Electron Delocalization in Mixed-Valence Keggin Polyoxometalates. Ab Initio Calculation of the Local Effective Transfer Integrals and Its Consequences on the Spin Coupling

Abstract

We present a quantitative evaluation of the influence of the electron transfer on the magnetic properties of mixed-valence polyoxometalates reduced by two electrons. For that purpose, we extract from valence-spectroscopy ab initio calculations on embedded fragments the value of the transfer integrals between W nearest-neighbor atoms in a mixed-valence alphaPW(12)O(40) polyoxowolframate Keggin anion. In contradiction with what is usually assumed, we show that the electron transfer between edge-sharing and corner-sharing WO(6) octahedra have very close values. Considering fragments of various ranges, we analyze the accuracy of calculations on fragments based on only two WO(5) pyramids which should allow a low cost general study of transfer parameters in polyoxometalates. Finally, these parameters are introduced in an extended Hubbard Hamiltonian that models the whole anion. It permits to prove that electron transfers induce a large energy gap between the singlet ground state and the lowest triplet states providing a clear explanation of the diamagnetic properties of the mixed-valence Keggin ions reduced by two electrons.

Intramolecular and Lattice Dynamics in V6-nIVVnVO7(OCH3)12Crystal

“…These metal-oxygen anionic clusters with metal ions in d 0 electronic configuration can accommodate several excess, usually strongly delocalized, electrons. As it has been shown by several theoretical studies, if two electrons reduce a polyoxoanion, the delocalization leads to a strong stabilization of singlet spin state [5][6][7][8][9]. This stabilization is due to the filling of the lowest non-degenerate delocalized molecular orbital by two electrons.…”

MVPACK: A package to calculate energy levels and magnetic properties of high nuclearity mixed valence clusters

“…This result was initially attributed to a very strong antiferromagnetic coupling via a multiroute superexchange mechanism, 39 but more recently it was suggested [17][18][19] that the electron delocalization can stabilize the diamagnetic ground state and it was proved 18,19,40,41 (see also review 2 ) that a combination of Coulomb repulsion between the electrons and electron delocalization is an efficient mechanism of a spin pairing in the ground state. The cited works 18,19,40,41 dealt with the case of 2e-reduced polyoxoanion containing delocalized electronic pair. Here we present the calculations of the energy patterns for a series of polyoxoanions with Keggin structure containing different even numbers P 5 2, 4, 6, 8, 10, 12 of itinerant electrons.…”

“…In these calculations we use the set of parameters obtained from the ab initio study of the double reduced polyoxoanion with delocalized electronic pair. 40,41 The set of parameters includes the three one-electron transfer integrals t 5 2530 meV, t The energy patterns calculated with MVPACK are shown in Figure 14. One can see that for all even numbers P except P 5 4 the ground state proves to be diamagnetic.…”

Electron Delocalization and Electrostatic Repulsion at the Origin of the Strong Spin Coupling in Mixed‐Valence Keggin Polyoxometalates: Ab Initio Calculations of the One‐ and Two‐Electron Processes

“…For that purpose, we chose the appealing case of a two-electron-reduced polyoxowolframate anion of Keggin structure as a testing ground for this procedure. A previous work [10] proved that very accurate values of the Abstract: We present here a general theoretical procedure to treat the problem of electron delocalization and magnetic interactions in high-nuclearity mixed valence clusters based on polyoxometalates. The main interactions between the delocalized electrons of mixed-valence polyoxometalate anions are extracted from valence spectroscopy ab initio calculations on embedded fragments.…”