Convenient Synthetic Route to Palladium Complexes of

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Convenient Synthetic Route to Palladium Complexes of Unconventional N‑Heterocyclic Carbenes Derived from Pyridazine and Phthalazine Tongxun Guo, Sebastian Dechert, Steffen Meyer, and Franc Meyer* Institute of Inorganic Chemistry, Georg-August-University Göttingen, Tammannstrasse 4, 37077 Göttingen, Germany S Supporting Information *

ABSTRACT: Several Pd(II) complexes with unconventional pyridazine- and phthalazine-derived carbene ligands were synthesized via direct oxidative addition of Cl derivatives of the alkylated diazine heterocycles to Pd(0) species. The alkylated ligand precursors are readily prepared from commercial starting materials, and oxidative addition is regioselective. DFT calculations confirm that the thermodynamically favored products are formed. Four complexes (1− 4) have been fully characterized, including by X-ray crystallography. Attractive intramolecular π−π stacking between the electron-poor N-alkylated diazine heterocycles and adjacent phenyl groups of the PPh3 coligands is revealed by the solid-state structures.

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INTRODUCTION Metal complexes of N-heterocyclic carbenes (NHCs), especially 2-imidazolylidene derivatives, have experienced an impressive development in organometallic chemistry and catalysis during the past two decades.1 In recent years, 4imidazolylidenes (so-called abnormal NHCs) and carbenes derived from N-heterocycles other than imidazole have been discovered and are now finding increasing attention as strong σdonor ligands.2,3 Such unconventional carbenes are not stabilized by two adjacent heteroatoms, and some of them do not necessarily have heteroatoms in a position α to the carbene carbon. Representative examples are the pyridylidenes (I),4 pyrimidinylidenes (II),5 triazolylidenes (III),6 and pyrazolylidenes (IV)7 (Figure 1).

Figure 2. Pyridazine/2-imidazolylidene hybrid ligands V9,10 and VI11 and phthalazinylidene metal carbene complexes VII.12

have likely been hampered by their low to moderate yields, since they were obtained via C−H metalation of phthalazinium salts with basic precursor complexes that had been generated in situ.12 Herein, we present a new and convenient synthetic route to palladium(II) pyridazinylidene- and phthalazinylidene-derived carbene complexes by direct oxidative addition of the readily available ligand precursors ([L1Cl]BF4, [L2Cl]BF4, and [L3Cl]BF4) to Pd(0) species.

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Figure 1. Examples of N-heterocyclic carbenes beyond the “classical” 2-imidazolylidenes, including representatives with reduced heteroatom stabilization.

RESULTS AND DISCUSSION Common methods for the synthesis of NHC complexes comprise the in situ deprotonation with internal or external bases12,13 or transmetalation via silver NHC complexes.14 The insertion of low-valent metal precursors into a C−X bond of the N-alkylated ligand precursor is an attractive and widely used alternative,15 especially for the formation of unconventional palladium carbene complexes;4b,5,16 this strategy has the advantage of neat reaction conditions and usually high product yields.

Pyridazines (1,2-diazines) are well established in coordination chemistry and usually act as potentially bridging N-donor ligands.8 They have been combined with 2-imidazolylidene units to give pyridazine/NHC hybrid ligands such as V and VI (Figure 2).9−11 Recently, some unconventional rhodium and iridium carbene complexes VII with M−C bonds directly involving a ring C atom of the phthalazine heterocycle have also been reported.12 However, to our knowledge this has remained the only publication about carbene complexes derived from 1,2diazines so far. Further studies and the use of complexes VII © 2012 American Chemical Society

Received: September 21, 2012 Published: December 3, 2012 8537

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pyridazinylidene ligand is oriented perpendicular to the palladium(II) coordination plane or undergoes rapid rotation around the Pd−C bond on the NMR time scale. 13C NMR signals for the carbene-C are observed at 192.6 (1) and 199.4 ppm (2). The molecular structures of complexes 1 and 2 determined by single crystal X-ray diffractometry are depicted in Figures 3 and 4. They confirm the conclusions from NMR spectroscopy:

In this work, treatment of 3,6-dichloropyridazine or 1,4dichlorophthalazine with Meerwein’s salt [Et3O]BF4 smoothly generated the corresponding alkylated products 1-ethyl-3,6dichloropyridazinium tetrafluoroborate ([L1Cl]BF4) and 2ethyl-1,4-dichlorophthalazinium tetrafluoroborate ([L2Cl]BF4), respectively (Scheme 1). In the 1H NMR spectra in CD3CN, Scheme 1. Synthesis of the Ligand Precursors [L1Cl]BF4 and [L2Cl]BF4 and Palladium NHC Complexes 1 and 2

signals of the aromatic protons of 3,6-dichloropyridazine shift from 7.71 ppm to 8.46 and 8.54 ppm after ethylation (AB pattern with JAB = 9.1 Hz), reflecting the reduced symmetry and the positive charge of the six-membered heterocycle in [L1Cl]BF4. In its 13C NMR spectrum the C6 (Cα) resonance shifts to high field from 155.8 ppm for 3,6-dichloropyridazine to 154.3 ppm for [L1Cl]BF4. Treatment of [L1Cl]BF4 and [L2Cl]BF4 with 1 equiv of Pd(PPh3)4 in acetone at room temperature for 30 min gave palladium(II) carbene complexes 1 and 2 in good yields (73% and 61%, respectively; Scheme 1). Complexes 1 and 2 have been purified and obtained as colorless crystals by slow diffusion of diethyl ether into solutions of the crude products in dichloromethane or acetone; these crystals were also used for X-ray crystallography. The crystalline material is soluble in dichloromethane, slightly soluble in acetone or acetonitrile, and insoluble in hexane, diethyl ether, THF, or toluene. Both 1 and 2 are reasonably stable in the solid state under aerobic conditions. Solutions of the complexes in dichloromethane gradually turn pale yellow upon prolonged standing. The integrity of 1 and 2 in solution was confirmed by positive ion ESI mass spectrometry, showing the base peaks at m/z 808.8 (1) and 858.8 (2) for the [M − BF4]+ cations. The two PPh3 ligands give rise to a single resonance at 20.9 ppm (1) or 21.1 ppm (2) in the 31P NMR spectra, suggesting that the PPh 3 ligands are trans to each other and that the

Figure 4. Molecular structure of 2. Anisotropic displacement ellipsoids are drawn at the 30% probability level. The solvent molecule, BF4−, and hydrogen atoms have been omitted for clarity. Selected bond lengths (Å) and angles (deg): Pd1−C1 = 1.989(5), Pd1−P1 = 2.3439(8), Pd1−P1′ = 2.3438(8), Pd1−Cl2 = 2.3513(12), C1−C2 = 1.423(7), N1−C1 = 1.340(6), N2−C8 = 1.278(8), N1−N2 = 1.377(6); C1−Pd1−P1 = 88.89(2), C1−Pd1−P1′ = 88.89(2), P1− Pd1−Cl2 = 91.08(2), P1′−Pd1−Cl2 = 91.08(2), N1−C1−C2 = 117.6(5), C1−N1−N2 = 125.4(5).

in both cases the palladium atoms adopt square-planar coordination geometries with the carbene-C and chloride in trans positions. The Pd−Ccarbene bond lengths are 1.996(2) Å (1) and 1.989(5) Å (2), respectively, which agrees well with values for other reported unconventional NHC palladium complexes.4b,6,7 Ccarbene−N(1)/CCl−N(2) bond lengths are 1.337(3)/1.301(3) Å in 1 and 1.340(6)/1.278(8) Å in 2, indicating that the heterocyclic rings are still delocalized, yet the Ccarbene−N(1) bond is somewhat elongated due to the backbonding from palladium to the carbene-C (cf. Scheme 2). The pyridazinylidene ligands are found almost perpendicular to the palladium coordination planes in both structures, with torsion

Figure 3. Different views of the molecular structure of 1 (middle and right views for emphasizing the π−π interaction). Anisotropic displacement ellipsoids drawn at the 30% probability level. BF4− and hydrogen atoms have been omitted for clarity. Selected bond lengths (Å) and angles (deg): Pd1−C1 = 1.996(2), Pd1−P1 = 2.3331(5), Pd1−P2 = 2.3343(5), Pd1−Cl1 = 2.3422(5), C1−C2 = 1.419(3), N1−C1 = 1.337(3), N2−C4 = 1.301(3), N1−N2 = 1.357(3); C1−Pd1−P1 = 91.50(6), C1−Pd1−P2 = 89.72(6), P1−Pd1−Cl1 = 88.962(19), P2−Pd1−Cl1 = 90.10(2), N1−C1− C2 = 115.8(2), C1−N1−N2 = 126.13(19). 8538

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To confirm the above considerations, relative energies (Erel) have been computed at the BP86/TZVPP level of theory for 1 (represented by A; Figure 5) and its congener B that has the Pd

Scheme 2. Selective Formation of Palladium C6-Carbene Bond

angles of around 87° (1) or 89° (2), which minimizes steric repulsion between the heterocycles and the phenyl groups on the phosphine ligands. An interesting feature of 1 is the intramolecular π−π interaction between the electron-poor N-alkylated pyridazine ring and two adjacent phenyl groups of the PPh3 coligands. This results in a remarkable bending of the respective phenyl rings toward the pyridazine moiety (Figure 3, middle); the pyridazine ring even appears to be sandwiched between two phenyl groups. This bending can be quantified by the angles between the P−C bond and the least-squares planes of the phenyl ring atoms, which are 10.5 and 11.2° in the case of the interacting rings and 1.3−2.8° for the other phenyl groups.17 The distance of the ring centroids is 3.4 Å, though both phenyl rings are not strictly coplanar with the pyridazine ring and are somewhat displaced laterally (Figure 3, middle and right). It is interesting to note that the reactions forming 1 and 2 proceed regioselectively via oxidative addition of the ligand precursor’s Cα−Cl bonds, while the other C−Cl bond remains intact and no such palladium carbene complex is detected. Steric effects obviously do not play a role, since they should favor Pd insertion into the C−Cl bond remote from the ethylated N atom. Related regioselectivity was previously observed in the reaction of 2,4-dichloropyridium salts with [Pd(PPh3)4], which exclusively gave the 4-pyridylidene complexes.4d That observation was rationalized against the background of established pyridine cross-coupling chemistry, where the electron-rich d10 metal is known to act as a nucleophile and attacks the most electron-deficient position.18 A distinct electrophilic character of the C4 atom of the 2,4dichloropyridium salts was experimentally correlated with its 13 C NMR chemical shift, which indeed indicated that C4 (δ 156.0 ppm) is more deshielded than C2 (δ 148.9 ppm).4d Since DFT calculations (BP86/TZVPP level) revealed that the 2pyridylidene complexes are more stable than the 4-congeners, it was concluded that formation of the 4-pyridylidene complexes proceeds via kinetic control to yield the less stable isomers.4d For the present ligand precursors [L1Cl]BF4 and [L2Cl]BF4 the situation appears to be different. 13C NMR data, which have been assigned on the basis of HMBC experiments (see the Supporting Information), clearly show that the C atoms of the remote C−Cl units are more deshielded (δC 156.7/156.4 ppm) than the Cα carbon atoms next to the ethylated N atoms (δC 154.3/155.6 ppm), though differences are much less pronounced than in the case of 2,4-dichloropyridium salts. For [L1Cl]BF4 and [L2Cl]BF4 insertion of Pd0 evidently occurs at the less deshielded site, namely the Cα−Cl bond. This formally generates a σ-bonded C−PdII−Cl complex with a positively charged nitrogen atom attached to the carbene-C, which may be favorably delocalized onto palladium according to the resonance structures shown in Scheme 2. No such valence structures can be drawn for the other possible isomer. One may thus assume that regioselective formation of 1 and 2 gives the thermodynamically most favorable products.

Figure 5. DFT models of the possible products formed upon Pd insertion into either the Cα−Cl bond (left) or the remote C−Cl bond of [L1Cl]BF4.

inserted in the remote C−Cl bond, as well as for a number of related models that vary with respect to the N-alkyl substituent (Et versus Me) or the phosphine ligand (PPh3 versus PMe3). The latter was chosen to obviate any potential effect from π−π interaction of the PPh3 phenyl groups and the pyridazine ring (see the structure discussion and Supporting Information for details). Computational results are compiled in Table 1. Table 1. Computational Results: Relative Minimized Energies (BP86/TZVPP) Erel (kcal/mol) A B

0 +8.4

Erel (kcal/mol) C D

0 +9.7

Erel (kcal/mol) E F

0 +7.5

Erel (kcal/mol) G H

0 +7.1

In all cases the isomer with the Pd−C(carbene) bond next to the alkylated N atom, which was also observed experimentally in 1, is clearly more stable than the other isomer that has the Pd−C(carbene) bond more remote from the alkylated N atom, by around 7−10 kcal/mol. Comparing the pairs A/B (with a more bulky Et group) and C/D (with a less demanding Me group) suggests that the effect of sterics is minor and attenuates the Erel value only slightly. Oxidative addition reactions of [L1Cl]BF4 and [L2Cl]BF4 apparently give the thermodynamic products in regioselective reactions. In order to shed further light on the rotational dynamics of the pyridazinylidene ligand, a complex with only one PPh3 ligand and reduced symmetry was sought. Treatment of [L1Cl]BF4 with 1/2 equiv of Pd2(dba)3, 1 equiv of NEt4Cl, and 1 equiv of PPh3 in acetone gave a gray precipitate of the neutral complex 3 (40%; Scheme 3). Pure material was obtained as colorless crystals by diffusing diethyl ether into a solution of the crude product in dichloromethane. Complex 3 is Scheme 3. Synthesis of the Neutral Pyridazinylidene Palladium Complex 3

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rafluoroborate ([L3Cl]BF4) as a major product. Reaction of [L3Cl]BF4 and Pd(PPh3)4 smoothly generated the complex 4 (80%; Scheme 4).

soluble in dichloromethane, slightly soluble in acetonitrile, and insoluble in acetone, diethyl ether, hexane, or toluene. Upon prolonged standing under aerobic conditions and under light the solution of complex 3 in dichloromethane gradually gives a palladium black sediment. In solid state under light complex 3 gradually turns green. The molecular structure of complex 3 determined by X-ray diffraction is shown in Figure 6. As in 1, the palladium atom in

Scheme 4. Synthesis of Palladium NHC Complex 4 from the Ligand Precursor [L3Cl]BF4

The structure of complex 4 was confirmed by X-ray diffraction (Figure 7). As expected, the palladium-centered Figure 6. Molecular structure of 3. Anisotropic displacement ellipsoids are drawn at the 30% probability level. Selected bond lengths (Å) and angles (deg): Pd1−C1 = 1.9772(18), Pd1−P1 = 2.2618(5), Pd1−Cl1 = 2.3693(5), Pd1−Cl2 = 2.3540(5), C1−C2 = 1.4329(24), N1−C1 = 1.333(2), N2−C4 = 1.318(3), N1−N2 = 1.355(2); C1−Pd1−P1 = 90.86(5), C1−Pd1−Cl1 = 89.15(5), P1−Pd1−Cl2 = 88.342(17), Cl2−Pd1−Cl1 = 91.700(18), N1−C1−C2 = 115.61(16), C1−N1− N2 = 125.49(16).

3 also adopts the expected square planar geometry with a chlorido ligand trans to the unconventional NHC, hence with the PPh3 cis to the NHC moiety. The Pd−Ccarbene bond length (1.977(2) Å) is essentially identical with that in 1, while the Pd−Cl2 bond trans to the pyridazinylidene ligand (2.3540(5) Å) is slightly shorter than the Pd−Cl1 bond trans to the phosphine ligand (2.3693(5) Å), suggesting a subtly stronger trans influence of PPh3. The pyridazinylidene ring is not strictly perpendicular to the palladium coordination plane, but the torsion angle C2−C1−Pd−P is roughly 76°; this reflects that steric congestion in 3 is reduced compared to the situation in 1. The carbene C of 3 shows up at 191.5 ppm in the 13C NMR spectrum, and the phosphine ligand gives rise to a 31P NMR signal at 26.1 ppm; the latter confirms the presence of a single isomer in solution. In 1H NMR spectroscopy, the proton bound to C2 resonantes at 7.95 ppm as a doublet of doublets (3JHH = 8.8 Hz, 4JHP = 1.1 Hz), while the proton bound to C3 is observed at 6.89 ppm (3JHH = 8.8 Hz). CH2 protons of the Nbound ethyl groups are diastereotopic: one of them is observed as a doublet/quartet at 5.38 ppm (J = 12.4/7.2 Hz) and the other as a multiplet at 4.62−4.48 ppm. This shows that the pyridazinylidene ligand is oriented roughly perpendicular to the palladium coordination plane and that rotation around the Pd− Ccarbene bond is hindered on the NMR time scale. While the phenyl groups of even a single adjacent phosphine ligand provide sufficient steric bulk to prevent rotation of the NHC, it may also indicate significant back-bonding and hence doublebond character of the Pd−Ccarbene linkage, as well as the effect of π−π stacking. The oxidative addition strategy could also be applied to 3chloro 6-substituted pyridazine derivatives. As an example, treatment of 3-chloro-6-(3,5-dimethylpyrazol-1-yl)pyridazine with [Et3O]BF4 in CH3CN at room temperature for 3 h gave 1-ethyl-3-chloro-6-(3,5-dimethylpyrazol-1-yl)pyridazinium tet-

Figure 7. Molecular structure of 4. Anisotropic displacement ellipsoids are drawn at the 30% probability level. The solvent molecule, BF4−, and hydrogen atoms have been omitted for clarity. Selected bond lengths (Å) and angles (deg): Pd1−C1 = 1.982(4), Pd1−P1 = 2.3208(9), Pd1−P2 = 2.3431(10), Pd1−Cl1 = 2.3463(10), C1−C2 = 1.433(6), N1−C1 = 1.336(5), N2−C4 = 1.305(5), N1−N2 = 1.350(4); C1−Pd1−P1 = 89.40(10), C1−Pd1−P2 = 89.39(10), P1−Pd1−Cl1 = 88.99(3), P2−Pd1−Cl1 = 92.24(4), N1−C1−C2 = 115.6(4), C1−N1−N2 = 126.0(3).

core of complex 4 is similar to that of 1: the metal ion adopts a square-planar geometry with two PPh3 trans to each other and a Pd−Ccarbene bond of 1.982(4)°, which is slightly shorter than that in 1. The torsion angle between the pyridazine ring and the appended pyrazole ring is 21°, with the pyrazole-N4 pointing away from the pyridazine-N2. However, facile rotation of the interring C−N bond might bring the N2 and N4 atoms on the same side, thus creating a potentially N,N′-bidentate chelating pocket for binding additional metal ions. Work toward the synthesis of such heterobimetallic complexes is in progress in our laboratories. π−π interactions similar to the situation in 1 are likely operative also for 3 and 4, since the PPh3 phenyl rings close to the pyridazine moiety are again bent by 8.6°(3) and 8.7/10.8° (4) toward the N-alkylated heterocycle, whereas angles between the P−C bond and the least-squares plane of the phenyl ring atoms are much smaller and in the range from 0.3 to 4.6° for the other phenyl groups. Distances of the relevant 8540

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centroids of the π−π interacting rings range from 3.4 to 3.6 Å. In the case of the phthalazinylidene complex 2 all such angles for the PPh3 ligands are found in the narrow range from 3.4 to 5.5° and the relevant centroid distance is 3.8 Å, indicating that any intramolecular π−π interaction is much less pronounced than for the pyridazinylidene system.

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CONCLUSIONS

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EXPERIMENTAL SECTION

Synthesis of [L2Cl]BF4.

A solution of 1,4-dichlorophthalazine (100 mg, 0.502 mmol) in dry DCM (5.0 mL) was treated with [Et3O]BF4 (104 mg, 0.553 mmol, 1.10 equiv). The colorless solution was stirred at room temperature for 1 h. Et2O (10.0 mL) was added to the solution to precipitate a white solid. This solid was collected by filtration, washed with Et2O (10.0 mL, twice), and dried under reduced pressure to give 128 mg of [L2Cl]BF4 (0.406 mmol, 81.0 %). The product is soluble in acetonitrile or acetone, slightly soluble in dichloromethane, and insoluble in diethyl ether, THF, toluene, or hexane. The product is unstable at room temperature and is better used freshly prepared. 1 H NMR (300 MHz, CD3CN, 298 K): δ 8.85−8.78 (m, 1H, CH), 8.71-8.66 (m, 2H, CH), 8.54 (J1 = 8.4 Hz, J2 = 7.0 Hz, J3 = 1.0 Hz, 1H, CH), 5.05 (q, J = 7.3 Hz, 2H, CH2CH3), 1.71 (t, J = 7.3 Hz, 3H, CH3). 13 C NMR (75 MHz, CD3CN, 298 K): δ 156.4 (s, C4Cl), 155.6 (s, C1Cl), 140.8 (s, C8H), 138.1 (s, C7H), 129.2 (s, C9H), 128.8 (s, C5), 128.0 (s, C10), 127.2 (s, C6H), 59.7 (s, CH2CH3), 12.3 (s, CH3). HR MS (ESI+): m/z calculated [M − BF4]+ 227.0137, observed 227.0137. Anal. Calcd for C10H9BCl2F4N2: C, 38.14; H, 2.88; N, 8.90. Found: C, 38.14; H, 2.81; N, 8.85. Synthesis of [L3Cl]BF4. A solution of 3-chloro-6-(3,5-dimethylpyrazol-1-yl)pyridazine (100 mg, 0.479 mmol) in dry CH3CN (8.0 mL) was treated with [Et3O]BF4 (109 mg, 0.575 mmol, 1.20 equiv). The colorless solution was stirred at room temperature for 3 h. Et2O (15.0 mL) was added to the solution to precipitate a white solid. This solid was collected by filtration, washed with Et2O (10.0 mL, twice), and dried under reduced pressure to give 105 mg of [L3Cl]BF4 (0.324 mmol, 67.6%). The product is soluble in acetonitrile or acetone, slightly soluble in dichloromethane, and insoluble in diethyl ether, THF, toluene, or hexane. The product is stable at room temperature in the solid state. 1 H NMR (300 MHz, CD3CN, 298 K): δ 8.85 (d, J = 9.5 Hz, 1H, CHPyri), 8.45 (d, J = 9.5 Hz, 1H, CHPyri), 6.33 (s, 1H, CHPyra), 4.91 (q, J = 7.2 Hz, 2H, CH2CH3), 2.68 (s, 3H, CH3Pyra), 2.31 (s, 3H, CH3Pyra), 1.67 (t, J = 7.2 Hz, 3H, CH2CH3). 13C NMR (75 MHz, CD3CN, 298 K): δ 155.2 (s, C), 154.7 (s, C), 149.9 (s, C), 143.6 (s, C), 138.7 (s, CHPyri), 130.3 (s, CHPyri), 112.7 (s, CHPyra), 59.7 (s, CH2CH3), 13.9 (s, CH3Pyra), 12.5 (s, CH2CH3), 12.2 (s, CH3Pyra). HR MS (ESI+): m/z calculated [M − BF4]+ 237.0902, observed 237.0901. Anal. Calcd for C11H14BClF4N4: C, 40.71; H, 4.35; N, 17.26. Found: C, 40.73; H, 4.23; N, 17.23. Synthesis of [L1Pd(PPh3)2Cl]BF4 (1). A mixture of the ligand precursor [L1Cl]BF4 (100 mg, 0.378 mmol) and Pd(PPh3)4 (436 mg, 0.378 mmol, 1.00 equiv) in a Schlenk tube was degassed three times. Degassed acetone (12.0 mL) was added through a syringe. The suspension was stirred for 30 min until a clear yellow solution was formed. Degassed toluene (15.0 mL) was added, and the mixture was stirred vigorously for 30 min; a white precipitate gradually appeared. The solid was collected by filtration, washed with toluene (8.0 mL) and diethyl ether (8.0 mL), and finally dried under reduced pressure to give 248 mg of an off-white powder (0.277 mmol, 73%). The product can be purified and obtained as colorless crystals by slow diffusion of diethyl ether into a solution of the crude material in dichloromethane or acetone. [L1Pd(PPh3)2Cl]BF4 is soluble in dichloromethane, slightly soluble in acetone or acetonitrile, and insoluble in THF, diethyl ether, toluene, or hexane. The product is reasonably stable in the solid state and slowly turns yellow as a solution in dichloromethane. 1 H NMR (300 MHz, CD2Cl2, 298 K): δ 7.72 (d, J = 8.8 Hz, 1H, CHPyri), 7.59−7.31 (m, 30H, CHPh), 6.70 (d, J = 8.8 Hz, 1H, CHPyri), 4.59 (q, J = 7.2 Hz, 2H, CH2CH3), 1.14 (t, J = 7.2 Hz, CH3). 13C NMR (75 MHz, CD2Cl2, 298 K): δ 192.6 (t, J = 6.9 Hz, PdC), 154.4 (s, ClC), 147.6 (t, J = 2.9 Hz, CHPyri), 134.5 (t, J = 6.3 Hz, CHPh), 132.2 (s, CHPh), 129.6 (s, CHPyri), 129.6 (t, J = 5.8 Hz, CHPh), 128.5

In conclusion, we present a convenient synthetic route to palladium pyridazinylidene- and phthalazinylidene-derived carbene complexes. These complexes are easily generated via direct oxidative addition of N-alkylated ligand precursors, obtained from cheap heterocyclic compounds, to common Pd0 reagents. A remarkable structural feature is the pronounced attractive π−π interaction of the electron-poor alkylated pyridazine ring with adjacent phenyl groups of the PPh3 ligands on Pd. DFT results confirm that the isomers with the Pd−C(carbene) bond next to the alkylated N atom, whose regioselective formation is observed experimentally, represent the thermodynamically favored products. However, the presence of the C3−Cl moiety at the pyridazinylidene and phthalazinylidene backbone in 1−3 might offer interesting perspectives for further functionalization, either before or after formation of the unconventional NHC complexes, as demonstrated for the pyrazole/pyridazinylidene hybrid 4. Studies in this direction and toward the use of these new NHC ligands in catalytic applications are ongoing.

General Remarks. The ligand precursors were synthesized under an anaerobic atmosphere, while all other organometallic reactions were carried out under an inert atmosphere of dry dinitrogen. All solvents were used in P.A. grade. 3-Chloro-6-(3,5-dimethylpyrazol-1-yl)pyridazine was synthesized according to the reported procedure.19 All other chemicals were used as purchased. NMR spectra were recorded on Bruker Avance 300 MHz or Bruker DRX 500 MHz spectrometers. EI mass spectra were recorded with a Finnigan MAT 8200 instrument. ESI mass spectra were recorded with an Applied Biosystems API 2000 and Bruker HCT ultra instruments. Elemental analyses were performed by the analytical laboratory of the Institute of Inorganic Chemistry at Georg-August-University using an Elementar Vario EL III instrument. Crystal data and refinement details of the Xray diffraction analyses are given in the Supporting Information. Synthesis of [L1Cl]BF4. A solution of 3,6-dichloropyridazine (1.00 g, 6.71 mmol) in dry DCM (20.0 mL) was treated with [Et3O]BF4 (1.40 g, 7.38 mmol, 1.10 equiv). The colorless solution was stirred at room temperature for 1 h. The product normally precipitated as a white powder when the reaction was complete. Otherwise, Et2O (10.0 mL) could be added to the solution to help precipitate the product. The white solid was collected after filtration, washed with Et2O (20.0 mL, twice), and dried under reduced pressure to give 1.50 g of [L1Cl]BF4 (5.68 mmol, 84.6 %). The product is soluble in acetonitrile or acetone, slightly soluble in dichloromethane, and insoluble in diethyl ether, THF, toluene, or hexane. The product is stable under aerobic and humid conditions. 1 H NMR (300 MHz, CD3CN, 298 K): δ 8.54 (d, JAB = 9.1 Hz, 1H, C4H), 8.46 (d, JAB = 9.1 Hz, C5H), 4.93 (q, J = 7.2 Hz, 2H, CH2CH3), 1.65 (t, J = 7.2 Hz, 3H, CH3). 13C NMR (75 MHz, CD3CN, 298 K): δ 156.7 (s, C3), 154.3 (s, C6), 139.6 (s, CH), 139.5 (s, CH), 60.3 (s, CH2CH3), 12.1 (s, CH3). HR MS (ESI+): m/z calculated [M − BF4]+ 176.9981, observed 176.9980. Anal. Calcd for C6H7BCl2F4N2: C, 27.21; H, 2.66; N, 10.58. Found: C, 26.92; H, 2.83; N, 10.43. 8541

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Organometallics

Article

(t, J = 25.3 Hz, PC), 64.3 (s, CH2CH3), 13.9 (s, CH3). 31P NMR (121 MHz, CD2Cl2, 298 K): δ 20.9 (s). MS (ESI+, CH2Cl2): m/z (%) 808.8 (100, [M − BF4]+). MS (ESI−, CH2Cl2): m/z (%) 87.0 (100, [BF4]−). HR MS (ESI+): m/z calculated [M − BF4]+ 807.0849, observed 807.0835. Anal. Calcd for C42H37BCl2F4N2P2Pd: C, 56.31; H, 4.16; N, 3.13. Found: C, 56.35; H, 4.01; N, 3.02. Synthesis of [L2Pd(PPh3)2Cl]BF4 (2). A mixture of the ligand precursor [L2Cl]BF4 (50.0 mg, 0.159 mmol) and Pd(PPh3)4 (167 mg, 0.159 mmol, 1.00 equiv) in a Schlenk tube was degassed three times. Degassed acetone (5.0 mL) was added through a syringe. The suspension was stirred for 30 min until a clear yellow solution was formed. Degassed diethyl ether (10.0 mL) was added to cause precipitation of a yellow solid. The solid was separated by filtration, washed with diethyl ether (8.0 mL), and dried under reduced pressure to give 91.0 mg of a yellow powder (0.096 mmol, 61%). [L2Pd(PPh3)2Cl]BF4 can be purified and obtained as colorless crystals by slow diffusion of diethyl ether into a solution of the crude material in acetone or dichloromethane. The product is soluble in dichloromethane, slightly soluble in acetone or acetonitrile, and insoluble in THF, diethyl ether, toluene, or hexane. The product is reasonably stable in the solid state and slowly turns yellow as a solution in dichloromethane. 1 H NMR (300 MHz, CD2Cl2, 298 K): δ 8.90 (d, J = 7.8 Hz, 1H, CHPhth), 8.09-7.92 (m, 3H, CHPhth), 7.55−7.28 (m, 30H, CHPh), 4.84 (q, J = 7.2 Hz, 4H, CH2CH3), 1.15 (t, J = 7.2 Hz, 3H, CH3). 13C NMR (75 MHz, CD2Cl2, 298 K): δ 199.4 (t, J = 6.3 Hz, PdC), 154.9 (s, CPhth), 138.5 (s, CHPhth), 136.5 (t, J = 1.8 Hz, CPhth), 136.0 (s, CHPhth), 135.2 (s, CHPhth), 134.2 (t, J = 6.2 Hz, CHPh), 132.2 (s, CPh), 129.3 (t, J = 5.4 Hz, CHPh), 128.2 (t, J = 25.3 Hz, CPh), 127.0 (s, CHPhth), 123.1 (s, CPhth), 63.4 (s, CH2CH3), 13.4 (s, CH3). 31P NMR (121 MHz, d6acetone, 298 K): δ 21.1 (s). MS (ESI+): m/z (%) 858.8 (83.0, [M − BF4]+). MS (ESI−): m/z (%) 87.0 (100, [BF4]−). Anal. Calcd for C46H39N2Cl2P2BF4Pd·1/3CH2Cl2: C, 57.12; H, 4.10; N, 2.88. Found: C, 57.03; H, 4.18; N, 2.60. Synthesis of L1Pd(PPh3)Cl2 (3). The ligand precursor [L1Cl]BF4 (100 mg, 0.378 mmol), NEt4Cl (62.6 mg, 0.378 mmol, 1.00 equiv), and Pd2(dba)3 (119 mg, 0.208 mmol, 0.55 equiv) were placed in a Schlenk tube that was then purged with N2. Degassed acetone (8.0 mL) was added through a syringe, and the suspension was stirred under N2 at room temperature for 30 min. During this time, the solution turned gradually from dark purple to greenish yellow. PPh3 (99.0 mg, 0.378 mmol, 1.0 equiv) was added in the Schlenk tube as one portion, followed by degassed acetone (5.0 mL), and the mixture was stirred for a further 15 min. The suspension was filtered, and the gray solid was collected after filtration. The solid was dissolved in CH2Cl2 (8.0 mL) and the solution, containing a small amount of palladium black, was filtered through Celite 545 to give an almost colorless solution, which was dried under reduced pressure to give L1Pd(PPh3)Cl2 as an off-white solid (yield 87.0 mg, 0.149 mmol, 40%). The product is soluble in dichloromethane, slightly soluble in acetone or acetonitrile, and insoluble in THF, diethyl ether, toluene, or hexane. Upon prolonged standing under aerobic conditions and under light solutions of the product in dichloromethane gradually give palladium black sediments. In the solid state under light the product gradually turns green. 1 H NMR (300 MHz, CD2Cl2, 298 K): δ 7.95 (dd, J1 = 8.8 Hz, J2 = 1.1 Hz, 1H, CHPyri), 7.63−7.23 (m, 15H, CHPh), 6.89 (d, J = 8.8 Hz, 1H, CHPyri), 5.38 (dq, J1 = 12.4 Hz, J2 = 7.2 Hz, 1H, CH2CH3), 4.55 (m, 1H, CH2CH3), 1.53 (t, J = 7.2 Hz, 3H, CH3). 13C NMR (75 MHz, CD2Cl2, 298 K): δ 191.5 (s, PdC), 153.6 (s, ClC), 147.0 (d, J = 3.7 Hz, CHPyri), 134.6 (d, J = 11.1 Hz, CHPh), 131.8 (d, J = 2.7 Hz, CHPh), 129.9 (d, J = 53.6 Hz, CPh), 129.1 (d, J = 11.1 Hz, CHPh), 123.3 (s, CHPyri), 64.7 (s, CH2CH3), 14.6 (s, CH3). 31P NMR (121 MHz, CD2Cl2, 298 K): δ 26.1 (s). HR MS (ESI+): m/z calculated [M − Cl]+ 544.9929, observed 544.9923. Anal. Calcd for C24H22Cl3N2PPd·1/8CH2Cl2: C, 48.88; H, 3.78; N, 4.73. Found: C, 48.75; H, 3.63; N, 4.93. Synthesis of [L3Pd(PPh3)2Cl]BF4 (4). A mixture of the ligand precursor [L3Cl]BF4 (30.0 mg, 0.092 mmol) and Pd(PPh3)4 (106.8 mg, 0.092 mmol, 1.0 equiv) in a Schlenk tube was degassed three

times. Degassed acetone (4.0 mL) was added through a syringe. The suspension was stirred for 30 min until a clear yellow solution was formed. Degassed toluene (15.0 mL) was added to cause precipitation of an off-white solid. The solid was separated by filtration, washed with toluene (5.0 mL) and diethyl ether (5.0 mL), and dried under reduced pressure to give 70.0 mg of a white powder (0.073 mmol, 80%). [L3Pd(PPh3)2Cl]BF4 can be purified and obtained as colorless crystals by slow diffusion of diethyl ether into a solution of the crude material in acetone or dichloromethane. The product is soluble in dichloromethane, slightly soluble in acetone or acetonitrile, and insoluble in THF, diethyl ether, toluene, or hexane. The product is reasonably stable in the solid state. 1 H NMR (300 MHz, CD2Cl2, 298 K): δ 7.62−7.28 (m, 32H, CHPh, CHPyri), 6.02 (s, 1H, CHPyra), 4.50 (q, J = 7.2 Hz, 2H, CH2CH3), 2.29 (s, 3H, CH3Pyra), 2.16 (s, 3H, CH3Pyra), 1.06 (t, J = 7.2 Hz, 3H, CH2CH3). 13C NMR (75 MHz, CD2Cl2, 298 K): δ 187.5 (t, J = Hz, PdC), 154.2 (s, Pyra-CPyri), 153.9 (s, CH3CPyra), 145.6 (t, J = Hz, CHPyri), 142.4 (s, CH3CPyra), 134.5 (t, J = 6.2 Hz, CHPh), 132.2 (s, CHPh), 129.5 (t, J = 5.3 Hz, CHPh), 129.0 (t, J = 25.1 Hz, PC), 121.7 (s, CHPyri), 112.5 (s, CHPyra), 63.9 (s, CH2CH3), 15.0 (s, CH3Pyra), 13.8 (s, CH2CH3), 13.7 (s, CH3Pyra). 31P NMR (121 MHz, CD2Cl2, 298 K): δ 21.5 (s). HR MS (ESI+): m/z calculated [M − BF4]+ 867.1774, observed 867.1774. Anal. Calcd for C47H44BClF4N4P2Pd·1/4CH2Cl2: C, 58.10; H, 4.59; N, 5.74. Found: C, 58.25; H, 4.46; N, 5.72.

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ASSOCIATED CONTENT

S Supporting Information *

Text, tables, figures, and CIF files giving NMR spectra of all new compounds, details of the DFT calculations, and details of the crystallographic studies. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Fax: +49 551 393063. Tel: +49 551 393012. E-mail: franc. [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge support by the State of Lower Saxony (Lichtenberg fellowship to T.G. within the framework of the international Ph.D. program Catalysis for Sustainable Synthesis; see www.casus.uni-goettingen.de) and by Umicore AG & Co. KG for providing chemicals.

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REFERENCES

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