On the Origin of Alternating Bond Distortions and the Emergence of Chirality in Polyoxometalate Anions

Abstract

Alternating short and long bond length (ABL) distortions observed within the ring structures of molecular metal oxide anions or polyoxometalates (POMs) are reminiscent of the cooperative linear ABL distortions in perovskite d(0) metal oxides. We show herein that these distortions have a common origin: a pseudo Jahn-Teller (PJT) vibronic instability. Four POM structural types with different M(n)O(n) ring sizes are investigated herein using density functional theoretical methods: Lindqvist [M6O19](q-) (n = 4), Keggin alpha-[XM12O40](q-) (n = 6), Wells-Dawson alpha-[X2M18O62](q-) (n = 8), and Preyssler [(Na)P5W30O110](14-) (n = 10), where M = Mo(VI) and W(VI) and X = Si(IV), Ge(IV), P(V), As(V), S(VI), and Se(VI). Chirality is induced within the latter three structural types by the ABL ring distortions. The calculations confirm the PJT vibronic origin of the ABL distortions with good agreement between calculated geometries and published single-crystal X-ray diffraction data. Both theory and experiment show that the vibronic interaction and distortion magnitude increase for (1) molybdates relative to that of tungstates, (2) larger M(n)O(n) ring sizes, (3) increases in negative charge of the internalized fragments (O(2-) or XO4(q-)), and (4) d(0) versus d(n) metal oxidation states. The PJT vibronic coupling model explains these observations in terms of the energy gap between Kohn-Sham frontier molecular orbitals (MOs) concomitant with the propensity for metal-oxygen pi-bonding within the M(n)O(n) rings. The frontier MOs for the undistorted nuclear configurations are largely nonbonding pi-O(p) (occupied) and pi-M(d) (unoccupied) in character, where smaller HOMO-LUMO (H-L) gap energies lead to greater metal-oxygen pi-orbital mixing under the influence of the nuclear distortion. A reduction in pi-bond order decreases the distortion in mixed-valence POMs. Of the tungstates examined, only the Preyssler anion shows pronounced ABL ring distortions, which derive from its large ring size and concomitant small H-L gap.

Which is the most basic oxygen in [Ta6O19]8−? — Synthesis and structural characterization of [H2Ta6O19]6−

“…It is virtually identical to the structure predicted by a DFT calculation (see Table 1) [24]. [14,17,20,23,41]. The calculations reproduced experimental structures reasonably well in those studies, but the fits were seldom perfect.…”

Polyoxometallate als Bausteine für funktionelle Nanosysteme

“…[28,43] Bei diesen Systemen werden hoch geladene "POM-Liganden" verwendet, um reaktive Oxo-, Peroxooder Superoxid-Gruppen zu stabilisieren, wovon auch die Entwicklung neuer Katalysatoren und Strukturmuster sowie mechanistische Untersuchungen profitieren können. [44] 2.3. Polyoxometallate und Chiralität [45] Das breite Anwendungsspektrum der Polyoxometallate, welches Katalyse, Biologie, Medizin, Magnetismus, Materialwissenschaften und Photochemie beinhaltet, führt zu einem großen Interesse an der Entwicklung chiraler Metalloxidstrukturen.…”

“…[58] Auch der Mechanismus der Reaktion von Polyoxoniobaten mit Wasserstoffperoxid wurde mit ESI-MS untersucht. [44] Bei einer aktuellen Arbeit unserer Forschungsgruppe wurde die Kryospray-Massenspektrometrie (CSI-MS) zur Untersuchung von instabilen POM-Komplexen [56] und Reaktionsmechanismen eingesetzt. Bei diesem massenspektrometrischen Verfahren kann die Temperatur der eingeführten Lösung und des erhaltenen Elektrosprays eingestellt werden.…”

TDDFT Studies on the Electronic Structures and Chiroptical Properties of Mono-Tin-Substituted Wells–Dawson Polyoxotungstates

“…8,9 Although experimental works on chiral POMs have been continuously developed, theoretical studies are few. 5 Chiral molecules can be detected through a variety of optical and spectroscopic effects, such as optical rotation (OR), optical rotatory dispersion (ORD), and circular dichroism (CD), which disclose the relationship between chirality and electronic structure. Electronic circular dichroism (ECD) measures the difference between the absorptions of left and right circularly polarized light and provides information such as the origin of optical activity and the electronic and geometric structures of chiral molecules.…”