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Cite this: Chem. Commun., 2013, 49, 11755 Received 24th September 2013, Accepted 22nd October 2013
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Synthesis of titanium and zirconium complexes supported by a p-terphenoxide ligand and their reactions with N2, CO2 and CS2† Takashi Kurogi, Yutaka Ishida and Hiroyuki Kawaguchi*
DOI: 10.1039/c3cc47284a www.rsc.org/chemcomm
The synthesis, characterization and reactions of a series of titanium and zirconium complexes that incorporate a p-terphenolate ligand are described.
Much work in coordination chemistry has been devoted to the study of early transition metal complexes with low oxidation states and low coordination numbers. Interest in this area stems from their potential for diverse applications ranging from organic synthesis1 to activation of small molecules such as N2.2 The design of ligand systems is of critical importance to gain access to low-valent, lowcoordinate complexes of early transition metals or their synthetic equivalents. One strategy is to utilize coordinated arenes or ligand systems containing a hemilabile p-arene unit. For example, some arene complexes have been shown to behave as a source of lowvalent metal fragments along with arene extrusion.3 Intramolecular coordination of ligand aryl groups has been found to mask coordinatively unsaturated metal centers.4 Recently, multidentate ligands flanking an arene ring with two more donor groups have been developed.5 We have previously reported the synthesis of the arenebridged dinuclear complex [{(OAr)2Zr}2(m-C6H6)], which serves to transfer the two-coordinate Zr(II) fragment.6 On the basis of this observation, we sought to extend this strategy by employing p-terphenoxide [OO]2 . The two aryloxide donors can bind strongly to the metal center, while the arene unit can coordinate reversibly to the metal center by virtue of its position in the chelate array and act as an electron reservoir through metal–arene p-interaction. Here we describe the use of this diaryloxide as an ancillary ligand for stabilization of reduced titanium and zirconium complexes. Treatment of TiCl3(THF)3 with Li2[OO] in THF followed by crystallization from DME gave pale red crystals identified as paramagnetic 1 in 54% yield (Scheme 1). The X-ray diffraction study of 1 confirmed a monomeric complex with Cs symmetry.7 The titanium center is best described as square pyramidal with a DME molecule
Department of Chemistry, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152-8551, Japan. E-mail: [email protected] † Electronic supplementary information (ESI) available: Experimental and characterization data and CIF data for 1 and 3–9. CCDC 962315–962322. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c3cc47284a
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Scheme 1
spanning apical and basal sites (t = 0.06). The [OO] ligand is a trans chelator with a bite angle of 166.4(1)1. This geometry allows two carbons of the central arene ring to be in close proximity to titanium (2.762(6), 2.718(4) Å), indicating weak p-interaction.8 Reduction of 1 with KC8 in toluene under N2 produced a dark yellow solution, from which 2 was isolated in 44% yield upon workup. Addition of pyridine to a toluene solution of dark yellow 2 resulted in an immediate color change to purple to generate 3 quantitatively. Complexes 2 and 3 are diamagnetic, and their NMR spectra are consistent with a centrosymmetric molecule in which each [(OO)Ti(L)] fragment is equivalent (L = DME, py). The 15 N-enriched compounds prepared analogously from 1 and 15N2 display a 15N NMR resonance at d 114.7 for 2-15N2 and d 178.1 for 3-15N2. The Raman spectra of 2 and 3 exhibit a highly reduced Chem. Commun., 2013, 49, 11755--11757
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Fig. 1 Molecular structures of 3 and 6. Carbon atoms of tert-butyl and methyl substituents and hydrogen atoms have been omitted for clarity. Thermal ellipsoids are at 50% probability.
nNN stretch at 1394 and 1362 cm 1, respectively, which are blue shifted when compared to their isotopologues (2-15N2, 1352 cm 1; 3-15N2, 1320 cm 1). The X-ray structure of 3 revealed two fivecoordinate titanium centers bridged by an end-on N2 ligand, with a crystallographic two-fold axis (Fig. 1). Each metal center adopts a distorted trigonal bipyramidal geometry with two pyridine molecules occupying the axial positions (N–Ti–N = 166.1(1)1). The [OO] bite angle of 150.7(1)1 is smaller than that of 1, and the long Ti–C(arene) distances (>3.15 Å) suggest no metal–arene interaction. The elongated N–N distance of 1.264(4) Å and the short Ti–N2 distances of 1.792(3) Å are comparable to those found in other reduced dinitrogen complexes of titanium.9 These spectroscopic and structural data provide evidence for significant N–N bond activation. When 1 was analogously reduced in the absence of N2, the reaction mixture turned dark brown. Attempts to isolate any compounds have met with difficulty, and characterization of the mixture by NMR spectroscopy did not aid in identifying any products. Complexes 2 and 3 are thermally stable and did not react with H2 and CO, according to their 1H NMR spectra. Next we are interested in the suitability of the [OO] ligand for larger second row metals. The p-terphenol H2[OO] ligand underwent protonolysis with Zr(CH2Ph)4 in toluene to generate 4 in 72%. Exchange of the benzyl ligands for chlorides took place upon reaction of 4 with NEt3HCl in THF, and 5 was obtained as a white power in 85% yield. Treating 5 with KC8 in THF under N2 resulted in formation of 6, which was isolated as green crystals in 50% yield. The structure of 6 was determined by an X-ray diffraction study (Fig. 1). What became evident is that the molecule contains no N2. Instead, the central arene ring is Z4-bonded to the metal (Zr–C = 2.388(5)–2.500(5) Å) and puckered with a dihedral angle of 44.71. The C–C bonds of the central ring vary, with C(2)–C(3) (1.378(6) Å) and C(5)–C(6) (1.346(6) Å) being significantly shorter than the other four (1.430(7)–1.478(7) Å), suggesting the cyclohexadienelike character.10,11 The 1H NMR spectrum of 6 exhibits a number of resonances for a C2v symmetric molecule, even down to 80 1C, consistent with the solid-state structure which is Cs symmetric, indicating a fluxional behavior in solution. The process likely involves the slippage of the arene ring. The central arene protons appear as singlets upfield shifted to d 5.61 indicating coordination. Complex 6 is stable either in the solid state or in a THF solution for prolonged periods of time. In contrast, upon dissolving 6 into toluene, the solution turned from green to brown in minutes. Workup of the reaction mixture gave 7 in quantitative yield. When 7 was dissolved in THF, no regeneration of 6 from 7 was observed. 11756
Chem. Commun., 2013, 49, 11755--11757
Fig. 2 Molecular structure of 7. Carbon atoms of tert-butyl/methyl substituents and THF and hydrogen atoms except for the hydride and the metalated arene protons have been omitted for clarity. Thermal ellipsoids are at 50% probability.
The connectivity of 7 was unambiguously provided by an X-ray diffraction study (Fig. 2). The two halves of the molecule are related by a crystallographic two-fold axis with a square Zr2C2 core containing metalated carbons. Each half unit has two diphenoxide ligands spanning two Zr centers. One ligand central ring displays no close contact to the metal center, while the other is metalated at C(3) and retains the cyclohexadiene character. The formation of 7 likely results from dissociation of THF from 6, which opens up a site for the central ring proton to undergo C–H activation, generating a [Zr(m-C6H3)(m-H)] moiety. The 1H NMR spectrum of 7 is consistent with the C2-symmetric solid-state structure. The resonances due to the metalated cyclohexadiene-type ring protons at 5.59, 5.75, and 6.97 are upfield shifted compared with those of the uncoordinated ring protons at 7.15, 7.39, 7.78, and 7.79. Additionally, diagnostic of 7 is the presence of a doublet at d 6.86 due to a hydride ligand, which is coupled to one of the metalated ring protons (d 5.59, 3 J = 12 Hz) and correlates with no 13C resonance according to 1 H–13C-HMQC spectroscopy. To investigate its reactivity, we exposed 6 to CO2 in THF. The solution immediately changed from green to yellow, and quantitative conversion to 8 was achieved based on NMR data. An X-ray diffraction study revealed a spiro-type structure of 8, wherein di-insertion of one molecule of CO2 into two Zr–C(arene) bonds generates two hexadienyl units linked by a geminal diolate (Fig. 3).12 Thus, each central ring binds the metal in a Z5-cyclohexadienyl fashion.11,13 This transformation represents a rare example of CO2 insertion into metal–arene bonds.14 Inspection of the reaction mixture by NMR spectroscopy indicated formation of a single C2-symmetric diastereomer consistent with the solid-state structure. The 1H NMR spectrum displays four upfield-shifted central ring resonances at d 4.12, 4.62, 5.62, and 5.86. When 13CO2 was used in the reaction, the 13C chemical shift of the geminal diolate unit was definitely identified to be d 173.0. We further explored the reactivity of 6 with CS2 in THF and found that an immediate color change occurred from green to yellow-orange upon addition of CS2. The reaction proceeded via head-to-head reductive coupling of CS2 to afford 9 in 74% yield, in which 6 behaves as a source of the diaryloxide Zr(II) fragment. This journal is
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Fig. 3 Molecular structures of 8 and 9. Carbon atoms of tert-butyl/methyl substituents and THF and hydrogen atoms have been omitted for clarity. Thermal ellipsoids are at 50% probability.
There have been a number of reports of reductive coupling of CS2 by metal complexes.15 The X-ray structure showed that 9 possesses a central Zr2S4C2 core situated about a crystallographic inversion center (Fig. 2). The C2S4 unit is planar, while the five-membered ZrS2C2 ring is puckered with a dihedral angle of 39.61. Additionally, the short C–C (1.379(6) Å) and the long C–S (1.778(5), 1.764(4) Å) distances corroborate the bonding description of the C2S4 ligand as an ethylenetetrathiolate form and agree well with those of reported [C2S4]4 complexes.15,16 The Zr–C(arene) distances are too long (>3.00 Å) to form a strong interaction. In solution, 8 exhibits a 1 H NMR spectrum indicative of average C2h symmetry rather than Ci symmetry in the solid state with two singlets of the central arene protons at d 7.25 and 8.42. This is attributed to the inversion process between the folded ZrS2C2 conformations on the NMR timescale. In summary, we demonstrated the viability of p-terphenoxide [OO]2 to support a series of titanium and zirconium complexes in various oxidation states. Efforts towards the installation of the [OO] ligand on other transition metals are ongoing. We are further investigating ancillary ligand modification and the reactivity of these complexes toward small molecules. This work was financially supported by a Grant-in-Aid for Scientific Research on Innovative Areas ‘‘Molecular Activation Directed toward Straightforward Synthesis’’ (MEXT KAKENHI Grant Number 22105008), Scientific Research (B) ( JSPS KAKENHI Grant Number 25288026) and the ACT-C program of Japan Science and Technology Agency.
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Notes and references 1 U. Rosenthal, P.-M. Pellny, F. G. Kirchbauer and V. V. Burlakov, Acc. ¨rstner and B. Bogdanovic´, Angew. Chem. Res., 2000, 33, 119; A. Fu Chem., Int. Ed. Engl., 1996, 35, 2442. 2 M. D. Fryzuk and S. A. Johnson, Coord. Chem. Rev., 2000, 200, 379; P. J. Chirik, Dalton Trans., 2007, 16. 3 Y.-C. Tsai, P.-Y. Wang, K.-M. Lin, S.-A. Chen and J.-M. Chen, Chem. Commun., 2008, 205; W. H. Monillas, G. P. A. Yap and K. H. Theopold, Angew. Chem., Int. Ed., 2007, 46, 6692; E. Kogut,
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H. L. Wiencko, L. Zhang, D. E. Cordeau and T. H. Warren, J. Am. Chem. Soc., 2005, 127, 11248; P. L. Diaconescu, P. L. Arnold, T. A. Baker, D. J. Mindiola and C. C. Cummins, J. Am. Chem. Soc., 2000, 122, 6108; I. Korobkov, S. Gambarotta and G. P. A. Yap, Angew. Chem., Int. Ed., 2003, 42, 4958; G. Bai, P. Wei and D. W. Stephan, Organometallics, 2005, 24, 5901. M. A. Lockwood, P. E. Fanwick and I. P. Rothwell, Chem. Commun., 1996, 2013; M. A. Lockwood, P. E. Fanwick, O. Eisenstein and I. P. Rothwell, J. Am. Chem. Soc., 1996, 118, 2762; T. R. Dugan, X. Sun, E. V. Rybak-Akimova, O. Olatunji-Ojo, T. R. Cundari and P. L. Holland, J. Am. Chem. Soc., 2011, 133, 12418; P. J. W. Deckers, B. Hessen and J. H. Teuben, Angew. Chem., Int. Ed., 2001, 40, 2516; S. M. Franke, B. L. Tran, F. W. Heinemann, W. Hieringer, D. J. Mindiola and K. Meyer, Inorg. Chem., 2013, 52, 10552; B. L. Tran, M. Singhal, H. Park, O. P. Lam, M. Pink, J. Krzystek, A. Ozarowski, J. Telser, K. Meyer and D. J. Mindiola, Angew. Chem., Int. Ed., 2010, 49, 9871. S. C. Bart, F. W. Heinemann, C. Anthon, C. Hauser and K. Meyer, Inorg. Chem., 2009, 48, 9419; A. Velian, S. Lin, A. J. M. Miller, M. W. Day and T. Agapie, J. Am. Chem. Soc., 2010, 132, 6296; S. T. Chao, N. C. Lara, S. Lin, M. W. Day and T. Agapie, Angew. Chem., Int. Ed., 2011, 50, 7529. T. Watanabe, Y. Ishida, T. Matsuo and H. Kawaguchi, Dalton Trans., 2010, 39, 484. See ESI†. M. G. Thorn, Z. C. Etheridge, P. E. Fanwick and I. P. Rothwell, J. Organomet. Chem., 1999, 591, 148; P. J. W. Deckers, A. J. van der Linden, A. Meetsma and B. Hessen, Eur. J. Inorg. Chem., 2000, 929. N. Beydoun, R. Duchateau and S. Gambarotta, J. Chem. Soc., Chem. Commun., 1992, 244; R. Baumann, R. Stumpf, W. M. Davis, L.-C. Liang and R. R. Schrock, J. Am. Chem. Soc., 1999, 121, 7822; J. R. Hagadorn and J. Arnold, Organometallics, 1998, 17, 1355; W. A. Chomitz and J. Arnold, Chem. Commun., 2007, 4797. T. L. Gianetti, G. Nocton, S. G. Minasian, N. C. Tomson, A. L. D. Kilcoyne, S. A. Kozimor, D. K. Shuh, T. Tyliszczak, R. G. Bergman and J. Arnold, J. Am. Chem. Soc., 2013, 135, 3224; C. G. Ortiz, K. A. Abboud, T. M. Cameron and J. M. Boncella, Chem. Commun., 2001, 247; T. W. Graham, J. Kickham, S. Courtenay, P. Wei and D. W. Stephan, Organometallics, 2004, 23, 3309; E. Otten, A. Meetsma and B. Hessen, J. Am. Chem. Soc., 2007, 129, 10100; D. J. Arney, P. A. Wexler and D. E. Wigley, Organometallics, 1990, 9, 1282; B. S. Buyuktas, M. M. Olmstead and P. P. Power, Chem. Commun., 1998, 1689; E. L. Muetterties, J. R. Bleeke, E. J. Wucherer and T. A. Albright, Chem. Rev., 1982, 82, 499. M. D. Fryzuk, C. M. Kozak, P. Mehrkhodavandi, L. Morello, B. O. Patrick and S. J. Rettig, J. Am. Chem. Soc., 2002, 124, 516. H. Yasuda, T. Okamoto, Y. Matsuoka, A. Nakamura, Y. Kai, N. Kanehisa and N. Kasai, Organometallics, 1989, 8, 1139; ¨rer and S. Berger, Organometallics, 2001, 20, 1703; N. E. Schlo M. A. Rankin and C. C. Cummins, J. Am. Chem. Soc., 2010, 132, 10021. R. Basta, B. G. Harvey, A. M. Arif and R. D. Ernst, Inorg. Chim. Acta, 2004, 357, 3883; A. Rajapakshe, N. E. Gruhn, D. L. Lichtenberger, R. Basta, A. M. Arif and R. D. Ernst, J. Am. Chem. Soc., 2004, 126, 14105. I. Korobkov and S. Gambarotta, Organometallics, 2004, 23, 5379; M. N. Bochkarev, A. A. Trifonov, E. A. Fedorova, N. S. Emelyanova, T. A. Basalgina, G. S. Kalinina and G. A. Razuvaev, J. Organomet. Chem., 1989, 372, 217. O. P. Lam, F. W. Heinemann and K. Meyer, Angew. Chem., Int. Ed., 2011, 50, 5965; H. A. Harris, A. D. Rae and L. F. Dahl, J. Am. Chem. Soc., 1987, 109, 4739. G. A. Holloway and T. B. Rauchfuss, Inorg. Chem., 1999, 38, 3018.
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COMMUNICATION
Cite this: Chem. Commun., 2013, 49, 11755 Received 24th September 2013, Accepted 22nd October 2013
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Synthesis of titanium and zirconium complexes supported by a p-terphenoxide ligand and their reactions with N2, CO2 and CS2† Takashi Kurogi, Yutaka Ishida and Hiroyuki Kawaguchi*
DOI: 10.1039/c3cc47284a www.rsc.org/chemcomm
The synthesis, characterization and reactions of a series of titanium and zirconium complexes that incorporate a p-terphenolate ligand are described.
Much work in coordination chemistry has been devoted to the study of early transition metal complexes with low oxidation states and low coordination numbers. Interest in this area stems from their potential for diverse applications ranging from organic synthesis1 to activation of small molecules such as N2.2 The design of ligand systems is of critical importance to gain access to low-valent, lowcoordinate complexes of early transition metals or their synthetic equivalents. One strategy is to utilize coordinated arenes or ligand systems containing a hemilabile p-arene unit. For example, some arene complexes have been shown to behave as a source of lowvalent metal fragments along with arene extrusion.3 Intramolecular coordination of ligand aryl groups has been found to mask coordinatively unsaturated metal centers.4 Recently, multidentate ligands flanking an arene ring with two more donor groups have been developed.5 We have previously reported the synthesis of the arenebridged dinuclear complex [{(OAr)2Zr}2(m-C6H6)], which serves to transfer the two-coordinate Zr(II) fragment.6 On the basis of this observation, we sought to extend this strategy by employing p-terphenoxide [OO]2 . The two aryloxide donors can bind strongly to the metal center, while the arene unit can coordinate reversibly to the metal center by virtue of its position in the chelate array and act as an electron reservoir through metal–arene p-interaction. Here we describe the use of this diaryloxide as an ancillary ligand for stabilization of reduced titanium and zirconium complexes. Treatment of TiCl3(THF)3 with Li2[OO] in THF followed by crystallization from DME gave pale red crystals identified as paramagnetic 1 in 54% yield (Scheme 1). The X-ray diffraction study of 1 confirmed a monomeric complex with Cs symmetry.7 The titanium center is best described as square pyramidal with a DME molecule
Department of Chemistry, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152-8551, Japan. E-mail: [email protected] † Electronic supplementary information (ESI) available: Experimental and characterization data and CIF data for 1 and 3–9. CCDC 962315–962322. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c3cc47284a
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Scheme 1
spanning apical and basal sites (t = 0.06). The [OO] ligand is a trans chelator with a bite angle of 166.4(1)1. This geometry allows two carbons of the central arene ring to be in close proximity to titanium (2.762(6), 2.718(4) Å), indicating weak p-interaction.8 Reduction of 1 with KC8 in toluene under N2 produced a dark yellow solution, from which 2 was isolated in 44% yield upon workup. Addition of pyridine to a toluene solution of dark yellow 2 resulted in an immediate color change to purple to generate 3 quantitatively. Complexes 2 and 3 are diamagnetic, and their NMR spectra are consistent with a centrosymmetric molecule in which each [(OO)Ti(L)] fragment is equivalent (L = DME, py). The 15 N-enriched compounds prepared analogously from 1 and 15N2 display a 15N NMR resonance at d 114.7 for 2-15N2 and d 178.1 for 3-15N2. The Raman spectra of 2 and 3 exhibit a highly reduced Chem. Commun., 2013, 49, 11755--11757
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Fig. 1 Molecular structures of 3 and 6. Carbon atoms of tert-butyl and methyl substituents and hydrogen atoms have been omitted for clarity. Thermal ellipsoids are at 50% probability.
nNN stretch at 1394 and 1362 cm 1, respectively, which are blue shifted when compared to their isotopologues (2-15N2, 1352 cm 1; 3-15N2, 1320 cm 1). The X-ray structure of 3 revealed two fivecoordinate titanium centers bridged by an end-on N2 ligand, with a crystallographic two-fold axis (Fig. 1). Each metal center adopts a distorted trigonal bipyramidal geometry with two pyridine molecules occupying the axial positions (N–Ti–N = 166.1(1)1). The [OO] bite angle of 150.7(1)1 is smaller than that of 1, and the long Ti–C(arene) distances (>3.15 Å) suggest no metal–arene interaction. The elongated N–N distance of 1.264(4) Å and the short Ti–N2 distances of 1.792(3) Å are comparable to those found in other reduced dinitrogen complexes of titanium.9 These spectroscopic and structural data provide evidence for significant N–N bond activation. When 1 was analogously reduced in the absence of N2, the reaction mixture turned dark brown. Attempts to isolate any compounds have met with difficulty, and characterization of the mixture by NMR spectroscopy did not aid in identifying any products. Complexes 2 and 3 are thermally stable and did not react with H2 and CO, according to their 1H NMR spectra. Next we are interested in the suitability of the [OO] ligand for larger second row metals. The p-terphenol H2[OO] ligand underwent protonolysis with Zr(CH2Ph)4 in toluene to generate 4 in 72%. Exchange of the benzyl ligands for chlorides took place upon reaction of 4 with NEt3HCl in THF, and 5 was obtained as a white power in 85% yield. Treating 5 with KC8 in THF under N2 resulted in formation of 6, which was isolated as green crystals in 50% yield. The structure of 6 was determined by an X-ray diffraction study (Fig. 1). What became evident is that the molecule contains no N2. Instead, the central arene ring is Z4-bonded to the metal (Zr–C = 2.388(5)–2.500(5) Å) and puckered with a dihedral angle of 44.71. The C–C bonds of the central ring vary, with C(2)–C(3) (1.378(6) Å) and C(5)–C(6) (1.346(6) Å) being significantly shorter than the other four (1.430(7)–1.478(7) Å), suggesting the cyclohexadienelike character.10,11 The 1H NMR spectrum of 6 exhibits a number of resonances for a C2v symmetric molecule, even down to 80 1C, consistent with the solid-state structure which is Cs symmetric, indicating a fluxional behavior in solution. The process likely involves the slippage of the arene ring. The central arene protons appear as singlets upfield shifted to d 5.61 indicating coordination. Complex 6 is stable either in the solid state or in a THF solution for prolonged periods of time. In contrast, upon dissolving 6 into toluene, the solution turned from green to brown in minutes. Workup of the reaction mixture gave 7 in quantitative yield. When 7 was dissolved in THF, no regeneration of 6 from 7 was observed. 11756
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Fig. 2 Molecular structure of 7. Carbon atoms of tert-butyl/methyl substituents and THF and hydrogen atoms except for the hydride and the metalated arene protons have been omitted for clarity. Thermal ellipsoids are at 50% probability.
The connectivity of 7 was unambiguously provided by an X-ray diffraction study (Fig. 2). The two halves of the molecule are related by a crystallographic two-fold axis with a square Zr2C2 core containing metalated carbons. Each half unit has two diphenoxide ligands spanning two Zr centers. One ligand central ring displays no close contact to the metal center, while the other is metalated at C(3) and retains the cyclohexadiene character. The formation of 7 likely results from dissociation of THF from 6, which opens up a site for the central ring proton to undergo C–H activation, generating a [Zr(m-C6H3)(m-H)] moiety. The 1H NMR spectrum of 7 is consistent with the C2-symmetric solid-state structure. The resonances due to the metalated cyclohexadiene-type ring protons at 5.59, 5.75, and 6.97 are upfield shifted compared with those of the uncoordinated ring protons at 7.15, 7.39, 7.78, and 7.79. Additionally, diagnostic of 7 is the presence of a doublet at d 6.86 due to a hydride ligand, which is coupled to one of the metalated ring protons (d 5.59, 3 J = 12 Hz) and correlates with no 13C resonance according to 1 H–13C-HMQC spectroscopy. To investigate its reactivity, we exposed 6 to CO2 in THF. The solution immediately changed from green to yellow, and quantitative conversion to 8 was achieved based on NMR data. An X-ray diffraction study revealed a spiro-type structure of 8, wherein di-insertion of one molecule of CO2 into two Zr–C(arene) bonds generates two hexadienyl units linked by a geminal diolate (Fig. 3).12 Thus, each central ring binds the metal in a Z5-cyclohexadienyl fashion.11,13 This transformation represents a rare example of CO2 insertion into metal–arene bonds.14 Inspection of the reaction mixture by NMR spectroscopy indicated formation of a single C2-symmetric diastereomer consistent with the solid-state structure. The 1H NMR spectrum displays four upfield-shifted central ring resonances at d 4.12, 4.62, 5.62, and 5.86. When 13CO2 was used in the reaction, the 13C chemical shift of the geminal diolate unit was definitely identified to be d 173.0. We further explored the reactivity of 6 with CS2 in THF and found that an immediate color change occurred from green to yellow-orange upon addition of CS2. The reaction proceeded via head-to-head reductive coupling of CS2 to afford 9 in 74% yield, in which 6 behaves as a source of the diaryloxide Zr(II) fragment. This journal is
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Fig. 3 Molecular structures of 8 and 9. Carbon atoms of tert-butyl/methyl substituents and THF and hydrogen atoms have been omitted for clarity. Thermal ellipsoids are at 50% probability.
There have been a number of reports of reductive coupling of CS2 by metal complexes.15 The X-ray structure showed that 9 possesses a central Zr2S4C2 core situated about a crystallographic inversion center (Fig. 2). The C2S4 unit is planar, while the five-membered ZrS2C2 ring is puckered with a dihedral angle of 39.61. Additionally, the short C–C (1.379(6) Å) and the long C–S (1.778(5), 1.764(4) Å) distances corroborate the bonding description of the C2S4 ligand as an ethylenetetrathiolate form and agree well with those of reported [C2S4]4 complexes.15,16 The Zr–C(arene) distances are too long (>3.00 Å) to form a strong interaction. In solution, 8 exhibits a 1 H NMR spectrum indicative of average C2h symmetry rather than Ci symmetry in the solid state with two singlets of the central arene protons at d 7.25 and 8.42. This is attributed to the inversion process between the folded ZrS2C2 conformations on the NMR timescale. In summary, we demonstrated the viability of p-terphenoxide [OO]2 to support a series of titanium and zirconium complexes in various oxidation states. Efforts towards the installation of the [OO] ligand on other transition metals are ongoing. We are further investigating ancillary ligand modification and the reactivity of these complexes toward small molecules. This work was financially supported by a Grant-in-Aid for Scientific Research on Innovative Areas ‘‘Molecular Activation Directed toward Straightforward Synthesis’’ (MEXT KAKENHI Grant Number 22105008), Scientific Research (B) ( JSPS KAKENHI Grant Number 25288026) and the ACT-C program of Japan Science and Technology Agency.
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Notes and references 1 U. Rosenthal, P.-M. Pellny, F. G. Kirchbauer and V. V. Burlakov, Acc. ¨rstner and B. Bogdanovic´, Angew. Chem. Res., 2000, 33, 119; A. Fu Chem., Int. Ed. Engl., 1996, 35, 2442. 2 M. D. Fryzuk and S. A. Johnson, Coord. Chem. Rev., 2000, 200, 379; P. J. Chirik, Dalton Trans., 2007, 16. 3 Y.-C. Tsai, P.-Y. Wang, K.-M. Lin, S.-A. Chen and J.-M. Chen, Chem. Commun., 2008, 205; W. H. Monillas, G. P. A. Yap and K. H. Theopold, Angew. Chem., Int. Ed., 2007, 46, 6692; E. Kogut,
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