A comparative kinetic study for the oxidation of 2‐mercaptoethanol by di‐μ‐oxo‐bis(1,4,7,10‐tetraazacyclododecane)‐dimanganese(III,IV) and di‐μ‐oxo‐bis(1,4,8,11‐tetraazacyclotetradecane)‐dimanganese(III,IV) complexes: Influence of copper(II)

A comparative kinetic study of the reactions of two mixed valence manganese(III,IV) complexes with macrocyclic ligands, [L¹Mn^IV(O)₂Mn^IIIL¹], 1 (L¹ = 1,4,7,10‐tetraazacyclododecane) and [L²Mn^IV(O)₂Mn^IIIL²], 2 (L² = 1,4,8,11‐tetraazacyclotetradecane) with 2‐mercaptoethanol (RSH) has been carried out by spectrophotometry in aqueous buffer at (30 ± 0.1)°C. Rate of the reactions between the oxidants and the reductant was found to be negligibly slow with no systematic dependence on either redox partners. Externally added copper(II) (usually 5 × 10^?7 mol dm^?3), however, increases the rate of the reduction of 1 and 2 significantly. In the presence of catalytic amount of copper(II), the rate of the reaction is nearly proportional to [RSH] at lower concentration of the reductant but follows a saturation kinetics at higher concentration of the latter for the reaction between 1 and the thiol. Reaction rate was found to be strongly influenced by the variation of acidity of the medium and the observed kinetics suggests that the two reductant species ([Cu(RSH)]²⁺ and [Cu(RS)]⁺) are significant for the reaction between 1 and the thiol. The dependence of the rate on [RSH] for the reduction of 2 by the thiol was complex and rationalized considering two equilibria involving the catalyst (Cu²⁺) and the reductant. The pH rate profile suggests that both the μ‐O protonated [Mn^III(O)(OH)Mn^IV] and the deprotonated [Mn^III(O)₂Mn^IV] forms of the oxidant 2 become important. The kinetic results presented in this study indicate the domination of outer‐sphere path. © 2003 Wiley Periodicals, Inc. Int J Chem Kinet 36: 129–137, 2004     

Kinetics and mechanism of oxidation of 2‐mercaptosuccinic acid by bis(μ‐oxo)‐ manganese(III,IV)‐cyclam complex in aqueous medium: Influence of externally added copper(II)

Kinetic studies on the oxidation of 2‐mercaptosuccinic acid by dinuclear [Mn₂^III/IV(μ‐O)₂(cyclam)₂](ClO₄)₃] ( 1 ) (abbreviated as Mn^III–Mn^IV) (cyclam = 1,4,8,11‐tetraaza‐cyclotetradecane) have been carried out in aqueous medium in the pH range of 4.0–6.0, in the presence of acetate buffer at 30°C by UV–vis spectrophotometry. In the pH region, two species of complex 1 (Mn^III–Mn^IV and Mn^III–Mn^IVH, the later being μ‐O protonated form) were found to be kinetically significant. The first‐order dependence of the rate of the reactions on [Thiol] both in presence and absence of externally added copper(II) ions, first‐order dependence on [Cu²⁺] and a decrease of rate of the reactions with increase in pH have been rationalized by suitable sequence of reactions. Protonation of μ‐O bridge of 1 is evidenced by the perchloric acid catalyzed decomposition of 1 to mononuclear Mn(III) and Mn(IV) complex observed by UV–vis and EPR spectroscopy. The kinetic features have been rationalized considering Cu(RSH) as the reactive intermediate. EPR spectroscopy lends support for this. The formation of a hydrogen bonded outer‐sphere adduct between the reductant and the complex in the lower pH range prior to electron transfer reactions is most likely to occur. © 2004 Wiley Periodicals, Inc. Int J Chem Kinet 36: 170–177 2004     

A mixed‐valence manganese(III)–manganese(IV) di‐μ‐oxo complex, [(cyclam)MnO]2(ClO4)2(NO3)

The title dinuclear di‐μ‐oxo‐bis­[(1,4,8,11‐tetra­aza­cyclo­tetra­decane‐κ⁴N)­manganese(III,IV)] diperchlorate nitrate complex, [Mn₂O₂(C₁₀H₂₄N₄)₂](ClO₄)₂(NO₃) or [(cyclam)Mn­O]₂(ClO₄)₂(NO₃), was self‐assembled by the reaction of Mn²⁺ with 1,4,8,11‐tetra­aza­cyclo­tetra­decane in aqueous media. The structure of this compound consists of a centrosymmetric binuclear [(cyclam)MnO]³⁺ unit, two perchlorate anions and one nitrate anion. While the low‐temperature electron paramagnetic resonance spectra show a typical 16‐line signal for a di‐μ‐oxo Mn^III/Mn^IV dimer, the magnetic susceptibility studies also confirm a characteristic antiferromagnetic coupling between the electronic spins of the Mn^IV and Mn^III ions.     

Bis[μ3‐1,8‐bis(triisopropylsilylamido)naphthalene]bis(tetrahydrofuran)di‐μ3‐oxido‐dimanganese(III)disodium

The solid‐state structure of the title compound, [Na₂Mn₂(C₃₂H₅₆N₂OSi₂)₂O₂] or [1,8‐C₁₀H₆(NSiⁱPr₃)₂Mn(μ₃‐O)Na(THF)]₂, which lies across a crystallographic twofold axis, exhibits a central [Mn₂O₂Na₂]⁴⁺ core, with two oxide groups, each triply bridging between the two Mn^III ions and an Na⁺ ion. Additional coordination is provided to each Mn^III centre by a 1,8‐C₁₀H₆(NSiⁱPr₃)₂ [1,8‐bis(triisopropylsilylamido)naphthalene] ligand and to the Na⁺ centres by a tetrahydrofuran molecule. The presence of an additional Na...H—C agostic interaction potentially contributes to the distortion around the bridging oxide group.     

Mechanistic Insight into the Nitric Oxide Dioxygenation Reaction of Nonheme Iron(III)–Superoxo and Manganese(IV)–Peroxo Complexes

Reactions of nonheme Fe^III–superoxo and Mn^IV–peroxo complexes bearing a common tetraamido macrocyclic ligand (TAML), namely [(TAML)Fe^III(O₂)]^2? and [(TAML)Mn^IV(O₂)]^2?, with nitric oxide (NO) afford the Fe^III–NO₃ complex [(TAML)Fe^III(NO₃)]^2? and the Mn^V–oxo complex [(TAML)Mn^V(O)]^? plus NO₂^?, respectively. Mechanistic studies, including density functional theory (DFT) calculations, reveal that M^III–peroxynitrite (M=Fe and Mn) species, generated in the reactions of [(TAML)Fe^III(O₂)]^2? and [(TAML)Mn^IV(O₂)]^2? with NO, are converted into M^IV(O) and ^.NO₂ species through O?O bond homolysis of the peroxynitrite ligand. Then, a rebound of Fe^IV(O) with ^.NO₂ affords [(TAML)Fe^III(NO₃)]^2?, whereas electron transfer from Mn^IV(O) to ^.NO₂ yields [(TAML)Mn^V(O)]^? plus NO₂^?.     

A novel heterometallic dinuclear compound: rac‐pentaaqua‐1κ5O‐(μ‐2‐sulfidoacetato‐1:2κ3O:O′,S)bis(2‐sulfidoacetato‐2κ2O,S)manganese(II)tin(IV)

The title racemic heterometallic dinuclear compound, [MnSn(C₂H₂O₂S)₃(H₂O)₅], (I), contains one main group Sn^IV metal centre and one transition metal Mn^II centre, and, by design, links the Mn^II centre to the building unit of the (Δ/Λ) [SnL₃]²⁻ complex anion (L is the 2‐sulfidoacetate dianion). In this cluster, the Sn^IV centre of the (Δ/Λ) [SnL₃]²⁻ unit is coordinated by three O atoms and three S atoms from three L ligands to form an [SnO₃S₃] octahedral coordination environment. The Mn^II centre is in an [MnO₆] octahedral coordination environment, with five O atoms from five water molecules and the sixth from the μ₂‐L ligand of the (Δ/Λ) [SnL₃]²⁻ unit. Between adjacent dinuclear molecules, there are many hydrogen‐bond interactions of O—H...O, O—H...S, C—H...O and C—H...S types. Of these, eight pairs of O—H...O hydrogen bonds fuse all the dinuclear molecules into two‐dimensional supramolecular sheets along the bc plane. Adjacent supramolecular sheets are further connected through O—H...S hydrogen bonds to give a three‐dimensional supramolecular network.     

A High‐Valent Non‐Heme μ‐Oxo Manganese(IV) Dimer Generated from a Thiolate‐Bound Manganese(II) Complex and Dioxygen

This study deals with the unprecedented reactivity of dinuclear non‐heme Mn^II–thiolate complexes with O₂, which dependent on the protonation state of the initial Mn^II dimer selectively generates either a di‐μ‐oxo or μ‐oxo‐μ‐hydroxo Mn^IV complex. Both dimers have been characterized by different techniques including single‐crystal X‐ray diffraction and mass spectrometry. Oxygenation reactions carried out with labeled ¹⁸O₂ unambiguously show that the oxygen atoms present in the Mn^IV dimers originate from O₂. Based on experimental observations and DFT calculations, evidence is provided that these Mn^IV species comproportionate with a Mn^II precursor to yield μ‐oxo and/or μ‐hydroxo Mn^III dimers. Our work highlights the delicate balance of reaction conditions to control the synthesis of non‐heme high‐valent μ‐oxo and μ‐hydroxo Mn species from Mn^II precursors and O₂.     

Bis(μ‐6‐hydroxy­picolinato)‐μ‐oxo‐bis­[di­pyridine­manganese(III)] monohydrate

The title compound, [Mn₂(μ‐O)(C₆H₃NO₃)₂(C₅H₅N)₄]·H₂O, was isolated from the reaction of 2,6‐pyridine­di­carboxylic acid with [Mn₁₂O₁₂(CH₃COO)₁₆(H₂O)₄] in pyridine. The dimanganese complex has twofold symmetry; the Mn^III atoms are bridged by one oxo and two amidate ligands and show compressed octahedral Jahn–Teller distortion. The molecular packing comprises a three‐dimensional structure constructed by means of extensive intermolecular interactions, including three kinds of hydrogen bonds and π–π interactions.     

Mononuclear Manganese–Peroxo and Bis(μ‐oxo)dimanganese Complexes Bearing a Common N‐Methylated Macrocyclic Ligand

Mononuclear Mn^III–peroxo and dinuclear bis(μ‐oxo)Mn^III₂ complexes that bear a common macrocyclic ligand were synthesized by controlling the concentration of the starting Mn^II complex in the reaction of H₂O₂ (i.e., a Mn^III–peroxo complex at a low concentration (≤1 mM ) and a bis(μ‐oxo)Mn^III₂ complex at a high concentration (≥30 mM )). These intermediates were successfully characterized by various physicochemical methods such as UV–visible spectroscopy, ESI‐MS, resonance Raman, and X‐ray analysis. The structural and spectroscopic characterization combined with density functional theory (DFT) calculations demonstrated unambiguously that the peroxo ligand is bound in a side‐on fashion in the Mn^III–peroxo complex and the Mn₂O₂ diamond core is in the bis(μ‐oxo)Mn^III₂ complex. The reactivity of these intermediates was investigated in electrophilic and nucleophilic reactions, in which only the Mn^III–peroxo complex showed a nucleophilic reactivity in the deformylation of aldehydes.     

Di‐μ‐oxido‐bis{bis[N,N′‐bis(2‐pyridylmethyl)ethane‐1,2‐diamine]manganese(III,IV)} tris(perchlorate) hexahydrate: clarification of an order–disorder phase transition

The title compound, [Mn₂O₂(C₁₄H₁₈N₄)₂](ClO₄)₃·6H₂O, contains a mixed‐valent Mn^III/Mn^IV complex. In accordance with a previous report [Collins, Hodgson, Michelsen & Towle (1987). J. Chem. Soc. Chem. Commun. pp. 1659–1660], the structure at 295 K is best described in the space group C2/c, with the complex exhibiting twofold rotational symmetry, and with half site occupancy for one perchlorate anion and several solvent water molecules. At 180 K, the structure is ordered in the subgroup P2₁/n and is clearly shown to be a hexahydrate, rather than the previously reported trihydrate. The origin of the order–disorder phase transition lies in the thermal motion of the perchlorate anions.     

A novel one‐dimensional complex: catena‐poly­[[manganese(III)‐di‐μ‐2‐[(2‐hydroxy­ethyl)­imino­methyl]­phenolato‐κ2O1,N:κO2;κO2:κ2O1] chloride]

In the title one‐dimensional complex, {[Mn^III(C₉H₁₀NO₂)₂]Cl}_n, the Schiff base ligand 2‐[(2‐hydroxy­ethyl)­imino­methyl]­phenolate (Hsae⁻) functions as both a bridging and a chelating ligand. The Mn^III ion is six‐coordinated by two N and four O atoms from four different Hsae⁻ ligands, yielding a distorted MnO₄N₂ octahedral environment. Each [Mn^III(Hsae)₂]⁺ cationic unit has the Mn atom on an inversion centre and each [Mn^III(Hsae)₂]⁺ cation lies about another inversion centre. The chain‐like complex is further extended into a three‐dimensional network structure through Cl⋯H—O hydrogen bonds and C—H⋯π contacts involving the Hsae⁻ rings.     

Poly[[bis(pyridin‐3‐ol)manganese(II)]‐di‐μ‐pyridin‐3‐olato]

In the title complex, [Mn(C₅H₄NO)₂(C₅H₅NO)₂]_n or [Mn(μ‐3‐PyO)₂(3‐PyOH)₂]_n (3‐PyO⁻ is the pyridin‐3‐olate anion and 3‐PyOH is pyridin‐3‐ol), the Mn^II atom lies on an inversion centre and has octahedral geometry, defined by two N atoms and two deprotonated exocyclic O atoms of symmetry‐related pyridin‐3‐olate ligands [Mn—N = 2.3559 (14) Å and Mn—O = 2.1703 (11) Å], as well as two N atoms of terminal 3‐PyOH ligands [Mn—N = 2.3482 (13) Å]. The Mn^II atoms are bridged by the deprotonated pyridin‐3‐olate anion into a layer structure, generating sheets in the (01) plane. These sheets are linked by O—H⋯O hydrogen bonds. There are also π–π and C—H⋯π interactions in the crystal structure.     

catena‐Poly[[bis(μ‐2‐sulfonatobenzoato)bis[triaquagadolinium(III)]]‐μ‐oxalato]

The asymmetric unit of the title coordination polymer, [Gd₂(C₇H₄O₅S)₂(C₂O₄)(H₂O)₆]_n or [Gd(2‐SB)(ox)_0.5(H₂O)₃]_2n (2‐SB is 2‐sulfonatobenzoate and ox is oxalate), (I), consists of one Gd^III ion, one 2‐SB anion, three coordinated water molecules and one half of an ox ligand. The ox ligand is located on a crystallographic inversion centre. The Gd^III centre shows a distorted tricapped trigonal–prismatic coordination formed by nine O atoms from two 2‐SB anions, one ox ligand and three coordinated water molecules. The carboxylate and sulfonate groups of the 2‐SB anions adopt μ₂‐η¹:η² and μ₁‐η⁰:η⁰:η¹ coordination modes to link two Gd^III ions, generating a centrosymmetric binuclear [Gd₂(2‐SB)₂(H₂O)₆]²⁻ subunit. The ox ligand acts as a bridge, linking the binuclear [Gd₂(2‐SB)₂(H₂O)₆]²⁻ subunits into a one‐dimensional chain structure parallel to the b axis. Furthermore, extensive O—H...O hydrogen bonds connect the chains into a three‐dimensional supramolecular architecture.     

Binuclear Homoleptic Manganese(III,III) and Manganese(IV,III) Complexes with Deprotonated D-Mannose from Aqueous Solution

Just a “reducing” sugar —namely, D -mannose—is a starting material in the synthesis of a mixed-valence complex of manganese in the oxidation states +III and +IV . Ba₂[Mn^IIIMn^IV(β-D -ManfH₋₅)₂]Cl⋅14 H₂O (Manf=mannofuranose; the structure of the anion is shown on the right) is prepared in aqueous solution by oxidation of an analogous Mn₂^III complex with oxygen. In neutral solutions the Mn^IIIMn^IV binuclear complex is formed by disproportionation of the Mn₂^III precursor.     

Facile and Reversible Formation of Iron(III)–Oxo–Cerium(IV) Adducts from Nonheme Oxoiron(IV) Complexes and Cerium(III)

Ceric ammonium nitrate (CAN) or Ce^IV(NH₄)₂(NO₃)₆ is often used in artificial water oxidation and generally considered to be an outer‐sphere oxidant. Herein we report the spectroscopic and crystallographic characterization of [(N4Py)Fe^III‐O‐Ce^IV(OH₂)(NO₃)₄]⁺ ( 3 ), a complex obtained from the reaction of [(N4Py)Fe^II(NCMe)]²⁺ with 2 equiv CAN or [(N4Py)Fe^IV=O]²⁺ ( 2 ) with Ce^III(NO₃)₃ in MeCN. Surprisingly, the formation of 3 is reversible, the position of the equilibrium being dependent on the MeCN/water ratio of the solvent. These results suggest that the Fe^IV and Ce^IV centers have comparable reduction potentials. Moreover, the equilibrium entails a change in iron spin state, from S =1 Fe^IV in 2 to S =5/2 in 3 , which is found to be facile despite the formal spin‐forbidden nature of this process. This observation suggests that Fe^IV=O complexes may avail of reaction pathways involving multiple spin states having little or no barrier.     

trans‐Bis(cyano‐κC)(1,4,8,11‐tetraazacyclotetradecane‐κ4N)manganese(III) perchlorate,a low‐spin manganese(III) complex

The crystal structure of the low‐spin (S = 1) Mn^III complex [Mn(CN)₂(C₁₀H₂₄N₄)]ClO₄, or trans‐[Mn(CN)₂(cyclam)](ClO₄) (cyclam is the tetradentate amine ligand 1,4,8,11‐tetra­aza­cyclo­tetra­decane), is reported. The structural parameters in the Mn(cyclam) moiety are found to be insensitive to both the spin and the oxidation state of the Mn ion. The difference between high‐ and low‐spin Mn^III complexes is that a pronounced tetragonal elongation of the coordination octahedron occurs in high‐spin complexes and a slight tetragonal compression is seen in low‐spin complexes, as in the title complex.     

Synthesis,characterization and α‐amylase and α‐glucosidase inhibition studies of novel vanadyl chalcone complexes

A series of chalcone ligands and their corresponding vanadyl complexes of composition [VO (L^I–IV)₂(H₂O)₂]SO₄ (where L^I = 1,3‐Diphenylprop‐2‐en‐1‐one, L^II = 3‐(2‐Hydroxy‐phenyl)‐1‐phenyl‐propenone, L^III = 3‐(3‐Nitro‐phenyl)‐1‐phenyl‐propenone, L^IV = 3‐(4‐Methoxy‐phenyl)‐1‐phenyl‐propenone) have been synthesized and characterized using various spectroscopic (Fourier‐transform infrared, electrospray ionization mass, nuclear magnetic resonance, electron paramagnetic resonance, thermogravimetric analysis, vibrating sample magnetometer) and physico‐analytic techniques. Antidiabetic activities of synthesized complexes along with chalcones were evaluated by performing in vitro and in silico α‐amylase and α‐glucosidase inhibition studies. The obtained results displayed moderate to significant inhibition activity against both the enzymes by vanadyl chalcone complexes. The most potent complexes were further investigated for the enzyme kinetic studies and displayed the mixed inhibition for both the enzymes. Further, antioxidant activity of vanadyl chalcone complexes was evaluated for their efficiency to release oxidative stress using 2,2‐diphenyl‐1‐picryl‐hydrazyl‐hydrate assay, and two complexes (Complexes 2 and 4 ) have demonstrated remarkable antioxidant activity. All the complexes were found to possess promising antidiabetic and antioxidant potential.     

Disproportionation Equilibrium of a μ‐Oxodiiron(III) Complex Giving Rise to C−H Activation Reactivity: Structural Snapshot of a Unique Oxoiron(IV) Adduct

μ‐Oxodiiron(III) species are air‐stable and unreactive products of autoxidation processes of monomeric heme and non‐heme iron(II) complexes. Now, the organometallic [(L^NHC)Fe^III‐(μ‐O)‐Fe^III(L^NHC)]⁴⁺ complex 1 (L^NHC is a macrocyclic tetracarbene) is shown to be reactive in C?H activation without addition of further oxidants. Studying the oxidation of dihydroanthracene, it was found that 1 thermally disproportionates in MeCN solution into its oxoiron(IV) ( 2 ) and iron(II) components; the former is the active species in the observed oxidation processes. Possible cleavage scenarios for 1 are shown by scrambling experiments and structural characterization of an unprecedented adduct of 1 and oxoiron(IV) complex 2 . Kinetic analysis gave an equilibrium constant for the disproportionation of 1 , which is very small (K_eq=7.5±2.5×10^?8 m ). Increasing K_eq might by a useful strategy for circumventing the formation of dead‐end μ‐oxodiiron(III) products during Fe‐based homogeneous oxidation catalysis.     

A High‐Valent Manganese(IV)–Oxo–Cerium(IV) Complex and Its Enhanced Oxidizing Reactivity

A mononuclear nonheme manganese(IV)–oxo complex binding the Ce⁴⁺ ion, [(dpaq)Mn^IV(O)]⁺–Ce⁴⁺ ( 1 ‐Ce⁴⁺), was synthesized by reacting [(dpaq)Mn^III(OH)]⁺ ( 2 ) with cerium ammonium nitrate (CAN). 1 ‐Ce⁴⁺ was characterized using various spectroscopic techniques, such as UV/Vis, EPR, CSI‐MS, resonance Raman, XANES, and EXAFS, showing an Mn?O bond distance of 1.69 Å with a resonance Raman band at 675 cm^?1. Electron‐transfer and oxygen atom transfer reactivities of 1 ‐Ce⁴⁺ were found to be greater than those of Mn^IV(O) intermediates binding redox‐inactive metal ions ( 1 ‐Mⁿ⁺). This study reports the first example of a redox‐active Ce⁴⁺ ion‐bound Mn^IV‐oxo complex and its spectroscopic characterization and chemical properties.     

Synthesis,Characterization, and Reactivity of Cobalt(III)–Oxygen Complexes Bearing a Macrocyclic N‐Tetramethylated Cyclam Ligand

Mononuclear metal–dioxygen species are key intermediates that are frequently observed in the catalytic cycles of dioxygen activation by metalloenzymes and their biomimetic compounds. In this work, a side‐on cobalt(III)–peroxo complex bearing a macrocyclic N‐tetramethylated cyclam (TMC) ligand, [Co^III(15‐TMC)(O₂)]⁺, was synthesized and characterized with various spectroscopic methods. Upon protonation, this cobalt(III)–peroxo complex was cleanly converted into an end‐on cobalt(III)–hydroperoxo complex, [Co^III(15‐TMC)(OOH)]²⁺. The cobalt(III)–hydroperoxo complex was further converted to [Co^III(15‐TMC‐CH₂‐O)]²⁺ by hydroxylation of a methyl group of the 15‐TMC ligand. Kinetic studies and ¹⁸O‐labeling experiments proposed that the aliphatic hydroxylation occurred via a Co^IV–oxo (or Co^III–oxyl) species, which was formed by O? O bond homolysis of the cobalt(III)–hydroperoxo complex. In conclusion, we have shown the synthesis, structural and spectroscopic characterization, and reactivities of mononuclear cobalt complexes with peroxo, hydroperoxo, and oxo ligands.