THE SYNTHESIS AND CHARACTERIZATION OF ISOCYANIDE COMPLEXES OF MOLYBDENUM(II), CHROMIUM(II), RHENIUM(I) AND RHENIUM(III): STUDIES OF THE ISOCYANIDE CLEAVAGE OF METAL-METAL QUADRUPLE BONDS
Abstract
The reactions of Mo(,2)(O(,2)CCH(,3))(,4) and K(,4)Mo(,2)Cl(,8) with alkyl isocyanides causes scission of the Mo-Mo quadruple bond to give {Mo(CNR)(,7)}('2+) ions, where R = CH(,3), (CH(,3))(,3)C, or C(,6)H(,11), which can be isolated by using large anions such as BF(,4)('-) and PF(,6)('-). An X-ray crystallographic study of {Mo(CNCH(,3))(,7)} (BF(,4))(,2) has determined the structure of the {Mo(CNCH(,3))(,7)}('2+) ion to be a capped octahedron and as such furnishes the first example of a homoleptic ML(,7) species with a capped octahedral geometry. The reactions of the tertiary phosphine complexes Mo(,2)Cl(,4)(dppm)(,2), Mo(,2)Cl(,4)(dppe)(,2) and Mo(,2)Cl(,4)(PR(,3))(,4), where dppm = bis(diphenylphosphino)methane and dppe = 1,2-bis(diphenylphosphino)ethane with alkyl isocyanides RNC, where R = CH(,3), C(CH(,3))(,3) or C(,6)H(,11), leads to the cleavage of the Mo-Mo quadruple bonds and the formation of the following cations: {Mo(CNR)(,5)(dppm)}('2+), {Mo(CNR)(,5)(dppe)}('2+), {Mo(CNR)(,5)(PR(,3))(,2)}('2+), and {Mo(CNR)(,6)(PR(,3))}('2+) which are all isolated as their PF(,6)('-) salts. The reactions of {Mo(CNR)(,7)}(PF(,6))(,2) with phosphines have been studied and found to give the same seven-coordinate mixed phosphine-alkyl isocyanide products mentioned above. The spectroscopic characterizations of these complexes support the mixed-ligand complexes possessing seven-coordinate geometries since there is no evidence for unbound phosphine donor atoms. The reaction of alkyl isocyanides with ethanolic solutions of Cr(II) produce the paramagnetic six-coordinate cations Cr(CNR)(,6)('2+) which have been prepared as their PF(,6)('-) salts (R = C(,6)H(,11) or C(CH(,3))(,3)). The treatment of {Cr(CNR)(,6)}(PF(,6))(,2) with neat alkyl isocyanide leads to the formation of diamagnetic seven-coordinate {Cr(CNR)(,7)}(PF(,6))(,2) providing an exceedingly rare example of a homoleptic seven-coordinate complex of a first row transition element and the first involving alkyl or aryl isocyanide ligands. The reactions of Re(,2)(O(,2)CR)(,4)Cl(,2) and {(n-Bu)(,4)N}(,2)Re(,2)X(,8), where R = CH(,3) or C(,6)H(,5) and X = Cl or Br, with t-butylisocyanide causes rupture of the Re-Re quadruple bond to give {Re(CNC(CH(,3))(,3))(,6)}('+) and {Re(CNC(CH(,3))(,3))(,5)X(,2)}('+), respectively, which can be isolated using large anions such as PF(,6)('-) or BPh(,4)('-). The cation {Re(CNC(CH(,3))(,3))(,6)}('+) reacts readily with halogens X(,2), where X = Cl, Br or I, to form the seven-coordinate cations {Re(CNC(CH(,3))(,3))(,6)X}('2+) which are also isolated as PF(,6)('-) salts. The species {Re(CNC(CH(,3))(,3))(,6)Br}('2+) can be converted back to {Re(CNC(CH(,3))(,3))(,6)}('+) by the action of tertiary phosphines and phosphites. The reactions of K(,2)ReX(,6), where X = Cl, Br or I, with t-butylisocyanide under various conditions have been investigated, and the following products have been isolated and characterized: {Re(CNC(CH(,3))(,3))(,6)}I(,3), {Re(CNC(CH(,3))(,3))(,5)I(,2)}I(,3) (for X = I), and {Re(CNC(CH(,3))(,3))(,6)}(PF(,6)) and {Re(CNC(CH(,3))(,5)Br(,2)}(PF(,6)) (for X = Br). No reaction occurred when X = Cl. The studies presented in this thesis have established cleavage of the quadruple bond as the usual reaction pathway when isocyanides, a class of strong (pi)-acceptor ligands, react with compounds containing metal-metal quadruple bonds.
Degree
Ph.D.
Subject Area
Chemistry
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