Subhash Padhye

Structural and physical correlations in the biological properties of transition metal heterocyclic thiosemicarbazone and S-alkyldithiocarbazate complexes

The nature of metal complexes of heterocyclic thiosemicarbazones reported in the literature through 1989 have been reviewed with an emphasis on variations in stoichiometry and stereochemistry. A brief introduction is followed by a survey of the biological activity of the uncomplexed thiosemicarbazones. Next is a section on the chemical nature of thiosemicarbazones including a discussion of preparative methods, modes of coordination and isomerism. This is followed by a discussion of structural information and biological activity of the iron(III), iron(II), copper(II), cobalt(III), cobalt(II) and nickel(II) complexes of heterocyclic thiosemicarbazones. Short sections on the biological activity of other chelating thiosemicarbazones and metal complexes are also included.

Manganese-histidine cluster as the functional center of the water oxidation complex in photosynthesis

The recent model of Kambara and Govindjee for water oxidation [Kambara T. and Govindjee (1985) Proc. Natl. Acad. Sci. U.S.A., 82:6119–6123] has been extended in this paper by examining all the data in order to identify the most likely candidate for the ‘redox-active ligand’ (RAL), suggested to operate between the water oxidizing complex (WOC) and Z, the electron donor to the reaction center P680. We have concluded that a very suitable candidate for RAL is the imidazole moiety of a histidine residue. The electrochemical data available on imidazole derivatives play heavily in this identification of RAL. Thus, we suggest that histidine might play the role of an electron mediator between the WOC and Z. A model of S-states in terms of their plausible chemical identity is presented here.

Iron(II) complexes of ortho-functionalized para-naphthoquinones 1. Synthesis, characterization, electronic structure and magnetic properties

Abstract A series of six high-spin ferrous complexes of ortho-functionalized paraquinone ligands of the type M(L)2(H2O)2, L = anion of the lawsone derivative, are synthesized where the bound quinone ligands are found to be in their fully oxidized form and the coordinating centers are the C-2 phenolate and the C-1 carbonyl. Thermal studies indicate that the two water molecules are coordinated and their loss leads to the decomposition of the chelates. Variable temperature magnetic susceptibility studies establish the high-spin state of the iron and confirm the quinone formulation of the ligands while Mossbauer data clearly establish the +2 oxidation state for the iron centers. Finally, the electronic absorption data indicate that the six high-spin ferrous complexes described herein are isostructural species including the O6 chromophore and the cyclic voltammetry confirms that the lawsone ligands are coordinated in their quinone form while the iron centre is at the +2 oxidation state.

Communication:: The first well characterized Fe (phen) Cl3 complex : structure of aquo mono(1,10-phenanthroline) iron (III) trichloride : [Fe (phen) Cl3 (H2O)]

Abstract The crystal structure of the neutralpseudo-octahedral mono phenanthroline iron (III) complex Fe (phen) Cl3H2O is reported which surprisingly is the first well-characterized Fe (phen) Cl3 complex (space group P1 a = 10224 (4) d b = 10603 (3) d c = 6628 (2) d alpha 10070 (2) °β = 10817 (2) °γ = 9206 (2) ° R = Rw = 0033) Of the three unidentate chloride ions two are coplanar with phenanthroline ligand while the third is out of plane and is trans to the water molecule The metal environment is distorted from regular octahedral due to the non-equivalent ligand donor set The N–Fe–N bite angle of 756° is compressed below right-angular with a corresponding opening of the other bond angles around the iron in this plane The Fe — Cl bonds are inequivalentranging from 225 to 235 d the axial Fe — Cl bond being elongated The axial aquo group is tilted towards the phenanthroline ringmaking an angle of 838° with N (1) and 818° with N (2) respectively Comparison with the Mn (III) analog yields the effect of changing from a high spin d 5 to a high spin d 4 center III) ; phenanthroline ; DNA intercalator.

Inequivalent coordination of thiosemicarbazone ligands in cobalt (III) and chromium (III) complexes

Chromium (III) and cobalt (III) complexes derived from 2-ketobutyric acid thiosemicarbazone (H,Kbtsc) have been prepared and characterized. Crystal structure analyses have shown both products to be mixed ligand complexes of the type M(HKbtsc) (Kbtsc) in which the thiosemicarbazone ligand is present in singly deprotonated thione and doubly deprotonated thiol forms. In each complex, the metal ion is bound to two tridentate ligands in a distorted octahedral geometry with meridional stereochemistry. Small but consistent structural differences between the two types of ligands are observed. Spectroscopic and electrochemical data are consistent with the formulations of these com- pounds as derived from the crystallographic results. Transition metal complexes of thiosemicarbazones, R,R,C=N-N(H)-C(S)NH, have been exten- sively studied and have been the subject of several recent reviews.lm3 Thiosemicarbazones have been shown to exhibit a range of types of biological activity which in a number of cases is thought to be related to their ability to chelate trace metals. Also, transition metal complexes of thiosemicarbazones have been found in some instances to have enhanced or modified activity in comparison to the uncom- plexed ligands.3 Simple thiosemicarbazones (R, = H ; RZ = alkyl or aryl) normally bind to transition metals as neutral bidentate ligands, with a five- membered chelate ring formed by metal-sulphur and metal-imine nitrogen bonds. A few cases of monodentate S-binding are known, as are instances of coordination as an anion, formed by depro- tonation at the imide nitrogen atom. Tridentate thiosemicarbazone derivatives may be obtained by

Synthesis, spectroscopic and structural characterization of the mer isomer of ammonium bis(phenylpyruvic acid thiosemicarbazone)-cobalt(III) hemihydrate

SummaryPhenyl pyruvate thiosemicarbazone (H2PPVATSC) coordinates in its binegative thiolate form to cobalt(III) in the complex isolated from basic medium. An X-ray structure determination of the resulting meridional isomer was found to contain mutually cis sulphur, trans nitrogen, and trans oxygen donor atoms, respectively. The two five-membered rings formed by each ligand are puckered towards each other, resulting in a distortion from regular octahedral geometry about the cobalt atom. The complex was found to exhibit only a metal-based one electron reversible reduction at -0.97 V, while the parent ligand yields a c.v. profile consisting of two irreversible reduction peaks at -0.75 and -1.05V respectively, the latter corresponding to the reduction of the azomethine group.

Electrochemical studies of transition metal complexes of 2-acetyl-pyridine thiosemicarbazones. Part 2. Correlations with spectral and antifungal properties of copper(II) complexes of 2-acetylpyridine 3-azacyclothiosemicarbazones

SummaryThe electrochemical behaviour of a series of copper(II) complexes of 2-acetylpyridine 3-azacyclothiosemicarbazones is reported. The complexes undergo a quasireversible one electron reduction in the range −0.400 to −0.450 Vversus Ag/AgCl, attributable to the copper(II)/-copper(I) redox couple. The electrochemical, as well as spectral characteristics of these complexes, can be correlated with their antifungal activity againstAspergillus niger, Paecilomyces variotti, Penicillum rubrum andAspergillus terreus.

Electrochemical, spectroscopic and structural characterization of the mer isomer of ammonium bis(pyruvic acid thiosemicarbazone)cobalt(III) - benzene

Abstract The crystal structure is reported of ammonium bis(pyruvic acid thiosemicarbazone)cobalt(III)·benzene, CoS 2 O 4 N 6 C 16 H 16 , which forms dark red triclinic crystals with unit cell parameters: a =9.307(2), b =9.555(2), c =14.729(3) A, α=67.04(1), β=118.89(2), γ=79.09(1)°, V =932.9(2) A 3 , space group P 1 . The final R factor for 3948 observed reflections is 3.1%. The distorted octahedral complex is held together by a hydrogen bonding system involving the ammonium and amine protons and the carboxylate oxygen and imine nitrogen atoms. The coordination polyhedron around the cobalt atom in the complex anion [Co(PVATSCH) 2 ] − is highly distorted from regular octahedral geometry as evidenced by the fact that the bond angles about the cobalt atom range from 83.40(6) to 96.75(5)°. The isolated meridional isomer contains a cis placement of the sulfur donor atoms with puckered ligand moieties arising from the non-planar hydrazinic chain with a folding angle of ca . 4° across the sulfur-nitrogen and oxygen-nitrogen atoms. The ligand H 2 PVATSC provides a strong ligand field and thus results in a low spin cobalt(III) complex. The complex undergoes a reversible one-electron reduction at the metal center.

High-spin iron (II) complexes of ortho-functionalized paraquinones as models for quinone binding sites in reaction centres of photosynthetic bacteria

Abstract In an attempt to mimic the iron–quinone couple of bacterial reaction center, spectral, magnetic and Mossbauer characterization of seven high-spin iron(II) complexes of the ortho -functionalized paraquinones is reported where the quinones are coordinated in their fully oxidized, monoanionic form.

Thermal and spectral properties of lanthanide(III) complexes of 3-amino-2-hydroxy-1,4-naphthoquinone

Octa-coordinated La( III), Ce( III), Pr( III), Nd( III), Sm( III), Gd( III) and Dy( III) complexes of 3-amino-2-hydroxy- 1,4_naphthoquinone (3A2HNQ) of general formula [ M( 3A2HNQ),( H20)J have been synthesized and characterized. The non-isothermal thermogravimetric profiles indicate the loss of one of the coordinated ligands along with one coordinated water molecule at a significantly higher energy of activation than that of the second step of decomposition with loss of two ligands and one water molecule. Such a difference in the energy of activation for the two steps can be correlated to the strong intermolecular hydrogen bonding interactions between quinone carbonyls and coordinated water molecules. These chelates exhibit slightly lowered magnetic moments probably due to their polymeric nature. IR and far-IR spectral data suggest that phenolic oxygen and amino nitrogen are the coordinating sites for 3A2HNQ, while coordinated water molecules are involved in the intermolecular hydrogen bonding interactions with free quinone carbonyls of 3A2HNQ molecules. The nephelauxetic ratio (fl), covalency factor (h ‘jr) and Sinha’s parameter (ci) evaluated from electronic spectral studies of Ce( III), Sm( III), Gd( III) and Dy( III) reveal a negligible amount of covalency in metallligand bonding.