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1. 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)

   null (Ed.)

   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, 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. 

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2. Magnetic properties of two Gd ^III Fe ^III ₄ metallacrowns and strategies for optimizing the magnetocaloric effect of this topology

   https://doi.org/10.1039/D1QI00207D  

   Salerno, Elvin V.; Kampf, Jeff W.; Pecoraro, Vincent L.; Mallah, Talal (May 2021, Inorganic Chemistry Frontiers)

   Two Gd III Fe III 4 metallacrown complexes are presented and analyzed for their magnetic properties. One of these species is newly identified and exhibits a bent ring geometry as opposed to the more conventional flatter conformation of the other. Both complexes are quite similar magnetically, exhibiting antiferromagnetic exchange coupling values ca . J (Fe III –N–O–Fe III ) = −7 cm −1 and J (Gd III –O–Fe III ) = −0.7 cm −1 . When analyzed for the molecular magnetocaloric effect, maximum −Δ S m values of 7.3 J K −1 kg −1 at 3 K and at 6.1 J K −1 kg −1 at 4 K were exhibited. A detailed structural-magnetic correlation is established and an assessment of several similar magnetic metallacrowns with diverse metal combinations is given with regards to their potential magnetocaloric properties. Strategies for improving the magnetocaloric properties within the Metaln + FeIII4 family of metallacrowns are proposed regarding the ratio between coupling parameters J (Fe III –N–O–Fe III )/ J (Metal n+ –O–Fe III ). 

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3. The role of defects in Fe(II)-goethite electron transfer

   Notini, L.; Latta, D.E.; Neumann, A.A.; Pearce, C.I.; Sassi, M.; N’Diaye, A.T.; Rosso, K.M.; Scherer, M.M. (February 2018, Environmental science & technology)

   Despite substantial experimental evidence for Fe(II)–Fe(III) oxide electron transfer, computational chemistry calculations suggest that oxidation of sorbed Fe(II) by goethite is kinetically inhibited on structurally perfect surfaces. We used a combination of 57Fe Mössbauer spectroscopy, synchrotron X-ray absorption and magnetic circular dichroism (XAS/XMCD) spectroscopies to investigate whether Fe(II)–goethite electron transfer is influenced by defects. Specifically, Fe L-edge and O K-edge XAS indicates that the outermost few Angstroms of goethite synthesized by low temperature Fe(III) hydrolysis is iron deficient relative to oxygen, suggesting the presence of defects from Fe vacancies. This nonstoichiometric goethite undergoes facile Fe(II)–Fe(III) oxide electron transfer, depositing additional goethite consistent with experimental precedent. Hydrothermal treatment of this goethite, however, appears to remove defects, decrease the amount of Fe(II) oxidation, and change the composition of the oxidation product. When hydrothermally treated goethite was ground, surface defect characteristics as well as the extent of electron transfer were largely restored. Our findings suggest that surface defects play a commanding role in Fe(II)–goethite redox interaction, as predicted by computational chemistry. Moreover, it suggests that, in the environment, the extent of this interaction will vary depending on diagenetic history, local redox conditions, as well as being subject to regeneration via seasonal fluctuations. 

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4. Encapsulation of tricopper cluster in a synthetic cryptand enables facile redox processes from Cu ^I Cu ^I Cu ^I to Cu ^II Cu ^II Cu ^II states

   https://doi.org/10.1039/D0SC05441K  

   Zhang, Weiyao; Moore, Curtis E.; Zhang, Shiyu (March 2021, Chemical Science)

   null (Ed.)

   One-pot reaction of tris(2-aminoethyl)amine (TREN), [Cu I (MeCN) 4 ]PF 6 , and paraformaldehyde affords a mixed-valent [ TREN4 Cu II Cu I Cu I (μ 3 -OH)](PF 6 ) 3 complex. The macrocyclic azacryptand TREN4 contains four TREN motifs, three of which provide a bowl-shape binding pocket for the [Cu 3 (μ 3 -OH)] 3+ core. The fourth TREN caps on top of the tricopper cluster to form a cryptand, imposing conformational constraints and preventing solvent interaction. Contrasting the limited redox capability of synthetic tricopper complexes reported so far, [ TREN4 Cu II Cu I Cu I (μ 3 -OH)](PF 6 ) 3 exhibits several reversible single-electron redox events. The distinct electrochemical behaviors of [ TREN4 Cu II Cu I Cu I (μ 3 -OH)](PF 6 ) 3 and its solvent-exposed analog [ TREN3 Cu II Cu II Cu II (μ 3 -O)](PF 6 ) 4 suggest that isolation of tricopper core in a cryptand enables facile electron transfer, allowing potential application of synthetic tricopper complexes as redox catalysts. Indeed, the fully reduced [ TREN4 Cu I Cu I Cu I (μ 3 -OH)](PF 6 ) 2 can reduce O 2 under acidic conditions. The geometric constraints provided by the cryptand are reminiscent of Nature's multicopper oxidases (MCOs). For the first time, a synthetic tricopper cluster was isolated and fully characterized at Cu I Cu I Cu I ( 4a ), Cu II Cu I Cu I ( 4b ), and Cu II Cu II Cu I ( 4c ) states, providing structural and spectroscopic models for many intermediates in MCOs. Fast electron transfer rates (10 5 to 10 6 M −1 s −1 ) were observed for both Cu I Cu I Cu I /Cu II Cu I Cu I and Cu II Cu I Cu I /Cu II Cu II Cu I redox couples, approaching the rapid electron transfer rates of copper sites in MCO. 

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5. Correlating magnetic anisotropy with [Mo(CN) ₇ ] ⁴⁻ geometry of Mn ^II –Mo ^III magnetic frameworks

   https://doi.org/10.1039/c9dt02164g  

   Magott, Michał; Dunbar, Kim R.; Pinkowicz, Dawid (January 2019, Dalton Transactions)

   Four new three-dimensional (3-D) coordination frameworks based on the heptacyanomolybdate( iii ) anion were prepared and characterised by magnetic measurements: {[Mn II (imH) 2 ] 2 [Mn II (H 2 O)(imH) 3 ][Mn II (imH) 4 ] [Mo III (CN) 7 ] 2 ·6H 2 O} n ( 1 ) (imH = imidazole), {[Mn II (H 2 O) 2 (imH)] 3 [Mn II (H 2 O)(imH) 2 ][Mo III (CN) 7 ] 2 ·5H 2 O} n ( 2 ), {[Mn II (Htrz)(H 2 O) 2 ][Mn II (Htrz) 0.7 (H 2 O) 2.3 ][Mo III (CN) 7 ]·5.6H 2 O} n ( 3 ) (Htrz = 1,2,4-triazole) and {[Mn II (H 2 O) 2 ] 3 [Mn II (H 2 O) 4 ][Mo III (CN) 7 ] 2 ·6H 2 O·2urea} n ( 4 ). All four compounds exhibit long-range ferrimagnetic ordering and exhibit an opening of their magnetic hysteresis loops at 1.8 K; 1 and 2 exhibit the highest coercive fields among all known [Mo III (CN) 7 ]-based assemblies, 5000 and 4500 Oe respectively. The coercivity of 1–4 is correlated with the geometry of the heptacyanomolybdate( iii ) anion and the cyanide bridging pattern. A paramagnetic analogue of compound 1 , {[Mn II (imH) 2 ] 2 [Mn II (H 2 O)(imH) 3 ][Mn II (imH) 4 ][Re III (CN) 7 ] 2 ·6H 2 O} n ( 1Re ), where the heptacyanomolybdate( iii ) anion is substituted by the diamagnetic heptacyanorhenate( iii ) anion is also reported which constitutes the first example of a coordination framework based on [Re III (CN) 7 ] 4− . 

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