Light-Driven Hydrogen Evolution by BODIPY-Sensitized Cobaloxime Catalysts

We
report four photocatalytically active cobaloxime complexes for
light-driven hydrogen evolution. The cobaloxime catalysts are sensitized
by different meso-pyridyl boron dipyrromethene (BODIPY)
chromophores, bearing either two bromo- or iodo-substituents on the
BODIPY core. The pyridine linker between the BODIPY and cobaloxime
is further modified by a methyl substituent on the pyridine, influencing
the stability and electronic properties of the cobaloxime catalyst
and thus the photocatalytic efficiency of each system. Four cobaloxime
catalyst complexes and three novel BODIPY chromophores are synthesized
and characterized by absorption, fluorescence, infrared (IR) and nuclear
magnetic resonance (NMR) spectroscopy, mass spectrometry, and electrochemistry.
Crystal structures for the BODIPY–cobaloxime complexes 2 and 3 are presented. In contrast to the photocatalytically
inactive, nonhalogenated reference complex 1, the four
newly reported molecules are active for photocatalytic hydrogen evolution,
with a maximum turnover number (TON) of 30.9 mol equiv of H2 per catalyst for the meso-methylpyridyl 2,6-diiodo
BODIPY-sensitized cobaloxime complex 5. We conclude that
accessing the photoexcited triplet state of the BODIPY chromophore
by introducing heavy atoms (i.e., bromine or iodine) is necessary
for efficient electron transfer in this system, enabling catalytic
hydrogen generation. In addition, relatively electron-donating pyridyl
linkers improve the stability of the complex, increasing the overall
TON for hydrogen production.