NHC-Gold-Alkyne Complexes: Influence of the Carbene Backbone on

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NHC-Gold-Alkyne Complexes: Influence of the Carbene Backbone on the Ion Pair Structure Gianluca Ciancaleoni,† Luca Biasiolo,†,‡ Giovanni Bistoni,†,§ Alceo Macchioni,§ Francesco Tarantelli,†,§ Daniele Zuccaccia,*,‡ and Leonardo Belpassi*,† †

Istituto di Scienze e Tecnologie Molecolari del CNR (CNR-ISTM), c/o Dipartimento di Chimica, Università degli Studi di Perugia, I-06123, Perugia, Italy ‡ Dipartimento di Chimica, Fisica e Ambiente, Università di Udine, Via Cotonificio 108, I-33100 Udine, Italy § Dipartimento di Chimica, Università degli Studi di Perugia, Via Elce di Sotto 8, I-06123, Perugia, Italy S Supporting Information *

ABSTRACT: We have studied the ion pair structure of [NHCAu(η2-3-hexyne)][BF4] (NHC = nitrogen-heterocyclic carbene) by solution NOE NMR spectroscopy and relativistic DFT calculation. The neutral complexes [NHC-AuCl] have been synthesized through an improved, silver-free one-pot synthesis, by reaction (in air and using solvents and substrates without any previous purification) of the appropriate [NHC(H)]Cl, gold precursor, and KHCO3. Ion pairs were generated in situ in NMR tubes. In our previous work, two main ion pair orientations were observed for unsaturated NHC ligands: one with the anion close to the carbene backbone (A, most populated) and another with the anion close to the alkyne (B). Here we focus on the effect of the carbene backbone on the ion pair structure, comparing the unsaturated NHC (1BF4) with two different variants: a saturated NHC (2BF4) and a polycyclic ligand with an extended aromatic system (3BF4). For 2BF4, the A:B ratio remains almost the same as for 1BF4, while the ion pair structure of 3BF4 becomes mainly nonspecific, with a slight preference for the orientation B. Both cases can be explained analyzing the DFT Coulomb potential map, which shows an attractive region on the backbone of 2BF4 and a flat weak potential around the whole 3BF4.

C

solution and its dependence on the ligand properties. For this reason, in the last years our group has been carrying out a systematic NMR/DFT study on one of the key intermediates in the functionalization of double and triple C−C bonds, the linear bis-coordinated [L-Au-(UHC)][BF4] ion pair. We have demonstrated by 19F,1H-HOESY NMR17 how the anion position can be finely tuned by the choice of the ligand8 and how the ion pair structure can be related to the electronic properties of the phosphorus-based ligand.9a The results show that the anion is mainly located on the ligand side when carbene ligands are used8 and on the UHC side in phosphinegold complexes.18 This is due to the charge distribution throughout the complex, which can be effectively rationalized by relativistic density functional theory (DFT) calculations and, in particular, maps of the Coulomb potential around the cation, useful to locate the most attractive regions.8,9 In this contribution, we continue exploring the relation between ligand properties and anion/cation relative position, focusing on NHC carbene ligands. We heavily modified the ligand structure with the aim to produce a different charge

arbene ligands are commonly used in organometallic chemistry,1 and interest in their properties is continuously increasing in many fields.2 In particular, Arduengo-type Nheterocyclic carbenes (NHC)3 show a great affinity toward gold(I),4 allowing chemists to isolate and study species such as gold carbonyls,5 multimetallic complexes,6 and linear [(NHC)Au(UHC)][X] complexes (UHC = unsaturated hydrocarbon).7−9 The driving force of this research field, besides the natural curiosity of organometallic chemists, is the high potential of carbene gold complexes as catalysts.10 In fact, nowadays most gold(I) catalysts bear carbene or phosphine molecules as ligands, with a plethora of “variations on a theme”. Such variations are very important in order to direct the outcome of the catalysis, but finding a rationale for the results is not always easy.11 Among the factors influencing the product distribution, the anion can be crucial to tune the performance of a catalyst. Recent papers show that the anion can influence the yield12 and the regio- and stereoselectivity13 of a catalyst. It is also observed that the anion influences the structure of the catalyst14 and important intermediates of the catalytic cycle.15 Unfortunately, explaining the details of the anion effect is complicated and, at the same time, its role is very difficult to study.16 One of the few pieces of experimentally accessible information is the most favored position of the anion in © XXXX American Chemical Society

Received: June 21, 2013

A

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complexes,23 even if the carbonate salt is highly hygroscopic and very dry conditions are required. We obtained the NHC carbene gold precursors 1−3Cl in high yield and purity and without successive crystallization with a simple one-pot methodology.24 Stirring [NHC(H)]Cl salt, KHCO3, and THT-AuCl (THT = tetrahydrothiophene) together in a CH2Cl2/CH3OH mixture in air without any distillation of solvents and without drying the KHCO3 (Scheme 1; see the Supporting Information for details) yields directly the desired gold precursors.

distribution and, consequently, different ion pair structures. We already know that for the “classical” unsaturated [(IPr)-Au(η22-hexyne)][BF4] [IPr = (1,3-bis(2,6-di-isopropylphenyl)imidazol-2-ylidene)] the anion is strongly attracted by the hydrogen atoms on the imidazole ring.9b The substitution of such hydrogen atoms with methyl groups only moderately reduces this interaction in [NHC-Au-(η2-alkene)][BF4] related complexes.8b Here we study the influence of eliminating the unsaturation using the saturated heterocyclic carbene [(SIPr)Au(η2-3-hexyne)][BF4] [SIPr = 1,3-bis(2,6-diisopropylphenyl)dihydroimidazol-2-ylidene] (Chart 1, 2BF4),

Scheme 1. One-Pot Synthesis of [NHC-AuCl] Precursors 1− 3Cl

Chart 1. Structure and Numbering of NHC Gold Complexes

Subsequently, cationic bis-coordinate gold(I) complexes 1− 3BF4 were generated in an NMR tube by the reaction of the parent [NHC-AuCl] complexes with AgBF4 in CD2Cl2, in the presence of 3-hexyne (see the Supporting Information for details). From 1H, 13C, 1H-COSY, 1H-NOESY, 1H,13C-HMQC, and 1H,13C-HMBC NMR spectroscopies all proton and carbon resonances belonging to the different fragments were assigned (see the Supporting Information and Scheme 1 for the numbering of carbon and proton resonances). The interionic structure has been studied combining the 19 1 F, H-HOESY NMR (Figures 1 and 2) technique and

in order to reduce the affinity of the hydrogen atoms of the imidazole ring toward the anion. We also explore the opposite direction, studying the effect of an extended aromatic π-system with the [IPr(BIAN)-Au(η2-3-hexyne)][BF4] ligand19 [IPr(BIAN) = bis(imino)acenaphthene-1,3-bis(2,6-diisopropylphenyl)dihydroimidazol-2-ylidene] (Chart 1, 3BF4). In fact, it is known that such a polycyclic aromatic system in Pd(II) and Pt(II) complexes delocalizes the charge so effectively as to make the ion pair structure completely unspecific.20 For a strict comparison between the complexes also [(IPr)Au(η2-3-hexyne)][BF4] (Chart 1, 1BF4) was synthesized and studied. Before the description of the NMR/DFT results, we briefly present an improved strategy to synthesize NHC gold chloride complexes. Generally, such species are prepared by deprotonation of imidazolium salts [NHC(H)]Cl with a strong base, followed by the addiction of a gold source.4,21 Since this method requires moisture- and air-free conditions, different approaches have been developed in the last years: one of them is the synthesis of [NHC-AgCl] complexes22 as carbene transfer agents, which are able to transfer the carbene to a gold(I) precursor via a transmetalation reaction, with precipitation of silver chloride. Decomposition of silver adducts can reduce the yield and purity of the final gold complexes, and recrystallization is always necessary. Recently [NHC(H)][HCO3] salts have been used for the synthesis of gold

Figure 1. (Top) Coulomb potential map (in au) of 2+, mapped on an electronic isodensity surface (0.007 e/Å3) (Bottom) 19F,1H-HOESY NMR spectrum (376.65 MHz, 297 K, CD2Cl2) of complex 2BF4. For the numbering of protons see Chart 1. * denotes the resonances of free 3-hexyne.27 B

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Table 1. Relative NOE Intensities Determined by Arbitrarily Fixing the Largest Intensity of the NOE(s) between the Anion Fluorines and the Cation Protons to 1 1BF4 H1 H2 H3 H5 H4 H6 H7 H8 H9 H10 A:B

2BF4a

2BF4

0.13 0.18 0.50b

0.10 0.39 0.40b

0.17 0.32 0.37b

0.23 0.17 0.08 1.00

0.32 0.10 0.04 1.00

0.27 0.10 0.05 1.00

85:15

72:28

76:24

3BF4 0.49 1.00 0.34 0.50 0.89 0.46c 0.26 0.24 0.46c 0.37 27:73d

a

Isolated ion pairs. bH3/F and H5/F contacts partially overlap. cH6/F and H9/F contacts partially overlap. dIn this case the conformations A and B are not enough to fully describe the ion pair structure; see text.

comparison of the potential maps of 2+ and 1+ (see the Supporting Information) shows that the partial saturation of the NHC ring only slightly reduces the accumulation of positive charge on the ligand backbone, thus giving a similar A:B ratio (Table 1). Figure 2 shows the 19F,1H-HOESY spectrum for 3BF4. In this case all the interionic interactions are of comparable intensity. In fact the weakest is H8/F (H8 is the most “internal” proton of the BIAN system), with relative intensity 0.24. This means that there is no single favored conformation and many anion/cation relative orientations are possible. The most intense contact is with H2, and, among the carbene ligands we have studied up to now, this is the first time that the conformation B is favored, even if slightly, over the A one. This aspecific ion pair structure is a direct consequence of the extended aromaticity on the backbone of the carbene, which makes the Coulomb potential quite flat throughout the cation. This is seen in the Coulomb map of 3+ (Figure 2), where the regions around the 3-hexyne, the carbene ring, and the aromatic fragment of NHC show essentially the same small potential value. In conclusion, we have described the ion pair structure for different [(NHC)-Au(η2-3-hexyne)][BF4] complexes, chosen with the aim of heavily altering the imidazole ring. We also successfully tested a simple one-pot procedure for the synthesis of [NHC-AuCl] neutral precursors, which is silver-free and moisture- and air-tolerant. The NOE results on the saturated NHC show that the removal of the unsaturation is not enough to markedly influence the ion pair structure, and the anion still prefers to stay on the carbene side. On the other hand, the extended aromaticity of an acenaphthene-based NHC makes the ion pair structure aspecific, with only a small preference for the UHC side. These results underline that the structure in solution of NHC gold(I)-UHC ion pairs cannot be easily tuned, and meaningful modifications can be achieved only through large alterations of the NHC backbone. Gold complexes containing phosphorusbased ligands show a more marked ligand influence on the ion pair structure in solution.9a,28

+

Figure 2. (Top) Coulomb potential map (in au) of 3 , mapped on an electronic isodensity surface (0.007 e/Å3). (Bottom) 19F,1H-HOESY NMR spectrum (376.65 MHz, 297 K, CD2Cl2) of complex 3BF4. For the numbering of protons see Chart 1. * denotes the resonances of free 3-hexyne.

relativistic DFT calculations. The latter were performed using the ADF (Amsterdam Density Functional) package, at the TZ2P/BLYP/ZORA25 level (see Computational Details in the Supporting Information), including explicitly the conductor-like screening model (COSMO,26 with ε = 8.93) to include solvent effects. From previous studies28 on [NHC-Au-(UHC)][BF4] ion pairs we found that two important ion pair relative orientations are possible, A and B, in which the anion is located near NHC and UHC, respectively. Other orientations seem to be unfavored.29 Since the protons of the imidazole ring of the NHC ligand are slightly acidic, they carry some additional positive charge and may act as an anchoring point for the anion, thus favoring orientation A. This is confirmed by analyzing30 the NOE contacts for 1BF4 (Figure S8 in the Supporting Information). The most intense contact is with H8 (Table 1), and it is representative for orientation A, whereas the contact H2/F (representative for the orientation B) is weaker. The weakness of H6/F and H7/F contacts indicates that the ion pair structure can be well described by either orientation A or B. The ratio between H8/F and H2/F is 1.00:0.18, leading to an A:B ratio of 85:15. As expected, it is very similar to that measured for [(IPr)-Au(η2-2hexyne)]BF4.9b Figure 1 shows the 19F,1H-HOESY spectrum for 2BF4. The most intense interaction is H8/F, and the general pattern is very similar to that for 1BF4. Performing a 19F,1H-HOESY NMR experiment on the isolated ion pairs 2BF4 (Table 1 and Supporting Information) gives essentialy the same results, within the experimental error. The A:B ratio is around 76:24. Mapping the Coulomb potential of the 2+ cation shows that H2 and H8 are indeed the most attractive regions of the cation, with a slight predominance of H8 (Figure 1).31 This map is in qualitative agreement with the experimental A:B ratio, and C

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(11) (a) Gorin, D. J.; Sherry, B. D.; Toste, F. D. Chem. Rev. 2008, 108, 3351−3378. (b) Wang, W.; Hammond, G. B.; Xu, B. J. Am. Chem. Soc. 2012, 134, 5697−5705. (12) (a) Brouwer, C.; He, C. Angew. Chem., Int. Ed. 2006, 45, 1744− 1747. (b) Gramage-Doria, R.; Bellini, R.; Rintjema, J.; Reek, J. N. H. ChemCatChem 2013, 5, 1084−1087. (13) LaLonde, R. L.; Sherry, B. D.; Kang, E. J.; Toste, F. D. J. Am. Chem. Soc. 2007, 129, 2452−2453. (14) Aikawa, K.; Kojima, M.; Mikami, K. Angew. Chem., Int. Ed. 2009, 48, 6073−6077. (15) Weber, D.; Jones, T. D.; Adduci, L.; Gagnè, M. R. Angew. Chem., Int. Ed. 2012, 51, 1−6. (16) Bandini, M.; Bottoni, A.; Chiarucci, M.; Cera, G.; Miscione, G. P. J. Am. Chem. Soc. 2012, 134, 20690−20700. (17) For the importance of this technique in organometallic chemistry see: (a) Bellachioma, G.; Ciancaleoni, G.; Zuccaccia, C.; Zuccaccia, D.; Macchioni, A. Coord. Chem. Rev. 2008, 252, 2224− 2238. (b) Macchioni, A. Eur. J. Inorg. Chem. 2003, 2, 195−205. (18) Only the presence of a very polarized proton, as the ortho-H of the P(3,5-(CF3)2-C6H3)3 ligand (PArF), can force the anion to be close to the gold atom; see ref 9b. (19) Vasudevan, K. V.; Butorac, R. R.; Abernethy, C. D.; Cowley, A. H. Dalton Trans. 2010, 39, 7401−7408. (20) Bellachioma, G.; Binotti, B.; Cardaci, G.; Carfagna, C.; Macchioni, A.; Sabatini, S.; Zuccaccia, C. Inorg. Chim. Acta 2002, 330, 44−51. (21) de Frémont, P.; Marion, N.; Nolan, S. P. Coord. Chem. Rev. 2009, 253, 862. (22) (a) Wang, H. M. J.; Lin, I. J. B. Organometallics 1998, 17, 972. (b) Lin, J. C. Y.; Huang, R. T. W.; Lee, C. S.; Bhattacharyya, A.; Hwang, W. S.; Lin, I. J. B. Chem. Rev. 2009, 109, 3561. (23) Fèvre, M.; Pinaud, J.; Leteneur, A.; Gnanou, Y.; Vignolle, J.; Taton, D.; Miqueu, K.; Sotiropoulos, J. J. Am. Chem. Soc. 2012, 134, 6776. (24) In concomitance with the preparation of this paper, Nolan and co-workers published a similar procedure. Please, see: Collado, A.; Gómez-Suárez, A.; Martin, A. R.; Slawin, A. M. Z.; Nolan, S. P. Chem. Commun. 2013, 49, 5541−5543. (25) van Lenthe, E.; Ehlers, A. E.; Baerends, E. J. J. Chem. Phys. 1999, 110, 8943. (26) Pye, C. C.; Ziegler, T. Theor. Chem. Acc. 1999, 101, 396. (27) Apparently, the anion interacts also with the protons of free 3hexyne, but this contact can be reasonably explained by a fast exchange between the coordinated and free 3-hexyne: the anion interacts only with the coordinated alkyne, but because of the exchange, the NOE is indirectly transferred also to the free one; the total Overhauser effect is the sum of the cross-peaks with free and coordinated alkyne. See: Neuhaus, D.; Williamson, M. P. The Nuclear Overhauser Effect in Structural and Conformational Analysis, 2nd ed.; Wiley-VCH, Inc.: New York, 2000. (28) Zuccaccia, D.; Belpassi, L.; Tarantelli, F.; Macchioni, A. Eur. J. Inorg. Chem. 2013, DOI: 10.1002/ejic.201300285. (29) The orientation in which the anion is located near the gold is unfavored because the aryl moieties with hindered ortho-substituents introduce steric hindrance above and below the metal center. See: (a) Zuccaccia, C.; Macchioni, A.; Orabona, I.; Ruffo, F. Organometallics 1999, 18, 4367−4372. (b) Macchioni, A.; Magistrato, A.; Orabona, I.; Ruffo, F.; Rothlisberger, U.; Zuccaccia, C. New J. Chem. 2003, 27, 455−458. (c) Binotti, B.; Bellachioma, G.; Cardaci, G.; Carfagna, C.; Zuccaccia, C.; Macchioni, A. Chem.Eur. J. 2007, 13, 1570−1582. (30) NOE intensities must be scaled considering that they are proportional to (nInS/nI + nS), where nI and nS are the number of equivalent I and S nuclei (that undergo a dipolar interaction), respectively. See: Macura, S.; Ernst, R. R. Mol. Phys. 1980, 41, 95. (31) A 76:24 ratio is consistent with a small difference in energy between the two conformations, around 0.7 kcal/mol.

ASSOCIATED CONTENT

S Supporting Information *

Experimental section and computational studies. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]; [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by a grant from the Ministero dell’ Istruzione dell’Università e della Ricerca (MIUR, Rome, Italy) through FIRB-futuro in ricerca (RBFR1022UQ, Novel Au(I)based molecular catalysts: from know-how to know-why, “AuCat”) and PRIN 2009 (LR88XR) programs.

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