Homoleptic Fe(III) and Fe(IV) Complexes of a Dianionic C3-Symmetric Scorpionate.

High-valent iron species have been implicated as key intermediates in catalytic oxidation reactions, both in biological and synthetic systems. Many heteroleptic Fe(IV) complexes have now been prepared and characterized, especially using strongly π-donating oxo, imido, or nitrido ligands. On the other hand, homoleptic examples are scarce. Herein, we investigate the redox chemistry of iron complexes of the dianonic tris-skatylmethylphosphonium (TSMP2-) scorpionate ligand. One-electron oxidation of the tetrahedral, bis-ligated [(TSMP)2FeII]2- leads to the octahedral [(TSMP)2FeIII]-. The latter undergoes thermal spin-cross-over both in the solid state and solution, which we characterize using superconducting quantum inference device (SQUID), Evans method, and paramagnetic nuclear magnetic resonance spectroscopy. Furthermore, [(TSMP)2FeIII]- can be reversibly oxidized to the stable high-valent [(TSMP)2FeIV]0 complex. We use a variety of electrochemical, spectroscopic, and computational techniques as well as SQUID magnetometry to establish a triplet (S = 1) ground state with a metal-centered oxidation and little spin delocalization on the ligand. The complex also has a fairly isotropic g-tensor (giso = 1.97) combined with a positive zero-field splitting (ZFS) parameter D (+19.1 cm-1) and very low rhombicity, in agreement with quantum chemical calculations. This thorough spectroscopic characterization contributes to a general understanding of octahedral Fe(IV) complexes.

Experimental assignment of long-range magnetic communication through Pd & Pt metallophilic contacts.

Record-breaking magnetic exchange interactions have previously been reported for 3d-metal dimers of the form [M(Pt(SAc)4)(pyNO2)]2 (M = Ni or Co) that are linked in the solid state via metallophilic Pt⋯Pt bridges. This contrasts the terminally capped monomers [M(Pt(SAc)4)(py)2], for which neither metallophilic bridges nor magnetic exchange interactions are found. Computational modeling has shown that the magnetic exchange interaction is facilitated by the pseudo-closed shell d8⋯d8 metallophilic interaction between the filled Pt2+ 5d z 2 orbitals. We present here inelastic neutron scattering experiments on these complexes, wherein the dimers present an oscillatory momentum-transfer-dependence of the magnetic transitions. This allows for the unequivocal experimental assignment of the distance between the coupled ions, which matches exactly the coupling pathway via the metallophilic bridges. Furthermore, we have synthesized and magnetically characterized the isostructural palladium-analogues. The magnetic coupling across the Pd⋯Pd bridge is found through SQUID-magnetometry and FD-FT THz-EPR spectroscopy to be much weaker than via the Pt⋯Pt bridge. The weaker coupling is traced to the larger radial extent of the 5d z 2 orbitals compared to that of the 4d z 2 orbitals. The existence of a palladium metallophilic interaction is evaluated computationally from potential surface cuts along the metal stretching direction. Similar behavior is found for the Pd⋯Pd and Pt⋯Pt-systems with clear minima along this coordinate and provide estimates for the force constant for this distortion. The estimated M⋯M stretching frequencies are found to match experimental observed, polarized bands in single-crystal Raman spectra close to 45 cm-1. This substantiates the existence of energetically relevant Pd⋯Pd metallophilic interactions. The unique properties of both Pt2+ and Pd2+ constitutes an orthogonal reactivity, which can be utilized for steering both the direction and strength of magnetic interactions.

Applying Unconventional Spectroscopies to the Single-Molecule Magnets, Co(PPh3 )2 X2 (X=Cl, Br, I): Unveiling Magnetic Transitions and Spin-Phonon Coupling.

Large separation of magnetic levels and slow relaxation in metal complexes are desirable properties of single-molecule magnets (SMMs). Spin-phonon coupling (interactions of magnetic levels with phonons) is ubiquitous, leading to magnetic relaxation and loss of memory in SMMs and quantum coherence in qubits. Direct observation of magnetic transitions and spin-phonon coupling in molecules is challenging. We have found that far-IR magnetic spectra (FIRMS) of Co(PPh3 )2 X2 (Co-X; X=Cl, Br, I) reveal rarely observed spin-phonon coupling as avoided crossings between magnetic and u-symmetry phonon transitions. Inelastic neutron scattering (INS) gives phonon spectra. Calculations using VASP and phonopy programs gave phonon symmetries and movies. Magnetic transitions among zero-field split (ZFS) levels of the S=3/2 electronic ground state were probed by INS, high-frequency and -field EPR (HFEPR), FIRMS, and frequency-domain FT terahertz EPR (FD-FT THz-EPR), giving magnetic excitation spectra and determining ZFS parameters (D, E) and g values. Ligand-field theory (LFT) was used to analyze earlier electronic absorption spectra and give calculated ZFS parameters matching those from the experiments. DFT calculations also gave spin densities in Co-X, showing that the larger Co(II) spin density in a molecule, the larger its ZFS magnitude. The current work reveals dynamics of magnetic and phonon excitations in SMMs. Studies of such couplings in the future would help to understand how spin-phonon coupling may lead to magnetic relaxation and develop guidance to control such coupling.

Spectroscopic Investigation of a Metal-Metal-Bonded Fe6 Single-Molecule Magnet with an Isolated S = 19/2 Giant-Spin Ground State.

The metal-metal-bonded molecule [Bu4N][(HL)2Fe6(dmf)2] (Fe6) was previously shown to possess a thermally isolated spin S = 19/2 ground state and found to exhibit slow magnetization relaxation below a blocking temperature of ∼5 K [J. Am. Chem. Soc. 2015, 137, 13949-13956]. Here, we present a comprehensive spectroscopic investigation of this unique single-molecule magnet (SMM), combining ultrawideband field-swept high-field electron paramagnetic resonance (EPR) with frequency-domain Fourier-transform terahertz EPR to accurately quantify the spin Hamiltonian parameters of Fe6. Of particular importance is the near absence of a 4th-order axial zero-field splitting term, which is known to arise because of quantum mechanical mixing of spin states on account of the relatively weak spin-spin (superexchange) interactions in traditional polynuclear SMMs such as the celebrated Mn12-acetate. The combined high-resolution measurements on both powder samples and an oriented single crystal provide a quantitative measure of the isolated nature of the spin ground state in the Fe6 molecule, as well as additional microscopic insights into factors that govern the quantum tunneling of its magnetization. This work suggests strategies for improving the performance of polynuclear SMMs featuring direct metal-metal bonds and strong ferromagnetic spin-spin (exchange) interactions.

A Synergy and Struggle of EPR, Magnetometry and NMR: A Case Study of Magnetic Interaction Parameters in a Six-Coordinate Cobalt(II) Complex.

Herein, we combine for the first time SQUID magnetometry, cw-EPR, THz-EPR, and paramagnetic NMR spectroscopies to study the magnetic properties of a high-spin cobalt(II) heteroscorpionate complex. Complementary information provided by these methods allowed precise determination of the magnetic interaction parameters, thereby removing the ambiguity inherit to single-method studies. We systematically investigate the extent to which information about the magnetic interaction parameters can be deduced from reduced data sets. The detailed study revealed significant different magnetic properties in solid state and solution. To further exploit the information content of the solution NMR experimental results, we introduce the new concept of reduced paramagnetic shift. It allows for the determination of the magnetic axes and, subsequently, full NMR signal assignment. It is shown that even in complicated cases, in which common NMR analytics (integral intensities, relaxation factors, etc.) fail, it yields robust results.

Advanced Paramagnetic Resonance Studies on Manganese and Iron Corroles with a Formal d4 Electron Count.

Metallocorroles wherein the metal ion is MnIII and formally FeIV are studied here using field- and frequency-domain electron paramagnetic resonance techniques. The MnIII corrole, Mn(tpfc) (tpfc = 5,10,15-tris(pentafluorophenyl)corrole trianion), exhibits the following S = 2 zero-field splitting (zfs) parameters: D = -2.67(1) cm-1, |E| = 0.023(5) cm-1. This result and those for other MnIII tetrapyrroles indicate that when D ≈ - 2.5 ± 0.5 cm-1 for 4- or 5-coordinate and D ≈ - 3.5 ± 0.5 cm-1 for 6-coordinate complexes, the ground state description is [MnIII(Cor3-)]0 or [MnIII(P2-)]+ (Cor = corrole, P = porphyrin). The situation for formally FeIV corroles is more complicated, and it has been shown that for Fe(Cor)X, when X = Ph (phenyl), the ground state is a spin triplet best described by [FeIV(Cor3-)]+, but when X = halide, the ground state corresponds to [FeIII(Cor•2-)]+, wherein an intermediate spin (S = 3/2) FeIII is antiferromagnetically coupled to a corrole radical dianion (S = 1/2) to also give an S = 1 ground state. These two valence isomers can be distinguished by their zfs parameters, as determined here for Fe(tpc)X, X = Ph, Cl (tpc = 5,10,15-triphenylcorrole trianion). The complex with axial phenyl gives D = 21.1(2) cm-1, while that with axial chloride gives D = 14.6(1) cm-1. The D value for Fe(tpc)Ph is in rough agreement with the range of values reported for other FeIV complexes. In contrast, the D value for Fe(tpc)Cl is inconsistent with an FeIV description and represents a different type of iron center. Computational studies corroborate the zfs for the two types of iron corrole complexes. Thus, the zfs of metallocorroles can be diagnostic as to the electronic structure of a formally high oxidation state metallocorrole, and by extension to metalloporphyrins, although such studies have yet to be performed.

Angular Momentum Flow During Ultrafast Demagnetization of a Ferrimagnet.

One of the key processes setting the speed of the ultrafast magnetization phenomena is the angular momentum transfer from and into the spin system. However, the way the angular momentum flows during ultrafast demagnetization and magnetization switching phenomena remains elusive so far. We report on time-resolved soft x-ray magnetic circular dichroism measurements of the ferrimagnetic GdFeCo alloy allowing us to record the dynamics of elemental spin and orbital moments at the Fe and Gd sites during femtosecond laser-induced demagnetization. We observe a complete transfer of spin and orbital angular momentum to the lattice during the first hundreds of femtoseconds of the demagnetization process.

Recent advances in ultrafast X-ray sources.

Over more than a century, X-rays have transformed our understanding of the fundamental structure of matter and have been an indispensable tool for chemistry, physics, biology, materials science and related fields. Recent advances in ultrafast X-ray sources operating in the femtosecond to attosecond regimes have opened an important new frontier in X-ray science. These advances now enable: (i) sensitive probing of structural dynamics in matter on the fundamental timescales of atomic motion, (ii) element-specific probing of electronic structure and charge dynamics on fundamental timescales of electronic motion, and (iii) powerful new approaches for unravelling the coupling between electronic and atomic structural dynamics that underpin the properties and function of matter. Most notable is the recent realization of X-ray free-electron lasers (XFELs) with numerous new XFEL facilities in operation or under development worldwide. Advances in XFELs are complemented by advances in synchrotron-based and table-top laser-plasma X-ray sources now operating in the femtosecond regime, and laser-based high-order harmonic XUV sources operating in the attosecond regime. This article is part of the theme issue 'Measurement of ultrafast electronic and structural dynamics with X-rays'.

Determination of Large Zero-Field Splitting in High-Spin Co(I) Clathrochelates.

Zero-field splitting (ZFS) of three high-spin Co(I) ( S = 1) clathrochelate complexes was determined by frequency-domain Fourier-transform THz-EPR (FD-FT THz-EPR). The following axial and rhombic ZFS values ( D and E, respectively) were determined: [N( n-Bu)4]CoI(GmCl2)3(BPh)2 (1, D/ hc = +16.43(1) cm-1, E/ hc = 0.0(1) cm-1), [P(Me2N)4]CoI(GmCl2)3(BPh)2 (2, D/ hc = +16.67(4) cm-1, E/ hc = 0.0(1) cm-1), and [P(C6H5)4]CoI(GmCl2)3(BPh)2 (3, D/ hc = +16.72(2) cm-1, E/ hc = 0.24(3) cm-1). Complementary susceptibility χ T measurements and quantum chemistry calculations on 1 revealed hard-axis-type magnetic anisotropy and allowed for a correlation of ZFS and the electronic structure. Increased rhombicity of 3 as compared to 1 and 2 was assigned to symmetry changes of the ligand structure induced by the change of the counterion. 1 and 3 exhibited temperature-dependent ZFS values. Possible reasons for this phenomenon, such as structural changes and weak chain-like intermolecular antiferromagnetic interactions, are discussed.

Ultrafast and Energy-Efficient Quenching of Spin Order: Antiferromagnetism Beats Ferromagnetism.

By comparing femtosecond laser pulse induced ferro- and antiferromagnetic dynamics in one and the same material-metallic dysprosium-we show both to behave fundamentally different. Antiferromagnetic order is considerably faster and much more efficiently reduced by optical excitation than its ferromagnetic counterpart. We assign the fast and extremely efficient process in the antiferromagnet to an interatomic transfer of angular momentum within the spin system. Our findings imply that this angular momentum transfer channel is effective in other magnetic metals with nonparallel spin alignment. They also point out a possible route towards energy-efficient spin manipulation for magnetic devices.

Recent progress in synchrotron-based frequency-domain Fourier-transform THz-EPR.

We describe frequency-domain Fourier-transform THz-EPR as a method to assign spin-coupling parameters of high-spin (S>1/2) systems with very large zero-field splittings. The instrumental foundations of synchrotron-based FD-FT THz-EPR are presented, alongside with a discussion of frequency-domain EPR simulation routines. The capabilities of this approach is demonstrated for selected mono- and multinuclear HS systems. Finally, we discuss remaining challenges and give an outlook on the future prospects of the technique.

Analysis of Magnetic Anisotropy and the Role of Magnetic Dilution in Triggering Single-Molecule Magnet (SMM) Behavior in a Family of CoII YIII Dinuclear Complexes with Easy-Plane Anisotropy.

Three new closely related CoII YIII complexes of general formula [Co(µ-L)(µ-X)Y(NO3 )2 ] (X- =NO3- 1, benzoate 2, or 9-anthracenecarboxylato 3) have been prepared with the compartmental ligand N,N',N''-trimethyl-N,N''-bis(2-hydroxy-3-methoxy-5-methylbenzyl)diethylenetriamine (H2 L). In these complexes, CoII and YIII are triply bridged by two phenoxide groups belonging to the di-deprotonated ligand (L2- ) and one ancillary anion X- . The change of the ancillary bridging group connecting CoII and YIII ions induces small differences in the trigonally distorted CoN3 O3 coordination sphere with a concomitant tuning of the magnetic anisotropy and intermolecular interactions. Direct current magnetic, high-frequency and -field EPR (HFEPR), frequency domain Fourier transform THz electron paramagnetic resonance (FD-FT THz-EPR) measurements, and ab initio theoretical calculations demonstrate that CoII ions in compounds 1-3 have large and positive D values (≈50 cm-1 ), which decrease with increasing the distortion of the pseudo-octahedral CoII coordination sphere. Dynamic ac magnetic susceptibility measurements indicate that compound 1 exhibits field-induced single-molecule magnet (SMM) behavior, whereas compounds 2 and 3 only display this behavior when they are magnetically diluted with diamagnetic ZnII (Zn/Co=10:1). In view of this, it is always advisable to use magnetically diluted complexes, in which intermolecular interactions and quantum tunneling of magnetism (QTM) would be at least partly suppressed, so that "hidden single-ion magnet (SIM)" behavior could emerge. Field- and temperature-dependence of the relaxation times indicate the prevalence of the Raman process in all these complexes above approximately 3 K.

Analysis of granular packing structure by scattering of THz radiation.

Scattering methods are widely used to characterize the structure and constituents of matter on small length scales. This motivates this introductory text on identifying prospective approaches to scattering-based methods for granular media. A survey to light scattering by particles and particle ensembles is given. It is elaborated why the established scattering methods using X-rays and visible light cannot in general be transferred to granular media. Spectroscopic measurements using terahertz radiation are highlighted as they probe the scattering properties of granular media, which are sensitive to the packing structure. Experimental details to optimize a spectrometer for measurements on granular media are discussed. We perform transmission measurements on static and agitated granular media using Fourier transform spectroscopy at the THz beamline of the Bessy II storage ring. The measurements demonstrate the potential to evaluate degrees of order in the media and to track transient structural states in agitated bulk granular media.

Spectroscopic and Computational Studies of Spin States of Iron(IV) Nitrido and Imido Complexes.

High-oxidation-state metal complexes with multiply bonded ligands are of great interest for both their reactivity as well as their fundamental bonding properties. This paper reports a combined spectroscopic and theoretical investigation into the effect of the apical multiply bonded ligand on the spin-state preferences of threefold symmetric iron(IV) complexes with tris(carbene) donor ligands. Specifically, singlet (S = 0) nitrido [{PhB(ImR)3}FeN], R = tBu (1), Mes (mesityl, 2) and the related triplet (S = 1) imido complexes, [{PhB(ImR)3}Fe(NR')]+, R = Mes, R' = 1-adamantyl (3), tBu (4), were investigated by electronic absorption and Mössbauer effect spectroscopies. For comparison, two other Fe(IV) nitrido complexes, [(TIMENAr)FeN]+ (TIMENAr = tris[2-(3-aryl-imidazol-2-ylidene)ethyl]amine; Ar = Xyl (xylyl), Mes), were investigated by 57Fe Mössbauer spectroscopy, including applied-field measurements. The paramagnetic imido complexes 3 and 4 were also studied by magnetic susceptibility measurements (for 3) and paramagnetic resonance spectroscopy: high-frequency and -field electron paramagnetic resonance (for 3 and 4) and frequency-domain Fourier-transform (FD-FT) terahertz electron paramagnetic resonance (for 3), which reveal their zero-field splitting parameters. Experimentally correlated theoretical studies comprising ligand-field theory and quantum chemical theory, the latter including both density functional theory and ab initio methods, reveal the key role played by the Fe 3dz2 (a1) orbital in these systems: the nature of its interaction with the nitrido or imido ligand dictates the spin-state preference of the complex. The ability to tune the spin state through the energy and nature of a single orbital has general relevance to the factors controlling spin states in complexes with applicability as single molecule devices.

Versatile soft X-ray-optical cross-correlator for ultrafast applications.

We present an X-ray-optical cross-correlator for the soft ([Formula: see text]) up to the hard X-ray regime based on a molybdenum-silicon superlattice. The cross-correlation is done by probing intensity and position changes of superlattice Bragg peaks caused by photoexcitation of coherent phonons. This approach is applicable for a wide range of X-ray photon energies as well as for a broad range of excitation wavelengths and requires no external fields or changes of temperature. Moreover, the cross-correlator can be employed on a 10 ps or 100 fs time scale featuring up to 50% total X-ray reflectivity and transient signal changes of more than 20%.

Multifaceted magnetization dynamics in the mononuclear complex [ReIVCl4(CN)2]2.

The mononuclear complex (Bu4N)2[ReIVCl4(CN)2]·2DMA (DMA = N,N-dimethylacetamide) displays intricate magnetization dynamics, implying Orbach, direct, and Raman-type relaxation processes. The Orbach relaxation process is characterized by an energy barrier of 39 K (27 cm-1) that is discussed based on high-field electron paramagnetic resonance (EPR), inelastic neutron scattering and frequency-domain THz EPR investigations.

Analysis of the halo background in femtosecond slicing experiments.

The slicing facility FemtoSpeX at BESSY II offers unique opportunities to study photo-induced dynamics on femtosecond time scales by means of X-ray magnetic circular dichroism, resonant and non-resonant X-ray diffraction, and X-ray absorption spectroscopy experiments in the soft X-ray regime. Besides femtosecond X-ray pulses, slicing sources inherently also produce a so-called `halo' background with a different time structure, polarization and pointing. Here a detailed experimental characterization of the halo radiation is presented, and a method is demonstrated for its correct and unambiguous removal from femtosecond time-resolved data using a special laser triggering scheme as well as analytical models. Examples are given for time-resolved measurements with corresponding halo correction, and errors of the relevant physical quantities caused by either neglecting or by applying a simplified model to describe this background are estimated.

Cyanide Single-Molecule Magnets Exhibiting Solvent Dependent Reversible "On" and "Off" Exchange Bias Behavior.

The syntheses, structures, and magnetic properties of four new complex salts, (PPN){[Mn(III)(salphen)(MeOH)]2[M(III)(CN)6]}·7MeOH (Mn2M·7MeOH) (M = Fe, Ru, Os and Co; PPN(+) = bis(triphenylphosphoranylidene)ammonium cation; H2salphen = N,N'-bis(salicylidene)-1,2-diaminobenzene), and a mixed metal Co/Os analogue (PPN){[Mn(III)(salphen)(MeOH)]2[Co(III)0.92Os(III)0.08(CN)6]}·7MeOH were undertaken. It was found that all compounds exhibit switchable single-molecule magnet (SMM) and exchange-bias behavior depending on the interstitial methanol content. The pristine (PPN){[Mn(salphen)(MeOH)]2[Os(CN)6]}·7MeOH (Mn2Os·7MeOH) behaves as an SMM with an effective barrier for the magnetization reversal, (Ueff/kB), of 17.1 K. Upon desolvation, Mn2Os exhibits an increase of Ueff/kB to 42.0 K and an opening of the hysteresis loop observable at 1.8 K. Mn2Os·7MeOH shows also exchange-bias behavior with magnetic hysteresis loops exhibiting a shift in the quantum tunneling to 0.25 T from zero-field. The Fe(III) and Ru(III) analogues were prepared as reference compounds for assessing the effect of the 5d versus 4d and 3d metal ions on the SMM properties. These compounds are also SMMs and exhibit similar effects but with lower energy barriers. These findings underscore the importance of introducing heavy transition elements into SMMs to improve their slow relaxation of the magnetization properties. The (PPN){[Mn(III)(salphen)(MeOH)]2[Co(III)(CN)6]}·7MeOH (Mn2Co·7MeOH) analogue with a diamagnetic Co(III) central atom and the mixed Co/Os (PPN){[Mn(III)(salphen)(MeOH)]2[Co(III)0.92Os(III)0.08(CN)6]}·7MeOH (Mn2Co/Os·7MeOH) "magnetically diluted" system with a 9:1 Co/Os metal ratio were prepared in order to further probe the nature of the energy barrier increase upon desolvation of Mn2Os. In addition, inelastic neutron scattering and frequency-domain Fourier-transform THz electron paramagnetic resonance spectra obtained on Mn2Os·7MeOH and Mn2Os in combination with the magnetic data revealed the presence of anisotropic exchange interactions between Mn(III) and Os(III) ions.

Thermal Treatment of Chromium(III) Oxide with Carbonates Analyzed by Far-Infrared Spectroscopy.

The chemical state of thermochemically treated chromium(III) oxide (Cr2O3) with various carbonates was analyzed by far-infrared (far-IR) spectroscopy (spectral region 700-25 cm(-1)). Non-toxic Cr2O3 was oxidized with potassium, sodium, and calcium carbonate, respectively, to toxic Cr(VI) and Cr(V) compounds during thermal treatment at 1000 °C. In reverse, thermochemical treatment of Cr2O3 with magnesium carbonate lead to the formation of the Cr(III) compound MgCr2O4. Higher temperatures (>1200 °C) or reducing atmospheric conditions prevent the formation of Cr(VI)/Cr(V) compounds, too. Additionally, it was found that polyethylene powder with a low particle size (<70 µm) is favorable for the collection of good far-IR spectra of inorganic powders.