Dinuclear NHC Gold(I) Allenyl and Propargyl Complexes: An

Article Cite This: Organometallics XXXX, XXX, XXX−XXX

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Dinuclear NHC Gold(I) Allenyl and Propargyl Complexes: An Experimental and Theoretical Study Poorya Zargaran, Florian. F. Mulks, Samuel Gall, Matthias Rudolph, Frank Rominger, and A. Stephen K. Hashmi* Organisch-Chemisches Institut, Heidelberg University, Im Neuenheimer Feld 270, 69120 Heidelberg, Germany

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ABSTRACT: The synthesis and isolation of the dinuclear NHC gold(I) allene1,3-diyl complex Ph(IPrAu)CCCH(AuIPr) and the dinuclear NHC gold(I) propyne-1,3-diyl complex Ph(IPrAu)CH−CC−Au(IPr) are presented. The monoprotodeauration reactions of these dinuclear complexes selectively led to the mononuclear organogold complexes Ph(IPrAu)CCCH2 and PhCH2−CC− Au(IPr), respectively. The experimental structures and the obtained analytical data of the synthesized complexes as well as the results of a computational DFT study of their thermodynamic stability are compared systematically.

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INTRODUCTION In the last few decades, homogeneous gold catalysis has developed into a unique tool in organic synthesis.1−16 Despite the various gold-catalyzed transformations known, the knowledge about the gold−carbon bond in the assumed intermediates is mostly based on the observed reactivity and not on the direct isolation of the corresponding organogold species. After the first isolation of a vinyl gold intermediate by Hammond et al. in 2008,17 many studies focused on the synthesis and isolation of organogold intermediates.18−29 In addition to alkynes and alkenes as substrates for gold-catalyzed transformations,3,30−33 allenes also play an important role and many versatile conversions of these substrates in gold catalysis are known.14,34−36 The substitution pattern of the allene substrate strongly affects the selectivity of these gold-catalyzed reactions.14,36 In order to shed more light on the selectivity issue of such reactions, the synthesis and isolation of allene gold species is of key importance. In 2008 Malacria et al. presented a comprehensive computational study on the interaction of (R)-1-acetoxy-3methylallene with gold(I) and gold(III) complexes. The σcoordinated allylic cation complexes 1 turned out to be the more stable form in comparison with the η1-coordinated bent allene form 2 and the η2-coordinated complex 3 (Scheme 1). For (R)-1,3-dimethylallene they found the η2 complex to be the most stable.37 Two years later, Widenhoefer and co-workers synthesized the first example of a gold(I) allene π complex which demonstrated that the gold was positioned at the less substituted CC bond of the allene.38,39 Although there has still been no consideration of σ-coordinated gold(I) allenyl intermediates in gold catalysis studies, the synthesis of such complexes deepens our knowledge about the structure and properties of such species in general and thus is fundamental to the understanding of more complex systems.40 © XXXX American Chemical Society

Scheme 1. Gold(I) Allene Complexes

The first isolated σ-coordinated gold(I) allene complex was reported by Gimeno and co-workers in 2014 (Scheme 2).41 They applied triphenyl(propargyl)phosphonium bromide as an Scheme 2. Synthesis of Allenylgold(I) Derivatives by Gimeno and Co-workersa

a

Legend: (a) X = Br, [AuCl(tht)], Cs2CO3; (b) X = Br, [Au(C6F5)(tht)], Cs2CO3; (c) X = OTf, [Au(acac)(PPh3)]; (d) [Au(PPh3)2]OTf. Received: December 31, 2018

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DOI: 10.1021/acs.organomet.8b00943 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

of the IPrAu units, respectively (Figure 1, right). In addition, HSQC and HMBC NMR techniques led to the assignment of Cγ and Cα at 62.4 and 115.8 ppm, respectively (Figure 1, right, and Table 1). The absorption for the allenyl CCC fragment appears at 1896 cm−1 in the IR spectrum. The collected data prove the allenic structure of the obtained product, which is in agreement with the reported data by Gimeno for a different monogold(I) allenyl complex.41 With the obtained set of characterization data, a reactivity study on the protodemetalation of the obtained complex 6 was conducted. The solution of 6 in an acidic environment (caused by the traces of acid and water in the applied deuterated CDCl3) led to a completely selective protodemetalation at the terminal position of the allene (Scheme 5). Complex 7 selectively resulted from the protodemetalation of complex 6, which indicates a high lability of the Cγ−Au bond of 6, while the second gold center was completely preserved in the molecule (Table 1). Complex 7 was isolated and characterized by 1H, 13C, DEPT 135, COSY, HMBC, and HSQC NMR techniques as well as IR spectroscopy and mass spectrometry. The two allenic hydrogen atoms appear as a singlet peak at 4.09 ppm, and the 13C NMR spectrum shows the two characteristic peaks at 208.3 and 194.6 ppm for the central allenic carbon and the NHC carbene atom, respectively. The DEPT 135, COSY, HMBC, and HSQC NMR results are in agreement with the suggested structure. The IR spectroscopic analysis of complex 7 shows a sharp peak at 1894 cm−1 which is characteristic for the allenyl moiety. The selective protodemetalation reaction was monitored by the conversion of 6 in CDCl3 containing residual acid and water. Under these conditions the protodemetalation process took place in approximately 33 h (Figure 2). This indicates that the solution is strongly hygroscopic and completely reacts with ambient moisture over 1.5 days. Filtration of the CDCl3 over basic aluminum oxide stopped this reaction. In CDCl3 that was saturated with distilled water even without acid at room temperature, the reaction was diffusion controlled and was complete in under 5 min. The benzylic IPrAu unit showed high stability under these conditions (Figure 3b), but when the amount of acid was increased to 0.64 M (HCl in H2O added to a 20 mM solution of 6 in 500 μL of CDCl3), protodemetalation of the benzylic IPrAu moiety was observed (as the second protodemetalation reaction) and 1-phenylallene resulted (Figure 3c). In NMR kinetic experiments with CDCl3 that was filtered over basic aluminum oxide and afterward saturated with distilled H2O at room temperature, we found the first protodemetalation to be very fast. It was complete in under 5 min to give 7 quantitatively. The second protodemetalation needed several days at room temperature. A series of 1H NMR spectra was taken at 22, 40, 60, and 80 °C to gain deeper insights into the reaction kinetics (Figure 4). 7 was isolated for this reaction and employed in 1.3 mM concentration. CDCl3 (freshly filtrated over aluminum oxide) was employed that was saturated with H2O at 22 °C to dissolve 7 and to immediately start the protodemetalation. We found a relatively low activation barrier of ΔG⧧ = 12.8 kcal/mol with a coefficient of determination R2 = 0.9913 in an Arrhenius plot for the first-order reaction (Figure 5). This seems feasible in the context of the known high relative basicities of vinylgold compounds in comparison to vinylic dihydrofurans, for which we published a computational analysis. It ranges in the lower end of the spectrum of

allene precursor; the resulting complex is not a pure gold(I) allenyl complex but due to the phosphonium substituent can be rationalized as the first isolated gold(I) vinylidenoid complex and thus was the first ever, but so far overlooked, gold carbenoid. Furthermore, despite the preceding studies on the synthesis of N-heterocyclic carbene (NHC) allene complexes,42,43 there has still been no report on the isolation of σ-coordinated NHC metal allene complexes in the literature. When we were writing up this paper, Widenhoefer et al. published a report which focuses on gold(I) allenylidene complexes and after nucleophilic addition provides one gold(I) allenyloid complex with an α-1-pyridinium or α-1-tetrahydrothiophenium substituent: i.e., again carbenoids.44 This overall lack of knowledge encouraged us to investigate potential new precursors for the synthesis and isolation of electronically undisturbed mono- and dinuclear σ-coordinated allenic complexes bearing NHC ligands at the gold(I) metal center.

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RESULTS AND DISCUSSION The first step of our study was access to a proper allene source. Klein and Brenner reported a comprehensive study on the metalation of 1-phenylpropyne (4) in 1970.45 The addition of n-butyllithium to a solution of 4 delivered a dilithiated allenyl species (5) as product (Scheme 3). Scheme 3. Lithiation of 1-Phenylpropyne Reported by Klein and Brenner45

This study encouraged us to apply 1-phenylpropyne (4) as the allene source for the generation of a σ-coordinated dinuclear allenic NHC gold(I) complex. By following this idea, species 6 was synthesized in situ and trapped by 2.0 equiv of IPrAuCl at −40 °C (Scheme 4). Scheme 4. Synthetic Route toward the Dinuclear NHCGold(I) Allenyl Complex 6

The 1H NMR of the resulting crude product showed the target complex in approximately 67% yield. The obtained product decomposed during column chromatography or Soxhlet extraction, but it was surprisingly unipolar. Filtration in n-pentane delivered 6 in acceptable yield (65%). Complex 6 is the first example of a dinuclear gold allenediyl complex. The product was characterized by 1H, 13C, DEPT 135, COSY, HMBC, and HSQC NMR techniques as well as IR spectroscopy and mass spectrometry. The 1H NMR spectrum shows a singlet at 4.09 ppm, which is characteristic of Hα (Figure 1, left). In the 13C NMR spectrum three peaks are detected at 208.2, 202.9, and 194.5 ppm that can be attributed to Cβ and the two carbene signals B


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Figure 1. Comparison of the 1H NMR and 13C NMR spectra of compounds 6 and 7 in CDCl3.

expected, by addition of a solution of 9 to a solution of 2.0 equiv of IPrAuCl in THF under an argon atmosphere, a dinuclear propargylic NHC gold(I) complex (10) was obtained (Scheme 6b). The obtained complex 10 was isolated in 71% yield by recrystallization from toluene. Complex 10 was also characterized by 1H, 13C, DEPT 135, COSY, HMBC, and HSQC NMR experiments as well as IR spectroscopy and mass spectrometry. As expected, the same mass as for complex 6 was detected. The 1H NMR spectrum shows a sharp singlet at 3.66 ppm in the same solvent (CDCl3) which is characteristic for Hα in this complex (Figure 6, left). The 13C NMR spectrum shows Cα, Cβ, and Cγ at 26.6, 100.8, and 119.1 ppm, respectively. For a better structural comparison, acetylide complex 11 was synthesized by adding 0.9 equiv of n-butyllithium to a solution of 8 followed by the addition of 0.85 equiv of IPrAuCl (Scheme 7). The 1H NMR spectrum shows the characteristic propargylic hydrogens at 3.41 ppm (Figure 6, left). 13C NMR spectroscopy detected Cα, Cβ, and Cγ in the same range as for complex 10 (Figure 6, right, and Table 2). This is in good agreement with our previously reported study on gold allenylidene complexes.53 The molecular structure of 11 in the solid state is presented in Figure 7. The protodemetalation reaction of complex 10 was also studied (Figure 8). The propargylic bond was extremely labile toward protodeauration. As had already occurred during the isolation of 10, the propargylic bond selectively protodeaurated within minutes in CDCl3 (freshly filtered over basic aluminum oxide) that was saturated with distilled H2O at 22 °C. The protodeauration of 11 was extremely slow, both with water and even with benzoic acid (Figure 8). We conducted T1/T2 1H NMR kinetic experiments with the more acidic pnitrobenzoic acid (pKa = 3.42). Five equiv of the acid was used to ensure a (pseudo) first-order reaction toward the acid. The reactions were conducted on a 8 μmol scale (1.3 mM) in CDCl3 with 2,2,5,5-tetramethyltetrahydrofuran as internal standard. The NMR tubes with J. Young valves were heated in the spectrometer during the measurements to 60, 40, and 25 °C. We found a rather high activation barrier of ΔG⧧ = 20.8 kcal/mol with a coefficient of determination R2 = 0.9988 in an Arrhenius plot for the first-order reaction (see Figure 9). This

Table 1. NMR Chemical Shifts of Allenyl Compounds allene compound

Hα

Cα

Cβ

Cγ

PhCHCCH246 Ph(IPrAu)CCCH2 (7) Ph(IPrAu)CCCH(AuIPr) (6)

5.15 4.09 4.09

93.9 116.0 115.8

209.0 208.4 208.2

78.9 62.6 62.4

Scheme 5. Protodemetalation Reaction of the Synthesized Complex 6

Figure 2. Protodemetalation reaction of 6 in CDCl3 monitored by 1H NMR.

activation energies that were found for different dihydrofurans (3.1−25.8 kcal/mol).47−49 Water was enough to conduct the first protodemetalation within minutes and the second one within days. The analogous protodeauration of the other gold moiety shows a higher barrier to the point that we could not see any competition. This reaction would induce interaction of the attacking proton (and by that also a charge) in the benzylic position of a styrene moiety, and thus stabilization of this charge would compete with the conjugation of the styryl system. Our study was continued by applying 3-phenyl-1-propyne (8) as a substrate under the same conditions (Scheme 6). It is known that 8 produces 1,3-dilithio-3-phenylpropyne (9) in the presence of 2.0 equiv of n-butyllithium (Scheme 6a).50−52 As C


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Figure 3. NMR monitoring of the protodemetalation reaction of 6 in CDCl3 ((a) at 0 h; (b) at 13 h) and the second protodemetalation in CDCl3 with 0.64 M HCl ((c) after 33 h).

Figure 4. First-order kinetics plot of the protodeauration of 7 in CDCl3 saturated with H2O at different temperatures.

under the influence of ambient air 10 seemed even more reactive toward water. The alkynyl gold group in 11, however, was shown to be very stable. The activation barrier was found

is comparable to those for average dihydrofuran vinylgold species.15 While both 6 and 10 were protodeaurated at extremely fast rates with water, judging from its decomposition D


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Scheme 7. Synthesis of Alkynyl NHC Gold(I) Complex 11

Table 2. NMR Chemical Shifts of Propargyl Compounds 8, 11, and 10 propargyl compound

Hα

Cα

Cβ

Cγ

Ph−CH2−CCH (8) Ph−CH2−CC(AuIPr) (11) Ph(IPrAu)CH−CC(AuIPr) (10)

3.61 3.66 3.66

24.8 26.6 26.6

82.0 100.8 100.8

70.5 119.0 119.1

Figure 5. Arrhenius plot of the protodeauration of 7 in CDCl3 saturated with H2O at different temperatures.

Scheme 6. Synthesis of a Dinuclear Propargylic NHC Gold(I) Complex

to be 8 kcal/mol higher than for the protodeauration of 7, resulting in the need to employ a much stronger acid to induce protodeauration on a comparable time scale. It is noteworthy that the selectivity of the protodeauration of 7 and 11 decreased under these much milder conditions in comparison to the case for HCl. Rearrangements and decomposition of the products on this time scale might be part of the reason for this. However, it is likely that these reactions were quite unselective because IPrAuOH or IPrAu(pOCO−C6H4−NO2) were generated, which are both much more reactive than IPrAuCl, with a basic hydroxide or a weakly coordinating carboxylate, respectively. We have experimentally observed that 6 and 10 in a hydrochloric acid environment lead to a selective protodemetalation reaction. This encouraged us to calculate the

Figure 7. Molecular structure of complex 11 in the solid state.

thermodynamics of the protodeauration of 6 and 10 with HCl, leading to the hydrocarbons and IPrAuCl. Kohn−Sham density functional theory calculations were performed at the B97D/SDD+f (Au) and 6-31G** (other atoms) levels of theory with the Gaussian 09 program package to investigate the thermodynamics of the investigated protodeaurations.54−60 Bourissou et al. have successfully used this method to reproduce experimental ΔH⧧ value of a silaauracyclobutene transition state and used it to investigate a five-membered ring and a carbene complex of their o-carborane diphosphine gold(I) precursor.61−63 Furthermore, we already did an indepth study on cyclopropenylgold(I) complexes in which we

Figure 6. Comparison of the 1H NMR and 13C NMR spectra of compounds 10 and 11 in CDCl3. E


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Figure 8. First-order kinetics plot of the protodeauration of 11 in CDCl3 saturated with H2O at different temperatures.

demetalations (ΔRGboth solvents ≈ 100 kJ/mol; see Table 3). Neither the protodeauration position nor the solvent polarity seemed to have any influence. In addition, we calculated the protodeauration reaction toward the pure hydrocarbons. The same reaction thermodynamics as in the corresponding diaurated systems were observed: i.e., virtually identical reaction energies. This indicates that the effect of the IMeAu unit on the bond strength of the opposite site of the allene unit is very limited. Thus, the experimental finding of selective protodeauration at the 3-position is kinetically controlled. For the propargyl complexes, the IMeAu unit at the propargyl position is thermodynamically highly exergonic toward protodeauration in comparison to that at the alkynyl position (ΔΔRGchloroform = 51.31 kJ/mol), which is in line with the experimentally observed data. Again, insignificant differences were observed for the thermodynamics of protodeauration on one AuIMe domain, when mono- and diaurated species are compared. This is a strong hint that AuIMe substituents do not have a significant effect on the electronic properties and stability of the domains on the other side of our allenyl and propargyl systems, allowing us to employ both in reactions individually.

Figure 9. Arrhenius plot of the protodeauration of 11 in CDCl3 saturated with H2O at different temperatures.

found this methodology to be very reliable.64 All geometries were optimized with tight optimization parameters and ultrafine integration grid. We used the polarizable conductor calculation model (CPCM) to model solvent influences in a self-consistent reaction field (SCRF).65−70 Structures were optimized in the respective solvent model and afterward subjected to frequency calculations with the same parameters to confirm them as local minima. The reaction of the isolated gold complexes with hydrogen chloride to give LAuCl and the protonated substituent was analyzed. The ligand IPr was simplified with IMe (methyl substituents in 1- and 3-positions of the imidazole unit). Both chloroform and dichloromethane were used to evaluate the polarity influences of the solvents used (Scheme 8). For the allenyl complexes virtually no thermodynamic energy differences were observed for the addressed proto-

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CONCLUSIONS

The first dinuclear NHC gold(I) allenyl complex, Ph(IPrAu)CCCH(AuIPr), and the first dinuclear NHC gold(I) propargyl-alkynyl complex, Ph(IPrAu)CH−CC−AuIPr, have been synthesized and fully evaluated. The selective protodemetalation reaction of Ph(IPrAu)C CCH(AuIPr) led to the new complex Ph(IPrAu)CC CH2. Our quantum chemical data on the thermodynamics of this reaction system shows no thermodynamic privilege for either the 1-position or the 3-position in this case and therefore suggest kinetic control of this highly selective protodeauration. Furthermore, by increasing the acidity of the environment, the protodemetalation of the remaining benzylic IPrAu moiety was observed. The propargyl complex Ph(IPrAu)CH−CC−AuIPr shows high selectivity for protodemetalation at the 3-position, releasing the propargylic gold center. In this case our DFT studies support thermodynamic control. In addition, the acetylide complex PhCH2−CC−AuIPr has been also synthesized and evaluated to give a better understanding of the effects of auration at different moieties of the alkyne.

Scheme 8. Reaction Pattern Used To Investigate the Thermodynamics of the Protodeauration Reactions of Complexes 6 and 10

F


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Table 3. Thermodynamic Data of the Reaction Displayed in Scheme 8 Calculated at the B97D/SDD+f (Au) and 6-31G** (Other Atoms) Level of Theory

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spectrometer. T1 (spin−lattice relaxation time constant) values of 6 and 7 were measured in NMR tubes with J. Young valves that were charged with 8 μmol of the compound and 8 μmol of 2,2,5,5tetramethyltetrahydrofuran in 600 μmol of CDCl3. This sample was used for locking and shimming. The reaction was performed in an NMR tube with a J. Young valve that was charged with the gold complex (8 μmol) and a solution of 8 μmol of 2,2,5,5tetramethyltetrahydrofuran in 600 μL of CDCl3 that was saturated with distilled H2O at room temperature. On the time scale of the necessary relaxation delay, we were unable to observe the reaction of 6 to 7. Only traces of 6 were measured in the first scan. Bis(1,3-bis(2,6-diisopropylphenyl)-2,3-imidazol-2-yl)(3-phenylprop-1-yn-1,3-yl)digold(I) (10). A Schlenk flask that was flamedried three times was charged with 40.0 mg (344.3 mmol) of 3phenylpropyne dissolved in 5.0 mL of dry THF. The solution was cooled to −78 °C, and 0.3 mL of a 2.5 M solution of n-butyllithium in hexane (750 mmol) was gently added under argon atmosphere. The solution was stirred for 2 h until it reached −40 °C. The obtained dark green solution was added to a prepared solution of IPrAuCl (427.7 mg, 688.7 μmol) in 10 mL of dry THF at −40 °C. The solution was stirred for 4 h while it was warmed to room temperature. The solvent was removed in vacuo, and the crude product was dissolved in dry toluene and filtered over flame-dried Celite under inert gas. The solvent was removed under inert conditions, and the orange crude product was dissolved in toluene under reflux and crystallized overnight at −21 °C. The solvent was filtered into a second Schlenk flask employing a cannula capped with a paper filter under positive nitrogen pressure. Then, the solvent was removed under vacuum and the title compound was obtained as an off-white

EXPERIMENTAL SECTION

Bis(1,3-bis(2,6-diisopropylphenyl)-2,3-imidazol-2-yl)(1-phenylallen-1,3-yl)digold(I) (6). A Schlenk flask that was flame-dried three times was charged with 40.0 mg (344.3 mmol) of 1phenylpropyne, and this compound was dissolved in 2.0 mL of dry THF. The solution was cooled to −78 °C, and 0.3 mL of a 2.5 M solution of n-butyllythium in hexane (757.5 mmol) was gently added under an argon atmosphere. The solution was stirred for 2 h until it reached −40 °C. The obtained dark red solution was added to a prepared solution of IPrAuCl (427.7 mg, 688.7 μmol) dissolved in 10 mL of dry THF at −40 °C. The solution was stirred for 4 h until it slowly reached room temperature. The solvent was removed in vacuo, and the crude product was dissolved in n-pentane and filtered over flame-dried Celite under inert gas. The solvent was removed on a rotary evaporator, and the title compound was yielded as a colorless powder: yield 65% (267.1 mg, 223.8 mmol); 1H NMR (300 MHz, CDCl3) δ 1.19−1.24 (m, 24H), 1.34−1.40 (m, 24H), 2.54−2.71 (m, 8H), 4.08 (s, 1H), 6.88−7.07 (m, 5H), 7.09 (s, 4H), 7.12−7.16 (m, 4H), 7.24−7.31 (m, 4H), 7.43−7.52 (m, 4H); 13C NMR (500 MHz, CDCl3) δ 14.15, 21.12, 24.08, 24.18, 24.39, 24.43, 25.74, 28.64, 28.82, 28.92, 30.44, 34.65, 62.40, 68.09, 115.89, 122.38, 122.88, 123.87, 124.13, 124.18, 124.26, 124.39, 124.43, 127.46, 127.85, 128.20, 129.37, 129.96, 130.32, 130.53, 130.58, 134.68, 135.07, 143.18, 145.90, 194.51, 202.97, 208.19; IR (ATR) ν̃ 3161, 2962, 2926, 2867, 2181, 1896, 1592, 1470, 1413, 1384, 1363, 1329, 1256, 1213, 1181, 1106, 1060, 974, 947, 803, 758, 743, 701, 673 cm−1; HR-MS (DART pos): C63H79AuN4 [M + H+] calcd 1285.5636, found 1285.5657. Protodeauration of Bis(1,3-bis(2,6-diisopropylphenyl)-2,3imidazol-2-yl)(1-phenylallen-1,3-yl)digold(I) (6). T1/T2 kinetic experiments were performed on a Bruker Avance DRX-300 NMR G


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Organometallics

CDCl3) δ 24.00, 24.57. 26.57, 28.78, 28.81, 100.77, 119.00, 123.07, 124.13, 124.27, 125.29, 125.54, 127.88, 128.06, 128.22, 129.03, 130.40, 134.36, 137.87, 139.10, 145.59, 191.38; IR (ATR) ν̃ 2962, 1735, 1494, 1469, 1416, 1385, 1364, 1330, 1261, 1330, 1093, 1060, 1006, 1029, 937, 686, 804, 761, 730, 406, 669, 613 cm−1; HR-MS (DART pos) C36H44AuN2+ [M + H+] calcd 701.3170, found 701.3164. Protodeauration of (1,3-Bis(2,6-diisopropylphenyl)-2,3-imidazol-2-yl)(3-phenylpropyn-1-yl)gold(I) (11). T1/T2 kinetic experiments were performed on a Bruker Avance DRX-300 NMR spectrometer. The T1 (spin−lattice relaxation time constant) value of 11 was measured in an NMR tube with a J. Young valve that was charged with 8 μmol of the compound and 8 μmol of 2,2,5,5tetramethyltetrahydrofuran in 600 μmol of CDCl3. The reaction was performed in an NMR tube with a J. Young valve that was charged with the gold complex (8 μmol) and a solution of 8 μmol of 2,2,5,5tetramethyltetrahydrofuran in 600 μL of CDCl3. This reaction sample was used for locking and shimming, before it was charged with 40 μmol of p-nitrobenzoic acid (∼40 μmol) and the kinetic experiment was started.

powder: yield 71% (314.2 mg, 144.4 mmol) 1H NMR (CDCl3, 500 MHz): δ 1.22−2.25 (m, 24H), 1.36−1.40 (m, 24H), 2.55−2.64 (m, 8H), 3.66 (s, 1H), 7.10−7.21 (m, 7H), 7.27−7.33 (m, 10H), 7.47− 7.53 (m, 4H); 13C NMR (500 MHz, CDCl3) δ 24.02, 24.03, 24.46, 26.58, 28.79, 28.82, 100.76, 119.05, 123.07, 123.10, 124.14, 124.27, 125.54, 127.87, 128.07, 130.40, 134.39, 139.14, 145.60, 145.61, 170.50, 191.43; IR (ATR) ν̃ 3421, 3073 2962, 2868, 1470, 1415, 1364, 1329, 1261, 1098, 1023, 946, 865, 803, 759, 700, 612 cm−1; HR-MS (DART pos) C63H79AuN4 [M + H+] calcd 1285.5636, found 1285.5634. Protodeauration of Bis(1,3-bis(2,6-diisopropylphenyl)-2,3imidazol-2-yl)(3-phenylprop-1-yn-1,3-yl)digold(I) (10). T1/T2 kinetic experiments were performed on a Bruker Avance DRX-300 NMR spectrometer. T1 (spin−lattice relaxation time constant) values of 10 and 11 were measured in NMR tubes with J. Young valves that were charged with 8 μmol of the compound and 8 μmol of 2,2,5,5tetramethyltetrahydrofuran in 600 μmol of CDCl3. This sample was used for locking and shimming. The reaction was performed in an NMR tube with a J. Young valve that was charged with the gold complex (8 μmol) and a solution of 8 μmol of 2,2,5,5tetramethyltetrahydrofuran in 600 μL of CDCl3 that was saturated with distilled H2O at room temperature. On the time scale of the necessary relaxation delay, we were unable to observe the reaction of 10 to 11. Only traces of 10 were measured in the first scan. (1,3-Bis(2,6-diisopropylphenyl)-2,3-imidazol-2-yl)(1-phenylallen-1-yl)gold(I) (7). A 160.27 mg portion (124.5 mmol) of complex 6 was placed in a round-bottom flask and dissolved in CH2Cl2. The solution was stirred for 24 h in air. The solvent was removed in vacuo, and the crude powder was purified by recrystallization in diethyl ether and n-hexane and kept at −28 °C. The title compound was obtained as colorless crystals: yield 35% (30.0 mg, 43.0 mmol); 1H NMR (300 MHz, CDCl3) δ 1.22 (d, 12H), 1.35 (d, 12H), 2.67 (sept., 4H), 4.08 (s, 2H), 6.88−6.90 (m, 1H), 6.94−6.99 (m, 4H), 7.04 (s, 2H), 7.16−7.31 (m, 4H), 7.50 (t, 2H); 13 C NMR (500 MHz, CDCl3) δ 24.3, 24.6, 29.0, 62.6, 116.0, 123.0, 124.6, 127.6, 129.5, 130.5, 134.8, 143.3, 146.0, 194.6, 208.3; IR (ATR) ν̃ 3154, 2996, 2928, 2869, 1895, 1592, 1469, 1413, 1384, 1363, 1257, 1182, 1108, 1063, 946, 801, 777, 758, 699, 656 cm−1; HR-MS (DART pos) C36H44AuN2+ [M + H+] calcd 701.3170, found 701.3164. Protodeauration of (1,3-Bis(2,6-diisopropylphenyl)-2,3-imidazol-2-yl)(1-phenylallen-1-yl)gold(I) (7). An NMR tube with a J. Young valve was charged with the gold complex (8 μmol). An 8 μmol portion of 2,2,5,5-tetramethyltetrahydrofuran was dissolved in 600 μL of CDCl3 that was saturated with distilled H2O at room temperature. As this reaction was slow, we chose to measure a series of 1H NMR experiments. A 1H NMR spectrum was taken for t = 0, before submitting the identical samples to heating at different temperatures. The time was taken until the samples were cooled to −28 °C to stop the reactions, before another 1H NMR spectrum was taken. This procedure was repeated until enough data points were gathered. (1,3-Bis(2,6-diisopropylphenyl)-2,3-imidazol-2-yl)(3-phenylpropyn-1-yl)gold(I) (11). A Schlenk flask that was flame-dried three times was charged with 45.0 mg (387.4 mmol) of 3-phenylpropyne in 5.0 mL of dry THF. The solution was cooled to −78 °C, and 0.15 mL of a 2.5 M solution of n-butyllithium in hexane (375 mmol) was added gently under an argon atmosphere. The solution was stirred for 2 h until it reached −40 °C. A prepared solution of IPrAuCl (228.5 mg, 368 μmol) in 5 mL of dry THF was cooled to −40 °C and was added to the lithiated solution dropwise. The solution was stirred for 4 h and was warmed to room temperature. The solvent was removed in vacuo, and the crude product was dissolved in toluene. The obtained suspension was filtered over a pad of Celite, and the solvent was removed. The obtained crude product was purified by recrystallization in THF and n-pentane. The title compound was obtained as off-white crystals: yield 93% (251.22 mg, 358.52 μmol); 1 H NMR (300 MHz, d8-THF) δ 1.11 (d, 12H), 1.27 (d, 12H), 2.55 (sept, 4H), 3.30 (s, 1H), 6.85−6.90 (m, 1H), 6.94−6.99 (m, 2H), 7.10−7.12 (m, 2H), 7.20−7.34 (m, 9H); 13C NMR (125 MHz,

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.8b00943. General information for synthetic and computational methods and spectral data (PDF) xyz coordinates of computed structures (XYZ) Accession Codes

CCDC 1824339 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 [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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AUTHOR INFORMATION

Corresponding Author

*E-mail for A.S.K.H.: [email protected]. ORCID

A. Stephen K. Hashmi: 0000-0002-6720-8602 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS P.Z. is grateful for a DAAD PhD fellowship. The authors acknowledge support by the state of Baden-Württemberg through bwHPC.

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REFERENCES

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Dinuclear NHC Gold(I) Allenyl and Propargyl Complexes: An

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