Methanol carbonylation via platinum group metal complexes
| dc.contributor.advisor | Roodt, Andreas | |
| dc.contributor.author | Kotze, Philippus Daniel Riekert | |
| dc.date.accessioned | 2017-07-11T05:49:28Z | |
| dc.date.available | 2017-07-11T05:49:28Z | |
| dc.date.issued | 2010-11 | |
| dc.description.abstract | The aim of this study, firstly, involved the synthesis of a range of S,O-functionalized thiourea ligands with systematically changing electro-steric properties and investigate these ligands coordination modes to rhodium complexes in an attempt to primarily synthesize a range of [Rh(S,O-thioureato)(CO)2] and [Rh(S,O-thioureato)(CO)(PR1R2R3)] complexes. Moreover, the aim also included the synthesis of [Rh(diphosphine)(CO)2]+ complexes using a range of diphosphine ligands. These complexes were then to be used to synthesize the corresponding Rh(III)-acyl complexes via iodomethane oxidative addition and study the carbonylation/hydrogenation of methanol to ethanol by investigating the kinetic and activation parameters of the iodomethane oxidative addition as well as reductive elimination/hydrogenation of acyl iodide/acyl species. Several S,O-functionalized thiourea ligands were successfully synthesized and characterized from which the X-ray crystallographic structures for several of the ligand systems are reported: N-benzoyl-N' -(2,4,6-trimethylphenyl)thiourea (Triclinic P 1, R1 = 5.60 %), N-benzoyl-N' -(2,6-dibromo- 4-fluorophenyl)thiourea (Triclinic P 1, R1 = 3.76 %), N-benzoyl-N' - (pentafluorophenyl)thiourea (Monoclinic C2/c, R1 = 3.69 %), N-benzoyl-N -(phenethyl)thiourea (Monoclinic P21/n, R1 = 3.91 %), N-benzoyl-N' -(naphthalene-1-ylmethyl)thiourea (Monoclinic C2/c, R1 = 5.37 %), N-benzoyl-N '-(cyclohexyl)thiourea (Triclinic P , R1 = 2.10 %) and Nbenzoyl- N' -(isopentyl)thiourea (Triclinic P , R1 = 5.06 %). It was established that these ligands exhibit a keto conformation in the solid state, where the carbonyl oxygen is trans to the sulphur atom and is stabilized by a hydrogen bond interaction with the terminal nitrogen atom of the thiourea moiety. The keto conformation was also confirmed in solution by NMR spectroscopy. Furthermore, hydrogen bond interactions exist between neighbouring molecules in the solid state, which leads to either dimer or polymer formation in the crystal packing of these thiourea compounds. The [Rh(diphosphine)(CO)2]+ complexes could not be successfully synthesized, however, during several attempts one synthetic route led to the formation of a cationic A-frame complex of the type [Rh2(µ -Cl)(diphosphine)2(CO)2]BF4. The X-ray crystallographic structure of [Rh2( µ-Cl)(dppm)2(CO)2]BF4 (Monoclinic P21/n, R1 = 8.84 %) is reported. Several [Rh(S,O-thioureato)(CO)2] complexes were synthesized and characterized, however, these complexes were unstable outside of solution. Several attempts were made to synthesize [Rh(S,O-thioureato)(CO)(PPh3)] complexes, which led to the isolation of rhodium thiourea complexes where the thiourea ligands exhibit S,O-, S- and N,S-coordination modes. The X-ray crystallographic structures of the following complexes are reported: [Rh(N,S-(N- 4h2mPT))(CO)(PPh3)2] (Triclinic P , R1 = 2.75 %), [Rh(N,S-(N-PT))(S,O-(N-PT))(PPh3)2] (Triclinic P , R1 = 4.44 %), [Rh(COD)(Cl)(S-(N-PTH))] (Triclinic P , R1 = 3.18 %), [Rh(COD)(Cl)(S-(N-tmPTH))] (Monoclinic C2/c, R1 = 6.74 %). [Rh(N,S-(N- 4h2mPT))(CO)(PPh3)2] is analogous to typical Vaska-type complexes, where the coordinated thiourea ligand is trans to the carbonyl ligand and the two PPh3 are trans to each other on the rhodium centre. [Rh(N,S-(N-PT))(S,O-(N-PT))(PPh3)2] is a Rh(III) species with an octahedral arrangement around the rhodium centre, where one of thiourea ligands coordinated in its enol conformation. In both [Rh(COD)(Cl)(S-(N-PTH))] and [Rh(COD)(Cl)(S-(N-tmPTH))] the preferred orientation of the free ligands translated to the orientation of the coordinated ligands. These complexes were also stabilized by hydrogen bond interactions between the chlorido ligand and the internal nitrogen atom of the thiourea moiety. A range of [Rh(S,O-(N-diPT))(CO)(PR1R2R3)] complexes were successfully synthesized using N-benzoyl-N' ,N' -(diphenyl)thiourea and a range of phosphine ligands with systematically changing electro-steric properties (PPh3, PPh2Cy, PPhCy2, PCy3). The X-ray crystallographic structures of the following complexes are reported: [Rh(S,O-(N-diPT))(CO)(PPh3)] (Monoclinic P21/c, R1 = 6.86 %), [Rh(S,O-(N-diPT))(CO)(PPh2Cy)] (Monoclinic P21/c, R1 = 6.32 %), [Rh(S,O-(N-diPT))(CO)(PCy3)] (Monoclinic P21/c, R1 = 6.86 %). The respective first order coupling constants (1JRh-P) and the carbonyl stretching frequencies ( VCO) were obtained, from which the expected order of electronic effects of the phosphine ligands was established. The effective cone angles ( E) for the different phosphine ligands were also calculated, which correlated well with the expected steric congestion of the ligands on the rhodium centre. The reactivity of the [Rh(S,O-(N-diPT))(CO)(PR1R2R3)] complexes towards the iodomethane oxidative addition was investigated. In general the reaction rate of the individual reactions increased in the order of [Rh(N-diPT)(CO)(PPhCy2)] < [Rh(N-diPT)(CO)(PCy3)] < [Rh(N-diPT)(CO)(PPh3)] < [Rh(N-diPT)(CO)(PPh2Cy)]. This order of reactivity was ascribed to a combinative effect of both the steric and electronic properties of the phosphine ligands. The activation parameters calculated for the individual reactions were found to be similar. The proposed mechanism for the iodomethane oxidative addition to complexes of the type [Rh(S,O-thioureato)(CO)(L)], where L = CO/PR1R2R3, is depicted in Scheme I. (Find scheme 1 on full text) The electro-steric effects of phosphine ligands in catalytic processes were further investigated by studying these effects in the phosphine exchange reactions of Vaska-type complexes [Rh(Cl)(CO)(PR1R2R3)2] with the corresponding PR1R2R3 ligand via NMR techniques. The reaction rate for the exchange reaction was almost two orders of magnitude faster for PPh3 than for PPh2Cy. Both exchange processes exhibited a large negative ∆ S≠ and a small ∆H≠ , which suggested an associative activation, where a stable 5-coordinated transition state is formed. | en_ZA |
| dc.description.sponsorship | SASOL | en_ZA |
| dc.identifier.uri | http://hdl.handle.net/11660/6456 | |
| dc.language.iso | en | en_ZA |
| dc.publisher | University of the Free State | en_ZA |
| dc.rights.holder | University of the Free State | en_ZA |
| dc.subject | Rhodium | en_ZA |
| dc.subject | S,O-thiourea | en_ZA |
| dc.subject | Phosphine | en_ZA |
| dc.subject | Oxidative addition | en_ZA |
| dc.subject | Exchange reaction | en_ZA |
| dc.subject | Homologation | en_ZA |
| dc.subject | Carbonylation | en_ZA |
| dc.subject | Platinum group | en_ZA |
| dc.subject | Thesis (Ph.D. (Chemistry))--University of the Free State, 2010 | en_ZA |
| dc.title | Methanol carbonylation via platinum group metal complexes | en_ZA |
| dc.type | Thesis | en_ZA |