Spin Crossover in a Series of Iron(II) Complexes of 2-(2-Alkyl-2<i>H</i>-tetrazol-5-yl)-1,10-phenanthroline: Effects of Alkyl Side Chain, Solvent, and Anion

2-(2H-Tetrazol-5-yl)-1,10-phenanthroline (HL0), its alkyl-substituted derivatives (Ln, where n = 1−8, 10, 12, 14, and 16, denoting the carbon atom number of the alkyl chain) at the 2H position of the tetrazole ring, and their iron(II) complexes (a for [Fe(L0)2], na for [Fe(Ln)2](ClO4)2, and nb for [Fe(Ln)2](BF4)2) were synthesized and characterized. The crystal structures of a, a·CH3OH, 1a·CH3OH, 1b·CH3OH·CH3CN, 2a·H2O, 2b·H2O, 4b·CH3OH, 5a·H2O, 5b·H2O, 6a, 6b, 7a, 7b, and 16a are described, along with thermal analyses. a undergoes an abrupt spin crossover (SCO) at 255 K with a hysteresis loop of 6 K. a·CH3OH, 2a·H2O, and 2b·H2O exhibit irreversible SCO behaviors due to the loss of solvent molecules upon heating. 3a, 3b, 4a, and 5a·H2O show simple spin transitions above 350 K. The desolvated samples of 4b·CH3OH and 5b·H2O undergo two-step spin transitions. 16a exhibits a two-step SCO behavior between 100 and 300 K, corresponding to sequential phase transitions from the low-spin (LS) phase to the intermediate phase and then to the high-spin phase, respectively, proved by crystal structure analysis and 57Fe Mössbauer spectroscopy. 1a·CH3OH, 10a, 10b, 12a, 12b, 14a, 14b, and 16b show gradual and incomplete SCO behaviors after cooling down from 400 K. 1b·CH3OH·CH3CN, 6a, 6b, 7a, 7b, 8a, and 8b remain in the LS state even at 400 K. This proves that the alkyl side chains, together with the solvent molecules and anions, play a crucial role in the complicated SCO behaviors in this system.2-(2H-四唑-5-基)-1,10-菲咯啉(HL0)、其四唑环2H位的烷基取代衍生物(Lₙ，其中n=1~8、10、12、14、16，分别代表烷基链的碳原子数)及其铁(II)配合物均已完成合成与表征：其中[Fe(L₀)₂]记为a，[Fe(Lₙ)₂](ClO₄)₂记为nₐ，[Fe(Lₙ)₂](BF₄)₂记为n_b。本文对配合物a、a·CH₃OH、1a·CH₃OH、1b·CH₃OH·CH₃CN、2a·H₂O、2b·H₂O、4b·CH₃OH、5a·H₂O、5b·H₂O、6a、6b、7a、7b及16a的晶体结构进行了解析，并开展了热分析研究。配合物a在255 K时发生突变型自旋交叉(SCO)，伴随6 K的磁滞回线。a·CH₃OH、2a·H₂O与2b·H₂O因受热过程中溶剂分子脱除，表现出不可逆自旋交叉行为。3a、3b、4a及5a·H₂O在350 K以上呈现简单自旋转变。4b·CH₃OH与5b·H₂O的脱溶剂样品则展现两步自旋转变。16a在100~300 K区间表现出两步自旋交叉行为，对应依次从低自旋(LS)相向中间相、再向高自旋相的相变，该结论通过晶体结构解析与57Fe穆斯堡尔谱学得到验证。1a·CH₃OH、10a、10b、12a、12b、14a、14b及16b从400 K降温后，呈现渐变性且不完全的自旋交叉行为。1b·CH₃OH·CH₃CN、6a、6b、7a、7b、8a及8b即便在400 K下仍维持低自旋状态。上述结果证实，烷基侧链与溶剂分子、阴离子共同对该体系中复杂的自旋交叉行为起到关键调控作用。