Raphael Marx

Direct measurement of dysprosium(III)···dysprosium(III) interactions in a single-molecule magnet.

Lanthanide compounds show much higher energy barriers to magnetic relaxation than 3d-block compounds, and this has led to speculation that they could be used in molecular spintronic devices. Prototype molecular spin valves and molecular transistors have been reported, with remarkable experiments showing the influence of nuclear hyperfine coupling on transport properties. Modelling magnetic data measured on lanthanides is always complicated due to the strong spin-orbit coupling and subtle crystal field effects observed for the 4f-ions; this problem becomes still more challenging when interactions between lanthanide ions are also important. Such interactions have been shown to hinder and enhance magnetic relaxation in different examples, hence understanding their nature is vital. Here we are able to measure directly the interaction between two dysprosium(III) ions through multi-frequency electron paramagnetic resonance spectroscopy and other techniques, and explain how this influences the dynamic magnetic behaviour of the system.

Comprehensive Spectroscopic Determination of the Crystal Field Splitting in an Erbium Single-Ion Magnet

The electronic structure of a novel lanthanide-based single-ion magnet, {C(NH2)3}5[Er(CO3)4]·11H2O, was comprehensively studied by means of a large number of different spectroscopic techniques, including far-infrared, optical, and magnetic resonance spectroscopies. A thorough analysis, based on crystal field theory, allowed an unambiguous determination of all relevant free ion and crystal field parameters. We show that inclusion of methods sensitive to the nature of the lowest-energy states is essential to arrive at a correct description of the states that are most relevant for the static and dynamic magnetic properties. The spectroscopic investigations also allowed for a full understanding of the magnetic relaxation processes occurring in this system. Thus, the importance of spectroscopic studies for the improvement of single-molecule magnets is underlined.

Spectroscopic determination of crystal field splittings in lanthanide double deckers

We have investigated the crystal field splitting in the archetypal lanthanide-based single-ion magnets and related complexes (NBu4)+[LnPc2]−·2dmf (Ln = Dy, Ho, Er; dmf = N,N-dimethylformamide) by means of far infrared and inelastic neutron scattering spectroscopies. In each case, we have found several features corresponding to direct crystal field transitions within the ground multiplet. The observation of three independent peaks in the holmium derivative enabled us to derive crystal field splitting parameters. In addition, we have carried out CASSCF calculations. We show that exploiting the interplay of CASSCF calculation (for the composition of the states) and advanced spectroscopic measurements (for accurate determination of the energies) is a very powerful approach to gain insight into the electronic structure of lanthanide-based single-molecule magnets.

Redox-Induced Spin-State Switching and Mixed Valency in Quinonoid-Bridged Dicobalt Complexes

The complexes [{(tmpa)Co(II) }2 (μ-L(1) )(2-) ](2+) (1(2+) ) and [{(tmpa)Co(II) }2 (μ-L(2) )(2-) ](2+) (2(2+) ), with tmpa=tris(2-pyridylmethyl)amine, H2 L(1) =2,5-di-[2-(methoxy)-anilino]-1,4-benzoquinone, and H2 L(2) =2,5-di-[2-(trifluoromethyl)-anilino]-1,4-benzoquinone, were synthesized and characterized. Structural analysis of 2(2+) revealed a distorted octahedral coordination around the cobalt centers, and cobalt-ligand bond lengths that match with high-spin Co(II) centers. Superconducting quantum interference device (SQUID) magnetometric studies on 1(2+) and 2(2+) are consistent with the presence of two weakly exchange-coupled high-spin cobalt(II) ions, for which the nature of the coupling appears to depend on the substituents on the bridging ligand, being antiferromagnetic for 1(2+) and ferromagnetic for 2(2+) . Both complexes exhibit several one-electron redox steps, and these were investigated with cyclic voltammetry and UV/Vis/near-IR spectroelectrochemistry. For 1(2+) , it was possible to chemically isolate the pure forms of both the one-electron oxidized mixed-valent 1(3+) and the two-electron oxidized isovalent 1(4+) forms, and characterize them structurally as well as magnetically. This series thus provided an opportunity to investigate the effect of reversible electron transfers on the total spin-state of the molecule. In contrast to 2(2+) , for 1(4+) the metal-ligand distances and the distances within the quinonoid ligand point to the existence of two low-spin Co(III) centers, thus showing the innocence of the quintessential non-innocent ligands L. Magnetic data corroborate these observations by showing the decrease of the magnetic moment by roughly half (neglecting spin exchange effects) on oxidizing the molecules with one electron, and the disappearance of a paramagnetic response upon two-electron oxidation, which confirms the change in spin state associated with the electron-transfer steps.

The solvent effect in an axially symmetric Fe-4(III) single-molecule magnet

A pair of enantiopure Fe(III)4 SMMs with axial symmetry was synthesized and characterized by magnetization and high-frequency electron paramagnetic resonance methods. The results reveal that the axial symmetry of the structure is broken by the interaction of Fe(III)4 with the disordered solvent molecules.

A Dicobalt Complex with an Unsymmetrical Quinonoid Bridge Isolated in Three Units of Charge: A Combined Structural, (Spectro)electrochemical, Magnetic and Spectroscopic Study.

Quinonoid ligands are excellent bridges for generating redox-rich dinuclear assemblies. A large majority of these bridges are symmetrically substituted, with examples of unsymmetrically substituted quinonoid bridges being extremely rare. We present here a dicobalt complex in its various redox states with an unsymmetrically substituted quinonoid bridging ligand. Two homovalent forms and one mixed-valent form have been isolated and characterized by single crystal X-ray diffraction. The complex displays a large comproportionation constant for the mixed-valent state which is three orders of magnitude higher than that observed for the analogous complex with a symmetrically substituted bridge. Results from electrochemistry, UV/Vis/NIR spectroelectrochemistry, SQUID magnetometry, multi-frequency EPR spectroscopy and FIR spectroscopy are used to probe the electronic structures of these complexes. FIR provides direct evidence of exchange coupling. The results presented here display the advantages of using an unsymmetrically substituted bridge: site specific redox chemistry, high thermodynamic stabilization of the mixed-valent form, isolation and crystallization of various redox forms of the complex. This work represents an important step on the way to generating heterodinuclear complexes for use in cooperative catalysis.

Measurement of Magnetic Exchange in Asymmetric Lanthanide Dimetallics: Toward a Transferable Theoretical Framework

Magnetic exchange interactions within the asymmetric dimetallic compounds [hqH2][Ln2(hq)4(NO3)3]·MeOH, (Ln = Er(III) and Yb(III), hqH = 8-hydroxyquinoline) have been directly probed with EPR spectroscopy and accurately modeled by spin Hamiltonian techniques. Exploitation of site selectivity via doping experiments in Y(III) and Lu(III) matrices yields simple EPR spectra corresponding to isolated Kramers doublets, allowing determination of the local magnetic properties of the individual sites within the dimetallic compounds. CASSCF-SO calculations and INS and far-IR measurements are all employed to further support the identification and modeling of the local electronic structure for each site. EPR spectra of the pure dimetallic compounds are highly featured and correspond to transitions within the lowest-lying exchange-coupled manifold, permitting determination of the highly anisotropic magnetic exchange between the lanthanide ions. We find a unique orientation for the exchange interaction, corresponding to a common elongated oxygen bridge for both isostructural analogs. This suggests a microscopic physical connection to the magnetic superexchange. These results are of fundamental importance for building and validating model microscopic Hamiltonians to understand the origins of magnetic interactions between lanthanides and how they may be controlled with chemistry.

Bimetallic MnIII–FeII hybrid complexes formed by a functionalized MnIII Anderson polyoxometalate coordinated to FeII: observation of a field-induced slow relaxation of magnetization in the MnIII centres and a photoinduced spin-crossover in the FeII centres

The synthesis and crystal structure of an Anderson POM functionalized with two 2,6-di(pyrazol-1-yl)-pyridine (1-bpp) ligands are reported (compound 1). High-frequency electron paramagnetic resonance (HF-EPR) and magnetic measurements show that it presents a significant negative axial zero-field splitting and field-induced slow relaxation of magnetization due to the presence of isolated MnIII anisotropic magnetic ions. Complexation of 1 with FeII gives rise to a 2D cationic network formed by Anderson POMs coordinated to two FeII ions through the two tridentate 1-bpp ligands and to other two FeII ions through two oxo ligands in compound 2, and to an anionic polymeric network formed by Anderson POMs coordinated through the 1-bpp ligands to two FeII, which are coordinated to two 1-bpp ligands from two neighbouring POMs, in compound 3. The crystal structure of 2 has been solved. Magnetic properties show that the FeII atoms of 3 remain in the low-spin state, while those of 2 remain in the high-spin state due to coordination to oxygen atoms from a neighbouring POM and dimethylformamide and water solvent molecules. Irradiation of 3 at 10 K with green light induces a spin-crossover (LIESST effect) with a small but significant photoconversion (∼8%). Finally, AC susceptibility measurements of 2, 3 and (C16H36N)3[MnMo6O18{(OCH2)3CNH2}2] (4) confirm field-induced slow relaxation of magnetization of MnIII Anderson POMs.

Quantum coherence in a triangular Cu3 complex

We report pulsed electron paramagnetic resonance investigations on [(CuL)3(OH)](ClO4)2 · H2O, where HL = (E)-2-((3-(methylamino)propylimino) methyl)phenol, to assess its suitability as a qubit. We measured spin-lattice relaxation times up to T1 = 698(2) μs at 5 K in methanolic frozen solution. The spin-lattice relaxation is driven by the direct process. The phase memory time was determined to be up to Tm = 1290(3) ns in methanolic frozen solution and Tm = 392(5) ns in powder at 5 K, respectively. By means of both continuous waves and pulsed ENDOR spectroscopy, interactions to protons, nitrogen and copper nuclei were observed. The couplings to the protons were shown to be due to dipolar and through bond interactions.

Control of Complex Formation through Peripheral Substituents in Click-Tripodal Ligands: Structural Diversity in Homo- and Heterodinuclear Cobalt-Azido Complexes

The azide anion is widely used as a ligand in coordination chemistry. Despite its ubiquitous presence, controlled synthesis of azido complexes remains a challenging task. Making use of click-derived tripodal ligands, we present here various coordination motifs of the azido ligands, the formation of which appears to be controlled by the peripheral substituents on the tripodal ligands with otherwise identical structure of the coordination moieties. Thus, the flexible benzyl substituents on the tripodal ligand TBTA led to the formation of the first example of an unsupported and solely μ1,1-azido-bridged dicobalt(II) complex. The more rigid phenyl substituents on the TPTA ligand deliver an unsupported and solely μ1,3-azido-bridged dicobalt(II) complex. Bulky diisopropylphenyl substituents on the TDTA ligand deliver a doubly μ1,1-azido-bridged dicobalt(II) complex. Intriguingly, the mononuclear copper(II) complex [Cu(TBTA)N3]+ is an excellent synthon for generating mixed dinuclear complexes of the form [(TBTA)Co(μ1,1-N3)Cu(TBTA)]3+ or [(TBTA)Cu(μ1,1-N3)Cu(TPTA)]3+, both of which contain a single unsupported μ1,1-N3 as a bridge. To the best of our knowledge, these are also the first examples of mixed dinuclear complexes with a μ1,1-N3 monoazido bridge. All complexes were crystallographically characterized, and selected examples were probed via magnetometry and high-field EPR spectroscopy to elucidate the electronic structures of these complexes and the nature of magnetic coupling in the various azido-bridged complexes. These results thus prove the power of click-tripodal ligands in generating hitherto unknown chemical structures and properties.