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Palladium(II) Hydrido Complexes Having a Primary Silyl or Germyl Ligand: Synthesis, Crystal Structures, and Dynamic Behavior Norio Nakata, Shun Fukazawa, Nanami Kato, and Akihiko Ishii* Department of Chemistry, Graduate School of Science and Engineering, Saitama University, Shimo-okubo, Sakura-ku, Saitama 338-8570, Japan
bS Supporting Information ABSTRACT: The first examples of palladium(II) hydrido complexes having a dihydrosilyl or dihydrogermyl ligand, [PdH(EH2Trip)(dcpe)] (E = Si (2), Ge (4); dcpe = 1,2-bis(dicyclohexylphosphino)ethane), were synthesized by oxidative addition of an overcrowded primary silane and germane, TripEH3 (E = Si (1), Ge (3); Trip = 9-triptycyl) with [(μ-dcpe)Pd]2 in toluene. The molecular structures of complexes 2 and 4 were established by single-crystal X-ray analysis, which showed a distorted-square-planar geometry around the palladium centers, probably due to the steric requirements of the chelating dcpe ligand and the bulky 9-triptycyl group on the silicon or germanium atom. A variabletemperature NMR experiment on the palladium silyl complex 2 indicated that the intramolecular interchange of coordination environments between the silyl and hydrido ligands through an Si H σ-complex intermediate occurs on the NMR time scale.
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xidative addition of hydrosilane with palladium(0) species is an efficient method for the generation of a palladium(II) hydride complex, which has been proposed as a key reaction intermediate in palladium-catalyzed hydrosilylations1 and bissilylations,2 as well as cross-coupling reactions to give arylsilanes.3 While a number of reactions of hydrosilanes with palladium(0) complexes affording mononuclear bis(silyl)4 and silyl-bridged multinuclear palladium complexes5 have been described so far, the isolation of mononuclear hydrido silyl complexes is quite limited, due to the high reactivity of a Pd H bond. In 2002, Fink et al. demonstrated the reactions of tertiary silanes R3SiH with a palladium(0) complex bearing a strongly electron-donating bidentate phosphine ligand, [(μ-dcpe)Pd]26 (dcpe = 1,2-bis(dicyclohexylphosphino)ethane), to produce the first examples of stable hydrido silyl complexes, [PdH(SiR3)(dcpe)].7a They also have reported that the reaction of the hindered secondary silane t Bu2SiH2 with [(μ-dcpe)Pd]2 afforded the corresponding hydrido hydrosilyl complex [PdH(SiHtBu2)(dcpe)], which is in an equilibrium with the starting hydrosilane.7b These hydrido silyl complexes displayed a fluxional NMR behavior in solution, indicating the formation of intermediate Si H σ-complexes.8 Recently, Iwasawa et al. described the synthesis and structure of a new type of η2-Si H palladium(0) complex utilizing a bis(phosphinophenyl)silyl ligand as a PSiP pincer-type scaffold and its dynamic behavior through the reaction with an allene.9 However, palladium(II) hydrido dihydrosilyl complexes, which are anticipated as the initial products in the Si H bond activation reactions of primary hydrosilanes with palladium(0) complexes, have not been previously synthesized or characterized. Furthermore, there have been a few reports of palladium(II) hydrido stannyl complexes [PdH(SnR3)L2] containing a chelating bidentate phosphine ligand, which are prepared by the oxidative addition of hydrostannane with palladium(0) complexes10 or HX (X = OH, OMe) r 2011 American Chemical Society
addition across a PddSn double bond in palladium(II) stannylene complexes, {L2PddSn[CH(SiMe3)2]2}.11 To the best of our knowledge, the synthesis and isolation of the simple palladium(II) hydrido germyl complex [PdH(GeR3)L2] has never been explored so far, whereas germyl-bridged multinuclear palladium complexes12 and unique germapalladacycles13 formed by the reactions of secondary germanes with palladium(0) complexes have been reported by Osakada et al. Meanwhile, we recently succeeded in the first isolation of a series of platinum(II) dihydrogermyl hydrido complexes, [PtH(GeH2Trip)(L)]14 (Trip = 9-triptycyl), using a bulky substituent, the 9-triptycyl group.15 In addition, we reported the conversion of the bis-germyl complex [Pt(GeH2Trip)2(dppe)] (dppe = Ph2PCH2CH2PPh2) to the digermyl hydrido complex [PtH{Ge(HTrip)GeH2Trip}(dppe)] via facile thermal rearrangement of the secondary germyl ligand.14 Inspired by these results, we extended our chemistry toward investigations concerning the preparation of palladium(II) hydrido species. Herein, we present the first syntheses and structural characterizations of palladium(II) hydrido complexes having a dihydrosilyl or dihydrogermyl ligand, [PdH(EH2Trip)(dcpe)] (E = Si (2), Ge (4)), formed by the oxidative addition reactions of an overcrowded primary silane or germane, TripEH3 (E = Si (1), Ge (3)), with the palladium(0) complex [(μ-dcpe)Pd]2. We also describe the unique fluxional process of silyl complex 2, which involves an intermediate Si H σ complex. The complex [PdH(SiH2Trip)(dcpe)] (2) was readily prepared as colorless crystals in 82% yield by the reaction of TripSiH3 (1)16 with [(μ-dcpe)Pd]2, which was generated in situ by the reduction of [PdCl2(dcpe)] with an excess amount Received: June 30, 2011 Published: August 18, 2011 4490
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Organometallics of LiBEt3H in THF at room temperature (Scheme 1). In a similar manner, the germanium analogue [PdH(GeH2Trip)(dcpe)] (4) was efficiently produced by the reaction of TripGeH3 (3)17 with [(μ-dcpe)Pd]2 in 84% yield as colorless crystals (Scheme 1). The structural feature of the silyl complex 2 brings about a unique dynamic behavior in solution. In the 1H NMR spectrum of 2 in C7D8 at room temperature, no peaks due to the hydride proton were observed and the Si H protons appeared as a very broad signal at 5.68 ppm. As the temperature was lowered, the hydride proton resonated as a doublet at 1.49 ppm with a 31P 1H coupling constant of 176 Hz, which is slightly shifted downfield relative to that of [PdH(SiPh3)(dcpe)] ( 1.81 ppm).7a The silicon atom of 2 gave rise to a resonance at 50.5 ppm accompanied by splitting due to 31P 29Si couplings (2JP(trans) Si = 171 Hz, 2 JP(cis) Si = 8 Hz) in the 29Si{1H} NMR spectrum at 253 K. The 31 1 P{ H} NMR spectrum of 2 measured at 333 K showed only a broad signal at 61.9 ppm, whereas that measured at 283 K gave two nonequivalent signals at 60.4 and 65.3 ppm. The former signal was assigned to the phosphorus trans to the hydrido ligand using a non-1H-decoupled 31P NMR technique at 278 K (2JP H = 175 Hz). Moreover, the value of 1JSi H (70 Hz), which was determined by 31Pdecoupled 1H NMR spectroscopy at 243 K, was in the range of 40 70 Hz for the typical η2-Si H complexes and larger than usual values were found for the H M Si linkage complexes (1JSi H < 20 Hz).8d,9 This fluxional behavior and 1JSi H value indicate that the intramolecular interchange of coordination environments between the silyl and hydrido ligands through the Si H σ-complex intermediate as proposed by Fink7 occurs on the NMR time scale (Scheme 2).18 The activation parameters were estimated by variable-temperature 31P{1H} NMR experiments in the range of 283 333 K (Ea = 77.5 ( 1.7 kJ mol 1, ΔHq = 74.9 ( 1.6 kJ mol 1, ΔSq = 46.1 ( 5.3 J mol 1 K 1, Tc = 311 K) (Figure 1). The activation energy Ea is considerably greater than those previously obtained for hydrido complexes [PdH(SiR3)(dcpe)] (Ea = 40.1 53.9 kJ mol 1)7a and [PdH(SiHtBu2)(dcpe)] (Ea = 59.0 kJ mol 1).7b In contrast to [PdH(SiR3)(dcpe)] and [PdH(SiHR2)(dcpe)], which readily dissociate to the corresponding starting palladium(0) precursor and hydrosilanes or decompose to mononuclear bis(silyl) complexes, complex 2 is stable in solution without the addition of excess silane 1. In the case of the germyl complex 4, the 1H NMR spectrum at room temperature exhibits a hydride proton resonance at 2.84 ppm split into a doublet with a 31P 1H coupling constant of 194 Hz. In addition, the spectrum also displays a multiplet at 5.22 ppm corresponding to the Ge H resonance. The 31P{1H}
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NMR spectrum of 4 showed two doublets at 62.1 (2JP P = 15 Hz) and 71.7 ppm (2JP P = 15 Hz), which were assigned to the phosphorus atoms lying trans to the hydrido and germyl ligands, respectively, in agreement with the NMR data reported for [PtH(GeH2Trip)(dcpe)] (69.4 (1JPt P = 1793 Hz) and 83.2 ppm (1JPt P = 2161 Hz)).14 Complex 4 is thermally stable in the solid state (mp 115 116 °C dec) or in solution, and no dissociation into the palladium(0) precursor and the hydrogermane was observed on the NMR time scale. Furthermore, in the solid-state IR spectra for 2 and 4, the E H and Pd H stretching vibrations were observed at 2031 and 1893 cm 1 for 2 (E = Si), and 1921 and 1867 cm 1 for 4 (E = Ge), respectively. The molecular structures of 2 and 4 were confirmed unambiguously by X-ray crystallography, as depicted in Figures 2 and 3.19 The X-ray analyses of 2 and 4 revealed that each palladium center has a distorted-square-planar environment, probably due to steric repulsion between the chelating dcpe ligand and the 9-triptycyl group. The P Pd P angles (2, P(1) Pd(1) P(2) = 86.95(5)°; 4, P(1) Pd(1) P(2) = 87.32(3)°) are reduced slightly from the ideal value of 90° and are close to the range typically observed for Pd(dcpe) complexes (85.70(3) 88.26(5)°).4a,7,20 Despite the introduction of the overcrowded 9-triptycyl substituent on the silicon center, the Pd Si bond
Scheme 1
Figure 1. Observed (left) and simulated (right) 31P{1H} NMR spectra of [PdH(SiH2Trip)(dcpe)] (2) in the range 283 333 K (Tc = 311 K) in C7D8.
Scheme 2
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Figure 2. ORTEP drawing of [PdH(SiH2Trip)(dcpe)] (2) (30% thermal ellipsoids; three solvated THF molecules and hydrogen atoms except H1, H2, and H3 omitted for clarity). Selected bond lengths (Å): Pd1 Si1 = 2.3184(14), Pd1 P1 = 2.3102(14), Pd1 P2 = 2.3319(14), Pd1 H1 = 1.52(6), Si1 C1 = 1.935(5). Selected bond angles (deg): P1 Pd1 P2 = 86.95(5), Si1 Pd1 P1 = 101.70(5), Si1 Pd1 H1 = 74(2), P2 Pd1 H1 = 98(2), Si1 Pd1 P2 = 170.44(5), P1 Pd1 H1 = 175(2).
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trans to the hydrido ligand in 4 (Pd(1) P(1) = 2.3119(9) Å) is longer than the bond trans to the germyl ligand (Pd(1) P(2) = 2.2935(10) Å), indicating that the germyl ligand has a larger trans influence than the hydrido ligand. This tendency is agreement with that in the hydrido stannyl complex [PdH(SnMe3)(dtbpe)] (Pd(1) P(1) = 2.3030(16) Å, Pd(1) P(2) = 2.2935(10) Å) (dtbpe = tBu2PCH2CH2PtBu2) reported by P€orschke.10 Thus, the trans influence of the ER3 ligand (E = Si, Ge, Sn) increases by the magnitude of Pauling electron negativities for group 14 elements in the order Ge (2.01) > Sn (1.96) > Si (1.90).21 In summary, we have demonstrated that the reactions of the overcrowded primary silane 1 or germane 3 with Pd(0) complex stabilized by a bidentate dcpe ligand resulted in the successful formation of the novel palladium(II) hydrido complexes 2 and 4, having a dihydrosilyl or dihydrogermyl ligand. Further investigations on the reactivity of 2 and 4 are currently in progress.
’ ASSOCIATED CONTENT Supporting Information. CIF files giving crystallographic data for 2 and 4 and text, tables, and a figure tiving experimental procedures and spectral data for 2 and 4 and kinetic data for 2. This material is available free of charge via the Internet at http://pubs.acs.org.
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’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected].
’ ACKNOWLEDGMENT This work was partially supported by Grants-in-Aid for Scientific Research (Nos. 18750026 and 19027014) from the Ministry of Education, Science, Sports, and Culture of Japan. We are grateful to Professor Kohtaro Osakada and Dr. Makoto Tanabe of the Tokyo Institute of Technology for their useful discussions and suggestions. ’ REFERENCES Figure 3. ORTEP drawing of [PdH(GeH2Trip)(dcpe)] 4 (30% thermal ellipsoids; three solvated THF molecules and hydrogen atoms except H1, H2, and H3 omitted for clarity). Selected bond lengths (Å): Pd1 Ge1 = 2.4012(4), Pd1 P1 = 2.3119(9), Pd1 P2 = 2.9935(10), Pd1 H1 = 1.68(5), Ge1 C1 = 2.015(3). Selected bond angles (deg): P1 Pd1 P2 = 87.32(3), Ge1 Pd1 P1 = 100.46(3), Ge1 Pd1 H1 = 82.4(16), P2 Pd1 H1 = 90.1(16), Ge1 Pd1 P2 = 171.31(3), P1 Pd1 H1 = 175.8(16).
length (2.3184(14) Å) of 2 is slightly shorter than those of the previously reported mononuclear palladium(II) hydrido complex [PdH(SiPh3)(dcpe)] (2.335(2) and 2.330(2) Å)7a and the palladium(II) bis(silyl) complex [Pd(SiHMe2)2(dcpe)] (2.3563(9) and 2.359(1) Å).4a The Pd Ge bond length (2.4012(4) Å) of 4 is also shorter than that of the related mononuclear palladium(II) bis(germyl) complex [Pd(GeHPh2)2(dmpe)] (2.4259(2) and 2.4266(3) Å) (dmpe = Me2PCH2CH2PMe2).12 The Pd P bond trans to the hydrido ligand in 2 (Pd(1) P(1) = 2.3102(14) Å) is shortened compared with the bond trans to the silyl ligand (Pd(1) P(2) = 2.3319(14) Å), due to the strong structural trans influence of the silyl ligand. In sharp contrast, the Pd P bond
(1) For reviews on palladium-catalyzed hydrosilylations, see: (a) Marciniec, B. Comprehensive Handbook on Hydrosilylation; Pergamon Press: Oxford, 1992. (b) Ojima, I.; Li, Z.; Zhu, J. Recent Advances in Hydrosilylation and Related Reactions. In The Chemistry of Organic Silicon Compounds; Patai, S., Rappoport, Z., Eds.; Wiley: New York, 1998; p 1687. (c) Marciniec, B., Maciejewski, H., Pietraszuk, C., Pawluc, P. Hydrosilylation: A Comprehensive Review on Recent Advances; Marciniec, B., Ed.; Springer: Berlin, 2009; Advances in Silicon Science Vol. 1. (2) For reviews on palladium-catalyzed bis-silylations, see: (a) Sharma, H.; Pannell, K. H. Chem. Rev. 1995, 95, 1351–1374. (b) Suginome, M.; Ito, Y. Chem. Rev. 2000, 100, 3221–3256. (3) (a) Yamanoi, Y. J. Org. Chem. 2005, 70, 9607–9609. (b) Yamanoi, Y.; Taira, T.; Sato, J.; Nakamula, I.; Nishihara, H. Org. Lett. 2007, 9, 4543–4546. (c) Yabusaki, Y.; Ohshima, N.; Kondo, H.; Kusamoto, T.; Yamanoi, Y.; Nishihara, H. Chem. Eur. J. 2010, 16, 5581–5585. (d) Lesbani, A.; Kondo, H.; Yabusaki, Y.; Nakai, M.; Yamanoi, Y.; Nishihara, H. Chem. Eur. J. 2010, 16, 13519–13527. (4) (a) Pan, Y.; Mague, J. T.; Fink, M. J. Organometallics 1992, 11, 3495–3497. (b) Pan, Y.; Mague, J. T.; Fink, M. J. J. Am. Chem. Soc. 1993, 115, 3842–3843. (c) Murakami, M.; Yoshida, T.; Ito, Y. Organometallics 1994, 13, 2900–2902. (d) Tanabe, M.; Mawatari, A.; Osakada, K. Organometallics 2007, 26, 2937–2940. (e) Li, Y. H.; Shimada, S. Organometallics 2010, 29, 4406–4409. 4492
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(5) For recent reviews on silyl-bridged multinuclear palladium complexes, see: (a) Osakada, K.; Tanabe, M. Bull. Chem. Soc. Jpn. 2005, 78, 1887–1898. (b) Shimada, S.; Tanaka, M. Coord. Chem. Rev. 2006, 250, 991–1011. (c) Tanabe, M.; Osakada, K. Organometallics 2010, 29, 4702–4710. (6) Reid, S. M.; Fink, M. J. Organometallics 2001, 20, 2959–2961. (7) (a) Boyle, R. C.; Mague, J. T.; Fink, M. J. J. Am. Chem. Soc. 2003, 125, 3228–3229. (b) Boyle, R. C.; Pool, D.; Jacobsen, H.; Fink, M. J. J. Am. Chem. Soc. 2006, 128, 9054–9055. (8) For reviews on Si H σ complexes, see: (a) Crabtree, R. H. Angew. Chem., Int. Ed. Engl. 1993, 32, 789–805. (b) Schneider, J. J. Angew. Chem., Int. Ed. Engl. 1996, 35, 1068–1075. (c) Corey, J. Y.; Braddock-Wilking, J. Chem. Rev. 1999, 99, 175–292. (d) Corey, J. Y. Chem. Rev. 2001, 111, 863–1071. (9) Takaya, J.; Iwasawa, N. Organometallics 2009, 28, 6636–6638. (10) Trebbe, R.; Schager, F.; Goddard, R.; P€orschke, K.-R. Organometallics 2000, 19, 521–526. (11) Schager, F.; Seevogel, K.; P€orschke, K.-R.; Kessler, M.; Kr€uger, C. J. Am. Chem. Soc. 1996, 118, 13075–13076. (12) Tanabe, M.; Ishikawa, N.; Osakada, K. Organometallics 2006, 25, 796–798. (13) Tanabe, M.; Ishikawa, N.; Hanzawa, M.; Osakada, K. Organometallics 2008, 27, 5152–5158. (14) Nakata, N.; Fukazawa, S.; Ishii, A. Organometallics 2009, 28, 534–538. (15) For our recent applications of the Trip group, see: (a) Ishii, A.; Nakata, N.; Uchiumi, R.; Murakami, K. Angew. Chem., Int. Ed. 2008, 47, 2661–2664. (b) Nakata, N.; Uchiumi, R.; Yoshino, T.; Ikeda, T.; Kamon, H.; Ishii, A. Organometallics 2009, 28, 1981–1984. (c) Nakata, N.; Yamamoto, S.; Hashima, W.; Ishii, A. Chem. Lett. 2009, 38, 400–401. (d) Nakata, N.; Yoshino, T.; Ishii, A. Phosphorus, Sulfur, Silicon Relat. Elem. 2010, 185, 992–999. (e) Ishii, A.; Kamon, H.; Murakami, K.; Nakata, N. Eur. J. Org. Chem. 2010, 1653–1659. (16) Brynda, M.; Bernardinelli, G.; Dutan, C.; Geoffroy, M. Inorg. Chem. 2003, 42, 6586–6588. (17) Brynda, M.; Geoffroy, M.; Bernardinelli, G. Chem. Commun. 1999, 961–962. (18) The platinum(II) silyl hydrido complexes cis-[PtH(SiR3)(PCy3)2] show unique thermal behavior through the Pt(η2-H SiR3) interaction, which includes a phosphine exchange process and reductive elimination of silane involving two distinct transition states:Chan, D.; Duckett, S. B.; Heath, S. L.; Khazal, I. G.; Perutz, R. N.; Sabo-Etienne, S.; Timmins, P. L. Organometallics 2004, 23, 5744–5756. (19) Crystal data for 2 at 100 K: C58H88O3P2PdSi, MW = 1029.71, monoclinic, space group P21/c, Z = 4, a = 16.4057(7) Å, b = 19.7682(9) Å, c = 17.2878(7) Å, β = 105.3740(10)°, V = 5406.0(4) Å3, Dcalcd = 1.265 g cm 3, 2θmax = 50.00°, R1 (I > 2σ(I)) = 0.0690, wR2 (all data) = 0.1583 for 9523 reflections, 644 parameters, and 6 restraints, GOF = 1.025. Crystal data for 4 at 100 K: C58H88GeO3P2Pd, MW = 1074.21, monoclinic, space group P21/c, Z = 4, a = 16.4583(11) Å, b = 19.8712(13) Å, c = 17.2632(12) Å, β = 105.5310(10)°, V = 5439.7(6) Å3, Dcalcd = 1.312 g cm 3, 2θmax = 51.00°, R1 (I > 2σ(I)) = 0.0450, wR2 (all data) = 0.1166 for 10 126 reflections, 645 parameters, and 4 restraints, GOF = 1.041. (20) (a) Landtiser, R.; Mague, J. T.; Fink, M. J.; Silvestru, C.; Haiduc, I. Inorg. Chem. 1995, 34, 6141–6144. (b) Reid., S. M.; Mague, J. T.; Fink, M. J. J. Organomet. Chem. 2000, 616, 10–18. (21) Shriver, D. F.; Atkins, P. W. In Inorg. Chem., 4th ed.; Oxford University Press: Oxford, U.K., 2006; p 503.
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