Revisiting the quasi-aromaticity in polynuclear metal chalcogenide clusters and their derivative "cluster-assembly" crystalline structures.

The concept of aromaticity is primarily invented to account for the high stability of conjugated organic compounds that possess a specific structural and chemical stability with (4 n + 2) π electrons. In 1988, quasi-aromaticity was theoretically proposed for the Mo 3 S 4 4+ core in the Mo 3 (μ 3 -S)(μ-S) 3 (χ-dtp) 3 (μ-dtp) L compound (χ: chelating ligand; dtp: (EtO) 2 PS 2 - ) illustrated by canonical molecular orbitals. However, the origin of the quasi-aromaticity and chemical bonding remains ambiguous, lacking a thorough analysis in terms of stability and quantitative measurement of the aromatic character. Thus, in this work, we systematically reported the electronic structure and aromaticity of a series of polynuclear metal chalcogenide clusters [M 3 X 4 (H 2 O) 9 ] 4+ (M = Cr, Mo, W, and Sg; X = O, S, Se, and Te) to explore an efficient tool of NICS index values at specific points to measure the quasi-aromaticity and to figure out the (d-p-d) π three-center bonding as the predominant origin from the arrangement of three Mo atoms and three bridged X atoms. Interestingly, derived from the Mo 3 ⋯S 3 quasi-plane, the extended sandwich cluster model of a S 3 ⋯Mo 3 ⋯S 3 (Mo 3 S 6 ) structure can be seen as the seed unit of the popular MoS 2 nanomaterials, with the resemblance between both molecular and periodic systems regarding geometries, electronic structures, and chemical bonding. Additionally, the highly symmetric Mo 3 S 4 core in [Mo 3 X 4 (H 2 O) 9 ] 4+ can be arranged in a staggered and stacked manner to create the Mo 6 S 8 2- building block, corresponding to the crystalline structures in BaMo 6 S 8 Chevrel phases, albeit with slight deformations. But the neutral Mo 6 S 8 cluster can be seen as the seed structure for the Mo 3 S 4 periodic materials for the high resemblance in terms of geometry, electronic structures and chemical bonding. Drawing upon the observed similarities between cluster models and materials, we propose a new concept termed "cluster-assembly" materials. This concept involves the expansion from a high-symmetry and/or aromatic stable cluster seed unit to form the corresponding derivative materials, presenting an alternative paradigm for investigating crystals and enriching our comprehension of the stabilities exhibited by both gas-phase clusters and solid-state materials. The concept of "cluster-assembly" materials not only contributes to the formulation of design strategies for novel materials or stable clusters but also provides valuable insights into the extension of periodic aromaticity.