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(N H)2-type ligand bis(2-mercap-tophenyl)-2,3-diaminobutane(mPhabH4), at neutral, acidic, and basic conditions, has been investigated. The mole cular structure of (mPhabH2)Mo(VI)O2 (1) has been determined: space gr oup P2(1)2(1)2, (No. 19), a = 8.667 (<1), b = 12.884(<1), c = 19.877(3 ) angstrom, V= 2220 angstrom3; Z = 4. The cis-dioxo complex 1 displays a highly distorted octahedral structure. Electrochemical, one-electro n 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 reduc tion. Upon addition of N-methylimidazole (NMI) the dehydration rate de creases and two separate reduction steps can be observed. Acidic condi tions also lead to a decrease of the dehydration rate and to two separ ate one-electron reductions. In basic solution the Mo(VI) complex 1 is reduced by methanol to [(mPhab)Mo(V)O]- (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(V)O]-(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 de hydration to the mononuclear Mo(V) complex 4. Neither mu-oxo-bridged M o(V) dimers nor mononuclear Mo(IV) species can be detected.