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

Abstract

The salt complex [Me₂NH₂][Ta(NMe₂)₂Cl₄] 4 has been isolated from the reaction of [Ta₂Cl₁₀] with Me₂NH and the anion shown to contain mutually cis-dimethylamido ligands. 4 reacts with Me₂NH and pyridine to produce the neutral adducts mer,cis-[Ta(NMe₂)₂Cl₃(HNMe₂)] 1 (known compound) and [Ta(NMe₂)₂Cl₃(py)] 5. The solid state structure of [Ta(NMe₂)₂Cl₃(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 1 in benzene was found to produce a mixture of two isomers each containing a residual Ta–NMe₂ and Ta–NHMe₂ group. These were formulated as [Ta(OAr)₂Cl₂(NMe₂)(NHMe₂)] with the coordinated amine trans to the Ta–NMe₂ group and aryloxide ligands either mutually cis or trans. The cis isomer was found to thermally convert to the trans form. The trans isomers [M(OAr)₂Cl₂(NMe₂)(NHMe₂)] (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 treatment of the corresponding tri(chlorides) [M(OAr)₂Cl₃] with excess Me₂NH. The isomorphous/isostructural compounds [M(OC₆HPh₄-2,3,5,6)₂Cl₂(NMe₂)(py)] (M = Nb, Ta) were structurally characterized and shown to contain the pyridine ligand trans to the Ta–NMe₂ group with mutually trans aryloxides. Addition of 2,2′-methylenebis(6-phenylphenol) {(HOC₆H₃Ph)₂CH₂} to 1 resulted in formation of the compound [Ta{(OC₆H₃Ph)₂CH₂}Cl₂(NMe₂)(HNMe₂)] 15. The solid state structure of 15 shows the nitrogen atoms to be mutually trans with cis aryloxide oxygen atoms. The eight-membered dioxametallacycle ring is puckered with the methylene bridge folded up towards the Ta–NMe₂ group. The reaction pathways leading to these products are discussed.