183W Nuclear Magnetic Resonance and Photocatalysis Studies of Two Ruthenium-Decorated Isopolyoxometalates {Ru2W10} and {Ru2W13} via pH-Induced Assemblies.

Abstract

Two structurally intriguing Ru-containing isopolyoxometalates [(Ru(OH))2O(W5O18)2]8- (1) and [(W5O18)(Ru2W8O31)]12- (2) were constructed from subtly different conditions. Single-crystal X-ray diffraction indicated that the precise pH modification has allowed us to trap a diruthenium-oxo core within different isopolyoxotungstate species. Compound 1 is the first sandwich-type ruthenium isopolyoxotungstate consisting of a linear {(HO)Ru-O-Ru(OH)} unit and two Lindqvist-type {W5} building blocks, while a ligand replacement of {W5} with an unusual {W8} ring in the case of compound 1 produced a unique embedded-type compound 2 with a quasi-linear {Ru-O-Ru} core. In addition to being determined in the solid state, crystal structures of 1 and 2 were also confirmed by 183W nuclear magnetic resonance (NMR) spectroscopy and electrospray ionization-mass spectrometry (ESI-MS) in solution. 183W NMR spectrum demonstrated that the two-line pattern of 1 (with approximately 4:1 relative intensities) is consistent with the pseudo-D2h symmetry observed in the solid state. However, two other lines were observed in 2 according to the C2v symmetry but not in accord with the expected 4/4/4/1 ratio in the crystalline state, which indicated that the structure of 2 could not be maintained completely in aqueous solution. After recrystallizing the solid sample of 2 in water, the crystal structure of 2 partly converted to the structure of 1, and the transformation was determined by the combined results of 183W NMR, ESI-MS and single-crystal X-ray diffraction measurements. Catalytic investigations showed that their sodium salts presented excellent photocatalytic activities toward the benzylamine oxidation reaction induced by visible light (λ > 400 nm). The structures of the two compounds can still be maintained without a significant yield decrease after five continuous reaction cycles. Furthermore, both catalysts 1 and 2 could proceed with the oxidative coupling reaction smoothly for most of the primary benzylamine derivatives bearing various functional groups (H, F, Cl, Br, and Me) in good to excellent yields.