Cooperative Effects Drive Water Oxidation Catalysis in Cobalt Electrocatalysts through the Destabilization of Intermediates.

A barrier to understanding the factors driving catalysis in the oxygen evolution reaction (OER) is understanding multiple overlapping redox transitions in the OER catalysts. The complexity of these transitions obscure the relationship between the coverage of adsorbates and OER kinetics, leading to an experimental challenge in measuring activity descriptors, such as binding energies, as well as adsorbate interactions, which may destabilize intermediates and modulate their binding energies. Herein, we utilize a newly designed optical spectroelectrochemistry system to measure these phenomena in order to contrast the behavior of two electrocatalysts, cobalt oxyhydroxide (CoOOH) and cobalt-iron hexacyanoferrate (cobalt-iron Prussian blue, CoFe-PB). Three distinct optical spectra are observed in each catalyst, corresponding to three separate redox transitions, the last of which we show to be active for the OER using time-resolved spectroscopy and electrochemical mass spectroscopy. By combining predictions from density functional theory with parameters obtained from electroadsorption isotherms, we demonstrate that a destabilization of catalytic intermediates occurs with increasing coverage. In CoOOH, a strong (∼0.34 eV/monolayer) destabilization of a strongly bound catalytic intermediate is observed, leading to a potential offset between the accumulation of the intermediate and measurable O2 evolution. We contrast these data to CoFe-PB, where catalytic intermediate generation and O2 evolution onset coincide due to weaker binding and destabilization (∼0.19 eV/monolayer). By considering a correlation between activation energy and binding strength, we suggest that such adsorbate driven destabilization may account for a significant fraction of the observed OER catalytic activity in both materials. Finally, we disentangle the effects of adsorbate interactions on state coverages and kinetics to show how adsorbate interactions determine the observed Tafel slopes. Crucially, the case of CoFe-PB shows that, even where interactions are weaker, adsorption remains non-Nernstian, which strongly influences the observed Tafel slope.

Photoinduced Oxygen Evolution Catalysis Promoted by Polyoxometalate Salts of Cationic Photosensitizers.

The insoluble salt Cs15K[Co9(H2O)6(OH)3(HPO4)2(PW9O34)3] (CsCo9) is tested as heterogeneous oxygen evolution catalyst in light-induced experiments, when combined with the homogeneous photosensitizer [Ru(bpy)3]2+ and the oxidant Na2S2O8 in neutral pH. Oxygen evolution occurs in parallel to a solid transformation. Post-catalytic essays indicate that the CsCo9 salt is transformed into the corresponding [Ru(bpy)3]2+ salt, upon cesium loss. Remarkably, analogous photoactivated oxygen evolution experiments starting with the [Ru(bpy)3](5+x)K(6-2x)[Co9(H2O)6(OH)3(HPO4)2(PW9O34)3]·(39+x)H2O (RuCo9) salt demonstrate much higher efficiency and kinetics. The origin of this improved performance is at the cation-anion, photosensitizer-catalyst pairing in the solid state. This is beneficial for the electron transfer event, and for the long-term stability of the photosensitizer. The latter was confirmed as the limiting process during these oxygen evolution reactions, with the polyoxometalate catalyst exhibiting robust performance in multiple cycles, upon addition of photosensitizer, and/or oxidant to the reaction mixture.

Polyoxometalate electrocatalysts based on earth-abundant metals for efficient water oxidation in acidic media.

Water splitting is a promising approach to the efficient and cost-effective production of renewable fuels, but water oxidation remains a bottleneck in its technological development because it largely relies on noble-metal catalysts. Although inexpensive transition-metal oxides are competitive water oxidation catalysts in alkaline media, they cannot compete with noble metals in acidic media, in which hydrogen production is easier and faster. Here, we report a water oxidation catalyst based on earth-abundant metals that performs well in acidic conditions. Specifically, we report the enhanced catalytic activity of insoluble salts of polyoxometalates with caesium or barium counter-cations for oxygen evolution. In particular, the barium salt of a cobalt-phosphotungstate polyanion outperforms the state-of-the-art IrO2 catalyst even at pH < 1, with an overpotential of 189 mV at 1 mA cm-2. In addition, we find that a carbon-paste conducting support with a hydrocarbon binder can improve the stability of metal-oxide catalysts in acidic media by providing a hydrophobic environment.

Preservation of electronic properties of double-decker complexes on metallic supports.

Single-molecule magnets based on lanthanide double-deckers are attracting significant attention due to their unrivaled single-ion anisotropy. To exploit their fascinating electronic and magnetic properties in devices for information storage or spin transport, studies on the preservation or variation of electronic and magnetic functionalities upon adsorption on surfaces are necessary. Herein, we introduced a comprehensive scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy (XPS) surface science study, complemented by density functional theory (DFT) simulations, of a recently synthesized single-molecule magnet based on porphyrazine deckers, conveniently equipped with ethyl moieties to make them soluble and sublimable. We demonstrated that the double-decker species were intactly adsorbed on Au(111), Ag(111), and Cu(111) in a flat-on fashion and self-assembled in hexagonal close-packed layers. Systematic multi- and monolayer XPS was performed on the surface-confined species, confirming the preservation of the electronic properties of the ligands and the lanthanide center upon adsorption.

Persistence of slow dynamics in Tb(OETAP)2 single molecule magnets embedded in conducting polymers.

UNLABELLED: The spin dynamics of Tb(OETAP)2 single ion magnets was investigated by means of muon spin relaxation (µSR) both in the bulk material as well as when the molecule is embedded into PEDOT: PSS polymer conductor. The spin fluctuation time is characterized by a high temperature activated trend, with an energy barrier around 320 K, and by a low temperature tunneling regime. When the single ion magnet is embedded into the polymer the energy barrier only slightly decreases and the fluctuation time remains of the same order of magnitude, even at low temperature. This finding shows that these single molecule magnets preserve their characteristics which, if combined with those of the conducting polymer, result in a hybrid material of potential interest for organic spintronics.

Hysteretic spin crossover above room temperature and magnetic coupling in trinuclear transition-metal complexes with anionic 1,2,4-triazole ligands.

The reaction of 4-(1,2,4-triazol-4-yl)ethanesulfonate (L) with Zn(2+), Cu(2+), Ni(2+), Co(2+), and Fe(2+) gave a series of analogous neutral trinuclear complexes with the formula [M3(µ-L)6(H2O)6] (1-5). These compounds were characterized by single-crystal X-ray diffraction, thermogravimetry, and elemental analysis. The magnetic properties of compounds 2-5 were studied. Complexes 2-4 show weak antiferromagnetic superexchange, with J values of -0.33 (2), -9.56 (3), and -4.50 cm(-1) (4) (exchange Hamiltonian H = -2 J (S1S2+S2S3)). Compound 5 shows two additional crystallographic phases (5 b and 5 c) that can be obtained by dehydration and/or thermal treatment. These three phases exhibit distinct magnetic behavior. The Fe(2+) centers in 5 are in high-spin (HS) configuration at room temperature, with the central one exhibiting a non-cooperative gradual spin transition below 250 K with T1/2 = 150 K. In 5 b, the central Fe(2+) stays in its low-spin (LS) state at room temperature, and cooperative spin transition occurs at higher temperatures and with the appearance of memory effect (T1/2↑ = 357 K and T1/2↓ = 343 K). In the case of 5 c, all iron centers remain in their HS configuration down to very low temperatures, with weak antiferromagnetic coupling (J = -1.16 cm(-1)). Compound 5 b exhibits spin transition with memory effect at the highest temperature reported, which matches the remarkable features of coordination polymers.

Hybrid molecular materials based upon organic pi-electron donors and metal complexes. Radical salts of bis(ethylenethia)tetrathiafulvalene (BET-TTF) with the octahedral anions hexacyanoferrate(III) and nitroprusside. The first kappa phase in the BET-TTF family.

The synthesis, structure, and physical characterization of two new radical salts formed with the organic donor bis(ethylenethia)tetrathiafulvalene (BET-TTF) and the octahedral anions hexacyanoferrate(III), [Fe(CN)(6)](3-), and nitroprusside, [Fe(CN)(5)NO](2-), are reported. These salts are (BET-TTF)(4)(NEt(4))(2)[Fe(CN)(6)] (1) (monoclinic space group C2/c with a = 38.867(7) A, b = 8.438(8) A, c = 11.239(6) A, beta = 90.994(9) degrees, V = 3685(4) A(3), Z = 4) and (BET-TTF)(2)[Fe(CN)(5)NO].CH(2)Cl(2) (2) (monoclinic space group C2/c with a = 16.237(6) A, b = 18.097(8) A, c = 12.663(7) A, beta = 106.016(9) degrees, V = 3576(3) A(3), Z = 4). In salt 1 the organic BET-TTF molecules are packed in orthogonal dimers, forming the first kappa phase observed for this donor. The analysis of the bond distances and the electronic and IR spectra suggests a degree of ionicity of 1/4 per BET-TTF molecule, in agreement with the stoichiometry of the salt. The electrical properties show that 1 is a semiconductor with a high room-temperature conductivity (11.6 S cm(-1)) and a low activation energy (45 meV), in agreement with the band structure calculations. The magnetic susceptibility of 1 shows, besides the paramagnetic contribution from the anion, a temperature-independent paramagnetism (TIP) of the Pauli type due to the electronic delocalization observed at high temperatures in the organic sublattice. This Pauli type paramagnetism is confirmed by the ESR spectra that also show a Dysonian line when the magnetic field is parallel to the conducting plane, typical of metallic and highly conducting systems. Salt 2 presents an unprecedented packing of the organic molecules that form zigzag tunnels where the anions and the solvent molecules are located. The stoichiometry indicates that all the BET-TTF molecules bear a charge of +1, and accordingly, 2 behaves as a semiconductor with a very low room-temperature conductivity. The magnetic properties of this salt indicate that the unpaired electrons on the organic molecules are strongly antiferromagnetically coupled, giving rise to a diamagnetic behavior of 2, as the nitroprusside anion is also diamagnetic.

Molecule-based magnets formed by bimetallic three-dimensional oxalate networks and chiral tris(bipyridyl) complex cations. The series [ZII(bpy)3][ClO4][MIICrIII(ox)3] (ZII = Ru, Fe, Co, and Ni; MII = Mn, Fe, Co, Ni, Cu, and Zn; ox = oxalate dianion).

The synthesis, structure, and physical properties of the series of molecular magnets formulated as [ZII(bpy)3][ClO4][MIICrIII(ox)3] (ZII = Ru, Fe, Co, and Ni; MII = Mn, Fe, Co, Ni, Cu, and Zn; ox = oxalate dianion) are presented. All the compounds are isostructural to the [Ru(bpy)3][ClO4][MnCr(ox)3] member whose structure (cubic space group P4(1)32 with a = 15.506(2) A, Z = 4) consists of a three-dimensional bimetallic network formed by alternating MII and CrIII ions connected by oxalate anions. The identical chirality (lambda in the solved crystal) of all the metallic centers determines the 3D chiral structure adopted by these compounds. The anionic 3D sublattice leaves some holes where the chiral [Z(bpy)3]2+ and ClO4- counterions are located. These compounds behave as soft ferromagnets with ordering temperatures up to 6.6 K and coercive fields up to 8 mT.

Bimetallic cyanide-bridged complexes based on the photochromic nitroprusside anion and paramagnetic metal complexes. Syntheses, structures, and physical characterization of the coordination compounds [Ni(en)2]4[Fe(CN)5NO]2[Fe(CN)6]x5H2O, [Ni(en)2][Fe(CN)5NO]x3H2O, [Mn(3-MeOsalen)(H2O)]2[Fe(CN)5NO], and [Mn(5-Brsalen)]2[Fe(CN)5NO].

The synthesis, crystal structure, and physical characterization of the coordination compounds [Ni(en)2]4[Fe(CN)5NO]2[Fe(CN)6]x5H2O (1), [Ni(en)2][Fe(CN)5NO]x3H2O (2), [Mn(3-MeOsalen)(H2O)]2[Fe(CN)5NO] (3), and [Mn(5-Brsalen)]2[Fe(CN)5NO] (4) are presented. 1 crystallizes in the monoclinic space group P2(1)/n (a = 7.407(4) A, b = 28.963(6) A, c = 14.744(5) A, alpha = 90 degrees, beta = 103.26(4) degrees, gamma = 90 degrees, Z = 2). Its structure consists of branched linear chains formed by cis-[Ni(en)2]2+ cations and ferrocyanide and nitroprusside anions. The presence of two kinds of iron(II) sites has been demonstrated by Mössbauer spectroscopy. 2 crystallizes in the monoclinic space group P2(1)/c (a = 11.076(3) A, b = 10.983(2) A, c = 17.018(5) A, alpha = 90 degrees, beta = 107.25(2) degrees, gamma = 90 degrees, Z = 4). Its structure consists of zigzag chains formed by an alternated array of cis-[Ni(en)2]2+ cations and nitroprusside anions. 3 crystallizes in the triclinic space group P1 (a = 8.896(5) A, b = 10.430(5) A, c = 12.699(5) A, alpha = 71.110(5) degrees, beta = 79.990(5) degrees, gamma = 89.470(5) degrees, Z = 1). Its structure comprises neutral trinuclear bimetallic complexes in which a central [Fe(CN)5NO]2- anion is linked to two [Mn(3-MeOsalen)]+ cations. 4 crystallizes in the tetragonal space group P4/ncc (a = 13.630(5) A, c = 21.420(8) A, Z = 4). Its structure shows an extended 2D neutral network formed by cyclic octameric [-Mn-NC-Fe-CN-]4 units. The magnetic properties of these compounds indicate the presence of quasi-isolated paramagnetic Ni2+ and Mn3+. Irradiated samples of the four compounds have been studied by differential scanning calorimetry to detect the existence of the long-lived metastable states of nitroprusside.

Coexistence of ferromagnetism and metallic conductivity in a molecule-based layered compound.

Crystal engineering--the planning and construction of crystalline supramolecular architectures from modular building blocks--permits the rational design of functional molecular materials that exhibit technologically useful behaviour such as conductivity and superconductivity, ferromagnetism and nonlinear optical properties. Because the presence of two cooperative properties in the same crystal lattice might result in new physical phenomena and novel applications, a particularly attractive goal is the design of molecular materials with two properties that are difficult or impossible to combine in a conventional inorganic solid with a continuous lattice. A promising strategy for creating this type of 'bi-functionality' targets hybrid organic/inorganic crystals comprising two functional sub-lattices exhibiting distinct properties. In this way, the organic pi-electron donor bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) and its derivatives, which form the basis of most known molecular conductors and superconductors, have been combined with molecular magnetic anions, yielding predominantly materials with conventional semiconducting or conducting properties, but also systems that are both superconducting and paramagnetic. But interesting bulk magnetic properties fail to develop, owing to the discrete nature of the inorganic anions. Another strategy for achieving cooperative magnetism involves insertion of functional bulky cations into a polymeric magnetic anion, such as the bimetallic oxalato complex [MnIICrIII(C2O4)3]-, but only insoluble powders have been obtained in most cases. Here we report the synthesis of single crystals formed by infinite sheets of this magnetic coordination polymer interleaved with layers of conducting BEDT-TTF cations, and show that this molecule-based compound displays ferromagnetism and metallic conductivity.