Dinuclear Palladium(II) and -(III) Compounds with O,O-Chelating

Article pubs.acs.org/Organometallics

Dinuclear Palladium(II) and -(III) Compounds with O,O-Chelating Ligands. Room-Temperature Direct 2‑Phenylation of 1‑Methylindole Susana Ibáñez,† Francisco Estevan,† Pipsa Hirva,§ Mercedes Sanaú,† and Ma Angeles Ú beda*,† †

Departament de Química Inorgànica, Universitat de València, Dr.Moliner 50, 46100-Burjassot, Valencia, Spain Department of Chemistry, University of Eastern Finland, Joensuu Campus, P.O. Box 111, FI-80101 Joensuu, Finland

§

S Supporting Information *

ABSTRACT: New dinuclear palladium(III) compounds of general formula Pd2[(C6H4)PPh2]2[O−O]2Cl2, O−O being chelating phenolates C6H4OC(O)R (R = CH3, 3a; R = C2H5, 3b; R = OPh, 3c) or acetylacetonates RC(O)CHC(O)R (R = CH3, 4a; R = CF3, 4b; R = C(CH3)3, 4c), have been obtained by oxidation with PhICl2 of the corresponding palladium(II) compounds. The stability of the new compounds has been studied by 31P NMR spectroscopy from 200 to 298 K. DFT calculations of the stability of the complexes have also been performed. In agreement with these calculations, only compound Pd2[(C6H4)PPh2]2[(CF3C(O)CHC(O)CF3]2Cl2, 6b, showed the highest thermal stability. 6b was characterized by X-ray diffraction methods, presenting the longest Pd−Pd distance, 2,6403(6) Å, observed among the already described discrete Pd26+ compounds. The isolated palladium(II) and -(III) compounds have been tested at room temperature in the catalytic 2-phenylation of 1-methylindole with [Ph2I]PF6. With 3a as precatalyst the reaction was completed in 2 h with a 93% isolated yield. The results were compared with those obtained with other orthometalated dinuclear and mononuclear palladium compounds.

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proposed in some −Pd−X−Pd−X− one-dimensional (1D) chains. Lahuerta and co-workers tested for the first time one dinuclear palladium(III) compound as precatalyst in the diboration of terminal and internal vinylarenes with excellent conversions and chemoselectivities. They have also reported the one-pot diboration−arylation reaction of alkenes, being the first example of the multifaceted properties of a palladium complex participating in different catalytic cycles with identical success.9 In these reactions the presence of bis(catecholato)diboron favored the reduction of the Pd(III) compound to the tetranuclear Pd(II) complex Pd4[(C6H4)PPh2]4Cl4. The synthetic accessibility of palladium(III) and -(IV) compounds has allowed the development of new strategies in which these compounds are considered as intermediates in catalytic processes. Sanford and Hickman described the advantages of “high-valent” organometallic palladium(III) and -(IV) intermediates over the more common “low-valent” analogues.10 Ritter and co-workers brought forward the first evidence of dinuclear palladium(III) intermediates in Pdcatalized C−H oxidation reactions.5,6,11 In contrast with the mononuclear palladium(IV) intermediates, dinuclear palladium(III) compounds with metal−metal bonds can lower the activation barriers for oxidation because of the metal−metal cooperation. Sanford and co-workers also proposed a dinuclear palladium(III) or Pd(II)/Pd(IV) compound as an intermediate in Pd-catalyzed ligand-directed C−H arylation with diaryliodonium

INTRODUCTION The chemistry of palladium in the III oxidation state is nowadays not well known. Only a few mononuclear and dinuclear palladium(III) compounds have been well characterized.1 Cotton and co-workers synthesized in low yield the first dinuclear paddlewheel palladium(III) compound, Pd2(hpp)4Cl2 (hpp = hexahidro-2H-pyrimido[1,2-α]pyridinate), with a metal−metal σ bond and the Pd26+ core.2 In 2006, Lahuerta and co-workers reported for the first time the synthesis in high yield of the first organometallic dinuclear palladium(III) compounds.3 These paddlewheel metalated complexes with a Pd−Pd σ bond and general formula Pd2[C6H4PPh2][O2CR]2X2 (X = Cl; R = CH3, CF3, CMe3; X = Br, R = CH3) were obtained by oxidation of the counterpart palladium(II) compounds with PhICl2 or Br2 and characterized by X-ray methods. Related palladium(III) complexes with N,N′-diarylformamidinate bridging ligands were also reported.4 Ritter and co-workers synthesized and characterized structurally new dinuclear palladium(III) compounds with a Pd26+ core. These compounds were obtained at low temperature by oxidation of the corresponding palladium(II) compound5 or by substitution of the axial chloride ligand in Cotton's palladium(III) compound.6 Recently they reported the first examples of 1D palladium wires with metal−metal bonds.7 Cationic dinuclear palladium(III) compounds, [(Me3tacn)PdIIIX2(μ-X)PdIIIX2(Me3tacn)]PF6 (Me3tacn = N,N′,N″-trimethyl-1,4,7-triazacyclononane; X = Cl, Br), have also been reported by Mirica and co-workers.8 They are the first group 10 d7−d7 dinuclear complexes bridged by a single halide ligand and a model of the delocalized PdIII−X−PdIII electronic structure © XXXX American Chemical Society

Received: May 11, 2012

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dx.doi.org/10.1021/om300400k | Organometallics XXXX, XXX, XXX−XXX

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salts.12 Mirica and co-workers detected by UV/vis the presence of dinuclear palladium(III) with halide as bridging ligand in the Kharasch addition of polyhaloalkanes to alkenes.8 Focused on exploring the chemistry of the palladium(III) compounds, we present in this paper the synthesis of new dinuclear palladium(III) with a Pd26+ core (Figure 1A) and the

with high yields and selectivities. Other orthometalated dinuclear palladium(II) and -(III) and mononuclear palladium(II) compounds have also been tested in this catalytic reaction, and the results have been compared.

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RESULTS AND DISCUSSION Dinuclear Palladium(II) Compounds with Chelated O,O-Donor Ligands. Synthesis. The reaction of the cationic solvated dinuclear compound 2 obtained from the tetranuclear compound 1 with potassium salts of different phenols and acetylacetones produced new Pd24+ compounds of general formula Pd2[(C6H4)PPh2]2[O−O]2, O−O being chelating phenolates C6H4OC(O)R (R = CH3, 3a; R = C2H5, 3b; R = OPh, 3c) or acetylacetonates RC(O)CHC(O)R (R = CH3, 4a; R = CF3, 4b; R = C(CH3)3, 4c) (Scheme 1). Compounds 3a,b and 4a−c were structurally characterized by single-crystal X-ray diffractions methods (Figures 2 and 3). Selected bond distances and angles for the five compounds are given in Table 1. The five compounds show similar structures, and the bimetallic unit is only supported by two metalated phosphines. Two O,O-donor chelating ligands, phenolates or acetylacetonates, complete the palladium square-planar coordination mode. The Pd−Pd distances are longer than those observed in other metalated dinuclear palladium(II) compounds with paddlewheel structure3,4 and also indicate the absence of a Pd−Pd interaction. The biggest distance observed in compound 4c is attributed to steric hindrance of the bulky C(CH3)3 groups. Electrochemical Studies. The new Pd24+ complexes, 3a−c and 4a−c, were investigated by cyclic voltammetry in CH2Cl2 as solvent at low (263 K) and room temperature. In all cases, the cyclic voltammograms obtained were similar, showing an irreversible oxidation peak at values close to 1 V (vs SCE). No reversible processes were observed even at high scan rates or low temperature. Figure SI 1 in the Supporting Information shows the voltammogram for compound 4a. The data obtained by cyclic voltammetry showed that the oxidation of all the palladium(II) compounds took place at

Figure 1. Dinuclear metalated palladium(III) (A) and palladium(II) (B) compounds.

study of their stability. These compounds were obtained by oxidation of new dinuclear metalated palladium(II) compounds with chelating O−O-donor ligands: acetylacetonates and phenolates (Figure 1B). DFT calculations have been developed for all the palladium(III) and -(II) compounds, and the results compared with the previously published palladium compounds with carboxylate ligands.9 Palladium is a suitable transition metal in some catalytic carbon−carbon cross-coupling reactions. The direct oxidative coupling of arene C−H has the potential to give products of carbon−carbon cross-coupling reactions without using prefunctionalized substrates.1 Sanford and co-workers reported a new method for the direct 2-arylation of indoles catalyzed by the mononuclear palladium(II) compound IMesPd(O2CCH3)2 (IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene) that proceeds under very mild conditions, generally at room temperature.13 The authors proposed a Pd(II)/(IV) pathway in these systems. Following this procedure, the described dinuclear metalated palladium compounds have been tested in the direct 2phenylation of 1-methylindole at room temperature, proceeding Scheme 1. Synthesis of Compounds 3a−c and 4a−c

B


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Figure 2. ORTEP views of Pd2[(C6H4)PPh2]2[C6H4OC(O)R]2 (R = CH3, 3a; R = C2H5, 3b) with ellipsoids representing 30% probability and H atoms omitted for clarity.

Figure 3. ORTEP views of Pd2[(C6H4)PPh2]2[(RC(O)CHC(O)R]2 (R = CH3, 4a; R = CF3, 4b; R = C(CH3)3, 4c) with ellipsoids representing 30% probability and H atoms omitted for clarity.

Table 1. Selected Distances (Å) and Angles (deg) for Compounds 3a,b and 4a−c Pd(1)−Pd(2) Pd(1)−P(1) Pd(1)−O(1) Pd(1)−O(2) Pd(1)−C(42) O(1)−Pd(1)−O(2) P(1)−Pd(1)−O(2)

3a

3b

4a

4b

4c

2.9762(5) 2.2436(12) 2.088(3) 2.047(3) 1.982(4) 87.54(12) 176.02(9)

2.9891(12) 2.249(3) 2.080(6) 2.059(6) 1.997(9) 87.3(3) 177.6(2)

2.9730(6) 2.2309(10) 2.082(3) 2.071(3) 1.995(4) 90.18(11) 174.54(9)

2.9740(4) 2.2426(10) 2.130(3) 2.094(3) 1.972(4) 87.37(11) 174.64(8)

3.0940(5) 2.2515(12) 2.086(3) 2.049(3) 1.980(4) 88.90(12) 171.47(10)

potentials easily reached by chemical methods. The use of PhICl2 not only allows the oxidation of Pd(II) → Pd(III) but

also brings chloride ligands that can stabilize the compounds obtained. The latter hypothesis was confirmed by DFT calculations. C


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compound.3,4 Compounds 5a−c and 6a,c were not stable, even at 200 K, and the spectra showed the formation of the wellcharacterized stable tetranuclear palladium(II) compound Pd4[(C6H4)PPh2]4Cl4 (7)14a (similar to 1 with Cl bridges) in addition to the corresponding palladium(III) compounds (Figure 4). At room temperature 5a,b and 6a,c evolved quantitatively to compound 7 (see Figure SI 2). In the case of 3c, an uncharacterized palladium(II) compound was obtained in addition to 7. The only stable palladium(III) compound at room temperature was 6b, Pd2[(C6H4)PPh2]2[CF3C(O)CHC(O)CF3]2Cl2 (Figure SI 3), which was synthesized and characterized by singlecrystal X-ray diffractions methods (Figure 5).

Dinuclear Palladium(III) Compounds with Chelated O,O-Donor Ligands. Chemical Oxidation of Compounds 3a−c and 4a−c. In order to get information about the synthesis and the stability of the resulting palladium(III) compounds 5a−c and 6a−c, we have monitored by 31P NMR spectroscopy from 200 to 298 K the reaction of compounds 3a−c and 4a−c with PhICl2 (Scheme 2). Scheme 2. Reaction of Compounds 3a−c and 4a−c with PhICl2 to Give Palladium(III) Compounds 5a−c and 6a−c

When a stoichiometric amount of PhICl2 was added to a solution of the corresponding palladium(II) compound in a NMR tube, the 31P NMR spectrum at 200 K showed the disappearance of the signal corresponding to the palladium(II) complex and the appearance of a new signal shifted toward high fields by at least 20 ppm (Table 2). In agreement with the literature this new signal corresponds to the palladium(III)

Figure 5. ORTEP view of 6b with ellipsoids representing 30% probability and H atoms omitted for clarity. Selected bond distances (Å) and bond angles (deg): Pd(1)−Pd(2), 2.6403(6); Pd(1)−P(1), 2.2626(16); Pd(1)−O(1), 2.154(4); Pd(1)−Cl(1), 2.4382(16); Pd(1)−O(2), 2.122(4); O(1)−Pd(1)−P(1), 95.73(12); P(1)− Pd(1)−O(2), 175.50(11).

Table 2. 31P NMR Spectroscopy Data at 200 K of the Reaction of 3a−c and 4a−c with PhICl2 and the Pd(III)/7 Ratio palladium(II) compounds

a

Table 3 collects all the Pd−Pd distances in discrete dinuclear palladium(III) compounds published in the literature. Comparing all these data, compound 6b shows the longest Pd−Pd distance observed among the described discrete Pd26+ compounds. Electrochemical Studies. The new Pd26+ complex 6b was also investigated by cyclic voltammetry in CH2Cl2 as solvent at 263 K and room temperature. In both cases, the cyclic voltammograms are similar, showing an irreversible reduction peak at values close to 0.6 V (vs SCE). Computational Studies of Pd24+ and Pd26+ Compounds. Optimized Geometries. The optimized geometries of the

palladium(III) compounds

compound

δ (ppm)

compound

δ (ppm)

Pd(III):7 ratio

3a 3b 3c 4a 4b 4c

27.15 26.50 25.62 26.76 26.72 22.30

5a 5b 5c 6a 6b 6c

−0.34 −0.51 −1.88 3.34 4.65 −19.10

3.4:1 3.1:1 1.0:7a 1.1:1 b

0.88:1

Pd(III): 7 + other compound. b7 was not observed.

Figure 4. 31P NMR spectrum of the reaction of 3a with PhICl2 at 200 K in CD2Cl2. D


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distance shortens considerably, which is an indication of a stronger Pd−Pd interaction. The same trend can also be seen in complexes 8 and 9 with carboxylate ligands, although with the chelating ligands the shortening is even more pronounced (about 0.3 Å in 3 to 5 or 4 to 6, compared to about 0.1 Å in 8 to 9). The data are also consistent with the available experimental Pd−Pd distances, although the computational method was found to overestimate the Pd−Pd distances for about 3% on average. Analysis of the Molecular Orbitals. Analysis of the frontier molecular orbitals shows interesting differences in their nature when the palladium(II) complexes are compared with the palladium(III) counterparts. Figures 6 and 7 show the appearance of

Table 3. Pd−Pd Distances in Å for All the Discrete Dinuclear Palladium(III) Compounds Structurally Characterized by Single-Crystal X-ray Diffraction Methodsa authorsref Cotton et al. Lahuerta et al.3,4

palladium(III) compound 2

Ritter et al.5,6

Pd2(hpp)4Cl2 Pd2[(C6H4)PPh2]2[O2CCH3]2Cl2 Pd2[(C6H4)PPh2]2[O2CCF3]2Cl2 Pd2[(C6H4)PPh2]2[O2CCMe3]2Cl2 Pd2[(C6H4)PPh2]2[O2CCH3]2Br2 Pd2[(C6H5)PPh2]2[CF3C(O)C(H)C(O)CF3]2Cl2 (6b) Pd2[bhq]2[O2CCH3]2Cl2 Pd2[bhq]2[O2CCH3]2[OAc]2 Pd2[ppy]2[O2CCH3]2[OAc]2 Pd2[hpp]4[OBz]2

d(Pd−Pd), Å 2.391(2) 2.5294(17) 2.5434(4) 2.5241(9) 2.5411(9) 2.6403 (6) 2.5672(5) 2.5681(5) 2.5548(5) 2.3991(2)

a

hpp = hexahydro-2H-pyrimido[1,2-α]pyridine; bhq = benzo[h]quinoline; ppy = 2-phenylpyridine.

palladium(II) and -(III) complexes 3, 4 and 5, 6, respectively, having chelating phenolate and acetylacetonate ligands with different substituents, were compared with the previously studied corresponding palladium complexes with bridging carboxylate ligands, Pd2[(C6H4)PPh2][O2CR]2 (R = CH3, 8a; R = CF3, 8b) and Pd2[(C6H4)PPh2][O2CR]2Cl2 (R = CH3, 9a; R = CF3, 9b). Table 4 and Table SI 1 list the optimized Pd−Pd distances of all calculated options. Complexes with both types of chelating ligands result in longer Pd−Pd distances than those with bridging carboxylates, which force the metal atoms into a stronger interaction. This behavior of bridging and chelating ligands has been observed previously with, for example, ruthenium extended metal atoms chains.15 The effect of substitution is rather small in all optimized complexes except for compounds with bulky substituents in the ligand, which for obvious steric reasons induce rather large lengthening of the Pd−Pd distance. Also, most probably owing to steric hindrance groups, a theoretic palladium(III) compound with substituted C(CH3)3 phenolate and axial chlorines could not be optimized. A somewhat stronger substituent effect can be obtained with the acetylacetonate ligand (complexes 4 and 6) than with the phenolate (complexes 3 and 5), since the former ligands are more symmetric and include two substituents instead of only one. When the formal oxidation state of palladium atoms changes from II to III with bonding of the axial chlorines, the Pd−Pd

Figure 6. Frontier molecular orbitals for 4 with varying substituents (R = CF3 (4b), CH3 (4a), and C(CH3)3 (4c)).

the FMOs in the case of acetylacetonate ligands (complexes 4 and 6), where selected substituents (R = CF3, CH3, and C(CH3)3) were compared. Figures SI 4−7 (Supporting Information) contain further data on the FMOs for other complexes, 3, 5, 8, and 9). In the palladium(II) complexes with acetylacetonate ligands, the highest occupied molecular orbital is concentrated on the antibonding d(z2) orbitals of the palladium atoms, with a small contribution from the surrounding acetylacetonate and

Table 4. Optimized Pd−Pd Distances (Å) of Synthesized Complexes 3−6, 8, and 9

E


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(and thus the most stable palladium(III) compound) for the complex with CF3-substituted acetylacetonate ligands. This is well in agreement with the experimental observations, since only compound 6b has been isolated and characterized by X-ray diffraction methods. Compared to the palladium(II) complexes, the nature of the LUMO orbitals in palladium(III) complexes is also different. Similarly to the HOMOs, LUMOs are concentrated on the MXorbitals instead of the surrounding ligands. Again, a large contribution from the antibonding palladium d-orbitals combined with the chlorine p-orbitals lowers the energy of the LUMOs to a large extent. It should be noted that the character of the FMOs in the palladium(III) and -(II) compounds with carboxylate ligands is very similar to the corresponding acetylacetonate or phenolate complexes, even though the carboxylates are bridging ligands. We can therefore expect similar energetic stability for all the complexes 3, 4, and 8 and on the other hand for 5, 6, and 9. Analysis of the Electron Density. In the topological analysis of the electron density according to the QTAIM theory, the detailed properties of the electron density at the bond critical points can give information on the nature of the bonding between the palladium atoms and, furthermore, how the nature will change when the formal oxidation state of the palladium atoms changes from Pd(II) to Pd(III). Table 5 lists the selected properties at the bond critical points (BCPs) between the two palladium atoms. Examples of the formed bond paths are shown in the Supporting Information, Figure SI 8. The total electron density at the bond critical points, ρ(BCP), reflects the strength of the Pd···Pd interaction. For the palladium(III) complexes 5, 6, and 9, the electron density increases and is about twice as large as the ρ(BCP) of the palladium(II) complexes 3, 4, and 8. The same conclusion can be reached by calculating the corresponding interaction energy Eint with the relation Eint = 1/2V(BCP). Again, the interaction between the two palladium atoms more than doubles when the oxidation state of Pd increases. The effect of ligand and ligand substitution on the strength of the Pd−Pd interaction is minimal, except in the case of C(CH3)3 substituents, which give lower interaction energy for both oxidation states. Additionally, the corresponding parameters describing the strength of the

Figure 7. Frontier molecular orbitals for 6 with varying substituents (R = CF3 (6b), CH3 (6a), and C(CH3)3 (6c)).

phosphine ligands. The orbital energy follows roughly the ratio of the metal contribution, so that the most stabilized HOMO can be found with the CF3 substituents. However, the largest substituent effect is obtained in the energy and composition of the lowest unoccupied molecular orbital, where the CF3 substituents shift the focus on the acetylacetonate ligands and the contribution of the palladium atomic orbitals is much smaller. This electronic effect was found to stabilize the LUMO orbital notably, indicating the importance of the strongly electronwithdrawing substituents in the behavior of the MOs. The same trends can also be seen in the FMOs of the phenolate complexes (Figures SI4 and SI5), with only slightly differing orbital compositions. When the interaction with the axial chlorines changes the oxidation state of the palladium atoms, the nature of the HOMO orbitals also changes from antibonding to bonding. Here the contribution of the metal d(z2) orbitals decreases in the order CF3 > CH3 > C(CH3)3, which is also the trend in the stability of the orbital energies. Both the large contribution of the combined metal d-orbitals and chlorine p-orbitals and the larger stability of the HOMO energy indicate the strongest Pd−Pd interaction

Table 5. Selected Properties of the Electron Density at the Pd−Pd Bond Critical Points for Complexes 3−6 complex

R

ρ(BCP), e Å−3

Laplacian, e Å−5

λ1/λ3

V, au

G, au

|V|/G

G/ρ

H, au

Eint, kJ mol−1

3b 3a 3c 4b 4a 4c 5b 5a 5c 6b 6a 6c 8b 8a 9b 9a

C2H5 CH3 OPh CF3 CH3 C(CH3)3 C2H5 CH3 OPh CF3 CH3 C(CH3)3 CF3 CH3 CF3 CH3

0.1571 0.1664 0.1585 0.1715 0.1636 0.1297 0.3635 0.3637 0.3651 0.3721 0.3465 0.3269 0.2936 0.3003 0.4400 0.4513

1.2689 1.3557 1.2721 1.3879 1.3469 1.0199 1.4596 1.4581 1.4609 1.5233 1.3607 1.2209 2.8025 2.9244 2.1436 2.2608

0.0343 0.0212 0.0297 0.0424 0.0327 0.0881 0.0055 0.0066 0.0076 0.0082 0.0038 0.0100 0.0069 0.0061 0.0031 0.0025

−0.0183 −0.0200 −0.0185 −0.0209 −0.0197 −0.0137 −0.0420 −0.0420 −0.0422 −0.0438 −0.0389 −0.0353 −0.0470 −0.0488 −0.0581 −0.0604

0.0158 0.0170 0.0176 0.0176 0.0168 0.0121 0.0286 0.0286 0.0287 0.0298 0.0265 0.0240 0.0380 0.0395 0.0402 0.0419

1.16 1.17 1.17 1.18 1.17 1.13 1.47 1.47 1.47 1.47 1.47 1.47 1.24 1.23 1.45 1.44

0.1003 0.1024 0.1000 0.1029 0.1028 0.0929 0.0786 0.0786 0.0801 0.0801 0.0765 0.0734 0.1296 0.1317 0.0913 0.0929

−0.0026 −0.0030 −0.0027 −0.0032 −0.0029 −0.0015 −0.0134 −0.0134 −0.0135 −0.0140 −0.0124 −0.0113 −0.0090 −0.0092 −0.0179 −0.0185

−24.1 −26.3 −24.3 −27.4 −25.8 −17.9 −55.1 −55.2 −55.4 −57.5 −51.0 −46.4 −61.7 −64.0 −76.3 −79.3

F


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interaction are slightly larger in complexes with the carboxylate ligands, as can be expected from the smaller bond lengths. Further information on the nature of the interaction can be obtained by investigating the values of the Laplacian of the electron density, the total energy density H, and the ratio between the potential energy density and the kinetic energy density, |V|/G. Generally, different bonding interactions can be divided into covalent, partially covalent, and pure shared shell (noncovalent) interactions. For covalent interactions, the conditions |V|/G > 2 and H < 0 should apply. Pure shared shell interactions give H > 0 and |V|/G < 1. By inspecting the corresponding parameters in Table 5, it can be seen that in our complexes the interaction is partially covalent (H < 0 and 1 < |V|/ G < 2), with increasing covalent nature, when the oxidation of the Pd atoms increases. The positive Laplacian means that the charge is locally depleted between the palladium atoms (at the bond critical point), which is typical for metal−metal bonds. The analysis of the electronic properties (FMOs, charge density) has shown that all the calculated complexes should be stable enough to be formed. All the palladium(II) compounds have been isolated and characterized; however in relation to palladium(III) compounds only 6b was stable enough to be isolated. Steric reasons may hinder the formation of palladium(III) compounds with chelating ligands with the most bulky substituents. The computational results also showed that even if the coordination of the ligands is different for carboxylates (bridging) and acetylacetonates and phenolates (chelating), the resulting electronic characteristics are very similar. The nature of the Pd···Pd interaction was found to be partially covalent, with the ratio of covalency increasing when the oxidation state of palladium atoms was increased from Pd(II) to Pd(III). Metalated Palladium(II) and -(III) Compounds As Precatalysts in the Direct Phenylation of 1-Methylindole. Compounds 3a−c, 4a−c, and 6b have been tested as precatalysts in the direct phenylation of 1-methylindole (10) at room temperature with [Ph2I]PF6 in acetic acid to give compound 11 (Scheme 3). The results are displayed in Table 6.

Table 6. 2-Phenylation of 1-Methylindole with 3a−c, 4a−c, and 6b As Precatalysts entry 1 2 3 4 5 6 7 8 9 10 11 a

conditions Scheme 3

addition of CH2Cl2 Scheme 3 [Ph2I]BF4 [Ph2I]BF4/CH2Cl2

precatalyst

reaction time (h)

11 yield (%)a

3a 3b 3c 4a 4b 4c 4b 6b black solid 3a 4b

2 9 4 6 30 4 19 192 12 6 96

93 85 93 91 98 93 94 96 0 92 60

Yields calculated from the isolated compound 11.

compound Pd4[(C6H4)PPh2]4Cl4 (7).14a The catalytic reaction was developed in CH2Cl2/acetic acid and gave 11 in high yield after 8 days (Table 6, entry 8). The progress with time of the 2-phenylation reaction of 1-methylindole with 3a−c and 4a−c has been studied by 1H and 31 P NMR spectroscopy. Figure 8 shows the 1H NMR spectra of

Scheme 3. Direct Phenylation of 1-Methylindole

Figure 8. 1H NMR spectra of the 2-phenylation of 1-methylindole with time using 4c as precatalyst.

With compounds 3a−c and 4a,c, the 2-phenylation of 1-methylindole proceeded in high isolated yields (98−85%) and short periods of times (Table 6, entries 1−4, 6). Results published by Sanford and co-workers showed a yield of 86% and a longer reaction time (18 h) for this reaction using [Ph2I]BF4 and IMesPd(OAc)2 (IMes = 1,3-bis(2,4,6-trimethylphenyl)imidazol-2-ylidene) as potential catalyst.13 Compound 4b, insoluble in acetic acid, gave also 11 in a high yield but with longer reaction time (Table 6, entry 5). The reaction proceeded also in high yield but in shorter time when CH2Cl2 was added to solvate compound 4b in the reaction medium (Table 6, entry 7). Palladium(III) compound 6b has also been tested in the catalytic reaction. When 6b was added to the reaction medium, a quick change of color from red to yellow was observed, and a yellow solid that was insoluble in acetic acid appeared. The compound was isolated and characterized by X-ray methods, showing that it corresponds to the tetranuclear palladium(II)

this evolution when 4c was used. The signal at 7.45 ppm corresponding to 2-H in the 1-methylindole decreases and disappears totally after 4 h of reaction. In the same way the 1-methylindole 3-H signal at 6.77 ppm is progressively substituted by the 2-phenyl-1-methylindole 3-H signal at 6.9 ppm. The spectra show that the reaction is completed in 4 h. The 31P NMR spectrum at 3 h when the reaction is about 80% complete still showed the presence of 4c (Figure SI 9). At 4 h, when the catalytic reaction was complete, signals from the evolution of PF6− were observed. When the reaction was developed without the addition of palladium complexes, compound 11 was not observed after 7 days of reaction. At the end of the each reaction a black solid was obtained. The electron microscopy spectrum of the isolated solid showed that it contains mostly fluorine with some palladium together with G


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other elements (Figure SI 10). The isolated black solid did not catalyze the phenylation reaction (Table 6 entry 9). The HPF6 obtained by reaction of [Ph2I]PF6 with acetic acid decomposed at room temperature, giving HF, which attacked the glass Schlenk tubes.16 Sanford and co-workers used [Ph2I]BF4,13 the HBF4 is stable until 403 K. When the reaction of 2-phenylation of 1-methylindole was performed using [Ph2I]BF4 as oxidant and 3a and 4b as precatalysts the results showed reaction times at least three times higher (Table 6, entries 10 and 11). In both cases the black solid recovered at the end of the reaction showed by electron microscopy that it did not contain fluorine (Figure SI 11). Investigation on the Phenylation of 1-Methylindole Reaction. At this point, we sought to gain an understanding of this catalytic C−H phenylation. The phenylation of 1-methylindole proceeded in the presence of the free radical inhibitor MEQH (25 mol %), thus suggesting that free radicals are not involved in the reaction mechanism. When the substrate 10 reacted at room temperature with [Ph2I]PF6 and 3a as precatalyst in the presence of MEQH, 11 was obtained with 81% isolated yield after 17 h. The reaction was followed by 1H NMR spectroscopy, and the spectra showed, in addition to 11, the formation of some new products by the interaction of MEQH with the oxidant. The inhibitor is not an innocent reactant in this reaction. This can explain the reaction time increase (17 vs 2 h) and the yield decrease (93 vs 81%). Trying to detect intermediate species in this catalytic process, we have studied the interaction of the precatalyst 4c with the substrate on one hand and the oxidant on the other. We have also monitored the catalytic reaction by UV−vis spectrophotometry with the precatalyst that has shown the lowest reaction time, 3a. When 4c and 10 were mixed in acetic acid, no change in the 31 P NMR spectrum of 4c was observed. The catalytic reaction started with the addition of [Ph2I]PF6. The reaction in acetic acid of 4c with [Ph2I]PF6 was followed by 31P NMR spectroscopy at 298 K using a D2O coaxial (Figures SI 12 and 13). The signal at 19.37 ppm, which corresponds to compound 4c in acetic acid medium, decreases with time. The new signals between 0 to −30 ppm observed after 4 h have been assigned to some phosphorus compounds obtained by the evolution of HPF6 (see in the Supporting Information the 31P NMR spectra of the oxidant in acetic acid, Figures SI 14 and 15). This spectrum is practically the same as the one registered in the presence of substrate (Figure SI 9). The catalytic reaction was initiated quickly with the addition of the substrate, and the formation of a black precipitate was also observed. These results indicated that the interaction of the precatalyst with the oxidant gave catalytically active oxidized intermediates whose instability did not allow them to be recorded by 31P NMR spectroscopy at 298 K; acetic acid as solvent prevents the use of low temperature. In this paper we have described the oxidation of palladium(II) compounds to more or less stable palladium(III) ones. Similar dinuclear palladium(II) and -(III) compounds were also tested in catalysis with similar results to those reported in this paper. Pd2[(C6H4)PPh2][O2CCH3]2 (8a) and Pd2[(C6H4)PPh2][O2CCF3]2 (8b) gave 11 in high yields and short reaction times, Table 7, entries 1 and 2. Pd2[(C6H4)PPh2][O2CCH3]2Cl2 (9a) and Pd2[(C6H4)PPh2][O2CCF3]2Cl2 (9b) in the catalytic conditions also evolved to Pd4[(C6H4)PPh2]4Cl4 (7), giving a high yield of 11 but with very high reaction times (Table 7, entries 3 and 4).

Table 7. 2-Phenylation of 1-Methylindole with Mononuclear and Dinuclear Palladium(II) and -(III) As Precatalysts entry 1

conditions Scheme 3

2 3

addition of CH2Cl2

4 5 6 a

addition of CH2Cl2

precatalyst

reaction time (h)

Pd2[(C6H4)PPh2][O2CCH3]2 Pd2[(C6H4)PPh2][O2CCF3]2 Pd2[(C6H4)PPh2][O2CCH3]2Cl2 Pd2[(C6H4)PPh2][O2CCF3]2Cl2 Pd[(C6H4)PPh2]BrPPh3 Pd[(C6H4)PPh2]Br[PPh2(4-BrC6H4)]

11 yield (%)a

9

95

11

90

216

96

216

98

240 144

96 98

Yields calculated from the isolated compound 11.

Moreover the phenylation of the 1-methylindole reaction has also been tested with mononuclear palladium(II) compounds that contain the same orthometalated phosphine ligand, Pd[(C6H4)PPh2]BrPPh3 (12) and Pd[(C6H4)PPh2]Br[PPh2(4-BrC6H4)] (13). The results are displayed in Table 7, entries 5 and 6, and show that the reaction proceeded with high yields but with slower rates. The data obtained by monitoring the reaction by UV−vis spectrophotometry are not conclusive about the formation of palladium(III) intermediates (see Supporting Information). At this point of the study, there are some facts to take into account: (a) the interaction of the precatalyst with the oxidant [Ph2I]PF6 giving unstable catalytic oxidized intermediates, (b) the accessibility to the dinuclear palladium(III) compounds by oxidation of the palladium(II) counterparts, (c) the high reaction time observed with compound 4b, whose oxidation gave the most stable palladium(III) compound, 6b, (d) the similarity of behavior to other dinuclear compounds whose accessibility to the oxidation state III has already been proven, and (e) the fact that the dinuclear compounds show better results than the mononuclear ones in which redox cooperation is not possible. In the absence of a detailed mechanistic study, with these results and in spite of the proven accessibility to the oxidation state III for all the synthesized dinuclear palladium(II) compounds, we cannot state that the phenylation of 1-methylindole reaction occurs through dinuclear palladium(III) species with Pd−Pd redox cooperation. Sanford and co-workers, who used a mononuclear palladium(II) compound as precatalyst, proposed that this oxidative reaction proceeds via a mononuclear Pd(IV)-phenyl intermediate.13 It is conceivable that the C−H oxidation takes place through “high-valent” palladium intermediates whose accessibility has been demonstrate in recent years1−9 and in this paper. Furthermore, Ritter and Sanford have developed experimental and computational studies that connected the dinuclear palladium(III) and mononuclear palladium(IV) chemistry.17 It is noteworthy that Sanford and Hickman described the advantages of “high-valent” catalysis over more common, lowvalent analogues: enhanced substrate range, milder reaction conditions, and improved chemioselectivity and regioselectivity.10

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CONCLUSIONS New dinuclear orthometalated palladium(II) and -(III) compounds with chelating O,O-donor ligands (phenolates and acetylacetonates) have been synthesized and characterized by H


Organometallics

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The composition of the black solid obtained in the catalytic reaction was determined by energy-dispersive X-ray analysis on a Philips XL-30 EDX scanning electron microscope. The operating voltage was 20 kV. 31 P NMR Spectroscopy Study between 200 and 298 K for the Oxidation Reactions of Compounds 3a−c and 4a−c with PhICl2. A solution of 3a−c and 4a−c (0.004 mmol) in CD2Cl2 (0.5 mL) was cooled in a NMR tube at 200 K, and the 1H and 31P NMR spectra were recorded. A stoichiometric amount of PhICl2 was added, and the color of the solution immediately changed from yellow to red. The 31P NMR spectra were recorded at different temperatures from 200 to 298 K. Preparation of Pd2[(C6H4)PPh2]2[C6H4OC(O)R]2 (R = CH3, 3a; R = C2H5, 3b; R = OPh, 3c). Potassium phenolates were prepared by reaction of the corresponding phenol (0.223 mmol) and potassium hydroxide (0.223 mmol) dissolved in the minimum amount of methanol. Each solution was added to [Pd2((C6H4)PPh2)2(NCMe)4][BF4]2 that was prepared by reaction of a suspension of [Pd((C6H4)PPh2)Br]4 (100 mg, 0.056 mmol) in 20 mL of CH2Cl2/NCMe (8:1) with AgBF4 (43 mg, 0.223 mmol). After 5 min of stirring, the solution was evaporated to dryness. The yellow crude product obtained was extracted with dichloromethane, filtered over a short plug of silica, and precipitated by addition of hexane to give a yellow microcrystalline powder, which was collected by filtration and washed with hexane. Yield: 74.2 mg, 66% (3a); 64.0 mg, 56% (3b); 71.6 mg, 55% (3c). Characterization data for 3a: 1H NMR (CDCl3, 500 MHz) δ 7.8 (m, 4H, ar), 7.4−7.1 (m, 20H, ar), 7.0 (m, 2H, ar), 6.8−6.7 (m, 6H, ar), 6.4 (m, 2H, ar), 6.3 (m, 2H, ar), 1.4 (s, 6H, CH3); 31P NMR (CDCl3, 202 MHz) δ 27.1 (s); 13C NMR (CDCl3, 125 MHz) δ 196.7 (s, CO), 170 (s, C phenolate), 161.7 (m, C metalated), 138−112.6 (ar), 25.6 (s, CH3). Anal. Calcd for C52H42O4P2Pd2: C, 62.10; H, 4.21. Found: C, 61.99; H, 4.26. X-ray crystal structure data for 3a: Empirical formula, C52H42O4P2Pd2; crystal system, monoclinic; space group P2(1)/n, a = 10.2830(2) Å, b = 10.8720(3) Å, c = 39.6050(9) Å, β = 94.8760(11)°, V = 4411.69(18)Å3, Z = 4 crystal dimensions: 0.23 × 0.23 × 0.26 mm3; Mo Kα radiation, 273(2) K; 16 318 reflections, 9931 independent (μ = 0.933 mm−1); refinement (on F2) with SHELXTL (version 6.1), 541 parameters, 0 restraints, R1= 0.0457 (I > 2σ) and wR2 (all data) = 0.1264, GOF = 1.056, max/min residual electron density 0.563/−0.786 e Å−3. Characterization data for 3b: 1H NMR (CDCl3, 500 MHz) δ 7.8 (m, 4H, ar), 7.4−7.1 (m, 20H, ar), 7.0 (m, 2H, ar), 6.8−6.7 (m, 6H, ar), 6.4 (m, 2H, ar), 6.3 (m, 2H, ar), 2.3 (dq, 2H; 2JH−H = 15 Hz, 3JH−H = 8 Hz, CH2), 1.4 (dq, 2H, 2JH−H = 15 Hz, 3JH−H = 8 Hz, CH2), 0.4 (t, 6H, 3JH−H = 8 Hz, CH3); 31P NMR (CDCl3, 202 MHz) δ 26.5 (s); 13C NMR (CDCl3, 125 MHz) δ 200.2 (s, CO), 169.8 (s, C phenolate), 161.8 (m, C metalated), 134.4−112.6 (ar), 31.8 (s, CH2), 9.9 (s, CH3). Anal.Calcd for C54H46O4P2Pd2: C, 62.74; H, 4.48. Found: C, 61.93; H, 4.43. X-ray crystal structure data for 3b: Empirical formula, C54H46O4P2Pd2; crystal system, triclinic; space group P1,̅ a = 10.552(2) Å, b = 12.401(3) Å, c = 18.432(4) Å, α = 86.65(3)°, β = 84.42(3)°, γ = 70.60(3)°, V = 2263.3(8)Å3, Z = 2, crystal dimensions 0.18 × 0.21 × 0.23 mm3; Mo Kα radiation, 273(2) K; 6136 reflections, 3825 independent (μ = 0.912 mm−1); refinement (on F2) with SHELXTL (version 6.1), 560 parameters, 0.1963 restraints, R1= 0.0999 (I > 2σ) and wR2 (all data) = 0.1963, GOF = 1.324, max/min residual electron density 0.697/−0.669 e Å−3. Characterization data for 3c: 1H NMR (CDCl3, 500 MHz) δ 7.8 (m, 4H, ar), 7.5 (m, 4H, ar), 7.4−7.0 (m, 20H, ar), 6.9 (m, 4H, ar), 6.7 (m, 8H, ar), 6.6 (m, 2H, ar), 6.4 (m, 2H, ar), 6.3 (m, 2H, ar); 31P NMR (CDCl3, 202 MHz) δ 26.1 (s); 13C NMR (CDCl3, 125 MHz) δ 171.1 (s, C phenolate), 170.1 (s, CO), 160.5 (m, C metalated), 156.0 (s, C no phenolate), 138.8−112.3 (ar), 31.8 (s, C quaternary). Anal. Calcd for C62H46O6P2Pd2: C, 64.09; H, 3.39. Found: C, 63.47; H, 4.45. Preparation of Pd2[(C6H4)PPh2]2[RC(O)CHC(O)R]2 (R = CH3, 4a; R = CF3, 4b; R = C(CH3)3, 4c). Potassium acetylacetonates were prepared by reaction of the corresponding acetylacetone (0.223 mmol) and potassium hydroxide (0.223 mmol) dissolved in the minimum account of methanol. Each solution was added to [Pd 2 [((C 6 H 4 )PPh2)2(NCMe)4][BF4]2 that was prepared by reaction of a suspension of [Pd((C6H4)PPh2)Br]4 (100 mg, 0.056 mmol) in 20 mL of CH2Cl2/ NCMe (8:1) with AgBF4 (43 mg, 0.223). After 5 min of stirring, the solution was evaporated to dryness. The yellow crude product obtained

NMR spectroscopy and X-ray diffraction methods. The analysis of electronic properties, FOMs, and charge density showed that energetically all the complexes should be stable enough; however steric reasons may hinder the formation of palladium(III) compounds. In agreement with the DFT calculations, only the palladium(III) compound Pd2[(C6H4)PPh2]2[CF3C(O)CHC(O)CF3]2Cl2, 6b, was stable at room temperature and was characterized by single-crystal X-ray diffractions methods. This compound shows the longest Pd−Pd distance observed in discrete Pd26+ compounds. Even though other palladium(III) compounds were obtained at 200 K, they evolved, even at low temperature, toward the more stable tetranuclear compound Pd4[(C6H4)PPh2]4Cl4 (7). The isolated palladium(II) and -(III) compounds were tested in the direct phenylation of 1-methylindole at room temperature with [Ph2I]PF6 in acetic acid. With soluble compounds 3a−c and 4a,c, the reaction proceeded with high yields and short times. In the case of the insoluble precatalysts, the addition of CH2Cl2 reduced the reaction times without decreasing the isolated yield of 11. Better results than the ones described in the literature by Sanford and co-workers with mononuclear palladium(II) compounds as precatalysts have been obtained. 12 Although dinuclear palladium(III) compounds were obtained by oxidation of their palladium(II) counterparts and high reaction times were observed with orthometalated mononuclear palladium(II) in which a cooperation redox is not possible, further mechanistic investigations would be necessary to determine if this catalytic reaction occurs through dinuclear palladium(III) intermediates with Pd−Pd cooperation redox.

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EXPERIMENTAL SECTION

All the reactions were carried out under a dry argon atmosphere using Schlenk techniques. Solvents were purified according to standard procedures.18 Commercially available reagents were used as purchased. [Pd((C6H4)PPh2)Br]4, 1,14 Pd2[(C6H4)PPh2][O2CR]2 (R = CH3, 8a; R = CF3, 8b),9 Pd2[(C6H4)PPh2][O2CR]2Cl2 (R = CH3, 9a; R = CF3, 9b),9 Pd[(C6H4)PPh2]BrPPh3 (12),14 Pd[(C6H4)PPh2]Br[PPh2(4-C6H4)] (13),14 iodobenzene dichloride (PhICl2),19 and [Ph2I]BF420 were synthesized according to literature procedures. Solvent mixtures are v/v mixtures. Column chromatography was performed on silica gel (35−70 mesh). NMR spectra were recorded on Bruker 400 and 500 AMX spectrometers as solutions in CDCl3 and CD2Cl2 at 298 K and low temperatures. Chemical shifts are reported in ppm, using TMS (1H, 13 C) and 85% H3PO4 (31P) as references. The coupling constants (J) are in hertz (Hz). Elemental analysis were provided by Centro Microanálisis Elemental of Madrid. UV−vis spectra and data were carried out with an Agilent 8453 UV− vis spectrophotometer equipped with a Peltier temperature-controlled sample cell and driver (Agilent 89090A). X-ray structure determinations were carried out on a Bruker-Nonius Kappa CCD diffractometer (Mo Kα radiation, λ = 0.71073 Å). The structures were solved by direct methods using the SHELXTL software package.21 The correct positions for the heavy atoms were deduced from an E map. Subsequent least-squares refinement and difference Fourier calculations revealed the positions of the remaining non-hydrogen atoms. Hydrogen atoms were placed in geometrically generated positions and refined riding on the atom to which they are attached. The electrochemical studies were carried out at both room temperature and 263 K with a 273 A PAR potentiostat in a threeelectrode cell. The working electrode was a Pt electrode with a surface of 3.1 mm2, the counter electrode a Pt wire, and the reference electrode Ag/AgCl (saturated KCl). The solvent was 0.2 M Bu4NPF6/CH2Cl2, the solution was 0.5 mM for compounds 3a−c, 4a−c, and 6b, and the scan rate was 50 mV/s unless specified otherwise. In these conditions Ea for the couple Fc/Fc+ was 0.550 V. I


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b = 12.9390(3) Å, c = 18.6140(4) Å, α = 107.1280(15)°, β = 101.8740(14)°, γ = 95.8470(15)°, V = 2641.81(11) Å3, Z = 2, crystal dimensions 0.20 × 0.22 × 0.25 mm3; Mo Kα radiation, 273(2) K; 20 841 reflections, 12 047 independent (μ= 1.130 mm−1); refinement (on F2) with SHELXTL (version 6.1), 667 parameters, 0 restraints, R1= 0.0581 (I > 2σ) and wR2 (all data) = 0.1722, GOF = 1.028, max/min residual electron density 1.379/−1.196 e Å−3. Computational Details. All models were fully optimized with the Gaussian09 program package22 at the DFT level of theory. The hybrid density functional B3PW9123,24 was utilized together with the basis set consisting of the Stuttgart−Dresden effective core potential basis set with an additional p-polarization function for Pd atoms (SDD(p)) and the standard all-electron basis set 6-31G(d) for all other atoms. Frequency calculations with no scaling were performed to ensure optimization to true minima. None of the optimized structures gave imaginary frequencies. To further study the electronic properties of the complexes, we performed topological charge density analysis with the QTAIM (Quantum Theory of Atoms in Molecules)25 methods, which allowed us to access the nature of the bonding via calculating different properties of the electron density at the bond critical points. The analysis was done with the AIMALL program26 using the wave functions obtained from the DFT calculations with the computationally optimized structures. This method has been previously used successfully in describing the nature of the metal−metal bonding in, for example, linear chain ruthenium complexes.27 Direct Catalytic Phenylation of 1-Methylindole. The procedure reported by Sanford and co-workers has been followed.12 1-Methylindole (65.6 mg, 0.5 mmol) and the precatalysts (0.0125 mmol, 2.5 mol % of Pd) were dissolved in CH3CO2H (5 mL) (4b, 6b, 9a, 9b, 12 and 13 were dissolved in a minimum volume of CH2Cl2), and the solution was stirred at 298 K for 5 min. [Ph2I]PF6 (426.0 mg, 1 mmol) or [Ph2I]BF4 (367.9 mg, 1 mmol) was added, and the resulting solution stirred at 298 K until no 1-methylindole was observed by 1H NMR. The reaction mixture was filtered through a plug of siliceous earth and evaporated to dryness. The resulting oil was dissolved in CH2Cl2 (25 mL) and extracted with aqueous NaHCO3 (2 × 40 mL). The organic phase was dried with Na2SO4, concentrated, and purified by chromatography on silica gel using as eluent hexanes/ethylacetate (96:4). The reactions were monitored by 31P and 1H NMR spectroscopy using a D2O coaxial. Compound 11: 1H NMR (CDCl3, 400 MHz) δ 7.75 (d, 1H, 3JH−H = 7.6 Hz), 7.63−7.60 (m, 2H), 7.56 (t, 2H, 3JH−H = 7.6 Hz), 7.51−7.45 (m, 2H), 7.36 (dt, 1H, 3JH−H = 8.0 Hz, 3JH−H = 1.2 Hz), 7.26 (m, 1H), 6.68 (s, 1H), 3.83 (s, 3H, CH3); 13C NMR (CDCl3, 100 MHz) δ 141.7 (s, C quaternary), 138.5 (s, C quaternary), 132.9 (s, C quaternary), 129.5 (s, 2 C−H), 128.6 (s, 2 C−H), 128.1 (s, C quaternary), 127.9 (s, C−H), 121.8 (s, C−H), 120.6 (s, C−H), 120.0 (s, C−H), 109.7 (s C−H), 101.8 (s, C−H), 31.2 (s, CH3).

was extracted with dichloromethane, filtered over a short plug of silica, and precipitated by addition of hexane to give a yellow microcrystalline powder, which was collected by filtration and washed with hexane. Yield: 69.2 mg, 66% (4a); 89.6 mg, 70% (4b); 72.5 mg, 59% (4c). Characterization data for 4a: 1H NMR (CDCl3, 500 MHz) δ 7.6 (m, 4H, ar), 7.3 (m, 8H, ar), 7.2 (m,4H, ar), 7.1 (m, 2H, ar), 7.0 (m, 4H, ar), 6.8 (m, 6H, ar), 4.9 (s, 2H, CH), 1.6 (s, 6H, CH3), 1.3 (s, 6H, CH3); 31P NMR (CDCl3, 202 MHz) δ 26.4 (s); 13C NMR (CDCl3, 125 MHz) δ 186, 185.3 (s, CO), 163.7 (m, C metalated), 139.5−122.5 (ar), 99.3 (s, C−H), 27.3 (s, CH3), 26.7 (s, CH3). Anal. Calcd for C46H42O4P2Pd2: C, 59.18; H, 4.53. Found: C, 59.80; H, 4.56. X-ray crystal structure data for 4a: Empirical formula, C46H42O4P2Pd2· C6H14; crystal system, monoclinic; space group C2/c, a = 16.8200(6) Å, b = 26.0490(7) Å, c = 10.7660(4) Å, β = 91.002(2)°, V = 4716.3(3) Å3, Z = 4 crystal dimensions 0.22 × 0.22 × 0.26 mm3; Mo Kα radiation, 273(2) K; 9341 reflections, 5367 independent (μ = 4716.3(3) mm−1); refinement (on F2) with SHELXTL (version 6.1), 275 parameters, 0 restraints, R1= 0.0445 (I > 2σ) and wR2 (all data) = 0.1359, GOF = 1.038, max/min residual electron density 0.737/−0.501 e Å−3. Characterization data for 4b: 1H NMR (CDCl3, 500 MHz) δ 7.4 (m, 4H, ar), 7.4 (m, 4H, ar), 7.3 (m, 4H, ar), 7.2 (m, 4H, ar), 7.1 (m, 2H, ar), 7.0 (m, 4H, ar), 6.9 (m, 4H, ar), 6.8 (s, 2H, ar), 5.7 (s, 2H, CH); 19F NMR (CDCl3, 282.4 MHz) δ −75.5 (m), −75.9 (m); 31P NMR (CDCl3, 202 MHz) δ 26.9 (s); 13C NMR (CDCl3, 125 MHz) δ 174.4 (m, 2JC−F = 34 Hz, CO), 158.9 (m, C metalated), 138.4−123.7 (ar), 117.6 (q, 1JC−F = 286 Hz, CF3), 89.5 (s, C−H). Anal. Calcd for C46H30F12O4P2Pd2: C, 48.06; H, 2.63. Found: C, 48.67; H, 2.83. X-ray crystal structure data for 4b: Empirical formula, C46H30F12O4P2Pd2; crystal system, triclinic; space group P1̅, a = 12.7000(2) Å, b = 13.6670(2) Å, c = 13.7970(3) Å, α = 87.2950(12)°, β = 81.3370(11)°, γ = 70.8280(12)°, V = 2263.13(7) Å3, Z = 2 crystal dimensions 0.20 × 0.22 × 0.24 mm3; Mo Kα radiation, 273(2) K; 17 178 reflections, 10 220 independent (μ = mm−1); refinement (on F2) with SHELXTL (version 6.1), 595 parameters, 0 restraints, R1 = 0.0427 (I > 2σ) and wR2 (all data) = 0.1141, GOF = 1.064, max/min residual electron density 0.957/ −0.669 e Å−3. Characterization data for 4c: 1H NMR (CDCl3, 500 MHz) δ 7.9 (m, 4H, ar), 7.8 (m, 4H, ar), 7.4−7.2 (m, 14H, ar), 6.7 (m, 2H, ar), 6.6 (m, 2H, ar), 6.4 (m, 2H, ar), 5.4 (s, 2H, CH), 1.1 (s, 18H, CH3), 0.8 (s, 18H, CH3); 31P NMR (CDCl3, 202 MHz) δ 22.7 (s); 13C NMR (CDCl3, 125 MHz) δ 194.9, 194.5 (s, CO), 162.3 (m, C metalated), 136.2−122.1 (ar), 88.3 (s, C−H), 41.1 (s, C quaternary), 40.9 (s, C quaternary), 28.7 (s, CH3), 28.5 (s, CH3). Anal. Calcd for C58H66O4P2Pd2: C, 63.21; H, 6.04. Found: C, 63.15; H, 5.97. X-ray crystal structure data for 4c: Empirical formula, C58H66O4P2Pd2; crystal system, triclinic; space group P1̅, a = 10.8140(3) Å, b = 12.9040(3) Å, c = 19.6090(5) Å, α = 86.8720 (18)°, β = 89.1930(17)°, γ = 77.1980(14)°, V = 2664.31(12) Å3, Z = 2, crystal dimensions 0.16 × 0.18 × 0.23 mm3; Mo Kα radiation, 273(2) K; 18 462 reflections, 12 017 independent (μ = 0.779 mm−1); refinement (on F2) with SHELXTL (version 6.1), 595 parameters, 0 restraints, R1= 0.0547 (I > 2σ) and wR2 (all data) = 0.1108, GOF = 1.084, max/min residual electron density 0.553/−0.715 e Å−3. Preparation of Pd2[(C6H4)PPh2]2[CF3C(O)CHC(O)CF3]2Cl2 (6b). To a solution at 223 K of 4b (50 mg, 0.044 mmol) in CH2Cl2 (5 mL) was added iodobenzene dichloride (18 mg, 0.066 mmol). The solution immediately changed from yellow to red. After 5 min of stirring, the solution was evaporated to dryness and hexane was added. The red microcrystalline precipitate obtained was isolated by filtration and washed with hexane. Yield: 43.6 mg (82%). Characterization data for 6b: 1H NMR (CD2Cl2, 400 MHz, 223 K) δ 8.1 (m, 4H, ar), 8.0 (m, 2H, ar), 7.6 (m, 4H, ar), 7.5 (m, 6H, ar), 7.3 (m, 2H, ar), 7.2 (m, 2H, ar), 7.1 (m, 2H, ar), 7.0 (m, 2H, ar), 6.9 (m, 2H, ar), 6.6 (s, 2H, ar), 5.7 (s, 2H, CH); 31P NMR (CD2Cl2, 162 MHz, 223 K) δ 4.6 (s); 13C NMR (CD2Cl2, 100 MHz, 223 K) δ 175.3 (q, 2JC−F = 34 Hz, CO), 174.7 (q, 2JC−F = 34 Hz, CO), 154.9 (m, C metalated), 141.9−121.3 (ar), 117.3 (q, 1JC−F = 287 Hz, CF3), 116.8 (dq, 1JC−F = 287 Hz, 4JC‑p = 10 Hz, CF3), 90.4 (s, C). X-ray crystal structure data for 6b: Empirical formula, C48H34Cl6F12O4P2Pd2; crystal system, triclinic; space group P1̅, a = 11.9100(3) Å,

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ASSOCIATED CONTENT

S Supporting Information *

Cyclic voltammogram, 31P NMR spectra of the reaction of 4c with PhICl2 from 200 to 298 K, 31P NMR spectra of the 6b from 200 to 298 K, frontier molecular orbitals for 3b,c, 5a−c, 8a,b, and 9a,b, 31P NMR spectra of the progress of the catalytic reaction with 4c, EDX spectra of the black solids obtained in the catalytic reaction, 31P NMR spectra of the reaction of 4c and [Ph2I]PF6 in acetic acid and the evolution of [Ph2I]PF6 in acetic acid, study by UV−vis spectroscopy of the catalytic reaction with 3a, and crystallographic data of compounds 3a,b, 4a−c, and 6b in CIF format. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel: 00 34 963543147. Fax: 00 34 963543929. E-mail: angeles. [email protected]. J


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Notes

Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision C.02; Gaussian, Inc.: Wallingford, CT, 2004. (b) It should be noted that, in order to correctly account for the ECP basis set of the metal atoms in the AIM analysis, it was necessary to recompute the wavefunctions with the latest version of Gaussian 09 (G09 revision C.01). (23) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (24) Perdew, J. P.; Wang, Y. Phys. Rev. B 1992, 45, 13244. (25) Bader, R. F. W. In Atoms in Molecules: A Quantum Theory; Clarendon Press: Oxford, 1990. (26) Keith, T. A. AIMAll (Version 12.06.03); TK Gristmill Software: Overland Park, KS, USA, 2012 (aim.tkgristmill.com). (27) Niskanen, M.; Hirva, P.; Haukka, M. Phys. Chem. Chem. Phys. 2010, 12, 9777.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Anja Podlogar for some preliminary work. We are grateful for financial support from MEC of Spain (CTQ200806466).

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DEDICATION This paper is dedicated to the memory of our beloved friend Prof. Dr. Purificación Escribano, who will be forever in our hearts.

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REFERENCES

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