Methanol carbonylation via platinum group metal complexes
Loading...
Date
2010-11
Authors
Kotze, Philippus Daniel Riekert
Journal Title
Journal ISSN
Volume Title
Publisher
University of the Free State
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.
Description
Keywords
Rhodium, S,O-thiourea, Phosphine, Oxidative addition, Exchange reaction, Homologation, Carbonylation, Platinum group, Thesis (Ph.D. (Chemistry))--University of the Free State, 2010