Organofunctionalized Borotungstate Polyoxometalates As Tunable Photocatalysts for Oxidative Dimerization of Amines

Organofunctionalized borotungstate Keggin polyoxometalates, (nBu4N)3H[HBW11O39(P(O)Ph)2] (PBW11), (nBu4N)3H[HBW11O39(As(O)Ph)2] (AsBW11), and (nBu4N)4[HBW11O39(PhSiOSiPh)] (SiBW11), were synthesized and structurally characterized. Cyclic voltammetry showed that the electronic properties of the clusters are dependent on the nature of the appended main group atoms (P, As, or Si). The first reduction potentials were found to shift positively with respect to that of the unmodified parent species (nBu4N)5[BW12O40], with PBW11 showing the largest shift at +100 mV. All clusters were evaluated as photocatalysts for the oxidative dimerization of amines where the organophosphonate hybrid PBW11 was found to be the most active. This study demonstrates how organofunctionalization of polyoxometalates may be used to tune and improve their performance as photocatalysts for organic reactions. The organic hybridization of borotungstate Keggin polyoxometalates is shown to modulate the electronic structure of the clusters, allowing the rational design of single-electron transfer photocatalysts that can be cycled under oxygen.