Homoleptic complexes of divalent metals bearing N,O-bidentate imidazo[1,5-a]pyridine ligands: Synthesis, X-ray characterization and catalytic activity in the Heck reaction

Article abstract of DOI:10.1016/j.ica.2017.11.039

Two imidazo[1,5-a]pyridine-based ligands, namely 2-(1-phenylimidazo[1,5-a]pyridin-3-yl)phenol (N,OH-L^Ph) and 2-(1-methylimidazo[1,5-a]pyridin-3-yl)phenol (N,OH-L^Me) have been reacted with different divalent metals (M = Co, Cu, Ni, Pd) to obtain the following complexes [Co(N,O-L^Ph)₂] (1), [Cu(N,O-L^Ph)₂] (2), [Ni(N,O-L^Ph)₂] (3), [Pd(N,O-L^Ph)₂] (4), [Co(N,O-L^Me)₂] (5), [Cu(N,O-L^Me)₂] (6), [Ni(N,O-L^Me)₂] (7) and [Pd(N,O-L^Me)₂] (8). These homoleptic complexes showed the monoanionic ligands to be coordinated in a N,O-bidentate mode via the pyridine-like nitrogen of the imidazo[1,5-a]pyridine skeleton and the phenolic oxygen. The coordination mode of the ligands was confirmed by the single-crystal X-ray structure analysis of [Co(N,O-L^Ph)₂] (1) and [Pd(N,O-L^Me)₂] (8). The palladium compounds [Pd(N,O-L^Ph)₂] (4) and [Pd(N,O-L^Me)₂] (8) proved to be good catalysts in the Heck reaction between iodobenzene and different olefins.

Full text of DOI:10.1016/j.ica.2017.11.039

Inorganica Chimica Acta 471 (2018) 384–390
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Inorganica Chimica Acta
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Research paper
Homoleptic complexes of divalent metals bearing N,O-bidentate imidazo
[1,5-a]pyridine ligands: Synthesis, X-ray characterization and catalytic
activity in the Heck reaction
G. Attilio Ardizzoia a, Daniel Ghiotti a, Bruno Therrien b, Stefano Brenna a,
⇑
a University of Insubria, Department of Science and High Technology and CIRCC, Via Valleggio 9, 22100 Como, Italy
b Institute of Chemistry, University of Neuchâtel, Avenue de Bellevaux 51, CH-2000 Neuchâtel, Switzerland
a r t i c l e i n f o
a b s t r a c t
Article history:
Two imidazo[1,5-a]pyridine-based ligands, namely 2-(1-phenylimidazo[1,5-a]pyridin-3-yl)phenol (N,
OH-LPh) and 2-(1-methylimidazo[1,5-a]pyridin-3-yl)phenol (N,OH-LMe) have been reacted with different
divalent metals (M = Co, Cu, Ni, Pd) to obtain the following complexes [Co(N,O-LPh)2] (1), [Cu(N,O-LPh)2]
(2), [Ni(N,O-LPh)2] (3), [Pd(N,O-LPh)2] (4), [Co(N,O-LMe)2] (5), [Cu(N,O-LMe)2] (6), [Ni(N,O-LMe)2] (7) and [Pd
(N,O-LMe)2] (8). These homoleptic complexes showed the monoanionic ligands to be coordinated in a N,O-
bidentate mode via the pyridine-like nitrogen of the imidazo[1,5-a]pyridine skeleton and the phenolic
oxygen. The coordination mode of the ligands was confirmed by the single-crystal X-ray structure anal-
ysis of [Co(N,O-LPh)2] (1) and [Pd(N,O-LMe)2] (8). The palladium compounds [Pd(N,O-LPh)2] (4) and [Pd(N,
O-LMe)2] (8) proved to be good catalysts in the Heck reaction between iodobenzene and different olefins.
Ó 2017 Elsevier B.V. All rights reserved.
Received 13 July 2017
Received in revised form 18 September
2017
Accepted 22 November 2017
Available online 24 November 2017
Keywords:
Heck reaction
N,O-ligands
Imidazo[1,5-a]pyridine
Crystal structure
Homogeneous catalysis
DFT calculations
1. Introduction
recently emerged as low cost emitters with large Stokes’ shift
[26] and as potential ligands in the synthesis of luminescent tran-
One of the most powerful CAC bond forming transformation is
the Mizoroki-Heck reaction [1,2] which was originally proposed as
the coupling between aryl or vinyl halides and activated alkenes, in
the presence of a base. Since then this reaction has been intensively
investigated and nowadays a broader range of both intermolecular
[3–5] and intramolecular [6] versions are available. The use of chi-
ral ligands also allowed for asymmetric [7–9] Heck couplings to
take place. The Heck reaction is usually catalyzed by palladium
and involves a Pd(0)/Pd(II) cycle [10,11]. However, recent exam-
ples on the use of different palladium(II) catalysts which work with
a Pd(II)/Pd(IV) cycle [12,13] or even other transition metals like
cobalt, copper or nickel [14] have been reported in the literature.
Imidazo[1,5-a]pyridines constitute a very important class of
heterocyclic compounds [15–19] whose interesting photochemical
and biological properties are widely known in the literature. They
find application in several fields of research, from pharmaceutical
uses based on their antiviral [20] or antibacterial [21] activities
to utilization in Organic Light Emitting Diodes (OLEDs) [22–24]
or thin-layer Field Effect Transistors (FET) [25]. In particular, they
sition metal compounds [27–31]. Finally, imidazo [1,5-a]pyridine-
based complexes have been recently employed as catalysts in ole-
fin epoxidation [32] or transfer hydrogenation of ketones [33].
In the past, we widely explored the coordination chemistry of
N-based multidentate ligands [34–37], with special attention to
pyrazole and its analogues (imidazole, triazole) both in their neu-
tral [38–40] and anionic azolate forms [41]. In this investigation
on N-based ligands, we lately conducted a study on the photophys-
ical properties of zinc(II) [42–43] and silver(I) [44] compounds
with 3-aryl-substituted 1-pyridylimidazo [1,5-a]pyridines. In these
luminescent complexes, the imidazo-pyridine ligands coordinate
to the metal centers with the pyridine ring (Npy) and the pyri-
dine-like nitrogen of the imidazo[1,5-a]pyridine group (Nim), thus
acting as N,N-bidentate neutral ligands. In this context, herein we
employed two similar ligands, 2-(1-phenylimidazo[1,5-a]pyridin-
3-yl)phenol (N,OH-LPh) and 2-(1-methylimidazo[1,5-a]pyridin-3-
yl)phenol (N,OH-LMe) to synthesize complexes of divalent metals.
As established by X-ray diffraction analysis, the anionic form of
N,OH-LPh and N,OH-LMe coordinate to the metal centers in a N,O-
bidentate mode. The synthesized compounds were used as cata-
lysts in the Heck reaction, showing good catalytic activity in the
⇑
Corresponding author.
E-mail address: stefano.brenna@uninsubria.it (S. Brenna).
https://doi.org/10.1016/j.ica.2017.11.039
0020-1693/Ó 2017 Elsevier B.V. All rights reserved.

G.A. Ardizzoia et al. / Inorganica Chimica Acta 471 (2018) 384–390
385
intermolecular coupling between iodobenzene and different ole-
fins in the case of Pd-containing derivatives.
C38H26CuN4O2 (%): C, 71.97; H, 4.13; N, 8.83. Found (%): C, 72.10;
H, 4.32; N, 8.88.
2.2.4. Synthesis of [Ni(N,O-LPh)2] (3)
2. Experimental
A suspension of NiCl2 6H2O (0.195 g, 0.820 mmol) and ligand N,
OH-LPh (0.495 g, 1.729 mmol) in acetonitrile (15 mL) was treated
2.1. Materials and methods
with Et3N (290
lL, 2.859 mmol). A yellow suspension formed,
which was stirred at room temperature for 12 h. Then it was fil-
tered and the solid washed with water, acetonitrile and diethy-
lether to give a yellow product. Yield: 0.370 g (72%). Anal. Calcd
for C38H26NiN4O2 (%): C, 72.52; H, 4.16; N, 8.90. Found (%): C,
72.66; H, 4.35; N, 8.93.
All reactions were carried out under purified nitrogen using
standard Schlenk techniques. Solvents were dried and distilled
according to standard procedures prior to use. NMR spectra were
recorded with an AVANCE 400 Bruker spectrometer at 400 MHz
for 1H NMR and 100 MHz for 13C{1H} NMR. Chemical shifts are
given as d values in ppm relative to residual solvent peaks as the
internal reference. Elemental analyses were obtained with a Per-
kin-Elmer CHN Analyzer 2400 Series II. Infrared Spectra were
acquired on a Shimadzu Prestige 21 instrument with a 1-cmÀ1 res-
olution. ATR-IR Spectra were acquired on a Nicolet iS10 FT-IR
instrument equipped with Smart iTR ATR accessory with a 1-
cmÀ1 resolution. Quantitative analyses of products in the catalytic
runs were performed on a Finningan Trace GC with a DB-5MS UI
capillary column (30 m, 0.25 mm) equipped with a Finningan Trace
Mass Spectrometer. Ligand 2-(1-phenylimidazo[1,5-a]pyridin-3-
yl)phenol (N,OH-LPh) was prepared according to literature methods
[45,27], whereas 2-(1-methylimidazo[1,5-a]pyridin-3-yl)phenol
(N,OH-LMe) was prepared by a slight modification of the same pro-
cedure. All other chemicals were of reagent grade quality from
commercial sources (Aldrich, TCI Chemicals) and used as received.
2.2.5. Synthesis of [Pd(N,O-LPh)2] (4)
A suspension of K2PdCl4 (0.150 g, 0.460 mmol) and ligand N,
OH-LPh (0.280 g, 0.978 mmol) in dichloromethane (10 mL) was
treated with Et3N (220 lL, 1.602 mmol). The addition of the base
slowly afforded a dark orange solution, which was stirred at room
temperature for 3 days. Then the white solid (KCl) was removed by
filtration over a pad of Celite, and the filtrate was evaporated to
dryness. The resulting crude product was suspended in water,
the suspension was filtered and the orange solid washed with
diethylether. Yield: 0.258 g (83%). Anal. Calcd for C38H26N4O2Pd
(%): C, 67.41; H, 3.87; N, 8.28. Found (%): C, 66.99; H, 4.60; N, 8.41.
2.2.6. Synthesis of [Co(N,O-LMe)2] (5)
A suspension of CoCl2 6H2O (0.100 g, 0.365 mmol) and ligand N,
OH-LMe (0.175 g, 0.780 mmol) in acetonitrile (15 mL) was treated
with Et3N (130 lL, 0.934 mmol). The resulting light brown suspen-
sion was stirred at room temperature for 2 h. Then it was filtered
2.2. Syntheses
and the solid washed with water, acetonitrile and diethylether to
2.2.1. Synthesis of ligand N,OH-LMe
give a light brown product. Yield: 0.120 g (66%). Anal. Calcd for C28
-
A mixture of 2-acetylpyridine (1 mL, 8.92 mmol, 1 eq), salicy-
laldehyde (1.9 mL, 17.83 mmol, 2 eq) and ammonium acetate
(3.44 g, 44.58 mmol, 5 eq) in glacial acetic acid (30 mL) was stirred
at room temperature for 7 days. Then the mixture was poured into
150 mL of water and extracted with CH2Cl2 (200 mL). The organic
phase was washed with a saturated aqueous solution of NaHCO3,
dried with Na2SO4 and the suspension was filtered. The solvent
was evaporated under vacuum, and the crude product was crystal-
lized with hexane. Yield: 1.915 g (75%). Anal. Calcd for C14H12N2O
(%): C, 74.98; H, 5.39; N, 12.49. Found (%): C, 74.66; H, 5.31; N,
12.08. 1H NMR (400 MHz, CDCl3): d 12.48 (s, 1H), 8.92 (d, 1H, J =
7,3 Hz), 8.28 (d, 1H, J = 7,8 Hz), 8.01 (d, 1H, J = 9,1 Hz), 7.87 –
7.78 (m, 1H), 7.73 (d, 1H, J = 8,1 Hz), 7.54 (t, 1H, J = 7,6 Hz), 7.30
(t, 1H, J = 6,3 Hz), 7.20 (t, 1H, J = 6,9 Hz), 3.15 (s, 3H).
H
22CoN4O2 (%): C, 66.54; H, 4.39; N, 11.08. Found (%): C, 66.78; H,
4.43; N, 11.17.
2.2.7. Synthesis of [Cu(N,O-LMe)2] (6)
A suspension of anhydrous CuCl2 (0.250 g, 1.859 mmol) and
ligand N,OH-LMe (0.875 g, 3.902 mmol) in acetonitrile (30 mL)
was treated with Et3N (0.660 mL, 4.744 mmol). After stirring the
suspension at room temperature for 2 h, it was filtered and the
solid washed with water, acetonitrile and diethylether to give a
light grey product. Yield: 0.650 g (68%). Anal. Calcd for
C28H22CuN4O2 (%): C, 65.94; H, 4.35; N, 10.98. Found (%): C,
65.72; H, 4.21; N, 10.71.
2.2.8. Synthesis of [Ni(N,O-LMe)2] (7)
A suspension of NiCl2 6H2O (0.195 g, 0.820 mmol) and ligand N,
2.2.2. Synthesis of [Co(N,O-LPh)2] (1)
OH-LMe (0.390 g, 1.739 mmol) in acetonitrile (15 mL) was treated
A suspension of CoCl2 6H2O (0.120 g, 0.504 mmol) and ligand N,
with Et3N (290
lL, 2.859 mmol). The resulting yellow suspension
OH-LPh (0.300 g, 1.048 mmol) in acetonitrile (15 mL) was treated
was stirred at room temperature for 12 h, then it was filtered
and the solid washed with water, acetonitrile and diethylether to
give a yellow product. Yield: 0.290 g (70%). Anal. Calcd for
28H22NiN4O2 (%): C, 66.57; H, 4.39; N, 11.09. Found (%): C,
66.38; H, 4.12; N, 10.87.
with Et3N (180 lL, 1.294 mmol). An orange-to-brown suspension
formed, which was stirred at room temperature for 2 h. Then it
was filtered and the solid washed with water, acetonitrile and
diethylether to give a light brown product. Yield: 0.304 g (96%).
Anal. Calcd for C38H26CoN4O2 (%): C, 72.49; H, 4.16; N, 8.90. Found
(%): C, 71.99; H, 4.44; N, 8.79. X-ray quality single crystals (1
C2H4Cl2) were grown by slow diffusion of hexane into a 1,2-
dichloroethane solution of 1.
C
2.2.9. Synthesis of [Pd(N,O-LMe)2] (8)
A suspension of K2PdCl4 (0.150 g, 0.460 mmol) and ligand N,
OH-LMe (0.220 g, 0.978 mmol) in dichloromethane (10 mL) was
treated with Et3N (220 lL, 1.602 mmol). The addition of the base
2.2.3. Synthesis of [Cu(N,O-LPh)2] (2)
afforded a dark solution, which was stirred at room temperature
for 3 days. Then the white solid (KCl) was removed by filtration
over a pad of Celite, and the filtrate was evaporated to dryness.
The resulting crude product was suspended in water, the suspen-
sion was filtered and the orange solid washed with diethylether.
Yield: 0.198 g (78%). Anal. Calcd for C28H22N4O2Pd (%): C, 60.82;
H, 4.01; N, 10.13. Found (%): C, 60.92; H, 4.33; N, 10.38. (NMR data
A suspension of anhydrous CuCl2 (0.340 g, 2.827 mmol) and
ligand N,OH-LPh (1.700 g, 5.937 mmol) in acetonitrile (40 mL)
was treated with Et3N (1.0 mL, 7.188 mmol). The suspension was
stirred at room temperature for 2 h, then it was filtered and the
solid washed with water, acetonitrile and diethylether to
give a light grey product. Yield: 1.344 g (75%). Anal. Calcd for

386
G.A. Ardizzoia et al. / Inorganica Chimica Acta 471 (2018) 384–390
refer to the major isomer, see text) 1H NMR (400 MHz, CD2Cl2): d
8.36 (d, 1H, J = 7.3 Hz), 7.68 (d, 1H, J = 8.0 Hz), 7.38 (d, 1H, J = 9.3
Hz), 7.18 (t, 1H, J = 8.0 Hz), 6.99 (d, 1H, J = 8.2 Hz), 6.90 (t, 1H, J =
7.5 Hz), 6.64 (dd, 1H, J = 9,1 Hz, J = 6.3 Hz), 6.55 (t, 1H, J = 6.8 Hz),
2.46 (s, 3H). 13C{1H} NMR (100 MHz, CD2Cl2): d 155.63, 133.47,
129.22, 126.62, 126.20, 123.84, 122.10, 118.75, 118.49, 117.87,
117.42, 113.71, 113.65, 11.87. X-ray quality single crystals were
grown by slow diffusion of diethylether into an ethanol solution
of 8.
2.2.10. Catalytic Heck reactions
In a schlenk tube, iodobenzene (30 mmol) and hexamethylben-
zene (100 mg, internal standard) were dissolved in 10 mL of DMF,
under an inert atmosphere. The solution was heated up to 100 °C,
then the olefin (36 mmol), K2CO3 (36 mmol) and the catalyst (0.3
mmol) were added and the progress of the reaction was monitored
at GC–MS.
Fig. 1. ORTEP representation of [Co(N,O-LPh)2] at 50% probability level. Selected
bond lengths (Å) and angles (°): Co(1)-N(1) 2.011(2), Co(1)-O(1) 1.894(2); N(1)-Co
(1)-N(1)i 104.48(9), N(1)-Co(1)-O(1) 95.95(6), O(1)-Co(1)-O(1)i 126.19(9), N(1)-C
(13)-C(14)-C(19)-39.2 (3), N(1)-C(7)-C(6)-C(1)-32.1(3), O(1)-C(1)-C(6)-C(7) 7.38(3)
(i = Àx, y, 0.5 À z).
2.3. X-ray diffraction analysis
Crystals of complexes [Co(N,O-LPh)2] C2H4Cl2 (1 C2H4Cl2) and
[Pd(N,O-LMe)2] (8) were mounted on a Stoe Image Plate Diffraction
system equipped with a / circle goniometer, using Mo-Ka graphite
monochromated radiation (k = 0.71073 Å) with / range 0-200°.
The structures were solved by direct methods using the program
SHELXS-97, while the refinement and all further calculations were
carried out using SHELXL-97 [46]. Examination of the structure of
complex 8 by PLATON [47] revealed a void of 87 Å3. This void was
considered to be a highly disordered ethanol molecule, which was
removed using the squeeze algorithm from PLATON. Otherwise, in
both structures the H-atoms were included in calculated positions
and treated as riding atoms using the SHELXL default parameters,
while the non-H atoms were refined anisotropically, using
weighted full-matrix least-square on F2. Crystallographic details
are summarized in Table 1. Figs. 1 and 2 were drawn with ORTEP
[48].
Fig. 2. ORTEP representation of [Pd(N,O-LMe)2] at 50% probability level. Selected
bond lengths (Å) and angles (°): Pd(1)-N(1) 2.034(4), Pd(1)-O(1) 2.011(3); N(1)-Pd
(1)-O(1) 86.6(1), O(1)-Pd(1)-N(1)i 93.4(1); N(1)-C(7)-C(6)-C(1) 32.1(3), N(1)-C(7)-N
(2)-C(8) 174.4(4) (i = Àx, 1 À y, Àz).
Table 1
Crystallographic and structure refinement parameters for complexes [Co(N,OLPh)2]
C2H4Cl2 (1 C2H4Cl2) and [Pd(N,O-LMe)2] (8).
1
C2H4Cl2
40H30Cl2CoN4O2
8
Chemical formula
Formula weight
Crystal system
Space group
Crystal color & shape
Crystal size
a (Å)
C
C26H22N4O2Pd
552.90
CCDC-1556974 [Co(N,O-LPh)2] C2H4Cl2 (1 C2H4Cl2) and 1556975
[Pd(N,O-LMe)2] (8) contain the supplementary crystallographic data
for this paper. These data can be obtained free of charge at www.
ccdc.cam.ac.uk/conts/retrieving.html [or from the Cambridge Crys-
tallographic Data Centre, 12, Union Road, Cambridge CB2 1EZ, UK;
fax: (internat.) +44-1223/336-033; E-mail: deposit@ccdc.cam.ac.
uk].
728.51
monoclinic
C2/c (No. 15)
brown block
0.18 Â 0.16 Â 0.16
23.8034(8)
12.9487(5)
11.3680(4)
90
monoclinic
P 21/c (No. 14)
orange block
0.21 Â 0.19 Â 0.18
11.0865(7)
7.5203(3)
16.6150(12)
90
b (Å)
c (Å)
(°)
(o)
113.368(2)
90
116.943(5)
90
c
(°)
V (Å3)
Z
3216.5(2)
4
1234.90(13)
2
2.4. Computational details
T (K)
173(2)
1.504
0.745
1.83 < < 25.10
2867
2406
0.0461
0.0317, wR2 0.0802
0.0401, wR2 0.0837
1.052
0.463, À0.463
173(2)
1.487
0.784
2.06 < < 25.15
2194
1537
0.0805
0.0406, wR2 0.0774
0.0780, wR2 0.0890
1.061
0.375, À0.515
All calculations were carried out at the density functional (DFT)
level of theory with the ADF2017.105 program package [49–51].
The hybrid functional PBE0 [52–54] was employed for geometry
optimization. The PBE0 exchange correlation functional is based
on the generalized gradient corrected exchange correlation func-
tional by Perdew-Burke-Ernzerhof (PBE) [52–54] with a fixed
amount of 25% of Hartree-Fock exchange energy. C, H, N, and O
atoms were described through TZ2P basis sets [triple-f Slater-type
orbitals (STOs) plus two polarization function]; the QZ4P basis set
(quadruple-f STO plus four polarization functions) was used for the
Pd atom. Scalar relativistic effects on palladium atoms were trea-
Dc (g.cmÀ3
)
(mmÀ1
)
Scan range (o)
Unique reflections
Reflections used [I2(I)]
Rint
Final R indices [I2(I)]*
R indices (all data)
Goodness-of-fit
Max, Min /e (ÅÀ3
)
*Structures were refined on F02: wR2 = [
R
[w(F20 À F2c)2]/
R
w(F02)2]1/2, where wÀ1 = [
R
(F20) + (aP)2 + bP] and P = [max(F02, 0) + 2Fc2]/3.

G.A. Ardizzoia et al. / Inorganica Chimica Acta 471 (2018) 384–390
387
ted by applying the zeroth-order regular approximation (ZORA)
[55,56]. Restricted formalism, no-frozen-core approximation (all-
electron) and no-symmetry constrains were used in all calcula-
tions. Complexes geometries were optimized simulating solvent
effects (ethanol, CH2Cl2) employing the conductor-like continuum
solvent model (COSMO) [57–59] as implemented in the ADF suite.
Frequency calculations were performed for all optimized struc-
tures to establish the minima nature of stationary points.
compounds 3 and 7 (Figs. S11 and S16). In this case, both square
planar and tetrahedral geometry around nickel(II) could be possi-
ble according to the proposed stoichiometry of the complexes.
The broad signals in the NMR spectrum suggest a paramagnetic
scenario for the nickel center, which consequently denotes a
tetrahedral geometry in both complexes. Also the NMR investiga-
tion on the palladium complexes revealed interesting information
about the nature of these compounds. The 1H NMR spectrum of
[Pd(N,O-LPh)2] (4) (Figs. S12–S13) is characterized by a complex
pattern in the aromatic region: we attributed such a behavior to
the presence of two isomers, respectively designated as cis-[Pd
(N,O-LPh)2], cis-(4), and trans-[Pd(N,O-LPh)2], trans-(4), which orig-
inate from the possible mutual disposition of nitrogen and oxygen
binding atoms of the two ligand molecules. Unfortunately, the
complexity of the pattern did not allow a real quantification of
the two isomers in solution. A similar (but better) situation has
been observed for the methyl-containing derivative [Pd(N,O-
LMe)2] (8). Again, the presence of two possible isomers, cis-(8)
and trans-(8), has been detected, this time with a definitely higher
predominance of one species. Indeed, a combined 1D and 2D
analysis including (Figs. S19–S20) allowed to identify all the sig-
nals belonging to the major isomer in solution (CD2Cl2, 25 °C).
From the integration of peaks area in 1H NMR, we approximately
calculated a major:minor isomer ratio of 4:1. DFT calculations
performed on complexes cis-(8) and trans-(8) (see section 3.3)
later supported the possibility to have a mixture of the two iso-
mers in solution.
3. Results and discussion
3.1. Syntheses and spectroscopic characterization
Ligand 2-(1-phenylimidazo[1,5-a]pyridin-3-yl)phenol (N,OH-
LPh) (Scheme 1) was obtained as a light yellow powder according
to a published procedure [45,27] and its purity was assessed by
1H NMR and elemental analysis. The same procedure was initially
applied to the synthesis of 2-(1-methylimidazo[1,5-a]pyridin-3-yl)
phenol (N,OH-LMe
) (Scheme 1), by simply replacing 2-ben-
zoylpyridine with 2-acetylpyridine, but a mixture of undefined
products was systematically isolated. This was probably due to
the high temperature involved. Therefore, we performed the reac-
tion at room temperature, and optimized the working conditions
by extending the reaction time to one week. By doing so, ligand
N,OH-LMe was obtained as a pure orange solid in very high yield.
Complexes 1–8 have been prepared following similar proce-
dures (Scheme 1), by reacting ligand N,OH-LR with the correspond-
ing metal(II) salt, in acetonitrile (M = Co, Cu, Ni) or in
dichloromethane (M = Pd), at room temperature, in the presence
of Et3N. They are formulated respectively as [Co(N,O-LPh)2] (1),
[Cu(N,O-LPh)2] (2), [Ni(N,O-LPh)2] (3), [Pd(N,O-LPh)2] (4), [Co(N,O-
LMe)2] (5), [Cu(N,O-LMe)2] (6), [Ni(N,O-LMe)2] (7), [Pd(N,O-LMe)2]
(8) (Scheme 1). All complexes are air and moisture stable, and have
been characterized by elemental analysis and infrared spec-
troscopy. In their infrared spectra recorded with ATR-IR technique
(Supplementary Material, Fig. S1–S8), they show the typical pat-
tern of the coordinated ligands, with sharp and intense stretching
bands in the region 1515–1615 cmÀ1 and an additional band of
3.2. X-ray crystal structures
Two complexes, namely [Co(N,O-LPh)2] (1) and [Pd(N,O-LMe)2]
(8), were characterized via single-crystal X-ray structure analysis
(Figs. 1 and 2). As expected, in both structures the two N,O-LR
ligands are bound in a N,O-bidentate coordination mode via the
deprotonated phenolic oxygen and the pyridine-like nitrogen of
the imidazo-part (Nim) of the ligand. The molecular structures of
complexes 1 and 8 are quite different from those observed for
other M(II) (M = Co, Zn) derivatives bearing a 2-(imidazo[1,5-a]
pyridin-3-yl)phenol ligand [60]. Indeed, in that case the absence
of any substituent in position 1 of the imidazo[1,5-a]pyridine scaf-
fold led to di- or trinuclear complexes, with the deprotonated
ligand often bridging two adjacent metal centers. In our case, the
presence of the methyl or the phenyl residue increased the steric
hindrance of the ligand, thus most probably forcing the formation
of mononuclear species. Consequently, in compounds 1 and 8 the
metal centers are in a four coordinated N,O-N,O environment. In
medium intensity around 1630 cmÀ1
(tC@N). Furthermore, in the
case of N,O-LMe derivatives 5–8, the
tC@H stretchings of the methyl
group are detected in the 2900–3100 cmÀ1 region.
As expected, due to the presence of the paramagnetic M(II) ion
the NMR spectra recorded for the cobalt (1 and 5) and copper (2
and 6) derivatives are characterized by broad and undefined sig-
nals, often spread out over a wide spectral window (Figs. S9–
S10 and S14–S15). A similar pattern is observed also for the nickel
OH
OH
N
N
N
N
CH3
Ph
N,OH-LPh
N,OH-LMe
Et3N
solvent
M(N,O-LR
)
2
2 N,OH-LR
+
2 Et3NHX
+
MX2
1
2
3
4
R = Ph, M = Co ( ), Cu ( ), Ni ( ), Pd ( )
5
6
7
8
R = CH3, M = Co ( ), Cu ( ), Ni ( ), Pd ( )
Fig. 3. Differences in Gibbs free energies for the optimized structures of cis-[Pd(N,
O-LMe)2] (cis-(8)) and trans-[Pd(N,O-LMe)2] (trans-(8)) in vacuum, in CH2Cl2 and in
ethanol.
Scheme 1. Imidazo[1,5-a]pyridine-based ligands used in this study (top) and
syntheses of the M(II) complexes (bottom).

388
G.A. Ardizzoia et al. / Inorganica Chimica Acta 471 (2018) 384–390
[Co(N,O-LPh)2], the Co-N and Co-O bond distances measure respec-
tively 2.011(2) and 1.894(2) Å, whereas angles around the cobalt
atom are representative of a tetrahedral geometry (N(1)-Co(1)-N
oxygen atoms of the phenolic residue are in a trans configuration
around the metal center. The Pd-N and Pd-O bond distances are
2.034(4) and 2.011(3) Å respectively, whereas basal angles desig-
nate a perfect square planar geometry around the palladium (N
(1)-Pd(1)-N(1) = 180.0(2)°, O(1)-Pd(1)-O(1) = 180.0(2)°, O(1)-Pd
(1)-N(1)i = 93.4(1)°, N(1)-Pd(1)-O(1)i = 86.6(1)°). Like complex 1,
the dihedral angles in 8 show that the phenyl-imidazo[1,5-a]pyri-
dine moiety of the N,O-LMe ligands is quite distorted from a planar
configuration, the angle between the plane of the phenolate and
the plane of the imidazo-pyridine skeleton being 37.7°.
(1) = 104.48(9)°,
N(1)-Co(1)-O(1) = 95.95(6)°,O(1)-Co(1)-O(1) =
126.19(9)°). Indeed, by applying the equation proposed by Houser
[61] to these angles, a s4 value of 0.917 is obtained, thus speaking
for a nearly ideal tetrahedral geometry for cobalt. Dihedral angles
show that the phenyl-imidazo[1,5-a]pyridine moiety of the N,O-
LPh ligand is quite far from a planar configuration: the C(12)-C
(13)-C(14)-C(19) torsion angle being 140.3(2)°. The phenolic ring
in position 3 of the imidazo[1,5-a]pyridine rotates along the C
(13)-C(14) bond, and the two planes containing the 3-aryl and
the imidazo-pyridine skeleton being twisted by 39.1°.
3.3. DFT study on Pd(II) derivatives
In [Pd(N,O-LMe)2] (8) (Fig. 2), the palladium center is in a square
planar environment comprising two phenolic oxygen and two
nitrogen atoms from the two imidazo[1,5-a]pyridine ligands. The
The molecular structure of both cis and trans isomers of com-
plexes [Pd(N,O-LMe)2] (8) and [Pd(N,O-LPh)2] (4), cis-(8), trans-(8),
cis-(4) and trans-(4), have been optimized at the Density Functional
Table 2
Outcomes of Heck reaction catalyzed by complexes [Pd(N,O-LPh)2] (4) and [Pd(N,O-LMe)2] (8).a
Entry
1
cat.
olefin
solvent
THF
T (°C)
t (h)
25
Olefin Conversion (%)b
<5%
A (%)
–
B (%)
–
C (%)
–
8
70
2
3
4
8
8
8
DMF
DMF
DMF
70
25
15
2.5
24%
95%
98%
94
6
5
–
–
–
–
100
100
95
100
O
O
5
8
DMF
100
8
99%
4
–
96
O
6c
7
8
4
4
DMF
DMF
DMF
100
100
100
15
30
20
95%
90%
95%
94
6
5
–
–
–
–
95
8
100
O
O
9
4
DMF
100
25
95%
7
–
93
O
10
11
5
5
DMF
DMF
100
100
30
30
<5%
10%
–
–
3
–
–
97
O
O
O
O
O
O
12
13
6
6
DMF
DMF
100
100
30
30
<5%
<5%
–
–
–
–
–
–
14
15
7
7
DMF
DMF
100
100
30
30
15%
32%
95
98
5
2
a
b
c
General conditions: cat.: iodobenzene: olefin = 0.3: 30: 36 (mmol). Base: K2CO3 (36 mmol). Solvent: 10 mL.
Olefin conversion determined by GC–MS analysis (internal standard: hexamethylbenzene).
Bulk reaction filtered on Celite after complete conversion.

G.A. Ardizzoia et al. / Inorganica Chimica Acta 471 (2018) 384–390
389
I
level of Theory (DFT) using the PBE0 [52–54] functional. All opti-
mized structures (Supplementary Information) are minima in the
potential surface of the molecule, as the calculation of the Hessian
gave no imaginary frequencies. Concerning cis-(8) and trans-(8),
there are no large differences between the gas-phase structures
and those calculated in the presence of solvent (CH2Cl2 and etha-
nol). The largest deviation among bond distances is about 0.01 Å,
while those of bond angles are less than 1°. Thermodynamic data
obtained from frequencies analysis demonstrated that in the
absence of any solvent interactions (i.e., in the gas phase) the trans
R
[Pd] (1%mol)
R
+
+
+
DMF
100°C
R
A
B
C
Scheme 2. Coupling of iodobenzene with different olefins catalyzed by complexes
[Pd(N,O-LPh)2] (4) and [Pd(N,O-LMe)2] (8).
1–4 and consequently supporting the active role played by com-
pound 8 in entries 1–4. Similar results were then reached using
complex [Pd(N,O-LPh)2] (4) as catalyst, with the exception of longer
reaction times needed than in previous runs. This effect is reason-
ably due to the residue in position 1 of the imidazo-pyridine skele-
ton in ligand N,O-LPh: the phenyl substituent is less electron-
donating than the methyl in N,O-LMe, and this disfavor both the
oxidative addition and the reductive elimination steps in the cat-
alytic cycle [14].
isomer is slightly more stable (Fig. 3), the difference (Gtrans À Gcis
)
at 298 K being about 12 kJ/mol. In solution the situation slightly
changes: indeed, in CH2Cl2 the cis- form becomes more stable by
6.6 kJ/mol (Fig. 3) and in the more polar ethanol the stabilization
of cis-(8) with respect to trans-(8) is even superior (9 kJ/mol). The
plausible reason for this variation is the high dipole moment of
cis-(8) (lcis = 14.2 D) compared to the absence of dipole moment
shown by trans-(8) (cis-(8) adopts a C1 symmetry, whereas trans-
(8) has a Ci symmetry). Hence, a greater stability is expected for
With these outcomes in hands, we next tested the catalytic
activity of the other methyl-containing complexes, [Co(N,O-LMe)2]
(5), [Cu(N,O-LMe)2] (6) and [Ni(N,O-LMe)2] (7), employing the same
conditions as before. Despite all the reactions were performed at
high temperatures (100 °C) and for prolonged times (30 h), com-
plexes 5, 6 and 7 showed poor catalytic activity. The cobalt deriva-
tive resulted only slightly active in the conversion of ethyl acrylate
(entry 11), whereas no activity was noticed with styrene as sub-
strate (entry 10). [Cu(N,O-LMe)2] did not show catalytic activity,
the products of the Heck reaction being recovered only in traces
(entries 12–13) regardless the olefin used. Finally, species 7 could
catalytically convert both the olefins in the desired products
(entries 14–15), but the yields was satisfactory only with ethyl
acrylate as substrate.
cis-(8) in polar solvents according to
DFT calculations.
l, as effectively attained by
However, the energy difference between cis-(8) and trans-(8) is
quite small, thus the isolation of the cis or trans isomer could be
attributed to small differences in the reaction and precipitation
conditions, with both isomers co-existing in solution. Indeed, a
careful inspection of the 1H NMR spectrum of complex 8 (see sec-
tion 3.1) reveals the presence of a mixture of isomers with one iso-
mer present only in an extremely minor fraction. Then, we can
conclude that the crystallographic characterization of the trans iso-
mer may merely be attributed to solid-state effects, with both iso-
mers co-existing in solution. A similar situation was also found for
the two possible isomers of [Pd(N,O-LPh)2] (4), namely cis-(4) and
trans-(4). In this case, cis-(4) appears to be the more stable species
in the absence or presence of solvents (Fig. S21). These observa-
tions are in accordance with the NMR outcomes discussed above.
4. Conclusions
In this study, two imidazo[1,5-a]pyridine-based ligands (desig-
nated as N,OH-LPh and N,OH-LMe) have been prepared and reacted
with different divalent metals (M = Co, Cu, Ni, Pd). The correspond-
ing complexes have been characterized by elemental analysis and
infrared spectroscopy. 1H NMR investigation allowed to confirm
the paramagnetic nature of Co, Cu and Ni compounds, whereas
1H and 13C NMR were used to shed light on the palladium deriva-
tives. Two of these complexes, namely [Co(N,O-LPh)2] (1) and [Pd
(N,O-LMe)2] (8), were characterized via X-ray single crystal diffrac-
tion. The imidazo[1,5-a]pyridine ligand is N,O-coordinated via the
pyridine-like nitrogen of the imidazo[1,5-a]pyridine skeleton
(Nim) and the phenolic oxygen. Differently from what was observed
for other reported M(II) compounds bearing similar N,O-bidentate
imidazo[1,5-a]pyridine ligands (with H in position 1), in our case
the ligands were not bridging two adjacent metal centers, but
forming mononuclear species. This is most likely related to the
increased steric hindrance given by the residue (Me or Ph) in posi-
tion 1 of the heterocyclic moiety. TD-DFT calculations performed
on the two possible isomers of compounds [Pd(N,O-LMe)2] and
[Pd(N,O-LPh)2] allowed to establish the relative energies of the dif-
ferent isomers. Finally, some compounds, namely [Co(N,O-LMe)2],
[Cu(N,O-LMe)2], [Ni(N,O-LMe)2], [Pd(N,O-LMe)2] and [Pd(N,O-LPh)2]
have been tested as catalysts in the Heck reaction between
iodobenzene and different olefins, giving high yields of conversion
in the case of palladium derivatives.
3.4. Catalytic Heck reactions
The catalytic activity in the Heck reaction of complexes 1–8 was
explored (Table 2), starting with the palladium(II) derivatives [Pd
(N,O-LPh)2] (4) and [Pd(N,O-LMe)2] (8). Iodobenzene was employed
as the reference aryl halide, whereas the olefin was varied from
styrene, 2-cyclohexen-1-one and ethyl acrylate (Scheme 2).
The reaction conditions (solvent, temperature) were optimized
using styrene as substrate (entry 1–3), potassium carbonate as
base and compound 8 as catalyst: DMF was chosen as solvent,
and the reaction temperature was fixed at 100 °C. Under these con-
ditions (entry 3), styrene was converted in the expected trans-stil-
bene with 95% selectivity, with no formation of biphenyl by-
product. A similar result was obtained using ethyl acrylate as sub-
strate (entry 4); in this case, the reaction time was shorter (2.5 h)
compared to entry 3 (15 h). A different behavior was observed in
the case of 2-cyclohexen-1-one (entry 5), with a scarce conversion
to the desired substituted olefin, biphenyl being the main by-prod-
uct. This can probably be ascribed to the higher steric hindrance of
the internal olefin with respect to the terminal ones. Worthy of
note, at the end of the reaction some palladium black was noticed
in the bulk. Thus, it is not excluded that the homo-coupling reac-
tion of iodobenzene leading to biphenyl was promoted by this Pd
(0) species somehow generated in the course of the reaction. Then,
an additional run was performed (entry 6) to rule out the possibil-
ity that also reactions in entry 1–4 were catalyzed by Pd(0). Indeed,
the same conditions as in entry 3 were applied, and after comple-
tion of the reaction the bulk was filtered over a pad of Celite. Nota-
bly, no traces of palladium black were recovered after filtration,
excluding the generation of palladium black in the catalytic runs
Acknowledgments
This work was supported by the Ministero dell’Università e
della Ricerca (MIUR) and the University of Insubria (grant CSR-12).

390
G.A. Ardizzoia et al. / Inorganica Chimica Acta 471 (2018) 384–390
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