Ni Center Research Articles

Adsorbate Configurations in Ni Single-Atom Catalysts during CO2 Electrocatalytic Reduction Unveiled by Operando XAS, XES, and Machine Learning

Nickel and nitrogen co-doped carbon (Ni-N-C) catalysts are attracting attention due to their exceptionally high performance in the electrocatalytic reduction of CO2(CO2RR) to CO. However, the direct experimental insight into the working mechanism of these catalysts is missing, hindering our fundamental understanding and their further improvement. This work sheds light on the nature of adsorbates forming under CO2RR at singly dispersed Ni sites. In particular, high energy resolution fluorescence detected x-ray absorption near edge structure (HERFD-XANES) at the Ni K-edge together with valence-to-core x-ray emission spectroscopy (vtc-XES) and x-ray absorption (XAS) at the Ni L3-edge were employed to unveil the structure and electronic properties of the reaction intermediates. These techniques, coupled with unsupervised and supervised machine learning methodologies and density functional theory, enabled a comprehensive characterization of the local atomistic and electronic structure of the working Ni-N-C catalysts. Specifically, we were able to distinguish between the structural and electronic changes of the Ni sites associated with the CO2RR functionality from the effect of radiation-induced damage, providing direct insight into the bond formation between the Ni centers and CO2RR intermediates such as CO adsorbates. Published by the American Physical Society 2024

Coordination Induced Spin State Transition Switches the Reactivity of Nickel (II) Porphyrin in Hydrogen Evolution Reaction

Electron spin plays a critical role in chemical processes, particularly in reactions involving metal complexes with unpaired electrons. However, more definitive state‐to‐state experiments are needed to better elucidate the role of electronic spin. Herein, we chose nickel (II) 5,10,15,20‐tetrakis(pentafluorophenyl) porphyrin 1 as a catalyst, which allows switching from a low spin to a high spin state of Ni(II) center through an axial pyridine coordination, for electrocatalytic hydrogen evolution reaction (HER). When pyridine is present, we observed β‐hydrogenation of porphyrin through electron transfer followed by proton transfer. In contrast, hydrogen evolution mainly occurs via the concerted proton‐coupling electron transfer without pyridine coordination. Similar distinct spin‐dependent selectivity was also observed in chemical reduction of 1 by CoCp2 with subsequent addition of pyridinium p‐toluenesulfonate. Computational calculations using density functional theory demonstrated that the transition from low spin to high spin state enriches the ligand’s electron density after one‐electron reduction, leading to preferential protonation of β‐periphery rather than meso‐position or metal center.

Coordination Induced Spin State Transition Switches the Reactivity of Nickel (II) Porphyrin in Hydrogen Evolution Reaction.

Electron spin plays a critical role in chemical processes, particularly in reactions involving metal complexes with unpaired electrons. However, more definitive state-to-state experiments are needed to better elucidate the role of electronic spin. Herein, we chose nickel (II) 5,10,15,20-tetrakis(pentafluorophenyl) porphyrin 1 as a catalyst, which allows switching from a low spin to a high spin state of Ni(II) center through an axial pyridine coordination, for electrocatalytic hydrogen evolution reaction (HER). When pyridine is present, we observed β-hydrogenation of porphyrin through electron transfer followed by proton transfer. In contrast, hydrogen evolution mainly occurs via the concerted proton-coupling electron transfer without pyridine coordination. Similar distinct spin-dependent selectivity was also observed in chemical reduction of 1 by CoCp2 with subsequent addition of pyridinium p-toluenesulfonate. Computational calculations using density functional theory demonstrated that the transition from low spin to high spin state enriches the ligand's electron density after one-electron reduction, leading to preferential protonation of β-periphery rather than meso-position or metal center.

Electrochemical Enantioselective Nickel-Catalyzed Cross-Coupling of Aldehydes with Aryl Iodides.

The preparation of enantioenriched diarylmethanol derivatives is described using nickel-catalyzed electrochemical cross-couplings between various alkyl/aryl aldehydes and aryl iodides. Performed in an electrochemical cell equipped with an iron anode and a nickel cathode, this electrocatalytic variant led to the scalemic targeted products in the presence of 2,2-bis((4R,5S)-4,5-diphenyl-4,5-dihydrooxazol-2-yl)acetonitrile (L2), as enantiopure cyano-bis(oxazoline) ligand. X-ray structure analysis of a pre-catalyst, for instance the [Ni(II)(L2)2] complex, with L2 as an anionic bisoxazolinate ligand, confirms the chemical formulation of one nickel surrounded by two ligands. The redox behavior of the new Ni complexes generated in situ was first assessed by cyclic voltammetry showing a redox wave at ca. -1.5 V that can be assigned to the two-electron reduction of the Ni(II) center to the Ni(0) state. Oxidative addition between the electrogenerated Ni(0) complex and aryl iodide was evidenced. An intense current was observed in presence of a mixture of the two substrates pertaining an electrocatalytic process. Interestingly, we found that the sacrificial iron anode plays a crucial role in the catalytic mechanism.

Copper-Based Electrochemical Sensor Derived from Thiosemicarbazide for Selective Detection of Neurotransmitter Dopamine.

This paper presents the synthesis of the ligand 1-picolinoyl-4-cyclohexyl-3-thiosemicarbazide (H2pctc) and new metal complexes [Ni(Hpctc)2] (1), [Cu(Hpctc)Cl] (2), and [Cd(Hpctc)2] (3). The synthesized metal complexes were precisely characterized using single crystal X-ray diffraction (SC-XRD). In addition, complexes 1-3 were also characterized by UV-vis, fluorescence, and infrared spectroscopy. SC-XRD data confirmed the distorted octahedral geometry around the Ni(II) and Cd(II) centers and the distorted square planar geometry around the Cu(II) center. Data derived from the emission spectra depict that higher fluorescence intensity was exhibited by complexes 1, 2, and 3 in comparison to that of the free ligand H2pctc, and complex 3 showed the maximum intensity. Further, these metal complex-modified GCEs (glassy carbon electrodes) were utilized for electrochemical sensing of dopamine (DPM). The electrochemical studies of these complexes were performed using modified glassy carbon electrodes supported by electrical impedance spectroscopy (EIS) and cyclic voltammetry (CV) methods. In contrast to complexes 1 and 3, complex 2 is a superior electrode material with a high effective surface area for the electrochemical oxidation of DPM, according to the electrochemical response results. The derived sensor had a wide linear detection range of 1 to 1400 μM, an acceptable sensitivity of 0.01531 μA cm-2 μM-1, and a low LOD of 0.38 μM. The proposed approach was free of the interfering effects of ascorbic acid, uric acid, aminophenol, and other substances. During the successive scans, no fouling of the electrode surface was observed. The proposed electrochemical sensor had excellent stability, sensitivity, and a low detection limit making it suitable for the analysis of a variety of real samples. Additionally, it was proven to be useful for analyzing biological fluids.

Ancillary Ligand-Promoted Charge Transfer in Bis-indole Pyridine Ligand-Based Nickel Complexes.

There is growing demand for the utilization of first-row transition metal complexes in light-driven processes instead of their conventional noble metal counterparts due to the greater sustainability of first-row transition metal complexes. However, their major drawback is the ultrafast lifetime of the electronic excited states of these first-row transition metal complexes, particularly those of d8 square-planar systems such as Ni(II) complexes, wherein low-lying metal-centered (MC) states provide the deactivation pathway. To increase the excited-state lifetime and broaden their applications, it is important to develop sterically bulky, strong field ligands with low-lying π* orbitals and a highly σ-donating nature to augment the energy of MC states. The current strategy relies on synthetically carbene-based ligands, which are substitutionally cumbersome and act as σ-donors only. In this work, we introduce a bis-indole pyridine (H2BIP) ligand framework, whose dianionic congener (BIP) demonstrates the ability to form stronger covalent bonds with a Ni(II) center compared to neutral donors like carbene and its effect on the complex to produce a less distorted excited-state structure. When conjoined with ancillary ligands such as pyridine or lutidine, the BIP ligand orchestrates the formation of low-energy 3CT states, which decay in ∼40 ps.

Unveiling active sites in FeOOH nanorods@NiOOH nanosheets heterojunction for superior OER and HER electrocatalysis in water splitting

The development of cost-effective, highly active, and stable electrocatalysts for water splitting to produce green hydrogen is crucial for advancing clean and sustainable energy technologies. Herein, we present an innovative in-situ synthesis of FeOOH nanorods@NiOOH nanosheets on nickel foam (FeOOH@NiOOH/NF) at an unprecedentedly low temperature, resulting in a highly efficient electrocatalyst for overall water splitting. The optimized FeOOH@NiOOH/NF sample, evaluated through time-dependent studies, exhibits exceptional oxygen evolution reaction (OER) performance with a low overpotential of 261 mV at a current density of 20 mA cm−2, alongside outstanding hydrogen evolution reaction (HER) activity with an overpotential of 150 mV at a current density of 10 mA cm−2, demonstrating excellent stability in alkaline solution. The water-splitting device featuring FeOOH@NiOOH/NF-2 electrodes achieves a voltage of 1.59 V at a current density of 10 mA cm−2, rivalling the state-of-the-art RuO2/NF||PtC/NF electrode system. Density functional theory (DFT) calculations unveil the efficient functionality of the Fe sites within the FeOOH@NiOOH heterojunction as the active OER catalyst, while the Ni centres are identified as the active HER sites. The enhanced performance of OER and HER is attributed to the tailored electronic structure at the heterojunction, modified magnetic moments of active sites, and increased electron density in the dx2-y2 orbital of Fe. This work provides critical insights into the rational design of advanced electrocatalysts for efficient water splitting.

Hierarchically Ordered Pore Engineering of Carbon Supports with High-Density Edge-Type Single-Atom Sites to Boost Electrochemical CO2 Reduction.

Metal sites at the edge of the carbon matrix possess unique geometric and electronic structures, exhibiting higher intrinsic activity than in-plane sites. However, creating single-atom catalysts with high-density edge sites remains challenging. Herein, the hierarchically ordered pore engineering of metal-organic framework-based materials to construct high-density edge-type single-atomic Ni sites for electrochemical CO2 reduction reaction (CO2RR) is reported. The created ordered macroporous structure can expose enriched edges, further increased by hollowing the pore walls, which overcomes the low edge percentage in the traditional microporous substrates. The prepared single-atomic Ni sites on the ordered macroporous carbon with ultra-thin hollow walls (Ni/H-OMC) exhibit Faraday efficiencies of CO above 90% in an ultra-wide potential window of 600mV and a turnover frequency of 3.4×104h-1, much superior than that of the microporous material with dominant plane-type sites. Theory calculations reveal that NiN4 sites at the edges have a significantly disrupted charge distribution, forming electron-rich Ni centers with enhanced adsorption ability with *COOH, thereby boosting CO2RR efficiency. Furthermore, a Zn-CO2 battery using the Ni/H-OMC cathode shows an unprecedentedly high power density of 15.9mW cm-2 and maintains an exceptionally stable charge-discharge performance over 100h.

Regulating the 3d-orbital occupancy on Ni sites enables high-rate and durable Ni(OH)2 cathode for alkaline Zn batteries

The capacity and cycling stability of β-Ni(OH)2-based cathodes in aqueous alkaline Ni-Zn batteries are still unsatisfactory due to their undesirable OH− adsorption/desorption dynamics during the electrochemical redox process. To settle this issue, we introduce a new atomic-level strategy to finely modulate the OH− adsorption/desorption of β-Ni(OH)2 through tailoring the 3d-orbital occupancy of Ni center by Co/Cu co-doping (denoted as Co-Cu-Ni(OH)2). Both experimental outcomes and density functional theory calculations validate that the co-doping of Co and Cu endows the Ni species in Co-Cu-Ni(OH)2 with appropriate proportion of the unoccupied 3d-orbital, leading to optimized adsorption/desorption strength of OH−. As anticipated, the Co-Cu-Ni(OH)2 electrode demonstrates superior performance, achieving an areal capacity of 0.83 mAh cm−2 and a gravimetric capacity of 164.3 mAh g−1 at ∼50 mA cm−2 (10 A g−1). Furthermore, it sustains an impressive capacity of 170.8 mAh g−1 (2.3 mAh cm−2) at a high mass loading of 13.5 mg cm−2, alongside a long-term cycling performance over 1000 cycles. The assembled Co-Cu-Ni(OH)2//Zn cell is able to provide a peak energy density of 0.98 mWh cm−2 and excellent durability. This work highlights the potential of an orbital engineering strategy in the development of next-generation high-capacity and durable energy storage materials.

Square-Planar Nickel Bis(phosphinopyridyl) Complexes for Long-Lived Photocatalytic Hydrogen Evolution.

Phosphinopyridyl ligands are used to synthesize a class of Ni(II) bis(chelate) complexes, which have been comprehensively characterized in both solid and solution phases. The structures display a square-planar configuration within the primary coordination sphere, with axially positioned labile binding sites. Their electrochemical data reveal two redox couples during the reduction process, suggesting the possibility of accessing two-electron reduction states. Significantly, these complexes serve as robust catalysts for homogeneous photocatalytic H2 evolution. In a system utilizing an organic photosensitizer and a sacrificial electron donor, an optimal turnover number of 27,100 is achieved in an alcohol-containing aqueous solution. A series of photophysical and electrochemical measurements were conducted to elucidate the reaction mechanism of photocatalytic hydrogen generation. Density function theory calculations propose a catalytic pathway involving two successive one-electron reduction steps, followed by two proton discharges. The sustained photocatalytic activity of these complexes stems from their distinct ligand system, which includes phosphine and pyridine donors that aid in stabilizing the low oxidation states of the Ni center.

Electrochemically Created Active Centers in a Bimetallic CoNi-Triazole Metal-Organic Framework for Enhanced Oxygen Evolution Reaction Activity.

Electrochemical water oxidation utilizing bimetallic CoNi-Tz (Tz=1,2,4-triazole) framework is explored. Initially, CoNi-Tz possesses active tetrahedral Co center and electron-mediated octahedral Ni chain. After performing an electrochemical activation, the partial structural transformation on the Ni center occurs. This leads to the generation of excessive active centers which can promote catalytic activity of the framework. The activated CoNi-Tz catalyst displays a remarkably low OER overpotential of 293 mV at a current density of 10 mA cm-2 with a small Tafel slope of 49.98 mV dec-1, outperforming the single metal Co-Tz and benchmark IrO2 catalysts.

Hemilabile and Redox-Active Quinone Ligands Unlock sp3-Rich Couplings in Nickel-Catalyzed Olefin Carbosulfenylation.

A three-component coupling approach toward structurally complex dialkylsulfides is described via the nickel-catalyzed 1,2-carbosulfenylation of unactivated alkenes with organoboron nucleophiles and alkylsulfenamide (N-S) electrophiles. Efficient catalytic turnover is facilitated using a tailored N-S electrophile containing an N-methyl methanesulfonamide leaving group, allowing catalyst loadings as low as 1 mol %. Regioselectivity is controlled by a collection of monodentate, weakly coordinating native directing groups, including sulfonamides, amides, sulfinamides, phosphoramides, and carbamates. Key to the development of this transformation is the identification of quinones as a family of hemilabile and redox-active ligands that tune the steric and electronic properties of the metal throughout the catalytic cycle. Density functional theory (DFT) results show that the duroquinone (DQ) ligand adopts different coordination modes in different stages of the Ni-catalyzed 1,2-carbosulfenylation-binding as an η6 capping ligand to stabilize the precatalyst/resting state and prevent catalyst decomposition, binding as an X-type redox-active durosemiquinone radical anion to promote alkene migratory insertion with a less distorted square planar Ni(II) center, and binding as an L-type ligand to promote N-S oxidative addition at a relatively more electron-rich Ni(I) center.

Hemilabile and Redox‐Active Quinone Ligands Unlock sp3‐Rich Couplings in Nickel‐Catalyzed Olefin Carbosulfenylation

A three‐component coupling approach toward structurally complex dialkylsulfides is described via the nickel‐catalyzed 1,2‐carbosulfenylation of unactivated alkenes with organoboron nucleophiles and alkylsulfenamide (N–S) electrophiles. Efficient catalytic turnover is facilitated using a tailored N–S electrophile containing an N‐methyl methanesulfonamide leaving group, allowing catalyst loadings as low as 1 mol%. Regioselectivity is controlled by a collection of monodentate, weakly coordinating native directing groups, including sulfonamides, amides, sulfinamides, phosphoramides, and carbamates. Key to the development of this transformation is the identification of quinones as a family of hemilabile and redox‐active ligands that tune the steric and electron properties of the metal throughout the catalytic cycle. DFT calculations show that the duroquinone (DQ) ligand adopts different coordination modes in different stages of the Ni‐catalyzed 1,2‐carbosulfenylation—binding as an η6 capping ligand to stabilize the precatalyst/resting state and prevent catalyst decomposition, binding as an X‐type redox‐active durosemiquinone radical anion to promote alkene migratory insertion with a less distorted square planar Ni(II) center, while binding as an η1 L‐type ligand to promote N–S oxidative addition at a relatively more electron‐rich and sterically less crowded Ni(I) center.

Helical Ni‐Cluster Organic Framework Constructed from Enantiopure Lactate Derivative: Synthesis, Structure and Properties

AbstractUnder hydrothermal condition, lactate derivative (R)‐4‐(1‐ carboxyethoxy)‐2‐methylbenzoic acid ((R)‐H2cma) as chiral synthon assembled with 4,4′‐di(1H‐imidazol‐1‐yl)‐1,1′‐biphenyl (4,4’‐dib) and Ni(II) ions to obtain complex {[Ni2((R)‐cma)2(4,4’‐dib)2(H2O)] ⋅ 2H2O}n (HU20‐R). In complex HU20‐R, carboxyl group from lactate unit of (R)‐cma2− anion and coordinating H2O molecule adopt a μ2‐coordination mode to obtain dinuclear [Ni2(CO2)2(H2O)]2+ cluster. (R)‐cma2− anions and Ni(II) ions build single left‐handed (R)‐cma‐Ni‐chain, while 4,4’‐dib ligands are bridged by different Ni(II) centers to form double left‐handed 4,4’‐dib‐Ni‐chain. These helical secondary structures are further connected together to result in an 8‐connected 3D framework. PXRD tests indicated that complex HU20‐R has well hydro‐stability. UV‐vis absorption experiment confirmed that HU20‐R is a semi‐conducting material with strong absorption capacity for ultraviolet‐visible (UV‐vis) light. Additionally, electrochemical experiment revealed that HU20‐R has low impedance and high charge transport capacity. Further test results confirm that HU20‐R has noticeable photo‐catalytic effect in photodegradation of Rhodamine B (RhB) and antiferromagnetic property.

Hydrosilylation of Dihydrosilylium Ion Stabilized by Coordination of a σ‐Donating Ni(0) Ligand

AbstractThe hydrosilylation reactions of dihydrosilylium ion 2, stabilized by coordination of a σ‐donating Ni(0) fragment, has been investigated. Complex 2 with two reactive sites, dihydrosilylium and Ni(0) centers, readily reacts with diphenylacetylene via a selective mono‐hydrosilylation reaction to afford the corresponding Ni(0)‐stabilized (hydro)(vinyl)silylium ion 6. In the case of ethylene, three equivalents of olefin are consumed to give a cationic Ni(II)‐complex 7 featuring a Bu‐Si+‐NiII‐Et moiety with a NHC‐supported Si atom. DFT calculations indicate that the hydrosilylation proceeds by a classical (Chalk‐Harrod type) mechanism with the assistance of NHC ligand moving between Si and Ni centers according to their stabilization requirements.

Electronic and Molecular Structures of a Series of Nickel Bis-1,1-Dithiolates.

A series of homoleptic Ni bis-1,1-dithiolates, [Ni(S2C2RR')2]2- (R=CN, R'=CN, CO2Et, CONH2, Ph, Ph-4-Cl, Ph-4-OMe, Ph-4-NO2, Ph-3-CF3, Ph-4-CF3, Ph-4-CN; R=NO2, R'=H; R=R'=CO2Et) have been synthesized from the reaction of the alkali metal salt of the ligand and nickel chloride, and isolated as tetraphenylphosphonium or tetrabutylammonium salts. The complexes were characterized by X-ray crystallography, high-resolution mass spectrometry, and infrared (IR), nuclear magnetic resonance (NMR) and electronic absorption spectroscopies. The molecular structures show a rigidly square planar Ni(II) center linking two four-membered chelate rings whose dimensions are constant across the series. The electronic effect of the ligand substituent is revealed in the 13C NMR and electronic spectra, and corroborated by density functional calculations. Electron withdrawing groups deshield the low-field CS2 resonance, and the signature charge transfer band in the visible region is red-shifted. These observables have been accurately reproduced computationally, and revealed the Ni contribution to the ground state diminishes with decreasing electron withdrawing capacity of the ligand substituent. In contrast to 1,2-dithiolates, the redox inactivity afforded by 1,1-dithiolates stems from the smaller chelate ring and substantially reduced sulfur content that is key to stabilizing the radical form.

Enhanced activity in industrial hydrogen evolution reaction on NiS/NiCoP by sulfur vacancy coupled with transition metal doping

Ni-based chalcogenide catalysts show the weak H2O molecule adsorption and sluggish kinetics, restricting their industrial hydrogen evolution reaction (HER) in a wide pH range. Herein, a controllable fabrication strategy of NiS/NiCoP nanoparticles by simultaneously constructing sulfur vacancies and introducing the heterometallic atoms (Zn, Fe, Mn) (M-NiCoSP-Sv) is proposed. The numerous uncoordinated surface Ni centers generated by sulfur vacancies are beneficial for cleaving the HO–H bond of H2O molecule, while S sites at Sv-NiS side and P sites at Zn-NiCoP side with rich electron aggregation are used as H2-evolution centers, synergistically facilitating HER kinetics over a wide pH range. As respected, Zn-NiCoSP-Sv exhibits excellent durability and outstanding HER activity such as 41, 78 and 34 mV of 10 mA cm–2 in alkaline, neutral and acidic electrolytes, respectively. And at the industrial current density, Zn-NiCoSP-Sv also possesses outstanding HER activity and stability. This work reveals a strategy for activating NiS-based catalyst to cleave H2O and optimize Gibbs free energy of H* by vacancy and doping co-engineering.

Metal-ligand dual-site single-atom nanozyme mimicking urate oxidase with high substrates specificity

In nature, coenzyme-independent oxidases have evolved in selective catalysis using isolated substrate-binding pockets. Single-atom nanozymes (SAzymes), an emerging type of non-protein artificial enzymes, are promising to simulate enzyme active centers, but owing to the lack of recognition sites, realizing substrate specificity is a formidable task. Here we report a metal-ligand dual-site SAzyme (Ni-DAB) that exhibited selectivity in uric acid (UA) oxidation. Ni-DAB mimics the dual-site catalytic mechanism of urate oxidase, in which the Ni metal center and the C atom in the ligand serve as the specific UA and O2 binding sites, respectively, characterized by synchrotron soft X-ray absorption spectroscopy, in situ near ambient pressure X-ray photoelectron spectroscopy, and isotope labeling. The theoretical calculations reveal the high catalytic specificity is derived from not only the delicate interaction between UA and the Ni center but also the complementary oxygen reduction at the beta C site in the ligand. As a potential application, a Ni-DAB-based biofuel cell using human urine is constructed. This work unlocks an approach of enzyme-like isolated dual sites in boosting the selectivity of non-protein artificial enzymes.

Theoretical and experimental demonstration of mononuclear Ni(II) and dinuclear Ni(III) complexes concerning their catalytic activity, DNA/ protein binding efficacy

The successful use of metal complexes in enzyme catalysis and biomedical applications boosts researchers to develop more and more metal-based compounds to inspect their said applications.In the present work one novel mononuclear Ni(II) [Ni(L1)2] (1) and one di nuclear Ni(III) complexes [Ni(L2)2(N3)2] (2) have been developed by utilizing N2O donor Schiff base ligand HL1 and N2O2 donor Schiff base ligand H2L2 respectively. The single crystal data analysis reveals that in complex 1 two deprotonated ligands coordinate to the Ni(II) forming a distorted octahedral geometry. The asymmetric unit of complex 2 comprises one deprotonated ligand (both the protons of phenolic –OH group were deprotonated), one azide anion, and one Ni(III). Two Ni(III) centers are connected via phenoxide bridging. Followed by the structural analysis the UV spectroscopic study is performed to understand the catecholase-like activity of the developed complexes by using 3,5-di-tertbutyl catechol (3,5-DTBC). The calculated turnover numbers (Kcat) for both complexes disclose the fact that complex2 is more susceptible to this catalytic process than 1. During the catalysis process, the production of Ni(II) in the case of complex2is favorable and this is the main factor to show its higher susceptibilitytowards the catalytic process. The biomedical applicability in terms of anticancer and antibacterial of complexes 1 and 2 is assessed by evaluating their interaction ability with DNA and HSA with the help of several spectroscopic approaches. The remarkably high complex-macromolecules binding constant values, obtained from electronic titration approve the binding efficacy of the target complexes (order ∼ 105). But if a tiny comparison is done then it is seen that complex 2 shows better DNA and HSA binding efficacy. The theoretical approach using molecular docking study fully validates the experimental findings.

Aroylhydrazone‐based nickel(II) complexes: Synthesis and their structural, density functional theory (DFT), biological and catalytic studies

Nickel(II) complexes of tridentate NNO donor aroylhydrazones, viz., di‐2‐pyridyl ketone‐4‐methoxybenzhydrazone monohydrate (DKMBH·H2O) and 2‐acetylpyrazine‐4‐methoxybenzhydrazone monohydrate (APMBH·H2O), were synthesized and characterized using various methods, including spectral techniques (Fourier transform infrared [FT‐IR], UV–vis) and analytical methods (elemental and X‐ray diffraction [XRD] analysis). Single‐crystal XRD was used to elucidate the crystal structures of nickel(II) complexes 1 and 2. The XRD analysis revealed that the respective ligands coordinated to the Ni(II) center through the deprotonated iminolate form in complex 1 and the neutral amido form in complex 2, resulting in distorted octahedral geometries. The ground state electronic configurations of the complexes were studied using the B3LYP/UB3LYP levels of DFT. Using absorption spectral titration methods, the interactions of complexes with CT‐DNA and bovine serum albumin (BSA) protein were examined. The observed data demonstrate that the complexes adopt an intercalative binding mode to bind to CT‐DNA and BSA protein. Additionally, the antioxidant activity of the nickel(II) complexes and their ligands was determined using 1,1‐diphenyl‐2‐picrylhydrazyl (DPPH). Complex 2 demonstrated the highest radical scavenging activity of the compounds studied (IC50 = 3.81 mM). Furthermore, molecular docking studies were performed to understand how the complexes can interact with the SARS‐CoV‐2 main protease. The results revealed that complex 2 had the highest docking score (−8.18 kcal/mol) when compared with the other complexes under study. Aside from their biological properties, the nickel complexes heterogenized on functionalized SBA‐15 showed promising catalytic activity, achieving 98% yield in the reduction of nitrobenzene, forming exclusively aniline as the end product.