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https://hdl.handle.net/2142/70315
Description
Title
Redox Reactions of Aqueous Molybdenum(iv)
Author(s)
Rahmoeller, Kenneth Mark
Issue Date
1985
Department of Study
Chemistry
Discipline
Chemistry
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Chemistry, Inorganic
Abstract
The redox chemistry of aqueous molybdenum(III), (IV), and (V) was investigated under a variety of conditions. Monomeric Mo(IV)(,aq), whether produced by the dissolution of "monomeric" Mo(IV) compounds in water or by the oxidation of Mo('3+), is unstable with respect to disproportionation. Mo(OR)(,4), (Et(,4)N)(,2)MoCl(,6), and the initial one-electron oxidation product of Mo('3+) disproportionate to give Mo('3+) and Mo(,2)O(,4)('2+), while (NH(,4))(,2)MoCl(,4)(SnCl(,3))(,2) and MoCl(,4) give Mo(,3)Cl(,12)('3-) and Mo(,2)O(,4)('2+). Mo(,3)Cl(,12)('3-) decomposes in water (t(, 1/2) = 8.3min) to produce monomeric Mo(III) chlorocomplexes in varying degrees of hydration. This instability of aqueous monomeric Mo(IV) precludes its use in the production of the stable trinuclear Mo(,3)O(,4)('4+) unit.
Oxidation of Mo(,2)(OH)(,2)('4+) leads to the initial production of Mo(,3)O(,4)('4+) and Mo(,2)O(,4)('2+) in a 2 to 3 ratio under all conditions except when large excesses of the oxidizing agent is used, thus causing Mo(,2)O(,4)('2+) to be the only product. The reaction is oxidant independent. Although the use of Mo(VI) as oxidant introduces another source of molybdenum, the resulting products remain the same. A similar reaction occurs with dimeric Mo(,2)('4+). After its initial oxidation to a Mo(III) dimer, continued oxidation yields the same 2Mo(,3)O(,4)('4+)/3Mo(,2)O(,4)('2+) product ratio.
A common intermediate (or activated state) is suspected for these reactions. The intermediate is probably a large (4 or more molybdenum atoms) molybdenum-oxygen oligomer produced by the combination of two or more molybdenum dimers resulting from the initial oxidation of Mo(,2)(OH)(,2)('4+) or Mo(,2)('4+). EDTA and 1,4,7-triazacyclononane complexes of Mo(,2)(OH)(,2)('4+), when oxidized yield only Mo(,2)O(,4)('2+) complexes. Removal of coordination sites from the original molybdenum starting materials with chelating agents changes the product stoichiometries, probably by inhibiting the formation of large oligomeric intermediates. Large intermediates such as Mo(,2)O(,2) (,n) could allow the necessary trimeric and dimeric structures found in the products to be performed with very few rearrangements. Thus the oxidation of noncomplexed Mo(III) dimers may be a necessary prerequisite for the production of Mo(,3)O(,4)('4+) in all cases.
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