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

Treatment of U(acac)4 with the hexadentate Schiff base H2L(i) gave the [UL(i)2] 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(L(i))(acac)2] compounds could not be isolated because of their ready disproportionation into [UL(i)2] and U(acac)4. Compounds 2 and 4 adopt a meridional configuration in the solid state and in solution, while 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.