Complexes of niobium and tantalum containing metal-metal bonds

Abstract

A new entry into the area of low valent niobium and tantalum complexes containing metal-metal bonds is described. M₂X₆(μ-THT)(THF)₂ complexes, where M = Nb or Ta, X = Cl or Br and THT = tetrahydrothiophene, are conveniently synthesized via the reaction of M₂M₆(THT)₃with THF (THF = tetrahydrofuran). Unlike M₂X₆(THT)₃, M₂X₆(μ-THT)(THF)₂ can be easily reduced by sodium amalgam. In THF and in the presence of THT, complexes of the type M₂X₆(μ-THT)₃²⁻form. Salts of these compounds can also be synthesized directly from the metal pentahalides in a "two step, one pot" reaction in which M₂X₆(THT)₃ is an intermediate. The M₂X₆(μ-THT)₃²⁻complexes have a face-sharing bioctahedral structure with metal-metal triple bonds, the first reported for niobium or tantalum. Surprisingly, these compounds are relatively inert. They do not react with phosphines or carboxylic acid salts, even under somewhat forcing conditions. Reaction of [Nb₂Cl₆(μ-THT)₃]²⁻ with pyridine gives mononuclear NbCl₂(py)₄. Reaction of [Nb₂Cl₆(μ-THT)₃]²⁻ with a solution of acetic acid/acetic anhydride results in a mixture of oxidized products. The sodium salts of M₂X₆(μ-THT)₃²⁻decompose slowly in THF to give tetrameric [M₂X₅(μ-THT)₃]₂²⁻. The four metal atoms in these compounds participate in two metal-metal triple bonds. When Nb₂Cl₆(μ-THT)(THF)₂ is reduced in THF and in the presence of PMe₂Ph or P[superscript n]Bu₃, trinuclear complexes of formula Nb₃Cl₇L₆, where L = PMe₂Ph or P[superscript n]Bu₃, form. Reaction of Nb₃Cl₇(P[superscript n]Bu₃)₆ with dmpe (dmpe = bis(dimethylphosphino)ethane) gives Nb₃Cl₇(dmpe)₃. In general, trinuclear complexes of niobium and tantalum are rare, and this type of discrete trinuclear complex, which contains one capping, three bridging and nine terminal ligands, has been seen only once before for the group 5 metals, although many examples of similar group 6 metal complexes are known. Full details of the synthesis and characterization of these complexes are described and an interpretation of their electronic structures is given.