Coordinative flexibility of the 2-amino-2-(hydroxymethyl)propane-1,3-diol ligand in the synthesis of polynuclear Fe(III) complexes

The synthesis, structure and magnetic properties of four polynuclear iron(III)-containing polynuclear complexes are reported, [Fe6NaO2(HL)2(H2L)2(PhCO2)9]·2MeCN (1·2MeCN), [Fe6NaO2(HL)2(H2L)2(O2CCMe3)9]·2MeCN (2·2MeCN), [Fe8O4(HL)2(O2CC6H4OPh)10(NO3)2]·6MeCN (3·6MeCN) and [Fe12O6(OH)2(HL)2(H2L)2(PhCO2)16(PhCO2H)2]·6MeCN (4·6MeCN) {H3L = 2-amino-2-(hydroxymethyl)propane-1,3-diol, H2NC(CH2OH)3 or Tris}. The flexibility of the ligand, in both binding mode and level of deprotonation, has allowed isolation of complexes with a range of core topologies. All four complexes were prepared using iron oxo-centred carboxylate (pivalate, benzoate or 2-phenoxybenzoate) triangles {Fe3O(O2CR)6}0/+1 as starting materials and combining these with the Tris ligand or the trisodium salt of the ligand. The cores of complexes 1 and 2 are composed of {Fe3O} units; 3 of {Fe4O2} units and 4 of fused {Fe3O} units. Complexes 1 and 2 display a previously unseen core topology in iron-oxo chemistry. The magnetic properties of the complexes are dominated by antiferromagnetic exchange interactions. Magnetic characterization reveals small non-zero spin ground states for 1, 2 and 4 that are not well isolated, while 3 has a spin ground state of zero.