A Polyoxometalate–Cyanometalate Multilayered Coordination Network

The reaction of the ε-Keggin polyoxometalate (POM) [PMo₁₂O₃₆(OH)₄{La­(H₂O)₄}₄]⁵⁺ with Fe^II(CN)₆^4– under typical bench conditions at room temperature and ambient pressure has afforded the novel [ε-PMo₁₂O₃₇(OH)₃{La­(H₂O)₅(Fe­(CN)₆)_0.25}₄] network, which exhibits a three-dimensional multilayered structure. The compound has been fully characterized by synchrotron-radiation X-ray crystallography, IR spectroscopy, elemental analysis, and thermogravimetric analysis. This coordination network constitutes the first example of a cyanometalate bonded to a POM unit

Water Substitution on Iron Centers: from 0D to 1D Sandwich Type Polyoxotungstates

Four novel polyoxotungstates have been synthesized by reaction of the sandwich type compound [FeIII4(H2O)10(B-β-SbW9O33)2]6− (noted Fe4(H2O)10Sb2W18) with ethylenediamine (en) and/or oxalate (ox) ligands under various conditions. The one-dimensional (1D) compound [enH2]3[FeIII4(H2O)8(SbW9O33)2]·20H2O (1) is isolated at 130 °C and results from the elimination of two water molecules and the condensation of the polyoxotungstate precursor. The reaction of Fe4(H2O)10Sb2W18 with oxalate ligands affords the molecular complex Na14[FeIII4(ox)4(H2O)2(SbW9O33)2]·60H2O (2) where two organic ligands substitute four water molecules, while the same reaction in the presence of en molecules at 130 °C leads to the formation of the functionalized 1D chain [enH2]7[FeIII4(ox)4(SbW9O33)2]·14H2O (3) with protonated ethylenediamine counterions. Finally, at 160 °C a rearrangement of the Fe4(H2O)10Sb2W18 polyoxotungstate is observed, and the sandwich type compound [enH2]5[FeII2FeII2(enH)2(FeIIIW9O34)2]·24H2O (4) crystallizes. In 4, the heteroelement is a FeIII ion, and the water molecules on the two outer FeII centers are bound to pendant monoprotonated en ligands. The four compounds have been characterized by IR spectroscopy, thermogravimetric analysis, and single crystal X-ray diffraction. A detailed study of the magnetic properties of the mixed-valent hexanuclear iron complex in 4 shows evidence of an S = 5 ground-state because of spin frustration effects. A quantification of the electronic parameters characterizing the ground state (D = +1.12 cm−1, E/D = 0.15) confirms that polyoxotungstate ligands induce large magnetic anisotropy

Selective Catalytic Electroreduction of CO₂ at Silicon Nanowires (SiNWs) Photocathodes Using Non-Noble Metal-Based Manganese Carbonyl Bipyridyl Molecular Catalysts in Solution and Grafted onto SiNWs

The electrocatalytic reduction of CO₂ to CO in hydroorganic medium has been investigated at illuminated (λ > 600 nm; 20 mW cm^–2) hydrogen-terminated silicon nanowires (SiNWs–H) photocathodes using three Mn-based carbonyl bipyridyl complexes as homogeneous molecular catalysts ([Mn­(L) (CO)₃(CH₃CN)]­(PF₆) and [Mn­(bpy) (CO)₃Br] with L = bpy = 2,2′-bipyridine and dmbpy = 4,4′-dimethyl-2,2′-bipyridine). Systematic comparison of their cyclic voltammetry characteristics with those obtained at flat hydrogen-terminated silicon and traditional glassy carbon electrodes (GCE) enabled us to demonstrate the superior catalytic efficiency of SiNWs–H in terms of cathodic photocurrent densities and overpotentials. For example, the photocurrent densities measured at −1.0 V vs SCE for [Mn­(bpy) (CO)₃(CH₃CN)]­(PF₆) at SiNWs–H exceeded 1.0 mA cm^–2 in CO₂-saturated CH₃CN + 5% v/v H₂O, whereas almost zero current was measured at this potential at GCE. Such characteristics have been supported by the energetic diagrams built for the different SiNWs|Mn-based catalyst interfaces. The fill factor FF and energy conversion efficiency η calculated under catalytic conditions were higher for [Mn­(bpy or dmbpy) (CO)₃(CH₃CN)]­(PF₆) (FF = 0.35 and 0.34; η = 3.0 and 2.0%, respectively). Further preparative-scale electrolysis at SiNWs–H photocathode with Mn-based complex catalysts in electrolytic solution evidenced the quantitative conversion of CO₂ to CO with a higher stability of the [Mn­(dmbpy) (CO)₃(CH₃CN)]­(PF₆) complex. Finally, in order to develop technologically viable electrocatalytic devices, the elaboration of SiNWs–H photoelectrodes modified with a Mn-based complex has been successfully achieved from an electropolymerizable catalyst, and it was shown that the electrocatalytic activity of the complex was retained after immobilization

Water Substitution on Iron Centers: from 0D to 1D Sandwich Type Polyoxotungstates

Four novel polyoxotungstates have been synthesized by reaction of the sandwich type compound [FeIII4(H2O)10(B-β-SbW9O33)2]6− (noted Fe4(H2O)10Sb2W18) with ethylenediamine (en) and/or oxalate (ox) ligands under various conditions. The one-dimensional (1D) compound [enH2]3[FeIII4(H2O)8(SbW9O33)2]·20H2O (1) is isolated at 130 °C and results from the elimination of two water molecules and the condensation of the polyoxotungstate precursor. The reaction of Fe4(H2O)10Sb2W18 with oxalate ligands affords the molecular complex Na14[FeIII4(ox)4(H2O)2(SbW9O33)2]·60H2O (2) where two organic ligands substitute four water molecules, while the same reaction in the presence of en molecules at 130 °C leads to the formation of the functionalized 1D chain [enH2]7[FeIII4(ox)4(SbW9O33)2]·14H2O (3) with protonated ethylenediamine counterions. Finally, at 160 °C a rearrangement of the Fe4(H2O)10Sb2W18 polyoxotungstate is observed, and the sandwich type compound [enH2]5[FeII2FeII2(enH)2(FeIIIW9O34)2]·24H2O (4) crystallizes. In 4, the heteroelement is a FeIII ion, and the water molecules on the two outer FeII centers are bound to pendant monoprotonated en ligands. The four compounds have been characterized by IR spectroscopy, thermogravimetric analysis, and single crystal X-ray diffraction. A detailed study of the magnetic properties of the mixed-valent hexanuclear iron complex in 4 shows evidence of an S = 5 ground-state because of spin frustration effects. A quantification of the electronic parameters characterizing the ground state (D = +1.12 cm−1, E/D = 0.15) confirms that polyoxotungstate ligands induce large magnetic anisotropy

Second-Order Nonlinear Optical Properties of Polyoxometalate Salts of a Chiral Stilbazolium Derivative

The synthesis of nonlinear optical (NLO) active salts with stilbazolium derivatives and polyoxometalate (POM) counterions has been investigated. With known nonchiral stilbazolium derivatives, such as MOMS+, compounds with centrosymmetric structures have been isolated, like for instance the centrosymmetric salt (MOMS)4[Mo8O26] (1), synthesized under hydrothermal conditions. A new chiral derivative of the known DAMS+ molecules, named here CHIDAMS+, has therefore been synthesized in order to force the crystallization of the hybrid ionic salts in noncentrosymmetric space groups. The CHIDAMS+ cation has been crystallized under two polymorphic PF6− salts, (CHIDAMS)PF6 (2a and 2b), and its reactivity with various POMs has been investigated. The ionic salt (CHIDAMS)2[Mo5O13(OEt)4(NO){Na(H2O)0.5(DMF)0.5}] (4) crystallizes in the noncentrosymmetric P21 group, but the push−pull axis of the CHIDAMS+ cations adopts a quasi-antiparallel alignment. The ionic salt (CHIDAMS)3[PW12O40]·2DMF (5) associating three CHIDAMS+ cations and a PW12O403− Keggin anion crystallizes also in the P21 space group, but the disposition of the cations in the solid state is far more favorable. Diffuse reflectance experiments have evidenced a charge transfer between the organic and inorganic components in 5, and Kurtz−Perry experiments show that this salt exhibits a second harmonic generation efficiency more than 10 times higher than those of the PF6− salts 2a and 2b, the hybrid salt 4, and all of the other NLO active POM molecular materials reported in the literature

Second-Order Nonlinear Optical Properties of Polyoxometalate Salts of a Chiral Stilbazolium Derivative

The synthesis of nonlinear optical (NLO) active salts with stilbazolium derivatives and polyoxometalate (POM) counterions has been investigated. With known nonchiral stilbazolium derivatives, such as MOMS+, compounds with centrosymmetric structures have been isolated, like for instance the centrosymmetric salt (MOMS)4[Mo8O26] (1), synthesized under hydrothermal conditions. A new chiral derivative of the known DAMS+ molecules, named here CHIDAMS+, has therefore been synthesized in order to force the crystallization of the hybrid ionic salts in noncentrosymmetric space groups. The CHIDAMS+ cation has been crystallized under two polymorphic PF6− salts, (CHIDAMS)PF6 (2a and 2b), and its reactivity with various POMs has been investigated. The ionic salt (CHIDAMS)2[Mo5O13(OEt)4(NO){Na(H2O)0.5(DMF)0.5}] (4) crystallizes in the noncentrosymmetric P21 group, but the push−pull axis of the CHIDAMS+ cations adopts a quasi-antiparallel alignment. The ionic salt (CHIDAMS)3[PW12O40]·2DMF (5) associating three CHIDAMS+ cations and a PW12O403− Keggin anion crystallizes also in the P21 space group, but the disposition of the cations in the solid state is far more favorable. Diffuse reflectance experiments have evidenced a charge transfer between the organic and inorganic components in 5, and Kurtz−Perry experiments show that this salt exhibits a second harmonic generation efficiency more than 10 times higher than those of the PF6− salts 2a and 2b, the hybrid salt 4, and all of the other NLO active POM molecular materials reported in the literature

Structural, Magnetic, EPR, and Electrochemical Characterizations of a Spin-Frustrated Trinuclear Cr^III Polyoxometalate and Study of Its Reactivity with Lanthanum Cations

The asymmetric CrIII polyoxometalate complex Cs10[(γ-SiW10O36)2(Cr(OH)(H2O))3]·17H2O (1) has been synthesized in water under atmospheric pressure from the trinuclear precursor [Cr3(CH3COO)7(OH)2] and the divacant ligand [γ-SiW10O36]8−. Complex 1 is built up of two [γ-SiW10O36]8− Keggin units sandwiching a trinuclear {(CrIII(OH)(H2O))3} fragment where the paramagnetic centers are bridged by three μ-OH ligands forming a nearly isosceles triangle. The magnetic properties of this spin-frustrated system have thus been interpreted considering a 2-J Hamiltonian showing that the CrIII ions are antiferromagnetically coupled and that 1 possesses an S = 3/2 ground state with an S = 1/2 first excited state located at 11 cm−1. These results have been confirmed by EPR spectroscopy measurements (Q-band), which have also enabled the quantification of the electronic parameters characterizing the quadruplet spin ground state. The magnitude of the magnetic exchange interactions and the nature of the ground state are discussed in light of previously reported isosceles triangular S = 3/2 clusters. UV−visible and electrochemical studies have shown that 1 is stable in aqueous media in a 1−7 pH range. This stability is chemically confirmed by the study of the reactivity of 1 with LaIII cations, which has allowed the isolation of the Cs4[(γ-SiW10O36)2(Cr(OH)(H2O))3(La(H2O)7)2]·20H2O compound (2). Indeed, during the synthetic process of this 3d−4f system, the integrity of the [(γ-SiW10O36)2(Cr(OH)(H2O))3]10− building unit constituting 1 is maintained despite the high oxophilic character of the LaIII ions. The single crystal X-ray diffraction study of 2 has revealed that in the solid state the rare earth cations connect these subunits, affording a 3d−4f double-chain monodimensional system

Structural, Magnetic, EPR, and Electrochemical Characterizations of a Spin-Frustrated Trinuclear Cr^III Polyoxometalate and Study of Its Reactivity with Lanthanum Cations

The asymmetric CrIII polyoxometalate complex Cs10[(γ-SiW10O36)2(Cr(OH)(H2O))3]·17H2O (1) has been synthesized in water under atmospheric pressure from the trinuclear precursor [Cr3(CH3COO)7(OH)2] and the divacant ligand [γ-SiW10O36]8−. Complex 1 is built up of two [γ-SiW10O36]8− Keggin units sandwiching a trinuclear {(CrIII(OH)(H2O))3} fragment where the paramagnetic centers are bridged by three μ-OH ligands forming a nearly isosceles triangle. The magnetic properties of this spin-frustrated system have thus been interpreted considering a 2-J Hamiltonian showing that the CrIII ions are antiferromagnetically coupled and that 1 possesses an S = 3/2 ground state with an S = 1/2 first excited state located at 11 cm−1. These results have been confirmed by EPR spectroscopy measurements (Q-band), which have also enabled the quantification of the electronic parameters characterizing the quadruplet spin ground state. The magnitude of the magnetic exchange interactions and the nature of the ground state are discussed in light of previously reported isosceles triangular S = 3/2 clusters. UV−visible and electrochemical studies have shown that 1 is stable in aqueous media in a 1−7 pH range. This stability is chemically confirmed by the study of the reactivity of 1 with LaIII cations, which has allowed the isolation of the Cs4[(γ-SiW10O36)2(Cr(OH)(H2O))3(La(H2O)7)2]·20H2O compound (2). Indeed, during the synthetic process of this 3d−4f system, the integrity of the [(γ-SiW10O36)2(Cr(OH)(H2O))3]10− building unit constituting 1 is maintained despite the high oxophilic character of the LaIII ions. The single crystal X-ray diffraction study of 2 has revealed that in the solid state the rare earth cations connect these subunits, affording a 3d−4f double-chain monodimensional system