B–C Bond Cleavage and Ru–C Bond Formation from a

Article pubs.acs.org/Organometallics

B−C Bond Cleavage and Ru−C Bond Formation from a Phosphinoborane: Synthesis of a Bis‑σ Borane Aryl-Ruthenium Complex Audrey Cassen,†,‡ Laure Vendier,†,‡ Jean-Claude Daran,†,‡ Amalia I. Poblador-Bahamonde,§ Eric Clot,§ Gilles Alcaraz,*,†,‡ and Sylviane Sabo-Etienne†,‡ †

CNRS, LCC (Laboratoire de Chimie de Coordination), BP44099, 205 route de Narbonne, F-31077 Toulouse cedex 4, France Université de Toulouse, UPS, INPT, F-31077 Toulouse, France § Institut Charles Gerhardt, CNRS 5253, Université Montpellier 2, cc 1501, Place Eugène Bataillon, 34095 Montpellier, France ‡

S Supporting Information *

ABSTRACT: Compared with the reactivity of o-Ph2P(C6H4− CH2)BH(NiPr2) with [RuH2(η2-H2)2(PCy3)2], the behavior of the phosphinoborane Ph2P(CH2−C6H4)BH(NiPr2) is radically different. No agostic σ-B−H complex could be observed, the reaction leading to the isolation of a new bis-σ borane arylruthenium complex via B−C bond cleavage and Ru−C bond formation. Reactivity studies of this complex with dihydrogen and/or HBArF4 as a proton source enable the formation of a tethered chiral-at-Ru piano-stool cationic complex.

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INTRODUCTION

Bifunctional molecules featuring both a Lewis basic phosphine and a borane function with an sp2 hybridized boron center (R2P∼BR′2) display very different behaviors depending on their intrinsic Lewis acido-basic properties. With a Lewis acidic boryl group, these compounds are Lewis pairs that can function as bidentate ambiphilic ligands in metal complexes, and their properties have been strongly investigated.1,2 With a non-Lewis acidic boryl group,3 their role as a bidentate ligand strongly depends on the substitution pattern at the boron atom. This peculiar class of bifunctional species acting as a bidentate ligand is mostly limited to molecules of the general formulation R2P∼BH(NiPr2) that integrate an aminoboryl group.4−6 Only three cases are reported to date for which the nature of the spacer linking the P and B atoms plays a key role in the coordination mode adopted by the phosphinoborane ligand, as illustrated upon reaction with the bis(dihydrogen) ruthenium complex [RuH2(η2-H2)2(PCy3)2] (1).4−6 In the case of a methylene4 (CH2) or an ortho-phenylene5 (oC6H4) spacer, we showed that the phosphinoborane is coordinated to the ruthenium center in a bidentate mode through the phosphine and an agostic σ-Bsp2−H bond7 (Figure 1). When a mixed o-phenylmethylene spacer (o-C6H4−CH2) was used, with the boron atom at the benzylic position, we recently disclosed the isolation of the bis-agostic complex [RuH2{η2-HB:η2-HC-HB(NiPr2)CH2−C6H4−PPh2}(PCy3)], in which the phophinoborane displays an additional η2-C−H bond: the complex is thus stabilized by two adjacent agostic bonds of different polarity, η2-C−H and η2-B−H (Figure 1).6 Here, we report a novel aspect of the chemistry of phosphinoborane Ph2P∼BH(NiPr2) compounds. While retain© 2014 American Chemical Society

Figure 1. Agostic and bis-agostic coordination modes of phosphinoboranes R2P∼BH(NiPr2) at a metal center.

ing the same o-phenylmethylene spacer, and by only swapping the phosphino and the boryl groups in the molecule, we show in this article that such a change has a tremendous influence on the reactivity of the new compound Ph2P(CH2−C6H4)BH(NiPr2) (2) with 1. Indeed, B−C bond cleavage is observed, leading to the isolation of a cyclo-metalated complex. Reactivity studies with dihydrogen and protic acid are also presented.

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RESULTS AND DISCUSSION Based on our previously reported procedure, ((o(diphenylphosphino)methyl)phenyl)borane 2 was prepared from o-Br-C6H4−CH2−PPh2 and HClBNiPr2.4,5 The reaction of 2 with [RuH2(η2-H2)2(PCy3)2] (1) carried out in pentane, at room temperature, for 1 min, led after workup to a brown solid analyzed as an equimolar mixture of complex 3 and PCy3 (Scheme 1). 3 was found to be unstable in solution at room temperature and had to be stored in the solid state, below −30 °C. However, 3 could be isolated and crystallized, as yellow Received: September 23, 2014 Published: November 26, 2014 7157

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Scheme 1. Synthesis of [RuH(η2:η2-H2B−NiPr2)(κ2-C,PC6H4−CH2−PPh2)(PCy3)] (3)

Table 1. Selected Geometrical Parameters (Distances in Å, Angles in deg) for the Experimental and Calculated Structures for 3 3 Ru1···B1 Ru1−Hy1 Ru1−Hy2 Ru1−Hy3 Ru1−C9 Ru1−P1 Ru1−P2 B1−Hy2 B1−Hy3 B1−N1 P1−Ru−P2 B1−Ru1−C9 Ru1−B1−N1 Hy1−Ru−C9

crystals, from a cold (−37 °C) pentane solution. 2 and 3 were fully characterized by multinuclear NMR spectroscopy and Xray diffraction crystallography. The 31P{1H} NMR spectrum of 3 in C7D8 at 193 K displays an AB pattern at δ 60.2 and 72.4 with a JPP coupling constant of 246 Hz, indicative of two different phosphorus in a trans position. The 11B{1H} NMR spectrum shows a broad signal at δ 41. The 1H NMR spectrum exhibits three signals with the same intensity, in the hydride zone. The more shielded signal appears as a pseudo triplet at δ −10.33 and collapses into a singlet upon 31P decoupling. The two signals at δ −8.54 and −4.67 only sharpened upon 11B decoupling. Selective decoupling experiments at different temperatures led to complete assignment of the hydrogen atoms around the ruthenium as a hydride and a bis(σ-B−H) ligand, respectively. In the 13C{1H} spectrum, the deshielded signal observed at δ 173.10 as a pseudo triplet, with an apparent J coupling constant of 6.7 Hz, is assigned to the ortho-metalated carbon coupling with the two phosphorus nuclei (Supporting Information, Figures S2−S5). The X-ray structure of [RuH(η2:η2-H2B−NiPr2)(κ2-C,PC6H4−CH2−PPh2)(PCy3)] (3) was determined at 110 K, and the hydrogen atoms around the metal were located (Figure 2). The ruthenium atom is in a distorted octahedral

exptl

calcd

1.997(3) 1.55(3) 1.72(3) 1.80(3) 2.108(2) 2.2591(5) 2.3757(5) 1.21(3) 1.18(3) 1.385(3) 155.64(2) 133.05(9) 173.74(18) 85.5(10)

1.960 1.614 1.809 1.800 2.085 2.258 2.345 1.340 1.334 1.394 155.73 129.66 171.27 89.70

very similar to those previously reported for the bis(σ-B−H) aminoborane ruthenium complex [RuH2(η2:η2-H2B−NiPr2)(PCy3)2].10 No noticeable difference could be observed in the B−H bond lengths in 3 (both by X-ray and DFT) despite the presence of two different trans ligands (H vs Aryl). This situation strictly differs from the one in the bis(σ-B−H) ruthenium complex [RuHCl(η2:η2-H2B−NMe2)(PiPr3)2] displaying an unsymmetrically coordinated aminoborane ligand with the B−H bond, trans to the chlorine, stretched to the limit (1.53(2) Å) and reaching the breaking point.11 The optimized geometry for 3 is in excellent agreement with the X-ray structure (Table 1). The calculations afford us to obtain more accurate values for the geometrical parameters associated with hydrogen atoms. Despite the different nature of the ligands trans to the coordinated B−H bonds (H and Ph), the latter are computed to have similar distances (Table 1). As expected for a bis-σ coordination of H2BNiPr2, the B−H bond distance is significantly longer than in the free ligand (1.340 and 1.334 Å in 3 vs 1.208 Å in free H2BNiPr2). Thus, complex 3 results from B−C bond cleavage within the starting phosphinoborane 2, and formation of the corresponding [Ru(η2:η2-H2B−NiPr2)] linkage. When monitoring the reaction of [RuH2(η2-H2)2(PCy3)2] (1) with the phosphinoborane 2, no agostic or bis(agostic) borane intermediate complex, such as A (Figure 3), could be observed, even at low temperature. This contrasts with the situation found in the case of the phosphinoborane o-Ph2P(C6H4−CH2)B(H)NiPr2, bear-

Figure 2. X-ray structure of 3. The hydrogen atoms not associated with the metal are omitted for clarity.

environment with the two phosphorus atoms in a pseudo axial position, bent away from the diisopropylaminoborane ligand. The equatorial plane is occupied by three hydrides and one coplanar carbon atom from the phenyl ligand cis to the terminal hydride Hy1 and bisecting the plane containing the P2, Ru1, and Hy3 atoms. The Ru1−C9 distance (2.108(2) Å) is in the range for Ru−C single bond lengths found in arylruthenium complexes.8,9 The B−H bond lengths, as well as the Ru1···B1, Ru1···Hy2, and Ru1···Hy3 distances (Table 1), are

Figure 3. Postulated intermediates in the reaction leading to 3. 7158

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ing the same spacer, o-C6H4−CH2, but reversely connected to P and B atoms. In the latter case, the corresponding bis-agostic complex (Figure 1) was isolated in high yield.6 The postulated agostic complexes B and/or C (Figure 3) likely to give 3 by B−C bond cleavage and ligand rearrangement were never detected.4,5 Calculations indicated that substitution of the two H2 ligands in 1 by ligand 2 affords complex B in an exoergic transformation with ΔG = −2.5 kcal mol−1.12 This complex features an agostic B−H bond that is elongated with respect to the free ligand (1.304 Å, B; 1.208 Å, 2). Dissociation of PCy3 from B to generate, after isomerization, complex C is computed to be an uphill process with ΔG = 5.9 kcal mol−1. In addition, no bis-agostic complex similar to A could be located computationally. Another 16-electron complex, C′, could be located after dissociation of PCy3 from B with ΔG = 6.0 kcal mol−1 with respect to B. This complex C′ features a significant reduction of the Ru−Cipso distance with the phenyl ring of the spacer (3.426 Å, B; 2.360 Å, C′). Also, the coordinated B−H bond is lying in the equatorial plane (defined by Ru and the two hydrides) in B, whereas, in C′, it is lying perpendicular to the equatorial plane. The new interaction between the phenyl ring of the spacer and the ruthenium center leads also to an elongation of the B−Cipso bond (1.587 Å, B; 1.605 Å, C′). Therefore, the geometry of C′ is adapted to B−C bond cleavage and the transition state TS-C′-D associated with this transformation has been located at Δ⧧G = 4.5 kcal mol−1 above C′. The product of the B−C bond cleavage, D, is computed to be more stable than C′ by ΔG = −1.4 kcal mol−1. D is an isomer of 3 with PPh2 coordinated in the equatorial plane defined by the hydride and the bis-σ adduct of H2BNiPr2 and the metalated phenyl ring in the axial position trans to PCy3. Decoordination of PPh2 and isomerization would allow the formation of complex 3 that is computed to be ΔG = −11.3 kcal mol−1 more stable than D. Figure 4 shows an energetic diagram of the various transformations from B to 3. The activation barrier from B to TS-C′-D, associated with B−C bond cleavage, is sufficiently low (Δ⧧G = 10.5 kcal mol−1) to afford easy reaction at room temperature. The reaction is computed to be exergonic by ΔG = −6.7 kcal mol−1, in

agreement with the experimental observations where 3 is formed quantitatively at room temperature and B is not observed. Pressurization of a cold (−73 °C) toluene-d8 solution of 3 with dihydrogen (3 bar) leads quantitatively to the formation of 4 that was fully characterized by multinuclear NMR spectroscopy as a dihydride complex featuring a diphenyl(benzyl)phosphine (Scheme 2, Supporting Information, Figures S6− S11). Scheme 2. Reactivity of 3 toward H2 and HBArF4

The pathway between 3 and 4 has been probed computationally, and two limiting cases have been considered: (i) C−H reductive elimination from 3 to form a Ru(0) intermediate, followed by H 2 oxidative addition to give 4; (ii) H 2 coordination to Ru, followed by formation of 4 along a σCAM pathway.13 Figure 4 (red pathway) indicates that the C− H reductive elimination in 3 to form E through TS-3-E is associated with a rather high activation barrier of Δ⧧G = 32.5 kcal mol−1. In TS-3-E, the new C−H bond is still long at 1.26 Å, whereas the two coordinated B−H bonds are now different: B−H = 1.28 Å cisoid to C−H and B−H = 1.379 Å transoid to C−H. The product of the reaction, E, is less stable than 3 by ΔG = 29.2 kcal mol−1. E is best described as a square-based Ru(II) complex with apical PCy3 and, within the basal plane, hydride trans to C−H on one hand, and α-agostic boryl trans to PPh2 on the other hand. Another pathway (in blue in Figure 4) has been found on the potential energy surface. Decoordination of one B−H bond and coordination of H2 to Ru generate complex F that is less stable than 3 by ΔG = 16.5 kcal mol−1. C−H bond formation from F is effective through TS-F-G with concomitant H−H bond cleavage in a typical σ-CAM mechanism. The H2 ligand, which is initially perpendicular to the equatorial plane in F, rotates in the equatorial plane in TSF-G, and one hydrogen atom is transferred to B, while the other one is transferred to the Ru−C bond. The formation of the new B−H bond is accompanied by the cleavage of the coordinated B−H bond initially present in F, thus generating two cis-hydrides in G. The activation barrier for this process is very low with Δ⧧G = 11.5 kcal mol−1, and the product of the reaction, G, is computed to be less stable than F by ΔG = +2.6 kcal mol−1. This complex features two hydrides in the equatorial plane, one coordinated trans to a σ(B−H) bond

Figure 4. Schematic energy diagram (kcal mol−1) of the various complexes studied computationally. 7159

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and the other one trans to an agostic C−H bond. Complex 4, which results from loss of the agostic C−H bond and formation of a typical bis-σ(B−H) adduct, is computed to be more stable than G by ΔG = −15.8 kcal mol−1. Overall, with respect to 3, TS-F-G is computed to be 4.5 kcal mol−1 lower in energy than TS-3-E, and therefore, formation of 4 is likely to occur through coordination of H2, followed by σ-CAM formation of the C−H bond and the dihydride moiety. The product 4 is computed to be less stable than 3 by 3.3 kcal mol−1, but this energy difference reduces to ΔG = 0.5 kcal mol−1 when the temperature 225 K is considered to estimate the Gibbs correction. Moreover, increasing the H2 pressure would certainly drive the equilibrium toward formation of 4 at the expense of 3. Protonation of 4 with HBArF4 in diethyl ether, at room temperature, results in the loss of diisopropylaminoborane and formation of the tethered chiral-at-ruthenium piano-stool cationic complex 5 as a racemic mixture.14 The constrainedgeometry of complex 5 was unambiguously deduced from multinuclear NMR spectroscopy and mass spectrum analysis. The 31P{1H} NMR spectrum of 5 in THF-d8 displays an AX pattern at δ 65.37 and −50.87 with a JPP coupling constant of 38 Hz, in agreement with two different phosphorus coordinated in a three-legged piano-stool configuration15 (Supporting Information, Figure S12). The highly shielded phosphorus resonance is indicative of a strained coordination pattern of the diphenylphosphine moiety due to the geometrical constraint imposed by the sole methylene group. In the 1H NMR, the terminal hydride resonates at δ −8.47 as a doublet of doublets (2JPH = 28.0 and 40.8 Hz) that collapses into a singlet upon phosphorus decoupling (Supporting Information, Figure S13). Moreover, the spectrum exhibits five characteristic shielded signals for the coordinated arene ring as well as two doublet of doublets for the diastereotopic hydrogens of the methylene group, each becoming a doublet upon phosphorus decoupling (2JPH = 14.4 Hz and 2JHH = 9.2 Hz) (Supporting Information, Figure S14). Direct protonation of 3 also led predominantly to the formation of 5 contaminated with the symmetrical ruthenium complex [RuH2(η2:η2-H2B−NiPr2)(PCy3)2]10 in a respective 4:1 ratio, as determined by 31P NMR integration experiments (Supporting Information, Figure S15).

this case, the integrity of the ligand is not retained and B−C bond cleavage is observed. The resulting five-membered ring η1-aryl-ruthenium complex 3 achieves an 18-electron configuration through the establishment of a bis-σ borane ligation of diisopropylaminoborane. The reason for this preferential B−C bond activation is currently examined. Pressurization of 3 with dihydrogen resulted selectively in Ru−C bond cleavage and formation of the dihydride ruthenium complex 4, the bis(σ-B− H) linkage being preserved. Subsequent reaction with HBArF4 as proton source promotes η6-coordination of the pendant benzyl group at the expense of the bis(σ-B−H) aminoborane. The resulting tethered η6-arene-κ-P ruthenium hydride cationic complex 5 displaying a constrained geometry could also be obtained directly from 3 and HBArF4. Efforts are currently underway to explore the reactivity of these polyfunctional aminoboranes to access a variety of organometallic architectures and coordination modes.

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EXPERIMENTAL SECTION

General Procedures. All experiments were performed under an atmosphere of dry argon using standard Schlenk and glovebox techniques. Unless stated, all chemicals were purchased from Aldrich and used without further purification. [RuH2(η2-H2)2(PCy3)2]16 was prepared according to the literature procedure. The solvents were purified and dried through an activated alumina purification system (MBraun SPS-800). NMR solvents were dried using appropriate methods and degassed prior to use. NMR samples of sensitive compounds were prepared under an argon atmosphere. Nuclear magnetic resonance spectra were recorded on Bruker AV 300, 400, or 500 spectrometers operating at 300.13, 400.13, or 500.33 MHz, respectively, for 1H; 121.5, 161.97, or 202.5 MHz, respectively, for 31P; 75.48, 100.62, or 125.81 MHz, respectively, for 13C; 96.29, 128.38, or 160.52 MHz, respectively, for 11B. 1H and 13C chemical shifts are reported in ppm referenced internally to residual protio-solvent, while 31 P are relative to 85% H3PO4, and 11B are relative to BF3.OEt2 external references. Chemical shifts are quoted in δ (ppm) and coupling constants in hertz. The following abbreviations are used: br, broad; s, singlet; d, doublet; t, triplet; m, multiplet; ψ, pseudo. Infrared spectra were recorded on a PerkinElmer Spectrum 100 FT-IR spectrometer fitted with ATR accessories. The crystal in the ATR module is Ge. High-resolution mass spectra were obtained at the ICT Mass Spectrometry Service at the Université Paul Sabatier (Toulouse) using a Waters Xevo G2 QTof spectrometer using electrospray ionization. X-ray Crystallographic Studies. Data for compound 2 were collected at low temperature (106 K) on a Gemini Agilent diffractometer using a graphite-monochromated Mo−Kα radiation (λ = 0.71073 Å) and equipped with an Oxford Instrument Cooler Device. All hydrogen atoms were placed geometrically and refined by using a riding model, except for H100, which was located by Fourier differences and isotropically refined. Data for complex 3 were collected at low temperature (100 K) on a Bruker Kappa Apex II diffractometer using a graphite-monochromated Mo−Kα radiation (λ = 0.71073 Å) and equipped with an Oxford Cryosystems Cryostream Cooler Device. The structures have been solved by Direct Methods using SIR92,17 and refined by means of least-squares procedures on an F2 with the aid of the program SHELXL9718 included in the software package WinGX, version 1.63.19 The Atomic Scattering Factors were taken from International Tables for X-ray Crystallography.20 All hydrogen atoms were placed geometrically and refined by using a riding model, except for the hydrides Hy1, Hy2, and Hy3 of 3, which were located by Fourier differences and isotropically refined. All non-hydrogen atoms were anisotropically refined, and in the last cycles of refinement, a weighting scheme was used, where weights are calculated from the following formula: w = 1/[σ2(Fo2) + (aP)2 + bP], where P = (Fo2 + 2Fc2)/3. Drawing of molecules was performed with the program

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CONCLUSION A few years ago, we launched a program aimed at developing a new class of Ph2P∼BH(NiPr2) phosphinoborane compounds. To evaluate the influence of the spacer on the B−H activation bond, we focused on their reaction with the bis(dihydrogen) ruthenium complex 1. We first disclosed a straightforward entry route to the synthesis of agostic complexes displaying a 3center-2-electron Ru−H−Bsp2 bond, and in which the integrity of the starting ligand was retained. With an o-phenylmethylene spacer displaying the boryl group in the benzylic position, the “noninnocent” character of the spacer was very recently illustrated. Stabilization of the ruthenium center requires an additional 3-center-2-electron interaction from an adjacent C− H bond to the boron, leading to the isolation of the first bisagostic η2-C−H/η2-B−H ruthenium complex (Figure 1). Mirroring this study by swapping the positions of the phosphino and the boryl groups at the o-phenylmethylene spacer, we have demonstrated that the reaction of 1 with o-iPr2N(H)B−C6H4−CH2-PPh2 (2) strongly differs with no sign of any agostic σ-B−H species, even as an intermediate. In 7160

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with BH3·THF (1 M in THF) at −50 °C for 1 h. Cy3P·BH3 incompletely precipitated and was filtered off. Pure 3 could be, however, obtained from a pentane solution (3 mL) containing 131.0 mg of the brown solid. After filtration on a 0.45 μm PTFE filter that was rinsed with additional pentane (1 mL), the resulting pentane solution was kept at −37 °C for 11 days, affording yellow crystals of pure 3 (25.7 mg). 1H NMR (C7D8, 193 K, 500.33 MHz): δ −10.33 (dd, 2JHPCy3 = 14.5 Hz, 2JHPPh2 = 29.0 Hz, 1H, RuH), −8.54 (br, 1H, BH), −4.67 (br, 1H, BH), 0.60 (br, 6H, CH3 iPr), 0.91 (br, 3H, CH3 i Pr), 1.08 (br, 3H, CH3 iPr), 0.82−2.90 (m, 33H, PCy3), 2.76 (br, 1H, CH iPr), 2.97 (br, 1H, CH iPr), 3.79 (m, 2H, CH2P), 6.82−6.90 (m, 3H, CHar), 7.22−7.32 (m, 4H, CHar), 7.36 (ψt, 3JHHapp= 7.5 Hz, 2H, CH PPh2), 7.45 (d, 3JHH= 6.5 Hz, 1H, CH C6H4), 8.03 (ψt, 3JHHapp= 8.0 Hz, 2H, CH PPh2), 8.34 (d, 3JHH= 5.5 Hz, 1H, CH C6H4). One aromatic CH is hidden by the solvent and could not be observed. 31P {1H} NMR (C7D8, 193 K, 202.55 MHz): δ 60.2 (d, 2JPP = 246 Hz, PCy3), 72.4 (d, 2JPP = 246 Hz, PPh2). 11B{1H} NMR (C7D8, 193 K, 160.53 MHz): δ 41 (br). 13C{1H} NMR (C7D8, 193 K, 125.81 MHz): δ 21.86, 22.59, 24.42, 24.93 (br, CH3 iPr), 27.04, 28.64, 31.92 (br, CH2, Cy), 40.83 (br, CH Cy), 45.59, 49.60 (br, CH iPr), 51.47 (d, 1JCP = 39.0 Hz, CH2P), 121.12 (s, CH C6H4), 122.93 (s, CH C6H4), 123.14 (d, 3JCP = 16.4 Hz, CH C6H4), 129.72 (s, p-CH PPh2), 130.76 (d, 2JCP = 8.8 Hz, CH PPh2), 134.95 (d, 2JCP = 10.1 Hz, CH PPh2), 139.18 (d, 1JCP = 44.8 Hz, CIV PPh2), 142.57 (d, 4JCP = 3.8 Hz, CH C6H4), 144.2 (d, 1JCP = 21.9 Hz, CIV PPh2), 149.1 (d, 2JCP = 16.4 Hz, CIV C6H4), 173.1 (ψt, JCP = 6.7 Hz, CIV−Ru C6H4). Interference between the sample and the solvent prevents the observation of some aromatic CH carbon atoms from the PPh2 group. RuH2(η2:η2-H2B−NiPr2)(PPh2Bz))(PCy3) (4). A 6.7 mg portion of 3 was introduced in a pressurizable NMR tube and dissolved in C7D8 to give a yellow solution. The tube was first immersed in a liquid nitrogen/ethanol cooling bath (−73 °C) and second pressurized with dihydrogen to 3 bar for 17 h, while the cooling bath was allowed to reach room temperature. 4 was generated quantitatively. After evaporation of the solvent, complex 4 was stored neat at −37 °C to avoid any rearrangement. 31P{1H} NMR (C7D8, 298 K, 161.98 MHz): δ 58.76 (d, 2JPP= 232.3 Hz, PPh2), 76.43 (d, 2JPP = 232.3 Hz, PCy3).11B{1H} (C7D8, 298 K, 128.38 MHz): δ 46 (br), 1H NMR (C7D8, 298 K, 400.13 MHz): δ −11.99 (ddd, 2H, 2JHP1 = 2JHP2 = 24.8 Hz, 2JHH = 6.0 Hz, 2H, RuH), −6.68 (br, 2H, BH), 0.94 (d, 3JHH = 6.8 Hz, 6H, CH3 iPr), 1.26 (d, 3JHH = 6.8 Hz, 6H, CH3 iPr), 0.87−2.18 (m, 33H, PCy3), 3.09 (h, 3JHH = 6.8 Hz, 2H, CH iPr), 3.75 (d, 3JPH = 8.8 Hz, 2H, CH2P), 4.39 (H2), 6.97−7.11 (m, 2H, CHar), 7.81 (Ψt, Japp = 8.4 Hz, 3H, CHar). 13C{1H} NMR (C6D6, 298 K, 100.62 MHz): δ 24.10 (s, CH3 iPr), 27.31, 28.32, 30.89 (CH2 Cy), 38.64 (m, CH Cy), 46.00 (d, 1JPC = 22.0 Hz, CH2P), 48.29 (s, CH iPr), 125.87 (d, JPC = 2.7 Hz, CHar Bz), 127.32 (d, JPC = 2.1 Hz, CHar Bz), 127.40 (d, JPC = 8.6 Hz, CHar Ph), 128.32 (s, CHar Ph) 131.48 (d, JPC = 4.2 Hz, CHar Bz), 133.88 (d, JPC = 11.4 Hz, CHar Ph), 137.32 (d, 2JPC = 2.4 Hz, CIV Bz), 143.90 (dd, 1JPC = 32.7 Hz, 3JPC = 1.8 Hz CIV Ph). [RuH(η6-C6H5-κ1-P-C6H5−CH2−PPh2)(PCy3)]BArF4 (5). a. At Room Temperature. HBArF4·2.75Et2O (14.7 mg, 13.8 μmol, 0.8 equiv) was added to a diethyl ether solution (1 mL) of pure 3 (13.5 mg, 17.5 μmol). The solution turned orange after 20 min. The solvent was then removed under vacuum, and the resulting oil was solubilized in THF-d8. After 17 h at room temperature, the solvent was evaporated and pentane was added to the obtained oil. After removal of the supernatant, the remaining oil was dried under vacuum to give 5 as an impure brown powder. b. At Room Temperature. HBArF4·2.23Et2O (22.3 mg, 22 μmol) was added to an ethereal solution (1 mL) of pure 4 (19.3 mg, 25 μmol), and the resulting solution was stirred for 17 h. The solvent was evaporated, and a small amount of pentane was added to the obtained oil. After stirring, the oil was allowed to settle, the supernatant was removed, and the oil was dried under vacuum to give complex 5 as an impure brown powder. 1 H NMR (THF-d8, 298 K, 400.13 MHz): δ −8.47 ppm (dd, 2JPH = 28.0 Hz, 2JPH = 40.8 Hz, 1H, RuH), 0.7−2.2 (m, 33H, Cy), 3.90 (dd, 2 JPH = 14.4 Hz, 2JHH = 9.2 Hz, 1H, CHHP), 4.56 (dd, 2JPH = 14.4 Hz, 2 JHH = 9.2 Hz 1H, CHHP), 5.21 (t, 3JHH = 6.0 Hz, 1H, CHar CH2Ph),

ORTEP3221 with 30% probability displacement ellipsoids for nonhydrogen atoms. o- iPr 2 NB(H)−C 6H 4−CH 2−PPh 2 (2). a. o-Br-C6 H 4 -CH 2PPh2 . Grignard reagent o-Br−C6H4−CH2MgBr was prepared following a modified literature procedure. 22 Under an inert atmosphere, magnesium chips were covered with diethyl ether (20 mL) and activated with iodine and 1,2-dibromoethane. A solution of obromobenzylbromide (5.64 g, 22.55 mmol) in 20 mL of diethyl ether was added dropwise to activated magnesium. After addition of half the volume, the solution of o-bromobenzylbromide was diluted with another 20 mL of diethyl ether, and this new solution was added slowly to the reaction mixture, which was refluxed for 20 min before being allowed to cool down to room temperature. The Grignard solution was carefully added to a solution of diphenylchlorophosphine (4.05 mL, 22.55 mmol) in 10 mL of diethyl ether under vigorous stirring. The resulting suspension was refluxed overnight. Once at room temperature, the mixture was filtered and the filtrate was dried under vacuum. The obtained yellow oil was coevaporated with pentane. Pentane was added, and the resulting suspension was filtered, leading to a white solid with 56% yield. 31P {1H} NMR (THF-d8, 298 K, 161.98 MHz): δ −12.77 (s). 1H{31P} NMR (THF-d8, 298 K, 400.13 MHz): δ 3.5 (s, 2H, CH2), 6.83−6.85 (m, 1H, CHar), 6.97− 7.04 (m, 2H, CHar), 7.29−7.31 (m, 6H, CHar), 7.39−7.43 (m, 4H, CHar), 7.49−7.52 (m, 1H, CHar). 13C{1H} NMR (THF-d8, 298 K, 100.61 MHz): δ 36.86 (d, 1JPC = 17.8 Hz, CH2), 125.49 (d, 3JPC = 4.7 Hz, CIV−Br), 127.66 (d, 4JPC = 1.3 Hz, CHar), 128.23 (d, 5JPC = 2.8 Hz, CHar), 128.94 (d, 3JPC = 6.4 Hz, CHar Ph), 129.30 (s, CHar Ph), 131.78 (d, 3JPC = 8.3 Hz, CHar), 133.38 (d, 4JPC = 1.4 Hz, CHar), 133.70 (d, 2JPC = 19.3 Hz, CHar Ph), 138.22 (d, 2JPC = 8.1 Hz, CIVCH2P), 139.14 (d, 1JPC = 17.3 Hz, CIV Ph). b. o-iPr2NB(H)−C6H4−CH2PPh2 (2). o-Li-C6H4−CH2PPh2 derived from o-Br−C6H4−CH2PPh2 (504.0 mg, 1.42 mmol) was prepared according a reported literature procedure.23 An ethereal solution (5 mL) of iPr2NBHCl24 (571.8 mg, 3.88 mmol) was added to a cooled (−37 °C) solution of the lithium reagent and stirred for 45 min at the same temperature. The solution was allowed to warm up to room temperature and kept under stirring for 17h. The solvent was then removed under vacuum, and the resulting oil was dissolved in 20 mL of toluene. After filtration over activated Celite, the filtrate was evaporated to dryness and coevaporated with 20 mL of pentane, three times. The resulting oil was purified by Kügelrhor distillation (89.3% yield). 31P {1H} NMR (THF-d8, 298 K, 161.99 MHz): δ −9.88 (s). 1H NMR (THF-d8, 298 K, 400.13 MHz): δ 1.18 (d, 3JHH = 6.7 Hz, 6H, CH3), 1.33 (d, 3JHH = 6.7 Hz, 6H, CH3), 3.40−3.48 (m, 3H, CH2 + CH), 4.09 (hept, 3JHH = 6.8 Hz, 1H, CH), 5.34 (br, 1H, BH), 6.74 (d, 3 JHH = 7.6 Hz, 1H, CHar), 6.92 (td, 3JHH = 7.2 Hz, 4JHH = 1.2, 1H, CHar), 7.01 (t,3JHH = 7.4 Hz, 1H, CHar), 7.14 (d, 3JHH = 7.2 Hz, 1H, CHar), 7.39−7.26 (m, 10H, CHar). 11B{1H} NMR (THF-d8, 298 K, 128.38 MHz): δ 38 (s). 13C{1H} NMR (THF-d8, 298 K, 100.62 MHz): δ 21.94 (d, CH3 iPr), 27.24 (s, CH3 iPr), 37.23 (d, 1JPC = 15.7 Hz, CH2), 45.07 (s, CH iPr), 50.82 (s, CH iPr), 125.50 (d, 5JPC = 2.8 Hz, CHar C6H4), 127.57 (d, 4JPC = 1.4 Hz, CHar C6H4), 128.79 (d, 3JPC = 6.4 Hz, CHar PPh2), 129.00 (s, CHar PPh2), 129.75 (d, 3JPC = 6.7 Hz, CHar C6H4), 131.21 (d, 4JPC = 1.9 Hz, CHar C6H4), 133.51 (d, 2JPC = 18.7 Hz, CHar PPh2), 139.78 (d, 1JPC = 17.3 Hz, CIV PPh2), 141.43 (d, 2 JPC = 8.7 Hz, CIV C6H4), 142.8 (br, CIV−B). IR (neat): ν = 2470 cm−1 (br, BH). HRMS ESI+ [M + H+] calcd. for C25H32BNP+: m/z 387.2402. Found 387.2395 (1.8 ppm), exact agreement between the experimental and theoretical isotopic peak distributions; the accurate mass is measured and calculated on the monoisotopic peak. RuH(η2:η2-H2B−NiPr2)(κ2-C,P-C6H4−CH2−PPh2)(PCy3) (3). A pentane solution of Ph2PCH2C6H4BH(NiPr2) (207.9 mg, 0.537 pentane was added to a pentane (3 mL) suspension of RuH2(η2H2)2(PCy3)2 (358.0 mg, 0.536 mmol) at room temperature. After stirring for 1 min, the solvent was removed under vacuum and the solid was cooled at −55 °C before addition of pentane (3 mL). The resulting suspension was filtered, and the filtrate was dried under vacuum. A brown solid containing complex 3 and PCy3 in a 1:1 integration ratio, in 66% overall yield, was obtained. PCy3 could be partially removed from the mixture by reaction of the crude filtrate 7161

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Organometallics

Article

6.10 (d, 3JHH = 5.6 Hz, 1H, CHar CH2Ph), 6.79 (t, 3JHH = 5.6 Hz, 1H, CHar CH2Ph), 6.96 (t, 3JHH = 6.0 Hz, 1H, CHar CH2Ph), 7.33 (d, 3JHH = 6.0 Hz, 1H, CHar CH2Ph), 7.40−7.65 (m, 8H, CHar PPh2), 7.57 (br, 4H, p-CHar BArF4), 7.78 (br, 8H, o-CHar BAr4F), 7.96 (dd, JPH = 7.2 Hz, JHH = 13.2 Hz, 2H, CHar PPh2). 31P NMR (THF-d8, 298 K, 400.13 MHz): δ −50.87 (d, 2JPP = 37.9 Hz, PPh2), 65.37 (d, 2JPP = 37.9 Hz, PCy3). 13C{1H} NMR (THF-d8, 273 K, 125.81 MHz): δ 27.05, 28.05, 28.18, 30.60, 30.71 (m, CH2 PCy3), 38.94 (m, CH PCy3), 42.43 (d, 1 JPC = 29.8 Hz, CH2P), 85.20 (d, 3JPC = 9.8 Hz, o-CHar CH2Ph), 86.60 (d, 3JPC = 8.8 Hz, p-CHar CH2Ph), 88.65 (d, 2JPC = 6.3 HZ, CIV-CH2), 94.95 (s, o-CHar CH2Ph), 98.16 (s, m-CHar CH2Ph), 103.56 (s, mCHar CH2Ph), 118.21 (m, p-CHar BArF4), 125.48 (q, 1JCF = 272.3 Hz, CF3 BArF4), 129.67 (d, JPC = 11.4 Hz, CHar PPh2), 129.78 (d, JPC = 10.8 Hz, CHar PPh2), 129.99 (Ψq, 1JCF = 2.9 Hz, CIV-CF3 BArF4), 131.53 (s, CHar PPh2), 132.58 (s, CHar PPh2), 133.12 (d, JPC = 10.4 Hz, CHar PPh2), 134.90 (br, CIV-PPh2), 135.54 (br s, o-CHar BArF4), 135.89 (d, JPC = 13.2 Hz, CHar PPh2), 162.80 (q, 1JBC = 49.8 Hz, ipsoCIV BArF4). Overlapping of signals prevents the observation of one CIV aromatic carbon from a PPh2 phenyl group. HRMS ESI± [M+] calcd. for C37H51P2Ru+: m/z 659.2509. Found 659.2515 (0.9 ppm). [M−] calcd. for C32H12BF24−: m/z 862.0685. Found 862.0687 (0.2 ppm), exact agreement between the experimental and theoretical isotopic peak distributions; the accurate mass is measured and calculated on the monoisotopic peak. Computational Details. Geometry optimizations have been performed with the Gaussian 0925 package at the B3PW9126,27 level of hybrid density functional theory with the D3(bj) dispersion correction28,29 included in the optimization process. The ruthenium atom was represented by the relativistic effective core potential (RECP) from the Stuttgart group and the associated basis sets,30 augmented by an f polarization function.31 The remaining atoms (P, C, H, N, B) were represented by an SVP basis set.32 Better energies were obtained using the PBE0 functional33 and the ORCA software,34 and describing all the atoms but Ru by the def2-tzvp basis set.35 For Ru, the SDD pseudo-potential and the associated basis set were kept. All energies reported in the present work are Gibbs free energies obtained by adding to the PBE0 electronic energy obtained by ORCA, the gasphase Gibbs contribution at 298 K (B3PW91+D3 optimizations), and the PBE0 D3(bj) correction.

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(2) Braunschweig, H.; Dewhurst, R. D.; Schneider, A. Chem. Rev. 2010, 110, 3924−3957. (3) Braunschweig, H.; Dirk, R.; Ganter, B. J. Organomet. Chem. 1997, 545−546, 257−266. (4) Gloaguen, Y.; Alcaraz, G.; Pécharman, A.-F.; Clot, E.; Vendier, L.; Sabo-Etienne, S. Angew. Chem., Int. Ed. 2009, 48, 2964−2968. (5) Gloaguen, Y.; Alcaraz, G.; Petit, A. S.; Clot, E.; Coppel, Y.; Vendier, L.; Sabo-Etienne, S. J. Am. Chem. Soc. 2011, 133, 17232− 17238. (6) Cassen, A.; Gloaguen, Y.; Vendier, L.; Duhayon, C.; PobladorBahamonde, A.; Raynaud, C.; Clot, E.; Alcaraz, G.; Sabo-Etienne, S. Angew. Chem., Int. Ed. 2014, 53, 7569−73. (7) For the first example of an agostic B−H bond, see: Braunschweig, H.; Dewhurst, R. D.; Herbst, T.; Radacki, K. Angew. Chem., Int. Ed. 2008, 47, 5978−5980. (8) Cordero, B.; Gomez, V.; Platero-Prats, A. E.; Reves, M.; Echeverria, J.; Cremades, E.; Barragan, F.; Alvarez, S. Dalton Trans. 2008, 2832−2838. (9) Jazzar, R. F. R.; Mahon, M. F.; Whittlesey, M. K. Organometallics 2001, 20, 3745−3751. (10) Alcaraz, G.; Chaplin, A. B.; Stevens, C. J.; Clot, E.; Vendier, L.; Weller, A. S.; Sabo-Etienne, S. Organometallics 2010, 29, 5591−5595. (11) Bénac-Lestrille, G.; Helmstedt, U.; Vendier, L.; Alcaraz, G.; Clot, E.; Sabo-Etienne, S. Inorg. Chem. 2011, 50, 11039−11045. (12) All the optimized structures discussed in this manuscript are available in a single .xyz file in the Supporting Information that allows one to view the geometries. (13) Perutz, R. N.; Sabo-Etienne, S. Angew. Chem., Int. Ed. 2007, 46, 2578−2592. (14) Kumar, P.; Gupta, R. K.; Pandey, D. S. Chem. Soc. Rev. 2014, 43, 707−733. (15) Jung, S.; Ilg, K.; Brandt, C. D.; Wolf, J.; Werner, H. Dalton Trans. 2002, 318−327. (16) Borowski, A. F.; Sabo-Etienne, S.; Christ, M. L.; Donnadieu, B.; Chaudret, B. Organometallics 1996, 15, 1427−1434. (17) Altomare, A.; Cascarano, G.; Giacovazzo, C.; Guagliardi, A. J. Appl. Crystallogr. 1993, 26, 343−350. (18) Sheldrick, G. M. SHELX97 [Includes SHELXS97, SHELXL97, CIFTAB]: Programs for Crystal Structure Analysis, (Release 97-2); Institü t fü r Anorganische Chemie der Universität: Gö ttingen, Germany, 1998. (19) Farrugia, L. J. J. Appl. Crystallogr. 1999, 32, 837−838. (20) INTERNATIONAL Tables for X-ray Crystallography; Kynoch Press: Birmingham, England, 1974; Vol. IV. (21) Farrugia, L. J. J. Appl. Crystallogr. 1997, 30, 565−565. (22) Rogers, H. R.; Hill, C. L.; Fujiwara, Y.; Rogers, R. J.; Mitchell, H. L.; Whitesides, G. M. J. Am. Chem. Soc. 1980, 102, 217−226. (23) Gossage, R. A.; McLennan, G. D.; Stobart, S. R. Inorg. Chem. 1996, 35, 1729−1732. (24) Maringgele, W.; Noltemeyer, M.; Schmidt, H.-G.; Meiler, A. Main Group Met. Chem. 1999, 22, 715−732. (25) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G. A.; Nakatsuji, H.; Caricato, M.; Li, X.; Hratchian, H. P.; Izmaylov, A. F.; Bloino, J.; Zheng, G.; Sonnenberg, J. L.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Vreven, T.; Montgomery, J. A., Jr.; Peralta, J. E.; Ogliaro, F.; Bearpark, M.; Heyd, J. J.; Brothers, E.; Kudin, K. N.; Staroverov, V. N.; Kobayashi, R.; Normand, J.; Raghavachari, K.; Rendell, A.; Burant, J. C.; Iyengar, S. S.; Tomasi, J.; Cossi, M.; Rega, N.; Millam, J. M.; Klene, M.; Knox, J. E.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Martin, R. L.; Morokuma, K.; Zakrzewski, V. G.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Dapprich, S.; Daniels, A. D.; Farkas, Ö .; Foresman, J. B.; Ortiz, J. V.; Cioslowski, J.; Fox, D. J. Gaussian 09, Revision D1; Gaussian, Inc.: Wallingford, CT, 2009. (26) Becke, A. D. J. Chem. Phys. 1993, 98, 5648−5652.

ASSOCIATED CONTENT

S Supporting Information *

NMR data; optimized structures as a single .xyz file; energy, Gibbs correction, and D3(bj) dispersion correction of the optimized structures. This material is available free of charge via the Internet at http://pubs.acs.org. The crystal structures have been deposited at the Cambridge Crystallographic Data Centre and allocated the deposition numbers CCDC 1025073 (3) and 1025074 (2).

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (G.A.). Notes

The authors declare no competing financial interests.

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ACKNOWLEDGMENTS We thank the CNRS and the ANR program ReBAB ANR-11BS07-0015 for support. REFERENCES

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