Plasma-Engineering of Oxygen Vacancies on NiCo2O4 Nanowires with Enhanced Bifunctional Electrocatalytic Performance for Rechargeable Zinc-air Battery.

Designing an efficient, durable, and inexpensive bifunctional electrocatalyst toward oxygen evolution reactions (OER) and oxygen reduction reactions (ORR) remains a significant challenge for the development of rechargeable zinc-air batteries (ZABs). The generation of oxygen vacancies plays a vital role in modifying the surface properties of transition-metal-oxides (TMOs) and thus optimizing their electrocatalytic performances. Herein, a H2/Ar plasma is employed to generate abundant oxygen vacancies at the surfaces of NiCo2O4 nanowires. Compared with the Ar plasma, the H2/Ar plasma generated more oxygen vacancies at the catalyst surface owing to the synergic effect of the Ar-related ions and H-radicals in the plasma. As a result, the NiCo2O4 catalyst treated for 7.5 min in H2/Ar plasma exhibited the best bifunctional electrocatalytic activities and its gap potential between Ej = 10 for OER and E1/2 for ORR is even smaller than that of the noble-metal-based catalyst. In situ electrochemical experiments are also conducted to reveal the proposed mechanisms for the enhanced electrocatalytic performance. The rechargeable ZABs, when equipped with cathodes utilizing the aforementioned catalyst, achieved an outstanding charge-discharge gap, as well as superior cycling stability, outperforming batteries employing noble-metal catalyst counterparts.

Catalytic Ammonia Synthesis Mediated by Molybdenum Complexes with PN3P Pincer Ligands: Influence of P/N Substituents and Molecular Mechanism.

Three molybdenum trihalogenido complexes supported by different PN3P pincer ligands were synthesized and investigated regarding their activity towards catalytic N2-to-NH3 conversion. The highest yields were obtained with the H-PN3PtBu ligand. The corresponding Mo(V)-nitrido complex also shows good catalytic activity. Experiments regarding the formation of the analogous Mo(IV)-nitrido complex lead to the conclusion that the mechanism of catalytic ammonia formation mediated by the title systems does not involve N-N cleavage of a dinuclear Mo-dinitrogen complex, but follows the classic Chatt cycle.

Tungsten and molybdenum dinitrogen complexes supported by a pentadentate tetrapodal phosphine ligand: comparative spectroscopic, electrochemical and reactivity studies.

The tungsten dinitrogen complex [W(N2)(PMe2PPPh2)] (2) (PMe2PPPh2 = [2-({bis[3-(diphenylphosphino)propyl]-phosphino}methyl)-2-methylpropane-1,3-diyl]bis(dimethylphosphine)) is synthesized and characterized by X-ray diffraction as well as IR and NMR spectroscopies, showing strong analogies to its molybdenum analogue [Mo(N2)(PMe2PPPh2)] (1). Whereas cyclic voltammetry studies indicate very similar redox potentials, detailed electrochemical and IR-spectroelectrochemical investigations reveal characteristic differences between 1 and 2 upon electrochemical oxidation in THF. Protonation of 2 with HBArF (BArF = tetrakis(3,5-bis(trifluoromethyl)-phenyl)borate) leads to the hydrazido(2-) derivative 3 which is spectroscopically characterized as well. In the presence of SmI2/H2O slightly overstoichiometric conversion of N2 to ammonia (2.75 equiv.) is observed. Although this is far below the activity of the Mo-complex 1, it renders 2 the first W complex to produce more than 2 equivalents of NH3 from N2 upon addition of protons and reductant.

A Chatt-Type Catalyst with One Coordination Site for Dinitrogen Reduction to Ammonia.

Invited for the cover of this issue is the group of Felix Tuczek at Christian-Albrechts-Universität zu Kiel. The image depicts the coordination geometry of the catalyst reported in this work. Read the full text of the article at 10.1002/chem.202003549.

A Chatt-Type Catalyst with One Coordination Site for Dinitrogen Reduction to Ammonia.

With [Mo(N2 )(P2 Me PP2 Ph )] the first Chatt-type complex with one coordination site catalytically converting N2 to ammonia is presented. Employing SmI2 as reductant and H2 O as proton source 26 equivalents of ammonia are generated. Analogous Mo0 -N2 complexes supported by a combination of bi- and tridentate phosphine ligands are catalytically inactive under the same conditions. These findings are interpreted by analyzing structural and spectroscopic features of the employed systems, leading to the conclusion that the catalytic activity of the title complex is due to the strong activation of N2 and the unique topology of the pentadentate tetrapodal (pentaPod) ligand P2 Me PP2 Ph . The analogous hydrazido(2-) complex [Mo(NNH2 )(P2 Me PP2 Ph )](BArF )2 is generated by protonation with HBArF in ether and characterized by NMR and vibrational spectroscopy. Importantly, it is shown to be catalytically active as well. Along with the fact that the structure of the title complex precludes dimerization this demonstrates that the corresponding catalytic cycle follows a mononuclear pathway. The implications of a PCET mechanism on this reactive scheme are considered.

Synthesis and Characterization of a Rare Transition-Metal Oxothiostannate and Investigation of Its Photocatalytic Properties.

The new transition-metal oxothiostannate [Ni(cyclen)(H2O)2]4[Sn10S20O4]·âˆ¼13H2O (1) was prepared under hydrothermal conditions using Na4SnS4·14H2O as the precursor in the presence of [Ni(cyclen)(H2O)2](ClO4)2·H2O. Compound 1 comprises the [Sn10S20O4]8- anion constructed by the T3-type supertetrahedron [Sn10S20] and the [Sn10O4] anti-T2 cluster. Channels host the H2O molecules, and the sample can be reversibly dehydrated and rehydrated without significantly affecting the crystallinity of the material. 119Sn NMR spectroscopy of an aqueous solution of Na4SnS4·14H2O evidences that between 25 and 120 °C only [SnS4]4- and [Sn2S6]4- anions are present. In further experiments, hints were found that the formation of tin oxosulfide ions depends on the Ni2+-centered complexes. Compound 1 exhibits promising photocatalytic properties for the visible-light-driven hydrogen evolution reaction, with 18.7 mmol·g-1 H2 being evolved after 3 h.

Molybdenum dinitrogen complex supported by a cyclohexane-based triphosphine ligand and dmpm.

We report the first molybdenum dinitrogen complex supported by a cyclohexane-based triphosphine ligand. By employing several spectroscopic methods we are able to pinpoint the pentaphosphine environment and evaluate the activation of the dinitrogen ligand. Furthermore, we conduct protonation experiments and obtain spectroscopic evidence for the formation of a hydrazido-species.

Influence of a Metal Substrate on Small-Molecule Activation Mediated by a Surface-Adsorbed Complex.

Activating small molecules with transition metal complexes adsorbed on metal surfaces is a novel approach combining aspects of homogeneous and heterogeneous catalysis. In order to study the influence of an Au(111) substrate on the activation of the small-molecule ligand carbon monoxide, a molybdenum tricarbonyl complex containing a PN3 P pincer ligand was synthesized and investigated in the bulk, in solution, and adsorbed on an Au(111) surface. By means of a platform approach, a perpendicular orientation of the molybdenum complex was achieved and confirmed by IRRAS and NEXAFS. By using vibrational spectroscopy (IR, Raman, IRRAS) coupled to DFT calculations, the influence of the metal substrate on the activation of the CO ligands bound to the molybdenum complex was determined. The electron-withdrawing behavior of gold causes an overall shift of the CO stretching vibrations to higher frequencies, which is partly compensated by dynamic charge transfer from the substrate to the molybdenum center, which increases its (dynamic) polarizability.

Design, synthesis, and evaluation of nickel dipyridylmethane complexes for Coordination-Induced Spin State Switching (CISSS).

In order to develop a new system capable of performing Ligand-Driven Coordination-Induced Spin State Switching (LD-CISSS), a new CISSS complex based on a nickel(ii) dipyridylmethane framework was rationally designed, synthesized, and investigated both experimentally and theoretically. In DFT calculations, a dipyridylmethane dicarboxylic acid ligand was determined to be most suitable for this purpose. The corresponding nickel(ii) complex was synthesized by an optimized procedure, and the CISSS effect was evidenced by crystallography as well as Evans NMR susceptibility measurements. In the solid state, the square planar nickel complex is diamagnetic and the octahedral bispyridine adduct is paramagnetic. Also, a dimer containing three ethanol ligands was obtained. Dimer formation occurs in solution as well. Upon addition of pyridine, a continuous transition from predominantly diamagnetic to paramagnetic behavior takes place. Equilibrium constants for the binding of pyridine as well as corresponding thermodynamic parameters indicate a high affinity for this ligand and show that the title system is a suitable candidate for LD-CISSS.

Molybdenum dinitrogen complexes facially coordinated by linear tridentate PEP ligands (E = N or P): impact of the central E donor in trans-position to N2.

The syntheses of molybdenum dinitrogen complexes supported by the tridentate PEP ligands (E = N, P) prPP(Ph)P = (Ph2PCH2CH2CH2)2P(Ph), prPPHP = (Ph2PCH2CH2CH2)2PH, PN(Ph)P = (Ph2PCH2CH2)2N(Ph) and prPN(Ph)P = (Ph2PCH2CH2CH2)2N(Ph) are reported. Together with the coligand dmpm = (CH3)2PCH2P(CH3)2 dinitrogen complexes of the type [Mo(N2)(PEP)(dmpm)] are formed. The new systems are characterized by IR and NMR spectroscopy and compared with the literature-known complex [Mo(N2)(dpepp)(dmpm)] (1) (dpepp = PhP(CH2CH2PPh2)2). The consequences of the substitution of the central P-donor of dpepp by N and the replacement of its C2 by C3 linkages as well as the exchange of the EPh by an EH function are investigated with respect to the stability of the corresponding N2-complexes. Importantly, the activation of the N2 ligand drastically increases upon replacing the trans-phosphine with a trans-amine donor.

Molybdenum(0) Dinitrogen Complexes Supported by Pentadentate Tetrapodal Phosphine Ligands: Structure, Synthesis, and Reactivity toward Acids.

The syntheses of two pentadentate tetrapodal phosphine (pentaPod(P)) ligands, P2(Ph)PP2(Ph) and P2(Me)PP2(Ph), are reported, which derive from the fusion of a tripod and a trident ligand. Reaction of the ligand P2(Ph)PP2(Ph) with [MoCl3(THF)3] followed by an amalgam reduction under N2 does not lead to well-defined products. The same reactions performed with the ligand P2(Me)PP2(Ph) afford the mononuclear molybdenum dinitrogen complex [MoN2(P2(Me)PP2(Ph))]. Because of the unprecedented topology of the pentaphosphine ligand, the Mo-P bond to the phosphine in the trans position to N2 is significantly shortened, explaining the very strong activation of the dinitrogen ligand (ν̃NN = 1929 cm(-1)). The reactivity of this complex toward acids is investigated.

Molybdenum complexes supported by mixed NHC/phosphine ligands: activation of N2 and reaction with P(OMe)3 to the first meta-phosphite complex.

Molybdenum(0) dinitrogen complexes, supported by the mixed NHC/phosphine pincer ligand PCP, exhibit an extreme activation of the N2 ligand due to a very π-electron-rich metal center. The low thermal stability of these compounds can be increased using phosphites instead of phosphines as coligands. Through an amalgam reduction of [MoCl3(PCP)] in the presence of trimethyl phosphite and N2 the highly activated and room-temperature stable dinitrogen complex [Mo(N2)(PCP)(P(OMe)3)2] is obtained. As a second product, the first transition metal complex containing the meta-phosphite ligand P(O)(OMe) originates from this reaction.

Molybdenum dinitrogen complexes supported by a silicon-centred tripod ligand and dppm or dmpm: tuning the activation of N2.

Two molybdenum dinitrogen complexes with a novel pentaphosphine environment have been synthesised and structurally characterised. A combination of the tripodal alkylphosphine ligand tris(dimethylphosphinomethyl)methylsilane (SiP3) and the coligands dppm/dmpm has been employed to tune the activation of the N2 ligand.

Bonding and activation of N2 in Mo(0) complexes supported by hybrid tripod ligands with mixed dialkylphosphine/diarylphosphine donor groups: interplay of steric and electronic factors.

Molybdenum dinitrogen complexes are presented which are supported by novel hybrid tripod ligands of the type Me-C(CH2PPh2)2(CH2P(i)Pr2) (trpd-1) and H-C(CH2PPh2)(CH2P(i)Pr2)2 (trpd-2) having mixed dialkylphosphine/diarylphosphine donor groups. Reaction of the ligand trpd-1 with [MoI3(thf)3] followed by sodium amalgam reduction in the presence of the dppm gives the dinitrogen complex [Mo(N2)(trpd-1)(dmpm)] where trpd-1 is coordinated in a κ(3) fashion. The complex exhibits a moderate activation of N2 which enables its protonation under retention of the pentaphosphine ligation. Replacement of dmpm by the sterically more demanding coligand dppm is found to hamper coordination of N2 and leads to [Mo(trpd-1)(dppm)], the first structurally characterized five-coordinate Mo(0) complex with a phosphine-only ligand sphere. Employing the ligand trpd-2 along with the diphosphines dmpm and dppm in an analogous synthetic route results in a mixture of the bis(dinitrogen) complexes trans-[Mo(N2)2(κ(2)-trpd-2)(diphosphine)] and trans-[Mo(N2)2(iso-κ(2)-trpd-2)(diphosphine)] where the tripod ligand trpd-2 coordinates with two phosphine arms and one phosphine group (PPh2 or P(i)Pr2, respectively) is free. Similar results are obtained with the pure alkyl- and arylphosphine tripod ligands H-C(CH2P(i)Pr2)3 (trpd-3) and H-C(CH2PPh2)3 (tdppmm), leading to trans-[Mo(N2)2(κ(2)-trpd-3)(diphos)] and trans-[Mo(N2)2(κ(2)-tdppmm)(dmpm)], respectively. The electronic and steric reasons for the experimental findings are considered, and the implications of the results for the area of synthetic nitrogen fixation with molybdenum phosphine systems are discussed.