Nickel Complexes of Pyridine-Functionalized N-Heterocyclic Carbenes: Syntheses, Structures, and Activity in Electrocatalytic Hydrogen Production

Article abstract of DOI:10.1002/ejic.201600917

Two bis(benz)imidazolium salts alkylated with pyridyl side arms, H₂L1Br₂(1,2-bis{[1-(2-pyridylmethyl)benzimidazolium-3-yl]methyl}benzene bromide) and H₂L2Br₂(1,2-bis{[1-(2-pyridylmethyl)imidazolium-3-yl]methyl}benzene bromide) have been prepared and were used as precursors in the synthesis of novel nickel compounds of N-heterocyclic carbenes (Ni-NHCs). The four Ni-NHC complexes [Ni(L1)Br]Br (1a), [Ni(L1)](PF₆)₂(1b), [Ni(L2)]Br₂(2a) and [Ni(L2)](PF₆)₂(2b) were isolated and characterized by various methods, and the X-ray crystal structures of 1a, 2a and 2b are reported. The nickel ion in 1a is in a square-pyramidal geometry with one of the bromide ions in the apical position, the nickel ions in 2a and 2b are in square-planar geometries. The compounds of the ligand with an imidazole-based carbene revealed much higher activity in electrocatalytic proton reduction and better acid tolerance, although their overpotentials are higher than those of the benzimidazole-based compounds. The presence of bromide ions has an adverse effect on the redox potentials as well as the overpotentials for proton reduction. Complex 2b, having the most planar coordination geometry, appeared to have the highest catalytic efficiency for proton reduction in DMF (i_c/i_p= 50, k_obs= 490 s^–1at 0.1 V/s) when using acetic acid as the proton source. To the best of our knowledge, this is the first report of Ni-NHC complexes that are active in electrocatalytic proton reduction.

Full text of DOI:10.1002/ejic.201600917

DOI: 10.1002/ejic.201600917
Full Paper
Electrocatalytic Ni Complexes
Nickel Complexes of Pyridine-Functionalized N-Heterocyclic
Carbenes: Syntheses, Structures, and Activity in Electrocatalytic
Hydrogen Production
Siyuan Luo,[a] Maxime A. Siegler,[b] and Elisabeth Bouwman*[a]
Abstract: Two bis(benz)imidazolium salts alkylated with pyridyl in square-planar geometries. The compounds of the ligand with
side arms, H2L1Br2 (1,2-bis{[1-(2-pyridylmethyl)benzimid- an imidazole-based carbene revealed much higher activity in
azolium-3-yl]methyl}benzene bromide) and H2L2Br2 (1,2-bis- electrocatalytic proton reduction and better acid tolerance, al-
{[1-(2-pyridylmethyl)imidazolium-3-yl]methyl}benzene bromide) though their overpotentials are higher than those of the benz-
have been prepared and were used as precursors in the synthe- imidazole-based compounds. The presence of bromide ions has
sis of novel nickel compounds of N-heterocyclic carbenes an adverse effect on the redox potentials as well as the overpo-
(Ni-NHCs). The four Ni-NHC complexes [Ni(L1)Br]Br (1a), tentials for proton reduction. Complex 2b, having the most pla-
[Ni(L1)](PF6)2 (1b), [Ni(L2)]Br2 (2a) and [Ni(L2)](PF6)2 (2b) were nar coordination geometry, appeared to have the highest cata-
isolated and characterized by various methods, and the X-ray lytic efficiency for proton reduction in DMF (ic/ip = 50, kobs
=
crystal structures of 1a, 2a and 2b are reported. The nickel ion 490 s–1 at 0.1 V/s) when using acetic acid as the proton source.
in 1a is in a square-pyramidal geometry with one of the brom- To the best of our knowledge, this is the first report of Ni-NHC
ide ions in the apical position, the nickel ions in 2a and 2b are complexes that are active in electrocatalytic proton reduction.
Introduction
though Ni-NHC complexes have found various applications in
organometallic chemistry,[15–21] the electrocatalytic properties
of this kind of compounds so far mostly have been neglected.
NHC ligands functionalized with pyridine groups have been
used to synthesize nickel and palladium complexes that were
found to be efficient catalysts for the Kumada cross-coupling[22]
and Heck-type coupling reactions.[23] Recently, the group of Ku-
biak reported a study of nickel compounds with four carbene
ligands including their electrochemical characterization, but
their activity in electrocatalysis was not discussed.[24] A series
of nickel complexes with NHC-pyridine ligands was reported by
Thoi et al.[25,26] to selectively reduce carbon dioxide to carbon
monoxide. Nevertheless, the number of studies in the area of
electrocatalysis using Ni-NHC compounds is limited. Herein, we
report four novel Ni-NHC complexes obtained with the ligand
precursors H2L1Br2 and H2L2Br2, which are based on
bis(benz)imidazolium salts bridged by a xylyl linker (Scheme 1).
Dihydrogen is an environmentally friendly energy carrier as
upon combustion it only produces H2O.[1,2] In order to enable
the establishment of a society based on dihydogen as an en-
ergy source, for many years researchers have focused on the
search for efficient proton-reduction catalysts, notably for cata-
lysts that are not based on noble metals.[3–7] In the field of
bioinorganic chemistry the synthesis of structural models of
hydrogenases is a common strategy to devise molecular cata-
lysts for proton reduction.[8,9] However, the so-called “functional
models” of hydrogenases, which do not exhibit exactly similar
structures, appear to result in more active and more stable cata-
lytic systems. The two-electron donor ligands such as phos-
phanes and amines have been widely used in combination with
Co[10–12] and Ni[7,13] metal centers. The N-heterocyclic carbene
(NHC) ligand is also a two-electron donor, with strong binding
properties comparable to that of phosphane ligands.[14] Al-
[a] Leiden Institute of Chemistry, Gorlaeus Laboratories, Leiden University,
P. O. Box 9502, 2300 RA Leiden, The Netherlands
E-mail: bouwman@chem.leidenuniv.nl
http://mcbim.lic.leidenuniv.nl/
[b] Department of Chemistry, Johns Hopkins University,
3400 N. Charles Street, Baltimore, Maryland 21218, USA
Supporting information and ORCID(s) from the author(s) for this article are
available on the WWW under http://dx.doi.org/10.1002/ejic.201600917.
© 2016 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA. ·
This is an open access article under the terms of the <xref href="http://
creativecommons.org/licenses/by/4.0/">Creative Commons Attribution Li-
cense</xref>, which permits use, distribution and reproduction in any me-
dium, provided the original work is properly cited.
Scheme 1. Complexes reported in this paper (linker = xylyl) and related com-
pounds previously reported (with linker = propyl[26] or methylquinoxaline[27]).
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We have studied their redox properties and electrocatalytic ac- diethyl ether). The crystallographic data are provided in Table 1,
tivity for proton reduction in an organic solvent.
selected bond lengths and bond angles are given in Table 2.
Table 2. Selected bond lengths (Å) and angles (°) of compounds 1a, 2a, and
2b.
Results and Discussion
1a
2a
2b
Syntheses
Ni1–N22
Ni1–N52
Ni1–C12
Ni1–C42
1.962(2)
1.963(2)
1.879(2)
1.882(2)
3.080(8)
87.42(9)
176.72(9)
87.86(8)
95.8(1)
1.946(7)
1.929(2)
1.877(5)
1.876(3)
1.942(3)
1.932(3)
1.879(4)
1.887(4)
The two bis(benz)imidazolium salts 1,2-bis{[1-(2-pyridylmeth-
yl)benzimidazolium-3-yl]methyl}benzene bromide (H2L1Br2)
and 1,2-bis{[1-(2-pyridylmethyl)imidazolium-3-yl]-methyl}benz-
ene bromide (H2L2Br2) were synthesized following published
procedures with small modifications.[28,29] The nickel bromide
compounds [Ni(L1)Br]Br (1a) and [Ni(L2)]Br2 (2a) were obtained
as yellow-colored powders from a melt of nickel acetate with
the ligand in tetrabutylammonium bromide (TBAB) in good
yields (74 and 55 %, respectively). The color of complex 1a
turned to orange once dried under vacuum overnight. Whereas
the benzimidazole-based complex 1a is poorly soluble in water,
the imidazole-based compound 2a is soluble in water. Com-
pounds [Ni(L1)](PF6)2 (1b) and [Ni(L2)](PF6)2 (2b) were obtained
from 1a and 2a via anion exchange by adding a solution of the
complex in methanol to a boiling methanol solution of NH4PF6.
All complexes are air and moisture stable in the solid state for
at least several months. The low-spin character of the NiII ion in
all compounds is evidenced by their diamagnetic 1H NMR spec-
tra, indicating that the square-planar geometry of the Ni cen-
ters is retained in solution. The ESI-MS analysis of the four com-
pounds shows the presence of monocationic (with one coordi-
nated bromide anion, or as an ion pair) as well as dicationic
fragment ions. Recrystallized samples of the Ni compounds
were dried in vacuo before elemental analysis was performed;
however, in some cases the analysis still showed the presence
of the solvents used in recrystallization.
Ni1–Br2
N22–Ni1–C12
N22–Ni1–C42
N22–Ni1–N52
C12–Ni1–C42
C12–Ni1–N52
C42–Ni1–N52
87.29(3)
171.7(3)
86.5(3)
99.1(2)
167.7(2)
88.2(1)
87.8(2)
172.7(1)
89.4(1)
95.0(2)
173.4(2)
88.5(1)
171.03(9)
88.86(9)
Compound 1a crystallizes as a monocationic complex with
one uncoordinated bromide ion and 1.5 water molecules per
asymmetric unit. The nickel ion in complex 1a is coordinated
in a square-pyramidal geometry, with the donor atoms of the
tetradentate ligand in the equatorial plane and one coordinat-
ing bromide ion at the apical position (Figure 1). The bond
length to the apical bromide (Ni1–Br2) is 3.0800(8) Å. This dis-
tance is rather long and could be considered as a normal van
der Waals contact; however, also the coordination angles (be-
tween 86–99°), the small torsion angle of the equatorial plane
and the low τ5 value (see below) are indicative of a stronger
interaction. The tetradentate ligand is wrapped around the
nickel ion with the aromatic groups orientated in a zig-zag fash-
ion with respect to each other. The ligand donor atoms form
an approximate plane with only a small torsion angle [N22–
N52–C42–C12 = 5.18(8)°], with Ni–N and Ni–C bond lengths in
the range of 1.882–1.963 Å, consistent with the typical distan-
ces reported for this type of structures.[21,27] The N–Ni–C and
C–Ni–C bond angles are slightly smaller than the ideal right
Single Crystal X-ray Crystallography
Single crystals of 1a, 2a, and 2b suitable for X-ray diffraction angles (87.42–88.86°), but the “open” N52–Ni1–N22 angle is
were obtained using the liquid-liquid diffusion method (MeCN/ slightly larger [95.85(9)°]. The five-coordinate geometry is char-
Table 1. Selected crystallographic and structure refinement data for complex 1a, 2a and 2b.
1a
2a
2b
Empirical formula
Formula weight
Temperature (K)
Crystal system
Space group
a (Å)
C34H28BrN6Ni, (Br), 1.5(H2O)
766.18
110(2)
monoclinic
C2/c
36.2037(12)
11.2404(2)
16.7135(6)
109.865(4)
6396.7(4)
8
C26H24N6Ni, 2(Br), 2.361(H2O), 0.431(O)
688.50
110(2)
monoclinic
C2/c
32.8374(6),
8.89992(13)
20.2657(4)
109.610(2)
5579.13(18)
8
C26H24N6Ni, 2(PF6)
769.16
110(2)
monoclinic
P21/c
23.0500(14)
9.5940(3)
14.7070(7)
105.683(6)
3131.3(3)
4
b (Å)
c (Å)
ꢀ (°)
V (Å3)
Z
Dcalc (g/cm3)
1.591
3.145
3096
1.639
4.65
2777
1.632
2.796
1552
μ (mm–1
F(000)
)
Crystal size (mm)
Reflections collected,
unique, I > 2σ(I)
0.27 × 0.17 × 0.10
0.26 × 0.09 × 0.02
0.25 × 0.14 × 0.01
25476, 7345, 6196
23382, 5423, 4625
18755, 6111, 5006
R-factor, I > 2σ(I)
R-factor (all data)
R1 = 0.0290, wR2 = 0.0635
R1 = 0.0400, wR2 = 0.0674
0.43, –0.49
R1 = 0.0298, wR2 = 0.0761
R1 = 0.0372, wR2 = 0.0810
0.61, –0.56
R1 = 0.0698, wR2 = 0.1823
R1 = 0.0832, wR2 = 0.1933
0.92, –0.71
Δρmax, Δρmin (e Å–3
)
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acterized with a τ5 value of 0.09, indicating that the nickel ion ligands with pyridyl side groups. The donor atoms of tetra-
is coordinated in a near ideal square pyramid.[30]
dentate ligands containing the more flexible pyridylmethyl
groups are arranged in a more planar fashion around the nickel
ions with lower τ4 values (τ4 = 0.09 for pyridylmethyl groups
and 0.14 for pyridyl groups). The different linkers in the di-
carbene chelating structure may also affect the planarity of the
arrangement of donor atoms around the nickel centers: Ligands
with rigid linkers containing an aromatic group such as xylene
(2a τ4 = 0.06; 2b τ4 = 0.04) or dimethylquinoxaline[27] (τ4 = 0.03)
generally present smaller τ4 values than the ligands with alkyl
linkers (τ4 = 0.12–0.14, see Table S1).[26]
Figure 1. Displacement ellipsoid plots (50 % probability level) of the cations:
a) [Ni(L1)Br]+ in 1a and b) [Ni(L1)]2+ in 2b at 110(2) K. Atom numbering
scheme for the first coordination sphere is provided. Hydrogen atoms, the
non-coordinated bromide ion, the hexafluoridophosphate ion and the lattice
water molecules are omitted for clarity.
UV/Vis Absorption Studies
Electronic absorption spectra of the four nickel compounds as
well as the precursor ligands H2L1Br2 and H2L2Br2 were re-
corded both in the solid state and in DMF solutions (see Table 3
and Figures S2–S5).
The solid-state absorption spectra of the ligand precursors
H2L1Br2 and H2L2Br2 show one absorption around 260 nm as-
cribed to π–π* transitions of the pyridyl groups.[39] The absorp-
tion bands in the solid-state spectrum of 1a at 305 and 355 nm
are assigned to ligand-to-metal charge-transfer (LMCT) transi-
tions, and the absorption at 433 nm is assigned to a d-d transi-
tion (Figure S3). Similar assignments are proposed for the spec-
tra of the other compounds (Figure S4).
Compounds 2a, 1b and 2b readily dissolve in dry DMF re-
sulting clear (pale) yellow solutions (within 1 min). However, the
solubility of 1a is lower, initially resulting in a cloudy solution;
only after stirring for 1 h the solution became clear. The limited
solubility of 1a in DMF may be related to the coordination of
one of the bromide ions; perhaps only upon dissociation of this
bromide ion the compound is fully soluble. The UV/Vis spectra
in DMF again show the intense π–π* transition of the pyridyl
groups at around 275 nm and an intense transition around 310
nm, which has been ascribed to a ligand-to metal charge trans-
fer (LMCT of carbene to nickel).[40] All compounds show a d-d
transition in the region of 360–380 nm common for square-
planar nickel(II) complexes (Figure S5).
The five-coordinate geometry of the nickel ion in 1a is not
rare; five-coordinate geometries can be divided into two cate-
gories: the trigonal bipyramid[31] and the tetragonal pyramid.[32]
The square-pyramidal geometry is commonly formed with
pentadentate chelating ligands,[33] or from a combination of
tridentate and bidentate chelating ligands.[34,35] Square-pyrami-
dal structures formed by a tetradentate ligand with an addi-
tional halide ion are rather rare.[36,37] The non-coordinating
bromide ion and the lattice water molecules are connected
with the coordinating bromide ion via hydrogen bonding inter-
actions.
Compounds 2a and 2b crystallize as dicationic complexes
with the non-coordinating bromide ions and the solvent mol-
ecules (2a) or the hexafluoridophosphate ions (2b) in the crys-
tal lattice. The nickel ion in these complexes is coordinated in
a square-planar geometry, with the donor atoms of the tetra-
dentate ligand wrapped around the nickel ion (Figure 1). The
Ni–N bond lengths in 2a and 2b are slightly shorter than those
in 1a (see Table 2), whereas the Ni–C bond lengths are rather
similar.
The square-planar geometry of the nickel ion in 2a was eval-
uated with an equation[24] derived from a report by Yang.[38]
The largest and second-largest L–Ni–L angles in 2a are 171.7(3)°
and 167.7(2)°, resulting in a τ4 value of 0.06. For compound 2b
a τ4 value of 0.04 is calculated, indicating that the geometry of
Electrochemical Studies
the central ion in both compounds is close to the ideal square- Electrochemical analysis of the four nickel complexes in dry
planar geometry. The structures of the nickel(II) compounds DMF was performed with 0.1 tetrabutylammonium hexafluor-
M
have been compared with those earlier reported (Table 2 and idophosphate (TBAP) as the supporting electrolyte under a
Table S1);[26] it appears that ligands containing pyridylmethyl stream of argon. An Ag/AgCl reference electrode was used, but
pendant arms result in longer Ni–C bond lengths compared to the potentials are reported vs. the ferrocene/ferrocinium (Fc0/+
)
Table 3. Parameters of the electronic spectra of Ni complexes and ligands.
Solid state optical spectra [nm]
Absorption bands, λmax nm (ε/M )
–1 cm–1 [a]
1a
1b
2a
2b
H2L1Br2
H2L2Br2
264, 305, 355, 433 sh
262 298, 324, 390
260, 308, 351
258, 299, 384
259
276 (4640), 311 (5230), 361 sh (1170)
280 (4450), 314 (5860), 389 sh (360)
274 (4000), 305 (4860), 378 sh (510)
273 (2860), 307 (3860), 381 sh (180)
275 (4750)
258
267 (4000)
[a] The spectra were recorded in DMF at 22 °C at a concentration of 1 mM.
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couple, which was added after each experiment. The electro- hexafluoridophosphate species 1b and 2b show similar pat-
chemical data of the nickel compounds are collected in Table 4. terns compared to their bromide analogs, but with the redox
processes occurring at less negative redox potentials (by about
Table 4. Electrochemical data of the nickel compounds.[a]
20–50 mV); the presence of the potentially coordinating anions
apparently stabilizes the NiII oxidation state.
Epc1 [V]
Epa1 [V]
E1/2 [V]
Epc2 [V]
Epa2 [V]
E1/2 [V]
Epc3 [V]
(ΔEp [mV])
(ΔEp [mV])
It appears that the nickel compounds with a benzimidazole-
based carbene ligand are more readily reduced with less nega-
tive half-wave potentials (–1.53 V, –1.55 V) than those with the
imidazole-based carbenes (–1.66 V, –1.71 V), indicating the im-
idazole-based ligands to be slightly more electron-donating.
The redox potentials of these nickel compounds with the rela-
tively hard pyridyl ligands occur at rather negative potentials
compared to Ni centers that are coordinated with softer phos-
phane ligands.[13,32]
1a –1.59
1b –1.58
2a –1.76
2b –1.70
–1.51
–1.49
–1.67
–1.63
–1.55 (82) –1.84
–1.53 (84) –1.80
–1.71 (82) –1.96
–1.66 (75) –1.90
–1.76
–1.72
–1.86
–1.82
–1.80 (79)
–1.76 (80)
–1.91 (100) –2.27
–1.86 (84) –2.23
–2.04
–2.00
[a] All voltammograms were recorded in DMF; the potentials are referenced
to the Fc+/0 couple. Conditions: scan rate = 0.1 V/s, compound (1 m
), TBAP
(0.1 ), glassy-carbon working electrode. Under these conditions we found
ΔEp (Fc+/0) = 78 mV.
M
M
The cyclic voltammogram of complex 1a shows the presence
of three reductive waves, two of which are reversible (see Fig-
ure 2). The two reversible waves are tentatively assigned to the
NiII/NiI and NiI/Ni0 redox couples (E1/2 = –1.55 and –1.80 V).
However, the formation of a NiI species with a ligand-centred
radical is also possible, although the voltammogram of the li-
gand does not have a reduction wave coinciding with the sec-
ond reversible reduction of the complex. Without a full spectro-
electrochemical study the true nature of the reduction process
occurring at –1.8 V is unknown. The first reversible redox wave
at –1.55 V vs. Fc0/+, is characterized by a peak current ratio
ipa/ipc of 0.99. A plot of the reductive peak current ipc displays
a linear relationships with the square root of the scan rate (Fig-
ure S6), which demonstrates this first electrochemical reduction
to be diffusion-controlled.[41] In addition, the current also in-
creases linearly with the concentration of the nickel compound
(Figure S7). The third small reductive peak at –2.04 V is tenta-
tively attributed to a ligand-based process, forming a ligand-
centered radical, as its position coincides with a reduction wave
observed in the CV of H2L1Br2 (see Figure 2).
Catalytic Activity for Proton Reduction in Non-Aqueous
Solution
The four compounds have been investigated for their catalytic
activity in the electrocatalytic proton reduction in DMF, using
either acetic acid (HOAc) or trifluoroacetic acid (Htfa) as the
proton source. For each scan the working electrode was freshly
polished.
When using the strong acid Htfa as the proton source (pKa
in DMF: 6.0[42]), two new reductive waves (–1.9 and –2.4 V)
appeared after two irreversible peaks at –1.6 and –1.8 V (Fig-
ure 3), which is consistent with the report of Canaguier et al.[6]
The catalytic current ic is the maximum current of the catalytic
proton-reduction peak, and ip is the plateau current of the non-
catalytic reduction waves. The current of this catalytic wave is
dependent on the acid concentration, but saturates at a con-
centration of 40 mM. As shown in Figure 3 and Figure S9, the
ic/ip ratio increases linearly with the acid concentration until the
acid-independent region is reached. This saturation indicates
that at this acid level the main rate-controlling process is no
longer acid diffusion, but rather the catalytic efficiency of the
Ni complex. The ic/ip value is 13 in the acid-independent region
(at a scan rate of 0.1 V/s) which corresponds to a rate constant
(kobs) of 33 s–1.[7,43]
Figure 2. Cyclic voltammograms of 1a (1 m
and H2L1Br2 (1 m , dashed line) recorded in DMF in different scan ranges.
Conditions: scan rate: 0.1 V/s, TBAP (0.1 ), glassy-carbon working electrode.
M, solid line and dash-dotted line)
M
M
Compound 2a reveals a similar cyclic voltammogram with
three reduction processes at –1.76, –1.96 and –2.27 V, and two
oxidation processes at –1.67 and –1.86 V. However, the redox
processes are significantly less reversible; the peak-current ratio
ipa/ipc of the first reduction event is 0.75 indicating a quasi-
reversible redox process (Figure S8). The voltammograms of the
Figure 3. Cyclic voltammogram of 1a (1 m
recorded in DMF. Conditions: scan rate: 0.1 V/s, TBAP (0.1
working electrode. Inset: plot of ic/ip vs. [Htfa]. The value for ic is the maxi-
mum current of the catalytic peak, and ip is the plateau current of the non-
catalytic wave, as indicated in the Figure.
M
) in presence of Htfa (10 m
M)
M), glassy-carbon
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Activity for proton reduction is also observed when the The linker type between the carbenes as well as the bridge to
weaker acetic acid is added to a solution of 1a in DMF. The two the pyridine group influences the geometry of the complexes,
reversible redox couples as well as the third irreversible reduc- as characterized by their τ4 values. It appears that a rigid linker
tive wave combine into one irreversible wave at –1.8 V. Mean- based on an aromatic group in combination with a pyridyl-
while only one new reduction peak is observed at –2.6 V. The methyl pendent group attributes to more ideal square-planar
current of this peak is dependent on the acid concentration geometries. We found that the use of imidazole-based carbenes
and thus is ascribed to the electrocatalytic reduction of protons result in much higher catalytic activity for electrocatalytic re-
by the nickel compound. The appearance of bubbles on the duction of protons, as well as better acid tolerance, although
GC working electrode during the experiment is another visible these compounds reveal more negative NiII/NiI and NiI/Ni0
indication for H2 production. No obvious current enhancement redox couples and higher overpotentials. The most planar com-
is observed in a blank experiment using the same quantity of plex 2b (with the smallest τ4 value) shows the highest catalytic
acid in the absence of nickel complex (see Figure S10).
Comparison of the catalytic activities of four Ni complexes
using acetic acid as the proton source, showed the ic/ip value
of compound 2b to be highest, reaching 50 (corresponding to
efficiency (ic/ip = 50, kobs = 490–1at 0.1 V/s) of all complexes
investigated in this study for the catalytic proton reduction us-
ing acetic acid, although the overpotential at which dihydrogen
is produced is sizeable. To the best of our knowledge, these are
the first Ni-NHC complexes reported to catalyze proton reduc-
tion. Only a few recent reports relate to proton reduction with
metal-NHC compounds, but these concern cobalt-NHC com-
plexes.[14,45]
kobs = 490–1 at 0.1 V/s) at an acid concentration of 80 m
M (Fig-
ure 4, Figure S11 and Table 5). The overpotential of the four
molecular catalysts for electrocatalytic proton reduction at an
acetic acid concentration of 10 mM has been calculated taking
homoconjugation of the acid into account (Table 5).[44] Imid-
azole-based compound 2b, the most efficient electrocatalyst of
the four compounds, unfortunately displays a higher overpo-
tential (1.28 V) than the benzimidazole-based carbene com-
plexes (ca. 1.15 V). Furthermore, the presence of bromide ions
in complexes results in slightly higher overpotentials (30–50 mV
difference).
Furthermore, it was shown that the presence of halide ions
results in more negative potentials for the NiII/NiI and NiI/Ni0
redox couples, and also leads to larger overpotentials for proton
reduction.
The experiments of catalytic proton reduction in organic sol-
vents provide a starting point for analysis; however, water is the
ideal solvent for H2 production. Considering the higher solubil-
ity of the hexafluoridophosphate salts in aqueous solutions and
the adverse effect of the bromide ions on the catalytic activity
of the nickel compounds, further investigations to proton re-
duction catalysis with this type of molecular catalysts will be
carried out in acid-buffered aqueous solutions. Although the
observed overpotentials are not outstanding, the reported
nickel compounds are attractive: the synthesis route of the
complexes is straightforward and the complexes are air and
moisture stable due to the presence of the bis-carbene donor.
Based on their ease of synthesis and overall stability this type of
molecular catalysts may thus be competitive with the diimide/
phosphane Ni catalysts.
Figure 4. Cyclic voltammograms of 1 m
amounts of acetic acid (0–120 equiv.) in 0.1
M
solutions of 2b with various
TBAP-DMF at 0.1 V/s. Inset:
M
plot of ic/ip vs. [CH3COOH] for 2b.
Experimental Section
Table 5. Summary of the electrocatalytic properties of the compounds.[a]
General: Commercial chemicals and solvent were used without fur-
ther purification. Dry solvent obtained from PureSolv MD5. All air-
sensitive reactions were performed under argon using standard
Schlenk techniques unless mentioned otherwise.
1a
1b
2a
2b
[HOAc] (mM)
16
16
80
80
ic/ip
kobs
5.4
5.7
1.17
4.5
4.0
1.14
35
240
1.33
50
490
1.28
1H, 13C and 31P NMR spectra were recorded on a Bruker 300 DPX
spectrometer. Mass spectra were obtained using a Finnigan Aqua
Mass Spectrometer (MS) with electro spray ionization (ESI). UV/Vis
spectra were obtained using either a transmission dip probe or a
solid-state reflection probe on a Avantes Avaspec-2048 spectrome-
ter with a Avalight-DH-S-BAL light source. Elemental analyses were
performed by the Kolbe Mikroanalytisches Laboratorium, Germany.
Overpotential (V)[b]
[a] The ic/ip value, kobs and overpotentials of the reductive waves of the four
complexes(1 m ) at the maximum concentration of acetic acid at a scan rate
of 0.1 V/s (vs. Fc+/0). [b] Overpotentials of complexes were calculated in the
presence of 10 m acetic acid.
M
M
X-ray Structural Determination: All reflection intensities were
measured at 110(2) K using a SuperNova diffractometer (equipped
with Atlas detector) with Mo-Kα radiation (λ = 0.71073 Å) (1a) or Cu-
Conclusions
In this paper we report four novel pyridine-functionalized Ni-
NHC complexes and their redox and electrocatalytic properties. Kα radiation (λ = 1.54178 Å) (2a, 2b) under the program CrysAlisPro
Eur. J. Inorg. Chem. 2016, 4693–4700
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Full Paper
(version 1.171.36.32 Agilent Technologies, 2013). The same program
was used to refine the cell dimensions and for data reduction. The
structure was solved with the program SHELXS-2013[46] and was
refined on F2 with SHELXL-2013.[46] Analytical numeric absorption
corrections based on a multifaceted crystal model were applied us-
ing CrysAlisPro. The temperature of the data collection was con-
trolled using the system Cryojet (manufactured by Oxford Instru-
ments). The H atoms were placed at calculated positions (unless
otherwise specified, see below) using the instructions AFIX 23 or
AFIX 43 with isotropic displacement parameters having values 1.2
times Ueq of the attached C atoms.
The apparent rate constant (kobs) can be regarded as turn-over fre-
quency (TOF) when a first-order or pseudo-first order H2 evolving
reaction is catalyzed by a freely diffusing molecular catalyst.[43] The
corresponding equation is provided below:[7,43]
Here, ic is the maximum current of the catalytic peak, and ip is the
plateau current of the non-catalytic reduction wave. F is Faraday's
constant, T is 295 K, R is the universal gas constant, v is the scan
rate, n is 2 in the case of the H2 evolution process.
1a: The H atoms attached to O1W and O2W (i.e., lattice water mol-
ecules) were found from difference Fourier maps, and their coordi-
nates were refined freely. The structure is ordered. The crystal lattice
also contains some amount of very disordered solvent molecules.
Their contribution has been taken out using the SQUEEZE proce-
dure[47] in the final refinement (SQUEEZE details are provided in the
CIF file).
Synthetic Procedures
Synthesis of the Ligands H2L1Br2 and H2L2Br2: N-Pyridyl-
[29]
methyl(benz)imidazole,[28] H2L1Br2 and H2L2Br2
were synthe-
sized following literature methods with small modifications.
H2L1Br2: N-Pyridylmethylbenzimidazole (4 mmol, 0.836 g) and α,α′-
dibromo-o-xylene (2 mmol, 0.57 g) were added into a 25 mL round-
bottomed flask containing 10 mL of toluene. The mixture was
stirred for 24 h at 110 °C, after which time the product was collected
by filtration. The white powder was washed with THF and ether,
and dried under vacuum, yield 1.32g (97 %). 1H NMR (300 MHz,
[D6]DMSO): δ = 10.07 (s, 2 H), 8.48 (d, J = 4.77 Hz, 2 H), 8.04 (s, 1
H), 8.02 (d, J = 3.17 Hz, 2 H), 7.98 (d, J = 2.60 Hz, 1 H), 7.95 (t, J =
2.18, 2.18 Hz, 2 H), 7.92 (d, J = 1.74 Hz, 1 H) 7.90 (d, J = 1.76 Hz, 1
H), 7.74 (s, 1 H), 7.72 (s, 1 H), 7.66 (m, 4 H), 7.44 (dd, J = 3.33, 5.70 Hz,
2 H), 7.38 (dd, J = 5.35, 7.05 Hz, 2 H), 7.19 (dd, J = 3.46, 5.58 Hz, 2
H), 6.16 (s, 2 H), 6.00 (s, 2 H) ppm. 13C NMR (75 MHz, [D6]DMSO):
δ = 152.87, 149.55, 148.05, 143.87, 137.58, 132.06, 131.55, 131.07,
129.37, 128.25, 127.07, 126.80, 123.74, 122.77, 114.07, 50.85, 47.53
ppm. ESI-MS found (calculated): [M – 2Br]2+ m/z 261.3 (261.1); [M –
Br]+ m/z 601.1 (601.2).
2a: The H atoms attached to O1W, O2W (O2W is found at sites of
twofold axial symmetry) and O3W were found from difference Fou-
rier maps, and their coordinates were refined freely. The H atoms
attached to the lattice water solvent molecules O3W′ (minor com-
ponent of the disordered water lattice molecule) and O4W [partially
occupied with an occupancy factor of 0.292(9)] could not be re-
trieved from difference Fourier maps. The structure is partly disor-
dered. One part of the Ni complex and one lattice water solvent
molecule (O3W/O3W′) is disordered over two orientations, and the
occupancy factors of the major components of the disorder refine
to 0.649(11) and 0.861(4), respectively. One of the two counterions
(Br2) is disordered over three orientations, and the occupancy fac-
tors of the three components refine to 0.779(2), 0.180(2) and
0.0410(8).
2b: The structure is ordered, except for some amount of disordered
lattice solvent molecules, whose contribution has been taken out
using the SQUEEZE procedure[47] in the final refinement (SQUEEZE
details are provided in the CIF file).
H2L2Br2: N-Pyridylmethylimidazole (4 mmol, 0.836 g) and α,α′-di-
bromo-o-xylene (2 mmol, 0.57 g) were added into a 25 mL round-
bottomed flask containing 10 mL of toluene. The mixture was
stirred for 48 h at 110 °C, after which time the product was collected
by filtration. The sticky solid was crushed and washed with THF
until it became a brown powder, which was dried under vacuum,
CCDC 1444939 (for 1a), 1444940 (for 2a), and 1444941 (for 2b) con-
tain the supplementary crystallographic data for this paper. These
data can be obtained free of charge from The Cambridge Crystallo-
graphic Data Centre.
1
yield 0.90 g (76 %). H NMR (300 MHz, [D6]DMSO): δ = 9.48 (s, 2 H),
8.55 (d, 2 H), 7.93 (d, J = 1.5 Hz, 1 H), 7.91 (d, J = 1.2 Hz, 1 H), 7.88
(s, 2 H), 7.82 (s, 2 H), 7.55 (d, J = 8.1 Hz, 2 H), 7.52–7.48 (m, 2 H),
7.41 (dd, J = 4.8, 2.1 Hz, 2 H), 7.32 (dd, J = 3.48, 5.44 Hz, 2 H), 5.73
(s, 4 H), 5.64 (s, 4 H) ppm. 13C NMR (75 MHz, [D6]DMSO): δ = 153.41,
149.57, 137.54, 137.48, 132.98, 129.64, 129.38, 123.75, 123.70,
122.69, 122.56, 53.09, 49.12 ppm. ESI-MS found (calculated): [M –
2Br]2+ m/z 211.2 (211.6); [M – Br]+ m/z 501.0 (502.1).
Electrochemistry: Cyclic voltammetry was recorded with an Auto-
lab PGstat10 potentiostat controlled by GPES4 software under ar-
gon. A 3 mm diameter glassy carbon electrode was used as working
electrode and a Pt wire as the counter electrode. The experimental
reference was an Ag/AgCl electrode in the electrolyte solution
(TBAP). Ferrocene was added at the end of each measurement as
an internal standard. All electrode potentials reported in this publi-
cation are given vs. the potential of the ferrocene/ferrocenium
redox couple ([Fc+/0], Eo = 0.00 V) as the reference.
Synthesis of [Ni(L1)Br]Br (1a) and [Ni(L2)]Br2 (2a): Tetrabutyl-
ammonium bromide (2 g), nickel acetate (0.18 g, 1 mmol) and
H2L1Br2 (0.69 g, 1 mmol) or nickel acetate (0.15 g, 0.84 mmol) and
H2L2Br2 (0.49 g, 0.84 mmol) were weighed into a 10 mL round-
bottomed flask. The mixture was heated at 80 °C under vacuum for
3 h. Then the temperature was increased to 120–130 °C and the
flask was heated for another 24 h under vacuum. After cooling to
room temperature the residue was triturated with 50 mL distilled
water, and kept overnight. The mixture was filtered yielding a yel-
low powder. The powder was thoroughly washed with diethyl ether,
yield 1a 0.55 g (74 %), 2a 0.29 g (45 %). Single crystals were ob-
tained by the liquid-liquid diffusion method (MeCN/diethyl ether).
Overpotentials were calculated using the following equation:
Here, η = ET1/2 is the thermodynamic standard potential computed
in the case of an electrochemically reversible process which consid-
ered the homoconjugation effect given by the equation:[44]
η = EE1/2 is the experimental reduction potential, which is defined
here as the maximum of the first derivative of the irreversible
wave.[44]
1a: 1H NMR (300 MHz, [D6]DMSO): δ = 8.49 (d, J = 5.50 Hz, 2 H),
8.13 (m, 6 H), 7.885 (dd, J = 3.66, 5.20 Hz, 2 H), 7.34–7.44 (m, 10 H),
7.25 (t, J = 7.70, 7.70 Hz, 2 H), 6.465 (d, J = 15.22 Hz, 2 H), 5.45 (d,
Eur. J. Inorg. Chem. 2016, 4693–4700
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Full Paper
J = 16.06 Hz, 2 H), 5.32 (d, J = 15.93 Hz, 2 H) ppm. 13C NMR (75 MHz,
[D6]DMSO): δ = 168.43, 154.56, 153.16, 141.00, 134.30, 134.04,
132.86, 132.78, 128.81, 124.81, 124.64, 124.34, 124.04, 112.77,
111.35, 50.76, 50.12 ppm. ESI-MS found (calculated): [M – 2Br]2+ m/z
289.0 (289.1); [M – Br]+ m/z 657.1 (657.1). C34H28Br2N6Ni (739.15):
calcd. C 55.25, H 3.82, N 11.37; found C 54.57, H 3.93, N 11.80.
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[4]
[5]
[6]
[7]
2a: 1H NMR (300 MHz, [D6]DMSO): δ = 8.30 (d, J = 5.27 Hz, 2 H),
8.11 (t, J = 7.62, 7.62 Hz, 2 H), 7.88 (d, J = 7.62 Hz, 2 H), 7.71 (d, J =
1.56 Hz, 2 H), 7.55 (m, 2 H), 7.35–7.37 (m, 4 H), 6.98 (m, 4 H), 5.81
(d, J = 14.99 Hz, 2 H), 5.01 (d, J = 15.46 Hz, 2 H), 4.69 (d, J = 15.30 Hz,
2 H) ppm. 13C NMR (75 MHz, DMSO): δ = 155.56, 154.67, 152.64,
140.83, 135.54, 131.42, 129.04, 124.97, 124.84, 124.67, 122.80, 53.39,
51.50 ppm. ESI-MS found (calculated): [M – 2Br]2+ m/z 239.1 (239.1);
[M – Br]+ m/z 557.0 (557.1). C26H24Br2N6Ni·H2O: calcd. C 47.53, H
3.99, N 12.79; found C 47.73, H 3.74, N 12.69.
[8]
[9]
[10]
[11]
[12]
Synthesis of [Ni(L1)](PF6)2 (1b) and [Ni(L2)](PF6)2 (2b): NH4PF6
(128 mg, 0.4 mmol) was dissolved in 15 mL of methanol and the
solution was brought to a boil. The compounds 1a (74 mg,
0.1 mmol) or 2a (64 mg, 0.1 mmol) were added into this boiling
methanol solution. The resulting suspension was stirred for 30 min
during which time it cooled down. The product was collected by
filtration and the pale yellow solid was washed with cold methanol
and diethyl ether, yield 1b 35 mg (55 %), 2b 55 mg (72 %). Single
crystals were obtained by the liquid-liquid diffusion method (MeCN/
diethyl ether).
[13]
[14]
[15]
[16]
[17]
[18]
[19]
1b: 1H NMR (300 MHz, [D6]DMSO): δ = 8.42 (s, J = 5.26 Hz, 2 H),
8.15–8.18 (m, 5 H), 7.87 (dd, J = 3.61, 5.23 Hz, 2 H), 7.40–7.46 (m, 6
H), 7.37 (s, 1 H), 7.27 (m, 2 H), 7.02 (d, J = 15.23 Hz, 2 H), 6.43 (d,
J = 15.32 Hz, 2 H), 5.48 (d, J = 15.99 Hz, 2 H), 5.17 (d, J = 15.83 Hz,
2 H) ppm. 13C NMR (75 MHz, [D6]DMSO): δ = 168.11, 154.54, 152.65,
141.23, 134.23, 132.89, 132.76, 128.82, 124.97, 124.89, 124.40,
124.06, 112.86, 111.42, 101.22, 50.41, 49.89 ppm. 31P NMR (122 MHz,
[D6]DMSO): δ = –144.88 ppm. ESI-MS found (calculated): [M –
[20]
[21]
[22]
[23]
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2PF6]2+ m/z 289.9 (289.9); [M
–
PF6]+ m/z 723.2 (723.1).
C34H28N6Ni(PF6)2·CH3CN: calcd. C 47.50, H 3.43, N 10.77; found C
47.78, H 3.43, N 10.80.
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1
2b: H NMR (300 MHz, [D6]DMSO): δ = 8.25 (d, J = 15.99 Hz, 2 H),
8.12 (t, J = 7.41, 7.41 Hz, 2 H), 7.89 (d, J = 7.57 Hz, 2 H), 7.71 (s, 2
H), 7.54 (m, 2 H), 7.40–7.46 (m, 4 H), 6.99 (s, 2 H), 6.77 (d, J =
14.93 Hz, 2 H), 5.80 (d, J = 15.01 Hz, 2 H), 5.04 (d, J = 15.39 Hz, 2
H), 4.61 (d, J = 15.26 Hz, 2 H) ppm. 13C NMR (75 MHz, [D6]DMSO):
δ = 155.45, 154.62, 152.33, 141.01, 135.47, 131.44, 129.10, 125.17,
124.99, 124.88, 122.93, 53.28, 51.34 ppm. 31P NMR (122 MHz,
[D6]DMSO): δ = –144.87 ppm. ESI-MS found (calculated): [M –
2PF6]2+ m/z 239.1 (239.6). C26H24N6Ni(PF6)2: calcd. C 40.60, H 3.15,
N 10.93; found C 40.64, H 3.18, N 10.92.
[27]
[28]
[29]
[30]
[31]
[32]
Acknowledgments
S. L. gratefully acknowledges a grant from the Chinese Scholar-
ship Council (grant number 201306410011). The authors are
grateful to Mr. John van Dijk for mass spectrometry.
[33]
[34]
[35]
[36]
[37]
[38]
Keywords: N-Heterocyclic carbenes · Carbenes · Nickel ·
Cyclic voltammetry · Electrocatalysis · Proton reduction
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Received: July 25, 2016
Published Online: August 29, 2016
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www.eurjic.org
4700
© 2016 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim