Acetylene and terminal alkyne complexes of copper(I) supported by fluorinated pyrazolates: syntheses, structures, and transformations

Devaborniny Parasar, Tharun T. Ponduru, Anurag Noonikara-Poyil, Naleen B. Jayaratna and H. V. Rasika Dias*
Department of Chemistry and Biochemistry, The University of Texas at Arlington, Arlington, Texas 76019, USA. E-mail: dias@uta.edu

Received 16th August 2019 , Accepted 5th September 2019

First published on 5th September 2019


Trinuclear {μ-[3,5-(CF3)2Pz]Cu}3 reacts with acetylene to produce the 2[thin space (1/6-em)]:[thin space (1/6-em)]1 copper(I) acetylene complex, Cu4(μ-[3,5-(CF3)2Pz])4(μ-HC[triple bond, length as m-dash]CH)2. Related Cu4(μ-[4-Br-3,5-(CF3)2Pz])4(μ-HC[triple bond, length as m-dash]CH)2 and Cu4(μ-[4-Cl-3,5-(CF3)2Pz])4(μ-HC[triple bond, length as m-dash]CH)2 have also been isolated using the corresponding copper(I) pyrazolate and acetylene. The 1[thin space (1/6-em)]:[thin space (1/6-em)]1 adducts Cu2(μ-[3,5-(CF3)2Pz])2(HC[triple bond, length as m-dash]CH)2 and Cu2(μ-[4-Br-3,5-(CF3)2Pz])2(HC[triple bond, length as m-dash]CH)2 are significantly less stable to the acetylene loss and can be observed in solution at low temperatures under excess acetylene. The X-ray crystal structures of 2[thin space (1/6-em)]:[thin space (1/6-em)]1 and 1[thin space (1/6-em)]:[thin space (1/6-em)]1 complexes, Cu4(μ-[3,5-(CF3)2Pz])4(μ-HC[triple bond, length as m-dash]CH)2 and Cu2(μ-[4-Br-3,5-(CF3)2Pz])2(HC[triple bond, length as m-dash]CH)2 are reported. Raman data show a reduction in [small nu, Greek, macron]C[triple bond, length as m-dash]C stretching frequency by about ∼340 and ∼163 cm−1 in the 2[thin space (1/6-em)]:[thin space (1/6-em)]1 and 1[thin space (1/6-em)]:[thin space (1/6-em)]1 Cu(I)/acetylene complexes, respectively, from that of the free acetylene. Copper(I) pyrazolate complexes of the terminal alkynes, phenylacetylene, 1,8-nonadiyne, and 1,7-octadiyne are also reported. They form adducts involving one copper atom on each alkyne moiety. The {μ-[3,5-(CF3)2Pz]Cu}3 is also a very versatile and competent catalyst for alkyne transformations as evident from its ability to catalyze the alkyne C(sp)–H bond carboxylation chemistry with CO2, azide–alkyne cycloadditions leading to 1,2,3-triazoles including the use of acetylene itself as a substrate, and thiol addition to phenylacetylene affording vinyl sulfides.


Introduction

Trinuclear copper pyrazolates are of significant interest due to their fascinating photophysical properties and tendency to self-assemble via Cu⋯Cu contacts.1–16 For example, the copper complex {μ-[3,5-(CF3)2Pz]Cu}3 (1) exhibits bright orange luminescence, which can be fine- and coarse-turned to multiple bright visible colors by varying solvents, concentration, temperatures, and excitation wavelength.1,2 Dendritic, trinuclear Cu(I) pyrazolate complexes carrying long alkyl chains have been utilized in the fabrication of rewritable phosphorescent paper.4 Trinuclear copper pyrazolates also serve as good precursors to produce various mixed ligand complexes or hybrid materials with different nuclearities and/or supramolecular structures.7,9,17–23 For example, dinuclear Cu2(μ-[3,5-(CF3)2Pz])2(2,4,6-collidine)2 obtained from {μ-[3,5-(CF3)2Pz]Cu}3 and 2,4,6-collidine show ligand based blue emissions that are different from the orange emissions of the precursor {μ-[3,5-(CF3)2Pz]Cu}3.24 Some of these copper complexes such as {μ-[3,5-(CF3)2Pz]Cu}3 with electron-withdrawing fluoroalkyl groups on the ligand backbones act as π-acidic units, and are known to associate with electron rich arenes (e.g., benzene, mesitylene) and C60 leading to 2D- or 3D-stacks.18,25,26 In addition, copper pyrazolates also show promise in catalysis,27–34 and adsorption and/or separation of small molecules,10,20,35,36 but these applications have received relatively less scrutiny.

Copper mediates many important transformations involving alkynes such as hetero atom-hydrogen bond additions, cycloaddition chemistry, cyclopropenation, Csp–H bond functionalizations, and alkyne coupling processes.37–53 Copper alkyne or alkynide complexes are believed to be key intermediates in most of these reactions. Copper alkynes are also used as precursors for the copper deposition.54,55 Despite the importance of copper in alkyne chemistry and the availability of a large number and a diverse group of binary copper(I) pyrazolates, coordination chemistry of copper pyrazolates with alkynes, or the use of copper-pyrazolates as catalysts in alkyne transformations remain virtually unexplored. There was an isolated report in 2002 relevant to this topic concerning the synthesis Cu2(μ-[3,5-(CF3)2Pz])2(Me3SiC[triple bond, length as m-dash]CSiMe3)2, as a chemical vapor deposition (CVD) precursor for copper.56 As the first part of a detailed investigation into this area, we reported the isolation of di- and tetra-nuclear copper complexes such as 2 and 3 (Fig. 1) resulting from the reaction between tri-nuclear {μ-[3,5-(CF3)2Pz]Cu}3 (1) and internal-alkynes, together with their structures (including molecules featuring bridged, μ2–η22-/4e-alkyne donors) and photophysical properties.57 Here we describe the coordination chemistry of copper(I) pyrazolates with acetylene and terminal-alkynes, as well as the use of {μ-[3,5-(CF3)2Pz]Cu}3 as a catalyst in the carboxylation, azide-cycloaddition and hydrothiolation of alkynes. Recently, a communication appeared on the use of {μ-[3,5-(CF3)2Pz]Cu}3 in click-chemistry, and the isolation of two mixed-valent copper alkyne complexes (resulting from the partial oxidation of Cu(I) in the precursor).58 Their report complements the azide–alkyne cycloaddition results described in this manuscript.


image file: c9dt03350e-f1.tif
Fig. 1 Diagram showing the structures of {μ-[3,5-(CF3)2Pz]Cu}3 (1), Cu2(μ-[3,5-(CF3)2Pz])2(EtC[triple bond, length as m-dash]CEt)2 (2), and Cu4(μ-[3,5-(CF3)2Pz])4(μ-EtC[triple bond, length as m-dash]CEt)2 (3).

Results and discussion

Coordination chemistry of copper pyrazolates with acetylene

 
image file: c9dt03350e-t1.tif(1)
 
image file: c9dt03350e-t2.tif(2)
The highly fluorinated {μ-[3,5-(CF3)2Pz]Cu}3 reacts with purified acetylene (∼1 atm)53,59 in CH2Cl2, affording Cu4(μ-[3,5-(CF3)2Pz])4(μ-HC[triple bond, length as m-dash]CH)2 (4) as a white, crystalline solid in >90% yield (Fig. 2, eqn (1)). The room temperature 1H NMR data of 4 in CDCl3 displayed the presence of a 2[thin space (1/6-em)]:[thin space (1/6-em)]1 Cu[thin space (1/6-em)]:[thin space (1/6-em)]acetylene complex, and a large, down-field shift of acetylenic proton resonances (observed at δ 6.16 ppm) relative to the corresponding signal of the free acetylene (δ 2.05 ppm). The [small nu, Greek, macron]C[triple bond, length as m-dash]C band of solid 4 in the Raman spectrum was observed at 1638 cm−1, representing a significant (336 cm−1) red shift relative to the corresponding stretching frequency of the free acetylene (1974 cm−1).60

image file: c9dt03350e-f2.tif
Fig. 2 Structures and synthetic routes to tetra-nuclear complexes Cu4(μ-[3,5-(CF3)2Pz])4(μ-HC[triple bond, length as m-dash]CH)2 (4), Cu4(μ-[4-Br-3,5-(CF3)2Pz])4(μ-HC[triple bond, length as m-dash]CH)2 (6), Cu4(μ-[4-Cl-3,5-(CF3)2Pz])4(μ-HC[triple bond, length as m-dash]CH)2 (7) and di-nuclear complexes and Cu2(μ-[3,5-(CF3)2Pz])2(HC[triple bond, length as m-dash]CH)2 (5) and Cu2(μ-[4-Br-3,5-(CF3)2Pz])2(HC[triple bond, length as m-dash]CH)2 (8) from tri-nuclear copper(I) pyrazolates and acetylene.

Although there is a long history of copper(I)–acetylene chemistry,61,62 copper(I) acetylene complexes with detailed structural and spectroscopic data are surprisingly scarce. This is perhaps due to challenges such as facile loss of coordinated acetylene, copper acetylide formation, and the potential explosion hazard associated with this work.62–64 The search of Cambridge Structural Database65 revealed eight structurally characterized copper–acetylene complexes. These include, [Cu{NH(Py)2}(HC[triple bond, length as m-dash]CH)]BF4 and [Cu(phen)(HC[triple bond, length as m-dash]CH)]ClO4 complexes with Cu(η2-HC[triple bond, length as m-dash]CH) moieties,63,66,67 and polymeric or octanuclear, chloride bridged copper(I) adducts containing μ2–η22-(HC[triple bond, length as m-dash]CH) moieties.68–70 Among these, compound [Cu{NH(Py)2}(HC[triple bond, length as m-dash]CH)]BF4 has 1H NMR and IR data available for comparison, and displays its acetylenic proton resonance and [small nu, Greek, macron]C[triple bond, length as m-dash]C band at δ 5.59 ppm and 1796 cm−1, respectively.66 The C[triple bond, length as m-dash]C stretch of [Cu(phen)(HC[triple bond, length as m-dash]CH)]ClO4 has been reported at 1800 cm−1.67 These two copper adducts featuring η2-acetylene moieties show only about 174–179 cm−1 lowering of their acetylene C[triple bond, length as m-dash]C stretching frequency upon coordination to Cu(I). Both the NMR shifts and [small nu, Greek, macron]C[triple bond, length as m-dash]C data of the acetylene ligands of Cu4(μ-[3,5-(CF3)2Pz])4(μ-HC[triple bond, length as m-dash]CH)2 (4) are however, significantly different from these adducts, and suggest the presence of a μ2–η22-acetylene group rather than the η2-acetylene moiety. Note that even larger down field shifts of acetylenic proton resonance71 and more significant reductions in C[triple bond, length as m-dash]C stretching frequency have been reported for coordinated acetylene (e.g., [small nu, Greek, macron]C[triple bond, length as m-dash]C band of Co2(CO)6(μ-HC[triple bond, length as m-dash]CH) in IR was observed at 1402 cm−1, which is about 570 cm−1 lower than that of free acetylene),72 but they involve earlier transition metal ions instead of Cu(I).

The X-ray crystal structure of Cu4(μ-[3,5-(CF3)2Pz])4(μ-HC[triple bond, length as m-dash]CH)2 (4) is illustrated in Fig. 3. It is a tetranuclear copper(I) complex with trigonal planar copper sites and sits on an inversion center. Unfortunately, this molecule shows positional disorder over two orientations (at 88%[thin space (1/6-em)]:[thin space (1/6-em)]12% occupancy), which was rather challenging initially to recognize (see ESI), but was resolved satisfactorily. Although metrical parameters (Table 1) are not ideal for a detailed analysis due to the disorder, the basic structural features and atom connectivities are clear and indicate the presence of μ2–η22-(HC[triple bond, length as m-dash]CH) moieties, consistent with the spectroscopic data. There are close intramolecular Cu⋯Cu contacts that are within the van der Waals separation of two copper atoms.73 It is also interesting to note that during the formation of 4, copper pyrazolate moieties of {μ-[3,5-(CF3)2Pz]Cu}3 rearrange to form two separate six-membered, Cu2N4 fragments, whereas in Cu4(μ-[3,5-(CF3)2Pz])4(μ-EtC[triple bond, length as m-dash]CEt)2 (3), they form a large 12-membered Cu4N8 metallacycle (see Fig. 1 and 2).57


image file: c9dt03350e-f3.tif
Fig. 3 Molecular structure of Cu4(μ-[3,5-(CF3)2Pz])4(μ-HC[triple bond, length as m-dash]CH)2 (4); ORTEP view with 50% probability ellipsoids are shown. Disordered atoms of the minor occupancy (12%) component have been removed for clarity.
Table 1 Selected bond distances (Å) and angles (°) for Cu2(μ-[3,5-(CF3)2Pz])2(EtC[triple bond, length as m-dash]CEt)2 (2), Cu4(μ-[3,5-(CF3)2Pz])4(μ-EtC[triple bond, length as m-dash]CEt)2 (3), Cu4(μ-[3,5-(CF3)2Pz])4(μ-HC[triple bond, length as m-dash]CH)2 (4), Cu2(μ-[4-Br-3,5-(CF3)2Pz])2(HC[triple bond, length as m-dash]CH)2 (8), Cu2(μ-[3,5-(CF3)2Pz])2(HC[triple bond, length as m-dash]CPh)2 (9), Cu2(μ-[3,5-(CF3)2Pz])2(HC[triple bond, length as m-dash]C(CH2)5C[triple bond, length as m-dash]CH) (10), Cu4(μ-[3,5-(CF3)2Pz])4(HC[triple bond, length as m-dash]C(CH2)4C[triple bond, length as m-dash]CH)2 (11), and Cu4(μ-[3,5-(CF3)2Pz])4(C2H5C[triple bond, length as m-dash]C(CH2)4C[triple bond, length as m-dash]CC2H5)2 (12). Average values are given below, and specific bond distances and angles are presented in CIF and ESI†
Parameter\complex 3 4a 2 8 9 10 11 12
a Metrical parameters of 4 should be used with due caution considering the disorder. Only the bond distance/angle values for major occupancy component are provided for compound 4.
Av. Cu–C(H)[triple bond, length as m-dash] 1.987 1.970 1.943 1.955 1.958
Av. Cu–C(C)[triple bond, length as m-dash] 2.016 1.978 1.996 2.008 1.992 1.981
Av. C[triple bond, length as m-dash]C 1.265 1.269 1.227 1.227 1.226 1.229 1.220 1.234
Av. Cu–N 1.978 1.961 1.985 1.972 1.971 1.978 1.975 1.982
Shortest Cu⋯Cu 2.646 2.647 3.051 3.200 3.161 3.136 3.128 3.074
Av. C–Cu–C 36.57 37.24 36.13 36.28 36.23 36.10 35.97 36.29
Av. C–C[triple bond, length as m-dash]C 155.2 161.2 161.4 161.1 163.7 162.4
Av. N–Cu–N 108.99 103.04 97.49 98.95 100.05 98.13 100.04 97.85
–C[triple bond, length as m-dash]C– bonding mode μ2η2,η2- μ2η2,η2- η2- η2- η2- η2- η2- η2-
Ref. 57 This work 57 This work This work This work This work This work


We could not isolate the 1[thin space (1/6-em)]:[thin space (1/6-em)]1 complex, Cu2(μ-[3,5-(CF3)2Pz])2(HC[triple bond, length as m-dash]CH)2 (5) involving non-bridging acetylene as a solid, from solutions containing even large excess of acetylene with {[3,5-(CF3)2Pz]Cu}3 (Fig. 2, eqn (2)). This is however, not surprising based on the similar findings reported by us and others involving alkynes like 2-butyne with {μ-[3,5-(CF3)2Pz]Cu}3 or MeC[triple bond, length as m-dash]CCO2Me with copper(I) trifluoroacetate.57,74 Only the tetranuclear copper complexes featuring bridging alkynes have been preferentially precipitated from these reactions, despite the presence of an excess of alkyne. The VT-NMR data of {μ-[3,5-(CF3)2Pz]Cu}3 in the presence of excess acetylene in CD2Cl2 indicated the likely presence of Cu2(μ-[3,5-(CF3)2Pz])2(HC[triple bond, length as m-dash]CH)2 (5) in solution, as evident from the appearance of a broad signal at δ 4.86 ppm corresponding to bound acetylene, starting at about −35 °C during the cooling process. It is still broad even at −70 °C indicating the presence of extremely labile copper-bound acetylene group in 5 that rapidly exchanges with free acetylene on the NMR time scale.

The work involving CO and fluorinated copper(I) pyrazolates indicates that more weakly coordinating pyrazolates such as [3,4,5-(CF3)3Pz] offer greater stability to, and allow relatively easier isolation of the dinuclear Cu–CO adducts such as Cu2(μ-[3,4,5-(CF3)3Pz])2(CO)2 relative to that of Cu2(μ-[3,5-(CF3)2Pz])2(CO)2.19 Accordingly, we also investigated the chemistry of {μ-[4-Br-3,5-(CF3)2Pz]Cu}3,75 and {μ-[4-Cl-3,5-(CF3)2Pz]Cu}375 with acetylene (Fig. 2). The treatment of {μ-[4-Br-3,5-(CF3)2Pz]Cu}3 and {μ-[4-Cl-3,5-(CF3)2Pz]Cu}3 with acetylene in CH2Cl2 followed by the cooling at −20 °C afforded white crystalline solids. The 1H NMR and Raman spectroscopic data of these samples however, indicated the formation of Cu4(μ-[4-Br-3,5-(CF3)2Pz])4(μ-HC[triple bond, length as m-dash]CH)2 (6) and Cu4(μ-[4-Cl-3,5-(CF3)2Pz])4(μ-HC[triple bond, length as m-dash]CH)2 (7) featuring bridging acetylenes, respectively, rather than molecules with 2e-donor, η2-(HC[triple bond, length as m-dash]CH) ligands.52,63,66,67 For example, copper-bound acetylene proton signal of 6 and 7 was observed at δ 6.03 and 6.04 ppm, respectively, whereas their C[triple bond, length as m-dash]C stretch in Raman spectra was detected at 1631 and 1632 cm−1, respectively. These values are similar to those observed for the tetranuclear Cu4(μ-[3,5-(CF3)2Pz])4(μ-HC[triple bond, length as m-dash]CH)2 (4). The solubility of 6 and 7 in less polar solvents like hexanes are rather limited once the solid is formed. Unfortunately, many attempts to grow acceptable crystals of 6 and 7 for X-ray crystallography failed. Twinning and the formation of badly inter-twined crystals are common. Interestingly, solids obtained from certain batches of Cu4(μ-[4-Br-3,5-(CF3)2Pz])4(μ-HC[triple bond, length as m-dash]CH)2 (6) in CH2Cl2 solutions at −20 °C indicated the presence of two types of crystals and new-band at 1810 and 1811 cm−1 in the IR and Raman spectra, respectively (in addition to the [small nu, Greek, macron]C[triple bond, length as m-dash]C band due to 6). These new peaks with relatively small changes in C[triple bond, length as m-dash]C frequency (compared to the free acetylene [small nu, Greek, macron]C[triple bond, length as m-dash]C) are indicative of a copper-bound, non-bridging acetylene moiety. Indeed, a careful analysis of a needle-shaped crystal fragment (found as a minor product amongst diamond shaped, and often badly twinned crystals) revealed the presence of 1[thin space (1/6-em)]:[thin space (1/6-em)]1 adduct, Cu2(μ-[4-Br-3,5-(CF3)2Pz])2(HC[triple bond, length as m-dash]CH)2 (8). It was later obtained as the major product (and as much better-quality single crystals) using toluene as the solvent of crystallization. X-ray crystal structure of this molecule is illustrated in Fig. 4. It crystallizes with molecules of toluene in the crystal lattice, and sits on a mirror plane. The copper sites adopt trigonal planar geometry and acetylenes bind to copper atoms in an η2-fashion. The average C[triple bond, length as m-dash]C bond distance of 8 (1.227 Å) is slightly longer than the corresponding distance of free acetylene (1.2033(2) Å).60


image file: c9dt03350e-f4.tif
Fig. 4 Molecular structure of Cu2(μ-[4-Br-3,5-(CF3)2Pz])2(HC[triple bond, length as m-dash]CH)2 (8); ORTEP view with 50% probability ellipsoids are shown.

The Raman spectrum of solid samples of 8 displays a strong band at 1811 cm−1, corresponding to the [small nu, Greek, macron]C[triple bond, length as m-dash]C. This represent a 163 cm−1 reduction in stretching frequency as a result of copper-coordination, relative to that of the free acetylene.60 Solid samples of 8 (even with a thin, hydrocarbon oil coating) lose acetylene upon standing at room temperature in air, as evident from the gradual intensity loss of 1811 cm−1 band over several hours, and a complete disappearance within a day. This process accompanies the gradual growth of the 1636 cm−1 band, pointing to the formation of a bridged-acetylene species 6, which is relatively more stable to the loss of acetylene. Notably, we could not observe the NMR signal of the coordinated acetylene in 8 in CD2Cl2 at room temperature, as it rapidly loses some of the acetylene during the dissolution (can even observe the release of bubbles when the NMR solvent is added to dissolve the crystals), and establishes a fast equilibrium with the free acetylene on the NMR time scale. We can however observe this resonance at low temperatures in the presence of excess acetylene. The copper bound acetylenic proton signal of 8 was observed as a broad peak at δ 4.75 ppm in CD2Cl2. We have not probed the acetylene chemistry of 4-chloro analog {μ-[4-Cl-3,5-(CF3)2Pz]Cu}3 in greater detail apart from the isolation of 7, although it could also show similar chemistry as the 4-bromo analog, {μ-[4-Br-3,5-(CF3)2Pz]Cu}3.

The solution of 4 (in CH2Cl2) is notably sensitive and changes color to brown on exposure to air or upon removal of the solvent under reduced pressure or if purged with nitrogen. It however, remains stable for a few days under acetylene atmosphere both in solution (in −20 °C freezer) and in solid state but slowly decomposes over time. In contrast, solutions of compounds 6 and 7 are comparatively stable, and do not show noticeable color changes upon exposure to air or if purged with nitrogen briefly. Both these complexes remain stable for days in solution (in a freezer) and in solid state under an acetylene atmosphere. In the presence of excess acetylene at room temperature, solutions of tetranuclear 4, 6 and 7 lose the bound acetylene signal in 1H NMR spectrum indicating fast exchange with free acetylene, and perhaps the formation of 1[thin space (1/6-em)]:[thin space (1/6-em)]1 Cu[thin space (1/6-em)]:[thin space (1/6-em)]alkyne adducts.

Coordination chemistry of copper(I) pyrazolates with terminal-alkynes

We have also explored the chemistry of phenylacetylene and 1,8-nonadiyne with {μ-[3,5-(CF3)2Pz]Cu}3. The treatment of the copper pyrazolate with slight excess of these terminal alkynes in CH2Cl2 led to Cu2(μ-[3,5-(CF3)2Pz])2(HC[triple bond, length as m-dash]CPh)2 (9) and Cu2(μ-[3,5-(CF3)2Pz])2(HC[triple bond, length as m-dash]C(CH2)5C[triple bond, length as m-dash]CH) (10) in high yield (Scheme 1). They show their C[triple bond, length as m-dash]C stretch in Raman at 1932 (average of three bands observed at 1918, 1928, 1950 cm−1) and 1946 cm−1, respectively, indicating the presence of typical, non-bridging, 2e donor, η2-alkynes on copper. They represent a ∼179 and 169 cm−1 reduction in stretching frequency upon Cu(I) coordination (free phenylacetylene and 1,8-nonadiyne display their C[triple bond, length as m-dash]C stretch in Raman at 2011 and 2015 cm−1, respectively). For comparison, the bis(pyrazolyl)borate complex [H2B(3,5-(CF3)2Pz)2]Cu(HC[triple bond, length as m-dash]CPh) exhibits its C[triple bond, length as m-dash]C band at 1927 cm−1,76 while the mixed-valent Cu{(μ-[3,5-(CF3)2Pz])2Cu(HC[triple bond, length as m-dash]CPh)}2 and Cu{(μ-[3,5-(CF3)2Pz])2Cu(HC[triple bond, length as m-dash]CC6H13)}2 show their C[triple bond, length as m-dash]C bands at 1910 and 1945 cm−1,58 respectively, in their IR spectra. Copper complexes featuring η2– and μ2–η22-bound, internal alkyne 3-hexyne, Cu2(μ-[3,5-(CF3)2Pz])2(EtC[triple bond, length as m-dash]CEt)2 (2) and Cu4(μ-[3,5-(CF3)2Pz])4(μ-EtC[triple bond, length as m-dash]CEt)2 (3), show their [small nu, Greek, macron]C[triple bond, length as m-dash]C bands in Raman at 2050 and 1874 cm−1, respectively, and about 210 and 386 cm−1 red shifts relative to the corresponding stretching frequency of the free EtC[triple bond, length as m-dash]CEt.57
image file: c9dt03350e-s1.tif
Scheme 1 Synthetic routes to di-nuclear complexes Cu2(μ-[3,5-(CF3)2Pz])2(HC[triple bond, length as m-dash]CPh)2 (9) and Cu2(μ-[3,5-(CF3)2Pz])2(HC[triple bond, length as m-dash]C(CH2)5C[triple bond, length as m-dash]CH) (10) from tri-nuclear {[3,5-(CF3)2Pz]Cu}3 and the corresponding alkyne.

X-ray crystal structures of 9 and 10 confirm the formation of dinuclear species containing alkyne ligands bonded to copper in an η2-fashion. Compound Cu2(μ-[3,5-(CF3)2Pz])2(HC[triple bond, length as m-dash]CPh)2 (9) crystallized in the P[1 with combining macron] space group with two chemically similar but crystallographically different molecules in the asymmetric unit. One of these molecules is depicted in Fig. 5. In these molecules, phenylacetylene ligands orient in a similar fashion with phenyl moieties pointing in the same direction. The Cu2N4 metallacycle adopts the familiar boat shape.


image file: c9dt03350e-f5.tif
Fig. 5 Molecular structure of Cu2(μ-[3,5-(CF3)2Pz])2(HC[triple bond, length as m-dash]CPh)2 (9); ORTEP view with 50% probability ellipsoids are shown.

The X-ray structure of Cu2(μ-[3,5-(CF3)2Pz])2(HC[triple bond, length as m-dash]C(CH2)5C[triple bond, length as m-dash]CH) (10) is shown in Fig. 6. The 1,8-nonadiyne serves as an intramolecular bridge for two copper sites of the dinuclear “Cu2(μ-[3,5-(CF3)2Pz])2” fragment, and the alkyne groups coordinate to copper atoms in an η2-fashion. 1H NMR resonances corresponding to H–C[triple bond, length as m-dash] protons of 9 and 10 in CDCl3 at room temperature appear as broad singlets at δ 3.17 and 4.33 ppm, respectively. The 13C NMR peaks assignable to the C[triple bond, length as m-dash]C carbons are also broad suggesting that the alkyne groups in these complexes are rather labile in solution at ambient temperatures.


image file: c9dt03350e-f6.tif
Fig. 6 Molecular structure of Cu2(μ-[3,5-(CF3)2Pz])2(HC[triple bond, length as m-dash]C(CH2)5C[triple bond, length as m-dash]CH) (10); ORTEP view with 50% probability ellipsoids are shown.

We have investigated the effect of alkyne moiety linker length on copper–alkyne adduct formation by using a terminal bis-alkyne 1,7-octadiyne, as well as an internal bis-alkyne 3,9-dodecadiyne ligands with {μ-[3,5-(CF3)2Pz]Cu}3 (Scheme 2). The X-ray crystal structures of the resulting molecules Cu4(μ-[3,5-(CF3)2Pz])4(HC[triple bond, length as m-dash]C(CH2)4C[triple bond, length as m-dash]CH)2 (11) and Cu4(μ-[3,5-(CF3)2Pz])4(C2H5C[triple bond, length as m-dash]C(CH2)4C[triple bond, length as m-dash]CC2H5)2 (12) are illustrated in Fig. 7. In these 11 and 12, the two alkyne moieties are linked by a four-carbon, –(CH2)2– linker, and they serve as a bridge to two-separate dinuclear “Cu2(μ-[3,5-(CF3)2Pz])2” fragments. In contrast, the di-alkyne HC[triple bond, length as m-dash]C(CH2)5C[triple bond, length as m-dash]CH with a five-carbon linker in 10 acts as in intramolecular bridge to a single “Cu2(μ-[3,5-(CF3)2Pz])2” moiety (Scheme 1). Thus, it appears that anything shorter than the five-carbon –(CH2)5– linker in these aliphatic bis-alkyne molecules is not long enough to bridge Cu atoms of dinuclear “Cu2(μ-[3,5-(CF3)2Pz])2” fragments in an intra-dimer fashion.


image file: c9dt03350e-s2.tif
Scheme 2 Synthetic routes to Cu4(μ-[3,5-(CF3)2Pz])4(HC[triple bond, length as m-dash]C(CH2)4C[triple bond, length as m-dash]CH)2 (11) and Cu4(μ-[3,5-(CF3)2Pz])4(C2H5C[triple bond, length as m-dash]C(CH2)4C[triple bond, length as m-dash]CC2H5)2 (12) from tri-nuclear {[3,5-(CF3)2Pz]Cu}3 and the corresponding alkyne.

image file: c9dt03350e-f7.tif
Fig. 7 Molecular structures of Cu4(μ-[3,5-(CF3)2Pz])4(HC[triple bond, length as m-dash]C(CH2)4C[triple bond, length as m-dash]CH)2 (11) (top) and Cu4(μ-[3,5-(CF3)2Pz])4(C2H5C[triple bond, length as m-dash]C(CH2)4C[triple bond, length as m-dash]CC2H5)2 (12) (bottom); ORTEP views with 50% probability ellipsoids are shown.

A comparison of metrical parameters of 3 and 4 containing 4e-donor, μ2–η22-alkynes to 2, 8–12 featuring 2e-donor, η2-alkyne ligands (Table 1) show that the former group of molecules have longer C[triple bond, length as m-dash]C bonds, which is expected and consistent with the vibration spectroscopic data.77 Also, compound 3 shows a greater alkyne bending-back angle (deviation from linearity of the alkyne C–C[triple bond, length as m-dash]C bond angle) as a result of the coordination of two copper atoms compared to those of 2 and 9–12 that have only one copper atom on each alkyne. The structural data from 9–11 with terminal alkynes indicate that the Cu–C(H)[triple bond, length as m-dash] bond is significantly shorter than the Cu–C(C)[triple bond, length as m-dash] bond length, which is probably a result of steric effects. The coordinated alkyne groups of 4 and 8–12 are co-planar with respect to the trigonal plane of copper (i.e., N2CuC2 atoms are in the same plane; see ESI). These molecules represent a rare group of structurally characterized, terminal alkyne complexes of copper, derived from binary copper(I) pyrazolates. The mix-valent, Cu{(μ-[3,5-(CF3)2Pz])2Cu(HC[triple bond, length as m-dash]CPh)}2 and Cu{(μ-[3,5-(CF3)2Pz])2Cu(HC[triple bond, length as m-dash]CC6H13)}2 featuring Cu(I) and Cu(II) sites are the only copper–alkyne-pyrazolates that are somewhat related to 8–11 in the literature.58

Copper(I) pyrazolates in alkyne transformations

Despite the importance of copper in alkyne chemistry,37–53 and the ease of synthesis and availability of many copper(I) pyrazolates, they have not been used widely as catalysts in alkyne transformations. We have been working on the chemistry of highly fluorinated copper pyrazolates,2,78 and copper–alkyne complexes for a number of years.76,79–81 Fluorinated copper pyrazolates serve as excellent Lewis acids and bind to molecules with lone pairs (e.g., CO, pyridines) or π-electrons (e.g., alkene, alkynes).19,20,23–26,57 During this work, we noticed that they are also competent catalysts for several processes that transform alkynes. Here we report the use of fluorinated copper pyraolates as a catalyst in the carboxylation, azide-cycloaddition and hydrothiolation of alkynes.
Carboxylation of the terminal alkynes. Incorporation of carbon dioxide into molecules is a significant current interest.45,47 One way to achieve this is via the insertion of CO2 into C(sp)–H bonds of terminal alkynes leading to carboxylic acids. Copper is turning out to be an important catalyst in this regard.45,47 For example, CuCl with various ligands (e.g., TMEDA) in the presence of K2CO3 have been reported to mediate CO2 insertion to phenylacetylene.47 We found that {μ-[3,5-(CF3)2Pz]Cu}3 is also effective in this process. It does not require an external base and the reactions proceed at 1 atm of CO2 at the room temperature (Table 2). For example, {μ-[3,5-(CF3)2Pz]Cu}3 at 2 mol% level, catalyzes the reaction between phenylacetylene and CO2 at room temperature and produces PhC2CO2H in 72% isolated yield (entry 1). In these reactions, molecules resulting from an alkyne coupling were also observed as a minor product.
Table 2 Carboxylation of terminal alkynes using {μ-[3,5-(CF3)2Pz]Cu}3 (2 mol%) as the catalyst

image file: c9dt03350e-u1.tif

Entry R Time (h) % Yield (product A) % Yield (product B)
1 H 12 72 10
2 CH3 12 80 2
3 Cl 12 64 5


Azide–alkyne cycloaddition. Copper catalyzed synthesis of 1,2,3-triazoles via the cycloaddition of azides to triple bonds of alkynes is perhaps the most well-known process involving copper and alkynes.38,40,82–84 The standard catalytic system uses copper(II) salts such as copper sulfate pentahydrate in the presence of a reducing agent, such as sodium ascorbate.83 During our work involving copper(I) pyrazolates and alkynes, we discovered that {μ-[3,5-(CF3)2Pz]Cu}3 is an excellent catalyst for the cycloaddition of azides to terminal alkynes. Furthermore, it also mediates similar chemistry with acetylene,85 which is rare (Scheme 3). It is important to note that a report appeared recently on the use of {μ-[3,5-(CF3)2Pz]Cu}3 in click chemistry with 1-octyne and phenylacetylene with ortho-fluorobenzyl azide.58 That work complements the findings reported below involving terminal alkynes.
image file: c9dt03350e-s3.tif
Scheme 3 The {μ-[3,5-(CF3)2Pz]Cu}3 catalyzed alkyne–azide cycloaddition involving acetylene or various terminal alkynes as the alkyne source and p-tolylazide.

Specifically, the trinuclear copper(I) pyrazolate {μ-[3,5-(CF3)2Pz]Cu}3 (1 mol%) catalyzes the cycloaddition of p-tolylazide to acetylene (1 atm) to form the desired 1-substituted-1,2,3-triazole in quantitative yield based on the NMR spectroscopic analysis of the product mixture (Scheme 1). Reaction proceeds in CH2Cl2 under mild conditions and no heating or base is required. The reaction ensue equally well with phenylacetylene or 1-octyne as the alkyne source affording the corresponding 1,4-disubstituted 1,2,3-triazole.86 Mild reaction conditions of the copper pyrazolate catalyzed process and the high yields are noteworthy, as also highlighted by Titov and co-workers.58

We have also successfully converted 1,8-nonadiyne to the corresponding 1,4-disubstituted 1,2,3-triazole using p-tolylazide (Scheme 3). Although it produces the bis-triazole product in quantitative yield, this reaction required the use of slightly elevated temperature (80 °C) and benzene as the solvent. The control reaction at this temperature (with no copper catalyst) produces the product in 19% yield. The {μ-[3,5-(CF3)2Pz]Cu}3 catalyzed process at room temperature also gives the product, but in low yield (13%). Overall, this work indicates that the trinuclear copper(I) pyrazolate {μ-[3,5-(CF3)2Pz]Cu}3 can be used as an efficient catalyst to prepare 1-substituted-1,2,3-triazoles and 1,4-disubstituted 1,2,3-triazoles using just the organic azide and an alkyne source. Some of the isolable copper–alkyne complexes described earlier (e.g., Table 1), resulting from the same alkynes and {μ-[3,5-(CF3)2Pz]Cu}3 combination may exist as intermediates in these processes.

Hydrothiolation of alkynes. Copper also plays an important role as a catalyst in the alkyne hydrothiolation chemistry which leads to an important class of compounds, vinyl sulfides.37,87–92 We found that {μ-[3,5-(CF3)2Pz]Cu}3 is an effective catalyst in this process involving phenylacetylene and thiophenol (Table 3). For example, it catalyzes (1 mol%) the addition of thiol group of PhSH to the alkyne moiety of PhCCH at room temperature under CO2 producing PhCHCHSPh in 70% isolated yield (entry 1). Note that both the E- and Z-isomers are obtained, which is not unusual for a copper catalyzed process.88 Interestingly, the control reaction without the catalyst also produces the product,91 albeit in lower yield (e.g., 42% yield (with almost opposite E[thin space (1/6-em)]:[thin space (1/6-em)]Z of 30[thin space (1/6-em)]:[thin space (1/6-em)]70) at room temperature after 3 h under CO2).
Table 3 Hydrothiolation of terminal alkynes using {μ-[3,5-(CF3)2Pz]Cu}3 (1 mol%) as the catalyst. Yields reported are isolated yields (average from two separate experiments). The E/Z ratio was determined by 1H NMR analysis

image file: c9dt03350e-u2.tif

Entry Temp (°C) Time (h) Under CO2 (E[thin space (1/6-em)]:[thin space (1/6-em)]Z) Under N2 (E[thin space (1/6-em)]:[thin space (1/6-em)]Z) Overall % yield under CO2 Overall % yield under N2
1 RT 3 63[thin space (1/6-em)]:[thin space (1/6-em)]37 50[thin space (1/6-em)]:[thin space (1/6-em)]50 70 56
2 0 3 26[thin space (1/6-em)]:[thin space (1/6-em)]74 10[thin space (1/6-em)]:[thin space (1/6-em)]90 65 48
3 90 3 30[thin space (1/6-em)]:[thin space (1/6-em)]70 56[thin space (1/6-em)]:[thin space (1/6-em)]44 78 62
4 90 16 78[thin space (1/6-em)]:[thin space (1/6-em)]22 34[thin space (1/6-em)]:[thin space (1/6-em)]66 90 81


The E/Z stereoselectivity of {μ-[3,5-(CF3)2Pz]Cu}3 catalyzed reaction is somewhat complicated, and dependent on reaction time, reaction temperature and the atmosphere. For example, at 0 °C and after 3 h of reaction time (entry 2), we have observed different stereoselectivities for reactions carried out under CO2 (E/Z ratio = 26[thin space (1/6-em)]:[thin space (1/6-em)]74) and N2 atmosphere (E/Z ratio = 10[thin space (1/6-em)]:[thin space (1/6-em)]90). At 90 °C and after 16 h of reaction time, a reversal in product stereoselectivity was observed (E/Z ratio = 78[thin space (1/6-em)]:[thin space (1/6-em)]22) in the reaction performed under CO2 atmosphere compared to the 3 h reaction at 0 °C (entries 4 and 2). The change in product ratio was less significant for the reaction carried out under N2 atmosphere (E/Z ratio = 34[thin space (1/6-em)]:[thin space (1/6-em)]66 vs. 10[thin space (1/6-em)]:[thin space (1/6-em)]90). The hydrothiolation chemistry reported by Y. Zhang and coworkers using CuI (5 mol%) catalyst (in DMSO with K2CO3 as a base)88 under similar reaction temperatures and time, also generated very different product ratios under CO2 and argon (E/Z ratio = 10[thin space (1/6-em)]:[thin space (1/6-em)]90 and 84[thin space (1/6-em)]:[thin space (1/6-em)]16, with 92% and 68% yields, respectively). Note also that the E/Z stereoselectivity is nearly opposite for the two copper catalysts (i.e., E/Z ratio = 10[thin space (1/6-em)]:[thin space (1/6-em)]90 and 78[thin space (1/6-em)]:[thin space (1/6-em)]22 for CuI and {μ-[3,5-(CF3)2Pz]Cu}3 catalyzed reactions, respectively). The isolated product yields of each {μ-[3,5-(CF3)2Pz]Cu}3 catalyzed experiment were relatively low, when the reactions were performed under N2 instead of CO2. We have thus far not probed various temperature, solvent, reaction time effects in detail. This work however, shows that copper pyrazolates are competent catalysts for alkyne hydrothiolations.

Summary and conclusions

Overall, we describe convenient routes to HC[triple bond, length as m-dash]CH and terminal alkyne (phenylacetylene, 1,8-nonadiyne, 1,7-octadiyne) complexes of copper(I) using readily available, fluorinated copper(I) pyrazolates and the corresponding alkynes. The 2[thin space (1/6-em)]:[thin space (1/6-em)]1 copper[thin space (1/6-em)]:[thin space (1/6-em)]acetylene complexes Cu4(μ-[3,5-(CF3)2Pz])4(μ-HC[triple bond, length as m-dash]CH)2 (4), Cu4(μ-[4-Br-3,5-(CF3)2Pz])4(μ-HC[triple bond, length as m-dash]CH)2 (6), and Cu4(μ-[4-Cl-3,5-(CF3)2Pz])4(μ-HC[triple bond, length as m-dash]CH)2 (7) are easier to isolate and relatively more stable in solid state and solution. They have bridging acetylene ligands as evident from the NMR and Raman spectroscopic data and confirmed for 4 by X-ray crystallography. Low temperature NMR data suggest the existence of 1[thin space (1/6-em)]:[thin space (1/6-em)]1 Cu[thin space (1/6-em)]:[thin space (1/6-em)]alkyne adducts, perhaps of the type Cu2(μ-[3,5-(CF3)2Pz])2(HC[triple bond, length as m-dash]CH)2 (5) and Cu2(μ-[4-Br-3,5-(CF3)2Pz])2(HC[triple bond, length as m-dash]CH)2 (8) in solutions containing excess acetylene. They are however, extremely labile in solution, and challenging to isolate than 4, 6, or 7. Compound 8 has been isolated in crystalline form, and characterized structurally but it loses acetylene even in the solid state. Raman data show a reduction in [small nu, Greek, macron]C[triple bond, length as m-dash]C stretching frequency from 1974 cm−1 in free acetylene to 1811 cm−1 in the η2-/2e-donor acetylene adduct 8 and, more drastically, to 1631 cm−1 in the bridged μ2–η22-/4e-donor adduct 6. The terminal alkynes, phenylacetylene, 1,8-nonadiyne, and 1,7-octadiyne easily form their η2-/2e-donor alkyne adducts Cu2(μ-[3,5-(CF3)2Pz])2(HC[triple bond, length as m-dash]CPh)2 (9), Cu2(μ-[3,5-(CF3)2Pz])2(HC[triple bond, length as m-dash]C(CH2)5C[triple bond, length as m-dash]CH) (10), and Cu4(μ-[3,5-(CF3)2Pz])4(HC[triple bond, length as m-dash]C(CH2)4C[triple bond, length as m-dash]CH)2 (11) that have 1[thin space (1/6-em)]:[thin space (1/6-em)]1 Cu/alkyne stoichiometry. Isolation of a copper(I)–pyrazolate 12 containing an internal alkyne is also reported. In addition to alkyne coordination, the {μ-[3,5-(CF3)2Pz]Cu}3 is also a very versatile and competent catalyst for alkyne transformations. We have presented its utility in C(sp)–H bond carboxylation with CO2, facile azide–alkyne cycloaddition leading to 1,2,3-triazoles including the rare chemistry involving acetylene itself, and S–H addition to alkyne moiety leading to vinyl sulfides. We are currently, exploring further details on these copper–alkyne chemistry and additional transformations of alkynes mediated by copper. Note that unlike the often used copper halide catalysts, these copper pyrazolates can be more easily fine-tuned sterically and electronically via changes to pyrazolyl ring substituents, which is a useful attribute for homogeneous catalysts.

Experimental details

General procedures

All manipulations were carried out under an atmosphere of purified nitrogen using standard Schlenk techniques unless otherwise noted. Solvents were purchased from commercial sources, and distilled prior to use. NMR spectra were recorded at 25 °C on a JEOL Eclipse 500 spectrometer (1H, 500.16 MHz; 13C, 125.78 MHz; 19F, 470.62 MHz), unless otherwise noted. Proton and carbon chemical shifts are reported in ppm versus Me4Si. 19F NMR values were referenced to external CFCl3. Melting points were obtained on a Mel-Temp II apparatus and were not corrected. Elemental analyses were performed using a PerkinElmer Model 2400 CHN analyzer. Infrared spectra were recorded on a Shimadzu IR Prestige-21 spectrometer. Raman data were collected on a Horiba Jobin Yvon LabRAM Aramin Raman spectrometer with a HeNe laser source of 633 nm. High-resolution mass spectra (HRMS) were recorded in Electron spray ionization time-of-flight (ESI/TOF) mode. Samples were introduced as a solution in dichloromethane. The {μ-[3,5-(CF3)2Pz]Cu}3, {μ-[4-Br-3,5-(CF3)2Pz]Cu}3, and {μ-[4-Cl-3,5-(CF3)2Pz]Cu}3 were prepared according to reported literature procedures with slight modifications.75,78 p-Tolyl azide was prepared according to literature procedure.93 Acetylene gas was freed from acetone and purified before use.59 All other reactants and reagents were purchased from commercial sources.

Warning. Due care must be taken when working with acetylene gas. It is known to produce explosive combinations with oxygen, and also form potentially explosive acetylides and other materials with copper salts.53

Synthesis of Cu4(μ-[3,5-(CF3)2Pz])4(μ-HC[triple bond, length as m-dash]CH)2 (4)

{μ-[3,5-(CF3)2Pz]Cu}3 (1) (0.25 g, 0.312 mmol) was dissolved in 10 mL of dichloromethane and stirred for ∼10 min while bubbling acetylene as a steady stream (1 atm) through the solution. The reaction mixture was concentrated with continuous flow of acetylene and kept at −20 °C to obtain X-ray quality colorless crystals of Cu4(μ-[3,5-(CF3)2Pz])4(μ-HC[triple bond, length as m-dash]CH)2. Yield: >90%. M.p.: 170 °C (decomp.). 1H NMR (in CDCl3): δ (ppm) 6.16 (s, 2H, CH), 6.84 (s, 2H, Pz–H). 19F NMR (in CDCl3): δ (ppm) −60.1 (s). 13C{1H} NMR (in CDCl3): δ (ppm) 79.2 (br s, C[triple bond, length as m-dash]C), 104.8 (s, C-4), 120.7 (q, 1JC–F = 268.7 Hz, CF3), 143.0 (br q, 2JC–F = 37.8 Hz, C-3/C-5). Raman (cm−1), selected peak: 1638 (C[triple bond, length as m-dash]C). Anal. calc. C24H8Cu4F24N8·0.4C15H3Cu3F18N6: C, 25.05%; H, 0.64%; N, 10.13%. Found: C, 25.68%; H, 0.74%; N, 10.02%. Compound 4 lose some acetylene under reduced pressure leading to 1. Signals corresponding to {μ-[3,5-(CF3)2Pz]Cu}3 were also observed in the solutions of resulting material. 1H NMR (in CDCl3): δ (ppm) 7.01 (br s, 2H, Pz–H). 19F NMR (in CDCl3): δ (ppm) −61.0 (br s).

Generation of Cu2(μ-[3,5-(CF3)2Pz])2(HC[triple bond, length as m-dash]CH)2 (5)

{μ-[3,5-(CF3)2Pz]Cu}3 (1) (4 mg) was dissolved in 1.5 mL CD2Cl2 in a NMR tube and acetylene gas was bubbled for 30 to 60 s. The NMR tube was quickly sealed and the 1H and 19F NMR data were collected at different temperatures (21 °C, −10 °C, −35 °C, −60 °C and −70 °C). A broad peak started to appear around 4.8 ppm at −35 °C which is likely the signal from bound acetylene of Cu2(μ-[3,5-(CF3)2Pz])2(HC[triple bond, length as m-dash]CH)2. 1H NMR (in CD2Cl2 at −70 °C): δ (ppm) 6.92 (s, Pz–H), 4.86 (br s, [triple bond, length as m-dash]CH). 19F NMR (in CD2Cl2 at −70 °C): δ (ppm) −59.7 (s). We could not isolate this molecule as a solid, as all attempts even in the presence of excess acetylene led to the precipitation of 4.

Synthesis of Cu4(μ-[4-Br-3,5-(CF3)2Pz])4(μ-HC[triple bond, length as m-dash]CH)2 (6)

{μ-[4-Br-3,5-(CF3)2Pz]Cu}3 (0.25 g, 0.241 mmol) was dissolved in 10 mL of dichloromethane and stirred for ∼10 min while bubbling acetylene as a steady stream (1 atm) through the solution. The reaction mixture was concentrated using an acetylene stream and kept at −20 °C to obtain Cu4(μ-[4-Br-3,5-(CF3)2Pz])4(μ-HC[triple bond, length as m-dash]CH)2 as a crystalline solid. Yield: 85%. M.p.: 180 °C (decomp.). 1H NMR (in CDCl3): δ (ppm) 6.03 (s, 2H, CH). 19F NMR (in CDCl3): δ (ppm) −59.5 (s). Solid 6 does not show good solubility in most NMR solvents to collect good 13C NMR data. 13C{1H} NMR (in (CD3)2CO): δ (ppm) 74.0 (br s, C[triple bond, length as m-dash]C), 92.8 (s, C-4), 121.7 (br, CF3), 142.6 (br, C-3/C-5). Raman (cm−1), selected peaks: 1631 (C[triple bond, length as m-dash]C). Anal. calc. C24H4Br4Cu4F24N8: C, 20.09%; H, 0.28%; N, 7.81%. Found: C, 20.87%; H, 0.31%; N, 8.01%. Indicates minor acetylene loss leading to the precursor copper pyrazolate. Also in solution, some Cu4(μ-[4-Br-3,5-(CF3)2Pz])4(μ-HC[triple bond, length as m-dash]CH)2 dissociates into {μ-[4-Br-3,5-(CF3)2Pz]Cu}3 and free acetylene as evident from 19F and 1H NMR data.

Generation and isolation of Cu2(μ-[4-Br-3,5-(CF3)2Pz])2(HC[triple bond, length as m-dash]CH)2 (8)

{μ-[4-Br-3,5-(CF3)2Pz]Cu}3 (4 mg) was dissolved in 1.5 mL CD2Cl2 in a NMR tube and acetylene gas was bubbled for 30 to 60 s. The NMR tube was quickly sealed and the 1H and 19F NMR data were collected at different temperatures (21 °C, −10 °C, −35 °C, −60 °C and −70 °C). A broad peak started to appear around 4.7 ppm at −35 °C which is likely the signal from bound acetylene of Cu2(μ-[4-Br-3,5-(CF3)2Pz])2(HC[triple bond, length as m-dash]CH)2. 1H NMR (in CD2Cl2 at −70 °C): δ (ppm) 4.75 (br s, 4H, CH). 19F NMR (in CD2Cl2 at −70 °C): δ (ppm) −59.7 (s). X-ray quality crystals of Cu2(μ-[4-Br-3,5-(CF3)2Pz])2(HC[triple bond, length as m-dash]CH)2 was obtained in toluene at −20 °C. For that, 50 mg of {μ-[4-Br-3,5-(CF3)2Pz]Cu}3 was dissolved in 8 mL of dry dichloromethane in 50 mL Schlenk flask and acetylene gas was bubbled until all dichloromethane was evaporated and white powder was obtained. This white powder was dissolved in 10 mL of dry toluene under acetylene atmosphere and kept at −20 °C refrigerator to obtain colorless crystals. Raman (cm−1), selected peak: 1811 (C[triple bond, length as m-dash]C). IR (cm−1), selected peak: 1810. This molecule loses acetylene easily in solution or in the solid.

Synthesis of Cu4(μ-[4-Cl-3,5-(CF3)2Pz])4(μ-HC[triple bond, length as m-dash]CH)2 (7)

{μ-[4-Cl-3,5-(CF3)2Pz]Cu}3 (0.25 g, 0.276 mmol) was dissolved in 10 mL of dichloromethane, and acetylene was bubbled as a steady stream (1 atm) through the solution for 10 min. The reaction mixture was concentrated using acetylene stream and kept at −20 °C to obtain Cu4(μ-[4-Cl-3,5-(CF3)2Pz])4(μ-HC[triple bond, length as m-dash]CH)2 as a white, crystalline solid. Yield: 87%. M.p.: 180 °C (decomp.). 1H NMR (in CDCl3): δ (ppm) 6.04 (s, 2H, CH). 19F NMR (in CDCl3): δ (ppm) −59.6 (s). Solid 7 does not show good solubility in most NMR solvents to collect good 13C NMR data. 13C{1H} NMR (in (CD3)2CO): δ (ppm) 74.1 (br s, C[triple bond, length as m-dash]C), 109.6 (s, C-4), 119.4 (br, CF3), 140.7 (br q, C-3/C-5). Raman (cm−1), selected peaks: 1632 (C[triple bond, length as m-dash]C). Anal. calc. C24H4Cl4Cu4F24N8: C, 22.93%; H, 0.32%; N, 8.92%. Found: C, 22.53%; H, 0.56%; N, 8.45%. Compound 7 tend to lose some acetylene under reduced pressure. Also in solution, some Cu4(μ-[4-Cl-3,5-(CF3)2Pz])4(μ-HC[triple bond, length as m-dash]CH)2 dissociates into {μ-[4-Cl-3,5-(CF3)2Pz]Cu}3 and free acetylene as evident from 19F and 1H NMR data.

Synthesis of Cu2(μ-[3,5-(CF3)2Pz])2(HC[triple bond, length as m-dash]CPh)2 (9)

Phenylacetylene (0.096 g, 0.936 mmol) was added dropwise to a solution of {μ-[3,5-(CF3)2Pz]Cu}3 (1) (0.25 g, 0.312 mmol) in dichloromethane (∼10 mL) at room temperature. The reaction mixture was stirred for 4 h under nitrogen, and the resulting solution was reduced to dryness by evaporation of solvent under vacuum to obtain Cu2(μ-[3,5-(CF3)2Pz])2(HC[triple bond, length as m-dash]CPh)2 as an off-white solid. X-ray quality crystals were grown from dichloromethane at −20 °C. Yield: >90%. M.p.: 55–57 °C. 1H NMR (in CDCl3): δ (ppm) 3.17 (br s, 2H, CH), 7.01 (s, 2H, Pz–H), 7.32 (m, J = 6.65 Hz, 6H, CH), 7.47 (d, J = 6.85 Hz, 4H, CH). 19F NMR (in CDCl3): δ (ppm) −61.0 (s). 13C{1H} NMR (in CDCl3): δ (ppm) 77.8 (br s, C[triple bond, length as m-dash]C), 104.6 (s, C-4), 120.5 (q, 1JC–F = 268.3 Hz, CF3), 122.4 (s, phenyl–C), 128.8 (s, m-CH), 129.4 (s, p-CH), 132.3 (s, o-CH), 144.4 (br q, 2JC–F = 37.2 Hz, C-3/C-5). The 13C{1H} NMR peak of the quaternary carbon atom of the triple bond of phenylacetylene broadens into the base line at around 84–85 ppm and as a result, it is difficult to pinpoint its exact position. Raman (cm−1), selected peaks: 1918, 1928 and 1950 (C[triple bond, length as m-dash]C). Anal. calc. C26H14Cu2F12N4: C, 42.34%; H, 1.91%; N, 7.60%. Found: C, 42.37%; H, 1.94%; N, 7.69%.

Synthesis of Cu2(μ-[3,5-(CF3)2Pz])2(HC[triple bond, length as m-dash]C(CH2)5C[triple bond, length as m-dash]CH) (10)

{μ-[3,5-(CF3)2Pz]Cu}3 (1) (0.25 g, 0.312 mmol) was dissolved in the dichloromethane (∼10 mL) under nitrogen at room temperature. To this solution, 3 equivalents of 1,8-nonadiyne (0.11 g, 0.936 mmol) was slowly added and stirred for ∼4 h. The solution was then filtered and dried under reduced pressure to obtain crude white Cu2(μ-[3,5-(CF3)2Pz])2(HC[triple bond, length as m-dash]C(CH2)5C[triple bond, length as m-dash]CH) as solid product. X-ray quality crystals were grown from dichloromethane at −20 °C. Yield: 82%. M.p.: 144 °C. 1H NMR (in CDCl3): δ (ppm) 1.31 (br s, 4H, CH2), 1.53 (m, J = 7.5 Hz, 2H, CH2), 2.61 (br s, 4H, CH2), 4.33 (br s, 2H, CH), 6.81 (s, 2H, Pz–H). 19F NMR (in CDCl3): δ (ppm) −60.54 (s). 13C{1H} NMR (in CDCl3): δ (ppm) 21.7 (br s, alkyne C-3/C-7), 26.4 (br s, alkyne C-4/C-5/C-6), 75.1 (br s, C[triple bond, length as m-dash]C), 96.4 (br s, C-2/C-8), 103.6 (s, C-4), 121.6 (q, 1JC–F = 268.3 Hz, CF3), 142.2 (br q, 2JC–F = 37.2 Hz, C-3/C-5). Raman (cm−1), selected peak: 1946 (C[triple bond, length as m-dash]C). Anal. calc. C19H14Cu2F12N4: C, 34.92%; H, 2.16%; N, 8.57%. Found: C, 34.89%; H, 2.01%; N, 8.56%.

Synthesis of Cu4(μ-[3,5-(CF3)2Pz])4(HC[triple bond, length as m-dash]C(CH2)4C[triple bond, length as m-dash]CH)2 (11)

1,7- Octadiyne (0.080 g, 0.750 mmol) was added dropwise to a solution of {μ-[3,5-(CF3)2Pz]Cu}3 (1) (0.20 g, 0.250 mmol) in dichloromethane (∼8 mL) in 3[thin space (1/6-em)]:[thin space (1/6-em)]1 molar ratio. The reaction mixture was stirred for ∼4 h under nitrogen at room temperature. The resulting mixture was filtered and dried under reduced pressure to obtain Cu4(μ-[3,5-(CF3)2Pz])4(HC[triple bond, length as m-dash]C(CH2)4C[triple bond, length as m-dash]CH)2 as white solid. It was recrystallized from dichloromethane at −20 °C to obtain X-ray quality crystals of Cu2(μ-[3,5-(CF3)2Pz])2(1,7-octadiyne). Yield: 81%. Raman (cm−1), selected peak: 1952 (C[triple bond, length as m-dash]C).

Synthesis of Cu4(μ-[3,5-(CF3)2Pz])4(C2H5C[triple bond, length as m-dash]C(CH2)4C[triple bond, length as m-dash]CC2H5)2 (12)

3,9-Dodecadiyne (0.091 g, 0.561 mmol) was added dropwise to a solution of {μ-[3,5-(CF3)2Pz]Cu}3 (1) (0.15 g, 0.187 mmol) in dichloromethane (∼8 mL) in 3[thin space (1/6-em)]:[thin space (1/6-em)]1 molar ratio. The reaction mixture was stirred for ∼4 h under nitrogen at room temperature. The solution was then filtered and dried under reduced pressure to obtain crude white Cu4(μ-[3,5-(CF3)2Pz])4(C2H5C[triple bond, length as m-dash]C(CH2)4C[triple bond, length as m-dash]CC2H5)2 as solid product. X-ray quality crystals were grown from dichloromethane at −20 °C. Yield: 78%. M.p.: 118 °C. 1H NMR (in CDCl3): δ (ppm) 1.11 (t, J = 7.2 Hz, 6H, CH3), 1.54 (br s, 4H, CH2), 1.57 (br s, 4H, CH2), 2.18 (br s, 4H, CH2), 7.01 (s, 2H, Pz–H). 19F NMR (in CDCl3): δ (ppm) −60.98 (s). 13C{1H} NMR (in CDCl3): δ (ppm) 13.1 (br s, CH3), 14.4 (br s, CH2), 19.1 (br s, CH2), 28.3 (br s, CH2), 80.3 (br s, C[triple bond, length as m-dash]C), 83.2 (br s, C[triple bond, length as m-dash]C), 104.3 (s, C-4), 121.3 (br q, CF3), 143.9 (br q, C-3/C-5). Raman (cm−1), selected peaks: 2040 and 2070 (C[triple bond, length as m-dash]C). Anal. calc. C44H40Cu4F24N8: C, 37.99%; H, 2.90%; N, 8.06%. Found: C, 37.82%; H, 2.82%; N, 8.15%.

General procedure for carboxylation of terminal alkynes

{μ-[3,5-(CF3)2Pz]Cu}3 (0.036 mmol, 30 mg, 2 mol% based on phenylacetylene) was dissolved in DMF (4 mL) in a reaction vessel. Phenylacetylene (2 mmol, 204 mg) was introduced into the reaction mixture while stirring under CO2, and stirred for 12 h at room temperature under a CO2 atmosphere using a balloon. After completion of the reaction, DMF was removed, as much as possible, under reduced pressure and treated with water/CH2Cl2 mixture. The product was extracted with dichloromethane (3 × 5 mL). The leftover aqueous layer was acidified with concentrated HCl to pH = 1 and then extracted with ethyl acetate (3 × 5 mL) to collect additional product. The organic layers (CH2Cl2 and ethyl acetate extracts) were combined, dried with anhydrous Na2SO4, filtered and the volatiles were removed in vacuo. The residue was purified by silica gel column chromatography (ethyl acetate/n-hexane gradient), affording the carboxylic acid product as a major product (72% yield) and alkyne coupling product as a minor product (10% yield).94,95

General procedure for alkyne – p-tolylazide cycloadditon reaction

{μ-[3,5-(CF3)2Pz]Cu}3 (0.018 mmol, 15 mg, 1 mol% based on p-tolylazide) was dissolved in CH2Cl2 (5 mL) under nitrogen atmosphere at room temperature. Acetylene (1 atm, balloon) and p-tolylazide (1.5 mmol, 200 mg) were introduced into the reaction mixture under stirring, while slowly closing the nitrogen line. The reaction mixture was stirred at room temperature for 12 h while maintaining the mixture under an acetylene atmosphere (using a balloon). After removal of the solvent under reduced pressure, the crude reaction mixture was analyzed using 1H NMR, which indicated the presence of the desired triazole product in 99% yield.96 Similarly, we have used phenylacetylene or 1-octyne (1.5 mmol, 0.18 mL) as the alkyne source along with p-tolylazide, and reacted it with a dichloromethane solution of {μ-[3,5-(CF3)2Pz]Cu}3 (0.018 mmol, 15 mg, 1 mol% based on alkyne) under nitrogen atmosphere at room temperature. After completion of the reaction, the crude reaction mixture was analyzed using 1H NMR indicating the formation of desired triazole product in 99% yield.97 We were also successful in isolating the bis-triazole using the terminal alkyne 1,8-nonadiyne. In this reactions, the alkyne (1.5 mmol) and p-tolylazide (1.5 mmol, 400 mg of 1,8-nonadiyne) were added to a solution of {μ-[3,5-(CF3)2Pz]Cu}3 (0.015 mmol, 12 mg, 1 mol% based on respective alkyne) in benzene (5 mL) under nitrogen and heated to 80 °C with continuous stirring for 12 h. After cooling the mixture to room temperature and removal of the solvent under reduced pressure, the crude reaction mixture was characterized using 1H NMR. The observed product yield was 99% (the control reaction without the copper catalyst under same conditions gave about 19% of the bis-triazole, and the Cu catalyzed reaction at room temperature in CH2Cl2 is slow and gave the bis-triazole product in about 13% yield after 12 h).

General procedure for hydrothiolation of alkynes

{μ-[3,5-(CF3)2Pz]Cu}3 (0.01 mmol, 7.99 mg, 1.0 mol% based on phenylacetylene) was dissolved in dry toluene (5 mL) in a reaction vessel. Phenylacetylene (1.0 mmol, 100 mg) was added to it under a CO2 atmosphere (1 atm, balloon). Then thiophenol (1.1 mmol, 118 mg) was introduced into the reaction mixture under stirring. The resulting mixture was stirred at room temperature for 3 h. After removal of the solvent and all the volatilities (including unreacted starting materials) under reduced pressure, the crude reaction mixture was analyzed using 1H NMR. The vinyl sulfide products were obtained at a yield of 70% (isolated yield) and an E/Z ratio of 63[thin space (1/6-em)]:[thin space (1/6-em)]37 (from NMR peak analysis).88 The reaction performed under the same conditions, except for the use of N2, gave similar products but with different stereoselectivity (E/Z ratio = 50[thin space (1/6-em)]:[thin space (1/6-em)]50). These results demonstrate that the stereoselectivity of this Cu(I)-catalyzed alkyne hydrothiolation reaction could be controlled with the presence or absence of the CO2 atmosphere. We have then performed several experiments by changing the reaction temperature and reaction time.

X-ray crystallographic data

A suitable crystal covered with a layer of hydrocarbon/Paratone-N oil was selected and 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 D8 Quest with a Photon 100 CMOS detector equipped with an Oxford Cryosystems 700 series cooler, a Triumph monochromator, and a Mo Kα fine-focus sealed tube (λ = 0.71073 Å). Intensity data were processed using the Bruker Apex3 program suite. Absorption corrections were applied by using SADABS. Initial atomic positions were located by direct methods using XT, and the structures of the compounds were refined by the least-squares method using SHELXL98,99 within Olex2100 GUI. All the non-hydrogen atoms were refined anisotropically. Hydrogen atoms were included at calculated positions and refined riding on corresponding carbons. X-ray structural figures were generated using Olex2. CCDC 1934981–1934986 files contain the supplementary crystallographic data. Additional details are provided in ESI. Further details are given in the CIF.

Conflicts of interest

There are no conflicts of interest to report.

Acknowledgements

This work was supported by the Robert A. Welch Foundation (Grant Y-1289 to H. V. R. D.). Partial support by Americal Floral Endowment for the acetylene chemistry is also acknowledged.

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Footnote

Electronic supplementary information (ESI) available: X-ray crystallographic data, additional figures and experimental details, NMR and Raman spectroscopic data of metal complexes, NMR data of reaction products, Raman data of free alkynes (phenylacetylene, 1,8-nonadiyne, 1,8-nonadiyne, 3,9-dodecadiyne). CCDC 1934981–1934986. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c9dt03350e

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