Article Cite This: Organometallics XXXX, XXX, XXX−XXX
Benzylic Complexes of Palladium(II): Bonding Modes and Pentacoordination for Steric Relief Blanca Martín-Ruiz, Ignacio Pérez-Ortega, and Ana C. Albéniz* IU CINQUIMA/Química Inorgánica, Universidad de Valladolid, E-47071 Valladolid, Spain S Supporting Information *
ABSTRACT: A large variety of α-(pentafluorophenylmethyl)benzylic palladium complexes with different ligands have been synthesized and characterized. Multinuclear NMR spectroscopic data allow to determine the σ- or η3-benzylic nature of the complexes in solution. The adoption of either coordination mode is a function of the number of ligands coordinated to palladium, and remarkably, the presence of bulky phosphines favors the adoption of a bidentate η3-benzylic mode and pentacoordinated palladium complexes. Experimental data and DFT calculations indicate that this five-coordination could alleviate the steric hindrance of two bulky cis phosphines. The benzylic complexes show a rich fluxional behavior that involves both ligand exchange and σ- to η3-benzylic interconversion.
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INTRODUCTION Benzylic palladium complexes are frequently formed in Pdcatalyzed transformations of benzyl reagents and of styrene derivatives.1−6 They can be considered weakly stabilized palladium alkyls by coordination of the aryl ring in a pseudoallylic (η3) form (Scheme 1). Yet, this η3 interaction, which is
that can be expected and how it can be correlated to the nature of the other ligands bound to the metal. We describe here a large family of palladium benzylic complexes with auxiliary ligands of different types and in different numbers and analyze how they influence the type of benzylic coordination, the coordination number of the metal, and the fluxional behavior of the complexes.
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Scheme 1. σ-Benzylic and Bidentate η3-Benzylic Coordination Modes
RESULTS AND DISCUSSION Palladium(II) benzylic complexes bearing a pentafluorophenylmethyl substituent were prepared from complex 1 and are collected in Scheme 2. The dimeric complex 1 was synthesized by insertion of styrene into the Pd−C6F5 bond of [PdBr(C6F5)(NCMe)2].7 It precipitates in CH2Cl2; therefore, it can be easily isolated in good yield and is a convenient starting material for the synthesis of other benzylic palladium derivatives. Upon addition of ligands to 1, different coordination modes were found depending on the [Pd]:ligand stoichiometric ratio and the solvent used. σ- and η3(pentafluorophenylmethyl)benzyl complexes were generated and characterized in solution, and trends in the NMR data were found that allow determination of the coordination mode of the benzylic moiety. The presence of the pentafluorophenyl group is most useful, since the simplicity and wide chemical shift range in its 19F NMR spectra allow the detection of new species and the composition of mixtures of isomers, if it is the case (see Figure S1 in the Supporting Information). Most complexes are fluxional and display a dynamic behavior that was evident in their NMR spectra, and this is described below. The stabilities
made at the expense of the aromaticity of the ring, is easily broken to return to the σ-benzylic form. In this way, benzylic metal derivatives show, in a more controlled way, the typical reactivity of metal alkyls. Also, by a shift from the bidentate η3benzylic to the monodentate σ-benzylic coordination mode an extra ligand can be accommodated, thus modifying the coordination sphere of the metal and influencing the dissociation−coordination processes that are important in any catalytic transformation (Scheme 1). The preference of a benzyl fragment for the σ- or η3-benzylic coordination in palladium complexes depends, in a subtle manner, on the steric and electronic properties of the other ligands present in the complex, resulting in a rich variety of coordination environments, including both the usual squareplanar as well as pentacoordinated geometries, infrequent for this metal. It would be useful to gather some information regarding the type of coordination mode (σ- or η3-benzylic) © XXXX American Chemical Society
Received: January 31, 2018
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DOI: 10.1021/acs.organomet.8b00065 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics
determined by X-ray diffraction and is shown in Figure 1a. The σ coordination of the benzylic group can be observed along with the anti conformation of both phenyl rings in the bis(phenylthio)ethane ligand. The Pd−S distances are quite different and show the greater trans influence of the benzylic group in comparison to the bromo ligand (Pd−S2 = 2.460 (4) Å vs Pd−S1 = 2.334 (4) Å). Selected distances and angles are given in Table S3. The bipyridine complex 2e is a mixture of two isomers in solution in a 5:1 ratio. Both show a typical ABX spin system for the Hα and Hβ protons of the benzylic group, but Hα of the minor isomer (2e2) is clearly shifted downfield (δ 5.57 vs δ 3.93 for the major isomer 2e1). We attribute both isomers to the two σ-benzylic derivatives shown in Scheme 3, where 2e2 is formed by β-H elimination and readdition from 2e1. The fluxional behavior of these complexes is consistent with the structures proposed, as will be described below. Exchange between Hα and Hβ in cationic methylbenzyl derivatives of palladium has been observed before and occurs by the same β-H elimination− readdition pathway.9 The crystal structure of the major isomer (2e1) has been determined (Figure 1b), and significant distances and angles are collected in Table S4. A small deviation from square-planar geometry is observed (the dihedral angle between the planes that contain Pd−Br−C2 and Pd−N1−N2 is 9.1°),10 and the σ-CH(CH2C6F5)Ph group shows features similar to those of complex 2g. When crystals of complex 2e1 are dissolved in CDCl3 at room temperature, a mixture of both isomers 2e1 and 2e2 is observed, whereas only 2e1 is present if the crystals are dissolved at 253 K. This confirms that the minor isomer is generated from 2e1. Complex 2h is formed when a large excess of the weakly coordinating acetonitrile is present: i.e., when 1 is dissolved in NCMe. The chemical shift of Cα (43.45 ppm) indicates a σ coordination of the benzylic group. When the bromo ligand is eliminated by precipitation with a silver salt, the cationic complex 3h also retains a σ-benzylic ligand (Scheme 2). The analogous 3d was obtained upon addition of an excess of pyridine to a solution of 3h in acetonitrile. In these cases, the electrophilic cationic metal center coordinates an extra σ-donor ligand to complete its coordination sphere rather than adopts an η3-benzylic coordination mode. The type of complex formed upon addition of a monodentate ligand in the ratio Pd:L = 1:1 is more difficult to anticipate. As can be seen in Scheme 2, both σ- and η3benzylic coordination are possible (4, 6). Complex 4 was formed upon addition of tetrahydrothiophene (tht) in the ratio Pd:tht = 1:1. Although the initial η3 coordination in 1 does not need to be changed to accommodate the new ligand, the formation of a σ-benzylic dimeric derivative is preferred to bridge cleavage. The chemical shift of Cα, 37.1 ppm, is characteristic of this arrangement. At 223 K, 4 is a mixture of two isomers derived from the relative cis and trans arrangement of the ligands in the dimer. These isomers undergo fast interconversion at temperatures above 283 K.11 η3-Benzylic Derivatives. The elimination of the bromide in complexes 2a−g by treatment with a silver salt leads to the cationic η3-pentafluorobenzylbenzyl complexes 5a−g (Scheme 2). NMR data for all η3-benzylic complexes are collected in Tables 2−4 (13C, 1H, and 31P, respectively) and Table S1 (19F) in the Supporting Information. The η3 coordination mode is characterized spectroscopically by the chemical shift for Cα in the 13C NMR spectra around 60 ppm and a value of 1JCα−H close to 155 Hz consistent with an sp2 carbon atom. Both
Scheme 2. Benzylic Palladium Complexes from Complex 1
of the complexes are very different, but all of them eventually decompose by β-H elimination. For this reason, many complexes were characterized at low temperature to guarantee sufficient stability and static spectra. In several cases the complexes were also isolated. σ-Benzylic Derivatives. These types of complexes can be obtained by addition of 2 equiv of a monodentate ligand per palladium or 1 equiv of a bidentate ligand per Pd to complex 1 in a noncoordinating solvent, such as chloroform or dichloromethane (2, Scheme 2). Thus, both bridge cleavage and η3 to σ coordination change occur for most ligands. NMR data are collected in Tables 1, 2, and 4 (1H, 13C, and 31P, respectively) and Table S1 (19F) in the Supporting Information. All complexes show characteristic spectroscopic features of the σ-benzylic coordination mode. The salient feature is the chemical shift of Cα in the 13C NMR spectra at about 30−44 ppm and 1JCα−H = 130−140 Hz, in the range of C(sp3)−H coupling (Figure S4). The 1H NMR signals in the aromatic region generally follow the trend: δ(Hpara) < δ(Hmeta) < δ(Hortho) with equivalent ortho hydrogens or meta hydrogens in a freely rotating phenyl group. The cis or trans stereochemistry of the complexes with monodentate ligands has been unequivocally determined by 31P NMR for the phosphino derivatives and, when possible (L = py), by the 1H NMR pattern of the ligand signals. Neither the cis nor the trans situation produces the equivalence of both L ligands, due to the chiral Cα carbon. However, very close chemical shifts for the active nuclei of L are observed for the trans isomers vs well-separated signals for the cis isomers (Figure S2). Trans σ-benzylic complexes are obtained as the only product for L = PMe3 (2a) or as the major species for L = P(OMe)3 (2b) and L = py (2d). In the last two cases the cis isomer is formed in a trans:cis ratio of 7:1 for 2b at 223 K in CDCl3 and 3.3:1 for 2d at 273 K in CDCl 3.8 The stereochemistry of the complexes for L = tht (2f) and L = NCMe (2h) could not be unequivocally assigned, but by analogy to the preceding examples, we assume a trans geometry. The crystal structure of complex 2g has been B
DOI: 10.1021/acs.organomet.8b00065 Organometallics XXXX, XXX, XXX−XXX
273
293
293
223 293
293
243
233
cis-2d
2e1
2e2
2f 2g
2h
3h
3dc
C
4.87
3.60t (7.6)
4.33 t (7.8)
3.83 t (7.3)
4.79 m 3.36 m
3.25 dd (13.9, 7.6) 3.22 dd (14.8, 8.2) 3.38dd (14.4, 7.5) 3.30
3.68 dd (14.1, 9.5) 3.13 dd (13.8, 10.3) 3.35 bt 3.72 t (14.3)
3.80 m 4.17 m (14.2, 11.5) 3.22 dd (14.0, 7.3) 3.55 m
3.19 m
Hβ (J)
2.70 dd (13.9, 7.6) 2.59 dd (14.8, 7.4) 2.66 dd (14.4, 8.8) 2.89
3.39 dd (14.1, 5.9) 3.02 dd (13.8, 5.2) 2.85 b 2.88 m
2.92 m 3.57 m (14.2, 4.8) 2.55 dd (14.0, 7.3) 3.10 m
3.19 m
Hβ’ (J)
7.21 a
7.50 d (7.5) 6.87 m
7.25 b
7.1−7.5 m 7.34 m
7.65 d (6.5) 7.1−8.1
6.79 d (6.7) 6.8−7.4
7.37 d (6.9) 7.30 m 7.2−7.9
H2/H6 other
6.97 (m, 2H, H3,5), 7.01 (t, 1H, H4), 7.22 (m, 2H, Hmeta py trans), 7.38 (m, 4H, Hmeta py cis), 7.66 (t, 1H, Hpara py trans), 7.84 (t, 2H, Hpara py cis), 8.33 (m, 2H, Hortho py trans), 8.57 (d, 4H, Hortho py cis) 1.5−3.3 (m, 8H, tht), 7.38 (a, 3H, H3,4,5)
7.29 (m, 2H, H3,5), 7.34 (m, 1H, H4)
2.01 (bs, 8H, H3′,4’ tht), 2.85 (a, 8H, H2′,5′ tht), 7.1−7.5 (m, 3H, H3,4,5) 2.6−3.1 (m, 4H, CH2S), 6.69 (t, J = 7.4, 1H, H4), 6.81 (t, J = 7.4, 2H, H3,5), 7.27 (m, 4H, Hmeta Ph), 7.32 (m, 2H, Hpara Ph), 7.58 (m, 4H, Hortho Ph) 7.4 (m, 3H, H3,4,5)
7.1−8.1 (m, 3H, H3,4,5), 7.39 (t, 1H, H5 bipy), 7.50 (t, 1H, H5′ bipy), 7.98 (m, 4H, H3,4,3′,4’ bipy), 8.75 (d, 1H, H6 bipy), 9.33 (d, 1H, H6’ bipy)
6.8−7.4 (m, 3H, H3,4,5), 7.20 (m, 2H, Hmeta py), 7.38 (m, 2H, Hmeta py), 7.65 (m, 2H, Hpara py), 8.12 (bs, 2H, Hortho py), 8.52 (bs, 2H, Hortho py) 7.12 (m, 3H, H3,4,5), 7.98 (m, 4H, H3,4,3′,4’ bipy), 7.50 (m, 2H, H5,5′ bipy), 9.10 (a, 2H, H6,6’ bipy)
7.01 (t, 2H, H3,5), 7.07 (t, 1H, H4), 7.30 (t, J = 6.7, 4H, Hmeta py), 7.79 (t, J = 6.7, 2H, Hpara py), 8.52 (t, 4H, Hortho py)
3.62 (b, 18H, OMe), 7.13 (m, 1H, H4), 7.23 (m, 2H, H3,5) 1.75−2.55 (m, 4H, CH2 dppe), 6.86 (b, 5H, H4, Ph dppe), 7.2−7.9 (m, 18H, H3, H5, Ph dppe)
1.25 (m, 18H, Me), 7.08 (t, J = 6.9, 1H, H ), 7.23 (t, J = 6.9, 2H, H3,5)
4
a
δ values 300 MHz except as noted otherwise. Spectra were recorded in CDCl3 except for 2h, 3h, and 3d (CD3CN). J values are given in Hz. bThe characterization of cis-2b was not possible due to the low intensity of the signals. c500 MHz.
223
3.93 dd (9.5, 5.9) 5.57 dd (10.4, 5.2) 4.35 b 5.47 dd (9.6, 5.2) 4.32 t (7.6)
223
trans-2d
4
3.83 m
223 293
trans-2bb 2c
4.26 m
233
2a
Hα (J)
T (K)
complex
Table 1. 1H NMR Data for the σ-Benzylic Palladium Derivativesa
Organometallics Article
DOI: 10.1021/acs.organomet.8b00065 Organometallics XXXX, XXX, XXX−XXX
Article
Organometallics Table 2. 13C NMR Data for the Palladium Benzylic Derivativesa complex
T (K)
Cα
2a trans-2b 2c trans-2d 2e1 2g 2hb 3hc,d 3d 4 5a 5c 5f 5gd 5i 5j 5k 6 7 8i 8j 8k
263 233 223 263 293 293 263 233 233 223 223 223 213 293 223 243 223 233 233 223 243 223
30.95 37.28 42.91 37.44 37.43 41.31 43.45 36.80 38.22 37.10 62.05 63.82 65.2 67.80 70.48 68.34 68.94 54.59 59.65 60.63 54.68 58.91
JCα−H
Cβ
134.1 142.6 140.1 140.4 * 150 * 145.2 137 * 160.2 156.4 * 155 156.0 158.5
27.54 26.60 29.95 26.05 30.55 28.42 24.06 26.72 30.7 26.18 22.21 21.52 22.59 23.14 21.19 21.59 20.60 * 21.78 24.84 23.27 25.80
1
154.7 155.4 155.0 * 150.0
1
2
JCα−P
C1
C2/C6
C3/C5
C4
132.4 131.3 135.5 130.6 * 132 131.9 139 131.4 136.0 137.3 133.0 137.3 136 132.5 135.8
