Terminal and Internal Alkyne Complexes and Azide-Alkyne Cycloaddition Chemistry of Copper(I) Supported by a Fluorinated Bis(pyrazolyl)borate

Copper plays an important role in alkyne coordination chemistry and transformations. This report describes the isolation and full characterization of a thermally stable, copper(I) acetylene complex using a highly fluorinated bis(pyrazolyl)borate ligand support. Details of the related copper(I) complex of HC≡CSiMe3 are also reported. They are three-coordinate copper complexes featuring η2-bound alkynes. Raman data show significant red-shifts in C≡C stretch of [H2B(3,5-(CF3)2Pz)2]Cu(HC≡CH) and [H2B(3,5-(CF3)2Pz)2]Cu(HC≡CSiMe3) relative to those of the corresponding alkynes. Computational analysis using DFT indicates that the Cu(I) alkyne interaction in these molecules is primarily of the electrostatic character. The π-backbonding is the larger component of the orbital contribution to the interaction. The dinuclear complexes such as Cu2(μ-[3,5-(CF3)2Pz])2(HC≡CH)2 display similar Cu-alkyne bonding features. The mononuclear [H2B(3,5-(CF3)2Pz)2]Cu(NCMe) complex catalyzes [3 + 2] cycloadditions between tolyl azide and a variety of alkynes including acetylene. It is comparatively less effective than the related trinuclear copper catalyst {μ-[3,5-(CF3)2Pz]Cu}3 involving bridging pyrazolates.


Results and Discussion
The bis(pyrazolyl)borate copper(I) complex [H2B(3,5-(CF3)2Pz)2]Cu(NCMe) [42] reacts with purified acetylene (~1 atm) [61,62] in CH2Cl2, affording [H2B(3,5-(CF3)2Pz)2]Cu(HCCH) (4) as a white solid in >90% yield (Scheme 1), which is quite amenable to detailed spectroscopic and structural studies. The room temperature 1 H NMR spectrum of 4 in CDCl3 displayed the acetylenic proton resonances at δ 4.70 ppm. That is a significant downfield shift relative to the corresponding signal of the free acetylene (δ 2.01 ppm) [63]. The 13 C resonance of the acetylenic carbons appears at δ 80.2 ppm, which is a downfield shift of 7.0 ppm relative to that of the free acetylene (δ 73.2 ppm) [63]. The ῡCC band of solid 4 in the Raman spectrum was observed at 1819 cm −1 , representing a 155 cm −1 red shift relative to the corresponding stretching frequency of the free HCCH (1974 cm −1 ) [64]. This red shift is not as high as that observed for Cu4(μ-[3,5-(CF3)2Pz])4(μ-HCCH)2 (3 with ῡCC of 1638 cm −1 ) containing a μη 2 η 2 -(HCCH) (which is a formally 4e-donor, bridging acetylene). Table 1 shows available, albeit limited, 1 H and 13 C NMR data and CC stretch of structurally characterized copper complexes featuring a formally 2e-donor η 2 -(HCCH). [H2B(3,5-(CF3)2Pz)2]Cu(HCCH) (4) shows the smallest downfield shift of the acetylenic NMR signal, and red-shift of CC stretching frequency relative to that of the free HCCH among these (although the differences are minor), suggesting relatively weaker σπ-interaction between the copper(I) and acetylene ligand in terms of the The X-ray crystal structure of [H 2 B(3,5-(CF 3 ) 2 Pz) 2 ]Cu(HC≡CH) (4) is illustrated in Figure 2. It is a three coordinate, trigonal planar copper-acetylene complex. The acetylene ligand is oriented parallel to the NCuN plane so as to maximize back-bonding interactions [73]. Selected bond distances and angles of 4 and copper complexes featuring 2e-donor, η 2 -acetylene ligand in the literature are given in Table 1. The key parameters involving the CuC 2 core are remarkably similar between these molecules. This suggests that cationic copper species [Cu{NH(Py) 2 }(HC≡CH)]BF 4 and [Cu(phen)(HC≡CH)]ClO 4 featuring relatively electron-rich supporting ligands and neutral copper complexes 4 and 2 involving weakly donating fluorinated ligands have similar effects on the Cu-C 2 H 2 alkyne moiety, or produce effects that are not large enough to be parsed out by routine X-ray crystallography. They both show slightly elongated C≡C bonds relative to the free acetylene (1.181(7) Å) [74], but these changes are overshadowed by the somewhat high esd associated with bond distance measurements. In addition to acetylene, we also tested the use of HC≡CSiMe3 as a substrate in CuACC chemistry. Considering that structurally authenticated metal complexes of η 2 -HC≡CSiMe3 are rare (a search of Cambridge Structural Database [75] disclosed only three such examples involving transition metal ions) [76][77][78] and unknown for copper to our knowledge [75], we also synthesized [H2B(3,5-(CF3)2Pz)2]Cu(HC≡CSiMe3) (5) for a detailed study. Treatment of [H2B(3,5-(CF3)2Pz)2]Cu(NCMe) with HC≡CSiMe3 in CH2Cl2 led to 5 in 91% yield (Scheme 1). It is a white solid and was characterized by NMR and Raman spectroscopy and X-ray crystallography. The ῡC≡C band of solid 5 in the Raman spectrum was observed at 1870 cm −1 , which is a 237 cm −1 red shift relative to the corresponding stretching frequency of the free HC≡CSiMe3 (2107 cm −1 ). This ῡ(C≡C) is similar to that reported for [HC{C(CF3)CO}2]Cu(HC≡CSiMe3) [41]. This suggests the presence of an η 2 -HC≡CSiMe3 bound alkyne moiety on copper(I) [46,51].
X-ray crystal structure of [H2B(3,5-(CF3)2Pz)2]Cu(HC≡CSiMe3) (5) is depicted in Figure 3. It is a monomeric, trigonal planar copper complex with an η 2 -HC≡CSiMe3 bound alkyne moiety. The HC≡CSiMe3 is bonded slightly asymmetrically as evident from the marginally longer Cu-C12, which is a carbon atom with the larger, silyl group. The alkyne group shows a significant deviation from the ideal 180° as evident from C≡C-Si angle, 160.64 (11)°. This is about 19° bending back of the alkyne group due to the metal ion coordination. As noted above, there are no structural data on related copper η 2 -HC≡CSiMe3 complexes for comparisons. The Cp2Nb(H)(HC≡CSiMe3) [76] and (NMe- In addition to acetylene, we also tested the use of HC≡CSiMe 3 as a substrate in CuACC chemistry. Considering that structurally authenticated metal complexes of η 2 -HC≡CSiMe 3 are rare (a search of Cambridge Structural Database [75] disclosed only three such examples involving transition metal ions) [76][77][78] and unknown for copper to our knowledge [75], we also synthesized [H 2 B(3,5-(CF 3 ) 2

Experimental Details
All manipulations except catalysis were carried out under an atmosphere of purified nitrogen using standard Schlenk techniques or in a MBraun glovebox equipped with a −25 • C refrigerator. Solvents were purchased from commercial sources, purified prior to use. NMR spectra were recorded at 25 • C on a JEOL Eclipse 500 spectrometer (Peabody, MA, USA) ( 1 H, 500.16 MHz; 13 19 F NMR values were referenced to external CFCl 3 . Melting points were obtained on a Mel-Temp II apparatus (Wayne, PA, USA) and were not corrected. Elemental analyses were performed using a Perkin-Elmer Model 2400 CHN analyzer (Waltham, MA, USA). IR spectra were collected at room temperature on a Shimadzu IR Prestige-21 FTIR (Kyoto, Japan) containing an ATR attachment using pure liquid or solid materials, with instrument resolution at 2 cm −1 . Raman data were collected on a Horiba Jobin Yvon LabRAM Aramin Raman spectrometer (Edison, NJ, USA) with a HeNe laser source of 633 nm, by placing pure liquid or solid materials on a glass slide/cuvette. Heating was accomplished by either a heating mantle or a silicone oil bath. Purification of reaction products was carried out by flash column chromatography using silica gel 60 (230-400 mesh). TLC visualization was accompanied by UV light or KMnO 4 stains. The [H 2 B(3,5-(CF 3 ) 2 Pz) 2 ]Cu(NCMe) (1) [42] and {µ-[3,5-(CF 3 ) 2 Pz]Cu} 3 [50] were prepared according to reported literature procedures. p-Tolyl azide was prepared according to the literature procedure [92]. All other reactants and reagents were purchased from commercial sources. Acetylene gas was freed from acetone and purified before use [61]. 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 [62]. [H 2 B(3,5-(CF 3 ) 2 Pz) 2 ]Cu(EtC≡CEt) (6): This was synthesized as reported earlier [6] and crystallized using dichloromethane at −20 • C to obtain crystals suitable for X-ray analysis.
Details of CuACC chemistry involving several alkynes and p-tolyl azide 1. General method I for the synthesis of triazoles: A 50 mL Schlenk flask was charged with the selected alkyne (1.0. mmol), [H 2 B(3,5-(CF 3 ) 2 Pz) 2 ]Cu(NCMe) (1 mol%) and toluene (5.0 mL) under a nitrogen atmosphere. p-tolyl azide (1.0 mmol) was added to the reaction and stirred at 110 • C for 12 h. The solvent was removed under reduced pressure. The residue was dissolved in dichloromethane and filtered through a celite. The dichloromethane was evaporated to get pure product.

X-ray Data Collection and Structure Determinations
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 (Billerica, MA, USA) 700 series cooler, a graphite monochromator, and a Mo Kα fine-focus sealed tube (λ = 0.71073 Å). Intensity data were processed using the Bruker Apex program suite. Absorption corrections were applied by using SADABS [93]. Initial atomic positions were located by SHELXT [94] and the structures of the compounds were refined by the leastsquares method using SHELXL [95] within Olex2 GUI [96]. All the non-hydrogen atoms were refined anisotropically. The hydrogen atoms of BH 2 moieties as well as acetylenic C≡CH were located in difference Fourier maps, included and refined freely with isotropic displacement parameters. The remaining hydrogen atoms were included in their calculated positions and refined as riding on the atoms to which they are joined. X-ray structural figures were generated using Olex2 [96]. The