Electron Spin Relaxation in Chromium-Nitrosyl Complexes

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• 作者：Lenny M. Carruthers ; Christina L. Closken ; Karen L. Link ; Surendra N. Mahapatro ; Malavosklish Bikram ; Jing-Long Du ; Sandra S. Eaton ; and Gareth R. Eaton
• 刊名：Inorganic Chemistry
• 出版年：1999
• 出版时间：July 26, 1999
• 年：1999
• 卷：38
• 期：15
• 页码：3529 - 3534
• 全文大小：82K
• 年卷期：v.38,no.15(July 26, 1999)
• ISSN：1520-510X

文摘

 A new method to prepare Cr(NO)(H₂O)₅²⁺ from dichromate and NH₂OH is reported. The chromium nitrosylsCr(NO)(EHBA)⁺ and Cr(NO)(EHBA)₂ (EHBA = 2-ethyl-2-hydoxybutyrate) were prepared by a literature reactionand characterized by continuous wave electron paramagnetic resonance and two-pulse electron spin echospectroscopy at X-band. The g values are characteristic of a single unpaired electron in a predominantly d_xyorbital. In fluid and glassy solutions Cr(NO)(EHBA)₂ is a mixture of cis and trans isomers. Rotation of the methylgroups in the EHBA ligands causes an increased rate of spin echo dephasing at temperatures between 40 and 120K. For the EHBA complexes echo envelope modulation is observed at temperatures below about 40 K that isattributed to inequivalent coupling to protons of the slowly rotating methyl groups. Both the effect of the methylrotation on spin echo dephasing and the depth of the proton modulation are dependent on the number of ethylgroups in the ligand, and thus the spin echo experiments provide confirmation of the number of EHBA ligandsin the complexes. The spin-lattice relaxation rates for the chromium-nitrosyl complexes at temperatures near100 K are similar to values reported previously for Cr(V) complexes, which also have a single unpaired electronin a predominantly d_xy orbital. For Cr(NO)(H₂O)₅²⁺, Cr(NO)(EHBA)⁺, and Cr(NO)(EHBA)₂ the dominantcontribution to spin-lattice relaxation between 12 and 150 K is the Raman process with a Debye temperature,mages/gifchars/theta.gif" BORDER=0 >_D, of 110-120 K. For Cr(NO)(CN)₅^3- the data are consistent with a Raman process (mages/gifchars/theta.gif" BORDER=0 >_D = 135 K) and acontribution from a local mode, which dominates above about 60 K. The formally low-spin d⁵ chromium nitrosylcomplexes relax about 5 orders of magnitude more slowly than low-spin d⁵ Fe(III) porphyrins, which is attributedto the absence of a low-lying excited state.