Multiple Polymorphic Phases of a Mononuclear Spin-Crossover Fe(II) Complex with Significant Coligand Effect

Abstract

Polymorphism is known to show a significant effect on the crystal packing structures and physical properties of molecular complexes. On the basis of our previous works on the spin-crossover (SCO) complex [Fe­(tpa)­(NCS)2] (tpa = tris­(2-pyridylmethyl)­amine), we have prepared the analogous complex [Fe­(tpa)­(NCSe)2] using a NCSe– coligand and obtained a series of crystalline polymorphs, namely SeI–SeIV, respectively, by controlled synthetic processes. Single-crystal X-ray diffraction studies reveal that these polymorphs show different packing modes and different intermolecular interactions. Various SCO behaviors are thus observed. Moreover, an obvious coligand effect on SCO behaviors is found by a comparison of the present study and the reported complex [Fe­(tpa)­(NCS)2]. The NCSe– complexes show a critical shift of T 1/2 values to a higher temperature range and a more gradual transition process in comparison to those for the isostructural complexes using NCS–, indicating a stronger ligand-field strength and a weaker cooperativity for the NCSe– coligand.

The Influence of the Substrate on the Functionality of Spin Crossover Molecular Materials

“…To better understand how different substrates can affect the functionality of SCO complexes, careful attention to other parameters that can potentially perturb SCO molecules should be considered. These include ligand types, film thickness, thin film fabrication method, surface properties of the substrates, and the probing methods used [91,[129][130][131][132][133][134][135][136][137][138]. 1500 nm thick [Fe(Htrz) 2 (trz)](BF 4 )] thin films on PVDF-TrFE polymer were fabricated as bilayer samples to be used potentially in MEMS devices [128].…”

Regulable Spin-Crossover Behaviors of a Family of Cyanido-Bridged Binuclear Iron(II) Compounds with a Thermally Induced Ultrafast High–Low Spin State Interconversion

“…The switch ability and memory conduction of functional molecular materials are of great significance for molecular electronic devices involving switches, − information storage, , and sensors. ,− Spin-crossover (SCO) complexes as a kind of controllable functional molecular material have been widely concerned due to their properties that can switch between the high-spin (HS) and the low-spin (LS) states in response to external stimuli such as temperature, light, and pressure. − Since Cambi and Szegö reported the first SCO complex in 1931, there has been a long investigation of this class of complexes with unique properties. To date, a great deal of novel and interesting SCO complexes have been synthesized and characterized. ,− To achieve the practical or potential applications in molecular devices, predicting and regulating the occurrence of SCO properties has always been the focus of current research. − …”

“…When both energies are comparable, the SCO phenomenon can easily occur with external stimuli such as temperature, pressure, light, electric field, guest molecules, etc. − Since the structure of the organic ligand directly determines the ligand field strength, regulating SCO behavior by modifying the organic ligand, which directly links the SCO metal center, is a common research method that has been extensively studied. − However, the effect of such a direct modification of ligands on SCO properties is complicated and is usually attributed to steric or electronic effects. Some earlier reports have also pointed out the differences between σ-donating and π-accepting properties of ligands to explain the complexity of the electronic effects of substituents on the SCO properties. ,− Therefore, the regulation of SCO properties by directly modifying the ligand, which directly links the SCO metal center is often unpredictable. To date, although there have been some reports on the regulation of the SCO properties of complexes, it remains a great challenge to fine-tune the properties of SCO, especially the spin transition temperature.…”

Remote H/F Tuning of Cooperativity in (NN′O)2-Coordinate Iron(II) Complexes: Dynamic, Hysteretic, and Continuous Spin Crossover

“…Spin crossover in the solid state can be affected by many parameters such as crystallinity, crystal mosaicity, anion conformation, etc . In the end, the spin transition for the very same material may be different if polymorphism occurs. − While the quantity and quality of interactions between the SCO active sites, solvent molecules, and/or anions , is certainly key to control the cooperativity among the centers, real a priori prediction of the magnetochemical feedback is out of reach. , This remains a major challenge in the solid state, albeit ongoing efforts are being made to establish rules a posteriori and to develop rational strategies (crystal engineering). , On the other hand, recent studies in the solution phase for different compound series have shown that the spin transition curves can be selectively affected by either solvent tuning, alkyl chain length, or the Hammett substitutent parameter …”