Rameshwer Shukla

Synthesis and studies of Cu(II)-thiolato complexes: bioinorganic perspectives

Abstract Ligation of thiolate sulfur to copper at the active sites of quite a number of copper proteins has been established either by X-ray crystallographic and/or by spectroscopic studies. In addition, for Cu(II)-substituted metalloproteins, the presence of Cu(II)-thiolate bonding at the active sites could be established spectroscopically. Cu(II)-thiolate bonding in different enzymes is not always very similar. Obviously, the bioinorganic significance of Cu(II)-thiolate bonding is enormous and has attracted a lot of attention to synthesize model Cu(II)-thiolato complexes as electronic structural analogues of the active sites of these biomolecules. The present review deals with (i) nature of Cu(II)-thiolate bonding present in different metalloproteins, (ii) difficulties involved in the synthesis of Cu(II)-thiolates and ways to surmount them, (iii) characterization of the Cu(II)-thiolate bonding by electronic and EPR spectroscopic techniques and (iv) electron transfer properties of the Cu(II)-thiolato complexes by cyclic voltammetric studies. The properties of the Cu(II)-thiolato complexes have been discussed as possible models for the active site(s) of copper proteins.

Mononuclear manganese(III) complexes with imine/amine and phenolate ligation: X-ray structural, spectroscopic and electrochemical studies

Abstract The tridentate Schiff base, formed by reacting salicylaldehyde with 2-dimethylaminoethyl amine, readily forms a 2:1 complex with manganese(III) in methanol where the Schiff base acts only as a bidentate ligand binding through the imine nitrogen and the phenolate oxygen thus occupying four coordination positions on the metal ion. The other two positions are occupied by two N-bonded thiocyanato groups. This complex (1) has been characterized by X-ray crystallography. This Schiff base, when reduced, acts simply as a tridentate ligand in the presence of a base forming a bis-complex (2) with the manganese(III) ion. Complex 2 has also been crystallographically characterized. The coordination geometry around manganese(III) in each is highly distorted octahedral consistent with a d4 ion showing strong Jahn-Teller distortion. In the cyclic voltammetric experiments at 298 K in DMF, complex 1 exhibits a cyclic response at Ef = −0.79 V (vs SCE) with ΔEp = 360 mV while 2 shows a quasi-reversible signal with Ef = −0.18 V (vs SCE) with ΔEp = 110 mV both at the scan rate of 100 mV s−1. The signal is due to the Mn(III)/Mn(II) couple. Solid state magnetic susceptibility experiments at 300 K indicate that both the complexes are high spin (S = 2) in nature. Electronic and infrared spectral data of the complexes are consistent with their structures.

Synthesis and characterization of copper(II) and nickel(II) complexes with ligands having N2S*2S2 (S* = thioether) donors: pseudo-reversible CuIII/CuII couple

Abstract Two new hexadentate ligands having thioether and thiolate groups in pairs along with two amide nitrogens have been synthesized. The thiols are protected as their tertiary butyl derivatives. On refluxing the ligands with a copper(II) or a nickel(II) salt in isopropanol, the S-tertiary butyl group was cleaved to form the corresponding hexa-coordinated neutral complex with the chromophore MIIN2S*2S2, where the four sulphurs are equatorial and the amide nitrogens are axial. Both the copper(II) complexes show pseudo-reversible responses (ΔEp = 100 mV) in cyclic voltammograms with E 1 2 = 0.5 V vs S.C.E. at room temperature in acetonitrile, attributable to a CuIII/CuII couple. The corresponding nickel(II) complex shows an irreversible response in the region −1.0 to +1.0 V. In their electronic spectra the copper(II) complexes show a strong band near 600 nm, attributable to a σ(thiolate) → CuII LMCT transition. Each of the copper(II) complexes show ligand field bands typical of a tetragonally distorted octahedral complex. The nickel(II) complexes show three ligand field bands typical of octahedral geometry.

Synthesis of cryptands having tritopic receptor sites by [2+3] Schiff base condensation using Cs(I) ion as the template

Synthesis of two tritopic receptors L1 and L2 by [2+3] Schiff base condensation of trialdehyde 1 or 2 and 1,2-diaminobenzene followed by reduction with NaBH4 are reported. The reactions undergo smoothly in presence of Cs(I) ion as the template.

Hexa-coordinated copper(II)-thiolates—II. Synthesis and characterization of a copper(II) complex with a reversible oxidative response

Abstract Two air-stable copper(II) complexes of homoleptic hexadentate ligands with the N2S2*S2 (S* = thioether, S = thiolate) donor set have been isolated. These complexes are dark blue in colour owing to the presence of a strong absorption peak around 600 nm, attributable to the σ(thiolate) → copper(II) LMCT transition. In acetonitrile, one complex provides the first example for a copper(II)-thiolate complex of a reversible cyclic voltammogram with E 1 2 = 0.50 V (vs SCE). This reversible response is attributable to the oxidation of the complex. Controlled experiments to explore possible ligand oxidations indicate that the reversible response is possibly metal-centred. Exhaustive, controlled potential coulometry experiments at 0.7 V at room temperature give n = 1.02 e−, signifying the cyclic response is a one-electron process. The other complex gives a quasi-reversible cyclic voltammogram with E 1 2 = 0.54 V (vs SCE).

MODELLING THE BLUE PROTEIN ACTIVE SITES : SYNTHESIS AND CHARACTERIZATION OF CUN2S2 COMPLEXES SHOWING RHOMBIC EPR SPECTRA AND HIGH CUII/CUI POTENTIAL

Abstract Two new tetradentate ligands have been synthesized by Schiff base condensation oof diisobutyraldehyde disulphide with 2-mercaptoethylamine (L1) and 2-aminothiophenol (L2) respectively and then reducing the imine linkages with NaBH4 in refluxing methanol. In the free ligands the thiolate sulphur is protected with tertiary butyl groups which are cleaved in the presence of CuII-salts to give neutral CuN2S2 complexes. The copper complexes show ligand field transitions at 815 and 760 nm at room temperature which are independent of the solvents used and are consistent with a pseudotetrahedral coordination around the CuII ion. The EPR spectrum of the aliphatic thiolate in MeCN glass shows significant rhombic splitting (gx−gx = 0.09 and Ax−Ay = 60 × 10−4 cm−1) attributable to dz2 mixing into the ground state wavefunction. For the aromatic thiolate complex, however, the EPR spectrum was not well resolved although the rhombic nature of the spectrum could easily be observed. Both the complexes exhibit well-defined cyclic responses in their cyclic voltammograms at RT and in acetonitrile for the CuII/CuI couple with E 1 2 = 0.5 V vs SCE. This high positive value for the redox couple is also consistent with a coordination geomttry much distorted from planarity. The active sites of the blue protein which contain copper in distorted geomtries exhibit CuII/CuI potential in the range 300–800 mV vs NHE at pH = 7.0.

Reactivity of (dtc)BiCl2 (dtc = diethyldithiocarbamate) towards phenyllithium : synthesis and characterization of novel organometallic tetradentate bismuth(III) complexes

Abstract Two tetradentate organometallic complexes of bismuth(III) have been synthesized in high yields from the reactions of (dtc)BiCl2 with phenyllithium in 1:1 and 1:2.5 molar ratios. These complexes were characterized by elemental analysis, IR and NMR spectroscopy. In Bi(dtc)PhCl (1) the bismuth(III) ion is coordinated to a dtc, a chloride and a phenyl group, thus providing the first example of a bismuth(III) complex having three different types of ligation. The other complex has ligation from two phenyl groups in addition to one dtc.

Synthesis and characterization of a mononuclear iron(III) complex with a tripodal triamide ligand

Abstract A new potentially heptadentate tripodal ligand has been synthesized by the condensation reaction between tris(2-aminoethyl)amine and acetylsalicyclic acid in a 1:3 molar ratio. The o -acetyl linkage is cleaved by treating with KOH to form the desired ligand with the donors that include three amide nitrogens, three phenolate oxygens and one bridgehead nitrogen. Fe(acac) 3 readily forms a dark red 1:1 complex with the ligand. The complex behaves as a non-electrolyte in acetonitrile. The electronic absorption spectrum in the visible region is dominated by a strong absorption with λ max at 425 nm, assignable to an LMCT transition from phenolate oxygen to iron(III) in an octahedral coordination geometry. The magnetic moment value at room temperature (5.5 μ eff /μ g ) and the EPR spectra in the solid state and in solution ( g = 4.06) are consistent with high-spin rhombically distorted octahedral iron(III). Room-temperature Mossbauer data provide the following values: isomer shift 0.37 mm s −1 and quadrupole splitting 0.82 mm s −1 . These data are consistent with a discrete, high-spin octahedral iron(III) complex.

A convenient route to iron(III)-thiolates: Synthesis and characterization of low-spin (S = 12) iron(III) complexes having the chromophore FeN2S*2S2 (S* = thioether)

Abstract Three air-stable iron(III) complexes with homoleptic hexadentate ligands with the donor set N2S*2S2 (S* = thioether, S = thiolate) have been isolated. The two thiol groups present in each ligand are initially protected as their t-butyl derivatives. When iron(III) chloride is allowed to reflux with any of these thiol-protected ligands the SBut linkages are cleaved forming the corresponding iron(III)-thiolates in high yields. The complexes are characterized by electronic absorption, EPR and room-temperature Mossbauer spectroscopic as well as room-temperature magnetic susceptibility studies. As the spectral data indicate each complex has a pseudo-octahedral coordination geometry around the metal ion. The effective magnetic moment values at 300 K lie in the range 2.24–2.30, corresponding to low-spin iron(III) (S = 1 2 ) complexes with significant orbital contribution. One of the complexes exhibits a quasi-reversible FeIII/FeII couple at Ef = 1.66 V (vs S.C.E.), while another complex shows a quasi-reversible FeIV/FeIII couple at Ef = 0.86 V (vs S.C.E.).