Abstract
A series of solvates based on a dinuclear Fe(II) complex [(TPA)2Fe2(μ-DHBQ)][BF4]2·S [S = CH3OH (1·CH3OH), 2CH3OH (2·2CH3OH), 4DMF (3·4DMF), 2Et2O (4·2Et2O), Et2O·MeCN (5·Et2O·MeCN), and 2CH2Cl2 (6·2CH2Cl2)] 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 4·0.5Et2O. Within the temperature range of 400–10 K, these compounds exhibited abundant variations in their spin crossover (SCO) properties. 2,5·Et2O·MeCN and 6·2CH2Cl2 displayed half high-spin (HS) to low-spin (LS) transitions, and 4·2Et2O underwent incomplete LS-to-HS conversion, whereas other solvatomorphs showed complete SCO. As all these solvatomorphs possessed the identical [(TPA)2Fe2(μ-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)2Fe2(μ-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·Et2O·MeCN and 6·2CH2Cl2, 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.
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