Preparation, X-ray Crystal Structure Determination, Lattice Potential Energy, and Energetics of Formation of the Salt S4(AsF6)2·AsF3Containing the Lattice-Stabilized Tetrasulfur [2+] Cation. Implications for the Understanding of the Stability of M42+and M2+(M = S, Se, and Te) Crystalline Salts

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Abstract
S4(AsF6)2·AsF3 was prepared by the reaction of sulfur with arsenic pentafluoride in liquid AsF3 (quantitatively) and in anhydrous HF in the presence of trace amounts of bromine. A single-crystal X-ray structure of the compound has been determined: monoclinic, space group P21/c, Z = 4, a = 7.886(1) Å, b = 9.261(2) Å, c = 19.191(3) Å, β = 92.82(1)°, V = 1399.9(4) Å3, T = 293 K, R1 = 0.052 for 1563 reflections (I > 2σ(I) 1580 total and 235 parameters). We report a term-by-term calculation of the lattice potential energy of this salt and also use our generalized equation, which estimates lattice energies to assist in probing the homopolyatomic cation thermochemistry in the solid and the gaseous states. We find S4(AsF6)2·AsF3 to be more stable (ΔfH°[S4(AsF6)2·AsF3,c] ≈ −4050 ± 105 kJ/mol) than either the unsolvated S4(AsF6)2 (ΔfH°[S4(AsF6)2,c] ≈ −3104 ± 117 kJ/mol) by 144 kJ/mol or two moles of S2AsF6 (c) and AsF3 (l) by 362 kJ/mol. The greater stability of the S42+ salt arises because of the greater lattice potential energy of the 1:2 solvated salt (1734 kJ/mol) relative to twice that of the 1:1 salt (2 × 541 = 1082 kJ/mol). The relative lattice stabilization enthalpies of M42+ ions relative to two M2+ ions (i.e., in M4(AsF6)2 (c) with respect to two M2AsF6 (c) (M = S, Se, and Te)) are found to be 218, 289, and 365 kJ/mol, respectively. Evaluation of the thermodynamic data implies that appropriate presently available anions are unlikely to stabilize M2+ in the solid phase. A revised value for ΔfH°[Se4(AsF6)2,c] = −3182 ± 106 kJ/mol is proposed based on estimates of the lattice energy of Se4(AsF6)2 (c) and a previously calculated gas-phase dimerization energy of 2Se2+ to Se42+.

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