Article pubs.acs.org/Organometallics
Coordination and Ligand Substitution Chemistry of Bis(cyclooctyne)copper(I) Animesh Das, Chandrakanta Dash, Muhammed Yousufuddin, and H. V. Rasika Dias* Department of Chemistry and Biochemistry, The University of Texas at Arlington, Arlington, Texas 76019, United States S Supporting Information *
ABSTRACT: Cationic bis(alkyne)copper(I) carbonyl and bis(alkyne)copper(I) isocyanide complexes have been synthesized from the precursor (cyclooctyne)2CuBr. [Cu(cyclooctyne)2(CO)][SbF6] and [Cu(cyclooctyne)2(CNtBu)][SbF6] have trigonal-planar and three-coordinate copper centers. The copper carbonyl complex [Cu(cyclooctyne)2(CO)][SbF6] displays its C−O stretching frequency in the “nonclassical” metal carbonyl region (2171 cm−1), and the analogous copper(I) isocyanide complex [Cu(cyclooctyne)2(CNtBu)][SbF6] also has an unusually high CN stretching band at 2230 cm−1. The reaction of 3,5-Me2C6H3NH2 and 4-tBuC6H4NH2 with [Cu(cyclooctyne)2(CO)][SbF6] led to CO displacement rather than addition to CO. CNtBu reacts with [Cu(cyclooctyne)2(CO)][SbF6] to afford [Cu(cyclooctyne)2(CNtBu)][SbF6]. The syntheses of [Cu(cyclooctyne)(CNtBu)2][SbF6] and [Cu(CNtBu)4][SbF6] from the (cyclooctyne)2CuBr precursor are also reported.
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investigate molecules involving alkynes (side-on-bound π-acid ligands) and various types of end-on bound π-acceptors (e.g., CO) on a metal center. Here we describe the synthesis of [Cu(cyclooctyne) 2 (CO)][SbF 6 ] and [Cu(cyclooctyne)2 (CNtBu)][SbF6] (Figure 1), which feature carbonyl and isocyanide ligands on a cationic bis(alkyne)copper template. Interestingly, these two metal complexes display notably high CO and CN stretching frequencies.
INTRODUCTION Alkyne adducts of coinage metals (Cu, Ag, Au) have attracted significant interest because of their importance as models for likely intermediates in many coinage-metal-mediated processes involving alkynes.1−18 Recently, we reported the isolation and structural data of the bis(alkyne) complexes (cyclooctyne)2AuCl19 and [Au(cyclooctyne)2][SbF6],20 as well as the related tris(alkyne) adducts [M(cyclooctyne)3]+ (M = Au, Ag, Cu).19,20 Such molecules of coinage metals featuring more than one regular alkyne moiety on the metal atom are rather limited. For example, copper(I) adducts of this type that have been structurally authenticated include (cyclooctyne)2CuX (X = Cl, Br, I) reported by Behrens and co-workers, 21 [Cu(cyclooctyne)3][SbF6] (Figure 1),20 and copper triflate coordinated di- and trialkynes such as 1,7-cyclododecadiyne,22 1,2:5,6:9,10-tribenzocyclododeca-1,5,9-triene-3,7,11-triyne,23 and 4,7-dioxa-1,10-diazabicyclo[8.6.6]docosa-13,19-diyne.24 Note that coinage-metal adducts of alkynyl metal groups (i.e., metal acetylides) of the type M′CCR, M′(CCR)2, etc. (M′ = transition metal, R = typically alkyl or aryl group),25−33 however, are more common, but we are interested in the former category (and especially those involving nonchelating monoalkyne ligands) because alkynes of these compounds are not influenced by additional metal ions (M′), thus providing useful information on preferred coordination modes/geometry and effects of a π-bound coinage metal on an alkyne moiety (or vice versa). The [M(cyclooctyne)3]+ complexes, for example, display interesting spoke-wheel structures.20 The tris(alkyne) adduct [Au(cyclooctyne)3]+ also shows facile [2+2+2] cycloaddition chemistry, affording arenes.19 As a continuation of our work on the σ-donor/π-acceptor ligand34 chemistry of coinage-metal ions, we set out to © 2014 American Chemical Society
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RESULTS AND DISCUSSION The copper(I) carbonyl complex [Cu(cyclooctyne)2(CO)][SbF6] was synthesized by treating a dichloromethane solution of (cyclooctyne)2CuBr and Ag[SbF6] with carbon monoxide (1 atm) (Scheme 1). Colorless crystals of [Cu(cyclooctyne)2(CO)][SbF6] were obtained in CH2Cl2/n-hexane under a CO atmosphere at −10 °C. [Cu(cyclooctyne)2(CO)][SbF6] is a moderately air stable solid and can be stored in a sealed container under nitrogen in a −10 °C refrigerator. It was characterized by NMR and IR spectroscopy, elemental analysis, and X-ray crystallography. The room-temperature 1H NMR spectrum of [Cu(cyclooctyne)2(CO)][SbF6] in CD2Cl2 shows that the methylene protons on the α carbons of the alkyne moiety appear as a multiplet at δ 2.49 ppm. These protons have been deshielded as a result of coordination of the alkyne moiety to a cationic copper center. For comparison, the corresponding resonance of the free cyclooctyne appears at an upfield position (δ 2.13 ppm). The carbon resonance of the metal-bound alkyne moiety of [Cu(cyclooctyne)2(CO)][SbF6] in the 13C{1H} NMR spectrum was observed at δ 98.47 ppm, which is Received: December 14, 2013 Published: March 21, 2014 1644
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Figure 1. Tris- and bis(alkyne) copper adducts [Cu(cyclooctyne)3]+, [Cu(cyclooctyne)2(CO)]+, and [Cu(cyclooctyne)2(CNtBu)]+.
Scheme 1. Synthesis of Copper(I) Adducts of Various Donors Starting from (cyclooctyne)2CuBr
Cu−C−O moiety is essentially linear with a Cu−C−O angle of 175.5(3)°. The Cu−CO distance of 1.929(3) Å is long and somewhat comparable to the corresponding distance observed in the three-coordinate copper(I) dicarbonyl complex [(IPr*)Cu(CO) 2 ][SbF 6 ] (1.948(7) Å; IPr* = 1,3-bis(2,6-bis(diphenylmethyl)-4-methylphenyl)imidazol-2-ylidene),37 but even longer Cu−CO distances have been reported in fourcoordinate [Cu(CO) 4 ][1-Et-CB 11 F 11 ] (1.968(3) Å) by Strauss.38 A search of three-coordinate copper−carbonyl adducts in the Cambridge Structural Database39 revealed that the Cu− CO distances spread between 1.749(9) Å in [Cu2(CO)2(TC5,5)] (TC = tropocoronand macrocycle)40 and 1.948(7) Å of [(IPr*)Cu(CO)2][SbF6]37 with an average of 1.828 Å (for 25 Cu−CO bonds). 3 9 The Cu−CO moiety of [Cu(cyclooctyne)2(CO)][SbF6] shows several van der Waals contacts with fluorine atoms of the antimony hexafluoride counterion. The IR spectrum of [Cu(cyclooctyne)2(CO)][SbF6] displays a strong absorption band at 2171 cm−1, which is due to the CO stretch. This band is substantially higher than that of the free CO (2143 cm−1). It is also one of the highest νC̅ O values that has been observed for a structurally characterized copper(I) monocarbonyl species (see Table S1 in the Supporting
marginally downfield of the corresponding peak of Cu(cyclooctyne)2Br (δ 97.85 ppm)21 but is shifted significantly downfield in comparison to the corresponding signal of the free cyclooctyne (δ 94.90 ppm). Much larger upfield and downfield 13 C shifts of the metal-bound alkyne carbons of cyclooctyne have also been reported, for example, in [K(18C6)][PtCl3(cyclooctyne)] (δ 76.6 ppm)35 or [Au(cyclooctyne)2][SbF6] (δ 110.08 ppm).20 We have not observed the 13C NMR signal of CO for [Cu(cyclooctyne)2(CO)][SbF6], which is not unusual for copper carbonyls.36 The X-ray structure of [Cu(cyclooctyne)2(CO)][SbF6] is shown in Figure 2. It crystallizes in the space group P21/c. The Cu center is trigonally coordinated by two alkyne moieties and CO, but the alkyne moieties are slightly distorted by the steric congestion between the in-plane ligands. For example, the torsion angles between the four alkyne carbon atom, carbonyl carbon, and copper (Cu−C1−C2−C9−C10-C17) mean plane and the Cu−C1−C2 and Cu−C9−C10 planes are 17.76 and 5.85°, respectively. Each alkyne is bonded to copper asymmetrically, as seen from Cu−C1 2.070(3) Å, Cu−C2 2.097(3) Å and Cu−C9 2.080(3) Å, Cu−C10 2.060(3) Å distances. The longer Cu−C bond is on the side facing the neighboring alkyne (see Figure 2), which is perhaps a result of steric effects. The 1645
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and Ag[SbF6] with 1 equiv of tert-butyl isocyanide in dichloromethane (Scheme 1). In contrast to the synthesis of the carbonyl adduct [Cu(cyclooctyne)2(CO)][SbF6], where excess CO is used, the use of more than 1 equiv of isocyanide with [Cu(cyclooctyne)2]+ (generated in situ from (cyclooctyne)2CuBr and Ag[SbF6]) leads to displacement of cyclooctyne ligands on the copper. For example, it is possible to isolate the bis(isocyanide) adduct [Cu(cyclooctyne)(CNtBu)2][SbF6] from a reaction between (cyclooctyne)2CuBr and Ag[SbF6] with approximately 2 equiv of tert-butyl isocyanide (Scheme 1). This is not surprising, as tert-butyl isocyanide is a much better and stronger σ-donor in comparison to carbon monoxide.34 Preliminary data suggest that the bis(carbonyl) adduct [Cu(cyclooctyne)(CO)2][SbF6] could be synthesized but it requires the use of the mono(alkyne) precursor [Cu(cyclooctyne)Br]2. [Cu(cyclooctyne)2(CNtBu)][SbF6] and [Cu(cyclooctyne)(CNtBu)2][SbF6] have been characterized by several analytical techniques, including X-ray crystallography. The bis(alkyne) copper adduct [Cu(cyclooctyne)2(CNtBu)][SbF6] is a colorless solid and moderately air stable. The 1H NMR chemical shifts of the methylene proton on the α-carbons of the alkyne moiety and tert-butyl group protons of isocyanide appear at δ 2.53 and 1.58 ppm, respectively, while the carbon resonance of the metal-bound alkyne moieties in 13C{1H} NMR is observed at δ 98.35 ppm. These are shifted downfield in comparison to the corresponding signal of the free cyclooctyne and tert-butyl isocyanide, suggesting the presence of copper(I)-bound ligands in solution. The infrared spectrum of [Cu(cyclooctyne)2(CNtBu)][SbF6] showed a strong absorption band at 2230 cm−1 due to a CN stretch, which is nearly 100 cm−1 higher than the corresponding value for the free ligand CNtBu (2136 cm−1). This is also higher than the ν̅CN values reported for neutral or cationic three-coordinate Cu(I) adducts such as [N{(C3F7)C(Dipp)N}2]Cu(CNtBu)] (ν̅CN 2176 cm−1)48,49 and [{(1-Me4,5-Ph2Imz)3COMe}Cu(CNtBu)][PF6] (ν̅CN 2186 cm−1).50 This rather high CN stretching frequency for [Cu(cyclooctyne)2(CNtBu)][SbF6] could be a result of a strong σ-type tBuNC→Cu interaction and the cationic copper site. The CC stretch of the alkyne moieties of [Cu(cyclooctyne)2(CNtBu)][SbF6] is observed at 2074 cm−1 as a weak band in the IR spectrum, which is not significantly different from the corresponding νC̅ C band for [Cu(cyclooctyne)2(CO)][SbF6] (2070 cm−1). The X-ray crystal structure of [Cu(cyclooctyne)2(CNtBu)][SbF6] is illustrated in Figure 3. It crystallizes in space group Pca21 with two [Cu(cyclooctyne)2(CNtBu)][SbF6] molecules in the asymmetric unit. The copper centers adopt a trigonalplanar geometry, but the alkyne moieties show a slight distortion, perhaps due to the steric congestion between the ligands on the trigonal plane. For example, torsion angles between the four alkyne carbon atom, isocyanide carbon, and copper (Cu1−C1−C2−C9−C10-C17) mean plane and the Cu−C1−C2 and Cu−C9−C10 planes are 9.14 and, 11.48°, respectively, for the adduct involving Cu1, while the related angles of the second molecule in the asymmetric unit are 8.53 and 8.70°. The Cu−C−N moiety is linear. The average Cu− CNtBu bond distance of 1.924(7) Å is at the longer end of the spectrum of Cu−CNR values observed for three-coordinate copper−isocyanide adducts, which span from 1.813(4) Å for L′CuCN(2,6-Me2C6H3) (L′H = 2-(S-2-phenylethyl)amino-3succinimido-4-(S-4-phenylethyl)iminopent-2-ene) 51 to
Figure 2. Molecular structure of [Cu(cyclooctyne)2(CO)][SbF6]. Ellipsoids are shown at the 50% probability level. Selected bond lengths (Ǻ ) and angles (deg): Cu−C17 1.929(3), O−C17 1.106(4), Cu−C1 2.070(3), Cu−C2 2.097(3), Cu−C9 2.080(3), Cu−C10 2.060(3), C1−C2 1.202(5), C9−C10 1.218(4), O−C17−Cu 175.5(3), C1−Cu−C2 33.52(13), C8−C1−Cu 128.4(2), C3−C2− Cu 132.8(2), C10−Cu−C9 34.21(12), C16−C9−Cu 134.6(2), C11− C10−Cu 129.7(2).
Information).41 For comparison, the recently reported [Cu(CO)(ClCH2Cl)3][Al{OC(CF3)3}4] displays two weak bands for CO at 2183 and 2172 cm−1.42 The next closest values are reported for [Cu(ttt-cdt)(CO)][SbF6] (ν̅CO 2160 cm−1; ttt-cdt = trans,trans,trans-1,5,9-cyclododecatriene)43 and (CO)CuAlCl4 (ν̅CO 2156 cm−1).44 However, they are lower than the CO stretch of “naked” [Cu(CO)]+ (observed in a Ne/CO matrix), which appears at 2234 cm−1.45 This high νC̅ O value of [Cu(cyclooctyne)2(CO)][SbF6] is a sign of dominant electrostatic effects and a σ-type Cu←CO bonding interaction with only a low level of Cu→CO π-back-donation. 42,46,47 Furthermore, it indicates that the copper site supported by two cyclooctynes in [Cu(cyclooctyne)2(CO)]+ is very electrophilic. The long Cu−CO bond is also not favorable for effective Cu→CO π-back-bonding. The CC stretch of the alkyne moieties of [Cu(cyclooctyne)2(CO)][SbF6] is observed at 2070 cm−1 as a weak band in the IR spectrum, which is lower than the value observed for free cyclooctyne (2216 cm−1) but not very different from that observed for Cu(cyclooctyne)2Br (2076 cm−1). In these adducts, (−CC−)→Cu σ-donation as well as (−CC−)←Cu π-back-donation weakens the CC bond, and both could contribute to the lower CC stretch in [Cu(cyclooctyne)2(CO)][SbF6]. The computational studies of cationic copper cyclooctyne complexes show that σ-donation is stronger than π-back-donation but that both of these orbital interaction components are relatively weaker in comparison to the electrostatic interactions.20 Isocyanides are closely related to carbonyls (CNH and CO are isoelectronic) and are also π-acceptor ligands that typically coordinate to metals in an end-on fashion.34 Therefore, we also probed the isocyanide coordination chemistry of the [Cu(cyclooctyne)2]+ moiety for a comparison using a common alkyl isocyanide, CNtBu. [Cu(cyclooctyne)2(CNtBu)][SbF6] was synthesized by treating a mixture of (cyclooctyne)2CuBr 1646
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ison, νC̅ C of [Cu(cyclooctyne)2(CO)][SbF6] is observed at 2070 cm−1. The X-ray crystal structure of [Cu(cyclooctyne)(CNtBu)2][SbF6] is shown in Figure 4. There is a plane of symmetry
Figure 3. Molecular structure showing [Cu(cyclooctyne)2(CNtBu)][SbF6]. Ellipsoids are shown at the 50% probability level. Only one of the two [Cu(cyclooctyne)2(CNtBu)][SbF6] molecules in the asymmetric unit is shown here. Selected bond lengths (Ǻ ) and angles (deg): for the first molecule, C17−Cu1 1.912(7), C1−Cu1 2.061(6), C2− Cu1 2.027(8), C9−Cu1 2.072(5), C10−Cu1 2.029(6), C1−C2 1.192(10), C9−C10 1.207(8), N1−C17 1.140(9), N1−C17−Cu1 178.2(5), C1−Cu1−C2 33.9(3), C8−C1−Cu1 135.1(5), C3−C2− Cu1 128.2(6), C9−Cu1−C10 34.2(2), C16−C9−Cu1 135.1(4), C11−C10−Cu1 127.9(4); for the second molecule, C38−Cu2 1.937(7), C22−Cu2 2.083(6), C23−Cu2 2.037(6), C30−Cu2 2.042(7), C31−Cu2 2.069(6), C22−C23 1.214(8), C30−C31 1.218(10), N2−C38 1.129(9), N2−C38−Cu2 178.4(6), C22−Cu2− C23 34.3(2), C29−C22−Cu2 135.1(4), C24−C23−Cu2 128.6(4), C30−Cu2−C31 34.4(3), C37−C30−Cu2 128.2(6), C32−C31−Cu2 136.2(5).
Figure 4. Molecular structure of [Cu(cyclooctyne)(CNtBu)2][SbF6]. Ellipsoids are shown at the 50% probability level. Selected bond lengths (Ǻ ) and angles (deg): C1−Cu 2.017(8), C2−Cu 1.988(8), C9−Cu 1.910(8), C13−Cu 1.916(8), C1−C2 1.225(12), C1−Cu−C2 35.6(3), C9−Cu−C13 112.0(3), C9−N1−C10 179.5(8), C13−N2− C14 179.1(8), N2−C13−Cu 179.0(7), N1−C9−Cu 178.2(7).
essentially bisecting both the cationic moiety and [SbF6]− counterion. Unfortunately, the cyclooctyne ligand backbone and [SbF6]− show positional disorder (see Figure S3 in the Supporting Information), somewhat lowering the overall quality of the structure. The most important core atoms surrounding the copper center (especially the (−CC−)Cu(CNC−)2 core), however, are not disordered and they lie on a plane forming trigonal-planar coordination geometry at copper. The CNtBu groups coordinate to the copper in a linear fashion, as is evident from the near 180° bond angles at C9, N1, C13, and N2. We have also investigated the chemistry of [Cu(cyclooctyne)2(CO)][SbF6]. In particular, we were mainly interested in nucleophilic addition reactions on coordinated CO as a route to carbene adducts, as illustrated in Scheme 1. However, the treatment of [Cu(cyclooctyne) 2 (CO)][SbF 6 ] with 3,5Me2C6H3NH2 and 4-tBuC6H4NH2 at −10 °C in dichloromethane led to the displacement of CO from the copper center (rather than addition to CO), forming [Cu(cyclooctyne)2(3,5Me2C6H3NH2)][SbF6] and [Cu(cyclooctyne)2(4-tBuC6H4NH2)][SbF6], respectively. The presence of cyclooctyne moieties and the corresponding aniline in the product is evident from the 1H and 13C NMR spectra. The amine protons of [Cu(cyclooctyne)2(3,5-Me2C6H3NH2)][SbF6] and [Cu(cyclooctyne)2(4-tBuC6H4NH2)][SbF6] appeared as a broad peak at δ 4.30 and 2.72 ppm, respectively. The IR spectra of the isolated products showed the absence of the CO signal, supporting the displacement of the carbonyl group. It is also possible to synthesize [Cu(cyclooctyne)2(CNtBu)][SbF6] by treating [Cu(cyclooctyne)2(CO)][SbF6] in dichloromethane with 1 equiv of tBuNC at room temperature (Scheme
1.945(14) Å for [Cu2(μ-L)3][PF6]2 (L = 1,1′-diisocyanoferrocene)52 with an average of 1.858 Å for 40 Cu−CNR distances.39 Four-coordinate and cationic copper(I) isocyanides such as [Cu(CNtBu)4][SbF6] (average 1.997 Å) feature even longer Cu−CNR bonds. The bis(isocyanide) copper adduct [Cu(cyclooctyne)(CNtBu)2][SbF6] was synthesized from (cyclooctyne)2CuBr and Ag[SbF6] and tert-butyl isocyanide as noted above. The protons on the α-carbons of the alkyne moiety and tert-butyl group in [Cu(cyclooctyne)(CNtBu)2][SbF6] appeared at δ 2.51 and 1.56 ppm, respectively, in the 1H NMR spectrum, whereas the 13C NMR signal of the metal-bound carbons of the cyclooctyne was observed at δ 100.76 ppm. These resonances are shifted downfield in comparison to the corresponding signals of the free ligand, which is similar to the trend observed with [Cu(cyclooctyne)2(CNtBu)][SbF6]. The 13C resonance of the alkyne carbons of [Cu(cyclooctyne)(CNtBu)2][SbF6] shows a slightly larger downfield shift in comparison to that of [Cu(cyclooctyne)2(CNtBu)][SbF6]. The IR spectrum of [Cu(cyclooctyne)(CNtBu)2][SbF6] shows two bands attributable to the CN stretch at 2229 and 2212 cm−1. These values may be compared to those reported for bis(isocyanide) adducts like [{H2B[3-(CF3)pz]2}Cu(CNtBu)2] (ν̅CN 2161 cm−1),53 [N{(CF3)C(C6F5)N}2]Cu(CNtBu)2 (ν̅CN 2175 cm−1),49 and [Cu(CNtBu)2(κ2-P,Pdppf][BF4] (ν̅CN 2175 cm−1).54 The νC̅ C band of [Cu(cyclooctyne)(CNtBu)2][SbF6] (2059 cm−1) shows an ∼15 cm−1 reduction in comparison to the corresponding stretch of [Cu(cyclooctyne)2(CNtBu)][SbF6] (2074 cm−1). For compar1647
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−10 °C (using an ice/acetone bath). To the reaction mixture was added CH2Cl2 (ca. 8 mL), and this mixture was stirred for 1 h at −10 °C and a further 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 ∼3 mL. The mixture was cooled to −10 °C (using an ice/acetone bath) and saturated with CO by bubbling CO through the solution for a few minutes. n-Hexane (∼5 mL) was slowly added to this concentrated filtrate and kept in a −10 °C freezer under CO atmosphere to obtain colorless crystals of [Cu(cyclooctyne)2(CO)][SbF6]: (0.018 g, 42% yield). It is moderately stable in air. However, it is best stored under nitrogen, protected from light, in a low-temperature freezer. 1H NMR (CD2Cl2, 500.16 MHz, 298 K): δ 1.71 (m, 8H, CH2, C5/6), 1.99 (m, 8H, CH2, C4/7), 2.49 (m, 8H, CH2, C3/8) ppm. 13C{1H} NMR (CD2Cl2, 125.77 MHz, 298 K): δ 24.32, 29.29, 34.17, 98.47 (C1/2) ppm. IR (Nujol, selected bands; cm−1): 2070 (CC stretch), 2171 (CO), 2119 (13CO isotopomer). Anal. Calcd for C17H24CuOSbF6·0.5CH2Cl2: C, 35.86; H, 4.30. Found: C, 35.94; H, 4.45. Synthesis of [Cu(cyclooctyne)2(CNtBu)][SbF6]. Cu(cyclooctyne)2Br (0.061 g, 0.169 mmol) and Ag[SbF6] (0.058 g, 0.169 mmol) 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 CH2Cl2 (ca. 8 mL), and the mixture was then stirred for 1 h at −10 °C. 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. The mixture was cooled to −10 °C (using an ice/acetone bath), tBuNC in CH2Cl2 (0.65 mL, 0.25 M solution in CH2Cl2; 0.163 mmol of tBuNC) was added, and this mixture was stirred for 1 h at −10 °C. n-Hexane (∼20 mL) was slowly added to this mixture, which was then kept in a −10 °C refrigerator. Plate-shaped crystals of [Cu(cyclooctyne)2(CNtBu)][SbF6] were obtained. Yield: 0.079 g (78%). 1H NMR (CD2Cl2, 500.16 MHz, 298 K): δ 1.58 (s, 9H, CH3, tBu), 1.71 (m, 8H, CH2, C5/6), 1.99 (m, 8H, CH2, C4/7), 2.53 (m, 8H, CH2, C3/8) ppm. 13C{1H} NMR (CD2Cl2, 125.77 MHz, 298 K): δ 25.01, 29.14, 30.17, 33.63, 59.72, 98.35 (C1/2) ppm. IR (Nujol, selected bands; cm−1): 2074 (CC stretch), 2230 (CN stretch). Anal. Calcd for C21H33CuNSbF6: C, 42.12; H, 5.55; N, 2.34. Found: C, 42.31; H, 5.07; N, 2.17. Synthesis of [Cu(cyclooctyne)(CNtBu)2][SbF6]. Cu(cyclooctyne)2Br (0.05 g, 0.139 mmol) and Ag[SbF6] (0.047 g, 0.139 mmol) 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 CH2Cl2 (ca. 8 mL), and this mixture was stirred for 1 h at −10 °C and 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 ∼1 mL. The mixture was cooled to −10 °C (using an ice/acetone bath), a tBuNC solution in CH2Cl2 (1.2 mL, 0.22 M solution in CH2Cl2; 0.264 mmol of tBuNC) was added, and this mixture was stirred for 30 min at that temperature. n-Hexane (∼5 mL) was slowly added to the mixture and kept in a −30 °C refrigerator. Plate-shaped crystals of [Cu(cyclooctyne)(CNtBu)2][SbF6] were obtained. Yield: 0.070 g (87%). 1H NMR (CD2Cl2, 500.16 MHz, 298 K): δ 1.56 (s, 18H, CH3, tBu), 1.70 (m, 4H, CH2, C5/6), 1.94 (m, 4H, CH2, C4/7), 2.51 (m, 4H, CH2, C3/8) ppm. 13 C{1H} NMR (CD2Cl2, 125.77 MHz, 298 K): δ 25.49, 29.35, 30.16, 33.38, 59.03, 100.76 (C1/2) ppm. IR (KBr, selected bands; cm−1): 2059 (CC stretch), 2212 (CN stretch), 2229 (CN stretch). Anal. Calcd for C18H30CuN2SbF6: C, 37.68; H, 5.27; N, 4.88. Found: C, 38.31; H, 5.35; N, 4.74. Synthesis of [Cu(CNtBu)4][SbF6]. Cu(cyclooctyne)2Br (0.05 g, 0.139 mmol) and Ag[SbF6] (0.047 g, 0.139 mmol) 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 CH2Cl2 (ca. 8 mL), and this mixture was stirred for 1 h at −10 °C and then stirred 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 ∼1 mL. The mixture was cooled to −10 °C (using
1), which displaces carbon monoxide on copper selectively. Treatment of [Cu(cyclooctyne)2(CO)][SbF6] with additional amounts of tBuNC, however, leads to displacement of both cyclooctyne and carbon monoxide from Cu(I), ultimately leading to the formation of [Cu(CNtBu)4][SbF6]. This was confirmed by comparing the IR and NMR spectroscopic data of the resulting product with those of [Cu(CNtBu)4][SbF6] (ν̅CN in IR 2175 cm−1), which was prepared independently from (cyclooctyne)2CuBr and Ag[SbF6] and ∼4 equiv of tertbutyl isocyanide. It is also possible to replace all the cyclooctyne ligands of [Cu(cyclooctyne)2(CNtBu)][SbF6] from the Cu(I) center, leading to [Cu(CNtBu)4][SbF6] using sufficient amounts of CNtBu. The crystal structure of [Cu(CNtBu)4][SbF6] shows the presence of a tetrahedral Cu(CNtBu)4 moiety, as expected (Supporting Information). The related [Cu(CNtBu)4]X adducts (X = Cl, Br, I; ν̅CN 2181 cm−1) have been reported.55
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SUMMARY AND CONCLUSION In summary, we have described the synthesis and isolation of cationic, three-coordinate copper(I) carbonyl and copper(I) isocyanide adducts supported by cyclooctyne ligands. [Cu(cyclooctyne)2(CO)][SbF6] and [Cu(cyclooctyne)2(CNtBu)][SbF6] adopt, planar “spoke-wheel” structures. The copper carbonyl complex shows high C−O stretching frequencies (in the “nonclassical” metal carbonyl region), implicating the dominance of electrostatic and OC→Cu σ-bonding with low Cu→CO π-back-bonding. The analogous copper(I) isocyanide complex also shows a significantly increased CN stretching vibration (∼94 cm−1) upon coordination to the [Cu(cyclooctyne)2]+ moiety, pointing to strong σ-type tBuNC→ Cu and electrostatic components. Overall, [Cu(cyclooctyne)2]+ provides a rather electron deficient copper site for CO and CNtBu binding. It is possible to displace CO selectively from [Cu(cyclooctyne)2(CO)][SbF6] using aniline derivatives or CNtBu. The use of additional amounts of CNtBu leads to displacement of cyclooctynes as well and the formation of [Cu(CNtBu)4][SbF6]. We are currently exploring the chemistry of various σ-donor/π-acid ligand combinations on Ag(I) and Au(I) and their cycloaddition chemistry.
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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 an 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 or JEOL Eclipse 300 spectrometer (1H, 500.16 or 300.53 MHz; 13C, 125.77 or 75.56 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. IR spectra were collected on a Bruker FT-IR instrument containing an ATR attachment or on a JASCO FT-IR 410 spectrometer. Cyclooctyne and Cu(cyclooctyne)2Br were prepared according to modified literature procedures.21,56 CuBr, Ag[SbF6], tBuNC, 3,5Me2C6H3NH2, and 4-tBuC6H4NH2 were purchased from commercial sources and used as received. Certain NMR peak assignments were based on reported values of related copper−cyclooctyne adducts.21 Synthesis of [Cu(cyclooctyne) 2 (CO)][SbF 6 ]. Cu(cyclooctyne)2Br (0.05 g, 0.139 mmol) and Ag[SbF6] (0.047 g, 0.139 mmol) were placed in a Schlenk flask, and the mixture was cooled to 1648
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in all calculations. The final R1 valuewas 0.0359 (I > 2σ(I)), and wR2 was 0.0829 (all data). Crystallographic data for [Cu(cyclooctyne)(CNtBu)2][SbF6]: formula C18H30N2F6CuSb (M = 573.73), orthorhombic, space group Pnma (No. 62), a = 23.146(5) Å, b = 7.8797(16) Å, c = 12.974(3) Å, V = 2366.3(8) Å3, Z = 4, T = 100(2) K, μ(Mo Kα) = 2.092 mm−1, Dcalcd = 1.610 g/mm3, 14630 reflections measured (3.52 ≤ 2θ ≤ 52.88°), 2596 unique reflections (Rint = 0.0355, Rσ = 0.0237) which were used in all calculations. The final R1 value was 0.0554 (I > 2σ(I)), and wR2 was 0.1286 (all data). The molecule sits on a mirror plane. The cyclooctyne ligand backbone and fluorine atoms of the SbF6 counterion show positional disorder, which was modeled reasonably well. Two parts of the cyclooctyne ligand backbone disorder were related by the reflection over the mirror plane. The disorder of the SbF6 counterion is rather complicated, since it lies on the mirror plane as three entities at 17%, 46%, and 37% occupancy, in three different orientations. This disorder was modeled using Part −1, −2, −3 commands and some restraints such as SADI for Sb−F distances and F- - -F separations and RIGU for Sb and F atoms. Overall, the metrical parameters of this molecule should be used with due caution. Crystallographic data for [Cu(CNtBu)4][SbF6]·2CH2Cl2: formula C22H40Cl4CuF6N4Sb (M = 801.67), monoclinic, space group P21/n (No. 14), a = 13.5180(6) Å, b = 11.3958(5) Å, c = 22.2579(10) Å, β = 91.1220(6)°, V = 3428.1(3) Å3, Z = 4, T = 100.0 K, μ(Mo Kα) = 1.771 mm−1, Dcalcd = 1.553 g/mm3, 29915 reflections measured (3.494 ≤ 2θ ≤ 53.998°), 7488 unique reflections (Rint = 0.0329, Rσ = 0.0292), which were used in all calculations. The final R1 value was 0.0588 (I > 2σ(I)), and wR2 was 0.1723 (all data). One of the dichloromethane molecules in the asymmetric unit shows positional disorder, which was modeled satisfactorily.
an ice/acetone bath), a tBuNC solution in CH2Cl2 (1.1 mL, 0.5 M solution in CH2Cl2; 0.550 mmol of tBuNC) was added, and this mixture was stirred for 1 h at that temperature. n-Hexane (∼5 mL) was slowly added to this mixture and kept in a −10 °C refrigerator. Needle-shaped crystals of [Cu(CNtBu)4][SbF6] were obtained. Yield: 0.072 g (82%). 1H NMR (CDCl3, 500.16 MHz, 298 K): δ 1.53 (s, 36H, CH3, tBu) ppm. 13C{1H} NMR (CDCl3, 125.77 MHz, 298 K): δ 30.26 (C(CH3)3), 56.96 (C(CH3)3) ppm. IR (neat, selected bands; cm−1): 2175 (CN stretch). Anal. Calcd for C20H36CuN4SbF6· 0.3CH2Cl2: C, 37.09; H, 5.61; N, 8.52. Found: C, 37.34; H, 5.19; N, 8.40. Synthesis of [Cu(cyclooctyne)2(NH2Ar)][SbF6] (Ar = 3,5Me2C6H3, 4-tBuC6H4). [Cu(cyclooctyne)2(CO)][SbF6] (0.035 g, 0.064 mmol) was placed in a Schlenk flask and cooled to −10 °C (using ice/acetone bath). A 2 mL portion of CH2Cl2 saturated with CO and 3,5-Me2C6H3NH2 (0.007 g, 0.064 mmol) or 4-tBuC6H4NH2 (0.009 g, 0.064 mmol) were added to the reaction mixture, and this mixture was stirred for 2 h at −10 °C under a CO atmosphere. The flask was covered with aluminum foil to protect it from the light. nHexane (∼3 mL) was slowly added to this reaction mixture and kept in a −10 °C refrigerator. The corresponding products [Cu(cyclooctyne)2(3,5-Me2C6H3NH2)][SbF6] (0.027 g, 67% yield) and [Cu(cyclooctyne)2(4-tBuC6H4NH2)][SbF6] (0.02 g, 51% yield) were obtained as colorless solids. Data for [Cu(cyclooctyne)2(3,5-Me2C6H3NH2)][SbF6] are as follows. 1H NMR (CDCl3, 500.16 MHz, 298 K): δ 1.65 (m, 8H, CH2, C5/6), 1.91 (m, 8H, CH2, C4/7), 2.25 (s, 6H, meta-CH3), 2.33 (m, 8H, CH2, C3/8), 4.30 (br, 2H, NH2), 6.65 (s, 2H, phenyl orthoCH), 6.77 (s, 1H, phenyl para-CH) ppm. 13C{1H} NMR (CDCl3, 125.77 MHz, 298 K): δ 21.33, 23.58, 28.88, 33.23, 98.25 (C1/2), 118.66, 127.25, 139.94 ppm. Anal. Calcd for C24H35CuF6NSb· 0.25CH2Cl2: C, 44.26; H, 5.44; N, 2.13. Found: C, 44.52; H, 6.05; N, 2.37. Data for [Cu(cyclooctyne)2(4-tBuC6H4NH2)][SbF6] are as follows. 1 H NMR (CDCl3, 500.16 MHz, 298 K): δ 1.28 (s, 9H, CH3), 1.65 (m, 8H, CH2, C5/6), 1.87 (m, 8H, CH2, C4/7), 2.30 (m, 8H, CH2, C3/8), 2.72 (br, 2H, NH2), 7.01 (d, 2H, phenyl ortho-CH), 7.33 (d, 2H, phenyl meta-CH) ppm. 13C{1H} NMR (CDCl3, 125.77 MHz, 298 K): δ 23.3, 28.82, 31.48, 32.91, 34.51, 98.47, 120.17, 126.8, 138.13, 148.21 ppm. Anal. Calcd for C26H39CuF6NSb·0.25CH2Cl2: C, 45.95; H, 5.80; N, 2.04. Found: C, 45.92; H, 5.75; N, 2.01. X-ray Crystallographic Data. A suitable crystal covered with a layer of Paratone-N oil was selected and mounted on a cryo-loop and immediately placed in a low-temperature nitrogen stream. Diffraction data were collected at T = 100(2) K. The data sets were collected on a Bruker SMART APEX CCD diffractometer with graphite-monochromated Mo Kα radiation (λ = 0.71073 Å). Intensity data were processed using the APEX2 (version 2013.10-0) program. Absorption corrections were applied by using SADABS. Initial atomic positions were located by direct methods or the intrinsic phasing method, and the structures of the compounds were refined by the least-squares method using Olex257 with the ShelXL-201358 refinement package. Hydrogen atoms were included at calculated positions and refined in a riding manner along with the attached carbons. Further details are given in the CIF files. Crystallographic data for [Cu(cyclooctyne)2(CO)][SbF6]: formula C17H24CuF6OSb (M = 543.65), monoclinic, space group P21/c (No. 14), a = 11.4781(10) Å, b = 12.2864(11) Å, c = 16.0681(11) Å, β = 117.088(5)°, V = 2017.4(3) Å3, Z = 4, T = 100.0 K, μ(Mo Kα) = 2.449 mm−1, Dcalcd = 1.790 g/mm3, 17648 reflections measured (3.986 ≤ 2θ ≤ 53.992°), 4409 unique reflections (Rint = 0.0359, Rσ = 0.0271) which were used in all calculations. The final R1 value was 0.0312 (I > 2σ(I)), and wR2 was 0.0828 (all data). Crystallographic data for [Cu(cyclooctyne)2(CNtBu)][SbF6]: formula C21H33CuF6NSb (M = 598.77), orthorhombic, space group Pca21 (No. 29), a = 21.9038(19) Å, b = 8.6153(7) Å, c = 25.675(2) Å, V = 4845.0(7) Å3, Z = 8, T = 100.0 K, μ(Mo Kα) = 2.046 mm−1, Dcalcd = 1.642 g/mm3, 43168 reflections measured (3.172 ≤ 2θ ≤ 54.998°), 11038 unique reflections (Rint = 0.0505, Rσ = 0.0478) which were used
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ASSOCIATED CONTENT
S Supporting Information *
Figures, tables, and CIF files giving X-ray crystallographic data for [Cu(cyclooctyne) 2 (CO)][SbF 6 ], [Cu(cyclooctyne) 2 (CNtBu)][SbF6], [Cu(cyclooctyne)(CNtBu)2][SbF6], and [Cu(CNtBu)4][SbF6]·2CH2Cl2, additional figures, and selected data on copper carbonyl and isocyanide adducts. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*H.V.R.D.: tel, (+1) 817 272 3813; e-mail,
[email protected]; web, http://www.uta.edu/chemistry/faculty/directory/Dias.php. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS This material is based upon work supported by the National Science Foundation under CHE-1265807 and the Robert A. Welch Foundation (Grant Y-1289). We thank Dr. Charles Campana (Bruker AXS) for providing advice for modeling the disorder found in [Cu(cyclooctyne)(CNtBu)2][SbF6].
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