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.

空气稳定配合物[(PNP)­FeCl₂]（1）[PNP = 二[2-二(异丙基膦基)-4-甲基苯基]胺阴离子（N[2-P(CHMe₂)₂-4-甲基苯基]₂⁻）]，通过ClCPh₃对[(PNP)­FeCl]进行单电子氧化制备得到。通过直流超导量子干涉装置（dc SQUID）的直流磁化率测试推断，该配合物在80 K以上会出现意料之外的S=3/2到S=5/2的自旋态转变。在零场下、频率范围10~1042 Hz时采集的交流超导量子干涉装置（ac SQUID）磁化强度数据清晰表明，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（对应温度区间47~52 K）。变温穆斯堡尔（Mössbauer）谱数据同样证实，配合物1的同质异能移位δ和四极裂分ΔEQ值均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）谱显示，S=3/2态大概率为基态，而S=5/2态的能量与之相近。多边缘X射线吸收谱（XAS）数据表明，配合物1的电子结构具有高度共价性，其有效铁氧化态比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30~300 K区间采集的配合物1变温单晶体X射线衍射研究同样显示，随着S=5/2态的占比提升，Fe-P键长逐渐伸长，Cl-Fe-Cl键角逐渐增大。理论计算研究表明，除d(z²)轨道外，其余轨道的整体图像基本相似；d(z²)轨道对几何结构和成键变化最为敏感，而四重态（4B）与六重态（6A）的能量较为接近。