A series of Fischer mono- and biscarbene complexes of the type [MLn{C=(XR)R'}] was
synthesized and characterized. The redox behavior of the complexes was studied by different
techniques, including cyclic voltammetry, spectroelectrochemistry, ESR and computational
methods. Different transition metals (M) and carbene substituents (XR, R') were employed to
compare both the effect of the central metal atom as well as the carbene substituent.
Thienyl, furyl and ferrocenyl chromium(0) mono- and biscarbene complexes with ethoxy and
amino substituents were electrochemically studied in CH2Cl2. Results were mutually
consistent with computational data showing that the carbene double bond of all complexes is
reduced pseudo reversibly to an anion radical, -Cr-C•. The Cr centers are oxidized in two
successive one electron transfer steps to Cr(II) via the Cr(I) intermediate. For all ferrocenyl
carbene complexes the Fe(II) is oxidized after the first oxidation of Cr. It was found that with
respect to the aryl substituents the donating effect decreases from Fc>Fu>Th. Stabilization
from the XR substituent, where XR = NHR, also resulted in lower redox potentials compared
to their OEt analogues.
The inclusion of ferrocene in the carbene substituent was done, as its redox activity and
increased donating effect are well known. Mono- and biscarbene complexes with ethoxy and
amino substituents of both chromium and tungsten were electrochemically studied. Again
experimental data were supported by computational studies. Similar to the ethoxy chromium
complexes, reduction of the W=C fragment to -W-C• was observed. However oxidation of the
Fc group occurred first before the electrochemically irreversible oxidation process for W(0) involving a three electron-mediated process as seen in chronocoulometric analyses. The
tungsten oxidation was restricted to a W0/II, consistent with computational studies, by the use
of the electrolyte [NnBu4][B(C6F5)4]. The short-lived W(II) species were calculated to be
stabilized by agostic CH···W interactions, similar to the chromium analogues. To extend
linkers between the metal-carbene termini and investigate metal-metal interaction,
biferrocenyl and 2,5-thienylbiferrocenyl tungsten(0) mono-and biscarbene complexes were
synthesized and studied by spectroelectrochemistry. A metal-metal charge transfer transition between the tungsten carbonyl increment and the biferrocenyl / 2,5-thienylbiferrocenyl unit
was confirmed by infrared spectroelectrochemical studies. The electronic interaction in the
corresponding cationic species can be described as weakly coupled class II systems according
to Robin and Day.
The cymantrenyl moiety, Mn(η5-C5H4)(CO)3, provides an interesting alternative to ferrocene
as an organometallic molecular tag, however the instability of the radical cation impairs its
use. Improved stability of the cation, monitored by electrochemical measurements, was
accomplished by substituting a carbonyl with a ferrocenyl Fischer carbene ligand resulting in
Mn(I) oxidation occuring at lower potentials than ferrocenyl oxidation. These uncommon
Fischer carbene complexes is the first organometallic multi-tags reported. The mono cationic
species, [CpMn(CO)2{=C(OEt)Fc}][PF6], could be isolated and characterized with ESR
analysis. Finally, transmetallation from the tungsten(0) Fischer carbene complexes yielded examples of
rare acyclic alkoxy- and aminocarbene complexes of gold(I). Single x-ray structures for all
complexes could be obtained including the novel ferrocenophane dinuclear biscarbene Au(I)
complex. All structures display unsupported aurophilic interactions, while the bridging
biscarbene shows a semi-supported Au-Au interaction. In the case of the furanyl/thienyl
methoxy monocarbene complexes, extended Au-Au interactions result in oligomeric
structures. Although this study is of a fundamental nature, it is imperative for the
understanding and design of gold compounds with specific applications. / Thesis (PhD)--University of Pretoria, 2013. / gm2014 / Chemistry / unrestricted
| Identifer | oai:union.ndltd.org:netd.ac.za/oai:union.ndltd.org:up/oai:repository.up.ac.za:2263/40242 |
| Date | January 2013 |
| Creators | Van der Westhuizen, Belinda |
| Contributors | Bezuidenhout, Daniela Ina |
| Publisher | University of Pretoria |
| Source Sets | South African National ETD Portal |
| Language | English |
| Detected Language | English |
| Type | Thesis |
| Rights | © 2013 University of Pretoria. All rights reserved. The copyright in this work vests in the University of Pretoria. No part of this work may be reproduced or transmitted in any form or by any means, without the prior written permission of the University of Pretoria. |
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