Photochemistry of Solar Energy Storing Dinickel Phosphazane Photocatalysts

Solar energy storage is a key technology for the proliferation of clean solar energy. Photochemically-driven HX splitting (where X = Cl, Br) is a desirable method of storing solar energy in the form of chemical bonds. A challenge to developing a catalytic photocycle involving HX splitting is halide oxidation to deliver X₂ upon photoexcitation. With successes in halogen photoelimination with Ni(III) complexes in the literature, we focused on delivering the second half of the cycle, namely hydrogen, by synthesizing candidate HX-splitting Ni phosphazane catalysts.  Currently, our most promising photocatalyst, a Ni₂(I,I) chloride complex, photochemically produced H₂ in ~1% yield, unprecedented for an earth-abundant first-row transition metal catalyst. Based on literature precedent and experimental data, we proposed a mechanism for the HX splitting catalytic cycle involving halide photoelimination followed by oxidative addition of HCl and finally hydrogen photoelimination, regenerating the starting material.