Multinuclear Complexes Bearing a Four-Electron-Reduced Dinitrogen

Jul 26, 2011 - The OCU Advanced Research Institute for Natural Science and Technology (OCARINA), Osaka City University, Sumiyoshi-ku,...
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Sulfur-Bridged Ta M (M = Mo, Cr) Multinuclear Complexes Bearing a Four-Electron-Reduced Dinitrogen Ligand Ryoichi Takada,† Masakazu Hirotsu,*,† Takanori Nishioka,†,‡ Hideki Hashimoto,†,‡,§ and Isamu Kinoshita†,‡,§ †

Graduate School of Science, Osaka City University, Sumiyoshi-ku, Osaka 558-8585, Japan CREST, Japan Science and Technology Agency, 4-1-8 Honcho, Kawaguchi, Saitama 332-0012, Japan § The OCU Advanced Research Institute for Natural Science and Technology (OCARINA), Osaka City University, Sumiyoshi-ku, Osaka 558-8585, Japan ‡

bS Supporting Information ABSTRACT: Sulfur-bridged Ta2M2 complexes (M = Mo, Cr) containing a four-electron-reduced dinitrogen ligand, [Cp*Ta(μ-SC6H4Me)2M(CO)4]2(μ-η1:η1-N2) (Cp* = η5C5Me5), were synthesized from the ditantalum complex [Cp*Ta(SC6H4Me)2]2(μ-η1:η1-N2), which was obtained by a one-pot reaction using [Cp*TaCl4], di-p-tolyl disulfide, and KC8 under dinitrogen. Crystal structures of the Ta2 and Ta2M2 complexes revealed the analogy of the Ta N N Ta moieties. This new route demonstrates the stability of the N24 ligand in M S bond formation reactions, which are applicable to the synthesis of sulfur-bridged complexes bearing reduced dinitrogen species.

B

iological reduction of dinitrogen to ammonia is catalyzed by nitrogenase enzymes under ambient conditions, which is an essential process in the biochemical nitrogen cycle. The N2 binding and reduction proceed at the FeMo-cofactor in nitrogenase, in which the iron centers are surrounded by the sulfur donor atoms.1 The structure of the FeMo-cofactor has been revealed by crystallographic studies,2 and synthetic approaches are currently underway to obtain the structural model.3 In addition, the reduction and functionalization of the coordinated N2 using early- to middle-transition-metal complexes have become an active research area.4 However, the mechanistic details of the N2 reduction on the FeMo-cofactor are still unclear. To elucidate the N2 reduction mechanism, sulfur-bridged multinuclear complexes having dinitrogen and related ligands have been investigated,5,6 in addition to dinitrogen complexes with terminal thiolato or thioether ligands.7 9 Although the coordination of N2 is a key step in N2 fixation, dinitrogen complexes with sulfur donor ligands are rarely prepared directly from molecular N2.8 Furthermore, S-donor complexes containing activated N2 ligands are usually prepared stepwise from other nitrogen sources.9 The thiolato tantalum complexes [Ta(SAr)3(THF)]2(μ-N2) contain a four-electron-reduced N2 ligand, which is derived from benzaldehyde azine.9a It seems to be possible to prepare this type of complex from molecular N2, since the activation and functionalization of N2 have been widely investigated by using Ta complexes.9 14 On the other hand, cyclopentadienyl complexes of early transition metals exhibit a high ability for N2 activation.4e,12 15 The combination of thiolate ligands and cyclopentadienyl derivatives would provide r 2011 American Chemical Society

a useful ligand system for S-donor complexes that activate N2. Here we communicate that a Cp* thiolato Ta complex bearing a four-electron-reduced dinitrogen species was synthesized directly from molecular N2 by a one-pot reaction. Furthermore, the complex formed would be a useful synthon for S-bridged multinuclear dinitrogen complexes. The reduction of [Cp*TaCl4] with KC8 (5 equiv) in the presence of di-p-tolyl disulfide was performed in THF under N2. The reaction mixture was stirred at 78 °C for 2 h and then warmed to 0 °C. The 1H NMR spectrum of the crude reaction mixture showed the presence of a major product and minor byproducts (Figure S1 (Supporting Information)). A singlet signal due to the Cp* ligand and three signals due to the 4-methylbenzenethiolato ligand were observed for the major product, where the molar ratio of Cp* and SC6H4Me was 1:2. These signals were not observed for the corresponding reaction in the absence of N2. The major product was isolated as red crystals, which proved to be the thiolato dinitrogen complex [Cp*Ta(SC6H4Me)2]2(μ-η1:η1-N2) (1; 18% yield) (Scheme 1). A single-crystal structure analysis of 1 confirmed the presence of the N2 ligand bound to two Ta atoms in an end-on fashion (Figure 1). Each Ta atom has a Cp* and two 4-methylbenzenethiolato ligands to form a three-legged piano-stool structure. The N N bond length of 1 is 1.281(7) Å, which is close to that of [(η5-C5Me4Et)TaCl2]2(μ-N2) (1.280(6) Å),13 and the Ta N bond lengths are 1.813(5) and 1.814(5) Å. The Ta N N Received: July 6, 2011 Published: July 26, 2011 4232

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Scheme 1. Synthesis of a Thiolato Ta2 Complex Bearing a Four-Electron-Reduced Dinitrogen Ligand

Figure 1. ORTEP drawing of 1 with thermal ellipsoids at the 50% probability level. Hydrogen atoms are omitted for clarity.

angles are slightly bent from linearity (166.9(4) and 165.8(4)°). These geometrical parameters exhibit characteristics of an endon bound N24 ligand, which are found in many tantalum(V) dinitrogen complexes.8,10b,13,14 In the one-pot synthesis of 1, KC8 reduces the Ta center (or the coordinated dinitrogen) and di-p-tolyl disulfide. Oxidative addition of the disulfide to the reduced Ta center is also possible.16 To elucidate the reaction mechanism, the reaction mixture was stirred at 78 °C for 2 h without N2 and then treated with N2. The 1H NMR spectrum of the products did not show signals attributed to complex 1. Therefore, the coordination of N2 should occur prior to the reaction of the disulfide. If the formation of the Ta S bonds takes place in the initial stage of the reaction, the thiolato S atoms readily occupy the vacant coordination sites by forming S-bridged structures and prevent the coordination of N2. The terminal thiolato ligands in 1 are available to form sulfurbridged structures. A red suspension of 1 and [M(CO)6] (M = Mo, Cr; 2 equiv) in benzene was irradiated with a high-pressure mercury lamp. Undissolved [M(CO)6] disappeared, and red crystals of [Cp*Ta(μ-SC6H4Me)2M(CO)4]2(μ-η1:η1-N2) (2, M = Mo; 3, M = Cr) were deposited (ca. 40%; Scheme 2). The X-ray structure analyses revealed that 2 and 3 have a thiolato-bridged tetranuclear structure, in which the N2 ligand is bound to two Ta atoms in an end-on fashion similar to that for 1 (Figure 2 and Figure S6 (Supporting Information)). The structures of 2 and 3 contain a crystallographically imposed center of inversion at the midpoint of the N N bond. The Cp*Ta and M(CO)4 units are bridged by the two thiolato ligands. The tantalum center adopts a three-legged piano-stool structure, while the molybdenum or chromium center of M(CO)4(SC6H4Me)2 is octahedral. The N(1) N(1)* distances are 1.308(9) Å for 2 and 1.295(4) Å for 3, which show the character of N24 found in

the parent complex 1, together with other structural parameters: 2, Ta N = 1.826(6) Å, Ta N N = 164.6(4)°; 3, Ta N = 1.833(3) Å, Ta N N = 165.6(2)°. In contrast to the similarity of the Ta N N Ta moieties, the structures of the Cp*Ta(SC6H4Me)2 units in 2 and 3 differ from that in 1. First, the p-tolylthiolato ligands bridging Ta and M in 2 and 3 are both oriented away from Cp* on the Ta center to reduce steric hindrance. The S Ta S angles in the tetranuclear complexes (2, 103.24(6)°; 3, 99.66(2)°) are decreased compared with those in ditantalum complex 1 (S(1) Ta(1) S(2) = 110.07(6)°, S(3) Ta(2) S(4) = 107.35(6)°). It is notable that the relative arrangements of the two Cp*Ta(SC6H4Me)2 units are quite different, as shown in Figure 3: complex 1 has an eclipsed conformation, while 2 and 3 have a staggered conformation along the Ta N N Ta axis. The 1H NMR spectrum of 1 shows four signals due to two Cp* and four 4-methylbenzenethiolato ligands, as described above. Thus, the two Cp*Ta(SC6H4Me)2 units in 1 rotate around the Ta N N Ta axis, and the four 4-methylbenzenethiolato ligands freely rotate at room temperature. Since complexes 2 and 3 were insoluble in most solvents, we could not observe NMR signals. The crystal structures suggest that the staggered conformations of 2 and 3 result from the steric interactions between the Cp* ligands and the p-tolyl groups on the rigid TaS2M framework. In monitoring the photochemical reactions forming 2 or 3 by 1 H NMR measurements, new signals due to two chemically inequivalent Cp* and tolyl groups appeared at the early stage of the reactions (Figure 4 and Figure S2 (Supporting Information)). Because insoluble 2 and 3 have pseudo C2h symmetry, the dissolved compounds are expected to be new S-bridged N2 complexes. As the formation of the TaS2M bridging structures is stepwise, the initial species in the reaction solution is a 1:1 reaction product, [Cp*Ta(μ-SC6H4Me)2M(CO)4](μ-η1:η1-N2)[Cp*Ta(SC6H4Me)2] (4, M = Mo; 5, M = Cr). We obtained a small amount of crystals of 5 from the reaction solution and determined the crystal structure by X-ray analysis (Figure 5). The Ta N N Ta structure is analogous to those of 1 3: N N = 1.283(6) Å, Ta N = 1.822(4), 1.822(4) Å, Ta N N = 168.1(3), 174.5(3)°. The Cp*Ta(μ-SC6H4Me)2Cr(CO)4 and Cp*Ta(SC6H4Me)2 units in 5 have geometries similar to those of 3 and 1, respectively, and show the staggered conformation as in 2 and 3. Density functional calculations of 1, 2, and 5 were performed at the B3LYP/LANL2DZ level to understand the electronic structures of thiolato-dinitrogen complexes. The optimized structures were quite consistent with the experimentally determined structures: 1, N N = 1.296 Å, Ta N = 1.812 Å; 2, N N = 1.307 Å, Ta N = 1.820 Å; 5, N N = 1.301 Å, Ta N = 1.816, 4233

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Scheme 2. Synthesis of S-Bridged Ta2M2 and Ta2M Complexes with N24 (M = Mo, Cr)

Figure 2. ORTEP drawing of 2 with thermal ellipsoids at the 50% probability level. Hydrogen atoms are omitted for clarity. Complex 3 is isostructural with 2.

Figure 4. 1H NMR spectral changes during the photochemical reaction of 1 with [Mo(CO)6] in C6D6: (a) 0 min; (b) 60 min; (c) 90 min; (d) 120 min; (e) 150 min. Signals for complex 1 are denoted by O, those for complex 4 by b, and those for residual solvent signals by *.

Figure 3. Views along the Ta N N Ta axis of (a) 1 and (b) 2. Methyl groups and carbonyl ligands are omitted for clarity.

1.820 Å. Isosurfaces calculated for HOMO, HOMO-1, HOMO2, and LUMO of 1, 2, and 5 are shown in Figures S9, S11, and S13 (Supporting Information), respectively. The HOMO, HOMO-1, and HOMO-2 of 1 are centered on Ta, N, and S atoms, indicating Ta N π bonding and N N π* antibonding. The thiolato S atoms act as π donors to the Ta centers in a high oxidation state. In 2, the HOMO and HOMO-1 are distributed over Mo C

Figure 5. ORTEP drawing of 5 with thermal ellipsoids at the 50% probability level. Hydrogen atoms are omitted for clarity.

bonds, which displays π back-bonding from Mo to CO. The HOMO of 2 is also distributed over N, Ta, and S atoms, showing Ta N π bonding and N N π* antibonding. Isosurfaces calculated for the HOMO and HOMO-1 of Ta2Cr complex 5 suggested that the Ta N π bonding and N N π* antibonding character of 4234

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Organometallics the unsymmetrical Ta2M complexes is similar to that of the Ta2 and Ta2M2 complexes. The Mulliken population analysis showed that the atomic chages of N atoms in 1, 2, and 5 are quite similar: 1, 0.41; 2, 0.44; 5, 0.41 (N bound to the TaCr unit), 0.45 (N bound to the Ta unit). These results suggest that a strong πdonor character of the end-on-bound N24 ligand decreases the basicity of the coordinated N atoms. In summary, we have developed a one-pot synthesis of a Ta2 dinitrogen complex with thiolato ligands. Although the N2 binding step proceeds at the sulfur-free metal center, the coordinated N2 ligand is placed in a sulfur-rich environment by the successive Ta S bond formation reactions. The thiolato Ta2 complex bearing a four-electron-reduced dinitrogen ligand reacted with Mo(CO)4 or Cr(CO)4 fragments to afford S-bridged tri- and tetranuclear complexes, in which the N24 ligand remained coordinated to the Ta centers. This M S bond formation reaction clearly demonstrated the higher reactivity of the terminal thiolato S atoms compared with that of the end-on bound N24 ligand. The new synthetic procedure presented here provides various types of S-bridged multinuclear N2 complexes, which should be useful for investigating the reactivity of N2 incorporated in sulfurrich metal clusters.

’ ASSOCIATED CONTENT

bS

Supporting Information. Text, tables, figures, and CIF files giving experimental details, crystallographic data for 1 3 and 5, and computational details for 1, 2, and 5. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Tel: +81-6-6605-2519. Fax: +81-6-6690-2753. E-mail: mhiro@ sci.osaka-cu.ac.jp.

’ ACKNOWLEDGMENT We gratefully acknowledge support from the Kinki-chiho Hatsumei Center. This work was supported by a Grant-in-Aid for Scientific Research (No. 18350033) from the Ministry of Education, Culture, Sports, Science, and Technology of Japan. ’ REFERENCES (1) (a) Howard, J. B.; Rees, D. C. Chem. Rev. 1996, 96, 2965–2982. (b) Howard, J. B.; Rees, D. C. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 17088–17124. (c) Seefeldt, L. C.; Hoffman, B. M.; Dean, D. R. Annu. Rev. Biochem. 2009, 78, 701–722. (2) (a) Kim, J.; Rees, D. C. Science 1992, 257, 1677–1682. (b) Mayer, S. M.; Lawson, D. M.; Gormal, C. A.; Roe, S. M.; Smith, B. E. J. Mol. Biol. 1999, 292, 871. (c) Einsle, O.; Tezcan, F. A.; Andrade, S. L. A.; Schmid, B.; Yoshida, M.; Howard, J. B.; Rees, D. C. Science 2002, 297, 1696–1700. (3) (a) Ohki, Y.; Ikagawa, Y.; Tatsumi, K. J. Am. Chem. Soc. 2007, 129, 10457–10465. (b) Vela, J.; Cirera, J.; Smith, J. M.; Lachicotte, R. J.; Flaschenriem, C. J.; Alvarez, S.; Holland, P. L. Inorg. Chem. 2007, 46, 60–71. (c) Chen, X.-D.; Duncan, J. S.; Verma, A. K.; Lee, S. C. J. Am. Chem. Soc. 2010, 132, 15884–15886. (4) (a) Fryzuk, M. D.; Love, J. B.; Rettig, S. J.; Young, V. G. Science 1997, 275, 1445–1447. (b) MacKay, B. A.; Fryzuk, M. D. Chem. Rev. 2004, 104, 385–401. (c) Gambarotta, S.; Scott, J. Angew. Chem., Int. Ed. 2004, 43, 5298–5308. (d) MacLachlan, E. A.; Fryzuk, M. D. Organometallics

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