Editorial pubs.acs.org/crystal
Bonding Matters
C
participants which were from Europe, Americas, and Asia. He sketched the specific and general objectives of the Project and underlined its close relationships with the two IUPAC Projects which recently issued Recommendations defining the HB2 and the XB.3 Antonio Frontera (Universitat des les Illes Balears, Spain) opened the workshop with a fairly comprehensive analyses of interactions wherein elements of Groups 14−16 are the electrophilic site. He addressed the issue from the point of view of modeling and calculations and discussed how the tetrel bond (TB), pnictogen bond (PB), chalcogen bond (CB), and XB all result from the anisotropic distribution of the electron density in bonded atoms. Regions of positive electrostatic potential are frequently located along the vectors of covalent bonds (σ-holes) or perpendicular to molecular frameworks (πholes), and these regions form attractive interactions with a variety of electron rich sites, either neutral or anionic. The attractive interactions between derivatives of heavier elements of Group 15 and aromatic electron donor systems were discussed as examples of PBs involving σ-hole at the pnictogen. Several examples of similar interactions are present in the Protein Data Bank (PDB) suggesting the possible use of such interactions in the design and synthesis of potential inhibitors of enzymatic reactions. Anthony C. Legon (University of Bristol, UK) discussed the structures of adducts formed in the gas phase at very low temperature. Calculations of the molecular electrostatic surface potential of SO2 identifies its most positive region at an area perpendicular to the SO2 nuclear plane, and rotational spectroscopy shows that SO2 forms dimers with ethylene and acetylene thanks to a CB between this area and the center of the π density of the unsaturated hydrocarbon. Similar pairings of positive and negative sites of interacting molecules are observed in the dimer between CO2 and HCN or HBr, assembled thanks to C···N or C···Br TBs, and in the dimer between N2O and HCl, assembled thanks to an N···Cl PB. The third lecture, delivered by David L. Bryce (University of Ottawa, Canada) focused on the potential of NMR spectroscopy to detect the presence of TBs, PBs, and CBs. Several isotopes are amenable to the direct or indirect study of the adducts formed by these interactions which may lead to observable changes in NMR parameters. Promising results have been reported for lead(II) metal organic frameworks showing TBs. Bryce also reported that when carbon is the TB donor, the sensitivity of the NMR parameters to the noncovalent interaction is demonstrated via an increase in δiso and in |1cJ(13C,Y)| (Y = 17O, 15N) as the TB shortens. The last lecture of the first session was given by Giancarlo Terraneo (Politecnico di Milano, Italy) who listed several cases where single crystal X-ray analyses revealed its effectiveness in detecting the presence of CBs and TBs. Consistent with the ability of electron withdrawing residues to promote the electrophilic character of adjacent atoms, the crystal packing
rystallography, affording unequivocal experimental information on the position of atoms in the solid, has played a major and pervasive role in the understanding of both strong and weak chemical bonds. A prototype example of this role is the assignment of the Nobel Prize to Odd Hassel in 1969: The motivation of the prize was Hassel’s “contribution to the development of the concept of conformation and its application in chemistry”, and his Nobel Lecture1 was a discussion of the single crystal structure of small molecules in order to establish their self-organization under control of the hydrogen bond (HB)2 and halogen bond (XB).3 It was thus considered of interest for the readers of Crystal Growth & Design to offer a report of the kick-off event of an IUPAP project having the specific objective “to develop a nonambiguous terminology for interactions formed by chalcogens, pnictogens, and tetrels, namely the elements of Groups 16, 15, and 14”.4 This objective is a part of a more general enterprise pursuing “the development of an unambiguous, systematic, and periodic naming of most interactions wherein it is possible to identify an element or moiety working as the electrophile”. The Project shall acknowledge that “Group 16−14 elements can form attractive interactions with both nucleophiles and electrophiles” but “consistent with the use of the terms hydrogen and halogen bond only for interactions where hydrogen and halogens are the electrophiles” it is expected the Project will propose “that the terms chalcogen bond, pnictogen bond, and tetrel bond are used exclusively for interactions wherein the respective elements are the electrophile”.4 The possible interest in this IUPAC Project might extend far from crystallography as the chemical bond is and has always been a central issue in all fields of chemistry, spanning inorganic, organic, and physical chemistry. Alfred Werner, who first proposed the octahedral configuration of transition metal complexes, was assigned the Nobel prize in 1913 “in recognition of his work on the linkage of atoms in molecules”; Linus C. Pauling, one of the founders of quantum chemistry and molecular biology, won the Nobel Prize in 1954 “for his research into the nature of the chemical bond”; and Robert S. Mulliken, who proposed, among other things, the electronegativity scale bearing his name, was assigned the Nobel Prize in 1966 “for his fundamental work concerning chemical bonds and the electronic structure of molecules”. These three cases pertain to the period when the basics for the understanding of stronger bonds were laid out, and in more recent years the focus shifted on weaker bonds, frequently named interactions. In 1987 Donald J. Cram, Jean-Marie Lehn, and Charles J. Pedersen were given the Nobel Prize “for their development and use of molecules with structure-specific interactions of high selectivity”. The kick-off event of an IUPAP project was held at the Department of Chemistry, Materials and Chemical Engineering “Giulio Natta” of the Politecnico di Milano (Italy) on February 6, 2017. Giuseppe Resnati (Politecnico di Milano, Italy) opened the sessions by welcoming the speakers and the © 2017 American Chemical Society
Published: April 5, 2017 1439
DOI: 10.1021/acs.cgd.7b00309 Cryst. Growth Des. 2017, 17, 1439−1440
Crystal Growth & Design
Editorial
and suggestions to the IUPAC Project from any reader of Crystal Growth & Design as these contributions and suggestions will benefit anyone in the community.
of 2,2,4,4-tetrafluorodithietane shows the presence of two S···F CBs, while sulfur is not involved in short contacts in the tetrachloro analogue, whose molecules are pinned in their position by Cl···Cl XBs. Terraneo also described how various 5,5-difluorobarbituric acid derivatives show F2C···O and OC··· O TBs, these latter interactions being formed on the entrance of the oxygen on the carbonyl after the Bürgi-Dunitz trajectory. The quite keen scientific discussion went on during the coffee break which came after, possibly thanks to the Italian espresso available to the participants which were nonetheless back in the lecture hall on time for the second session. The session was opened by the presentation of Gautam R. Desiraju (Indian Institute of Science, India), and the focus of his lecture was on the information afforded by single crystal X-ray analyses. Various structures containing arsenic···π and bismuth···π PBs were discussed. The modularity of interactions and their potential in the formation of extended nets or discrete dimers were considered. Some crystals wherein the PB acceptor was a lone pair possessing atom were also presented. The successive talk, given by Kari Rissanen (University of Jyväskylä, Finland), examined how to search HB, XB, CB, PB, and TB in the Cambridge Structure Database (CSD) when no other information about the interacting atoms is available, except the atom types. It was shown that a multitude of examples is obtained for the HB and XB, but those for CB, PB, and TB are much less numerous. This is related to the differences in the general chemical structure of the electrophilic site of the different interactions. In the last presentation Christer B. Aakeröy (Kansas State University, USA) reported some chalcogen bonded adducts formed by selenocarbamate and selenourea derivatives and some pnictogen bonded complexes given by antimony tricyanide and its bismuth analogue. A discussion of the structural similarities and differences between HB, XB, CB, PB, and TB followed and it was a particularly interesting part of the lecture. The adopted holistic stance was particularly inspiring and seminal, and it was nicely mirroring, at an experimental level, of the similar generalizations based on modeling and calculations proposed by Tony Frontera in the opening lecture. In the concluding remarks Resnati acknowledged that the meeting was the beginning of the activity of the IUPAC Project, a two year story. He also reminded everyone that contributions and suggestions from the whole chemical community will be helpful in reaching Project’s objectives, namely, to propose CB, PB, and TB definitions which conveniently balance generality and specificity, wide applicability, and robust descriptive power. In the afternoon a round table was organized for those having a role in the IUPAC Project, namely, Giuseppe Resnati and Pierangelo Metrangolo, who are serving as cochairs of the Project, the speakers of the meeting, and Steve Scheiner (Utah State University), a Task Group member who was unable to come to Milano and attended the round table via videoconference. It was agreed to pursue recommendations defining CB, PB, and TB. These recommendations should heavily refer to the experimental evidence on the three interactions and should parallel definitions recently issued for HB2 and XB.3 In this way three pieces would be added to the body of a systematic and periodic terminology naming any interaction wherein an electrophilic partner can be identified, from the Group of the Periodic Table to which the electrophilic atom belongs. This terminology might finally span most of the attractive interactions formed by the elements of Groups 1, 2, 13−18. One aim of this Editorial is to prompt contributions
Giancarlo Terraneo, Guest Editor Giuseppe Resnati, Guest Editor
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Laboratory of Nanostructured Fluorinated Materials (NFMLab), Department of Chemistry, Materials, and Chemical Engineering, “Giulio Natta,” Politecnico di Milano
AUTHOR INFORMATION
ORCID
Giuseppe Resnati: 0000-0002-0797-9296 Notes
Views expressed in this editorial are those of the authors and not necessarily the views of the ACS.
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REFERENCES
(1) Hassel, O. Science 1970, 170, 497−502. (2) Sadlej, J.; Scheiner, S.; Alkorta, I.; Clary, D. C.; Crabtree, R. H.; Dannenberg, J. J.; Hobza, P.; Kjaergaard, H. G.; Legon, A. C.; Mennucci, B.; Nesbitt, D. J.; Arunan, E.; Desiraju, G. R.; Klein, R. A. Pure Appl. Chem. 2011, 83, 1637−1641. (3) Desiraju, G. R.; Ho, P. S.; Kloo, L.; Legon, A. C.; Marquardt, R.; Metrangolo, P.; Politzer, P.; Resnati, G.; Rissanen, K. Pure Appl. Chem. 2013, 85, 1711−1713. (4) Metrangolo, P.; Resnati, G. Chem. Int. 2016, 38 (6), 22−24.
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DOI: 10.1021/acs.cgd.7b00309 Cryst. Growth Des. 2017, 17, 1439−1440
