Reactions of Molybdenum and Tungsten Oxide Tetrafluoride with Sulfur(IV) Lewis Bases: Structure and Bonding in [WOF], MOF(OSO), and [SF][MOF] (M = Mo, W).

The structure of [WOF] has been reinvestigated by low-temperature X-ray crystallography and DFT (MN15/def2-SVPD) studies. Whereas the WF ring of the tetramer is planar and disordered in the solid state, the optimized gas-phase geometry prefers a disphenoidally puckered WF ring and demonstrates asymmetric fluorine bridging. Dissolution of MOF (M = Mo, W) in SO and SF results in the formation of MOF(OSO) and [SF][MOF], respectively. Both SO adducts and [SF][MoOF] have been characterized by X-ray crystallography. The crystal structure of [SF][MoOF] reveals dimerization of the ion pair that results in a rare heptacoordinate sulfur center. Optimization of the {[SF][MOF]} dimers in the gas phase, however, results in the elongation of one contact such that the sulfur centers are effectively hexacoordinate. Meanwhile, the crystal structure of [SF][WOF]·HF instead demonstrates hexacoordinate sulfur centers and a highly unusual coordination to [SF] from [WOF] through an oxido ligand. While [SF][WOF] does not decompose at ambient temperature, MOF(OSO) and [SF][MoOF] are unstable toward evolution of SO or SF. Computational studies reveal that the monomerization of [WOF] in the gas phase at 25 °C is thermodynamically unfavorable using SO, but favorable using SF, consistent with the relative thermal stabilities of WOF(OSO) and [SF][WOF].