Studies on the Reactivity of the Aminophosphino [W 3 S 4 Br 3 (edpp ) 3 ] + (edpp= (2 - aminoethyl) diphenylphosphine) Cluster Cation towards Bases Reveal ing a Non - Passive Role of the Amino Group

Abstract

Detailed studies using stopped-flow, 31 P{ 1 H} NMR, a nd ESI - MS have been carried on the reaction of the [W 3 S 4 Br 3 (edpp) 3 ] + (edpp = ( 2 - aminoethyl)diphenylphosphine ) cluster with NaOH and Et 3 N in acetonitrile. The study has allowed identification of h ydroxo species during the reactivity studies an d the development of a procedure for the synthesis of the [W 3 S 4 (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 Et 3 N is quite different and essentially leads to the formation of species resulting from proton abstraction at the NH 2 groups of the edpp ligands, the amine acting as a base. DFT calculations provide a rationalization of the experimental findings by showing that Et 3 N 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 NH 2 groups of edpp and Et 3 N, which is confirmed by the NMR and ESI - MS data. The formation of intermediate cluster species containing the conjugate base o f 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 [W 3 S 4 (OH) 3 (edpp) 3 ] + with respect to other [W 3 S 4 X 3 (edpp) 3 ] + (X= Cl, F, SCN) clusters.