Hemilability of phosphine-thioether ligands coordinated to trinuclear Mo3S4 cluster and its effect on hydrogenation catalysis

Abstract

Ligand-exchange reactions of [Mo₃S₄(tu)₈(H₂O)]Cl₄·4H₂O (tu = thiourea) with (PhCH₂CH₂)₂PCH₂CH₂SR ligands, where R = Ph (PS1), pentyl (PS2) or Pr (PS3), afford new complexes isolated as [Mo₃S₄Cl₃(PS1)₃]PF₆ ([1]PF₆), [Mo₃S₄Cl₃(PS2)₃]PF₆ ([2]PF₆) and [Mo₃S₄Cl₃(PS3)₃]PF₆ ([3]PF₆) salts in 30–50% yields as the major reaction products. The crystal structures of [1]PF₆ and [2]PF₆ were determined by X-ray diffraction (XRD) analysis. Each of the three phosphine-thioether ligands is coordinated in a bidentate chelating mode to a different molybdenum atom of the Mo₃S₄ trinuclear cluster; herein, all the phosphorus atoms of the phosphino-thioether ligand are located trans to the capping sulfur (μ₃-S). A second product that forms in the reaction of [Mo₃S₄(tu)₈(H₂O)]Cl₄·4H₂O with PS1 corresponds to the neutral [Mo₃S₄Cl₄(PS1)₂(PS1*)] complex. Its XRD analysis reveals both bidentate (PS1) and monodentate (PS1*) coordinating modes of the same ligand. In the latter mode the phosphine-thioether is coordinated to a Mo atom only via the P atom. All compounds were characterized using ¹H and ³¹P{¹H} NMR spectroscopy, electrospray-ionization (ESI) mass spectrometry and cyclic voltammetry (CV). Reactions of [1]PF₆, [2]PF₆ and [3]PF₆ with an excess of Bu₄NCl in CD₂Cl₂ were followed by ³¹P{¹H} NMR spectroscopy. The spectra indicate equilibrium between cationic [Mo₃S₄Cl₃(PSn)₃]⁺ and neutral [Mo₃S₄Cl₄(PSn)₂(PSn*)] (n = 1, 2) species. The equilibrium constants were determined as 2.5 ± 0.2 × 10³ M⁻¹, 43 ± 2 M⁻¹ and 30 ± 2 M⁻¹ (at 25 °C) for [1]PF₆, [2]PF₆ and [3]PF₆, respectively, indicating quantitative differences in the hemilabile behaviors of the phosphino-thioether ligands, depending on the substituent at the sulfur. Clusters [1]PF₆, [2]PF₆ and [3]PF₆ were tested as catalysts in the reduction of nitrobenzene to aniline with Ph₂SiH₂ under mild conditions. Significant differences in the catalytic activities were observed, which can be attributed to different hemilabile behaviors of the PS1 and PS2/PS3 ligands.