Versatile Coordination Mode of a New Pyridine-Based Ditopic Ligand with Transition Metals: From Regular Pyridine to Alkyne and Alkenyl Bindings and Indolizinium Formation

Article abstract of DOI:10.1021/acs.inorgchem.5b01096

The new BPMPB ligand, namely, bis[1-bis(2-pyridylmethyl),1 (pyridyl)]butyne, can be very easily obtained as a side product in the known reaction of picolyl chloride and sodium acetylide (which major product is the known terminal alkyne-substituted tripod). This symmetrical ligand contains two identical coordination sites with two methylenepyridines and one pyridyl group on each side, linked by an alkyne function providing a semirigid segment. Together with the molecular structure of the ligand which is reported, we describe the preparation of complexes with Fe(II)Cl₂, Co(II)Cl₂, Ni(II)Cl₂, Cu(I)Cl, and Zn(II)Cl₂ salts. All complexes have been characterized by X-ray diffraction studies as well as by standard spectroscopic techniques. The striking point in this work is the diversity of the structures that are obtained. Co(II) and Zn(II) provide isostructural dinuclear complexes in which both coordination sites are occupied within a tetrahedral symmetry. The Cu(I) complex is also a dinuclear compound, but in that case, the copper atom is coordinated to the alkyne moiety, two pyridines, and a bridging chloride. The ¹³C NMR spectrum of the copper complex confirms that the metal center is coordinated to the alkyne in solution. The coordination of Ni(II) results in the formation of a mononuclear complex in which a pyridine has fused with the alkyne moiety to generate an indolizinium group; the structure of the corresponding alkenyl complex is reported. Finally, the addition of FeCl₂ to the ligand results in the formation of a mononuclear complex with a free, noncoordinated indolizinium. The sequence developed in the present work illustrates the possibility for the metal centers to adopt various coordination modes which may be relevant to the conversion of an alkyne and a pyridyl unit into indolizinium.

Full text of DOI:10.1021/acs.inorgchem.5b01096

Article
pubs.acs.org/IC
Versatile Coordination Mode of a New Pyridine-Based Ditopic Ligand
with Transition Metals: From Regular Pyridine to Alkyne and Alkenyl
Bindings and Indolizinium Formation
Sushil Kumar and Dominique Mandon*
́ ́
Laboratoire de Chimie, Electrochimie Moleculaires et Chimie Analytique, UMR 6521, CNRS−Universite de Bretagne Occidentale, 6
Avenue Victor Le Gorgeu CS 93837, F-29238 Brest cedex 3, France
S
* Supporting Information
ABSTRACT: The new BPMPB ligand, namely, bis[1-bis(2-pyridyl-
methyl),1 (pyridyl)]butyne, can be very easily obtained as a side product
in the known reaction of picolyl chloride and sodium acetylide (which
major product is the known terminal alkyne-substituted tripod). This
symmetrical ligand contains two identical coordination sites with two
methylenepyridines and one pyridyl group on each side, linked by an
alkyne function providing a semirigid segment. Together with the
molecular structure of the ligand which is reported, we describe the
preparation of complexes with Fe(II)Cl2, Co(II)Cl2, Ni(II)Cl2, Cu(I)Cl,
and Zn(II)Cl2 salts. All complexes have been characterized by X-ray
diffraction studies as well as by standard spectroscopic techniques. The
striking point in this work is the diversity of the structures that are
obtained. Co(II) and Zn(II) provide isostructural dinuclear complexes in
which both coordination sites are occupied within a tetrahedral symmetry. The Cu(I) complex is also a dinuclear compound, but
in that case, the copper atom is coordinated to the alkyne moiety, two pyridines, and a bridging chloride. The 13C NMR
spectrum of the copper complex confirms that the metal center is coordinated to the alkyne in solution. The coordination of
Ni(II) results in the formation of a mononuclear complex in which a pyridine has fused with the alkyne moiety to generate an
indolizinium group; the structure of the corresponding alkenyl complex is reported. Finally, the addition of FeCl2 to the ligand
results in the formation of a mononuclear complex with a free, noncoordinated indolizinium. The sequence developed in the
present work illustrates the possibility for the metal centers to adopt various coordination modes which may be relevant to the
conversion of an alkyne and a pyridyl unit into indolizinium.
a high flexibility at the coordination site. In these systems, a
pure pyridine-based tripod would be linked through the central
carbon atom to a fourth arm, with this latter being potentially
functionalized, and able to subsequently react with various
substrates, or reactive groups. The alkyne-substituted tripod A
in Figure 1, namely, the bis(2-pyridylmethyl)(pyridyl)propyne,
would be supposed to fulfill these requirements. Indeed, its
synthesis was reported together with the formation of
compound B, the tris(2-pyridylmethyl)pyridylmethane tetra-
pod, obtained as a minor component.14,15
Ligand A was prepared according to the procedure reported
in 1974.14 We got this compound as the main product of the
reaction together with the new symmetrical ditopic ligand L =
BPMPB drawn in Figure 1, and some 1,2-trans-bis(2-pyridyl)-
ethylene. We thus decided to investigate the reactivity of
BPMPB. This new ligand, being very easy to obtain, turned out
to be very stable and reacted with various metal salts of the first
series to provide several new compounds. In particular, those
included complexes in which part of the ligand was converted
INTRODUCTION
■
Tripodal tetraamines with pyridine-containing arms constitute a
well-known family of ligands. Both σ-donating tertiary amine
and π-accepting pyridyl groups make these ligands excellent
chelators that typically coordinate to metal centers in a
tetradentate, or tridentate, fashion. With the emblematic
representative of this class of compounds, namely the TPA
ligand (tris(2-pyridylmethyl)amine), the variety of metal
centers range from transition metal complexes to lanthanides
and actinides, with applications in various area such as
biomimetic chemistry, or material chemistry for instance.1,2
Chemical modification of existing ligands is an interesting
area, since it can afford compounds with new and sometimes
unexpected properties. Within the family of pyridyl-containing
ligands, modified compounds providing various coordination
modes, or ligand field modulation, for instance, are still under
current investigation.3 These include the family of bispidine,4−7
N3-Py-R or N4-Py ligands, or hexapyridine dinucleating
ligands,8−13 for example. Considering capping ligands, we
were interested in studying simple tripodal ligands in which the
amine nitrogen of the tetraamine tripod would be replaced by a
carbon atom as is the case for some tripyridine ligands,13 having
Received: May 15, 2015
© XXXX American Chemical Society
A
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Figure 1. Bis(2-pyridylmethyl)(pyridyl)propyne (A) and tris(2-pyridylmethyl)pyridylmethane (B) compounds as reported in the literature,14
together with the new bis[1-bis(2-pyridylmethyl),1 (pyridyl)]butyne ligand (BPMPB) reported in the present study (right, in frame).
Figure 2. Versatile coordination chemistry of the BPMPB ligand reported in this Article.
Press: Oxford, 1997]. Alternatively, they could be obtained from a
solvent purification device (MBRAUN). Preparation and handling of
all the compounds were performed under an argon atmosphere by
using the Schlenk technique following standard procedures. Mass
spectrometric experiments were carried out by the “Service Commun
to an indolizinium cation, the formation of which involves
fusion of the alkyne part of the ligand with one pyridine, which
in the present case is metal-dependent.
In the present contribution, we wish to disclose our results
on the coordination chemistry of the BPMPB ligand with Fe,
Co, Ni, Cu, and Zn salts (Figure 2). Following the description
of the structure of this new ligand, XRD studies of several
complexes are reported. These include mononuclear or
dinuclear compounds containing bridging-alkyne, alkenyl ligand
resulting from fused alkyne/pyridyl units, or the free
indolizinium ring. In the last case, the indolizinium could
represent the ultimate step in the conversion of an alkyne and
pyridyl unit into such an heterocycle.16
́
de Spectrometrie de Masse de l’UBO”, Brest. All XRD experiments
were carried out by the “Service Commun de Diffraction X de la
Faculte des Sciences et Techniques de l’UBO”, Brest. The elemental
analyses were carried out by the “Service de Microanalyses de l’ICSN,
Gif sur Yvette”, and the “Service Central d’ Analyses de l’ISA, Lyon”.
1
Physical Methods. The H NMR data were recorded in CDCl3,
CD2Cl2 or (CD3)2SO for the ligand as well as metal complexes at
ambient temperature on a Bruker AC 300 spectrometer at 300.1300
MHz with the residual signal of CHCl3, CDHCl2 and [(CD2H)2SO]
as a reference for calibration. The IR spectra were recorded on a
Bruker Vertex 70 spectrometer. The UV−vis spectra were recorded on
a Varian Cary 05 E UV−vis−NIR spectrophotometer equipped with
an Oxford instrument DN1704 cryostat with optically transparent
Schlenk cells.
EXPERIMENTAL SECTION
■
General Considerations. Chemicals were purchased from Aldrich
Chemicals and used as received. Unless otherwise stated, all the
solvents used during the metalation reactions and workup were
distilled and dried according to standard methods [Armarego, W. L. F.,
Perrin, D. D. Purification of Laboratory Chemicals, 4th ed.; Pergamon
The effective magnetic moments (μeff’s) for paramagnetic species
1
were determined in solution by H NMR (Avance 400 BRUKER, or
DRX 300 BRUKER) at room temperature using the Evans
B
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method,17,18 with tetramethylsilane (TMS) as an internal standard.
Diamagnetic corrections have been applied using Pascal constants.19
Reaction of Picolyl Chloride with Sodium Acetylide, and
Synthesis of the Bis[1-bis(2-pyridylmethyl),1 (pyridyl)]butyne
Ligand (BPMPB). A slurry of 24 mmol of sodium acetylide (6.4 g of
suspension in xylene/mineral oil, 18 wt %) was introduced in a well-
dried Schlenk tube. An ammonia gas cylinder was fitted to the Schlenk
tube, and about 50 mL of liquid NH3 was condensed maintaining the
temperature around −70 °C. After removal of the cylinder, 3.0 g of 2-
chloromethylpyridine (24 mmol) was added dropwise to ammoniacal
solution in the course of 5 min which resulted in the formation of a
white precipitate. The reaction mixture was stirred for 3−4 h while the
liquid ammonia was evaporated slowly. The resulting dark brown pasty
residue was quenched with ice-cold water and was filtered using a
suction pump. The light brown precipitate was washed 2−3 times with
pentane and diethyl ether and was recrystallized from dichloro-
methane/hexane mixture to get white crystals of compound BPMPB
with an 18.5% yield.
1H NMR, 298 K, CD2Cl2, δ, ppm: 9.08, 1H, α-py; 9.03, 2H, α-py;
8.87, 1H, α-py; 8.27, 2H, α-py; 7.94−7.23, m, 15H, aromatics; 6.70,
1H, aromatics; 6.20, 2H, aromatics; 4 AB systems corresponding to the
methylene protons, δ = 3.50 ppm (HHJ = 14.2 Hz), δ = 3.44 ppm (HH
J
= 13.8 Hz), δ = 3.37 ppm (HHJ = 14.1 Hz), and δ = 3.36 ppm (HHJ =
13.9 Hz).
DMSO-d6, δ, ppm: 8.58, 3H, α-py; 8.35, 1H, α-py; 8.30, 2H, α-py;
8.11, 1H, aromatics; 7.80−6.92, aromatics + broad signals at 298 K; 2
AB systems corresponding to the methylene protons, δ = 3.55 ppm
(
HHJ = 12.7 Hz) and δ = 3.43 ppm (HHJ = 13.1 Hz).
Variable temperature spectra displayed in the Supporting
Information.
IR: 1607, 1571, 1487, 1469, 1440, 1325, 1160, 1027, 774, 577, and
424 cm−1.
UV−vis, 298 K, CHCl3, λmax/nm, ε, M−1 cm−1: 264 (13 378), 272
(9622).
DMSO, λmax/nm, ε, M−1 cm−1: 263 (11 000), 270 (8293).
MS, ESI, positive, m/z = 671.1737; [BPMPBZnCl]+ and 573.2822;
[BPMPBH]+.
1H NMR, 298 K, CDCl3, δ, ppm: 8.66 (d, 2H), 8.32 (d, 4H), 7.27−
6.93 (m, 14H), 6.56 (d, 4H), 3.60 (AB system, −CH2− protons, HHJ =
12.7 Hz).
Anal. Calcd C38H32Zn2Cl4N6·CH2Cl2, %: C, 50.5; H, 3.7; N, 9.0;
Found: C, 50.9; H, 3.9; N, 8.8.
13C NMR, 298 K, CDCl3, δ, ppm: 48.68, 48.78, 89.40, 120.65,
120.93, 122.77, 124.35, 134.68, 135.16, 148.00, 148.28, 157.91, and
160.57.
[(BPMPB)Co2Cl4]. Blue, yield 72%. To obtain the single crystals of
the complex for X-ray diffraction, a dichloromethane solution of
complex was layered with hexane for 2 days at room temperature.
1H NMR, 298 K. CDCl3, δ, ppm (Δν1/2, Hz): 62 (73), 61 (98), 57
(55), 56 (55), 43 (73), 41 (73), 40 (83), 39 (114), 36 (65), 17 (140),
14 (100), 13 (115), 11 (100), + sharper signals between 8 and 0 ppm.
Spectrum displayed in the Supporting Information.
IR: 1655, 1605, 1569, 1484, 1433, 1296, 1102, 1027, 772, and 576
cm−1.
UV−vis, 298 K, CH2Cl2, λmax/nm, ε, M−1 cm−1: 262 (20 800).
IR: 1587, 1566, 1467, 1431, 1339, 1088, 1050, 994, 785, 770, 745,
and 578 cm−1.
MS, ESI, positive, m/z = 573, [M + H]+; 595, [M + Na]+.
Anal. Calcd C38H32N6, %: C, 79.7; H, 5.6; N, 14.7. Found: C, 79.4;
H, 5.7; N, 14.4.
UV−vis, 298 K, CHCl3, λmax/nm, ε, M−1 cm−1: 264 (8885), 574
(423), 611 (615), 633 (615).
The filtrate was extracted with diethyl ether, dried over sodium
sulfate, and concentrated under vacuum. The dark crude material was
placed on an alumina column. A very light yellow compound was
eluted as the first fraction with diethyl ether. White single crystals of
trans-1,2-dipyridyl ethene were obtained by slow evaporation of the
solution of compound in dichloromethane/diethyl ether mixture. Yield
20.8%.
MS, ESI, positive, m/z = 702.1448; [BPMPBHCoCl2]+, 666.1689;
[BPMPBCoCl]+ and 573.2750; [BPMPBH]+; m/2z = 315.6020;
[BPMPBCo]2+.
Anal. Calcd C38H32Co2Cl4N6·1/2CH2Cl2, %: C, 52.9; H, 3.8; N, 9.6;
Found: C, 53.0; H, 3.9; N, 9.2.
Magnetic susceptibility (DRX 300 BRUKER): μeff = 6.06 μB
(CD2Cl2, Evans method). Δν = 39.61 Hz, for a 2 mM solution.
Diamagnetic correction: −0.000 391 emu/mol.
1H NMR, 298 K, CDCl3, δ, ppm: 8.60−7.10 ppm (10H, all
expected aromatic and alkene protons).
13C NMR, 298 K, CDCl3, δ, ppm: 122.16, 122.86, 131.24, 136.17,
and 149.29.
[(BPMPB)Cu2Cl]Cl. Colorless, yield 66% from 3 equiv of CuCl per
ligand. In this case, the complex [(BPMPB)Cu2Cl]CuCl2 is obtained,
and it displays the same spectroscopic data. To obtain single crystals of
the complex for X-ray diffraction, diethyl ether was diffused slowly into
a dichloromethane solution of complex at room temperature.
1H NMR, 298 K, CDCl3, δ, ppm: 8.57, 6H, α-py; 7.80−7.08, 18H,
aromatics; 1 AB system, 8H, centered at δ = 3.72 ppm (HHJ = 13.9
Hz). Spectrum displayed in the Supporting Information.
IR: 1601, 1567, 1476, 1439, 1259, 1055, 791, 701, 661, and 452
cm−1.
MS, ESI, positive, m/z = 183, [M + H]+.
A yellow liquid was eluted as the second component from a column
with a diethyl ether/acetone (1:1) mixture, to yield an oily compound
which was identified as compound A of Figure 1 and characterized by
comparison of its 1H and 13C NMR spectra with those already
published. Yield 41.7%.
1H NMR, 298 K, CD2Cl2, δ, ppm: 8.66 (d, 1H), 8.38 (d, 2H),
7.52−6.90 (m, 9H), 3.71 (AB system, −CH2−, 2H), 3.46 (AB system,
−CH2−, 2H) and 2.51 (s, 1H) ppm. 13C NMR, 298 K, CD2Cl2, δ,
ppm: 49.02, 49.30, 76.71, 85.89, 121.60, 122.04, 123.00, 124.95,
135.47, 136.22, 148.92, 149.15, 158.36, and 160.96 ppm.
Preparation of the BPMPBM2Cl4 Complexes. All the complexes
were prepared by following a general procedure. Details are given for
(BPMPB)Zn2Cl4, but the following procedure applies for all
complexes. The reactions were carried out under inert atmosphere
in all the cases except complex (BPMPB)Zn2Cl4.
UV−vis, 298 K, CH2Cl2, λmax/nm, ε, M−1 cm−1: 263 (20 583), 308
(sh, 5458).
Anal. Calcd C38H32Cu3Cl3N6·1/2CH2Cl2, %: C, 50.7; H, 3.6; N, 9.2.
Found: C, 50.9; H, 3.8; N, 8.7.
[(InBPMPB)NiCl]Cl. Green, yield 66%. To obtain the single crystals
of the complex for X-ray diffraction, diethyl ether was allowed to
slowly diffuse into a solution of complex in a dichloromethane/
methanol mixture.
1H NMR, 298 K, CDCl3, δ, ppm (Δν1/2, Hz): 212 (900), 184 (900),
62 (315), 57 (310), 45 (310), 39 (320), 26 (305), 23 (300), −5 (290),
−8 (315) + sharper signals between 11 and 3 ppm. Spectrum available
in the Supporting Information.
[(BPMPB)Zn2Cl4]. A batch of 0.029 g (0.05 mmol) of ligand
(BPMPB) was dissolved in 20 mL of distilled dichloromethane to
obtain a colorless solution. A 0.014 g portion of anhydrous ZnCl2 (0.1
mmol) was introduced to the reaction vessel, and the mixture was
stirred at room temperature for 5−6 h. A white suspension was
obtained. The medium was concentrated to 10 mL under vacuum, and
20−30 mL of diethyl ether was added to obtain the desired compound
as white precipitate (0.025 g, 0.029 mmol, 58% yield), which was dried
under vacuum.
IR: 1623, 1605, 1589, 1568, 1476, 1434, 1261, 1102, 995, 759, 581,
and 462 cm−1.
UV−vis, 298 K, CH2Cl2, λmax/nm, ε, M−1 cm−1: 264 (27 042), 371
(4667).
MS, ESI, positive, m/z = 665.1680; [InBPMPBNi-Cl]+, 573.2728;
[InBPMPB]+; m/2z = 315.1014; [InBPMPBNi]2+.
Anal. Calcd C38H32NiCl2N6·CH2Cl2, %: C, 59.5; H, 4.3; N, 10.7.
Found: C, 59.2; H, 4.6; N, 10.4.
Single crystals of complex for X-ray diffraction were grown by slow
diffusion of diethyl ether into a solution of compound in dichloro-
methane/methanol mixture at room temperature.
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Scheme 1. Access to the BPMPB Ligand by Reaction of Picolyl Chloride with Sodium Acetylide
Magnetic susceptibility (Avance 400 BRUKER): μeff = 2.89 μB
(CD2Cl2, Evans method). Δν = 5.22 Hz, for a 1.6 mM solution.
Diamagnetic correction: −0.000 335 emu/mol).
reaction is outlined in Scheme 1. At the first step of the workup,
a white solid could easily be separated from an oily residue, as
mentioned in the original paper.
[(HInBPMPB)FeCl2][Cl]. Light brown, yield 60%. To obtain the
single crystals of complex [(HInBPMPB)FeCl2]2[FeCl4] for X-ray
diffraction, a solution of complex in dichloromethane/methanol
mixture was layered with hexane for 2−3 days under inert atmosphere.
1H NMR, 298 K, CDCl3, δ, ppm (Δν1/2, Hz): 77 (720), 46 (390), +
a series of broad signals at δ = 13 (26), 10 (47), 8 (52), 6 (70), 5 (58),
4 (43), 3 (29) ppm. Spectrum available in the Supporting Information.
IR: 1626, 1588, 1473, 1434, 1261, 1151, 1097, 1052, 994, 857, 780,
751, 567, and 467 cm−1.
The oily liquid was treated and turned out to contain some
ligand A that was recovered by chromatographic treatment,
with a yield of 42% with respect to the pyridine. This
compound, already known, was first characterized by ESI MS,
1
and then by H NMR and 13C NMR spectroscopy with the
data matching those already reported.14,15 In the course of the
workup, the known (and commercially available) 1,2-bis(2-
pyridyl)ethylene compound was also isolated as a white solid in
UV−vis, 298 K, CH2Cl2, λmax/nm, ε, M−1 cm−1: 261 (23 250), 296
(sh,11 250), 364 (4450).
21% yield,21 and characterized by H and 13C NMR, and
1
eventually by XRD (the crystal structure was already published
and will not be shown here).22 Finally, it should be mentioned
here that, in our hands, tetrapod B, drawn in Figure 1 and
mentioned as a minor product in the original paper, could not
be obtained.
MS, ESI, positive, m/z = 573.2828; [BPMPBH]+.
Anal. Calcd C38H33FeCl3N6·2CH2Cl2, %: C, 53.0; H, 3.9; N, 9.7.
Found: C, 53.0; H, 4.1; N, 9.3.
Magnetic susceptibility (DRX 300 BRUKER) on single crystals
dissolved in CD2Cl2: μeff = 8.71 μB (CD2Cl2, Evans Method). Δν =
47.31 Hz, for a 1.2 mM solution. Diamagnetic correction: −0.000 726
emu/mol.
X-ray Analysis. The single crystals were mounted on a Xcalibur-2
diffractometer from Oxford Diffraction (Mo Kα, λ = 0.710 73 Å).
Quantitative data were obtained at 170 K for all the compounds. The
crystal data collection and processing were performed with CRYSALIS
software (CrysAlis CCD and CrysAlis RED; Oxford Diffraction Ltd.:
Abingdon, Oxford, United Kingdom, 2009). The cell parameters were
determined using all the frames. All structures were solved using
SIR9220 and SHELXS-97 (Sheldrick, G. M. SHELXS-97: Program for
Crystal Structure Solution; University of Gottingen: Gottingen,
Germany, 1997), and were refined by full-matrix least-squares on F2
using the SHELXL-97. The absorption was corrected empirically and
analytically. All non-hydrogen atoms were refined anisotropically.
Hydrogen atoms were generated according to stereochemistry and
refined using a riding model in SHELXL97. Molecular graphics were
created with the DIAMOND program (Brandenburg, K. DIAMOND:
Program for Crystal and Molecular Structure Visualization; Crystal
Impact GbR: Bonn, Germany, 2011).
The white solid which separated from the oily residue is
particularly easy to obtain. It turned out to be the BPMPB
ligand, obtained as the minor product of this reaction since the
yield is 18.5%. It was characterized by mass spectroscopy with
1
an m/z signal at 573.269 for BPMPBH+, and by H and 13C
NMR spectroscopy. In principle, BPMPB is a symmetrical
molecule: this is evidenced by the observation of identical
aromatic resonances for all pyridylmethyl, and pyridyl, protons,
respectively. The methylene groups, however, are magnetically
nonequivalent, and appear as an AB system, centered at δ =
3.60 ppm (JHH = 12.7 Hz). The 13C spectrum does not lead to
any particular comment, with the alkyne carbon appearing at δ
= 89.4 ppm. The UV−vis spectrum of the free BPMPB ligand
in methylene chloride consisted of a single absorption at λ =
262 nm (ε = 20 800 L mol−1 cm−1). All traces are given in the
Supporting Information.
Single crystals of the BPMPB ligand could be obtained by
slow evaporation of a solution of the compound in dichloro-
methane/hexane mixture, and the structure was solved by X-ray
analysis. The ORTEP diagram is displayed in Figure 3.
As expected, a short distance dC1−C′1 = 1.191(2) Å is
found for the triple bond. The distance dC1−C2 = 1.472(2) Å
is relatively short and indicates some degree of delocalization of
the triple bond between these atoms. This makes the heart of
the ligand somewhat rigid, with a distance of 4.134(2) Å
between both quaternary carbon atoms C2 and C′2. Addition-
ally, short contacts dN2−H9B = 2.591(2) Å and dN2−H3B =
2.538 (2) Å indicate hydrogen bonding between the pyridine
and the adjacent methylene groups, which assign the
conformation to this ligand in the solid state.
RESULTS
■
BPMPB Ligand: Preparation and Structure. The early
report of the preparation of the (2-pyridylmethyl)(pyridyl)-
propyne ligand (A in Figure 1) involved the reaction of
equimolar amounts of 2-chloromethylpyridine and sodium
acetylide generated in situ in liquid ammonia.14 Small amounts
of the tris(2-pyridylmethyl)pyridylmethane tetrapod (B of
Figure 1) were also obtained. The yield of A + B was reported
as being quantitative. The ligands were characterized by the
available techniques such as melting point, infrared spectros-
1
copy, elemental analysis, and H and 13C NMR spectroscopy
with the resolution available at this time (1974). We
reproduced the published procedure, with the only modifica-
tion being that we used sodium acetylide from a commercial
source, provided as a slurry in a mixture of mineral oil and
xylenes. The reaction was carried out in a Schlenk tube under
controlled atmosphere, in liquid ammonia. The result of this
Preparation of the Metal Complexes. All metal
complexes reported in this Article were obtained by reaction
of 2 or 3 equiv of MCl2 (M = Fe, Co, Ni, Zn) or MCl (M =
Cu) with 1 equiv of BPMPB. The nickel and cobalt salts were
in the hexahydrated form NiCl2·6 H2O and CoCl2·6 H2O. The
solvent was methylene chloride, and all preparations were
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appeared as slightly broadened, measurements were performed
at lower temperature, 273, 263, and 243 K. As a result, from
273 to 243 K, the resolution was found to be significantly
improved, with no other modification in the position of the
signals, suggesting the occurrence of some limited dynamic
effect in solution at ambient temperature. Data could also be
obtained in DMSO-d6, solvent which provided a significantly
different spectrum, with some broadened signals in the
aromatic region, along with well-resolved resonances. In this
case, only two AB systems were observed at δ = 3.55 ppm (HHJ
= 12.7 Hz) and δ = 3.43 ppm (HHJ = 13.1 Hz). The spectrum
was also recorded at 333 K, the temperature at which the broad
resonances were replaced by highly resolved signals. DMSO is a
polar and highly dissociating solvent. It is likely that the
chloride ligands may exchange, at room temperature with the
solvent or with water present in the solution. At the time scale
of the NMR at room temperature, this exchange would affect
some signals in the aromatic region while it would be invisible
at higher temperature. To conclude, both UV−vis and NMR
studies suggest a solvent-dependent coordination for this
complex. The room temperature spectra are displayed in
Figure 4. All UV−vis and variable temperature NMR traces are
given in the Supporting Information.
Figure 3. ORTEP diagram of the BPMPB ligand, with partial
numbering. This compound has an inversion symmetry: bond
distances and angles are identical for the inversion counterpart
(identical labeling with a ′ superscript). Selected distances in Å: dC1−
C′1, 1.191(2); dC1−C2, 1.472(2); dC2−C2′, 4.134(2).
The cobalt complex is a blue solid, with absorptions in the
carried out under inert atmosphere. After reaction, the mixture
was concentrated and the solid complex obtained by
precipitation with diethyl ether, a solvent from which they
were generally recrystallized. Depending on the preparation,
the yields in crystalline material were ca. 60−70%.
UV−vis range at λ = 264, 574, 611, and 633 nm (ε = 8885, 423,
1
615, and 615 L mol−1 cm−1, respectively). The H NMR
spectrum of this complex exhibited paramagnetically shifted
and broadened signals, together with sharper ones in the
diamagnetic region. The spectrum, which could not be easily
assigned, is displayed in the Supporting Information. Magnetic
susceptibility measurements were carried out by the Evans
method. The magnetic moment μeff = 6.06 μB was found to be
slightly higher than the expected spin-only moment for a two
independent center species (μeff = 5.48 μB).
Tetragonal Complexes with Unmodified Ligand: Zinc
and Cobalt Complex. The zinc complex was obtained as a
white solid. With a main absorption at λ = 264 nm (ε = 13 378
M−1 cm−1), the UV−vis spectrum does not qualitatively differ
1
from that of the free ligand. In H NMR in CD2Cl2 at room
temperature, four AB systems appear in the methylene region,
Single crystals of both compounds could be obtained by
diffusion of diethyl ether in a solution of methylene chloride/
methanol for the zinc complex, and hexane in a solution of
methylene chloride for the cobalt complex. Structural
determination was possible for these two compounds. The
ORTEP diagrams are displayed in Figure 5.
at δ = 3.50 ppm (JHH = 14.2 Hz), δ = 3.44 ppm (JHH = 13.8
Hz), δ = 3.37 ppm (JHH = 14.1 Hz), and δ = 3.36 ppm (JHH
=
13.9 Hz). Obviously, the symmetry is broken in this complex,
and this is confirmed by the presence of four distinct signals
corresponding to the two distinct α protons of the pyridines (δ
= 9.1 and 8.9 ppm, 1H each) and two pairs of α protons of the
pyridyl groups bound by a methylene bridge (δ = 9.0 and 8.3
ppm, 2H each). Since the signals in the aromatic region
At first glance both compounds look isostructural: this is true
at the molecular level only, and is obvious from the ORTEP
Figure 4. Comparison between the room temperature 1H NMR spectra of BPMPBZn2Cl4 in CD2Cl2 (lower) and DMSO-d6 (upper trace). At this
temperature, the CD2Cl2 spectrum exhibits slightly broadened signals in the aromatic region. All variable temperature spectra are shown in the
Supporting Information. The asterisk corresponds to the presence of water in DMSO.
E
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Figure 5. ORTEP diagrams of BPMPBZn2Cl4 (left) and BPMPBCo2Cl4 (right).
Table 1. Metal-to-Ligand Distances in the BPMPBZn2Cl4 and BPMPBCo2Cl4 Complexes
M1−Cl1
M1−Cl2
M1−N1
M1−N2
M2−Cl3
M2−Cl4
M2−N21/M2−N5
M2−N2/M2−N4
BPMPBZn2Cl4
BPMPBCo2Cl4
2.2252(13)
2.234(2)
2.2312(13)
2.225(2)
2.042(4)
2.042(6)
2.049(3)
2.038(7)
2.2365(14)
2.224(2)
2.2174(15)
2.243(2)
2.036(4)
2.046(6)
2.049(3)
2.046(6)
diagrams which display identical structures. However, they have
different symmetry and space groups.
Single crystals of this complex could be obtained by slow
diffusion of diethyl ether into a methylene chloride solution,
and the structure could be resolved. The first thing to note is
that the complex is an ionic compound: the well-known cuprate
The structure consists of the BPMPB ligand in which the
pyridines of the methylenepyridine arms coordinate to the
metal dichloride. The pyridyl groups which are directly linked
to the carbon skeleton are free, but in interaction with two
protons of an adjacent methylene, as this was the case for the
free BPMPB ligand. For both complexes, the corresponding
contacts lie in the range 2.56 < dNpy−CH2 < 2.76 Å. The
central triple bond is short, with dC19−C20 = 1.191(5) and
1.180(9) Å for the zinc and cobalt complex, respectively. The
principal metric parameters of both complexes are summarized
in Table 1.
−
CuCl2 anion is present in the structure together with the
dinuclear BPMPBCu2Cl+ cation. The ORTEP diagram of the
cationic complex is displayed in Figure 6.
Coordination of the Metal to Methylenepyridines and
the Triple Bond: [BPMPBCu2Cl]Cl. The Cu(I) complex was
obtained as a white solid, and characterized using different
spectroscopic techniques. Its UV−vis spectrum in methylene
chloride mainly consists of the ligand centered absorption at λ
= 263 (ε = 20 583 L mol−1 cm−1) and a shoulder near 308 nm
(ε = 5458 L mol−1 cm−1). The 1H NMR spectrum of
[BPMPBCu2Cl]Cl in CD2Cl2 looks simple: the methylene
groups appear as a broadened AB system, centered at δ = 3.72
ppm (JHH = 13.9 Hz), and the aromatic protons between 7.0
and 8.6 ppm as slightly broadened signals. By contrast with the
zinc complex, a symmetry similar to that of the free ligand is
observed in this complex. In 13C NMR, the most striking
feature is the important upfield shift of the alkyne carbons
which appear at δ = 54.9 ppm whereas they were observed at δ
= 89.4 ppm in the free ligand. This supports a strong
interaction between the metal and the triple bond in solution.
Finally, we may mention that, in the former preparations, 2
equiv of Cu(I)Cl were reacted with BPMPB to afford a cationic
species with Cl− as the counteranion. At this stage, we decided
to use 3 equiv of metal salt which allowed us to get easily, by
simple precipitation, a microcrystalline material with the
Figure 6. ORTEP diagram of the BPMPBCu2Cl+ complex with partial
labeling.
In this complex, each copper atom coordinates to two
pyridines from different segments (i.e., from either side of the
alkyne bond), to a bridging chlorine atom, and to the triple
bond through the carbons C19 and C20. If one approximates
that the copper atom is being directly linked to the middle of
the triple bond (instead of carbons C19 and C20), the
geometry at each metal center could be considered as distorted
tetrahedral, in line with stabilization of Cu(I) sites with this
geometry. This molecule is asymmetric in the solid state as can
be deduced from the dCu1−Cl1 = 2.61 Å and dCu2−Cl1 =
2.49 Å (Table 2). The Cu1−Cu2 separation of 2.853 Å (with
−
CuCl2 anion, having the same spectroscopic properties.
F
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Table 2. Metal-to-Ligand Distances in the BPMPBCu2Cl+ Complex
N1
N2
Cl1
C19
C20
N3
N4
1.988(3)
Cu1
Cu2
1.984(3)
1.993(3)
2.6079(10)
2.4898(10)
2.054(3)
2.053(3)
2.053(3)
2.058(3)
1.989(3)
Figure 7. ORTEP diagram of the (InBPMPB)NiCl+ complex with partial labeling: in this drawing, the uncoordinated pyridyl units are in gray. An
axial view of the nickel center, and a view of the coordination polyhedron with Cl1NiC19 and N4NiN1 angles. In these two drawings, the
uncoordinated pyridyl units have been removed for clarity. Also, finally (lower right), the drawing of the molecule.
Table 3. Selected Distances in the (InBPMPB)NiCl+ Complex
Molecule 1
N1Ni1
N4Ni1
C19Ni1
1.859(5)
Cl1Ni1
C19C20
C20N3
N3Cl4
C14C1
C1C19
a
a
a
a
a
a
a
1.936(4)
1.905(4)
2.253(1)
1.337(6)
1.461(6)
1.359(6)
1.510(6)
1.500(6)
Molecule 2
N7Ni2
N10Ni2
1.903(4)
C59Ni2
1.846(5)
Cl2Ni2
C60C59
N9C60
C54N9
C41C54
C59C41
a
a
a
1.947(4)
2.263(1)
1.346(6)
1.463(6)
1.376(6)
1.505(7)
1.504(6)
a
Indicates the presence of the indolizinium ring.
dC19-C20 = 1.265(5) Å) is close to the distances (ca. 2.80 Å)
found in other alkyne-bridged dicopper(I) complexes.23,24
Again, the orientation of the pyridyl groups is due to the
hydrogen bonds between the nitrogen atom and adjacent
methylene groups, dN5−H6B, dN5−H13B, dN6−H33B,
dN6−H26B, being in the 2.4−2.8 Å range.
that part of the ligand only is bound to a paramagnetic center.
The trace is displayed in the Supporting Information. Magnetic
susceptibility measurements were carried out by the Evans
method. The magnetic moment μeff = 2.89 μB was found to be
almost identical to that of the expected spin-only moment for a
single nickel center (μeff = 2.83 μB).
Single crystals were obtained from a slow diffusion of diethyl
ether into a solution of the complex in dichloromethane in
which some drops of methanol were added. The structure was
solved, and the ORTEP diagram is displayed in Figure 7. Three
main characteristics can be evidenced from this structure: (i)
the ligand has been modified, and one pyridine has fused with
the alkyne to yield an indolizinium moiety, which is
coordinated to the metal through a metal−alkenyl bond; (ii)
the BPMPB ligand has become InBPMPB+, and a chloride
atom (not shown in Figure 7) is present in the structure to
balance the positive charge; (iii) this is a mononuclear complex,
and one nickel only is linked to the ligand.
Tetragonal Complexes with an Alkenyl Indolizinium
Modified Ligand InBPMPB. The addition of NiCl2·6H2O to
BPMPB in methylene chloride produced a light green color.
The addition of diethyl ether to the reaction mixture resulted in
the isolation of green precipitate in 66% yield. The complex was
characterized with all the spectral studies including ESI-MS,
NMR, IR, and electronic absorption spectroscopies. The
molecular ion peak of the complex was found at m/z
=665.1680 for (InBPMPBNiCl)+. The electronic spectrum for
this complex in dichloromethane solution exhibited absorption
bands at λ = 264 nm (ε = 27 042 L mol−1 cm−1) and 371 nm (ε
= 4667 L mol−1 cm−1). This compound is paramagnetic: its 1H
NMR spectrum in CD2Cl2 clearly showed very broad (δ = 212
and 184 ppm) to broad (δ = 62, 57, 45, 39, 26, 23, −5, and −8
ppm) signals along with a few sharper signals in the
diamagnetic region. This latter observation strongly suggests
Two independent molecules are present in the unit cell, and
the metric parameters do not significantly differ. All data are
summarized in Table 3. The analysis of the structure reveals
relatively short metal-to-ligand distances with dN1−Ni1 =
G
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+
Figure 8. ORTEP diagram of one of the HInBPMPBFeCl2 complexes with partial labeling.
+
Table 4. Selected Distances in the HInBPMPBFeCl2 Complex
Molecule 1
N5Fe1
N6Fe1
Cl1Fe1
Cl2Fe1
C19C20
C20N1
N1C2
C2C1
C1C19
a
a
a
a
a
2.107(3)
2.116(3)
2.2656(12)
2.2361(13)
1.320(5)
1.467(5)
1.355(5)
1.509(5)
1.506(5)
Molecule 2
N45Fe2
2.116(3)
N46Fe2
2.120(3)
Cl3Fe2
Cl4Fe2
C59C60
C60N41
N41C42
C42C41
C41C59
a
a
a
a
a
2.2581(12)
2.2439(13)
1.329(5)
1.450(5)
1.374(5)
1.492(6)
1.503(5)
a
Indicates the presence of the indolizinium ring.
1.936(4) Å, dN4−Ni1 = 1.905(4) Å, and dC19−Ni1 =
1.859(5) Å. The indolizinium ring is characterized by dC19−
C20 = 1.337(6) Å, dC20−N3 = 1.461(6) Å, dN3−C14 =
1.359(6), dC14−C1 = 1.510(6) Å, and dC1−C19 = 1.500(6)
Å.
It is noteworthy that the geometry at the nickel site cannot
rigorously be considered as square planar with Cl1NiC19 and
N4NiN1 angles of 171° and 174°, respectively. The picture
would be that of an almost flat tetrahedron, in line with the
paramagnetic character of the complex in solution.
was uncoordinated. As already mentioned and as the
consequence of the formation of an indolizinium, the ligand
itself is a charged molecule. In the present structure the charge
is equilibrated by a FeCl42−. The ORTEP diagram of one
independent molecule of the complex is displayed in Figure 8.
The metal lies in a pseudotetrahedral environment, with two
coordinated methylenepyridines and two chloride ions. The
metal-to-ligand distances are summarized in Table 4. As already
mentioned for the other structures, the conformation of the
uncoordinated pyridines is due to intramolecular contacts. In
particular, the indolizinium hydrogen interacts with two
neighboring N atoms of uncoordinated pyridines with dNH
= 2.58 (molecule 1) and dNH = 2.55 Å (molecule 2).
To summarize these results, a variety of different structures
could be established from the same ligand, by coordination with
different metals. These results will be discussed in the following
section.
Tetragonal Complexes with a Free Indolizinium
Modified Ligand HInBPMPB. Iron Complex. Upon
addition of FeCl2 to a solution of BPMPB ligand in CH2Cl2,
a pale green color developed, rapidly followed by a yellow
color. The workup under inert atmosphere yielded a caramel
brown solid, and the UV−vis spectrum in dichloromethane
solution exhibited absorptions at λ = 261 (ε = 23 250 L mol−1
cm−1), 296 (ε = 11 250 L mol−1 cm−1), and 364 nm (ε = 4450
1
L mol−1 cm−1). The H NMR of this complex in CDCl3
DISCUSSION
■
exhibited two broad signals at 77 and 46 ppm, with the
presence of some sharper resonances in the diamagnetic region.
Magnetic susceptibility measurements were carried out by the
Evans method on single crystals dissolved in CD2Cl2. The
magnetic moment μeff = 8.71 μB was found to be slightly higher
than the expected spin-only moment for a three independent
tris-Fe(II) center (μeff = 8.48 μB).
Single crystals of this compound could be obtained, and the
structure was resolved. The unit cell contains two independent
molecules. The complex termed [(HInBPMPB)FeCl2]2[FeCl4]
was obtained. Here, also, an indolizinium ring, resulting of the
fusion of a pyridine with the triple bond, was formed. However,
in contrast with the nickel complex, the indolizinium fragment
The mechanism of formation of ligand A of Scheme 1 has been
proposed, and it involves electrophilic attack of picolyl chloride
on stabilized anionic forms of mono- and disubstituted
alkyne.14 In our hands, from A a similar mechanism will
ultimately yield BPMPB. By comparison with the 1974 paper,
our experiments differ only by the nature of the sodium
acetylide, a commercial slurry in our case versus an in situ
preparation in the former paper. In the present study, we obtain
the BPMPB ligand, plus some dipyridylethene, and ligand A, as
shown in the scheme displayed in Supporting Information.
Such a difference might arise from differences in the
preparation mode of sodium acetylide, which was formerly
obtained from sodium amide generated in situ, with different
H
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a
Table 5. Metric Parameters of the Indolizinium Cycle in the Five Complexes Currently Known
+
Ru([9ane]S3Ind 1
Ru([9ane]S3Ind 2
Ru([9ane]S3Ind 3
IndBPMPBNiCl2 (#1)
HInBPMPBFeCl2 (#1)
C1−C2
C2−N3
N3−C4
C4−C5
C5−C1
1.341(3)
1.441(3)
1.346(3)
1.529(3)
1.550(3)
ref 35
1.342(3)
1.443(3)
1.350(3)
1.521(3)
1.541(3)
ref 35
1.338(4)
1.444(4)
1.336(4)
1.516(4)
1.543(4)
ref 36
1.337(6)
1.461(6)
1.359(6)
1.510(6)
1.500(6)
this work
1.320(5)
1.467(5)
1.355(5)
1.509(6)
1.506(5)
this work
a
For clarity, the numbering of the cycle is displayed on the drawing above the table.
Scheme 2. General Scheme Summarizing the Different Steps Involved in the Preparation of the Complexes Reported in This
a
Article
a
The nickel line has been adapted from ref 35. The complexes in brackets are intermediates.
basic properties and possibly containing trace amounts of
sodium, responsible for some radical-based reactions, as
mentioned in the original paper. Anyway, this point goes
beyond the scope of this Article and will not be addressed in
the present study.
Regarding coordination chemistry, in none of the complexes
reported in the present work does the pyridine coordinate.
Only methylenepyridine is bound to the metal. This ligand is
well-suited for tetrahedral coordination, at least in methylene
chloride which is our solvent.
The coordination of ZnCl2 and CoCl2 affords isostructural
complexes in the solid state. The coordination is classical and
does not lead to particular comments. For the zinc complex, the
1
striking point is the break of symmetry deduced from the H
NMR in CD2Cl2. Our low temperature measurements rule out
any possibility of temperature-dependent dynamic process. The
only explanation is an asymmetrical conformation of the
complex in solution. This is not true in DMSO-d6, for which a
higher symmetry is observed. However, DMSO is a dissociating
and potentially coordinating solvent, and the structure might be
I
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Inorganic Chemistry
Article
different from that seen in the solid state. The cobalt complex is
blue and exhibits d−d transitions in UV−vis, in the region
where these transitions are generally observed for TPA Co(II)
complexes.25−27 This complex displays a typical paramagnetic
1H NMR spectrum, indicating the absence of magnetic
exchange between the two metal centers within this structure.
Cu(I) was initially chosen to test the ability of the three
pyridines of each side of the ligand to provide a tetrahedral
environment to a metal−chloride complex. Indeed, this is the
case, but we found that the metal coordinated to the alkyne
part. This was evidenced in the solid state, but also in solution,
as shown from our 13C NMR measurements which indicate an
important downshift of the resonance of the alkyne carbon.
nickel complex. In the case of a d10 Cu(I) complex, the alkyne
complex is stable, which is not the case for the more Lewis
acidic d8 Ni(II) center. This is summarized in Scheme 2.
During the preparation of the iron complex, we mentioned a
transient light green color, prior to a yellow one. The iron
complex in the solid state is similar to the nickel one, in which
the indolizinium part has been uncoordinated, and the metal
center rearranged. The green transient might probably
correspond to one of the putative intermediates drawn in
Scheme 2, iron line. The origin of the hydrogen atom present
on the indolizinum part is uncertain, and could be due to H
abstraction from the solvent, or some protonation by small
amounts of methanol during the workup.
To conclude, we have reported the coordination chemistry of
the new BPMPB ligand, serendipitously obtained as a
byproduct of a known reaction. In spite of a modest yield,
the facility by which BPMPB is obtained makes it easy to work
with. The presence of pyridyl and alkyne coordination sites
confers to this ligand some potential in the study of
fundamental processes: difference of coordination depending
on the nature of the metal, preferred geometry of a given
center, or its Lewis acidity leading to ligand modification.
Applications of the BPMPB ligand are under study in our
group.
1
Additionally, the H NMR indicated that the symmetry is not
broken in solution. Yet, despite being not so common, alkyne-
bridged Cu(I) complexes have been known for years,23,24 and
the topic has recently been reviewed.28 It is noteworthy that the
Cu(I) complex is perfectly stable, and does not lead to the
formation of indolizinium, as had sometimes been reported.29
In the case of NiCl2, we were unable to isolate any simple
pyridyl-coordinated, or alkyne-coordinated, complexes. Instead,
we obtained the InBPMPBNiCl+ cation, in which the ligand
had been converted into the indolizinium cation, which
coordinates through the alkenyl fragment. This complex,
which can be seen as an indolizine zwitterion Ni-based system,
is a mononuclear one, with a Cl− anion to balance the charge.
At first glance, the geometry of the metal within this compound
looks to be square planar. A careful analysis however shows that
some distortion toward a tetrahedral geometry occurs, as
displayed in Figure 7: this is probably the reason why this
complex is paramagnetic, considering that the environment
maybe even be more flexible in solution. Any comparison of the
UV−vis data with those of the methylenepyridine-containing
nickel complex might be daring, since these complexes
generally exhibit weak absorption in the visible region30 and
that this is the first Ni−indolizinium complex known today. We
might simply postulate that the 371 nm band is due to a charge
transfer dπ(NiII) → π*(indolizine) MLCT, as is the case for
the three related ruthenium complexes (vide infra).
ASSOCIATED CONTENT
* Supporting Information
■
S
All NMR and UV−vis spectra. For the reported structures, the
crystallographic files in CIF format. The Supporting Informa-
tion is available free of charge on the ACS Publications website
at DOI: 10.1021/acs.inorgchem.5b01096.
AUTHOR INFORMATION
Corresponding Author
*E-mail: dominique.mandon@univ-brest.fr. Phone: ++ 33 (0)
298 016 028. Fax: ++ 33 (0)298 017 001.
■
Notes
The authors declare no competing financial interest.
Indolizines, with partially or wholly reduced rings, constitute
an important class of compounds due to their biological
potential, which includes antimicrobial activity, antioxidant
activity, anti-inflammatory activity, anticonvulsant activity,
enzyme inhibition activity, and activity as calcium entry
blocker.31,32 The formation of indolizine (not indolizinium)
cycles is known in metal-assisted organic chemistry using
Cu(I), Cu(II), Au(I), Au(III), Ru(II), Pt(II), Pd(II), Ag(I) and
generally involves different pathways.29,33,34 Formation of
indolizinium unit from pyridine-alkyne coupling has very
recently been observed in the ruthenium chemistry.35,36 So
far, only three crystal structures were reported on metal-
indolizinium complexes, within ruthenium complexes. The
present work describes the fourth structure of a metal-bound
indolizinium. The metric parameters of the indolizinium ring,
including those of the free heterocyle found in the Fe complex,
are reported in Table 5 and compared to those of the related
ruthenium complexes.
ACKNOWLEDGMENTS
■
We thank the Reg
CNRS, and the Universite
́
ion Bretagne (BioActO2 programm), the
de Bretagne Occidentale in Brest.
́
The CEMCA Laboratory in Brest (UMR 6521) is also
gratefully acknowledged. We thank all the colleagues from
the Service de RMN and Service de Diffraction des Rayons X,
of the Faculte des Sciences et Techniques at UBO, Brest. We
thank the Agence Nationale de la Recherche (ANR
CATHYMETOXY) for financial support. We finally thank
Prof. Franco̧ is Petillon (UBO Brest) for helpful discussions and
his critical reading of our manuscript.
REFERENCES
■
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DOI: 10.1021/acs.inorgchem.5b01096
Inorg. Chem. XXXX, XXX, XXX−XXX