Special Issue on Molecular Magnetism

The vibrational dynamics of the iron centres in 1D and 3D spin crossover Fe(II) 4-alkyl-urea triazole chains have been investigated by synchrotron based nuclear inelastic scattering. For the 1D system, the partial density of phonon states has been modelled with density functional theory methods. Furthermore, spin dependent iron ligand distances and vibrational modes were obtained. The previously introduced intramolecular cooperativity parameter H_coop (Rackwitz et al, Phys. Chem. Chem. Phys. 2013, 15, 15450) has been determined to −31 kJ mol⁻¹ for [Fe(n-Prtrzu)₃(tosylate)₂] and to +27 kJ mol⁻¹ for [Fe(n-Prtrzu)₃(BF₄)₂]. The change of sign in H_coop is in line with the incomplete and gradual character of the spin transition for the former as well as with the sharp transition for the latter reported previously (Rentschler and von Malotki, Inorg. Chem., Act. 2008, 361, 3646). This effect can be ascribed to the networks of intramolecular interactions in the second coordination sphere of the polymer chains, depending on the spin state of the iron centres. In addition, we observe a decreased coupling and coherence when comparing the system which displays a sharp spin transition to the system with an incomplete soft transition by analyzing molecular modes involving a movement of the iron centres.

Structural, magnetic and electromechanical changes resulting from spin crossover between the spin quintet and spin triplet forms of a mononuclear Mn³⁺ complex embedded in six lattices with different charge balancing counterions are reported. Isostructural ClO₄⁻ and BF₄⁻ salts (1) and (2) each have two unique Mn³⁺ sites which follow different thermal evolution pathways resulting in a crossover from the spin quintet form at room temperature to a 1:1 spin triplet:quintet ratio below 150 K. The PF₆⁻ (3) and NO₃⁻ (4) salts which each have one unique Mn³⁺ site show a complete conversion from spin quintet to spin triplet over the same temperature range. A complete two step spin crossover is observed in the CF₃SO₃⁻ lattice (5) with a 1:1 ratio of spin quintet and spin triplet forms at intermediate temperature, while the BPh₄⁻ lattice (6) stabilizes the spin triplet form over most of the temperature range with gradual and incomplete spin state switching above 250 K. An electromechanical piezoresponse was detected in NO₃⁻ complex 4 despite crystallization in a centrosymmetric space group. The role of deformations associated with stress-induced spin triplet-spin quintet switching in breaking the local symmetry are discussed and computational analysis is used to estimate the energy gap between the two spin states.

The iron(II) spin crossover complex Fe(1,10-phenanthroline)₂(NCS)₂, dubbed Fe-phen, has been studied with scanning tunneling microscopy, after adsorption on the 'herringbone' reconstructed surface of Au(111) for sub-monolayer coverages. The Fe-phen molecules attach, through their NCS-groups, to the Au atoms of the fcc domains of the reconstructed surface only, thereby lifting the herringbone reconstruction. The molecules stack to form 1D chains, which run along the Au[110] directions. Neighboring Fe-phen molecules are separated by approximately 2.65 nm, corresponding to 9 atomic spacings in this direction. The molecular axis, defined by the two phenanthroline groups, is aligned perpendicular to the chain axis, along the Au $\left[22\overline{1}\right]$ direction, thereby bridging over 5 atomic spacings, in this direction. Experimental evidence suggests that the molecular spins are locked in a mixed state in the sub-monolayer regime at temperatures between 100 K and 300 K.

Nuclear spin levels play an important role in understanding magnetization dynamics and implementation and control of quantum bits in lanthanide-based single-molecule magnets. We investigate the hyperfine and nuclear quadrupole interactions for ¹⁶¹Dy and ¹⁶³Dy nuclei in anionic DyPc₂ (Pc  =  phthalocyanine) single-molecule magnets, using multiconfigurational ab initio methods (beyond density-functional theory) including spin–orbit interaction. The two isotopes of Dy are chosen because the others have zero nuclear spin. Both isotopes have the nuclear spin I  =  5/2, although the magnitude and sign of the nuclear magnetic moment differ from each other. The large energy gap between the electronic ground and first-excited Kramers doublets, allows us to map the microscopic hyperfine and quadrupole interaction Hamiltonian onto an effective Hamiltonian with an electronic pseudo-spin that corresponds to the ground Kramers doublet. Our ab initio calculations show that the coupling between the nuclear spin and electronic orbital angular momentum contributes the most to the hyperfine interaction and that both the hyperfine and nuclear quadrupole interactions for ¹⁶¹Dy and ¹⁶³Dy nuclei are much smaller than those for the ¹⁵⁹Tb nucleus in TbPc₂ single-molecule magnets. The calculated separations of the electronic-nuclear levels are comparable to experimental data reported for ¹⁶³DyPc₂. We demonstrate that hyperfine interaction for the Dy Kramers ion leads to tunnel splitting (or quantum tunneling of magnetization) at zero field. This effect does not occur for TbPc₂ single-molecule magnets. The magnetic field values of the avoided level crossings for ¹⁶¹DyPc₂ and ¹⁶³DyPc₂ are found to be noticeably different, which can be observed from the experiment.

The complex dielectric permittivity of a series of spin crossover complexes, with variable ligand stoichiometry [Fe(Htrz)_1+y−x(trz)_2−y(NH₂trz)_x](BF₄)_y·nH₂O, has been investigated as a function of temperature in a wide frequency range. In each compound, a substantial drop of the conductivity and permittivity is evidenced when going from the low spin to the high spin state, albeit with decreasing amplitude for increasing ligand substitution (i.e. for increasing x). The deconvolution of the dielectric spectra using the Havriliak–Negami equation allowed to extract the dipole and conductivity relaxation times, their distributions as well as the dielectric strengths in both spin states. Remarkably, no clear correlation appears between the conductivity changes and the lattice properties (Debye temperature) in the dilution series. We rationalize these results by considering the dimensionality of the system (1D), wherein the charge transport occurs most likely by hopping along the [Fe(Rtrz)₃]_nⁿ⁺ chains.

An accurate experimental characterization of finite antiferromagnetic (AF) spin chains is crucial for controlling and manipulating their magnetic properties and quantum states for potential applications in spintronics or quantum computation. In particular, finite AF chains are expected to show a different magnetic behaviour depending on their length and topology. Molecular AF rings are able to combine the quantum-magnetic behaviour of AF chains with a very remarkable tunability of their topological and geometrical properties. In this work we measure the ⁵³Cr-NMR spectra of the Cr₈Cd ring to study the local spin densities on the Cr sites. Cr₈Cd can in fact be considered a model system of a finite AF open chain with an even number of spins. The NMR resonant frequencies are in good agreement with the theoretical local spin densities, by assuming a core polarization field A_C = −12.7 T μ_B⁻¹. Moreover, these NMR results confirm the theoretically predicted non-collinear spin arrangement along the Cr₈Cd ring, which is typical of an even-open AF spin chain.

Local moment molecular systems have now been used as the conduction channel in gated spintronics devices, and some of these three terminal devices might even be considered molecular spin transistors. In these systems, the gate voltage can be used to tune the molecular level alignment, while applied magnetic fields have an influence on the spin state, altering the magnetic properties, and providing insights to the magnetic anisotropy. More recently, the use of molecular spin crossover complexes, as the conduction channel, has led to devices that are both nonvolatile and have functionality at higher temperatures. Indeed, some devices have now been demonstrated to work at room temperature. Here, several molecular transistors, including those claiming to use single molecule magnets (SMM), are reviewed.

Multilayer crossbar junctions composed of ITO/[Fe(HB(1,2,4-triazol-1-yl)₃)₂]/M (with M  =  Al or Ca) were fabricated and investigated for their resistance switching properties. Current–voltage–temperature maps revealed ON/OFF resistance ratios as high as 400, with the ON and OFF states defined, respectively, as the low-resistance, low spin state and the high-resistance, high spin state of the spin crossover layer. Similar results were obtained with Al and Ca cathodes indicating that the charge transport in the insulating spin crossover film is at the origin of the resistance switching instead of electron injection at the electrodes. The reproducibility and stability of the device properties were also studied.

Luminescent oligo(p-phenylene ethynylene) (OPE) and spin-crossover (SCO) active Fe(II)-2,6-di(pyrazol-1-yl) pyridine (BPP) systems are prominent examples proposed to develop functional materials such as molecular wires/memories. A marriage between OPE and Fe(II)-BPP systems is a strategy to obtain supramolecular luminescent ligands capable of metal coordination useful to produce novel spin-switchable hybrids with synergistic coupling between spin-state of Fe(II) and a physical property associated with the OPE skeleton, for example, electronic conductivity or luminescence. To begin in this direction, two novel ditopic ligands, namely L¹ and L², featuring OPE-type backbone end-capped with metal coordinating BPP were designed and synthetized. The ligand L² tailored with 2-ethylhexyloxy chains at the 2 and 5 positions of the OPE skeleton shows modulated optical properties and improved solubility in common organic solvents relative to the parent ligand L¹. Solution phase complexation of L¹ and L² with Fe(BF₄)₂·6H₂O resulted in the formation of insoluble materials of the composition [Fe(L¹)]_n(BF₄)_2n and [Fe(L²)]_n(BF₄)_2n as inferred from elemental analyses. Complex [Fe(L¹)]_n(BF₄)_2n underwent thermal SCO centred at T_1/2  =  275 K as well as photoinduced low-spin to high-spin transition with the existence of the metastable high-spin state up to 52 K. On the other hand, complex [Fe(L²)]_n(BF₄)_2n, tethered with 2-ethylhexyloxy groups, showed gradual and half-complete SCO with 50% of the Fe(II)-centres permanently blocked in the high-spin state due to intermolecular steric interactions. The small angle x-ray scattering (SAXS) pattern of the as-prepared solid complex [Fe(L¹)]_n(BF₄)_2n revealed the presence of nm-sized crystallites implying a possible methodology towards the template-free synthesis of functional-SCO nanostructures.

Three isostructural complexes with the formula [Fe(L_5Me)(NCE)₂]: L_5Me  =  N,N'-bis(5-methyl-2-pyridylmethyl)ethane-1,2-diamine and E  =  S (1-S), E  =  Se (1-Se), E  =  BH₃ (1-BH₃) have been synthesized and characterized by single-crystal x-ray diffraction, magnetic susceptibility and DSC studies. All the three derivatives are spin crossover (SCO) active, showing complete one-step spin conversion. The SCO midpoint temperatures (T_1/2) are 193 K for 1-S, 226 K for 1-Se, and 330 K for 1-BH₃, which are among the highest values for the homologous Fe(II)-NCE complexes with comparable tetradentate ligands. The almost linear Fe–N  ≡  C(E) angles are consistent with the strong ligand field (LF) strength imposed by these NCE⁻ co-ligands. Strong hydrogen-like bonding N–H...E was observed to connect the molecules into 2D supramolecular sheets parallel to the bc plane. However, such supramolecular interaction is not sufficient enough to transmit strong cooperativity. A discussion on the factors governing the LF strength and the cooperativity has been made, based on the comparison of analogous complexes and also based on UV–vis spectroscopy studies of the Ni(II) complexes.

X-ray absorption spectroscopy investigations of the spin-state switching of spin-crossover (SCO) complexes adsorbed on a highly-oriented pyrolytic graphite (HOPG) surface have shown so far that HOPG is a promising candidate to realize applications such as spintronic devices because of the stability of SCO complexes on HOPG and the possibility of highly efficient thermal and light-induced spin-state switching. Herein, we present the spin switching of several Fe(II) SCO complexes adsorbed on an HOPG surface with particular emphasis on the thermally induced spin transition behaviour with respect to different structural modifications. The complexes of the type [Fe(bpz)₂(L)] (bpz  =  dihydrobis(pyrazolyl)borate, L  =  1,10-phenanthroline, 2,2'-bipyridine) and their methylated derivatives exhibit SCO in the solid state with some differences regarding cooperative effects. However, in the vacuum-deposited thick films on quartz, complete and more gradual spin transition behavior is observable via UV/vis spectroscopy. In contrast to that, all complexes show large differences upon direct contact with HOPG. Whereas the unmodified complexes show thermal and light-induced SCO, the addition of e.g. two or four methyl groups leads to a partial or a complete loss of the SCO on the surface. The angle-dependent measurement of the N K-edge compared to calculations indicates that the complete SCO and HS-locked molecules on the surface exhibit a similar preferential orientation, whereas complexes undergoing an incomplete SCO exhibit a random orientation on the surface. These results are discussed in the light of molecule-substrate interactions.

The adsorption of two transition metal adatoms, Fe and Co added to a coronene molecule were studied by means of a full potential local orbital method in the framework of relativistic density functional theory. A sequence of fixed spin moment calculations based on the Hubbard-like interaction (U) were carried out on the basis of the generalized gradient approximation (GGA) of the exchange–correlation functional. We found a transition from low-spin to high-spin state upon increasing the transition metal-coronene distance. Furthermore, the magnetism of both Fe-coronene and Co-coronene complexes revealed sensitivity to the magnitude of the U values. In our GGA+U (U  =  2 eV) calculations, a low-spin ground state was found with in-plane magnetic anisotropy for both Fe-coronene and Co-coronene, whereas the GGA+U (U  =  4 eV) calculations resulted in high-spin state with out-of-plane and in-plane magnetic anisotropy for Fe-coronene and Co-coronene, respectively.

The new complex [Fe(H₂B(4-CH₃-pz)₂)₂(bipy)] (1, pz  =  pyrazole, bipy  =  2,2'-bipyridine) is synthesized and investigated by temperature-dependent magnetic measurements, Mößbauer, electronic absorption and vibrational spectroscopy as well as single crystal x-ray diffraction. In the crystal structure of 1 the complexes are pairwise linked to dimers by intermolecular π–π interactions between their bipyridine ligands, with a shortest intradimer distance between two neighboring pyridine rings of 3.575 Å. Analysis of the crystal structures of related iron(II) bis(dihydrobis(pyrazoyl)borate) complexes reveals that most of them contain similar dimers, and that at short π–π intra-dimer distances the complexes are locked in the high-spin state whereas at long distances complete thermal spin crossover (SCO) is observed. Compound 1 with an intermediate π–π intra-dimer distance shows incomplete SCO in the bulk but complete SCO in vacuum-deposited thin films where intermolecular interactions are absent. The implications of this remarkable structure-property relationship are discussed.

The addition of various dipolar molecules is shown to affect the temperature dependence of the spin state occupancy of the much studied spin crossover Fe(II) complex, [Fe{H₂B(pz)₂}₂(bipy)] (pz  =  pyrazol-1-yl, bipy  =  2,2'-bipyridine). Specifically, the addition of benzimidazole results in a re-entrant spin crossover transition, i.e. the spin state starts in the mostly low spin state, then high spin state occupancy increases, and finally the high spin state occupancy decreases with increasing temperature. This behavior contrasts with that observed when the highly polar p -benzoquinonemonoimine zwitterion C₆H₂(...NH₂)₂(...O)₂ was mixed with [Fe{H₂B(pz)₂}₂(bipy)], which resulted in locking [Fe{H₂B(pz)₂}₂(bipy)] largely into a low spin state while addition of the ethyl derivative C₆H₂(...NHC₂H₅)₂(...O)₂ did not appear to perturb the spin crossover transition of [Fe{H₂B(pz)₂}₂(bipy)].

Three new unique mononuclear iron(II) pincer complexes were synthesized using 1,2-bis(pyridin-2-ylethynyl)benzene as axially coordinating pincer ligand and N₂O₂ coordinating Schiff base-like equatorial ligands. Magnetic susceptibility measurements reveal that all three complexes remain in the high spin state throughout the entire temperature range investigated. Reasons for this are restraining sterical interactions revealed in the single crystal x-ray structure analysis and extended DFT-computational studies of one of the pincer complexes. Those interactions also lead to the formation of unexpected side products during the synthesis such as a complex with two ethanol molecules as axial ligand, whose x-ray structure was determined.

We present the results of muon-spin relaxation (SR) measurements on antiferromagnetic and ferromagnetic spin chains. In antiferromagnetic CuF₂(pyz) we identify a transition to long range magnetic order taking place at K, allowing us to estimate a ratio with the intrachain exchange of and the ratio of interchain to intrachain exchange coupling as . The ferromagnetic chain [Sm(hfac)₃(boaDTDA)]_n undergoes an ordering transition at K, seen via a broad freezing of dynamic fluctuations on the muon (microsecond) timescale and implying . The ordered radical moment continues to fluctuate on this timescale down to 0.3 K, while the Sm moments remain disordered. In contrast, the radical spins in [La(hfac)₃(boaDTDA)]_n remain magnetically disordered down to T  =  0.1 K suggesting .