Deeply Reduced Chiral Mo^IV₆-Polyoxometalates with Highly Enhanced Electronic Conductivity

A series of deeply reduced Mo^IV₆-polyoxometalates (POMs), [Mo^IV₆(bpy)₆Mo^VI₂O₁₆S₂]·10H₂O (1) (bpy = 2,2'-bipyridine), [Mo^IV₆(bpy)₄(L-TrA)₂Mo^V₄(bpy)₂O₁₄S₄]₂·36.5H₂O (2), and [Mo^IV₆(bpy)₄(D-TrA)₂Mo^V₄(bpy)₂O₁₄S₄]₂ (3) (TrA = tartaric acid), were obtained from one-pot hydrothermal syntheses. They feature the bioxo-bridged [Mo^IV₃O₃S]⁴⁺ incomplete cuboidal cores stabilized by strong triangular metal-metal bonds and datively chelated bpy π-ligands. The two apical Mo^VIO₄ in 12e-Mo₈ (1) and two [Mo^V₂O₃STrAbpy] in 16e-Mo₁₀ (2, 3) complete the octahedral coordination geometry of the deeply reduced Mo^IV, not counting the Mo^IV-Mo^IV d²-d² bonds. The aromatic planar bpy π ligands form a two-dimensional face-to-face π stacking supramolecular structure, which are further strengthened by point-to-face C-H···π interactions in the π stacked layers. 16e-Mo₁₀ (2, 3) has stronger intramolecular face-to-face π stacking interactions, which are interconnected by intermolecular C-H···π into a three-dimensional framework structure. These structural features account for their highly enhanced electronic conductivities (1, 6.19 × 10^-8 S cm^-1; 2, 2.18 × 10^-7 S cm^-1), providing a new way of thinking to improve electronic conductivity of POMs. 1-3 have been described by first-principles density function theory (DFT) calculations and also characterized by elemental analyses, powder X-ray diffraction (PXRD), X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA), solid UV-visible spectra, and circular dichroic (CD) spectra.