Hello there, everyone.
When I compare the melting points of different lanthanides - Schiff base complexes, I discover that they are all lower than the free Schiff base? Is it because of the large effective nuclear charge of Ln+3 that I can show this?
Hello Abdelaziz,
is this a literature comparison, or are you talking about preparations you made yourself? If this is the case, how well can you guarantee the purity of the samples? It could simply be that impurities (e.g. residues of the free Schiff base) lower the melting point to such an extent that the mixture melts at lower temperatures than the reactants on their own.
That would be a first question from my side, without being familiar with your complexes.
yes, it was synthesized in my lab. and I check the purity by TLC and the sharpness of melting point in addition to recrystallization to bring x-ray for them (I have the x-ray single crystal of three of them)
Hello Abdelaziz,
this is a very interesting technical question. I absolutely agree with Florian in that even minor impurities such as small amounts of unreacted Schiff base ligand could lead to significantly decreased melting points. Even a successful X-ray diffraction study does not say much about the purity of the bulk material. Are ypu sure that the Schiff base complexes can be deteced on the TLC plate? Quite often lanthanide complexes decompose on silica and you will then see only the free ligand. Did you check if the complexes contain solvent of crystallization? This could also lead to lower melting points. The higher melting point of the ligand could e.g. result from a more densely packed crystal structure. In any case, I would strongly suggest not to over-interprete the melting point data.
Good luck with your research and best wishes, Frank Edelmann