Temperature- and Solvent-Induced Multistep Single-Crystal to Single-Crystal Conversions of Mn(II) Complexes with Diverse Magnetic Properties

A series of Mn(II) complexes, namely, {(H-2-MI)2[Mn(C5O5)2]}n (1), (H-2-MI)2[Mn(C5O5)2(H2O)2] (2), [Mn(C5O5)(CH3OH)2(H2O)]n (3), [Mn(H2O)3(C5O5)]n (4), and [Mn(2-MI)(C5O5)(H2O)2]n (5), (2-MI = 2-methylimidazole, H2C5O5 = croconic acid), was obtained through a series of multistep single-crystal to single-crystal conversions by modulating external conditions. Notably, 1 and 2 demonstrate reversible interconversion driven by temperature variations, whereas interconversions among 2, 3, 4, and 5 are achieved by altering solvent environment. To assess the feasibility of these conversions, crystal free energy and ligand binding energy calculations were performed to elucidate the underlying mechanism driving the pronounced structural changes, which involve the breaking and reformation of coordination bonds. The conversion mechanism of 1 and 2 is related to free energy, the dispersion-corrected DFT+U calculation shows a 21.075 eV increase in free energy during the conversion. The spontaneous crystal transformations within 2, 3, 4, and 5 are consistent with the ligand binding energy trend. Magnetic measurements were conducted on the stable crystals of 2, 4, and 5. Interestingly, as Mn(II) complexes, 2 exhibited field-induced slow magnetic relaxation behaviors, whereas 4 and 5 displayed no magnetic relaxation dynamics. Further research shows that this slow magnetic relaxation originates from the phonon bottleneck process. The difference of magnetic properties comes from the difference of dimensions and geometric configuration.