Pentagonal-bipyramidal dysprosium(iii) complexes with two apical phosphine oxide ligands and equatorial pentadentate N3O2 Schiff-base ligands: breakdown of the apical magnetic axiality by a strong equatorial crystal field

Abstract

A series of three new seven-coordinate pentagonal-bipyramidal (PBPY-7) Dy(III) complexes, [Dy(L^CH₃)(Cy₃PO)₂]ClO₄·CH₃CN (1), [Dy(L^2(t-Bu))(Ph₃PO)₂]ClO₄·0.63C₂H₅OH (2), and [Dy(L^OCH₃)(Ph₃PO)₂]ClO₄·2H₂O (3), including various chelating pentadentate ligands with [N₃O₂]²⁻ binding node in the equatorial plane, L^CH₃ = [2,6-diacetylpyridine bis(acetylhydrazone)]²⁻, L^2(t-Bu) = [2,6-diacetylpyridine bis(3,5di-tert-butylbenzoylhydrazone)]²⁻, and L^CH₃ = [2,6-diacetylpyridine bis(4-methoxybenzoylhydrazone)]²⁻, and two apical ligands Cy₃PO and Ph₃PO were synthesized and characterized structurally and magnetically. The ac magnetic measurements indicated the single-molecule-magnet (SMM) behavior of 1–3 with energy barriers of U_eff ≈ 318–350 K. Ab initio calculations and crystal-field (CF) analysis showed that the ground states of 1–3 were a nearly pure Ising type Kramers doublet (KD₀) |±15/2〉_eq with the long magnetic axis lying in the equatorial plane of N₃O₂, which was the opposite of high-performance PBPY-7 Dy(III) SMMs (U_eff > 1000 K), where the long magnetic axis of KD₀ |±15/2〉 invariably pointed toward apical ligands. This difference is due to competition between the apical and equatorial CFs, which have been quantitatively examined with CF calculations. We show that the turning of the long magnetic axis (g_z ∼ 19.6) from apical ligands (z) to the equatorial plane (xy) is due to crossover between the oblate |±15/2〉 and prolate |±1/2〉 ground states of the Dy(III) ion, which occurs at the negative ratio of B₂₀/B₄₀ < −0.07 of the two axial CF parameters B₂₀ and B₄₀. Complexes 1–3 correspond to this case due to the strong equatorial CF of the negatively charged chelate node of [N₃O₂]²⁻ producing a large positive CF parameter B₄₀ and negative B₂₀. In this case, the SMM properties of 1–3 arise from distortions of the PBPY-7 complex (namely, from a large O1–Dy–O2 bond angle of ∼100° in the N₃O₂ pentagon of 1–3) that mix the lowest |±1/2〉 state and low-lying low-m_J states to produce the equatorial KD₀ |±15/2〉_eq. This highlights a breakdown of the apical magnetic axiality, since the SMM performances of 1–3 are governed by a strong equatorial CF and distortions rather than by high D_5h symmetry and strong apical ligands. Some ways to improve the SMM efficiency of 1–3 and related PBPY-7 Dy(III) complexes are discussed.