Slow Magnetic Relaxation in Trigonal-Planar Mononuclear Fe(II) and Co(II) Bis(trimethylsilyl)amido Complexes—A Comparative Study
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- 作者：Andreas Eichh?fer ; Yanhua Lan ; Valeriu Mereacre ; Tilmann Bodenstein ; Florian Weigend
- 刊名：Inorganic Chemistry
- 出版年：2014
- 出版时间：February 17, 2014
- 年：2014
- 卷：53
- 期：4
- 页码：1962-1974
- 全文大小：478K
- ISSN：1520-510X
文摘
Alternating current magnetic investigations on the trigonal-planar high-spin Co2+ complexes [Li(15-crown-5)]鈥塠Co{N(SiMe3)2}3], [Co{N(SiMe3)2}2(THF)] (THF = tetrahydrofuran), and [Co{N(SiMe3)2}2(PCy3)] (Cy = 鈭扖6H13 = 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鈥?) and relaxation times 蟿0 (3.5(3) 脳 10鈥?, 9.3(8) 脳 10鈥?, and 3.0(8) 脳 10鈥? s) are almost the same, despite distinct differences in the ligand properties. In contrast, the isostructural high-spin Fe2+ complexes [Li(15-crown-5)]鈥塠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鈥搊rbit 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鈥揹 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 57Fe M枚脽bauer spectra of the three high-spin iron complexes exhibit a doublet at 3 K with small and similar values of the isomer shifts (未), ranging between 0.57 and 0.59 mm/s, as well as an unusual small quadrupole splitting (螖EQ = 0.60 mm/s) in [Li(15-crown-5)][Fe{N(SiMe3)2}3].