Transformations of aryl isothiocyanates on tetraphosphine tungsten complexes and reactivity of the resulting dithiocarbonimidate ligand

Abstract

Treatment of [WH₄(κ⁴-P4)] (3: P4 = meso-o-C₆H₄(PPhCH₂CH₂PPh₂)₂) with aryl isothiocyanate ArNCS at 50 °C afforded the dithiocarbonimidate-isocyanide complex [W(κ²-S₂CNAr)(CNAr)(κ⁴-P4)] (4) in moderate yields. The reaction also produced ArNHCH₃ and a small amount of ArNH₂. The yield of the hydrodesulfurization product ArNHCH₃ increased when the reaction was conducted under H₂ (up to 0.65 equiv. to 3 for Ar = p-MeC₆H₄ (Tol)). Complex 4 was proposed to be formed via reductive disproportionation of two ArNCS molecules on a zero-valent W species generated by dissociation of H₂ from 3. The reaction of W(0) complex [W(dppe)(κ⁴-P4)] (dppe = Ph₂PCH₂CH₂PPh₂) with ArNCS also yielded 4 accompanied by free dppe, in contrast to that of [Mo(dppe)(κ⁴-P4)], which had been previously reported to undergo sulfur-atom transfer to phosphine ligands. The dithiocarbonimidate ligands in 4a (Ar = Tol) received the addition of electrophiles [PhMe₂NH][BF₄], MeI, and PhCOCl selectively at the N atom to afford the cationic dithiocarbamate complexes [W(κ²-S₂CNHTol)(CNTol)(κ⁴-P4)][BF₄] (6), [W{κ²-S₂CN(Me)Tol}(CNTol)(κ⁴-P4)]I (7), and [W{κ²-S₂CN(COPh)Tol}(CNTol)(κ⁴-P4)]Cl (8). Complexes 4a, 6, 7, and 8 have been characterized by spectroscopic and crystallographic methods, and the donor strengths of their κ²-dithio ligands are discussed.