7)
78.6(2) 78.8(3) 79.2(3)
Synthesis of (PyPyr)M(CO)2 Complexes. Carbonyl complexes of (PyPyr)Rh and (PyPyr)Ir were of interest as molecules that should help to characterize the electronic properties of the PyPyr ligand. The reaction of 1 with [RhCl(CO)2]251 in benzene provided a modest yield (17%) of (PyPyr)Rh(CO)2 (6) after workup and crystallization from a benzene/hexane solvent mixture (Scheme 2). Thus, an alternative route to 6 was sought, and bubbling 1 atm of CO through a solution of 3 in benzene for 1 h quantitatively gave 6 after removal of the volatile components under vacuum (eq 1). Similarly, 4 was treated with 1 atm of CO to quantitatively generate 6 by 1H NMR spectroscopy.
The analogous Ir carbonyl complex (PyPyr)Ir(CO)2 (7) was obtained in low yield (10%) by reaction of 1 with IrCl(CO)352 in benzene (eq 2). Complex 7 was isolated from toluene as analytically pure red crystals. An alternative route to 7 is based (51) McCleverty, J. A.; Wilkinson, G. Inorg. Syn. 1990, 28, 84. (52) Ginsberg, A. P.; Koepke, J. W.; Sprinkle, C. R. Inorg. Syn. 1979, 19, 220.
on treatment of 5 with 1 atm of CO, to quantitatively generate 7 (by 1H NMR spectroscopy).
The IR spectrum of 6 displays two carbonyl stretching frequencies at 2004 and 2067 cm-1 and these values are lower than those associated with the related complex [(bipy)Rh(CO)2]ClO4 (2050, 2108 cm-1).53 Similarly, the carbonyl stretching frequencies for 7 (1986 and 2050 cm-1) are at lower frequencies than those for [(bipy)Ir(CO)2]ClO4 (2024, 2089 cm-1).54 The lower frequency carbonyl stretches of 6 and 7 might reflect more electron-donating properties for PyPyr (vs bipy), but this difference may also be primarily attributed to the difference in charge on the complexes.55 Synthesis of (PyPyr)M(H)2(SiR3)2 Complexes. To compare the behaviors of (PyInd)M42,43 and (PyPyr)M fragments in Si-H activations, reactions of the PyPyr complexes (M ) Rh, Ir) with hydrosilanes were explored. Treatment of 3 with three equivs of HSiEt3, HSiPh3, or HSiPh2tBu in benzene resulted in formation of the 16-electron Rh(V) bis(silyl)dihydrides (PyPyr)Rh(H)2(SiEt3)2 (8), (PyPyr)Rh(H)2(SiPh3)2 (9), and (PyPyr)Rh(H)2(SiPh2tBu)2 (10), respectively, in good yields (Scheme 3). Under these conditions, the C2H4 ligands are lost as the free molecule and as the hydrosilylation product (R3SiEt). It is worth nothing that 8-10 can be generated with only 2 equiv of HSiR3; however, under these reaction conditions, the reaction times exceed 2 d. The 1H NMR spectra of 8-10 in benzene-d6 exhibit two doublets of doublets in the hydride regions, with observed coupling between the inequivalent hydride ligands and rhodium. For 8, the hydride resonances appear at -15.43 (JRhH ) 25.5 Hz, JHH ) 9.5 Hz) and -14.64 ppm (JRhH ) 25.0 Hz) and the coupling constants are consistent with classical rhodium hydrides resulting from complete Si-H oxidative addition (Table 2).56,57 A 2D NOESY experiment revealed that the hydride resonance at -15.43 ppm corresponds to a site cis to the pyrrolide group. Although the T1 value of 0.64 s for both hydrides is not particularly long, it is within the range expected for a classical hydride complex.58-60 Only one silyl resonance was found in (53) Reddy, G. K. N.; Ramesh, B. R. J. Organomet. Chem. 1974, 67, 443– 447. (54) Mestroni, G.; Camus, A. Inorg. Nucl. Chem. Lett. 1973, 9, 261–263. (55) Goldman, A. S.; Krogh-Jespersen, K. J. Am. Chem. Soc. 1996, 118, 12159–12166. (56) Heinrich, H.; Giernoth, R.; Bargon, J.; Brown, J. M. Chem. Comm. 2001, 14, 1296–1297. (57) Ezhova, M. B.; Patrick, B. O.; Sereviratne, K. N.; James, B. R.; Waller, F. J.; Ford, M. E. Inorg. Chem. 2005, 44, 1482–1491. J. AM. CHEM. SOC.
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Table 2. NMR Parameters for Complexes 8-11(a) 1
H (ppm)
8 9 10 11
-15.43 -14.64 -13.20 -11.90 -14.64 -13.02 -15.80 -13.98
29
Si (ppm) JHH (Hz) JRhH (Hz) JSiH (Hz) JRhSi (Hz) T1 (min) (s) 78.8
9.5
49.1
8.0
56.6
9.6
8.3
2.7
25.5 25.0 22.0 23.0 19.2 24.4
4.8 11.6 7.4 19.4
