Efficient Chemisorption of Organophosphorous Redox Probes on Indium Tin Oxide Surfaces under Mild Conditions

We report a mild and straightforward one-step chemical surface functionalization of indium tin oxide (ITO) electrodes by redox-active molecules bearing an organophosphoryl anchoring group (i.e., alkyl phosphate or alkyl phosphonate group). The method takes advantage of simple passive adsorption in an aqueous solution at room temperature. We show that organophosphorus compounds can adsorb much more strongly and stably on an ITO surface than analogous redox-active molecules bearing a carboxylate or a boronate moiety. We provide evidence, through quantitative electrochemical characterization (i.e., by cyclic voltammetry) of the adsorbed organophosphoryl redox-active molecules, of the occurrence of three different adsorbate fractions on ITO, exhibiting different stabilities on the surface. Among these three fractions, one is observed to be strongly chemisorbed, exhibiting high stability and resistance to desorption/hydrolysis in a free-redox probe aqueous buffer. We attribute this remarkable stability to the formation of chemical bonds between the organophosphorus anchoring group and the metal oxide surface, likely occurring through a heterocondensation reaction in water. From XPS analysis, we also demonstrate that the surface coverage of the chemisorbed molecules is highly affected by the degree of surface hydroxylation, a parameter that can be tuned by simply preconditioning the freshly cleaned ITO surfaces in water. The lower the relative surface hydroxide density on ITO, the higher was the surface coverage of the chemisorbed species. This behavior is in line with a chemisorption mechanism involving coordination of a deprotonated phosphoryl oxygen atom to the non-hydroxylated acidic metal sites of ITO

Synthesis and Reactivity of Tripodal Complexes Containing Pendant Bases

The synthesis of a new tripodal ligand family that contains tertiary amine groups in the second-coordination sphere is reported. The ligands are tris­(amido)­amine derivatives, with the pendant amines attached via a peptide coupling strategy. They were designed to function as new molecular catalysts for the oxygen reduction reaction (ORR), in which the pendant acid/base group could improve the catalyst performance. Two members of the ligand family were each metalated with cobalt­(II) and zinc­(II) to afford trigonal-monopyramidal complexes. The reaction of the cobalt complexes <b>[Co­(L)]</b>^<b>–</b> with dioxygen reversibly generates a small amount of a cobalt­(III) superoxo species, which was characterized by electron paramagnetic resonance (EPR) spectroscopy. Protonation of the zinc complex Zn­[N­{CH₂CH₂NC­(O)­CH₂N­(CH₂Ph)₂}₃)]⁻ (<b>[Zn­(TN</b>^<b>Bn</b><b>)]</b>^<b>–</b>) with 1 equiv of acid occurs at a primary-coordination-sphere amide moiety rather than at a pendant basic site. The addition of excess acid to any of the complexes <b>[M­(L)]</b>^<b>–</b> results in complete proteolysis and formation of the ligands <b>H</b>_<b>3</b><b>L</b>. These undesired reactions limit the use of these complexes as catalysts for the ORR. An alternative ligand with two pyridyl arms was also prepared but could not be metalated. These studies highlight the importance of the stability of the primary-coordination sphere of ORR electrocatalysts to both oxidative <i>and</i> acidic conditions