Discrete Iridamacrocycle Formation via C–H Activation of an N

NOTE pubs.acs.org/Organometallics

Discrete Iridamacrocycle Formation via C H Activation of an N-Heterocycle Wei-Bin Yu, Yue-Jian Lin, and Guo-Xin Jin* Shanghai Key Laboratory of Molecular Catalysis and Innovative Material, Department of Chemistry, Fudan University, Shanghai, 200433, People's Republic of China

bS Supporting Information ABSTRACT:

An N-heterocycle (pyrazine) utilized as a precursor was employed to construct multinuclear metallamacrocycle 1 though C H activation and metal metal bond formation under mild conditions. This iridamacrocycle was fully characterized by 1H NMR, IR, and elemental analysis. In order to further confirm its structure, X-ray analysis was carried out, proving the hexanuclear iridium macrocyclic backbone of the complex.

’ INTRODUCTION The abundance of N-heterocycles in biologically active molecules has occasioned many efforts for their synthesis and functionalization,1 resulting in attracting many chemists' focus on the field. In previous documents, direct approaches toward their functionalization have become competitive with more traditional protocols based on substrate preactivation.2 Indeed, methods involving C H bond cleavage and subsequent metal carbon bond formation are emerging as attractive alternatives for functionalizing N-heterocycles. In fact, the efficient activation and functionalization of C H bonds by transition metal compounds is one of the most important, but at the same time difficult, challenges in chemistry.3 In 1982 Bergman4 and Graham5 observed for the first time the intermolecular activation of saturated hydrocarbons by iridium complexes, and these results encouraged numerous efforts that have culminated in some instances in the catalytic functionalization of these molecules.6 So far, some remarkable discoveries pointing toward the possibility of using transition metals to cleave C H bonds have been found, giving rise to strong motivation of chemists to obtain more numerous achievements in the C H activation field.7 Recently, our group has successfully explored Cp* fragments utilized as catalysis to cleave C H bonds of aromatic carboxylic acid to build metallamacrocycles.8 However, a case of formation of a metallamacrocycle via cleavage of a C H bond from an N-heterocycle has not been reported until now. r 2011 American Chemical Society

On the other hand, a metal metal bond in a di- or polynuclear complex tends to be its most characteristic structural feature and at the same time bears a reactive potential that sets it apart from the other components of the molecule.9 11 As a result, molecular compounds featuring direct metal metal bonds have stirred up interest for many decades now, with the nature of the bonding between the two metal centers generally being the focus of attention.12 Additionally, in the coordination chemistry of transition metals, single and multiple metal metal bonds have been well-established13 since the mid 1960s.14 In contrast to the rapid development in transition metal chemistry, metal metal bonds in macrocyclic complexes are virtually unknown. Herein, we report thus a novel Ir-metallamacrocycle that is formed through cleaving C H bonds of an N-heterocycle (pyrazine) and metal metal bonds. Motivated by the successful construction of metallamacrocycles via C H activation of aromatic carboxylic acids,8 we have attempted to assemble a novel metallamacrocycle involving a metal metal bond and C H bond activation on the basis of an N-heterocycle. Here, we describe in detail an efficient one-pot method to synthesize a new metallamacrocycle of half-sandwich iridium via C H activation and metal metal bonds under mild conditions. Received: May 13, 2011 Published: June 24, 2011 3905

dx.doi.org/10.1021/om200399b | Organometallics 2011, 30, 3905–3907

Organometallics

NOTE

Scheme 1. Synthesis of Complex 1

Table 1. Evaluation of Reaction Conditions for 1a temperature (°C)

yieldb

none none

room temperature 60

none 55%

1,4-H2chdcc

room temperature

none

KOBu

1,4-H2chdc

60

61%

5

NaOBut

none

60

none

6

NaOBut

1,4-H2chdc

60

none

entry

base

additive

1 2

KOBut KOBut

3

KOBut

4

t

a

Reactions were conducted on a 0.025 mmol scale. Conditions: 0.05 mmol of [Cp*IrCl2]2, 1 equiv of pyrazine, 4 equiv of base, 1 equiv of additive, 24 h. b Yields: Isolated yield. c 1,4-Cyclohexanedicarboxylic acid.

Figure 1. Molecular structure of 1. Anions, H atoms, and guest molecules are omitted for clarity. Selected bonds (Å) and angles (deg): Ir1 C1, 2.008(7); Ir1 Ir2, 2.965(8); Ir2 N1, 2.092(7); Ir3 Cl1, 2.394(8); C1 Ir1 C14, 81.912(2); C1 Ir1 Ir2, 67.297(2); N1 Ir2 N8, 81.060(2); N1 Ir2 Ir1, 66.359(2). C, gray; N, blue; Cl, green; Ir, dark red.

’ RESULTS AND DISCUSSION As indicated in Scheme 1, [Cp*IrCl2]2 was first treated with pyrazine in methanol at room temperature to afford μ-pyrazinebridged dinuclear complexes,15 to which 4 equiv of AgOTf (OTf = O3SCF3) was then added. After the mixture was stirred at room temperature overnight, KOBut was added and the mixture was kept stirring for an additional 12 h at 60 °C. Red compound 1 was obtained after extracting with methanol. Suitable crystals were obtained by diffusion of ether into the methanol solution of 1 for several days. X-ray structural analysis revealed, interestingly, a novel iridium metallamacrocycle involving formation of a metal metal bond was built though C H bond activation based on pyrazine. As Figure 1 shows, cation 1 consists of six Cp*Ir fragments, four pyrazine molecules, and a pair of chloride ions, resulting in a hexanuclear distorted square architecture with dimension of 6.95 Å  6.92 Å more or less. The distance of the Ir Ir bond is 2.9651(8) Å, suggesting that a classical single bond was formed between the iridium atoms. The length of the Ir C bond is 2.008

Å, which is similar to our previous works.8 The backbone of 1 in solution is confirmed by 1H NMR, proving that the backbone of 1 still remains the macrocyclic structure as well. Considering that the newly synthetic approach is usually required to achieve C H activation of aromatic carboxylic acids to construct metallamacrocycles, we decided to further explore this direct reaction pathway to obtain 1 in a one-pot method. To our delight, 1,4-H2chdc (1,4-cyclohexanedicarboxylic acid) utilized as an additive takes part in the reaction of C H activation for building metallamacrocycles. As a result, complex 1 was also obtained in higher yield (Table 1). With this set of conditions in hand, the scope of metallamacrocyclic formation was demonstrated with a variety of bases. As shown in Table 1, the reaction cannot provide complex 1 when the base was changed to NaOBut regardless of changes of temperature and additive. Additionally, when terephthalic acid was used as additive, the C H activation of aromatic carboxylic acid occurred, providing a metallamacrocycle that has been published under similar conditions.8a

’ CONCLUSION In summary, we have developed a new approach to metallamacrocycle formation involving C H activation and a metal metal bond from an N-heterocycle (pyrazine). The synthesis of the metallamacrocycle operates under quite mild conditions and does not require an additive when KOBut is used as a base to take part in the reaction at 60 °C. This interesting reactivity should find a broader use in the formation and functionalization of other N-heterocycles. 3906

dx.doi.org/10.1021/om200399b |Organometallics 2011, 30, 3905–3907

Organometallics

’ EXPERIMENTAL SECTION General Considerations. All manipulations were performed under an atmosphere of nitrogen using standard Schlenk techniques. Dichloromethane and methanol were distilled. The starting material [Cp*IrCl(μ-Cl)]2 was prepared according to the literature method,16 while other chemicals were obtained commercially and used without further purification. Elemental analyses were performed on an Elementar III Vario EI analyzer. 1H NMR (400 MHz) spectra were obtained on a Bruker DMX-500 spectrometer in CDCl3 solution. IR spectra are measured on a Nicolet Avatar-360 spectrophotometer. X-ray Crystal Structure Determinations. A single crystal was immersed in mother solution and sealed in thin-walled glass. Data were collected on a CCD-Bruker SMART APEX system. All the determinations of the unit cell and intensity data were performed with graphitemonochromated Mo KR radiation (λ = 0.71073 Å). All the data were collected at room temperature using the ω scan technique. These structures were solved by direct methods, using Fourier techniques, and refined on F2 by a full-matrix least-squares method. All calculations were carried out with the SHELXTL program.17 A summary of the crystallographic data and selected experimental information are given in ref 18. Synthesis of Complex 1. To a solution of [Cp*IrCl(μ-Cl)]2 (80 mg, 0.1 mmol) in CH3OH (20 mL) was added pyrazine (8 mg, 0.1 mmol) at room temperature. After vigorous stirring for 4 h, AgOTf (102 mg, 0.4 mmol) was then added to the solution, and the reaction was carried out in the dark. Finally, 1,4-cyclohexanedicarboxylic acid (17.2 mg, 0.1 mmol) was added to the solution with KOBut (0.2 mmol), and vigorous stirring was continued for 24 h at 60 °C. After the reaction was complete, the solution was filtered to remove undissolved compounds. The filtrate was concentrated and further purified via neutral alumina gel chromatography (CH2Cl2, CH3OH). Red compounds were obtained by concentration via vacuum: 1, 59.3 mg, 61%. Suitable crystals 1 were obtained via diffusion of ether into the solution of complex 1 in methanol for several days. Data for complex 1 are as follows. Anal. Calcd for C80H102Cl2F12Ir6N8O12S4: C, 32.59; H, 3.49; N, 3.80. Found: C, 32.44; H, 3.40; N, 3.83. 1 H NMR (400 MHz, CDCl3, TMS): 1.94(s, 30H, Cp*), 1.97(s, 30H, Cp*), 2.04(s, 30H, Cp*), 7.40(m, 8H, pyrazine), 7.67(m, 4H, pyrazine). IR (liquid paraffin, cm 1): 2957(s), 2920(s), 2853(s), 2475(w), 2661(w), 1716(w), 1637(w), 1454(s), 1372(m), 1259(w), 1174(w), 1095(w), 1027(w), 802(w), 720(w).

’ ASSOCIATED CONTENT

bS

Supporting Information. Crystallographic information files of complex 1. This material is available free of charge via the Internet at http://pubs.acs.org.

NOTE

Heterocycles; Wiley-VCH: Weinheim, 2003. (c) Katrizky, A. R.; Pozharskii, A. F. Handbook of Heterocyclic Chemistry, 2nd ed.; Pergamon: Amsterdam, 2000. (2) Guimond, N.; Gouliaras, C.; Fagnou, K. J. Am. Chem. Soc. 2010, 132, 6908. (3) (a) Hall, C.; Perutz, R. Chem. Rev. 1995, 96, 3125. (b) Labinger, J. A.; Bercaw, J. E. Nature 2002, 417, 507. (c) Bergman, R. G. Nature 2007, 446, 391. (4) Janowicz, A. H.; Bergman, R. H. J. Am. Chem. Soc. 1982, 104, 352. (5) Hoyano, J. K.; Graham, W. A. G. J. Am. Chem. Soc. 1982, 104, 3723. (6) (a) Chen, H.; Schlecht, S.; Semple, T. C.; Hartwig, J. F. Science 2000, 287, 1995. (b) Kakiuchi, F.; Murai, S. Acc. Chem. Res. 2002, 35, 826. (7) (a) Johansson, L.; Tilset, M. J. Am. Chem. Soc. 2001, 123, 739. (b) Thomas, J. C.; Peters, J. C. J. Am. Chem. Soc. 2001, 123, 5100. (c) Johansson, L.; Ryan, O. B.; Rømming, C.; Tilset, M. J. Am. Chem. Soc. 2001, 123, 6579. (d) Ritleng, V.; Sirlin, C.; Pfeffer, M. Chem. Rev. 2002, 102, 1731. (e) Whited, M. T.; Grubbs, R. H. Acc. Chem. Res. 2009, 42, 1607. (f) Grounds, H.; Anderson, J. C.; Hayter, B.; Blake, A. J. Organometallics 2009, 28, 5289. (8) (a) Yu, W.-B.; Han, Y.-F.; Lin, Y.-J.; Jin, G.-X. Organometallics 2010, 29, 2827. (b) Yu, W.-B.; Han, Y.-F.; Lin, Y.-J.; Jin, G.-X. Chem.— Eur. J. 2011, 17, 1863. (c) Yu, W.-B.; Han, Y.-F.; Lin, Y.-J.; Jin, G.-X. Organometallics 2011, 30, 3090. (9) (a) Lewis, J.; Nyholm, R. S. Sci. Prog. 1964, 52, 557. (b) Cotton, F. A. Q. Rev. Chem. Soc. 1966, 20, 389. (c) Chini, P.; Longoni, G.; Albano, V. G. Adv. Organomet. Chem. 1976, 14, 285. (10) Herherhold, M.; Jin, G.-X. Angew. Chem., Int. Ed. 1994, 33, 964. (11) Braunstein, P.; Oro, L. A.; Raithby, P. R. Metal Clusters in Chemistry; Wiley-VCH: Weinheim, 1999. (12) Schnepf, A.; Himmel, H.-J. Angew. Chem., Int. Ed. 2005, 44, 3006. (13) Shriver, D. F.; Kaesz, H. D.; Adams, R. D. The Chemistry of Metal Cluster Complexes; Wiley: New York, 1990. (14) Cotton, F. A.; Curtis, N. F.; Harris, C. B.; Johnson, B. F. G.; Lippard, S. J.; Mague, J. T.; Robinson, W. R.; Wood, J. S. Science 1964, 145, 1305. (15) Wang, J.-Q.; Ren, C.-X.; Jin, G.-X. Organometallic 2006, 25, 74. (16) White, C.; Yates, A.; Maitles, P. M. Inorg. Synth. 1992, 29, 228. (17) Sheldrick, G. M. SHELXL-97; Universit€at G€ottingen: Germany, 1997. (18) Crystal data for 1: C80H102Cl2F12Ir6N8O12S4, Mr = 2948.04, monoclinic space group P2(1)/c, a = 15.821(6) Å, b = 31.134(12) Å, c = 21.920(8) Å, β = 109.851(5)o, V = 10156(7) Å3, Z = 4, Fcalcd = 1.928 g cm 3. A total of 45 832 reflections, of which 19 962 were independent (Rint = 0.0844). The structure was refined to a final R1 = 0.0529 [I > 2σ(I)], wR2 = 0.1261 for all data, GOF = 0.844, and residual electron density max./min. = 2.450 and 1.402 e Å 3.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Fax: (+86)-21-65643776.

’ ACKNOWLEDGMENT This work was supported by Shanghai Science and Technology Committee (08DZ2270500, 08DJ1400103), Shanghai Leading Academic Discipline Project (B108), and the National Basic Research Program of China (2009CB825300, 2010DFA41160). ’ REFERENCES (1) (a) Joule, J. A.; Mills, K. Heterocyclic Chemistry, 4th ed.; Blackwell: Oxford, 2000. (b) Eicher, T.; Hauptmann, S. The Chemistry of 3907

dx.doi.org/10.1021/om200399b |Organometallics 2011, 30, 3905–3907