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.