Multiple Temperature Electrochemical Studies of the Oxidatively Catalyzed E-Z Isomerization Reactions in Iron Alkenyl Complexes. Improved Resolution of Closely Spaced Redox Systems Using Deconvolution Voltammetry

Robert H. Philp, Daniel L. Reger, Alan M. Bond

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Abstract

The one-electron electrochemical oxidation of the eighteen-electron (18e) Z and E isomers of CpFeCO[P(OPh)3](η1-C(Ph)=C(Ph)Me) to their seventeen-electron (17e) counterparts has been studied by cyclic voltammetry and by deconvolution (semidifferential) voltammetry in dichloromethane in the temperature range-87 to 22 °C. At room temperature the 17e cations appear to be unstable and the electrochemistry is that of an EC system with a half-life of 0.4 s. At temperatures below ca.-60 °C two reversible redox couples are resolved in mixtures, whereas at temperatures around-20 °C the electrochemical response is indistinguishable from that of a single simple reversible system. This is attributable to the temperature dependent, but rapid E+ to Z+ isomerization and associated cross redox reaction. The electrochemistry at-78 °C may be described by the following equations: Z ⇌Z++ eE1/2= 0.51 V vs Ag/AgCl E⇌ E+ + e-E1/2 = 0.65 V vs Ag/AgCl E+ + Z ⇌ Z+ + E Keq= 4 X 103 By combining the E1/2 values measured at low temperature with NMR data for the neutral species an equilibrium ratio [CpFeCO[P(OPh)3](η1-(Z)-C(Ph)=C(Ph)Me)]+/[CpFeCO[P(OPh)3](η1-(E)-C(Ph)= CMe(Ph)]+ of 9 X 102 at-78 °C is calculated. In the 18e configuration the E rather than the Z isomer is favored by a factor of about 5:1 so the isomer distribution is strongly modified by oxidation to the 17e configuration. The change in isomer distribution may have implications in the synthetic chemistry of this class of compounds.

Original languageEnglish
Pages (from-to)1714-1718
Number of pages5
JournalOrganometallics
Volume8
Issue number7
DOIs
Publication statusPublished - 1 Jul 1989
Externally publishedYes

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