Molybdenum(VI) imidodisulfur complexes formed via double sulfur–carbon bond cleavage of dithiocarbamates

Abstract

Thermolysis of the bis(oxo) complexes [MoO₂(S₂CNR′₂)₂]1 with organic isocyanates afforded imidodisulfur complexes [Mo(NR)(η²-S₂)(S₂CNR′₂)₂]2 in moderate yields. Three of the latter, [Mo(NPh)(η²-S₂)-(S₂CNEt₂)₂]2a, [Mo(NBu^t)(η²-S₂)(S₂CNMe₂)₂]2j and [Mo(NC₆H₃Pr₂ⁱ-2,6)(η²-S₂)(S₂CNMe₂)₂]2k have been characterised by X-ray crystallography. All contain a distorted pentagonal-bipyramidal metal centre, the disulfur ligand lying in the equatorial plane while the linear imide ligand occupies an axial site. Reaction of 2a with P(OEt)₃ afforded the dinuclear molybdenum(V) complex [Mo₂O₂(µ-NPh)(µ-S)(S₂CNEt₂)₂]3 which has also been crystallographically characterised. Thermolysis of 2a with an excess of the activated alkyne dimethyl acetylene dicarboxylate yielded three isomeric insertion products formulated as [Mo{SC(R)=C(R)NPh}-{η⁴-SC(R)=C(R)C(NEt₂)S}(S₂CNEt₂)]4 and 5(differing in the relative orientations of sulfur) and [Mo{SC(R)=C(R)NPh}{η³-SC(R)=C(R)SC(NEt₂)}(S₂CNEt₂)]6(R = CO₂Me). The nature of the sulfur–carbon bond cleavage reaction was investigated in a series of experiments centred around the formation of 2a which is concomitant with that of [{MoO(µ-NPh)(S₂CNEt₂)}₂]7a. Thermolysis of [MoO₂(S₂CNEt₂)₂]1a alone afforded the known molybdenum(V) complex [Mo₂O₂(µ-O)(µ-S)(S₂CNEt₂)₂] and, while addition of PhNCO to [MoO(η²-S₂)(S₂CNEt₂)₂] afforded 2a, imido substitution was not noted with Bu^tNCO, thus ruling out the general intermediacy of [MoO(η²-S₂)(S₂CNEt₂)₂] in the dithiocarbamate cleavage process. Thermolysis of bis(imido) complexes [Mo(NR)₂(S₂CNEt₂)₂]8(R = Ph or C₆H₄Me-o) did not afford the respective disulfur complexes, however thermolysis of [MoO(NPh)(S₂CNEt₂)₂] did give 2a and 7a, thus implicating the oxoimidomolybdenum(VI) complexes as key reaction intermediates. These observations allow a general reaction scheme to be formulated.