MS₂(Me₂PC₂H₄PMe ₂)₂ (M = Mo, W): Acid-base properties, proton transfer, and reversible protonolysis of sulfido ligands

The acid-base reactivity of MS₂(dmpe)₂, where M = Mo (1) and W (2) and dmpe = Me₂PCH₂CH₂PMe ₂, was examined. Compounds 1 and 2 arise via the one-pot reaction of (NH₄)₂MS₄ and dmpe. Protonation of these species gives the stable salts [MS(SH)(dmpe)₂]X. The pK_a's of the Mo and W compounds are estimated to be 16.5 and 15.5, respectively. Protonation causes the M=S distances to diverge from 2.24 Å to 2.06 and 2.57 Å, whereas the Mo-P distances do not change appreciably. ¹H and ³¹P NMR studies for [1H]BAr^F₄ reveal that the proton exchange is competitive with the NMR time scale; at low temperatures, individual signals for both the parent disulfide and its conjugate acid can be observed. Treatment of 1 with excess HOTf liberates H₂S to afford [MoS(OTf)(dmpe)₂]OTf, which forms an adduct with CD ₃CN and regenerates 1 upon treatment with SH^-/Et ₃N solutions. Consistent with its ready protonation, complex 1 is methylated, and the use of excess MeOTf gives [MoS(OTf)(dmpe)₂] ⁺ and Me₂S in a rare example of double alkylation at a sulfido ligand.