Abundant Solvatomorphism-Tuned Spin Crossover in a Dinuclear Fe(II) Compound: Computational Insights on Molecular Distortion and Packing Effects

A series of solvates based on a dinuclear Fe(II) complex [(TPA)(2)Fe-2 (mu-DHBQ)][BF4](2)center dot S [S = CH3OH (1 center dot CH3OH), 2CH(3)OH (2 center dot 2CH(3)OH), 4DMF (3 center dot 4DMF), 2Et(2)O (4 center dot 2Et(2)O), Et2O center dot MeCN (5 center dot Et2O center dot MeCN), and 2CH(2)Cl(2) (6 center dot 2CH(2)Cl(2))] were synthesized. Upon solvent removal, single-crystal-to-single-crystal (SC-to-SC) transitions could occur for the first four solvates to give 1, 2, 3, and 4 center dot 0.5Et(2)O. Within the temperature range of 400-10 K, these compounds exhibited abundant variations in their spin crossover (SCO) properties. 2,5 center dot Et2O center dot MeCN and 6 center dot 2CH(2)Cl(2) displayed half high-spin (HS) to low-spin (LS) transitions, and 4 center dot 2Et(2)O underwent incomplete LS-to-HS conversion, whereas other solvatomorphs showed complete SCO. As all these solvatomorphs possessed the identical [(TPA)(2)Fe-2(mu-DHBQ)][BF4](2) core, these variations in SCO behavior emphasized the critical role of the crystal lattice contributions. With the aim of deciphering their origin, periodic DFT+U+D3 computations were performed on these solvatomorphs to quantify the importance of all possible intramolecular and intermolecular effects on their spin-state energetics. Computationally, the isolated [(TPA)(2)Fe-2(mu-DHBQ)](2-) molecule in the gas phase would undergo SCO around 350 K in one step intrinsically. However, distinctive roles of the crystal-lattice effects in the solid state resulted in varying SCO behaviors. Different reasons were discovered for the incomplete spin transitions of different solvatomorphs. For 2, although serious volume shrinkage of the LS state caused efficient packing of the overall crystal-lattice organization, the originally close-packing SCO cations got loose and thus strongly destabilized its LS state. For 5 center dot Et2O center dot MeCN and 6 center dot 2CH(2)Cl(2), severe molecular distortions kinetically trapped their LS state . These computationally magneto-structural studies on dinuclear solvatomorphs have great importance for designing SCO compounds with selected properties, which is critical for their actual application.

Automated summary

A series of solvates based on a dinuclear Fe(II) complex were synthesized and showed abundant variations in spin crossover properties, emphasizing the critical role of the crystal lattice contributions. Computational studies on these solvatomorphs revealed varying spin state behaviors due to different crystal lattice effects and molecular distortions, providing insights for designing compounds with selected properties.