Ross Inglis

Magnetic quantum tunneling: insights from simple molecule-based magnets

This perspectives article takes a broad view of the current understanding of magnetic bistability and magnetic quantum tunneling in single-molecule magnets (SMMs), focusing on three families of relatively simple, low-nuclearity transition metal clusters: spin S = 4 Ni(II)(4), Mn(III)(3) (S = 2 and 6) and Mn(III)(6) (S = 4 and 12). The Mn(III) complexes are related by the fact that they contain triangular Mn(III)(3) units in which the exchange may be switched from antiferromagnetic to ferromagnetic without significantly altering the coordination around the Mn(III) centers, thereby leaving the single-ion physics more-or-less unaltered. This allows for a detailed and systematic study of the way in which the individual-ion anisotropies project onto the molecular spin ground state in otherwise identical low- and high-spin molecules, thus providing unique insights into the key factors that control the quantum dynamics of SMMs, namely: (i) the height of the kinetic barrier to magnetization relaxation; and (ii) the transverse interactions that cause tunneling through this barrier. Numerical calculations are supported by an unprecedented experimental data set (17 different compounds), including very detailed spectroscopic information obtained from high-frequency electron paramagnetic resonance and low-temperature hysteresis measurements. Comparisons are made between the giant spin and multi-spin phenomenologies. The giant spin approach assumes the ground state spin, S, to be exact, enabling implementation of simple anisotropy projection techniques. This methodology provides a basic understanding of the concept of anisotropy dilution whereby the cluster anisotropy decreases as the total spin increases, resulting in a barrier that depends weakly on S. This partly explains why the record barrier for a SMM (86 K for Mn(6)) has barely increased in the 15 years since the first studies of Mn(12)-acetate, and why the tiny Mn(3) molecule can have a barrier approaching 60% of this record. Ultimately, the giant spin approach fails to capture all of the key physics, although it works remarkably well for the purely ferromagnetic cases. Nevertheless, diagonalization of the multi-spin Hamiltonian matrix is necessary in order to fully capture the interplay between exchange and local anisotropy, and the resultant spin-state mixing which ultimately gives rise to the tunneling matrix elements in the high symmetry SMMs (ferromagnetic Mn(3) and Ni(4)). The simplicity (low-nuclearity, high-symmetry, weak disorder, etc.) of the molecules highlighted in this study proves to be of crucial importance. Not only that, these simple molecules may be considered among the best SMMs: Mn(6) possesses the record anisotropy barrier, and Mn(3) is the first SMM to exhibit quantum tunneling selection rules that reflect the intrinsic symmetry of the molecule.

Magnetism in metal–organic capsules

Nickel and cobalt seamed metal-organic capsules have been isolated and studied using structural, magnetic and computational approaches. Antiferromagnetic exchange in the Ni capsule results from coordination environments enforced by the capsule framework.

Enhancing SMM properties via axial distortion of Mn-3(III) clusters

Replacement of carboxylate and solvent with facially capping tripodal ligands enhances the single-molecule magnet (SMM) properties of [Mn(III)3] triangles.

Enhancing U-eff in oxime-bridged [Mn(6)(III)Ln(2)(III)] hexagonal prisms

The first 3d-4f clusters built using derivatised salicylaldoximes (R-saoH(2)) describe unusual hexagonal prisms. Replacement of the paramagnetic Gd(III) ions with diamagnetic Ln(III) ions allows for a more thorough understanding of the magnetic properties, whilst replacement with Tb(III) doubles U(eff).

Constructing clusters with enhanced magnetic properties by assembling and distorting Mn3 building blocks

The reaction of Mn(ClO4)(2).6H2O with Naphth-saoH2 (Naphth-saoH2=2-hydroxy-1-napthaldoxime) in pyridine (py) forms the complex [MnIII3O(Naphth-sao)3(py)3](ClO4).0.5py (.0.5py) in very good yields. Reaction of with NaO2CPh in EtOH produces the complex [MnIII6O2(Naphth-sao)6(O2CPh)2(EtOH)6].[MnIII6O2(Naphth-sao)6(O2CPh)2(EtOH)4].2.5Et2O.0.5H2O (.2.5Et2O.0.5H2O). Further reaction of complex with 1 equivalent of both NaN3 and Mn(ClO4)(2).6H2O in MeOH produces the complex [MnII2MnIII6O2(Naphth-sao)6(N3)6(MeOH)8].10MeOH (.10MeOH) that displays an S approximately 0 ground state. Ligand substitution of Naphth-saoH2 with Me-saoH2 in CH2Cl2-MeOH for the latter complex (Me-saoH2= 2-hydroxyphenylethanone oxime) forms the complex [MnII2MnIII6O2(Me-sao)6(N3)6(MeOH)8].10MeOH (.10MeOH) that displays an S=7 ground state with Ueff= 44.6 K. In all four complexes the main building block is the triangular {MnIII3O(R-sao)3} unit (R=Naphth for , and ; R=Me for ). The ligand substitution in triggers a structural distortion in the [Mn6] sub-core as observed by the increased (Mn-N-O-Mn) torsion angles in , switching the interactions from antiferro- to ferromagnetic, dramatically changing the ground-state of the octanuclear complexes from S=0 to 7.

Wheel-like MnII6 and NiII6 complexes from the use of 2-pyridinealdoxime and carboxylates

The employment of 2-pyridinealdoxime, (py)C(H)NOH, in nickel(II) and manganese(II) carboxylate chemistry under solvothermal conditions is reported. The syntheses, crystal structures and magnetochemical characterization (for two representative compounds) are described for [Ni(6)(O(2)CMe)(6){(py)C(H)NO}(6)].H(2)O (1.H(2)O), [Ni(6)(O(2)CPh)(6){(py)C(H)NO}(6)].2EtOH (2.2EtOH), [Ni(6){(4-Cl)O(2)CPh}(6){(py)C(H)NO}(6)].2EtOH (3.2EtOH) and [Mn(6)(O(2)CMe)(6){(py)C(H)NO}(6)].H(2)O (4.H(2)O), where (4-Cl)PhCO(2)(-) is 4-chlorobenzoate. The reactions of M(O(2)CMe)(2).4H(2)O (M = Ni, Mn) with one equivalent of (py)C(H)NOH in EtOH at 120 degrees C under autogenous pressure give isostructural 1.H(2)O and 4.H(2)O. Complexes 2.2EtOH and 3.2EtOH were obtained from the 1 : 1 : 1 Ni(O(2)CMe)(2).4H(2)O/{(py)C(H)NOH/(X)PhCO(2)H reaction mixtures in EtOH under solvothermal conditions (X = H, 4-Cl). The structurally similar clusters 1-4 have a wheel-like topology with the six metal ions in a chair conformation. Each metal site is bound to four oxygen and two nitrogen atoms; the donor atoms come from two carboxylate oxygens, two oximate oxygens, one pyridyl nitrogen and one oximate nitrogen atom. The carboxylate ligands show the syn, syn eta(1):eta(1):mu mode, while the (py)C(H)NO(-) ions behave as eta(1):eta(1):eta(2):mu(3) ligands. Each metal...metal vector is bridged by one carboxylate group, one mu-O derived from a (py)C(H)NO(-) ligand and by one diatomic oximate-NO- group from an adjacent (py)C(H)NO(-) group. The IR spectra of the complexes are discussed in terms of the coordination modes of the ligands. Variable-temperature, solid-state dc magnetic susceptibility studies were carried out on polycrystalline samples of 1 and 4. The data in the 2.0-300 K range have been fit to a model with one J value revealing moderate (1) or weak (2) antiferromagnetic M(II)...M(II) exchange interactions. This work demonstrates the synthetic potential of combining (py)C(H)NOH with carboxylate ligands and the usefulness of solvothermal techniques in 3d-metal cluster chemistry.

Chiral single-molecule magnets: a partial Mn(III) supertetrahedron from achiral components

A [Mn(III)(9)] partial supertetrahedron is a Single-Molecule Magnet (SMM) with an energy barrier to magnetisation reversal of ~30 K and represents the first chiral SMM obtained from achiral starting materials.

New structural types and different oxidation levels in the family of Mn6-oxime single-molecule magnets.

The synthesis and magnetic properties of three new members of a family of salicyaldoxime based [Mn6] single-molecule magnets possessing new structural types, core topologies and Mn oxidation state distributions are reported. The isostructural complexes [MnIII6O2(R-sao)6(X)2(EtOH)6] (R = Et, X = Br (1); R = Me, X = I (2)) exhibit single-molecule magnet behaviour with spin Hamiltonian parameters S = 12, g = 1.98 and D = -0.36 cm(-1) in both cases. The hexametallic cluster [MnIII4MnIV2O2(OMe)(4-)(Et-sao)6(MeOH)2].MeOH (3.MeOH) possesses a planar rod-like topology and a mixed valent [MnIV4MnIII2] core, which is unprecedented in this family of [Mn6] SMMs.

Rare Oxidation-State Combinations and Unusual Structural Motifs in Hexanuclear Mn Complexes Using 2-Pyridyloximate Ligands

The use of phenyl-2-pyridyl ketone oxime and di-2-pyridyl ketone oxime in Mn chemistry has led to hexanuclear clusters with unprecedented (Mn(II)(4)Mn(III)Mn(IV)) or extremely rare (Mn(II)Mn(III)(5) and Mn(II)(3)Mn(III)(3)) metal oxidation-state combinations and uncommon structural motifs.

Addressing the magnetic properties of sub-monolayers of single-molecule magnets by X-ray magnetic circular dichroism

We report on a comparative study of electronic and magnetic properties of Mn6 single-molecule magnets (SMMs) grafted on gold surface. Two derivatives with spin-ground states S=4 and S=12 have been functionalized with 3-tp-CO2- (3-thiophene carboxylate, tpc) ligands and characterized as thick films (TFs) as well as sub-monolayers (sMLs) by synchrotron based techniques. X-ray absorption spectroscopy at the Mn L2,3 edges shows the modification of the spectral lineshape in the sMLs with respect to the TFs suggesting that the local symmetry at the Mn sites changes once the molecules are deposited on gold surface. In spite of this, the expected MnIII oxidation state is preserved. X-ray magnetic circular dichroism (XMCD) spectra show that the total magnetic moment is only given by spin part because of the quenched orbital moment. Moreover, variable temperature and variable field XMCD spectra reveal an effective decrease of the Mn spin moment for both derivatives.