一、 Introduction
Since the discovery of xenon
hexafluoroplatinate,
XePtF6, the first noble gas compound, by Bartlett in
1962[1], numerous analogue species of xenon and
krypton have become accessible as chemical reagents in the condensed
phase; many neutral and ionic species containing noble gas elements,
including light argon, neon, and helium, have also been discovered by
gas-phase
spectroscopy[2-14].
However, only neutral species containing a heavy noble gas, such as
krypton, xenon, and radon, have been chemically synthesized to date.
Light noble gases, namely, argon,
neon, and helium, are the last three elements on the periodic table for
which no stable compound is known.
The synthesis of HArF[15,16], the first chemically
bonded neutral argon compound, in the year 2000 received great
attention. The molecule was prepared by photolyzing HF embedded in solid
argon at low temperature. This molecule could be identified by comparing
its experimentally observed infrared spectrum with those obtained from
quantum chemical calculations of its vibrational modes. Dong et al.
recently reported in a combined experimental and computational study
that a stable helium and sodium compound,
Na2He, can be
synthesized in a diamond anvil
cell[17]. Such remarkable work was based on the
idea that high pressure can alter the bonding characteristics of noble
gases such as xenon, krypton, and argon[18-21].
Although HArF and Na2He evidently form chemical bonds
with the noble gas, they are only stable under unconventional
environments. The former is stable only up to 27 K in an argon matrix,
while the latter must be kept in a diamond anvil cell at
>113 GPa. Both
compounds are not genuinely
accessible as chemical reagents. Thus,
the synthesis of a genuine bulk salt
compound (i.e., as a chemical reagent) containing
a noble gas lighter than krypton
remains a fascinating
challenge.
To the best of our knowledge, only
two theoretical papers addressing this challenge. The first of these
reports was published by Frenking et
al.[22], who
predicted that bulk salt compounds
of ArF+ cations are isolable in principle because the
calculated bond dissociation energy of the cation is very high (49
kcal/mol). The second report was
from Aschi et
al.[23], who predicted thermochemically stable
FBeNg+ (Ng=helium,
neon, argon) cations with energies of dissociation into
BeF+ and Ng sufficiently large to suggest that these
cations could be suitable to prepare
the bulk salt compounds of light noble gases.
To date, these two theoretical predictions have yet to be verified. Our
interest in the existence of
these bulk salt compounds led us to
perform a different set of calculations on the basis of a more
comprehensive consideration of all factors related to the stability of
these salts compared with previous studies
(Supporting Information). In
contrast to the previous predictions of Franking et al. and Aschi et
al., our results show that the outlooks for synthesizing bulk salt
compounds \(\text{ArF}^{+}\text{MF}_{6}^{-}\) and
FBeNg+NO2− are
obscure. This contradiction can be attributed to the fact that, when the
aforementioned authors studied the stability of ionic solid salts
(A+ B− type), they only focused on
the adiabatic electron affinity energy of the A+cation, the adiabatic first ionization energy of the
B− anion, and the
lattice energy of ionic solid salts
A+B−. In fact, the stability of solid
salts
A+B− is solely determined by the sum
of the adiabatic electron affinity of the A+ cation, the adiabatic first
ionization potential of the B− anion, and the lattice
energy of the solid salt A+ B−, only
if the corresponding neutral species A(g) and B(g) are kinetically
stable species possessing an activation energy barring further reaction.
However, if the corresponding neutral species A(g) and B(g) are
vibrationally unstable and undergo further spontaneous decomposition
without a barrier, the first stable reaction products must be used
instead of A(g) or B(g). Our results reveal that, during their study on
the existence of the bulk salt compounds\(\text{ArF}^{+}\text{MF}_{6}^{-}\) and
FBeNg+NO2−,
Franking et al. and Aschi et al. used the unstable states F(g) and
BeF(g) as stable starting materials
and ignored some important factors, such as the bond energy of the F–F
bond, BE (F2,g), and the enthalpy of the
disproportionation reaction of BeF(g),
∆Hf298(1/2Be(s)+1/2BeF2(g)→BeF(g)).
These issues may account for the failure to prepare the aforementioned
bulk salt compounds.
Another challenge is presented by
the synthesis of the salt
compound[24].
This advancement was a remarkable breakthrough because it proved the
existence of the first bulk salt
compound between Au and Xe. Both elements were previously considered
archetypes of chemical inertia.
Other noble metal bulk salt
compounds containing the noble gas Xe, were subsequently
reported[25-27].
Chemical intuition indicates that noble metals and gases are inert
toward forming chemical bonds, and the existence of these bulk salt
compounds with a Xe–Au bond may surprise some chemists. Today, the
chemistry of noble metals and gases
is recognized to be rich. In 1995, Pyykk[28]predicted the presence of significant covalent interactions between Au
and noble gases by analyzing the stability of NgAu+and NgAuNg+ species. Since then, unceasing progress
has been made. A series of compounds has been theoretically and
experimentally investigated[29-39].
However, the synthesis
of noble metal bulk salt compounds
containing noble gases other than Xe remains a fascinating challenge.
The following question is intriguing to ask: Is it possible to
synthesize
salt
compounds?
The answer to this question may
address two other fascinating challenges: (1) the synthesis of a bulk
salt compound containing a noble gas lighter than krypton and (2) the
synthesis of the congeners of containing noble gases other than Xe.
Our study is inspired by the results reported on the
cation(40,58), which are
light congeners of the synthesized
Au–Xe cation in the gas phase.
In this work, we performed a systematic and comprehensive
study on the stability of bulk salts
and predicted their stability by
using Born–Haber cycles. This method is well established and widely
used to evaluate the stability of ionic salts if reliable values for the
lattice energy of the salts and ancillary thermochemical data are
available.
This article is outlined as follows: In the next section, we first
illustrate the employed methods.
Thereafter, we present and discuss the obtained results. We perform
preliminary calculations to accurately evaluate the reliability of our
results with respect to the method and basis set. Subsequently, we
discuss the stability of salt compounds. Finally, a summary and brief
chemical significance of our study are disclosed.