Heterooctamolybdate-Based Clusters H3[(Cp*Rh)4PMo8O32] and H5[Na2(Cp*Ir)4PMo8O34] and Derived Hybrid Nanomaterials with Efficient Electrocatalytic Hydrogen Evolution Reaction Activity.

Polyoxometalates (POMs), emerging as a new class of porous molecular materials, play a promising role in homo- and heterogeneous catalysis. Among them, noble-metal-decorated POMs have a profound impact as catalytic materials. Thus, it is imperative to design and structurally explore new catalysts including noble metals. Herein, two new clusters, H3[(Cp*Rh)4PMo8O32]·14H2O (1) and H5[Na2(Cp*Ir)4PMo8O34]·13H2O (2) (Cp* = pentamethylcyclopentadienyl), based on a heterooctamolybdate anionic core were successfully obtained via a one-pot reaction using [Cp*MCl2]2 [M = Rh (1) and Ir (2)] and Na2MoO4 in acidic conditions. Compounds 1 and 2 were well characterized in the solid state by single-crystal X-ray diffraction, IR, and thermogravimetric analysis and in solution by UV-vis, electrospray ionization mass spectrometry, and electrochemistry. Compounds 1 and 2 represent an important class of structurally isolated organometallic POM-based clusters that were successfully nanostructured onto Ni foam and electrochemically reduced after 48 h of electrolysis to M/MoO2, where M = Rh (3) and Ir (4), nanocomposite hybrid materials on a Ni foam surface in a 0.1 M KOH solution. The modified electrocatalysts (3 and 4) show efficient hydrogen evolution reaction activities almost comparable to those of high-grade Pt/C at 0.1 M KOH. The nanostructured POMs [1- and 2@NF (Ni foam)] and their corresponding reduced products (3 and 4) were observed by scanning electron microscopy, energy-dispersive X-ray spectroscopy, powder X-ray diffraction, and X-ray photoelectron spectroscopy and further proven by transmission electron microscopy (TEM) and high-resolution TEM.