A new hybrid inorganic-organometallic salt has been obtained from the reaction of the Keggin-type silicotungstate anion with ferrocene in a water/methanol mixture as a result of the partial oxidation of ferrocene molecules to ferrocenium cations. Single-crystal X-ray diffraction analysis reveals the presence of four ferrocenium (Fe^III) cations and one ferrocene (Fe^II) molecule per plenary Keggin anion in the crystal structure of [Fe^III (Cp)₂]₄[SiW₁₂O₄₀]·[Fe^II(Cp)₂]·2CH₃OH (1). Compound 1 thus constitutes the first example in the literature in which ferrocenium and ferrocene species coexist in the structure of a polyoxometalate-based salt. The two crystallographically independent ferrocenium species in the asymmetric unit of 1 exhibit different configurations: One displays an eclipsed conformation with ideal D_5h symmetry, whereas the conformation in the other one is staggered D_5d. The crystal packing of 1 can be best described as an organometallic sub-lattice of ferrocenium and ferrocene species linked by a network of π-π interactions that generates rectangular cavities of about 14 × 10 Å in which strings of Keggin anions and methanol molecules are hosted, further connected to each other via weak O_POM···C_MeOH-O_MeOH···O_POM type hydrogen bonds. The charge-transfer nature of the salt has been studied by solid-state diffuse reflectance UV-Vis spectroscopy and the presence of magnetically isolated Fe^III/Fe^II centres has been confirmed by Mössbauer spectroscopy. A topological study carried out on all of the pristine ferrocenyl species deposited in the Cambridge Structural Database (CSD) has allowed two main conclusions to be drawn: (1) these species tend to adopt extreme conformations (either eclipsed or staggered) with less than a 15% of examples showing intermediate states and (2) the oxidation state of the iron centres can be unequivocally assigned on the basis of a close inspection of the Fe···Cp distances, which allows ferrocene neutral molecules and ferrocenium cations to be easily distinguished.

New organophosphonate derivatives of monovacant Keggin-type polyoxotungstates that contain naphthyl groups have been synthesized and characterized in both solid state and solution. Single-crystal structural analysis shows that two phosphonate groups occupy the vacant position of the lacunary cluster unit in the isostructural compounds [N(C₄H₉)₄]₃[H(POC₁₁H₉)₂(α-HBW₁₁O₃₉)] (TBA-1) and [N(C₄H₉)₄]₃[H(POC₁₁H₉)₂(α-SiW₁₁O₃₉)] (TBA-2). Liquid-solution UV–Vis transmittance and solid-state diffuse reflectance spectroscopy studies reveal the presence of a new absorption band in the visible region, the charge transfer character of which has been further confirmed by time-dependent density functional theory (TD-DFT) calculations. The latter evidence that the charge transfer process is dominated by transitions from the highest occupied molecular orbital (HOMO), localized in the aromatic ring of the organic group, to the lowest unoccupied molecular orbital (LUMO), localized in the Keggin anion. Photoluminescence studies show that the fluorescent properties of the 1-naphthylmethylphosphonate group are quenched upon its incorporation into the inorganic oxo-tungstate skeleton. The solution stability of the hybrid clusters has been evaluated by a combination of ¹H-, ¹³C- and ³¹P-Nuclear Magnetic Resonance spectroscopy and Electrospray Ionization-Mass Spectrometry. The hybrid polyanion [H(POC₁₁H₉)₂(α-HBW₁₁O₃₉)]³⁻ (1) herein constitutes the first structurally characterized organo-p-block containing borotungstate, and hence it confirms that this strategy for the organic functionalization of polyoxometalate clusters can be applied to new platforms belonging to the family of group-13 heteropolyoxotungstates.

Experimental magnetic studies performed on the [{CuLX}₂] system (HL = pyridine-2-carbaldehyde thiosemicarbazone, X = Cl⁻, Br⁻, I⁻) point to the larger electronegativity in X, the lower magnitude of the antiferromagnetic interactions. In order to confirm this and other trends observed and to dip into them, computational studies on the [{CuLX}₂] (X = Cl⁻ (1), I⁻ (2)) compounds are here reported. The chemical and structural comparisons have been extended to the compounds obtained in acid medium. In this regard, chlorido ligands yield the [Cu(HL)Cl₂]∙H₂O (3) complex, whose crystal structure shows that thiosemicarbazone links as a tridentate chelate ligand to square pyramidal Cu(II) ions. On the other hand, iodido ligands provoke the formation of the [{Cu(H₂L)I₂}₂] (4) derivative, which contains pyridine-protonated cationic H₂L⁺ as a S-donor monodentate ligand bonded to Cu(I) ions. Crystallographic, infrared and electron paramagnetic resonance spectroscopic results are discussed. Computational calculations predict a greater stability for the chlorido species, containing both the neutral (HL) and anionic (L⁻) ligand. The theoretical magnetic studies considering isolated dimeric entities reproduce the sign and magnitude of the antiferromagnetism in 1, but no good agreement is found for compound 2. The sensitivity to the basis set and the presence of interdimer magnetic interactions are debated.

The well-known compound tetrakis(tert-butylammonium)-cyclo-tetrametavanadate (V), [(CH₃)₃CNH₃]₄[V₄O₁₂] (1h_RT), which crystallizes in the tetragonal I4/m space group, undergoes an irreversible solid state transformation upon heating, constituting one of the few examples in which the initial and the final stages are structurally characterized by sc-XRD. Now, we observed the ability of the same compound to undergo an additional single-crystal-to-single-crystal (SCSC) transformation upon thermal stimuli, but this time at low temperatures (153 K). Compound 1h_RT contains a discrete unprotonated [V₄O₁₂]⁴⁻ tetrahedral anion in which V and O bridging atoms are coplanar. In both phases, these tetrameric anions are linked through tert-butylammonium cations in an extensive network of hydrogen bonds, but at low temperatures, this phase loses its characteristic O-V-O coplanarity, with the resulting rearrangement of the crystal packing and hydrogen-bond network which provide its reversibility at low temperatures. Again, the initial and final stages have been characterized structurally by sc-XRD.