Photoreduction of CO to Methanol. The Photolysis of Aqueous Solutions of [Os(NH3)5(CO)]2+

If the reduction of CO could be achieved photochemically it would represent a rather new type of artificial photosynthesis since in distinction to H2 as a result of photochemical water splitting, methanol is well suited for storage and transportation. As an energy-rich liquid it offers many benefits [1]. Metal carbonyl complexes should be attractive candidates for this purpose. Unfortunately, methanol has not yet been observed as a photoproduct of metal carbonyls [4, 5]. Generally, the photolysis of such compounds leads simply to the release of a CO ligand originating from LF excited states. In order to reduce the CO ligand, a MLCT state should be populated [6, 7]. However, a reduction to methanol will be only facilitated if the MLCT (M→ π∗CO) transition as a one-electron process is followed by a multielectron transfer from the central metal to CO. We now examined this possibility and selected the complex [Os(NH3)5(CO)] [8] for this purpose. This choice was based on the close relationship between [Os(NH3)5(CO)] and [Os(NH3)5(N2)]. The latter complex has been shown to react according to Eq. 2 following (OsII→ πN2) MLCT excitation [9]. [Os(NH3)5(N2)] +H2O→ [Os(NH3)4(N)] +N2H4 +OH− (2)