Monomeric Ferrocene Bis-Imidazoline Bis-Palladacycles: Variation of

Article pubs.acs.org/Organometallics

Monomeric Ferrocene Bis-Imidazoline Bis-Palladacycles: Variation of Pd−Pd Distances by an Interplay of Metallophilic, Dispersive, and Coulombic Interactions Manuel Weber, Johannes E. M. N. Klein, Burkhard Miehlich,* Wolfgang Frey, and René Peters* Institut für Organische Chemie, Universität Stuttgart, Pfaffenwaldring 55, D-70569 Stuttgart, Germany S Supporting Information *

ABSTRACT: Monomeric ferrocene bis-imidazoline bis-palladacycles (FBIP) have recently been reported to be efficient bimetallic catalysts in different sorts of asymmetric reactions by the cooperation of two Pd(II) centers. A crucial parameter regarding the efficiency of reactions catalyzed in a bimetallic mode isin generalthe intermetallic distance of both catalytically relevant metal centers. In this article we describe the structural elucidation of the monomeric FBIP catalyst type (usually generated in situ from a catalytically inactive dimeric chloride bridged precatalyst) by X-ray crystal structure analysis. Two dicationic monomeric complexes are compared to a neutral complex. The solid-state structures reveal varying Pd− Pd distances ranging from 3.15 to 5.27 Å for the doubly charged complexes, whereas for the neutral complex a distance of 3.28 Å has been found. This variability is supposed to be one of the key advantages of a ferrocene backbone in a bimetallic catalyst system, since the Fe−Cp bonds allow the bimetallic complex to readily open and close like a pair of scissors, employing just a few degrees of rotational freedom. In addition, on the basis of the nature of the reported catalyst species, we suggest that a permanent switch among neutral and mono- and dicationic catalyst species by a Brønsted acid such as acetic acid might facilitate different elementary steps in a catalytic cycle. By DFT calculations we have found that weak d8−d8 interactions contribute to short Pd−Pd distances but are less important than dispersive interactions, which can even overcome the Coulombic repulsion of two cationic palladium centers.

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INTRODUCTION Recently, we have reported the first catalytic enantio- and diastereoselective 1,4-additions of azlactones 3 to enones 4, providing a rapid and divergent access to quaternary amino acid derivatives.1 In these studies isolated and in situ generated azlactones 3 (formed in situ from unprotected or Nbenzoylated amino acids 1 or 2) were almost equally effective for the conjugate addition (Scheme 1).2 For reactions proceeding via 2-Ph-substituted azlactones 3, a bis-Pd catalyst was found to be necessary for high product yields and enantioselectivity, pointing to a preferred bimetallic cooperative activation mode,3,4 in which both the enone substrate and the azlactone pronucleophile are activated by the same catalyst.1c,e For the related 1,4-addition of α-cyanoacetates to enones using the same precatalyst, extensive kinetic and spectroscopic studies have recently confirmed a mechanism, which operates by cooperation of both Pd(II) centers.5 The catalyst precursor [FBIP-Cl]2 is in both cases a dimeric chloride-bridged ferrocene bis-imdazoline bis-palladacycle.6−8 To achieve catalytic activity, precatalyst activation by a silver salt such as silver trifluoromethanesulfonate in combination with MeCN is crucial to remove the chloride bridges and to generate monomeric complexes in order to facilitate the substrate coordination. © 2013 American Chemical Society

Herein we report the formation and structural analysis of two dicationic monomeric complexes of the FBIP type, which is based on X-ray crystal structure analysis and density functional theory (DFT) calculations, and compare these structures with that of a neutral FBIP complex. We have found that the preferred conformations of the different complexes and thus the preferred intermetallic Pd−Pd distances are caused by an interplay of Coulombic, intermetallic, and dispersive interactions. Furthermore, we suggest that the temporary generation of mono- and bis-cationic FBIP catalyst species similar to those described might have a positive impact on the activity in bimetal-catalyzed reactions such as the aforementioned 1,4additions.

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RESULTS AND DISCUSSION Structure Elucidations and Theoretical Explanations. Treatment of [FBIP-Cl]2 with silver triflate (AgOTf, 4.0 equiv per dimeric precatalyst molecule) was performed in acetonitrile at room temperature to achieve a chloride ligand exchange. Special Issue: Ferrocene - Beauty and Function Received: April 25, 2013 Published: May 28, 2013 5810

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Scheme 1. Catalytic Asymmetric 1,4-Addition of (in Situ Generated) Azlactones to Enones

Scheme 2. Synthesis of Dicationic Bis-Palladacycles 6 and 7

Figure 1. X-ray single crystal structure analysis of 6. Color code: C (gray); N (blue); O (red); S (yellow); Fe (orange); Pd (magenta). Hydrogen atoms, both triflate counterions, and ethyl acetate (one per unit cell) are omitted for clarity. Two different views are shown (ORTEP plots, ellipsoids at 50% probability level): (left) view nearly perpendicular to the ferrocene axis; (right) view along the ferrocene axis.13 Selected bond lengths (Å) and angles (deg) for the upper half in the left view: C−Pd, 1.947(2); N′imidazoline−Pd, 2.010(2); NMeCN−Pd, 2.003(2); O−Pd, 2.183(2); C−Pd−N′, 80.20(7); C−Pd−N, 91.87(7); N−Pd−O, 96.32(7); N′−Pd−O, 91.56(6); N−Pd−N′, 172.05(7); C−Pd−O, 170.94(7). Selected bond lengths (Å) and angles (deg) for the bottom half in the left view: C−Pd, 1.949(2); N′imidazoline−Pd, 2.019(2); NMeCN−Pd, 1.997(2); O−Pd, 2.156(1); C−Pd−N′, 80.44(7); C−Pd−N, 94.42(7); N−Pd−O, 89.62(6); N′−Pd−O, 95.53(6); N−Pd−N′, 171.85(6); C−Pd−O, 175.82(7).

distorted square planar coordination geometry.10,11 Per bispalladacycle two triflate counterions were observed (not depicted in Figure 1), which do not coordinate to the Pd centers but form hydrogen bonds to the H2O ligands. In the solid state the ferrocene unit adopts a nearly eclipsed conformation of both Cp ligands.12 Both imidazoline moieties are in a pseudo-antiperiplanar arrangement. This results in a relatively large Pd−Pd distance of ca. 5.2698(3) Å and a torsion angle of 86.4(1)° for the C−Pd bonds. It is noteworthy that both tosyl moieties unexpectedly point in the same direction in the solid state. A different bis-cationic complex 7 carrying two acetic acid ligands was obtained after chloride ligand exchange and crystallization from acetic acid (Figure 2). Again both Pd

Crystals were grown from (wet) EtOAc and AcOH, respectively (Scheme 2). The X-ray crystal structure analyses confirmed in both cases that monomeric complexes were formed in which acetonitrile is coordinated at the trans-positions to the imidazoline donors (Figures 1 and 2).9 Such a coordination geometry was previously suggested by us based on NOE studies for monomeric FBIP complexes bearing aromatic sulfonate counterions.6b Since the Lewis-basicity of the triflate counterions is relatively poor, neutral ligands bind to the cis-positions in the presented cases. As Figure 1 shows, H2O is coordinated after crystallization from wet ethyl acetate, resulting in the monomeric bis-cationic bis-palladacycle 6, in which both Pd centers adopt a slightly 5811

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Figure 2. X-ray single crystal structure analysis of 7. Color code: C (gray); N (blue); O (red); S (yellow); Fe (orange); Pd (magenta). Hydrogen atoms, both triflate counterions, and acetic acid (two molecules per unit cell) are omitted for clarity. Two different views are shown (ORTEP plots, ellipsoids at 25% probability level): (left) view nearly perpendicular to the ferrocene axis; (right) view along the ferrocene axis. Selected bond lengths (Å) and angles (deg): C−Pd, 1.950 (8); N′imidazoline−Pd, 2.036(7); NMeCN−Pd, 2.003(9); O−Pd, 2.121(7); C−Pd−N′, 80.5(3); CO, 1.075(15); C−O, 1.477(10); C−Pd−N, 95.3(3); N−Pd−O, 90.1(3); N′−Pd−O, 94.5(3); N−Pd−N′, 172.9(3); C−Pd−O, 173.5(3).

dispersion correction D3.19 The structural parameters are in good agreement in this case with the rather short distance determined by X-ray structure analysis. The previous overestimation of the Pd−Pd distances and torsion angles on applying the other methods are thus due to a poor description of dispersive effects by DFT, which has been reported in related systems.20,15n Analysis of the MOs allowed us to locate HOMO-21, which possesses a moderate bonding character. In addition, we also located HOMO-3, which exhibits antibonding character and is typical for such bimetallic complexes (Figure 3).

centers possess a slightly distorted square planar coordination geometry. An intramolecular distance of both cationic Pd centers of 3.160(1) Å was found. On the basis of the van der Waals radii the interatomic Pd−Pd distance would be expected to be ≥3.26 Å, if there is no bonding character between both Pd centers.14 Like 6, 7 also prefers a nearly eclipsed conformation of the ferrocene fragment in the solid state, resulting in a torsion angle of 21.5(4)° between the corresponding C−Pd bonds in the Xray structure. The intermetallic Pd−Pd distance is considerably shorter than the distance of the Cp rings, which has been determined to be ca. 3.34(1) Å between the two C atoms carrying the Pd centers.14 This means that the C−Pd bonds are significantly bent toward each other. We assumed that d8−d8 interactions may be the reason for this effect. This kind of intermetallic interaction has been suggested for many metals, resulting in metal−metal distances below the sum of the van der Waals radii.15 As the Pd−Pd distance is only slightly below the sum of the van der Waals radii, judgment of the origin purely based on the structural parameters obtained from the X-ray crystallographic data would be speculative. For that reason, we investigated this matter further using DFT calculations. The structure of 7 was fully optimized using density functional theory (DFT). The TPSS16 functional in combination with the split valence basis set def2SVP17 by Ahlrichs was chosen using an effective core potential (ECP) on Pd, replacing 28 inner electrons.18 Initial attempts to optimize the above structure in the gas phase resulted in severely overestimated Pd−Pd distances and Pd−C−C−Pd torsion angles. In addition, other combinations of functionals and basis sets were similarly unsuccessful. A Pd−Pd distance of 3.097 Å and a Pd−C−C−Pd torsion angle of 22.1° was obtained when we included Grimme’s third-generation

Figure 3. Characteristic antibonding and bonding MOs for 7. Hydrogen atoms are omitted for clarity.

Mayer and Löwdin Pd−Pd bond orders were computed to be 0.10 and 0.25, respectively. Topological analysis of the density following Bader’s “atoms in molecules” 21 was performed using the XAIM22 software. A (3, −1) bond critical point could be identified in 7 with an ellipticity of 0.049, suggesting a bonding interaction of the cationic Pd centers in agreement with σ bonding. However, only a very low density of ρ = 0.021 was determined, reflected in the low Mayer/Löwdin bond orders and ultimately with a weakly bonding interaction. 5812

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This evidence suggests that the d8−d8 interactions play a minor role in the short Pd−Pd distance. In support of this is the calculation that the removal of phenyl groups attached to the imidazoline rings in the ligand would theoretically lead to a significantly larger preferred Pd− Pd distance and torsion angle (4.523 Å and −44.7°; see the Supporting Information for details) due to rotation along the ferrocene axis. In contrast, the N-tosyl groups in 7 proved to be less important in the calculations and a truncated complex showed a Pd−Pd distance of 3.205 Å and a Pd−C−C−Pd torsion angle of 27.1° (for structure calculations, see the Supporting Information). Short intermetallic distances in d8−d8 complexes have also been suggested to rely on dispersive effects rather than on strong metal−metal interactions.23 The fact that omission of Grimme’s dispersion correction leads to significantly larger calculated Pd−Pd distances, the calculated low d8−d8 contribution, and the theoretical results from the truncated structures all support that the origin of the observed short distance is in the presented case mainly due to the properties of the ligand system.23 It is noteworthy that the sum of the dispersive effects and the weak d8−d8 interaction can overcome the Coulomb repulsion expected from two cationic metal centers. For comparison with a related neutral bis-palladacycle, we have also prepared the corresponding monomeric acac complex FBIP-acac (Scheme 3).

In order to understand the significantly larger Pd−Pd distance in complex 6 possessing two H2O ligands, we have also studied this system by DFT. Geometry optimization starting from the X-ray structure leads to an equilibrium structure with a larger Pd−Pd distance of 4.525 Å and a Pd− C−C−Pd torsion angle of 73.0° (for details see the Supporting Information). The tendency is thus in agreement with the X-ray structure. In both the X-ray and computed structures, one tosyl group faces in the direction of the ferrocene unit. Usually the FBIP systems prefer a conformation in which the N-sulfonyl groups both point away from the ferrocene core.6 Rotation of the tosyl group, which points toward the ferrocene core in 6, results in the identical sulfonyl group arrangement (trans) as in complexes 7 and FBIP-acac. Geometry optimization of this modified structure leads to an equilibrium structure featuring a short Pd−Pd distance of 3.048 Å and a Pd−C−C−Pd torsion angle of 9.1°. In this structure an intramolecular hydrogen bond between the H2O ligands is present. It was found that the aquo complex 6 is ca. ΔE = 27 kJ/mol more stable when adopting a trans arrangement of the tosyl groups, which results in a short Pd−Pd distance and a narrow torsion angle. We therefore assume that the large Pd−C−C−Pd torsion angle and Pd−Pd distance in the X-ray structure of 6 originate from crystalpacking effects alongside one untypically arranged tosyl group. Nevertheless, it is a good indication that there is still a high level of mobility regarding the Pd−C−C−Pd torsion angles, which results in a facile variation of the Pd−Pd distances and should be important for bimetal-catalyzed processes. We have attempted to identify the most significant dispersive contributions in this system resulting in narrow torsion angles and short Pd−Pd distances. Most contacts of the aromatic moieties are within the top or bottom half and do not directly affect the torsion angle. A pronounced interaction between the Ph rings (pointing toward the ferrocene core and highlighted by red color) and MeCN (6 (closed conformation) and 7) or acac (FBIP-acac) is present in these complexes, contributing to a narrow torsion angle and a short Pd−Pd distance. Contributions were quantified via fragment-based analysis using Grimme’s dftd3-program V3.0 Rev 0 (Table 1).19 The total energies from dispersive effects, interactions between the top (green) and bottom (blue) fragments, and interactions between Ph and L (red) are listed. For structures possessing narrow torsion angles, dispersive interactions between the top and bottom fragments have been determined to be in the range of −18 to −23 kJ/mol (entries 2−4). Within the top to bottom interactions the major contributions are provided by the Ph/L interactions (marked in red) which are in the range of −5.2 to −6.2 kJ/mol. Mechanistic Considerations Regarding the Possible Roles of Acetic Acid/NaOAc in FBIP-Catalyzed 1,4Addition Reactions. We have previously found that the use of acetic acid as solvent is essential for high efficiency in terms of turnover and enantioselectivity for the bis-palladacyclecatalyzed 1,4-addition of azlactones to enones.1 Moreover, we have found that NaOAc as an additive is necessary for high product yields and has a positive impact on the enantioselectivity. We have speculated that NaOAc could increase the reactivity by reversible deprotonation of an enol species to generate a more potent nucleophile. On the other hand, regarding the ready formation of dicationic FBIP complexes, the acetic acid/NaOAc buffer system might also allow for a permanent switch of the charge of the bis-Pd catalyst. By that switch, different elementary steps of the catalytic cycle would be

Scheme 3. Synthesis of the Neutral Monomeric BisPalladacycle FBIP-acac

The X-ray crystal structure analysis of FBIP-acac has revealed an eclipsed conformation very similar to that found for 7 with an identical torsion angle of 21.5(2)° between the corresponding C−Pd bonds (Figure 4). In comparison to 7 the Pd−Pd distance is 3.2779(5) Å and is thus slightly larger than the sum of the van der Waals radii.24 Again, the C−Pd bonds are bent toward each other, as the C atoms directly connected to the Pd centers have a distance of 3.370(6) Å. As expected, both Pd centers adopt a slightly distorted square planar geometry. This structure was also computed using DFT and is in similarly good agreement with the X-ray structure. A Pd−Pd distance of 3.114 Å and a Pd−C−C−Pd torsion angle of 13.4° were calculated, which underestimates the structural parameters by 0.17 Å and 8.7° in comparison with the X-ray data obtained. Mayer and Löwdin Pd−Pd bond orders were computed, giving 0.21 and 0.27, respectively. This again suggests a minor contribution from a d8−d8 interaction. In addition, a (3, −1) bond critical point was identified with an ellipticity of 0.061 and a density of ρ = 0.021. Careful inspection of the molecular orbitals of FBIP-acac allowed us to identify both bonding (HOMO-8) and antibonding (HOMO-1) orbitals with regard to the Pd−Pd interaction (Figure 5). 5813

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Figure 4. X-ray single crystal structure analysis of FBIP-acac. Color code: C (gray); N (blue); O (red); S (yellow); Fe (orange); Pd (magenta). Hydrogen atoms and n-pentane (1 molecule per unit cell) are omitted for clarity. Two different views are shown (ORTEP plots, ellipsoids at 50% probability level): (left) view nearly perpendicular to the ferrocene axis; (right) view along the ferrocene axis. Selected bond lengths (Å) and angles (deg): C−Pd, 1.964(4); N−Pd, 1.994(4); Ocis to N−Pd, 2.049(3); Otrans to N−Pd, 2.009(3); C−Pd−N, 80.65(16); C−Pd−Otrans to N, 94.05(16); N− Pd−Ocis to N, 91.31(13); O−Pd−O, 93.90(12); N−Pd−Otrans to N, 173.14(13); C−Pd−Ocis to N, 171.92(16).

accelerated. One possible role of acetic acid could thus be to (reversibly) generate mono- and dicationic catalyst species/ intermediates with an enhanced Lewis acidity, which should result in a more efficient activation of the enone electrophile. The temporary generation of a (di)cationic catalyst would also facilitate the enolization of the azlactone pronucleophile in the anticipated bimetallic activation mode (Scheme 4). In contrast, the enol nucleophilicity would be expected to be higher for a neutral Pd center generated by NaOAc.

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Figure 5. Characteristic antibonding and bonding MOs for FBIP-acac. Hydrogen atoms are omitted for clarity.

Table 1. Analysis of the Dispersive Interactions in FBIP Complexes

CONCLUSION

The structurally diverse complexes obtained show that the ferrocene ligand serves as a backbone which in solution should allow the bimetallic complex to readily open and close like a Scheme 4. Proposed Bimetallic Activation of 2-Ph-azlactones and Enones in the FBIP-Catalyzed 1,4-Additiona

entry

complex

L

total dispersive energy (kJ/mol)

1 2 3 4

6 (open) 6 (closed) 7 FBIP-acac

MeCN MeCN MeCN acac

−129 −133 −145 −148

a

interaction of the “blue” and “green” fragments (kJ/mol)

contribution from Ph−L interactionsa (kJ/mol)

−9 −18 −18 −23

−6.2 −6.0 −5.2b

a In comparison to neutral Pd centers, cationic Pd centers in 8 should (1) facilitate the azlactone enolization and (2) withdraw more electron density from the enone electrophile, resulting in a more reactive Michael acceptor. Formation of the monocationic bis-palladacycle 9 by NaOAc would increase the nucleophilicity of the enol.

b

Indicated by the red arrows. The C(O)Me moiety facing toward the Ph ring fragment was considered for the fragment-based analysis.

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21.6, 3.0. 19F NMR (CDCl3, 235 MHz): δ −78.0. IR (solid): ν 3374, 2931, 2377, 2292, 2034, 1731, 1594, 1551, 1494, 1466, 1453, 1365, 1324, 1222, 1168, 1098, 1084, 1049, 1026, 989, 971, 887, 868, 819, 758, 716, 700, 671, 636, 597. MS (ESI): m/z 1295.0 (M − 2 AcOH − 2 MeCN − OTf; 100%), 1191.0 (15%), 1147.0 (30%). HRMS (ESI): m/z calcd for [M − 2 H2O − 2 MeCN − OTf]+ (C55H44F3FeN4O7Pd2S3) 1294.9747, found 1294.9773. Anal. Calcd for [FBIP-H2O*OTf + EtOAc]: C, 46.58; H, 3.79; N, 5.09. Found, C, 46.30; H, 3.86; N, 5.05. Bis{[cis-(acetic acid-κO1)(acetonitrile-κN1)][η5-(4R,5R)-(Sp)(2−4,5-dihydro-4,5-diphenyl-1-tosyl-1H-imidazoyl-κN 7 )cyclopentadienyl-κC1]palladium(II)}iron(II) Bis(trifluoromethanesulfonate) (FBIP-HOAc*OTf, 7). [FBIP-Cl]2 (18.4 μmol, 44.7 mg) was monomerized using silver triflate (73.3 μmol, 18.8 mg) according to the general procedure. The obtained material was dissolved in AcOH (1200 μL) and was stirred for 18 h at room temperature. All volatiles were then removed by rotatory evaporation, and the solid material was dried under reduced pressure to deliver FBIP-HOAc*OTf (7) as a red crystalline solid (36.6 μmol, 60.3 mg, 99%). Crystals suitable for X-ray analysis formed when the reaction mixture was not stirred.9 Molecular weight for C64H58F6FeN6O14Pd2S4: 1646.11. Mp: >250 °C. [α]D23 = −215.4° (c = 0.013 g dL−1, CH2Cl2). 1H NMR (CDCl3, 300 MHz): δ 7.63−7.45 (m, 10 H), 7.44−7.35 (m, 4 H), 7.25−7.19 (m, 6 H), 7.17−7.09 (m, 4 H), 6.77−6.18 (m, 4 H), 5.54 (b, 2 H), 5.26 (d, J = 2.7, 2 H), 5.14 (d, J = 4.0, 2 H), 5.00 (d, J = 3.6, 2 H), 4.49 (b, 2 H), 2.44 (s, 6 H), 2.42 (s, 6 H). 13C NMR (CDCl3, 125 MHz): δ 170.2, 145.9, 139.4, 139.2, 134.3, 130.3, 129.50, 129.48, 128.8, 128.2, 127.3, 127.1, 125.6, 122.2, 95.9, 77.9, 74.5, 73.7, 72.6, 72.4, 70.8, 21.6, 3.1. 19F NMR (CDCl3, 235 MHz): δ −78.0. IR (solid): ν 2932, 2320, 1675, 1593, 1554, 1493, 1464, 1377, 1362, 1291, 1216, 1164, 1095, 1082, 1049, 1024, 969, 888, 867, 812, 759, 715, 700, 670, 655, 636, 608, 597. MS (ESI): m/z 1295.0 (M − 2 AcOH − 2 MeCN − OTf; 20%), 1205.0 (90%), 1191.0 (100%), 1147.0 (15%). HRMS (ESI): m/ z calcd for [M − 2 AcOH − 2 MeCN − OTf]+ (C55H44F3FeN4O7Pd2S3) 1294.9747, found, 1294.9792. Anal. Calcd for FBIP-HOAc*OTf: C, 46.70; H, 3.55; N, 5.11. Found, C, 46.36; H, 3.80; N, 5.40. Bis{[2,4-pentanedionato-κO1,κO5][η5-(4R,5R)-(Sp)-(2−4,5-dihydro-4,5-diphenyl-1-tosyl-1H-imidazoyl-κN7)cyclopentadienyl-κC1]palladium(II)}iron(II) (FBIP-acac). [FBIPCl]2 (6.16 μmol, 15.0 mg) was monomerized using silver triflate (24.7 μmol, 6.34 mg) according to the general procedure. The obtained material was dissolved in CH2Cl2/methanol (1/1, 6 mL). Na(acac) (6 equiv, 37.0 μmol, 5.18 mg) was added, and the resulting mixture was stirred at room temperature overnight. Subsequently, the mixture was diluted with CH2Cl2 (10 mL) and washed twice with water. The organic layer was separated and dried over Na2SO4. Removal of the volatiles afforded FBIP-acac as a crystalline red-purple solid (12.9 μmol, 17.7 mg, 100%). Crystals suitable for X-ray analysis formed when n-pentane was slowly diffused into a concentrated solution of racemic FBIP-acac in acetone.9 Molecular weight for C64H58FeN4O8Pd2S2: 1343.99. Mp: >200 °C dec. [α]D23 = −672.4° (c = 0.0116 g dL−1, CH2Cl2). 1H NMR (CDCl3, 500 MHz): δ 7.76−7.69 (m, 4 H), 7.55−7.48 (m, 8 H), 7.44−7.39 (m, 2 H), 7.44−7.39 (m, 4 H), 7.21−7.15 (m, 4 H), 7.11−7.06 (m, 2 H), 7.00−6.94 (m, 4 H), 6.59−6.53 (m, 4 H), 5.24 (d, J = 1.9, 2 H), 5.04 (d, J = 1.4, 2 H), 4.93 (d, J = 5.0, 2 H), 4.86 (d, J = 5.0, 2 H), 4.77 (t, J = 1.9, 2 H), 4.74 (s, 2 H), 2.44 (s, 6 H), 1.57 (s, 6 H), 1.15 (s, 6 H). 13 C NMR (CDCl3, 125 MHz): δ 185.3, 183.9, 170.8, 145.0, 141.5, 140.4, 134.0, 130.1, 129.1, 128.7, 127.9, 127.7, 127.5, 127.0, 125.9, 99.2, 77.9, 77.8, 74.6, 73.1, 70.9, 68.6, 27.0, 25.9, 21.6. IR (solid): ν 3030, 2922, 2853, 2360, 1737, 1586, 1565, 1513, 1494, 1453, 1399, 1374, 1357, 1329, 1283, 1260, 1187, 1166, 1153, 1098, 1085, 1048, 1025, 966, 928, 908, 885, 866, 810, 756, 713, 697, 670, 658, 599. MS (ESI): m/z 1344.1 (M + H, 100%), 1081.0 (10%), 923.0 (90%), 869.1 (50%), 817.1 (13%). HRMS (ESI): m/z calcd for [M] + (C64H58FeN4O8Pd2S2) 1344.1140, found 1344.1118. Anal. Calcd for (FBIP-acac + n-pentane): C, 58.52; H, 4.98; N, 3.96. Found: C, 58.17; H, 4.59; N, 3.84.

pair of scissors. It seems that the ferrocene complexes might even overcome the repulsive Coulomb interactions in the dicationic species 7 through dispersive interactions. The implications for catalytic reactions are yet to be fully elucidated. Nevertheless, it can easily be imagined that such rotation may assist a bond-forming process in a catalytic reaction by selfadjustment of the most beneficial intermetallic distance for a bimetallic catalyzed reaction. While we conclude that d8−d8 interactions are of minor importance in the present groundstate systems, the preferred Pd−Pd distances achievable rely on the substitution pattern of the ligand system and could enhance cooperative effects during catalytic turnover. Moreover, we suggest a permanent switch among neutral and mono- and dicationic catalyst species caused by a Brønsted acid such as HOAc in the bis-palladacycle-catalyzed 1,4-addition of azlactones to enones, which might facilitate different elementary steps of the catalytic cycle.

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EXPERIMENTAL SECTION

General Considerations. All reactions were performed in ovendried glassware under a positive pressure of nitrogen. All glassware used was washed with demineralized water to remove any traces of chloride. Acetonitrile was dried under N2 over molecular sieves in a solvent purification system. Acetic acid was dried and distilled over P2O5. [FBIP-Cl]26a was synthesized according to a literature protocol. Silver triflate and sodium acetylacetonate as well as the solvents dichloromethane, chloroform, methanol, and ethyl acetate were used as purchased from commercial suppliers. Solvents were usually removed at 30−40 °C by rotary evaporation at 600−10 mbar pressure, and nonvolatile compounds were dried in vacuo at 0.1 mbar. Yields refer to isolated, pure compounds and are calculated in mole percent of the used starting material. NMR spectra were recorded at 21 °C operating at 300 or 500 MHz (1H), 125 MHz (13C), and 235 MHz (19F). Chemical shifts are referred in terms of ppm, and J coupling constants are given in Hz. Abbreviations for multiplicities are as follows: s (singlet), d (doublet), t (triplet), m (multiplet), b (broad signal). IR spectra were recorded on an ATR unit, and the signals are given in wavenumbers (cm−1). Optical rotation was measured at the sodium D line in a cell with 100 mm path length. Melting points were measured in open glass capillaries and are uncorrected. Mass spectra were measured on an ESI spectrometer. Single-crystal X-ray analysis was performed by Dr. Wolfgang Frey (Universität Stuttgart). General Procedure for the Monomerization of [FBIP-Cl]2. A suspension of [FBIP-Cl]2 (1 equiv) and silver triflate (4 equiv) in MeCN (1 mL per 5 mg of [FBIP-Cl]2) was placed in an ultrasonic bath for 30 min. The resulting mixture was filtered through Celite/ CaH2 to remove precipitated AgCl. The filtrate was subsequently concentrated to dryness. Bis{[cis-(acetonitrile-κN1)(aqua-κO1)][η5-(4R,5R)-(Sp)-(2−4,5dihydro-4,5-diphenyl-1-tosyl-1H-imidazoyl-κN7)cyclopentadienyl-κC1]palladium(II)}iron(II) Bis(trifluoromethanesulfonate) (FBIP-H2O*OTf, 6). Racemic [FBIPCl]2 (12.3 μmol, 30 mg) was monomerized using silver triflate (49.3 μmol, 12.7 mg) according to the general procedure. The obtained material was dissolved in wet EtOAc (ca. 1 mL). n-Pentane was slowly diffused into this solution. Generally after 2 days at ambient temperature crystals formed which were suitable for X-ray analysis.9 The supernatant was discarded and the crystals collected, subsequently washed once with petroleum ether, and then dried under reduced pressure to give FBIP-H2O*OTf (6) as red crystals (10.6 mmol, 35.1 mg, 86%). Molecular weight for C60H54F6FeN6O12Pd2S4: 1562.04. Mp: >250 °C dec. 1H NMR (CDCl3, 300 MHz): δ 7.63−7.45 (m, 10 H), 7.44− 7.37 (m, 5 H), 7.26−7.19 (m, 5 H), 7.17−7.08 (m, 4 H), 6.78−6.67 (m, 4 H), 5.41 (b, 2 H), 5.13 (d, J = 2.6, 2 H), 5.13 (d, J = 3.9, 2 H), 4.98 (d, J = 3.5, 2 H), 4.60 (b, 2 H), 2.44 (s, 6 H), 2.27 (s, 6 H). 13C NMR (CDCl3, 125 MHz): δ 145.9, 139.4, 134.2, 130.4, 129.5, 129.4, 128.8, 128.2, 127.3, 127.0, 125.6, 122.3, 95.9, 78.1, 74.4, 72.6, 70.8, 5815

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Organometallics

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

Article

Asian J. 2010, 5, 1770. (d) Eitel, S. H.; Bauer, M.; Schweinfurth, D.; Deibel, N.; Sarkar, B.; Kelm, H.; Krüger, H.-J.; Frey, W.; Peters, R. J. Am. Chem. Soc. 2012, 134, 4683. (9) CCDC-891037 (6), CCDC-928395 (7), and CCDC-928394 (FBIP-acac) contain supplementary crystallographic data. These data can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif. (10) Reviews about ferrocene palladacycles in asymmetric catalysis: (a) Richards, C. J. In Chiral Ferrocenes in Asymmetric Catalysis; Dai, L.X., Hou, X.-L., Eds.; Wiley-VCH: Weinheim, Germany, 2010; pp 337− 368. (b) Peters, R.; Fischer, D. F.; Jautze, S. Top. Organomet. Chem. 2011, 33, 139. Representative examples for studies of planar chiral palladacycles in asymmetric catalysis: (c) Watson, M. P.; Overman, L. E.; Bergman, R. G. J. Am. Chem. Soc. 2007, 129, 5031. (d) Nomura, H.; Richards, C. J. Chem. Asian J. 2010, 5, 1726. (e) Hollis, T. K.; Overman, L. E. J. Organomet. Chem. 1999, 576, 290. (f) Overman, L. E.; Carpenter, N. E. Org. React. 2005, 66, 1. (g) Anderson, C. E.; Overman, L. E. J. Am. Chem. Soc. 2003, 125, 12412. (h) Kirsch, S. F.; Overman, L. E.; Watson, M. P. J. Org. Chem. 2004, 69, 8101. (i) Nomura, H.; Richards, C. J. Chem. Eur. J. 2007, 13, 10216. (j) Overman, L. E.; Owen, C. E.; Pavan, M. M.; Richards, C. J. Org. Lett. 2003, 5, 1809. (k) Prasad, R. S.; Anderson, C. E.; Richards, C. J.; Overman, L. E. Organometallics 2005, 24, 77. (l) Anderson, C. E.; Donde, Y.; Douglas, C. J.; Overman, L. E. J. Org. Chem. 2005, 70, 648. (m) Overman, L. E.; Remarchuk, T. P. J. Am. Chem. Soc. 2002, 124, 12. (n) Kirsch, S. F.; Overman, L. E. J. Org. Chem. 2005, 70, 2859. (o) Moyano, A.; Rosol, M.; Moreno, R. M.; López, C.; Maestro, M. A. Angew. Chem., Int. Ed. 2005, 44, 1865. (p) Kang, J.; Yew, K. H.; Kim, T. H.; Choi, D. H. Tetrahedron Lett. 2002, 43, 9509. (11) (a) Dupont, J.; Pfeffer, M. Palladacycles; Wiley-VCH: Weinheim, Germany, 2008. (b) Djukic, J.-P.; Hijazi, A.; Flack, H. D.; Bernardinelli, G. Chem. Soc. Rev. 2008, 37, 406. (12) (a) Ferrocenes; Hayashi, T., Togni, A.; Eds.; VCH: Weinheim, Germany, 1995. (b) Richards, C. J.; Locke, A. J. Tetrahedron: Asymmetry 1998, 9, 2377. (c) Chiral Ferrocenes in Asymmetric Catalysis. Synthesis and Applications; Dai, L.-X., Hou, X.-L., Eds.; Wiley-VCH: Weinheim, Germany, 2009. (13) Complex 6 was crystalized in its racemic form and is depicted in the enantiomeric form for indication. (14) Bondi, A. J. Phys. Chem. 1964, 68, 441. (15) For a representative review on metallophilic interactions in d10− d10 systems, see: (a) Sculfort, S.; Braunstein, P. Chem. Soc. Rev. 2011, 40, 2741. For reviews on palladium−palladium interactions, see: (b) Murahashi, T.; Kurosawa, H. Coord. Chem. Rev. 2002, 231, 207. (c) Mirica, L. M.; Khusnutdinova, J. R. Coord. Chem. Rev. 2013, 257, 299. For selected recent examples of d8−d8 interactions in palladium complexes, see: (d) Chiarella, G. M.; Cotton, F. A.; Ibragimov, S. A.; Murillo, C. A.; Wilkinson, C. C.; Young, M. D. Polyhedron 2013, DOI: 10.1016/j.poly.2012.06.002. (e) Suess, D. L.; Peters, J. C. Chem. Commun. 2010, 46, 6554. (f) Kajitani, Y.; Tsuge, K.; Sasaki, Y.; Kato, M. Chem. Eur. J. 2012, 18, 11196. (g) Bercaw, J. E.; Durrell, A. C.; Gray, H. B.; Green, J. C.; Hazari, N.; Labinger, J. A.; Winkler, J. R. Inorg. Chem. 2010, 49, 1801. (h) Eerdun, C.; Hisanaga, S.; Setsune, J.-i. Angew. Chem., Int. Ed. 2013, 52, 929. (i) Sluch, I. M.; Miranda, A. J.; Elbjeirami, O.; Omary, M. A.; Slaughter, L. M. Inorg. Chem. 2012, 51, 10728. (j) Luo, J.; Khusnutdinova, J. R.; Rath, N. P.; Mirica, L. M. Chem. Commun. 2012, 48, 1532. (k) Nebra, N.; Ladeira, S.; Maron, L.; Martin-Vaca, B.; Bourissou, D. Chem. Eur. J. 2012, 18, 8474. (l) Nebra, N.; Saffon, N.; Maron, L.; Martin-Vaca, B.; Bourissou, D. Inorg. Chem. 2011, 50, 6378. (m) Murahashi, T.; Takase, K.; Oka, M.-a.; Ogoshi, S. J. Am .Chem. Soc. 2011, 133, 14908. (n) Marino, N.; Fazen, C. H.; Blakemore, J. D.; Incarvito, C. D.; Hazari, N.; Doyle, R. P. Inorg. Chem. 2011, 50, 2507. (o) Maestri, G.; Motti, E.; Della Ca’, N.; Malacria, M.; Derat, E.; Catellani, M. J. Am. Chem. Soc. 2011, 133, 8574. (p) Herbert, D. E.; Ozerov, O. V. Organometallics 2011, 30, 6641. (q) Bennett, M. A.; Kar, G.; Mirzadeh, N.; Privér, S. H.; Rae, A. D.; Wagler, J.; Willis, A. C.; Bhargava, S. K. Organometallics 2011, 30, 2749. (16) Tao, J.; Perdew, J. P.; Staroverov, V. N.; Scuseria, G. E. Phys. Rev. Lett. 2003, 91, 146401.

S Supporting Information *

Text, figures, tables, and CIF files giving further experimental procedures, characterization data, details of the calculations, and crystallographic data. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Author

*E-mail: [email protected] (B.M.); rene. [email protected] (R.P.). Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the Deutsche Forschungsgemeinschaft (DFG, PE 818/4-1). REFERENCES

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