No abstract

Inelastic neutron scattering (INS) has been used to investigate the crystal field (CF) magnetic excitations of the analogs of the most representative lanthanoid-polyoxometalate single-molecule magnet family: Na[Ln(WO)] (Ln = Nd, Tb, Ho, Er). Ab initio complete active space self-consistent field/restricted active space state interaction calculations, extended also to the Dy analog, show good agreement with the experimentally determined low-lying CF levels, with accuracy better in most cases than that reported for approaches based only on simultaneous fitting to CF models of magnetic or spectroscopic data for isostructural Ln families. In this work we demonstrate the power of a combined spectroscopic and computational approach. Inelastic neutron scattering has provided direct access to CF levels, which together with the magnetometry data, were employed to benchmark the ab initio results. The ab initio determined wave functions corresponding to the CF levels were in turn employed to assign the INS transitions allowed by selection rules and interpret the observed relative intensities of the INS peaks. Ultimately, we have been able to establish the relationship between the wave function composition of the CF split Ln ground multiplets and the experimentally measured magnetic and spectroscopic properties for the various analogs of the Na[Ln(WO)] family.

Correlating blocking temperatures with relaxation mechanisms in single-molecule magnets.

Magnetic exchange interactions within the asymmetric dimetallic compounds [hqH][Ln(hq)(NO)]·MeOH, (Ln = Er(III) and Yb(III), hqH = 8-hydroxyquinoline) have been directly probed with EPR spectroscopy and accurately modeled by spin Hamiltonian techniques. Exploitation of site selectivity via doping experiments in Y(III) and Lu(III) matrices yields simple EPR spectra corresponding to isolated Kramers doublets, allowing determination of the local magnetic properties of the individual sites within the dimetallic compounds. CASSCF-SO calculations and INS and far-IR measurements are all employed to further support the identification and modeling of the local electronic structure for each site. EPR spectra of the pure dimetallic compounds are highly featured and correspond to transitions within the lowest-lying exchange-coupled manifold, permitting determination of the highly anisotropic magnetic exchange between the lanthanide ions. We find a unique orientation for the exchange interaction, corresponding to a common elongated oxygen bridge for both isostructural analogs. This suggests a microscopic physical connection to the magnetic superexchange. These results are of fundamental importance for building and validating model microscopic Hamiltonians to understand the origins of magnetic interactions between lanthanides and how they may be controlled with chemistry.

Combining Ising-type magnetic anisotropyw ith collinear magnetic interactions in single-molecule magnets (SMMs) is as ignificant synthetic challenge.H erein we report aD y[15-MC Cu-5] (1-Dy)S MM, where aD y III ion is held in ac entral pseudo-D 5h pocket of ar igid and planar Cu 5 metallacrown (MC). Linking two Dy[15-MC Cu-5] units with as ingle hydroxideb ridge yields the doubledecker {Dy[15-MC Cu-5]} 2 (2-Dy)S MM where the anisotropya xes of the two Dy III ions are nearly collinear,resulting in magnetic relaxation times for 2-Dy that are approximately 200 000 times slower at 2Kthan for 1-Dy in zero external field. Whereas 1-Dy and the Y III-diluted Dy@2-Y analogue do not show remanence in magnetic hysteresis experiments,t he hysteresis data for 2-Dy remain open up to 6Kwithout asudden drop at zero field. In conjunction with theoretical calculations,these results demonstrate that the axial ferromagnetic Dy-Dy coupling suppresses fast quantum tunneling of magnetization (QTM). The relaxation profiles of both complexes curiously exhibit three distinct exponential regimes,a nd hold the largest effective energy barriers for any reported d-f SMMs up to 625 cm À1 .

We discuss a cost-effective approach to understand magnetic relaxation in the new generation of rare-earth single-molecule magnets. It combines ab initio calculations of the crystal field parameters, of the magneto-elastic coupling with local modes, and of the phonon density of states with fitting of only three microscopic parameters. Although much less demanding than a fully ab initio approach, the method gives important physical insights into the origin of the observed relaxation. By applying it to high-anisotropy compounds with very different relaxation, we demonstrate the power of the approach and pinpoint ingredients for improving the performance of single-molecule magnets.

The discovery of ferrocene nearly 70 years ago marked the genesis of metallocene chemistry; although the ferrocenium cation was discovered soon afterwards, a derivatized ferrocenium dication was only isolated in 2016 and the monoanion of ferrocene has only been observed in low temperature electrochemical studies. Here we report the isolation of a derivatized ferrocene anion in the solid state as part of an isostructural family of 3 d metallocenates which consist of anionic complexes of a metal centre (manganese, iron or cobalt) sandwiched between two bulkyCp ttt ligands (where Cp ttt is {1,2,4-C 5 H 2 t Bu 3 }). These air- and thermally-sensitive complexes rapidly decompose above -30 °C, however, we were able to characterise all metallocenates by a wide range of physical techniques and ab initio calculations. These data have allowed us to map the electronic structures of this metallocenate family, including an unexpected high-spin S = 3/2 ground state for the 19e - derivatized ferrocene anion.

Ab initio calculations carried out on the Tb analogue of the single-molecule magnet family Na9[Ln(W5O18)2] (Ln = Nd, Gd, Ho and Er) have allowed interpretation of the inelastic neutron scattering spectra. The combined experimental and theoretical approach sheds new light on the sensitivity of the electronic structure of the Tb(III) ground and excited states to small structural distortions from axial symmetry, thus revealing the subtle relationship between molecular geometry and magnetic properties of the two isostructural species that comprise the sample.