NJC
Letter
Table 1 Chemical and faradaic yields for tetrahydroquinaldine dehydrogena-
tion experiments at a Pt electrode 0.5 mol% catalysts 1 and 2
Table 2 Chemical and faradaic yields for tetrahydroquinaldine dehydrogena-
tion at a Pt electrode (entry 1), SS60 (2) and RVC (3) by 1 (0.5%). Plots of current
vs. time shown in Fig. 4
Electrode
material
Chemical
yield (%)
Faradaic
efficiency (%)
Entry
1
2
3
Pt
SS60
RVC
78
30
55
80
86
95
Chemical
yielda (%)
Faradaic
Entry
a
Catalyst
yieldb (%)
Conclusions
78
80
Dehydrogenation of 1,2,3,4-tetrahydroquinaldine is not only
possible with quinone catalyst 1, but also with free quinone
added to the non-quinoid catalyst 2 (entries c and d in Table 1)
that in the absence of the quinone is inactive. Our CpNi(NHC)
catalyst is one of the first examples of first-row transition metal
complexes exhibiting dehydrogenative electrocatalytic activity
at room temperature and shows that a molecular catalyst
precursor can be viable in the electrode-driven H2 release step
of ‘‘virtual hydrogen storage’’.1
b
N/A
N/A
c
51
68
71
90
Experimental
Synthesis and characterization of compound 1
d
To a 20 mL solution of 1.1 equivalents of nickelocene (41 mg,
0.22 mmol) in anhydrous THF, the yellow quinone-annulated
dimesitylimidazolium chloride ImQ (0.2 mmol) was added as a
solid as shown in eqn (1). The resulting suspension was then
refluxed for 4 hours. 1 was isolated as a red solid in 69% yield
by column chromatography in 4 : 1 hexanes–ethylacetate as a
brown-red solid. FT-ICR MS analysis was performed at the Yale
Keck Proteomics facility on a 9.4 T Bruker Qe FT-ICR MS.
Elemental Analysis was performed by Robertson Microlit.
1H NMR (500 MHz, CD2Cl2) d 8.01 (s, 2H, ArCH), 7.74 (s, 2H,
ArCH), 7.20 (s, 4H, MesCH), 4.58 (s, 5H, CpCH), 2.50 (s, 6H,
e
f
None
0
0
0
0
5 mol% benzoquinone
a
b
Isolated yields. Determined from the ratio of current used for
current used for efficient product formation vs. total passed current.
Mesp-CH3), 2.08 (s, 12H, Meso,o -CH3). 13C{1H} (126 MHz, CD2Cl2)
0
d 141.19, 135.85, 135.35, 133.00, 130.66, 128.25, 94.34, 22.46,
19.66. FT ICR MS: [M-Cl] calculated 557.1734, found 557.1721.
Elemental analysis. Expected: C: 68.77%, H: 5.26%, N: 4.72%
Found: C: 68.59%, H: 5.23%; N: 4.44%.
Acknowledgements
We thank Daryl Smith for suggestions for our electrochemical
cell design. The work was supported as part of the Center for
Electrocatalysis, Transport Phenomena, and Materials (CETM)
for Innovative Energy Storage, an Energy Frontier Research
Center funded by the U.S. Department of Energy, Office of
Basic Energy Sciences under Award Number DE-SC00001055.
We thank Ms. Julie Thomsen for useful discussions.
Fig. 4 Plots of charge passed vs. time during 4 h of electrolysis at 1 V vs. NHE
with the three different electrode materials. Background runs with just the model
fuel (200 mL 1,2,3,4-tetrahydroquinaldine) in the absence of catalyst are shown in
short dash-dot—SS60, dash-dot—RVC, dot-dot—Pt. Catalytic runs were per-
formed at 0.5 mol% 1. The top dashed gray line indicates the level of charge
expected to pass for 100% efficient current conversion for the reaction depicted
in eqn (2). Background current consumptions observed in the absence of catalyst
1 (at the bare electrode) in the presence of substrate does not correlate with
formation of desired product and are depicted by dashed lines. Benzoquinone
was unable to mediate the heterocycle desaturation in the absence of 2 at
5 mol% (entry f in Table 1).
Notes and references
1 R. H. Crabtree, Energy Environ. Sci., 2008, 1, 134.
2 R. Crabtree and O. Luca, WO Pat. 2,012,112,758, 2012.
c
This journal is The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2013
New J. Chem.