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 using stopped-flow, 31P{1H} NMR, and ESI-MS have been carried on the reaction of the [W3S4Br3(edpp)3]+ (edpp = (2-aminoethyl)diphenylphosphine) cluster with bases as NaOH and Et3N in acetonitrile. For the reaction with OH- the study allowed 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 bromides by OH- ligands, and the associated kinetics is several orders of magnitude faster than that 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, the amine acting as a base. DFT calculations provide a rationalization of the experimental findings by showing that Et3N coordination does not occur even in the presence of a vacant coordination site. Computed pKa values for the different species involved in these reactions indicate the possibility of proton exchange between the NH2 groups of edpp and Et3N, which is confirmed by the NMR and ESI-MS data. The formation of intermediate cluster species containing the conjugate base of edpp has been also found to facilitate bromide dissociation, thus providing 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.