A Mononuclear Fe(III) Single Molecule Magnet with a 3/2↔5/2 Spin Crossover

The air stable complex [(PNP)­FeCl2] (1) (PNP = N[2-P­(CHMe2)2-4-methylphenyl]2–), prepared from one-electron oxidation of [(PNP)­FeCl] with ClCPh3, displays an unexpected S = 3/2 to S = 5/2 transition above 80 K as inferred by the dc SQUID magnetic susceptibility measurement. The ac SQUID magnetization data, at zero field and between frequencies 10 and 1042 Hz, clearly reveal complex 1 to have frequency dependence on the out-of-phase signal and thus being a single molecular magnet with a thermally activated barrier of Ueff = 32–36 cm–1 (47–52 K). Variable-temperature Mössbauer data also corroborate a significant temperature dependence in δ and ΔEQ values for 1, which is in agreement with the system undergoing a change in spin state. Likewise, variable-temperature X-band EPR spectra of 1 reveals the S = 3/2 to be likely the ground state with the S = 5/2 being close in energy. Multiedge XAS absorption spectra suggest the electronic structure of 1 to be highly covalent with an effective iron oxidation state that is more reduced than the typical ferric complexes due to the significant interaction of the phosphine groups in PNP and Cl ligands with iron. A variable-temperature single crystal X-ray diffraction study of 1 collected between 30 and 300 K also reveals elongation of the Fe–P bond lengths and increment in the Cl–Fe–Cl angle as the S = 5/2 state is populated. Theoretical studies show overall similar orbital pictures except for the d­(z2) orbital, which has the most sensitivity to change in the geometry and bonding, where the quartet (4B) and the sextet (6A) states are close in energy.

空气稳定的配合物[(PNP)­FeCl₂]（1，其中PNP = N[2-P(CHMe₂)₂-4-甲基苯基]²⁻）通过[(PNP)­FeCl]与ClCPh₃发生单电子氧化制备得到。经直流超导量子干涉装置（dc SQUID）磁化率测试表征，该配合物在80 K以上会出现意外的S=3/2至S=5/2自旋态转变。在零场且频率范围为10至1042 Hz的条件下采集的交流超导量子干涉装置（ac SQUID）磁化数据清晰显示，配合物1的异相信号具有频率依赖性，因此属于单分子磁体，其热激活能垒Ueff为32–36 cm⁻¹（对应47–52 K）。变温穆斯堡尔（Mössbauer）谱数据进一步证实，配合物1的同质异能位移δ与四极裂分ΔEQ值存在显著的温度依赖性，这与该体系发生自旋态转变的结论相符。同样，配合物1的变温X波段电子顺磁共振（X-band EPR）光谱表明，其基态大概率为S=3/2，而S=5/2态的能量与之相近。多边缘X射线吸收光谱（XAS）数据显示，配合物1的电子结构具有高度共价性，其有效铁氧化态较典型三价铁配合物更低，这源于PNP配体中的膦基团与Cl配体均与铁原子存在较强相互作用。在30至300 K范围内开展的变温单晶X射线衍射研究还发现，随着S=5/2态的布居占比提升，Fe-P键长发生伸长，且Cl-Fe-Cl键角随之增大。理论计算结果显示，除d(z²)轨道外，其余轨道的整体图像较为相似；d(z²)轨道对几何结构与成键变化的敏感性最高，且四重态（⁴B）与六重态（⁶A）的能量较为接近。