Synthesis and Properties of a Triruthenium Hydrido Complex Capped

Article Cite This: Organometallics XXXX, XXX, XXX−XXX

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Synthesis and Properties of a Triruthenium Hydrido Complex Capped by a μ3‑Oxoboryl Ligand Takeshi Kaneko, Hayato Ninagawa, Moe Matsuoka, and Toshiro Takao* Department of Chemical Science and Engineering, School of Materials and Chemical Technology, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8552, Japan

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S Supporting Information *

ABSTRACT: A triruthenium hydrido complex in which one of the Ru3 planes is capped by a μ3-BO ligand, [{Cp*Ru(μH)}3(μ3-BO)(μ3-H)] (3; Cp* = η5-C5Me5), was synthesized by the reaction of the μ3-borylene complex [{Cp*Ru(μH)}3(μ3-BH)] (2a) with water in the presence of Et2NH. The face-capping coordination of the oxoboryl ligand was unambiguously established by X-ray diffraction and exhibited short B−O (1.231(6) Å) and long Ru−B bonds (average 2.31 Å). Density functional theory (DFT) calculations of 3 reproduced the observed structure well, and the multiplicity of the BO bond suggested by the value of the Wiberg bond index is 1.62. The four hydrido ligands in 3, three μ-hydrides and one μ3-hydride, underwent site exchange on the NMR time scale via the formation of a μ3-hydroxyborylene intermediate, [{Cp*Ru(μ-H)}3(μ3-BOH)] (2c). The DFT calculations showed that 2c lies above 3 by 10.5 kcal mol−1 at 25 °C. The basic oxygen atom in 3 allowed the formation of a B(C6F5)3 adduct, [{Cp*Ru(μ-H)}3{μ3-BO···B(C6F5)3}(μ3-H)] (5), in which the site exchange of hydrides was retarded considerably. Complex 3 reacted with PMe3 and CO to afford [(Cp*Ru)3(μ-BO)(μ-H)3(μ3-H)(PMe3)] (6) and [{Cp*Ru(CO)}3(μ-BO)(μ-H)2] (7), respectively, which demonstrated that the coordination mode of the BO ligand changed from a face-capping to an edge-bridging mode at the Ru3 site.

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INTRODUCTION Carbon monoxide is widely used as a supporting ligand or a reactant in organometallic chemistry. An extensive understanding of the coordination chemistry of CO analogues, such as NO+ and CN−, has also been developed, and it has been demonstrated that substituting CO by these diatomic ligands allows the fine adjustment of the electronic environment of a transition-metal center with less steric impact. In this regard, novel diatomic ligands that are isoelectronic with CO are of great importance and have attracted considerable attention.1 However, because of the instability of the free ligands, the coordination chemistry of CO alternatives is limited. Baerends and co-workers examined the metal-binding capabilities of boron-containing CO alternatives (BE, where E = F, NR2, O−) theoretically and demonstrated that both steric protection to stabilize the highly polar B−E bond and efficient π backdonation to balance the strong σ donation by a BE ligand are important for stable complexation.2,3 They also noted that binuclear complexes could be good candidates to stabilize a BE ligand because the higher lying π*(M−M) orbital acts as an excellent π donor to the π*(BE) orbital. In particular, dinuclear complexes containing a bridging aminoborylene and fluoroborylene ligand have been synthesized by Braunschweig’s4 and Aldridge’s groups,5 respectively. Among these BE ligands, theoretical studies have also suggested that BO− substantially differs from isolobal BF and © XXXX American Chemical Society

BNR2 because of its negative charge, which shifts the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energies upward significantly. Thus, BO− acts as an extraordinarily strong σ donor and a very poor π acceptor. In 2010, Braunschweig and co-workers reported the first synthesis of a terminal oxoboryl complex, [Pt(BO)Br(PCy3)2], in which the terminal coordination of BO is effectively stabilized by both bulky phosphine groups and a π-donating Br atom at the trans position.6 The importance of the Br ligand is exemplified by the immediate cyclodimerization to [{Pt(PCy3)2}2(μ-B2O2)]2+ upon abstraction of the Br ligand.7 Braunschweig and co-workers also identified a large trans influence of the σ-donating BO group in [Pt(BO)Br(PCy3)2] and the high basicity of the oxygen atom in the BO ligand, which can form an adduct with a Lewis acid.8 We previously reported the synthesis of a triruthenium alkyne complex capped by a triply bridging oxoboryl ligand, [(Cp*Ru)3(μ3-η2:η2(⊥)-PhCCH)(μ3-BO)(μ-H)2] (1; Cp* = η5-C5Me5), by reaction of [(Cp*Ru)3(μ3-η2:η2(⊥)-PhCCH)(μ3-BH)(μ-H)] with water.9 The μ3-BO group seemed to be stabilized effectively by the surrounding bulky Cp* groups. However, the influence of the μ3-BO group on the multimetallic site could not be verified, owing to the presence of an Received: March 18, 2019

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DOI: 10.1021/acs.organomet.9b00182 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

philicity, which leads to facile ethanolysis of the B−Cl bond.16,17 Complex 2a also seems to react with amine to form an adduct, which would limit the number of H2O molecules that can react. Thus, we examined the reactions in the presence of various amines (6 equiv to 2a). In all cases, the reactions were accelerated by the addition of an amine (Table 1, entries 4−8) and remarkable accelerations were achieved by the addition of the less bulky amines EtNH2 and Et2NH. This is likely due to the rearrangement of the borylene ligand from the μ3 to the μ fashion, which facilitates the access of H2O molecules. In particular, the selectivity was also improved by the addition of Et2NH, which gave 3 in 86% yield. Although the reaction was also accelerated, the selectivity of 3 remained at 73% on the addition of EtNH2. This implies that access of multiple H2O molecules was not suppressed by the formation of an EtNH2 adduct. On the other hand, the selectivity of 3 was also improved by the addition of larger secondary amines, (i-Pr)2NH and (n-Pr)2NH, but the conversion of 2a did not increase as for the reaction with Et2NH (Table 1, entries 7 and 8). The observed sensitivity to the amine steric bulk seems to arise from the location of the μ3-BH ligand, which is surrounded by three Cp* groups. When NaOH was added to the reaction mixture, neither the conversion nor the selectivity was improved (Table 1, entry 9). This indicates that the hydrolysis of the B−H bond is promoted only by the formation of the amine adduct. A plausible mechanism for the hydrolysis of the μ3-BH ligand is shown in Scheme 1. The H2O molecule coordinated to the μ3-

alkyne ligand on the opposite face of the Ru3 plane. Thus, we have attempted to synthesize a triruthenium μ3-BO complex without an alkyne ligand to evaluate the electronic influence of the μ3-BO group on the reactivity of a Ru3 cluster. Herein, we report herein the synthesis and properties of a novel triruthenium hydrido complex capped by a μ3-BO ligand.

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RESULTS AND DISCUSSION Synthesis of μ3-Oxoboryl Tetrahydrido Complex 3. We found that the hydrogen atom at the μ3-BH ligand in [{Cp*Ru(μ-H)}3(μ3-BH)] (2a) is replaced by an alkoxy group upon treatment with alcohols such as methanol and ethanol.10,11 In addition, we synthesized a μ3-BO ligand by the reaction of [(Cp*Ru)3(μ3-η2:η2(⊥)-PhCCH)(μ3-BH)(μH)] with water.9 According to these reactions, we examined the reaction of 2a with water. The desired μ3-oxoboryl complex [{Cp*Ru(μ-H)}3(μ3-BO)(μ3-H)] (3) was formed by the reaction, but the reaction proceeded sluggishly at 25 °C (Table 1, entry 1), unlike the reaction with ethanol, which Table 1. Results of Hydrolysis of {Cp*Ru(μ-H)}3(μ3-BH) (2a)

Scheme 1. Plausible Mechanism for Hydrolysis of the μ3-BH Ligand in 2a

yield (%) a

entry

temp (°C)

additive

1 2 3 4b 5b 6b 7b 8b 9c

25 50 80 25 25 25 25 25 25

none none none EtNH2 Et2NH Et3N (i-Pr)2NH (n-Pr)2NH NaOH

conversn of 2a (%)

3

4

20 63 99 97 97 33 55 82 7

10 31 0 73 86 28 50 73 4

10 32 99 24 11 5 5 9 1

The reactions were carried out using 2a (6.9 μmol) and H2O (20 μL, 1.1 equiv) in THF (0.5 mL) for 48 h. The conversion of 2a and yields of 3 and 4 were determined by 1H NMR spectroscopy. bThe reactions were carried out with amine (38 μmol, 5 equiv to 2a). cThe reaction was carried out using 2a (25.4 μmol) and 0.1 M NaOH(aq) (1.4 mL, 6 equiv) in THF (14 mL) for 48 h. In addition to 3 and 4, formation of a small amount of paramagnetic species was observed. a

BH ligand is deprotonated readily by the neighboring amine, as shown in B. Subsequent dehydrogenation from C is also promoted by R3NH+, which yields the μ3-hydroxyborylene intermediate 2c. Complex 3 is formed by a proton transfer from the hydroxy group in 2c to the Ru3 center. The equilibrium between 2c and 3 will be discussed later. Using Et2NH, complex 3 was obtained in ca. 90% yield. However, the formation of 4 could not be suppressed completely by modifying the reaction period and the amounts of water and Et2NH. Complex 3 decomposed on alumina, unlike 1, and the solubility of 3 to common organic solvents is fairly similar to that of 4. Therefore, complex 3 could not be

yielded [{Cp*Ru(μ-H)}3(μ3-BOEt)] (2b) quantitatively. In addition, a considerable amount of [{Cp*Ru(μ-H)}3(μ3-H)2] (4) was formed by the reaction simultaneously, in which the borylene moiety was eliminated from the Ru3 cluster as B(OH)3. Although the conversion of 2a increased upon heating at 50 °C, the selectivity for 3 was not improved and only 4 was obtained at 80 °C (Table 1, entries 2 and 3). Terminal borylene ligands are known to be stabilized by the coordination of nucleophiles,12−15 and it has also been shown that base-stabilized borylene complexes still show electroB


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Organometallics

and [Co2(μ-BO)2(CO)6(μ-CO)] (1.240 Å),20 but these values are still shorter than the calculated BO length in the free BO− molecule (1.247 Å).2 In contrast to alkyne complex 1, the μ3-BO ligand lies above the center of the Ru3 triangle; the three Ru−B distances are almost equal (Ru(1)−B(1) 2.302(5) Å, Ru(2)−B(1) 2.320(6) Å, and Ru(3)−B(1) 2.305(5) Å). These values are considerably larger than the sum of the covalent radii of the Ru and B atoms (2.10 Å).21 This is in contrast with the Pt−B lengths in [Pt(BO)(SPh)(PCy3)2] (1.983(3) Å)6 and [Pt(BO)(MeCN) (PCy3)2]− (1.971(5) Å),8 which are slightly smaller than the sum of the covalent radii of Pt and B (2.08 Å). The average Ru−B lengths observed in the known μ3borylene ligand on Ru3 clusters (average 2.12 Å) is close to the sum of the covalent radii.9,10,22−27 The averages of the Ru−Al and Ru−Ga lengths in congeneric [{Cp*Ru(μ-H)}3(μ3-AlEt)] (2.481 Å)28,29 and [{Cp*Ru(μ-H)}3(μ3-GaMe)] (2.475 Å) also correspond to the sum of the covalent radii, respectively (RuAl 2.51 Å, RuGa 2.49 Å).21 We previously reported triruthenium complexes capped by various one-electron donors M, [{Cp*Ru(μ-H)}3(μ3-H)(μ3M)] (M = Li, Mg(i-Pr), ZnEt).28 In these complexes, the Ru− M lengths are observed to be greater than the sum of the covalent radii by 0.13−0.23 Å. The μ3-borylene ligands act as two-electron donors, whereas the μ3-BO ligand donates only one electron to the Ru3 core. Thus, the difference in the number of the donating electrons to the Ru3 core could be the reason for the Ru−B bond elongation in 3. Terminal oxoboryl ligands have been shown to have extremely strong σ-donating abilities. Sakaki and co-workers found that substantial charge transfer occurs from the lone pair of BO− to the dσ orbital of the [Pt(PMe3)2Br]+ fragment in the model complex trans-[Pt(PMe3)2(BO)Br].30 In contrast, the contribution of π back-donation from the filled dπ orbitals to the π*(BO) orbitals is very small in the Pt−BO bond. If this behavior of the BO− ligand was also operative in 3, the electron density at the Ru3 core should increase. However, the HOMO of 3 was shown to be stabilized remarkably in comparison to those of 44-electron complexes 2a and 4. The cyclic voltammogram (CV) of 3 showed a quasireversible oxidation wave at a half-wave potential (E1/20/+) of −420 mV (vs ferrocene) (peak potential of oxidation, Ep,a0/+ = −366 mV) (Figures S39 and S40 in the Supporting Information). The E1/20/+ value shifted in the positive direction considerably in comparison to that of pentahydrido complex 4 (E1/20/+ = −731 mV, Ep,a0/+ = −673 mV). Although μ3borylene complex 2a displayed an irreversible oxidation wave, the Ep,a0/+ value (−694 mV) was more negative than that of 3 and close to that of 4. Notably, the E1/20/+ value of 3 is even more positive than that of the mono(μ-carbonyl) complex [(Cp*Ru)3(μ-H)2(μ3-H)(μ-CO)] (E1/20/+ = −578 mV).31 The CV of 3 displayed a quasi-reversible reduction wave at E1/20/− = −2386 mV, which also shifted in the positive direction in comparison to the E1/20/− value of 2a (−2591 mV). This indicates that the LUMO of 3 is also stabilized in comparison to that of 2a. The CV data showed that the Ru3 center of 3 becomes electron deficient owing to the presence of the μ3-BO ligand, unlike the case for mononuclear oxoboryl complexes. In comparison to the platinum complexes with a short Pt−B bond, donation from the lone pair of BO− seems to decrease in 3 owing to the long Ru−B bonds.

separated from 4. Thus, the obtained mixture was treated with 1,3-cyclohecadiene to convert 4 to a face-capping benzene complex, [{Cp*Ru(μ-H)}3(μ3-η2:η2:η2-C6H6)].18 Unlike 4, complex 3 did not react with 1,3-cyclohexadiene at 25 °C. Owing to the insolubility of the μ3-benzene complex to polar solvents, 3 could be separated from [{Cp*Ru(μ-H)}3(μ3η2:η2:η2-C6H6)] by extraction with acetonitrile. Analytically pure 3 was obtained by subsequent recrystallization. The formation of a μ3-oxoboryl ligand was confirmed by Xray diffraction (XRD), as shown in Figure 1. Although two

Figure 1. Molecular structure and labeling scheme of 3 with thermal ellipsoids at 30% probability. Selected bond lengths (Å) and angles (deg): Ru(1)−Ru(2) 2.7113(4), Ru(1)−Ru(3) 2.7170(4), Ru(2)− Ru(3) 2.7205(5), Ru(1)−B(1) 2.302(5), Ru(2)−B(1) 2.320(6), Ru(3)−B(1) 2.305(5), B(1)−O(1) 1.231(6); Ru(2)−Ru(1)−Ru(3) 60.155(11), Ru(1)−Ru(2)−Ru(3) 60.027(11), Ru(1)−Ru(3)− Ru(2) 59.818(11), Ru(1)−B(1)−Ru(2) 71.85(16), Ru(1)−B(1)− Ru(3) 72.29 (16), Ru(1)−B(1)−O(1) 137.5(4), Ru(2)−B(1)− Ru(3) 72.07(16), Ru(2)−B(1)−O(1) 136.5(4), Ru(3)−B(1)− O(1) 137.6(4).

independent molecules with similar structural parameters are present in the unit cell, only one molecule is depicted. The three Ru−Ru distances are nearly equal (Ru(1)−Ru(2) 2.7113(4) Å, Ru(1)−Ru(3) 2.7170(4) Å, and Ru(2)−Ru(3) 2.7205(5) Å). These values lie in the middle of the Ru−Ru bond lengths of [{Cp*Ru(μ-H)}3(μ3-H)2] (4; 2.754 Å)19 and [{Cp*Ru(μ-H)}3(μ3-BOEt)] (2b; 2.677 Å),10 which adopt the same 44-electron configuration as 3. One of the four hydrides caps the Ru3 plane from the face opposite to the μ3-BO ligand. The remaining three hydrides bridge each Ru−Ru bond. The μ-hydrides are located slightly above the Ru3 plane (0.07−0.26 Å), and the Cp* groups are inclined toward the μ3-BO ligand group by ca. 7° to avoid steric repulsion with the μ-hydrides. The B(1)−O(1) distance of 1.231(6) Å is similar to that of 1 (1.229(9) Å),9 and this value is significantly smaller than the B−O distance in μ3-ethoxyborylene complex 2b (1.374(13) Å).10 This strongly indicates multiple-bonding character between B(1) and O(1). On the other hand, this value is slightly larger than the B−O lengths in a terminal BO ligand (1.210(3) and 1.197(6) Å).6,8 This is likely due to the enhanced back-donation to the π*(BO) orbital from the Ru3 core. The density functional theory (DFT) calculations for [Fe(BO)(CO)4]− and [Fe2(μ-BO)(CO)8]− by Baerends and co-workers showed that the B−O distance extends from 1.234 to 1.240 Å upon bridging coordination.2 Similar elongation is also reported for the hypothetical [Co(BO)(CO)4] (1.231 Å) C


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Organometallics Figure S7 in the Supporting Information shows the optimized structure of 3, which agrees well with the observed structure. The optimized BO length (1.242 Å) closely matches the observed value (1.231(6) Å), and the Wiberg bond index (1.62) strongly suggests the multiplicity of the BO bond. The elongated Ru−B distance is also reproduced by the DFT calculations (2.320 Å). The NBO charges at the boron and oxygen atoms were estimated to be +0.968 and −0.882, respectively. These values are comparable to those of the B−H bond in 2a (B, +0.525; H, −0.075) (Figure S10a in the Supporting Information) and suggest the highly polarized nature of the BO bond. Because the HOMO and HOMO-1 have almost the same energy, these orbitals appear to be degenerate. Although the HOMO and HOMO-1 mainly consist of π*(Ru−Ru) and the lone pairs at the oxygen atoms, they also have partial π*(BO) character (Figure 2). Although the π*(BO) orbital is mainly

Figure 3. Variable-temperature 1H NMR spectra of 3 showing hydrido signals (400 MHz, THF-d8:toluene-d8 = 4:1). Signals with asterisks are derived from the residual protons of toluene-d8.

bridging hydrides at the Ru−Ru edge. Although the values differed by ca. 4 ppm, the tendency of the μ-H signal to show a remarkable downfield shift was reproduced by gaugeindependent atomic orbital DFT (GIAO-DFT) calculations (μ3-H, δ −23.63; μ-H, δ 10.95 ppm). Pentahydrido complex 4 also contains both μ- and μ3hydrides like 3, but owing to the rapid site exchange between these hydrides, the chemical shifts of the individual hydride of 4 could not be confirmed; the hydride signal of 4 was observed to be equivalent at δ −7.22 ppm even at −110 °C.19 The GIAO-DFT calculation indicates that the μ3- and μ-hydrides of 4 resonate at δ −25.91 and δ 11.53 ppm, respectively, which are very close to the values estimated for 3. This similarity implies that the downfield shift observed for the μ-H signal of 3 does not arise from the μ3-BO ligand but, presumably owing to the location of the hydride ligand, is nearly coplanar with the Ru3 plane. Tsipis and co-workers examined the aromaticity/antiaromaticity of triangular ruthenium core structures by DFT calculations and noted that [cyclo-Ru3(μ-H)3]3+ species, which forms a 24-valence-electron system, exhibits highly antiaromatic character, whereas [cyclo-Ru3(μ-H)3]+ shows a considerable aromatic character.32 The Ru3 core of [{Cp*Ru(μ-H)}3(μ3-BO)(μ3-H)] (3) and [{Cp*Ru(μ-H)}3(μ3-H)2] (4) can be written as [cyclo-Ru3(μ-H)3]5+ according to the Tsipis description and forms a 22-valence-electron system. On the basis of the Hückel rule, the Ru3H3 core is also expected to exhibit aromatic properties. Therefore, the large downfield shift observed for the μ-hydrides in 3, which are placed nearly coplanar with the Ru3 plane, would arise from the ring current deshielding effect of the Ru3H3 core. As discussed later, the site exchange of the hydrides in 3 is retarded by the formation of a borane adduct, [{Cp*Ru(μH)}3(μ3-H){μ3-BO····B(C6F5)3}] (5). This fact strongly indicates that the site exchange proceeds via formation of the μ-hydroxyborylene intermediate 2c, as shown in Scheme 2a. This motion is different from the proposed mechanism of the hydride site exchange in 4 (Scheme 2b). Morokuma and co-workers proposed that the coordination mode of the μ3-H ligands readily changes to the μ mode with a small Gibbs free energy of activation (0.2 kcal mol−1) and forms an isomer, [(Cp*Ru)3(μ-H)5] (D), in which two Ru−Ru bonds are doubly bridged by μ-hydrides.33 In contrast to the calculation at the B3LYP level showing that D is more stable than 4 by 1.4 kcal mol−1,33 D is shown to be less stable than 4 at the ωB97XD level; however, the difference is only 0.55 kcal mol−1.

Figure 2. Shapes of the (a) LUMO+1 (0.028 eV), (b) LUMO (0.027 eV), (c) HOMO (−0.236 eV), and (d) HOMO-1 (−0.237 eV) of 3 with an isovalue of 0.03.

involved in LUMO and LUMO+1, which are also degenerate, the μ3 coordination of the BO fragment would be stabilized by the interaction of the multiple d orbitals with the π*(BO) orbital as seen in HOMO and HOMO-1. In the IR spectrum of 3, two sharp absorption bands originating from ν(μ3-10BO) and ν(μ3-11BO) were observed at 1699 and 1650 cm−1, respectively. These signals showed a slight blue shift in comparison to those of 1 (1672 and 1639 cm−1). This implies that the replacement of a μ3-alkyne ligand by two hydrides reduces back-donation from the Ru3 core to the π*(BO) orbital, likely owing to the increase in the formal oxidation state of the Ru centers. Site Exchange of Hydrides in 3. In the 1H NMR spectrum recorded at 25 °C, the signals for the four hydrides were observed to be equivalent as a broad signal resonating at δ −1.98 ppm (Figure 3). This shows that the hydrides undergo rapid site exchange between the μ3 and μ positions. With decreasing temperature, the hydride signal broadened further and sank beneath the baseline. Although the low-temperature limiting spectrum was not obtained, the spectrum recorded at −110 °C displays two broad signals resonating at δ −24.02 and δ 6.99 ppm with an intensity ratio of 1:3, which are assigned to the μ3-H and μ-H signals, respectively. Although the signal of the μ3-hydride appeared in the normal region for a metal hydride, the μ-hydride resonated in the extremely low magnetic field region, unlike common D


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Organometallics Scheme 2. Plausible Mechanism for the Site Exchange between the μ3- and μ-Hydrides

5 could be purified using column chromatography on alumina, unlike 3.

Although we did not investigate the transition state, the small energy difference suggests that a considerable amount of D is present in solution and is consistent with the rapid site exchange of hydrides in 4. The geometry of D would facilitate bond formation between the two μ-H ligands and ease the H/D exchange with D2 at ambient temperature. In contrast, 3 does not react with D2 at 25 °C, which is likely due to the difference in the motion of the hydride ligands at the Ru3 core. Although the rate was slow, the incorporation of deuterium was observed upon treatment with CD3OD at 25 °C (ca. 20% in 6 days); thus, the formation of 3-d1 and 3-d2 was confirmed. This result strongly supports the proposed mechanism in Scheme 2a. At 70 °C, complex 3 began to react with C6D6, leading to incorporation of deuterium into the hydrido positions (ca. 16% in 24 h). This result may indicate that an η2-H2 intermediate is formed at elevated temperatures via mechanism B shown in Scheme 2. However, mechanism A, the reaction of μ3-borylene intermediate 2c with C6D6, cannot be excluded at present. Although we could not observe 2c directly, the structure was optimized by DFT calculations (Figure S10c in the Supporting Information). The optimized structure resembles the previously reported structure of 2b, and 2c was found to be less stable than 3 by 10.5 kcal mol−1 at 25 °C. This indicates that the equilibrium largely shifts toward 3 and is consistent with the fact that 2c was not observed. The optimized B−O distance in 2c (1.367 Å) is significantly larger than the B−O length in 3 and close to the B−O length in 2b (1.374(13) Å). The Wiberg bond index (0.889) shows the single-bond character of the B−O bond. On the proton migration at the oxygen atom, the B−O bond is further polarized in comparison to the B−O bond in 3; the positive charge at the boron atom slightly increases to +0.997 from +0.968 in 3 and the negative charge at the oxygen atom decreases to −0.923 from −0.882 in 3. Although 2c is considered to be initially formed in the reaction of 2a with H2O, as shown in Scheme 1, the reduction of polarization in the B−O bond would be the driving force for the subsequent proton migration to the Ru3 core. Formation of a Borane Adduct. The basic oxygen atom in the μ3-BO ligand renders the formation of a Lewis acid adduct as seen in the terminal BO ligand.8 The addition of 1.5 equiv of B(C6F5)3 to a toluene solution of 3 resulted in an immediate color change of the solution from dark brown to dark purple (eq 2). The formation of a borane adduct, [{Cp*Ru(μ-H)}3{μ3-BO···B(C6F5)3}(μ3-H)] (5), was confirmed by XRD measurements, as shown in Figure 4. Notably,

Figure 4. Molecular structures and labeling schemes of 5 with thermal ellipsoids at 30% probability. Selected bond lengths (Å) and angles (deg): molecule 1, Ru(1)−Ru(2) 2.7041(3), Ru(1)−Ru(3) 2.7402(3), Ru(2)−Ru(3) 2.7119(3), Ru(1)−B(1) 2.395(4), Ru(2)−B(1) 2.196(4), Ru(3)−B(1) 2.450(4), B(1)−O(1) 1.260(4), B(2)− O(1) 1.511(3), Ru(2)−Ru(1)−Ru(3) 59.748(7), Ru(1)−Ru(2)−Ru(3) 60.788(7), Ru(1)−Ru(3)−Ru(2) 59.464(7), Ru(1)−B(1)−Ru(2) 72.02(10), Ru(1)−B(1)−Ru(3) 68.86(9), Ru(2)−B(1)−Ru(3) 71.18(10), B(1)−O(1)−B(2) 178.9(3); molecule 2, Ru(4)−Ru(5) 2.6886(3), Ru(4)−Ru(6) 2.7364(3), Ru(5)− Ru(6) 2.7479(3), Ru(4)−B(3) 2.144(3), Ru(5)−B(3) 2.316(3), Ru(6)···B(3) 2.665(3), B(3)−O(2) 1.259(4), B(4)−O(2) 1.509(3), Ru(5)−Ru(4)−Ru(6) 60.858(7), Ru(4)−Ru(5)−Ru(6) 60.432(7), Ru(4)−Ru(6)−Ru(5) 58.711(8), Ru(4)−B(3)−Ru(5) 74.02(10), B(3)−O(2)−B(4) 177.7(3).

In the unit cell, there were two independent molecules with different coordination modes of the BO···B(C6F5)3 moiety in a 1:1 ratio. Molecule 1 contains a μ3-BO ligand adducted by B(C6F5)3, whereas molecule 2 contains a μ-BO···B(C6F5)3 group. In molecule 1, all C6F5 groups of B(C6F5)3 are tilted with respect to the Ru3 plane to reduce the steric repulsion with the Cp* groups. In contrast, one of the C6F5 groups in molecule 2 lies nearly normal to the Ru3 plane. Owing to the steric repulsion of the C6F5 group with the Cp* group at Ru(6), the BO group moved away from the Ru(6) atom and adopted a doubly bridging ligation at the Ru(4)−Ru(5) edge. The interatomic distance between Ru(6) and B(3) was elongated to 2.665(3) Å. As shown later, the 1H NMR spectrum of 5 arose only from the C3v structure, even at −80 °C. Although we cannot exclude the rapid interconversion of the BO···B(C6F5)3 group between the μ3 and μ positions, these two structures seem to arise from packing effects in the lattice. In other words, this observation indicates that the coordination mode of the BO···B(C6F5)3 E


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Organometallics

H)3(μ3-H)(PMe3)] (6), quantitatively (eq 3). The most salient feature of 6 is its μ-BO ligand, which was

group at the Ru3 site can be readily changed by subtle steric effects. The Ru−Ru bond lengths in molecule 1 (2.7041(3), 2.7119(3), and 2.7402(3) Å) are comparable to those of 3 (average 2.72 Å). The Ru3 core of molecule 2 forms an isosceles triangle with sides of 2.6886(3), 2.7364(3), and 2.7479(3) Å, in which the Ru(4)−Ru(5) bond is noticeably shortened because of the presence of the μ-BO···B(C6F5)3 group. As seen in trans-[PtBr(BO···BArf3)(PCy3)2] (Arf = C6H33,5-(CF3)2),8 the B(1)−O(1) (1.260 (4) Å) and B(3)−O(2) (1.259 (4) Å) bonds were elongated by 0.03 Å upon the coordination of a Lewis acid. The B(2)−O(1) (1.511 (3) Å) and B(4)−O(2) (1.509 (3) Å) lengths are remarkably larger than the B−O single bond in 2b (1.374 (13) Å),10 which shows weak interaction between the oxygen atom and B(C6F5)3. These values are comparable to that in the platinum complex (1.581(3) Å).8 In the IR spectrum of the borane adduct 5, broad absorptions assignable to ν(11BO) and ν(10BO) were observed at 1585 and 1542 cm−1, which shift considerably to lower wavenumbers in comparison to those of 3 (1699 and 1650 cm−1). These bathochromic shifts indicate that the strength of the BO bond is considerably weakened by the coordination of borane. The magnitude of the bathochromic shift is similar to that reported for the platinum complex.8 Although the Pt−B bond in trans-[PtBr(BO···BArf3) (PCy3)2] is shortened by 0.04 Å in comparison to that of the parent oxoboryl complex,8 the average Ru−B length in molecule 1 of 5 (2.349 Å) became slightly larger than that of 3 (2.310 Å). This implies that the enhanced back-donation owing to the borane coordination is canceled by the steric repulsion. In contrast, the Ru−B length in molecule 2 (average 2.233 Å) became significantly shorter owing to the μ coordination. Such shortening in the Ru−B bond upon μ coordination is also seen in [(Cp*Ru)3(μ-BO)(μ-H)3(μ3H)(PMe3)] (6) and [{Cp*Ru(CO)}3(μ-BO)(μ-H)2] (7) (vide infra). In the 1H NMR spectrum of 5 recorded at −80 °C, the signal of the Cp* groups was observed at δ 1.50 ppm as a sharp singlet. At this temperature, two singlets assignable to μ- and μ3-hydrides were observed at δ 6.97 and −27.63 ppm, respectively (Figure S25 in the Supporting Information). This contrasts starkly with the 1H NMR spectrum of 3 recorded at −90 °C, in which the hydride signal was buried beneath the baseline because of the rapid site exchange. On the other hand, although the time-averaged hydrido signal was observed at 25 °C for 3, the hydride signals of 5 broadened severely even at 50 °C. These observations indicate that the rate of site exchange of hydrides decreases considerably, owing to the suppression of the formation of 2c by the coordination of B(C6F5)3 at the oxygen atom (Scheme 2a), and the coordination of B(C6F5)3 to the BO ligand was maintained at 50 °C. Reaction of 3 with PMe3. Complex 3 adopts the same 44electron configuration as [{Cp*Ru(μ-H)}3(μ3-H)2] (4). Thus, it was anticipated that 3 would show reactivity similar to that of 4, particularly for the addition of a simple 2e donor. We previously showed that 4 reacts with PR3 (R = Me, OPh) to afford a phosphine adduct, [(Cp*Ru)3(μ-H)4(μ3-H)(PR3)].19 Thus, the reaction of 3 with PMe3 was examined. As expected, complex 3 immediately reacted with PMe3 to yield a similar phosphine adduct, [(Cp*Ru)3(μ-BO)(μ-

Figure 5. Molecular structure and labeling scheme of 6 with thermal ellipsoids at 30% probability. Selected bond lengths (Å) and angles (deg): Ru(1)−Ru(2) 2.9953(3), Ru(1)−Ru(3) 3.0002(4), Ru(2)− Ru(3) 2.6459(3), Ru(1)−P(1) 2.2884(9), Ru(2)−B(1) 2.174(4), Ru(3)−B(1) 2.164(4), B(1)−O(1) 1.230(5); Ru(2)−Ru(1)−Ru(3) 52.375(8), Ru(1)−Ru(2)−Ru(3) 63.908(9), Ru(1)−Ru(3)−Ru(2) 63.72(1), Ru(2)−Ru(1)−P(1) 94.50(3), Ru(3)−Ru(1)−P(1) 95.17(3), Ru(1)−Ru(2)−B(1) 90.86(11), Ru(3)−Ru(2)−B(1) 52.24(10), Ru(1)−Ru(3)−B(1) 90.93(10), Ru(2)−Ru(3)−B(1) 52.59(10), Ru(2)−B(1)−Ru(3) 75.17(13), Ru(2)−B(1)−O(1) 141.8(3), Ru(3)−B(1)−O(4) 142.9(3).

unambiguously confirmed by XRD (Figure 5). One PMe3 molecule was coordinated to the Ru(1) atom from the opposite face of the BO ligand. The Cp* group at Ru(1) bent away from PMe3, which pushes the BO ligand from the facecapping position to the Ru(2)−Ru(3) edge. Although the bridging coordination of a BO ligand has been sometimes proposed on the basis of DFT calculations,20 complex 6 is the first discrete example showing the bridging coordination of a BO ligand. This result demonstrates that a BO ligand can alter its coordination mode at a multimetallic site in a way similar to that of a CO ligand. In spite of the different coordination mode, the B(1)−O(1) length (1.230(5) Å) is almost the same as that of 3. On the other hand, the Ru−B distance (average 2.17 Å) is remarkably shortened from that of 3. The sum of the bond angles around B(1) is 359.9°, which clearly indicates the sp2 character of the B(1) atom. A set of ν(10BO) and ν(11BO) bands was observed at 1709 and 1656 cm−1, which are slightly hypsochromically shifted in comparison to those of μ3-BO in 3 (1699 and 1650 cm−1). Although these values are considerably smaller than those reported for the terminal BO ligand in [trans-Pt(BO)Br(PCy3)2] (1853 and 1797 cm−1), the difference between ν(μ3BO) and ν(μ-BO) is significantly smaller than that between ν(μ3-CO) and ν(μ-CO), which is normally reported to be 20− 200 cm−1. F


Article

Organometallics The crystal structure of 6 also shows that the positions of the hydrides are the same as those found for [(Cp*Ru)3(μH)4(μ3-H){P(OPh)3}],19 except for the presence of a μ-BO ligand at the Ru(2)−Ru(3) edge. The 31P{1H} NMR spectrum of 6 contains a signal at δ 11.0 ppm, which is similar to that of [(Cp*Ru)3(μ-H)4(μ3-H)(PMe3)] (δ 12.0).19 Unlike [(Cp*Ru)3(μ-H)4(μ3-H)(PMe3)], 6 gradually decomposes to 3 with liberation of PMe3. Upon heating at 60 °C for 3 h, 3 was regenerated in ca. 10% yield. This is likely due to reduced back-donation to the σ*(P−C) orbitals, owing to the presence of the electron-withdrawing BO ligand at the Ru3 core. Reaction of 3 with CO. King and co-workers showed that the most preferable isomer of [Co2(BO)2(CO)7] is that which contains two μ-BO ligands,20 whereas the global minimum structure of [Cp2Fe2(BO)2(CO)3] is predicted to contain one μ-CO and two terminal BO ligands with an Fe−Fe single bond.34 Thus, we examined the reaction of 3 with CO to verify which site the BO ligand prefers. Complex 3 smoothly reacted with CO (1 atm) in methanol at 0 °C to afford a tricarbonyl complex, [{Cp*Ru(CO)}3(μBO)(μ-H)2] (7), quantitatively (eq 4). Complex 7 was only

Figure 6. Molecular structure and labeling scheme of 7 with thermal ellipsoids at 30% probability. Selected bond lengths (Å) and angles (deg): Ru(1)−Ru(2) 3.0140(4), Ru(1)−Ru(3) 3.0167(4), Ru(2)− Ru(3) 2.9578(4), Ru(1)−C(1) 1.845(4), Ru(2)−B(1) 2.194(4), Ru(2)−C(2) 1.850(4), Ru(3)−B(1) 2.174(5), Ru(3)−C(3) 1.853(4), B(1)−O(4) 1.236(5); Ru(2)−Ru(1)−Ru(3) 58.741(9), Ru(1)−Ru(2)−Ru(3) 60.675(9), Ru(1)−Ru(3)−Ru(2) 60.585(9), Ru(3)−Ru(2)−B(1) 47.09(12), Ru(2)−Ru(3)−B(1) 47.66(11), Ru(2)−B(1)−Ru(3) 85.26(16), Ru(2)−B(1)−O(4) 137.9(3), Ru(3)−B(1)−O(4) 136.9(3), Ru(2)−B(1)−Ru(3) 72.07(16), Ru(2)−B(1)−O(1) 136.5(4), Ru(3)−B(1)−O(1) 137.6(4).

adsorptions around 1900 cm−1 (νsym(CO) 1939 cm−1, νasym(CO) 1910 cm−1, and ν(CO) 1889 cm−1) in the IR spectrum. The BO ligand bridges the Ru(2)−Ru(3) edge and is slightly bent away from the Ru3 plane by 14° toward the two terminal CO ligands. The Ru(2)−B(1) (2.194(4) Å) and Ru(3)−B(1) bonds (2.174(5) Å) are noticeably shortened in comparison to the Ru−B bond in 3, as seen in 6. The B(1)− O(4) distance in 7 (1.236(5) Å) is almost the same as that in 3 (1.231(6) Å). This is also reflected in the ν(BO) values; two adsorptions for the μ-BO ligand were observed at 1690 and 1640 cm−1, which were respectively assigned to ν(10BO) and ν(11BO). These absorptions underwent a slight bathochromic shift in comparison to those of 3 (1699 and 1650 cm−1) despite its μ-coordination mode. These results indicate that effective back-donation to the π*(μ-BO) orbital occurs in spite of the presence of π-acidic CO ligands at the Ru3 core. This is likely due to the suitable overlap of the π*(Ru−Ru) orbital with the π*(BO) orbital. Such strong electronic interactions are clearly seen in the HOMO of 7 (Figure 7), as Baerends and co-workers proposed for a dinuclear μ-BO system.2 The fact that BO rather than CO occupies the bridging coordination site is likely due to the substantial stabilization arising from the efficient back-donation to the π*(BO) orbital, which should balance the strong σ donation of the BO moiety. Although the polarization of the B−O bond was still large in 7, the positive charge at the B atom considerably decreased to +0.889 from +0.968 in 3.

stable below 0 °C in toluene and gradually decomposed to a mixture of unidentified complexes even in the solid state. However, it was stable in methanol for hours at 25 °C, probably because of the stabilization by the hydrogen bond between the oxygen atom of the BO ligand and methanol. We have shown that [{Cp*Ru(μ-H)}3(μ3-H)2] (4) reacts with four molecules of CO to yield a paramagnetic tetracarbonyl complex, [{Cp*Ru(μ-CO)}3(μ3-CO)].19 The formation of the tetracarbonyl complex suggests that dihydrogen elimination can take place in the plausible intermediate, [{Cp*Ru(CO)(μ-H)}3], thus allowing the incorporation of a fourth CO molecule. On the other hand, the hydrides in 7 were not removed from 7 cleanly; thus, it is likely that 7 undergoes decomposition. The molecular structure of 7 was determined by XRD analysis, as shown in Figure 6, which shows the μ coordination of the BO ligand, as well as the presence of three terminal CO ligands. Accompanied by the formation of a coordinatively saturated 48-electron configuration, the Ru−Ru distances in 7 (average 3.00 Å) become larger than those of 3. The two hydrides were successfully located at the Ru(1)−Ru(2) and Ru(1)−Ru(3) edges, nearly coplanar with the Ru3 plane. In contrast to the μhydrides in 3, the hydrido signal was observed in the normal region and resonated at δ −17.03 ppm in the 1H NMR spectrum. The [cyclo-Ru3(μ-H2)(μ-BO)]3+ core of 7 forms a 24-valence-electron system, which is expected to exhibit antiaromatic character on the basis of Tsipis’s description.32 Three CO molecules are coordinated to each Ru atom in a terminal fashion. The CO molecules at the Ru(2) and Ru(3) atoms are located on the same side, but the remaining CO molecule is coordinated to the Ru(1) atom from the opposite direction with respect to the Ru3 plane. Terminal coordination of the CO ligands is also inferred by the three strong

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CONCLUSION In summary, we synthesized the novel triruthenium hydrido complex 3 supported by a μ3-BO ligand by the reaction of the μ3-borylene complex 2a with water in the presence of an amine. In addition to the steric protection stemming from the surrounding three Cp* groups, an overlap of the π*(Ru−Ru) orbitals with the π*(BO) orbitals effectively stabilizes the G


Article

Organometallics

according to a published method.10 1H, 13C, 11B, 19F, and 31P NMR spectra were recorded on Varian INOVA-400 and Varian 400MR spectrometers. 1H NMR spectra were referenced to tetramethylsilane as an internal standard. 13C NMR spectra were referenced to the natural-abundance carbon signal of the solvent employed. 11B{1H} NMR spectra were referenced to boron trifluoride diethyl etherate as an external standard (δ 0 ppm). 19F{1H} NMR spectra were recorded with trifluoroacetic acid as an external standard (δ −76.0 ppm referenced to CFCl3). The 31P{1H} NMR spectrum was referenced to H3PO4 (85% in water) as an external standard (δ 0 ppm). IR spectra were recorded on a JASCO FT/IR-4200 spectrophotometer with a Ge-ATR cell or by the diffuse reflection method using KBr. Elemental analyses were performed on a PerkinElmer 2400II CHN analyzer. X-ray Diffraction Studies. Single crystals of 2a, 3, 5−7, and [{Cp*Ru(μ-H)}3(μ3-AlEt)] for the X-ray analyses were obtained directly from the preparations described below and mounted on nylon Cryoloops with Paratone-N (Hampton Research Corp.). Diffraction experiments were performed on a Rigaku R-AXIS RAPID imaging plate diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.71069 Å). Cell refinement and data reduction were performed using the PROCESS-AUTO program.35 Intensity data were corrected for Lorentz−polarization effects and for numerical and empirical absorption. The structures were solved by the direct method using SHELXT-2014/5 and further refined with the SHELXL-2018/1 program package.36 All non-hydrogen atoms were found by a difference Fourier synthesis and were refined anisotropically except for the disordered carbon atoms in the Cp* moieties attached to Ru(6) in 3 and Ru(3) in 6. The refinements were carried out by leastsquares methods based on F2 with all measured reflections. In the case of 3, the carbon atoms in the Cp* group attached to Ru(6) (C(51)− C(60)) were disordered and refined with 68%:32% occupancy. The carbon atoms in the Cp* group attached to Ru(3) of 6 (C(24)− C(33)) were disordered and refined with 52%:48% occupancy. The carbon atoms in the Cp* group attached to Ru(4) of 2a (C(31)− C(40)) were disordered and refined with 71%:29% occupancy. For the structure of 7, checkCIF reported an A alert due to the large residual peaks. However, the peaks are close to the Ru atoms (6.18 e Å−3, 0.95 Å from Ru(1); 2.65 e Å−3, 0.70 Å from Ru(3)) and can be ascribed to the summation terminated error (typical for heavy-atom structures). The metal-bound hydrogen atoms in 3, 5−7, and [{Cp*Ru(μ-H)}3(μ3-AlEt)] were found by a difference Fourier synthesis and refined isotropically. The hydrogen atoms attached to the Ru(4), Ru(5), Ru(6), and B(2) atoms in 2a could not be located. Although the hydrogen atom attached to B(1) in 2a was refined isotropically without restraint, the hydrogen atoms attached to the Ru(1), Ru(2), and Ru(3) atoms were refined using the SADI command. Crystal data and results of the analyses are given in Table S1 in the Supporting Information. The CIF data of 2a, 3, 5−7, and [{Cp*Ru(μ-H)}3(μ3-AlEt)] are deposited with the Cambridge Crystallographic Data Center with deposition numbers of 1901653 (3), 1901654 (5), 1901655 (6), 1901656 (7), 1901657 (2a), and 1901658 ([{Cp*Ru(μ-H)}3(μ3-AlEt)]). Computational Details. DFT calculations for 2a, 2b′, 2c, 3, 4, D, and 7 were carried out at the ωB97X-D level in conjunction with the Stuttgart/Dresden ECP55 basis set associated with triple-ζ SDD basis sets for Ru.37,38 For H, B, C, and O, 6-31G(d) basis sets were employed. No simplified model compounds were used for the calculations. For 2a, 3, 4, and 7, initial geometries for the optimization were based on the crystallographically determined structure. For 2b′ and 2c, initial geometries for the optimization were based on the crystallographically determined structure of [{Cp*Ru(μ-H)}3(μ3BOEt)] (2b). The molecular structures were drawn using the GaussView version 6.0 program.39 Frequency calculations at the same level of theory as geometry optimizations were performed on the optimized structure to ensure that minima exhibit only positive frequency. For 3 and 4, chemical shifts of the hydrido ligands were calculated using the gauge-independent atomic orbital method at the ωB97X-D level for Ru with SDD and the 6-31G(d) level for C, H, B, and O. The reference shielding was set at 0 ppm (SiMe4, B3LYP/6311+G(2d,p) GIAO). Information on the atom coordinates (xyz file)

Figure 7. Shape of the HOMO of 7 with an isovalue of 0.03.

ligation of the BO ligand to the Ru3 core. The nucleophilic attack of H2O at the μ3-BH group in 2a would afford a μ3hydroxyborylene complex 2c initially, in which Et2NH not only accelerates the reaction with H2O but also suppresses the overreaction with further H2O molecules. The following proton migration from μ3-B−OH to the Ru core leads to the formation of a μ3-BO ligand. Although 2c was not directly observed, considerable deceleration of the hydride site exchange in the borane adduct 5 strongly suggests equilibrium between 3 and 2c. Although the μ3-oxoboryl complex 3 adopts the same 44electron configuration as the pentahydrido complex 4, cyclic voltammetry analysis showed that the Ru3 core of 3 becomes remarkably electron deficient in comparison to that of 4. This fact suggests that efficient back-donation to the π*(BO) orbitals is operative, unlike the terminal BO ligand. Thus, a BO− ligand is expected to act as a unique XZ2- or XZ-type ligand at multimetallic sites. Because of its coordinatively unsaturated nature, 3 readily took up PMe3 to form 6, like the pentahydrido complex 4. Upon the incorporation of PMe3, the BO ligand migrated from the μ3 position to the Ru−Ru edge where the PMe3 was not coordinated. The formation of 6 clearly demonstrated that a BO ligand can alter its coordination mode at the multimetallic center, like CO. The preference of the μ coordination of BO rather than CO is exemplified by the formation of tricarbonyl μ-BO complex 7, which is also due to the substantial stabilization of BO− by the efficient overlap of the π*(Ru− Ru) with π*(BO) orbitals. We are currently studying the reactivity of 3 toward unsaturated hydrocarbons to verify the effect of the μ3-BO ligand more precisely.

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EXPERIMENTAL SECTION

General Procedures. All compounds were manipulated using standard Schlenk and high-vacuum-line techniques under an atmosphere of argon. Dehydrated toluene, tetrahydrofuran (THF), pentane, hexane, and methanol used in this study were purchased from Kanto Chemicals and stored under an atmosphere of argon. Diethyl ether was dried over sodium−benzophenone ketyl and distilled under an atmosphere of argon. C6D6 and THF-d8 were dried over sodium−benzophenone ketyl and stored under an atmosphere of argon. Acetone-d6 was dried over MS-4A and stored under an atmosphere of argon. Other reagents were used as received. The μ3borylene complex {Cp*Ru(μ-H)}3(μ3-BH) (2a) was prepared H


Article

Organometallics

RuH), −10.76 (br, 3H, RuH), 1.01 (d, JPH = 8.5 Hz, 9H, PMe3), 1.77 (d, JPH = 1.5 Hz, 15H, C5Me5), 2.01 ppm (s, 30H, C5Me5). 13C NMR (100 MHz, C6D6, 25 °C): δ 11.9 (q, 1JCH = 126 Hz, C5Me5), 12.6 (q, 1 JCH = 126 Hz, C5Me5), 24.1 (dq, JPC = 30 Hz, JCH = 126 Hz, PMe3), 86.1 (s, C5Me5), 92.4 ppm (s, C5Me5). 11B{1H} NMR (129 MHz, C6D6, 25 °C): δ 43.4 ppm (br s, μ-BO). 31P{1H} NMR (162 MHz, C6D6, 25 °C): δ 11.0 ppm (PMe3). IR (ATR, cm−1): 2977, 2954, 2899, 1709 (ν(10BO)), 1656 (ν(11BO)), 1456, 1370, 1272, 1024, 948, 933. Because of the decomposition of 6 to 3 with liberation of PMe3, the elemental analysis did not match the calculated values. Preparation of {Cp*Ru(CO)}3(μ-BO)(μ-H)2 (7). A 50 mL Schlenk tube was charged with 3 (55.4 mg, 74.9 μmol) and toluene (3.0 mL). After the Schlenk tube was evacuated using a liquid nitrogen bath, 1 atm of CO was introduced into the flask at 0 °C. The solution immediately turned from dark brown to dark purple. After the solution was stirred for 10 min at 0 °C, the solvent was removed under reduced pressure at 0 °C. The residual solid was then dissolved in acetone (2.0 mL) at 0 °C and stored at −30 °C. A dark purple precipitate of 7 was formed after few days, and an 18.8 mg amount of 7 was obtained by decantation (22.5 μmol, 23%). A black single crystal used for the diffraction study was prepared by slow evaporation of a pentane/CH2Cl2 solution (100/1) of 7 at −30 °C. 1H NMR (400 MHz, CD3OD, 25 °C): δ −17.03 (s, 2H, RuH), 1.91 (s, 15H, C5Me5), 1.96 ppm (s, 30H, C5Me5). 13C NMR (100 MHz, toluene-d8, 0 °C): δ 11.0 (q, 1JCH = 127 Hz, C5Me5), 11.1 (q, 1JCH = 127 Hz, C5Me5), 95.7 (s, C5Me5), 98.1 (s, C5Me5), 206.4 (s, CO), 209.0 ppm (s, CO). 11B{1H} NMR (129 MHz, CD3OD, 25 °C):δ 19.0 ppm (br s, μ-BO). IR (ATR, cm−1): 2965, 2910, 1939 (νsym(CO)), 1910 (νasym(CO)), 1889 (ν(CO)), 1690 (ν(10BO)), 1640 (ν(11BO)), 1455, 1423, 1374, 1151, 1073, 1025, 951, 799, 685, 618. Anal. Calcd for C33H47BO4Ru3: C, 48.23; H, 5.77. Found: C, 48.32; H, 5.95.

for all optimized structures are given in the Supporting Information. Selected bond distances and angles are given in Tables S8−S11 in the Supporting Information. Preparation of [{Cp*Ru(μ-H)}3(μ3-BO)(μ3-H)] (3). After a 100 mL Schlenk tube equipped with a Teflon valve was charged with 2a (0.993 g, 1.37 mmol), THF (30 mL), and H2O (0.48 mL, 26.7 mmol, 19 equiv), 6 equiv of Et2NH (0.78 mL, 8.19 mmol) was added to the mixture. The solution was stirred for 7 days at 25 °C. The solution turned from dark orange to dark brown. The solvent was then removed under reduced pressure. The 1H NMR spectrum of the residual solid showed complete consumption of 2a and formation of a mixture of 3 and {Cp*Ru(μ-H)}3(μ3-H)2 (4) in a ratio of 87:13. After the residual solid including 3 and 4 was dissolved in toluene (50 mL), 1,3-cyclohexadiene (1.2 mL, 13 mmol) was added to the solution to convert 4 into {Cp*Ru(μ-H)}3(μ3-η2:η2:η2-C6H6). After the mixture was stirred for 9 h at 25 °C, the solvent was removed under reduced pressure. The mixture was extracted with toluene and filtered through a short column packed with Celite to remove B(OH)3. After the solvent was removed under reduced pressure, the residue was then extracted with acetonitrile at 0 °C and filtered through a short column packed with Celite again to remove {Cp*Ru(μ-H)}3(μ3-η2:η2:η2-C6H6). Drying in vacuo gave crude 3, and analytically pure 3 was obtained by recrystallization from a cold toluene solution (ca. 4 mL) stored at −30 °C (0.432 g, 0.59 mmol, 43%). 1H NMR (400 MHz, C6D6, 25 °C): δ −1.69 (br s, w1/2 = 61 Hz, 4H, RuH), 2.04 ppm (s, 45H, C5Me5). 1H NMR (400 MHz, THF-d8:toluene-d8 = 4:1, −110 °C): δ −24.02 (br s, w1/2 = 36 Hz, 1H, μ3-H), 1.99 (s, 45H, C5Me5), 6.69 ppm (br s, w1/2 = 18 Hz, 3H, μ-H). 13C NMR (100 MHz, C6D6, 25 °C): δ 12.6 (q, 1JCH= 126 Hz, C5Me5), 89.8 ppm (s, C5Me5). 11B{1H} NMR (129 MHz, THF-d8, 25 °C): δ 33.2 ppm (br s, μ3-BO). IR (ATR, cm−1): 2987, 2943, 2898, 1699 (ν(10BO)), 1650 (ν(11BO)), 1449, 1369, 1071, 1024, 729. Anal. Calcd for C30H49BORu3: C, 48.71; H, 6.68. Found: C, 48.78; H, 6.90. Preparation of [{Cp*Ru(μ-H)}3{μ3-BO···B(C6F5)3}(μ3-H)] (5). A 50 mL Schlenk tube was charged with 3 (39.0 mg, 52.7 μmol) and toluene (3.0 mL). Tris(pentafluorophenyl)borane (40.5 mg, 79.1 μmol) was added to the solution, and the solution was vigorously stirred for 5 min at 25 °C. The solution was then purified by column chromatography on alumina (Merck, Art. No. 1097) with toluene. The purple band was collected, and the solvent was removed under reduced pressure. The residual solid was dissolved into THF (ca. 1 mL) and stored at −30 °C. Borane adduct 5 was obtained from the solution as a black single crystal (27.6 mg, 22.0 μmol, 42%). 1H NMR (400 MHz, THF-d8, 25 °C): δ 1.83 ppm (s, 45H, C5Me5). Signals derived from hydrides were not observed due to severe broadening arising from site exchange. 1H NMR (400 MHz, toluene-d8, −80 °C): δ −27.63 (s, 1H, μ3-H), 1.50 (s, 45H, C5Me5), 6.97 ppm (s, 3H, μH).1.96 ppm (s, 30H, C5Me5). 13C NMR (100 MHz, THF-d8, 25 °C): δ 11.7 (q, 1JCH = 127 Hz, C5Me5), 92.0 (s, C5Me5), 121.5 (s, ipso-C6F5), 137.4 (d, JCF = 249 Hz, m-C6F5), 139.9 (d, JCF = 253 Hz, p-C6F5), 148.7 ppm (d, JCF = 244 Hz, o-C6F5). 11B{1H} NMR (129 MHz, THF-d8, 25 °C): δ −4.2 (br s, B(C6F5)3), 50.6 ppm (br s, BO··· B(C6F5)3). 19F NMR (375 MHz, C6D6, 25 °C): δ −163.91 (m, 6F, mPh), −158.39 (t, JFF = 20.5 Hz, 3F, p-Ph), −130.28 ppm (dd, JFF = 24.9, 8.7 Hz, 6F, o-Ph). IR (ATR, cm−1): 2984, 2906, 1642, 1585 (ν(10BO)), 1542 (ν(11BO)), 1514, 1449, 1395, 1278, 1092, 1025, 972, 877, 808. Anal. Calcd for C48H49B2F15ORu3: C, 46.06; H, 3.95. Found: C, 46.47; H, 3.99. Preparation of [(Cp*Ru)3(μ-BO)(μ-H)3(μ3-H)(PMe3)] (6). A 50 mL Schlenk tube was charged with 3 (30.0 mg, 40.6 μmol) and THF (3.0 mL). A stock solution of PMe3 in toluene (0.45 mL, 95.7 mmol mL−1, 43.1 μmol) was added to the solution, and the solution immediately turned from dark brown to dark purple. After the solution was vigorously stirred for 10 min at 25 °C, the solvent and remaining PMe3 were removed under reduced pressure. The residual solid was rinsed three times with 3 mL of pentane. Drying under reduced pressure gave 6 as a black crystalline solid (28.2 mg, 34.6 μmol, 85%). A dark brown single crystal used for the diffraction study was obtained by recrystallization from the toluene solution stored at −30 °C. 1H NMR (400 MHz, C6D6, 25 °C): δ −21.56 (br, 1H,

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.9b00182. Crystal data and results of XRD studies of 3, 5−7, 2a, and {Cp*Ru(μ-H)}3(μ3-AlEt)], results of DFT calculations for 2a, 2b′, 2c, 3, 4, D, and 7, and NMR and IR spectra of 3 and 5−7 (PDF) Atom coordinates of the optimized structures of 2a, 2b′, 2c, 3, 4, D, and 7 (XYZ) Accession Codes

CCDC 1901653−1901658 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail for T.T.: [email protected]. ORCID

Toshiro Takao: 0000-0002-5393-112X Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a Grant-in-Aid for Scientific Research in Innovative Areas “Stimuli-Responsive Chemical Species for the Creation of Functional Molecules” from MEXT (Japan). T.K. also acknowledges the JSPS (Grant-in-Aid for I


Article

Organometallics

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JSPS fellows) for support. The numerical calculations were carried out on the TSUBAME3.0 supercomputer at the Tokyo Institute of Technology, Tokyo, Japan.

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Synthesis and Properties of a Triruthenium Hydrido Complex Capped

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