Strategic design of VO2 encased in N-doped carbon as an efficient electrocatalyst for the nitrogen reduction reaction in neutral and acidic media.

Electrocatalytic nitrogen fixation to ammonia (NH3), a precursor for fertilizer production and a promising energy carrier, has garnered widespread interest as an environment-friendly and sustainable alternative to the energy-intensive fossil-feedstock-dependent Haber-Bosch process. The large-scale deployment of this process is contingent on the identification of inexpensive, Earth-abundant systems that can operate efficiently, irrespective of the electrolyte pH for the selective production of NH3. In this regard, we discuss the scalable synthesis of VO2 anchored on N-doped carbon (VO2@CN), and its applicability as a robust electrocatalyst for the nitrogen reduction reaction (NRR). Benefitting from the presence of exposed VO2, which presumably acts as the active site for nitrogen reduction, and its activity over a broad pH range (from acidic to neutral), VO2@CN exhibits a high NH3 yield of 0.31 and 0.52 µmol h-1 mgcat-1 and a maximum Faradaic efficiency (FE) of 67.9% and 61.9% at -0.1 V vs. RHE, under neutral and acidic conditions, respectively. The obscured reaction intermediates of the NRR were identified from in situ ATR-IR studies under both electrolyte conditions. Additionally, the high selectivity of the catalyst was ascertained from the absence of hydrazine production and the competing hydrogen evolution reaction (HER). However, ammonia production underwent a reduction over 12 h of continuous operation presumably owing to the leaching of catalyst under these electrolysis conditions, which was more pronounced in electrolytes with acidic pH. Overall, the present report unveils the performance of an earth-abundant vanadium oxide-based system as an efficient electrocatalyst for the NRR under acidic and neutral pH conditions.

Amine-rich Nickel(II)-Xerogel as a Highly Active Bifunctional Metallo-organo Catalyst for Aqueous Knoevenagel Condensation and Solvent-free CO2 Cycloaddition.

Metallogels formed from supramolecular interactions of low-molecular-weight gelators (LMWGs) combine the qualities of heterogeneous catalysts and offer the advantages of multifunctionality owing to the facile installation of desired task-specific moieties on the surface and along the channels of the gels. We discuss the applications of a triazole-based Ni(II) gel-derived xerogel (NiXero) having a high density of Ni(II)-nodes and appended primary amines as a recyclable heterogeneous catalyst for Knoevenagel condensation of aldehyde and malononitrile in water and the solvent-free cycloaddition of CO2 to form a series of cyclic carbonates with near-quantitative conversion of the respective epoxides, with low catalyst loading (0.59 mol %), high catalyst stability, and recyclability. The structural advantages of NiXero, due to the concurrent presence of bifunctional Lewis acid-base sites on the channels, open Ni(II) nodes, Ntriazole, pendant -NH2 and its chemical stability, are conducive to the cooperative heterogeneous catalytic activity under mild conditions. This work emphasizes the effective amalgamation of metals with purpose-built ligand systems for the construction of metallogels and their utility as heterogeneous catalysts for desired organic transformations.

Exploring amine-rich supramolecular silver(I) metallogels for autonomous self-healing and as catalysts for a three component coupling reaction.

A series of Ag(I) supramolecular organo-aqueous gels have been synthesized in the presence of an amine-rich triazole ligand as a gelator. Judicious choice of the triazole derivative and counter anion allows a desired spatial orientation of the pendant amine functionality to accentuate the gelation ability and autonomous self-healability via hydrogen bonding. In addition, the hydrogen bond donors, i.e. pendant -NH2 groups, offer a critical proximity of counter anions to the Lewis acidic Ag(I) and the reactants for promoting a three component coupling reaction of an aldehyde, a terminal alkyne and an amine, giving expedient access to propargyl amines, with remarkable functional group tolerance for both aromatic and aliphatic aldehydes, and aryl acetylenes. Experiments substantiate the pivotal role of counter anions and H-bonding interactions in the observed preference for propargylamines over the diacetylene by-product. Our catalyst is robust, bench-stable, and recyclable, and demonstrates a catalytic efficiency comparable to or better than those of reported systems. The catalyst was found equally effective for the gram-scale synthesis of propargylamines. Our approach lies at the intersection of metal-based, H-bond-mediated counter anion-tuned catalysis, evincing a potential for the development of purpose-built supramolecular gels for desired catalytic applications in the future.

Melamine-Based Hydrogen-bonded Systems as Organoelectrocatalysts for Water Oxidation Reaction.

Applications of small organic molecules and hydrogen-bonded aggregates, instead of traditional transition-metal-based electrocatalysts, are gaining momentum for addressing the issue of low-cost generation of H2 to power a sustainable environment. Such systems offer the possibility to integrate desired functional moieties with predictive structural repetition for modulating their properties. Despite these advantages, hydrogen-bonded organic systems have largely remained unexplored, especially as electrocatalysts. Melamine and adipic acid-based hydrogen-bonded organic ionic (BMA) and co-crystal systems developed under varying temperatures are explored as electrocatalysts for water oxidation reaction (WOR). These systems are easily modifiable with precisely designed molecular architecture and judiciously positioned nitrogen atoms. Combined effect of charge-assisted hydrogen bonding stabilizes the ionic BMA system under corrosive alkaline conditions and augments its remarkable electrocatalytic WOR activity, achieving a current density of 10 mA cm-2 at an overpotential of 387 mV and Faradaic efficiency ∼94.5 %. The enhanced electrocatalytic ability of BMA is attributed to its hydrophilic nature, unique molecular composition with complementary hydrogen-bonded motifs and a high density of positively charged nitrogen atoms on the surface, that facilitates electrostatic interactions and accelerate charge and mass transport processes culminating in a turnover frequency of ∼0.024 s-1 . This work validates the potential of hydrogen-bonded molecular organo-electrocatalysts towards WOR.

In situ Generated Ru(0)-HRO@Na-ß From Hydrous Ruthenium Oxide (HRO)/Na-ß: An Energy-Efficient Catalyst for Selective Hydrogenation of Sugars.

A green process for the hydrogenation of sugars to sugar alcohols was designed in aqueous medium using hydrous ruthenium oxide (HRO) as a pre-catalyst supported on Na-ß zeolite. Under optimized reaction conditions, sugars such as xylose, glucose, and mannose converted completely to the corresponding sugar alcohols xylitol, sorbitol, and mannitol with 100% selectivity. The pre-catalyst (HRO) is converted in situ to active Ru(0) species during the reaction under H2, which is responsible for the hydrogenation. The catalyst was recyclable up to five cycles with no loss in activity. The reduction of HRO to the active Ru(0) species is dependent on the reaction temperature and H2 pressure. Ru(0) formation increased and consequently an increased hydrogenation of sugars was observed with an increase in reaction temperature and hydrogen pressure. Further, in situ generation of Ru(0) from HRO was assessed in different solvents such as water, methanol, and tetrahydrofuran; aqueous medium was found to be the most efficient in reducing HRO. This work further demonstrates the use of supported HRO as an efficient pre-catalyst for biomass-based hydrogenation reactions.

Multistimuli-Responsive Self-Healable and Moldable Nickel(II)-Based Gels for Reversible Gas Adsorption and Palladium Sequestration via Gel-to-Gel Transformation.

We report the in situ formation of Ni-based supramolecular organogel and organic-aqueous gels using amine appended triazole ligand, having varying morphology and rheological properties. These gels are self-healable and moldable or injectable respectively depending on the absence or presence of water in the gelation medium. Our studies reveal that the formation and rupture of hydrogen bonds assisted by the solvent movement is responsible for the self-healing nature of the gels. The porous structure of the gel has been observed from the migration of dye molecules on the self-healed gel. In addition, the gels show dual function of reversible adsorption of toxic gases and sequestration of heavy metal ions, especially palladium via  gel-to-gel transformation. It is imperative to stress that such transformation is extremely rare for small molecule based metallogels. The dynamic nature of Ni-Ntriazole interactions has been utilized in achieving the reversible gas/vapor responsive behavior of the metallogels, which could be suitable in developing colorimetric probes for the detection of toxic gases and heavy metal ions. Such multifunctional gels are exceptional in contemporary literature and are expected to find utility in fabricating smart multistimuli-responsive gel-based materials in the future.

Diiron Dithiolate Hydrides Complemented with Proton-Responsive Phosphine-Amine Ligands.

The reaction of Fe2(pdt)(CO)6 with two equivalents of Ph2PC6H4NH2 (PNH2) affords the amido hydride HFe2(pdt)(CO)2(PNH2)(PNH) {[H1H]0, pdt2- = CH2(CH2S-)2}. Isolated intermediates in this conversion include Fe2(pdt)(CO)5-(κ1-PNH2) and Fe2(pdt)(CO)4(κ2-PNH2). X-ray crystallographic analysis of [H1H]0 shows that the chelating amino/amido-phosphine ligands occupy trans-dibasal positions. The 31P NMR spectrum indicates that [H1H]0 undergoes rapid proton exchange between the amido and amine centers. No exchange was observed for the hydride. Protonation of [H1H]0 gives [HFe2(pdt)(CO)2(PNH2)2]+ ([H21H]+), which contains two equivalent amino-phosphine ligands. Single-crystal X-ray crystallographic analysis of [H21H]+ also reveals hydrogen bonds between the exo amine protons with a THF molecule and BF4. Deprotonation of [H1H]0 with potassium tert-butoxide gave [HFe2(pdt)(CO)2(PNH)2]- ([1H]-), which was characterized spectroscopically. The complex has time-averaged C2 symmetry with two amido-phosphine ligands. FTIR spectroscopic measurements show that υCO shifts by approximately 20 cm-1 in the series [1H]-, [H1H]0, and [H21H]+. These shifts are comparable to those seen for the S-protonation of the (NC)2(CO)Fe-(µ-Scys)2Ni(Scys)2 site in the [NiFe]-hydrogenases.[1].

Ni(I)/Ru(II) model for the Ni-L state of the [NiFe]hydrogenases: synthesis, spectroscopy, and reactivity.

This study describes the characterization of a mixed-valence Ru(II)/Ni(I) complex, a structural model for the Ni-L state of the [NiFe]hydrogenases. One-electron oxidation of (cymene)Ru(µ-pdt)Ni(diphos) ([1](0), diphos = dppe, C2H4(PPh2)2; [2](0), diphos = dcpe, C2H4(P(C6H11)2)2] affords the mixed-valence cations [(cymene)Ru(pdt)Ni(diphos)](+) ([1](+) and [2](+)). Crystallographic and spectroscopic measurements indicate that these cations are described as Ru(II)/Ni(I). Although [1](0) and [1](+) are very similar structurally, the following changes are notable: the Ni-P distances elongate upon oxidation, and the Ru-Ni distance changes insignificantly. The molecular and electronic structures of the Ni center in [1](+) approaches that observed in the [NiFe]hydrogenases. Density functional theory calculations indicate that [1](0) is best described as Ru(II)/Ni(0), consistent with its oxidation to Ru(II)/Ni(I) in [1](+). The fast electron self-exchange rate of 10(7) M(-1) s(-1) between [1](0) and [1](+) suggests minor reorganization, more consistent with a Ni(0)/Ni(I) oxidation state change than a Ni(I)/Ni(II) couple. In solution, [1](+) slowly converts to [H1](+) and [1-H](+), with the latter being a complex of the thioaldehyde SCHCH2CH2S arising from C-H activation of the pdt backbone. Treatment of [1](+) with the H-atom abstracting reagent 2,2,6,6-tetramethylpiperidine-1-oxy also gives [1-H](+).

Lignol cleavage by Pd/C under mild conditions and without hydrogen: a role for benzylic C-H activation?

The cleavage of C-O bonds in lignin model compounds without hydrogen was developed using the commercially available Pd/C. Hydrogen donor solvents are helpful for this reaction through transfer hydrogenation, but not necessary. A redox neutral process that utilizes the internal hydrogen source for the cleavage is also possible. An initial mechanistic study indicates that the ß-benzylic-H atom in the substrate plays a critical role and that the present system undergoes a process different from previous reports.

Second order non-linear optical activity of arsenic and antimony dithiolene complexes.

Synthesis and characterization of dithiolene complexes of arsenic and antimony in trivalent state have been reported. A four coordinated structural motif results in a ladder like arrangement in the arsenic complex due to the inter-anionic As-S interaction which is replaced by Sb-π interaction with the counter cation in the solid state structure of the similar antimony complex. Electronic structure calculations on ground state geometries and the time-dependent density functional theoretical calculations were performed in order to characterize the absorption spectra incorporating solvent effects. Notably, both the complexes display intense second order optical non-linearity as has been determined using hyper-Rayleigh scattering technique in dichloromethane solution and the results are corroborated by DFT calculations.

DFT studies of structural and some spectral parameters of copper(II) complexes with N,N,N',N″-tetrakis (2-hydroxyethyl/propyl) ethylenediamine and tris(2-hydroxyethyl)amine.

The structures and some spectral parameters of three copper(II) complexes; [Cu(THEEN)(H2O)](PIC)2 (1), [Cu(THPEN)] (PIC)2 C3H8O (2) and [Cu(TEAH3)(PIC)] (PIC)⋅(H2O) (3), previously synthesized and characterized by X-ray diffraction, are here computationally studied by using density functional theory (DFT) in its hybrid form B3LYP. In these complexes, THEEN is N,N,N',N″-tetrakis(2-hydroxyethyl) ethylenediamine and THPEN is N,N,N',N″-tetrakis(2-hydroxypropyl) ethylenediamine, tetrapodal ligands and TEAH3 is tris(2-hydroxyethyl)amine, a tripodal ligand. The primary coordination sphere of copper(II) ion in complexes (1), (2) and (3) are optimized, structural parameters are calculated, vibrational bands are assigned and energy gaps of frontier orbital (HOMO-LUMO) have been calculated with B3LYP/6-31G/LANL2DZ level of theory using DMSO as solvent. The calculated geometric and spectral results reproduced the experimental data with well agreement. Theoretical calculated molecular orbitals (HOMO-LUMO) and their energies have been calculated that suggest charge transfer occurs within the complexes.

Oxo-Mo(IV)(dithiolene)thiolato complexes: analogue of reduced sulfite oxidase.

A series of [Mo(IV)O(mnt)(SR)(N-N)](-) (mnt = maleonitriledithiolate; R = Ph, nap, p-Cl-Ph, p-CO2H-Ph, and p-NO2-Ph; N-N = 2,2'-bipyridine (bipy) and 1,10-phenanthroline (phen)) complexes analogous to the reduced active site of enzymes of the sulfite oxidase family has been synthesized and their participation in electron transfer reactions studied. Equatorial thiolate and dithiolene ligations have been used to closely simulate the three sulfur coordinations present in the native molybdenum active site. These synthetic analogues have been shown to participate in electron transfer via a pentavalent EPR-active Mo(V) intermediate with minimal structural change as observed electrochemically by reversible oxidative responses. The role of the redox-active dithiolene ligand as an electron transfer gate between external oxidants and the molybdenum center could be envisaged in one of the analogue systems where the initial transient EPR signal with = 2.008 is replaced by the appearance of a typical Mo(V)-centered EPR ( = 1.976) signal. The appearance of such a ligand-based transient radical at the initial stage has been supported by the ligand-centered frontier orbital from DFT calculation. A stepwise rationale has been provided by computational study to show that the coupled effects of the diimine bite angle and the thiolato dihedral angle determine the metal- or ligand-based frontier orbital occupancy. DFT calculation has further supported the similarity between the reduced, semireduced, and oxidized resting state of the molybdenum center in Moco of SO with the synthesized complexes and their corresponding one-electron and fully oxidized species.

Modelling the reduced xanthine oxidase in active sulfo and inactive desulfo forms.

Synthesis of two complexes, [NBu(n)(4)][Mo(IV)O(mnt)(S-Tol)(N-N)] (N-N = 2,2'-bipyridine (1a) or 1,10-phenanthroline (1b); mnt = maleonitriledithiolate; S-Tol = toluenethiol) are reported. These on treatment with H(2)S generate the corresponding [NBu(n)(4)][Mo(IV)O(mnt)(SH)(N-N)] (2a and 2b) complexes bearing the susceptible hydrosulfide coordination. 2a (and 2b) upon chemical oxidation show EPR spectra with the appearance of a Mo(V) signal at = 1.976 (for 2a and 2b). Such EPR signal changed to another slow Mo(V) signal at = 1.949 at the expense of the initial signal. This conversion is accelerated in the presence of trace amount of moisture in the solvent. These data are similar to the Moco isolated from the xanthine oxidase (XO) as reported by Bray. 2a (and 2b) responds to one electron electrochemical oxidation and the generated pentavalent species responds to proton coupled electron transfer reaction. A predominantly metal centered HOMO is observed in 2a (and 2b) from DFT calculations. In the inhibited Moco of XO bearing a -SH moiety, the HOMO shows considerably less electron density residing on the Mo d(xy) orbital and maximum electron density is distributed to the phospho-ester group rendering the Mo(IV) center incapable of participating in the electron transfer process manifesting inhibition.

Mono-oxo bis(dithiolene) Mo(IV)/W(IV) complexes as building blocks for sulfide bridged Bi- and Tri-nuclear complexes.

A new electron precise, six-electron, sulfide-bicapped trinuclear cluster complex [Et4N]4[Mo(IV)3(mu3-S)2(mnt)6] (1) has been synthesized, where each Mo(IV) atom is seven coordinated. Identical reaction conditions yielded a dimeric complex, [PNP]2[W(V)2(mu2-S)2(mnt)4] (2) from the starting W(IV) analogue due to oxidation by sulfur formed by the auto-oxidation of H2S. Two stepwise reversible reductions and no oxidation of 2 as observed by cyclic voltammetry are correlated with the nonbonding nature of the lowest unoccupied molecular orbital and deeply buried highest occupied molecular orbital by theoretical calculations at the density-functional theory level.