and Bis(alkyne) Complexes of Coinage Metals - American Chemical

Mar 6, 2013 - 96.1 a. The values in parentheses are the percentage contributions to the ..... (70) Diefenbach, A.; Bickelhaupt, F. M.; Frenking, G. J...
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Tris(alkyne) and Bis(alkyne) Complexes of Coinage Metals: Synthesis and Characterization of (cyclooctyne)3M+ (M = Cu, Ag) and (cyclooctyne)2Au+ and Coinage Metal (M = Cu, Ag, Au) Family Group Trends Animesh Das,† Chandrakanta Dash,† Mehmet Ali Celik,‡ Muhammed Yousufuddin,† Gernot Frenking,*,‡ and H. V. Rasika Dias*,† †

Department of Chemistry and Biochemistry, The University of Texas at Arlington, Arlington, Texas 76019, United States Fachbereich Chemie, Philipps-Universität Marburg, D-35039 Marburg, Germany

‡

S Supporting Information *

ABSTRACT: The tris(alkyne) copper complex [(cyclooctyne)3Cu][SbF6] has been synthesized using cyclooctyne and in situ generated CuSbF6. Tris(alkyne) silver complexes [(cyclooctyne)3Ag]+ involving weakly coordinating counterions such as [SbF6]− and [PF6]− have also been isolated in good yield using cyclooctyne and commercially available AgSbF6 and AgPF6. These coinage metal tris(alkyne) adducts have trigonal-planar metal sites. The alkyne carbon atoms and the metal site form distorted spoke-wheel (rather than upright trigonal-prismatic) structures in the solid state. In [(cyclooctyne)3Cu][SbF6], these distortions result in a propellerlike arrangement of alkynes. A cationic gold(I) complex having two alkynes has been prepared by a reaction of equimolar amounts of Au(cyclooctyne)2Cl and AgSbF6 in dichloromethane. The gold atom of [(cyclooctyne)2Au]+ coordinates to the cyclooctynes in a linear fashion, while the carbon atoms of the alkyne groups form a tetrahedron around gold(I). Optimized geometries of cationic [(cyclooctyne)3M]+, [(cyclooctyne)2M]+, and [(cyclooctyne)M]+ and neutral [(cyclooctyne)2MCl] and [(cyclooctyne)MCl] adducts (M = Cu, Ag, Au) using density functional theory (DFT) at the BP86/def2-TZVPP level of theory and a detailed analysis of metal−alkyne bonding interactions are also presented.

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INTRODUCTION Coinage metal (Cu, Ag, Au) ions are some of the best reagents to activate and mediate reactions involving alkynes, including heteroatom−hydrogen bond additions, cycloaddition chemistry, Csp−H bond functionalizations, and alkyne coupling processes and hydrogenations.1−17 Most of these processes are believed to be initiated by the π complexation of alkyne to the electrophilic coinage metal(I) center.18,19 Therefore, isolable coinage metal−alkyne adducts and their structures, bonding, and properties are of considerable interest. An area of research focus in our laboratory has been the study of gold(I) complexes of simple, unsaturated hydrocarbons (in particular, regular alkenes and alkynes)20−30 and the related adducts involving lighter coinage metal family members (copper and silver).20−23,25−27,31−40 Recently, we reported the synthesis and structural data of [(cyclooctyne)3Au][SbF6] and [(cyclooctyne)2AuCl], which represent two rare molecules featuring more than one π-alkyne moiety on gold(I).41 The search for structurally characterized coinage metal complexes with more than one π-alkyne moiety bonded to a Cu, Ag, or Au center in Cambridge Structural Database shows that the vast majority involve alkynyl metal groups (metal © 2013 American Chemical Society

acetylides) of the type M′CCR, M′(CCR)2, etc. (M′ = transition metal, R = typically alkyl or aryl group)42−50 and, to a lesser degree, cyclic and often chelating polyalkyne ligands serving as π donors.51−57 In contrast, structurally authenticated coinage metal bis- and tris(alkyne) adducts based on regular monoalkyne hydrocarbon donors are very rare and they include [Ag(C2H2)3][Al{OC(CF3)3}4],58 [(cyclooctyne)2CuX] (X = Cl, Br, I),59 [M(L′)(OTf)]n (M = Cu, Ag; L′ = 1,6-cyclodecadiyne, 1,7-cyclododecadiyne, 1,8-cyclotetradecadiyne; note that these are dialkynes but here they serve as monoalkynes for a given metal)60 and the two adducts we communicated recently, [(cyclooctyne)3Au][SbF6] and [(cyclooctyne)2AuCl].41 We are particularly interested in the last group, because alkynes of these compounds are not influenced by additional metal ions (M′) as in metal acetylides and coordinate to or dissociate from coinage metal freely and independently of the other alkyne (in contrast to the case for chelating polyalkyne systems), thus providing useful information on preferred coordination modes/geometry and the effects of a π-bound coinage metal on an alkyne moiety. Received: January 29, 2013 Published: March 6, 2013 3135

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[(cyclooctyne)3Ag][SbF6], and [(cyclooctyne)3Au][SbF6] represent three closely related isoleptic coinage metal tris(alkyne) complexes and are ideal for investigating coinage metal family group trends. 1 H NMR signals of the −CH2CC protons attached to α-carbons of the alkyne moiety of [(cyclooctyne)3M]+ complexes (M = Cu, Ag, Au) in CD2Cl2 appear in the δ 2.42−2.64 ppm region, which is a downfield shift relative to the corresponding resonance of the free cyclooctyne (δ 2.13 ppm). The gold adduct shows the largest downfield shift (from δ 2.13 to 2.64 ppm), while the silver analogue displays the smallest shift. The 13 C NMR spectra of [(cyclooctyne)3M]+ at room temperature showed a carbon resonance shift of the metal-bound alkyne moiety at δ 99.00 and 102.15 ppm for the copper and gold adducts, respectively. These are shifted downfield in comparison to the corresponding signal of the free cyclooctyne (δ 94.90 ppm). while the analogous silver complex shows corresponding signals at δ 95.00 and 95.62 ppm for the [SbF6]− and [PF6]− derivatives. A similar trend was observed in the monoalkyne adducts [N{(C3F7)C(Dipp)N}2]M(EtCCEt) (M = Cu, Ag, Au), where the 13C resonance of the acetylenic carbons of the gold (δ 91.3 ppm) and copper (δ 99.2 ppm) adducts were observed downfield of the free EtCCEt resonance (δ 80.7 ppm) while the silver analogue (δ 80.6 ppm) displayed essentially no change.26 The chemical shift values of [(cyclooctyne)3M]+ can also be compared to those of three-coordinate bis(cyclooctyne) adducts such as [(cyclooctyne)2CuCl]59 and [(cyclooctyne)2AuCl] (see Table S1, Supporting Information).41 The 13C NMR signal of the acetylenic carbons of cationic [(cyclooctyne)3Cu][SbF6] in CD2Cl2 (which appears at δ 99.00 ppm) shows a relatively small downfield shift in comparison to the neutral complex Cu(cyclooctyne)2Cl (δ 97.2 ppm). In the related gold adducts, the difference is much larger. For example, [(cyclooctyne)3Au][SbF6] and [(cyclooctyne)2AuCl] display acetylenic carbons at δ 102.15 and 93.80 (or 90.81 at 193K) ppm, respectively. The [(cyclooctyne)2AgCl] adduct has not been reported yet for a comparison. The addition of excess cyclooctyne to a CD2Cl2 solution of [(cyclooctyne)3M][SbF6] complexes (M = Cu, Ag, Au) leads to the coalescence of corresponding 1H and 13C NMR resonances, suggesting that in these adducts, free and coordinated cyclooctyne exchange quite rapidly in solution at room temperature on the NMR time scale. We have reported IR data on [(cyclooctyne)3Au][SbF6].41 Unfortunately, despite several attempts, we could not detect the IR stretching bands of CC stretch of [(cyclooctyne)3Cu][SbF6] and [(cyclooctyne)3Ag][SbF6]. The bis(alkyne) gold(I) adduct [(cyclooctyne)2Au][SbF6] has been synthesized by using equimolar amounts of [(cyclooctyne)2AuCl] and AgSbF6 in dichloromethane (Figure 3). Interestingly, the same product crystallized out (but in much lower yield) from a CH2Cl2/hexane solution of [(cyclooctyne)3Au][SbF6] after 30 days in a 0 °C freezer. The 1H NMR spectrum of [(cyclooctyne)2Au][SbF6] in CD2Cl2 at room temperature showed signals corresponding to the methylene protons of α-carbons of the alkyne moiety as a multiplet at δ 2.93 ppm, which is a significant downfield shift in comparison to the corresponding signal of the free cyclooctyne or [(cyclooctyne)3Au][SbF6]. The 13C NMR spectrum in CD2Cl2 at room temperature exhibits a resonance at δ 110.08 ppm, which can be assigned to the carbons of the gold-bound alkyne moiety. The corresponding signal of the tris(alkyne) adduct [(cyclooctyne)3Au][SbF6] was observed at δ 102.15 ppm, while the alkyne carbon of the free cyclooctyne appears at δ 94.90 ppm. Thus, alkyne

In this paper, we report the successful isolation and characterization of tris(alkyne) species [(cyclooctyne)3Ag]+ and [(cyclooctyne)3Cu]+ as well as the bis(alkyne) gold(I) adduct [(cyclooctyne)2Au]+ (Figure 1) using readily available weakly

Figure 1. Tris- and bis(alkyne) complexes of coinage metals: [(cyclooctyne)3M]+ (M = Cu, Ag, Au), Au(cyclooctyne)2Cl, and [(cyclooctyne)2Au]+.

coordinating counterions such as [SbF6]− and [PF6]−. In addition, optimized geometries of [(cyclooctyne)3M]+, [(cyclooctyne)2M]+, and [(cyclooctyne)M]+ as well as neutral [(cyclooctyne)2MCl] and [(cyclooctyne)MCl] (M = Cu, Ag, Au) using density functional theory (DFT) at the BP86/def2-TZVPP level of theory and a detailed analysis of the M−alkyne bonding interactions are also presented.

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RESULTS AND DISCUSSION The reaction of CuSbF6 (prepared in situ from the metathesis reaction between CuBr and AgSbF6) with slightly more than 4 equiv of cyclooctyne in dichloromethane afforded the tris(alkyne) adduct [(cyclooctyne)3Cu][SbF6] in 51% yield (Figure 2). The analogous silver adduct [(cyclooctyne)3Ag]-

Figure 2. Synthesis of [(cyclooctyne)3Cu][SbF6].

[SbF6] can be synthesized via a similar route using AgSbF6 as the silver(I) source in 54% yield. We have also synthesized [(cyclooctyne)3Ag][PF6] because, in our hands, it gave better crystals for X-ray structure determination. These copper and silver adducts are moderately air stable compounds. In comparison to the tris(alkyne) gold adduct [(cyclooctyne)3Au][SbF6],41 the analogous copper and silver adducts are relatively more thermally stable. [(cyclooctyne)3Cu][SbF6], 3136

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Table 1. Summary of Experimental X-ray Crystallographic Data and Optimized Structural Parametersa for [(L)3M][X] (L = Cyclooctyne; M = Cu, Ag, Au; X = SbF6, PF6) and [(L)3M]+ param

[(L)3Cu] [SbF6] [[(L)3Cu]+]

d(CC), Å

1.200(16) [1.243] 1.222(16) [1.243] 1.238(15) [1.236] av d(CC), Å 1.220(16) [1.241] d(M−C), Å 2.090(12) [2.100] 2.084(11) [2.086] 2.100(10) [2.086] 2.105(10) [2.100] 2.117(10) [2.171] 2.134(10) [2.171] av d(M−C), Å 2.105(10) [2.119] ∠C−M−C, deg 33.4(4) [34.6]

Figure 3. Synthesis of [(cyclooctyne)2Au][SbF6].

carbon atoms of [(cyclooctyne)2Au][SbF6] show a much larger downfield shift in comparison to those of [(cyclooctyne)3Au][SbF6] as a result of coordination to the Au(I) center. The addition of excess cyclooctyne to a CD2Cl2 solution of [(cyclooctyne)2Au][SbF6] at room temperature leads to the coalescence of the corresponding 1H and 13C NMR resonances. These data indicate that the coordinated cyclooctynes in [(cyclooctyne)2Au][SbF6] exchange quite rapidly with free cyclooctyne in solution at room temperature on the NMR time scale. The νCC stretch in [(cyclooctyne)2Au][SbF6] was observed as a weak and broad band at 2030 cm−1, which is about 186 cm−1 lower than the value observed for the free cyclooctyne (2216 cm−1). For comparison, the corresponding signal in [(cyclooctyne)3Au][SbF6] was observed at a higher value of 2116 cm−1 (Table S1, Supporting Information). This suggests that the Au−cyclooctyne interactions are relatively stronger and CC bonds are much weaker in the bis(cyclooctyne) adduct [(cyclooctyne)2Au][SbF6] in comparison to those of [(cyclooctyne)3Au][SbF6]. The X-ray crystal structure of [(cyclooctyne)3Cu][SbF6] is depicted in Figure 4. Selected bond lengths and angles are

33.8(4) [34.6] 33.9(4) [33.1] twist angle between 26.2, 27.3, 35.5 MC6 and each MC2 (av 29.6) plane, deg

[(L)3Ag][PF6] [(L)3Au] molecule 1, molecule [SbF6]b + 2 [[(L)3Ag] ] [[(L)3Au]+] 1.203(4), 1.218(4) [1.240] 1.192(4) 1.204(3), [1.231] 1.205(3). 1.210(4) [1.240] 1.205(4) [1.237] 2.340(2), 2.320(2) [2.313] 2.319(2), 2.336(2) [2.283] 2.371(2), 2.415(2); [2.429] 2.370(3), 2.383(2); [2.429] 2.363(2), 2.339(2) [2.283] 2.371(2), 2.346(2); [2.313] 2.356(2) [2.342] 29.93(9), 30.33(9) [31.3] 29.11(9), 29.07(8) [31.3] 29.48(8), 29.94(9) [29.4] 8.0, 31.8, 35.9 (av 25.2) 4.9, 43.7, 17.7 (av 22.1)

1.217(7) [1.255] 1.213(7) [1.236] 1.217(7) [1.255] 1.216(7) [1.249] 2.205(4) [2.197] 2.216(5) [2.229] 2.341(5); [2.407] 2.336(4) [2.407] 2.198(5); [2.197] 2.218(5) [2.229] 2.252(5) [2.278] 31.96(17) [32.9] 30.07(16) [29.7] 32.00(17) [32.9] 6.3, 43.9, 3.0 (av 17.7)

a

Results of DFT calculations (at the BP86/def2-TZVPP level of theory) on [(L)3M]+ are given in brackets. bReference 41.

for the X-ray crystallographic study either. Nevertheless, the molecular structure of [(cyclooctyne)3Cu][SbF6] shows that the cationic [(cyclooctyne)3Cu]+ moiety adopts a distorted spoke-wheel structure (more like a propeller). The steric congestion perhaps prevents achieving the coplanar geometry of the three alkyne groups. The twist angles between the six alkyne carbon atom and copper (CuC1C2C9C10C17C18) mean plane and the CuC1C2, CuC9C10, CuC17C18 planes are 26.2, 27.3, and 35.5°, respectively. Such out-of-plane twisting could affect the orbital overlap and bonding (especially the π component) between Cu and the alkyne moiety. [(cyclooctyne)3Cu]+ adduct does not have close Cu···F or Cu···Cu contacts in the solid state. The silver(I) adduct [(cyclooctyne)3Ag][SbF6] is a crystalline solid, but [(cyclooctyne)3 Ag][PF 6 ], with a [PF 6 ]− counterion, gave relatively better quality crystals for the X-ray crystallographic study. It crystallizes in space group P21 with two [(cyclooctyne)3Ag][PF6] molecules in the asymmetric unit (for a Z value of 4). Both [(cyclooctyne)3Ag]+ moieties adopt distorted spoke-wheel structures (Figure 5). Key structural parameters are summarized in Table 1. The torsion angles between the six alkyne carbon atom and silver (Ag1C1C2C9C10C17C18) mean plane and Ag1C1C2, Ag1C9C10, and Ag1C17C18 planes are 8.0, 31.8, and 35.9°,

Figure 4. Molecular structure showing [(cyclooctyne)3Cu]+ of [(cyclooctyne) 3 Cu][SbF6 ]. Ellipsoids are shown at the 40% probability level.

given in Table 1. Crystals of [(cyclooctyne)3Cu][SbF6] diffracted poorly. The use of alternative counterions such as [BF4]− and [PF6]− did not produce better quality crystals 3137

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Figure 5. Molecular structure showing [(cyclooctyne)3Ag]+ of [(cyclooctyne)3Ag][PF6]. Ellipsoids are shown at the 40% probability level. There are two [(cyclooctyne)3Ag][PF6] molecules in the asymmetric unit, but only one is illustrated here.

respectively. The related torsion angles of the second molecule in the asymmetric unit involving Ag2 are 4.9, 43.7, and 17.7°. We now have data on closely related isoleptic series of adducts [(cyclooctyne)3M]+ (M = Cu, Ag, Au) for examining coinage metal family group trends. X-ray structural data show that in the gold adduct [(cyclooctyne)3Au]+ one of the cyclooctynes shows a noticeable out-of-plane twist (torsion angles are 6.3, 43.9, and 3.0°).41 In the silver and copper analogues, two out of the three cyclooctynes on silver and all three cyclooctynes on the copper atom show significant deviation from planarity (Figure 6). Thus, in these cationic tris(alkyne) adducts, steric congestion is too severe to attain the “ideal” coplanar geometry of the three alkyne groups that facilitates the strongest metal−alkyne bonding interaction. Computational studies also show that a planar spoke-wheel arrangement is not the lowest energy configuration, as a result of steric repulsions between the alkyne moieties (Figures S1−S3, Supporting Information). Theory and experiment agree that the complexes [(cyclooctyne)3M]+ possess two equivalent cyclooctyne ligands which have slightly longer M−C bonds and shorter CC distances than the third cyclooctyne (Table 1). The steric effects are most pronounced in the copper adduct, since it features the smallest metal ion of the coinage metal family. Experimental data and computational studies of tris(alkene) adducts of coinage metal ions such as [(ethylene)3M]+ and [(norbornene)3M]+ have been reported and they, however, show that the lower energy structure is the planar spoke-wheel arrangement rather than the trigonal-prismatic structure.25,28,61−63 The M−C bond distances involving the alkyne group in [(cyclooctyne)3M]+ vary somewhat depending on the out-ofplane torsion angle. Usually longer M−C(alkyne) distances for a given metal adduct are associated with the alkyne showing a larger torsion angle. For example, in [(cyclooctyne)3Au]+, Au−C distances to one of the cyclooctynes (which is twisted by 43.9°) are significantly larger in comparison to the corresponding distances involving the other two cyclooctynes (that are essentially in plane). In [(cyclooctyne)3Cu]+, which features three significantly out-of-plane-twisted cyclooctynes, all the Cu−C distances are similar (within the esd), but they

Figure 6. Side view showing deviation from the coplanar spoke-wheel arrangement of [(cyclooctyne)3M]+ (from top to bottom, M = Cu, Ag, Au).

are shorter than the average Au−C or Ag−C distances of [(cyclooctyne)3Au]+ and [(cyclooctyne)3Ag]+. For example, average M−C distances of [(cyclooctyne)3M]+ adducts are 2.105, 2.356, and 2.252 Å for M = Cu, Ag, Au, respectively (Table 1). They agree well with the computed average M−C distances of 2.119, 2.342, and 2.278 Å (Table 1). For comparison, the three-coordinate coinage metal mono(alkyne) adducts [N{(C3F7)C(Dipp)N}2]M(EtCCEt) show average M−C distances of 1.984, 2.237, and 2.070 Å for M = Cu, Ag, Au, respectively.26 Overall, M−C distances of these [(cyclooctyne)3M]+ adducts (and [N{(C3F7)C(Dipp)N}2]M(EtC CEt)) follow the trend expected on the basis of covalent radii of coinage metal ions: Cu (1.13 Å) < Au (1.25 Å) < Ag (1.33 Å).64,65 The structural data of [(norbornene)3M][SbF6] involving all three coinage metal ions are also known and can be compared to those of [(cyclooctyne)3M]+ (M = Cu, Ag, and Au). The tris(alkene) adducts [(norbornene)3M][SbF6] adopt spokewheel structures with average M−C distances of 2.198, 2.408, and 2.291 Å for M = Cu, Ag, Au, respectively.25 Thus, these alkene adducts have relatively longer M−C distances than their alkyne counterparts, which is not surprising, as alkenes feature sp2-hybridized carbons. This is despite the likely bond lengthening due to adverse steric interactions between in-plane alkyne moieties of tris(alkyne) adducts [(cyclooctyne)3M]+. The CC distances are similar and within esd values for all three [(cyclooctyne)3M]+ adducts (average CC distances of Cu, Ag, and Au adducts are 1.220, 1.205, and 1.216 Å, 3138

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respectively). The X-ray data for free cyclooctyne are not available for comparison. Keep in mind, however, that often errors associated with CC bond distance determination via routine X-ray crystallography overshadow the small changes resulting from coinage metal ion coordination. Furthermore, there are even instances where the metal adducts show CC distances “shorter” than that of free alkyne, which is unrealistic. [Ag(C2H2)3][Al{OC(CF3)3}4] is a case in point, where Ag(I)bound acetylene has an average distance of 1.123 Å while the free acetylene in the gas phase has a distance of 1.2033(2) Å and the neutron diffraction studies on solid acetylene yield a distance of 1.193(6) Å.58 Similar observations have been noted for other cases, such as silver−ethylene adducts.21,63,66 Such “unusually short” distances between multiply bonded light atoms resulting from X-ray diffraction data have been attributed to libration effects and the anisotropy of the electron density.63 Therefore, changes observed in CC distances (or lack of them) as a result of metal coordination on the basis of routine X-ray crystallography should be treated with due caution. The average CC distances of [(cyclooctyne) 3 M] + calculated for Cu, Ag, and Au adducts are 1.241, 1.237, and 1.249 Å, respectively. They all show a lengthening upon coordination in comparison to the calculated free cyclooctyne CC distance of 1.214 Å. As noted earlier, X-ray data for free cyclooctyne are not available for a comparison with computed values. Data from electron diffraction studies are available and point to a CC distance as high as 1.228 Å for the free cyclooctyne, but they may be less accurate.59,67,68 The CC distance based on X-ray data of free cyclododecyne is known: 1.196(4) Å.69 In any event, computational data indicate that the longest CC is found in the gold adduct [(cyclooctyne)3Au]+, while the silver adduct [(cyclooctyne)3Ag]+ displays the smallest change in CC bond distance upon metal ion coordination. This is in agreement with the trend of the metal−ligand interaction energies, which are discussed later in the paper. In addition to the tris(cyclooctyne) metal complexes noted above, we have also synthesized the bis(alkyne)gold(I) adduct [(cyclooctyne)2Au][SbF6]. It crystallizes in the space group P21/c with two molecules in the asymmetric unit. The molecular structure is depicted in Figure 7. Table 2 gives selected bond distances and angles. Each gold atom coordinates to two cyclooctyne molecules in an η2 fashion, forming a distorted-tetrahedral arrangement. The torsion angle between the mean planes of C1−C2−Au and C9−C10−Au is 89.1° (83.3° for the second molecule). This indicates that two alkyne moieties are almost perpendicular to each other. The average CC bond length of [(cyclooctyne)2Au][SbF6] (1.230 Å) is slightly longer in comparison to the corresponding parameters observed for [(cyclooctyne)3Au][SbF6] (1.216 Å). In contrast, the Au−C bond distances are significantly shorter in [(cyclooctyne)2Au][SbF6] (average 2.150 Å) in comparison to those of [(cyclooctyne)3Au][SbF6] (average 2.252 Å). These bond distances and large chemical shift values perhaps point to a relatively stronger alkyne−gold interaction in [(cyclooctyne)2Au][SbF6]. Optimized geometries of cationic [(cyclooctyne)2M]+ (M = Cu, Ag, Au) using density functional theory (DFT) at the BP86/def2-TZVPP level of theory (Figure S1−S3, Supporting Information) show that these adducts adopt a tetrahedral arrangement of the cyclooctyne ligands, which bind in a η2 fashion. The predicted geometry and calculated bond lengths and angles of [(cyclooctyne)2Au]+ are in good agreement with the experimental data (Table 2). The related copper and silver

Figure 7. (top) Molecular structure showing [(cyclooctyne)2Au]+ of [(cyclooctyne)2Au][SbF6] Ellipsoids are shown at the 40% probability level. There are two [(cyclooctyne)2Au][SbF6] molecules in the asymmetric unit, but only one of those is shown here. (bottom) Side view showing the tetrahedral conformation of two alkyne moieties.

Table 2. Summary of Experimental X-ray Crystallographic Data and Optimized Structural Parametersa for Bis(alkyne) Adducts [(L)2M]+ (M = Cu, Ag, Au; L = Cyclooctyne) parameter d(CC), Å

[(L)2Au][SbF6] [(L)2Cu]+ [(L)2Ag]+ [(L)2Au]+ molecule 1, molecule 2

[1.248] [1.248] av d(CC), Å [1.248] d(M−C), Å [2.011] [2.011] [2.011] [2.011] av d(M−C), Å [2.011] ∠C−M−C, deg [36.1] [36.1]

[1.243] [1.243] [1.243] [2.225] [2.225] [2.225] [2.225] [2.225] [32.4] [32.4]

[1.257] [1.257] [1.257] [2.144] [2.144] [2.144] [2.144] [2.144] [34.1] [34.1]

1.243(11), 1.222(10) 1.221(10), 1.235(10) 1.232(11) 2.143(7), 2.135(7) 2.139(7), 2.170(7) 2.133(7), 2.149(7) 2.148(7), 2.181(7) 2.150(7) 33.8(3), 33.0(3) 33.1(3), 33.1(3)

a

Results of DFT calculations (at the BP86/def2-TZVPP level of theory) on [(L)2M]+ are given in brackets.

adducts have not yet been reported. However, computed structural parameters of [(cyclooctyne)2M]+ can be compared to those of the corresponding mono- and tris(cyclooctyne) adducts (Tables 1−3). For example, the Au−C bond lengths Table 3. Summary of Optimized Structural Parameters for Mono(alkyne) Adducts [(L)M]+ (M = Cu, Ag, Au; L = Cyclooctyne)a param d(CC), Å d(M−C), Å ∠C−M−C, deg a

3139

[(L)Cu]+

[(L)Ag]+

[(L)Au]+

1.256 1.993 1.993 36.7

1.249 2.237 2.237 32.4

1.270 2.128 2.128 34.7

Calculated free cyclooctyne CC distance 1.214 Å. dx.doi.org/10.1021/om400073a | Organometallics 2013, 32, 3135−3144

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−5.3 (11.2) 16.8 17.1 −5.8 (10.2) 23.0 17.2 restb Eprep De

The values in parentheses are the percentage contributions to the total attractive interactions ΔEelstat + ΔEorbital. bThe values in parentheses are the percentage contributions to the total orbital interactions ΔEorbital.

−2.4 (5.1) −3.5 (6.2) ΔEπ⊥b((−CC−)→M)

interacting fragments

a

−8.6 (7.6) 4.6 96.1

−11.4 (9.8) −8.5 (9.2)

−9.1 (9.8) 3.5 66.7 −4.5 (9.3) 20.2 7.7

−1.9 (3.9) −3.6 (4.6)

−6.2 (7.9) 36.8 7.7 −9.2 (14.9) 2.1 60.5

−4.4 (7.1) −4.4 (8.4)

−6.8 (12.9) 1.6 48.1 −3.8 (11.9) 10.7 13.4

−1.5 (4.7) −2.4 (5.6)

−4.4 (10.2) 17.8 13.3 −11.8 (14.1) 2.9 84.8

−6.2 (7.4)

(60.2) (39.9) (55.4) (22.6) (63.4) (36.6) (50.6) (28.1)

−62.6 91.9 −92.9 −61.6 −34.1 −13.9 −49.7 94.1 −91.1 −52.6 −26.6 −14.8 −87.7 106.2 −110.3 (56.9) −83.6 (43.1) −40.4 (48.3) −25.2 (30.1) −65.1 101.6 −100.0 (60.0) −66.6 (40.0) −29.2 (43.8) −22.9 (34.4) ΔEint ΔEPauli ΔEelstata ΔEorbitala ΔEσb((−CC−)→M) ΔEπ∥b((−CC−)←M)

−7.6 (11.4) 2.3 62.8

[(L)2Au] L−A −44.5 164.6 −131.0 (62.7) −78.1 (37.3) −36.9 (47.2) −31.4 (40.2) Ag L

[(L)Ag] L

[(L)2Ag] L−B −24.1 64.9 −57.2 (64.3) −31.8 (35.7) −19.6 (61.6) −6.9 (21.7) [(L)2Ag] L−A −31.1 91.6 −79.6 (64.8) −43.2 (35.2) −22.6 (52.3) −13.8 (31.9) Cu L

[(L)Cu] L

[(L)2Cu] L−B −33.9 89.1 −75.6 (61.5) −47.4 (38.5) −26.6 (56.1) −13.1 (27.6) [(L)2Cu] L−A −40.2 108.9 −92.1 (61.8) −56.9 (38.2) −26.4 (46.4) −21.2 (37.3)

−6.9 (10.4)

−100.7 178.7 −162.6 (58.2) −116.8 (41.8) −68.8 (58.9) −28.0 (24.0) −70.2 169.1 −146.9 (61.4) −92.4 (38.6) −46.5 (50.30) −28.3 (30.6)

Au+ L [(L)Au] L

[(L)2Au] L−B −27.9 98.8 −78.5 (62.0) −48.2 (38.0) −29.2 (60.6) −12.6 (26.1)

+

[(L)2Au]+ [(L)3Au]+

+ +

[(L)3Au]+ [(L)Ag]+

+ +

[(L)2Ag]+ [(L)3Ag]+

+ +

[(L)3Ag]+ [(L)Cu]+

+ +

[(L)2Cu]+ [(L)3Cu]+

+ +

[(L)3Cu]+

Table 4. EDA-NOCV Results of [(L)3M]+, [(L)2M]+, and [(L)M]+ at the BP86/TZ2P+//BP86/def2-TZVPP Level in kcal/mol (M = Cu, Ag, Au; L = Cyclooctyne)

in [(cyclooctyne) 2 Au] + (2.144 Å) are larger than in [(cyclooctyne)Au]+ (2.128 Å) but they are shorter than the average bond length in [(cyclooctyne)3Au]+ (2.278 Å). The copper adducts show the same trend. Interestingly, the trend in the Ag−C distances is not the same as that observed in the copper or gold homologues. The Ag−C bond length in [(cyclooctyne)Ag]+ (2.237 Å) is longer than in [(cyclooctyne)2Ag]+ (2.225 Å). However, just as in the tris(cyclooctyne) copper and gold analogues, [(cyclooctyne)3Ag]+ (average 2.342 Å) has the longest M−C bond distance of the series. Calculations also show that the CC distances of cyclooctyne ligands in [(cyclooctyne)3M]+, [(cyclooctyne)2M]+, and [(cyclooctyne)M]+ become longer in that order in comparison to the CC distance (1.214 Å) of free cyclooctyne (Tables 1−3). Shorter Au−C distances and longer CC bond lengths in [(cyclooctyne)2Au]+ in comparison to the corresponding values of [(cyclooctyne)3Au]+ are consistent with the IR data of these compounds. Table 4 shows the EDA-NOCV results of the cationic coinage metal complexes of tris-, bis-, and mono(cyclooctyne). For the [(cyclooctyne)3M]+ adducts, EDA results are presented for the [(cyclooctyne)2M]+−cyclooctyne(A) and [(cyclooctyne)2M]+−cyclooctyne(B) interactions, where A and B denote the ligands with short and long metal−ligand bonds, respectively (see Figures S1−S3 in the Supporting Information for cyclooctyne(A) and cyclooctyne(B)). Figure 8 depicts the key orbital interactions of the M−alkyne moiety using the d orbitals of the metal as valence orbitals. Since the coinage metal ions M+ have a d10 valence configuration, there are no vacant d orbitals which may serve as acceptors. The alkyne→M+ σ donation takes place into the vacant valence s orbital of M+, which may mix with the dz2 orbital, yielding two sdz2 hybrids. One of them is occupied and enhances the tubular-shaped part of the dz2 orbital. The vacant sdz2 hybrid has a larger extension toward the alkyne ligand, which makes it a very effective acceptor orbital. The alkyne→M+ π⊥ donation takes place into the vacant p(π) AO of the metal, which therefore should be small. However, it should be noted that a small part of ΔEorb comes from the relaxation of the orbitals within the fragments rather than from genuine orbital interactions between the ligand and metal moieties. This has been discussed in a detailed EDA analysis of metal−carbonyl bonding.70 The intrinsic interaction energy for [(cyclooctyne)3Cu]+ between the copper fragment and cyclooctyne(A) is larger (ΔEint = 40.2 kcal/mol) than for the cyclooctyne(B) ligand (ΔEint = 33.9 kcal/mol)). The breakdown of ΔEorb into contributions of orbitals with different symmetries shows that the (−CC−)→Cu σ donation is the largest component for both the strongly bonded ligand cyclooctyne(A) (46.4%) and the weakly bound cyclooctyne(B) (56.1%). The in-plane C C←Cu π∥ back-donation contributes 37.3% (cyclooctyne(A)) and 27.6% (cyclooctyne(B)) to ΔEorb, respectively. The contribution of the (−CC−)→Cu out-of-plane π⊥ donation is very small. The contribution of ΔEorb is slightly higher for [(cyclooctyne)2Cu]+ (40.0%) and [(cyclooctyne)Cu]+ (43.1%) than for [(cyclooctyne)3Cu]+ (38.2%). The intrinsic interaction energy ΔEint of [(cyclooctyne)3Cu]+ (40.2 kcal/mol) is lower than that of [(cyclooctyne)2Cu]+ (65.1 kcal/mol) and [(cyclooctyne)Cu]+ (87.7 kcal/mol) because both electrostatic and orbital interactions are stronger for the latter complexes than for [(cyclooctyne)3Cu]+. The data in Table 4 show that the preparation energies ΔEprep are sometimes rather large. This holds in particular for

[(L)Au]+

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C−) π back-donation. The former interactions nicely explain the trend of the CC bond lengths, which for all metals possesses the order [(cyclooctyne)3M]+ < [(cyclooctyne)2M]+ < [(cyclooctyne)M]+. The (−CC−)→M σ donation weakens the triple bond, and therefore, the CC bond becomes longer when the σ donation gets stronger. It could be assumed that the lengthening of the CC bond might also be caused by the M→(−CC−) π back-donation, where charge is donated into the antibonding π* orbitals. Inspection Table 4 shows that this holds true for the copper complexes but not for the silver and gold homologues, where the strength of the π back-donation has the order [(cyclooctyne)2M]+ > [(cyclooctyne)M]+, which does not agree with a CC bond lengthening in the latter species. We have also investigated the nature and strength of the M−cyclooctyne interactions in [(cyclooctyne)2MCl] and [(cyclooctyne)MCl] with the EDA-NOCV method. The numerical results are presented in Table S2 (Supporting Information). A comparison of the latter data with the EDANOCV results for the positively charged complexes (Table 4) shows that the orbital interactions ΔEorb in the latter systems feature a larger percentage contribution of (−CC−)→[M]+ donation and less (−CC−)←[M]+ back-donation in comparison with the (−CC−)→[M] donation and (−C C−)←[M] back-donation in the former complexes. This is in agreement with the calculated NBO partial charges. Table S3 (Supporting Information) shows that the cyclooctyne ligands are always electron donors in the complexes [(cyclooctyne)nM]+ and are very weak charge donors or acceptors, depending on the nature of M, in the corresponding [(cyclooctyne)nMCl] complexes. Figures S1−S3 (Supporting Information) display the calculated structures of [(cyclooctyne)2MCl] and [(cyclooctyne)MCl]. Note that X-ray crystal structures of only [(cyclooctyne)2CuCl], [(cyclooctyne)2AuCl], and [(cyclooctyne)CuCl]2 (which is a chloride-bridged dimer) have been reported.41,59 The calculated bond lengths and angles of [(cyclooctyne)2CuCl] and [(cyclooctyne)2AuCl] are in good agreement with the experimental data (Table S4, Supporting Information). The data show that the M-bound cyclooctyne ligands have longer CC bonds than in free cyclooctyne (1.214 Å). In agreement with experiment, the calculations give lower wavenumbers for the CC stretching frequencies of the ligands in the metal complexes in comparison with free cyclooctyne. The calculated frequency shifts are 188 and 241 cm−1 for [(cyclooctyne)2CuCl] and [(cyclooctyne)2AuCl], respectively, using the asymmetric mode, which has a higher intensity (see the Supporting Information). IR/Raman data of [(cyclooctyne)2CuCl] and [(cyclooctyne)2AuCl] show 143 and 180−190 cm−1 reductions in CC stretching value upon coordination. In summary, we have reported the synthesis and isolation of tris(cyclooctyne) adducts of copper(I) and silver(I) and a comparison to its gold(I) analogue. We have also reported data for a rare bis(cyclooctyne)gold(I) adduct. The EDA-NOCV results show that the intrinsic interactions exhibit the trend in the bond strength, Au > Cu > Ag, which is in agreement with previous studies of complexes of the coinage metals.71−76 The stronger bonds of the gold complexes come from relativistic effects. The most important orbital interactions come in all complexes from the (−CC−)→M σ donation and the in-plane M→(−CC−) π∥ back-donation, while the (-C C−)→M out-of-plane π⊥ donation is very small. Reactivity studies of bis- and tris(cyclooctyne) adducts of coinage metals

Figure 8. General scheme for orbital interactions between a transitionmetal atom (TM) and an alkyne moiety (RCCR): (1) ΔEσ ((−C C−)→M); (2) ΔEπ∥ ((−CC−)←M); (3) ΔEπ⊥ ((−CC−)→M); (4) rest term that includes ΔEδ ((−CC−)←M) and numerous small contributions from lower-lying orbitals.

the complexes [(cyclooctyne)3M]+, where the relaxation of the fragments [(cyclooctyne)2M]+ + cyclooctyne(A) is the largest for each metal M. This is because the changes in the geometries of the latter system are particularly large (see the geometries in the Supporting Information). The ΔEprep values exhibit the same trend, Au > Cu > Ag, as the ΔEint values, because the stronger interaction energy induces larger geometry changes. Note that the bond dissociation energies De do not reveal the strength of the metal−ligand interactions, because they are calculated as the absolute value of the difference between ΔEint and ΔEprep. The EDA-NOCV results of the silver complexes show features similar to those for the copper complexes. The intrinsic interaction energies of the former complexes are roughly 10−20 kcal/mol lower than for the corresponding copper complexes. The breakdown of ΔEint into the EDA-NOCV terms shows that the percentage contributions of the electrostatic attraction and the orbital interactions remain nearly the same for all species. The breakdown of ΔEorb into orbitals which possess different symmetries shows that the Ag→(−CC−) π back-donation (22.6%) is lower for [(cyclooctyne)Ag]+ than in [(cyclooctyne)3Ag]+ (31.9%) and [(cyclooctyne)2Ag]+ (28.1%). The energy decomposition analysis results in Table 4 also show that the intrinsic interaction energies are higher for the gold complexes than for the corresponding copper and silver cyclooctyne complexes, because both electrostatic interactions and orbital interactions are stronger in the Au systems than in the Ag and Cu homologues. The intrinsic interaction energy for [(cyclooctyne)2Au]+ (70.2 kcal/mol) between the gold fragment and cyclooctyne is stronger than for [(cyclooctyne)3Au]+ (44.5 kcal/mol) but weaker than for [(cyclooctyne)Au]+ (100.7 kcal/mol), which shows the same trend as the homologous Cu and Ag complexes. The ΔEint values of the gold complexes are always greater than the values for the Cu and Ag adducts. The V-shaped trend for the bond strength of the first-, second-, and third-row transition metals is well-known.71−76 Chemical bonding of the third-row transition metals is enhanced by relativistic effects, which are particularly strong for gold.77−82 Overall, electrostatic interactions (ranging from 64.8 to 56.9%) are significantly stronger than the orbital interactions (ranging from 43.1 to 35.2%) of cationic tris-, bis-, and mono(cyclooctyne) complexes of coinage metal ions. Further analysis of the orbital component shows that the (−CC−)→M σ donation is stronger than the M→(−C 3141

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Organometallics

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are under way. Preliminary data show that [(cyclooctyne)3Au]+ catalyzes the [2 + 2 + 2] cycloaddition of cyclooctyne to afford the corresponding arene.41

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with several pieces of 4 Å molecular sieves. A mixture of AgSbF6 (0.075 g, 0.22 mmol) and CH2Cl2 (ca. 10 mL) was stirred in another Schlenk flask for a few minutes at room temperature. The mixture was cooled to −10 °C (using ice/acetone bath), degassed cyclooctyne (0.1 g, 0.92 mmol) solution was added dropwise, and this mixture was stirred for 1 h at −10 °C and then stirred overnight at room temperature. The flask was covered with aluminum foil to protect it from the light. The resulting mixture was filtered through a bed of Celite, and the filtrate was collected and concentrated under reduced pressure to ∼2 mL. n-Hexane (∼10 mL) was slowly added to this concentrated filtrate and kept in a −10 °C freezer to obtain colorless crystals of the product. It is a fairly air stable compound. However, it is best stored under nitrogen, protected from light, in a low-temperature freezer. Yield: 0.08 g (54%). Mp: 110 °C. 1H NMR (CD2Cl2, 500.16 MHz, 298 K): δ 1.72 (m, 12H, CH2, C5/6), 2.01 (m, 12H, CH2, C4/7), 2.44 (m, 12H, CH2, C3/8) ppm. 13C{1H} NMR (CD2Cl2, 125.77 MHz, 298 K): δ 23.04, 29.35, 34.96, 95.0 (C1/2) ppm. 1H NMR (CD2Cl2 + excess cyclooctyne, 500.16 MHz, 298 K): δ 1.66 (m, 12H, CH2, C5/6), 1.93 (m, 12H, CH2, C4/7), 2.29 (m, 12H, CH2, C3/8) ppm. 13C{1H} NMR (CD2Cl2 + excess cyclooctyne, 125.77 MHz, 298 K): δ 22.25, 29.81, 35.02, 94.63 (C1/2) ppm. Anal. Calcd for C24H36AgSbF6: C, 43.14; H, 5.43. Found: C, 42.79; H, 5.40. Synthesis of [Ag(cyclooctyne)3][PF6]. This complex was obtained in a manner similar to that described for [Ag(cyclooctyne)3][SbF6], but using AgPF6 instead of AgSbF6. Cyclooctyne (0.1 g, 0.92 mmol) and CH2Cl2 (ca. 5 mL) were placed in a Schlenk flask together with several pieces of 4 Å molecular sieves. A mixture of AgPF6 (0.056 g, 0.22 mmol) and CH2Cl2 (ca. 10 mL) was stirred in another Schlenk flask for a few minutes at room temperature. The mixture was cooled to −10 °C (using ice/acetone bath), degassed cyclooctyne (0.1 g, 0.92 mmol) solution was added dropwise, and this mixture was stirred for 1 h at −10 °C and then stirred overnight at room temperature. The flask was covered with aluminum foil to protect it from the light. The resulting mixture was filtered through a bed of Celite, and the filtrate was collected and concentrated under reduced pressure to ∼2 mL. n-Hexane (∼10 mL) was slowly added to this concentrated filtrate, and this mixture was kept in a −10 °C freezer to obtain colorless crystals of the product. It was stored under nitrogen at −10 °C. This compound is sensitive to air and light as well. Yield: 0.06 g (47%). Mp: 112 °C. 1H NMR (CD2Cl2, 500.16 MHz, 298 K): δ 1.71 (m, 12H, CH2, C5/6), 2.00 (m, 12H, CH2, C4/7), 2.42 (m, 12H, CH2, C3/8) ppm. 13C{1H} NMR (CD2Cl2, 125.77 MHz, 298 K): δ 23.92, 30.45, 35.94, 95.62 (C1/2) ppm. 1H NMR (CD2Cl2 + excess cyclooctyne, 500.16 MHz, 298 K): δ 1.66 (m, 12H, CH2, C5/6), 1.93 (m, 12H, CH2, C4/7), 2.29 (m, 12H, CH2, C3/8) ppm. 13C{1H} NMR (CD2Cl2 + excess cyclooctyne, 125.77 MHz, 298 K): δ 22.25, 29.81, 35.02, 94.63 (C1/2) ppm. Anal. Calcd for C24H36AgPF6: C, 49.93; H, 6.28. Found: C, 49.79; H, 6.25. Synthesis of [Au(cyclooctyne)2][SbF6]. Au(cyclooctyne)2Cl (0.067 g, 0.15 mmol) and CH2Cl2 (ca. 5 mL) were placed in a Schlenk flask, and the mixture was cooled to −10 °C (using an ice/ acetone bath). To the reaction mixture was added dropwise a solution of AgSbF6 (0.052 g, 0.15 mmol) in CH2Cl2 (ca. 8 mL), and this mixture was stirred for 1 h at −10 °C and then for 2 h at room temperature. The flask was covered with aluminum foil to protect it from the light. The resulting mixture was filtered through a bed of Celite, and the filtrate was collected and concentrated under reduced pressure to ∼2 mL. n-Hexane (∼8 mL) was slowly added to this concentrated filtrate, and this mixture was kept in a −10 °C freezer to obtain colorless crystals of the product. It was stored under nitrogen at −10 °C. This compound is sensitive to air and light as well. Yield: 0.025 g (34%). Mp: 92 °C. 1H NMR (CD2Cl2, 500.16 MHz, 298 K): δ 1.77 (m, 8H, CH2, C5/6), 2.07 (m, 8H, CH2, C4/7), 2.93 (m, 8H, CH2, C3/8) ppm. 13C{1H} NMR (CD2Cl2, 125.77 MHz, 298 K): δ 23.67, 28.73, 33.70, 110.08 (C1/2) ppm. 1H NMR (CD2Cl2 + excess cyclooctyne, 500.16 MHz, 298 K): δ 1.67 (m, 12H, CH2, C5/6), 1.93 (m, 12H, CH2, C4/7), 2.37 (m, 12H, CH2, C3/8) ppm. 13C{1H} NMR (CD2Cl2 + excess cyclooctyne, 125.77 MHz, 298 K): δ 22.24, 25.97, 27.51, 29.46, 34.30, 42.16, 97.14 (br, C1/2) ppm. IR (KBr, selected band): 2030 cm−1 (br, CC stretch).

EXPERIMENTAL SECTION

All manipulations were carried out under an atmosphere of purified nitrogen using standard Schlenk techniques or in a Vacuum Atmosphere single-station drybox equipped with a −10 °C refrigerator. Solvents were purchased from commercial sources, purified by using the Innovative Technology SPS-400 PureSolv solvent drying system or by distilling over conventional drying agents, and degassed by the freeze−pump−thaw method prior to use. Glassware was oven-dried at 150 °C overnight. NMR spectra were recorded on a JEOL Eclipse 500 spectrometer (1H, 500.16 MHz; 13C, 125.77 MHz). Proton and carbon chemical shifts are reported in ppm and referenced using the residual proton or carbon signals of the deuterated solvent. Elemental analyses were performed using a Perkin-Elmer Series II CHNS/O analyzer. Raman spectra were recorded at room temperature on Horiba Jobin Yvon LabRAM Aramis instrument using a 473 nm laser source. Crystals of the samples were placed on a glass slide for Raman analysis. Different experimental settings (laser intensity, level of magnification, time of exposure, number of cycles) were used for each compound in order to obtain the best signal-to-noise ratio. IR spectra were collected on a Bruker FT-IR instrument containing an ATR attachment or on a JASCO FT-IR 410 instrument. Cyclooctyne83 was prepared according to the literature procedure. CuBr, AuCl, AgPF6, and AgSbF6 were purchased from commercial sources and used as received. Certain NMR peak assignments were based on reported values of somewhat related copper−cyclooctyne adducts. Cyclooctyne (C8H12).

H NMR (CD2Cl2, 500.16 MHz, 298 K): δ 1.61 (m, 4H, CH2, C5/6), 1.85 (m, 4H, CH2, C4/7), 2.13 (m, 4H, CH2, C3/8) ppm. 13C{1H} NMR (CD2Cl2, 125.77 MHz, 298 K): δ 21.29, 30.32, 35.13, 94.90 (C1/2) ppm. IR (Nujol, selected band): 2216 cm−1 (CC stretch). Raman (neat): 2217 cm−1 (CC stretch). Synthesis of [Cu(cyclooctyne)3][SbF6]. Cyclooctyne (0.1 g, 0.92 mmol) and CH2Cl2 (ca. 5 mL) were placed in a Schlenk flask together with several pieces of 4 Å molecular sieves. A mixture of AgSbF6 (0.075 g, 0.22 mmol) and CuBr (0.032 g, 0.22 mmol) in CH2Cl2 (ca. 8 mL) was stirred in another Schlenk flask for 10 min at room temperature. The mixture was cooled to −10 °C (using ice/ acetone bath), degassed cyclooctyne (0.1 g, 0.92 mmol) solution was added, and this mixture was stirred for 1 h at −10 °C and then stirred overnight at room temperature. The flask was covered with aluminum foil to protect it from the light. The resulting mixture was filtered through a bed of Celite, and the filtrate was collected and concentrated under reduced pressure to ∼2 mL. n-Hexane (∼8 mL) was slowly added to this concentrated filtrate and kept in a −10 °C freezer to obtain colorless prism-shaped crystals of the product. It is a moderately air stable compound. However, it is best stored under nitrogen, in a low-temperature freezer. Yield: 0.07 g (51%). Mp: 136 °C. 1H NMR (CD2Cl2, 500.16 MHz, 298 K): δ 1.71 (m, 12H, CH2, C5/6), 2.0 (m, 12H, CH2, C4/7), 2.48 (m, 12H, CH2, C3/8) ppm. 13C{1H} NMR (CD2Cl2, 125.77 MHz, 298 K): δ 24.5, 29.36, 34.40, 99.0 (C1/2) ppm. 1 H NMR (CD2Cl2 + excess cyclooctyne, 500.16 MHz, 298 K): δ 1.66 (m, 12H, CH2, C5/6), 1.92 (m, 12H, CH2, C4/7), 2.3 (m, 12H, CH2, C3/8) ppm. 13C{1H} NMR (CD2Cl2 + excess cyclooctyne, 125.77 MHz, 298 K): δ 22.79, 29.85, 34.81, 96.54 (C1/2) ppm. Anal. Calcd for C24H36CuSbF6: C, 46.21; H, 5.82. Found: C, 45.97; H, 5.77. Synthesis of [Ag(cyclooctyne)3][SbF6]. Cyclooctyne (0.1 g, 0.92 mmol) and CH2Cl2 (ca. 5 mL) were placed in a Schlenk flask together 1

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Notes

Anal. Calcd for C16H24AuSbF6: C, 29.61; H, 3.73. Found: C, 29.45; H, 3.74. Alternative Synthetic Route for [Au(cyclooctyne)2][SbF6]. [Au(cyclooctyne)3][SbF6] was recrystallized from CH2Cl2/hexane. After about 30 days in a freezer at 0 °C, this mixture afforded crystalline [Au(cyclooctyne)2][SbF6], but in lower yield (estimated 10%). X-ray Crystallographic Data. A suitable crystal covered with a layer of hydrocarbon/Paratone-N oil was selected, mounted on a Cryo-loop, and immediately placed in the low-temperature nitrogen stream. The X-ray intensity data were measured at 100(2) K on a Bruker SMART APEX CCD area detector system equipped with a Oxford Cryosystems 700 series cooler, a graphite monochromator, and a Mo Kα fine-focus sealed tube (λ = 0.71073 Å). Intensity data were processed using the Saint Plus program. All the calculations for the structure determinations were carried out using the SHELXTL package (version 6.14).84 Initial atomic positions were located by direct methods using XS, and the structures of the compounds were refined by least-squares methods using XL. Absorption corrections were applied by using SADABS. Hydrogen atoms were placed at calculated positions and refined riding on the corresponding carbons. Crystals of [Cu(cyclooctyne)3][SbF6] diffracted poorly. The compound crystallizes in the Pbca space group. Four fluorine atoms of the SbF6− counterion show positional disorder, which was modeled reasonably well. One of the cyclooctyne ring backbones also demonstrated disorder, which was modeled satisfactorily. All the non-hydrogen atoms except disordered fluorine atoms were refined anisotropically. [Ag(cyclooctyne)3][PF6] crystallizes in the P21 space group with two molecules in the asymmetric unit (for a Z value of 4). One of the cyclooctyne ring backbones shows positional disorder, which was modeled satisfactorily. All the non-hydrogen atoms were refined anisotropically. [Au(cyclooctyne)2][SbF6] crystallizes in the P21/c space group with two molecules in the asymmetric unit (for a Z value of 8). Four fluorine atoms of one of the SbF6− counterions show disorder over two sites, which was modeled satisfactorily. All the non-hydrogen atoms were refined anisotropically. Computational Details. The geometry optimizations of all complexes have been carried out using gradient-corrected density functional theory at the BP8685 level in conjunction with def2-TZVPP basis sets86,87 as implemented in Gaussian09.88 Relativistic effects were considered by using relativistic effective core potentials for Ag and Au.89 The nature of the metal−ligand interactions has been investigated by means of an energy decomposition analysis (EDA) developed independently by Morokuma90 and by Ziegler and Rauk91 in conjunction with the natural orbitals for chemical valence (NOCV).92−94 The EDA-NOCV calculations were carried out at the BP86 level using TZ2P+95 basis sets which have uncontracted Slater-type orbitals (STOs) as basis functions using the program package ADF2009.01b.96 Relativistic effects have been considered in the latter calculations by using the zeroth-order regular approximation (ZORA).97

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Foundation (No. CHE-0845321) and the Robert A. Welch Foundation (Grant No. Y-1289).

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ASSOCIATED CONTENT

S Supporting Information *

Tables, figures, and CIF files giving additional experimental, crystallographic, and computational data. This material is available free of charge via the Internet at http://pubs.acs.org. The CCDC files 911939−911941 also contain supplementary crystallographic data. These data can be obtained free of charge via http://www.ccdc.cam.ac.uk/conts/retrieving.html (or from the Cambridge Crystallographic Data Centre (CCDC), 12 Union Road, Cambridge, CB2 1EZ, U.K.).

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (H.V.R.D.); [email protected] (G.F.). 3143

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