Incorporation of expanded organic cations in dysprosium(III) borohydrides for achieving luminescent molecular nanomagnets

Wojciech Wegner,Szymon Chorazy,Jakub J Zakrzewski,Mikolaj Zychowicz

doi:10.1038/s41598-021-88446-7

Wojciech Wegner, Szymon Chorazy + Show 2 more

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https://doi.org/10.1038/s41598-021-88446-7

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Abstract

Luminescent single-molecule magnets (SMMs) constitute a class of molecular materials offering optical insight into magnetic anisotropy, magnetic switching of emission, and magnetic luminescent thermometry. They are accessible using lanthanide(III) complexes with advanced organic ligands or metalloligands. We present a simple route to luminescent SMMs realized by the insertion of well-known organic cations, tetrabutylammonium and tetraphenylphosphonium, into dysprosium(III) borohydrides, the representatives of metal borohydrides investigated due to their hydrogen storage properties. We report two novel compounds, [n-Bu4N][DyIII(BH4)4] (1) and [Ph4P][DyIII(BH4)4] (2), involving DyIII centers surrounded by four pseudo-tetrahedrally arranged BH4– ions. While 2 has higher symmetry and adopts a tetragonal unit cell (I41/a), 1 crystallizes in a less symmetric monoclinic unit cell (P21/c). They exhibit yellow room-temperature photoluminescence related to the f–f electronic transitions. Moreover, they reveal DyIII-centered magnetic anisotropy generated by the distorted arrangement of four borohydride anions. It leads to field-induced slow magnetic relaxation, well-observed for the magnetically diluted samples, [n-Bu4N][YIII0.9DyIII0.1(BH4)4] (1@Y) and [Ph4P][YIII0.9DyIII0.1(BH4)4] (2@Y). 1@Y exhibits an Orbach-type relaxation with an energy barrier of 26.4(5) K while only the onset of SMM features was found in 2@Y. The more pronounced single-ion anisotropy of DyIII complexes of 1 was confirmed by the results of the ab initio calculations performed for both 1–2 and the highly symmetrical inorganic DyIII borohydrides, α/β-Dy(BH4)3, 3 and 4. The magneto-luminescent character was achieved by the implementation of large organic cations that lower the symmetry of DyIII centers inducing single-ion anisotropy and separate them in the crystal lattice enabling the emission property. These findings are supported by the comparison with 3 and 4, crystalizing in cubic unit cells, which are not emissive and do not exhibit SMM behavior.

Highlights

Single-molecule magnets (SMMs) form an extraordinary class of d- or f-block metal complexes exhibiting strong magnetic anisotropy which results in the slow relaxation of magnetization[1,2,3,4]
While the luminescent property was recognized in this family, the generation of magnetic anisotropy by using lanthanide borohydrides is a more challenging task as they usually crystallize in space groups of high symmetry with a rather isotropic coordination environment around 4f metal center
In 2, there is only one independent BH4– group according to symmetry restrains, while in 1, there are four of them, which leads to the deformed geometry of [Dy(BH4)4]– in 1 (Fig. 1)

Summary

Introduction

Single-molecule magnets (SMMs) form an extraordinary class of d- or f-block metal complexes exhibiting strong magnetic anisotropy which results in the slow relaxation of magnetization[1,2,3,4]. There is an attractive perspective in searching for simpler organic or inorganic molecular systems involving lanthanide(III) centers that can serve as luminescent molecular nanomagnets In this context, we decided to test lanthanide(III) borohydrides as a possible source of luminescent SMMs. While the luminescent property was recognized in this family, the generation of magnetic anisotropy by using lanthanide borohydrides is a more challenging task as they usually crystallize in space groups of high symmetry with a rather isotropic coordination environment around 4f metal center. We decided to employ expanded organic cations, tetrabutylammonium (n-Bu4N+) and tetraphenylphosphonium ( Ph4P+) for the construction of organic lanthanide(III) borohydrides to decrease the overall crystal symmetry and distort the geometry of 4f metal complexes These are expected to generate distinct single-ion anisotropy and better magnetic isolation within the crystal lattice[81,82,83]. We decided to explore Dy(III) complexes which are yellow-to-white emissive as a result of the characteristic f-f electronic transitions[25,84], and found to be highly anisotropic in diverse coordination environments

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