Multiple Bistability in Quinonoid-Bridged Diiron(II) Complexes: Influence of Bridge Symmetry on Bistable Properties

Quinonoid bridges are well-suited for generating dinuclear assemblies that might display various bistable properties. In this contribution we present two diiron­(II) complexes where the iron­(II) centers are either bridged by the doubly deprotonated form of a symmetrically substituted quinonoid bridge, 2,5-bis­[4-(isopropyl)­anilino]-1,4-benzoquinone (<b>H</b>_<b>2</b><b>L2′</b>) with a [O,N,O,N] donor set, or with the doubly deprotonated form of an unsymmetrically substituted quinonoid bridge, 2-[4-(isopropyl)­anilino]-5-hydroxy-1,4-benzoquinone (<b>H</b>_<b>2</b><b>L5′</b>) with a [O,O,O,N] donor set. Both complexes display temperature-induced spin crossover (SCO). The nature of the SCO is strongly dependent on the bridging ligand, with only the complex with the [O,O,O,N] donor set displaying a prominent hysteresis loop of about 55 K. Importantly, only the latter complex also shows a pronounced light-induced spin state change. Furthermore, both complexes can be oxidized to the mixed-valent iron­(II)–iron­(III) form, and the nature of the bridge determines the Robin and Day classification of these forms. Both complexes have been probed by a battery of electrochemical, spectroscopic, and magnetic methods, and this combined approach is used to shed light on the electronic structures of the complexes and on bistability. The results presented here thus show the potential of using the relatively new class of unsymmetrically substituted bridging quinonoid ligands for generating intriguing bistable properties and for performing site-specific magnetic switching

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

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 μ_1,1-azido-bridged dicobalt­(II) complex. The more rigid phenyl substituents on the TPTA ligand deliver an unsupported and solely μ_1,3-azido-bridged dicobalt­(II) complex. Bulky diiso­propyl­phenyl substituents on the TDTA ligand deliver a doubly μ_1,1-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(μ_1,1-N₃)­Cu­(TBTA)]³⁺ or [(TBTA)­Cu­(μ_1,1-N₃)­Cu­(TPTA)]³⁺, both of which contain a single unsupported μ_1,1-N₃ as a bridge. To the best of our knowledge, these are also the first examples of mixed dinuclear complexes with a μ_1,1-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