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

Treatment of UCl4 with the hexadentate Schiff bases H2Li in thf gave the expected [(ULCl2)-Cl-i(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 [UL4 Cl-2(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) Cl-2(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) [U-4(L-6)(2)(H2L6)(2)Cl-6] (6a) and [Hpy](2) [U-4(L-7)(2)(H2L7)(2)Cl-6] [U-4(L-7)(2)(H2L7)(2) Cl-4(py)(2)] (7), respectively. Treatment of UCl4 with [Zn(H2L6)] led to the formation of the neutral compound [U-4(L-6)(2)(H2L6)(2)Cl-4(py)(2)] (6b). The hexanuclear complex [Hpy](2) [U-6(L-8)(4)Cl-10(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.