Coordination Complexes of Molybdenum with 3,6-Di-<i>tert</i>-butylcatechol. Addition Products of DMSO, Pyridine <i>N</i>-oxide, and Triphenylarsine Oxide to the Putative [Mo<sup>VI</sup>O(3,6-DBCat)<sub>2</sub>] Monomer and Self-Assembly of the Chiral [{Mo<sup>VI</sup>O(3,6-DBCat)<sub>2</sub>}<sub>4</sub>] Square
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Molybdenum complexes of 3,6-di-tert-butylcatechol have been prepared from the reaction between [Mo(CO)6] and 3,6-di-tert-butyl-1,2-benzoquinone. A putative “[MoO(3,6-DBCat)2]” monomer is assumed to form initially by reaction with trace quantities of oxygen. Condensation of the reaction mixture leads to the formation of oligomeric products, including the [{MoO(3,6-DBCat)2}4] chiral square isolated by chromatographic separation. Molybdenum centers at the corner of the square are bridged by oxo ligands centered along edges. Four-fold and inversion crystallographic symmetry gives tetramers as either ΛΛΛΛ or ΔΔΔΔ isomers, and the crystal structure consists of parallel columns of squares with the same chirality. Addition of O-Subst (O-Subst = dmso, pyridine N-oxide, triphenylarsine oxide) ligands to [MoO(3,6-DBCat)2] occurs selectively to give cis-[MoO(O-Subst)(3,6-DBCat)2] products. All three addition complexes are fluxional in solution. The temperature-dependent stereodymanic behavior of [MoO(dmso)(3,6-DBCat)2] has been shown to occur via a trigonal prismatic intermediate (Bailar twist) that conserves the cis disposition of oxo and dmso ligands. Electrochemical and chemical reduction reactions have been investigated for [MoO(dmso)(3,6-DBCat)2] with interest in displacement of SMe2 with formation of cis-[MoO2(3,6-DBCat)2]2-. Cyclic voltammetry shows an irreversible two-electron reduction for the complex at −0.852 V (vs Fc/Fc+). Chemical reduction using CoCp2 was observed to give a product with an electronic spectrum that is generally associated with cis-[MoO2(Cat)2]2- complexes. Structural characterization revealed that the product was [CoCp2][MoO(3,6-DBCat)2], possibly formed as the product of dmso displacement upon one-electron reduction of [MoO(dmso)(3,6-DBCat)2].
