Influence of the Ligand Alkyl Chain Length on the Solubility, Aqueous Speciation, and Kinetics of Substitution Reactions of Water-Soluble M3S4 (M = Mo, W) Clusters Bearing Hydroxyalkyl Diphosphines

Water-soluble [M3S4X3(dhbupe)3]+ diphosphino complexes (dhbupe = 1,2-bis­(bis­(hydroxybutyl)­phosphino)­ethane), 1+ (M = Mo, X = Cl) and 2+ (M = W; X = Br), have been synthesized by extending the procedure used for the preparation of their hydroxypropyl analogues by reaction of the M3S4(PPh3)3X4(solvent)x molecular clusters with the corresponding 1,2-bis­(bishydroxyalkyl)­diphosphine. The solid state structure of the [M3S4X3(dhbupe)3]+ cation possesses a C3 symmetry with a cuboidal M3S4 unit, and the outer positions are occupied by one halogen and two phosphorus atoms of the diphosphine ligand. At a basic pH, the halide ligands are substituted by hydroxo groups to afford the corresponding [Mo3S4(OH)3(dhbupe)3]+ (1OH+) and [W3S4(OH)3(dhbupe)3]+ (2OH+) complexes. This behavior is similar to that found in 1,2-bis­(bis­(hydroxymethyl)­phosphino)­ethane (dhmpe) complexes and differs from that observed for 1,2-bis­(bis­(hydroxypropyl)­phosphino)­ethane (dhprpe) derivatives. In the latter case, an alkylhydroxo group of the functionalized diphosphine replaces the chlorine ligands to afford Mo3S4 complexes in which the deprotonated dhprpe acts in a tridentate fashion. Detailed studies based on stopped-flow, 31P­{1H} NMR, and electrospray ionization mass spectrometry techniques have been carried out in order to understand the solution behavior and kinetics of interconversion between the different species formed in solution: 1 and 1OH+ or 2 and 2OH+. On the basis of the kinetic results, a mechanism with two parallel reaction pathways involving water and OH– attacks is proposed for the formal substitution of halides by hydroxo ligands. On the other hand, reaction of the hydroxo clusters with HX acids occurs with protonation of the OH– ligands followed by substitution of coordinated water by X–.