1. Introduction
In recent times, the scientific community has shown an increased interest in non–halogen electronegative moieties (NHEMs) due to their diverse applications and novel properties. NHEMs act as oxidizing agents in chemical reactions [1–3], play an active role in the formation of novel salts and other compounds [4,5], and are also expected to be useful in future designing of efficient energy storage devices [6]. The general structure of these moieties isϺ ϒN , where, N represents the coordination number satisfying the relation N × vn= vm + 1; vn is normal valence of the ligand atom ϒ and vm is maximal valence of the central atom Ϻ . Electron affinity (EA) of such chemical species was predicted by Gutsev and Boldyrev [7] to be even higher than the highest atomic EA [5] corresponding to chlorine (3.617 ± 0.003 eV). Due to their ability to display halogen like behavior, they were aptly termed as superhalogens . These superhalogensusually consist of a central atom Ϻ , which is a metal corresponding to s or d block elements of the periodic table. The s block elements are capable of binding with a limited number of electronegative atoms due to their fixed valency. However, transition metal atoms having variable oxidation states due to the presence of d orbital, are able to combine with relatively larger number of electronegative species. Workers [5–13] have thoroughly investigated such type of superhalogens containing transition metal atoms as the central atom Ϻ . In such compounds, the transition metal atom combines with other atoms, usually, halogens leading to the formation of strongly bound electronegative species. Current research efforts are directed towards finding novel structural configurations involving alternate atomic combinations, which also satisfy the superhalogen criteria [7].
Pseudohalogens are polyatomic molecules that have the capability to form stable negative ions [14,15], just like the halogens. Due to their electronegative nature, these species can acquire an extra electron leading to the formation of negatively charged ions. Hence, these compounds display chemical behavior similar to that of halogens [14,15]. They are constituted of two or more atoms whose bonding remains unchanged during a chemical reaction. The concept of pseudohalogens was introduced in 1925 by Birckenbach [14], who predicted that they have halogen like characteristics and satisfy certain conditions [16,17]. The criteria for a polyatomic molecule to be classified as a pseudohalogen is that, it should be a strongly bound monovalent chemical group, must have the ability to form singly charged anion, and also must be able to combine with hydrogen nucleus to form pseudohalogen–hydrogen acid [16,17]. Workers [18] have particularly concentrated their efforts towards the study of pseudohalogen species such as CN, SCN, OCN, etc. whose anions are more stable as compared to halogens. Superacids corresponding to such chemical groups are combinations of pseudohalogen anions and the hydrogen nucleus [19]. Acidity of superacids was described by Gillespie [20] to be greater than that of 100% sulfuric acid and the Hammett acidity function (H0) to be lower than –12 [20,21]. Sulfuric acid is considered to be the standard while designating compounds as superacids [22,23]. A technically more accurate description for the strength of superacid is described in terms of Gibbs’ free energy change for deprotonation [24,25] which should be lower as compared to that of pure sulfuric acid (302.2 kcal/mol) [26]. As a general measure, compounds having Gibbs’ free energy change for deprotonation values lesser than 300 kcal/mol are termed as superacids [27–29]. Olah et al. [30] were the first scientists to report synthesis of a superacid (FSO3H·SbF5) known as “magic acid”. It was composed of a mixture of antimony pentafluoride (SbF5) and fluorosulfuric acid (HSO3F) in the molar ratio of 1:1. It has the ability to interact with even hydrocarbons and satisfies the criteria of superacids. Currently, fluoroantimonic acid (HSbF6) is considered to be the strongest superacid [25,26]. Researchers [26,31–38] are actively engaged in the exploration of new superacids which can react even with very weak bases. Such superacids have application in the catalysis and synthesis of compounds particularly those which are useful in medical science [29,39–41]. Research is still ongoing in the search for better superacids. An alternate strategy maybe to use pseudohalogen containing transition metal superhalogens for designing superacids by interacting them with hydrogen nucleus.
The pseudohalogen ligand CN is of particular interest to researchers for use in superhalogens due to their simple structure and ability to combine with various elements. Molecules containing the CN pseudohalogen are found to be having high values of vertical detachment energy (VDE) and hence, have greater probability of displaying superhalogen properties [3]. Samanta et al. [15] have explored the superhalogen properties of Au(CN)n using theoretical calculations. Smuczyńska et al. [18] have used ab initio quantum chemical calculations to predict the properties of LiX2¯, NaX2¯, BeX3¯, MgX3¯, CaX3¯, BX4¯, and AlX4¯, where, X represents the CN group. A major benefit of such calculations is their ability to accurately predict superhalogen properties, along with the strength and other traits of corresponding superacid. Experimental laboratory assessment of such superacid is a difficult proposition due to the necessity of heavy–duty apparatus and expertise required in handling them. Computational prediction and analysis of superacid provides important information that maybe useful for chemists involved in chemical synthesis [15]. Previous theoretical investigations [42] have shown that Pt atom can combine with F and Cl to produce superhalogens. Continuing with this line of investigation, the current work aims to study whether the interactions of Pt atom with pseudohalogen CN moieties leads to the formation of superhalogens. It also deals with the in–depth theoretical study of the properties of Pt(CN)n complexes and their corresponding supersalts and superacids.