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A Cu I complex of 3-ethynyl-phenanthroline covalently immobilized to an azide-modified glassy carbon surface is an active electrocatalyst for the 4-electron reduction of O 2 to H 2 O. The rate of O 2 reduction is 2 nd order in Cu coverage at moderate overpotential, suggesting that two Cu I species are necessary for efficient 4-electron reduction of O 2 . Mechanisms for O 2 reduction are proposed that are consistent with the observations for this covalently immobilized system and previously reported results for a similar physisorbed Cu I system. Discrete copper complexes are potential catalysts for the 4-electron reduction of O 2 to water in ambient temperature fuel cells as evidenced by Cu-containing fungal laccase enzymes that rapidly reduce O 2 directly to water at a trinuclear Cu active site at remarkably positive potentials. [1][2][3][4][5] Several groups have studied molecular Cu complexes immobilized onto electrode surfaces as an entry into the study of 4-electron O 2 reduction. [6][7][8][9][10][11][12][13][14][15][16][17][18][19] In particular, physisorbed Cu I (1,10-phenanthroline), Cu(phen P ), reduces O 2 quantitatively by 4 electrons and 4 protons to water. [8][9][10] Anson, et al., determined that this reaction was 1 st order in Cu coverage, suggestive of a mononuclear Cu site as the active catalyst. 8,10 In the present study, similar Cu I complexes are covalently attached to a modified glassycarbon electrode surface to form a species denoted Cu(phen C ), and the effect of Cu coverage on the kinetics of electrocatalytic O 2 reduction is investigated. At low overpotentials, we observe a 2 nd order dependence of the O 2 -reduction rate on the coverage of Cu(phen C ), from which we infer that two physically proximal Cu(phen C ) bind O 2 to form a binuclear Cu 2 O 2 species required for 4-electron reduction. We suggest that a similar binuclear species also forms in the case of Cu(phen P ) 8,10 but that rate-limiting binding of O 2 to the first Cu(phen P ) NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manuscript followed by rapid surface diffusion of a second Cu(phen P ) has, until now, obscured the binuclear nature of the reaction.The covalent attachment of 3-ethynyl-1,10-phenanthroline to an azide-modified glassy carbon electrode to form Cu(phen C ) relies on the Cu I -catalyzed cycloaddition of azide and ethynyl groups to form a triazole linker, commonly referred to as the click reaction. 20,21 The electrode is azide terminated by treating a roughly-ground, heat-treated glassy carbon surface with a solution of IN 3 in hexanes, a procedure modified from that first described by Devadoss and Chidsey. 22 An XPS survey of the azide-modified surface shows two N 1s peaks at 399 eV and 403 eV in a 2:1 ratio attributable to the azide nitrogens. [22][23][24] Upon exposure to 3-ethynyl-1,10-phenanthroline under the click reaction conditions 25 , the 403-eV peak disappears and the 399-eV peak broadens, consistent with the formation of the 1,2,3-triazole linker. 22,24 XPS peaks at 934 and ...

We have synthesized and characterized bis(mu-oxo)dicopper(III) dimers 1b-4b (Os) based on a core family of peralkylated trans-(1R,2R)-cyclohexanediamine (CD) ligands, self-assembled from the corresponding [LCu(MeCN)]CF3SO3 species 1a-4a and O2 at 193 K in aprotic media; additional Os based on peralkylated ethylenediamine and tridentate polyazacyclononane ligands were synthesized analogously for comparative purposes (5b-7b and 8b-9b, respectively). Trigonal-planar [LCu(MeCN)]1+ species are proposed as the active O precursors. The 3-coordinate Cu(I) complexes [(L(TE))Cu(MeCN)]CF3SO3 (4a) and [(L(TB))Cu(MeCN)]CF3SO3 (10a) were structurally characterized; the apparent O2-inertness of 10a correlates with the steric demands of its four benzyl substituents. The rate of O formation, a multistep process that likely proceeds via associative formation of a 1:1 [LCu(O2)]1+ intermediate, exhibits significant dependence upon ligand sterics and solvent: oxygenation of 4a-the slowest-reacting O precursor of the CD series-is first-order with respect to [4a] and proceeds at least 300 times faster in tetrahydrofuran than in CH2Cl2. The EPR, UV-vis, and resonance Raman spectra of 1b-9b are all characteristic of the diamagnetic bis(mu-oxo)dicopper(III) core. The intense ligand-to-metal charge transfer absorption maxima of CD-based Os are red-shifted proportionally with increasing peripheral ligand bulk, an effect ascribed to a slight distortion of the [Cu2O2] rhomb. The well-ordered crystal structure of [(L(ME))2Cu2(mu-O)2](CF3SO3)2.4CH2Cl2 ([3b. 4CH2Cl2]) features the most metrically compact [Cu2O2]2+ core among structurally characterized Os (av Cu-O 1.802(7) A; Cu...Cu 2.744(1) A) and exemplifies the minimal square-planar ligation environment necessary for stabilization of Cu(III). The reported Os are mild oxidants with moderate reactivity toward coordinating substrates, readily oxidizing thiols, certain activated alkoxides, and electron-rich phenols in a net 2e-, 2H+ process. In the absence of substrates, 1b-9b undergo thermally induced autolysis with concomitant degradation of the polyamine ligands. Ligand product distribution and primary kinetic isotope effects (kobsH/kobsD approximately 8, 1b/d24-1b, 293 K) support a unimolecular mechanism involving rate-determining C-H bond cleavage at accessible ligand N-alkyl substituents. Decomposition half-lives span almost 3 orders of magnitude at 293 K, ranging from approximately 2 s for 4b to almost 30 min for d(24)-1b, the most thermally robust dicationic O yet reported. Dealkylation is highly selective where ligand rigidity constrains accessibility; in 3b, the ethyl groups are attacked preferentially. The observed relative thermal stabilities and dealkylation selectivities of 1b-9b are correlated with NC(alpha)-H bond dissociation energies, statistical factors, ligand backbone rigidity, and ligand denticity/axial donor strength. Among the peralkylated amines surveyed, bidentate ligands with oxidatively robust NC(alpha)-H bonds provide optimal stabilization for Os. Fortuitously, the le...

The aim and scope of this review is to show the general validity of the 'complex-as-ligand' approach for the rational design of metallosupramolecular assemblies of increasing structural and magnetic complexity. This is illustrated herein on the basis of our recent studies on oxamato complexes with transition metal ions looking for the limits of the research avenue opened by Kahn's pioneering research twenty years ago. The use as building blocks of mono-, di- and trinuclear metal complexes with a novel family of aromatic polyoxamato ligands allowed us to move further in the coordination chemistry-based approach to high-nuclearity coordination compounds and high-dimensionality coordination polymers. In order to do so, we have taken advantage of the new developments of metallosupramolecular chemistry and in particular, of the molecular-programmed self-assembly methods that exploit the coordination preferences of metal ions and specifically tailored ligands. The judicious choice of the oxamato metal building block (substitution pattern and steric requirements of the bridging ligand, as well as the electronic configuration and magnetic anisotropy of the metal ion) allowed us to control the overall structure and magnetic properties of the final multidimensional nD products (n = 0-3). These species exhibit interesting magnetic properties which are brand-new targets in the field of molecular magnetism, such as single-molecule or single-chain magnets, and the well-known class of molecule-based magnets. This unique family of molecule-based magnetic materials expands on the reported examples of nD species with cyanide and related oxalato and dithiooxalato analogues. Moreover, the development of new oxamato metal building blocks with potential photo or redox activity at the aromatic ligand counterpart will provide us with addressable, multifunctional molecular materials for future applications in molecular electronics and nanotechnology.

First organic radicals, now metal complexes: A successful extension to metal complexes of a well‐known organic radical approach to ferromagnetism is exemplified by the triplet ground‐state molecule containing two CuII centers connected by a double m‐phenylenediamide skeleton of the cyclophane type shown in the scheme.