Abstract
We report a computational study on the structures and bonding of a charged molecular alloy D2h [Pd2As14]4− (1), as well as a model D2h [Au2Sb14]4− (2) cluster. Our effort makes use of an array of quantum chemistry tools: canonical molecular orbital analysis, adaptive natural density partitioning, natural bond orbital analysis, orbital composition analysis, and nucleus independent chemical shift calculations. Both clusters consist of two X7 (X = As, Sb) cages, which are interconnected via a M2 (M = Pd, Au) dumbbell, featuring two distorted square-planar MX4 units. Excluding the Pd/As or Au/Sb lone-pairs, clusters 1 and 2 are 50- and 44-electron systems, respectively, of which 32 electrons are for two-center two-electron (2c-2e) As-As or Sb-Sb σ bonds and an additional 16 electrons in 1 for 2c-2e Pd-As σ bonds. No covalent Pd-Pd or Au-Au bond is present in the systems. Cluster 1 is shown to possess two globally delocalized σ electrons, whereas 2 has two σ sextets (each associated with an AuSb4 fragment). Thus, 1 and 2 conform to the (4n + 2) Hückel rule, for n = 0 and 1, respectively, rendering them σ-aromaticity.
Highlights
The advances in cluster science have led to a large body of intermetallic and multinary clusters, which are blurring the lines between nanomaterials and cluster chemistry[1]
We have performed a detailed computational study on the [Pd2As14]4− (1) cluster at the PBE0 level[39] of density-functional theory (DFT)[40] and elucidated its nature of chemical bonding using a range of state-of-the-art quantum chemistry tools: canonical molecular orbital (CMO) analysis, adaptive natural density partitioning (AdNDP)[41], natural bond orbital (NBO)[42] analysis, orbital composition analysis, and nucleus independent chemical shift (NICS)[43]
To further ensure the computational reliability for a naked multiply charged anion, conductor-like polarizable continuum mode (C-PCM)[44,45,46,47] calculation as an alternative method has been carried out for 1 to take into account the solvation effects, whose results many people believe should be closer to the truth for a synthetic cluster compound in the bulk: a multiply charged anion being stabilized by bulky ligands including counter-ions[24]
Summary
The advances in cluster science have led to a large body of intermetallic and multinary clusters, which are blurring the lines between nanomaterials and cluster chemistry[1]. We shall report on a quantum chemical study on the structure and bonding of a “charged molecular alloy” cluster: D2h [Pd2As14]4− (1). The Pd centers are coordinated by As in a distorted square-planar fashion, featuring two PdAs4 fragments In this bulk compound, the [Pd2As14]4− tetraanion is effectively stabilized by four K+ counter-ions. The computational data allows an in-depth understanding of the nature of bonding in 1, which turns out to possess two delocalized σ electrons within the square-planar PdAs4 fragments, rendering σ-aromaticity for the charged molecular alloy according to the (4n + 2) Hückel rule, for n = 0. The concept of σ-aromaticity, in particular σ sextets, was discussed recently in a synthetic [Au2Sb16]4− compound[38]
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