Photochemistry of Solar Energy Storing Dinickel Phosphazane Photocatalysts

Sunday, February 14, 2016
James Lawniczak, University of Michigan, Shelby Township, MI
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.