Nickel Complexes for Robust Light-Driven and Electrocatalytic Hydrogen Production from Water

A series of nickel bis­(chelate) complexes
having square planar
coordination are studied for light-driven and electrocatalytic hydrogen
production from water. The complexes Ni­(abt)2 (abt = 2-aminobenzenethiolate),
Ni­(mp)2 (mp = 2-mercaptophenolate) and Ni­(mpo)2 (mpo = 2-mercaptopyridyl-N-oxide) are found to
be active catalysts under light-driven conditions, using fluorescein
(Fl) as the photosensitizer (PS) and triethanolamine (TEOA) as the
sacrificial electron donor in water under basic pH (pH = 9.8). These
molecular systems achieve a turnover number (TON) of ∼6000
(relative to catalyst) and are stable for more than 100 h under H2-generating conditions. When water-soluble CdSe quantum dots
with tripodal S-donor capping agents are employed as PS and ascorbic
acid (AA) is used as the sacrificial electron donor at pH 4.5, an
active and robust system is obtained for the light-driven generation
of H2 from aqueous protons. A TON of over 280 000
is achieved for the three active catalysts. These complexes are also
examined electrochemically in organic solvents with weak organic acids
as the proton source and in aqueous and aqueous/organic media for
proton reduction. The most active photochemical catalysts also show
excellent electrocatalytic activity in neutral pH water, achieving
Faradaic yields close to 100% under anaerobic conditions and ∼80%
under aerobic conditions.