ENHANCING THE MAGNETIC ANISOTROPY OF TRANSITION METAL COMPLEXES BY LIGAND FIELD ENGINEERING

Molecule-based magnets incorporating heavy transition metal ions display fascinating electronic properties, which remain much less explored than those of 3d counterparts. XAS and XMCD were employed in this work, which reports an element-selective characterisation of the magnetic properties of unprecedented trans-[MIVF4(CN)2]2- complexes (M: a 5d metal ion).

ELECTRONIC STRUCTURE, MAGNETISM AND DYNAMICS

102 ESRF

Magnets involving heavy 5d transition metals have recently gained increasing attention due to their great promise in data storage, quantum computing and molecule-based spintronic devices [1,2]. Due to strong spin-orbit interactions, 5d metal ions usually display a large magnetic anisotropy. The combination of this property and the diffuse nature of the 5d orbitals leads to stronger exchange interactions with spin carriers than first row metal ions with more contracted 3d orbitals. The intrinsic magnetic properties of the 5d metal ions and, therefore, their potential applications depend on the electronic structure, which can be controlled by the coordination geometry and the ligand field. Keeping this in mind, the present study reports the first transition metal complexes featuring mixed fluorido-cyanido ligands, trans- [MIVF4(CN)2]2- (M = Re, Os), which are obtained by partial silicon-mediated fluoride abstraction from [MIVF6]2- anions (Figure 83) [3,4]. Even though the displacement of fluoride ions by organo-silicon reagents has already been reported for main-group elements, this is the first time that such a reaction has been successfully applied to a heavy transition metal complex.

The resulting compounds feature elongated axial M-C bonds (Figure 83), reflecting a higher deviation from an ideal octahedral environment than their [MIVF6]2- precursor. Magnetic susceptibility measurements for the Os analogue reveal a non-magnetic Jeff = 0 ground state

(low-spin d4), in agreement with the properties measured for the [OsIVF6]2- complex [4]. On the other hand, the Re counterpart (d3) displays more exciting magnetic properties, which differ significantly from those observed for the related [ReIVF6]2- and trans-[ReIVCl4(CN)2]2- species [3,5]. Thus, even though all of these ReIV complexes exhibit slow relaxation of the magnetisation (i.e., single-molecule magnet properties), the paramagnetic relaxation in trans-[ReIVF4(CN)2]2- is governed by direct and Raman mechanisms, while the Orbach process dominates in the others.

In order to gain insights into the electronic structure of this unique trans-[ReIVF4(CN)2]2- anion and compare with analogous ReIV complexes, X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) experiments were performed at beamline ID12. XAS spectra collected at the L2,3 edges (probing 5d states directly via dipole allowed 2p g 5d transitions) display intense resonant absorption at both edges, with XMCD signals of opposite signs (Figure 84).

A quantitative analysis of these X-ray spectra allows a determination of the spin and orbital contributions to the magnetic moment, revealing a surprising almost complete quenching of the orbital angular momentum by the ligand field (Mspin = 1.6 µB and Morbital = 0.01 µB).

Fig. 83: Schematic illustrating the novel

silicon-mediated fluoride abstraction method and crystal

structure of the anionic complex [ReIVF4(CN)2]2-.

Colour code: Re: orange, F: green, C: dark grey, N: blue.

Fig. 84: Experimental XAS (top) and XMCD (bottom) spectra of (PPh4)2[trans-ReIVF4(CN)2]·H2O at the L2,3 edges obtained in a magnetic field of ±17 T at 3 K.