Estimation of the reducing power and electrochemical behavior of few flavonoids and polyhydroxybenzophenones substantiated by bond dissociation energy: a comparative analysis.

Oxidative stress that damages cellular components affects various organs including the brain. It is thus believed to play an active role in neurodegenerative diseases, wherein the intrinsic antioxidant enzymes metabolize toxic intermediates. For therapeutic purpose, instead of antioxidant enzymes, small organic compounds as antioxidants may be more effective. Here, reducing power and electrochemical behavior of some flavanols, flavanonols, flavones, flavonols and O-methylated flavonols have been estimated and confirmed by the calculated bond dissociation energy. Compared to other classes, flavonols exhibited increased reducing power that decreased with methylation of the oxygen atom in the B-ring. Gossypetin emerged as the most effective of these flavonols. Generally, compounds with two hydroxyl groups in two consecutive positions of the phenyl ring and an enolic group in the C-ring with more preference for the hydroxyl group in the ortho position with respect to each other in the catechol moiety showed major activity. 5 position of the A-ring showed the least effect on the activity. The present understanding therefore may be applied for identifying compounds to be used as scaffold for designing potent antioxidants.

Synthesis of Ni(ii)-Mn(ii) complexes using a new mononuclear Ni(ii) complex of an unsymmetrical N2O3 donor ligand: structures, magnetic properties and catalytic oxidase activity.

A new Ni(ii) complex [NiL] (complex 1) of an asymmetrically di-condensed N2O3 donor Schiff base ligand, N-salicylidene-N'-3-methoxysalicylidene-1,3-propanediamine (H2L), has been synthesized and utilized for the synthesis of three heterometallic complexes, [(NiL)2Mn(NCS)2(CH3OH)2]·CH3OH (2) [(NiL)2Mn(N(CN)2)2(CH3OH)2]·CH3OH (3) and [(NiL)2Mn2(N3)2(µ1,1-N3)2(CH3OH)2] (4). Single crystal X-ray diffraction analyses show that complexes 2 and 3 have linear trinuclear structures where two tridentate O3 donor (NiL) units are coordinated to the central octahedral Mn(ii) centre, whereas complex 4 has a centrosymmetric tetranuclear structure where two binuclear (NiL)Mn units are linked via two phenoxido and two µ1,1-N3 bridges. Among the heterometallic complexes (2-4), only 4 is active towards the catalytic oxidation of 3,5-di-tert-butylcatechol to the corresponding quinone. The turnover number for the aerobic oxidation of 3,5-DTBC is 935 h-1. ESI-mass spectra have been recorded to scrutinize the mechanistic pathway of this catalytic reaction. Variable temperature magnetic susceptibility measurements suggest that complexes 2-4 are antiferromagnetically coupled with coupling constants (J) of -4.84 and -5.23 cm-1 for complexes 2 and 3, respectively and J1 = -2.20 cm-1, J2 = 1.13 cm-1 and J3 = -1.12 cm-1 for complex 4. DFT calculations have been used to rationalize the magnetic super-exchange in complexes 2-4, by computing the theoretical coupling constants and analyzing the spin density plots.

Joining of Trinuclear Heterometallic CuII2-MII (M = Mn, Cd) Nodes by Nicotinate to Form 1D Chains: Magnetic Properties and Catalytic Activities.

In the present work, four new heterometallic coordination complexes, {[(CuL)2Mn(nic)(H2O)2](ClO4)(0.5H2O)}n (1), {[(CuL)2Cd(nic)(H2O)2](ClO4)(H2O)}n (2), [(CuL)2Mn(nic)2]·2CH3OH (3), and [(CuL)2Cd(nic)2]·2CH3OH (4) (where H2L = N,N'-bis(α-methylsalicylidene)-1,3-propanediamine and nic = nicotinate ion), have been synthesized and characterized by single-crystal X-ray crystallography. In complexes 1 and 2, the nicotinate ion acts as a bifunctional linker (N,O donor) and joins the linear trinuclear nodes to form 1D polymeric chains. However, in complexes 3 and 4, the nicotinate ion uses only the oxygen atoms of the carboxylic acid (O donor) to bind to the metal centers, forming discrete linear trinuclear units, while the pyridyl nitrogen (N donor atom) remains free. The dc magnetic susceptibility measurements show that the CuII and MnII ions are antiferromagnetically coupled in both 1 and 3, with exchange coupling constants (JMn-Cu) of -20.57 ± 0.08 and -9.38 ± 0.08 cm-1, respectively. Among the four complexes, 1 and 3 show catechol oxidase and phenoxazinone synthase like catalytic activities. The turnover numbers (kcat) of complexes 1 and 3 for catecholase activity are 1121 and 720 h-1, respectively, at an optimum pH of 8.0 and for phenoxazinone synthase activity are 429 and 398 h-1, respectively, at an optimum pH of 9.7. The higher kcat values of 1 for both reactions are attributable to a water molecule coordinated to the central MnII atom that facilitates the substrate-catalyst binding. An ESI-mass spectral analysis indicates that trinuclear heterometallic species, e.g., [(CuL)2Mn(nic)(H2O)]+ for 1 and [(CuL)2Mn(nic)]+ for 3, are the active species that bind to the substrate, and on that basis, probable mechanisms through the formation of radical intermediates have been proposed.

Correction: Tri- and hexa-nuclear NiII-MnII complexes of a N2O2 donor unsymmetrical ligand: synthesis, structures, magnetic properties and catalytic oxidase activities.

Correction for 'Tri- and hexa-nuclear NiII-MnII complexes of a N2O2 donor unsymmetrical ligand: synthesis, structures, magnetic properties and catalytic oxidase activities' by A. Ghosh et al., Dalton Trans., 2018, 47, 13957-13971.

The Effect of Guest Metal Ions on the Reduction Potentials of Uranium(VI) Complexes: Experimental and Theoretical Investigations.

Two mononuclear uranyl complexes, [UO2 L1 ] (1) and [UO2 L2 ]⋅0.5 CH3 CN⋅0.25 CH3 OH (2), have been synthesized from two multidentate N3 O4 donor ligands, N,N'-bis(5-methoxysalicylidene)diethylenetriamine (H2 L1 ) and N,N'-bis(3-methoxysalicylidene)diethylenetriamine (H2 L2 ), respectively, and have been structurally characterized. Both complexes 1 and 2 showed a reversible UVI /UV couple at -1.571 and -1.519 V, respectively, in cyclic voltammetry. The reduction potential of the UVI /UV couple shifted towards more positive potential on addition of Li+ , Na+ , K+ , and Ag+ metal ions to acetonitrile solutions of complex 2, and the resulting potential was correlated with the Lewis acidity of the metal ions and was also justified by theoretical DFT calculations. No such shift in reduction potential was observed for complex 1. All four bimetallic products, [UO2 L2 Li0.5 ](ClO4 )0.5 (3), [UO2 L2 Na(ClO4 )]2 (4), [UO2 L2 Ag(NO3 )(H2 O)] (5), and [(UO2 L2 )2 K(H2 O)2 ]PF6 (6), formed on addition of the Li+ , Na+ , Ag+ , and K+ metal ions, respectively, to acetonitrile solutions of complex 2, were isolated in the solid state and structurally characterized by single-crystal X-ray diffraction. In all the species, the inner N3 O2 donor set of the ligand encompasses the equatorial plane of the uranyl ion and the outer open compartment with O2 O'2 donor sites hosts the second metal ion.

Tri- and hexa-nuclear NiII-MnII complexes of a N2O2 donor unsymmetrical ligand: synthesis, structures, magnetic properties and catalytic oxidase activities.

A new mononuclear Ni(ii) complex, [NiL] (1) of an unsymmetrically dicondensed N2O2 donor ligand, H2L (where H2L = N-α-methylsalicylidene-N'-salicylidene-1,3-propanediamine), has been synthesized. Complex 1, on reaction with Mn(ii) in the presence of different co-anions, yielded three heterometallic NiII-MnII complexes, [(NiL)2Mn(NCS)2]·CH3CN (2), [(NiL)2Mn(N3)(H2O)](ClO4)·H2O (3) and [{(NiL)2Mn(H2O)}2(µ1,3-N3)](ClO4)3 (4). Single crystal structural analyses show that complexes 2 and 3 contain similar bent trinuclear NiII2MnII structures, with the difference that in complex 2 two SCN- ions and in 3, one N3- and one H2O molecule are coordinated to the Mn(ii) centre. Complex 4 possesses a hexanuclear structure, in which two trinuclear NiII2MnII units are connected via a single µ1,3-azido bridge. Variable temperature dc molar magnetic susceptibility measurements reveal that the two Mn(ii) centres in complex 4 are antiferromagnetically coupled with an exchange coupling constant (J) of -3.73 cm-1. Among the three heterometallic complexes, only 3 and 4 show bio-mimetic catalytic oxidase activities. For catecholase like activity, the turnover numbers (Kcat) of complexes 3 and 4 are 984 and 2081 h-1, respectively, whereas for phenoxazinone synthase-like activity, the turnover numbers of complexes 3 and 4 are 6351 and 10 545 h-1, respectively. The high catalytic oxidase activities of complexes 3 and 4 in contrast to the inactivity of complex 2 are attributed to the coordination of a water molecule to the Mn(ii) centre in the former structures, which facilitates the binding of the substrate with the catalyst. Mass spectral analyses suggest the probable formation of a complex-substrate intermediate in these catalytic reactions and cyclic voltammetry measurements show that Ni(ii) is reducible to Ni(i).

Ni(II) Complex of N2O3 Donor Unsymmetrical Ligand and Its Use for the Synthesis of NiII-MnII Complexes of Diverse Nuclearity: Structures, Magnetic Properties, and Catalytic Oxidase Activities.

A new mononuclear Ni(II) complex [NiL] (1) of an unsymmetrically dicondensed N2O3 donor ligand, H2L ( N-α-methylsalicylidene- N'-3-methoxysalicylidene-1,3-propanediamine), has been synthesized. Complex 1 on reaction with Mn(ClO4)2·6H2O and NaN3 in different molar ratios yielded three novel heterometallic NiII-MnII complexes, [(NiL)2Mn(N3)](ClO4) (2), [(NiL)2Mn2(N3)2(µ1,1-N3)2(CH3OH)2] (3), and [{(NiL)2Mn}2(µ1,3-N3)(H2O)]·(CH3OH),(ClO4)3 (4). The single crystal structure analyses show a trinuclear NiII2MnII structure for complex 2 and a tetranuclear NiII2MnII2 structure where two dinuclear NiIIMnII units are connected via µ1,1-azido and phenoxido bridges for complex 3. Complex 4 possesses a hexanuclear structure where two trinuclear NiII2MnII units are connected via a µ1,3-azido bridge. The temperature-dependent dc molar magnetic susceptibility measurements reveal that complexes 3 and 4 are antiferromagnetically coupled with the exchange coupling constants ( J) of -4.97, -0.14, -0.55 cm-1 for 3 and -3.94 cm-1 for 4. All complexes 2-4 show biomimetic catalytic oxidase activities. For catecholase like activity, the turnover numbers ( Kcat) are 768, 1985, and 2309 h-1 for complexes 2-4, respectively, whereas for phenoxazinone synthase like activity, the turnover numbers are 3240, 3360, and 13 248 h-1 for complexes 2-4, respectively. This difference in catalytic efficiencies is attributed to the variations in structures of the complexes and formation of active NiII-MnII species in solution during catalysis. The mass spectral analyses suggest the probable intermediate formation and cyclic voltammetry measurement suggest the reduction of Ni(II) to Ni(I) during catalytic reaction. The very high catalytic efficiencies for aerial dioxygen activation of all these heterometallic complexes as well as the highest activity of 4 is attributed to the coordinatively unsaturated penta-coordinated geometry or hexa-coordinated geometry with a solvent water molecule around Mn(II).

Control of nuclearity in heterometallic CuII-MnII complexes derived from asymmetric Schiff bases: structures and magnetic properties.

Two new N2O3 donor ligands N-(3-methoxysalicylidene)-N'-(salicylidene)-1,3-propanediamine (H2L2) and N-(3-methoxysalicylidene)-N'-(α-methylsalicylidene)-1,3-propanediamine (H2L3) and their Cu(ii) complexes have been synthesized. Using these complexes and another Cu-complex with one asymmetric N2O2 donor ligand N-(α-methylsalicylidene)-N'-(salicylidene)-1,3-propanediamine (H2L1) as metalloligands, four new heterometallic Cu(ii)-Mn(ii) complexes [(CuL1)2Mn2Cl4]·3CH3OH (1), [(CuL1)2MnCl2]·2CH3CN (2), [CuL2MnCl2]·H2O (3) and [CuL3MnCl2] (4) have been synthesized with MnCl2·4H2O. The tetranuclear (1) and trinuclear (2) complexes were prepared by varying the ratio of the metalloligand [CuL1] and MnCl2 whereas dinuclear complexes 3 and 4 are the sole products irrespective of the metalloligand : MnCl2 ratio. All the complexes 1-4 were structurally characterized by single crystal X-ray diffraction. The tetranuclear (CuMn) structure of 1 is formed by the dimerization of phenoxido bridged CuIIMnII units whereas the bent CuMnII trinuclear species 2 is formed by the coordination of two metalloligands (CuL1) to Mn(ii) via a diphenoxido bridge. Complexes 3 and 4 possess a similar dinuclear CuIIMnII structure in which two phenoxido groups form a bridge between the two metal centres. The variable temperature dc magnetic susceptibility measurements reveal that the Cu(ii) and Mn(ii) ions are antiferromagnetically coupled in all the complexes, 1-4. The values of exchange coupling constants (JMn-Cu) are -45.54, -39.66, -71.54 and -48.40 cm-1 for complexes 1-4, respectively. A DFT study of all the complexes has also been performed to rationalize the experimental results of magnetic measurements.

Structural variations in (CuL)2Ln complexes of a series of lanthanide ions with a salen-type unsymmetrical Schiff base(H2L): Dy and Tb derivatives as potential single-molecule magnets.

A new series of heterometallic trinuclear Cu2Ln complexes [lanthanide ions Ln = Gd (1), Tb (2), Dy (3), Ho (4) and Er (5)] has been synthesized using a Cu(ii)-metalloligand derived from a N2O2 donor unsymmetrical Schiff base, H2L (where H2L = N-α-methylsalicylidene-N'-salicylidene-1,3-propanediamine), and structurally characterized. Among these complexes, [(CuL)2Gd(NO3)3(CH3CN)2] (1), [(CuL)2Tb(NO3)3(CH3CN)2] (2) and [(CuL)2Dy(NO3)3(CH3CN)2] (3) are isomorphic and isostructural. In these complexes two metalloligands coordinate to the central Ln(iii) (Ln = Gd, Tb and Dy respectively) ion in a transoid fashion viaµ2-phenoxido oxygen atoms. The Ln(iii) ions are deca-coordinated with a distorted tetradecahedron geometry. The two terminal Cu(ii) ions of the complexes possess a hexa-coordinated distorted octahedral geometry. In contrast, in complexes [(CuL)2Ho(NO3)3(CH3CN)], (4) and [(CuL)2Er(NO3)3(CH3CN)]·0.5(CH3CN) (5), the two metalloligands coordinated to the Ln(iii) ions in a cisoid fashion. The Ho(iii) ion in 4 is nona-coordinated with a distorted tricapped trigonal prismatic geometry and the Er(iii) ion in 5 is octa-coordinated with a distorted square antiprismatic geometry. The two terminal Cu(ii) ions in complexes 4 and 5 are penta-coordinated with a distorted square-pyramidal geometry. The dc magnetic susceptibilities and field dependent magnetization measurement of complex 1 reveal the occurrence of ferromagnetic interactions between Cu(ii) and Gd(iii) ions as well as intermolecular antiferromagnetic interactions. Both complexes 2 and 3 show ferromagnetic interactions between Cu(ii) and Ln(iii) ions. The ac magnetic susceptibilities of all the complexes were also recorded and it was found that only complexes 2 and 3 exhibit slow relaxation of magnetization reorientation below 10 K at 2000 Oe applied dc field, this being characteristic of single molecule magnets.

Subtle Structural Changes in (CuIIL)2MnII Complexes To Induce Heterometallic Cooperative Catalytic Oxidase Activities on Phenolic Substrates (H2L = Salen Type Unsymmetrical Schiff Base).

A new Cu(II) complex of an asymmetrically dicondensed Schiff base (H2L = N-(2-hydroxyacetophenylidene)-N'-salicylidene-1,3-propanediamine) derived from 1,3-propanediamine, salicylaldehyde, and o-hydroxyacetophenone has been synthesized. Using this complex, [CuL] (1), as a metalloligand, two new trinuclear Cu-Mn complexes, [(CuL)2Mn(N3)(H2O)](ClO4)·H2O (2) and [(CuL)2Mn(NCS)2] (3), have been prepared. Single-crystal structural analyses reveal that complexes 2 and 3 both have the same bent trinuclear {(CuL)2Mn}2+ structural unit in which two terminal bidentate square-planar (CuL) units are chelated to the central octahedral Mn(II) ion. This structural similarity is also evident from the variable-temperature magnetic susceptibility measurements, which suggest that compounds 2 and 3 are both antiferromagnetically coupled with comparable exchange coupling constants (-21.8 and -22.3 cm-1, respectively). The only difference between 2 and 3 lies in the coordination around the central Mn(II) ion; in 3, two SCN- groups are coordinated to the Mn(II), leaving a neutral complex, but in 2, one N3- group and one H2O molecule are coordinated to give a positively charged species. The presence of such a labile H2O coligand makes 2 catalytically active in mimicking two well-known polynuclear copper proteins, catecholase and phenoxazinone synthase. The turnover numbers (kcat) for the aerial oxidation of 3,5-di-tert-butylcatechol and o-aminophenol are 1118 and 6581 h-1, respectively, values which reflect the facility of the heterometallic catalyst in terms of both efficiency and catalytic promiscuity for aerial dioxygen activation. The mechanisms of these biomimetic oxidase reactions are proposed for the first time involving any heterometallic catalyst on the basis of mass spectral analysis, EPR spectroscopy, and cyclic voltammetry. The evidence of the intermediates indicates possible heterometallic cooperative activity where the substrates bind to a Mn(II) center and Cu(II) plays the role of an electron carrier for transformation of the phenolic substrates to their respective products with the reduction of aerial dioxygen.