Methylplatinum(II) and Molecular Oxygen: Oxidation to Methylplatinum

Jun 19, 2014 - Methylplatinum(II) and Molecular Oxygen: Oxidation to Methylplatinum(IV) in Competition with Methyl Group Transfer To Form ...
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Methylplatinum(II) and Molecular Oxygen: Oxidation to Methylplatinum(IV) in Competition with Methyl Group Transfer To Form Dimethylplatinum(IV) Jason D. Prantner, Werner Kaminsky, and Karen I. Goldberg* Department of Chemistry, University of Washington, Box 351700, Seattle, Washington 98195-1700, United States S Supporting Information *

ABSTRACT: Reaction of molecular oxygen with the methylplatinum(II) complex K[(κ2-NNO)PtII(CH3)(OH)] (3; NNO = bis(3,5dimethylpyrazol-1-yl)acetate) in D2O results in oxidation to (κ3NNO)PtIV(CH3)(OD)2 along with competitive methyl transfer to produce (κ3-NNO)PtIV(CH3)2(OD). Methyl transfer is favored under more alkaline conditions and at higher temperatures. Mechanistic studies are consistent with the direct reaction of 3 with molecular oxygen as a common first reaction step on the path to both products.

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Herein, we describe the aqueous aerobic oxidation of a PtII monomethyl system with a different type of facially coordinating ligand. Similar to the recently reported finding by Vedernikov et al. in their related PtII/PtIV system,7b,c in addition to direct oxidation to form a PtIV monomethyl complex, a rare competitive methyl transfer reaction to form a PtIV dimethyl complex was observed. Notably, the reaction conditions for the oxidation can be modified to favor one PtIV product over the other. The known NNO ligand (NNO = bis(3,5-dimethylpyrazol-1yl)acetate (1); Figure 1)11 features two pyrazolyl groups which

atalytic oxidation of methane to methanol with PtII as a catalyst and PtIV as the oxidant was reported by Shilov more than 40 years ago.1 Since then, research efforts have focused both on developing an understanding of the mechanism of the functionalization reaction and on finding active and efficient systems that employ more practical oxidants.2,3 The most promising oxidant for a commercially viable process, considering cost, availability, and environmental impact, is molecular oxygen/air. Hindering development of an oxygen-based process, however, has been an incomplete understanding of how the various platinum species that are involved in the reaction, e.g. PtII methyl complexes, react with oxygen.4 PtII methyl complexes are key proposed intermediates in both the Shilov methane oxidation system2b and in the more recent Catalytica Pt methane oxidation system (which uses SO3 as the terminal oxidant).3 In both systems, it is proposed that PtIV oxidizes the PtII−Me species to form a PtIV−Me complex. Ideally, oxygen could be used for this oxidation reaction, and this has inspired investigations of the reactions of PtII−Me species with O2.5−7 In studies of model PtII complexes with bpy and diimine ligands, Labinger, Bercaw, and Goldberg were able to demonstrate oxidation of PtII dimethyl to PtIV dimethyl complexes in methanol using molecular oxygen.6 However, subsequent work by Labinger and Bercaw showed that PtII monomethyl complexes, the type of species that would be generated by activation of methane at a PtII center, were significantly more difficult to oxidize.8 Sarneski’s observation that oxidation of PtII to PtIV by O2 occurs in the presence of the potentially fac-coordinating cis,cis-1,3,5-triaminocyclohexane (tach) but not on replacement of tach by bidentate cis-1,3diaminocyclohexane led to the proposal that facially coordinating ligands could aid in the oxidation of PtII to PtIV.9 Vedernikov used this facially coordinating ligand hypothesis in the design of the ligand dpms (bis(2-pyridyl)methanesulfonate);10a dpms acts as a bidentate ligand in the PtII−Me complex, and after exposure to O2, dpms becomes tridentate in the oxidized PtIV−Me product. © 2014 American Chemical Society

Figure 1. NNO ligand (1) and ORTEP drawing of 2. Ellipsoids are given at the 50% probability level, and H atoms are omitted for clarity.

readily bind to a PtII center and a carboxylate group expected to coordinate upon oxidation to PtIV. Reaction of K[NNO] with [Pt(CH3)2(S(CH3)2)]2 in toluene yields the PtII dimethyl complex K[(κ2-NNO)Pt(CH3)2] (2). Complex 2 was characterized by NMR spectroscopy, elemental analysis, and X-ray crystallography (Figure 1). As expected, the NNO ligand is bidentate with the two pyrazolyls, but not the carboxylate group, coordinated to the PtII center. Dissolution of 2 in H2O results in protonation of a Pt−Me group with release of methane to produce the monomethyl PtII complex K[(κ2-NNO)PtII(CH3)(OH)] (3; Scheme 1). Similar protonolysis reactions with other Received: March 7, 2014 Published: June 19, 2014 3227

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nitrogen-ligated PtII dimethyl complexes have been reported in water and methanol.2f,10,12

Scheme 3

Scheme 1

Dissolution of isolated 3 (4 mM) in D2O yields an alkaline solution (pD 10). This is attributed to a small degree of protonation of 3, presumably at the hydroxide position, to generate 3D+ and KOD, as shown in Scheme 2. A pKa value of 8.5 ± 0.2 was measured for 3D+. Scheme 2

Figure 2. ORTEP of 4 and 5 with ellipsoids at 50% probability level and H atoms omitted for clarity.

Complex 3 was stable in D2O under anaerobic conditions at room temperature, but upon exposure to air a slow reaction (ca. 5% conversion of 3 after 3 h) was observed by 1H NMR spectroscopy. With a high O2 pressure (8.2 atm), only minutes were required to see a similar amount of reaction. However, the reaction with O2 slowed considerably over time, with full conversion of 3 (95%) requiring ca. 2 weeks (Table 1, entry 1). The monomethyl PtIV complex (κ3-NNO)PtIV(CH3)(OD)2 (4), the expected product on the basis of the previously reported aerobic oxidations of PtII methyl and dimethyl complexes,5−7 was identified as the major product of the reaction (89%; Scheme 3). Complex 4 exhibits a PtIV−CH3 signal in the 1H NMR spectrum at 2.48 ppm (2JPt−H = 72.4 Hz). Four singlets corresponding to the methyl groups on the pyrazolyls of NNO (2.53, 2.50, 2.42, and 2.41 ppm) are indicative of a structure with C1 symmetry, as depicted in Scheme 3. The increase in alkalinity of the solution to pD 12 at the end of the reaction is consistent with the generation of KOD. An independent synthesis of complex 4 was achieved by oxidation of 3 with H2O2. The 1H NMR spectrum of 4 is consistent with the product from the reaction of 3 with O2, confirming its identity. Crystals suitable for X-ray diffraction were grown by slow evaporation of H2O. Analysis reveals octahedral coordination about the platinum center with the carboxylate arm of the NNO ligand now bound to the metal (Pt− O = 2.0706(12) Å; Figure 2). The Pt−Me group resides trans to one of the pyrazolyls of the NNO ligand, consistent with the C1

symmetry noted in the solution characterization. There are two inequivalent hydroxide ligands; the OH trans to the carboxylate has a slightly shorter Pt−O bond length (1.9587(12) Å) than the OH group trans to the pyrazolyl arm (1.9806(12) Å). Along with resonances for complex 4, signals for two minor Pt products were apparent in the 1H NMR spectrum of the aqueous reaction of 3 and O2. These minor products, identified as (κ3NNO)PtIV(CH3)2(OD) (5) (3%) and (κ2-NNO)PtII(OD)2 (6) (3%), were confirmed by independent synthesis. The PtIV dimethyl product 5 was prepared by the addition of MeI to 3. Complex 5 is distinguished by singlets for two inequivalent PtIV−Me groups (2.01 and 1.75 ppm, 1:1 ratio) with 195 Pt satellites (77.1 and 69.8 Hz, respectively). The solid-state structure of 5 was determined by X-ray crystallography (Figure 2). The two platinum-bound Me groups are inequivalent, with one trans to the carboxylate arm and the other trans to a pyrazolyl group, but they exhibit similar Pt−C bond lengths (2.0310(18) and 2.0368(18) Å, respectively). The second minor product, identified as (κ2-NNO)PtII(OD)2 (6), notably has no Pt−CH3 groups. As a consequence of the methyl transfer that produced 5, for each dimethyl complex 5 an equivalent of “demethylated” NNOPt must be formed. This demethylated product, formed in a yield identical with that for 5, was observed in the 1H NMR spectrum with a singlet at 6.07 ppm for the two pyrazolyl hydrogens and two singlets at 2.41 and 2.33 ppm for the four pyrazolyl methyl groups, suggesting a Cs-

Table 1. Effects of Temperature, Alkalinity, and Pressure of O2 on the Rate and Product Distribution of the Reaction of 3 with O2a yield (%)

a

entry

temp (°C)

1 2 3 4 5 6 7 8

∼22 40 80 80 80 80 80 80

pD

p(O2) (atm)

first half-life (h)

conversn of 3 (%)

4

5

6

7

b 6.5c 8.6c 11.2d 11.2d

8.2 8.2 8.2 8.2 3.4 3.4 8.2 4.1

30 14 1 2.4 0.29 0.067 1.2 2.4

95 99 99 98 98 99 99 98

89 89 72 51 91 95 62 63

3 6 12 23 2 2 17 16

3 5 9 16 3 2 9 10

0 0 4 7 0 0 8 6

[3] = 4 mM. b4 mM KOH added. cH3PO4/H3BO3/KOH buffered. dH3BO3/KOH buffered. 3228

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observed during the third half-life and may indicate a secondorder contribution.7c A similar peak in the reaction rate with respect to pH was observed in the Vedernikov dpms system.7c A first-order dependence on 3/3D+ and first-order dependence on O2 are consistent with an initial rate-determining reaction of 3/3D+ with dioxygen. The similarity in the product ratios of [4]/[5] (and thus the rates of formation) when the pressure of oxygen is doubled is suggestive of a shared initial reaction path for the two types of PtIV products, although two separate paths cannot be ruled out.7b,c The increase in the rate of the reaction with increasing pD at pD values lower than the pKa is consistent with a faster reaction of 3 with O2 relative to 3D+. A similar explanation was offered by Vedernikov with respect to the relative reactivity of the dpms hydroxide and aquo complexes.7b,c As shown in Scheme 4, we propose that reaction of 3/3D+ and

symmetric species. Complex 6 was independently synthesized by addition of excess HBF4 to a solution of 3, generating 6 and methane. The 1H NMR signals of independently synthesized 6 matched those attributed to 6 from the reaction of 3 with O2. Modifications of the reaction conditions for the reaction of 3 with oxygen were investigated. As the temperature was raised, reaction times expectedly decreased, but an increase in the proportion of the methyl transfer product 5 relative to 4 was also observed. At 80 °C, the reaction was complete within 24 h, and 5 accounted for 12% of the Pt-containing product (Table 1, entry 3). Along with higher yield of 5, a new NNO product was observed in the 1H NMR spectrum of the products with signals at 6.32 and 2.57 ppm. These signals are consistent with the PtIV complex (κ3-NNO)PtIV(OD)3 (7). Complex 7 was independently synthesized from the addition of H2O2 to 6. Complex 6 is likely the source of 7, as the sum of the concentrations of 6 and 7 was consistently equal to the concentration of 5 (Table 1). Continued heating for 24 h after complete conversion of 3 had no effect on the ratio 6:7, but addition of H2O2 to the products resulted in rapid conversion of 6 to 7. Higher reaction temperatures (>80 °C) resulted in decomposition of 4, and thus, further studies were limited to 80 °C. Increasing the alkalinity of the solution through the addition of KOH (4 mM) resulted in increased methyl transfer with a 23% yield of 5 (Table 1, entry 4). Thus, under these conditions, almost 50% of the material undergoes methyl transfer. The addition of KOH also slowed the reaction, as seen by the first half-life more than doubling relative to the reaction without added KOH (Table 1, entry 3). This inhibition of the reaction by hydroxide explains the slowing of the reaction described above, as KOD is produced in the reaction of 3 in D2O with O2. In buffered alkaline solutions (pD 11; Table 1, entries 7 and 8), well-behaved first-order kinetic behavior with respect to 3 was observed (Figure S3, Supporting Information). The reaction was also found to be first order in O2; kobs increased from [8.2(0.1)] × 10−5 s−1 at 4.1 atm of O2 to [1.6(0.2)] × 10−4 s−1 at 8.2 atm of O2. There was no significant change in the product distribution with a change in oxygen pressure.14 There are few reported aerobic oxidations of monomethyl or dimethyl PtII species to produce their corresponding PtIV monomethyl or dimethyl complexes, respectively, and mechanistic understanding of this reaction class is still emerging.5−7 Notably, previous studies by Labinger, Bercaw, and co-workers determined that the oxidation of L2PtMe2 (L2 = α-diimine) with oxygen in methanol to form L2PtMe2(OH)(OCH3) was first order in oxygen and first order in Pt.6b Furthermore, a PtIV hydroperoxide intermediate was observed which was proposed to then react with the PtII dimethyl starting material to form 2 equiv of the dimethyl hydroxide product. A slower competitive disproportionation of the hydroperoxide species to form the same hydroxide product was also proposed. Vedernikov and coworkers reported that, in their dpms system, the oxidation reaction was first order in the PtII−Me complex at pH ≤8 and second-order at pH ≥10.7 The effect of the pD on the rate of reaction of 3 with O2 was examined in buffered D2O (pD 6.5−11.2). The reaction was found to be first order in [3D+] or [3] at both low pD (6.5, kobs = [7.3(0.4)] × 10−5 s−1) and high pD (11.2, kobs = [8.2(0.1)] × 10−5 s−1), respectively.18 Notably at pD 8.6 (ca. the pKa of 3D+), the reaction was faster than those under either the more acidic or more alkaline conditions. Under these conditions, excellent fit to first order in [3/3D+] was observed through the first two halflives with kobs = [2.6(0.4)] × 10−4 s−1. However, slowing was

Scheme 4

dioxygen forms the hydroperoxo intermediate I1, which decomposes by competing oxidation and methyl transfer pathways. Notably, to form the hydroperoxo intermediate I1 from 3 and O2, a proton is also needed, presenting an explanation for the observed decrease in reaction rate at high pD. Reaction of I1 with 3/3D+ is proposed to lead to 2 equiv of 4 in a mechanism analogous to that suggested by Labinger and Bercaw for their PtII−Me2 oxidation6b,15 or by Vedernikov for the dpms system.7 The oxidative methyl transfer reaction has less precedent than the direct oxidation of PtII−Me with oxygen.7b,c Notably, no reaction was observed between 3 and 4 under anaerobic conditions that were otherwise optimized for methyl transfer (80 °C, 4 mM added KOH), eliminating 4 as the source of the additional methyl group in 5. However, it has been shown that, for efficient nucleophilic attack on a PtIV methyl group, an open site or good leaving group in the trans position is required.16 To enable efficient methyl group transfer, we propose that I1 isomerizes to I2, which then reacts with 3 to produce 5. The position of the CH3 group trans to the carboxylate moiety (a competent leaving group) in I2 should facilitate the nucleophilic attack by 3, leading to methyl transfer.7a,c In support of this proposal, methyl transfer between 3 and 5 was demonstrated via isotopically labeled methyl groups.13 While first-order rate dependence with respect to both 3 and O2 is observed at both low and high pD values, the product distribution (the ratio of [4] to [5]) clearly decreases with increasing pD. We propose that, subsequent to the ratedetermining step to form I1, partitioning occurs wherein I1 reacts with 3 or undergoes isomerization to I2.17 Notably, the rate of isomerization of I1 is expected to positively correlate with pD; in the related dpms system Vedernikov found that isomerization of monomethyl PtIV hydroxide was considerably faster at higher pH.7a An increase in the rate of isomerization of I1 with increasing pD can explain the increased contribution of the methyl transfer pathway at high pD. The production of complex 7 occurs by 6 competing with 3 for reaction with I1. In contrast to our results, in the related dpms system, a change from first- to second-order kinetics was observed with respect to 3229

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oxygen oxidation of the PtII−Me species when the pH was increased above 10.7b,c A change in the rate-determining step from the reaction of the PtII-Me with O2 to the reaction of the PtIV−OOH intermediate with the PtII−Me species was suggested. Meanwhile the methyl transfer reaction was noted to maintain first-order kinetic behavior and dominated at high pH. The authors conclude that two distinct pathways are responsible for the oxidation and methyl transfer. While we cannot rule out two pathways in our system, our results are consistent with the mechanism shown in Scheme 4. In summary, reaction of the monomethyl κ2-NNO ligated PtII species 3/3D+ with dioxygen in D2O resulted in direct oxidation, yielding the monomethyl κ3-NNO PtIV complex 4 in competition with a rare methyl transfer reaction to form the dimethyl κ3NNO PtIV complex 5. The methyl transfer product is favored at high temperatures and high pD. A mechanism is proposed in which 3/3D+ reacts with O2 in the rate-determining step to generate the hydroperoxo species I1. The reaction of 3 with oxygen is apparently faster than that of 3D+, as the reaction rate increases as the pD value approaches the pKa value of 3D+. Subsequent reaction of I1 with 3/3D+ or isomerization to I2 controls the ratio of the observed PtIV products. Methyl transfer is proposed to occur from I2. Oxidation by oxygen to produce PtIV methyl hydroxide complexes from PtII methyl species has potential application in methane to methanol catalysis,2 and this work extends the generality and mechanistic understanding of such reactions.5−7 The observed methyl group transfer to produce a PtIV dimethyl species upon oxidation of a PtII monomethyl complex with O2 is highly unusual7b,c,19 and represents a reaction pathway that could have novel applications, for example, in methane oligomerization.20 C−H activation of methane to yield PtII-Me is known,2 and C−C reductive elimination of ethane from PtIV is also precedented.16 The knowledge gained from the studies described herein, particularly with respect to the dependence on pH and temperature, may be applicable to the development of methane functionalization catalysts using an environmentally benign oxidant and solvent (dioxygen and water). Mechanistic studies of these intriguing aqueous oxidation reactions are continuing.

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AUTHOR INFORMATION

Nature 2002, 417, 507. (e) Fekl, U.; Goldberg, K. I. Adv. Inorg. Chem. 2003, 54, 259. (f) Lersch, M.; Tilset, M. Chem. Rev. 2005, 105, 2471. (g) Vedernikov, A. N. Chem. Commun. 2009, 32, 4781. (3) (a) Periana, R. A.; Douglas, R. J.; Taube, D. J.; Gamble, S.; Taube, H.; Satoh, T.; Fujii, H. Science 1998, 280, 560. (b) Mironov, O. A.; Bischof, S. M.; Konnick, M. M.; Hashiguchi, B. G.; Ziatdinov, V. R.; Goddard, W. A.; Ahlquist, M.; Periana, R. A. J. Am. Chem. Soc. 2013, 135, 14644. (4) Boisvert, L.; Goldberg, K. I. Acc. Chem. Res. 2012, 45, 899 and references therein. (5) Prokopchuk, E. M.; Jenkins, H. A.; Puddephatt, R. J. Organometallics 1999, 18, 2861. (6) (a) Rostovtsev, V. V.; Lasseter, T. L.; Goldberg, K. I.; Labinger, J. A.; Bercaw, J. E. Organometallics 1998, 17, 4530. (b) Rostovtsev, V. V.; Henling, L. M.; Labinger, J. A.; Bercaw, J. E. Inorg. Chem. 2002, 41, 3608. (7) (a) Binfield, S. A.; Zavalij, P. Y.; Khusnutdinova, J. R.; Vedernikov, A. N. J. Am. Chem. Soc. 2006, 128, 82. (b) Liu, W. G.; Sberegaeva, A. V.; Nielsen, R. J.; Goddard, W. A.; Vedernikov, A. N. J. Am. Chem. Soc. 2014, 136, 2335. (c) Sberegaeva, A. V.; Liu, W. G.; Nielsen, R. J.; Goddard, W. A.; Vedernikov, A. N. J. Am. Chem. Soc. 2014, 136, 4761. (8) Scollard, J. D.; Day, M.; Labinger, J. A.; Bercaw, J. E. Helv. Chim. Acta 2001, 84, 3247. (9) Sarnski, J. E.; McPhail, A. T.; Onan, K. D.; Erickson, L. E.; Reilley, C. N. J. Am. Chem. Soc. 1977, 99, 7376. (10) (a) Fettinger, J. C.; Mohr, F.; Vedernikov, A. N. J. Am. Chem. Soc. 2004, 126, 11160. (b) Stahl, S. S.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc. 1996, 118, 5961. (11) Otero, A. J.; Fernandez-Baeza, J.; Tejada, J.; Antinolo, A.; CarrilloHermosilla, F.; Diez-Barra, E.; Lara-Sanchez, A.; Fernandez-Lopez, M.; Lanfranchi, M.; Pellinghelli, M. A. J. Chem. Soc., Dalton Trans. 1999, 3537. (12) Similar to previously reported PtII−Me systems2f,10 when 2 was dissolved in D2O, deuterium exchange into the PtII−Me moiety precedes loss of the methyl group as methane. Notably, dissolution of 3 in D2O does not result in any H/D exchange in the PtII−Me group. (13) See Supporting Information. (14) At a significantly lower pressure of O2 (2.1 atm), a slightly higher yield of 4 (71%) and lower yields of 5−7 (14, 7, and 6%, respectively) were noted. (15) When [3/3D+] is small (third half-life), the bimolecular steps (k2/ k4 in Scheme 4) or alternatively a disproportionation route to 4 may be slow enough to become rate determining, which may explain the slowing of the reaction observed at late reaction times (see Figures S5 and S6, Supporting Information). (16) Grice, K. A.; Scheuermann, M. L.; Goldberg, K. I. Top. Organomet. Chem. 2011, 35, 1 and references therein. (17) The difference in the propensity of I1 versus 4 to undergo isomerization may be attributed to the greater trans influence of OOH versus OH.6b (18) Reasonable fits of the data to second order in [3/3D+] were observed when experiments were carried out with insufficient buffer (see Figures S7 and S8, Supporting Information). (19) Related aerobic oxidation of monomethyl PdII to monomethyl PdIII followed by methyl group transfer and C−C coupling producing ethane has been reported: (a) Khusnutdinova, J. R.; Rath, N. P.; Mirica, L. M. J. Am. Chem. Soc. 2010, 132, 7303. (b) Khusnutdinova, J. R.; Rath, N. P.; Mirica, L. M. J. Am. Chem. Soc. 2012, 134, 2414. (20) Lanci, M. P.; Remy, M. S.; Lao, D. B.; Sanford, M. S.; Mayer, J. M. Organometallics 2011, 30, 3704 and references therein.

* Supporting Information S

Full experimental details and CIF files. This material is available free of charge via the Internet at http://pubs.acs.org.

Corresponding Author

*E-mail for K.I.G.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation (CHE-1012045). REFERENCES

(1) Gol’dshleger, N. F.; Es’kova, V. V.; Shilov, A. E.; Shteinman, A. A. Zh. Fiz. Khim. 1972, 46, 1353. (2) (a) Shilov, A. E.; Shul’pin, G. B. Russ. Chem. Rev. 1987, 56, 442. (b) Shilov, A. E.; Shul’pin, G. B. Activation and Catalytic Reactions of Saturated Hydrocarbons in the Presence of Metal Complexes; Kluwer Academic: Boston, MA, 2000. (c) Stahl, S. S.; Labinger, J. A.; Bercaw, J. E. Angew. Chem., Int. Ed. 1998, 37, 2180. (d) Labinger, J. A.; Bercaw, J. E. 3230

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