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(NBut)(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 ButNCO, 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.