Spin Equilibria in Monomeric Manganocenes: Solid-State Magnetic and EXAFS Studies

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• 作者：Marc D. Walter ; Chadwick D. Sofield ; Corwin H. Booth ; Richard A. Andersen*
• 刊名：Organometallics
• 出版年：2009
• 出版时间：April 13, 2009
• 年：2009
• 卷：28
• 期：7
• 页码：2005-2019
• 全文大小：395K
• 年卷期：v.28,no.7(April 13, 2009)
• ISSN：1520-6041

文摘

Magnetic susceptibility measurements and X-ray data confirm that tert-butyl-substituted manganocenes [(Me₃C)_nC₅H_5−n]₂Mn (n = 1, 2) follow the trend previously observed with the methylated manganocenes; that is, electron-donating groups attached to the Cp ring stabilize the low-spin (LS) electronic ground state relative to Cp₂Mn and exhibit higher spin-crossover (SCO) temperatures. However, introducing three CMe₃ groups on each ring gives a temperature-invariant high-spin (HS) state manganocene. The origin of the high-spin state in [1,2,4-(Me₃C)₃C₅H₂]₂Mn is due to the significant bulk of the [1,2,4-(Me₃C)₃C₅H₂]⁻ ligand, which is sufficient to generate severe inter-ring steric strain that prevents the realization of the low-spin state. Interestingly, the spin transition in [1,3-(Me₃C)₂C₅H₃]₂Mn is accompanied by a phase transition resulting in a significant irreversible hysteresis (ΔT_c = 16 K). This structural transition was also observed by extended X-ray absorption fine-structure (EXAFS) measurements. Magnetic susceptibility studies and X-ray diffraction data on SiMe₃-substituted manganocenes [(Me₃Si)_nC₅H_5−n]₂Mn (n = 1, 2, 3) show high-spin configurations in these cases. Although tetra- and hexasubstituted manganocenes are high-spin at all accessible temperatures, the disubstituted manganocenes exhibit a small low-spin admixture at low temperature. In this respect it behaves similarily to [(Me₃C)(Me₃Si)C₅H₃]₂Mn, which has a constant low-spin admixture up to 90 K and then gradually converts to high-spin. Thermal spin-trapping can be observed for [(Me₃C)(Me₃Si)C₅H₃]₂Mn on rapid cooling.