Gummadi Durgaprasad

Influence of the substituents on the electronic and electrochemical properties of a new square-planar nickel-bis(quinoxaline-6,7-dithiolate) system: synthesis, spectroscopy, electrochemistry, crystallography, and theoretical investigation.

We describe the synthesis, crystal structures, electronic absorption spectra, and electrochemistry of a series of square-planar nickel-bis(quinoxaline-6,7-dithiolate) complexes with the general formula [Bu(4)N](2)[Ni(X(2)6,7-qdt)(2)], where X = H (1a), Ph (2a), Cl (3), and Me (4). The solution and solid-state electronic absorption spectral behavior and electrochemical properties of these compounds are strongly dependent on the electron donating/accepting nature of the substituent X, attached to the quinoxaline-6,7-dithiolate ring in the system [Bu(4)N](2)[Ni(X(2)6,7-qdt)(2)]. Particularly, the charge transfer (CT) transition bands observed in the visible region are greatly affected by the electronic nature of the substituent. A possible explanation for this influence of the substituents on electronic absorption and electrochemistry is described based on highest occupied molecular orbital (HOMO) to lowest unoccupied molecular orbital (LUMO) gaps, which is further supported by ground-state electronic structure calculations. In addition to this, the observed CT bands in all the complexes are sensitive to the solvent polarity. Interestingly, compounds 1a, 2a, 3, and 4 undergo reversible oxidation at very low oxidation potentials appearing at E(1/2) = +0.12 V, 0.033 V, 0.18 V, and 0.044 V vs Ag/AgCl, respectively, in MeOH solutions, corresponding to the respective couples [Ni(X(2)6,7-qdt)(2)](-)/[Ni(X(2)6,7-qdt)(2)](2-). Compounds 1a, 3, and 4 have been characterized unambiguously by single crystal X-ray structural analysis; compound 2a could not be characterized by single crystal X-ray structure determination because of the poor quality of the concerned crystals. Thus, we have synthesized the tetraphenyl phosphonium salt of the complex anion of 2a, [PPh(4)](2)[Ni(Ph(2)6,7-qdt)(2)]·3DMF (2b) for its structural characterization.

Modeling the active site of [FeFe]-hydrogenase: Electro-catalytic hydrogen evolution from acetic acid catalysed by [Fe2(μ-L)(CO)6] and [Fe2(μ-L)(CO)5(PPh3)] (L=pyrazine-2,3-dithiolate, quinoxaline-2,3-dithiolate and pyrido[2,3-b]pyrazine-2,3-dithiolate)

AbstractCompounds [Fe2{ μ-pydt}(CO)6] (pydt = pyrazine-2,3-dithiolate) (1), [Fe2{ μ-qdt}(CO)6] (qdt = quinoxaline-2,3-dithiolate) (2), [Fe2{ μ-ppdt}(CO)6] (ppdt = pyrido[2,3-b]pyrazine-2,3-dithiolate) (3), [Fe2 { μ-pydt}(CO)5PPh3] (4), [Fe2{ μ-qdt}(CO)5PPh3] (5) and [Fe2{ μ-ppdt}(CO)5PPh3] (6) have been synthesized in order to model the active sites of ‘[FeFe]-hydrogenase’. Compounds 1–6 have been characterized by routine spectral studies and unambiguously by single crystal X-ray crystallography. Supramolecular chemistry of compounds 1–6 have been described in terms of intermolecular interactions, observed in their respective crystal structures. Electro-catalytic hydrogen evaluation studies (from acetic acid) have been performed using compounds 1–6 as electro-catalysts. The mechanistic aspects of relevant electro–catalytic proton reductions have been discussed in detail. Graphical AbstractIn an attempt to model the active sites of [FeFe]-hydrogenase, a series of 1,2-enedithiolate-bridged di–iron complexes have been synthesized and structurally characterized. The electrochemical generation of di-hydrogen from an weak acid, mediated by these model complexes, has been demonstrated.

6,7,6′,7′-Tetra­phenyl-2,2′-bi[1,3-dithia-5,8-diaza­cyclo­penta­[b]naphthalenyl­idene] chloro­form disolvate

The title compound, C42H24N4S4·2CHCl3, a symmetrical tetrathiafulvalene (TTF) derivative, was prepared by a triethylphosphite-mediated self-coupling reaction of 6,7-diphenyl-1,3-dithia-5,8-diazacyclopenta[b]napthalen-2-one. The asymmetric unit contains two TTF molecules and four chloroform solvent molecules. Cl⋯Cl interactions [contact distances = 3.263 (1)–3.395 (2) Å] are present between the solvent molecules, resulting in a tape along the bc plane. The crystal packing features weak C—H⋯Cl and C—H⋯N hydrogen bonds, resulting in the formation of a two-dimensional supramolecular network.