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shown to catalyze the HER with an excellent Faradaic efficiency
and high TON even under the low overpotential conditions.
More importantly, the use of such ligand-based PCET routes
allows us to provide a rational tunability in the overpotential for
the HER. Extended studies are actively in progress.
This work was supported by JSPS KAKENHI Grant Numbers
JP15H03786, JP18H01996, JP16K17879, JP18K05150 and
JP18H05171. This was also supported by the International
Institute for Carbon Neutral Energy Research (WPI-I2CNER),
sponsored by the World Premier International Research Center
Initiative (WPI), MEXT, Japan.
Conflicts of interest
There are no conflicts to declare.
Notes and references
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Fig. 5 Geometries and molecular orbitals relevant to the PCET-based
reduction processes of [Ni(pypzdt)2]2À
, optimized at the UB3P86/6-
311+G(d,p) level of DFT with solvation in water taken into consideration
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[Ni(pypzdt)(pypzdtHÀꢀ)]2À is also shown.
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naphthalene units (Fig. 5 and Fig. S15, ESI†). The difference in the
one-electron reduction potential between [Ni(pypzdt)2]2À and
[Ni(qdt)2]2À (DE = 57 mV) is almost consistent with the difference
in their overpotentials for the HER (DZ(Ecat/2) = 54 mV) at pH = 9.0.
Thus, the stabilization of the LUMO by increasing the number of
nitrogen atoms in the polyazanaphthalene has a major contribu-
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Finally, the Pourbaix diagram of the original catalyst
[Ni(dcpdt)2]2À at a higher pH domain (pH 4 6.4), which was
not established in our previous communications,8a is also
reported herein in order to allow comparison of the electro-
chemical parameters among [Ni(pypzdt)2]2À, [Ni(qdt)2]2À and
[Ni(dcpdt)2]2À. As shown in Fig. 4B, mono- and non-protonated
singly reduced intermediates (i.e., [Ni(dcpdt)(dcpdtHÀꢀ)]2À and
[Ni(dcpdt)2]3À) are generated at 6.4 o pH o 9.0 and pH 4 9.0,
respectively. Notably, we previously reported that one-electron
reduction coupled with two-proton transfer for [Ni(dcpdt)2]2À
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À
proceeds to afford [Ni(dcpdt)(dcpdtH2 ꢀ)]À at 3.5 o pH o
6.4.8a Thus, the pKa values for di- and mono-protonated singly
reduced species of [Ni(dcpdt)2]2À can be estimated to be 6.4
and 9.0, respectively. More importantly, one-electron reduction
potentials of [Ni(pypzdt)2]2À (À0.945 V vs. SCE) and [Ni(qdt)2]2À
(À0.999 V vs. SCE) at pH = 9.0 are significantly more positive than
that of [Ni(dcpdt)2]2À (À1.260 V vs. SCE) (see orange arrows in
Fig. 4A and B), indicating that extended p systems derived from
the polyazanaphthalene moieties greatly contribute to lowering
´
12 N. M. Markovic, B. N. Grgur and P. N. Ross, J. Phys. Chem. B, 1997,
101, 5405.
the overpotential for the HER by [Ni(pypzdt)2]2À and [Ni(qdt)2]2À
.
13 Although deposition of this catalyst at pH = 8 occurs under the
CV conditions ([catalyst] = 0.5 mM, Fig. 2), it is well soluble at this
concentration (1 mM). The saturated concentration was roughly
determined as 0.1 mM under these conditions.
14 R. K. Szilagyi, B. S. Lim, T. Glaser, R. H. Holm, B. Hedman,
K. O. Hodgson and E. I. Solomon, J. Am. Chem. Soc., 2003, 125, 9158.
In summary, [Ni(pypzdt)2]2À and [Ni(qdt)2]2À electrocatalyze
H2 evolution from water with lower overpotentials by employing
the ligand-centered PCET processes that trigger the subsequent
paths leading to H2 evolution. Furthermore, [Ni(pypzdt)2]2À is
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Chem. Commun., 2018, 54, 12820--12823 | 12823