Redox Properties and Catalytic Ability toward Electrochemical Proton Reduction of Sulfur-Bridged Trinuclear Mo3S4 Complexes Containing Acetate, Trifluoroacetate, and/or Dithiophosphate as Bridging Ligands
Authors: Kawamoto Keisuke , Ichimura Akio , Hashimoto Hideki , Kinoshita Isamu , Hirotsu Masakazu , Nishioka Takanori 
Published: January 2015

Abstract
A novel complex with the terminal acetonitrile and bridging trifluoroacetate (tfa) ligands of [Mo3S4(dtp)3(μ-tfa)(NCCH3)] (dtp: diethyl dithiophospate) (1-tfa) was prepared from the substitution reaction of [Mo3S4(dtp)3(μ-dtp)(NCCH3)] (1-dtp) with trifluoroacetate anhydride in acetonitrile. The reaction of 1-dtp with trifluoroacetic acid in dichloromethane also afforded 1-tfa. Complex 1-tfa reacts with pyridine in ethanol resulting in the formation of a complex with the terminal pyridine ligand (py), [Mo3S4(dtp)3(μ-tfa)(py)] (2-tfa). Structures of 1-tfa and 2-tfa were determined by single-crystal X-ray structural analysis showing that one of three Mo–Mo distances depend on a kind of the bridging ligands, dtp, OAc, and tfa, while no trends in the other two Mo–Mo distances in each of the complexes with the different bridging ligands. Each of the cyclic voltammograms of 1-dtp, [Mo3S4(dtp)3(μ-OAc)(NCCH3)] (OAc: acetate) (1-OAc), 1-tfa, [Mo3S4(dtp)3(μ-OAc)(py)] (2-OAc), and 2-tfa showed two consecutive one-electron reduction waves and an oxidation wave, which are formally assigned to the Mo(IV IV IV)/Mo(III IV IV), Mo(III IV IV)/Mo(III III IV), and Mo(V IV IV)/Mo(IV IV IV) couples, respectively. The re-oxidation and re-reduction peaks for the second reduction and the oxidation processes were not observed for all of the complexes. The redox potentials of the first reduction processes were shifted up to ca. 200 mV with the bridging and terminal ligands, for example, the redox potentials appeared at −0.98 and −1.19 V vs. Fc/Fc+2-OAc and 1-tfa, respectively, due to the lower electron-donating ability of CH3CN and tfa ligands compared to that of the py and OAc ligands. The re-oxidation peak for the second reduction process of 2-OAc was observed at −80 °C suggesting that this is an EC process and the chemical reaction following the second electrochemical reduction is probably inhibited at low temperature. On the other hand, no re-reduction peak for the oxidation process appeared even at −80 °C implying that the oxidation process is accompanied by a significant structural change. This argument is supported by the results of the DFT calculation. The HOMO component of the complex contains a bonding character between the two acetate-bridged Mo centers and the other acetate-unbridged one, and the spin densities of the 1-electron oxidized complex mostly located at the one of two acetate-bridged Mo centers and the other acetate-unbridged one. Cyclic voltammograms of these complexes in the presence of trifluoroacetic acid as a proton source exhibited catalytic currents around the first reduction wave for each complex. The DFT calculations for 2-OAc and 2-tfa demonstrated that the HOMO components are also distributed over the doubly bridged sulfur ligands in addition to the Mo centers in the Mo3S4 core. These result suggests that a protonation reaction of 2-OAc and 2-tfa probably occurs on the doubly bridged sulfur ligand in the Mo3S4 core.