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

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 trinuclear μ3-oxo-linked or hexanuclear μ-OH-linked Fe(III) pivalate clusters with rare earth nitrates, H3tea, azide ligands in MeOH/MeCN EtOH/MeCN media under ultrasonic irradiation. Single-crystal X-ray diffraction showed that wheels 1a–3a prepared from solutions crystallize triclinic space group P1̅ have Ci symmetry, whereas 1b–3b received solution trigonal R3̅ C3i symmetry. Magnetic studies reveal medium antiferromagnetic exchange interactions within Fe3 trimeric unit (with coupling parameters JFe3 −13.1 cm–1 for 1a, −11.6 1b) weak intermolecular (λmf −0.366 mol cm–3 −0.368 (zJmf −0.19 cm–1), respectively). This leads to spin ground states S 5/2 each {Fe3} unit. Substitution diamagnetic Y(III) centers paramagnetic Ln(III) (in 2a/2b Gd(III) 3a/3b Dy(III) centers) results ferromagnetic between addition predominant Fe···Fe 1a/1b.