Comparison of the Magnetic Anisotropy and Spin Relaxation Phenomenon of Dinuclear Terbium(III) Phthalocyaninato Single-Molecule Magnets Using the Geometric Spin Arrangement

Herein we report the synthesis and characterization of a dinuclear Tb^III single-molecule magnet (SMM) with two [TbPc₂]⁰ units connected via a fused-phthalocyaninato ligand. The stable and robust complex [(obPc)­Tb­(Fused-Pc)­Tb­(obPc)] (<b>1</b>) was characterized by using synchrotron radiation measurements and other spectroscopic techniques (ESI-MS, FT-IR, UV). The magnetic couplings between the Tb^III ions and the two π radicals present in <b>1</b> were explored by means of density functional theory (DFT). Direct and alternating current magnetic susceptibility measurements were conducted on magnetically diluted and nondiluted samples of <b>1</b>, indicating this compound to be an SMM with improved properties compared to those of the well-known [TbPc₂]^−/0/+ and the axially symmetric dinuclear Tb^III phthalocyaninato triple-decker complex (Tb₂(obPc)₃). Assuming that the probability of quantum tunneling of the magnetization (QTM) occurring in one TbPc₂ unit is <i>P</i>_QTM, the probability of QTM simultaneously occurring in <b>1</b> is <i>P</i>_QTM², meaning that QTM is effectively suppressed. Furthermore, nondiluted samples of <b>1</b> underwent slow magnetic relaxation times (τ ≈ 1000 s at 0.1 K), and the blocking temperature (<i>T</i>_B) was determined to be ca. 16 K with an energy barrier for spin reversal (<i>U</i>_eff) of 588 cm^–1 (847 K) due to <i>D</i>_4<i>d</i> geometry and weak inter- and intramolecular magnetic interactions as an exchange bias (<i>H</i>_bias), reducing QTM. Four hyperfine steps were observed by micro-SQUID measurement. Furthermore, solution NMR measurements (one-dimensional, two-dimensional, and dynamic) were done on <b>1</b>, which led to the determination of the high rotation barrier (83 ± 10 kJ/mol) of the obPc ligand. A comparison with previously reported Tb^III triple-decker compounds shows that ambient temperature NMR measurements can indicate improvements in the design of coordination environments for SMMs. A large <i>U</i>_eff causes strong uniaxial magnetic anisotropy in <b>1</b>, leading to a χ_ax value (1.39 × 10^–30 m³) that is larger than that for Tb₂(obPc)₃ (0.86 × 10^–30 m³). Controlling the coordination environment and spin arrangement is an effective technique for suppressing QTM in TbPc₂-based SMMs