A Ni-2,2′-bisdipyrrinato complex as a potential sensitizer: Synthesis and photoelectrochemical characterization

Article abstract of DOI:10.1039/c7nj03372a

A novel Ni-based 2,2′-bisdipyrrinato complex bearing peripheral benzoic acid groups (Ni-bisdpmCOOH) has been synthesized and characterized. Electrochemical investigations revealed a high energy LUMO level that, combined with its absorption properties extending up to the NIR region, is compatible with its use as a sensitizer in thermodynamically uphill photocatalytic reactions, such as proton and carbon dioxide photoreduction. A sensitized photo-anode was prepared by adsorbing Ni-bisdpmCOOH onto a TiO₂ film, which showed an incident photon to current efficiency, measured at 0.62 V vs. RHE, extending up to the visible light region. This demonstrates that electronically excited Ni-bisdpmCOOH is effectively able to inject electrons into the conduction band of TiO₂, thus opening the way for effective TiO₂ sensitization or MOF implementation of the molecule itself for energetic applications.

Full text of DOI:10.1039/c7nj03372a

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ISSN 1144-0546
PAPER
Jason B. Benedict et al.
The role of atropisomers on the photo-reactivity and fatigue of
diarylethene-based metal–organic frameworks
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A Ni-2,2’-bisdipyrrinato complex as a potential sensitizer:
synthesis and photoelectrochemical characterization
Alessandro Poma,a Ivan Grigioni,a Maria Vittoria Dozzi,a Stéphane A. Baudron,b Lucia Carlucci,a Mir
Wais Hosseinib and Elena Sellia*
Received 00th January 20xx,
Accepted 00th January 20xx
DOI: 10.1039/x0xx00000x
www.rsc.org/
A novel Ni-based 2,2’-bisdipyrrinato complex bearing peripheral benzoic acid groups (Ni-bisdpmCOOH) has been
synthesized and characterized. Electrochemical investigations revealed a high energy LUMO level that, combined with its
absorption properties extending up to the NIR region, are compatible with its use as sensitizer in thermodynamically uphill
photocatalytic reactions, such as proton and carbon dioxide photoreduction. A sensitized photo-anode was prepared by
adsorbing Ni-bisdpmCOOH onto a TiO2 film, which showed an incident photon to current efficiency, measured at 0.61 V vs
RHE, extending up to the visible light region. This demonstrates that electronically excited Ni-bisdpmCOOH is effectively
able to inject electrons into the conduction band of TiO2, thus opening the way to effective TiO2 sensitization or MOF
implementation of the molecule itself for energetic applications.
into more complex architectures and materials has, to the best of
our knowledge, not been achieved.
Introduction
In this contribution, we report on the synthesis of a novel Ni-
based 2,2’-bisdipyrrin complex Ni-bisdpmCOOH (Scheme 1) bearing
While the most studied class of dyes based on the dipyrrin
backbone is that of boron-dipyrromethene (BODIPY) characterized
by high chemical and photochemical stability,1 several examples of
luminescent dipyrrin metal complexes have also been recently
reported,2 e.g. dipyrrin complexes containing Rh(III),3 Cu(I),4 Pt(II),5
Ir(III)6 and Zn(II).7 For these systems, the rigidity of the bispyrrolic
backbone as well as the steric hindrance of the peripheral aryl
groups have been shown to affect their photophysical properties
such as the fluorescence quantum yield and the excited state
lifetime.2,6,7 The majority of these compounds is based on ligands
with a single dipyrrin moiety functionalized by various groups at the
pyrrolic and/or at the meso positions.
peripheral benzoic groups allowing potential grafting on metal
oxide semiconductor surfaces, such as TiO2. Ni-bisdpmCOOH
displays absorption properties extending up to the near infrared
(NIR) region of the spectrum. The photophysical and
electrochemical characterization of this complex evidenced
a
suitable positioning of its LUMO energy level with respect to the
H+/H2 redox couple, which makes it a potential sensitizer for the
hydrogen evolution reaction.
Experimental details
2,2’-bis-dipyrrins comprising two dipyrrin fragments connected Syntheses
by a single C-C bond can be regarded as an acyclic analogue of
All commercial chemicals were used as received from the suppliers
corroles.8 Even though this class of molecules has been described
long ago,9 so far its use has been mostly limited to the preparation
of helicate-type complexes.10 Although these systems have
interesting photophysical and electrochemical properties with
potential applications in several fields, a systematic and detailed
investigation of such features is still lacking, with only few reports
published so far.11 Furthermore, the introduction of such species
(Sigma-Aldrich, TCI Europe and Fluorochem) without further
purification. Solvents were dried prior to use. The Ni-dpmCOOMe
and Ni-bisdpmCOOMe complexes (see Scheme 1) and the free
bisdpmCOOH ligand were prepared according to already reported
procedures.10g,12
Complex Ni-bisdpmCOOH: Ni-bisdpmCOOMe (100.0 mg, 0.164
mmol) was dissolved in a mixture of THF, NaOH 1.0 M and H2O
(3/1/1, 25 mL total volume) and refluxed overnight. After cooling to
room temperature (RT), the organic solvent was removed under
reduced pressure and the aqueous phase was acidified by addition
of a 1.0 M HCl solution down to pH 6.5. This caused the
precipitation of Ni-bisdpmCOOH as a black powder which was
recovered by filtration and the solid was washed with water and
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finally dried in air (69.6 mg, 72.5%). 1H-NMR (300 MHz, DMSO-d6) δ:
The TiO2 – covered FTO photoelectrodes modified with NiTCPP
8.09 (d, J = 8.1 Hz, 5H), 7.69 (d, J = 8.1 Hz, 5H), 6.83 (d, J = 4.2 Hz, was obtained following an identical procedure, using a 0.5 mM
2H), 6.74 (d, J = 4.4 Hz, 2H), 6.71 (d, J = 4.2 Hz, 2H), 6.56 (d, J = 4.2 solution of NiTCPP in DMF.
Hz, 2H), 6.02 (s, 2H).13 ESI-MS: m/z = 581.37 [M–H+].
[5,10,15,20-tetrakis(4-carboxyphenyl)porphyrinato]-Ni(II)
(NiTCPP): Step 1 consisted in the synthesis of [5,10,15,20-tetrakis(4-
methoxycarboxyphenyl)porphyrin] (TCPPCOOMe). Pyrrole (3.0 mL,
0.043 mol) and methyl 4-formylbenzoate (6.9 g, 0.042 mol) were
added to propionic acid (100 mL) in a 500 mL 3-necked flask and the
solution was refluxed overnight in the dark. After cooling to RT,
TCPPCOOMe precipitated as a microcrystalline dark powder. This
solid was collected by vacuum filtration, first washed with propionic
acid, then with water and finally dried in an oven at 80 °C. Yield =
Structural and photo(electro)chemical characterization
Nuclear Magnetic Resonance (NMR) data were collected on a
Bruker AVANCE 300 instrument (300 MHz) at RT. The chemical shift
and coupling constant values are given in ppm and Hz, respectively.
ESI-MS analyses were performed using a LCQ Fleet ion trap
mass spectrometer with an electrospray ionisation source and an
‘Ion Trap’ mass analyser. The sample was solubilized in methanol
and traces of DMF were added in order to promote its solubility.
The MS spectra were obtained by direct infusion of the sample
solution under ionisation, ESI negative mode. Full-scan mass spectra
were recorded in the 50–2000 mass/charge (m/z) range.
1
2.5 g (17.5%). H-NMR (300 MHz, CDCl3) δ: 8.82 (s, 8H), 8.45 (d, J =
8.3 Hz, 8H), 8.29 (d, J = 8.25 Hz, 8H), 4.11 (s, 12H), -2.81 (bs, 2H).
For the synthesis of [5,10,15,20-tetrakis(4-methoxycarbonyl
phenyl) porphyrinato]-Ni(II) (NiTCPPCOOMe) (step 2), a solution of
UV-Vis-NIR studies were performed on a Perkin-Elmer Lambda
650 S spectrometer with a 2 nm resolution and a 200 nm min-1
scanning rate. UV-Vis-NIR reflectance was measured on a Perkin–
Elmer Lambda 950 spectrometer (spectra recorded in the reflection
mode, using a 150 mm integrating sphere) with a 2 nm resolution
and a 200 nm min-1 scanning rate.
.
TPPCOOMe (500.0 mg, 0.59 mmol) and NiCl2 6H2O (1.80 g, 7.57
mmol) in 60 mL of DMF was refluxed for 5 h. After cooling to RT, the
addition of ca. 150 mL of water caused the precipitation of
NiTCPPCOOMe as a dark powder. The solid was filtered, and
subsequently washed with water and with 1.0 M HCl until the
filtrate became colourless. The crystalline powder thus obtained
(quantitative yield) was dried in an oven at 80 °C and directly used,
without further purification, in the final step 3, yielding [5,10,15,20-
tetrakis(4-carboxyphenyl)porphyrinato]-Ni(II) (NiTCPP).
Cyclic voltammetry was performed in a conventional three-
electrode cell using an Autolab PGSTAT 12 potentiostat (EcoChemie,
Utrecht, The Netherlands), controlled by the NOVA (version 2.1)
software. The working electrode was a glassy carbon electrode with
a 0.071 cm2 surface area, a Pt wire was the counter electrode, with
Ag/AgCl as the reference electrode.
In step 3, NiTPPCOOMe (500 mg) was stirred in a mixture of THF
and methanol (25 mL each), and a solution of NaOH (2.4 g, 60
mmol) in water (25 mL) was added. This mixture was refluxed
overnight. After cooling to RT, THF and methanol were removed
under vacuum and the volume was restored by addition of water.
The mixture was refluxed for ca. 30 min in order to obtain a
homogeneous solution, which was acidified with 1.0 M HCl up to
complete precipitation. The solid was then filtered, washed with
water until the filtrate became colourless and dried in an oven at 80
°C.
The observed peak potential (Ep) was expressed with respect to
the Fc+|Fc redox couple by subtracting the Fc+|Fc half-height peak
value, calculated as Ep/2(Fc+|Fc) = (Ep,a(Fc+|Fc) + Ep,c(Fc+|Fc))/2, with Ep,a
and Ep,c referring to the anodic and cathodic peaks, respectively.
Moreover, Ep values were converted into the NHE scale by using the
following equation: ENHE = (Ep - Ep/2(Fc+|Fc)) + E°Fc+|Fc - E°H+/H2 , where
E°Fc+|Fc is the potential value of the Fc+|Fc couple (4.8 V) and E°H+/H2
is the potential of the H+/H2 couple (4.44 V), both referred to
vacuum.
Photoelectrochemical measurements were performed using a
home-made 3-arms cell and the same potentiostat employed for
Photoelectrode preparation
The photoanodes were prepared by depositing a commercial TiO2 cyclic voltammetry analysis, with the photoelectrode used as
paste (Ti-Nanoxide T/SP, Solaronix) on a fluorine tin oxide (FTO) working electrode (0.25 cm2 active surface), a Pt gauze as counter
electrode through the so called doctor-blade technique, followed by electrode and an Ag/AgCl (3 M NaCl) as reference electrode. The
adsorption of Ni-bisdpmCOOH through electrostatic interaction photoelectrodes were tested under back side illumination (through
between the carboxylic groups of the Ni complex and the -OH units the FTO/TiO2/Ni-bisdpmCOOH interface). The light source was an
on the TiO2 surface. In detail, the FTO electrode was covered with a Oriel, Model 81172 Solar Simulator with different cut-off filters (420
scotch tape (3M), thus obtaining a 0.25 cm2 exposed surface. A and 550 nm). The power intensity of the unfiltered AM 1.5G light,
small amount of the TiO2 paste was spread onto the surface with measured by means of a Thorlabs PM200 power meter equipped
the aid of a microscope slide. The scotch tape was then removed with a S130VC power head with Si detector, was 100 W cm−2. A 0.5
and the electrodes were kept at 70 °C for 1 h and finally annealed at M Na2SO3 aqueous solution buffered at pH 7 with potassium
500 °C for 1 h. A few drops of a Ni-bisdpmCOOH solution in DMF phosphate was used in photoelectrochemical measurements. The
(concentration ca. 10-3 M) were deposited on the so-obtained TiO2 potential vs. Ag/AgCl was converted into the RHE scale using the
– covered FTO electrodes and left to dry in air. The non-adsorbed following equation: ERHE = EAg/AgCl + 0.059 pH + E°Ag/AgCl, with
dye was removed by washing with DMF and the absorption E°Ag/AgCl(3.0 M NaCl) = 0.210 V at 25 °C.
spectrum of the clean electrode was recorded. This procedure was
Incident photon-to-current efficiency (IPCE) measurements
repeated until no further increase in absorbance was detected. A were carried out using a 300 W Lot-Oriel Xe lamp equipped with a
total of 10 cycles were performed to attain the final, highest Lot-Oriel Omni-λ 150 monochromator and a Thorlabs SC10
absorbance.
automatic shutter. A 0.62 V bias vs. RHE was applied and the
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current was measured with a 20 nm step, within the 400 to 900 nm bisdpmCOOMe complex. On the contrary, Ni-dpmCOOMe shows an
wavelength range. The incident light power was measured at each absorption edge at 570 nm with a strong absorption band peaking
wavelength using the above-mentioned calibrated Thorlabs at 464 nm and shoulders at ca. 440 and 500 nm. The different
photodiode and power meter. The IPCE was calculated at each absorption properties of Ni-bisdpmCOOH and Ni-bisdpmCOOMe
wavelength by the following equation:
complexes with respect to Ni-dpmCOOMe are clearly related to the
increased conjugation and molecular rigidity ensured by the 2,2’-
pyrrolic C-C bond. This difference is also evidenced by a comparison
between the Ni coordination geometry in the two reported crystal
structures.10g In fact, while Ni-dpmCOOMe shows a distorted
tetrahedral geometry with an angle of ca 56° between the two
bipyrrolic moieties, in Ni-bisdpmCOOMe the same angle is ca. 23°,
indicating a distorted square planar geometry.10g
ꢅꢆꢇꢈ ∙ ꢉ
ꢀꢁꢂꢃ ꢄ
∙ ꢅꢈꢈ
ꢁꢊ ∙ ꢊ
where j is the photocurrent density (mA cm-2) and Pλ (nW cm-2) is
the power of the monochromatic light at wavelength (nm).
λ
The internal quantum efficiency (IQE) at each wavelength was
calculated by combining the IPCE spectrum with the absorption (A)
spectrum of the photoanode:
ꢀꢁꢂꢃ
ꢀꢋꢃ ꢄ
ꢅ ꢌ ꢅꢈꢍꢎ
X-ray powder diffraction (XRPD) was measured on a BRUKER D8-
Focus Bragg-Brentano X-ray Powder Diffractometer equipped with
a Cu sealed tube (λ = 1.54178 Å) at 40 kV and 40 mA.
Results and discussion
Synthesis of Ni-bisdpmCOOH
The synthetic route to Ni-bisdpmCOOH is depicted in Scheme 1. The
saponification of the Ni-bisdpmCOOMe complex was carried out as
described by Brückner et al.,14 with a modification concerning the
acidification step; indeed the acid was added until a pH value of ca.
6 was reached. Because of the poor solubility of the Ni-
bisdpmCOOH complex, the 13C-NMR spectrum could not be
recorded. The successful saponification of the ester groups was
however confirmed by the absence of the methyl group signals in
the upfield region of the 1H-NMR spectrum of complex Ni-
bisdpmCOOH (see Fig. S1 in the ESI). On the other hand, evidence of
the retention of Ni in the coordinating environment of the molecule
was provided by mass spectrometry analysis, which showed a peak
at m/z = 581.37, corresponding to the desired [M–H+] structure.
Fig. 1 Absorption spectrum of Ni-dpmCOOMe (green line), Ni-bisdpmCOOMe (red line)
and Ni-bisdpmCOOH (black line) in DMF.
Electrochemical Characterization
The redox properties of the Ni-bisdpmCOOH complex were
determined by cyclic voltammetry measurements in a DMF solution
(3
×
10-4 M) containing 0.1 M tetra-N-butylammonium hexafluoro
phosphate (TBAPF6) as supporting electrolyte under an inert
atmosphere, obtained by bubbling N2 in the solution for 10 min.
The cyclic voltammetry curves obtained at different potential
scanning rate are shown in Fig. 2.
The shape of the curve in the anodic part (positive potentials)
evidences an irreversible bi-electronic oxidation process at 0.382 V
vs. Fc+|Fc for the Ni-bisdipyrrin metalloligand. Moreover, the shift
of the anodic oxidation potential with the scan rate implies an
electrochemically irreversible electron transfer. Differently, in the
cathodic region (negative potentials), the complex displays a
chemically and electrochemically reversible one-electron reduction
at -1.21 V vs. Fc+|Fc. These electrochemical processes are assigned
to ligand-centred electron transfer processes, by analogy with the
redox properties already demonstrated for other similar Ni-
bisdipyrrin complexes.11a Furthermore, with respect to a similar
substituted Ni bisdipyrrin complex investigated by Bröring et al.,11b
containing fully alkylated pyrrolic fragments, Ni-bisdpmCOOH
displays a significant shift of both cathodic and anodic peaks
towards more positive potentials. This effect can be related to the
electron depletion induced by the absence of alkyl groups on the
Scheme 1 Synthesis of the 2,2’-bisdipyrrin Ni(II) metalloligand.
Optical properties
The absorption spectrum of Ni-bisdpmCOOH, recorded in DMF
(concentration of 2.92 ∙ 10-5 M) is shown in Fig. 1, together with the
absorption spectra of the Ni-dpmCOOMe and Ni-bisdpmCOOMe
complexes, for comparison. Ni-bisdpmCOOH displays a maximum
absorption peak at 416 nm, accompanied by less intense and
broader peaks at 563, 770 and 846 nm. This absorption spectrum is
similar to those previously reported for 2,2’-bisdipyrrin based
species10,11 and is comparable to that of the starting Ni-
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bisdipyrrin core,11b which usually behave as electron-donating
groups.
Fig. 3 HOMO and LUMO energy levels of Ni-bisdpmCOOH and NiTCPP.
Photoelectrochemical characterization
Fig. 2 Anodic (positive potentials) and cathodic (negative potentials) scans of Ni- The UV-Vis-NIR absorption spectra of TiO2-based electrodes with
bisdpmCOOH around the first redox potentials, performed at different potential adsorbed Ni-bisdpmCOOH molecules are shown in Fig. 4. Each
scanning rates.
absorption profile is the difference between the spectrum of the
electrode containing the adsorbed dye and that of the electrode
containing only TiO2.
On the basis of the oxidation and reduction peaks shown in Fig.
A comparison between these spectra and that reported in Fig. 1
indicates that bisdpmCOOH adsorbed on TiO2 has an absorption
spectrum almost identical to that of the free molecule in solution.
The slight blue-shift of the main peak with a maximum at 416 nm
could be a consequence of the subtracting procedure of the bare
TiO2 spectrum, having a residual absorption tail up to 450 nm (Fig.
S3).
2, the HOMO and the LUMO energy levels of Ni-bisdpmCOOH are
located at 0.74 and -0.85 V vs. NHE, respectively, with a HOMO-
LUMO gap of 1.59 eV, which is slightly lower than the 1.62 eV
calculated for the fully alkylated dipyrrin analogue.11b This
difference can again be ascribed to the presence of alkyl chains,
which make the bisdipyrrin complex investigated by Bröring et al.11b
more electron rich, but less planar and conjugated than Ni-
bisdpmCOOH. On the other hand, the presence of electron
attracting carboxyl substituents on the phenyl rings of Ni-
bisdpmCOOH should not affect its electronic properties, the phenyl
groups being twisted with respect to the bisdipyrrin core.10g
The cyclic voltammetry curves of nickel tetracarboxyphenyl
porphyrin (NiTCPP), which was expressly synthesised in the present
work to compare its properties with those of Ni-bisdpmCOOH,
displays, in the anodic part, a chemically reversible (at high scan
rates) and electrochemically irreversible oxidation peak at 0.66 V vs.
Fc+|Fc. In the cathodic part, NiTCPP shows a chemically reversible
and electrochemically quasi-reversible reduction peak at -1.60 V vs.
Fc+|Fc (see Fig. S2). The HOMO and LUMO energy levels are at 1.03
and -1.24 V vs. NHE, respectively (Fig. 3).
Interestingly, the LUMO position of both Ni-bisdpmCOOH and
NiTCPP are higher in energy with respect to both the TiO2
conduction band (CB) position (i.e. -0.16 V vs. NHE)15 and the H+/H2
reduction potential.16 Therefore, according to the relative energy
level positions reported in Fig. 3, the electron injection from the
LUMO of both photoexcited complexes into the TiO2 CB is allowed,
which makes these systems potentially able to reduce protons from
water into hydrogen under visible light irradiation.
Fig.
4
UV-vis absorption spectra of Ni-bisdpmCOOH adsorbed on a TiO2-covered
electrode, recorded after each adsorption cycle. The absorption spectrum of bare TiO2
was always subtracted.
The photoelectrochemical (PEC) performance of the modified
electrode was explored by means of different photoelectrochemical
analyses. Fig. 5a shows the linear sweep voltammetry curves
recorded with the Ni-bisdpmCOOH on TiO2 electrode in the 0.22 to
0.62 V vs. RHE potential range, demonstrating the onset of
photocurrent upon illumination with simulated solar irradiation
with the 420 nm cut-off filter. The chronoamperometric curves
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a)
recorded at 0.62 V vs. RHE shown in Fig. 5b clearly indicate the
presence of photocurrent for the Ni-bisdpmCOOH – sensitized
photoanode also when employing the 550 nm cut-off filter, while
no photocurrent was detected with a bare TiO2 electrode even with
the 420 nm filter. By comparison, Fig. 5b also shows that when a
NiTCPP – sensitized TiO2 electrode was employed as photoanode
under the same conditions, a negligible photocurrent response was
detected with the 420 nm cut-off filter, although this molecule
absorbs in this spectral region, due to the intense Soret band and
the single Q band centred at 532 nm (see Fig. S4). The molecular
structure and the adsorption conditions of the sensitizer may have
a large impact on molecular packing, geometry and aggregation of
porphyrin molecules onto the TiO2 surface, eventually affecting the
photocurrent response.17 Giving this, the photocurrent response of
the Ni-bisdipyrrin molecule could be further increased by b)
optimization of the adsorption conditions, thus achieving higher
output values.
The IPCE curve shown in Fig. 5c clearly demonstrates that Ni-
bisdpmCOOH adsorbed on TiO2 is a rather good sensitizer and that
electron injection from photoexcited Ni-bisdpmCOOH to the TiO2
conduction band may efficiently occur up to 620 nm. Moreover, the
IQE curve is similar to the IPCE curve, due to the relatively high
absorbance of Ni-bisdpmCOOH in the prepared photoanodes (see
Fig. 4).
A possible way to avoid the dependence of the photocurrent
performance on the adsorption conditions is to implement the Ni-
bisdipyrrin molecule into a robust Metal Organic Framework (MOF)
by taking advantage of its functionalization by peripheral carboxylic
acid groups. In this way, the increased rigidity of the whole system
c)
could reduce the excited state distortion of the molecule, thus
increasing the separation between the photogenerated electron-
hole couples and the electron injection ability of the molecule, in a
way similar to that observed with other doped MOF structures.18
Conclusions
In conclusion, we have investigated in detail the photophysical and
electrochemical properties of Ni-bisdpmCOOH as a representative
example of an intriguing class of bisdipyrrin complexes and
demonstrate its effective ability to act as a rather good sensitizer of
TiO2-based photoanodes, enabling photocurrent production up to
620 nm irradiation. If successful doping of stable MOF with this
visible light sensitizer can be achieved, one may get a robust system
for solar-into-chemical energy conversion applications. Research in
this direction is currently under way in our laboratories.
Fig. 5 (a) Linear sweep voltammetry curves of Ni-bisdpmCOOH/TiO2 under
irradiation with the 420 nm cut off filter and in the dark; (b)
Chronoamperometric analysis of the Ni-bisdpmCOOH/TiO2 photoanode
employing the 420 nm (red thick line) and the 550 nm (black thick line) cut-off
filters and of the NiTCPP/TiO2-based photoanode with the 420 nm cut-off filter
(thin line); (c) IPCE and IQE curves obtained with the Ni-bisdpmCOOH/TiO2
electrode.
Conflicts of interest
Authors declare no conflict of interests
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Nishimura, R. Akuta, K. Takaishi, M. Uchiyama and A.
Muranaka, Chem. Sci., 2013, , 1204. (f) H. Ruffin, S. A.
Acknowledgements
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The contribution of the Regione Lombardia – Fondazione
Cariplo (Fondazione Cariplo project 2013-1766) is gratefully
acknowledged. We thank the Doctorate School in Chemistry of
the University of Milano, the Université de Strasbourg, the
C.N.R.S, the International centre for Frontier Research in
Chemistry (icFRC), the Labex CSC (ANR-10-LABX- 0026 CSC)
within the Investissement d’Avenir program ANR-10-IDEX-
0002-02, the Institut Universitaire de France, the Ministère de
l’Enseignement Supérieur et de la Recherche for financial
support.
12 M. Artigau, C. Bonnet, S. Ladeira, P. Hoffmann and A.
Vigroux, CrystEngComm., 2011, 13, 7149.
13 Due to solubility issues, the proton integration in the 1H NMR
spectrum is hampered.
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