Hydroformylation of alkenes using "heterogenized" complexes of rhodium

Joubert, Cornelia Maria (2018-12)

Thesis (PhD)--Stellenbosch University, 2018.

Thesis

ENGLISH ABSTRACT: This thesis describes the synthesis of a range of siloxane functionalized rhodium complexes with chelating ligands. The complexes were immobilized onto mesoporous silica, thereby producing novel MCM-41 and SBA-15 immobilized rhodium catalysts that could be applied in the hydroformylation of alkenes. Neutral and cationic model and siloxane functionalized complexes containing the Schiff base ligand scaffolds N-(2-diphenylphosphino)benzylidene-alkylamine, N-(2-pyridinylmethylene)-1-alkylimine and N-alkylsalicylaldimine (where alkyl equals n-propyl or 3-(triethoxysilyl)propyl-) were prepared by reacting the appropriate ligand with the rhodium precursors [RhCl(CO)2]2 or [RhCl(COD)]2. These ligands differed with regard to the chelating atoms utilized. Ligands with N,P, N,N and N,O chelating units were synthesized. Analogous cationic complexes were prepared using 2-(1-alkyl-1H-1,2,3-triazol-4-yl)pyridine, where alkyl equals octyl or 3-(triethoxysilyl)propyl as chelating ligands. The ligands were prepared by using copper-assisted azide-alkyne Huisgen cycloaddition. The complexes were characterized by Fourier transform infrared spectroscopy, nuclear magnetic resonance (NMR) (1H, 13C and 31P) spectroscopy, mass spectrometry analysis and elemental analysis. During characterization of the complexes by 1H NMR, two unusual phenomena were observed which were further investigated using advanced NMR techniques. Firstly, an unusually large shift of the imine signal in the 1H NMR spectrum of the cationic N,N Schiff base complex was shown to be due to close ion-pairing of the complex with the tetraphenylborate counter-ion in chloroform. The ion-pairing effect as well as the influence of the solvent was investigated using nuclear Overhauser spectroscopy, heteronuclear Overhauser spectroscopy, and diffusion ordered spectroscopy. Secondly, a chemical exchange process involving the olefinic protons of the 1,5-cyclooctadiene ligand on the cationic N,N Schiff base complex and the pyridyl-triazole complex was investigated using variable temperature NMR as well as some computational methods. The dynamic behaviour was tentatively attributed to the breaking a metal-ligand bond and subsequent rotation of the ligand around the remaining bond. The immobilized complexes were characterized by a range of solid-state techniques namely Fourier transform infrared spectroscopy, nitrogen adsorption/desorption surface analysis, powder X-ray diffraction, scanning and transmission electron microscopy, inductively coupled plasma optical emission spectroscopy (ICP-OES), and solid-state 29Si NMR. These analyses gave insight into the chemical and physical properties of the immobilized catalysts. The metal loading of the immobilized systems were determined using ICP-OES, and this allowed for the rhodium loadings during catalysis being kept constant. In this way was therefore possible to compare the model and immobilized systems directly. The model complexes and their immobilized counterparts were applied as catalyst precursors in the hydroformylation of 1-octene. The conversion and selectivity of the reaction were found to be influenced by reaction parameters such as temperature and pressure, with an increase in pressure leading to an increase in both conversion and aldehyde selectivity. In general it was observed that the model catalysts gave moderate conversions, with the pyridyl-triazole complex converting 85% of the starting material after 2 hours. The cationic complexes were generally more active than the neutral complexes, likely due to the metal centre on the cationic complexes being more electron deficient, leading to faster alkene coordination. Initially, a high selectivity towards internal octenes was obtained in these reactions. This is due to the rapid rate of isomerization mediated by these catalysts. As the reactions progressed, the internal octenes were hydroformylated to give branched aldehydes, leading to an increase in aldehyde selectivity with a concomitant decrease in aldehyde regioselectivity. The highest regioselectivity obtained was a ratio of nonanal to 2-methyloctanal of 3 to 1, using the neutral Schiff base chlorocarbonyl complex. The complexes were therefore not very regioselective. The immobilized catalysts gave higher activity than their model counterparts in all cases, with only slight differences being observed between the immobilized MCM-41 and SBA-15 systems. The recyclability of the systems were however poor, with dramatic losses in turnover number being observed upon the re-use of the catalysts. The filtrate obtained after recovering the solid catalysts was analyzed by ICP-OES and it was found that the rhodium leaching was quite significant, which explained the loss of activity. From some preliminary mechanistic studies it was found that the replacement of the 1,5-cyclooctadiene ligand on the cationic N,N Schiff base complex by two carbonyl ligands occurred within seconds at room temperature and atmospheric pressure. Attempts to observe the rhodium hydride (a known step in the hydroformylation reaction mechanism) using high- pressure 1H NMR was unsuccessful, leading to the conclusion that the rhodium hydride species was either present only in low abundance or that it had a relatively short life-time. When the complex was heated in a high-pressure NMR tube in the presence of 30 bar syngas and 1-octene, the disappearance of the terminal alkene proton signals and the formation of new signals in the aldehyde region could be observed in the 1H NMR spectrum over a time period of two hours, confirming the formation of major hydroformylation products.

AFRIKAANSE OPSOMMING: Hierdie tesis beskryf die sintese van ‘n reeks siloksaan-gefunksioneerde rhodiumkomplekse met chelerende ligande. Die komplekse is op mesoporieuse silika geïmmobiliseer en op so ‘n manier is nuwe MCM-41 en SBA-15 geïmmobiliseerde katalisatore wat in die hidroformilering van alkene toegepas kon word geproduseer. Neutrale en kationiese model en siloksaan-gefunksioneerde komplekse gebasseer op die Schiff basis ligand raamweke N-(2-difenielfosfien)bensielidien-alkielamien, N-(2- pyridienmetileen)-1-alkielamine en N-alkielsalisielaldimien (waar alkiel wys na n-propiel of 3- (triëtoksiesiliel)propiel-) is berei deur die gepaste ligand met òf [RhCl(CO)2]2 òf [RhCl(COD)]2 te reageer. Die ligande verskil ten opsigte van die chelaatatome wat in die struktuur teenwoordig is. Ligande met N,P, N,N en N,O chelaatsisteme is gesintetiseer. Kationiese komplekse is berei deur 2-(1-alkiel-1H-1,2,3-triasool-4-iel)piridien, waar alkiel wys na oktiel of 3-(triëtoksiesiliel)propiel, as chelaat-ligande te gebruik. Die ligande is berei deur koper- aangedrewe asied-alkyn Huisgen siklo-addisie. Die komplekse is gekarakteriseer deur van FT-IR spektroskopie, KMR (1H, 13C en 31P) spektroskopie, ESI-MS en mikroanalise gebruik te maak. Tydens die karakterisering van die komplekse deur 1H KMR is daar twee onverwagse verskynsels, naamlik ‘n onverwags groot verskuiwing van die imien-sein in die 1H KMR spektrum van die kationiese N,N Schiff basis komples asook ‘n opmerklike verskil tussen die vorms van die 1,5-siklo-oktadiëen olefien protone in die kationiese Schiff basis N,N kompleks en die piridieltriasool kompleks se sein in die 1H KMR, opgelet. Gevorderde KMR tegnieke is gebruik om die bogenoemde te ondersoek. Daar is gewys dat eersgenoemde te danke was aan intieme ioonparing tussen die katioon en die tetrafenielboraat teenioon in chloroform. Beide die ioonparingseffek en die invloed van die oplosmiddel is deur NOESY, HOESY en DOSY eksperimente ondersoek. Die tweede verskynsel was ‘n chemiese uitruilingsproses, waarby die olefien-protone van die 1,5-siklo- oktadiëen ligand in die kationiese Schiff basis N,N kompleks en die piridiel-triasoolkompleks betrokke was. Hierdie is deur veranderlike temperatuur KMR en berekeningsmetodes ondersoek. Die dinamiese gedrag is aan die splitsing van ‘n ligand-metaal binding en daaropvolgende rotasie van die ligand om die oorblywende binding toegeskryf. Die geïmmobiliseerde komplekse is deur ‘n reeks vaste fase tegnieke naamlik FT-IR, stikstof adsorbsie/desorpsie oppervlakanalise, poeier X-straal diffraksie, skandeer- en transmissie- elektronmikroskopie, IKP-OES en vastetoestand 29Si KMR gekarateriseer. Hierdie analises het meer insig oor die chemiese en fisiese eienskappe van die geïmmobiliseerde katalisatore verskaf. Die hoeveelheid metaal in die geïmmobiliseerde sisteme is deur ICP-OES bepaal, en sodoende was dit moontlik om die rhodiumladings tydens katalitiese reaksies konstant te hou. Op hierdie manier was dit dus moontlik om die model en geïmmobiliseerde sisteme direk (by dieselfde metaalkonsentrasie) te vergelyk. Die modelkomplekse en hulle geïmmobiliseerde ewekniëe is as katalisator-voorlopers in die hidroformilering van 1-okteen gebruik. Daar is gevind dat die omsetting en selektiwiteit van die reaksie beïnvloed kon word deur reaksieparameters soos temperatuur en druk. ‘n Verhoging in druk lei byvoorbeeld tot ‘n hoër omsetting en aldehied-selektiwiteit. Oor die algemeen is daar gesien dat die modelkatalisatore matige omsetting gee, met die piridiel- triasoolkompleks wat 85% van die 1-okteen na 2 ure na produkte omgeskakel het. Die kationiese komplekse was oor die algemeen meer aktief as die neutral komplekse, waarskynlik as gevolg van die feit dat die metaal in die kationiese komplekse minder elektronryk is en dat die koördinering van die alkeen dus vinniger kan plaasvind. Aanvanklik is daar meer interne oktene gevorm in hierdie reaksies. Met verloop van tyd word die interne oktene gehidroformileer na vertakte aldehiede, wat gelei het tot ‘n hoër aldehied-selektiwiteit en te gelyketyd tot ‘n afname in regioselektiwiteit. Die hoogste regioselektiwiteit wat verkry is (met die gebruik van die neutrale chloorkarboniel Schiff basis kompleks), was ‘n verhouding van nonanal tot 2-metieloktanaal van 3 tot 1. Die komplekse toon dus nie hoë regioselektiwiteit nie. Die geïmmobiliseerde katalisatore het hoër aktiwiteit as hulle model-ewekniëe getoon, alhoewel daar slegs klein verskille tussen die geïmmobiliseerde MCM-41 en SBA-15 sisteme. Die herwinbaarheid van die sisteme was egter teleurstellend in dat groot afnames in die omsettingsgetal gesien is met die hergebruik van die katalisatore. Na die vastetoestand katalisatore herwin is, is die filtraat deur IKP-OES geanaliseer en daar is gevind dat ‘n beduidende hoeveelheid van die rhodium uitgeloog is. Die afname in aktiwiteit kan dus hieraan toegeskryf word. Vanaf voorlopige meganistiese studies is daar gesien dat die verplasing van die 1,5-siklo- oktadiëenligand op die kationiese Schiff basis N,N kompleks met twee karbonielligande baie maklik was en dat dit binne sekondes by kamertemperatuur en atmosferiese druk gebeur. Pogings om die rhodiumhidried (‘n bekende stap in die hidroformuleringsmeganisme) met hoë-druk KMR waar te neem was onsuksesvol, wat tot die gevolgtrekking gelei het dat die hidried spesie òf in lae konsentrasies teenwoordig was, òf dat die spesie ‘n relatiewe kort leeftyd het. Wanneer die kompleks die kationiese Schiff basis N,N kompleks in ‘n hoë-druk KMR buis in die teenwoordigheid van 30 bar sintese-gas en 1-okteen verhit is, kon die verdwyning van die terminale alkeen se protonseine, asook die vorming van nuwe seine in die aldehied-omgewing in die 1H KMR spektrum oor ‘n tydperk van twee ure opgemerk word, wat die vorming van hidroformuleringsprodukte bevestig het.

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