Oxidative addition and co insertion of rhodium cupferrate complexes containing arsine ligands
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Date
2008-11
Authors
Kahsai, Fessahaye Tekeste
Journal Title
Journal ISSN
Volume Title
Publisher
University of the Free State
Abstract
Results obtained for the reaction between [Rh(cupf)(CO)(AsPh3)] and CH3I in a range
of solvents indicated that the second-order rate constants are comparable with its
phosphine analogue. A likely explanation given was that steric factors are probably
overshadowed by electronic ones. This suggestion is supported by literature findings
specifying the σ-donor order of group 15 ligands as PR3 > AsR3 > SbR3.
Rhodium complexes with phosphine ligands should thus have more electron density
than those with corresponding arsine ligands. The less steric crowding at the Rh
centre of the starting complex was proved by rate enhancement (relative to other
similar rhodium complexes) of oxidative addition of the bulky substrate, iodoethane,
to the complex. The oxidative addition kinetics showed insignificant intercepts for
the plots of kobs vs [CH3I] which implied a reaction thermodynamically favoured
towards the Rh(III)-alkyl complex product.
Electron density and steric constraints of the rhodium(I) species were manipulated in
the study by using different arsine ligands. [Rh(cupf.CH3)(CO)(AsPh3)] and
[Rh(cupf)(CO)(AsMePh2)] were thus prepared and analysed. Both complexes
enhanced the rate of oxidative addition, which is in agreement with the higher
electron density at the metal due to the more efficient electron donating property of
the methyl group compared to the phenyl group. The increased reactivity of
[Rh(cupf)(CO)(AsMePh2)] could also be ascribed to the smaller cone angle of
AsMePh2 compared to AsPh3. The rate of oxidative addition of iodoethane was
lower, which indicated that the first step of the reaction was a nucleophilic attack by
the metal centre (rhodium) on the α-carbon of the haloalkanes.
The first-order rate constant for CO-insertion was determined from IR-multi-scans in
acetonitrile and chloroform. Although UV/VIS spectra of the reaction showed
biphasic kinetics in acetonitrile and methanol, the second step after the initial
oxidative addition reaction was not only CO-insertion but, as revealed from IR-multiscanned
spectra, also isomerisation from a Rh(III)-acyl to alkyl species in the same
step. In other words, the UV/VIS spectra did not show clearly for which species the
specific rate was measured in the second part of the biphasic plots. However, IRmulti-
scanned spectra of the reaction at fixed [CH3I] for the oxidative addition step
(Rh(I)–CO peak disappearance) was possible since it was proved that the graph of kobs
vs [CH3I] was linear with insignificant intercepts. The IR-multi-scanned spectra
together with the UV/VIS rate data indicated that the fast equilibrium for the
formation of the alkyl complex was followed by a second slower CO-insertion
reaction. This was consistent with the proposed mechanism of the title reaction as
shown below.
See Scheme in full text.
The observed rate enhancement due to the increase in electron density of the starting
complex together with the activation parameters found (relatively large negative ΔS#)
and the type of IR-multi-scans spectra obtained are all indicative of an associative
mechanism. Furthermore, the activation of the second-order rate constants from
non-polar to highly polar solvents could be taken as an indication of a polar transition
state. Although activation parameters and IR multi-scanned spectra were performed
in different solvents, i.e. in acetone and chloroform, it would not be expected to affect
the proposed mechanism. On the basis of a combined solvent, temperature and
pressure dependence study conducted, activation parameters for the reaction between
CH3I and [Rh(cupf)(CO)(PPh3)] in acetone and chloroform were very close to those
obtained for the title reaction in this investigation.
In this study, the effect of organic halide substrates on the rate of the reaction by
changing the substrate from CH3I to CH3Br and C2H5I was also investigated. In
comparison to literature data, the rate of the reaction with C2H5I was faster, i.e. only a
40-fold deactivation was found. Both CH3Br and C2H5I retarded the reaction.
Possible explanations could be due to electronic factors (bond energy, BE, of CH3–Br
= 70 vs CH3–I = 56 kcal/mol). Since BE of CH3CH2–I is
53 kcal/mol, decreased reactivity observed could be interpreted in terms of the large
size of the ethyl group. With regard to the mode of addition, oxidative addition of
CH3I to [Rh(cupf)(CO)(AsPh3)] is expected to proceed by addition of iodide (I-) and
CH3 adjacently (cis to one another) like in its phosphine analogue.
Another aspect of this study was kinetic runs of the oxidative addition product,
[IRh(cupf)(CO)(CH3)(AsPh3)], performed in a range of solvents. Most kinetic studies
only employ Rh(I) or Ir(I) complexes as a starting material, but in this study the
Rh(III)-alkyl product was also utilized as a starting complex. In all solvents
employed, the product was depleted and gave rise to a small peak at ca. 1712 cm-1
corresponding to a Rh(III)-acyl species. The rate of acyl formation was thus
determined by the slow appearance of this peak.
Description
Keywords
Ligands, Complex compounds, Rhodium compounds -- Synthesis, Dissertation (M.Sc. (Chemistry))--University of the Free State, 2008