A Bridging Peroxide Complex of Platinum(IV) - Inorganic Chemistry

Article Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX

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A Bridging Peroxide Complex of Platinum(IV) S. Jafar Hoseini,*,† Roghayeh Hashemi Fath,‡ Mahmood A. Fard,§ Ava Behnia,§ and Richard J. Puddephatt*,§ †

Professor Rashidi Laboratory of Organometallic Chemistry, Department of Chemistry, College of Sciences, Shiraz University, Shiraz 7194684795, Iran ‡ Department of Chemistry, Faculty of Sciences, Yasouj University, Yasouj 7591874831, Iran § Department of Chemistry, University of Western Ontario, London N6A 5B7, Canada Downloaded via TUFTS UNIV on July 19, 2018 at 12:49:40 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

S Supporting Information *

ABSTRACT: The photolysis of the allylplatinum(IV) complex [PtBr(C3H5)(4-MeC6H4)2(bipy)], 1, bipy = 2,2′-bipyridine, in air yielded [{PtBr(4-MeC6H4)2(bipy)}2(μ-O2)], 2, the first diplatinum(IV) complex containing a single bridging peroxide ligand. The PtO− OPt bond distance in 2 is 1.481(3) Å. Complex 2 is thought to be formed by homolysis of the allyl-platinum bond of 1, followed by reaction of the platinum(III) intermediate [PtBr(4-MeC6H4)2(bipy)] with oxygen.

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INTRODUCTION

RESULTS AND DISCUSSION As part of a recent study on oxidative addition of allyl bromide to the ditolylplatinum(II) complex [Pt(4-MeC6H4)2(bipy)], the allylplatinum(IV) complex [PtBr(C3H5)(4-MeC6H4)2(bipy)], 1, was prepared.10 In an attempt to recrystallize complex 1 from CH2Cl2/hexane in the open laboratory, an unexpected reaction occurred, and crystals containing the new peroxide [{PtBr(4-MeC6H4)2(bipy)}2(μ-O2)], 2, were formed instead. A systematic study then established that solutions of 1 in dichloromethane in the dark were stable, but that reaction with oxygen to give 2 occurred on exposure to light. Complex 2 is very sparingly soluble in dichloromethane, so it crystallized from solution as it formed over a photolysis period of 2 d. The reaction is more complex than shown in Scheme 2. Crystals of 2 were pink, and they were separated under the microscope and isolated in 25% yield. Also formed was the known compound [PtBr2(bipy)], 3, which formed yellow crystals in ∼10% yield and other unidentified material.11 Parallel studies of the photolysis of complex 1 in CD2Cl2 solution in an NMR tube in the presence or absence of air were performed. In air, the same crystals formed, and the 1H NMR spectrum identified the major organic products as 1,5-hexadiene (formed by dimerization of allyl radicals), allyl alcohol (formed by reaction of allyl radicals with oxygen), and 4,4-dimethylbiphenyl (formed by coupling of two 4-tolyl groups).12,13 Under nitrogen, 1,5-hexadiene and 4,4-dimethylbiphenyl were again

The vital role of platinum catalysis in oxidation reactions has stimulated much research on the reactions of organoplatinum compounds with molecular oxygen.1−8 This research is especially topical because of continuing efforts to develop selective hydrocarbon oxidation chemistry using platinum complexes as catalyst and oxygen or hydrogen peroxide as oxidant.1,2 Platinum(0) complexes often react rapidly with oxygen to give complexes, such as [Pt(PPh3)2(O2)], A, Scheme 1, which are considered as chelating peroxide complexes of platinum(II).3 Analogous η2-peroxide complexes of platinum(IV) are not known, but several other peroxide derivatives of platinum(IV) have been reported (Scheme 1).4−9 Hydroperoxide complexes such as B can be prepared by insertion of oxygen into the Pt(IV)−H bond or by oxidation of platinum(II) complexes in the presence of a proton donor.4 Alkyl peroxide complexes such as C can be prepared, for example, by oxidative addition of an alkyl halide to platinum(II) in the presence of oxygen.5 Superoxide complexes of platinum(IV), such as the anionic [Pt2(μ-O2)(μ-OH)(OH)8]3−, are also known.9 Surprisingly, there is only one welldefined peroxide complex of platinum(IV), namely, the complex D (Scheme 1), which contains two bridging peroxide groups and which is formed by reaction of oxygen with a platinum(II) precursor complex in the presence of triazacyclononane. This article reports a unique diplatinum(IV) complex with a single peroxide bridge. © XXXX American Chemical Society

Received: April 5, 2018

A

DOI: 10.1021/acs.inorgchem.8b00888 Inorg. Chem. XXXX, XXX, XXX−XXX

Article

Inorganic Chemistry Scheme 1. Some Peroxide Complexes of Platinuma

Figure 1. 1H NMR spectrum (600 MHz, DMSO-d6) of complex 2 in the aryl region at 25 °C (below) and 60 °C (above).

HB(NN)3 = tris(3,5-dimethylpyrazolyl)borate in B.

a

Scheme 2. Synthesis of Complex 2

Figure 2. Structure of complex 1, showing 30% probability ellipsoids. Selected bond distances: Pt(1)C(1) 2.103(4), Pt(1)C(4) 2.029(4), Pt(1)C(11) 2.030(5), Pt(1)N(1) 2.146(4), Pt(1)N(2) 2.155(4), Pt(1)Br(1) 2.6009(14), C(1)C(2) 1.467(6), C(2)C(3) 1.307(7) Å.

formed, and [PtBr2(bipy)] was formed along with unidentified platinum complexes. The absence of 4-allyltoluene and p-cresol from the photolysis products may indicate that the 4,4dimethylbiphenyl formation does not occur by radical coupling. The 1H NMR spectrum of complex 2 in deuterated dimethyl sulfoxide (DMSO-d6) is shown in Figure 1. There is a single set of peaks for the protons of the 2,2′-bipyridine ligands, as expected. At room temperature, a resonance for the meta protons (H3a) of the 4-tolyl groups was observed, but no resonance for the ortho protons (H2a) was observed. At 60 °C, a broad resonance for these H2a protons was present, and the resonance for the H3a protons was sharper and appeared as the expected doublet. It was not possible to obtain a limiting lowtemperature spectrum in DMSO-d6, but clearly these unusual observations arise because there is a barrier to rotation about the Pt−C bonds of the 4-tolyl groups. The structures of complexes 1, 2·2CH2Cl2, and 3·CH2Cl2 were determined and are shown in Figures 2, 3, and S1. Complex 1 has the expected structure arising from trans oxidative addition of allyl bromide.10 The allyl−platinum bond is significantly longer than the aryl−platinum bonds (Figure 2). In the structure of the peroxide complex 2 (Figure 3) each platinum atom has octahedral stereochemistry, and there is a

Figure 3. Structure of complex 2, showing 30% probability ellipsoids. Selected bond parameters: Pt(1)O(1) 2.011(2), Pt(1)Br(1) 2.5034(9), O(1)O(1a) 1.481(3), Pt(1)C(1) 2.022(3), Pt(1)C(8) 2.029(3), Pt(1)N(1) 2.172(2), Pt(1)N(2) 2.164(3) Å; Pt(1)O(1)O(1A) 106.5(1)°. Symmetry equivalent atoms: x, y, z; 1 − x, 1 − y, 1 − z.

crystallographic inversion center at the midpoint of the O−O bond. The Pt−Br distance is shorter than in 1, as a result of the lower trans influence of the peroxide group in 2 compared to the allyl group in 1 (Figures 2 and 3). The O−O distance of 1.481(3) Å is in the expected range for peroxide complexes but significantly longer than in the complex D (Scheme 1), which B

DOI: 10.1021/acs.inorgchem.8b00888 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry has O−O = 1.39(1) Å,6 superoxides (O−O ca. 1.33 Å), or dioxygen (O = O 1.21 Å).1,14 A probable mechanism of formation of complex 2 is illustrated in Scheme 3 and Figure 4. Complex 1 is colorless

other compounds derived from initially formed allylperoxy radicals (only the dimerization is shown in Scheme 3, for simplicity).12 The platinum radical E can react with oxygen to form the radical F, and combination of E and F can then give the product 2. The calculated energies (Figure 4) of all reactions following the initial photochemical step are favorable. Figure 5 shows some key molecular orbitals. For complex 2 the HOMO has mostly π*(OO) character, while the LUMO

Scheme 3. A Probable Mechanism of Formation of Complex 2

Figure 5. Calculated frontier orbitals for complex 2 and spin density for E and F.

has π*(bipy) character. The UV−visible spectrum of 2 contained intense bands at ∼300 nm, attributed to π−π* transitions, with a shoulder that stretched into the visible region, but no distinct band could be attributed to the HOMO−LUMO transition. In complex 2 the peroxide group is sterically protected from external attack, and this is probably important in enhancing its thermal stability. Cyclic voltammetry (CV) of a solution of 2 in dimethyl sulfoxide showed an oxidation at ∼0.62 V versus standard calomel electrode, corresponding to loss of one electron from the π*(OO) molecular orbital (MO) to form a superoxide complex, but the CV indicates that the oxidation is not fully reversible, so isolation of a bridging superoxide complex is unlikely. The calculated spin density for proposed intermediate complexes E and F indicated that the unpaired electron in E is in an orbital with Pt(5dz2)-Br(4pz) antibonding character, while in F it is in a molecular orbital with mostly π*(OO) character (Figure 5).

Figure 4. Calculated structures and relative energies (kJ mol−1) for the compounds in Scheme 3 (C3H5 = allyl radical, C6H10 = biallyl). Selected distances: 2, Pt−O 2.08, Pt−Br 2.58, O−O 1.56; E, Pt−Br 2.73; F, Pt−O 2.13, Pt−Br 2.58, O−O 1.41 Å.

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and absorbs only in the UV region (λmax 300 nm, ε 4200 dm3 mol−1 cm−1). The density functional theory (DFT) calculations indicate that the highest occupied molecular orbital (HOMO) has mostly n(Br,4p) character, that the lowest unoccupied molecular orbital (LUMO) has mostly π*(bipy) character, and that the lowest energy transition is expected to have mostly HOMO to LUMO character. The highest occupied orbital with σ(allyl−platinum) character lies close in energy to a series of filled π-orbitals associated with the unsaturated tolyl and bipy groups. It is not surprising, therefore, that the photolytic cleavage of the allyl−platinum bond is not efficient. Product analysis indicates that the primary photochemical step gives homolysis of the allylplatinum bond, to form the allyl radical and platinum(III) complex E. The allyl radical can then either dimerize to form biallyl, C6H10, or react with oxygen eventually to give allyl alcohol and

CONCLUSIONS

The first diplatinum(IV) complex, 2, containing a single bridging peroxide group is reported. Together with the complex D (Scheme 1), which contains two peroxide groups, it may act as a paradigm for future studies of bridging peroxide complexes of platinum(IV).6 Complex D was prepared by direct oxidation of a platinum(II) precursor by oxygen,6 while 2 was prepared by reaction of a platinum(IV) complex with oxygen. This new reaction is proposed to occur by a mechanism involving an initial photochemical step to form a five-coordinate platinum(III) radical complex E followed by reaction with oxygen to give F and then combination of E and F to give 2. The structure confirms that 2 is a peroxide, having a typical O−O single-bond distance. C

DOI: 10.1021/acs.inorgchem.8b00888 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

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EXPERIMENTAL SECTION

Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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Complex 1 was prepared as reported previously, and single crystals were grown from CH2Cl2/hexane. NMR spectra were recorded using Inova 400 and Inova 600 spectrometers. UV−vis absorption spectra were measured with a Varian Cary 100 spectrophotometer. DFT calculations were performed by using the Amsterdam Density Functional program based on the BP functional, with double-ζ basis set and first-order scalar relativistic corrections.15 Single-crystal X-ray diffraction measurements were made using a Bruker APEX-II CCD diffractometer with graphite-monochromated Mo Kα (λ = 0.710 73 Å) radiation. Single crystals of the complexes were immersed in paraffin oil and mounted on MiteGen micromounts. The structures were solved using direct methods and refined by the full-matrix leastsquares procedure of SHELXTL.16 Crystallographic data are given in Table S1, and more complete X-ray data are given in the CIF files (CCDC 1833335−1833337). CV experiments were performed using a BAS 100B\W Electrochemical Analyzer, using a glassy carbon (GC, Tokai GC20) working electrode and platinum wires as the reference and counter electrodes. Electrochemical experiments were performed in 1:1 CH2Cl2−DMSO solvent containing 0.1 M tetrabutylammonium hexafluorophosphate as the supporting electrolyte. Potentials are referenced internally to ferrocene (0.342 V vs standard calomel electrode (SCE)) added at the end of the experiments. [{PtBr(4-C6H4Me)2(bipy)}2(μ-O2)], 2. A solution of [PtBr(4C6H4Me)2(CH2CHCH2)(bipy)], 1, (0.10 g, 0.15 mmol) in CH2Cl2 (5 mL) in a Pyrex flask under air was irradiated by UV lamp for 48 h to yield a yellow solution and a mixture of block crystals of [{PtBr(4-C6H4Me)2(bipy)}2(μ-O2)], 2, and plate yellow crystals of [PtBr2(bipy)], 3 (as the dichloromethane solvates). The solution was decanted, and the crystals were washed with pentane (2 × 5 mL). The pink crystals of 2 were separated manually under the microscope. Yield: 0.024 g, 25%. Anal. Calcd for C48H44Br2N4O2Pt2: C, 45.80; H, 3.52; N, 4.45. Found: C, 45.47; H, 3.31; N, 4.16%. NMR in DMSOd6: δ(1H) (600 MHz, 60 °C) = 8.63 (d, 4H, 3J(HH) = 5 Hz, H6), 8.21 (t, 4H, 3J(HH) = 7 Hz, H4), 8.16 (d, 4H, 3J(HH) = 7 Hz, H3), 7.67 (dd, 4H, 3J(HH) = 5 Hz, 7 Hz, H5), 6.73 (br, 8H, H2a), 6.48 (d, 8H, 3J(HH) = 7 Hz, H3a), 2.19 (s, 12H, Me); δ(13C) (150 MHz, 25 °C) = 55.2 (C2), 147.7 (C3), 140.1 (C4), 132.3 (C4a), 127.3 (C5, C1a, C2a), 127.2 (C3a), 126.1 (C2a), 124.4 (C6), 20.8 (Me). UV− vis in 1:1 CH2Cl2−DMSO: λmax 300 nm, ε = 2450 dm3 mol−1 cm−1. Simultaneously with the above experiment, a solution of complex 1 (0.020 g) in CD2Cl2 (1 mL) in an NMR tube was irradiated under oxygen, under air, or under nitrogen atmosphere. The organic products 1,5-hexadiene, allyl alcohol, and 4,4′-ditolyl and platinum product [PtBr2(bipy)] were identified by comparison of their 1H NMR spectra with those of authentic compounds,11,12 with NMR yields of these products 15%, 55%, 45%, 20% under oxygen; 44%, 40%, 56%, 21% under air; 74%, 0%, 44%, 20% under nitrogen. The yield of [PtBr2(bipy)] is a minimum value, since some precipitated. It was not possible to determine the NMR yield of the insoluble product 2.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. (R.J.P.) *E-mail: [email protected]. (S.J.H.) ORCID

Richard J. Puddephatt: 0000-0002-9846-3075 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank the NSERC (Canada) for financial support. DEDICATION Dedicated to the memory of Professor Mehdi Rashidi, a master of organoplatinum chemistry. REFERENCES

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00888. Illustrated structure of 3·CH2Cl2, UV−vis spectrum and CV data of complex 2, tabulated X-ray crystallographic data for complexes 1−3 (PDF) Atomic coordinates from DFT calculations (XYZ) Accession Codes

CCDC 1833335−1833337 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 D

DOI: 10.1021/acs.inorgchem.8b00888 Inorg. Chem. XXXX, XXX, XXX−XXX

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Inorganic Chemistry

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(h) Prantner, J. D.; Kaminsky, W.; Goldberg, K. I. Methylplatinum(II) and Molecular Oxygen: Oxidation to Methylplatinum(IV) in Comp eti tion wit h Me thyl G roup T ransf er To Form Dimethylplatinum(IV). Organometallics 2014, 33, 3227−3230. (i) Keith, J. M.; Ye, Y.; Wei, H.; Buck, M. R. Mechanistic Examination of Aerobic Pt Oxidation: Insertion of Molecular oxygen into Pt−H Bonds Through a Radical Chain Mechanism. Dalton Trans. 2016, 45, 11650−11656. (5) Pt(IV)-O-O-R: (a) Ferguson, G.; Monaghan, P. K.; Parvez, M.; Puddephatt, R. J. Alkylperoxoplatinum(IV) Complexes Formed by Oxidative Addition of Alkyl Halides in the Presence of Oxygen: the Mechanism of Reaction and the Structure of Trans-iodo(isopropylperoxo)dimethyl(1,10-phenanthroline)platinum(IV). Organometallics 1985, 4, 1669−1674. (b) Hill, R. H.; Puddephatt, R. J. A Mechanistic Study of the Photochemically Initiated Oxidative Addition of Isopropyl Iodide to Dimethyl(1,10-phenanthroline)platinum(II). J. Am. Chem. Soc. 1985, 107, 1218−1225. (c) Nabavizadeh, S. M.; Habibzadeh, S.; Rashidi, M.; Puddephatt, R. J. Oxidative Addition of Ethyl Iodide to a Dimethylplatinum(II) Complex: Unusually Large Kinetic Isotope Effects and Their Transition-State Implications. Organometallics 2010, 29, 6359−6368. (d) Scheuermann, M. L.; Fekl, U.; Kaminsky, W.; Goldberg, K. I. Metal−Ligand Cooperativity in O2 Activation: Observation of a “Pt−O−O−C” Peroxo Intermediate. Organometallics 2010, 29, 4749−4751. (e) Scheuermann, M. L.; Luedtke, A. T.; Hanson, S. K.; Fekl, U.; Kaminsky, W.; Goldberg, K. I. Reactions of Five-Coordinate Platinum(IV) Complexes with Molecular Oxygen. Organometallics 2013, 32, 4752−4758. (6) Pt(IV)-O-O-Pt(IV) Davies, M. S.; Hambley, T. W. [Pt2Cl2(μ2O2)2([9]aneN3)2]Cl2: A Novel Platinum(IV) Dimer with Two Bridging Peroxo Ligands that Provides Insight into the Mechanism of Aerial Oxidation of Platinum(II). Inorg. Chem. 1998, 37, 5408− 5409. (7) Other Pt-O-O-H or Pt-O-O-R: (a) Sicilia, V.; Baya, M.; Borja, P.; Martin, A. Oxidation of Half-Lantern Pt2(II,II) Compounds by Halocarbons. Evidence of Dioxygen Insertion into a Pt(III)−CH3 Bond. Inorg. Chem. 2015, 54, 7316−7324. (8) Other Pt-O-O-Pt: (a) Bhaduri, S.; Casella, L.; Ugo, R.; Raithby, P. R.; Zuccaro, C.; Hursthouse, M. B. Zerovalent Platinum Chemistry. Part 11. A Peroxo-bridged Binuclear Platinum Complex Obtained by Protonation of [Pt(O2)(PPh3)2]; the Crystal Structure of [Pt2(O2)(OH)(PPh3)4][ClO4]·2C6H6. J. Chem. Soc., Dalton Trans. 1979, 1624−1629. (b) Kurosawa, H.; Achiha, T.; Kajimaru, H.; Ikeda, I. Formation of μ-Peroxo-platinum Complexes via Attack of Metallic and Related Electrophiles at η2-Dioxygen-platinum Complexes. Inorg. Chim. Acta 1991, 190, 271−277. (c) Bould, J.; Kilner, C. A.; Kennedy, J. D. The Capture of Dioxygen, Carbon Monoxide and Sulfur Dioxide by [(PMe2Ph)4Pt2B10H10]. Dalton Trans. 2005, 1574−1582. (9) Superoxide complexes: (a) Pankratov, D. A.; Dement’ev, A. I.; Kiselev, Y. M. Ab Initio Calculations of Hydroxoplatinum Compounds: II. Binuclear Platinum(IV) Superoxo Complexes. Russ. J. Inorg. Chem. 2008, 53, 247−253. (b) Kiselev, Y. M.; Pankratov, D. A.; Shundrin, L. A.; Kiseleva, I. N. New Platinum Superoxocomplex. Zh. Neorg. Khim. 1996, 41, 2069−2072. (10) Hoseini, S. J.; Fath, R. H.; Hashemian, F.; Rashidi, M. Oxidative Addition Reaction of Allyl or Propargyl Bromide with a Diarylplatinum(II) Complex. J. Organomet. Chem. 2016, 822, 5−12. (11) (a) Momeni, B. Z.; Hamzeh, S.; Hosseini, S. S.; Rominger, F. Hydrogen-halide Versus Alkyl-halide Oxidative Addition in Dimethyl Platinum(II) Complexes: Crystal Structure of [PtBr2(bpy)]. Inorg. Chim. Acta 2007, 360, 2661−2668. (b) Momeni, B. Z.; Moradi, Z.; Rashidi, M.; Rominger, F. Insertion of Tin(II) Bromide into Pt−X (X = Cl, Br) Bond of Dimethylplatinum(IV) Complexes: A Novel Polymorphic form of [PtBr2(bpy)]. Polyhedron 2009, 28, 381−385. (12) (a) Selby, T. M.; Meloni, G.; Goulay, F.; Leone, S. R.; Fahr, A.; Taatjes, C. A.; Osborn, D. L. Synchrotron Photoionization Mass Spectrometry Measurements of Kinetics and Product Formation in the Allyl Radical (H2CCHCH2) Self-Reaction. J. Phys. Chem. A 2008, 112, 9366−9373. (b) Boyd, A. A.; Noziere, B.; Lesclaux, R. Kinetic E

DOI: 10.1021/acs.inorgchem.8b00888 Inorg. Chem. XXXX, XXX, XXX−XXX