Divalent Silicon-Assisted Activation of Dihydrogen in a Bis(N

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Divalent Silicon-Assisted Activation of Dihydrogen in a Bis(Nheterocyclic silylene)xanthene Nickel(0) Complex for Efficient Catalytic Hydrogenation of Olefins Yuwen Wang, Arseni Kostenko, Shenglai Yao, and Matthias Driess* Metalorganics and Inorganic Materials, Department of Chemistry, Technische Universität Berlin, Straße des 17. Juni 135, Sekr. C2, 10623 Berlin, Germany S Supporting Information *

ABSTRACT: The first chelating bis(N-heterocyclic silylene)xanthene ligand [SiII(Xant)SiII] as well as its Ni complexes [SiII(Xant)SiII]Ni(η2-1,3-cod) and [SiII(Xant)SiII]Ni(PMe3)2 were synthesized and fully characterized. Exposing [SiII(Xant)Si II ]Ni(η 2 -1,3-cod) to 1 bar H 2 at room temperature quantitatively generated an unexpected dinuclear hydrido Ni complex with a four-membered planar Ni2Si2 core. Exchange of the 1,3-COD ligand by PMe3 led to [SiII(Xant)SiII]Ni(PMe3)2, which could activate H2 reversibly to afford the first SiII-stabilized mononuclear dihydrido Ni complex characterized by multinuclear NMR and single-crystal X-ray diffraction analysis. [SiII(Xant)SiII]Ni(η2-1,3-cod) is a strikingly efficient precatalyst for homogeneous hydrogenation of olefins with a wide substrate scope under 1 bar H2 pressure at room temperature. DFT calculations reveal a novel mode of H2 activation, in which the SiII atoms of the [SiII(Xant)SiII] ligand are involved in the key step of H2 cleavage and hydrogen transfer to the olefin.

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INTRODUCTION Success in homogeneous transition metal catalysis greatly depends upon the development of well-designed ligand systems.1 Among the diverse ligand systems, chelating ligands are prominent owing to their significant advantages in controlling the electronic and geometric properties of metal complexes. Since the first isolation of the N-heterocyclic silylene (NHSi) iron complex by Welz and Schmid in 1977,2 NHSi ligands with strong σ-donating nature3 have fueled tremendous interest in organometallic chemistry.4 Recently, we have introduced an NHSi moiety5 into chelating ligand scaffolds, leading to the discovery of various and versatile bis(NHSi) ligands (Chart 1),6 whose transition metal complexes have shown excellent catalytic performance in borylation, hydrosilylation, and other homogeneous catalytic transformations.4 Nonetheless, thorough catalytic and mechanistic studies on the organometallic systems bearing diverse bis(NHSi) ligands are still in their infancy compared with their N-heterocyclic carbene (NHC) counterparts. Homogeneous hydrogenation of olefins, a field mainly dominated by precious metal (Rh,8 Ir,9 Ru10)-based catalysts, is one of the most powerful and atom-economical methods in organic synthesis.7 More recently, hydrogenation catalyzed by earth-abundant first-row transition metal complexes is emerging as an efficient alternative way due to their environment-friendly and cost-effective properties.11,13 While heterogeneous nickel catalysts have been widely used in hydrogenation of olefins, e.g., Raney nickel,12 studies of homogeneous hydrogenation catalyzed by nickel complexes are still rare.13 Bouwman et al. pioneered studies on the nickel-catalyzed homogeneous hydro© 2017 American Chemical Society

Chart 1. Examples of Chelating Bis(NHSi) Ligands

genation of 1-octene under high pressure (50 bar).13a−c In 2012, Hanson et al. reported a nickel hydride complex [(CyPNHPCy)NiH](BPh4) {CyPNHPCy = HN[CH2CH2P(Cy)2]2} catalyzed alkene hydrogenation under 4 bar H2 pressure at 80 °C.13d Two nickel−borane species, (MesDPBPh)Ni [MesDPBPh = MesB(oPPh2C6H4)2] and pincer-type (tBuPBPtBu)NiH, were synthesized by Peters and co-workers, both of which were successfully Received: July 10, 2017 Published: August 31, 2017 13499

DOI: 10.1021/jacs.7b07167 J. Am. Chem. Soc. 2017, 139, 13499−13506

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Journal of the American Chemical Society applied as active catalysts for olefin hydrogenation under 1 bar H2 pressure at room temperature.13e,f In 2015, Lu et al. conducted the hydrogenation of unhindered olefins with a galliumsupported bimetallic Ni(0) complex under mild conditions.13g Despite all these significant contributions, the substrate scope is still largely limited to less sterically hindered terminal and internal alkenes. Therefore, the development of new ligands for nickel-catalyzed homogeneous hydrogenation of olefins is highly desirable. Herein, we report the synthesis of a novel type of chelating bis(NHSi) ligand bearing a xanthene scaffold as well as its Ni complexes [SiII(Xant)SiII]Ni(η2-1,3-cod) 2 and [SiII(Xant)SiII]Ni(PMe3)2 3 (Scheme 1). Remarkably, H2 can be activated by 2

Scheme 3. Reversible H2 Activation by Complex 3 to Give Dihydrido Ni Complex 5

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RESULTS AND DISCUSSION Synthesis of the Bis(NHSi)xanthene 1 and Its Ni Complexes 2 and 3. The bis(NHSi) chelating ligand 1 is readily accessible in a one-pot synthesis (Scheme 1). Dilithiation of 4,5-dibromo-9,9-dimethylxanthene with 2 molar equiv of secBuLi in Et2O, followed by salt-metathesis reaction with the chlorosilylene [PhC(NtBu)2]SiCl, afforded the chelating ligand [SiII(Xant)SiII] 1 as yellow crystals in 70% isolated yields. Its molecular structure was unambiguously confirmed by NMR spectroscopy and X-ray diffraction analysis (Figure 1). The 29Si NMR spectrum of 1 shows a singlet at δ 17.3 ppm comparable to that reported for a related carborane-bridged bis(NHSi) (δ 18.9 ppm).6c The Si−Si distance of 4.316 Å reveals a suitable space for coordination of transition metals. Treatment of 1 with Ni(cod)2 (cod = 1,5-cyclooctadiene) in Et2O at RT led to the formation and isolation of unprecedented [SiII(Xant)SiII]Ni(η2-1,3-cod) (1,3-cod = 1,3-cyclooctadiene) complex 2 as dark red crystals with a coordinatively unsaturated 16-valence-electron Ni(0) center, where isomerization of 1,5cyclooctadiene occurred (Scheme 1). While monitoring the coordination process by 1H NMR spectroscopy, we could observe the rapid generation of the symmetrical species [SiII(Xant)SiII]Ni(η4-1,5-cod) and “free” 1,5-cyclooctadiene after 15 min. Four hours later, the 1H NMR spectrum of the reaction mixture exhibited a new set of unsymmetrical signals, which could be assigned to the final products [SiII(Xant)SiII]Ni(η2-1,3-cod) 2 and “free” 1,3-cyclooctadiene (Figure S8); such Ni-mediated olefin isomerizations are well-known.14 The 29Si NMR spectrum of 2 reveals a singlet at δ 61.4 ppm, which is downfield shifted compared with the precursor 1 (δ 17.3 ppm). Single crystals of 2 suitable for X-ray diffraction analysis were obtained from a concentrated Et2O solution at RT (Figure 1). The nickel center adopts a trigonal-planar geometry and is disordered due to the rapid coordination exchange between the two CC bonds (C51C52, C46C53), which also accounts for the two broad resonances of the olefin protons in the 1H NMR spectrum (Figure S5). The C51C52 distance (1.360(7) Å) in 2 is shorter than the C46C53 distance (1.415(6) Å) because of the back-donation from the Ni center to the π* orbital of the C46C53 bond. The reaction of [SiII(Xant)SiII]Ni(η2-1,3-cod) 2 with PMe3 in Et2O at RT furnished red crystals of [SiII(Xant)SiII]-Ni(PMe3)2 3 in 62% isolated yields (Scheme 1). NMR spectroscopy and X-ray diffraction analysis of the latter display a symmetric tetrahedral geometry around the nickel center (Figure 1). Activation of H2 by the Ni Complexes 2 and 3. With all these encouraging results in hand, we proceeded to examine if H2 could be activated by 2 and 3. Treatment of 2 with 1 bar H2 at room temperature afforded the unexpected diamagnetic dinuclear Ni complex 4 as yellow-brown crystals in 67% isolated yields (Scheme 2), with its molecular structure shown in Figure

Scheme 1. Synthesis of Bis(NHSi)xanthene 1 and Nickel Complexes [Si(Xant)Si]Ni(η2-1,3-cod) 2 and [Si(Xant)Si]Ni(PMe3)2 3

and 3 under very mild reaction conditions to generate the unexpected dinuclear Ni complex 4 (Scheme 2) and the first Scheme 2. H2 Activation by 2 to Give the Dinuclear Ni2H2 Complex 4

structurally characterized dihydrido Ni complex 5 (Scheme 3) stabilized by the aforementioned bis(NHSi) ligand, respectively. Complex 2 is a strikingly efficient catalyst in the hydrogenation of olefins with a broad substrate scope under smooth reaction conditions (1 bar H2, RT). Density functional theory (DFT) calculations suggest an unprecedented hydrogenation mechanism where the SiII atoms in the [SiII(Xant)SiII] ligand play a significant role in assisting the H2 cleavage, stabilizing the dihydrido Ni intermediate, and hydrogen transferring to the olefin. 13500

DOI: 10.1021/jacs.7b07167 J. Am. Chem. Soc. 2017, 139, 13499−13506

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Figure 1. Molecular structures of 1, 2, and 3. Thermal ellipsoids are drawn at the 50% probability level. H atoms and solvent molecules are omitted for clarity.

presence of η2-(Si−H) interaction.15d The 29Si{1H} NMR spectrum displays two signals at δ 114.0 and 54.9 ppm, one at lower magnetic field (δ 114.0 ppm) for a Ni−H−Si moiety akin to other dinuclear η2-(Si−H)→Ni complexes,17 and the signal at higher field (δ 54.9 ppm) stems from the silylene group. The 29Si DEPT NMR spectrum shows one resonance at δ 113.9 ppm, further confirming the existence of Si−H bonds. The in situ 1H NMR study of the H2 activation process clearly indicates that [SiII(Xant)SiII]Ni(coe) (coe = cyclooctene) was initially formed via the monohydrogenation of cod, and the following hydrogenation of the coe ligand led to the release of “free” cyclooctane and 4. Exposing [SiII(Xant)SiII]Ni(PMe3)2 3 to 1 bar H2 at room temperature in C6D6 resulted in an immediate color change from red to orange (Scheme 3). The resulting 1H, 31P, and 29Si NMR spectra revealed the formation of the new diamagnetic dihydrido Ni complex 5 together with 10% residual complex 3, free “PMe3”, and a trace amount of Ni(PMe3)4. Complex 5 regenerates 3 in 29% NMR yield upon removal of H2 by three freeze−pump− thaw cycles, consistent with a reversible H2 activation process (Figure S20). The low conversion could be attributed to the loss of the volatile PMe3 during the degassing process. The 1H NMR spectrum of 5 displays a doublet at δ −1.51 ppm (2 H, 2JPH = 15.4 Hz) with 29Si satellite signals (1JSiH = 44.2 Hz) attributed to the two Ni−H bonds. The 1JSiH coupling constant indicates a significant bonding interaction between the silicon and hydrogen atom. The doublet at δ 1.78 ppm (2JPH = 4.3 Hz) is assigned to the coordinated PMe3, while the corresponding 31P signal appears at δ −28.3 ppm. In the 29Si{1H} NMR spectrum, 5 features a doublet (δ 9.7 ppm, 2JPSi = 19.4 Hz) owing to the coupling with one 31P nucleus, which shows a high field shift relative to complex 3 (δ 70.1 ppm). A doublet at δ 9.7 ppm is also observed in the 29Si DEPT NMR spectrum, further confirming a strong interaction between the Si and H atoms. In the 1Hcoupled 29Si NMR spectrum, the resonance (δ 9.7 ppm) splits into a triplet of doublets (2JPSi = 19.1 Hz, 1JHSi = 44.1 Hz) because of coupling with two H atoms and one P atom. Since only one signal was observed for these two seemingly distinct SiII atoms, we propose fast exchange of the two SiII atoms with respect to dihydrido ligand interactions. Although the T1 (min) value was not determined in the temperature range of 183−273 K due to the freezing point of toluene-d8 (178 K), the T1 value (766 ms, 500 MHz) at 183 K was much larger than the typical values (99% 32% 65% 6% >99% >99%

a

Reaction conditions: norbornene (0.054 mmol), ferrocene (0.022 mmol, internal standard), and 2 mol % Ni catalyst in 0.45 mL C6D6 under 1 bar H2 at RT for 24 h. bConversion was monitored by 1H NMR with ferrocene as an internal standard. cHg to catalyst ratio 250:1.

was treated with 3 molar equiv of norbornene in C6D6. No reaction was observed after 12 h at RT or at 50 °C, and higher temperature (75 °C) caused decomposition of 4 instead of the formation of [SiII(Xant)SiII]Ni(η2-norbornene). In addition, under the same catalytic conditions, the hydrogenation catalyzed by complex 4 gave only 65% conversion after 24 h (Table 1, entry 3). Taken all together, it is reasonable to suggest that 4 is not an active intermediate in the [SiII(Xant)SiII]Ni(η2-1,3-cod) catalyzed hydrogenation. The usage of complex 3 as catalyst afforded low conversion (32%) presumably due to the stronger coordination of PMe3 to the Ni center compared with that of the olefin (Table 1, entry 2). When the commercially available Xantphos ligand supported Ni complex was applied as catalyst for the hydrogenation reaction, only 6% conversion was achieved after 24 h (Table 1, entry 4). For comparison, the 1:1 mixture of Ni(cod)2 and [SiII(Xant)SiII] 1 was tested as catalyst, and no decrease in catalytic ability was observed (>99% conv) under the same reaction conditions (Table 1, entry 5). Furthermore, the hydrogenation of norbornene catalyzed by 2 was unaffected in the presence of excess Hg in accordance with a homogeneously catalyzed process (Table 1, entry 6).23 Olefin Hydrogenation Catalyzed by the Ni Complex 2. Inspired by the efficient hydrogenation of norbornene catalyzed by 2, we expanded the substrate scope to a variety of olefins (Scheme 4). Unless otherwise noted, all of the hydrogenation reactions were conducted in a sealed J. Young NMR tube. Under the optimized reaction conditions (2 mol % catalyst, 1 bar H2 in C6D6 at RT), terminal alkenes were readily reduced to the corresponding alkanes in quantitative yield (7a−7f), and isomerizations of these alkenes were observed during the hydrogenation process. Complete hydrogenation of internal alkenes such as 2-hexene 6g and cyclic substrates 6h−6k could be achieved, although longer time was required for more sterically hindered olefins (6j). For the bulky tetrasubstituted olefin tetramethylethylene 6l, only 11.5% conversion could be reached after 12 h at RT. The olefins containing one (6m−6r) or two (6s, 6t) phenyl groups were quantitatively hydrogenated under the same conditions. The hydrogenation of 1,1-diphenylethylene 6s was conducted on a larger scale (0.27 mmol) under the standard conditions, affording pure 7s in 91% isolated yields only after filtration. Functional groups such as methoxy (6n), trifluoromethyl (6o), and cyano (6p) were well tolerated, although the cyano group may interact with the Ni active site of the catalyst more strongly, resulting in a much larger reaction time (110 h).

Figure 3. Molecular structure of 5. Thermal ellipsoids are drawn at the 50% probability level. H atoms (except H1 and H5) are omitted for clarity.

crystallizes in the space group P21/c with the nickel(II) site adopting a distorted square pyramidal coordination geometry. Two H atoms at the Ni center had been located in the electron density map and refined isotropically with a tilt toward Si2. The Ni−H distances (1.57(8) and 1.62(6) Å) lie in the typical hydrido nickel bond range (1.30−1.70 Å),21 while Ni−Si distances (2.1469(8), 2.1666(8) Å) are slightly shorter than those observed in [SiII(Xant)SiII]Ni(η2-1,3-cod) (2.1998(16), 2.2108(18) Å) and [SiII(Xant)SiII]Ni(PMe3)2 (2.2134(4), 2.2254(4) Å) probably due to the higher oxidation state of Ni in 5. Based on these evidence, 5 can be described as a SiIIstabilized dihydrido Ni complex. DFT calculations at the M06L/ SDD-def2/SVP level of theory show that the reaction of 3 with H2 to form complex 5 and “free” PMe3 is energy favorable by 4.0 kcal mol−1. According to previous reports, mononuclear dihydrido Ni complexes, usually considered as the initial intermediate of homolytic H2 cleavage, are highly unstable, which can only be observed at low temperature or in solutions with the assistance of a boron atom.13e,22 As far as we know, complex 5 is the first structurally characterized SiII-stabilized dihydrido Ni complex. Catalysts Screening for Hydrogenation. Given the high reactivity of 2 and 3 toward dihydrogen, we next investigated their catalytic behavior in hydrogenation of olefins. Choosing norbornene as the standard substrate, we compared the catalytic hydrogenation competencies of 2−4 as well as with that of the related Xantphos (4,5-bis(diphenylphosphino)-9,9-dimethylxanthene) supported Ni complex (Table 1). When 2 was mixed with norbornene (50 equiv) in C6D6 at RT, a new set of resonances appeared in the 1H NMR spectrum, corresponding to the immediate formation of [SiII(Xant)SiII]Ni(η2-norbornene) and “free” 1,3-COD ligand (Table 1, entry 1). Then exposing of this mixture to 1 bar H2 resulted in the complete conversion of norbornene to norbornane after 24 h. After completion, the color of the solution changed from red to yellow-brown and 4 could be observed as the only nickel species according to the resulting 1H NMR spectrum. However, 4 could not be detected during the catalytic reaction process. To find out whether 4 can be transformed into [SiII(Xant)SiII]Ni(η2-norbornene), complex 4 13502

DOI: 10.1021/jacs.7b07167 J. Am. Chem. Soc. 2017, 139, 13499−13506

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Journal of the American Chemical Society Scheme 4. Substrate Scope of Complex 2 Catalyzed Olefin Hydrogenationa

a

Reaction conditions: olefin (0.054 mmol), ferrocene (0.022 mmol, internal standard), and 2 mol % Ni catalyst in 0.45 mL C6D6 under 1 bar H2 at RT. The catalytic reactions were performed in a sealed J. Young NMR tube without stirring. Conversion was monitored by 1H NMR with ferrocene as the internal standard. bReaction condition: olefin (1.07 mmol), ferrocene (0.43 mmol, internal standard), and 0.1 mol % Ni catalyst in 3 mL C6D6 under 1 bar H2 at RT. The catalytic reactions were performed in a Schlenk tube with stirring. TON = turnover number, TOF = turnover frequency. c Isolated yields (6s 0.27 mmol).

Figure 4. Calculated potential energy surface (PES) of the proposed mechanism of hydrogenation of ethylene catalyzed by 2b.

4), which is an excellent result among preceding Ni-catalyzed homogeneous hydrogenations of olefins.13 Mechanism. H2 cleavage on transition metal centers usually occurs via a homolytic or heterolytic mechanism.24 For the homolytic process, oxidative addition of H2 to an electron-rich metal center affords a dihydrido metal complex. A heterolytic process generally occurs in a Lewis base−transition metal system, in which the metal center accepts a hydride and the assisted Lewis base ligand traps a proton. Recently, H2 cleaved by a Lewis acid assisted transition metal system has been presented as a new mode of H2 activation.25 For example, the Peters group found that (PhDPBiPr)Ni [PhDPBiPr = PhB(o-iPr2PC6H4)2] can activate H2 to generate a bridging hydrido-borohydrido Ni complex, where the B and Ni atoms cooperatively cleave H2 in a concerted mechanism with a lower activation energy compared with the homolytic mechanism.20d,26 Since the mechanistic studies of H2 activation by Ni complexes are rare, here we use DFT

Worthy to note, 4-vinylpyridine (6u) could also be hydrogenated quantitatively after 66 h. Unsaturated carbonyl complexes (6v− 6x) could also be fully converted to ketones (7v−7w) and ester (7x) with excellent chemoselectivity. Furthermore, nonconjugated and conjugated dienes (6y, 6ab−6ad), serving as active substrates, yielded the corresponding alkanes quantitatively. Lower conversion (68%) was afforded for the hydrogenation of 2,3-dimethyl-1,3-butadiene (6z) due to its isomerization to tetramethylethylene after monohydrogenation. Alkynes such as phenylacetylene and 2-methyl-1-hexen-3-yne turned out to be unreactive likely due to the strong coordination ability of a CC bond toward the Ni active site. In order to determine the performance of the catalyst with respect to turnover number (TON) and turnover frequency (TOF), we have chosen 1dodecene and styrene as substrates and carried out the catalytic reactions in a Schlenk tube. The TON and TOF values could reach up to 1000 and 250 h−1, respectively, with styrene (Scheme 13503

DOI: 10.1021/jacs.7b07167 J. Am. Chem. Soc. 2017, 139, 13499−13506

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and the H atoms of the dihydrogen. In complex D, as expected for a metal dihydrido complex, significant shortening of the Ni− H [r(Ni−H) = 1.598, 1.668 Å] bonds is observed, with WBIs of 0.19 and 0.14 and NBO charges on the hydrides of −0.22 and −0.24. Additionally, D also exhibits a strong interaction between the silylene ligand and the hydrides, based on r(Si−H) = 1.798 and 1.726 Å and WBIs of 0.50 and 0.58. NBO analysis of the transition state TS(C−D) suggests that the dihydrogen activation is achieved by interaction of the Ni 3dxy orbital and the σ(Ni−Si) orbital with the σ*(H−H) (Figure 6). Seminal

calculations to gain insight into the mechanism of complex 2 catalyzed hydrogenation of olefins. The proposed mechanism of ethylene hydrogenation is presented in Figure 4. The calculations of the mechanism were performed at the B3LYP-D3/SDD-def2SVP level of theory, on a model system (2b) in which the Ph and tBu substituents of the silylene ligand were replaced by methyls and the methyls of the xanthene backbone were replaced by hydrogens. In the first step of the proposed mechanism, complex A [SiII(Xant)SiII]Ni(η2-CH2CH2)] coordinates to a molecule of dihydrogen without a barrier to form complex B. Complex B converts, through a low-barrier TS(B−C), to the dihydrogen complex C at 8.6 kcal mol−1, which then undergoes a dihydrogen splitting via a TS(C−D) found at 15.4 kcal mol−1, generating complex D (at 9.9 kcal mol−1) in an endergonic step of 1.3 kcal mol−1. The next step is a migratory insertion of the coordinated ethylene to the Ni−H bond via TS(D-E) to afford E (at 7.6 kcal mol−1). In complex E the hydride binds directly to the Si atom of the silylene ligand, causing the ligand N-heterocyclic ring opening (see details below). Complex E isomerizes to complex F (at 3.7 kcal mol−1) via TS(E-F), bringing the hydride back to the Ni center. Further isomerization from intermediate F to G brings the hydride to a position that will allow reductive elimination of the hydrogenated product in the following step. This isomerization is endergonic by 2.5 kcal mol−1 and proceeds via the highest transition state TS(F−G) found at 24.8 kcal mol−1 on the PES, making this transformation the rate-determining step in the proposed mechanism. G coordinates to an additional molecule of olefin, yielding complex H, which reductively eliminates to release ethane and regenerates the catalytic species A. This step is exergonic by 40.5 kcal mol−1. One of the most interesting features about the proposed mechanism is the activation of the dihydrogen through formation of the intermediate complex D from C (Figure 5). This activation

Figure 6. NBOs of the TS(C−D). σ*(H−H), occ. 0.55 el., 3dxy(Ni), occ. 1.72 el., σ (Ni−Si), occ. 1.65 el.

Figure 5. B3LYP-D3/SDD-def2SVP optimized geometries of the intermediates C and D.

work by Sabo-Etienne et al. revealed that SiIV could assist hydrogen exchange in which (η 2-H−SiIV )Ru complexes converted to (η2-H−H)Ru complexes via a σ-complex-assisted metathesis (σ-CAM) process.27 However, cooperative SiII atom assisted H2 cleavage at a Ni center has never been reported as yet. The experimental support for this type of cooperative activation is observed in complex 5 (Figure 3), which is formed upon reaction of [SiII(Xant)SiII]Ni(PMe3)2 with dihydrogen (Scheme 3). DFT calculations show that 5, similarly to proposed intermediate D, contains a strong interaction between one of the Si atoms and the bound hydrides. This is supported by short Si− H bond distances (1.646, 1.657 Å) and high WBI(Si−H) (0.67, 0.67) (Table 2). Additionally, the WBI(Ni−H) of 0.1 and 0.12 and q(H) = −0.22 and −0.23 are similar to those in D. The interaction of the Ni center with the hydrides is achieved via donation of electrons from Ni d orbitals to σ*(Si−H). Complex D transfers a hydride to the coordinated olefin to form complex E, in which the second hydride migrates to the silicon atom of the silylene, resulting in opening of the Nheterocyclic ring by dissociation of the N−Si bond, with a Si−N distance of 2.83 Å compared with r(Si−N) of 1.89 and 1.90 Å in the closed ring (Figure 7). This type of ring opening is also observed in complex 4 (Figure 2), which is generated by exposing 2 to dihydrogen (Scheme 2).

step of the dihydrogen is presumably assisted by the SiII atoms of the bis(NHSi) ligand. In the dihydrogen complex C, the Ni−H bond distances are 1.710 and 1.711 Å with Ni−H Wiberg bond indexes (WBIs) of 0.10, 0.10, and −0.02, −0.02 el. charges on the H atoms, suggesting only a weak interaction between the Ni center and the coordinated dihydrogen molecule (Table 2). The Si−H distances of 2.932 and 2.944 Å and WBI(Si−H) = 0.04 and 0.04 imply almost no interaction between the silylene Si centers

CONCLUSIONS We could synthesize and fully characterize a new strong σdonating bis(NHSi)xanthene ligand whose coordination behavior toward Ni was explored. Treatment of the bis(NHSi)xanthene ligand 1 with Ni(cod)2 led to [SiII(Xant)SiII]Ni(η2-1,3cod) 2, and its subsequent reaction with PMe3 resulted in [SiII(Xant)SiII]Ni(PMe3)2 3. Unprecedentedly, the dinuclear Ni2H2 complex 4, with two bridging Si−H→Ni, was furnished

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Table 2. Calculated Parameters of C, D, and 5

C D 5

r(Ni−H) (Å)

r(Si−H) (Å)

WBI(Ni−H)

WBI(Si−H)

q(H)

1.710, 1.711 1.598, 1.668 1.614, 1.652

2.932, 2.944 1.798, 1.726 1.646, 1.657

0.10, 0.10 0.19, 0.14 0.11, 0.12

0.04, 0.04 0.50, 0.58 0.67, 0.67

−0.02, −0.02 −0.22, −0.24 −0.22, −0.23

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DOI: 10.1021/jacs.7b07167 J. Am. Chem. Soc. 2017, 139, 13499−13506

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Journal of the American Chemical Society

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Figure 7. B3LYP-D3/SDD-def2SVP optimized geometry of the intermediate E.

through the reaction of 2 with 1 bar H2 at room temperature. Mixing complex 3 and H2 led to a reversible H2 activation and afforded the first silylene-assisted dihydrido Ni complex confirmed by in situ NMR spectroscopy and single-crystal Xray diffraction analysis. Complex 2 can act as a strikingly efficient hydrogenation catalyst for olefins: 27 examples involving terminal and bulky internal olefins are reported which are hydrogenated quantitatively under very mild conditions (1 bar H2, RT). Based on DFT calculations, the bis(NHSi) ligand 1 can assist the Ni center to cleave dihydrogen during the hydrogenation process, which is a new mode for H2 activation. Studies aimed at applying these bis(NHSi) Ni complexes in other catalytic transformations are ongoing.

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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/jacs.7b07167. Experimental procedures, characterizations, crystallographic analyses, and computational data (PDF) Crystallographic data (CIF)

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AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

Matthias Driess: 0000-0002-9873-4103 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to the Cluster of Excellence UniCat (EXC 314-2, sponsored by the Deutsche Forschungsgemeinschaft and administered by the TU Berlin) for financial support. Y.W. gratefully acknowledges financial support by the China Scholarship Council. A.K. is grateful to the Alexander von Humboldt Foundation for a postdoctoral fellowship. We thank Dr. Zhenbo Mo and Yu-Peng Zhou for helpful discussions, Dr. Somenath Garai for XRD structural determinations, and Paula Nixdorf for assistance in the XRD measurements.

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REFERENCES

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DOI: 10.1021/jacs.7b07167 J. Am. Chem. Soc. 2017, 139, 13499−13506