Cobalt, nickel, and iron complexes of 8-hydroxyquinoline-di(2-picolyl)amine for light-driven hydrogen evolution

Article abstract of DOI:10.1039/c7dt02666h

Novel cobalt, nickel, and iron complexes based on the pentadentate 8-hydroxyquinoline-di(2-picolyl)amine ligand were synthesized and thoroughly characterized. X-ray structures of both the cobalt and iron complexes were also obtained, showing the tendency

Full text of DOI:10.1039/c7dt02666h

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Cobalt, nickel, and iron complexes of 8-hydroxyquinoline-di(2-
picolyl)amine for light-driven hydrogen evolution
Nadia Alessandra Carmo dos Santos,a Mirco Natali,*b Elena Badetti,a Klaus Wurst,c Giulia Licinia
and Cristiano Zonta*a
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
Novel cobalt, nickel, and iron complexes based on the pentadentate 8-hydroxyquinoline-di(2-picolyl)amine ligand have
been synthesized and thoroughly characterized. X-ray structures of both the cobalt and iron complexes have been also
obtained showing the tendency to adopt a pseudo-octahedral geometry by coordination of an additional sixth ligand. The
series of metal complexes have been then studied as potential hydrogen evolving catalysts (HECs) under both
electrochemical and light-driven conditions. In particular, two different photochemical systems have been tested involving
either the Ru(bpy)32+/ascorbic acid or the Ir(ppy)2(bpy)+/TEA sensitizer/sacrificial donor couples. The electrochemical
results show that these metal complexes may behave as competent HECs. However, under photochemical conditions, only
the cobalt compound display substantial hydrogen evolving activity in both ruthenium- and iridium-based systems. The
nickel and iron complexes, on the other hand, exhibit appreciable photocatalytic activity only in the iridium-based
photochemical system, while showing negligible hydrogen evolution ability when employed in the ruthenium-based one.
www.rsc.org/
aqueous solutions, and enhanced stability under turnover
conditions. Due to our recent interest in the molecular
recognition and catalytic properties of these systems9l,10,11 we
Introduction
One major issue that human society has to address in the
current century is the identification of renewable energy
sources that are abundant, widespread, and cheap.1 Solar-
driven water splitting, with production of hydrogen gas as a
clean fuel, represents a viable solution, although posing as a
major challenge.2 Huge efforts have been indeed dedicated in
the last years toward the preparation and characterization of
molecular hydrogen evolving catalysts (HECs),3 with particular
emphasis on their implementation into light-driven
photoreaction schemes.4 Special attention in this research field
has been mainly paid toward the investigation of HECs based
on first-row transition metals,5,6,7,8 being cheaper, more
abundant, and less toxic with respect to noble-metals, and
thus compatible with a sustainable energy approach.3c Among
the HECs studied, metal complexes based on polypyridine
ligands have received substantial consideration,9 since they
combine several key features such as synthetic ease and
flexibility, efficient catalytic activity particularly in purely
decided to move our attention toward the synthesis and
characterization of four metal complexes 3a-d obtained by
coordination of the new pentadentate ligand
2, the 8-
hydroxyquinoline-di(2-picolyl)amine, with first-row transition
metals such as cobalt, nickel, iron, and zinc (Scheme 1).
Scheme 1. General synthetic procedure for the preparation of ligand 2 and metal
complexes 3a-d.
Once assessed their potential ability to act as competent HECs,
the new complexes have been tested under photoactivated
conditions within two different photochemical systems,
namely (i) an aqueous solution (1 M acetate buffer, pH 5) with
a.Department of Chemical Sciences, University of Padova, Via F. Marzolo 1, 35131
Padova, Italy. E-mail: cristiano.zonta@unipd.it
b.Department of Chemical and Pharmaceutical Sciences, University of Ferrara, and
Centro Interuniversitario per la Conversione Chimica dell’Energia Solare
(SolarChem), sez. di Ferrara, via L. Borsari 46, 44121 Ferrara, Italy. E-mail:
mirco.natali@unife.it
Ru(bpy)3Cl2·6H2O (where bpy
= 2,2’-bipyridine) as the
sensitizer and ascorbic acid as the sacrificial electron donor
and (ii) a 50/50 acetonitrile/water mixture (pH 10) using
Ir(ppy)2(bpy)PF6 (where ppy = 2-phenylpyridine and bpy = 2,2’-
bipyridine) as the chromophore and triethylamine (TEA) as the
sacrificial reagent. Positive and negative aspects of the new
complexes studied in relation to state-of-the-art polypyridine
c. Institute of General, Inorganic and Theoretical Chemistry, University of Innsbruck,
A-6020 Innsbruck, Austria.
Electronic Supplementary Information (ESI) available: detailed synthetic procedure
for the preparation of Ir(ppy)2(bpy)PF6, characterization of ligand 2 and complexes
3a-d relevant crystallographic data of complexes 3a,c electrochemistry of
,
complexes 3a-d, comparison of absorption spectra before/after photocatalysis.
,
See DOI: 10.1039/x0xx00000x
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metal complexes used in photocatalytic hydrogen evolving this latter observation may be potentially relevant in order to
reaction (HER) schemes will be discussed.
enable those processes (namely ligand detachment and proton
coordination) which are typically expected in the catalytic HER
mechanism by a six-coordinated metal complex.5,9
Results and discussion
Synthesis and Coordination Chemistry
Ligand
2 has been prepared in two steps by oxidation of
commercially available 2-methyl-8-quinolinol with selenium
dioxide in dioxane for 21 hour at 95°C to give 8-
hydroxyquinoline-2-carboxaldehyde,12 followed by reductive
amination with di(2-picolyl)amine (Scheme 1, top). The
compound is a light yellow oil and can be obtained in good
yield. The general procedure for the synthesis of the metal
complexes requires the mixing of an equimolar amount of
ligand
2
to the corresponding dicationic perchlorate salt (Co2+,
Ni2+, Fe2+, or Zn2+) in acetonitrile. The solution was stirred at
room temperature (15 minutes for the nickel and zinc cases,
24 hours for both cobalt and iron) and the desired compounds
were obtained either as crystalline powders or by
recrystallization with diethyl ether (Scheme 1, bottom). While
in the case of the nickel and zinc metal centres the complexes
are stable in their original oxidation state (i.e., +2), in the case
of cobalt and iron the crystallisation process in air leads to the
formation of the cobalt(III) and iron(III) complexes (see below).
The obtained products have been characterized by ESI-MS and
Elemental Analysis (see ESI). In the case of complexes 3a and
3c it was also possible to obtain crystals suitable for X-ray
diffraction. The molecular structure of both complexes are
displayed in Figure 1, while relevant crystallographic data are
reported in Table S1. In the crystal structure of complex 3a
(Figure 1, top; relevant bond lengths and angles are gathered
in Table S2 and S3, respectively), the central cobalt ion is
observed to be six-coordinated with a distorted octahedral
geometry wherein the pentadentate 8-hydroxyquinoline-di(2-
picolyl)amine ligand is shown to chelate the metal centre and
an acetonitrile solvent molecule is placed in the apical position
to complete the pseudo-octahedral coordination motif. The
distortion with respect to the ideal octahedral geometry is
expected to be largely imparted by the structural constrain of
the chelating ligand, as demonstrated by the bond angles
between the cobalt centre and the nitrogen atoms which
result systematically lower than 90° (see Table S3).
Importantly, the average Co-N bond length is 1.907(4) Å and
falls within the range typically observed for related cobalt(III)
polypyridine complexes,13 thus confirming the occurrence of a
Fig. 1. Crystal structure of complexes 3a (top) and 3c (bottom) with selected atom
Co(II)-to-Co(III) oxidation during the crystallization process labeling.
under air-equilibrated conditions.¥ Interestingly, the shortest
length is observed for the Co-N(2) bond (1.734 Å, see Table S2)
which might be related to both the better -donation and π-
Interestingly, in the case of the iron analogue 3c, the
crystallization process leads to the formation of a dinuclear
structure as determined by X-ray diffraction (Figure 1, bottom;
relevant bond lengths and angles are collected in Table S4 and
S5 of the ESI, respectively). Both iron centres are surrounded
by one 8-hydroxyquinoline-di(2-picolyl)amine ligand and a
hydroxo group trans to the quinolinic nitrogen, with the latter
acting as a bridging ligand and completing a six-coordinating
environment around the metal centres in a distorted pseudo-
σ
back-bonding abilities of the quinoline moiety with respect to
the amine and pyridine ligands.14 These enhanced electronic
effects exerted by the quinoline moiety on the cobalt centre
are also seen to remarkably influence the length of the bond
between the metal and the acetonitrile solvent molecule,
resulting in the elongation of the Co-N bond (2.012 Å, see
Table S2) with respect to the average distance. Importantly,
2 | J. Name., 2012, 00, 1-3
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octahedral geometry. The two hydroxyquinoline moieties are Finally, in the investigated potential range, the zinc complex 3d
oriented toward opposite directions and lie within the same displays an irreversible cathodic process with a peak potential
plane of the Fe-O(H)-Fe bridge. Similarly to the cobalt case, the of E = −1.97 V vs. SCE (Figure S4), which can be assigned to
observed distortion appears largely determined by the reduction of the coordinated 8-hydroxyquinoline-di(2-
structural constrain exerted by the chelating pentadentate picolyl)amine ligand, thus confirming the attributions of the
ligand. The two iron complexes within the dimeric species are metal-based redox processes previously made with complexes
similar in terms of coordination motif and geometry, although 3a-c.§
they are not completely identical since slightly different bond
Table 1. Electrochemical data of complexes 3a-d.a
distances and angles are measured (Table S4 and S5), likely
attributable to different intramolecular interactions in the
solid state.15 More importantly, the average Fe-N bond length
(2.147 Å, see Table S4) and the mean Fe-O distance (1.944 Å)
are comparable to those found for related iron(III) complexes
featuring a similar ligand environment15,16 and thus confirm
the presence of a 3+ metal centre. Also, the Fe(1)···Fe(2)
distance (3.710 Å, see Table S4) and the Fe(1)-O(3)-Fe(2) bond
angle (142.07°, see Table S5) fall within the range typically
Complex
E [M(III)/M(II)], V
E [M(II)/M(I)], V
E [L/L−], V b
3a
3b
3c
3d
−0.17 c,d
−1.62
−1.44
−1.79 c,d
-
-
-
−0.01
-
-
-
−1.97 c,d
a
Obtained by cyclic voltammetry on 1 mM 3a-d solutions in acetonitrile (0.1 M
TBAPF6) at r.t., scan rate v = 100 mV/s, potentials given vs. SCE reference
b
c
d
electrode; L is the coordinated ligand 2; irreversible wave; cathodic peak
potential given.
observed for unsupported
ꢀ-OH-bridged Fe(III) dimers
featuring similar coordination motifs.15 While showing up as a
dinuclear species in the solid state, complex 3c appears as a
monomeric compound in fluid solution, as demonstrated by
ESI-MS analysis. In fact, this is not unexpected in view of the
potential liability of the hydroxo bridge in a strongly
coordinating environment.
Cyclic voltammetry experiments on the three redox active
metal complexes 3a-c were then performed upon addition of
several aliquots of acetic acid, in order to preliminarily assess
their potential ability to act as competent hydrogen evolving
catalysts (HECs) under light-activated conditions.
Addition of acetic acid to an acetonitrile solution containing
complex 3a (Figure 2a) triggers the appearance of intense
waves with an onset at ca E = −1.3 V vs. SCE, which increase in
intensity with increasing acid concentration and can be
ascribed to catalytic hydrogen evolution promoted by the
cobalt centre.18,19 The observed behaviour (onset potential at
E > E[Co(II)/Co(I)], constant onset potential with increasing
acid concentration) is similar to those observed with related
cobalt(II) polypyridine complexes.9b,g,l Accordingly, a similar
mechanism can be envisioned for proton reduction to
dihydrogen involving Co(II)/Co(I) reduction and protonation, to
yield a Co(III)-H intermediate, with the latter undergoing a
subsequent reduction to a Co(II)-H species prior to hydrogen
evolution.18,20
Electrochemical Studies
Electrochemical measurements were performed in acetonitrile
solutions (0.1 M TBAPF6) in order to obtain information on the
redox properties of the 3a-c complexes. Complex 3d was also
studied and used as
a reference compound for the
straightforward attribution of the metal-based redox events.
All the potentials are gathered in Table 1. Upon cathodic scan,
complex 3a (Figure S1) shows an irreversible process with a
cathodic peak at E = −0.17 V vs. SCE, which is attributable to a
Co(III)/Co(II) reduction. The irreversibility is clearly observable
from the absence of the anodic peak in the return scan (at
least in the potential window examined) and can be attributed
either to the expected structural rearrangement occurring
upon reduction from low-spin Co(III) to high-spin Co(II), as
already observed for related cobalt complexes,9b or to the
detachment of a ligand from the cobalt centre, most likely
involving the coordinated solvent molecule.17,18 At more
negative potential, a reversible redox process is also observed
with an E1/2 = −1.62 V vs. SCE, which is attributable to a
Co(II)/Co(I) reduction step. In the potential window
investigated, complex 3b (Figure S2) shows only a quasi-
reversible redox process at an E1/2 = −1.44 V vs. SCE, that is
ascribable to a Ni(II)/Ni(I) reduction. A poorly intense anodic
peak is also observed at ca E = −0.8 V vs. SCE upon subsequent
scans, likely associated to some adsorption phenomena
occurring at highly cathodic potentials.
In the case of complex 3b (Figure 2b), upon addition of
increasing aliquots of acetic acid the Ni(II)/Ni(I) reduction
process shifts progressively toward less negative potentials
and becomes also irreversible, while maintaining comparable
cathodic peak current values. Current enhancement is then
observed with an onset at ca E = −1.5 V vs. SCE featuring acid-
dependent intensity, thus attributable to catalytic proton
reduction to dihydrogen. The positive shift of the Ni(II)/Ni(I)
process is consistent with the occurrence of a proton-coupled
electron-transfer step involving formation of a Ni(III)-H
intermediate.21,22 However, the observation that the catalytic
wave starts at more negative potentials than the Ni(II)/Ni(III)-H
PCET process is consistent with the requirement of an
additional Ni(III)-H/Ni(II)-H reduction step which, contrarily to
the related process in the cobalt case (see above), happens to
be more difficult (i.e., thermodynamically more demanding)
than the first electron transfer.23 Subsequent protonation or
coupling between two M(II)-hydride species, as in the case of
complex 3a, are then expected to promote hydrogen release.24
Complex 3c exhibits a redox process with an E1/2 = −0.01 V vs.
SCE (Figure S3) that is compatible with a reversible Fe(III)/Fe(II)
interconversion, while at more negative potentials a not-
perfectly reversible redox process is observed with a peak
potential of E = −1.79 V vs. SCE, possibly attributable to an
additional metal-based Fe(II)/Fe(I) reduction.
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observed current enhancement in the CV experiments to
electrocatalytic hydrogen generation. However, low faradaic
efficiencies (FE) were found in all cases (~30 % for 3a, ~20 %
for 3b, and ~15 % for 3c after one hour electrolysis), possibly
attributable to concomitant, competitive catalyst degradation
and deactivation.26 In summary, the electrochemical studies
show that among the 3a-c series, all complexes may behave as
competent HECs, although the potentials required to power
hydrogen evolution appear substantially negative in all cases.
As a matter of fact, a comparison of the onset potentials of the
catalytic wave with respect to those of other polypyridine
metal complexes recently reported8e,f,9b,h shows that all the
complexes 3a-c always display more negative onset potentials
for the HER than typically observed. For instance, in the case of
3a, a difference by ca 200 mV is seen under comparable
experimental conditions (acetonitrile solution, acetic acid as
the proton source) with respect to a related pentapyridine
cobalt complex.9b The reason for this substantial shift can be
very likely ascribed to the electron donating effect of the
hydroxyquinoline fragment which enhances the electronic
density on the metal centre thus moving the metal-based
reduction events toward more negative potentials.
0.10
0.08
0.06
0.04
0.02
0.00
no AcH
4 mM
6 mM
10 mM
14 mM
(a)
ꢀ1.0
ꢀ1.0
ꢀ1.0
ꢀ1.2
ꢀ1.4
ꢀ1.6
ꢀ1.6
ꢀ1.6
ꢀ1.8
ꢀ1.8
ꢀ1.8
E, V vs. SCE
no AcH
0.08
0.06
0.04
0.02
0.00
2 mM
4 mM
6 mM
10 mM
(b)
Photocatalytic Hydrogen Evolution
ꢀ1.2
ꢀ1.4
The potential ability of complexes 3a-c to act as HECs was then
assessed in light-driven experiments which are of fundamental
importance toward the identification of suitable
sensitizer/catalyst couples to be exploited in artificial
photosynthetic schemes. These experiments are typically
accomplished in aqueous solutions or organic/aqueous
mixtures combining three molecular components, namely a
HEC, a photosensitizer (P), and a sacrificial electron donor
(D).2e,4c,5c The inactive zinc complex 3d was tested as well, in
order to provide a suitable blank experiment for the sake of
comparison. Complexes 3a-d were tested under two different
photoreaction conditions: (i) in a purely aqueous solution (1 M
E, V vs. SCE
no AcH
4 mM
8 mM
12 mM
16 mM
0.06
0.04
0.02
0.00
(c)
2+
acetate buffer, pH 5) in combination with Ru(bpy)3 as the
ꢀ1.2
ꢀ1.4
sensitizer (P) and ascorbic acid as the sacrificial electron donor
(D) and (ii) in a 50/50 acetonitrile/water mixture (pH 10) using
Ir(ppy)2(bpy)+ as the chromophore (P) and triethylamine as the
sacrificial agent (D). According to literature precedents on both
the ruthenium-5f,9,27 and iridium-based28,29 systems,
photoinduced hydrogen evolution by a suitable HEC (eq 6) is
expected to occur after the following (photo)chemical steps
(protonation steps are herein neglected for simplicity reasons):
light absorption (eq 1), reductive quenching of the excited
chromophore by the sacrificial electron donor (eq 2) with the
latter undergoing irreversible transformation upon oxidation
(eq 3), two consecutive one-electron transfers to the HEC from
the photogenerated reduced sensitizer (eqs 4,5).‡
E, V vs. SCE
Fig. 2. Cyclic voltammetry of (a) 1 mM 3a, (b) 1 mM 3b, and (c) 1 mM 3c in acetonitrile
solution (0.1 TBAPF6) upon addition of several aliquots of acetic acid (AcH).
M
Experimental conditions: GC as WE, Pt as CE, SCE as reference, scan rate v = 100 mV/s.
The electrochemical profile of complex 3c changes as well
upon addition of several aliquots of acetic acid (Figure 2c) with
a current enhancement observed at E < −1.5 V vs. SCE which is
dependent on acid concentration and thus compatible with
catalytic proton reduction by the iron complex. The onset is
more positive than the Fe(II)/Fe(I) reduction process,
suggesting that proton reduction proceeds upon Fe(II)-to-Fe(I)
reduction and protonation,8 most likely followed, similarly to
3a,b, by an additional reduction step prior to hydrogen
elimination.18,20 §§ For all complexes 3a-c, hydrogen formation
was detected in controlled potential electrolysis experiments
(1 mM 3a-c, 0.1 M TBAPF6 acetonitrile solution, 20 mM acetic
acid, constant potential of −1.7 V and −1.8 V vs. SCE for 3a,c
and 3b, respectively) thus confirming the attribution of the
P + h
*P + D
D+
ν
→
→
products
*P
P− + D+
(1)
(2)
(3)
(4)
(5)
(6)
→
P− + HEC
P− + HEC−
HEC2− + 2H+
→
→
P + HEC−
P + HEC2−
→
HEC2− + H2
4 | J. Name., 2012, 00, 1-3
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studied shows that (i) the cobalt complex 3a is always more
Apart from effects on photocatalysis arising from the different active than the nickel and iron complexes 3b,c, (ii) with the
environments used (presence of an organic co-solvent, same metal complex the iridium-based photochemical system
solution pH, etc.)30 and the different absorption properties of is always more efficient than the ruthenium-based one. This
the two sensitizers throughout the visible spectrum (where observation can be explained, at least qualitatively, on simple
irradiation
is
performed,
see
Figure
S7),
the thermodynamic basis considering that the cobalt complex 3a is
Ru(bpy)32+/ascorbic acid and the Ir(ppy)2(bpy)+/TEA systems capable of promoting hydrogen evolution at a lower potential
differ substantially in that the reducing agents that are than the parent complexes 3b,c (see above and Figure 2) and
photogenerated via reductive quenching by the donor (eq 2) thus catalyst activation from the photogenerated reducing
have different redox potentials, resulting in different agent (eqs 4,5) is expected to be more favourable in the
thermodynamics for the HEC activation steps (eqs 4,5). Indeed, former case.ǁ
the reduced ruthenium dye has a reduction potential of E =
−1.28 vs. SCE in aqueous solution,31 while the reduced iridium
chromophore has a reduction potential of E = −1.43 vs. SCE,29
10
8
20
16
12
8
with the latter thus resulting a more powerful reducing agent
by 150 mV than the former.
(a)
Photocatalytic hydrogen evolution experiments were thus
performed upon visible light irradiation of solutions containing
2+
6
(i) 3a-d, Ru(bpy)3 , and ascorbic acid and (ii) 3a-d,
Ir(ppy)2(bpy)+, and TEA. Experimental conditions (type of
buffer and concentration, solvent mixture and composition,
solution pH, etc.) were taken according to previous reports on
photochemical hydrogen evolution systems based on such
chromophore/donor couples.9,29 In particular, in the
ruthenium-based system the concentrations of the reactants
in the photolysis mixture were as follows: 0.1 mM 3a-d, 0.5
mM Ru(bpy)3Cl2·6H2O, 0.1 M ascorbic acid in 1 M acetate
buffer at pH 5. In the iridium-based three-component system
the experimental conditions were as described herein: 0.1 mM
4
2
4
0
0
0
20
40
60
80
100 120
Time, min
20
18
16
14
12
10
8
20
18
16
14
12
10
8
TON
TOF
(b)
3a-d
,
0.5 mM Ir(ppy)2(bpy)PF6, 45/45/10 acetonitrile/
water/TEA mixture at pH 10. All the kinetic traces (average of
two independent experiments) are reported in Figure 3a, while
relevant photocatalytic data (TONs and maximum TOFs) are
collected in Figure 3b for comparison (in the latter instance the
parameters are reported with respect to the amount of
complex 3a-d in solution regardless of the metal present).
The cobalt complex 3a is active as a HEC when employed in
both the ruthenium- and iridium-based photochemical
systems achieving turnover number (TON) of 10 and 19 after 2
hour irradiation with maximum turnover frequency (TOF) of
7.2 and 16.2 h-1, respectively (Figure 3b). The nickel and iron
complexes 3b,c, on the other hand, behave similarly, exhibiting
a lower hydrogen evolution activity with only a TON = 4 and 5,
respectively, after 2 hour irradiation with a maximum TOF =
3.0 and 2.4 h-1, respectively, when studied in the iridium-based
photochemical system (Figure 3b), while they are practically
inactive (TON < 1, low TOF) when employed in the ruthenium-
based one. As expected, the zinc complexes 3d displays an
almost negligible hydrogen production yield (low maximum
TOF, consistent with negligible quantum efficiency for the
reaction) when employed in both the ruthenium- and iridium-
based photochemical systems, with the trace amounts of
hydrogen detected likely arising from slow, direct reduction of
protons by the photogenerated reduced sensitizer species.
Interestingly, when the activity of the metal complexes 3a-c is
compared, the trend observed in terms of both TON and
maximum TOF with respect to the photocatalytic system
6
6
4
4
2
2
0
0
3a/Ru 3a/Ir 3b/Ru 3b/Ir 3c/Ru 3c/Ir 3d/Ru 3d/Ir
Fig. 3. (a) Photocatalytic hydrogen evolution kinetics (average of two independent
experiments) using complexes 3a (blue dots), 3b (green dots), 3c (red dots), and 3d
(grey dots) in the ruthenium- (full dots) and iridium-based (empty dots) photochemical
systems and (b) relevant photocatalytic data. Experimental conditions: Ru system: 0.1
mM 3a-d, 0.5 mM Ru(bpy)3Cl2, 0.1 M ascorbic acid in 1 M acetate buffer at pH 5; Ir
system: 0.1 mM 3a-d, 0.5 mM Ir(ppy)2(bpy)PF6, 45/45/10 acetonitrile/water/TEA
mixture at pH 10.
Also, the use of a more powerful reductant (as it is in the case
of the iridium sensitizer) turns out to be a key factor to
improve the photocatalytic efficiency with the same metal
complex. This latter point is exemplified by the abatement of
the light-driven hydrogen evolution activity mediated by both
the nickel and iron complexes 3b,c when the iridium-based
photochemical system is replaced by the ruthenium-based
one, where photocatalytic parameters close to those observed
with the inactive zinc complexes 3d are recorded (Figure 3). It
is worth pointing out, however, that the photocatalytic
performances of the investigated metal complexes 3a-c are
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rather modest when compared to other polypyridine
Journal Name
7.34 (dd, 1H, HAr). 13C NMR (62 MHz, CDCl3):
δ (ppm) = 192.94,
complexes reported in the literature, in particular as far as 137.72, 131.17, 118.26, 111.43. ESI-MS m/zcalc: 173.1 Found
[MH]+ m/z: 174.1.
cobalt-based ones are concerned, which typically display TONs
even up to ~103 and TOFs in the “min-1” regime.9 In order to
General procedure for synthesis of 2-((bis(pyridin-2-
find possible explanations we compared the absorption
spectra before and after 2 h photolysis (Figure S8, S9, and S10
ylmethyl)amino)methyl)quinolin-8-ol (2):
We adopted a reported synthetic procedure with minor
modifications 32
. 2-dipicolylamine (0.945 mL, 5.3 mmol) and 8-
for 3a, 3b, and 3c, respectively) and observed that with all 3a-c
complexes a major abatement of the absorption patterns of
both the ruthenium and iridium chromophores takes place (up
to ca 60% loss of the initial absorbance in the case of the
ruthenium sensitizer) comparable with what observed in the
blank experiments using the zinc complex 3d (Figure S11). This
thus indicates that, regardless of the metal complex present in
solution, both sensitizers undergo a substantial degradation
under light irradiation, which most likely proceeds via
disproportionation of and/or proton attack to the reduced
chromophore species.32 This clearly means that under the
experimental conditions adopted side-reactions potentially
involving the photogenerated reduced sensitizers are very
hydroxyquinoline-2-carbaldehyde (1g, 5.8 mmol, 1.1 eq) were
stirred in anhydrous dichloromethane (30 mL) under N2
atmosphere. After 3 h, sodium triacetoxy borohydride (1.446
g, 6.8 mmol, 1.3 eq) was added and the mixture was stirred
under N2 atmosphere overnight at room temperature. The
mixture was washed three times with saturated NaHCO3
solution and the organic phase was dried over Na2SO4. The
solvent was removed under reduced pressure and gave the
1
pure product as a light yellow oil. Yield: 98%. H NMR (200
MHz, CD3CN):
7.70 (m, 5H, HAr), 7.43 (m, 2H, HAr), 7.18 (m, 3H, HAr), 4.03 (s,
(ppm) =
δ (ppm) = 8.54 (dq, 2H, HAr), 8.24 (d, 1H, HAr),
likely to compete favourably with the forward electron 2H, CH2), 3.93 (s, 4H, CH2). 13C NMR (62 MHz, CDCl3):
δ
transfer pathways involving the HEC (eqs 4,5) thereby leading
to enhanced dye degradation and fast, concurrent
photocatalysis inactivation. The reasons for the observed
behaviour can be mostly attributed to the considerably
negative potentials required to trigger the HER by metal
complexes 3a-c and thus the poor driving force for the forward
electron transfer processes (eqs 4,5) which may have a
profound impact onto the related kinetics.
160.71, 158.62, 153.88, 150.63, 137.92, 128.90, 124.74,
123.60, 118.92, 111.56, 61.72, 61.17. ESI-MS m/zcalc: 356.4
Found [MH]+ m/z: 357.4.
General procedure for the synthesis of metal complexes 3a-d
2-((bis(pyridin-2-ylmethyl)amino)methyl)quinolin-8-ol (50 mg,
0.14 mmol) was dissolved in the minimum amount of
anhydrous acetonitrile and the desired metal salt (1 eq) was
added. The solution was let stirring at room temperature for
15 min and then, diethyl ether was slowly added to the
mixture to obtain the desired products by crystallization.
Caution! Perchlorate salts of metal complexes with organic
ligands are potentially explosive. They should be handled in
small quantities and with caution.
Experimental Section
Materials
Milli-Q Ultrapure water and spectroscopic grade acetonitrile
(Sigma-Aldrich) were used for the photocatalysis experiments.
Anhydrous acetonitrile (Alfa Aesar) was used for the
electrochemical measurements. Ru(bpy)3Cl2·6H2O was
purchased from Sigma-Aldrich and used as received. All other
reagents were purchased from commercially available
suppliers and used as received.
3a Dark brown crystal [74%]. ESI-MS m/zcalc: 513.0 Found
[MClO4]+ m/z: 512.9
3b Ochre crystal [83%]. ESI-MS m/zcalc: 413.1 Found [M]+ m/z:
413.1
3c Dark green crystal [71%]. ESI-MS m/z calc : 456.1 Found
[MHCOO] + m/z: 456.1
3d Dark yellow solid [87%]. 1H NMR (200 MHz, CD3CN):
δ
(ppm) = 8.81 (d, 2H, HAr), 8.52 (d, 1H, HAr), 8.12 (dt, 2H, HAr),
7.63 (m, 9H, HAr), 4.47 (d, 4H, CH2), 4.39 (s, 2H, CH2).Esi-MS
m/zcalc: 419.1 Found [M]+ m/z: 419.1
Synthetic procedures
General procedure for synthesis of 8-hydroxyquinoline-2-
carbaldehyde (1):
We adopted a reported synthetic procedure with minor
modifications.31 Commercially available 8-hydroxy-2-methyl-
quinoline (25.1 mmol) was added to a suspension of selenium
oxide (8.364 g, 75.4 mmol, 3 eq) in dioxane (100 ml) under N2
atmosphere. The mixture was heated at 95 °C during 21 h.
After cooling at room temperature, the mixture was filtered on
celite. The filtrate was concentrated under vacuum.
Purification of the resulting crude product on silica column
chromatography (dichloromethane) afforded the desired
product as a yellow solid. Yield: 65%. 1H NMR (200 MHz,
Synthesis of Ir(ppy)2(bpy)PF6. The cyclometalated iridium(III)
complex Ir(ppy)2(bpy)PF6 was synthesized according to slight
variations on reported literature procedures.33 A two-step
protocol was adopted, involving (i) reaction of IrCl3·xH2O with
a slight excess of ppy (where ppy = 2-phenylpyridine) to obtain
the [Ir(ppy)2Cl]2 dimer and (ii) subsequent reaction of the latter
with the bpy ligand (where bpy = 2,2’-bipyridine) followed by
precipitation with KPF6 (see ESI for additional details).
Crystallography. The supplementary crystallographic data
were deposited as CCDC 1556115-1556116. Copies of the data
CDCl3): δ (ppm) = 10.26 (s, 1H, CHO), 8.36 (d, 1H, HAr), 8.21 (s,
1H, OH), 8.09 (d, 1H, HAr), 7.67 (t, 1H, HAr), 7.48 (dd, 1H, HAr),
6 | J. Name., 2012, 00, 1-3
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ARTICLE
can be obtained, free of charge, at the Cambridge New first-row transition metal complexes 3a-d based on the 8-
Crystallographic Data Centre. hydroxyquinoline-di(2-picolyl)amine ligand have been
prepared and those based on redox active metal centres (3a-c
)
Apparatus and procedures. UV-Vis absorption spectra were studied as potential hydrogen evolving catalysts under both
recorded on a Jasco V-570 UV/Vis/NIR spectrophotometer. electrochemical and light-driven conditions. In particular, two
Cyclic Voltammetry (CV) measurements were carried out with different photochemical systems have been tested involving
a PC-interfaced Eco Chemie Autolab/Pgstat 30 Potentiostat. A either the Ru(bpy)32+/ascorbic acid or the Ir(ppy)2(bpy)+/TEA
conventional three-electrode cell assembly was adopted: a sensitizer/sacrificial donor couples. The electrochemical results
saturated calomel electrode (SCE, Amel) and a platinum show that all redox-active metal complexes 3a-c may actually
electrode were used as reference and counter electrodes, behave as competent HECs. Under photochemical conditions,
respectively; a glassy carbon (GC) electrode (7 mm2 surface only the cobalt compound display substantial hydrogen
area) was used as the working electrode. All experiments were evolving activity in both the ruthenium- and iridium-based
performed in nitrogen-purged acetonitrile solutions. The systems achieving TONs up to 10 and 19, respectively, with
TBAPF6 supporting electrolyte was dried overnight before use. maximum TOFs of 7.2 and 16.2 h-1, respectively. The nickel and
The GC electrode was accurately polished with alumina slurry iron complexes, on the other hand, exhibit appreciable
after every scan. Potential controlled electrolysis experiments photocatalytic activity only in the iridium-based photochemical
were performed in a gas-tight custom-made electrochemical system, while showing negligible hydrogen evolution in the
cell using a large surface area carbon foil as the working ruthenium one, similarly to what observed with the inactive
electrode, a platinum grid as the counter electrode (separated zinc complex 3d
.
Complexes 3a-c
,
however, display
from the test solution by a frit) and a Ag/AgCl (3 M NaCl) as the considerably lower activity toward proton reduction with
reference electrode (in the text, potentials are converted to respect to state-of-the-art polypyridine metal complexes
SCE for uniformity with CV experiments). The head-space of reported in the literature. Indeed, although the chelating
the cell was connected to the gas-chromatography (GC) properties of the new 8-hydroxyquinoline-di(2-picolyl)amine
apparatus described below for the determination and ligand are favourable toward the stabilization of the metal
quantification of hydrogen. The photochemical hydrogen complexes in their coordination geometry also leaving a labile
evolution experiments were carried out upon continuous (and thus potentially available) coordination site to enable
visible light irradiation with a 175 W Xe arc-lamp (CERMAX proton binding, the enhanced electron donating effect of the
PE175BFA) of a reactor containing the solution (a 10 mm hydroxyquinoline moiety apparently increases the electronic
pathlength pyrex glass cuvette with head space obtained from density on the metal centre thus shifting the metal-based
a round-bottom flask). A cut-off filter at
λ < 400 nm and a hot reduction processes toward highly negative potentials. In this
mirror (IR filtering) have been used in order to provide the respect the formation of the nucleophilic M(I) species which is
useful wavelength range (400-800 nm). The gas phase of the required to favour protonation as well as the subsequent
reaction vessel was analyzed on an Agilent Technologies 490 reduction of the M(III)-H intermediate turn out to be
microGC equipped with a 5 Å molecular sieve column (10 m), a thermodynamically more demanding than commonly observed
thermal conductivity detector, and using Ar as carrier gas. with other polypyridine complexes. Therefore catalyst
Additional details of the setup and procedures used for the activation by the photogenerated reducing agent, either in the
hydrogen evolution experiments can be found in previous ruthenium- and iridium-based photochemical systems, may
reports.9g,l In a typical photocatalytic experiment, samples of 5 actually compete unfavourably with side-reaction phenomena
mL were prepared in 20 mL scintillation vials in the following thereby leading to rapid abatement of the photocatalytic
manner depending on the sample studied: (i) by dilution in 1 M activity. This work, however, still outlines the relevance of
acetate buffer (pH 5) of Ru(bpy)3Cl2·6H2O (0.5 mL from a 5 mM carefully controlling all the stereoelectronic effects in catalyst
mother solution in 1 M acetate buffer), 3a-d (0.05 mL, from a design in order to achieve sustained and efficient hydrogen
0.01 M mother solution in acetonitrile), and ascorbic acid (88 production.
mg, added as solid); (ii) by adding Ir(ppy)2(bpy)PF6 (0.5 mL
from a 5 mM mother solution in acetonitrile), 3a-d (0.05 mL,
Acknowledgements
from a 0.01 M mother solution in acetonitrile), triethylamine
(0.5 mL), acetonitrile (1.7 mL), and water (2.25 mL) and
adjusting the pH to 10 upon addition of concentrated HCl.
Once prepared, the solution was put in the reactor, degassed
by bubbling Ar for 20 min, and thermostated at 17°C. The cell
was then irradiated and the solution continually stirred during
the photolysis. The gas phase of the reaction was analyzed
through microGC and the amount of hydrogen quantified.
Dr. Paulina Dreyse (Universidad Técnica Federico Santa María,
Valparaiso, Chile) is gratefully acknowledged for providing the
Ir(ppy)3(bpy)PF6 complex used. We thank Prof. Andrea Sartorel
(University of Padova) for help with the bulk electrolysis
experiments. Financial support from the University of Padova
(PRAT CPDA CPDA153122, Progetto Attrezzature Scientifiche
finalizzate alla Ricerca 2014) is gratefully acknowledged.
Conclusions
Notes and references
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 7
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Journal Name
¥ It should be also noted that the average Co-N bond length
observed herein is also considerably lower than that measured
in the case of related polypyridine cobalt(II) complexes used as
HECs.9
§ The failure to observe a ligand-based reduction in the case of
complexes 3a-c can be related to the fact that this redox event
likely occurs at more negative potentials than experimentally
investigated. On the basis of simple electrostatic considerations,
metal-based reduction processes are indeed expected to
cathodically shift the ligand-based redox event.
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7
§§ Blank experiments with 3d and no metal complex were also
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the bare GC electrode25 (see Figure S5 and S6, respectively).
These results confirm that the observed current enhancement in
the presence of acid is fully related to metal-based catalysis in
the case of complexes 3a,b, while direct contribution from the
bare GC electrode cannot be completely ruled out in the case of
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the HER mechanism. This is, however, out of the scope of the
present manuscript.
‡ An alternative photochemical mechanism, namely oxidative
quenching of the excited sensitizer by the HEC (and HEC−) and
subsequent hole shift from the oxidized dye to the sacrificial
donor, might be in principle operative. However, based on
simple thermodynamic considerations, the reducing power of
the excited states, particularly in the ruthenium case, are not
sufficient to favour direct HEC (and even more HEC−) reduction,
the latter processes being indeed occurring at very cathodic
potentials. Also, based on kinetic considerations, the typically
high donor concentration employed in the photolysis conditions
is such that the reductive quenching pathway is expected to be
kinetically dominating over the competitive, if any, oxidative
quenching.
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DOI: 10.1039/C7DT02666H
TOC
Novel first-row transition metal complexes based on the 8-hydroxyquinoline-di(2-picolyl)amine
ligand were prepared and tested as potential HECs in light-driven experiments.
26 Fe
27Co
2
8
14
2
8
15
2
2
Iron
Cobalt
55.847
58.933
28 Ni
30 Zn
2
8
16
2
2
8
18
2
Nickel
Zinc
58.693
65.409