Dalton Transactions PAPER Cite this: Dalton Trans., 2016, 45, 14725 Received 13th May 2016, Accepted 2nd June 2016 DOI: 10.1039/c6dt01903j www.rsc.org/dalton Heterodinuclear Ni(II) and Cu(II) Schiff base complexes and their activity in oxygen reduction† Sara Realista, Priscila Ramgi, Bernardo de P. Cardoso, Ana I. Melato, Ana S. Viana, Maria José Calhorda* and Paulo N. Martinho* New Cu(II)/Ni(II) heterodinuclear complexes with salphen-type ligands were synthesised via a stepwise template method. DFT studies were performed to understand their electronic properties, showing locali- sation of the HOMO on the Ni(II) fragment, while in the oxidised species the spin density was high at some carbon phenolate atoms. These new complexes were potentiodynamically electropolymerised on glassy carbon and platinum. Atomic force microscopy was used to evaluate the influence of the metal centres on the morphology of the polymers, revealing how the presence of Cu(II) increased the surface roughness. The oxygen reduction reaction was observed on both glassy carbon and platinum modified electrodes in neutral medium. Introduction The versatility of salen-type (salen = N,N-bis(salicylidene)ethy- lenediamine) complexes has been shown, among others, in their extensive applications in electrochemistry owing to their electrochromic, 1 sensor 2 and catalytic 3 properties. These fea- tures are complemented by their easy electropolymerisation without significant modifications of the metal environment. 4 Indeed, other complexes containing ligands such as porphyr- ins 5 and phtalocyanines 6 are often used as monomers, but they require the presence of an additional functionality in their structure in order to afford electropolymers. 7 Among them, aniline, pyrrole and thiophene have been commonly applied to produce electrosynthesised polymers with metallic conductivity. 8 In addition, when metal complexes based on these monomers are used, their polymers often display not only the properties as the organic based polymers, but additional features related to the metal centre. 4 Metal salen-type electropolymers of transition metal ions such as Co(II), 9,10 Fe(III) 11 and Ni(II) 12 have produced exciting results in the activation of small molecules, particularly electrocatalytic reduction of oxygen. Poly-Co(salen) 9,10 showed indeed an electrocatalytic performance similar to that of plati- num, while poly-Ni(salen) 12 behaved as a good oxygen sensor. The oxygen reduction reaction (ORR) is one of the most impor- tant steps in biological processes and energy conversion, and is used in several industrial electrocatalytic applications such as water electrolysis, batteries and fuel cells. 13 The electro- catalytic reduction of oxygen has been widely studied and can occur either by a two or a four electron mechanism. While the four electron reduction pathway produces water as a sub- product, the two electron reduction leads to the more pollutant hydrogen peroxide sub-product. 14 Precious metals (platinum, palladium, iridium, and ruthenium) have been used as electro- catalysts 15 to overcome the slow kinetics of oxygen reduction, but their combined low availability and high-cost are major concerns for large scale applications. The desire to find new electrocatalysts based on low-cost materials, which addition- ally show high stability, high durability and high activity, 15 led to modification of electrodes by self-assembled monolayers, 16 Langmuir–Blodgett deposition 17 and organic 18 and in- organic 19 polymeric thin-films. Within our interest in engineering materials with synergic properties arising from different catalytic centres, we applied our stepwise synthetic strategy to develop two asymmetric heterodinuclear Ni(II) and Cu(II) monomers (1 and 2). The possibility to have several monomers with different substitu- ents and metal centres (Scheme 1) allows obtaining new elec- tropolymeric films and evaluating their performance towards ORR. In a previous work we described the synthesis of asym- metric homodimetallic complexes of Ni(II)(3) and Cu(II)(4) (Scheme 1). 20 The combination of the two metals in these four monomers, associated with the ligand environment provided by salphen substituents, makes possible the fine tuning of the function/activity of the new materials. † Electronic supplementary information (ESI) available: HR-ESI/MS, cyclic vol- tammograms and peak current vs. square root of the scan rate plots (Fig. S1– S11); coordinates of all calculated structures. See DOI: 10.1039/c6dt01903j Centro de Química e Bioquímica, DQB, Faculdade de Ciências, Universidade de Lisboa, 1749-016 Lisboa, Portugal. E-mail: mjc@ciencias.ulisboa.pt, pnmartinho@ciencias.ulisboa.pt This journal is © The Royal Society of Chemistry 2016 Dalton Trans. , 2016, 45, 14725–14733 | 14725