ISOCYANIDE COMPLEXES OF THE EARLY TRANSITION METALS: SYNTHESIS, REDOX AND SUBSTITUTION CHEMISTRY
Abstract
The treatment of ethanolic solutions of chromium(II) (which are derived from the reduction of CrCl(,3)(.)6H(,2)O solutions) with alkyl isocyanides RNC in the presence of KPF(,6) produces the homoleptic complexes {Cr(CNR)(,6)}(PF(,6))(,2). These chromium(II) species are easily converted to diamagnetic seven-coordinate {Cr(CNR)(,7)}(PF(,6))(,2) by treatment with neat RNC. Phenyl isocyanide reacts with ethanolic chromium(II) solutions to form {Cr(CNPh)(,5)Cl}PF(,6). The homoleptic complexes {Cr(CNR)(,6)}(PF(,6))(,2) react with tertiary phosphines PR(,3) and dppe to yield trans-{Cr(CNR)(,4)(PR(,3))(,2)}(PF(,6))(,2) and cis-{Cr(CNR)(,4)(dppe)}(PF(,6))(,2), while {Cr(CNPh)(,5)Cl}PF(,6) reacts with these same reagents to form trans-{Cr(CNPh)(,4)(PR(,3))Cl}PF(,6) and {Cr(CNPh)(,3)(dppe)Cl}PF(,6). Cyclic voltammetric measurements of these chromium(II) derivatives reveal the electrochemical accessibility of the sequence of oxidation states Cr(III) (DBLARR) Cr(II) (DBLARR) Cr(I) (DBLARR) Cr(0). Nitric oxide reacts with the homoleptic complexes {Cr(CNR(,6)}(PF(,6))(,2) to generate the chromium(I) series {Cr(NO)(CNR)(,5)}(PF(,6))(,2). This formal reductive substitution provides these 17-electron species which themselves are easily reduced to their 18-electron chromium(0) congeners {Cr(NO)(CNR)(,5)}PF(,6). A wide variety of substitution products of {Cr(NO)(CNR)(,5)}(PF(,6))(,n) (n = 1 or 2) has been synthesized, including various isomers of {Cr(NO)(CNR)(,4)L}(PF(,6))(,n) (L = amine, n = 1; L = Cl or Br, n = 1; L = PR(,3), n = 1 or 2) and {Cr(NO)(CNR)(,3)(L-L)}(PF(,6))(,n) (where L-L = dppm, dppe, or arphos, n = 1 or 2). Only for the PR(,3) and L-L derivatives are stable, crystalline complexes readily obtainable in both chromium(I) and chromium(0) oxidation states. Cyclic voltammetric measurements of these compounds reveal a single, reversible couple. The series {Cr(NO)(CNR)(,3)(L-L)}('n+) (n = 1 or 2) are observed to isomerize in dichloromethane solution for the mer to the fac isomeric forms. These molecules are activated to isomerization by oxidation, as this process is much faster in the chromium(I) complexes than in the corresponding chromium(0) species. A series of syntheses directed toward the formation of homoleptic isocyanide complexes of the group 5 metals niobium and tantalum have been undertaken. The reactions of alkyl isocyanides with the halides of rhenium K(,2)ReI(,6) and Re(,3)X(,9) (X = Cl, Br, or I) have also been examined. The cluster adducts Re(,3)X(,9)(CNR)(,3), as well as various mononuclear products, have been isolated and characterized.
Degree
Ph.D.
Subject Area
Chemistry
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