Organometallics 2010, 29, 4749–4751 DOI: 10.1021/om1003946
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Metal-Ligand Cooperativity in O2 Activation: Observation of a “Pt-O-O-C” Peroxo Intermediate§ Margaret L. Scheuermann, Ulrich Fekl,† Werner Kaminsky,‡ and Karen I. Goldberg* Department of Chemistry, Box 351700, University of Washington, Seattle, Washington 98195-1700. † Current address: University of Toronto, ON, Canada. ‡X-ray Crystallographic Facility, University of Washington Received April 30, 2010 Summary: The Pt(IV ) complex (tBuMe2nacnac)PtMe3 (1, tBuMe2nacnac=[((4-tBu-2,6-Me2C6H2)NC(CH3))2CH]) reacts with molecular oxygen to form a peroxo complex, 2, in which one oxygen atom is bound to the platinum center and the other to the central carbon of the nacnac ligand backbone. Over time, 2 converts to a new Pt(IV) species (3) wherein the oxygen-oxygen bond has been cleaved. Aryl-substituted β-diketiminate ligands (commonly referred to as “nacnac” ligands) are used extensively in inorganic and organometallic chemistry.1 Complexes containing nacnac ligands have been reported for elements across the periodic table from transition metals to main group elements to lanthanides. These ligands have been found to stabilize metals in unusual oxidation states and/or coordination environments and also to complex metals in active catalytic systems. However, a complication with the use of these ligands is that the central carbon in the β-diketiminate backbone is potentially reactive. For example, formal methyl cation migration to the central carbon has been observed for a nacnac Pt(IV) trimethyl complex.2 Another well-documented reaction involving this site is cycloaddition to form a ring structure involving the central carbon of the nacnac ligand, the metal center, and an exogenous multiple bond.3 Cycloadditions with alkenes, alkynes, ketenes, ketones, phosphaalkynes, and diazo compounds to form bicyclic structures have § Part of the Dietmar Seyferth Festschrift. The authors dedicate this contribution to Professor Dietmar Seyferth in appreciation of his many years of distinguished service to Organometallics. *To whom correspondence should be addressed. E-mail: goldberg@ chem.washington.edu. (1) For reviews on β-diketiminate chemistry see: (a) Bourget-Merle, L.; Lappert, M. F.; Severn, J. R. Chem. Rev. 2002, 102, 3031. (b) Mindiola, D. J. Angew. Chem., Int. Ed. 2009, 48, 6198. (c) Holland, P. L. Acc. Chem. Res. 2008, 41, 905. (d) Roesky, H. W.; Singh, S.; Jancik, V.; Chandrasekhar, V. Acc. Chem. Res. 2004, 37, 969. (e) Piers, W. E.; Emslie, D. J. H. Coord. Chem. Rev. 2002, 233-234, 131. (2) Fekl, U.; Kaminsky, W.; Goldberg, K. I. J. Am. Chem. Soc. 2001, 123, 6423. (3) (a) Radzewich, C. E.; Coles, M. P.; Jordan, R. F. J. Am. Chem. Soc. 1998, 120, 9384. (b) West, N. M.; White, P. S.; Templeton, J. L.; Nixon, J. F. Organometallics 2009, 28, 1425. (c) Phillips, A. D.; Laurenczy, G.; Scopelliti, R.; Dyson, P. J. Organometallics 2007, 26, 1120. (d) Yao, S.; van Wullen, C.; Driess, M. Chem. Commun. 2008, 5393. (e) Gregory, E. A.; Lachicotte, R. J.; Holland, P. L. Organometallics 2005, 24, 1803. (f) Basuli, F.; Huffman, J. C.; Mindiola, D. J. Inorg. Chem. 2003, 42, 8003. (g) Carey, D. T.; Cope-Eatough, E. K.; Vilaplana-Mafe, E.; Mair, F. S.; Pritchard, R. G.; Warren, J. E.; Woods, R. J. Dalton Trans. 2003, 1083. (4) For example see: (a) Aboelella, N. W.; Kryatov, S. V.; Gherman, B. F.; Brennessel, W. W.; Young, V. G., Jr.; Sarangi, R.; RybakAkimova, E. V.; Hodgson, K. O.; Hedman, B.; Solomon, E. I.; Cramer, C. J.; Tolman, W. B. J. Am. Chem. Soc. 2004, 126, 16896. (b) Dai, X.; Kapoor, P.; Warren, T. H. J. Am. Chem. Soc. 2004, 126, 4798.
r 2010 American Chemical Society
Scheme 1. Reaction of 1 with Dioxygen
all been reported. In addition, amid reports of nacnac complexes reacting with oxygen exclusively at the metal center,4 there is a report of dioxygen oxidizing the central carbon of the ligand backbone to a ketone moiety.5 This oxidation was proposed to proceed through a bicyclic peroxo intermediate in which dioxygen is bound both to the metal center and to the central carbon of the ligand backbone.5 This binding would be analogous to that observed in cycloadditions of other exogenous multiple bonds.3 However, such an O2 adduct involving nacnac has never been directly observed and unambiguously characterized. We report herein the observation and spectroscopic solution-phase characterization of such a peroxo intermediate formed by the reaction of a Pt(IV) nacnac complex with molecular oxygen. Further conversion of this intermediate species to a product in which the oxygen-oxygen bond has been cleaved was also observed. This reaction scheme may provide a model for the reported formation of a ketone moiety in Cu and Zn nacnac systems5 and in a related Co bis-nacnac macrocycle system.6 Upon exposure to molecular oxygen, the previously reported five-coordinate Pt(IV) complex (tBuMe2nacnac)PtMe3 (1, tBuMe2nacnac=[((4-tBu-2,6-Me2C6H2)NC(CH3))2CH])7 was found to undergo functionalization of the nacnac ligand as shown in Scheme 1. The peroxo complex 2 was observed as a long-lived intermediate in this reaction and was characterized by 1H and 13C NMR spectroscopies. When dry dioxygen was added to a toluene-d8 solution of 1, the solution immediately faded from bright yellow-orange to pale yellow. A 1H NMR spectrum revealed the complete disappearance of the resonances associated with 1 and the presence of a new set of resonances consistent with structure 2. The conversion of 1 to 2 was complete within minutes, even at temperatures as low as -40 °C. Complex 2 was (5) Yokota, S.; Tachi, Y.; Itoh, S. Inorg. Chem. 2002, 41, 1342. (6) Weiss, M. C.; Goedken, V. L. J. Am. Chem. Soc. 1976, 98, 3389. (7) Fekl, U.; Kaminsky, W.; Goldberg, K. I. J. Am. Chem. Soc. 2003, 125, 15286. Published on Web 09/27/2010
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Scheuermann et al. Scheme 2. Preparation of Complexes 6 and 7
Figure 1. C-Ha coupling constants for the central carbon of a variety of nacnac complexes and cyclometalated derivatives.
formed in >90% yield by 1H NMR spectroscopy (integration against an internal standard). The subsequent conversion of the peroxo complex 2 to complex 3, wherein the O-O bond has been cleaved, proceeded in >65% yield. The reaction time for the conversion of 2 to 3 was variable; complete disappearance of 2 was observed by 1H NMR spectroscopy in as little as 20 min but sometimes required several days at room temperature. This conversion of 2 to 3 occurs with or without an oxygen atmosphere. At room temperature, the 1H NMR resonances of the nacnac ligand in the peroxo complex 2 are all sharp and wellresolved, yet the Pt-Me groups appear together as a broad feature in the baseline centered at 0.83 ppm (in toluene-d8). At -40 °C in toluene-d8, the 1H NMR resonances for the Pt-Me groups of 2 were observed as two distinct singlets with Pt satellites: a resonance for the axial Me at 0.55 ppm (2JPt-H = 68 Hz, 3H) and a resonance for the equatorial methyl groups at 1.10 ppm (2JPt-H = 71 Hz, 6H). At 50 °C in C6D6, a single resonance for the methyl groups at 0.77 ppm (2JPt-H = 70 Hz, 9H) was obtained. The variable-temperature NMR data indicate that complex 2 undergoes a fluxional process whereby the Pt-Me groups exchange their positions. This type of facile Me group exchange is commonly observed among five-coordinate Pt(IV) trimethyl complexes.2,7,8a The exchange of Me positions in 2 likely proceeds through a fluxional five-coordinate intermediate generated by the reversible dissociation of a peroxo or imine ligand. Reversible chelate opening to generate fivecoordinate intermediates has been proposed to explain similar exchange processes in other six-coordinate Pt(IV) complexes.9 Notably, complex 2 is six-coordinate at least at -40 °C, where the 2 JPt-H for the axial methyl group has been measured to be 68 Hz. A significantly higher value for 2JPt-H would be observed if the axial methyl group was trans to an open site or even a very weakly coordinating group.8b The 2JPt-H is comparable to those observed for Pt-Me groups trans to hydroxide or methoxide ligands.10 To confirm that molecular oxygen is bound to the central carbon of the nacnac ligand, the one-bond coupling constant between the central carbon and its proton (Ha in Figure 1) (8) (a) Luedtke, A. T.; Goldberg, K. I. Inorg. Chem. 2007, 46, 8496. (b) Zhao, S.-B.; Wu, G.; Wang, S. Organometallics 2008, 27, 1030. (9) Crumpton-Bregel, D. M.; Goldberg, K. I. J. Am. Chem. Soc. 2003, 125, 9442. (10) Smythe, N. A.; Grice, K. A.; Willams, B. S.; Goldberg, K. I. Organometallics 2009, 28, 277.
was measured. The change in the hybridization of this carbon from sp2 in 1 to sp3 in 2 is expected to result in a decrease of the C-H coupling constant (1JC-H).11 From 1D HMQC experiments, it was found that upon conversion of 1 to 2 the 1JC-H of the central carbon on the ligand backbone decreased from 157 to 144 Hz. To establish that complexing a ligand exclusively to the open site at Pt (with no binding to the nacnac ligand) does not substantially influence the 1JC-H value, the related Pt(IV) CO and tert-butylisocyanide complexes (4 and 5, respectively, Figure 1) were prepared and characterized. Complexes 4 and 5 were formed in high yield upon the direct reaction of 1 with CO or tBuNC, respectively. It was necessary to characterize 4 under an atmosphere of CO, as the CO binding is reversible and removing the CO gas results in regeneration of 1. At room temperature, a single broad resonance for the Pt-Me groups of 4 was observed; however at -40 °C, distinct, sharp resonances for the axial and equatorial Me groups were seen. This temperature dependence of spectral data is likely the result of dissociation of CO and generation of the fluxional 1 on the NMR time scale. In contrast, 5 exhibits two distinct Pt-Me resonances at room temperature, indicative of a static sixcoordinate geometry on the NMR time scale. The 1JC-H values of complexes 4 and 5 were measured to be 158 and 155 Hz, respectively. Thus, changing from a five-coordinate to a six-coordinate Pt(IV) center without reaction at the central carbon of the nacnac ligand has little effect on the coupling constant of the central carbon of the nacnac ligand. The bicyclic peroxo structure of complex 2 is reminiscent of the metallacycles formed upon reaction of nacnac metal complexes with small molecules containing multiple bonds.3 We determined that complex 1 also reacts with ethylene or 3,3-dimethylbutyne to form metallacyclic structures. The reactions yielded complex 6 and 7 (Scheme 2) in 75% and >95% yield, respectively (integration relative to an internal standard in the 1H NMR spectrum). The ethylene binding is reversible, requiring that characterization of 6 by 1H and 13C NMR spectroscopies be performed under excess ethylene. A COSY experiment showed a correlation between the proton on the central carbon of the ligand backbone and the protons of the adjacent methylene group, confirming the metallacyclic structure. The relatively small 2JPt-H observed for the axial methyl group (46 Hz) is consistent with an alkyl ligand (11) Silverstein, R. M.; Bassler, G. C.; Morrill, T. C. Spectrometric Identification of Organic Compounds, 5th ed.; John Wiley & Sons, Inc.: New York, 1991.
Communication
Figure 2. Thermal ellipsoid representations of 3 (left) and 7 (right) at the 50% level of probability with hydrogen atoms and the aryl rings omitted for clarity. Selected bond lengths (A˚) and angles (deg) for 3: Pt1-C2=2.034(6), Pt1-C3=2.021(6), Pt1-C1= 2.023(6), Pt1-O1 = 2.146(4), Pt1-N1 = 2.206(5), Pt1-N2 = 2.180(5), O1-C6 = 1.397(7), O2-C6 = 1.384(7), C3-Pt1-C1 = 85.8(3), C3-Pt1-C2 = 87.6(2), O1-Pt1-N2 = 75.72(17), O1-Pt1-N1 = 74.16(16), N2-Pt1-N1 = 88.19(18), C6-O1Pt1 = 100.2(3). Selected bond lengths (A˚) and angles (deg) for 7: Pt1-C1 = 2.126(2), Pt1-C2 = 2.053(2), Pt1-C3 = 2.0461(19), Pt1-C45=2.073(2), Pt1-N2=2.1894(15), Pt1-N1=2.1806(17), C3-Pt1-C45=92.35(8), C2-Pt1-C45=93.83(8), C3-Pt1-C1= 86.02(9), C2-Pt1-C1=85.98(9), C45-Pt1-N1=84.07(7), C45Pt1-N2=84.48(7), N1-Pt1-N2=85.73(6).
occupying the trans position,9 supporting the binding of the other CH2 group to the platinum center. Complex 7 (formed from 1 and 3,3-dimethylbutyne) was characterized by 1H and 13 C NMR spectroscopies, elemental analysis, and X-ray crystallography (Figure 2). The 1JC-H values of the central C-H unit of the ligand in 6 and 7 are 131 and 129 Hz, respectively, consistent with sp3 hybridization of the central carbon.11 These coupling constants are also similar to those found in other bicyclic structures formed from the cycloaddition reaction between nacnac metal complexes and alkenes or alkynes.3a,b Complexes 6 and 7 serve as structural models for the binding of the dioxygen in 2. The higher coupling constant of 2 relative to those of 6 and 7 is readily explained by the presence of an electronegative substituent in 2.11 The oxygen-oxygen bond of the peroxo moiety in 2 is cleaved as it converts into complex 3. The central carbon of the nacnac ligand in 3 is bound to a hydroxyl group and through an oxygen atom bridge to the platinum center. Complex 3 has been characterized by 1H and 13C NMR spectroscopies, elemental analysis, and X-ray crystallography (Figure 2). The distorted octahedral geometry is reminiscent of complexes reported by Puddephatt and co-workers that (12) Zhang, F.; Broczkowski, M. E.; Jennings, M. C.; Puddephatt, R. J. Can. J. Chem. 2005, 83, 595.
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contain similar bicyclic Pt hemiketal ring systems.12 The 1H NMR resonance for the hydroxyl group of 3 in toluene-d8 (11.25 ppm) shifts upfield and broadens in the presence of water. Addition of D2O to the sample results in similar upfield shifting and broadening and then in the disappearance of this signal. The 13 C resonance for the adjacent carbon at 109.8 ppm is comparable to that reported in a similar Ru-O-C-OH structure.13 At room temperature, in deuterated arene solvent, the two Pt-Me resonances in the 1H NMR spectrum of 3 are somewhat broad, and at 50 °C no Pt-Me resonance could be distinguished from the baseline. These resonances sharpen at lower temperatures, indicating a fluxional behavior similar to that observed for 2. In summary, the reaction of molecular oxygen with a fivecoordinate Pt(IV) nacnac complex leads to the peroxo complex 2 with one oxygen atom bound to the metal center and the other to the central carbon of the nacnac ligand. On standing, complex 2 converts to complex 3, wherein the oxygen-oxygen bond has been cleaved. The formation of 2 upon reaction of 1 with oxygen and its further transformation to 3 provide insight into the mechanism of oxidation of the metal-bound nacnac ligand. The formation of the bicyclic metallacycle peroxo species appears to be directly analogous to the formation of metallacycles from the formal cycloaddition of nacnac metal complexes with unsaturated organic compounds such as alkenes and alkynes. Notably, the peroxo-type structure found in 2 wherein the oxygen is bound on one end to a carbon on the ligand and on the other end to the metal is also reminiscent of intermediates that have been proposed for nonheme iron dioxygenases and heme oxygenase.14,15 This cooperative relationship of the ligand and the metal in the binding of the molecular oxygen is intriguing, and current investigations are exploring the importance of such cooperative interactions in the oxygen reactivity of other metal-ligand combinations.
Acknowledgment. This work was supported by the NSF (CHE-00719372). We are grateful to the ARCS Foundation and the NSF for graduate fellowships (to M.L.S) and to the German Academic Exchange Service (DAAD) for a postdoctoral fellowship (to U.F). We thank Dr. Steven Matthews for assistance with the 1D HMQC experiments. Supporting Information Available: Experimental information and NMR characterization data for all complexes and CIF files giving the crystal data for 3 and 7. This material is available free of charge via the Internet at http://pubs.acs.org. (13) Toyama, M.; Nakahara, M.; Nagao, N. Bull. Chem. Soc. Jpn. 2007, 80, 937. (14) Kovaleva, E. G.; Lipscomb, J. D. Science 2007, 316, 453. (15) Evans, J. P.; Niemevz, F.; Buldain, G.; Ortiz de Montellano, P. J. Biol. Chem. 2008, 283, 19530.
