Visible Light-Driven Hydrogen Production from Aqueous Protons Catalyzed by Molecular Cobaloxime Catalysts

A series of cobaloxime complexes([Co(dmgH)2pyCl]
(1), [Co(dmgH)2(4-COOMe-py)Cl] (2), [Co(dmgH)2(4-Me2N-py)Cl] (3), [Co(dmgH)(dmgH2)Cl2] (4), [Co(dmgH)2(py)2](PF6) (5), [Co(dmgH)2(P(n-Bu)3)Cl] (6),
and [Co(dmgBF2)2(OH2)2] (7), where dmgH = dimethylglyoximate monoanion, dmgH2 = dimethylglyoxime, dmgBF2 = (difluoroboryl)dimethylglyoximate
anion, and py = pyridinewere synthesized and studied as molecular
catalysts for the photogeneration of hydrogen from systems containing
a Pt terpyridyl acetylide chromophore and triethanolamine (TEOA) as
a sacrificial donor in aqueous acetonitrile. All cobaloxime complexes 1−7 are able to quench the luminescence
of the Pt(II) chromophore [Pt(ttpy)(CCPh)]ClO4 (C1) (ttpy = 4′-p-tolyterpyridine).
The most effective electron acceptor for hydrogen evolution is found
to be complex 2, which provides the fastest luminescence
quenching rate constant for C1 of 1.7 × 109 M−1 s−1. The rate of hydrogen
evolution depends on many factors, including the stability of the
catalysts, the driving force for proton reduction, the relative and
absolute concentrations of system components (TEOA, Co molecular catalyst,
and sensitizer), and the ratio of MeCN/water in the reaction medium.
For example, when the concentration of TEOA increases, the rate of
H2 photogeneration is faster and the induction period is
shorter. Colloidal cobalt experiments and mercury tests were run to
verify that the system is homogeneous and that catalysis does not
occur from in situ generated colloidal particles during photolysis.
The most effective system examined to date consists of the chromophore C1 (1.1 × 10−5 M), TEOA (0.27 M), and
catalyst complex 1 (2.0 × 10−4 M) in a MeCN/water mixture (24:1 v/v, total 25 mL); this system
has produced ∼2150 turnovers of H2 after only 10
h of photolysis with λ > 410 nm.