Kinetico-Mechanistic Insight into the Platinum-Mediated CâC Coupling

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Organometallics 2009, 28, 5096–5106 DOI: 10.1021/om9004934

Kinetico-Mechanistic Insight into the Platinum-Mediated C-C Coupling of Fluorinated Arenes Teresa Calvet,† Margarita Crespo,*,‡ Merce Font-Bardia,† Kerman G omez,§ § ,‡ Gabriel Gonzalez, and Manuel Martı´ nez* †

Departament de Crystal 3 lografia, Mineralogia i Dip osits Minerals, Universitat de Barcelona, Martı´ i Franqu es s/n, E-08028 Barcelona, Spain, ‡Departament de Quı´mica Inorg anica, Universitat de Barcelona, Martı´ i Franqu es, 1-11, E-08028 Barcelona, Spain, and §Institut Catal a d’Investigaci o Quı´mica, Avinguda dels Paı¨sos Catalans 16, E-43007 Tarragona, Spain Received June 11, 2009

The reaction of compound cis-[Pt(C6F5)2(SEt2)2] with the imine ligands ArCHdNCH2CH2NMe2 (Ar = 2-BrC6H4 (1a) or 2,6-Cl2C6H3 (1b)) in toluene produces coordination compounds cis-[Pt(C6F5)2(ArCHdNCH2CH2NMe2)] (2a, 2b) containing a bidentate [N,N0 ] ligand. No further reactivity has been observed from this point. Compound 2b has been characterized by single-crystal XRD. Under analogous conditions, imine ligand 2-BrC6H4CHdNCH2(40 -ClC6H4) (1c) produced the PtII-metalated compound [PtBr{6-(C6F5)(2-C)C5H3CHNCH2(40 -ClC6H4)SEt2] (2c), which contains a five-membered metallacycle with a biaryl linkage involving a C6F5 group. The derivative compounds [PtBr{6-(C6F5)(2-C)C5H3CHNCH2(40 -ClC6H4)L] (L = SMe2 (3c), L = PPh3 (4c)) were also prepared, and compound 4c has also been characterized by XRD. The kinetico-mechanistic study of the formation of compound 2c has also been pursued in view of the previously published data, leading to seven-membered metallacycles. The time monitoring via UV-vis of the full process allowed the detection and NMR characterization of two intermediate species. An initial PtIV complex is present in steady-state low-concentration conditions, and formation of a non-cyclometalated intermediate PtII compound is also detected during the process. The latter already contains the C-C coupled ligand arising from a reductive elimination of the former. Intramolecular C-H activation from the latter produces the final characterized compound 2c along with C6F5H. The full process has been studied as a function of temperature and pressure as well as at varying nonstoichiometric concentrations of SEt2 and free imine ligand. The results agree with the quenching of the process at important excesses of SEt2 (for stoichiometric reasons) or free imine (avoiding the formation of the final complex from the C-C reductively coupled intermediate). The thermal and pressure activation parameters measured indicate that the mechanism operating in this case lies out of the continuum existing for the series of C-H bond activations studied so far. The more than probable associative shift of the reactivity of the PtII complex containing the electron-withdrawing C6F5 ligands is held responsible for this fact. Introduction The involvement of PtII and PdII d8 organometallic complexes in the activation of organic substrates is a fairly developed field; its implication in an important number of value-added catalytic cycles is well known.1 Good examples of this reactivity are the Shilov, Wacker, and Heck reactions.2 The majority of these processes involve an initial C-X (X = Cl, Br, I) oxidative addition (PtII, PdII) or a C-H *Corresponding authors. E-mail: [email protected]; manel. [email protected]. (1) Elschenbroich, Ch.; Salzer, A. Organometallics. A Concise Introduction; VCH: Weinheim, 1999. (2) Hartwig, J. F. Nature 2008, 455, 314–322. (3) Anderson, C. M.; Crespo, M.; Jennings, M. C.; Lough, A. J.; Ferguson, G.; Puddephatt, R. J. Organometallics 1991, 10, 2672–2679. (4) Rendina, L. M.; Puddephatt, R. J. Chem. Rev. 1997, 97, 1735–1754. (5) Ryabov, A. D. Chem. Rev. 1990, 90, 403–424. (6) Canty, A. J. Acc. Chem. Res. 1992, 25, 83–90. (7) Ducker-Benfer, C.; van Eldik, R.; Canty, A. J. Organometallics 1994, 13, 2412–2414. pubs.acs.org/Organometallics

Published on Web 08/20/2009

activation via oxidative addition (PtII) or electrophilic substitution (PdII).3-7 Similarly, most of the overall processes end up with a reductive elimination reaction from these metal centers. Nevertheless, these are very seldom C-X reductive eliminations and much more often C-C reductive couplings.8-13 In this respect, although both biaryl linkages14 and fluorine substituents15 are entities frequently (8) Amatore, C.; Catellani, M.; Deledda, S.; Jutand, A.; Motti, E. Organometallics 2008, 27, 4549–4554. (9) Vigalok, A. Angew. Chem., Int. Ed. 2009, 14, 5102–5106. (10) Yahav-Levi, A.; Goldberg, I.; Vigalok, A.; Vedernikov, A. N. J. Am. Chem. Soc. 2008, 130, 724–731. (11) Koizumi, T.; Yamazaki, A.; Yamamoto, T. Dalton Trans. 2008, 3949–3952. (12) Gracia, C.; Marco, G.; Navarro, R.; Romero, P.; Soler, T.; Urriolabeitia, E. P. Organometallics 2003, 22, 4910–4921. (13) Yahav-Levi, A.; Goldberg, I.; Vigalok, A.; Vedernikov, A. N. J. Am. Chem. Soc. 2008, 130, 724–731. (14) Hassan, J.; Sevignon, M.; Gozzi, C.; Schulz, E.; Lemarie, M. Chem. Rev 2002, 102, 1359–1469. (15) M€ uller, K.; Faeh, C.; Diederich, F. Science 2007, 317, 1881– 1886. r 2009 American Chemical Society

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incorporated in new biologically active pharmaceuticals, the cross-coupling of aryl fluorides has received less attention than that of other aryl halides.11 In particular, it is only recently that the first examples of platinum-catalyzed crosscoupling of polyfluoroarylimines have been reported,16,17 which can be associated with the challenge of activating the strong C-F bond.18-20 We have been involved for some time in developing preparative strategies for the activation of some C-X (X = H, F, Cl, Br) bonds on PtII simple complexes,3,18,19,21-24 as well as in the kinetico-mechanistic study of the oxidative addition process.18,23,25 From the oxidatively added PtIV complexes, reductive aryl-aryl coupling has also been observed in some cases;21,26-28 furthermore, the reductive eliminated complexes normally underwent an oxidative addition process, leading to the final characterized species. The kinetico-mechanistic details of these processes have been determined either directly or indirectly.24,29 The presence of bulky ligand arrangements around the PtII center have been shown to be crucial, as well as the increased Lewis acidity of the metal center. In this respect, the presence of electron-withdrawing ligands attached to the PtIV center has allowed the observation of an interesting unprecedented associative shift of the general reactivity of these organometallic complexes30,31 having an important number of Pt-C bonds, which usually lead to a dissociatively activated general reactivity32-35 despite some differentiating cases.36 With this background in mind, in order to overcome the difficulty associated with C-C coupling with perfluorinated ligands and following our recent work using diarylplatinum compounds as precursors for C-C coupling processes,21,24,26-29 we present in this paper a new preparative strategy based on the reaction of cis-[Pt(C6F5)2(SEt2)2] with (16) Wang, T.; Love, J. A. Organometallics 2008, 27, 3290–3296. (17) Buckley, H. L.; Wang, T.; Love, J. A. Organometallics 2009, 28, 2356–2359. (18) Crespo, M.; Martı´ nez, M.; Sales, J. Organometallics 1993, 12, 4297–4304. (19) Crespo, M.; Martı´ nez, M.; Sales, J. J. Chem. Soc., Chem. Commun. 1992, 822–823. (20) Vicente, J.; Chicote, M. T.; Fernandez-Baeza, J.; FernandezBaeza, A.; Jones, P. H. J. Am. Chem. Soc. 1993, 115, 794–796. (21) Capape, A.; Crespo, M.; Granell, J.; Vizcarro, A.; Zafrilla, J.; Font-Bardı´ a, M.; Solans, X. Chem. Commun. 2006, 4128–4130. (22) Crespo, M.; Font-Bardı´ a, M.; Granell, J.; Martı´ nez, M.; Solans, X. Dalton Trans. 2003, 3763–3769. (23) Crespo, M.; Martı´ nez, M.; Sales, J. Organometallics 1992, 11, 1288–1295. (24) Capape, A.; Crespo, M.; Granell, J.; Font-Bardia, M.; Solans, X. Dalton Trans. 2007, 2030–2039. (25) Crespo, M.; Martı´ nez, M.; de Pablo, E. J. Chem. Soc., Dalton Trans. 1997, 1231–1235. (26) Font-Bardia, M.; Gallego, C.; Martı´ nez, M.; Solans, X. Organometallics 2002, 21, 3305–3307. (27) Martı´ n, R.; Crespo, M.; Font-Bardia, M.; Calvet, T. Organometallics 2009, 28, 587–597. (28) Crespo, M.; Font-Bardia, M.; Solans, X. Organometallics 2004, 23, 1708–1713. (29) Gallego, C.; Martı´ nez, M.; Safont, V. S. Organometallics 2007, 26, 527–537. (30) Bernhardt, P. V.; Gallego, C.; Martı´ nez, M.; Parella, T. Inorg. Chem. 2002, 41, 1747–1754. (31) Gallego, C.; Gonzalez, G.; Martı´ nez, M.; Merbach, A. E. Organometallics 2004, 23, 2434–2438. (32) Frey, U.; Helm, L.; Merbach, A. E.; Romeo, R. J. Am. Chem. Soc. 1989, 111, 8161–8165. (33) Felk, U.; Kaminsky, W.; Goldberg, K. I. J. Am. Chem. Soc. 2001, 123, 6423–6424. (34) Luedtke, A. T.; Goldberg, K. I. Inorg. Chem. 2007, 46, 8496–8498. (35) Smythe, N. A.; Grice, K. A.; Williams, B. S.; Goldberg, K. I. Organometallics 2009, 28, 277–288. (36) Edelbach, B. L.; Lachicotte, R. J.; Jones, W. D. J. Am. Chem. Soc. 1998, 120, 2843–2853.

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aryl imines. The desired outcome is the occurrence of aryl-aryl coupling between the Pt-bound pentafluorophenyl group and the aryl group of the incoming imine. The process has been found to be plausible despite the increased energetic demands needed, which has allowed for the kinetic and spectroscopic characterization of intermediate species not detected in similar systems. The full characterization by XRD of the final cyclometalated complex, having the C-C coupled ligand, has been achieved and the full kinetico-mechanistic study has also been carried out. The results obtained allow for the establishment of a very important associative shift of the processes involving PtII complexes with C6F5 ligands that also explains the quenching of the reactivity when a significant excess of any ligand is present in the reaction medium.

Results Preparation of Compounds. Imines Me2N(CH2)2NCHR (R= 2-BrC6H4 (1a); 2,6-Cl2C6H3 (1b)) and (4-ClC6H4)CH2NCHR (R= 2-BrC6H4 (1c); 2,6-Cl2C6H3 (1d); 4ClC6H4 (1e)) were obtained from the condensation reaction between Me2N(CH2)2NH2 or 4-ClC6H4CH2NH2 and the corresponding aldehydes following well-established procedures.3,19,24 The reactions of cis-[Pt(C6F5)2(SEt2)2] with potentially N-N-C terdentate (after intramolecular C-Br bond activation) ligands 1a and 1b were initially tested in toluene at room temperature. After 15 h, the starting materials were recovered unchanged. When the reactions were carried out in refluxing toluene, coordination compounds [Pt(C6F5)2{Me2NCH2CH2NCH(2-BrC6H4)}] (2a) and [Pt(C6F5)2{Me2NCH2CH2NCH(2,6-Cl2C6H3)}] (2b) were formed within 4 h. These compounds were characterized by 1H and 19F NMR spectra, mass spectrometry, and elemental analyses, and the corresponding structures are shown in Chart 1. For both compounds evidence of coordination of the two nitrogen atoms to platinum is obtained from the fact that both methylamino and imine protons are coupled to 195 Pt. The imine adopts an E conformation according to a cross-peak observed between the imine and the methylene protons in a 2D-NOESY NMR carried out for 2a. In the 19F NMR spectra, the obtained values for δ(F), J(F-F), and J(F-Pt) are consistent with those described for analogous cis-[Pt(C6F5)2L2] compounds.37-40 For both compounds, the two pentafluoro groups are nonequivalent, as indicated by the presence of two para F signals; in each case, the presence of only two ortho F and two meta F signals is consistent with free rotation of the C6F5 groups around the Pt-C bond. The ortho F are downfield shifted (δ ca. -119 to -120 ppm) and coupled to 195Pt (J(F-Pt) in the range 453-476 Hz), and the higher J(F-Pt) values (458.5 Hz (2a) and 475.6 Hz (2b)) are assigned to the C6F5 trans to the NMe2, in agreement with the lower trans influence of the amine moiety. Compound 2b was also characterized crystallographically, and the molecular structure confirms the geometry predicted from spectroscopic data. Attempts to produce intramolecular activation of C-Br or C-Cl bonds were carried out by heating under reflux (37) Casas, J. M.; Fornies, J.; Martı´ n, A.; Menj on, B. Organometallics 1993, 12, 4376–4380. (38) Us on, R.; Fornies, J.; Tomas, M.; Menj on, B. Organometallics 1986, 5, 1581–1584. (39) Casas, J. M.; Diosdado, B. E.; Fornies, J.; Martin, A.; Rueda, A. J.; Orpen, A. G. Inorg. Chem. 2008, 47, 8767–8775. (40) Gelling, A.; Orrell, K. G.; Osborne, A. G.; Sik, V. J. Chem. Soc., Dalton Trans. 1998, 937–945.

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Calvet et al. Chart 1

toluene solutions of compounds 2a or 2b for extended periods (several days). Although analogous reactions have been reported for [PtMe2L] or [PtAr2L] compounds (L = 1a or 1b; Ar = Ph, 4-CH3C6H4),3,27,28 in this case compounds 2a and 2b were recovered unaltered. This fact can be related to the reduced electronic density on the platinum center, having two coordinated pentafluorophenyl groups. Furthermore, the presence of two nitrogen atoms in these ligands may account for the formation of very stable chelate complexes, which could hamper further reaction. In order to promote intramolecular C-X bond activation (X=Br, Cl, or H), the reactions of cis-[Pt(C6F5)2(SEt2)2] with ligands 1c, 1d, and 1e, which contain only one nitrogen atom, were pursued. No reaction between cis-[Pt(C6F5)2(SEt2)2] and ligand 1c was observed in toluene at room temperature;

however, compound 2c was obtained after 4 h in refluxing toluene. Under the same conditions, no reaction occurs with ligands 1d and 1e. The difference is consistent with the lower reactivity expected for C-Cl or C-H versus C-Br bonds, although these bonds have been activated for analogous ligands when platinum substrate [{PtMe2(μ-SMe2)}2] was used.23 As indicated above, the presence of electron-withdrawing C6F5 groups on the platinum center clearly reduces its reactivity toward intramolecular C-X bond activation. Given the fact that the necessary initial substitution of diethylsulfide for the imine should also be affected by the strong inductive effect of the pentafluorophenyl groups,41,42 (41) Minniti, D. J. Chem. Soc., Dalton Trans. 1993, 1343–1345. (42) Deacon, G. B.; Nelson-Reed, K. T. J. Organomet. Chem. 1987, 322, 257–268.

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Table 1. Selected Bond Lengths (A˚) and Angles (deg) for Compounds 2b and 4c with Estimated Standard Deviations compound 2b

compound 4c

Pt-C(12) Pt-C(18) Pt-N(1) Pt-N(2) N(1)-C(7)

2.006 (4) 2.007 (4) 2.044 (4) 2.130 (4) 1.287 (6)

Pt(1)-C(1) Pt(1)-N Pt(1)-P Pt(1)-Br C(5)-C(7)

2.072 (5) 2.098 (3) 2.2448 (15) 2.4799 (14) 1.489 (7)

C(12)-Pt-C(18) C(18)-Pt-N(1) C(12)-Pt-N(2) N(1)-Pt-N(2)

89.16 (17) 94.53 (15) 94.15 (17) 82.01 (15)

C(1)-Pt(1)-N C(1)-Pt(1)-P N-Pt(1)-Br P-Pt(1)-Br

80.74 (16) 95.74 (13) 90.99(11) 92.70(6)

reaction of 1c with cis-[Pt(C6F5)2(SMe2)2] was also tested. The results indicate that, even with this lower basicity ligand (SMe2), the reaction to produce compound 3c did not proceed at room temperature and required analogous conditions to those reported above for 2c. Compounds 2c and 3c were characterized by 1H, 19F, and 195 Pt 1D-NMR spectra, mass spectrometry, and elemental analyses (2c); in addition, 2D-NMR experiments (1H-1H COSY, 1H-1H NOESY, 1H-19F HOESY, and 1H-13C gHSQC) were carried for 2c. In the 1H NMR spectra a signal at 7.77-7.78 ppm coupled to 195Pt (J(H-Pt) = 121.6-125.2 Hz) is assigned to the imine resonance, and the value of the coupling to platinum is consistent with the presence of a bromine atom trans to the imine in a platinum(II) compound. The methylene protons are also coupled to platinum, and a cross-peak observed in the 2D-NOESY NMR spectra between the imine and the methylene protons suggests an E conformation about the CHdN moiety. In the 19F NMR spectra, one set of signals for the ortho, meta, and para F is observed; in this case, both the δ value and the lack of coupling to 195Pt for the ortho F suggest that the C6F5 group is uncoordinated to platinum.43 In addition, only five crosspeaks were observed in the aromatic region for the 1H-13C heterocorrelation spectrum, and cross-peaks between ortho F and both the imine proton and one aromatic proton (He) are observed in the 1H-19F HOESY spectrum. The signals observed in the 195Pt NMR spectra are in the range expected for a platinum(II) coordinated to a [C,N,S,X] donor atoms set.44 All these results are consistent with the structure shown in Chart 1 for compounds 2c and 3c, in which a fivemembered metallacycle with an “exo” Caryl-Caryl bond is formed. This result is in contrast with those previously reported26-29 in which seven-membered metallacycles, which include the biaryl linkage, are formed. In order to confirm this proposal, the reaction of 2c with triphenylphosphine was studied, and the resulting compound 4c was characterized crystallographically. The reaction of 2c with triphenylphosphine was carried out in acetone at room temperature and led to substitution of the coordinated SEt2 ligand for PPh3. The obtained compound 4c was characterized by 1H, 19F, and 31P NMR spectra, mass spectrometry, elemental analyses, and crystal structure. The 1H NMR spectrum shows the presence of the methylene protons coupled to 195Pt, but the imine proton could not be observed since it is overlapped by the PPh3 resonances. As for 2c and 3c, one set of signals for the ortho, meta, and para F is observed in the 19F NMR spectra, none of them coupled (43) Espinet, P.; Albeniz, A. C.; Casares, J. A.; Martı´ nez-Ilarduya, J. M. Coord. Chem. Rev. 2008, 252, 2180–2208. (44) Pregosin, P. S. Coord. Chem. Rev. 1982, 44, 247–291.

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to 195Pt. The large value observed for J(P-Pt) (4164 Hz) in the 31P NMR spectrum indicates that the PPh3 is trans to the imine. Crystal Structure of Compounds 2b and 4c. Suitable crystals of 2b and 4c were grown by slow diffusion of a CH2Cl2 solution into MeOH. The crystal structures are composed of discrete molecules separated by van der Waals interactions. Selected bond lengths and angles are given in Table 1, and the molecular views are shown in Figure 1. For 2b, square-planar coordination of the platinum(II) is achieved with a bidentate [N,N0 ] and two pentafluorophenyl ligands. While the chelate ligand is nearly coplanar with the coordination plane, the dihedral angle being 8.3(2)°, the two pentafluorophenyl rings are twisted from that plane (angles of 67.4(2)° and 80.0(2)°), as reported for similar compounds.45 The imine adopts an E configuration, which is suitable for intramolecular activation of the C-Cl bonds; moreover, one of the ortho chloro substituents is close enough to the platinum center as to be activated (Cl(2) 3 3 3 Pt = 3.624 A˚). Therefore, the failure to react could be related to disfavored electronic effects arising from the presence of two electron-withdrawing pentafluorophenyl rings. In 4c, a C6F5 group is bound to the benzal ring of the imine ligand in an adjacent position to the imine group, thus leading to a biphenyl system with a dihedral angle between both aryl rings of 62.8(3)°. The ligand behaves as [C,N,]-bidentate, and a five-membered endo metallacycle is formed. A bromo and a triphenylphosphine ligand complete the square-planar coordination of the platinum atom. The metallacycle is approximately planar, as suggested by the sum of internal angles, which is close to 540° and nearly coplanar with the coordination plane, the dihedral angle between the mean planes being 3.62(15)°. In both cases, bond lengths and angles are well within the range of values obtained for analogous compounds. Bond angles at platinum are close to the ideal value of 90°, and the smallest angle corresponds to the bidentate ligand, with the “bite” angles N(1)-Pt-N(2) (2b) and C(1)-Pt-N (4c) being 82.01(15)° and 80.74(16)°, respectively. For 2b, in agreement with the lower ligating ability of amines for platinum, the Pt-amine distance (2.130(4) A˚) is longer than the Pt-imine distance (2.044(4) A˚). Kinetico-Mechanistic Study. The chemical reactivity occurring during the preparation of complex 2c described above from cis-[Pt(C6F5)2(SEt2)2] plus imine 1c is dramatically different from that observed for the formation of similar cyclometalated complexes from [{Pt(Me)2(μ-SMe2)}2] and [Pt(Ph)2(SMe2)2] with the same imines.26,29 In the present case, the reaction sequence leading to the formation of the final cyclometalated complex, where a C-C coupling has taken place, occurs at a much slower time scale. When a toluene solution of cis-[Pt(C6F5)2(SEt2)2] is mixed with ligand 1c in a 1 ( 30% ratio, no changes in the UV-vis spectrum are detected at room temperature. When the temperature is raised to the 70-90 °C margin, the expected reactivity, leading to the final highly colored compound obtained on the preparative scale, 2c, is observed. Furthermore, the presence of large excesses of SEt2 produces a quenching of the process, leading to compound 2c, despite the fact that for the formation of cyclometalated complexes with [{Pt(Me)2(μ-SMe2)}2] and [Pt(Ph)2(SMe2)2] as starting material limiting kinetics have been observed.18,23,25 The addition of the cyclometalating imine in (45) Nishida, J.; Maruyama, A.; Iwata, T.; Yamashita, Y. Chem. Lett. 2005, 34, 592–593.

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Figure 1. Molecular structure of compounds 2b (left) and 4c (right).

Figure 2. Changes with time of the ortho fluorine signals for the 19F NMR spectrum of the different complexes involved in the reaction of 1 10-3 M solution of 1c and cis-[Pt(C6F5)2(SEt2)2] at 75 °C in toluene solution: (a) 19F NMR spectra; (b) changes of the relative integral with time.

large excesses also produces the quenching of the formation of compound 2c. With these observations in hand, the kinetico-mechanistic study of the reactivity at high temperatures (70-90 °C) for 5-24 h and under stoichiometric conditions ([Pt] ≈ [imine] ≈ 0.5 10-3 M) was further pursued. The monitoring of the changes in the UV-vis spectrum of the solutions was carried out in m-xylene to avoid solvent evaporation during the study. The absorbance versus time profiles obtained (Figure S1) show, at 425 nm, an induction period, indicating the presence of a complex reaction sequence occurring under the preparative conditions for 2c. 19F NMR time monitoring at 75 °C of equimolar (1 10-3 M) toluene-d8 solutions of platinum starting material and imine 1c proved to be an excellent handle for the establishment of the reaction sequence. Figure 2 shows the changes with time of the ortho fluorine signals of the different compounds present in the solution. While the signal associated with the initial (46) Donghi, D.; Maggioni, D.; Beringhelli, T.; D’Alfonso, D. Eur. J. Inorg. Chem. 2008, 3606–3613.

compound at -119.56 ppm decreases exponentially, the appearance of the final 2c complex, -140.34 ppm (as well an C6F5H group, -142.10 ppm),46 has an induction period. Furthermore, from the data it is evident that there is an initial formation and further depletion of a new species with signals at -117.50 (J(Pt-F) = 470 Hz) and -142.00 ppm. These signals are indicative of the formation of a compound having a PtII-C6F5 group, as well as an organic pentafluorophenyl substituent (R-C6F5).43 The presence of a very small signal at -122.05 ppm with a steady-state intensity is associated with the presence of a putative PtIV.47,48 No other species whatsoever is observed in the 19F NMR spectrum of the samples studied. From the plots it is clear that, under these reaction conditions, no relevant buildup of any PtII-coordinated imine or PtIV-metalated imine complexes occurs. Consequently the reaction sequence corresponds to eq 1, which (47) Casas, J. M.; Martı´ n, A.; Oliva, J.; Tomas, M. Inorg. Chim. Acta 1995, 229, 291–298. (48) Menj on, B.; G omez-Saso, M. A.; Fornies, J.; Falvello, L. R.; Martı´ n, A.; Tsipis, A. Chem.;Eur. J. 2009, 15, 6371–6382.

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Figure 3. Fitting of the UV-vis spectral changes observed for the reaction of 0.48 10-3 M cis-[Pt(C6F5)2(SEt2)2] with 0.62 10-3 M imine 1c at 97 °C in m-xylene solution: (a) absorbance versus time fitting to an {A + B f C f D} profile at 400 nm where B (1c) does not absorb; (b) concentration versus time profiles extracted from the fitting. Table 2. Summary of the Kinetic and Thermal Activation Parameters Obtained for the Reactions of cis-[Pt(C6F5)2(SEt2)2] with Imine 1c in m-xylenea process

k (T /K)

{Pt(C6F5)2} + 1c f {PtII(C6F5)}{R-C6F5} {PtII(C6F5)}{R-C6F5} f 2c

0.038 M-1 s-1 (361) 8.5 10-5 s-1 (361)

a

ΔHq/kJ mol-1

ΔSq/J K-1 mol-1

ΔVq(T)/cm3 mol-1(/K)

135 ( 13 86 ( 8

100 ( 40 -85 ( 22

12 ( 2(363) -34 ( 4(363)

Stoichiometric [Pt] ≈ [imine] ≈ 5.0 10-4 ( 30% M.

Figure 4. (a) Eyring plots for the processes observed during the solution chemistry study of the reactions of cis-[Pt(C6F5)2(SEt2)2] with imine 1c (see text for details). (b) ln k versus P plots for the same reactions.

has a {A + B f C f D} kinetic profile.

fPtðC6 F5 Þ2 g þ 1c f fPtII ðC6 F5 ÞgfR-C6 F5 g f 2c þ C6 F5 H ð1Þ

data the extremely different nature of the two processes occurring under high temperatures and long time conditions, (A+B f C) and (C f D), is evident.

Discussion Fitting of the UV-vis spectral changes to this reaction sequence produced rather good results, as seen in Figure 3, where the parallelism with the 19F NMR experiments shown in Figure 2 is evident. The values of the corresponding second- (A + B f C) and first-order (C f D) rate constants, as well as the derived activation parameters, are collected in Table 2. Figure 4 shows the Eyring and ln k versus P plots obtained for the activation parameters for the studied processes. From these

Intermediate {PtII(C6F5)}{R-C6F5} Species Formation. Comparison of the data obtained in the present kineticomechanistic study (Table 2) with that available in the literature (Table 3) for similar systems represents the best approach to the determination of the nature of compound C ({PtII(C6F5)}{R-C6F5}) in the kinetic sequence shown in Figures 2 and 3 and eq 1. The only reaction studied so far on these types of complexes showing positive ΔS q and ΔV q,

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Table 3. Relevant Data for Other C-X (X = Br, H) Bond Activation, Pt-C Insertion, and C-C Coupling Processes for Similar Systems to That Studied in This Worka process

k/s-1

ΔHq/kJ mol-1

ΔSq/J K-1 mol-1

ΔVq/cm3 mol-1

[{PtII(Me)2(μ-SMe2)}2] + 2-BrC6H4CHNBzl C-Br oxidative additionb [{PtII(Me)2(μ-SMe2)}2] + C6H5CHNBzl C-H oxidative additionb [PtIIPh2(SMe2)2] + 2-BrC6H4CHNCH2(40 -ClC6H4) C-Br oxidative additionc [PtIIPh2(SMe2)2] + 2-PhC6H4CHNBzl C-H oxidative additionc [PtIVPh2Br(2-CC5H4CHNBzl)] C-C reductive elimination couplingd [PtIVPh2Br(2-CC5H4CHNCH2Me)] C-C reductive elimination couplingd [PtIVPh2Br(2-CC5H4CHNBzl)] Ph insertion on the Pt-C bondd [PtIVPh2Br(2-CC5H4CHNCH2Me)] Ph insertion on the Pt-C bondd

5.9 10-3 -3

48 ( 7

-125 ( 23

-22 ( 1

3.4 10

60 ( 5

-100 ( 10

-17 ( 1

1.4 10-3

87 ( 4

-11 ( 13

-4.1 ( 0.6

-3

2.2 10

75 ( 3

-52 ( 12

-7.0 ( 0.7

3.6 10-3

96 ( 3

29 ( 10

13 ( 1

7.3 10-4

94 ( 3

12 ( 9

10 ( 1

3.0 10-4

91 ( 7

-9 ( 24

∼0

7.8 10-5

100 ( 4

10 ( 12

∼0

a Unless stated [Pt] = (0.5-2.1) 10-4 M; [imine] = (4.5-120) 10-4 M; acetone solution; 298 K. b Limiting rate data.25 c This work; toluene solution; limiting rate data. d Chloroform solution.29

Scheme 1

as well as fairly large ΔH q, corresponds to reductive C-C coupling on cyclometalated PtIV compounds.29 The 19F NMR data indicated in the previous section agree with the reaction sequence A+B f C shown in Scheme 1. The low stability of the PtIV oxidatively added intermediate (Figure 2 and Scheme 1) can be held responsible for its lack of significant buildup concentration in the reaction medium and determines the general features observed. The positive ΔV q and ΔS q values agree with a late transition state for the reductive elimination C-C coupling, thus involving also a fairly large activation enthalpy. Although reductive elimination C-C coupling on PtIV complexes is well known1 and some new interesting contributions have been appearing in the last years,13,34,35,49 there are not many studies involving the determination of the activation parameters for the process.29,50-53 In all cases the values of the parameters found agree with those presented here, despite (49) Pawlikowski, A. V.; Getty, A. D.; Goldberg, K. I. J. Am. Chem. Soc. 2007, 129, 10382–10393. (50) Felk, U.; Zahl, A.; van Eldik, R. Organometallics 1999, 18, 4156– 4164. (51) Wik, B. J.; Ivanovic-Burmazovic, I.; Tilset, M.; van Eldik, R. Inorg. Chem. 2006, 45, 3613–3621. (52) Stahl, S. S.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc. 1996, 118, 5961–5976. (53) Crumpton, D. M.; Goldberg, K. I. J. Am. Chem. Soc. 2003, 125, 9442–9456.

the larger errors involved due to the second-order conditions that have had to be used for the study. It is clear that large excesses of SEt2 should prevent the advance of the process stoichiometrically competing with imine 1c in an initial substitution reaction, as observed. Even more, the strong electron-withdrawing features of the C6F5 groups should drive the substitution processes on the starting compound to be associatively activated,54 contrarily to what is expected for the presence of two Pt-C cis bonds,32 thus favoring reentry of the SEt2 ligand. In fact, vacuum heating of cis-[PtII(C6F5)2(SR2)2] does not produce the expected [{Pt(C6F5)2(μ-SR2)}2] (R = Me, Et) compounds, which is indicative of the nonoccurrence of dissociative pathways.55 Furthermore, the low electron density on the platinum center has to make difficult the oxidative addition reaction of the 1c imine C-Br bond and reduce the stability of the PtIV complex formed. As a consequence of all these facts, the equilibrium producing this compound would be very little displaced, as observed (Figure 2, -122.05 ppm signal). As for the quenching of the formation of compound 2c by large excesses of imine, there is no reason to believe that an (54) Tobe, M. L.; Burgess, J. Inorganic Reaction Mechanisms; Longman: White Plains, NY, 1999. (55) Rashidi, M.; Fakhroeian, Z.; Puddephatt, R. J. J. Organomet. Chem. 1991, 406, 261–267.

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Scheme 2

increased difficulty in the oxidative addition process is responsible for this fact, which has not been observed in similar systems.18,19,23,25 Time 19F NMR monitoring of the reaction of a toluene-d8 solution being [1c] = 6 [Pt] = 1 10-3 M under the conditions indicated in Figure 2 proved to be revealing. Under these concentration ratio conditions no signal at -122.05 ppm is observed, indicating that the PtIV compound in Scheme 1 is not present in measurable amounts. Furthermore, a signal that agrees with the presence of the free, C-C coupled (2-C6F5)C6H4CHNCH2(20 ClC6H4) ligand56 emerges very slowly in the 19F NMR spectrum together with a complex pattern of PtII-coupled signals in the ortho-fluorine zone of the C6F5 groups. The full disappearance of the starting material occurs during these changes (Figure S2). The important difference in bulk for imines 1c and (2-C6F5)C6H4CHNCH2(40 -ClC6H4) plus the associatively activated characteristics of the substitution on the intermediate PtII complex (C, Scheme 1), showing only a Pt-C bond, should force a (2-C6F5)C6H4CHNCH2(40 -ClC6H4) by 1c substitution, thus leading to a mixture of unresolved complexes that produce the quenching of the reaction leading to compound 2c under excess imine conditions (see below). At this point a note should be raised about the reductive elimination C-C coupling reaction occurring on a fully saturated PtIV 18-electron complex and not on a pentacoordinated intermediate. Although the presence of a pentacoordinated intermediate has been found necessary in most of the oxidative addition/reductive elimination equilibria on platinum complexes,16,34,51,57-59 some examples of reactions occurring on fully saturated octahedral 18-electron [PtIV(Ph)2Br{2-CC5H4CHNBzl}(SMe2)] or [PtIV(Ph)2Br{2-CC5H4CHN(CH2)2N(Me)2}] complexes have already been reported.9,10,27-29,35,53 Final [PtIIBr{6-C6F5,2-CC5H3CHNCH2(40 -ClC6H4)}(SEt2)], 2c, Complex Formation. It is clear from the results discussed above that the appearance of the final 2c compound from cis-[Pt(C6F5)2(SEt2)2]+1c is related to the C f D kinetic profile indicated (Scheme 2). The process thus corresponds to a C-H oxidative addition to the intermediate PtII complex C indicated in Scheme 1, (56) 19F NMR signals at -140.76 (Fortho) [dd, J=29.6; 11.2]; -154.77 (Fpara) [t, J=51.7]; -162.01 (Fmeta) [m]. (57) Arthur, K. L.; Wang, Q. L.; Bregel, D. M.; Smythe, N. A.; O’Neil, B. A.; Goldberg, K. I.; Moloy, K. G. Organometallics 2005, 24, 4624–4628. (58) Procelewska, J.; Zhal, A.; Liehr, G.; van Eldik, R.; Smythe, N. A.; Williams, B. S.; Goldberg, K. I. Inorg. Chem. 2005, 44, 7732–7742. (59) Wik, B. J.; Lersch, M.; Krivokapic, A.; Tilset, M. J. Am. Chem. Soc. 2006, 128, 2682–2896.

[PtIIBrPh{(2-C6F5)C6H4CHNCH2(40 -ClC6H4)}(SEt2)], followed by a fast C6F5-hydride reductive elimination to form the final complex as found for similar phenyl PtII complexes.26,29 It is interesting to note, though, that C-H bond activation had not been observed on trans-[PtIIBrPh(SMe2)2] with the 2-PhC6H4CHNBzl ligand. This fact had been associated with the lack of preliminary coordination of the imine (due to its low Lewis basicity and bulky character, as well as to the associativeness of the substitution on a PtII center having only a Pt-C bond).32,60,61 In the present study the same factors apply, especially considering the abovementioned electron-withdrawing character of the C6F5 ligand. Dissociation of the already coordinated C-C coupled imine ligand (in the absence of a better ligand, see above) is impeded, contrarily to what has been observed from [PtIIBrPh(2-PhC6H4CHNBzl)(SMe2)] in the presence of large excesses of SMe2.29 As a result, C-H bond activation is made feasible under these rather comparatively strong conditions. Table 3 collects the kinetic and activation parameters available in the literature for C-Br and C-H oxidative addition reactions on similar PtII complexes, as well as some new values determined in this work.62 For all the systems studied having an organometallic cis-{PtII(R)2} (R = Me, Ph) arrangement, the reaction mechanism had been found equivalent.18,25 Figure 5 collects simplified isokinetic relationships obtained. From the plots it is clear that the substitution of two PtII-Me by PtII-Ph bonds produces an increase of ca. 25 kJ/mol in the ΔH q values, but this higher enthalpic demand is compensated by a less negative ΔS q value, so producing equivalent values of k at the isokinetic temperature (ca. 298 K).54,63 The linearity observed for the ΔH q versus ΔV q plots also ascertains the equivalence of the mechanism. From the same plots it is clear that the C-H bond activation process indicated in Scheme 2 produces a set of activation parameters (Table 2, / in Figure 5) that deviate from the relationship, indicating a discontinuity on the (60) Romeo, R.; Plutino, M. R.; Scolaro, L. M.; Stoccoro, S.; Minghetti, G. Inorg. Chem. 2000, 39, 4749–4755. (61) Alibrandi, G.; Bruno, G.; Lanza, S.; Minniti, D.; Romeo, R.; Tobe, M. L. Inorg. Chem. 1987, 26, 185–190. (62) Experiments were carried out at varying high concentration of imine (0.018-0.025 M) with a concentration of cis-[Pt(Ph)2(SMe2)2] of (2-4) 10-4 M. Under these conditions, and according to the trend observed in previous work, no significant variation of kobs with the imine concentration was observed (Supporting Information), and these values were used for the derivation of the thermal and pressure activation parameters collected in Table 3. (63) Wilkins, R. G. Kinetics and Mechanisms of Reactions of Transition Metal Complexes; VCH: Weinheim, 1991.

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Figure 5. (a) Simplified isokinetic relationship for C-X concerted oxidative addition reactions on PtII compounds of this family. (b) Equivalent ΔHq versus ΔVq plot, except for the data indicated in the text extracted from refs 25 and 18. Arrows indicate the shift observed on going from a cis-{PtII(Me)2} to a cis-{PtII(Ph)2} complex; / corresponds to the C-H bond activation related to the cis-{PtII(C6F5)2} moiety complexes studied (Scheme 2).

mechanism for the process. The fact that the PtII compound responsible for this bond activation has only a PtII-C bond with bad donor characteristics (Scheme 2) should be held responsible for the difference. For the compound we are dealing with in this study the standard preliminary ligand dissociation to produce a vacant coordination site, observed for systems of the same family, is very much impeded.23 A similar type of associative shift has already been established for the substitution processes occurring on PtIV organometallic complexes with ligands having a high electron-withdrawing character.30,31 Having this in mind a possible oxidative addition reaction occurring directly from the square-planar 16-electron PtII complex may be possible. In fact the relative accessibility of such 14/16-electron species has recently been discussed for PdII compounds, a key point being the electron-donating capabilities of the ligands attached to the metal center.64 Thus if the process does not require the dissociation of any ligand, the ordering/volume collapse on going from the reactants to the transition state has to be much more important, as seen in Figure 5. Alternatively a putative C6F5 assistance for the hydrogen abstraction might be also considered as part of the mechanistic continuum for bond activation reactions.65-67 The process will then be a formal electrophilic substitution, as those observed and accepted on PdII centers,68-71 which have also been claimed for certain C-H bond activation processes at PtII.22 (64) Moncho, S.; Ujaque, G.; Lled os, A.; Espinet, P. Chem.;Eur. J. 2008, 14, 8986–8994. (65) Vastine, B. A.; Hall, M. B. J. Am. Chem. Soc. 2007, 129, 12068– 12069. (66) Oxgaard, J.; Tenn, W. J.; Nielsen, R. B.; Periana, R. A.; Goddard, W. A. Organometallics 2007, 26, 1565–1567. (67) Perutz, R. N.; Sabo-Etienne, S. Angew. Chem., Int. Ed. 2007, 46, 2578–2592. (68) Aull on, G.; Chat, R.; Favier, I.; Font-Bardı´ a, M.; G omez, M.; Granell, J.; Martı´ nez, M.; Solans, X. Dalton Trans. 2009, DOI 10.10391/ b905134a. (69) Favier, I.; G omez, M.; Granell, J.; Martı´ nez, M.; Font-Bardı´ a, M.; Solans, X. Dalton Trans. 2005, 123–132. (70) G omez, M.; Granell, J.; Martı´ nez, M. J. Chem. Soc., Dalton Trans. 1998, 37–44. (71) G omez, M.; Granell, J.; Martı´ nez, M. Organometallics 1997, 16, 2539–2546.

Conclusions The work reported here shows that compound cis-[Pt(C6F5)2(SEt2)2] is an efficient substrate to produce C-C coupling between a pentafluorophenyl ligand and imine 2-BrC6H4CHdNCH2(4-ClC6H4) (1c). Although previous work carried out for platinum substrates cis-[Pt(Ph)2(SMe2)2] or [{Pt(4-MeC6H4)2(μ-SEt2)}2] produced sevenmembered metallacycles containing a biaryl linkage, in this case a five-membered metallacycle with an “exo Caryl-Caryl” bond is obtained, as for other systems where direct metalation of the ((2-C6H5)C6H4)CHNCH2Ph biaryl imine has been tried. The kinetico-mechanistic study carried out on the system indicates the presence of very small amounts of an initial PtIV compound arising from the C-Br bond activation of imine 1c that thus undergoes a C-C reductive elimination to produce a ligand already having the arylpentafluorophenyl coupling. Further reaction from this point produces a C-H activation on the phenyl ring leading to the final five-membered cyclometalated PtII compound after elimination of C6F5H. In this respect reaction of the same starting material with imines RCHNCH2(4-ClC6H4) (R = 2,6-Cl2C6H3 (1d); 4-ClC6H4 (1e)) having stronger C-Cl or C-H bonds in the positions to be initially activated do not undergo such reaction. Quenching of the process by excess amounts of imine or SEt2 ligands indicates that the initial presence of the PtIV cyclometalated material and its preservation during the time needed for the formation of the final thermodynamically stable PtII form are crucial points for the reactivity observed.

Experimental Section General Procedures. Microanalyses were performed at the Serveis Cientifico-Tecnics (Universitat de Barcelona). Mass spectra were performed at the Servei d’Espectrometria de Masses (Universitat de Barcelona). Electrospray mass spectra were carried out in a LC/MSD-TOF spectrometer using H2OCH3CN (1:1) to introduce the sample. NMR spectra were obtained at the Unitat de RMN d’Alt Camp (Universitat de Barcelona) and at the Institut Catal a d’Investigaci o Quı´ mica. 1 H, 19F, 31P, and 195Pt NMR spectra were recorded by using

Article Bruker 250 (195Pt, 54 MHz), Varian Unity 300 (1H, 300 MHz; 19 F, 282.2 MHz; 31P, 121.42 MHz), Varian Inova 300 (HOESY, 1 H-19F), Mercury 400 (1H, 400 MHz; 19F, 376.5 MHz; 1H-1HCOSY; 1H-1H-NOESY), Varian Inova DMX-500 (1H, 500 MHz; 1H-13C-gHSQC), or Bruker Avance 500 (19F, 470.3 MHz) spectrometers and referenced to SiMe4 (1H, internal), H3PO4 (31P, external), CFCl3 (19F, external), and H2PtCl6 in D2O (195Pt, external). δ values are in ppm and J values in Hz; s= singlet; d = doublet; t=triplet; m = multiplet; NMR labeling is as shown in Chart 1. Preparation of Compounds. cis-[Pt(C6F5)2(SR2)2] (R = Me; Et), cis-[Pt(Ph)2(SMe2)2], and 1a-1e and 2-PhC6H4CHNBzl ligands were prepared as reported elsewhere.3,23,24,26,72,73 [Pt(C6F5)2{Me2NCH2CH2NCH(2-BrC6H4)] (2a) was obtained from 100 mg (0.14 mmol) of cis-[Pt(C6F5)2(SEt2)2] and 36 mg (0.14 mmol) of ligand 1a, which were allowed to react in refluxing toluene for 4 h. The solvent was removed in a rotary evaporator, and the residue was treated with hexane. A yellow solid was isolated by filtration in vacuo. Yield: 50 mg (45.5%). 1 H NMR (400 MHz, CDCl3): δ 2.85 [s, 3J(Pt-H)= 21.6, 6H, Ha]; 2.90 [t, J(H-H)=5.6, 2H, Hb]; 4.26 [td, J(H-H)=5.2; 1.6, 2H, Hc]; {7.19 [td, J(H-H) = 8.0; 2.0, 1H]; 7.31[d, J(H-H)= 8.0, 1H]; 7.40-7.46 [m, 1H]; 7.91-7.93 [m, 1H], aromatic protons}; 8.92 [s, 3J(Pt-H)=62.8, 1H, Hd]. 19F NMR (376.48 MHz, CDCl3): δ -120.09 [m, J(F-Pt) = 453.3, 2F, Fortho]; 120.17 [m, J(F-Pt) = 458.5, 2F, Fortho]; -162.50 [t, J(F-F) = 19.9, 1F, Fpara]; -164.31 [t, J(F-F)=19.9, 1F, Fpara]; -164.95 [m, 2F]; -165.61 [m, 2F]. ESI-MS, m/z: 616.99 [M - C6F5]þ. Anal. Found (calc for C23H15BrF10N2Pt): C: 36.1 (35.22); H: 2.0 (1.93); N: 4.0 (3.57). [Pt(C6F5)2{Me2NCH2CH2NCH(2,6-Cl2C6H3)] (2b) was obtained from 50 mg (0.07 mmol) of cis-[Pt(C6F5)2(SEt2)2] and 20 mg (0.08 mmol) of ligand 1b, which were allowed to react in refluxing toluene for 4 h. The solvent was removed in a rotary evaporator, and the residue was treated with hexane. A light yellow solid was isolated by filtration in vacuo and recrystallized in dichloromethane-methanol. Yield: 25 mg (46.0%). 1H NMR (400 MHz, CDCl3): δ 2.83 [s, 3J(Pt-H)=27.6, 6H, Ha]; 2.92 [t, J(H-H)=5.7, 2H, Hb]; 4.32 [td, J(H-H)=6.0; 1.8, 2H, Hc]; 7.14 [m, 3H, aromatic protons]; 8.76 [s, 3J(Pt-H)=72.0, 1H, Hd]. 19F NMR (376.48 MHz, CDCl3): δ -119.11 [d, J(F-F) = 31.7, J(F-Pt) = 475.6, 2F, Fortho]; -119.77 [d, J(F-F) = 31.7, J(FPt) = 459.8, 2F, Fortho]; -162.61 [t, J(F-F)=19.8, 1F, Fpara]; 164.54 [t, J(F-F)=19.8, 1F, Fpara]; -165.00 [m, 2F]; -165.04 [m, 2F]. ESI-MS, m/z: 792.03 [MþNH4]þ. Anal. Found (calc for C23H14Cl2F10N2Pt): C: 35.6 (35.67); H: 2.1 (1.82); N: 4.0 (3.62). (2c) [PtBr{6-(C6F5)(2-C)C5H3CHNCH2(40 -ClC6H4)SEt2] was obtained from 100 mg (0.14 mmol) of cis-[Pt(C6F5)2(SEt2)2] and 43.5 mg (0.14 mmol) of ligand 1c, which were allowed to react in refluxing toluene for 4 h. The solvent was removed in a rotary evaporator, and the residue was treated with hexane. An orange solid was isolated by filtration in vacuo. Yield: 55 mg (52.0%). 1H NMR (400 MHz, CDCl3): δ 1.40 [t, J(H-H)=7.6, 6H, Ha]; 3.02 [m, 2H, Hb]; 3.40 [m, 2H, Hb]; 5.40 [s, 3J(Pt-H)= 28.8, 2H, Hc]. {6.99 [d, J(H-H)=7.6, He, 1H]; 7.29-7.33 [m, 5H], 7.78 [d, J(H-H) = 7.6, 1H], aromatic protons}; 7.77 [s, 3 J(Pt-H)=125.2, 1H, Hd]. 1H NMR (400 MHz, toluene-d8): δ 1.06 [t, J(H-H)=7.6, 6H, Ha]; 2.43 [m, 2H, Hb]; 3.24 [m, 2H, Hb]; 5.19 [s, 3J(Pt-H) = 34.0, 2H, Hc]. {6.79 [d, J(H-H) = 8.4, 1H]; 6.93 [m, 1H]; 7.09 [m, 4H], 7.86 [d, J(H-H)=7.2, 3J(Pt-H)= 47.6, 1H], aromatic protons}; 7.51 [s, 3J(Pt-H) = 121.6, 1H, Hd]. 13C NMR (1H-13C gHSQC, 500 MHz, CDCl3): δ 13.0 [Ca]; 33.0 [Cb]; 62.5 [Cc]; {126.0 [1C], 129.0 [2C], 131.0 [2C], 132.0 [1C], 133.0 [1C], aromatic carbon}; 176.5 [Cd]. 19F NMR (376.48 MHz, CDCl3): δ -140.81 [dd, J(F-F) = 23.7; 7.9, 2F, Fortho]; -153.61 [t, J(F-F)=21, 1F, Fpara]; -161.21 [td, J(F-F)=21.0; (72) Steele, B. R.; Vrieze, K. Transition Met. Chem. 1977, 2, 140–144. (73) Us on, R.; Fornies, J.; Tomas, M.; Menj on, B.; Navarro, R. Inorg. Chim. Acta 1989, 162, 33–37.


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Table 4. Crystallographic and Refinement Data for Compounds 2b and 4c compound 2b formula fw temp, K wavelength, A˚ cryst syst space group a, A˚ b, A˚ c, A˚ R, deg β, deg γ, deg V, A˚3; Z d(calcd), mg/m3 abs coeff, mm-1 F(000) rflns coll/unique data/restraints/ params GOF on F2 R1(I > 2σ(I)) wR2 (all data) peak and hole, e 3 A˚-3

compound 4c

C23H14Cl2F10N2Pt C38H25BrClF5NPPt 774.35 932.01 293(2) 293(2) 0.71073 0.71073 monoclinic triclinic P1 P21/c 12.271(5) 12.233(7) 13.321(3) 12.259(5) 15.371(5) 13.842(6) 90 111.65(3) 95.64(2) 98.54(3) 90 109.89(3) 2500.4(14); 4 1722.8(16); 2 2.057 1.797 5.916 5412 1472 900 25 342/6954 [R(int) = 21 069/10 973[R(int) = 0.0483] 0.0628] 3829/0/221 10 973/0/433 1110 0.0349 0.1034 1.688 and -1.681

1.054 0.0468 0.1214 0.926 and -1.135

7.1, 2F, Fmeta]. 19F NMR (376.48 MHz, toluene-d8): δ -141.13 [dd, J(F-F)=23.3; 8.2, 2F, Fortho]; -153.87 [t, J(F-F)=21.0, 1F, Fpara]; -161.38 [td, J(F-F)=21.8; 6.8, 2F, Fmeta]. 195Pt NMR (54 MHz, CDCl3): δ -3947.20. ESI-MS, m/z: 680.05 [M - Br]þ. Anal. Found (calc for C24H20BrClF5NPtS): C: 37.9 (37.93); H: 2.6 (2.65); N: 2.3 (1.84); S 4.0 (4.22). [PtBr{6-(C6F5)(2-C)C5H3CHNCH2(40 -ClC6H4)SMe2] (3c) was obtained using an analogous procedure from cis-[Pt(C6F5)2(SMe2)2]. Yield: 55 mg (49.0%). 1H NMR (400 MHz, CDCl3): δ 2.75 [s, J(H-Pt)=52.4, 3H, Ha]; 5.39 [s, 3J(Pt-H)= 28.0, 2H, Hc]; {7.01 [d, J(H-H) =7.6, He, 1H]; 7.29-7.33 [m, 5H], 7.71 [d, J(H-H) = 8.0, 1H], aromatic protons}; 7.78 [s, 3 J(Pt-H) = 121.6, 1H, Hd]. 19F NMR (376.48 MHz, CDCl3): δ -140.88 [dd, J(F-F)=22.6; 8.3, 2F, Fortho]; -153.55 [t, J(F -F) = 20.7, 1F, Fpara]; -161.17 [td, J(F-F) = 22.6; 8.0, 2F, Fmeta]. 195Pt NMR (54 MHz, CDCl3): δ -3861.7. ESI-MS, m/z: 731.04 [M]; 652.02 [M - Br]þ. [PtBr{6-(C6F5)(2-C)C5H3CHNCH2(40 -ClC6H4)PPh3] (4c) was obtained from 20 mg (0.03 mmol) of compound 2c and 8 mg (0.03 mmol) of triphenylphosphine, which were allowed to react in acetone at room temperature for 2 h. The solvent was removed in a rotary evaporator, and the residue was washed with pentane. An orange solid was isolated by filtration in vacuo. Yield: 20 mg (81.0%). 1H NMR (400 MHz, CDCl3): δ 5.49 [s, 3 J(Pt-H) = 15.3, 2H, CH2]; {6.64 [m, 2H]; 6.78 [m, 1H], 7.387.41 [m, 13H], 7.73-7.77 [m, 7H], aromatic þ imine protons}. 19 F NMR (282.23 MHz, CDCl3): δ -140.68 [dd, J(F-F)=21.8; 7.9, 2F, Fortho]; -153.90 [t, J(F-F)=19.8, 1F, Fpara]; -161.41 [td, J(F-F) = 21.8; 7.9, 2F, Fmeta]. 31P NMR (121.42 MHz, CDCl3): δ 22.72 [s, J(P-Pt) = 4164]. ESI-MS, m/z: 852.09 [M Br]þ. Anal. Found (calc for C38H25BrClF5NPPt): C: 49.9 (48.97); H: 2.8 (2.70); N: 2.0 (1.50). X-ray Structure Analysis. Prismatic crystals were selected and mounted on a MAR345 diffractometer with an image plate detector. Intensities were collected with graphite-monochromatized Mo KR radiation. The structures were solved by direct methods using the SHELXS computer program74 and refined by the full-matrix least-squares method, with the SHELXL97 computer program using 25 342 (2b) and 21 069 (4c) reflections (very negative intensities were not assumed). All hydrogen (74) Sheldrick, G. M. SHELXS; Universit€at of G€ottingen: Germany, 1997.

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atoms were computed and refined using a riding model, with an isotropic temperature factor equal to 1.2 times the equivalent temperature factor of the atom to which they are linked. Further details are given in Table 4. Kinetics. The kinetic profiles for the reactions were followed in general by UV-vis spectroscopy in the 500-330 nm range. Some nonkinetic, but time-dependent NMR experiments were also conducted (Figure 2). Atmospheric pressure runs were recorded on an HP8452A or Cary50 instrument equipped with thermostated multicell transports. Observed rate constants were derived from absorbance versus time traces at the wavelengths where a maximum increase and/or decrease of absorbance was observed. For runs at variable pressure, a previously described pressurizing system and pill-box cell were used;71 the system was connected to a J&M TIDAS spectrophotometer, which was used for the absorbance measurements. The calculation of the observed rate constants from the absorbance versus time monitoring of reactions, studied under second- or first-order concentration conditions, was carried out using the SPECFIT software.75 (75) Binstead, R. A.; Zuberbuhler, A. D.; Jung, B. SPECFIT32, [3.0.34]; Spectrum Software Associates, 2005.

Calvet et al. Due to the larger errors involved in the method, all experiments were run at least in duplicate and special care was taken in the global analysis results of the fitting. The general kinetic technique is that previously described.25,26,30 Table S1 collects the obtained kobs values for all the systems studied as a function of the starting complex, process studied, platinum and imine concentrations, pressure, and temperature. All post-run fittings were carried out by the standard available commercial programs.

Acknowledgment. We would like to thank the Ministerio de Educaci on y Ciencia (CTQ2006-02007/ BQU, CTQ2006-14909-C02-02) for financial support. Supporting Information Available: Crystallographic data in CIF format; values of kobs determined as a function of the complex, solvent, temperature, and pressure; and observed UV-vis and NMR spectral changes for one of the reactions studied. This material is available free of charge via the Internet at http://pubs.acs.org.

Kinetico-Mechanistic Insight into the Platinum-Mediated CâC Coupling

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