Spontaneous metathesis of bis-chelated PdII(L-S,O)2 complexes in solution : a rp-HPLC study
Date
2008-03
Authors
Van der Molen, Lynndal
Journal Title
Journal ISSN
Volume Title
Publisher
Stellenbosch : Stellenbosch University
Abstract
N,N-dialkyl-N-acyl(aroyl)thiourea ligands form stable chelated complexes in a cis
configuration with the platinum group metals. Such chelated complexes are generally
considered substitutionally “inert” in solution, however, it was found that cis-bis(N,Ndialkyl-
N-acyl(aroyl)thioureato)M(II) complexes (M = Ni(II), Pd(II) or Pt(II)) readily
undergo facile chelate metathesis reactions in solution at room temperature. Upon
mixing two different parent complexes, a mixed-ligand product formed in solution, with
an equilibrium, or steady state, between the two parent complexes and the mixed-ligand
product being attained after a period of time: M(LA)2 + M(LB)2 M(LA)(LB). All three
complexes remained in solution even with a ten-fold excess of one parent complex.
The presence of the mixed-ligand products in solution was confirmed by
liquid chromatography-mass spectrometry (LC-MS), nuclear magnetic resonance
(NMR) spectra and a crystal structure of the mixed-ligand complex
cis-Pd(L3-S,O)(L4-S,O). Though a number of attempts were made, it was not possible
to either isolate or synthesise the mixed-ligand complexes exclusively.
The equilibrium distribution and the rate of the metathesis reaction were influenced by a
number of factors, including the central metal ion, the substituents on the complexed
ligands and the reaction medium. In addition to these, a number of other factors, some
unexpected, also played a role in the rate of the reaction. Initial concentration of the
parent complexes, the age of the solutions upon mixing and the presence of impurities
or additives all contributed to the overall rate of reaction. The results from these rate
studies highlighted the necessity for extensively purified compounds.
In addition to chelate metathesis reactions, the exchange between a cis-Pd(L-S,O)2
complex and an unbound HL ligand in solution was also investigated. Again, even with
an excess of unbound ligand, all three possible complexes were present in solution.
It has been shown previously that these complexes undergo a photoinduced cis-trans
isomerisation under intense light, and it has been proposed that the reverse trans-cis
process, which occurs in the dark, may be a metathesis reaction. In light of this, the
relationship between these chelate metathesis reactions and the reverse trans-cis
reaction was briefly investigated.
Though the metathesis reactions were a general phenomenon in the Ni(II), Pd(II) and
Pt(II) complexes of the aforementioned ligands, the experiments focused mainly on the
cis-Pd(L-S,O)2 complexes due to the favourable timescales of their metathesis
reactions. The primary technique to observe these reactions was reversed-phase
high-performance liquid chromatography (rp-HPLC). The timescales involved in the
cis-Pd(L-S,O)2 metathesis reactions as well as the stability of the Pd(II) complexes
under the HPLC conditions made this technique ideal.
Description
Thesis (MSc (Chemistry and Polymer Science))--Stellenbosch University, 2008.
Keywords
Chelate metathesis, Palladium(II), High-performance liquid chromatography, Platinum(II), Nickel(II) chelates, Dissertations -- Chemistry, Theses -- Chemistry