An air-stable Dy(iii) single-ion magnet with high anisotropy barrier and blocking temperature

Abstract

A mononuclear Dy(iii) complex assembled just from five water molecules and two phosphonic diamide ligands combines the advantages of high anisotropy barrier, high blocking temperature and significant coercivity, apart from its remarkable air- and moisture-stability.

Mononuclear Dysprosium(III) Complexes with Triphenylphosphine Oxide Ligands: Controlling the Coordination Environment and Magnetic Anisotropy

“…This in turn weakens the P=O bond, leading to a large charge polarization, with a residual positive charge on P and concomitant negative charge on the oxygen atom, reflecting a P + -O − scenario. This is similar to other reported complexes possessing the P=O coordinating moiety [12]. The presence of three TPPO ligands and a nitrate in the equatorial plane (~90 • to each other) enforces the rather strong equatorial ligation (shortest bond lengths, with the largest negative charges on the donor ligand atoms), which competes with the axial interactions of the two chelating nitrate groups (bond lengths of which are slightly longer), leading to a very small ground to first excited state gap and a significant transverse anisotropy.…”

“…At a field of 1000 Oe the relaxation is slower; thus, a larger effective barrier is obtained compared to the barrier at 2000 Oe and a reasonable pre-exponential factor (10 −6 and 10 −5 s) allows confidence in the extracted values for an SMM [3]. The U eff parameters revealed here are significantly smaller than the above-mentioned complex containing axially ligated phosphine oxide ligands (U eff~6 50 cm −1 ) [12], coupled with the fact that slow magnetic relaxation is only observable in an applied magnetic field (fast quantum tunneling of the magnetization (QTM)), indicating the importance of the number and position of the phosphine oxide ligand(s) and possibly the choice of the "co-ligand" (see later).…”

“…Since the ground KD are mostly ±15/2 state with small contributions from other states, these U calc values of 49.1 and 61.4 cm −1 for a and b, respectively, can be easily achieved upon application of a dc field. The absence of zero field SMM character can be directly attributed to the unfavorable ground state KD g zz alignment along the Dy-Cl bonds rather than the shorter Dy-O bonds of the TPPO ligands [12][13][14][15]. This is a consequence of the large number of Cl − ligands that would interact with the prolate Dy III electron density if g zz pointed along the desired Dy-O direction.…”

“…It has been reported that high symmetry environments (i.e., quasi-D 5h ) around the Dy III ion are preferred in order to exhibit very large anisotropy barrier heights [6,12,[25][26][27]. We have therefore modelled two fictitious complexes related to this work, containing the TPPO and chloride ligands- The calculations again predict large transverse components and QTM values in the ground KD for the two model complexes, which is primarily due to relatively strong equatorial donation by the Cl − ligand (note four and five Cl − ligands are present in the equatorial plane, compared to complexes 1-5).…”

“…Our calculations, however, suggest some important points to improve the SIM characteristics: (a) it is an established fact that the oblate Dy III ion requires a strong axial ligand, but our calculations reveal that weak equatorial ligation is also an equally important criterion. For example, seven water ligands in the equatorial plane, with P=O ligands at the axial position possess blocking temperatures in the range of 7-12 K [12]. Replacing the equatorial Cl − /NO 3 − ions by a much weaker ligand is therefore a desired step; (b) With this is mind, substitution of the chloride by other halide ions such as iodide (longer bond lengths and more diffuse charge, therefore "weaker ligation") is perhaps the easiest choice available to incrementally improve the SIM characteristics in these systems; (c) Some of the systems studied clearly possess stronger equatorial ligation compared to axial interactions.…”