Transition-Metal-Mediated Germanium–Fluorine Activation: Inverse

Feb 23, 2016 - We report the first example of Ge–F activation using a transition metal complex, in which σ-bond metathesis between Ge–F and Ir–...
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Transition-Metal-Mediated Germanium−Fluorine Activation: Inverse Electron Flow in σ‑Bond Metathesis Hajime Kameo,*,† Koki Ikeda,† Didier Bourissou,‡,§ Shigeyoshi Sakaki,∥ Shin Takemoto,† Hiroshi Nakazawa,⊥ and Hiroyuki Matsuzaka† †

Department of Chemistry, Graduate School of Science, Osaka Prefecture University, Gakuen-cho 1-1, Naka-ku, Sakai, Osaka 599-8531, Japan ‡ UPS, Laboratoire Hétérochimie Fondamentale Appliquée, Université de Toulouse, 118 Route de Narbonne, F-31062 Toulouse, France § CNRS, LHFA UMR 5069, F-31062 Toulouse, France ∥ Fukui Institute for Fundamental Chemistry, Kyoto University, Takano-nishihiraki-cho 34-4, Sakyo-ku, Kyoto 606-8103, Japan ⊥ Department of Chemistry, Graduate School of Science, Osaka City University, Sugimoto 3-3-138, Sumiyoshi-ku, Osaka 558-8585, Japan S Supporting Information *

ABSTRACT: We report the first example of Ge−F activation using a transition metal complex, in which σ-bond metathesis between Ge−F and Ir−H σ-bonds takes place with a specific electron flow. Unexpected selectivity for Ge−F over Ge−CPh σbond activation is observed. Density functional theory suggests that a strong dative Ir→Ge interaction, which efficiently weakens the σ-bond, helps electrostatically the coupling between the (δ−) fluorine on Ge and the (δ+) hydrogen on Ir, making Ge−F bond activation very easy.

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INTRODUCTION

The bond activation process by transition metals represents the first step of many catalytic cycles.1 Oxidative addition is emblematic of such a process, especially for late transition metals. Another important mechanism is σ-bond metathesis, in which the bond activation is generally achieved by cooperation between a Lewis acidic transition metal and a Lewis basic ligand. In a typical σ-bond metathesis between M(δ+)−X1(δ−) and E(δ+)−X2(δ−) σ-bonds, the positively charged M center approaches the X2 atom, and the negatively charged X1 approaches the E atom (Figure 1a), giving rise to electron flow from X2 to M and from X1 to E.2 In contrast, σ-bond metathesis involving dative M→Z (Z = B, Al, In, Si, Ge, Sn, etc.) interactions3 induces a formal inverse electron flow (Figure 1b). Here, the electron-positive fragment (Z) withdraws electron density from the transition metal (M) while the electron-negative fragment (X2) interacts with the electrondepleted ligand X1.3m,4,5 This feature potentially enables activating strong and polar Z−F bonds (Z = Si, Ge), because the fluorine can bind to X1 (X1 = H, B), which can form a stronger bond with fluorine than the transition metal. In fact, several C−F bond activation reactions have been, thus far, achieved by involving fluoride transfer to a metal-bonded ligand (such as a phosphine, boryl, or silyl moiety).6 The implication of M→Z interactions can open a new avenue, and σ-bond metathesis with inverse electron flow might enable the activation of σ-bonds even stronger than C−F bonds (such © XXXX American Chemical Society

Figure 1. (a) Typical σ-bond metathesis. (b) σ-Bond metathesis involving a dative M→Z interaction and inverse electron flow.

as Si−F and Ge−F), which are considered very difficult to activate with transition metals.7−9 Recently we reported σ-bond metathesis of Si−F σ-bonds with an Ir−H σ-bond accompanying an inverse electron flow in the reaction of {o-(Ph2P)C6H4}2Si(F)2 (1a) with the iridium hydride Ir(H)(CO)(PPh3)3 (2), affording complex 3 (Scheme 1).10 In this reaction, the formation of a strong H−F bond Received: December 9, 2015

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Organometallics Scheme 1. Activation of Si−F and Si−CPh Bonds of Diphosphine-fluorosilanes 1a,b

would be one of the driving forces for the activation of the strong Si−F σ-bond. This result prompted us to investigate the activation of the Ge−F bond of the germane analogue {o(Ph2P)C6H4}2Ge(F)2 (4a). To our knowledge, the cleavage of Ge−F σ-bonds by transition metals is still unknown. Note that the Ge−F is, similar to the Si−F σ-bond, one of the strongest single bonds. Its bond dissociation energy (125 kcal/mol) is higher than that of the C−F σ-bond (123 kcal/mol).11 Recently, we reported that fluorogermane can behave as a stronger σ-acceptor (Z-type) ligand than fluorosilane.12,13 This feature may lead to stronger inverse electron flow in σ-bond metathesis, and thus the E−F bond cleavage reaction may be accelerated if the inverse electron flow plays a crucial role in the σ-bond activation process. Comparison of fluorosilane and -germane activation can thus provide valuable insight into our strategy of strong σ-bond activation based on inverse electron flow.

Figure 2. Molecular structures of 5a (left) and 5b-trans (right). X-ray diffraction structures are shown with thermal ellipsoids at 40% probability. The H atoms, the phenyl groups at phosphorus except the ipso carbons, and solvent molecules are omitted for clarity.

Surprisingly, similar Ge−F σ-bond activation took place upon reaction of 2 with the diphosphine germane {o(Ph2P)C6H4}2Ge(F)(Ph) (4b), featuring Ph and F substituents at Ge (Scheme 3). Under slightly more severe conditions than for 4a, 2 reacted with 4b to afford the germyl complex {o(Ph2P)C6H4}2(Ph)Ge{Ir(CO)(PPh3)} (5b) in quantitative spectroscopic yield (31P NMR).15,16 No intermediate was detected by 31P NMR monitoring of the reaction. The 31P NMR spectrum of 5b showed the presence of two isomers, 5btrans and 5b-cis, in an 89:11 ratio in solution (trans and cis refer here to the relative positions of CO and Ge around Ir).17 Each isomer gives in the 31P NMR spectrum a doublet (5b-trans: δ 31.2 ppm; 5b-cis: δ 42.6 ppm) and a triplet (5b-trans: δ 5.5 ppm; 5b-cis: δ 8.9 ppm). Notably, the 31P signals show no coupling to fluorine, indicating that the Ge−F bond of 5b was cleaved. The relatively smaller 2JP−P coupling constant of 5b-cis (22.0 Hz vs 103.4 Hz for 5b-trans) indicates that the PPh3 ligand occupies the position cis to the two phosphines of the [PGeP] ligand. The other isomer, 5b-trans, was assigned to the complex bearing the carbonyl ligand trans to the germyl ligand, which is consistent with the X-ray structure (Figure 2, right). Here it should be noted that the stronger Ge−F σ-bond (BDE: 129 kcal/mol) is cleaved preferentially to the much weaker Ge−CPh σ-bond (BDE: 91 kcal/mol).18 This selectivity is in marked contrast with that observed previously for the Si analogue {o-(Ph2P)C6H4}2Si(F)(Ph) (1b), for which the Si− CPh σ-bond is cleaved preferentially to the Si−F σ-bond (Scheme 1), in line with the larger bond dissociation energy of Si−F vs Si−CPh σ-bonds (BDE of 147 and 101 kcal/mol, respectively).18 The unexpected selectively for Ge−F vs Ge− CPh activation is discussed later on. Next, we performed the reaction of 2 with {o-(Ph2P)C6H4}2Ge(Ph)2 (4c), bearing two phenyl groups at Ge, and found that the germyl complex 5b was formed quantitatively as a result of Ge−CPh σ-bond cleavage.19 This reaction does not occur under the same conditions as the Ge−F activation reactions of 4a and 4b (see Experimental Section “Estimate of t1/2 Concerning the Reaction of 4 with Ir(H)(CO)(PPh3)3 (2)”). No reaction occurred between 4c and 2 over 48 h at 60 °C. More severe conditions (100 °C) were required, and complete conversion was achieved in 12 h, leading to complex 5b (76% isolated yield), again as an 81:19 mixture of trans and cis isomers. Note that t1/2 (time to achieve 50% conversion of 2) largely differs for the Ge−F activation reactions of 4a and 4b: the t1/2 value is 5 times larger for 4b (250 min) than for 4a

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RESULTS AND DISCUSSION The diphosphine-difluorogermane {o-(Ph2P)C6H4}2Ge(F)2 (4a) was prepared by reacting Ge(OEt)4 with 2 equiv of Li{o-(Ph2P)C6H4} followed by OEt/F exchange using HFpyridine, and the possibility of Ge−F activation was investigated.14 Upon reaction of 4a with the iridium hydride complex 2, σ-bond activation took place to afford the germyl compound {o-(Ph 2 P)C 6 H 4 } 2 (F)Ge{Ir(CO)(PPh3 )} (5a) quantitatively according to 31P NMR spectroscopy (Scheme 2).15 The reaction of the fluorogermane 4a is cleaner than that Scheme 2. Ge−F Activation of {o-(Ph2P)C6H4}2Ge(F)2 (4a) at Iridium

of the fluorosilane 1a (which was obtained in 84% NMR yield) and smoother (Ge−F activation is complete within 12 h, while Si−F σ-bond activation requires 24 h to reach completion under the same conditions). The 19F{1H} NMR spectrum of 5a displays a doublet of triplets (dt, JP−F = 20.1 Hz, JP−F = 7.0 Hz) at δ = −176.0 ppm. The 31P{1H} NMR spectrum shows mutually coupled signals at δ = 10.1 ppm (td, 1P, JP−P = 85.9 Hz, JP−F = 20.1 Hz) and δ = 45.7 ppm (dd, 2P, JP−P = 85.9 Hz, JP−F = 7.0 Hz). It should be noted that the 31P NMR signals couple to only one fluorine atom, indicating that one of two Ge−F bonds of 4a was cleaved. An X-ray diffraction study confirmed this feature, and the structure of germyl complex 5a is depicted in Figure 2, left. B

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Scheme 3. Ge−F and Ge−CPh Activation of {o-(Ph2P)C6H4}2Ge(F)(Ph) (4b) and {o-(Ph2P)C6H4}2Ge(Ph)2 (4c), Leading to the Germyl Complex 5b

Figure 3. Geometry and Gibbs energy changes in the σ-bond metathesis. Distances are in angstroms. Hydrogen atoms are omitted for clarity. The Gibbs energies relative to that of {o-(Me2P)C6H4}2(F)2Ge{Ir(H)(CO)(PMe3)} (A1) are provided in parentheses (kcal/mol).

(50 min). Furthermore, the t1/2 values increase significantly when the reactions are performed in the presence of 10 equiv of PPh3 (4a: 380 min; 4b: 1840 min), suggesting that the initial exchange of PPh3 at Ir for the phosphine moieties of the diphosphine-germanes has a noticeable impact on the reaction rate. To shed light on the mechanism of Ge−F cleavage at Ir, density functional theory with the B3PW91 functional was carried out. A thorough investigation taking into consideration various pathways20 suggested that Ge−F activation takes place through σ-bond metathesis starting from the 18-electron species {o-(Me2P)C6H4}2(F)2Ge{Ir(H)(CO)(PMe3)} (A1)21 (Figure 3), which is reminiscent of that previously reported for Si−F activation (see pages S7−S12 in the Supporting Information). In A1, the two phosphines of the [PGeP] framework are in cis arrangement. The Ir−Ge distance (2.720 Å) is longer than the sum of covalent radii (2.61 Å)22 but significantly shorter than the sum of van der Waals radii (4.10 Å),23 suggesting the presence of a dative Ir→Ge interaction. The apical Ge−F bond is longer than the one in equatorial position (1.871 vs 1.836 Å). This can be attributed to the presence of significant d(Ir)→σ*(Ge−F) interaction,24 something that is confirmed by NBO analyses (a donor−acceptor interaction is found at the second-order perturbation level, ΔE = 47.9 kcal/mol). A1 readily isomerizes into intermediate A2 (pseudorotation, ΔG°⧧ = 1.8 kcal/mol),25 which is slightly more stable than A1 (by 0.5 kcal/mol). Subsequently, σ-bond metathesis between the Ge−F and Ir−H bonds takes place with ΔG°⧧ and ΔG° of 9.4 and −8.3 kcal/mol, respectively, to afford

the germyl complex A3. This reaction profile parallels that found for Si−F bond activation, but the activation barrier for the cleavage of the Ge−F bond is noticeably lower (ΔG°⧧ = 9.4 vs 15.1 kcal/mol).10 It is likely that the easiness of the Ge−F bond activation plays a role in the unexpected selectivity for Ge−F vs Ge−CPh activation observed in the reaction of 2 with 4b. To discuss the origin of the low Gibbs activation energy of the Ge−F bond activation, we investigated the variations of the Wiberg bond index (WBI) of Ge/Si−F bonds along the reaction profile (Figure 4a). In A1, the WBI is smaller for Ge−F2 than Ge−F1, because F1 and F2 occupy equatorial and apical positions, respectively. The situation is reversed in A2. The apical Ge−F1 bond, which is weaker than the equatorial Ge−F2 bond according to WBI, is cleaved upon σ-bond metathesis with the Ir−H bond. The tendency of changes of the Wiberg bond index in the Si system is similar to that in the Ge systems, but it should be noted that the Ge−F2 bond in A1 and Ge−F1 bond in A2 are apparently more weakened than the corresponding Si−F σ-bonds in Si analogues. This would indicate that stronger Ir→Ge donation weakens the Ge−F σ-bond more efficiently than the Si system. Next, we looked at the variation of the atomic charges (Figure 4b). In A1, the atomic charges of F1 and F2 are more negative than in F2GePh2, in agreement with a transfer of electron density from Ir to Ge. Note also that the atomic charge is more negative for F2 than for F1 due to the electron transfer associated with the Ir→σ*(Ge−F2) interaction. In A2, the apical F1 atom becomes more negative than the equatorial F2 atom. In addition, the hydrogen atom at Ir C

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Figure 4. (a) Changes in WBIs (Wiberg bond indexes) of E−F1 and E−F2 σ-bonds (E = Si, Ge). WBIs are relative to those of Ph2EF2. (b) Changes in NBO charges on F1, F2, and H atoms. Natural charges on the F atoms and the H ligand are relative to those of Ph2EF2 and A1, respectively.

novel and efficient strategy for the activation of strong and polar σ-bonds including not only E−F bonds (E = Si, Ge) but maybe also other E−Hal bonds. Although the Ge−F σ-bond activation reported here is assisted by phosphine chelation, the strategy can be applied to more general systems without specific donor buttresses. In summary, activation of a Ge−F bond by a transition metal complex has been achieved for the first time. The reaction proceeds under mild and neutral conditions. DFT calculations support mechanisms with an inverse electron flow (which is actually larger with Ge than with Si). The transfer of electron density from Ir to Ge weakens the Ge−F bonds and promotes electrostatically the coupling between the fluoride on Ge and the hydrogen on Ir. The strengthened inverse electron flow found for Ge compared to Si is likely to explain the unexpected selectivity, namely, preferential Ge−F over Ge−CPh bond activation. The strategy based on the inverse electron flow may become a new general avenue for transition-metal-mediated activation and transformation of strong and polar σ-bonds.

becomes positively charged in the order A1 < TSA1/A2 < A2, due to electron transfer from the iridium center to the F2GeAr2 moiety. Thus, the Ir→Ge interaction results in a positively charged H atom and a negatively charged F atom, favoring formation of hydrogen fluoride. It can be noted that the changes in the atomic charges at F by inverse electron flow are smaller with Si than with Ge.26 More negative atomic charge of F1 in A2 should make the formation of hydrogen−fluorine more facile electrostatically, which is another important factor for lower Gibbs activation energy. To better understand the unexpected selectively for Ge−F cleavage in the reaction of 2 with 4b, we then investigated pathways for the activations of the Ge−F and Ge−CPh bonds of 4b (see pages S13−S18 in the Supporting Information for details). As for 4a, Ge−F activation occurs via σ-bond metathesis with a low Gibbs activation energy (ΔG°⧧ = 3.5 kcal/mol) and negative Gibbs reaction energy (ΔG° = −12.3 kcal/mol). (The isomerization prior to σ-bond metathesis requires in this case higher Gibbs activation energy, ΔG°⧧ = 13.6 kcal/mol: see page S13 in the Supporting Information.) As far as the Ge−CPh activation is concerned, oxidative addition followed by reductive elimination is the lowest activation pathway: Gibbs activation energies (ΔG°⧧) of oxidative addition and reductive elimination are calculated to be 21.1 and 16.8 kcal/mol, respectively. σ-Bond metathesis of the Ge− CPh and Ir−H bonds involves a much larger barrier (ΔG°⧧ = 35.7 kcal/mol), which can be attributed, at least in part, to the lower polarity of the Ge−CPh bond. Note that the Gibbs activation energy is lower for the activation of the stronger Ge− F bond than for the weaker Ge−CPh bond, in agreement with our experimental observation.27 These results suggest that σbond metathesis initiated by M→Z interaction may represent a

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EXPERIMENTAL SECTION

General Procedures. All experiments were performed under a dry nitrogen atmosphere using standard Schlenk techniques. Benzene, diethyl ether, tetrahydrofuran, toluene, benzene-d6, and toluene-d8 were dried over sodium and distilled under a dinitrogen atmosphere. Chloroform-d and dichloromethane were dried over 4 Å molecular sieves. The other reagents used in this study were purchased from commercial sources and used without further purification. 1H, 13C{1H}, 19F{1H}, and 31P{1H} NMR spectra were recorded with a JEOL JNM-AL 400 spectrometer. The 1H and 13C{1H} NMR data were analyzed with reference to the residual peaks of the solvent, and the 19 1 F{ H} and 31P{1H} NMR chemical shifts were referenced to external hexafluorobenzene (−164.9 ppm) and 85% H3PO4 (0 ppm) samples, respectively. Elemental analyses were conducted using a J-Science Lab D

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afford a white solid. The residue was washed with hexane (4 mL × 2) and dried under vacuum to afford 4c (372.2 mg, 0.497 mmol) in 87% yield as a white powder. 1H NMR (400 MHz, C6D6): δ 6.88−7.05 (m, 30H, Harom), 7.57 (dd, 2JH−H = 7.3 and 4.6 Hz, 2H, Harom), 7.79 (d, 2 JH−H = 7.6 Hz, 4H, Harom), 8.00 (d, 2JH−H = 7.3 Hz, 2H, Harom). 31 1 P{ H} NMR (162 MHz, C6D6): δ −10.2 (s). 13C{1H} NMR (100 MHz, C6D6): δ 128.0 (s), 128.2 (d, JP−C = 6.6 Hz), 128.4 (s), 128.5 (s), 128.6 (s), 129.5 (d, JP−C = 53.9 Hz), 133.6 (d, JP−C = 18.3 Hz), 136.5 (m), 137.0 (m), 138.3 (dd, JP−C = 15.8 Hz, 2.5 Hz), 139.1 (d, JP−C = 13.3 Hz), 139.8 (t, JP−C = 4.1 Hz), 144.0 (d, JP−C = 11.6 Hz), 147.6 (dd, JP−C = 51.4 Hz, 4.4 Hz). Anal. Calcd for C48H38GeP2: C, 76.93; H, 5.11. Found: C, 77.08; H, 5.39. Preparation of {(o-Ph2PC6H4)2(F)Ge}Ir(CO)(PPh3) (5a). A Schlenk tube was charged with {o-(Ph2P)C6H4}2Ge(F)2 (4a) (152.6 mg, 0.2410 mmol), Ir(H)(CO)(PPh3)3 (2) (243.0 mg, 0.2410 mmol), and toluene (10 mL). The mixture was stirred at 60 °C for 12 h. The solution was filtered, and the volatile materials were removed under vacuum to afford a yellow solid. The residue was washed with hexane (3 mL × 3) and dried under vacuum to afford 5a (169.9 mg, 0.155 mmol) in 64% yield as a yellow powder. 1H NMR (400 MHz, C6D6): δ 6.69−6.73 (m, 4H, Harom), 6.77−6.90 (m, 17H, Harom), 6.96 (s, 1H, Harom), 7.00−7.04 (m, 2H, Harom), 7.12−7.24 (m, 4H, Harom), 7.34− 7.39 (m, 7H, Harom), 7.60−7.67 (m, 6H, Harom), 8.39 (d, 2JH−H = 6.8 Hz, 2H, Harom). 31P{1H} NMR (162 MHz, CDCl3): δ 10.1 (td, 2JP−P = 85.9 Hz, 3JP−F = 20.1 Hz, PPh3), 45.7 (dd, 2JP−P = 85.9 Hz, 3JP−F = 7.0 Hz, PPh2). 19F{1H} NMR (376 MHz, CDCl3): δ −176.0 (dt, 3JP−F = 20.1 Hz, 3JP−F = 7.0 Hz). 13C{1H} NMR (100 MHz, CDCl3): δ 127.4 (d, J = 9.9 Hz), 127.8 (m), 128.3 (m), 128.9 (m), 130.1 (s), 131.5 (m), 132.3 (t, J = 9.9 Hz), 133.0 (d, J = 13.3 Hz), 133.4 (t, J = 7.5 Hz), 138.0 (t, J = 25.7 Hz), 138.7 (dt, J = 41.5 Hz, 2.5 Hz), 140.6 (m), 141.8 (m), 159.0 (m), 188.5 (m, CO). IR (KBr, cm−1) ν(CO): 1962. Anal. Calcd for C55H43FGeIrOP3: C, 60.23; H, 3.95. Found: C, 60.44; H, 4.09. Preparation of {(o-Ph2PC6H4)2(Ph)Ge}Ir(CO)(PPh3) (5b). A Schlenk tube was charged with {o-(Ph2P)C6H4}2Ge(Ph)2 (4c) (172.1 mg, 0.230 mmol), Ir(H)(CO)(PPh3)3 (2) (231.3 mg, 0.229 mmol), and toluene (6 mL). The mixture was stirred at 100 °C for 12 h. The solution was filtered, and the volatile materials were removed under vacuum to afford a yellow solid. The residue was washed with hexane (3 mL × 2) and dried under vacuum to afford 5b (200.3 mg, 0.173 mmol) in 76% yield as a yellow powder. At 20 °C, the ratio of 5b-trans:5b-cis was calculated to be 81:19, and the 31P NMR spectra ranging from −80 to 100 °C did not show any change, indicating that there is no fast interconversion between 5b-trans and 5b-cis. 5b-trans: 1 H NMR (400 MHz, CDCl3): δ 6.64−6.89 (m, 24H, Harom), 6.93− 7.00 (m, 8H, Harom), 7.05−7.39 (m, 7H, Harom), 7.51−7.53 (m, 2H, Harom), 7.65 (m, 1H, Harom), 7.76 (m, 4H, Harom), 7.95 (d, 2JH−H = 7.1 Hz, 2H, Harom). 31P{1H} NMR (162 MHz, CDCl3): δ 8.9 (t, 2JP−P = 103.4 Hz, PPh3), 42.6 (d, 2JP−P = 103.4, PPh2). IR (KBr, cm−1) ν(CO): 1963. 5b-cis: 1H NMR (400 MHz, CDCl3): δ 6.64−6.89 (m, 24H, Harom), 6.93−7.00 (m, 8H, Harom), 7.05−7.39 (m, 7H, Harom), 7.51−7.53 (m, 2H, Harom), 7.65 (m, 1H, Harom), 7.76 (m, 4H, Harom), 8.39 (d, JH−H = 7.8 Hz, 2H, Harom). 31P{1H} NMR (162 MHz, CDCl3): δ 5.5 (t, 2JP−P = 22.0 Hz, PPh3), 31.2 (d, 2JP−P = 22.0 Hz, PPh2). IR (KBr, cm−1) ν(CO): 1919. Anal. Calcd for C61H48GeIrOP3: C, 63,44; H, 4.19. Found: C, 63.28; H, 4.30. Reaction of Ir(H)(CO)(PPh3)3 (2) with {o-(Ph2P)C6H4}2Ge(F)(Ph) (4b). A Schlenk tube was charged with {o-(Ph2P)(C6H4)}2Ge(F)(Ph) (4b) (161.8 mg, 0.234 mmol), Ir(H)(CO)(PPh3)3 (2) (235.7 mg, 0.234 mmol), and toluene (5 mL). The mixture was stirred at 100 °C for 12 h. The solution was filtered, and the volatile materials were removed under vacuum to afford a yellow solid. The residue was washed with hexane (3 mL × 2) and dried under vacuum to afford 5b (171.5 mg, 0.149 mmol) in 64% yield as a yellow powder. Determination of NMR Yield. (a) Reaction of 4a with 2: An NMR tube was charged with 4a (5.4 mg, 0.0085 mmol), 2 (8.5 mg, 0.0085 mmol), and toluene (0.60 mL), and a capillary filled with a toluene solution of trimesitylphosphine was placed in the NMR tube as an internal standard. The reaction was performed at 60 °C for 12 h to afford 5a in >99% NMR yield (within the detection of 31P NMR

JM-10 or Fisons Instrument EA1108 elemental analyzer. {o-(Ph2P)C6H4}Li·Et2O28 and Ir(H)(CO)(PPh3)3 (2)29 were prepared as described in the literature. Preparation of {o-(Ph2P)C6H4}2GeF2 (4a). A Schlenk tube was charged with {o-(Ph2P)C6H4}Li·Et2O (435.7 mg, 1.27 mmol) and THF (10 mL), and the solution was cooled to −78 °C. Ge(OEt)4 (117 μL, 0.571 mmol) was added slowly to the prepared reaction solution, and the mixture was gradually allowed to reach room temperature. The reaction mixture was stirred at room temperature for 12 h. The solution was filtered, and the volatile materials were removed under vacuum to afford a white solid. The residue was washed with hexane (4 mL × 3) and dried under vacuum to afford a white powder. After the white power was dissolved in CH2Cl2 (6 mL), hydrogen fluoride-pyridine (130 μL, 0.976 mmol) was added to the solution, and the mixture was stirred at room temperature for 2 h. The solution was filtered, and the volatile materials were removed under vacuum to afford a white solid. The white residue was washed with hexane (2 mL × 2) and dried under vacuum to afford 4a (162.6 mg, 0.257 mmol) in 45% yield as a white powder. 1H NMR (400 MHz, C6D6): δ 6.90−6.99 (m, 16H, Harom), 7.08−7.36 (m, 10H, Harom), 8.44 (d, 2JH−H = 7.6 Hz, 2H, Harom). 13C{1H} NMR (100 MHz, C6D6): δ 128.7 (d, JP−C = 4.1 Hz), 128.8 (d, JP−C = 14.1 Hz), 130.4 (s), 131.7 (s), 133.5 (d, JP−C = 18.3 Hz), 134.4 (s), 135.4 (d, JP−C = 23.2 Hz), 136.7 (m), 143.1 (m), 146.5 (m). 19F{1H} NMR (376 MHz, C6D6): δ −158.2 (t, 3JP−F = 31.6 Hz). 31P{1H} NMR (162 MHz, C6D6): δ −4.5 (t, 3JP−F = 31.6 Hz). Anal. Calcd for C36H28F2GeP2: C, 68.29; H, 4.46. Found: C, 68.21; H, 4.58. Preparation of {o-(Ph2P)C6H4}2Ge(F)(Ph) (4b). A Schlenk tube was charged with {o-(Ph2P)C6H4}Li·Et2O (460.7 mg, 1.35 mmol) and toluene (10 mL), and the solution was cooled to −78 °C. GePhCl3 (0.673 mmol), a 1 M solution in toluene, was added slowly to the prepared reaction solution, and the mixture was allowed to reach room temperature. The reaction mixture was stirred at 100 °C for 15 h. The solution was filtered, and the volatile materials were removed under vacuum to afford a white solid. The white residue was washed with hexane (4 mL × 3) and dried under vacuum to afford {o(Ph2P)C6H4}2Ge(Cl)(Ph) (370.6 mg, 0.523 mmol) in 78% yield as a white powder. 1H NMR (400 MHz, CDCl3): δ 6.81−6.85 (m, 4H, Harom), 6.99−7.03 (m, 4H, Harom), 7.09−7.18 (m, 11H, Harom), 7.20− 7.25 (m, 4H, Harom), 7.31−7.36 (m, 2H, Harom), 7.38−7.43 (m, 4H, Harom), 7.66−7.69 (m, 2H, Harom), 8.07−8.10 (m, 2H, Harom). 13C{1H} NMR (100 MHz, CDCl3): δ 128.0 (s), 128.2 (m), 128.4 (m), 129.5 (s), 129.6 (s), 130.4 (s), 133.3 (m), 135.0 (m), 135.7 (s), 136.3 (dd, JP−C = 17.1 Hz, 3.0 Hz), 137.5 (dd, JP−C = 48.2 Hz, 10.0 Hz), 139.0 (t, JP−C = 5.0 Hz), 142.8 (dd, JP−C = 12.0 Hz, 2.0 Hz), 146.7 (dd, JP−C = 51.2 Hz, 5.0 Hz). 31P{1H} NMR (162 MHz, C6D6): δ − 11.1 (s). Anal. Calcd for C42H33ClGeP2: C, 71.28; H, 4.70. Found: C, 71.26; H, 4.56. A Schlenk tube was charged with {o-(Ph2P)C6H4}2Ge(Cl)(Ph) (460.7 mg, 0.651 mmol), CsF (1.589 g, 10.5 mmol), and THF (10 mL). The mixture was stirred at 50 °C for 2 h. The solution was filtered, and the volatile materials were removed under vacuum to afford a white solid. The residue was washed with hexane (4 mL × 2) and dried under vacuum to afford 4b (240.3 mg, 0.347 mmol) in 53% yield as a white powder. 1H NMR (400 MHz, C6D6): δ 6.87−7.16 (m, 23H, Harom), 7.21−7.26 (m, 4H, Harom), 7.37−7.40 (m, 2H, Harom), 7.99−8.02 (m, 2H, Harom), 8.30 (d, 2JH−H = 7.6 Hz, 2H, Harom). 31 1 P{ H} NMR (162 MHz, CDCl3): δ −9.1 (t, 3JP−F = 42.1 Hz). 19 1 F{ H} NMR (376 MHz, CDCl3): δ −187.8 (t, 3JP−F = 42.1 Hz). 13 C{1H} NMR (100 MHz, CDCl3): δ 128.1 (s), 128.3 (m), 128.5 (s), 129.4 (s), 129.8 (m), 130.4 (s), 132.3 (m), 133.0 (m), 134.9 (s), 136.0 (m), 137.4 (m), 137.4 (m), 143.0 (m), 146.9 (m). Anal. Calcd for C42H33FGeP2: C, 72.97; H, 4.81. Found: C, 72.65; H, 4.76. Preparation of {o-(Ph2P)C6H4}2Ge(Ph)2 (4c). A Schlenk tube was charged with {o-(Ph2P)C6H4}Li·Et2O (415.7 mg, 1.21 mmol) and toluene (10 mL), and the solution was cooled to −78 °C. Ph2GeCl2 (117 μL, 0.571 mmol) was added slowly to the prepared reaction solution, and the mixture was allowed to reach room temperature. The reaction mixture was stirred at 110 °C for 12 h. The solution was filtered, and the volatile materials were removed under vacuum to E

DOI: 10.1021/acs.organomet.5b01000 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

Aid for Scientific Research on Innovative Areas “StimuliResponsive Chemical Species for the Creation of Functional Molecules” (Nos. 15H00940, 15H00957, and 15H00958) from the Ministry of Education, Science, Sports, and Culture of Japan (MEXT). H.K. wishes to acknowledge the financial support from the Kyoto Technoscience Center. Also, we acknowledge financial support from Toyota Motor Corporation.

spectroscopy). (b) Reaction of 4b with 2: An NMR tube was charged with 4b (5.9 mg, 0.0085 mmol), 2 (8.5 mg, 0.0085 mmol), and toluene (0.60 mL), and a capillary filled with a toluene solution of trimesitylphosphine was placed in the NMR tube as an internal standard. The reaction was performed at 100 °C for 12 h to afford 5b in >99% NMR yield (within the detection of 31P NMR spectroscopy). The reaction in a Teflon vessel was performed under the same condition (the sample was prepared under a dry nitrogen atmosphere using a VAC inert-gas glovebox), and the exclusive formation of 5b was confirmed. (c) Reaction of 4c with 2: An NMR tube was charged with 4c (6.4 mg, 0.0085 mmol), 2 (8.5 mg, 0.0085 mmol), and toluene (0.60 mL), and a capillary filled with a toluene solution of trimesitylphosphine was placed in the NMR tube as an internal standard. The reaction was performed at 100 °C for 12 h to afford 5b in >99% NMR yield (within the detection of 31P NMR spectroscopy). Estimate of t1/2 Concerning the Reaction of 4 with Ir(H)(CO)(PPh3)3 (2). An NMR tube was charged with 4a (5.4 mg, 0.0085 mmol), 2 (8.5 mg, 0.0085 mmol), and toluene (0.60 mL). A sealed capillary containing a toluene solution of trimesitylphosphine was placed in the NMR tube to serve as the external standard. The reaction was performed at 60 °C, and the reaction was monitored by 31 1 P{ H} NMR spectroscopy. t1/2 was calculated on the basis of the time conversion of 2: t1/2 = 50 min. t1/2 concerning the reactions of 4b and 4c with 2 were calculated by using 4b (5.9 mg, 0.0085 mmol) and 4c (6.4 mg, 0.0085 mmol) instead of 4a: t1/2 = 250 min for 4b; unavailable for 4c due to no conversion. t1/2 concerning these reactions with excess PPh3 (22.8 mg, 0.087 mmol): t1/2 = 380 min for 4a; t1/2 = 9840 min for 4b; unavailable for 4c due to no conversion. Structure Determination by X-ray Diffraction. Suitable single crystals of 5a were obtained from the slow diffusion of n-hexane into a benzene solution. Single crystals of 5b-trans were grown by the slow diffusion of n-hexane into a dichloromethane solution. Diffraction intensity data were collected with a Rigaku/MSC Mercury CCD diffractometer at 200 K(2), and a semiempirical multiscan absorption30 correction was performed. The space groups were chosen based on the systematic absences in the diffraction data. The structures were solved using SIR9731 by subsequent difference Fourier synthesis and refined by full matrix least-squares procedures on F2. All non-hydrogen atoms were refined with anisotropic displacement coefficients. The hydrogen atoms were treated as idealized contributions and refined in a rigid group model. All software and sources of scattering factors are contained in the SHELXL97 program package.32 CCDC 1062023 and 1062024 contain supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via www. ccdc.cam.ac.uk/data_request/cif.

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(1) (a) Crabtree, R. H. In Organometallic Chemistry of the Transition Metals, 5th ed.; John Wiley & Sons, Inc.: NJ, 2009. (b) Hartwig, J. F. Organotransition Metal Chemistry from Bonding to Catalysis; University Science Books: Sausalito, 2010. (2) Guan, W.; Sayyed, F. B.; Zeng, G.; Sakaki, S. Inorg. Chem. 2014, 53, 6444. (3) Selected reviews for chemistry of σ-acceptor (Z-type) ligands. (a) Green, M. L. H. J. Organomet. Chem. 1995, 500, 127. (b) Hill, A. F. Organometallics 2006, 25, 4741. (c) Parkin, G. Organometallics 2006, 25, 4744. (d) Kuzu, I.; Krummenacher, I.; Meyer, J.; Armbruster, F.; Breher, F. Dalton Trans. 2008, 5836. (e) Fontaine, F.-G.; Boudreau, J.; Thibault, M.-H. Eur. J. Inorg. Chem. 2008, 2008, 5439. (f) Braunschweig, H.; Dewhurst, R. D.; Schneider, A. Chem. Rev. 2010, 110, 3924. (g) Bouhadir, G.; Amgoune, A.; Bourissou, D. Adv. Organomet. Chem. 2010, 58, 1. (h) Amgoune, A.; Bourissou, D. Chem. Commun. 2011, 47, 859. (i) Braunschweig, H.; Dewhurst, R. D. Dalton Trans. 2011, 40, 549. (j) Owen, G. R. Chem. Soc. Rev. 2012, 41, 3535. (k) Kameo, H.; Nakazawa, H. Chem. - Asian J. 2013, 8, 1720. (l) Mingos, D. M. P. J. Organomet. Chem. 2014, 751, 153. (m) Bouhadir, G.; Bourissou D. Chem. Soc. Rev. 2016, 45, 106510.1039/C5CS00697J. (4) (a) Devillard, M.; Bouhadir, G.; Bourissou, D. Angew. Chem., Int. Ed. 2015, 54, 730. (b) Tsoureas, N.; Kuo, Y.-Y.; Haddow, M. F.; Owen, G. R. Chem. Commun. 2011, 47, 484. (c) Harman, W. H.; Peters, J. C. J. Am. Chem. Soc. 2012, 134, 5080. (d) Fong, H.; Moret, M. E.; Lee, Y.; Peters, J. C. Organometallics 2013, 32, 3053. (e) MacMillan, S. N.; Harman, W. H.; Peters, J. C. Chem. Sci. 2014, 5, 590. (f) Harman, W. H.; Lin, T. P.; Peters, J. C. Angew. Chem., Int. Ed. 2014, 53, 1081. (g) Zeng, G.; Sakaki, S. Inorg. Chem. 2013, 52, 2844. (h) Khusnutdinova, J. R.; Milstein, D. Angew. Chem., Int. Ed. 2015, 54, 12236. (5) For the activation of E−H bonds (E = N, O) by metal−borane cooperation with transfer of E to the borane moiety and transfer of the H atom to the transition metal, see: (a) Barnett, B. R.; Moore, C. E.; Rheingold, A. L.; Figueroa, J. S. J. Am. Chem. Soc. 2014, 136, 10262. (b) Cowie, B. E.; Emslie, D. J. E. Chem. - Eur. J. 2014, 20, 16899. (6) (a) Blum, O.; Frolow, F.; Milstein, D. J. Chem. Soc., Chem. Commun. 1991, 258. (b) Grushin, V. V.; Marshall, W. J. J. Am. Chem. Soc. 2004, 126, 3068. (c) Jasim, N. A.; Perutz, R. N.; Whitwood, A. C.; Braun, T.; Izundu, J.; Neumann, B.; Rothfeld, S.; Stammler, H.-G. Organometallics 2004, 23, 6140. (d) Macgregor, S. A.; Roe, D. C.; Marshall, W. J.; Bloch, K. M.; Bakhmutov, V. I.; Grushin, V. V. J. Am. Chem. Soc. 2005, 127, 15304. (e) Erhardt, S.; Macgregor, S. A. J. Am. Chem. Soc. 2008, 130, 15490. (f) Nova, A.; Erhardt, S.; Jasim, N. A.; Perutz, R. N.; Macgregor, S. A.; McGrady, J. E.; Whitwood, A. C. J. Am. Chem. Soc. 2008, 130, 15499. (g) Teltewskoi, M.; Panetier, J. A.; Macgregor, S. A.; Braun, T. Angew. Chem., Int. Ed. 2010, 49, 3947. (h) Raza, A. L.; Panetier, J. A.; Teltewskoi, M.; Macgregor, S. A.; Braun, T. Organometallics 2013, 32, 3795. (7) Prediction of difficulty in oxidative addition of the Si−F σ-bond. (a) Sakaki, S.; Ieki, M. J. Am. Chem. Soc. 1993, 115, 2373. (b) Kameo, H.; Sakaki, S. Chem. - Eur. J. 2015, 21, 13588. (8) The B−F σ-bond is another strong and polar σ-bond, and the chemistry of transition-metal-mediated cleavage of the B−F σ-bond has been developed by Braunschweig et al. (a) Bauer, J.; Braunschweig, H.; Kraft, K.; Radacki, K. Angew. Chem., Int. Ed. 2011, 50, 10457. (b) Bauer, J.; Braunschweig, H.; Dewhurst, R. D.; Radacki, K. Chem. Eur. J. 2013, 19, 8797.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.5b01000. Crystallographic data and computational details (PDF) Crystallographic data (CIF) (TXT)

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was supported by a Grant-in-Aid for Scientific Research (C) (Nos. 15K05458 and 15K05459) from Japan Society for the Promotion of Science (JSPS) and by a Grant-inF

DOI: 10.1021/acs.organomet.5b01000 Organometallics XXXX, XXX, XXX−XXX

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Organometallics (9) For rare catalytic transformations supposed to involve Si−X oxidative addition, see: (a) Yamashita, H.; Kobayashi, T.; Hayashi, T.; Tanaka, M. Chem. Lett. 1991, 761. (b) Chatani, N.; Amishiro, N.; Murai, S. J. Am. Chem. Soc. 1991, 113, 7778. (c) McAtee, J. R.; Martin, S. E. S.; Ahneman, D. T.; Johnson, K. A.; Watson, D. A. Angew. Chem., Int. Ed. 2012, 51, 3663. (10) Kameo, H.; Kawamoto, T.; Sakaki, S.; Bourissou, D.; Nakazawa, H. Chem. - Eur. J. 2016, 22, 2370. (11) Luo, Y. R. In Comprehensive Handbook of Chemical Bond Energies; CRC Press: Boca Raton, FL, 2007. (12) (a) Kameo, H.; Kawamoto, T.; Bourissou, D.; Sakaki, S.; Nakazawa, H. Organometallics 2015, 34, 1440. For synthesis of related ligand precursors, see: (b) Kameo, H.; Kawamoto, T.; Sakaki, S.; Nakazawa, H. Organometallics 2014, 33, 5960. (c) Kameo, H.; Kawamoto, T.; Bourissou, D.; Sakaki, S.; Nakazawa, H. Organometallics 2014, 33, 6557. (13) Heavier group 14 elements act as stronger electron acceptor ligands. (a) Gualco, P.; Lin, T. P.; Sircoglou, M.; Mercy, M.; Ladeira, S.; Bouhadir, G.; Perez, L. M.; Amgoune, A.; Maron, L.; Gabbaï, F. P.; Bourissou, D. Angew. Chem., Int. Ed. 2009, 48, 9882. (b) Gualco, P.; Mercy, M.; Ladeira, S.; Coppel, Y.; Maron, L.; Amgoune, A.; Bourissou, D. Chem. - Eur. J. 2010, 16, 10808. (c) Gualco, P.; Ladeira, S.; Kameo, H.; Nakazawa, H.; Mercy, M.; Maron, L.; Amgoune, A.; Bourissou, D. Organometallics 2015, 34, 1449. (14) Milstein et al. have developed chemistry of inert σ-bond activation based on phosphine chelation. (a) Gozin, M.; Weisman, A.; Ben-David, Y.; Milstein, D. Nature 1993, 364, 699. (b) Gozin, M.; Aizenberg, M.; Liou, S.-Y.; Weisman, A.; Ben-David, Y.; Milstein, D. Nature 1994, 370, 42. (15) Although the formation of HF and HF2− was not confirmed by 1 H and 19F NMR spectroscopy, several intractable products were detected by 19F and 31P NMR spectroscopy. This observation implies that PPh3 acts as a scavenger of HF. This would be supported by the experimental observation that the reaction of PPh3 with hydrofluoric acid provided the same intractable mixture. (16) The reaction of 4b with 2 in a Teflon vessel resulted in exclusive formation of Ge−F bond activation product in a similar way to that in a glass vessel, and thus the kind of vessel has no influence on the selectivity of Ge−F bond activation. (17) Spectroscopic analysis on 5b demonstrated the presence of two isomers, 5b-trans and 5b-cis, while that on 5a indicated only one isomer with trans form, which is probably attributed to steric repulsion between the PPh3 ligand and the substituent at the Ge atom. Accordingly, the replacement of the Ph group by the F atom resulted in alleviation of the repulsion, exclusively affording the trans form. A similar phenomenon was previously observed. See: Kameo, H.; Ishii, S.; Nakazawa, H. Dalton Trans. 2013, 42, 4663. (18) Bond dissociation energies of E−F and E−CPh σ-bonds (E = Si, Ge) were calculated with Ph3EF in the gas phase at the B3PW91/6311G** level of theory. (19) Similar Ge−C σ-bond activation was reported previously: Kameo, H.; Ishii, S.; Nakazawa, H. Dalton trans. 2012, 41, 11386. (20) Although one may consider the possibility of an ionic dissociation (SN2-type) pathway because of the highly polar character of the Ge−F bond, fluorine dissociation modeled by the elongation of the Ge−F bonds in possible intermediates did not provide any stable species (the details are shown on pages S7−S12 in the Supporting Information). Hence, the possibility of an ionic dissociation (SN2type) mechanism was ruled out. This is attributed, at least in part, to the high activation barrier for fluoride dissociation from fluorosilane and fluorogermane. According to B3PW91/6-311G** calculations, the activation barriers for fluoride dissociations from Ph3SiF and Ph3GeF were calculated to be 255 and 236 kcal/mol, respectively. These energies are slightly larger than that from Ph3CF (225 kcal/mol). (21) Iridium hydride 2 reacts with the diphosphine-borane ligand {o(Ph2P)C6H4}2B(Ph) to afford {o-(Ph2P)C6H4}2(Ph)B{Ir(H)(CO) (PPh3)}, featuring a dative Ir→B interaction. See: Kameo, H.; Nakazawa, H. Organometallics 2012, 31, 7476.

(22) Cordero, B.; Gómez, V.; Platero-Prats, A. E.; Revés, M.; Echeverría, J.; Cremades, E.; Barragán, F.; Alvarez, S. Dalton Trans. 2008, 37, 2832. (23) Batsanov, S. S. Inorg. Mater. 2001, 37, 871. (24) Examples of transition metal complexes bearing a M→Ge interaction: Kano, N.; Yoshinari, N.; Shibata, Y.; Miyachi, M.; Kawashima, T.; Enomoto, M.; Okazawa, A.; Kojima, N.; Guo, J.-D.; Nagase, S. Organometallics 2012, 31, 8059 and references therein. (25) An example is that a transition metal occupies the equatorial position of a pentacoordiate Si atom: Sun, J.; Ou, C.; Wang, C.; Uchiyama, M.; Deng, L. Organometallics 2015, 34, 1546. (26) As previously reported in ref 12a, an energy level of an antibonding orbital is lower for the σ*(Ge−F) orbital in Ph3GeF than the σ*(Si−F) orbital in Ph3SiF, which is probably responsible for a stronger Ir→Ge interaction relative to the Ir→Si interaction. (27) We also investigated pathways for the activations of the Si−F and Si−CPh bonds of 1b (see pages S13−S18 in the Supporting Information). (28) Kameo, H.; Ishii, S.; Nakazawa, H. Organometallics 2012, 31, 2212. (29) Wilkinson, G. Inorg. Synth. 1972, 13, 126. (30) Rigaku. REQAB, Version 1.1; Rigaku Coporation: Tokyo, Japan, 1998. (31) Altomare, A.; Burla, M. C.; Camalli, M.; Cascarano, G.; Giacovazzo, C.; Guagliard, A.; Moliterni, A. G. G.; Spagna, R. J. Appl. Crystallogr. 1999, 32, 115. (32) Sheldrick, G. M. SHELXL97: Program for the Refinement of Crystal Structures; University of Göttingen: Germany, 1997.

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DOI: 10.1021/acs.organomet.5b01000 Organometallics XXXX, XXX, XXX−XXX