Studies on the Reactivity of the Aminophosphino [W3S4Br3(edpp)3]+ (edpp= (2‐aminoethyl)diphenylphosphine) Cluster Cation towards Bases Revealing an Active Role of the Amino Group

Detailed studies through stopped-flow P-31{H-1} NMR spectroscopy and ESI-MS were performed for the reactions of the [W3S4Br3(edpp)(3)](+) [edpp = (2-aminoethyl)diphenylphosphine] cluster with bases such as NaOH and Et3N in acetonitrile. For the reaction with OH- ions, the study allowed the identification of hydroxo species and the development of a procedure for the synthesis of the [W3S4(OH)(3)(edpp)(3)](+) cluster in high yields. This involves the substitution of the coordinated bromide ligands by OH- ligands, and the associated kinetics is several orders of magnitude faster than those with Cl-, F- and SCN- anions. The reactivity with Et3N is quite different and essentially leads to the formation of species resulting from proton abstraction at the NH2 groups of the edpp ligands, and the amine acts as a base. DFT calculations provided a rationalization of the experimental findings by showing that Et3N coordination does not occur even in the presence of a vacant coordination site. The computed pK(a) values for the different species involved in these reactions indicate the possibility of proton exchange between the NH2 groups of edpp and Et3N, and this behaviour was confirmed by the NMR spectroscopy and ESI-MS data. The formation of intermediate cluster species containing the conjugate base of edpp also facilitates bromide dissociation and, thus, provides a reaction pathway with a lower energy barrier that justifies the faster formation of [W3S4(OH)(3)(edpp)(3)](+) with respect to other [W3S4X3(edpp)(3)](+) (X = Cl, F, SCN) clusters.