s-Hole Opposite to a Lone Pair: Unconventional Pnicogen Bonding Interactions between ZF 3 (Z = N, P, As, and Sb) Compounds and Several Donors Antonio Bauzµ, [a] Tiddo J. Mooibroek, [b] and Antonio Frontera* [a] 1. Introduction Supramolecular chemists rely on in-depth comprehension of noncovalent forces, which are pillars of modern chemistry. A proper understanding and intelligent utilization of such forces is essential to achieve progress in fields such as supramolecular chemistry, [1] molecular recognition, [2] and materials science. [3] The best known supramolecular force that is ubiquitous in many chemical and biological systems is hydrogen bonding. [4] In recent years, another noncovalent force that shares strength and directionality features with hydrogen bonding is the halo- gen bonding interaction. [5] A series of studies using the Cam- bridge Structural Database (CSD) have been carried out to shed light on the impact of this interaction in crystal struc- tures. [6] The interest among the scientific community has ex- panded exponentially because of the recognition of its impor- tance in biological systems and the design of new materials, which has led to a plethora of theoretical and experimental studies devoted to this topic. [7–9] In addition, apart from the well-established amount of noncovalent forces, an emerging group of so called “unconventional” noncovalent interactions has attracted the interest of the scientific community during the past years. [10] It refers to the interaction between group IV to VII bearing compounds (s-hole donor) and nucleophiles (s- hole acceptor). [11–13] Frequently, the name of the group of atoms that provides the s-hole (from IV to VII) is used to speci- fy the name of the interaction; indeed, they can be considered as subgroups of a general definition of s-hole bonding interac- tions given by Politzer and co-workers. [8, 9] Recently, a large number of computational studies [14–16] have been devoted to studying these interactions. It has become clear that the inter- actions are moderately strong, highly directional, and gov- erned by the same physical principles as halogen bonding. The directional character of s-hole interactions is due to the locali- zation of the s-hole at a limited region of space along the vector of the covalent bond between the s-hole bearing atom and another atom within the donor molecule. [17–21] In recent years, pnicogen (Z) bonding interactions have at- tracted considerable attention. The nature of this interaction has been widely studied theoretically, [22] concluding that the physical nature of pnicogen bonding is explained by an elec- trostatic attraction between the positive s-hole on the tip of the pnicogen atom (opposite to the s-bond) and a negative charge or Lewis base. Experimentally, it has been widely used in crystal engineering, [23] and supramolecular chemistry in the solid state. [24] Moreover, its importance in several biological sys- tems has been demonstrated, including ligand binding and enzyme inhibition. [25] In this regard, Alkorta and co-workers have extensively studied s-hole interactions involving pnico- gen atoms to rationalize and understand in a deeper way the physical nature and properties of these particular interac- tions. [26] Also related to the present investigation, Alkorta and co-workers have studied the interactions of NF 3 and PF 3 with neutral ambidentate electron-donor and electron-acceptor molecules using both s-holes. [27] Moreover, Li and co-workers have recently studied the dual role of pnicogen as Lewis acid and base and they have found an unexpected interplay be- tween the pnicogen bond and coordination interaction in H 3 N···FH 2 Z·MCN (Z = P and As; M = Cu, Ag, and Au). [28] In the present study, our objective was to evaluate the abili- ty of nitrogen, phosphorus, arsenium, and antimonium sp 3 compounds to establish pnicogen bonding interactions by using the opposite face of the lone pair (lp) as electron accept- or. This counterintuitive binding mode is certainly conditioned The ability of several pnicogen sp 3 derivatives ZF 3 (Z = N, P, As, Sb) to interact with electron-rich entities by means of the op- posite face to the lone pair (lp) is investigated at the RI-MP2/ aug-cc-pVQZ level of theory. The strength of the interaction ranges from 1 to 87 kJ mol 1 , proving its favorable nature, especially when the lp is coordinated to a metal center, where- by the strength of the interaction is significantly enhanced. NBO analysis showed that orbital effects are modest contribu- tors to the global stabilization of the pnicogen s-hole bonded complexes studied. Finally, a selection of Cambridge Structural Database examples are shown that demonstrate the impact of this counterintuitive binding mode in the solid state. [a] A. Bauzµ, Prof. A. Frontera Departament de Química, Universitat de les Illes Balears Crta. de Valldemossa km 7.5, 07122 Palma de Mallorca (Baleares) (Spain) E-mail : toni.frontera@uib.es [b] T. J. Mooibroek School of Chemistry of the University of Bristol Cantock’s Close, BS8 1TS, Bristol (UK) Supporting Information and the ORCID identification number(s) for the author(s) of this article can be found under http://dx.doi.org/10.1002/ cphc.201600073. ChemPhysChem 2016, 17, 1608 – 1614 # 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 1608 Articles DOI: 10.1002/cphc.201600073