Insertion of Molecular Oxygen into the Metal–Methyl Bonds of

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Insertion of Molecular Oxygen into the Metal−Methyl Bonds of Platinum(II) and Palladium(II) 1,3-Bis(2-pyridylimino)isoindolate Complexes Hannah E. Zeitler,#,† Werner A. Kaminsky,# and Karen I. Goldberg*,#,† #

Department of Chemistry, University of Washington, Seattle, Washington 98195, United States Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania 19104, United States

†

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S Supporting Information *

ABSTRACT: Upon exposure of [(BPI)M(CH3)] (M = Pd, Pt, BPI = 1,3-bis(2-pyridylimino)isoindole) complexes to O2, direct insertion was observed to generate the O2 insertion products (BPI)M(OOCH3). Kinetic and mechanistic analyses of the O2 insertion reaction support a radical chain substitution mechanism. Reproducible kinetics were observed in the presence of a radical initiator. The release of methanol was subsequently observed from the M−OOCH3 products.

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commercially viable method for the direct partial oxidation of methane to methanol remains an elusive holy grail in chemistry.1,2 While one of the significant obstacles is promoting selective C−H activation without overoxidation, another critical challenge is that, in large-scale industrial oxidation processes, the oxidant employed should be inexpensive, readily available, and environmentally benign. It has been argued that oxygen, the oxidant used in most large scale commercial organic oxidations, is likely to be required.3−5 Although the challenge of selective C−H activation has attracted much attention in the research community,6,7 there has been considerably less effort directed at understanding the reactivity of organometallic species with molecular oxygen.8,9 Of particular relevance to selective methane oxidation are investigations of the reactivity of oxygen with metal−methyl species, the primary products of methane C−H activation. Recently some remarkably clean and well-characterized examples of the direct insertion of molecular oxygen into late metal−methyl bonds have been reported. In 2009, Grice and Goldberg found that the reaction of O2 with [(PN)Pt(CH3)2] (PN = 2-((di-tert-butylphosphino)methyl)pyridine) resulted in the formation of [(PN)Pt(CH3)(OOCH3)] (Figure 1).10 Similarly, [(bpy)Pd(CH3)(OOCH3)] (bpy = 2,2′-bipyridine) (Figure 1) was produced when [(bpy)Pd(CH3)2] reacted with O2.11 Detailed kinetic and mechanistic studies of this reaction provided strong evidence in support of a radical chain substitution pathway. Notably, with sufficient O2 present, the rate was independent of O2 concentration, and a half-order dependence of the rate with respect to both the PdII starting material and the radical initiator was observed. Insertion of O2 into M−CH3 bonds of substituted terpyridine (2,6-bis(2-pyridyl)pyridine) Pt/Pd methyl species (Figure 1) in the presence of light have also been reported. However, © XXXX American Chemical Society

Figure 1. M−OOCH3 systems formed by reaction of M−CH3 complexes with O2.

mechanistic studies of these systems did not support a radical chain mechanism.12,13 Although an experimental article proposed a mechanism where excited triplet-state dinuclear intermediates are generated upon exposure to light and then undergo reaction with O2 to form the M−OOCH3 species,13 a recent computational paper has suggested that the reaction proceeds instead through a triplet-state excited monomer that Received: August 10, 2018

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DOI: 10.1021/acs.organomet.8b00573 Organometallics XXXX, XXX, XXX−XXX

Communication

Organometallics

corresponding to the palladium methyl protons. Crystals of 2 suitable for X-ray diffraction were crystallized from the reaction mixture (see Supporting Information). Complex 2 has a distorted square planar geometry (τ4 = 0.1768(6))18 and a palladium methyl bond length of 2.054(2) Å. When a C6D6 solution of 1 was pressurized with 5 atm O2 and heated at 60 °C (Scheme 1), the appearance of a new Pt

reacts with oxygen to form superoxo and peroxo intermediates on their way to the M−OOCH3 products.14 With such a small number of reported examples of oxygen insertion into M−C bonds, it is difficult to generalize or make predictions concerning the potential reactivity of other late metal alkyl complexes with oxygen. Recent investigations of the reactions of [(PNP)Pd(CH3)]Cl (PNP = (2,6-bis[(di-tertbutylphosphino)methyl]pyridine) with O2 demonstrated the importance of oxidation-resistant ligands as oxidation of the ligand backbone was observed.15 Furthermore, studies with the related complex [(PCP)Pd(CH3)] (PCP = 2,6-bis[(di-tertbutylphosphino)methyl]phenyl) and O2 suggested that aerobic reactions may be inhibited by steric blocking of the axial approach to the metal in the square planar complex.15 On this basis, O2 reactions of M−CH3 complexes bearing the ligand 1,3-bis(2-pyridylimino)isoindole (BPI) were targeted for study (Figure 1). The planar BPI ligand has been shown to be stable in the presence of oxygen and at elevated temperatures.16,17 Herein, we report the synthesis of [(BPI)M(CH3)] (M = Pd, Pt) complexes and investigations of their reactions with molecular oxygen. The [(BPI)M(CH3)] complexes react cleanly with O2 to generate the oxygen insertion products [(BPI)M(OOCH3)]. Kinetic and mechanistic studies of these O2 insertion reactions are reported along with observations of further reactivity of the M−OOCH3 species to generate methanol. Reaction of a BPI-H ligand precursor bearing p-methyl groups on the pyridyl groups (BPI-H-4-methyl) with half an equivalent of [{Pt(CH3)2(μ-S(CH3)2)}2] in benzene at 60 °C afforded the novel Pt−CH3 complex [(BPI-4-methyl)Pt(CH3)] (1) in high yield. The 1H NMR spectrum of 1 (C6D6) contains a doublet at 9.18 ppm (2H, 3JPt−H = 50.8 Hz) corresponding to the protons attached to the position ortho to the N in the pyridine ring and a singlet at 1.51 ppm (3H, 2JPt−H = 70.4 Hz) corresponding to the protons of the Pt-bound methyl group. Crystals of 1 suitable for X-ray diffraction crystallized directly from the reaction mixture. The solid-state structure of 1 (Figure 2) shows a distorted square planar environment (τ4 = 0.1269(5))18 with a platinum methyl bond length of 2.09(3) Å, which is comparable to those of other platinum methyl complexes.10,13

Scheme 1. Reaction of 1 and 2 with O2

species was detected by 1H NMR spectroscopy. After 5 h, the starting material had been fully consumed. The 1H NMR spectrum of the product species (3) (Scheme 1) featured a new methyl signal (3H) at 4.16 ppm, and the signal corresponding to the protons ortho to the N in the pyridine rings appeared at 10.78 ppm. Similar reactivity with oxygen was observed with the palladium analogue 2 to form complex 4 with signals for the analogous protons at 4.09 and 10.13 ppm, respectively, in the 1H NMR spectrum. No reaction was observed when 1 or 2 was heated at 60 °C in the absence of oxygen. The methyl signal at 4.16 ppm in complex 3 and 4.09 ppm in complex 4 suggested that either a methylperoxo species or a methoxide species had formed in each of these reactions.10−13 To more easily characterize the products, the solubilities of 3 and 4 in hydrocarbon solvents were enhanced by replacing the methyl substituents of the pyridyl moieties with tert-butyl groups. Addition of triphenylphosphine to a C6D6 solution of 3-tBu resulted in the formation of OPPh3 (26.4 ppm, 31P NMR) and a new methyl signal (shifted from 4.21 for 3-tBu to 4.17 ppm, 1H NMR). These results are consistent with the assignment of 3-tBu as [(BPI-4-tert-butyl)Pt(OOCH3)] and the methoxide complex [(BPI-4-tert-butyl)Pt(OCH3)] (6-tBu) as the product formed in the PPh3 reaction.19 Similar results were obtained when the product 4-tBu was treated with PPh3, supporting the identification of the product of the reaction of 2- tBu with oxygen as [(BPI-4-tert-butyl)Pd(OOCH 3)] (4-tBu). Crystals of 3-tBu suitable for X-ray diffraction were grown by slow evaporation of a pentane solution. The solidstate structure shows that the product is indeed a Pt−OOCH3 complex, [(BPI-4-tert-butyl)Pt(OOCH3)] (3-tBu), as shown in Figure 3. There have only been two previously reported Pt− OOCH3 solid-state structures. 10,12 Compared to these structures, the Pt−O bond length of 3-tBu is of similar length at 1.985(8) Å, as is the O−O bond length (1.419(10) Å). Investigations to determine the mechanism of the reaction of 1 with oxygen were carried out. The times required for complete reaction were shorter in the presence of light, and it was noted that the rate of the reaction was generally irreproducible. Furthermore, upon addition of butylated hydroxytoluene (BHT), a known radical chain inhibitor, the yield and rate of reaction dramatically decreased: 16.0% (with

Figure 2. ORTEP of 1 with thermal ellipsoids set at 50% probability. Hydrogens omitted for clarity.

Similarly, the Pd analogue [(BPI-4-methyl)Pd(CH3)] (2) was generated upon reaction of BPI-H-4-methyl with [Pd(CH3)2(TMEDA)] (TMEDA = tetramethylethylenediamine) in benzene at 60 °C. By 1H NMR spectroscopy (C6D6), a doublet at 8.58 ppm (2H), corresponding to the protons attached to the position ortho to the N in the pyridine ring, was observed as well as a singlet at 0.99 ppm (3H), B

DOI: 10.1021/acs.organomet.8b00573 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

Figure 3. ORTEP of 3-tBu with thermal ellipsoids set at 50% probability. Hydrogens omitted for clarity.

BHT) vs 81.7% (without BHT) of 1 converted after 4 h. These results are suggestive of a radical chain mechanism. Indeed, it was possible to achieve highly reproducible reaction rates with the addition of azobis(isobutyronitrile) (AIBN), a radical initiator. Using this initiator, a detailed kinetic study of the reaction of 1 with O2 was then carried out. Kinetic studies of the conversion of Pt−CH3 (1) to Pt− OOCH3 (3) in the presence of O2 and AIBN were conducted at 60 °C in C6D6 in the dark. Notably, no reaction was observed when 1 was heated with AIBN in C6D6 without O2. The reactions with O2, AIBN, and 1 were run under pseudo first-order conditions with respect to 1 (high relative concentrations of O2 and AIBN) and were monitored by 1H NMR spectroscopy. As shown in Figure 4, the rate of the reaction exhibited half-order dependences on both 1 and AIBN and a zero-order dependence on O2 (eq 1) (see Supporting Information for additional kinetic plots). 1/2

rate = k[1]

1/2

[AIBN]

0

[O2 ]

Figure 4. Graphs showing rate dependence on 1, AIBN, and O2. (A) Pt−CH3 = 4.66 mM; AIBN = 284 mM; O2 pressure = 5 atm. (B) Pt− CH3 = 4−5 mM; O2 pressure = 7.5 atm. (C) Pt−CH3 = 4−5 mM; AIBN = 27 mM. All: internal standard = hexamethylbenzene, temperature = 60 °C.

(1)

The empirical rate law (eq 1) is identical in form to that found for the reaction of [(bpy)Pd(CH3)2] with oxygen,11 and the data can be similarly accounted for by a radical chain substitution mechanism as shown in Scheme 2. After initiation, the PtII starting material is attacked by CH3OO· to form a fivecoordinate PtIII species (I). A CH3· is then released, resulting in the Pt−OOCH3 product and CH3·. Under the reaction conditions ([O2] > 22 mM),20 CH3· reacts rapidly with O2 to produce CH3OO·, the radical chain carrier. Finally, termination takes place by the reaction of the PtIII radical (I) and CH3OO· to yield nonchain propagating species. Results of preliminary mechanistic studies for the oxygen reaction with the palladium analogue [(BPI-4-methyl)Pd(CH3)] (2) were quite similar to the data collected for the reaction of the platinum complex 1 with oxygen. For example, when BHT was added to the Pd reaction with oxygen, only 7.3% conversion of 2 was observed within 2 h as opposed to 48% in the absence of BHT. Kinetics experiments in the presence of AIBN also found the analogous hallmark half-order dependence on 2. These results suggest that the oxygen reaction with the palladium methyl analogue follows a similar mechanism to the platinum complex. With the addition of the BPI Pd/Pt complexes, there are now four complexes that appear to react with oxygen to form M−OOCH3 products via a radical chain substitution mechanism.10,11 A different mechanism has been proposed for the (terpy)Pd/Pt complexes under photolysis conditions.13,14 Requirements for oxygen insertion in late metal−methyl bonds by either mechanism appear to be redox-stable ligands and access to the open site at

Scheme 2. Proposed Radical Chain Mechanism

the metal either for an incoming oxygenated radical or for oxygen itself. The platinum and palladium methylperoxo species, 3 and 4, respectively, were observed to undergo further reaction. Continued heating and monitoring of the C6D6 solution after methylperoxo species 3 had formed led to the observation of methanol (3.10 ppm, 1H NMR) in up to 53% observed yield as well as signals corresponding to two new Pt-containing species. The methanol was confirmed by 1H NMR spectroscopy via spiking the product mixture with an authentic sample as well as by GC-MS and 13C NMR spectroscopy. The Pt products were C

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Organometallics more difficult to analyze due to their low solubility. When the 4-methyl substituents of the pyridyl moieties were replaced with 4-tert-butyl groups, one of the new metal species was confirmed to be the Pt−OH complex [(BPI-4-tert-butyl)Pt(OH)] (5-tBu) by comparison with independently prepared 5-tBu. The other metal species was also confirmed by comparison with an authentic sample as the corresponding Pt−OCH3 complex [(BPI-4-tert-butyl)Pt(OCH3)] (6-tBu). The source of the hydrogens needed to produce methanol from 3 has not yet been confirmed. Investigations to determine if it is adventitious water or another source are currently in progress. Signals for both the Pt−OH (5-tBu) and Pt−OCH3 (6-tBu) appear simultaneously in the 1H NMR spectrum of the reaction. It is difficult to determine if one of them forms first and undergoes conversion to the other as independently prepared samples of 5-tBu and MeOH or 6-tBu and H2O demonstrate that these species are in equilibrium (Scheme 3). t

Experimental details regarding the synthesis and characterization of the metal complexes used, including spectroscopic analysis as well as additional kinetic plots (PDF) Accession Codes

CCDC 1861494−1861497 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Karen I. Goldberg: 0000-0002-0124-1709

t

Notes

Scheme 3. Equilibrium between 5- Bu and 6- Bu

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation (CHE-1464661) and the University of Pennsylvania. We thank Dr. Karena Smoll for helpful discussions.

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The equilibrium constant at 298 K was determined to be 2.81 ± 0.09 for the reaction shown in Scheme 3. This value is comparable to other Keq determined for the equilibria between PtII hydroxide and methoxide complexes.21,22 Complex 4 also undergoes reaction to form MeOH (up to 60% yield). Multiple palladium species are observed by 1H NMR, and the identity of the palladium products of this reaction have not been established. In conclusion, novel platinum and palladium methyl complexes bearing the BPI ligand have been synthesized, characterized, and allowed to react with oxygen. Both compounds react with oxygen cleanly to form methylperoxo species. The results of kinetic studies support that a radical chain mechanism similar to that previously proposed by Goldberg for the reaction of [(bpy)Pd(CH3)2] with oxygen is followed.11 The methylperoxo complexes undergo further reaction to form methanol. This work helps to build general understanding in the field of how molecular oxygen can react productively with late transition metal−methyl complexes. Furthermore, the production of methanol in these reactions is promising for the design of catalysts for the conversion of methane to methanol. Work to understand the methanol production reactions from the M−OOCH3 complexes and to optimize yields is underway.

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REFERENCES

(1) Caballero, A.; Pérez, P. J. Methane as raw material in synthetic chemistry: the final frontier. Chem. Soc. Rev. 2013, 42, 8809−8820. (2) Goldberg, K. I.; Goldman, A. S. Large-Scale Selective Functionalization of Alkanes. Acc. Chem. Res. 2017, 50, 620−626. (3) Cavani, F.; Teles, J. H. Sustainability in Catalytic Oxidation: An Alternative Approach or a Structural Evolution? ChemSusChem 2009, 2, 508−534. (4) Stahl, S. S. Palladium-Catalyzed Oxidation of Organic Chemicals with O2. Science 2005, 309, 1824−1826. (5) Munz, D.; Strassner, T. Alkane C−H Functionalization and Oxidation with Molecular Oxygen. Inorg. Chem. 2015, 54, 5043− 5052. (6) Lersch, M.; Tilset, M. Mechanistic Aspects of C-H Activation by Pt Complexes. Chem. Rev. 2005, 105, 2471−2526. (7) Labinger, J. A. Platinum-Catalyzed C−H Functionalization. Chem. Rev. 2017, 117, 8483−8496. (8) Scheuermann, M. L.; Goldberg, K. I. Reactions of Pd and Pt Complexes with Molecular Oxygen. Chem. Eur. J. 2014, 20, 14556− 14568. (9) Boisvert, L.; Goldberg, K. I. Reactions of Late Transition Metal Complexes with Molecular Oxygen. Acc. Chem. Res. 2012, 45, 899− 910. (10) Grice, K. A.; Goldberg, K. I. Insertion of Dioxygen into a Platinum(II)-Methyl Bond To Form a Platinum(II) Methylperoxo Complex. Organometallics 2009, 28, 953−955. (11) Boisvert, L.; Denney, M. C.; Hanson, S. K.; Goldberg, K. I. Insertion of Molecular Oxygen into a Palladium(II) Methyl Bond: A Radical Chain Mechanism Involving Palladium(III) Intermediates. J. Am. Chem. Soc. 2009, 131, 15802−15814. (12) Taylor, R. A.; Law, D. J.; Sunley, G. J.; White, A. J. P.; Britovsek, G. J. P. Towards Photocatalytic Alkane Oxidation: The Insertion of Dioxygen into a Platinum(II)−Methyl Bond. Angew. Chem., Int. Ed. 2009, 48, 5900−5903. (13) Petersen, A. R.; Taylor, R. A.; Vicente-Hernández, I.; Mallender, P. R.; Olley, H.; White, A. J. P.; Britovsek, G. J. P. Oxygen Insertion into Metal Carbon Bonds: Formation of Methylperoxo Pd(II) and Pt(II) Complexes via Photogenerated Dinuclear Intermediates. J. Am. Chem. Soc. 2014, 136, 14089−14099.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.8b00573. D

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Organometallics (14) Fernández-Alvarez, V. M.; Ho, S. K. Y.; Britovsek, G. J. P.; Maseras, F. A DFT-based mechanistic proposal for the light-driven insertion of dioxygen into Pt(II)−C bonds. Chem. Sci. 2018, 9, 5039− 5046. (15) Smoll, K. A.; Kaminsky, W.; Goldberg, K. I. Photolysis of Pincer-Ligated PdII−Me Complexes in the Presence of Molecular Oxygen. Organometallics 2017, 36, 1213−1216. (16) Siegl, W. O. Metal Ion Activation of Nitriles. Syntheses of 1,3Bis(arylimino)isoindolines. J. Org. Chem. 1977, 42, 1872−1878. (17) Gagne, R. R.; Gall, R. S.; Lisensky, G. C.; Marsh, R. E.; Speltz, L. M. Reaction of Molecular Oxygen with Copper(I). Strucutral Characterization of Dimeric μ-Carbonato and Tetrameric μ-Hydroxo Complexes Resulting from the Autoxidation of a Copper(I) Carbonyl Complex. Inorg. Chem. 1979, 18, 771−781. (18) Yang, L.; Powell, D. R.; Houser, R. P. Structural variation in copper(I) complexes with pyridylmethylamide ligands: structural analysis with a new four-coordinate geometry index, τ4. Dalton Trans. 2007, 955−964. (19) Michelin, R. A.; Ros, R.; Strukul, G. Synthesis and Reactivity of Hydroperoxo Complexes of Platinum(II). Inorg. Chim. Acta 1979, 37, L491−L492. (20) Look, J. L.; Wick, D. D.; Mayer, J. M.; Goldberg, K. I. Autoxidation of Platinum(IV) Hydrocarbyl Hydride Complexes To Form Platinum(IV) Hydrocarbyl Hydroperoxide Complexes. Inorg. Chem. 2009, 48, 1356−1369. (21) Bryndza, H. E.; Fong, L. K.; Paciello, R. A.; Tam, W.; Bercaw, J. E. Relative Metal-Hydrogen, -Oxygen, -Nitrogen, and -Carbon Bond Strengths for Organoruthenium and Organoplatinum Compounds; Equilibrium Studies of CP*(PMe3)2RuX and (DPPE)MePtX Systems. J. Am. Chem. Soc. 1987, 109, 1444−1456. (22) Smythe, N. A.; Grice, K. A.; Williams, B. S.; Goldberg, K. I. Reductive Elimination and Dissociative β-Hydride Abstraction from Pt(IV) Hydroxide and Methoxide Complexes. Organometallics 2009, 28, 277−288.

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DOI: 10.1021/acs.organomet.8b00573 Organometallics XXXX, XXX, XXX−XXX