| Summary: | Catalytic properties of the cluster compound (TBA)<sub>2</sub>[Mo<sub>6</sub>I<sup>i</sup><sub>8</sub>(O<sub>2</sub>CCH<sub>3</sub>)<sup>a</sup><sub>6</sub>] (TBA = tetrabutylammonium) and a new hybrid material (TBA)<sub>2</sub>Mo<sub>6</sub>I<sup>i</sup><sub>8</sub>@GO (GO = graphene oxide) in water photoreduction into molecular hydrogen were investigated. New hybrid material (TBA)<sub>2</sub>Mo<sub>6</sub>I<sup>i</sup><sub>8</sub>@GO was prepared by coordinative immobilization of the (TBA)<sub>2</sub>[Mo<sub>6</sub>I<sup>i</sup><sub>8</sub>(O<sub>2</sub>CCH<sub>3</sub>)<sup>a</sup><sub>6</sub>] onto GO sheets and characterized by spectroscopic, analytical, and morphological techniques. Liquid and, for the first time, gas phase conditions were chosen for catalytic experiments under UV–Vis irradiation. In liquid water, optimal H<sub>2</sub> production yields were obtained after using (TBA)<sub>2</sub>[Mo<sub>6</sub>I<sup>i</sup><sub>8</sub>(O<sub>2</sub>CCH<sub>3</sub>)<sup>a</sup><sub>6</sub>] and (TBA)<sub>2</sub>Mo<sub>6</sub>I<sup>i</sup><sub>8</sub>@GO) catalysts after 5 h of irradiation of liquid water. Despite these remarkable catalytic performances, “liquid-phase” catalytic systems have serious drawbacks: the cluster anion evolves to less active cluster species with partial hydrolytic decomposition, and the nanocomposite completely decays in the process. Vapor water photoreduction showed lower catalytic performance but offers more advantages in terms of cluster stability, even after longer radiation exposure times and recyclability of both catalysts. The turnover frequency (TOF) of (TBA)<sub>2</sub>Mo<sub>6</sub>I<sup>i</sup><sub>8</sub>@GO is three times higher than that of the microcrystalline (TBA)<sub>2</sub>[Mo<sub>6</sub>I<sup>i</sup><sub>8</sub>(O<sub>2</sub>CCH<sub>3</sub>)<sup>a</sup><sub>6</sub>], in agreement with the better accessibility of catalytic cluster sites for water molecules in the gas phase. This bodes well for the possibility of creating {Mo<sub>6</sub>I<sub>8</sub>}<sup>4+</sup>-based materials as catalysts in hydrogen production technology from water vapor.
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