Synergistic construction of highly symmetrical lanthanide single-ion magnets using neutral phosphoryl and anionic thiocyanate ligands.

A series of lanthanide single-ion compounds, [Dy(TMPO)(SCN)(HO)] (1), [Dy(TMPO)(SCN)(HO)]Dy(TMPO) (SCN)(HO)·(HO) (2), [Ho(TMPO)(SCN)(HO)] (3) (TMPO = trimorpholinophosphine oxide) and Dy(HMPA)(SCN) (4) (HMPA = hexamethylphosphoramide), were synthesized by assembling neutral phosphoryl and anionic thiocyanate ligands with lanthanide central ions. Compounds 1 and 3 were revealed to be isostructural with distorted axially compressed pentagonal bipyramidal (PBP, pseudo- symmetry) coordination environments, which consist of two TMPO ligands on the apical sites and three thiocyanate anions and two coordinated water molecules in the equatorial plane. Compound 2 contains two molecular fragments, among which the Dy1 molecular fragment has a composition and structure very similar to compound 1, except for a lower degree of local distortion; there are slight differences between Dy2 and Dy1 molecular fragments, with one SCN replaced by a coordinating water molecule to become a counter anion, resulting in lower local distortion and shorter axial bond lengths. Compound 4 has a slightly distorted axially elongated octahedral ( symmetry) coordination geometry, with two SCN anions occupying the apical sites and four HMPA ligands occupying the equatorial plane. The synthesis of this compound demonstrates the effectiveness of the strategy of synergistically constructing highly symmetrical lanthanide single-ion complexes using large steric hindrance phosphoryl ligands and SCN anions. Due to the high similarity in the coordination environments between compounds 1 and 2, a comparative study was conducted on their magnetic properties through experimental measurements and theoretical calculations. Compound 1 exhibits only a QTM (quantum tunneling of the magnetization) effect under zero dc field due to its high degree of local distortion, which masks the magnetic relaxation processes. Under the application of a 2 kOe dc field, the QTM effect was eliminated, resulting in a magnetic relaxation effective energy barrier of 45 K through the first excited KDs (Kramers doublets). The Dy1 and Dy2 of compound 2 have low local distortion and high symmetry, but the axial bond lengths of the latter are noticeably shorter than those of the former, and there is one less negatively charged SCN coordination in the equatorial plane of Dy2 than that of Dy1, indicating that Dy2 has stronger axial anisotropy and a weaker transverse field than Dy1. Consistent with the above structural analysis, compound 2 exhibits a coexistence of QTM and magnetic relaxation through the second excited state under zero dc field, with a relaxation energy barrier of 58 K, higher than that of compound 1, and exhibits butterfly-shaped hysteresis loops below 7 K, in contrast to the weak hysteresis at 2 K for 1. Therefore, we utilized 1 and 2 as a class of model complexes to explore the significant impact of local distortion of the axially compressed pentagonal bipyramidal coordination geometry on single-ion magnetic performance under extremely similar coordination environments.