Controlled Synthesis of a Vacancy-Defect Single-Atom Catalyst for Boosting CO2 Electroreduction

The reaction of precursors containing both nitrogen and oxygen atoms with Ni^II under 500 °C can generate a N/O mixing coordinated Ni-N₃O single-atom catalyst (SAC) in which the oxygen atom can be gradually removed under high temperature due to the weaker Ni−O interaction, resulting in a vacancy-defect Ni-N₃-V SAC at Ni site under 800 °C. For the reaction of Ni^II with the precursor simply containing nitrogen atoms, only a no-vacancy-defect Ni-N₄ SAC was obtained. Experimental and DFT calculations reveal that the presence of a vacancy-defect in Ni-N₃-V SAC can dramatically boost the electrocatalytic activity for CO₂ reduction, with extremely high CO₂ reduction current density of 65 mA cm⁻² and high Faradaic efficiency over 90 % at −0.9 V vs. RHE, as well as a record high turnover frequency of 1.35×10⁵ h⁻¹, much higher than those of Ni-N₄ SAC, and being one of the best reported electrocatalysts for CO₂-to-CO conversion to date.     

Acetatodiaqua[3‐(salicyloylhydra­zono)butan‐2‐one oximato]nickel(II) ethanol solvate

In the title compound, [Ni(C₁₁H₁₂N₃O₃)(CH₃CO₂)(H₂O)₂]·C₂H₅OH, the coordination geometry of the Ni^II atom is a distorted octahedron, with one carbonyl O and two imino N atoms of the hydrazone ligand, together with an acetate O atom, comprising the basal plane, and the two water O atoms occupying axial positions.     

A novel two‐dimensional nickel(II) coordination polymer with 1,4‐bis(1,2,4‐triazol‐1‐ylmethyl)benzene and azide ligands

The coordination geometry of the Ni^II atom in the title complex, poly[diazidobis[μ‐1,4‐bis(1,2,4‐triazol‐1‐ylmethyl)benzene‐κ²N⁴:N^4′]nickel(II)], [Ni(N₃)₂(C₁₂H₁₂N₆)₂]_n, is a distorted octahedron, in which the Ni^II atom lies on an inversion centre and is coordinated by four N atoms from the triazole rings of two symmetry‐related pairs of 1,4‐bis(1,2,4‐triazol‐1‐ylmethyl)benzene (bbtz) ligands and two N atoms from two symmetry‐related monodentate azide ligands. The Ni^II atoms are bridged by four bbtz ligands to form a two‐dimensional (4,4)‐network.     

Regulating the Coordination Environment of MOF‐Templated Single‐Atom Nickel Electrocatalysts for Boosting CO2 Reduction

The general synthesis and control of the coordination environment of single‐atom catalysts (SACs) remains a great challenge. Herein, a general host–guest cooperative protection strategy has been developed to construct SACs by introducing polypyrrole (PPy) into a bimetallic metal–organic framework. As an example, the introduction of Mg²⁺ in MgNi‐MOF‐74 extends the distance between adjacent Ni atoms; the PPy guests serve as N source to stabilize the isolated Ni atoms during pyrolysis. As a result, a series of single‐atom Ni catalysts (named Ni_SA‐N_x‐C) with different N coordination numbers have been fabricated by controlling the pyrolysis temperature. Significantly, the Ni_SA‐N₂‐C catalyst, with the lowest N coordination number, achieves high CO Faradaic efficiency (98 %) and turnover frequency (1622 h^?1), far superior to those of Ni_SA‐N₃‐C and Ni_SA‐N₄‐C, in electrocatalytic CO₂ reduction. Theoretical calculations reveal that the low N coordination number of single‐atom Ni sites in Ni_SA‐N₂‐C is favorable to the formation of COOH* intermediate and thus accounts for its superior activity.     

[N,N′‐Bis­(salicyl­idene)‐2,2‐di­methyl‐1,3‐propane­diaminato]­nickel(II) and [N,N′‐bis­(salicyl­idene)‐2,2‐di­methyl‐1,3‐propane­diaminato]copper(II)

In the title compounds, {2,2′‐[2,2‐di­methyl‐1,3‐propane­diyl­bis­(nitrilo­methyl­idyne)]­diphenolato‐κ⁴N,N′,O,O′}nickel(II), [Ni(C₁₉H₂₀N₂O₂)], and {2,2′‐[2,2‐di­methyl‐1,3‐propane­diyl­bis­(nitrilo­methyl­idyne)]­diphenolato‐κ⁴N,N′,O,O′}copper(II), [Cu(C₁₉H₂₀N₂O₂)], the Ni^II and Cu^II atoms are coordinated by two iminic N and two phenolic O atoms of the N,N′‐bis­(salicyl­idene)‐2,2‐di­methyl‐1,3‐propane­diaminate (SALPD^2?, C₁₇H₁₆N₂O₂^2?) ligand. The geometry of the coordination sphere is planar in the case of the Ni^II complex and distorted towards tetrahedral for the Cu^II complex. Both complexes have a cis configuration imposed by the chelate ligand. The dihedral angles between the N/Ni/O and N/Cu/O coordination planes are 17.20 (6) and 35.13 (7)°, respectively.     

Synthesis,Spectral, Thermal Studies of Co(II), Ni(II), Cu(II) and Zn(II)‐arginato Complexes. Crystal Structure of Monoaquabis(arginato‐κO,κN)copper(II). [Cu(arg)2(H2O)].NaNO3

Transition metal complexes of arginine (using Co(II), Ni(II), Cu(II) and Zn(II) cations separately) were synthesized and characterized by FTIR, TG/DTA‐DrTG, UV‐Vis spectroscopy and elemental analysis methods. Cu(II)‐Arg complex crystals was found suitable for x‐ray diffraction studies. It was contained, one mole Cu^II and Na⁺ ions, two arginate ligands, one coordinated aqua ligand and one solvent NO₃^? group in the asymmetric unit. The principle coordination sites of metal atom have been occupied by two N atoms of arginate ligands, two carboxylate O atoms, while the apical site was occupied by one O atom for Cu^II cation and two O atoms for Co^II, Ni^II, Zn^II atoms of aqua ligands. Although Cu^II ion adopts a square pyramidal geometry of the structure. Co^II, Ni^II, Zn^II cations have octahedral due to coordination number of these metals. Neighbouring chains were linked together to form a three‐dimensional network via hydrogen‐bonding between coordinated water molecule, amino atoms and O atoms of the bridging carboxylate groups. Cu^II complex was crystallized in the monoclinic space group P2₁, a = 8.4407(5) Å, b = 12.0976(5) Å, c = 10.2448(6) Å, V = 1041.03(10) ų, Z = 2. Structures of the other metal complexes were similar to CuII complex, because of their spectroscopic studies have in agreement with each other. Copper complex has shown DNA like helix chain structure. Lastly, anti‐bacterial, anti‐microbial and anti‐fungal biological activities of complexes were investigated.     

A new one‐dimensional nickel metal–organic framework: catena‐poly[[[diaquabis(4‐{[(1‐phenyl‐1H‐tetrazol‐5‐yl)sulfanyl]methyl}benzoato‐κO)nickel(II)]‐μ‐4,4′‐bipyridine‐κ2N:N′] monohydrate]

The title complex, {[Ni(C₁₅H₁₁N₄O₂S)₂(C₁₀H₈N₂)(H₂O)₂]·H₂O}_n, was synthesized by the reaction of nickel chloride, 4‐{[(1‐phenyl‐1H‐tetrazol‐5‐yl)sulfanyl]methyl}benzoic acid (HL) and 4,4′‐bipyridine (bpy) under hydrothermal conditions. The asymmetric unit contains two half Ni^II ions, each located on an inversion centre, two L⁻ ligands, one bpy ligand, two coordinated water molecules and one unligated water molecule. Each Ni^II centre is six‐coordinated by two monodentate carboxylate O atoms from two different L⁻ ligands, two pyridine N atoms from two different bpy ligands and two terminal water molecules, displaying a nearly ideal octahedral geometry. The Ni^II ions are bridged by 4,4′‐bipyridine ligands to afford a linear array, with an Ni...Ni separation of 11.361 (1) Å, which is further decorated by two monodentate L⁻ ligands trans to each other, resulting in a one‐dimensional fishbone‐like chain structure. These one‐dimensional fishbone‐like chains are further linked by O—H...O, O—H...N and C—H...O hydrogen bonds and π–π stacking interactions to form a three‐dimensional supramolecular architecture. The thermal stability of the title complex was investigated via thermogravimetric analysis.     

Aquachlorobis[4,4,5,5‐tetramethyl‐2‐(2‐pyridyl)‐4,5‐dihydro‐1H‐imidazol‐1‐oxyl 3‐oxide‐κ2O1,N2]nickel(II) nitrate methanol disolvate monohydrate

The title complex, [NiCl(C₁₂H₁₆N₃O₂)₂(H₂O)]NO₃·2CH₄O·H₂O, was obtained from a methano­lic solution of Ni(NO₃)₂·6H₂O, 2‐pyridyl nitro­nyl nitro­xide (2‐NITpy) and (NEt₄)₂[CoCl₄]. The equatorial coordination sites of the octahedral Ni^II centre are occupied by two chelating radical ligands, with the axial positions occupied by the Cl^? and water ligands. The H₂O—Ni—Cl axis of the complex lies along a crystallographic twofold axis, so that only half the cation is present in the asymmetric unit. The Ni—Cl bond length [2.3614 (17) Å] is significantly shorter than distances typical of octahedral Ni^II centres [2.441 (5) Å]. However, with only one nitrate anion per formula unit, the oxidation state of the metal must be assigned as Ni^II. The 2‐NITpy ligands bend away from the equatorial plane, forming a hydro­phobic region around the Cl atoms. Conversely, the ligated water mol­ecule forms moderately strong hydrogen bonds with the disordered methanol solvent mol­ecules, which in turn form interactions with the water of crystallization and the disordered nitrate anion. These interactions combine to give hydro­philic regions throughout the crystal structure.     

{4‐Bromo‐2‐[2‐(5‐bromo‐2‐oxido­benzyl­idene­amino­methyl)­phenyl­imino­methyl]­phenolato‐κ4O,N,N′,O′}nickel(II)

In the title complex, [Ni(C₂₁H₁₄Br₂N₂O₂)], the Ni^II atom is coordinated by the two imine N and two phenolate O atoms of the Schiff base ligand in a tetrahedrally distorted square‐planar geometry. The Ni—N and Ni—O distances are within the ranges expected for Ni–Schiff base derivatives. Intermolecular C—H⋯O hydrogen bonds link the mol­ecules into centrosymmetric dimers, forming (12) (A) and (10) (B) rings. These dimers combine to form a supramolecular ABAB… aggregate which propagates along the [100] direction.     

Hydrothermal Synthesis,Structure, and Magnetic Properties of a Three‐dimensional Polymeric NiII Complex, [Ni(bpp)(NIP)(H2O)]n (bpp = 1,3‐di(4‐pyridyl)propane,NIP = 5‐nitroisophthalate)

A three‐dimensional polymeric Ni^II complex, [Ni(bpp)(NIP)(H₂O)]_n (bpp = 1,3‐di(4‐pyridyl)propane and NIP = 5‐nitroisophthalate), has been synthesized and characterized. The coordination number of the nickel atom is six (NiN₂O₄) and the coordination environment around the Ni^II atom may be described as a distorted octahedron in which two nitrogen atoms of “bpp” ligand occupy the cis positions. The effective magnetic moment for this complex indicate that the interactions between two Ni^II atoms through the effective exchange media are antiferromagnetic. Self‐assembly of these compounds in the solid state via π–π‐stacking interactions is discussed.     

A tetrahedrally coordinated AgI and NiII complex with a two‐dimensional framework containing one‐dimensional helical chains

The title complex, poly[bis(μ₆‐pyridine‐2,6‐dicarboxylato N‐oxide)nickel(II)disilver(I)], [Ag₂Ni(C₇H₃NO₅)₂]_n or [Ag₂Ni(pydco)₂]_n (H₂pydco = pyridine‐2,6‐dicarboxylic acid N‐oxide), has a two‐dimensional sheet structure. The two carboxylate groups adopt two coordination modes. The Ni^II ion displays a distorted octahedral geometry, bonded to two carboxylate O atoms of two different pydco ligands and four O donors from another two ligands, i.e. two carboxylate O atoms and two N‐oxide O atoms. The Ag^I ion adopts a tetrahedral coordination, linked by three O atoms of three different carboxylate groups and an N‐oxide O atom.     

Elucidating the Electrocatalytic CO2 Reduction Reaction over a Model Single‐Atom Nickel Catalyst

Designing effective electrocatalysts for the carbon dioxide reduction reaction (CO₂RR) is an appealing approach to tackling the challenges posed by rising CO₂ levels and realizing a closed carbon cycle. However, fundamental understanding of the complicated CO₂RR mechanism in CO₂ electrocatalysis is still lacking because model systems are limited. We have designed a model nickel single‐atom catalyst (Ni SAC) with a uniform structure and well‐defined Ni‐N₄ moiety on a conductive carbon support with which to explore the electrochemical CO₂RR. Operando X‐ray absorption near‐edge structure spectroscopy, Raman spectroscopy, and near‐ambient X‐ray photoelectron spectroscopy, revealed that Ni⁺ in the Ni SAC was highly active for CO₂ activation, and functioned as an authentic catalytically active site for the CO₂RR. Furthermore, through combination with a kinetics study, the rate‐determining step of the CO₂RR was determined to be *CO₂^?+H⁺→*COOH. This study tackles the four challenges faced by the CO₂RR; namely, activity, selectivity, stability, and dynamics.     

A one‐dimensional ladder‐like coordination polymer derived from chains formed via hydrogen bonds: catena‐poly­[[aqua­di­pyridine­nickel(II)]‐μ‐2,2′‐di­thio­dibenzoato‐κ3O,O′:O′′]

The title one‐dimensional chain nickel(II)–di­sulfide complex, [Ni(C₁₄H₈O₄S₂)(C₅H₅N)₂(H₂O)]_n, has each Ni^II cation coordinated by two N atoms from two pyridine ligands, three carboxyl­ate O atoms from two different di­thio­dibenzoate ligands and one O atom from a coordinated water mol­ecule, in a distorted octahedral coordination geometry. Each di­thio­dibenzoate ion links two Ni^II cations through its carboxyl­ate O atoms, making the structure polymeric. Hydro­gen‐bond interactions between two shoulder‐to‐shoulder chains lead to the formation of a ladder‐like structure.     

Poly[[bis{μ2‐N2,N6‐bis[(pyridin‐3‐yl)methyl]pyridine‐2,6‐dicarboxamide}dichloridonickel(II)] N,N‐dimethylformamide tetrasolvate]

In the title compound, {[NiCl₂(C₁₉H₁₇N₅O₂)₂]·4C₃H₇NO}_n, the Ni^II atom is located on an inversion centre and is in a six‐coordinated octahedral geometry, formed by four pyridine N atoms from four N²,N⁶‐bis[(pyridin‐3‐yl)methyl]pyridine‐2,6‐dicarboxamide (BPDA) ligands occupying the equatorial plane and two chloride anions at the axial sites. The bidentate bridging BPDA ligands link the Ni^II atoms into a two‐dimensional corrugated grid‐like flexible layer with a (4,4)‐connected topology, which consists of left‐ and right‐handed helical chains sharing the common Ni^II atoms. Investigation of the thermal stability shows that the network is stable up to 573 K.     

μ‐Aqua‐penta­aqua­[μ‐3,6‐bis(6‐methyl‐2‐pyridyl)­pyridazine]­chloro­dinickel(II) trichloride trihydrate

The title compound, μ‐aqua‐1:2κ²O‐penta­aqua‐1κ²O,2κ³O‐μ‐3,6‐bis(6‐methyl‐2‐pyridyl)­pyridazine‐1κ²N¹,N⁶:2κ²N²,N³‐chloro‐1κCl‐dinickel(II) trichloride trihydrate, [Ni₂Cl(C₁₆H₁₄­N₄)(H₂O)₆]Cl₃·3H₂O, consists of two Ni^II atoms, a 3,6‐bis(6‐methyl‐2‐pyridyl)­pyridazine mol­ecule, four Cl atoms and nine water mol­ecules. The two Ni atoms are octahedrally coordinated by N and Cl atoms, and by water mol­ecules, and the three six‐membered rings, a pyridazine and two picolines, are planar to within 0.181 (3) Å. The crystal structure is stabilized by an intra‐ and intermolecular hydrogen‐bonding scheme involving water–water and water–chlorine interactions.     

High‐Efficiency Oxygen Reduction to Hydrogen Peroxide Catalyzed by Nickel Single‐Atom Catalysts with Tetradentate N2O2 Coordination in a Three‐Phase Flow Cell

Carbon‐supported Ni^II single‐atom catalysts with a tetradentate Ni‐N₂O₂ coordination formed by a Schiff base ligand‐mediated pyrolysis strategy are presented. A Ni^II complex of the Schiff base ligand (R,R)‐(?)‐N,N′‐bis(3,5‐di‐tert‐butylsalicylidene)‐1,2‐cyclohexanediamine was adsorbed onto a carbon black support, followed by pyrolysis of the modified carbon material at 300 °C in Ar. The Ni‐N₂O₂/C catalyst showed excellent performance for the electrocatalytic reduction of O₂ to H₂O₂ through a two‐electron transfer process in alkaline conditions, with a H₂O₂ selectivity of 96 %. At a current density of 70 mA cm^?2, a H₂O₂ production rate of 5.9 mol g_cat.^?1 h^?1 was achieved using a three‐phase flow cell, with good catalyst stability maintained over 8 h of testing. The Ni‐N₂O₂/C catalyst could electrocatalytically reduce O₂ in air to H₂O₂ at a high current density, still affording a high H₂O₂ selectivity (>90 %). A precise Ni‐N₂O₂ coordination was key to the performance.     

Tris(ethyl­ene­diamine)­nickel(II) bis­[2‐cyano‐2‐(oxidoimino)­acetamidato]­nickelate(II) monohydrate

The title compound, [Ni(C₂H₈N₂)₃][Ni(C₃HN₃O₂)₂]·H₂O, appears to be a modular associate consisting of two complex counter‐ions, containing bivalent nickel as the central atom in both cases, and a solvent water mol­ecule. The Ni^II ion in the complex cation lies on the C₂ crystallographic axis. Its coordination environment is formed by six N atoms of three ethyl­ene­diamine (en) mol­ecules, representing a distorted octa­hedral geometry. The Ni^II ion in the complex anion occupies a position at the center of inversion. It exhibits a distorted square‐planar coordination geometry formed by four N atoms belonging to the deprotonated oxidoimine and amide groups of the two doubly charged 2‐cyano‐2‐(oxidoimino)acetamidate anions, situated in trans positions with respect to each other. In the crystal packing, the complex anions are linked by water mol­ecules via hydrogen bonds between the amide O atoms and water H atoms, forming chains translated along the a direction. The [Ni(en)₃]²⁺ cations fill empty spaces between the translational chains, connecting them by hydrogen bonds between the oxime and amide O atoms of the anions and the amine H atoms of the cations, forming layers along the ac plane. The water mol­ecules provide connection between layers through N atoms of the cations, thus forming a three‐dimensional modular structure.     

A comparative study of the packing of two polymorphs of the nickel(II) pincer complex [2,6‐bis(di‐tert‐butylphosphinoyl)‐4‐(3,5‐dinitrobenzoyloxy)phenyl‐κ3P,C1,P′]chloridonickel(II)

Pincer complexes can act as catalysts in organic transformations and have potential applications in materials, medicine and biology. They exhibit robust structures and high thermal stability attributed to the tridentate coordination of the pincer ligands and the strong σ metal–carbon bond. Nickel derivatives of these ligands have shown high catalytic activities in cross‐coupling reactions and other industrially relevant transformations. This work reports the crystal structures of two polymorphs of the title Ni^II POCOP pincer complex, [Ni(C₂₉H₄₁N₂O₈P₂)Cl] or [NiCl{C₆H₂‐4‐[OCOC₆H₄‐3,5‐(NO₂)₂]‐2,6‐(OP^tBu₂)₂}]. Both pincer structures exhibit the Ni^II atom in a distorted square‐planar coordination geometry with the POCOP pincer ligand coordinated in a typical tridentate manner via the two P atoms and one arene C atom via a C—Ni σ bond, giving rise to two five‐membered chelate rings. The coordination sphere of the Ni^II centre is completed by a chloride ligand. The asymmetric units of both polymorphs consist of one molecule of the pincer complex. In the first polymorph, the arene rings are nearly coplanar, with a dihedral angle between the mean planes of 27.9 (1)°, while in the second polymorph, this angle is 82.64 (1)°, which shows that the arene rings are almost perpendicular to one another. The supramolecular structure is directed by the presence of weak C—H…O=X (X = C or N) interactions, forming two‐ and three‐dimensional chain arrangements.     

Di­aqua­bis(4,5‐di­hydro‐1,2,4‐triazolo[1,5‐a]­pyrimidin‐5‐one‐N3)bis­(thiocyanato‐N)­nickel(II)

The title compound, [Ni(NCS)₂(C₅H₄N₄O)₂(H₂O)₂], crystallizes in the triclinic space group P. The molecular unit contains two neutral mol­ecules of 4,5‐di­hydro‐1,2,4‐triazolo[1,5‐a]­pyrimidin‐5‐one (5HtpO) coordinated through the N atom in position 3, two thio­cyanate ligands coordinated through their N atoms and two water mol­ecules completing an octahedral environment around the Ni^II ion, which lies on a centre of inversion. The structure is stabilized by hydrogen bonding. Distances in the coordination sphere are Ni—N3(5HtpO) 2.132 (2), Ni—O(water) 2.085 (2) and Ni—N(thio­cyanato) 2.040 (2) Å.     

mer‐Di­aqua­bis(1H‐imidazole‐κN3)(orotato‐κ2N3,O4)­nickel(II)

The title mononuclear complex, [Ni(C₅H₂N₂O₄)(C₃H₄N₂)₂(H₂O)₂] or [Ni(HOr)(im)₂(H₂O)₂] (im is imidazole and H₃Or is orotic acid, or 2,6‐dioxo‐1,2,3,6‐tetra­hydro­pyrimidine‐4‐carboxylic acid), has been synthesized and the crystal structure determination is reported. The Ni^II ion in the complex has a distorted octahedral coordination geometry comprised of one deprotonated pyrimidine N atom and the adjacent carboxyl­ate O atom of the orotate ligand, two tertiary imidazole N atoms and two aqua ligands. An extensive three‐dimensional network of OW—H⋯O and N—H⋯O hydrogen bonds, and π–π and π–ring interactions are responsible for crystal stabilization.