C–H and O2 Activation at a Pt(II) Center Enabled by a Novel

Sep 21, 2016 - Mahmood Azizpoor Fard , Ava Behnia , and Richard J. Puddephatt ... David Watts , Daoyong Wang , Peter Y. Zavalij , Andrei N. Vedernikov...
0 downloads 0 Views 2MB Size
Download PDF
This is an open access article published under an ACS AuthorChoice License, which permits copying and redistribution of the article or any adaptations for non-commercial purposes.

Article pubs.acs.org/Organometallics

C−H and O2 Activation at a Pt(II) Center Enabled by a Novel Sulfonated CNN Pincer Ligand David Watts, Daoyong Wang, Mackenzie Adelberg, Peter Y. Zavalij, and Andrei N. Vedernikov* Department of Chemistry and Biochemistry, University of Maryland, College Park, Maryland 20742, United States S Supporting Information *

ABSTRACT: A novel sulfonated CNN pincer ligand has been designed to support CH and O2 activation at a Pt(II) center. The derived cycloplatinated aqua complex 7 was found to be one of the most active reported homogeneous Pt catalysts for H/D exchange between studied arenes (benzene, benzene-d6, toluene-d8, p-xylene, and mesitylene) and 2,2,2-trifluoroethanol (TFE) or 2,2,2trifluoroethanol-d; the TON for C6D6 as a substrate is >250 after 48 h at 80 °C. The reaction is very selective; no benzylic CH bond activation was observed. The per-CH-bond reactivity diminishes in the series benzene (19) > toluene (p-CH:m-CH:o-CH = 1:0.9:0.2) > xylene (2.9) > mesitylene (1.1). The complex 7 reacts slowly in TFE solutions under ambient light but not in the dark with O2 to selectively produce a Pt(IV) trifluoroethoxo derivative. The H/D exchange reaction kinetics and results of the DFT study suggest that complex 7, and not its TFE derivatives, is the major species responsible for the arene CH bond activation. The reaction deuterium kinetic isotope effect, kH/kD = 1.7, the reaction selectivity, and reaction kinetics modeling suggest that the CH bond cleavage step is rate-determining.

1. INTRODUCTION The homogeneous C−H functionalization of hydrocarbons mediated by PtII complexes has an impressive history spanning almost 50 years from its discovery1 to subsequent mechanistic studies1b,2,3 and up to current developments.4−7 While some substantial progress in Pt-catalyzed “non-oxidative” C−H functionalization has been achieved,7 the oxidative C−H functionalization remains limited to the use of more or less “impractical” oxidizing agents such as H2PtCl6,1 CuCl2,5 and SO3 (oleum).6 The use of a more economically attractive oxidant, O2, has only been reported for mediator-containing PtII systems such as O2/heteropolyacids8,9 and O2/CuCl2,4 among others, but these systems suffer from their low selectivity. The use of O2 as an inexpensive oxidant for selective oxidative C−H functionalization mediated by PtII compounds remains desirable and challenging. Potentially, such processes could be realized in mediator-free systems where both C−H and O2 activation occur at the same metal center. Toward this goal, previously we have demonstrated a facile mediator-free PtII− C(sp3) bond functionalization in water or alcohols using O2 from air.10 Such transformations include aerobic PtII(Alk) to PtIV(Alk) oxidation (e.g., 1 to 2 in Scheme 1) and subsequent C(sp3)−O elimination from PtIV(Alk) intermediates such as 3, all enabled by the tripod facially chelating ligand bis(2pyridyl)methanesulfonate (dpms). The pendant sulfonate group of the dpms ligand serves as a surprisingly good donor facilitating the formation of PtIV intermediates such as 2.11,12 The sulfonate also acts as a good leaving group, thus making the subsequent C(sp3)−O elimination from the PtIV center facile.13 © XXXX American Chemical Society

In spite of the demonstration of this important reactivity, the catalytic aerobic C−H functionalization of alkanes or arenes by the aqua complex 4, [(dpms)PtII(OH2)2]+ (Scheme 1), could not be developed. One of the reasons for that is a bimolecular decomposition of 4 leading to the robust dinuclear complex (dpms)2Pt2(μ-OH)2 5.13 Fortunately, the bimolecular decomposition pathway can be shut down via a covalent immobilization of 4. Using this strategy we recently were able to achieve catalytic aerobic epoxidation of some olefins with an impressive TON exceeding 40000.14 Another important factor contributing to the lack of the C− H bond activation reactivity of 4 toward alkanes and arenes may be related to the difficulty of coordinating alkane and arene substrates to the Pt(II) center, which is a step required for subsequent aerobic functionalization of the substrate in the (dpms)PtII systems.10 In general, coordination of olefins is thermodynamically more favorable and kinetically more facile in comparison to alkanes and arenes, thus accounting for the success of the aerobic epoxidation of olefins by immobilized complex 4.14 In particular, olefins but not arenes can displace the DMSO ligand in the DMSO complex (dpms)Pt(OH)(DMSO) (6) and be engaged in CH activation by this complex even in aqueous media.15 To enhance the reactivity of (dpms)PtII(OH2)- type complexes in reactions requiring displacement of the aqua ligand, one can attempt diminishing the aqua ligand Special Issue: Hydrocarbon Chemistry: Activation and Beyond Received: July 31, 2016

A

DOI: 10.1021/acs.organomet.6b00613 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics Scheme 1. Aerobic PtII−C(sp3) Functionalization in Water

Scheme 2. New Sulfonated CNN Pincer Preligand 8 and Derived PtII Complexes 7 and 9−14

bimolecular decomposition pathway leading to the formation of bis-μ-hydroxo complexes such as 5. In this work we have tested the hypotheses above. The novel sulfonated CNN pincer preligand 8 and the derived PtII complexes K[(C6H4-dpms)PtCl] (9) and 7 have been prepared and fully characterized. In contrast to the aqua complex 4, solutions of 7 in 2,2,2-trifluoroethanol (TFE) allow for catalytic H/D exchange between benzene as well as some (poly)methylbenzenes and TFE solvent at 21−80 °C and exhibit the highest reactivity in this reaction, in comparison to other reported homogeneous PtII-based systems, some of which require a temperature of 150−180 °C to operate.16−19 Interestingly, our data indicate that the aqua complex 7 and not its TFE derivatives, such 10 and 12, is responsible for the

dissociation energy. At this point, on the basis of the results of a molecular modeling of 1 and 4, we hypothesized that a strong intramolecular hydrogen bond, SO3−···H2O−PtII, may be the factor contributing to the high energy of the aqua ligand dissociation. Expectedly, the hydrogen bonding can be diminished if the sulfonate group and the PtII center are slightly pulled apart, as in the more rigid pincer complex (C6H4-dpms)Pt(OH2) (7), supported by a novel sulfonated CNN pincer ligand derived from the preligand 8 (Scheme 2). The presence of a close-by pendant sulfonate group in 7 would allow for facile aerobic PtII to PtIV oxidation,10 whereas the presence of a rigid CNN pincer scaffold would also allow weakening of the Pt−OH2 bonding/facilitation of hydrocarbon substrate coordination to the metal and would shut down the B

DOI: 10.1021/acs.organomet.6b00613 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics Scheme 3. Aerobic Oxidation of 7 in TFE Solutions

Scheme 4. Preparation of Complexes 9 and 7

Figure 1. ORTEP drawings (50% probability ellipsoids) of (a) 7 and (b) 9 (cation is not shown; hydrogen atoms are omitted for clarity). The cocrystallized solvent is not shown.

(Scheme 4). Reaction of 9 with ethanolic KOH followed by solvent removal and extraction of the solid product with ethanol afforded K[(C6H4-dpms)Pt(OEt)] (16). The latter was dissolved in water and converted to the water-insoluble aqua complex 7, which crystallized upon acidification of the solution with H2SO4. The target aqua complex was isolated as the tetrahydrate 7·4H2O in analytically pure form in 51% yield. Slow crystallization of 9 from water−methanol mixtures afforded 9·H2O·MeOH. In both cases the crystalline products so obtained were suitable for single-crystal X-ray characterization (Figure 1). According to the XRD characterization, in their solid state both 9 and 7 feature the κ3-C,N,N coordination mode of the sulfonated pincer ligand bound to the PtII center. In both complexes the phenylpyridine unit is almost planar. The least-squares mean plane of this fragment forms a 35−40° angle with the plane of the second pyridine ring. In the crystalline tetrahydrate 7·4H2O the aqua ligand is not engaged in H bonding with the sulfonate group of the same unit of 7 but instead is involved in hydrogen bonding with the sulfonate group of the adjacent unit of 7 indirectly, via a chain comprised of two H-bonded water molecules. 2.1.1. Transformations of 7 Involving the TFE for H2O Substitution and the Change of the Pincer Ligand Coordination Mode from CNN to CNO. While in the solid state 7·4H2O and 9·H2O·MeOH have the pincer ligand coordinated to the metal in a κ3-C,N,N fashion, in solutions these complexes may coexist in equilibrium with their

CH activation in our systems. Finally, compound 7 also allows for O2 activation to selectively produce the corresponding trifluorethoxo PtIV derivative 15, presumably via the intermediacy of the TFE derivative 14 (Scheme 3). The oxidation reaction occurs only under light, representing a simple tool to control this type of reactivity. One can expect that the transient (L)PtIIR species resulting from hydrocarbon RH activation are much more reactive than 7 thanks to the presence of the second hydrocarbyl group at the metal center resulting from a CH bond cleavage event, so that they can be engaged in the reaction with O2 more readily. The analysis of such reactivity is currently under investigation in our laboratory. In this work we report the benzene and some (poly)methylbenzene CH activation chemistry with TFE solutions of 7, including mechanistic analysis of benzene C−H activation based on experimental and computational (DFT) data, as well as some details of photochemical oxidation of 7 with O2.

2. RESULTS AND DISCUSSION 2.1. Preparation and Characterization of Novel Sulfonated CNN-Pincer Complexes (C6H4-dpms)Pt(OH2) (7) and K[(C6H4-dpms)PtCl] (9) (Scheme 4). The preligand 8 was prepared from 6-phenylbis(2-pyridyl)methane by sulfonation of the derived carbanion with Me3NSO3.20 Reaction of 8 with 1 equiv of K2PtCl4 in aqueous acetic acid led to its quantitative conversion to the chloroplatinate(II) complex 9, which was isolated in 77% yield in analytically pure form C

DOI: 10.1021/acs.organomet.6b00613 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics respective κ3-C,N,O-coordinated isomers: e.g., 7 vs 11 (Scheme 2). The presence of C,N,O-coordinated Pt(II) complexes in TFE solutions of 7 is indirectly supported (vide infra) by the fact that aerobic oxidation of these solutions produces 15 (Scheme 3), featuring a mer-CNO arrangement of the ligand donor atoms. In addition to the coordination isomerism, both 7 and 11 may be engaged in a ligand exchange with the TFE solvent, resulting in the formation of TFE adducts 10 and 12, respectively. Finally, the CNO-coordinated complexes 11 and 12 having a basic pendant pyridyl group may undergo an intramolecular proton transfer from a relatively acidic coordinated solvent ligand to the pyridine nitrogen atom to produce zwitterionic species 13 and 14, respectively. The pyridinium proton, in turn, may be involved in a weak hydrogen bonding with the PtII center, as was observed earlier in similar systems.21 Dissolution of yellow crystalline 7·4H2O in TFE produces yellow solutions which rapidly change their color to dark orange, thus suggesting that the identity of the PtII complex changes upon its dissolution. The removal of the solvent under vacuum or rapid precipitation of all of the solute upon addition of diethyl ether to “aged” TFE solutions of 7 leads to a product that, on the basis of its 1H NMR spectra in DMSO-d6, contains two to three water molecules per LPtII unit and 0.2 equiv of TFE. The electrospray ionization mass spectrometry of aged TFE solutions of 7 with additives of NaBArF4 shows a strong signal corresponding to a 13·Na+ adduct (or its isomers) and a weak signal corresponding to a 14·Na+ adduct (or its isomers). No signal assignable to 13·H+ is observed in the positive mode; the only signal seen in the negative mode mass spectrum is assigned to the anion (14-H)+. Our data presented below suggest that 13 is the major Pt-containing species present in TFE solutions of 7·4H2O at 21 °C (e.g., the fraction of 13 is 85% at 21 °C and [7·4H2O] = 13 mM), whereas the minor component is, predominantly, a mixture of TFE complexes LPt(TFE) existing in a fast (on the NMR time scale) equilibrium. The fraction of 13 changes reversibly as a function of the [TFE]:[H2O] ratio and temperature, as described below. Two sets of signals are observed in 1H NMR spectra of aged solutions of 7 in CF3CH2OH. In the first set of signals assigned to 13 all but one of its signals remain sharp in the temperature range of 21−80 °C. The second component of “aged” solutions of 7 exhibits broad 1H NMR signals at lower temperatures of this range which become somewhat sharper at 80 °C. Notably, the subspectrum associated with 13 features a slightly broadened singlet at 7.61 ppm integrating as 1H and assigned to the NH+ group proton. This signal disappears upon addition of TFE-d to the above solutions due to an H/D exchange with the additive. Notably, 14, the trifluoroethoxo analogue of 13, also has an acidic NH+ fragment in it. Using solutions with different [TFE]:[H2O] ratios turned out to be an efficient way to distinguish between 13 and 14 as the source of the 7.61 ppm signal. The use of water-poorer samples 7·xH2O where x = 0.6, 0 leads to a progressively lower fraction of 13 (sharp signals) in such solutions at 21 °C: 66% and 55%, respectively. In turn, addition of water to the above solutions leads to an almost complete disappearance of broad signals (97% fraction of 13 at [H2O] = 1 M) in their 1H NMR spectra. Our experiments with varying concentrations of water performed at 80 °C, when the integral intensities of both sharp and broad signals can be comparable, allowed us to find a linear correlation between the ratio of intensity of broad signals assigned to LPt(TFE) to the intensity of sharp signals assigned

to 13 and the [TFE]:[H2O] ratio (Figure 2). The linear relationship observed suggests that the Pt-containing compo-

Figure 2. Linear correlation between the integral intensity of broad signals associated with LPt(TFE) (14 and its isomers): integral intensity of sharp signals associated with 13 and the [TFE]:[H2O] ratio at 80 °C. The [H2O] value ranges from 30 mM to 1.0 M, whereas [TFE] = 12.2−12.4 M.

nents of the solution are likely to be related by a ligand exchange equilibrium (eq 1): LPt(H 2O) + TFE → LPt(TFE) + H 2O

(1)

Here LPt(H2O) denotes 13 and LPt(TFE) designates some mixture of TFE complexes that are rapidly equilibrating: e.g., 10, 12, and 14. The corresponding equilibrium constant KTFE value KTFE =

[LPt(TFE)][H 2O] [LPt(H 2O)][TFE]

was found to be (1.32 ± 0.06) × 10−2 at 80 °C, with ΔGrxn(1) = 3.0 ± 0.1 kcal/mol. Consistent with the analysis above, our VT 1H NMR experiments with 13 mM TFE solutions of partially hydrated 7· xH2O, x = 2, show that, as the temperature increases from 21 to 80 °C, the fraction of 13 decreases from 85% (the equilibrium constant value KTFE = 4.4 × 10−4) to 15% (KTFE = 1.4 × 10−2). From the van’t Hoff plot corresponding to the interconversion of LPt(H2O) (13) and LPt(TFE) in reaction 1 the reaction thermodynamic parameters are as follows: ΔH = 11.9 ± 0.1 kcal/mol, ΔS = 25.2 ± 0.4 cal/K mol, and ΔG = 4.5 ± 0.2 kcal/ mol at 25 °C and 3.0 ± 0.2 kcal/mol at 80 °C (Figure 3). The latter value matches the reaction 1 Gibbs energy of 3.0 kcal/ mol found using the plot in Figure 2.

Figure 3. van’t Hoff plot for reaction 1 in the temperature range of 21−80 °C at [LPt(solv)] = 13 mM and [TFE]/[H2O] = 415. D

DOI: 10.1021/acs.organomet.6b00613 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics Finally, our observations of aged TFE solutions of 7 showed that, on exposure to a prolonged (several hours) heating at 80 °C, they slowly produce a new derived compound, the dark precipitate 17. This solid dissolves in TFE in the presence of HBF4 or KCl. Accordingly, the formation of 17 can be precluded in the presence of acid additives: e.g., HBF4. Compound 17 can also be dissolved in DMSO to form (C6H4-dpms)Pt(DMSO), which could be prepared independently from complex 7, as judged by 1H NMR spectroscopy (Scheme 5). These observations are consistent with the

TON of 100 could be achieved after 8 h of reaction at 80 °C. An analysis of forward and reverse (C6H6−CF3CH2OD system) reaction 2 kinetics allowed us to determine the deuterium kinetic isotope effect for (2). The observed reaction 2 rate constants were estimated as described below. 2.2.1. C−H Activation of Benzene. 2.2.1.1. Modeling the Reaction Kinetics. To calculate the observed rate constants for reaction 2, a numerical integration method22 was used along with the simple model presented in Scheme 6. An example of fitting experimental TONs is given in Figure 4.

Scheme 5. “Dimerization” Reactivity of TFE Solutions of 7

Scheme 6. Simple Kinetics Model Used in This Work To Account for the H/D Exchange Kinetics between Benzene-d6 and TFE and the Catalyst Decomposition

hypothesis that 17 is a product of a reversible “oligomerization” of 7 that may occur by displacement of solvento ligands in LPt(solv) species with a pendant donor group belonging to another such species. ESI-MS spectra of aged TFE solutions of 7 containing NaBArF4 show the presence of a weak signal attributable to a Na+ adduct with L2PtII2, one of the possible components of 17. 2.2. Aromatic C−H Bond Activation in Benzene and Methylarenes by 7 and 9 in 2,2,2-Trifluoroethanol. As was mentioned earlier, the (dpms)PtII aqua complex 4 (Scheme 1) does not react with alkanes or arenes in water or TFE solvent.13 Hence, it was exciting for us to discover that TFE solutions of the PtII aqua complex 7 can catalyze the H/D exchange between benzene-d6 and CF3CH2OH (eq 2): C6D6 + nCF3CH 2OH ⇌ C6D6 − nHn + nCF3CH 2OD

The reaction model in Scheme 6 assumes a slow formation of PtIV aryl hydrido intermediates16 which are involved in a fast hydride ligand H atom exchange with the TFE solvent following a deprotonation−protonation reaction sequence. A slow decomposition of LPt(solv) leading to 17 was taken into account as follows. The best least-squares fit to the plot of the total concentration of all LPt(solv) species vs time allowed us to find the reaction rate law, −d[LPt(solv)]/dt = k2[LPt(solv)]4 − k−2, and the rate constants for the forward and reverse reactions, k2 = 1.1 ± 0.2 M−3 s−3 and k−2 = 0. The reaction order 4 may reflect the complexity of the process of coordination polymerization of LPt(solv). When the catalyst loss due to formation of 17 was accounted for, the reaction 2 profile could be fully modeled as shown in Figure 4 using the rate law in eq 3

(2)

The reaction progress could be monitored by means of 1H NMR spectroscopy. The H/D exchange occurs at a noticeable rate already at 21 °C and is reasonably fast at 80 °C (Figure 4). No H/D exchange was noticed in the absence of the catalyst. A

[ROH] −d[CD] = k1obs,D[Pt(solv)][CD] dt [ROD] + [ROH] [ROD] − k −1obs,H[Pt(solv)][CH] [ROD] + [ROH] (3)

where [CD] and [CH] are the molar concentrations of the substrate CD and CH bonds in solution and [ROH] and [ROD] are the concentrations of water and/or TFE, serving as a source of “acidic” H and D, respectively.20 Using the reaction model in Scheme 6 and the rate law (3), we were able to calculate the observed per-bond second-order rate constant corresponding to the C−D bond cleavage in benzene-d6, k1obs,D, equal to (7.2 ± 0.2) × 10−6 s−1 M−1 (initial TOF 4.9 × 10−5 s−1) at 21 °C. At 80 °C the k1obs,D value is (1.28 ± 0.05) × 10−3 s−1 M−1 (initial TOF 8.7 × 10−3 s−1). The k1obs,D values can be compared with those reported in the literature for some other Pt-based systems used for

Figure 4. Reaction profile for the H/D exchange between benzene-d6 (10 vol %)−CF3CH2OH catalyzed by 13 mM 7·4H2O at 80 °C: black circles denote experimental data; the red line corresponds to the TONs predicted by the reaction model in Scheme 6. E

DOI: 10.1021/acs.organomet.6b00613 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics Scheme 7. Reactivity of Previously Reported Systems for Arene CH Bond Activation at a PtII Center

stoichiometric benzene CH activation16 or catalytic H/D exchange involving benzene−TFE-d or benzene−CF3COOD systems (Scheme 7).17−19 The highest rate constants for benzene CH bond cleavage at a PtII center have been reported by Bercaw and Labinger for a series of cationic complexes (N,N′-diaryldiimine)PtMe(OH2)+ such as 18.16 In TFE solution at 20 °C the estimated second-order rate constant reported is 6.9 × 10−4 s−1 M−1. Neglecting the reaction 2 kinetic isotope effect at this point (vide infra), this value is almost 100 times greater than k1obs,D for 7, which may be attributed to the presence of a methyl ligand at the PtII center and, therefore, an electron-richer metal complex, a factor recognized as very important in this type of reaction.16 The next most active series of complexes, (N,N′-diaryldiimine)PtII species such as 19, catalyze the H/D exchange between C6H6 and CF3CH2OD (TFE-d) with comparable rate constants of (1−8) × 10−3 s−1 but only at the much higher temperature of 150 °C.17 One might expect that TFE solutions of 7 would demonstrate a much higher reactivity toward benzene under comparable conditions. Two more examples of PtII-catalyzed H/D exchange with C6H6 (catalysts 20 and 21) involve trifluoroacetic acid and also exhibit comparable (20) or slower rates of catalysis (21). 2.2.1.2. Effect of Water Additives on the Rate of CH Activation. A more detailed mechanistic analysis of reaction 2 could be carried out using the model presented in Scheme 8. This model considers concurrent participation in the reaction with benzene of two solvento species of different reactivity, LPt(TFE) and LPt(H2O). For simplicity, only the initial reaction period ( 1, the reaction rate-determining step is the substrate CH bond cleavage. Our experiments were performed in aqueous TFE− benzene−complex 7 mixtures with a 0.040−1.16 M total concentration of water at 80 °C. Using 1H NMR spectroscopy the concentrations of LPt(TFE) (broader 1H NMR signals) F

DOI: 10.1021/acs.organomet.6b00613 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics and LPt(H2O) (sharper 1H NMR signals) could be determined experimentally. As the water concentration increases, the [LPt(TFE)]/[LPt(H2O)] ratio and the observed initial H/D exchange rates decrease, as shown in Table 1. Table 1. Effect of Water Additives on the Initial Rates of reaction 3 Measured in the System H2O−CF3CH2OH− Benzene-d6 (10 vol %)−Complex 7 (0.00515 M) at 80 °C [H2O], M

[LPt(TFE)]/[LPt(H2O)]

0.038 0.110 0.275 0.477 0.765 1.16

3.71 1.77 0.454 0.171 0.065 0.029

initial rate, M h−1 0.178 0.174 0.132 0.097 0.073 0.060

± ± ± ± ± ±

0.003 0.003 0.001 0.001 0.001 0.002

Figure 5. Eyring plot for the second-order rate constant k1obs,D for the H/D exchange in the system benzene-d6 (10 vol %)−CF3CH2OH− complex 7.

6. The effective reaction Gibbs activation energy is 25.4 kcal/ mol at 80 °C and 24.2 kcal/mol at 25 °C. A more detailed analysis of the CH bond cleavage reactivity of LPt(solv) species is given in section 2.4. Finally, the reactivity of the chloro complex 9 in TFE solutions in the H/D exchange in a 1/9 C6D6/TFE mixture was also analyzed. Due to its activity being lower than that of 7, a higher temperature of 120 °C was used along with a 15 mM HBF4 additive to prevent formation of 17. The second-order reaction rate constant k1obs,D estimated using the simple reaction model in Scheme 6 was found to be (3.5 ± 0.1) × 10−5 s−1 M−1. The much lower reactivity of 9 in comparison to 7 is, most likely, due to the relatively high energy cost required for the chloride ligand substitution with benzene (eq 5):

The experimental data fitting20 led us to the value of k‑1,TFE/ k2 = 3.8 × 10−5, thus suggesting that the LPt(TFE) species are virtually unreactive. A repeated data-fitting procedure taking this conclusion into account leads to the reaction parameter values k1,H2O = 8.67 × 10−2 M−1 s−1 (the ligand exchange activation energy ΔG353⧧ = 22.5 kcal/mol) and k2k1,H2O/k‑1,H2O = (3.0 ± 0.2) × 10−3 M−1 s−1, corresponding to the overall reaction 2 activation energy ΔG353⧧ = 24.8 kcal/mol and k‑1,H2O/k2 = 28.7. The conclusion that the LPt(TFE) species are less reactive in CH bond activation in comparison to their LPt(H2O) analogues is important.16 It means that the effect of the TFE as a cosolvent present in aqueous solutions is just to serve the role of an “inert” diluting agent, diminishing the high water concentration and the associated inhibiting effect of water on the CH activation. In turn, the k‑1,H2O/k2 value well above 1 suggests realization of the rate-determining CH activation step in reaction 2 involving the aqua complex LPt(H2O). On the basis of this value, the difference in the Gibbs energy of the transition states corresponding to the benzene displacement by H2O in LPt(H2O) and CH bond cleavage in the resulting LPt(PhD) is −RT ln(28.7) = −2.4 kcal/mol. 2.2.1.3. Deuterium Kinetic Isotope Effect. The deuterium kinetic isotope effect is a standard reaction parameter used to analyze the mechanism of CH activation by transition metals23 and, in particular, arenes at a PtII center.16,17 To estimate the reaction deuterium kinetic isotope effect (KIE) in our system, in addition to C6D6−CF3CH2OH systems, C6H6−CF3CH2OD (TFE-d1) mixtures were analyzed at 80 °C. The KIE value found for 7 as a catalyst was derived from the initial H/D exchange rate ratio, rate(H)/rate(D) = 1.7, and falls into the range of 1.6−2.2 typical for benzene CH activation at PtII center where the substrate C−H bond cleavage is ratedetermining.16,17 The reaction 2 activation parameters for the system benzened6 (10 vol %)−CF3CH2OH−complex 7·4H2O in the temperature range of 21−80 °C were also determined. The resulting linear dependence of −ln(k1h/kBT) vs 1/T (Eyring plot coordinates) is shown in Figure 5. The high quality of the linear correlation suggests the predominant involvement of one active PtII species, LPt(H2O), according to our analysis above, in reaction 2 in this temperature range. The reaction activation parameters, ΔH⧧ = 16.8 ± 0.5 kcal/mol and ΔS⧧ = −25 ± 2 cal/(K mol), are consistent with a stepwise bimolecular transformation corresponding to the steps k1 and k2 in Scheme

K[LPtCl] + PhH → KCl + LPt(PhH)

(5)

Indeed, 9 is much more robust than 7; mixing TFE solutions of 7 with LiCl produces 9 virtually instantaneously and quantitatively. 2.3. CH Activation of Toluene, p-Xylene, and Mesitylene. The selectivity of CH activation of hydrocarbon substrates possessing CH bonds of different type is of prime interest for practical applications of CH activation.16b Hence, we analyzed the selectivity of CH bond activation in some (poly)methylbenzenes using TFE solutions of 7 at 80 °C. Using toluene-d8 instead of benzene-d6 as a substrate in our H/D exchange experiments, we calculated the following TONs after 18 h of reaction at 80 °C: 120 for the meta C(sp2)−D bonds with the initial TOF of 1.8 × 10−3 s−1 and 85 for the ortho/para C−D bonds altogether with the initial TOF of 1.31 × 10−3 s−1 (Table 2). Notably, when TOF values normalized per number of equivalent C−D bonds involved in reaction 2 are compared, the value for the toluene-d8 meta C−D bonds, 0.98 × 10−3 s−1, is lower in comparison to that for benzene-d6, 1.28 × 10−3 s−1. Remarkably, the benzylic C(sp3)−D bonds of toluene were not detectably involved in the reaction. To estimate the initial reaction selectivity more accurately by resolving the 1H NMR signals corresponding to the toluene para and ortho C−H bonds with close values of chemical shifts, the H/D exchange between toluene-d8 and TFE was carried out to achieve only near-stoichiometric (TON ≈ 1) yield of the protio isotopologues of the substrate so that each of the signals would appear as a virtual singlet originating, predominantly, from isomeric monoprotio species CD3C6D4H (Figure 6). The difference in the integral intensity of various CH bond signals in Figure 6 before and after a short-term heating of the mixture allowed us to find the relative selectivity per bond for G

DOI: 10.1021/acs.organomet.6b00613 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics

Table 2. Observed Aromatic CH Bond Activation Rate Constants k1 (Per-Bond Values) and Initial TOF for the Reaction between 7 and Aromatic Substrates (10 vol %) in TFE Solutiona substrate C6D6 C6D6 C6H6 CD3C6D5, o-CD CD3C6D5, p-CD CD3C6D5, m-CD 1,4-(CH3)2C6H4 1,3,5-(CH3)3C6H3 a

k1, M−1 s−1 (7.2 ± 0.2) × (1.28 ± 0.05) (1.89 ± 0.05) (0.19 ± 0.02) (1.01 ± 0.05) (0.98 ± 0.05) (3.5 ± 0.2) × (1.53 ± 0.05)

initial TOF, s−1 −6

(4.9 ± 0.1) × (8.7 ± 0.3) × (1.27 ± 0.03) (1.31 ± 0.10) (1.85 ± 0.10)

10 × 10−3 × 10−3 × 10−3 × 10−3 × 10−3 10−4 × 10−4

−5

10 10−3 × 10−2 × 10−3 × 10−3

(1.14 ± 0.06) × 10−3 (3.3 ± 0.1) × 10−4

T, °C

method

21 80 80 80 80 80 80 80

eq 3 eq 3 eq 3 init rates init rates init rates init rates init rates

The rate constants were found using the simple model in Scheme 6 and the rate law (3) or initial reaction rates.

(1.14 ± 0.06) × 10−3 and (3.3 ± 0.1) × 10−4 s−1 for p-xylene and mesitylene, respectively, at 80 °C. If per-bond TOF values are compared, (0.29 ± 0.02) × 10−3 (p-xylene), (0.11 ± 0.1) × 10 −3 (mesitylene), and 2.1 × 10 −3 s −1 (C 6 H 6 ), the methylarenes are noticeably less reactive than benzene, presumably, for both electronic and steric reasons. A DFT computational analysis of the CH bond activation of benzene with TFE solutions of complex 7 is presented next. 2.4. Mechanism of Benzene C−H Bond Activation in TFE Solutions of 7: DFT Analysis (Schemes 9 and 10). In the Supporting Information we present some results of the DFT analysis of the distribution of major PtII-containing species in the system 7−TFE. The gas-phase calculations for this system predict the predominance of 13−TFE, a hydrogenbond-stabilized adduct of 13 and TFE, and a low population for the TFE complexes 10, 12, and 14 that all have approximately same Gibbs energy. In turn, for TFE solutions the solvation model used in this work fails to predict the predominance of 13 in this solvent with a slightly positive Gibbs energy of 1.0 kcal/ mol for the reaction leading from TFE and 7 to the 13−TFE adduct. As such, the “ground state” species we consider in Schemes 9 and 10 is the aqua complex 7. The error appears to be of the magnitude of a few kcal/mol, which is not uncommon for such calculations. The uncertainty in determining the most stable species in TFE solutions of 7 implies an underestimation by a few kcal/mol of the reaction 2 effective activation energy discussed below. In the following analysis (Schemes 9 and 10) the Gibbs energies were calculated for TFE solutions under standard conditions, at 1 M concentration of each component except for the solvent: [TFE] = 13.8 M. To adjust the calculated ΔG values to our typical working conditions corresponding to the use of about 15 mM 7·4H2O and the associated [H2O] = 60 mM, one needs to subtract 1.7 kcal/mol from any reaction energy value in Schemes 9 and 10 that corresponds to processes leading to the formation of H2O. According to our experimental observations, the CNOcoordinated complex 13 is the major Pt-containing species in TFE solutions at 21 °C. As a consequence, we started our analysis with the LPt(solv) complexes featuring a CNO coordination of the sulfonated pincer ligand (Scheme 9). The CNN to CNO isomerization and the TFE for H2O ligand exchange are facile (ΔG298⧧ < 12 kcal/mol) for 13 and all the LPt(solv) species considered in Scheme 2.20 On the basis of our analysis of the reaction 2 kinetics in TFE−H2O mixtures, complex 11 is one of two aqua complexes that may be responsible for CH arene activation. According to the DFT, the associative benzene for aqua ligand substitution in 11 is predicted to be a relatively slow process with a Gibbs

Figure 6. 1H NMR spectrum of a toluene-d8 (10 vol %)−CF3CH2OH (90 vol %)−complex 7 reaction mixture after a short heating at 80 °C corresponding to TON ≈ 1, showing well-resolved individual aromatic C−H bond signals (left). The toluene methyl CHD2 signal intensity remained virtually unchanged.

ortho, meta, and para C−D bonds as 0.19:0.92:1, respectively. The observed much higher preference of C(sp2)−H vs C(sp3)−H bonds is very similar to that found earlier using cationic (N,N′-diaryldiimine)PtIIMe(H2O)+ complexes with a reduced diimine ligand steric bulk (e.g., aryl = 3,5-bis(trifluoromethyl)phenyl).24 The reduced steric bulk, in turn, favors a lower arene for solvento ligand exchange activation barrier and the C−H bond cleavage being rate determining, as follows from the notable deuterium kinetic isotope effect kH/kD of 2.2 in the reaction of complex 18 (Scheme 7) with benzene.16a In contrast, bulkier ligands at the metal (aryl = 2,6dimethylphenyl) favor a 21% selective benzylic CH activation24 and exhibit a low deuterium kinetic isotope effect kH/kD of 1.1 for the reaction of this PtII complex with benzene. The arene for solvento ligand exchange step was proposed to be ratedetermining in this system.16a On the basis of this comparison, it appears that the PtII center in the LPt(H2O) species responsible for CH activation in our systems is sterically uncrowded so that LPt(H2O) can react with benzene via ratedetermining substrate C−H bond cleavage. Interestingly, the use of an even bulkier and electron-richer substrate, p-xylene, allowed the authors24 to invert the reaction selectivity in favor of the benzylic C−H activation with 90% selectivity. We also tested p-xylene and mesitylene in the H/D exchange experiments using 10 vol % of the appropriate arene in its mixtures with TFE-d. The initial rate method was used (Table 2). Only C(sp2)−H bonds of the substrates were detectably involved in the reaction, showing initial TOFs of H

DOI: 10.1021/acs.organomet.6b00613 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics

Scheme 9. DFT Calculated Gibbs Energy Profile for Benzene CH Bond Activation by LPtII(solv) Species Supported by a C,N,O-Coordinated Pincer Ligand L in Neat TFE at 25 °C

Summarizing results of this analysis, we conclude that the reaction sequences involving either 11 or 12 have the ratedetermining benzene coordination step, which is not consistent with our experimental observations. It is interesting to note here that there is another kinetically competitive reaction sequence leading to the transient CH benzene σ complex 23 and the CH bond cleavage transition state TS23‑−24 (vide infra). This sequence begins with the CNNcoordinated aqua complex 7 and involves a CNN to CNO isomerization of the σ-CH benzene complex 26 via TS26‑23 (Scheme 10, bottom). Our analysis of reactivity of the CNN-coordinated LPt(solv) species toward benzene is illustrated in Scheme 10. An associative ligand substitution in the CNN-coordinated TFE complex 10 with benzene as an incoming ligand is a kinetically viable process with a transition state energy of 22.8 kcal/mol (TS10‑26). The reaction between 10 and benzene leads to 26, a benzene σ-CH complex. It is worth noting that the corresponding π complex appears to be nonexistent for CNNcoordinated LPt species, presumably because of excessive steric interactions between the arene and the pincer ligand C−H bonds near the Pt atom in the expected LPt(PhH) derivative. The last reaction step in Scheme 10 following the formation of 26 corresponds to the coordinated arene CH bond cleavage to form the PtIV phenyl hydride 27. Importantly, the CNN-coordinated aqua complex 7 can form 26 via a lower energy path with an overall activation barrier of

activation energy of 24.2 kcal/mol (TS11‑22). Even less competitive is the similar benzene for TFE ligand substitution in the CNO-coordinated TFE analogue 12 with the corresponding transition state (TS12‑22) Gibbs energy of 30.6 kcal/mol. The resulting η2-benzene π complex 22 can undergo a low-barrier π to σ-complex isomerization (TS22‑23) to form the CH σ-complex 23 with subsequent low-barrier CH bond oxidative addition (TS23‑24). The resulting PtIV phenyl hydride 24 may be involved in an isomerization reaction leading to the even more stable zwitterionic structure 25, a phenyl analogue of the hydroxo and trifluoroethoxo species 13 and 14, respectively (Scheme 2). Though it was not calculated, we presume that the activation barrier for such isomerization should not exceed the activation energy needed for the reverse reaction leading from 24 back to 23. While looking for alternative routes for the formation of the zwitterionic phenyl complex 25, we have also been able to find a direct route to it from the CH σ complex 23 employing a cyclometalation−deprotonation mechanism (transition state TSCMD; Scheme 9, bottom). A water molecule is needed to shuttle the proton of the Pt-coordinated benzene CH bond to the nitrogen atom of the ligand pendant pyridyl group. At the same time, with the transition state TSCMD Gibbs energy of 29.9 kcal/mol this pathway is virtually out of competition with the CH bond oxidative addition to Pt(II) center, with the transition state TS23‑24 Gibbs energy of only 22.3 kcal/mol. I

DOI: 10.1021/acs.organomet.6b00613 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics

energy than TS26‑27 (Scheme 10), the lowest energy path for benzene CH activation in our system appears to be by complex 7 via the transition states TS23‑24 and TS26‑23 (Scheme 10) and TS23‑24 (Scheme 9) with an overall barrier of 22.3 kcal/mol for C6H6 and 23.1 kcal/mol for C6D6. The latter value is a close enough match to the experimentally determined effective ΔG298⧧ = 24.2 kcal/mol for this substrate. The predicted reaction path suggests that the PtIV hydride 24 and the zwitterionic PtII phenyl complex 25 are also expected to appear as prominent transient products of benzene CH activation in reaction 2 catalyzed by 7. 2.5. Oxidation of 7 by O2 in 2,2,2-Trifluoroethanol Solutions. Solutions of 7 in TFE are sufficiently stable under air and show no signs of degradation for at least 10 h at 21 °C. Under pure oxygen gas at a partial pressure of 2.4 bar and vigorous stirring under ambient light the TFE solutions of 7 react with O2 with 41% conversion after 5 days to form a single new product which was identified as the PtIV trifluoroethoxide complex 15 (Scheme 3). No reaction was observed when an O2-pressurized reaction mixture was left in the dark for 5 days. Our preliminary kinetics observations suggest that the oxidation reaction is second order in concentration of 7.20 Single crystals of 15·TFE suitable for X-ray diffraction analysis (Figure 7) could be grown from TFE−ether mixtures. In this product the sulfonated pincer ligand adopts a meridional C,N,O arrangement of three donor atoms.

Scheme 10. DFT Calculated Gibbs Energy Profile for Benzene CH Bond Activation by LPtII(solv) Species Supported by a C,N,N-Coordinated Pincer Ligand L in Neat TFE at 25 °C

only 17.7 kcal/mol (TS7‑26), in comparison to the reaction of the TFE analogue 10 (TS10‑26). Hence, DFT predicts the predominant involvement of 7 in the benzene CH activation. This conclusion is consistent with the results of our modeling of the reaction 2 kinetics in the presence of water additives (Scheme 8, eq 4). Notably, the reaction sequence involving 7 and TS7‑26 has a rate-limiting CH bond cleavage step (TS26‑27), which is also consistent with our experimental observations. The noticeably lower energy of the benzene for aqua ligand substitution transition state TS7‑26, according to our analysis, may be due to the participation of the pincer ligand sulfonate group in two hydrogen-bonding interactions with two protons of the “departing” aqua ligand, which are absent in the ground state (complex 7) and which help “pull out” the aqua ligand from the metal coordination sphere. This effect is unique for TS7‑26, in comparison to other benzene for solvento ligand exchange transition states shown in Schemes 9 and 10. To complete the analysis of reactions shown in Scheme 10, it is worth adding a few more comments on the facile isomerization of the benzene σ-CH complex 26 having CNN coordination of the pincer ligand (Scheme 10) to its CNOcoordinated analogue 23 (Scheme 9) via the low-energy transition state TS26‑23. This transformation leaves the ligand− metal bonding of the σ-coordinated arene virtually intact as the intramolecular sulfonate for pyridyl fragment substitution occurs. The transition state TS26‑23 energy of 19.9 kcal/mol is lower than that of either of the CH bond oxidative addition transition states, TS23‑24 (Scheme 9) and TS26‑27 (Scheme 10). Since the transition state TS23‑24 (Scheme 9) is slightly lower in

Figure 7. ORTEP drawing (50% probability ellipsoids) for 15. Hydrogen atoms, except that of the OH group, are omitted for clarity; the cocrystallized solvent is not shown.

The results of our 1H−1H COSY and 1H NOE NMR experiments20 are consistent with the retention in TFE solutions of the structure found in the solid state. Considering the possible mechanism of formation of 15, it is worth noting that this complex has the same meridional arrangement of C, N, and O donor atoms as in the complexes 13 and 14, which may be expected to be involved in the oxidation reaction in Scheme 3. It is our experimental observation10 and a result of a previous theoretical (DFT) analysis of the mechanism of aerobic oxidation of (dpms)PtIIR complexes11 that reactions between O2 and (dpms)PtIIR species usually occur without altering the donor atom arrangement in the PtII coordination plane. Therefore, 13 and/or 14 (Scheme 2) could be directly involved in the photochemical oxidation. These complexes are also expected to be more reactive toward O2 in comparison to 10, 11, or 12 since they bear a formal negative charge on the metal, as J

DOI: 10.1021/acs.organomet.6b00613 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics

As a continuation of this work, we expect that the novel sulfonated pincer ligand and its analogues will play an important role in the development of new Pt-based catalysts for selective aerobic CH bond functionalization.

opposed to the case for 10, 11, and 12, an important factor affecting this type of reactivity.25 When distinguishing between 13 and 14 as the most likely direct participants of the reaction with O2, it is worth mentioning that 14 is expected to be the more acidic of these two and would produce reactive anionic PtIIOCH2CF3 intermediates25 more readily. Hence, we hypothesize that complex 14 is directly involved in the oxidation reaction, as shown in Scheme 3. An alternative reaction sequence when 13 is oxidized first and subsequently one of the resulting OH ligands of the anticipated PtIV dihydroxo analogue of the complex 15 is selectively displaced by CF3CH2O seems unlikely. The kinetics of such ligand exchange reactions at a PtIV center are usually very slow, and the observed exceptionally selective formation of 15 would be difficult to rationalize. The spin-forbidden character of this transformation and its photochemical nature require additional efforts in the mechanistic investigation of the transformation in Scheme 3. An experimental investigation and theoretical analysis of this and similar aerobic oxidation reactions is ongoing in our laboratory.

4. EXPERIMENTAL SECTION 4.1. General Considerations. All manipulations were carried out under an argon atmosphere unless otherwise noted. All reagents for which the synthesis is not given were purchased from Aldrich, Acros, Alfa-Aesar, TCI America, or Pressure Chemicals and were used without further purification except where stated. Deuterium-labeled solvents were purchased from Cambridge Isotope Laboratories (C6D6, CDCl3, CD3OD, and DMSO-d6) or Sigma-Aldrich (CF3CH2OD). 1H (400 MHz) and 13C NMR (125 MHz) spectra were recorded on a Bruker AVANCE 400 or Bruker DRX-500 instrument. Chemical shifts are reported in parts per million (ppm) (δ) and referenced to residual solvent resonance peaks. Multiplicities are reported as follows: br (broad signal), s (singlet), d (doublet), t (triplet), q (quartet), quin (quintet), sex (sextet), m (multiplet), dd (doublet of doublets), ddd (doublet of doublets of doublets), qd (quartet of doublets). Coupling constants (J) are reported in Hz. High-resolution mass spectrometry (HRMS) experiments were performed using a JEOL AccuTOF-CS instrument. 4.2. Li(Ph-dpms) (8). 4.2.1. (6-Phenylpyrid-2-yl)(2-pyridyl)methane. A solution of 2-phenyl-6-methylpyridine (1.69 g, 10 mmol) in 20 mL of dry THF was cooled to −78 °C in a 50 mL Schlenk flask using a dry ice−acetone bath. n-BuLi (2.5 M, 4.0 mL, 10 mmol) was slowly added to the stirred solution, resulting in a color change from light yellow to dark red. After the solution was stirred for an additional 1 h, 2-fluoropyridine (0.58 g, 6 mmol) was added dropwise via syringe. The dry ice bath was subsequently removed, and the reaction mixture was warmed to room temperature and stirred overnight. A 30 mL portion of water was added to quench the reaction. The organic and aqueous layers of the resulting yellow solution were separated, the aqueous layer was extracted with Et2O (3 × 30 mL), and the combined organic phases were dried over MgSO4. After filtration, the solvents were removed under reduced pressure, and the residual oil was purified by flash chromatography (silica, hexanes/EtOAc/TEA (1/1/0.01)) to yield 0.70 g of the product (57% yield based on 2-fluoropyridine used). 1H NMR (400 MHz, 22 °C, CDCl3): δ 4.66 (s, 2H), 7.17 (ddd, 3JHH = 7.6, 4.8 Hz, 4JHH = 1.6 Hz, 1H), 7.44 (d, 3JHH = 8.0 Hz, 1H), 7.52 (d, 3JHH = 8.0 Hz, 1H), 7.62− 7.76 (m, 4H), 7.79 (d, 3JHH = 8.8 Hz, 1H), 7.91 (dd, 3JHH = 7.2 Hz, 4 JHH = 2.0 Hz, 1H), 8.60 (ddd, 3JHH = 4.8, 2.0 Hz, 4JHH = 0.8 Hz, 1H), 9.36 (dd, 3JHH = 8.0 Hz, 4JHH = 0.6 Hz, 1H). 13C NMR (125 MHz, 22 °C, DMSO-d6): δ 47.1, 118.3, 122.0, 122.6, 124.0, 127.0, 129.1, 129.4, 137.0, 138.1, 139.1, 149.5, 155.9, 159.6, 159.8. ESI-MS: solution in methanol, [M + H]+ 247.137; calcd for [M + H]+ C17H15N2, 247.124. 4.2.2. Li(Ph-dpms) (8). (6-Phenylpyrid-2-yl)(2-pyridyl)methane (0.70 g, 2.8 mmol) was mixed with 3 mL of dry THF in a 25 mL Schlenk flask under argon. The solution was cooled to −78 °C, and 2.5 M n-BuLi (1.1 mL, 2.8 mmol) was added dropwise over 10 min with stirring. The resulting dark red solution was stirred for an additional 1/ 2 h before being removed from the cold bath and brought into an argon atmosphere glovebox. NMe3·SO3 complex (0.39 g, 2.8 mmol) was added. The Schlenk flask was then closed with a Teflon seal, and the mixture was heated in an oil bath for 24 h at 120 °C with rapid stirring. The next day a large amount of precipitate had formed. The mixture was then cooled to 5 °C, and the reaction was quenched with 10 mL of deionized water. The product was extracted in water and washed with diethyl ether (3 × 10 mL). The water layer was concentrated, and the residue was dried under vacuum. The crude product was dissolved in dry trifluoroethanol and filtered through a paper filter to remove inorganic impurities. The filtrate was concentrated and dried. The resulting residue was purified by flash chromatography (silica, EtOAc/CH2Cl2/MeOH/Et3N (2/2/1/0.01)). The eluent was treated with K2CO3 to convert any triethylammonium salts to TEA and then filtered and dried to yield 0.56 g of the product (61% yield). The formation of the disulfonated product can be

3. CONCLUSIONS The design of the novel sulfonated CNN pincer ligand L and derived PtII complexes was shown to be successful: a derived aqua complex was demonstrated experimentally and computationally (DFT) to be a kinetically competent catalyst for H/D exchange reactions between protio- and deuteriobenzene, some (poly)methylarenes, and TFE-d or TFE. The new pincer ligand also supports a clean PtII to PtIV aerobic oxidation chemistry which is manifested by a slow photochemical oxidation of the H/D exchange reaction catalyst. The latter fact may be considered as a limitation of this first-generation sulfonated pincer ligand based system, but the ability to turn off the reaction in the dark may be beneficial for reaction control. Interestingly, the most stable of the LPtII(H2O) derivatives present in the catalyst TFE solutions appears to be a zwitterionic (HL)PtII(OH) compound featuring a CNO coordination to PtII of the sulfonated pincer ligand L and the presence of a protonated pendant pyridine NH+ group on the ligand. In spite of the relatively high abundance of the zwitterionic derivative in the TFE solutions at 21 °C, the species most active in the benzene CH bond cleavage is the CNN-coordinated LPt(H2O) complex, which is connected through a series of low-barrier reaction steps to other LPt(solv) and (HL)PtII(OH) species. The CH bond activation reaction by LPt(H2O) is characterized by a noticeable deuterium kinetic isotope effect kH/kD of 1.7, which suggests the realization of a rate-determining substrate CH bond cleavage and a low steric crowding around the metal center in the reactive LPt(H2O) species. The latter conclusion also follows from the analysis of the CH bond cleavage selectivity in toluene, p-xylene, and mesitylene; in all of these cases exclusive aromatic CH bond activation was observed. Computationally it was also shown that the kinetically accessible reaction intermediates resulting from the substrate CH bond cleavage are isomeric PtIV phenyl hydrides and a derived zwitterionic PtII phenyl complex featuring a mer-CNO arrangement of the pincer ligand donor atoms. The DFT analysis suggests that these products are accessible through a benzene reaction with the LPtII(H2O) having the CNN coordinated pincer ligand through the CNN to CNO isomerization of the transient LPt(σ-benzene) complexes and subsequent CH bond cleavage in the CNO isomer via the lowest energy CH bond cleavage transition state. K

DOI: 10.1021/acs.organomet.6b00613 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics

126.9, 129.3, 130.7, 134.5, 140.5, 141.2, 140.0, 145.9, 152.6, 152.7, 153.7. ESI-MS: dilute solution of 7 in H2O, [2-H]−, 536.028; calcd for [7-H]−, C17H13N2O4PtS, 536.025. Anal. Calcd for C17H14N2O4PtS· 4H2O: C, 33.50; H, 3.64; N, 4.60. Found: C, 33.47; H, 3.79; N, 4.58. 4.5. VT NMR Experiments. An NMR Young tube equipped with a sealable Teflon cap and a sealed capillary tube containing D2O were taken into an argon-filled glovebox and charged with 0.5 mL of a TFE solution containing 7·2H2O (13.1 mM), C6D6 (0 or 1.13 M), and H2O (60 mM from the hydrate). The solution was monitored by 1H NMR over 21−80 °C. The bridging methine CH singlets at 5.66 and 6.01 ppm were used to monitor the ratio between the two (C6H4dpms)Pt(solv) species in solution at 40 °C and above (Figure S4 in the Supporting Information). In the 21 °C experiments extreme broadening of the 6.01 ppm signal was observed and the LPt(solv) ratio was found by integrating the whole aromatic region of the spectrum (assignable to both solv = TFE and H2O species); a sharp multiplet standing alone at 5.66 ppm was assigned to 13. Virtually the same LPt(TFE)/LPt(H2O) ratios were found whether the temperature was ramping up or down (demonstrating reversibility) and whether or not benzene was present or absent in the mixtures. The reported Keq values are given in Table S3 in the Supporting Information. 4.6. General Procedure for H/D Exchange Reactions. In a scintillation vial was placed 7·4H2O (4.0 mg, 6.7 μmol), and the vial was then brought into an argon-filled glovebox. Degassed TFE (0.45 mL) and C6D6 (0.05 mL) were then added. After about 10 min all had dissolved to form a reddish brown solution, which was then transferred to an NMR Young tube equipped with a sealable Teflon cap. For locking and shimming purposes, a D2O-containing capillary tube that had been flame-sealed on both ends was inserted. The NMR tube was removed from the glovebox and charged with 15 psi of argon on a Schlenk line to prevent refluxing at 80 °C. After an initial 1H NMR spectrum was taken, the NMR tube was submerged in a preheated bath at 80 °C and periodically removed for 1H NMR analysis. All reactions were run at least twice. The TFE CH2 quartet at 3.9 ppm was used as an internal standard to monitor the growth of the benzene CH singlet at 7.48 ppm (Figure S7 in the Supporting Information). To track the disappearance of 13, either the ortho pyridyl CH doublet at 8.87 ppm or the bridging methine singlet at 5.74 ppm was monitored. All reactions were run at least twice for at least 20 h. A control reaction containing only 10% v/v C6D6 in TFE showed no detectable H/D exchange. 4.7. Oxidation of 7 in TFE Solutions To Form 15. Only the reactions run under ambient fluorescent lighting showed oxidation of 7 to 15. In a 25 mL Schlenk tube was placed 7·4H2O (40 mg, 66 μmol) dissolved in 5 mL of TFE. A stir bar was added, followed by purging of the tube headspace with O2 and pressurization to 20 psi of O2 before being sealed. The solution was stirred vigorously at room temperature over the course of 5 days. The mixture initially turned red and then slowly turned yellow. A 0.5 mL aliquot was removed and submitted to a 1H NMR analysis in DMSO-d6, which showed one new species, identified as complex 15 (vide infra) in a 0.7:1 ratio with (C6H4dpms)Pt(DMSO-d6) (41% NMR yield of 15). The TFE solution was dried in vacuo to yield a light yellow residue, which was extracted with 3 × 1 mL of MeOH. The MeOH was removed to yield 19 mg of an off-white powder. The powder was dissolved in 0.5 mL of TFE, layered with Et2O, and placed in the freezer. The next day small thin needle crystals suitable for XRD analysis had formed. The structure of 15 in DMSO-d6 solution was additionally confirmed by 1H−1H COSY and 1H NOE NMR experiments and was found to be consistent with the observed crystal structure. Samples of complex 15 are stable on storage in the freezer but slowly decompose at room temperature over the course of a few days. 1H NMR (400 MHz, 22 °C, DMSO-d6): δ 1.53 (s, 1H), 3.77 (dq, 2JHH = 12.7 Hz, 3JHF = 10.1 Hz, 1H), 4.01 (dq, 2 JHH = 12.7 Hz, 3JHF = 10.1 Hz, 1H), 6.88 (s, 1H), 7.35 (m, 2H), 7.62 (m, 1H), 7.78 (ddd, 3JHH = 8.1, 5.8 Hz, 4JHH = 1.3 Hz, 1H), 7.86 (dd, 3 JHH = 7.2 Hz, 4JHH = 1.4 Hz, 1H), 7.90 (m, 1H), 8.01 (dd, 3JHH = 8.0 Hz, 4JHH = 0.7 Hz, 1H), 8.23−8.32 (m, 3H), 8.67 (dd, 3JHH = 5.9 Hz, 4 JHH = 1.5 Hz, 1H). 19F NMR (376 MHz, 21 °C, DMSO-d6): δ −73.49 (bt, 3JHF = 10.1 Hz). ESI-MS: solution of 15 in TFE, [15-H]−,

minimized by running the reaction at high concentrations (≥0.5 M), which promotes precipitation of the desired monosulfonated product. 1 H NMR (400 MHz, 22 °C, MeOD): δ 5.80 (s, 1H), 7.34 (ddd, 3JHH = 7.2, 4.8 Hz, 4JHH = 0.8 Hz, 1H), 7.38−7.45 (m, 3H), 7.75 (dd, 3JHH = 7.6 Hz, 4JHH = 0.8 Hz, 1H), 7.82−7.92 (m, 3H), 8.00−8.02 (m, 2H), 8.26 (d, 3JHH = 8.0 Hz, 1H), 8.47 (dd, 3JHH = 4.0 Hz, 4JHH = 0.4 Hz, 1H). 13C NMR (125 MHz, 22 °C, MeOD): δ 73.6, 117.3, 121.1, 121.6, 124.0, 125.1, 126.7, 127.0, 135.0, 135.5, 137.8, 146.3, 154.0, 154.7, 154.9. ESI-MS: solution of Li(Ph-dpms) in water, [Ph-dpms]− 325.085; calcd for [Ph-dpms]−, C17H13N2O3S, 325.065. 4.3. K[(C6H4-dpms)PtCl] (9). Li(Ph-dpms) (100 mg, 0.3 mmol) and K2PtCl4 (125 mg, 0.3 mmol) were dissolved in 4.0 mL of deionized H2O and transferred to a 25 mL Schlenk tube. The red solution was diluted with 8.0 mL of glacial acetic acid, and a stir bar was added. The tube was sealed and brought to 100 °C for 18 h with stirring. The resulting yellow solution was concentrated and dried under vacuum, and the residue was extracted with warm trifluoroethanol (8 mL). The product was obtained as a 1:1 mixture of 9 and LiCl by removing trifluoroethanol (163 mg, 91%), appearing as a yellow solid. This mixture could be used in the preparation of 7. To remove LiCl, the yellow solid was recrystallized from H2O/EtOH. The yellow crystalline powder was filtered off, washed with cold EtOH, and then dried in a vacuum oven at 60 °C for 12 h (137 mg, 77%). XRD-quality crystals of 9 were prepared by slow evaporation of a saturated methanolic solution. 1H NMR (125 MHz, 22 °C, DMSOd6): δ 5.933 (s, 1H), 7.05 (m, 2H), 7.49−7.57 (m, 2H), 7.61 (dd, 3JHH = 7.1 Hz, 4JHH = 1.9 Hz, 1H), 7.81 (vd, 3JHH = 7.7 Hz, 1H), 7.93 (vd, 3 JHH = 7.8 Hz, 2H), 8.04 (vt, 3JHH = 7.98 Hz, 1H), 8.07 (ddd, 3JHH = 9.5, 7.6 Hz, 4JHH = 1.9 Hz, 1H), 9.6 (dd, 3JHH = 6.0 Hz, 4JHH = 1.3 Hz, 1H). 13C NMR (400 MHz, 22 °C, DMSO-d6): δ 76.6, 117.8, 123.3, 123.7, 124.0, 126.7, 128.5, 129.6, 135.2, 138.0, 138.1, 143.0, 145.7, 149.8, 152.3, 153.1, 166.8. ESI-MS: solution of 1 in water, [1-K]− 553.973; calcd for [9-K]−, C17H12ClN2O3PtS 553.990. Anal. Calcd for C17H16ClKN2O5PtS·2H2O: C, 32.41; H, 2.56; N, 4.45. Found: C, 32.35; H, 2.84; N, 4.46. 4.4. (C6H4-dpms)Pt(H2O) (7). In a Schlenk flask was placed K[(C6H4-dpms)PtCl] (9; 200 mg, 0.337 mmol), along with 10 mL of EtOH. Then KOH pellets (∼85% KOH, ∼15% H2O, 115 mg, 1.75 mmol, ∼5 equiv) were added. The flask was sealed and heated to 60 °C with vigorous stirring overnight. The resulting dark red solution containing 16 was then filtered through Celite. When 9 contaminated with LiCl was used as the starting material, the solution was then passed through a K+ cation exchange column and filtered again followed by solvent removal in vacuo. The dark red residue containing crude 16 was characterized by 1H NMR and used without purification. The residue was redissolved in 3 mL of EtOH, diluted to 9 mL with DCM, and then placed in the refrigerator. After 1 h a light-colored precipitate had formed, which was removed by filtration through Celite. The filter cake was washed with 2 × 5 mL of a 2/1 DCM/ EtOH solution. The filtrate was stripped of solvent under vacuum ,and the red residue was dispersed in 10 mL of deionized argon-sparged H2O to which a few drops of TFE were added until all had dissolved. With vigorous stirring, the pH was adjusted to 2 using a 1 M H2SO4 solution. The mixture turned light yellow followed by the formation of a large amount of fluffy precipitate. The flask was placed in the refrigerator overnight. The next day small yellow crystals had formed on the walls of the flask which were suitable for XRD along with a yellow silty precipitate. The solids were filtered off, washed with deionized H2O, and dried under vacuum to yield 7·4H2O (146 mg, 73%). Product prepared in this manner usually contained a 1−5% impurity of 9; however, this does not affect the rate of CH activation by 7. Analytically pure 7·4H2O could be obtained by recrystallization from TFE/H2O to yield yellow crystals (105 mg, 51%). Complex 7 is soluble in TFE and DMSO, slightly soluble in MeOH, and insoluble in H2O/DCM/THF/acetone. The NMR data below are for the soluble DMSO derivative LPt(DMSO). 1H NMR (400 MHz, 22 °C, DMSOd6): δ 6.25 (s, 1H), 7.24−7.27 (m, 2H), 7.66−7.72 (m, 2H), 7.76 (dd, 3 JHH = 6.0 Hz, 4JHH = 2.8 Hz, 1H), 7.88 (m, 1H), 8.00 (d, 3JHH = 7.6 Hz, 1H), 8.18−8.22 (m, 3H), 9.04 (d, 3JHH = 4.8 Hz, 1H). 13C NMR (125 MHz, 22 °C, DMSO-d6): δ 75.3, 119.4, 125.3, 125.7, 125.9, L

DOI: 10.1021/acs.organomet.6b00613 Organometallics XXXX, XXX, XXX−XXX

Article

Organometallics 633.98; calcd for [15-H]−, C19H16F3N2O5PtS−, 634.02. The oxidation of 7 dissolved in TFE was monitored by 1H NMR spectroscopy. Complex 7·4H2O (4.0 mg, 6.7 μmol) was dissolved in 0.5 mL of TFE and transferred to an NMR Young tube equipped with a sealable Teflon cap. After an initial NMR spectrum was taken, the headspace was flushed with dry O2 and then pressurized to 20 psi with additional O2 (35 psi of O2 total). The NMR tube was then placed on a rotator to ensure adequate O2/solution mixing. The disappearance of the starting material 7 was monitored using signals of the solvent as an internal standard, resulting in the reaction profile shown below. The rate was found to follow a second-order rate law (Figure S10 in the Supporting Information). 4.8. DFT Calculations. Theoretical calculations in this work have been performed using the density functional theory (DFT) method,26 specifically the functional PBE27 and LACVP relativistic basis set with two polarization functions implemented in the Jaguar program28 package. Full geometry optimization has been performed without constraints on symmetry in 2,2,2-trifluoroethanol as a solvent using a Poisson−Boltzmann continuum solvation model (PBF).28 For all species under investigation frequency analysis has been carried out. All energy minima have been checked for the absence of imaginary frequencies. All transition states possessed just one imaginary frequency. Using the intrinsic reaction coordinate method, reactants, products, and the corresponding transition states were proven to be connected by a single minimum energy reaction path.

■

(h) Zhang, F.; Kirby, C. W.; Hairsine, D. W.; Jennings, M. C.; Puddephatt, R. J. J. Am. Chem. Soc. 2005, 127, 14196−7. (i) West, N. M.; Templeton, J. L. Can. J. Chem. 2009, 87, 288−96. (j) Vedernikov, A. N. Curr. Org. Chem. 2007, 11, 1401−16. (3) For a review on Pt-mediated CH activation see: Lersch, M.; Tilset, M. Chem. Rev. 2005, 105, 2471−526. (4) Lin, M. R.; Shen, C. Y.; Garcia-Zayas, E. A.; Sen, A. J. Am. Chem. Soc. 2001, 123, 1000−1. (5) Dangel, B. D.; Johnson, J. A.; Sames, D. J. Am. Chem. Soc. 2001, 123, 8149−50. (6) Periana, R. A.; Taube, D. J.; Gamble, S.; Taube, H.; Satoh, T.; Fujii, H. Science 1998, 280, 560−564. (7) Hartwig, J. F. J. Am. Chem. Soc. 2016, 138, 2−24. (8) Geletii, Yu. V.; Shilov, A. E. Kinet. Catal. 1983, 24, 413−416. (9) Bar-Nahum, I.; Khenkin, A. M.; Neumann, R. J. Am. Chem. Soc. 2004, 126, 10236−7. (10) Vedernikov, A. N. Acc. Chem. Res. 2012, 45, 803−13. (11) Liu, W.-G.; Sberegaeva, A. V.; Nielsen, R. J.; Goddard, W. A.; Vedernikov, A. N. J. Am. Chem. Soc. 2014, 136, 2335−41. (12) Sberegaeva, A. V.; Liu, W.-G.; Nielsen, R. J.; Goddard, W. A., III; Vedernikov, A. N. J. Am. Chem. Soc. 2014, 136, 4761−8. (13) Vedernikov, A. N.; Binfield, S. A.; Zavalij, P. Y.; Khusnutdinova, J. R. J. Am. Chem. Soc. 2006, 128, 82−3. (14) Munz, D.; Wang, D.; Moyer, M. M.; Webster-Gardiner, M.; Kunal, P.; Watts, D.; Trewyn, B. G.; Vedernikov, A. N.; Gunnoe, T. B. ACS Catal. 2016, 6, 4584−93. (15) Khusnutdinova, J. R.; Zavalij, P. Y.; Vedernikov, A. N. Organometallics 2011, 30, 3392−9. (16) (a) Zhong, H. A.; Labinger, J. a; Bercaw, J. E. J. Am. Chem. Soc. 2002, 124, 1378−99. (b) Driver, T.; Day, M. W.; Labinger, J. A.; Bercaw, J. E. Organometallics 2005, 24, 3644. (17) Hickman, A. J.; Cismesia, M. A.; Sanford, M. S. Organometallics 2012, 31, 1761−6. (18) Ziatdinov, V. R.; Oxgaard, J.; Mironov, O. A.; Young, K. J. H.; Goddard, W. A.; Periana, R. A. J. Am. Chem. Soc. 2006, 128, 7404−5. (19) Young, K.; Meier, S.; Gonzales, J.; Periana, R. A. Organometallics 2006, 25, 4734−4737. (20) See the Supporting Information for details. (21) Vedernikov, A. N.; Pink, M.; Caulton, K. G. Inorg. Chem. 2004, 43, 3642−6. (22) Khusnutdinova, J. R.; Zavalij, P. Y.; Vedernikov, A. N. Organometallics 2007, 26, 3466−83. (23) Jones, W. D. Acc. Chem. Res. 2003, 36, 140−6. (24) Johansson, L.; Ryan, O. B.; Romming, C.; Tilset, M. J. Am. Chem. Soc. 2001, 123, 6579−90. (25) Khusnutdinova, J. R.; Zavalij, P. Y.; Vedernikov, A. N. Can. J. Chem. 2009, 87, 110−120. (26) Parr, R. G.; Yang, W. Density-functional Theory of Atoms and Molecules; Oxford University Press: Oxford, U.K., 1989. (27) Perdew, J. P.; Burke, K.; Ernzerhof, M. Phys. Rev. Lett. 1996, 77, 3865−8. (28) Jaguar, version 8.4; Schrödinger, LLC, New York, 2014.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.6b00613. Experimental details regarding synthesis and characterization of the preligand 8, derived metal complexes, reactivity studies, kinetics modeling, X-ray characterization data for 7, 9, and 15 and DFT calculations (PDF) Crystallographic data (CIF) Cartesian coordinates for the calculated structures (XYZ)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail for A.N.V.: [email protected]. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS This work was supported by the National Science Foundation (CHE-1464772) and, in part, by the Center for Catalytic Hydrocarbon Functionalization, an Energy Frontier Research Center Funded by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Award Number DE-SC0001298.

■

REFERENCES

(1) (a) Gol’dshleger, N. F.; Es’kova, V. V.; Shilov, A. E.; Shteinman, A. A. Zh. Fiz. Khim. 1972, 46, 1353−4. (b) Shilov, A. E.; Shulpin, G. B. Activation and Catalytic Reactions of Saturated Hydrocarbons in the Presence of Metal Complexes; Kluwer: Boston, MA, 2000. (2) (a) Zamashchikov, V. V.; Rudakov, E. S.; Mitchenko, S. A.; Pekhtereva, T. M. Koord. Khim. 1985, 11, 69−72. (b) Crabtree, R. H. Chem. Rev. 1995, 95, 987−1007. (c) Arndtsen, B. A.; Bergman, R. G.; Mobley, T. A.; Peterson, T. H. Acc. Chem. Res. 1995, 28, 154−62. (d) Shilov, A. E.; Shu'lpin, G. B. Chem. Rev. 1997, 97, 2879−2932. (e) Stahl, S.; Labinger, J. A.; Bercaw, J. E. Angew. Chem., Int. Ed. 1998, 37, 2180−92. (f) Labinger, J. A.; Bercaw, J. E. Nature 2002, 417, 507− 14. (g) Fekl, U.; Goldberg, K. I. Adv. Inorg. Chem. 2003, 54, 259−320. M

DOI: 10.1021/acs.organomet.6b00613 Organometallics XXXX, XXX, XXX−XXX