Peculiarities of the reactions between early lanthanide(III) ions and an anionic porphyrin

Abstract

Insertion of early lanthanide(III) ions into the coordination cavity of porphyrins is a slow and complicated process in aqueous solution, originating from the stability of their aqua complexes and their possible oligomerization. The presence of potential axial ligands can accelerate the coordination of the first porphyrin, but it can hinder the connection of a further porphyrin. Lanthanide ions may coordinate not only to the pyrrolic nitrogens of porphyrins but, under kinetic control, also to the peripheral substituents possessing O-donor atoms. In the case of such anionic porphyrins, the coordination position of metal ions can be influenced by the change of temperature. Keywords: simultaneous metal and ligand control, out-of-plane metalloporphyrins, early lanthanide(III) ions, axial enhancement or hindrance, tail-to-tail oligomerization.

Peculiar photoinduced properties of water-soluble, early lanthanide(III) porphyrins

“…This kinetic barrier is vanquishable, e.g. by heating: in our case to ~60 o C [22]. The presence of bidentate, O-donor acetate ion enhances the coordination of the first porphyrin ligand, due to its trans effect, but it hinders the connection of a further porphyrin, i.e.…”

“…In this work, we studied the photophysical and primary photochemical properties of the complexes between a water-soluble, anionic porphyrin, the 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin anion (H2TSPP 4-, abbreviated as H2P 4-) and early lanthanide(III) ions (Ln=La,Ce,Nd,Sm). Lanthanide(III) ions are hard Lewis acids, due to the classification of Pearson, therefore, their insertion into the coordination cavity of the tetradentate, N-donor porphyrin ligand is a slow and complicated process in aqueous solution, originating partly from the strong bond of the solvent water molecules to the metal ions [13,[19][20][21][22]. This kinetic barrier is vanquishable, e.g.…”

“…The presence of bidentate, O-donor acetate ion enhances the coordination of the first porphyrin ligand, due to its trans effect, but it hinders the connection of a further porphyrin, i.e. the formation of bisporphyrins, because it remains on the lanthanide(III) ion in axial position [9,22]. Insertion of these larger metal ions into the porphyrin cavity can be spectrophotometrically followed due to the redshifts of UVVis, intraligand ππ* absorption bands (Fig.…”

“…4). While the lifetimes for mono-and bisporphyrin complexes are almost equal (~2 ns), only the fluoresence quantum yield decreases slightly because the formation of bisporphyrin results only in the deceleration of radiative decay, as a consequence of a special type of aggregation, probably through the peripheral, sulfonato substituents [16,22,24,25] (tail-to-tail or perpendicular head-to-tail dimerization) without strong π-π interactions between the macrocycles [22,23]. If these interactions were stronger, the absorption bands should show much larger redshifts and hyperchromicities, and the fluorescence should be much weaker, nearly disappear.…”

“…the significant parts of the overall photochemical quantum yields originate from these photoinduced transformations of complexes to each other in this photostationary state [9]. Light accelerates these reactions by orders of magnitude, compared to their very low rate in dark at room temperature [22].…”