New Insights into the Electrochemistry of Molybdenum(VI) Dioxo Complexes Containing a Tetradentate S₂(NH)₂-Type Ligand

The electrochemistry of dioxomolybdenum(VI) with the tetradentate S₂(NH)₂-type ligand N,N′-bis(2-mercaptophenyl)-2,3-diaminobutane (mPhabH₄), at neutral, acidic, and basic conditions, has been investigated. The molecular structure of (mPhabH₂)Mo^VIO₂ (1) has been determined: space group P2₁2₁2₁ (No. 19), a = 8.667(<1), b = 12.884(<1), c = 19.877(3) Å, V= 2220 ų; Z = 4. The cis-dioxo complex 1 displays a highly distorted octahedral structure. Electrochemical, one-electron reduction of the Mo(VI) compound 1 is followed by a fast dehydration reaction leading to [(mPhab)MoO]⁻ (4), which can be further reduced in a subsequent one-electron reduction step to the Mo(IV) dianion. At neutral conditions this process appears as a single two-electron reduction. Upon addition of N-methylimidazole (NMI) the dehydration rate decreases and two separate reduction steps can be observed. Acidic conditions also lead to a decrease of the dehydration rate and to two separate one-electron reductions. In basic solution the Mo(VI) complex 1 is reduced by methanol to [(mPhab)Mo^vO]⁻ (4), which shows both a quasi-reversible one-electron reduction wave to form the Mo(IV) dianion and a quasi-reversible oxidation wave. Synthetically, [(mPhab)Mo^vO]⁻ (4) can also be obtained via deoxygenation of 1 by Ph₂PMe in the presence of N-methylimidazole, presumably via formation of a Mo(IV)-NMI adduct that undergoes a comproportionation reaction with 1 and subsequent dehydration to the mononuclear Mo(V) complex 4. Neither μ-oxo-bridged Mo(V) dimers nor mononuclear Mo(IV) species can be detected.