C−C, C−S, and C−N Coupling versus Dealkylation Processes in the Cationic Tris(thiolato)dimolybdenum(III) Complexes [Mo 2 Cp 2 (μ-SMe) 3 L 2 ] + (L = xylNC, t -BuNC, CO, MeCN)

International audience; Reductive coupling between isocyanide ligands attached to adjacent molybdenum atoms in the bis(aryl isocyanide) complex [Mo2Cp2(μ-SMe)3(xylNC)2](BF4) (1) was initiated by addition of the hydrosulfide anion (HS−) to 1 under reflux in tetrahydrofuran, affording, in nearly quantitative yields, the dimetallaimidoyl(amino)carbene derivative [Mo2Cp2(μ-SMe)3{μ-η1(C):η1(C)-C(NHxyl)C(Nxyl)}] (7), in which both isocyanide groups of 1 are now linked by a new C−C bond. When the previous reaction was conducted in the presence of a large excess of hydrosulfide (20 equiv), under similar experimental conditions, the dithiocarbonimidate compound [Mo2Cp2(μ-SMe)2(μ-S2CNxyl)] (8) was obtained in quantitative yields. This product results from subsequent thiolate exchange and substitution reactions to afford a quadruply sulfur bridged intermediate, followed by free isonitrile addition to sulfido S atoms. Treatment of the coupled isocyanide derivative 7 with HBF4 led exclusively to the formation of the starting complex 1 by reversible isocyanide ligand decoupling. The heating of a tetrahydrofuran solution of 1 with the base NaC≡CH gave, in high yields, the μ-alkylidyne and μ-acetylide product [Mo2Cp(μ-SMe)2{μ-(η5-C5H4)(xylN)CN(xyl)C}(μ-CCH)] (9), in which a deprotonated Cp and both isocyanide ligands of 1 are now linked by new C−C and C−N bonds, and in addition an acetylide has replaced a thiolate ligand of 5. Reaction of the μ-alkylidyne complex [Mo2Cp(μ-SMe)3{μ-(η5-C5H4)(xylN)CN(xyl)C}] (5) with NaC≡CH under reflux in tetrahydrofuran produced, in high yields, the μ-alkylidyne and bis(μ-acetylide) cationic compound [Mo2Cp(μ-SMe)2{μ-(η5-C5H4)(xylN)CN(xyl)C}(μ-CCH)2]Br (10), in which two xylNC ligands have replaced a thiolate group of 5. Further reactions of the bis(μ-isocyanide) complex 10 with NaOH (suspension), on the one hand, and HBF4, on the other hand, under reflux in tetrahydrofuran or at room temperature in dichloromethane, respectively, led to the formation in high yields of the known mixed μ-alkylidyne and μ-amino-oxycarbene species [Mo2Cp(μ-SMe)2{μ-(η5-C5H4)(xylN)CN(xyl)C}{μ-η1(O):η1(C)-OCNHxyl}] (12) and the new tetrakis(isocyanide) dicationic derivative [Mo2Cp2(μ-SMe)2(xylNC)4](BF4)2 (11), respectively. The latter complex results from successive protonation at one carbon atom of the cyclopentadienyl ligand linked with isocyanides of 10 and decoupling between deprotonated Cp and isocyanide ligands. The heating of a tetrahydrofuran solution of a mixture of the bis(alkyl isocyanide) complex [Mo2Cp2(μ-SMe)3(t-BuNC)2](BF4) (2) and NaSH proceeded exclusively through the dealkylation of one tert-butyl isocyanide ligand to afford the known product [Mo2Cp2(μ-SMe)3(t-BuNC)](CN)] (13). Similar reaction of the dicarbonyl derivative [Mo2Cp2(μ-SMe)3(CO)2]Cl (3) with NaSH proceeded through dealkylation of one thiolate group to yield the new neutral sulfido species [Mo2Cp2(μ-SMe)2(μ-S)(CO)2] (14). Further heating of 14 in dichloromethane led to the formation of the chloromethanethiolate derivative [Mo2Cp2(μ-SMe)2(μ-SCH2Cl)(CO)2]Cl (15). Treatment of the bis(nitrile) species [Mo2Cp2(μ-SMe)3(MeCN)2]BF4 (4) under reflux in tetrahydrofuran with NaSH afforded, in valuable yields, the thio-amidato derivative [Mo2Cp2(μ-SMe)3(μ-MeCSNH)] (18). All new complexes have been characterized by elemental analyses and spectroscopic methods, supplemented for 7−10, 11′, 14, and 15 by X-ray diffraction studies.