Mononuclear Transition Metal Complexes Research Articles

Approaching the Complete Basis Set Limit for Spin-State Energetics of Mononuclear First-Row Transition Metal Complexes.

Convergence to the complete basis set (CBS) limit is analyzed for the problem of spin-state energetics in mononuclear first-row transition metal (TM) complexes by taking under scrutiny a benchmark set of 18 energy differences between spin states for 13 chemically diverse TM complexes. The performance of conventional CCSD(T) and explicitly correlated CCSD(T)-F12a/b calculations in approaching the CCSD(T)/CBS limits is systematically studied. An economic computational protocol is developed based on the CCSD-F12a approximation and (here proposed) modified scaling of the perturbative triples term (T#). This computational protocol recovers the relative spin-state energetics of the benchmark set in excellent agreement with the reference CCSD(T)/CBS limits (mean absolute deviation of 0.4, mean signed deviation of 0.2, and maximum deviation of 0.8 kcal/mol) and enables performing canonical CCSD(T) calculations for mononuclear TM complexes sized up to ca. 50 atoms, which is illustrated by application to heme-related metalloporphyrins. Furthermore, a good transferability of the basis set incompleteness error (BSIE) is demonstrated for spin-state energetics computed using CCSD(T) and other wave function methods (MP2, CASPT2, CASPT2/CC, NEVPT2, and MRCI + Q), which justifies efficient focal-point approximations and simplifies the construction of multimethod benchmark studies.

Tridentate xanthene-based hydrazone ligands and their mononuclear transition metal complexes: synthesis, anti-malarial, antimicrobial and molecular docking studies

Tridentate xanthene-based hydrazone ligands and their mononuclear transition metal complexes: synthesis, anti-malarial, antimicrobial and molecular docking studies

Facile synthesis of ZnO nanoparticles by thermal decomposition of Schiff base complexes for the effective removal of organic pollutants in an aqueous medium

A series of mononuclear Schiff base transition metal complexes of the type [M(LS)(temed)] (where, M = Co(II), Ni(II), Cu(II), and Zn(II); LS = Schiff base ligand derived from L-leucine and Salicylaldehyde and co-ligand temed = N,N,N’N’- tetramethylethylenediamine) have been synthesized and characterized by spectroscopic studies such as Mass, NMR, UV–Visible, FT-IR, and TGA analysis. Spectral results reveal that the metal ions are coordinated to imine nitrogen, phenolic oxygen, and oxygen atom present in the carboxylate group of the Schiff base ligand and two nitrogen atoms from ligand (L2) temed. The scavenging activity of the complexes was carried out by the DPPH method with different concentrations, and all the complexes possesses good antioxidant activity in the range 58.93 to 78.79 %. Among them Zn(II) complex showed higher scavenging activity 78.79 % using 10 mg concentration. Furthermore, an investigation was conducted to assess the antibacterial properties of the complexes. It was observed that all of the metal complexes exhibited significant antimicrobial activity. Notably, the copper complex demonstrated enhanced activity, particularly against S. aureus and Mucor species, with zone of inhibition measurements of 19 mm and 20 mm, respectively. The Zn(II) complex has been used as a precursor to produce zinc oxide nanoparticles by the thermal decomposition method. ZnO nanoparticles have been characterized by powder X-ray diffraction (PXRD), UV–Vis spectroscopy techniques, scanning electron microscopy (SEM), FT-IR spectroscopy. The photocatalytic activity of zinc oxide nanoparticles was investigated in the presence of ultraviolet light. ZnO nanoparticles can efficiently catalyze the degradation of methyl orange, methylene blue, and rhodamine B in the aqueous suspension, and the highest percentage of degradation was observed against methylene blue (99 %). Furthermore, the impact of altering dye concentration, pH, catalyst quantity and kinetics on the photodegradation of model pollutant methylene blue was investigated.

Co(II), Ni(II), Cu(II) and Zn(II) complexes of 1,8-naphthalimide-based ligand: syntheses, characterizations and evaluation of antibacterial activities

Mononuclear later first-row transition metal complexes, [M(L)2] (1–4), where M = Co(II) (1), Ni(II) (2), Cu(II) (3), and Zn(II) (4) incorporating 1,8-naphthalimide based ligand were designed. The complex preparation was achieved by reacting metal nitrates with ligand (L) in 1:2 stoichiometry. The ligand and complexes were characterized with analytical and photophysical techniques. Single crystal X-ray diffraction determines that 3 is square planar. The complexes are highly fluorescent compared to the free ligand. The synthesized ligand (L) and complexes (1–4) were screened for their antibacterial activities against gram-positive [Streptococcus mutans (SM), Staphylococcus aureus (SA)] and gram-negative [Escherichia coli (EC), Klebsiella pneumoniae (KL)] bacterial strains. Ni(II) and Cu(II) complexes exhibited good activity against K. pneumonia and S. aureus compared to the free ligand. Particularly effective against gram-negative bacteria (Klebsiella pneumoniae), Ni(II) complex (2) has a MIC value of 6.25 µM and exhibits similar activity to standard ciprofloxacin. Cu(II) complex (3) is two times more effective than the common antibiotic ciprofloxacin with a MIC value of 3.125 µM against gram-positive bacteria (S. aureus).

Identifying Underexplored and Untapped Regions in the Chemical Space of Transition Metal Complexes.

We survey more than 240 000 crystallized mononuclear transition metal complexes (TMCs) to identify trends in preferred geometric structure and metal coordination. While we observe that an increased level of d filling correlates with a lower coordination number preference, we note exceptions, and we observe undersampling of 4d/5d transition metals and 3p-coordinating ligands. For the one-third of mononuclear TMCs that are octahedral, analysis of the 67 symmetry classes of their ligand environments reveals that complexes often contain monodentate ligands that may be removable, forming an open site amenable to catalysis. Due to their use in catalysis, we analyze trends in coordination by tetradentate ligands in terms of the capacity to support multiple metals and the variability of coordination geometry. We identify promising tetradentate ligands that co-occur in crystallized complexes with labile monodentate ligands that would lead to reactive sites. Literature mining suggests that these ligands are untapped as catalysts, motivating proposal of a promising octa-functionalized porphyrin.

Gamma ray irradiated binuclear and mononuclear transition metal complexes with polydentate ligand: Template synthesis, spectral, XRD, morphology, solid electrical conductivity and antimicrobial activity

In this work, new binuclear Fe(III), Ni(II) and mononuclear Cu(II) Schiff base complexes that obtained via template condensation of Fe(III), Ni(II) and Cu(II) chlorides with N1-(3-aminopropyl) propane-1,3-diamine and 1,3-diphenylpropane-1,3‑dione (1B, 2B, 3B), respectively were prepared and well investigated. The structure of the complexes was elucidated by means of microanalyses, various spectroscopic techniques (FT–IR, UV/Vis, X–ray diffraction), thermal (TG/DTG), molar conductivity and magnetic moment measurements, Fe(III), Ni(II) complexes have an octahedral structure, while Cu(II) complex adopts distorted square pyramidal geometry. To study γ– irradiation induced modifications, powder sample of these complexes was exposed to high energetic γ–ray irradiation at a dose of 20 kGy, (hereafter referred to as 1A, 2A, 3A, respectively). Physico-chemical properties of irradiated samples were also investigated by the aforementioned tools. The results revealed no remarkable change in the molar conductivity values and little shift of the peak position and the intensity for FT–IR and electronic spectra as a result of the distortion of lattice planes. Also, TG/DTG studies revealed decreasing in the thermal stability for all irradiated samples. From the powder XRD data for non-irradiated and γ–irradiated Ni(II) (2B, 2A) and Cu(II) (3B, 3A) complexes, average crystallite size (D) and dislocation density (δ) were calculated, the γ–irradiation resulted in decrease in the crystallite size for Ni(II) (2A) complex. The irradiated (3A) showed changes in the surface morphology compared to its non-irradiated one. The solid state electrical conductivity of non-irradiated and γ–irradiated Ni(II) complexes have been investigated. Furthermore, the non-irradiated samples (1B, 3B) and their γ–irradiated (1A, 3A) were assayed for their antibacterial property against Gram–positive as Staphylococcus aureus, Gram– negative as Escherichia coli and antifungal property against fungi as Candida albicans.

Mononuclear half-sandwich nd7 metallo drug complexes based on bidentate N∩N dendritic scaffolds: DFT (B3LYP; BP86 and B3PW91) examination

Through an use of three functionals (B3PW91, B3LYP and BP86) associated to a generic basis set LanL2DZ for transition metals (as well as halogen atoms) and 6-311+G (d,p) for others atoms, an examination of the bonding properties of a series of mononuclear half-sandwich nd7 transition metal (anticancer) complexes based on N∩N dendritic scaffolds (L) has been done. Collectively, complexes studied have adopted the piano-stool environment. An examination of the performance of each functional has shown that for the most reliable geometrical analysis of Metal-Nitrogen and Metal-Halogen bonds, the B3LYP and B3PW91 functionalities are suitable respectively. Regardless of the halogen ligand adopted, the B3LYP metal-nitrogen bond lengths are the most widely overestimated. A correlation has been built between the retained charge on each divalent transition metal cation and its metal ion affinity (MIA). Topological examinations reveal the higher instability of metal-N bonds compared to metal-X ones (X = Cl and Br). By the mean of the energy decomposition analysis, a predominant electrostatic character of metal … halogen and [LCP]- … [MX]+ interaction has been demonstrated. The transition metal atom … (hydrophobic) surface (Cp*) interaction is most pronounced for the chloride rhodium complexes of rhodium (combined with (E)-N-(pyridin-2-ylmethylene) Propan-1-amine and 2,2′- dipyridylketone ligands and iridium combined with 2,2′- dipyridylketone ligand. The charge decomposition analysis displays the weakening of the [MLCP*]+−X− bonds in the studied complexes.

PH-Switching of the luminescent, redox, and magnetic properties in a spin crossover cobalt(ii) molecular nanomagnet.

The ability of mononuclear first-row transition metal complexes as dynamic molecular systems to perform selective functions under the control of an external stimulus that appropriately tunes their properties may greatly impact several domains of molecular nanoscience and nanotechnology. This study focuses on two mononuclear octahedral cobalt(ii) complexes of formula {[CoII(HL)2][CoII(HL)L]}(ClO4)3·9H2O (1) and [CoIIL2]·5H2O (2) [HL = 4'-(4-carboxyphenyl)-2,2':6',2''-terpyridine], isolated as a mixed protonated/hemiprotonated cationic salt or a deprotonated neutral species. This pair of pH isomers constitutes a remarkable example of a dynamic molecular system exhibiting reversible changes in luminescence, redox, and magnetic (spin crossover and spin dynamics) properties as a result of ligand deprotonation, either in solution or solid state. In this last case, the thermal-assisted spin transition coexists with the field-induced magnetisation blockage of "faster" or "slower" relaxing low-spin CoII ions in 1 or 2, respectively. In addition, pH-reversible control of the acid-base equilibrium among dicationic protonated, cationic hemiprotonated, and neutral deprotonated forms in solution enhances luminescence in the UV region. Besides, the reversibility of the one-electron oxidation of the paramagnetic low-spin CoII into the diamagnetic low-spin CoIII ion is partially lost and completely restored by pH decreasing and increasing. The fine-tuning of the optical, redox, and magnetic properties in this novel class of pH-responsive, spin crossover molecular nanomagnets offers fascinating possibilities for advanced multifunctional and multiresponsive magnetic devices for molecular spintronics and quantum computing such as pH-effect spin quantum transformers.

Structure and Properties of Heterometallics Based on Lanthanides and Transition Metals with Methoxy-β-Diketonates

The possibility of obtaining volatile polynuclear heterometallic complexes containing lanthanides and transition metals bound by methoxy-β-diketonates was studied. New compounds were prepared by cocrystallization of monometallic complexes from organic solvents. Ln(tmhd)3 were used as initial monometallic complexes (Ln = La, Pr, Sm, Gd, Tb, Dy, Lu; tmhd = 2,2,6,6-tetramethylheptane-3,5-dionate) in combination with TML2 in various ratios (TM = Cu, Co, Ni, Mn; L: L1 = 1,1,1-trifluoro-5,5-dimethoxypentane-2,4-dionate, L2 = 1,1,1-trifluoro-5,5-dimethoxy-hexane-2,4-dionate, L3 = 1,1,1-trifluoro-5-methoxy-5-methylhexane-2,4-dionate). Heterometallic complexes of the composition [(LnL2tmhd)2TM(tmhd)2] were isolated for light lanthanides Ln= La, Pr, Sm, Gd, and L= L1 or L2. By single crystal XRD, it has been established that heterometallic compounds containing La, Pr, Cu, Co, and Ni are isostructural linear coordination polymers of alternating mononuclear transition metal complexes and binuclear heteroleptic lanthanide complexes, connected by donor-acceptor interactions between oxygen atoms of the methoxy groups and transition metal atoms. A comparison of powder XRD patterns has shown that all heterometallic complexes obtained are isostructural. Havier lanthanides Ln = Tb, Dy, Lu did not form heterometallics. Instead, homometallic complexes Ln(L3)3 were identified for Ln = Dy, Lu as well as for Ln = La. The thermal properties of the complexes were investigated by TG-DTA and vacuum sublimation tests. The heterometallic complexes were found to be not volatile and decomposed under heating to produce inorganic composites of TM oxides and Ln fluorides. In contrast, Ln(L3)3 is volatile and may be sublimed in a vacuum. Results of magnetic measurements are discussed for several heterometallic and homometallic complexes.

Reaching the Maximal Unquenched Orbital Angular Momentum L = 3 in Mononuclear Transition-Metal Complexes: Where, When and How?

The conditions for achieving the maximal unquenched orbital angular momentum L = 3 and the highest magnetic anisotropy in mononuclear 3d complexes with axial coordination symmetry are examined in terms of the ligand field theory. It is shown that, apart from the known linear two-coordinate 3d7 complex CoII(C(SiMe2ONaph)3)2 characterized by record magnetic anisotropy and single-molecule magnet (SMM) performance (with the largest known spin-reversal barrier Ueff = 450 cm−1), the maximal orbital angular momentum L = 3 can also be obtained in linear two-coordinate 3d2 complexes (V3+, Cr4+) and in trigonal-prismatic 3d3 (Cr3+, Mn4+) and 3d8 (Co+, Ni2+) complexes. A comparative assessment of the SMM performance of the 3d2, 3d3 and 3d8 complexes indicates that they are unlikely to compete with the record linear complex CoII(C(SiMe2ONaph)3)2, whose magnetic anisotropy is close to the physical limit for a 3d metal.

Ligand additivity relationships enable efficient exploration of transition metal chemical space.

To accelerate the exploration of chemical space, it is necessary to identify the compounds that will provide the most additional information or value. A large-scale analysis of mononuclear octahedral transition metal complexes deposited in an experimental database confirms an under-representation of lower-symmetry complexes. From a set of around 1000 previously studied Fe(II) complexes, we show that the theoretical space of synthetically accessible complexes formed from the relatively small number of unique ligands is significantly (∼816k) larger. For the properties of these complexes, we validate the concept of ligand additivity by inferring heteroleptic properties from a stoichiometric combination of homoleptic complexes. An improved interpolation scheme that incorporates information about cis and trans isomer effects predicts the adiabatic spin-splitting energy to around 2 kcal/mol and the HOMO level to less than 0.2eV. We demonstrate a multi-stage strategy to discover leads from the 816k Fe(II) complexes within a targeted property region. We carry out a coarse interpolation from homoleptic complexes that we refine over a subspace of ligands based on the likelihood of generating complexes with targeted properties. We validate our approach on nine new binary and ternary complexes predicted to be in a targeted zone of discovery, suggesting opportunities for efficient transition metal complex discovery.

Transition metals of arene derivatives with functionalized ionic liquid: DFT investigation, biological applications and electrochemical behavior of complexes

Mononuclear transition metal complexes based on ionic liquid have been prepared and characterized in detail. The biological properties of the three complexes were evaluated using radical scavenging activity, reducing power, antibacterial effect, DNA binding and cleavage activity. Among the complexes, [3-[(2R)-2-({[dichloro(η6-benzene)ruthenium]diphenylphosphanyl}oxy)-2-phenylethyl]-1-methyl-1H-imidazol-3-ium chloride] (4), demonstrated the highest radical scavenging (64.7 %) and reducing power activity (0.467) at 200 μg/ml concentration. The highest zone of inhibition was obtained from [3-[(2R)-2-({[dichloro(η6-p-cymene)ruthenium]diphenyl phosphanyl}oxy)-2-phenylethyl]-1-methyl-1H-imidazol-3-ium chloride] (3), against Bacillus cereus as 14 mm. Furthermore, all complexes were determined to have DNA binding and cleavage activities. Furthermore, theoretical DFT computations have also been carried out for the cationic complexes, to obtain minimum energy configuration of molecules. The effects of the chemical structures of three cationic complexes were also examined in relation to the variable property of electron-donating ligands for ruthenium-based complexes and iridium complex and their potential energy levels in ground and excited states HOMO and LUMO were determined.

Exploiting Ligand Additivity for Transferable Machine Learning of Multireference Character across Known Transition Metal Complex Ligands.

Accurate virtual high-throughput screening (VHTS) of transition metal complexes (TMCs) remains challenging due to the possibility of high multireference (MR) character that complicates property evaluation. We compute MR diagnostics for over 5,000 ligands present in previously synthesized octahedral mononuclear transition metal complexes in the Cambridge Structural Database (CSD). To accomplish this task, we introduce an iterative approach for consistent ligand charge assignment for ligands in the CSD. Across this set, we observe that the MR character correlates linearly with the inverse value of the averaged bond order over all bonds in the molecule. We then demonstrate that ligand additivity of the MR character holds in TMCs, which suggests that the TMC MR character can be inferred from the sum of the MR character of the ligands. Encouraged by this observation, we leverage ligand additivity and develop a ligand-derived machine learning representation to train neural networks to predict the MR character of TMCs from properties of the constituent ligands. This approach yields models with excellent performance and superior transferability to unseen ligand chemistry and compositions.

Synthesis, Characterization and Chemical Behavior of Disulfide-bearing Mononuclear Transition Metal Complexes with the Rigid Structure

A novel rigid macrocyclic ligand based on the bis(pyridylimino)isoindole framework bearing the disulfide moiety was designed and synthesized to investigate the behavior of the disulfide in mononuclear transition metal complexes. Intrinsically high reactivity of the Co(II) disulfide complex was clearly found by observation of the progress of the ring-contraction reaction to form a thioether complex in the presence of cyclooctene. On the other hand, the Fe(II) disulfide complex was found to be stable even when heated at 60 °C under the same conditions. This difference in reactivity of disulfide complexes might be derived from the difference in the ionic radii of Co(II) and Fe(II); when the smaller Co(II) ion was installed into the disulfide-bearing macrocyclic ligand, the structural demand for square pyramidal geometry around the Co center drove ring-contraction, eliminating a sulfur atom from the strained disulfide complex.

Molecular and Electronic Structures and Single-Molecule Magnet Behavior of Tris(thioether)-Iron Complexes Containing Redox-Active α-Diimine Ligands.

Incorporating radical ligands into metal complexes is one of the emerging trends in the design of single-molecule magnets (SMMs). While significant effort has been expended to generate multinuclear transition metal-based SMMs with bridging radical ligands, less attention has been paid to mononuclear transition metal-radical SMMs. Herein, we describe the first α-diiminato radical-containing mononuclear transition metal SMM, namely, [κ2-PhTttBu]Fe(AdNCHCHNAd) (1), and its analogue [κ2-PhTttBu]Fe(CyNCHCHNCy) (2) (PhTttBu = phenyltris(tert-butylthiomethyl)borate, Ad = adamantyl, and Cy = cyclohexyl). 1 and 2 feature nearly identical geometric and electronic structures, as shown by X-ray crystallography and electronic absorption spectroscopy. A more detailed description of the electronic structure of 1 was obtained through EPR and Mössbauer spectroscopies, SQUID magnetometry, and DFT, TD-DFT, and CAS calculations. 1 and 2 are best described as high-spin iron(II) complexes with antiferromagnetically coupled α-diiminato radical ligands. A strong magnetic exchange coupling between the iron(II) ion and the ligand radical was confirmed in 1, with an estimated coupling constant J < -250 cm-1 (J = -657 cm-1, DFT). Calibrated CAS calculations revealed that the ground-state Fe(II)-α-diiminato radical configuration has significant ionic contributions, which are weighted specifically toward the Fe(I)-neutral α-diimine species. Experimental data and theoretical calculations also suggest that 1 possesses an easy-axis anisotropy, with an axial zero-field splitting parameter D in the range from -4 to-1 cm-1. Finally, dynamic magnetic studies show that 1 exhibits slow magnetic relaxation behavior with an energy barrier close to the theoretical maximum, 2|D|. These results demonstrate that incorporating strongly coupled α-diiminato radicals into mononuclear transition metal complexes can be an effective strategy to prepare SMMs.

Speciation study involving mononuclear binary transition metal (CoII, NiII and CuII) complexes of L-methionine in non-ionic micellar medium

The present work aims to evaluate the binding capacities of binary complexes of L-Methionine with transition metal ions CoII, NiII and CuII in Triton X 100-water mixtures, a non-ionic micellar media of different compositions (0.0–2.5% (v/v)), investigated under the experimental conditions of 0.16 mol/dm3 ionic strength using NaNO3 at a temperature of 303.0 ± 0.1 K Potentiometrically. The potentiometric data was assessed by Irving-Rossotti pH metric technique with the least-squares method of MINIQUAD75, a computer program. The selectivity of the best fit model obtained by continuous exhaustive chemical modelling studies and the accuracy of the results is assessed based on statistical parameters. MLH, ML and ML2 are the identified species for the three M(CoII, NiII and CuII) - Met systems. The shift in the equilibrium of binary metal-methionine complexes with variation in composition (0.0–2.5%) of the solvent can be explained by electrostatic together with non-electrostatic forces contributed by the interaction of solute with solvent.

MONO-NUCLEATED TRANSITION METAL (M2+) COMPLEXES OF A NEW SCHIFF-BASE AS LIGAND: SYNTHESIS, STRUCTURAL CHARACTERIZATION AND ANTI-MICROBIAL ACTIVITY

We have synthesized a new class of Schiff-base Ligands with m-pthaladehyde and 3-amino benzoic acid. This adduct act as bi-dentate (N8#x38;O) ligand in the formation of mononuclear transition metal complexes of nickel (II), zinc (II), copper (II), cobalt (II), and cadmium (II). Ligand and complexes are characterized and their formation was confirmed by spectroscopic techniques i.e. 1HNMR, FTIR, and UV-Vis spectroscopy. We have synthesized Ligand-metal coordination sphere [M2+(L)2] (where M = Cu(II), Co(II), Cd(II),Ni(II)and Zn(II)) and L=3,3'-((1E,1'E)-(1,3 phenylenebis (methanylylidene)) bis (azanylylidene)) dibenzoic acid and (E)-N-((1H-indol-3-yl) methylene)-4H-1,2,4-triazole-4- amine. Both Ligand and their metal complexes showed magnificent antimicrobial property against Pseudomonas aeruginosa (MTCC 741) which causes nosocomial infections and E. coli (MTCC 1610) which causes intestinal infection.

Reciprocating thermal behavior in the family of single ion magnets

Among the mononuclear transition metal and lanthanide complexes showing the slow magnetic relaxation, as a prerequisite of the single ion magnetism, there are examples of a non-traditional thermal behavior when at the low temperature the relaxation time, referring to the high-frequency relaxation channel, is shortened on further cooling instead of an expected prolongation. This reciprocating thermal behavior has been identified in a number of examples: for the Mn(II), Co(II), Ni(II), Cu(II), {Co(II)Co(III)}, {Gd(III)Ni(II)} and Dy(III) systems. The relaxation time in the low-temperature range can be recovered by applying equation τ–1 = CpbTl + FT–k where the phonon bottleneck process (rather than Raman process) and the “strange” process participate in the inverse relaxation time. The exponents of the temperature variable typically vary as l = 2–4 and k = 0.5–1.

Designing, spectroscopic characterization, biological screening and antioxidant activity of mononuclear transition metal complexes of bidentate Schiff base hydrazones

In this article, a series of coordination complexes of Co(II), Ni(II), Cu(II) and Zn(II) metals were prepared by four novel Schiff base ligands (E)-N’-(4-(prop-2-yn-1-yloxy)benzylidene)benzohydrazide (HL1), (E)-4-chloro-N’-(4-(prop-2-yn-1-yloxy)benzylidene)benzohydrazide (HL2), (E)-N’-((2-(benzyloxy)naphthalen-1-yl)methylene)benzohydrazide (HL3), (E)-N’-((2-(benzyloxy)naphthalen-1-yl)methylene)-4-chlorobenzohydrazide (HL4) obtained by the condensation reaction of benzoic acid hydrazide/4-chloro benzoic acid hydrazide with oxy derivatives of 2-hydroxy-1-napthaldehyde/4-hydroxy benzaldehyde. The proposed structure of the complexes have been established by spectroscopic and analytical techniques like C, H, N and metal analysis, FT-IR, 1H and 13C NMR, mass, UV-Vis, EPR, fluorescence, molar conductance, magnetic susceptibility and TGA which concluded that Schiff bases act as bidentate NO donor ligands coordinating through nitrogen atom of azomethine linkage and uninegative oxygen atom of carbonyl group in enolic form and resulting in complexes of the type [M(L1–4)·(CH3COO)·3H2O] in 1:1 molar ratio. The synthesized compounds were screened for their in vitro antimicrobial activities against two gram +ve bacteria i.e. S. aureus and S. gordonii; two gram -ve bacteria i.e. E. coli and P. aeruginosa and two fungal strains i.e. A. niger and C. albicans and found that metal chelates were more noxious than their parent Schiff bases. The copper(II) complex 19 was found to be the highest noxious antimicrobial active compound. The compounds were also assessed for their antioxidant activity and found that the complexes are more potent and show excellent efficiency in decolorizing the DPPH purple colored solution than their free ligands. The complexes 6, 7, 10, 11, 14, 15, 18, 19 were found to be excellent antioxidants and copper(II) complexes were the most potent antioxidant compounds with lowest IC50 values (2.04–2.56 µM)) among all the tested compounds. Total 20 compounds of Schiff bases transition metal complexes were synthesized and characterized. The copper complex (19) was found to be highest antimicrobial against Streptococcus gordonii, Staphylococcus aureus, Escherichia coli, Pseudomonas aeruginosa, Aspergillus niger, Candida albicans. The copper(II) complex (11) exhibits outstanding and highest antioxidant activity (IC50 = 2.04 µM).

Synthesis, crystallographic characterization and Hirshfeld surface analysis of metal complexes of conformationally constrained β-amino acid, 2-(1-aminocyclohexyl)acetic acid with CoII, NiII, CuII and ZnII ions

2-(1-aminocyclohexyl)acetic acid (β 3,3-Ac6cH) is selected as a lead compound to synthesize a new series of mononuclear transition metal complexes of the type [MII-(Κ2-O,N- β 3,3-Ac6c)2(H2O)2] (M = CoIIC1, NiIIC2, CuIIC3, and ZnIIC4. All the complexes C1-C4 have been suitably characterized by relevant spectroscopic techniques, viz. NMR (1H, 13C, DEPT 135), UV–visible, IR, and HRMS (for C1) spectroscopies. These complexes displayed ν(M-N) and ν(M-O) bands in the region of 417–448 and 395–410 cm−1, respectively having maximum absorbance in the UV region. A clear blue shifts up to 20–51 nm of n→π* electronic transition band (360–390 nm), compared to its position in β3,3-Ac6cH, clearly demonstrates the participation of amine/carboxylate moieties in the complex formation. In addition, complexes C1, C2, and C3 exhibit weak d-d transition bands in the visible region at 560 nm, 630 nm, and 602 nm, respectively. The X-ray analysis of both complexes C3 and C4 confirms the distorted octahedral geometry around Cu and Zn centers. In both complexes, the equatorial positions are occupied by N, O donor sites of coordinated ligands, while the axial positions are occupied by oxygen atoms of the two water molecules. The molecules of C3 and C4 are predominantly stabilized by several intermolecular bifurcated non-conventional hydrogen bonding interactions, viz. NH…O, and OH…O in the solid state. Notably, the space fill model demonstrates the formation of rectangular voids in the molecular packing of C3, showing 2D sheet like architecture in ab-plane. The Hirshfeld surface and subsequent fingerprint plots were calculated to quantify the intermolecular contacts present within the crystal structure of compounds C3 and C4.