Coordination complexes that possess large magnetic anisotropy (otherwise known as zero-field splitting, ZFS) have possible applications in the field of magnetic materials, including single molecule magnets (SMMs). Previous studies have explored the role of coordination number and geometry in controlling the magnetic anisotropy and SMM behavior of high-spin (S = 3/2) Co(II) complexes. Building upon these efforts, the present work examines the impact of ligand oxidation state and structural distortions on the spin states and ZFS parameters of pentacoordinate Co(II) complexes. The five complexes included in this study (1−5) have the general formula, [Co(Tp Ph2 )(L X,Y )] n+ (X = O, S; Y = N, O; n = 0 or 1), where Tp Ph2 is the scorpionate ligand hydrotris(3,5-diphenyl-pyrazolyl)borate(1−) and L X,Y are bidentate dioxolenetype ligands that can access multiple oxidation states. The specific L X,Y ligands used herein are 4,6-di-tert-butyl substituted oaminophenolate and o-aminothiophenolate (1 and 2, respectively), o-iminosemiquinonate and o-semiquinonate radicals (3 and 4, respectively), and o-iminobenzoquinone (5). Each complex exhibits a distorted trigonal bipyramidal geometry, as revealed by singlecrystal X-ray diffraction. Direct current (dc) magnetic susceptibility experiments confirmed that the complexes with closed-shell ligands (1, 2, and 5) possess S = 3/2 ground states with negative D-values (easy-axis anisotropy) of −41, −78, and −30 cm −1 , respectively. For 3 and 4, antiferromagnetic coupling between the Co(II) center and o-(imino)semiquinonate radical ligand results in S = 1 ground states that likewise exhibit very large and negative anisotropy (−100 > D > −140 cm −1 ). Notably, ZFS was measured directly for each complex using far-infrared magnetic spectroscopy (FIRMS). In combination with high-frequency and -field electron paramagnetic resonance (HFEPR) studies, these techniques provided precise spin-Hamiltonian parameters for complexes 1, 2, and 5. Multireference ab initio calculations, using the CASSCF/NEVPT2 approach, indicate that the strongly negative anisotropies of these Co(II) complexes arise primarily from distortions in the equatorial plane due to constrictions imposed by the Tp Ph2 ligand. This effect is further amplified by cobalt(II)-radical exchange interactions in 3 and 4.
Zeolitic imidazolate frameworks (ZIFs) with open-shell transition metal nodes represent a promising class of highly ordered light harvesting antennas for photoenergy applications. However, their charge transport properties within the framework, the key criterion to achieve efficient photoenergy conversion, are not yet explored. Herein, we report the first direct evidence of a charge transport pathway through node-to-node communication in both ground state and excited state ZIFs using the combination of paramagnetic susceptibility measurements and time-resolved optical and X-ray absorption spectroscopy. These findings provide unprecedented new insights into the photoactivity and charge transport nature of ZIF frameworks, paving the way for their novel application as light harvesting arrays in diverse photoenergy conversion devices.
Large Spin-Orbit Coupling (SOC) is an intrinsic property of the heavy-elements that directly affects the electronic structures of the compounds. Herein we report the synthesis and characterization of a mono-coordinate bismuthinidene featuring a rigid and bulky ligand. All magnetic measurements (SQUID, NMR) point to a diamagnetic compound. However, multiconfigurational quantum chemical calculations predict the ground state of the compound to be dominated (76%) by a spin-triplet. The apparent diamagnetism is explained by an extremely large SOC induced positive zero-field-splitting of more than 4500 cm −1 that leaves the M S = 0 magnetic sublevel thermally isolated in the electronic ground state.
Iron-based extended metal atom chains (EMACs) are potentially high-spin molecules with axial magnetic anisotropy and thus candidate single-molecule magnets (SMMs). We herein compare the tetrairon(II), halide-capped complexes [Fe4(tpda)3Cl2] (1Cl) and...
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