A nickel(II) di-μ2-phenolato bridged dinuclear complex: Weak antiferromagnetic interactions in nickel(II) dimers

Improving the potential of paraCEST through magnetic-coupling induced line sharpening

Magnetic coupling between paramagnetic centers is a crucial phenomenon in the design of efficient MRI contrast agents. In this study, we investigate the paraCEST properties and magnetic coupling effects of a novel homodinuclear Ni(ii) complex, 1, containing a Robson type macrocyclic ligand. A thorough analysis of the complex's electronic and magnetic properties revealed that the magnetic coupling effect reduces the transverse relaxation rate and enhances the sharpness of the proton resonances, leading to enhanced CEST efficiency. This novel mechanism, which we coined "magnetic-coupling induced line sharpening" (MILS), can be crucial for optimizing the performance of paramagnetic metal complexes in paraCEST imaging. Moreover, magnetic coupling plays a critical role in the relaxation properties of homodinuclear complexes. Our study not only paves the way for the creation of advanced paraCEST agents with enhanced CEST capabilities and sensitivity but also provides valuable guidance for the design of other MRI contrast agents utilizing dinuclear metal complexes.

Theoretical study on the inhibition mechanisms of heavy metal ions on urease activity

Soil urease is highly sensitive to soil heavy metal pollution, and thus its activity can be used as bio-indicator of soil health. However, little is known about the inhibition mechanisms of heavy metals on urease. The effects of dimetallic substitution (i.e., Cd, Co, Cu, Hg, and Zn) on the binding of urea in the urease and its subsequent decomposition were studied using quantum chemical methodologies with a urease mimic (phthalazine-dinickel complex). The dimetallic substitution altered the structural features of the dimetal complexes and the M-O bond length between the dimetals and the carbonyl-O of coordinated urea molecules, weakening the binding energies of urea in dimetal complexes, which further affected the transformation of urea. In the urea decomposition via intra-molecular proton transfer, all dimetal complexes have a high activation barrier due to the weak binding of urea in complexes and hydrogen bonding within urea molecules, which are therefore difficult to occur spontaneously. In the urea decomposition via water-assisted inter-molecular proton transfer, the addition of water molecules decreased the energy barrier of urea decomposition. Regardless of the urea decomposition pathway, the dimetallic substitution altered the M-O bond length and hydrogen bond pattern of intermediates and transition states, and also affected the leave of the resulting NH3 from the dimetal complexes by regulating the C-N bond length within the decomposed urea molecule. Overall, the theoretical study provided insight into the molecular mechanisms of the inhibitory effects of heavy metals on urease activity.

An unexpected single crystal structure of nickel(II) complex: Spectral, DFT, NLO, magnetic and molecular docking studies

This work explores the study of a synthesized nickel complex as a possible inhibitor against the main protease (Mpro) of the recent emerging coronavirus disease (COVID-19). Overall, the template reaction of 3-acetyl-2-hydroxy-6-methyl-4H-pyran-4-one with nickel(II) chloride hexahydrate in N,N-dimethylformamide (DMF) medium leads to the formation of neutral nickel complex. This resulting complex is formulated as [Ni(DHA)₂(DMF)₂] on the basis of FT-IR, UV-Vis., single-crystal X-ray diffraction analysis, magnetic susceptibility and CV measurements as well as DFT quantum chemical calculations. Its single crystal suggests was found to be surrounded by the both pairs of molecules of DHA and DMF through six oxygen atoms with octahedral coordination sphere. The obtained magnetic susceptibilities are positive and agree with its paramagnetic state. In addition to the experimental investigations, optimized geometry, spectroscopic and electronic properties were also performed using DFT calculation with B3LYP/6-31G(d,p) level of theory. The nonlinear optical (NLO) properties of this complex are again examined. Some suitable quantum descriptors (E_HOMO, E_LUMO, Energy gap, Global hardness), Milliken atomic charge, Electrophilic potion and Molecular Electrostatic Potential) have been elegantly described. Molecular docking results demonstrated that the docked nickel complex displayed remarkable binding energy with Mpro. Besides, important molecular properties and ADME pharmacokinetic profiles of possible Mpro inhibitors were assessed by in silico prediction.

Insight into non-covalent interactions in two triamine-based mononuclear iron(iii) Schiff base complexes with special emphasis on the formation of Br⋯π halogen bonding

The nature and characteristics of noncovalent interactions that play important roles in the crystal packing of two iron(iii) complexes have been discussed.

Phenoxido mediated antiferromagnetic and azide mediated ferromagnetic coupling in two dinuclear ferromagnetic nickel(ii) complexes with isomeric Schiff bases: a theoretical insight on the pathway of magnetic interaction

Two relatively rare mixed phenoxo and azide bridged dinuclear nickel(ii) complexes have been discussed. Both complexes show strong intermolecular hydrogen bonding interactions and exhibit ferromagnetic exchange coupling.

Crystal structure and Hirshfeld surface analysis of 2,2'-{(1E,1'E)-[ethane-1,2-diylbis(aza-nylyl-idene)]bis-(methanylyl-idene)}bis-[4-(tri-fluoro-meth-oxy)phenol]copper(II) hydro-quinone hemisolvate

In the title com-plex, [Cu(C18H12F6N2O4)]·0.5C6H6O2, the CuII ion has a square-planar coordination geometry, being ligated by two N and two O atoms of the tetra-dentate open-chain Schiff base ligand 6,6'-{(1E,1'E)-[ethane-1,2-diylbis(aza-nylyl-idene)]bis-(methanylyl-idene)}bis-[2-(tri-fluoro-meth-oxy)phenol]. The crystal packing is stabilized by intra-molecular O-H⋯O and inter-molecular C-H⋯F, C-H⋯O and C-H⋯π hydrogen bonds. In addition, weak π-π inter-actions form a three-dimensional structure. Hirshfeld surface analysis and two-dimensional fingerprint plots were performed and created to analyze the inter-molecular inter-actions present in the crystal, indicating that the most important contributions for the crystal packing are from F⋯H/H⋯F (25.7%), H⋯H (23.5%) and C⋯H/H⋯C (12.6%) inter-actions.

Synthesis, Magnetic Properties, and Catalytic Properties of a Nickel(II)-Dependent Biomimetic of Metallohydrolases

A dinickel(II) complex of the ligand 1,3-bis(bis(pyridin-2-ylmethyl)amino)propan-2-ol (HL1) has been prepared and characterized to generate a functional model for nickel(II) phosphoesterase enzymes. The complex, [Ni2(L1)(μ-OAc)(H2O)2](ClO4)2·H2O, was characterized by microanalysis, X-ray crystallography, UV-visible, and IR absorption spectroscopy and solid state magnetic susceptibility measurements. Susceptibility studies show that the complex is antiferromagnetically coupled with the best fit parameters J = -27.4 cm-1, g = 2.29, D = 28.4 cm-1, comparable to corresponding values measured for the analogous dicobalt(II) complex [Co2(L1)(μ-OAc)](ClO4)2·0.5 H2O (J = -14.9 cm-1 and g = 2.16). Catalytic measurements with the diNi(II) complex using the substrate bis(2,4-dinitrophenyl)phosphate (BDNPP) demonstrated activity toward hydrolysis of the phosphoester substrate with K m ~10 mM, and k cat ~0.025 s-1. The combination of structural and catalytic studies suggests that the likely mechanism involves a nucleophilic attack on the substrate by a terminal nucleophilic hydroxido moiety.

Stabilization of Ni2+ dimers in hexacyano Mo6 cluster-based Prussian blue derivatives: experimental and theoretical investigations of magnetic properties

Herein, two new octahedral molybdenum cyanide cluster compounds, namely [{Ni(NH₃)₆}₄{Ni₂(NH₃)₈}₁][Mo₆Br₆Q₂(CN)₆]₃·12H₂O, Q = S (1) and Se (2), have been synthesized as single crystals by slow diffusion of a solution of nickel chloride into aqueous ammonia solutions of a K₂Cs₂[Mo₆Br₆Q₂(CN)₆] molybdenum cyanide cluster-based compound. Both 1 and 2 were structurally characterized by single-crystal X-ray diffraction. They are isostructural and crystallize in the cubic system (Im3[combining macron]m (no. 229); Z = 2, a = 18.147(1) Å, and V = 5976(1) ų and a = 18.188(2) Å and V = 6016(2) ų for 1 and 2, respectively). 1 and 2 are based on the association of [Mo₆Brⁱ₆Qⁱ₂(CN)^a₆]^4- (Q = S, Se) cluster anions with Ni²⁺ dimer-based cubic [Ni₂(NH₃)₈]⁴⁺ and octahedral [Ni(NH₃)₆]²⁺ cations. The structure is based on 2-fold interpenetrated [{Ni(NH₃)₆}₄{Ni₂(NH₃)₈}₁][Mo₆Br₆Q₂(CN)₆]₃ frameworks related to each other by [½, ½, ½] translation. The unit cell is based on a body-centered cubic framework of cubic [Ni₂(NH₃)₈]⁴⁺. The [Mo₆Brⁱ₆Qⁱ₂(CN)^a₆]^4- (Q = S, Se) cluster units are located in the middle of the edges and at the center of the faces of the cell. The [{Ni(NH₃)₆}]²⁺ cations are located at the center of the cubes of the a/2 edge. The dimers [Ni₂(NH₃)₈]⁴⁺ are stabilized by hydrogen bonds between the cyanide ligands of the cluster unit and the hydrogen atoms of the ammonia molecules. Both compounds exhibit a weak antiferromagnetic coupling within the [Ni₂(NH₃)₈]⁴⁺ dimer entities at low temperatures together with a paramagnetic behavior originating from the cations of the octahedral [{Ni(NH₃)₆}]²⁺ complexes.

Hydrogen Evolution by FeIII Molecular Electrocatalysts Interconverting between Mono and Di-Nuclear Structures in Aqueous Phase

A new FeL/Fe₂ L₂ manifold, with HL=2-({[di(2-pyridyl)methyl](methyl)amino}methyl)phenol, was prepared in gram scale (>50 % yield) and characterized in solution and solid state. The monomer/dimer interconversion is controlled in aqueous phase, upon varying the pH conditions. The electrocatalytic hydrogen evolution reaction (HER) occurs through the FeL monomer with added trifluoroacetic acid (TFA) and through the Fe₂ L₂ μ-oxo dimer in acetate buffer (pH 4.9), with an overpotential of about 1 V and faradaic yield up to 75 %. The resulting i_cat /i_p values in the range 15-28 are among the highest reported for Fe-based electrocatalysts (i_cat is the catalytic current, whereas i_p is the current of an Fe-based redox event).

Isolation of a hexanuclear chromium cluster with a tetrahedral hydridic core and its catalytic behavior for ethylene oligomerization

A chromium complex [2-(NHCH2PPh2)C5H4N]CrCl3·THF2 (1) of the ligand PyNHCH2PPh2 has been synthesized, characterized, and examined for its catalytic behavior toward ethylene oligomerization. When complex 1 was treated with (i-Bu)3Al, an unprecedented divalent polyhydride chromium cluster μ,κ(1),κ(2),κ(3)-N,N,P-{[2-(NCH2PPh2)C5H4N]Cr(μ-H)}4[(μ-Cl)Cr(μ-Cl)Al(i-Bu)2Cl]2 (2) was obtained. The complex contains a Cr4H4 core, which is expected to be diamagnetic, and which remains coordinated to two additional divalent high-spin Cr atoms via bridging interactions. Two aluminate residues remain bonded to the peripheral chromium atoms. The structure, magnetism, and electronic configuration are herein discussed.

Limiting nuclearity in formation of polynuclear metal complexes through [2 + 3] cycloaddition: synthesis and magnetic properties of tri- and pentanuclear metal complexes

The cycloaddition reaction between [Ni(L)(N₃)] and different organonitriles afforded tetrazolate bridged tri- and pentanuclear nickel(ii) complexes.

Binuclear ruthenium complexes of a neutral radical bridging ligand. A new "spin" on mixed valency

The electronic structures of (LX)2Ru(Vd)Ru(LX)2 complexes (Vd = 1,5-diisopropyl-3-(4,6-dimethyl-2-pyrimidinyl)-6-oxoverdazyl radical; LX = acac (acetylacetonate) or hfac (hexafluoroacetylacetonate)) in multiple charge states have been investigated experimentally and computationally. The main focus was to probe the consequences of the interplay between the ruthenium ions and the redox-active verdazyl ligand for possible mixed-valent behavior. Cyclic voltammetry studies reveal one reversible reduction and one reversible oxidation process for both complexes; in addition the acac-based derivative possesses a second reversible oxidation. Analysis of a collection of experimental (X-ray structures, EPR, electronic spectra) and computational (TD-DFT (PCM)) data reveal that the ruthenium ancillary ligands (acac vs hfac) have dramatic consequences for the electronic structures of the complexes in all charge states studied. In the hfac series, the neutral complex is best regarded as a binuclear Ru(II) species bridged by a neutral radical ligand. Reduction to give the anionic complex takes place on the verdazyl ligand, whereas oxidation to the cation (a closed shell species) is shared between Vd and ruthenium. For the acac-based complexes, the neutral species is most accurately represented as a Ru(II)/Ru(III) mixed valent complex containing a bridging verdazyl anion, though some bis(Ru(II))-neutral radical character remains. The monocation complex contains a significant contribution from a "broken symmetry" singlet diradical structure, best represented as a bis-Ru(III) system with an anionic ligand, with significant spin coupling of the two Ru(III) centers via the Vd(-1) ligand (calculated J = -218 cm(-1)). The dication, a spin doublet, consists of two Ru(III) ions linked (and antiferromagnetically coupled) to the neutral radical ligand. Computed net σ- and π-back-donation, spin densities, and orbital populations are provided. Time dependent DFT is used to predict the optical spectra and assign experimental data.

The importance of an additional water bridge in making the exchange coupling of bis(μ-phenoxo) dinickel(II) complexes ferromagnetic

Two new nickel(II) complexes [Ni(2)L(2)(PhCOO)(2)(H(2)O)] (1), [Ni(2)L(2)(PhCH(2)COO)(2)(H(2)O)] (2) have been synthesized using a tridentate Schiff base ligand, HL (2-[(3-dimethylamino-propylimino)-methyl]-phenol) and the carboxylate monoanions, benzoate and phenylacetate, respectively. The complexes have been characterized by spectral analysis, variable temperature magnetic susceptibility measurement and crystal structure analysis. The structural analyses reveal that both complexes are dinuclear in which the distorted octahedral Ni(2+) ions share a face, bridged by one water molecule and two μ(2)-phenoxo oxygen atoms. A monodentate benzoate or phenylacetate anion and two nitrogen atoms of the chelating deprotonated Schiff base (L) complete the hexa-coordination around the metal ion. Variable-temperature magnetic susceptibility studies indicate the presence of dominant ferromagnetic exchange coupling in complexes 1 and 2 with J values of 11.1(2) and 10.9(2) cm(-1) respectively. An attempt has been made to rationalize the observed magneto-structural behavior considering the importance of the additional water bridge in the present two complexes and also in other similar species.

Binuclear copper(II) complexes with N4O3 coordinating heptadentate ligand: synthesis, structure, magnetic properties, density-functional theory study, and catecholase activity

The N4O3 coordinating heptadentate ligand afforded binuclear complex [Cu 2(H 2L)(mu-OH)](ClO4)2 (1) and [Cu2(L)(H2O)2]PF6 (2). In complex 1, two copper ions are held together by mu-phenoxo and mu-hydroxo bridges, whereas in complex 2, the copper centers are connected only by a mu-phenoxo bridge. In 1, both the Cu(II) centers have square pyramidal geometry (tau=0.01-0.205), whereas in the case of 2, one Cu(II) center has square pyramidal (tau=0.2517) and other one has square based pyramidal distorted trigonal bipyramidal (tau=0.54) geometry. Complexes 1 and 2 show an strong intramolecular and very weak antiferromagnetic interaction, respectively. Density-functional theory calculations were performed to establish the magneto structural correlation between the two paramagnetic copper(II) centers. Both of the complexes display a couple of one-electron reductive responses near -0.80 and -1.10 V. The complexes show significant catalytic activity at pH 8.5 on the oxidation of 3,5-di- tert-butylcatechol (3,5-DTBC) to 3,5-di- tert-butylquinone (3,5-DTBQ), and the activity measured in terms of kcat=29-37 h(-1).