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

English: Two methods were used for the synthesis of the mixed-metal carboxylatido complexes. The first method involves the reaction of one equivalent of [Pd3 II(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 10 or [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]+, with yields between 66 to 99 %. The mono-metal and mixed-metal complexes were characterised using ATR-FTIR. This study indicated that the mixed-metal complexes have more than one binding mode, namely the unidentate, 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 group P21/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 modes observed in the ATR-FTIR studies. Selected complexes were subjected to thermal analysis using DSC and TGA-MS. Liquid crystal properties 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 processes observed using DSC. TGA-MS analysis indicated volatile decomposition products were methane, hydroxide ions, water, carbon monoxide, oxygen, methanol, propyne, carbon dioxide and other products. Non-volatile decomposition product residues obtained were metal oxides. Cyclic Voltammetry, Osteryoung Square Wave Voltammetry and Linear sweep voltammetry was performed on selected complexes and electronic communication between the metals was observed. The length of the carbon chain had an influence on the position of the oxidation wave of the palladium cerium paddlewheel carboxylatido complexes. By increasing the carboxylatido carbon chain length from two to ten, the Epa decreased from 514 mV 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 wafers using either acetone or DCM as solvent. The pre-catalyst was activated by oxidation in a stream of oxygen at 450 °C. This results in palladium oxide and metal oxide being deposited on the modified 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 carbonyl stretching 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 prepared from short-chain mixed-metal complexes. TOF’s between 4 to 7 molecules s-1 were obtained for catalysts prepared from long-chain mixed-metal complexes. XPS analysis of the catalysts revealed that the PdO and MO (metal oxide) ratio was close to 1:1 and also 1:1.5.