Direct synthesis of a new class of N,N,N ligands based on 1,2-dihydro-1,10-phenanthroline backbone and their coordination to Pd complexes

Article abstract of DOI:10.1039/b817726k

Reactions of acetylpyridine derivatives with 8-aminoquinolines provide a general and simple access to an unexpected class of versatile N,N,N ligands, which offer interesting perspectives in coordination chemistry.

Full text of DOI:10.1039/b817726k

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Direct synthesis of a new class of N,N,N ligands based on
1,2-dihydro-1,10-phenanthroline backbone and their
coordination to Pd complexes†
Claudine Rangheard,a David Proriol,a He´le`ne Olivier-Bourbigou*a and Pierre Braunsteinb
Received 8th October 2008, Accepted 26th November 2008
First published as an Advance Article on the web 12th December 2008
DOI: 10.1039/b817726k
Reactions of acetylpyridine derivatives with 8-aminoqui-
nolines provide a general and simple access to an unexpected
class of versatile N,N,N ligands, which offer interesting
perspectives in coordination chemistry.
Tridentate N,N,N ligands centered on N-heterocycles, such as
bis(imino)pyridines, have been extensively studied in the last
few years because they provide interesting and tunable ligands
for coordination to transition metals, especially iron, nickel
and cobalt, and lead to highly active catalytic systems for
olefin polymerization or oligomerization.1–3 In the search for
alternative multidentate systems, Sun and Bortoluzzi recently
explored ligands containing Schiff bases resulting from the con-
densation of 8-aminoquinoline with aldehydo- or keto-pyridine
derivatives.2e,4,5 Whereas they did not succeed in isolating the pure
imino ligands, these authors employed a one-pot metal-assisted
synthesis to obtain the N-((pyridin-2-yl)methylidene)quinoline-8-
amine metal complexes in good yields. Herein, we report the first
successful direct synthesis and isolation of a pure N-((pyridin-2-
yl)methylidene)quinoline-8-amine derivative by condensation re-
action between 2,2-dimethyl-1-(pyridin-2-yl)-propane-1-one with
8-aminoquinoline (see Scheme 1, path A, 1). Moreover, the
reaction of 2-acetylpyridine with 8-aminoquinoline derivatives did
not yield the expected Schiff bases but instead an unprecedented
class of versatile N,N,N ligands based on a 2,4-bis(pyridin-2-yl)-2-
methyl-1,2-dihydro-1,10-phenanthroline scaffold (Scheme 1, path
B). This new ligand backbone provides not only a favourable
environment for the coordination to various metal centres but
also a structure which can be fine-tuned by changing the R
substituents and thus offers the opportunity to easily control
the metal coordination environment and therefore the catalytic
behaviour of the resulting complexes.
Scheme 1 Condensation reaction (path A and B) and complexation
to Pd.
=
a C N stretching vibration. Ligand 1 was also characterized by
GC-MS, NMR and X-ray diffraction (Fig. 1).
The reaction in Scheme 1, path A, was carried out in refluxing
toluene for 5 days in the presence of an acid catalyst. Such
conditions are needed because of the low reactivity of the hindered
ketone precursor. Compound 1 was obtained in ca. 35 wt% yield.
Its IR spectrum shows an absorption at 1644 cm-1, consistent with
aIFP-Lyon, Rond-point de l’e´changeur de Solaize, BP3, 69360, Solaize,
France. E-mail: helene.olivier-bourbigou@ifp.fr; Fax: +33 478022066
bLaboratoire de Chimie de Coordination, Institut de Chimie (UMR 7177
CNRS), Universite´ Louis Pasteur, 4 rue Blaise Pascal, 67070, Strasbourg,
France. E-mail: braunstein@chimie.u-strasbg.fr; Fax: +33 390241322
† Electronic supplementary information (ESI) available: Synthetic details
for all ligands and complexes described, crystallographic data for 1, 4, 6, 9.
CCDC reference numbers 701153–701156. For ESI and crystallographic
data in CIF or other electronic format see DOI: 10.1039/b817726k
Fig. 1 ORTEP plot of the molecular structure of 1 in CH2Cl2. Ellipsoids
are represented at 50% probability level.
770 | Dalton Trans., 2009, 770–772
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˚
=
The N(2)–C(10) distance of 1.276(2) A is typical for a C N
double bond. The imino-pyridine derivative 1 exists in the solid
=
state as the Z isomer at the C N bond, a configuration that is
clearly unfavourable for tridentate coordination to a metal centre.
The pyridine and 8-quinoline planes are nearly perpendicular
to the imine plane, with torsion angles N(2)–C(10)–C(15)–N(3)
of 93.70(19)◦ and C(9)–C(8)–N(2)–C(10) of 92.30(18)◦, and the
1
nitrogen atoms are placed on either side of this plane. H NMR
analysis of 1 indicates the presence of a single isomer in solution.
The condensation reactions of 2-acetylpyridine derivatives with
8-aminoquinoline were performed in refluxing methanol with
formic acid as a catalyst (Scheme 1, path B). The presence of both
the enolizable 2-acetylpyridine ketone and the 8-aminoquinoline
enables a Mannich-type reaction to take place, followed by a
cyclization under mild conditions which involves aromatic C–H
activation and C–C bond formation (Scheme 2). This method was
readily generalized for the synthesis of various 2,4-bis(pyridin-
2-yl)-2-methyl-1,2-dihydro-1,10-phenanthroline derivatives. Only
ligand 7 was obtained by a Suzuki coupling of ligand 6 with phenyl
boronic acid.
Fig. 2 ORTEP plot of the molecular structure of 4 grown in CH2Cl2.
Ellipsoids are represented at 50% probability.
The structural characterization of these new ligands has been
achieved by several analytical methods, such as 1H and 13C NMR,
1
mass spectroscopy and IR spectroscopy. The H and 13C NMR
spectra contain characteristic peaks: a singlet at 2.9 ppm for the
methyl group and a doublet around 6.2 ppm for the ethylene
proton. A broad signal at 6.9–7.1 ppm shows the presence of
the amine proton. In 13C NMR, the aliphatic quaternary carbon
peak is observed at 60 ppm.
In addition, the IR spectra of all compounds show a band
between 3380 cm-1 and 3360 cm-1, consistent with a N–H
stretching vibration. Ligands 4 and 6 were further characterized
by X-ray diffraction analysis.
Single crystals were obtained by slow evaporation of their
dichloromethane solution. The molecular structure of 4, shown
in Fig. 2, unambiguously established the existence of the 2,2,4-
substituted 1,2-dihydro-1,10-phenanthroline scaffold. The pres-
ence of a quaternary aliphatic carbon leads to a racemic mixture.
The pyridine ring at C10 forms adihedralangle of71.62(5)◦ with the
phenanthroline ring. The crystal structure of 6 is similar (Fig. 3).
These new, bi- or tridentate N,N,N ligands have obvious
potential in coordination chemistry and their stereoelectronic
environment could be readily modified by varying the substituents
in the precursors.
Fig. 3 ORTEP plot of the molecular structure of 6 grown in CH2Cl2.
Ellipsoids are represented at 50% probability.
Palladium complexes are of considerable importance in homo-
geneous catalysis6 and Pd(II) diimine systems are, for example,
capable of polymerizing ethylene and other a-olefins to form
high molecular weight polymers.7–9 Ligand 7 was reacted with
Scheme 2 Proposed mechanism for the formation of 2,4-bis(pyridin-2-yl)-2-methyl-1,2-dihydro-1,10-phenanthroline derivatives.
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[PdCl2(NCPh)2] in dichloromethane at room temperature and
˚
of the palladium complex (H–N(3), 2.16(6) A). The pyridine
ring on C(10) is nearly orthogonal to the phenanthroline ring
(C(9)–C(10)–C(14)–N(3), 88.0(5)◦). There is no direct interaction
between the palladium centre and the nitrogen pyridine atom with
complex 9 was isolated as an air-stable, red solid. The shifts
1
of its H NMR peaks provide evidence for the coordination of
the quinoline and amine nitrogen atoms to the palladium centre.
Compared with the free ligand 7, 9 displays a large downfield
shift for the aromatic proton attached to the carbon alpha to the
quinoline nitrogen. The amine proton shows a similar downfield
shift. The molecular structure of 9 was determined by X-ray
diffraction (Fig. 4).
˚
a distance of 3.79(8) A.
In conclusion, we have discovered a new, versatile class of N,N,N
ligands based on the 2,4-bis(pyridin-2-yl)-substituted 1,2-dihydro-
1,10-phenanthroline structure. They were readily synthesized
by the direct and general reaction of 2-acetylpyridine with 8-
aminoquinoline derivatives. These ligands coordinate to Pd(II) in a
bidentate mode. We are currently investigating their coordination
to iron and nickel and the application of these complexes for the
catalytic oligomerization and polymerization of ethylene.
Acknowledgements
We would like to thank IFP-Lyon and ANRT for financial support
and Drs R. Pattacini (LCC, Strasbourg) and L. Brelot (Institut de
Chimie, Strasbourg) for the X-ray analyses.
Notes and references
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Fig. 4 ORTEP plot of the molecular structure of 9. Ellipsoids are
represented at 50% probability level. H atoms and solvent molecules
omitted for clarity.
The palladium centre is coordinated by two cis nitrogen and
two chlorine atoms. The average deviation from the mean plane
C(12)–N(1)–Pd(1)–N(2) is 8.1(3)◦, which indicates a nearly planar
˚
coordination geometry. The Pd–Cl distances (2.291(1) A) and
(2.298(1) A) are within the usual range for such bonds,10,11 and the
˚
˚
Pd–Namine bond length (2.111(4) A) is slightly longer than Pd–Nphen
◦
˚
(2.028(3) A). The N(1)–Pd(1)–N(2) bite angle of 82.1(2) results in
a slightly distorted square planar structure, also observed in other
palladium complexes ligated by pyridyl-imine or bis(pyridine)
ligands.12
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The coordination of Pd to the amine nitrogen creates a second
stereogenic centre on N(2). Only two enantiomers, RR and SS,
are observed in the crystal lattice, although four diastereoisomers,
i.e. RR, RS, SS and SR, could have been present. We suggest that
steric hindrance imposes the bonds N(2)–H and C(10)–C(13)H3
to be placed on opposite sides of the phenanthroline plane. The
configuration of C(10) thus induces that of N(2). This also allows
a potential H-bonding with N(3) which improves stabilization
772 | Dalton Trans., 2009, 770–772
This journal is
The Royal Society of Chemistry 2009
©