Synthesis, Characterization, and Reactivity of Alkylphosphomolybdates: Oxidation, Dehydration, and Isomerization

Wang, Ren-Chain

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Description

Title

Synthesis, Characterization, and Reactivity of Alkylphosphomolybdates: Oxidation, Dehydration, and Isomerization

Author(s)

Wang, Ren-Chain

Issue Date

1988

Doctoral Committee Chair(s)

Klemperer, Walter G.

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 alkyl and silyl phosphomolybdates (P$\sb3$O$\sb9$)MoO$\sb2$OR((n-C$\sb4$H$\sb9)\sb4$N) $\sb2$ $\{$R = CH$\sb3$, CH$\sb2$CH$\sb3$, 1, CH(CH$\sb3)\sb2$, CH$\sb2$CH=CH$\sb2$, CH$\sb2$CH$\sb2$C$\sb6$H$\sb5$, and Si(CH$\sb3)\sb3\}$ have been prepared and characterized. A single crystal X-ray diffraction study reveals that in 1, the P$\sb3$O$\sb9\sp{3-}$ ligand acts as a tridentate ligand coordinated to MoO$\sb2$OCH$\sb2$CH$\sb3\sp+$ unit. Two short intramolecular CH$\cdots$OMo contacts between the methylene hydrogens and the oxygens of P-O and Mo=O are found in anion 1. Based on $\sp{31}$P, $\sp $O, and $\sp1$H NMR studies, hydrolysis of (P$\sb3$O$\sb9$)MoO$\sb2$OCH$\sb3\sp{2-}$ produces the anhydride ((P$\sb3$O$\sb9$)MoO$\sb2$) $\sb2$O$\sp{4-}$ and the acid (P$\sb3$O$\sb9$)MoO$\sb3$H$\sp{2-}$. In the presence of excess methanol, the methyl ester (P$\sb3$O$\sb9$)MoO$\sb2$OCH$\sb3\sp{2-}$ can be regenerated from the anhydride and the acid. The acid slowly dimerizes to form the dimer, (P$\sb3$O$\sb $HMoO$\sb2)\sb2\sp{4-}$, which has been isolated in crystalline form. An X-ray crystal structural analysis of this dimer reveals the presence of a tetradentate linear triphosphate ligand. The hydrogens in this anion were located by $\sp{31}$P NMR study.
• Thermolysis of (P$\sb3$O$\sb9$)MoO$\sb2$OCH$\sb2$CH$\sb3$((n-C$\sb4$H$\sb9)\sb4$N) $\sb2$, 1, in CH$\sb3$CN produces acetaldehyde, ethanol, and unidentified soluble inorganic products. According to deuterium labeling experiments, the rate-determining step of the reaction is the methylene C-H cleavage. An intramolecular mechanism is proposed based on these kinetic data and the short CH$\cdots$OMo contact found in the structure. Photolysis of 1 gives ethylene, acetaldehyde, ethanol, and unidentified soluble inorganic products. A deuterium labeling experiment revealed that the cleavage of methylene C-H bond and methyl C-H bond are the rate-determining steps for acetaldehyde and ethylene formation, respectively. Intramolecular mechanisms which are analogous to the well-known organic Norrish Type II reaction have been proposed for these photoreactions.
• The allyl ester (P$\sb3$O$\sb9)$MoO$\sb2$OCH$\sb2$CH=CH$\sb2$((n-C$\sb4$H$\sb9)\sb4$N) $\sb2$ isomerizes in CH$\sb3$CN at 80$\sp\circ$C to form the enol ester cis-(P$\sb3$O$\sb9$)MoO$\sb2$OCH=CHCH$\sb3$((n-C$\sb4$H$\sb9)\sb4$N) $\sb2$. Under the same reaction conditions, allyl-1,1-d$\sb2$ ester (P$\sb3$O$\sb9$)MoO$\sb2$OCD$\sb2$CH=CH$\sb2$((n- C$\sb4$H$\sb9)\sb4$N) $\sb2$ isomerizes to yield enol-1,3-d$\sb2$ ester cis-(P$\sb3$O$\sb9$)MoO$\sb2$OCD=CHCH$\sb2$D((n-C$\sb4$H$\sb9)\sb4$N) $\sb2$. The isomerization kinetics for both allyl esters are first order. The rate-determining step in the isomerization is cleavage of the methylene C-H bond according to the deuterium labeling experiments. Deuterium labeling crossover experiments indicate the isomerization is intramolecular. A mechanism is proposed for this isomerization that involves a 1,3 proton shift via an intermediate in which the proton is bonded to a molybdenyl (Mo=O) oxygen.

Graduation Semester

1988

Type of Resource

text

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