Reactions of Nitrogen Donors with ... - ACS Publications

Article pubs.acs.org/Organometallics

Reactions of Nitrogen Donors with Cycloheptatrienylidene Complexes: Metal Coordination versus Nucleophilic Attack on the Carbene Ligand Kevser Mantas-Ö ktem, Karl Ö fele, Alexander Pöthig, Bettina Bechlars, Wolfgang A. Herrmann, and Fritz E. Kühn* Chair of Inorganic Chemistry/Molecular Catalysis, Catalysis Research Center, Technische Universität München, Ernst-Otto-Fischer-Strasse 1, D-85747 Garching b. München, Germany S Supporting Information *

ABSTRACT: Cycloheptatrienylidene (CHT)−palladium complexes may react with N-donor molecules, showing two different pathways of reaction, either nucleophilic attack on the CHT ligand or coordination to the metal center. The first variant leads to a formation of water-soluble η3-cycloheptatrienyl complexes, as in the case of 3,5-lutidine or 3-chloropyridine. Reaction with 2,6-lutidine, on the other hand, yields monomeric pyridine-substituted CHT−Pd compounds comparable to NHC−PEPPSI complexes. Reaction with 1-methylimidazole yields both a dimeric water-soluble cycloheptatrienyl palladium complex and a monomeric CHT−Pd compound, depending on the conditions of the reaction. Furthermore, a subsequent formation of a Pd3-sandwich type complex was observed, which has been determined by single-crystal X-ray diffractometry. The nucleophilic attack of morpholine on the CHT ligand reveals another possible reaction path: removal of the CHT ligand from the metal under formation of a tropylidenimmonium cation.

■

INTRODUCTION Carbocyclic carbenes, such as cycloheptatrienylidene (CHT), have already been applied as ligands for a variety of transition metals.1 However, only few details are reported about their reactivity toward donor molecules, especially under preservation of the metal−carbene bond. A well-established class of precursors, the dinuclear halogene-bridged Pd complexes [Pd(CHT)X2]2, can react with CH3CN or phosphines, forming donor-substituted mononuclear CHT complexes (Scheme 1). The latter are also known to be active catalysts for several C−C and C−N cross-coupling reactions.2−4 The reaction with DMSO, however, results in the substitution of the CHT ligand and its conversion to tropone,5 which was observed earlier when treating a pentacarbonyl tungsten CHT complex with DMSO.6 Another rare example of a reaction with CHT ligands that does not break the metal−carbene bond is the deprotonation of the methylene bridge in a P-C(CHT)-P pincer iridium complex. Its hydrogen atoms become acidic due to the positively charged CHT ring, leading to an extended π system.7 The same iridium CHT complex is able to coordinate a molybdenum tricarbonyl moiety by the seven-membered ring, yielding a heterobimetallic carbene complex. It indicates that the metal bound CHT ring and the tropylium ion are comparable systems with respect to their reactivity.7 In general, the reactivity of CHT complexes toward nitrogen donor molecules is not well-examined. It was found, however, that the compound 2a efficiently catalyzes α-ketoarylations.8 © 2012 American Chemical Society

However, as far as we know, no systematic investigation on the reactivity of CHT complexes toward nucleophiles has been conducted thus far. We, therefore, examined the reactions of A and B with different secondary and tertiary amines and now report the isolation and characterization of the reaction products.

■

RESULTS AND DISCUSSION We synthesized the precursors A and B according to the procedures described in the literature.3 The formation of B from 1,1′-dichlorocycloheptatriene and Pd0 proceeds through a dinuclear neutral palladium complex H (Scheme 2).3 For unsubstituted cycloheptatrienyl ligands, the general motif of a η3-coordination has been proposed already in 1989,9 but was confirmed by single-crystal diffraction experiments only recently.4 Reaction of [Pd(CHT)X2]2 with Pyridine Derivatives. The treatment of suspensions of complex A in THF with 3,5lutidine or 3-chloropyridine at 50 °C produces red crystalline air-stable products in good yields (Scheme 3, 1a/2a). These compounds are not soluble in THF, but, unlike the known mononuclear complexes, such as C−G, they dissolve in water. The subsequent addition of NH4PF6 to the aqueous solutions resulted in almost quantitative formation of deep purple precipitates, which are soluble in acetone and Received: September 7, 2012 Published: November 7, 2012 8249

dx.doi.org/10.1021/om300867g | Organometallics 2012, 31, 8249−8256

Organometallics

Article

Scheme 1. Reaction of Donor Ligands with Dinuclear Pd−CHT Complexes2,3

Scheme 2. Synthesis of an η3-Cycloheptatrienyl Complex H and Its Conversion into a Carbene Complex B3

Scheme 3. Synthesis of Dinuclear Palladium Complexes with Pyridine-Substituted Cycloheptatrienyl Ligands

acetonitrile. Single-crystal X-ray diffraction analysis and 1H NMR spectroscopy revealed the formation of compound 1b, whose structure is similar to that of complex H, but, due to the pyridine-substituted cycloheptatrienyl ligands, it is an ionic compound (Scheme 3). Owing to the similarity of the proton spectra, we assume that the structure of 2b is analogous. Unlike phosphines and acetonitrile, the pyridine nucleophiles did not react by coordination to the metal but instead attack the sevenmembered CHT ring. The analogous chloro-substituted complexes were synthesized by reaction of compound B with 3,5-lutidine or 3-chloropyridine (Scheme 3, 3a/3b and 4a/4b). The molecular structure of the C2-symmetric dinuclear cationic complex in 1b is depicted in Figure 1. The Pd2(μ-Br)2 core is stabilized by η3-coordinated lutidine-substituted cycloheptatrienyl ligands to bind to each palladium center. The palladium atom Pd1 adopts a distorted trigonal-planar coordination sphere formed by Cg1 and the bromine atom Br1. The center of the triangle is formed by the palladiumbound carbon atoms C2−C4. The equivalent positions for the atoms Pd1a, Br1a, and C2a−C4a as well as for Cg1a are generated by symmetry operations (1 − x, y, −z). The planes formed by the described triangles, including Pd1 and Pd1a, respectively, form an angle of 69.83°, while the Pd2X2 core is almost planar in the neutral complexes H and B.4 The 1H NMR spectrum of 1b shows three signals for 12 protons of two cycloheptatrienyl ligands in the region from 4.6 to 6.2 ppm, while the signals of the lutidine protons appear downfield at 8.3 and 8.7 ppm. This indicates that the centers of the positive charge are situated at the heterocyclic substituents.

Furthermore, the relatively simple pattern of the cycloheptatrienyl signals points to a fluctuation of the palladium center over the η3-coordinated tropylium-3,5-lutidine unit. This behavior can be observed even at temperatures as low as −90 °C. The same phenomenon has been reported for the abovementioned chlorocycloheptatrienyl palladium complex H.4 In the cases of 1a and 2a, the conversion of the carbene ligand into a cycloheptatrienyl moiety takes place during the initial reaction (Scheme 3) of the N base with the CHT complex A and not after dissolution of the primary products 1a and 2a in water. This is confirmed by the 13C NMR solid-state spectra of 1a and 2a, which show signals between 72 and 79 ppm, typical for allylic compounds.10 All attempts to stimulate a rearrangement of the pyridine-substituted cycloheptatrienyl complexes 1a and 2a to the corresponding mononuclear CHT complexes C−G (Scheme 1) analogous to the rearrangement of complex H to B (Scheme 2) failed. The observed nucleophilic attack on the CHT ring can be explained by considering that the Fischer-type carbene atom is electrophilic and the tropylium resonance form of the coordinated CHT ring is favored for low-valent metal complexes,11 since comparable reactions of free tropylium salts with amines are reported in the literature.12−14 Sterically hindered pyridines, such as 2,6-lutidine, react with A or B under the same conditions as described above, yielding different greenish yellow products. These easily prepared airand moisture-stable compounds were found to be the mononuclear complexes 5a and 5b (Scheme 4), which are the first CHT analogues to the Pd−NHC PEPPSI complexes.15 8250


Organometallics

Article

Figure 2. ORTEP view of 5a. Thermal ellipsoids are shown at a 50% probability level. The hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): Pd1−C1 1.960(4), Pd1−N1 2.155(3), Pd1−Br1 2.4369(5), Pd1−Br2 2.4546(5), C1−C2 1.397(4); C1−Pd1−N1 178.39(14), C1−Pd1−Br1 88.22(11), N1−Pd1−Br1 90.16(9), C1−Pd1−Br2 89.36(11), N1−Pd1−Br2 92.26(9), Br1− Pd1−Br2 177.582(19), C5−N1−Pd1 119.84(17). Symmetry operations for the equivalent atom positions a: x, 1/2 − y, z.

Figure 1. ORTEP view of the cation of 1b in the solid state. Thermal ellipsoids are shown at the 50% probability level. The PF6− anions and the hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): Pd1−Br1 2.5665(4), Pd1−Br1a 2.5786(4), Pd1a− Br1 2.5786(4), Pd1−C3 2.073(3), Pd1−C4 2.166(3), Pd1−C2 2.201(3), C1−N1 1.457(4); Pd1−Br1−Pd1a 71.393(11), Br1−Pd1− Br1a 89.200(13), C3−Pd1−C4 39.26(13), C3−Pd1−C2 38.15(13), C4−Pd1−C2 69.20(12), C3−Pd1−Br1 134.90(9), C4−Pd1−Br1 98.29(9), C2−Pd1−Br1 163.11(9), C3−Pd1−Br1a 134.22(10), C4− Pd1−Br1a 172.11(9), C2−Pd1−Br1a 102.96(9), C7−C1−C2 130.3(3), C7−C1−N1 116.6(3), C2−C1−N1 113.1(3), C8−N1− C1 119.5(3), C12−N1−C1 119.0(3). Symmetry operations for the equivalent atompositions a: 1 − x, y, −z. Cg1 defines the mass center of the allylic system C2, C3, and C4.

employing the bulky 2,6-lutidine, a nucleophilic attack on the ligand is sterically hindered, only allowing the coordination of 2,6-lutidine to the Pd center. Accordingly, when treating 5a with an excess of the less bulky 3,5-lutidine, the dinuclear complex 1a was formed. DFT calculations (B3LYP using 6-31G(d) for C, H, and N; LANL2DZ for Pd; 6-311(s) for Br) for the different isomers support this observation. We found that the observed monocarbene complex 5a is slightly more stable by Δ(ΔG) = 0.5 kcal/mol than its (non-observed) isomer with a 3,5-lutidine ligand. On the other hand, the observed cationic fragment of dinuclear complex 1a or 1b is Δ(ΔG) = 6.9 kcal/mol lower in energy than its (non-observed) isomer with 2,6-lutidinium substituents, which corresponds to the increased steric demand of the ortho-disubstituted pyridine. Reaction of [Pd(CHT)X2]2 with 1-Methylimidazole. Another tertiary amine with a pyridine-like nitrogen donor atom, 1-methylimidazole, revealed a more complex reactivity toward the dinuclear complex A, depending on the reaction conditions. When a suspension of A in THF was treated with an excess of 1-methylimidazole at 50 °C for 18 h, a red microcrystalline water-soluble compound 6a was obtained. After adding NH4PF6 to the aqueous solution, a purple precipitate 6b soluble in acetone and acetonitrile was formed (Scheme 5(I)). The elemental analysis of 6a corresponds to the dinuclear formula depicted in Scheme 5. The formation of an imidazolesubstituted cycloheptatrienyl complex 6b analogous to 1b and 2b was confirmed by the 1H NMR spectrum. Multiplets at 4.46, 4.99, and 6.03 ppm were observed for the cycloheptatrienyl protons, almost identical to the corresponding signals of 1b and 2b. Compared to the free heterocycle, the imidazole signals are shifted downfield to 7.19, 7.44, and 8.68 ppm, as in the case of the pyridine signals in 1b and 2b. When the reaction between A and 1-methylimidazole was carried out without an excess of imidazole, a yellow waterinsoluble product 7 was isolated (Scheme 5(II)). Its 1H NMR spectrum is comparable to the spectrum of the 2,6-lutidine− CHT complex 5a and corresponds to a mononuclear CHT complex with 1-methylimidazole coordinated to the palladium center. Additionally, in the 13C NMR solid-state spectrum of 7, a typical signal for metal coordinated carbene-C atoms at 223.5 ppm is observed, similar to complex 5a. Subsequent treatment

Scheme 4. Synthesis of the 2,6-Lutidine-Substituted Carbene Complexes 5a and 5b

Contrary to 1b and 2b, 5a and 5b are neutral complexes and insoluble in water. Recrystallization of 5a in dichloromethane afforded single crystals suitable for X-ray diffraction analysis (Figure 2). The palladium atom in 5a is coordinated by two transoriented bromine atoms (Br1 and Br2), one carbon atom (C1) of the CHT ligand, and the nitrogen atom (N1) of the 2,6lutidine ligand, adopting a slightly distorted square-planar geometry. The molecule contains a mirror plane that includes the atoms Pd1, Br1, Br2, C1, and N1, so that the CHT and the lutidine ligands exhibit a coplanar orientation. Compound 5b is stable against further addition of 2,6lutidine. When adding less sterically hindered 3,5-lutidine, it reacts to form the dinuclear complex 1a. This transformation supports our presumption that the reaction products occur according to the ability of the pyridine derivatives to act as nucleophiles, due to steric reasons. The reactivity of these nitrogen-donating pyridine compounds toward A indicates that they preferably attack the carbene ligand, rather than coordinating to the metal center. Therefore, when treating A with 3-chloropyridine or 3,5lutidine, the dinuclear species 1a or 1b with substituted cycloheptatrienyl ligands are obtained. On the contrary, when 8251


Organometallics

Article

Scheme 5. Reactions of 1-Methylimidazole with [Pd(CHT)Br2]2a

a

(I) Nucleophilic attack at the carbene ligand. (II) Coordination to the metal center. (III) Rearrangement of the metal-coordinated complex.

of a solution of complex 7 in THF with an excess of 1methylimidazole at 50 °C for 18 h led to complete conversion into the water-soluble dinuclear cycloheptatrienyl complex 6a (Scheme 5(III)). Single crystals of 7 suitable for X-ray diffraction analysis were grown from a solution in dichloromethane and n-hexane. As shown in Figure 3, the palladium atom in 7 is coordinated by

Figure 3. ORTEP view of 7. Thermal ellipsoids are shown at a 50% probability level. Hydrogen atoms are omitted for clarity. Selected bond lengths (Å) and angles (deg): Pd1−C1 1.9460(16), Pd1−N1 2.1225(15), Pd1−Br1 2.4413(3), Pd1−Br2 2.4602(2), C1−C2 1.403(3), C1−C7 1.407(3); N1−Pd1−C1 177.09(7), Br1−Pd1−C1 86.91(5), Br2−Pd1−C1 87.44(5), Br1−Pd1−N1 93.33(4), Br2− Pd1−C1 87.44(5), Br2−Pd1−N1 92.74(4), Br1−Pd1−Br2 169.93(1), C2−C1−C7 124.59(17), Pd1−C1−C7 118.78(14), Pd1−N1−C9 126.78(13).

Figure 4. ORTEP view of the cation of the complex 8a. Thermal ellipsoids are shown at a 50% probability level. Hydrogen atoms and solvent molecules are omitted for clarity. Selected bond lengths (Å) and angles (deg): Pd1−Pd2 2.7767(4), Pd2−Pd3 2.7825(4), Pd1− Pd3 2.7319(5), Pd1−Br1 2.5648(6), Pd2−Br2 2.5556(5), Pd3−Br3 2.5570(6), Pd1−C6 2.244(4), Pd1−C7 2.202(4), Pd1−C15 2.180(4), Pd1−C16 2.221(4), Pd2−C1 2.537(4), Pd2−C2 2.149(4), Pd2−C3 2.447(4), Pd2−C17 2.238(4), Pd2−C18 2.166(4), Pd3−C4 2.158(4), Pd3−C5 2.251(4), Pd3−C13 2.185(4), Pd3−C14 2.392(4), N1−C1 1.442(5), N3−C12 1.439(6); Pd2−Pd1−Pd3 60.67(1), Pd1−Pd2− Pd3 58.87(1), Pd1−Pd3−Pd2 60.46(1), Pd2−Pd1−Br1 148.17(2), Pd1−Pd3−Br3 148.11(2), Pd3−Pd2−Br2 160.85(2), C1−N1−C8 126.8(4), C12−N3−C19 124.9(3).

two trans-oriented bromine atoms (Br1 and Br2), one carbon atom (C1) of the CHT ligand, and the nitrogen atom (N1) of the 1-methylimidazole ligand, also adopting a slightly distorted square-planar geometry as was the case for 5a. The formation of a different reaction product was observed after leaving a solution of 6a in D2O at room temperature without being protected from air. Red crystals formed within 3 days, and single-crystal X-ray diffraction analysis revealed the cycloheptatrienyl complex 8a based on a triangular tripalladium center (Figure 4), which is the first example of this compound class with substituted CHT rings. Only a few examples of such tripalladium sandwich complexes are known. As Murahashi et al. first reported in 2006,11 these complexes only possess unsubstituted cycloheptatrienyl ligands like complex 9 (Scheme 6). Additionally, mechanistic investigations on the formation of such compounds have been carried out, using different metals in the cluster.16 In contrast to all of these complexes, the

bis(cycloheptatrienyl) tripalladium complex moiety in 8a is cationic due to the positively charged imidazolium substituents at the cycloheptatrienyl ligands. The Pd3 core of complex 8a is located between two planar cycloheptatrienyl ligands as a sandwich compound. With bond lengths between 2.782 and 2.731 Å, the Pd−Pd contacts are in the range of typical Pd−Pd bonds.11 The two cycloheptatrienyl rings are positioned staggered to each other with a distance of 2.100(1) and 2.095(1) Å, respectively, to the centroid of the central Pd3 core. The C−C bond lengths of the cycloheptatrienyl ligands (1.39−1.43 Å) correspond to an electronic delocalization in the seven-membered carbocyclic ring systems. A 5-fold hydrogen−halogen bonding to Br3 can be observed, including two contacts to the acidic imidazolium protons at C19 and C8′ of a second symmetry equivalent (Figure 5I), one contact to the imidazolium backbone proton C20″ of a third symmetry equivalent, and two hydrogen bonds to CHT protons of C2′ and C18″ (not shown). Further hydrogen 8252


Organometallics

Article

Scheme 6. Left: Conversion of the Dinuclear Complex 6a into Tripalladium Sandwich Complexes 8a and 8b. Right: Analogous C7H7 Complex 911

Figure 5. (I) Hydrogen bonding in complex 8a between imidazolium protons of C19 and C8′ and the bromo ligand (Br3) of the central Pd3Br3 moiety. (II) Top view of complex 8a showing the distortion of the Pd3−Br3−Br2 angle from ideal 120°. Figure 6. Solid-state structure of compound 10. Thermal ellipsoids are shown at a 50% probability level. Selected bond lengths (Å) and angles (deg): Br1−Pd1 2.4296(4), Br2−Pd1 2.4403(4), Br3−Pd1 2.4414(4), Br4−Pd1 2.4453(4), N1−C1 1.340(4), N1−C11 1.477(4), N1−C8 1.484(4), O1−C10 1.426(4), O1−C9 1.435(4), C8−C9 1.507(5), C10−C11 1.500(5), C7−C1 1.427(4), C1−C2 1.443(4), C2−C3 1.357(5), C3−C4 1.409(5), C4−C5 1.362(5), C5−C6 1.416(5), C6− C7 1.359(5); C1−N1−C11 125.5(2), C1−N1−C8 124.2(2), C11− N1−C8 110.3(2), C10−O1−C9 109.6(2), N1−C8−C9 109.5(3), O1−C9−C8 112.0(3), O1−C10−C11 111.1(3), N1−C11−C10 110.7(3), N1−C1−C7 119.5(3), N1−C1−C2 118.1(3), C7−C1−C2 122.4(3), C3−C2−C1 130.2(3), C2−C3−C4 131.3(3), C5−C4−C3 126.6(3), C4−C5−C6 127.1(3), C7−C6−C5 131.5(3), C6−C7−C1 129.7(3).

bonding can be found to Br1 and Br2, but only to less acidic CHT protons of neighboring complexes. This is accompanied by a distortion of the Pd3−Pd2−Br2 angle (Figure 5II). The somewhat obscure generation of a formally Pd(I) species 8a (Scheme 6(I)) from the Pd(II) complex 6a could be reproduced by treatment of the latter with 1 equiv of Pd0(dba)2 in acetonitrile(Scheme 6(II)). This reaction led to an almost quantitative conversion of the dinuclear complex into the tripalladium sandwich, which could be further transformed into complex 8b by exchange of the anion. Both compounds 8a and 8b are well characterized by elemental analysis and 1H and 13C NMR. Presumably, the observed conversion of the dinuclear complex 6a into the tripalladium sandwich 8a by air and in water is initiated by its partial decomposition, yielding Pd0 fragments that may react with remaining molecules of 6a, forming the Pd3 core of 8a. Reaction of [Pd(CHT)X2]2 with Morpholine. The efficient catalysis of the Hartwig−Buchwald coupling reaction of aryl halides with morpholine is one of the few reported catalytic applications of CHT−palladium complexes.2 To obtain further information about the mechanism of this process, A was treated with morpholine using the same conditions as those in the experiments with pyridine derivatives. It resulted in the formation of colloidal Pd and a brown precipitate 10, which is insoluble in water. Single crystals of 10 suitable for X-ray diffraction analysis were grown from a solution in dichloromethane. It revealed the formation of a salt consisting of a square-planar tetrabromopalladium(II) anion and a planar tropylidenimmonium cation shown in Figure 6. A reaction pathway is suggested in Scheme 7. As observed in the cases of 3,5-lutidine and 3-chloropyridine, morpholine could engage in a nucleophilic attack on the CHT ligand to form the unstable dinuclear complex 10′ with morpholine-substituted cycloheptatrienyl ligands as an intermediate. It may decompose to 10 in a still obscure reaction sequence, while generating dihydrogen and Pd(0). Inves-

tigation into whether this behavior plays a role during the catalytic application of CHT−Pd complexes in C−N coupling reactions is warranted. Tropylidenimmonium cations similar to compound 10 have been generated from addition products of secondary amines to tropylium salts by bromination or hydride abstraction.13 Catalytic Activity. All of the compounds described in this work are catalytically active in C−C or C−N coupling reactions. Our ongoing investigations are focused on improving the catalytic application of these synthesized compounds. Exemplarily, the results of 2a in the α-arylation of propiophenone (Scheme 8) are described in Table 1.

■

CONCLUSION We were able to show that, unlike other donor molecules, such as phosphines and acetonitrile, the reactions of N-donor molecules, such as pyridines, 1-methylimidazole, and morpholine, with CHT−Pd complexes exhibit two different reaction pathways. One of them is the attack of sterically less hindered N donors on the CHT ligand, followed by its rearrangement to an η3-coordinated cycloheptatrienyl system. The second possible variant is the coordination of sterically demanding 8253


Organometallics

Article

Scheme 7. Reaction of CHT Complex B with Morpholine with Proposed Intermediate

Scheme 8. α-Arylation of Propiophenone Catalyzed by 2a

Table 1. Catalytic Runs of α-Arylation of Propiophenone catalysisa

T [°C]

yieldb [%]

1 2

rt 65

60 74

(CPMAS), and the rotation frequency of the probe head was 15 kHz. Elemental analyses were performed at the Microanalytical Laboratory of TU Munich. Compounds 1a/1b. 3,5-Lutidine (0.7 mmol, 75 mg, 70.3 μL) was added to an orange suspension of A (0.28 mmol, 199 mg) in THF (10 mL). The reaction mixture was stirred at room temperature overnight. Within a few minutes, a color change was observed from orange to red. The resulting red product was filtered off, washed with THF (1 × 5 mL), Et2O (1 × 5 mL), and n-pentane (1 × 5 mL), and dried in vacuo. The resulting red product 1a is soluble in water and less soluble in D2O. Yield of 1a: 179 mg, 90%. Elemental analysis (%) calcd for C28H30Br4N2Pd2: C, 36.28; H, 3.26; N, 3.02; Pd, 22.96. Found: C, 36.06; H, 3.50; N, 2.96; Pd, 22.82. 13C solid-state NMR of 1a (75.468 MHz): δ = 146.2, 144.3, 140.3, 138.9, 134.3, 126.3, 124.9, 79.5, 79.2, 21.05, 19.3. After dissolution of 1a in water and addition of a saturated NH4PF6 solution, a brown precipitation was quantitatively formed. The brown product 1b was filtered, washed several times with H2O, dried in vacuo, and recrystallized from acetone by cooling in a freezer. 1H NMR of 1b (500 MHz, (CD3)2CO): δ = 8.77 (s, 4H), 8.37 (s, 2H), 6.22 (d, 3J = 7 Hz, 4H), 5.14 (m, 4H), 4.62 (m, 4H), 2.48 (s, 12H). 13 C {1H} NMR of 1b (500 MHz, (CD3)2CO): δ = 148.4, 142.5, 138.8, 79.6, 70.8, 68.3, 17.6. Compounds 2a/2b. 3-Chloropyridine (0.6 mmol, 68 mg, 78 μL) was added to an orange suspension of A (0.24 mmol, 171 mg) in THF (10 mL). The reaction mixture was stirred at room temperature overnight. Within a few minutes, a color change was observed from orange to red. The resulting red product was filtered off, washed with THF (1 × 5 mL), Et2O (1 × 5 mL), and n-pentane (1 × 5 mL), and dried in vacuo. The resulting red product 2a is soluble in water and less soluble in D2O. Yield of 2a: 150 mg, 88%. Elemental analysis (%) calcd for C24H20Br4Cl2N2Pd2: C, 30.61; H, 2.35; N, 2.97; Pd, 22.6. Found: C, 30.43; H, 2.22; N, 2.86; Pd, 22.6. 13C solid-state NMR of 2a (75.468 MHz): δ = 162.7, 145.9, 138.8, 135.3, 132.3, 128.7, 125.4, 79.1, 73.9. After dissolution of 2a in water and addition of a saturated NH4PF6 solution, a brown precipitation was quantitatively formed. The brown product 2b was filtered, washed several times with H2O, and dried in vacuo. 1H NMR of 2b (500 MHz, CD3CN): δ = 8.94 (m, 2H), 8.49 (m, 2H), 8.34 (m, 2H), 7.87 (m, 2H), 6.06 (d, 3J = 8 Hz, 4H), 5.15 (m, 4H), 4.65 (m, 4H). Compound 5a. 2,6-Lutidine (2.5 mmol, 267 mg, 268 μL) was added to a suspension of A (1.0 mmol, 712 mg) in THF (10 mL). The reaction mixture was stirred at room temperature overnight. The resulting suspension was filtered off, washed with THF (1 × 5 mL), Et2O (2 × 10 mL), and n-pentane (2 × 10 mL), and dried in vacuo. After recrystallization from dichloromethane, 5a was obtained as

a

Reaction conditions: 1.0 mmol p-bromoanisole, 1.1 mmol propiophenone, 1.5 mmol NaOtBu, 1 mol % 2a, 1 mL of THF, 65 °C, 18 h reaction time. bMonitored by GC with benzophenone as internal standard.

nucleophiles to the metal center under formation of mononuclear substituted carbene complexes. Contrary to pyridine derivatives, 1-methylimidazole can both perform the coordination to the metal as well as attack the CHT ligand. The observed inclusion of Pd0 in a dinuclear cycloheptatrienyl palladium complex opens an easy way to Pd3-sandwich compounds with N-donor-substituted cycloheptatrienyl ligands, granting access to an interesting class of metal complexes. The ability of CHT ligands to transform into cycloheptatrienyl ligands through a nucleophilic attack presumably plays an important role in the catalytic application of CHT−Pd complexes involving N-donor molecules as solvents, supporting ligands, or substrate components. During the reaction of morpholine with a CHT−Pd complex, the carbene ligand is transformed into a tropylidenimmonium cation by a nucleophilic attack and removed from the metal. This reaction, which is accompanied by the formation of colloidal palladium, might have a crucial effect on the catalytic activity of CHT−Pd complexes in C−N coupling reactions, currently being studied in our laboratories.

■

EXPERIMENTAL SECTION

Materials. All experiments were carried out under dry argon, using standard Schlenk or glovebox techniques if it is not otherwise noted. Dry and oxygen-free solvents were used. Solvents were dried by standard procedures, distilled, and kept under argon over molecular sieves. Instruments. 1H and 13C spectra were recorded on a Bruker DPX 500 and referenced to the residual solvent signals.17 13C NMR spectra in the solid state were measured at room temperature with a Bruker Avance 300 and a 4 mm MAS probe head, and the samples were packed in 4 mm ZrO2. 13C solid-state NMR chemical shifts are reported relative to external adamantane (referenced to TMS). 13C solid-state NMR spectra were measured with cross-polarization 8254


Organometallics

■

yellow needles. Yield: 655 mg, 92%. Elemental analysis (%) calcd for C14H15Br2NPd: C, 36.28; H, 3.26; N, 3.02; Pd, 22.96. Found: C, 35.77; H, 3.66; N, 2.78; Pd, 21.8. 1H NMR (500 MHz, CD2Cl2): δ = 9.89 (d, 3J = 10 Hz, 2H), 8.21 (m, 2H), 7.97 (m, 2H), 7.56 (t, 3J = 8/7 Hz, 1H), 7.10 (d, 3J = 8 Hz, 2H), 3.31 (s, 6H). 13C {1H} NMR (500 MHz, CD2Cl2): δ = 228.5 (carben C), 164.4, 159.6, 145.6, 138.0, 123.3, 27.3. 13C solid-state NMR (75.468 MHz): δ = 220.5, 163.6, 160.7, 158.3, 147.8, 139.2, 123.6, 28.2. Compounds 6a/6b. 1-Methylimidazole (1.22 mmol, 100.1 mg, 97.8 μL) was added to an orange suspension of A (0.49 mmol, 350 mg) in THF (10 mL). The reaction mixture was stirred at a temperature of 50 °C overnight. Within a few minutes, a color change was observed from orange to red. The resulting red product was filtered off, washed with THF (1 × 5 mL), Et2O (1 × 5 mL), and npentane (1 × 5 mL), and dried in vacuo. The resulting red compound 6a is soluble in water and less soluble in D2O. Yield of 6a: 386 mg, 90%. Elemental analysis (%) calcd for C22H24Pd2Br4N4: C, 30.13; H, 2.76; N, 6.39; Pd, 24.27. Found: C, 30.63; H, 2.76; N, 6.23; Pd, 25.46. 13 C solid-state NMR of 6a (75.468 MHz): δ = 137.89, 132.88, 126.21, 120.57, 116.96, 91.27, 78.80, 74.82, 39.17. After dissolution of 6a in water and addition of a saturated NH4PF6 solution, a brown precipitation was quantitatively formed. The brown product 6b was filtered, washed several times with H2O, and dried in vacuo. 1H NMR of 6b (500 MHz, CD3CN): δ = 8.68 (s, 2H), 7.44 (m, 2H), 7.19 (m, 2H), 6.03 (d, 3J = 8 Hz, 4H), 4.99 (m, 4H), 4.46 (s, 4H), 3.73 (s, 6H). 13C {1H} NMR of 6b (500 MHz, CD3CN): δ = 134.21, 124.39, 123.70, 96.30, 77.93, 71.10, 68.58, 36.99. Compound 7. 1-Methylimidazole (1.14 mmol, 93.6 mg, 90.8 μL) was added to a suspension of A (0.57 mmol, 409 mg) in THF (10 mL). The reaction mixture was stirred at 50 °C overnight. The resulting dark yellow suspension was filtered off, washed with THF (1 × 5 mL), Et2O (1 × 5 mL), and n-pentane (1 × 5 mL), and dried in vacuo. Yield: 204 mg, 82%. Elemental analysis (%) calcd for C22H24Pd2Br4N4: C, 30.13; H, 2.76; N, 6.39; Pd, 24.27. Found: C, 30.31; H, 2.98; N, 6.40; Pd, 23.2. 1H NMR (500 MHz, CD2Cl2): δ = 9.80 (d, 3J = 11 Hz, 2H), 8.17 (m, 2H), 8.06 (s, 1H), 7.90 (m, 2H), 7.49 (m, 1H), 6.84 (m, 1H), 3.68 (s, 3H). 13C {1H} NMR (500 MHz, CD2Cl2): δ = 227.0 (carben C), 164.0, 159.6, 145.5, 140.6, 137.8, 130.6, 120.4, 31.1. 13C solid-state NMR (75.468 MHz): δ = 223.5 (carben C), 166.4, 160.3, 149.3, 139.8, 131.6, 121.1, 37.3, 25.7. Compounds 8a/8b. 6a was suspended (0.283 mmol, 0.240 g) in acetonitrile, and 0.340 mmol of Pd(dba)2 was added to the red suspension. The reaction mixture was stirred at room temperature overnight. The red product was washed several times with diethyl ether. Yield: 233 mg, 84%. Elemental analysis (%) calcd for C22H24Pd3Br4N4: C, 26.87; H, 2.46; N, 5.70; Pd, 32.47. Found: C, 25.70; H, 2.38; N, 4.91; Pd, 31.60. 1H NMR (500 MHz, D2O): δ = 8.43 (s, 2H), 7.34 (m, 4H), 6.04 (m, 4H), 5.04 (m, 4H), 4.66 (m, 4H), 6.84 (m, 1H), 3.83 (s, 6H). 13C solid-state NMR of 8a (75.468 MHz): δ = 139.0, 129.6, 126.0, 123.3, 94.0, 78.5, 74.6, 37.0. The precipitation of 8a was quantitatively carried out with NH4PF6 in water. 1H NMR of 8b (500 MHz, CD3CN): δ = 8.68 (s, 2H), 7.44 (m, 2H), 7.19 (m, 2H), 6.04 (d, 3J = 8 Hz, 4H), 4.99 (m, 4H), 4.47 (s, 4H), 3.73 (s, 6H). Elemental analysis (%) calcd for C22H24Pd3Br3N4PF6: C, 25.20; H, 2.31; N, 5.34; Pd, 30.45. Found: C, 24.90; H, 2.31; N, 5.01; Pd, 29.18. Compound 10. Morpholine (0.263 mmol, 22.8 mg, 22.8 μL) was added to a suspension of A (0.105 mmol, 75.3 mg) in THF (10 mL). The reaction mixture was stirred at room temperature overnight. A brown product was filtered off, washed with THF (1 × 5 mL), Et2O (2 × 10 mL), and (2 × 10 mL) n-pentane, and dried in vacuo. The product 10 could be recrystallized from dichloromethane by addition of diethyl ether. Yield: 0.15 g, 92%. Elemental analysis (%) calcd for C22H28Br4N2O2Pd: C, 33.94; H, 3.63; N, 3.60; Pd, 13.67. Found: C, 32.14; H, 4.13; N, 3.54; Pd, 13.5.

Article

ASSOCIATED CONTENT

S Supporting Information *

Supplementary crystallographic data of 1b, 5a, 7, 8a, and 10 in CIF format and experimental details of all synthesized compounds are described in the Supporting Information. This material is available free of charge via the Internet at http://pubs.acs.org. Crystallographic data (excluding structure factors) for the structures reported in this paper have been deposited with the Cambridge Crystallographic Data Centre as supplementary publication Nos. CCDC-873305, 873306, 873307, 873308, and 873309. These can be obtained free of charge from The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.uk/data_request/cif or CCDC, 12 Union Road, Cambridge CB2 1EZ, U.K. (fax: (+44)1223-336-033; email: [email protected]).

■

AUTHOR INFORMATION

Corresponding Author

*Tel: +49 89 289 13081. Fax: +49 89 289 13473. E-mail: fritz. [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The authors are grateful to Dr. Gabriele Raudaschl-Sieber for the measurement of solid-state NMR and Dr. Markus Drees for performing the DFT calculations. K.M.-Ö . thanks the TUM Graduate School for financial support.

■

REFERENCES

(1) Ö fele, K.; Tosh, E.; Taubmann, C.; Herrmann, W. A. Chem. Rev. 2009, 109, 3408. (2) Taubmann, C.; Tosh, E.; Ö fele, K.; Herdtweck, E.; Herrmann, W. A. J. Organomet. Chem. 2008, 693, 2231. (3) (a) Herrmann, W. A.; Ö fele, K.; Schneider, S. K.; Herdtweck, E.; Hoffmann, S. D. Angew. Chem. 2006, 118, 3943−3947;(b) Angew. Chem., Int. Ed. 2006, 45, 3859−3862. (4) Schneider, S. K. Ph.D. Thesis, Technische Universität München (DE), Munich, Germany, 2005. (5) Herrmann, W. A.; Ö fele, K.; Taubmann, C.; Herdtweck, E.; Hoffmann, S. D. J. Organomet. Chem. 2007, 692, 3846. (6) Kawada, Y.; Jones, W. M. J. Organomet. Chem. 1980, 192, 87. (7) Winter, A. M.; Eichele, K.; Mack, H.-G.; Kaska, W. C.; Mayer, H. A. Organometallics 2005, 24, 1837. (8) (a) Taubmann, C. Ph.D. Thesis, Technische Universität München (DE), Munich, Germany, 2009. (b) Mantaş, K., Masters Thesis, Technische Universität München (DE), Munich, Germany, 2009. (9) Lin, S.; Boudjouk, P. J. Chinese Chem. Soc. 1989, 36, 35. (10) Murahashi, T.; Fujimoto, M.; Oka, M.; Hashimoto, Y.; Uemara, T.; Tatsumi, Y.; Nakao, Y.; Ikeda, A.; Sakaki, S.; Kurosawa, H. Science 2006, 313, 1104. (11) (a) Allison, N. T.; Kawada, Y.; Jones, W. M. J. Am. Chem. Soc. 1978, 100, 5224. (b) Riley, P. E.; Davis, R. E.; Allison, N. T.; Jones, W. M. Inorg. Chem. 1982, 21, 1321. (12) Von E. Doering, W.; Knox, L. H. J. Am. Chem. Soc. 1957, 79, 352. (13) (a) Jutz, C. Chem. Ber. 1964, 97, 2050. (b) Bauld, N. L.; Rim, Y. S. J. Am. Chem. Soc. 1967, 89, 6763. (c) Dauben, H. J.; Rhoades, D. F. J. Am. Chem. Soc. 1967, 89, 6764. (14) McGeachin, S. G. Can. J. Chem. 1969, 47, 151. (15) For example, see: Nasielski, J.; Hadei, N.; Achonduh, G.; Kantchev, E. A. B.; O’Brien, C. J.; Lough, A.; Organ, M. G. Chem. Eur. J. 2010, 16, 10844. 8255


Organometallics

Article

(16) Murahashi, T.; Usui, K.; Tachibana, Y.; Kimura, S.; Ogoshi, S. Chem.Eur. J. 2012, 18, 8886. (17) Fulmer, G. R.; Miller, A. J. M.; Sherden, N. H.; Gottlieb, H. E.; Nudelman, A.; Stoltz, B. M.; Bercaw, J. E.; Goldberg, K. I. Organometallics 2010, 29, 2176.

8256


Reactions of Nitrogen Donors with ... - ACS Publications

Recommend Documents