Water-Soluble Mo₃S₄ Clusters Bearing Hydroxypropyl Diphosphine Ligands: Synthesis, Crystal Structure, Aqueous Speciation, and Kinetics of Substitution Reactions

Abstract

The [Mo₃S₄Cl₃(dhprpe)₃]⁺ (<b>1</b>⁺) cluster cation has been prepared by reaction between Mo₃S₄Cl₄(PPh₃)₃ (solvent)₂ and the water-soluble 1,2-bis­(bis­(hydroxypropyl)­phosphino)­ethane (dhprpe, L) ligand. The crystal structure of [<b>1</b>]₂[Mo₆Cl₁₄] has been determined by X-ray diffraction methods and shows the typical incomplete cuboidal structure with a capping and three bridging sulfides. The octahedral coordination around each metal center is completed with a chlorine and two phosphorus atoms of the diphosphine ligand. Depending on the pH, the hydroxo group of the functionalized diphosphine can substitute the chloride ligands and coordinate to the cluster core to give new clusters with tridentate deprotonated dhprpe ligands of formula [Mo₃S₄(dhprpe-H)₃]⁺ (<b>2</b>⁺). A detailed study based on stopped-flow, ³¹P­{¹H} NMR, and electrospray ionization mass spectrometry techniques has been carried out to understand the behavior of acid–base equilibria and the kinetics of interconversion between the <b>1</b>⁺ and the <b>2</b>⁺ forms. Both conversion of <b>1</b>⁺ to <b>2</b>⁺ and its reverse process occur in a single kinetic step, so that reactions proceed at the three metal centers with statistically controlled kinetics. The values of the rate constants under different conditions are used to discuss on the mechanisms of opening and closing of the chelate rings with coordination or dissociation of chloride