Graduate Thesis Or Dissertation
 

Methanolysis of tetrakis(trichlorophosphine)nickel(0)

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  • The methanolysis of phosphorus trichloride coordinated to nickel (0) has been studied. Free phosphorus trichloride reacts with methanol to give dimethyl hydrogen phosphonate, methyl chloride, and hydrogen chloride. The reaction involves a rearrangement on phosphorus using the lone pair, in which the coordination number increases from three to four. In the complex Ni(PC1₃)₄, tetrakis(trichlorophosphine)-nickel(0), the lone pair is already involved in coordinate bonding and the coordination number is already four. Attack of methanol might be on the complex itself, with the metal-phosphorus bond retained throughout the reaction, or might be on free phosphorus ligand in equilibrium with a partly dissociated complex. The reaction of methanol with tetrakis (trichlorophosphine)nickel(0) in hydrocarbon solvents was found to be quite slow, to involve a series of various colored heterogeneous intermediates, and to give eventually a clear green solution. Hydrogen chloride, methyl chloride, dimethyl hydrogen phosphonate, and nickel in the plus two oxidation state were identified as the major reaction products. These were identified by vacuum line separation and characterization, gas chromatography, and spectroscopic techniques. No specific reduction product was identified. It was established that hydrogen was not formed. It is postulated that a reduced organo-phosphorus species was formed, such as a hypophosphite, possibly through the intermediate formation of a hydrido-nickel complex. Such a species could be formed by oxidative addition of hydrogen chloride to a nickel(0) complex. Experimental evidence of an indirect nature is presented to support this suggestion. With free phosphorus trichloride, methanolysis in the presence of a tertiary amine (which consumes hydrogen chloride as it is formed, but is otherwise inert) permits synthesis of the unrearranged tertiary phosphite ester. An attempt to use this technique in studying methanolysis of the nickel(0) complex was unsuccessful. An unexpected direct reaction took place between triethylamine and tetrakis-(trichlorophosphine)nickel(0). Methanolysis of the unidentified product of this reaction also gave oxidized nickel(II) species. Reaction of tetrakis(trichlorophosphine)nickel(0) with sodium methoxide in methanol causes hydrogen chloride to be consumed as fast as it is formed and also involves methoxide ion, a more reactive nucleophile than methanol. The product of this reaction, in hydrocarbon solvents, was tetrakis(trimethyl phosphite)nickel(0). Conditions for obtaining this product in the pure state, conveniently, and in good yield were developed. The method is recommended as the preferred method of synthesis of this compound.
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