Ion-exchange voltammetry of tris(2,2′-bipyridine) nickel(II), cobalt(II), and Co(salen) at polyestersulfonated ionomer coated electrodes in acetonitrile: Reactivity of the electrogenerated low-valent complexes

Abstract

The electrochemical behaviour of [Ni(bpy) 3(BF 4) 2], [Co(bpy) 3(BF 4) 2], and Co(salen) (where bpy = 2,2′-bipyridine, and salen = N, N′-bis(salicylidene)ethylenediamine) is studied at a glassy carbon electrode modified with the poly(estersulfonate) ionomer Eastman AQ 55 in acetonitrile (MeCN). It is shown that the nickel complex is strongly incorporated into the polymer. The reduction of the divalent nickel compound features a two-electron process leading to a nickel(0) species which is released from the coating because of the lack of electrostatic attraction with the ionomer. Yet, the neutral zerovalent nickel-bipyridine complex is reactive towards ethyl 4-iodobenzoate and di-bromocyclohexane despite the presence of the polymer. The activation of the aryl halide occurs through an oxidative addition, whereas, an electron transfer is involved in the presence of the alkyl halide making the catalyst regeneration much faster in the latter case. The electrochemical study of [Co(bpy) 3(BF 4) 2] shows that incorporation of the cobalt complex into the polymer is efficient, provided excess bpy is used. This excess bpy does not interfere with the electrocatalytic activity of the cobalt complex incorporated in the AQ coating and efficient electrocatalysis is observed towards di-bromocyclohexane and benzyl-bromide as substrates. Finally, replacement of the bpy ligand with the macrocycle N, N′-bis(salicylidene)ethylenediamine, salen, leads to the incorporation of the non-charged Co II(salen) complex into the AQ 55 polymer showing the relevancy of hydrophobic interactions. The reaction between the electrogenerated [Co I(salen)] − with 1,2-dibromocyclohexane exhibits a fast inner sphere electron transfer.