The (Calix[4]arene)chloromolybdate(IV) Anion [MoCl(Calix)]^-: A Convenient Entry into Molybdenum Calix[4]arene Chemistry

The complex (HNEt3)[MoCl(NCMe)(Calix)] (1), prepared from the reaction of [MoCl4(NCMe)2] with p-tert-butylcalix[4]arene, H4Calix, in the presence of triethylamine, has been used as a source of the d2-[Mo(NCMe)(Calix)] fragment. Complex 1 is readily oxidized with PhICl2 to afford the molybdenum(VI) dichloro complex [MoCl2(Calix)] (2). Both complexes are a convenient entry point into molybdenum(VI) and molybdenum(IV) calixarene chemistry. The reaction of 1 with trimethylphosphine and pyridine in the presence of catalytic amounts [Ag(OTf)] led to the formation of neutral d2 complexes [Mo(PMe3)(NCMe)(Calix)] (3) and [Mo(NC5H5)(NCMe)(Calix)] (4). The role of the silver salt in the reaction mixture is presumably the oxidation of the chloromolybdate anion of 1 to give a reactive molybdenum(V) species. The same reactions can also be initiated with ferrocenium cations such as [Cp2Fe](BF4). Without the presence of coordinating ligands, the dimeric complex [{Mo(NCMe)(Calix)}2] (5) was isolated. The reaction of 1 with Ph2CN2 led to the formation of a metallahydrazone complex [Mo(N2CPh2)(NCMe)(Calix)] (6), in which the diphenyldiazomethane has been formally reduced by two electrons. Molybdenum(VI) complexes were also obtained from reaction of 1 with azobenzene and sodium azide in the presence of catalytic amounts of silver salt. The reaction with azobenzene led under cleavage of the nitrogen nitrogen bond to an imido complex [Mo(NPh)(NCMe)(Calix)] (7), whereas the reaction with sodium azide afforded the mononuclear molybdenum(VI) nitrido complex (HNEt3)[MoN(Calix)] (8).