77Se NMR Spectroscopic, DFT MO, and VBT Investigations of the Reversible Dissociation of Solid (Se6I2)[AsF6]2•2SO2 in Liquid SO2 to Solutions Containing 1,4-Se6I22+ in Equilibrium with Sen2+ (n = 4, 8, 10) and Seven Binary Selenium Iodine Cations: Preliminary Evidence for 1,1,4,4-Se4Br42+ and cyclo-Se7Br+

Abstract

The composition of a complex equilibrium mixture formed upon dissolution of (Se6I2)[AsF6]2·2SO2 in SO2(l) was studied by 77Se NMR spectroscopy at −70 °C with both natural-abundance and enriched 77Se-isotope samples (enrichment 92%). Both the natural-abundance and enriched NMR spectra showed the presence of previously known cations 1,4-Se6I22+, SeI3+, 1,1,4,4-Se4I42+, Se102+, Se82+, and Se42+. The structure and bonding in 1,4-Se6I22+ and 1,1,4,4-Se4I42+ were explored using DFT calculations. It was shown that the observed Se−Se bond alternation and presence of thermodynamically stable 4pπ−4pπ Se−Se and 4pπ−5pπ Se−I bonds arise from positive charge delocalization from the formally positively charged tricoordinate Se+. The 77Se chemical shifts for cations were calculated using the relativistic zeroth-order regular approximation (ZORA). In addition, calculations adding a small number of explicit solvent molecules and an implicit conductor-like screening model were conducted to include the effect that solvent has on the chemical shifts. The calculations yielded reasonable agreement with experimental chemical shifts, and inclusion of solvent effects was shown to improve the agreement over vacuum values. The 77Se NMR spectrum of the equilibrium solution showed 22 additional resonances. These were assigned on the basis of 77Se−77Se correlation spectroscopy, selective irradiation experiments, and spectral simulation. By combining this information with the trends in the chemical shifts, with iodine, selenium, and charge balances, as well as with ZORA chemical shift predictions, these resonances were assigned to acyclic 1,1,2-Se2I3+, 1,1,6,6-Se6I42+, and 1,1,6-Se6I3+, as well as to cyclic Se7I+ and (4-Se7I)2I3+. A preliminary natural-abundance 77Se NMR study of the soluble products of the reaction of (Se4)[AsF6]2 and bromine in liquid SO2 included resonances attributable to 1,1,4,4-Se4Br42+. These assignments are supported by the agreement of the observed and calculated 77Se chemical shifts. Resonances attributable to cyclic Se7Br+ were also observed. The thermal stability of (Se6I2)[AsF6]2·2SO2(s) was consistent with estimates of thermodynamic values obtained using volume-based thermodynamics (VBT) and the first application of the thermodynamic solvate difference rule for nonaqueous solvates. (Se6I2)[AsF6]2·2SO2(s) is the first example of a SO2 solvate for which the nonsolvated parent salt, (Se6I2)[AsF6]2(s), is not thermodynamically stable, disproportionating to Se4I4(AsF6)2(s) and Se8(AsF6)2(s) (ΔG° for the disproportion reaction is estimated to be −17 ± 15 kJ mol−1 at 298 K from VBT theory).