Interplay of Spin Crossover and Coordination-Induced Spin State Switch for Iron Bis(pyrazolyl)methanes in Solution

Abstract

Bis(pyrazolyl)bipyridinylmethane iron(II) complexes show a versatile spin state switching behavior in different solvents. In the solid, the magnetic properties of the compounds have been characterized by X-ray diffraction, Moßbauer spectroscopy, and SQUID magnetometry and point toward a high spin state. For nitrilic solvents, the solvation of the complexes leads to a change of the coordination environment from {N 5 O} to {N 6 } and results in a temperature-dependent SCO behavior. Thermodynamic properties of this transformation are obtained via UV/vis spectroscopy, SQUID measurements, and the Evans NMR method. Moreover, a coordination-induced spin state switch (CISSS) to low spin is observed by using methanol as solvent, triggered through a rearrangement of the coordination sphere. The same behavior can be observed by changing the stoichiometry of the ligand-to-metal ratio in MeCN, where the process is reversible. This transformation is monitored via UV/vis spectroscopy, and the resulting new bis-meridional coordination motif, first described for bis(pyrazolyl)methanes, is characterized in the solid state via X-ray diffraction, Moßbauer spectroscopy, and SQUID measurements. The sophisticated correlation of these switchable properties in dependence on different types of solvents reveals that the influence of the solvent on the coordination environment and magnetic properties should not be underestimated. Furthermore, careful investigation is necessary to differentiate between a thermally-induced spin crossover and a coordination-induced spin state switch.

Influence of Coordinated Triflate Anions on the Solution Magnetic Properties of a Neutral Iron(II) Complex

“…The larger HS iron­(II) might be expected to lead to a smaller conductivity value due to the higher viscosity from the stronger van der Waals interactions between the cationic and anionic species. , Because of this, we correlate the higher conductivity observed in CH 3 CN to 1 being in the LS state, whereas the lower value in CH 3 OH is consistent with 1 in the HS state. This change in the conductivity values due to the spin state has been observed previously for bis­(pyrazolyl)­methane iron­(II) complexes in solution …”

“…This change in the conductivity values due to the spin state has been observed previously for bis(pyrazolyl)methane iron(II) complexes in solution. 25 Spin and Speciation in Noncoordinating Environments. When 1 is dissolved in the noncoordinating solvent CH 2 Cl 2 , the magnetic properties are similar to what is observed in the solid state, but the speciation is more complicated.…”

“…In addition, ligand exchange in solution can impact the spin-state properties of a complex. Labile ligands can be replaced by solvent and counteranions or leave open coordination sites when dissolved, resulting in the formation of new species. − Such ligand exchanges can generate the conditions under which coordination-induced spin-state switching (CISSS) can occur. ,, It is often difficult to differentiate CISSS from classical spin-state switching in solution, but the simultaneous analysis of several solution-based analytical techniques can help with this determination.…”

Triflate vs Acetonitrile: Understanding the Iron(II)-Based Coordination Chemistry of Tri(quinolin-8-yl)amine

“…A species with coordinating triflate anions becomes preferred in weaker or non-coordinating solvents like dichloromethane. In the moderately coordinating solvent acetonitrile, studies often suggest that a species with acetonitrile coligands is present in solution since acetonitrile is known to replace such anions. ,,,,, This behavior can be explained by the stronger ligand field of the acetonitrile ligand compared to the triflate anion. ,, …”

Vapochromic effect in switchable molecular-based spin crossover compounds

“…[86][87][88][89][90][91][92][93][94][95] This phenomenon is defined as a switch in the spin state of the metal centre, due to a change of the coordination number of the central ion. In most cases, the switch in the spin state is achieved by light irradiation (ligand-driven coordinationinduced spin state switching, LD-CISSS) 87,88,90,[94][95][96][97] or takes place in solution [86][87][88][89]91,92,[94][95][96][97][98][99][100][101] or in thin films/monolayers. 93 There are only a limited number of examples of solid-state CISSS vapochromism, where the inclusion of a vapour leads to a change in the spin state of the metal centre, [102][103][104] but we have decided to focus this review on purely spin crossover examples, and have excluded those resulting from coordination-induced spin state switching.…”