A HydroâFluoro Hybrid Compound ... - ACS Publications

Downloaded by UNIV MASSACHUSETTS AMHERST on September 27, 2012 | http://pubs.acs.org Publication Date: January 11, 2007 | doi: 10.1021/bk-2007-0949.ch010

Chapter 10

A Hydro-Fluoro Hybrid Compound, Hermaphrodite, Is Fluorophilic or Fluorophobic? A New Concept for the Fluorine-Based Crystal Engineering 1

1

2

1

T. Ono , H . Hayakawa , N. Yasuda , H . Uekusa , and Y. Ohashi

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1

Research Institute of Instrumentation Frontier, National Institute of Advanced Industrial Science and Technology (AIST), 2266-98, Anagahora, Shimoshidami, Moriyama-ku, Nagoya, Japan Department of Chemistry, Tokyo Institute of Technology, 1-1, Ookayama, Meguro-ku, Tokyo, Japan 2

We proposed a new concept of the fluorine-based crystal engineering based on the hydro-fluoro hybrid compounds, R X-R , of which structures were featured by the symbiotic name Hermaphrodite having a hydro-segment (R ;Hermes) and perfluoro-segment (R ;Aphrodite) connected by a junction (X). Hermaphrodites aligned in a way to face Hermes to Hermes and Aphrodite to Aphrodite, resulting in the segregated hydro -and fluoro-layers in the crystal packing. The origin of this nano-scale phase separation was discussed from both fluorophobic and fluorophilic interactions. A systematic study of the Hermaphrodites having functional groups with stronger intermolecular interactions revealed the universal nature of the Hermaphrodites for the high potential to the material design. H

F

H

F

170

© 2007 American Chemical Society

In Current Fluoroorganic Chemistry; Soloshonok, V., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2007.

171


Introduction The term crystal engineering was coined by Schmidt about three decades ago (7). His seminar work published in 1971 (2) gives an idea that too difficult task for the prediction of the crystal packing makes the solid state chemistry as empirical. The overwhelming difficulty has been emphasized in many ways and the simplest abstract composed of ' N o ' appeared in the wellknown journal. (3). The famous example is the packing of trimesic acid which totally betray our expectation which is aesthetically driven by the molecular symmetry, but the reality is a complex supramolecular assemblage with an intertwined zigzag catenane-like structure with a very large Z of 48 (space group C2/c) (4). Even a simplest shape with perfect symmetry, sphere, packed into fee or bec in the pressurized core in the earth is just recently discussed (5). As is stated by Schmidt, the crystal engineering has developed since then by mainly focusing on strong molecular interactions such as ionic or hydrogen bonds. To use such strong intermolecular interactions for the molecular assemblage is quite reasonable because the crystallmity of the concerned materials is essentially important and the substance having only weak intermolecular interactions is generally in poor of crystallinity. With the increasing attention of the supramolecular assemblage, the crystal engineering has been recognized as one of the branch of the supramolecular chemistry. The main player for the packing design has been hydrogen bonds due to a rather strong interaction and at the same time the directionality of the bond. The directionality of the hydrogen bond gives the bases for the life to do many sophisticated orthogonal reactions in a very complex soup of organic substances like a blood or a body fluid where the three dimensional architecture of the hydrogen bond doners and acceptors gives a molecule the ability of the highly sophisticated recognition in a high specific manner such as the bioactive molecule-to-receptor, enzyme-to-substarate, or antigen-to-antibody, the base-pair in D N A . This kind of beauty of recognition through hydrogen bond architecture has fascinated many researchers and driven the scientists toward the crystal engineering. From the directionality point of view, one more directional bond, a coordinate bond, has been intensively investigated in connection with the supramolecular chemistry because this bond is strong enough to construct a molecular architecture. The latest addition to this category, directional and strong intermolecular interactions, is halogen bonds of which use for the crystal engineering was initiated by Resnati (6). This technology has been expanding rapidly into various fields of more general supramolecular chemistry (7). The nature of the halogen bonding is recently reviewed by Resnati as a world parallel to hydrogen bonding (#) and by others (9). This innovative supramolecular technology based on the halogen bonding is involved elsewhere in this book.



172 We described the new concept of the fluorine-based crystal engineering in the previous A C S series (70) in which how the new concept works is presented by an example of the design of the topochemically controlled solid state polymerization of the fluorine-containing hybrid diyne derivatives. With this successful example in hand, here we would like to shed a light on the phenomena, solid state nano-scale fluorous phase separation, which underlies in the new concept of the fluorine-based crystal engineering. Before entering into the crystal engineering, we will see the present technological circumstances in the next section, where we will confront with the wave of the revolution of X-ray instruments.

The technical revolution of X-ray analysis for the crystal engineering: The X-ray crystallography is undoubtedly the essential technique for the crystal engineering. The unceasing efforts to develop this technology have shortened the time required for the analysis. The computational aspect is, of course, important but the most recent eye-opening development is found in the data collection method. The data collection time changed from months order in the film age to the minutes-to-hours order in the recent imaging plate (IP) or C C D detector age (77), and the latest feat comes in the second order by microstrip gas chamber (MSGC) (72). The first analysis by laboratory level equipment with an IP detector appeared in 1996, claiming only one hour data collection time for the standard cytidine crystal (75). The M S G C originally proposed in 1988 as a new type of a proportional gaseous detector as a neutron detector was applied for the rapid analysis of X-ray crystal structure. The champion data by this new M S G C detector is only 2 seconds which was obtained on the ammonium bitartrate crystal by using continuous rotation photograph method (14). The R value was a little bit large but in an acceptable level of 7.9 (I>2c(I)). As is mentioned above, the X-ray work was once a very time-consuming process, thus this process was apparently in the rate-determining step, hampering the crystal engineering as a practical approach. However, now both X-ray and synthetic processes are comparable in the time scale so that the X-ray results feed back to the design and synthesis of the molecules and the back-and-forth cycles are repeated in a reasonable time scale until the desired goal is attained. The appearance of the high sensitivity detector has one more important meaning in the crystal engineering because the size of the crystals amenable for the X-ray analysis becomes as small as 100 micron order which was not possible before. The advancement on these two factors, rapidness and a crystal size, are very important in a practical sense of crystal engineering. It has been increasingly recognized that the rational design for the functional materials by nano-level


173 control with the supramolecular assemblage (15). In this regards it should be again emphasized here that the time is ripe for the crystal engineering.


Supramolecular assemblage of the Hermaphrodite molecules We have recently devised new fluorine-based crystal engineering which based on the hydro-fluoro hybrid compounds of which structures are represented by R - X - R . The hybrid compounds have both natures originated from hydro (R ) and fluoro (R ) segments at the same time so that the fluorophobic nature coming from the hydro segment and the fluorophilic nature from the fluoro segment. This kind of dichotomous combination of nature for which we call such hybrid compounds as Hermaphrodites meaning both sexes both anti in one body (Scheme 1). H

H

F

F

Scheme 1. The structure of the hydro-fluoro hybrid compounds, Hermaphrodites, Rh-X-R . F

Due to these unique structures of the compounds, Hermaphrodites aligned in the crystal, resulting in the segregation of hydro and fluoro segments like a nanoscale fluorous phase. We have intensively investigated the topology-packing relationships by analyzing X-ray structures of a series of Hermaphrodites and showed the topochemical control of solid phase polymerization. We would like to propose that such a nano-scale fluorous phase formation could be used as a main driving force for the supramolecular alignment in the crystals. The most essential schemes to explain our idea are recited here for convenience (Scheme 2,3). In Current Fluoroorganic Chemistry; Soloshonok, V., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2007.

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R -0=, H

CHO


(97.5%)

(94.8%)

(98.3%)

(41.2%)

(quantit)

(97.7%)

(quantit.) f

y

Y

(90%)

(99.2%)

(99 8%)

6

(97.8%)

(quantit.)

)

^ (96.0%^^

{W9%

F 3

Q

Sf

X F 3

R = F

(quantit)

(quantit.)

CF3 Af

3

Scheme 2. Structures of the Hermaphrodies, Rp-O-Rfr

Scheme 3. Packing Motifs found in the Hermaphrodites, Rp-0-R , where the Aphrodite part is perfluoropropen trimers and the Hermes parts are described in Scheme 2. H


175

One of the representatives Type 1 packing of the Hermaphrodites is shown below (Fig.l).


Predominant F-F intermolecular interactions in the molecules: Fluorophilic Nature is more important?

Hermaphrodite

The intermolecular interaction is undoubtedly a main player for the supramolecular assemblage, and thus, no exception for the Hermaphrodites too. There are two parts in Hermaphrodites, H-segment (Hermes part) and F-segment (Aphrodite part) so that there are three combinations for the intermolecular interactions, two homolytic ones on H-segments and F-segments, and one heterolytic one between H - and F-segments. The terms, lipophilic and fluorophilic, are the terms for between the H-segments and for between the Fsegments and both are favorouble for the nano-scale phase separation. These words focus on the philic nature of Hermes and Aphrodite, but there are reverse expressions from the phobia world, fluorophobic and lipophobic, in the heterolytic relations between Hermes and Aphrodite parts. Therefore, there could be a phobic relationship between the Hermes and Aphrodite. However, the electron distributions on the C-H and C-F bonds are polarized in a mirror image, thus, with a positive surface on the Hermes and a negative surface on the Aphrodite, suggesting the philic interctions. Moreover, the recent development of the understanding of the hydrogen bonding has diversified into a variety of unconventional hydrogen bondings, including X - H — F - C type (76). Therefore, the polar effect and new type hydrogen bonding suggest the circumstantial affairs between Hermes and Aphrodite, but still controversial and thus set aside at the moment and the conventional rather obscure phobic ideas of fluorophobicity and lipophobicity are taken up as they are. A l l in all, both philic and phobic ideas work in unison, thus Hermes and Aphrodite separate to create the hydro and fluoro layers, eventually occurring in the nano-scale fluorous phase separation. Recently, intermolecular F-F interactions are attracting a wide attention because they play increasingly important roles in various ways for the molecular recognition (77) and the supramolecular assemblage of the molecules, especially in relation to the perfluoro system. The new crystal engineering developed by Resnati is based on the halogen bonding, but the technological base is in the perfluoro system so that the F-F interactions play an important role accordingly. The recent upsurging demand for understanding the F-F interactions in crystal engineering and also in the fluorous chemistry should be behind the scenes for the publication of the review titled "Fluorine in crystal engineering" (18). According to the review, there are 788 compounds with F-F distances within 0.30 nm in the CSD and only 13 of them show F-F contacts of type II which is claimed to be more attractive than type I. Furthermore, only two of them have short F-F contacts with distances smaller than the van der Waals radii (0.294 nm) but appear to be driven by the general packing and not by dispersion forces



Figure J. The Type 1 packing of the Hermaphrodites.



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between F atoms. In summary, in contrast to the other heavier halogens (79), fluorine is more involved in X - H than in X - X interactions. The other recent view is also in this line (20). In our new crystal engineering concept using Hermaphrodites, homolytic contact F-F and H-H are both philic, fluorophilic and lipophilic, and heterolytic contact H-F is phobic, fluorophobic viewed from H-side and lipophobic viewed from F-side, thus why not for nano-scale fluorous phase separation. But still remained is the question "which interactions of H-H and F-F is the main cause for the nano-scale phase separation phenomenon?" The answer may be very simple, both cooperatively, not large difference to be precise. However, in connection to this question we would like to draw your attention to an interesting reverse disorder structure found in the crystal packing of Af-(4-methoxyphenyl)F-2-(2,6-dimethylmorpholino)- propanamide (MPMPA) which gives a hint for understanding the underlying rules (Fig. 2). The disorder structure is ubiquitous in the X-ray structures of fluoro compounds, especially notoriously in the trifluoromethyl group which is rarely ordered. However, in contrast with the usual fluoro compounds, no disorder is found in the fluoro segment of M P M P A , but surprisingly in the hydro segment of phenyl group. Thus, the scaffold for the packing of this molecule is apparently in the fluoro segment, but not in the hydro segment. To the most of our knowledge, this is the second example for the reverse order-disorder structure of fluoro compounds (27). These two reverse order-disorder structures are consistent with the idea of the predominancy of the F-F intermolecular interactions in the packing of the hybrid compounds, Hermaphrodites, at least in our cases. But the story may be a little bit more complex. We believe the cumulative effect is the main cause of the segregation. It is very well known about the aggregation of the carbon nanotubes. The carbon nanotubes does not behave one by one, always act in the aggregated form due to a very strong intermolecular forces between the large surface of the carbon nanotubes. If the carbon nanotube molecule has a hydrophilic surface, then it can interact with hydrophilic solvent, thus, disperse into the hydrophilic solvent, but a carbon nanotube molecule has no ability to form such strong interactions with any other molecules so that they have no choice but aggregate each other. Fluoro- and hydro-segregation of the Hermaphrodite molecules is very much the case of this kind. Of course, there is no analogy between the molecular structures of the Hermaphrodites and carbon nanotubes, but the underlying rule for the aggregation of nanotube and segregation of Hermaphrodites are the same. If we select C , which is also notorious of aggregation, as an example instead of the carbon nanotubes, most people agree with our idea due to the very analogous shapes between the fluoro segment used in the Hermaphrodite and C60, both soccer balls, one fluoro soccer 6 0



structure.

Disordered in Hydro-moiety

Ordered in Fluoro-moiety

Hydro-Phase

Figure 2. The unusual hydro-disorderedfluoro-ordered

3

•

Fluoro-Phase

Hydro-Phase


^4 00

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ball, the other carbon soccer ball, as is seen in Fig. 3. We will see some evidence for this kind of mass effect of a large surface contact of F-F interactions in the later section of a cluster formation based on the F-F interactions.

ca. 1 nm

ca. 1 nm

Figure 3. The similarity in shape and dimension between F-part of Hermaphrodite and C ; the former is a fluoro soccer ball and the latter is a hydro soccer ball. 60

We have discussed on the predominacy of F-F interactions based on the phenomenological reverse order-disorder observations, but finally here presented is a very simple evidence for that the homolytic F-F intermolecular interactions are stronger than the H-H homolytic intermolecular interactions. The boiling point of tetrafluoromethane (b.p. -128.1°C) (22) is about 34°C higher than the one of methane (b.p. -162°C), supporting the F-F predominancy. Against such simplicity, here should be pointed out is that the phase separation should be understood as a whole by the combination of enthalpy and entropy terms.

Hermaphrodites with a hydrogen doner-acceptor functional group We showed a unique packing nature of the Hermaphrodite molecules in the previous sections and claimed its uniqueness comes from the predominant F-F interactions with the emphasis on the analogy to the aggregational nature of the carbon nanotubes and C . We would like to see how the incorporation of the stronger-bondage-creating group(s) into the Hermaphrodite linkage takes effect on the packing patterns of the Hermaphrodites. One example is already shown in the previous section as for the explanation of very rare reverse order-disorder structures about the F-segment and H segment. The nano-scale F- and H-segregations are well preserved in this 6 0



180 example in a consistent manner with the hydrogen bonding network. We would like to see i f this coexistence, satisfying the demands for the segregation and hydrogen bond net work formation, could be found by the nature in general. Otherwise, we will come across something syrupy or intractable matters for X ray analysis. We chose an amido group as the linkage between Hermes and Aphrodite because this functional group is very famous in the crystal engineering field to design the packing and at the same time from the curiosity due to the biological importance in the peptide or protein back bones. The structures investigated are rather diverse in the F-parts and a little bit monotonous in the H-parts by the freak lack of consideration at the outset. Ten compounds, la-5a, l c , 6a-6c, and 7a, out of the possible 21 compounds by the combination of seven F-parts and three H-parts, were prepared (Scheme 4). A l l the F-parts come from the perfluoro carboxylic acid fluorides which were prepared by the in-house electrochemical fluorination equipment (23). The other H-parts are commons.

FFFFF

-CONH-

HHHHH

Scheme 4. Hermaphrodite with a polar functional group, -CONH-, having ability to make hydrogen bonding. F- inside the ring denotes the ring is perfluorinated.

Unfortunately all the crystals prepared have very thin needle-shapes. Therefore, the data collection was done by the high energy X-ray beam in SPring 8. Only 7a gives somewhat fat needle crystals so that the IP detector was used for this crystal. The crystal packing of 6b was shown as the representative (Fig. 4). The amide group forms the hydrogen bond network perpendicular to the molecules. The direction of the hydrogen bond net work is along the b axis which is along the needle. One more hydrogen net work forms between the phenolic OH groups,


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parallel to the amide net work. The molecules are aligned head-to-head perpendicular to both hydrogen net works with the demand for the fluoro- and hydro-segregation structure satisfied. A l l the packing structures follows the same kind of packing patterns, thus both requirements for the nano-scale fluorous phase separation and the haydrogen bond net work formed by the amide linkage are satisfactorily found by nature in quite a general manner if considering of the diversity of the Hermaphrodite structures.

Fluoro -Phase

Hydro -Phase

Fluoro -Phase

Figure 4. The crystal packing of the representative Hermaphrodite, 6b, RpCONH-R having an amide function as a linkage group. H

It should be noteworthy to point out the morphology of the crystals. It is easily expected that the crystal grows at much a higher rate in the direction along the the hydrogen net work formed by the amide groups than in the directions perpendicular to this hydrogen bond net work because the latter directions are only driven by weak dispersion forces. This is the reason for the needle shape seen for all the Hermaphrodites having the amide linkage. In one extreme case, 6b, a few centimeters long needle-shape crystals are formed due to the two hydrogen bond net works paralleled each other, both driving the crystal growth in the same direction. This crystal morphology-Hermaphrodite structure relationship will be suggestive for designing the functional materials. Finally, the crystal packing of a Hermaphrodite molecule which is known as perfluoroalkylating reagent, so-called FITS-8, is shown as an example of the Hermaphrodites having hypervalent cationic iodine function as the linkage part which is expected for stronger intermolecular interactions (Fig. 5). The type 2 packing, but in a reverse manner on the F- and H - parts due to the topological



182

Fluorous phase

Ionic Channel

] •

]

Fluorous phase

A HydroâFluoro Hybrid Compound ... - ACS Publications

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