The thermodynamic favorability of an alkaline solution for the oxidation of water suggests the need for developing hydrogen evolution reaction (HER) catalysts that can function in basic aqueous solutions so that both of the half reactions in overall water splitting can occur in mutually compatible solutions. Although photocatalytic HERs have been reported mostly in acidic solutions and a few at basic pHs in mixed organic aqueous solutions, visible-light driven HER catalyzed by molecular metal complexes in purely alkaline aqueous solutions remains largely unexplored. Here, we report a new cobalt complex with a tetrapyridylamine ligand that catalyzes photolytic HER with turnover number up to 218 000 in purely aqueous solutions at pH 9.0. Density functional theory (DFT) calculations suggested a modified electron transfer (E)-proton transfer (C)-electron transfer (E)-proton transfer (C) (mod-ECEC) pathway for hydrogen production from the protonation of CoII-H species. The remarkable catalytic activity resulting from subtle structural changes of the ligand scaffold highlights the importance of studying structure-function relationships in molecular catalyst design. Our present work significantly advances the development of a molecular metal catalyst for visible-light driven HER in more challenging alkaline aqueous solutions that holds substantial promise in solar-driven water-splitting systems.

An efficient one-pot direct dihydroxylation of alkenes has been achieved. This reaction proceeds under transition-metal-free conditions and in water solution, producing the corresponding 1,2-diols in good to excellent yields. The discovery of an optimal pH environment is identified as a key factor in the current system, which ensures the generality and good functional group compatibility of this reaction. Moreover, the gram-scale dihydroxylation of alkenes was demonstrated and without significant decrease of the yield of product, making this protocol very practical.

The conductivity changes associated with optical excitations and changing temperature in cobalt valence tautomer molecular thin films were investigated. Conductance switching in the presence of illumination is observed, with occasional locking in a higher conductance state, depending on the temperature, the photon energy of the illumination, and the bias voltage. Light of sufficiently short wavelengths is needed to ensure the light enhanced conductance switching, consistent with the optical absorption, but bias voltage clearly plays a role as well. The conductance switching is associated with excitations to the ligand to metal charge transfer state. The bistability and the conductivity changes associated with optical excitations in cobalt valence tautomer molecular thin films were investigated.

Research on the magnetoelectric (ME) effect (or spin-electric coupling) in molecule-based magnetic materials is a relatively nascent but promising topic. Molecule-based magnetic materials have diverse magnetic functionalities that can be coupled to electrical properties. Here we investigate a realization of ME coupling that is fundamental but not heavily studied-the coupling of magnetic spin level crossings to changes in electric polarization. A mixed-valence Mn4Na complex with a total ground-state spin S = 5/2 under zero magnetic field and S = 17/2 under high magnetic field undergoes a cascade of ground-state level crossings of the S z states with increasing magnetic field. Magnetization and electrical polarization measurements under pulsed magnetic fields up to 73 T show that each spin level crossing is accompanied by a significant change in electric polarization that is an even function of the applied magnetic field. A molecular Hamiltonian describing antiferromagnetic exchange in a distorted tetrahedron of three MnIII and one MnII ions matches the data well. We conclude that the ME coupling is caused by magnetostriction within the polar molecule as it distorts to lower its magnetic exchange energy.

Here we report the controlled self-assembly of vanadium-seamed metal-organic nanocapsules with specific metal oxidation state distributions. Three supramolecular assemblies composed of the same numbers of components including 24 metal centers and six pyrogallol[4]arene ligands were constructed: a VIII24L6 capsule, a mixed-valence VIII18VIV6L6 capsule, and a VIV24L6 capsule. Crystallographic studies of the new capsules reveal their remarkable structural complexity and geometries, while marked differences in metal oxidation state distribution greatly affect the photoelectric conversion properties of these assemblies. This work therefore represents a significant step forward in the construction of intricate metal-organic architectures with tailored structure and functionality.

Future molecular microelectronics require the electronic conductivity of the device to be tunable without impairing the voltage control of the molecular electronic properties. This work reports the influence of an interface between a semiconducting polyaniline polymer or a polar poly-D-lysine molecular film and one of two valence tautomeric complexes, i.e., [CoIII(SQ)(Cat)(4-CN-py)2] ↔ [CoII(SQ)2(4-CN-py)2] and [CoIII(SQ)(Cat)(3-tpp)2] ↔ [CoII(SQ)2(3-tpp)2]. The electronic transitions and orbitals are identified using X-ray photoemission, X-ray absorption, inverse photoemission, and optical absorption spectroscopy measurements that are guided by density functional theory. Except for slightly modified binding energies and shifted orbital levels, the choice of the underlying substrate layer has little effect on the electronic structure. A prominent unoccupied ligand-to-metal charge transfer state exists in [CoIII(SQ)(Cat)(3-tpp)2] ↔ [CoII(SQ)2(3-tpp)2] that is virtually insensitive to the interface between the polymer and tautomeric complexes in the CoII high-spin state.

We report a series of heteroleptic cobalt complexes of the general formula [Co(tBu2sq)2(L)2], where tBu2sq = 3,5-di(tert-butyl)-o-semiquinonato ligand and L represents pyridines functionalized with sulfur-containing substituents. The octahedral coordination of the Co metal center in these mononuclear complexes is formed by two chelating tBu2sq ligands in the equatorial plane and two axial pyridines in the axial positions. All complexes exhibit layered crystal packings, with S-containing substituents providing cohesion within the layers and the tBu substituents of dioxolenes protruding into the interlayer space, where disordered solvent molecules are located. The presence of thienyl substituents in the para position of the pyridine ring leads to the formation of molecular chains within the layers, due to efficient pi-pi interactions between the L ligands. Even more efficient packing with two-dimensional intermolecular pi-pi interactions is achieved when two thienyl substituents are present in the meta positions of the pyridine ring. Introduction of a terminal cyano or 1,3-dithiole-2-one substituent on the opposite end of the pyridyl-bound thienyl group leads to the disruption of the pi-pi interactions, thus decreasing the crystal packing efficiency. Such disruption, however, is not observed when the thienyl group is terminated with methyl-carboxylate. Magnetic measurements reveal an onset of valence-tautomeric spin-crossover (VT-SCO) from the [LS-CoIII(tBu2sq center dot)(tBu2cat)(L)2] state (low-spin, S= 1/2) to the [HS-CoII(tBu2sq center dot)2(L)2] state (high-spin, S= 5/2) above 300 K for the complexes with para-thienyl-substituted pyridines, while the complexes with a bithienyl substituent in the para position or two thienyl substituents in the meta positions exhibit only LS state up to 400 K. Adding a terminal group to the para-thienyl substituent lowers the VT-SCO temperature, leading to two-step spin-state conversion.

AbstractWe report the direct observation of an electrically-induced magnetic susceptibility in the molecular nano- magnet [Fe3O(O2CPh)6(py)3]ClO4·py, an Fe3 trimer. This magnetoelectric effect results from the breaking of spatial inversion symmetry due to the spin configurations of the antiferromagnetic trimer. Both static and very low frequency electric fields were used. Fractional changes of the magnetic susceptibility of 11 ppb$$\pm 2$$ ± 2 per kVm-1 for the temperature range 8.5 < T < 13.5 K were observed for applied electric fields up to 62 kV m−1. The changes in susceptibility were measured using a tunnel diode oscillator operating at liquid helium temperatures while the sample is held at a higher regulated temperature.

We investigated modifications driven by 7,7,8,8-tetracyanoquinodimethane (TCNQ) to the spin state configuration of [Fe(3-bpp)2](TCNQ)2 co-crystal and both spin state and electric conductivity of [Fe{H2B(pz)2}2(bipy)] and TCNQ mixtures. The Fe2+ site in the [Fe(3-bpp)2](TCNQ)2 co-crystal has a sizable orbital moment. During X-ray absorption measurements, the iron ion is partially excited to the high spin state and strong surface effects are indicated. Mixing TCNQ with the [Fe{H2B (pz)2}2(bipy)] spin crossover complex leads to a molecular combination with increased conductivity and drift carrier lifetimes. [Fe{H2B(pz)2}2(bipy)] thin films with TCNQ, grown using dimethylformamide (DMF), are to great extent locked mainly in the low spin (LS) state across a broad temperature range and exhibit drift carrier lifetimes approaching 0.5 s. When deposited onto a ferroelectric polyvinylidenefluoride-hexafluoropropylene thin film substrate, [Fe{H2B(pz)2}2(bipy)], shows enhanced transistor carrier mobility, likely associated with the increasing cationic character of [Fe{H2B(pz)2}2(bipy)] thin films with TCNQ.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

An entry from the Cambridge Structural Database, the world’s repository for small molecule crystal structures. The entry contains experimental data from a crystal diffraction study. The deposited dataset for this entry is freely available from the CCDC and typically includes 3D coordinates, cell parameters, space group, experimental conditions and quality measures.

The salt [K(18-crown-6)]2[Ru(CN)2(CO)3] ([K(18-crown-6)]2[1]) was generated by the reaction of Ru(C2H4)(CO)4 with [K(18-crown-6)]CN. An initial thermal reaction gives [Ru(CN)(CO)4]-, which, upon ultraviolet (UV) irradiation, reacts with a second equiv of CN-. Protonation of [1]2- gave [HRu(CN)2(CO)3]- ([H1]-), which was isolated as a single isomer with mutually trans cyanide ligands. The complex cis,cis,cis-[Ru(pdt)(CN)2(CO)2]2- ([2]2-) was prepared by the UV-induced reaction of [1]2- with propanedithiol (pdtH2). The corresponding iron complex cis,cis,cis-[Fe(pdt)(CN)2(CO)2]2- ([3]2-) was prepared similarly. The pdt complexes [2]2- and [3]2- were treated with Fe(benzylideneacetone)(CO)3 to give, respectively, [RuFe (μ-pdt)(CN)2(CO)4]2- ([5]2-) and [Fe2(μ-pdt)(CN)2(CO)4]2- ([4]2-). The pathway from [3]2- to Fe2 complex [4]2- implicates intermetallic migration of CN-. In contrast, the formation of [5]2- leaves the Ru(CN)2(CO) center intact, as confirmed by X-ray crystallography. The structure of [5]2- features a "rotated" square-pyramidal Fe(CO)2(μ-CO) site. NMR measurements indicate that the octahedral Ru site is stereochemically rigid, whereas the Fe site dynamically undergoes turnstile rotation. 57Fe Mössbauer spectral parameters are very similar for rotated [5]2- and unrotated Fe2 complex [4]2-, indicating the insensitivity of that technique to both the geometry and the oxidation state of the Fe site. According to cyclic voltammetry, [5]2- oxidizes at E1/2 ∼ -0.8 V vs Fc+/0. Electron paramagnetic resonance (EPR) measurements show that 1e- oxidation of [5]2- gives an S = 1/2 rhombic species, consistent with the formulation Ru(II)Fe(I), related to the Hox state of the [FeFe] hydrogenases. Density functional theory (DFT) studies reproduce the structure, 1H NMR shifts, and infrared (IR) spectra observed for [5]2-. Related homometallic complexes with both cyanides on a single metal are predicted to not adopt rotated structures. These data suggest that [5]2- is best described as Ru(II)Fe(0). This conclusion raises the possibility that for some reduced states of the [FeFe]-hydrogenases, the [2Fe]H site may be better described as Fe(II)Fe(0) than Fe(I)Fe(I).

The [Co(SQ)2(4-CN-py)2] complex exhibits dynamical effects over a wide range of temperature. The orbital moment, determined by X-ray magnetic circular dichroism (XMCD) with decreasing applied magnetic field, indicates a nonzero critical field for net alignment of magnetic moments, an effect not seen with the spin moment of [Co(SQ)2(4-CN-py)2].

In this work, the differences in catalytic performance for a series of Co hydrogen evolution catalysts with different pentadentate polypyridyl ligands (L), have been rationalized by examining elementary steps of the catalytic cycle using a combination of electrochemical and transient pulse radiolysis (PR) studies in aqueous solution. Solvolysis of the [CoII-Cl]+ species results in the formation of [CoII(κ4-L)(OH2)]2+. Further reduction produces [CoI κ4-L)(OH2)]+, which undergoes a rate-limiting structural rearrangement to [CoI(κ5-L)]+ before being protonated to form [CoIII -H]2+. The rate of [CoIII-H]2+ formation is similar for all complexes in the series. Using E1/2 values of various Co species and pKa values of [CoIII-H]2+ estimated from PR experiments, we found that while the protonation of [CoIII-H]2+ is unfavorable, [CoII-H]+ reacts with protons to produce H2. The catalytic activity for H2 evolution tracks the hydricity of the [CoII-H] + intermediate.

Hydrogen production from sunlight and water represents one promising solution to resolve the environmental problems caused by the consumption of fossil fuels and to meet the increasing global energy demands. Catalysts based on transition metal complexes have been extensively studied for electro- and photocatalytic production of hydrogen. Among the reported catalysts, molecular cobalt complexes have received special attention for hydrogen production over the past years. In this review, the recent progresses for electro- and photocatalytic production of hydrogen in aqueous solutions catalyzed by Co complexes with pentadentate ligands, with an emphasis of those developed in our lab, are discussed. Mechanistic investigations as well as future directions for hydrogen production by Co complexes are also discussed.

To explore the structure-function relationships of cobalt complexes in the catalytic hydrogen evolution reaction (HER), we studied the substitution of a tertiary amine with a softer pyridine group and the inclusion of a conjugated bpy unit in a Co complex with a new pentadentate ligand, 6-[6-(1,1-di-pyridin-2-yl-ethyl)-pyridin-2-ylmethyl]-[2,2 ']bipyridinyl (Py3Me-Bpy). These modifications resulted in significantly improved stability and activity in both electro- and photocatalytic HER in neutral water. [Co(Py3Me-Bpy)(OH2)](PF6)(2) catalyzes the electrolytic HER at -1.3 V (vs. SHE) for 20 hours with a turnover number (TON) of 266 300, and photolytic HER for two days with a TON of 15 000 in pH 7 aqueous solutions. The softer ligand scaffold possibly provides increased stability towards the intermediate Co-I species. DFT calculations demonstrate that HER occurs through a general electron transfer/proton transfer/electron transfer/proton transfer pathway, with H-2 released from the protonation of Co-II-H species.