Six-coordinate Metal Research Articles

Development of a Fluctuating Charge Model for Zinc-Containing Metalloproteins.

Metalloproteins widely exist in biology and play important roles in various processes. To accurately simulate metalloprotein systems, modeling polarization and charge transfer effects is vital. The fluctuating charge (FQ) model can efficiently generate atomic charges and simulate the charge transfer effect; it has been developed for a wide range of applications, but few models have been specifically tailored for metalloproteins. In this study, we present a fluctuating charge model specifically for zinc-containing metalloproteins based on the extended charge equilibration (EQeq) scheme. Our model was parametrized to reproduce CM5 charges instead of RESP/CHELPG charges because the former is less dependent on the conformation or basis set, does not suffer from unphysical charges for buried atoms, and is still being able to well reproduce the molecular dipoles. During our study, we found that adding the Pauling-bond-order-like term (referred to as the "+C term" in a previous study) between the zinc ion and ligating atoms significantly improves the model's performance. Although our model was trained for four-coordinated zinc sites only, our results indicated it can well describe the atomic charges in diverse zinc sites. Morever, our model was used to generate partial charges for the metal sites in three different zinc-containing metalloproteins (with four-, five-, and six-coordinated metal sites, respectively). These charges exhibited performance comparable to that of the RESP charges in molecular dynamics (MD) simulations. Additional tests indicated our model could also well reproduce the CM5 charges when geometric changes were involved. Those results indicate that our model can efficiently calculate the atomic charges for metal sites and well simulate the charge transfer effect, which marks an important step toward developing versatile polarizable force fields for metalloproteins.

Front Cover: Heterobridged Hexa‐ and Octanuclear Ruthenium‐Thiolate/Halide Wheels: Synthesis, X‐ray Crystal Structure, and Spectroscopic Characterization (Eur. J. Inorg. Chem. 33/2022)

The Front Cover shows the structure of an octanuclear heterobridged molecular wheel featuring six-coordinate ruthenium ions, terminal CO ligands, and thiolate/iodide mixed bridging ligands. This wheel represents one of the crystallographically characterized {Run(μ-SR)n(μ-X)n}(CO)2n (X=Cl, Br, n=6; X=I, n=8) wheels composed of edge-shared RuC2S2X2 (X=halide) octahedra. The present work demonstrates the construction of hexa- and octanuclear wheels which feature fused Mn(μ-SR)n/Mn(μ-X)n (n=6, 8) metallamacrocycles as well as six-coordinate metal molecular wheels with their metal ions linked by mixed S/X-donor ligands. More information can be found in the Research Article by J.-S. Huang, C.-M. Che and co-workers.

Two substituted tetrazole-based isomers of [M2(PPT)(OH)(SO4)(H2O)]n (M = Zn, Cd): The in-situ syntheses and characterization

Two new isotypic substituted tetrazolates of d10 metals, [M2(PPT)(OH)(SO4)(H2O)]n, (M = Zn, 1 and Cd, 2) (PPT = 5-(4-pyridylphenyl)tetrazole) have been in-situ synthesized using the metal sulfates, 4-(4-pyridyl)benzonitrile (PBN) and sodium azide under mild solvothermal conditions. The prepared materials have been characterized by single crystal X-ray diffraction, powder XRD, elemental analyses, IR & Raman spectroscopy, SEM-EDS, solid state fluorescence, and TG analyses. The PPT ligand in both compounds is generated via the in situ [2 + 3] cycloaddition in the reaction of the cyano groups of the organonitrile and sodium azide. Single crystal X-ray diffraction reveals both materials possess 3D framework of six-coordinated metal polyhedrons that are further connected by the PPT linkers. The BV calculation confirms the existence of extremely long ZnO bond distance of 2.663 Å in compound 1. In addition, the solid-state photoluminescence indicates the cadmium-based material has a more intensive photoluminescent emission than the zinc one upon excitation at room temperature.

Thermal steam reduction etching to construct POM@CuICuII-BTC with hierarchical porosity for adsorption property enhancement

Host MOFs with atomically precise characteristics and functional-oriented guest molecules are the key to assemble multifunctional host–guest MOF composites. We selected O−rich POM tungstophosphoric acid (TPA) and CuII-BTC with high stability and adjustable structure to synthesize host–guest TPA@CuII-BTC. Then thermal steam reduction etching was used to construct TPA@CuICuII-BTC with hierarchical pores at molecular scale, enlarging the pore space and making partially inaccessible sites of TPA@CuII-BTC more expose. Under the thermal methanol atmosphere, partial six-coordinated metal node Cu2+ was reduced slowly into Cu+ with lower coordination number, accompanied by the fracture of partial Cu−O coordination bonds. The generated TPA@CuICuII-BTC not only obtained larger pore and exposed more space in contrast to TPA@CuII-BTC, but also possessed more free hydroxyl oxygen sites due to the breaking of partial coordination bonds, promoting the contact and interaction of the composite with external substances. The Pb2+ adsorption experiment showed that TPA@CuICuII-BTC exhibited best adsorption ability to external Pb2+, and the adsorption capacity was 614.95 mg·g−1, which was 180.25 mg·g−1 and 243.45 mg·g−1 higher than that of TPA@CuII-BTC and CuII-BTC. This work has great reference significance for the construction of MOF composites with hierarchical porosity and the study of MOF composites on adsorption performance improvement.

Crystal and Molecular Structures of 2,2′-bipyridine-5,5′-dicarboxylate Complexes: [M(II)(O2CC10H6N2CO2)(H2O)4] (M = Co, Ni)

The reaction of 2,2′-bipyridine-5,5′-dicarboxylic acid (HO2CC12H6N2CO2H, H2bpydicarb) with the appropriate metal salt in water provided the mononuclear complexes [M(II)(bpydicarb)(H2O)4]·2H2O where M(II) = Co and Ni. The isomorphous materials exhibit neutral six-coordinate metal–ligand complexes with MN2O4 geometry. Charge balance is achieved through deprotonation of the carboxylate groups. The Co compound crystallizes in the monoclinic space group C2/c with a = 12.696(4) A, b = 9.195(4) A, c = 13.207(5) A, β = 93.78(1)°; the isomorphous Ni compound exhibits cell parameters a = 12.477(4) A, b = 9.818(3) A, c = 12.658(4) A, β = 92.099(6)°. The six coordinate structure of [Co(II)(bpydicarb)(H2O)4] achieves charge balance through deprotonation of the carboxylate residues of the bipyridine-based ligand.

The magnetism change by Jahn-Teller distortionin octahedral hexa-coordinate complex

Metal complexes exhibit significant physical properties such as electrical conductivity, magnetism and optical properties. They can be controlled by changing the ligand field splitting of the d-orbital of the central metal ion. We have investigated the reversible spin crossover (SCO) phenomenon by changing the ligand field splitting in six-coordinated metal complexes ML6. In this study, we focused on the change of the spin states by uniaxial Jahn-Teller distortion in the case of an octahedral hexa-coordinated metal complexes ML6, involving two types of different ligand molecules. In addition, we have used DV-Xα and DVME methods in order to calculate the electronic states. In order to reproduce the effect of the Jahn-Teller distortion in the calculation, a uniaxial anisotropic distortion has been applied into the octahedral hexa-coordinate metal complex. The purpose of this research is to study the mechanism and phenomenon of the spin state changing by discussing the calculated electronic states of the spin crossover phenomenon.

A Hexagonal Planar Metal Complex.

A six-coordinate [ML3 Z3 ]-type transition-metal complex with a hexagonal planar geometry has been isolated and characterized, extending the scope of six-coordinate metal coordination compounds to those with a geometry beyond octahedral and trigonal prismatic.

Template Synthesis of Tin(IV) Complexes with Tridentate Iminopyridine Ligands

Six-coordinate metal complexes containing tridentate Schiff bases with an ONN-chelating moiety were prepared by template synthesis on tin tetrachloride using various aminophenols and α-carbonyl-substituted pyridines. The structures of five compounds were studied by X-ray diffraction (CIF files CCDC nos. 849152–1849156). The UV/Vis spectra of the obtained series of compounds were found to be highly sensitive to the nature of substituents in the moieties of organic ligand.

Functional Modifications and Electronic Influences on Tetra-aza Macrocyclic Cu(II) Complexes

A library of novel tetra-aza macrocyclic molecules, specifically 3,6,9,15-tetraazabicyclo [9.3.1]pentadeca-1(15),11,13-trienederivatives, capable of chelating metal ions in vivo have been synthesized. Applications of these complexes are currently being pursued as a 1) therapeutic, focusing on radical scavenging and metal chelation, and 2) diagnostic tool such as magnetic resonance imaging (MRI) contrast agents when complexed with specific metal ions. However, a full study of the electronic effects imparted by substitution to the pyridyl moiety and the subsequent impact on the metal center have not been explored. The objective of the present study is to characterize metal complexes of four tetra-aza macrocyclic metal chelating molecules. The pyridyl functional groups studied include: A) uper (II) complexes. Analysis of the resulting copper (II) complex of the p-nitrile tetra-aza macrocycle (L6) indicate a six-coordinate metal center based on X-ray diffraction. UV-visible spectroscopy and electrochemistry help to confirm donor strength among the ligand series as well as a comparison to other tetra-aza macrocycles of interest. modified pyridyl (L1), B) p-hydroxyl (L2), and C)p-nitrile modified pyridyls (L6) on a pyclen base structure. Notable progress has been made in developing an optimal procedure for obtaining cop.
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Tuning Packing, Structural Flexibility, and Porosity in 2D Metal–Organic Frameworks by Metal Node Choice

Understanding the key features that determine structural flexibility in metal–organic frameworks (MOFs) is key to exploiting their dynamic physical and chemical properties. We have previously reported a 2D MOF material, CuL1, comprising five-coordinate metal nodes that displays exceptional CO2/N2 selectively (L1=bis(4-(4-carboxyphenyl)-1H-pyrazolyl)methane). Here we examine the effect of utilising six-coordinate metal centres (CoII and NiII) in the synthesis of isostructural MOFs from L1, namely CoL1 and NiL1. The octahedral geometry of the metal centre within the MOF analogues precludes an ideal eclipse of the 2D layers, resulting in an offset stacking, and in certain cases, the formation of 2-fold interpenetrated analogues β-CoL1 and β-NiL1. We used a combination of thermogravimetric analysis (TGA), and powder and single crystal X-ray diffraction (PXRD and SCXRD) to show that desolvation is accompanied by a structural change for NiL1, and complete removal of the coordinated H2O ligands results in a reduction in long-range order. The offset nature of the 2D layers in combination with the structural changes impedes the adsorption of meaningful quantities of gases (N2, CO2), highlighting the importance of a five-coordinate metal centre in achieving optimal pore accessibility for this family of flexible materials.

Synthesis and characterization of metal-diaminobipyridine complexes as low-cost co-catalysts for photo-sensitized hydrogen evolution

We synthesized novel 4,4′-diamino-2,2′-bipyridine (dabpy) complexes of Cu(II), Co(III), and Ni(II), and determined their molecular structures by single-crystal XRD analysis. As a result, we found that [Cu(dabpy)2]2+ had a four-coordinated Cu center and distorted square-planar geometry, while the Co(III) and Ni(II) complexes contained six-coordinated metal centers and exhibited an octahedral geometry. The prepared complexes were investigated as co-catalysts for H2 evolution from aqueous media in the presence of erythrosin B as a photosensitizer. Notably, the Cu(II) complex demonstrated a much higher co-catalyst activity than the corresponding Co(III) and Ni(II) species. Additionally, the H2 evolution mechanism and the redox potentials of the metal dabpy complexes were elucidated using electrochemical studies, and it was found that the reduction of protons to H2 was accompanied by a Cu(II)/Cu(I) redox reaction. In addition, it was suggested that the high co-catalyst activity of the Cu(II) complex was due to the low protonation-induced overpotential compared to those of Co(III) and Ni(II). Furthermore, the photo-sensitized H2 evolution activity of the metal complex/K2PtCl4 mixed systems were shown to exceed that of the K2PtCl4-only control system. The Cu(II) complex was found to be low-cost and effective co-catalyst for homogeneous H2 evolution system.

Metal Coordination to Ligand-Modified Peptide Nucleic Acid Triplexes.

A challenging goal in nanotechnology is the precise and programmable arrangement of specific elements in nanosystems in the three-dimensional space. The use of ligand-modified nucleic acids represents an accurate and selective tool to achieve this goal when it comes to metal ion organization. The synthesis of peptide nucleic acid (PNA) monomers that contain ligands instead of nucleobases makes possible the creation of metal-mediated alternative base pairs and triplets at specific locations in PNA duplexes and triplexes, respectively. We report the formation of four- and six-coordinate metal complexes between PNA triplexes modified with 2,2'-bipyridine (Bpy) or 8-hydroxyquinoline (Q) ligands and 3d metal ions. These metal complexes function as alternative base triplets or pairs in that they increase the thermal stability of the triplexes if the stability constants of the metal complexes are relatively high. The increase in the triplex melting temperature correlates with the stability constants of the metal complexes with ligand-containing PNA determined by UV-vis titrations. The metal complexes coordinate two or three ligands although three bidentate ligands are in close proximity of each other within a triplex. Metal coordination to ligand-modified PNA triplexes was further studied by electron paramagnetic resonance (EPR) spectroscopy and circular dichrosim (CD) spectroscopy. EPR spectroscopy indicated the formation of a square planar [CuQ2] complex between Cu2+ and Q-containing PNA triplex. Taken together, the spectroscopic results indicate that in the presence of 1 equiv of Fe2+ or Ni2+ the majority, but not all, of the Bpy-containing PNA triplexes contain [MBpy3] complexes, with a minority of them being metal free. We attribute this behavior to a supramolecular chelate effect exerted by the triplex, which favors the formation of tris-ligand complexes, that is balanced by the steric interactions between the metal complex and the adjacent nucleobase triplets, which decrease the stability of the complex and triplex. In contrast, the very high stability of square planar [MQ2] complexes of Cu2+ and Ni2+ leads to formation of bis-ligand complexes instead of tris-ligand complexes with Q3-containing PNA triplexes. The metal-containing PNA triplexes have a terminal l-lysine and adopt a left-handed chiral structure in solution. The handedness of the PNA triplex determines that of the metal complexes formed with the Bpy-containing PNA triplexes.

Synthesis, crystal structure, theoretical studies and biological properties of three novel trigonal prismatic Co(II), Ni(II) and Cu(II) macroacyclic Schiff base complexes incorporating piperazine moiety

Abstract Trigonal-prismatic structures among different transition metal complexes according to the Cambridge Structural Database (CSD) are rare. The number of trigonal prisms throughout the transition-metal series is less than 1% of all six-coordinate metal centers and only 0.4% of cobalt complexes. Herein we report a novel macroacyclic Schiff base complexes incorporating piperazine moiety with Trigonal-Prismatic coordination were prepared via the metal templated of the 2,2′-(piperazine-1,4-diylbis(methylene))dianiline (A) and 2-formylpyridinein the presence of appropriate Co(II), Ni(II) and Cu(II) metal ions in methanolic/ethanolic solution. The complexes were characterized by elemental analysis, mass spectrometry and FT-IR. Also, the crystal structure of [CoL](ClO4)2·(H2O) complex was obtained by single-crystal X-ray crystallography. An interesting point in this report is the trigonal prismatic coordination for complexes that is the first for Schiff-base complexes containing piperazine moiety. Furthermore, these newly synthesized complexes were evaluated for their in vitro antioxidant and antibacterial activities by using scavenging effects on 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical and disc diffusion methods, respectively. Based on the resulting experimental data, these compounds exhibited good biological activities. The geometry of [ML]2+ (M = Co, Ni, Cu) complexes have been also optimized at the BP86/def2-SVP level of theory. Furthermore the nature of M ← L (M = Co, Ni, Cu) bonds in the complexes have been studied with the help of NBO and Energy decomposition analysis (EDA). Results showed that the nature of metal-ligand bonds is more covalent (orbital term) with a contribution of about 53.4–62.8% in total interaction energy.

Incorporation of Sb5+into CeO2: local structural distortion of the fluorite structure from a pentavalent substituent

Hydrothermal crystallisation of CeO2 from aqueous sodium hydroxide solution at 240 °C using CeCl3·7H2O in the presence of hydrogen peroxide with addition of either SbCl3 or SbCl5 yields polycrystalline samples of antimony-containing ceria directly from solution. Powder X-ray diffraction shows a contraction of the cubic lattice parameter with increasing Sb content, and also a broadening of Bragg peaks, from which Scherrer analysis yields crystallite domain sizes of 5-20 nm. Scanning transmission electron microscopy provides consistent results with observation of highly crystalline particles of a few nm in diameter. X-ray absorption near edge structure spectroscopy at the Ce LIII- and Sb K-edges reveals the presence of Ce4+ and Sb5+ in the solids. To balance charge the presence of co-included Na is proposed, corroborated by elemental analysis. The general chemical formula of the materials can thus be written as (Ce1-xSbx)1-yNayO2-δ (where x < 0.4 and y ≥ x/3). Sb K-edge extended X-ray absorption fine structure spectroscopy of the substituted ceria samples shows that the local structure of Sb resembles that in NaSbO3, where six-coordinate metal sites are found, but with evidence of a longer interatomic correlation due to surrounding Ce/Sb atoms in the fluorite structure; this implies that the Sb is displaced from the ideal eight-coordinate site of the fluorite structure. This structural distortion gives materials that are unstable under reducing conditions, coupled by the ease of reduction to elemental antimony, which is extruded leading to phase separation.

Manganese(II) chloride complexes with pyridine N-oxide (PNO) derivatives and their solid-state structures.

Three manganese(II) N-oxide complexes have been synthesized from the reaction of manganese(II) chloride with either pyridine N-oxide (PNO), 2-methyl-pyridine N-oxide (2MePNO) or 3-methyl-pyridine N-oxide (3MePNO). The compounds were synthesized from methano-lic solutions of MnCl2·4H2O and the respective N-oxide, and subsequently characterized structurally by single-crystal X-ray diffraction. The compounds are catena-poly[[aqua-chlorido-manganese(II)]-di-μ-chlorido-[aqua-chlorido-manganese(II)]-bis-(μ-pyridine N-oxide)], [MnCl2(C5H5NO)(H2O)] n or [MnCl2(PNO)(H2O)] n (I), catena-poly[[aqua-chlorido-man-gan-ese(II)]-di-μ-chlorido-[aqua-chlorido-manganese(II)]-bis-(μ-2-methyl-pyridine N-oxide)], [MnCl2(C6H7NO)(H2O)] n or [MnCl2(2MePNO)(H2O)] n (II), and bis-(μ-3-methyl-pyridine N-oxide)bis-[di-aqua-dichlorido-manganese(II)], [Mn2Cl4(C6H7NO)2(H2O)4] or [MnCl2(3MePNO)(H2O)2]2 (III). The MnII atoms are found in pseudo-octa-hedral environments for each of the three complexes. Compound I forms a coordination polymer with alternating pairs of bridging N-oxide and chloride ligands. The coordination environment is defined by two PNO bridging O atoms, two chloride bridging atoms, a terminal chloride, and a terminal water. Compound II also forms a coordination polymer with a similar metal cation; however, the coordination polymer is bridged between MnII atoms by both a single chloride and 2MePNO. The distorted octahedrally coordinated metal cation is defined by two bridging 2MePNO trans to each other, two chlorides, also trans to one another in the equatorial (polymeric) plane, and a terminal chloride and terminal water. Finally, complex III forms a dimer with two bridging 3MePNOs, two terminal chlorides and two terminal waters forming the six-coordinate metal environment. All three compounds exhibit hydrogen bonding between the coordinating water(s) and terminal chlorides.

Warfarin inhibits allosterically the reductive nitrosylation of ferric human serum heme-albumin

Human serum heme-albumin (HSA-heme-Fe) displays heme-based ligand binding and (pseudo-)enzymatic properties. Here, the effect of the prototypical drug warfarin on kinetics and thermodynamics of NO binding to ferric and ferrous HSA-heme-Fe (HSA-heme-Fe(III) and HSA-heme-Fe(II), respectively) and on the NO-mediated reductive nitrosylation of the heme-Fe atom is reported; data were obtained between pH5.5 and 9.5 at 20.0°C. Since warfarin is a common drug, its effect on the reactivity of HSA-heme-Fe represents a relevant issue in the pharmacological therapy management. The inhibition of NO binding to HSA-heme-Fe(III) and HSA-heme-Fe(II) as well as of the NO-mediated reductive nitrosylation of the heme-Fe(III) atom by warfarin has been ascribed to drug binding to the fatty acid binding site 2 (FA2), shifting allosterically the penta-to-six coordination equilibrium of the heme-Fe atom toward the low reactive species showing the six-coordinated metal center by His146 and Tyr161 residues. These data: (i) support the role of HSA-heme-Fe in trapping NO, (ii) highlight the modulation of the heme-Fe-based reactivity by drugs, and (iii) could be relevant for the modulation of HSA functions by drugs in vivo.

TQOPEN (N,N,N',N'-Tetrakis(2-quinolylmethyl)-3-oxa-1,5-pentanediamine) Family as Heptadentate Fluorescent Cd2+ Sensors.

A quinoline-based heptadentate ligand, N,N,N',N'-tetrakis(2-quinolylmethyl)-3-oxa-1,5-pentanediamine (TQOPEN), exhibits a fluorescence increase (ICd/I0 = 25, ϕCd = 0.017) at 428 nm upon addition of 1 equiv of Cd2+. In contrast, 1 equiv of Zn2+ induces a negligible fluorescence change due to weak interaction (IZn/I0 = 2.5, IZn/ICd = 10%). In comparison with TQOPEN, the thia and aza derivatives TQSPEN and TQNPEN exhibit improved Cd2+/Zn2+ selectivity and higher Cd2+-binding affinity, respectively. The solid-state structures of mononuclear Cd2+ and hydroxide-bridged dinuclear Zn2+ complexes of TQOPEN were elucidated by X-ray crystallography. Although the crystal structure of the TQOPEN-Cd2+ complex exhibits a six-coordinate metal center, in which one quinoline weakly interacts with the Cd center (Cd···Nquinoline = 3.303(3) Å), a 1H NMR study at 233 K suggests that all quinolines interact with the Cd center to form a symmetrical seven-coordinate structure in solution. Theoretical calculations (TDDFT) support the flexible coordination environment around the Cd center, leading to intramolecular excimer formation with two quinoline moieties in the excited state. The importance of a heptadentate structure was further demonstrated by the lack of Cd2+ specificity with hexadentate ligands TriQOPEN (N,N,N'-tris(2-quinolylmethyl)-3-oxa-1,5-pentanediamine) and TQCPEN (N,N,N',N'-tetrakis(2-quinolylmethyl)-1,5-pentanediamine).

Crystal structure of catena-poly[[copper(II)-μ2-salicylato-[diaqua-copper(II)]-μ2-salicylato] dihydrate

The title compound, {[Cu2(C7H4O3)2(H2O)2]·2H2O} n , contains two copper(II) cations in special positions (one on a twofold rotation axis and one on an inversion centre) and the the salicylate ligand in its dianionic form. By four- and six-coordinate metal coordination, chains are formed parallel to [001], which are extended by O-H⋯O hydrogen bonding into sheets extending parallel to (100). These sheets are weakly connected by O-H⋯O hydrogen bonding via the non-coordinating lattice water mol-ecules into a three-dimensional network.

Toward inert paramagnetic Ni(ii)-based chemical exchange saturation transfer MRI agents.

The Ni2+ complexes with hexadentate ligands containing two 6-methylpicolinamide groups linked by ethane-1,2-diamine (dedpam) or cyclohexane-1,2-diamine (chxdedpam) spacers were investigated as potential contrast agents in magnetic resonance imaging (MRI). The properties of the complexes were compared to that of the analogues containing 6-methylpicolinate units (dedpa2- and chxdedpa2-). The X-ray structure of the [Ni(dedpam)]2+ complex reveals a six-coordinated metal ion with a distorted octahedral environment. The protonation constants of the dedpa2- and dedpam ligands and the stability constants of their Ni2+ complexes were determined using pH-potentiometry and spectrophotometric titrations (25 °C, 0.15 M NaCl). The [Ni(dedpa)] complex (log KNiL = 20.88(1)) was found to be considerably more stable than the corresponding amide derivative [Ni(dedpam)]2+ (log KNiL = 14.29(2)). However, the amide derivative [Ni(chxdedpam)]2+ was found to be considerably more inert with respect to proton-assisted dissociation than the carboxylate derivative [Ni(chxdedpa)]. A detailed 1H NMR and DFT study was conducted to assign the 1H NMR spectra of the [Ni(chxdedpa)] and [Ni(chxdedpam)]2+ complexes. The observed 1H NMR paramagnetic shifts were found to be dominated by the Fermi contact contribution. The amide resonances of [Ni(chxdedpam)]2+ at 91.5 and 22.2 ppm were found to provide a sizeable chemical exchange saturation transfer effect, paving the way for the development of NiCEST agents based on these rigid non-macrocyclic platforms.

New cobalt(II) and nickel(II) complexes of benzyl carbazate Schiff bases: Syntheses, crystal structures, in vitro DNA and HSA binding studies

In the present study, new Schiff base complexes with the composition [M(NCS)2(L1)2]·nH2O, where M=Co (n=0) (1) and Ni (n=2) (2); [M(NCS)2(L2)2], M=Co (3) and Ni (4) as well as [M(NCS)2(L3)2], M=Co (5) and Ni (6); (L1=benzyl 2-(propan-2-ylidene)hydrazinecarboxylate, L2=benzyl 2-(butan-2-ylidene)hydrazinecarboxylate and L3=benzyl 2-(pentan-3-ylidene)hydrazinecarboxylate) have been synthesized by a template method. The complexes were characterised by analytical methods, spectroscopic studies, thermal and X-ray diffraction techniques. The structures of all the complexes explore that the metal(II) cation has a trans-planar coordination environment, the monomeric units containing a six-coordinated metal center in octahedral geometry with N-bound isothiocyanate anions coordinated as terminal ligands. Furthermore, the binding of the two Schiff base ligands to the metal centers involves the azomethine nitrogen and the carbonyl oxygen in mutually trans configuration. The binding interactions of all the complexes with Calf thymus-deoxyribonucleic acid (CT-DNA) and human serum albumin (HSA) have been investigated using absorption and emission spectral techniques. The CT-DNA binding properties of these complexes reveal that they bind to CT-DNA through a partial intercalation mode and the binding constant values were calculated using the absorption and emission spectral data. The binding constant values (~10×106moldm−3) indicate strong binding of metal complexes with CT-DNA. HSA binding interaction studies showed that the cobalt and nickel complexes can quench the intrinsic fluorescence of HSA through static quenching process. Also, molecular docking studies were supported out to apprehend the binding interactions of these complexes with DNA and HSA which offer new understandings into the experimental model observations.