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|Title:||Base Catalysis of Ligand Substitution in Metal Carbonyls|
|Author(s):||Bellus, Peter Alexander|
|Department / Program:||Chemistry|
|Degree Granting Institution:||University of Illinois at Urbana-Champaign|
|Abstract:||Manganese(I) carbonyls can undergo substitution by a variety of mechanisms. Three different mechanisms have been found for the substitution reactions of Mn(CO)(,5)CH(,3)CN('+).
Mn(CO)(,5)CH(,3)CN('+) reacts with P(C(,6)H(,5))(,3) in CH(,3)CN by a dissociative process. CH(,3)CN dissociates and P(C(,6)H(,5))(,3) takes its place to form Mn(CO(,5))P(C(,6)H(,5))(,3)('+). First order overall, and first order in metal carbonyl, kinetics are observed.
Mn(CO)(,5)CH(,3)CN('+) reacts with pyridine in CH(,3)CN by an associative process that is best explained by a carbonyl base-attack mechanism, whereby a pyridine attacks the carbon of a CO to generate an adduct-ligand of the formula, ('-)C(O)NC(,5)H(,5). This adduct ligand labilizes the complex toward loss of the ligands cis to it. Pyridine or CH(,3)CN can fill the vacancy thus generated. The substitution process continues until the adduct-ligand breaks up, regenerating free pyridine and a coordinated CO, in the observed product, Mn(CO)(,3)(CH(,3)CN) (pyridine)(,2)('+). The kinetics observed are second order overall and first order each in metal carbonyl and pyridine.
Mn(CO)(,5)CH(,3)CN('+) reacts with pyridine in CH(,3)NO(,2) also by a base-attack mechanism. The nucleophile is CH(,2)NO(,2)('-), formed by the deprotonation of CH(,3)NO(,2) by pyridine. The adduct-ligand formed is C(O)ON(O)CH(,2), which labilizes the complex towards loss of ligand cis to it, and substitution occurs. The adduct ligand breaks down to give CO(,2) and, presumably, CH(,2)NO('-). Mn(CO)(,3)(pyridine)(,3)('+) is the metal containing product. The kinetics show first order dependences on both the metal carbonyl and pyridine, and an inverse dependence on pyridinium ion.
Mn(CO)(,5)CH(,3)CN('+) also undergoes a complex series of reactions in CH(,3)CN containing both pyridine and water. Mn(O) binuclear species, Mn(CO)(,5)('-), Mn(II) species and Mn(I) carbonyls are among the products. Formation of a hydroxycarbonyl species is thought to be the key step, leading to substitution by virtue of the cis-labilizing ability of the COOH moiety, and leading to Mn(CO)(,5)('-) formation via the hydroxycarbonyl. Electron transfer steps involving these species generate Mn(O) radicals and Mn(II).
The application of the base-attack mechanism of ligand substitution to other metal carbonyl systems is discussed.
Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 1980.
|Date Available in IDEALS:||2014-12-13|