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

A series of three new seven-coordinate pentagonal-bipyramidal (PBPY-7) Dy(III) complexes, [Dy(L-3(CH))(Cy3PO)(2)]ClO4 center dot CH3CN (1), [Dy(L2(t-Bu))(Ph3PO)(2)]ClO4 center dot 0.63C(2)H(5)OH (2), and [Dy(L-3(OCH))(Ph3PO)(2)]ClO4 center dot 2H(2)O (3), including various chelating pentadentate ligands with [N3O2](2-) binding node in the equatorial plane, L-3(CH) = [2,6-diacetylpyridine bis(acetylhydrazone)](2-), L2(t-Bu) = [2,6-diacetylpyridine bis(3,5di-tert-butylbenzoylhydrazone)](2-), and L-3(CH) = [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 approximate to 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) similar to 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 B-20/B-40 < -0.07 of the two axial CF parameters B-20 and B-40. 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 B-40 and negative B-20. 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 similar to 100 degrees 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.