The σ-arsolido complex [Mo(AsC4Me4)(CO)3(η5-C5H5)] is alkylated at arsenic by MeOTf to afford the pentamethylarsole complex [Mo(MeAsC4Me4)(CO)3(η5-C5H5)](OTf) while iodomethane affords a mixture of [Me2AsC4Me4]I, [MoMe(CO)3(η5-C5H5)], [MoI(CO)3(η5-C5H5)] and the arsole complexes cisoid- and transoid-[MoI(MeAsC4Me4)(CO)2(η5-C5H5)] and transoid-[Mo{C(O)Me}(MeAsC4Me4)(CO)2(η5-C5H5)], The arsole ligand in [Mo(MeAsC4Me4)(CO)3(η5-C5H5)](OTf) is readily liberated by NaI in acetone to afford free MeAsC4Me4 and [MoI(CO)3(η5-C5H5)]. In a similar manner, the reaction of [Mo(AsC4Ph4)(CO)3(η5-C5H5)] with MeI affords MeAsC4Ph4 and [MoI(CO)3(η5-C5H5)], while [Mo{AsC4(SiMe3)-2-Me2-3,4}(CO)3(η5-C5H5)] with MeOTf affords [Mo{MeAsC4(SiMe3)-2-Me2-3,4}(CO)3(η5-C5H5)](OTf). The reaction of [Mo(AsC4Me4)(CO)3(η5-C5H5)] with activated alkynes (RCCR: R = CF3, CO2Me) does not proceed via [4 + 2] cyclo-addition but rather electrophilic attack at arsenic followed by metallacyclisation with incorporation of a carbonyl ligand in the spirocyclic complexes [Mo{As(C4Me4)CRCRCO}(CO)2(η5-C5H5)].
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