CHEMICAL AND ELECTROCHEMICAL STUDIES OF EARLY TRANSITION METAL CLUSTERS OF NIOBIUM AND TANTALUM AND ISOCYANIDE COMPLEXES OF MOLYBDENUM AND TUNGSTEN

DOUGLAS DEAN KLENDWORTH, Purdue University

Abstract

Part I. Tertiary phosphines have been found to stabilize low oxidation state octahedral clusters of niobium, Nb(,6)('n+), and tantalum, Ta(,6)('n+). Electrochemical studies on dichloromethane and acetonitrile solutions of the phosphine derivatives {(M(,6)Cl(,12))Cl(,2)(PR(,3))(,4)}(PF(,6))(,n) (where M = Nb or Ta; PR(,3) = PEt(,3), PPr(,3)('n), or PEt(,2)Ph; n = 0, 1, or 2) have yielded the first evidence for the cluster core, {M(,6)Cl(,12)}('+). Chemical oxidations involving the neutral phosphine derivative {(Ta(,6)Cl(,12))Cl(,2)(PPr(,3)('n))(,4)}, have produced the first complete series with the {Ta(,6)Cl(,12)}('n+) core stabilized in all three oxidation states, n = 2, 3, and 4, by an organic ligand, viz., {(Ta(,6)Cl(,12))Cl(,2)(PPr(,3)('n))(,4)}(PF(,6))(,n), where n = 0, 1, or 2. Part II. The first homoleptic, seven-coordinate aryl isocyanide cation of Mo(II), {Mo(CNPh)(,7)}('2+), has been prepared by oxidation of Mo(CNPh)(,6). Exchange of one of the isocyanide ligands by a halide ion (X('-)) was found to be quite facile, yielding the seven-coordinate mixed halo-aryl isocyanide species {Mo(CNPh)(,6)X}('+). The substitution chemistry of the new seven-coordinate complex {Mo(CNPh)(,7)}(PF(,6))(,2) was investigated with mono- and bidentate phosphines, yielding seven-coordinate mixed phosphine-aryl isocyanide complexes {Mo(CNPh)(,5)(PR(,3))(,2)}(PF(,6))(,2) (where PR(,3) = PEt(,3), PPr(,3)('n), PEt(,2)Ph, or PEtPh(,2)), {Mo(CNPh)(,6)PPh(,3)}(PF(,6))(,2), {Mo(CNPh)(,5)dppe}(PF(,6))(,2), and {Mo(CNPh)(,5)dppm}(PF(,6))(,2). Similar substitution chemistry was observed for {W(CNPh)(,7)}(PF(,6))(,2). The cyclic voltammograms of all the seven-coordinate complexes contained a two-electron, irreversible reduction with E(,p,c) at ca. -1.0V vs SCE. The cyclic voltammogram of {Mo(CNPh)(,7)}(PF(,6))(,2) after bulk electrolysis at a potential cathodic of this reduction established that Mo(CNPh)(,6) is the chemical product. Chemical reduction of {Mo(CNPh)(,7)}(PF(,6))(,2), {W(CNPh)(,7)}(PF(,6))(,2), and {Mo(CNPh)(,5)dppe}(PF(,6))(,2) with magnesium produced Mo(CNPh)(,6), W(CNPh)(,6), and Mo(CNPh)(,4)dppe, respectively, in good yield.

Degree

Ph.D.

Subject Area

Chemistry

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