Effect of One- and Two-Electron Reduction of Terbium(III) Double-Decker Phthalocyanine on Single-Ion Magnet Behavior and NIR Absorption

Reduction of terbium(III) double-decker phthalocyanine, TbIIIPc2 (1), by sodium fluorenone ketyl in the presence of bis(triphenylphosphoranylidene)ammonium cations yields one-electron-reduced (PPN+){TbIII(Pc2-)2}-·2.5C6H4Cl2 (2) containing the dianionic Pc2- macrocycles, whereas a stronger NaCpCo(CO)2 reductant in the presence of an excess of cryptand yields two-electron-reduced {Cryptand(Na+)}2{(Pc2-)TbIII(Pc•3-)}2- (3) containing the Pc•3- radical trianionic macrocycle. Isolated pairs of the {TbIII(Pc2-)2}- anions are formed in 2, whereas compound 3 has unique 3D packing of the macrocycles with weak π-orbitals overlapping in all three directions. This is the first example of the two-electron-reduced lanthanide double-decker containing Pc•3- radical trianion studied in solid state. Compound 2 manifests single-ion magnet (SIM) behavior with a large effective spin-reversal energy barrier of Ueff = 538 cm-1 in comparison with previously studied (Bu4N+){TbIII(Pc2-)2}- ( Ueff = 230 cm-1). Thus, changes in cation size and shape affect the molecular packing of {TbIII(Pc2-)2}- and increase the spin-reversal energy barrier. At the same time, two-electron-reduced species 3 containing TbIII and Pc•3- show no signs of SIM such as hysteresis loop at 1.9 K, and no peaks are observed on the temperature dependencies of in-phase (χ') and out-of-phase (χ″) signals. In contrast to EPR silent 2, both compounds 1 and 3 manifest broad signals from paramagnetic TbIII ions. Narrower signals attributed to Pc•3- are of high intensity only in 3. In addition to the absorption bands of Pc in the UV and visible spectral range, compound 3 manifests the lowest energy absorption band in solid-state spectra even in the near IR range at 4700 cm-1 (2130 nm), whereas such bands are not observed in the spectrum of 2. These data show that the reduction of the Pc macrocycles in the lanthanide double-deckers leads to the appearance of new very low-energy new transitions associated with Pc•3- whose energy is the lowest among known reduced metal phthalocyanines.