Synthesis of [F3SXeF][AsF6] and structural study by multi-NMR and Raman spectroscopy, electronic structure calculations, and X-ray crystallography

The salt, [F3S equivalent to NXeF][AsF6], has been synthesized by the reaction of [XeF][AsF6] with liquid NSF3 at -20 degrees C. The Xe-N bonded cation provides a rare example of xenon bound to an inorganic nitrogen base in which nitrogen is formally sp-hybridized. The F3S equivalent to NXeF+ cation was characterized by Raman spectroscopy at -150 degrees C and by Xe-129, F-19, and N-14 NMR spectroscopy in HF solution at -20 degrees C and in BrF5 solution at -60 degrees C. Colorless [F3S equivalent to NXeF][AsF6] was crystallized from HF solvent at -45 degrees C, and its low-temperature X-ray crystal structure was determined. The Xe-N bond is among the longest Xe-N bonds known (2.236(4) angstrom), whereas the Xe-F bond length (1.938(3) angstrom) is significantly shorter than that of XeF2 but longer than in XeF+ salts. The Xe-F and Xe-N bond lengths are similar to those of HCNXeF+, placing it among the most ionic Xe-N bonds known. The nonlinear Xe-N-S angle (142.6(3)(o)) contrasts with the linear angle predicted by electronic structure calculations and is attributed to close N center dot center dot center dot F contacts within the crystallographic unit cell. Electronic structure calculations at the MP2 and DFT levels of theory were used to calculate the gas-phase geometries, charges, bond orders, and valencies of F3S equivalent to NXeF+ and to assign vibrational frequencies. The calculated small energy difference (7.9 kJ mol(-1)) between bent and linear Xe-N-S angles also indicates that the bent geometry is likely the result of crystal packing. The structural studies, natural bond orbital analyses, and calculated gas-phase dissociation enthalpies reveal that F3S equivalent to NXeF+ is among the weakest donor-acceptor adducts of XeF+ with an Xe-N donor-acceptor interaction that is very similar to that of HC equivalent to NXeF+, but considerably stronger than that of F3S equivalent to NAsF5. Despite the low dissociation enthalpy of the donor-acceptor bond in F3S equivalent to NXeF+, Xe-129, F-19, and N-14 NMR studies reveal that the F3S equivalent to NXeF+ cation is nonlabile at low temperatures in HF and BrF5 solvents.