Syntheses and Redox Properties of Complexes with Mo3S4 Cores and Tridentate Schiff Base Ligands

Abstract

Abstract A series of potentially tridentate Schiff base ligands, {HOPhC=N-(CH2)3-XMe: X = S (HLSMe), Se (HLSeMe)}, was prepared by condensation reactions of salicylaldehyde and the corresponding functionalized propylamines. These ligands were used to form the monocationic complexes, [Mo3S4(LXMe)3]+ (X = S (1+), Se (2+)), through treatment of [Mo3S4(H2O)9]4+ with the appropriate Schiff base ligand in methanol. Single-crystal X-ray structural analysis of 1-PF6 revealed that one Schiff base ligand coordinates to each Mo via the O, N, and SMe donor atoms, and then the mono cationic cluster [Mo3S4(LSMe)3]+ has a pseudo C3 symmetry if the chirality around the coordinated sulfur atoms is not taken into account. Because of the arrangements of the asymmetric Schiff base ligand, this monocationic cluster possibly adopts one of two axial chiralities. Furthermore, each sulfur atom of the three coordinated SMe groups in the complex cation is also chiral affording the two enantiomers observed in the unit cell. Dynamic behaviors of complexes 1-PF6 and 2-PF6 in solution were examined using line shape analyses of variable temperature NMR spectra revealing that the rate constant of the chiral inversion at the S atoms in 1-PF6 is greater than that at the Se atoms in 2-PF6 and the difference in the ΔH‡ values of 1-PF6 (47.1 kJ mol−1) and 2-PF6 (56.7 kJ mol−1) and the negative ΔS‡ values (−39.9 and −57.5 J mol−1 K−1 for 1-PF6 and 2-PF6, respectively) suggest that the inversion processes involve bond cleavage between the metal centers and S or Se atoms followed by coordination of solvent molecules. The electrochemical properties of 1-PF6 and 2-PF6 were evaluated using cyclic voltammetry and this revealed that both 1+ and 2+ exhibit two consecutive, reversible one-electron reduction waves that are assigned to formal Mo(IV IV IV)/Mo(III IV IV) and Mo(III IV IV)/Mo(III III IV) couples, respectively. The ability of 1-PF6 and 2-PF6 to catalyze the electroreduction of H+ was also examined using CH3CO2H or CF3CO2H as a proton source. Noteworthy, the redox potentials for the catalytic wave depend on the acidity of the added acids. Thus, the catalytic current around the first reduction wave is observed in the presence of the stronger trifluoroacetic acid as a proton source, while the current around the second reduction wave appears when the weaker acetic acid is used.