Electronic and Steric Effects in Rollover C–H Bond Activation

Article pubs.acs.org/Organometallics

Electronic and Steric Effects in Rollover C−H Bond Activation Luca Maidich,†,⊥,§ Gavina Dettori,†,⊥ Sergio Stoccoro,†,⊥ Maria Agostina Cinellu,†,⊥ Jonathan P. Rourke,‡ and Antonio Zucca*,†,⊥ †

Dipartimento di Chimica e Farmacia, Università degli Studi di Sassari, Via Vienna 2, 07100 Sassari, Italy Consorzio Interuniversitario Reattività Chimica e Catalisi (CIRCC), Italy ‡ Department of Chemistry, University of Warwick, Coventry CV4 7AL, United Kingdom ⊥

S Supporting Information *

ABSTRACT: Steric and electronic factors in rollover C−H bond activation of substituted 2,2′-bipyridines, mediated by platinum(II), have been investigated by comparing the influence of two substituents, CH3 and CF3, on the progress of the reaction. The substituents were chosen to have similar steric hindrance but different electronic effects and were placed in position 6 (i.e., near one of the nitrogen atoms) or in position 5, which allows, in part, electronic and steric influence to be distinguished. The ligands studied, 6-methyl-2,2′-bipyridine, 5-methyl-2,2′-bipyridine, 6-trifluoromethyl-2,2′-bipyridine, and 5-trifluoromethyl2,2′-bipyridine, were compared to unsubstituted 2,2′-bipyridine in the reaction with the electron-rich complex [Pt(Me)2(DMSO)2]. The electron-withdrawing CF3 group was found to have a significant effect in accelerating the cyclometalation reaction. The substituent in position 6 influences the stability of the intermediate adduct [Pt(N,N)(Me)2] (N,N = chelated bipyridine), as indicated by density functional theory calculations. The steric hindrance of substituted bipyridines was also evaluated by defining and measuring the angle ζ in [Pt(N,N)(Me)2] adducts. The presence of a substituent in position 6 causes destabilization of the adduct, acceleration of the cyclometalation reaction, and regioselectivity of C−H bond activation.

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INTRODUCTION The chemistry of cyclometalated complexes is undoubtedly one of the most advanced areas in organometallic chemistry.1 The long-standing interest in cyclometalated compounds derives from the fact that their properties can be easily tuned, for example by modification of the cyclometalated ligand or the ancillary ligands. Furthermore, cyclometalated compounds show a wide range of potential applications in many areas, such as organic synthesis, catalysis, photochemistry, biomedicine, and advanced materials.1,2 It is noteworthy that in recent years a variety of catalytic carbon−carbon coupling processes have been developed that involve cyclometalation as a key step.3 Cyclometalation allows for the simultaneous investigation of the aspects governing the metal-mediated activation of C−H bonds,4 which in general encounters two fundamental challenges: the inert nature of carbon−hydrogen bonds and the desire to control the selectivity in molecules containing several C−H bonds. Cyclometalation reactions address both © XXXX American Chemical Society

challenges: the ligands bind to the metal through a heteroatom and deliver a specific C−H bond in proximity to the metal center, facilitating activation and providing selectivity. Despite many efforts, the C−H bond activation reaction has been elucidated only in part, and several aspects remain unclear; even subtle structural differences in the backbone of the complex may direct the reaction toward different, often unpredictable, results.5 Among the great number of cyclometalated complexes appearing in the literature, a particular and growing family of compounds consists of the so-called “rollover complexes”.6 Rollover cyclometalation has been defined as a peculiar case of cyclometalation arising from bidentate heterocyclic donors rather than monodentate ligands.6a The mechanism of the C− H bond activation is different in the two processes because rollover cyclometalation requires a chelated ligand that partially Received: June 27, 2014

A

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Organometallics

and complex 2 can be obtained. It is worthy of note, from the electronic point of view, that as the mechanism is nucleophilic in nature, the electron-releasing alkyl substituent should disfavor the cyclometalation reaction, making the nitrogen a better donor and the C−H bond to be activated less prone to nucleophilic attack. For this reason, the global effect seems to be essentially steric, as the presence of the substituent in position 6 destabilizes adduct 1, facilitating nitrogen detachment, i.e., the first act of the second reaction. In the case of 6substituted bipyridines, the reaction is always regiospecific. As a part of our continuous efforts in elucidating aspects of cyclometalation reactions11 and, in particular, rollover C−H bond activation,12 we decided to extend our study to 2,2′bipyridines bearing an electron-releasing or electron-withdrawing substituent (CH3 or CF3, respectively) having similar bulkiness in two different positions (6 or 5) in order to elucidate steric and electronic contributions to rollover C−H bond activation and cyclometalation. The CF3 substituent was also chosen because of the unique properties of fluorine, as cyclometalated complexes containing fluorine are attracting interest due to their properties in several areas.13

decomplexes in the course of the reaction, generating a vacant coordination site. After internal rotation of the ligand, a remote C−H bond is activated in a coordinatively unsaturated complex (Scheme 1, path a). In contrast, classical cyclometalation occurs Scheme 1

in a coordinatively saturated complex (Scheme 1, path b). Once formed, a rollover complex has additional potential, with respect to classical cyclometalates, due to the presence of an uncoordinated donor (usually nitrogen), which may provide further reactivity. The growing interest in rollover metalation is proved by a recent review of the subject6a and by the design of catalytic cycles based on rollover and retro-rollover reactions7 with the ultimate aim of activating internal positions in heteroaromatic rings. Rollover complexes have also been found to promote C− C bond formation in the gas phase8 and in solution.9 In the case of platinum(II), rollover metalation usually occurs only with electron-rich derivatives such as [Pt(Me)2(DMSO)2].10 The reaction proceeds through the formation of the adduct [Pt(κ2N,N)(Me)2] (e.g., κ2N,N = 2,2′-bipyridine), from which nucleophilic attack of the metal generates the cyclometalated complex, likely through a Pt(IV) intermediate. Even though it has not been demonstrated, the reaction is thought to proceed through an oxidative addition/ reductive elimination process.6b,10a The reaction profile is that typical of a consecutive reaction: in the case of unsubstituted 2,2′-bipyridine, step I (Scheme 2) is significantly faster than step II, so the intermediate adduct 1 can be isolated and easily characterized. In the case of 2,2′-bipyridine, step II occurs only under harsh conditions (e.g., refluxing toluene) to give complex 2a (R = H). When an alkyl, aryl, or benzyl substituent is present in position 6, i.e., adjacent to a nitrogen atom, step II is faster and occurs under milder conditions, even at room temperature. The same reaction occurs, less easily, with the corresponding complex [Pt(Ph)2(DMSO)2]. Our earlier studies showed the dominance of steric factors: 6-alkyl-substituted 2,2′-bipyridines (e.g., 6methyl-2,2′-bipyridine, 6-tert-butyl-2,2′-bipyridine) show a rate for step II comparable to that for step I.10a In these cases it is not possible to isolate the adduct 1, as it converts into the metalated complex 2 during its formation: at intermediate reaction times, only a mixture of starting materials, complex 1,

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RESULTS AND DISCUSSION Synthesis of the Ligands. The ligands were chosen in order to evaluate electronic and steric effects in rollover C−H bond activation and cyclometalation. One electron-donating (CH3) and one electron-withdrawing (CF3) substituent were chosen, and they were placed in position 6 or 5 of the 2,2′bipyridine scaffold in order to evaluate and differentiate electronic and steric contributions. The ligands studied, 6methyl-2,2′-bipyridine (bpy6CH3), 5-methyl-2,2′-bipyridine (bpy5CH3), 6-trifluoromethyl-2,2′-bipyridine (bpy6CF3), and 5trifluoromethyl-2,2′-bipyridine (bpy5CF3) (Chart 1), were synthesized following different synthetic approaches (see the Experimental Section). The numbering scheme followed in this paper is reported below:

Steric Factors. The two concepts of steric hindrance and steric ef fect describe different properties: the former indicates the physical volume occupied by a certain group, whereas the latter is defined as “the effect on a chemical or physical property (structure, rate or equilibrium constant) upon introduction of substituents having different steric requirements”.14 CH3 and CF3 were considered in the past to be similar groups with respect to steric hindrance, even if in some cases trifluoromethyl was thought to be more closely related to the isopropyl group.15 Recently it has been suggested that CF3

Scheme 2

B

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Organometallics Chart 1

Another way to look at the sterics is to analyze the effect of a group on the reactivity of a class of compounds. In this respect, a quantity called the A value has been defined that corresponds to the ΔG° of the equilibrium between the two chair conformations of monosubstituted cyclohexanes.14 This value gives an idea of the conformational preference of a substituent for the more stable equatorial position. Values of A are also reported in Table 1 for comparison. Electronic Factors: Basicity versus Proton Affinity. Evaluation of the donor capabilities of the nitrogen atoms in substituted 2,2′-bipyridines is not an easy task either experimentally or theoretically because of problems ranging from solubility in water (or other solvents) to the presence of cisoid and transoid conformations.17 It is clear that with an unsymmetrically substituted bpy the situation becomes even more complicated to describe. From a theoretical point of view, proton affinities (ΔH) and gas-phase basicities (ΔG) can be calculated in different ways.18 Given these issues, in order to have an idea about the different donor abilities of the nitrogen atoms present in the ligands described here, we checked pKa and proton affinity values of the corresponding pyridines. Literature data are summarized in Table 2.19 A clear correlation between the acid

most closely resembles an ethyl group and is smaller than the isopropyl group.16 From a purely geometrical point of view, we can estimate the steric hindrance of a methyl group versus a trifluoromethyl group in position 6 of 2,2′-bipyridine by evaluating how much coordination plane is required by each of the two bipyridines. In a square-planar complex [Pt(κ2N,N)X2] (κ2N,N = chelated 2,2′-bipyridine, X = anionic ligand), assuming that the bpy ligand is coplanar with the PtX2 fragment and also that the bpy substituent lies in the same plane, we can define an angle ζ between two planes perpendicular to the coordination plane: one bisects the N−Pt−N angle, and the other is tangent to the outermost van der Waals surface of the substituent and passes through the metal center, as depicted in Figure 1 for [Pt(bpy6CH3)(CH3)2].

Table 2. Acid Dissociation Constant and Proton Affinity Values for Different Pyridine Derivatives py py2CH3 py3CH3 py4CH3 py2CF3 py3CF3 py4CF3 bpy

Figure 1. Evaluation of the ζ angle for [Pt(bpy6CH3)(CH3)2].

The problem can be solved in two dimensions (see the Supporting Information), and it is possible to derive the data reported in Table 1. Unsubstituted bipyridine occupies 8.8° Table 1. Steric Requirements (ζ Anglesa) and A Values (Relative to the Substituent) of Various Substituted 2,2′Bipyridines bpy bpy6F b bpy6CH3 bpy6CF3 a b

substituent

ζ (deg)

A (kcal/mol)15

H F CH3 CF3

98.8 107.3 125.1 137.6

0 0.15 1.70 2.10

pKa at 25 °C20

ΔH (kJ/mol)21

5.23 6.00 5.70 5.99 − 3.36 3.59 −

930.0 949.1 943.4 947.2 887.1 892.5 893.9 933.422

dissociation constant and proton affinity can be drawn. It is pertinent to note that we did not find any experimental value for py2CF3 in the literature. As expected, the trifluoromethyl group enhances the acidity of the pyridinium cation (pyRH+) by two pKa units, and this corresponds to a ca. 60 kJ/mol difference in the proton affinities. As a consequence, the donor properties of py2CF3, and hence of the corresponding nitrogen in bpy6CF3, are expected to be poor. A survey of literature data showed only one result for coordination of py2CF3,23 versus 888 results for py2CH3.24 A clue concerning the electronic influence of these substituents on the pyridine ring is also given by comparison of Hammett σ values: CF3 has a marked electron-attracting effect, both inductive and mesomeric, whereas the opposite effect is provided by CH3: σm(CF3) = +0.43, σm(CH3) = −0.07; σp(CF3) = +0.54, σp(CH3) = −0.17. As for the inductive effect, σI(CF3) = +0.42, σI(CH3) = −0.01.25 Reactivity of 6-Substituted 2,2′-Bipyridines. Reactions involving bpy6R and cis-[Pt(CH3)2(DMSO)2] were followed in

The angle ζ is defined in Figure 1 and the Supporting information. bpy6F = 6-fluoro-2,2′-bipyridine.

more than the theoretical 90° arising from a symmetrical division of the plane; substitution of the hydrogen next to the nitrogen with a fluorine has the effect of increasing the steric hindrance by a further ∼10°. As we would intuitively expect, a methyl group is more demanding (ζ ≈ 125°, i.e., 35° more than 90°), but the fact that the CF3 occupies more than “a quarter and a half” of the coordination plane (ζ = 137.6°) is particularly noteworthy. C

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Organometallics Scheme 3. Observed Steps in the Rollover Cyclometalation of bpy6CH3

Scheme 4. Plausible Steps in the Cyclometalation of bpy6CF3

second nitrogen does not coordinate and only the κ1N adduct [Pt(κ1N-bpy6CF3)(Me)2(DMSO)] is formed. This species would presumably not be detectable in solution because of its extremely fast conversion to the cyclometalated complex 2c at room temperature. If this is the operating mechanism, the reaction is a simple cyclometalation, like that reported for 2phenylpyridine, and not a true rollover cyclometalation. Complex 2c could be obtained in pure form and isolated in the solid state at room temperature with prolonged reaction times or in refluxing toluene. Complex 2c was thoroughly characterized by means of mono- and bidimensional NMR spectroscopy (1H, 13C, 19F, 1H−1H COSY, and 1H−13C HSQC). In particular, coordination of the four donors N1′, C3, CH3, and DMSO was supported by the presence of four signals coupled to platinum: H6′ (9.73 ppm, 3JPt−H = 14 Hz), H4 (8.15 ppm, 3JPt−H = 56 Hz), DMSO (3.26 ppm, 6H, 3JPt−H = 19 Hz, S-coordinated), and CH3 (0.73 ppm, 3H, 2JPt−H = 82 Hz, Pt−CH3). Cyclometalation of the substituted pyridyl ring was also evident from the absence of proton H3 and by the multiplicity of the signals of the pyridine rings, and it was confirmed by a 1H−1H COSY spectrum, which showed two spin systems having four and two protons, respectively. In agreement with the proposed formulation, the 13C NMR spectrum showed signals for the coordinated CH3 (−13.4 ppm, 1 JPt−C = 761 Hz), C3 (149.5 ppm, 1JPt−C = 1100 Hz), and DMSO (43.8 ppm, 2JPt−C = 42 Hz). The high value of the 1 JPt−C3 coupling constant, 1100 Hz, is indicative of a strong Pt− C3 bond. In general, all of the coupling constants of the metalated pyridyl ring were larger than those of the Ncoordinated one, as previously observed for 2a.10b Comparison of the 13C NMR data for 2a and 2c shows similar 195Pt−13C and 195Pt−1H coupling constant values (e.g., 2JPt−C2 = 55 Hz in both 2a and 2c; 1JPt−CH3 = 763 Hz in 2a and 761 Hz in 2c; 3 JPt−H4 = 56 Hz in both 2a and 2c). The NMR data for bpy6CH3 and bpy6CF3, which are presented in Table 3, allowed us to evaluate the electronic influence of the substituent while keeping the steric effects almost comparable.27 The chemical shift differences reflect the electronic properties of the ligands, but the 195Pt−1H coupling constant values are very similar (e.g., 2JPt−H = 82 Hz and 3JPt−H = 18.5 Hz for the three complexes). Because of the peculiar behavior of bpy6CF3 with [Pt(Me)2(DMSO)2], we decided to investigate its reactivity with

acetone-d6 by means of NMR spectroscopy. The reagents were mixed at low temperature (i.e., acetone/dry ice bath), and the spectra were recorded from −40 to 25 °C at 10 °C intervals. In the first instance, we were interested in checking the lowest temperature at which the first signs of cyclometalation started to become detectable. We defined this temperature as the activation temperature (Tact). In the case of bpy6CH3, the initial formation of the κ2N,N adduct [Pt(bpy6CH3)(CH3)2] (1b) was very rapid at room temperature, even though completeness was only reached slowly. As 1b was formed, its conversion into [Pt(bpy6CH3H)(CH3)(DMSO)] (2b) occurred through a subsequent reaction (Scheme 3). The formation of 1b was very slow at low temperatures (below −10 °C), and 2b started to become detectable at around 0 °C. A 195Pt−1H HMQC spectrum at room temperature confirmed the presence in solution of the starting material, cis-[Pt(CH3)2(DMSO)2], and 1b, which showed signals at −4150 and −3334 ppm, respectively, along with the expected correlations. When the reaction was carried out in refluxing acetone, the cyclometalated species 2b was rapidly formed and could be isolated in the solid state in high yield and characterized as previously reported.10a Under the same conditions with bpy6CF3, no sign of reaction was visible at −40 °C. As the sample was warmed to −20 °C, the first evidence of reaction became visible as tiny singlets with satellites in the region of the coordinated DMSO (four sets of signals between 3.41 and 2.95 ppm). Unfortunately, we were not able to identify these species. Around 0 °C, the appearance of a new set of signals and free methane confirmed that the cyclometalation was taking place; in agreement with this evidence, a singlet at −67.8 ppm grew in the 19F NMR spectrum. At room temperature, the metalation reaction to give the complex [Pt(bpy6CF3-H)(CH3)(DMSO)] (2c) was very fast, even though some of the starting materials ([Pt(Me)2(DMSO)2] and free ligand) remained in solution and almost 24 h was needed for complete conversion. This may be ascribable not to slow C−H bond activation, i.e., step II of the reaction, but to the poor coordinating ability of bpy6CF3, which slows down the first act of the reaction, i.e., the DMSO displacement (Scheme 4). Furthermore, no sign of the κ2N,N adduct [Pt(bpy6CF3)Me2] was visible in solution. It is likely that as a result of the electron-withdrawing properties of CF3 in addition to its steric hindrance,26 the D

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Organometallics Table 3. Selected 1H and 19F NMR Chemical Shifts (in ppm) and [in brackets] 195Pt Coupling Constants (in Hz) for Complexes 2a−c 2a H3′ H4′ H5′ H6′ H4 H5 H6 DMSO Pt−CH3 R on bpy

8.29 7.95 7.36 9.71 8.01 7.17 8.36 3.25 0.70 −

[14] [56]

[18.5] [82]

2b 8.31 7.91 7.31 9.68 7.88 7.04 − 3.24 0.69 2.52

[14] [53] [19] [18.5] [82]

2c 8.40 7.98 7.40 9.73 8.15 7.48 − 3.26 0.73 −67.6

[14] [56]

visible, clearly demonstrating that no C−H activation took place; moreover, the presence of satellites on only one signal (9.15 ppm, 3JPt−H = 40 Hz, 3JH−H = 5.9 Hz, H6′) and the broad deshielded doublet due to H3 at 8.99 ppm (Δδ = 0.35 ppm) support our interpretation and may also suggest an anagostic interaction 28 between H 3 and the metal center. The diastereotopicity of the DMSO methyls is in agreement with a nonrotating κ1N-coordinated bpy6CF3, generating planar chirality.11a The two chlorides should be considered in this case as different ligands: being trans to N and S, they should have, inter alia, different Pt−Cl bond lengths. The isolation of 3 provides strong evidence for our suggestion of the formation of a κ1N adduct with bpy6CF3: it is plausible that the κ1N adduct forms even with R = Me or Ph and that in general the ligand acts more like a phenylpyridine than a bipyridine. 5-Substituted 2,2′-Bipyridines. In the 5-substituted 2,2′bipyridines, the substituent is not close enough to the metal center to sterically influence the regioselectivity of the process. The substituent in position 5 is located meta to both the nitrogen and the C3−H bond and thus might have a different impact on the C−H activation process than the substituent in position 6, allowing us, at least in principle, to distinguish between electronic and steric effects. In some of our earlier work it was observed that under conditions where bpy6CH3 gave cyclometalation, the corresponding 5-CH3-substituted bpy afforded only the adduct [Pt(bpy5CH3)(Me)2].10 We decided here to explore a bit more the reactivity of this ligand and found that bpy5CH3 does indeed undergo cyclometalation with longer reaction times (several weeks at room temperature) or under harsher conditions (refluxing toluene for at least 2 h). Addition of bpy5CH3 to an acetone-d6 solution of cis[Pt(Me)2(DMSO)2] in an NMR tube rapidly afforded the κ2N,N adduct [Pt(bpy5CH3)(Me)2] (1d) (in ca. 5 min). After 1 week at room temperature, the appearance of a set of small signals (ca. 2% of the total) suggested cyclometalation. Quantitative conversion to the cyclometalated product 2d was obtained in refluxing toluene; the reaction led to the isolation of a mixture of the two cyclometalated products 2d-cis and 2d-trans (with the Pt-coordinated methyl group cis or trans to the bpy substituent, respectively, arising from activation of either the substituted or unsubstituted pyridine ring) in a 1:1

[18.5] [82]

the less-electron-rich complex [Pt(Ph)2(DMSO)2], which is usually less prone to cyclometalate than the PtMe2 complex, and with the electron-poor complex [Pt(Cl)2(DMSO)2]. The diphenyl complex reacted rapidly: in acetone at room temperature, the rollover complex [Pt(bpy6CF3-H)(Ph)(DMSO)] (2f) was rapidly formed with no evidence for the intermediacy of the κ2N,N adduct, in analogy to what was observed for 2c. Complex 2f was isolated in the solid state and characterized. In particular, the 1H NMR spectrum showed a coordinated DMSO (2.98 ppm, 3JPt−H = 18.5 Hz) shifted to lower frequency with respect to complex 2c (δ = 3.26 ppm, 3 JPt−H = 19 Hz), likely because of the presence of an adjacent coordinated phenyl ring. The H6′ signal (9.70 ppm, 3JPt−H = 10.2 Hz, 3JH−H = 5.1 Hz) is in agreement with this formulation: the greater trans influence of the phenyl group compared with the methyl group makes 3JPt−H6′ smaller in 2f than in 2c (i.e., 10 vs 14 Hz).

In contrast, no metalation was observed with [Pt(Cl)2(DMSO)2], even at high temperatures, and only the monodentate adduct [Pt(κ1N-bpy6CF3)(Cl)2(DMSO)] (3) was observed in solution and isolated in the solid state. The 1H NMR spectrum of 3 showed two singlets with satellites in the aliphatic region (2.82 ppm, 3H, 3JPt−H = 22.3 Hz; 3.37 ppm, 3H, 3JPt−H = 24.8 Hz) ascribable to the two diasterotopic methyl groups of an S-bound DMSO. The relatively high Pt−H coupling constant is in agreement with a DMSO trans to Cl. In the aromatic region all seven resonances of the ligand were Scheme 5

E

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contrast, the 1JPt−P values follow an interesting trend for the substituents: H ≈ 6CH3 ≈ 5CH3 < 6CF3 < 5CF3 (1JPt−P = 2229, 2226, 2224−2216, 2279, and 2312 Hz, respectively). A similar trend was observed by Clark and co-workers for Pt(II)− phosphane complexes with 4-substituted pyridines.29 They observed that factors making the platinum 6s orbital more available for the Pt−P bond increase the Pt−P coupling constant. As a consequence, substituents with high electronegativity on the pyridine ligands decrease the electron donation of the pyridine ligand to the metal and allow the platinum 6s orbital to contribute more to the Pt−P bond, resulting in larger Pt−P coupling constants. As a result, the 1 JPt−P values are greater for the CF3-substituted bipyridines. The electron-withdrawing substituent is nearer to the Pt−C bond in bpy5CF3 (in 4e) than in bpy6CF3 (in 4c) and hence is more effective. DFT Calculations. Adducts [Pt(κ2N,N)(CH3)2] (1). Density functional theory (DFT) calculations were performed on the adducts [Pt(κ2N,N)(CH3)2] (1a−e) (κ2N,N = bpy derivative) in order to understand their reactivity. [Pt(bpy6CF3)(CH3)2] (1c), which was not observed experimentally, was included in the study for completeness. Geometrical parameters resulting from the optimizations as well as a more complete discussion are provided in the Supporting Information. A search of the Cambridge Crystallographic Data Centre (CCDC)30 for [Pt(κ2N,N)(CH3)2] structures gave only seven results, including [Pt(bpy)(CH3)2] (1a).31 Comparison of the X-ray structure of 1a with that obtained by DFT calculations highlights the suitability of the chosen model: the mean absolute errors (MAEs) lie in the intervals 0.8−1.6 pm and 1.4−1.7° for bonds and angles, respectively. (The evaluated bonds were those involving the metal center and those of the chelate ring, while only the angles involving Pt as the central atom were used in the comparison.) The substituent in position 6 distorts the geometry, affecting mainly the Pt−N1 bonds and N1−Pt−CH3 angles (Figure 2). In particular, the Pt−N1 bonds

molar ratio (Scheme 5). Complexes 2d-cis and 2d-trans were characterized mainly on the basis of their NMR spectra (see the Experimental Section). As suggested by these data, no regioselectivity in the C−H bond activation was observed, indicating an almost negligible electronic effect of the Me substituent in position 5. In the case of bpy5CF3, the reaction was followed by 1H and 19 F NMR spectroscopy. The 1H NMR spectra showed the initial formation of the coordination compound [Pt(bpy5CF3)(Me)2] (1e). The sample was left at room temperature for almost 3 weeks before heavy decomposition prevented further analysis. During this time it was evident that cyclometalation was taking place on both pyridyl rings. The striking difference with respect to bpy5CH3 was that the ratio of the cyclometalated products was not 1:1 but approximately 5:1 (Scheme 5). In acetone at 50 °C (24 h) and in refluxing toluene (2 h), similar molar ratios were observed. In all cases, the predominant isomer arose from activation of the substituted ring with clear regioselectivity due to electronic factors. The electron-withdrawing CF3 group may act in two ways: by making the nitrogen a worse donor or by facilitating nucleophilic attack on the C−H group (N1 and C3−H, both meta to CF3).

Substitution Reactions. In complexes 2, the labile DMSO can be easily substituted by two-electron-donor ligands under mild conditions; in order to gain information on the influence of the CH3 and CF3 substituents, we reacted complexes 2b−e with PPh3 at room temperature. Thus, we obtained in high yields the new series of complexes [Pt(bpyR-H)(Me)(PPh3)] (R = 6CH3 (4b), 6CF3 (4c), 5CH3 (4d), 5CF3 (4e)) and were able to compare them with the analogous bpy complex [Pt(bpy-H)(Me)(PPh3)] (4a). No isomerization was observed in the conversion of species 2 to 4, so for 4b and 4c only the P-trans-C(sp2) isomer was obtained. 4d was present in two forms, 4d-trans and 4d-cis, in a 1:1 molar ratio, whereas for 4e only the dominant trans isomer 4e-trans was isolated (Chart 2). Comparison of the 31P data for species 4a−e showed very small chemical shift differences that seem to be insignificant. In Chart 2

Figure 2. Calculated Pt−N1 and N1−Pt−CH3 angles in 1a−e.

are comparable when R = H (ca. 210 pm), and both elongate to ca. 217 pm for R = CH3 or CF3. The same trend is observed for the N1−Pt−CH3 angles (ca. 96° for R = H and ca. 102° for R = CH3 or CF3). Overall, a considerable distortion is found for the bpy6CF3 and bpy6CH3 adducts. Another interesting comparison involves the isomers with respect to the position of the bpy substituent, i.e., [Pt(bpy5R)(CH3)2] versus [Pt(bpy6R)(CH3)2] (1b vs 1d and 1c vs 1e). From the in vacuo zero-point energy (ZPE)-corrected ΔH values, it is evident how the steric hindrance of the substituent influences the stability of the coordination compound that forms in the first step of the rollover cyclometalation process: F

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Organometallics an 18.3 kJ/mol difference was found in favor of bpy5CH3, and the value for the CF3 couple was almost double that (Figure 3). The greater destabilization experienced by the bpy6CF3 species 1c correlates well with the fact that we were not able to detect this compound during our experiments.

approximation) along with the zeroth-order regular approximation (ZORA) at the PBE0/def2-SVP level. The geometries (shown in Figure 4) were confirmed as minima on the potential energy surface by the absence of imaginary frequencies in the Hessian evaluated at the same level of theory. The resulting energy differences obtained are in good agreement with the experimental findings. The enthalpy difference (ΔH in kJ/mol in vacuo) predicts that when bpy5CH3 is used, a rough 1:1 ratio between the two activation products is to be expected. It is also worth noting that the most stable isomer is the one with the substituted ring N-bound to platinum, as would be expected on the basis of the electronic effects of the methyl group. On the other hand, with bpy5CF3 a bigger ΔH is obtained, and now the most stable isomer is the one in which the metalated ring is the one bearing the substituent. Again the theory reproduces well the experimental observations. The enthalpy differences (ΔH in vacuo) between the cyclometalated species of bpy5R and bpy6R were also evaluated, and it turned out that the more stable complexes are now those obtained using 6substituted bpys, i.e., a reverse trend compared to the [Pt(κ2N,N)(CH3)2] species. Moreover, we note that when the methyl group is in position 5, its destabilizing effect is bigger than that of the CF3 group in the same position (see Table 4).

Figure 3. Equilibrium geometries for the adducts [Pt(κ2N,N)(CH3)2] with κ2-N,N = bpy6CH3 (1b), bpy6CF3 (1c), bpy5CH3 (1d), or bpy5CF3 (1e). ΔH values were calculated in vacuo and ZPE-corrected and are expressed in kJ/mol.

Energy decomposition analysis (EDA)32 was performed on the equilibrium geometries. The analysis indicated that the interactions of bpy5CF3 and bpy5CH3 with the Pt(CH3)2 fragment are comparable with that of bpy and are influenced only to a small extent by the electronic nature of the group attached in position 5. On the other hand, bpy6CF3 and bpy6CH3 have smaller interaction energies, in fair agreement with the experimental finding that they give cyclometalation more easily than bpy. Cyclometalated Species. Products of the cyclometalation of the bpy ligands with cis-[Pt(CH3)2(DMSO)2], namely, [Pt(κ2N′,C)(CH3)(DMSO)] (2a−e), were also analyzed with the same DFT methods used in the previous section. All of the complexes display a planar structure with the coordination plane including the cyclometalated ligand, i.e., coplanarity is observed. In all cases an interaction is detected between the oxygen of DMSO and the proton next to the nitrogen in the Nbound pyridyl, in agreement with the NMR deshielding of proton H6′. Localized molecular orbital EDA (LMO-EDA) was also performed in this case, and the results are reported in the Supporting Information. In brief, the interaction energies regarding the DMSO ligand reflect the relative donor capabilities of the trans metalated carbons. The sequence followed (bpy6CF3 > bpy ≈ bpy6CH3) is in agreement with the electronic characteristics of the metalated groups. It was found experimentally that activation of both the substituted and unsubstituted pyridyl rings occurs with bpy5CH3 and bpy5CF3 as the cyclometalating ligand. In order to gain insight into the different stabilities of these isomers, we performed DFT calculations for all four species (2d and 2e, two for each ligand bpy5CH3 and bpy5CF3) using the resolution of the identity approximation for the Coulomb energy (RI-J

Table 4. Relative Stabilities of the Cyclometalated Products [Pt(κ2N,C)(CH3)(DMSO)]a [Pt(bpy6R-H)(CH3)(DMSO)] [Pt(bpy5R-H)(CH3)(DMSO)] (R cis to CH3) [Pt(bpy5R-H)(CH3)(DMSO)] (R trans to CH3)

R = CH3

R = CF3

0.00 10.97 10.59

0.00 4.98 8.52

a

Geometries were obtained at the PBE0/def2-SVP level using the ZORA formalism with the RI-J approximation. Shown are in vacuo ZPE-corrected ΔH values in kJ/mol.

In order to shed some more light on the experimental observations, we performed DFT calculations on some of the species involved in the C−H activation process. A complete study of the mechanism, evaluating all of the possible pathways for the rollover activation both in solution and in the gas phase, would be beyond the scope of this article; therefore, we concentrated our attention on the first two intermediate species we found. For each of the κ2N,N coordination compounds, i.e., [Pt(κ2N,N)(CH3)2], we explored the potential energy surfaces (PESs) obtained by rotation of each of the pyridyl rings (see Chart 3). Obviously, when R = H (i.e., in the case of bpy), the situation is the same in both cases. Full optimization without

Figure 4. Equilibrium geometries for the isomers of [Pt(bpy5R-H)(CH3)(DMSO)] (R = CH3 (2d), CF3 (2e)). ΔH values were calculated in vacuo and ZPE-corrected and are expressed in kJ/mol. G

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similar to that of bpy is taken into account and the inductive effects of the substituent are considered. The same trend is observed for the substituted ring but with a bigger energy difference, and this agrees with the observed ratio of cyclometalated complexes we detected by NMR spectroscopy. Second, when the substituent is in position 6 (i.e., bpy6R), the energy difference is almost halved, with a larger decrease when the substituent is the electron-withdrawing trifluoromethyl group. The energy difference observed for the rotation of the unsubstituted ring for bpy6CH3 can be compared with that of bpy in order to obtain a clue about the destabilization of the κ2N,N coordination compound. This can be roughly estimated as 25 kJ/mol, in reasonable agreement with the value obtained from the analysis of the coordination compounds (vide supra). In summary, from the preliminary data reported here, we can conclude that when the substituent is in close proximity to the metal, steric effects overcome the electronic ones, which apparently remain constant (ca. 10 kJ/mol), as can be seen by comparing the energy differences for bpys having the substituent in the same position. Further insights into the energetics of the reaction from the coordination compound [Pt(κ2N,N)(CH3)2] to the agostic complex [Pt(κ1N,N)(Me)2] were pursued by evaluating the Gibbs free energy and enthalpy differences in solution (Table 6). Toluene was used as the solvent because only in this solvent

Chart 3

constraints at the PBE0/def2-SVP level using ZORA led to the data in Table 5, which gives ΔH values for the reaction from Table 5. ΔH Values (ZPE-Corrected, in Vacuo, in kJ/mol) for the Reaction from the Coordination Compound to the Agostic Complex bpy bpy5CF3 bpy5CH3 bpy6CF3 bpy6CH3

unsubstituted ring rotates

substituted ring rotates

87.3 83.3 84.9 46.1 54.5

− 74.8 82.6 40.8 53.9

Table 6. ΔH and ΔG Values (ZPE-Corrected, in Vacuo and in Toluene, in kJ/mol) for the Reaction from the Coordination Compound to the Agostic Complex

the coordination compound to the agostic complex. Figure 5 shows the optimized structures of the most stable agostic intermediates of the five ligands studied (bpy, bpy5CF3, bpy5CH3, bpy6CF3, and bpy6CH3). First of all, in every case we were able to locate an intermediate geometry with an agostic interaction between the C3−H or the C3′−H and the metal center. The presence of this intermediate points toward a two-step activation mechanism where the first transition state (TS) is connected to the rotation of one of the pyridyl rings while the second is relative to the C−H bond rupture. (Investigation on the second step will be the subject of future work.) From a comparison of the ZPE-corrected enthalpy values, it is clear that the discriminating factor, whether steric or electronic, already operates in the rotation of one of the heteroaromatic rings. Globally, the presence of a substituent favors the rollover C−H activation process, as can be seen from the fact that the difference is highest for bpy and the other bpys follow the order bpy6CF3 < bpy6CH3 ≪ bpy5CF3 < bpy5CH3 ≈ bpy5CH3 (unsubstituted ring). The theoretical data nicely reflect the experimental findings, but some points are worth noting. In the first instance, the activation of the unsubstituted ring in bpy5R is very close in energy to that in bpy, but when R = CF3 the energy is lower. This is what is expected when the fact that the ring is very

bpy bpy6CH3 (unsub) bpy6CH3 (sub) bpy6CF3 (unsub) bpy6CF3 (sub)

ΔH (gas-phase)

ΔH (toluene)

ΔG (toluene)

87.3 54.5 53.9 46.1 40.8

67.6 61.1 52.0 42.4 28.6

54.2 55.6 45.3 46.8 34.1

was cyclometalation observed for all of the bipyridines, therefore allowing a sensible comparison with the theoretical results to be made. At first glance, the enthalpy differences in toluene are always lower than those in the gas phase, but the overall trend is the same, i.e., solvation does not influence the relative stability of the species. However, further calculations are needed to assess whether the cyclometalation of bpy6CF3 is a true rollover process.

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CONCLUSIONS The influence of the substituents studied on the chemical behavior of 2,2′-bipyridine is striking. With bpy6CF3, the substituent, besides an expected steric effect, provides a dramatic acceleration of the C−H bond activation compared with simple bpy. Furthermore, with bpy6CF3, there is no evidence for the presence of the chelated adduct [Pt(bpy6CF3)-

Figure 5. Equilibrium geometries for the agostic intermediates of (a) bpy, (b) bpy5CF3, (c) bpy5CH3, (d) bpy6CF3, and (e) bpy6CH3. H

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HMBC pulse sequence, and the quoted 195Pt chemical shifts are taken from here (referenced to external Na2PtCl6). DFT Calculations. DFT calculations were carried out using the PBE0 hybrid functional developed by Perdew, Burke, and Ernzerhof38 and modified in its hybrid version by Adamo and Barone,39 with ZORA40−43 using the SV-ZORA basis sets44 along with the RI-JONX approximation as implemented in the ORCA 2.9.1 package.45 Convergence criteria used were tightened compared with the default ones: i.e., the keywords “tolmaxg 1.0e-5”, “tolrmsg 0.3e-5”, and “convergence tight” were added to the input file. Harmonic analysis at the same level of theory (RI-JONX/PBE0/def2-SVP) was carried out on each of the equilibrium geometries to confirm the nature of the minimum (i.e., the absence of imaginary frequencies) on the PES. The influence of the solvent was evaluated using the COSMO model46 as implemented in ORCA. Optimizations were carried out using the same convergence criteria as for the gas-phase structures, and the nature of each minimum on the PES was checked for the absence of imaginary frequencies in the Hessian evaluation. Relevant parameters used by the software for toluene and atomic radii are reported in the Supporting Information. LMO-EDA analysis was performed with GAMESS version 2013 (R1)47 using the def2-SVP basis sets of Ahlrichs and co-workers,48 as found in the EMSL basis set library,49 for all lighter atoms (H, C, N, O, and S), while for platinum the same basis set was integrated with an effective core potential (ECP) removing 60 core electrons. 19 F NMR Data for the Ligands (376.4 MHz, CDCl3, 298 K, ppm). bpy5CF3. δ = −62.34 ppm (s). A 1H−19F HOESY experiment showed NOE contacts between the CF3 group and the adjacent protons H6 (8.95 ppm) and H4 (8.07 ppm). bpy6CF3. δ = −68.0 ppm (s). Syntheses and Characterization Data for Complexes. [Pt(bpy6CH3)(CH3)2] (1b). The species was detected and characterized in solution (see ref 10). 1H NMR (700 MHz, acetone-d6, 298 K, ppm): 9.14 (d sat, 1H, 3JPt−H = 22.6 Hz, JH−H = 5.4 Hz, H6′); 8.35 (d, 1H, JH−H = 8.1 Hz, H3′); 8.26 (d, 1H, JH−H = 7.6 Hz, H3); 8.10 (t, 1H, JH−H = 7.8 Hz, H4′); 7.62 (ddd, 1H, JH−H = 7.4, 5.6, 1.0 Hz, H5′); 7.58 (d, 1H, JH−H = 7.6 Hz, H4); 7.21 (d, 1H, JH−H = 9.1 Hz, H5); 2.84 (s, 3H, CH3); 1.04 (s sat, 3H, 3JPt−H = 87.3 Hz, Pt−CH3); 1.03 (s sat, 3H, 3 JPt−H = 89.5 Hz, Pt−CH3). 195Pt−1H HMQC (500 MHz, acetone-d6, 298 K, ppm): −3334 (s). The signal is correlated with 1H signals at 9.14, 1.04, and 1.03 ppm. [Pt(bpy5CH3)(CH3)2] (1d). cis-[Pt(CH3)2(DMSO)2] (372.1 mg, 0.97 mmol, 1 equiv) was added to an acetone solution of bpy5CH3 (183.8 mg, 1.08 mmol, 1.11 equiv) at room temperature, and the mixture was left stirring for 1 h. The color quickly turned red, and a precipitate formed. 1H NMR (300 MHz, CDCl3, 298 K, ppm): 9.24 (d sat, 1H, 3 JPt−H = 21.6 Hz, JH−H = 6.0 Hz, H6′); 9.06 (s sat, 1H, 3JPt−H = 22.0 Hz, H6); 8.07 (td, 1H, JH−H = 8.9, 1.6 Hz, H4′); 7.95 (d, 1H, JH−H = 7.8 Hz H3′); 7.89 (m, 2H, H3 + H4); 7.49 (dd, 1H, JH−H = 7.1, 1.3 Hz, H5′); 2.52 (s, 3H, CH3(bpy)); 1.12 (s sat, 3H, 2JPt−H = 85.5 Hz, Pt−CH3); 1.11 (s sat, 3H, 2JPt−H = 85.4 Hz, Pt−CH3). [Pt(bpy5CF3)(CH3)2] (1e). cis-[Pt(CH3)2(DMSO)2] (22.3 mg, 0.0585 mmol, 1 equiv) was added to an acetone solution of bpy5CF3 (22.5 mg, 0.1 mmol, 1.7 equiv) at room temperature, and the mixture was left stirring for 24 h. The color quickly turned red and did not change for the whole period. Yield: 50%. 1H NMR (300 MHz, CDCl3, 298 K, ppm): 9.58 (s sat, 1H, 3JPt−H = 23.5 Hz, H6); 9.35 (d sat, 1H, 3JPt−H = n.r., JH−H = 5.0 Hz, H6′); 8.39 (d, 1H, JH−H = 6.8 Hz, H3′); 8.19 (td, 1H, JH−H = 7.9, 1.5 Hz, H4′); 8.10 (m, 2H, H3 + H4); 7.64 (m, 1H, H5′); 1.24 (s sat, 3H, 2JPt−H = 85.7 Hz, Pt−CH3); 1.20 (s sat, 3H, 2 JPt−H = 86.5 Hz, Pt−CH3). 1H NMR (700 MHz, acetone-d6, 298 K, ppm): 9.49 (s sat, 1H, 3JPt−H = 21.6 Hz, H6); 9.30 (d sat, 1H, 3JPt−H = 21.3 Hz, JH−H = 5.3 Hz, H6′); 8.75 (dd, 1H, JH−H = 8.5, 1.8 Hz, H4); 8.66 (d, 1H, JH−H = 8.5 Hz, H3); 8.58 (d, 1H, JH−H = 8.1 Hz, H3′); 8.41 (td, 1H, JH−H = 7.8, 1.5 Hz, H4′); 7.82 (ddd, 1H, JH−H = 7.5, 5.4, 1.1 Hz, H5′); 1.08 (s sat, 3H, 2JPt−H = 87.2 Hz, Pt−CH3); 1.02 (s sat, 3H, 3 JPt−H = 86.2 Hz, Pt−CH3). A 1H−1H COSY experiment (400 MHz, acetone-d6, 298 K, ppm) helped in the assignment. [Pt(bpy6CH3-H)(CH3)(DMSO)] (2b). bpy6CH3 (174.0 mg, 1.01 mmol, 1.28 equiv) was added to a solution of cis-[Pt(CH3)2(DMSO)2]

(CH3)2] in solution. These results can be rationalized in terms of the strongly electron-withdrawing character of the CF3 group, in conjunction with its steric hindrance (greater than that of CH3), having an effect on the donor properties of the adjacent nitrogen atom and thus affecting its coordinating abilities. The proposed κ 1 N intermediate was not observed, presumably because of its tendency to undergo C−H activation; this very high reactivity can be explained by the fact that the C−H bond is para to the CF3 substituent, a position that enhances the electron deficiency at the C3−H bond and presumably makes the nucleophilic attack of the platinum center easier. Even though the C−H activation is very fast, the reaction takes some time to reach completion, as the limiting step of the reaction is probably the substitution of DMSO by a bpy nitrogen (now not assisted by chelation), after which the second step, i.e. C−H bond activation, is extremely fast. By contrast, the bpy6CH3 ligand cyclometalates following the steps already described for bpy, i.e. following a true rollover cyclometalation pathway. The substituent is likely to have a destabilizing effect on the ground state of the κ2N,N adduct, facilitating the detachment of the substituted pyridine and thus favoring the C−H activation on that ring. In both cases, the substituent in position 6 has the dominant influence on the regioselectivity of the activation. A rigorous comparison between the two ligands and the two substituents is hampered by the possibility that the operating mechanisms are different. In the first instance, it is possible to say that the two ligands show similar steric effects but very different electronic effects on the C−H activation process. The observed difference in the temperatures at which cyclometalation becomes detectable (263 K for bpy6CF3 vs 273 K for bpy6CH3) is reflected also in the time required to reach completion at room temperature (a couple of hours for bpy6CF3 vs at least 24 h for bpy6CH3). In the case of 5-substituted bipyridines, the steric bulkiness at the platinum center can be assumed to be almost the same as for unsubstituted 2,2′-bipyridine, and thus only electronic contributions affect the reaction. The presence of a methyl group in the position meta to both the nitrogen and the C3−H does not have substantial effect on the rate and selectivity of the reaction. Conversely, the CF3 group facilitates the activation of the C−H bond in the meta position, promoting regioselectivity.

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EXPERIMENTAL SECTION

Materials and Characterization Methods. All of the solvents were purified and dried according to standard procedures.33 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 synthesis of bpy6CH3 was accomplished following a modification of the procedure reported by Garber and co-workers34 involving selective alkylation of 2,2′-bipyridine with CH3Li in position 6. bpy5CH3 was obtained by means of Negishi coupling,35,36 and bpy5CF3 and bpy6CF3 were obtained by Suzuki−Miyaura coupling.37 The detailed synthesis of bpy6CF3 is reported in the Supporting Information. 1 H, 13C, 19F, and 31P NMR spectra were obtained on Bruker Avance III 400, 500, 600, or 700 spectrometers. Chemical shifts are given in parts per million relative to internal tetramethylsilane for 1H and 13 C{1H} spectra and external 85% H3PO4 for 31P{1H} spectra. 1H NOE difference, 1H−1H-COSY, 1H−19F HOESY, and 1H−13C HSQC experiments were performed by means of standard pulse sequences. 195 Pt−1H correlation spectra were recorded using a variant of the I

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Organometallics

0.234 mmol, 1 equiv) in anhydrous toluene (6 mL). The mixture was heated to reflux for 3 h, and a color change was observed: the colorless solution of the starting complex became suddenly red upon addition of bpy5CF3 and then slowly turned yellow. The volume of the mixture was then reduced to ca. 3 mL, and n-hexane was added until complete precipitation of the product occurred. The product was then filtered, washed with n-hexane, and vacuum-dried to give the analytical sample as a yellow solid. Yield: 75%. Melting point: 200 °C (dec). 1H NMR (300 MHz, CDCl3, 298 K, ppm): Major species: 9.76 (m sat, 1H, 3 JPt−H = n.r., H6′); 8.59 (s, 1H, H6); 8.36 (d, 1H, JH−H = 7.9 Hz, H3′); 8.21 (s sat, 1H, 3JPt−H = 55.2 Hz, H4); 8.00 (m, 1H, H4′); 7.44 (m, 1H, H5′); 3.27 (s sat, 6H, 3JPt−H = 19.0 Hz, CH3(DMSO)); 0.73 (s sat, 3H, 2 JPt−H = 81.2 Hz, Pt−CH3). Minor species: 3.23 (s sat, 6H, 3JPt−H = n.r., CH3(DMSO)); 0.75 (s sat, 3H, 2JPt−H = 78.0 Hz, Pt−CH3). [Pt(bpy6CF3-H)(Ph)(DMSO)] (2f). bpy6CF3 (31.4 mg, 0.140 mmol, 1.2 equiv) was added to a solution of cis-[Pt(Ph)2(DMSO)2] (59.3 mg, 0.117 mmol, 1 equiv) in acetone (6 mL). The mixture was stirred at room temperature for 4 h, and a color change was observed: the colorless solution of the starting complex became suddenly orange upon addition of bpy6CF3. The volume of the mixture was then reduced to ca. 3 mL, and n-hexane was added until complete precipitation of the product occurred. The product was then filtered, washed with nhexane, and dried under vacuum to give the analytical sample. Yield: 66%. 1H NMR (400 MHz, CDCl3, 298 K, ppm): 9.70 (d sat, 1H, 3 JPt−H = 10.2 Hz, JH−H = 5.1 Hz, H6′); 8.40 (d, 1H, JH−H = 7.1 Hz, H3′); 8.00 (td, 1H, JH−H = 7.7, 0.9 Hz, H4′); 7.60−7.00 (m, 8H, H5′ + H4 + H5 + Ho(Ph) + Hm(Ph) + Hp(Ph)); 2.98 (s sat, 1H, 3JPt−H = 18.4 Hz, CH3(DMSO)). [PtCl2(bpy6CF3)(DMSO)] (3). Under stirring, bpy6CF3 (54.6 mg, 0.243 mmol, 1.7 equiv) was added to a solution of [PtCl2(DMSO)2] (60.5 mg, 0.149 mmol) in ethanol. The solution was refluxed for 5 h and then concentrated to a small volume, and diethyl ether was added. The precipitate formed was filtered off and washed with dimethyl ether to give a pale-yellow solid. The reaction was not complete, and some of the starting complex [PtCl2(DMSO)2] was observed (ca. 20%) in addition to complex 3 (ca. 80%). 1H NMR (300 MHz, CDCl3, 298 K, ppm): 9.15 (dd, 1H, 3JPt−H = 40 Hz, 3JH−H = 5.8, 1.3 Hz, H6′); 8.99 (d, 1H, 3JH−H = 7.9 Hz, H3), 8.24 (t, 1H, JH−H = 7.9 Hz, H4); 8.05 (td, 1H, JH−H = 7.8 Hz, H4′); 7.95−7.86 (m, 2H, H5 + H3′), 7.53 (m, 1H, H5′); 3.37 (s with sat, 3H, 3JPt−H = 24.8 Hz, CH3(DMSO)); 2.82 (s with sat, 3H, 3JPt−H = 22.3 Hz, CH3(DMSO)). [Pt(bpy6CF3-H)(CH3)(PPh3)] (4c). To a solution of [Pt(bpy6CF3H)(CH3)(DMSO)] (2c) (114.9 mg, 0.225 mmol, 1 equiv) in acetone (10 mL) was added PPh3 (89.3 mg, 0.340 mmol, 1.5 equiv). After 3 h the mixture was concentrated to a small volume and treated with nhexane to yield the product as a yellow solid. Yield: 74%. Melting point: dec 200−205 °C. 1H NMR (600 MHz, CDCl3, 298 K, ppm): 8.44 (d, 1H, JH−H = 7.7 Hz, H3′); 8.39 (dd sat, 1H, 3JPt−H = 47.8 Hz, JH−H = 7.7, 5.4 Hz, H4); 7.83 (td, 1H, JH−H = 7.7, 1.4 Hz, H4′); 7.76 (m, 7H, H6′ + Hm(PPh3) or Ho(PPh3)); 7.56 (dd sat, 1H, 3JPt−H = 14.3 Hz, 4JP−H = 1.5 Hz, JH−H = 7.9 Hz, H5); 7.46 (m, 3H, Hp(PPh3)); 7.42 (m, 6H, Ho(PPh3) or Hm(PPh3)); 6.75 (ddd, 1H, JH−H = 7.5, 5.5, 1.5 Hz, H5′); 0.79 (d sat, 3H, 2JPt−H = 83.0 Hz, 3JP−H = 7.9 Hz, Pt−CH3). A 1H−1H COSY experiment (600 MHz, CDCl3, 298 K, ppm) helped in the assignments. 31P NMR (242.9 MHz, CDCl3, 298 K, ppm): 32.1 (s sat, JPt−P = 2279 Hz, PPh3). 195Pt−1H HMQC (600 MHz, CDCl3, 298 K, ppm): −4202 (d, JPt−P ≈ 2395 Hz). The signal is correlated with signals at 8.39, 7.77, 7.56, and 0.79 ppm. [Pt(bpy5CH3-H)(CH3)(PPh3)] (4d-cis and 4d-trans). To a solution of [Pt(bpy5CH3-H)(CH3)(DMSO)] (2d) (38.1 mg, 0.090 mmol, 1 equiv) in acetone (10 mL) was added PPh3 (35.0 mg, 0.133 mmol, 1.48 equiv). After 3 h the mixture was concentrated to a small volume and treated with n-hexane to yield the product as a yellow solid. Yield: 63%. Selected 1H NMR data for the two isomers (300 MHz, CDCl3, 298 K, ppm): 2.35 (s, 6H, CH3(bpy), both isomers); 0.76 (s sat, 3H, 2 JPt−H = n.r., Pt−CH3); 0.73 (s sat, 3H, 2JPt−H = n.r., Pt−CH3). 31P NMR (121.4 MHz, CDCl3, 298 K, ppm): 33.8 (s sat, JPt−P = 2216 Hz, PPh3); 33.6 (s sat, JPt−P = 2224 Hz, PPh3).

(301.2 mg, 0.790 mmol, 1 equiv) in acetone (15 mL). The mixture was heated to reflux for 4 h, and a color change was observed: the colorless solution of the starting complex became suddenly red upon addition of bpy6CH3 and then slowly turned yellow. The volume of the mixture was then reduced to ca. 3 mL, and n-pentane was added to complete the precipitation of the product, which was washed with npentane and dried to give the analytical sample as a yellow solid. Yield: 81%. Melting point: 180 °C. 1H NMR (300 MHz, CDCl3, 298 K, ppm): 9.67 (d sat, 1H, 3JPt−H ≈ 14 Hz, JH−H = 5.9 Hz, H6′); 8.31 (d, 1H, JH−H = 7.6 Hz, H3′); 7.91 (m, 1H, H4′); 7.88 (d sat, 1H, 3JPt−H = 53.0 Hz, JH−H = 8.0 Hz, H4); 7.31 (m, 1H, H5′); 7.04 (d sat, 1H, 3JPt−H = 18.6 Hz, JH−H = 7.9 Hz, H5); 3.24 (s sat, 6H, 3JPt−H = 18.5 Hz, CH3(DMSO)); 2.52 (s, 3H, CH3(bpy)); 0.69 (s sat, 3H, 2JPt−H = 82.1 Hz, Pt−CH3). 195Pt−1H HMQC (300 MHz, acetone-d6, 298 K, ppm): −4072 (s). The signal is correlated with 1H signals at 9.79, 7.84, 7.05, 3.23, and 0.67 ppm. [Pt(bpy6CF3-H)(CH3)(DMSO)] (2c). bpy6CF3 (104.4 mg, 0.466 mmol, 1.7 equiv) was added to a solution of cis-[Pt(CH3)2(DMSO)2] (101.9 mg, 0.267 mmol, 1 equiv) in acetone (6 mL). The mixture was heated to reflux (5 h), whereupon the volume was reduced and n-hexane was added until complete precipitation of the product occurred. The product was collected, washed with n-hexane, and dried to give the analytical sample as a yellow solid. Yield: 66%. The same reaction was carried out in anhydrous toluene, refluxing for 3 h and elaborating in the same way. Yield: 82%. Melting point: 150 °C. Chemical formula: C14H15F3N2OPtS, Anal. Calcd: C, 32.88; H, 2.96; N, 5.48. Found: C, 32.96; H, 3.12; N, 5.63. 1 H NMR (400 MHz, CDCl3, 298 K, ppm): 9.73 (d sat, 1H, 3JPt−H = 14 Hz, JH−H = 4.8 Hz, H6′); 8.40 (d, 1H, JH−H = 7.8 Hz, H3′); 8.15 (d sat, 1H, 3JPt−H = 56 Hz, JH−H = 8.2 Hz, H4); 7.98 (m, 1H, H4′); 7.48 (d, 1H, JH−H = 8.1 Hz, H5); 7.40 (m, 1H, H5′); 3.26 (s sat, 6H, 3JPt−H = 18.5 Hz, CH3(DMSO)); 0.75 (s sat, 3H, 3JPt−H = 82 Hz, Pt−CH3). 1H NMR (700 MHz, acetone-d6, 298 K, ppm): 9.85 (d sat, 1H, 3JPt−H = 194 Hz, JH−H = 4.8 Hz, H6′); 8.36 (d, 1H, JH−H = 7.6 Hz, H3′); 8.22 (d sat, 1H, 3JPt−H = 55.1 Hz, JH−H = 7.8 Hz, H4); 8.22 (td, 1H, JH−H = 7.6, 1.4 Hz, H4′ or H5′); 7.62 (ddd, 1H, JH−H = 7.4, 5.6, 1.5 Hz, H5′ or H4′); 7.59 (d sat, 1H, 4JPt−H = 16.4 Hz, H5); 3.29 (s sat, 6H, 3JPt−H = 18.5 Hz, CH3(DMSO)); 0.73 (s sat, 3H, 3JPt−H = 83.1 Hz, Pt−CH3). 1 H−1H COSY (700 MHz, acetone-d6, 298 K, ppm): the signal at 9.85 ppm is correlated with those at 7.62, 8.21 and 8.36 ppm; the signal at 8.22 ppm is correlates with that at 7.59 ppm. 13 C NMR (100.6 MHz, CDCl3, 298 K, ppm): 165.1 (s sat, 2JPt−C = 29.0 Hz, C2′); 161.3 (s sat, 2JPt−C = 55.0 Hz, C2); 150.2 (s sat, 2JPt−C ≈ 7 Hz, C6′); 149.5 (s sat, JPt−C = 1100 Hz, C3); 144.2 (s sat, 4JPt−C = 34.0 Hz, C6); 141.2 (s sat, 2JPt−C = 88.4 Hz, C4); 138.8 (s, C4′); 125.2 (s sat, 2JPt−C ≈ 10 Hz, C5′); 122.2 (q, JF−C = 273.4 Hz, CF3); 122.0 (s sat, 3JPt−C = 22.6 Hz, C3′); 119.7 (q sat, 3JPt−C ≈ 65 Hz, 3JF−C = 3.0 Hz, C5); 43.8 (s sat, 2JPt−C = 43.4 Hz, CH3(DMSO)); −13.4 (s sat, JPt−C = 761 Hz, Pt−CH3). 195 Pt−1H HMQC (700 MHz, acetone-d6, 298 K, ppm): −4059 (s). The signal is correlated with signals at 9.85, 8.22, 7.59, 3.29, and 0.73 ppm. [Pt(bpy5CH3-H)(CH3)(DMSO)] (2d). To a solution of [Pt(bpy5CH3)(CH3)2] (1d) (80 mg, 0.202 mmol, in 3 mL of anhydrous toluene) was added 30 μL of anhydrous DMSO (d = 1.101 g·mL−1, 0.420 mmol, 2.08 equiv). The mixture was refluxed for 3 h and during this time changed color from red to yellow. The volume of the mixture was then reduced, and n-pentane was added until complete precipitation of the product occurred. The product was then collected, washed with npentane, and vacuum-dried to give the analytical sample as a yellow solid. The complex was present as a mixture of isomers 2d-cis and 2dtrans. Selected 1H NMR data for the two isomers (300 MHz, CDCl3, 298 K, ppm): 9.54 (s sat, 1H, 3JPt−H = 15.0 Hz, JH−H = 6.0 Hz, Pt−H6′ (substituted py activated)); 9.42 (s sat, 1H, 3JPt−H = 15.0 Hz, Pt−H6 (unsubstituted py activated)); 3.08 (s sat, 12H, 3JPt−H = 17.7 Hz, CH3(DMSO) both isomers); 2.25 (s, 3H, CH3(bpy)); 2.21 (s, 3H, CH3(bpy)); 0.55 (s sat, 3H, 2JPt−H = 82.2 Hz, Pt−CH3); 0.54 (s sat, 3H, 2JPt−H = 81.4 Hz, Pt−CH3). [Pt(bpy5CF3-H)(CH3)(DMSO)] (2e). bpy5CF3 (89.8 mg, 0.40 mmol, 1.7 equiv) was added to a solution of cis-[Pt(CH3)2(DMSO)2] (89.4 mg, J

DOI: 10.1021/om500681u Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics [Pt(bpy5CF3-H)(CH3)(PPh3)] (4e). To a solution of [Pt(bpy5CF3H)(CH3)(DMSO)] (2e) (39.1 mg, 0.076 mmol, 1 equiv) in acetone (10 mL) was added PPh3 (29.6 mg, 0.113 mmol, 1.49 equiv). After 3 h the mixture was concentrated to a small volume and treated with nhexane to yield the product as a yellow solid. Yield: 81%. Melting point: 200 °C (dec). 1H NMR (300 MHz, CDCl3, 298 K, ppm): 8.63 (m, 1H, H6); 8.46 (d sat, 1H, 3JPt−H = 39.0 Hz, 4JP−H = 4.0 Hz, H4); 8.38 (d, 1H, JH−H = 8.1 Hz, H3′); 7.85−7.38 (m, 17H, H6′ + H4′ + Ho(PPh3) + Hm(PPh3) + Hp(PPh3)); 6.76 (m, 1H, H5′); 0.77 (d sat, 3H, 2JPt−H = 85.6 Hz, 3JP−H = 7.7 Hz, Pt−CH3). 31P NMR (121.4 MHz, CDCl3, 298 K, ppm): 33.2 (s sat, JPt−P = 2312 Hz, PPh3).

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* Supporting Information S

Text, tables, and figures giving details of DFT calculations and evaluation of ζ; synthesis and characterization of bpy6CF3. This material is available free of charge via the Internet at http:// pubs.acs.org. Corresponding Author

*E-mail: [email protected]. Present Address §

L.M.: Dipartimento di Fisica, Università degli Studi di Pavia, via Bassi 6, 27100 Pavia, Italy.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from Università di Sassari (FAR) is gratefully acknowledged. L.M. gratefully acknowledges a Ph.D. fund, financed on POR/FSE 2007-2013, from Regione Autonoma della Sardegna, and INFN section in Pavia for granting computational time.

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DOI: 10.1021/om500681u Organometallics XXXX, XXX, XXX−XXX

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DOI: 10.1021/om500681u Organometallics XXXX, XXX, XXX−XXX