Cyclopalladation of meta-(Diphenylthiophosphoryloxy)benzaldimines

Apr 12, 2010 - Complexes 5 present the products of formal oxidation of the P═S ... Ming-Jun Yang , Yan-Jing Liu , Jun-Fang Gong , and Mao-Ping Song...
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Organometallics 2010, 29, 2054–2062 DOI: 10.1021/om9009894

Cyclopalladation of meta-(Diphenylthiophosphoryloxy)benzaldimines: NCS and Unexpected NCO 5,6-Membered Pincer Palladium Complexes V. A. Kozlov,† D. V. Aleksanyan,† Yu. V. Nelyubina,† K. A. Lyssenko,† A. A. Vasil’ev,‡ P. V. Petrovskii,† and I. L. Odinets*,† †

A.N. Nesmeyanov Institute of Organoelement Compounds, Russian Academy of Sciences, 28 Vavilova Street, Moscow 119991, Russian Federation, and ‡N.D. Zelinsky Institute of Organic Chemistry, Russian Academy of Sciences, 47 Leninsky Prospect, 117913 Moscow, Russian Federation Received November 13, 2009

Unsymmetrical NCS-pincer ligands of a new type, namely, m-(diphenylthiophosphoryloxy)benzaldimines 3 (1-[Ph2P(S)O]-3-[CHdNR]-C6H4, R = OMe (3a), Ph (3b), tBu (3c)), were obtained in two steps starting from commercially available 3-hydroxybenzaldehyde. These ligands easily underwent cyclopalladation at the C-2 position of the central benzene ring in the reaction with PdCl2(PhCN)2 in benzene or benzene-methanol solutions to afford the corresponding hybrid pincer complexes 4a-c with five- and six-membered fused metallacycles in moderate to good yields. The same reaction in dichloromethane followed by treatment with alcohol resulted in unexpected formation of the related NCO-palladacycles 5a,b, along with the above NCS-complexes. Complexes 5 present the products of formal oxidation of the PdS group in the starting ligand, which apparently proceeds in the metal ion coordination sphere. Realization of κ3-NCS and κ3-NCO coordination in 4a-c and 5a,b, respectively, was unambiguously confirmed by X-ray diffraction analysis as well as multinuclear (1H, 13C, 31P) NMR, IR, and Raman spectroscopy. The NCS-pincer complexes 4a-c demonstrated excellent catalytic activity for the Suzuki cross-coupling reactions of aryl halides with phenylboronic acid.

Introduction Pincer complexes with a terdentate monoanionic backbone are of particular interest due to the readiness of finetuning their steric and electronic properties via variation of the metal center nature and its immediate and distant surroundings. An extending scope of applications of pincer complexes, particularly during the last decades, has led to more and more investigations in this area.1 Among the strategies used to improve the chemical and physical properties of these compounds, desymmetrization of a pincer structure holds a special position. Unsymmetrical pincer complexes were shown to exhibit unique reactivity and *Corresponding author. E-mail: [email protected]. (1) (a) Albrecht, M.; van Koten, G. Angew. Chem., Int. Ed. 2001, 40, 3750. (b) Singleton, J. T. Tetrahedron 2003, 59, 1837. (c) van der Boom, M. E.; Milstein, D. Chem. Rev. 2003, 103, 1759. (d) Peris, E.; Crabtree, R. H. Coord. Chem. Rev. 2004, 248, 2239. (e) Morales-Morales, D. Rev. Soc. Quı´m. M ex. 2004, 48, 338. (f) Dupont, J.; Consorti, C. S.; Spencer, J. Chem. Rev. 2005, 105, 2527. (g) Pughl, D.; Danopoulos, A. A. Coord. Chem. Rev. 2007, 251, 610. (h) Morales-Morales, D.; Jensen, C. M. Eds. The Chemistry of Pincer Compounds; Elsevier: New York, 2007. (2) (a) Wang, Z.; Eberhard, M. R.; Jensen, C. M.; Matsukawa, S.; Yamamoto, Y. J. Organomet. Chem. 2003, 681, 189. (b) Poverenov, E.; Gandelman, M.; Schimon, L. J. W.; Rozenberg, H.; Ben-David, Y.; Milstein, D. Organometallics 2005, 24, 1082. (c) Poverenov, E.; Leitus, G.; Schimon, L. J. W.; Milstein, D. Organometallics. 2005, 24, 5937. (d) Gagliardo, M.; Selander, N.; Mehendale, N. C.; van Koten, G.; Klein Gebbink, R. J. M.; Szabo, K. J. Chem.;Eur. J. 2008, 14, 4800. (3) For the first review on unsymmetrical palladium pincer complexes see: Moreno, I.; SanMartin, R.; Ines, B.; Herrero, M. T.; Domı´ nguez, E. Curr. Org. Chem. 2009, 13, 878. pubs.acs.org/Organometallics

Published on Web 04/12/2010

physical properties (including catalytic activity, photoluminescence, etc.) compared to their symmetric analogues.2,3 As a part of our research on unsymmetrical pincer complexes with organothiophosphorus ligands, recently we reported the syntheses of pincer-type palladacycles with coordinated thiophosphoryl and thiocarbamoyl (I)4 and thiophosphoryl and thiophosphoryloxy donor moieties (II).5 Note that complexes II appeared to be the first example of cyclometalation involving a thiophosphoryloxy group. While the palladacycles of both series may be attributed to the κ3-SCS0 unsymmetrical type, compounds II present the rare examples of pincer complexes containing two fused metallacycles of different size. In continuation of this investigation, it was of interest to design unsymmetrical pincer complexes containing five- and six-membered metallacycles formed by coordination sites of completely different natures. Azomethynes represent one of the most popular types of ligands in organometallic and coordination chemistry. A literature survey has revealed a few monocyclic and pincer palladium complexes with participation of the imine functionality. As for monopalladacycle species, particular attention should be drawn to Milstein’s imine palladacycle III,6 which exhibited exceptional activity in the Heck reaction of (4) Kozlov, V. A.; Aleksanyan, D. V.; Nelyubina, Yu. V.; Lyssenko, K. A.; Gutsul, E. I.; Puntus, L. N.; Vasil’ev, A. A.; Petrovskii, P. V.; Odinets, I. L. Organometallics 2008, 27, 4062. (5) Kozlov, V. A.; Aleksanyan, D. V.; Nelyubina, Yu. V.; Lyssenko, K. A.; Gutsul, E. I.; Vasil’ev, A. A.; Petrovskii, P. V.; Odinets, I. L. Dalton Trans. 2009, 8657. r 2010 American Chemical Society

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Scheme 1. Unsymmetrical Pincer Complexes with Organothiophosphorus Ligands

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Scheme 3. Synthesis of meta-Thiophosphoryloxybenzaldimines and Their NCS-Palladacycles

Scheme 2. Palladacycles with an Imine Functionality

bromobenzene and methyl acrylate, providing turnover numbers up to 106. This pioneering work promoted numerous investigations in this area, and a series of imine- and oxime-based palladacycles were further developed and tested as catalytic precursors for various chemical processes.7,1f Among symmetric pincer complexes, only a few palladacycles of general formula IV based on bis(imino)benzenes have been described.8,9 These complexes were obtained through direct cyclometalation,8 a “ligand-introduction” route,9 or oxidative addition.10 It should be noted that unsymmetrical 5,5-membered pincer complexes V containing imine and phosphinite moieties were recently developed by two independent research groups11 in parallel with our investigations. Therefore, the elaboration of a new hybrid pincer system containing a thiophosphoryloxy group along with an imine moiety anchored on the same benzene backbone and investigation of its ability to undergo cyclopalladation seems reasonable. Herein, we report the convenient synthesis of m-(thiophosphoryloxy)benzaldimines bearing thiophosphinite and imine coordination sited and their direct cyclopalladation to form unsymmetrical 5,6-membered palladacycles.

Results and Discussion A convenient synthetic approach to m-(thiophosphoryloxy)benzaldimines was devised starting from commercially available 3-hydroxybenzaldehyde (Scheme 3). The ligands were (6) Ohff, M.; Ohff, A.; Milstein, D. Chem. Commun. 1999, 357. (7) Beletskaya, I. P.; Cheprakov, A. V. J. Organomet. Chem. 2004, 689, 4055. (8) Vila, J. M.; Gayoso, M.; Pereira, M. T.; Torres, M. L.; Fernandez, J. J.; Fernandez, A.; Ortigueira, J. M. J. Organomet. Chem. 1996, 506, 165. (9) Takenaka, K.; Minekawa, M.; Uozumi, Y. J. Am. Chem. Soc. 2005, 127, 12273. (10) Fossey, J. S.; Russell, M. L.; Malik, K. M. A.; Richards, C. J. J. Organomet. Chem. 2007, 692, 4843. (11) (a) Zhang, B.-C.; Wang, C.; Gong, J.-F.; Song, M.-P. J. Organomet. Chem. 2009, 694, 2555. (b) Yorke, J.; Sanford, J.; Decken, A.; Xia, A. Inorg. Chim. Acta 2010, ASAP. doi: 10.1016/j.ica.2009.12.031

obtained by two reciprocal routes differing by the sequence of thiophosphorylation and Schiff base formation depending on the nature of the amino component. Thus, in the case of O-methylhydroxylamine the synthesis of the oxime 1 was accomplished first followed by thiophosphorylation to afford the corresponding product 3a. In contrast, N-phenyl- and tert-butyl-substituted m-(thiophosphoryloxy)benzaldimines 3b,c were prepared from thiophosphorylated benzaldehyde 2. The synthesis of 3c was readily carried out in dichloromethane solution at room temperature, while 3b was obtained only upon prolonged heating. In all cases thiophosphorylation with Ph2P(S)Cl was carried out under phase transfer catalysis conditions using triethylbenzylammonium chloride (TEBA) as a catalyst. Structures of ligands 3a-c were confirmed by IR and 31P, 1 H, and 13C NMR spectra. Thus, the IR spectra of 3a-c show the intensive absorption bands at 1605-1638 cm-1 relating to the valent vibrations of the CdN groups. The singlet signals at 82-83 ppm in the 31P NMR spectra for 3a-c are typical for the phosphorus atom of the thiophosphinic acid aryl esters. Upon transformation of 2 into corresponding aldimines 3b,c, the aldehyde proton signal observed at 9.87 ppm in the 1H NMR spectrum of 2 disappeared and the signals assigned to the protons of the CHdN moieties were observed at ca. 8 ppm. Moreover, the structures of 3b and 3c were confirmed by a single-crystal X-ray diffraction analysis. In line with the molecular geometry examination, the bond lengths and angles of 3b and 3c (Figure 1) fall into the range typical for compounds of this type. The ligands 3a-c underwent direct cyclopalladation in the reaction with (PhCN)2PdCl2 in benzene solution to give the corresponding NCS-pincer complexes in moderate yields (reflux, 2 h, yield ca. 30% for 4a and ca. 50% for 4b,c). More prolonged heating of the reaction mixture did not provide significant increase of the palladacycle yield, as shown using ligand 3c as a representative example. The yield of 4a was increased to 66% by performing the reaction in the presence of methanol as a cosolvent. Such an effect can be explained by homogenization of the reaction mixture, which is not achieved in neat benzene even under heating. Despite the close nature of the complexes 4a-c, their isolation

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Figure 1. General view of 3b and one of the independent molecules of 3c in a representation of atoms by thermal ellipsoids (p = 50%). The disorder in 3b is omitted for clarity.

Figure 2. General view of 4a with representation of atoms by thermal ellipsoids (p = 50%).

differed depending on the substituent at the nitrogen atom of the imine functionality (see Experimental Section). The rate of cyclopalladation of 3a-c was found to be significantly slower in dichloromethane solution (room temperature), and it took 5 days to achieve about 30-40% yield of 4b and 3 weeks to produce 4a in a yield of only ca. 20%. The complexes 4a-c are yellow crystalline solids that are air and moisture stable both in the solid state and in solutions. Complexes 4a,c are characterized with rather high thermal stability and do not undergo decomposition up to 230 and 265 °C, respectively, while the decomposition point for 4b is substantially lower (135 °C). The 31P NMR spectra of the complexes 4a-c displayed singlets at 75-76 ppm, upfield shifted (Δδ = 6.2-7.7 ppm) relative to the signals of the corresponding free ligand. The shifting of phosphorus resonances indicates the coordination by the metal center at the sulfur atom of the PdS group in the ligand.4,5,13 In turn, the shift of proton signals of the imine group in the 1H NMR spectra of 4a-c by 0.2-0.6 ppm confirms the coordination of the aldimine moiety. The (12) Poverenov, E.; Efremenko, I.; Frenkel, A. I.; Ben-David, Y.; W. Shimon, L. J.; Leitus, G.; Konstantinovski, L.; Martin, J. M. L.; Milstein, D. Nature 2008, 455, 1093. (13) (a) Kanbara, T.; Yamamoto, T. J. Organomet. Chem. 2003, 688, 15. (b) Meguro, H.; Koizumi, T.; Yamamoto, T.; Kanbara, T. J. Organomet. Chem. 2008, 693, 1109. (c) Doux, M.; Bouet, C.; Mezailles, N.; Ricard, L.; Le Floch, P. Organometallics 2002, 21, 2785. (d) Doux, M.; Le Floch, P.; Jan, Y. J. Mol. Struct. (THEOCHEM) 2005, 724, 73. (e) Doux, M.; Piechaczyk, O.; Cantat, T.; Mezailles, N.; Le Floch, P. C. R. Chim. 2007, 10, 1.

direction of the shift for 4a is inverse to that for 4b,c. Finally, the absence of the C2-H proton resonance gives unambiguous evidence for C2-metalation. In the 13C NMR spectrum of 4b, the most significant changes compared to that of the corresponding ligand 3b comprise the downfield shifts of the resonances corresponding to C2 (Δδ = ca. 14 ppm) and C3 (Δδ = 10 ppm) carbon atoms. The almost 2-fold increase of the 3JPC(2) coupling constant was observed. Nevertheless, the chemical shifts of the C1 carbon atoms are practically the same for both 3b and NCS-pincer complex 4b obtained from this ligand. The CdN absorption bands in the IR spectra of 4b,c were found to be shifted to the low-frequency region by ∼30 cm-1 relative to the band in the corresponding free ligands. Comparing the Raman and IR spectra of 3 and 4, the lines and bands at ca. 640 cm-1 in the spectra of the ligands and ca. 620 cm-1 in the spectra of the corresponding complexes were assigned to free and coordinated PdS groups, respectively. Furthermore, X-ray diffraction analysis performed for 4a-c (Figure 2-4) unambiguously confirmed formation of the palladium pincer complexes in all cases. The 31P NMR monitoring of the cyclopalladation in dichloromethane solution demonstrated a set of signals at 85-86 ppm of different intensities downfield shifted relative to the signals of free ligands along with the upfield shifted signals of the final NCS-pincer complexes. These downfield shifted signals may be assigned to the nonmetalated palladium(II) complexes with thiophosphoryloxybenzaldimines 3a-c and that with thiophosphoryloxybenzaldehyde 2. The latter is apparently formed via the partial hydrolysis of an imine moiety occurring under the action of HCl generated in the course of cyclopalladation, in the presence of the solvent residual moisture. Indeed, when 3b and (PhCN)2PdCl2 were reacted in wet dichloromethane, bis(aniline)palladium dichloride, (PhNH2)2PdCl2, was isolated and characterized, in particular, by means of X-ray diffraction analysis (see Figure S1). Note that application of nonpolar solvents such as diethyl ether or hexane, commonly used for isolation of pincer complexes via precipitation, did not result in pure samples of 4a,b formed in the corresponding CH2Cl2 reaction mixtures. At the same time, surprising results were obtained when reaction mixtures were treated with alcohol (either methanol or ethanol). Thus, addition of methanol led to the decomposition of the above-mentioned intermediate nonmetalated palladium complexes with signals in the 85-86 ppm region followed by precipitation of PdCl2. Correspondingly, the

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Figure 3. General view of 4b with representation of atoms by thermal ellipsoids (p = 50%). The disorder of the phenyl ring is omitted for clarity.

Figure 4. General view of 4c with representation of atoms by thermal ellipsoids (p = 50%).

disappearance of these signals and concomitant appearance of the resonances of the NCS-complexes 4a,b along with the signal of thiophosphorylated aldehyde 2 were observed in the 31 P NMR spectra. Besides the above signals, the 31P NMR spectra of dichloromethane reaction mixtures treated with alcohol showed new resonances at ca. 43.5 ppm, the region typical for compounds bearing a coordinated PdO moiety. Moreover, after evaporation of the reaction mixtures and recrystallization of the resulting residues from CH2Cl2-Et2O (1:2), the 31P NMR spectra of the crystalline samples obtained also revealed signals at ∼43.5 ppm along with the signals of the NCS-pincer complexes 4a,b. Fortunately, the crystalline sample isolated via workup of the reaction mixture in the case of the N-phenyl-substituted ligand 3b was found to be suitable for X-ray analysis, which revealed a superposition of two pincer complexes with NCS (complex 4b) and NCO (complex 5b) coordination in a 7:3 ratio (see Figure 2S in the Supporting Information). In other words, the alcohol treatment of the dichloromethane reaction mixtures resulted in unexpected side formation of a palladacycle bearing a phosphoryloxy moiety coordinated to the Pd(II) ion instead of the thiophosphoryloxy one (Scheme 4). It should be noted that according to the 31P NMR data the ratio of NCS- and NCO-complexes in the crude reaction mixtures (approximately 7:3) was the same as was observed in the crystalline sample isolated. The pairs of complexes bearing coordinated thiophosphoryl- and phosphoryloxy moieties 4a,b and 5a,b, respectively, were further successfully separated using thin-layer chromatography; however the yield of the pure NCO-complexes 5a,b isolated in such a manner did not exceed 5%. Such low yields

Figure 5. General view of 5b with representation of atoms by thermal ellipsoids (p = 50%). Scheme 4. Synthesis of NCS- and NCO-Pincer Complexes upon MeOH Treatment of 3a,b and (PhCN)2PdCl2 Dichloromethane Reaction Mixtures

may be explained by partial decomposition of NCO-complexes on a solid support. The structure of complex 5b after purification was elucidated by X-ray diffraction technique (Figure 5). Concerning the spectral data for NCO-complexes 5a,b, the resonances of coordinated OP(O) groups in the 31P NMR spectra are observed at ca. 44 ppm. Their 1H NMR spectra are similar to those for the complexes 4a,b bearing a thiophosphoryloxy moiety and unambiguously confirm the occurrence of C-metalation and coordination of an aldimine group by the palladium ion. The IR spectra of the NCO-complex 5b demonstrates the intensive absorption bands of PdO and CdN groups at 1210 and 1587 cm-1, respectively. Although the NCO-complex 5a, with an oxime moiety, was characterized only by means of 31P and 1H NMR spectroscopy, the similarity of spectral pattern for 5a and 5b allows suggesting for 5a the same structure with κ3-NCO coordination.

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Table 1. Selected Bond Lengths (A˚) and Angles (deg) in 4a-c and 5b

Pd(1)-Cl(1) Pd(1)-C(2) Pd(1)-N(1) Pd(1)-X(1) (X = S(1), O(1)) P(1)-S(1)a N(1)-C(19)b C(2)Pd(1)N(1) C(2)Pd(1)X(1) (X = S(1), O(1)) N(1)Pd(1)Cl(1) X(1)Pd(1)Cl(1) (X = S(1), O(1)) b

4a

4b

4c

5b

2.39289(17) 2.0160(6) 2.0432(6) 2.29535(18)

2.3585(5) 2.006(2) 2.0560(18) 2.3013(6)

2.3974(6) 1.9988(16) 2.0949(15) 2.3031(6)

2.3647(10) 1.984(4) 2.018(3) 2.059(3)

1.9786(2) 1.2902(9)

1.9746(7) 1.291(3)

1.9547(7) 1.279(2)

1.501(3) 1.298(5)

80.09(2) 98.767(17)

81.10(7) 99.56(6)

81.11(6) 96.76(5)

80.87(15) 93.57(14)

97.046(17) 84.125(6)

94.56(5) 101.76(4) 97.72(9) 84.732(18) 80.777(18) 87.91(8)

a PdS bonds in 3b and 3c are 1.9289(8) and 1.9351(7) A˚, respectively. NdC bonds in 3b and 3c are 1.253(5) and 1.269(2) A˚, respectively.

Table 1 summarizes the main geometrical parameters for NCS- and NCO-pincer complexes 4a-c and 5b, respectively. As one can see, the principal bond lengths in these complexes are almost the same independent from the coordination pattern. Furthermore, the conformation of five- and sixmembered palladacycles along with distortions of the coordinated palladium polyhedron (with the only exception of the most sterically hindered 4c) are also rather close to each other. Thus, according to the X-ray data, complex 5b is almost isostructural to the related 4b, which in turn can explain the formation of the above-mentioned solid solution (see Figure S2). In 4a, 4b, and 5b the five-membered rings are flat, while the six-membered ones are characterized by a sofa conformation with the deviation of the P(1) atom by 0.63-0.78 A˚. In 4c the introduction of the bulky tBu substituent leads to the increase of cycle puckering for both five-membered (envelope with the deviation of the C(3) atom by 0.16 A˚) and six-memebred (distorted sofa with the deviation of P(1) and O(1) atoms by 1.06 and 0.32 A˚, respectively) rings. The steric effects are also reflected in the distortion of the square-planar configuration of the Pd(1) atom. The latter is characterized by a tetrahedral configuration with the folding along the S(1) 3 3 3 N(1) line in 4a-4c and the O(2) 3 3 3 N(1) line in 5b. The maximum folding that is reflected in the dihedral angle between the Pd(1)C(2)S(1)N(1) and Pd(1)Cl(1)S(1)N(1) planes was also observed for sterically hindered 4c (10.2°), while in the other structures the corresponding angle was almost the same, at ca. 4°. Furthermore, the significant puckering of the above palladacycles in 4c leads to the rather high value (17.7°) of the dihedral angle between the PdONClC square and the central aromatic cycle. It should be noted that Pd-C and Pd-Cl bond lengths in 4a-c and 5b are almost equal to the corresponding ones (2.008(2) and 2.4013(6) A˚) in the relative 5,6-membered SCS0 -pincer complexes II.5 Thus, we may conclude that Pd-C and Pd-Cl bond lengths and consequently the energy of these bonds are almost independent of the nature of the coordinated heteroatoms in the ligand as well as the puckering of the palladacycles and dihedral angles between the PdX2ClC square and the central aromatic cycle. The mechanism of formation of these PdO palladacycles is still obscure since the preformed NCS-complexes 4a,b are stable in CH2Cl2-MeOH solution and do not transform into the corresponding phosphoryl analogues 5a,b in time. Moreover, forced blowing of air and addition of water to the dichloromethane reaction mixtures at different time points

Scheme 5. First Example of a PdO-Containing Platinum Pincer Complex

also did not afford the formation of the NCO-complexes 5. The reaction of 3a with (PhCN)2PdCl2 in methanol as a sole solvent resulted in NCS-complex 4a along with the PdO analogue of the starting ligand, namely, N-methoxy-(m-diphenylphosphoryloxy)benzaldimine (6) [see Supporting Information] in a 9:1 ratio rather than NCO-complex 5a. Furthermore, unlike the ligand 3a, the above-mentioned PdO ligand 6 did not form the corresponding NCO-pincer complex 5b upon interaction with (PhCN)2PdCl2 either in dichloromethane solution or under heating in benzene. It should be emphasized that such an oxidation of the thiophosphoryloxy group in the presence of alcohol was not observed in the cyclopalladation of 1-thiophosphoryloxy-3-thiophosphorylbenzenes.5 On the basis of these results, we may suggest that the unexpected NCO-complexes 5 are formed in the oxidation of the PdS group in the metal ion coordination sphere in the intermediate noncyclopalladated species formed due to the starting PdS-containing pincer ligand. However, the mechanism of such oxidation requires additional detailed investigation. An important point to emphasize is that the complexes 5a, b present only the second example of the noble metal pincer complexes bearing a coordinated phosphoryl group. The first one, recently reported by Milstein’s group,12 represents the platinum unsymmetrical NCO-pincer complex resulting from insertion of the terminal oxo-ligand into the Pt-P bond of the Pt(IV) PCN-pincer complex containing a phosphine moiety (Scheme 5). Such an intramolecular oxygen transfer occurred in the absence of an external oxygen acceptor due to the instability of the initial oxo-complex at room temperature. However, no close analogy can be drawn with the formation of the complexes 5a,b, as the starting platinum oxo-complex was preformed under action of a strong oxidant, namely, dioxirane. Catalytic Studies. The application of symmetrical pincer complexes as catalysts is well studied and presents the most important sphere of their usage. At the same time, far less information about the catalytic activity of their unsymmetrical analogues is available. These data mostly concern the palladium-catalyzed cross-coupling reactions, in particular the Suzuki-Miyaura cross-coupling of aryl halides with arylboronic acids,1h,4,5,11,14 being one of the most powerful methods for CAr-CAr bond formation. Indeed, due to commercial availability of the starting materials, the relatively mild reaction conditions, tolerance of a broad range of functionalities, easy handling, and removal of the nontoxic boron-containing byproduct, as well as the possibility of using water as a solvent (or cosolvent) and solid-support materials, this cross-coupling reaction has gained prominence in recent years at an industrial level, mainly in the synthesis of pharmaceuticals and fine chemicals. The recent advances involving also nonconventional methodologies (14) Serrano-Becerra J. M., Hernandez-Ortega S., Morales-Morales D. Inorg. Chim. Acta, 2010, ASAP. doi: 10.1016/j.ica.2010.02.003.

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Table 2. Palladacycles 4-Catalyzed Suzuki Cross-Coupling

Table 3. Influence of Different Solvents on the Conversion in the Suzuki Cross-Coupling Catalyzed by 4b entry 1 2 3 4 5 6 7

X/yield, %a catalyst

mol %

R

C(O)Me

OMe

NMe2

4a 4a 4a 4b 4b 4b 4c 4c 4c

1 0.1 0.01 1 0.1 0.01 1 0.1 0.01

OMe OMe OMe Ph Ph Ph t Bu t Bu t Bu

100 100 93 100 100 100 100 99 91

86 99 0 79 95 3 91 84 3

35 23

a

25 30 40 40

Reaction conditions: K3PO4, Bu4NBr, DMF, 120 °C, 5 h.

that have been applied to the Suzuki-Miyaura reaction are well documented in the literature.15 The main challenges in this cross-coupling pertain to the use of less reactive chloroarenes and bromoarenes bearing electron-donating groups, and in these cases only a few pincer complexes displayed catalytic activity. Noteworthy, the best results for coupling of chloroarenes were achieved just with phosphorus-containing pincer complexes or with palladacycles modified by carbenes. That was related to the extra stability provided by these ligands toward the low-ligated catalytically active Pd(0) species involved in the catalytic cycle.1h Hence, the catalytic activity of complexes 4a-c was examined in a standard Suzuki-Miyaura reaction as a benchmark evaluation to compare with previously reported data for the other above-mentioned pincer complexes having at least one thiophosphoryl coordinating group. First, we performed the experiments with aryl bromides differing in the electronic properties of substituents in the para-position in DMF solution at 120 °C for 5 h similar to the procedure used for testing of pincer palladium complexes formed by 3-thiophosphorylbenzoic acid thioamides (I)4 and 1-thiophosphoryloxy-3-thiophosphorylbenzenes(II).5 Table2 summarizes the results obtained. In all experiments the normal dependence on the electronic properties of the substituent X at the aryl bromide was observed. For 4-bromoacetophenone activated by an electronwithdrawing group, complete conversions were reached even using 0.01 mol % of 4a-c. For the least active 4-bromo-N,Ndimethylaniline the yields decreased to ca. 25-40% using both 1 mol % and 0.1 mol % of catalyst. In the case of intermediate 4-bromoanisole, the yields of the corresponding biphenyl product were in the range 79-91% and 84-99% with 1 mol % and 0.1 mol % of a catalyst, respectively. However, none of the catalysts 4a-c were active in the reaction with this substrate when used at 0.01 mol %. In general, all the catalysts 4a-c demonstrated similar activity under these conditions; that is, no pronounced effect was observed depending on the substituent at the nitrogen atom of the CdN group. Nevertheless, they were more effective in this reaction compared with reference catalysts I and II. Thus, the reaction with bromoanisole complexes I having 5,5membered palladacycles and differing in substituents at the (15) (a) Alonso, F.; Beletskaya, I. P.; Yus, M. Tetrahedron 2008, 64, 3047 (and referencies therein). (b) Schneider, F.; Stolle, A.; Ondruschka, B.; Hopf, H. Org. Process Res. Dev. 2009, 13, 44. (c) Barder, T. E.; Walker, S. D.; Martinelli, J. R.; Buchwald, S. L. J. Am. Chem. Soc. 2005, 127, 4685.

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c

X Ac OMe

cat. mol %

solvent

reaction temperature

yield, %a

0.1 0.1 0.1 1 1 1 1

toluene dioxane EtOHb toluene dioxane dioxanec 90% EtOH

105 105 80 105 105 105 80

91 86 82 79 67 55 79

a Reaction conditions: K3PO4, Bu4NBr, 5 h. Cs2CO3 as a base.

b

Without Bu4NBr.

nitrogen atom of the thiocarbamide group provided 80-84% yield when used in the amount of 3 mol % even at more prolonged reaction times.4 The best catalyst of type II having 5,6-membered palladacycles similar to compounds 4a-c gave 93% and 68% of 4-methoxybiphenyl when used in 1 and 0.1 mol % amounts, respectively. Therefore, the higher asymmetry for a pincer palladium complex with at least one PdS coordinating group may serve as a factor of its higher catalytic activity in the Suzuki-Miyaura cross-coupling. Furthermore, using N-phenyl-substituted complex 4b as a representative example, we investigated the influence of a few common solvents for the above cross-coupling reaction on the conversion. Independently from the solvent in use, the yields varied in a relatively narrow range for the same starting substrate and were slightly lower compared with those obtained in DMF. Such a decrease of the yields can be explained by the lower reaction temperature. Nevertheless, promoting the coupling of 4-chloroacetophenone, the catalyst 4c demonstrates yields of the product equal to 22% and 8% at 120 °C in DMF and xylene, respectively. In contrast to iminophosphite PCN palladium pincer complexes V,11 replacement of K3PO4 by Cs2CO3 resulted in a decrease of the product yield. However, comparing the catalytic activity of PCN-complexes V with those for NCS-complex 4b, we may mention that the latter provided similar yields of 4-methoxybiphenyl already after 5 h (vs 18 h11) with other conditions being equal. The higher catalytic efficiency of the complexes described may be attributed to their hemilabile properties, as they contain rather weakly bonded thiophosphoryloxy groups that may facilitate substrate interaction with the metal center. According to Hermann’s hypothesis,16 palladacycles may serve as a reservoir providing the catalytic cycle with highly catalytically active Pd0 particles. Furthermore, in the absence of tetrabutylammonium bromide;a salt known to stabilize palladium nanoparticles17;only approximately 10% decrease of the yield of the final biaryl product was observed under catalysis by 4b. Therefore, we cannot exclude the catalytic mechanism comprising the cleavage of the palladium-carbon bond and/or formation of palladium nanoparticles as the active form of the catalysts.

Conclusions To conclude the results presented, we developed a convenient approach to meta-diphenylthiophosphoryloxybenzaldimines (16) Hermann, W. A.; B€ ohm, V. P. W.; Reisinger, C.-P. J. Organomet. Chem. 1999, 576, 23. (17) Bolliger, J. L.; Blacque, O.; Frech, C. M. Angew. Chem. 2007, 46, 6514.

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as NCS-pincer ligands bearing a thiophosphoryloxy group and imine moieties as coordination sites. These ligands undergo direct cyclopalladation, affording unsymmetrical NCS-pincer complexes with fused five- and six-membered metallacycles under reaction with (PhCN)2PdCl2 in benzene at elevated temperature. When the same reaction was performed in dichloromethane, addition of an alcohol surprisingly resulted in oxidation of a PdS group in the metal ion coordination sphere to yield NCO-pincer complexes along with the expected NCS ones. These 5,6-membered palladium NCS-pincer complexes demonstrated the highest catalytic activity in the Suzuki crosscoupling reaction among known ECS-analogues.

Experimental Section If not noted otherwise, all manipulations were carried out without taking precautions to exclude air and moisture. Benzene was distilled over sodium benzophenone ketyl. Dichloromethane was distilled from P2O5. The starting 3-hydroxybenzaldehyde was recrystallized from water prior to use. 3-Hydroxy-N-methoxybenzaldimine 118 and Ph2P(S)Cl19 were obtained according to the literature procedures. All other chemicals and solvents were used as purchased without further purification. NMR spectra were recorded on Bruker Avance-300 and Bruker Avance-400 spectrometers, and the chemical shifts (δ) were internally referenced by the residual solvent signals relative to tetramethylsilane (1H and 13C) or externally to H3PO4 (31P). The 13C NMR spectra were registered using the JMODECHO mode; the signals for the C atom bearing odd and even numbers of protons have opposite polarities. The numeration for carbon atoms of the central benzene ring in the descriptions of the 1H and 13C spectral data is in agreement with IUPAC nomenclature used for the ligands. The same principle of numbering was used for the description of solid-state molecular structures characterized by X-ray crystallography. Column chromatography was carried out using Merck silica gel 60 (230-400 mesh ASTM). Analytical TLCs were performed with Merck silica gel 60 F254 plates. IR spectra were recorded on a “Magna-IR750” Fourier spectrometer (Nicolet), resolution 2 cm-1, 128 scans. The assignment of the absorption bands in the IR spectra was made according to ref 20. Raman spectra were recorded with a LabRAM Jobin-Yvon Raman spectrometer with an exciting He-Ne laser line of 632.8 nm. Melting points were determined with an Electrothermal IA9100 digital melting point apparatus and are uncorrected. 3-(Diphenylthiophosphoryl)oxybenzaldehyde, 2. A solution of Na2CO3 (0.90 g, 8.2 mmol) and TEBA (4 mol %) in 8 mL of water was slowly dropwise added to a stirred solution of 3-hydroxybenzaldehyde (1.00 g, 8.2 mmol) and Ph2P(S)Cl (2.07 g, 8.2 mmol) in 14 mL of benzene at 0-5 °C. The resulting reaction mixture was stirred for 2.5 h at 50-55 °C and after cooling to room temperature diluted with 20 mL of water and benzene. After separation of the organic layer, the water phase was additionally washed with benzene (25 mL). The combined benzene solution was dried over anhydrous Na2SO4 and evaporated to dryness. The resulting residue was purified by silica gel column chromatography (eluent: hexane-EtOAc (5:1)) to give 1.90 g of 2 as a white solid. Yield: 72%. Mp: 70-71 °C (Et2O). 31P{1H} NMR (161.98 MHz, CDCl3): δ 83.75 ppm. 1H NMR (400.13 MHz, CDCl3): δ 7.32-7.40 (m, 2H, HAr), 7.45-7.53 (m, 7H, HAr), 7.62 (d, 1H, HAr, 3JHH = 7.3 Hz), 7.98 (dd, 4H, o-H in P(S)Ph2, 3 JHH = 7.5 Hz, 3JPH = 13.8 Hz), 9.87 (s, 1H, CHO). IR (KBr, ν/cm-1): 637 (PdS), 692, 725, 737, 796, 849, 1107, 1118, 1135, (18) Boehringer, C. H. Patent DE 1934443 (A1); Chem. Abstr. 1971, 74, 99676. (19) Ko, E. C. F.; Robertson, P. E. Can. J. Chem. 1973, 51, 597. (20) Bellamy, L. J. The Infrared Spectra of Complex Molecules; Wiley: New York, 1975.

Kozlov et al. 1223, 1239, 1437, 1700 (CdO), 2744, 2847, 3066. Anal. Calcd for C19H15O2PS: C, 67.44; H, 4.47. Found: C, 67.58; H, 4.55. O-{3-[(Methoxyimino)methyl]phenyl} Diphenylthiophosphinate, 3a. A solution of Na2CO3 (0.77 g, 7.3 mmol) and TEBA (4 mol %) in 8 mL of H2O was slowly dropwise added to a stirred solution of 1 (1.10 g, 7.3 mmol) and Ph2P(S)Cl (1.84 g, 7.3 mmol) in 14 mL of benzene at 0-5 °C. The resulting mixture was stirred for 2.5 h at 50-55 °C and, after cooling to room temperature, diluted with 20 mL of water and benzene. After separation of the organic layer, the water phase was additionally washed with benzene (25 mL). The combined benzene solution was dried over anhydrous Na2SO4 and evaporated to dryness. The resulting residue was crystallized from hexane-Et2O (2:1, 15 mL) and recrystallized from hexane (15 mL) to give 1.70 g of 3a as a white solid. Yield: 64%. Mp: 79-81 °C (hexane). 31P{1H} NMR (161.98 MHz, CDCl3): δ 82.49 ppm. 1H NMR (400.13 MHz, CDCl3): δ 3.93 (s, 3H, OCH3), 7.04 (d, 1H, H-C4, 3JHH = 8.3 Hz), 7.17-7.25 (m, 2H, HAr), 7.33 (d, 1H, H-C6, 3JHH = 7.7 Hz), 7.45-7.55 (m, 6H, HAr), 7.92 (s, 1H, CHdN), 7.97 (dd, 4H, o-H in P(S)Ph2, 3JPH = 13.8 Hz, 3JHH = 7.1 Hz). IR (KBr, ν/cm-1): 643 (PdS), 691, 721, 745, 792, 845, 978, 1054, 1117, 1142, 1233, 1437, 1603 (CdN), 2816, 2933. Raman (solid, Δν/ cm-1): 239, 644 (PdS), 998, 1232, 1597, 1605 (CdN). Anal. Calcd for C20H18NO2PS: C, 65.38; H, 4.94, N 3.81. Found: C, 65.60; H, 4.84, N, 3.67. O-[3-[(Phenylimino)methyl]phenyl] Diphenylthiophosphinate, 3b. A mixture of 2 (1.90 g, 5.6 mmol), aniline (0.52 g, 5.6 mmol), and MgSO4 (0.34 g, 2.8 mmol) in 15 mL of dichloromethane was refluxed for 6 h under stirring and left for 2 days. After evaporation to dryness, the resulting residue was crystallized from hexane and recrystallized from EtOH to yield 1.5 g (65%) of 3b as a white solid. Mp: 87-88 °C (EtOH). 31P{1H} NMR (121.49 MHz, CDCl3): δ 82.80 ppm. 1H NMR (300.13 MHz, CDCl3): δ 7.15-7.25 (m, 4H, HAr), 7.30-7.41 (m, 3H, HAr), 7.46-7.68 (m, 8H, HAr), 7.96-8.04 (m, 4H, HAr), 8.33 (s, 1H, CHdN). 13C{1H} NMR (100. 61 MHz, CDCl3): δ, 120.68 (s, o-C in NPh), 121.69 (d, C6, 3JCP = 5.1 Hz), 124.30 (d, C2, 3JCP = 4.8 Hz), 125.05 (d, C4, 4JCP = 1.3 Hz), 125.95 (s, p-C in NPh), 128.42 (d, m-C in P(S)Ph2, 3JCP = 13.5 Hz), 128.96 (s, m-C in NPh), 129.45 (d, C5, 4JCP = 1.0 Hz), 131.23 (d, o-C in P(S)Ph2, 2 JCP = 11.5 Hz), 132.05 (d, p-C in P(S)Ph2, 4JCP = 2.9 Hz), 133.98 (d, ipso-C in P(S)Ph2, 1JCP = 110.9 Hz), 137.60 (s, C3), 150.87 (d, C1, 2JCP = 8.1 Hz), 151.51 (s, ipso-C in NPh), 159.00 (s, CHdN). IR (KBr, ν/cm-1): 641 (PdS), 692, 721, 728, 840, 875, 960, 1104, 1115, 1197, 1253, 1436, 1579, 1630 (CdN), 2863, 3050. Anal. Calcd for C25H20NOPS: C, 72.62; H, 4.88: N, 3.39. Found: C, 72.74; H, 4.81; N, 3.31. O-[3-[(tert-Butylimino)methyl]phenyl] Diphenylthiophosphinate, 3c. 3c was obtained analogously to 3b, except that the reaction was carried out at room temperature. Yield: 86%. Mp: 75-77 °C (hexane). 31P{1H} NMR (121.49 MHz, CDCl3): δ 82.26 ppm. 1H NMR (300.13 MHz, CDCl3): δ 1.25 (s, 9H, C(CH3)3), 7.04-7.08 (m, 1H, H-C4), 7.23 (t, 1H, H-C5, 3JHH = 7.9 Hz), 7.43-7.56 (m, 8H, HAr), 7.95-8.03 (m, 4H, HAr), 8.15 (s, 1H, CHdN). 13C{1H} NMR (75.47 MHz, CDCl3): δ 29.46 (s, CH3), 57.21 (s, C(CH3)3), 120.92 (s, C2), 122.95 (s, C6), 124.15 (s, C4), 128.38 (d, 3JCP = 13.2, m-C in P(S)Ph2), 129.16 (s, C5), 131.23 (d, o-C in P(S)Ph2, 2 JCP = 11.5,), 131.98 (s, p-C in P(S)Ph2), 134.05 (d, ipso-C in P(S)Ph2, 1JCP = 113.1 Hz), 138.64 (s, C3), 150.73 (d, C1, 2JCP = 7.1 Hz), 153.89 (s, CHdN). IR (KBr, ν/cm-1): 644 (PdS), 688, 719, 736, 791, 846, 981, 1107, 1117, 1135, 1206, 1245, 1437, 1478, 1580, 1638 (CdN), 2961, 3072. Raman (solid, Δν/ cm-1): 239, 644 (PdS), 999, 1589, 1605, 1638 (CdN), 3059. Anal. Calcd for C23H24NOPS: C, 70.21; H, 6.15: N, 3.56. Found: C, 70.35; H, 6.21; N, 3.51. Preparation of the Palladium Complexes 4a-c in Benzene/ Benzene-Methanol. [2-[(Diphenylthiophosphoryl)oxy]-6-[(methoxyimino)methyl]phenyl]palladium Chloride, 4a. A solution of PdCl2(PhCN)2 (42.2 mg, 0.110 mmol) in 5 mL of benzene was slowly dropwise added to a solution of 3a (40.4 mg, 0.110 mmol) in 6 mL of C6H6 under stirring. Then, the reaction mixture was diluted with

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Table 4. Crystal Data and Structure Refinement Parameters

formula MW T, K cryst syst space group Z(Z0 ) a, A˚ b, A˚ c, A˚ β, deg V, A˚3 dcalc, g 3 cm-3 μ, cm-1 F(000) 2θmax, deg reflns collected indep reflns (Rint) obsd reflns [I > 2σ(I)] R1 wR2 GOF ΔFmax/ΔFmin, e A˚-3

3b

3c

4a

4b

4c

5b

C25H20NOPS 413.45 100 monoclinic P21/n 4 10.108(2) 16.831(4) 12.130(3) 91.406(8) 2063.1(8) 1.331 2.51 864 56 15 717 5136 3687 0.0486 0.1271 1.017 0.414/-0.341

C23H24NOPS 393.46 120 monoclinic P21/c 8(2) 26.116(2) 9.5744(7) 17.7198(14) 107.978(2) 4214.4(6) 1.240 2.42 1664 58 45 054 11 184 7297 0.0538 0.1379 1.005 0.841/-0.271

C20H17ClNO2PPdS 508.23 100 monoclinic P21/c 4(1) 13.3324(3) 10.8818(3) 14.5024(4) 114.223(1) 1918.77(9) 1.759 13.14 1016 100 15 2546 20 127 17 438 0.0231 0.0700 1.004 1.398/-1.705

C26H21Cl3NOPPdS 639.22 100 monoclinic C2/c 8(1) 31.0174(14) 9.9746(5) 17.4994(8) 109.130(1) 5115.1(4) 1.660 12.04 2560 58 30 322 6788 5937 0.0290 0.1000 1.001 1.289/-0.740

C23H23ClNOPPdS 534.30 100 orthorhombic Pbca 8(1) 15.550(3) 16.818(4) 16.818(4) 90.00 4398.2(17) 1.614 11.48 2160 58 51 336 5857 5041 0.0212 0.0673 1.001 0.535/-0.388

C26H21Cl3NO2PPd 623.16 100 monoclinic C2/c 8(1) 30.418(7) 9.9067(18) 17.049(4) 104.959(9) 4963.5(18) 1.668 11.61 2496 58 19 719 6597 5000 0.0440 0.1329 1.008 0.898/-1.319

4.5 mL of MeOH and heated at 50-60 °C for 1.5 h. After cooling to room temperature the resulting mixture was filtered and evaporated to dryness to give 55.9 mg of 4a as a yellow crystalline solid. Yield: 66%. Mp: dec > 230 °C. 31P{1H} NMR (161.98 MHz, CDCl3): δ 76.26 ppm. 1H NMR (400.13 MHz, CDCl3): δ 4.00 (s, 3H, OCH3), 7.09 (d, 1H, H-C4, 3JHH = 8.1 Hz), 7.14-7.18 (m, 1H, H-C5), 7.27 (d, 1H, H-C6, 3JHH = 7.0 Hz), 7.55-7.68 (m, 6H, HAr), 7.94 (dd, 4H, o-H in P(S)Ph2, 3JHH = 7.7 Hz, 3JPH = 13.9 Hz), 8.49 (s, 1H, CHdN). IR (KBr, ν/cm-1): 626 (PdS), 694, 715, 727, 790, 912, 1030, 1108, 1118, 1199, 1439, 1582 (CdN), 2927. Raman (solid, Δν/ cm-1): 170, 239, 276 (Pd-Cl), 391, 624 (PdS), 994, 1216, 1548, 1584 (CdN),1598, 3051. Anal. Calcd for C20H17ClNO2PPdS: C, 47.26; H, 3.37; N, 2.76. Found: C, 47.11; H, 3.31; N, 2.65. [2-[(Diphenylthiophosphoryl)oxy]-6-[(phenylimino)methyl]phenyl]palladium Chloride, 4b. A benzene (8 mL) solution of PdCl2(PhCN)2 (72.2 mg, 0.188 mmol) was slowly dropwise added to a solution of 3b (77.8 mg, 0.188 mmol) in 8 mL of C6H6 3 The resulting reaction mixture was refluxed for 2 h under stirring. After cooling to room temperature, the precipitate was filtered off and recrystallized from CH2Cl2-Et2O (1:2, 12 mL) to give 4b as a yellow crystalline solid. Yield: 53.1 mg (51%). Mp: dec > 135 °C. 31P{1H} (161.98 MHz, CDCl3): δ 76.48 ppm. 1H NMR (400.13 MHz, CDCl3): δ 7.03 (dd, 1H, H-C4, 3JHH = 8.0 Hz, 4 JHP = 1.2 Hz), 7.13-7.17 (m, 1H, H-C5), 7.25-7.38 (m, 6H, HAr), 7.49-7.54 (m, 4H, HAr), 7.58-7.62 (m, 2H, HAr), 7.96 (dd, 4H, o-H in P(S)Ph2, 3JHH = 7.2 Hz, 3JPH = 14.0 Hz), 8.03 (s, 1H, CHdN). 13C{1H} NMR (100. 61 MHz, CDCl3/DMSO-d6): δ 121.63 (d, C6, 3JCP = 8.1 Hz), 122.58 (s, o-C in NPh), 125.61 (s, C4), 125.79 (s, C5), 126.28 (s, p-C in NPh), 126.84 (s, m-C in NPh), 126.93 (d, ipso-C in P(S)Ph2, 1JCP = 108.6 Hz), 128.02 (d, m-C in P(S)Ph2, 3JCP = 13.9 Hz), 130.67 (d, o-C in P(S)Ph2, 2 JCP = 11.7 Hz), 132.67 (d, p-C in P(S)Ph2, 4JCP = 2.2 Hz), 138.17 (d, C2, 3JCP = 10.3 Hz), 147.21 (s, C3), 148.51 (s, i-NPh), 150.82 (d, C1, 2JCP = 8.1 Hz), 175.31 (d, CHdN, 4JCP = 5.1 Hz). IR (KBr, ν/cm-1): 628 (PdS), 690, 716, 744, 924, 1108, 1117, 1204, 1437, 1587, 1603 (CdN), 3052. Anal. Calcd for C25H19ClNOPPdS 3 0.20CH2Cl2: C, 52.98; H, 3.42; N, 2.45. Found: C, 53.03; H, 3.35; N, 2.39. [2-[(Diphenylthiophosphoryl)oxy]-6-[(tert-butylimino)methyl]phenyl]palladium Chloride, 4c. A benzene (8 mL) solution of PdCl2(PhCN)2 (76.2 mg, 0.199 mmol) was slowly dropwise added to a solution of 3c (78.2 mg, 0.199 mmol) in 8 mL of C6H6. The resulting reaction mixture was refluxed for 2 h under stirring. After cooling to room temperature the resulting mixture was filtered and evaporated to dryness. The obtained residue was recrystallized from CH2Cl2-Et2O (1:2, 12 mL) to

give 4c as a yellow crystalline solid. Yield: 55.8 mg (53%). Mp: dec > 265 °C. 31P{1H} NMR (161.97 MHz, CDCl3): δ 74.61 ppm. 1H NMR (400.13 MHz, CDCl3): δ 1.60 (s, 9H, C(CH3)3), 6.88-6.90 (m, 1H, H-C4), 7.07-7.15 (m, 2H, H-C5,6), 7.47-7.52 (m, 5H, HAr), 7.56-7.60 (m, 2H, HAr), 7.93 (s, 1H, CHdN), 7.95-7.98 (m, 4H, HAr). IR (KBr, ν/cm-1): 632 (PdS), 696, 719, 786, 933, 1108, 1120, 1200, 1438 (P-Ph), 1610 (CdN), 2965, 3055. Anal. Calcd for C23H23ClNOPPdS: C, 51.70; H, 4.34; N, 2.62. Found: C, 51.87; H, 4.44; N, 2.64. Preparation of the Complexes 4a/5a and 4b/5b in Dichloromethane-Methanol Solutions. A solution of PdCl2(PhCN)2 (0.222 mmol) in 4 mL of dichloromethane was slowly dropwise added to a solution of 3a,b (0.222 mmol) in 2 mL of CH2Cl2. The reaction mixture was left under ambient conditions for 5 days in the case of 3b and 3 weeks in the case of 3a. Then, 3 mL of MeOH was added to the half-evaporated reaction mixture to give a 1:1 CH2Cl2-MeOH solution. In 5-6 h the resulting mixture was filtered and evaporated to dryness. The obtained residue was washed with Et2O (15 mL) and purified by thin-layer chromatography (eluent CH2Cl2-EtOAc (4:1)) to give the complexes 4a,b and 5a,b. Complexes 4a and 5a (R = OMe). Yields: 22% (4a), 2% (5a). Spectral data for 5a: 31P{1H} NMR (161.98 MHz, CDCl3): δ 43.83 ppm. 1H NMR (400.13 MHz, CDCl3): δ 4.00 (s, 3H, OCH3), 6.87 (dd, 1H, H-C4, 3JHH = 6.0 Hz, 4JPH = 2.9 Hz), 7.11-7.12 (m, 2H, H-C5,C6), 7.49-7.64 (m, 6H, HAr), 7.96 (dd, 4H, o-H in P(S)Ph2, 3JHH = 7.5 Hz, 3JPH = 13.1 Hz), 8.01 (s, 1H, CHdN). Complexes 4b and 5b (R = Ph). Yields: 38% (4b), 5% (5b). 5b: Mp: dec > 180 °C. 31P{1H} NMR (121.97 MHz, CDCl3): δ 43.98 ppm. 1H NMR (300.13 MHz, CDCl3): δ 6.94 (d, 1H, H-C4, 3 JHH = 7.8 Hz), 7.14 (m, 1H, H-C5), 7.22 (d, 1H, H-C6, 3JHH = 7.1 Hz), 7.28-7.37 (m, 5H, HAr), 7.48-7.65 (m, 6H, HAr), 7.85 (s, 1H, CHdN), 7.97 (dd, 4H, o-H in P(S)Ph2, 3JHP = 13.1 Hz, 3 JHH = 7.4 Hz). IR (KBr, ν/cm-1): 691, 734, 926, 1134, 1152, 1210 (P-O-Ar), 1439 (P-Ph), 1587 (CdN), 3050. Anal. Calcd for C25H19ClNO2PPd 3 0.33CH2Cl2: C, 53.70; H, 3.50: N, 2.47. Found: C, 53.68; H, 3.54; N, 2.41. Catalytic Experiments. In a typical experiment a solution of 0.25 mmol of aryl bromide, 0.375 mmol of PhB(OH)2, 2 mmol of K3PO4, 0.5 mmol of Bu4NBr, and the mentioned amount of the corresponding palladium complex (used as titrated solutions in DMF) in 1 mL of DMF was heated at 120 °C over 5 h. After cooling the reaction mixture was immediately filtered, treated with water, extracted with benzene, and analyzed by GC and 31P NMR.

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X-ray Crystallography. Single crystals were grown by recrystallization from EtOH (3b) and hexane (3c) or by slow evaporation from CH2Cl2-hexane (1:3), (4b, 5b), CHCl3 (4a), and CH2Cl2 (4c). All diffraction data were collected on a Bruker SMART APEX II CCD diffractometer [λ(Mo KR) = 0.71072 A˚, ω-scans] at 100 K. The substantial redundancy in data allows empirical absorption correction to be performed with SADABS,21 using multiple measurements of equivalent reflections. The structures were solved by direct methods and refined by the full-matrix least-squares technique against F2 in the anisotropicisotropic approximation. All calculations were performed with (21) Sheldrick, G. M. SADABS; University of G€ottingen, 1996. (22) SHELXTL, version 6.1; Bruker AXS Inc.: Madison, WI, 2005.

Kozlov et al. the SHELXTL software package.22 Crystal data and structure refinement parameters are listed in Table 2.

Acknowledgment. The authors are grateful to the Russian Basic Research Foundation (grant 08-03-00508) for financial support. Supporting Information Available: Synthesis of the comparative ligand 6, i.e., O-[3-[(methoxyimino)methyl]phenyl] diphenylphosphonate, bearing an O-PdO group, as well as crystal data and structure refinement parameters for crystals (PhNH2)2PdCl2 and solid solution formed by 4b and 5b are available free of charge via the Internet at http://pubs.acs.org.