Tridentate Cyclometalated Ruthenium(II) Complexes of “Click” Ligand

Mar 22, 2011 - A tridentate cyclometalating ligand, 1,3-di(1,2,3-triazol-4-yl)benzene (dtab), has been prepared and used for the syntheses of a number...
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Tridentate Cyclometalated Ruthenium(II) Complexes of “Click” Ligand 1,3-Di(1,2,3-triazol-4-yl)benzene Wen-Wen Yang, Lei Wang, Yu-Wu Zhong,* and Jiannian Yao* Beijing National Laboratory for Molecular Sciences, CAS Key Laboratory of Photochemistry, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China

bS Supporting Information ABSTRACT: A tridentate cyclometalating ligand, 1,3-di(1,2,3triazol-4-yl)benzene (dtab), has been prepared and used for the syntheses of a number of cyclometalated RuII complexes. A comparison of the electrochemical and spectroscopic properties of cyclometalated complexes made from dtab or 1,3-di(2pyridyl)benzene is presented as well.

’ INTRODUCTION Polyazine transition-metal complexes have been the subject of intense research activities because of their distinguished electrochemical and photophysical properties.1 One of the mostly studied prototype complexes is [Ru(tpy)2](PF6)2 (tpy = 2,20 :60 ,200 -terpyridine). This complex can be readily incorporated into supramolecular architectures with well-defined structures via the easy and reliable functionalization at the 4 and 40 -position of the tpy ligand.2 Recently, much attention has been drawn to the studies of cyclometalated transition-metal complexes.3 Compound 1 (Figure 1) is the simplest cyclometalated version of [Ru(tpy)2](PF6)2, with a RuC bond present between the metal center and the 1,3-di(2-pyridyl)benzene (dpb) ligand.1d The anionic nature of the cyclometalating ligand significantly changes the properties of these complexes, as compared to noncyclometalated ones. It has been reported that cyclometalated complexes can greatly enhance electronic coupling between metal centers in mixed-valence systems and have high potential to be used as molecular wires.4 Besides, recent investigations have proved that cyclometalated ruthenium complexes could be used as efficient sensitizers for solar cell applications,5 and there is still large room for further improvement. In this contribution, we report the synthesis and studies of new tridentate cyclometalated RuII complexes derived from 1,3-di(1,2,3-triazol-4-yl)benzene (dtab), which could be accessed via click chemistry.6 This complex exhibits similar electrochemical and spectroscopic properties to complex 1, but with more positions available for further elaboration and fine-tuning its properties. ’ RESULTS AND DISCUSSION Huisgen 1,3-dipolar cycloaddition, one of the so-called click reactions, could readily afford a 1,2,3-triazole ring system in high r 2011 American Chemical Society

yields and has received a tremendous degree of recent attention.7 We are particularly interested in the application of this reaction to coordination chemistry.8 It should be noted that a number of 1,2,3-triazole-containing ligands, such as 2,6-bis(1,2,3-triazol-4yl)pyridines,9 2-(1H-1,2,3-triazol-4-yl)-pyridine,10 and phenyl1H-1,2,3-triazoles,11 have been synthesized and reported to form stable complexes with Ru, Ir, Re, etc. However, to the best of our knowledge, 1,2,3-triazole-containing cyclometalated ruthenium complexes have not been documented. As outlined in Scheme 1, dtab ligands 2 and 3 were prepared through the Cu-catalyzed 1,3-dipolar cycloaddition between 1,3diethynylbenzene and 1-butylazide or phenylazide in good yields (see the Experimental Section for details). Subsequent reaction of 1 equiv of Ru(tpy)Cl3 with 2 or 3 in the presence of silver triflate afforded cyclometalated RuII complexes 4 and 5 in 19% and 5% yield, respectively. The lower yield for the formation of complex 5 is partly because ligand 3 has more coordination sites available due to the presence of two additional phenyl rings. Complex 4 could be regio-exclusively brominated on the para position of the RuC bond to give complex 6.12 On the other hand, complex 8, with a bromine substituent on the tpy ligand side, could be prepared from the known compound 74a and the above-synthesized ligand 2. In addition, complex 10, with a tolyl group on the tpy ligand, was prepared in order to examine the influence of substituents on the electronic properties of these complexes. All of these efforts illustrated that multiple positions are available for structural variations and modifications of the new cyclometalated complex with a “click” ligand and brominated complexes 6 and 8 could be Received: January 17, 2011 Published: March 22, 2011 2236

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Table 1. Electrochemical and Electronic Absorption Data of Complexes Studied E1/2a complex

anodic

λmax nm c

E1/2a cathodic

ΔE (eV)

ε (105 M1 cm1) 316(0.43), 367(0.14) 490(0.10), 537(0.077)

b

4

þ0.53 1.57 þ1.60 1.95

2.10

5

þ0.58 1.53

2.11

þ1.64 1.89 6

þ0.59 1.55

2.14

þ1.65 1.97 8

Figure 1. Chemical structures of cyclometalated ruthenium complexes. The arrows point to the sites that can be easily functionalized.

þ0.56 1.35, 1.58

2.14

316(0.31), 369(0.13) 500(0.10), 548(0.065)

10

þ0.53 1.53 þ1.55 1.92

2.06

1

þ0.56 1.51

2.07

þ1.60 1.92 [Ru(tpy)2]2þ þ1.32 1.22, 1.46

315(0.36), 366(0.12) 483(0.083), 529(0.066)

þ1.66 1.96

Scheme 1. Synthesis of 210

315(0.41), 378(0.21) 485(0.079), 524(0.07)

314(0.35), 366(0.17) 497(0.12), 541(0.098) 315(0.37), 372(0.086) 423(0.095), 499(0.14)

2.54

307(0.78), 475(0.17)

a

The potential is reported as the E1/2 value vs Ag/AgCl. b The electrochemical energy gap is determined by the potential difference between the first oxidation and first reduction wave. c The absorption spectra were recorded in acetonitrile.

Figure 2. Cyclic voltammograms of 4 (red line), 1 (black line), and [Ru(tpy)2](PF6)2 (blue line) in acetonitrile containing 0.1 M Bu4NClO4 as the supporting electrolyte at a scan rate of 100 mV/s. The working electrode was glassy carbon, and the counter electrode was a platinum wire.

used as basic building blocks for the construction of more complex systems. The electronic properties of these complexes were studied by electrochemical analysis (Table 1). The cyclic voltammetric (CV) profile of cyclometalated ruthenium complex 4 is shown in Figure 2, together with those of complex 1 and [Ru(tpy)2] (PF6)2 for comparison. The RuII/III redox process of 4 occurs at þ0.53 V vs Ag/AgCl. This is a typical value for a cyclometalated RuII/III redox process.4,5 The anodic scan to a more positive

potential led to the appearance of an irreversible oxidation at þ1.60 V (Figure S2 in the Supporting Information). This peak is attributable to a RuIII/IV process or ligand-based decomposition.4,5 The first reduction wave of 4 occurs at 1.57 V, followed by an irreversible reduction peak at 1.95 V. The first reversible redox couple is assigned to the reduction of the tpy ligand,4,5 and the second irreversible peak is probably due to the reduction of the cyclometalating ligand. The electrochemical energy gap 2237

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complex 10 leads to the increase of absorption of the MLtpyCT transition.

Figure 3. UV/vis absorption spectra of 4 (red line), 5 (olive line), 10 (magenta line), 1 (black line), and [Ru(tpy)2](PF6)2 (blue line) in acetonitrile.

determined by the potential difference between the first oxidation and reduction wave is 2.10 eV for complex 4. In comparison to 1, both oxidation and reduction waves of complex 4 shift negatively slightly. It should be noted that the electrochemical properties of cyclometalated complexes are significantly different from those of noncyclometalated complexes, such as [Ru(tpy)2]2þ (blue line in Figure 2). This complex displays a Ru-based oxidation at þ1.32 V vs Ag/AgCl and two reduction couples at 1.22 and 1.46 V. The electrochemical behaviors of complexes 5 and 6 (Figures S3 and S4) are similar to that of 4. However, the cathodic scan of complex 8 exhibits an additional peak at 1.35 V (Figure S5), which could be assigned to the reduction of bromide followed by irreversible loss of bromine anion. In contrast, complex 6, with a bromine substituent on the cyclometalating ligand side, does not show this process. This fact is in accordance with the assignment of the first cathodic process of complexes with a “click” ligand to the reduction of the tpy ligand instead of the cyclometalating ligand (dtab). The electrochemical properties of 10 also support this assignment. The introduction of a tolyl group on the tpy ligand in complex 10 results in a 40 mV positive shift of the first cathodic wave as compared to complex 4, while their RuIII/IV processes take place at exactly the same potential (þ0.53 V). The presence of a tolyl group enhances the delocalization degree of the tpy ligand and makes it easier to be reduced.13 The UV/vis absorption spectra of the above compounds were recorded and compared with those of 1 and [Ru(tpy)2](PF6)2 (Figure 3, Table 1, and Figure S7 in the Supporting Information) to further probe their electronic properties. Complex [Ru(tpy)2](PF6)2 shows two strong intraligand absorptions in the UV region and a metal-to-ligand charge-transfer (MLCT) transition at 475 nm (blue line).14 In comparison, the MLCT absorptions of cyclometalated Ru complex 1 are bathochromically shifted (499 nm), broadened, and slightly decreased in absorptivity (black line). The absorptions of cyclometalated complexes 46, 8, and 10 are significantly different from those of [Ru(tpy)2](PF6)2 and 1. In addition to the intraligand transitions below 340 nm, an intense absorption band between 340 and 420 nm is evident. This band is attributable to the metal to N∧C∧N ligand CT transitions (MLNCNCT).3c,e In addition, the metal to tpy liand CT transitions (MLtpyCT) around 490 nm exhibit distinct lower energy shoulders. This feature has also been documented for cyclometalated Ru complexes with an ester substituent on the tpy ligand.3c Complex 5, with two lateral phenyl groups on the triazole rings, display a more intense MLNCNCT transition than those of 4 and 10. On the other hand, the introduction of a tolyl group to the tpy ligand side on

’ CONCLUSION In summary, two new tridentate cyclometalating ligands, 2 and 3, were designed and synthesized through click chemistry. A number of cyclometalated ruthenium complexes could be accessed from reactions of 2 or 3 with various tpy ligands with different substituents. Electrochemical and spectroscopic studies demonstrated that electronic properties of these complexes could be fine-tuned by changing the substituents on either the cyclometalating or noncyclometalating ligand sides. This preliminary study confirms the effectiveness of the “click” compound dtab as an alternative tridentate ligand to the commonly used dpb ligand, but with distinctive favorable differences, such as the simple approach to the “click” ligand and more available positions for structural variations and modifications of the complexes. We are currently working on the optimization of the cyclometalation reaction conditions and the construction of supromolecular coordination systems from these ligands and complexes. ’ EXPERIMENTAL SECTION General Procedures. NMR spectra were recorded in the designated solvent on a Bruker Avance 400 M spectrometer. Spectra are reported in ppm values from residual protons of deuterated solvent for 1H NMR (δ 7.26 ppm for CDCl3 and 1.92 ppm for CD3CN) and 13 C NMR (δ 77.00 ppm for CDCl3). MS data were obtained with a Bruker Daltonics Inc. ApexII FT-ICR or Autoflex III MALDI-TOF mass spectrometer. The matrix for MALDI-TOF measurements is 2,5dihydroxybenzoic acid or R-cyano-4-hydroxycinnamic acid. Microanalysis was carried out using a Flash EA 1112 or Carlo Erba 1106 analyzer at the Institute of Chemistry, CAS. Compound 2. To a solution of 345 mg (5.3 mmol) of sodium azide in a mixture of water (14 mL) and methanol (7 mL) was added 685 mg (5.0 mmol) of n-butyl bromide at room temperature. The resulting mixture was heated at 95 °C for 24 h, during which time the bottom layer of butyl bromide disappeared and a top layer of butyl azide appeared. The butyl azide layer (3 mL) was subsequently isolated using a separatory funnel and used in the next transformation without further purification. A mixture of 1,3-diethynylbenzene (1 mmol, 126 mg), butyl azide (8 mmol, 792 mg), sodium ascorbate (0.2 mmol, 39.6 mg), and CuSO4 3 5H2O (0.02 mmol, 5 mg) in a 1:1 mixture of EtOH and H2O (20 mL) was stirred at room temperature for 24 h. After removal of solvents under vacuum, the crude product was purified by column chromatography (eluent: CH2Cl2/ethyl acetate, 10:1) to afford 2 as a white solid (99%). 1 H NMR (400 MHz, CDCl3): δ 0.99 (t, J = 7.4 Hz, 6H), 1.40 (dd, J = 14.9, 7.4 Hz, 4H), 1.95 (m, 4H), 4.42 (t, J = 7.1 Hz, 4H), 7.49 (t, J = 7.7 Hz, 1H), 7.84 (t, J = 6.2, 7.8 Hz, 4H), 8.30 (s, 1H). 13C NMR (100 MHz, CDCl3): δ 147.4, 131.3, 129.4, 125.3, 122.8, 119.7, 50.2, 32.3, 19.7, 13.5. EI-HRMS: calcd for C18H24N6 324.2062, found 324.2066. Anal. Calcd for C18H24N6: C, 66.64; H, 7.46; N, 25.90. Found: C, 66.66; H, 7.62; N, 25.85. Compound 3. To an ethanol/water mixture (20 mL, 7:3 ratio) were added bromobenzene (0.84 mL, 8 mmol), N,N0 -dimethylethylenediamine (0.13 mL, 1.2 mmol), CuI (154 mg, 0.8 mmol), and sodium L-ascorbate (80 mg, 0.4 mmol). The flask was then flushed with nitrogen, followed by the addition of NaN3 (260 mg, 4 mmol). The reaction mixture was heated to reflux for 3 h. After cooling to room temperature, 1,3-diethynylbenzene (0.13 mL, 1.0 mmol) was added into the slightly yellow mixture. After stirring for another 24 h at room temperature, the 2238

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Organometallics solution was concentrated under reduced pressure. The residue was then dissolved in 100 mL of dichloromethane. After filtration, the organic solution was washed with water and dried over MgSO4. The product was purified by flash column chromatography on silica gel (eluent: CH2Cl2/ ethyl acetate, 10:1) to afford the product as a white solid (344 mg, 95%). 1 H NMR (400 MHz, CDCl3): δ 7.48 (d, J = 7.4 Hz, 2H), 7.58 (t, J = 7.5 Hz, 5H), 7.82 (d, J = 7.8 Hz, 4H), 7.94 (d, J = 7.6 Hz, 2H), 8.33 (s, 2H), 8.47 (s, 1H). 13C NMR (100 MHz, CDCl3): δ 148.0, 137.0, 130.9, 129.8, 129.6, 128.8, 125.7, 123.1, 120.5, 117.9. Anal. Calcd for C22H16N6: C, 72.51; H, 4.43; N, 23.06. Found: C, 72.09; H, 4.45; N, 22.74. Complex 4. To 15 mL of dry acetone were added Ru(tpy)Cl3 (78.1 mg, 0.2 mmol) and AgOTf (154.3 mg, 0.6 mmol), and the mixture was refluxed for 2 h. After cooling to room temperature, the mixture was filtered to remove unwanted precipitates, and the filtrate was concentrated to dryness. To the residue were added ligand 2 (48.8 mg, 0.15 mmol), DMF (5 mL), and t-BuOH (5 mL). The mixture was heated to reflux for 24 h. After cooling to room temperature, the solvent was removed under reduced pressure, and the residue was dissolved in the proper amount of methanol. After addition of an excess of KPF6, the resulting precipitate was collected by filtering and washing with water and Et2O. The obtained solid was subjected to flash column chromatography on silica gel (eluent: CH2Cl2/CH3CN, 10:1) to give 22.5 mg of complex 4 (19%). 1H NMR (400 MHz, CDCl3): δ 0.71 (t, J = 7.2 Hz, 6H), 1.04 (m, 4H), 1.61 (m, 4H), 4.02 (t, J = 7.0 Hz, 4H), 6.88 (t, J = 6.3 Hz, 2H), 7.29 (m, 3H, overlapped), 7.63 (t, J = 7.6 Hz, 2H), 7.75 (s, 2H), 7.77 (d, J = 4.3 Hz, 2H), 8.10 (t, J = 7.4 Hz, 1H), 8.22 (d, J = 7.9 Hz, 2H), 8.46 (d, J = 8.2 Hz, 2H). 13C NMR (100 MHz, CDCl3): δ 12.4, 18.8, 31.3, 50.8, 118.3, 120.2, 120.7, 121.3, 122.9, 126.2, 132.0, 134.8, 135.0, 154.2, 154.5, 158.8, 159.6. MALDI-MS: 658.2 for [M  PF6]þ. ESIHRMS: calcd for C33H34N9Ru 658.1978, found 658.1985. Anal. Calcd for C33H34F6N9PRu 3 H2O: C, 48.29; H, 4.42; N, 15.36. Found: C, 48.20; H, 4.31; N, 15.10. Complex 5. To 10 mL of dry acetone were added Ru(tpy)Cl3 (44 mg, 0.1 mmol) and AgOTf (77 mg, 0.3 mmol), and the mixture was refluxed for 3 h. After cooling to room temperature, the mixture were filtered to afford a purple-black solution, and the filtrate was concentrated to dryness. To the residue were added ligand 3 (36 mg, 0.1 mmol), DMF (5 mL), and t-BuOH (5 mL), and the mixture was refluxed in a sealed tube for 24 h. After cooling to room temperature, the solvent was removed under reduced pressure, and the residue was dissolved in the proper amount of methanol. After addition of an excess of KPF6, the resulting precipitate was collected by filtering and washing with water and Et2O. The obtained solid was subjected to flash column chromatography on silica gel (eluent: CH2Cl2/CH3CN, 15:1) to afford 4.3 mg of complex 5 as a purple-black solid (5%). 1H NMR (400 MHz, CD3CN): δ 7.05 (t, J = 6.4 Hz, 2H), 7.39 (d, J = 4.2 Hz, 5H), 7.44 (d, J = 4.2 Hz, 7H), 7.72 (t, J = 7.8 Hz, 3H), 7.94 (d, J = 7.4 Hz, 2H), 8.23 (t, J = 8.0 Hz, 1H), 8.38 (d, J = 8.0 Hz, 2H), 8.62 (d, J = 5.9 Hz, 3H), 8.65 (s, 1H). ESI-MS: 698.4 for [M  PF6]þ. HRESI-MS: calcd for C37H26N9Ru 698.1360, found 698.1350. Anal. Calcd for C37H26F6N9PRu 3 CH3CN: C, 63.40; H, 3.96; N, 18.96. Found: C, 63.33; H, 4.09; N, 19.37. Complex 6. Complex 4 (16.7 mg, 0.02 mmol) was treated with 1.1 equiv of N-bromosuccinimide (NBS, 5 mg) in CH3CN at room temperature, and the reaction mixture was allowed to stir for 6 h. The crude product was purified by column chromatography (CH2Cl2/ CH3CN, 10:1) on neutral Al2O3 to afford 6 as a purple-black solid (11 mg, 63%). 1H NMR (400 MHz, CD3CN): δ 0.64 (t, J = 7.3 Hz, 6H), 0.94 (m, 4H), 1.53 (m, 4H), 4.03 (t, J = 7.1 Hz, 4H), 7.00 (t, J = 6.5 Hz, 2H), 7.26 (d, J = 4.6 Hz, 2H), 7.71 (t, J = 7.8 Hz, 2H), 7.97 (s, 2H), 8.17 (s, 2H), 8.19 (t, J = 8.1 Hz, 1H), 8.34 (d, J = 8.0 Hz, 2H), 8.60 (d, J = 8.0 Hz, 2H). ESI-MS: 738.5 for [M  PF6]þ, 662.5 for [M  PF6  Br]þ. ESI-HRMS: calcd for C33H33BrN9Ru 736.1081, found 736.1068.

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Complex 8. To 10 mL of dry acetone were added Ru(tpy-Br)Cl3 (52 mg, 0.1 mmol) and AgOTf (77 mg, 0.3 mmol), and the mixture was refluxed for 2 h. After cooling to room temperature, the mixture were filtered to afford a purple-black solution, and the filtrate was concentrated to dryness. To the residue were added ligand 2 (32 mg, 0.1 mmol), DMF (4 mL), and t-BuOH (8 mL). The mixture was heated to reflux in a sealed tube for 24 h. After cooling to room temperature, the solvent was removed under reduced pressure, and the residue was dissolved in the proper amount of methanol. After addition of an excess of KPF6, the resulting precipitate was collected by filtering and washing with water and Et2O. The obtained solid was subjected to flash column chromatography on silica gel (eluent: CH2Cl2/CH3CN, 10:1) to give 5.4 mg of complex 8 (6%). 1H NMR (400 MHz, CD3CN): δ 0.64 (t, J = 7.2 Hz, 6H), 0.92 (m, 4H), 1.53 (m, 4H), 4.02 (t, J = 6.7 Hz, 4H), 7.04 (t, J = 6.6 Hz, 2H), 7.30 (d, J = 5.6 Hz, 2H), 7.34 (t, J = 8.5 Hz, 1H, overlapped with the peak at 7.30 ppm), 7.72 (t, J = 8.1 Hz, 2H), 7.82 (d, J = 7.5 Hz, 2H), 8.04 (s, 2H), 8.36 (d, J = 8.1 Hz, 2H), 8.83 (s, 2H). ESI-MS: 736.3 for [M  PF6]þ, 658.4 for [M  PF6  Br]þ. Anal. Calcd for C33H33BrF6N9PRu 3 H2O: C, 44.06; H, 3.92; N, 14.01. Found: C, 43.96; H, 3.80; N, 14.19. Complex 10. To 10 mL of dry acetone were added Ru(ttpy)Cl3 (79.9 mg, 0.2 mmol) and AgOTf (120.3 mg, 0.6 mmol), and the mixture was refluxed for 3 h. After cooling to room temperature, the mixture was filtered to afford a purple-black solution, and the filtrate was concentrated to dryness. To the residue were added ligand 2 (34.2 mg, 0.1 mmol) and 10 mL of DMF, and the mixture was heated to reflux for 24 h. After cooling to room temperature, the solvent was removed under reduced pressure, and the residue was dissolved in the proper amount of methanol. After addition of an excess of KPF6, the resulting precipitate was collected by filtering and washing with water and Et2O. The obtained solid was subjected to flash column chromatography on silica gel (eluent: CH2Cl2/CH3CN, 10:1) to give 27.4 mg of complex 10 (31%). 1H NMR (400 MHz, CD3CN): δ 0.63 (t, J = 7.4 Hz, 6H), 0.92 (m, 4H), 1.53 (m, 4H), 2.49 (s, 3H), 4.02 (t, J = 7.1 Hz, 4H), 7.00 (t, J = 6.6 Hz, 2H), 7.28 (d, J = 5.5 Hz, 2H), 7.36 (s, 1H), 7.50 (d, J = 7.9 Hz, 2H), 7.70 (t, J = 7.8 Hz, 2H), 7.82 (d, J = 7.5 Hz, 2H), 8.05 (d, J = 6.3 Hz, 4H), 8.46 (d, J = 8.1 Hz, 2H), 8.84 (s, 2H). ESI-MS: 748.5 for [M  PF6]þ. ESI-HRMS: calcd for C40H40N9Ru 748.2453, found 748.2443. Anal. Calcd for C40H40F6N9PRu: C, 53.81; H, 4.52; N, 14.12. Found: C, 53.62; H, 4.54; N, 14.12.

’ ASSOCIATED CONTENT

bS

Supporting Information. CV profiles of cyclometalated complexes and electronic absorption spectra of 6 and 8, and NMR and MS spectra of new compounds. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected] (Y.-W.Z.), [email protected] (J.Y.).

’ ACKNOWLEDGMENT We thank the National Natural Science Foundation of China (No. 21002104), Institute of Chemistry, Chinese Academy of Sciences (“100 Talent” Program), and 973 program (2011CB932300) for funding support. 2239

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Organometallics

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dx.doi.org/10.1021/om200039j |Organometallics 2011, 30, 2236–2240