Synthesis and Characterization of Palladium (II) and Platinum (II

Jun 24, 2014 - Sina P. Zucker , Friedrich Wossidlo , Manuela Weber , Dieter Lentz , and C. Christoph Tzschucke. The Journal of Organic Chemistry 2017 ...
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Synthesis and Characterization of Palladium(II) and Platinum(II) Adducts and Cyclometalated Complexes of 6,6′-Dimethoxy-2,2′bipyridine: C(sp3)−H and C(sp2)−H Bond Activations Fabio Cocco,† Antonio Zucca,† Sergio Stoccoro,† Maria Serratrice,† Annalisa Guerri,‡ and Maria A. Cinellu*,† †

Dipartimento di Chimica e Farmacia, Università degli Studi di Sassari, CIRC, Via Vienna 2, I-07100 Sassari, Italy Dipartimento di Chimica, Università di Firenze, Via della Lastruccia 3, I-50019 Sesto Fiorentino, Italy

‡

S Supporting Information *

ABSTRACT: Reactions of 6,6′-dimethoxy-2,2′-bipyridine (bipy2OMe) with palladium(II) and platinum(II) chlorides or acetates afforded either neutral, [M(bipy2OMe)Cl2], and cationic, [Pt(bipy2OMe)2][Pt(DMSO)Cl3]2, adducts or cyclometalated derivatives [M(bipyOMe,OCH2)X] and [M(bipy2OMeH)X]2 (M = Pd, X = OAc, Cl; M = Pt, X = Cl) arising from respectively C(sp3)−H and C(sp2)−H bond activation, depending on the metal precursor, solvent, and other reaction conditions. Cyclometalated complexes of the new ligand 6hydroxy-6′-methoxy-2,2′-bipyridine (bipyOMe,OH), arising from monodemethylation of bipy2OMe, were also formed as coproducts under certain reaction conditions. The crystal structures of [Pt(bipy2OMe)2][Pt(DMSO)Cl3]2 and of [Pd(bipy2OMe-H)(OAc)]2 have been solved by X-ray crystallography.

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vs sp3 C−H bond activation include metal precursors,8d,f−h solvent,8a,9e,f reaction temperature8b,f and the ring size of the metallacycle.8b,f,i Five-membered metallacycles are usually preferred over six-membered ones, although specific features of the preligands can override this trend; this is the case of imines which show a strong preference for the formation of endo complexesthe so-called endo effectover the alternative exo derivatives.10 In the course of our studies in the field of cyclometalation reaction of heterocyclic nitrogen ligands with d8 metal ions, mainly palladium(II), platinum(II) and gold(III), we obtained a variety of cyclometalated complexes of 2-substituted pyridines,11 6-substituted 2,2′-bipyridines,8f,h,12 2,2′-bipyridine,13 2,2′:6′,2″terpyridine,14 1,3-disubstituted benzenes,15 and other nitrogen donor ligands.16 Some of these complexes arise from activation of a C(sp2)−H bond or, to a lesser extent, of a C(sp3)−H bond on the substituent of the pyridine or bipyridine ligand, while others have been obtained from activation of the C(3)−H bond of a pyridine ring of unsubstituted 2,2-bipyridine,13 as well as of 2,2′:6′,2″-terpyridine14in this case both C(3)−H bonds of the central pyridine ring have been activated -, or even of 6substituted 2,2′-bipyridines. This peculiar metalation, known as “rollover” cyclometalation,17 was observed when electron-rich platinum(II) intermediates, typically [PtR2L2] (R = Me, Ph), and

INTRODUCTION Metal-mediated C−H bond activation, by paving the way for subsequent functionalization of hydrocarbons, is a process of paramount importance in chemical industry.1 Cyclometalation, i.e. the intramolecular C−H bond activation facilitated by coordination of a donor heteroatom,2 is undoubtedly the most popular organometallic reaction leading to organometallic compounds featuring metal−carbon σ bonds.3 Nearly all transition metals are able to activate C−H bonds in cyclometalation reactions, and a great variety of organic molecules, bearing at least one heteroatom, have been employed to create stable cyclometalated complexes and to investigate aspects underlying the metal-mediated activation of C−H bonds. Among the plethora of cyclometalated derivatives, those made of a metal of the platinum group and a heterocyclic nitrogen ligand have attracted considerable attention due to their potential applications in many fields including organic synthesis,4 homogeneous catalysis,5 novel materials6 and medicinal chemistry.7 Control of selectivity in C−H bond activation is a challenging issue in chemical transformations of organic molecules. For the intramolecular C−H bond activation by palladium(II) and platinum(II), it is generally observed that aromatic over aliphatic C−H bond activation is preferred. Nevertheless, examples have been reported of preferred or selectively directed sp3 C−H over sp2 C−H bond activation by palladium(II) and platinum(II).8,9 Factors which can influence the preference in the competing sp2 © XXXX American Chemical Society

Received: March 21, 2014

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Pd(OAc)2 were employed as metal precursors. Under these conditions, in the case of 6-substituted 2,2′-bipyridines, activation of the C(3)−H bond could be selectively achieved not only over activation of sp3 C−H bonds but also of other sp2 C−H bonds of the substituent in 6.8f,12 Following our continuous efforts in the comprehension of the behavior of cyclometalated and rollover derivatives of d8 metal ions, we decided to investigate the coordinating behavior of 6,6′dimethoxy-2,2′-bipyridine (bipy2OMe). This ligand is characterized by a low basicity18indeed, for example, it is not protonated by HCl in waternevertheless, a more pronounced nucleophilic character, with respect to 2,2′-bipyridine, is expected, taking into account the mesomeric effect (+M) of the methoxy groups in 6,6′. The coordinating behavior of bipy2OMe is almost unexplored,19 although it has been used as a useful precursor for the synthesis of 6,6′-dihydroxy-2,2′-bipyridine (bipy2OH).19a,20 Some years ago, we studied the reaction of bipy2OMe with gold(III) salts and found that, while the reaction with Na[AuCl4] afforded the trichloride adduct [Au(bipy2OMe)Cl3], reaction with Au(OAc)3 resulted in the C(3)−H bond activation of a pyridine ring to give the cycloaurated complex [Au(bipy2OMe-H)(OAc)2].21 We report here the reaction of bipy2OMe with palladium(II) and platinum(II) inorganic salts and electron-poor precursors to give species in which bipy2OMe behaves as an N^N classical neutral chelated ligand, as an N^N^C cyclometalated anionic ligand, originated from activation of a sp3 C−H bond of one methoxy substituent, and as a N^C roll-over cyclometalated anionic ligand, originated from activation of an aromatic C(3)− H bond of one of the two pyridine rings. The kind of product formed depends on: (i) metal precursor, (ii) solvent, (iii) temperature, and (iv) reaction time. Cyclometalated complexes of the new ligand 6-hydroxy-6′-methoxy-2,2′-bipyridine (bipyOMe,OH), arising from monodemethylation of bipy2OMe, were also formed as coproducts under certain reaction conditions.

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Chart 1. Palladium(II) and Platinum(II) Adducts and Cyclometalated Derivatives of bipy2OMe and Cyclometalated Derivatives of bipyOMe,OH/bipyOMe,O

[Pt(bipy 2OMe)2 ][Pt(DMSO)Cl3]2 9 toluene

⎯⎯⎯⎯⎯⎯→ 2[Pt(bipy 2OMe)Cl 2] + Pt(DMSO)2 Cl 2 Δ

6

(1)

The two adducts 1 and 6 show a low solubility in most organic solvents, likely due to π-stacking interactions between pyridine rings. The 1H NMR spectrum of 1 in CD3CN is almost superimposable to that of the free ligand in the same solvent (see Experimental Details and Table S1 in the Supporting Information [SI]), which suggests that displacement of the coordinated ligand by two CD3CN molecules readily occurs in this coordinating solvent, while in the case of 6 no free ligand was detected, and large downfield shifts of all aromatic protons and upfield shift of the methyl protons are observed. The electrolytic nature of complex 9 was unambiguously shown by its high conductivity value in MeCN solution (see Experimental Details). Its 1H NMR spectrum in CD3CN shows only one set of signals for the aromatic and methyl protons. A singlet at δ 3.27 ppm, flanked by satellites (3JPt−H = 22.0 Hz), is consistent with an Sbonded DMSO trans to a chlorine atom.b Its structure was confirmed by an X-ray crystallographic analysis on single crystal. Yellowish crystals of 9 were obtained by evaporation of an acetonitrile solution. The asymmetric unit (a.u.) of compound 9 holds half of the dication and one [PtCl3(DMSO)]− counterion. The platinum atom of the cationic adduct lies in a special position, and its occupancy factor is 0.5. The platinum atom in the dicationic fragment of complex 9 is tetracoordinated in a square planar fashion (Figure 1); it is bound to the two nitrogen atoms of the bipyridine moiety, the bond lengths and anglesa selection of which is given in caption for Figure 1are comparable to those found for analogous complexes, e.g., [Pt(4,4′Me2bipy)2](BF4)2 (9a).23 The bipyridine ligand results in being very distorted compared to the ideal planar geometry featured by the free molecule, and also found in platinum complexes of the type [Pt(bipy)Cl2] (where bipy = 2,2′bipyridine or 4,4′- and 5,5′-disubstituted 2,2′-bipyridine).23 In complex 9 the so-called bowing angle23 is 25.4(2)° (it is 24.8(2) in 9a) calculated as the angle between the mean planes of the two

RESULTS AND DISCUSSION

Synthesis and Characterization of the Complexes. The ligand 6,6′-dimethoxy-2,2′-bipyridine (bipy2OMe) was synthesized by 6-methoxy-2-bromopyridine homocoupling via nickelcatalyzed Negishi cross-coupling, according to literature methods.22 Palladium(II) and platinum(II) adducts, cyclometalated complexes of bipy2OMe, and cyclometalated complexes of the new ligand 6-hydroxy-6′-methoxy-2,2′-bipyridine (bipyOMe,OH), arising from monodemethylation of bipy2OMe, are shown in Chart 1. Reaction of bipy2OMe with trans-[Pd(PhCN)2Cl2] in dichloromethane at room temperature afforded the adduct [Pd(bipy2OMe)Cl2] (1) in high yield. Reaction of the ligand with various platinum(II) chlorides (e.g., K 2 [PtCl 4 ], [Pt(4MeC6H4CN)2Cl2], cis-[Pt(DMSO)2Cl2]) under mild conditions failed to give the analogous platinum(II) adduct, [Pt(bipy2OMe)Cl2] (6), which could be obtained only by rearrangement of the cationic adduct [Pt(bipy2OMe)2][Pt(DMSO)Cl3]2 (9) in refluxing toluene, according to eq 1. Complex 9, in its turn, was obtained as a coproduct of the reaction of bipy2OMe with cis[Pt(DMSO)2Cl2] in EtOH at reflux (see below; Scheme 4, entry c). B

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chloride complex [Pd(bipyOCH2,OH)Cl] (3oh) was obtained as coproduct of the reaction of 1 in refluxing AcOH: in this case the integral ratio between 3 and 3oh was 1:0.8. Analogous cycloplatinated complexes [Pt(bipyOMe,OCH2)Cl] (7) and [Pt(bipyOCH2,OH)Cl] (7oh) have been obtained under various reaction conditions, although in some cases these complexes were accompanied by at least another unidentified species (Scheme 3). Reaction of bipy2OMe with cis-[Pt(DMSO)2Cl2] in toluene at reflux afforded complex 7 in low to moderate yields. Best results were obtained from the reaction with K2[PtCl4] in AcOH at reflux for 5 days. In this case, a small amount of bipyOMe,OH was recovered from the mother liquor and fully characterized. On prolonging the reaction time, mixtures of 7 and 7oh were obtained. Mixtures of 7, 7oh, and other unidentified products were also obtained when the reaction with K2[PtCl4] was carried out in water in the presence of HCl at reflux. Flash column chromatographic separations of these mixtures resulted in the isolation of 7, while fractions containing 7oh were always contaminated by 7 or by other products. The 1H NMR spectra of the cyclometalated complexes [M(bipyOMe,OCH2)X] (M = Pd, X = OAc, 2; M = Pd, X = Cl, 3; M = Pt, X = Cl, 7) are characterized by a singlet at 6.30 ppm, in the case of 2 and 3, and at 6.86 for complex 7, which integrates for two protons, relative to the methylenic protons of the metallacycle; in the case of the platinum complex 7, this signal is flanked by satellites, due to coupling to 195Pt, with 2JPt−H of 65.2 Hz. In all cases, the signal of the methyl protons, which integrates for three protons, is slightly shielded with respect to the free ligand. The aromatic protons of the two different pyridine rings give rise to two sets of signals which could be fully assigned by means of 2D-COSY and NOESY experiments (see Experimental Details and Table S1 in the SI). As seen before, when the syntheses of the cyclometalated derivatives 2, 3, and 7 were carried out in refluxing acetic acid, these were accompanied by similar cyclometalated complexes [M(bipyOCH2,OH)X], respectively 2oh, 3oh, and 7oh, likely arising from bipyOMe,OH. In one case the free ligand was isolated

Figure 1. ORTEP drawing of the dication in complex 9 with ellipsoids probability of 30%. Hydrogen atoms are not shown for clarity. Selected bond lengths (Å) and angles (deg.): Pt(1)−N(1) 2.021(5), Pt(1)− N(2) 2.010(5); N(1)−Pt(1)−N(2) 77.50(18), N(1)−Pt(1)−N(2)′ 102.50(18); N(2)′ reported by the symmetry operation −x+1, −y+1, −z+1.

pyrine rings. The distortion of the ligand is mainly due to the O− Me substituents of the rings which hindered the coordination. Reaction of bipy2OMe with the more efficient metalating agent Pd(OAc)224 resulted in the activation of either a Csp3−H (Scheme 1, entries a and b) or a Csp2−H (entry c) bond, depending on the reaction solvent and temperature. When the reaction was carried out in acetic acid at room temperature, palladation occurs at the aliphatic carbon of one of the methoxy substituents to give the cyclometalated derivative [Pd(bipyOMe,OCH2)(OAc)] (2) which features a N,N,C terdentate ligand. Treatment of 2 with LiCl in acetone afforded the chloride complex [Pd(bipyOMe,OCH2)Cl] (3); this could also be obtained from the adduct 1 when refluxed in toluene or in AcOH (Scheme 2). Notably, when the reaction of bipy2OMe and Pd(OAc)2 in AcOH was carried out under reflux and prolonged reaction times, a 2:1 mixture of 2 and a second cyclometalated complex, formulated as [Pd(bipyOCH2,OH)(OAc)] (2oh), was obtained (1H NMR criterion). Attempts to separate the two complexes failed to give any pure sample of this species. The

Scheme 1. Products Obtained from Reaction of bipy2OMe with Pd(OAc)2

C

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Scheme 2. Cyclopalladated Complexes Obtained from Thermal Activation of Adduct 1

Scheme 3. Products Obtained from Reaction of bipy2OMe with K2[PtCl4]

Scheme 4. Products obtained from reaction of bipy2OMe with cis-[Pt(DMSO)2Cl2]

refluxed in AcOH in the presence of catalytic amounts (10%) of Pd(OAc)2 and K2[PtCl4], respectively, while a third amount was refluxed without any metal salt. The result is that bipyOMe,OH was slowly formed in all three cases, although a pronounced catalytic effect of the metal was observed (e.g.: after 3 days, the bipy2OMe/ bipyOMe,OH molar ratio, according to the 1H NMR of the reaction mixture, was 0.2:1, in the platinum(II)-catalyzed reaction, and 2.7:1 in the metal-free reaction). Pure samples of 2oh and 7oh could be finally obtained from bipyOMe,OH, and their 1H NMR spectra compared to those

and thoroughly characterized by means of analytical and spectroscopic methods (see Experimental Details). A detailed NMR study (1H, 13C, 2D-NOESY experiments) strongly suggested the proposed molecular structure.25 Surprisingly, no data were available for this compound,26 while it has been reported that 6,6′-dihydroxy-2,2-bipyridine, bipy2OH, can be obtained by refluxing bipy2OMe in a mixture of HBr and acetic acid.27 Thus, thinking that demethylation of only one methoxy group in acetic acid could have been triggered off by palladium28 and platinum salts,29 amounts of bipy2OMe were D

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previously obtained by subtracting the signals of their respective coproducts 2 and 7. These are characterized by sharp singlets relative to the OH substituent in 6′ at 10.24 (2oh), 10.15 (3oh), and 10.67 ppm (7oh), one singlet integrating for two protons at 6.53 (2oh), 6.27 (3oh), and 6.84 ppm (7oh), the latter flanked by satellites with 2JPt−H = 64.0 Hz, relative to the methylenic protons of the metallacycle, and six signals for the aromatic protons; (see Experimental Details and Table S1 in the SI). A completely different cyclometalated complex was obtained when the reaction between bipy2OMe and Pd(OAc)2 was carried out in toluene, both at room temperature and at reflux (Scheme 1, entry c). The product, formed in moderate yield, is the rollover cyclometalated dinuclear complex [Pd(bipy2OMe-H)(OAc)]2 (4) arising from activation of the sp2 C(3)−H bond of one of the two pyridine rings. This kind of activation in bipy2OMe was previously observed in the case of gold(III): reaction of the ligand with Au(OAc)3 in AcOH at reflux yielded the mononuclear complex [Au(bipy2OMeH)(OAc)2].21 Treatment of 4 with LiCl in acetone solution afforded the chloro complex [Pd(bipy2OMe-H)Cl]2 (5). The analogous platinum(II) complex [Pt(bipy2OMe-H)Cl]2 (8) was obtained from the reaction of bipy2OMe with cis-[Pt(DMSO)2Cl2] in dichloromethane at reflux (Scheme 4, entry b). The same complex was also obtained when the reaction was carried out in ethanol at reflux, but in this case a second product was formed in 1:1 molar ratio, which could be separated after treatment of a dichloromethane suspension of the mixture with CO. Under these conditions, only the dinuclear cyclometalated complex 8 reacted with CO to give the mononuclear complex [Pt(bipy2OMe-H)(CO)Cl] (10), that was separated from the solid residue, due to its solubility in dichloromethane. The remaining insoluble product was fully characterized by means of analytical, spectroscopic, and crystallographic methods as the cationic trinuclear adduct [Pt(bipy2OMe)2][Pt(DMSO)Cl3]2 (9) described above. Separation of 8 and 9 was also accomplished after treatment of the mixture with PPh3.30 Surprisingly, also in this case PPh3 reacted selectively only with 8 to give the mononuclear cyclometalated complex [Pt(bipy2OMe-H)(PPh3)Cl] (11). Reaction of the mixture with excess PPh3 resulted in further reaction of 11 which underwent displacement of the pyridine ligand to give the bis-phosphine complex [Pt(bipy2OMeH)(PPh3)2Cl] (12). The cyclometalated molecular structure of compounds 4, 5, and 8 is apparent from the 1H NMR spectra by observation of two singlets, each integrating for three protons, at δ 3.66 and 3.82 (4), 3.90 and 3.96 (5), 3.91 and 4.01 (8), relative to the two methoxy groups. Particularly diagnostic of the coordination of the metal ion with the C(3) atom is an AB system due to H4 and H5: e.g. in complex 8 these resontate at δ 6.55 (H5) and 8.03 (H4) with JAB = 8.5 Hz, both protons are coupled to 195Pt, with H4 showing the larger JPt−H value (36.8 Hz vs 16.8 Hz). X-ray-quality crystals of the palladium complex 4 were obtained by slow evaporation of a mixture of chloroform/diethyl ether. The solid state molecular structure of complex 4 is depicted in Figure 2 with principal bond lengths and angles reported in the caption. The asymmetric unit (a.u.) of compound 4 contains the whole metal complex. The two palladium atoms of complex 4 have a classic square planar coordination, the four positions occupied by two oxygen atoms belonging to two different acetate anions, a nitrogen atom of one ring of the bipyridine ligand, and a carbon atom of the other ring of the same ligand. The bond lengths and

Figure 2. ORTEP drawing of complex 4 with ellipsoids at 50% probability. Hydrogen atoms are not shown. Selected bond lengths (Å) and angles (deg): Pd(1)−N(101) 2.078(4), Pd(1)−C(108) 1.960(5), Pd(1)−O(1) 2.154(3), Pd(1)−O(3) 2.033(3), Pd(2)−N(201) 2.070(4), Pd(2)−C(208) 1.949(5), Pd(2)−O(2) 2.027(3), Pd(2)− O(4) 2.144(3), Pd(1)−Pd(2) 2.8473(5); N(101)−Pd(1)−C(108) 80.70(17), N(101)−Pd(1)−O(3) 171.59(14), N(101)−Pd(1)−O(1) 102.52(14), C(108)−Pd(1)−O(3) 90.90(16), C(108)−Pd(1)−O(1) 174.93(16), O(3)−Pd(1)−O(1) 85.89(13).

angles are in the range found for analogous [Pd(C^N)(OAc)]2 complexes.31 The two acetate ligands bridge the two palladium atoms keeping them 2.8472(1) Å apart; this distance is close to the lower value of the range found in related structures, 3.0457(5)−2.864(1) Å,31 and may be regarded as weakly bonding.32 The coordination planes containing the metal atoms, the bipyridine ligands, and the oxygen atoms of the acetate anions are almost parallel, forming an angle of 24°. The distance between the ring centroids of the pyridine moieties are 3.76 and 3.80 Å. These distances and the angles formed by the planes are consistent with a π-stacking interaction even though the d8−d8 interaction plays a major role in determining the overall geometry of the complex.33 The mononuclear platinum(II) cyclometalated complexes 10, 11, and 12 (Chart 2) have been fully characterized. The carbonyl Chart 2. Mononuclear Platinum(II) Cyclometalated Complexes

derivative 10 was obtained as a 1:2 mixture of the kinetic (10k)34 and thermodynamic (10t) isomers; these could be separated due to their slightly different solubility in dichloromethane. The IR spectra of 10k and 10t show strong absorption peaks (flanked by shoulders) respectively at 2084 and 2100 cm−1 relative to the CO stretching vibration, and peaks relative to the Pt−Cl stretching vibration at 341 cm−1 (Pt−Cl trans to N) and E

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289 cm−1 (Pt−Cl trans to C). In the 1H NMR spectra in CDCl3 of the two isomers rather different chemical shifts are found for the H4 and H5 protons: in particular, the H4 proton of 10k is highly downfield shifted with respect to that of 10t (δ 8.12 vs 7.59) due its proximity to the chloride ligand. In both isomers the signal of this proton is flanked by satellites with 3JPt−H of 26.8 Hz (10k) and 62.4 Hz (10t); coupling constants clearly reflect the different trans influence of CO and Cl in 10k and 10t, respectively. At variance with the carbonyl derivative, only one isomer was formed in the case of the triphenylphosphine derivative 11, likely the thermodynamic one: this isomer should be more stable also for steric reasons. The 31P NMR spectrum shows one singlet at δ 22.8 ppm flanked by satellites with JPt−P = 4557.9 Hz, in line with a N−Pt−P trans arrangement. The bis-phosphine complex 12, which is formed both by reaction of 8 with excess PPh3 and by reaction of 11 with one equiv of PPh3, features a trans arrangement of the two PPh3, as shown by its 31P NMR spectrum where only one signal is observed, at δ 17.0 ppm, flanked by satellites with JPt−P = 3186.0 Hz. Factors Affecting C(sp3)−H or C(sp2)−H Bond Activation. As shown, C(sp3)−H or C(sp2)−H bonds of bipy2OMe can be selectively activated by both palladium(II) and platinum(II) precursors. The resulting cyclometalated complexes, respectively [M(bipyOMe,OCH2)X] (M = Pd, X = OAc, 2; M = Pt, X = Cl, 7) and [M(bipy2OMe-H))X]2 (M = Pd, X = OAc, 4; M = Pt, X = Cl, 8) feature five-membered metallacycles. In the case of palladium, for which the same precursor namely Pd(OAc)2was used, selectivity is primarily controlled by the solvent. Reaction in AcOH, in which the typical electrophilic character of Pd(OAc)2 is depressed,3a resulted in the activation of the more challenging aliphatic C−H bond, of the MeO substituent in 6, to give the [Pd(N^N^C)(OAc)] cyclometalated complex 2. Thus, the effect of the bidentate ligand as the directing group for initial metal coordination and the geometrical constraints imposed by the resulting chelate ring seem to play a crucial role in directing the C−H activation on this site.35 Notably, the intermolecular C−H bond activation of ethers has been reported for many late transition metal ions36 including palladium(II)37 and platinum(II);38 in all these cases the alkoxomethyl(alkyl) intermediates rearrange to stable carbene complexes by a rapid α-hydrogen migration.39 By contrast, few examples of the intramolecular C−H activation of methoxy(alkoxy-) substituted substrates have been found, a relevant example being the C-platination of 2-alkoxyphenylphosphines, reported many years ago by Shaw and co-workers.29 In that case, a four-center intermediate was tentatively suggested to be involved in C-metalation, with the metal acting essentially as an electrophile.29 In toluene, in which Pd(OAc)2 exhibits electrophilic properties, activation of the more reactive aromatic C−H occurred, affording the dimeric [Pd(N^C)(OAc)]2 cyclometalated complex 4; in this case, activation of the C(3)−H of one of the pyridine rings is facilitated by the presence of a strong donor in para position. C(3)−H activation, the so-called rollover cyclometalation, of 6-substituted 2,2′-bipyridines by Pd(OAc)2 in benzene or toluene was previously observed by some of us.8f,12a Deeming that the formation of 4 was kinetically controlled and that of 2 thermodynamically controlled, a sample of complex 4 was refluxed in AcOH, wherein isomerization to 2 proceeded

rather quickly and after 30 min a 2:1 mixture of 2 and 4 was formed (after 1 h 2:4 = 8.3:1). In the case of platinum(II), due to its lower electrophilicity, activation of the sp3 C−H bond was shown to be more favored over that of the sp2 C−H. Indeed, the [Pt(N^N^C)Cl] complex 7 was obtained either from K2[PtCl4] in AcOH and in H2O−HCl and, albeit in small amount, also from [Pt(DMSO)2Cl2] in toluene. Thus, also in this case, N,N-chelate control of the selectivity is strongly suggested.35 Reaction of [Pt(DMSO)2Cl2] in CH2Cl2 or in ethanol, in which a more electrophilic character of platinum(II) is expected, resulted in the activation of the sp2 C(3)−H to give the [Pt(N^C)Cl]2 cyclometalated complex 8. An electrophilic pathway for the C(3)−H bond activation of bipy2OMe can be postulated involving the dichloride adduct 6.40 A plausible pathway for the conversion of 6 to the N′,C(3) cyclometalated species is shown in Scheme 5. Scheme 5

The steric hindrance due to the methoxy substituent in 6 and the electronic repulsion between lone pairs in OMe and in Cl make the adduct 6 unstable and allow the cleavage of a Pt−N bond weakened by the poor donating ability of the N atom, as a result of the ortho offect of the OMe group.41 Rotation of a pyridine ring around the C(2)−C(2′) bond brings the C(3)−H bond close to the metal.42 The OMe group in para activates this position to the electrophilic substitution by the PtCl2 fragment, which is followed by loss of HCl. Notably, up to now, in the case of platinum(II), C(3)−H activation of 2,2′-bipyridine,13 6-substituted 2,2′-bipyridines,12 and related ligands,11b,14 to give cyclometalated rollover complexes had been accomplished only when electron-rich precursors such as [Pt(DMSO)2R2] (R = Me, Ph) were used. The postulated mechanism for this reaction involves an oxidative-addition/reductive-elimination pathway.12c With those substrates no C(3)−H activation was observed with less electron-rich precursorseven [Pt(DMSO)2MeCl] was ineffective. Moreover, we found that, although N′,C(3) metalation is highly facilitated by substituents in the 6 position (due to steric hindrance), reaction of [Pt(DMSO)2Me2] with 6,6′-dimethyl2,2′-bipyridine failed to give any cyclometalated rollover complex.12c The same behavior is found here for bipy2OMe; no reaction occurs in acetone solution at room temperature,43 whereas heavy decomposition is observed on heating at reflux. F

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CONCLUSION The still almost unexplored coordination chemistry of bipy2OMe was revealed to be very attractive, and a variety of palladium(II) and platinum(II) complexes were obtained, depending on the metal precursor, solvent, temperature, and reaction time. Besides neutral and cationic adducts, in which bipy2OMe behaves as an N^N classical neutral chelated ligand, two kinds of cyclometalated complexes were selectively obtained featuring, respectively, an N^N^C cyclometalated anionic ligand, originating from activation of an sp3 C−H bond of one methoxy substituent, and an N^C roll-over cyclometalated anionic ligand, originating from activation of an aromatic C(3)−H bond of one of the two pyridine rings. Regioselectivity in the C−H activation is primarily controlled by the solvent, which modulates the electrophilic properties of the metal precursor, although other factors play an important role. This is particularly evident in the case of Pd(OAc)2. In acetic acid, in which the electrophilic character of this precursor is depressed, activation of the aliphatic C−H bond is accomplished, as the N,N-chelate-control enforces the position of the methoxy substituent in 6(6′), and cyclometalation can then occur regiospecifically at the methyl group. On the other hand in toluene, where Pd(OAc)2 is a typical electrophile, electrophilic activation of the aromatic C−H bond occurs, the reaction being accelerated by the electron-donating OMe group in the para position. The resulting complex [Pd{C(sp2)^N}(OAc)]2 can be converted in refluxing acetic acid into [Pd{C(sp3)^N^N}(OAc)], which suggests that selectivity can be attributed to kinetic or thermodynamic control under different conditions. The aliphatic C−H is easily activated by the less electrophilic platinum(II), either by K2[PtCl4] in polar solvents (H2O, AcOH) and by [Pt(DMSO)2Cl2] in toluene, while activation of the aromatic C(3)−H occurs when this precursor is used in CH2Cl2 and in ethanol. Notably, this is the first time that rollover cyclometalation is observed for electron-poor platinum(II) starting complexes. Reaction in refluxing AcOH, causing demethylation of one methoxy group, led to the new ligand 6-hydroxy-6′-methoxy2,2′-bipyridine (bipyOMe,OH); this undergoes activation of an sp3 C−H bond of the remaining methoxy group to give N^N^C cyclometalated complexes analogous to those obtained from bipy2OMe. Further studies will be devoted to the improvement of the yields of the cyclometalated complexes of bipy2OMe, to the coordinating behavior of bipyOMe,OH, and to the reactivity and catalytic properties of all the cyclometalated complexes.

conductivity meter. Infrared spectra were recorded with an FTIR Jasco 480P using nujol mulls. 1H, 31P{1H}, and 19F{1H} NMR spectra were recorded with a Bruker Avance III 400 and with a Varian VXR 300 spectrometers at 298 K. Chemical shifts are given in ppm relative to internal TMS for 1H and external 85% H3PO4 for 31P{1H}; J values are given in Hz. 2D-NOESY and 2D-COSY experiments were performed by means of standard pulse sequences.

Spectroscopic Data of bipy2OMe. Selected IR bands: (ν/ cm−1, nujol mull): 1578, 1303, 1266, 1026, 796, and 722. 1H NMR (CDCl3): δ 4.04 (s, 6H, CH3), 6.75 (dd, JH−H = 8.2, 0.8 Hz, 2H, H5,5′), 7.68 (pseudo-t, JH−H = 8.2, 7.4 Hz, 2H, H4,4′), 8.01 (dd, JH−H = 7.4, 0.8 Hz, 2H, H3,3′); (CD3CN): δ 4.00 (s, 6H, CH3), 6.81 (dd, JH−H = 8.2, 0.8 Hz, 2H, H5,5′), 7.77 (pseudo-t, JH−H = 8.5, 7.4 Hz, 2H, H4,4′), 8.03 (dd, JH−H = 7.4, 0.8 Hz, 2H, H3,3′). 13C NMR (CD2Cl2): δ 53.4 (OCH3), 111.2, 113.8, and 139.6 (Ar CH), 153.8 and 163.9 (Ar Cipso). bipyOMe,OH/bipyOMe,O. Anal. Calcd for C11H10N2O2 (202.21): C, 65.34; H, 4.98; N, 13.85. Found: C, 65.45; H, 5.03; N, 13.80. GC/MS spectrum, m/z: 202.10 (100%) [M.+], 173.10 (32%) [M+ − CHO], 144.10 (14%) [M+ − 2CHO], 117.10 (5%) [M+ − 2CHO − HCN]. IR (ν/cm−1, nujol): 1656, 1615, 1571, 1335, 1263, 1151, 1028, 997, 950, 862, 793, 722. 1H NMR (CDCl3): δ 4.03 (s, 3H, OMe), 6.62 (d, JH−H = 9.2 Hz, 1H, H5′), 6.80 (d, JH−H = 7.2 Hz, 1H, H3′), 6.82 (d, JH−H = 8.4 Hz, 1H, H5), 7.43 (d, JH−H = 7.6 Hz, 1H, H3), 7.48 (dd, JH−H = 9.2, 6.8 Hz, 1H, H4′), 7.69 (t, JH−H = 8.0 Hz, 1H, H4), 10.41 (broad, 1H, OH or NH). Assignments based on 2D-NOESY spectrum. 13C NMR (CDCl3): δ 54.0 (OCH3), 103.16, 112.9, 113.0, 121.7, 125.2, 139.9, and 140.9 (Ar CH), 142.0, 145.7, 163.0, and 164.0 (Ar Cipso). [Pd(bipy2OMe)Cl2] (1). To a solution of bipy2OMe (238.0 mg, 1.1 mmol) in dichloromethane (35 mL) was added trans-[Pd(PhCN)2Cl2] (421.9 mg, 1.1 mmol). The mixture was stirred for 4 days at room temperature. The precipitate obtained was filtered off and recrystallized from acetonitrile/diethyl ether to give the analytical sample as an orange solid. Yield 80%. Mp 165 °C. Anal. Calcd for C12H12Cl2N2O2Pd (393,56): C, 36.62; H, 3.07; N, 7.12. Found: C, 36.35; H, 2.82; N, 7.56. IR (ν/cm−1, nujol): 1602, 1572, 1147, 1029, 786, 721, and 342 (ν Pd−Cl). 1H NMR (CD3CN): δ 4.01 (s, 6H, CH3), 6.81 (dd, JH−H = 8.2, 0.7 Hz, 2H, H5,5′), 7.78 (pseudo-t, JH−H = 8.2, 7.4 Hz, 2H, H4,4′), 8.03 (dd, JH−H = 7.4, 0.7 Hz, 2H, H3,3′).

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EXPERIMENTAL DETAILS General Procedures. All starting materials were used as received from commercial sources; K2PtCl4 and Pd(OAc)2 were purchased from Johnson Matthey and Alfa Aesar, respectively; trans-[Pd(PhCN)2Cl2], [Pt(p-MeC6H4CN)2Cl2] and cis-[Pt(DMSO)2Cl2] were synthesized according to the literature;44 6,6′-dimethoxy-2,2′-bipyridine (bipy2OMe) was synthesized by 6methoxy-2-bromopyridine homocoupling via nickel-catalyzed Negishi cross-coupling, according to literature methods.22 Elemental analyses were performed with a PerkinElmer 240B elemental analyzer by Mr. Antonello Canu (Dipartimento di Chimica e Farmacia, Università degli Studi di Sassari, Italy). The MS spectrum of bipyOMe,OH was performed on a combined Agilent 6850−Agilent 5973 GC/MS apparatus. Conductivity measurements were performed with a Philips PW 9505

[Pd(bipyOMe,OCH2)(OAc)] (2). A solution of bipy2OMe (216.2 mg, 1.0 mmol) and Pd(OAc)2 (224.5 mg, 1.0 mmol) in glacial AcOH (25 mL) was stirred for 7 days at room temperature, and then diluted with water (15 mL). The resulting solution was extracted with dichloromethane (3 × 15 mL) and the combined G

dx.doi.org/10.1021/om5003057 | Organometallics XXXX, XXX, XXX−XXX

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organic extracts treated with K2CO3 and Na2SO4. The filtered solution was concentrated to a small volume and treated with diethyl ether. The precipitate formed was filtered off, washed with diethyl ether, and vacuum-pumped to give the analytical sample as a yellow solid. Yield 87%. The same compound (yield 87.5%) was obtained when the reaction was carried out under reflux for 30 min. (Under the same reaction conditions, i.e. refluxing AcOH, but prolonged reaction times, a ∼2:1 mixture of [Pd(bipyOMe,OCH2)(OAc)] (2) and [Pd(bipyOCH2,OH)(OAc)] (2oh) was obtained.) Mp 127 °C. Anal. Calcd for C14H14N2O4Pd (380.69): C, 44.17; H, 3.71; N, 7.36. Found: C, 44.28; H, 3.80; N, 7.15. IR (ν/cm−1, nujol): 1707, 1660, 1574, 1278, 1136, 1063, 790, 722. 1H NMR (CDCl3): δ 2.10 (s broad, 3H, CH3C(O)O), 3.95 (s, 3H, CH3O), 6.30 (s, 2H, CH2O), 6.84 (d, JH−H = 8.4 Hz, 1H, H5), 6.94 (d, JH−H = 8.4 Hz, 1H, H5′), 7.40 (d, JH−H = 7.6 Hz, 1H, H3), 7.55 (d, JH−H = 7.6 Hz, 1H, H3′), 7.74 (t, JH−H = 8.0 Hz, 1H, H4), 7.92 (t, JH−H = 8.0 Hz, 1H, H4′). Assignments based on 2D- COSY and NOESY spectra. [Pd(bipyOCH2,OH)(OAc)] (2oh). A solution of bipyOMe,OH (10.7 mg, 0.05 mmol) and Pd(OAc)2 (11.9 mg, 0.05 mmol) in glacial AcOH (5 mL) was refluxed for 1 h under stirring. The resulting solution was evaporated to dryness and the residue taken up with CHCl3; the filtered solution was evaporated to dryness to give the analytical sample. Yield 90%. Mp 147 °C, dec. Anal. Calcd for C13H12N2O4Pd (366.67): C, 42.58; H, 3.30; N, 7.64. Found: C, 42.48; H, 3.25; N, 7.58. IR (ν/cm−1, nujol): 3387, 2666, 1655, 1604, 1570, 1491, 1307, 1280, 1250, 1166, 1128, 1007, 725. 1H NMR (CDCl3): δ 2.07 (s, 3H, CH3C(O)O), 6.53 (s, 1H, CH2O), 6.83 (d, JH−H = 8.4 Hz, 1H, H5), 6.92 (d, JH−H = 8.4 Hz, 1H, H5′), 7.35 (d, JH−H = 7.6 Hz, 1H, H3), 7.41 (d, JH−H = 7.6 Hz, 1H, H3′), 7.74 (pseudo-t, JH−H = 8.0, 7.6 Hz, 1H, H4), 7.78 (pseudo-t, JH−H = 8.0, 7.6 Hz, 1H, H4′), 10.24 (s, 1H, OH). [Pd(bipyOMe,OCH2)Cl] (3). An acetone solution of [Pd(bipyOMe,OCH2)(OAc)] (190.3 mg, 0.5 mmol) was treated with LiCl (35.2 mg, 0.83 mmol) and the mixture stirred for 12 h at room temperature. Then the filtered solution obtained was concentrated to a small volume and diethyl ether added. The precipitate obtained was filtered off, washed with diethyl ether, and vacuum-dried to give the analytical sample as a yellow solid. Yield 88%. Mp 244 °C. Anal. Calcd for C12H11ClN2O2Pd (357.10): C, 40.36; H, 3.10; N, 7.84. Found: C, 40.23; H, 2.96; N, 7.80. IR (ν/cm−1, nujol): 1600, 1569, 1484, 1277, 1142, 1059, 786, 320, 267. 1H NMR CDCl3: δ 4.02 (s, 3H, CH3O), 6.30 (s, 2H, CH2O), 6.91 (d, JH−H = 8.4 Hz, 1H, H5), 6.95 (d, JH−H = 8.4 Hz, 1H, H5′), 7.42 (d, JH−H = 8.0 Hz, 1H, H3), 7.55 (d, JH−H = 7.6 Hz, 1H, H3′), 7.77 (t, JH−H = 8.0 Hz, 1H, H4), 7.94 (t, JH−H = 8.0 Hz, 1H, H4′); [Pd(bipyOCH2,OH)Cl] (3oh). 1H NMR (CDCl3): δ 6.27 (s, 2H, CH2O), 6.90 (d, JH−H = 8.4 Hz, 1H, H5), 6.96 (d, JH−H = 8.4 Hz, 1H, H5′), 7.37 (d, JH−H = 8.0 Hz, 1H, H3), 7.45 (d, JH−H = 7.2 Hz, 1H, H3′), 7.78 (t, JH−H = 8.0 Hz, 1H, H4), 7.81 (t, JH−H = 8.0 Hz, 1H, H4′), 10.15 (s, 1H, OH). Data obtained by subtraction of the signals of 3 from the 1H NMR of the mixture. [Pd(bipy2OMe-H)(OAc)]2 (4). To a solution of bipy2OMe (216.2 mg, 1.0 mmol) in toluene (35 mL) was added solid Pd(OAc)2 (224.5 mg, 1.0 mmol). The resulting mixture was refluxed for 10 min under stirring, after which a dark orange solution and metallic Pd were formed. (When the reaction was carried out at room temperature for 9 days a suspension was obtained that was filtered off and the filtrate concentrated to a small volume; addition of diethyl ether afforded a yellow solid which was filtered off and vacuum-dried to give the analytical sample. Yield 22%.) The suspension was filtered over Celite and the filtered

solution concentrated to a small volume; addition of hexanes afforded an orange precipitate which was filtered off and vacuumdried to give the analytical sample. Yield 145.3 mg, 38%. Mp 176 °C. Anal. Calcd for C28H30N4O8Pd2 (763.40): C, 44.05; H, 3.96; N, 7.34. Found: C, 44.10; H, 3.98; N, 7.31. IR (ν/cm−1, nujol): 1585, 1562, 1481, 1441, 1404, 1346, 1323, 1298, 1254, 1178, 1129, 1064, 1022, 935, 799, 755, 691, 666. 1H NMR (CDCl3): δ 2.19 (s, 3H, CH3C(O)O), 3.66 (s, 3H, CH3O), 3.82 (s, 3H, CH3O), 6.02 (d, JH−H = 8.4 Hz, 1H, H5), 6.29 (d, JH−H = 8.4 Hz, 1H, H5′), 7.19 (d, JH−H = 8.4 Hz, 1H, H4), 7.21 (d, JH−H = 8.0 Hz, 1H, H3′), 7.44 (t, JH−H = 8.0 Hz, 1H, H4′). X-ray quality crystals of the complex were obtained by slow diffusion of diethyl ether into a chloroform solution. [Pd(bipy2OMe-H)Cl]2 (5). To a solution of [Pd(bipy2OMeH)(OAc)]2 (5) (47.3 mg, 0.062 mmol) in acetone (25 mL) was added LiCl (10.6 mg, 0.25 mmol). The mixture was stirred for 12 h at room temperature, and then the solution was evaporated to dryness. The residue was taken up with water, filtered off, and washed with ethanol and diethyl ether, to give the analytical sample as a yellow solid (38.4 mg, yield 86.7%). Mp 220 °C dec. Anal. Calcd for C24H22Cl2N4O4Pd2 (714.20): C, 40.36; H, 3.10; N, 7.84. Found: C, 40.34; H, 3.08; N, 7.87. IR (ν/cm−1, nujol): 1608, 1567, 1409, 1305, 1265, 1132, 1070, 1032, 1013, 937, 814, 793, 746, 312. 1H NMR (CD3CN): δ 3.90 (s, 3H, CH3O), 3.96 (s, 3H, CH3O), 6.46 (d, JH−H = 8.4 Hz, 1H, H5), 6.91 (d, JH−H = 8.4 Hz, 1H, H5′), 7.69 (d, JH−H = 7.6 Hz, 1H, H3′), 7.99 (pseudo t, JH−H = 8.4, 7.6 Hz, 1H, H4′), 8.03 (d, JH−H = 8.4 Hz, 1H, H4). [Pt(bipy2OMe)Cl2] (6). A suspension of [Pt(bipy2OMe)2][Pt(DMSO)Cl3]2 (9) (see below) (103.4, mg 0.07 mmol) in toluene (40 mL) was stirred for 3 days at reflux and then filtered under vacuum. The crude product was recrystallized from acetonitrile/diethyl ether to give 70 mg of the analytical sample as a yellow solid. Mp 137 °C. Anal. Calcd for C12H12Cl2N2O2Pt (482.22): C, 29.89, H, 2.51, N, 5.81. Found: C, 29.85, H, 2.47, N, 5.78. IR (ν/cm−1, nujol): 1606, 1573, 1490, 1351, 1277, 1145, 1068, 1.38, 1009, 796, 724, 445, 380, 357. 1H NMR (DMSO-d6): δ 3.83 (s, 6H, 2OMe), 7.40 (d, JH−H = 8.7 Hz, 2H, H5,5′), 8.27 (d, JH−H = 7.8 Hz, 2H, H3,3′), 8.48 (pseudo-t, JH−H = 8.5, 7.8 Hz, 2H, H4,4′). [Pt(bipyOMe,OCH2)Cl] (7). 1. From cis-[Pt(DMSO)2Cl2] in Toluene. To a solution of bipy2OMe (108.1 mg, 0.5 mmol) in toluene (35 mL) was added cis-[Pt(DMSO)2Cl2] (211.0 mg, 0.5 mmol) and the mixture stirred for 48 h at reflux. The precipitate obtained was filtered off, washed with toluene and diethyl ether, and dried under vacuum. The crude product was extracted with chloroform and the filtered solution concentrated to a small volume; addition of diethyl ether and n-hexane gave a precipitate that was filtered off and purified by flash column chromatography on silica gel using dichloromethane as eluent. The analytical sample was a yellow solid. Yield 21%. Mp > 220 °C. Anal. Calcd for C12H11ClN2O2Pt (445.76): C, 32.33; H, 2.49; N, 6.28. Found: C, 32.45; H, 2.31; N, 6.24. IR (ν/cm−1, nujol): 1603, 1567, 1486, 1400, 1277, 1145, 1062, 1001, 952, 780, 727, 325. 1H NMR (CDCl3, 293 K): δ 4.08 (s, 3H, OMe), 6.86 (s with sat, 2 JPt−H = 65.2 Hz, 2H, CH2), 6.91 (d, JH−H = 8.4 Hz, 1H, H5), 7.08 (d, JH−H = 8.4 Hz, 1H, H5′), 7.33 (d, JH−H = 7.6 Hz, 1H, H3), 7.57 (d, JH−H = 7.6 Hz, 1H, H3′), 7.85 (t, JH−H = 8.0 Hz, 1H, H4), 8.03 (t, JH−H = 8.0 Hz, 1H, H4′). Assignments based on 2D-COSY and NOESY spectra. 2. From K2PtCl4 in Acetic Acid. To a suspension of K2PtCl4 (207.5 mg, 0.5 mmol) in AcOH (50 mL) was added bipy2OMe (108.1 mg, 0.5 mmol); the resulting mixture was refluxed for 5 days under stirring. A precipitate obtained was filtered off and H

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extracted with dichloromethane; the filtered solution was concentrated to small volume, and diethyl ether and n-hexane were added to give a yellow precipitate of [Pt(bipyOMe,OCH2)Cl] (164.5 mg, 73.8%). The mother liquor was evaporated to dryness to give a solid orange product which was analytically and spectroscopically characterized as bipyOMe,OH (see below). When the reaction mixture was refluxed for a more prolonged time, a ∼1:1 mixture of [Pt(bipyOMe,OCH2)Cl] (7) and [Pt(bipyOCH2,OH)Cl] (7oh) (see below) was obtained. Attempts to separate the two products failed to give any pure sample of this species. 3. From K2PtCl4 in H2O/HCl. To a solution of K2[PtCl4] (207.5 mg, 0.5 mmol) in water (20 mL) were added bipy2OMe (108.1 mg, 0.5 mmol) and 1.5 mL of 2 N HCl. The resulting mixture was stirred at reflux for 2 days and at room temperature for 5 days. A pale-yellow precipitate was filtered off and washed with water, ethanol, and diethyl ether to give 151.5 mg of a mixture of [Pt(bipyOMe,OCH2)Cl] (7), [Pt(bipyOCH2,OH)Cl] (7oh), and other unidentified species. The mixture was extracted with chloroform and dichloromethane, and the combined solutions were subjected to flash column chromatography on silica gel using dichloromethane as eluent, a fraction of which gave 46.4 mg (20.8%) of [Pt(bipyOMe,OCH2)Cl] (7); this compound was also contained in the other fractions. [Pt(bipyOCH2,OH)Cl] (7oh). A mixture of bipyOMe,OH (108.1 mg, 0.5 mmol) and K2[PtCl4] (207.5 mg, mmol) in glacial AcOH (20 mL) was refluxed for 7 days under stirring. The resulting solution was evaporated to dryness and the residue taken up with CHCl3; the filtered solution was concentrated to a small volume; addition of diethyl ether gave a yellow solid that was filtered off and vacuum-dried to give the analytical sample. Yield 30%. No decomposition or melting to 240 °C. Anal. Calcd for C11H9ClN2O2Pt (431.73): C, 30.60; H, 2.10; N, 6.49. Found: C, 30.58; H, 2.05; N, 6.43. IR (ν/cm−1, nujol): 3089, 1608, 1566, 1481, 1284, 1223, 1161, 1007, 845, 791, 324. 1H NMR (CDCl3): δ 6.84 (s with sat, 2JPt−H = 64.0 Hz, 2H, CH2), 6.92 (d, JH−H = 8.4 Hz, 1H, H5), 7.09 (d, JH−H = 8.4 Hz, 1H, H5′), 7.28 (d, JH−H = 7.6 Hz, 1H, H3), 7.46 (d, JH−H = 7.6 Hz, 1H, H3′), 7.85 (t, JH−H = 8.0 Hz, 1H, H4), 7.89 (pseudo-t, JH−H = 8.0, 7.6 Hz, 1H, H4′), 10.67 (s, 1H, OH). [Pt(bipy2OMe-H)Cl]2 (8). To a solution of bipy2OMe (131.9, mg, 0.61 mmol) in dichloromethane (40 mL) was added cis(DMSO)2PtCl2 (257.6 mg, 0.61 mmol). The mixture was stirred for 48 h at reflux and then evaporated to dryness. The residue was treated with chloroform to extract the starting material. The insoluble residue was recrystallized from acetone/diethyl ether and n-hexane to give the analytical sample as a white solid. Yield 25%. Anal. Calcd for C24H22Cl2N4O4Pt2 (890.03): C, 32.33; H, 2.49; N, 6.28. Found: C, 32.58; H, 2.16; N, 6.22. IR (ν/cm−1, nujol): 1614, 1568, 1490, 1410, 1307, 1131, 1032, 938, 814, 791, 749, 341. 1H NMR (CD3CN, 293 K): δ 3.91 (s, 3H, OMe), 4.01 (s, 3H, OMe), 6.55 (d, JH−H = 8.5 Hz, 1H, H5), 6.90 (dd, JH−H = 8.5, 1.2 Hz, 1H, H5′), 7.67 (dd, JH−H = 7.5, 1.2 Hz, 1H, H3′), 8.00 (t, JH−H = 7.5 Hz, 1H, H4′), 8.03 (d with sat, JH−H = 8.5 Hz, 3JPt−H = 36 Hz, 1H, H4). [Pt(bipy2OMe-H)Cl]2 (8) + [Pt(bipy2OMe)2][Pt(DMSO)Cl3]2 (9). To a solution of bipy2OMe (70.0 mg, 0.32 mmol) in ethanol (30 mL) was added cis-[Pt(DMSO)2Cl2] (136.7 mg, 0.32 mmol). The resulting mixture was stirred for 24 h at reflux. The precipitate obtained was filtered off, washed with ethanol and diethyl ether, and dried under vacuum. The 1H NMR shows two sets of signals with integral ratio of ∼1:1, corresponding to two

different species, one of which was identified as the cyclometalated derivative [Pt(bipy2OMe-H)Cl]2. The two species were separated after reaction with PPh3 or CO. Separation of the Products. Reaction with CO. CO was bubbled into a suspension of the mixture (145.0 mg) in CH2Cl2 (35 mL) at room temperature for 2 h. An insoluble product was filtered off and dried under vacuum. The compound was a yellow solid (85 mg) which analyzed for [Pt(bipy2OMe)2][Pt(DMSO)Cl3]2 (see below). The solution was concentrated to a small volume, and diethyl ether and n-hexane were added to give a white solid which was filtered off and vacuum-dried to give 58.0 mg of the analytical sample which analyzed for [Pt(bipy2OMe -H)(CO)Cl] (see below). Reaction with PPh3. A suspension of the mixture of the two products (151.0 mg) in CH2Cl2 (35 mL) was treated with PPh3 (34.6 mg, 0.132 mmol). After 5 min the remaining yellow solid product was filtered off and dried under vacuum to give an analytical sample of [Pt(bipy2OMe)2][Pt(DMSO)Cl3]2 (90 mg, 0.065 mmol). The solution was concentrated to a small volume, and diethyl ether and n-hexane were added to give a white precipitate that was filtered off and vacuum-dried to give an analytical sample of [Pt(bipy2OMe-H)(PPh3)Cl] (91 mg, 0.128 mmol) (see below). [Pt(bipy2OMe)2][Pt(DMSO)Cl3]2 (9). Mp 193 °C. Anal. Calcd for C28H36Cl6N4O6Pt3S2 (1386.69): C, 24.25; H, 2.62; N, 4.04. Found: C, 24.33; H, 2.64; N, 3.98. ΛM (5 × 10−4 mol L−1, MeCN): 262 Ω−1 cm2 mol−1. IR (ν/cm−1, nujol): 1606, 1573, 1491, 1420, 1277, 1145, 1068, 1038, 1010, 896, 796, 444, 379, 338, 306. 1H NMR (CD3CN, 293 K): δ 3.27 (s, 3JPt−H = 22.0 Hz, 6H, CH3-DMSO), 3.81 (s, 6H, CH3O), 7.14 (d, JH−H = 8.2 Hz, 2H, H5,5′), 7.90 (d, JH−H = 7.8 Hz, 2H, H3,3′), 8.30 (dd, JH−H = 8.2, 7.8 Hz, 2H, H4,4′). [Pt(bipy2OMe-H)(CO)Cl] (10). Mp 184 °C. Anal. Calcd for C13H11ClN2O3Pt (474.77): C, 32.96; H, 2.34; N, 5.91. Found: C, 33.19; H, 2.45; N, 6.07. IR (ν/cm−1, nujol): 2100 (νCO trans to N) and 2084 (νCO trans to C), 1617, 1570, 1415, 1267, 1140, 1027, 936, 802, 757, 341 (νPt−Cl trans to N), 288 (νPt−Cl trans to C). 10k: 1H NMR (CDCl3, 293 K): δ 3.95 (s, 3H, OMe), 4.05 (s, 3H, OMe), 6.67 (d with sat, JH−H = 8.4 Hz, 4JPt−H = 3.2 Hz, 1H, H5), 6.74 (dd with sat, JH−H = 8.4, 0.8 Hz, 4JPt−H = 12.8 Hz, 1H, H5′), 7.68 (dd, JH−H = 7.6, 1.2 Hz, 1H, H3′), 7.90 (dd, JH−H = 8.3, 7.7 Hz, 1H, H4′), 8.12 (d with sat, JH−H = 8.4 Hz, 3JPt−H = 26.8 Hz, 1H, H4). 13C NMR (CD2Cl2): 56.9 (CH3), 106.1, 112.2, 115.1, 143.4, and 143.8 (CH), Cipso not observed. 10t: 1H NMR (CDCl3, 293 K): δ 3.96 (s, 3H, OMe), 4.07 (s, 3H, OMe), 6.54 (d with sat, JH−H = 8.4 Hz, 4JPt−H = 8.7 Hz, 1H, H5), 6.80 (d with sat, JH−H = 8.1 Hz, 4JPt−H = 14.4 Hz, 1H, H5′), 7.59 (d with sat, JH−H = 8.4 Hz, 3JPt−H = 62.4 Hz, 1H, H4), 7.80 (d, JH−H = 7.5 Hz, 1H, H3′), 7.96 (pseudo t, JH−H = 8.1, 7.5 Hz, 1H, H4′). Assignments based on 2D-COSY and NOESY spectra. 13C NMR (CDCl3): 53.3 and 56.8 (CH3), 107.3, 113.7, 113.8, 144.3, and 145.7 (CH), 127.7, 157.8, 160.2, 161.9, 162.7, and 166.8 (Cipso). [Pt(bipy2OMe-H)(PPh3)Cl] (11). Mp 207 °C. Anal. Calcd for C30H26ClN2O2PPt (708.04): C, 50.89; H, 3.70; N, 3.96. Found: C, 50.72; H, 3.57; N, 3.83. IR (ν/cm−1, nujol): 1616, 1566, 1303, 1266, 1254, 1131, 1030, 338, 307. 1H NMR (CDCl3): δ 3.88 (s, 3H, OMe), 3.99 (s, 3H, OMe), 5.87 (d, JH−H = 8.6 Hz, 1H, H5), 6.72 (dd, JH−H = 8.6, 3.2 Hz, 1H, H5′), 6.79 (td, JH−H = 5.1, 1.8 Hz, 1H, H4′), 7.31−7.47 (m, 9H, Hm,p-PPh3), 7.75−7.86 (m, 7H, H4 + Ho-PPh3), 7.89 (d, JH−H = 5.1 Hz, 1H, H3′). 31P NMR (CD3CN, 293 K): δ 22.8 (s with sat, JPt−P = 4557.9 Hz,). I

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[Pt(bipy2OMe-H)(PPh3)2Cl] (12). To a solution of 11 (50.0 mg, 0.07 mmol) in dichloromethane (25 mL) was added PPh3 (18.5 mg, 0.07 mmol). The solution was stirred for 5 min at room temperature, then filtered, concentrated to a small volume; addition of diethyl ether and n-hexane gave a white precipitate that was filtered off, washed with n-hexane, and vacuum-dried to give the analytical sample. Yield 86%. Mp 207 °C. Anal. Calcd for C48H41ClN2O2P2Pt (970.33): C, 59.41; H, 4.26; N, 2.89. Found: C, 58.92; H, 3.98; N, 2.74. IR (ν/cm−1, nujol): 1563, 1400, 1284, 1250, 1097, 1025, 744, 694, 521. 1H NMR (CDCl3): δ 3.69 (s, 3H, OMe), 3.71 (s, 3H, OMe), 5.60 (d, JH−H = 8.0 Hz, 1H, H5), 6.62 (d, JH−H = 8.0 Hz, 1H, H5′), 6.95 (d, JH−H = 7.6 Hz, 1H, H3′), 7.15 (t, JH−H = 7.6 Hz, 1H, H4′), 7.20 (t, JH−H = 7.6 Hz, 12H, Hm), 7.32 (t, JH−H = 7.2 Hz, 6H, Hp), 7.43−7.49 (m, 13H, Ho+H4). 31P NMR (CDCl3): δ = 17.0 (s with sat, JPt−P = 3186.0 Hz). X-ray Data Collection and Structure Determination. Xray diffraction data for 4 and 9 were collected on an Oxford Diffraction XCalibur Diffractometer with CCD area detector and equipped with Mo Kα radiation (λ = 0.7107 Å) and a low temperature device (data collection for 9 was performed at 150 K). Data were collected and reduced with the program CrysAlis (CCD and RED).45 Absorption correction to both data collections was applied through the program SCALE3 ABSPACK implemented in the CrysAlis suite. The structures were determined by the program SIR9746 and refined against F2 by full-matrix least-squares techniques using SHELXL-201347 with anisotropic displacement parameters for all non-hydrogen atoms. All hydrogen atoms in 4 and 9 were introduced in calculated positions and refined according to a riding model with isotropic thermal parameters. Due to the large absorption factor in 9, large residual density peaks are located close to the platinum ions Pt(1) and Pt(2). All calculations were performed by using the program PARST,48 and molecular plots were produced with ORTEP3,49 both implemented in the Crystal Structure crystallographic software package WINGX.50 The crystal data collections and refinement parameters are listed in the SI. CCDC 991178 and 991179 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge from the Cambridge Crystallographic Data Centre via http://www.ccdc. cam.ac.uk/Community/Requestastructure.

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REFERENCES

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S Supporting Information *

Crystal data collections and refinement parameters. NMR data. This material is available free of charge via the Internet at http:// pubs.acs.org.

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*E-mail: [email protected] Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS M.A.C. and S.S. gratefully aknowledge the Regione Autonoma della Sardegna (RAS) for the Grants “Premialità Regionale 2011” and “Premialità Regionale 2012″, respectively. F.C. gratefully acknowledges a research grant (assegno di ricerca) from the Fondazione Banco di Sardegna. J

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dx.doi.org/10.1021/om5003057 | Organometallics XXXX, XXX, XXX−XXX