Synthesis, Detailed Characterization, and Theoretical Understanding of Mononuclear Chromium(III)-Containing Polyoxotungstates [CrIII(HXVW7O28)2]13– (X = P, As) with Exceptionally Large Magnetic Anisotropy

Arsenic(III)-Capped 12-Tungsto-2-Arsenates(III) [M2(AsIIIW6O25)2(AsIIIOH)x]n- (M = CrIII, FeIII, ScIII, InIII, TiIV, MnII) and Their Magnetic Properties

Six arsenic(III)-capped 12-tungsto-2-arsenates(III) of the type [M₂(As^IIIW₆O₂₅)₂(As^IIIOH)_x]^n- (M = Cr^III, 1; Fe^III, 2; Sc^III, 3; In^III, 4; Ti^IV, 5; Mn^II, 6) have been synthesized in aqueous medium by direct reaction of the elements using a one-pot strategy and structurally characterized by FT-IR spectroscopy, single-crystal XRD, and elemental analysis. Polyanions 1-6 are comprised of two octahedrally coordinated guest metal ions M sandwiched between two {AsW₆} units, resulting in a structure with C_2h point-group symmetry. Polyanions 1-5 contain tri- and tetravalent metal ion guests M (M = Cr^III, Fe^III, Sc^III, In^III, and Ti^IV, respectively), and they have one {As^IIIOH} group grafted on each {AsW₆} unit, whereas the divalent Mn^II-containing derivative 6 has two such {As^IIIOH} groups grafted on each {AsW₆} unit. Magnetic studies on polyanions 3-5 over the temperature range 1.8-295 K and magnetic fields of 0-7 T confirmed that they are diamagnetic. On the other hand, polyanions 1, 2, and 6 are strongly magnetic and follow the Curie-Weiss law above 30 K. The susceptibility plots of 1 and 6 exhibit broad peaks suggesting short-range antiferromagnetic ordering, while the very weak antiferromagnetic ordering of 2 is overshadowed by traces of a paramagnetic impurity. The magnetization data of 1, 2, and 6 at 1.8 K over 0-7 T were analyzed by using the Heisenberg exchange procedure. Small (negative) values of the obtained J values help in understanding the absence of long-range antiferromagnetic ordering.

Incorporation of CrIII into a Keggin Polyoxometalate as a Chemical Strategy to Stabilize a Labile {CrIIIO4} Tetrahedral Conformation and Promote Unattended Single-Ion Magnet Properties

Polyoxometalates (POMs) provide rigid and highly symmetric coordination sites and can be used as a strategy for the stabilization of magnetic ions. Herein, we report a new member of the Keggin archetype, the Cr-centered Keggin anion [α-CrW12O40]5- (CrW12), with the unusual tetrahedral coordination of CrIII reported for the first time in POMs conferring unattended magnetic properties. POM chemistry has recently presented excellent examples of single-molecule and single-ion magnets (SMMs and SIMs) as well as molecular spin qubits; however, the majority of POM-based SIMs reported to date contain lanthanoid ions. CrW12, as the first example of a chromium(III) SIM, exhibits slow relaxation of magnetization and quantum tunneling with a single-ion magnetic behavior even above 10 K with an energy barrier for the reversal of the magnetization of 3.0 K. The first 3d-metal SIM based on a nonlacunary Keggin anion is the foundation for a new research area in POM chemistry.

CrIII as an alternative to RuII in metallo-supramolecular chemistry

Semi-labile [Cr(tpy)(CF₃SO₃)₃] precursors can be exploited for building rod-like dinuclear dyads, in which paramagnetic and luminescent trivalent Cr^III replace diamagnetic Ru^II for unravelling intermetallic communication in supramolecular architectures.

CrIII-Substituted Heteropoly-16-Tungstates [CrIII2(B-β-XIVW8O31)2]14- (X = Si, Ge): Magnetic, Biological, and Electrochemical Studies

The dichromium(III)-containing heteropoly-16-tungstates [Cr^III₂(B-β-Si^IVW₈O₃₁)₂]^14- (1) and [Cr^III₂(B-β-Ge^IVW₈O₃₁)₂]^14- (2) were prepared via a one-pot reaction of the composing elements in aqueous, basic medium. Polyanions 1 and 2 represent the first examples of Cr^III-containing heteropolytungstates comprising the octatungstate unit {XW₈O₃₁} (X = Si, Ge). Magnetic studies demonstrated that, in the solid state, the two polyanions exhibit a weak antiferromagnetic interaction between the two Cr^III centers with J = -3.5 ± 0.5 cm^-1, with no long-range ordering down to 1.8 K. The ground-state spin of polyanions 1 and 2 was thus deduced to be 0, but the detection of a complex set of EPR signals implies that there are thermally accessible excited states containing unpaired spins resulting from the two S = ³/₂ Cr^III ions. A comprehensive electrochemistry study on 1 and 2 in solution was performed, and biological tests showed that both polyanions display significant antidiabetic and anticancer activities.

Enhancing the Magnetic Anisotropy of Linear Cr(II) Chain Compounds Using Heavy Metal Substitutions

Magnetic properties of the series of three linear, trimetallic chain compounds Cr2Cr(dpa)4Cl2, 1, Mo2Cr(dpa)4Cl2, 2, and W2Cr(dpa)4Cl2, 3 (dpa = 2,2'-dipyridylamido), have been studied using variable-temperature dc and ac magnetometry and high-frequency EPR spectroscopy. All three compounds possess an S = 2 electronic ground state arising from the terminal Cr(2+) ion, which exhibits slow magnetic relaxation under an applied magnetic field, as evidenced by ac magnetic susceptibility and magnetization measurements. The slow relaxation stems from the existence of an easy-axis magnetic anisotropy, which is bolstered by the axial symmetry of the compounds and has been quantified through rigorous high-frequency EPR measurements. The magnitude of D in these compounds increases when heavier ions are substituted into the trimetallic chain; thus D = -1.640, -2.187, and -3.617 cm(-1) for Cr2Cr(dpa)4Cl2, Mo2Cr(dpa)4Cl2, and W2Cr(dpa)4Cl2, respectively. Additionally, the D value measured for W2Cr(dpa)4Cl2 is the largest yet reported for a high-spin Cr(2+) system. While earlier studies have demonstrated that ligands containing heavy atoms can enhance magnetic anisotropy, this is the first report of this phenomenon using heavy metal atoms as "ligands".

Six-coordinate ferric porphyrins containing bidentate N-t-butyl-N-nitrosohydroxylaminato ligands: structure, magnetism, IR spectroelectrochemisty, and reactivity

NONOates (diazeniumdiolates) containing the [X{N2O2}](-) functional group are frequently employed as nitric oxide (NO) donors in biology, and some NONOates have been shown to bind to metalloenzymes. We report the preparation, crystal structures, detailed magnetic behavior, redox properties, and reactivities of the first isolable alkyl C-NONOate complexes of heme models, namely (OEP)Fe(η(2)-ON(t-Bu)NO) (1) and (TPP)Fe(η(2)-ON(t-Bu)NO) (2) (OEP = octaethylporphyrinato dianion, TPP = tetraphenylporphyrinato dianion). The compounds display the unusual NONOate O,O-bidentate binding mode for porphyrins, resulting in significant apical Fe displacements (+0.60 Å for 1, and +0.69 Å for 2) towards the axial ligands. Magnetic susceptibility and magnetization measurements made from 1.8-300 K at magnetic fields from 0.02 to 5 T, yielded magnetic moments of 5.976 and 5.974 Bohr magnetons for 1 and 2, respectively, clearly identifying them as high-spin (S = 5/2) ferric compounds. Variable-frequency (9.4 GHz and 34.5 GHz) EPR measurements, coupled with computer simulations, confirmed the magnetization results and yielded more precise values for the spin Hamiltonian parameters: g(avg) = 2.00 ± 0.03, |D| = 3.89 ± 0.09 cm(-1), and E/D = 0.07 ± 0.01 for both compounds, where D and E are the axial and rhombic zero-field splittings. IR spectroelectrochemistry studies reveal that the first oxidations of these compounds occur at the porphyrin macrocycles and not at the Fe-NONOate moieties. Reactions of 1 and 2 with a histidine mimic (1-methylimidazole) generate RNO and NO, both of which may bind to the metal center if sterics allow, as shown by a comparative study with the Cupferron complex (T(p-OMe)PP)Fe(η(2)-ON(Ph)NO). Protonation of 1 and 2 yields N2O as a gaseous product, presumably from the initial generation of HNO that dimerizes to the observed N2O product.

An open chain carboxyethyltin functionalized sandwich-type tungstophosphate based on a trivacant Dawson subunit: synthesis, characterization and properties

A Dawson sandwich-type polyoxometalate {C(NH2)3}12H4[αββα-{(Sn(C3H4O2))2Mn2(P2W15O56)2}]·22H2O (abbreviated as SnR-Mn-P2W15), functionalized by open chain carboxyethyltin groups, was first prepared in aqueous solution under conventional reaction conditions, and then structurally characterized by physicochemical and spectroscopic methods. Single crystal X-ray diffraction analysis revealed that two Mn(2+) cations and two [Sn(CH2CH2COO)](2+) groups are located in the internal and external positions in the so-called equatorial region of SnR-Mn-P2W15, respectively. Intriguingly, two exposed carboxyl groups act as stretching-arm brackets, which provide a favorable structure for potential further functionalization. The electrocatalytic activity of SnR-Mn-P2W15 towards the reduction of hydrogen peroxide and nitrite was studied. Additionally, its acid catalysis and oxidation catalysis activities in organic synthesis were investigated.

Cr(i)Cl as well as Cr+ are stabilised between two cyclic alkyl amino carbenes

Complexes with two and three coordinate chromium(i).

Cr(i)Cl is a very unstable species. The present work describes the stabilisation of Cr(i)Cl in the low coordinate environment of cyclic alkyl(amino) carbene ligands and its synthetic application to yield an unprecedented cationic complex with a two coordinate Cr(i). One electron reduction of (cAAC)₂CrCl₂ (1) with equivalent amount of KC₈ results in the formation of (cAAC)₂CrCl (2), with a distorted trigonal planar configuration at the metal centre. SQUID, EPR and theoretical studies reveal a Cr(i) centre with S = 5/2 spin ground state for 2. It represents the first example of a mononuclear Cr complex showing slow relaxation of magnetisation under an applied magnetic field. The chlorine atom in 2 is expected to be prone to further reactions with appropriate reagents. This qualifies 2 as a promising precursor for the preparation of various interesting complexes with Cr(i) in a low coordinate environment. The first example of this metathesis reaction is observed when 2 is treated with Na[B(C₆H₃(CF₃)₂)₄] resulting in [(cAAC)₂Cr]⁺[B(C₆H₃(CF₃)₂)₄]^–, a linear cationic complex with two coordinate Cr(i) and an S = 5/2 spin ground state.

Norfloxacin-derivative functionalized octamolybdate: unusual carbonyl coordination and acidity sensitive luminescence

The first γ-type octamolybdate functionalized by decarboxylated norfloxacin, (dNF)₂[γ-Mo₈O₂₆(dNF)₂]·10H₂O, has been synthesized featuring unusual carbonyl coordination. The fluorescence is sensitive to acidity and can be used as an acid–base switch.