C3-symmetric ligands carrying a rigid triaminoguanidinium backbone are important building blocks for the preparation of supramolecular coordination cages as tetrahedra or trigonal bipyramides. Coordination of Eu(III)- or Gd(III)-ions leads to 1,2,4-triazole formation, which has been reported only rarely. Using Pd(II)-complexes as a model system, this triazole formation could be analyzed in more detail. The preparation of Pd(II)-coordination compounds can be easily done under stoichiometric control. These complexes could be transformed into 1,2,4-triazoles using O2 or H2O2 as an oxidation reagent. The steric demand of the PR3-coligand seems to play a key role in the cyclisation reaction.
Tris-(2-hydroxybenzylidene)triaminoguanidinium salts having six alkyl chains with proper spacing served as new molecular building blocks for the formation of porous honeycomb networks by van der Waals interaction between interdigitated alkyl chains at the liquid/graphite interfaces.
A three-part experiment that leads
to the synthesis of palladium(II)
complex starting from a C
3-symmetric triaminoguanidinium-based
ligand is presented. In the first part, the preparation of tris-benzylidenetriaminoguanidinium
chloride ([H6
Br
3
L]Cl) by an acidic catalyzed 3-fold imine
formation reaction of 5-bromo-2-hydroxybenzaldehyde and triaminoguanidinium
chloride is described. Starting from the second part, the reaction
procedures are performed under inert gas atmosphere. The conversion
of PdCl2 with acetonitrile to give [Pd(MeCN)4]Cl2 as a precursor is performed at 80 °C. The third
part of the experiment is a three-step procedure that begins with
deprotonation of [H6
Br
3
L]Cl, followed by transfer of [Pd(MeCN)4]Cl2 to the reaction flask, where the chelation of Pd(II)-precursor
leads to the in situ species [Pd(MeCN)H3
Br
3
L]. Addition of PPh3 as the
final reaction step yields [Pd(PPh3)H3
Br
3
L]. This sequence of experiments
provides an excellent example of ligand and transition metal preparation
and its subsequent complexation to form an oxidant-sensitive organometallic
species. Undergraduate students get the opportunity to learn Schlenk
techniques and to investigate 1H NMR spectra of organic
and organometallic compounds starting from relatively simple to more
advanced spectra. The distinct result of the chemical shift of the 4
J
P–H-affected doublet mirrors
the preparative success of the student and demonstrates the utility
of 1H NMR experiments.
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