Synthesis, Structure, and Magnetic Properties of [(CH₃CN)₅V−O−V(CH₃CN)₅][BF₄]₄

The reaction of vanadium(III) acetylacetonate with HBF4 in acetonitrile yields [(CH3CN)5V−O−V(CH3CN)5][BF4]4, a material that serves as a convenient precursor to other [V−O−V]4+ species such as [(bipy)2(CH3CN)V−O−V(CH3CN)(bipy)2][BF4]4 (bipy = 2,2‘-bipyridine). Single-crystal X-ray diffraction shows that the V−O−V linkage of [(CH3CN)5V−O−V(CH3CN)5]4+ is linear. An Evans method measurement of the solution-phase magnetic susceptibility indicates strong ferromagnetic coupling between the vanadium centers. Magnetic susceptibility (χ) and magnetization (M(H)) data for a powdered sample and for a single crystal oriented with its V−O−V axis parallel to the applied field were measured over 1.8−300 K. The results suggest that the V(III) centers are ferromagnetically coupled with J ≈ 72 K (∼50 cm-1) yielding a ground state with a total spin Stotal = 2. Theoretical fit to the M(H) plot for the single crystal yielded g|| = 2.01 ± 0.01 and the zero-field splitting parameter D = 0.60 ± 0.04 K (0.42 ± 0.03 cm-1). EPR measurements at 34 and 101.6 GHz are consistent with the Stotal = 2 ground state and yield g|| = 1.9825, g⊥ = 1.9725 and D = 0.57 ± 0.03 K

Synthesis, Structure, and Magnetic Properties of [(CH₃CN)₅V−O−V(CH₃CN)₅][BF₄]₄

The reaction of vanadium(III) acetylacetonate with HBF4 in acetonitrile yields [(CH3CN)5V−O−V(CH3CN)5][BF4]4, a material that serves as a convenient precursor to other [V−O−V]4+ species such as [(bipy)2(CH3CN)V−O−V(CH3CN)(bipy)2][BF4]4 (bipy = 2,2‘-bipyridine). Single-crystal X-ray diffraction shows that the V−O−V linkage of [(CH3CN)5V−O−V(CH3CN)5]4+ is linear. An Evans method measurement of the solution-phase magnetic susceptibility indicates strong ferromagnetic coupling between the vanadium centers. Magnetic susceptibility (χ) and magnetization (M(H)) data for a powdered sample and for a single crystal oriented with its V−O−V axis parallel to the applied field were measured over 1.8−300 K. The results suggest that the V(III) centers are ferromagnetically coupled with J ≈ 72 K (∼50 cm-1) yielding a ground state with a total spin Stotal = 2. Theoretical fit to the M(H) plot for the single crystal yielded g|| = 2.01 ± 0.01 and the zero-field splitting parameter D = 0.60 ± 0.04 K (0.42 ± 0.03 cm-1). EPR measurements at 34 and 101.6 GHz are consistent with the Stotal = 2 ground state and yield g|| = 1.9825, g⊥ = 1.9725 and D = 0.57 ± 0.03 K

An ENDOR and HYSCORE Investigation of a Reaction Intermediate in IspG (GcpE) Catalysis

IspG is a 4Fe–4S protein that carries out an essential reduction step in isoprenoid biosynthesis. Using electron–nuclear double resonance (ENDOR) and hyperfine sublevel correlation (HYSCORE) spectroscopies on labeled samples, we have specifically assigned the hyperfine interactions in a reaction intermediate. These results help clarify the nature of the reaction intermediate, supporting a direct interaction between the unique fourth Fe in the cluster and C2 and O3 of the ligand

Sandwich-Type Germanotungstates: Structure and Magnetic Properties of the Dimeric Polyoxoanions [M₄(H₂O)₂(GeW₉O₃₄)₂]^12- (M = Mn²⁺, Cu²⁺, Zn²⁺, Cd²⁺)

The novel dimeric germanotungstates [M4(H2O)2(GeW9O34)2]12- (M = Mn2+, Cu2+, Zn2+, Cd2+) have been synthesized and characterized by IR spectroscopy, elemental analysis, magnetic measurements, and 183W-NMR spectroscopy. X-ray single-crystal analyses were carried out on Na12[Mn4(H2O)2(GeW9O34)2]·38H2O (Na12-1), which crystallizes in the monoclinic system, space group P21/n, with a = 13.0419(8) Å, b = 17.8422(10) Å, c = 21.1626(12) Å, β = 93.3120(10)°, and Z = 2; Na11Cs2[Cu4(H2O)2(GeW9O34)2]Cl·31H2O (Na11Cs-2) crystallizes in the triclinic system, space group P1̄, with a = 12.2338(17) Å, b = 12.3833(17) Å, c = 15.449(2) Å, α = 100.041(2)°, β = 97.034(2)°, γ = 101.153(2)°, and Z = 1; Na12[Zn4(H2O)2(GeW9O34)2]·32H2O (Na12-3) crystallizes in the triclinic system, space group P1̄, with a = 11.589(3) Å, b = 12.811(3) Å, c = 17.221(4) Å, α = 97.828(6)°, β = 106.169(6)°, γ = 112.113(5)°, and Z = 1; Na12[Cd4(H2O)2(GeW9O34)2]·32.2H2O (Na12-4) crystallizes also in the triclinic system, space group P1̄, with a = 11.6923(17) Å, b = 12.8464(18) Å, c = 17.616(2) Å, α = 98.149(3)°, β = 105.677(3)°, γ = 112.233(2)°, and Z = 1. The polyanions consist of two lacunary B-α-[GeW9O34]10- Keggin moieties linked via a rhomblike M4O16 (M = Mn, Cu, Zn, Cd) group leading to a sandwich-type structure. 183W-NMR studies of the diamagnetic Zn and Cd derivatives indicate that the solid-state polyoxoanion structures are preserved in solution. EPR measurements on Na12-1 at frequencies up to 188 GHz and temperatures down to 4 K yield a single, exchange-narrowed peak, at giso = 1.9949, typical of Mn systems, and an upper limit of |D| = 20.0 mT; its magnetization studies still await further theoretical treatment. Detailed EPR studies on Na11Cs-2 over temperatures down to 2 K and variable frequencies yield g∥ = 2.4303 and g⊥ = 2.0567 and A∥ = 4.4 mT (delocalized over the Cu4 framework), with |D| = 12.1 mT. Magnetization studies in addition yield the exchange parameters J1 = −11 and J2 = −82 cm-1, in agreement with the EPR studies

Structure and Magnetism of the Tetra-Copper(II)-Substituted Heteropolyanion [Cu₄K₂(H₂O)₈(α-AsW₉O₃₃)₂]^8-^†

The novel heteropolyanion [Cu4K2(H2O)8(α-AsW9O33)2]8- (1) has been synthesized and characterized by IR spectroscopy, elemental analysis, and magnetic studies. Single-crystal X-ray analysis was carried out on {K7Na[Cu4K2(H2O)6(α-AsW9O33)2]·5.5H2O}n (K7Na-1), which crystallizes in the tetragonal system, space group P4̄21m, with a = 16.705(4) Å, b = 16.705(4) Å, c = 13.956(5) Å, and Z = 2. Interaction of the lacunary [α-AsW9O33]9- with Cu2+ ions in neutral, aqueous medium leads to the formation of the dimeric polyoxoanion 1 in high yield. Polyanion 1 consists of two α-AsW9O33 units joined by a cyclic arrangement of four Cu2+ and two K+ ions, resulting in a structure with C2v symmetry. All copper ions have one terminal water molecule, resulting in square-pyramidal coordination geometry. Three of the copper ions are adjacent to each other and connected via two μ3-oxo bridges. EPR studies on K7Na-1 and also on Na9[Cu3Na3(H2O)9(α-AsW9O33)2]·26H2O (Na9-2) over 2−300 K yielded g values that are consistent with a square-pyramidal coordination around the copper(II) ions in 1 and 2. No hyperfine structure was observed due to the presence of strong spin exchange, but fine structure was observed for the excited (ST = 3/2) state of Na9-2 and the ground state (ST = 1) of K7Na-1. The zero-field (D) parameters have also been determined for these states, constituting a rare case wherein one observes EPR from both the ground and the excited states. Magnetic susceptibility data show that Na9-2 has antiferromagnetically coupled Cu2+ ions, with J = −1.36 ± 0.01 cm-1, while K7Na-1 has both ferromagnetically and antiferromagnetically coupled Cu2+ ions (J1 = 2.78 ± 0.13 cm-1, J2 = −1.35 ± 0.02 cm-1, and J3 = −2.24 ± 0.06 cm-1), and the ground-state total spins are ST = 1/2 in Na9-2 and ST = 1 in K7Na-1

Sandwich-Type Germanotungstates: Structure and Magnetic Properties of the Dimeric Polyoxoanions [M₄(H₂O)₂(GeW₉O₃₄)₂]^12- (M = Mn²⁺, Cu²⁺, Zn²⁺, Cd²⁺)

The novel dimeric germanotungstates [M4(H2O)2(GeW9O34)2]12- (M = Mn2+, Cu2+, Zn2+, Cd2+) have been synthesized and characterized by IR spectroscopy, elemental analysis, magnetic measurements, and 183W-NMR spectroscopy. X-ray single-crystal analyses were carried out on Na12[Mn4(H2O)2(GeW9O34)2]·38H2O (Na12-1), which crystallizes in the monoclinic system, space group P21/n, with a = 13.0419(8) Å, b = 17.8422(10) Å, c = 21.1626(12) Å, β = 93.3120(10)°, and Z = 2; Na11Cs2[Cu4(H2O)2(GeW9O34)2]Cl·31H2O (Na11Cs-2) crystallizes in the triclinic system, space group P1̄, with a = 12.2338(17) Å, b = 12.3833(17) Å, c = 15.449(2) Å, α = 100.041(2)°, β = 97.034(2)°, γ = 101.153(2)°, and Z = 1; Na12[Zn4(H2O)2(GeW9O34)2]·32H2O (Na12-3) crystallizes in the triclinic system, space group P1̄, with a = 11.589(3) Å, b = 12.811(3) Å, c = 17.221(4) Å, α = 97.828(6)°, β = 106.169(6)°, γ = 112.113(5)°, and Z = 1; Na12[Cd4(H2O)2(GeW9O34)2]·32.2H2O (Na12-4) crystallizes also in the triclinic system, space group P1̄, with a = 11.6923(17) Å, b = 12.8464(18) Å, c = 17.616(2) Å, α = 98.149(3)°, β = 105.677(3)°, γ = 112.233(2)°, and Z = 1. The polyanions consist of two lacunary B-α-[GeW9O34]10- Keggin moieties linked via a rhomblike M4O16 (M = Mn, Cu, Zn, Cd) group leading to a sandwich-type structure. 183W-NMR studies of the diamagnetic Zn and Cd derivatives indicate that the solid-state polyoxoanion structures are preserved in solution. EPR measurements on Na12-1 at frequencies up to 188 GHz and temperatures down to 4 K yield a single, exchange-narrowed peak, at giso = 1.9949, typical of Mn systems, and an upper limit of |D| = 20.0 mT; its magnetization studies still await further theoretical treatment. Detailed EPR studies on Na11Cs-2 over temperatures down to 2 K and variable frequencies yield g∥ = 2.4303 and g⊥ = 2.0567 and A∥ = 4.4 mT (delocalized over the Cu4 framework), with |D| = 12.1 mT. Magnetization studies in addition yield the exchange parameters J1 = −11 and J2 = −82 cm-1, in agreement with the EPR studies

Structure, Electrochemistry, and Magnetism of the Iron(III)-Substituted Keggin Dimer, [Fe₆(OH)₃(<i>A-</i>α<i>-</i>GeW₉O₃₄(OH)₃)₂]^11-

The iron(III)-substituted tungstogermanate [Fe6(OH)3(A-α-GeW9O34(OH)3)2]11- (1) has been synthesized and characterized by IR, elemental analysis, SQUID magnetometry, electron paramagnetic resonance (EPR), and electrochemistry. Single-crystal X-ray analysis was carried out on Cs4Na7[Fe6(OH)3(A-α-GeW9O34(OH)3)2]·30H2O, which crystallizes in the monoclinic system, space group C2/m, with a = 36.981(4) Å, b = 16.5759(15) Å, c = 16.0678(15) Å, β = 95.311(3)°, and Z = 4. Polyanion 1 consists of two (A-α-GeW9O34) Keggin moieties linked via six Fe3+ ions, leading to a double-sandwich structure. The equivalent iron centers represent a trigonal prismatic Fe6 fragment, resulting in virtual D3h symmetry for 1. Electrochemistry studies revealed that 1 is stable in solution from pH 3 to at least pH 7. In pH = 3 media the reduction of the six Fe3+ centers was featured by a single voltammetric wave for most supporting electrolytes used. In that case, whatever the scan rate from 1000 mV·s-1 down to 2 mV·s-1, no splitting of the single Fe-wave of 1 was observed. The acetate medium induced a partial splitting of the wave, and this separation is enhanced with increasing pH. Remarkable efficiency of 1 in the electrocatalytic reduction of nitrite, nitric oxide, and nitrate is demonstrated. Magnetic susceptibility (χ) measurements indicate a diamagnetic (ST = 0) ground state, with an average J = −12 cm-1 and g = 2.00. EPR studies confirm that the ground state is indeed diamagnetic, since the EPR signal intensity steadily decreases without any line broadening as the temperature is lowered and becomes unobservable below about 50 K. The signal is a single broad peak at all frequencies (90−370 GHz), ascribed to the thermally accessible excited states. Its giso is 1.992 51, as expected for a high-spin Fe3+-containing species, and supports the χ data analysis

Structure, Electrochemistry, and Magnetism of the Iron(III)-Substituted Keggin Dimer, [Fe₆(OH)₃(<i>A-</i>α<i>-</i>GeW₉O₃₄(OH)₃)₂]^11-

The iron(III)-substituted tungstogermanate [Fe6(OH)3(A-α-GeW9O34(OH)3)2]11- (1) has been synthesized and characterized by IR, elemental analysis, SQUID magnetometry, electron paramagnetic resonance (EPR), and electrochemistry. Single-crystal X-ray analysis was carried out on Cs4Na7[Fe6(OH)3(A-α-GeW9O34(OH)3)2]·30H2O, which crystallizes in the monoclinic system, space group C2/m, with a = 36.981(4) Å, b = 16.5759(15) Å, c = 16.0678(15) Å, β = 95.311(3)°, and Z = 4. Polyanion 1 consists of two (A-α-GeW9O34) Keggin moieties linked via six Fe3+ ions, leading to a double-sandwich structure. The equivalent iron centers represent a trigonal prismatic Fe6 fragment, resulting in virtual D3h symmetry for 1. Electrochemistry studies revealed that 1 is stable in solution from pH 3 to at least pH 7. In pH = 3 media the reduction of the six Fe3+ centers was featured by a single voltammetric wave for most supporting electrolytes used. In that case, whatever the scan rate from 1000 mV·s-1 down to 2 mV·s-1, no splitting of the single Fe-wave of 1 was observed. The acetate medium induced a partial splitting of the wave, and this separation is enhanced with increasing pH. Remarkable efficiency of 1 in the electrocatalytic reduction of nitrite, nitric oxide, and nitrate is demonstrated. Magnetic susceptibility (χ) measurements indicate a diamagnetic (ST = 0) ground state, with an average J = −12 cm-1 and g = 2.00. EPR studies confirm that the ground state is indeed diamagnetic, since the EPR signal intensity steadily decreases without any line broadening as the temperature is lowered and becomes unobservable below about 50 K. The signal is a single broad peak at all frequencies (90−370 GHz), ascribed to the thermally accessible excited states. Its giso is 1.992 51, as expected for a high-spin Fe3+-containing species, and supports the χ data analysis

The Satellite-Shaped Co-15 Polyoxotungstate, [Co₆(H₂O)₃₀{Co₉Cl₂(OH)₃(H₂O)₉(<i>β</i><i>-</i>SiW₈O₃₁)₃}]^5-

The 15-cobalt-substituted polyoxotungstate [Co6(H2O)30{Co9Cl2(OH)3(H2O)9(β-SiW8O31)3}]5- (1) has been characterized by single-crystal XRD, elemental analysis, IR, electrochemistry, magnetic measurements, and EPR. Single-crystal X-ray analysis was carried out on Na5[Co6(H2O)30{Co9Cl2(OH)3(H2O)9(β-SiW8O31)3}]·37H2O, which crystallizes in the hexagonal system, space group P63/m, with a = 19.8754(17) Å, b = 19.8754(17) Å, c = 22.344(4) Å, α = 90°, β = 90°, γ = 120°, and Z = 2. The trimeric polyanion 1 has a core of nine CoII ions encapsulated by three unprecedented (β-SiW8O31) fragments and two Cl- ligands. This central assembly {Co9Cl2(OH)3(H2O)9(β-SiW8O31)3}17- is surrounded by six antenna-like CoII(H2O)5 groups resulting in the satellite-like structure 1. Synthesis of 1 is accomplished in a simple one-pot procedure by interaction of CoII ions with [γ-SiW10O36]8- in aqueous, acidic NaCl medium (pH 5.4). Polyanion 1 was studied by cyclic voltammetry as a function of pH. The current intensity of its CoII centers was compared with that of free CoII in solution. Our results suggest that 1 keeps its integrity in solution. Magnetic susceptibility results show the presence of both antiferro- and ferromagnetic coupling within the (CoII)9 core. A fully anisotropic Ising model has been employed to describe the exchange-coupling and yields g = 2.42 ± 0.01, J1 = 17.0 ± 1.5 cm-1, and J2 = −13 ± 1 cm-1. Variable frequency EPR studies reveal an anisotropic Kramer's doublet

Magnetism, Electron Paramagnetic Resonance, Electrochemistry, and Mass Spectrometry of the Pentacopper(II)-Substituted Tungstosilicate [Cu₅(OH)₄(H₂O)₂(A<i>-</i>α-SiW₉O₃₃)₂]^10-, A Model Five-Spin Frustrated Cluster

The dimeric, pentacopper(II)-substituted tungstosilicate [Cu5(OH)4(H2O)2(A-α-SiW9O33)2]10- (1) has been characterized by single-crystal X-ray diffraction, elemental analysis, IR, electrochemistry, magnetic measurements, electron paramagnetic resonance (EPR), and mass spectrometry (MS). Magnetization and high-field EPR measurements reveal that the pentameric copper core {Cu5(OH)4(H2O)2}6+ of 1 exhibits strong antiferromagnetic interactions (Ja = −51 ± 6 cm-1, Jb = −104 ± 1 cm-1, and Jc = −55 ± 3 cm-1) resulting in a spin ST = 1/2 ground state. EPR data show that the unpaired electron spin density is localized on the spin-frustrated apical Cu2+ ion with gzz = 2.4073 ± 0.0005, gyy = 2.0672 ± 0.0005, gxx = 2.0240 ± 0.0005, and Azz = −340 ± 20 MHz (−0.0113 cm-1). 1 can therefore be considered as a model system for a five-spin, electronically coupled, spin-frustrated system. Polyanion 1, which is stable over a wide pH domain (pH 1−7), was characterized by cyclic voltammetry (CV) in a pH 5 medium. Its CV was constituted by an initial two-step reduction of the Cu2+ centers to Cu0 through Cu+, followed at more negative potential by the redox processes of the W centers. Controlled potential coulometry of 1 allows for the reduction of the five Cu2+ centers, as seen by consumption of 10.05 ± 0.05 electrons per molecule. Polyanion 1 triggers efficiently the electrocatalytic reduction of nitrate and nitrite, and it also catalyzes the reduction of N2O. To our knowledge, this is the first example of N2O catalytic reduction by a polyoxoanion. Fourier transform ion cyclotron resonance MS was used to unambiguously assign the molecular weight of the solution-phase species 1 and the oxidation states of the Cu atoms in the central {Cu5(OH)4(H2O)2}6+ core. Infrared (IR) multiphoton dissociation MS/MS of 1 showed evidence of a condensation process similar to bronze formation at low irradiation intensity. Higher IR intensity resulted in the formation of stable fragments consistent with those previously observed in the solution chemistry of polyoxoanions