Electronic Properties of Polyoxometalates:  Electron and Proton Affinity of Mixed-Addenda Keggin and Wells−Dawson Anions

Abstract

A series of systematic DFT calculations were conducted on Keggin [SiW(9)M(3)O(40)](n-), M = Mo, V, and Nb; and Wells-Dawson anions [P(2)M(18)O(62)],(6-) M = W and Mo; [P(2)M(15)M(3)'O(62)](m-), M = W and Mo, M' = W, Mo, and V to analyze the redox properties and the basicity of the external oxygen sites in polyoxometalates with nonequivalent addenda metals. The energy and composition of the lowest unoccupied orbitals, formally delocalized over the addenda atoms, determine the redox properties of a polyoxometalate. When a Mo(6+) substitutes one W(6+) in the 1:12 tungstate, the energy of the LUMO decreases and the cluster is more easily reduced. The tungstoniobates behave differently because the niobium orbitals insert into the tungsten band and the reduction of [SiW(9)Nb(3)O(40)](7-) yields the blue species SiW(9)Nb(3) 1e and not the cluster SiW(9)Nb(2)Nb(IV). In Wells-Dawson structures, the polar and equatorial sites have different electron affinities and the reduction preferentially occurs in the equatorial sites. Inserting ions with larger electron affinities into the polar sites can modify this traditional conduct. Hence, the trisubstituted [P(2)W(15)V(3)O(62)](9-) anion is reduced in the vanadium polar sites. By means of molecular electrostatic potential maps and the relative energy of the various protonated forms of [SiW(9)V(3)O(40)](7-) and [SiW(9)Mo(3)O(40)](4-), we established the basicity scale: OV(2) > OMo(2) > OW(2) > OV > OW > OMo. Finally, a continuum model for the solvent enabled us to compare anions with different total charges.

Gas-Phase Fragmentation Pathways of Mixed Addenda Keggin Anions: PMo12-nWnO403– (n = 0–12)

“…Our results indicate that the metal atoms are not distributed randomly in the mixed addenda Keggin anions. This observation is consistent with previous results from the literature, which indicate that W atoms as well as other early transition metals substitute preferentially at the crown sites of mixed Keggin POMs [7,24,57,58]. The formation of segregated islands of W within mixed addenda POMs may explain the non-statistical behavior presented in Figure 4.…”

“…These results indicate that regardless of the charge state, the electron binding energy of W 12 POM is higher than that of Mo 12 POM. In addition, Poblet and co-workers demonstrated that the energy of the LUMO is substantially decreased when only one W atom in the W 12 POM is replaced with Mo [24]. It is reasonable to assume that the electron binding energies of the mixed addenda POMs described herein fall between these two extremes.…”

“…Such a proton transfer process is possible depending on the proton affinities of the precursor POMs, presence of water molecules or other acidic neutral species in the mass spectrometer, and the residence time of the ions in the quadrupole ion trap. POMs are known to be basic species [1,24] and previous studies have estimated the proton affinities of Mo 12 POM and W 12 POM to be 17.1 eV (394.3 kcal/mol) and 16.6 eV (382.8 kcal/mol), respectively [20,61]. By comparison, the gas-phase acidity of water is in a range of 387.7-390.3 kcal/ mol (http://webbook.nist.gov/chemistry/).…”

“…Mixed Keggin POMs with Mo and W have been investigated previously by several research groups both theoretically [24] and experimentally [25,26]. In the present study, we use a straightforward method of preparing mixed addenda Keggin POMs, PMo 12-n W n O 40 3-(n = 0-12), for analysis by electrospray ionization mass spectrometry (ESI-MS) and examine their dissociation pathways and relative stability towards fragmentation by means of collision-induced dissociation (CID) experiments.…”

Simulating the Favorable Aggregation of Monolacunary Keggin Anions

“…49 If, on the one hand, static QM calculations have proven essential to investigate intrinsic properties such as structural stability, 50-55 electronic structure, [56][57][58][59][60] reactivity, 61,62 and spectroscopic signature; 63 it is equally true that classical MD simulations of Keggin salt solutions are an obligatory choice if one wishes to better elucidate their dynamic properties at the nanoscale. As such, their use alongside experiment is becoming increasingly common.…”