A structural, electrochemical and thermal study of new mono- and bimetallic long chain carboxylates

[摘要] English: Two methods were used for the synthesis of the mixed-metal carboxylatido complexes. The firstmethod involves the reaction of one equivalent of [Pd3II(μ-OOC(CH2)nCH3)6] where n = 4, 6, 8,or 10 with three equivalents of the relevant [MII(OOC(CH2)nCH3)2] where n = 4, 6, 8, or 10 or[CeIII(OOC(CH2)8CH3)3], which results in [PdIIMII(μ-OOC(CH2)nCH3)4] where n = 4, 6, 8, or 10or [PdIICeIII(μ-OOC(CH2)8CH3)4]+ with yields between 56 to 95 %.The second method involves a ligand exchange type of reaction where [PdIIMII(μ-OOCH3)4] or[PdIICeIII(μ-OOCH3)4]+ is reacted with the desired long chain carboxylic acid. This results in[PdIIMII(μ-OOC(CH2)nCH3)4] where n = 4, 6, 8, or 10 or [PdIICeIII(μ-OOC(CH2)8CH3)4]+, withyields between 66 to 99 %.The mono-metal and mixed-metal complexes were characterised using ATR-FTIR. This studyindicated that the mixed-metal complexes have more than one binding mode, namely theunidentate, bidentate, tridendate, bridging (syn-syn) binding mode and ionic binding mode.The single crystal X-ray structures of [PdIICoII(μ-OOC(CH2)8CH3)4] [21] (Z = 2, space groupP21/c), [PdIIZnII(μ-OOC(CH2)8CH3)4] [36] (Z = 4, space group P21/c), and [PdIINiII(μ-OOC(CH2)8CH3)4] [38] (Z = 2, space group P-1) were solved and confirmed the binding modesobserved in the ATR-FTIR studies.Selected complexes were subjected to thermal analysis using DSC and TGA-MS. Liquid crystalproperties was observed for PdCo(C8)4 [20], PdCo(C10)4 [21] and PdZn(C10)4 [36].Polymorphism was observed for PdCd(C10)4 [34], PdMn(C10)4 [39] and PdCu(C10)4 [40].Variable temperature polarized light microscopy studies was used to shed light on the processesobserved using DSC. TGA-MS analysis indicated volatile decomposition products weremethane, hydroxide ions, water, carbon monoxide, oxygen, methanol, propyne, carbon dioxideand other products. Non-volatile decomposition product residues obtained were metal oxides.Cyclic Voltammetry, Osteryoung Square Wave Voltammetry and Linear sweep voltammetrywas performed on selected complexes and electronic communication between the metals wasobserved. The length of the carbon chain had an influence on the position of the oxidation waveof the palladium cerium paddlewheel carboxylatido complexes.By increasing the carboxylatido carbon chain length from two to ten, the Epa decreased from 514mV for [PdIICeIII(μ-OOCCH3)4]+ [32], to 297 mV for [PdIICeIII(μ-OOC(CH2)8CH3)4]+ [41].Selected mixed-metal paddlewheel complexes were spin coated onto modified silicon wafersusing either acetone or DCM as solvent. The pre-catalyst was activated by oxidation in a streamof oxygen at 450 °C. This results in palladium oxide and metal oxide being deposited on themodified silicon wafer surface.The catalysts were tested in the solvent-free aerobic oxidation of 1-octadecanol to 1-octadecanoic acid. The reaction was monitored by following the appearance of the carbonylstretching frequencies at 1730 and 1710 cm-1 using ATR-FTIR.Turn over frequencies (TOF) between 0.8 to 2 molecules s-1 were obtained for catalysts preparedfrom short-chain mixed-metal complexes. TOF's between 4 to 7 molecules s-1 were obtained forcatalysts prepared from long-chain mixed-metal complexes. XPS analysis of the catalystsrevealed that the PdO and MO (metal oxide) ratio was close to 1:1 and also 1:1.5.

[发布机构] University of the Free State