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In this study, we investigated reversible intermolecular proton shifting (IPS) coupled with spin transition (ST) in a novel Fe II complex. The host Fe II complex and the guest carboxylic acid anion were connected by intermolecular hydrogen bonds (IHBs). We extended the intramolecular proton transfer coupled ST phenomenon to the intermolecular system. The dynamic phenomenon was confirmed by variable-temperature single-crystal X-ray diffraction, neutron crystallography, and infrared spectroscopy. The mechanism of IPS was further validated using density functional theory calculations. The discovery of IPS-coupled ST in crystalline molecular materials provides good insights into fundamental processes and promotes the design of novel multifunctional materials with tunable properties for various applications, such as optoelectronics, information storage, and molecular devices., (© 2024 Wiley-VCH GmbH.)

Two isostructural Ni II compounds, bis-{ N -[1-(pyridin-2-yl-κ N )eth-ylidene]pyridine-4-carbohydrazonato-κ 2 N ', O }nickel(II)-2,5-di-chloro-terephthalic acid (1/1), [Ni(C 13 H 11 N 4 O) 2 ](C 8 H 4 Cl 2 O 4 ), and bis-{ N -[1-(pyridin-2-yl-κ N )eth-ylidene]pyridine-4-carbohydrazonato-κ 2 N ', O }nickel(II)-2,5-di-bromo-terephthalic acid (1/1), [Ni(C 13 H 11 N 4 O) 2 ](C 8 H 4 Br 2 O 4 ), were synthesized and their crystal structures determined. The pair of N , N ', O -tridentate N -[1-(pyridin-2-yl-κ N )eth-yl]pyridine-4-carbohydrazonate L ligands result in a cis -NiO 2 N 4 octa-hedral coordination sphere for the metal ions. The asymmetric units consist of two half-mol-ecules of the di-carb-oxy-lic acids, which are completed by crystallographic inversion symmetry. In the respective crystals, the 2,5-di-chloro-terephthalic acid (H 2 Cl 2 TPA, 1-Cl ) mol-ecules form zigzag hydrogen-bonded chains with the [Ni( L ) 2 ] mol-ecules, with the hydrogen-bond distances in 1-Br slightly longer than those in 1-Cl . The packing is consolidated by aromatic π-π stacking between the di-carb-oxy-lic acid mol-ecules and terminal pyridine rings in [Ni( L ) 2 ] and short halogen-halogen inter-actions are also observed. The qualitative prediction of the H-atom position from the C-N-C angles of the terminal pyridine rings in L and the C-O distances in the carboxyl groups show that 1-Cl and 1-Br are co-crystals rather than salts.