Synthesis and crystal structure of uranium(iv) complexes with compartmental Schiff bases: from mononuclear species to tri- and tetranuclear clusters

Abstract

Treatment of U(acac)₄ with the hexadentate Schiff base H₂Lⁱ gave the [ULⁱ₂] complexes 1–4 [H₂L¹ = N,N′-bis(3-methoxysalicylidene)-2-methyl-1,2-propanediamine, H₂L² = N,N′-bis(3-methoxysalicylidene)-1,2-phenylenediamine, H₂L³ = N,N′-bis(3-methoxysalicylidene)-2-aminobenzylamine and H₂L⁴ = N,N′-bis(3-methoxysalicylidene)-2,2-dimethyl-1,3-propanediamine for 1–4, respectively]. The [U(Lⁱ)(acac)₂] compounds could not be isolated because of their ready disproportionation into [ULⁱ₂] and U(acac)₄. Compounds 2 and 4 adopt a meridional configuration in the solid state and in solution, while 3 exists in solution as the two equilibrating meridional and sandwich isomers and crystallizes in the meridional isomeric form. Reaction of U(acac)₄ with H₄L⁵ afforded the expected compound [U(H₂L⁵)(acac)₂] (5) [H₄L⁵ = N,N′-bis(3-hydroxysalicylidene)-2-methyl-1,2-propanediamine] but, in the presence of H₄L⁶ and H₄L⁷, U(acac)₄ was transformed in a serendipitous and reproducible manner into the tri- and tetranuclear U(IV) complexes [U₃(L⁶)(HL⁶)₂(acac)₂] (6) and [U₄(HL⁷)₄(H₂L⁷)₂] (7) [H₄L⁶ = N,N′-bis(3-hydroxysalicylidene)-1,2-phenylenediamine and H₄L⁷ = N,N′-bis(3-hydroxysalicylidene)-2-aminobenzylamine]. The crystal structures of 6·3thf and 7·5thf show the assembling role of the Schiff-base ligands.