Ruthenium(II) complexes of amino-substituted polypyridine ligands: : synthesis and photophysical studies with a potential application for molecular switches

In this PhD thesis, numerous synthetic strategies towards new ruthenium(II) complexes of polypyridine ligands with a long amino-substituted side chain are reported. The first approach involves the complexation of methyl 4'-(pyridin-4-yl)-[2,2':6',2''-terpyridine]-4-carboxylate (L2), followed by trans-esterification with a long side chain. However this reaction gives the desired complexes in very poor yields, along with the complex with a free carboxylic acid – a product of the side chain cleavage.
The next attempted route to obtain Ru(II) complexes of pytpy ligands with an amino side-chain appended the side chain to the ligand first and subsequently formed the complex. Several ways to synthetize pytpy ligands with the long amino side-chain linked via an ester L10-14 were tried. Hydrolysis of L2 to free carboxylic acid L8, followed by esterification via the corresponding acid chloride was successful whereas direct Steglich esterification of carboxylic acid L8 did not work due to the insolubility of L8. Further complexation of ligands L10-14 was the more successful alternative to the original idea of complex synthesis by the trans-esterification of C7. Heteroleptic and homoleptic Ru(II) complexes with pytpy ligands with an amino-substituted side chain (diethylamine, piperidine and morpholine), linked via an ester or an ether group were successfully synthesized. In order to have model compounds for use in the photophysical studies, Ru(II) complexes with 4'-phenyl-2,2':6',2''-terpyridine ligands (Phtpy) substituted in 4-possition with the same side chains were also prepared. Pytpy ligands with short (3 and 5 carbon atoms) amino-substituted side chains linked via an ester group or an ether group were synthesized and characterised. Numerous attempts to synthesize Ru(II) complexes of these ligands failed and led only to trace amounts of the desired products or to decomposition (i.e., side chain cleavage). Several reactions under mild conditions were attempted. One worked well for the synthesis of the model complex C17, but similar reaction conditions failed when applied to complexation of the ligands with short side chains. These results point to the lability of the short side chain during exposure to acidic or basic conditions.
Detailed photophysical of the new Ru(II) complexes were carried out in collaboration with the photochemistry research group of Prof. Alberto Credi from the University of Bologna in Italy. Ru(II) complexes were analysed in terms of their photophysical properties and compared in groups according to the pendant substituents of the tpy unit, the linker group of the side chain and also considering substituents on the side chains. The complexes with the ester linker exhibit similar behaviour in the absorption and emission spectra as reported for the model complex C8. Complexes with the ether linker are very weak emitters. However they exhibit a continuous red shift in their emission spectra during titration with acid, in contrast to complexes with ester linkers or C8. Complexes of Phtpy ligands were also found to be very weak emitters, which corresponds with known Ru(II) complexes of tpy ligands. Homoleptic Ru(II) complexes of ester-substituted ligands exhibit are the strongest emitters of all the analysed complexes.
Unfortunately, the actual effect of the amino-substituent on the side chain was not investigated, due to the fact that such complexes were isolated already in their side-chain protonated form. Unfortunately, this fact was discovered nearly at the end of this work, after all the photophysical studies were carried out. Therefore, there was not enough time to develop a sufficient methodology to convert the mono-protonated Ru(II) complexes to the de-protonated form and examine the complexes in terms of the originally suggested full-adder.
NMR studies of Ru(II) complexes and of their free ligands under acidic and basic conditions were carried out and the processes monitored by 1H NMR spectroscopy. Protonation possibilities of the nitrogen atoms were investigated and the stability of the side amino chains under these acidic conditions was examined. It was observed that the side chains stay attached to the Ru(II) complex. The results of these NMR studies reveal and confirm that all Ru(II) complexes with three protonation sites were synthesized and isolated in the mono-protonated form - on the amino group of the side chains (which most likely occurred during the work-up with NH4PF6). Therefore, the only protonation happening during the NMR titrations of the Ru(II) complexes was on the pendant pyridyl units. The ligands were titrated with TFA-d to monitor the side chain protonation and prove that this amino unit is protonated first. From these processes we calculated pK values which points at basicity of the side chain amines.
In this PhD thesis, 25 new substituted pytpy or Phtpy ligands and 30 new Ru(II) complexes of these ligands were synthesized and characterised.