Organometallics 2009, 28, 953–955
953
Insertion of Dioxygen into a Platinum(II)-Methyl Bond To Form a Platinum(II) Methylperoxo Complex Kyle A. Grice and Karen I. Goldberg* Department of Chemistry, Box 351700, UniVersity of Washington, Seattle, Washington 98195-1700 ReceiVed NoVember 25, 2008 Summary: The platinum(II) complex (PN)PtMe2 (1; PN ) 2-((ditert-butylphosphino)methyl)pyridine) reacts with molecular oxygen in benzene or methylene chloride solutions to form (PN)PtMe(OOMe) (2), a platinum(II) methylperoxo complex. The structure of the methylperoxo species 2 was determined by X-ray crystallography. The last several decades have seen significant progress in the area of homogeneous, transition metal catalyzed alkane functionalization reactions.1 Notably, several of the reported advances have involved platinum species as catalysts.2 This has led to an increased level of research in the area of C-H bond activation by platinum complexes, and the number of platinum complexes that have been shown to stoichiometrically activate C-H bonds to form stable alkyl and aryl complexes has grown dramatically in the last several years.3 Particularly desirable in catalytic alkane functionalization and other oxidations would be the use of molecular oxygen as an oxidant,4,5 since molecular oxygen is inexpensive, abundant, and environmentally benign. In work toward achieving platinumcatalyzed alkane functionalization with oxygen as the oxidant, several examples of stoichiometric reactions of platinum alkyl complexes with oxygen have recently been reported.6,7 Platinum(II) dialkyl species can be oxidized in protic solvents to platinum(IV) hydroxide complexes, with Pt(IV) hydroperoxide species having been detected as intermediates in some cases.6c The reaction of platinum(IV) dihydrocarbyl hydride species with * To whom correspondence should be addressed. E-mail: goldberg@ chem.washington.edu. (1) (a) Crabtree, R. H. Dalton Trans. 2001, 2437. (b) Labinger, J. A.; Bercaw, J. E. Nature 2002, 417, 507. (c) Goldman, A., Goldberg, K. I., Eds. ActiVation and Functionalization of C-H Bonds; American Chemical Society: Washington, DC, 2004. (d) Periana, R. A.; Bhalla, G.; Tenn, W. J., III.; Young, K. J. H.; Liu, X. Y.; Mironov, O.; Jones, C.; Ziatdinov, V. R. J. Mol. Catal. A: Chem. 2004, 220, 7. (2) (a) Shilov, A. E.; Shulpin, G. B. ActiVation and Catalytic Reactions of Saturated Hydrocarbons in the Presence of Metal Complexes, Kluwer: Boston, MA, 2000. (b) Periana, R. A.; Taube, D. J.; Gamble, S.; Taube, H.; Satoh, T.; Fujii, H. Science 1998, 280, 560. (c) Yamakawa, T.; Fujita, T.; Shinoda, S. Chem. Lett. 1992, 905. (3) (a) Fekl, U.; Goldberg, K. I. AdV. Inorg. Chem. 2003, 54, 259. (b) Lersch, M.; Tilset, M. Chem. ReV. 2005, 105, 2471. (c) Vedernikov, A. N. Curr. Org. Chem. 2007, 11, 1401. (4) (a) Bar-Nahum, I.; Khenkin, A. M.; Neumann, R. J. Am. Chem. Soc. 2004, 126, 10235. (b) Weinberg, D. R.; Labinger, J. A.; Bercaw, J. E. Organometallics 2007, 26, 167, and references therein. . (5) (a) Stahl, S. S. Angew. Chem., Int. Ed. 2004, 43, 3400. (b) Stahl, S. S. Science 2005, 309, 1824. (c) Gligorich, K. M.; Sigman, M. S. Angew. Chem., Int. Ed. 2006, 46, 6612. (d) Bercaw, J. E.; Hazari, N.; Labinger, J. A. J. Org. Chem. 2008, 73, 8654. (6) (a) Rostovtsev, V. V.; Labinger, J. A.; Bercaw, J. E.; Lasseter, T. L.; Goldberg, K. I. Organometallics 1998, 17, 4530. (b) Prokopchuk, E. M.; Jenkins, H. A.; Puddephatt, R. J. Organometallics 1999, 18, 2861. (c) Rostovtsev, V. V.; Henling, L. M.; Labinger, J. A.; Bercaw, J. E. Inorg. Chem. 2002, 41, 3608. (d) Vedernikov, A. N.; Binfield, S. A.; Zavalij, P. Y.; Khusnutdinova, J. R. J. Am. Chem. Soc. 2006, 128, 82. (7) (a) Wick, D. D.; Goldberg, K. I. J. Am. Chem. Soc. 1999, 121, 11900. (b) Look, J. L.; Wick, D. D.; Mayer, J. M.; Goldberg, K. I. Inorg. Chem.,in press.
molecular oxygen to form stable Pt(IV) hydroperoxide complexes has also been reported.7 Notably, in each of these reactions, oxygen did not react with the alkyl or aryl group on the metal. Instead, either oxidation of the Pt(II) center or, if the starting complex was a Pt(IV) dihydrocarbyl hydride, insertion of dioxygen into the Pt-hydride bond was observed. Despite the perception that many organometallic complexes are sensitive to oxygen, there are relatively few examples in the literature of late transition metal-alkyl groups reacting with molecular oxygen. In comparison, there have been numerous reports of the reaction of molecular oxygen with early metaland main group metal-alkyl bonds to form metal peroxides or alkoxides and eventually alcohols.8 However, for the late metals, the precedent is limited to the first row, where several examples of insertion of oxygen into Co-alkyl or Ni-alkyl bonds to generate alkylperoxo species have been reported.9,10 The lack of reports of this type of reactivity with second- and third-row late transition metals is interesting, as these heavier metals are known to activate the C-H bonds of alkanes to generate stable metal alkyls.1-3 The insertion of molecular oxygen into an alkane-derived metal-alkyl bond could have significant potential in developing new catalytic alkane functionalization reactions using molecular oxygen. In this communication, we report the synthesis of a new platinum(II) dimethyl complex, (PN)PtMe2 (1; PN ) 2-((ditert-butylphosphino)methyl)pyridine), which reacts with dioxygen to form a platinum methylperoxo species, 2, illustrating the first example of insertion of molecular oxygen into a platinum-alkyl bond. (8) For selected examples of oxygen insertion into main group metaland early-transition-metal-alkyl bonds, see the following and references within: (a) Davies, A. G.; Roberts, B. P. J. Chem. Soc. B 1968, 1074. (b) Davies, A. G.; Ingold, K. U.; Roberts, B. P.; Tudor, R. J. Chem. Soc. B 1971, 698. (c) Brindley, P. B.; Scotton, M. J. J. Chem. Soc., Perkin Trans. 2 1981, 419. (d) Rensch, R.; Friebolin, H. Chem. Ber. 1977, 110, 2189. (e) Ryan, D. A.; Espenson, J. H. J. Am. Chem. Soc. 1982, 104, 704. (f) Lubben, T. V.; Wolczanski, P. T. J. Am. Chem. Soc. 1987, 109, 424. (g) Cleaver, W. M.; Barron, A. R. J. Am. Chem. Soc. 1989, 111, 8966. (h) Han, R.; Parkin, G. J. Am. Chem. Soc. 1990, 112, 3662. (i) Atkinson, J. M.; Brindley, P. B. J. Organomet. Chem. 1991, 411, 131. (j) Atkinson, J. M.; Brindley, P. B. J. Organomet. Chem. 1991, 411, 139. (k) Power, M. B.; Cleaver, W. M.; Apblett, A. W.; Barron, A. R.; Ziller, J. W. Polyhedron 1992, 11, 477. (l) Lewinski, J.; Zachara, J.; Gos, P.; Grabska, E.; Kopec, T.; Madura, I.; Marciniak, W.; Prowotorow, I. Chem. Eur. J. 2000, 6, 3215. (m) Mo¨ller, M.; Husemann, M.; Boche, G. J. Organomet. Chem. 2001, 624, 47. (n) Bailey, P. J.; Coxall, R. A.; Dick, C. M.; Fabre, S.; Henderson, L. C.; Herber, C.; Liddle, S. T.; Lorono-Gonzalez, D.; Parkin, A.; Parsons, S. Chem. Eur. J. 2003, 9, 4820. (o) Lewinski, J.; Sliwinski, W.; Dranka, M.; Justyniak, I.; Lipkowski, J. Angew. Chem., Int. Ed. 2006, 45, 4826. (9) For oxygen insertion into Co(III) alkyl complexes, see: (a) Jensen, F. R.; Kiskis, R. C. J. Am. Chem. Soc. 1975, 97, 5825. (b) Deniau, J.; Gaudemer, A. J. Organomet. Chem. 1980, 191, C1. (c) Kendrick, M. J.; Al-Akhdar, W. Inorg. Chem. 1987, 26, 3971. (d) Sauer, A.; Cohen, H.; Meyerstein, D. Inorg. Chem. 1989, 28, 2511. (e) Gupta, B. D.; Kanth, V. V.; Singh, V. J. Organomet. Chem. 1998, 570, 1. (f) Dutta, G.; Moitree, L.; Gupta, B. D. Organometallics 2008, 27, 3338. (10) Observation of a Ni(III)-OOCH3 complex: Sauer, A.; Cohen, H.; Meyerstein, D. Inorg. Chem. 1988, 27, 4578.
10.1021/om8011272 CCC: $40.75 2009 American Chemical Society Publication on Web 01/20/2009
954 Organometallics, Vol. 28, No. 4, 2009
Communications
Scheme 1. Synthesis of Pt(II) Complex 1
Reaction of the known bidentate ligand 2-((di-tert-butylphosphino)methyl)pyridine11 (3) with the Pt(II) dimethyl complex [(µ-SEt2)PtMe2]212 in toluene at 100 °C for 90 min, followed by removal of the volatiles and washing with cold pentane, yields compound 1 as a white solid (Scheme 1). The 1H NMR spectrum of 1 in CD2Cl2 at 298 K contains one signal for the tert-butyl protons and one for the methylene protons, as well as four for the inequivalent pyridyl protons on the ligand, the furthest downfield of which has 195Pt satellites. The presence of two inequivalent Pt-Me groups is evident from the Pt-CH3 signals of equal intensity (0.84 and 0.41 ppm), both with coupling to one 31P nucleus as well to 195Pt (3JP-H ) 6.1 Hz, 2JPt-H ) 87.3 Hz and 3JP-H ) 7.2 Hz, 2JPt-H ) 64.2 Hz, respectively). The 31P{1H} NMR spectrum of 1 shows one signal with 195Pt satellites characteristic of a phosphorus trans to a methyl on Pt(II) (64.3 ppm, 1JPt-P ) 1970 Hz).13 Similar spectral data are obtained in C6D6. In order to verify the structure, X-ray-quality crystals of 1 were grown by slow evaporation of a THF solution of 1.14 The Pt-C bond lengths for the two methyl groups are within the expected range for a Pt(II)-Me moiety.15 The Pt-C bond trans to the phosphorus is longer (2.087(7) Å) than that trans to nitrogen (2.038(8) Å), as would be expected based on trans influence arguments. When a solution of 1 in CD2Cl2 or C6D6 was pressurized with 5 atm of O2 and the sample was monitored by 1H NMR spectroscopy, signals for a new platinum species were observed over the course of hours at room temperature.16 The ligand signals for this new platinum complex are similar to those of 1, but only one Pt-Me signal was apparent (0.86 ppm, 2JPt-H ) 75.4 Hz in CD2Cl2) with no resolvable coupling to 31P. A singlet with no coupling to 195Pt or to 31P and accounting for 3H by integration was observed at 3.75 ppm in CD2Cl2. A singlet for the new species appeared in the 31P{1H} NMR spectrum (50.9 ppm, 1JPt-P ) 3740 Hz in CD2Cl2) with coupling to 195Pt indicative of phosphorus trans to a weaker trans influence ligand than the alkyl group found in 1. Similar spectral data for this complex were obtained in C6D6. The identity of the new species was established by X-ray crystallography as (PN)PtMe(OOMe) (2), the product of insertion of molecular oxygen into a Pt-Me bond (Scheme 2). X-rayquality crystals were grown by vapor diffusion of pentane into a CD2Cl2 solution of 2 at -25 °C. The ORTEP diagram of 2 is shown in Figure 1, with selected bond lengths and angles listed in the figure caption. The characterization of 2 as a Pt methylperoxo rather than a Pt methoxo species is consistent (11) Edwards, P. G.; Fallis, I. A.; Yong, B. S. Preparation of heterocyclecontaining phosphines and corresponding palladium complexes and use there of in palladium catalyzed coupling reactions. British Patent Application GB 2378182 A, 2003. (12) Bancroft, D. P.; Cotton, F. A.; Falvello, L. R.; Schwotzer, W. Inorg. Chem. 1986, 25, 763. (13) (a) Mhinzi, G. S.; Litster, S. A.; Redhouse, A. D.; Spencer, J. L. J. Chem. Soc., Dalton Trans. 1991, 2769. (b) Achar, S.; Catalano, V. J. Polyhedron 1997, 16, 1555. (c) Haar, C. M.; Nolan, S. P.; Marshall, W. J.; Moloy, K. G.; Prock, A.; Giering, W. P. Organometallics 1999, 18, 474. (14) See the Supporting Information. (15) See refs 13b,c. (16) A high pressure of oxygen (5 atm) was used to ensure that a sufficient concentration of oxygen would be available in solution throughout the reaction. See safety note in the Supporting Information.
Figure 1. ORTEP diagram of 2 with ellipsoids at the 50% probability level. Hydrogen atoms are excluded for clarity. Selected bond lengths (Å) and angles (deg): Pt1-P1 ) 21830(18), Pt1-N1 ) 2.111(5), Pt1-C15 ) 2.055(7), Pt1-O1 ) 2.083(5), O1-O2 ) 1.502(8), O2-C16 ) 1.403(10); N1-Pt1-P1 ) 84.59(17), P1-Pt1-C15 ) 99.7(2), C15-Pt1-O1 ) 89.0(3), O1-Pt1-N1 ) 86.8(2), Pt1-O1-O2 ) 111.3(4), O1-O2-C16 ) 104.3(6). Scheme 2. Reaction of 1 with O2 To Form 2
with the absence of 195Pt satellites for the signal assigned to the oxygenated methyl group of 2 in the 1H NMR spectrum.17 Notably, only one isomer of 2, that with the methylperoxo ligand trans to phosphorus, is observed in the crystal. This is consistent with the evidence for the solution structure, since only one signal is observed in the 31P{1H} NMR spectrum and the 31P-195Pt coupling is characteristic of phosphorus trans to a ligand with a weaker trans influence than that of an alkyl group. There are no reports of platinum methylperoxo species in the Cambridge Crystallographic Database,18 and the closest structurally characterized platinum(II) analogue to 2 is a tertbutylperoxo species, trans-(PPh3)2PtPh(OOtBu).19 Unfortunately, the peroxy group in the structure of this tert-butylperoxo complex was disordered, making comparison of the structure to that of 2 challenging. There is a structurally characterized Pt(IV) isopropylperoxo complex in the literature.20 This Pt(IV) isopropylperoxo group has Pt-O and O-O bonds that are somewhat shorter (by 0.05 and 0.04 Å, respectively) than those in 2. The O-C bond in the isopropylperoxo group is slightly longer than that in the methylperoxo group (by 0.05 Å). (17) Monomeric Pt(II) and Pt(IV) methoxide complexes typically show JPt-H values of ca. 20-60 Hz for the 195Pt coupling to the Pt-OCH3 signals in the 1H NMR spectrum. For examples, see: (a) Bryndza, H. E.; Tam, W. Chem. ReV. 1988, 88, 1163, and references therein. (b) Ref 6c. (c) Smythe, N. A.; Grice, K. A.; Williams, B. S.; Goldberg, K. I. Organometallics 2009, 28, 277. (18) CSD version 5.29 (Nov 2007), Conquest 1.10 (Build 3), Copyright CCDC 2008. Bruno, I. J.; Cole, J. C.; Edgington, P. R.; Kessler, M.; Macrae, C. F.; McCabe, P.; Pearson, J.; Taylor, R. Acta Crystallogr. 2002, B58, 389 Consulted on Nov 11, 2008. . (19) Strukul, G.; Michelin, R. A.; Orbell, J. D.; Randaccio, L. Inorg. Chem. 1983, 22, 3706. (20) Ferguson, G.; Monaghan, P. K.; Parvez, M.; Puddephatt, R. J. Organometallics 1985, 4, 1669. 3
Communications
When the reaction of 1 with O2 (5 atm) was carried out in CD2Cl2 in ambient light at room temperature, the product 2 formed in 64-79% yield, as determined by integration against an internal standard in the 1H NMR spectrum. No intermediates were observed by 1H or 31P NMR spectroscopy as the reaction progressed. One additional Pt species (not an intermediate, vide infra) was observed by NMR spectroscopy to form over the course of the reaction and was present in 13-20% yield at the end of the reaction. The 1H NMR spectrum of this species has one Pt-Me signal (0.92 ppm, 2JPt-H ) 72 Hz) and a full set of ligand signals. As the reaction progressed, the solution became slightly cloudy, suggesting precipitation of a product. This insoluble material could account for the remaining 4-22% of Pt-containing species that were not detected in the NMR spectra. No decomposition to platinum black was observed. The platinum complex detected as a minor product in the reaction of 1 with O2 was identified as (PN)PtMe(Cl) (4) by X-ray crystallography.14 When left to stand at room temperature in CD2Cl2, the methylperoxo complex 2 was observed to convert over a period of days to complex 4, enabling isolation and characterization of this species. The thermal conversion of 2 to 4 confirms that complex 4 is not an intermediate in the formation of 2. Notably, when complex 1 stood in CD2Cl2 at room temperature in the absence of oxygen overnight, no conversion to species 4 was observed. The reaction of 1 with O2 was noticeably faster when exposed to light than when the experiments were carried out in the absence of light. In ambient light, the reaction of 1 (24 mM) with 5 atm of O2 in CD2Cl2 proceeded to completion at room temperature with reaction times ranging from 7-11 h. When the same reaction was carried out in a NMR tube wrapped in foil, the reaction was only 58% complete after 9.5 h. When the reaction (in ambient light) was carried out in the presence of 4-methyl-2,6-di-tert-butylphenol (BHT, 8 mM, 0.3 equiv), a known radical inhibitor, only 29% conversion of 1 was noted after 11 h. A parallel experiment without added BHT showed
Organometallics, Vol. 28, No. 4, 2009 955
complete conversion by 11 h. The effects of light and BHT on these reactions are consistent with a radical mechanism. Notably, radical chain mechanisms have been implicated in the oxygen insertion reactions of early metal- and main group metal-alkyls.21 Experiments are currently underway to investigate the mechanism of this platinum-methyl oxygen insertion reaction in greater detail. In summary, the Pt(II) dimethyl compound 1 reacts with oxygen to form a novel Pt(II) methylperoxo species, 2. The structure of 2 has been confirmed by X-ray crystallography. To our knowledge, 2 is the first example of a platinum methylperoxo complex and is a rare example of a structurally characterized platinum(II) alkylperoxo complex. This observation of insertion of oxygen into a Pt-alkyl bond is of special interest, as numerous Pt complexes are known to form stable metal-alkyl complexes through alkane C-H activation.3 Investigations of the mechanism of this unusual oxygen insertion reaction are planned, as are studies of the reactivity of 2.
Acknowledgment. We thank the NSF (Grant No. CHE00719372) for funding. In addition, Rodney Swartz, John Freudenthal, and Dr. Werner Kaminsky are acknowledged for collecting crystallographic data and solving the structures of 1, 2, and 4. Dr. Luc Boisvert is acknowledged for insightful suggestions and discussion, and Dr. Susan KloekHanson is acknowledged for synthesis of 3. Supporting Information Available: Text and figures giving experimental information and spectral data and CIF files giving crystal data for 1, 2, and 4. This material is available free of charge via the Internet at http://pubs.acs.org. OM8011272 (21) See refs 8a,b,f for examples of systems in which radical chain mechanisms have been implicated.
