Pronounced Magnetic Bistability in Highly Cooperative Mononuclear [Fe(Lnpdtz)2(NCX)2] Complexes.

Molecular materials that exhibit stimuli-responsive bistability are promising candidates for the development of molecular switches and sensors. We herein report on the coexistence of a wide thermal hysteretic spin crossover (SCO) effect and a thermally inducible metastable high-spin state at low temperatures achieved with the two new complexes [FeII(Lnpdtz)2(NCX)2] (X = S; Se), with Lnpdtz being (2-naphthyl-5-pyridyl-1,2,4-thiadiazole) and X = S (1) and Se (2). Pronounced π-π-stacking of the aromatic side residues of the ligands enables strong intermolecular interactions, leading to abrupt SCO properties and broad magnetic hysteresis of 10 K for X = S and 58 K for X = Se. In this paper, we also present the pressure-induced spin-state switching around 0.8 GPa. A pronounced thermally induced excited spin state trapping (TIESST effect) is observed for the highly cooperative SCO compounds, which was experimentally followed by low-temperature single crystal structure analysis (20 K) and temperature-dependent Mössbauer spectroscopy.

Chiral Resolution of Spin-Crossover Active Iron(II) [2x2] Grid Complexes.

Chiral magnetic materials are proposed for applications in second-order non-linear optics, magneto-chiral dichroism, among others. Recently, we have reported a set of tetra-nuclear Fe(II) grid complex conformers with general formula C/S-[Fe4 L4 ]8+ (L: 2,6-bis(6-(pyrazol-1-yl)pyridin-2-yl)-1,5-dihydrobenzo[1,2-d : 4,5-d']diimidazole). In the grid complexes, isomerism emerges from tautomerism and conformational isomerism of the ligand L, and the S-type grid complex is chiral, which originates from different non-centrosymmetric spatial organization of the trans type ligand around the Fe(II) center. However, the selective preparation of an enantiomerically pure grid complex in a controlled manner is difficult due to spontaneous self-assembly. To achieve the pre-synthesis programmable resolution of Fe(II) grid complexes, we designed and synthesized two novel intrinsically chiral ligands by appending chiral moieties to the parent ligand. The complexation of these chiral ligands with Fe(II) salt resulted in the formation of enantiomerically pure Fe(II) grid complexes, as unambiguously elucidated by CD and XRD studies. The enantiomeric complexes exhibited similar gradual and half-complete thermal and photo-induced SCO characteristics. The good agreement between the experimentally obtained and calculated CD spectra further supports the enantiomeric purity of the complexes and even the magnetic studies. The chiral resolution of Fe(II)- [2×2] grid complexes reported in this study, for the first time, might enable the fabrication of magneto-chiral molecular devices.

Exploring the Vibrational Side of Spin-Phonon Coupling in Single-Molecule Magnets via 161 Dy Nuclear Resonance Vibrational Spectroscopy.

Synchrotron-based nuclear resonance vibrational spectroscopy (NRVS) using the Mössbauer isotope 161 Dy has been employed for the first time to study the vibrational properties of a single-molecule magnet (SMM) incorporating DyIII , namely [Dy(Cy3 PO)2 (H2 O)5 ]Br3 ⋅2 (Cy3 PO)⋅2 H2 O ⋅2 EtOH. The experimental partial phonon density of states (pDOS), which includes all vibrational modes involving a displacement of the DyIII ion, was reproduced by means of simulations using density functional theory (DFT), enabling the assignment of all intramolecular vibrational modes. This study proves that 161 Dy NRVS is a powerful experimental tool with significant potential to help to clarify the role of phonons in SMMs.

Supramolecular Iron Metallocubanes Exhibiting Site-Selective Thermal and Light-Induced Spin-Crossover.

Treatment of Fe[BF4]2·6H2O with 4,6-di(pyrazol-1-yl)-1H-pyrimid-2-one (HL1) or 4,6-di(4-methylpyrazol-1-yl)-1H-pyrimid-2-one (HL2) affords solvated crystals of [{FeIII(OH2)6}⊂FeII8(µ-L)12][BF4]7 (1, HL = HL1; 2, HL = HL2). The centrosymmetric complexes contain a cubic arrangement of iron(II) centers, with bis-bidentate [L]- ligands bridging the edges of the cube. The encapsulated [Fe(OH2)6]3+ moiety templates the assembly through 12 O-H···O hydrogen bonds to the [L]- hydroxylate groups. All four unique iron(II) ions in the cages are crystallographically high-spin at 250 K, but they undergo a gradual high → low spin-crossover on cooling, which is predominantly centered on one iron(II) site and its symmetry-related congener. This was confirmed by magnetic susceptibility data, light-induced excited spin state trapping (LIESST) effect measurements, and, for 1, Mössbauer spectroscopy and diffuse reflectance data. The clusters are stable in MeCN solution, and 1 remains high-spin above 240 K in that solvent. The cubane assembly was not obtained from reactions using other iron(II) salts or 4,6-di(pyrazol-1-yl)pyrimidine ligands, highlighting the importance of hydrogen bonding in templating the cubane assembly.

161 Dy Time-Domain Synchrotron Mössbauer Spectroscopy for Investigating Single-Molecule Magnets Incorporating Dy Ions.

Time-domain synchrotron Mössbauer spectroscopy (SMS) based on the Mössbauer effect of 161 Dy has been used to investigate the magnetic properties of a DyIII -based single-molecule magnet (SMM). The magnetic hyperfine field of [Dy(Cy3 PO)2 (H2 O)5 ]Br3 ⋅2 (Cy3 PO)⋅2 H2 O⋅2 EtOH is with B0 =582.3(5) T significantly larger than that of the free-ion DyIII with a 6 H15/2 ground state. This difference is attributed to the influence of the coordinating ligands on the Fermi contact interaction between the s and 4f electrons of the DyIII ion. This study demonstrates that 161 Dy SMS is an effective local probe of the influence of the coordinating ligands on the magnetic structure of Dy-containing compounds.

Vibrational properties of 1D- and 3D polynuclear spin crossover Fe(II) urea-triazoles polymer chains and quantification of intrachain cooperativity.

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 parameterHcoop(Rackwitzet al, Phys. Chem. Chem. Phys. 2013,15,15450) has been determined to -31 kJ mol-1for [Fe(n-Prtrzu)3(tosylate)2] and to +27 kJ mol-1for [Fe(n-Prtrzu)3(BF4)2]. The change of sign inHcoopis 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.

Vibrational properties and cooperativity of the 3D spin crossover network [Fe(pyrazine)][Pt(CN)4].

Nuclear inelastic scattering of synchrotron radiation has been used to determine the phonon density of vibrational states (pDOS) for the high-spin and low-spin phases of the hydrated and dehydrated isomer of the spin crossover polymer [Fe(pyrazine)][Pt(CN)4]. Density functional theory calculations have been performed for molecular models of the 3D polymeric system. The models contain 15 Fe(ii)/Zn(ii) centres and allowed the assignment of the observed bands to the corresponding vibrational modes. Thermodynamic parameters like the mean force constant and the vibrational entropy but also sound velocities of the molecular lattices in both spin states have been derived from the pDOS. Modelling of the low-spin and high-spin centres in the environment or matrix of different spins has revealed the enthalpic and entropic components of the intramolecular cooperativity. In contrast to the 1D spin crossover systems (Rackwitz, et al., Phys. Chem. Chem. Phys., 2013, 15, 15450) based on the rigid 1,2,4-triazole derivatives the distortion of the low-spin iron Fe(ii) centre by the matrix of high-spin Fe(ii) (modelled as Zn(ii)) occurs only in two dimensions, defined by the [M(CN)4]2- sheets, rather than concerning all six Fe-N bonds, as in 1D systems. The enthalpic intramolecular cooperativity has been determined to be 15 kJ mol-1 which is lower than that in 1D systems (20-30 kJ mol-1). Yet, the entropic contribution stabilizes the low-spin state in a low-spin matrix, a behaviour which is opposite to what was found for the 1D systems.