Influence of Selenocyanate Ligands on the Transition Temperature and Cooperativity of bapbpy-Based Fe(II) Spin-Crossover Compounds

Abstract

Coordination of the ligand bapbpy (1, bapbpy = N,N′-di(pyrid-2-yl)-2,2′-bipyridine-6,6′-diamine), of one of its four dimethyl-substituted analogues 2−5 (R 2 bapbpy = N,N′-di(methylpyrid-2-yl)-2,2′-bipyridine-6,6′-diamine), or of one of its three bis(iso)quinoline analogues 6−8 (R 2 bapbpy= N,N′-di(quinolyl)-2,2′bipyridine-6,6′-diamine), to Fe(NCSe) 2 , afforded eight new iron(II) compounds of the type [Fe(R 2 bapbpy)(NCSe) 2 ] (9−16). Three of these compounds (11, 13, and 16) were structurally characterized by single crystal X-ray diffraction, which showed similar molecular geometry and packing compared to their thiocyanate analogues. Magnetic susceptibility measurements were carried out for all iron compounds and revealed thermal spin-crossover (SCO) behavior for compounds 9, 11, 13, 15, and 16. Compounds 11, 13, 15, and 16 show an increased transition temperature compared to the thiocyanate analogues. [Fe(bapbpy)(NCSe) 2 ] (9) shows a gradual, one-step SCO, whereas its thiocyanate analogue [Fe(bapbpy)-(NCS) 2 ] is known for its cooperative two-step SCO. To discuss the influence of Sto-Se substitution on the cooperativity of the SCO, heat capacity measurements were carried out for compounds 9, 11, 13, 15, and 16, and fitted to the Sorai domain model. The number n of like-spin SCO centers per interacting domain, which is a quantitative measure of the cooperativity of the spin transition, was found to be high for compounds 11 and 15, and low for compounds 9, 11, and 13. Compound 15 is one of the few known SCO compounds that is more cooperative than its thiocyanate analogue. Altogether, X-ray diffraction, calorimetry, and magnetic data give a consistent structure−property relationship for this family of compounds: hydrogen-bonding networks made of intermolecular N−H•••Se interactions are of paramount importance for the cooperativity of the SCO.

Thermal spin-crossover with a large hysteresis spanning room temperature in a mononuclear complex

“…This replacement is commonly reported in the literature without a clear trend on the effect of this substitution on the cooperativity. 18 In the present case, the retaining of the hysteresis width is in favour of a conservation of cooperativity. Additionally, light irradiation was applied at low temperature on the compound and no lightinduced effect was observed.…”

Development of Isoselenocyanate Compounds’ Syntheses and Biological Applications

“…77 Zheng et al conducted a comparison of thermal spin crossover (SCO) behavior, transition temperature, and cooperativity of SCO for various iron(II) complexes containing bis(ITC) or bis(ISC) ligands. 78 Several of the ISC analogues were found to have similar packing and molecular geometry to their corresponding ITC analogue. In addition, thermal SCO behavior was displayed, and the ISC iron(II) complexes had a higher SCO transition temperature than their corresponding ITC iron(II) complex.…”

Spin Interconversion of Heme-Peroxo-Copper Complexes Facilitated by Intramolecular Hydrogen-Bonding Interactions

“…33–36 Fundamental chemical insight into how these components impact electronic structure, which dictates magnetic, structural, and physical properties, is essential for promising fields of study such as molecular switching materials for nanotechnology applications and thin-film (or metal–organic framework) studies of SCO complexes for molecular electronics and spintronics applications. 16,17,21,25,27,37–47…”

“…Metalloenzymes with d 4 –d 7 metal centers can manifest both low- and high-spin (LS and HS, respectively) electronic states, which can be nearly degenerate and result in spin crossover (SCO) phenomena; these occur most prominently in protein-bound hemes. − SCO has also been implicated in multiple reaction mechanisms, in some contexts referred to as two-state reactivity. − For example, during the hydroxylation of C–H substrates by a high-valent non-heme metal-oxo species, SCO from the LS to the HS potential energy surface is said to be required due to the high steric activation barrier of the former. − Many divalent first-row transition-metal complexes also exhibit SCO behavior, and these have been extensively studied, particularly for non-heme iron­(II) complexes. ,,− …”

“…A complex’s predilection for SCO behavior may be influenced and controlled by external factors including temperature, pressure, light, and magnetic/electric fields as well as properties (often interdependent) such as ligand type, coordination geometry, intermolecular interactions, and secondary bonding interactions. ,,,,,− Thus, synthetic model coordination complexes inspired by metalloenzymes are designed for systematically and independently probing the influence of each aspect through synthetic input of ligand and/or metal ion and variations in environment. − Fundamental chemical insight into how these components impact electronic structure, which dictates magnetic, structural, and physical properties, is essential for promising fields of study such as molecular switching materials for nanotechnology applications and thin-film (or metal–organic framework) studies of SCO complexes for molecular electronics and spintronics applications. ,,,,,− …”