Halogen Bonding: Where We Are and Where We ... - ACS Publications

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Halogen Bonding: Where We Are and Where We Are Going Published as part of the Crystal Growth & Design virtual special issue on Halogen Bonding in Crystal Engineering: Fundamentals and Applications Pierangelo Metrangolo*,†,‡ and Giuseppe Resnati*,†,‡ †

NFMLab, Department of Chemistry, Materials, and Chemical Engineering “Giulio Natta”, Politecnico di Milano, Via L. Mancinelli 7, 20131 Milan, Italy ‡ CNST-IIT@POLIMI, Via Pascoli 70/3, 20133 Milan, Italy ABSTRACT: The state of the art of the theory and practice of halogen bonding in crystal engineering is sketched and some future prospects are highlighted.

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shortly after,6 and the use of complex organic compounds as XB donors or acceptors was first reported in 1883 by O. Roussopoulos who described the adduct between iodoform and quinoline7 and other analogous systems. After that, numerous scientists reported chemical phenomena where we are now acknowledging the role played by the interaction. However, while the occurrence of single experimental results was conveniently described and analyzed through different techniques, the common features were not recognized. Single findings were understood within different conceptual frames and designated by different and erratic terms, varying from the imaginative “bumps in hollow” to the chemically meaningful “charge-transfer interaction”.8 The iodine−benzene complexes is solution were identified via their UV−vis spectra by H. A. Benesi and J. H. Hildebrand9 in 1948, and one year later other aromatic solvents were reported to behave analogously. In 1950, ether, thioether, and carbonyl solvents were described to afford similar complexes by R. Mulliken10 who rationalized the adducts as a subclass of the electron donor−acceptor molecular complexes two years later.11 In 1954 O. Hassel12 described the solid-state structure of the 1:1 complex between dibromine and 1,4-dioxane and then reported several related adducts in rapid sequence.13 In 1968 H. A. Bent8 published a review on the structural chemistry of donor−acceptor adducts. Murray-Rust, Desiraju, and Parthasarathy afforded further information on halogen bonded (X-bonded) systems in the solid analyzing statistically the structures in the Cambridge Structure Database (CSD).14 In 1983 J.-M. Dumas, M. Gomel, and M. Guerin15 put forward

eneral conceptual frames are typically developed in chemistry only when a sufficiently wide and diverse set of single instances and occurrences relevant to the topic has been reported.1 This is what recently happened for halogen bonding (XB), the attractive interaction between an electrophilic region on a halogen atom and a nucleophilic region of a molecule or molecular fragment.2 While an operative consensus spontaneously emerged on what an XB is, a formal consensus was timely and an IUPAC project has pursued a definition of the term XB.3 As a part of the attempt to promote a unified conceptual frame among the scientists interested in the field, Crystal Growth & Design realized a virtual special issue entitled “Halogen Bonding in Crystal Engineering: Fundamentals and Applications”. The specific aim was to give a snapshot of the state of the art on XB from the viewpoint of structural chemistry, an area which has had a nucleation role for the understanding and development of the XB. The gathering of experimental findings on XB and their conceptualization is very instructive relative to the way concepts are framed in science. A brief history of major findings and rationalizations in the field is sketched in the following paragraph and may help to better understand the present status and future prospects of the interaction.

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THE HISTORY XB is a very general and basic phenomenon. The first report describing adducts whose formation is now understood to occur under XB control dates back to 1814 when M. Colin4 illustrated the behavior of iodine reacting with ammonia and reported the formation of a diiodine/ammonia system which was purified and formulated as NH3·I2 by F. Guthrie5 in 1863. Similar adducts involving bromine and chlorine were obtained © 2012 American Chemical Society

Received: September 28, 2012 Revised: October 27, 2012 Published: November 5, 2012 5835

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specific subsets of the phenomena encompassed by the term XB. Relative to these subsets, the XB concept gives the additional plus that single experimental and theoretical results can also be understood thanks to the trends observed by varying the halogen atom. Unfortunately, the terms fluorine bond, chlorine bond, bromine bond, and iodine bond have also been used in some cases to name interactions involving the halogen atom as a nucleophilic site, for instance, when in close contact with hydrogen atoms.28 While the terminology used to name noncovalent interactions given by halogen atoms has to be as unifying as possible, it has always to be in keeping with the electrophile/nucleophile role that the halogen atom plays29 and the latter contacts, wherein the halogen atoms function as the electron donor partners, must be addressed by using terms different from halogen bond, e.g., hydrogen bond (HB), as indeed it has frequently been the case.30

several features of XB in the liquid phase, while the features of the interaction in the gas phase were summarized in two reviews published by A. Legon in 1998 and 1999.16 A valuable tool for the theoretical understanding of the XB was proposed in 1994 by P. Politzer and J. S. Murray who identified the anisotropic distribution of the electron density around covalently bound halogens and proposed the concept of the σ-hole.17 The results summarized above, and other related findings, were never connected to each other in a single and comprehensive model till the seminal paper entitled “Halogen Bonding: A Paradigm in Supramolecular Chemistry” was published in 2001.18 This Concept Article proposed for the first time a unified model where previously unrelated phenomena were comprehensively recollected in order to propose the electrophilic behavior of halogen atoms as a general phenomenon impacting on “all the fields where design and manipulation of aggregation processes play a key role”. Indeed, an exponential growth of the interest of the scientific community on the topic immediately followed thereafter (Figure 1). P. Metrangolo and G. Resnati19 clearly identified

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HALOGEN BONDING IN STRUCTURAL CHEMISTRY This virtual special issue collects results on XB in order to provide a more accurate or more general understanding of the role of the interaction in affecting molecular aggregation in the solid. The role of XB based supramolecular synthons in controlling, or influencing, the overall packing of crystalline solids is the main focus of several papers of the virtual special issue. G. R. Desiraju et al. (DOI: 10.1021/cg2007815) describe how the crystalline structure of polychlorophenols is characterized by the presence of well insulated domains wherein HB and XB are selectively present, while C. B. Aakeröy et al. (DOI: 10.1021/ cg2009013) show that the two interactions (specifically the C− I···N XBs and the N−H···N HBs) can mix and alternate along one-dimensional chains. Experimental and theoretical charge density analyses have been performed by T. N. Guru Row et al. (DOI: 10.1021/cg2000415) on a dichlorobenzoquinone, and both approaches unequivocally establish the attractive nature of the C−Cl···O synthon present in the crystalline lattice. The overall crystal packing of some tetraarylethylene halobenzoyl esters (F. C. Pigge et al., DOI: 10.1021/cg200986v) results from the subtle balance of various XBs, namely, the established C−Br···O and C−I···O interactions and the less recognized C− X···π and C−X···X−C short contacts (X = Br, I). A similar interplay of different XBs has been observed by M. F. Roll et al. (DOI: 10.1021/cg200312g) in the structure of some iodinated and brominated phenylsilsesquioxanes. Crystal structures of the free forms and some solvates of tris(4-halophenoxy)methylbenzenes (B. K. Saha et al., DOI: 10.1021/cg2009144) also show a recurrent presence of C−X···π and C−X···X-C XBs; the frequency of occurrence of these interactions decreases gradually from iodine to fluorine, the latter preferring weak C−F···H−C HBs. Interestingly, P. Politzer et al. (DOI: 10.1021/cg200888n) show computationally, through a series of examples, that fluorine can have positive σ-holes when linked to strong electron withdrawing residues and demonstrate, through a statistical analysis of the CSD data, that such fluorines do also halogen bond in the solid state. At the other extreme of a strength scale of XB donor moieties, P. D. Beer et al. (DOI: 10. 1021/cg200811a) report that a 5-iodo-1,2,3-triazol-3-ium derivative forms short and directional XBs with different halide anions. Several papers focus on the nature and role of the XB acceptor tectons. The geometrical features of dinitrogen moieties tailored to give rise to symmetrical bifurcated XB is investigated computationally and experimentally by B. Ji et al.

Figure 1. Papers per year citing the phrase “halogen bonding” (Source: SciFinder).

the key role of residues close to covalently bound halogen atoms in determining their ability to work as electrophilic sites. More important, they showed how anions work similar to, and better than, lone pair possessing heteroatoms as XB acceptors in the solid and in solution.20 On the experimental side, these findings allowed selfassembly processes under XB control to be designed and tuned. On the conceptual side, they made a major breakthrough when developing a comprehensive process of unification of previously unrelated findings. The first book specifically devoted to the interaction and its applications appeared in 2008.21

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THE HALOGEN BONDING AND ITS SUBCLASSES Iodine, bromine, and chlorine are traditionally accepted XB donors, and a consensus emerged recently that also fluorine, under some circumstances, can work as an XB donor both in chemical22 and biological23 environments. The terms fluorine bond,24 chlorine bond,25 bromine bond,26 and iodine bond27 have been used to address cases in which one of the four halogens behaved as electrophilic sites. These terms indicate 5836

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electronic devices. The strength and character of XB interactions in DNA junctions and protein−ligand complexes are addressed by P. S. Ho et al. (DOI: 10.1021/cg200991v) and K. E. Riley et al. (DOI: 10.1021/cg200882f), respectively. In both cases particular attention has been paid to interactions involving brominated donors.

(DOI: 10.1021/cg200603z). W. T. Pennington et al. (DOI: 10. 1021/cg201348u) report that the selenium atom of triphenylphosphine selenide can work both as a mono- and bidentate XB acceptor as a function of the XB donor partner. W.-W. du Mont et al. (DOI: 10.1021/cg200923q) discuss how the iodophosphane selenide functional group comprises an XB donor and an XB acceptor site in the same moiety and can thus be considered the XB analogue of phosphinic or carboxylic acids (which similarly contain an HB donor and an HB acceptor site in the same moiety). The general ability of selenium, and sulfur, moieties to work as acceptor sites has been confirmed by F. A. Devillanova et al. (DOI: 10.1021/cg201328y) through a statistical analysis of structures in the CSD. An extensive computational study by T. W. Hanks et al. (DOI: 10.1021/ cg201231t) suggests that when pseudopolyhalides are formed on the interaction of halide anions and halocarbons, the charge transfer contribution to the XB interaction can be quite large and generally increases with the XB strength. Computational studies have been used by C. Mealli et al. (DOI: 10.1021/ cg201154n) to investigate the bonding nature in polyiodides. A thorough crystallographic study of mono- and dihalopyridinium cyanometallate performed by L. Brammer et al. (DOI: 10. 1021/cg200942u) indicates predominant interaction with the exo lone pair of the nitrogen atom (CN···X > 145°) or with the π-bond of the cyano group (CN···X < 105°), and a study by F. Awwadi et al. (DOI: 10.1021/cg200893n) found the presence of XB interactions also in the crystalline structure of polyhalopyridine/copper(II) halides adducts. P. Metrangolo et al. (DOI: 10.1021/cg200840m) describe the ability of tetrahedral mononegative oxyanions (e.g., perchlorate and periodate anions) to drive the formation of two-dimensional supramolecular anions, and M. Fourmigué et al. (DOI: 10. 1021/cg200934r) report that sulfonate anions also work as effective XB acceptor sites. Polymeric one-dimensional zigzag chains are formed, by K. Rissanen et al. (DOI: 10.1021/ cg201170w), on self-assembly of chloride and bromide anions with diiodoperfluorocarbons, while the formation of one-and two-dimensional nets is reported by J. McMurtrie et al. (DOI: 10.1021/cg201017r) when chloride anions coordinate di- and triiodoperfluorocarbons, respectively. Papers in the virtual special issue also widen the palette of techniques and methodologies affording structural and energetic information on XB interactions. D. L. Bryce et al. (DOI: 10.1021/cg200889y and 10.1021/cg201683p) used multinuclear solid-state magnetic resonance spectroscopy of 15 N/77Se nuclei and of 35Cl/81Br/127I nuclei to study a series of X-bonded adducts involving thio- or selenocyanates and monohaloanilinium halides, respectively. Molecular beam scattering experiments performed by D. Cappelletti et al. (DOI: 10.1021/cg200890h) characterized the nature and strength of water/halomethane adducts. Finally, some papers addressed the functional properties of molecular materials targeted to XB driven self-assembly. E. Cariati et al. (DOI: 10.1021/cg201194a) describe the high hyperpolarizabilities at the molecular level in solution of some push−pull molecules wherein the electron acceptor ending is a 4-iodotetrafluorophenyl group. J. Lieffrig et al. (DOI: 10.1021/ cg200843w) study the magnetic properties of some networks containing the bis(propylenedithio)tetrathiafulvalene residue. P. Sgarbossa et al. (DOI: 10.1021/cg201073m) present some X-bonded organometallic materials which exhibit dielectric properties more interesting than those of SiO2 so that may be very promising for application in next generation micro-

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SELECTED FUTURE PROSPECTS The fundamental aspects of XB are now fairly understood as it is documented in some of the papers of this virtual special issue. The coming years will probably witness the tailored use of unique and useful features of XB in fields where specific advantages are secured by the interaction. Cation binding and coordination played a major role in the development of the concept and practice of supramolecular chemistry. Despite the relevance of anions in many chemical and biological processes, their coordination chemistry had a later and slower start than the coordination of cations. The first paper describing the purposeful formation of self-assembled and X-bonded supramolecular anions in solution and in the solid31 appeared in 1999. The interaction is emerging now as a promising complement to hydrogen bonding32 in anion coordination and useful applications, e.g., in the transmembrane anion transport,33 are beginning to appear. XB has found numerous applications in the structural control of molecular materials. The XB formed by iodinated tetrathiafulvalene derivatives and congeners has been extensively used to tune conductive and magnetic properties of these compounds.34 The appearance of liquid crystalline properties in adducts formed on XB driven self-assembly of non-mesogenic components was reported first for lyotropic systems35 and then for thermotropic molecular36 and polymeric37 systems. Activity in both fields can be expected to continue, and many other new applications of the interaction in the field of functional materials are expected to appear.38 Biosciences will probably witness a tremendous increase of interest in the interaction in the coming years. Conformational control of biomacromolecules (e.g., peptides and polynucleotides)39 and optimization of small molecule-protein interactions40 are two of the specific fields where a great potential can already be foreseen. The possible final acceptance of the definition of halogen bonding by IUPAC41 can be expected to conclude the pioneering period and start a prosperous maturity for the interaction.

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AUTHOR INFORMATION

Corresponding Author

*(P.M.) Fax: +39.02.2399.3180; tel: +39.02.2399.3041; e-mail: [email protected]. (G.R.) Tel: +39.02.2399.3032; e-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Authors thank Fondazione Cariplo for financial support under projects 2009-2550 and 2010-1351.

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REFERENCES

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(35) Bertani, R.; Metrangolo, P.; Moiana, A.; Perez, E.; Pilati, T.; Resnati, G.; Rico-Lattes, I.; Sassi, A. Adv. Mater. 2002, 14, 1197. (36) Bruce, D. W.; Metrangolo, P.; Meyer, F.; Pilati, F.; Prasang, C.; Resnati, G.; Terraneo, G.; Wainwright, S. G.; Whitwood, A. C. Chem. Eur. J. 2010, 16, 9511. (37) Xu, J.; Liu, X.; Kok-Peng, J.; Lin, T.; Huang, J.; He, C. J. Mater. Chem. 2006, 16, 3540−3545. (38) Priimagi, A.; Cavallo, G.; Forni, A.; Gorynsztejn−Leben, M.; Kaivola, M.; Metrangolo, P.; Milani, R.; Shishido, A.; Pilati, T.; Resnati, G.; Terraneo, G. Adv. Funct. Mater. 2012, 22, 2572−2579. (39) Voth, A. R.; Hays, F. A.; Ho, P. S Proc. Natl. Acad. Sci. U. S. A. 2007, 104, 6188−6193. Tatko, C. D.; Waters, M. L. Org. Lett. 2004, 6, 3969−3972. (40) Riley, K. E.; Hobza, P. Cryst. Growth Des. 2011, 11, 4272. Xu, Z.; Liu, Z.; Chen, T.; Chen, T. T.; Wang, Z.; Tian, G.; Shi, J.; Wang, X.; Lu, Y.; Yan, X.; Wang, G.; Jiang, H.; Chen, K.; Wang, S.; Xu, Y.; Shen, J.; Zhu, W. J. Med. Chem. 2011, 54, 5607. (41) A provisional recommendation for the definition of the halogen bond in now posted and open for public comments at http://www. iupac.org/home/publications/provisional-recommendations/ currently-under-public-review/currently-under-public-reviewcontainer/definition-of-the-halogen-bond.html

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