Pavan Mathur

Monomeric and dimeric copper(II) complexes of a redox active Schiff base ligand bis(2,5-dihydroxyacetophenone) ethylenediamine

Abstract Monomeric and dimeric copper(II) complexes with a tetradentate Schiff base, bis(2,5-dihydroxyacetophenone) ethylenediamine have been synthesized and characterized by spectroscopic techniques. Solution EPR of the complexes in DMSO shows less than four g| lines and a broadening of the g⊥ component suggesting a lowered site symmetry clear five-line pattern emerges with AN ⋍ 15 ± 1 G, supporting the binding of a tetradentate ligand with two N and two O donor sites. For the monomeric complexes g| is in the range of 2.18–2.20, suggesting a near square-planar arrangement of N2O2 donor groups (θ) = 0). The complexes exhibit a visible band with λmax in the 500–600 nm region in solution. This is found to split into two components in the solid state reflectance spectra. The spectral data suggests a distorted tetragonal geometry for copper(II) ions in the present complexes. The copper complexes prepared by using CuCl2 analyses for the stoichiometry 2 Cu : 1 DAen, i.e. [Cu(DAen)CuCl2], suggesting the formation of a dimeric species. This is supported by low λeff of 1.49 B.M., implying an exchange coupled complex. Further, the coupling constant evaluated for the peaks involved in g| region is 118.75 G, a value which is nearly half of that found for the monomeric complexes.

Copper(II) complexes of tetradentate N,N′,N″-tris(2-benzimidazolylmethyl) and its N-ethyl derivative

Abstract The tripodal ligand N,N′,N″-tris(2-benzimidazolylmethyl)amine and its N-ethyl derivative have been used to synthesize copper(II) complexes along with an exogenous ligand X (where X = OAc, HCOO, N3 and benzimidazole). Solution EPR spectra suggest the complexes to be tetragonal in nature with g| > g⊥. The low A| values are indicative of a tetrahedral distortion of the basal plane. The bonding parameters α2, β2, γ2 and K have been evaluated. The decreasing trend of covalency as reflected in the value of K has been found to be acetate ⩾, azide > formate ⩾ benzimidazole. These complexes may have potential relevance to copper coordination sites in the metalloprotein Rhus Laccase and superoxide dismutase.

Monomeric Cu (II) complexes with tetradentate bis (benzimidazole) ligand ; synthesis, EPR, spectral and electrochemical studies

Abstract A series of 1 : 1 copper (II) complexes with the tetradentate ligand 1,2bis (2 benzimidazolyloxamethyl) benzene (L) have been prepared with stoichiometry of Cu (L) X2 · nH2O (X = Cl−, NO−3, OAc− and HCOO−) . X-band EPR spectra of the above complexes examined as a frozen DMSO\DMF solution indicate g‖ > g↑ > 2.0 and show four or less than four g‖ lines and a broadening of the g↑ component suggesting a lowered site symmetry for the copper (II) ion. The g↑ component of all the complexes show nitrogen superhyperfine structure. A clear five line pattern is found in the g↑ region with BN sime; 15G±1 for Cu (L) X2 · nH2O complexes indicating that two nitrogen from the benzimidazole ring are coordinated to the central CuII in the equatorial basal plane while the other two positions in the sphere are occupied by the anionic ligand (X) . Interestingly with ClO−4 as the anion, a 1 : 2 complex results with stoichiometry of [Cu (L) 2] (ClO4) 2. The X-band EPR spectra of [Cu (L) 2] (ClO4) 2 complex shows a clear nine line SHF structure on the g↑ component indicating the presence of four nitrogen atoms coordinated to the central CuII atom ; this confirms the retention of two ligands around CuII in solution. Molecular orbital coefficients β1 and β1′ have been evaluated from the expression used by Manoharan and Rogers. The trends of β1 and β1′ indicate increasing electron delocalization on nitrogen atom as we change the exogenous ligand from formate\acetate-chloride to nitrate. The covalency parameter, α2, has also been evaluated and reflects the decreasing trend of covalency in the order, nitrate > chloride ≥ acetate > formate for complexes containing these ligands. This trend of order is in conformity with that obtained from MO coefficients. All the complexes exhibit a visible band with λmax in the 625–690 nm region, in DMF ; suggesting a distorted geometry for copper (II) ion in the present series of complexes. Cyclic voltammetric studies of copper (II) complexes reveal that the E1\2 for the CuII\CuI couple shifts to most negative value with HCOO− and OAc− anionic ligands implying that these anion stabilize the CuII state whereas E1\2 data for NO−3 reveals that this anion destabilizes the CuII state. A plot of visible band energy vs E1\2 shows that as the energy of the dx2−y2 orbital decreases, an anodic shift in CuII → CuI reduction potential is observed.

E.s.r. spectra of mixed ligand manganese(II) dithiocarbamates

SummaryE.s.r. spectra of mixed ligand manganese(II) complexes of the type Mn(dtc)2(B), where dtc = pipyridyl dithiocarbamate (pip-dtc) or morpholyl dithiocarbamate (morph-dtc) and B = 2,2′-dipyridyl (dipy) or 1,10-phenanthroline (phen), are described. The results are comparable to those calculated for D=0.19 cm−1, λ=0.04, giso = 2.00 and for magnetic field directions parallel to principal D-tensor axes.

Mononuclear and binuclear manganese(II) complexes with tridentate bis-benzimidazolyl ligands

SummaryManganese(II) complexes of bis(2-benzimidazolylmethyl) ether (DGB), bis(2-benzimidazolylmethyl) sulphide (TGB) and the n-butyl derivative of DGB (BDGB) were prepared and characterised. The solution e.p.r. spectrum of [Mn(TGB)Cl2] in DMF at 143 K is commensurate with an axially distorted monomeric manganese(II) complex, room temperature magnetic moment (6.04 B.M.) per manganese(II) atom being in the range found for other d5 monomeric manganese(II) complexes. The solution e.p.r. spectrum of [Mn(BDGB)Cl2]-2H2O in DMF at 143 K indicates the presence of two equivalent manganese(II) ions coupled by an exchange interaction, fostered by bridging chlorides. Evidence for this is provided by a nearly isotropic 11 line hyperfine structure of 55Mn, with a coupling constant 45 ± 5G. Contact-shifted 1H n.m.r. data also supports an exchange coupled dimeric manganese complex. The room temperature magnetic moment, 5.64 B.M., per manganese(II) indicates quenching of the magnetic moment below that of monomeric manganese(II) ion. The [Mn(DGB)Cl2]·H2O complex exhibits a magnetic moment of 6.02 B.M. per manganese, indicating a monomeric manganese complex. E.p.r. data of the complex diluted in an analogous Zn-DGB complex (1∶20) correlates well for D = 0.22cm−1 and λ ∼- 0.267. The [Mn(DGB)-(C1O4)2] and [Mn(BDGB)(ClO4)2] complexes, diluted in analogous Zn-DGB and Zn-BDGB complexes (1∶20), show a strong single e.p.r. line at geff ∼- 2. The complexes have low magnetic moments; 4.44 B.M./Mn and 4.39 B.M./Mn, at room temperature.

Synthesis and characterization of iron(III) complexes with tridentate ligands containing N2O/N2S donor sites

Abstract Iron(III) complexes of tridentate ligands bis)2-benzimidazolylmethyl) ether, sulphide and their N-butyl derivatives have been synthesized and characterized. X -band EPR spectra in frozen DMF reveal the presence of a number of signals in the region g eff = ca 4 to g eff = ca 9. The departure of signals away from g eff = ca 4.3 is indicative of distortion from purely rhombic to axial geometry. The 1 H NMR spectra of iron(III) depict dipolar, broadened, isotropically contact shifted resonances in the range −10 to +40 ppm, with respect to TMS, typical of high spin iron(III). Cyclic voltammograms of these complexes have been recorded which show complexes containing chloride as an inner sphere ligand have a consistently lower redox potential, which suggests that the Cl − anion is more effective in reducing the Lewis acidity of the iron(III) centre compared with the nitrate anion. These complexes may have relevance to iron coordination sites in phenylalanine hydroxylase and soyabean lipoxygenase.

Di-iron(III) complex of a hexadentate ligand with inequivalent iron coordination sites

A binuclear iron(III) complex [ClFe(N6)FeCl3]Cl2·3H2O has been synthesized, where N6 is the N-ethyl derivative of the hexadentate ligandN,N,N′,N′-tetrakis[(2-benzimidazolyl) methyl-1,2-ethanediamine. Variable temperature magnetic susceptibility in the temperature range of 80 <T < 300 K was least squares fitted, withJ = −7.0 cm−1, supporting weak exchange coupling between the iron atoms. Mo¨ssbauer spectra for the di-iron complex at room temperature consist of two quadruple doublets of nearly equal area. The isomer shifts lie within the range for high spin ferric ions, while the small quadruple splittings are not consistent with the presence of an oxo bridge. Mo¨ssbauer data support the presence of distinct coordination environments for the two iron atoms. Such coordination environment asymmetry is also found for iron sites in purple acid phosphatase.

Copper(II) complexes as catalyst for the aerobic oxidation of o-phenylenediamine to 2,3-diaminophenazine.

Two new mononuclear copper(II) complexes [Cu (L) (NO3)2] (1) and [Cu (L) Br2] (2) where (L=bis(1-(pyridin-2-ylmethyl)-benzimidazol-2-ylmethyl)ether) are synthesized and characterized by single-crystal X-ray diffraction analysis, elemental analysis, UV-Visible, IR spectroscopy, EPR and cyclic voltammetry. The complexes exhibit different coordination structures; the E1/2 value of the complex (1) is found to be relatively more cathodic than that of complex (2). X-band EPR spectra at low temperature in DMF supports a tetragonally distorted complex (1) while complex (2) shows three different g values suggesting a rhombic geometry. These complexes were utilized as a catalyst for the aerobic oxidation of o-phenylenediamine to 2,3-diaminophenazine assisted by molecular oxygen. The initial rate of reaction is dependent on the concentration of Cu(II) complex as well as substrate, and was found to be higher for the nitrate bound complex, while presence of acetate anion acts as a mild inhibitor of the reaction, as it is likely to pick up protons generated during the course of reaction. The inhibition suggests that the generated protons are further required in another important catalytic step.

Copper(II) complexes of a new tetradentate bis-benzimidazolyl diamide ligand with disulfanediyl linker: synthesis, characterization, and oxidation of some pyridyl, napthyl, and benzyl alcohols

The bis-benzimidazolyl diamide ligand 3,3′-disulfanediyl-bis-(N-((1H-benzo[d]imidazol-2-yl)methyl)propanamide) (L) having N, S, and O donors has been synthesized and utilized to prepare copper(II) complexes of general composition [Cu(L)X]·X·nH2O, where X is Cl– or . Ligands and complexes were characterized by elemental analysis, UV–visible, IR spectroscopy, 1H NMR, mass spectrometry, conductance, electron paramagnetic resonance, cyclic voltammetry, fluorescence, single-crystal, and PXRD. [Cu(L)(NO3)]NO3·H2O crystallizes in the monoclinic C2/c space group with one molecule in the asymmetric unit. Copper adopts a distorted octahedral geometry with four coordination sites occupied by the ligand and two sites by a bidentate nitrate. [Cu(L)(NO3)]NO3·H2O has been utilized as a catalyst to oxidize alcohols with tert-butyl hydroperoxide as an alternative source of oxygen. The oxidized products have been characterized by GCMS.

A Mössbauer and electrochemical study of mononuclear iron(III) complexes with benzimidazole-based flexible bidentate ligands

A series of FeIII complexes of stoichiometry [FeLX3].nH2O have been synthesized, where X=Cl−, HCO−2 and L is a flexible bidentate ligand, 1,3(bis-benzimidazolyl) propane and 1,4(bis-benzimidazolyl)butane. Mössbauer data reveals that the isomer shift values lie in the range typically observed for high spin FeIII complexes, while the slightly large quadrupole splitting parameter indicates a rhombically distorted FeIII centre. Cyclic voltammetric studies reveal that the E1/2 for the FeIII/FeII couple for the formate complex shifts more cathodically than for the chloride complex; implying that HCO−2 is more effective at lowering the Lewis acidity of the FeIII centre.