Magda Pascual Research

Microwave Synthesis of Alkali-Free Hexaniobate, Decaniobate, and Hexatantalate Polyoxometalate Ions
Mark A. Rambaran, Magda Pascual‐Borràs and C. André Ohlin

Microwave preparation of polyoxoniobates and ‐tantalates afford a more rapid alternative to conventional hydrothermal methods of synthesis, in addition to allowing for the use of anhydrous niobium pentoxide in lieu of niobic acid, albeit with diminished yields. Limitations associated with the pH at which different oxides can be activated and how this affects the accessibility of different products are also discussed.

European Journal of Inorganic Chemistry, 2019, 35, 3913-3918

DOI: 10.1002/ejic.201900750

Direct Single- and Double-Side Triol-Functionalization of the Mixed Type Anderson Polyoxotungstate [Cr(OH)₃W₆O₂₁]^6-
Nadiia I. Gumerova, Tania Caldera Fraile, Alexander Roller, Gerald Giester, Magda Pascual-Borràs, C. André Ohlin and Annette Rompel

Since the first successful triol-functionalization of the Anderson polyoxometalates, the six protons of their central octahedron X(OH)₆ (X—heteroatom, p- or d-element) have been considered as a prerequisite for their functionalization with tripodal alcohols, and therefore, the functionalization of Anderson structures from the unprotonated sides have never been reported. Here, we describe the triol-functionalization of [Cr(OH)₃W₆O₂₁]^6– leading to the single-side grafted anions [Cr(OCH₂)₃CRW₆O₂₁]^6– (CrW₆-tris-R, R = −C₂H₅, −NH₂, −CH₂OH) and the unprecedented double-side functionalized anion [Cr((OCH₂)₃CC₂H₅)₂W₆O₁₈]^3– (CrW₆-(tris-C₂H₅)₂), despite the lack of protons in the parent anion in the solid state. CrW₆-(tris-C₂H₅)₂ demonstrates the first example of double-side functionalized Anderson POT with the partially one-side protonated corresponding parent anion. The new heteropolytungstates were characterized by single-crystal X-ray diffraction, elemental analysis, Fourier-transform infrared spectroscopy, thermal gravimetric analysis, cyclic voltammetry, and electrospray ionization mass spectrometry. Density functional theory calculations were performed to investigate and compare the stability among the different isomers of the parent anion [Cr(OH)₃W₆O₂₁]^6–.

Inorganic Chemisty, 2019, 58, 1, 106-113

DOI: 10.1021/acs.inorgchem.8b01740

PMID: 30543282

Protonation and water exchange kinetics in sandwich polyoxometalates
C. André Ohlin and Magda Pascual Borràs

Density functional theory is used to explore the locus and consequences of protonation in [Zn₄(HO)₂(PW₉O₃₄)₂]¹⁰⁻. The results are used to explain recent observations regarding the contrasting pH effects on the water-ligand exchange in [Mn₄(H₂O)₂(P₂W₁₅O₅₆)₂]¹⁶⁻ and [Co₄(H₂O)₂(P₂W₁₅O₅₆)₂]¹⁶⁻, and the general effect of protonation on solvent exchange in metal oxides is discussed.

Dalton Transactions, 2018, 47, 13602-13607

DOI: 10.1039/c8dt02342e

PMID: 30207367

Bonding Insights from Structural and Spectroscopic Comparisons of {SnW₅} and {TiW₅} Alkoxido- and Aryloxido-Substituted Lindqvist Polyoxometalates
Dr. Balamurugan Kandasamy, Prof. Peter G. Bruce, Prof. William Clegg, Dr. Ross W. Harrington, Dr. Antonio Rodríguez‐Fortea, Dr. Magda Pascual‐Borrás and Dr. R. John Errington

Incorporation of {MX}^{n +} groups into polyoxometalates (POMs) provides the means not only to introduce reactivity and functionality but also to tune the electronic properties of the oxide framework by varying M, X and n. In order to elucidate the factors responsible for differences in reactivity between {TiW₅} and {SnW₅} Lindqvist‐type hexametalates, a series of alkoxido‐ and aryloxido‐tin substituted POMs (n Bu₄N)₃[(RO)SnW₅O₁₈] (R=Me, Et, i Pr and t Bu) and (n Bu₄N)₃[(ArO)SnW₅O₁₈] (Ar=C₆H₅, 4‐MeC₆H₅, 4‐t BuC₆H₅, 4‐HOC₆H₄, 3‐HOC₆H₄ and 2‐CHOC₆H₄) has been structurally characterised and studied by multinuclear NMR (¹H, ¹³C, ¹⁷O, ¹¹⁹Sn and ¹⁸³W) and FTIR spectroscopy. Spectroscopic and structural parameters were compared with those of titanium‐substituted homologues and, when coupled with theoretical studies, indicated that Sn−OR and Sn−OAr bonds are ionic with little π‐contribution, whereas Ti−OR and Ti−OAr bonds are more covalent with π‐bonding that is more prevalent for Ti−OR than Ti−OAr. This experimental and theoretical analysis of bonding in a homologous series of reactive POMs is the most extensive and detailed to date, and reveals factors which account for significant differences in reactivity between tin and titanium congeners.

Chemistry European Journal, 2018, 24, 2750-2757

DOI: 10.1002/chem.201705547

PMID: 29315880

Synthesis, Structure, and Antibacterial Activity of a Thallium(III)-Containing Polyoxometalate, [Tl₂{B-β-SiW₈O₃₀(OH)}₂]^12-
Wassim W. Ayass, Tamás Fodor, Zhengguo Lin, Rachelle M. Smith, Xiaolin Xing, Khaled Abdallah, Imre Tóth, László Zékány, Magda Pascual-Borràs, Antonio Rodríguez-Fortea, Josep M. Poblet, Linyuan Fan, Jie Cao, Bineta Keita, Matthias S. Ullrich and Ulrich Kortz

We have synthesized and structurally characterized the first discrete thallium-containing polyoxometalate, [Tl₂{B-β-SiW₈O₃₀(OH)}₂]^12– (1). Polyanion 1 was characterized in the solid-state and shown to be solution-stable by ^203/205Tl NMR, electrospray ionization mass spectrometry, and electrochemical studies. The antibacterial activity of 1 was also investigated.

Inorganic Chemistry, 2016, 55, 20, 10118-10121

DOI: 10.1021/acs.inorgchem.6b01921

PMID: 27704800

Accurate calculation of (³¹)P NMR chemical shifts in polyoxometalates
Magda Pascual-Borràs, Xavier López and Josep M. Poblet

We search for the best density functional theory strategy for the determination of ³¹P nuclear magnetic resonance (NMR) chemical shifts, δ(³¹P), in polyoxometalates. Among the variables governing the quality of the quantum modelling, we tackle herein the influence of the functional and the basis set. The spin–orbit and solvent effects were routinely included. To do so we analysed the family of structures α-[P₂W_18−xM_xO₆₂]ⁿ⁻ with M = Mo^VI, V^V or Nb^V; [P₂W₁₇O₆₂(M′R)]ⁿ⁻ with M′ = Sn^IV, Ge^IV and Ru^II and [PW_12−xM_xO₄₀]ⁿ⁻ with M = Pd^IV, Nb^V and Ti^IV. The main results suggest that, to date, the best procedure for the accurate calculation of δ(³¹P) in polyoxometalates is the combination of TZP/PBE//TZ2P/OPBE (for NMR//optimization step). The hybrid functionals (PBE0, B3LYP) tested herein were applied to the NMR step, besides being more CPU-consuming, do not outperform pure GGA functionals. Although previous studies on ¹⁸³W NMR suggested that the use of very large basis sets like QZ4P were needed for geometry optimization, the present results indicate that TZ2P suffices if the functional is optimal. Moreover, scaling corrections were applied to the results providing low mean absolute errors below 1 ppm for δ(³¹P), which is a step forward in order to confirm or predict chemical shifts in polyoxometalates. Finally, via a simplified molecular model, we establish how the small variations in δ(³¹P) arise from energy changes in the occupied and virtual orbitals of the PO₄ group.

Physical Chemistry Chemical Physics, 2015, 17, 8723-8731

DOI: 10.1039/c4cp05016a

PMID: 25738630

¹⁷O NMR chemical shifts in oxometalates: from the simplest monometallic species to mixed-metal polyoxometalates
Magda Pascual-Borràs, Xavier López, Antonio Rodríguez-Fortea, R. John Errington and Josep M. Poblet

We report a theoretical analysis on ¹⁷O NMR chemical shifts for a family of prototypical polyoxometalate anions. The huge diversity of structures and compositions in this family of oxometalates provides a unique resource for evaluating the influence of the metal type and connectivity over the resonance of ¹⁷O nuclei. For a set of 75 signals, we show that DFT calculations performed with the GGA-type OPBE functional, including spin–orbit and scaling corrections, provide a mean absolute error <30 ppm, a small value considering that the range of δ(¹⁷O) values in these systems is ∼1200 ppm. For terminal M=O oxygens, the chemical shifts primarily depend on the energy gap between π^*_M–O and σ_M–O orbitals. When M is in its highest oxidation state, the energy of π^*_M–O increases as we replace M going to the left and down in the periodic table. Consequently, we must expect large energy gaps and upfield shifts for O atoms linked to more electropositive ions. Although there is not a direct relationship between δ(¹⁷O) and the negative charge of the oxygen, it is not entirely wrong to correlate atomic charge and chemical shift because the ionicity of the M–O bond, the orbital energy gap and the charge density of oxygen are related. The ¹⁷O NMR chemical shifts move upfield with an increasing number of bound metal ions because of the larger energy gap in the involved orbitals. Finally, we explored the effect of protonation on δ(¹⁷O) in oxometalates and demonstrated that ¹⁷O NMR can be a powerful tool to identify the site(s) of protonation at low pH.

Chemical Science, 2014, 5, 2031-2042

DOI: 10.1039/c4sc00083h

Part of ISSN: 2041-6520