Communication pubs.acs.org/Organometallics
A Trigold(I) Ketenylidene Cation Nicholas T. Daugherty,† John Bacsa,‡ and Joseph P. Sadighi*,† †
School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia 30332-0400, United States X-ray Crystallography Center, Department of Chemistry, Emory University, 1515 Dickey Drive, Atlanta, Georgia 30322, United States
‡
S Supporting Information *
ABSTRACT: An (N-heterocyclic carbene)gold(I) acetate reacts with acetic anhydride and triethylamine to form a ketenylidene-bridged trigold(I) cation as its dihydrogen triacetate salt. Ion exchange with tetrafluoroborate affords a more stable compound. The cation reacts with borohydride to form (NHC)gold hydride and with benzenethiol to form (NHC)gold benzenethiolate and S-phenyl thioacetate.
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etate (SIMes = 1,3-bis(2,4,6-trimethylphenyl)imidazolin-2ylidene) with excess acetic anhydride and triethylamine, under conditions that generate ketene at equilibrium,30 afforded a product that precipitated upon addition of diethyl ether to a concentrated acetonitrile solution. The infrared spectrum of this product displayed a strong absorbance at 2000 cm−1, which we initially attributed to the doubly bridging ketenide of the expected product. The 1H NMR spectrum of the product in CD3CN solution appeared consistent with this assignment, except for an additional broad singlet at δ 1.88 ppm. This resonance appeared acetate-derived, and integration suggested the presence of one acetate per SIMes ligand. To our surprise, the ESI mass spectrum of the isolated complex displayed a strong parent ion peak at m/z 1549.4, corresponding to the trigold(I) ketenylidene cation {[(SIMes)Au]3(μ3-CCO)}+. Because the formation of this cation from (SIMes)gold(I) acetate and ketene requires liberation of 2 equiv of acetic acid plus acetate anion, we formulate the initially isolated product as {[(SIMes)Au]3(μ3-CCO)}+[AcO·2HOAc]− (1a), consistent with the 1H NMR signals observed for both SIMes and acetate. The IMes-supported analogue of 1a (IMes = 1,3-dimesitylimidazol-2-ylidene) may be similarly prepared but is accompanied by a significant fraction of homoleptic [(IMes)2Au]+,31 which proved impractical to remove. Use of the more sterically demanding IDipp 32 (Dipp = 2,6diisopropylphenyl) did not give rise to clean product formation. Scheme 1 depicts a plausible sequence leading to 1a, similar to that invoked for the formation of binary silver ketenide [Ag2CCO]n.20 Complex 1a proved slightly unstable on prolonged standing in the solid form or in the course of repeated manipulations in solution. Reasoning that this instability might arise from the protic and potentially nucleophilic anion, we carried out anion
ransition-metal ketenides and ketenylidenes have been studied as models for plausible intermediates in carbon monoxide chemistry.1 The first well-defined transition-metal ketenylidene complex was the highly electrophilic [Co3(CO)9(CCO)]+, described by Seyferth et al.2−4 Shriver and co-workers studied a series of generally nucleophilic, anionic ketenylidenes,5−10 including one featuring a mixedmetal [Fe3Cu] core.11 Several of these complexes display remarkable C−C coupling and cleavage reactions.12 Low-valent early metals such as Zr and Hf can convert coordinated CO to a doubly bridging ketenylidene fragment.13,14 Mononuclear ketenylidenes are formed from Ta(III) via reductive coupling of CO15 and from W(II), transiently, by reaction with carbon suboxide (C3O2).16 Beginning in 1970 a series of reports described the coinagemetal ketenides ([M2CCO]n; M = Cu,17 Ag,18 Au19), prepared by generating ketene in the presence of suitable metal precursors and amines. These insoluble shock-sensitive explosives were characterized by infrared spectroscopy, which revealed distinctive strong absorbances near 2000 cm−1 arising from the ketenide stretch. X-ray powder diffraction studies of the silver complex indicated a μ4-bridging mode for the ketenide in a two-dimensional coordination polymer.20 The copper and silver ketenides serve as precatalysts for olefin oxidation reactions.21−24 Fürstner et al. have described a neutral gold(I) chloride complex of Ph3PCCO, in which the Lewis basic ylide carbon acts as a donor to gold.25 A surface-bound ketenylidene has been observed in the growth of thin copper films from β-diketonates.26 Recently, Yates and co-workers have identified and studied surface-bound [Au2CCO] as a reactive intermediate in the aerobic oxidation of acetic acid and related species on Au/TiO2 surfaces.27−29 We now report the synthesis and structure of a stable trigold ketenylidene cation. Reaction of this cation with soft Lewis bases leads to Au−C bond cleavage. We initially sought the digold(I) ketenylidene {[(SIMes)Au]2(μ2-CCO) as a precursor to the trinuclear cation {[(SIMes)Au]3(μ3-CCO)}+. Reaction of (SIMes)gold(I) ac© XXXX American Chemical Society
Received: June 24, 2017
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DOI: 10.1021/acs.organomet.7b00481 Organometallics XXXX, XXX, XXX−XXX
Communication
Organometallics
radii33 at 3.2904(8) Å, suggestive of minimal aurophilic interaction.34 In solution, all three SIMes ligands are symmetric and equivalent on the 1H NMR time scale, suggesting small energy differences both for distortion from 3-fold symmetry and for rotation of the NHC about the C−Au bond. Table 1 compares
Scheme 1. Proposed Sequence for [Au3CCO]+ Formation
Table 1. Key Metrics and IR Data in Ketenylidene-Related Species compound 1b (this work) [Au2CCO]na [Cu2CCO]nb [Ag2CCO]nc [(OC)9Co3(CCO)]+ d [(OC)9Fe3(CCO)]2− e Ph3PCCOf H3CCO+SbF6− g H2CCOh
exchange with sodium tetrafluoroborate to obtain the stable {[(SIMes)Au]3(μ3-CCO)}+BF4− (1b). Careful layering of hexanes onto a solution of 1b in CH2Cl2 afforded crystals suitable for X-ray diffraction. The solid-state structure (Figure 1) revealed a linear ketenylidene moiety
C−C (Å)
C−O (Å)
1.318(8)
1.183(7)
1.28(3) 1.210(10) 1.419(4) 1.3165(15)
1.18(3) 1.185(9) 1.108(4) 1.1614(14)
ν (cm−1) 2013 2015 2030 2060 2260 1924 2110 2302 2152
Reference 19. bReference 17. cReference 18. dPF6− counterion; references 2 and 3. e[Ph4As]+ salt; reference 6. fReferences 35 and 37. g Reference 38. hDerived from microwave rotational spectroscopy; reference 39. a
key metrics and stretching frequencies for 1b to those of selected transition-metal ketenylidenes, the main-group ketenylidene Ph3PCCO,35 acetylium ion, and ketene itself. Although in idealized 3-fold symmetry the trigold(I) ketenylidene cation is isolobal36 with acetylium ion, the bond metrics and infrared stretching frequency for the ketenylidene are closer to those of neutral ketene. These data suggest an arrangement closer to net double bonds than to a single and a triple bond. Consistent with this interpretation, the [(LAu)3(μ3-CCO)]+ cation displayed no obvious electrophilic behavior at the carbonyl. Treatment with sodium azide or sodium trimethylsiloxide, in attempts to produce triaurated acetyl derivatives, gave little apparent reaction after several hours as judged by 1H NMR spectroscopy. Attempted aza-Wittig reactions40 to form the complexes [(LAu)3(μ3-CCNR)]+, using N-phenyl- or Nbenzyliminotriphenylphosphorane, led to some phosphine oxide formation as judged by 31P NMR spectroscopy, in addition to a number of yet-unidentified byproducts. Treatment with a solution of the Tebbe reagent [(η5-C5H5)2Ti(μ-Cl)(μCH2)Al(CH3)2],41 in the hope of forming the allenylidene [(LAu)3(μ3-CCCH2)]+,42,43 gave complex product mixtures. The reaction of 1b with excess sodium borohydride in THF suspension, which we had hoped would lead to C−H bond formation, led instead to the formation of (SIMes)AuH (2), reflecting nucleophilic addition of hydride to gold rather than to carbon. This reaction proceeds rather cleanly, with a single set of SIMes resonances in the 1H NMR spectrum, and a singlet resonance integrating to one hydrogen for the hydride. Like that of (IDipp)AuH,44 the hydride resonance for 2 exhibits large solvent shifts, appearing at δ 3.79 ppm in THF-d8 but δ 5.28 ppm in C6D6 solution. The yield was assessed at 88% relative to an internal standard (4,4′-dimethylbiphenyl), and a preparative-scale reaction starting from 1a afforded the hydride in 81% isolated yield, ruling out the possibility that only one or two of the three gold centers is converted to hydride (Scheme 2a).
Figure 1. Solid-state structure of 1b (50% probability ellipsoids). H atoms, BF4− anion, and cocrystallized CH2Cl2 are omitted for clarity. Selected interatomic distances (Å) and angles (deg): C1−C2, 1.318(8); C2−O1, 1.183(7); Au1−C1, 2.062(6); Au2−C1, 2.080(6); Au3−C1, 2.075(6); C1−C2−O1, 178.5(6); Au1−C1−C2, 116.6(4); Au2−C1−C2, 103.2(4); Au3−C1−C2, 105.0(4); C45− Au1−C1, 178.8(2); C3−Au2−C1, 171.5(2); C24−Au3−C1, 172.5(2).
bridging three gold centers in a cation distorted from 3-fold symmetry. The plane of the heterocycle bound to Au1 forms a torsion angle of 87.9(5)° with respect to the ketenylidene C−C vector, in comparison to 18.2(5) and 18.0(5)° for the other two NHC ligands. The Au−C−C angle is significantly larger for Au1 than for Au2 or Au3, placing Au1 farther from the carbonyl carbon C2: 2.903(6) Å versus 2.704(6) Å for Au2 and 2.733(6) Å for Au3. The Au−C1 distances range narrowly from 2.062(5) to 2.080(6) Å. The Au−CNHC distances are likewise quite similar, ranging from 1.998(5) to 2.007(5) Å. The shortest Au···Au distance is only trivially smaller than two van der Waals B
DOI: 10.1021/acs.organomet.7b00481 Organometallics XXXX, XXX, XXX−XXX
Organometallics Scheme 2. Reactions of [(LAu)3CCO]+ with Lewis Bases
Communication
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ACKNOWLEDGMENTS
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REFERENCES
We thank the U.S. National Science Foundation (CHE1300659 to J.P.S.) and the Georgia Institute of Technology for generous support of this research. Professor Jake D. Soper kindly allowed us the use of his group’s FTIR and UV−vis spectrometers.
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We could not account by IR, 1H, or 11B NMR spectroscopy for the fate of the ketenylidene fragment in this reaction. The reaction between equimolar 1b and NaBH4 would afford three (SIMes)AuH plus [HBCCO], not expected to persist in solution. With excess borohydride, anionic ketenide−borane adducts or their decomposition products might be expected. We therefore examined a reaction chosen to give both gold− carbon bond cleavage and conversion of the ketenide to a readily identifiable product (Scheme 2b). The reaction of 1b in CD2Cl2 solution with a mixture of thiophenol and sodium thiophenolate resulted in the clean formation of 3 equiv of (SIMes)AuSPh and 1 equiv of S-phenyl thioacetate,45 as judged by 1H NMR spectroscopy. The identity of the thioacetate ester was confirmed by GC−mass spectrometry. In summary, the reaction of an (NHC)gold(I) acetate with ketene and base leads directly to a trigold ketenylidene cation, rather than to neutral monogold ketenyl or digold ketenide. When this cation is paired with a suitable anion, it is stable, permitting spectroscopic and structural characterization. Reaction with soft Lewis bases leads to facile gold−carbon bond cleavage and, in the case of a protic substrate, conversion of the ketenylidene fragment to an acetyl group.
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ASSOCIATED CONTENT
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The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.7b00481. Experimental procedures and spectral and crystallographic data (PDF) Accession Codes
CCDC 1556767 contains the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing data_
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AUTHOR INFORMATION
Corresponding Author
*E-mail for J.P.S.:
[email protected]. ORCID
John Bacsa: 0000-0001-5681-4458 Joseph P. Sadighi: 0000-0003-1304-1170 Notes
The authors declare no competing financial interest. C
DOI: 10.1021/acs.organomet.7b00481 Organometallics XXXX, XXX, XXX−XXX
Communication
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DOI: 10.1021/acs.organomet.7b00481 Organometallics XXXX, XXX, XXX−XXX
