SYNTHESIS AND CHARACTERIZATION OF LANTHANIDE [LN(L)]2 COMPLEXES OF N4O3 AMINE PHENOL LIGANDS WITH PHENOLATE OXYGEN BRIDGES - EVIDENCE FOR VERY WEAK MAGNETIC EXCHANGE BETWEEN LANTHANIDE IONS

The synthesis and characterization of three potentially heptadentate (N4O3) amine phenols and of a series of their mononuclear and dinuclear lanthanide complexes are reported. The heptadentate ligands are KBH4 reduction products of the Schiff bases derived from the condensation reactions of tris(2-aminoethyl)amine with 3 equiv of one of salicylaldebyde (tris(((2-hydroxybenzyl)amino)ethyl)amine (H3L1)), 5-chlorosalicylaldehyde (tris(((2-hydroxy-5-chlorobenzyl)amino)ethyl)amine (H3L2)), and 5-bromosalicylaldehyde (tris(((2-hydroxy-5-bromobenzyl)amino)ethyl)amine (H3L3)). The reaction of an amine phenol with 1 equiv of a lanthanide (Ln) nitrate produces the mononuclear complex [Ln(H3L)(NO3)3]; homodinuclear complexes [Ln(L)]2 were obtained from the reaction of lanthanide salts with the respective ligand in the presence of a base (hydroxide or acetate). The X-ray structure of [Gd(L1)]2.2CHCl3 (H3L1 = tris(((2-hydroxybenzyl)amino)ethyl)amine) has been determined; it is the first structurally characterized example of an [Ln(L)]2 complex, a homodinuclear eight-coordinate lanthanide complex [Ln(L)]2 with heptadentate ligands only. The complex crystallizes as a chloroform solvate in the triclinic space group P1BAR with a = 12.002 (2) angstrom, b = 12.248 (2) angstrom, c = 11.889 (2) angstrom, alpha = 109.22 (1)-degrees, beta = 108.29 (1)-degrees, gamma = 71.64 (1)-degrees, Z = 1, and V = 1522.5 (4) angstrom3. The structure was solved by conventional heavy-atom methods and was refined by full-matrix least-squares procedures to R = 0.034 and R(w) = 0.040 for 8771 reflections with I greater-than-or-equal-to 3-sigma-(I). The dinuclear complex [Gd(L1)]2.2CHCl3 bas a distorted bicapped octahedral (or cubic) coordination geometry around each gadolinium atom which is coordinated by an N4O4 donor set. The two gadolinium atoms are bridged by two phenolate O atoms, one from each heptadentate ligand. Angles at the oxygen bridgeheads are 113.12 (8)-degrees, and the Gd-Gd separation is 3.9841 (2) angstrom. The Gd-N distances range from 2.615 (3) to 2.813 (3) angstrom. The bridging O-Gd bond lengths are 2.384 (2) and 2.391 (2) angstrom, while the terminal Gd-O distances are 2.269 (2) and 2.237 (2) angstrom. Room-temperature magnetic susceptibilities of some of the mononuclear and homodinuclear complexes give no indication of magnetic interaction between the two Ln3+ ions and are discussed in relation to their structures. A variable-temperature (4.2-82 K) magnetic susceptibility study of the [Gd(L1)]2.2CHCl3 complex revealed a good fit of the data to two very weakly antiferromagnetically coupled S = 7/2 ions (g = 2.00, -J = 0.045 (1) cm-1).