Properties of Prussian Blue Materials Manifested in Molecular Complexes: Observation of Cyanide Linkage Isomerism and Spin-Crossover Behavior in Pentanuclear Cyanide Clusters

Pentanuclear, cyanide-bridged clusters [M(tmphen)2]3[M‘(CN)6]2 (M/M‘ = Zn/Cr (1), Zn/Fe (2),
Fe/Fe (3), Fe/Co (4), and Fe/Cr (5); tmphen = 3,4,7,8-tetramethyl-1,10-phenanthroline) were prepared by
combining [M‘III(CN)6]3- anions with mononuclear complexes of MII ions with two capping tmphen ligands.
The clusters consist of a trigonal bipyramidal (TBP) core with three MII ions in the equatorial positions and
two M‘III ions in the axial positions. Compounds 1−4 are isostructural and crystallize in the monoclinic
space group P21/c. Complex 5 crystallizes in the enantiomorphic space group P3221. The magnetic
properties of compounds 1 and 2 reflect the contributions of the individual [CrIII(CN)6]3- and [FeIII(CN)6]3-
ions. The FeII ions in compounds 3 and 4 exhibit a gradual, temperature-induced spin transition between
high spin (HS) and low spin (LS), as determined by the combination of Mössbauer spectroscopy, magnetic
measurements, and single-crystal X-ray studies. The investigation of compound 5 by these methods and
by IR spectroscopy indicates that cyanide linkage isomerism occurs during cluster formation. The magnetic
behavior of 5 is determined by weak ferromagnetic coupling between the axial CrIII centers mediated by
the equatorial diamagnetic FeII ions. Mössbauer spectra collected in the presence of a high applied field
have allowed, for the first time, the direct experimental observation of uncompensated spin density at
diamagnetic metal ions that bridge paramagnetic metal ions.

五核氰基桥联簇[M(tmphen)₂]₃[M‘(CN)₆]₂（其中M/M‘分别为Zn/Cr (1)、Zn/Fe (2)、Fe/Fe (3)、Fe/Co (4)及Fe/Cr (5)；tmphen为3,4,7,8-四甲基-1,10-邻菲啰啉（3,4,7,8-tetramethyl-1,10-phenanthroline)）通过将六氰合金属(III)阴离子[M‘III(CN)₆]³⁻与带有两个盖帽型tmphen配体的M(II)单核配合物复合制备得到。
该类簇合物具有三角双锥（TBP）核心结构，三个M(II)离子位于赤道位置，两个M‘(III)离子位于轴向位置。
化合物1~4为同构结构，结晶于单斜晶系空间群P2₁/c；化合物5则结晶于对映异构空间群P3221。
化合物1和2的磁性质反映了单独的[CrIII(CN)₆]³⁻与[FeIII(CN)₆]³⁻离子的磁贡献。
化合物3和4中的Fe(II)离子呈现出渐进的、温度诱导的高自旋（HS）与低自旋（LS）自旋转换行为，该结论通过穆斯堡尔光谱（Mössbauer spectroscopy）、磁性能测试以及单晶X射线衍射研究联合验证得到。
通过上述方法结合红外光谱（IR spectroscopy）对化合物5进行的研究表明，簇合物形成过程中发生了氰基键合异构现象。
化合物5的磁行为由轴向Cr(III)中心之间的弱铁磁耦合决定，该耦合通过赤道位置的抗磁性Fe(II)离子介导。
首次通过高外场下采集的穆斯堡尔光谱，直接实验观测到了桥连顺磁性金属离子的抗磁性金属离子位点处存在未补偿自旋密度。