Communication pubs.acs.org/Organometallics
Gold(I) Complexes Bearing Sterically Imposing, Saturated Six- and Seven-Membered Expanded Ring N-Heterocyclic Carbene Ligands Jay J. Dunsford,† Kingsley J. Cavell,* and Benson M. Kariuki School of Chemistry, Main Building, Cardiff University, Cardiff CF10 3AT, U.K. S Supporting Information *
ABSTRACT: The synthesis and characterization of novel six- and sevenmembered expanded ring N-heterocyclic carbene (NHC) complexes of the general formula [Au(NHC)Cl] are described. The key structural parameters of complexes 1, 2, and 4 have been evaluated by solid-state analysis and by means of the percent buried volume (%Vbur) analysis. Complex 4 is found to demonstrate the largest %Vbur value of any gold(I) NHC complex reported to date, with a value of 52.6. All complexes 1−4 have been evaluated in a preliminary catalytic survey of the hydration of internal alkynes.
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Scheme 1. Synthesis of Expanded Ring NHC Gold(I) Complexes 1−46
ver the past 20 years N-heterocyclic carbene (NHC) ancillary ligands have become an extremely successful subset of compounds in organometallic chemistry with regard to transition metal mediated transformations.1 In more recent years our group2 and others3 have detailed the synthesis and application of a new series of NHC compounds coined expanded ring NHCs in light of their heterocyle ring size being >5. These expanded ring NHC ligands demonstrate significantly altered steric and electronic properties in relation to their five-membered counterparts,2j and as such, we are interested in their application toward a number of important transition metal mediated reactions. Recently, there has been a great deal of interest in gold(I)-mediated transformations,4 with a particular bias toward complexes bearing sterically imposing NHC ancillary ligands.5 With these previous studies in mind we are looking to expand the scope and utility of expanded ring NHC ligands into this area. Herein, the synthesis, full characterization, and a structural survey of new six- and seven-membered NHC gold(I) complexes is reported along with a preliminary catalytic survey of the hydration of internal alkynes. The six- and seven-membered NHC complexes 1−4 of the general formula [Au(NHC)Cl] are accessed in a facile manner via the addition of the desired free carbene solution (generated in situ) to a stirred THF suspension of the commercially available [Au(SMe2)Cl] under inert conditions (Scheme 1).6 After workup the complexes are isolated as stable white solids in good yields, ranging from 62% to 85%. Analysis of the compounds by 1H NMR reveals the absence of the characteristic NCHN proton of the NHC·HX salt and a subtle shift of the NCH2 protons of the heterocycle backbone. 13C{1H} NMR provides a useful diagnostic feature in confirming complex formation through the downfield shift of the N−CNHC−N carbon resonance. In the cases of complexes 1−4 the N− © 2012 American Chemical Society
CNHC−N resonances were found to lie in the range 192.55 to 200.85 ppm. We have previously noted that in Pd(0) complexes bearing five-, six-, and seven-membered NHC ligands the carbenic carbon resonance provides a useful indication of the σ-donor function of the ancillary NHC ligand as increasing heterocycle ring size furnishes a pronounced downfield shift.2h This does not appear to be the case in these Au(I) examples, with values being comparable for the five-, six-, and seven-membered NHC complexes (SIMes (195.03 ppm),7 1 (192.55 ppm), 3 (200.85 ppm)). Crystals suitable for X-ray diffraction were grown overnight by the slow vapor diffusion of n-pentane into a concentrated chloroform solution, and the solid-state structures of all complexes 1−4 were determined.8 Through analysis of the solid-state structures it is clear that the ancillary ligands (especially in complexes 2 and 4) are extremely sterically imposing (Figures 1, 2, and 3). The CNHC−Au bond lengths are comparable in complexes 1, 2, and 4 (1.993(8)−2.012(6) Å), although complexes 2 and 4, bearing 2,6-diisopropylphenyl Received: May 3, 2012 Published: May 25, 2012 4118
dx.doi.org/10.1021/om300351s | Organometallics 2012, 31, 4118−4121
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Communication
trimethylphenyl (Mes) N-aryl substituents is in line with previously described five-membered systems.7 The Au−Cl bond distances are found to lie within the range 2.281(12)− 2.303(2) Å. These lengths are slightly longer than those observed in the previously described five-membered systems,7 possibly as a consequence of the trans-effect of the stronger σdonor NHC ligands.2j,7 A structural feature of particular importance in such complexes is the N−CNHC−N angle, which affords increasingly hindered complexes with increasing heterocycle ring size as the N-aryl substituents are “pushed” toward the coordinated metal center.2j The N−CNHC−N bond angles in complexes 2 and 4 are 118.6(7)° and 121.2(5)°, respectively, in comparison to 109.1(2)° for the related saturated five-membered derivative (SIPr),7 a clear and substantial increase with heterocycle ring size. The CNHC− Au−Cl bond angles are all found to be close to linearity and lie within the range of 175.6(3)° and 178.03(17)°. Analysis of the steric demand imparted by the ancillary ligands by means of the percent buried volume analysis (%Vbur)10 reveals the least sterically demanding complex detailed in this study (1) to demonstrate a calculated %Vbur value of 42.2, 5.2 units greater than that of its five-membered derivative (SIMes, 36.9) (Table 1, entries 2 and 5). The values for complexes bearing DIPP N-
Figure 1. ORTEP9 representation of [Au(6-Mes)Cl], 1. Ellipsoids are shown at 50% probability with all hydrogen atoms omitted for clarity. Selected bond lengths (Å) and angles (deg): C(1)−Au(1) 1.993(8), Au(1)−Cl(1) 2.303(2), N(1)−C(1)−N(2) 118.6(7), C(1)−N(1)− C(5) 120.9(7), C(1)−N(2)−(C14) 118.8(7), C(1)−Au(1)−Cl(1) 176.4(2).
Table 1. Calculated Percent Buried Volume (%Vbur) Values for a Select Range of Gold(I) Complexes of the General Formula [Au(NHC)Cl]10,13 entry 1 2 3 4 5 6 7 8 9 10 11
Figure 2. ORTEP9 representation of [Au(6-DIPP)Cl], 2. Ellipsoids are shown at 50% probability with all hydrogen atoms omitted for clarity. Selected bond lengths (Å) and angles (deg): C(1)−Au(1) 2.004(8), Au(1)−Cl(1) 2.281(12), N(1)−C(1)−N(2) 118.6(7), C(1)−N(1)−C(5) 119.9(6), C(1)−N(2)−(C17) 120.2(6), C(1)− Au(1)−Cl(1) 175.6(3).
(DIPP) N-aryl substituents (Figures 2 and 3), are found to exhibit a slight elongation of the bond as a consequence of the increase in steric demand associated with these bulky ligands. The value demonstrated by complex 1 bearing 2,4,6-
complex 7
[Au(ICy)Cl] [Au(IMes)Cl]7 Au(SIMes)Cl]7 [Au(IAd)Cl]7 [Au(6-Mes)Cl] (this work) [Au(IPr)Cl]7 [Au(SIPr)Cl]7 [Au(IPr*)Cl]12 [Au(6-DIPP)Cl] (this work) [Au(CAAC)Cl]11 [Au(7-DIPP)Cl] (this work)
%Vbur value 27.4 36.5 36.9 39.8 42.2 44.5 47.0 50.4 50.8 51.2 52.6
aryl substituents 2 and 4 were found to be greater still, with values of 50.8 and 52.6, respectively (Table 1, entries 10 and 12). To the best of our knowledge complex 4 has the largest calculated %Vbur value of any gold(I) NHC complex reported to date, surpassing the 51.2 of the CAAC ancillary ligand reported of Bertrand et al.12 (Table 1, entry 10). Again these values were found to be substantially larger than the related SIPr five-memberd system (Table 1, entry 7).7 In order to evaluate the influence of the increased steric demand imparted by the expanded ring NHC ligands, the alkyne hydration reaction was investigated.14 This reaction was settled upon as it has previously been the subject of a number of reports utilizing a range of [Au(NHC)Cl] complexes.15 The [Au(IPr)Cl] system of Nolan et al. has demonstrated particular promise in this area, allowing for the hydration of a number of terminal and internal alkyne substrates at exceedingly low catalytic loadings (in one example as low as 10 ppm).15a We were interested in investigating how expanded ring NHC ancillary ligands would perform under analogous conditions; hence all reactions herein are carried out according to the previously described protocol of Nolan and are nonoptimized for these specific systems. Our initial studies investigated the
Figure 3. ORTEP9 representation of [Au(7-DIPP)Cl], 4. Ellipsoids are shown at 50% probability with all hydrogen atoms omitted for clarity. Selected bond lengths (Å) and angles (deg): C(1)−Au(1) 2.012(6), Au(1)−Cl(1) 2.3048(14), N(1)−C(1)−N(2) 121.2(5), C(1)−N(1)−C(6) 119.9(5), C(1)−N(2)−(C18) 118.7(5), C(1)− Au(1)−Cl(1) 178.03(17). 4119
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Communication
systems incorporating DIPP N-aryl substituents (entries 10 and 12). This observation correlates well with the observed selectivity in the previously described five-membered examples, which demonstrate a ratio of 2.2:1 in favor of isomer II in the hydration of 2-octyne.15a In summary, new six- and seven-membered gold(I) complexes have been synthesized. The complexes represent some of the most sterically demanding gold(I) complexes reported to date, with complex 4, to the best of our knowledge, demonstrating the largest reported %Vbur value of any Au(I) NHC complex. The complexes have been employed in a preliminary survey of the hydration of internal alkynes with mixed success. Complexes 1 and 3, bearing mesityl N-aryl substituents, have been shown to be effective catalysts for the hydration of 4-octyne, in contrast to their five-membered counterparts, which were previously found to be inactive with related substrates. At present in our laboratory, work in this area is ongoing with regard to mechanism, substrate scope, and other coinage metal-based systems, the results of which will be reported in due course.
hydration of the terminal alkyne, phenylacetylene, employing complexes 1−4. However, upon analysis of the reaction mixture after 17 h, only the phenylacetylene starting material was observed by GCMS.16 This observation was surprising in that the related five-membered system15a and a gold(I) complex bearing a seven-membered DAC ligand previously reported by Bielawski15d both facilitated this transformation under similar or identical reaction conditions. We then investigated internal alkyne substrates, which could be hydrated successfully under analogous conditions (Table 2). The hydration of a symTable 2. Hydration of Internal Alkynes with [Au(NHC)Cl] Complexes 1−4a
entry
complex
substrate
% yieldb
1 2 3 4 5 6 7 8 9 10 11 12
1 2 3 4 1 2 3 4 1 2 3 4
4-octyne 4-octyne 4-octyne 4-octyne diphenylacetylene diphenylacetylene diphenylacetylene diphenylacetylene 2-hexyned 2-hexyned 2-hexyned 2-hexyned
100 100 100 100 trace trace trace trace 100 79.9 100 66.7
selectivity I:IIc
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ASSOCIATED CONTENT
S Supporting Information *
Full experimental and physical data for complexes 1−4, experimental details for the hydration of alkynes, molecular structure of 3, structure refinement data for compounds 1, 2, and 4, and CIF files for complexes 1, 2, and 4. This material is available free of charge via the Internet at http://pubs.acs.org.
37:63 32:68 39:61 35:65
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AUTHOR INFORMATION
Corresponding Author
a
Reaction conditions: alkyne (1 mmol), decane (1 mmol, internal standard), catalyst (0.1 mol %, THF stock solution), 1,4-dioxane (660 μL), H2O (330 μL), 80 °C 17 h. bPercentage yields based upon consumption of alkyne substrate by GCMS. cSelectivity determined by integration of GCMS peaks and 1H NMR. dR = CH3(CH2)2, R′ = CH3.
*E-mail:
[email protected]. Present Address †
School of Chemistry, University of Nottingham, Nottingham, NG7 2RD, UK. Author Contributions
The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.
metrical substrate, 4-octyne, resulted in complete conversion to the corresponding ketone for all complexes 1−4 (Table 2, entries 1−4). The successful application of systems bearing mesityl N-aryl substituents (1 and 3) is notable given that the analogous five-membered system reported by Nolan (IMes) was found to be completely inactive in related transformations.15a The increase in steric constraint imparted by the 6-Mes and 7-Mes ancillary ligands appears to be beneficial in such transformations. The 6-Mes ancillary ligand demonstrates a calculated %Vbur value of 42.2, which falls between the SIMes and SIPr systems (36.9 and 47.0, respectively).11 The diphenylacetylene substrate proved challenging; only trace amounts of the desired ketone product were observed after 17 h, presumably as a consequence of the unfavorable steric demand imparted by both the substrate and the ancillary NHC ligands (Table 2, entries 5−8).17 Finally we were interested in the effect of increasing steric demand on selectivity in the hydration of unsymmetrical internal alkyne substrates (Table 2, entries 9−12). The hydration of 2-hexyne proceeded efficiently with the less sterically demanding complexes 1 and 3, while complexes 2 and 4 demonstrated a decrease in overall percentage yield with increasing heterocycle ring size and steric demand (entries 10 and 12). The selectivity imparted by the expanded ring NHC ancillary ligands remains moderate in all cases, with an approximate ratio of 2:1 in favor of isomer II. Selectivity does however appear to be marginally better in the
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We would like to acknowledge the Engineering and Physical Sciences Research Council (EPSRC) for their financial support and the Cardiff Catalysis Institute for J.J.D.’s Ph.D. studentship.
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REFERENCES
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Dervisi, A.; Fallis, I.; Cavell, K. J. Eur. J. Inorg. Chem. 2010, 5426. (f) Dunsford, J. J.; Cavell, K. J.; Kariuki, B. J. Organomet. Chem. 2011, 696, 188. (g) Newman, P. D.; Cavell, K. J.; Kariuki, B. M. Organometallics 2010, 29, 2724. (h) Dunsford, J. J.; Cavell, K. J. Dalton Trans. 2011, 40, 9131. (i) Newman, P. D.; Cavell, K. J.; Hallett, A. J.; Kariuki, B. M. Dalton Trans. 2011, 40, 8807. (j) Lu, W. Y.; Cavell, K. J.; Wixey, J. S.; Kariuki, B. Organometallics 2011, 30, 5649. (3) (a) Alder, R. W.; Blake, M. E.; Bortolotti, C.; Bufali, S.; Butts, C. P.; Linehan, E.; Oliva, J. M.; Orpen, A. G.; Quayle, M. J. Chem. Commun. 1999, 241. (b) Bazinet, P.; Yap, G. P. A.; Richeson, D. S. J. Am. Chem. Soc. 2003, 125, 13314. (c) Mayr, M.; Wurst, K.; Ongania, K-H; Buchmeiser, M. R. Chem. Eur. J. 2004, 10, 1256. (d) Herrmann, W. A.; Schneider, S. K.; Ofele, K.; Sakamoto, M.; Herdtweck, E. J. Organomet. Chem. 2004, 689, 2441. (e) Scarborough, C. C.; Grady, M. J. W.; Guzei, I. A.; Gandhi, B. A.; Bunel, E. E.; Stahl, S. S. Angew. Chem., Int. Ed. 2005, 44, 5269. (f) Moerdyk, J. P.; Bielawski, C. W. Organometallics 2011, 30, 2278. (g) Armstrong, R.; Ecott, C.; MasMarza, E.; Page, M. J.; Mahon, M. F.; Whittlesey, M. K. Organometallics 2010, 29, 991. (h) Davies, C. J. E.; Page, M. J.; Ellul, C. E.; Mahon, M. F.; Whittlesey, M. K. Chem. Commun. 2010, 46, 5151. (4) For reviews see: (a) Hashmi, A. S. K.; Hutchings, G. J. Angew. Chem., Int. Ed. 2006, 45, 7896. (b) Boorman, T.; Larrosa, I. Chem. Soc. Rev. 2011, 40, 1910. (5) For a reviews see: (a) Marion, N.; Nolan, S. P. Chem. Soc. Rev. 2008, 37, 1776. (b) Nolan, S. P. Acc. Chem. Res. 2011, 44, 91. (6) For isostructural complexes of Ag(I) and Cu(I) see: ref 2b and Kolychev, E. L.; Portnyagin, I. A.; Shuntikov, V. V.; Khrustalev, V. N.; Nechaev, M. S. J. Organomet. Chem. 2009, 694, 2454. (7) de Frémont, P.; Scott, N. M.; Stevens, E. D.; Nolan, S. P. Organometallics 2005, 24, 2411. (8) The molecular structure of compound 3 has also been determined, confirming its connectivity; however, the quality of data obtained prevents its direct inclusion in this article. An OTREP representation of the molecular structure of 3 may be found in the Supporting Information, and the corresponding cif file is available by request from the CCDC (CCDC 882660). (9) Farrugia, L. J. J. Appl. Crystallogr. 1997, 30, 565. (10) (a) Huang, J.; Schanz, H. J.; Stevens, E. D.; Nolan, S. P. Organometallics 1999, 18, 2370. (b) Poater, A.; Cozenza, B.; Correa, A.; Giudice, S.; Ragone, F.; Scarano, V.; Cavallo, L. Eur. J. Inorg. Chem. 2009, 1759. Parameters applied for SambVca calculations: 3.50 Å was selected as the value for the sphere radius, 2.00 Å was used as distances for the metal−ligand bond, hydrogen atoms were omitted, and Bondi radii scaled by 1.17 were used. The above parameters applied are identical to those of all literature examples discussed, allowing a direct comparison of calculated values. (11) (a) Frey, G. D.; Dewhurst, R. D.; Kousar, S.; Donnadieu, B.; Bertrand, G. J. Organomet. Chem. 2008, 693, 1674. (12) Gomez-Suarez, A.; Ramon, R. S.; Songis, O.; Slawin, A. M. Z.; Cazin, C. S. J.; Nolan, S. P. Organometallics 2011, 30, 5463. (13) For a comprehensive review of %Vbur values of NHC complexes see: Clavier, H.; Nolan, S. P. Chem. Commun. 2010, 46, 841. (14) For a review on alkyne hydration see: Hintermann, L.; Labonne, A. Synthesis 2007, 1121. (15) (a) Marion, N.; Ramon, R. S.; Nolan, S. P. J. Am. Chem. Soc. 2009, 131, 448. (b) Almassy, A.; Nagy, C. E.; Benyei, A. C.; Joo, F. Organometallics 2010, 29, 2484. (c) Nun, P.; Ramón, R. S.; Gaillard, S.; Nolan, S. P. J. Organomet. Chem. 2011, 696, 7. (d) Hudnall, T. W.; Tennyson, A. G.; Bielawski, C. W. Organometallics 2010, 29, 4569. (16) The reason behind the complete lack of activity concerning terminal alkyne substrates is under current investigation. (17) The [Au(IPr)Cl] complex of Nolan demonstrates a %Vbur value of 44.5 and was found to be efficient in the hydration of phenylacetylene.15a This observation suggests that on the basis of a steric argument complexes 1 and 3 should also be effective, with complex 1 bearing a value of 42.2. This observation is under current investigation.
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