Juan M. Clemente-Juan

MAGPACK1 A package to calculate the energy levels, bulk magnetic properties, and inelastic neutron scattering spectra of high nuclearity spin clusters

M agnetic molecular clusters, i.e., molecular assemblies formed by a finite number of exchange-coupled magnetic moments, are currently receiving much attention in several active areas of research as molecular chemistry, magnetism, and biochemistry. A reason for this interest lies in the possibility to use simple molecular clusters as magnets of nanometer size exhibiting unusual magnetic properties as superparamagnetic like behavior or quantum tunneling of magnetization.2 – 4 Organic molecules of increasing sizes and large number of unpaired electrons are being explored as a means of obtaining building blocks for molecule-based magnets.5 Magnetic clusters of metal ions are also relevant in biochemistry.6 This area between molecule and bulk will require new theoretical concepts and techniques for investigation of their peculiar properties. Still, the theoretical treatment required to understand the magnetic and spectroscopic properties of this wide variety of compounds is a challenging problem in molecular magnetism.7 For a long time, this problem has been mostly restricted to treat comparatively simple clusters comprising a reduced number of exchange-coupled centers and special spin topologies, for which solutions can be obtained either analytically or numerically. However, on increasing the spin nuclearity of the cluster, the problem rapidly becomes unapproachable because the lack of translational symmetry in the clusters. An additional complication is the spin anisotropy of the cluster. Until now only the isotropic-exchange case has been treated, so as to take full advantage of the spin symmetry of the cluster.8 In this article we present a very powerful and efficient computational approach to solve the exchange problem in high nuclearity spin clusters with all kind of exchange interactions (isotropic and anisotropic), including the single-ion anisotropic effects. The clusters are formed by an arbitrary number of exchangecoupled centers that combine different spin values and arbitrary topology. This approach is based on the use of the irreducible tensor operators (ITO) technique.7, 9 – 12 It allows evaluation of both eigenvalues and eigenvectors of the system, and then, calculation of the magnetic susceptibility, magnetization, or heat capacity, and also the inelastic neutron scattering spectra. In the following sections we will present both the theory and the four different implemented FORTRAN programs that integrate a package called MAGPACK . In the last section some examples are presented in order to show the possibilities of the programs.

Mononuclear lanthanide single-molecule magnets based on polyoxometalates.

[ErW10O36]9- is the first polyoxometalate behaving as a single-molecule magnet (SMM). It shows frequency-dependent out-of-phase magnetization and a thermally activated single relaxation process with an effective barrier of 55.8 K. This single lanthanide ion polyoxometalate is the inorganic analogue of the bis(phthalocyaninato)lanthanide SMMs, both exhibiting very similar ligand field symmetries around the lanthanide ion (idealized D4d). It is chemically stable and offers new avenues for organization and processing of single-molecule magnets. Furthermore, it can be made free from nuclear spins and opens the possibility to be used for studies of decoherence on unimolecular qubits.

Magnetic polyoxometalates: from molecular magnetism to molecular spintronics and quantum computing.

In this review we discuss the relevance of polyoxometalate (POM) chemistry to provide model objects in molecular magnetism. We present several potential applications in nanomagnetism, in particular, in molecular spintronics and quantum computing.

Mononuclear Lanthanide Single Molecule Magnets Based on the Polyoxometalates [Ln(W5O18)2]9− and [Ln(β2-SiW11O39)2]13−(LnIII = Tb, Dy, Ho, Er, Tm, and Yb)

The first two families of polyoxometalate-based single-molecule magnets (SMMs) are reported here. Compounds of the general formula [Ln(W(5)O(18))(2)](9-) (Ln(III) = Tb, Dy, Ho, and Er) and [Ln(SiW(11)O(39))(2)](13-) (Ln(III) = Tb, Dy, Ho, Er, Tm, and Yb) have been magnetically characterized with static and dynamic measurements. Slow relaxation of the magnetization, typically associated with SMM-like behavior, was observed for [Ln(W(5)O(18))(2)](9-) (Ln(III) = Ho and Er) and [Ln(SiW(11)O(39))(2)](13-) (Ln(III) = Dy, Ho, Er, and Yb). Among them, only the [Er(W(5)O(18))(2)](9-) derivative exhibited such a behavior above 2 K with an energy barrier for the reversal of the magnetization of 55 K. For a deep understanding of the appearance of slow relaxation of the magnetization in these types of mononuclear complexes, the ligand-field parameters and the splitting of the J ground-state multiplet of the lanthanide ions have been also estimated.

Magnetic clusters from polyoxometalate complexes

Abstract The present article highlights the increasing interest of polyoxometalates in molecular magnetism, providing at the same time a perspective of the state-of-the-art in this area. The main focus is the polyoxotungstates. The first aspect we discuss is that of the coordination chemistry of these metal–oxide ligands. We show that this chemistry leads to remarkable examples of well-insulated magnetic clusters of controlled nuclearity and topology. In these clusters detailed information on the nature of the magnetic exchange interactions can be extracted by using, in addition to the classical magnetic techniques (magnetic susceptibility, magnetization and EPR spectroscopy), other physical techniques as the inelastic neutron scattering (INS) spectroscopy, which provides more direct information on the lower lying energy levels of the magnetic cluster. The second aspect we discuss is that of the interplay between electron delocalization and exchange interactions in the mixed-valence polyoxometalates. We show that these high-nuclearity multielectronic clusters are model systems for the development of new theories in the mixed valence area.

Lanthanoid single-ion magnets based on polyoxometalates with a 5-fold symmetry: the series [LnP5W30O110]12- (Ln3+ = Tb, Dy, Ho, Er, Tm, and Yb).

A robust, stable and processable family of mononuclear lanthanoid complexes based on polyoxometalates (POMs) that exhibit single-molecule magnetic behavior is described here. Preyssler polyanions of general formula [LnP(5)W(30)O(110)](12-) (Ln(3+) = Tb, Dy, Ho, Er, Tm, and Yb) have been characterized with static and dynamic magnetic measurements and heat capacity experiments. For the Dy and Ho derivatives, slow relaxation of the magnetization has been found. A simple interpretation of these properties is achieved by using crystal field theory.

Rational Design of Single-Ion Magnets and Spin Qubits Based on Mononuclear Lanthanoid Complexes

Here we develop a general approach to calculating the energy spectrum and the wave functions of the low-lying magnetic levels of a lanthanoid ion submitted to the crystal field created by the surrounding ligands. This model allows us to propose general criteria for the rational design of new mononuclear lanthanoid complexes behaving as single-molecule magnets (SMMs) or acting as robust spin qubits. Three typical environments exhibited by these metal complexes are considered, namely, (a) square antiprism, (b) triangular dodecahedron, and (c) trigonal prism. The developed model is used to explain the properties of some representative examples showing these geometries. Key questions in this area, such as the chemical tailoring of the superparamagnetic energy barrier, tunneling gap, or spin relaxation time, are discussed. Finally, in order to take into account delocalization and/or covalent effects of the ligands, this point-charge model is complemented with ab initio calculations, which provide accurate information on the charge distribution around the metal, allowing for an explanation of the SMM behavior displayed by some sandwich-type organometallic compounds.

A mixed-valence polyoxovanadate(III,IV) cluster with a calixarene cap exhibiting ferromagnetic V(III)-V(IV) interactions.

A series of compounds (cat)[V6O6(OCH3)8(calix)(CH3OH)] was obtained under anaerobic conditions and solvothermal reaction of VOSO4 with p-tert-butylcalix[4]arene (calix) in methanol using different types of bases (Et4NOH, NH4OH, pyridine, Et3N). All compounds contain the same polyoxo(alkoxo)hexavanadate anion [V6O6(OCH3)8(calix)(CH3OH)]- (1) exhibiting a mixed valence {VIIIVIV5O19} core with the so-called Lindqvist structure coordinated to a calix[4]arene macrocycle and cocrystallizing with the conjugated acid of the base (cat = Et4N+, NH4(+), pyridinium, Et3NH+) involved in the synthesis process. The structures have been fully established from X-ray diffraction on single crystals and the mixed valence state has been confirmed by bond valence sum calculations. The magnetic behavior of all compounds are the same because of the polyalkoxohexavanadate anion [V6O6(OCH3)8(calix)(CH3OH)]- (1) and have been interpreted by DFT calculations. Thus the V(III)...V(IV) interactions are found to be weakly ferromagnetic (<5.5 cm(-1)) while the V(IV)...V(IV) are antiferromagnetic (-17.6; -67.6 cm(-1)). The set of the coupling exchange parameters allows a good agreement with the magnetic experimental data.

A unique example of structural and magnetic diversity in four interconvertible copper(II)-azide complexes with the same schiff base ligand: a monomer, a dimer, a chain, and a layer.

Four new Cu(II)-azido complexes of formula [CuL(N(3))] (1), [CuL(N(3))](2) (2), [Cu(7)L(2)(N(3))(12)](n) (3), and [Cu(2)L(dmen)(N(3))(3)](n) (4) (dmen = N,N-dimethylethylenediamine) have been synthesized using the same tridentate Schiff base ligand HL (2-[1-(2-dimethylaminoethylimino)ethyl]phenol, the condensation product of dmen and 2-hydroxyacetophenone). The four compounds have been characterized by X-ray structural analyses and variable-temperature magnetic susceptibility measurements. Complex 1 is mononuclear, whereas 2 is a single mu-1,1 azido-bridged dinuclear compound. The polymeric compound 3 possesses a 2D structure in which the Cu(II) ions are linked by phenoxo oxygen atoms and two different azide bridges (mu-1,1 and mu-1,1,3). The structure of complex 4 is a double helix in which two mu-1,3-azido-bridged alternating one-dimensional helical chains of CuL(N(3)) and Cu(dmen)(N(3))(2) are joined together by weak mu-1,1 azido bridges and H-bonds. The complexes interconvert in solution and can be obtained in pure form by carefully controlling the conditions. The magnetic properties of compounds 1 and 2 show the presence of very weak antiferromagnetic exchange interactions mediated by a ligand pi overlap (J = -1.77) and by an asymmetric 1,1-N(3) bridge (J = -1.97 cm(-1)), respectively. Compound 3 presents, from the magnetic point of view, a decorated chain structure with both ferro- and antiferromagnetic interactions. Compound 4 is an alternating helicoidal chain with two weak antiferromagnetic exchange interactions (J = -1.35 and -2.64 cm(-1)).

A Family of Enneanuclear Iron(II) Single‐Molecule Magnets

Complexes [Fe9(X)2-(O2CMe)8{(2-py)2CO2}4] (X(-)=OH(-) (1), N3(-) (2), and NCO(-) (3)) have been prepared by a route previously employed for the synthesis of analogous Co(9) and Ni(9) complexes, involving hydroxide substitution by pseudohalides (N3(-), NCO(-)). As indicated by DC magnetic susceptibility measurements, this substitution induced higher ferromagnetic couplings in complexes 2 and 3, leading to higher ground spin states compared to that of 1. Variable-field experiments have shown that the ground state is not well isolated from excited states, as a result of which it cannot be unambiguously determined. AC susceptometry has revealed out-of-phase signals, which suggests that these complexes exhibit a slow relaxation of magnetization that follows Arrhenius behavior, as observed in single-molecule magnets, with energy barriers of 41 K for 2 (tau 0=3.4 x 10(-12) s) and 44 K for 3 (tau 0=2.0 x 10(-11) s). Slow magnetic relaxation has also been observed by zero-field 57Fe Mössbauer spectroscopy. Characteristic integer-spin electron paramagnetic resonance (EPR) signals have been observed at X-band for 1, whereas 2 and 3 were found to be EPR-silent at this frequency. 1H NMR spectrometry in CD3CN has shown that complexes 1-3 are stable in solution.