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Letter
The Catalytic Effect of Fluoroalcohol Mixtures Depends on Domain Formation Oldamur Holloczki, Albrecht Berkessel, Julian Mars, Markus Mezger, Anton Wiebe, Siegfried R. R Waldvogel, and Barbara Kirchner ACS Catal., Just Accepted Manuscript • DOI: 10.1021/acscatal.6b03090 • Publication Date (Web): 31 Jan 2017 Downloaded from http://pubs.acs.org on February 1, 2017
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The Catalytic Effect of Fluoroalcohol Mixtures Depends on Domain Formation Oldamur Holl´oczki,a,∗ Albrecht Berkessel,b Julian Mars,c,d Markus Mezger,c,d Anton Wiebe,e Siegfried R. Waldvogele and Barbara Kirchnera,∗ a
Mulliken Center for Theoretical Chemistry, University of Bonn, Beringstr. 4+6, D-53115 Bonn, Germany
[email protected] [email protected] b
c
Universit¨at zu K¨oln, Greinstrasse 4, D-50939 K¨oln, Germany
Institute of Physics, Johannes Gutenberg University Mainz, 55128 Mainz, Germany d
Max Planck Institute for Polymer Research, Ackermannweg 10, 55128 Mainz, Germany
e
Institute of Organic Chemistry, Johannes Gutenberg University Mainz, Duesbergweg 10-14, 55128 Mainz, Germany
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Abstract: In the present contribution we investigated catalytically active mixtures of 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) and aqueous H2 O2 by molecular dynamics simulations. It is clearly observable that the HFIP molecule strongly binds to the H2 O2 , which is necessary for the desired catalytic reaction to occur. Upon the addition of the substrate cyclooctene to the solution, this interaction is enhanced, which suggests that the catalytic activity is increased by the presence of the hydrocarbon. We could clearly observe the microheterogeneous structure of the mixture, which is the result of the separation of the hydroxyl groups, water, and H2 O2 from the fluorinated alkyl moiety in the form of large domains, which span through large areas of the system. The hydrocarbon, however, does not fit into either one of these two microphases, and it forms separate aggregates in the macroscopically homogeneous liquid; creating thereby a triphilic mixture. The latter kinds of aggregates are mostly surrounded by the fluorous moieties, and therefore the H2 O2 has to move from the polar through the fluorous domain to be able to react with the cyclooctene. Accordingly, the present reaction should be described figuratively as a phase transfer or an interfacial reaction, rather than a homogeneous liquid phase process.
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Epoxidation reactions are often carried out with H2 O2 as oxidant. Since this reagent is abundantly available, and it offers the convenience of having only water as side product in the process, this reaction is desirable for designing sustainable synthetic pathways. In this approach the catalytic activity of fluoroalcohols, e.g. 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) is of great interest (Figure 1),1–4 as in the presence of this substance an up to 105 fold acceleration of the reaction has been observed.2 This allows more efficient synthetic approaches that can be performed under milder conditions compared to the alternatives.
Figure 1: Reaction of alkenes with H2 O2 (HFIP = 1,1,1,3,3,3-hexafluoro-2-propanol).
The mechanism of the HFIP-catalyzed epoxidation has been investigated in a series of DFT studies,2, 3, 5 which together with detailed reaction kinetics measurements2, 3 revealed that the catalytic effect lies in the strong hydrogen bond donating ability of the HFIP’s hydroxyl group. In fact, the formation of clusters around the reactant H2 O2 by three HFIP molecules is a decisive factor for the effective decrease of the reaction barrier in the rate-limiting step (for an example, see Figure 2).2, 3 Regarding these well-defined structural motifs, and keeping in mind that H2 O2 is applied in aqueous solutions, it is important to point out that HFIP-water mixtures exhibit a particularly intriguing microscopic structure.6 It has been observed that the polar -OH groups of the alcohol and the water segregate from the (CF3 )2 CH- moieties.6, 7 This separation results in a “microheterogeneous” liquid structure of two kinds of domains (polar and fluorous); while macroscopically the liquid is still homogeneous. Although the structure of HFIP-water mixtures have been investigated,6, 7 no work has been devoted to connect this curious liquid structure to the reaction mechanism. However, considering that highly ordered clusters have to occur in the solution if the alkene is to undergo a facile oxidation, characterizing the local and the long range structure of these systems — especially the relative location of the substrate, reactant and catalyst — is crucial to underACS Paragon3Plus Environment
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Table 1: Composition (number of each molecule types, N; molar percentage n/n %), physical size and density (ρ) of the investigated systems I-V, and the calculated diffusion coefficients of each components (D) in these simulations. I
II
III
IV
V
N (HFIP)
1980
1980
1980
1980
1980
N (H2 O)
0
1980
1782
1584
1782
N (H2 O2 )
0
0
198
396
198
N (cyclooctene)
0
0
0
0
198
n/n % (HFIP)
100
50
50
50
47.6
n/n % (H2 O)
0
50
45
40
42.9
n/n % (H2 O2 )
0
0
5
10
4.8
n/n % (cyclooctene)
0
0
0
0
4.8
cell vector /˚ A
70.94
75.58
75.60
75.71
78.27
ρ /g cm−3
1.55
1.42
1.43
1.43
1.36
D (HFIP) /10−10 m2 s−1
1.0
4.8
3.9
3.5
4.4
D (H2 O) /10−10 m2 s−1
−
6.6
5.2
4.4
5.8
D (H2 O2 ) /10−10 m2 s−1
−
−
3.9
2.9
4.0
D (cyclooctene) /10−10 m2 s−1
−
−
−
−
6.2
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Figure 3: Radial Distribution Functions (RDFs) of the hydrogen bonds between the hydrogen bond donor and acceptor molecules in the investigated systems (A: O(HFIP)· · · H(HFIP); B: O(water)· · · H(HFIP); C: O(water)· · · H(water); D: O(H2 O2 )· · · H(water or HFIP)). The arrows show the increase in the first peaks in the analogous RDFs by the addition of cyclooctene, thus the difference in first peak heights between systems III and V. (HFIP: 1,1,1,3,3,3-hexafluoro-2-propanol. System I: pure HFIP, system II: HFIP-water mixture; system III and IV: HFIP-water-H2 O2 mixtures, system V: HFIP-water-H2 O2 -cyclooctene mixture. For the exact compositions, see Table 1.)
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Applying our recently developed domain analysis,13 it is possible to assign groups of atoms to different subsets (see Figure 4), and then to observe if these subsets are neighboring each other in a systematic manner by forming larger domains in the liquid. The most important information that we can obtain from the domain analysis is the existence and the size of the domains formed by the defined subsets. According to the data presented in Table 2, the fluorinated moieties — defined as composed of only the (CF3 )2 CH- units (see Figure 4) — form a continous microphase. The polar -OH substituents of the HFIP molecule are much smaller, hence they cannot form an uninterrupted microphase, only smaller clusters of ca. 10 hydroxyl groups.
Figure 4: Definition of the subsets that form the domains in system I-V (left); and a snapshot of the simulation box of system V as an example, with the domain subsets colored accordingly (right). On the right it is clearly visible that the cyclooctene molecules form small clusters in the liquid, which are surrounded by the fluorous moieties.
By the addition of water, a hydrogen bond acceptor stronger than HFIP2, 3, 5 is present in system II, and therefore the occurrence of the HFIP-HFIP hydrogen bonds drops (Figure 3A). Instead, the O(water)-H(HFIP) interactions rise, showing a sharp and high peak in the corresponding RDF (Figure 3B). Comparing the height of the first peaks in all possible RDFs, the occurrence of the hydrogen bonds decreases in the following order: O(water)H(HFIP) >> O(HFIP)-H(HFIP) > O(HFIP)-H(water) > O(water)-H(water). Considering ACS Paragon7Plus Environment
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Table 2: Average number of the three kinds of domains, and the average number of molecules in each domain (in parenthesis) over the simulation of systems I-V. For the definition of the domain subsets, see Figure 4. I
II
III
IV
V
polar
193.10 (10)
7.24 (547)
8.28 (478)
8.98 (441)
13.54 (292)
fluorous
1.00 (1980)
1.00 (1980)
1.00 (1980)
1.00 (1980)
1.00 (1980)
–
–
–
–
48.91 (4)
non-polar
Figure 5: Radial Distribution Functions (RDFs) for inspecting the interactions with the cyclooctene molecules in system V (CoM: Center of Mass).
the weak hydrogen bond acceptor property of HFIP, it seems rather unexpected that the water-water interplay exhibits the lowest first peak. This is most likely due to the extreme hydrogen bond donor strength of HFIP, which occupy the hydrogen bond acceptor sites of the water, leaving only the HFIP’s oxygen atoms as acceptors, with which the water molecules can interact. The extent of the hydrogen bond network can be shown by the size of the corresponding polar domains (Table 2). The fluorous domain shows no change in size compared to that in system I, but at the same time, the number of individual polar domains decreases, showing an increased clustering of polar groups. Regarding this structural change from system I to system II, it is interesting to see the changes in the diffusion constants ACS Paragon8Plus Environment
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of the HFIP molecules. Interestingly, the addition of the water facilitates the movement of the fluoroalcohol molecules by a factor of ca. 5 (Table 1). This interesting finding might be explained by the more extensive and more flexible hydrogen bonding network that the presence of the water introduces to the system. Systems III and IV are already of practical relevance, since they provide direct information on how the oxidizing agent H2 O2 is solvated in the solution. The occurrence of the HFIPH2 O2 interactions are necessary for the desired catalytic effect of the fluoroalcohol. The peaks for the O(H2 O2 )-H(HFIP) RDFs are indeed high (Figure 3D), but those representing the O(H2 O2 )-H(water) interplay are as well, although at slightly larger distances. Since replacing some of the water molecules by H2 O2 introduces new hydrogen bond acceptor sites, all hydrogen bonds between the water and the HFIP are decreased. Similarly, upon increasing peroxide concentration, all first peaks in the hydrogen bond-related RDFs show a slight decrease. Similarly to the previous cases, in systems III and IV the fluorous domain is in both cases a single entity throughout the whole simulation. Compared to system II, systems III and IV show slightly more dispersed polar domains, as their average count exhibits a slight increasing trend by increasing H2 O2 concentration. Nevertheless, the polar microphases are still very large (small domain count), and they incorporate many of the hydroxyl groups of the HFIP, water and H2 O2 molecules. It is well-known that in the presence of a non-polar (e.g. hydrophobic) particle, the hydrogen bond network in the solution is altered.14 This is observable in our case as well, as the presence of cyclooctene increases all the first peaks of the H-bond related RDFs in system V compared to system III (Figure 3A-D). In fact, this effect makes the O(H2 O2 )-H(HFIP) the most occurring hydrogen bond of the H2 O2 , which is outperforming the O(H2 O2 )-H(water) over two times in g(r) (Figure 3D). Considering the importance of the HFIP-H2 O2 interplay,2, 3, 5 these interesting results show that the cycooctene itself enhances the catalytic effect of the HFIP, and therefore the term “polarophobic support” can be coined. The extent of this polarophobic support is indicated by the arrows in Figure 3, as the difference between the first peak heights of the analogous RDFs in systems III and IV. This effect also has ACS Paragon9Plus Environment
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Table 3: Neighbor counts and surface coverages (in parenthesis) of the molecules in system V (e.g.: HFIP@H2 O: the number of HFIP molecules that on average neighbor a water molecule, and the average percentage of the surface of H2 O, which is covered by HFIP molecules). HFIP@HFIP
11.0 (66.9%)
H2 O@HFIP
5.4 (22.2%)
H2 O2 @HFIP
0.6 (2.3%)
cycoct@HFIP
1.2 (8.6%)
HFIP@H2 O
6.0 (76.8%)
H2 O@H2 O
2.2 (14.6%)
H2 O2 @H2 O
0.4 (4.4%)
cycoct@H2 O
0.4 (4.1%)
HFIP@H2 O2
6.3 (56.1%)
H2 O@H2 O2
4.0 (31.2%)
H2 O2 @H2 O2
0.9 (10.1%)
cycoct@H2 O2
0.4 (2.5%)
HFIP@cycoct
12.1 (71.2%)
H2 O@cycoct
3.7 (9.9%)
H2 O2 @cycoct
0.4 (0.9%)
cycoct@cycoct
2.4 (18.1%)
a notable influence on the dynamics of the system, since the diffusion constants of each component slightly increase by the addition of the cyclooctene (cf. the values for system III and V in Table 1). However, for the reaction to occur the substrate must also collide often with the oxidantHFIP assemblies, thus, the solvation of the cyclooctene molecule must also be characterized. Considering the RDFs between the center of mass (CoM) of the cyclooctene and all components in the mixture (Figure 5), the highest peak was provided by the cyclooctenecyclooctene interactions, which suggests a certain aggregation in the liquid. Apart from this,
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only the interplay with the HFIP molecules exhibits a peak in the corresponding RDF at short distances that reaches to a value higher than g(r) = 1, see red curves in Figure 5. Analyzing the domains by defining the cyclooctene as a non-fluorous, but non-polar subset, confirms that the cyclooctene molecules tend to form small aggregates in the solution, in agreement with the aformentioned high peak in the CoM-CoM RDF (Figure 5). The average number of molecules in these domains of the system at a given time is on average four (see domain count data in Table 2). Accordingly, system V, representing a realistic reaction mixture, shows a triphilic character, having three kinds of clearly separated microphases. Interestingly, this behavior also results in a more significant dispersion of the polar moieties, shown by their higher domain counts compared to systems II-IV (Table 2).
Figure 6: The structure of compound 1, an example for TEFDDOLs.
TEFDDOLs15 are highly fluorinated analogs of the tartrate-derived TADDOLs.16 TEFDDOLs such as compound 1 (Figure 6) harbor, in an intramolecular fashion, highly polar hydroxyl groups, the fluorinated substituents (here CF3 ) at the tertiary alcohol functionality, and non-polar alkyl residues at the acetal carbon (here CH3 ). It is interesting to note that in the crystal structure of 1, a very distinct separation of these three domains is clearly visible (Figure 7). The hydroxyl groups form a long H-bonded ribbon, coated by the fluorous domains, the aggregation of which results from attractive CF3 -CF3 dispersion interaction.2, 17, 18 The fluorous domain is followed by the low-polarity acetal/hydrocarbon layer. Overall, an infinite ABC-CBA-ABC-CBA arrangement results. Accordingly, although the collision of the cyclooctene substrate and the H2 O2 -HFIP oxidantcatalyst assembly is crucial for the reaction to occur, these two entities are situated in separated microphases of the system. Hence the process can be described figuratively as ACS Paragon11 Plus Environment
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Figure 7: The segregation of polar (hydroxyl groups, red), non-polar (the CH3 units and the rings, grey) and fluorous (CF3 -C-CF3 units, green) phases in the crystal structure of 1 (cf. the liquid structure of system V in Figure 4, for the Lewis structure see Figure 6).15
a phase transfer or an interfacial reaction, rather than a homogeneous catalytic process. Thus, characterizing the local environment of the molecules involved in the reaction is not enough to understand the process in details, and to that end the interplay between these microphases at their interfaces has to be considered. The Voronoi analysis (see Applied Methods section) provides information also on the neighbor counts (number of the certain kind of molecules sharing a face with the reference molecule at a time) and surface coverage (the percentage of the surface of each chosen cell that is shared with a certain kind of subset or molecule) for all particles in the system, based solely on the atomic positions, see Table 3. Clearly, it should be kept in mind that the number of neighboring Voronoi cells holds geometrical information on the system, which does not necessarily corresponds to the bonding situation of these molecules. Considering these numbers together with the surface coverages, however, can provide information on the arrangement
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of the molecules with respect to each other in the liquid, which is clearly crucial for understanding the catalytic process in details. It is interesting to observe that water and H2 O2 covers only 10% of the surface of cyclooctene (Table 3). Instead, the clusters of the cyclooctene molecules are surrounded mainly by the HFIP molecules. According to the RDFs in Figure 5 (cf. red dashed and red dotted-dashed lines), the HFIP molecules are pointing with their fluorous moieties toward to cyclooctene clusters, forming a micelle-like structure. The hydroxyl groups of the HFIP molecules are, on the other hand, pointing outside of the micelle, and are covered by water and H2 O2 molecules at a larger portion of their surface (Table 3). According to these results, a layer-like structure emerges, where the polar oxidant has to penetrate through a layer of the fluorinated microphase to the non-polar domain, in order to allow the ocurrence of the reaction (Figure 8).
Figure 8: Proposed solvation pattern for cyclooctene in system V.
Figure 9 shows the SAXS patterns of pure and cyclooctene saturated water/HFIP mixtures. At low momentum transfer q, the scattered intensity I(q) of both samples monotonously decreases. A similar behavior has been observed by SANS and SAXS for water/HFIP mixtures with HFIP concentrations below 35%.19, 20 They are attributed to composition fluctuations in the multi-component mixtures, forming polar and fluorous domains. For quantitative analysis, a Lorenzian I (q) = I0 1−ξ12 q2 + const. was fitted to the low q < 4.5 1/nm part of the experimental data.21 From the fits we extract a correlation length ξ of 0.95 nm and 0.58 nm for the pure and cyclooctene saturated water/HFIP mixtures respectively. This ACS Paragon13 Plus Environment
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and fluorinated alcohols in general. It is worth mentioning that this kind of structuring on the nanoscale might be also a rationale for the unusual anodic conversions in such solvents, as the recently reported C,C and N,N coupling reactions can only be performed in this unique solvent system.10–12
Applied methods The simulations were performed by the LAMMPS program,22, 23 using previously established models for the HFIP6 and H2 O2 ,24 the SPC/E model for water,25 and the OPLS parameters for cyclooctene.26 In the simulations a 1 fs timestep was applied. The temperature and the pressure were regulated by Nose–Hoover chain thermostats and barostats, respectively.27, 28 The initial simulation boxes were created to have a density of 1 g cm−3 . Then a 1 ns long N pT simulation was performed (T = 298 K, τT = 100 fs, p = 1 bar, τp = 1000 fs), in which the volume was averaged over the last half nanosecond. The resulting cell vectors and the corresponding densities of the simulation boxes are listed in Table 1. After 1 ns of further equilibration in the N V T ensemble (T = 298 K, τT = 100 fs), 10 ns of production run was conducted, along which the atomic coordinates were saved in every 1 ps. The obtained trajectories were analyzed by the TRAVIS program.13, 29 The microheterogeneous structure of aqueous HFIP mixtures was studied by small angle X-ray scattering (SAXS). Samples saturated with cyclooctene were prepared at 40◦ C, and subsequently equilibrated for 12h at the measurement temperature of 21◦ C to allow segregation of excess cyclooctene. Data was obtained on a home-built multi-purpose diffractometer in transmission geometry. The monochromatic X-ray beam (Cu Kα , (λ = 0.154nm), flux 4 107 phot./s) from a rotating anode X-ray generator (Rigaku MicroMax 007) with Cu target was focused on the sample position by a confocal curved multilayer optic (Osmic) and collimated by a set of two 4-jaw apertures. Samples were contained in 0.7 mm thick sealed glass capillaries. Scattering data was recorded at a sample-detector distance of 57 cm by a microstrip solid-state detector (Dectris Mythen) using 8000 s illumination time per measurement. SAXS patterns (I (q)) vs. total scattering vector (q = ACS Paragon15 Plus Environment
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angle 2θ, were corrected for the empty cell background. The domain analysis is performed via radical Voronoi tesselation, where each atom is assigned to a cell, composed of all points in space that are closer to the given atom minus an atom specific distance, than to the other atoms in the system. By merging the atomic cells for each subsets a polyhedron is obtained. Two subsets are defined as neighbors if they share a face. By assessing if these subsets are connected to each other in a systematic manner, microheterogeneity can be observed and characterized.
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