Synthesis and crystal structure of tetra- and hexanuclear uranium(iv) complexes with hexadentate compartmental Schiff-base ligands

Abstract

Treatment of UCl₄ with the hexadentate Schiff bases H₂Lⁱ in thf gave the expected [ULⁱCl₂(thf)] complexes [H₂Lⁱ = N,N′-bis(3-methoxysalicylidene)-R and R = 2,2-dimethyl-1,3-propanediamine (i = 1), R = 1,3-propanediamine (i = 2), R = 2-amino-benzylamine (i = 3), R = 2-methyl-1,2-propanediamine (i = 4), R = 1,2-phenylenediamine (i = 5)]. The crystal structure of [UL⁴Cl₂(thf)] (4) shows the metal in a quite perfect pentagonal bipyramidal configuration, with the two Cl atoms in apical positions. Reaction of UCl₄ with H₄Lⁱ in pyridine did not afford the mononuclear products [U(H₂Lⁱ)Cl₂(py)_x] but gave instead polynuclear complexes [H₄Lⁱ = N,N′-bis(3-hydroxysalicylidene)-R and R = 1,3-propanediamine (i = 6), R = 2-amino-benzylamine (i = 7) or R = 2-methyl-1,2-propanediamine (i = 8)]. In the presence of H₄L⁶ and H₄L⁷ in pyridine, UCl₄ was transformed in a serendipitous and reproducible manner into the tetranuclear U(IV) complexes [Hpy]₂[U₄(L⁶)₂(H₂L⁶)₂Cl₆] ( 6a) and [Hpy]₂[U₄(L⁷)₂(H₂L⁷)₂Cl₆][U₄(L⁷)₂(H₂L⁷)₂Cl₄(py)₂] (7), respectively. Treatment of UCl₄ with [Zn(H₂L⁶)] led to the formation of the neutral compound [U₄(L⁶)₂(H₂L⁶)₂Cl₄(py)₂] ( 6b). The hexanuclear complex [Hpy]₂[U₆(L⁸)₄Cl₁₀(py)₄] (8) was obtained by reaction of UCl₄ and H₄L⁸. The centrosymmetric crystal structures of 6a·2HpyCl·2py, 6b·6py, 7·16py and 8·6py illustrate the potential of Schiff bases as associating ligands for the design of polynuclear assemblies.