New diruthenium (II,III) compounds bearing terminal olefin groups

Inorganica Chimica Acta 396 (2013) 144–148

Contents lists available at SciVerse ScienceDirect

Inorganica Chimica Acta journal homepage: www.elsevier.com/locate/ica

New diruthenium (II,III) compounds bearing terminal olefin groups Julia Savchenko a, Phillip E. Fanwick a, Håkon Hope b, Yang Gao a, Charu K. Yerneni a, Tong Ren a,⇑ a b

Department of Chemistry, Purdue University, West Lafayette, IN 47907, USA Department of Chemistry, University of California, Davis, CA 95616, USA

a r t i c l e

i n f o

Article history: Received 26 July 2012 Received in revised form 14 November 2012 Accepted 15 November 2012 Available online 24 November 2012 Keywords: Diruthenium Olefin metathesis Structure Electrochemistry Si surface

a b s t r a c t The reaction between Ru2(DmAniF)3(OAc)Cl (DmAniF is N,N0 -di(m-methoxyphenyl)formamidinate) and HO2C(CH2)mCH@CH2 (m = 3, 4 and 8) under reflux afforded new diruthenium species Ru2(DmAniF)3 (O2C(CH2)mCH@CH2)Cl (m = 3, 1a; 4, 1b; and 8, 1c). Similarly, the reaction between cis-Ru2(DmAniF)2 (OAc)2Cl and HO2C(CH2)mCH@CH2 resulted in Ru2(DmAniF)2(O2C(CH2)mCH@CH2)2Cl (m = 3, 2a; and 8, 2c). Compounds 2 subsequently underwent an olefin ring closing metathesis reaction catalyzed by (Cy3P)2Cl2Ru(@CHPh) to afford the dimerized compounds Ru2(DmAniF)2(l-O2C(CH2)mCH@)2Cl (m = 3, 3a; and 8, 3c). All compounds reported herein were analyzed by voltammetry, high resolution mass spectrometry and Vis–NIR spectroscopy, with the structures of 1c and 2c established through X-ray single crystal diffraction. Ó 2012 Elsevier B.V. All rights reserved.

1. Introduction Metal-catalyzed olefin metathesis has become one of the most widely used carbon–carbon bond formation reactions in organic synthesis. It has been demonstrated with a large number of successful examples that olefin cross metathesis (CM) can be utilized as an elegant synthetic tool to either link two units together or to achieve intramolecular cyclization (ring closing metathesis or RCM) [1,2]. The Gladysz group has described the RCM assembly of a series of complex structures containing diverse metallic supports as templates, including Pt, Pd, Rh, Re, and W [3–7]. Several recent reports include employment of olefin metathesis to prepare metallosalens, metallocenes (Ni, Fc) and phosphine chelate chromium complexes [8–12]. Efforts from our laboratory focus on the modular nature of bimetallic paddlewheel species, in which dimeric and oligomeric assemblies can be achieved by modifying the ligand periphery [13–18]. Among the previously reported Ru2 species are a series of compounds containing one or two terminal olefins and their olefin metathesis products [14,15]. These diruthenium species are particularly attractive as building blocks for supramolecular materials as well as active components for molecular devices due to their robust redox chemistry over a broad potential window, net molecular spin and the possibility of ligand engineering. A potential application of Ru2 species bearing a peripheral olefin is the incorporation of molecules onto Si surfaces to realize hybridmolecule CMOS devices (Scheme 1) [19]. In order to achieve ⇑ Corresponding author. E-mail address: [email protected] (T. Ren). 0020-1693/$ - see front matter Ó 2012 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.ica.2012.11.008

effective and dense passivation of H–Si surface with these types of compounds, the diruthenium species needs to contain an extended hydrocarbon tether due to the bulky nature of Ru2 species (Scheme 1). Described in this contribution are the synthesis and structural study of Ru2(DmAniF)3(l-O2C(CH2)mCH@CH2)Cl (m = 3, 1a; 4, 1b; 8, 1c; DmAniF is N,N0 -di(m-methoxyphenyl)formamidine) and cis-Ru2(DmAniF)2(l-O2C(CH2)mCH@CH2)2Cl (m = 3, 2a; 8, 2c), the latter of which underwent the ring closing metathesis reaction to afford cis-Ru2(DmAniF)2(l-O2C(CH2)mCH@)2Cl (m = 3, 3a; 8, 3c). 2. Results and discussion The syntheses of compounds 1–3 are based on the Ru2(DArF)4n (OAc)n type synthons developed in the laboratories of Cotton [20–24], Jiménez-Aparicio [25–30] and Ren [14,15,18,31–33]. As shown in Scheme 2, the species containing one or two x-alkenea-carboxylate ligands can be prepared from carboxylate exchange reactions by following a procedure that has been reported in literature [14,15]. Gentle reflux of Ru2(DmAniF)3(OAc)Cl in the presence of excess carboxylic acid such as 5-hexenoic, 6-heptenoic or 10-undecylenic affords the new compounds Ru2(DmAniF)3 (l-O2C(CH2)mCH@CH2)Cl (m = 3, 1a; 4, 1b; 8, 1c) in good yields. In contrast to the preparation of compound 1, the carboxylate exchange reaction between cis-Ru2(DmAniF)2(OAc)2Cl and either 10-undecylenic or 5-hexenoic acids required more rigorous conditions. The synthesis of 2 was achieved by refluxing in toluene aided by an acetic acid scrubbing apparatus as described previously [34]. The two x-alkene-a-carboxylates in the coordination sphere of compounds 2 underwent ring closing metathesis (RCM) in the

J. Savchenko et al. / Inorganica Chimica Acta 396 (2013) 144–148

υ Δ

145

° Si

Scheme 1. Functionalization of Si surface with olefin-capped molecule, sphere represents diruthenium coordination sphere.

presence of the first generation Grubbs catalyst to give compounds 3. Compounds 3 were identified as the ring RCM product based on the ESI-MS data. Interestingly, refluxing compound 2a under the same conditions resulted in the formation of two new compounds that are distinct on TLC (Rf = 0.45 and 0.35, THF/Hex 1:1) but yield the same 940 ([3a-Cl]+) peak in ESI-MS. These compounds are likely the Z/E isomers of Ru2(DmAniF)2(l-O2C(CH2)3CH@)2Cl. Similar to their precursors, compounds 1–3 have effective magnetic moments at room temperature in a narrow range of 3.93–3.96 lB (Bohr magneton), which is consistent with a S = 3/2 ground state [35].  and compound Compound 1c crystallizes in the space group P 1, 2c in C2/c. The asymmetric unit of 1c contains two complete molecules, while that of 2c contains one. The structural plot of 1c (Fig. 1) shows three DmAniF ligands and one 10-undecyleonate around the diruthenium core, with a chloro ligand in the axial position. It is clear from the structural plot of 2c (Fig. 2) that the coordination sphere of the Ru2 unit consists of two DmAniF and two 10-undecyleonate bidentate ligands in a cis-arrangement, with chloro and water ligands occupying opposite axial positions. Listed in Table 1 are the selected bond lengths and angles for compounds 1c and 2c. Specifically, the Ru–Ru bond length of 1c (2.3202(4) Å) is nearly identical to that of the parent compound Ru2(DmAniF)3(O2CMe)Cl (2.3220(7) Å) [33]. The Ru–Ru bond length in 2c is 2.3194(6) Å, which is similar to that of Ru2(DmAniF)2(O2CMe)2Cl (2.3219(4) Å) [33]. The averaged equatorial Ru–O and Ru–N bond lengths in both 1c and 2c are comparable to those reported for Ru2 compounds containing x-alkene-a-

Fig. 1. Structural plot of 1c. Hydrogen atoms were omitted for clarity.

carboxylate ligand [15,33,36,37]. The Ru–N bond lengths on Ru1 in 1c are slightly elongated compared to those on Ru2, which is attributed to the attachment of an axial chloro ligand to Ru1. Less variation is noticed in the Ru–N bond lengths of 2c, since a water molecule is coordinated to the axial site of the second Ru center. Similar to other Ru2 paddlewheel species previously reported from our laboratory, compounds 1–3 exhibit multiple reversible or quasi-reversible one-electron redox couples as shown in Fig. 3 and the electrode potentials in Table 2. The reversible 1e- oxidation, A, is a Ru2(III,III)/Ru2(III,II) couple. As discussed in details previously [15], the first reduction couple, B, is irreversible due to a fast dissociation of the axial Cl ligand upon reduction, yielding an axial-ligand-free Ru2(II,II) species (Scheme 3). The further reduction of the axial-ligand-free Ru2(II,II) species results in the reversible couple, D. Oxidation of the axial-ligand-free Ru2(II,II) species on the return sweep yielded wave C at a potential far more positive than Epc(B). The cyclic voltammograms measured for compounds 2 (Fig. 3) reveal three Ru2-based couples similar to those recorded for related Ru2(DmAniF)2(OAc)2Cl type compounds [14,15,33]. In general, the redox couples in the type 2 compounds

Scheme 2. Preparation of diruthenium-x-olefin-a-carboxylate compounds.

146

J. Savchenko et al. / Inorganica Chimica Acta 396 (2013) 144–148

Fig. 2. Structural plot of 2c. Hydrogen atoms were omitted for clarity.

Table 1 Selected bond lengths (Å) and angles (s) for 1c and 2c. 1c

2c

Ru1–Ru2 Ru1–Cl1 Ru1–N3 Ru1–N1 Ru1–O1 Ru1–N5 Ru2–N2 Ru2–N6 Ru2–O2 Ru2–N4 C10–C11

2.3202(4) 2.4101(8) 2.073(3) 2.088(3) 2.086(2) 2.093(3) 2.013(3) 2.045(2) 2.057(2) 2.048(3) 1.311(19)

Ru2–Ru1–Cl1

175.64(2)

Ru1–Ru2 Ru1–Cl1 Ru1–N3 Ru1–N1 Ru1–O1 Ru1–O3 Ru2–N2 Ru2–O4 Ru2–O2 Ru2–N4 C40–C41 C51–C52 Ru2–O9 Ru2–Ru1–Cl1 Ru1–Ru2–O9

2.3194(6) 2.5015(16) 2.053(6) 2.058(5) 2.093(4) 2.066(5) 2.030(5) 2.069(5) 2.046(4) 2.031(6) 1.38(2) 1.37(2) 2.359(4) 73.30(4) 166.77(12)

are less reversible compared to that of compounds 1, which is typical of cis-Ru2L2(O2CR)2 type compounds [15,38]. Compounds 3 display voltammograms very similar to those of the corresponding parent compounds 2. 3. Conclusion Several diruthenium species bearing one or two terminal olefin functional groups have been prepared, and those bearing two olefin groups in the cis-position undergo intramolecular ring-closing metathesis reactions. Because of the long saturated hydrocarbon tethers present in compounds 1c/2c/3c, these compounds are suitable candidates for silicon surface passivation through hydrosilylation reactions [19], which are being investigated in our laboratory. 4. Experimental

Fig. 3. Cyclic voltammograms of compounds 1–3 recorded in 0.2 M THF solution of n-Bu4NPF6 at a scan rate of 0.10 V/s.

Table 2 Electrode potentials of compounds 1–3. E1/2(A), V

Epc(B), V

E1/2(D), V

1a 1b 1c

0.73 0.73 0.74

0.58 0.57 0.55

1.39 1.40 1.39

2a 2c 3a 3c

Epa(A), V 0.80 0.86 0.80 0.85

Epc(B), V 0.54 0.58 0.58 0.60

Epc(D), V 1.58 1.70 1.64 1.68

4.1. General remarks 10-Undecylenic acid and (Cy3P)2Cl2Ru(@CHPh) were purchased from Aldrich. 5-Hexenoic and 6-heptenoic acids were obtained from Alfa Aesar. Silica gel was purchased from Merck. Ru2

J. Savchenko et al. / Inorganica Chimica Acta 396 (2013) 144–148

Scheme 3. Assignments of observed Ru2-based redox couples in compounds 1–3.

(DmAniF)3(OAc)Cl and cis-Ru2(DmAniF)2(OAc)2Cl were prepared according to a literature procedure [33]. THF was distilled over Na/benzophenone under a N2 atmosphere. All reactions were carried out using Schlenk techniques under nitrogen unless stated otherwise. TLC analyses were carried out using Whatman 250 lm flexible polyester-backed TLC plates. Vis–NIR spectra were recorded in THF with a JASCO V-670 UV–Vis–NIR spectrophotometer. Magnetic susceptibility was measured at 294 K with a Johnson Matthey Mark-I magnetic susceptibility balance. Infrared spectra were obtained on a JASCO FT-IR 6300 spectrometer via ATR on a ZnSe crystal. All HR-nESI-MS spectra were performed on a prototype version of a QqTOF tandem mass spectrometer in CH2Cl2 (Q-Star Pulsar XL; Applied Biosystems/MDS Sciex, Concord, ON, Canada). Masses were calculated by isotopic distribution utilizing Analyst 1.5 software (Applied Biosystems/MDS Sciex, Concord, ON, Canada). Cyclic voltammograms were recorded in 0.2 M n-Bu4 NPF6 solution (THF, N2-degassed) on a CHI620A voltammetric analyzer with a glassy carbon working electrode (diameter = 2 mm), a Pt-wire auxiliary electrode, and a Ag/AgCl reference electrode. The concentration of the diruthenium species is always 1.0 mM. The ferrocenium/ferrocene couple was observed at 0.58 V (versus Ag/AgCl) under experimental conditions. 4.2. Ru2(DmAniF)3(l-O2C(CH2)3CH@CH2)Cl (1a) Ru2(DmAniF)3(OAc)Cl (200 mg, 0.190 mmol) and 5-hexenoic acid (0.22 mL, 1.90 mmol) were dissolved in 50 mL THF and refluxed in air overnight. The reaction progress was monitored by TLC. Upon completion, the solvent volume was reduced and purification was carried out by recrystallization from THF/n-pentane (1:20, v/v). The precipitate was collected by filtration and washed with ethanol to yield a purple crystalline material. Yield: 162 mg (75% based on Ru). Rf = 0.5 (THF/hexanes 1:1). Anal. Calc. for C51H59N6O10.5ClRu2 (1a2.5H2O): C, 52.73; H, 5.11; N, 7.23. Found: C, 53.17; H, 4.95; N, 7.14%. Vis–NIR (nm, e (M1 cm1)): 528(5300); leff = 3.93 lB; HR-nESI-MS: m/z 1082.221 (calc. 1082.212), corresponding to [1a-Cl]+; IR (cm1): C@C, 1598(m); Cyclic voltammogram [E1/2/V, DEp/V, ibackward/iforward]: A, 0.73, 0.030, 0.84; Epc (B), 0.58; D, 1.39, 0.030, 0.81. 4.3. Ru2(DmAniF)3(l-O2C(CH2)4CH@CH2)Cl (1b) Synthesis of 1b was analogous to that of 1a with 5-hexenoic being replaced by 6-heptenoic acid. Yield 140 mg (65% based on Ru). Rf = 0.5 (THF/hexanes 1:1). Anal. Calc. for C56H66N6O10ClRu2 (1b1H2O1THF): C, 55.10; H, 5.45; N, 6.88. Found: C, 55.73; H, 5.19; N, 6.77%. Vis–NIR, (nm, e (M1 cm1)): 528(5300); leff = 3.93 lB; HR-nESI-MS: m/z 1096.245 (calc. 1096.227), corresponding to [1b-Cl]+; IR (cm1): C@C, 1598(m); Cyclic voltammogram [E1/2/V, DEp/V, ibackward/iforward]: A, 0.73, 0.030, 0.85; Epc (B), 0.57; D, 1.40, 0.039, 0.81. 4.4. Ru2(DmAniF)3(l-O2C(CH2)8CH@CH2)Cl (1c) Ru2(DmAniF)3(OAc)Cl (270 mg, 0.26 mmol) and 10-undecylenic acid (0.50 ml, 2.6 mmol) were dissolved in 50 mL of THF and refluxed in air overnight. Upon completion, the solvent volume was reduced and purification was carried out by recrystallization from

147

THF/n-pentane (1:20, v/v). The precipitate was collected by filtration and washed with ethanol to yield a purple crystalline material (250 mg, 81% based on Ru). Rf = 0.7 (THF/hexanes, 1:1). Anal. Calc. for C56H64N6O8ClRu2 (1c): C, 56.67; H, 5.44; N, 7.08. Found: C, 56.38; H, 5.53; N, 7.04%. Vis–NIR, (nm, e (M1 cm1)): 530 (6000); leff = 3.95 lB; HR-nESI-MS: m/z 1152.300 (calc. 1152.290), corresponding to [1c-Cl]+; IR (cm1): C@C, 1598(m); Cyclic voltammogram [E1/2/V, DEp/V, ibackward/iforward]: A, 0.74, 0.030, 0.94; Epc(B), 0.55; D, 1.39, 0.035, 0.94. 4.5. cis-Ru2(DmAniF)2(l-O2C(CH2)3CH@CH2)2Cl (2a) A 200 mL Schlenk flask was charged with cis-Ru2(DmAniF)2(OAc)2Cl (200 mg, 0.19 mmol), 5-hexenoic acid (0.22 mL, 1.9 mmol) and toluene (50 mL). The flask was mounted with a MicroSoxhlet extractor containing a thimble filled with K2CO3 and the mixture was refluxed under N2 for 2 days. The reaction progress was monitored by TLC, which ultimately resulted in 2a as the major product with a trace amount of 1a. Upon the removal of toluene under reduced pressure, the residue was purified on a silica column (EtOAc/ hexanes (linear gradient 1:4 to 2:1, v/v) to yield a dark green sticky material (96.4 mg, 52% based on Ru), which was recrystallized from THF/pentane (1:10, v/v) at room temperature. Rf = 0.50 (CH2Cl2/acetone 2:1). Anal. Calc. for C46.5H58.5N4O8ClRu2 (2a0.75hexane): C, 53.75; H, 5.67; N, 5.39. Found: C, 53.16; H, 5.85; N, 5.44%. Vis–NIR, kmax(nm, e (M1 cm1)): 568 (4000), 474 (3600), leff = 3.91 lB, HR-nESI-MS: 940.164 (calc. 940.158), corresponding to [2a-Cl]+. IR (cm1): C@C, 1598(m); Cyclic voltammogram: Epa(A), 0.86; Epc(B), 0.54; Epc(D), 1.58. 4.6. cis-Ru2(DmAniF)2(l-O2C(CH2)8CH@CH2)2Cl (2c) A 200 mL Schlenk flask was charged with cis-Ru2(DmAniF)2 (OAc)2Cl (380 mg, 0.44 mmol), 10-undecylenic acid (0.8 ml, 4.4 mmol) and toluene (80 mL). The flask was mounted with a MicroSoxhlet extractor containing a thimble filled with K2CO3 and the mixture was refluxed under N2 for 5 days. The reaction progress was monitored by TLC (EtOAc/hexanes, 1:1, v/v) which revealed the appearance of the product 2c and a trace amount of 1c. Upon the removal of toluene under reduced pressure, the residue was purified on a silica column (EtOAc/hexanes (linear gradient 1:3 to 2:1, v/v) to yield a dark green sticky material (220 mg, 44% based on Ru), which was recrystallized from THF/pentane (1:10, v/v) at room temperature. Rf = 0.45 (EtOAc/Hex 1:1). Anal. Calc. for C77.5H123.5N4O11ClRu2 (2c2.25hexane3THF): C, 61.04; H, 8.16; N, 3.67. Found: C, 61.84; H, 7.80; N, 3.68%. Vis–NIR, kmax (nm, e (M1 cm1)): 567 (3900), 472 (3500), leff = 3.96 lB, HRnESI-MS: m/z Z080.330 (calc. 1080.315), corresponding to [2cCl]+; IR (cm1): C@C, 1598(m); Cyclic voltammogram [E1/2/V, DEp/V, ibackward/iforward]: Epa(A), 0.86; Epc(B), 0.58; Epc(D), 1.80. 4.7. Preparation of cis-Ru2(DmAniF)2(l-O2C(CH2)3CH@C)2Cl (3a) To 50 mL CH2Cl2 was added 140 mg of 2a (0.144 mmol) and 5 mg of (Cy3P)2Cl2Ru(@C CHPh), and the mixture was refluxed for 5 days under N2. The reaction progress was monitored by TLC (CH2Cl2/acetone, 2:1, v/v), and two new products were detected. The separation of the two compounds was achieved by column chromatography (CH2Cl2/acetone, 9:1 v/v) and resulted in the isolation of two dark green materials (15 mg and 110 mg, 92% total, based on Ru). These two species appeared to be a mixture of E/Z isomers as was found from the analysis of its mass spectrometer data. Rf = 0.35 and 0.45 (THF/Hex 1:1). Anal. Calc. for C64H94N4O15ClRu2 (3a6THF1H2O): C, 55.02; H, 6.78; N, 4.01. Found: C, 55.42; H, 6.49; N, 3.84%. Vis–NIR, kmax (nm, e (M1 cm1)): 601 (5500), 453 (5100); IR (cm1): C@C, 1598(m); HR-nESI-MS: m/z 912.110

148

J. Savchenko et al. / Inorganica Chimica Acta 396 (2013) 144–148

(calc. 912.126), corresponding to [3a-Cl]+; Cyclic voltammogram [E1/2/V, DEp/V, ibackward/iforward]: Epa(A), 0.80; Epc(B), 0.58; Epc(D), 1.64. 4.8. Preparation of cis-Ru2(DmAniF)2(l-O2C(CH2)8CH@)2Cl (3c) To 50 mL CH2Cl2 were added 220 mg of 2c (0.20 mmol) and 5 mg of (Cy3P)2Cl2Ru(@CHPh), and the resultant mixture was refluxed under N2 for 2 days. The reaction progress was monitored by TLC (CH2Cl2/acetone, 2:1, v/v), and the product 3c was detected (205 mg, 94% based on Ru). Rf = 0.40 (CH2Cl2/acetone 2:1). Anal. Calc. for C50H68N4O10ClRu2 (3c2H2O): C, 53.49; H, 6.10; N, 4.99. Found: C, 52.78; H, 5.79; N, 4.86%. Vis–NIR, kmax (nm, e (M1 cm1)): 565 (5900), 472 (5200); IR (cm1): C@C, 1598 (m); leff = 3.94 lB; HR-nESI-MS: m/z 1052.275 (calc. 1052.284), corresponding to [3c-Cl]+; Cyclic voltammogram [E1/2/V, DEp/V, ibackward/ iforward]: Epa(A), 0.85; Epc(B), 0.60; Epc(D), 1.68. 4.9. X-ray diffraction study of compounds 1c and 2c Single crystals of compounds 1c and 2c were obtained by slow diffusion of pentane into a saturated THF solution. X-ray diffraction data of 1c was collected on a Bruker diffractometer with an Apex 2 detector using Mo Ka radiation (k = 0.71073 Å) at 10 K (liquid helium cooler by Cryo Industries of America). Standard datareduction methods were applied, including an empirical absorption correction (SADABS). The structure was solved by direct methods (SHELXS) and refined via cycled least-squares and difference map techniques (SHELX programs) [39]. The structure refinement of 1c was complicated by indications of stacking faults. Shadows of the heavy atoms (about 5%) are seen at a displacement of 0.85 Å from the main atoms. These shadows were included in the structure factor calculations. Shadows of the remaining atoms were ignored. The hydrocarbon chains are disordered, with a main component well defined, and a minor component less well defined. Several of the methoxy groups in the DmAniF ligands are also disordered over two positions. The pentane molecules are situated around inversion centers, leading to solvent disorder. Crystal data for  a = 13.0162(6), b = 1c: C56H64N6O8Ru2Cl, M = 1186.8, triclinic, P 1, 16.8728(7), c = 27.4565(12) Å, a = 88.7030(10)°, b = 82.6050(10)°, c = 67.7390(10)°, V = 5531.9(4) Å3, Z = 4, Dcalc = 1.347 g cm3, R1 = 0.0838, wR2 = 0.2501. X-ray diffraction data of 2c was collected on a Rigaku Rapid II image plate diffractometer using Cu Ka radiation (k = 1.54184 Å) at 150 K, and the structures were solved using the structure solution program PATTY in DIRDIF99, and refined using SHELX-07 [39]. Crystal data for 2c: C52H68N4O8Ru2Cl, M = 1114.7, monoclinic, C2/ c, a = 35.6160(16), b = 14.1010(6), c = 28.6790(15) Å, a = 90.0(3), b = 127.61(3), c = 90.0°, V = 11409.97(9) Å3, Z = 8, Dcalc = 1.316 g cm3, R1 = 0.0720, wR2 = 0.2330. These data, CCDC 893688 (1c) and CCDC 893687 (2c), can be obtained from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. Acknowledgments We thank both Purdue University and the USAF Asian Office of Aerospace Research & Development (Grant FA2386-12-1-4006) for

support. T. Ren acknowledges the IR/D support from the US National Science Foundation during the preparation of this manuscript. References [1] R.H. Grubbs, Angew. Chem., Int. Ed. 45 (2006) 3760. [2] R.H. Grubbs (Ed.), Handbook of Metathesis, vol. vols. 1–3, Wiley-VCH, Weinheim, Germany, 2003. [3] J. Han, C. Deng, R. Fang, D.Y. Zhao, L.Y. Wang, J.A. Gladysz, Organometallics 29 (2010) 3231. [4] T. Shima, E.B. Bauer, F. Hampel, J.A. Gladysz, Dalton Trans. (2004) 1012. [5] E.B. Bauer, J. Ruwwe, J.M. Martin-Alvarez, T.B. Peters, J.C. Bohling, F.A. Hampel, S. Szafert, T. Lis, J.A. Gladysz, Chem. Commun. (2000) 2261. [6] E.B. Bauer, F. Hampel, J.A. Gladysz, Organometallics 22 (2003) 5567. [7] E.B. Bauer, J.A. Gladysz (Eds.), Handbook of Metathesis, vol. 2, Wiley-VCH, Weinheim, Germany, 2003, pp 403–431. [8] R.M. Haak, A.M. Castilla, M.M. Belmonte, E.C. Escudero-Adan, J. BenetBuchholz, A.W. Kleij, Dalton Trans. 40 (2011) 3352. [9] W. Buchowicz, M. Szmajda, Organometallics 28 (2009) 6838. [10] M. Ogasawara, S. Watanabe, K. Nakajima, T. Takahashi, Organometallics 27 (2008) 6565. [11] M. Ogasawara, W.-Y. Wu, S. Arae, S. Watanabe, T. Morita, T. Takahashi, K. Kamikawa, Angew. Chem., Int. Ed. 51 (2012) 2951. [12] M. Ogasawara, W.Y. Wu, S. Arae, T. Morita, S. Watanabe, T. Takahashi, K. Kamikawa, J. Organomet. Chem. 696 (2011) 3987. [13] T. Ren, Chem. Rev. 108 (2008) 4185. [14] W.Z. Chen, J.D. Protasiewicz, A.J. Shirar, T. Ren, Eur. J. Inorg. Chem. (2006) 4737. [15] W.Z. Chen, J.D. Protasiewicz, S.A. Davis, J.B. Updegraff, L.Q. Ma, P.E. Fanwick, T. Ren, Inorg. Chem. 46 (2007) 3775. [16] G.-L. Xu, T. Ren, Organometallics 24 (2005) 2564. [17] G.-L. Xu, T. Ren, Inorg. Chem. 45 (2006) 10449. [18] W.Z. Chen, T. Ren, Inorg. Chem. 45 (2006) 8156. [19] S.P. Cummings, J. Savchenko, T. Ren, Coord. Chem. Rev. 255 (2011) 1587. [20] F.A. Cotton, C. Lin, C.A. Murillo, Acc. Chem. Res. 34 (2001) 759. [21] F.A. Cotton, C.A. Murillo, R.A. Walton (Eds.), Multiple Bonds between Metal Atoms, Third ed., Springer Science and Business Media, Inc., New York, 2005. [22] P. Angaridis, J.F. Berry, F.A. Cotton, C.A. Murillo, X. Wang, J. Am. Chem. Soc. 125 (2003) 10327. [23] P. Angaridis, J.F. Berry, F.A. Cotton, P. Lei, C. Lin, C.A. Murillo, D. Villagrán, Inorg. Chem. Commun. 7 (2004) 9. [24] G.M. Chiarella, F.A. Cotton, C.A. Murillo, M.D. Young, Q. Zhao, Inorg. Chem. 49 (2010) 3051. [25] M.C. Barral, T. Gallo, S. Herrero, R. Jiménez-Aparicio, M.R. Torres, F.A. Urbanos, Inorg. Chem. 45 (2006) 3639. [26] M.C. Barral, S. Herrero, R. Jiménez-Aparicio, M.R. Torres, F.A. Urbanos, Angew. Chem., Int. Ed. 44 (2005) 305. [27] M.C. Barral, T. Gallo, S. Herrero, R. Jiménez-Aparicio, M.R. Torres, F.A. Urbanos, Chem. Eur. J. 13 (2007) 10088. [28] M.C. Barral, S. Herrero, R. Jiménez-Aparicio, M.R. Torres, F.A. Urbanos, J. Organomet. Chem. 693 (2008) 1597. [29] M.C. Barral, D. Casanova, S. Herrero, R. Jiménez-Aparicio, M.R. Torres, F.A. Urbanos, Chem. Eur. J. 16 (2010) 6203. [30] H. Agarwala, T.M. Scherer, S. Maji, T.K. Mondal, S.M. Mobin, J. Fiedler, F.A. Urbanos, R. Jimenez-Aparicio, W. Kaim, G.K. Lahiri, Chem. Eur. J. 18 (2012) 5667. [31] T. Ren, V. DeSilva, G. Zou, C. Lin, L.M. Daniels, C.F. Campana, J.C. Alvarez, Inorg. Chem. Commun. 2 (1999) 301. [32] W.-Z. Chen, T. Ren, Organometallics 23 (2004) 3766. [33] W.Z. Chen, T. Ren, Organometallics 24 (2005) 2660. [34] G. Zou, J.C. Alvarez, T. Ren, J. Organomet. Chem. 596 (2000) 152. [35] F.A. Cotton, A. Yokochi, Inorg. Chem. 36 (1997) 567. [36] Y. Fan, P.E. Fanwick, T. Ren, Polyhedron 28 (2009) 3654. [37] M.C. Barral, S. Herrero, R. Jiménez-Aparicio, M.R. Torres, F.A. Urbanos, Inorg. Chem. Commun. 7 (2004) 42. [38] T. Ren, C. R. Chim. 11 (2008) 684. [39] G.M. Sheldrick, Acta Crystallogr., Sect. A 64 (2008) 112.