The reduction of protons or water to form hydrogen using solar energy is an attractive option for alternative, renewable fuels. Importantly, transformation of sunlight into H2 stores energy in chemical bonds that can be released in a carbon-free combustion reaction as necessary. In order to use solar energy for proton reduction, light adsorption must be coupled to catalysis. Current research has focused on the attachment of cobalt diglyoxime proton reduction catalysts to either molecular or semiconductor-based photosensitizers. The design of these constructs involves tethering the catalyst to the photosensitizer through a labile, monodentate, axial ligand.  The catalytic mechanism of proton reduction involves a labile Co(II) intermediate and requires that an axial ligand dissociate in order to allow the formation of the Co(III) hydride necessary for catalysis. As such, attachment using an axial ligand leads to photocatalyst dissociation on the photocatalytic timescale. We are interested in a new photosensitization motif that involves direct incorporation of the light harvesting element into the macrocyclic ligand framework of the proton reduction catalyst. The synthesis of a diimine dioximate cobalt complex that contains a binding pocket to facilitate this new approach to photosensitization will be discussed, along with preliminary results on photocatalytic hydrogen evolution by this system using both molecular and quantum dot photosensitizers.

Cobalt diimine dioximate proton reduction catalysts with direct attachment to photosensitizers within a macrocyclic framework

248th National Meeting (2014)

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