The isolation and chemistry of niobium and tantalum dimethylamides containing mono- and di-aryloxide ancillary ligands

The salt Complex [Me2NH2][Ta(NMe2)(2)Cl-4](4) has been isolated from the re action of [Ta2Cl10] with Me2NH and the anion shown to contain mutually cis- dimethylamido ligands. 4 reacts with Me2NH and pyridine to produce the neut ral adducts mer,cis-[Ta(NMe2)(2)Cl-3(HNMe2)] 1 (known compound) and [Ta(NMe 2)(2)Cl-3(py)] 5. The solid state structure of [Ta(NMe2)(2)Cl-3(py-4Ph)] 7 shows a mer,cis arrangement of Cl and NMe2 groups. The addition of 2,3,5,6- tetraphenyl- or 2,6-di-isopropyl-phenol (2 equiv) to solutions of I in benz ene was found to produce a mixture of two isomers each containing a residua l Ta-NMe2 and Ta-NHMe2 group. These were formulated as [Ta(OAr)(2)Cl-2(NMe2 )-(NHMe2)] with the coordinated amine trans to the Ta-NMe2 group and arylox ide ligands either mutually cis or trans. The cis isomer was found to therm ally convert to the trans form. The trans isomers [M(OAr)(2)Cl-2(NMe2)(NHMe 2)] (M = Ta, OAr = 2,3,5,6-tetraphenyl- or 2,6-di-isopropyl-phenoxide; M = Nb, OAr = 2,3,5,6-tetraphenylphenoxide) are obtained in high yield by treat ment of the corresponding tri(chlorides) [M(OAr)(2)Cl-3] with excess Me2NH. The isomorphous/isostructural compounds [M(OC6HPh4-2,3,5,6)(2)Cl-2(NMe2)(p y)] (M = Nb, Ta) were structurally characterized and shown to contain the p yridine ligand trans to the Ta-NMe2 group with mutually trans aryloxides. A ddition of 2,2'-methylenebis(6-phenylphenol) {(HOC6H3Ph)(2)CH2} to 1 result ed in formation of the compound [Ta{(OC6H3Ph)(2)CH2}Cl-2(NMe2)(HNMe2)] 15. The solid state structure of 15 shows the nitrogen atoms to be mutually tra ns with cis aryloxide oxygen atoms. The eight-membered dioxametallacycle ri ng is puckered with the methylene bridge folded up towards the Ta-NMe2 grou p. The reaction pathways leading to these products are discussed.