Syntheses and Redox Properties of Complexes with Mo<sub>3</sub>S<sub>4</sub> Cores and Tridentate Schiff Base Ligands
作者:Keisuke Kawamoto、Akio Ichimura、Hideki Hashimoto、Isamu Kinoshita、Takanori Nishioka
DOI:10.1246/bcsj.20140275
日期:2015.2.15
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.
通过水杨醛和相应的官能化丙胺的缩合反应,制备了一系列潜在的三叉席夫碱配体 HOPhC=N-(CH2)3-XMe: X = S (HLSMe), Se (HLSeMe)} 。通过在甲醇中用适当的希夫碱配体处理 [Mo3S4(H2O)9]4+,这些配体被用来形成单配位配合物 [Mo3S4(LXMe)3]+(X = S (1+),Se (2+))。1-PF6 的单晶 X 射线结构分析表明,一个希夫碱配体通过 O、N 和 SMe 供体原子与每个 Mo 配位,如果不考虑配位硫原子周围的手性,则单阳离子簇[Mo3S4(LSMe)3]+ 具有伪 C3 对称性。由于不对称席夫碱配体的排列,这个单配位簇可能具有两种轴向手性中的一种。此外,络合物阳离子中三个配位 SMe 基团中的每个硫原子也是手性的,因此在单胞中观察到两种对映体。通过对变温核磁共振波谱进行线形分析,研究了复合物 1-PF6 和 2-PF6 在溶液中的动态行为,发现 1-PF6 中 S 原子的手性反转速率常数大于 2-PF6 中 Se 原子的手性反转速率常数,而且 1-PF6 和 2-PF6 的 ΔH‡ 值(47.1 kJ mol-1)和 2-PF6 (56.7 kJ mol-1)的差异以及负 ΔS‡ 值(1-PF6 和 2-PF6 分别为 -39.9 和 -57.5 J mol-1 K-1)表明,反转过程涉及金属中心和 S 或 Se 原子间的键裂解,然后是溶剂分子的配位。使用循环伏安法评估了 1-PF6 和 2-PF6 的电化学特性,结果表明 1+ 和 2+ 都表现出两个连续、可逆的单电子还原波,分别属于形式 Mo(IV IV)/Mo(III IV IV) 和 Mo(III IV IV)/Mo(III III IV) 偶合。此外,还使用 CH3CO2H 或 CF3CO2H 作为质子源,考察了 1-PF6 和 2-PF6 催化 H+ 电还原的能力。值得注意的是,催化波的氧化还原电位取决于添加酸的酸性。因此,在使用较强的三氟乙酸作为质子源时,可以观察到围绕第一个还原波的催化电流,而在使用较弱的乙酸时,则会出现围绕第二个还原波的电流。