Synthesis and Physicochemical Characterization of Protonated and Deprotonated Forms in Heteroleptic Lanthanide(III) Porphyrinate Double-Deckers. X-ray Structure of Gd^IIIH(oep)(tpp) at 298 and 21 K

Abstract

The synthesis, spectroscopic characterization, and electrochemical study of eleven heteroleptic and their corresponding homoleptic lanthanide sandwiches are reported. Studies in solution have been carried out in solvents of different basicity, in order to elucidate the equilibrium between the protonated and deprotonated form of these complexes. The investigated compounds are represented by the formulas Ln(III)H(oep)(tpp) and [Ln(III)(oep)(tpp)](-) corresponding to the protonated and deprotonated forms, respectively (in the case of heteroleptic), and the formulas Ln(III)H(tpp)(2) and [Ln(III)(tpp)(2)](-) (in the case of the homoleptic porphyrin double-deckers), where Ln Nd,., Lu (except Pm), oep = 2,3,7,8,12,13,17,18-octaethylporphyrinate, and tpp = 5,10,15,20-tetraphenylporphyrinate). Various spectroscopic methods are used for the physicochemical characterization of the title complexes. The electronic spectra of the complexes above present different features in CH(2)Cl(2) and in DMF. In the latter solvent they reveal features similar to those of the analogous actinide(IV) porphyrin double-decker. The electrochemical studies carried out in CH(2)Cl(2) and THF demonstrate clearly that the redox behavior of the double-deckers, heteroleptic or homoleptic, is strongly dependent on the proton on the porphyrinic core. In CH(2)Cl(2), four reversible oxidation processes and two quasi-reversible waves are observed for the protonated species in both homo- and heteroleptic double-deckers. In contrast, two oxidations and two reductions are observed in THF for the homoleptic derivatives, while the corresponding heteroleptic ones undergo three oxidations and one reduction process. The structure of the new heteroleptic double-decker Gd(III)H(oep)(tpp) was determined by X-ray diffraction at 298 and 21 K. Both structures are compared with the first analogous structure of Sm(III)H(oep)(tpp). According to the spectroscopic and structural data reported for the heteroleptic protonated derivatives, the oep macrocycle is the favored binding site of the proton in solutions as well as in the solid state.