Stereoselectivity in the Outer-Sphere Reduction of Nickel(IV) Bis(oxime-imine) Complexes by [Co(phen)3]2+

A. Graham Lappin, Daniel P. Martone, Peter Osvath, Rosemary A. Marusak

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Abstract

The kinetics and mechanisms of oxidation of [Co(phen)3]2+, [Co(bpy)3]2+, [Co(4,7-Me2phen)3]2+, and [Co(5,6-Me2phen)3]2+ (phen = 1,10-phenanthroline, bpy = 2, 2′-bipyridyl) by seven bis(oxime-imine) nickel(IV) complexes have been examined at 25.0 °C and 0.10 M ionic strength by stopped flow spectrophotometry. The nickel(IV) species are substituted methyl and benzyl derivatives of [NiIVMe2L(1)]2+, [NiIVMePhL(1)]2+, and [NiIVPhMeL(1)]2+ (Me2L(1)H2 = 3,14-dimethyl-4,7,10,13-tetraazahexadeca-3,13-diene-2,15-dione dioxime; MePhL(1)H2 and PhMeL(l)H2 have methyl groups on the oxime-imine chromophore replaced by phenyl groups). Reactions are biphasic with an initial, rapid, pH-independent reduction of nickel(IV) to give nickel(III) species that are subsequently reduced to nickel(II) in the pH-dependent second reaction. Consideration of the reaction rates and comparisons with rates calculated by using Marcus theory lead to the conclusion that the electron transfers are outer sphere in nature. Stereoselectivities in the oxidations of [Co(phen)3]2+ have been investigated with a series of chiral nickel(IV) derivatives formed stereospecifically with the A configuration from optically active ligands, Me2L(2)H2 and Me2L(3)H2, which have respectively (S)-methyl and (S)-benzyl groups attached to the 5- and 12-positions of the amine backbone. Except for the reaction with [⋀-NiIVPhMeL(2)]2+, the preferred product is [⋀-Co(phen)3]3+ with optical purities ranging from 19% to 36%. For [⋀-NiIVPhMeL(2)]2+, the preferred product is 9% [Δ-Co(phen)3]3+. It is argued that the magnitude of the stereoselectivities indicates an intimate interaction between the reactants, which are both cationic species. The dependence on structure can be explained by considering hydrophobic interactions and reflects precursor ion pair structure rather than a stereoelectronic effect associated with the electron-transfer act.

Original languageEnglish (US)
Pages (from-to)1863-1868
Number of pages6
JournalInorganic chemistry
Volume27
Issue number11
DOIs
StatePublished - Jun 1 1988
Externally publishedYes

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