Synthesis, crystal structure, aqueous speciation, and kinetics of substitution reactions in a water-soluble Mo3S4 cluster bearing hydroxymethyl diphosphine ligands

The Mo3S4Cl3(dhmpe)3]Cl (1]Cl) cluster has been prepared from Mo3S7Cl6]2- and the water-soluble 1,2-bis(bis(hydroxymethyl)-phosphino)ethane (dhmpe, L) ligand. The crystal structure has been determined by X-ray diffraction methods and shows the incomplete cuboidal structure typical of the M3Q4 clusters (M=Mo, W; Q=S, Se), with a capping sulfide ligand to the three metal centers and the other three sulfides acting as bridges between two Mo atoms. The octahedral coordination around each metal center is completed with a chlorine and two phosphorus atoms of one L ligand. The chemistry of aqueous solutions of 1]Cl is dominated by the formation of the Mo3S4L(L-H)2(H2O)]2+ complex (2]2+), where the three chlorides have been replaced by one water molecule and two alkoxo groups of two different dhmpe ligands, thus leading to a solution structure where the three metal centers are not equivalent. 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 2]2+ in aqueous solution. In this way, it has been established that the addition of an excess of X- (Cl-, SCN-) leads to Mo3S4X3(dhmpe)3]+ complexes in three resolved kinetic steps that correspond to the sequential coordination of X- at the three metal centers. However, whereas the first two steps involve the opening of the chelate rings formed with the alkoxo groups of the dhmpe ligands, the third one corresponds to the substitution of the coordinated water molecule. These results demonstrate that the asymmetry introduced by the closure of chelate rings at only two of the three Mo centers makes the kinetics of the reaction deviate significantly from the statistical behavior typically associated with M3Q4 clusters. The results obtained for the reaction of 2]2+ with acid and base are also described, and they complete the picture of the aqueous speciation of this cluster.