Reductive Elimination of C6F5âC6F5 from Pd(II) Complexes: Influence

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Reductive Elimination of C6F5−C6F5 from Pd(II) Complexes: Influence of α‑Dicationic Chelating Phosphines Lianghu Gu,† Lawrence M. Wolf,‡ Walter Thiel,§ Christian W. Lehmann,§ and Manuel Alcarazo*,† †

Institut für Organische und Biomolekulare Chemie, Georg-August-Universität Göttingen, Tammannstraße 2, 37077 Göttingen, Germany ‡ Department of Chemistry, University of Massachusetts Lowell, Lowell, Massachusetts 01854, United States § Max-Planck-Institut für Kohlenforschung, Kaiser-Wilhelm-Platz 1, 45470 Mülheim an der Ruhr, Germany S Supporting Information *

ABSTRACT: We report the synthesis and characterization through NMR and X-ray techniques of a series of [Pd(C6F5)2(P∧P′)] complexes constituted by diphosphine chelating ligands of different nature and evaluate the rates for the challenging reductive elimination of C6F5−C6F5. By virtue of their very weak donor properties, dicationic ancillary ligands effectively promote the desired transformation. Density functional theory (DFT) calculations were performed to rationalize these findings. The Pd(0)-complexes formed after the elimination step could not be isolated because the Pd(0) center has a tremendous tendency to insert into one of the P− C+ bonds of the α-cationic ligands rendering Pd(II)phosphinidene complexes. The same behavior was observed for Ni(0) species.

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INTRODUCTION The rates of reductive elimination, the product-forming step in many catalytic cycles, are affected by many factors such as the nature of the metal center, the presence of coordination vacancies, the stereoelectronic properties of the ancillary ligands employed, and the type of groups participating directly in the reaction.1 For a specific complex of general structure [LnM(Ar)(Ar′)], reductive elimination of biaryl Ar−Ar′ tends to be faster when the ancillary ligands L are bulky and strong πacceptors and when their geometries favor a cis orientation of the two aromatic groups undergoing the elimination.2 In addition, faster reductive eliminations are often observed from complexes in which both aryl groups are electron-rich or when they have very dissimilar electronic properties, that is, one very electron-rich and the other one quite electron-poor.3,4 When both aryl groups are strongly electron-deficient, reductive elimination is, in general, much more difficult because of the stronger M−Ar bonds present in these compounds.5 During the last years, our group has been actively involved in the design and synthesis of cationic phosphines, in which the positively charged group(s) is/are directly attached to the phosphorus atom.6 Due to this particular architecture, the resulting α-cationic phosphines depict enhanced π-acceptor character, a property that has been used to develop a series of Au, Pt, and Rh catalysts where the augmented Lewis acidity induced at the metal has been transduced into higher catalytic activities.7 The work described herein addresses the question of whether such ligand architectures are also able to promote © XXXX American Chemical Society

another mechanistically distinct, but again difficult process: the reductive elimination of C6F5−C6F5 from Pd(II) centers. For this purpose, we employ dicationic chelating ligands 1 and 2, both containing a −PPh2 moiety and a strong π-acceptor −[P(H2Im)2]+2 unit (H2Im = 1,3-dimethyl-4,5-dihydroimidazol-2-ylidene) connected by o-phenylene or 2−2′-biphenylene linkers, respectively (Figure 1). These ancillary ligands were

Figure 1. Molecular structures of dicationic ligands 1 and 2.

chosen because infrared studies on their Mo(CO)4-derivatives indicate that they exhibit a pronounced acceptor character, which clearly surpasses that of neutral analogues.8,9 Moreover, the influence of the ligand bite angle can be also studied by comparison of the kinetic results obtained employing 1 and 2. Special Issue: Organometallic Chemistry in Europe Received: June 6, 2017

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DOI: 10.1021/acs.organomet.7b00418 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

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RESULTS AND DISCUSSION Synthesis and Characterization of Bis(pentafluorophenyl) Palladium Complexes. The preparation of the bis(pentafluorophenyl) palladium complexes derived from phosphines 1 and 2 is summarized in Scheme 1. Pd(C6F5)2(COD) (3), the necessary bis-aryl Pd precursor,

bidentate phosphorus-based ancillary ligands covering a broad spectrum of electronic properties (see the Experimental Section). In addition, compound 10 incorporates a biaryl Buchwald ligand decorated with a dimethylamino moiety at the pendant phenyl group, and known compounds 11 and 12 were also synthesized and included in our study.5 All complexes were isolated in analytically pure form as stable solids. The structure of 10 in the solid state deserves further consideration. As can be seen in Scheme 2, the biaryl

Scheme 1. Synthesis and Molecular Structures of 4 and 5 in the Solid State

Scheme 2. Synthesis of 6−12 and Molecular Structures of 6 (Left) and 10 (Right) in the Solid State

Hydrogen atoms, anions, and solvent molecules were removed for clarity; ellipsoids are set at 50% probability.11 Reagents and conditions: (a) 3 (1 equiv), 1 (1 equiv), CH2Cl2, r.t., 12 h, 84%; (b) 3 (1 equiv), 2 (1 equiv), acetone, r.t., 48 h, 73%.

was obtained in 94% yield by reaction of PdCl2(COD) with 2 equiv of in situ generated LiC6F5.10 Subsequent exchange of COD by ligands 1 and 2 in dichloromethane or acetone formed the corresponding derivatives 4 and 5, respectively, which were isolated as air-stable solids and completely characterized by spectroscopic techniques. The 31P NMR spectra of both compounds consist of two multiplets due to long-range JPF coupling (δP= 49.0 (P2), 11.9 (P1) ppm for 1 and δP= 15.3 (P2), −12.6 (P1) ppm for 2), which can be certainly attributed to their −PPh2 and [−P(H2Im)2]+2 moieties, respectively. Moreover, after recrystallization by slow diffusion of Et2O into CH2Cl2 solutions of the title compounds, crystals suitable for X-ray diffraction analyses were obtained (Scheme 1). The geometry around the Pd atom in 4 is nearly planar; the sum of the four angles is 360.4°. In this complex, the bite angle of 1, P(1)−Pd(1)−P(2), is 85.1°, while the C(1)−Pd(1)−C(7) bond angle is slightly larger, 87.7°. As might be expected from the larger biphenylene backbone of 2, the P(1)−Pd(1)−P(2) bite angle increases significantly to 95.1° in 5, and consequently, the bond angle between the perfluoroaromatic rings C(1)−Pd(1)−C(7) is slightly reduced to 84.1°. In order to compare the reactivity of 4 and 5 with that of Pd complexes containing more typical ancillary ligands, we prepared in an analogously manner a set of model compounds of general formula cis-[LPd(C6F5)2] 6−9, in which L are

Hydrogen atoms, anions, and solvent molecules were removed for clarity; ellipsoids are set at 50% probability.11 Reagents and conditions: (a) 1,2-bis(diphenylphosphino)benzene (1 equiv), CH2Cl2, r.t., 12h, 95%; (b) 1-diphenylphosphino-2-[bis(pentrafluorophenyl)phosphine]benzene (1 equiv), CH2Cl2, r.t., 12h, 96%; (c) 1diphenylphosphino-2-[di(pyrro-1-yl)phosphine]benzene (1 equiv), CH2Cl2, r.t., 12 h, 93%; (d) diphosphonite (1 equiv), CH2Cl2, r.t., 12h, 93%; (e) 2-diphenylphosphino-2′-(N,N-dimethylamino)biphenyl (1 equiv), CH2Cl2, r.t., 48 h, 92%; (f) bipyridine (1 equiv), THF, r.t., 12h, 98%; (g) 2-dicyclohexylphosphino-2′,6′-(dimethoxy)biphenyl (1 equiv), CH2Cl2, r.t., 48 h, 92%.

phosphine coordinates to the central Pd atom not only through the electron pair at the phosphorus, but the electronrich aryl group also gets involved. The Pd(1)−C(19) and Pd(1)−C(20) distances are 2.353 and 2.470 Å, respectively, clearly indicating η2 coordination of the aromatic ring. These bonds lengths are significantly larger than those expected for C−Pd σ-bonds but are similar to those found for other weak arene−Pd interactions assisted by chelation,12 which suggests B


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Organometallics

dicationic moiety. In the latter case the acceleration observed could be attributed to the electron poor character of the [−P(H2Im)2]+2 group. DFT Calculations on the Reaction Mechanism. To address this question and to better understand the origin of the enhanced reactivity observed, the free energy profiles for the reductive elimination of 4−6 were computed at the M06L(SMDCH3CN)/def2-TZVP//BP86-D3/def2-TZVP level of DFT (see Figure 2 for 4 and 6 and the Supporting Information

that a vacant coordination site can be easily created on Pd just by moderate heating. Reductive eliminations are fostered not only by strong acceptor ancillary ligands but also by coordinately unsaturated metal centers13 or by large bite angles of the ancillary ligands used.14 Hence, a comparative analysis of the relative rates of C6F5−C6F5 elimination in 4−12 will allow us to assess the influence of these parameters on the chosen model transformation. Reductive Elimination from Bis(pentafluorophenyl) Palladium Complexes. With compounds 4−12 in hand, the rate constants for the first-order reductive elimination of decafluorobiphenyl were determined from plots of the decreasing concentration of the Pd complexes versus time obtained from 19F NMR data. The measured values of kobs are listed in Table 1. Chelating neutral complexes 6−9 and 11 are Table 1. First-Order Rate Constants for the Thermal Decomposition of Pd Complexesa complex 4 5 10 12

kobs × 103 (s‑1) 1.7 11.2 4.3 9.3

± ± ± ±

0.2 0.2 0.1 0.1

Figure 2. Gibbs free energy profile for the reductive elimination of decafluorobiphenyl from 4 (black) and 6 (red) calculated at the M06L(SMDCH3CN)/def2-TZVP//BP86-D3/def2-TZVP level.

a

for 5). The direct reductive elimination from 4 is predicted to be rather facile, with a barrier of 22.9 kcal mol−1 and exergonic by 8.8 kcal mol−1. We find that ligand 1 is not hemilabile since monodentate intermediate 4-monoP is higher in energy (by 3.4 kcal mol−1) than transition state TS-4, which contains 1 as bidentate ligand. The calculated (H2Im)2P−Pd bond distance in TS-4 (2.235 Å) is basically identical to the one in 4 (2.237 Å). In contrast, the analogous neutral complex 6 has a high barrier to reductive elimination (9.3 kcal mol−1 larger than that of 4), and the reaction is endergonic by 11.3 kcal mol−1. The difference in the reaction free energies suggests that 1,2bis(diphenylphosphino)benzene forms a much stronger complex with Pd(II) than with Pd(0), while for ligand 1 this preference is not so dramatic. The barrier for complex 5 is predicted to be 1.3 kcal mol−1 lower than that for 4 (see the Supporting Information). This is in agreement with the faster rate measured for this compound and can be traced back to the wider bite angle of 2. To understand the origin of the different activation barriers for 4 and 6, the details of the ligand−Pd binding were deconstructed with the aid of energy decomposition analysis in Pd(II) complexes 4 and 6 and in the respective transition states TS-4 and TS-6 (Table 2).18 In terms of the so-called interaction−distortion/activation strain model,19 the barrier is caused to a similar extent by the intrinsic electronic contributions (ΔΔEint) and the distortion contributions (ΔΔEdist) in the case of 4, while the former strongly dominate in the case of 6. The energy decomposition analysis further shows that the higher barrier for TS-6

Measured at 70 °C, [Pd]0 = 0.015 M.

completely ineffective to reductively eliminate decafluorobiphenyl under the relatively soft conditions employed (acetonitrile, 70 °C); in contrast, decafluorobiphenyl is readily formed from dicationic complexes 4 and 5 under identical conditions. These results already highlight the enhanced activation toward reductive elimination exerted on the Pd center by dicationic ligands 1 and 2 when compared with structurally similar but neutral (and thus electron richer) analogues. The rate constant for reductive elimination from 5 is approximately 7 times higher than that for its analogue, 4. This faster rate can be attributed to the larger bite angle of 2 compared with that of 1;15 as consequence, less distortion is required in 5 to reach the necessary transition state, which gives rise to a lower overall barrier. However, complexes 10 and 12, both neutral but derived from bis-aryl Buchwald-type phosphines, give rate constants for the reductive elimination that are intermediate in magnitude to those observed for 4 and 5. The remarkable ease of reductive elimination in these systems is attributed to the (pseudo-)three-coordinated Tshaped configuration imposed by the biaryl ligands, with only weak interactions between the hanging arene and the Pd atom.16 Our kinetic analysis raises the question of whether 1 and 2 act as hemilabile ligands in the transition state of the reductive elimination from 4 and 5 (dissociative mechanism)17 or whether the elimination proceeds without dissociation of the C


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Organometallics Table 2. Energy Decomposition Analysis and NBO Charge Transfer (L → Pd(C6F5)2)a 4 TS-4 TS-4−4 (ΔΔE) 6 TS-6 TS-6−6 (ΔΔE)

ΔEPauli

ΔEelstat

ΔEorb

ΔEdisp

ΔEint

ΔEdist

281.3 358.3 −23.0 280.3 242.1 −38.19

−205.7 −201.4 4.3 −227.2 −210.2 16.98

−134.9 −112.1 22.8 −135.2 −93.4 41.73

−42.5 −37.6 4.9 −38.4 −29.5 8.86

−101.9 −92.7 9.17 −120.4 −91.0 29.38

20.1 30.9 10.8 19.5 20.5 0.96

Ea

20.01

30.34

ΔΔq −0.77 −0.42 −0.35 −0.97 −0.56 −0.41

Energy values in kcal/mol. Ea = ΔΔEint + ΔΔEdist. Results were determined at the ZORA-BP86-D3/TZ2P level and are reported as the energy difference between the transition state and reactant.

a

Figure 3. Frontier fragment orbitals calculated at the geometries of 4, TS-4, 6, and TS-6 at the BP86/TZ2P//BP86/def2-TZVP level. Also given are the orbital energies and the HOMO−LUMO overlaps (s). Teal and orange: Pd(C6F5)2 LUMO. Red and blue: Left: Ligand 1 HOMO in 4 and TS-4. Right: neutral ligand HOMO in 6 and TS-6. See text.

geometries in the reactant complexes (4 and 6) and the reductive elimination transition states (TS-4, TS-6) are shown in Figure 3, along with their energies and overlaps. The fragment orbital energy gap is larger for 4 (Δε = 4.94 eV) than for 6 (Δε= −0.89 eV), which implies a much stronger donation from the ligand to the metal in the latter. In addition, the HOMO of the ligand fragment in 4 is mainly localized on the neutral phosphorus atom and delocalized over both phosphorus atoms in 6, which favors a more efficient HOMO−LUMO overlap in 6. Thus, both the energy and the shape of the interacting orbitals point to a stronger ligand−metal bond in 6. The approach to the transition state involves a more pronounced shortening of the (Ar)C−C(Ar) bond distance for 6 compared to 4 (values in TS-6 and TS-4: 1.72 vs 1.88 Å). Consequently, the LUMO energy of the Pd(C6F5)2 fragment in TS-6 increases more strongly, and the orbital overlap between

compared with TS-4 is due to the electronic contributions (ΔΔEint), in particular the orbital interaction term (ΔΔEorb), and to a lesser extent the electrostatic term (ΔΔEelec). Moreover, upon going from the reactant complexes to the reductive elimination transition states, there is less charge transfer (ΔΔq) from the ligand to the Pd(C6F5)2 fragment for 4 than for 6, which will tend to reduce the barrier for 4 since charge transfer from the ligand is detrimental to Pd(II) → Pd(0) reduction. The main orbital interaction governing the ligand-to-metal electron transfer is between the HOMO of the ligand and the LUMO of the Pd(C6F5)2 moiety; thus, as first approximation, the different amount of charge transfer en route to the corresponding transition states can be explained through this interaction. The frontier orbitals of the free ligands (HOMO) and the Pd(C6F5)2 fragments (LUMO) evaluated at their D


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Organometallics the metal and ligand fragments is reduced by 26% (TS-6 vs 6) compared with only 10% when progressing from 4 to TS-4. Combined with the larger HOMO energy of the metal fragment in TS-6, this implies that the electron transfer from the ligand to the metal will be reduced more strongly when going from 6 to TS-6 rather than from 4 to TS-4. In summary, 1 and 2 behave as bidentate ligands during the reductive elimination of decafluorophenyl, and they both facilitate the process mainly by reducing the thermodynamic stability of the Pd(II) precursors compared with neutral bidentate phosphines, as supported by the greater difference in ligand−Pd(C6F5)2 bond strengths (ΔE = ΔEint + ΔEdist) between 4 and 6 (19.1 kcal/mol) and TS-4 and TS-6 (8.7 kcal/ mol). Furthermore, the reduction of Pd(II) → Pd(0) is favored with limited electron flow from the ligand to the metal. Orbital overlap between the ligand HOMO and Pd(C6F5)2 inhibits the reduction, which can be accelerated if the overlap and interaction are limited. The orbital overlap between the HOMO of 1 and the LUMO of Pd(C6F5)2 in complex 4 (s = 0.20) is already much lower than the analogous overlap in complex 6 (s = 0.27). Consequently, the overlap reduction required to reach the Pd(II) → Pd(0) TS is much less in 4 than in 6 which translates to a greater distortion requirement in 6 → TS-6 (Table 2). Attempts to Isolate Pd(0) Complexes. Finally, in an attempt to characterize the Pd(0) complex resulting from the reductive elimination of decafluorobiphenyl from 4, the reaction mixtures from the kinetic experiments were filtered and layered with diethyl ether. This resulted in the formation of a few yellow crystals of a new compound, 13, which depicts two singlets in its 31P NMR spectrum, one at δP = 49.4 ppm that surely corresponds to the R−PPh2 fragment, and a quite indicative signal at δP = 15.9 ppm., which cannot be attributed to the original [−P(H2Im)2]+2 group. Moreover, in contrast to 4, the 1H NMR spectrum of 13 indicates that its four methyl groups are not equivalent. X-ray crystallography was used to determine the atom connectivity in this complex (Scheme 3). Compound 13 is constituted by a bidentate phosphine− phosphinidene ligand coordinated to a PdII atom. An Nheterocyclic carbene and an acetonitrile molecule taken from the solvent of crystallization complete the coordination sphere of the metal. Presumably, 13 is formed by oxidative addition into one of the C−P(H2Im) bonds of the Pd0 center formed after the reductive elimination process.20 This hypothesis is supported by the isolation of 13 in much better yields by treatment of 1 with an equivalent amount of Pd2(dba)3 in dichloromethane. Very similar reactivity has been also observed by reaction of 1 with Ni(cod)2; although in this case the exchange of acetonitrile by 2,6-(dimethyl) phenylisonitrile was necessary to obtain a crystalline material, 14 (Scheme 3). Unfortunately, these results suggest that the use of ancillary ligands 1 and 2 is not feasible in catalytic cycles where Pd(0) or Ni(0) species are formed.

Scheme 3. Synthesis and Molecular Structures of 13 and 14 in the Solid State

Hydrogen atoms, anions and solvent molecules were removed for clarity; ellipsoids are set at 50% probability. Reagents and conditions: (a) Pd(dba)2 (1 equiv), CH2Cl2, r.t., 2 h, 21%; (b) Ni(cod)2 (1 equiv), CH2Cl2, r.t., 12 h, and then 2,6-dimethyphenyl isocyanide (2 equiv), 37%.

subsequently with the ligand through an oxidative addition of the Pd-center into one of the C−P(H2Im) bonds. This undesired process, which affords quite stable Pd(II) phosphinylidene complexes, precludes the use of ligands 1 and 2 in catalytic cycles involving low-valent Pd species.

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EXPERIMENTAL SECTION

General Information. All reactions were carried out in flame-dried glassware under Ar. All solvents were purified by distillation over the appropiate drying agents and were transferred under Ar. IR: Nicolet FT-7199 spectrometer, wavenumbers in cm−1. MS (EI): Finnigan MAT 8200 (70 eV), ESIMS: Finnigan MAT 95, accurate mass determinations: Bruker APEX III FT-MS (7 T magnet). NMR: Spectra were recorded on a Bruker AV 600, AV 400 or DPX 300; 1H and 13C chemical shifts (δ) are given in ppm relative to TMS, coupling constants (J) in Hz. Solvent signals were used as references and the chemical shifts converted to the TMS scale. Column chromatographies were performed on Merck 60 silica gel (40−63 μm), and for thin-layer chromatography (TLC) analyses, Merck silica gel 60 F254 TLC plates were used. All commercially available compounds (ABCR, Acros, Aldrich, Fischer) were used as received.21 Compound 4. Pd complex 3 (100.0 mg, 0.182 mmol) was added to a CH2Cl2 (4 mL) solution of 1 (120.3 mg, 0.182 mmol) and the mixture obtained stirred overnight. After removal of the solvent in vacuo the solid residue washed with CH2Cl2 and dried, affording 4 as a white solid (168.8 mg, 84%). Colorless crystals suitable for X-ray crystallography were obtained by slow diffusion of Et2O into CH3CN/ CH2Cl2 solutions of 4. 1H NMR (CD3CN, 400 MHz): δ = 8.25−8.20 (m, 1H), 8.16−8.11 (m, 1H), 8.10−8.05 (m, 2H), 7.68−7.65 (m, 2H), 7.53−7.50 (m, 8H), 4.14−4.10 (m, 8H), 3.03 (s, 12H); 13C NMR (CD3CN, 125 MHz): δ = 157.2 (dd, J = 26.2 Hz; 1.1 Hz), 147.6 (dm, J = 194.2 Hz), 146.0 (dm, J = 191.3 Hz), 144.2 (dd, J = 52.0 Hz; 44.5 Hz), 140.6 (br), 138.8 (dd, J = 5.6 Hz; 2.3 Hz), 138.3 (d, J = 20.2 Hz), 137.8 (dm, J = 256.1 Hz), 137.5 (d, J = 13.4 Hz), 137.2 (dd, J = 7.2 Hz; 1.7 Hz), 134.4 (d, J = 12.5 Hz), 134.1 (d, J = 2.8 Hz), 130.6 (d, J = 11.4 Hz), 127.9 (d, J = 53.4 Hz), 126.3 (dd, J = 50.3 Hz, J = 33.7 Hz), 54.3 (d, J = 2.1 Hz), 38.3 (d, J = 3.4 Hz); 31P NMR (CD3CN, 121

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CONCLUSIONS The influence of chelating α-dicationic phosphines on the reductive elimination of decafluorobiphenyl from (P∧P′)Pd(C 6 F5 )2 complexes has been studied. This process is significantly facilitated by cationic ligands. Due to their weak donor properties, these ligands reduce the thermodynamic stability of the reaction precursors and thus decrease the barriers of the corresponding transition states. Unfortunately, the Pd(0) intermediates obtained after the elimination react E


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Organometallics MHz): δ = 49.0 (m), 11.9 (m); 11B NMR (CD3CN, 96 MHz): δ = −1.1; 19F NMR (CD3CN, 282 MHz): δ = −116.8 (m), −117.6 (m), −157.9 (t, J = 19.7 Hz), −159.9 (t, J = 19.2 Hz), −161.8 (dt, J = 19.7; 8.7 Hz), −163.6 (dt, J = 20.3; 8.3 Hz); HRMS calcd for C40H34N4BF14P2Pd+: 1015.119220; found, 1015.115674; IR ν̃ = 465, 499, 518, 536, 643, 691, 735, 775, 1300, 1363, 1440, 1458, 1501, 1589, 1600 cm−1. Compound 5. Acetone (3 mL) was added to a mixture of 3 (50.0 mg, 0.065 mmol) and 2 (35.8 mg, 0.065 mmol), and the mixture stirred for 48 h. After removal of the solvent in vacuo, the solid residue was washed with CH2Cl2 and dried, affording 5 as a light yellow solid (57.5 mg, 73%). 1H NMR (CD3COCD3, 400 MHz): δ = 8.08−8.04 (m, 1H), 7.97 (d, J = 7.8 Hz, 1H), 7.93−7.78 (m, 3H), 7.71−7.62 (m, 2H), 7.56−7.44 (m, 3H), 7.43−4.40 (m, 1H), 7.36−7.29 (m, 3H), 7.20−7.12 (m, 2H), 4.49−4.31 (m, 4H), 3.84−3.74 (m, 8H), 3.59− 3.54 (m, 2H), 3.25 (s, 3H), 3.23 (s, 3H), 1.89 (s, 3H), 1.50 (s, 3H); 13 C NMR (CD3COCD3, 100 MHz): δ = 158.3 (dd, J = 24.4 Hz; 3.6 Hz), 156.6 (d, J = 14.3 Hz), 144.3 (d, J = 8.3 Hz), 144.0 (d, J = 10.8 Hz), 142.3 (dd, J = 23.3 Hz; 3.0 Hz), 138.1 (d, J = 2.0 Hz), 137.7 (dd, J = 13.5 Hz, J = 5.5 Hz), 136.8 (dd, J = 12.5 Hz; 2.8 Hz), 136.4 (d, J = 2.2 Hz), 134.7 (d, J = 10.0 Hz), 134.2 (d, J = 4.5 Hz), 133.9 (d, J = 2.4 Hz), 132.1 (d, J = 7.3 Hz), 131.9 (d, J = 2.5 Hz), 131.6 (d, J = 41.8 Hz), 131.3 (d, J = 8.7 Hz), 130.5 (d, J = 8.6 Hz), 130.4 (d, J = 11.3 Hz), 129.2 (d, J = 10.7 Hz), 128.6 (t, J = 24.6 Hz), 126.5 (dd, J = 51.8 Hz; 1.8 Hz), 125.3 (d, J = 48.6 Hz), 56.1 (d, J = 1.5 Hz), 54.1 (d, J = 2.2 Hz), 53.6 (d, J = 2.2 Hz), 53.4, 42.5 (dd, J = 6.2 Hz; 5.1 Hz), 39.7 (t, J = 9.4 Hz), 37.6, 37.3 (d, J = 6.3 Hz), 20.9 (d, J = 2.5 Hz), 19.9 (d, J = 1.6 Hz); 31P NMR (CD3COCD3, 121 MHz): δ = 15.3 (m), 12.6 (m); 11B NMR (CD3COCD3, 96 MHz): δ = −1.0; 19F NMR (CD3COCD3, 282 MHz): δ = −110.8 (m), −111.0 (m), −113.7 (m), −114.2 (m), −156.7 (t, J = 19.9 Hz), −160.7 (m), −161.6 (t, J = 19.9 Hz), −163.4 (m), −163.9 (m); HRMS calcd for C48H42N4BF14P2Pd+: 1119.178050; found, 1119.178274; IR ν̃ = 420, 458, 468, 501, 521, 544, 696, 747, 766, 783, 924, 956, 1056, 1298, 1442, 1504, 1580 cm−1. Compound 6. CH2Cl2 (2 mL) was added to a mixture of 3 (50.0 mg, 0.091 mmol) and 1,2-bis(diphenylphosphino)benzene (40.4 mg, 0.091 mmol) and the mixture stirred overnight. After removal of the solvent in vacuo, the solid residue was washed with pentane and dried, affording the desired product as a white solid (76.8 mg, 95%). Colorless crystals suitable for X-ray crystallography were obtained from CH2Cl2. 1H NMR (CD2Cl2, 600 MHz): δ = 7.75−7.72 (m, 2H), 7.61−7.60 (m, 2H), 7.51−7.49 (m, 4H), 7.47−7.44 (m, 8H), 7.39− 7.37 (m, 8H); 13C NMR (CD2Cl2, 150 MHz): δ = 146.3 (dm, J = 230.4), 142.5 (t, J = 43.0 Hz), 137.5 (dm, J = 241.9 Hz), 136.5 (dm, J = 237.4 Hz), 133.9 (t, J = 8.6 Hz), 133.7 (t, J = 8.6 Hz), 133.1, 131.8, 130.6 (d, J = 47.9 Hz), 129.2 (t, J = 4.5 Hz); 31P NMR (CD2Cl2, 121 MHz): δ = 52.3; 19F NMR (CD2Cl2, 282 MHz): δ = −113.7 (m), −161.2 (t, J = 20.7 Hz), −163.1 (tm, J = 20.7 Hz); MS-EI calcd for C42H24F10P2Pd: 886.02; found, 886.90; IR ν̃ = 411, 422, 445, 498, 544, 602, 617, 668, 687, 741, 760, 774, 950, 1000, 1027, 1055, 1098, 1159, 1186, 1254, 1281, 1308, 1346, 1432, 1496, 1608, 1633, 3062 cm−1. Compound 7. A solution of the corresponding free diphosphine (50.0 mg, 0.080 mmol) in CH2Cl2 (2 mL) was added to 3 (43.8 mg, 0.080 mmol) and stirred overnight. After removal of the solvent in vacuo, the solid residue was washed with pentane and dried, affording 7 as a white solid (81.7 mg, 96%). 1H NMR (CD2Cl2, 400 MHz): δ = 8.07−7.96 (m, 1H), 7.84−7.82 (m, 1H), 7.76−7.65 (m, 2H), 7.63− 7.54 (m, 2H), 7.36−7.52 (m, 8H); 13C NMR (CD2Cl2, 100 MHz): δ = 148.5 (m), 147.3 (dm, J = 21.0 Hz), 147.2 (dm, 22.4 Hz), 145.9 (m), 145.7 (m), 144.9 (dm, J = 29.2 Hz), 143.1 (m), 141.4 (dd, J = 50.8 Hz; 44.5 Hz), 139.7 (m), 139.2 (m), 138.9, 138.0 (m), 137.2 (m), 135.5 (m), 134.7 (dd, J = 19.9 Hz, J = 1.1 Hz), 134.5 (dd, J = 5.5 Hz, J = 2.0 Hz), 134.3 (dd, J = 6.3 Hz; 1.8 Hz), 133.7 (d, J = 12.5 Hz), 133.3 (dm, J = 15.7 Hz), 132.3 (d, J = 2.6 Hz), 129.8, 129.4 (d, J = 11.1 Hz); 31 P NMR (CD2Cl2, 121 MHz): δ = 51.0 (m), 16.9 (m); 19F NMR (CD2Cl2, 282 MHz): δ = −115.0 (m), −118.1 (m), −127.1 (m), −145.7 (m), −159.0 (m), −160.7 (t, J = 19.7 Hz), −161.4 (t, J = 19.7 Hz), −163.5 (td, J = 20.1 Hz, J = 9.4 Hz), −164.0 (td, J = 20.1 Hz, J = 10.4 Hz); HRMS calcd for C42H14F20P2Pd1Na1+: 1088.917860; found, 1088.917765; IR ν̃ = 458, 483, 519, 536, 631, 670, 692, 745, 797, 954,

977, 1017, 1091, 1260, 1297, 1360, 1455, 1475, 1499, 1519, 1642, 2963 cm−1. Compound 8. (Dipyrrolylphosphino)-2-diphenylphosphine (38.7 mg, 0.091 mmol) and 3 (50.0 mg, 0.091 mmol) were dissolved in CH2Cl2 (1 mL) and stirred overnight. Then, the solvent was evaporated in vacuo and washed with Et2O to afford the desired compound as a white solid (73.3 mg, 93%). 1H NMR (CD2Cl2, 500 MHz): δ = 7.50−7.91 (m, 1H), 7.85−7.81 (m, 1H), 7.80−7.75 (m, 2H), 7.53−7.49 (m, 2H), 7.46−7.33 (m, 8H), 6.86−6.84 (m, 5H), 6.40−6.38 (m, 4H); 13C NMR (CD2Cl2, 125 Mz): δ = 147.2 (dm, JC−F = 68.6 Hz), 145.3 (dm, JC−F = 72.8 Hz), 141.8 (dd, JC−P = 49.6, JC−P = 37.0 Hz), 140.8 (dd, JC−P = 52.1, JC−P = 43.2 Hz), 138.1 (dm, JC−F = 241.6 Hz), 136.8 (d, JC−F = 250.6 Hz), 135.62 (d, JC−P = 6.0 Hz), 134.23 (d, JC−P = 19.3 Hz), 133.59 (d, JC−P = 12.6 Hz), 133.3 (dd, JC−P = 5.9 Hz, JC−P = 1.6 Hz), 132.2 (d, JC−P = 2.5 Hz), 132.1 (dd, JC−P = 15.8, JC−P = 2.4 Hz), 129.4 (d, JC−P = 10.9 Hz), 128.9 (d, JC−P = 49.7 Hz), 124.1 (d, JC−P = 8.2 Hz), 114.8 (d, JC−P = Hz); 31P NMR (CD2Cl2, 162 MHz): δ = 109.1 (br), 47.9 (br); 19F NMR (CD2Cl2, 282 MHz): −115.02 (m), −161.59 (m), −163.57 (dm, JF−P = 139.0 Hz); HRMS calcd for C38H22N2F10P2PdNa+: 887.002710; found, 887.002477; IR ν̃ = 421, 450, 478, 511, 537, 566, 608, 627, 672, 702, 725, 776, 953, 1001, 1055, 1100, 1115, 1237, 1350, 1360, 1436, 1498, 1531, 3060 cm−1. Compound 9. Palladium complex 3 (53.8 mg, 0.098 mmol) was added to a solution of the corresponding diphosphonite (45.0 mg, 0.098 mmol) in CH2Cl2 (2 mL) and the resulting mixture stirred overnight. After removal of the solvent in vacuo, the solid residue was washed with pentane and dried to afford 9 as a white solid (82.1 mg, 93%). 1H NMR (CD2Cl2, 400 MHz): δ = 7.48−7.45 (m, 4H), 7.36− 7.34 (m, 8H), 7.15−7.13 (m, 4H), 2.41 (d, J = 23.0 Hz, 4H); 13C NMR (CD2Cl2, 100 MHz): δ = 148.2 (m), 146.1 (dm, J = 226.3 Hz), 137.4 (dm, J = 245.2 Hz), 136.1 (dm, J = 252 Hz), 130.5, 129.6, 129.2, 126.6, 121.0, 26.8 (t, J = 23.2 Hz); 31P NMR (CD2Cl2, 121 MHz): δ = 202.2 (m); 19F NMR (CD2Cl2, 282 MHz): δ = −114.5 (m), −161.9 (t, J = 19.9 Hz), −163.1 (td, J = 19.9 Hz, J = 9.1 Hz); HRMS calcd. for C38H20O4F10P2PdNa+: 920.960310; found 920.960340; IR ν̃ = 493, 523, 536, 595, 654, 716, 755, 772, 823, 871, 912, 954, 1012, 1045, 1094, 1191, 1248, 1274, 1361, 1403, 1456, 1498, 1532, 1606, 1633, 2916, 3067 cm−1. Compound 10. 2−Diphenylphosphino−2′−(N,N−dimethylamino) biphenyl (30.0 mg, 0.055 mmol) and 3 (20.8 mg, 0.055 mmol) were stirred in CH2Cl2 (2 mL) for 2 days. Then, the solvent was evaporated in vacuo and the crude product washed with Et2O to afford compound 10 as a pale yellow solid (41.4 mg, 92%). Yellow crystals suitable for X-ray analysis were obtained from saturated CH2Cl2/ pentane solution. 1H NMR (CD2Cl2, 300 MHz): δ = 8.05−7.99 (m, 2H), 7.70−7.65 (m, 1H), 7.55−7.46 (m, 4H), 7.42−7.32 (m, 2H), 7.23−7.16 (m, 3H), 6.98−6.75 (m, 5H), 6.59−6.56 (m, 1H), 3.04 (s, 6H) ppm. 13C NMR (CD2Cl2, 125 Mz): δ = 155.33 (m), 150.5 (d, JC−P = 22.0 Hz), 147.2 (m), 145.8 (m), 144.6 (m), 138.2 (m), 135.5 (d, JC−P = 13.3 Hz), 133.7, 132.5 (m), 131.9, 131.6, 131.4 (d, JC−P = 11.4 Hz), 130.5, 129.4 (d, JC−P = 10.6 Hz), 128.4 (d, JC−P = 10.2 Hz), 128.2 (d, JC−P = 5.6 Hz), 122.4 (br), 116.6 (br), 47.3 (m); 31P NMR (CD2Cl2, 162 MHz): δ = 23.2 (m); 19F NMR (CD2Cl2, 282 MHz): δ = −112.92 (m), − 114.28 (m), − 115.26 (m), − 117.93 (m), − 162.35 (t, JF−F = 19.8 Hz), − 162.95 (t, JF−F = 19.8 Hz), − 163.64 (m), − 163.88, − 164.43 (m), − 164.94 (m); HRMS calcd. for C38H24NF10PPdNa+: 844.041910, found 884.041289. IR ν̃ = 433, 450, 494, 538, 692, 760, 788, 852, 949, 1041, 1058, 1099, 1213, 1274, 1344, 1362, 1435, 1493, 1577, 2965, 3067 cm−1. Compound 13. Diphosphine 1 (20.0 mg, 0.021 mmol) and Pd(dba)2 (12.0 mg, 0.021 mmol) were stirred in CH2Cl2 (2 mL) at r.t. for 2 h and the solvent evaporated in vacuo. The resulting solid was extracted with CH3CN and recrystallized from CH3CN, CH2Cl2, and Et2O to afford the desired compound 13 as a yellow solid (4.9 mg, 21%). Colorless crystals suitable for X-ray analysis were obtained from CH3CN/CH2Cl2/Et2O. 1H NMR (CD3CN, 600 MHz): δ = 7.69− 7.55 (m, 13H), 7.55−7.50 (m, 1H), 3.90−3.84 (m, 2H), 3.83−3.76 (m, 2H), 3.75−3.66 (m, 4H), 3.35 (s, 3H), 3.04 (s, 3H), 2.94 (s, 6H); 13 C NMR (CD3CN, 125 Mz): δ = 196.5 (dd, JC−P = 126.7 Hz,, JC−P = F


Article

Organometallics 16.4 Hz), 176.6 (dd, JC−P = 80.7 Hz, JC−P = 1.7 Hz), 146.2 (dd, JC−P = 40.9 Hz, JC−P = 20.9 Hz), 135.4 (dd, JC−P = 56.0 Hz, JC−P = 15.8 Hz), 135.1 (d, JC−P = 2.4 Hz), 134.6 (dd, JC−P = 4.4 Hz, JC−P = 1.8 Hz), 134.21, 134.20 (d, JC−P = 9.3 Hz), 134.1 (d, JC−P = 11.3 Hz), 134.0 (d, JC−P = 21.8 Hz), 133.4 (d, JC−P = 2.6 Hz), 130.8 (dd, JC−P = 11.0 Hz, JC−P = 2.4 Hz), 130.7 (d, JC−P = 1.7 Hz), 130.4 (d, JC−P = 47.5 Hz), 129.7 (d, JC−P = 96.8 Hz), 128.6 (d, JC−P = 48.2 Hz), 52.8 (d, JC−P = 5.2 Hz), 52.5 (d, JC−P = 4.6 Hz), 52.4 (d, JC−P = 1.0 Hz), 37.7, 37.6, 37.4, 37.3, 37.2 (m); 31P NMR (CD3CN, 162 MHz): δ = 49.4, 15.9; 19 F NMR (CD3CN, 282 MHz): δ = −124.0 (sextet, JF−Sb(I=5/2) = 1933 Hz, octet, J F − S b ( I = 7 / 2 ) = 1049 Hz); HRMS calcd for C28H34N4F6P2SbPd+: 829.022380; found, 829.022889; IR ν̃ = 426, 495, 507, 534, 591, 652, 699, 752, 774, 920, 940, 1103, 1203, 1291, 1333, 1407, 1438, 1546, 1567, 2301, 2929 cm−1. Compound 14. Diphosphine 1 (50.0 mg, 0.052 mmol) and Ni(cod)2 (14.3 mg, 0.052 mmol) were stirred overnight in CH2Cl2 (2 mL). The yellow precipitate formed was filtered. Then, 2,6dimethylphenyl isocyanide (16.7 mg, 0.128 mmol) in CH2Cl2 (2 mL) was added, and the resulting mixture was stirred overnight. After evaporation of the solvent, the solid obtained was washed with Et2O and recrystallized from CH2Cl2/Et2O to afford desired compound 14 as a yellow solid (22.1 mg, 37%). Yellow crystals suitable for X-ray analysis were obtained from a saturated solution of the title compound in CH2Cl2/Et2O. 1H NMR (CD2Cl2, 600 MHz): δ = 7.74−7.48 (m, 13H), 7.37 (t, J = 8.2 Hz, 1H), 7.32 (t, J = 7.7 Hz, 1H), 7.14 (d, J = 7.7 Hz, 2H), 3.98 (s, J = 4H), 3.88−3.71 (m, 4H), 3.33 (s, 6H), 3.01 (s, 6H), 1.98 (s, 6H); 13C NMR (CD2Cl2, 125 Mz): δ = 199.2 (dd, JC−P = 71.0 Hz,, JC−P = 22.3 Hz), 176.7 (dd, JC−P = 76.9 Hz, JC−P = 3.7 Hz), 146.2 (m), 144.7 (dd, JC−P = 39.7 Hz, JC−P = 15.4 Hz), 136.2, 135.4 (dd, JC−P = 59.0 Hz, JC−P = 20.0 Hz), 135.0, 134.1, 133.5, 133.3, 133.2, 132.8 (dd, JC−P = 35.4 Hz, JC−P = 17.5 Hz), 131.7, 130.7 (d, JC−P = 11.4 Hz), 130.4 (d, JC−P = 8.3 Hz), 129.0, 125.6, 52.7, 51.7, 37.43, 37.41, 37.34, 37.28, 18.3 (m); 31P NMR (CD2Cl2, 162 MHz): δ = 56.9 (d, JP−P = 4.3 Hz), 23.7 (d, JP−P = 4.3 Hz); 19F NMR (CD2Cl2, 282 MHz): δ = −124.0 (sextet, JF−Sb(I=5/2) = 1933 Hz, octet, JF−Sb(I=7/2) = 1049 Hz); HRMS calcd for C37H43N5F6P2Sb+: 912.128170; found, 912.128332; IR ν̃ = 442, 487, 515, 530, 651, 693, 713, 752, 773, 791, 939, 957, 1097, 1205, 1287, 1438, 1536, 1566, 2164 cm−1. Computational Methods. All geometry optimizations were performed using the BP8622 and M06L23 functionals with BP86 being augmented by the D3 dispersion correction with BJ-damping (BP86-D3).24 The def2-SVP25 basis set was used for all atoms. The 28 inner-shell core electrons of the palladium atom were described by the corresponding def2 effective core potential26 accounting for scalar relativistic effects (def2-ecp). For the purpose of computational efficiency, the resolution-of-identity (RI) approximation27 was applied using auxiliary basis sets to approximate Coulomb potentials in conjunction with the multipole-accelerated resolution of the identity approximation (MA-RI) method during geometry optimizations using the BP86-D3 method.28 Stationary points were characterized by evaluating the harmonic vibrational frequencies at the optimized geometries. Zero-point vibrational energies (ZPVE) were computed from the corresponding harmonic vibrational frequencies without scaling. Relative free energies (ΔG) were determined at standard pressure (1 bar) and at an elevated temperature (343 K). The thermal and entropic contributions were evaluated within the rigid-rotor harmonic-oscillator approximation.29 Solvation contributions were included for acetonitrile on the optimized gas-phase geometries employing the SMD solvation model30 using the same functional and the def2-TZVP basis set. Geometry optimizations at the BP86-D3 level were performed with TURBOMOLE (version-6.4)31 and single-point SMD solvation calculations were performed using Gaussian09.32 The energy decomposition was performed on BP86-D3/TZVP optimized geometries using the ADF201633 program package at the BP86-D3 level in conjunction with a triple-ζ-quality basis set of uncontracted Slater-type orbitals (STOs)34 augmented with two sets of polarization functions for all atoms; all electrons were included (i.e., inner core electrons were not described by a frozen core). Scalar

relativistic effects were accounted for using the zeroth-order regular approximation (ZORA).35

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.7b00418. NMR spectra and detailed computational results (PDF) Cartesian coordinates for crystallographic structures (XYZ) Accession Codes

CCDC 1033320−1033322 and 1536494−1536496 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/ data_request/cif, or by emailing [email protected]. uk, or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Walter Thiel: 0000-0001-6780-0350 Manuel Alcarazo: 0000-0002-5491-5682 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Generous financial support from the Fonds der Chemischen Industrie (Dozentenstipendium to M.A.), the European Research Council (ERC Starting Grant to M.A.), the Deutsche Forschungsgemeinschaft (AL 1348/5-1), and the Chinese Scholarship Council (doctoral fellowship to L.G.) is gratefully acknowledged.

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REFERENCES

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