Modulation of Slow Magnetic Relaxation in Gd(III)‐Tetrahalosemiquinonate Complexes

Abstract

Incorporating lanthanoid(III)‐radical magnetic exchange coupling is a possible route to improving the performance of lanthanoid (Ln) single‐molecule magnets (SMMs), molecular materials that exhibit slow relaxation and low temperature quantum tunnelling of the magnetization. Complexes of Gd(III) can conveniently be used as model systems to study the Ln‐radical exchange coupling, thanks to the absence of the orbital angular momentum that is present for many Ln(III) ions. Two new Gd(III)‐radical compounds of formula [Gd(18‐c‐6)X4SQ(NO3)].I3 (18‐c‐6=18‐crown‐6, X4SQ⋅−=tetrahalo‐1,2‐semiquinonate, 1: X=Cl, 2: X=Br) have been synthesized, and the presence of the dioxolene ligand in its semiquinonate form confirmed by X‐ray crystallography, UV‐Visible‐NIR spectroscopy and voltammetry. Static magnetometry and EPR spectroscopy indicate differences in the low temperature magnetic properties of the two compounds, with antiferromagnetic exchange coupling of JGd‐SQ∼−2.0 cm−1 (Hex=−2JGd‐SQ(SGdSSQ)) determined by data fitting. Interestingly, compound 1 exhibits slow magnetic relaxation in applied magnetic fields while 2 relaxes much faster, pointing to the major role of packing effects in modulating slow relaxation of the magnetization.

Elucidation of LMCT Excited States for Lanthanoid Complexes: A Theoretical and Solid-State Experimental Framework

“…To date, lanthanoid complexes with redox-active ligands have been primarily investigated from the perspective of catalysis and reaction chemistry . Some of us have been investigating ortho -dioxolene ligands that can exist in three accessible forms: catecholate (Cat), radical semiquinonate (SQ), and quinonate (Q). ,,, While numerous examples of complexes of these ligands with 3d metals are known, including many cobalt species that exhibit valence tautomerism, complexes with f-block metals are rarer. ,,− The reaction of hydrated europium nitrate with 18- crown -6 and deprotonated tetrahalocatechol (X 2 Cat 2– , where X = Cl, Br) affords neutral mononuclear complexes of general formula [Eu­(18- c -6)­(X 4 Cat)­(NO 3 )] in high yield (Scheme S1). While the previously reported lanthanoid analogues were colorless (for X = Cl) or yellow (for X = Br), the trivalent europium compounds are much darker: dark purple for 1-Eu (X = Cl) and dark green for 2-Eu (X = Br) (Figure ).…”

“…Measurements of the Eu analogues were pursued to provide further experimental insights into the LMCT origin of the broad band observed in the visible region of the reflectance spectra. While Gd III -semiquinonate analogues have been investigated in solution, the catecholate complexes presented herein are generally insoluble. Therefore, the redox properties of the 1-Ln and 2-Ln series were investigated using solid-state cyclic voltammetry on powder samples immobilized on a glassy carbon working electrode.…”

Strong magnetic exchange coupling in Ln2 metallocenes attained by the trans-coordination of a tetrazinyl radical ligand

“…For 1 , a peak of the χ ′′ susceptibility was observed only in the presence of a static dc field, which is expected for Gd( iii )-based complexes. 56–58 Despite our best efforts, the frequency-dependent studies of the ac susceptibility, both with varying fields (Fig. S17, S18 and Table S17†) and temperatures (Fig.…”

Ab initio-based determination of lanthanoid–radical exchange as visualised by inelastic neutron scattering

“…44 Despite a few INS studies, experimental determination of the magnetic exchange coupling in Ln(III)-radical systems is typically only achieved unambiguously by magnetometry for spin-only Gd(III), due to the large unquenched orbital angular momentum of many Ln(III). 45 Examples detailing the strength of exchange and modelling method have been tabulated in Table S1. † Approaches to Ln(III)radical complex design include coordination of stable radical ligands, 46 utilizing diffuse radical orbitals to offer better spatial overlap with Ln(III) electron density [47][48][49] and electron/Ln encapsulation in endohedral fullerenes.…”