Large Magnetic Polyoxometalates Containing the Cobalt Cubane '[CoIIICo 3II (OH)3(H2O)6-m(PW9O34)]3-' (m = 3 or 5) as a Subunit

Structure and Electronic Properties of Large Oligomeric Heterometallic 3d/CeIV Polyoxometalates

Merging the rich chemistry of Ce(IV) polyoxometalates (POMs) with that of 3d polyanions remains a challenge due to the strong competition between these highly oxophilic lanthanide cations and 3d metallic ions for coordination to lacunary molecular metal oxides. We report herein the characterization of an unprecedented water stable hexameric CeIV/CoII POM (Ce12Co6) made of two {(SiW9)2Ce6} units connected to a {(SiW10)2Co6(PO4)2} core. In addition, the pentameric CeIV/NiII compound Ce6Ni8, where two {PW9Ni3W} and a {PW10Ni2} fragments are grafted on a {(PW9)2Ce6} moiety, has been obtained. Magnetic studies of Ce6Ni8 revealed ferromagnetic interactions between the NiII centers constituting the {Ni3PW10} fragments, in agreement with the geometry of such a trinuclear cluster. Related insoluble barium salts of Ce12Co6 and Ce6Ni8 were also prepared, allowing their solid-state electrochemical investigations and showing in particular that in Ce12Co6, both the cobalt, cerium, and silicotungstate moieties are electroactive. Finally, photophysical studies demonstrate the formation of long-lived reduced POMs photosensitized by [Ru(bpy)3]2+, suggesting that Ce12Co6 and Ce6Ni8 could be used as efficient reservoirs of reduction equivalents for photocatalytic reactions.

Coordination of Pt(IV) by {P8W48} Macrocyclic Inorganic Cavitand: Structural, Solution, and Electrochemical Studies

Hydrothermal reaction of a macrocyclic inorganic POM cavitand Li₁₇(NH₄)₂₁H₂[P₈W₄₈O₁₈₄] with [Pt(H₂O)₂(OH)₄] results in coordination of up to six {Pt(H₂O)_x(OH)_4-x} fragments to the internal surface of the polyoxoanion. The product was isolated as K₂₂(NH₄)₉H₃[{Pt(OH)₃(H₂O)}₆P₈W₄₈O₁₈₄]·79H₂O (1) and characterized by multiple techniques in the solid state (SCXRD, XRPD, XPS, FTIR, and TGA) and in solution (NMR, ESI-MS, and HPLC-ICP-AES). Electrochemical properties were studied both in solution and as components of the paste electrode. The complex shows electrocatalytic activity in water oxidation.

Keggin-type polyoxometalate 1 : 1 complexes of Pb(II) and Bi(III): experimental, theoretical and luminescence studies

Bi3+ and Pb2+ uptake by a monolacunary Keggin-type [PW11O39]7- anion leads to the formation of [PW11O39Bi]4- and [PW11O39Pb]5- complexes with a stereochemically active lone pair at the incorporated heterometal. The two complexes were isolated as (TBA)4[PW11O39Bi] (1) and (TBA)5[PW11O39Pb] (2) and characterized by 31P and 183W NMR spectroscopy, high-resolution electrospray mass-spectrometry (HR-ESI-MS) and cyclic voltammetry (CV). EXAFS and XANES data confirm the unchanged oxidation state and ψ-square pyramidal geometry of Bi3+ and Pb2+ in 1 and 2. DFT calculations were used in order to (i) confirm the absence of ligands attached to the heterometal sites in both complexes and localize the lone pair, and (ii) assign all signals in the 183W NMR spectra. Complexes 1 and 2 demonstrate photoluminescence (PL). A reversible change in the PL spectra of both complexes in the presence of water vapor has been detected. On the contrary, PL data for sandwich-type ((CH3)4N)4K3[H4(PW11O39)2Bi]·25H2O (3) do not show sensitivity to water vapor.

A trapped hexaaqua CoII complex between the polyanionic sheets of decavanadate reveals high axial anisotropy and field induced SIM behaviour

In this work, we report an inorganic compound [{Co(H₂O)₆}²⁺{Na₄V₁₀O₂₈}^2-] (1) in which the polyanionic sheets of decavanadate play the role of a diamagnetic matrix that reduces the dipolar-dipolar and spin-spin interactions between [Co(H₂O)₆]⁺² units to suppress the fast tunnelling of magnetization. Structural analysis reveals that each [Co(H₂O)₆]⁺² complex is surrounded by four decavanadates and separated by a large internuclear distance (9 Å). It was also found that the adjacent decavanadates are connected via sodium ions and form a 2D sheet of the inorganic layer in which the [Co(H₂O)₆]²⁺ ions are present in between two layers. Detailed dc (direct current) and ac (alternating current) magnetic measurements disclose the presence of large easy-axis anisotropy (D = -102 cm^-1) and field induced slow magnetic relaxation behaviour with a spin reversal barrier of U_eff = 50 K. Additionally, the temperature dependence of the relaxation time reveals that the Raman and QTM processes mainly play an important role rather than the thermally activated Orbach process in the overall relaxation dynamics of the studied compound. To analyse the electronic structure and magnetic properties of compound 1, ab initio calculations were performed which further support the experimental observations. Notably, the U_eff value of 1 represents the highest energy barrier reported for POM based SMMs with transition metal ions to date.

Isolation of a Bimetallic Cobalt(III) Nitride and Examination of Its Hydrogen Atom Abstraction Chemistry and Reactivity toward H2

Room temperature photolysis of the bis(azide)cobaltate(II) complex [Na(THF)_x][(^ketguan)Co(N₃)₂] (^ketguan = [(tBu₂CN)C(NDipp)₂]^-, Dipp = 2,6-diisopropylphenyl) (3a) in THF cleanly forms the binuclear cobalt nitride Na(THF)₄{[(^ketguan)Co(N₃)]₂(μ-N)} (1). Compound 1 represents the first example of an isolable, bimetallic cobalt nitride complex, and it has been fully characterized by spectroscopic, magnetic, and computational analyses. Density functional theory supports a Co^III═N═Co^III canonical form with significant π-bonding between the cobalt centers and the nitride atom. Unlike other group 9 bridging nitride complexes, no radical character is detected at the bridging N atom of 1. Indeed, 1 is unreactive toward weak C-H donors and even cocrystallizes with a molecule of cyclohexadiene (CHD) in its crystallographic unit cell to give 1·CHD as a room temperature stable product. Notably, addition of pyridine to 1 or photolyzed solutions of [(^ketguan)Co(N₃)(py)]₂ (4a) leads to destabilization via activation of the nitride unit, resulting in the mixed-valent Co(II)/Co(III) bridged imido species [(^ketguan)Co(py)][(^ketguan)Co](μ-NH)(μ-N₃) (5) formed from intermolecular hydrogen atom abstraction (HAA) of strong C-H bonds (BDE ∼ 100 kcal/mol). Kinetic rate analysis of the formation of 5 in the presence of C₆H₁₂ or C₆D₁₂ gives a KIE = 2.5 ± 0.1, supportive of a HAA formation pathway. The reactivity of our system was further probed by photolyzing benzene/pyridine solutions of 4a under H₂ and D₂ atmospheres (150 psi), which leads to the exclusive formation of the bis(imido) complexes [(^ketguan)Co(μ-NH)]₂ (6) and [(^ketguan)Co(μ-ND)]₂ (6-D), respectively, as a result of dihydrogen activation. These results provide unique insights into the chemistry and electronic structure of late 3d metal nitrides while providing entryway into C-H activation pathways.

Self-Assembly of Ag+/[PW11NbO40]4- Complexes in Nonaqueous Solutions

Self-assembly between Ag⁺ and [PW₁₁NbO₄₀]^4- in N- and O-donor solvents (nitriles and amides) has been studied. In the case of dimethylformamide (DMF), formation of a yellow [Ag₄(DMF)₁₂][PW₁₁NbO₄₀] (1a) metastable phase and a colorless [Ag₄(DMF)₁₀][PW₁₁NbO₄₀] (1) stable phase was observed. In acetonitrile (CH₃CN), the product was [Ag(CH₃CN)₄]₂{[Ag(CH₃CN)₃]₂[PW₁₁NbO₄₀]} (2a). By contrast, [SiW₁₂O₄₀]^4- of the same size and charge as [PW₁₁NbO₄₀]^4- produces [Ag(CH₃CN)₃]₄[SiW₁₂O₄₀] (3a). Partial desolvation of 2a and 3a leads to Ag₄[PW₁₁NbO₄₀]·7.5CH₃CN (2) and Ag₄[SiW₁₂O₄₀]·7.5CH₃CN (3), respectively. The CH₃CN molecules in the structure of 2 are labile, and this compound was used as the starting material to study solvent-exchange processes in N-methyl-2-pyrrolidone (NMP), dimethylacetamide (DMA), diethylformamide (DEF), and benzonitrile (PhCN) solutions. These solvent reactions yield [Ag(DMA)₄][Ag₃(DMA)₆][PW₁₁NbO₄₀] (4), [Ag₂(NMP)₄(CH₃CN)]₂[PW₁₁NbO₄₀]·1.3NMP (5a), [Ag₂(NMP)₅]₂[PW₁₁NbO₄₀] (5b), Ag₄[PW₁₁NbO₄₀]·9.5DEF (6), and [Ag(PhCN)₄]₂[{Ag(PhCN)₃}₂PW₁₁NbO₄₀] (7). All products were isolated and characterized by single-crystal X-ray diffraction (except for 2 and 3), IR, elemental analysis, and thermogravimetric analysis techniques. The O-donor solvents favor polynuclear, solvent-bridged cationic aggregates. In the case of DMF, DMA, and DEF discrete, tri- and tetranuclear polycations are observed, while in the case of NMP, the formation of infinite polycationic structures takes place. By contrast, the N-donor solvents (CH₃CN and PhCN) favor mononuclear cations, which can exist either as distorted tetrahedral, isolated [Ag(Solv)₄]⁺ cations or as pseudotriangular {Ag(Solv)₃}⁺ units, additionally coordinated to a polyoxometalate. Screening of the luminescent properties for solid samples of 1-7 revealed that only 5a/5b and 7 are emissive. In particular, the sample containing 5a and 5b demonstrates long-lived phosphorescence with a 30 ms lifetime.