Synthesis, structure, and redox behaviour of a novel cis-dioxomolybdenum(VI) dinuclear complex with a quadradentate dithiocarbamate

A novel dinuclear cis-dioxomolybdenum(VI) complex [{MoO2(Bz2endtc)}2] coordinated with a quadradentate dithiocarbamate (Bz2endtc2−: ((2-(dithiocarboxybenzylamino)ethyl)benzylamino)-methanedithioate(2−)) has been synthesised. The structural features of [{MoO2(Bz2endtc)}2] have been elucidated by X-ray crystal analysis, elemental analysis and 13C NMR, IR and FAB+ mass spectroscopy: two almost identical cis-dioxomolybdenum(VI) centres are bridged by the two Bz2endtc2− ligands and each molybdenum(VI) centre has a distorted octahedral geometry with four sulphur atoms and two terminal oxo ligands lying in a cis position to each other. There is unlikely to be electronic interaction between the two cis-dioxomolybdenum(VI) centres in [{MoO2(Bz2endtc)}2] because the MoMo distance is long (=7.337 Å). In the [{MoO2(Bz2endtc)}2]/PPh3 system, the oxygen atom transfer reaction (Eq. (A)) occurs to give a tetranuclear oxomolybdenum(VI,V) complex formulated as [MoO2(Bz2endtc)2Mo2O3(Bz2endtc)2MoO2] which has one μ-oxomolybdenum(V) moiety. equation(A) The absence of a disproportionation reaction of a μ-oxomolybdenum(V) complex which accompanies the oxygen atom transfer reaction involving bidentate dithiocarbamate cis-dioxomoloybdenum(VI) complexes [MoO2(R,R′dtc)2] and PPh3 is attributed to steric rather than electronic properties of [{MoO2(Bz2endtc)}2]. [MoO2(Bz2endtc)2Mo2O3(Bz2endtc)2MoO2] also reacts with PPh3 to give an oligomeric oxomolybdenum(V,VI) complex which is postulated to be [MoO2(Bz2endtc)2Mo2O3(Bz2endtc)2Mo2O3(Bz2endtc)2Mo2O3(Bz2endtc)2MoO2]. Under fast scan rate conditions, [{MoO2(Bz2endtc)}2] is electrochemically reduced in two closely spaced one-electron charge transfer processes at −1.32 and −1.47 V versus Fc/Fc+ in N,N-dimethylformamide (DMF). At slow scan rates, homogeneous chemical reactions are coupled to the electron transfer step in DMF. The formal potential of the [{MoO2(Bz2endtc)}2]0/−1 couple (Eo′=−1.33 V vs. Fc/Fc+) in CH2Cl2 is the same as that for [MoO2(BzMedtc)2]0/−1 (BzMedtc−=benzylmethydithiocarbamate) couple in the same solvent, which is expected since [MoO2(BzMedtc)2] can be regarded as a half unit of [{MoO2(Bz2endtc)}2].