When SF₅ outplays CF₃: effects of pentafluorosulfanyl decorated scorpionates on copper

Abstract

Polyfluorinated, electron-withdrawing, and sterically demanding supporting ligands are of significant value in chemistry. Here we report the assembly and use of a bis(pyrazolyl)borate, [Ph₂B(3-(SF₅)Pz)₂]⁻ that combines all such features, and involves underutilized pentafluorosulfanyl substituents. The ethylene and carbonyl chemistry of copper(I) supported by [Ph₂B(3-(SF₅)Pz)₂]⁻, a comparison to the trifluoromethylated counterparts involving [Ph₂B(3-(CF₃)Pz)₂]⁻, as well as copper catalyzed cyclopropanation of styrene with ethyl diazoacetate and CF₃CHN₂ are presented. The results from cyclopropanation show that SF₅ groups dramatically improved the yields and stereoselectivity compared to the CF₃.

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Introduction

Substituents are the key to modulating the chemical and physical properties of molecules, including those of metal complexes and catalysts. The number of electron-withdrawing substituents that can be utilized for this purpose that are also relatively inert and practical, however, are quite limited. Fluorinated substituents such as the trifluoromethyl (CF₃) group are especially useful in this regard as they often drastically alter the properties of a molecule compared to their hydrocarbon counterparts.¹ The pentafluorosulfanyl (SF₅) is a noticeably underutilized fluorinated substituent compared to the CF₃ group in chemistry.² It is, however, gaining increasing attention due to its unique and attractive properties including large size (marginally smaller than a tert-butyl group), strong electron-withdrawing capabilities, high lipophilicity and excellent chemical and thermal stability, and showing great promise in agrochemical, medicinal and materials chemistry applications.^2,3 Furthermore, molecules with pentafluorosulfanyl groups are also becoming more accessible via effective and convenient routes.⁴ A number of derivatization reactions of SF₅-group containing molecules are also known.^3a,3b,4e,5

The metal complexes featuring SF₅ groups are quite limited,^3a,6 although it was a substituent first introduced in 1960.⁷ Promising outcomes noted in recent reports suggest that pentafluorosulfanyl moiety merits more closer scrutiny and wider utility. For example, recent work by Mecking and co-workers illustrated the benefits of SF₅ over CF₃ groups on Ni(II) salicylaldiminato complexes in ethylene polymerization catalysis (to get more linear and higher molecular weight polymers),^6a as well as on tetraphenylborate ions in Ni(II) mediated butadiene polymerizations.⁸ In addition, SF₅ group has been utilized in luminescent transition metal complexes to minimize the aggregation in the solid-state, improve the solubility, and alter the emission features such as blue shifting of the phosphorescent emissions more significantly relatively to CF₃ bearing analogs.^2b,3a,9

Poly(pyrazolyl)borates, commonly referred to as scorpionates,¹⁰ are very valuable class of ligands in coordination chemistry and catalysis, and form complexes with most metals of the periodic table. Here we report the first metal scorpionates decorated with pentafluorosulfanyl groups. In particular, we describe the synthesis of [Ph₂B(3-(SF₅)Pz)₂]⁻ and the effects of this ligand support on copper(I) as reflected in the structures and bonding of ethylene and CO complexes (which represent two classes of organometallic complexes with significant fundamental and practical significance),¹¹ and catalytic alkene cyclopropanation, as well as a direct comparison to the related trifluoromethylated analogs (Fig. 1). It is also notable that there is only an isolated example of a copper complex involving a 4-SF₅C₆H₄-substituted ligand to our knowledge,¹² whereas CF₃-bearing ligands with copper are more common and valued in many applications.^11,13

Fig. 1 Bis(pyrazolyl)boratocopper(I) complexes decorated with pentafluorosulfanyl (SF₅) and trifluoromethyl (CF₃) groups.

Results and discussion

Synthesis of the SF₅-pyrazole

In 1964, Hoover and Coffman reported that a reaction of alkyne 2 with diazomethane in diethyl ether at 0 °C led to the formation of a mixture of isomeric products 1 and 3 (3 : 2) (Scheme 1).¹⁴ The authors also mentioned that: “these pyrazoles were not separated.” Therefore, we needed to develop a robust practical protocol for the SF₅-prazole 1. After some optimization, we found that the reaction of alkene 4 with diazomethane at −10 °C gave pyrazoline 5 in 85% yield. Oxidation of the latter with MnO₂ followed by crystallization of the resulting material from hexane gave the needed compound SF₅-pyrazole (1) in 38% yield. This product was obtained in 11 g scale in one run (Scheme 1).

Scheme 1 Synthesis of SF₅-pyrazole 1.

Synthesis of fluorinated scorpionate ligands and copper–ethylene complexes

The fluorinated bis(pyrazolyl)borate [Ph₂B(3-(SF₅)Pz)₂]⁻ ligand possessing SF₅ groups at the pyrazolyl ring 3-positions was prepared by a reaction of SF₅-pyrazole (1) with NaBPh₄via a benzene elimination pathway (Scheme 2, see ESI† section). This resulting sodium salt was converted to [Ph₂B(3-(SF₅)Pz)₂]Tl (6) through metathesis using TlOAc, and utilized in the synthesis of [Ph₂B(3-(SF₅)Pz)₂]Cu(C₂H₄) (7) by treating with CuOTf and ethylene (Scheme 2). The related copper–ethylene, complex [Ph₂B(3-(CF₃)Pz)₂]Cu(C₂H₄) was also synthesized for a comparison. They are colorless crystalline solids, and stable to loss of ethylene in a nitrogen atmosphere at room temperature. The ¹⁹F NMR spectra of the two adducts are very different due to the unique square pyramidal arrangement of fluorine atoms in SF₅ moieties vs. trigonal pyramidal array in CF₃ groups, leading to a doublet and a pentet in the former and a singlet in the latter.

Scheme 2 Synthesis of [Ph₂B(3-(SF₅)Pz)₂]Cu(C₂H₄).

The ethylene ¹³C NMR signal in [Ph₂B(3-(SF₅)Pz)₂]Cu(C₂H₄) was observed at δ = 86.4 ppm. This resonance in [Ph₂B(3-(CF₃)Pz)₂]Cu(C₂H₄) was observed at δ 82.7 ppm, which is an even larger upfield shift from the free C₂H₄ (δ 123.1 ppm) signal. Larger upfield shift of the metal bound ethylene ¹³C resonance from the free C₂H₄ (δ 123.1 ppm) signal has been attributed to the increased shielding resulting from metal-to-ethylene π-back-donation.¹⁵ Thus [Ph₂B(3-(SF₅)Pz)₂]Cu(C₂H₄), with a smaller upfield shift points to relatively lower Cu → ethylene π-backbonding. This is reasonable considering the presence of more electron withdrawing SF₅ groups (with an estimated electronegativity of 3.65 vs. 3.36, and Hammett substituent constant σ_m of 0.61 vs. 0.43 for SF₅vs. CF₃)^2,16 on the scorpionate ligand backbone of this copper complex. For comparison, three coordinate [t-Bu₂P(NSiMe₃)₂]Cu(C₂H₄) with a more strongly backbonding copper site displays its ethylene carbon shift at δ 73.0 ppm.¹⁷ The ¹³C NMR data are particularly useful for such bonding analysis since they are less affected by the ring current effects.

The ethylene protons of [Ph₂B(3-(R)Pz)₂]Cu(C₂H₄) (R = –SF₅, –CF₃) in the ¹H NMR spectrum appear at δ 3.72 and 3.69 ppm, respectively. These protons are most likely affected by the ring currents of flanking phenyl groups sitting over ethylene moieties (see molecular structures below). The presence of additional ethylene in CDCl₃ solutions at room temperature leads to separate broad signals of free and coordinated ethylene in [Ph₂B(3-(CF₃)Pz)₂]Cu(C₂H₄) whereas these signals remain sharp for the –SF₅ analog 7, suggesting a quite rapid olefin exchange only in the former at room temperature on the NMR time scale.

X-ray crystal structures of Cu–ethylene complexes

Molecular structure of [Ph₂B(3-(R)Pz)₂]Cu(C₂H₄) (R = SF₅, CF₃) were unambiguously established by single-crystal X-ray diffraction (Fig. 2). Compound [Ph₂B(3-(SF₅)Pz)₂]Cu(C₂H₄) crystallizes with two chemical identical but crystallographically distinct molecules in the asymmetric unit. Selected bond distance and angles are given in Table S3 (ESI†). They are three-coordinate, trigonal planar copper complexes with an η²-bound C₂H₄ moieties. The bis(pyrazolyl)borate ligands coordinate to copper in κ² fashion via nitrogen atoms of two pyrazolyl arms and adopt a boat configuration. One of the phenyl groups on boron sits above the ethylene group. Most of the key features are similar between the two adducts, although the [Ph₂B(3-(SF₅)Pz)₂]Cu(C₂H₄) has slightly longer Cu–C and Cu–N distances compared to those of the CF₃ analog. This could be a result of either greater steric demand or more weakly donating nature of scorpionate in [Ph₂B(3-(SF₅)Pz)₂]Cu(C₂H₄).

Fig. 2 Molecular structures of [Ph₂B(3-(SF₅)Pz)₂]Cu(C₂H₄) (7) and [Ph₂B(3-(CF₃)Pz)₂]Cu(C₂H₄), from left to right.

Analysis of the topographic steric maps of the two metal complexes using SambVca¹⁸ and the X-ray crystallographic data indicate percent buried volumes of 69.9% and 64.0% for [Ph₂B(3-(SF₅)Pz)₂]Cu(C₂H₄) and [Ph₂B(3-(CF₃)Pz)₂]Cu(C₂H₄), respectively (Fig. 3), clearly indicating more protected copper sites in the former as a result of having sterically more demanding SF₅ groups at the periphery of the coordination pocket. Sluggish ethylene exchange in [Ph₂B(3-(SF₅)Pz)₂]Cu(C₂H₄) noted above is probably a result of having greater steric protection at the copper site of this –SF₅ bearing molecule.

Fig. 3 Steric maps of [Ph₂B(3-(SF₅)Pz)₂]Cu (left) and [Ph₂B(3-(CF₃)Pz)₂]Cu (right) moieties based on the calculations using SambVca tool and X-ray data from the ethylene complexes [Ph₂B(3-(SF₅)Pz)₂]Cu(C₂H₄) and [Ph₂B(3-(CF₃)Pz)₂]Cu(C₂H₄). The resulting % buried volume values are 69.9% (average for the two molecules in the asymmetric unit) and 64.0%, respectively.

Computational analysis of copper–ethylene complexes

We have also investigated alkene–copper(I) bonding of [Ph₂B(3-(SF₅)Pz)}₂]Cu(C₂H₄), [Ph₂B(3-(CF₃)Pz)₂]Cu(C₂H₄), and the hypothetical [Ph₂B(3-(CH₃)Pz)₂]Cu(C₂H₄) via density functional calculations. The calculated interaction energy (ΔE_int) between the ethylene and Cu(I) center remains similar, ranging from −44.9, −45.9, to −45.2 kcal mol⁻¹ (Table 1), respectively, which is further dissected in different contributions within the Ziegler–Rauk energy decomposition analysis (EDA).¹⁹ It shows that these interactions are primarily electrostatic in nature for all three [Ph₂B(3-(R)Pz)₂]Cu(C₂H₄) complexes as evident from ΔE_elstat of about ∼60%, with the remainder consists of ∼36% orbital contributions (ΔE_orb) and ∼4% dispersion-type interactions (ΔE_disp). The ΔE_orb of [Ph₂B(3-(SF₅)Pz)}₂]Cu(C₂H₄) composed of σ-donation and π-backdonation ascribed to the π₁-C₂H₄ → Cu and (Fig. S43, ESI†) in a 29.9% and 54.9% contribution, respectively, which is similar in trend but shows a gradual decrease and an increase in the two components going from –SF₅ to the –CF₃ and –CH₃ analogs, culminating in 24.5% and 63.7%, σ/π-contributions in the most electron rich scorpionate ligand analog [Ph₂B(3-(CH₃)Pz)₂]Cu(C₂H₄). These interactions cause a more red-shifted [small upsilon, Greek, macron](C=C) as evident from the computed values of 1516.3, 1513.5 and 1509.3 cm⁻¹, for [Ph₂B(3-(R)Pz)₂]Cu(C₂H₄) (R = –SF₅, –CF₃, –CH₃, respectively). These numbers follow the order of Hammett substituent constant σ_m/σ_p (0.61/0.68, 0.43/0.54, and −0.07/−0.17 for –SF₅, –CF₃, –CH₃, respectively),^2,16 and are inversely related to the backbonding contribution (Table 1). This trend is also consistent with computed proton affinities of the [Ph₂B(3-(R)Pz)₂]⁻ ligands (and therefore, the donor features of the scorpionate nitrogen sites; see ESI Table S17†), and indicate that [Ph₂B(3-(SF₅)Pz)₂]Cu(C₂H₄) features the most weakly donating scorpionate and least backbonding copper site in this series.

Table 1 Energy decomposition analyses for the C₂H₄–Cu interaction for different [Ph₂B(3-(R)Pz)₂]Cu(C₂H₄) complexes, with R = –SF₅, –CF₃, and –CH₃. Values in kcal mol⁻¹. In addition, π-backbonding and σ-donation components are given as and π₁-C₂H₄ → Cu, respectively. Calculated [small upsilon, Greek, macron](C=C) (in cm⁻¹) values are also given

Parameter	[Ph2B(3-(SF5)Pz)2]Cu(C2H4)		[Ph2B(3-(CF3)Pz)2]Cu(C2H4)		[Ph2B(3-(CH3)Pz)2]Cu(C2H4)
a Percentage contribution to the total attractive interactions ΔEelstat + ΔEorb + ΔEdisp. b Percentage contribution to the total orbital interactions ΔEorb.
ΔEint	−44.9		−45.9		−45.2
ΔEPauli	114.0		112.5		125.4
ΔEdisp	−6.5	4.1%^a	−5.6	3.5%^a	−5.3	3.1%^a
ΔEelstat	−95.2	59.9%^a	−95.0	59.9%^a	−102.1	59.9%^a
ΔEorb	−57.2	36.0%^a	−57.9	36.6%^a	−63.2	37.1%^a
π1-C2H4 → Cu	−17.1	29.9%^b	−16.5	28.5%^b	−15.5	24.5%^b
	−31.4	54.9%^b	−33.5	57.9%^b	−40.2	63.7%^b
	−8.7		−7.8		−7.5
[small upsilon, Greek, macron](C=C) calc.	1516.3		1513.5		1509.3

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Olefin displacement leading to Cu-carbonyl complexes

Some reactivities and catalytic features of [Ph₂B(3-(SF₅)Pz)₂]Cu(C₂H₄), [Ph₂B(3-(CF₃)Pz)₂]Cu(C₂H₄) have also been investigated. Upon treatment with CO in CH₂Cl₂, both adducts afford the corresponding copper carbonyl complexes. They do not lose CO under reduced pressure. The CO stretching frequencies of [Ph₂B(3-(SF₅)Pz)₂]Cu(CO) and [Ph₂B(3-(CF₃)Pz)₂]Cu(CO) were observed at 2121 and 2117 cm⁻¹, respectively. For comparison, the [small upsilon, Greek, macron](CO) for the highly fluorinated [H₂B(3,5-(CF₃)₂Pz)₂]Cu(CO)²⁰ and relatively electron rich [(Ph₃B)CH(3,5-(CH₃)₂Pz)₂]Cu(CO)²¹ appear at 2127 and 2092 cm⁻¹, respectively. These data indicate that [Ph₂B(3-(SF₅)Pz)₂]Cu(CO) has a notably Lewis acidic copper site, and a relatively weakly donating supporting scorpionate, consistent with the observed carbon chemical shifts and DFT analysis of the corresponding ethylene complex. DFT calculations show that the Cu–CO interaction (ESI†) is slightly less favorable than Cu–C₂H₄ (Table 1) in the corresponding [Ph₂B(3-(R)Pz)₂]Cu(CO) (ΔE_int = −39.9 (R = SF₅), −39.8 (R = CF₃), and −40.9 kcal mol⁻¹ (for hypothetical R = CH₃)). The thermochemical parameters for the observed C₂H₄ → CO replacement in 7 and its –CF₃ counterpart were also estimated computationally, which show that the free-energy change at room temperature (ΔG²⁹⁸ K) for these reactions are very small at +1.30 and +0.59 kcal mol⁻¹, respectively. They are essentially thermo-neutral processes. Indeed, it is possible to treat CHCl₃ solutions of [Ph₂B(3-(SF₅)Pz)₂]Cu(CO) and [Ph₂B(3-(CF₃)Pz)₂]Cu(CO) with ethylene at room temperature to re-generate the corresponding ethylene complexes. The Cu–CO bonding features of [Ph₂B(3-(R)Pz)₂]Cu(CO) were also investigated using DFT and found to vary systematically along the R = –SF₅, –CF₃, and –CH₃ series, with the lowest 2π* ← Cu backbonding observed for [Ph₂B(3-(SF₅)Pz)₂]Cu(CO), leading to the calculated [small upsilon, Greek, macron](CO) of 2110, 2099, and 2080 cm⁻¹, respectively (ESI†).

Molecular structures of [Ph₂B(3-(SF₅)Pz)₂]Cu(CO), [Ph₂B(3-(CF₃)Pz)₂]Cu(CO) are illustrated in Fig. 4. There are two chemically identical molecules of [Ph₂B(3-(SF₅)Pz)₂]Cu(CO) in its asymmetric unit. Selected bond distances and angles are presented in Table S3 (ESI†). The Cu–CO moieties are essentially linear. The scorpionate coordinates to the metal ion in κ² fashion and adopts a boat configuration. One of the phenyl groups on boron sits above the copper center. The metal to ipso-carbon distances are 2.58 and 2.78 Å in [Ph₂B(3-(R)Pz)₂]Cu(C₂H₄) (R = –SF₅, –CF₃), respectively. These separations are within the sum of van der Waals radii of Cu and C (3.10 Å). However, these contacts do not appear to be significant enough to distort the coordination geometry at the metal center because these molecules feature trigonal planar metal sites as evident from the sum of angles at the metal center (∼360°). Furthermore, the [small upsilon, Greek, macron](CO) values suggest that the copper sites remain quite Lewis acidic despite the close approach of the phenyl groups. Note that three-coordinate, trigonal planar copper carbonyls are very limited.²²

Fig. 4 Molecular structures of [Ph₂B(3-(SF₅)Pz)₂]Cu(CO) and [Ph₂B(3-(CF₃)Pz)₂]Cu(CO), from left to right.

Analysis of the topographic steric maps of the two metal complexes using SambVca¹⁸ and the X-ray crystallographic data indicate percent buried volumes (%V_bur) of 72.8% and 66.3% for [Ph₂B(3-(SF₅)Pz)₂]Cu(CO) and [Ph₂B(3-(CF₃)Pz)₂]Cu(CO), respectively clearly indicting more protected copper sites in the former as a result of having sterically more demanding SF₅ groups at the periphery of the coordination pocket (Fig. S41, ESI†). These percent buried volume values are larger than those observed for the related ethylene analogs (described above), indicating the adaptability of the scorpionate to accommodate organometallic fragments of different sizes.

Catalytic activity of copper-complexes

Finally, we have also investigated the catalytic potential of [Ph₂B(3-(R)Pz)₂]Cu(C₂H₄) (R = –SF₅, –CF₃) in cyclopropanation via a carbene transfer process. It was found that on reaction of styrene with ethyl diazoacetate (EDA), both copper(I) complexes serve as carbene transfer agents providing the expected cyclopropane as a diastereomeric mixture (Scheme 2). However, [Ph₂B(3-(SF₅)Pz)₂]Cu(C₂H₄) gave dramatically higher cyclopropane product yields (99% vs. 62%) and greater cis-selectivity (3 : 2 vs. 1 : 1) compared to the –CF₃ substituted analog (Scheme 2).

These results are consistent with the previous reports by Perez and co-workers involving tris(pyrazolyl)boratocopper complexes and EDA, which indicate that the higher cis-selectivities are associated with bulkier supporting ligands.²³ Interestingly, when CF₃CHN₂ was used as the carbene source,²⁴ [Ph₂B(3-(SF₅)Pz)₂]Cu(C₂H₄) again gave notably higher product yields than the [Ph₂B(3-(CF₃)Pz)₂]Cu(C₂H₄) catalyzed process, but this time, the trans-isomer was the major product. It is also known that the cis-isomer is the kinetic product while the trans-isomer is the thermodynamically favored product.²³ Therefore, it is possible that the greater steric bulk of the diazo reagent CF₃CHN₂ (compared to EDA) favors the latter, causing this interesting reversal in diastereoselectivity. Indeed, Doyle et al. has observed high trans-selective cyclopropanations in rhodium chemistry with bulky diazo reagents (Scheme 3).²⁵

Scheme 3 Cyclopropanation of styrene with N₂CHCO₂Et (EDA) and CF₃CHN₂.

Conclusions

Overall, we have described the preparation and characterization of the first pentafluorosulfanyl decorated scorpionate [Ph₂B(3-(SF₅)Pz)₂]⁻ and some of its copper chemistry, as well as a new, regioselective route to SF₅-pyrazole. The [Ph₂B(3-(SF₅)Pz)₂]⁻ is a more sterically demanding and weakly donating ligand compared to the [Ph₂B(3-(CF₃)Pz)₂]⁻, as evident from the copper ethylene and carbonyl chemistry and computational analysis. Moreover, the [Ph₂B(3-(SF₅)Pz)₂]Cu(C₂H₄) (7) complex displays significantly better efficacy in cyclopropanation of styrene with EDA and CF₃CHN₂ compared to that of [Ph₂B(3-(SF₅)Pz)₂]Cu(C₂H₄). Fluorinated ligands are important as they often provide metal complexes with certain beneficial features relative to the non-fluorinated, hydrocarbon group bearing ligands. Given the common appearance of CF₃-ligands in various areas of chemistry,^1f we believe that with this work, the SF₅-analogues will also become popular. Further studies on metal complexes supported by SF₅ containing ligands and practical approaches to other SF₅-heterocycles are currently underway.

Data availability

All data associated with this article can be found in the ESI.†

Author contributions

Conceptualization: HVRD, PM; investigation: AN-P, AM-C, AB; writing and validation: AN-P, AM-C, AB, PM, HVRD; project administration: HVRD.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This work was supported by the Robert A. Welch Foundation (Grant Y-1289) and Ministry of Education and Science of Ukraine for perspective development of a scientific direction “Mathematical sciences and natural sciences” at Taras Shevchenko National University of Kyiv. AB and PKM are grateful to Oleksandr I. Khyzhan (Enamine) for setting up a reaction of alkene 4 with CH₂N₂.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental details, synthesis, catalysis, spectroscopic and structural data, steric maps, computational data, additional figures and tables. The CCDC 2104704–2104707 files contain the supplementary crystallographic data. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/d1sc04846e

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