Coordination Motifs Research Articles

Ionic γ-FeO(OH) Nanocrystal Stabilized by Small Isopolymolybdate Clusters as Reactive Core for Water Oxidation.

At near neutral to basic pH, hydrolysis-induced aggregation to insoluble bulk iron-oxide is often regarded as the pitfalls of molecular iron clusters. Iron-oxide nanocrystals are encouragingly active over the molecular clusters and/or bulk oxides albeit, stabilizing such nanostructures in aqueous pH and under turnover condition remain a perdurable challenge. Herein, an Anderson-type [Mo7 O24 ]6- isopolyanion, a small (dimension ca. 0.85 nm) isolable polyoxometalate (POM) possessing only {31} atoms, has been introduced for the first time as a covalent linker to stabilize an infinitely stable and aqueous-soluble γ-FeO(OH) nanocore. During the hydrothermal isolation of the material, a partial dissociation of the parent [Mo7 O24 ]6- may lead to the in situ generation of few analogous [Mox Oy ]n- clusters, proved by Raman study, which can also participate in stabilizing the γ-FeO(OH) nanocore, Mox Oy @FeO(OH). However, due to high ionic charge on {Mo=O} terminals of the [Mox Oy ]n- , they are covalently linked via MoVI -μ2 O-FeIII bridging to γ-FeO(OH) core in Mox Oy @FeO(OH), established by numerous spectroscopic and microscopic evidence. Such bonding mode is more likely as precedent from the coordination motif documented in the transition metal clusters stabilized by this POM. The γ-FeO(OH) nanocore of Mox Oy @FeO(OH) behaves as potent active center for electrochemical water oxidation with a overpotential, 263 mV @ 10 mA cm-2 , lower than that observed for bare γ-FeO(OH). Despite of some molybdenum dissolution from the POM ligands to the electrolyte, residual anionic POM fragments covalently bound to the OER active γ-FeO(OH) core of the Mox Oy @FeO(OH) makes the surface predominantly ionic that results in an ordered electrical double layer to promote a better charge transport across the electrode-electrolyte junction, less likely in bulk γ-FeO(OH).

Nitrate and Nitrite Reductions at Copper(II) Sites: Role of Noncovalent Interactions from Second-Coordination-Sphere.

Reductions of nitrate and nitrite (NOx-) are of prime importance in combatting water pollution arising from the excessive use of N-rich fertilizers. While examples of NOx- reductions are known, this report illustrates hydrazine (N2H4)-mediated transformations of NOx- to nitric oxide (NO)/nitrous oxide (N2O). For nitrate reduction to NO, initial coordination of the weakly coordinating NO3- anion at [(mC)CuII]2+ cryptate has been demonstrated to play a crucial role. A set of complementary analyses (X-ray diffraction and Fourier-transform infrared spectroscopy (FTIR), UV-vis, and NMR spectroscopies) on NO3--bound metal-cryptates [(mC)MII(NO3)](ClO4) (1-M, M = Cu/Zn) demonstrates the binding of NO3- through noncovalent (NH···O, CH···O, and anion···π) and metal-ligand coordinate interactions. Subsequently, reactions of [(mC)CuII(14/15NO3)](ClO4) (1-Cu or 1-Cu/15N) with N2H4·H2O have been illustrated to reduce 14/15NO3- to 14/15NO. Intriguingly, in the absence of the second-coordination-sphere interactions, a closely related coordination motif [(Bz3Tren)CuII]2+ (in 3-Cu) does not bind NO3- and is unable to assist in N2H4·H2O-mediated NO3- reduction. In contrast, nitrite coordinates at the tripodal CuII sites in both [(mC)CuII]2+ and [(Bz3Tren)CuII]2+ irrespective of the additional noncovalent interactions, and hence, the N2H4 reactions of the copper(II)-nitrite complexes [(mC)CuII(O14/15NO)]+ and [(Bz3Tren)CuII(O14/15NO)]+ (in 2-Cu/4-Cu) result in a mixture of 14/15NO and N14/15NO.

Citrus VASCULAR PLANT ONE-ZINC FINGER1 (VOZ1) interacts with CuFT1 and CuFT3, affecting flowering in transgenic Arabidopsis

Understanding the flowering process is essential for developing methods to shorten the breeding period and regulate the yield of tree fruits, such as Satsuma mandarin (Citrus unshiu Marc.). In the flowering network system in Arabidopsis, FLOWERING LOCUS T (FT) acts as a transmissible floral inducer. In this study, a citrus ortholog of the transcription factor (TF) VASCULAR PLANT ONE-ZINC FINGER1 (VOZ1)-like gene, CuVOZ1, and two FT-like genes, CuFT1 and CuFT3, were isolated from the Satsuma mandarin ‘Aoshima’. Protein-protein interaction between CuVOZ1 and CuFTs was confirmed in vitro with a yeast two-hybrid (Y2H) system. N-terminal 400 amino acids of CuVOZ1, consisting of three motifs, namely, domain of unknown function 4749 (DUF4749), no apical meristem (NAM), and a zinc coordination motif, were assumed to be involved in the CuVOZ1–CuFT1 and CuVOZ1–CuFT3 complexes. Docking simulation suggested that only the zinc coordination motif of CuVOZ1 was possibly involved in the CuVOZ1–CuFT3 interaction and that the phosphatidylethanolamine-binding protein (PBP) motif in exon 4 of CuFTs was predicted to be crucial for the interaction between CuVOZ1 and CuFTs. The distance between the amino acid residues involved in docking was predicted to be from 2.69 to 3.37 Å, suggesting that the forces between CuVOZ1 and CuFTs in the CuVOZ1–CuFT complexes were weak Van der Waals forces. Constitutive expression of CuVOZ1 in Arabidopsis affected the flowering time, plant size, morphology of inflorescence, number of flowers and siliques, and formation of flower buds on the elongated stem. It was suggested that CuVOZ1 might act as a trigger for early flowering and might be involved in the elongation and branching of the inflorescence. The CuVOZ1–CuFT complexes might regulate cellular proliferation and the formation of new tissues and affect both vegetative and reproductive development.

Directed Conformational Switching of a Zinc Finger Analogue Regulates the Mechanosensing and Differentiation of Stem Cells.

The dynamic conformational changes in the secondary structures of proteins are essential to their functions and can regulate diverse cellular events. Herein we report the design of a synthetic polymer-based secondary structure analogue of a zinc finger (ZnF) by introducing a zinc coordination motif to overcome the free energy barrier predicted by theoretical calculations and fold-free polymer chains. The conformational switching between unfolded and folded state of the ZnF analogue can be triggered in situ to drastically manipulate the accessibility of conjugated cell adhesive ligands to the cell membrane receptors, thereby effectively controlling the adhesion, spreading, mechanosensing, and differentiation of stem cells. We believe that emulating the dynamic secondary structures of proteins via rational design of a folded synthetic polymer-cation complex is a promising strategy for developing bioactive materials to mediate desired cellular functions.

Directed Conformational Switching of a Zinc Finger Analogue Regulates the Mechanosensing and Differentiation of Stem Cells

The dynamic conformational changes in the secondary structures of proteins are essential to their functions and can regulate diverse cellular events. Herein we report the design of a synthetic polymer-based secondary structure analogue of a zinc finger (ZnF) by introducing a zinc coordination motif to overcome the free energy barrier predicted by theoretical calculations and fold-free polymer chains. The conformational switching between unfolded and folded state of the ZnF analogue can be triggered in situ to drastically manipulate the accessibility of conjugated cell adhesive ligands to the cell membrane receptors, thereby effectively controlling the adhesion, spreading, mechanosensing, and differentiation of stem cells. We believe that emulating the dynamic secondary structures of proteins via rational design of a folded synthetic polymer–cation complex is a promising strategy for developing bioactive materials to mediate desired cellular functions.

Design of a Flexible, Zn-Selective Protein Scaffold that Displays Anti-Irving-Williams Behavior.

Selective metal binding is a key requirement not only for the functions of natural metalloproteins but also for the potential applications of artificial metalloproteins in heterogeneous environments such as cells and environmental samples. The selection of transition-metal ions through protein design can, in principle, be achieved through the appropriate choice and the precise positioning of amino acids that comprise the primary metal coordination sphere. However, this task is made difficult by the intrinsic flexibility of proteins and the fact that protein design approaches generally lack the sub-Å precision required for the steric selection of metal ions. We recently introduced a flexible/probabilistic protein design strategy (MASCoT) that allows metal ions to search for optimal coordination geometry within a flexible, yet covalently constrained dimer interface. In an earlier proof-of-principle study, we used MASCoT to generate an artificial metalloprotein dimer, (AB)2, which selectively bound CoII and NiII over CuII (as well as other first-row transition-metal ions) through the imposition of a rigid octahedral coordination geometry, thus countering the Irving-Williams trend. In this study, we set out to redesign (AB)2 to examine the applicability of MASCoT to the selective binding of other metal ions. We report here the design and characterization of a new flexible protein dimer, B2, which displays ZnII selectivity over all other tested metal ions including CuII both in vitro and in cellulo. Selective, anti-Irving-Williams ZnII binding by B2 is achieved through the formation of a unique trinuclear Zn coordination motif in which His and Glu residues are rigidly placed in a tetrahedral geometry. These results highlight the utility of protein flexibility in the design and discovery of selective binding motifs.

Self-Assembly of Mechanoluminochromic Ladder-Shaped Gold(I) Clusters Promoted Using Cooperative Aurophilicity.

The self-assembly of mechanoluminochromic polynuclear gold(I) complexes has attracted more and more attention in the field of supramolecular gold(I) chemistry. In this work, we adopted a stepwise self-assembly strategy to precisely synthesize two polynuclear gold(I) supramolecular clusters. Through cooperative AuI···AuI and Au-N interactions, the gold(I) clusters 1+•BF4- and 24+•4BF4- with Au4 and Au16 cores, respectively, were successfully constructed. In these supramolecular clusters, (dppm)Au2Cl2 coordination motifs and trithiocyanuric linkers were stepwise assembled via sequential thiolate-chloride/phosphine coordination substitution and Au-S/Au-N coordination bond rearrangement. Two well-defined gold(I) supramolecular clusters displayed intense emission both in the solid state and in solution. Furthermore, the ladder-shaped cluster 24+•4BF4- exhibited reversible mechanochromic luminescence behavior in the solid state as well as aggregation-caused redshifted emission in solution. Upon mechanical grinding, the emission of the cluster 24+•4BF4- changed from yellow at 582 nm to red at 612 nm. The initial emission could be fully recovered by treatment with acetonitrile.

Identification of the Highly Active Co-N4 Coordination Motif for Selective Oxygen Reduction to Hydrogen Peroxide.

Electrosynthesis of hydrogen peroxide (H2O2) through oxygen reduction reaction (ORR) is an environment-friendly and sustainable route for obtaining a fundamental product in the chemical industry. Co–N4 single-atom catalysts (SAC) have sparkled attention for being highly active in both 2e– ORR, leading to H2O2 and 4e– ORR, in which H2O is the main product. However, there is still a lack of fundamental insights into the structure–function relationship between CoN4 and the ORR mechanism over this family of catalysts. Here, by combining theoretical simulation and experiments, we unveil that pyrrole-type CoN4 (Co–N SACDp) is mainly responsible for the 2e– ORR, while pyridine-type CoN4 catalyzes the 4e– ORR. Indeed, Co–N SACDp exhibits a remarkable H2O2 selectivity of 94% and a superb H2O2 yield of 2032 mg for 90 h in a flow cell, outperforming most reported catalysts in acid media. Theoretical analysis and experimental investigations confirm that Co–N SACDp—with weakening O2/HOO* interaction—boosts the H2O2 production.

Structural Investigation of Magnesium Complexes Supported by a Thiopyridyl Scorpionate Ligand.

Herein, we report the synthesis of a series of heteroleptic magnesium complexes stabilized with the scorpionate ligand tris(2-pyridylthio)methanide (Tptm). The compounds of the general formula [Mg(Tptm)(X)] (1-X; X = Cl, Br, I) were obtained via protonolysis reaction between the proligand and selected Grignard reagents. Attempts to isolate the potassium derivative K(Tptm) lead to decomposition of Tptm and formation of the alkene (C5H4N-S)2C=C(C5H4N-S)2, and this degradation was also modelled using DFT methods. Compound 1-I was treated with K(CH2Ph), affording the degradation product [Mg(Bptm)2] (2; Bptm = {CH(S-C5NH3)2}−). We analyzed and quantified the steric properties of the Tptm ligand using the structural information of the compounds obtained in this study paired with buried volume calculations, also adding the structural data of HTptm and its CF3-substituted congener (HTptmCF3). These studies highlight the highly flexible nature of this ligand scaffold and its ability to stabilize various coordination motifs and geometries, which is a highly desirable feature in the design of novel organometallic reagents and catalysts.

Assessing the Influence of Zeolite Composition on Oxygen-Bridged Diamino Dicopper(II) Complexes in Cu-CHA DeNOx Catalysts by Machine Learning-Assisted X-ray Absorption Spectroscopy.

Cu-exchanged chabazite is the catalyst of choice for NOx abatement in diesel vehicles aftertreatment systems via ammonia-assisted selective catalytic reduction (NH3–SCR). Herein, we exploit in situ X-ray absorption spectroscopy powered by wavelet transform analysis and machine learning-assisted fitting to assess the impact of the zeolite composition on NH3-mobilized Cu-complexes formed during the reduction and oxidation half-cycles in NH3–SCR at 200 °C. Comparatively analyzing well-characterized Cu-CHA catalysts, we show that the Si/Al ratio of the zeolite host affects the structure of mobile dicopper(II) complexes formed during the oxidation of the [CuI(NH3)2]+ complexes by O2. Al-rich zeolites promote a planar coordination motif with longer Cu–Cu interatomic distances, while at higher Si/Al values, a bent motif with shorter internuclear separations is also observed. This is paralleled by a more efficient oxidation at a given volumetric Cu density at lower Si/Al, beneficial for the NOx conversion under NH3–SCR conditions at 200 °C.

Capturing the geometry, function, and evolution of enzymes with 3D templates.

Structural templates are 3D signatures representing protein functional sites, such as ligand binding cavities, metal coordination motifs, or catalytic sites. Here we explore methods to generate template libraries and algorithms to query structures for conserved 3D motifs. Applications of templates are discussed, as well as some exemplar cases for examining evolutionary links in enzymes. We also introduce the concept of using more than one template per structure to represent flexible sites, as an approach to better understand catalysis through snapshots captured in enzyme structures. Functional annotation from structure is an important topic that has recently resurfaced due to the new more accurate methods of protein structure prediction. Therefore, we anticipate that template‐based functional site detection will be a powerful tool in the task of characterizing a vast number of new protein models.

Metal-organic coordination networks on a titanium carbide MXene: DFT based grand canonical Monte Carlo simulation

The self-assembly of 2D metal–organic networks comprising 1,3,5-tris (pyridyl)benzene (TPyB) molecules and copper atoms on the oxygen-terminated titanium carbide MXene surface was theoretically investigated. We have developed a lattice model of the TPyB-Cu networks on the energetically heterogeneous Ti2CO2(0001) surface. The model based on DFT calculations of the structure and energy of key adsorption complexes and metal–organic structures. Using the grand canonical Monte Carlo method, we have calculated and analyzed adsorption isotherms, structure, potential energy, and heat capacity of the adlayer. Due to steric hindrances in the three-fold Cu-TPyB junction, metal–organic structures consisting of only two-fold Cu-TPyB coordination motifs predominantly emerge on the Ti2CO2(0001) surface: honeycomb (HON), honeycomb filled with Cu3TPyB (HON + Cu3TPyB) and zigzag (ZZ) phases. These phases differ in the local environment of the copper adatoms. Thermal stability of the phases decreases in the following series: ZZ, HON + Cu3TPyB and HON. Self-assembly of these structures offers the opportunity to stabilize and “tune” properties of the single-atom Cu/Ti2CO2(0001) catalyst. We hope that our results will stimulate further experimental studies of hybrid “metal–organic network/MXene” catalysts.

Front Cover: Synthesis of Geminal Bis‐ and Tetrakisphosphonate Ester Derivatives and Their Coordination Behavior Towards Ca(II) Ions (Eur. J. Inorg. Chem. 17/2022)

The Front Cover shows an illustration of a hip joint flanked by calcium- and phosphorus-containing capsules, representing the most essential elements of bone tissue. A geminal bisphosphonate (BP) ester-supported calcium complex forming a solvent-separated ion pair in the solid state seems to be a convenient and bio-available combination of these two components. As BPs and derived Ca(II) complexes are the leading class of therapeutics in osteoporosis treatment, the presented results contribute to the availability of differently substituted geminal BPs and Ca(II) compounds for application as potential future multi-delivery systems in osteoporosis therapy. Additionally, comprehensive X-ray- and DOSY NMR-based structure elucidation in comparison to structures and coordination motifs known in the literature offers an extended insight into their coordination behavior in the solid as well as in the liquid phase. More information can be found in the Research Article by I. Koehne and R. Pietschnig.

Cross-Talks Between Sulfane Sulfur and Thiol at a Zinc(II) Site.

Transformations of sulfane sulfur compounds (e. g. organic polysulfides (R-Sn -R, n>2) and elemental sulfur (S8 )) play pivotal roles in the biochemical landscape of sulfur, and thus supports signaling activities of H2 S. Although a number of previous reports illustrate amine mediated reactions of S8 and thiol (RSH) yielding R-Sn -R, this report illustrates that a tripodal [ZnII ] complex [(Bn3 Tren)ZnII -OH2 ](ClO4 )2 (1) facilitates the reactions of sulfane sulfur and thiol (RSH), thereby offering an amine-free biologically relevant complementary route. UV-vis monitoring of the reactions and a set of control experiments underline the definitive role of [ZnII ] coordination motif in the reactions of sulfane sulfur (e. g. S8 and R-Sn -R) with RSH. Detailed investigations (UV-vis, NMR, ESI-MS, intermediate trapping, and TEMPO radical interference experiments) disclose the key differences in the [ZnII ] versus previously known amine mediated routes. Moreover, the persulfide (RSS- ) trapping experiments using 1-fluoro-2,4-dinitrobenzene (F-DNB) reveal the intermediacy of RSS- species in the [ZnII ] mediated reactions of sulfane sulfur and thiol, thereby demonstrating [ZnII ] assisted persulfidation of thiol in the presence of sulfane sulfur species. Of broader impact, this study underscores the feasible influence of biologically relevant [ZnII ] coordination motifs (e. g. carbonic anhydrase) on the sulfane sulfur chemistry in biology.

Incorporation of a Phosphino(pyridine) Subcomponent Enables the Formation of Cages with Homobimetallic and Heterobimetallic Vertices.

Biological systems employ multimetallic assemblies to achieve a range of functions. Here we demonstrate the preparation of metal–organic cages that contain either homobimetallic or heterobimetallic vertices. These vertices are constructed using 2-formyl-6-diphenylphosphinopyridine, which forms ligands that readily bridge between a pair of metal centers, thus enforcing the formation of bimetallic coordination motifs. Two pseudo-octahedral homometallic MI12L4 cages (MI = CuI or AgI) were prepared, with a head-to-head configuration of their vertices confirmed by X-ray crystallography and multinuclear NMR for AgI. The phosphino-pyridine subcomponent also enabled the formation of a class of octanuclear CdII4CuI4L4 tetrahedral cages, representing an initial example of self-assembled cages containing well-defined heterobimetallic vertices.

Revisiting the Bonding Model for Gold(I) Species: The Importance of Pauli Repulsion Revealed in a Gold(I)-Cyclobutadiene Complex.

Understanding the bonding of gold(I) species has been central to the development of gold(I) catalysis. Herein, we present the synthesis and characterization of the first gold(I)-cyclobutadiene complex, accompanied with bonding analysis by state-of-the-art energy decomposition analysis methods. Analysis of possible coordination modes for the new species not only confirms established characteristics of gold(I) bonding, but also suggests that Pauli repulsion is a key yet hitherto overlooked element. Additionally, we obtain a new perspective on gold(I)-bonding by comparison of the gold(I)-cyclobutadiene to congeners stabilized by p-, d-, and f-block metals. Consequently, we refine the gold(I) bonding model, with a delicate interplay of Pauli repulsion and charge transfer as the key driving force for various coordination motifs. Pauli repulsion is similarly determined as a significant interaction in AuI -alkyne species, corroborating this revised understanding of AuI bonding.

Revisiting the Bonding Model for Gold(I) Species: The Importance of Pauli Repulsion Revealed in a Gold(I)‐Cyclobutadiene Complex

AbstractUnderstanding the bonding of gold(I) species has been central to the development of gold(I) catalysis. Herein, we present the synthesis and characterization of the first gold(I)‐cyclobutadiene complex, accompanied with bonding analysis by state‐of‐the‐art energy decomposition analysis methods. Analysis of possible coordination modes for the new species not only confirms established characteristics of gold(I) bonding, but also suggests that Pauli repulsion is a key yet hitherto overlooked element. Additionally, we obtain a new perspective on gold(I)‐bonding by comparison of the gold(I)‐cyclobutadiene to congeners stabilized by p‐, d‐, and f‐block metals. Consequently, we refine the gold(I) bonding model, with a delicate interplay of Pauli repulsion and charge transfer as the key driving force for various coordination motifs. Pauli repulsion is similarly determined as a significant interaction in AuI‐alkyne species, corroborating this revised understanding of AuI bonding.

Coordination motifs of binary neodymium(III) D-gluconate, D-galactonate and L-gulonate complexes and the transition from inner- to outer-sphere coordination in neutral to strongly alkaline medium

The structural aspects of the complexation of Nd(III) in aqueous solutions with D-gluconate, D-galactonate and L-gulonate in the neutral to hyperalkaline pH range have been studied by using 1H and 13C NMR, CD and Raman spectroscopies. Beside the identification of the binding sites, an attempt was made to reveal whether the structure of the complexes depends on the type of the ligand. It was concluded that the carboxylate group of each ligand has an essential role in the coordination to Nd(III). In solutions of pH > 5, other hydroxyl or alkoxide groups are also coordinated. Based on the NMR spectra, the coordination of several C-OH groups can be inferred, and CD measurements strongly suggest that beside the carboxylate, the C4-OH group takes part in the metal ion coordination in the Gluc– and Gal– complexes. On the basis of our observations from all the three spectroscopic methods, Nd(OH)3 units covered by the polyhydroxy carboxylate is the most appropriate model for describing the systems at pH > 12.

Bis(pyrazolyl)thioether/amine‐chromium(III) catalysts bearing pendantO‐ andN‐donor group for oligomerization and polymerization of ethylene

In this work, a new family of chromium complexes supported by bis(pyrazolyl)thioether/amine ligands containing pendantO‐ andN‐donor groups CrCl3{S‐bis[(3,5‐DMPz)methyl]sulfide} (2a), CrCl3{N‐bis[(3,5‐DMPz)benzylamine]} (2b), CrCl3{N‐bis[(3,5‐DMPz)butylamine]} (2c), [CrCl2{N‐bis[(3,5‐DMPz)methyl][(2‐pyridinyl)methyl]amine}]Cl (2d), [CrCl2{N‐bis[(3,5‐DMPz)methyl][quinoline]amine}]Cl (2e), [CrCl2{N‐bis[3,5‐DMPz‐methyl][EtNMe2]amine}]Cl (2f), CrCl3{N‐bis[(3,5‐DMPz)methyl][2‐methoxyphenyl]amine} (2g), and CrCl3{N‐bis[2‐(3,5‐DMPz)methyl][(2‐pyridyl)ethyl]amine} (2h) were synthetized and characterized by elemental analysis and Fourier transform infrared spectroscopy (FTIR) spectroscopy. Density functional theory (DFT) calculations demonstrated that the pendant group as well as the spacer between theN‐donor group and the central amine nitrogen have strong influence on the coordination motif of the ligand. Thus,1d–1fact as tetradentate ligands while1gand1hpreferably coordinated to the chromium metal center in a tridentate [N,N,N] mode fashion. The catalytic performance of2a–2his strongly influenced by the nature of the central bridging atom (S, N) as well as the pendantO‐ andN‐donor group. Upon activation with methylaluminoxane (MAO), chromium precatalysts2a–2c,2e, and2fgenerated active systems producing oligomers ranging from C4to C12+with a high selectivity for α‐olefins (>82.4%). On the other hand, precatalysts bearing pyridine (2dand2h) and methoxy (2g) functionalized amine bis (pyrazolyl) ligands exclusively produce polyethylene (PE) with activities in the range of 107.2–372.0 kg of PE·mol[Cr]−1 h−1.

MoOxSy/Ni3S2 Microspheres on Ni Foam as Highly Efficient, Durable Electrocatalysts for Hydrogen Evolution Reaction

Hydrogen energy derived from water splitting is the cleanest renewable energy source, but it is also very challenging to achieve because the hydrogen evolution reaction (HER) requires highly efficient and low-cost electrocatalysts. We have fabricated a novel hierarchical system of amorphous molybdenum oxy/sulfide microspheres with crystalline Ni3S2 intergrown in situ on Ni foam (MoOxSy/Ni3S2/NF) as an outstanding electrocatalyst for HER. The MoOxSy/Ni3S2/NF demonstrates an ultra-low overpotential of 58 mV at a current density of 10 mA cm–2 and extremely durable stability (>200 h), suggesting superior performance comparable to that of Pt-C/NF under acidic conditions. The X-ray absorption fine structure (XAFS) determines the average valence state of Mo to be +(5 + δ), with a coordination motif by O and S. To explain such high HER activity, a [Mo2O2(S,O)4] dimer-based periodic model structure with the average composition of [Mo4O8S4] interfaced with the Ni3S2(101) surface is proposed. The interactions between the Ni of Ni3S2 and bridging S/O of [Mo4O8S4] result in an average formal Mo charge state between +5 and +6, and significant charge transfer from Ni3S2 to [Mo4O8S4] activates the Mo═O bonds. The calculated |ΔGH*| of less than 50 meV suggests that the double-bonded O is the most active site. This work points to the importance of oxy/sulfides with Mon+ (+5 < n<+6) as exceptional electrochemical catalysts for HER.