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

Abstract

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.

Afrikaans: Twee metodes is gebruik vir die sintese van gemengde-metaal karboksilatido komplekse. Die eerste metode behels die reaksie tussen een ekwivalent van [Pd3 II(μ-OOC(CH2)nCH3)6] waar n = 4, 6, 8 of 10 en drie ekwivalente van die relevante [MII(OOC(CH2)nCH3)2] waar n = 4, 6, 8 of 10 of [CeIII(OOC(CH2)8CH3)3], wat [PdIIMII(μ-OOC(CH2)nCH3)4] waar n = 4, 6, 8 of 10 of [PdIICeIII(μ-OOC(CH2)8CH3)4]+ met n opbrengs tussen 56 en 95 % gee. Die tweede metode behels ‘n liganduitruilreaksie waar [PdIIMII(μ-OOCH3)4] of [PdIICeIII(μ- OOCH3)4]+ gereageer word met die gekose langketting karbosielsuur. Dit lei tot die vorming van [PdIIMII(μ-OOC(CH2)nCH3)4] waar n = 4, 6, 8 of 10 of [PdIICeIII(μ-OOC(CH2)8CH3)4]+ met opbrengste tussen 66 en 99 %. Die mono-metaal en gemengde-metaal komplekse is gekarakteriseer met ATR-FTIR. Die studie het aangedui dat die gemengde-metaal komplekse meer as een bindingsmodus besit, naamlik unidentaat, bidentaat, tridentaat, gebrugde (syn-syn) en ioniese bindings. Die enkelkristal X-straal strukture van [PdIICoII(μ-OOC(CH2)8CH3)4] [21] (Z = 2, ruimte groep P21/c), [PdIIZnII(μ-OOC(CH2)8CH3)4] [36] (Z = 4, ruimte groep P21/c), en [PdIINiII(μ- OOC(CH2)8CH3)4] [38] (Z = 2, ruimte groep P-1) is bepaal en het die bindingsmodusse soos deur ATR-FTIR waargeneem, bevestig. DSC en TGA-MS is gebruik om termiese analise van geselekteerde komplekse te doen. Vloeikristal eienskappe is waargeneem vir PdCo(C8)4 [20], PdCo(C10)4 [21] en PdZn(C10)4 [36]. Polimorfisme is waargeneem vir PdCd(C10)4 [34], PdMn(C10)4 [39] en PdCu(C10)4 [40]. Verstelbare temperatuur, gepolariseerde lig mikroskoopstudies is gebruik om lig te werp op die prosesse waargeneem tydens DSC. TGA-MS analise het vlugtige ontbindings produkte aangetoon: metaan, hidroksied ione, water, koolstofmonoksied, suurstof, metanol, propyn, koolstofdioksied en ander produkte. Nie-vlugtige ontbindingsprodukte wat verkry is, is metaal oksiede. Sikliese voltammetrie, Osteryoung vierkant golf voltammetrie en lineêre skanderings voltammetrie van geselekteerde komplekse het elektroniese komunikasie tussen die verskillende metale aangetoon. Die lengte van die koolstofketting het ‘n invloed op die posisie van oksidasie koppel van die palladium cerium skepwiel karboksilatido komplekse. As die kettinglengte van die karboksilatido ligand van twee na tien vermeerder word, neem Epa af van 514 mV vir [PdIICeIII(μ-OOCCH3)4]+ [32] na 297 mV vir [PdIICeIII(μ-OOC(CH2)8CH3)4]+ [41]. Lagies van geselekteerde gemengde-metaal komplekse is op aangepaste silikonplaatjies gespin met behulp van asetoon of dichlorometaan as oplosmiddel. Die voor-katalisator is geaktiveer deur oksidasie in ‘n suurstofstroom by 450 °C. Dit het gelei tot palladiumoksied en metaaloksied op die aangepaste silikonplaatjie se oppervlakte. Die kataliste is getoets vir die oplosmiddel-vrye aerobiese oksidasie van 1-oktadekanol na 1-oktadekanoësuur. Die reaksie is gemoniteer deur die verskuiwing van karboniel strekkingsfrekwensies by 1730 en 1710 cm-1 te volg, met behulp van ATR-FTIR. Omskakelingsfrekwensies tussen 0.8 en 2 molekules s-1 is verkry vir kataliste wat vanaf kort-ketting gemengde-metaal komplekse berei is. Omskakelingsfrekwensies tussen 4 en 7 molekules s-1 is vir kataliste verkry wat vanaf lang-ketting gemengdemetaal komplekse berei is. XPS analise van die kataliste het getoon dat die PdO en MO (metaaloksied) verhouding ongeveer 1:1 en ook 1:1.5 is.