A novel iron-enhanced pathway for base-catalyzed catechol oxidation by dioxygen

Summary The kinetics and mechanisms of the catecholase-type biomimetic activation of O₂ by the new dioximatoiron(II) complexes [Fe(Hdmed)]⁺, [Fe(Hdmpd)]⁺ and [Fe(H₂dmdt)]²⁺ have been studied in methanol. Kinetic measurements reveal first order behavior with respect to catalyst and O₂ and a saturation type dependence on the 3,5-di-tert-butylcatechol (H₂dtbc) substrate. Added triethylamine increases the rate by changing the reaction mechanism. An important, hitherto unknown feature is iron(II)-enhanced base catalysis of H₂dtbc oxidation, via coordination of HdtbcO₂^- to the Fe(II) complex present, resulting in a significant acceleration of oxidation. A mechanism involving free radicals is suggested on grounds of ESR evidence. The activity pattern of the catalyst complexes correlates with coordination number and symmetry as revealed by M?ssbauer spectra.     

Unique example of flexible phenol coordination in mononuclear manganese compounds

The synthesis and characterization of six novel mononuclear Mn(II) and Mn(III) complexes are presented. The tripodal ligands 2-((bis(pyridin-2-ylmethyl)amino)methyl)-4-nitrophenol (HL1), 2-[[((6-methylpyridin-2-yl)methyl)(pyridin-2-ylmethyl)amino]methyl]-4-nitrophenol (HL2), (2-pyridylmethyl)(6-methyl-2-pyridylmethyl)(2-hydroxybenzyl)amine (HL3) and 2-((bis(pyridin-2-ylmethyl)amino)methyl)-4-bromophenol were used. All ligands provide an N3O donor set. The compounds [Mn(II)(HL1)Cl2].CH3OH (1), [Mn(III)(L1)Cl2] (2), [Mn(II)(HL2)(EtOH)Cl2] (3), [Mn(II)(HL3)Cl2].CH3OH (4), [Mn(III)(HL4)Br2] (5) and [Mn(III)(L1)(tcc)] (6), with tcc = tetrachlorocatecholate dianion, were synthesized and characterized by various techniques such as X-ray crystallography, mass spectrometry, IR and UV-vis spectroscopy, cyclic voltammetry, and elemental analysis. Compound 1 crystallizes in the triclinic space group P1, compounds 2, 3 and 4 were solved in the monoclinic space group P2(1)/c, whereas the structure determination of and succeeded in the orthorhombic space groups Pbca and P2(1)2(1)2(1), respectively. Notably, the crystal structures of 1 and 3 are the first Mn(II) complexes featuring a non-coordinating phenol moiety. Compound 2 oxidizes 3,5-di-tert-butylcatechol to 3,5-di-tert-butylquinone exhibiting saturation kinetics at high substrate concentrations with a turnover number of kcat = 173 h(-1). The electronic influence of different substituents in para position of the phenol group is lined out.     

Mixed-valence tetra- and hexanuclear manganese complexes from the flexibility of pyridine-containing beta-diketone ligands

The reactions of [Mn3O(O2CCCl3)6(H2O)3] with 1-phenyl-3-(2-pyridyl)propane-1,3-dione (HL(1)) and 1-(2-pyridly)-3-(p-tolyl)propane-1,3-dione (HL(2)) in CH2Cl2 afford the mixed-valence Mn(II)2Mn(III)2 tetranuclear complexes [Mn4O(O2CCCl3)6(L(1))2] (1) and [Mn4O(O2CCCl3)6L2(2)] (2), respectively. Similar reactions employing [Mn3O(O2CPh)6(H2O)(py)2] with HL(1) and HL(2) give the Mn(II)3Mn(III)3 hexanuclear complexes [Mn6O2(O2CPh)8(L(1))3] (3) and [Mn6O2(O2CPh)8L3(2)] (4), respectively. Complexes 1.2CH2Cl2, 2.2CH2Cl2.H2O, 3.1.5CH2Cl2.Et2O.H2O, and 4.2CH2Cl2 crystallize in the triclinic space group P1, monoclinic space group P2(1)/c, monoclinic space group P2 1/ n, and monoclinic space group P2(1)/n, respectively. Complexes 1 and 2 consist of a trapped-valence tetranuclear core of [Mn(II)2Mn(III)2(mu4-O)](8+), and complexes 3 and 4 represent a new structural type, possessing a [Mn(II)3Mn(III)3(mu4-O)2](11+) core. The magnetic data indicate that complexes 3 and 4 have a ground-state spin value of S = 7/2 with significant magnetoanisotropy as gauged by the D values of -0.51 cm (-1) and -0.46 cm (-1), respectively, and frequency-dependent out-of-phase signals in alternating current magnetic susceptibility studies indicate their superparamagnetic behavior. In contrast, complexes 1 and 2 are low-spin molecules with an S = 1 ground state. Single-molecule magnetism behavior confirmed for 3 the presence of sweep-rate and temperature-dependent hysteresis loops in single-crystal M versus H studies at temperatures down to 40 mK.     

The formation of a luminescent Mn(III,IV) intermediate of bis(2-pyridylmethyl)amine and acetone assistant its intramolecular C-H oxidation

The dinuclear Mn(II) complexes of bis(2-pyridylmethyl)amine (dpa) reacted with H(2)O(2) producing a fluorescent dioxodimanganese(III,IV) intermediate [(dpa)Mn(2)Cl(2)(μ-O(2))(OHdpa)](3+), which was characterized by IR, UV, ESR, ES-MS and fluorescence spectra. ES-MS data show that this intermediate could bind an acetone molecule forming dioxodimanganese(III,IV)-acetone adduct [(dpa)Mn(2)Cl(2)(μ-O)(CH(3)COCH(3))(OHdpa)](3+). The emission of dioxodimanganese(III,IV)-acetone at 378 nm was stronger than that of dioxodimanganese(III,IV) complex. Excess acetone molecules promoted the intramolecular C-H oxidation and the formation of one dimensional chain Mn(II) complex [(2-picolinic-acid)Mn(H(2)O)(2)Cl(O)](n) through possible intramolecular oxygen transfer reaction.     

Synthesis, structure and catalase-like activity of dimanganese(III) complexes of 1,5-bis(X-salicylidenamino)pentan-3-ol (X = 3- and 5-methyl). Influence of phenyl-ring substituents on catalytic activity

The diMn(III) complexes [Mn2(5-Me-salpentO)(mu-MeO)(mu-AcO)(H2O)Br] (1) and [Mn2(3-Me-salpentO)(mu-MeO)(mu-AcO)(MeOH)2]Br (2), where salpentOH = 1,5-bis(salicylidenamino)pentan-3-ol, were synthesised and structurally characterized. The two complexes include a bis(micro-alkoxo)(micro-acetato) triply-bridged diMn(III) core with an Mn...Mn separation of 2.93-2.94 A, the structure of which is retained upon dissolution. Complexes 1 and 2 show catalytic activity toward disproportionation of H2O2, with first-order dependence on the catalyst, and saturation kinetics on [H2O2], in methanol and DMF. In DMF, the two complexes are able to disproportionate at least 1500 eq. of H2O2 without significant decomposition, while in methanol, they rapidly lose activity with formation of a non-coupled Mn(II) species. Electrospray ionisation mass spectrometry, EPR and UV/vis spectroscopy used to monitor the reaction suggest that the major active form of the catalyst occurs in the Mn2(III) oxidation state during cycling. The correlation between log(k(cat)) and the redox potentials of 1, 2 and analogous complexes of other X-salpentOH derivatives indicates that, in this series, the oxidation of the catalyst is probably the rate-limiting step in the catalytic cycle. It is also noted that formation of the catalyst-peroxide adduct is more sensitive to steric effects in DMF than in methanol. Overall, kinetics and spectroscopic studies of H2O2 dismutation by these complexes converge at a catalytic cycle that involves the Mn2(III) and Mn2(IV) oxidation states.     

主（脱铝Y分子筛纳米腔）-客（Co，Mn和Cox/Mnx三核金属簇合物）纳米复合物催化剂高效催化环己烯环氧化反应

采用模板合成法制备了在脱铝Y分子筛纳米腔中含有Co, Mn,和Co/Mn混合物的三核金属簇合物的复合物催化剂,包括[Mn3(O)(CH3COO)6-(py)3]-Y,[Co3(O)(CH3COO)6-(py)3],[Co2Mn(O)(CH3COO)6-(py)3]-Y和[CoMn2(O)(CH3COO)6-(py)3]-Y],并运用傅里叶变换红外光谱、紫外-可见光光谱、前场扫描电镜、X射线衍射和原子吸收光谱对其进行了表征.包裹的金属簇合物在环己烯环氧化反应中表现出较高的催化活性,反应以H2O2/O2为氧化剂,在加热机械搅拌下进行,所得环氧化物产率在82.5%?90.7%.在该催化体系中, NaBr用作氧化助剂可以提高反应性能.在所制多相催化体系中,含单核的金属簇合物催化活性更高,其中以Mn3(O)-DAZY化合物的效率最高.各催化剂活性顺序为Mn3(O)-DAZY> Co3(O)-DAZY> Mn2Co(O)-DAZY> MnCo2(O)-DAZY.     

Methane oxidation at redox stable fuel cell electrode La0.75Sr0.25Cr0.5Mn0.5O(3-delta)

Because of its widespread availability, natural gas is the most important fuel for early application of stationary fuel cells, and furthermore, methane containing biogases are one of the most promising renewable energy alternatives; thus, it is very important to be able to efficiently utilize methane in fuel cells. Typically, external steam reforming is applied to allow methane utilization in high temperature fuel cells; however, direct oxidation will provide a much better solution. Recently, we reported good electrochemical performance for an oxide anode La0.75Sr0.25Cr0.5Mn0.5O3 (LSCM) in low moisture (3% H2O) H2 and CH4 fuels without significant coking in CH4. Here, we investigate the catalytic activity of this oxide with respect to its ability to utilize methane. This oxide is found to exhibit fairly low reforming activity for both H2O and CO2 reforming but is active for methane oxidation. LSCM is found to be a full oxidation catalyst rather than a partial oxidation catalyst as CO2 production dominates CO production even in CH4-rich CH4/O2 mixtures. X-ray adsorption spectroscopy was utilized to confirm that Mn was the redox active species, clearly demonstrating that this material has the oxidation catalytic behavior that might be expected from a Mn perovskite and that the Cr ion is only present to ensure stability under fuel atmospheres.     

Aroylhydrazone derivative as fluorescent sensor for highly selective recognition of Zn2+ ions: syntheses, characterization, crystal structures and spectroscopic properties

A new Zn(2+) fluorescent chemosensor N'-(3,5-di-tert-butylsalicylidene)-2-hydroxybenzoylhydrazine (H(3)L(1)) and its complexes [Zn(HL(1))C(2)H(5)OH](∞) (1) and [Cu(HL(1))(H(2)O)]CH(3)OH (2) have been synthesized and characterized in terms of their crystal structures, absorption and emission spectra. H(3)L(1) displays high selectivity for Zn(2+) over Na(+), K(+), Mg(2+), Ca(2+) and other transition metal ions in Tris-HCl buffer solution (pH = 7.13, EtOH-H(2)O = 8?:?2 v/v). To obtain insight into the relation between the structure and selectivity, a similar ligand 3,5-di-tert-butylsalicylidene benzoylhydrazine (H(2)L(2)), which lacks the hydroxyl group substituent in salicyloyl hydrazide compared with H(3)L(1), and its complex [Zn(2)(HL(2))(2)(CH(3)COO)(2)(C(2)H(5)OH)] (3), [Co(L(2))(2)][Co(DMF)(4)(C(2)H(5)OH)(H(2)O)] (4), [Fe(HL(2))(2)]Cl·2CH(3)OH (5), have also been investigated as a reference. H(3)L(1) exhibits improved selectivity for Zn(2+) compared to H(2)L(2). The findings indicate that the hydroxyl group substituent exerts an effect on the spectroscopic properties, complex structures and selectivity of the fluorescent sensor.     

Syntheses, crystal structures, and magnetic characterization of five new dimeric manganese(III) tetradentate Schiff base complexes exhibiting single-molecule-magnet behavior

Tetradentate Schiff base ligands H2L (H2saltmen, H2salen, H2-5-Brsalen, and H2-3,5-Brsalen), derived from the condensation of the corresponding salicylaldehyde or its derivatives with 1,1,2,2-tetramethylethyldiamine or 1, 2-diaminoethane, reacted with Mn(III) acetate or perchlorate salts and sodium azide or sodium cyanate to produce five Mn(III) dimer complexes, [Mn(saltmen)(O2CCH3)]2.2CH3CO2H (1), [Mn(saltmen)(N3)]2 (2), [Mn(salen)(NCO)]2 (3), [Mn(3,5-Brsalen)(3,5-Brsalicylaldehyde)]2 (4), and [Mn(5-Brsalen)(CH3OH)]2(ClO4)2 (5). These new complexes have been characterized by IR, elemental analyses, crystal structural analyses, and magnetic studies. Within these Mn(III) dimeric complexes, two Mn(III) ions are connected by phenolate oxygen atoms with acetate, azide, cyanate, a 3,5-Brsalicyladehyde anion, and a neutral methanol molecule as the axial ligands for complexes 1-5, respectively. Complexes 1-4 exhibit intradimer ferromagnetic exchange and display frequency dependence of ac magnetic susceptibility, possibly showing single-molecule-magnet (SMM) behavior. In contrast, complex 5 shows an intradimer antiferromagnetic coupling probably originating from the relatively shorter Mn-O distance, compared to those of complexes 1-4.     

Synthesis and spectroscopic studies on iron(III) complexes of 1-benzotriazol-1-yl-1-[(p-X-phenyl)hydrazono]propan-2-one

A new series of iron(III) complexes are synthesized from the reaction of the polyfunctional ligands 1-benzotriazol-1-yl-1-[p-X-phenyl]hydrazono]propan-2-one (X=H, Cl, NO(2), CH(3) or OCH(3) corresponding to HL(1),HL(2), HL(3), HL(4) or HL(5), respectively, with iron(III) chloride in the presence of LiOH by the conventional and microwave induced energy methods. The conventional method led to the formation of [FeL(3)].nH(2)O but the microwave induced energy gave [FeLCl(2)], n=1-3 and L is the anion of HL(1)-HL(5). The complexes are characterized by the elemental analysis, molar conductivity, magnetic and spectral (FT-IR, UV-vis and ESR) studies. The magnetic and spectral studies showed that [FeLCl(2)] are polymeric octahedral, [Fe(L(1))(3)].H(2)O is a low spin octahedral and (d(xz),d(yz))(4) (d(xy))(1) ground state, [FeL(3)].nH(2)O, L=anion of HL(4) or HL(5) and are octahedral with intermediate spin (S=32) with ground state (d(xy))(2)(d(xz),d(yz))(3) electronic configuration while for the anions of HL(2) and HL(3), they have (t(2g))(3)(e(g))(5) admixed with (d(xy))(2)(d(xz),d(yz))(3) configurations. From the ESR data, the contribution of the high spin (S=52) and low spin (S=32) to the quantum mechanical spin intermediate (QMS), and the crystal field parameters Delta and V are calculated and related to the electronic and steric effects of the ligands. The electronic spectral data confirm that obtained from the ESR, and the different ligand field parameters as well as the pi-->t(2g), t(2g)-->e(g), e(g)-->pi*, pi-->pi* transitions are estimated and compared with that experimentally obtained.     

Porous metalloporphyrinic frameworks constructed from metal 5,10,15,20-tetrakis(3,5-biscarboxylphenyl)porphyrin for highly efficient and selective catalytic oxidation of alkylbenzenes

We incorporate metal 5,10,15,20-tetrakis(3,5-biscarboxylphenyl)porphyrin (M-H(8)OCPP), for the first time, into porous metal-organic frameworks. The self-assembled porous metalloporphyrinic frameworks [Mn(5)Cl(2)(MnCl-OCPP)(DMF)(4)(H(2)O)(4)]·2DMF·8CH(3)COOH·14H(2)O (ZJU-18; ZJU = Zhejiang University), [Mn(5)Cl(2)(Ni-OCPP)(H(2)O)(8)]·7DMF·6CH(3)COOH·11H(2)O (ZJU-19), and [Cd(5)Cl(2)(MnCl-OCPP)(H(2)O)(6)]·13DMF·2CH(3)COOH·9H(2)O (ZJU-20) are isostructural as revealed by their single X-ray crystal structures. The metalloporphyrin octacarboxylates (M-OCPP) (M = Mn(III)Cl for ZJU-18 and ZJU-20, M = Ni(II) for ZJU-19) are bridged by binuclear and trinuclear metal carboxylate secondary building units to form a 3-periodic, binodal, edge-transitive net with Reticular Chemistry Structure Resource symbol tbo with pore windows of about 11.5 ? and pore cages about 21.3 ? in diameter. The porous nature of these metalloporphyrinic frameworks is further established by sorption studies in which different substrates such as ethanol, acetonitrile, acetone, cyclohexane, benzene, toluene, ethylbenzene, and acetophenone can readily have access to the pores. Their catalytic activities for the oxidation of alkylbenzenes were examined at 65 °C using tert-butyl hydroperoxide as the oxidant. The results indicate that ZJU-18 is much superior to ZJU-19, ZJU-20, and homogeneous molecular MnCl-Me(8)OCPP, exhibiting highly efficient and selective oxidation of ethylbenzene to acetophenone in quantitative >99% yield and a turnover number of 8076 after 48 h.     

Si取代Keggin型多酸催化光解水产氢机理

本文采用DFT和TD-DFT方法研究了Keggin型多酸[SiW12O40]4-光催化劈裂水产氢气机理。计算结果显示反应主要包括四个步骤：(i) 光激发,(ii) 电荷转移和生成单电子还原(OER)中间体,(iii) 生成双电子还原(TER)中间体,(iv)氢气从多酸表面解离和催化剂重生。当第一个电子从甲醇转移到多酸后,后续反应存在均为热力学上有利的放热途径,并推动第二个电子从甲醇自由基,H[SiW12O40]4-或[SiW12O40]5-转移到OER中间体H[SiW12O40]4-或[SiW12O40]5-生成TER中间体[SiW12O40]6-,H[SiW12O40]5-或H2[SiW12O40]4-,并伴随着H2产生。耦合的电子和质子转移路径在能量上最有利。甲醇和水分子的参与有利于H2产生。多酸在整个催化循环中,扮演了光敏剂、催化剂、电子的受体和给体。     

Iron(III)-catecholato complexes as structural and functional models of the intradiol-cleaving catechol dioxygenases

The structural and spectroscopic characterization of mononuclear iron(III)-catecholato complexes of ligand L4 (methyl bis(1-methylimidazol-2-yl)(2-hydroxyphenyl)methyl ether, HL4) are described, which closely mimic the enzyme-substrate complex of the intradiol-cleaving catechol dioxygenases. The tridentate, tripodal monoanionic ligand framework of L4 incorporates one phenolato and two imidazole donor groups and thus well reproduces the His2Tyr endogenous donor set. In fact, regarding the structural features of [FeIII(L4)(tcc)(H2O)] (5.H2O, tcc = tetrachlorocatechol) in the solid state, the complex constitutes the closest structural model reported to date. The iron(III)-catecholato complexes mimic both the structural features of the active site and its spectroscopic characteristics. As part of its spectroscopic characterization, the electron paramagnetic resonance (EPR) spectra were successfully simulated using a simple model that accounts for D strain. The simulation procedure showed that the observed g = 4.3 line is an intrinsic part of the EPR envelope of the studied complexes and should not necessarily be attributed to a highly rhombic impurity. [FeIII(L4)(dtbc)(H2O)] (dtbc = 3,5-di-tert-butylcatechol) was studied with respect to its dioxygen reactivity, and oxidative cleavage of the substrate was observed. Intradiol- and extradiol-type cleavage products were found in roughly equal amounts. This shows that an accurate structural model of the first-coordination sphere of the active site is not sufficient for obtaining regioselectivity.     

Vanadium-based, extended catalytic lifetime catechol dioxygenases: evidence for a common catalyst

In 1999, a catechol dioxygenase derived from a V-polyoxometalate was reported which was able to perform a record >100 000 total turnovers of 3,5-di-tert-butylcatechol oxygenation using O2 as the oxidant (Weiner, H.; Finke, R. G. J. Am. Chem. Soc. 1999, 121, 9831). An important goal is to better understand this and other vanadium-based catechol dioxygenases. Scrutiny of 11 literature reports of vanadium-based catechol dioxygenases yielded the insight that they all proceed with closely similar selectivities. This, in turn, led to a "common catalyst hypothesis" for the broad range of vanadium based catechol dioxygenase precatalysts presently known. The following three classes of V-based compounds, 10 complexes total, have been explored to test the common catalyst hypothesis: (i) six vanadium-based polyoxometalate precatalysts, (n-Bu4N)4H5PV14O42, (n-Bu4N)7SiW9V3O40, (n-Bu4N)5[(CH3CN)(x)Fe(II).SiW9V3O40], (n-Bu4N)9P2W15V3O62, (n-Bu4N)5Na2[(CH3CN)(x)Fe(II).P2W15V3O62], and (n-Bu4N)4H2-gamma-SiW10V2O40; (ii) three vanadium catecholate complexes, [V(V)O(DBSQ)(DTBC)]2, [Et3NH]2[V(IV)O(DBTC)2].2CH3OH, and [Na(CH3OH)2]2[V(V)(DTBC)3]2.4CH3OH (where DBSQ = 3,5-di-tert-butylsemiquinone anion and DTBC = 3,5-di-tert-butylcatecholate dianion), and (iii) simple VO(acac)2. Product selectivity studies, catalytic lifetime tests, electron paramagnetic resonance spectroscopy (EPR), negative ion mode electrospray ionization-mass spectrometry (negative ion ESI-MS), and kinetic studies provided compelling evidence for a common catalyst or catalyst resting state, namely, Pierpont's structurally characterized vanadyl semiquinone catecholate dimer complex, [VO(DBSQ)(DTBC)]2, formed from V-leaching from the precatalysts. The results provide a considerable simplification and unification of a previously disparate literature of V-based catechol dioxygenases.     

New types of metal squarato-phosphonates: condensation of aminodiphosphonate with squaric acid under hydrothermal conditions

The syntheses and crystal structures of the first copper(I) phosphonate, Cu2(H3L)(bipy)(2).2H2O 1 (H5L = C4HO3N(CH2PO3H2)2), which is also the first example of metal phosphonates formed by a type of organic reaction, and a novel luminescent Mn(II) squarate diphosphonate, {Mn[NH(CH2PO3H)2](H2O)2}2{Mn(C4O4)(H2O)4}.(C4H2O4) 2, have been reported. The structure of 1 features a layer architecture in which the Cu(I) centers are three coordinated, and the newly formed ligand acts as a bidentate metal linker. Compound 2 is composed of 1D chains of Mn[NH(CH2PO3H)2](H2O)2, 1D chains of {Mn(C4O4)(H2O)4}, as well as the neutral squaric acid molecules. These three types of building units are interconnected via hydrogen bonding.     

Orientational isomers of alpha-cyclodextrin [2]semi-rotaxanes with asymmetric dicationic threads

Two series of novel dicationic threading molecules [Quin(CH2)10R]2+ and [3,5-Lut(CH2)10R]2+, where Quin+ = quinuclidinium, 3,5-Lut+ = 3,5-lutidinium, and R+ = N(CH3)3+ and N(CH3)2CH2CH3+, form [2]semi-rotaxanes with [small alpha]-cyclodextrin (alpha-CD) in aqueous solution. The quinuclidinium and 3,5-lutidinium are sufficiently bulky to prevent threading while the R+ groups allow for slow threading by alpha-CD at 25 degrees C. The resulting [2]semi-rotaxanes exist in two orientational isomers owing to the asymmetry of both the alpha-CD cavity and the threading molecules. Two-dimensional 1H NMR spectroscopy and kinetics experiments reveal that the isomer in which the narrower rim (primary OHs) is positioned near the R+ group is the kinetically preferred isomer, while the other isomer is the thermodynamically preferred product. The kinetics and mechanism of the formation, dissociation, and interconversion of the two isomers have been determined at 25 degrees C.     

Synthesis, spectral and catalytic activity of some manganese(II) bis-benzimidazole diamide complexes

Four Mn(II) complexes bound to a neutral bis-benzimidazole diamide ligand N,N'-bis(2-methyl benzimidazolyl 2,2'-oxy-diethanamide) (GBOA) have been synthesized and characterized. Anionic ligand associated with the complexes varies as Cl- CH3COO-, SCN- and ClO4-. X-ray structure of one of the complexes [Mn(GBOA)2(H2O)2]Cl(2)·4H2O was solved and shows that the Mn(II) ion is hexacoordinate. Two equatorial positions are occupied by benzimidazole imine nitrogen atoms while the other two sites are occupied by amide carbonyl oxygens. The imine nitrogen and carbonyl oxygens are bound to Mn(II) by different arms of the two ligands while axial sites are occupied by two water molecules. Two Cl- anions are outside the coordination sphere and form an extensive 3D H-bonded network. Axially distorted octahedral geometry is confirmed for all the four complexes by low temperature EPR spectroscopy. Distortion parameter D was found to be similar for [Mn(GBOA)2(H2O)2]Cl(2)·4H2O and [Mn(GBOA)2(H2O)2]·(CH3COO)2·H2O. Cyclic voltammograms have been obtained for all the four complexes and E(1/2) values are dependent on the anionic ligand being in the coordination sphere or outside. [Mn(GBOA)2(H2O)2]Cl(2)·4H2O and [Mn(GBOA)2(H2O)2]·(CH3COO)2·H2O carry out the selective oxidation of N-benzyldimethylamine, and 1-methyl-pyrollidine to their respective carbonyl products with catalytic efficiency of 35-50%.     

Synthesis, structure, and catalytic properties of V(IV), Mn(III), Mo(VI), and U(VI) complexes containing bidentate (N, O) oxazine and oxazoline ligands

Synthesis of seven complexes containing oxazoline ([(L(1))(2)V=O] (4), [(L(1))(2)MoO(2)] (5), [(L(1))(2)UO(2)] (6); HL(1) (1) [HL(1) = 2-(4',4'-dimethyl-3'-4'-dihydroxazol-2'-yl)phenol]), chiral oxazoline ([(L(2))(2)UO(2)] (7); HL(2) (2) [HL(2) = (4'R)-2-(4'-ethyl-3'4'-dihyroxazol-2'-yl)phenol]), and oxazine ([(L(3))(2)V=O] (8), [(L(3))(2)Mn(CH(3)COO(-))] (9), [(L(3))(2)Co] (10); HL(3) (3) [HL(3) = 2-(5,6-dihydro-4H-1,3-oxazolinyl)phenol]) and their characterization by various techniques such as UV-vis, IR, and EPR spectroscopy, mass spectrometry, cyclic voltammetry, and elemental analysis are reported. The novel oxazine (3) and complexes 4, 5, 8 and 9 were also characterized by X-ray crystallography. Oxazine 3 crystallizes in the monoclinic system with the P2(1)/n space group, complexes 4 and 9 crystallize in the monoclinic system with the P2(1)/c space group, and complexes 5 and 8 crystallize in the orthorhombic system with the C222(1) space group and the P2(1)2(1)2(1) chiral space group, respectively. The representative synthetic procedure involves the reaction of metal acetate or acetylacetonate derivatives with corresponding ligand in ethanol. Addition of Mn(OAc)(2).4H(2)O to an ethanol solution of 3 gave the unexpected complex Mn(L(3))(2).(CH(3)COO(-)) (9) where the acetate group is coordinated with the metal center in a bidentate fashion. The catalytic activity of complexes 4-9 for oxidation of styrene with tert-butyl hydroperoxide was tested. In all cases, benzaldehyde formed exclusively as the oxidation product.     

Radical pathway in catecholase activity with zinc-based model complexes of compartmental ligands

Four dinuclear and three mononuclear Zn(II) complexes of phenol-based compartmental ligands (HL(1)-HL(7)) have been synthesized with the aim to investigate the viability of a radical pathway in catecholase activity. The complexes have been characterized by routine physicochemical studies as well as X-ray single-crystal structure analysis: [Zn(2)(H(2)L(1))(OH)(H(2)O)(NO(3))](NO(3))(3) (1), [Zn(2)L(2)Cl(3)] (2), [Zn(2)L(3)Cl(3)] (3), [Zn(2)(L(4))(2)(CH(3)COO)(2)] (4), [Zn(HL(5))Cl(2)] (5), [Zn(HL(6))Cl(2)] (6), and [Zn(HL(7))Cl(2)] (7) [L(1)-L(3) and L(5)-L(7) = 2,6-bis(R-iminomethyl)-4-methylphenolato, where R= N-ethylpiperazine for L(1), R = 2-(N-ethyl)pyridine for L(2), R = N-ethylpyrrolidine for L(3), R = N-methylbenzene for L(5), R = 2-(N-methyl)thiophene for L(6), R = 2-(N-ethyl)thiophene for L(7), and L(4) = 2-formyl-4-methyl-6-N-methylbenzene-iminomethyl-phenolato]. Catecholase-like activity of the complexes has been investigated in methanol medium by UV-vis spectrophotometric study using 3,5-di-tert-butylcatechol as model substrate. All complexes are highly active in catalyzing the aerobic oxidation of 3,5-di-tert-butylcatechol (3,5-DTBC) to 3,5-di-tert-butylbenzoquinone (3,5-DTBQ). Conversion of 3,5-DTBC to 3,5-DTBQ catalyzed by mononuclear complexes (5-7) is observed to proceed via formation of two enzyme-substrate adducts, ES1 and ES2, detected spectroscopically, a finding reported for the first time in any Zn(II) complex catalyzed oxidation of catechol. On the other hand, no such enzyme-substrate adduct has been identified, and 3,5-DTBC to 3,5-DTBQ conversion is observed to be catalyzed by the dinuclear complexes (1-4) very smoothly. EPR experiment suggests generation of radicals in the presence of 3,5-DTBC, and that finding has been strengthened by cyclic voltammetric study. Thus, it may be proposed that the radical pathway is probably responsible for conversion of 3,5-DTBC to 3,5-DTBQ promoted by complexes of redox-innocent Zn(II) ion. The ligand-centered radical generation has further been verified by density functional theory calculation.     

Synthesis,spectroscopic characterization and EPR studies on electron transfer reactions of bis[N-(2,5-di-tert-butylphenyl)salicylaldiminato]copper complexes with PPh3

New bidentate N-(2,5-di-tert-butylphenyl)salicylaldimines bearing X = H, HO, CH3O, Br, NO2, 3,5-di-Br, 3-NO2-5-Br and 5,6-benzo substituents on the salicylaldehyde moiety, LxH, and their mononuclear bis[N-(2,5-di-tert-butylphenyl)salicylaldiminato]copper(II) complexes, Cu(Lx)2, have been prepared and investigated by IR, UV-Visible, 1H NMR, ESR spectroscopy, magnetic measurements, as well as reactions of Cu(LX)2 with PPh3 were studied. It has been found that some complexes with X = HO and CH3O unlike their electron-withdrawing and unsubstituted analogues are readily reduced by PPh3 via intramolecular electron transfer from ligand to copper(II) to give Cu-stabilized radical intermediates. The spectra of the primary radicals interpreted in terms of couplings of unpaired electron with (63,65)Cu, 31P, 14N nuclei and aromatic protons.