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1. Semiquinone radical-bridged M ₂ (M = Fe, Co, Ni) complexes with strong magnetic exchange giving rise to slow magnetic relaxation

   https://doi.org/10.1039/d0sc03078c  

   Chakarawet, Khetpakorn ; Harris, T. David ; Long, Jeffrey R. ( August 2020 , Chemical Science)

   The use of radical bridging ligands to facilitate strong magnetic exchange between paramagnetic metal centers represents a key step toward the realization of single-molecule magnets with high operating temperatures. Moreover, bridging ligands that allow the incorporation of high-anisotropy metal ions are particularly advantageous. Toward these ends, we report the synthesis and detailed characterization of the dinuclear hydroquinone-bridged complexes [(Me 6 tren) 2 MII2(C 6 H 4 O 2 2− )] 2+ (Me 6 tren = tris(2-dimethylaminoethyl)amine; M = Fe, Co, Ni) and their one-electron-oxidized, semiquinone-bridged analogues [(Me 6 tren) 2 MII2(C 6 H 4 O 2 − ˙)] 3+ . Single-crystal X-ray diffraction shows that the Me 6 tren ligand restrains the metal centers in a trigonal bipyramidal geometry, and coordination of the bridging hydro- or semiquinone ligand results in a parallel alignment of the three-fold axes. We quantify the p -benzosemiquinone–transition metal magnetic exchange coupling for the first time and find that the nickel( ii ) complex exhibits a substantial J < −600 cm −1 , resulting in a well-isolated S = 3/2 ground state even as high as 300 K. The iron and cobalt complexes feature metal–semiquinone exchange constants of J = −144(1) and −252(2) cm −1 ,more »respectively, which are substantially larger in magnitude than those reported for related bis(bidentate) semiquinoid complexes. Finally, the semiquinone-bridged cobalt and nickel complexes exhibit field-induced slow magnetic relaxation, with relaxation barriers of U eff = 22 and 46 cm −1 , respectively. Remarkably, the Orbach relaxation observed for the Ni complex is in stark contrast to the fast processes that dominate relaxation in related mononuclear Ni II complexes, thus demonstrating that strong magnetic coupling can engender slow magnetic relaxation.« less
2. Actinide arene-metalates: ion pairing effects on the electronic structure of unsupported uranium–arenide sandwich complexes

   https://doi.org/10.1039/D1SC03275E  

   Murillo, Jesse ; Bhowmick, Rina ; Harriman, Katie L. ; Gomez-Torres, Alejandra ; Wright, Joshua ; Meulenberg, Robert W. ; Miró, Pere ; Metta-Magaña, Alejandro ; Murugesu, Muralee ; Vlaisavljevich, Bess ; et al ( October 2021 , Chemical Science)

   Addition of [UI 2 (THF) 3 (μ-OMe)] 2 ·THF (2·THF) to THF solutions containing 6 equiv. of K[C 14 H 10 ] generates the heteroleptic dimeric complexes [K(18-crown-6)(THF) 2 ] 2 [U(η 6 -C 14 H 10 )(η 4 -C 14 H 10 )(μ-OMe)] 2 ·4THF (118C6·4THF) and {[K(THF) 3 ][U(η 6 -C 14 H 10 )(η 4 -C 14 H 10 )(μ-OMe)]} 2 (1THF) upon crystallization of the products in THF in the presence or absence of 18-crown-6, respectively. Both 118C6·4THF and 1THF are thermally stable in the solid-state at room temperature; however, after crystallization, they become insoluble in THF or DME solutions and instead gradually decompose upon standing. X-ray diffraction analysis reveals 118C6·4THF and 1THF to be structurally similar, possessing uranium centres sandwiched between bent anthracenide ligands of mixed tetrahapto and hexahapto ligation modes. Yet, the two complexes are distinguished by the close contact potassium-arenide ion pairing that is seen in 1THF but absent in 118C6·4THF, which is observed to have a significant effect on the electronic characteristics of the two complexes. Structural analysis, SQUID magnetometry data, XANES spectral characterization, and computational analyses are generally consistent with U( iv ) formal assignments for the metal centres in bothmore »118C6·4THF and 1THF, though noticeable differences are detected between the two species. For instance, the effective magnetic moment of 1THF (3.74 μ B ) is significantly lower than that of 118C6·4THF (4.40 μ B ) at 300 K. Furthermore, the XANES data shows the U L III -edge absorption energy for 1THF to be 0.9 eV higher than that of 118C6·4THF, suggestive of more oxidized metal centres in the former. Of note, CASSCF calculations on the model complex {[U(η 6 -C 14 H 10 )(η 4 -C 14 H 10 )(μ-OMe)] 2 } 2− (1*) shows highly polarized uranium–arenide interactions defined by π-type bonds where the metal contributions are primarily comprised by the 6d-orbitals (7.3 ± 0.6%) with minor participation from the 5f-orbitals (1.5 ± 0.5%). These unique complexes provide new insights into actinide–arenide bonding interactions and show the sensitivity of the electronic structures of the uranium atoms to coordination sphere effects.« less
3. High-temperature magnetic blocking and magneto-structural correlations in a series of dysprosium( iii ) metallocenium single-molecule magnets

   https://doi.org/10.1039/c8sc03907k  

   Randall McClain, K. ; Gould, Colin A. ; Chakarawet, Khetpakorn ; Teat, Simon J. ; Groshens, Thomas J. ; Long, Jeffrey R. ; Harvey, Benjamin G. ( November 2018 , Chemical Science)

   A series of dysprosium( iii ) metallocenium salts, [Dy(Cp iPr4R ) 2 ][B(C 6 F 5 ) 4 ] (R = H ( 1 ), Me ( 2 ), Et ( 3 ), iPr ( 4 )), was synthesized by reaction of DyI 3 with the corresponding known NaCp iPr4R (R = H, iPr) and novel NaCp iPr4R (R = Me, Et) salts at high temperature, followed by iodide abstraction with [H(SiEt 3 ) 2 ][B(C 6 F 5 ) 4 ]. Variation of the substituents in this series results in substantial changes in molecular structure, with more sterically-encumbering cyclopentadienyl ligands promoting longer Dy–C distances and larger Cp–Dy–Cp angles. Dc and ac magnetic susceptibility data reveal that these structural changes have a considerable impact on the magnetic relaxation behavior and operating temperature of each compound. In particular, the magnetic relaxation barrier increases as the Dy–C distance decreases and the Cp–Dy–Cp angle increases. An overall 45 K increase in the magnetic blocking temperature is observed across the series, with compounds 2–4 exhibiting the highest 100 s blocking temperatures yet reported for a single-molecule magnet. Compound 2 possesses the highest operating temperature of the series with a 100 s blocking temperature ofmore »62 K. Concomitant increases in the effective relaxation barrier and the maximum magnetic hysteresis temperature are observed, with 2 displaying a barrier of 1468 cm −1 and open magnetic hysteresis as high as 72 K at a sweep rate of 3.1 mT s −1 . Magneto-structural correlations are discussed with the goal of guiding the synthesis of future high operating temperature Dy III metallocenium single-molecule magnets.« less
4. Tetrairon( ii ) extended metal atom chains as single-molecule magnets

   https://doi.org/10.1039/d1dt01007g  

   Nicolini, Alessio ; Affronte, Marco ; SantaLucia, Daniel J. ; Borsari, Marco ; Cahier, Benjamin ; Caleffi, Matteo ; Ranieri, Antonio ; Berry, John F. ; Cornia, Andrea ( June 2021 , Dalton Transactions)

   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 [Fe 4 (tpda) 3 Cl 2 ] ( 1Cl ) and [Fe 4 (tpda) 3 Br 2 ] ( 1Br ), obtained by reacting iron( ii ) dihalides with [Fe 2 (Mes) 4 ] and N 2 , N 6 -di(pyridin-2-yl)pyridine-2,6-diamine (H 2 tpda) in toluene, under strictly anhydrous and anaerobic conditions (HMes = mesitylene). Detailed structural, electrochemical and Mössbauer data are presented along with direct-current (DC) and alternating-current (AC) magnetic characterizations. DC measurements revealed similar static magnetic properties for the two derivatives, with χ M T at room temperature above that for independent spin carriers, but much lower at low temperature. The electronic structure of the iron( ii ) ions in each derivative was explored by ab initio (CASSCF-NEVPT2-SO) calculations, which showed that the main magnetic axis of all metals is directed close to the axis of the chain. The outer metals, Fe1 and Fe4, have an easy-axis magnetic anisotropy ( D = −11 to −19 cm −1 , | E / D | = 0.05–0.18), while the internal metals,more »Fe2 and Fe3, possess weaker hard-axis anisotropy ( D = 8–10 cm −1 , | E / D | = 0.06–0.21). These single-ion parameters were held constant in the fitting of DC magnetic data, which revealed ferromagnetic Fe1–Fe2 and Fe3–Fe4 interactions and antiferromagnetic Fe2–Fe3 coupling. The competition between super-exchange interactions and the large, noncollinear anisotropies at metal sites results in a weakly magnetic non-Kramers doublet ground state. This explains the SMM behavior displayed by both derivatives in the AC susceptibility data, with slow magnetic relaxation in 1Br being observable even in zero static field.« less
5. [Ni ₃₀ S ₁₆ (PEt ₃ ) ₁₁ ]: an open-shell nickel sulfide nanocluster with a “metal-like” core

   https://doi.org/10.1039/D2SC00960A  

   Touchton, Alexander J. ; Wu, Guang ; Hayton, Trevor W. ( May 2022 , Chemical Science)

   Reaction of [Ni(1,5-cod) 2 ] (30 equiv.) with PEt 3 (46 equiv.) and S 8 (1.9 equiv.) in toluene, followed by heating at 115 °C for 16 h, results in the formation of the atomically precise nanocluster (APNC), [Ni 30 S 16 (PEt 3 ) 11 ] (1), in 14% isolated yield. Complex 1 represents the largest open-shell Ni APNC yet isolated. In the solid state, 1 features a compact “metal-like” core indicative of a high degree of Ni–Ni bonding. Additionally, SQUID magnetometry suggests that 1 possesses a manifold of closely-spaced electronic states near the HOMO–LUMO gap. In situ monitoring by ESI-MS and 31 P{ 1 H} NMR spectroscopy reveal that 1 forms via the intermediacy of smaller APNCs, including [Ni 8 S 5 (PEt 3 ) 7 ] and [Ni 26 S 14 (PEt 3 ) 10 ] (2). The latter APNC was also characterized by X-ray crystallography and features a nearly identical core structure to that found in 1. This work demonstrates that large APNCs with a high degree of metal–metal bonding are isolable for nickel, and not just the noble metals.