Intriguing C-H?Cu interactions in bis-(phenanthroline)Cu(i) redox mediators for dye-sensitized solar cells

Article abstract of DOI:10.1039/c7dt04045h

We have synthesized and characterized a series of bis-(phenanthroline)Cu(i) complexes of interest as redox mediators for dye-sensitized solar cells. This study led to the discovery of intriguing anagostic interactions between the hydrogen atom and the copper center as evidenced by X-ray diffraction studies on a single crystal. Remarkably, an anagostic interaction was found between a H atom of a methyl group and a copper site.

Full text of DOI:10.1039/c7dt04045h

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Intriguing C–H···Cu interactions in bis-(phenanthroline)Cu(I) redox
mediators for dye-sensitized solar cells
Received 00th January 20xx,
Accepted 00th January 20xx
Alessia Colombo,a,b Rachele Ossola,c Mirko Magni,a Dominique Roberto,a,b Denis Jacquemin,d,e
Carlo Castellano,a Francesco Demartina * Claudia Dragonetti a,b,
*
DOI: 10.1039/x0xx00000x
www.rsc.org/
We have synthesized and characterized
a series of bis-
(phenanthroline)Cu(I) complexes of interest as redox mediators
for dye-sensitized solar cells. This study led to the discovery of
intriguing anagostic interactions between the hydrogen atom and
the copper center as evidenced by X-ray diffraction studies on
single crystal. Remarkably, an anagostic interaction was found
between a H atom of a methyl group and a copper site.
1
2
3
An important challenge of humanity is to replace fossil fuels with
renewable energy sources to answer the growing worldwide energy
demand, minimizing negative environmental and climate effects.
Indeed, sunlight remains the cleanest, the most abundant and the
cheapest energy source. 1 Dye-sensitized solar cells (DSSCs)2–5 are a
realistic solution for harnessing the sun energy and converting it
into electrical energy. 6
4
5
Fig.1. Chemical structures of the investigated Cu complexes.
The use of copper complexes with a distorted tetrahedral
geometryin which the structural change between the Cu(I) and
Cu(II) complexes is minimized, provides a promising strategy to
electron mediators for DSSCs, allowing a very fast electron
transfer and a redox potential of the Cu(I)/Cu(II) couple high
enough to increase open-circuit voltage (Voc) values.9-11
−
The traditional redox mediator in DSSCs is the I−/I3 couple as it
presents ideal kinetic properties. However it has some
disadvantages, e.g., I2 in equilibrium with I3− is volatile, complicating
−
long-term cell sealing, I3 is darkly colored and limits the light
harvesting efficiency of the dye, the couple presents large
photovoltage loss due to the non-optimal matching with the dye
redox potential, and I3−/I− is corrosive and corrodes most metals.7
Therefore, during the recent years, there has been numerous
efforts to search for new electron transfer mediators, in particular
based on transition metal complexes.7 In this context, copper
complexes are particularly appealing, as they stand as promising
efficient and cheap electron mediators for solar cells.8
In this framework, we have synthesized a series of different copper
complexes (1-5)10-12 (see Fig.1, all with PF6 counterion), with
-
phenanthrolines that bear, in
α position, a substituent with
different steric hindrance, in order to clarify its contribution in
achieving simultaneously: (i) an appropriate redox potential; (ii) a
distorted tetrahedral geometry; and (iii) an effective shield of the
metal core.
We have synthesized the proper phenanthroline ligand and the
corresponding Cu(I) complex, with simple and fast reactions
(Scheme 1). Ligands were prepared thanks to a reaction developed
by Sauvage 13 as an effective and quick way to obtain 2-substituted
and
2,9-disubstituted-1,10-phenanthrolines
starting
from
commercially available 1,10-phenanthroline and organolithium
reagents.14
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1
2
1 unit
3
4
5
[Cu(phe)2]+
-1.0
-0.5
0.0
0.5
1.0
E vs Fc+|Fc /V
Fig. 2 Cyclovoltammetric, CV, patterns of Cu-based complexes 1-5, in acetonitrile with
0.1 M TBAPF6 on glassy carbon electrode at 0.2 Vs-1
.
Table 1. Oxidation half-wave potential, E1/2, in acetonitrile.
Complex
E1/2 vs Fc+|Fc /V
1
2
3
4
5
Scheme 1. Synthesis of ligands and complexes.
-0.02
0.09
0.34
0.10
0.50
[Cu(1,10-phenanthroline)2][PF6] shows E1/2 = -0.38 V
The reaction is a two-step addition-oxidation. The 2,9-disubstituted
derivative is obtained using an excess of organolithium reagent in
the first stage. In the second step re-aromatization is achieved
treating the intermediate with activated manganese dioxide. In
more details, copper complexes were prepared by reaction of the
appropriate ligand with the stoichiometric amount of
[Cu(CH3CN)4][PF6] (2:1) in the minimal amount of dichloromethane
in order to favor the formation of the product that in some cases
directly precipitates from the reaction environment in a pure form,
avoiding the subsequent crystallization step. The starting white salt
[Cu(CH3CN)4][PF6] was synthesized from Cu2O according to
literature15 (Scheme 1).
Table 2. Visible absorption spectra of 1-5 complexes in acetonitrile.
Complex
λ (nm)
103 ε (M-1cm-1)
1
2
3
445
455
4.4
4.6
356
426
2.1
2.2
4
5
437
470
4.6
4.5
The knowledge of metal complexes potentials is crucial to ascertain
their applicability as redox mediators in DSSCs. Therefore the
electrochemical properties of the copper coordination compounds
were studied by cyclic voltammetry in acetonitrile, focusing on the
electron transfer process centered on the metal core (see Fig. 2). In
all complexes the anodic window showed one chemically reversible
peak attributed to a Cu(I) to Cu(II) redox switching; a quite high
In agreement with
a
previous work,10 complex
5
with 2,9-
disubtituted phenanthrolines exhibits E1/2 positively shifted with
respect to the 2-substituted analogue (complex 2) due to an
increased steric hindrance around the copper atom that
progressively destabilizes the electrogenerated Cu(II) product that
is forced to maintain a geometry similar to the preferred Cu(I) one.
Remarkably, all 2-substituted complexes (3 being borderline) have
redox potentials suitable for energetic application as electron
mediators in DSSCs, being thermodynamically able to regenerate
many common photosensitizers with oxidation potentials higher
than 0.30-0.35 V vs Fc+|Fc.
Electronic absorption spectra of the investigated copper complexes
were recorded in acetonitrile (see Table 2 and ESI). They follow the
pattern reported in literature, with a MLCT band between 426 and
470 nm,18 associated with charge transfer from Cu(dπ) to
phenanthroline (π*) orbitals. The extinction coefficients of the
MLCT band decrease with increasing steric bulk at position 2, with
compound 3 possessing the weakest visible absorption (2200 M−1
cm−1 ). To verify the influence of various substituents in α position
of the phenanthrolines on the distorted tetrahedral geometry of
the corresponding complexes, we have prepared crystals of
complexes 2-4 suitable for X-ray characterization using a simple
double layer procedure (slow diffusion in CH3CN/hexane).‡ The
structures are displayed in Figs. 3 to 6, for compounds 1 to 4. The
structure of 5 has also been determined but is not detailed here
because it is already known.12a Unexpectedly, the synthesized
complexes show a “peculiar” interaction between a hydrogen of a
C-H bond (of the CH3 or of a CH of phenanthroline or of a tolyl ring
ring) and the copper site, as shown in Table 3.
electrochemical reversibility, i.e.
a small potential difference
between forward and backward peaks, indicative of a fast electron
transfer occurs, is found in all the compounds indicating a limited
rearrangement of the coordination sphere. The determination of
the heterogeneous electron transfer rate constant, kheter, can be a
valuable parameter to quantitatively assess the impact of the steric
bulkiness of α-substituents on the self-exchange rate.16 For the sake
of comparison bulky tert-butyl chains in compound 3 induce a ca.
20 times faster heterogeneous electron transfer toward glassy
carbon electrode surface with respect to the unsubstituted
bis(1,10-phenanthroline)copper complex; kheter, being 9∙10–3 and
5∙10–4 cm s–1 respectively, with and without the bulky substituents,
respectively. These values were estimated from charge transfer
resistance, Rct, by electrochemical impedance spectroscopy
according to the following relationship:17
RT
F2RctC
kheter
=
where R is the gas constant, T the absolute temperature, F the
Faraday constant and C the concentration of the copper complex (in
mol cm–3).
The Cu(I)/Cu(II) half-wave potentials, E1/2, are reported in Table 1.
Electronic and steric effects of the substituents finely control the
oxidation potentials.
2 | J. Name., 2012, 00, 1-3
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Table 3. Shortest Cu∙∙∙H distances in Å in the synthesized complexes
Complexa
Shortest Cu…H distances (Å)
H belongs to:
1 (ref 11)
3.18
3.05
CH3 of mesityl ring
CH3 of tolyl ring
CH3 of tertbutyl
2
3
4
2.49; 2.53
3.11;(3.27)
CH of phenanthroline
(CH3 of phenyl ring)
CH of tolyl ring
5 (ref. 12a)
2.72; 2.86
a In the complex [Cu(1,10-phen)2][PF6]19 the shortest Cu…H distance is 3.203 Å.
These interactions awoke our curiosity because in the literature
there is some controversy regarding the agostic and anagostic
interaction in copper complexes.20 The strength, nonpolar nature,
and low polarizability of C-H bonds make that they are generally
considered as chemically inert. However, significant interactions
between C-H bonds of σ-bound alkyl groups and metal atoms can
occur.18,21 The origin of the agostic interaction lies in the
identification in the 1960s of hydridic H atoms in transition metal
complexes by Trofimenko.22 The term agostic was proposed in 1983
by Brookhart and Green in their seminal contribution23 for a case in
which a C–H group interacts with a transition metal with the
formation of a two-electron three-center bond. In that case, the
metal centre behaves like a Lewis acid. The term anagostic was
coined in 1990 by Lippard and co-workers to distinguish sterically
enforced M∙∙∙H-C contacts (M=Pd, Pt) in square-planar transition
metal d8 complexes from attractive agostic interactions.24 In this
case, the availability of an empty orbital is not required. Today the
theme of agostic and anagostic interactions remains a hot topic, as
evidenced, for example, in a recent paper on anagostic interactions
under pressure.25 It is evident that the assignment of an agostic or
anagostic interaction requires caution.26 In the case of an ephedrine
copper complex, for example, an interaction initially described as
agostic27 was next disputed by other authors. 20
Fig.3 Structure of the cation in (1) (see ref. 11)
Fig.4 Structure of the cation in (2)
In the investigated complexes the Cu∙∙∙H lengths, superior to 2.3 Å,28
preclude any agostic bonds; in fact as far as bond lengths are
concerned agostic interactions are characterized by relatively short
M–H distances (1.8–2.3 Å) whereas anagostic interactions are
characterized by relatively longer M-H distances (2.3-2.9 Å).The
shortest bonds between a H atom of a methyl group (complex 3) or
of a CH in ortho position of tolyl group (complex 5) and copper can
be consequently attributed to anagostic interactions. To our
knowledge, this is the first time that an anagostic interaction is
found between a H atom of a methyl group and a copper site, as
happened for complex 3.
Fig.5 Structure of the cation in (3)
DFT calculations were performed on all cations (see the ESI for
technical details) to verify that the existence of “short” C–H•••Cu
distances is not induced by crystal packing effects. We have found
the following distances: for complex 1, 2.74 Å (between Cu and the
H of the CH3) for complex 2, 2.60 Å (between Cu and H of CH3 of
tolyl ring); for complex 3, 2.37 and 2.69 Å (between Cu and the H of
the CH3 of tertbutyl); for complex 4, 2.90 and 3.20 Å (respectively
between Cu and H of phenyl ring, and between Cu and H of
phenantroline); and for complex 5, 2.36 and 2.41 Å (between Cu
and the H of the tolyl ring). Concerning complex 4, different H
atoms are involved in the shortest C–H•••Cu contact: in the X-ray
structure the closest H atom belongs to the phen ligand whereas in
DFT calculations the closest H atom belongs to the phenyl group:
this difference is likely due to crystal packing effects that tune the
dihedral angle between the phenyl ring and the phenantroline. As
Fig.6 Structure of the cation in (4)
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‡ X-ray crystallographic data in CIF format for 2-4 have been
deposited with the Cambridge Crystallographic Data Centre,
CCDC 1564905-1564907.
expected, the smallest distances obtained through DFT are in
complexes 3 and 5 which is consistent with the solid-state results.
This hints that the XRD finding should pertain in other environment
than the crystals.
1
2
A.J. Nozik, J. Miller, Chem. Rev., 2010, 110, 644.
a) B. O’Regan, M. Graetzel, Nature, 1991, 353, 737; b) M.
Graetzel, Nature, 2001, 414, 338.
The anagostic interactions evidenced in the present work protect
the copper site and improve the stability of the structure by further
minimizing the change of geometry between Cu (I) and Cu (II). This
protection is of crucial importance for the design of efficient redox
couples for DSSCs. Therefore our complexes, whose potential can
be easily tuned by adequate substituents on the phenanthrolines,
have a great potential as redox couple of an electron transfer
mediator electrolyte in DSSC applications, along with a proper dye
with adequate HOMO and LUMO levels, where the HOMO level lies
under the potential of the redox couple. In this framework, we
underline that the most popular redox couple, I–/I3–, has several
limitations (see Introduction), and, in particular it presents a too
3
A. Hagfeldt, G. Boschloo, L. Sun, L. Kloo, H. Pettersson, Chem.
Rev., 2010, 110, 6595.
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Roberto, R. Ugo, A. Valore, A. Valsecchi, P. Salvatori, M.G.
Lobello, F. De Angelis, Inorg. Chem. 2013, 52, 10723.
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negative redox potential (–0.28
V
vs Fc+|Fc in acetonitrile,
converted from 0.35 V vs NHE)29 that precludes its use in DSSCs
relying on some of the most promising dyes that have a relatively
high potential. Copper complexes, and specifically complex 3, are
expected to be adequate redox mediators (along with the related 10 M. Magni, A. Colombo, C. Dragonetti, P. Mussini,
Electrochim. Acta, 2014, 141, 324.
Cu(II) complex) for DSSCs upon combination with organic dyes such
as C218 30a , LEG4 30b or Y123.30c
11 a) A. Colombo, C. Dragonetti, M. Magni, D. Roberto, F.
Demartin, S. Caramori, C.A. Bignozzi, ACS Appl. Mater.
Interfaces, 2014, 6, 13945; b) M. Magni, R. Giannuzzi, A.
It is known that copper complexes, which versatile coordination
chemistry has significant influence over their optical and redox
properties,26 are of great interest as catalysts because the use of
complexes deriving from earth-abundant elements is desirable from
the standpoint of scalability and sustainability. Besides, agostic31 or
anagostic32 interactions are often used to activate inert C-H bonds,
facilitating new reactions. For example, recently the catalytic
activity of bis(pyrazolyl)borate copper complexes toward
carbenoide insertion into N-H bonds was reported.31 In this system
weak intramolecular C-H∙∙∙Cu interactions are of great importance
and act as a switch which is turned “on” if interacting with the
substrate and “off” if eliminating the product and regenerating the
weak interaction.31 Moreover the steric hindrance of groups in the
positions 2,9 of 1,10-phenanthroline copper (I) complexes plays an
important role in the regioselective addition of CBr4 to styrene,
substituted phenyl rings leading to a better catalytic activity then
methyl groups.33 The complexes investigated in the present work
are therefore of potential interest for such catalysis studies.
Colombo, M.P. Cipolla, C. Dragonetti, S. Caramori, S. Carli,
R. Grisorio, G.P. Suranna, C.A. Bignozzi, D. Roberto, M.
Manca, Inorg. Chem. 2016, 55, 5245.
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In summary we have fully characterized variously substituted 1,10-
phenanthroline copper(I) complexes. Their crystal structure,
determined by single-crystal X-ray analysis, confirms a distorted
tetrahedral geometry and reveals interesting anagostic interactions
for complexes 3 and 5. Clearly the investigated copper complexes
are excellent candidates as electron transfer mediators for DSSCs
and are appealing for catalysis studies.
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C.D., A.C. and D.R deeply thank the bilateral project Italy-India
“Cromofori a forma di Y coniugati al ferrocene come potenziali
sensibilizzatori in celle DSSC in combinazione con innovativi
mediatori redox” (Prot. nr. MAE0104617), “Con il contributo del
Ministero degli Affari Esteri e della Cooperazione Internazionale,
Direzione Generale per la Promozione del Sistema Paese”. This work
used computational resources from the CCIPL (Centre de Calcul
Intensif des Pays de la Loire) installed in Nantes.
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Table of Contents
For the first time an anagostic interaction is found between a H atom of a methyl group and a copper site in bis-2-
tertbutyl(phenanthroline)Cu(I) complex.