Lu Le Laboratory

Sunday, July 13, 2025

Dual XH–π Interaction of Hexafluoroisopropanol with Arenes: A Comprehensive Review

1,1,1,3,3,3-Hexafluoroisopropanol (HFIP) exhibits a unique dual hydrogen-bond-donating ability, engaging both its O–H and C–H bonds in strong π interactions with aromatic surfaces. These dual XH–π interactions result in higher binding energies than traditional CH–π interactions, significantly lowering the molecular orbital energy levels of arenes and enhancing solvent effects in organic transformations.

Background

HFIP is widely used as a solvent and additive in organic synthesis due to its high acidity (pKa ≈ 9.3) and strong electron-withdrawing CF₃ groups. It can form both classical O–H–π and unconventional C–H–π hydrogen bonds, leading to micelle-like aggregation and unique solvation properties.

Computational Methods

Geometries of HFIP conformers and their benzene complexes were optimized at the M06-2X/6-311++G(2d,2p) level, with counterpoise corrections for basis-set superposition error. Post-Hartree–Fock interaction energies were computed via CCSD(T)/CBS extrapolation. Density of states (DOS), partial DOS (PDOS), and overlap population DOS (OPDOS) analyses were carried out with Multiwfn, and charge decomposition analysis (CDA) dissected orbital contributions.

Results and Discussion

Three HFIP/benzene conformers were identified. The synperiplanar conformer exhibited a dual XH–π interaction energy of –7.49 kcal mol⁻¹, surpassing comparable CHCl₃/benzene and isopropanol/benzene complexes by over 1.8 kcal mol⁻¹. Correlation energy contributions (~ –7.47 kcal mol⁻¹) underscore the dominance of dispersion forces in dual-binding.

**Figure 1.** Optimized HFIP/benzene conformers showing dual XH–π, CH–π, and OH–π interactions and their complexation energies.

DOS analysis reveals a significant lowering (~0.73 eV) of benzene HOMO levels upon complexation, indicating non-superpositional orbital mixing. PDOS and OPDOS profiles confirm constructive overlap in HOMO–1, LUMO, and higher unoccupied orbitals, while antibonding character appears in certain deeper MOs.

**Figure 2.** (a) DOS of isolated benzene, HFIP, and their complex; (b) PDOS of the HFIP/benzene complex; (c) OPDOS indicating bonding (positive) and antibonding (negative) interactions.

Across 24 aromatic substrates, HFIP forms complexes 1.48× stronger than isopropanol and 1.61× stronger than chloroform, except with electron-deficient arenes. Single-crystal X-ray diffraction of HFIP benzoate (CCDC 2083861) validates the C–H hydrogen-donating capability.

Conclusions

This review elucidates the mechanism and energetics of dual XH–π interactions between HFIP and arenes. The synergy of O–H and C–H binding explains HFIP’s remarkable solvent effects and informs its strategic application in organic synthesis and materials science.

References

Lu, L.; Hua, R. “Dual XH–π Interaction of Hexafluoroisopropanol with Arenes: A Comprehensive Review.” Molecules 2021, 26(15), 4558. doi.org/10.3390/molecules26154558
Zhao, Y.; Truhlar, D. G. “The M06 suite of density functionals for main group thermochemistry, thermochemical kinetics, noncovalent interactions, excited states, and transition elements.” Theor. Chem. Acc. 2008, 120, 215–241. doi.org/10.1007/s00214-007-0310-x
Helgaker, T.; Klopper, W.; Koch, H.; Noga, J. “Basis-set convergence of correlated calculations on water.” J. Chem. Phys. 1997, 106, 9639–9646. doi.org/10.1063/1.473863
Lu, T.; Chen, F. “Multiwfn: A multifunctional wavefunction analyzer.” J. Comput. Chem. 2012, 33, 580–592. sobereva.com/multiwfn/
Frisch, M. J.; et al. Gaussian 16 Revision C.01; Gaussian, Inc.: Wallingford, CT, 2016. gaussian.com/g16
Ostojić, B. D.; Stilinović, V.; Glumac, B.; et al. “Aromatic CH as a hydrogen-bond donor: crystallographic evidence.” Chem. Commun. 2008, 48, 6546–6548. doi.org/10.1039/B810699A
Rocher-Casterline, B. E.; Vázquez, J.; Zwier, T. S. “Determination of the (H₂O)₂ bond dissociation energy.” J. Chem. Phys. 2011, 134, 211101. doi.org/10.1063/1.3562767
Colomer, I.; Chamberlain, A. E. R.; Haughey, M. B.; Donohoe, T. J. “Hexafluoroisopropanol as a highly versatile solvent.” Nat. Rev. Chem. 2017, 1, 0088. doi.org/10.1038/s41570-017-0088
Nakagawa, K.; et al. “Characterizing dipole–quadrupole CH–π interactions.” Chem. Sci. 2011, 2, 1234–1240. doi.org/10.1039/C1SC00045B
Li, H.-Y.; Zhao, D.; Cui, H.-L.; Lai, T.-T.; Zhao, P. “Hexafluoroisopropanol in organic synthesis: mechanistic insights and applications.” Tetrahedron 2016, 72, 8593–8604. doi.org/10.1016/j.tet.2016.10.030

HNO3/HFIP Nitration System: Unveiling the Arene–Nitronium π-Complex

HNO₃/HFIP Nitration System

Introduction

In 2018, Le Lu, Liu H., and Hua R. reported a mild, metal-free nitration of arenes using equimolar HNO₃ in 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) at room temperature, with direct UV–vis observation of the arene–nitronium π-complex.¹

The protocol avoids harsh mixed-acid conditions, employs only stoichiometric HNO₃, and delivers yields up to 98 % with high para-selectivity across both electron-rich and electron-poor arenes.¹

Background & Significance

Traditional “mixed-acid” nitration (conc. HNO₃/H₂SO₄) generates the nitronium ion at elevated temperatures but produces large acidic waste and often poor selectivity on deactivated substrates.²

Electrophilic aromatic substitution (S_EAr) encompasses nitration, halogenation, sulfonation, and Friedel–Crafts reactions, proceeding via σ-complex intermediates whose stability dictates regioselectivity.³

Role of HFIP

HFIP ((CF₃)₂CHOH) is a highly polar, strongly hydrogen-bond-donating solvent that stabilizes cationic intermediates.³

With a dielectric constant ε ≈ 16.7 and strong H-bond donor ability, HFIP creates a unique solvation shell that promotes arene–[NO₂]⁺ complexation at ambient temperature.³

The Nitronium Ion

In HFIP, [NO₂]⁺ is further stabilized by hydrogen-bond networks, enhancing its electrophilicity toward arenes.⁴

Experimental Methodology

Arenes and equimolar HNO₃ are stirred in HFIP at 20–25 °C under air. Reactions complete in 1–4 h, and nitroarenes are isolated by extraction and silica chromatography in yields up to 98 %.¹

Mechanistic & Computational Insights

In situ UV–vis spectra display absorptions at ≈327, 408, and 525 nm, characteristic of the arene–[NO₂]⁺ π- and σ-complexes.¹

DFT and TD-DFT studies (Gaussian 09; M06-2X/6-311G* for geometry optimizations, CCSD(T)/aug-cc-pVDZ benchmarking, and TD-DFT M06-2X/6-311+G(2d,2p) with explicit HFIP molecules + PCM ε = 16.7) reproduced these UV–vis bands and assigned them to specific electronic transitions within the π-complex.⁵

Substrate Scope & Selectivity

Electron-rich arenes (e.g., anisole → p-nitroanisole) and deactivated substrates (e.g., chlorobenzene) undergo clean mononitration with para-selectivity > 90 % and yields up to 98 %.¹

Sustainability & Practical Implications

Only stoichiometric HNO₃ is required, reducing acidic waste compared to mixed-acid protocols, and HFIP can be recovered by distillation for reuse, enhancing green metrics.⁶

References

Le Lu, H. Liu, R. Hua. “HNO₃/HFIP: A Nitrating System for Arenes with Direct Observation of π-Complex Intermediates.” Org. Lett. 2018, 20(11), 3197–3201. doi.org/10.1021/acs.orglett.8b01028
“Nitration.” Wikipedia. en.wikipedia.org/wiki/Nitration
“Hexafluoro-2-propanol.” Wikipedia. en.wikipedia.org/wiki/Hexafluoro-2-propanol
“Nitronium ion.” Wikipedia. en.wikipedia.org/wiki/Nitronium_ion
Gaussian 09 Users Guide; M06-2X/6-311G* optimizations; TD-DFT M06-2X/6-311+G(2d,2p) with explicit HFIP + PCM. (See Supporting Information of Lu et al.)
Green Chemistry principle applied: HFIP recovery by distillation reduces solvent waste. Green Chem. DOI: 10.1039/d5gc02232k

Monomer-Polymer-Monomer Method - From Monomers to High-Value Molecules

Introduction

Functionalization of the benzene ring of benzofuran is synthetically challenging because direct electrophilic aromatic substitution (S_EAr) is directed to the 2- and 3-positions of the furan ring. The Monomer-Polymer-Monomer (MPM) method overcomes this bias via a concise three-step cycle—cationic polymerization, S_EAr on the polymer, and thermal depolymerization—enabling regioselective access to 5-substituted benzofurans with high theoretical atom economy.

Concept of the MPM Strategy

In the MPM approach, benzofuran monomers are first “locked” into solvent-soluble, linear polybenzofuran (PBF) chains by cationic polymerization in hexafluoro-2-propanol (HFIP) using trifluoromethanesulfonic acid (TfOH) as the initiator. This polymeric intermediate uniformly activates the benzene ring for subsequent S_EAr reactions.

Graphical Abstract

Polymerization in HFIP

Under optimized conditions (0.2 mol % TfOH in HFIP at 0 °C), benzofuran undergoes rapid cationic polymerization to yield soluble, linear PBF. Real-time UV–vis spectroscopy and TD-DFT studies reveal cationic π- and σ-complex intermediates, and ¹³C NMR shows two characteristic signals at 83.7 ppm and 49.0 ppm.

Regioselective S_EAr on PBF

The soluble PBF intermediate undergoes S_EAr with various electrophiles—acylation, halogenation, nitration, etc.—installing substituents para to the alkoxy linkage in each repeat unit with excellent regiocontrol and broad functional-group tolerance.

Thermal Depolymerization

Substituted PBF chains cleanly depolymerize at 320 °C under nitrogen to regenerate 5-substituted benzofuran monomers in high yield, completing the MPM cycle with minimal waste and high selectivity.

References

R. Hua & L. Lu, “A Monomer-Polymer-Monomer (MPM) Organic Synthesis Strategy: Synthesis and Application of Polybenzofuran for Functionalizing Benzene Ring of Benzofuran,” Asian Journal of Organic Chemistry 2021, 10(8), 2137–2142. https://www.sciencedirect.com/science/article/pii/S2193580722023868

Friday, August 18, 2017

Preparation of η5-pentamethylcyclopentadienyl rhodium(III) dichloride [Cp*RhCl2]2 - Lu Le Laboratory

Preparation of η⁵-pentamethylcyclopentadienyl rhodium(III) dichloride

Pentamethylcyclopentadienyl rhodium dichloride is an organometallic compound with the formula [(C₅(CH₃)₅RhCl₂)]₂, commonly abbreviated [Cp*RhCl₂]₂ This dark red air-stable diamagnetic solid is a reagent in organometallic chemistry.¹

Cp*RhCl₂dimer has been widely used as a homogeneous catalyst for C-H bond activation,²C-C bond formation,³ and alkenylation and annulation of arenes and heteroarenes with alkynes.^{4, 5} The synthesis of the catalyst can be readily achieved by reacting rhodium trichloride trihydrate with 1,2,3,4,5-pentamethylcyclopentadiene in hot methanol.

Chemicals

1. Rhodium trichloride trihydrate (2.0 g, 8.4 mmol, 国药集团 Sinopharm Group Co. Ltd.)

2. pentamethylcyclopentadiene (1.2 g or 1.380 mL, 8.8 mmol, Alfa Aesar)

3. methanol (60 mL)

Procedure

Rhodium trichloride trihydrate (2.0 g, 8.4 mmol, 国药集团 Sinopharm Group Co. Ltd.), pentamethylcyclopentadiene (1.2 g or 1.380 mL, 8.8 mmol, Alfa Aesar), methanol (60 mL) and a magnetic stirring bar are placed in a 100-mL round-bottomed flask fitted with a reflux condenser.

The mixture is refluxed gently under nitrogen for 48h with stirring. The reaction mixture is allowed to cool to room temperature and the dark red precipitate is filtered off in air through a glass sinter.

The red filtrate is reduced involume to 10-mL using a rotary evaporator to give more red crystals that were combined with the first crop and washed with diethyl ether (3 x 10 mL).

Air drying gives 2.25g (95% yield) of [Rh(η⁵-C₅Me₅)C1₂]₂, which is pure enough for most purposes. If required, the product may be recrystallized by dissolving in a minimum volume of chloroform, filtering if necessary, and slowly adding twice that volume of hexane.

(The procedure was modified form White et. al. )

Note: The procedure was modified from White’s et. al. work¹

References

(1) White, C.; Yates, A.; Maitlis, Peter M. (1992). "(η⁵-Pentamethylcyclopentadienyl)Rhodium and -Iridium Compounds". Inorg. Synth., 29, 2007, 228-234. doi:10.1002/9780470132609.ch53.

(2) Colby, D. A., Tsai, A. S., Bergman, R. G., & Ellman, J. A., "Rhodium catalyzed chelation-assisted C–H bond functionalization reactions." Acc.Chem. Res., 2012, 45 (6), pp 814–825

(3) Colby, D. A., Bergman, R. G., & Ellman, J. A., “Rhodium-catalyzed C− C bond formation via heteroatom-directed C− H bond activation.” Chem.Rev., 2010, 110 (2),pp 624–655

(4) Boyarskiy, V. P., Ryabukhin, D. S., Bokach, N. A., & Vasilyev, A. V., “Alkenylation of arenes and heteroarenes with alkynes.” Chem. Rev., 2016, 116 (10),pp 5894–5986

(5) Manan, R. S., & Zhao, P., “Merging rhodium-catalysed CH activation and hydroamination in a highly selective [lsqb] 4+ 2 [rsqb] imine/alkyne annulation.” Nat. Commun., 2016, 7, 11506. doi:10.1038/ncomms11506

Wednesday, May 7, 2014

BZ Reaction - Oscillating Reaction - Physical Chemistry

Purpose

1. To understand the mechanism of Belousov–Zhabotinsky reaction.

2. To determine the apparent activation energy of the reaction by potentiometry.

Computer simulation of the Belousov–Zhabotinsky

reaction occurring in a Petri dish (From Wikipedia)

B-Z reaction with indicator

Principles

Belousov–Zhabotinsky reaction is a very complex reaction and is thought to involve about 18 different steps. So it is difficult to use simple way to describe the reaction. The FKN mechanism is usually introduced to simplify the problem on description of mechanism.

FKN Mechanism

(R1) HOBr + Br^- + H⁺ → Br₂ + H2O

(R2) HBrO₂ + Br^- + H⁺ → 2HOBr

(R3) BrO₃^- +Br^- +2H⁺ → HBrO₂ + HOBr

(R4) 2HBrO₂ → BrO₃^- + HOBr + H⁺

(R5) BrO₃^- + HBrO₂ + H⁺ → 2BrO₂ + H₂O

(R6) BrO₂ + Ce³⁺ + H⁺ → HBrO₂ + Ce⁴⁺

(R7) BrO₂ + Ce⁴⁺ + H₂O → BrO₃^- + Ce³⁺ + 2H⁺

(R8) Br₂ + MA → BrMA + Br^- + H⁺

(R9) 6Ce⁴⁺ + MA + 2H₂O → 6Ce³⁺ + HCOOH + 2CO₂ + 6H⁺

(R10) 4Ce⁴⁺ + BrMA + 2H₂O → Br^- + 4Ce³⁺ + HCOOH + 2CO₂ + 5H⁺

When the concentration of [Br^-] is “higher” the main reaction path is R1-R2-R3. The total reaction equation can be represented as follow：

BrO₃^- +5Br^- +6H⁺ → 3Br₂ + 3H₂O

The product ,Br₂, is consumed through R8. The route, R1-R2-R3-R8, is called Chain A, and its total reaction equation can be written as follow：

BrO₃^- + 2Br^- + 3CH₂(COOH)₂ + 3H⁺ → 3BrCH(COOH)₂+ 3H₂O

When the concentration of [Br^-] is “lower” the main reaction path is R5-R6. The total reaction equation can be represented as follow：

2Ce³⁺ + BrO₃^- + HBrO₂ + 3H⁺→ 2Ce⁴⁺ + 2BrO₂ + H₂O

The product HBrO₂ could autocatalysis the reaction，but the concentration of HBrO₂

is restricted with R4. We call the path, R4-R5-R6, Chain B. Its total reaction equation：

BrO₃^- + 4Ce³⁺ + 5H⁺→ HOBr + 4Ce⁴⁺ + 2H₂O

Finally, the route R9-R10 is called Chain C. Its total reaction equation is as follow：

HOBr + 4Ce⁴⁺ + 3BrCH(COOH)₂+ H₂O → 2Br^-+ 4Ce³⁺ +3CO₂ + 6H⁺

After the analysis, we notice the concentration of [Br^-], [HBrO₂] and [Ce⁴⁺]/[ Ce³⁺] are periodically changing according to time. So we can use ion selective electrode to determine the concentration of bromine ion [Br^-], and use platinum electrode with SCE (standard calomel electrode) to determine the ration of [Ce⁴⁺]/[ Ce³⁺]. The apparent activation energy of the reaction can be determined by the measurement of the length of induction time at different temperature.

Chemicals

1. Cerium ammonium nitrate solution: 0.02M

2. Malonic acid solution: 0.5M

3. Sulfuric acid: 0.8M

4. Potassium bromate: 0.2M

Apparatus

1. Computer

2. HS-4 Thermostatic water bath

3. Reactor

4. Platinum electrode

5. Standard Calomel Electrode

6. Washing bottle

Procedure

1. Turn on the computer, the recorder and the circulating water of the thermostatic water bath.

2. Set up the reactor as the following picture:

3. Set the temperature of the circulating water at 20.00℃. Add 7mL of malonic acid solution,15 mL of potassium bromate solution, 18 mL sulfuric acid solution in the clean reactor. Turn on the stirrer and put the electrodes in the reacting mixture. After the read of the potential is stable, add 2 mL of cerium ammonium nitrate solution in to the reactant.

4. Observe the color changing. After the oscillations appear 6~8 times, stop recording and save the data. Raise up the temperature of the circulating water for 3℃ and repeat the step 3~step 4 until finish records.

Experimental Record

Raw Data

Figure 1. 20.00℃

Figure 2. 23.00℃

Figure 3. 26.00℃

Figure 4. 29.00℃

Data Process

Table 1. Data Process

Reaction T(K)	Induction Time t(s)	Oscillating Period t’(s)
293.15	625.02	123.69
296.15	493.17	79.80
299.15	407.08	73.80
302.15	330.67	54.21

Draw the diagrams of ln(1/t_induction)-1/T and ln(1/t_oscillating)-1/T and do linear fit to find the slopes. And then find the E_induction、E_oscillating from the slopes.

Figure 5. ln(1/t_induction)-1/T

Figure 6. ln(1/t_oscillating)-1/T

According to Arrhenius equation, the apparent activation energy can be deduced：

Table 2. Apparent Activation Energy

Slope_induction	Slope_oscillating	Apparent E_{a (}_induction₎ (kJ/mol )	Apparent E_{a (}_oscillating₎(kJ/mol )
-6207	-7545	51.605	62.729

References

[1] 傅献彩, 沈文霞, 姚天扬. 物理化学, 上册欧4 版. 北京:高等教育出版社, 1990:144.

[2] 清华大学化学系物理化学实验编写组. 物理化学实验. 北京：清华大学出版社, 1991.

[3] Robert C. Wcast Handbook of Chemistry and Physics. Physics. 58^th ed. Ohio: CRC Press, 1977.

[4] 朱文涛. 物理化学. 北京：清华大学出版社，1995.

Monday, April 28, 2014

Acid Catalyzed Iodination of Acetone - Physical Chemistry - Lu Le Laboratory

Purpose

1. To determine the order of the reaction of iodine-acetone.

2. To determine the rate constant at an assigned temperature

Principles

Acid catalyzed iodination of acetone is a complex reaction. The rate law for overall reaction cannot be determined from the balanced equation for the reaction but from experiments.

When an aqueous iodine solution is reacted with acetone in the prescence of an acid, the yellow color slowly fades as the iodine, I₂, is consumed. The products of the reaction are iodoacetone and hydrogen iodide. Hydrogen ion is a catalyst for this reaction. The mechanism of the reaction is as follow:

Hydrogen ion participate in the reaction as a catalyst in step on and step two and also be produced as a product in step three. This kind of reaction is called an autocatalysis reaction. The rate equation can be represented as follow：

The reaction progress can be tracked by the determination of the concentration of iodine and triiodide ion.

For the reaction, the K^-=700 and the absorbance of the solution can be represented as follow：

A = A(I₃^-)+A(I₂) = ε(I₃^-)L[I₃^-] +ε(I₂)L[I₂]

When we set the wavelength of the light source of the spectrometer at 565nm，the molar absorbance of I₂and I₃^- is equal: ε(I₃^-) = ε(I₂)：

Absorbance = ε(I₃^-)L[I₃^-+ I₂]

Since the concentration of acetone and hydrochloric acid is much larger than the concentration of iodine/triiodide ion so we can assume the concentration of acetone and acid as a constant at the beginning of the reaction:

r = k[A]^α[I₃^-]^β[H⁺]^δ = k[A]^α[H⁺]^δ = constant

Finally, we can figure out the reaction order and the activation energy of the reaction from Arrhenius equation:

Chemicals

1. Iodine/KI solution (standardized): 0.02134M

2. Acetone aqueous: 3.3738M

3. Hydrochloric acid: 1.436M

4. Distilled water

Apparatus

1. Computer

2. 722S Spectrometer

3. Cuvette

4. Thermostatic water bath

5. Pipette

6. Dropper

7. Washing bottle

Procedure

1. Calibration the spectrometer with distilled water before use.

2. Turn on the thermostatic water bath and set the temperature at 25℃.

Put the vessels with distilled water, acetone aqueous, hydrochloric acid, iodine solution in the water bath for at least 10 minutes.

3. Measurement the εL value of iodine solution:

Turn on the 722S spectrometer and warm it up for at least 10 minutes. Put the cuvette with d.d. water into the spectrometer as a blank. Pour 25.00mL iodine solution into a 25mL volumetric flask and dilute to the mark line with distilled water. Rinse the cuvette with the solution for twice, and add the solution to the two-third full of the cuvette, and determine the absorbance with the spectrometer.