Synthesis and characterization of rhenium (vii) and rhuthenium (ii) dendritic catalysts: oxidative cleavage and epoxidation of alkenes
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Date
2012
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Abstract
Herein we report the successful synthesis of a class of stable and flexible Schiff-base chelators capable of coordinating both ruthenium (II) and rhenium (VII) and which would be catalytically active for oxo-transfer reactions. The synthesis of bidentate (L1), tetradentate (L2-L3), and multidendate ligands (DL1-DL4) of nitrogen was a result of a reaction of primary amine with 2-pyridinecarboxaldehyde. Ligand (L3) is reported herein for the first time. The amines (n-propylamine, ethylenediamine, butanediamine, diaminobutane,
propylene iminopyridyl (DAB-PPI) dendrimer) were varied as to afford metal complexes that exhibit different physical and chemical properties. The ligands were isolated and fully characterized by IR, NMR spectroscopy and elemental (H, C, N) analysis.The Schiff-base complexes of methyltrioxorhenium (MTO): Methyl(n-pyridin-2-yl)methylene)propan-1-amine)trioxorhenium (C1), Methyl([bis(pyridin-2-
yl)formylidene]butane-1,4-diamine)trioxorhenium (C2), Methyl(diaminobutane propylene imonopyridyl)trioxorhenium G1(DC1) and G2(DC2) have exhibited sensitivity to water than MTO itself. Rapid ligand-exchange reactions in solution are observed at elevated temperatures. The MTO Schiff-base complexes are also slightly sensitive to light and slowly decompose as they are exposes to air. These complexes were isolated and fully characterized by IR, NMR, UV-Vis, EA and MS. In the ESI mass spectra of compound C1-C2 and DC1-DC2 show the peaks of the Schiff-base ligand and the MTO moiety separately, without a traceable fragmentation pattern. The isotopic cluster and the molecular ion peak were
observed.The mononuclear ruthenium compounds (B1 and B3) were prepared from
dichlorotetrakis(dimethyl sulfoxide)ruthenium (II) metal precursor by reacting the
synthesized ligands (L2 and L3) with the metal precursor. Compounds (B2 and B4) were obtained by subsequently stabilizing the neutral compounds (B1 and B2) as
hexaflourophosphate salts via metathesis employing thallium (I) hexafluorophosphate (V).The homobimetallic cationic compound (B5) was synthesized by reacting the dinuclear complex [(p-cymene)2RuCl2]2 with ligand (L4).The neutral tetranuclear (V1 and V3) and octanuclear (V2 and V4) (N,N) ruthenium(II) metallodendrimers were synthesized mimicking the same route as for the neutral mononuclear compounds (B1 and B3). The compounds (V1-V4) were prepared from the dichlotetrakis(dimethyl sulfoxide)ruthenium(II) based on the synthesized dendritic scaffolds (DL1-DL4). Compounds (V5 and V6) were fashioned in a similar manner to compound (B5),by reacting the iminopyridyl dendritic scaffolds (DL1 and DL3) with the dinuclear precursor[(p-cymene)2RuCl2]2 to afford two complexes of the type [{(p-cymene)RuCl2}4G1, V5] and [{p-cymene)RuCl2}8G2, V6]. Electronic spectra of the prepared complexes were obtained (in a Sharpless Biphasic solvent system: CCl4:MeCN:H2O) in order to understand the nature of the active species in the catalytic cycle and to propose a mechanism for the catalytic cycle .Confirmation of the prepared complexes (B1-V6) was done using several spectroscopic techniques (IR, NMR, UV-Vis, ESI-MS) in conjuction with elemental analysis.The compounds C1-DC2 were then tested towards the epoxidation of selected cyclic alkenesi.e cyclohexene and cis-cyclooctene, respectively and straight chain alkenes. The catalyzed epoxidation reactions were carried out at room temperature employing using Urea hydrogen
peroxide adduct (UHP) as the oxidant and dichlomethane (DCM) as the solvent. The
complexes displayed high catalytic activity and selectivity when applied to the epoxidation of cyclohexene and cis-cyclooctene with urea hydrogen peroxide adduct (UHP) as oxidant in dicholoromethane. The epoxidation reaction was quantified using gas chromatography.Conversions reached 100% for all the complexes within 6 hours. The catalytic activity of complex C1 and C2 was relatively low compared to the catalytic activity of complex DC1 and DC2.
Description
>Magister Scientiae - MSc
Keywords
Ruthenium, Rhenium, Oxo-transfer reactions