Eichhöfer Andreas, Lan Yanhua, Mereacre Valeriu, Bodenstein Tilmann, Weigend Florian
Institut für Nanotechnologie, Karlsruher Institut für Technologie (KIT), Campus Nord , Hermann-von-Helmholtz-Platz 1, 76344, Eggenstein-Leopoldshafen, Germany.
Inorg Chem. 2014 Feb 17;53(4):1962-74. doi: 10.1021/ic401677j. Epub 2014 Jan 27.
Alternating current magnetic investigations on the trigonal-planar high-spin Co(2+) complexes [Li(15-crown-5)] [Co{N(SiMe3)2}3], [Co{N(SiMe3)2}2(THF)] (THF = tetrahydrofuran), and [Co{N(SiMe3)2}2(PCy3)] (Cy = -C6H13 = cyclohexyl) reveal that all three complexes display slow magnetic relaxation at temperatures below 8 K under applied dc (direct current) fields. The parameters characteristic for their respective relaxation processes such as effective energy barriers Ueff (16.1(2), 17.1(3), and 19.1(7) cm(-1)) and relaxation times τ0 (3.5(3) × 10(-7), 9.3(8) × 10(-8), and 3.0(8) × 10(-7) s) are almost the same, despite distinct differences in the ligand properties. In contrast, the isostructural high-spin Fe(2+) complexes [Li(15-crown-5)] [Fe{N(SiMe3)2}3] and [Fe{N(SiMe3)2}2(THF)] do not show slow relaxation of the magnetization under similar conditions, whereas the phosphine complex [Fe{N(SiMe3)2}2(PCy3)] does, as recently reported by Lin et al. (Lin, P.-H.; Smythe, N. C.; Gorelsky, S. I.; Maguire, S.; Henson, N. J.; Korobkov, I.; Scott, B. L.; Gordon, J. C.; Baker, R. T.; Murugesu, M. J. Am. Chem. Soc. 2011, 135, 15806.) Distinctly differing axial anisotropy D parameters were obtained from fits of the dc magnetic data for both sets of complexes. According to density functional theory (DFT) calculations, all complexes possess spatially nondegenerate ground states. Thus distinct spin-orbit coupling effects, as a main source of magnetic anisotropy, can only be generated by mixing with excited states. This is in line with significant contributions of excited determinants for some of the compounds in complete active space self-consistent field (CASSCF) calculations done for model complexes. Furthermore, the calculated energetic sequence of d orbitals for the cobalt compounds as well as for [Fe{N(SiMe3)2}2(PCy3)] differs significantly from the prediction by crystal field theory. Experimental and calculated (time-dependent DFT) optical spectra display characteristic d-d transitions in the visible to near-infrared region. Energies for lowest transitions range from 0.19 to 0.35 eV; whereas, for [Li(15-crown-5)][Fe{N(SiMe3)2}3] a higher value is found (0.66 eV). Zero-field (57)Fe Mößbauer spectra of the three high-spin iron complexes exhibit a doublet at 3 K with small and similar values of the isomer shifts (δ), ranging between 0.57 and 0.59 mm/s, as well as an unusual small quadrupole splitting (ΔEQ = 0.60 mm/s) in [Li(15-crown-5)][Fe{N(SiMe3)2}3].
对三角平面高自旋Co(2+)配合物[Li(15-冠-5)][Co{N(SiMe3)2}3]、[Co{N(SiMe3)2}2(THF)]（THF = 四氢呋喃）和[Co{N(SiMe3)2}2(PCy3)]（Cy = -C6H13 = 环己基）进行的交流磁性研究表明，在施加直流（direct current）场的情况下，所有这三种配合物在温度低于8 K时都表现出缓慢的磁弛豫。它们各自弛豫过程的特征参数，如有效能垒Ueff（16.1(2)、17.1(3)和19.1(7) cm(-1)）和弛豫时间τ0（3.5(3)×10(-7)、9.3(8)×10(-8)和3.0(8)×10(-7) s）几乎相同，尽管配体性质存在明显差异。相比之下，同结构的高自旋Fe(2+)配合物[Li(15-冠-5)][Fe{N(SiMe3)2}3]和[Fe{N(SiMe3)2}2(THF)]在类似条件下并未表现出磁化强度的缓慢弛豫，而膦配合物[Fe{N(SiMe3)2}2(PCy3)]则表现出缓慢弛豫，正如Lin等人最近所报道的（Lin, P.-H.; Smythe, N. C.; Gorelsky, S. I.; Maguire, S.; Henson, N. J.; Korobkov, I.; Scott, B. L.; Gordon, J. C.; Baker, R. T.; Murugesu, M. J. Am. Chem. Soc. 2011, 135, 15806.）。从这两组配合物的直流磁性数据拟合中获得了明显不同的轴向各向异性D参数。根据密度泛函理论（DFT）计算，所有配合物都具有空间非简并基态。因此，作为磁各向异性主要来源的明显的自旋-轨道耦合效应，只能通过与激发态混合产生。这与对模型配合物进行的完全活性空间自洽场（CASSCF）计算中某些化合物的激发行列式的显著贡献一致。此外，钴化合物以及[Fe{N(SiMe3)2}2(PCy3)]的d轨道计算能量顺序与晶体场理论的预测有显著差异。实验和计算（含时DFT）的光谱在可见光到近红外区域显示出特征性的d-d跃迁。最低跃迁的能量范围为0.19至0.35 eV；而对于[Li(15-冠-5)][Fe{N(SiMe3)2}3]，发现其值更高（0.66 eV）。三种高自旋铁配合物的零场(57)Fe穆斯堡尔谱在3 K时呈现出一个双峰，其同质异能位移（δ）值较小且相似，范围在0.57至0.59 mm/s之间，并且在[Li(15-冠-5)][Fe{N(SiMe3)2}3]中存在不寻常的小四极分裂（ΔEQ = 0.60 mm/s）。
