CoII complexes of triazine-based tridentate ligands with positive and attractive CoII/III redox couples

Article abstract of DOI:10.1002/ejic.200400831

Co^II complexes based on the triazine ligand 2,4-di(2′- pyridyl)-6-(p-R-C₆H₄)-1,3,5-triazine have been synthesized and characterized. Cyclic voltammetry shows that the Co^II/III redox couple is more positive than

Full text of DOI:10.1002/ejic.200400831

SHORT COMMUNICATION
CoII Complexes of Triazine-Based Tridentate Ligands with Positive and
Attractive CoII/III Redox Couples
Elaine A. Medlycott,[a] Isabelle Theobald,[a] and Garry S. Hanan*[a]
Keywords: Tridentate N ligands / Cobalt complexes / Electrochemistry / Triazine
CoII complexes based on the triazine ligand 2,4-di(2Ј-pyri-
may be varied depending on the nature of the substituent on
dyl)-6-(p-R-C6H4)-1,3,5-triazine have been synthesized and the phenyl ring of the triazine ligand.
characterized. Cyclic voltammetry shows that the CoII/III re-
dox couple is more positive than previously described Co
complexes of tridentate ligands. The CoII/III redox couple
(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2005)
lowed by precipitation of complexes 2a–c from the reaction
mixture by the addition of NH4PF6 [see (a) in Scheme 1].[11]
Heteroleptic complexes 3a–c may be obtained directly by
treating ligands 1a–c with one equivalent of CoCl2·6H2O in
the presence of tetrabutylammonium chloride (Bu4NCl)
[see (b) in Scheme 1] or indirectly when Bu4NCl is intro-
duced into an acetone solution of complexes 2a–c [see (c)
in Scheme 1].[12] It has previously been established that an
equilibrium exists between the homo- and heteroleptic state
of CoII complexes of 2,4,6-tripyridyl-s-triazine in acidic
solution.[13] In this case, heteroleptic CoII complexes 3a–c
could be obtained by the addition of Bu4NCl to complexes
2a–c.
Introduction
Dye-sensitized solar cells (DSSCs) have been intensely
studied in the last fifteen years since Grätzel published his
landmark work.[1,2] In order to advance the field, many
groups have focused their efforts on improving the excited-
state properties of the polypyridine RuII complexes that act
as the photosensitizers in the Grätzel cell.[3] Much less at-
tention has been paid to the redox mediator, even though
it contributes directly to the maximum photocurrent attain-
able by the DSSC.[2,3] Whereas the classic redox mediator
–
in such systems is the volatile and corrosive I–/I3 couple,
its redox potential is fixed and, therefore, cannot be varied
to improve the efficiency of the DSSC.[4] Recently, CoII
complexes of heterocyclic ligands have been shown to be
effective redox mediators in DSSCs, as they are non-corros-
ive, non-volatile and modifiable.[5–7] Herein we report on a
new family of CoII complexes based on tridentate triazine
ligands that are easily synthesized and modified. Their con-
siderably more positive CoII/III redox couples than pre-
viously reported CoII complexes of polypyridine ligands fa-
vors their use in DSSCs.[5,6]
Results and Discussion
The synthesis of triazines ligands 1a[8] and 1b[9] has pre-
viously been reported. Ligand 1c was synthesized by the
addition of p-bromobenzonitrile to a solution of LiNMe2
in anhydrous diethyl ether.[9,10] The amidinate intermediate
that forms reacts with two equivalents of 2-cyanopyridine
to afford ligand 1c after loss of dimethylamide. A series of
homoleptic CoII complexes of ligands 1a–c were synthe-
sized by allowing the ligands to react with CoCl2·6H2O fol-
Scheme 1. Synthesis of CoII homo- and heteroleptic complexes.
1
[a] Département de Chimie, Université de Montréal,
2900 Edouard-Montpetit, Montreal, Quebec, H3T 1J4, Canada
Fax: +001-514-343-7586
The H NMR chemical shifts of the homoleptic com-
plexes are typical of paramagnetic CoII complexes that are
low spin, with chemical shifts ranging over approximately
E-mail: garry.hanan@umontreal.ca
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DOI: 10.1002/ejic.200400831
© 2005 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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E. A. Medlycott, I. Theobald, G. S. Hanan
SHORT COMMUNICATION
110 ppm (Table 1).[14] The signals are broadened consider- angles of 151.79°. The Co–N bond length to the central
ably, and therefore no coupling constants could be obtained triazine (1.986 Å) is shorter than the Co–N bond length in
from the spectra. The greatest shifts are observed for the homoleptic terpyridine complex (2.022 Å) and the homo-
protons ortho to the pyridine nitrogen atoms and, therefore, leptic triazine complex of 2,4,6-tris(2Ј-pyridyl)-1,3,5-tri-
closest to the paramagnetic CoII center. The protons far- azine ligand (2.058 Å).[16,17] The Co–N bond lengths to the
thest from the paramagnetic center, those on the phenyl peripheral pyridine rings range from 2.134 Å to 2.158 Å,
ring, have the smallest shift as compared to the free li- similar to those found in terpyridine complexes of CoII.
gands.[9]
The phenyl ring is twisted by 13.3° relative to the central
triazine ring (Figure 1). This near planar arrangement is
due to favorable N-to-CH hydrogen bonding interactions
between the N lone pairs and the hydrogen atoms ortho to
the interannular bond. The slight twist in the interannular
bond is a result of edge-to-face packing between the phenyl
rings in different molecules of 2c, which are separated by
3.514 Å (Figure 2). A planar arrangement should easily be
adopted in solution which would permit greater electron
delocalization between the triazine and the phenyl ring. The
two PF6 counterions are found above and below the planes
of the triazine rings and do not interfere with the phenyl
packing arrangement, consistent with other extended pack-
ing arrangements in the solid state.[18] There is also a short
Br1···Br2Ј distance of 3.395 Å between adjacent molecules
which is indicative of favorable van der Waals interactions
which give rise to a linear one-dimensional tape in the solid
state (Figure 2).
1
Table 1. H NMR chemical shifts of homoleptic complexes 2a, 2b
and 2c in CD3CN.
δ [ppm]
Pyridyl
8.99 8.87 8.74
Phenyl
CH3
5.83
1c
7.60
8.74 7.73
2a 115.08 80.09 37.54 22.87 13.51 6.17
2b 113.39 78.56 37.13 21.99 13.27 6.13
2c 111.88 79.38 36.28 22.96 13.36 6.66
The X-ray crystal structure of 2c is shown in Figure 1.[15]
The CoII ion is bound by two orthogonal ligands 1c in a
pseudo-octahedral coordination environment as the periph-
eral pyridines in each ligand pinch in to form N–Co–N
In the electronic spectra of complexes 2a–c, a very weak
metal-centered transition is found at approximately 470 nm
(Figure 3). The λmax. for these d–d transitions are difficult
to identify due to the low energy tail from the ligand-based
transitions but the extinction coefficients are in the order
of 500 m–1 cm–1 for all three complexes. For applications in
DSSC, a low extinction coefficient is desirable so as to mini-
mise the competition between the redox mediator and sensi-
tiser for absorption in the visible region.[5–7] Intense ligand-
based πǞπ* transitions are observed in the UV region.
Lower energy πǞπ* transitions are observed for the Me-
and Br-substituted complexes as compared to the complex
with R = H. In 2b, the electron donating methyl groups
destabilise the ligand π orbitals to a greater extent than the
ligand π* orbitals. Conversely, in 2c the ligand π* orbitals
Figure 1. Ball-and-stick representation of the X-ray crystal struc-
ture of 2c. The counteranions and solvent of crystallization have
been omitted for clarity.
Figure 2. Intermolecular packing forces in the solid-state structure of 2c. Phenyl-to-phenyl distance = 3.514 Å and the Br1···Br2Ј distance
= 3.395 Å. The PF6 counteranions and solvent of crystallization have been removed for clarity.
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CoII Complexes of Triazine-Based Tridentate Ligands
SHORT COMMUNICATION
are stabilized to a greater extent than ligand π orbitals due
to the electron withdrawing nature of the bromine substitu-
ent.
Conclusions
CoII complexes of tridentate triazine-based ligands,
either heteroleptic or homoleptic, are readily synthesized
and isolated. The electrochemical properties of the homo-
leptic complexes indicate that the CoII/III redox couples are
more positive and more favorable for DSSC than those of
CoII terpyridine-based complexes. The CoII/III couple exhib-
its small shifts in oxidation potential upon modification of
the phenyl ring substituents, whereas the CoII/I reduction
potential is more readily modified. Further modifications
to substitute electron withdrawing and donating groups di-
rectly on the triazine ring will be reported in due course.
Acknowledgments
We thank the Natural Sciences and Engineering Research Council
(NSERC) of Canada and the Université de Montréal for financial
support. E. A. M. thanks the Canadian Commonwealth Scholar-
ship and Fellowship Program. I. T. thanks NSERC of Canada for
an undergraduate research award.
Figure 3. Electronic absorption spectra of complexes 2a (solid line),
2b (dashed line) and 2c (dotted line).
The electrochemical parameters of complexes 2a, 2b and
2c were measured in acetonitrile vs. tetrabutylammonium
PF6 using a Pt electrode and ferrocene as the internal stan-
dard. The data (reported vs. SCE) are gathered in Table 2.
The CoII/III couples are centered at approximately +0.75 V,
which is considerably more positive than previously re-
ported CoII of heterocyclic ligands.[5–7] In each case, oxidat-
ive processes are irreversible, which may be a result of lim-
ited solubility of the CoIII species. It had previously been
noted that irreversibility of the CoII/III couple does not nec-
essarily translate into inefficient redox mediators, but some-
times give rise to the most efficient redox mediators.[6] The
more positive oxidation potential of 2a–c as compared to
terpyridine complexes of CoII is favorable with respect to
an increased photocurrent in DSSCs.[7] The CoII/III couple
is only slightly affected by substituent on the phenyl ring,
with complex 2c being the most difficult to oxidise as a
result of the electron withdrawing Br substituent and 2b
being the easiest as a result of the electron donating Me
substituent. The CoII/I couple is also affected by the various
substitutents, with 2b being the hardest to reduce and 2c
the easiest to reduce. The greater variability in the CoII/I
reduction potential is a reflection of the increased back-
bonding of the metal center to the ligand upon reduction.
The two remaining reductions are independent triazine-
based reductions and follow the electron-withdrawing effect
of the substitutents.
[1] B. O’Regan, M. Grätzel, Nature 1991, 353, 737.
[2] A. Hagfeldt, M. Grätzel, Acc. Chem. Res. 2000, 33, 269.
[3] C. A. Bignozzi, E. Argazzi, C. J. Kleverlaan, Chem. Soc. Rev.
2000, 29, 87.
[4] M. K. Nazeeruddin, A. Kay, I. Rodicio, R. Humphry-Baker,
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Chem. Soc. 1993, 115, 6382.
[5] H. Nusbaumer, J.-E. Moser, S. M. Zakeeruddin, M. K. Na-
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[10] Synthesis of 2,4-di(2Ј-pyridyl)-6-(p-bromophenyl)-1,3,5-tri-
azine 1c: para-bromobenzonitrile (0.68 g, 3.73 mmol) was
added to a stirred mixture of LiNMe2 (0.19 g, 3.73 mmol) in
anhydrous diethyl ether. After 30 min, 2-cyanopyridine (0.77 g,
7.46 mmol) was added to the mixture. After an hour the reac-
tion mixture was diluted with H2O (250 mL) and the precipi-
tate was collected. Recrystallization of the solid from ethanol
1
afforded 1c (0.90 g, 62%) as a white solid. H NMR (CDCl3):
δ = 8.99 (d, J = 4.1 Hz, 2 H), 8.87 (d, J = 7.8 Hz, 2 H), 8.74
(d, J = 8.4 Hz, 2 H), 8.0 (t, J = 7.1 Hz, 2 H), 7.73 (d, J =
8.4 Hz, 2 H), 7.60 (t, 2 H, J = 5.0 Hz) ppm.
[11] General procedure for the synthesis of homoleptic CoII com-
plexes 2a, 2b, 2c: The triazine ligand (0.42 mmol) was stirred
in acetone (25 mL) and added to a solution of CoCl2·6H2O
(0.38 mmol, 91 mg). After heating at reflux for 75 min and pre-
cipitation in aqueous NH4PF6, complexes 2a (110 mg, 54%),
2b (127 mg, 61%) and 2c (103 mg, 43%) were collected.
Table 2. Electrochemical redox potentials for complexes 2a–c in
argon-purged acetonitrile solutions (vs. SCE).
[12] Synthesis of heteroleptic complexes 3a, 3b and 3c followed the
same procedure for complexes 2. After heating for 75 min the
acetone/water mix was removed under reduced pressure and
the solution was dissolved in a small amount of acetone and
precipitated by the addition of Bu4NCl. Complexes 3a (44%),
3b (33%) and 3c (23%) were collected. 3a·2H2O: calcd. C
47.82, H 3.59 N 14.68; found C 47.67, H 3.33, N 14.66.
3b·H2O: calcd. C 50.76, H 3.62, N 14.80; found C 51.38, H
E1/2 (V) [ΔEp (mV)]
Compound
CoII/III
CoII/I
Triazine reductions
2a
2b
2c
0.76 [irr]
0.74 [irr]
0.77 [irr]
–0.42 [73]
–0.47 [64]
–0.40 [52]
–1.06 [72]
–1.11 [66]
–1.04 [56]
–1.47 [85]
–1.52 [64]
–1.46 [66]
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E. A. Medlycott, I. Theobald, G. S. Hanan
SHORT COMMUNICATION
3.46, N 15.09. 3c·4H2O: calcd: C 38.54 H 3.40; N 11.83, found
C 38.55, H 2.78, N 11.81.
[13] J. Prasad, N. C. Peterson, Inorg. Chem. 1971, 10, 88.
[14] E. C. Constable, C. E. Housecroft, T. Kulke, C. Lazzarini,
E. R. Schofield, Y. Zimmermann, J. Chem. Soc., Dalton Trans.
2001, 2864.
[15] Orange crystals of 2c suitable for X-ray analysis could be
grown by slow diffusion of isopropyl ether into a solution of
the complex in acetonitrile. Crystal data for 2c:
C38H24CoN10·2(PF6) was collected with a Bruker APEX at
100 K using Cu-Kα radiation (λ = 1.54178 Å). Full-matrix, le-
ast-squares refinements on F2 using all data. 3198 reflections,
M = 1129.36, monoclinic, space group C2/c, a = 14.7467(4), b
= 20.8588(6), c = 16.6451(5) Å, α = 90°, β = 96.2640(10)°, γ =
90°, V = 5089.4(3) Å3, Z = 4, R1 [I Ͼ 2σ(I)] = 4.28, wR2 =
8.36. CCDC-252138 contains the supplementary crystallo-
graphic data for this paper. These data can be obtained free of
charge from The Cambridge Crystallographic Data Centre via
otherwww.ccdc.cam.ac.uk/data_request/cif.
[16] T. Yutaka, I. Mori, M. Kurihara, N. Tamai, H. Nishihara, In-
org. Chem. 2003, 42, 6306.
[17] B. N. Figgis, E. S. Kucharski, S. Mitra, B. W. Skelton, A. H.
White, Aust. J. Chem. 1990, 43, 1269.
[18] N. W. Alcock, P. R. Barker, J. M. Haider, M. J. Hannon, C. L.
Painting, Z. Pikramenou, E. A. Plummer, K. Rissanen, P. Saar-
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Received: October 06, 2004
Published Online: February 22, 2005
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