Theoretical Studies on Hexanuclear Oxometalates [M₆L₁₉]^q−(M = Cr, Mo, W, Sg, Nd, U). Electronic Structures, Oxidation States, Aromaticity, and Stability

Abstract

We here report a systematic theoretical study on geometries, electronic structures, and energetic stabilities of six hexanuclear polyoxometalates [M6O19](2-) of the six-valence-electron metals including the d-elements M = Cr, Mo, W, Sg from group 6 and the f-elements M = Nd, U. Scalar relativistic density functional theory was applied to these clusters in vacuum and in solution. It is shown that the Oh Lindqvist structure of the isolated [M6O19](2-) units with hexavalent M elements (M(+6)) is only stable for the three heavy transition metals M = Mo, W, and Sg. The rare Th symmetry is predicted for M = U both in vacuum and in solution, owing to pseudo-Jahn-Teller distortion of these closed-shell systems. The Oh and Th structures correspond to cyclic "aromatic" U-̇O-̇U and alternating U=O-U bonding of cross-linked U4O4 rings, respectively. The reduced [U6O19](8-) cluster with pentavalent U(+5) also shows Th symmetry in vacuum, but Oh symmetry in a dielectric environment. The occurrence of different structures for varying fractional oxidation states in different environments is rationalized. Theoretical investigation of the recently synthesized U(+5) complex [U6O13L6](0) (L6 = tetracyclopentadienyl dibipyridine) shows a distorted Th-type symmetry, too. The stabilities of these complexes of different metal oxidation states are consistent with the general periodic trends of oxidation states.

Protactinium and the intersection of actinide and transition metal chemistry

“…It was proposed by Jiang that the cause of the asymmetry observed for the uranium clusters was due to weaker U(5 f )–O(2 p ) interactions in comparison to the stronger Mo, W, Sg ( d )–O(2 p ) interactions in the transition metals. This does not explain the observed O h symmetry of the Pa hexametalate and its symmetric bridging metal oxygen bonds like those observed in Nb and Ta 36 . Because the 5 f orbitals of Pa would are expected to be higher in energy than the U 5 f orbitals based on relativistic periodic trends, we might expect greater 6 d , or rather less 5 f character in the Pa–O bonds vs. the U system.…”

“…The calculated U(V) and U(VI) hexametalate geometries distort forming species with T h symmetry. Attempts to force the U complexes to octahedral symmetry resulted in unstable complexes as identified by imaginary vibrational frequencies for these anionic clusters as previously observed by Jiang et al in their study of uranium hexametalates 36 . Inspection of the bond distances, QTAIM parameters, and the natural localized molecular orbitals in these molecules also reveals a break in the nature of the bonding between the transition metals and within the actinides, Table 3 , and Fig.…”

“…Interestingly however, the value of the Laplacian for the Pa hexametalate is smaller than that calculated for the transition metals and is accompanied by a larger delocalization index. Though tempting to attribute this to increased covalent bonding in the Pa hexametalate versus the transition metals, it could also be a consequence of the larger radial extent of the 5 f and 6 d orbitals in Pa vs. the less expansive 4 d and 5 d in Nb and Ta, respectively 36 . Similar behavior is observed in the metal bonds to the central oxygen atom, which by the QTAIM metrics all indicate a weak and ionic like metal–oxygen interaction and, in a similar manner to the central oxygen atom in the synthesized tetranuclear Pa–peroxo complex, can be thought of as an unhybridized O 2− atom.…”

Periodic Trends in Actinyl Thio-Crown Ether Complexes

“…[9][10][11][12][13] Published structural data reveal a smooth actinide contraction of actinyl bond lengths from R(U-O) = 180 pm to R(Am-O) = 175 pm. [14][15][16][17] The existence of CmO 2 2+ in solution has not yet been reliably demonstrated and this species is considered unimportant in curium chemistry. [18,19] The actinyl moiety is remarkable due to the fact that it is, in contrast to similar d-element compounds, linear and has very short metal-oxygen bond lengths, suggesting strong multiple bond character.…”

TGMin: An efficient global minimum searching program for free and surface‐supported clusters

“…Until now, several global optimization programs based on the BH algorithm, in conjunction with density functional theory (DFT), tight‐binding DFT (DFTB), or (semi‐) empirical methods have been proposed . Tsinghua Global Minimum (TGMin), developed in 2011, is one of the most successful BH program as TGMin has already helped researchers identify tens of new clusters, including the hexagonal B 36 − cluster, the all‐boron borophenes and borospherenes B 40 − , B 26 − , B 28 − , B 30 − , B 35 − , B 39 − , metallo‐ borophenes, ‐borospherenes, and ‐nanotubes CoB 18 − , MnB 16 − , RhB 18 − , TaB 20 − , as well as the PrB 7 − , LnB 8 Ln (Ln = La, Pr) sandwich complexes, and others . TGMin was also used to identify atmospheric aerosol clusters and surface‐adsorbed species .…”