Communication pubs.acs.org/Organometallics
Isolation of a Dicationic Platinum Complex with Two Accessible Coordination Sites Miriam A. Bowring, Robert G. Bergman,* and T. Don Tilley* Department of Chemistry, University of California, Berkeley, California 94720, United States Chemical Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, California 94720, United States S Supporting Information *
ABSTRACT: The dicationic Pt complex [(tBu2bpy)Pt](NTf2)2·1/2C6H6 (tBu2bpy = 4,4′-di-tert-butyl-2,2′-bipyridine, NTf2− = N(SO2CF3)2−) has been isolated and characterized. The new complex is supported by a simple bidentate bipyridine ligand and weakly coordinating, easily displaced triflimidate counterions. The Pt salt is competent for allylic C−H activation in aprotic, nonpolar media.
D
icationic platinum complexes are rare1−3 but show promising reactivity toward hydrocarbon substrates. Platinum dications supported by tridentate pincer ligands activate alkenes and alkynes toward nucleophilic attack1 and thereby mediate catalytic hydroarylation and hydroalkylation reactions.2 Tridentate ligand systems limit the possibilities for reactivity because they leave only a single accessible coordination site at a square-planar Pt(II) center. Bercaw, Labinger, and co-workers have shown that platinum complexes supported by bidentate diimine ligands activate C−H bonds in the presence of added strong acids.4 These authors have proposed, and observed in situ, dicationic platinum diimine intermediates bearing coordinated alcohols occupying two coordination sites. The isolation and characterization of a dicationic platinum complex with two accessible coordination sites could confirm the ability of such proposed intermediates to activate C−H bonds. Isolated platinum salts of this type would also allow reactivity studies in the absence of strong acids, which limit the use of basic reagents and have been shown to complicate or supersede metal-mediated reactions.5 Herein, we report the synthesis, isolation, and characterization of the platinum complex [(tBu2bpy)Pt](NTf2)2·1/2C6H6 (1(NTf2)2·1/2C6H6, tBu2bpy = 4,4′-di-tert-butyl-2,2′-bipyridine, NTf2− = N(SO2CF3)2−). The triflimidate counterions are very weakly coordinating6 and are easily displaced by poor donors such as perfluoropyridine. We demonstrate that the new platinum salt activates allylic C−H bonds. Treatment of (tBu2bpy)PtPh2 with 2 equiv of HNTf2 at 24 °C in benzene resulted in precipitation of the complex 1(NTf2)2·1/2C6H6, which was isolated in 92% yield (eq 1).
The 1H NMR spectrum of the isolated solid dissolved in THFd8 consistently indicates the presence of 1/2 equiv of free benzene. Elemental analysis data are also consistent with the empirical formula [(tBu2bpy)Pt]2[NTf2]4(C6H6). The coordination environment of platinum in the pure solid 1(NTf2)2·1/2C6H6 was not conclusively determined. Single crystals for X-ray diffraction studies of this material were only obtained in the presence of solvents that coordinate to the metal center. The benzene may be either coordinated to platinum7 or simply cocrystallized with the platinum complex, to which the NTf2− counterions could be weakly bound. The platinum salt takes the doubly solvated form [(tBu2bpy)Pt(solvent)2][NTf2]2 (1(solvent)2(NTf2)2) when dissolved in weakly coordinating solvents, including THF, nitromethane, and perfluoropyridine. This structure is evidenced by 1H NMR and 19F NMR spectroscopy. The 19F NMR spectrum in THFd8 reveals a single resonance corresponding to free NTf2− at −79 ppm.8 The platinum complex 1(C5F5N)2(NTf2)2 retains its solvated form in the solid state when crystallized from perfluoropyridine (Figure 1).
Figure 1. Molecular structure of 1(C5F5N)2(NTf2)2 determined by single-crystal X-ray diffraction. The thermal ellipsoids are shown at the 50% probability level. Hydrogen atoms are omitted for clarity. Received: August 19, 2013
© XXXX American Chemical Society
A
dx.doi.org/10.1021/om4008357 | Organometallics XXXX, XXX, XXX−XXX
Organometallics
Communication
hydrocarbon activation and functionalization reactivity that would be incompatible with acidic media.
The crystallographic characterization of the isolated dicationic platinum complex 1(C5F5N)2(NTf2)2 provides evidence to support the structure of the dicationic intermediates proposed by Bercaw, Labinger, and co-workers. Further, the isolated complex 1(NTf2)2·1/2C6H6 provides the opportunity to study the reactivity of dicationic platinum complexes with two available coordination sites under nonacidic conditions and with aprotic and noncoordinating solvents. Several solvents were evaluated for their ability to dissolve, react with, o r co ordinate to 1(NTf 2 ) 2 · 1 / 2 C 6 H 6 . 1(NTf2)2·1/2C6H6 reacts with numerous common organic solvents, including ethers and halogenated arenes and alkanes, to form complex product mixtures, providing early evidence that it is a reactive species that can activate small molecules. The weakly coordinating solvents nitromethane and perfluoropyridine easily dissolve the complex and cleanly coordinate to the Pt center (as discussed above). 1(NTf2)2·1/2C6H6 is very sparingly soluble in the noncoordinating aromatic solvents benzene and toluene even at elevated temperatures (80 °C), on the basis of visual inspection and variable-temperature NMR studies. The dicationic complex does not appear to activate these solvents. No solvent was identified that readily dissolves 1(NTf2)2·1/2C6H6 without coordinating to it. Benzene was chosen as a solvent for reactivity studies due to its nonreactivity, in spite of its poor solvation ability. To determine its potential for allylic C−H activation, the complex 1(NTf2)2·1/2C6H6 was treated with cyclohexene. A heterogeneous mixture of 1(NTf2)2·1/2C6H6 with a solution of cyclohexene in benzene, when heated to 70 °C for 4 h, generates the allylic C−H activation product 2 and 1 equiv of HNTf2 in quantitative yield by 1H NMR (eq 2). The 1H NMR
■
ASSOCIATED CONTENT
■
AUTHOR INFORMATION
S Supporting Information *
Text, tables, and a CIF file giving experimental details and X-ray crystallographic data for 1(C5F5N)2(NTf2)2. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Authors
*E-mail for R.G.B.:
[email protected]. *E-mail for T.D.T.:
[email protected]. Notes
The authors declare no competing financial interest.
■
ACKNOWLEDGMENTS We gratefully acknowledge financial support from the Director of the Office of Energy Research, Office of Basic Energy Sciences, Chemical Sciences Division, of the U.S. Department of Energy under Contract DE-AC02-05CH11231. For technical assistance and helpful discussion, we thank Dr. Anthony T. Iavarone of the QB3/Chemistry Mass Spectrometry Facility and Dr. Antonio DiPasquale of the College of Chemistry X-ray Crystallography Facility, both at the University of California, Berkeley. For additional assistance with X-ray crystallography, we thank Michael I. Lipschutz.
■
REFERENCES
(1) (a) Carlisle, S.; Matta, A.; Valles, H.; Bracken, J. B.; Miranda, M.; Yoo, J.; Hahn, C. Organometallics 2011, 30, 6446. (b) Hahn, C. Organometallics 2010, 29, 1331. (c) Hahn, C. Chem. Eur. J. 2004, 10, 5888. (d) Hahn, C.; Morvillo, P.; Herdtweck, E.; Vitagliano, A. Organometallics 2002, 21, 1807. (e) Hahn, C.; Cucciolito, M. E.; Vitagliano, A. J. Am. Chem. Soc. 2002, 124, 9038. (2) (a) Cucciolito, M. E.; D’Amora, A.; Vitagliano, A. Organometallics 2010, 29, 5878. (b) Cucciolito, M. E.; Vitagliano, A. Organometallics 2008, 27, 6360. (c) Cucciolito, M. E.; D’Amora, A.; Tuzi, A.; Vitagliano, A. Organometallics 2007, 26, 5216. (d) Cucciolito, M. E.; D’Amora, A.; Vitagliano, A. Organometallics 2005, 24, 3359. (3) (a) Braunschweig, H.; Brenner, P.; Dewhurst, R. D.; JimenezHalla, J. O. C.; Kupfer, T.; Rais, D.; Uttinger, K. Angew. Chem., Int. Ed. 2013, 52, 2981. (b) Parmene, J.; Krivokapic, A.; Tilset, M. Eur. J. Inor. Chem. 2010, 1381. (c) Bertani, R.; Michelin, R. A.; Mozzon, M.; Sassi, A.; Basato, M.; Biffis, A.; Martinati, G.; Zecca, M. Inorg. Chem. Commun. 2001, 4, 281. (d) Siegmann, K.; Pregosin, P. S.; Venanzi, L. M. Organometallics 1989, 8, 2659. (4) (a) Williams, T. J.; Caffyn, A. J. M.; Hazari, N.; Oblad, P. F.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc. 2008, 130, 2418. (b) Driver, T. G.; Williams, T. J.; Labinger, J. A.; Bercaw, J. E. Organometallics 2007, 26, 294. (c) Wong-Foy, A. G.; Henling, L. M.; Day, M.; Labinger, J. A.; Bercaw, J. E. J. Mol. Catal. A: Chem. 2002, 189, 3. (5) (a) Bowring, M. A.; Bergman, R. G.; Tilley, T. D. Organometallics 2011, 30, 1295. (b) Taylor, J. G.; Adrio, L. A.; Hii, K. K. Dalton Trans. 2010, 39, 1171. (c) Hashmi, A. S. K. Catal. Today 2007, 122, 211. (d) Rosenfeld, D. C.; Shekhar, S.; Takemiya, A.; Utsunomiya, M.; Hartwig, J. F. Org. Lett. 2006, 8, 4179. (e) Wabnitz, T. C.; Yu, J.-Q.; Spencer, J. B. Chem. Eur. J. 2004, 10, 484. (6) Antoniotti, S.; Dalla, V.; Duñach, E. Angew. Chem., Int. Ed. 2010, 49, 7860. (7) (a) Reinartz, S.; White, P. S.; Brookhart, M.; Templeton, J. L. J. Am. Chem. Soc. 2002, 124, 7249. (b) Johansson, L.; Tilset, M.; Labinger, J. A.; Bercaw, J. E. J. Am. Chem. Soc. 2000, 122, 10846. (c) Hubig, S. M.; Lindeman, S. V.; Kochi, J. K. Coord. Chem. Rev. 2000, 200, 831. (d) Wadepohl, H. Angew. Chem., Int. Ed. Engl. 1992, 31, 247.
spectrum of 2 contains characteristic triplets bearing Pt satellites at 5.6 and 5.1 ppm in THF-d8, corresponding to the allyl protons. Product 2 was isolated from the acid byproduct under aqueous workup conditions in 35% yield. This demonstrates that the isolated dicationic complex is competent for C−H activation and that allylic activation by a Pt dication occurs even in solvents that do not readily participate in acid− base reactions. The Pt allyl complex 2 is more soluble in benzene than is the precursor 1 and is unusually stable. No reaction of 2 is observed even with excess aqueous potassium hydroxide at 70 °C, although preliminary reactivity studies reveal that 2 does react with stronger bases such as lithium dimethylamide. The generation of complex 2 in the absence of excess acid or protic solvents will facilitate the further study of its reactivity with nucleophiles and bases. In conclusion, an isolable platinum dication bearing two available coordination sites has been synthesized and characterized. The observed allylic activation of a simple hydrocarbon by this well-defined platinum dication is a promising development, opening the possibility for further B
dx.doi.org/10.1021/om4008357 | Organometallics XXXX, XXX, XXX−XXX
Organometallics
Communication
(e) Muetterties, E. L.; Bleeke, J. R.; Wucherer, E. J.; Albright, T. Chem. Rev. 1982, 82, 499. (8) Bowring, M. A.; Bergman, R. G.; Tilley, T. D. J. Am. Chem. Soc. 2013, 135, 13121.
C
dx.doi.org/10.1021/om4008357 | Organometallics XXXX, XXX, XXX−XXX
