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.