Subrata Mandal

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.

Hexa-coordinated copper(II) complexes having CuN2S2*S2 (S* = thioether) chromophore: synthesis, electrochemistry and spectroscopy

Abstract Four stable, neutral hexa-coordinated copper(II) complexes with the N 2 S 2 * S 2 (S * = thioether) donor set have been isolated in the solid state. A dichloro complex of copper(II) has also been prepared with the chromophore CuN 2 S 2 * Cl 2 , where the two thiolates are replaced by two chlorides. These complexes have tetragonal coordination geometry around copper(II) with g | > g hu > g e and A | values within 101–115 G in acetonitrile glass, and show an intense absorption band around 600 nm attributable to thiolate → copper(II) LMCT transition. In acetonitrile the complexes show well-defined cyclic responses in their cyclic voltammograms at room temperature.

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.

A hexadentate nickel(II) complex with a tripodal ligand bearing S3N3 donors: Synthesis, spectroscopic and X-ray crystal structural investigation

Abstract A tripodal ligand, tris[2-(2-aminoethyl)thio]ethane (L 1 ), bearing three thioether sulphurs and three primary amino nitrogens has been synthesized in high yield. Reaction with nickel(II) perchlorate hexahydrate yielded the complex [Ni(L 1 )](ClO) 4 , which has been characterized by X-ray crystallography. The structure consists of two independent hexacoordinate cage cations and four independent perchlorate anions. The Ni II ion is bound equatorially to two nitrogen and two sulphur atoms, while the other two donor atoms occupy the acial positions. Both the independent cage cations are disordered, with thioether sulphurs occupying two alternative sets of ligand sites. One of the four perchlorate anions is also disordered. IR, conductivity, RT magnetic susceptibility and electronic absorption spectral data are consistent with the solid state structure. The corresponding aromatic analogue, tris[2-(2-aminophenyl)thio]ethane (L 2 ), forms the complex [Ni(L 2 )](ClO 4 ) 2 , which has been characterized spectroscopically.

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 some cuprous salts towards tenmembered macrocyclic disulphides: A new route to the synthesis of Cu(II)-thiolates

Cuprous ion is found to be quite reactive towards disulphide present in a macrocycle. Thus, when [Cu(MeCN)4] BF4 or [Cu(MeCN)4]ClO4 is allowed to reflux in acetonitrile with a ten-membered macrocycle containing two amino nitrogens and one disulphide donor in an atmosphere of nitrogen, electron transfer takes place from the metal centre to the disulphide group. As a result, copper is oxidised to the + 2 state and the disulphide reduced to thiolates. The thiolates then bind the Cu(II) and form discrete and neutral CuN2S2 complexes. Syntheses of two new CuN2 S2 complexes have been achieved following this route. The reactions take place smoothly and give the desired product in excess of 25% yield with respect to the macrocycle. Electronic absorption spectral results are consistent with a nearly square planar geometry. Each of the two complexes isolated exhibits quasireversible Cu(II)/Cu(I) couple with Ef near — 0.3 V vs SCE. Both the complexes are further characterised by room temperature magnetic susceptibility, EPR spectroscopy at 298 and 77 K and elemental analyses.