Communication pubs.acs.org/Organometallics
NHC Gold(III) Triflimidate Complexes Béatrice Jacques, Jonathan Kirsch, Pierre de Frémont,* and Pierre Braunstein* Laboratoire de Chimie de Coordination, Institut de Chimie (UMR 7177 CNRS), Université de Strasbourg, 4 rue Blaise Pascal, CS 90032, 67081 Strasbourg, France S Supporting Information *
ABSTRACT: Substitution of a trans chloride ligand in the Nheterocyclic carbene (NHC) AuIII complexes (NHC)AuCl3 in the presence of AgNTf2 yields a new family of (NHC)AuCl2(NTf2) complexes. They display a moderate stability in solution but are stable once crystallized. These new organogold(III) complexes could be fully characterized by NMR, IR, and single crystal X-ray diffraction. They exhibit in 13C{1H} NMR an unusually upfield-shifted resonance associated with the carbene center.
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Various efficient precatalysts in synthetic chemistry have been accessed in this way, most of them being patented or commercialized, such as the cationic complexes [(NHC)AuI(MeCN)]+ 11 and [(NHC)AuI(THF)]+[BF4 or PF6 or SbF6]− .12 In contrast, the use of AuIII NHCs in homogeneous catalysis is still scarce and is generally limited to the neutral [(NHC)AuIII(trihalide)] complexes,13 even though a pincer NHC−pyridine−proline cationic AuIII complex was recently employed in asymmetric hydrogenation.14 Here we describe the synthesis and characterization of a stable, electrophilic AuIII version of the widely used [(NHC)AuI(NTf2)] complexes. The complex [(IPr)AuCl 3 ] 15 (IPr = N,N′-bis(2,6diisopropylphenyl)imidazol-2-ylidene) was dissolved in CD2Cl2, and a stoichiometric amount of silver triflimidate16 was added to replace one chloride with NTf2−. The solution was stirred at room temperature for 15 min, leading to the appearance of a white precipitate of AgCl. The 1H NMR spectrum of the solution revealed the complete formation of a new complex, 1, with only slight changes, between 0.02 and 0.08 ppm, of its proton chemical shifts compared to those of [(IPr)AuCl3]. In order to confirm the existence of a new complex, further characterizations were undertaken. The solution was filtered, and addition of pentane precipitated a pale yellow powder. Whereas the far-IR spectrum of [(IPr)AuCl3] displays two strong ν(Au−Cl) absorption bands at 330 and 371 cm−1 for [(IPr)AuCl2Cltrans] and [(IPr)AuCl2,cisCl] with respect to the NHC moiety,17 that of 1 displays three strong absorption bands at 371, 379, and 508 cm−1. The disappearance of the ν(Au−Cltrans) absorption band and its replacement by a new band at 508 cm−1 accounts for the formation of [(IPr)AuCl2(NTf 2)] and its assignment to ν(AuNTf2,trans). For comparison, the far-IR spectrum of [(IPr)Au(NTf2)] was recorded and found to display a very similar absorption band at 506 cm−1 for the Au-NTf2 bond. The bands at 371 and 379 cm−1 are obviously absent (Scheme 1).
he significance of gold in homogeneous catalysis has undergone a spectacular growth during the past decade.1 Most of the catalytic systems employed range from commercial inorganic salts, such as AuI and AuIII halides, to more versatile complexes stabilized with soft σ-donor ligands, typically phosphines or N-heterocyclic carbenes (NHCs). Remarkable reactions catalyzed by gold include alkynes/alkene cycloisomerizations, hydroaminations, glycosylations,2 aldehyde hydrosilylations, cyanations, and direct aminations of arenes.3 AuI complexes can accommodate incoming ligands arising from elemental halogens, pseudohalogens, or even silyl groups by a formal oxidative addition pathway.4 In such cases, the filled d10 orbital shell of the AuI center requires π-acidic ligands for retrodonation. In contrast, the AuIII complexes are nonreactive toward oxidative addition. The only AuV species known are the inorganic salts (AuF5)2 and [AuF6−][O2+ or KrF+], prepared by direct fluorination of gold metal with F2/O2 or KrF2 under extremely severe conditions.5 Generally, the lack of reactivity toward oxidative addition/reductive elimination cycles narrows the scope of available Au-mediated C−C coupling reactions, even though some can be carried out using a strong, sacrificial two-electron oxidant to access and regenerate a reactive AuIII intermediate.6 Generally, Selectfluor is employed to form phosphine/NHC difluoro-gold(III) alkyl/aryl complexes undergoing reductive elimination during the catalytic cycle.7 Interestingly, cationic mono-NHC/phosphine AuI complexes are exceptional alkyne and alkene activators in catalytic single or cascade nucleophilic additions. They exhibit a limited oxophilicity, which renders them tolerant toward moisture and most of the common organic functional groups. 8 By comparison, AuIII complexes are slightly more oxophilic and Lewis acidic, as expected for their higher oxidation state.4f In contrast to the situation with the neutral complexes [(NHC)Au I (OH)] 9 and [(NHC)Au I (NTf 2 )] 1 0 ( N T f 2 − = (trifluoromethylsulfonyl)imidate or triflimidate) the use of an halide abstractor (typically a AgI or TlI salt with a noncoordinating anion) is required to unveil the catalytic potential of mono-NHC AuI halide complexes and introduce a new, weakly coordinated and thus labile ligand on the gold center. © 2012 American Chemical Society
Received: May 17, 2012 Published: June 26, 2012 4654
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Organometallics
Communication
The synthesis of [(ItBu)AuCl2(NTf2)] (2; ItBu = N,N′-ditert-butylimidazol-2-ylidene) was attempted with the aim of providing an increased stability for the AuIII center with a bulkier NHC ligand. Following a route similar to that for 1, complex 2 was cleanly obtained. Its 1H NMR chemical shifts are virtually unchanged (less than 0.02 ppm) with respect to [(ItBu)AuCl3].15 Gratifyingly, a comparison between the far-IR spectra of [(ItBu)AuCl3] and 2 reveals the replacement of the absorption band of [(ItBu)AuCl2Cltrans] at 320 cm−1 by a new band at 508 cm−1, assigned to [(ItBu)AuCl2(NTf 2,trans)]. The absorptions assigned to ν(Au−Clcis) remained between 340 and 363 cm−1. The 13C{1H} NMR spectrum of 2 exhibits an extremely upfield shifted signal for the carbenic carbon at 117.2 ppm, confirmed with an HMBC experiment, and the characteristic quartet of the CF3 groups at 119.6 ppm (1J(13C−19F) = 325 Hz). There is no evidence of superior stability due to the replacement of IPr by ItBu, and 2 starts to decompose overnight in CH2Cl2 but can be kept indefinitely as a solid in the freezer. Importantly, the addition of 3 equiv of AgNTf2 did not result in the replacement of more than one chloride ligand from 1 or 2. To unambiguously characterize these complexes, X-rayquality crystals were grown from a solution mixture of CH2Cl2 and pentane. Stable, pale yellow crystals were obtained, and interestingly, crystals of 2 could be kept for weeks in a vial under ambient conditions without any sign of decomposition. The AuIII center exhibits the expected square-planar geometry in both complexes (Figure 1). The Ccarbene−Au−N angles are almost linear and are equal to 179.5(3) and 178.7(4)°, respectively. All other interligand angles are very close to 90° with maximum deviations of −2.4 and +1.6°. The Au−Ccarbene distances in 1 and 2 of 1.982(6) and 2.036(11) Å, respectively, are in good agreement with other AuIII−CNHC distances15,20 and remain almost unchanged in comparison to those in [(IPr)Au(NTf2)]10 and [(ItBu)Au(NTf2)].21 The NTf2 ligand is bound to the AuIII centers in a η1-N mode,22 and the Au−N distance of 2.112(5) Å in 1 is similar to that in [(IPr)Au(NTf2)]. The Au−N distance of 2.165(8) Å in 2 is slightly longer (0.08 Å) than in [(ItBu)Au(NTf2)]. All the Au−Clcis distances are between 2.259(2) and 2.287(3) Å and are similar to those in [(IPr)AuCl3] and [(ItBu)AuCl3]13d,15 and also in the [AuICl2]− salts (2.257(4) Å).23 In order to extend the synthetic strategies to access [(NHC)AuIIIX2(NTf2)] complexes, the oxidation of [(ItBu)Au(NTf2)] by I2 was undertaken. Elemental iodine is known to
Scheme 1. Synthesis of [(IPr)AuCl2(NTf2)] (1) Starting from [(IPr)AuCl3] by Substitution of the Trans Chloride with AgNTf2
The 13C{1H} NMR chemical shift of the carbenic carbon is very indicative of the environment around a [(NHC)AuI/III] moiety.18 The 13C{1H} NMR spectrum of 1 was compared to that of [(IPr)AuCl3] and contained all the expected signals for the IPr moiety, plus the characteristic quartet of the CF3 groups at 119.2 ppm (1J(13C−19F) = 324 Hz). The resonance of the carbenic carbon of 1 is shifted significantly upfield (−16 ppm vs [(IPr)AuCl3)])15 with an unprecedented value of 129.6 ppm.19 This value is surprisingly lower than the value of 132.2 ppm for the parent (IPr)HCl carbene precursor imidazolium salt.18b The assignment was confirmed by DEPT 135 and HMBC experiments revealing a small C−H coupling between the quaternary carbenic carbon atom and the hydrogen atoms of the imidazole backbone. In the search for optimization, the synthesis of 1 was performed in different solvents. In acetone or THF, 1 is formed, but not as cleanly as in CH2Cl2. Using dry solvents appears important to avoid premature decomposition in solution. No reaction took place in acetonitrile, even after 6 h reaction time. We also considered the possibility of isolating first the new complexes [(IPr)AuCl2(MeCN)][PF6] and [{(IPr)AuCl}2(μ-Cl)2][PF6]2,13b and then react them with NTf2−, but clean abstraction of a chloride ligand by the TlI or AgI salts used failed in acetonitrile or CH2Cl2. Unreacted [(IPr)AuCl3] was recovered, or uncharacterized decomposition products formed.20 Complex 1 has limited stability in solution and starts to decompose slowly (15%) overnight in CH2Cl2. In the solid state, it decomposes slowly within a few days at room temperature under light but can be kept indefinitely in a freezer. Interestingly, the stability of the complex in solution could be increased by addition of an excess of AgNTf2, which may indicate the occurrence of some exchange process between free and weakly Au coordinated triflimidate.
Figure 1. Ball and stick representations of the structures of [(IPr)AuCl2(NTf2)] (1) and [(ItBu)AuCl2(NTf2)] (2). Hydrogen atoms have been omitted for clarity. 4655
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oxidize [(IPr)AuMe] to [(IPr)AuI2Me]24 and was preferred to Br2 or Cl2 because of the easier control of a strict stoichiometry of 1 equiv of oxidant/equiv of [(ItBu)Au(NTf2)], thus avoiding the possible formation of [(ItBu)AuI3]. The 1H NMR spectrum of the reaction mixture indicated the formation of a new complex, 3, and the presence of some unreacted [(ItBu)Au(NTf2)]. The 13C{1H} NMR spectrum of 3 displays the characteristic signal of the CF3 groups from the triflimidate fragment. The signal for the carbenic carbon is shifted downfield in comparison to that of the precursor [(ItBu)Au(NTf2)], ruling out the formation of the desired [(ItBu)AuI2(NTf2)] and hinting at the formation of the cationic complex [(ItBu)2Au](NTf2) (3).25 This was further confirmed by an X-ray analysis of crystals grown from a mixture of CH2Cl2 and pentane (Figure 2). The AuI cation adopts a linear
Communication
ASSOCIATED CONTENT
S Supporting Information *
Text, tables, and CIF files giving experimental procedures, characterization data, and crystallographic data for 1−3 and [(ItBu)Au(NTf2)]. This material is available free of charge via the Internet at http://pubs.acs.org. Crystallographic information files (CIF) have also been deposited with the CCDC, 12 Union Road, Cambridge CB2 1EZ, U.K., and can be obtained on request free of charge, by quoting the publication citation and deposition numbers 802671−802674.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected] (P.B.);
[email protected] (P.d.F.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The Centre National de la Recherche Scientifique (CNRS) and the Ministère de la Recherche are gratefully acknowledged for financial support of this work. Johnson Matthey PLC is also gratefully acknowledged for a generous loan of gold salts.
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REFERENCES
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Figure 2. Ball and stick representation of the structure of [(ItBu)2Au](NTf2) (3). Hydrogen atoms have been omitted for clarity.
coordination geometry with a Ccarbene−Au−Ccarbene angle of 176.9(1)°. The Ccarbene−Au bond distances of 2.048(2) and 2.053(6) Å are similar to those in cationic (NHC)2AuI complexes.25 There is no interaction between the NTf2− anion and the AuI cation. It is relevant that the reduction and ligand scrambling of [(IBn)AuBr3] upon reaction with an excess of AgNO3 was reported recently by Monkowius et al.20a (IBn = N,N′-dibenzylimidazol-2-ylidene). In conclusion, the facile substitution of a halide from a neutral [(NHC)AuCl3] complex with AgNTf2 provides a straightforward entry into a new family of [(NHC)AuIIICl2(NTf2)] complexes. These complexes display moderate stability in solution but are fully stable once crystallized. These results contrast with the failed attempts of replacing a halide from a [(NHC)AuX3] complex with an acetate, alcoholate, or oxalate group using the corresponding AgI salts.20a They also suggest that, in contrast to commonly used procedures in homogeneous catalysis, it may not be necessary to use more than 1 equiv of silver salt to activate NHC-Au(III) complexes. The 13C{1H} NMR spectra of the [(NHC)AuIIICl2(NTf2)] complexes reveal carbenic chemical shifts shifted strikingly upfield. To the best of our knowledge, these are the first example of NHC-complexes having a carbenic signal shifted more upfield than the corresponding carbon atom of the parent imidazolium salt. Finally, this AuIII version of the commercial Gagosz catalysts is expected to be highly electrophilic and we are presently assessing its catalytic activity in our laboratory. 4656
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