Synthesis and Characterization of K0.19Ba3.81Mo22O34: A Quaternary Reduced Molybdenum Oxide Containing Metal-Metal Bonded Oligomers with Five Trans Edge-Shared Molybdenum Octahedra

Abstract

Crystals of K0.19Ba3.81Mo22O34, a metal-metal bonded oligomer in the series Mn-xMO4n+2O6n+4 with n = 5, have been synthesized. The Mo22 clusters consist of five trans edge-shared Mo octahedra. The dominant features of K0.19Ba3.81Mo22O34 include four short and four long apical-apical Mo bond distances within the cluster, relatively long intercluster metal-metal bond distances (>2.99 Å), and cluster interconnections which have a stair-step arrangement from one Mo22 unit to the next. K0.19Ba3.81Mo22O34 crystallizes in the centrosymmetric space group P21/a, with lattice parameters of a = 9.908(2) Å, b = 9.353(2) Å, c = 15.951(3) Å, β = 98.78(2)°, Vol = 1460.8(4) Å3, Z = 2, and R = 0.0493 (Rw = 0.0603) for 2768 reflections with I > 3.5σ(I). Five cations per formula unit can be accommodated in the pockets created by the interconnected Mo22O50 clusters. However, this phase has four fully occupied cation sites and one vacancy. The number of electrons available for metal-metal bonding (MCE) was derived from Mo-O bond valence sums around the molybdenum atoms (72(2)) and is in good agreement with the value calculated from format oxidation states (71.8e). Mo-Mo bond order sums, excluding intercluster metal-metal interactions, although slightly high (MCE = 72.8(5)), indicate maximum utilization of electrons for Mo-Mo bonding. The magnetic susceptibility of this compound indicates that it is spin-paired with a small paramagnetic tail at low temperatures and a large residual χTIP above 50 K. Electron spin resonance results indicate that the paramagnetic tail may be arising from molybdenum containing impurities, or defects, on which electrons are trapped. Structural and electronic comparisons to two known pentameric oligomers are discussed.