Redox properties of cobalt(II) complexes with azole-pyridines

Article abstract of DOI:10.1016/j.ica.2013.07.057

We report nine new homoleptic octahedral cobalt(II) complexes [Co(N^N) ₃]²⁺ and [Co(N^N^N)₂]²⁺ with bidentate or tridentate azole-pyridine ligands (where azole is 1,2,4-triazole or a substituted pyrazole). The complexes are air- and moisture-stable solids of pale pink to pale brown color that exhibit quasi-reversible redox processes in polar organic solvents. By changing azole heterocycle and by adding methyl groups, we tune redox potentials of the complexes from 0.05 to 0.5 V for the Co(III/II) couple (its cyclic voltammogram depends on the electrode history) and from -1.08 to -1.57 V for the Co(II/I) couple to give a redox gap of 1.37-1.82 V. Replacing a pyrazole with electron-deficient 1,2,4-triazole strongly positively shifts redox potentials of the complexes; in contrast, adding electron-donor methyl groups shifts the potentials negatively.

Full text of DOI:10.1016/j.ica.2013.07.057

Accepted Manuscript
Redox properties of cobalt(II) complexes with azole-pyridines
Nail M. Shavaleev, Florian Kessler, Michael Grätzel, Mohammad K.
Nazeeruddin
PII:
S0020-1693(13)00437-4
http://dx.doi.org/10.1016/j.ica.2013.07.057
ICA 15600
DOI:
Reference:
To appear in:
Inorganica Chimica Acta
Received Date:
Revised Date:
Accepted Date:
24 June 2013
30 July 2013
31 July 2013
Please cite this article as: N.M. Shavaleev, F. Kessler, M. Grätzel, M.K. Nazeeruddin, Redox properties of cobalt(II)
complexes with azole-pyridines, Inorganica Chimica Acta (2013), doi: http://dx.doi.org/10.1016/j.ica.2013.07.057
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1
Redox properties of cobalt(II) complexes with azole-pyridines
a,
a,b
a
a,
Nail M. Shavaleev *, Florian Kessler , Michael Grätzel , Mohammad K. Nazeeruddin *
a
Laboratory of Photonics and Interfaces, École Polytechnique Fédérale de Lausanne, CH-
1
015 Lausanne, Switzerland.
b
Present address: Siemens AG, Corporate Technology, CT RTC MAT MPV, Günther-
Scharowsky Strasse 1, D-91058 Erlangen, Germany.
*Corresponding authors. Tel.: +41 21 693 6124; fax: +41 21 693 4111. E-mail addresses:
shava@mail.ru (N.M. Shavaleev), mdkhaja.nazeeruddin@epfl.ch (M.K. Nazeeruddin).
Keywords: Azole; Ligand; Cobalt; Complex; Electrochemistry; Voltammetry

2
Highlights
>
>
>
Five new bidentate and tridentate azole-pyridine ligands.
Nine new homoleptic octahedral cobalt(II) complexes with azole-pyridines.
How to control redox and optical properties of cobalt(II) complexes.
Graphical Abstract
Redox and optical properties of homoleptic octahedral cobalt(II) complexes with bidentate
and tridentate azole-pyridine ligands.
N N
X
N
N
N
Co N
N N N

3
Abstract
We report nine new homoleptic octahedral cobalt(II) complexes [Co(N^N)3] and
2
+
2
+
[
Co(N^N^N)2] with bidentate or tridentate azole-pyridine ligands (where azole is 1,2,4-
triazole or a substituted pyrazole). The complexes are air- and moisture-stable solids of pale
pink to pale brown color that exhibit quasi-reversible redox processes in polar organic
solvents. By changing azole heterocycle and by adding methyl groups, we tune redox
potentials of the complexes from 0.05 V to 0.5 V for the Co(III/II) couple (its cyclic
voltammogram depends on the electrode history) and from −1.08 V to −1.57 V for the
Co(II/I) couple to give a redox gap of 1.37–1.82 V. Replacing a pyrazole with electron-
deficient 1,2,4-triazole strongly positively shifts redox potentials of the complexes; in
contrast, adding electron-donor methyl groups shifts the potentials negatively.

4
1
. Introduction
Redox processes in cobalt polypyridyl complexes give access to Co(III/II/I) metal
oxidation states and to reduced coordinated ligand(s) [1–10]. Cobalt polypyridyl complexes
are used as redox catalysts for electro-reduction/oxidation of carbon dioxide, oxygen and
water [11–15]; as redox shuttles [16–20] and p-dopants [21] for dye-sensitized solar cells; and
as electrochemical energy storage materials for batteries [22]. These applications are likely to
be developed further because cobalt is an abundant element [11], but to do that, one has to
have control over redox properties of its complexes.
Easy-to-make azole ligands are used to replace/complement polypyridyls in
coordination chemistry [23–35], but few studies have investigated cobalt–azole complexes
[
36–40]. Here, we report synthesis and electrochemistry of nine new octahedral cobalt(II)
complexes with bidentate or tridentate azole-pyridine ligands (Scheme 1).
2
. Results and discussion
2
.1. Synthesis and characterization
The azole-pyridine ligands L1–L9 were prepared by non-catalyzed C–N coupling of
an azole with fluoro-, chloro- or bromo-pyridine in DMSO, in the presence of potassium tert-
butoxide as a base (Scheme 1) [31,41]. Ligands L2 and L6–L9 are new. L1 was reported by
us before [31]. L3–L5 were reported [32–35], but we have made their synthesis faster and
more user-friendly. With 1,2,4-triazole, only the major product was isolated, the N1/N2-
derivative L6 and L9.

5
Electron-donor methyl groups were added to L1–L9 to make the ligand more electron-
rich to increase its affinity to cobalt, and to improve solubility and change redox properties of
its cobalt complexes.
Nine new homoleptic octahedral cobalt(II) complexes 1–9 were prepared as
hexafluorophosphate salts by the reaction of CoCl ·6H O with an excess of ligand L1–L9 in
2
2
hot methanol and by the anion exchange with KPF (Scheme 1). 1–9 are air- and moisture-
6
stable pale pink to pale brown solids that are soluble in polar organic solvents. 1–9 were
+
characterized by C, H, N elemental analysis; their ESI TOF mass-spectra exhibit peaks of the
+
2+
cations {M – PF } and {M – 2PF } .
6
6
NMR spectra of 1–9 are solvent-independent in acetone-d and acetonitrile-d
6
3
1
(
Supplementary material). Because cobalt(II) is paramagnetic [20,42], H NMR signals of 1–9
1
are observed in a wide chemical-shift range. H NMR spectra of 1 and 2 are broad because of
fast fac-mer isomerisation of non-symmetric bidentate ligands L1 and L2 at labile Co(II) ion
1
[
43]. In contrast, H NMR spectra of complexes 3–9 with tridentate ligands exhibit sharp
1
9
singlets and confirm C symmetry of 3–9. F NMR spectra of 1–9 confirm the presence of
2
−
PF anions that are not bound to Co(II).
6
Outward-facing N4-atom in 1,2,4-triazole of ligands L6 and L9 does not interfere with
2+
the synthesis of mono-nuclear complexes [Co(N^N^N) ] , 6 and 9, that is, it does not give
2
rise to coordination polymers. Sterically-hindering 3,5-dimethyl groups in the pyrazole of
ligands L5 and L8 do not interfere with the synthesis of complexes 5 and 8.
2
.2. Absorption spectroscopy
Ligands L1–L9 give colorless solutions in organic solvents. For L1–L9, we observe a
long-wavelength absorption shoulder in dichloromethane that is absent in acetonitrile and that

6
may originate from an azole-to-pyridine charge-transfer transition (Tables 1 and S3 and Fig.
S7–S15).
Complexes 1–9 give colorless or pale yellow solutions in dichloromethane and in
acetonitrile. Electronic absorption spectra of 1–9 are solvent-independent (Tables 1 and S3
and Fig. 1 and S16–S24); therefore, we consider that the coordination environment around
cobalt(II) is the same in dichloromethane and in acetonitrile (recall that NMR spectra of 1–9
are solvent-independent too, Supplementary material). In the visible range, absorption spectra
3
of 1–9 exhibit a shoulder that goes up to 500 nm with molar absorption coefficients of ≤10
−
1
−1
M cm at λ > 400 nm (Table 1 and Fig. 1).
Electronic absorption spectra of the ligands and their cobalt(II) complexes exhibit a
small red-shift from acetonitrile to dichloromethane; a large red-shift and a gain in intensity
when a conjugated π-system increases from bidentate to tridentate ligands; but a blue-shift
when electron-donor methyl groups are added to the 4,4′-positions of the 2,2′-bipyridine
and/or when a pyrazole is replaced by electron-deficient 1,2,4-triazole to destabilize azole-to-
pyridine charge-transfer transition (Tables 1 and S3 and Fig. S7–S24).
2
.3. Electrochemistry
The redox potentials of cobalt(II) complexes 1–9 were measured with cyclic
voltammetry on gold and on glassy-carbon electrodes in acetonitrile (ACN) and in DMF and
are reported relative to ferrocene couple [44] (Tables 2, S1 and S2 and Fig. 2–4 and S1–S6).
The one-electron redox processes of 1–9 were assigned by comparison with the literature [1–
1
0]. In cyclic voltammograms (CV) of 1–9, we observe Co(III/II) oxidation and Co(II/I)
−
reduction, followed by reduction(s) of the coordinated ligand(s) L/L [1–10]. Redox processes
in 1–9 are quasi-reversible with peak-to-peak separation that is larger than that for the

7
ferrocene and that increases at faster scan rates, indicating a slow electron-transfer for 1–9 on
the electrode (Tables 2, S1 and S2). Redox potentials of 1–9 do not depend on the electrode
material and vary only by ≤50 mV within the scan-rate range of 0.05–5 V/s; however, Co(II/I)
−
and L/L couples exhibit higher reversibility (smaller peak-to-peak separation) on glassy-
carbon electrode, while Co(III/II)—on gold (Tables 2, S1 and S2 and Fig. 2, 3 and S1–S6).
Others have previously noted too that the reversibility of Co(III/II) couple depends on the
electrode material [10,17].
The redox processes of 1–9 become more reversible from DMF to ACN; from
bidentate to tridentate ligands (complexes with higher denticity ligands often have higher
electrochemical stability); from 2,6-bis-azole-pyridine to 6-azole-2,2′-bipyridines (azoles are
weaker ligands than pyridines are [23,25,38]); and from 1,2,4-triazole to pyrazole ligands
(
1,2,4-triazole may interact with the electrode through the N4-atom) (Tables 2, S1 and S2).
The redox potential of the Co(II/I) couple in 1 and 2 and that of the Co(III/II) couple in 4 and
becomes less positive by ≤100 mV from ACN to DMF, indicating that the solvent interacts
5
to some extent with the cobalt (Tables S1 and S2).
In the complexes with bidentate ligands (1 and 2), the reduction that follows Co(II/I)
couple can be a two-electron process [1,2], but because it is irreversible, we do not discuss it.
−
In the complexes with tridentate ligands (3–9), one-electron ligand-centered L/L reductions
that follow Co(II/I) couple [1–10] are quasi-reversible at fast scan rate (1 V/s), but are often
irreversible at slow scan rate (0.1 V/s), indicating that these electrochemical processes are
accompanied by chemical reactions, for example, octahedral Co(I) complexes [5,6] can eject
coordinated ligands [1,2,8].
In 1–9, cyclic voltammogram of the Co(III/II) couple depends on the electrode history,
that is, it is quasi-reversible and reproducible only when the potential stays sufficiently
positive to avoid Co(II/I) reduction. After cycling the potential to the negative range beyond

8
the Co(II/I) couple, on return scan, CV of the Co(III/II) couple becomes irreproducible and
broad and its current decreases (Fig. 4). We conclude that the Co(II/I) couple produces
‘reduced Co species’ that adsorb on the electrode and passivate it towards the Co(III/II)
couple (recall that all redox processes in 1–9 are quasi-reversible). The electrode can be
reactivated by multiple cycling of the potential in the positive range or by mechanical
−
cleaning. In contrast, cyclic voltammograms of the Co(II/I) and L/L couples in 1–9 do not
depend on the electrode history (Fig. 4).
Cyclic voltammogram of the Co(III/II) couple in 1–9 is often non-symmetric with a
broad Co(II)→Co(III) wave, especially on a glassy-carbon electrode, but a narrow
7
Co(III)→Co(II) wave (Fig. 2 and 3). In octahedral polypyridyl complexes, Co(II), a d ion, is
5
g
2
2g
6
1
usually high-spin, t e , the contribution of low-spin t e component depends on the
2
2g 2g
6
environment (we did not investigate magnetism of 1–9); in contrast, Co(III), a d ion, is low-
6
0
spin, t e
2g
[20,42]. A spin change required by the Co(III/II) couple may cause non-
2
g
symmetric CV of 1–9 [10,17].
In acetonitrile, on Pt/FTO electrode, non-substituted analog of 1 and 2, reference
ox
red
complex R1 (Chart 1), exhibits E1/2 = 0.32 V and E1/2 = −1.45 V [19], while non-
ox
substituted analog of 4, 5, 7 and 8, reference complex R2 (Chart 1), exhibits E1/2 = 0.22 V
red
and E1/2 = −1.20 V [19]. It is clear, from comparison of R1 and R2 with the new complexes,
that by adding electron-donor methyl groups to the ligand we make it stronger σ-donor and
weaker π-acceptor, and we negatively shift redox potentials of its cobalt complex by ≤170
mV (Table 2).
By replacing 4-methylpyrazole (4, 7) with more electron-rich 3,5-dimethylpyrazole (5,
8
), we negatively shift redox potentials of the complexes by ≤60 mV (Table 2). By replacing
,2′-bipyridyl in 4–6 with more electron-rich 4,4′-dimethyl-2,2′-bipyridyl in 7–9, we
2
ox
red
negatively shift E1/2 by ≤80 mV and E1/2 by ≤120 mV (Table 2). In contrast, by replacing

9
pyrazole (R2, 4, 5, 7, 8) with electron-deficient 1,2,4-triazole (6, 9), we strongly positively
ox
red
shift E1/2 by ≤340 mV and E1/2 by ≤180 mV (Table 2).
In metal complexes, neutral azoles are weaker σ-donors/π-acceptors and, therefore,
weaker ligands than pyridines are [23,25,38] that explains the trends in redox potentials and
the often irreversible electrochemistry observed for 1–9 (Table 2). Others have previously
2+
noted too that the Co(III/II) couple in [Co(N^N^N) ] complexes became less reversible and
2
its potential shifted positively when the pyridines in 2,2′;6′,2′′-terpyridine were replaced by
pyrazoles [38].
We change redox potentials of complexes 1–9 from 0.05 V to 0.5 V for the Co(III/II)
couple, from −1.08 V to −1.57 V for the Co(II/I) couple, and from −2.04 V to −2.14 V for the
−
L/L couple (Table 2). The potential of the Co(III/II) couple (oxidation) increases when there
are more of weak σ-donor pyrazoles and less of strong σ-donor pyridines coordinated to
ox
cobalt(II) from 4, 5, 7 and 8 (2 pyrazoles and 4 pyridines per Co, E1/2 of 0.05–0.17 V) to 1
and 2 (3 pyrazoles and 3 pyridines per Co, 0.19–0.21 V) and to 3 (4 pyrazoles and 2 pyridines
per Co, 0.50 V). The potential of the Co(II/I) couple (first reduction) is largely determined by
red
π-acceptor strength of the ligand and increases from bidentate ligands (1 and 2, E1/2 of
−1.53 V to −1.57 V) to stronger π-acceptor tridentate ligands (3–9, −1.08 V to −1.37 V) and
from pyrazole ligands (4, 5, 7 and 8, −1.24 V to −1.37 V) to more electron-deficient and
stronger π-acceptor 1,2,4-triazole ligands (6 and 9, −1.08 V to −1.19 V). The potential of the
−
L/L couple (second reduction for 4, 5, 7 and 8) shifts negatively by 10–40 mV when 4-
methylpyrazole (4, 7) is replaced by more electron-rich 3,5-dimethylpyrazole (5, 8) and by
6
0–90 mV when 2,2′-bipyridyl (4, 5) is replaced by more electron-rich 4,4′-dimethyl-2,2′-
bipyridyl (7, 8).
ox
red
Redox gap of the complex, ΔE = E1/2 [Co(III/II)] − E1/2 [Co(II/I)], increases (i)
when there are more of weak σ-donor/π-acceptor azoles and less of strong σ-donor/π-acceptor

1
0
pyridines coordinated to the cobalt from 4–9 (2 azoles and 4 pyridines per Co, ΔE of 1.37–
.58 V) to 1 and 2 (3 azoles and 3 pyridines per Co, 1.76 V) and to 3 (4 azoles and 2 pyridines
per Co, 1.82 V) and (ii) when a pyrazole is replaced by 1,2,4-triazole from 4, 5, 7 and 8
1.37–1.45 V) to 6 and 9 (1.51–1.58 V) (Table 2).
1
(
3
. Conclusions
We control optical and redox properties of cobalt(II) complexes 1–9 with bidentate
and tridentate azole-pyridines by changing azole heterocycles and by adding methyl groups.
–9 do not have strong optical transitions (are transparent) in the visible range. 4, 5, 7 and 8
1
have low Co(III/II) potentials of 0.05–0.17 V and can be used as redox shuttles in dye-
sensitized solar cells [16–20]. 3, 6 and 9 have high Co(III/II) potentials of 0.37–0.50 V and
can be applied, in the Co(III) oxidation state, as p-dopants [21] for organic electronics and as
oxidizers. Electron-deficient 1,2,4-triazole strongly positively shifts redox potentials of cobalt
complexes.
One can incorporate azole heterocycles in polydentate [14,15] and bridging [36,45–48]
ligands for cobalt; for example, cobalt complexes with quadridentate [14] and pentadentate
[
15] ligands and hetero-polynuclear cobalt–ruthenium [47] and cobalt–iridium [48]
complexes find use in electrical/solar-to-chemical energy conversion. One can coordinate
easy-to-make azole-pyridines L1–L9 and their analogs to a variety of d- and f-metals [23–35].
4
. Experimental
Purification and handling of all compounds were carried out under air. All products
were stored in the dark. Chemicals from commercial suppliers were used without purification.

1
1
Chromatography was performed on a column with an i.d. of 30 mm on silica gel 60 (Fluka,
Nr 60752) or on silica gel (ultra pure, 40-60 µm, 60A; Acros, Nr 36005). The progress of
reactions and the elution of products were followed on TLC plates (silica gel 60 F254 on
aluminum sheets, Merck).
Elemental analyses were performed by Dr. E. Solari, Service for Elemental Analysis,
1
13
19
Institute of Chemical Sciences and Engineering (ISIC EPFL). H, C, and F NMR spectra
were recorded with AV400 and AVIII-400 spectrometers (400 MHz, Bruker). Mass spectra
were recorded with Q-TOF Ultima (Waters) and TSQ7000 (Thermo Fisher) spectrometers
(Mass-Spectroscopy Service, ISIC EPFL).
Electronic absorption spectra were recorded with an HP/Agilent 8453 Diode Array
UV/VIS spectrophotometer under air in optical cells of 2 or 10 mm path length. The solutions
in CH Cl (Sigma-Aldrich, puriss p.a., ACS reagent) or acetonitrile (Fisher Scientific, Far
2
2
UV, HPLC Gradient Grade) were freshly prepared before experiment.
4
.1. Synthesis of ligands
The structures of L1–L9 are shown in Scheme 1. The synthesis of L1 was reported by
us before [31]. The reaction was performed under argon. Substituted azole (excess) and
t
potassium tert-butoxide ( BuOK excess, Acros) were dissolved at RT in dry and degassed
®
DMSO (3.5–4 mL, Acros, 99.8%, ExtraDry, over Molecular Sieves, AcroSeal ). An
exothermic reaction occurred. The mixture was stirred for 10 min to allow the reaction to
finish and cool. Then, a substituted halopyridine was added. The reaction mixture was stirred
for 24 h at either 110 °C (L2) or 140 °C (L3–L9) to give a suspension ranging in colour from
white to dark-red. It was cooled to RT. Water (50 mL) was added: the product precipitated as
solid or as oil that solidified on stirring/sonication. The solid was filtered, washed with water,

1
2
and extracted with ether and water (L2) or with dichloromethane and water (L3–L9). The
organic layer was washed with water to extract DMSO. Purification by chromatography on
silica (20 g) removed the starting materials and by-products on elution with 0–0.4% CH OH
3
in CH Cl , and provided the pure product on elution with 0.4–1.0 % CH OH in CH Cl .
2
2
3
2
2
Further details are provided below.
t
L2: 4-Methylpyrazole (0.9 mL, 0.89 g, 10.8 mmol, Acros), BuOK (1.21 g, 10.8
mmol), and 2-fluoro-5-methylpyridine (1 g, 9.0 mmol, Acros) gave white crystalline solid:
1
.26 g (7.3 mmol, 81%). Anal. Calc. for C H N (MW 173.21): C, 69.34; H, 6.40; N, 24.26.
10 11 3
1
Found: C, 69.99; H, 6.24; N, 24.89%. H NMR (400 MHz, CDCl ): δ = 8.29 (s, 1H), 8.20 (s,
3
1
H), 7.82 (d, J = 8.4 Hz, 1H), 7.61 (d, J = 8.0 Hz, 1H), 7.53 (s, 1H), 2.35 (s, 3H, CH ), 2.17
3
1
3
(
s, 3H, CH ) ppm. C NMR (100 MHz, CDCl ): δ = 150.0, 148.0, 142.7, 139.3, 130.6, 125.5,
3
3
+
+
1
18.3, 111.7, 18.0, 9.2 ppm. ESI MS: m/z 174.2 ({M + H} , 100%).
t
L3: 4-Methylpyrazole (1.68 mL, 1.66 g, 20 mmol, Acros), BuOK (2.27 g, 20 mmol),
and 2,6-dichloropyridine (1 g, 6.8 mmol, Aldrich) gave white solid: 1.12 g (4.7 mmol, 69%).
Anal. Calc. for C H N (MW 239.28): C, 65.25; H, 5.48; N, 29.27. Found: C, 65.04; H,
1
3
13
5
1
5
.51; N, 29.87%. H NMR (400 MHz, CDCl ): δ = 8.35 (s, 2H), 7.89 (t, J = 8.0 Hz, 1H), 7.76
3
1
3
(
d, J = 8.0 Hz, 2H), 7.58 (s, 2H), 2.20 (s, 6H, CH ) ppm. C NMR (100 MHz, CDCl ): δ =
3
3
+
+
1
2
50.2, 143.5, 141.3, 125.6, 118.7, 108.6, 9.2 ppm. ESI MS: m/z 240.3 ({M + H} , 100%),
+
62.2 ({M + Na} , 16%).
t
L4: 4-Methylpyrazole (0.53 mL, 0.52 g, 6.3 mmol, Acros), BuOK (0.72 g, 6.4 mmol),
and 6-bromo-2,2'-bipyridine (1 g, 4.3 mmol, ABCR) gave white solid: 0.92 g (3.9 mmol,
1%). Anal. Calc. for C H N (MW 236.27): C, 71.17; H, 5.12; N, 23.71. Found: C, 71.44;
9
1
4
12
4
1
H, 5.10; N, 24.05%. H NMR (400 MHz, CDCl ): δ = 8.71 (d, J = 4.4 Hz, 1H), 8.51 (s, 1H),
3
8
1
.48 (d, J = 8.0 Hz, 1H), 8.29 (dd, J = 7.2, 1.2, 1H), 8.00–7.90 (m, 2H), 7.86 (td, J = 7.6, 1.6,
13
H), 7.59 (s, 1H), 7.35 (dd, J = 6.8, 5.2 Hz, 1H), 2.21 (s, 3H, CH ) ppm. C NMR (100 MHz,
3

1
3
CDCl ): δ = 155.6, 154.6, 151.2, 149.4, 143.2, 139.7, 137.0, 125.6, 124.1, 121.3, 118.5,
3
+
+
+
1
18.2, 112.2, 9.3 ppm. ESI MS: m/z 237.3 ({M + H} , 100%), 259.2 ({M + Na} , 8%).
t
L5: 3,5-Dimethylpyrazole (0.61 g, 6.3 mmol, Fluka), BuOK (0.72 g, 6.4 mmol), and
6
7
-bromo-2,2'-bipyridine (1 g, 4.3 mmol, ABCR) gave pale-yellow solid: 0.82 g (3.3 mmol,
6%). Anal. Calc. for C H N (MW 250.30): C, 71.98; H, 5.64; N, 22.38. Found: C, 72.04;
1
5
14
4
1
H, 5.48; N, 22.48%. H NMR (400 MHz, CDCl ): δ = 8.71 (d, J = 4.4 Hz, 1H), 8.37 (d, J =
3
8
1
=
.0 Hz, 1H), 8.38–8.30 (br, 1H), 7.97–7.90 (m, 2H), 7.86 (t, J = 7.6 Hz, 1H), 7.40–7.31 (m,
1
3
H), 6.05 (s, 1H), 2.81 (s, 3H, CH ), 2.34 (s, 3H, CH ) ppm. C NMR (100 MHz, CDCl ): δ
3
3
3
155.9, 154.3, 153.2, 150.0, 149.4, 141.5, 139.5, 137.1, 124.0, 121.2, 117.9, 115.8, 103.4,
+
+
+
1
5.4, 13.9 ppm. ESI MS: m/z 251.1 ({M + H} , 100%), 273.1 ({M + Na} , 12%).
t
L6: 1,2,4-Triazole (0.44 g, 6.4 mmol, Aldrich), BuOK (0.72 g, 6.4 mmol), and 6-
bromo-2,2'-bipyridine (1 g, 4.3 mmol, ABCR) gave white solid: 0.57 g (2.6 mmol, 60%).
Anal. Calc. for C H N (MW 223.23): C, 64.56; H, 4.06; N, 31.37. Found: C, 64.13; H, 3.96;
1
2
9
5
1
N, 32.08%. H NMR (400 MHz, CDCl ): δ = 9.33 (s, 1H), 8.73 (d, J = 4.4 Hz, 1H), 8.50–8.42
3
(m, 2H), 8.15 (s, 1H), 8.04 (t, J = 8.0 Hz, 1H), 7.95 (d, J = 8.0 Hz, 1H), 7.90 (td, J = 8.0, 1.6
13
Hz, 1H), 7.39 (dd, J = 6.4, 4.8 Hz, 1H) ppm. C NMR (100 MHz, CDCl ): δ = 155.3, 154.8,
3
+
1
53.1, 149.5, 149.0, 141.6, 140.4, 137.2, 124.5, 121.3, 120.4, 113.1 ppm. ESI MS: m/z 224
+
+
(
{M + H} , 100%), 246 ({M + Na} , 25%).
t
L7: 4-Methylpyrazole (0.57 mL, 0.56 g, 6.8 mmol, Acros), BuOK (0.77 g, 6.9 mmol),
and 6-chloro-4,4'-dimethyl-2,2'-bipyridine [49] (1 g, 4.6 mmol) gave white solid: 1.09 g (4.1
mmol, 89%). Anal. Calc. for C H N (MW 264.33): C, 72.70; H, 6.10; N, 21.20. Found: C,
16
16
4
1
7
1
6
3.08; H, 6.25; N, 21.59%. H NMR (400 MHz, CDCl ): δ = 8.56 (d, J = 4.8 Hz, 1H), 8.52 (s,
3
H), 8.28 (s, 1H), 8.13 (s, 1H), 7.81 (s, 1H), 7.58 (s, 1H), 7.18 (d, J = 4.4 Hz, 1H), 2.50 (s,
13
H, CH ), 2.22 (s, 3H, CH ) ppm. C NMR (100 MHz, CDCl ): δ = 155.6, 154.5, 151.4,
3
3
3

1
4
+
1
51.3, 149.1, 148.2, 143.0, 125.8, 125.0, 122.2, 119.5, 118.3, 112.6, 21.6, 21.5, 9.3 ppm. ESI
+
MS: m/z 265 ({M + H} , 100%).
t
L8: 3,5-Dimethylpyrazole (0.66 g, 6.9 mmol, Fluka), BuOK (0.77 g, 6.9 mmol), and
6
-chloro-4,4'-dimethyl-2,2'-bipyridine [49] (1 g, 4.6 mmol) gave pale yellow solid: 0.876 g
(
3.1 mmol, 68%). Anal. Calc. for C H N (MW 278.35): C, 73.35; H, 6.52; N, 20.13. Found:
17 18 4
1
C, 73.25; H, 6.64; N, 20.66%. H NMR (400 MHz, CD Cl ): δ = 8.54 (d, J = 4.8 Hz, 1H),
2
2
8
.22 (s, br, 2H), 7.76 (s, 1H), 7.21 (s, br, 1H), 6.07 (s, 1H), 2.80 (s, 3H, CH ), 2.52 (s, 3H,
3
1
CH ), 2.47 (s, 3H, CH ), 2.30 (s, 3H, CH ) ppm. H NMR (400 MHz, CDCl ): δ = 8.58 (s,
3
3
3
3
1
H), 8.23 (s, br, 2H), 7.77 (s, 1H), 7.23 (br, 1H), 6.05 (s, 1H), 2.78 (s, 3H, CH ), 2.51 (s, 6H,
3
1
3
CH ), 2.34 (s, 3H, CH ) ppm. C NMR (100 MHz, CDCl ): δ = 155.9, 154.2, 153.2, 151.0,
3
3
3
1
49.8, 149.1, 148.2, 141.5, 124.9, 122.3, 119.2, 116.4, 109.2, 21.63, 21.57, 15.2, 13.9 ppm.
+
+
ESI MS: m/z 279 ({M + H} , 100%).
t
L9: 1,2,4-Triazole (0.47 g, 6.8 mmol, Aldrich), BuOK (0.77 g, 6.9 mmol, Acros), and
6
-chloro-4,4'-dimethyl-2,2'-bipyridine [49] (1 g, 4.6 mmol) gave pale pink solid: 0.39 g (1.6
mmol, 34%). The reaction did not went to completion: 6-chloro-4,4'-dimethyl-2,2'-bipyridine
was still present in the reaction mixture when the reaction was terminated. Anal. Calc. for
C H N (MW 251.29): C, 66.92; H, 5.21; N, 27.87. Found: C, 66.68; H, 5.28; N, 27.98%.
14
13
5
1
H NMR (400 MHz, CD Cl ): δ = 9.35 (s, 1H), 8.56 (d, J = 4.8 Hz, 1H), 8.33 (s, 1H), 8.31 (s,
2
2
1
H), 8.11 (s, 1H), 7.79 (s, 1H), 7.23 (d, J = 4.4 Hz, 1H), 2.57 (s, 3H, CH ), 2.50 (s, 3H, CH )
3 3
1
ppm. H NMR (400 MHz, CDCl ): δ = 9.41 (s, br, 1H), 8.65 (s, br, 1H), 8.35 (s, br, 1H), 8.29
3
13
(
s, 1H), 8.14 (s, 1H), 7.82 (s, 1H), 1H obscured by solvent signal, 2.58 (s, 6H, CH ) ppm. C
3
NMR (100 MHz, CDCl ): δ = 155.0, 154.7, 152.9, 152.2, 149.2, 149.1, 148.4, 141.7, 125.4,
3
+
+
+
1
5
22.2, 121.3, 113.6, 21.6, 21.5 ppm. ESI MS: m/z 252 ({M + H} , 100%), 274 ({M + Na} ,
%).

1
5
4
.2. Synthesis of cobalt(II) complexes
The structures of 1–9 are shown in Scheme 1. The reactions were performed under
argon. The solvents were neither deoxygenated nor dried. The ligand (small excess) was
dissolved in methanol (5 mL) at 60 °C. Solid CoCl ·6H O (Fluka) was added to this solution.
2
2
The colour of the solution changed immediately from colourless to red. The ligand L7 on its
own is not soluble in methanol; however, addition of CoCl ·6H O gives a solution of the
2
2
complex. For complexes 3 and 6, the solution shortly after mixing (5–10 min) turned into a
suspension; therefore, water (0.5–1 mL) was added to dissolve the precipitate. Reaction
mixture was stirred at 60°C for 90 min to give pink (1) or red (2–9) solution. The solution was
cooled to RT. It was added drop-wise to a stirred solution of KPF (1–1.3 g, 5.4–7.1 mmol,
6
large excess) in 30 mL of water, in order to convert the complex to hexafluorophosphate salt.
The precipitate immediately formed. The suspension was stirred at RT for 30 min. The solid
was filtered; washed with water (to remove inorganic salts) and ether (to remove free ligand);
and dried under vacuum. 1–9 are moisture- and air-stable solids. Complexes 3, 6, and 9 are
1
insoluble in dichloromethane. All of the observed H NMR signals of 1–9 were singlets
(
information on J-coupling is lost); integration of the signals was often not informative.
Further details are provided below.
: CoCl ·6H O (150 mg, 0.63 mmol) and L1 (350 mg, 2.02 mmol) gave pale yellow-
1
2
2
pink solid: 507 mg (0.58 mmol, 93%). Anal. Calc. for C H CoF N P (MW 868.51): C,
3
0
33
12
9 2
1
4
1.49; H, 3.83; N, 14.51. Found: C, 41.77; H, 3.97; N, 14.65%. H NMR spectrum (400 MHz,
1
9
CD CN or acetone-d ) was not informative. F NMR (376 MHz, acetone-d ): δ = −72.0 (d,
3
6
6
+
2+
JP–F = 707 Hz, 6F, PF ) ppm. ESI TOF MS: m/z 289.0 ({M – 2PF } , 90%), 550.0 ({M – L
6
6
+
–
PF } , 5%).
6

1
6
2
: CoCl ·6H O (150 mg, 0.63 mmol) and L2 (349 mg, 2.0 mmol) gave pale pink solid:
2 2
4
83 mg (0.56 mmol, 88%). Anal. Calc. for C H CoF N P (MW 868.51): C, 41.49; H, 3.83;
30 33 12 9 2
1
N, 14.51. Found: C, 41.68; H, 3.71; N, 14.54%. H NMR spectrum (400 MHz, CD CN or
3
1
9
acetone-d ) was not informative. F NMR (376 MHz, acetone-d ): δ = −72.1 (d, J = 707
6
6
P–F
+
2+
Hz, 6F, PF ) ppm. ESI TOF MS: m/z 289.1 ({M – 2PF } ).
6
6
3
: CoCl ·6H O (150 mg, 0.63 mmol) and L3 (317 mg, 1.3 mmol) gave pale brownish-
2 2
yellow solid: 343 mg (0.41 mmol, 66%). Anal. Calc. for C H CoF N P (MW 827.41): C,
2
6
26
12 10 2
1
3
7.74; H, 3.17; N, 16.93. Found: C, 37.87; H, 3.25; N, 16.96%. H NMR (400 MHz, CD CN):
3
1
δ = 75.0, 61.0, 56.3, 14.3, 8.2 (CH ) ppm. H NMR (400 MHz, acetone-d ): δ = 74.7, 61.7,
3
6
1
9
5
6
6.8, 14.1, 8.3 (CH ) ppm. F NMR (376 MHz, acetone-d ): δ = −72.08 (d, J = 707 Hz,
3 6 P–F
+
+
F, PF ) ppm. ESI TOF MS: m/z 682.1 ({M – PF } ).
6
6
4
: CoCl ·6H O (150 mg, 0.63 mmol) and L4 (313 mg, 1.3 mmol) gave pale brownish-
2 2
yellow solid: 397 mg (0.48 mmol, 77%). Anal. Calc. for C H CoF N P (MW 821.41): C,
2
8
24
12
8 2
1
4
0.94; H, 2.95; N, 13.64. Found: C, 40.83; H, 3.06; N, 13.78%. H NMR (400 MHz, CD CN):
3
1
δ = 110.1, 94.3, 91.2, 75.2, 70.8, 43.2, 38.9, 19.1, 14.6, 3.1 (CH ) ppm. H NMR (400 MHz,
3
acetone-d ): δ = 109.8 (2H), 94.6 (2H), 91.5 (2H), 75.3 (2H), 71.0 (2H), 43.3 (2H), 39.4 (2H),
6
1
9
1
9.3 (2H), 14.8 (2H), 3.1 (6H, CH ) ppm. F NMR (376 MHz, acetone-d ): δ = −71.7 (d, J
3 6 P–F
+
+
=
707 Hz, 6F, PF ) ppm. ESI TOF MS: m/z 676.1 ({M – PF } ).
6 6
5
: CoCl ·6H O (150 mg, 0.63 mmol) and L5 (331 mg, 1.3 mmol) gave pale brown
2 2
solid: 464 mg (0.55 mmol, 87%). Anal. Calc. for C H CoF N P (MW 849.46): C, 42.42;
30
28
12
8 2
1
H, 3.32; N, 13.19. Found: C, 42.13; H, 3.33; N, 13.29%. H NMR (400 MHz, CD CN): one
3
set of aromatic H was not found at 135 ppm to −50 ppm; δ = 83.1, 78.0, 77.2, 71.0, 51.2, 15.8,
1
1
2.7, 7.2, −2.1 ppm. H NMR (400 MHz, acetone-d ): one set of aromatic H was not found at
6
1
35 ppm to −50 ppm; δ = 83.1 (2H), 78.2 (2H), 78.1 (2H), 72.0 (2H), 51.1 (2H), 15.9 (6H,
1
9
CH ), 12.9 (2H), 7.8 (2H), −0.1 (6H, CH ) ppm. F NMR (376 MHz, acetone-d ): δ = −71.9
3
3
6

1
7
+
+
(
d, JP–F = 707 Hz, 6F, PF ) ppm. ESI TOF MS: m/z 704.1 ({M – PF } , 100%), 279.6 ({M –
6
6
2
+
2
PF } , 90%).
6
6
: CoCl ·6H 0 (120 mg, 0.50 mmol) and L6 (236 mg, 1.06 mmol) gave pale pink-
2 2
yellow solid: 301 mg (0.38 mmol, 76%). Anal. Calc. for C H CoF N P (MW 795.33): C,
2
4
18
12 10 2
1
3
6.24; H, 2.28; N, 17.61. Found: C, 36.05; H, 2.35; N, 17.82%. H NMR (400 MHz, CD CN):
3
1
9
δ = 118.8, 96.7, 93.0, 77.9, 43.4, 32.0, 20.3, 17.1, 12.3 ppm. F NMR (376 MHz, CD CN): δ
3
1
=
−72.8 (d, JP–F = 705 Hz, 6F, PF ) ppm. H NMR (400 MHz, acetone-d ): δ = 118.5, 96.7,
6
6
19
9
3.9, 78.6, 43.2, 31.8, 20.8, 17.8, 12.4 ppm. F NMR (376 MHz, acetone-d ): δ = −72.2 (d,
6
JP–F = 707 Hz, 6F, PF ) ppm.
6
7
: CoCl ·6H 0 (120 mg, 0.50 mmol) and L7 (280 mg, 1.06 mmol) gave pale brown
2 2
solid: 248 mg (0.28 mmol, 57%). Anal. Calc. for C H CoF N P (MW 877.51): C, 43.80;
32
32
12
8 2
1
H, 3.68; N, 12.77. Found: C, 44.03; H, 3.43; N, 13.13%. H NMR (400 MHz, CD CN): only 6
3
out of 7 expected sets of aromatic signals were found; δ = 113.8, 91.5, 77.9, 65.9, 43.9, 41.1,
1
9
8
7
4
−
.0 (CH ), 2.8 (CH ), −2.2 (CH ) ppm. F NMR (376 MHz, CD CN): δ = −72.6 (d, J
=
3
3
3
3
P–F
1
06 Hz, 6F, PF ) ppm. H NMR (400 MHz, acetone-d ): δ = 112.1, 92.1, 91.5, 77.0, 67.1,
6
6
19
3.4, 41.0, 7.4 (CH ), 2.7 (CH ), −2.3 (CH ) ppm. F NMR (376 MHz, acetone-d ): δ =
3
3
3
6
+
2+
71.6 (d, JP–F = 707 Hz, 6F, PF ) ppm. ESI TOF MS: m/z 293.6 ({M – 2PF } , 20%).
6
6
8
: CoCl ·6H 0 (120 mg, 0.50 mmol) and L8 (295 mg, 1.06 mmol) gave pale pinkish-
2 2
yellow solid: 439 mg (0.48 mmol, 97%). Anal. Calc. for C H CoF N P (MW 905.57): C,
3
4
36
12
8 2
1
4
5.09; H, 4.01; N, 12.37. Found: C, 45.13; H, 3.98; N, 12.63%. H NMR (400 MHz, CD CN):
3
only 5 out of 6 expected sets of aromatic signals were found; δ = 83.2, 80.5, 73.5, 70.3, 49.2,
19
1
5.6 (CH ), 2.1 (CH ), −1.2 (CH ), −9.3 (CH ) ppm. F NMR (376 MHz, CD CN): δ = −72.8
3 3 3 3 3
1
(
d, JP–F = 705 Hz, 6F, PF ) ppm. H NMR (400 MHz, acetone-d ): only 5 out of 6 expected
6
6
sets of aromatic signals were found; δ = 83.2, 80.2, 74.5, 70.9, 49.0, 15.6 (CH ), 1.9 (CH ),
3
3

1
8
1
9
−
1.2 (CH ), −7.4 (CH ) ppm. F NMR (376 MHz, acetone-d ): δ = −71.9 (d, J = 707 Hz,
3 3 6 P–F
+
+
2+
6
F, PF ) ppm. ESI TOF MS: m/z 760.2 ({M – PF } , 5%), 307.6 ({M – 2PF } , 100%).
6 6 6
9
: CoCl ·6H 0 (90 mg, 0.38 mmol) and L9 (200 mg, 0.80 mmol) gave pale pink solid:
2 2
2
3
1
71 mg (0.32 mmol, 84%). Anal. Calc. for C H CoF N P (MW 851.44): C, 39.50; H,
2
8
26
12 10
2
1
.08; N, 16.45. Found: C, 39.45; H, 2.86; N, 16.46%. H NMR (400 MHz, CD CN): δ =
3
19
23.4, 92.2, 91.7, 80.1, 41.3, 26.8, 21.4, 10.6 (CH ), −1.3 (CH ) ppm. F NMR (376 MHz,
3
3
1
CD CN): δ = −72.7 (d, J = 705 Hz, 6F, PF ) ppm. H NMR (400 MHz, acetone-d ): δ =
3
P–F
6
6
19
1
22.3, 92.5, 92.3, 80.1, 41.1, 27.2, 21.6, 10.6 (CH ), −1.4 (CH ) ppm. F NMR (376 MHz,
3 3
acetone-d ): δ = −71.8 (d, J = 707 Hz, 6F, PF ) ppm.
6
P–F
6
4
.3. Electrochemistry
Electrochemical experiments were conducted in DMF (99.8%, Extra Dry, over
®
Molecular Sieves, AcroSeal , Acros) or acetonitrile (99.9%, Extra Dry, over Molecular
®
Sieves, AcroSeal , Acros), with 0.1 M tetra-n-butylammonium hexafluorophosphate (Fluka,
electrochemical grade) as the supporting electrolyte, with a PC controlled AutoLab PSTAT10
electrochemical workstation.
The experiments were carried out under argon in an electrochemical cell through
which a stream of Ar was passed (the flow of Ar was stopped during the measurement to
prevent stirring of the solution). Gold disk or glassy-carbon disk (3 mm diameter), platinum
spiral, and platinum wire served as working, counter, and quasi-reference electrodes. To
clean, the working electrodes were polished on a pad in ethanol slurry of abrasive paste
(Metrohm), briefly sonicated in ethanol, and rinsed with ethanol. At the end of each
experiment, ferrocene was added as an internal reference [44]. Estimated error: ±50 mV.
Cyclic voltammetry was performed at scan rates of 10–0.05 V/s. Peak-to-peak separation for

1
9
the ferrocene couple (reference) in acetonitrile was 83–107 mV at 1 V/s and 68–82 mV at 0.1
V/s [44].
Acknowledgements
European Union (FP7-ENERGY-2010-FET, MOLESOL, Ref.: 256617,
http://www.molesol.eu/). MKN acknowledges the World Class University program
(Photovoltaic Materials, Department of Material Chemistry, Korea University) funded by the
Ministry of Education, Science and Technology through the National Research Foundation of
Korea (Ref.: R31-2008-000-10035-0).
Appendix A. Supplementary material
Redox properties (Tables S1 and S2; Fig. S1–S6); absorption spectra (Table S3; Fig.
S7–S24); NMR spectra. Supplementary data associated with this article can be found, in the
online version, at http://dx.doi.org/...

2
0
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2
5
Table 1
a
UV-Vis absorption maxima in acetonitrile.
3
−1
−1
λ /nm (ε/10 M cm )
abs
L1
L2
L3
L4
L5
L6
L7
L8
L9
1
263 (11), 282 (13)
261 (14), 288 (13)
214 (11), 252 (26), 277 (12), 308 (21)
237 (20), 267 (19), 306 (15)
236 (21), 264 (21), 303 (12)
231 (19), 297 (14)
243 (21), 268 (18), 289 (13), 304 (14)
242 (22), 265 (19), 289 (12), 300 (12)
237 (21), 295 (13)
259 (36), 284 (32)
2
231 (21), 261 (42), 292 (31)
3
231 (24), 254 (31), 272 (39), 316 (25), 402 (0.8)
237 (29), 272 (34), 323 (24)
4
5
236 (35), 269 (35), 326 (24)
6
225 (40), 257 (24), 312 (25)
7
209 (69), 270 (35), 317 (24)
8
208 (65), 268 (36), 320 (24)
9
213 (69), 259 (26), 309 (26)
a
Room temperature. At 200–700 nm. Errors: ±2 nm for λ ; ±5% for ε. Absorption maxima
abs
in dichloromethane are listed in Table S3.

2
6
Table 2
Redox potentials of cobalt(II) complexes in acetonitrile.
a
ox
red
b
Complex Electrode E1/2 /V
E1/2 /V
ΔE/V
1.76
1.76
1.82
−
Co(III/II)
Co(II/I)
L/L
c
c
1
2
3
4
5
6
7
8
9
Gold
GC
0.19 (130)
−1.57 (100)
−1.56 (93)
−1.55 (110)
−1.53 (98)
d
Gold
GC
0.21 (110)
d
c
Gold
GC
0.50 (93)
−1.32 (88)
d
−1.33 (98)
−1.24 (93)
−1.24 (88)
−1.25 (83)
−1.25 (88)
−1.08 (93)
−1.09 (83)
−1.35 (78)
−1.34 (78)
−1.37 (73)
−1.37 (78)
−1.19 (93)
−1.19 (83)
Gold
GC
0.17 (100)
0.17 (140)
0.13 (83)
−2.04 (98)
−2.04 (98)
−2.05 (88)
−2.06 (88)
1.41
1.41
1.38
1.37
1.52
1.51
Gold
GC
0.12 (110)
0.44 (180)
0.42 (160)
0.10 (88)
Gold
GC
Gold
GC
−2.10 (150) 1.45
0.11 (160)
0.05 (88)
1.45
Gold
GC
−2.14 (88)
−2.14 (88)
1.42
1.42
1.56
1.58
0.05 (150)
0.37 (130)
0.39 (350)
Gold
GC
a
On gold or glassy-carbon (GC) working electrode. 0.1 M (NBu )PF . Scan rate: 100 mV/s.
4
6
red
ox
+
Quasi-reversible processes; E1/2 and E1/2 are reported relative to Fc /Fc [44]; the
+
anodic/cathodic peak separation is given in brackets (for the Fc /Fc it was 68–82 mV). Error:
±
50 mV. The one-electron redox processes of 1–9 were assigned by comparison with the
literature [1–10]. Redox potentials in DMF and at other scan rates in ACN are listed in Tables
S1 and S2.

2
7
b
c
d
ox
red
Redox gap: ΔE = E1/2 [Co(III/II)] − E1/2 [Co(II/I)].
Not completely reversible process.
Irreversible process or wide peak-to-peak separation.

2
8
R
a
b
X
Hal + HN
R
L1-L9
1-9
N
N
N
N
N
N
N
N
L1
L2
[
[
[
Co(L1) ](PF ) , 1
[Co(L2) ](PF ) , 2
3 6 2
3
6 2
N
N
N
N
N
L3
Co(L3) ](PF ) , 3
2
6 2
N
N
N
N
N
N
N
N
N
N
N
N
N
L4
L5
L6
Co(L4) ](PF ) , 4
[Co(L5) ](PF ) , 5
[Co(L6) ](PF ) , 6
2 6 2
2
6 2
2
6 2
N
N
N
N
N
N
N
N
N
N
N
N
N
L7
Co(L7) ](PF ) , 7
L8
[Co(L8) ](PF ) , 8
L9
[Co(L9) ](PF ) , 9
[
2
6 2
2
6 2
2
6 2
t
Scheme 1. Synthesis of ligands L1–L9 and octahedral cobalt(II) complexes 1–9: (a) BuOK,
dry DMSO, under Ar, 110–140 °C; (b) CoCl ·6H O (solid), methanol, under Ar, 60 °C, KPF .
2
2
6

2
9
N
N
N
N
N
N
N
LR1
LR2
[
Co(LR1) ](PF ) , R1
[Co(LR2) ](PF ) , R2
2 6 2
3
6 2
Chart 1. Reference ligands and cobalt(II) complexes [19].