Rational Synthesis and Electronic Structure of Functionalized

Article pubs.acs.org/Organometallics

Rational Synthesis and Electronic Structure of Functionalized Trinuclear Pd Metal Sheet Sandwich Complexes Christian Jandl,† James R. Pankhurst,‡ Jason B. Love,‡ and Alexander Pöthig*,† †

Department of Chemistry & Catalysis Research Center, Technische Universität München, Ernst-Otto-Fischer-Straße 1, D-85747 Garching, Germany ‡ EaStCHEM School of Chemistry, University of Edinburgh, Joseph Black Building, David Brewster Road, Edinburgh EH9 3FJ, United Kingdom S Supporting Information *

ABSTRACT: In this article we present the rational synthesis and full characterization of molecular Pd3 clusters sandwiched by imidazolium- and pyridinium-functionalized cycloheptatrienide ligands, of general formula [Pd3Br3(C7H6R)2]+. From functionalized η3-cycloheptatrienide Pd complexes as starting materials, PdBr2(η3-C7H6R), a number of synthetic routes were tested and a generally applicable strategy was developed on the basis of the formation of dimeric “preclusters”, [{PdBr(η3-C7H6R)}2(μ-Br)]+, which exhibit a Pd−Pd interaction. Spectroscopic and electrochemical results indicate a noninteger, common oxidation state of Pd in the trinuclear clusters, and crystallographic analysis reveals distinct differences in the binding properties of the Pd atoms in the solid state, suggesting that in certain environments there are even two discernible oxidation states, which is supported by computational analysis of the charge distribution.

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applications.1 Its acetonitrile derivative also exhibits surprising reactivity upon reduction, which leads to the coupling of two clusters via a Pd−Pd bond.8 Furthermore, Hurst and coworkers were able to obtain polymeric structures in which the Pd3 clusters are linked via bridging halides.7 This shows that Pd sandwich clusters could be viable building blocks for nanoarchitectures, especially in combination with known chain-type multinuclear Pd sandwich complexes.13−16 In 2012, the first derivative of a Pd3 sandwich complex bearing imidazolium substituents on the cycloheptatrienyl ligands was discovered serendipitously; after air exposure of the solution of a mononuclear, imidazolium-substituted η3cycloheptatrienide Pd complex (the structure of which was not fully elucidated at that time) crystals of the compound depicted in Scheme 1 (right) were obtained.17 As functionalization of the sandwiching ligands opens up a great opportunity to further modify and tailor this class of complexes, we became interested in developing a rational synthesis of such functionalized trinuclear Pd sandwich clusters. As a basis for that, we have recently reported a detailed study of imidazolium- and pyridinium-functionalized η3-cycloheptatrienide Pd complexes (compounds 1 and 2, respectively; Scheme 2), which serve as suitable starting materials.18 It was postulated that the partial decomposition of 1a (R = Me) to Pd(0) on exposure to air and moisture, and a subsequent reaction of Pd(0) with the remaining amounts of

INTRODUCTION In 2006, Murahashi, Kurosawa, and co-workers discovered the first Pd metal sheet sandwich complex consisting of a Pd3 triangle capped by two cycloheptatrienyl ligands (see Scheme 1 Scheme 1. Structures of the First Pd3 Metal Sheet Sandwich Complex (Left) and the First Imidazolium-Functionalized Derivative (Right)1,17

(left)).1 In the following years the same group reported several variations of this interesting sandwich cluster motif, stabilizing both trimetallic triangles and tetrametallic squares between capping ligands of ring sizes between five and nine atoms.2−6 Derivatization of the trimetallic cluster is also feasible by changing the equatorial ligands with different halides, acetonitrile, phosphines, or ethylene.7−10 In addition, Pt analogues and even mixed Pd/Pt cluster complexes of this type have been synthesized.10−12 Thus far, the largest Pd sheet sandwich complex is pentanuclear and is capped by naphthacene ligands.1 The original Pd3 complex has been shown to be active in cross-coupling reactions, thus highlighting the potential of defined cluster complexes for catalytic © XXXX American Chemical Society

Received: April 12, 2017

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DOI: 10.1021/acs.organomet.7b00276 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

one bromide ligand in 1 and 2 for a noncoordinating anion yields dimeric complexes with greatly enhanced solubility.18 To test their benefit in this context, we first tried the methylsubstituted dimer 4a in the reaction with Pd2(dba)3 (see Scheme 4 (top)). We expected a functionalized Pd3 sandwich

Scheme 2. General Lewis Structures of Imidazolium- and Pyridinium-Substituted η3-Cycloheptatrienide-Pd Complexesa

a

Scheme 4. Synthesis of Imidazolium-Substituted Pd3 Sandwich Complexes 3-BF4 from Dimeric Precursor 4 with and without Addition of an External Bromide Source (Top) and Attempted Syntheses of Dimeric Cycloheptatrienide Precursors with a Pd:Br Ratio of 2:3 (Bottom)

R = organic group.

1a, formed the prototypic functionalized cluster complex and that therefore the addition of Pd(0) sources to compounds of type 1 should lead to Pd3 cluster formation.17 As such, we report the syntheses of a variety of functionalized Pd3 cluster complexes using this Pd(0) method. These compounds have been characterized by NMR spectroscopy, elemental analysis, mass spectrometry, and single-crystal X-ray diffraction. As the structure and charge balance in these compounds raise interesting questions about the oxidation state of the palladium clusters, we also describe density functional theory (DFT) and electrochemical studies which provide further insights into their electronic structure.

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RESULTS AND DISCUSSION Direct Conversion of Monomeric Precursors. Initial attempts to form trimetallic Pd clusters involved the direct reaction of the monomeric cycloheptatrienide complexes 1 and 2 with a Pd(0) source, for which we chose the readily available Pd2(dba)3·CH2Cl2 (dba = dibenzylideneacetone; the cocrystallized dichloromethane will be omitted in the following). This reaction works well for derivatives of 1 with bulky substituents (R = Mes, R = DiPP) but fails in the presence of less sterically demanding imidazolium substituents (see Scheme 3). This can

complex containing two bromide ligands and one solvent ligand (acetonitrile) in the equatorial positions as the product, but instead we obtained complex 3a-BF4, which contains three equatorial bromide ligands, in moderate yield (34%). It seems that bromide is the preferred equatorial ligand and consequently precursors such as 4 bearing two bromides are suboptimal, as the bromide stoichiometry is incorrect. To fix the bromide stoichiometry, we repeated the reaction with addition of tetrabutylammonium bromide as an external bromide source (see Scheme 4 (top)), which greatly improved the yield of 3a-BF4 (81%). This procedure is also applicable to other substitution patterns, but it transpired that the byproduct, tetrabutylammonium tetrafluoroborate, is not easily separable in the case of complexes bearing bulkier substituents. Purification by fractional precipitation then reduces the yield significantly. Therefore, the next step in improving the synthetic strategy was to avoid such byproduct formation by using a precursor that already contains the correct number of bromide ligands. Conversion of Dimeric Precursors with Correct Bromide Stoichiometry. Attempting to prepare a dimeric precursor with a Pd:Br ratio of 2:3, we first tried to precipitate 1 equiv of bromide per two monomers of 1 by addition of silver tetrafluoroborate. Using the reverse method, we also reacted the dimer 4 (2:2 Pd:Br ratio) with 1 equiv of tetrabutylammonium bromide. However, in both cases, we observed only 50% conversion, leading to a mixture of 1 and 4 (see Scheme 4 (bottom)). In the case of the DiPP-substituted complex 1d only, NMR spectroscopy and elemental analysis indicated that a new species formed, but the bromine content of that material was still lower than expected, and unreacted 1d was still present. As this indicated that the problem again has to do with solubility, the reaction of 1d and silver tetrafluoroborate in a 2:1 stoichiometry was repeated in the more polar mixture of dimethyl sulfoxide/acetonitrile (see Scheme 5 (bottom)) and we were indeed able to isolate the envisioned dimeric complex 5d with a Pd:Br ratio of 2:3. Crystallographic analysis (see

Scheme 3. Synthesis of Imidazolium-Substituted Pd3 Sandwich Complexes from Monomeric Precursors

be attributed to the poor solubility of the latter compounds, as it is known that the solubility of 1 correlates with the steric bulk of the substituents.18 The solubilities of the pyridiniumsubstituted compounds 2 are similarly low, and therefore this route is also not viable for them. For soluble precursors such as 1c,d, however, this is the most straightforward and therefore preferable route. The reaction is very fast at room temperature, and the rate is mainly limited by the solubility of the starting materials. A slight excess of Pd2(dba)3 ensures a complete conversion and can conveniently be removed by washing. Conversion of Dimeric Precursors. In order to gain access to a broader variety of substitution patterns on 3, it is obviously necessary to use more soluble starting materials. In our previous research we have already shown that exchanging B


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Organometallics

Scheme 5. Generally Applicable Synthesis of Imidazolium-Substituted Pd3 Sandwich Complexes via Dimeric “Preclusters” with a Pd:Br Ratio of 2:3

synthetic route to imidazolium-substituted cluster complexes of type 3-BF4, we also applied this protocol to a selection of pyridinium-substituted derivatives. 2a,b mixtures with the respective literature-known dimeric compound (6a,b) react in the same way with Pd2(dba)3 to form the pyridiniumfunctionalized Pd sheet sandwich complexes 7a,b-BF4 (see Scheme 6).18

below) shows that an interaction between the two Pd atoms takes place. Thus, they are prearranged for the incorporation of the third Pd (“precluster”), which makes this compound an ideal precursor for Pd3 cluster complexes. Unfortunately, it was observed that 5d is only stable in very polar solvents, such as DMSO, which are inconvenient for further reactions. In less polar solvents only dilute solutions are stable, with anion redistribution followed by precipitation of 1d observed at higher concentrations, leaving mainly 4d in solution (see equilibrium depicted in Scheme 5 (top)). Generally Applicable Synthetic Route. From all of the observations discussed above, we conclude that complexes of type 5 generally exist in equilibrium with a mixture of 1 and 4 (Scheme 5 (top)). Due to its poor solubility, 1 readily precipitates from solution, thereby shifting the equilibrium away from 5. This would explain why only the DiPP derivative 5d could be isolated, as 1d features the highest solubility among complexes of type 1. As a consequence of this equilibrium, it was deemed most convenient to prepare complexes of type 5 in situ, before adding the additional Pd(0) source to prepare the final Pd3 complexes. Using this method, a mixture of 1 and 4, which can be prepared more readily than 5, can be used as the starting material for the cluster synthesis. This synthetic route is also preferable over building up the bromide content by addition of [Bu4N][Br], in that the tetrabutylammonium tetrafluoroborate byproduct is not formed. Indeed, we found this method to be generally applicable and had the same success using mixtures of 1a−c and 4a−c as we did for 5d (see Scheme 5). The reaction is best performed with a slight excess of Pd2(dba)3 to ensure a complete reaction; because it is not stable in acetonitrile, the excess Pd(0) precursor decomposes to Pd black, which can easily be separated by filtration. The products 3-BF4 are precipitated from the reaction mixture by addition of diethyl ether and washed with more diethyl ether to remove free dba. Compounds 3-BF4 are obtained in high yield (typically ca. 90%) and purity without any further workup. If necessary, purification by fractional precipitation from acetonitrile/diethyl ether can be undertaken. Having developed this general

Scheme 6. Synthesis of Pyridinium-Substituted Pd3 Metal Sheet Sandwich Complexes

The strategy of using a dimeric precursor that prearranges two Pd atoms is, of course, not limited to functionalized derivatives. Reacting the known dimeric unsubstituted η3cycloheptatrienide Pd complex 8 (accessible from Pd(0) and tropylium bromide) with Pd2(dba)3 in the presence of tetraphenylphosphonium bromide leads to the unfunctionalized anionic Pd sheet complex 9, previously reported by Hurst and co-workers (see Scheme 7 (top)).7,19 For preparative purposes it is advantageous to perform this synthesis as a one-pot reaction, starting from Pd2(dba)3, tropylium bromide, and tetraphenylphosphonium bromide without isolation of the intermediate product 8 (see Scheme 7 (bottom)). This is quite C


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Organometallics

than the α protons. A comparable high-field shift is also observed in the 13C{1H} NMR spectra recorded for 3 and 7. These observations are consistent with the conclusion that the sandwiching cycloheptatrienyl ligands are more electron rich than in the Pd allyl precursor complexes. The fact that a highfield shift is also observed for the resonances of the substituents on the cycloheptatrienyl rings makes NMR spectroscopy a useful technique to probe the completeness of the cluster formation. To unambiguously prove the structures proposed above, Xray crystallographic studies were conducted. These confirm the “precluster” arrangement of the Pd2Br3 core of compound 5d, featuring one bridging μ2-bromide and two terminal bromides, as well as the aforementioned Pd−Pd contact (Pd1−Pd2 = 3.0661(6) Å) (see Figure 1). In accordance with most

Scheme 7. Synthesis of Unfunctionalized Pd3 Metal Sheet Sandwich Complex 9 from Dimeric Precursor 8 (Top) and in a One-Pot Reaction (Bottom)

similar to literature-known procedures, which use Pd2(dba)3, tropylium tetrafluoroborate, and a correspondingly increased amount of tetraphenylphosphonium halide.1,7 As a synthetic advantage, the formation of tetraphenylphosphonium tetrafluoroborate byproduct is avoided in our case. From a mechanistic point of view, the stepwise synthesis is interesting, as it indicates that complexes of type 8 with allyl-bound cycloheptatrienide ligands are the key intermediate in the assembly of Pd3 sheet sandwich complexes from tropylium salts and Pd(0) precursors. Such reactions can therefore be divided into two steps: first, an oxidative addition of the tropylium cation to Pd(0) forming a dimeric η3-cycloheptatrienide Pd(II) complex (8), and second, cluster formation by reaction with the remaining Pd(0) precursor in analogy to the syntheses presented in this work. This is similar to the two-step mechanism proposed by Murahashi and co-workers for such reactions in the absence of halides, although in this case the dinuclear intermediates have not been isolated.12 All isolated compounds described above have been characterized and confirmed pure by 1H and 13C NMR spectroscopy, mass spectrometry, and elemental analysis. As the only exception, 5d contains ca. 2% of silver bromide, which is due to the fact that AgBr has some solubility in dimethyl sulfoxide. Its presence cannot be avoided, as any attempted precipitation would also affect the equilibrium between 5d and 1d/4d. However, AgBr does not interfere in the follow-up reaction to form 3d-BF4, and during the workup it is straightforwardly removed by filtration. Compounds 3 and 7, just as their precursors, are air-stable as dry solids but slightly sensitive to moisture in solution.18 It must also be mentioned that they are not completely stable in coordinating solvents, even if anhydrous. For example, in acetonitrile, a small but visible degree of decomposition to Pd black is observed after several days. A very unsuitable solvent is represented by DMF, in which both the cluster complexes and the precursor compounds decompose significantly within hours. The 1H NMR resonances of the cycloheptatrienyl moiety show the pattern of a monosubstituted, symmetric, sevenmembered ring, indicating a rapid spinning motion of the ligands. In the case of 3a-BF4 for example, a doublet at 6.05 ppm for the α protons (next to the imidazolium substituent), a multiplet between 4.49 and 4.55 ppm for the β protons, and a multiplet between 4.92 and 4.96 ppm for the γ protons are observed in deuterated DMSO. Similar data are obtained for all other Pd3 compounds of types 3 and 7. Whereas the resonance pattern is exactly the same as for the precursor compounds 1, 2, 4, and 6, the resonances in 3 and 7 are shifted to higher field by up to 1 ppm.18 This shift affects the protons away from the electron-withdrawing imidazolium/pyridinium group more

Figure 1. Molecular structure of 5d in the solid state with ellipsoids at the 50% probability level. Substituents are simplified as wireframes, and hydrogen atoms, counterions, and cocrystallized solvent molecules are omitted for clarity. Selected distances (Å) and angles (deg): Pd1− Br1 2.5241(7), Pd1−Br3 2.5828(6), Pd2−Br2 2.5124(6), Pd2−Br3 2.5784(6), Pd1−Pd2 3.0661(6), Pd1−C1 2.053(4), Pd1−C2 2.189(4), Pd1−C7 2.188(3), Pd2−C23 2.065(3), Pd2−C24 2.186(3), Pd2−C29 2.186(3); Br1−Pd1−Br3 95.46(2), Br2−Pd2− Br3 94.54(2), Pd1−Br3−Pd2 72.89(2), Br1−Pd1−Pd2 102.44(2), Br2−Pd2−Pd1 103.09(2).

literature-known examples, the cycloheptatrienide ligands bind in the β position (with respect to the position of the central allyl carbon to the substituent) and feature the typical buckle at the atoms involved in the allyl bond.17,18,20 The analogous compound 10, which is a Pd2Br3 “precluster” complex that bears a p-(dimethylamino)pyridinium substituent instead of a DiPP-imidazolium substituent, could not be isolated as a pure bulk material, but a small number of single crystals were obtained for crystallographic analysis (see Figure 2). The crystals were obtained from a solution of 6b in wet acetonitrile, and the formation of 10 is attributed to the decomposition of 6b in the presence of water, liberating bromide ions which would react with remaining amounts of 6b to form 10. This is another indication that dimeric complexes with three bromides such as 5 and 10 are easily formed in an equilibrium in solution, although their isolation is challenging. Structurally, 10 is very similar to 5d. Notably, the Pd−Pd contact is slightly shorter (Pd1−Pd1a = 2.9596(3) Å), while the bonds with the bridging bromide ligand are slightly elongated. Crystal structure analysis also confirmed the formation of the cluster complexes 3 and 7 (see Figure 3; for pertinent distances D


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Organometallics

vary significantly, which shows that changes on the substituents have no significant influence on the organometallic bonds. The differences in the orientation of the substituents themselves can mostly be attributed to crystal-packing effects, but also to interactions with the bromide ligands, which will be discussed in more detail below. On the basis of the Pd−C distances and the positions of the ligands with respect to the Pd3Br3 cores, coordination of the cycloheptatrienyl ligands to the cluster can formally be divided into one η3 and two η2 coordination modes.1 Unlike the structures of the precursor compounds (see above), the seven-membered ring does not feature a distinct buckle at the formally η3-coordinated Pd but instead can be regarded as planar. Inspection of the clusters reveals that there are some deviations from the ideal geometry of an equilateral triangle, which systematically appear in all structures. Both the Pd−Pd and the Pd−Br distances within a single compound are markedly unequal. In most cases (3b-BF4 and 7b-BF4 are exceptions) it is also observed that the shorter two of the Pd− Pd distances also have similar values, while the third is significantly longer; the geometry can be described as an approximate isosceles triangle. As the most visually obvious feature, the Pd−Br bonds do not exactly coincide with the external bisectors of the triangle (see visualization with Br− Pd−Pd angles in Figure 4). There is always one Pd−Br bond which is close to the bisector (always arranged on the top in Figure 4), and this one remarkably involves the Pd atom with the two shorter Pd−Pd bonds (apex of the isosceles triangle). The Pd−Br bonds of the other Pd atoms deviate more strongly from the bisectors, both being either bent toward or away from the apex Pd (i.e., both to the top or bottom in

Figure 2. Molecular structure of 10 in the solid state with ellipsoids at the 50% probability level. Substituents are simplified as wireframes, and hydrogen atoms as well as counterions are omitted for clarity. Selected distances (Å) and angles (deg): Pd1−Br1 2.5032(3), Pd1− Br2 2.6116(3), Pd1−Pd1a 2.9596(3), Pd1−C1 2.068(2), Pd1−C2 2.212(3), Pd1−C7 2.159(2); Br1−Pd1−Br2 95.28(1), Pd1−Br2− Pd1a 69.03(1), Br1−Pd1−Pd1a 101.79(1). Symmetry operation to create equivalent positions: 1 − x, y, 1/2 − z.

and angles see Table 1). As a crystal structure of 9 has been previously reported, it will not be included here.7 The core structural features of the Pd3 sandwich clusters are comparable to examples from the literature; as to be expected for functionalized derivatives, however, the ranges of the Pd−Pd bond lengths (2.7385(11)−2.8270(10) Å) and Pd−Br bond lengths (2.5281(13)−2.5917(7) Å) are more broad.1,7,17 Drawing comparisons within the set of functionalized complexes, the average Pd−C and C−C bond lengths do not

Figure 3. Molecular structures of 3a-BF4 (one out of two molecules in the asymmetric unit), 3b-BF4 (one out of two molecules in the asymmetric unit), 3d-Br, 3d-BF4, 7a-BF4 (symmetry code to create equivalent atoms: −x + 3/4, y, −z + 3/4) and 7b-BF4 in the solid state with ellipsoids at the 50% probability level. Substituents are simplified as wireframes, and hydrogen atoms, counterions, and cocrystallized solvent molecules are omitted for clarity. For distances and angles see Table 1. E


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Organometallics Table 1. Selected Distances (Å) and Angles (deg) for the Molecules Depicted in Figure 3 Pd1−Pd2 Pd1−Pd3 Pd2−Pd3 Pd2−Br1 Pd2−Br2 Pd3−Br3 Pd−C (min) Pd−C (max) Pd−C (av) C−C (min) C−C (max) C−C (av) Br1−Pd1−Pd2 Br1−Pd1−Pd3 Br2−Pd2−Pd1 Br2−Pd2−Pd3 Br3−Pd3−Pd1 Br3−Pd3−Pd2

3a-BF4

3b-BF4

3d-Br

3d-BF4

7a-BF4

7b-BF4

2.7464(12) 2.8165(7) 2.7417(12) 2.5740(11) 2.5501(13) 2.5631(11) 2.147(7) 2.559(7) 2.278 1.405(10) 1.429(10) 1.417 163.53(4) 137.36(4) 146.84(3) 151.39(3) 134.38(4) 166.30(4)

2.7573(13) 2.7630(12) 2.7433(12) 2.5110(15) 2.5596(15) 2.5521(14) 2.137(10) 2.548(11) 2.269 1.365(14) 1.432(15) 1.411 148.66(4) 151.73(4) 141.02(4) 158.31(4) 143.36(5) 155.90(5)

2.7495(11) 2.7385(11) 2.8050(11) 2.5765(13) 2.5281(13) 2.5540(12) 2.146(10) 2.578(11) 2.276 1.396(13) 1.459(14) 1.416 147.96(4) 150.31(4) 163.27(4) 137.63(4) 171.24(5) 128.87(4)

2.7428(6) 2.8270(10) 2.7465(8) 2.5369(7) 2.5917(7) 2.5407(9) 2.153(5) 2.582(5) 2.289 1.402(7) 1.425(7) 1.416 167.36(4) 133.31(3) 150.64(3) 147.18(3) 134.72(2) 166.23(3)

2.7555(7) 2.7555(7) 2.7941(8) 2.5376(8) 2.5435(8) 2.5435(8) 2.143(3) 2.578(3) 2.280 1.402(4) 1.424(4) 1.415 149.54(1) 149.54(1) 140.32(3) 159.89(3) 140.32(3) 159.89(3)

2.7599(3) 2.7695(4) 2.7535(4) 2.5425(4) 2.5665(4) 2.5540(4) 2.149(3) 2.608(3) 2.272 1.400(4) 1.430(4) 1.418 153.17(1) 146.60(1) 148.16(1) 151.17(1) 144.94(1) 153.87(1)

Figure 4. Pd3Br3 core fragments from the crystal structures of the title compounds with the values of all Br−Pd−Pd angles (deg). In the case of 3aBF4 and 3b-BF4 both crystallographically independent molecules in the asymmetric unit are considered. Symmetry code to create equivalent atoms in the case of 7a-BF4: −x + 3/4, y, −z + 3/4.

Pd3 cluster (Scheme 1 (right)).17 Furthermore, the apex Pd is usually not involved in the η3 coordination; exceptions are 7bBF4 and the second molecule in the crystal structure of 3a-BF4. In these latter cases, however, the longest Pd−C bond lengths occur (Pd3−C7 = 2.608(3) Å for 7b-BF4 and Pd4−C29 = 2.656(6) Å for 3a-BF4(2)) and thus make the formal η3 coordination more akin to a η2 coordination with an additional contact. Although it must be kept in mind that there are small exceptions from every trend described above, they are otherwise observed systematically and we therefore conclude that there is a marked nonequivalence of the three Pd atoms forming the cluster. This is closely related to the oxidation state of Pd in the sandwich cluster, for which in our opinion no satisfactory description has yet been given.

Figure 4). In which direction this distortion occurs is in most cases related to the orientation of the substituents; substituents located on the same side of the cluster tend to favor bromides bending away from the apex (e.g., 3d-Br), whereas substituents on opposite sides tend to favor bromides bending toward it (e.g., 7a-BF4). This can be attributed to the interactions between the cationic imidazolium/pyridinium groups and the bromides, which appear both in the form of hydrogen bonds and Coulombic interactions (see Figure S27 in the Supporting Information for illustrations). Both interactions serve to draw the bromides in toward the substituents. It should also be mentioned that such interactions play an important role in the intermolecular interactions in the crystal lattice, as has previously been observed for the prototypical functionalized F


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we employ no reducing agents and the η3-cycloheptatrienide ligands are, as discussed, not oxidized to tropylium ligands, the oxidation state 0 can be ruled out for Pd. For a more accurate interpretation of the electron distribution there are two possibilities to describe the ionic limit of bonding, on which formal oxidation states are based: (i) the formal oxidation states of the precursor compounds are maintained in the cluster complexes, i.e. the cycloheptatrienide ligands are monoanionic and the Pd centers share a formal oxidation state of +1.333 (II in Scheme 8), or (ii) the cycloheptatrienide ligands are triply anionic in analogy to their mononuclear complexes (see above), which then gives the Pd centers a formal oxidation state of +2.666 each (III in Scheme 8). Therefore, in both formalisms Pd has an intermediate, noninteger oxidation state, which immediately raises the question if different oxidation states can be discerned in the clusters. This is somewhat implicit on the basis of the differences in bonding observed in the crystal structure analyses. However, to gain further insight into the electronic structure, we performed DFT calculations and electrochemical studies. The electrochemical behavior of 1d, 3d-Br, 3d-BF4, and 3aBF4 was investigated by a range of methods in CH2Cl2 and THF, using a glassy-carbon working electrode. The compounds were also studied in DMF but were found to be unstable in this solvent as mentioned before. In the cyclic voltammogram (CV) of the mononuclear Pd(II) complex 1d, an irreversible oxidation wave was seen at Epa = +0.71 V versus the ferrocenium/ferrocene redox couple (CH2Cl2). No associated reduction wave was observed for this oxidation process, even at faster scan rates of 500 mV s−1. Similar oxidation processes were also observed for the trinuclear complexes containing DiPP groups on the imidazolium moiety (3d-Br and 3d-BF4, at Epa = +0.32 V and +0.75 V, respectively), but not for 3a-BF4, which instead has a methyl group. These oxidation processes are therefore attributed to ligand-based oxidations. All four complexes featured an irreversible reduction wave in their CVs, at Epc = −1.76 V for 1d, −1.31 V for 3d-Br, −1.26 V for 3d-BF4, and −1.53 V for 3a-BF4 (Figure 5). In order to ascertain whether these reduction processes were metalcentered, controlled-potential electrolysis (CPE) was conducted using a Pt-basket electrode, where the charge required

In their original publication, Murahashi, Kurosawa, and coworkers described the cycloheptatrienyl ligands as cationic tropylium ligands and the cluster as composed of three Pd(0) atoms (see I in Scheme 8), corresponding to the oxidation Scheme 8. Possible Fragmentations of Pd3 Metal Sheet Sandwich Complexes with Their Formal Oxidation States of Pd and Chargesa

a

X = halide.

states of the starting materials.1 This interpretation has been adapted in the literature since then.7−12,21 η7-C7H7 ligands bound to a single metal can serve as a helpful analogy here, as they are well-known and were also originally regarded as tropylium ligands, mainly on the basis of aromaticity of C7H7+.22,23 However, several studies of early-transition-metal cycloheptatrienyl complexes (e.g., [Ti(C7H7)(C5H5)]) concluded (on the basis of reactivity, NMR spectroscopy, computational data, and photoelectron spectroscopy) that the cycloheptatrienyl ring is indeed aromatic, but also anionic with a resulting formal charge of −3.23−32 It should be noted that there are exceptions to this assignment (e.g., [Mo(CO)3(C7H4(t-Bu)3)][BF4]), where the ligands are more consistent with bearing a positive charge and are better regarded as tropylium ligands.33,34 In the present case of Pd3 sandwich clusters, the data fit best with the seven-membered ring bearing anionic charge and we therefore regard the Pd3 sheet as cationic (see II and III in Scheme 8). As discussed above, the high-field shifts associated with the ligand NMR resonances in comparison to the allylbound precursors (in which the ligand is already formally anionic) shows that they are more electron rich: i.e., not cationic tropylium ligands. In contrast, the Mo tropylium complex mentioned above features 1H NMR resonances at around 7 ppm.33 The hitherto existing interpretation of the cluster as Pd(0) also seems counterintuitive to us, because Pd(0) would not be expected to bear halide ligands. It could also not explain the metal-based reduction observed in the electrochemical characterization (see below). Finally, a reduction of the Pd(II) precursors would be required, but as

Figure 5. Cyclic voltammograms of the mononuclear complex 1d and the trinuclear complexes 3d-Br, 3d-BF4, and 3a-BF4, all measured at 100 mV s−1 in CH2Cl2 vs Fc+/Fc, using a glassy-carbon working electrode. The relative peak heights between CVs are arbitrary. G


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Table 2. NBO Charges (Atomic Units) for Pd and Pd-Bound Atoms in Model Compounds 3a+, 7a+, 9−, and Reference Compounds with Unambiguous Oxidation States of Pd Pd1 Pd2 Pd3 av Br av C av N a

9−

3a+

7a+

Pd(cod)2a

Pd2(CO)2Br42−

0.074 0.076 0.079 −0.582 −0.221

0.017 0.106 0.107 −0.521 −0.206

0.008 0.065 0.065 −0.495 −0.170

−0.044

−0.042 −0.042

0.054

−0.563 0.599

−0.514

−0.213

PdBr42−

1a 0.045

Pd(tacn)23+ b 0.676

−0.503 −0.104c −0.516

cod = cycloocta-1,5-diene. btacn = 1,4,7-triazacyclononane. cThis average includes also the nonbonded C atoms of the seven-membered ring.

This may be due to a conformational change in the molecule that shifts the reduction potential to more negative values. The combination of CV, SWV, and CPE analyses indicates strongly that the Pd ions in the trinuclear complexes share a common charge density and a comparison of their reduction potentials with that of 1d indicates that this shared charge density is higher than that of the Pd(II) ion in 1d. A mixture of formal oxidation states is not supported by the electrochemical data; thus, at least in solution the three Pd ions seem equivalent. This might be due to the dynamic behavior of the complexes already mentioned in the NMR discussion. For further elucidation of the electron distribution, we made use of DFT calculations, which allowed us to obtain localized charge information in the Pd3Br3 core without interference from averaging that might be caused by the dynamics in solution. It should first be noted that, already in previously reported crystal structures of unsubstituted Pd3 sandwich complexes, differences between the coordination environments of the Pd atoms were observed, although these were less significant than in our examples.1,7 The initial geometry optimizations performed for all model compounds therefore are apt tools to determine whether these effects are only due to crystal packing or also exist for single molecules in the “gas phase” and in solution (modeled by means of a continuum solvent model). As model compounds we chose the cluster fragments of 3a-BF4 (3a+), 7a-BF4 (7a+), and 9 (9−). It turned out that, for the imidazolium- and pyridinium-substituted models 3a+ and 7a+, the geometries with deviations of the Pd− Br bonds from the external bisectors of the triangle (as observed in the crystal structures, see above) are representative minima in solution as well as in the gas phase. For unsubstituted 9−, however, such geometries do not represent minima and the optimizations (in solution and in the gas phase) converged at structures which have the Pd−Br bonds almost exactly on the external bisectors. Consequently, the observed distortions in the geometry of unsubstituted Pd3 sandwich cluster complexes arise from crystal-packing effects. As the obtained geometries of 3a+ and 7a+ support the idea of distinguishable Pd atoms, we studied the electron distribution in more detail by means of NBO charges, to determine whether these geometric features arise due to discernible differences in partial charges on Pd.38 The molecular orbital (MO) situation has been studied before; thus, we will not discuss it here and instead focus on concepts that translate MOs into localized electron densities.1,21 Because such charge concepts do not directly translate into formal oxidation states, we carried out the same calculations for a range of reference compounds with unambiguous oxidation states between 0 and +3. Where possible, the reference compounds chosen were chemically similar to the compounds in question (i.e., containing only Pd−Br and Pd−C bonds; for

to quantitatively reduce the complexes was determined by integration of the amperogram. From these experiments, it was found that one electron is involved in the reduction of 1d, while three electrons are involved in the reduction of the trimetallic complexes 3d-Br and 3d-BF4. The number of electrons involved in the reduction process is equal to the nuclearity of the complex, and these processes therefore describe oneelectron reduction of each metal center. For 1d, where the formal oxidation state is unambiguously +2, this reduction at −1.76 V describes the Pd(II)/Pd(I) couple and is similar to previously reported Pd(II)/Pd(I) reduction potentials, which lie between −1.35 and −1.48 V in phosphine coordination environments and around −2.3 V in thiolate environments.35−37 For the cluster complexes, this implies that all three metal centers share a common charge density, as the three-electron reduction appears concerted and occurs at a single potential. On comparison of the potentials of these metal-based reductions, the trinuclear complexes all undergo reduction at potentials 200−500 mV more positive than the formal Pd(II)/ Pd(I) reduction measured for 1d. This indicates that the Pd centers in the trinuclear complexes possess higher positive charge densities in comparison with the Pd(II) complex 1d. However, this does not necessarily translate directly into a higher formal oxidation state on Pd. Further comparison of the reduction potentials between the complexes highlights that the effect of the accompanying counteranion has only a minor influence on the reduction of the metal. Changing the counterion from bromide in 3d-Br to tetrafluoroborate in 3dBF4 causes the reduction to be shifted anodically by only 50 mV. On the other hand, the influence of the R group on the imidazole was much more pronounced. Changing the substituent from DiPP (3d-BF4) to methyl (3a-BF4) results in a cathodic shift of 270 mV. In complexes where there is electronic communication between two or more metal centers, changes in oxidation state are often observed as stepwise processes, and we were interested to try and observe such a process in the metal sheet complexes that feature direct Pd−Pd bonding. We attempted to resolve individual, stepwise Pd reduction in 3dBF4 using square-wave voltammetry (SWV) due to its higher resolution in comparison to CV. However, at room temperature, in both THF and CH2Cl2, only a single wave was observed in the SWV (potentials given in the Supporting Information). In THF, resolution of the individual Pd reductions was not observed even at 233 K, although at this temperature the peak started to broaden significantly. In CH2Cl2, the individual processes were also not resolved at low temperature. Interestingly, in this latter case the intensity of the peak observed at −1.22 V at room temperature decreased on cooling, while a second peak at −1.45 V increased in intensity. H


Article

Organometallics +3 no such compound could be found), as the directly bonded atoms can have just as great an influence on the calculated charge as the formal oxidation state of Pd. The reference compounds are Pd(cod)2 (cod = cycloocta-1,5-diene) for Pd(0), Pd2(CO)2Br42− for Pd(I), PdBr42− and 1a for Pd(II), and Pd(tacn)33+ (tacn = 1,4,7-triazacyclononane) for Pd(III). The results from these calculations are summarized in Table 2 (for full details see the Supporting Information).13,39,40 The NBO charges on Pd in the reference compounds illustrate that a higher formal oxidation state also gives a more positive charge on Pd; this is unsurprising but does indicate that an assessment of the charge distribution will provide information on the Pd oxidation states. The increase in positive partial charge when changing the oxidation state from +2 to +3 is very large in comparison to the differences between the other complexes, which certainly can be attributed to the different donor atoms in Pd(tacn)33+. With respect to the reference set the average NBO charges of Pd in the trinuclear sandwich compounds are closest to Pd(II). In 7a+ they match almost exactly, while in 3a+ and 9− they are more positive. This latter observation agrees well with the electrochemical results, which suggest a more positive charge density than for Pd(II). Moreover, the average NBO charge on the cycloheptatrienide C atoms of the trinuclear complexes is more negative than in the case of 1a, confirming the NMR results (see above) that the ligand is more electron rich than in the mononuclear complex. It should be mentioned here that a former theoretical study of M3X3(C7H7)2− (M = Ni, Pd, Pt; X = F, Cl) gave comparable NBO results (although no reference compounds were used), but these results were still interpreted as concordant with structure I, most probably because of the opinion in the literature at that time.21 In that study, strong back-donation was used as an explanation for the observations. On the basis of our data presented here, we argue that structure III is a more accurate description of the electronic structure. A more detailed description of the electronic structure is given after inspection of the NBO charges. The substituted model compounds feature two decisively different NBO charges on the Pd atoms that coincide with the differences in their coordination environments. The apex Pd atom has a comparably low charge, while the remaining two Pd atoms possess similar and significantly higher charges (see Figure 6 for

clusters within a single compound (see Table 3) confirm that the apex Pd atom is considerably less positive than the other Table 3. ESP-Derived Charges for the Pd Atoms in Model Compounds 3a+, 7a+, and 9− 9−

a

a

Pd

MK

1 2 3

0.089 0.111 0.144

3a+ HLY

b

0.056 0.063 0.065

7a+

MK

HLY

MK

HLY

−0.195 0.115 0.116

−0.202 0.033 0.033

−0.377 0.061 0.067

−0.545 −0.030 −0.030

MK = Kollman’s method.41 bHLY = Yang’s method.42

two in the case of the substituted derivatives 3a+ and 7a+, whereas there are only minor differences for 9−. This is strong evidence that the electron density is not distributed equally in the functionalized clusters, giving the impression that the Pd ions are of mixed valency in the static geometry; due to fluxionality in solution, they appear to have an averaged, intermediate charge density. In the crystalline state, the static, anisotropic environment creates preferential orientations for the cycloheptatrienide ligands and the different Pd atoms, and therefore the solid-state structure depicts the mixed-valence calculated geometry. The fact that this is not observed for 9− emphasizes how important the electron-withdrawing imidazolium/pyridinium substituents are as they polarize the sandwiching ligands. They also serve as “anchors” in the crystal structures, preventing disorder. Considering the above results that support structure III with the formal net Pd oxidation state +2.666, we conclude that the closest description based on integer oxidation numbers is a mixed-valence cluster composed of one Pd(II) ion and two Pd(III) ions. Consequently, the cluster formation from a Pd(II)−Pd(II) precluster and one Pd(0) is to be considered as an oxidative step formally transferring four electrons to the sandwiching cycloheptatrienide ligands.

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CONCLUSION On the basis of the concept of dimeric precluster formation we have developed a generally applicable synthesis of imidazoliumand pyridinium-functionalized trinuclear Pd sandwich cluster complexes. The precluster complex is formed from monomeric and dimeric η3-cycloheptatrienide Pd starting materials that are straightforwardly prepared. Because the halide stoichiometry in the bimetallic precluster complex is prearranged, the Pd3Br3 cluster is formed through simple addition of a Pd(0) source. In the complexes reported here, the imidazolium groups do not act as ligands but offer possibilities for future work, as these groups could be metalated further, thus providing a framework to build up higher nuclearity, possibly heteronuclear clusters such that cooperative reactivity can be explored. Conversion of the imidazolium side arm to a coordinating N-heterocyclic carbene has recently been shown for the precursor compound 1d.43 From spectroscopic, electrochemical, and computational analysis of the Pd3 motif, we found evidence for the Pd centers formally having an intermediate oxidation state of +2.666 and the cycloheptatrienide ligands being triply anionic. Closer crystallographic and computational examination of the Pd3Br3 core indicates that it is formed of inequivalent Pd atoms and therefore carries mixed-valence character; the cluster can formally be regarded as consisting of one Pd(II) ion and two

Figure 6. Pd3Br3 fragment from 3a+ with NBO charges (atomic units) showing the distribution of the different Pd charges.

3a+ as an example). To confirm this, we applied two more methods for the calculation of atomic charges, which are based on the electrostatic potential (ESP).41,42 It must be noted that these ESP models were inconsistent with the reference system: i.e., a higher oxidation state did not in all cases correspond to a more positive metal charge on comparison of different compounds. Nevertheless, the charges calculated for the Pd3 I


Article

Organometallics

101.13, 100.26, 27.91, 24.10, 23.80. ESI-MS (m/z): 504.95 [1/2 (M − BF4 − Br)]+, 1088.16 (M − BF4)+, 1193.97 (M − BF4 + Pd)+. General Synthesis of 3-Br. A 0.53 equiv amount of Pd2(dba)3· CH2Cl2 in dichloromethane was slowly added to 2 equiv of 1 in dichloromethane with constant stirring. After the mixture was stirred for another 15 min, the precipitate was collected by filtration, washed with dichloromethane and diethyl ether, and dried in vacuo. 3c-Br. Red-brown solid, 93%. Anal. Calcd for C38H40Br4N4Pd3: C, 38.30; H, 3.38; N, 4.70. Found: C, 38.03; H, 3.38; N, 4.70. 1H NMR (400 MHz, CD3CN, 298 K): δ (ppm) 8.91 (virt t, 3J = 4J = 1.8 Hz, 2H), 7.66 (virt t, 3J = 4J = 1.8 Hz, 2H), 7.26 (virt t, 3J = 4J = 1.8 Hz, 2H), 7.06 (s, 4H), 6.15 (d, 3J = 8.1 Hz, 4H), 4.99−5.03 (m, 4H), 4.56−4.63 (m, 4H), 2.31 (s, 6H), 2.00 (s, 12H). 13C NMR (100 MHz, DMSO-d6, 298 K): δ (ppm) 140.54, 135.91, 134.46, 130.70, 129.25, 124.22, 122.09, 93.99, 75.50, 70.81, 68.16, 20.59, 17.26. ESI-MS (m/ z): 1109.95 (M − Br)+. 3d-Br. Dark red solid containing 0.25 equiv of diethyl ether, 93%. Anal. Calcd for C44H52Br4N4Pd3·1/4C4H10O: C, 41.76; H, 4.24; N, 4.33. Found: C, 41.46; H, 4.36; N, 4.23. 1H NMR (400 MHz, CD3CN, 298 K): δ (ppm) 8.90 (virt t, 3J = 4J = 1.7 Hz, 2H), 7.55−7.60 (m, 4H), 7.57−7.60 (m, 6H), 6.13 (d, 3J = 8.1 Hz, 4H), 4.99−5.04 (m, 4H), 4.50−4.56 (m, 4H), 2.42 (septet, 3J = 6.8 Hz, 4H), 1.15 (d, 3J = 6.8 Hz, 12H), 1.08 (d, 3J = 6.8 Hz, 12H). 13C NMR (100 MHz, DMSO-d6, 298 K): δ (ppm) 145.40, 136.35, 131.60, 130.17, 125.09, 124.47, 123.35, 92.80, 76.25, 70.34, 68.53, 27.69, 24.13, 24.11. ESI-MS (m/z): 1193.95 (M − Br)+. Synthesis of 3a-BF4: Route A. A 200 mg portion (0.225 mmol, 1 equiv) of 4a and 84.3 mg (0.0842 mmol, 0.375 equiv) of Pd2(dba)3· CH2Cl2 were dissolved/suspended in 8 mL of acetonitrile and stirred for 3 h. The solid was collected by filtration and washed with 30 mL of acetonitrile. Then it was extracted with 55 mL of warm acetonitrile. The extract was evaporated in vacuo, and the residue was washed with diethyl ether (2 × 5 mL) and dried in vacuo. General Synthesis of 3-BF4: Route B. A 1 equiv amount of 4, 1 equiv of tetrabutylammonium bromide, and 0.53 equiv of Pd2(dba)3· CH2Cl2 were dissolved/suspended in acetonitrile and stirred for 3−4 h. The solution was separated by filtration, concentrated in vacuo, and treated with diethyl ether. The precipitate was collected by filtration, washed with diethyl ether, and dried in vacuo. If necessary, further purification can be achieved by fractioned precipitation from acetonitrile/diethyl ether. General Synthesis of 3-BF4: Route C. A 1 equiv amount of 1, 0.5 equiv of 4 (or instead of these 1 equiv of 5), and 0.55 equiv of Pd2(dba)3·CH2Cl2 were dissolved/suspended in acetonitrile and stirred for 3−4 h. The solution was separated by filtration, concentrated in vacuo, and treated with diethyl ether. The precipitate was collected by filtration, washed with diethyl ether, and dried in vacuo. If necessary, further purification can be achieved by fractioned precipitation from acetonitrile/diethyl ether. 3a-BF4. Dark red solid containing 0.5 equiv of acetonitrile: route A, 34%; route B, 81%; route C, 95%. Anal. Calcd for C22H24BBr3F4N4Pd3·1/2C2H3N: C, 27.33; H, 2.54; N, 6.24. Found: C, 27.37; H, 2.57; N, 6.31. 1H NMR (400 MHz, DMSO-d6, 298 K): δ (ppm) 9.42 (virt t, nr, 2H), 7.60 (virt t, 3J = 4J = 1.9 Hz, 2H), 7.47 (virt t, 3J = 4J = 1.9 Hz, 2H), 6.05 (d, 3J = 8.1 Hz, 4H), 4.92−4.96 (m, 4H), 4.49−4.55 (m, 4H), 3.79 (s, 4H). 13C NMR (100 MHz, DMSOd6, 298 K): δ (ppm) 135.86, 123.73, 122.00, 94.47, 75.90, 70.15, 68.20, 36.16. ESI-MS (m/z): 902.18 (M − BF4)+. 3b-BF4. Intense red solid: route C, 85%. Anal. Calcd for C34H32BBr3F4N4Pd3: C, 35.75; H, 2.82; N, 4.90. Found: C, 35.35; H, 2.85; N, 4.94. 1H NMR (400 MHz, CD3CN, 298 K): δ (ppm) 8.77 (virt t, nr, 2H), 7.42 (virt t, 3J = 4J = 1.9 Hz, 2H), 7.38−7.40 (m, 6H), 7.29−7.32 (m, 4H), 7.11 (virt t, 3J = 4J = 1.9 Hz, 2H), 6.02 (d, 3J = 8.0 Hz, 4H), 5.19 (m, 4H), 4.94−4.98 (m, 4H), 4.46−4.52 (m, 4H). 13C NMR (100 MHz, CD3CN, 298 K): δ (ppm) 136.00, 134.04, 130.23, 130.17, 129.57, 123.93, 123.38, 95.18, 77.75, 71.58, 68.70, 54.14. ESIMS (m/z): 1053.96 (M − BF4)+. 3c-BF4. Dark red solid: route B, 86%; route C, 93%. Anal. Calcd for C38H40BBr3F4N4Pd3: C, 38.08; H, 3.36; N, 4.67. Found: C, 38.22; H, 3.58; N, 4.65. 1H NMR (400 MHz, CD3CN, 298 K): δ (ppm) 8.74

Pd(III) ions. However, due to their highly dynamic nature in solution, all three Pd atoms share an averaged charge density, as observed in their electrochemistry. Further modification of the sandwiching ligands might help tune the compounds more toward actual mixed-valence behavior, but even without this the intermediate oxidation state could be an interesting feature for Pd catalysis. Very recently, a Pd3 cluster stabilized by phosphine ligands has been introduced as an efficient catalyst for Suzuki− Miyaura reactions, which underlines the potential of defined molecular clusters for catalytic applications.44 Finally, the stepwise synthesis via preclusters could also be modified to incorporate a metal different from Pd, which would give access to a variety of Pd2M metal sheet complexes with a high potential for cooperative metal−metal interactions.

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EXPERIMENTAL SECTION

General Procedures. All reactions were performed under argon using standard Schlenk techniques or in a glovebox with dried solvents. Monomeric and dimeric imidazolium- and pyridiniumsubstituted η3-cycloheptatrienide Pd complexes (1a−d, 2a−d, 4a,b, 6a,b), [PdBr(η3-C7H7)]2 (8), Pd2(dba)3 (recrystallized from dichloromethane), and tropylium bromide were synthesized according to literature procedures.18,19,45,46 NMR spectra were recorded on a Bruker AVIII 400US spectrometer (400.13 MHz for 1H, 100.61 MHz for 13C), and chemical shifts are referenced to the solvent residual signals with respect to tetramethylsilane; 13C spectra are protondecoupled. 1H NMR data are reported as follows: chemical shift in ppm (multiplicity (s = singlet, d = doublet, t = triplet, q = quartet, m = multiplet, virt = virtual, br = broad, nr = not resolved), coupling constant in Hz, integral; for detailed assignments see the Supporting Information). Elemental analyses were performed by the microanalytical laboratory of the Technische Universität München. ESI mass spectra were recorded on a Q-TOF micro (micromass) instrument, with the samples prepared as solutions in acetonitrile. For more details, including electrochemistry and X-ray crystallography, see the Supporting Information. Computational Details. All calculations were performed using the Gaussian 09 software package with methods and basis sets as implemented, if not stated otherwise.47 The basis sets def2-TZVP with a corresponding ECP and def2-TZVPD were obtained from EMSL Basis Set Exchange.48−52 If not stated otherwise, the ωB97X-D functional and the basis sets TZVP for C, H, N, def2-TZVP (with corresponding ECP) for Pd, and def2-TZVPD for Br were used. For calculations in solution the solvent model SMD with acetonitrile as solvent was employed.53 Geometry optimizations were performed for all compounds, and calculations of vibrational frequencies were used to confirm that the structures represent minima on the potential energy surface. For all details see the Supporting Information. Synthesis of 5d. A 33.3 mg portion (0.171 mmol, 1 equiv) of silver tetrafluoroborate in 5 mL of acetonitrile was slowly added to 200 mg (0.342 mmol, 2 equiv) of 1d in 5 mL of dimethyl sulfoxide with constant stirring. After the mixture was stirred for 10 min, the white precipitate was filtered off and the filtrate was evaporated in vacuo overnight. The residue was pulverized, washed with diethyl ether (3 × 10 mL), and dried in vacuo at 60 °C. After this procedure was repeated, 191 mg (0.152 mmol, 89%) of 5d containing 1 equiv of DMSO were obtained as an intense red solid. The product also contains ca. 2% of silver bromide, which does not interfere in the following reactions. Anal. Calcd for C44H52BBr3F4N4Pd2·C2H6SO: C, 44.04; H, 4.66; N, 4.47; S, 2.56, Br, 19.1; Pd, 17.0; Ag, 0.0. Found: C, 42.74; H, 4.85; N, 4.35; S, 2.31; Br, 19.7; Pd, 16.5, Ag, 1.4. 1H NMR (400 MHz, CD3CN, 298 K): δ (ppm) 9.60 (virt t, nr, 2H), 8.18 (virt t, 3 J = 4J = 1.8 Hz, 4H), 7.60−7.64 (m, 4H), 7.44 (d, 3J = 7.8 Hz, 4H), 6.24 (d, 3J = 8.3 Hz, 4H), 6.07−6.12 (m, 4H), 5.38−5.44 (m, 4H), 2.49 (septet, 3J = 6.8 Hz, 4H), 1.21 (d, 3J = 6.8 Hz, 12H), 1.17 (d, 3J = 6.8 Hz, 12H). 13C NMR (100 MHz, DMSO-d6, 298 K): δ (ppm) 145.23, 136.74, 132.29, 131.60, 130.39, 125.50, 124.43, 122.47, 104.99, J


Organometallics

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(virt t, nr, 2H), 7.67 (virt t, 3J = 4J = 1.8 Hz, 2H), 7.25 (virt t, 3J = 4J = 1.8 Hz, 2H), 7.06 (s, 4H), 6.13 (d, 3J = 8.0 Hz, 4H), 5.00−5.04 (m, 4H), 4.56−4.63 (m, 4H), 2.31 (s, 6H), 1.99 (s, 12H). 13C NMR (100 MHz, CD3CN, 298 K): δ (ppm) 142.50, 136.40, 135.64, 131.43, 130.48, 125.13, 123.75, 95.15, 77.92, 71.98, 68.47, 21.09, 17.88. ESIMS (m/z): 1109.92 (M − BF4)+. 3d-BF4. Intense red solid,: route C, 88%. Anal. Calcd for C44H52BBr3F4N4Pd3: C, 41.20; H, 4.09; N, 4.37. Found: C, 41.43; H, 4.38; N, 4.66. 1H NMR (400 MHz, CD3CN, 298 K): δ (ppm) 8.85 (virt t, 3J = 4J = 1.6 Hz, 2H), 7.55−7.60 (m, 4H), 7.37−7.39 (m, 6H), 6.13 (d, 3J = 8.0 Hz, 4H), 4.99−5.04 (m, 4H), 4.50−4.56 (m, 4H), 2.42 (septet, 3J = 6.8 Hz, 4H), 1.15 (d, 3J = 6.8 Hz, 12H), 1.08 (d, 3J = 6.8 Hz, 12H). 13C NMR (100 MHz, CD3CN, 298 K): δ (ppm) 146.65, 136.79, 132.93, 130.77, 125.92, 125.58, 124.58, 93.77, 78.20, 71.53, 68.92, 29.13, 24.54, 24.31. ESI-MS (m/z): 1193.82 (M − BF4)+. General Synthesis of 7-BF4: Route C. A 1 equiv amount of 2, 0.5 equiv of 6, and 0.55 equiv of Pd2(dba)3·CH2Cl2 were dissolved/ suspended in acetonitrile and stirred for 3−4 h. The solution was separated by filtration, concentrated in vacuo, and treated with diethyl ether. The precipitate was collected by filtration, washed with diethyl ether, and dried in vacuo. If necessary, further purification can be achieved by fractional precipitation from acetonitrile/diethyl ether. 7a-BF4. Intense red solid, 92%. Anal. Calcd for C28H30BBr3F4N2Pd3: C, 32.33; H, 2.91; N, 2.69. Found: C, 32.40; H, 2.84; N, 2.82. 1H NMR (400 MHz, CD3CN, 298 K): δ (ppm) 8.37 (s, 4H), 8.12 (s, 2H), 6.07 (d, 3J = 8.0 Hz, 4H), 5.10−5.14 (m, 4H), 4.54−4.59 (m, 4H), 2.39 (s, 12H). 13C NMR (100 MHz, DMSO-d6, 298 K): δ (ppm) 147.69, 141.61, 137.62, 103.89, 78.53, 71.05, 68.60, 17.63. ESI-MS (m/z): 952.08 (M − BF4)+. 7b-BF4. Red-brown solid, 90%. Anal. Calcd for C28H32BBr3F4N4Pd3: C, 31.42; H, 3.01; N, 5.23. Found: C, 31.25; H, 3.06; N, 5.16. 1H NMR (400 MHz, CD3CN, 298 K): δ (ppm) 7.84−7.88 (m, 4H), 6.66−6.70 (m, 4H), 6.01 (d, 3J = 8.0 Hz, 4H), 4.95−4.99 (m, 4H), 4.45−4.50 (m, 4H), 3.14 (s, 12H). 13C NMR (100 MHz, DMSO-d6, 298 K): δ (ppm) 157.16, 142.33, 108.05, 106.11, 79.62, 71.24, 68.77, 41.05. ESI-MS (m/z): 982.08 (M − BF4)+. Synthesis of 9 from 8. A 9 mg portion (0.016 mmol, 1 equiv) of 8, 6.8 mg (0.016 mmol, 1 equiv) of tetraphenylphosphonium bromide, and 8.5 mg (0.016 mmol, 0.53 equiv) of Pd2(dba)3·CH2Cl2 were dissolved/suspended in 4 mL of dichloromethane and stirred at room temperature for 2 h. The solution was concentrated to ca. 1.5 mL and treated with 6 mL of diethyl ether. The precipitate was collected by filtration, washed with diethyl ether (3 × 4 mL), and dried in vacuo. One-Pot Synthesis of 9. A 342 mg portion (2.00 mmol, 2 equiv) of tropylium bromide and 419 mg (1.00 mmol, 1 equiv) of tetraphenylphosphonium bromide were suspended in 10 mL of dichloromethane and cooled to 0 °C. Then 500 mg (0.500 mmol, 0.5 equiv) of Pd2(dba)3·CH2Cl2 was added and the reaction mixture was stirred at this temperature. After 30 min another 1.00 g (1.00 mmol, 1 equiv) of Pd2(dba)3·CH2Cl2 was added together with 4 mL of dichloromethane, and stirring was continued with warming to room temperature overnight. The precipitate was filtered off and extracted with 5 mL of dichloromethane. The combined filtrate and extract were concentrated to ca. 10 mL and treated with 20 mL of diethyl ether. The precipitate was collected by filtration, washed with diethyl ether (2 × 10 mL), acetonitrile (4 × 5 mL), and diethyl ether (2 × 10 mL), and dried in vacuo at elevated temperature (70−80 °C). 9: dark red solid: from 8, 91%; one pot, 82%. Anal. Calcd for C38H34Br3PPd3: C, 42.24; H, 3.17; N, 0.0. Found: C, 41.92; H, 3.18; N, 0.0. 1H NMR (400 MHz, CD2Cl2, 298 K): δ (ppm) 7.90−7.95 (m, 4H), 7.73−7.78 (m, 8H), 7.59−7.64 (m, 8H), 4.69 (s, 14H). 13C NMR (100 MHz, CD2Cl2, 298 K): δ (ppm) 136.38 (d, 4JC−P = 3.1 Hz), 134.00 (d, 3JC−P = 10.3 Hz), 131.22 (d, 2JC−P = 12.9 Hz), 118.02 (d, 1JC‑P = 89.6 Hz), 75.14. ESI-MS (m/z): 339.20 (PPh4)+, 741.18 (M − PPh4)−.

Article

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.7b00276. Detailed experimental, computational, and crystallographic data (PDF) Cartesian coordinates for the calculated structures (XYZ) Accession Codes

CCDC 1541696−1541703 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], 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 for A.P.: [email protected]. ORCID

Alexander Pöthig: 0000-0003-4663-3949 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank H. Banh, M. Bitzer, and E. Hahn for measuring ESI mass spectra and the Leibniz Rechenzentrum of the Bavarian Academy of Sciences for providing computing resources. C.J. gratefully acknowledges support from the TUM graduate school. We further thank Dr. K. Ö fele and Dr. M. Drees for helpful discussions and Prof. F. E. Kühn and Prof. W. A. Herrmann for their continued support. J.B.L. and J.R.P. thank the University of Edinburgh and the Principal’s Career Development Scholarship scheme for support.

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ABBREVIATIONS Bn, benzyl; Bu, (n-)butyl; dba, dibenzylideneacetone; cod, cycloocta-1,5-diene; CPE, controlled potential electrolysis; CV, cyclic volammetry; DFT, density functional theory; DiPP, 2,6diisopropylphenyl; DMF, dimethylformamide; DMSO, dimethyl sulfoxide; ESP, electrostatic potential; Me, methyl; Mes, mesityl (=2,4,6-trimethylphenyl); MO, molecular orbital; NBO, natural bond orbital; NHC, N-heterocyclic carbene; NMR, nuclear magnetic resonance; SWV, square-wave voltammetry; tacn, 1,4,7-triazacyclononane; THF, tetrahydrofuran

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REFERENCES

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