Transfer hydrogenation reactions catalyzed by diimine and salicylaldimine ruthenium complexes

Thesis (MSc)--Stellenbosch University, 2020.ENGLISH ABSTRACT: This thesis reports on the synthesis, characterization, and application of N^N diimine and N^O salicylaldimine ruthenium complexes as transfer hydrogenation catalysts. N^N imino-pyridyl complexes were cationic in nature while N^O salicylaldimato- complexes were isolated as neutral species. The cationic species were stabilized by non- coordinating counterions, BF4ˉ, PF6ˉ and BPh4ˉ. The effect of the nature of the anion as well as the type solvent are reported. The salicylaldimine complexes were found to be photochromic, changing colour when irradiated with visible light. No visible formation of metallic ruthenium is detected during this transformation indicating that the process is solely a chemical transformation rather than a decomposition event. The product of the chemical transformation was identified by both 1H NMR and mass spectrometry as a bis(salicylaldimine) complex of Ruthenium. NMR studies of the cationic N,N complexes revealed an interesting counterion effect in solvents of different dielectric constants. Ion-pairing was observed in low dielectric constant solvents while ion separation is prevalent in high dielectric constant solvents. This was also confirmed using conductivity measurements. The N^N diimine Ru system with the BPh4- counterion was used to establish the optimized catalytic parameters. A comparative study was done to determine the performance of the series of N^N diimine Ru systems as transfer hydrogenation catalysts. The cationic complexes, C1.1B—C1.1D were evaluated for their catalytic ability in transfer hydrogenation of acetophenone. Results seem to indicate that the nature of the counterion has an impact on the catalytic behaviour of these complexes. The BF4ˉ counterion is thought to be small enough to fit in the organo-ligand framework preventing access to the metal site. This lowers the activity of complex. The PF6ˉ ion is larger than the BF4ˉ ion and cannot fit into the organo-ligand framework thus enhancing catalytic performance. The BPh4ˉ is much larger and thus because of its steric bulk, as well as its intermolecular interaction with the ligand frame, preventing the substrate from approaching the active site, leading to a lowering of activity. As for series C1.2B—C1.2D the same conclusion can be made, however, C1.2B, had a higher conversion compared to C1.1B. This could be due the terminal phenyl ring on the 3-phenyl-propyl substituent, forcing the BF4ˉ counterion further away from the metal centre, therefore enhancing catalysis. The N^O ruthenium arene complexes appeared to perform poorly in the presence of a base. On the other hand, these complexes showed excellent activity in the absence of a base. This led to an alternative catalytic mechanism being proposed. Preliminary NMR investigations showed the presence of a Ru-H species, which is thought to be the active species in the absence of base. The alternative mechanism proposed is thought to be facilitated by the presence of a hemi-labile imine moiety which can dissociate easily. This leads to a coordinatively unsaturated metal complex which promotes the oxidative addition of isopropanol. This Ru-H species is only formed at 80 °C.AFRIKAANS OPSOMMING: Hierdie tesis raporteer die sintese, karakterisering en toepassing van N^N diimien en N^O salisielaldemien-rutenium komplekse as oordrag hidrogenerings katalisators. N^N imino-piridiel komplekse was kationies van aard, terwyl N^O salisielaldimato- komplekse as neutrale spesies geïsoleer is. Die kationiese spesies was deur nie-koördinerende teenione gestabiliseer, naamlik BF4ˉ, PF6ˉ en BPh4ˉ. Die effek van die aard van die anioon sowel as die tipe oplosmiddel word geraporteer. Daar is gevind dat die salisielaldimienkomplekse fotokromies is, wat van kleur verander wanneer dit met sigbare lig bestraal word. Geen sigbare vorming van metaal-rutenium is tydens hierdie transformasie opgemerk nie, wat aandui dat die proses slegs ʼn chemiese transformasie eerder as ʼn ontbindingsproses is. Die produk van die chemiese transformasie is deur beide 1H KMR en massaspektrometrie as ʼn bis(salisielaldimien) kompleks van rutenium identifiseer. KMR-studies van die kationiese N,N-komplekse het ʼn interessante teenioon effek onthul in oplosmiddels van verskillende diëlektriese konstantes. Ioon-pare is in oplosmiddels met lae diëlektriese konstantes waargeneem, terwyl ioonskeiding oorwegend in oplosmiddels met hoë diëlektriese konstantes voorkom. Dit was ook met behulp van konduktometriese meetings bevestig. Die N^N diimien Ru sisteme met die BPh4ˉ teenioon was gebruik om die optimale katalitiese parameters te bevestig. ʼn Vergelykbare studie is gedoen om die werkverrigting van die reeks N^N diimien Ru sisteme as oordrag hidrogenerings kataliste te bepaal. Die kationiese komplekse C1.1B-C1.1D was vir hul katalitiese vermoë in die oordrag hidrogenering van asetofenoon geëvalueer. Resultate dui blykbaar daarop aan dat die aard van die teenioon ʼn impak het op die katalitiese gedrag van hierdie komplekse. Die BF4ˉ teenioon is vermoedelik klein genoeg om in die organo-ligandraamwerk te pas en belemmer sodoende toegang van die substraat tot die metaal. Hierdie verlaag die aktiwiteit van die kompleks. Die PF6ˉ ioon is groter as die BF4ˉ ioon en kan nie binne die organo-ligandraamwerk pas nie, en verbeter dus die katalitiese verrigting van hierdie kompleks. Die BPh4ˉ is baie groter en dus danksy sy steriese massa, sowel as die intermolekulêre interaksie met die ligandraamwerk, word die substraat verhoed om die aktiewe terein te nader, wat lei tot ʼn afname in aktiwiteit. Dieselfde gevolgtrekkings is gemaak vir die reeks C1.2B-C1.2D, alhoewel C1.2B het ʼn hoër omskakeling getoon in vergelyking met C1.1B. Dit kan moontlik wees as gevolg van die terminale feniel ring op die 3-fenielpropiel substituent, wat die BF4ˉ teenioon verder wegdryf van die sentrale metaal en sodoende katalise bevorder. Dit blyk asof die N^O rutenium arenekomplekse swak presteer in die teenwoordigheid van ʼn basis. Aan die ander kant het hierdie komplekse uitstekende aktiwiteit getoon in die afwesigheid van ʼn basis. Dit het gelei na die voorstelling van ʼn alternatiewe katalitiese meganisme. Voorlopige KMR ondersoeke het aangedui op die teenwoordigheid van ʼn Ru-H spesies, wat vermoedelik die aktiewe spesie is in die afwesigheid van die basis. Die alternatiewe meganisme wat voorgestel is word vermoedelik bevorder deur die teenwoordigheid van ʼn hemi-labiele imien-eenheid wat maklik kan dissosieer. Dit lei tot ʼn onversadigde koördinerings metaalkompleks wat die oksidatiewe toevoeging van isopropanol bevorder. Hierdie Ru-H spesie word slegs by 80 °C gevorm.Master