Article pubs.acs.org/Organometallics
Syntheses, Characterization, and Reactivity of Diruthenium Hydrido Complexes Heather R. Wiltse,† Alyssa N. Johnson,† Raphael J. Durand,§ William Brennessel,‡ and Robert M. Chin*,† †
Department of Chemistry and Biochemistry, University of Northern Iowa, Cedar Falls, Iowa 50614-0423, United States Department of Chemistry, University of Rochester, Rochester, New York 14627, United States § Sciences Chimiques de Rennes, Ecole Nationale Supérieure de Chimie de Rennes, 35708 Rennes Cedex 7, France ‡
S Supporting Information *
ABSTRACT: The reaction of [cis-{(η 5 -C 5H 3 ) 2 (CMe 2) 2}Ru 2 (κ2-4,4′-di-tert-butyl-2,2′-bipyridine)2(MeCN)2][OTf]2, 1, with H2 yields a μ-dihydrido complex, [cis-{(η 5 -C 5 H3 ) 2(CMe 2) 2 }Ru 2 (κ2-4,4′-di-tert-butyl-2,2′-bipyridine)2(μ-H)2][OTf]2, 2, in 79% yield. The reaction of 2 with Et3SiH affords a mono-μ-hydrido complex, cis-{(η5-C5H3)2(CMe2)2}Ru2(κ2-4,4′-di-tert-butyl-2,2′bipyridine)2(μ-H)][OTf], 3. The reaction of either 2 or 3 (1−2 mol %) with C6H6 and Et3SiH results in the catalytic cleavage of the C−H bond in benzene along with the cleavage of the Si−Et bond to form PhEt2SiH (23−27% conversion) and C2H6. The reaction of 2 or 3 (1 mol %) with THF and Et3SiH results in the cleavage of the C−H bond in THF followed by the insertion of the SiEt2 group into the C−O bond, forming 2,2-diethyl-1-oxa-2-silacyclohexane (14% conversion) as one of the products.
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in place.7 Herein, we report the preparation, characterization, and reactivity of a series of diruthenium complexes that cleave both Si−C and C−H bonds to form new Si−C bonds.
INTRODUCTION Over the past two decades there have been tremendous advances in the activation and functionalization of C−H bonds in arenes and alkanes.1 In particular, the replacement of the hydrogen atom in a C−H bond with a heteroatom such as B or Si holds great promise since the resulting product could be further functionalized to provide more complex molecules.2 The use of R3SiH reagents, such as Et3SiH, usually results in the cleavage of the Si−H bond, with the formation of tetraorganosilane and dihydrogen as the products (eq 1).3 R′−H + R3Si−H →
R3Si−R′ tetraorganosilane
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RESULTS AND DISCUSSION Synthesis and Characterization of [cis-{(η 5 C5H3)2(CMe2)2}Ru2(κ2-4,4′-di-tert-butyl-2,2′-bipyridine)2(MeCN)2][OTf]2 (1). We have previously reported that the photolysis of a diruthenium naphthalene complex leads to the unhinging of the naphthalene ligand8 and coordination of labile acetonitrile ligands to the ruthenium metal center. This transformation then allows for the easy substitution of the naphthalene ligand with other ligands. The synthesis of a diruthenium 2,2′-bipyridine complex, [cis-{(η5-C5H3)2(CMe2)2}Ru2(κ2-4,4′-di-tert-butyl-2,2′-bipyridine)2(MeCN)2][OTf]2 (1), was achieved in a two-step process, starting with the diruthenium naphthalene complex [cis-{(η5-C5H3)2(CMe2)2}Ru2(μ,η6,η6C10H8)][OTf]2. Photolysis of the naphthalene complex with visible light yielded cis-{(η5-C5H3)2(CMe2)2}Ru2(MeCN)3 (η6-C10H8) ][OTf]2, a complex where the bound acetonitrile and naphthalene ligands are easily displaced by 4,4′-di-tert-butyl2,2′-bipyridine ligands to form 1 in an 80% yield (Scheme 1). The 1H NMR spectrum of 1 in CD3CN has the characteristic doublet and triplet resonances for the dicyclopentadienyl ligand along with three resonances in the aromatic region for the bipyridine ligands. The resonance for the bound MeCN ligands is not observed since they are easily exchanged for the deuterated
+ H2 (1)
These reactions sometimes require a hydrogen acceptor such as tert-butylethylene or cyclohexene to help drive the reaction to completion.4 Less observed is the cleavage of the Si−C bond in the silane (eq 2) instead of the cleavage of the Si−H bond, which would give rise to the free alkane, R−H, after a C−H bond cleavage step. A triorganosilane would also form in the reaction instead of a tetraorganosilane. R′−H + R 2HSi−R → R 2HSi−R′ + R−H triorganosilane
(2)
There have been previously reported examples of Si−C bond cleavage in silanes, which includes work by Sabo-Etienne and co-workers.5,6 However, these reactions did contain a C−H bond activation component to form a free alkane (eq 2). Over the past three years we have been looking at the cooperative reactivity of two ruthenium metal centers next to each other using a dicyclopentadienyl ligand to hold the metals © XXXX American Chemical Society
Received: January 26, 2016
A
DOI: 10.1021/acs.organomet.6b00067 Organometallics XXXX, XXX, XXX−XXX
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Organometallics Scheme 1. Synthesis of 1
Figure 1. ORTEP of 1 (50% probability). Hydrogens, anions, and solvent molecules are omitted for clarity. Selected bond distances (Å) and angles (deg): Ru(1)···Ru(2), 4.9125(4), Ru(1)−Cp(centroid), 1.80, Ru(2)−Cp(centroid), 1.81, Ru(1)−N(5), 2.060(2), Ru(2)−N(6), 2.080(2), Ru(1)−N(5)−C(37), 168.6(2), Ru(2)−N(6)−C(39), 176.3(2).
solvent, CD3CN. The structure of 1 was also confirmed by an X-ray crystallographic study (Figure 1). The dicyclopentadienyl (diCp) ligand in 1 is folded and twisted to allow for the steric crowding between the two metal centers caused by the bound acetonitrile ligands. The overall “w” shape of the ligand (Figure 1) places the ruthenium atoms at a nonbonding distance of 4.9125(4) Å. The angle between the two Cp rings of 192.61(7)° is exaggerated by the slight twists to accommodate the acetonitrile ligands, as seen by the angles between the Ru(1)−Cp(centroid)−Cp(centroid)−Ru(2) plane and each Cp plane of 93.2° and 93.1°. The Ru(1)−Cp(centroid) and Ru(2)−Cp(centroid) distances are 1.80 and 1.81 Å, respectively. Reaction of 1 with H2 to Form [cis-{(η5-C5H3)2(CMe2)2}Ru2(κ2-4,4′-di-tert-butyl-2,2′-bipyridine)2(μ-H) 2][OTf]2 (2). The reaction of 1 with H2 (1 atm, rt, THF solvent, 5 min) is rapid, forming the μ-dihydrido oxidative addition product [cis-{(η 5 -C 5 H 3 ) 2 (CMe 2 ) 2 }Ru 2 (κ 2 -4,4′-di-tert-butyl-2,2′bipyridine)2(μ-H)2][OTf]2 (2), in a 79% yield (eq 3). The 1H NMR spectrum has a single hydride resonance at −5.51 ppm in addition to the dicyclopentadienyl and bipyridine resonances. The bridging nature of the hydrides was determined by X-ray crystallography using the difference Fourier map (Figure 2). The ruthenium dication in 2 lies on a crystallographic mirror plane that coincides with C(Me)2 groups of the dicyclopentadienyl ligand and the bridging hydride ligands. The Ru−Ru distance in 2 is 3.0183(4) Å, which is in the same range as other Ru−Ru single bonds (2.927−3.1107 Å) in complexes with a
Figure 2. ORTEP of 2 (50% probability). Hydrogens (except for the hydrides), anions, tert-butyl groups, and solvent molecules are omitted for clarity. Selected bond distances (Å) and angles (deg): Ru(1)···Ru(1A), 3.0183(4), Ru(1)−Cp(centroid), 1.83, Ru(1)−H(1), 1.71(3), Ru(1)−H(2), 1.79(3), H(1)−Ru(1)−H(2), 60(2), Cp−Cp fold angle, 139.50(7).
bridging hydride and a doubly linked Cp ligand.9 The assignment of a Ru−Ru bond helps explain the diamagnetic nature of the complex given that the formal oxidation state of each ruthenium is +3 with a d5 electron configuration. For the dicyclopentadienyl ligand, the angle between Cp planes is 139.45(7)°. The Ru−Cp(centroid) distance is 1.83 Å. 2 will quickly revert back to 1 in MeCN solvent (less than 5 min) along with the formation of free H2 gas. This is consistent with the observation that there is no reaction between 1 and H2 (1 atm) in CD3CN. However, 2 is stable in less coordinating solvents such as tetrahydrofuran (THF) and acetone with no observed decomposition of 2 in these less coordinating solvents (24 h, rt). The reaction of 1 with HD gas affords the HD variant of 2 with a triplet (1:1.3:1) at δ −5.48 with an H−D coupling constant of 1.7 Hz. The small coupling constant confirms the classical hydride nature of 2 and would suggest that the B
DOI: 10.1021/acs.organomet.6b00067 Organometallics XXXX, XXX, XXX−XXX
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Organometallics bridging hydride assignment is the correct one rather than having terminal hydrides at each ruthenium center.10 Reaction of 2 with Et 3 SiH to Form [cis-{(η 5 C5H3)2(CMe2)2}Ru2(κ2-4,4′-di-tert-butyl-2,2′-bipyridine)2(μ-H)][OTf] (3). Heating a mixture of 2 in C6H6 and Et3SiH (120 °C, 20 h) results in the formation of black-purple crystals of [cis-{(η 5 -C 5 H 3 ) 2 (CMe 2 ) 2 }Ru 2 (κ 2 -4,4′-di-tert-butyl-2,2′bipyridine)2(μ-H)][OTf] (3), a compound with one less hydride ligand and triflate anion in the overall structure (eq 4).
The hydride resonance in 3 is observed at −16.18 ppm in the H NMR spectrum, along with the normal doublet and triplet resonances for the dicyclopentadienyl rings and two resonances (“up” and “down”) for the linking C(CH3)2 groups. The symmetrical 1H NMR spectrum suggests that either the bridging hydride sits in the bisecting Cp(centroid)−Ru−Ru−Cp(centroid) plane or the bridging hydride is fluxional in solution, rapidly moving from side to side of the bisecting plane. The structure of 3 was confirmed by X-ray diffraction shown in Figure 3. As in 2, the bridging hydride ligand in 3 was located using the difference Fourier map and refined freely. Interestingly, the hydride ligand sits off to one side of the plane that bisects the bipyridine ligands (Figure 3) with an angle between the Ru(1)−Cp(centroid)−Cp(centroid)−Ru(2) and Ru(1)−H(1)−Ru(2) planes of 27.5°. Therefore, the bridging hydride must be moving from side to side of the bisecting plane in solution, given the asymmetry of the solidstate structure and the symmetric nature of the 1H NMR spectrum. Angelici and co-workers have observed this same type of fluxional behavior in their bridging hydride in the previously reported [cis-{(η 5 -C 5 H 3 ) 2 (SiMe 2 ) 2 }Ru 2 (CO) 2 (μ-H)][OTf] complex.11 The Ru···Ru distance in 3 is 3.163 Å, which is at the long end for a metal−metal bond. This distance is similar to the Ru···Ru distance (3.103 Å) in the [cis-{(η5-C5H3)2(SiMe2)2}Ru2(CO)2(μ-H)][OTf] complex reported by Angelici. We have shown the structure of 3 without a formal Ru−Ru bond as well, since a metal−metal bond is not necessary for each metal to achieve an 18-electron configuration, but we do not discount the possibility of some interaction between the two ruthenium metal centers. Angelici and co-workers do not assign a Ru−Ru bond either in their [cis-{(η5-C5H3)2(SiMe2)2}Ru2(CO)2(μ-H)][OTf] complex. The dicyclopentadienyl ligand essentially has a single fold at the C(Me)2 junction of 144.6(2)°, and the angle between Cp planes is 145.0(2)°. The Ru(1)−Cp(centroid) and Ru(2)−Cp(centroid) distances are both 1.80 Å. Et3SiH is necessary in order for transformation of 2 into 3 to occur. Heating a mixture of just 2 in C6D6 (125 °C, 20 h) without any Et3SiH results in the formation of the bis(benzene) complex7 [cis-{(η5-C5H3)2(CMe2)2}Ru2(η6-C6H6)2][OTf]2 and free 4,4′-di-tert-butyl-2,2′-bipyridine, with no 3 being observed in the 1H NMR spectrum. The amount of bis(benzene) 1
Figure 3. ORTEP of 3 (50% probability). Hydrogens (except for the bridging hydride), anions, tert-butyl, and solvent molecules are omitted for clarity. Selected bond distances (Å) and angles (deg): Ru(1)···Ru(2), 3.1626(5), Ru−Cp(centroid) both, 1.80, Ru(1)−H(1), 1.77(5), Ru(2)−H(1), 1.71(5), Ru(1)−H(1)−Ru(2), 131(3), Cp−Cp fold angle, 145.0(2).
complex that formed in the control experiment was not quantified. The reaction of 2 with Et3SiH in C6D6 also results in the formation of free H2 (detected in the 1H NMR spectrum, not quantified) and Et3SiOTf (identified by comparing the 1 H NMR spectrum of the reaction with the 1H NMR spectrum of commercially available Et3SiOTf). Reaction of 2 or 3 with C6H6 and Et3SiH. In the reaction of 2, C6H6, and Et3SiH to form 3, we also noticed new aromatic resonances in the 1H NMR spectrum. These new resonances suggested that the C−H bond activation of benzene had occurred. However, the GC/MS spectrum of the reaction mixture did not have any peaks that matched PhEt3Si as a possible product. We postulated that the organic product was PhEt2SiH, where one of the ethyl groups in Et3SiH was substituted for a phenyl group since the organic product had an m/z of 164. Further investigation showed that the reaction of either 2 or 3 in C6H6 with Et3SiH (180 °C, 20 h) resulted in the catalytic formation of PhEt2SiH, Ph2EtSiH, and C2H6 (eq 5). It is particularly interesting to note that the Si−C bond from the Si−Et group is cleaved instead of the usual Si−H bond.3b,e,d Free C2H6 is one of the other products that results from the cleavage of the Si−Et bond. The presence of C2H6 was confirmed by both 1H NMR spectroscopy and GC/MS data, but the amount of C2H6 produced was not quantified. C
DOI: 10.1021/acs.organomet.6b00067 Organometallics XXXX, XXX, XXX−XXX
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Organometallics
THF and C6H6 since, at the time, we were unsure of how soluble a cationic complex like 2 would be in pure C6H6. As it turned out, 3 is slightly soluble in C6H6, which negated the need for THF as a cosolvent in the reaction between 2 or 3, C6H6, and Et3SiH. However, we took a closer look at the reaction of 2 or 3 with THF and Et3SiH since the GC/MS data from our earlier reactions indicated that THF was also undergoing C−H bond activation and silylation. The reaction of 2 or 3 with THF and Et3SiH results in many more products than the analogous C6H6 reaction. In the GC/MS trace, there are about 11 minor products and four major products along with the ever present Et3SiOSiEt3. We have only been able to identify one of the products in the reaction mixture. In the GC/MS data, there was a peak with m/z of 158, which would indicate the linking of Et2SiH with THF, either at the 2- or 3-position (Chart 1).
The formation of PhEt2SiH and Ph2EtSiH was confirmed by comparing the spectroscopic data (1H NMR spectra and GC/MS) from the reaction mixture with the spectral data from the independent syntheses of PhEt2SiH and Ph2EtSiH (see Experimental Section). The GC/MS data did not show the formation of any PhEt3Si in the reaction. The use of C6D6 confirmed that the source of the phenyl group in PhEt2SiH was from the benzene solvent. The reaction of Et2SiH2 and C6H6 with 2 did not produce any of the same products as the reaction of Et3SiH and C6H6 with 2. This ruled out the possibility of trace amounts of Et2SiH2 in the Et3SiH reagent being responsible for the formation of PhEt2SiH and Ph2EtSiH. Another point to note is that the preference for Si−C bond cleavage over Si−H bond cleavage continued in the formation of Ph2EtSiH, where a second phenyl group is added to the silane reagent. At this point, we are unsure as to why the Si−Et bond is cleaved over the Si−H bond but postulate that a σ-Si−H complex maybe responsible for the selectivity.12,13 We are currently working on delineating the mechanism for the various catalytic steps. We also detect trace amounts (
