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Interactions and Positive Electrostatic Potentials of N-Heterocycles Arise from the Cite This: Chem ChemComm View Article Online COMMUNICATION View Journal | View Issue Anion–p interactions and positive electrostatic potentials of N-heterocycles arise from the Cite this: Chem. Commun., 2014, 50, 11118 positions of the nuclei, not changes in the Received 9th July 2014, p-electron distribution† Accepted 5th August 2014 DOI: 10.1039/c4cc05304d Steven E. Wheeler* and Jacob W. G. Bloom www.rsc.org/chemcomm We show that the positive electrostatic potentials and molecular the molecule. As a result, a positive Qzz value can indicate the quadrupole moments characteristic of p-acidic azines, which depletion of electron density above the ring centre (e.g., underlie the ability of these rings to bind anions above their centres, reduction of the p-electron density) or the movement of nuclear arise from the position of nuclear charges, not changes in the charge towards the ring centre. Similarly, a positive ESP above p-electron density distribution. an aromatic ring can arise from changes in either the electronic or nuclear charge distributions.7 These distinctions are rarely Anion–p interactions1 are attractive non-covalent interactions made in discussions of anion–p interactions,1,2 which focus between anions and the faces of p-acidic rings.2 They often almost exclusively on the concepts of p-acidity and p-electron- involve azabenzenes (azines) such as s-triazine and s-tetrazine, deficiency. and have emerged as powerful tools for anion binding, recogni- Previously, Wheeler and Houk showed8 that substituent- tion and transport, and even catalysis.3 Despite rapidly-growing induced changes in the ESPs of substituted arenes are domi- interest in these non-covalent interactions, there is a dearth of nated by through-space effects, not changes in the p-electron rigorous explanations of their origin.4 Most authors2g,h ascribe density. Similarly, they also demonstrated4a that anion-binding anion–p interactions to a combination of electrostatic and by substituted benzenes is due to interactions of the anion with induction effects (anion-induced polarization of the arene).5 the substituents; the interaction of the anion with the phenyl Published on 06 August 2014. Downloaded 27/08/2014 08:09:54. The electrostatic component of these interactions is generally ring itself remains repulsive regardless of the substituents. characterized by the Qzz component of the quadrupole Unfortunately, neither of these studies addressed the impact moments of the arenes, which is correlated with the strength of heteroatoms, despite the importance of N-heterocycles in of anion–p interactions.2g,h Alternatively, the electrostatic supramolecular chemistry. Consequently, key questions remain component of these interactions can be described in terms of regarding the origin of the positive ESPs and Qzz values of electrostatic potentials (ESPs), which are widely employed in p-acidic N-heterocycles, as well as the ability of these arenes to analyses of a variety of other non-covalent interactions.6 bind anions. Here, we tackle these questions by separating the Although the correlation of the strength of anion–p interac- impact of s-andp-electron density on ESPs and Qzz values based tions with arene Qzz and ESP values seems straightforward, on the symmetry of the underlying molecular orbitals (see ESI† for questions remain regarding the origin of the positive Qzz and more details). The results are contrary to conventional descrip- ESP values that are characteristic of p-acidic N-heterocycles. tions of anion–p interactions involving N-heterocycles,1,2 as well Many equate positive Qzz and ESP values with p-electron as prevailing descriptions of p-acidic azines. deficiency,1b–d,3a–f and ‘‘p-acidic’’ is often implicitly defined in The azines pyridine, pyrazine, s-triazine, s-tetrazine, and the 3f terms of positive ESP and Qzz values. The problem is that both hypothetical molecule (planar) hexazine provide a convenient ESPs and molecular quadrupole moments reflect the balance platform for understanding the origin of anion–p interactions between the large and opposing effects of the distribution of and the nature of p-acidic N-heterocycles. Notably, these five positive (nuclear) and negative (electronic) charge throughout arenes exhibit drastically different ESPs and Qzz quadrupole moments (see Fig. 1a and Table 1), despite being s-and p-isoelectronic with benzene. Moreover, the isoelectronic nature Department of Chemistry, Texas A&M University, College Station, TX, USA. of these rings enables the direct evaluation of the impact of E-mail: [email protected] † Electronic supplementary information (ESI) available: Additional computa- nuclear and electronic charge distributions on ESPs and Qzz À tional details, additional figures and data, and Cartesian coordinates. See DOI: values. When an anion such as Cl is constrained to lie above 10.1039/c4cc05304d the ring centroid, these arenes provide a systematic and linear 11118 | Chem. Commun., 2014, 50, 11118--11121 This journal is © The Royal Society of Chemistry 2014 View Article Online Communication ChemComm Fig. 1 (a) Total ESPs of benzene and five azines, mapped onto the corresponding electron density isosurfaces; (b) fictitious EPSs resulting from the replacement of only the p-electron density of benzene with that of the corresponding azine, while leaving the s-electron density and nuclear charges unchanged. (c) Comparison of the proximity of nuclear charges to an anion above the centres of benzene and triazine.‡ Table 1 Qzz quadrupole moments for the five azines, relative to benzene (DQ, in Buckinghams), as well as nuclear and electronic contributions to Fig. 2 (a) Differences in ESPs of five azines, relative to benzene (DESP),aswellas DQzz (DQN and DQe)ands-andp-contributions to DQe [DQe(s) and (b) the nuclear (DESPN), (c) s-electronic [DESPe(s)], and (d) p-electronic [DESPe(p)] a DQe(p)] contributions to DESP, all mapped onto total electron density isosurfaces. DQ DQN DQe DQe(s) DQe(p) Pyridine +3.0 +17.8 À14.9 À14.6 À0.3 with the classic description of the N-heterocycles as p-electron- Pyrazine +6.1 +35.0 À29.0 À28.5 À0.5 deficient. However, closer examination of the components of s-Triazine +9.2 +53.2 À44.0 À43.3 À0.7 these ESPs reveals a starkly different picture. Differences in Published on 06 August 2014. Downloaded 27/08/2014 08:09:54. s-Tetrazine +11.4 +63.8 À52.4 À51.8 À0.6 Hexazine +15.9 +86.9 À71.0 À70.4 À0.6 computed ESPs (DESP) for the five azines are plotted in Fig. 2a, a relative to the ESP of benzene. These ESP differences arise from For benzene, Qzz = À8.9 B. For pyridine, Qzz is computed relative to the centre of mass. three sources: differences in the position of the nuclei (DESPN), differences in the s-electron density [DESPe(s)], and differences in the p-electron density [DESPe(p)]. These contributions to progression from unfavourable anion–p interactions (for benzene), DESP are plotted in Fig. 2b–d, respectively. Analogous plots in to strongly attractive interactions for triazine, tetrazine and hex- the plane bisecting these rings are provided for selected systems azine.4d Because of the strong correlation between interaction in ESI.† The only one of these components that leads to more energies for such model anion–p complexes and ESP and Qzz positive ESPs above the centres of these rings is the nuclear values for the rings (see ESI†), explaining the impact of N-sub- contribution, DESPN;boththes-andp-electronic contributions stitutions on anion–p interactions is equivalent to explaining their lead to more negative ESPs and weaker anion–p interactions, 9 impact on ESPs and Qzz values. Of course, cation–p interactions, compared to benzene. Additionally, the impact of the s-electron which are vital to sundry chemical and biological processes,10 are density differences on the ESPs is much more substantial than 4b,11 also strongly correlated with ESP and Qzz values. Indeed, variations in p-electron distributions. electrostatic effects play a key role in many non-covalent interac- The reason for the positive values of DESPN above the ring tions involving N-heterocycles. As such, the analysis below has centres is simple: each time a CH group is replaced with a nitrogen important implications for our understanding of many non- atom, there is effectively a movement of +1e charge about 1 Å covalent interactions.4c,d towards the ring centre, arising from the consolidation of the ESP maps for benzene and the five azines are depicted in hydrogen and carbon nuclei into a +7 nitrogen nucleus (see Fig. 1a, and show a clear progression from the negative ESP Fig. 1c). The proximity of a greater quantity of nuclear charge near above benzene to positive ESPs above triazine, tetrazine, and the centres of the azines leads to substantially more positive ESP hexazine. This is consistent with the well-established trend in values above the ring, and much more favourable anion–p inter- the ability of these rings to bind anions, and seemingly jives actions, as compared to benzene. This journal is © The Royal Society of Chemistry 2014 Chem. Commun., 2014, 50, 11118--11121 | 11119 View Article Online ChemComm Communication The effects of the nuclear positions are tempered by the benzene. Moreover, the s-andp-electron clouds of the azines accompanying changes in the electron distribution. That is, actually repel anions to a greater extent than those of benzene. In changes in both the s- and p-electron density distribution other words, N-heterocycles do not bind anions because of resulting from incorporation of nitrogen atoms depress the
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