Supramolecular Catalysis of Orthoformate Hydrolysis in Basic Solution: An Enzyme-Like Mechanism

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Magnetic susceptibility measurements and X-ray data confirm that tert-butyl substituted manganocenes [(Me{sub 3}C){sub n}C{sub 5}H{sub 5-n}]{sub 2}Mn (n= 1, 2) follow the trend previously observed with the methylated manganocenes, i.e., electron donating groups attached to the Cp ring stabilize the low-spin (LS) electronic ground state relative to Cp{sub 2}Mn 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-(Me{sub 3}C){sub 3}C{sub 5}H{sub 2}]{sub 2}Mn is due to the significant bulk of the [1,2,4-(Me{sub 3}C){sub 3}C{sub 5}H{sub 2}]{sup -} ligand, which ... continued below

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Pluth, Michael D.; Bergman, Robert G. & Raymond, Kenneth N. April 17, 2008.

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Magnetic susceptibility measurements and X-ray data confirm that tert-butyl substituted manganocenes [(Me{sub 3}C){sub n}C{sub 5}H{sub 5-n}]{sub 2}Mn (n= 1, 2) follow the trend previously observed with the methylated manganocenes, i.e., electron donating groups attached to the Cp ring stabilize the low-spin (LS) electronic ground state relative to Cp{sub 2}Mn 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-(Me{sub 3}C){sub 3}C{sub 5}H{sub 2}]{sub 2}Mn is due to the significant bulk of the [1,2,4-(Me{sub 3}C){sub 3}C{sub 5}H{sub 2}]{sup -} 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-(Me{sub 3}C){sub 2}C{sub 5}H{sub 3}]{sub 2}Mn is accompanied by a phase transition resulting in a significant irreversible hysteresis ({Delta}T{sub 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 SiMe{sub 3} substituted manganocenes [(Me{sub 3}Si){sub n}C{sub 5}H{sub 5-n}]{sub 2}Mn (n= 1, 2, 3) show high-spin configuration 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 similarly to [(Me{sub 3}C)(Me{sub 3}Si)C{sub 5}H{sub 3}]{sub 2}Mn, 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 [(Me{sub 3}C)(Me{sub 3}Si)C{sub 5}H{sub 3}]{sub 2}Mn on rapid cooling.

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  • Journal Name: Journal of the American Chemical Society

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  • Report No.: LBNL-1136E
  • Grant Number: DE-AC02-05CH11231
  • Office of Scientific & Technical Information Report Number: 963648
  • Archival Resource Key: ark:/67531/metadc933976

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  • April 17, 2008

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  • Nov. 13, 2016, 7:26 p.m.

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  • Nov. 18, 2016, 3:23 p.m.

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Pluth, Michael D.; Bergman, Robert G. & Raymond, Kenneth N. Supramolecular Catalysis of Orthoformate Hydrolysis in Basic Solution: An Enzyme-Like Mechanism, article, April 17, 2008; Berkeley, California. (digital.library.unt.edu/ark:/67531/metadc933976/: accessed June 22, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.