Synthesis of new dipyridinylamine and dipyridinylmethane ligands and their coordination chemistry with Mg(II) and Zn(II)

Article abstract of DOI:10.1039/b807258b

Sterically hindered, symmetrically 2,2′-substituted bispyridylamines 1a-c and methylene-bispyridine 4 were prepared in a three-step procedure from commercially available 2,6-dibromopyridine, via a Cu-mediated alkylation followed by two consecutive Buchwald-Hartwig N-arylation reactions or two Negishi cross-coupling reactions, respectively, in the presence of Pd catalysts. Deprotonation of the NH and CH₂ bridge of 1a and 4, respectively, enables the formation of Zn(II) and Mg(II) complexes, whose structures have been determined by single-crystal diffraction studies and/or NMR spectroscopy. A magnesium-isopropyl complex stabilized by a bispyridyldiiminate ligand derived from 4 is shown to be an active catalyst for the isotactic polymerization of methyl methacrylate. The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2008.

Full text of DOI:10.1039/b807258b

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PAPER
www.rsc.org/njc | New Journal of Chemistry
Synthesis of new dipyridinylamine and dipyridinylmethane ligands and
their coordination chemistry with Mg(II) and Zn(II)w
a
b
a
´
Zhanjiang Zheng, Mohammed Kamal Elmkaddem, Cedric Fischmeister,
Thierry Roisnel,c Christophe M. Thomas,*ad Jean-Francois Carpentier*a
and Jean-Luc Renaud*be
¸
Received (in Montpellier, France) 29th April 2008, Accepted 10th June 2008
First published as an Advance Article on the web 28th August 2008
DOI: 10.1039/b807258b
Sterically hindered, symmetrically 2,20-substituted bispyridylamines 1a–c and methylene-
bispyridine 4 were prepared in a three-step procedure from commercially available 2,6-
dibromopyridine, via a Cu-mediated alkylation followed by two consecutive Buchwald–Hartwig
N-arylation reactions or two Negishi cross-coupling reactions, respectively, in the presence of Pd
catalysts. Deprotonation of the NH and CH2 bridge of 1a and 4, respectively, enables the
formation of Zn(II) and Mg(II) complexes, whose structures have been determined by single-
crystal diffraction studies and/or NMR spectroscopy. A magnesium–isopropyl complex stabilized
by a bispyridyldiiminate ligand derived from 4 is shown to be an active catalyst for the isotactic
polymerization of methyl methacrylate.
Introduction
Nitrogen ligands are able to bind strongly a large variety of
metal centers and can stabilize both very low and high oxida-
tion states.1–6 Ubiquitous examples of neutral nitrogen ligands
are diimines (including bipyridines),7–11 which have been in-
tensively studied in asymmetric catalysis with almost all
transition metals.2,12 Other neutral multidentate N-donor
Chart 1
ligands have also found use in various processes. For instance,
catalytic applications of mainly late transition metal com-
plexes have been reported for tridentate bis(pyrazolyl)amine,13
triazacyclononane,14 and pyridine-2,6-diimine15 ligands. A
notable achievement in this line was reported by Gibson
and Brookhart with the discovery of highly active iron
and cobalt ethylene polymerization catalysts containing bulky
pyridinediimine ligands.16
namely, monoanionic aza-acac ligands. Metal complexes of
ligands A and B have been successfully applied to a broad
range of industrially important catalytic reactions.18,19
However, minor structural variations within substrate
molecules may dramatically change the outcome of the
catalytic reaction. Basically, each substrate needs its own
optimized catalyst and ligand system. Hence, it is
significant advantage of a given ligand class if it allows easy
a
Other members of privileged nitrogen ligands that have
emerged over the past several years are b-diiminate ligands
A and bisoxazolines B (Chart 1).17 The common feature of
these ligands is that they usually act as three-electron chelates,
steric and electronic tuning of the structure.
As recently pointed out by Chelucci and Thumel,20 the
chemistry of those N-donor ligands mostly relies on the
formation of stable five- or (to a lesser extent) six-membered
chelates upon metal binding. Only a few pyridine-based
ligands with the potential to form six-membered chelates with
metals have been introduced. Wright’s di(pyridyl)silane,21
a
´
Sciences Chimiques de Rennes, Catalyse et Organometalliques,
UMR 6226: CNRS-Universite de Rennes 1, Campus de Beaulieu,
´
35042 Rennes Cedex, France.
E-mail: jean-francois.carpentier@univ-rennes1.fr;
christophe.thomas@univ-rennes1.fr; Fax: 33(0)223236939
b Sciences Chimiques de Rennes, Chimie Organique et
Chelucci’s dipyridylpropane,22 Na
´
jera’s dihydropyridyl-
methane23 and Kwong’s dipyridyl ketone24 are examples of
such neutral ligands. Recently, the groups of Kempe,25 Bolm26
and Kim27 showed the applicability of 2,20-dipyridylamines as
neutral ligands in transition metal-catalyzed polymerization,
allylic oxidation and transesterification, respectively. Burns
and Jordan also prepared olefin polymerization catalysts that
contain neutral bidentate dipyridylmethane.28 Despite these
interesting examples, the chemistry and catalytic applica-
tions of 2,20-dipyridylamines and 2,20-dipyridylmethane
remains limited, and, to our knowledge, no sterically hindered
bispyridyl derivative has been described in the literature.
´
Supramoleculaire, UMR 6226: Ecole Nationale Superieure de
Chimie de Rennes, 35700 Rennes, France.
´
E-mail: jean-luc.renaud@univ-rennes1.fr; Fax: 33(0)223238108
c Sciences Chimiques de Rennes, Diffraction Rayons-X, UMR 6226:
´
CNRS-Universite de Rennes 1, Campus de Beaulieu, 35042 Rennes
Cedex, France
d Ecole Nationale Supe´rieure de Chimie de Paris, 11 rue Pierre et
Marie Curie, 75231 Paris Cedex 05, France.
E-mail: christophe-thomas@enscp.fr
e
´
Ecole Nationale Superieure d’Ingenieur de Caen, 6 Boulevard du
Marechal Juin, 14050 CAEN, France
´
w CCDC reference numbers 682251–682253. For crystallographic data
in CIF or other electronic format see DOI: 10.1039/b807258b
ꢀc
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Scheme 3 Synthesis of 2-aryl-6-bromopyridines.
transmetallate aryl halides, we turned our attention to copper
chemistry.30 Among different copper sources, cuprates, and
more specifically those arising from CuI and CuCNꢁ2LiCl,31
provided better yields than neutral organocopper reagents.
Thus, treatment of 2,6-dibromopyridine with 1.5 equiv. of
in situ-generated tBu2CuLi or tBuCuCNꢁLiCl at ꢂ78 1C in
THF led to 2a in 66 and 94% yields, respectively (Scheme 2).
Neither reduction of the bromopyridine nor bis addition of the
alkyl substituent was observed. To the best of our knowledge,
this is the shortest procedure to prepare efficiently 2a.32
The introduction of aryl substituents was carried out via a
nickel-catalyzed cross-coupling reaction between a sterically
hindered aryl Grignard reagent and 2,6-dibromopyridine at
50 1C in dioxane.33 The corresponding 2-aryl-6-bromopyridines
2b–c were isolated in (non-optimized) yields of 46 and 35%,
respectively (Scheme 3).
Scheme 1 Strategy used for the synthesis of bispyridines 1 and 4.
Based on our studies in allylic substitutions catalyzed by
ruthenium complexes8a–d and in polymerization reactions
catalyzed by rare earth complexes bearing nitrogen ligands,19
we envisioned to prepare new modular dipyridylamines 1 and
dipyridylmethane (pro)ligands 4, starting from simple, com-
mercially available 2,6-dibromopyridine (Scheme 1). Due to
the formation of six-membered chelates, bulky and/or chiral
substituents at the 2,20-positions shall be closer to the metal
center.
Synthesis of bispyridylamine 2 and methylene bispyridine 4
Primary arylamines are typically prepared from ammonia
surrogate.34 Thus, by using benzophenone imine as the
ammonia source and bromopyridines 2a–c, in the presence
of a catalytic amount of palladium(0) and dppp, followed by
hydrolysis, the amino derivatives 3a–c were isolated in good
yields (68–73%, Scheme 4). The last step in the synthesis of the
new symmetrically-substituted 2,20-dipyridylamines 1a–c was
a second N-arylation between the corresponding aminopyri-
dine 3a–c and bromopyridine 2a–c via a palladium-catalyzed
We report here the synthesis of 6-tert-butyl-2-bromopyri-
dine 2 and of new symmetrically 2,20-substituted-dipyridyl-
amine 1 and 2,20-dipyridylmethane derivatives 4 (Scheme 1).
Preliminary studies on the coordination of those molecules as
monoanionic ligands onto Zn(II) and Mg(II) centers, and on
the reactivity of some of these complexes, are described
as well.
Results and discussion
Synthesis of 6-substituted-2-bromopyridines
The first step of our strategy required the addition of a
sterically hindered substituent onto 2,6-dibromopyridine.
For this purpose, we chose a tert-butyl group and two
differently substituted aryl derivatives as ortho-pyridine sub-
stituents. Two important issues have to be addressed: (i) this
addition must proceed without any (or significant) reduction
of the halopyridines, and (ii) no double addition of the
nucleophile must occur.
With alkyl derivatives, palladium and nickel catalysts often
led to elimination products from secondary and tertiary
Grignard reagents.29 As lithium reagents could also
Scheme 2 Synthesis of 2-bromo-6-tert-butylpyridine.
Scheme 4 Synthesis of bispyridylamines 1a–c.
ꢀc
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Scheme 5 Synthesis of dipyridylmethane 4.
Buchwald–Hartwig coupling reaction. In the presence of
2.5 mol% of Pd2(dba)3, 4 mol% of rac-BINAP35 and
1.5 equiv. of tBuOK in toluene at 90 1C, compounds 1a–c
were isolated in high yields (74–98%, Scheme 4).36,37
The crystal structure of the bulky derivative 1c has been
determined by an X-ray diffraction study (see ESIw).
The 2,20-tert-butyl-substituted dipyridylmethane
4 was
Scheme 6 Preparation of zinc complexes from L1H (1a).
prepared via two consecutive palladium-catalyzed Negishi
cross-coupling reactions (Scheme 5). Thus, the reaction of
in situ-generated methylzinc chloride and bromopyridine 2a,
in the presence of a catalytic amount of palladium(0) in THF at
room temperature, led to the corresponding 2-tert-butyl-6-
methylpyridine (5) in 76% isolated yield. Subsequent deproto-
nation, followed by a transmetallation step with zinc(II) chloride
and a coupling reaction in the presence of palladium(II)–dppf in
refluxing THF, provided the desired methylene-bridged dipyri-
dine 4 in a moderate yield of 48% (overall yield of 36%).
in the solid state with coordination of the ethyl residue,
nitrogen of one of the two pyridyl groups and the central N
atom which bridges between the two Zn centers; there is a
crystallographically imposed twofold symmetry (Fig. 1). One
pyridyl group of the ligand does not coordinate and its
nitrogen and tert-butyl substituent point in the opposite
direction to zinc, to minimize steric crowding. On the other
hand, the bis(ligand) complex 7 is monomeric in the solid state
(Fig. 2). Each ligand is coordinated to the zinc center via one
pyridyl nitrogen atom and the central bridging nitrogen atom.
As observed in 6, the nitrogen atoms of the coordinated and
free pyridine rings are arranged in a trans conformation, with
Coordination chemistry of bispyridylamine 1a and methylene
bispyridine 4
To explore the potential of these new dipyridines as mono-
anionic (pro)ligands, preliminary studies of their coordination
chemistry onto Zn(II) and Mg(II) centers were conducted. The
2,20-tert-butyl-substituted amino- and methylene-bridged
derivatives 1a (hereafter referred as L1H) and 4 (L2H) were
selected for this purpose.
Reaction of the amino-bridged pro-ligand L1H (1a) with
1 equiv. of ZnEt2 at 0 1C in toluene affords via ethane
elimination the ethyl-zinc complex [L1]ZnEt (6), which was
isolated after recrystallization in low yield (20%) as colorless
crystals (Scheme 6). Complex 6 is unstable in toluene solution
and disproportionates slowly at room temperature to the
bis(ligand) complex [L1]2Zn (7) and ZnEt2 (identified by 1H
NMR). When the alkane elimination reaction between ZnEt2
and 1 equiv. of L1H was performed at 30 1C, only 7 was
isolated as colorless crystals in moderate yield (36% vs. Zn).
As expected, reducing the amount of ZnEt2 to 0.5 equiv. vs.
L1H (i.e., using the exact stoichiometry) and carrying out the
alkane elimination reaction at 20 1C led to 7 in higher isolated
yield (60%).
Fig. 1 Solid-state structure of complex 6 (ellipsoids drawn at the 60%
probability, H atoms omitted for clarity) (the ‘‘_2’’ symbol in the atom
labels indicates that these atoms are at equivalent position (1 ꢂ x, y, 1/2
ꢂ z)). Selected bond distances (A) and angles (1): Zn(1)–C(1),
1.9703(19); Zn(1)–N(10), 2.2794(15); Zn(1)–N(10_2), 2.0386(16);
Zn(1)–N(20), 2.1363(15); Zn(1)–Zn(1_2), 2.9951(4); N(10)–C(11),
1.406(2). C(1)–Zn(1)–N(10), 119.08(8); C(1)–Zn(1)–N(20), 115.98(7);
C(1)–Zn(1)–N(10_2), 133.85(8); N(10)–Zn(1)–N(10_2), 92.25(6);
N(20)–Zn(1)–N(10), 62.07(5), Zn(1)–N(10)–C(11), 120.78(12).
Complexes 6 and 7 were authenticated by elemental ana-
lyses, 1H and 13C NMR spectroscopy, and single-crystal X-ray
diffraction studies (see Experimental section). In both
complexes, the Zn centers adopt four-coordinated, slightly
distorted tetrahedral geometries. Complex 6 exists as a dimer
ꢀc
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Scheme 7 Preparation of magnesium complex 8 from L1H (1a).
with 1 equiv. of iPrMgCl to generate the corresponding
low-coordinated magnesium complexes. Applying this pro-
cedure to the amino-bridged pro-ligand L1H (1a) led syste-
matically to the isolation of the bis(ligand) magnesium
complex [L1]2Mg (10) (Scheme 7). Modification of the reaction
temperature and the sequential order of addition of reagents
apparently had no influence on the reaction outcome. This
suggests that ‘‘[L1]Mg(iPr)’’ is not stable in solution and
rapidly undergoes disproportionation to form 10 (and
Mg(iPr)2), an observation which can be related to the afore-
mentioned disproportionation of the ethyl-zinc complex
[L1]ZnEt (6) (Scheme 6).
Fig. 2 Solid-state structure of complex 7 (ellipsoids drawn at the 50%
probability, H atoms omitted for clarity; only one of the four independent
molecules is depicted). Selected bond distances (A) and angles (1) (data in
parentheses refer to the range observed in the other three inde-
pendent molecules, to illustrate the slight deviations): Zn(1)–N(5),
2.117(6) (2.110(6)–2.118(6)); Zn(1)–N(7), 2.086(6) (2.088(6)–2.102(6));
Zn(1)–N(8), 1.964(5) (1.947(6)–1.963(6)); Zn(1)–N(9), 1.943(5)
(1.943(6)–1.951(6)); N(5)–C(53), 1.379(8); N(8)–C(53), 1.360(9);
N(7)–C(32), 1.343(9); N(9)–C(32), 1.395(8); N(5)–Zn(1)–N(8), 66.0(2)
(65.8(2)–66.4(2)); N(7)–Zn(1)–N(9), 66.2(2) (66.3(2)); Zn(1)–N(5)–C(53),
89.0(4); Zn(1)–N(8)–C(53), 96.2(4); N(8)–C(53)–N(5), 108.7(5);
Zn(1)–N(7)–C(32), 95.6(4); Zn(1)–N(9)–C(32), 91.0(4); N(7)–C(32)–N(9),
107.1(5).
More satisfactory results were obtained with the methylene-
bridged pro-ligand 4. When the reaction was carried out using
toluene as the solvent, the base-free three-coordinated com-
plex [L2]Mg(iPr) (11) was isolated in 20% yield. This complex
is, however, quite unstable in solution, decomposing to uni-
1
dentified products, as revealed by H NMR monitoring. On
the other hand, using diethyl ether as the solvent in the above
procedure led to the stable ether-adduct [L2]Mg(iPr)(OEt2)
(12) in 31% yield (Scheme 8). Attempts to grow crystals
suitable for X-ray diffraction studies have failed thus far,
the tert-butyl substituents pointing in opposite directions. The
Zn–N distances involving the central bridging
N atom
[Zn(1)–N(8), 1.964(5) A; Zn(1)–N(9); 1.943(5) A] compare well
with those observed in related complexes, e.g. the tri-coordinated
imidinate zinc complex [dipp-NacNac]ZnMe (dipp = 2,6-diiso-
propylphenyl) (N–Zn, 1.9480 and 1.9429 A)38a and in the
terminal imidinate ligand of the tetra-coordinated dinuclear
1
and complexes 11 and 12 were identified on the basis of H
and 13C NMR spectroscopy. Key 1H NMR data for 11 include
a set of three resonances in the range d 6.2–7.0 for the pyridine
hydrogens, one singlet at d 5.16 for the bridge CH hydrogen,
and a doublet at d 1.70 and a multiplet at d 0.42 for the
isopropyl group. Those isopropyl resonances are shifted up-
field compared with those in Gibson’s isopropylmagnesium
complex
bis(m2-N-(2-pyridyl)aniline-N,N0)-bis(N-(2-pyridyl)-
aniline-N,N0)dizinc (N(Ph)–Zn, 2.010 A).38b The N(pyr)-
Zn–N(Ph) angle of 65.141 in the latter complex38b is very similar
to that observed in 7 (66.0(2)1). Also, the Zn–N distances
involving the pyridyl-N atom in 7 [Zn(1)–N(5), 2.117(6) A;
Zn(1)–N(7), 2.086(6) A] compare well with that observed in the
aforementioned imidinate dizinc complex (N(pyr)–Zn,
2.127 A).38b
Coordination of the methylene-bridged pro-ligand 4 (L2H)
onto Zn was next attempted. When 4 was reacted with 1 equiv.
of ZnEt2 in toluene either at room temperature or up to 60 1C
for 3 days, no reaction took place. This inertness most likely
reflects the much weaker acidity of pro-ligand L2H, as com-
pared to that of NH-bridged ligand L1H (1a) (Scheme 5).
Direct metallation of L2H with nBuLi (vide infra), followed by
reaction with 1 equiv. of ZnCl2 or MeZnCl, afforded crude
materials that could not be purified sufficiently to establish
their identity.
The synthesis of alkyl-magnesium complexes was carried
out in a two-step procedure: (i) reaction of the bispyridines 1a
and 4 with 1 equiv. of nBuLi to generate the corresponding Li
salts [Ln]Li (L1, 8; L2, 9), and (II) reaction of the latter salts
Scheme 8 Preparation of alkyl-magnesium complexes from L2H (4).
ꢀc
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ligand acts as a bidentate ligand toward Zn(II) and Mg(II),
leaving a pendant pyridine group, consistent with recent
observations on related late transition metal complexes.25–27
On the other hand, the methine-bridged dipyridine appears to
adopt an diiminate-like coordination mode towards Mg(II).
Further work is currently under way to explore the coordina-
tion abilities of these dipyridines onto diverse metal centers, as
well as their performance in catalytic processes.
Experimental
General
All experiments involving metal complexes were carried out
under purified argon using standard Schlenk techniques or in a
glove box (o1 ppm O2, 5 ppm H2O). Hydrocarbon solvents,
diethyl ether and tetrahydrofuran were distilled from Na/
benzophenone, toluene and pentane were distilled from Na/K
alloy under nitrogen and degassed by freeze–thaw–vacuum
cycles prior to use. 2,6-dibromopyridine, benzophenone imine,
t-BuLi (1.5 M in pentane), MeLi (1.6 M in diethyl ether),
1,3-bis(diphenylphosphine)propane (dppp), tricyclohexyl-
phosphine (P(Cy)3), rac-Binap, Pd2(dba)3, t-BuOK,
Pd(dppf)Cl2, CuCN, LiBr, 2,4,6-Me3C6H2COMe, ZnCl2,
ZnEt2 (15 wt% solution in hexane) and i-PrMgCl (2.0 M
solution in diethyl ether) were purchased from Acros or
Aldrich Co. and used as received. Methyl methacrylate
(MMA, Acros) was distilled twice under argon over CaH2
before use.
Scheme 9 Enolysis reaction of isopropyl-magnesium complexes 11
and 12.
imidinate (d 0.86 and 0.13, respectively);39 this is probably due
to the anisotropic shielding effect of the pyridyl systems and
the weaker ability of the latter rings to act as electron donors
as compared with usual imidinate frameworks. The NMR
data for 12 are similar to those of 11, except an additional set
of resonances for the diethyl ether molecule.
Preliminary investigations showed that 12 is an active
catalyst for the polymerization of methyl methacrylate at
low temperature (ꢂ78 1C) in toluene ([MMA]/[Mg] = 100;
65% conversion in 3 h). The PMMA formed had a unimodal
distribution (Mw/Mn = 1.97, Mn = 27 000 g molꢂ1 vs. PS
standards, as determined by GPC) and an isotactic-enriched
microstructure (42% mm, 43% mr, as determined by
1H NMR).
In this context, we were interested in preparing [L2]Mg-
enolate species. Indeed, Gibson and co-workers have reported
that low-coordinate magnesium–enolate complexes supported
by imidinate ligands polymerize MMA in a living manner to
give highly syndiotactic PMMA under mild conditions (92%
rr at ꢂ30 1C).39 Following the procedure of Gibson, the alkyl
Mg complexes 11 and 12 were reacted with 1 equiv. of the
bulky enolizable ketone 2,4,6-Me3C6H2COMe under a variety
of conditions (toluene solvent, ꢂ20 1C for 24 h, 20 1C for 6 h,
and 60 1C for 3 h) (Scheme 9); however, monitoring of these
reactions by NMR revealed the rapid formation, in all cases,
of an intense, sharp resonance at d 4.50 ppm (in toluene-d8),
which was assigned unambiguously to free pro-ligand L2H (4).
These observations indicate that, surprisingly enough, enolysis
of 11/12 by the bulky ketone takes place at the Mg-[L2] rather
than at Mg–C(iPr) bond.
NMR spectra were recorded on Bruker AC-200, AC-300
and AM-500 spectrometers (in Teflon-valved NMR tubes for
complexes) at 20 1C otherwise stated. 1H and 13C NMR
chemical shifts were determined using residual solvent
resonances and are reported vs. SiMe4. Assignment of signals
was made from 2D 1H–1H COSY and 1H–13C HMQC and
HMBC NMR experiments. Elemental analyses (C, H, N) were
performed using a Flash EA1112 CHNS Thermo Electron
apparatus and are the average of two independent determina-
tions. HRMS spectra were obtained on a high resolution MS/
MS spectrometer Micromass ZABSpecTOF (EI, ESI-methods).
Molecular weights of PMMA were determined by GPC at
20 1C on a Waters apparatus equipped with five PL gel columns
and a differential refractometer Shimadzu RID 6A. THF was
used as eluent at a flow rate of 1.0 mL minꢂ1; PS standards were
used for molecular weight calibration. The microstructure of
1
PMMA was determined by H NMR in CDCl3.
Syntheses
6-tert-Butyl-2-bromopyridine 2a. In a flame-dried Schlenk
flask, CuCN (1.02 g, 12.0 mmol) was added to a solution of
LiCl (dried under vacuum) (1.07 g, 24.0 mmol) in THF
(80 mL). The resulting pale green solution was cooled to
ꢂ78 1C and tBuLi in THF (8.0 mL of a 1.5 M solution,
12.0 mmol) was added. After an additional 30 min of stirring
at ꢂ78 1C, 2,6-dibromopyridine (2.37 g, 10.0 mmol) was
added. The mixture was slowly warmed up to room tempera-
ture and stirred overnight. Et2O (180 mL) was added and the
reaction mixture was filtered on Celites. Organic layers were
washed with aqueous ammonia solution (2 ꢃ 200 mL), brine
Conclusions
In conclusion, we have prepared efficiently, by using a combi-
nation of catalytic methods, a series of new dipyridylamines
and dipyridylmethane that have bulky substituents at the 2,20
positions. Preliminary investigations show that these mole-
cules can be deprotonated at the amino and methylene bridge,
respectively, to form monoanionic ligands capable to coordi-
nate onto Zn(II) and/or Mg(II) centers. The dipyridylamines
ꢀc
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(2 ꢃ 200 mL), dried over Na2SO4, filtered and concentrated
under vacuum. The crude product was used without purifica-
tion to yield the 6-tert-butyl-2-bromopyridine (2.02 g, 94%).
1H NMR (400 MHz, CDCl3): d 7.44 (dd, J = 7.7, 7.8 Hz, 1H,
H4), 7.25 (dd, J = 0.8, 7.8 Hz, 1H, H3), 7.24 (dd, J = 0.8, 7.6
Hz, 1H, H5), 1.34 (s, 9H, tBu). 13C{1H} NMR (50 MHz,
CDCl3) d 171.2 (pyr-C6), 141.2 (pyr-C2) 138.5 (pyr-C4), 124.9
(pyr-C3), 117.8 (pyr-C5) 37.6 (CMe3), 29.9 (CMe3). IR (neat)
n/cmꢂ1 3074, 2962, 2867, 1580, 1553, 1434, 1398, 1166, 1114,
983, 852, 794, 756, 742, 653 cmꢂ1. HRMS: m/z calc. for
C9H12BrN [M+]: 213.0153; found: 213.0169.
heated at 90 1C for 24 h. The solution was cooled to room
temperature, diluted with diethyl ether (80 mL) and filtered
through Celite. Solvents were removed under vacuum to give
an oily residue which was redissolved in aqueous 2 M HCl
(30 mL), stirred for 1 h, then diluted with 0.5 M HCl (30 mL).
The aqueous phase was treated with a saturated aqueous
solution of Na2CO3 and extracted with diethyl ether. The
organic phase was dried over anhydrous Na2SO4, filtered and
concentrated under vacuum. The residue was washed with
hexane–diethyl ether = 1 : 1 (100 mL) to give 3b as a pale
yellow solid (0.78 g, 72%). 1H NMR (CDCl3, 300 MHz): d
7.52 (t, 3J = 7.5, 1H, pyr-H, p), 6.93 (s, 2H, Ph-H, m), 6.61 (d,
2J = 7.0, 1H, pyr-H, m), 6.47 (d, 2J = 8.0, 1H, pyr-H, m), 4.48
(s, 2H, NH2), 2.33 (s, 3H, Ph-Me, p), 2.09 (s, 6H, Ph-Me, o).
13C{1H} NMR (C6D6, 75 MHz): d 158.46 (pyr-C2), 158.16
(pyr-C6), 137.97 (pyr-C4), 137.10 (Ph-C4), 135.58 (Ph-C2),
128.21 (Ph-C3,5), 114.78 (pyr-C5), 106.25 (pyr-C3), 21.07 (Me,
Ph-p), 20.04 (Me, Ph-o).
Dipyridylamine 1a
To a 100 mL Schlenk flask were added successively Pd2(dba)3
(0.18 g, 0.20 mmol), dppp (0.10 g, 0.25 mmol), 2-bromo-6-tert-
butylpyridine (2a, 1.07 g, 5.0 mmol), benzophenone imine
(1.085 g, 6.0 mmol) and tBuOK (0.84 g, 7.5 mmol), and then
toluene (80 mL). The mixture was heated under nitrogen at
90 1C for 24 h. The solution was cooled to room temperature,
diluted with diethyl ether (ca. 80 mL) and filtered through
Celite. Solvents were removed under vacuum to give an oily
residue which was redissolved in aqueous 2 M HCl (30 mL),
stirred for 1 h, and then diluted with 0.5 M HCl (30 mL). The
aqueous phase was treated with a saturated aqueous solution
of Na2CO3 and extracted with diethyl ether (100 mL). The
organic phase was dried over anhydrous Na2SO4, filtered and
concentrated. The residue was purified by flash chromato-
graphy (heptane–ethyl acetate = 5 : 1) to give 3a as a yellow
Compound 1b was prepared following the procedure
described above for 1a starting from Pd2(dba)3 (80 mg,
0.087 mmol), BINAP (64 mg, 0.10 mmol), 2b (0.62 g,
2.24 mmol), 3b (0.50 g, 2.40 mmol), and tBuOK (0.38 g,
3.36 mmol), and then toluene (30 mL). 1b was isolated after
purification by chromatography as a yellow liquid (0.74 g,
1
80%). H NMR (CDCl3, 200 MHz): d 7.75 (t, J = 15.6, 2H,
pyr-H, p), 7.65 (s, 2H, Pyr-H, m), 7.42 (s, 1H, NH), 6.98 (s, 4H,
Ph-H), 6.84 (d, J = 7.2, 2H, pyr-H, m), 2.37 (s, 6 H, Ph-CH3,
p), 2.12 (s, 12 H, Ph-CH3), m). 13C{1H} NMR (C6D6,
300 MHz): d 20.204 (phenyl-2,6-CH3), 21.140 (phenyl-4-
CH3), 109.22 (pyr-5C), 117.06 (pyr-3C), 127.05, 128.30 (Ph-
3,5 C), 135.79 (Ph-1C), 137.22 (Ph-4C), 137.88 (Ph-2,6-C),
138.03 (pyr-4C), 153.69 (pyr-6C), 157.94 (pyr-2C).
1
liquid (0.55 g, 73%). H NMR (400 MHz, CDCl3): d 1.29 (s,
9H), 4.25–4.35 (br s, 2H), 6.30 (dd, J = 0.5, 7.8 Hz, 1H), 6.68
(dd, J = 0.5, 7.8 Hz, 1H), 7.35 (t, J = 7.8 Hz, 1H). 13C{1H}
NMR (50 MHz, CDCl3) d 30.1 (q ꢃ 3), 37.0 (s), 105.5 (d),
108.9 (d), 137.8 (d), 157.5 (s), 168.1 (s). IR (neat) n/cmꢂ1 3475,
3378, 2956, 1577, 1457, 1257, 1043, 800, 665 cmꢂ1
.
Dipyridylamine 1c
To a 100 mL Schlenk flask were added successively Pd2(dba)3
(0.080 g, 0.087 mmol), BINAP (0.064 g, 0.10 mmol), 2-bromo-
6-tert-butylpyridine (2a, 0.34 g, 1.69 mmol), 2-amino-
6-tert-butylpyridine (3a, 0.28 g, 1.86 mmol), and tBuOK
(0.31 g, 2.76 mmol), and then toluene (30 mL). The mixture
was heated under nitrogen at 90 1C for 12 h. The solution was
cooled to room temperature, diluted with diethyl ether (ca.
30 mL) and filtered through celite. The residue was purified by
flash chromatography (heptane–ethyl acetate = 5 : 1) to give 1a
as a yellow liquid (0.387 g, 82%). 1H NMR (CDCl3, 300 MHz):
d 7.58 (t, 3J = 7.8, 2H, pyr-H, p), 7.45 (d, 2J = 8.0, 2H,
pyr-H, m), 7.18 (s, 1H, NH), 6.93 (d, 2J = 7.4, 2 H, pyr-H, m),
1.38 (s, 18H, tBu). 13C{1H} NMR (CDCl3, 75 MHz): d 167.74
(pyr-C6), 153.21 (pyr-C2), 137.65 (pyr-C4), 110.94 (pyr-C5),
108.29 (pyr-C3), 37.41 (CMe3), 30.20 (CMe3). HRMS: calc.
for C18H25N3 283.2048, found 283.2034; [M ꢂ H]+: found
282.1969, calc. 282.1970; [M ꢂ CH3]+: found 268.1821, calc.
268.1814.
Compound 3c was prepared following the procedure described
above for 3a, starting from Pd2(dba)3 (0.12 g, 0.13 mmol), dppp
(70 mg, 0.17 mmol), 2c (1.60 g, 4.4 mmol), benzophenone imine
(0.87 g, 4.8 mmol) and tBuOK (0.74 g, 6.6 mmol), to give 3c as a
pale yellow solid (890 mg, 68%). 1H NMR (300 MHz, CDCl3):
d 7.49 (t, 3J = 7.9, 1H, pyr-H, p), 7.06 (s, 2H, Ph-H, m), 6.64 (d,
2J = 7.0, 1H, pyr-H, m), 6.47 (d, 2J = 8.3, 1H, pyr-H, m), 4.43
(s, 2H, NH2), 2.94 (sept, 1 H, CH(CH3)2), 2.63 (sept, 2 H,
CH(CH3)2), 1.00–1.30 (m, 18 H, CH(CH3)2).
Compound 1c was prepared following the procedure
described above for 1a, starting from Pd2(dba)3 (0.070 g,
0.076 mmol), BINAP (0.056 g, 0.087 mmol), 2b (0.62 g, 1.73
mmol), 3b (0.54 g, 1.82 mmol), and tBuOK (0.29 g, 2.60 mmol),
to give 1c as a white solid (0.77 g, 76%). 1H NMR (500 MHz,
CDCl3): d 7.59 (t, 3J = 15.5, 2H, pyr-4H), 7.55 (d, 2J = 9.2, 2H,
pyr-5H), 7.42 (s, 1H, NH), 7.11 (s, 4H, Ph-H), 6.81 (d, 2J = 7.8,
2H, pyr-3H), 2.94 (sept, 2 H, CH(CH3)2), 2.70 (sept, 4 H,
CH(CH3)2), 1.10–1.40 (m, 36 H, CH(CH3)2). 13C{1H} NMR
(C6D6, 300 MHz): d 24.12 (CH(CH3)2), 24.22 (CH(CH3)2),
24.42 (CH(CH3)2), 30.29 (phenyl-o-C(CH3)2), 34.39 (phenyl-p-
C(CH3)2), 109.17 (pyr-5C), 117.58 (pyr-3C), 120.68 (Ph-3,5C),
136.39 (Ph-1C), 137.38 (pyr-4C), 146.27 (Ph-2,6C), 148.44
(Ph-4C), 153.43 (pyr-6C), 158.13 (pyr-2C). HRMS: calc. for
C40H53N3 [M + H]+: 576.4318, found: 576.4317.
Dipyridylamine 1b
To a 100 mL Schlenk flask were added successively Pd2(dba)3
(0.18 g, 0.20 mmol), dppp (103 mg, 0.25 mmol), 2b (1.38 g,
5.0 mmol), benzophenone imine (1.085 g, 6.0 mmol), tBuOK
(0.84 g, 7.5 mmol), and then toluene (80 mL). The mixture was
ꢀc
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Dipyridylmethane 4
residue was recrystallized from pentane at ꢂ34 1C to give 6 as
colorless crystals (0.055 g, 20%). 1H NMR (C6D6, 300 MHz):
d 7.35 (m, 4H, pyr-H, m and p), 6.70 (d, 2J = 7.2, 2H, pyr-H,
m), 1.36 (m, 21H, tBu and CH3), 0.75 (q, 4J = 1.4, 2H,
CH2CH3). 13C{1H} NMR (C6D6, 300 MHz): d 166.84
(pyr-C6), 138.75 (pyr-C4), 128.14 (pyr-C5), 128.06 (pyr-C4),
112.20 (pyr-C2), 36.88 (CMe3), 29.99 (CMe3), 12.08
(Zn–CH2CH3), 2.21 (Zn–CH2). Anal. Calc. for C20H29N3Zn:
C, 63.74; H, 7.76; N, 11.15. Found: C, 63.8; H, 7.8; N, 11.0%.
To a 100 mL Schlenk flask was added a dried solution of
ZnCl2 (2.05 g, 15.0 mmol) in THF (40 mL). The solution was
cooled to ꢂ78 1C, then CH3Li (10 mL of 1.6 M solution in
diethyl ether, 16.0 mmol) was slowly added. After stirring for
30 min at this temperature, 6-tert-butyl-2-bromopyridine (2a,
2.14 g, 10.0 mmol) and Pd(PPh3)4 (0.23 g, 0.20 mmol) were
added. The mixture was warmed to room temperature
and stirred overnight. The solution was diluted with Et2O
(150 mL), washed with a saturated solution of NH4Cl and
then with a saturated solution of NaCl. The organic phase was
dried over anhydrous Na2SO4, filtered and concentrated, and
purified by chromatography (heptane–diethyl ether = 100 : 1)
[L1]2Zn (7)
Procedure A. To a 50 mL Schlenk flask were added
bis(6-tert-butyl-2-pyridyl)amine (1a, 0.200 g, 0.706 mmol) in
toluene (10 mL), and ZnEt2 (0.58 g of a 15 wt% solution in
hexane, 0.706 mmol). The solution was stirred at 30 1C over-
night. After evaporation of volatiles under vacuum, the
residue was recrystallized from pentane at ꢂ34 1C to give 7
as colorless crystals (0.096 g, 36%).
1
to give 5 as a yellow liquid (1.13 g, 76%). H NMR (CDCl3,
300 MHz): d 7.49 (t, 3J = 7.6, 1H, pyr-H, p), 7.15 (d, 2J = 7.8,
2
1H, pyr-H, m), 6.96 (d, J = 7.6, 1H, pyr-H, m), 2.56 (s, 2H,
CH3), 1.35 (s, 9H, tBu). 13C{1H} NMR (CDCl3, 75 MHz): d
167.74 (pyr-C6), 153.2 (pyr-C2), 137.65 (pyr-C4), 110.94
(pyr-C5), 108.29 (pyr-C3), 37.41 (CMe3), 30.20 (CMe3).
Bis(6-tert-butyl-2-pyridyl)methane (4, L2H). To a 100 mL
Schlenk flask containing a solution of 2-methyl-6-tert-butyl-
pyridine (5, 0.46 g, 3.00 mmol) in THF (20 mL) placed at
ꢂ78 1C, t-BuLi (2.1 mL of a 1.5 M solution in hexane,
3.15 mmol) was added. The mixture was stirred at this
temperature for 30 min, then warmed to room temperature
for another 2 h, and again cooled to ꢂ78 1C. Then, a
suspension of anhydrous ZnCl2 (0.45 g, 3.3 mmol) in THF
(10 mL) was added under argon. The mixture was warmed to
room temperature for 1 h. Pd(dppf)Cl2 (0.044 g, 0.060 mmol)
and 2-bromo-6-tert-butylpyridine (2a, 0.32 g, 1.50 mmol) were
added at ꢂ78 1C, the mixture was warmed to room tempera-
ture and heated to reflux with vigorous stirring for 15 h.
Volatiles were removed under vacuum, the residue was sus-
pended in CHCl3, and an aqueous 5 M solution of NaOH
(2 mL) and a saturated aqueous solution of Na2S (2 mL) were
added. The mixture was stirred for 1 h to give a white
precipitate, which was removed by filtration and washed with
CHCl3. The filtrate and washings were transferred to a se-
paratory funnel, and the CHCl3 layer was separated and dried
over anhydrous Na2SO4, and concentrated to dryness. The
residue was purified by column chromatography on silica
(EtOAc–CH2Cl2 first 1 : 10 and then 1 : 4) to give 4 as a light
yellow liquid (0.203 g, yield: 48%). 1H NMR (CDCl3,
Procedure B. To a 50 mL Schlenk flask were added
bis(6-tert-butyl-2-pyridyl)amine (1a, 0.200 g, 0.706 mmol) in
toluene (10 mL), and ZnEt2 (0.290 g of a 15 wt% solution in
hexane, 0.353 mmol). The solution was stirred at 20 1C over-
night. After evaporation of volatiles under vacuum, the resi-
due was recrystallized from pentane at ꢂ34 1C to give 7 as
colorless crystals (0.080 g, 60%). 1H NMR (C6D6, 500 MHz):
d 7.79 (d, 2J = 8.2, 4H, pyr-H, o), 7.33 (t, 3J = 15.8, 4H, pyr-
2
H, p), 6.61 (d, J = 8.2, 4H, pyr-H, m), 1.40 (s, 36H, tBu).
13C{1H} NMR (C6D6, 125 MHz): d 166.32 (pyr-C6), 161.28
(pyr-C4), 139.34 (pyr-C5), 111.18 (pyr-C4), 108.92 (pyr-C2),
36.85 (CMe3), 30.12 (CMe3). Anal. Calc. for C36H48N6Zn: C,
68.61; H, 7.68; N, 13.34. Found: C, 68.4; H, 7.7; N, 13.4%.
[L1]2Mg (10)
To a 50 mL Schlenk flask containing bis(6-tert-butyl-2-pyridyl)-
amine (1a, 0.200 g, 0.706 mmol) in toluene (10 mL), nBuLi
(0.57 mL of a 1.6 M solution in hexane, 0.920 mmol) was
added slowly in at ꢂ78 1C. The solution was stirred and gently
warmed to 20 1C. After 2 h stirring at 20 1C, volatiles were
removed under vacuum and the resulting yellow solid residue
was washed with pentane (ca. 3 mL) to afford the lithium salt
[L1]Li (8) as a white powder. 1H NMR (C6D6, 200 MHz) of 8:
d 7.7 (m, 4H, pyr-H, m and p), 7.0 (d, 2H, pyr-H, m), 1.52
(s, 18H, tBu). This powder was dissolved in toluene (5 mL)
and iPrMgCl (0.35 mL of a 2.0 M solution in diethyl ether,
0.70 mmol) was added. The resulting solution was stirred at
20 1C overnight and the solvent was removed under vacuum.
Then, toluene (5 mL) was added to dissolve the residue, the
solution was filtered, and the filtrate concentrated in vacuum
3
2
300 MHz): d 7.50 (t, J = 7.8, 2H, pyr-H, p), 7.15 (d, J =
8, 2H, pyr-H, m), 7.06 (d, 2J = 7.4, 2H, pyr-H, m), 4.30 (s, 2H,
CH2), 1.37 (s, 18H, tBu). 13C{1H} NMR (CDCl3, 75 MHz): d
168.55 (pyr-C6), 158.60 (pyr-C2), 136.21 (pyr-C4), 120.13
(pyr-C3), 116.07 (pyr-C5), 47.79 (CH2), 37.37 (CMe3), 30.21
(CMe3). HRMS: calc. for C19H26N2 [M+]: 282.2096, found
282.2104, calc. for [M ꢂ CH3]+: 267.1861, found 267.1875.
1
to give 10 as a white solid (0.091 g, 22%). H NMR (C6D6,
300 MHz): d 7.20–7.32 (m, 8H, pyr-H, m and p), 6.79 (d, J =
8.3, 4H, pyr-H, m), 1.39 (s, 36H, tBu). 13C{1H} NMR (C6D6,
[L1]ZnEt (6)
To a 50 mL Schlenk flask were added bis(6-tert-butyl-2-
pyridyl)amine (1a, 0.200 g, 0.706 mmol) in toluene (10 mL),
and ZnEt2 (0.087 g of a 15 wt% solution in hexane, 0.706
mmol). The solution was stirred at room temperature
overnight. After evaporation of volatiles under vacuum, the
125 MHz):
d 30.21 (C(CH3)3), 37.32 (C(Me)3), 108.59
(pyr-C3), 110.94 (pyr-C5), 128.14, 137.66 (pyr-C4), 153.35
(pyr-C2), 167.52 (pyr-C6). Anal. Calc. for C36H48N6Mg: C,
73.40; H, 8.21; N, 14.27. Found: C, 73.4; H, 8.3; N, 14.4%.
ꢀc
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2156 | New J. Chem., 2008, 32, 2150–2158

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[L2]Mg(iPr) (11)
independent molecules. However, this appears to be a pseudo-
ꢀ
symmetry: refinements in the P4 space group led to a
To a 50 mL Schlenk flask containing bis(6-tert-butyl-2-pyridyl)-
methane (4, 0.31 g, 1.10 mmol) in diethyl ether (15 mL), nBuLi
(0.89 mL of a 1.6 M solution in hexane, 1.43 mmol) was added
slowly in at ꢂ78 1C. The solution was stirred and gently
warmed to 20 1C. After 2h stirring at 20 1C, volatiles were
removed under vacuum and the resulting yellow solid was
washed with pentane (ca. 3 mL) to afford the lithium salt
[L2]Li (9) as a yellow powder; 1H NMR (C6D6, 500 MHz) of 9:
disordered structure, especially pronounced at the tert-butyl
groups, with several non-positive-definite anisotropic displace-
ment parameters. The structure is clearly best described in the
P1 space group, with four independent molecules, which
allows satisfactory refinements, with all atomic displacement
parameters refined anisotropically.
CCDC reference numbers 682251–682253.
For crystallographic data in CIF or other electronic format
see DOI: 10.1039/b807258b
3
d 6.81 (t, J = 15.5, 2H, pyr-H, p), 6.21 (br s, 2H, pyr-H, m),
6.03 (d, 2J = 6.8, 2H, pyr-H, m), 4.72 (s, 1H, CH), 1.38
(s, 1.36, 18H, tBu). This powder was dissolved in diethyl ether
(ca. 5 mL) and iPrMgCl (0.55 mL of a 2.0 M solution in
diethyl ether, 1.10 mmol) was added. The resulting mixture
was stirred at 20 1C overnight and volatiles were removed
under vacuum. Then, toluene (5 mL) was added to dissolve the
residue, the solution was filtered and concentrated in vacuum
to give 11 as a dark red solid (0.077 g, 20%). 1H NMR (C6D6,
Acknowledgements
The authors thank the Agence Nationale pour la Recherche
(ANR) for a grant ‘‘Jeunes Chercheurs-Jeunes Chercheuses’’
(ANR-06-JCJC-0013). Z. Z. is grateful to Arkema Co. for a
post-doctoral fellowship.
3
2
500 MHz): d 6.91 (t, J = 15.7, 2H, pyr-H, p), 6.53 (d, J =
8.4, 2H, pyr-H, m), 6.20 (d, 2J = 7.9, 2H, pyr-H, m), 5.16
(s, 1H, CH), 1.70 (d, 2J = 7.6, 6H, C(CH3)2, 1.38 (s, 18H,
CMe3), 0.42 (m, 1H, CHMe2. Compound 11 was found to be
too unstable in benzene or toluene solutions to run 13C{1H}
NMR spectroscopy and also to obtain reliable elemental
analyses.
References
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[L2]Mg(iPr)(OEt2) (12)
Complex 12 was prepared following the procedure described
above for 11, using diethyl ether instead of toluene as the
solvent when Li-2 was reacted with iPrMgCl. Complex 12 was
1
recovered as a white solid (0.144 g, 31%). H NMR (C6D6,
3
2
500 MHz): d 6.91 (t, J = 15.8, 2H, pyr-H, p), 6.53 (d, J =
8.5, 2H, pyr-H, m), 6.20 (d, J = 7.9, 2H, pyr-H, m), 5.16 (s,
2
2
1H, CH), 1.70 (d, J = 7.8, 6H, C(CH3)2, 1.38 (s, 18H, tBu),
0.42 (m, 1H, CHMe2). 13C{1H} NMR (C6D6, 125 MHz): d
164.03 (pyr-C2), 158.30 (pyr-C6), 135.44 (pyr-C4), 119.16
(pyr-C5), 106.33 (pyr-C3), 84.27 (bridge-C), 36.23 (CMe3),
30.40 (CMe3), 24.44 (CH(Me)2), 9.66 (CH(Me)2). Anal. Calc.
for C26H42MgN2O: C, 73.84; H, 10.01; N, 6.62. Found: C,
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Suitable single crystals of 1c, 6 and 7 were mounted onto a
glass fiber using the ‘‘oil-drop’’ method. Diffraction data were
collected at 100 K using an APEXII Bruker-AXS diffrac-
tometer with graphite monochromatized Mo-Ka radiation
(l = 0.71073 A). The structure was solved by direct methods
using the SIR97 program,40 and then refined with full-matrix
least-squares methods based on F2 (SHELX-97)41 with the aid
of the WINGX42 program. All carbon-bound hydrogen atoms
were placed at calculated positions and forced to ride on
the attached carbon atom. In ligand 1c, the hydrogen atom
at N(3) was found from Fourier difference maps analysis.
All non-hydrogen atoms were refined with anisotropic
displacement parameters.
For complex 7, metric parameters and PLATON symmetry
checks suggested that the structure should be described in
ꢀ
the higher symmetry tetragonal P4 space group, with two
ꢀc
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19 A. Alaaeddine, A. Amgoune, C. M. Thomas, T. Roisnel,
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Inorg. Chem., 2006, 3652, and references cited therein.
32 For the previous synthesis of 2a (5 steps, 5% overall yield), see:
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