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Cite this: Chem. Commun., 2014, 50, 820 Received 1st October 2013, Accepted 8th November 2013
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Bis-cyclooctatetraene tripalladium sandwich complexes† Tetsuro Murahashi,*abc Seita Kimura,ad Kohei Takase,d Tomohito Uemura,d Sensuke Ogoshid and Koji Yamamotoab
DOI: 10.1039/c3cc47515h www.rsc.org/chemcomm
We report synthesis and structural characterization of biscyclooctatetraene trinuclear palladium sandwich complexes. The non-planar cyclooctatetraene ligands flank an isosceles Pd3 triangle, which is supported by one equatorial ligand.
Chemistry of mononuclear sandwich complexes has developed extensively, since ferrocene was discovered and structurally elucidated in 1951–1952.1 The mononuclear metallocenes and their derivatives such as the half-sandwich complexes are now widely used in catalysis and materials science.2 Recently, multinuclear sandwich complexes, which contain a metal chain or a metal sheet between two unsaturated hydrocarbon ligands, were discovered.3,4 It has been shown that tropylium (Tr), C7H7+, serves as an excellent facial m3-binder for the triangular M3L3 core (M = Pd, Pt).4a,d,f,g Later, it has also been revealed that neutral 6p electron donor ligands, [2.2]paracyclophane and cycloheptatriene (CHT), form stable trinuclear sandwich complexes containing a M3L3 core (M = Pd) (Scheme 1).4c,d However, it has not been verified whether larger p-conjugated carbocycles are able to form a simple triangular trimetal sandwich complex. 1,3,5,7-Cyclooctatetraene (COT) or substituted COTs are known to show unique coordination behavior toward metals, with variable hapticity and geometrical changes, due to the greater number of CQC bonds and a more flexible electron-donating/back-donating nature.5–10 While the potential ability of COT as the facially capping p-coordinating ligand has been recognized since early preparations
a
Research Center of Integrative Molecular Systems (CIMoS), Institute for Molecular Science, National Institutes of Natural Sciences, Myodaiji, Okazaki, Aichi, 444-8787, Japan. E-mail: [email protected]; Fax: +81-564-59-5582 b Department of Structural Molecular Science, The Graduate University for Advanced Studies, Myodaiji, Okazaki, Aichi, 444-8787, Japan c PRESTO, Japan Science and Technology Agency (JST), Japan d Department of Applied Chemistry, Graduate School of Engineering, Osaka University, Suita, Osaka, 565-0871, Japan † Electronic supplementary information (ESI) available: Experimental details of the preparation and characterization of 1-H2O. CCDC 904367 and 962478. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/ c3cc47515h
820 | Chem. Commun., 2014, 50, 820--822
Scheme 1 The known ligand types of triangular trimetal sandwich complexes: bis-tropylium sandwich complexes, bis-[2.2]paracyclophane sandwich complexes and bis-cycloheptatriene sandwich complexes.
of m-COT dinuclear sandwich and half-sandwich complexes,8,9 as well as m3-COT half-sandwich complexes,10 recent findings represent the versatility of the COT ligand for a metal sheet complex, i.e., a homoleptic trimetal tris-COT complex, Fe3(m-COT)3, was recently isolated through a catalytic method,11 and the facial m4-coordination of COT to a square- or rhombic tetranuclear metal sheet was revealed recently (Scheme 2).4b,e Herein, we report successful isolation and structural characterization of the first discrete bis-COT trimetal sandwich complexes. The m3-COT ligands in the bis-COT Pd3 sandwich complex were proven to be substitutionally labile. The redox-condensation of a substitutionally labile PdI–PdI complex [Pd2(CH3CN)6][BF4]2 (ref. 12) and Pd2(dba)3 in the presence of COT (3 equiv.) in CH2Cl2 afforded a dark brown precipitate, which is soluble in CD3NO2. 1H NMR spectra of the product showed two sharp singlet signals at d 5.42 ppm for the COT ligand, and at d 2.45 ppm for the CH3CN ligand, which are similar to those of the bis-COT sandwich complex [Pd3(m3-COT)2(CH3CN)][BF4]2 (1-CH3CN) (see below for the isolation of 1-CH3CN). However, relative intensity of the acetonitrile proton signal with reference to that of COT in the
Scheme 2 Recently isolated metal sheet complexes of COT; Fe3(m-COT)3 and [Pd4(m4-COT)(m4-C9H9)][B(ArF)4].
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crude samples was always smaller than that expected for the composition with one CH3CN ligand and two COT ligands.13 Thus, treatment of a crude product with CH3CN (3 equiv.) in CH3NO2 afforded 1-CH3CN in 77% yield (eqn (1)).
(1) Upon recrystallization of 1-CH3CN in CH3NO2–benzene under aerobic conditions, a dark brown single crystal suitable for X-ray diffraction analysis was obtained. The crystal contains [Pd3(m3-COT)2(H2O)][BF4]2 (1-H2O) without a CH3CN ligand (Fig. 1).‡ The Pd3 moiety in the bis-COT sandwich framework adopts an isosceles triangle in which two Pd–Pd lengths (Pd1–Pd2 = 2.7321(8) Å; Pd2– Pd3 = 2.7359(8) Å) are in the range of normal Pd–Pd bond lengths while the Pd–Pd distance of the long side (Pd1 Pd3 = 3.0604(8) Å) is out of the range of normal Pd–Pd bond lengths. Thus, the Pd3 geometry can also be attributed to a bent chain with an acute Pd–Pd–Pd angle (Pd1–Pd2–Pd3 = 68.07(2)1). A Pd3 isosceles triangle is found in a tBu-CHT Pd3 sandwich complex [Pd3(m3-tBu-CHT)2(CH3CN)3][BF4]2 (2) in which the long side Pd–Pd is much shorter (2.7905(8) Å).4d The m3-COT ligands in 1-H2O coordinate to a Pd3 core through the Z3:Z2:Z3-mode, and the COT rings are not perfectly planar (the dihedral angle [C2, C3, C6, C7]/[C1, C2, C7, C8] = 251; [C10, C11, C14, C15]/[C9, C10, C15, C16] = 241) to make the Pd1–C1, Pd3–C8, Pd1–C9, Pd3–C16 bond lengths relatively short (2.29– 2.32 Å). The Pd3 moiety possesses an aqua ligand (H2O) attached to the Pd2 atom. There is another H2O molecule in a unit cell, which is not coordinated directly to the Pd3 core but hydrogen bonded with the coordinated H2O ligand (O O distance = 2.67 Å).14 The fact that the Pd3 core in the bis-COT Pd3 sandwich complex has only one equatorial ligand while that in 2 has three equatorial ligands might reflect the number of p-electrons donated by the carbocyclic ligands
Fig. 1 ORTEPs of [Pd3(m3-Z3:Z2:Z3-COT)2(H2O)][BF4]2 (1-H2O) (30% probability ellipsoids, BF4 anions and hydrogen-bonded H2O are omitted for clarity). Selected bond lengths (Å) and angles (deg): Pd1–Pd2 2.7321(8), Pd2–Pd3 2.7359(8), Pd1 Pd3 3.0604(8), Pd1–C1 2.306(8), Pd1–C2 2.130(7), Pd1–C3 2.291(7), Pd2–C4 2.204(7), Pd2–C5 2.193(6), Pd2–O1 2.228(6), Pd3–C6 2.291(6), Pd3–C7 2.140(7), Pd3–C8 2.312(7), C1–C2 1.420(11), C2–C3 1.414(11), C3–C4 1.438(10), C4–C5 1.413(10), C5–C6 1.422(10), C6–C7 1.418(10), C7–C8 1.404(10), C8–C1 1.464(10), Pd1–Pd2–Pd3 68.07(2).
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Fig. 2 Variable temperature 1H NMR spectra of [Pd3(m3-C8H8)2(PPh3)][B(ArF)4]2 (10 -PPh3). = impurity, + = CH2Cl2.
to the Pd3 core; i.e., simple electron counting suggested that each m3-Z3:Z2:Z3-COT ligand in 1-H2O donates eight electrons to a [Pd3L]2+ core, while each m3-Z2:Z2:Z2-tBu-CHT ligand in 2 donates six electrons to a [Pd3L3]2+ core. It is noted that the total valence electron count of 1-H2O (46e) is the same as that of 2. A deep purple PPh3 complex [Pd3(m3-COT)2(PPh3)][BF4]2 (1-PPh3) or a PCy3 complex [Pd3(m3-COT)2(PCy3)][BF4]2 (1-PCy3) was obtained by treatment of 1-CH3CN with PPh3 or PCy3 (1 equiv.). In solution, 1-CH3CN or 1-PPh3 showed a sharp singlet NMR signal for C8H8 protons or carbons at 25 1C (Fig. 2). Lowering the temperature down to 80 1C in CD2Cl2 resulted in significant broadening of the C8H8 proton signal for 10 -PPh3 having B{(3,5-CF3)2C6H3}4 counter anions instead of BF4, suggesting a dynamic fluxional rotation of the COT ligands (Fig. 2). The structure of 1-L seems to be related to that of a linear tripalladium sandwich complex of 1,8-diphenyloctatetraene (DPOT) [Pd3{m3-Ph(CHQCH)4Ph}2][B(ArF)4]2 (30 ),15 in view of the fact that the m3-Z3:Z2:Z3-coordination mode of COT in 1-L is the same as that of DPOT in 30 . However, the Pd3 chain in 30 is linear and thus has no ‘‘equatorial’’ ligands, leading to the different total electron count of 30 from that of 1-L. This difference might arise from both steric and electronic influences of the Pd3 geometry (i.e., isosceles vs. linear). Ligand coordination space in 30 appears narrow at three Pd atoms arranged in a linear form so rigidly. In 1, the steric restriction would not be so remarkable as in 30 . Furthermore, qualitative MO consideration suggests that among MOs dominated by d orbitals of [Pd3]2+ one vacant MO has such a unique hybridization at the central Pd atom in the case of isosceles [Pd3]2+ unit to yield good overlap with a s-donor lobe of the equatorial ligand (H2O). Such hybridization may have originated from s-antibonding interaction between the d orbitals of the central and terminal Pd atoms, being more effective with the greater degree of the bending of a [Pd3]2+ chain. Indeed, ca. 1201 bending of a Pd chain does not lead to acceptance of an equatorial ligand; i.e., coordination of CH3CN and the other ligand to the equatorial position was not observed for [Pd5{Ph(CHQCH)2(p-C6H4)(CHQCH)2Ph}2]2+ having a 1221 bent Pd5 chain.3c We confirmed that the COT ligands in 1-CH3CN can be smoothly replaced with DPOT in CD3NO2–1,2-C2D4Cl2 to afford [Pd3{m3-Ph(CHQCH)4Ph}2][BF4]2 (3)15 as a mixture of two isomers (Scheme 3, 80% yield, meso : rac = 34 : 66), while the reverse reaction of 30 with COT (5 equiv.) and CH3CN (3 equiv.) did not proceed at all even at 60 1C for 1 day.
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Scheme 3 Facile dissociation of the COT ligand in [Pd3(m3-C8H8)2(CH3CN)][BF4]2 (1-CH3CN).
It was also confirmed that the COT sandwich Pd3 complex 1-CH3CN decomposed in the presence of free COT (5 equiv.) in CD3NO2. Furthermore, 1-CH3CN immediately decomposed in CD3CN to form a mixture containing a dinuclear half-sandwich complex [Pd2(m-COT)(CD3CN)4][BF4]2 (4-CD3CN) (Scheme 3),16 while 1-CH3CN was stable in the presence of 5 equiv. of free CH3CN in CD3NO2. Similarly, addition of PPh3 (5 equiv.) to 4-CH3CN in CD3NO2 resulted in the decomposition of the trimetal sandwich framework to afford a dinuclear half-sandwich complex [Pd2(m-COT)(PPh3)4][BF4]2 in 71% yield (4-PPh3) (Scheme 3). The X-ray structural analysis of 4-PPh3 showed that the two PdII moieties are bound on the COT ring in an antifacial manner (see ESI†). The reaction of 1-CH3CN with 1,10-phenanthroline (3 equiv.) afforded the intriguing half-sandwich COT–Pd3 complex [Pd3(m3-COT)(phen)3][BF4]2 (5) in 72% yield (Scheme 3), which was recently prepared by a different preparative method.17 The reactivity of 1-CH3CN summarized in Scheme 3 showed the substitutionally labile nature of the m3-COT ligands in the bis-COT Pd3 sandwich complex. In summary, it has been proven that the bis-COT trimetal sandwich complexes of palladium are isolable. The present results unveiled the unique coordination behavior of COT on a trinuclear palladium assembly. Further reactivity study of the bis-COT Pd3 sandwich complexes is underway. This work was supported by the Japan Science and Technology Agency (JST), the Ministry of Education, Science, Sports, and Technology (MEXT), Japan, and Tokuyama Science Foundation. We thank Prof. H. Kurosawa for helpful discussion.
Notes and references ‡ Crystal data for 1-H2OH2OC6H6: C22H26B2F8O2Pd3, Mr = 815.25, monoclinic, space group P21/n (no. 14), a = 10.0392(6), b = 16.352(1), c = 14.6824(8) Å, b = 93.420(2)1, V = 2405.9(3) Å3, Z = 4, F(000) = 1576, Dc = 2.251 g cm3, m(MoKa) = 23.025 cm1, T = 123 K, 22 510 reflections collected, 5472 unique (Rint = 0.0545), 334 variables refined with 4221 reflections with I > 3s(I) to R = 0.054. CCDC 904367. 1 (a) T. J. Kealy and P. L. Pauson, Nature, 1951, 168, 1039; (b) S. A. Miller, J. A. Tebboth and J. F. Tremaine, J. Chem. Soc., 1952, 632; (c) G. Wilkinson, M. Rosenblum, M. C. Whiting and R. B. Woodward, J. Am. Chem. Soc., 1952, 74, 2125. 2 (a) C. Elschenbroich, Organometallics, Wiley-VCH, Weinheim, 3rd edn, 2006; (b) R. H. Crabtree, The Organometallic Chemistry of the Transition Metals, Wiley, 5th edn, 2009.
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Communication 3 (a) T. Murahashi, E. Mochizuki, Y. Kai and H. Kurosawa, J. Am. Chem. Soc., 1999, 121, 10660; (b) T. Murahashi, T. Uemura and H. Kurosawa, J. Am. Chem. Soc., 2003, 125, 8436; (c) Y. Tatsumi, K. Shirato, T. Murahashi, S. Ogoshi and H. Kurosawa, Angew. Chem., Int. Ed., 2006, 45, 4799; (d) Y. Tatsumi, T. Murahashi, M. Okada, S. Ogoshi and H. Kurosawa, Chem. Commun., 2008, 477; (e) T. Murahashi, K. Shirato, A. Fukushima, K. Takase, T. Suenobu, S. Fukuzumi, S. Ogoshi and H. Kurosawa, Nat. Chem., 2012, 4, 52. 4 (a) T. Murahashi, M. Fujimoto, M. Oka, Y. Hashimoto, T. Uemura, Y. Tatsumi, Y. Nakao, A. Ikeda, S. Sakaki and H. Kurosawa, Science, 2006, 313, 1104; (b) T. Murahashi, N. Kato, T. Uemura and H. Kurosawa, Angew. Chem., Int. Ed., 2007, 46, 3509; (c) T. Murahashi, M. Fujimoto, Y. Kawabata, R. Inoue, S. Ogoshi and H. Kurosawa, Angew. Chem., Int. Ed., 2007, 46, 5440; (d) T. Murahashi, Y. Hashimoto, K. Chiyoda, M. Fujimoto, T. Uemura, R. Inoue, S. Ogoshi and H. Kurosawa, J. Am. Chem. Soc., 2008, 130, 8586; (e) T. Murahashi, R. Inoue, K. Usui and S. Ogoshi, J. Am. Chem. Soc., 2009, 131, 9888; ( f ) T. Murahashi, K. Usui, R. Inoue, S. Ogoshi and H. Kurosawa, Chem. Sci., 2011, 2, 117; (g) T. Murahashi, K. Usui, Y. Tachibana, S. Kimura and S. Ogoshi, Chem.–Eur. J., 2012, 18, 8886. 5 H. Feng, H. Wang, Y. Xie, R. B. King and H. F. Schaefer, J. Organomet. Chem., 2010, 695, 2461, and references therein. 6 For selected examples of mononuclear bis-COT sandwich complexes: (a) G. Wilke, Angew. Chem., 1960, 72, 581; (b) H. Breil and G. Wilke, Angew. Chem., Int. Ed., 1966, 5, 898; (c) A. Carbonaro, A. Greco and G. Dall’Asta, Tetrahedron Lett., 1967, 22, 2037; (d) A. Streitwieser and U. Muller-Westerhoff, J. Am. Chem. Soc., 1968, 90, 7364; (e) D. Gourier, E. Samuel, B. Bachmann, F. Hahn and J. Heck, Inorg. Chem., 1992, 31, 86; ( f ) W. W. Brennessel, V. G. Young and J. E. Ellis, Angew. Chem., Int. Ed., 2002, 41, 1211. 7 For mononuclear Group 10 metal complexes of COT: (a) K. A. Jensen, Acta Chem. Scand., 1953, 7, 868; (b) H. P. Fritz and H. Keller, Chem. Ber., 1962, 95, 158; (c) G. Wilke, Pure Appl. Chem., 1978, 50, 677; (d) I. Bach, K.-R. Porschke, B. Proft, R. Goddard, C. Kopiske, C. Kruger, A. Rufinska and K. Seevogel, J. Am. Chem. Soc., 1997, 119, 3773; (e) F. Schager, K.-J. Haack, R. Mynott, A. Rufinska and K.-R. Porschke, Organometallics, 1998, 17, 807; ( f ) B. F. Straub and C. Gollub, Chem.–Eur. J., 2004, 10, 3081. 8 Only a few dinuclear sandwich complexes of COT have been reported: (a) D. J. Brauer and C. Kruger, J. Organomet. Chem., 1976, 122, 265; (b) W. Gausing, G. Wilke, C. Kruger and L. K. Liu, J. Organomet. Chem., 1980, 199, 137; (c) A. H. Connop, F. G. Kennedy, S. A. R. Knox and G. H. Riding, J. Chem. Soc., Chem. Commun., 1980, 520; (d) M. Horacek, V. Kupfer, U. Thewalt, P. Stepnicka, M. Polasek and K. Mach, J. Organomet. Chem., 1999, 584, 286. 9 For early examples of m-COT M2 complexes: (a) C. E. Keller, G. F. Emerson and G. F. Pettit, J. Am. Chem. Soc., 1965, 87, 1388; (b) E. B. Fleischer, A. L. Stone, R. B. K. Dewar, J. D. Wright, C. E. Keller and R. Pettit, J. Am. Chem. Soc., 1966, 88, 3158; (c) S. R. Ely, T. E. Hopkins and C. W. DeKock, J. Am. Chem. Soc., 1976, 122, 1624; (d) W. J. Evans, R. D. Clark, M. D. Ansari and J. W. Ziller, J. Am. Chem. Soc., 1998, 120, 9555. 10 For the m3-COT carbonyl clusters: (a) B. H. Robinson and J. Spencer, J. Organomet. Chem., 1971, 33, 97; (b) J. L. Davidson, M. Green, F. G. A. Stone and A. J. Welch, J. Chem. Soc., Dalton Trans., 1979, 506; (c) M. I. Bruce, P. A. Humphrey, B. W. Skelton and A. H. White, J. Organomet. Chem., 1996, 526, 85; (d) H. Wadepohl, S. Gebert, H. Pritzkow, F. Grepioni and D. Braga, Chem.–Eur. J., 1998, 4, 279; (e) H. Wadepohl, S. Gebert, R. Merkel and H. Pritzkow, Eur. J. Inorg. Chem., 2000, 783. 11 (a) V. Lavallo and R. H. Grubbs, Science, 2009, 326, 559; (b) V. Lavallo, A. El-Batta, G. Bertrand and R. H. Grubbs, Angew. Chem., Int. Ed., 2011, 50, 268. 12 (a) T. Murahashi, T. Nagai, T. Okuno, T. Matsutani and H. Kurosawa, Chem. Commun., 2000, 1689; (b) T. Murahashi, H. Nakashima, T. Nagai, Y. Mino, T. Okuno, M. A. Jalil and H. Kurosawa, J. Am. Chem. Soc., 2006, 128, 4377. 13 The observed relative ratios of CH3CN/C8H8 spanned (0.40.7)/2. At this stage, it cannot be concluded whether an unsaturated species [Pd3(m3-COT)2][BF4]2 exists in solution. 14 The Pd O distance (2.89 Å) is out of the range of the Pd–O bond. 15 T. Murahashi, Y. Higuchi, T. Katoh and H. Kurosawa, J. Am. Chem. Soc., 2002, 124, 14288. 16 T. Murahashi, N. Kato, S. Ogoshi and H. Kurosawa, J. Organomet. Chem., 2008, 693, 894. 17 T. Murahashi, K. Takase, K. Usui, S. Kimura, M. Fujimoto, T. Uemura, S. Ogoshi and K. Yamamoto, Dalton Trans., 2013, 42, 10626.
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Cite this: Chem. Commun., 2014, 50, 820 Received 1st October 2013, Accepted 8th November 2013
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Bis-cyclooctatetraene tripalladium sandwich complexes† Tetsuro Murahashi,*abc Seita Kimura,ad Kohei Takase,d Tomohito Uemura,d Sensuke Ogoshid and Koji Yamamotoab
DOI: 10.1039/c3cc47515h www.rsc.org/chemcomm
We report synthesis and structural characterization of biscyclooctatetraene trinuclear palladium sandwich complexes. The non-planar cyclooctatetraene ligands flank an isosceles Pd3 triangle, which is supported by one equatorial ligand.
Chemistry of mononuclear sandwich complexes has developed extensively, since ferrocene was discovered and structurally elucidated in 1951–1952.1 The mononuclear metallocenes and their derivatives such as the half-sandwich complexes are now widely used in catalysis and materials science.2 Recently, multinuclear sandwich complexes, which contain a metal chain or a metal sheet between two unsaturated hydrocarbon ligands, were discovered.3,4 It has been shown that tropylium (Tr), C7H7+, serves as an excellent facial m3-binder for the triangular M3L3 core (M = Pd, Pt).4a,d,f,g Later, it has also been revealed that neutral 6p electron donor ligands, [2.2]paracyclophane and cycloheptatriene (CHT), form stable trinuclear sandwich complexes containing a M3L3 core (M = Pd) (Scheme 1).4c,d However, it has not been verified whether larger p-conjugated carbocycles are able to form a simple triangular trimetal sandwich complex. 1,3,5,7-Cyclooctatetraene (COT) or substituted COTs are known to show unique coordination behavior toward metals, with variable hapticity and geometrical changes, due to the greater number of CQC bonds and a more flexible electron-donating/back-donating nature.5–10 While the potential ability of COT as the facially capping p-coordinating ligand has been recognized since early preparations
a
Research Center of Integrative Molecular Systems (CIMoS), Institute for Molecular Science, National Institutes of Natural Sciences, Myodaiji, Okazaki, Aichi, 444-8787, Japan. E-mail: [email protected]; Fax: +81-564-59-5582 b Department of Structural Molecular Science, The Graduate University for Advanced Studies, Myodaiji, Okazaki, Aichi, 444-8787, Japan c PRESTO, Japan Science and Technology Agency (JST), Japan d Department of Applied Chemistry, Graduate School of Engineering, Osaka University, Suita, Osaka, 565-0871, Japan † Electronic supplementary information (ESI) available: Experimental details of the preparation and characterization of 1-H2O. CCDC 904367 and 962478. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/ c3cc47515h
820 | Chem. Commun., 2014, 50, 820--822
Scheme 1 The known ligand types of triangular trimetal sandwich complexes: bis-tropylium sandwich complexes, bis-[2.2]paracyclophane sandwich complexes and bis-cycloheptatriene sandwich complexes.
of m-COT dinuclear sandwich and half-sandwich complexes,8,9 as well as m3-COT half-sandwich complexes,10 recent findings represent the versatility of the COT ligand for a metal sheet complex, i.e., a homoleptic trimetal tris-COT complex, Fe3(m-COT)3, was recently isolated through a catalytic method,11 and the facial m4-coordination of COT to a square- or rhombic tetranuclear metal sheet was revealed recently (Scheme 2).4b,e Herein, we report successful isolation and structural characterization of the first discrete bis-COT trimetal sandwich complexes. The m3-COT ligands in the bis-COT Pd3 sandwich complex were proven to be substitutionally labile. The redox-condensation of a substitutionally labile PdI–PdI complex [Pd2(CH3CN)6][BF4]2 (ref. 12) and Pd2(dba)3 in the presence of COT (3 equiv.) in CH2Cl2 afforded a dark brown precipitate, which is soluble in CD3NO2. 1H NMR spectra of the product showed two sharp singlet signals at d 5.42 ppm for the COT ligand, and at d 2.45 ppm for the CH3CN ligand, which are similar to those of the bis-COT sandwich complex [Pd3(m3-COT)2(CH3CN)][BF4]2 (1-CH3CN) (see below for the isolation of 1-CH3CN). However, relative intensity of the acetonitrile proton signal with reference to that of COT in the
Scheme 2 Recently isolated metal sheet complexes of COT; Fe3(m-COT)3 and [Pd4(m4-COT)(m4-C9H9)][B(ArF)4].
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crude samples was always smaller than that expected for the composition with one CH3CN ligand and two COT ligands.13 Thus, treatment of a crude product with CH3CN (3 equiv.) in CH3NO2 afforded 1-CH3CN in 77% yield (eqn (1)).
(1) Upon recrystallization of 1-CH3CN in CH3NO2–benzene under aerobic conditions, a dark brown single crystal suitable for X-ray diffraction analysis was obtained. The crystal contains [Pd3(m3-COT)2(H2O)][BF4]2 (1-H2O) without a CH3CN ligand (Fig. 1).‡ The Pd3 moiety in the bis-COT sandwich framework adopts an isosceles triangle in which two Pd–Pd lengths (Pd1–Pd2 = 2.7321(8) Å; Pd2– Pd3 = 2.7359(8) Å) are in the range of normal Pd–Pd bond lengths while the Pd–Pd distance of the long side (Pd1 Pd3 = 3.0604(8) Å) is out of the range of normal Pd–Pd bond lengths. Thus, the Pd3 geometry can also be attributed to a bent chain with an acute Pd–Pd–Pd angle (Pd1–Pd2–Pd3 = 68.07(2)1). A Pd3 isosceles triangle is found in a tBu-CHT Pd3 sandwich complex [Pd3(m3-tBu-CHT)2(CH3CN)3][BF4]2 (2) in which the long side Pd–Pd is much shorter (2.7905(8) Å).4d The m3-COT ligands in 1-H2O coordinate to a Pd3 core through the Z3:Z2:Z3-mode, and the COT rings are not perfectly planar (the dihedral angle [C2, C3, C6, C7]/[C1, C2, C7, C8] = 251; [C10, C11, C14, C15]/[C9, C10, C15, C16] = 241) to make the Pd1–C1, Pd3–C8, Pd1–C9, Pd3–C16 bond lengths relatively short (2.29– 2.32 Å). The Pd3 moiety possesses an aqua ligand (H2O) attached to the Pd2 atom. There is another H2O molecule in a unit cell, which is not coordinated directly to the Pd3 core but hydrogen bonded with the coordinated H2O ligand (O O distance = 2.67 Å).14 The fact that the Pd3 core in the bis-COT Pd3 sandwich complex has only one equatorial ligand while that in 2 has three equatorial ligands might reflect the number of p-electrons donated by the carbocyclic ligands
Fig. 1 ORTEPs of [Pd3(m3-Z3:Z2:Z3-COT)2(H2O)][BF4]2 (1-H2O) (30% probability ellipsoids, BF4 anions and hydrogen-bonded H2O are omitted for clarity). Selected bond lengths (Å) and angles (deg): Pd1–Pd2 2.7321(8), Pd2–Pd3 2.7359(8), Pd1 Pd3 3.0604(8), Pd1–C1 2.306(8), Pd1–C2 2.130(7), Pd1–C3 2.291(7), Pd2–C4 2.204(7), Pd2–C5 2.193(6), Pd2–O1 2.228(6), Pd3–C6 2.291(6), Pd3–C7 2.140(7), Pd3–C8 2.312(7), C1–C2 1.420(11), C2–C3 1.414(11), C3–C4 1.438(10), C4–C5 1.413(10), C5–C6 1.422(10), C6–C7 1.418(10), C7–C8 1.404(10), C8–C1 1.464(10), Pd1–Pd2–Pd3 68.07(2).
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Fig. 2 Variable temperature 1H NMR spectra of [Pd3(m3-C8H8)2(PPh3)][B(ArF)4]2 (10 -PPh3). = impurity, + = CH2Cl2.
to the Pd3 core; i.e., simple electron counting suggested that each m3-Z3:Z2:Z3-COT ligand in 1-H2O donates eight electrons to a [Pd3L]2+ core, while each m3-Z2:Z2:Z2-tBu-CHT ligand in 2 donates six electrons to a [Pd3L3]2+ core. It is noted that the total valence electron count of 1-H2O (46e) is the same as that of 2. A deep purple PPh3 complex [Pd3(m3-COT)2(PPh3)][BF4]2 (1-PPh3) or a PCy3 complex [Pd3(m3-COT)2(PCy3)][BF4]2 (1-PCy3) was obtained by treatment of 1-CH3CN with PPh3 or PCy3 (1 equiv.). In solution, 1-CH3CN or 1-PPh3 showed a sharp singlet NMR signal for C8H8 protons or carbons at 25 1C (Fig. 2). Lowering the temperature down to 80 1C in CD2Cl2 resulted in significant broadening of the C8H8 proton signal for 10 -PPh3 having B{(3,5-CF3)2C6H3}4 counter anions instead of BF4, suggesting a dynamic fluxional rotation of the COT ligands (Fig. 2). The structure of 1-L seems to be related to that of a linear tripalladium sandwich complex of 1,8-diphenyloctatetraene (DPOT) [Pd3{m3-Ph(CHQCH)4Ph}2][B(ArF)4]2 (30 ),15 in view of the fact that the m3-Z3:Z2:Z3-coordination mode of COT in 1-L is the same as that of DPOT in 30 . However, the Pd3 chain in 30 is linear and thus has no ‘‘equatorial’’ ligands, leading to the different total electron count of 30 from that of 1-L. This difference might arise from both steric and electronic influences of the Pd3 geometry (i.e., isosceles vs. linear). Ligand coordination space in 30 appears narrow at three Pd atoms arranged in a linear form so rigidly. In 1, the steric restriction would not be so remarkable as in 30 . Furthermore, qualitative MO consideration suggests that among MOs dominated by d orbitals of [Pd3]2+ one vacant MO has such a unique hybridization at the central Pd atom in the case of isosceles [Pd3]2+ unit to yield good overlap with a s-donor lobe of the equatorial ligand (H2O). Such hybridization may have originated from s-antibonding interaction between the d orbitals of the central and terminal Pd atoms, being more effective with the greater degree of the bending of a [Pd3]2+ chain. Indeed, ca. 1201 bending of a Pd chain does not lead to acceptance of an equatorial ligand; i.e., coordination of CH3CN and the other ligand to the equatorial position was not observed for [Pd5{Ph(CHQCH)2(p-C6H4)(CHQCH)2Ph}2]2+ having a 1221 bent Pd5 chain.3c We confirmed that the COT ligands in 1-CH3CN can be smoothly replaced with DPOT in CD3NO2–1,2-C2D4Cl2 to afford [Pd3{m3-Ph(CHQCH)4Ph}2][BF4]2 (3)15 as a mixture of two isomers (Scheme 3, 80% yield, meso : rac = 34 : 66), while the reverse reaction of 30 with COT (5 equiv.) and CH3CN (3 equiv.) did not proceed at all even at 60 1C for 1 day.
Chem. Commun., 2014, 50, 820--822 | 821
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Scheme 3 Facile dissociation of the COT ligand in [Pd3(m3-C8H8)2(CH3CN)][BF4]2 (1-CH3CN).
It was also confirmed that the COT sandwich Pd3 complex 1-CH3CN decomposed in the presence of free COT (5 equiv.) in CD3NO2. Furthermore, 1-CH3CN immediately decomposed in CD3CN to form a mixture containing a dinuclear half-sandwich complex [Pd2(m-COT)(CD3CN)4][BF4]2 (4-CD3CN) (Scheme 3),16 while 1-CH3CN was stable in the presence of 5 equiv. of free CH3CN in CD3NO2. Similarly, addition of PPh3 (5 equiv.) to 4-CH3CN in CD3NO2 resulted in the decomposition of the trimetal sandwich framework to afford a dinuclear half-sandwich complex [Pd2(m-COT)(PPh3)4][BF4]2 in 71% yield (4-PPh3) (Scheme 3). The X-ray structural analysis of 4-PPh3 showed that the two PdII moieties are bound on the COT ring in an antifacial manner (see ESI†). The reaction of 1-CH3CN with 1,10-phenanthroline (3 equiv.) afforded the intriguing half-sandwich COT–Pd3 complex [Pd3(m3-COT)(phen)3][BF4]2 (5) in 72% yield (Scheme 3), which was recently prepared by a different preparative method.17 The reactivity of 1-CH3CN summarized in Scheme 3 showed the substitutionally labile nature of the m3-COT ligands in the bis-COT Pd3 sandwich complex. In summary, it has been proven that the bis-COT trimetal sandwich complexes of palladium are isolable. The present results unveiled the unique coordination behavior of COT on a trinuclear palladium assembly. Further reactivity study of the bis-COT Pd3 sandwich complexes is underway. This work was supported by the Japan Science and Technology Agency (JST), the Ministry of Education, Science, Sports, and Technology (MEXT), Japan, and Tokuyama Science Foundation. We thank Prof. H. Kurosawa for helpful discussion.
Notes and references ‡ Crystal data for 1-H2OH2OC6H6: C22H26B2F8O2Pd3, Mr = 815.25, monoclinic, space group P21/n (no. 14), a = 10.0392(6), b = 16.352(1), c = 14.6824(8) Å, b = 93.420(2)1, V = 2405.9(3) Å3, Z = 4, F(000) = 1576, Dc = 2.251 g cm3, m(MoKa) = 23.025 cm1, T = 123 K, 22 510 reflections collected, 5472 unique (Rint = 0.0545), 334 variables refined with 4221 reflections with I > 3s(I) to R = 0.054. CCDC 904367. 1 (a) T. J. Kealy and P. L. Pauson, Nature, 1951, 168, 1039; (b) S. A. Miller, J. A. Tebboth and J. F. Tremaine, J. Chem. Soc., 1952, 632; (c) G. Wilkinson, M. Rosenblum, M. C. Whiting and R. B. Woodward, J. Am. Chem. Soc., 1952, 74, 2125. 2 (a) C. Elschenbroich, Organometallics, Wiley-VCH, Weinheim, 3rd edn, 2006; (b) R. H. Crabtree, The Organometallic Chemistry of the Transition Metals, Wiley, 5th edn, 2009.
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Communication 3 (a) T. Murahashi, E. Mochizuki, Y. Kai and H. Kurosawa, J. Am. Chem. Soc., 1999, 121, 10660; (b) T. Murahashi, T. Uemura and H. Kurosawa, J. Am. Chem. Soc., 2003, 125, 8436; (c) Y. Tatsumi, K. Shirato, T. Murahashi, S. Ogoshi and H. Kurosawa, Angew. Chem., Int. Ed., 2006, 45, 4799; (d) Y. Tatsumi, T. Murahashi, M. Okada, S. Ogoshi and H. Kurosawa, Chem. Commun., 2008, 477; (e) T. Murahashi, K. Shirato, A. Fukushima, K. Takase, T. Suenobu, S. Fukuzumi, S. Ogoshi and H. Kurosawa, Nat. Chem., 2012, 4, 52. 4 (a) T. Murahashi, M. Fujimoto, M. Oka, Y. Hashimoto, T. Uemura, Y. Tatsumi, Y. Nakao, A. Ikeda, S. Sakaki and H. Kurosawa, Science, 2006, 313, 1104; (b) T. Murahashi, N. Kato, T. Uemura and H. Kurosawa, Angew. Chem., Int. Ed., 2007, 46, 3509; (c) T. Murahashi, M. Fujimoto, Y. Kawabata, R. Inoue, S. Ogoshi and H. Kurosawa, Angew. Chem., Int. Ed., 2007, 46, 5440; (d) T. Murahashi, Y. Hashimoto, K. Chiyoda, M. Fujimoto, T. Uemura, R. Inoue, S. Ogoshi and H. Kurosawa, J. Am. Chem. Soc., 2008, 130, 8586; (e) T. Murahashi, R. Inoue, K. Usui and S. Ogoshi, J. Am. Chem. Soc., 2009, 131, 9888; ( f ) T. Murahashi, K. Usui, R. Inoue, S. Ogoshi and H. Kurosawa, Chem. Sci., 2011, 2, 117; (g) T. Murahashi, K. Usui, Y. Tachibana, S. Kimura and S. Ogoshi, Chem.–Eur. J., 2012, 18, 8886. 5 H. Feng, H. Wang, Y. Xie, R. B. King and H. F. Schaefer, J. Organomet. Chem., 2010, 695, 2461, and references therein. 6 For selected examples of mononuclear bis-COT sandwich complexes: (a) G. Wilke, Angew. Chem., 1960, 72, 581; (b) H. Breil and G. Wilke, Angew. Chem., Int. Ed., 1966, 5, 898; (c) A. Carbonaro, A. Greco and G. Dall’Asta, Tetrahedron Lett., 1967, 22, 2037; (d) A. Streitwieser and U. Muller-Westerhoff, J. Am. Chem. Soc., 1968, 90, 7364; (e) D. Gourier, E. Samuel, B. Bachmann, F. Hahn and J. Heck, Inorg. Chem., 1992, 31, 86; ( f ) W. W. Brennessel, V. G. Young and J. E. Ellis, Angew. Chem., Int. Ed., 2002, 41, 1211. 7 For mononuclear Group 10 metal complexes of COT: (a) K. A. Jensen, Acta Chem. Scand., 1953, 7, 868; (b) H. P. Fritz and H. Keller, Chem. Ber., 1962, 95, 158; (c) G. Wilke, Pure Appl. Chem., 1978, 50, 677; (d) I. Bach, K.-R. Porschke, B. Proft, R. Goddard, C. Kopiske, C. Kruger, A. Rufinska and K. Seevogel, J. Am. Chem. Soc., 1997, 119, 3773; (e) F. Schager, K.-J. Haack, R. Mynott, A. Rufinska and K.-R. Porschke, Organometallics, 1998, 17, 807; ( f ) B. F. Straub and C. Gollub, Chem.–Eur. J., 2004, 10, 3081. 8 Only a few dinuclear sandwich complexes of COT have been reported: (a) D. J. Brauer and C. Kruger, J. Organomet. Chem., 1976, 122, 265; (b) W. Gausing, G. Wilke, C. Kruger and L. K. Liu, J. Organomet. Chem., 1980, 199, 137; (c) A. H. Connop, F. G. Kennedy, S. A. R. Knox and G. H. Riding, J. Chem. Soc., Chem. Commun., 1980, 520; (d) M. Horacek, V. Kupfer, U. Thewalt, P. Stepnicka, M. Polasek and K. Mach, J. Organomet. Chem., 1999, 584, 286. 9 For early examples of m-COT M2 complexes: (a) C. E. Keller, G. F. Emerson and G. F. Pettit, J. Am. Chem. Soc., 1965, 87, 1388; (b) E. B. Fleischer, A. L. Stone, R. B. K. Dewar, J. D. Wright, C. E. Keller and R. Pettit, J. Am. Chem. Soc., 1966, 88, 3158; (c) S. R. Ely, T. E. Hopkins and C. W. DeKock, J. Am. Chem. Soc., 1976, 122, 1624; (d) W. J. Evans, R. D. Clark, M. D. Ansari and J. W. Ziller, J. Am. Chem. Soc., 1998, 120, 9555. 10 For the m3-COT carbonyl clusters: (a) B. H. Robinson and J. Spencer, J. Organomet. Chem., 1971, 33, 97; (b) J. L. Davidson, M. Green, F. G. A. Stone and A. J. Welch, J. Chem. Soc., Dalton Trans., 1979, 506; (c) M. I. Bruce, P. A. Humphrey, B. W. Skelton and A. H. White, J. Organomet. Chem., 1996, 526, 85; (d) H. Wadepohl, S. Gebert, H. Pritzkow, F. Grepioni and D. Braga, Chem.–Eur. J., 1998, 4, 279; (e) H. Wadepohl, S. Gebert, R. Merkel and H. Pritzkow, Eur. J. Inorg. Chem., 2000, 783. 11 (a) V. Lavallo and R. H. Grubbs, Science, 2009, 326, 559; (b) V. Lavallo, A. El-Batta, G. Bertrand and R. H. Grubbs, Angew. Chem., Int. Ed., 2011, 50, 268. 12 (a) T. Murahashi, T. Nagai, T. Okuno, T. Matsutani and H. Kurosawa, Chem. Commun., 2000, 1689; (b) T. Murahashi, H. Nakashima, T. Nagai, Y. Mino, T. Okuno, M. A. Jalil and H. Kurosawa, J. Am. Chem. Soc., 2006, 128, 4377. 13 The observed relative ratios of CH3CN/C8H8 spanned (0.40.7)/2. At this stage, it cannot be concluded whether an unsaturated species [Pd3(m3-COT)2][BF4]2 exists in solution. 14 The Pd O distance (2.89 Å) is out of the range of the Pd–O bond. 15 T. Murahashi, Y. Higuchi, T. Katoh and H. Kurosawa, J. Am. Chem. Soc., 2002, 124, 14288. 16 T. Murahashi, N. Kato, S. Ogoshi and H. Kurosawa, J. Organomet. Chem., 2008, 693, 894. 17 T. Murahashi, K. Takase, K. Usui, S. Kimura, M. Fujimoto, T. Uemura, S. Ogoshi and K. Yamamoto, Dalton Trans., 2013, 42, 10626.
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