CO-Reduction Chemistry: Reaction of a CO-Derived

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CO-Reduction Chemistry: Reaction of a CO-Derived Formylhydridoborate with Carbon Monoxide, with Carbon Dioxide, and with Dihydrogen Zhongbao Jian,† Gerald Kehr,† Constantin G. Daniliuc,† Birgit Wibbeling,† Thomas Wiegand,§ Melanie Siedow,‡ Hellmut Eckert,‡,⊥ Markus Bursch,¶ Stefan Grimme,¶ and Gerhard Erker*,† †

Organisch-Chemisches Institut and ‡Institut für Physikalische Chemie, Westfälische Wilhelms-Universität Münster, 48149 Münster, Germany § Laboratorium für Physikalische Chemie, ETH Zürich, Vladimir-Prelog-Weg 1-5/10, 8093 Zürich, Switzerland ⊥ Institute of Physics in Sao Carlos, University of Sao Paulo, CEP 369, Sao Carlos, Sao Paulo 13566-590, Brazil ¶ Mulliken Center for Theoretical Chemistry, Institut für Physikalische und Theoretische Chemie, Rheinische Friedrich-Wilhelms Universität Bonn, Beringstr. 4, 53115 Bonn, Germany S Supporting Information *

ABSTRACT: Treatment of the bulky metallocene hydride Cp*2Zr(H)OMes (Cp* = pentamethylcyclopentadienyl, Mes = mesityl) with Piers’ borane [HB(C6F5)2] and carbon monoxide (CO) gave the formylhydridoborate complex [Zr]−OCH− BH(C6F5)2 ([Zr] = Cp*2Zr−OMes). From the dynamic NMR behavior, its endergonic equilibration with the [Zr]−O−CH2− B(C6F5)2 isomer was deduced, which showed typical reactions of an oxygen/boron frustrated Lewis pair. It was trapped with CO to give an O−[Zr] bonded borata-β-lactone. Trapping with carbon dioxide (CO2) gave the respective O−[Zr] bonded cyclic boratacarbonate product. These reaction pathways were analyzed by density functional theory calculation. The formylhydridoborate complex was further reduced by dihydrogen via two steps; it reacted rapidly with H2 to give Cp*2Zr(OH)OMes and H3C−B(C6F5)2, which then slowly reacted further with H2 to eventually give [Zr]−O(H)−B(H)(C6F5)2 and methane (CH4). Most complexes were characterized by X-ray diffraction.

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INTRODUCTION

Scheme 1. Formylborane and Borates

Trialkylboranes insert carbon monoxide (CO) into the boron− carbon bond. Some useful organic syntheses are based on this reaction.1 The behavior of [B]−H boranes toward CO is usually different. Catalyzed by [BH4]− diborane may reduce CO to trimethylboroxine.2 In contrast, the uncatalyzed reaction of diborane with CO leads to the formation of “borane carbonyl” [H3B−CO], a low-boiling liquid that readily dissociates to the borane and carbon monoxide upon lowering the CO pressure.3,4 We have recently shown that Piers’ borane [HB(C6F5)2]5 reacts analogously with CO at low temperature. It formed “Piers’ borane carbonyl” [(C6F5)2B(H)−CO], which we were able to characterize by X-ray diffraction.6 The formation of genuine formylboranes or the closely related formylborate anions has seldom been achieved. Our synthesis of the pyridine stabilized neutral formylborane 1 (Scheme 1) comes closest to this synthetic goal. We obtained it by HB(C6F5)2 reaction of CO at a variety of vicinal P/B frustrated Lewis pairs (P/B FLPs)7 followed by the reaction with pyridine.8 The pyridine stabilization is essential since a DFT study has shown that the free (C6F5)2B−CHO is endergonic relative to its components HB(C6F5)2 and CO © 2017 American Chemical Society

and would consequently dissociate upon removal of the pyridine donor.8 The formylborane 1 was characterized spectroscopically, by X-ray diffraction and by some chemical reactions.9 This thermodynamic behavior makes it understandable that the formylborates 2 and 3 are isolable molecules. The system 2 was obtained by Stephan et al. by treatment of the 2 B(C6F5)3/PtBu3 FLP with CO/H2;10 complex 3 was formed by carbonylation of the Cp*2ScH/B(C6F5)3 mixture.11 Compound 2 was characterized by X-ray diffraction. Both compounds reacted further by formation of boron/aldehyde Received: March 14, 2017 Published: April 13, 2017 6474

DOI: 10.1021/jacs.7b02548 J. Am. Chem. Soc. 2017, 139, 6474−6483

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Journal of the American Chemical Society

resonances, which indicates hindered rotation around both the Zr1−O1 and O1−C21 vectors on the NMR time scale. We then reacted the mixture of complex Cp*2Zr(H)OMes (5) and Piers’ borane [HB(C6F5)2] with carbon monoxide. For that purpose, a red solution of the 5/HB(C6F5)2 mixture was exposed to CO gas (1.5 bar) in bromobenzene at r.t. The color of the mixture turned to yellow within 20 min, and the subsequent workup involving crystallization from a toluene/ pentane mixture at −35 °C gave the formyl complex 6 in about 80% yield. It shows a B−H IR(KBr) feature at ṽ = 2289 cm−1. Complex 6 was characterized by X-ray diffraction. We used two different types of single crystals for the structure determination. Both were reproducibly obtained either by crystallization from a toluene/pentane mixture at −35 °C (6A, the crystal contains 0.5 molar equiv of toluene) or by layering a toluene solution of 6 with pentane at +26 °C (6B, this crystal is toluene-free). Both the structures were similar with regard to their essential structural parameters, but they differed in their conformational features. The 6A structure is depicted in Figure 2; see the Supporting Information for structure 6B and for a comparison of the two conformers.

products involving C6F5 migration from boron to the formyl carbon atom. We have now developed a new formylborate system that was easily available and avoided the unfavorable migration reaction involving the C6F5 group at boron. Our system contained a zwitterionic [Zr]−OCH−BH(C6F5)2 moiety, and it reacted by means of equilibration with its [Zr]−OCH2−B(C6F5)2 isomer in unique pathways with carbon monoxide, carbon dioxide, and it was further reduced under mild conditions just by exposure to dihydrogen. The first results of this chemistry are described in this account.

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RESULTS AND DISCUSSION Formylhydridoborate Complex 6. For this study, we used the hydrido zirconocene complex 5 as the CO-reducing agent. Complex 5 was readily obtained by treatment of Cp*2ZrH2 (4)12,13 with 2,4,6-trimethylphenol (i.e., hydroxymesitylene) in pentane solution at room temperature (r.t.) with evolution of dihydrogen gas.14 Complex 5 was crystallized from the pentane solution at −35 °C and obtained crystalline in 85% yield (see Scheme 2). The X-ray crystal structure analysis shows Scheme 2. CO Reduction with Zirconium Hydride Complex 5a)

With DFT calculated Gibbs free energies {ΔG} in kcal mol−1 at 298 K relative to complex 5 and activation energies [ΔG≠] in kcal mol−1 for the individual reaction step. a

the typical bent metallocene complex geometry with a hydride and the −OMes ligand attached in the σ-plane. The Zr1−O1 linkage is short (1.979(3) Å; O1−C21:1.351(5) Å). The mesityl plane is rotated slightly from the bent metallocene σligand plane. The [Zr]−O−Mes unit is only slightly bent at oxygen (angle Zr1−O1−C21:167.6(3)°; dihedral angle Zr1− O1−C21−C22:55.4(2)°, see Figure 1). The 1H NMR spectrum of complex 5 shows a sharp Cp* singlet (of rel. 30 H intensity) and the [Zr]−H hydride signal (δ 6.21 in benzened6). It features three methyl 1H/13C NMR signals each for the O-mesityl ligand plus respective pairs of mesityl m-CH

Figure 2. View of the molecular structure of 6A (thermal ellipsoids are shown with 30% probability). Selected bond lengths (Å) and angles (deg): Zr1−O1, 1.969(2); O1−C21, 1.355(3); Zr1−O2, 2.179(2); O2−C30, 1.251(3); C30−B1, 1.586(4); Zr1−O1−C21, 177.6(2); Zr1−O2−C30, 146.4(2); O2−C30−B1, 126.4(3); Zr1−O2−C30− B1, −158.8(2); Zr1−O2−C30−H30, 9.8; O2−C30−B1−H1, 84.7; O1−Zr1−O2−C30, 145.0.

Figure 1. Molecular structure of complex 5 (thermal ellipsoids are shown with 15% probability).

The 6A structure features the Cp*2Zr bent metallocene with the −OMes σ-ligand almost linearly attached to it. It shows the formyl−[B] unit bonded to zirconium via the carbonyl oxygen in a bent arrangement. The Zr1−O2 linkage is markedly longer than the Zr1−O1(Mes) connection. The O2−C30 bond is short, it is in the typical carbaldehyde range. The boron atom B1 has the pair of C6F5 groups bonded to it and a hydrogen atom. The structure 6A shows a conformational arrangement that has the carbonyl carbon atom (C30) oriented away from the −OMes group, and it has the [B]−H vector oriented almost normal to the formyl−[B] plane. Since the NMR spectra of complex 6 in solution showed a remarkable dynamic behavior (see below), we first determined some essential NMR features by MAS NMR spectroscopy in 6475


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Journal of the American Chemical Society the solid state. The 1H Hahn echo MAS NMR spectrum of complex 6 was acquired at 20.0 T with a spinning frequency of 60.0 kHz. The spectrum (see Figure 3) shows the resonance of

Scheme 3. Equilibration between Formylhydridoborate complex 6 and Its Isomer 7a

With DFT calculated Gibbs free energies {ΔG} in kcal mol−1 at 298 K relative to complex 6 and activation energies [ΔG≠] in kcal mol−1 for each individual reaction step. a

Figure 3. 1H Hahn echo MAS solid-state NMR spectrum of complex 6 (measured at 20.0 T with a spinning frequency of 60.0 kHz; signals marked + are due to a minor impurity).

Information). The computational studies reproduce the endergonic equilibrium between complex 6 (0.0 kcal/mol) and the invisible open intermediate 7o (6.5 kcal/mol) with a small barrier of 7.4 kcal/mol well (Figure 4). This is in accord

the Cp*2Zr- subunit and the mesityl methyl groups as an intense broad signal at about δ 1.5. Aside from it we located the relatively sharp 1H NMR signals of the nonequivalent pair of mesityl methine CH groups at δ 6.7 and δ 5.3. The formyl O CH− signal appears in the typical aldehyde region at δ 11.3, and we could even locate the [B]−H hydridoborate signal at δ 3.5. This spectrum is as expected for the zirconium formylhydridoborate structure as we had characterized it by the X-ray crystal structure analysis. The 1H NMR spectrum of complex 6 (in benzene-d6 solution) shows the large Cp*2Zr resonance, three methyl signals and the pair of mesityl CH resonances, indicating hindered rotation of the [Zr]−OMes moiety, and it shows a 1:1:1:1 intensity quartet at δ 7.74 with rel. intensity 2H. The apparent J(11B−1H) coupling constant obtained from this signal amounts to 47 Hz. Analogously, the 11B NMR spectrum shows a triplet at δ − 17.1 (i.e., in the typical borate region) with the same apparent J(11B−1H) = 47 Hz coupling constant. We note that this coupling constant is only about one-half of that expected for a typical 1JBH value for a RB(H)(C6F5)2− borate situation.15 11B{1H} heteronuclear J-resolved MAS NMR confirms the expected 1JBH coupling in the order of 95 Hz in the solid state (see the Supporting Information). Consequently, we assume that the observation in solution indicates a rapid equilibrium of the [Zr]−formylhydridoborate structure with a nonvisible [Zr]−OCH2−boryl situation of higher energy content on the NMR time scale (Scheme 3). The 13C NMR resonance (δ 270.4) and the 19F NMR spectrum [o,m,p-C6F5 signals at δ − 130.2, − 159.2, − 164.2 (Δδ19Fm,p = 5.0 ppm)] were not affected by the dynamic behavior. The spectral assignments of complex 6 were secured by using the respective 13CO labeled system and by recording the NMR spectra at variable temperature (see the Supporting Information for further details). This special structural situation was evaluated by a density functional theory (DFT) calculation. Computational DFT-D3 investigation of the observed equilibrium was conducted at PW6B95-D3(BJ)/def2-QZVP+COSMO-RS(toluene)//PBEh3c level of theory (for computational details, see the Supporting

Figure 4. Relative Gibbs free energy diagram of the equilibrium between complex 6 and intermediate 7o. All energy values in kcal mol−1 (at 298 K) are given relative to complex 6. Most hydrogen atoms are omitted for clarity. White = H; gray = C; red = O; green = F; pink = B; teal = Zr.

with the situation assumed from the NMR behavior in solution. According to this calculation, the open isomer 7o is in an equilibrium situation with a “closed” structural isomer (7c), which features an internal oxygen−boron interaction. Even though the structure of 7c with a closed B−C−O threemembered ring is energetically slightly favored over 7o by 1.3 kcal/mol, both structures are linked by a quite high energy barrier of 20.4 kcal/mol. This high barrier might be explained by the initial deplanarization of the boron atom and steric hindrance before a stabilization by a B−O bond formation can take place. Therefore, we conclude that any further chemical reactions (see below) occur from the open structure 7o. The critical involvement of the open isomer 7o in the chemistry of the formylhydridoborate complex 6 was further 6476


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Journal of the American Chemical Society indicated by the outcome of the reaction of 6 with pyridine. Exposure of 6 to this N-donor compound (r.t., toluene, 5 min) resulted in a rapid formation of the [Zr]-O−CH2−[B] pyridine adduct 7-pyr (see Scheme 4 and Figure 5). Complex 7-pyr was Scheme 4. Trapping of Isomer 7o with Pyridine

Figure 6. Projection of the molecular structure of the CO addition product 9 (thermal ellipsoids are shown with 15% probability).

Scheme 5. Assumed Reaction Pathway of the Formation of Complex 9a

Figure 5. View of the molecular structure of the pyridine adduct 7-pyr (thermal ellipsoids are shown with 15% probability).

isolated in 84% yield. It was characterized by C,H elemental analysis, by spectroscopy, and by X-ray diffraction. In the crystal complex 7-pyr shows the presence of the open Zr1−O2− C30(H2)-B1 unit (Zr1−O2:2.014(1) Å, O2−C30:1.434(2) Å, C30−B1:1.641(3) Å, angle Zr1−O2−C30:143.4(1)°, O2− C30−B1:118.6(2)°, dihedral angle Zr1−O2−C30B1:171.7(2)°). The boron atom B1 is tetra-coordinated with the pyridine bound to it (B1−N61:1.625(3) Å). In solution, complex 7-pyr shows the 1H NMR singlet of the O−CH2− group at δ 5.10 (13C: δ 72.3); it features a 11B NMR resonance at δ − 1.2. The single set of 19F NMR resonances shows a small Δδ19Fm,p = 6.1 chemical shift difference as it is typical of tetracoordinated boron in such a situation. Reaction of [Zr]−Formylhydridoborate Complex 6 with Carbon Monoxide. The [Zr]−OCH−[B]H system 6 reacts within minutes at near to ambient conditions (1.5 bar, r.t.) with carbon monoxide under preparative conditions. We isolated the product 9 by crystallization from toluene/pentane solution under CO atmosphere in about 70% yield on a 100 mg scale. The X-ray crystal structure analysis revealed that carbon monoxide had apparently trapped the [Zr]−O−CH2−[B] isomer 7o and formed a borata-β-lactone type structure at the [Cp*2Zr−OMes]+ unit (see Figure 6 and Scheme 5). It shows the bent metallocene with the close to linear [Zr]−O−Mes unit (Zr1−O1:1.968(2) Å, O1−C21:1.355(2) Å, angle Zr1−O1− C21:163.1(2)°). In the σ-plane, it has the newly formed fourmembered borata-β-lactone unit16 coordinated by means of the carbonyl oxygen atom (Zr1−O2:2.718(1) Å, O2− C30:1.250(2) Å, angle Zr1−O2−C30:144.7(2)°). The fourmembered heterocycle is planar (sum of the internal bonding angles: 359.9(2)°). The four-membered ring lies practically

a With DFT calculated Gibbs free energies {ΔG} in kcal mol−1 at 298 K relative to complex 6 and activation energies [ΔG≠] in kcal mol−1 for each individual reaction step.

coplanar with the bent metallocene σ-plane. It features a conformational arrangement that has the lactone oxygen atom (O3) oriented away from the −OMes ligand. We assume a reaction course that involves coordination of the CO molecule to the boron Lewis acid of the in situ formed reactive intermediate 8,4 followed by nucleophilic attack by the zirconium bound formyl oxygen atom and binding of the carbonyl oxygen atom to the adjacent metal center (see Scheme 5).10,17 This was supported by the results of a DFT calculation (for methods, see above). It confirmed that the borane-carbonyl 8 is 6477


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Journal of the American Chemical Society a likely intermediate that is easily formed by CO addition to the intermediate 7o. Complex 7o, in principle, functions as an intramolecular oxygen/boron FLP.8 Consequently, its oxygen Lewis base traps the borane activated carbonyl to form the complex 9i. This step is then followed by rearrangement to give the observed product 9. The 9/6 energy difference and the calculated activation energies involved are small enough to account for the observed slow reversibility of the CO coupling sequence. In solution, complex 9 shows the typical NMR signals of the [Cp*2Zr−OMes] subunit. The borata-β-lactone moiety is characterized by 13C NMR features at δ 216.6 (CO) and δ 72.2 (CH2, 1H: δ 4.69). The 11B NMR signal of 9 is at δ − 10.8, and it shows a single set of 19F NMR resonances (Δδ19Fm,p = 5.7 ppm). We have confirmed the assignments by isotopic labeling. The 13 C labeled formylhydridoborate system 6-13CO was treated with “normal” CO. The 13C label was then exclusively found in the CH2 group inside the four-membered ring. Removal of CO from the atmosphere above the solution of this experiment reformed the 13C labeled starting material 6-13CO, indicating reversibility of the formation of the CO addition product 9. Eventually, exposure of 6-13CO to a 13CO atmosphere gave the doubly labeled [Zr]−borata-β-lactone product 9-13CO-13CO. The respective 13C NMR signals showed 1J(13C−B) = 61.0 Hz (13CO) and 1J(13C−B) = 52.0 Hz (13CH2) coupling constants (for further details and the depicted NMR spectra, see the Supporting Information). Reaction of [Zr]−Formylhydridoborate Complex 6 with Carbon Dioxide. We next reacted the [Zr]− formylhydridoborate 6 with carbon dioxide. The reaction was carried out in toluene solution at r.t. using 1.5 bar of CO2. Workup after 12 h reaction time, involving crystallization of the product from a toluene/pentane mixture at −35 °C, gave complex 11 as a yellow crystalline product in about 85% yield. The X-ray crystal structure analysis revealed that in this case the reaction product had apparently also been formed via the [Zr]−O−CH2−[B] intermediate 7o (see Scheme 6 and Figure 7). Complex 11 has a cyclic boratacarbonate unit,18 attached at the [Cp*2Zr−OMes] moiety. It has been formed by oxygen attack at the CO2 carbonyl carbon atom and formation of a new boron−oxygen bond.19,20 The Zr−O−C sections of both the Zr1−O1−C21 (168.9(2)°) and the Zr1−O2−C30 (166.9(2)°) units are close to linear, and the planes of both their cyclic substituents are oriented almost coplanar in the σ-ligand plane in front of the bent metallocene wedge. The newly formed fivemembered carbonate heterocycle features a distorted twist conformation. The O2−C30 linkage is short (1.245(3) Å), as is the adjacent O3−C30 bond (1.278(3) Å). The O4−C30 bond is markedly longer (1.317(3) Å). In solution, complex 11 exhibits the carbonyl 13C NMR resonances at δ 168.8 and the endocyclic CH2 group at boron at δ 73.9 (1H: δ 4.64). The 13C labeled complex 11-13CO, derived from the reaction of Cp*2Zr(H)OMes (5) with HB(C6F5)2 and 13CO followed by treatment with CO2, shows the labeled 13CH2 group. In this complex the carbonyl carbon atom shows splitting to a doublet with 2J(13C−13C) = 3.3 Hz. The DFT analysis (for the methods, see above) indicates that the formation of the product 11 can be regarded as a concerted CO2 addition to the O/B pair of the geminal FLP 7o. We calculated a reasonable activation barrier for this cooperative C−O/B−O bond formation to give the intermediate 11i. The

Scheme 6. Assumed Reaction Pathway of the Formation of Complex 11a

With DFT calculated Gibbs free energies {ΔG} in kcal mol−1 at 298 K relative to complex 6 and activation energies [ΔG≠] in kcal mol−1 for each individual reaction step. a

Figure 7. Molecular structure of the CO2 addition product 11 (thermal ellipsoids are shown with 30% probability).

reaction is then continued by a rapidly proceeding internal isomerization to give the observed product 11 (for details of the DFT analyses of this study, see the Supporting Information). The 11/6 energy difference and the calculated activation energies involved are large enough to account for the observed irreversibility of the CO2 coupling sequence. Complex 6 reacts in a similar way with the Ph-NSO reagent. We have observed oxygen attack at the central sulfur atom with N−B and O−Zr bond formation.21 Complex 12 was selectively formed by treating a solution of 6 in toluene with one molar equiv of N-sulfinylaniline at r.t. within 20 min 6478


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Journal of the American Chemical Society reaction time. Workup with crystallization of the product from toluene/pentane gave crystalline 12 in 87% yield (Scheme 7).

Scheme 8. Reduction of Carbon Dioxide with Zr−Hydride Complex 5

Scheme 7. Reaction of the Formylhydridoborate Complex 6 with N-Sulfinylaniline

90% yield. It was subsequently converted to 14 by treatment with HB(C6F5)2. Both the products 13 and 14 were characterized by C,H elemental analysis, by spectroscopy, and by X-ray diffraction (for details, see the Supporting Information). Note that, in contrast to complex 6, complex 14 did not react with CO or CO2 under our typical conditions (1.5 bar, r.t.). Reaction of [Zr]−Formylhydridoborate Complex 6 with Dihydrogen: Methane Formation. The reduction of carbon monoxide is usually affected with the aid of metal complexes.11,13,24 CO reduction with main group element derived systems is rare, especially beyond the formyl stage.8−10,25 We had recently found that several η2-formyl borane FLP adducts, which are readily formed by CO reduction with Piers’ borane [HB(C6F5)2] at a variety of P/B FLP frameworks, react further with dihydrogen (H2, 60 bar) to reduce the formyl group to CH2. Scheme 9 shows a typical

The X-ray crystal structure analysis revealed the formation of the five-membered heterocycle, which is bonded to the [Cp*2Zr−OMes] unit by means of the SO oxygen O2 (Zr1−O2:2.207(4) Å, S1−O2:1.511(4) Å, angle Zr1−O2− S1:142.2(2)°). The sulfur atom shows a trigonal−pyramidal coordination geometry (ΣS1NOO = 310.4(2)°, S1−N1:1.631(5) Å, S1−O3:1.587(4) Å). The nitrogen atom N1 is found bonded to boron (N1−B1:1.582(9) Å), and its coordination geometry is close to planar−tricoordinate (ΣN1 SCB = 355.0(4)°). The mean plane of the five-membered heterocycle is oriented almost perpendicular to the bent metallocene σligand plane bisecting the Cp*−Zr−Cp* angle (Figure 8).

Scheme 9. Metal-Free Formyl Hydrogenation

Figure 8. View of the molecular structure of complex 12 (thermal ellipsoids are shown with 15% probability).

example.8,9,26 The reaction is thought to proceed by means of H2 splitting by an intermediate oxygen/boron FLP.8−10,27 Because of the conceptual similarity of our new [Zr]−O− CH2−B(C6F5)2 (7o) FLP, we investigated its reactivity toward dihydrogen and found that the formyl group in 6 was further reduced under mild conditions to eventually give methane.28,29 The [Zr]−formylhydridoborate complex 6 was exposed to dihydrogen (1.5 bar) at r.t. in benzene-d6 solution, and the progress of the reaction was directly followed by in situ NMR spectroscopy (the spectra are depicted in the Supporting Information). A rapid reaction was observed within 1 h, and a near to complete conversion to the hydroxy zirconocene complex 20 and H3C−B(C6F5)2 (21)30 (1H NMR: δ 1.33, 11B: δ 71.7, 19F: δ −130.0 (o), −147.0 (m), −161.3 (p)) was observed (see Scheme 10). The composition of the [Zr]product 20 was secured by an independent synthesis. For that purpose, we heated the zirconocene hydride precursor 5 with N2O. The slow oxidation reaction (80 °C, 14 d) gave the [Zr]− OH complex 20 (see Scheme 11), which we isolated in 82% yield [1H NMR in benzene-d6: δ 6.87 and 6.84 (s, each 1H, − OMes), 4.20 (s, [Zr]−OH)]. Complex 20 was also

The 11B NMR spectrum showed a signal at δ − 4.0. In this case, we observed two equal intensity sets of 19F NMR resonances of the pair of diastereotopic C6F5 groups at boron due to the presence of the sulfur chirality center in complex 12.22 Similarly, the 1H NMR spectrum of 12 exhibits an AX type pattern of the diastereotopic hydrogens of the endocyclic CH2 group (single corresponding 13C NMR signal at δ 83.7), and there are the 1H /13C NMR resonances of a pair of diastereotopic Cp* ligands at zirconium in addition to the typical 1H /13C NMR signals of the rigid [Zr]−OMes moiety (each three CH3− and two CH− resonances). For a comparison, we have reacted Cp*2Zr(H)OMes (5) with HB(C6F5)2 and CO2. The reaction takes a different, more conventional course.23 The reaction was carried out at r.t. in bromobenzene with 1.5 bar of CO2. It went to completion within 10 min, and we isolated the product 14 in 82% yield after crystallization from toluene/pentane at −35 °C (Scheme 8). The reaction probably proceeded via the reduction product 13 of CO2 by the zirconium hydride. From the reaction of Cp*2Zr(H)OMes (5) with CO2, we isolated this product 13 in 6479


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methane [δ 0.15 (1H NMR), δ −4.4 (13C)]. Complex 22 was also positively identified from an independent synthesis by reacting 20 with HB(C6F5)2 (r.t., toluene, 20 min). Complex 22 was isolated in 76% yield. It was characterized by X-ray diffraction (see Figure 10), by C,H elemental analysis, and by

Scheme 10. Uncatalyzed Reduction of Formylhydridoborate 6 by Dihydrogen to Methane

Scheme 11. Independent Syntheses of Products 20 and 22

Figure 10. View of the molecular structure of the complex 22 (thermal ellipsoids are shown with 30% probability). Only one molecule (molecule A) of the two found in the asymmetric unit is shown. Selected bond lengths (Å) and angles (deg): Zr1−O1, 1.972(2); O1− C21, 1.362(3); Zr1−O2, 2.237(2); O2−B1, 1.541(3); O1−Zr1−O2, 91.3(1); Zr1−O1−C21, 175.0(2); Zr1−O2−B1, 137.0(2); ΣB1OCC = 332.6.

characterized by X-ray diffraction (see Figure 9). It features the typical almost linear Zr−O−C(mesityl) σ-ligand at zirconium and a bent Zr−O−H arrangement. The mixture of 20 plus H3C−B(C6F5)2 (21) turned out to slowly react further with dihydrogen. In the course of a time period of 8 d at r.t. (1.5 bar H2), it was converted to about 80% to the [Zr]−O(H)−[B](H) complex 22 with formation of

spectroscopy (1H NMR in benzene-d6: δ 6.74 and 6.69 (s, each 1H, −OMes), 4.36 (br s, BH), 3.57 (OH), 11B: δ −4.8, 19F: Δδ19Fm,p = 4.5 ppm; for further details, see the Supporting Information). In the crystal complex, 22 shows a typical close to linear Zr−O−C(mesityl) unit with a short Zr1−O1 bond (1.972(2) Å). The adjacent [Zr]−O(H)−[B](H) bond (Zr1− O2:2.237(2) Å) is much longer (see Figure 10), and we note a marked angle (137.0(2)°) at that oxygen atom. The analogous reaction of 6 with D2 gave an equimolar mixture of Cp*2Zr(OMes)OD (20-D) and H2DC-B(C6F5)2 (21-D: 1H NMR: δ 1.29). The reaction continued under the D2 atmosphere (1.5 bar, r.t.) and gave a about 78% conversion after 7 d to Cp*2Zr(OMes)(μ-OD)B(D)(C6F5)2 (22-D2) with liberation of CH2D2 [1H NMR: δ 0.12 (quintet, 2JDH = 1.9 Hz)]. Both products were also characterized from the reaction mixture by 2H NMR spectroscopy (for details including the depicted NMR spectra, see the Supporting Information). We assume a reaction pathway that utilizes the rapid equilibrium of the formylhydridoborate complex 6 with the active zirconoxy borane isomer 7o (see Scheme 10). The system seems to effectively function as an intramolecular geminal oxygen/boron FLP and thus is able to serve as an active reagent for the heterolytic cleavage of dihydrogen to generate the intermediate 19. This is set to use the hydridoborate nucleophile to cleave the remaining adjacent carbon−oxygen σ-bond in a SNi process to yield the observed primary products 20 and 21. We assume a similar subsequent

Figure 9. Molecular structure of the [Zr]−OH complex 20 (thermal ellipsoids are shown with 15% probability). Selected bond lengths (Å) and angles (deg): Zr1−O1, 1.999(2); O1−C21, 1.342(3); Zr1−O2, 2.016(2); O1−Zr1−O2, 99.6(1); Zr1−O1−C21, 170.5(1); Zr1−O1− H1, 118(3). 6480


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Journal of the American Chemical Society intermolecular O/B FLP dihydrogen splitting reaction to give the salt 23. Protonolytic cleavage of the H3C−[B]− bond by the strong [Zr]−OH2+ Brønsted acid then offers an attractive pathway to the formation of the observed [Zr]−O(H)−[B](H) product 22 and methane (see Scheme 10).

Scheme 13. Sulfur Dioxide Addition to O/B FLP

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CONCLUSIONS We had recently shown that the reaction of CO with Piers’ borane [HB(C6F5)2] in the presence of a variety of P/B frustrated Lewis pairs results in the formation of unique FLP bonded η2-formylboranes.6,8,9 The reaction was thought to involve P/B-addition of “Piers’ borane carbonyl” followed by internal borohydride reduction of the activated carbonyl group (see Scheme 12). In a variety of cases, subsequent treatment with pyridines led to liberation of the genuine pyridine stabilized formylborane.

as shown by our contemporary DFT analysis33 and the respective trapping experiments. We expect that these findings will help to further expand and develop the chemistry of CO reduction processes with boron reagents and its related O/B FLP chemistry.

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EXPERIMENTAL SECTION

For general information and the spectroscopic and structural data of these new compounds, see the Supporting Information. Preparation of Complex 6. In a Schlenk tube, complex 5 (498 mg, 1.0 mmol) and HB(C6F5)2 (346 mg, 1.0 mmol) were mixed and dissolved in bromobenzene (5 mL) at room temperature. The obtained red solution was evacuated at −78 °C and then refilled by CO atmosphere (1.5 bar) at room temperature. After stirring for 20 min, the red solution became yellow. The solvent was removed in vacuo to give a pale red solid, which was recrystallized from a mixture of toluene and pentane at −35 °C to afford the resulting yellow crystalline product 6. Yield: 697 mg, 80%. Crystals of complex 6 suitable for the X-ray crystal structure analysis were obtained from a solution of complex 6 in toluene and pentane at −35 °C. Anal. calcd. for C42H43BF10O2Zr (871.8 g mol−1): C, 57.86; H, 4.97. Found: C, 58.37; H, 5.00. Preparation of the CO Addition Complex 9. Complex 6 (174.4 mg, 0.2 mmol) was dissolved in toluene (2 mL) in a glass vial (10 mL). The vial was then placed in a bigger Schlenk tube (50 mL), and pentane was added to the Schlenk tube outside of the vial. The Schlenk tube was evacuated at −78 °C and then refilled by CO atmosphere (1.5 bar) at room temperature. The Schlenk tube was stored at −15 °C for several days to afford yellow crystals of product 9, which were suitable for X-ray crystal structure analysis. Yield: 133 mg, 74%. Anal. calcd. for C43H43BF10O3Zr (899.8 g mol−1): C, 57.40; H, 4.82. Found: C, 57.45; H, 4.77. Preparation of the CO2 Addition Complex 11. In a Schlenk tube (10 mL), complex 6 (174.4 mg, 0.2 mmol) was dissolved in toluene (3 mL) at r.t. The Schlenk tube was evacuated at −78 °C and then refilled by a CO2 atmosphere (1.5 bar) at r.t. After stirring for 12 h, the solvent was removed in vacuo to give a pale yellow solid, which was recrystallized from a mixture of toluene and pentane at −35 °C to afford the resulting yellow crystal product 11. Yield: 152 mg, 83%. Crystals of complex 11 suitable for the X-ray crystal structure analysis were obtained from a solution of complex 11 in toluene and pentane at −35 °C. Anal. calcd. for C43H43BF10O4Zr (915.8 g mol−1): C, 56.39; H, 4.73. Found: C, 56.77; H, 4.96. Preparation of Complex 20. In a Schlenk tube (50 mL), complex 5 (249.0 mg, 0.5 mmol) was dissolved in toluene (5 mL) at r.t. The Schlenk tube was evacuated under −78 °C and then refilled by N2O atmosphere (1.5 bar) at r.t. After stirring for 14 d at 80 °C, the solvent was removed under vacuum to give a pale brown solid, which was recrystallized from pentane at −35 °C to afford the resulting colorless crystal product 20. Yield: 211 mg, 82%. Anal. calcd. for C29H42O2Zr (514.0 g mol−1): C, 67.78; H, 8.24. Found: C, 67.92; H, 8.42. Preparation of Complex 22. In a glass vial (10 mL), complex 20 (206 mg, 0.4 mmol) and HB(C6F5)2 (138 mg, 0.4 mmol) were mixed and dissolved in toluene (3 mL) at r.t. After stirring for 20 min, the original colorless solution became clear yellow. The resulting solution was layered with pentane (5 mL) and then directly stored at −35 °C to afford the resulting yellow crystal product 22. Yield: 261 mg, 76%. Anal. calcd. for C41H43BF10O2Zr (859.8 g mol−1): C, 57.27; H, 5.04. Found: C, 57.47; H, 5.00.

Scheme 12. Reduction Pathways of Piers’ Borane Carbonyl

The reaction described in this study provides an alternative. Here we assume that in situ formed “Piers’ borane carbonyl” [(C6F5)2B(H)−CO] becomes reduced by the added zirconocene hydride reagent to form the O-metalated formylhydridoborate 6. Complex 6 is a rather special formyl boron compound, different from its congeners 1−3 (see Scheme 1) by having an internal hydridoborate present. This actually determines the chemical features of the new formylborate system 6 to a considerable extent. The hydride functionality apparently reduces the adjacent carbonyl group in an endergonic equilibrium situation to generate the metaloxymethylborane species 7o. Although this was not directly observed in our experiments, it determines the chemistry of complex 6 to a great extent, since it becomes efficiently trapped from the equilibrium by a variety of reagents (CO, CO2, PhNSO, pyridine), and it even serves as an active dihydrogen splitting agent. The reaction with CO leads to a unique borataβ-lactone structure. The CO2 and PhNSO reactions are characterized by nucleophilic attack of the oxygen atom of 7o to the electrophilic central atoms of the respective heterocumulenes. This reaction type resembles the recently observed SO2 addition to the η2-formylborane FLP system 16b, which is thought to proceed through a similar open [B]−CHR−O structure (see Scheme 13).31,32 Our here reported reaction represents a convenient new entry to formylborate-type compounds and it adds a unique [B]−H functionalized variety to the small number of previously reported formylboranes and formylborates (see above Scheme 1), and the system opens a convenient pathway of generating a unique intramolecular geminal [Zr]O/B frustrated Lewis pair 6481


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(11) Berkefeld, A.; Piers, W. E.; Parvez, M.; Castro, L.; Maron, L.; Eisenstein, O. J. Am. Chem. Soc. 2012, 134, 10843−10851. (12) Miller, F. D.; Sanner, R. D. Organometallics 1988, 7, 818−825. (13) For the reaction of Cp*2ZrH2 with CO, see: Wolczanski, P. T.; Bercaw, J. E. Acc. Chem. Res. 1980, 13, 121−127. (14) For a comparison, see: (a) Metters, O. J.; Forrest, S. J. K.; Sparkes, H. A.; Manners, I.; Wass, D. F. J. Am. Chem. Soc. 2016, 138, 1994−2003. (b) Flynn, S. R.; Metters, O. J.; Manners, I.; Wass, D. F. Organometallics 2016, 35, 847−850. (15) Examples of typical 1JBH values (∼80−90 Hz) of RB(H) (C6F5)2− borate systems, see, for example: (a) Welch, G. C.; Juan, R. R. S.; Masuda, J. D.; Stephan, D. W. Science 2006, 314, 1124−1126. (1JBH = 85 Hz). (b) Spies, P.; Erker, G.; Kehr, G.; Bergander, K.; Fröhlich, R.; Grimme, S.; Stephan, D. W. Chem. Commun. 2007, 5072−5074 (1JBH = 88 Hz).. (16) Sudhakar, P. V.; Chandrasekhar, J. J. Mol. Struct. 1989, 194, 135−147. (17) (a) Xu, X.; Kehr, G.; Daniliuc, C. G.; Erker, G. Angew. Chem., Int. Ed. 2013, 52, 13629−13632. (b) Ye, K.-Y.; Kehr, G.; Daniliuc, C. G.; Liu, L.; Grimme, S.; Erker, G. Angew. Chem., Int. Ed. 2016, 55, 9216−9219. (c) Wang, T.; Wang, L.; Daniliuc, C. G.; Samigullin, K.; Wagner, M.; Kehr, G.; Erker, G. Chem. Sci. 2017, 8, 2457−2463. (18) Carry, B.; Zhang, L.; Nishiura, M.; Hou, Z. Angew. Chem., Int. Ed. 2016, 55, 6257−6260. (19) CO2 addition to FLPs, see, for example: (a) Mömming, C. M.; Otten, E.; Kehr, G.; Fröhlich, R.; Grimme, S.; Stephan, D. W.; Erker, G. Angew. Chem., Int. Ed. 2009, 48, 6643−6646. (b) Appelt, C.; Westenberg, H.; Bertini, F.; Ehlers, A. W.; Slootweg, J. C.; Lammertsma, K.; Uhl, W. Angew. Chem., Int. Ed. 2011, 50, 3925− 3928. (c) Peuser, I.; Neu, R. C.; Zhao, X.; Ulrich, M.; Schirmer, B.; Tannert, J. A.; Kehr, G.; Fröhlich, R.; Grimme, S.; Erker, G.; Stephan, D. W. Chem. - Eur. J. 2011, 17, 9640−9650. (d) Bertini, F.; Lyaskovskyy, V.; Timmer, B. J. J.; de Kanter, F. J. J.; Lutz, M.; Ehlers, A. W.; Slootweg, J. C.; Lammertsma, K. J. Am. Chem. Soc. 2012, 134, 201−204. (e) Neu, R. C.; Ménard, G.; Stephan, D. W. Dalton Trans. 2012, 41, 9016−9018. (f) Ménard, G.; Gilbert, T. M.; Hatnean, J. A.; Kraft, A.; Krossing, I.; Stephan, D. W. Organometallics 2013, 32, 4416− 4422. (g) Courtemanche, M.-A.; Larouche, J.; Légaré, M.-A.; Bi, W.; Maron, L.; Fontaine, F.-G. Organometallics 2013, 32, 6804−6811. (h) Pu, M.; Privalov, T. Chem. - Eur. J. 2013, 19, 16512−16517. (i) Stephan, D. W.; Erker, G. Chem. Sci. 2014, 5, 2625−2641. (20) CO2 reduction at FLPs, see also (a) Ashley, A. E.; Thompson, A. L.; O’Hare, D. Angew. Chem., Int. Ed. 2009, 48, 9839−9843. (b) Ashley, A. E.; O’Hare, D. Top. Curr. Chem. 2012, 334, 191−217. (c) Bontemps, S. Coord. Chem. Rev. 2016, 308, 117−130. (21) (a) Xu, X.; Kehr, G.; Daniliuc, C. G.; Erker, G. J. Am. Chem. Soc. 2014, 136, 12431−12443. (b) Longobardi, L. E.; Wolter, V.; Stephan, D. W. Angew. Chem., Int. Ed. 2015, 54, 809−812. (22) SO2 addition to P/B FLPs, see for a comparison: Sajid, M.; Klose, A.; Birkmann, B.; Liang, L.; Schirmer, B.; Wiegand, T.; Eckert, H.; Lough, A. J.; Fröhlich, R.; Daniliuc, C. G.; Grimme, S.; Stephan, D. W.; Kehr, G.; Erker, G. Chem. Sci. 2013, 4, 213−219. (23) CO2 reduction by metal hydrides: (a) Chapman, A. M.; Haddow, M. F.; Wass, D. F. J. Am. Chem. Soc. 2011, 133, 18463− 18478. (b) Bontemps, S.; Vendier, L.; Sabo-Etienne, S. Angew. Chem., Int. Ed. 2012, 51, 1671−1674. (c) Jiang, Y.; Blacque, O.; Fox, T.; Berke, H. J. Am. Chem. Soc. 2013, 135, 7751−7760. (24) For CO reduction by metal hydrides, see selected examples: (a) Manriquez, J. M.; McAlister, D. R.; Sanner, R. D.; Bercaw, J. E. J. Am. Chem. Soc. 1976, 98, 6733−6735. (b) Manriquez, J. M.; McAlister, D. R.; Sanner, R. D.; Bercaw, J. E. J. Am. Chem. Soc. 1978, 100, 2716− 2724. (c) Maatta, E. A.; Marks, T. J. J. Am. Chem. Soc. 1981, 103, 3576−3578. (d) Moloy, K. G.; Marks, T. J. J. Am. Chem. Soc. 1984, 106, 7051−7064. (e) Roddick, D. M.; Fryzuk, M. D.; Seidler, P. F.; Hillhouse, G. L.; Bercaw, J. E. Organometallics 1985, 4, 97−104. (f) Matsuo, T.; Kawaguchi, H. J. Am. Chem. Soc. 2005, 127, 17198− 17199. (g) Shima, T.; Hou, Z. J. Am. Chem. Soc. 2006, 128, 8124− 8125. (h) Werkema, E. L.; Maron, L.; Eisenstein, O.; Andersen, R. A. J. Am. Chem. Soc. 2007, 129, 2529−2541. (i) West, M. M.; Miller, A. J.;

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/jacs.7b02548. Experimental and analytical details; DFT calculation details (PDF) Crystallographic data and CIF files (CIF)

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AUTHOR INFORMATION

Corresponding Author

*[email protected] ORCID

Gerald Kehr: 0000-0002-5196-2491 Hellmut Eckert: 0000-0002-6536-0117 Stefan Grimme: 0000-0002-5844-4371 Gerhard Erker: 0000-0003-2488-3699 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the European Research Council and the Deutsche Forschungsgemeinschaft is gratefully acknowledged. We thank Prof. Beat Meier and the ETH Zürich for supporting the high field solid-state NMR measurements.

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REFERENCES

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CO-Reduction Chemistry: Reaction of a CO-Derived

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