Article pubs.acs.org/Organometallics
Dinuclear Palladium Complexes of Pyrazole-Bridged Bis(NHC) Ligands: A Delicate Balance between Normal and Abnormal Carbene Coordination Tongxun Guo, Sebastian Dechert, and Franc Meyer* Institute of Inorganic Chemistry, Georg-August-University Göttingen, Tammannstrasse 4, 37077 Göttingen, Germany S Supporting Information *
ABSTRACT: Pyrazole-bridged bis(imidazolium) salts [H3LR](PF6)2 (R = Et, nBu, C6H2Me3-2,4,6, and tBu), which are precursors to the corresponding pyrazole-bridged bis(Nheterocyclic carbene) ligands, were found to give, upon reaction with Pd(OAc)2, dinuclear palladium complexes with either normal or abnormal binding modes of the two NHC groups: via C2 in [LR2Pd2](PF6)2 (except R = tBu) and via C4/5 in [aLR2Pd2](PF6)2. It has been shown that the course of the reaction crucially depends on the amount of NH4OAc added, suggesting an acetate-assisted pathway leading to [LR2Pd2](PF6)2. Further reaction of [LR2Pd2](PF6)2 and [aLR2Pd2](PF6)2 with PdCl2 and NEt4Cl gave the corresponding neutral dinuclear complexes LEtPd2Cl3 and aLEtPd2Cl3 selectively, without any normal/abnormal rearrangement occurring during transmetalation. Only aLtBuPd2Cl3 is accessible directly from [H4LtBu]Cl3 and Pd(OAc)2. All complexes have been characterized by NMR spectroscopy and elemental analysis, and several of them have also been characterized by ESI mass spectrometry and single-crystal X-ray diffraction. The observed binding modes and structural features have been rationalized by density functional theory calculations, which evidence that for a given complex the thermodynamically favored conformer is found in the solid state.
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INTRODUCTION N-Heterocyclic carbenes (NHCs) have evolved into one of the most prominent ligand classes in organometallic chemistry, with a multitude of applications in catalysis and material sciences.1 Among the NHCs, the imidazol-2-ylidenes, which represent the prototype of this rapidly expanding ligand class,2 are still the most prominent and most widely used representatives. Consequently imidazol-2-ylidenes have been incorporated into manifold types of chelating,3,4 binucleating,5,6 and macrocyclic7,8 ligand scaffolds and have been decorated with various substituents for tuning their electronic and steric properties.9 The versatility of imidazolium-derived carbenes has further increased by the discovery of imidazolylidenes that bind via their C4 (or C5) atom instead of the C2 atom in classical NHC complexes.10 These less-stabilized carbenes with only one N atom next to the carbene-C have been termed abnormal NHCs (aNHCs).11 While imidazol-2-ylidenes are already considered very strong σ-donor ligands, aNHCs with their partial vinylic character even have substantially enhanced σdonor power compared to the corresponding NHCs.12 A considerable number of aNHC complexes have meanwhile been reported, and systematic approaches toward their targeted synthesis have been sought.13−15 Few examples are also known where 2-fold metalation of imidazolium salts at both C2 and C4/5 has been achieved.16 However, the factors that govern the formation of either normal (C2-bound) or abnormal (C4/5© XXXX American Chemical Society
bound) imidazolylidene complexes when starting from simple imidazolium-based proligands that have both sites potentially available for metalation (i.e., without any blocking substituents at C2 and C4/5) are in most cases poorly understood. In imidazolium salts the proton at C2 is much more acidic than the C4/5 proton, and free aNHCs have much lower thermodynamic stability than NHCs.12 On the other hand bulky substituents at the N1 and N3 atoms favor aNHC formation.14,15,17 The choice of solvent and ion-pairing effects, a dependence on anions, and the possibility of anion-assisted proton transfer pathways, as well the presence of base for deprotonation of the imidazolium salt, have been identified as factors that may determine the site of metalation, with the aNHC binding usually occurring in kinetic rather than thermodynamic reaction products.11,15 We18 and others19,20 have appended two NHC units to the 3- and 5-position of a central pyrazole in order to generate compartmental hybrid ligands [LR]− (Figure 1) that provide two bidentate mixed-donor C/N binding sites for hosting in close proximity two metal ions, poised for cooperative reactivity. The palladium chemistry of ligands [LR]− has been limited to a few complexes containing two allylpalladium(II) fragments, [LRPd2(allyl)2]+.18a During the course of our studies Received: April 2, 2014
A
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Scheme 1. Reaction of [H4LtBu]Cl3 with Pd(OAc)2 to Give a tBu L Pd2Cl3 Instead of the Expected LtBuPd2Cl3 Figure 1. Binucleating monoanionic pyrazole-bridged bis(NHC) ligands.
toward the synthesis and use of dinuclear palladium(II) complexes of the type LRPd2Cl3, we experienced that complexes with abnormally bound NHC moieties were unexpectedly obtained in many cases. We thus set out to investigate in more detail the factors that determined the course of these reactions, which in the end allowed for the development of selective synthetic routes to the sought-after complexes LRPd2Cl3 and their analogues with abnormal NHC coordination, aLRPd2Cl3. The results of those studies are reported here.
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RESULTS AND DISCUSSION Pyrazole-bridged imidazolium compounds [H4LR]Cl3 and [H3LR](PF6)2 were used as proligands in this work (Figure 2). Compounds [H4LR]Cl3 and [H3LR](PF6)2 derived from four different N-substituted imidazoles were prepared according to the published procedures.18−20
selective formation of the abnormal carbene complex L Pd2Cl3 to the steric bulk of the appended tert-butyl groups that come into steric congestion with the terminal chloride ligands of the square-planar palladium(II) and thus prevent the formation of LtBuPd2Cl3 (Scheme 1). Ample precedence exists in the literature that the wingtip substituents on the imidazolium salt determine the regioselectivity of metalation, with large substituents promoting C4 bonding.11,14 In order to explore whether less bulky N-substituents would lead to the corresponding normal carbene complexes (i.e., with imidazole-2-ylidene donors), [H4LMes]Cl3 bearing mesityl groups at the wingtips was reacted with two equivalents of Pd(OAc)2 under the same conditions. Unexpectedly, however, ESI mass spectrometry suggested that a bis(ligand) complex [LMes2Pd2]2+ (or [aLMes2Pd2]2+) had been formed, irrespective of the 1:2 ligand to Pd(OAc)2 stoichiometry employed in the reaction. In situ NMR spectroscopy revealed that the NHC moieties were bound in the normal mode: the 1H NMR spectrum shows two singlets at 7.77 and 7.39 ppm, typical for H4 and H5 at the back side of the imidazol-2-ylidene rings. We thus set out to investigate in more detail the scope and the conditions of the reaction leading to such [LR2Pd2]2+/ [aLR2Pd2]2+ products. In situ monitoring of the reactions of [H3LEt](PF6)2, Pd(OAc)2 (1.0 equiv), and variable equivalents of NH4OAc by 1H NMR spectroscopy (Figure 3) indicated that the products, namely, normal or abnormal carbene complexes [LEt2Pd2](PF6)2 and [aLEt2Pd2](PF6)2, respectively, can be selectively formed depending on the amount of NH4OAc added to the reaction system. On the basis of these findings, the reactions were carried out on a preparative scale in the presence of different amounts of NH4OAc. When treated with one equivalent of Pd(OAc)2 in DMSO and heating the reaction mixture to 95 °C for 1.5 h without any NH4OAc added, all four proligands [H3LR](PF6)2 (R = Et, nBu, Mes, tBu) gave the respective bis(pyrazolato)-bridged complexes [aLR2Pd2](PF6)2 with C4/5-bound (abnormal) NHC groups in moderate yields (42−62% after workup, Scheme 2). In the case of R = tBu single crystals could be grown and the proposed molecular structure confirmed for the salt [aLtBu2Pd2](BPh4)2 that was obtained after anion exchange (Figure 4). The cationic molecule [aLtBu2Pd2]2+ has a “saddle-shaped” structure, with the tert-butyl groups oriented cis to each other with respect to the {Pd2N4} core (approximately C2v symmetry a tBu
Figure 2. Ligand precursors, viz., pyrazole-bridged bis(imidazolium) salts, used in this work.
Deprotonation of the ligand precursors, i.e., the imidazolium salts, by strong bases (e.g., potassium hexamethyldisilazane or potassium tert-butoxide) to generate the free carbene species and subsequent addition of palladium chloride is a potential strategy to synthesize the sought-after dinuclear complexes LRPd2Cl3. However, initial studies showed that deprotonation of the above precursors resulted in rapid decomposition; the presence of the pyrazole-NH, which have to be deprotonated as well, adds to the complications for the present system. Therefore, two other strategies were considered: (A) synthesis of the corresponding silver complexes by the reactions of [H3LR](PF6)2 and silver oxide, then transmetalation reactions of the silver complexes to give the corresponding palladium complexes;21 and (B) direct reaction of the proligands and Pd(OAc)2. Strategy A was previously applied for the successful synthesis of dinuclear allyl palladium complexes with this type of ligand precursor, [LRPd2(allyl)2]+.18 In the present work, strategy B was adopted for the synthesis of complexes LRPd2Cl3. Reaction of [H4LtBu]Cl3 with two equivalents of Pd(OAc)2 in DMSO at 105 °C overnight gave a yellow solid besides trace amounts of palladium black. However, instead of the expected normal carbene complex LtBuPd2Cl3, NMR spectroscopy as well as X-ray diffraction of crystalline material (see below) revealed that an abnormal (or remote) carbene complex aLtBuPd2Cl3 had been formed (Scheme 1). Its 1H NMR spectrum shows two characteristic singlets at 8.97 ppm for H2 at the back side of the imidazol-4-ylidene rings and 7.06 ppm for H5. We attribute the B
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Figure 3. In situ 1H NMR spectra of the reactions of [H3LEt](PF6)2 (1.0 equiv), Pd(OAc)2 (1.0 equiv), and variable amounts of NH4OAc (n equiv) in d6-DMSO. Reaction conditions: (a) n = 0, at 95 °C for 1.5 h; (b) n = 1, at 105 °C overnight, (c) n = 2, at 105 °C overnight; (d) n = 3, at 105 °C overnight.
Scheme 2. Syntheses of Proligands [H3LR](PF6)2 with Pd(OAc)2 Leading to [aLR2Pd2](PF6)2 (Yield 42−62%) or [LR2Pd2](PF6)2 (Yield 44−52%) Depending on the Amount of NH4OAc Added
[LRPd2(allyl)2]+ cations (2.08−2.09 Å).18a Within the imidazol4-ylidene ring the remote N−C bonds (e.g., d(N4−C7) = 1.385(6) Å) are slightly shorter than the N−C bonds involving the carbene-C (e.g., d(C6−N3) = 1.408(5) Å). 1 H NMR data for complexes [aLR2Pd2](PF6)2 showed two characteristic doublets, one at around 7.04−7.21 ppm for H4/5 next to the Pd-bound site of the imidazol-4/5-ylidene ring and one at 8.26−8.46 ppm for H2. 13C NMR resonances of the carbene-C atoms were observed at unusually high field around
of the core structure). Both palladium(II) ions are found in a roughly square-planar environment, as expected; the dihedral angle between the Pd1−C6 bond and the plane formed by C44/N12/N1/Pd1 is 84.7°. All four six-membered N,Cchelating rings are boat-shaped rather than chair-shaped. The Pd−Ccarbene bond lengths are 1.98−1.99 Å, which is in the typical range for Pd-NHC complexes but shorter than those in the reported [LRPd2(allyl)2]+ cations (2.03−2.04 Å). Pd−Npz bond lengths are 2.05−2.07 Å, again shorter than those in C
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Figure 4. Molecular structure of the cation of [aLtBu2Pd2](BPh4)2; all H atoms are omitted for clarity. Anisotropic displacement ellipsoids are drawn at the 30% probability level.
139.0−139.3 ppm, and the bulkiness of the R groups has little effect on these 13C NMR chemical shifts. The same reactions of [H3LR](PF6)2 and Pd(OAc)2 were then carried out with one equivalent of NH4OAc added, for two reasons: (i) three equivalents of base are required to fully deprotonate the proligands [H3LR](PF6)2, but only two equivalents of acetate (which may serve as an internal base) are provided by the added stoichiometric amount of Pd(OAc)2; (ii) the reaction of [H4LMes]Cl3 with two equivalents of Pd(OAc)2, as described above, took a different course and yielded [LMes2Pd2]2+ with normal NHC binding via C2; this suggested that the additional acetate might effect the course of the reaction. However, as was suggested by the results of the in situ 1H NMR monitoring described above (Figure 3), the reaction of [H3LR](PF6)2 and Pd(OAc)2 in the presence of one equivalent of NH4OAc led to a complex mixture of unidentified products, which could not be separated. Addition of two (or even more) equivalents of NH4OAc, however, cleanly yielded the bis(pyrazolato)-bridged complexes [LR2Pd2](PF6)2 (R = Et, n Bu, Mes) with all NHC groups coordinating via their C2 atoms, i.e., in the normal mode (44−52%, Scheme 2). Only in the case of bulky tert-butyl groups at the wingtip imidazole-N atoms (R = tBu) was this reaction not successful, likely because of steric congestion. Crystalline material suitable for X-ray diffraction could be obtained for R = nBu and Mes (Figure 5). Characteristic NMR data reflecting the normal NHC binding mode are two 1H NMR doublets at around 7.15−7.24 and 7.33−7.37 ppm for the H4 and H5 protons at the NHC back side as well as a 13C NMR signal in the range 154.8−161.8 ppm for the C2 carbene atom. In contrast to [aLtBu2Pd2]2+, which adopted a “saddle-shaped” structure with all R substituents on the same side of the central six-membered ring (approximately C2v; isomer B in Figure 6), in [LnBu2Pd2]2+ and [LMes2Pd2]2+ the wingtip R substituents of the two ligand strands are found on opposite sides of the central {Pd2N4} core (approximately C2h; isomer A in Figure 6). [LMes2Pd2]2+ even has crystallographic inversion symmetry. Other possible isomers that might have the two R substituents of one ligand strand on different sides of the {Pd2N4} core (two possible combinations with C2h and D2 symmetry, respectively)22 were not observed in this work. The coordination environment of the palladium ions in [LnBu2Pd2]2+ and [LMes2Pd2]2+ is distorted from square planar. For example, in
Figure 5. Molecular structure of the cations of [LnBu2Pd2](PF6)2 (top) and [LMes2Pd2](PF6)2 (bottom); all H atoms are omitted for clarity. Anisotropic displacement ellipsoids are drawn at the 30% probability level. Symmetry operation used to generate equivalent atoms: 1−x, 1− y, 1−z.
Figure 6. Different stereoisomers of approximate (noncrystallographic) C2h and C2v symmetry observed for the [LR2Pd2]2+ and [aLR2Pd2]2+ platforms, respectively; the balls represent the R substituents.
[LnBu2Pd2]2+ the dihedral angle between the Pd1−C5 bond and the plane formed by C25/N11/N1/Pd1 is 72.1°. Pd−Ccarbene bond lengths are found in the narrow range 1.97−1.98 Å, slightly shorter than in the case of [aLtBu2Pd2]2+. The dramatic effect of added NH4OAc on the course of the above reactions and on the resulting NHC binding mode is intriguing. Despite the recent interest in abnormal and remote NHC ligands, factors that lead to selective metalation of imidazolium salts at either the C2 or C4/5 positions are still not well understood. In the absence of protecting groups at the C2 position, abnormal NHC binding is often seen as a kinetic rather than thermodynamic reaction product.11,15 Since the acidity of the C4−H bond in imidazolium salts is much lower than the acidity of the C2−H bond,23 it has been proposed that distinctly different reaction trajectories lead to either C2- or C4-bonding. In the case of iridium or osmium polyhydride species the type of imidazolium counteranion was found to have a significant effect, favoring either heterolytic C2−H bond cleavage or C4−H oxidative addition.10,24 However, these correlations do not hold for reactions of imidazolium salts with group 10 metal compounds such as Pd(OAc)2. Since the same D
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PF6− counteranion has been used in the syntheses of [aLR2Pd2](PF6)2 and [LR2Pd2](PF6)2 according to Scheme 2 and since oxidative addition pathways seem unlikely when using Pd(OAc)2 either with or without additional NH4OAc, the above considerations cannot account for the observed regioselectivity. Interestingly, for the metalation of N,N′dimesitylimidazolium chloride (IMes·HCl) with Pd(OAc)2 it was reported that in the presence of Cs2CO3 as a base the C2bound complex trans-Pd(IMes)2Cl2 is selectively formed, while in the absence of Cs2CO3 one of the NHC ligands binds abnormally via C4.25 It has been proposed that the CO32− anion promotes heterolysis of the most acidic C−H bond, hence favoring of the normal C2-bound species. A similar scenario may be operative in the present case. We assume that the pyrazole-NH, after formation of acetic acid, is the initial anchoring site for palladium. Involvement of additional acetate then triggers acetate-assisted26,27 deprotonation at the adjacent C2. Plausible scenarios for the reaction mechanism are proposed in Scheme 3; one should note though that
H−C4/5 (B1) because it is less shielded by the wingtip R substituent than H−C2; this then leads to the abnormal NHC intermediate C1 and finally to [aLR2Pd2](PF6)2. If the reaction is carried out with additional acetate (two equivalents), however, because the H−C2 is much more acidic than the H−C4/5, the exterior base forms strong H-bonding with the H−C2, eventually leading to activation of the H−C2 (B2) and the formation of normal NHC intermediate C2, which in the end gives [LR2Pd2](PF6)2. Selective formation of either [LR2Pd2](PF6)2 or [aLR2Pd2](PF6)2 has subsequently allowed for the controlled synthesis of isomeric LRPd2Cl3 and aLRPd2Cl3 complexes with either normal (LRPd2Cl3) or abnormal (aLRPd2Cl3) NHC coordination according to Scheme 4. Except for aLtBuPd2Cl3 (see above) Scheme 4. Synthesis of aLEtPd2Cl3 (yield 81%) and LEtPd2Cl3 (yield 55%) from [aLEt2Pd2](PF6)2 and [LEt2Pd2](PF6)2
Scheme 3. Tentative Scenario for the Selective Formation of [aLEt2Pd2](PF6)2 or [LEt2Pd2](PF6)2 Mediated by Different Amounts of NH4OAc Used in the Reactionsa
a
these complexes with 1:2 ligand to metal ratio, which had been the initial targets of this study, were not accessible directly from [H4LtBu]Cl3 and Pd(OAc)2.28 As a representative example, reaction of [LEt2Pd2](PF6)2 or [aLEt2Pd2](PF6)2 with NEt4Cl and PdCl2 gave the corresponding neutral dinuclear complexes LEtPd2Cl3 (yield 55%) and aLEtPd2Cl3 (yield 81%), respectively (Scheme 4). Both LEtPd2Cl3 and aLEtPd2Cl3 are poorly soluble in common solvents such as dichloromethane, acetonitrile, or acetone and just slightly soluble in DMSO. They have been crystallized by slow diffusion of acetonitrile into DMSO solutions of the crude products, and their molecular structures have been elucidated by X-ray diffractometry (see below). It is particularly noteworthy that the NHC binding mode, either normal via C2 or abnormal via C4/5, is fully retained in these reactions, even though the cleavage of some Pd−Ccarbene bonds and formation of new Pd−Ccarbene bonds obviously have to occur upon transmetalation of one of the ligand strands. We conclude that free carbenes (normal or abnormal) are mechanistically not relevant, but that ligand transfer proceeds via a more complex sequence involving oligometallic intermediates. LEtPd2Cl3 and aLEtPd2Cl3 are spectroscopically well distinguished by characteristic 1H NMR signals at 7.73 (d, J = 1.9 Hz, 2H, H4/5)/7.62 (d, J = 1.9 Hz, 2H, H4/5) ppm for LEtPd2Cl3 and 8.88 (d, J = 1.7 Hz, 2H, H2)/7.02 (d, J = 1.7 Hz, 2H, H5) ppm for aLEtPd2Cl3 as well as by their 13C NMR data showing the carbene signals at 153.1 and 132.8 ppm, respectively. Molecular structures of LEtPd2Cl3 and aLEtPd2Cl3 as well as a tBu L Pd2Cl3 (prepared according to Scheme 1) are shown in Figures 7−9. Interestingly, two crystallographically independent
solv = solvent molecule (DMSO).
mechanistic considerations remain purely speculative in the absence of any direct spectroscopic insight or any density functional theory (DFT) calculations of energy profiles of the proposed trajectories. According to the scenario proposed in Scheme 3, treatment of [H3LR](PF6)2 with Pd(OAc)2 initially leads to the formation of intermediate A, with the “Pd(OAc)” fragment bound to one of the pyrazole-N’s. If the reaction is carried out without exterior base, the palladium-bound acetate prefers to attack the E
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LEtPd2Cl3, 2.38/2.38 Å in conformer C of aLEtPd2Cl3, and 2.41/ 2.41 Å in conformer D of aLEtPd2Cl3. This reflects that the effect of conformational strain predominates over any possible differences in trans influence of the normal versus abnormal NHC; hence no conclusion about the relative trans influence of the [LR]− and [aLR]− ligands can be derived from the structural data. Anyway, several recent studies have shown that bonds trans to NHC or aNHC donors in closely related palladium(II) complexes do not show any significant differences.15,29 As in the case of the bis(ligand) complexes, distinct conformers that differ by the relative position of the R substituents with respect to the {Pd2N2} core are observed also for complexes LRPd2Cl3 and aLRPd2Cl3 (Figure 10). While
Figure 7. Molecular structure of LEtPd2Cl3; all H atoms are omitted for clarity. Anisotropic displacement ellipsoids are drawn at the 30% probability level.
Figure 10. Different conformers of idealized (noncrystallographic) Cs and C2 symmetry observed for the LRPd2Cl3 and aLRPd2Cl3 complexes, respectively; the balls represent the R substituents.
LEtPd2Cl3 and aLtBuPd2Cl3 adopt conformation D with approximate (noncrystalllographic) Cs symmetry of the core, one crystallographically independent molecule of aLEtPd2Cl3 (Figure 8) is found in conformation C and the other in conformation D. This suggests that both conformers C and D are close in energy, which has been confirmed by DFT calculations (see below). Pd···Pd distances of the two crystallographically independent molecules of aLEtPd2Cl3 are significantly different, namely, 3.49 Å in conformer D versus 3.75 Å in conformer C. In the case of conformer D the coordination planes of the two palladium ions are severely tilted with respect to each other, and the bridging chloride atom is located high above the plane of the pyrazole moiety (0.78 Å in D vs 0.15 Å in C). X-ray crystallographic characterization, in the present work, of an extensive series of closely related complexes with the same type of ligand scaffold in both normal and abnormal NHC binding modes suggests some comparison of the effect of the different binding mode on the metric parameters of the NHC moieties. C−N distances in the C−C−N parts of the imidazolylidene rings are always somewhat longer than in the N−C−N parts, and this is even more pronounced in the case of the abnormal (C4/5-bound) coordination. A slight elongation of the C−N distances of the N−C−N part is also observed upon coordination in the normal (C2-bound) NHC mode. A more pronounced difference can be observed for the N−C−N angles in the normal and abnormal coordination modes: in the normal mode the angle lies around 105°, while in the abnormal mode it is around 108°. The corresponding N−C−C angle in the abnormal coordination mode of the NHC is about 104°, but it is 106° to 107° in the normal coordination mode. In summary the coordination to palladium makes the N−C−N or N−C−C angle slightly more acute at the coordinating carbeneC atom. In order to assess the relative thermodynamic stabilities of the normal versus abnormal NHC binding modes, i.e., the [LR2Pd2]2+ versus [aLR2Pd2]2+ and the LRPd2Cl3 versus a R L Pd2Cl3 type of complexes, DFT calculations have been
Figure 8. Molecular structures of the two crystallographically independent molecules of aLEtPd2Cl3; all H atoms are omitted for clarity. Anisotropic displacement ellipsoids are drawn at the 30% probability level.
Figure 9. Molecular structure of aLtBuPd2Cl3; all H atoms are omitted for clarity. Anisotropic displacement ellipsoids are drawn at the 30% probability level.
molecules were present in the crystal lattice of aLEtPd2Cl3 that adopt significantly different conformations (C and D); this will be discussed in more detail below. For crystallographic details and selected bond lengths and angles see the Supporting Information. In this series of complexes LEtPd2Cl3, aLEtPd2Cl3, and a tBu L Pd2Cl3 the normal or abnormal coordination mode of the carbene moiety has only a minor influence on the Pd−C and Pd−N bond lengths, if at all (Table S16, SI). However, in the case of the LRPd2Cl3 and aLRPd2Cl3 complexes the Pd−N bond lengths are clearly shorter than in the [LR2Pd2]2+ and [aLR2Pd2]2+ cations because the trans influence of (normal and abnormal) NHC ligands is much stronger than of chloride. A related trend can be observed for the Pd···Pd distances. Pd−Cl distances involving the bridging chloride Cl2 are 2.37/2.38 Å in F
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Figure 11. Different views (top and bottom) of the calculated [LMe2Pd2]2+ and [aLMe2Pd2]2+ structures. Color code: Pd = red, N = blue, C = black.
Figure 12. Different views (top and bottom) of the calculated [LMePd2Cl3] and [aLMeLPd2Cl3] structures. Color code: Pd = red, Cl = green, N = blue, C = black.
the case of the cationic bis(ligand) complexes [LMe2Pd2]2+ and [aLMe2Pd2]2+ the Pd atoms are almost located within the plane with a long Pd···Pd distance in all A conformers, while the Pd··· Pd distance is shorter and the Pd atoms are located outside the plane in all B conformers. In line with this the shortest Pd···Pd distance has been found experimentally for the only B conformer [aLtBu2Pd2](BPh4)2 (3.82 Å). Likely due to the steric influence of the N-bound wingtip R substituents, the distance of the metal atom to the plane of its coordinating atoms is less well reproduced in the DFT calculations (in which R = Me was used). In the case of the neutral complexes LMePd2Cl3 and aLMePd2Cl3 all calculated Pd···Pd distances are slightly longer than found by X-ray crystallography, but the trends are well reproduced; Pd···Pd distances of conformers D are significantly shorter than for conformers C. When comparing the normal and abnormal binding modes for all individual conformers, in general the normal NHC complexes are thermodynamically favored except for the B conformers of bis(ligand) complexes [L Me 2 Pd 2 ] 2+ and [aLMe2Pd2]2+, where the B conformer of the latter is slightly favored by 1 kcal/mol over the B conformer of the former (likely within the error limits of the DFT calculations). One should note that steric effects caused by the wingtip R substituents are significantly more relevant for the complexes featuring normal C2-bound carbenes, since in all abnormally C4/5-bound complexes the R groups are pointing to the back side. In fact, steric repulsion between R groups of the two opposing ligand strands will be most relevant for B[LMe2Pd2]2+, which explains that this species is disfavored over its abnormally bound isomer.
performed. Geometry optimization and calculation of the single-point energies have been carried out with the ORCA program package30 at the BP86/RI/TZVPP level of theory. To largely exclude effects from steric congestion, the ligands [LMe]− and [aLMe]−, respectively, have been used in the calculated models.31 Both the C2h (A) and C2v (B) conformers (idealized geometry) in the case of [LMe2Pd2]2+ versus [aLMe2Pd2]2+ as well as the C2 (C) and Cs (D) conformers (idealized geometry) in the case of LMePd2Cl3 versus a Me L Pd2Cl3 have been considered. DFT-calculated structures are shown in Figures 11 and 12. Selected bond lengths and angles are compiled in Tables S17 and S18. Relative energies are summarized in Table 1. Table 1. Relative Energies Obtained by Single-Point Calculations Erel [kcal/mol] A-[LMe2Pd2]2+ B-[LMe2Pd2]2+ A-[aLMe2Pd2]2+ B-[aLMe2Pd2]2+
Erel [kcal/mol] 0 +3.9 +12.7 +2.9
Me
C-[L Pd2Cl3] D-[LMePd2Cl3] C-[aLMePd2Cl3] D-[aLMePd2Cl3]
+5.0 0 +22.0 +20.4
Metric parameters of the calculated molecular structures are in good agreement with experimental ones determined by single-crystal X-ray diffraction. In all cases the Pd−Ccarbene distances of the calculated structures are slightly longer, while the N−C(−Pd)−N and C−C(−Pd)−N or N−C−N and C− C−N angles are reproduced well. The Pd···Pd distances as well as the distance of the Pd atom from the least-squares plane of its coordinating atoms depend strongly on the conformation. In G
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in cooperative bimetallic catalysis; experiments in that direction are planned.
It should also be noted that steric effects will become more important in the case of larger R substituents, and hence the energetic ordering of the various isomers may well reverse in the case of the experimentally studied complexes with R = Et, n Bu, Mes, tBu. For individual types of complexes with R = Me, either [LMe2Pd2]2+ or [aLMe2Pd2]2+, differences in energy between conformers A and B or between conformers C and D, respectively, are rather small (≤5 kcal/mol), except for [aLMe2Pd2]2+ (9.8 kcal/mol). In essentially all cases, the thermodynamically favored isomers were indeed found experimentally in the solid state: A-[L nBu 2 Pd 2 ] 2+ , A[L Et 2 Pd 2 ] 2+ , and A-[L Mes 2 Pd 2 ] 2+ ; B-[ a L tBu 2 Pd 2 ] 2+ ; D[LEtPd2Cl3]; and D-[aLtBuPd2Cl3]. In the unique case of [aLEtPd2Cl3] the cocrystallization of C and D conformers reflects their small energetic difference (
