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

Abstract

A novel dinuclear cis-dioxomolybdenum(VI) complex [{MoO 2(Bz 2endtc)} 2] coordinated with a quadradentate dithiocarbamate (Bz 2endtc 2−: ((2-(dithiocarboxybenzylamino)ethyl)benzylamino)-methanedithioate(2−)) has been synthesised. The structural features of [{MoO 2(Bz 2endtc)} 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 Bz 2endtc 2− 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 [{MoO 2(Bz 2endtc)} 2] because the MoMo distance is long (=7.337 Å). In the [{MoO 2(Bz 2endtc)} 2]/PPh 3 system, the oxygen atom transfer reaction (Eq. (A)) occurs to give a tetranuclear oxomolybdenum(VI,V) complex formulated as [MoO 2(Bz 2endtc) 2Mo 2O 3(Bz 2endtc) 2MoO 2] which has one μ-oxomolybdenum(V) moiety. (A) 2[{ MoO 2( Bz 2 endtc)} 2]+ PPh 3 →[ MoO 2( Bz 2 endtc) 2 Mo 2 O 3( Bz 2 endtc) 2 MoO 2]+ OPPh 3 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 [MoO 2(R,R′dtc) 2] and PPh 3 is attributed to steric rather than electronic properties of [{MoO 2(Bz 2endtc)} 2]. [MoO 2(Bz 2endtc) 2Mo 2O 3(Bz 2endtc) 2MoO 2] also reacts with PPh 3 to give an oligomeric oxomolybdenum(V,VI) complex which is postulated to be [MoO 2(Bz 2endtc) 2Mo 2O 3(Bz 2endtc) 2Mo 2O 3(Bz 2endtc) 2Mo 2O 3(Bz 2endtc) 2MoO 2]. Under fast scan rate conditions, [{MoO 2(Bz 2endtc)} 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 [{MoO 2(Bz 2endtc)} 2] 0/−1 couple ( E o′=−1.33 V vs. Fc/Fc +) in CH 2Cl 2 is the same as that for [MoO 2(BzMedtc) 2] 0/−1 (BzMedtc −=benzylmethydithiocarbamate) couple in the same solvent, which is expected since [MoO 2(BzMedtc) 2] can be regarded as a half unit of [{MoO 2(Bz 2endtc)} 2].