Rhodium(I) Silyl Complexes for CâF Bond ... - ACS Publications

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Rhodium(I) Silyl Complexes for C−F Bond Activation Reactions of Aromatic Compounds: Experimental and Computational Studies Anna Lena Raza,† Julien A. Panetier,‡,§ Michael Teltewskoi,† Stuart A. Macgregor,*,‡ and Thomas Braun*,† †

Department of Chemistry, Humboldt-Universität zu Berlin, Brook-Taylor-Strasse 2, 12489 Berlin, Germany Institute of Chemical Sciences, School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh EH14 4AS, U.K.

‡

S Supporting Information *

ABSTRACT: The rhodium(I) silyl complexes [Rh{Si(OEt)3}(PEt3)3] (2a) and [Rh{Si(OMe)3}(PEt3)3] (2b) were synthesized by treatment of [Rh(CH3)(PEt3)3] (1) with the corresponding silanes HSi(OEt)3 and HSi(OMe)3 at low temperature. The intermediate oxidative addition products fac-[Rh(H)(CH3){Si(OR)3}(PEt3)3] (R = Et, 6a; R = Me, 6b) were observed by low-temperature NMR spectroscopy. A reaction of 2a with CO afforded trans-[Rh(CO){Si(OEt)3}(PEt3)2] (7) by the replacement of the phosphine ligand in the position trans to the silyl group. Treatment of 2a,b with pentafluoropyridine led to C−F activation reactions at the 2-position, yielding [Rh(2-C5F4N)(PEt3)3] (11). The silyl complexes [Rh{Si(OR)3}(PEt3)3] (2a,b) gave with 2,3,5,6-tetrafluoropyridine the C−F activation product [Rh(2-C5F3HN)(PEt3)3] (10), whereas complex 7 reacted by C−H activation to furnish trans-[Rh(CO)(4-C5F4N)(PEt3)2] (12). The C−F activation of pentafluoropyridine at 2b was studied with density functional theory calculations using a [Rh{Si(OMe)3}(PMe3)3] model complex (2′). The calculations indicate that a silyl-assisted C−F activation mechanism, analogous to related ligand-assisted processes at metal−phosphine and metal−boryl bonds, is more accessible than a C−F oxidative addition/Si−F reductive elimination pathway. The silyl-assisted process also proceeds with a kinetic preference for activation at the 2-position, as the transition state in this case derives extra stabilization through a Rh···N interaction. The C−F oxidative addition transition states show a significant degree of phosphine-assisted character and are not only higher in energy than the silyl-assisted process but also favor activation at the 4-position.

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INTRODUCTION Fluorinated organic compounds and building blocks play a key role in a wide range of applications and can be found, for example, in agrochemicals, pharmaceuticals, and refrigerants.1 The unique properties of these compounds can in part be attributed to the strength of the carbon−fluorine bond.2 An organometallic approach for the synthesis of fluorinated compounds consists of a metal-mediated selective fluorination of nonfluorinated precursors.3 However, a promising alternative route to access fluorinated building blocks is represented by selective C−F bond cleavage reactions at highly fluorinated molecules.4 There are a variety of strategies to induce a C−F activation step, such as an oxidative addition of a C−F bond at a transition-metal center.5 In an alternative reaction pathway hydrido complexes can also react with fluoroorganics to give metal fluoro compounds and hydrodefluorination products.4,6 Other approaches involve the employment of metal hydrido, boryl, or silyl complexes to yield on treatment with highly fluorinated organics HF, fluoroboranes, or fluorosilanes, respectively, as well as the corresponding organyl metal complexes.5,7 Thus, a catalytic C−F activation and hydrodefluorination of pentafluoropyridine was reported on using [Rh(H)(PEt3)3] as catalyst. The stoichiometric reaction of the heterocycle with the hydrido complex yielded the C−F © 2013 American Chemical Society

activation product [Rh(4-C5NF4)(PEt3)3]. In the presence of H2 2,3,5,6-tetrafluoropyridine and [Rh(H)(PEt3)3] were formed. The reaction is characterized by a selective C−F activation at the 4-position at pentafluoropyridine.8 In contrast, a unique reaction of the boryl complex [Rh(Bpin)(PEt3)3] (HBpin = 4,4,5,5-tetramethyl-1,3,2-dioxaborolane, pinacolborane) with pentafluoropyridine afforded the activation of pentafluoropyridine at the 2-position to give [Rh(2-C5NF4)(PEt3)3] and FBpin. Catalytic studies led to the development of a process which involves the formation of the boronate ester 2C5N(Bpin)F4.9 Density functional theory (DFT) calculations have played a significant role in both revealing the mechanistic complexity and rationalizing the selectivity of aromatic C−F activation at transition-metal complexes.4d,6m,q,10,11 For example, calculations showed that the C−F activation of pentafluoropyridine at [Rh(Bpin)(PEt3)3] proceeds via a four-centered boryl-assisted transition state in which C−F bond cleavage occurs over the Rh−B bond. The unusual selectivity for the 2-position results from the extra stabilization due to a Rh···N interaction involving the pentafluoropyridine nitrogen (see Scheme 1).9 Received: February 22, 2013 Published: July 8, 2013 3795

dx.doi.org/10.1021/om400150p | Organometallics 2013, 32, 3795−3807

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pentafluoropyridine proceeds via a silyl-assisted mechanism. This process adds to the range of the general class of ligandassisted C−F activation processes.

Scheme 1. Boryl-Assisted C−F Activation Transition State for the Reaction of Pentafluoropyridine at [Rh(Bpin)(PR3)3] (R = Me, Computational; R = Et, Experimental)9

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RESULTS AND DISCUSSION Experimental Studies. Synthesis of Rhodium(I) Silyl Complexes. The methyl complex [Rh(CH3)(PEt3)3] (1) was treated with a variety of silanes at room temperature as well as at low temperatures. Reactions of 1 with HSiEt3, HSiPh3, HSi(OEt)2Me, or HSiMePh2 led to the formation of the hydrido complex [Rh(H)(PEt3)3] (4) and MeSiEt3, MeSiPh3, Me2Si(OEt)2, or Me2SiPh2 (Scheme 2). The formation of 4 was also observed on using HSi(OEt)3 or HSi(OMe)3 at room temperature. However, the latter reactions at 223 K gave the rhodium(I) silyl complexes [Rh{Si(OEt)3}(PEt3)3] (2a) and [Rh{Si(OMe)3}(PEt3)3] (2b) as well as methane (Scheme 2). The complexes 2a,b could not be isolated and were only characterized in solution, because of their high reactivity. The 31 1 P{ H} NMR spectrum of 2a shows at room temperature a doublet at δ 13.6 ppm for the phosphine ligands. The rhodium−phosphorus coupling constant is 144 Hz. In addition, silicon satellites with a coupling constant of 47 Hz can be seen shouldering the main peak. The presence of only one signal for the phosphine ligands can be explained by a dynamic behavior which involves an intramolecular exchange of phosphine ligands. Variable-temperature NMR measurements showed that the presence of free phosphine has no influence on the signal pattern and therefore on the dynamic behavior of [Rh{Si(OEt)3}(PEt3)3] (2a). At 183 K the 31P{1H} NMR spectrum shows the expected splitting patterns: a doublet of doublets at δ 20.4 ppm (JRh,P = 151 Hz, JP,P = 34 Hz) for the phosphine ligands in mutually trans positions and a doublet of triplets at δ 3.2 ppm (JRh,P = 118 Hz, JP,P = 34 Hz) for the phosphorus atom in the position trans to the silyl ligand (Figure 1). The rhodium−phosphorus coupling constants are characteristic for a rhodium(I) species.18 The coalescence temperature is 233 K (121.5 MHz), at which the estimated Gibbs activation energy is 9.3 kcal mol−1.19 A comparable fluxional behavior has been observed for [Rh(H)(PEt3)3] (4) and [Rh(Bpin)(PEt3)3],8,9,20 as well as for a range of [Rh(X)(PPh3)3] complexes (X = H, CH3, CF3, Ph). In the latter cases DFT calculations propose a role for an intermediate with local C3 symmetry at Rh, the accessibility of which reflects the trans influence of the ligand X.21 In the 29Si NMR spectrum of 2a at 300 K a doublet of quartets at δ −11.3 ppm with a

Related phosphine-assisted C−F activation of fluoroaromatics, ArF−F (ArF = C6F5, C5F4N), had previously been characterized computationally at [LnM(PR3)] complexes (M = Ir,12a Pt,12b,c Ni13). In such cases C−F activation has led to the location of metallophosphorane intermediates, [LnM(ArF)(PR3F)],14 from which subsequent R group transfer results in fluorophosphine products.12 Alternatively, F transfer can take place to give apparent oxidative addition products, [LnM(ArF)(F)(PR3)]. Such processes have been termed “phosphine-assisted C−F oxidative addition” to reflect the masked role of the ligand in promoting the reaction. An example was seen in the reaction of [Ni(PEt3)2] with pentafluoropyridine to give trans-[Ni(2C5NF4)(F)(PEt3)2],13 in which DFT calculations accounted for the unusual selectivity for the 2-position by a phosphineassisted C−F activation pathway. Examples of the application of rhodium(I) silyl complexes for C−F activation reactions are rare.15 Milstein et al. reported the activation of hexafluorobenzene and pentafluorobenzene at [Rh(SiR3)(PMe3)3] (R3 = Ph3, Me2Ph).16 Perutz and Marder et al. published additional studies on the reactivity of [Rh(SiPh3)(PMe3)3] toward fluorinated pyridines.17 They showed that the reaction with pentafluoropyridine gives C−F activation products with the rhodium center at the 2- or 4-position of the heterocycle in a 3:1 ratio. Treatment of [Rh(SiPh3)(PMe 3 ) 3 ] with 2,3,5,6-tetrafluoropyridine led to C−H activation at the 4-position and C−F activation at the 2position in a ratio of 1:1.3. In this paper we present the synthesis of the rhodium(I) silyl complexes [Rh{Si(OR)3}(PEt3)3] (R = Et, 2a; R = Me, 2b) by treatment of the methyl complex [Rh(CH3)(PEt3)3] (1) with HSi(OR)3. Studies on the reactivity of the complexes 2a,b toward fluorinated aromatics, 2,3,5,6-tetrafluoropyridine, and pentafluoropyridine showed a unique reaction pattern. DFT calculations revealed that the C−F activation step at Scheme 2. Reactivity of the Methyl Complex 1 toward Silanes

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Figure 1. 31P{1H} NMR spectra of [Rh{Si(OEt)3}(PEt3)3] (2a) at room temperature and at 183 K (121.5 MHz, [D8]toluene).

Figure 2. 31P{1H} NMR spectrum of fac-[Rh(H)(CH3){Si(OEt)3}(PEt3)3] (6a) at 203 K (121.5 MHz, [D8]toluene).

rhodium−silicon coupling of 60 Hz and silicon−phosphorus coupling of 47 Hz was observed. The NMR data of 2b are comparable to those of 2a. Monitoring the formation of [Rh{Si(OR)3}(PEt3)3] (2a,b) by low-temperature NMR spectroscopy at 193 K revealed that the methyl complex [Rh(CH3)(PEt3)3] (1) reacted with HSi(OR)3 to give initially the oxidative addition products fac[Rh(H)(CH3){Si(OR)3}(PEt3)3] (R = Et, 6a; R = Me, 6b) (Scheme 3). At 223 K the reductive elimination of methane and the formation of 2a or 2b was observed. Addition of free phosphine has no influence on the rate of the methane elimination. Note also that in similar reactions of the rhodium trimethylphosphine complexes [Rh(CH3)(PMe3)3,4] with other silanes such as HSiEt3 or HSiPh3, intermediates such as 6a,b were not observed.22 The 31P{1H} NMR spectrum of 6a displays three doublets of doublets of doublets at δ 12.2, 4.0, and −3.3 ppm with rhodium−phosphorus coupling constants of 71−94 Hz which reveal the presence of a rhodium(III) compound (Figure 2).18 This pattern is compatible with a fac configuration for 6a. A 31 29 P, Si HMBC NMR spectrum, which was optimized on a large phosphorus−silicon coupling, shows that the signal at δ −3.3 ppm can be assigned to the phosphorus atom in the position trans to the silyl group. The 1H NMR spectrum shows a broad multiplet at δ 0.04 ppm that can be assigned to the methyl ligand. 31P decoupling experiments revealed a rhodium−hydrogen coupling constant of 1.8 Hz. A doublet of doublets of multiplets at δ −10.8 ppm confirms the presence of a hydrido ligand. The coupling constant to the phosphorus atom in the trans position is 144 Hz. 31P decoupling experiments revealed a rhodium−hydrogen coupling constant of 18 Hz. A 1H,29Si HMBC NMR spectrum exhibits a resonance at δ −6.6 ppm in the 29Si domain, which correlates with the signals for the hydrido ligand and the CH2 groups of the ethoxy moieties. The coupling constant of the hydrido ligand to the silicon atom is very small and could not be

observed in the HMBC NMR spectrum. The NMR data of 6b are comparable to those of 6a. The reaction of [Rh(CH3)(PEt3)3] (1) with an excess of HSi(OEt)3 gave the oxidative addition products 3 and 5a, which were characterized by NMR spectroscopy. At low temperature cis,fac-[Rh(H){Si(OEt)3}2(PEt3)3] (3) was furnished, whereas at room temperature cis,fac-[Rh(H)2{Si(OEt)3}(PEt3)3] (5a) was generated (Scheme 2). In the reaction of 1 with HSi(OMe)3 at room temperature the formation of cis,fac-[Rh(H)2{Si(OMe)3}(PEt3)3] (5b) was observed.23 The NMR data of 3 and 5a are in accordance with the data of comparable complexes but bearing PMe3 or Si(OMe)3 ligands, which were reported previously.22b,23 Treatment of a solution of [Rh{Si(OEt)3}(PEt3)3] (2a) in C6D6 with CO afforded the replacement of the phosphine ligand in the position trans to the silyl moiety by a carbonyl ligand (Scheme 3). The silyl complex [Rh(CO){Si(OEt)3}(PEt3)2] (7) is stable in the solid state for a short time, but under vacuum decomposition was observed. The 31P{1H} NMR spectrum of 7 displays a doublet at δ 14.8 ppm with a rhodium−phosphorus coupling constant of 108 Hz. The 29 Si{1H} NMR spectrum shows a doublet of triplets (JRh,Si = 25 Hz, JP,Si = 66 Hz,) at −10.3 ppm. This pattern is compatible with a square-planar configuration with both phosphine ligands in mutually trans positions. The 13C{1H} NMR spectrum displays a doublet of triplets at δ 200.3 ppm with a coupling constant of 67 Hz to the rhodium atom and a rather small coupling constant of 0.21 Hz to the phosphorus atoms. The IR spectrum of 7 shows an absorption band for the CO stretching vibration of ν̃ 1922 cm−1. C−F and C−H Activation of Fluorinated Aromatics. A reaction of the silyl complex [Rh{Si(OEt)3}(PEt3)3] (2a) with hexafluorobenzene led after 40 min at 303 K to the quantitative generation of the C−F activation product [Rh(C6F5)(PEt3)3] (8) and the fluorosilane FSi(OEt)3 (Scheme 4). As expected,

Scheme 3. Formation of Rhodium Silyl Complexes

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Scheme 4. C−F and C−H Activation of Fluorinated Aromatics at 2a,b

the reaction of [Rh{Si(OMe)3}(PEt3)3] (2b) with hexafluorobenzene gave also 8. As already mentioned in the introduction, Milstein et al. reported the C−F activation of hexafluorobenzene at [Rh(SiR3)(PMe3)3] (R3 = Ph3, Me2Ph).16 Note that on reaction of hexafluorobenzene with 2a in the presence of free phosphine no C−F activation was observed. Although the phosphine exchange in 2a seems to be intramolecular (see above), we suggest that phosphine dissociation plays a crucial role along the reaction pathway. This might occur either prior to or more likely after a precoordination of the fluorinated substrate. This assumption is also compatible with the behavior of the silyl carbonyl complex 7, which showed no activation of hexafluorobenzene at room temperature. Phosphine dissociation is here more difficult, because there is no phosphine ligand located in the position trans to the strongly donating silyl group. Compound 8 was characterized by 1H, 31P{1H}, and 19F NMR spectroscopy. The 31P{1H} spectrum shows a doublet of multiplets with a rhodium−phosphorus coupling of 132 Hz at δ 18.6 ppm for the phosphorus atom in the position trans to the fluorinated ligand. The multiplet splitting is due to couplings to the fluorine and phosphorus atoms. The phosphine ligands in a mutually trans arrangement give a doublet of doublets at δ 14.1 ppm with coupling constants of 141 Hz to the rhodium atom and of 40 Hz to the other phosphorus atom. The 19F NMR spectrum displays three multiplets at δ −108.1, −164.1, and −164.7 ppm, which integrate in the ratio 2:1:2. The molecular structure of 8 in the solid state was determined by X-ray diffraction analysis at 100 K (Figure 3). Suitable crystals were grown from a benzene[D6] solution at room temperature. Selected bond lengths and angles are summarized in Table 1. The compound reveals a distorted-square-planar structure similar to that reported by Milstein et al. for [Rh(C6F5)(PMe3)3].16 The Rh−P distances (Rh(1)−P(1) = 2.3304(3) Å, Rh(1)−P(2) = 2.3000(3) Å, Rh(1)−P(3) = 2.2840(3) Å) and the Rh−C distance to the pentafluorophenyl ligand (Rh(1)− C(1) = 2.0863(13) Å) are in a range similar to that found for [Rh(C6F5)(PMe3)3]. The reaction of silyl complex [Rh{Si(OEt)3}(PEt3)3] (2a) and pentafluorobenzene gave the C−H activation product

Figure 3. ORTEP diagram of complex 8. Ellipsoids are drawn at the 50% probability level; hydrogen atoms are omitted for clarity.

Table 1. Selected Bond Lengths (Å) and Angles (deg) in [Rh(C6F5)(PEt3)3] (8) with Standard Deviations in Parentheses Rh(1)−C(1) Rh(1)−P(1) Rh(1)−P(2) Rh(1)−P(3) C(1)−C(2) C(1)−C(6) C(2)−F(1) C(2)−C(3) P(1)−Rh(1)−P(2) P(2)−Rh(1)−P(3) P(1)−Rh(1)−P(3) C(1)−Rh(1)−P(1) C(1)−Rh(1)−P(2) C(1)−Rh(1)−P(3) C(2)−C(1)−Rh(1)

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2.0863(13) 2.3304(3) 2.3000(3) 2.2840(3) 1.3859(19) 1.3888(18) 1.3595(16) 1.386(2) 163.403(12) 96.436(12) 95.372(13) 83.71(4) 86.79(4) 169.35(4) 119.85(10)

C(3)−F(2) C(3)−C(4) C(4)−F(3) C(4)−C(5) C(5)−F(4) C(5)−C(6) C(6)−F(5) C(6)−C(1)−Rh(1) C(1)−C(2)−C(3) C(2)−C(3)−C(4) C(3)−C(4)−C(5) C(4)−C(5)−C(6) C(5)−C(6)−C(1) C(2)−C(1)−C(6)

1.3476(17) 1.380(2) 1.3443(18) 1.373(2) 1.3538(17) 1.384(2) 1.3645(17) 127.60(10) 125.02(13) 119.32(14) 118.60(14) 119.58(14) 124.89(14) 112.50(13)


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[Rh(C6F5)(PEt3)3] (8), even at 383 K (Scheme 4). In contrast, Milstein et al. observed a selective C−F activation of pentafluorobenzene at [Rh(SiMe2Ph)(PMe3)3] in the position para to the hydrogen atom at higher temperatures (383 K).16 Note that we consider the silyl ligand in [Rh(SiMe2Ph)(PMe3)3] to be electronically very different in comparison to the silyl ligand in 2a, which might be a reason for the difference in selectivities. Treatment of a solution of 2a or 2b with perfluorotoluene affords C−F activation in the position para to the CF3 group to yield [Rh(4-C6F4CF3)(PEt3)3] (9) (Scheme 4). The 31P{1H} NMR spectrum of complex 9 displays a doublet of doublets at δ 17.7 ppm (JRh,P = 140 Hz, JP,P = 41 Hz) for the phosphine ligands in mutually trans positions and a doublet of triplet of multiplets at δ 13.4 ppm (JRh,P = 130 Hz, JP,P = 41 Hz) for the phosphorus atom in the position trans to the fluorinated ligand. The 19F NMR spectrum exhibits a triplet at δ −54.9 ppm for the CF3 group, a multiplet at δ −107.8 ppm (JF,F = 20 Hz), and a triplet of multiplets at δ −145.8 ppm (JF,F = 20 Hz) which integrate in a ratio of 3:2:2. On treatment of [Rh{Si(OR)3}(PEt3)3] (2a,b) with 2,3,5,6tetrafluoropyridine the C−F activation product [Rh(2C5F3HN)(PEt3)3] (10) was generated (Scheme 5). Complex

Note that the activation of 2,3,5,6-tetrafluoropyridine at 2a,b is a spontaneous reaction at room temperature. In contrast, the silyl carbonyl complex 7 reacted very slowly with 2,3,5,6tetrafluoropyridine and after 7 days the C−H activation product [Rh(CO)(4-C5F4N)(PEt3)2] (12) was generated in 78% yield according to the 31P NMR spectrum (Scheme 6). An alternative synthesis of 12 was previously reported by treatment of [Rh(4-C5F4N)(PEt3)3] with CO.8a Scheme 6. C−H Activation of Pentafluoropyridine at the Silyl−Carbonyl Complex 7

A reaction of [Rh{Si(OEt)3}(PEt3)3] (2a) with pentafluoropyridine at 243 K furnished the C−F activation product [Rh(2-C5F4N)(PEt3)3] (11), which displays the rhodium center at the position ortho to the nitrogen atom (Scheme 5). At room temperature activation of the C−F bond in the position para to the nitrogen atom was observed as well. ortho and para activation products [Rh(2-C5F4N)(PEt3)3] (11) and [Rh(4-C5F4N)(PEt3)3] were generated in a 9:1 ratio according to the 31P{1H} NMR spectrum. In contrast, the reaction of [Rh{Si(OMe)3}(PEt3)3] (2b) with pentafluoropyridine at room temperature as well as at low temperature afforded selectively the activation product in an ortho position. The presence of free phosphine led to C−F activation at 2b at a comparable rate to yield 11. Thus, the reaction is not inhibited by free phosphine. The generation of fluorophosphoranes and 2,3,5,6-tetrafluoropyridine was also observed, but at a slower rate.24 However, the activation of pentafluoropyridine at 2a at 243 K is considerably slower in the presence of free phosphine, suggesting that phosphine dissociation might play a role in the mechanism of activation prior to or during the rate-determining step. [Rh(2-C5F4N)(PEt3)3] (11) and [Rh(4-C5F4N)(PEt3)3] were identified by comparison of their NMR spectroscopic data with the literature.8,9 Note that metalation of pentafluoropyridine at the 2-position is usually very difficult to achieve. However, this selectivity was previously observed in the reaction of [Rh(Bpin)(PEt3)3] with pentafluoropyridine.9 A comparison with a similar reaction at [Rh(SiPh3)(PMe3)3] shows again that complexes 2a,b display a more selective conversion. Treatment of [Rh(SiPh3)(PMe3)3] with pentafluoropyridine yields a mixture of the activation products in the 2- and 4-positions in a 3:1 ratio. 17,24 Activation of pentafluoropyridine at the 2-position was also reported at Ni, Ti, and Zr complexes, whereas organic nucleophiles can show a competitive pathway.13,25 However, in these cases the oxidative addition of the C−F bond or fluoro complex formation was observed. Similar to the reactivity pattern with hexafluorobenzene, no C−F activation of pentafluoropyridine at the silyl carbonyl complex 7 was observed. Computational Studies. Density functional theory (DFT) calculations have been performed to model the reaction of 2b with C5F5N to give 11, and in particular, the competition between C−F activation at the 2- and 4-positions. Two possible

Scheme 5. C−F Activation of 2,3,5,6-Tetrafluoropyridine and Pentafluoropyridine at 2a,b

10 could not be isolated and was only characterized in solution because of its low stability. The 31P{1H} NMR spectrum of [Rh(2-C5F3HN)(PEt3)3] (10) displays a doublet of doublets at δ 16.1 ppm for the phosphine ligands in mutually trans positions and a doublet of triplets at δ 20.0 ppm with rhodium−phosphorus couplings of 153 and 120 Hz and coupling constants of 38 Hz between the phosphorus atoms. The 19F NMR spectrum exhibits three doublets of multiplets at δ −93.0, −103.0, and −153.8 ppm which integrate in a ratio of 1:1:1. In the 1H NMR spectrum a multiplet at δ 6.45 ppm can be assigned to the hydrogen atom at the trifluoropyridyl ligand. The formation of 10 is rather remarkable, because in comparable reactions of 2,4,5,6-tetrafluoropyridine activation of the C−H bond takes place. Thus, treatment of [Rh(H)(PEt3)3] (4) or [Rh(Bpin)(PEt3)3] with 2,3,5,6-tetrafluoropyridine led in both cases to the formation of [Rh(4-C5F4N)(PEt3)3].8,9 As already mentioned in the introduction, Marder and Perutz et al. investigated the reactivity of [Rh(SiPh3)(PMe3)3] toward fluorinated pyridines. They observed a product mixture of C−H and C−F activation products in a ratio of 1:1.3.17 3799


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Scheme 7. General Mechanisms Considered for C−F Activation of C5F5N at Rh Silyl Complexes (L = PMe3)

general pathways have been considered for these processes, as illustrated in Scheme 7 for reaction at the 2-position. Along pathway A initial C−F oxidative addition yields a Rh(III) intermediate from which Si−F reductive elimination gives 11 and FSiR3. Alternatively, pathway B features a silyl-assisted process in which C−F activation occurs over the Rh−Si bond to give the products directly. Similar ligand-assisted processes have been characterized for reactions of fluoroarenes12a,c and fluoropyridines12b,13 at metal−phosphine bonds and for the activation of C5F5N over the Rh−boryl bond in [Rh(Bpin)(PMe3)3].9 Note that for 2b the rate of C−F activation is not affected by added phosphine and so we have not considered alternative routes based on phosphine dissociation. The calculations employed [Rh{Si(OMe)3}(PMe3)3] (2′) as a model reactant, primarily to avoid the extreme conformational flexibility of the PEt3 ligands in 2b (although extensive conformational searching was still performed for each stationary point; see Computational Details). The computed geometry of 2′ is shown in Figure 4 and shows good agreement with the experimentally determined structure of the related [Rh(SiPh3)(PMe3)3] system reported by Thorn and Harlow.22a

In particular, the observed deviation away from square-planar geometry is reproduced in 2′, with average computed trans-L− Rh−L angles of 151° in comparison to the experimental value of 146°. While both the Rh−Si and the Rh−P distances are somewhat overestimated, the elongation of the Rh−P1 bond trans to Si(OMe)3 is well reproduced, reflecting the higher trans influence of this ligand in comparison to PMe3. In the following all energies are quoted relative to the combined energies of 2′ and C5F5N set to 0.0 kcal/mol. The computed profiles for pathway A are shown in Figure 5 for the reactions of 2′ at both the 2- and 4-positions of C5F5N to give 2-11 and 4-11, respectively. In principle, the initial concerted C−F oxidative addition at 2′ can lead to three different isomers of the [Rh(C5F4N)(F){Si(OMe)3}(PMe3)3] intermediate: two mer forms, with either F (A2mer) or C5F4N (A2mer′) trans to Si(OMe)3, and a fac form (A2fac). For both C−F activations the most favored pathway, both kinetically and thermodynamically, leads to A2mer. These pathways are discussed here and shown in Figure 5, with the key C−F activation transition states in Figure 6; details of all other structures are provided in the Supporting Information. Throughout the focus will be on the zero-point energy corrected energies, although the computed free energies are also provided in italics. As the key transition states in all pathways involve an associative step, the inclusion of entropic effects does not affect which pathway is preferred. Activation at the 2-position proceeds via initial formation of a trigonal-bipyramidal intermediate, 2-A1 (E = +0.4 kcal/mol), with a barrier of only 5.9 kcal/mol. 2-A1 has Si(OMe)3 in an equatorial position and features an η2-bound C5F5N ligand with an elongated N−C2 bond (1.39 Å; cf. 1.33 Å in free C5F5N) oriented in the equatorial plane, consistent with a degree of π back-donation from the Rh center.26 C−F activation then occurs via 2-TS(A1-A2mer) (E = +17.3 kcal/mol) to give 2A2mer at −27.1 kcal/mol. Isomerization of 2-A2mer is then required to permit Si−F reductive elimination, and we have characterized one possible mechanism (omitted from Figure 5) involving dissociation of the PMe3 trans to the 2-C5F4N ligand. This proceeds through a transition state at −12.5 kcal/mol and is accompanied by spontaneous movement of F into the vacated site. PMe3 reassociation then gives A2mer″ (where PMe3 is now trans to silyl; E = −24.4 kcal/mol), from which Si−F

Figure 4. Computed structure of [Rh{Si(OMe)3}(PMe3)3] (2′) with selected distances (Å) and angles (deg). Hydrogen atoms are omitted for clarity, and experimental data for [Rh(SiPh3)(PMe3)3] are shown in italics.22a 3800


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Figure 5. Computed profiles (kcal/mol) for the reactions of 2′ with C5F5N to give 2-11 and 4-11 and FSiR3 via pathway A (L = PMe3 and R = OMe). Values in plain text are zero-point energy corrected energies, and those in italics are free energies.

Figure 6. Computed structures for (a) 2-TS(A1-A2mer) and (b) 4-TS(2′-A2mer) with selected distances (Å) and angles (deg). Relative energies (plain text, zero-point energy corrected energies; italics, free energies) are in kcal/mol, and H atoms are omitted for clarity.

bond formation readily occurs via 2-TS(A2mer″ -11) at −16.9 kcal/mol to give 2-11 and FSi(OMe)3 at −32.6 kcal/mol. The isomerization/Si−F reductive elimination steps are therefore much more accessible than the initial C−F oxidative addition.27 This, along with the exothermicity (and hence irreversibility) of C−F oxidative addition, indicates that the C−F activation step will control the overall reactivity (and selectivity) of the system. The C−F oxidative addition transition state 2-TS(A1-A2mer) is shown in Figure 6a. This structure exhibits a marked deviation from that expected for a concerted oxidative addition at a square-planar ML4 complex.28 Normally, the approach of a substrate, X−Y, induces a narrowing of one trans-L−M−L angle to accommodate the new M−X/M−Y bonds (see

Scheme 8): in the transition state the X−M−Y and L−M−L planes are nearly coplanar with the angle between these planes being close to 0°. In contrast, in 2-TS(A1-A2mer) the angle, α, between the F2−Rh−C2 and the Si−Rh−P1 planes is 71.5°. This and other features suggest a degree of phosphine Scheme 8. Expected Geometric Changes upon Concerted Oxidative Addition

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Figure 7. Computed reaction profiles (kcal/mol) for the reactions of 2′ to give 2-11 and 4-11 and FSiR3 via pathway B (L = PMe3 and R = OMe). Values in plain text are zero-point energy corrected energies, and those in italics are free energies.

Figure 8. Computed structures for (a) 2-TS(B1-B2) and (b) 4-TS(2′-B2) with selected distances (Å) and angles (deg). Relative energies (plain text, zero-point energy corrected energies; italics, free energies) are in kcal/mol, and H atoms are omitted for clarity.

addition transition state. The same trans-influence argument explains why 2-TS(A1-A2mer) is the most stable C−F oxidative addition transition state: 2-TS(2′-A2mer′) (E = +23.9 kcal/mol) shows a similar square-pyramidal geometry at Rh but has PMe3 in the axial position and Si(OMe)3 trans to the developing Rharyl bond; in 2-TS(2′-A2fac) (E = +20.4 kcal/mol) Si(OMe)3 lies trans to a PMe3 ligand (N.B.: no η2 intermediate was located in these cases and so reaction proceeds directly from 2′) . The greater stability of intermediate 2-A2mer also reflects the positioning of Si(OMe)3 trans to the weakly donating fluoride (cf. 2-A2mer′, Si(OMe)3 trans to C5F4N, E = −24.4 kcal/mol, and 2-A2fac, Si(OMe)3 trans to PMe3, E = −25.3 kcal/mol). Reaction at the 4-position of C5F5N via pathway A follows a pattern similar to that described above for the 2-position. In this case no η2 intermediate equivalent to 2-A1 was located and C−F oxidative addition instead proceeded directly via 4-TS(2′A2mer) (E = +13.2 kcal/mol) to 4-A2mer (E = −34.9 kcal/mol).

assistance in the C−F activation: 2-TS(A1-A2mer) exhibits a short Rh−C2 distance of 2.08 Å but a very long Rh···F2 distance of 2.85 Å; F2 also shows a short contact of 2.46 Å with P2, while the F2−C2−Rh−P2 torsion angle is only +23.3°. No interaction with the pyridine N is apparent in this case, however (Rh···N = 2.57 Å). Previous examples of phosphine-assisted oxidative addition have involved metallophosphorane intermediates,13,14 and a subsequent F → M transfer step has been necessary to complete the overall oxidative addition process. In the current case, however, characterization of 2-TS(A1-A2mer) shows that it leads directly to 4-A2mer, and all attempts to locate a metallophosphorane species were unsuccessful. A further feature of 2-TS(A1-A2mer) is the distorted-square-pyramidal geometry around the Rh center, in which Si(OMe)3 occupies the axial position. The high trans influence of this ligand favors placing it opposite a vacant trans site, and the strong Si→Rh donation can compensate for (and may even drive) the movement of F2 toward P2 in the phosphine-assisted oxidative 3802


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position, while in contrast, both pathways A and B for activation at the 2-position feature Rh−η2-CFN intermediates. Interestingly, we were able to locate an alternative silylassisted C−F activation transition state for reaction at the 2position which does not feature a short Rh···N contact. This structure has an energy of +14.6 kcal/mol (G = +31.4 kcal/ mol) and resembles 4-TS(2′-B2), with a wide P2−Rh−P3 angle of 152.3° and a long Rh···N distance of 2.85 Å. This in turn implies that the Rh···N stabilization in 2-TS(B1-B2) is worth 4.4 kcal/mol, sufficient to take this structure 1.7 kcal/mol below 4-TS(2′-B2). The reaction along pathway B is therefore computed to be kinetically favored at the 2-position; moreover, pathway B is more accessible than pathway A, and so the calculations successfully rationalize the experimentally observed selectivity for the 2-position by invoking a silyl-assisted C−F activation process. The characterization of silyl-assisted C−F activation adds to the range of ligand-assisted C−F activation processes previously reported at metal−phosphine12−14 and −boryl9 bonds. Both silyl (X = Si(OMe)3) and boryl assistance (X = Bpin) have now been characterized for the activation of pentafluoropyridine at the 2-position at [Rh(X)(PMe3)3] species, allowing for direct comparison of these two processes. Boryl-assisted C−F activation also proceeds through an η2 intermediate, although this is more stable (E = −2.3 kcal/mol) than 2-B1 located here (E = +5.0 kcal/mol). The ligand-assisted C−F activation step then proceeds with a very similar barrier (X = Bpin, ΔE⧧ = +6.4 kcal/mol; X = Si(OMe)3, ΔE⧧ = +5.2 kcal/mol), and this is reflected in comparable transition state geometries. The overall barrier to boryl assistance is therefore lower (+4.1 kcal/mol; cf. +10.2 kcal/mol for silyl assistance), although this reflects the energy of the η2 intermediate rather than different intrinsic abilities of the two ligands to assist the C−F cleavage step. A degree of Rh···N interaction is computed in both transition states (X = Bpin, Rh···N = 2.22 Å; X = Si(OMe)3, Rh···N = 2.16 Å), and this accounts for the novel selectivity for the 2position in both cases.

Isomerization and Si−F reductive elimination then give 4−11 and FSi(OMe)3 with a combined energy of −45.6 kcal/mol, 13.0 kcal/mol more stable than 2-11 and FSi(OMe)3. The overall reaction at the 4-position is therefore strongly favored thermodynamically. Moreover the initial C−F activation step also favors this process, both thermodynamically, 4-A2mer being 7.8 kcal/mol more stable than 2-A2mer, and kinetically, 4TS(2′-A2mer) being 4.1 kcal/mol more stable than 2-TS(A1A2mer). This is also reflected in an earlier transition state geometry for 4-TS(2′-A2mer), which exhibits a shorter cleaving C−F bond (1.61 Å) and a longer Rh···C2 distance (2.16 Å, see Figure 6b). Phosphine assistance is still apparent, with F4 closer to P2 (2.63 Å) than Rh (2.75 Å), and the twisting of the {C5F5N} moiety (α = 64.0°; F4−C4−Rh−P2 = +29.2°). Overall, all the computed results indicate that reaction via pathway A should result in the formation of 4-11, a conclusion clearly inconsistent with the observation of 2-11 experimentally. Computed reaction profiles for the activation of C5F5N at 2′ via pathway B are shown in Figure 7. The reaction at the 2position again proceeds by formation of an η2-bound adduct, 2B1 (E = +5.0 kcal/mol), an isomer of 2-A1 with Si(OMe)3 now in an axial position. This ligand can now therefore participate in a four-centered silyl-assisted C−F activation transition state, 2TS(B1-B2) (E = +10.2 kcal/mol, see also Figure 8a), in which the C2−F2 bond (1.71 Å) adds over the Rh−Si bond (Rh−C = 2.07 Å, Si···F2 = 2.21 Å, F2−C2−Rh−Si = −14.3°). Importantly, the Rh center maintains an interaction with the {C5F4N} nitrogen, the Rh−N distance shortening to 2.17 Å (cf. 2.20 Å in 2-B1). The nature of the {C5F4N} ligand changes as the C−F activation progresses, from an η2-arene in 2-B1 to having benzyne character in 2-B2, the initial intermediate formed (E = −5.2 kcal/mol; Rh−C2 = 2.02 Å, Rh−N = 2.21 Å). This change can be quantified by the angle between the best-fit plane through the C5N ring and the C2−Rh−N plane (2-B1, 56.4°; 2-TS(B1-B2), 30.6°; 2-B2, 3.2°). 2-B2 also features a new Si−F2 bond (1.75 Å) and a residual Rh···Si interaction (2.57 Å), resulting in the Si center exhibiting a trigonal-bipyramidal geometry. Loss of FSi(OMe)3 and formation of 2-11 entails a minimal barrier through 2-TS(B211) (E = −4.1 kcal/mol) in which both the Rh−N and Rh−Si distances lengthen to 2.69 and 3.01 Å, respectively. Silyl-assisted C−F activation at the 4-position proceeds in one step through 4-TS(2′-B2) (E = +11.9 kcal/mol). This links directly to intermediate 4-B2 (E = −33.2 kcal/mol), in which the newly formed {FSi(OMe)3} moiety interacts strongly with the Rh center, through both Si (Rh−Si = 2.42 Å) and one methoxy oxygen (Rh−O = 2.19 Å). 4-B2 is 27.0 kcal/mol more stable than 2-B2, and these interactions also mean that FSi(OMe)3 dissociation and product formation has a distinct barrier of 8.7 kcal/mol via 4-TS(B2-11) at −24.4 kcal/mol. The most important difference between the two reaction profiles in Figure 7 lies in the two silyl-assisted C−F activation transition states. The structure of 4-TS(2′-B2) (Figure 8b) shows a four-centered geometry very similar to that of 2TS(B1-B2), with C4−F4 (1.70 Å) adding over the Rh−Si bond (Rh−C4 = 2.08 Å; Si···F4 = 2.24 Å; F4−C4−Rh−Si = −14.6°). One key difference, however, is the lack of any additional interaction with the C5F4N ring: e.g., through a Rh−CFCF π interaction. This is also reflected in a much wider P2−Rh−P3 angle of 148.4° (cf. 104.5° in 2-TS(B1-B2)) and in the lack of any η2 intermediate prior to the silyl-assisted C−F activation step. This was also a feature of C−F oxidative addition at the 4-

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CONCLUSIONS The rhodium(I) silyl complexes 2a,b and 7 were synthesized from the methyl complex 1. Reactions of the complexes 2a,b with hexafluorobenzene and fluorinated pyridines result in C−F activation products. Treatment of 2a or 2b with pentafluoropyridine led to the activation product at the 2-position. This conversion is selective at 243 K for the triethoxysilyl-substituted complex 2a and at room temperature for the trimethoxysilyl complex 2b. Whereas the activation of hexafluorobenzene and pentafluoropyridine at 2a is inhibited in the presence of free phosphine, the activation of pentafluoropyridine at 2b is not retarded by free phosphine. DFT calculations using [Rh{Si(OMe)3}(PMe3)3] (2′) as a model complex for 2b were performed. C−F oxidative addition transition states are higher in energy and would imply that activation at the 4-position of pentafluoropyridine is favored. In contrast, a silyl-assisted C−F activation pathway is more accessible and in this case the reaction at the 2-position is preferred due to a stabilizing Rh···N interaction in the transition state. The computational studies are in agreement with the experimental results. A pathway based on phosphine dissociation may also be close in energy to the silyl-assisted reaction route, but for simplicity we only considered the activation reaction at 2b computationally. Silyl-assisted C−F activation is another example of the general process of ligand3803


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Formation of [Rh(H){Si(OEt)3}2(PEt3)3] (3). HSi(OEt)3 (12 μL, 0.065 mmol) was added to a solution of [Rh(CH3)(PEt3)3] (1; 0.053 mmol) in n-pentane (0.5 mL) at 183 K. The solution was warmed to 233 K and then stirred for 4 h at this temperature. After the reaction solution was warmed to room temperature, the solvent was removed under vacuum. The red residue was dissolved in 0.3 mL of [D6]benzene and the solution placed in a PFA NMR tube. The NMR data reveal the formation of 2a and 3 in a ratio of 4:1. NMR data for 3: 1H NMR (300.1 MHz, [D6]benzene, 300 K) δ −15.6 (dm, d in the 1H{31P} NMR spectrum 1J(H,Rh) = 29 Hz, 1H, RhH). The signals for the ethyl groups in 3 are covered by resonances of 2a; 31P{1H} NMR (121.5 MHz, [D6]benzene, 300 K) δ 4.6 (dt, 1J(Rh,P) = 97 Hz, 2 J(P,P) = 29 Hz, 1P), −2.4 (dd, 1J(Rh,P) = 71 Hz,2 J(P,P) = 29 Hz, 2P). Formation of [Rh(H)(PEt3)3] (4). HSiEt3 (8.5 μL, 0.053 mmol) was added to a solution of [Rh(CH3)(PEt3)3] (1; 0.053 mmol) in [D6]benzene (0.5 mL). A quantitative conversion of 1 into complex 4 occurred. Complex 4 was identified by its NMR data.8 Formation of [Rh(H)2{Si(OEt)3}(PEt3)3] (5a). HSi(OEt)3 (9.8 μL, 0.053 mmol) was added to a solution of [Rh(H)(PEt3)3] (4; 0.053 mmol) in [D6]benzene (0.5 mL). The NMR spectroscopic data of the reaction solution revealed the formation of 5a. NMR data for 5a: 1H NMR (300.1 MHz, [D6]benzene, 300 K) δ 4.1 (q, 3J(H,H) = 6.9 Hz, 6H, SiOCH2CH3), 1.9 (m, 6H, PCH2CH3), 1.6 (m, 12H, PCH2CH3), 1.4 (t, 3J(H,H) = 6.9 Hz, 9H, SiOCH2CH3), 1.1 (m, 18H, PCH2CH3), 1.0 (m, 9H, PCH2CH3), −11.3 (ddm, d in the 1H{31P} NMR spectrum, 1J(H,Rh) = 18 Hz, 2J(H,Ptrans) = 108 Hz, 1H, RhH); 31 1 P{ H} NMR (121.5 MHz, [D6]benzene, 300 K) δ 16.0 (dd, 1J(Rh,P) = 101 Hz, 2J(P,P) = 23 Hz, 2P), 7.2 (dt, 1J(Rh,P) = 83 Hz, 2J(P,P) = 23 Hz, 1P); 1H,29Si HMBC NMR (300.1/59.6 MHz, [D6]benzene, 300 K) δ{29Si} 4/−3 (SiOCH2CH3), −11/−3 (HRhSi). Formation of [Rh(H)2{Si(OMe)3}(PEt3)3] (5b). HSi(OMe)3 (6.8 μL, 0.053 mmol) was added to a solution of [Rh(H)(PEt3)3] (4; 0.053 mmol) in [D6]benzene (0.5 mL). A quantitative conversion of 4 into complex 5b occurred. The latter was identified by its NMR data.8 Formation of fac-[Rh(H)(CH3){Si(OEt)3}(PEt3)3] (6a). HSi(OEt)3 (9.8 μL, 0.053 mmol) was added to a solution of [Rh(CH3)(PEt3)3] (1; 0.053 mmol) in [D8]toluene (0.5 mL) at 183 K. After 15 min the quantitative formation of 6a was observed by NMR spectroscopy. NMR data for 6a: 1H NMR (300.1 MHz, [D8]toluene, 233 K) δ 4.0− 4.2 (m, 6H, SiOCH2CH3), 1.8−2.3 (m, br, 18H, PCH2CH3), 1.5−1.6 (m, 9H, SiOCH2CH3), 1.3−1.5 (m, 27H, PCH2CH3), 0.0 (m, br, d in the 1H{31P} NMR spectrum 2J(H,Rh) = 1.8 Hz, 3H, RhCH3), −10.8 (ddm, d in the 1H{31P} NMR spectrum, 1J(H,Rh) = 18 Hz, 2J(H,Ptrans) = 144 Hz, 1H, RhH), the assignment of the signals is supported by a 1 1 H, H COSY NMR spectrum; 31P{1H} NMR (121.5 MHz, [D8]toluene, 233 K) δ 12.2 (ddd, 1J(Rh,P) = 90 Hz, 2J(P,P) = 25 Hz, 2 J(P,P) = 22 Hz, 1P), 4.0 (ddd, 1J(Rh,P) = 94 Hz, 2J(P,P) = 25 Hz, 2 J(P,P) = 22 Hz, 1P), −3.3 (ddd, 1J(Rh,P) = 71 Hz, 2J(P,P) = 25 Hz, 2 J(P,P) = 22 Hz, 1P, P trans Si,); 1H,29Si HMBC NMR (300.1/59.6 MHz, [D8]THF, 220 K) δ{29Si} 4/−7 (SiOCH2CH3), −11/−7 (HRhSi). Formation of fac-[Rh(H)(CH3){Si(OMe)3}(PEt3)3] (6b). HSi(OMe)3 (6.8 μL, 0.053 mmol) was added to a solution of [Rh(CH3)(PEt3)3] (1; 0.053 mmol) in [D14]methylcyclohexane (0.5 mL) at 183 K. After 15 min the quantitative formation of 6b was observed by NMR spectroscopy. NMR data for 6b: 1H NMR (300.1 MHz, [D14]methylcyclohexane, 223 K) δ 3.1 (s, 9H, SiOCH3), 1.3− 1.5 (m, br, 6H, PCH2CH3), 1.3−1.1 (m, br, 12H, PCH2CH3), 0.7−0.8 (m, 27H, PCH2CH3), −0.7 (m, br, d in the 1H{31P} NMR spectrum 2 J(H,Rh) = 1.8 Hz, 3H, RhCH3), −11.2 (ddm, d in the 1H{31P} NMR spectrum, 1J(H,Rh) = 18 Hz,2J(H,Ptrans) = 144 Hz, 1H, RhH), the assignment of the signals is supported by a 1H,1H COSY NMR spectrum; 13C{1H} NMR (75.5 MHz, [D14]methylcyclohexane, 223 K) δ 55.4 (s, SiOCH3), 26.9 (m, PCH2CH3), 25.6 (m, PCH2CH3), 23.4 (m, PCH2CH3), 14.8 (s, PCH2CH3), 14.5 (s, PCH2CH3), 13.4 (s, PCH2CH3), −14.3 (m, CH3); the assignment of the signals is supported by a 1H,13C HMQC NMR spectrum; 31P{1H} NMR (121.5 MHz, [D14]methylcyclohexane, 223 K) δ 11.2 (ddd, 1J(Rh,P) = 90 Hz,

assisted C−F activation which has been reported at phosphine and boryl ligands.9,12−14 The favored four-membered transition state that features a stabilizing Rh···N interaction and accounts for activation at the 2-position of pentafluoropyridine is comparable to previously reported boryl-assisted and phosphine-assisted oxidative addition C−F activation processes. Note that radical-pair mechanisms for C−F activation reactions have been discussed for conversions at ruthenium, rhodium and iridium, but pentafluoropyridine should then be activated at the para position.5b,29 In addition, we do not have any experimental indication for such a mechanism. Thus, radical trapping experiments with 9,10-dihydroanthracene showed no formation of anthracene or tetrafluoropyridines.

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EXPERIMENTAL SECTION

General Considerations. The synthetic work was carried out on a Schlenk line. All solvents were dried and purified by conventional methods and distilled under argon before use. [D6]Benzene, [D8]toluene, [D14]methylcyclohexane, and [D8]THF were purified by distillation from Na/K under argon. [Rh(CH3)(PEt3)3] (1) was prepared according to the literature.30 In all experiments 1 was employed in solution with a concentration of 0.106 mmol mL−1. PFA NMR tubes were used for highly sensitive compounds to inhibit reactions with OH groups at glass surfaces. PFA NMR tubes were obtained from Roland Vetter Laborbedarf OHG. The NMR spectra were acquired on Bruker DPX 300, Bruker Avance 300, and Bruker AV 400 NMR spectrometers. The 1H, 13C, and 29Si NMR spectra were referenced to external TMS at δ 0 ppm. 19 F NMR spectra were referenced externally to C6F6 at δ −162.9 ppm, and 31P NMR spectra were referenced externally to H3PO4 at δ 0 ppm. Infrared spectra were recorded on a Bruker Vektor 22 spectrometer which was equipped with a diamond ATR unit. Microanalyses were carried out using a HEKAtech EURO EA 3000 elemental analyzer. Synthesis of [Rh{Si(OEt)3}(PEt3)3] (2a). HSi(OEt)3 (9.8 μL, 0.053 mmol) was added to a solution of [Rh(CH3)(PEt3)3] (1; 0.053 mmol) in n-pentane (0.5 mL) at 183 K. The solution was warmed to 233 K and then stirred for 4 h at this temperature. After the reaction solution was warmed to room temperature, the solvent was removed under vacuum. The red residue was dissolved in 0.3 mL of [D6]benzene and the solution placed in a PFA NMR tube. The 31 1 P{ H} spectrum reveals a quantitative conversion of the methyl complex 1 to 2a. NMR data for 2a: 1H NMR (300.1 MHz, [D6]benzene, 300 K) δ 4.1 (q, 3J(H,H) = 7.6 Hz, 6H, SiOCH2CH3) 1.8 (m, q in the 1H{31P} NMR spectrum, 3J(H,H) = 7.6 Hz, 18H, PCH2CH3), 1.4 (t, 3J(H,H) = 7.6 Hz, 9H, SiOCH2CH3), 1.0 (m, t in the 1H{31P} NMR spectrum, 3J(H,H) = 7.6 Hz, 27H, PCH2CH3); 31 1 P{ H} NMR (121.5 MHz, [D8]toluene, 300 K) δ 13.6 (d, 1J(Rh,P) = 144 Hz); 31P{1H} NMR (161.9 MHz, [D8]toluene, 183 K) δ 20.4 (dd, 1 J(Rh,P) = 151 Hz, 2J(P,P) = 34 Hz,2P), 3.2 (dt, 1J(Rh,P) = 118 Hz, 2 J(P,P) = 34 Hz, 1P); 29Si{1H} NMR (59.6 MHz, [D6]benzene, 300 K) δ −11.3 (dq, 1J(Rh,Si) = 60 Hz, 2J(Si,P) = 47 Hz). Synthesis of [Rh{Si(OMe)3}(PEt3)3] (2b). HSi(OMe)3 (6.8 μL, 0.053 mmol) was added to a solution of [Rh(CH3)(PEt3)3] (1; 0.053 mmol) in n-pentane (0.5 mL) at 183 K. The solution was warmed to 218 K and then stirred for 4 h at this temperature. After the reaction solution was warmed to room temperature, the solvent was removed under vacuum. The red residue was dissolved in 0.3 mL of [D14]methylcyclohexane and the solution placed in a PFA NMR tube. The 31P{1H} spectrum reveals a quantitative conversion of the methyl complex 1 to 2b. NMR data for 2b: 1H NMR (300.1 MHz, [D14]methylcyclohexane, 300 K) δ 3.2 (s, 9H, SiOCH3), 1.5 (m, q in the 1H{31P} NMR spectrum, 3J(H,H) = 7.6 Hz, 18H, PCH2CH3), 0.8 (m, t in the 1H{31P} NMR spectrum, 3J(H,H) = 7.6 Hz, 27H, PCH2CH3); 31P{1H} NMR (121.5 MHz, [D14]methylcyclohexane, 300 K) δ 13.7 (d, 1J(Rh,P) = 144 Hz); 31P{1H} NMR (121.5 MHz, [D8]toluene, 183 K) δ 20.6 (dd, 1J(Rh,P) = 150 Hz, 2J(P,P) = 34 Hz, 2P), 2.7 (dt, 1J(Rh,P) = 119 Hz, 2J(P,P) = 34 Hz, 1P); 1H,29Si HMBC NMR (30.1/59.6 MHz, [D14]methylcyclohexane, 300 K) δ 3/−8. 3804


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[D8]toluene, 300 K) δ 6.5 (m, 1H, C5F3HN); 1.4 (m, br, 18H, PCH2CH3), 1.0 (m, br, 27H, PCH2CH3); 31P{1H} NMR (121.5 MHz, [D8]toluene, 300 K) δ 20.0 (dt, 1J(Rh,P) = 120 Hz, 2J(P,P) = 38 Hz, 1P), 16.1 (dd, 1J(Rh,P) = 153 Hz, 2J(P,P) = 38 Hz, 2P); 19F NMR (282.4 MHz, [D8]toluene, 300 K) δ −93.0 (dm, J(F,F) = 32 Hz, 1F), −103.0 (dm, J(F,F) = 32 Hz, 1F), −153.8 (dm, J(F,F) = 32 Hz, 1F). Formation of [Rh(2-C5F4N)(PEt3)3] (11). (a) Pentafluoropyridine (5.8 μL, 0.053 mmol) was added to a solution of [Rh{Si(OEt)3}(PEt3)3] (2a; 0.053 mmol) in [D8]toluene (0.4 mL) at 243 K. After 3 h the quantitative formation of 11 and FSi(OEt)3 was observed by 31 P{ 1H} NMR spectroscopy. Complex 11 was identified by comparison of its NMR data with the literature.9 (b) Pentafluoropyridine (5.8 μL, 0.053 mmol) was added to a solution of [Rh{Si(OMe)3}(PEt3)3] (2b; 0.053 mmol) in [D8]toluene (0.4 mL). The quantitative formation of 11 and FSi(OMe)3 was observed by 31P{1H} NMR spectroscopy. Complex 11 was identified by comparison of its NMR data with the literature.9 Formation of [Rh(CO)(4-C5F4N)(PEt3)3] (12). 2,3,5,6-Tetrafluorobenzene (5.3 μL, 0.053 mmol) was added to a solution of [Rh(CO){Si(OEt)3}(PEt3)2] (4; 0.053 mmol) in [D6]benzene (0.4 mL). After 7 days 78% of [Rh(CO){Si(OEt)3}(PEt3)2] (4) was converted into 12. Compound 12 was identified by its NMR data.8 Structure Determination. Red crystals of 8 were obtained at room temperature from a solution of complex 8 in [D6]benzene. The diffraction data were collected with a Stoe IPDS 2 Θ diffractometer for a fragment with the dimensions of 0.36 × 0.28 × 0.16 mm3: C24H45F5P3Rh, Mr = 624.42, monoclinic, space group P21/n, a = 9.3377(3) Å, b = 18.07909(4) Å, c = 16.7460(5) Å, V = 2826.55(14) Å3, ρcalcd = 1.467 g cm−3, T = 100(2) K, Z = 4, μ(Mo Kα) = 0.818 mm−1, 30069 reflections measured, 7512 unique (R int = 0.0572); final R1 and wR2 values on all data 0.0273 and 0.0652; R1 and wR2 values for 7512 reflections with I0 > 2σ(I0) 0.0245 and 0.0639; residual electron density +1.168/−1.515 e Å−3. CCDC-924071 contains the supplementary crystallographic data. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www. ccdc.cam.ac.uk/data_request/cif. Computational Details. DFT calculations were run with Gaussian 09 (Revision A.02)31 using the BP86 functional.32 Rh, P, and Si centers were described with the Stuttgart RECPs and associated basis sets,33 with added d-orbital polarization on P (ζ = 0.387) and Si (ζ = 0.284).34 6-31G** basis sets were used for all other atoms.35 All stationary points were fully characterized via analytical frequency calculations as either minima (all positive eigenvalues) or transition states (one negative eigenvalue), and IRC calculations and subsequent geometry optimizations were used to confirm the minima linked by each transition state. Each stationary point was subjected to conformational searching using our published protocol, and the most stable conformation is reported in each case.36 All energies are corrected for zero-point energy, while free energies are quoted at 298.15 K and 1 atm.

2

J(P,P) = 26 Hz, 2J(P,P) = 23 Hz, 1P), 4.3 (ddd, 1J(Rh,P) = 95 Hz, J(P,P) = 26 Hz, 2J(P,P) = 23 Hz, 1P), −3.3 (ddd, 1J(Rh,P) = 72 Hz, 2 J(P,P) = 26 Hz, 2J(P,P) = 23 Hz, 1P); 1H,29Si HMBC NMR (300.1/ 59.6 MHz, [D 14 ]methylcyclohexane, 223 K) δ{ 29 Si} 3/−4 (SiOCH2CH3), −11/−4 (HRhSi). Synthesis of [Rh(CO){Si(OEt)3}(PEt3)2] (7). Carbon monoxide was bubbled for 30 s into a solution of [Rh{Si(OEt)3}(PEt3)3] (2a; 0.053 mmol) in n-pentane (0.5 mL). The solvent was removed by applying a moderate vacuum, and the resulting yellow oil was dissolved in [D6]benzene. The NMR spectroscopic data revealed the quantitative conversion of the silyl complex 2a into 7. NMR data for 7: 1H NMR (300.1 MHz, [D6]benzene, 300 K) δ 4.7 (q, 3J(H,H) = 6.9 Hz, 6H, SiOCH2CH3), 1.5 (m, q in the 1H{31P} NMR spectrum,3J(H,H) = 7.7 Hz, 12H, PCH2CH3), 1.4 (t, 3J(H,H) = 6.9 Hz, 9H, SiOCH2CH3), 0.9 (m, t in the 1H{31P} NMR spectrum, 3 J(H,H) = 7.7 Hz, 18H, PCH2CH3); 13C{1H} NMR (75.5 MHz, [D6]benzene, 300 K) δ 200.3 (d, 1J(C,Rh) = 67 Hz, CO), 57.8 (m, SiOCH2CH3), 21.0 (m, 1J(C,P) = 10.2 Hz, PCH2CH3), 18.4 (SiOCH2CH3), 7.7 (PCH2CH3), the assignment of the signals is supported by a 1H,13C HMBC NMR spectrum; 31P{1H} NMR (121.5 MHz, [D6]benzene, 300 K) δ 14.8 (d, 1J(Rh,P) = 108 Hz). 29Si{1H} NMR (59.6 MHz, [D6]benzene, 300 K) δ −10.3 (dt, 1J(P,Si) = 67 Hz, 2 J(Rh,Si) = 25 Hz). IR (ATR, ν̃/cm−1): 1922. Synthesis of [Rh(C6F5)(PEt3)3] (8) via C−H Activation of Pentafluorobenzene. [Rh{Si(OEt)3}(PEt3)3] (2a; 0.053 mmol) was dissolved in benzene (0.3 mL) in a PFA NMR tube. Pentafluorobenzene (20 μL, 0.180 mmol) was added. After 12 h the formation of 8 was observed via NMR spectroscopy. Evaporation of the solvent under vacuum gave a yellow solid. Yield: 31 mg (94%). NMR data for 8: 1H NMR (300.1 MHz, [D6]benzene, 300 K) δ 1.4 (m, q in the 1H{31P} NMR spectrum, 3J(H,H) = 7.6 Hz, 6H, PCH2CH3), 1.2 (m, q in the 1H{31P} NMR spectrum, 3J(H,H) = 7.6 Hz, 12H, PCH2CH3), 1.0 (m, t in the 1H{31P} NMR spectrum, 3 J(H,H) = 7.6 Hz, 9H, PCH2CH3), 0.9 (m, t in the 1H{31P} NMR spectrum, 3J(H,H) = 7.6 Hz, 18 H, PCH2CH3); 31P{1H} NMR (121.5 MHz, [D6]benzene, 300 K) δ 18.6 (dtm, 1J(Rh,P) = 132 Hz, 2J(P,P) = 40 Hz, 1P), 14.1 (dd, 1J(Rh,P) = 141 Hz, 2J(P,P) = 40 Hz, 2P). 19F NMR (282.4 MHz, [D6]benzene, 300 K) δ −108.1 (m, 2F), −164.1 (m, 1F), −164.7 (m, 2F). Anal. Calcd for C24H45F5P3Rh: C, 46.16; H, 7.26. Found: C, 46.57; H, 7.30. Formation of [Rh(C6F5)(PEt3)3] (8) via C−F Activation of Hexafluorobenzene. [Rh{Si(OR)3}(PEt3)3] (2a or 2b; 0.053 mmol) was dissolved in hexafluorobenzene (0.3 mL) in a PFA NMR tube. The NMR data revealed the quantitative formation of 8 and FSi(OR)3. Evaporation of the solvent under vacuum gave a yellow solid. Yield: 32 mg (98%). Synthesis of [Rh(4-C6F4CF3)(PEt3)3] (9). A solution of octafluorotoluene (37 mg, 0.157 mmol) in [D14]methylcyclohexane (0.1 mL) was added to a solution of [Rh{Si(OR)3}(PEt3)3] (2a or 2b; 0.053 mmol) in hexane (0.5 mL). The formation of 9 and fluorosilane was observed via NMR spectroscopy. Evaporation of the solvent under vacuum gave a yellow oil. Yield: 35 mg (98%). NMR data for 9: 1H NMR (300.1 MHz, [D6]benzene, 300 K) δ 1.4 (m, q in the 1H{31P} NMR spectrum, 3J(H,H) = 7.5 Hz, 6H, PCH2CH3), 1.2 (m, q in the 1 H{31P} NMR spectrum, 3J(H,H) = 7.5 Hz, 12H, PCH2CH3), 1.0 (m, t in the 1H{31P} NMR spectrum, 3J(H,H) = 7.5 Hz, 9H, PCH2CH3), 0.9 (m, t in the 1H{31P} NMR spectrum, 3J(H,H) = 7.5 Hz, 18 H, PCH2CH3); 31P{1H} NMR (121.5 MHz, [D6]benzene, 300 K) δ 17.7 (dtm, 1J(Rh,P) = 130 Hz, 2J(P,P) = 41 Hz, 1P), 13.4 (dd, 1J(Rh,P) = 140 Hz, 2J(P,P) = 41 Hz, 2P); 19F NMR (282.4 MHz, [D6]benzene, 300 K) δ −54.9 (t, J(F,F) = 20.4 Hz, 3F, CF3), −107.8 (m, 2F), −145.8 (tm, J(F,F) = 20.4 Hz, 2F). Formation of [Rh(2-C5F3HN)(PEt3)3] (10). 2,3,5,6-Tetrafluoropyridine (5.3 μL, 0.053 mmol) was added to a solution of [Rh{Si(OR)3}(PEt3)3] (2a or 2b; 0.053 mmol) in [D8]-toluene (0.4 mL). After 10 min the solvent was removed under vacuum. The yellow residue was dissolved in 0.5 mL of [D8]toluene. The NMR spectroscopic data of the reaction solution reveal the quantitative formation of 10. NMR data for 10: 1H NMR (300.1 MHz, 2

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ASSOCIATED CONTENT

* Supporting Information S

Text, tables, and a CIF file giving crystallographic data, computational details, and the full ref 32. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (T.B.); s.a. [email protected] (S.A.M.). Present Address §

Department of Chemistry, University of California, Berkeley, CA 94720, United States. Notes

The authors declare no competing financial interest. 3805


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ACKNOWLEDGMENTS Support from the research training group GRK 1582 “Fluorine as a Key Element” funded by the Deutsche Forschungsgemeinschaft is gratefully acknowledged, as well as Heriot-Watt University and the EPSRC for provision of a DTA studentship (J.A.P.). We thank Anna Eißler for the X-ray crystallographic analysis.

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