Halogen Bonding in Iodo-perfluoroalkanePyridine Mixtures

Halogen Bonding in Iodo-perfluoroalkanePyridine Mixtures

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Neither the thermodynamicnor microscopicpicture emerging from this study is entirely clear. Nonetheless, taken together, the data presented here lead us to the following hypothesis: 2-Iodo-perfluoropropane has a more ordered liquid structure

Figure 5. x-Marginal spectra for a Xpy ) 0.5 mixture of pyridine and 1-iodo-perfluorohexane collected at several different temperatures. As the temperature is increased, the equilibrium shifts in favor of the free pyridine relative to the halogen bonded complex. This trend with temperature indicates ∆(1)HL < 0.

Figure 6. x-Marginal spectra for a Xpy ) 0.6 mixture of pyridine and 2-iodo-perfluoropropane collected at several different temperatures. As the temperature is increased, the equilibrium shifts in favor of the free pyridine relative to the halogen bonded complex, but not nearly to the degree for the 1-iodo-perfluoroalkanes. This trend with temperature indicates ∆(2)HL < 0 but |∆(2)HL| < |∆(1)HL|.

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due to the nature and degree of self-halogen bonding than the 1-iodo-perfluoroalkanes at the molecular leVel. This manifests itself thermodynamicallysuch that 2-iodo-perfluoropropanehas both an oVerall less faVorable (but not unfaVorable) enthalpic and an oVerall less unfaVorable entropic contribution to the free energy of halogen bonding for the bulk solutions. This ultimately leads to a significantly more favorable free energy for halogen bonding between pyridine and 2-iodo-perfluoropropane versus pyridine and the 1-iodo-perfluoroalkanes. Here, self-halogen bonding means halogen bond formation between the electron-rich fluorine on one iodo-perfluoroalkane molecule and the σ-hole of the iodine on an adjacent iodo-perfluoroalkane molecule of the same type.

There are three main contributors to the overall free energy of halogen bonding for these systems. These are illustrated in Figure 7, which shows the Hess’ law cycle used to develop the above hypothesis, as (i) free (nonhalogen bonded to pyridine) iodo-perfluoroalkane and pyridine taken from solution to the

∆hb (x)SL), and (i) solvation of the halogen bonded complex

(x)HL is slightly more negative for 2-iodo-perfluoropropane than for the 1-iodoperfluoroalkanes, as implied by the blue shift in the ring breathing mode; however, ∆hb(2)HL = ∆hb(1)HL. It also follows for

∆s(1)HL and ∆s(2)SL = ∆s(1)SL because the pyridine/iodo-perfluoroalkane complex results in an occupied iodine, not allowing for a self-halogen bond interaction. Accepting these arguments as reasonableleaves∆vHL and ∆vSL to determinethe differences in thermodynamic behavior of the 2- versus 1-iodo-perfluoro- alkanes.The temperaturestudiessuggestboth ∆(2)HL and ∆(1)HL

Since ∆vHL and ∆vSL determine the difference in thermodynamic behavior, and since ∆v(2)HL > ∆v(1)HL, the right-hand side is positive. Thus,

> ∆v(1)SL > 0) provide the basis for our conjecture about the molecular level structure. They suggest that there is stronger self-halogen bonding for 2-iodo-perfluoropropane than for the 1-iodo-perfluoroalkanes. At first, there does not appear to be a physically intuitive reason for this conclusion. Perhaps the lone R-fluorine in 2-iodo-perfluoropropane predominately serves as the halogen bond acceptor for the iodine on an adjacent molecule. This could potentially produce an ordered chain. Figure 8 illustrates this idea and the following discussion. The positioning of the iodine and the R-fluorine might work synergistically to produce this chain ordering. It is reasonable that the R-fluorine would be the primary beneficiary of the increased electron density coming from the iodine. This, in turn, would allow the R-fluorine to become a better electron donor

Figure 7. The Hess’ law cycle used to interpret the spectroscopic and thermodynamic information about these systems. The overall halogen bonding process is complicated by the fact that it is taking place in the liquid state. Moreover,in these binary liquids,relativelyhigh concentrations of each component are present. This makes it difficult to directly comparethermodynamicresultswith thoseobtainedfrom measurements done when the halogen bond participantsare highly diluted by a neutral solvent. Here, three basic contributions are considered: solvation of the free components, the halogen bond itself, and solvation of the complex. The spectroscopic information in the form of the blue shift directly probes the halogen bond strength. The temperature studies shown in Figures 4-6 provide information about the overall enthalpy and entropy contributions.

Figure 8. The working hypothesis used to interpret the entirety of the data presented in the current work. For the 2-iodo-perfluoropropane there is (a) relatively stronger R fluorine directed self-halogen bonding than (b) -based self-halogen bonding. Our hypothesis suggests the I··· RF is the dominant self-interaction among the 2-iodo-perfluoropropane molecules. (c) There is less significant and less R-directed selfhalogen bonding for the 1-iodo-perfluoroalkanes. This leads to a more ordered state for the free (not halogen-bonded to pyridine) 2-iodoperfluoropropanethan for the 1-iodo-perfluoroalkanes.Although speculative, this hypothesis is consistent with the blue shift results, temperature studies, and boiling point information (see text for discussion).

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to coordinate with the σ-hole on the adjacent molecule. Thus, most of the self-halogen bonding would be of the I··· RF type rather than I · F.

For the case of the 1-iodo-perfluoroalkanes, the self-halogen bonding would be less directed to the I··· RF because the two R-fluorines would compete to some degree over the influx of electron density from the iodine. So a small energy difference between I··· RF and I··· F combined with the two R fluorine sites that allow for branching of the self-halogen bonding network could conceivably give rise to a higher basal entropy

independent of the I(2) CARS data supports the current hypothesis. It is interesting to consider the boiling points of the iodoperfluoroalkanes and their simple iodo-alkane counterparts. 2-Iodo-perfluoropropane and 1-iodo-perfluoropropane have essentially the same boiling point (40 and 41 °C, respectively), whereas 2-iodo-propane has a significantly lower boiling point than 1-iodo-propane(89.5 and 102 °C respectively).The simple iodo-alkanes are devoid of self-halogen bonding, and one sees, as expected, that an iodine in the 2 position on propane allows for less effective intermolecular interaction than does an iodine on an iodine at the 1 position. Hence, the boiling point of 2-iodopropane is lower than for 1-iodopropane. Barring any additional interaction, one would expect the 2-iodo-perfluoropropane and the 1-iodo-perfluoropropane to follow the same trend (Tboil(2) < Tboil(1) ), but this is not the case. This suggests there must be some additional intermolecular interaction and that it is more significant for 2-iodo-perfluoropropane.Consistent with the proposed hypothesis, this could be self-halogen bonding betweenthe R-fluorineon one 2-iodo-perfluoropropanemolecule and the iodine on an adjacent 2-iodo-perfluoropropane.

The clear evidence for relatively strong halogen bonding between pyridine and iodo-perfluoroalkanes suggests that there should be a reasonably strong halogen bond interactionbetween pyridineand bromo-perfluoroalkanes.Interestingly,pyridineand 1-bromo-perfluorohexane do not mix at room temperature. Additionally, pyridine and perfluorohexane are immiscible at room temperature. These observations suggest that the major driving factor for the miscibility of pyridine and the iodoperfluoroalkanes is the halogen bond interaction. Further, it suggests the halogen bond interaction between pyridine and bromo-perfluoroalkanes,althoughlikely present,is not sufficient to overcome the generic interactions contributing to the free energy of mixing.

VII. Conclusion

The I(2) CARS studyrevealsthat the halogenbondingbetween 1-iodo-perfluorobutane, 1-iodo-perfluorohexane, or 2-iodo-perfluoropropane and pyridine is strong. Indeed, peak shifts of around 7-10 cm-1 are observed, which is comparable to the ∼8c m-1 shift observed in pyridine/water systems. Despite similar halogen bond strengths between the 1-iodo-perfluoroalkanes/pyridineand the 2-iodo-perfluoropropane/pyridine,qualitatively different thermodynamic behavior is observed. Taken together, these data support the hypothesis that the 1-iodoperfluoroalkanes and 2-iodo-perfluoropropane have different molecularinteractions.A greater degree of self-halogenbonding is believed to be taking place in 2-iodo-perfluoropropane,which leads to a more ordered local molecular structure. Disruption of this ordering provides an increase in entropy that ultimately leads to different equilibrium constants for the overall halogen bonding process in these binary mixtures. The 1-iodo-perfluo- roalkane/pyridine systems have equilibrium constants of order unity and the 2-iodo-perfluoropropane/pyridine system has a much larger equilibrium constant.

It is hoped these data and their interpretation presented here might stimulate further study of both halogen bonding to pyridine-based molecules and the potential intermolecular interactions between iodo-perfluoro compounds. Understanding the fundamental dynamics and interactions in iodo-perfluoroalkanes and their mixtures with pyridine is important for applications as varied as crystal engineering and drug design.

From the experimental point of view, more traditional vibrational spectroscopy than I(2) CARS, such as conventional Raman and FTIR, is an obvious direction. Ultraviolet spectroscopy has been employed to identify halogen bonding79 and will be an avenue for further study, as well. Beyond that, perhaps studies on clusters of these systems could provide additional insight and serve as an empirical bridge between full liquid phase behavior and small cluster properties gleaned from computationalstudies.This has been done for hydrogenbonding systems.80

To our knowledge, the behavior of the R and fluorines have not been explored via computational chemistry. This could provide insight into the idea of self-halogen bonding proposed in the current work and perhaps even on its contribution to the thermodynamic properties of the iodo-perfluoroalkanes.

Acknowledgment. This work was supported by NSF CA-

REER Grant CHE-0341087, the Dreyfus Foundation, and the Concordia College Chemistry Research Endowment.

References and Notes

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