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|Title:||Part I. Studies of the substitution and formation reactions of rhenium carbonyl o-semiquinone compounds. Part II. Molecular mechanics studies of the steric interactions in chromium pentacarbonyl phosphine compounds|
|Author(s):||Lee, Kevin James|
|Doctoral Committee Chair(s):||Brown, Theodore L.|
|Department / Program:||Chemistry|
|Degree Granting Institution:||University of Illinois at Urbana-Champaign|
|Abstract:||In part I, the thermal synthesis of (3,5-di-t-butyl-1,2-benzosemiquinone)Re(CO)$\sb4$ from the sodium semiquinone radical anion and Re(CO)$\sb5$Cl is reported. The substitution kinetics of (3,5-di-t-butyl-1,2-benzosemiquinone)Re(CO)$\sb4$ with P(n-Bu)$\sb3$ was studied by stopped-flow methods. The observed kinetics data shows considerable variability, both within and among different sets of runs. The results are best explained by an electron-transfer catalyzed chain mechanism.
To investigate the photochemical formation of (3,5-di-t-butyl-1,2-benzosemiquinone)Re(CO)$\sb3$L (L = CO, PR$\sb3$), laser and xenon flash photolysis experiments were carried out on solutions containing Re$\sb2$(CO)$\sb8$L$\sb2$ and 3,5-di-t-butylbenzoquinone. The reaction of $\cdot$Re(CO)$\sb4$L with 3,5-di-t-butylbenzoquinone is very fast; the large magnitude of the bimolecular rate constants suggest that the reaction is close to the diffusion-controlled limit. More than one process leading to (3,5-di-t-butyl-1,2-benzosemiquinone)Re(CO)$\sb3$L is seen in the xenon flash photolysis experiments under argon; one of the processes is absent and the yield of this substance is reduced under one atmosphere of CO. These results are consistent with a mechanism in which both $\cdot$Re(CO)$\sb4$L and Re$\sb2$(CO)$\sb7$L$\sb2$ lead to (3,5-di-t-butyl-1,2-benzosemiquinone)Re(CO)$\sb3$L. The rate constants for the ring closure reactions in the mechanism are sensitive to the nature of L.
In part II, the interactions of 19 different trialkylphosphine ligands of varying steric requirement have been modeled using molecular mechanics methods. It was necessary to establish appropriate force field parameters for the Cr(CO)$\sb5$ fragment, and to properly account for the effects of complex formation on the strain-free bond angles about the chromium and phosphorus atoms. The optimum molecular mechanics structures provide insight into how steric forces are accommodated as the phosphine becomes increasingly bulky. Variations in the molecular mechanics energy change for the process PR$\sb3$ + Cr(CO)$\sb5$ $\to$ Cr(CO)$\sb5$PR$\sb3$, denoted $\Delta$E, were calculated for the series of phosphines. Analysis of rate data in terms of a two-parameter linear free energy relationship using $\Delta$E as the steric parameter suggest that $\Delta$E is an appropriate measure of phosphine steric requirements. However, because the components comprising $\Delta$E vary from one class of ligands to another and because the calculated van der Waals energy component contains contributions from an attractive term, the use of $\Delta$E as a steric parameter in linear free energy relationships is limited to a particular class of ligands.
|Rights Information:||Copyright 1991 Lee, Kevin James|
|Date Available in IDEALS:||2011-05-07|
|Identifier in Online Catalog:||AAI9210887|