Weak Ferromagnetic Interaction at the Surface of the Ferrimagnetic Rb2Co4[Fe(CN)6]3.3·11H2O Photoexcited State

Interplay between transition-metal K-edge XMCD, slight structural distortions and magnetism in a series of trimetallic (CoxNi(1-x))4[Fe(CN)6]3/8 Prussian blue analogues

The magnetic properties of a series of trimetallic (Co,Ni)Fe Prussian blue analogues (PBAs) were investigated by SQUID magnetometry and X-ray magnetic circular dichroism (XMCD) at the three transition metal (TM) K-edges. In turn, the PBA trimetallic series was used as a tool in order to better understand the information contained in TM K-edge XMCD and particularly the chemical nature of the probed species (extended sub-lattice or localized entities). The results show that the magnetic behavior of the compounds is dictated by competing exchange interactions between the Co-Fe and Ni-Fe pairs, without spin frustration. They also show that XMCD at the TM K-edge is a local atomic probe of the element at the N side of the cyanide bridge and a local probe of the absorbing atom and its first magnetic neighbors on the C side of the bridge. At last, XMCD at the TM K-edge turns out to be highly sensitive to very small structural distortions.

Orbital Magnetic Moment and Single-Ion Magnetic Anisotropy of the S = 1/2 K3[Fe(CN)6] Compound: A Case Where the Orbital Magnetic Moment Dominates the Spin Magnetic Moment

The potassium hexacyanoferrate(III), K3[FeIII(CN)6], is known for its exceptional magnetic anisotropy among the 3d transition metal series. The Fe(III) ions are in the S = 1/2 low spin state imposed by the strong crystal field of the cyanido ligands. A large orbital magnetic moment is expected from previous publications. In the present work, X-ray magnetic circular dichroism was recorded for a powder sample, allowing direct measurement of the Fe(III) orbital magnetic moment. A combination of molecular multiconfigurational ab initio and atomic ligand field multiplets calculations provides the spin and orbital magnetic moments for the [FeIII(CN)6]3- isolated cluster, the crystallographic unit cell, and the powder sample. The calculations of the angular dependencies of the spin and orbital magnetic moments with the external magnetic induction direction reveal easy magnetization axes for each S = 1/2 molecular entity and the crystal. It also shows that the orbital magnetic moment dominates the spin magnetic moment for all directions. Our measurements confirm that the orbital magnetic moment contributes to 60% of the total magnetization for the powder, which is in excellent agreement with our theoretical predictions. An orbital magnetic moment greater than the spin magnetic moment is exceptional for 3d transition metal ions. The impact of crystal field strength and distortion, π back-bonding, spin-orbit coupling, and external magnetic induction was analyzed, leading to a deeper understanding of the spin and orbital magnetic anisotropies.

Toward Quantitative Magnetic Information from Transition Metal K-Edge XMCD of Prussian Blue Analogs

Two series of Prussian blue analogs (PBA) were used as model compounds in order to disentangle the information contained in X-ray magnetic circular dichroism (XMCD) at the K-edges of transition metals. The number of 3d electrons on one site of the bimetallic cyanide polymer has been varied by associating to the [Fe(CN)₆]^3- or the [Cr(CN)₆]^3- precursors various divalent metal ions A²⁺ (Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, and Cu²⁺). The compounds were studied by X-ray diffraction and SQUID magnetometry, as well as by X-ray absorption spectroscopy and XMCD at the K-edges of the A²⁺ transition metal ion. The study shows that the 1s → 4p contribution to the A K-edge XMCD signal can be related to the electronic structure and the magnetic behavior of the probed A²⁺ ion: the shape of the signal to the filling of the 3d orbitals, the sign of the signal to the direction of the magnetic moment with respect to the applied magnetic field, the intensity of the signal to the total spin number S_A, and the area under curve to the Curie constant C_A. The whole study hence demonstrates that PBAs are particularly well-adapted for understanding the information contained in the transition metals K-edge XMCD signals. It also offers new perspectives toward the full disentangling of the information contained in these signals and access to new insights into materials magnetic properties.

Controlling dynamic magnetic properties of coordination clusters via switchable electronic configuration

Large-sized coordination clusters have emerged as a new class of molecular materials in which many metal atoms and organic ligands are integrated to synergize their properties. As dynamic magnetic materials, such a combination of multiple components functioning as responsive units has many advantages over monometallic systems due to the synergy between constituent components. Understanding the nature of dynamic magnetism at an atomic level is crucial for realizing the desired properties, designing responsive molecular nanomagnets, and ultimately unlocking the full potential of these nanomagnets for practical applications. Therefore, this review article highlights the recent development of large-sized coordination clusters with dynamic magnetic properties. These dynamic properties can be associated with spin transition, electron transfer, and valence fluctuation through their switchable electronic configurations. Subsequently, the article also highlights specialized characterization techniques with different timescales for supporting switching mechanisms, chemistry, and properties. Afterward, we present an overview of coordination clusters (such as cyanide-bridged and non-cyanide assemblies) with dynamic magnetic properties, namely, spin transition and electron transfer in magnetically bistable systems and mixed-valence complexes. In particular, the response mechanisms of coordination clusters are highlighted using representative examples with similar transition principles to gain insights into spin state and mixed-valence chemistry. In conclusion, we present possible solutions to challenges related to dynamic magnetic clusters and potential opportunities for a wide range of intelligent next-generation devices.

On the CN-K coordination modes in Kn[M6-n(CN)6]·xH2O: first evidence of CN-K electron-deficient bonding

Based on the determination of single crystal XRD structures of potassium hexacyanidometallates and on IR, and Raman data, here we propose for the first time the occurrence of an electron-deficient bonding between the N end of the CN^- ligand and the K⁺ metal center. The crystal structures of K_n[M^6-n(CN)₆]·xH₂O (M = Fe(ii), Ru(ii), Os(ii), Co(iii), Rh(iii), Ir(iii), Pt(iv)) reveal the presence of four types of CN^-K interactions: (i) a linear CN-K bond, (ii) the N ends in a bipodal coordination involving two K atoms, (iii) the N ends in a tripodal coordination mode involving three K atoms and (iv) the N ends and the K atoms with the largest K-N distances within the subseries that can be attributed to the electrostatic interactions. The bi- and tripodal coordination modes between the N end of the CN^- ligand and K⁺ ions are atypical and their nature is discussed in this contribution. The CN^- ligand N end can behave as a two-electron donor that participates in a three-center two-electron bonding (i.e. Class II μ-L 3c-2e) for a N-bipodal coordination mode or as a two-electron donor that participates in a four-center two-electron bonding (4c-2e) for an N-tripodal coordination mode. Such a possibility is closely related to the π-back donation ability of the CN^- ligand, which results in a charge density accumulation on the N end, which could be partially donated to the K atom through an σ-mechanism. For the divalent metals (Fe, Ru, Os), the solids crystallize with a monoclinic unit cell in the C2/c space group, while for the trivalent ones (Co, Rh, Ir), the crystal structure corresponds to an orthorhombic unit cell in the Pbcn space group. Potassium hexacyanidoplatinate(iv) crystallizes with a trigonal unit cell, in the P3[combining macron]1m space group, where each N end is always found coordinating two K atoms. The finding of these novel coordination modes of the CN^- ligands, relying on an electron-deficient bonding behavior, paves the way for the design of functional materials based on hexacyanidometallates. The experimental results and the proposed electron-deficient bonding model herein discussed were appropriately supported by the computational calculations.

Direct Observation of Charge Transfer and Magnetism in Fe4Co4 Cyanide-Bridged Molecular Cubes

We have studied the zero-dimensional cubane molecular correspondent of a Prussian blue analogue Cs-Fe₄Co₄ at low temperature and high magnetic field by means of L-edge X-ray absorption spectroscopy and X-ray magnetic circular dichroism. We probe the magnetic and electronic structures of Fe and Co separately upon light irradiation, which allows us to observe directly the electron transfer coupled to a spin transition phenomenon within the molecular cubes and to investigate the nature of the metastable photoexcited state. The magnetic moments in the photoexcited state are found to be M = 1.3μ_B ( M_spin = 0.59μ_B with large orbital moment, M_orbit = 0.74μ_B) for low-spin Fe^III and M = 1.5μ_B ( M_spin = 1.08μ_B with orbital moment, M_orbit = 0.41μ_B) for high-spin Co^II at 2 K and 6.8 T. From our results, we evidence that a strong antiferromagnetic coupling between the metal ions can be ruled out.