Heterometallic {Fe18M6} (M = Y, Gd, Dy) Pivalate Wheels Display Solvent-Induced Polymorphism

Abstract

A new series of heterometallic wheels, isolated as [Fe18M6(piv)12(Htea)18(tea)6(N3)6]·n(solvent) (MIII = Y (1a, 1b), Gd (2a, 2b), and Dy (3a, 3b); Hpiv = pivalic acid, H3tea = triethanolamine), forms by the reaction of trinuclear μ3-oxo-linked or hexanuclear μ-OH-linked Fe(III) pivalate clusters with rare earth nitrates, H3tea, and azide ligands in MeOH/MeCN or EtOH/MeCN media under ultrasonic irradiation. Single-crystal X-ray diffraction showed that wheels 1a-3a prepared from MeOH/MeCN solutions crystallize in the triclinic space group P1¯ and have Ci symmetry, whereas wheels 1b-3b received from EtOH/MeCN solution crystallize in the trigonal space group R3¯ and have C3i symmetry. Magnetic studies reveal medium antiferromagnetic exchange interactions within the Fe3 trimeric unit (with the exchange coupling parameters of JFe3 = -13.1 cm-1 for 1a, JFe3 = -11.6 cm-1 for 1b) and weak intermolecular antiferromagnetic exchange interactions (λmf = -0.366 mol cm-3 and -0.368 mol cm-3 (zJmf = -0.19 cm-1), respectively). This leads to spin ground states of S = 5/2 for each {Fe3} unit. Substitution of diamagnetic Y(III) centers by paramagnetic Ln(III) centers (in 2a/2b by Gd(III) and in 3a/3b by Dy(III) centers) results in ferromagnetic exchange interactions between the Fe(III) and Ln(III) centers in addition to the predominant antiferromagnetic Fe···Fe interactions in 1a/1b.