(β-Diketiminato)dimethylgold(III): Synthesis, Structure, and Reactivity

Apr 23, 2010 - 2248 Organometallics 2010, 29, 2248–2253 ... University of Oslo, P.O. Box 1033 Blindern, 0315 Oslo, Norway, and §Department of Chemi...
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Organometallics 2010, 29, 2248–2253 DOI: 10.1021/om100038f

(β-Diketiminato)dimethylgold(III): Synthesis, Structure, and Reactivity Ajay Venugopal,† Manik Kumer Ghosh,† Hannes J€ urgens,‡ Karl W. T€ ornroos,§ † ‡ ,† Ole Swang, Mats Tilset, and Richard H. Heyn* †

SINTEF Materials and Chemistry, P.O. Box 124, Blindern, 0314 Oslo, Norway, ‡Department of Chemistry, University of Oslo, P.O. Box 1033 Blindern, 0315 Oslo, Norway, and §Department of Chemistry, University of Bergen, All egaten 41, 5007 Bergen, Norway Received January 15, 2010

The reaction between [Me2AuCl]2 and the lithium salt of a β-diketimine, [HC{C(Me)N(C6H3-2, 6-Me2)}2Li], yields the neutral Au(III) compound [HC{C(Me)N(C6H3-2,6-Me2)}2AuMe2] (1). Compound 1 reacts with 1 equiv of triflic acid (HOTf) at -78 °C to provide the cationic Au(III) compound [H2C{C(Me)N(C6H3-2,6-Me2)}2AuMe2][OTf] (2). Treatment of 1 with 2 equiv of HOTf results in the displacement of Me2AuOTf and formation of the β-diketiminato triflate salt [HC{C(Me)NH(C6H3-2,6-Me2)}2][OTf] (3). Compound 1 reacts with I2 to yield the cation [HC(I){C(Me)N(C6H3-2,6-Me2)}2AuMe2]þ, which is isolated as the triflate salt [HC(I){C(Me)N(C6H3-2,6Me2)}2AuMe2][OTf] (4). The reaction of 1 with (PPh3)AuCl in the presence of AgOTf provides [HC(AuPPh3){C(Me)N(C6H3-2,6-Me2)}2AuMe2][OTf] (5). Molecular structures of 1, 4, and 5 were elucidated by single-crystal X-ray diffraction experiments; that of 4 was additionally confirmed by quantum-chemical calculations. The reactivity experiments indicate that the methine carbon in the β-diketiminato unit of 1 is the exclusive site of electrophilic attack, while the Au-C bonds are not perturbed.

Introduction The catalytic functionalization of alkanes is a particularly challenging and active field of research, with the goal of a sustainable, industrially viable route for the oxidation of methane to methanol.1 Since Shilov and co-workers demonstrated the ability of Pt(II)/Pt(IV) salts to catalytically convert methane to methanol in aqueous media,2 there has been increasing interest in both understanding the mechanism of the Pt C-H activation process3 and finding new homogeneous oxidation catalysts for light alkanes.4 Particularly relevant are the Pt system developed by Periana, which operates in concentrated H2SO4 and elevated temperatures,5 *To whom correspondence should be addressed. Tel: þ 47 9824 3927. Fax: þ 47 2206 7350. E-mail: [email protected]. (1) Arakawa, H.; Aresta, M.; Armor, J. N.; Barteau, M. A.; Beckman, E. J.; Bell, A. T.; Bercaw, J. E.; Creutz, C.; Dinjus, E.; Dixon, D. A.; Domen, K.; DuBois, D. L.; Eckert, J.; Fujita, E.; Gibson, D. H.; Goddard, W. A.; Goodman, D. W.; Keller, J.; Kubas, G. J.; Kung, H. H.; Lyons, J. E.; Manzer, L. E.; Marks, T. J.; Morokuma, K.; Nicholas, K. M.; Periana, R.; Que, L.; Rostrup-Nielson, J.; Sachtler, W. M. H.; Schmidt, L. D.; Sen, A.; Somorjai, G. A.; Stair, P. C.; Stults, B. R.; Tumas, W. Chem. Rev. 2001, 101, 953–996. (2) Goldshlegger, N. F.; Eskova, V. V.; Shilov, A. E.; Shteinman, A. A. Zh. Fiz. Khim. 1972, 46, 1353. (3) Lersch, M.; Tilset, M. Chem. Rev. 2005, 105, 2471–2526. (4) (a) Shilov, A. E. In Activation of Saturated Hydrocarbons by Transition Metal Complexes; D. Reidel: Dordrecht, The Netherlands, 1984. (b) Shilov, A. E.; Shul'pin, G. B. Chem. Rev. 1997, 97, 2879–2932. (c) Crabtree, R. H. Dalton Trans. 2001, 2951–2951. (5) Periana, R. A.; Taube, D. J.; Gamble, S.; Taube, H.; Satoh, T.; Fujii, H. Science 1998, 280, 560–564. (6) Jones, C. J.; Taube, D.; Ziatdinov, V. R.; Periana, R. A.; Nielsen, R. J.; Oxgaard, J.; Goddard, W. A. Angew. Chem., Int. Ed. 2004, 43, 4626–4629. pubs.acs.org/Organometallics

Published on Web 04/23/2010

and an analogous system based on Au.6 Other Au complexes are reported to catalyze alkane oxygenation with hydrogen peroxide.7 Theoretical investigations point out that C-H bond activation in alkanes can be catalyzed by gold complexes with a low energy barrier8 and that [AuCl2(H2O)2]þ can activate methane C-H bonds via a mechanism analogous to that of the original Shilov system.9 On the basis of our longstanding activities within the field of alkane C-H activation on electrophilic Pt(II) complexes,10 the similarities between Periana’s Pt and Au catalytic systems, and isoelectronic analogies between d8 Pt(II) and Au(III) metal centers, we have initiated a program to investigate the potential for C-H bond activation and eventual catalytic functionalization of light alkanes in Au systems. In a continuation of the analogies between Pt(II) and Au(III), neutral Au(III) complexes which could provide coordinatively labile Au(III) cationic species seemed to be a reasonable initial target for establishing Au-based C-H bond activation systems. Of the myriad of potentially fruitful systems, our previous positive experience with cationic, methane-activating diimine Pt(II) compounds10 led us to choose N-ligated Au(III) complexes as our starting point, (7) Shul’pin, G. B.; Shilov, A. E.; Suss-Fink, G. Tetrahedron Lett. 2001, 42, 7253–7256. (8) Pichugina, D. A.; Kuz’menko, N. E.; Shestakov, A. F. Gold Bull. 2007, 40, 115–120. (9) Pichugina, D. A.; Shestakov, A. F.; Kuz’menko, N. E. Russ. Chem. Bull., Int. Ed. 2006, 55, 195–206. (10) (a) Heiberg, H.; Johansson, L.; Gropen, O.; Ryan, O. B.; Swang, O.; Tilset, M. J. Am. Chem. Soc. 2000, 122, 10831–10845. (b) Johansson, L.; Tilset, M. J. Am. Chem. Soc. 2001, 123, 739–740. (c) Wik, B. J.; Lersch, M.; Tilset, M. J. Am. Chem. Soc. 2002, 124, 12116–12117. r 2010 American Chemical Society

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Scheme 1. Synthesis and Reactivity of 1

particularly since a range of Au(III) compounds bearing nitrogen-based ligands are known.11 Of the potential candidates, anionic β-diketiminato ligands, one of the most widely used classes of nitrogen-donor ligands in organometallic chemistry,12 were particularly intriguing. These ligands have recently been employed in Au(I) chemistry. Shi et al. reported the use of β-diketiminato Au(I) species in the selective aerobic oxidation of alcohols,13 and Dias et al. have synthesized and structurally characterized the thermally stable, dimeric gold(I) complexes {[HC{(H)C(2,4,6-Br3C6H2)N}2]Au}2 and {[HC{(H)C(Dipp)N}2]Au}2.14 Au(III) complexes supported by β-diketiminato ligands are, however, unknown. Herein, we present the synthesis, structure, and reactivity studies on the first (β-diketiminato)gold(III) compound, [HC{C(Me)N(C6H3-2,6-Me2)}2AuMe2].

Results and Discussion The salt metathesis reaction between [Me2AuCl]215 and [HC{C(Me)N(C6H3-2,6-Me2)}2Li]16 resulted in the formation of [HC{C(Me)N(C6H3-2,6-Me2)}2AuMe2] (1) in 55% yield (Scheme 1). Compound 1 is the first example of a Au(III) compound bearing a β-diketiminato ligand, and it has been characterized by NMR spectroscopy, mass spectrometry, and single-crystal X-ray diffraction studies. It is soluble in hydrocarbon, ethereal, and chlorinated solvents. It is air-stable but thermally sensitive, decomposing at room temperature after 3 h both in the solid state and in solution. It is, however, stable for more than 6 months when stored at temperatures below -20 °C. The 1H NMR spectrum of 1 in CD2Cl2 shows a single resonance at δ 0.02 for the protons of the two Au-methyl groups, while the signal for the proton on the methine carbon of the β-diketiminato backbone appears at δ 4.94. In the 13C{1H} NMR spectrum, the signal (11) (a) Str€ ahle, J., Gold Compounds of Nitrogen. In Gold-Progress in Chemistry, Biochemistry and Technology; Schmidbaur, H., Ed.; Wiley: Chichester, U.K., 1999; pp 311-348. (b) Abdou, H. E.; Mohamed, A. A.; Fackler, J. P., Gold(I) Nitrogen Chemistry. In Gold Chemistry, Applications and Future Directions in the Life Sciences; Mohr, F., Ed.; Wiley-VCH: Weinheim, Germany, 2009; pp 1-45. (c) Cinellu, M. A., Chemistry of Gold(III) Complexes with Nitrogen and Oxygen Ligands. In Gold Chemistry, Applications and Future Directions in the Life Sciences; Mohr, F., Ed.; Wiley-VCH: Weinheim, Germany, 2009; pp 47-92. (12) Bourget-Merle, L.; Lappert, M. F.; Severn, J. R. Chem. Rev. 2002, 102, 3031–3065. (13) Guan, B. T.; Xing, D.; Cai, G. X.; Wan, X. B.; Yu, N.; Fang, Z.; Yang, L. P.; Shi, Z. J. J. Am. Chem. Soc. 2005, 127, 18004–18005. (14) Dias, H. V. R.; Flores, J. A. Inorg. Chem. 2007, 46, 5841–5843. (15) Paul, M.; Schmidbaur, H. Z. Naturforsch., B 1994, 49, 647–649. (16) Stender, M.; Wright, R. J.; Eichler, B. E.; Prust, J.; Olmstead, M. M.; Roesky, H. W.; Power, P. P. Dalton Trans. 2001, 3465–3469.

for the carbon atoms of the Au-methyl groups appears at δ 4.8, while the signal for the methine carbon of the β-diketiminato backbone appears at δ 97.5. Compound 1 crystallizes from a concentrated pentane solution in the monoclinic space group P21/n. The gold atom in 1 has a slightly distorted C2N2 square-planar coordination geometry (Figure 1). The Au-C bond distances are 2.051(2) and 2.055(2) A˚, which are in the range for typical AuIII-C bonds,17-19 and the C7-Au1-C6 angle is 84.13(9)° (see Table 1 for selected bond distances and angles). The Au1N1 and Au1-N2 distances are 2.1144(14) and 2.1210(15) A˚, respectively, which are similar to other Au-N distances in complexes where the N atom is trans to a C-based ligand.20 The atoms of the β-diketiminato backbone together with the gold atom make a planar six-membered ring. The two aromatic rings on the nitrogen atoms of the β-diketiminato unit are perpendicular to this six-membered ring and face the Au-bound methyl groups. On the basis of our previous experience in related Pt systems, it was of obvious interest to react 1 with acids, with the goal of obtaining an open coordination site on Au via protonolysis of a Au-Me bond. However, addition of 1 equiv of HOTf to 1 at -78 °C in acetonitrile led exclusively to the protonation of the methine carbon of the β-diketiminato unit, providing [H2C{C(Me)N(H3C6-2,6-Me2)}2AuMe2][OTf] (2) (Scheme 1). This compound is thermally sensitive and decomposes above 0 °C within 2 h both in the solid state (17) Komiya, S.; Huffman, J. C.; Kochi, J. K. Inorg. Chem. 1977, 16, 2138–2140. (18) (a) Canty, A. J.; Minchin, N. J.; Patrick, J. M.; White, A. H. Aust. J. Chem. 1983, 36, 1107–1113. (b) Byers, P. K.; Canty, A. J.; Mills, K.; Titcombe, L. J. Organomet. Chem. 1985, 295, 401–405. (c) Schouteeten, S.; Allen, O. R.; Haley, A. D.; Ong, G. L.; Jones, G. D.; Vicic, D. A. J. Organomet. Chem. 2006, 691, 4975–4981. (d) Schuster, O.; Schmidbaur, H. Z. Naturforsch., B 2006, 61, 1–5. (e) Wile, B. M.; Burford, R. J.; McDonald, R.; Ferguson, M. J.; Stradiotto, M. Organometallics 2006, 25, 1028–1035. (19) (a) Zharkova, G. I.; Baidina, I. A.; Igumenov, I. K. J. Struct. Chem. 2006, 47, 1117–1126. (b) Bessonov, A. A.; Baidina, I. A.; Morozova, N. B.; Semyannikov, P. P.; Trubin, S. V.; Gelfond, N. V.; Igumenov, I. K. J. Struct. Chem. 2007, 48, 282–288. (c) Zharkova, G. I.; Baidina, I. A.; Igumenov, I. K. J. Struct. Chem. 2007, 48, 906–913. (d) Zharkova, G. I.; Baidina, I. A.; Igumenov, I. K. J. Struct. Chem. 2007, 48, 108–113. (e) Bessonov, A. A.; Morozova, N. B.; Gelfond, N. V.; Semyannikov, P. P.; Baidina, I. A.; Trubin, S. V.; Shevtsov, Y. V.; Igumenov, I. K. J. Organomet. Chem. 2008, 693, 2572–2578. (f) Bessonov, A. A.; Morozova, N. B.; Kurat'eva, N. V.; Baidina, I. A.; Gel'fond, N. V.; Igumenov, I. K. Russ. J. Coord. Chem. 2008, 34, 70–77. (g) Zharkova, G. I.; Baidina, I. A. Russ. J. Coord. Chem. 2008, 34, 395–399. (h) Zharkova, G. I.; Baidina, I. A. Russ. J. Coord. Chem. 2009, 35, 36–41. (20) Yam, V. W.-W.; Choi, S. W.-K.; Lai, T.-F.; Lee, W.-K. J. Chem. Soc., Dalton Trans. 1993, 1001–1002.

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Figure 1. Crystal structures of compounds 1, 4, and 5 3 CH2Cl2. The triflate anions (CF3SO3-) in 4 and 5 and the solvent molecule (CH2Cl2) in 5 are omitted for clarity. Atomic displacement parameters are shown at the 50% probability level. Table 1. Selected Bond Lengths (A˚) and Bond Angles (deg) for Compounds 1, 4, and 5 3 CH2Cl2 1

4

5

2.041(2) 2.050(2) 2.1616(16) 2.1607(16)

2.045(2) 2.0435(19) 2.1349(14) 2.1285(14) 2.1381(16) 2.2691(4)

Bond Lengths Au1-C6 Au1-C7 Au1-N1 Au1-N2 Au2-C2 Au2-P1 I1-C2

2.055(2) 2.051(2) 2.1144(14) 2.1210(15)

C7-Au1-C6 N1-Au1-N2 C6-Au1-N1 C7-Au1-N2 C1-N1-Au1 C3-N2-Au1 C2-Au2-P1

84.13(9) 90.56(6) 92.48(7) 93.00(8) 124.50(12) 124.04(12)

2.1763(18) Bond Angles 84.07(9) 88.90(6) 92.81(8) 94.20(8) 123.77(13) 124.43(13)

84.89(8) 89.03(5) 93.43(7) 92.65(7) 123.19(12) 122.92(11) 172.11(5)

and in solution. There are only a few examples of latetransition-metal complexes where the methine carbon of the β-diketiminato unit is protonated as in 2.21 Another potential Me- abstraction reagent,22 B(C6F5)3, did not react with 1. Treatment of 1 with excess trifluoroethanol in the presence of acetonitrile also did not lead to the protonation of the Au-bound methyl group. The identity of 2 was deduced by 1H and 13C{1H} NMR spectroscopy. In the 1H NMR spectrum, the signal for the protons of the Au-methyl groups is at δ 0.56, at lower field as compared to the corresponding signal in 1. The signal for the protons of the methyl groups on the β-diketiminato ligand also appear at lower field as compared to those in 1, at δ 2.30. The signal for the two methylene protons in 2 appears as a sharp singlet at δ 4.34, and no broadening is (21) Feldman, J.; McLain, S. J.; Parthasarathy, A.; Marshall, W. J.; Calabrese, J. C.; Arthur, S. D. Organometallics 1997, 16, 1514–1516. (22) Erker, G. Dalton Trans. 2005, 1883–1890.

observed even at -55 °C. Since the spectrum for a static boat conformation should display nonequivalent methylene protons, this suggests that ring inversion of this presumed conformation for 2 is fast on the NMR time scale. The 13 C{1H} NMR spectrum of 2 shows a peak at δ 50.0 for the methylene carbon on the β-diketiminato ligand. The presence of the triflate anion in 2 was confirmed by a single resonance at δ -78.3 in the 19F NMR spectrum. Treatment of 1 with 2 equiv of HOTf resulted in the formation of dimethylgold(III) triflate and the β-diketiminato triflate salt [HC{C(Me)NH(H3C6-2,6-Me2)}2][OTf] (3) (Scheme 1). The products crystallized from a dichloromethane solution as a mixture and could not be separated. The identity of 3 was confirmed from comparison of the 1H NMR spectrum of the mixture with that from an independent reaction of the β-diketimine (2,6-Me2H3C6)NHC(CH3)CHC(CH3)N(C6H32,6-Me2) and triflic acid. The signal for the methyl protons of Me2AuOTf appears at δ 1.38 in the 1H NMR spectrum, in agreement with the literature.17 Since I2 is reported to react with (β-diketiminato)metal alkyls to give the corresponding (β-diketiminato)metal iodide,23 1 was treated with 1 equiv of I2 in diethyl ether at -55 °C. This led to the formation of an orange precipitate, which was further reacted in situ with 1 equiv of AgOTf to provide [HC(I){C(Me)N(C6H3-2,6-Me2)}2AuMe2][OTf] (4) (Scheme 1). Decomposition occurs if the temperature of the reaction mixture exceeds 10 °C. However, once isolated, 4 is stable at ambient temperature in the solid state, and it was characterized by multinuclear NMR spectroscopy, elemental analysis, mass spectrometry, and single crystal X-ray diffraction studies and quantum-chemical calculations. To the best of our knowledge, this is the first example of a reaction where a halogen atom binds to the γ-carbon in a metal β-diketiminate complex. (23) (a) Schulz, S.; Eisenmann, T.; Westphal, U.; Schmidt, S.; Florke, U. Z. Anorg. Allg. Chem. 2009, 635, 216–220. (b) Stender, M.; Eichler, B. E.; Hardman, N. J.; Power, P. P.; Prust, J.; Noltemeyer, M.; Roesky, H. W. Inorg. Chem. 2001, 40, 2794–2799.

Article

The composition of the cation [HC(I){C(Me)N(C6H3-2,6Me2)}2AuMe2]þ was confirmed by mass spectrometry with the presence of a molecular ion peak at m/z 659.2. The chemical shift for the proton on the iodine-bearing γ-carbon atom is δ 6.05, and there is no detectable signal broadening in the 1H spectrum of 4, even upon cooling to -55 °C. Unlike the rapid ring inversion in 2, which equilibrates the environments of the ostensibly inequivalent methylene protons, this observation suggests that the AuN2C3 ring in 4 is restricted to one conformation, although rapid interconversion between two conformers cannot be ruled out. Optimization of a hypothetical conformer with the I atom in an equatorial position converged to a well-defined minimum with an energy 7.4 kcal/ mol higher than the conformer with I in an axial position. This may be taken as the minimum value for the activation energy for conformer interconversion, although the activation energy is most probably considerably higher. A transition state optimization was not carried out. Correspondingly, the signals for the aromatic methyl groups appear as two singlets of equal intensity, due to the inequivalence between the top and bottom halves of 4. The 13C signal of the γ-carbon appears at δ 26.8, and this strong shift of 23.2 ppm to higher field, in comparison to 2, is consistent with the substituent chemical shift for CR observed upon substitution of I for H.24 Compound 4 crystallizes in the orthorhombic space group P212121 (Figure 1). The atom Au1 is, as expected, bonded to two nitrogen atoms of the diketimine ligand and two methyl carbon atoms in a square-planar arrangement. Gratifyingly, a quantum-chemical geometry optimization and a singlecrystal X-ray diffraction experiment lead to very similar structures for 4. In the following, geometry parameters are given as xx {yy}, where xx and yy are the experimental and calculated values, respectively. The six-membered AuN2C3 ring adopts a boat conformation, with I1 in an axial position on the apical C2 atom. The I1-C2-Au1 angle is 92.88° {90.1°}, and the Au1-I1 distance is 4.074 A˚ {4.05 A˚}, which is 0.48 A˚ longer than the sum of the van der Waals radii. The I1-C2 distance is 2.1763(18) A˚ {2.24 A˚}. The angle between the two planes formed by the basal atoms N1, N2, C1, and C3 is 4.10° {0.7°}. Au1 and the γ-carbon C2 lie above this distorted plane with torsion angles of 36.7(3)° {44.5°} for N1-C1-C2-C3 and 19.5(2)° {27.4°} for N1-Au1-N2-C3. The reactivity of 1 with HOTf and I2 demonstrates that the methine carbon of the β-diketiminato ligand is nucleophilic enough to be the primary site of electrophilic attack, while the Au-methyl bonds remain inert. This observation suggested the potential of attaching electrophilic inorganic fragments to the methine carbon in 1. Since (phosphino)gold(I) cations [(R3P)Au]þ are isolobal with Hþ,25 1 was reacted with PPh3AuCl in the presence of AgOTf. This reaction provided [HC{AuPPh3}{C(Me)N(C6H3-2,6-Me2)}2AuMe2][OTf] (5), in which [PPh3Au]þ is indeed bound to the aforementioned methine carbon (Scheme 1). While the vast majority of known AuI 3 3 3 AuIII compounds are isomers of dimeric AuII-AuII ylide compounds,26 5 more closely resembles the structurally uncharacterized complex [{(Ph2P)2CH(AuPPh3)}Au{C6F5}2][ClO4].27 Unlike the other gold compounds (24) Wiberg, K. B.; Pratt, W. E.; Bailey, W. F. J. Org. Chem. 1980, 45, 4936–4947. (25) Schmidbaur, H. Chem. Soc. Rev. 1995, 24, 391–400. (26) Fackler, J. P. Inorg. Chem. 2002, 41, 6959–6972. (27) Fernandez, E. J.; Gimeno, M. C.; Jones, P. G.; Laguna, A.; Laguna, M.; Lopez-de-Luzuriaga, J. M. J. Chem. Soc., Dalton Trans. 1992, 3365–3370.

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reported in this paper, compound 5 has a high thermal stability and shows no decomposition even after heating for 8 h at 70 °C in acetonitrile. Compound 5 was characterized by NMR spectroscopy, elemental analysis, and singlecrystal X-ray diffraction studies. The most diagnostic feature in the NMR spectra is the coupling of the phosphorus atom to both the γ-carbon and its accompanying proton, with coupling constants of 2JPC=48.2 Hz and 3JPH= 9.8 Hz. Compound 5 crystallizes along with one molecule of dichloromethane in the triclinic space group P1 (Figure 1). The six-membered AuN2C3 ring in 5 also adopts a boat conformation like that of 4, with the (PPh3)AuI moiety also in the axial position of the apical C2 atom. The coordination geometry around Au2 is nearly linear, with a P1-Au2-C2 angle of 172.1(1)°. The Au2-P1 and Au2-C2 distances are within the range of Au-P and Au-C distances found in the literature.28 The Au2 atom is tilted even more toward Au1 than that observed with the I atom in 4; the Au1-C2-Au2 angle is acute (83.72°), about 10° less than the analogous angle in 4. The Au-Au distance is 3.742 A˚, which is, however, far greater than the typical Au-Au interactions (3.00 ( 0.25 A˚).29 Despite the cationic nature of 5, numerous attempts to abstract the proton on the γ-carbon atom with a variety of bases failed. Reaction between 5 and triethylamine in dichloromethane-d2 led to the formation of [PPh3Au(NEt3)]þ and 1. Similarly, 5 reacted with KOtBu to displace (PPh3)AuOtBu. However, no reactivity was observed when 5 was mixed with 2,6-di-tert-butylpyridine in dichloromethane-d2 and heated to 75 °C for 8 h in a sealed tube. Reaction of 5 with other bases in a variety of solvents and at a variety of temperatures (from ambient to -78 °C) gave either no reaction or loss of the (PPh3)Au moiety.

Conclusion We have successfully introduced the β-diketiminato ligand onto a gold(III) center and obtained the complex [HC{C(Me)N(C6H3-2,6-Me2)}2AuMe2] (1), which is the first example of a β-diketiminato-ligated Au(III) compound. Given the hypothesis that a coordinatively labile, electrophilic Au(III) center is capable of alkane C-H activation, reactivity studies focused on synthesis of a cationic gold(III) species of the type [(β-diketiminato)Au(Me)(L)]þ, via reaction of 1 with electrophiles. However, all results showed that the site of electrophilic attack is not the Au-C bond but rather the γ-carbon of the diketiminato ligand. Thus, triflic acid protonated the γ-carbon of the diketiminato unit, whereas reaction of 1 with both I2 and the inorganic electrophile PPh3AuCl provided the unique cationic gold(III) compounds 4 and 5. Crystallographic studies indicate the six-membered AuN2C3 ring in 1 is planar while the corresponding rings in 4 and 5 assume boat conformations, with the attacking electrophiles oriented axially from the apical γ-carbon. Due to the inability to obtain any data indicating even a hint of Au-C bond cleavage in 1, other studies, such as the thermal stability of 1 in acidic solvents or the reactivity of 1 directly with hydrocarbons, have not been carried out. We (28) Laguna, A., Gold Compounds of Phosphorus and the Heavy Group V. In Gold-Progress in Chemistry, Biochemistry and Technology; Schmidbaur, H., Ed.; Wiley: Chichester, U.K., 1999; pp 349-427. (29) Schmidbaur, H.; Schier, A. Chem. Soc. Rev. 2008, 37, 1931–1951.

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have rather turned our investigations toward other potentially fruitful Au(III) systems for alkane C-H activation, and the results of these studies will be presented in due course.

Experimental Section General Considerations. All manipulations were performed under an argon atmosphere using Schlenk techniques. All solvents were dried and distilled by standard methods prior to use. The starting materials [Me2AuCl]2,15 [{(2,6-Me2H3C6)N(CH3)C}2CH]Li,16 and (Ph3P)AuCl30 were synthesized according to literature procedures. HOTf, I2, and AgOTf were purchased from Sigma-Aldrich and used as received. NMR measurements were made on a 300 MHz Gemini spectrometer, and chemical shifts are represented in ppm, referenced to the residual proton signals of the deuterated solvents. The mass spectra were obtained using a Micromass Q-TOF 2 spectrometer. Elemental analyses were carried out by the Mikroanalytisches Labor H. Kolbe, M€ ulheim, Germany. The samples for elemental analysis were dried under vacuum (10-3 mbar) for 12 h. Compound 1. A solution of [Me2AuCl]2 (0.100 g, 0.38 mmol) in 15 mL of pentane was added to a suspension of [{(2,6Me2H3C6)N(CH3)C}2CH]Li (0.116 g, 0.38 mmol) in 25 mL of pentane at -78 °C. The reaction mixture was warmed to 10 °C over 5 h and was stirred for a further 15 h at this temperature. The resulting mixture was filtered, and the solution was concentrated to precipitate pale yellow crystals of 1. Yield: 55%. Thermal instability prevented collection of elemental analysis data. 1H NMR (CD2Cl2; 300 MHz): δ 7.16 (d, J=8.1 Hz, 4H, m-Ar H), 6.98 (t, J=8.1 Hz, 2H, p-Ar H), 4.94 (s, 1H, γ-CH), 2.18 (s, 12H, Ar-CH3), 1.60 (s, 6H, C-CH3), 0.02 (s, 6H, Au-CH3). 13C{1H} NMR (CD2Cl2; 75 MHz): δ 159.0, 148.5, 133.2, 128.0, 125.6, 97.5 (γ-CH), 25.0 (C-CH3), 18.6 (ArCH3), 4.8 (Au-CH3). TOF MS ES: m/z 533.2 [Mþ], 503.2 [Mþ - 2 Me], 279.0, 98.5, 89.5. Compound 2. A 1 mL portion of a 0.1 M acetonitrile solution of triflic acid (0.1 mmol) was added dropwise to a dichloromethane solution of 1 (0.053 g, 0.1 mmol) at -78 °C. The reaction mixture was warmed to 10 °C over 5 h and was stirred for a further 5 h at this temperature. The solvent was removed under vacuum, leaving behind a pale yellow residue of 2. Yield: 71%. Thermal instability prevented collection of elemental analysis data. 1H NMR (CD2Cl2; 300 MHz): δ 7.19 (br, 6H, Ar H), 4.34 (s, 2H, γ-CH2), 2.30 (s, 6H, C-CH3), 2.21 (s, 12H, Ar-CH3), 0.56 (s, 6H, Au-CH3). 13C{1H} NMR (CD2Cl2; 75 MHz): δ 180.2, 143.0, 130.3, 129.8, 128.1, 50.0 (γ-CH2), 25.2 (C-CH3), 18.4 (Ar-CH3), 6.2 (Au-CH3). TOF MS ES: m/z 534.3 [Mþ - OTf], 533.2, 503.2, 307.2. Compound 3. A 2 mL portion of a 0.1 M acetonitrile solution of triflic acid (0.2 mmol) was added dropwise to a dichloromethane solution of 1 (0.053 g, 0.1 mmol) at -78 °C. The reaction mixture was warmed to 10 °C over 5 h and was stirred for a further 5 h. The solvent was removed under vacuum, leaving behind a pale yellow residue of 3 and [Me2AuOTf], a mixture of which was crystallized from a mixture of dichloromethane and diethyl ether at 10 °C. 1H NMR (CD2Cl2; 300 MHz): δ 8.74 (br s, 2H, NH), 6.98 (t, 3JHH=8.8 Hz, 2H, p-Ar H), 6.82 (d, 3JHH=8.8 Hz, 4H, m-Ar H), 4.25 (s, 1H, γ-CH), 2.61 (s, 6H, C-CH3), 1.60 (s, 12H, Ar-CH3), 1.38 (s, 6H, [(CH3)2AuOTf]). 19F NMR (CD2Cl2; 282 MHz): δ -78.5. Compound 4. Iodine (0.023 g, 0.1 mmol) was added to a diethyl ether solution of 1 (0.053 g, 0.1 mmol) at -78 °C. The reaction mixture was warmed to 0 °C over 3 h, and the solvent was evaporated under vacuum. The residue was dissolved in

(30) Braunstein, P.; Lehner, H.; Matt, D. Inorg. Synth. 1990, 27, 218–221.

Venugopal et al. Table 2. Selected Crystallographic Data and Refinement Parameters for Compounds 1, 4, and 5 3 CH2Cl2 1

4

5 3 CH2Cl2

formula

C23H31AuN2

fw cryst syst space group a (A˚) b (A˚) c (A˚) R (deg) β (deg) γ (deg) V (A˚3) Z dcalcd (Mg m-3) μ (mm-1) θmax (deg) R(int) no. of rflns with I >2σ(I) R1 (I >2σ(I)) wR2 (all data)

532.46 monoclinic P21/n 13.3580(13) 13.1952(13) 13.473(13)

C24H31AuF3IN2O3S 808.43 orthorhombic P212121 11.1621(8) 12.8960(9) 19.3602(13)

2127.3(4) 4 1.663 6.924 33.24 0.0369 7470

2786.8(3) 4 1.927 6.509 31.65 0.0397 8991

C43H48Au2Cl2F3N2O3PS 1225.70 triclinic P1 10.4621(3) 13.3464(4) 15.9891(4) 92.5025(30) 92.2262(3) 101.4406(3) 2183.56(11) 2 1.864 6.972 31.54 0.0254 13 452

0.0207 0.0556

0.0139 0.0402

0.0180 0.0468

116.401(1)

15 mL of dichloromethane at 0 °C. AgOTf (0.025 g, 0.1 mmol) was then added to this solution, and the resulting reaction mixture was stirred at 0 °C for 1 h. The reaction mixture was thereafter filtered, concentrated to 3 mL, layered with 12 mL of diethyl ether, and stored at -45 °C, which provided yellow crystals of 3. Yield: 67%. 1H NMR (CD2Cl2; 300 MHz): δ 7.21 (br, 6H, Ar H), 6.05 (s, 6H, γ-CH), 2.38 (s, 6H, C-CH3), 2.24 (s, 6H, Ar-CH3), 2.21 (s, 6H, Ar-CH3), 0.58 (s, 6H, Au-CH3). 13 C{1H} NMR (CD2Cl2; 75 MHz): δ 180.9, 143.0, 131.2, 130.3, 130.0, 129.4, 128.4, 26.8 (γ-CH), 23.8 (C-CH3), 18.8 (Ar-CH3), 18.5 (Ar-CH3), 7.1 (Au-CH3). 19F NMR (CD2Cl2; 282 MHz): δ -78.3. TOF MS ESþ: m/z 659.2 [Mþ - OTf]. Anal. Calcd for C24H31AuF3IN2O3S: C, 35.7; H, 3.83; N, 3.46. Found: C, 35.5; H, 3.84; N, 3.46. Compound 5. A 15 mL portion of THF was added to a mixture of 1 (0.053 g, 0.1 mmol), (Ph3P)AuCl (0.049 g, 0.1 mmol), and AgOTf (0.025 g, 0.1 mmol), and the reaction mixture was stirred at room temperature for 15 h while being protected from light. The reaction mixture was then filtered, and the solvent was removed under vacuum. The resulting oily residue was dissolved in 2 mL of dichloromethane and layered with 10 mL of diethyl ether, after which 5 crystallized at room temperature as colorless needles. Yield: 65%. 1H NMR (CD2Cl2; 300 MHz): δ 7.64-7.43 (15 H, PPh3), 7.19 (t, 3JHH=8.5 Hz, 2H, p-Ar H), 7.10 (d, 3JHH= 8.5 Hz, 4H, m-Ar H), 4.49 (d, 3JHP=9.8 Hz, 1H, γ-CH), 2.22 (s, 6H, C-CH3), 2.09 (s, 6H, Ar-CH3), 1.90 (s, 6H, Ar-CH3), 0.38 (s, 6H, Au-CH3). 31P NMR (CD2Cl2; 121 MHz): δ 42.0 (br). 13 C{1H} NMR (CD2Cl2; 75 MHz): δ 186.4, 144.1, 134.6 (d, 2 JPC = 13.6 Hz, Co, PPh3), 133.1 (d, 4JPC = 2.6 Hz, Cp, PPh3), 131.6, 130.1 (d, 3JPC =11.5 Hz, Cm, PPh3), 129.1(d, 1JPC =65.0 Hz, Ci, PPh3), 128.3, 127.6, 127,2, 69.7 (d, 2JPC=48.2 Hz, γ-CH), 25.6 (C-CH3), 19.5 (Ar-CH3), 19.3 (Ar-CH3), 5.9 (Au-CH3). 19 F NMR (CDCl2; 282 MHz): δ -78.2. TOF MS ESþ: m/z 991.2 [Mþ - OTf], 500.1. Anal. Calcd for C42H46Au2F3N2O3PS: C, 44.2; H, 4.03; N, 2.45. Found: C, 44.5; H, 3.95; N, 2.46. Single-Crystal X-ray Diffraction Experiments. Single crystals of 1, 4, and 5 3 CH2Cl2 suitable for X-ray diffraction measurements were obtained by repeated crystallization of the corresponding products isolated from the reaction mixtures. Selected crystallographic data and refinement parameters are collected in Table 2. The crystals were suspended in Paratone-N oil (Hampton Research), mounted on a glass fiber and thereafter placed onto the goniometer under a cold stream. The measurements were carried out on a Bruker APEX-II CCD ULTRA rotating anode diffractometer using graphite-monochromated Mo KR radiation (λ=0.710 73 A˚), performing 182° ω scans in

Article four j positions. Raw data were collected using the APEX2 software packages,31 reduced, and scaled with the program SAINT. The structures were solved by direct methods and refined by fullmatrix least-squares cycles (SHELXS97, SHELXTL-97).32 The structures in this article are represented using the program ORTEP-III.33 Crystallographic data (excluding structure factors) for the structures reported in this paper have been deposited with the Cambridge Crystallographic Data Centre as supplementary publications no. 759373 (1), 759374 (4), and 759375 (5 3 CH2Cl2). Copies of the data can be obtained from the CCDC, 12 Union Road, Cambridge CB2 1EZ, U.K. (fax, (þ44)1223-336-033; e-mail, [email protected]). Quantum-Chemical Calculations. Geometry optimizations were carried out using the B3LYP density functional as implemented in the Gaussian 03 program system.34 For gold, a (31) APEX Software Suite, v. 2.1-4 and SAINT v. 7.53A, Data Integration Software for Bruker AXS CCD; Bruker AXS, Inc., Madison, WI, 2008. (32) Sheldrick, G. M. SHELXS97 and SHELXTL-97. Acta Crystallogr. 2008, A64, 112–122. (33) Burnett, M. N.; Johnson, C. K. ORTEP-III: Oak Ridge Thermal Ellipsoid Plot Program for Crystal Structure Illustrations; Report ORNL6895; Oak Ridge National Laboratory, Oak Ridge, TN, 1996. (34) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; Montgomery, J. A., Jr.; Vreven, T.; Kudin, K. N.; Burant, J. C.; Millam, J. M.; Iyengar, S. S.; Tomasi, J.; Barone, V.; Mennucci, B.; Cossi, M.; Scalmani, G.; Rega, N.; Petersson, G. A.; Nakatsuji, H.; Hada, M.; Ehara, M.; Toyota, K.; Fukuda, R.; Hasegawa, J.; Ishida, M.; Nakajima, T.; Honda, Y.; Kitao, O.; Nakai, H.; Klene, M.; Li, X.; Knox, J. E.; Hratchian, H. P.; Cross, J. B.; Bakken, V.; Adamo, C.; Jaramillo, J.; Gomperts, R.; Stratmann, R. E.; Yazyev, O.; Austin, A. J.; Cammi, R.; Pomelli, C.; Ochterski, J. W.; Ayala, P. Y.; Morokuma, K.; Voth, G. A.; Salvador, P.; Dannenberg, J. J.; Zakrzewski, V. G.; Dapprich, S.; Daniels, A. D.; Strain, M. C.; Farkas, O.; Malick, D. K.; Rabuck, A. D.; Raghavachari, K.; Foresman, J. B.; Ortiz, J. V.; Cui, Q.; Baboul, A. G.; Clifford, S.; Cioslowski, J.; Stefanov, B. B.; Liu, G.; Liashenko, A.; Piskorz, P.; Komaromi, I.; Martin, R. L.; Fox, D. J.; Keith, T.; Al-Laham, M. A.; Peng, C. Y.; Nanayakkara, A.; Challacombe, M.; Gill, P. M. W.; Johnson, B.; Chen, W.; Wong, M. W.; Gonzalez, C.; Pople, J. A. Gaussian 03, revision E.01; Gaussian, Inc., Wallingford, CT, 2003.

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multielectron adjusted quasirelativistic effective core potential covering 60 electrons ([Kr]4d104f14) and an 8s7p6d/[6s5p3d]GTO valence basis set (31111, 22111, 411, 21) was used.35 To further improve the quality of the basis set, f-type polarization functions for gold were optimized by choosing three exponents √ in an even-tempered fashion: i.e., separated by factors of 5. CCSD(T) calculations were carried out for the gold atom in its doublet S ground state, and the energy was minimized by varying the f-type exponents, keeping the ratios between them constant. Hence, only one parameter was optimized. Finally, the three optimized primitive functions were contracted to two using a (21) scheme, taking the contraction coefficients from the atomic calculation (see the Supporting Information). C, H, and N atoms were described with the Dunning correlation consistent cc-pVDZ and basis sets,36 while the LANL2DZ ECP and valence basis37 were employed for iodine.

Acknowledgment. We thank the GASSMAKS program of the Norwegian Research Council (Grant No. 185513/I30) for generous financial support, the NOTUR project for a grant of computing resources (http://www. notur.no, Acc. No. NN2147k), Aud Mjærum Bouzga for assistance with the NMR spectroscopy, and Dr. Anthony P. Shaw for helpful discussions. Supporting Information Available: A table giving optimized F-type basis functions for gold and CIF files for the crystal structures of 1, 4, and 5 3 CH2Cl2. This material is available free of charge via the Internet at http://pubs.acs.org. (35) Andrae, D.; H€aussermann, U.; Dolg, M.; Stoll, H.; Preuss, H. Theor. Chim. Acta 1990, 77, 123–141. (36) (a) Woon, D. E.; Dunning, T. H. J. Chem. Phys. 1993, 98, 1358– 1371. (b) Kendall, R. A.; Dunning, T. H.; Harrison, R. J. J. Chem. Phys. 1992, 96, 6796–6806. (c) Dunning, T. H. J. Chem. Phys. 1989, 90, 1007– 1023. (37) Hay, P. J.; Wadt, R. R. J. Chem. Phys. 1985, 82, 284–299.