Non-symmetrical, potentially redox non-innocent imino NHC pyridine 'pincers': Via a zinc ion template-assisted synthesis

Article abstract of DOI:10.1039/c7dt01014a

New non-symmetrical, redox-active imino NHC pyridine pincer ligands, 2-(R¹-imidazol-2-ylidene)-6-(R²NCH)-pyridine (R¹ = 2,6-diisopropylphenyl (DiPP), R² = 2,4,6-trimethylphenyl (Mes), 4B; R¹ = R² = DiPP, 4C), have been accessed by a Zn^II-promoted modular synthesis involving the quaternization of R¹-imidazole by [Zn(κN^imineκN^pyridine)(2-(R²NCH)-6-bromo-pyridine)Cl₂], followed by Zn^II removal and deprotonation of imidazolium pro-ligands. Redox active forms of 4B were implicated in the two complexes obtained by its reaction with FeBr₂/KC₈; metrical data analysis pointed to the occurrence of radical anionic and dianionic redox states of 4B.

Full text of DOI:10.1039/c7dt01014a

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PAPER
Joseph T. Hupp, Omar K. Farha et al.
Efficient extraction of sulfate from water using
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Non-symmetric, potentially redox non-innocent imino NHC
pyridine ‘pincers’ via a zinc ion template-assisted synthesis†
Thomas Simler,a Andreas A. Danopoulos,*a,b and Pierre Braunstein*a
Received 00th January 20xx,
Accepted 00th January 20xx
The new non-symmetrical, redox-active imino NHC pyridine pincer ligands, 2-(R1-imidazol-2-ylidene)-6-(R2N=CH)-pyridine
DOI: 10.1039/x0xx00000x
(R1 = 2,6-diisopropylphenyl (DiPP), R2 = 2,4,6-trimethylphenyl (Mes), 4B; R1 = R2 = DiPP, 4C) have been accessed by a ZnII-
www.rsc.org/
promoted modular synthesis involving the quaternization of R1-imidazole by [Zn( imineκNpyridine)(2-(R2N=CH)-6-bromo-
κN
pyridine)Cl2], followed by ZnII removal and deprotonation of the imidazolium pro-ligands. Redox active forms of 4B were
implicated in the two complexes obtained by its reaction with FeBr2/KC8; metrical data analysis pointed to the occurrence
of radical anionic and dianionic redox states of 4B.
rendering the formal assignment of metal OS difficult and
sometimes misleading. With related base metal complexes of
Introduction
pincer pyridine dicarbenes (CNHCNpyrCNHC
, 1 in Fig. 1), bearing
The intensive study of the catalytic chemistry of pincer
complexes of earth abundant base metals has recently focused
on the concept of ligand-metal cooperation.1,2 Frequently,
metal-ligand synergism can be achieved by ‘proton-borrowing’
from the backbone of ‘proton responsive’ ligands through
strong NHC donors, catalytic activities in alkene
hydrogenations are higher although redox non-innocence has
been rarely implicated.8 The electronic consequences of
formally substituting the two imines in NiminNpyrNimin by NHCs
in CNHCNpyrCNHC are still under debate.9
deprotonation/reprotonation,
leading
to
electronic
Recent attempts in pincer design have focused on non-
symmetrical structures i.e. disubstituted 1,3-benzen-2-ides and
2,6-pyridines bearing disparate functional groups at the
wingtips, e.g. phosphine, phosphinate, amine, pyridine, imine
and NHC donors.10 This strategy can combine ligand-metal
cooperation with electronic and steric tuning at the metal
and/or modify ligand characteristics relevant to the
cooperation (i.e. pKa, redox potential, C-H homolytic BDE etc.).
modifications of the reactive site(s) and activation of the
metal-ligand assembly.2 Alternatively, a ‘redox active’ ligand
can act as (temporary) reservoir of an integral number of
electrons, promoting during the catalytic cycle electronic
changes that cannot be attained solely by changes in the metal
oxidation state (OS).3 In contrast, spectator ligand(s) only
provide electronic tuning of the metal centre through σ- and π-
effects. Proton responsiveness and redox non-innocence can
occur on the same non-biological ligand; these cases are very
interesting but rare and may involve Hydrogen Atom Transfer
(HAT) or Proton Coupled Electron Transfer (PCET) steps.4
Base metal complexes with redox non-innocent aryl-
substituted 2,6-bis(imino)pyridines NiminNpyrNimin have provided
catalysts for diverse transformations, e.g. hydrogenation of
unactivated alkenes and hydrosilylation,5 cycloadditions of
alkenes,6 oligomerizations and polymerizations of ethylene.7 It
is noteworthy that occasionally both the metal and
NiminNpyrNimin may adopt open-shell electronic structures,
a.Université de Strasbourg, CNRS, CHIMIE UMR 7177, Laboratoire de Chimie de
Coordination, 4 rue Blaise Pascal, 67081 Strasbourg Cedex, France.
b.Institute for Advanced Study (USIAS), Université de Strasbourg, 4 rue Blaise
Pascal, 67081 Strasbourg Cedex, France.
E-mail: braunstein@unistra.fr, danopoulos@unistra.fr
† Dedicated to Prof. John Gladysz on the occasion of his 65th birthday, with our
congratulations and best wishes.
Electronic Supplementary Information (ESI) available: NMR spectra and X-ray
crystallographic data. CCDC 1530405–1530407. See DOI: 10.1039/x0xx00000x
Fig. 1 The symmetrical pincer pyridine dicarbene CNHCNpyrCNHC
symmetrical ligands ( 4A) that are relevant to 4B and 4C described in this
work
(1) and the non-
2, 3,
.
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For example, the α-CH2 acidity of the proton-responsive
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2
Results and discussion
(Fig. 1) is higher than that of bis-(dialkylphosphinomethyl)-
pyridines by ca. 5 units.10h An analogous strategy based on
non-symmetrical, redox non-innocent pincers can be
Plausible Synthetic Strategies Based on Retrosynthetic Analysis.
A retrosynthetic analysis of plausible synthetic strategies to
access the target pro-ligand imidazolium salts 5B and 5C is
shown in Scheme 1. In the paths A1-A2, the C=N imine bond
formation occurs as the last step of the pro-ligand synthesis.
envisaged, where
potentially redox non-innocent substructure such as α-imino-
pyridine. The recently described pyridine imino phosphine
a tunable donor is attached to the
3
(Fig. 1),10c and the related pyridine imino phosphinates,10a
bipyridine phosphines10b and bipyridine NHCs11 could fall in
this category. Interestingly, pyridine imino phosphine or
bipyridine phosphine in Fe0 complexes appear to be good π-
acceptors but redox innocent.10b,10c The coordinated 2-
(arylketimino)-6-(imidazol-2-ylidene) pyridine 4A (Fig. 1), was
obtained by reaction of the tautomeric 2-(arylketimino)-6-
imidazol-pyridine with IrI precursors;12 however, the
applicability of this method and the scope of 4A may be
limited owing to the reduced tendency of spontaneous
imidazole C2-H activation by simple base metal precursors. In
The parent aldehyde
quaternization of N-DiPP-imidazole (
2-carbaldehyde (path A1), similarly to the synthesis of
quaternization of
with 6-bromopyridine-2-methanol.10f,11
In contrast to the efficient synthesis of 9 10f,11
heating
stoichiometric amounts of and in a sealed flask at 150-
160 °C for 3 days resulted in a mixture of intractable products
and unreacted (Scheme 2). Thermal decomposition of the
6
can potentially be obtained by
) with 6-bromopyridine-
by
8
7
9
8
,
7
8
8
aldehyde may be responsible for the failure, and thus, initial
protection of the aldehyde was considered. The 1,3-dioxolane
derivative 11, obtained easily from
7 and ethylene glycol in
toluene, was heated with 8 at high temperature for several
order to expand the design principle of 4A
,
the 2-
4B
days resulting in low conversion to 12; unreacted starting
materials were also recovered. However, purification of 12
failed. Furthermore, cleavage of the 1,3-dioxolane protective
group in the crude 12 under various conditions was also
unsuccessful.
(arylaldimino)-6-(arylimidazol-2-ylidene)-pyridine ligands
(
and 4C, Fig. 1) were targeted. Herein, we describe the
difficulties we encountered to access these targets following
routes devised by rational retrosynthetic analysis, provide a
synthetic methodology to 4B and 4C, of broader relevance,
and describe preliminary results on their Fe coordination
chemistry that point to redox non-innocence.
Alternatively, protection of the aldehyde as an acyclic
diethylacetal was considered (Scheme 3). The product 13 was
readily obtained by reaction of
ethanol under acid-catalyzed conditions. However, the
quaternization of with 13 did not proceed neatly and
resulted only in very low conversion under forcing conditions.
7 with triethyl orthoformate in
8
Scheme 1 Retrosynthetic analysis of synthetic routes leading to the imidazolium pro-ligands 5B and 5C.
Scheme 2 Attempted synthesis of 14 without (top) and with (bottom) cyclic acetal protection of the aldehyde.
2 | J. Name., 2012, 00, 1-3
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Scheme 3 Attempted synthesis of the imidazolium salt 14 with acyclic aldehyde protection.
The use of the 2-iodopyridine derivative 15, obtained by the
‘aromatic Finkelstein reaction’ as described by Buchwald and
co-workers,13 surprisingly, did not provide any improvement,
again leading to low yields and purity of 14 (Scheme 3). Finally,
functional group interconversion (FGI) via controlled oxidation
of the known 910f,11 was also carried out (Scheme 1, path A2)
using IBX,14 TPAP15 and TEMPO,16 without any success.17
However, small amounts of
6 were obtained by the oxidation
of
9
using the Swern18 or Mukaiyama19 methodologies but the
latter presented reproducibility issues.17
Since the synthesis of 10B and 10C (R2 = Mes and DiPP,
respectively; Scheme 1, path B and Scheme 4) proceeded in
very good yield by the acid-catalyzed reaction of
corresponding aniline in ethanol, attempts focused on
facilitating the quaternization of by the aldimines 10B/10C
7 with the
8
.
Scheme 4 The synthesis of the pro-ligands 10B/10C and the ligands 4B/4C (Mes = 2,4,6-
trimethylphenyl, DiPP = 2,6-diisopropylphenyl); reagents and conditions: (i) 1.1 equiv.
R2NH2, HCOOH (5.0 mol%), ethanol reflux, 2-3 h; (ii) 1.0 equiv. ZnCl2, THF 60 oC, 1h; (iii)
2 equiv. DiPP-imidazole (8), 150-160 oC, 4 d; (iv) ca. 2 equiv. aq. K2C2O4 followed by
excess of aq. NaBr; (v) 1.0 equiv. KN(SiMe3)2, ether, -78 oC to rt.
Indeed, such quaternization reactions do not proceed neatly
under forcing conditions,17 probably due to the low thermal
stability of 10B/10C
.
Zinc Ion Template-Assisted Synthesis.
To circumvent the issues of thermal decomposition and low
reactivity, pre-coordination of the 2-(arylaldimino)-6-bromo-
pyridine 10B/10C to an electrophilic metal was considered, as
a
means of activating the heterocyclic ring towards
nucleophilic attack, while simultaneously suppressing
decompositions thanks to chelating and template effects. Note
that 2-halo-pyridinium salts are more susceptible to
nucleophilic attack than the free pyridines as a result of the
attachment of the electrophilic H+ on the N atom.20 The choice
of ZnII was based on its low cost and toxicity, diamagnetism,
high charge and ease of removal. A number of α-diimine and
pyridine imine complexes of zinc chloride have been
described.21 The successful synthetic strategy is outlined in
Scheme 4. The formation of the ZnII complexes of the easily
accessible bromo-pyridines 10B and 10C proceeded smoothly,
the products 16B and 16C, respectively, precipitated as yellow
powders from the reaction mixture and were characterized by
spectroscopic and analytical methods; 16C was also
characterized crystallographically (Fig. 2). The quaternization
Fig. 2 The structure of 16C; H atoms are omitted for clarity. Important bond
distances (Å) and angles (deg.): C1-N1 = 1.34(1), C1-C2 = 1.35(1), C1-Br1 =
1.914(9), C2-C3 = 1.39(2), C5-N1 = 1.34(1), C5-C6 = 1.48(1), C6-N2 = 1.30(1), C3-
C4 = 1.40(1), C4-C5 = 1.37(1), N1-Zn1 = 2.071(7), N2-Zn1 = 2.068(7), Cl1-Zn1 =
2.189(3), Cl2-Zn1 = 2.196(3); C5-N1-Zn1 = 113.5(6), C1-N1-Zn1 131.0(6), C6-N2-
C7 = 117.6(7), C6-N2-Zn1 = 114.8(6), C7-N2-Zn1 = 127.1(5), N2-Zn1-N1 = 79.4(3),
N2-Zn1-Cl1 = 118.7(2), N1-Zn1-Cl1= 114.5(2), N2-Zn1-Cl2 = 110.1(2), N1-Zn1-Cl2
= 114.2(2), Cl1-Zn1-Cl2 = 115.1(1).
of N-DiPP-imidazole (8) with 10B and 10C was carried out in
the melt at 150-160 °C using excess (2 mole equiv.) of the
imidazole to full conversion. Removal of the ZnII template was
achieved by addition of aqueous potassium oxalate to a CH2Cl2
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solution of the crude complex;22 excess NaBr was also added
to avoid counter-anion mixtures. The pro-ligands 5B and 5C
were subsequently converted by deprotonation with
Scheme 5 Formation of complexes 18 and 19 by reduction of 4B/FeBr2 with KC8.
KN(SiMe3)2 to 4B and 4C
, respectively, which were
characterized spectroscopically. They exhibit signatures
characteristic of the NHC functionality (i.e. disappearance of
the azolium NCHN signal in the 1H-NMR spectra and
appearance of the CNHC signal at ca. δ 220 in the 13C-NMR
spectra); the signals assignable to the imine functionality were
also diagnostic (HCimn=N at ca. δ 7.80 and HCimn=N at ca. δ
163.0 in the in the 1H- and 13C-NMR spectra, respectively).
Synthesis of reduced Fe complexes and metrical data analysis.
Preliminary coordination studies focused on the reaction of 4B
with one equivalent of FeBr2 in THF, resulting in the formation
of a sparingly soluble paramagnetic turquoise-blue precipitate,
which resisted further characterization due to its low solubility
in non-reactive solvents (THF, ether, hydrocarbons) and air
sensitivity. Relevant work23 has provided evidence that the
reaction of FeX2 (X = halide) with ligands of type
1 and 2 in THF
gives products the nature of which is sensitive to the bulk of
the wingtip functional groups, the solvent and the nature of X;
Fig. 3 The structure of the cation in 18 with thermal ellipsoids at the 30% probability
they include 5- or 6-coordinate, neutral or ionic species e.g. level (one anion Br- in the unit cell is not depicted); H-atoms have been omitted; only
[FeII(pincer)X2], [{FeII(pincer)2X}X],
[{FeII(pincer)2}X2] or
the ipso-C of the DiPP rings is shown. Important bond distances (Å) and angles (deg.):
[{FeII(pincer)2}{FeIIX4}], respectively. Subsequent reduction of
the aforementioned turquoise-blue precipitate with one or
two equiv. of KC8 in ether gave the air sensitive complexes 18
and 19 (Scheme 5), respectively, which were characterized
crystallographically (Figures 3 and 4).
Fe1-N3 = 1.882(3), Fe1-N4 = 2.007(3), Fe1-N7 = 1.944(3), Fe1-C1 = 1.946(4), Fe1-C31 =
1.908(4), C21-N4 = 1.339(4), C21-C20 = 1.398(5), C20-N3 = 1.388(5), C51-N8 = 1.276(5),
C51-C50 = 1.455(6), C50-N7 = 1.366(4); N3-Fe1-C31 = 96.4(1), N3-Fe1-N7 = 177.3(1),
C31-Fe1-N7 = 81.7(1), N3-Fe1-C1 = 79.5(1), C31-Fe1-C1 = 97.0(1), N7-Fe1-C1 = 98.8(1),
N3-Fe1-N4
155.8(1).
= 79.8(1), C31-Fe1-N4 = 97.6(1), N7-Fe1-N4 = 102.2(1), C1-Fe1-N4 =
The salt 18 features a cation with a distorted square
pyramidal (τ = 0.36)24 iron, coordinated by one tridentate
(
κN , , κ κN ,
imin κNpyr CNHC) and one bidentate ( pyr κCNHC) ligands,
both based on the 4B framework; the anion in 18 is a bromide.
Thus, in the bidentate ligand the imine group is dangling (Fe-N
= 2.708 Å), despite its Z-configuration suitable for metal
bonding; the Fe centre is ca. 0.085 Å above the square base of
the pyramid. In view of the redox non-innocent potential of
4B, it proved instructive to peruse the metrical data within the
framework of the two ligands in order to establish their exact
nature and OS. It is conceivable that 4B and 4C can adopt
three redox states (Scheme 6), featuring different bond
Mes
Br
N
Mes
N
1 equiv. KC8
(12%)
N
Fe
N
N
N
N
N
N
R1
R1
N
N
N
2
18
Fig. 4 The structure of 19 with thermal ellipsoids at the 30% probability level; H-atoms
have been omitted; only the ipso-C of the DiPP rings is shown. Important bond
N
N
DiPP
Mes
Mes
4B
Mes
N
distances (Å) and angles (deg.): Fe1-N3
= 1.878(3), Fe1-N4 = 1.949(4), Fe1-N7 =
+ FeBr2
2 equiv. KC8
(32%)
1.944(3), Fe1-C1 = 1.951(4), Fe1-C31 = 1.864(4), C21-N4 = 1.384(5), C21-C20 = 1.366(6),
C20-N3 = 1.383(5), C51-N8 = 1.275(5), C51-C50 = 1.475(5), C50-N7 = 1.373(5); N3-Fe1-
C31 = 109.0(2), N3-Fe1-N7 = 169.3(1), C31-Fe1-N7 = 81.5(1), N3-Fe1-C1 = 80.3(2), C31-
Fe1-C1 = 94.7(2), N7-Fe1-C1 = 97.2(1), N3-Fe1-N4 = 79.5(1), C31-Fe1-N4 = 107.8(2), N7-
Fe1-N4 = 99.5(1), C1-Fe1-N4 = 153.6(1).
N
N
N
Fe
N
R1
N
R1
19
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Table 1 Comparison of bond distances of the ligands in the complexes 18 and 19.a
distances within the ligand skeleton. Comparable redox states
have been put forward for bis-imino-pyridine25 and α-imino-
pyridine ligands,26 rendering 4B
/4C NHC-functionalized α-
iminopyridines. Based on this analogy, in the redox state of
Complex 18
Complex 19
Bidentate
Tridentate
1.339(4)
1.398(5)
1.388(5)
Bidentate
Tridentate
1.384(5)
1.366(6)
1.383(5)
a
b
c
1.276(5)
1.455(6)
1.366(4)
1.275(5)
1.475(5)
1.373(5)
a Bond distances a, b, c (Å) as defined in Scheme 3.
Scheme 6 Possible redox states of 4B/4C including neutral (4)0, mono-anionic (4.)1- and
di-anionic (4)2-; only one resonance form is depicted.
Experimental
(4 ) 4)
)0, (4. 1- and ( 2- the distances a, b and c should be similar to
Synthetic and Characterization Procedures
those in α-iminopyridine (e.g. ca. a = 1.28 Å, b = 1.47 Å, c =
1.35 Å), the open shell amido pyridine radical anion (e.g. a =
1.34 Å, b = 1.41 Å, c = 1.39 Å) and the amido piperidinyl
dianion (e.g. a = 1.46 Å, b = 1.35 Å, c = 1.40 Å), respectively.
The relevant bond distances in the two ligands of 18 are given
General methods. All air- and moisture-sensitive
manipulations were performed under dry argon atmosphere
using standard Schlenk techniques or in an MBraun glove-box
under N2. THF and diethyl ether were dried by refluxing over
sodium/benzophenone ketyl and distilled under an argon
atmosphere prior use. Other solvents (pentane, toluene and
CH2Cl2) were dried by passing through columns of activated
alumina and subsequently purged with argon. C6D6 was
distilled over KH, CD2Cl2 and CDCl3 were dried over 3 Å or 4 Å
molecular sieves and all were degassed by freeze-pump-thaw
in Table 1. It can be inferred that the bidentate and tridentate
0
ligands of 18 conform to the redox states (
4)
and (
4
.)1-,
respectively, and consequently, the Fe OS should be FeII. The
Fe-Nimine, Fe-Npyridine and Fe-CNHC bond distances fall in the
range reported in the literature for similar bond types.27 It thus
appears that the reduction to 18 is exclusively ligand based.
Although the coordination geometry of Fe in the overall
charge neutral 19 is broadly similar to that of 18, i.e. distorted
square pyramidal (τ = 0.26), with the Fe centre ca. 0.246 Å
above the square base of the pyramid, one tridentate pincer
and one bidentate ligands, the latter with dangling imine
functionality, careful inspection reveals an interesting
difference: the bond distances within the tridentate ligand
(Table 1) are closer to the values associated with the dianionic
amido piperidinato redox state. The bidentate and tridentate
cycles. The compounds 7 28 829 and 1128a were synthesized
,
from their literature procedure. All other chemicals were
obtained from commercial sources and used without further
purification. NMR spectra were recorded on Bruker
spectrometers (AVANCE I – 300 MHz, AVANCE III – 400 MHz or
AVANCE I – 500 MHZ equipped with a cryogenic probe).
Downfield shifts are reported as positive and referenced using
signals of the residual protio solvent (1H) or the solvent (13C).
All NMR spectra were measured at 298 K. The multiplicity of
the signals is indicated as s = singlet, d = doublet, t = triplet,
q=quartet, sept = septet, m = multiplet and br = broad.
Assignments were determined either on the basis of
unambiguous chemical shifts, coupling patterns or 2D
correlations (1H - 1H COSY, 1H - 13C HSQC, 1H - 13C HMBC).
Elemental analyses were performed by the “Service de
microanalyses”, Université de Strasbourg. Electrospray mass
spectra (ESI-MS) were recorded on a microTOF (Bruker
Daltonics, Bremen, Germany) instrument using nitrogen as
drying agent and nebulizing gas.
ligands most likely conform to the redox states (4 4
)0 and ( )2-,
respectively, and consequently, the Fe OS is FeII. Metrical data
within the bidentate ligand and Fe-CNHC and Fe-Npyr of the
bidentate ligand are very similar in both complexes, in
agreement with them featuring the same metal OS.
Conclusions
In conclusion, we have developed a ZnII template-based
synthesis of non-symmetrical 2-arylaldimino-6-arylimidazolium
pyridine pro-ligands and the NHC ligands obtained thereof.
The method has promising scope and acceptable overall yield
(based on the pyridine derivative). The determination of the
‘metrical oxidation state’ of the metal and ligands in the two
Fe complexes that were characterized crystallographically
Synthesis of 6-bromopyridine-2-carbaldehyde (7). This
compound was synthesized according to the literature
procedure.28 To a solution of 2,6-dibromopyridine (23.7 g, 100
mmol) in Et2O (500 mL), cooled at -78 °C, was added dropwise
a solution of nBuLi in hexane (63 mL of a 1.59 M hexane
solution, 100 mmol) over 15 min. After stirring at -78 °C for 10
min, a solution of DMF (9.3 mL, 8.77 g, 120 mmol) in Et2O (40
mL) was slowly added. The resulting mixture was stirred at -78
°C for 20 min, let warm up to -40 °C and quenched by addition
of 10% aqueous HCl until acidic (ca. 80 mL). The resulting
solution was made alkaline by addition of aqueous saturated
NaHCO3 solution. The aqueous layer was separated and
extracted with ethyl acetate (3 x 50 mL). The combined organic
manifests
a
redox non-innocent nature.
Further
characterization and electrochemical studies of the redox
properties of the ligand and iron complexes are aims of future
studies. The scope, coordination chemistry and catalytic
applications of
4 as redox-active, strong field ligands obtained
by the desymmetrization of the successful and popular
NiminNpyrNimin or CNHCNpyrCNHC will be the subject of future work.
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extracts were washed with brine and dried over MgSO4. (CHarom.), 102.7 (OCHO), 65.9 (OCH2), 29.0 (CH(CH3)2), 24.6
Evaporation of the volatiles under reduced pressure gave the (CH(CH3)2), 24.3 (CH(CH3)2).
crude product as a light brown solid. Recrystallization from
Et2O/pentane afforded the title compound as colourless Synthesis of 2-bromo-6-(diethoxymethyl)pyridine (13).
1
A
prisms. Yield: 17.0 g (91.4 mmol), 91%. H NMR (300.17 MHz, mixture of
7 (2.00 g, 10.8 mmol), triethylorthoformate (2.15
CDCl3): δ 9.98 (d, JHH = 0.7 Hz, 1H, CHO), 7.91 (dd, 3JHH = 6.6 Hz, mL, 1.91 g, 12.9 mmol) and p-toluenesulfonic acid
JHH = 1.8 Hz, 1H, CHpyr.), 7.78-7.68 (m, 2H, CHpyr.). 13C{1H} NMR monohydrate (0.205 g, 1.08 mmol, 10mol%) in ethanol (20 mL)
(75.49 MHz, CDCl3): δ 191.7 (CHO), 153.6 (Cpyr), 142.7 (Cpyr.), was stirred at room temperature. After 3 h, full conversion was
139.5 (CHpyr.), 132.8 (CHpyr.), 120.5 (CHpyr.). These data are confirmed by TLC analysis (Rf = 0.82, Et2O/petroleum, 50:50).
consistent with those described in literature.28
All the volatiles were evaporated under reduced pressure and
the resulting oil was redissolved in Et2O, successively washed
Synthesis of 2-bromo-6-(1,3-dioxolan-2-yl)pyridine (11). This with aqueous NaHCO3 solution and brine, and dried over
compound was synthesized according to the literature MgSO4. Evaporation of the solvent under reduced pressure
procedure.28a A mixture of
7 (7.50 g, 40.3 mmol), ethylene gave the title compound as a yellow oil. Yield: 2.73 g (10.5
glycol (3.8 mL, 4.25 g, 68.5 mmol), p-toluenesulfonic acid mmol), 97%. 1H NMR (400.13 MHz, CDCl3): δ 7.61-7.54 (m, 2H,
monohydrate (0.77 g, 4.0 mmol, 10 mol%) in toluene was CHpyr.), 7.43 (dd, 3JHH = 6.3 Hz, JHH = 2.5 Hz, 1H, CHpyr.), 5.39 (s,
heated to reflux overnight with azeotropic removal of the 1H, OCHO), 3.77-3.67 (m, 2H, OCH2), 3.64-3.55 (m, 2H, OCH2),
3
water (Dean-Stark). To the resulting solution, cooled to room 1.24 (t, JHH = 7.1 Hz, 6H, CH3). 13C{1H} NMR (100.62 MHz,
temperature, was added aqueous NaHCO3 solution. The CDCl3): δ 160.1 (Cpyr.), 141.3 (Cpyr.), 139.2 (CHpyr.), 128.0 (CHpyr.),
aqueous layer was separated and extracted with ethyl acetate 119.9 (CHpyr.), 102.4 (OCHO), 62.9 (OCH2), 15.3 (CH3).
(3 x 20 mL). The combined organic extracts were washed with
brine and dried over MgSO4. Evaporation of the solvent under Synthesis of 2-(diethoxymethyl)-6-iodopyridine (15). Halide
reduced pressure gave the crude product as a brown oil. exchange according to Buchwald’s procedure:13 a Schlenk flask
Distillation under reduced pressure (fraction boiling at ca. 100 was charged with 13 (1.53 g, 5.87 mmol), CuI (0.056 g, 0.294
°C / 0.1 mbar) afforded the title compound as a pale yellow oil. mmol, 5.0 mol%) and KI (1.95 g, 11.7 mmol). The system was
Yield: 7.68 g (33.4 mmol), 83%. 1H NMR (300.17 MHz, CDCl3): δ evacuated and backfilled with argon. Dioxane (10 mL) and
3
3
4
7.59 (t, JHH = 7.7 Hz, 1H, CHpyr.), 7.50 (dd, JHH = 7.6 Hz, JHH
=
N,N’-dimethylethylenediamine (63 μL, 0.59 mmol, 10 mol%)
3
4
1.1 Hz, 1H, CHpyr.), 7.48 (dd, JHH = 7.7 Hz, JHH = 1.1 Hz, 1H, were added and the reaction mixture was heated at 110 °C for
CHpyr.), 5.81 (s, 1H, OCHO), 4.21-4.02 (m, 4H, OCH2). 13C{1H} 1 d. To the resulting suspension, at rt, 30% aq. NH3 (25 mL) and
NMR (75.49 MHz, CDCl3): δ 158.6 (Cpyr.), 141.7 (Cpyr.), 139.2 water (100 mL) were added. The mixture was extracted with
(CHpyr.), 128.5 (CHpyr.), 119.5 (CHpyr.), 102.8 (OCHO), 65.7 CH2Cl2 (3 x 80 mL) and the combined organic layers were
(OCH2). These data are consistent with those described in washed with brine and dried over MgSO4. Evaporation of the
literature.28a
solvent under reduced pressure gave the crude product as a
1
yellow oil in almost quantitative yield. H NMR (400.13 MHz,
Synthesis
of
1-(6-(1,3-dioxolan-2-yl)pyridin-2-yl)-3-(2,6- CDCl3): δ 7.65 (d, 3JHH = 7.7 Hz, 1H, CHpyr.), 7.55 (d, 3JHH = 7.7 Hz,
diisopropylphenyl)-1H-imidazol-3-ium bromide (12).
mixture of 11 (4.69 g, 20.4 mmol) and 1-(2,6- 3.76-3.51 (m, 4H, OCH2), 1.22 (t, 3JHH = 7.1 Hz, 6H, CH3). 13C{1H}
diisopropylphenyl)-1H-imidazole ( ) (5.10 g, 22.3 mmol) was NMR (100.62 MHz, CDCl3): δ 160.7, 138.2, 134.7, 120.2, 117.0,
A
1H, CHpyr.), 7.33 (t, 3JHH = 7.7 Hz, 1H, CHpyr.), 5.37 (s, 1H, OCHO),
8
heated in a sealed flask at 150 °C for 5-6 d. After cooling to 102.4 (OCHO), 62.8 (OCH2), 15.3 (CH3). MS (ESI+): m/z 308.01
room temperature, the reaction mixture was dissolved in [M+H]+ with the corresponding isotopic pattern.
CH2Cl2 and a light brown solid (1.35 g) containing ca. 20-30% of
an unidentified impurity (probably originating from a side Synthesis of 1-(6-bromopyridin-2-yl)-N-mesitylmethanimine
reaction involving ring-opening of the 1,3-dioxolane protecting (10B).
A mixture of 7 (1.65 g, 8.87 mmol), 2,4,6-
group) was precipitated by the addition of Et2O. The Et2O trimethylaniline (1.37 mL, 9.75 mmol) and formic acid (17 μL,
fraction was found to consist of clean unreacted material, 0.444 mmol, 5.0 mol%) in ethanol was heated to reflux for 2-3
indicating a reaction with low/modest conversion. 1H NMR h. All volatiles were evaporated under reduced pressure to
(400.13 MHz, CDCl3): δ 11.05 (t, 4JHH = 1.6 Hz, 1H, CHimid.), 9.32 dryness and the resulting solid was recrystallized from hexane
3
3
4
(d, JHH = 7.7 Hz, 1H, CHpyr.), 9.31 (t, JHH = JHH = 1.8 Hz, 1H, to afford the 10B as yellow prisms. Yield: 2.11 g (6.96 mmol),
3
CHimid.), 8.19 (t, JHH = 8.0 Hz, 1H, CHpyr.), 7.73 (d, JHH = 7.7 Hz, 78%. Anal. Calcd for C15H15BrN2 (303.20): C, 59.42; H, 4.99; N,
3
3
1H, CHpyr.), 7.58 (t, JHH = 7.9 Hz, 1H, p-CHDiPP), 7.37 (t, JHH
3
=
9.24. Found: C, 59.57; H, 4.97; N, 9.45. 1H NMR (400.13 MHz,
4JHH = 1.8 Hz, 1H, CHimid.), 7.36 (d, JHH = 7.9 Hz, 2H, m-CHDiPP), CDCl3): δ 8.27 (s, 1H, NCH), 8.25 (dd, 3JHH = 7.7 Hz, 4JHH = 0.8 Hz,
3
3
5.87 (s, 1H, OCHO), 4.22-4.07 (m, 4H, OCH2), 2.41 (sept, JHH
=
1H, CHpyr.), 7.69 (t, 3JHH = 7.8 Hz, 1H, CHpyr.), 7.59 (dd, 3JHH = 7.8
6.8 Hz, 2H, CH(CH3)2), 1.29 (d, 3JHH = 6.8 Hz, 6H, CH(CH3)2), 1.17 Hz, JHH = 0.8 Hz, 1H, CHpyr.), 6.90 (s, 2H, CHmes.), 2.29 (s, 3H, p-
4
3
(d, JHH = 6.8 Hz, 6H, CH(CH3)2). 13C{1H} NMR (75.49 MHz, CH3 mes.), 2.13 (s, 6H, o-CH3 mes.). 13C{1H} NMR (100.62 MHz,
CDCl3): δ 157.3 (Carom.), 145.5 (Carom.), 145.2 (Carom.), 142.3 CDCl3): δ 162.0 (CHN), 155.9 (Carom.), 147.5 (Carom.), 141.9
(CHarom.), 135.8 (CHarom.), 132.4 (CHarom.), 130.1 (Carom.), 125.3 (Carom.), 139.1 (CHarom.), 133.9 (Carom.), 129.8 (CHarom.), 129.0
(CHarom.), 125.0 (CHarom.), 122.5 (CHarom.), 121.1 (CHarom.), 117.0
6 | J. Name., 2012, 00, 1-3
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Journal Name
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DOI: 10.1039/C7DT01014A
ARTICLE
(CHarom.), 126.9 (Carom.), 119.9 (CHarom.), 20.9 (p-CH3 mes.), 18.4 8.21 (s, 1H, NCH), 7.95 (t, 3JHH = 7.8 Hz, 1H, CHpyr.), 7.85 (d, 3JHH
3
= 7.8 Hz, 1H, CHpyr.), 7.17 (2 overlapping d, JHH = 8.4, 6.4 Hz,
(o-CH3 mes.).
3
2H, m-CHDiPP), 7.11 (dd, JHH = 8.6, 6.3 Hz, 1H, p-CHDiPP), 2.84
3
3
Synthesis of
1-(6-bromopyridin-2-yl)-N-(2,6-diisopropyl- (sept, JHH = 6.9 Hz, 2H, CH(CH3)2), 1.11 (d, JHH = 6.9 Hz, 12H,
(4.65 g, 25.0 CH(CH3)2). 13C{1H} NMR (100.62 MHz, DMSO-d6):
161.6
mmol), 2,6-diisopropylaniline (4.95 mL, 26.3 mmol) and formic (CHN), 154.6 (Carom.), 147.7 (Carom.), 141.4 (Carom.), 140.6
acid (47 L, 1.25 mmol, 5.0 mol%) in ethanol was heated to (CHarom.), 136.4 (Carom.), 130.4 (CHarom.), 124.6 (CHarom.), 122.9
phenyl)methanimine 6C (10C). A mixture of
7
δ
ꢀ
reflux for 2-3 h. All volatiles were evaporated to dryness and (CHarom.), 120.6 (CHarom.), 27.5 (CH(CH3)2), 23.2 (CH(CH3)2).
the resulting solid was recrystallized from Et2O/hexane to
afford 10C as yellow-green prisms. Yield: 8.44 g (24.4 mmol), Synthesis of 5B and 5C. A mixture of 16B (1.0 g, 2.3 mmol)
98%. Anal. Calcd for C18H21BrN2 (345.28): C, 62.61; H, 6.13; N, (resp. 16C (0.87 g, 1.8 mmol)) and 8 (2 equiv.) was heated in a
1
8.11. Found: C, 62.73; H, 6.14; N, 8.29. H NMR (400.13 MHz, sealed flask at 150-160 °C for 4 d. After cooling to rt,
3
4
CDCl3): δ 8.26 (dd, JHH = 7.5 Hz, JHH = 1.0 Hz, 1H, CHpyr.), 8.25 decoordination of ZnII of was carried out using a slightly
(d, 5JHH = 0.6 Hz, 1H, NCH), 7.71 (td, 3JHH = 7.7 Hz, 5JHH = 0.6 Hz, modified literature procedure.22 To a CH2Cl2 solution (50 mL) of
3
1H, CHpyr.), 7.61 (dd, JHH = 7.8 Hz, JHH = 1.0 Hz, 1H, CHpyr.), the crude zinc complex in a separating funnel was added a
4
3
7.20-7.13 (m, 3H, CHDiPP), 2.92 (sept, JHH = 6.9 Hz, 2H, solution of potassium oxalate monohydrate (ca. 2 equiv.) in
3
CH(CH3)2), 1.17 (d, JHH = 6.9 Hz, 12H, CH(CH3)2). 13C{1H} NMR water. After shaking the mixture for 5-10 min, the aqueous
(100.62 MHz, CDCl3): δ 161.6 (CHN), 155.6 (Carom.), 148.0 phase, containing
a white suspension of Zn(C2O4), was
(Carom.), 142.0 (Carom.), 139.1 (CHarom.), 137.2 (Carom.), 130.0 separated and the organic phase was washed twice with water
(CHarom.), 124.8 (CHarom.), 123.2 (CHarom.), 120.0 (CHarom.), 28.1 (2 x 25 mL). To avoid a possible mixture of counteranions in
(CH(CH3)2), 23.6 (CH(CH3)2).
the imidazolium salt product, saturated aqueous NaBr solution
was added to the organic phase and the resulting biphasic
Synthesis of [{1-(6-bromopyridin-2-yl)-N-mesitylmethan- mixture was vigorously stirred for 30 min. The organic phase
imine}ZnCl2] (16B). To a solution of 10B (0.552 g, 1.82 mmol) was separated, dried over anhydrous Na2SO4 and evaporated
in dry THF (5 mL) was added anhydrous ZnCl2 (0.248 g, 1.82 under reduced pressure. The solid residue was triturated with
mmol). The mixture was heated to 60 °C for 1h, during which hot hexane (4 x 20 mL), filtered and dried under vacuum to
time a yellow precipitate appeared from the light yellow afford
solution. After cooling down to rt Et2O (5 mL) was added. The and 0.79 g (1.4 mmol), 76% (5C).
precipitate formed was collected by filtration, washed with For 5B: Anal. Calcd for C30H35BrN4 (531.54): C, 67.79; H,
Et2O and dried in vacuo affording 16B as a yellow powder. 6.64; N, 10.54. Found: C, 66.35; H, 6.51; N, 10.36. No better
Yield: 0.779 (1.77 mmol), 97%. Anal. Calcd for analyses could be obtained and the experimental values
5 as a yellow solid. Yield: 0.83 g (1.6 mmol), 69% (5B)
g
C15H15BrCl2N2Zn (439.48): C, 40.99; H, 3.44; N, 6.37. Found: C, suggest the presence of residual NaBr: C30H35BrN4·0.1NaBr
41.05; H, 3.50; N, 6.41. 1H NMR (400.13 MHz, DMSO-d6): δ (541.83): C, 66.50; H, 6.51; N, 10.34. The purity of 5B can be
3
4
8.24 (s, 1H, NCH), 8.21 (dd, JHH = 7.6 Hz, JHH = 0.8 Hz, 1H, assessed from its NMR spectra (see ESI). 1H NMR (500.13 MHz,
CHpyr.), 7.94 (t, 3JHH = 7.7 Hz, 1H, CHpyr.), 7.83 (dd, JHH = 7.9 Hz, CD2Cl2): δ 11.53 (t, JHH = 1.5 Hz, 1H, CHimid.), 9.41 (d, 3JHH = 8.1
3
4
4JHH = 0.8 Hz, 1H, CHpyr.), 6.90 (s, 2H, CHmes.), 2.23 (s, 3H, p-CH3 Hz, 1H, CHpyr.), 9.21 (t, JHH = JHH = 1.6 Hz, 1H, CHimid.), 8.46 (t,
3
4
mes.), 2.05 (s, 6H, o-CH3 mes.). 13C{1H} NMR (100.62 MHz, DMSO- 3JHH = 7.7 Hz, 1H, CHpyr.), 8.33 (s, 1H, NCH), 8.28 (d, JHH = 7.9
3
d6): δ 161.8 (CHN), 155.0 (Carom.), 147.1 (Carom.), 141.2 (Carom.), Hz, 1H, CHpyr.), 7.63 (t, 3JHH = 7.8 Hz, 1H, p-CHDiPP), 7.43 (t, 3JHH
=
140.5 (CHarom.), 132.9 (Carom.), 130.1 (CHarom.), 128.6 (CHarom.), 4JHH = 1.7 Hz, 1H, CHimid.), 7.41 (d, JHH = 7.8 Hz, 2H, m-CHDiPP),
3
3
126.2 (Carom.), 120.3 (CHarom.), 20.3 (p-CH3 mes.), 17.8 (o-CH3 mes.). 6.92 (s, 2H, CHmes.), 2.45 (sept, JHH = 6.8 Hz, 2H, CH(CH3)2),
3
2.28 (s, 3H, p-CH3 mes.), 2.14 (s, 6H, o-CH3 mes.), 1.29 (d, JHH
=
3
Synthesis of [{1-(6-bromopyridin-2-yl)-N-(2,6-diisopropyl- 6.8 Hz, 6H, CH(CH3)2), 1.20 (d, JHH = 6.8 Hz, 6H, CH(CH3)2).
phenyl)methanimine}ZnCl2] (16C). To a solution of 10C (1.94 13C{1H} NMR (125.77 MHz, CD2Cl2): δ 161.7 (NCH), 154.3 (Cpyr.)
,
g, 5.60 mmol) in dry THF (25 mL) was added anhydrous ZnCl2 147.8 (Cmes.), 146.3 (Cpyr.), 145.6 (CDiPP), 141.8 (CHpyr.), 136.8
(0.764 g, 5.60 mmol). The mixture was heated for 1h to 60 °C, (CHimid.), 134.4 (Cmes.), 132.5 (CHDiPP), 130.4 (CDiPP), 129.2
during which time a yellow precipitate appeared from the light (CHmes.), 127.1 (Cmes.), 125.5 (CHimid.), 125.2 (CHDiPP), 122.8
brown solution. After cooling down to rt Et2O (20 mL) was (CHpyr.), 120.9 (CHimid.), 118.2 (CHpyr.), 29.3 (CH(CH3)2), 24.43
added and the formed precipitate was collected by filtration, (CH(CH3)2), 24.36 (CH(CH3)2), 20.9 (p-CH3 mes.), 18.4 (o-CH3 mes.).
washed with Et2O and dried in vacuo affording 16C·THF as a
For 5C: Anal. Calcd for C33H41BrN4 (573.62): C, 69.10; H,
yellow powder (THF adduct based on the NMR data). Heating 7.20; N, 9.77. Found: C, 68.47; H, 7.13; N, 9.71. No better
the THF adduct at 100 °C under vacuum for several hours analyses could be obtained and the experimental values
afforded 16C as a yellow powder. Single crystals suitable for X- suggest the presence of residual NaBr: Calcd for
ray diffraction analysis were obtained by slow diffusion of C33H41BrN4·0.05NaBr (578.77): C, 68.48; H, 7.14; N, 9.68. The
pentane in a dilute solution of the complex. Yield: 2.80 g (5.06 purity of 5C can be assessed from its NMR spectra (see ESI). 1H
mmol), 90%. Anal. Calcd for C18H21BrCl2N2Zn (481.56): C, 44.89; NMR (500.13 MHz, CD2Cl2): δ 11.58 (t, 4JHH= 1.5 Hz, 1H, CHimid.),
3
4
3
H, 4.40; N, 5.82. Found: C, 44.94; H, 4.32; N, 5.72. 1H NMR 9.46 (dd, JHH= 8.1 Hz, JHH ≈ 0.6 Hz, 1H, CHpyr.), 9.21 (t, JHH
≈
3
3
4
(400.13 MHz, DMSO-d6):
δ
8.22 (d, JHH = 7.7 Hz, 1H, CHpyr.), 4JHH ≈ 1.8 Hz, 1H, CHimid.), 8.46 (dd, JHH = 7.7 Hz, JHH ≈ 0.6 Hz,
This journal is © The Royal Society of Chemistry 20xx
J. Name., 2013, 00, 1-3 | 7
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DOI: 10.1039/C7DT01014A
PAPER
Dalton Transactions
3
1H, CHpyr.), 8.31 (br t, JHH = 7.9 Hz, 1H, CHpyr.), 8.30 (s, 1H, 6.9 Hz, 12H, CH(CH3)2), 1.11 (d, JHH = 6.9 Hz, 6H, CH(CH3)2).
3
3
3
4
NCH), 7.64 (t, JHH = 7.8 Hz, 1H, p-CHDiPP), 7.43 (t, JHH ≈ JHH
≈
13C{1H} NMR (125.77 MHz, C6D6): δ 220.0 (CNHC), 163.2 (NCH),
1.8 Hz, 1H, CHimid.), 7.41 (d, 3JHH = 7.8 Hz, 2H, m-CHDiPP), 7.19 (2 154.2 (Carom.), 152.9 (Carom.), 149.4 (Carom.), 146.2 (Carom.), 139.0
overlapping d, 3JHH = 8.6, 6.6 Hz, 2H, m-CHDiPP), 7.14 (dd, 3JHH
=
(p-CHpyr.), 138.7 (Carom.), 137.5 (Carom.), 129.3 (p-CHDiPP), 125.1
3
8.6, 6.6 Hz, 1H, p-CHDiPP), 2.95 (sept, JHH = 6.8 Hz, 2H, (p-CHDiPP), 123.8 (m-CHDiPP), 123.6 (m-CHDiPP), 123.0 (CHimid.),
3
3
CH(CH3)2), 2.46 (sept, JHH = 6.8 Hz, 2H, CH(CH3)2), 1.30 (d, JHH 118.6 (CHpyr.), 116.6 (CHpyr.), 116.3 (CHimid.), 28.6 (CH(CH3)2),
3
= 6.8 Hz, 6H, CH(CH3)2), 1.20 (d, JHH = 6.8 Hz, 6H, CH(CH3)2), 28.5 (CH(CH3)2), 24.5 (CH(CH3)2), 24.1 (CH(CH3)2), 23.6
1.18 (d, JHH = 6.8 Hz, 12H, CH(CH3)2). 13C{1H} NMR (125.77 (CH(CH3)2).
3
MHz, CD2Cl2): δ 161.4 (NCH), 153.9 (Carom.), 148.5 (Carom.), 146.4
(Carom.), 145.6 (Carom.), 141.9 (CHarom.), 137.5 (Carom.), 136.9 Synthesis of 18. To a suspension of FeBr2 (0.027 g, 0.125
(CHarom.), 132.5 (CHarom.), 130.4 (Carom.), 125.5 (CHarom.), 125.21 mmol) in THF (5 mL) was added a solution of 4B (0.113 g, 0.250
(CHarom.), 125.17 (CHarom.), 123.5 (CHarom.), 123.0 (CHarom.), 120.9 mmol) in THF (5 mL) at -78 °C. A paramagnetic turquoise blue
(CHarom.), 118.4 (CHarom.), 29.3 (CH(CH3)2), 28.4 (CH(CH3)2), precipitate was formed immediately with very low solubility in
24.43 (CH(CH3)2), 24.37 (CH(CH3)2), 23.6 (CH(CH3)2).
THF. The supernatant was discarded and the resulting solid
Synthesis of 4B. To a suspension of 5B (0.12 g, 0.23 mmol) in was dried under vacuum. The crude product was suspended in
Et2O (5 mL), precooled at -78 °C, was added a solution of THF (5 mL), cooled to -78 °C, and KC8 (0.017 g, 0.125 mmol)
KN(SiMe3)2 (0.050 g, 0.25 mmol) in Et2O (5 mL). The resulting was added. Filtration of the solution and evaporation of the
mixture was allowed to reach rt and was stirred for 2 h, giving solvent afforded an air-sensitive dark blue-green solid. No
a suspension of KBr in a light brown solution. Filtration and satisfactory elemental analysis could be obtained. Single
evaporation of the solvent gave 4B as a cream-colored solid. crystals suitable for X-ray diffraction studies were grown by
Yield: 0.086 g (0.19 mmol), 83%. Satisfactory elemental slow evaporation of a Et2O/THF solution of the complex. Yield
analysis data could not be obtained but the purity of the of the crystals: 0.015 g (0.014 mmol), 12% based on the metal.
1
compound can be assessed from its NMR spectra (see ESI). H We prefer to report the yield based on the crystalline solid
3
NMR (500.13 MHz, C6D6): δ 8.75 (dd, JHH = 8.2 Hz, JHH = 0.9 isolated since we cannot state how much was still present in
4
5
Hz, 1H, CHpyr.), 8.35 (d, JHH ≈ 0.5 Hz, 1H, NCH), 8.29 (d, JHH
3
=
the paramagnetic mother liquor.
3
4
1.7 Hz, 1H, CHimid.), 8.08 (dd, JHH = 7.5 Hz, JHH = 0.9 Hz, 1H,
CHpyr.), 7.28 (dd, 3JHH = 8.3, 7.2 Hz, 1H, p-CHDiPP), 7.17 (br d, 3JHH Synthesis of 19. To a suspension of FeBr2 (0.005 g, 0.023
3
5
= 7.7 Hz, 2H, m-CHDiPP), 7.12 (ddd, JHH = 8.2, 7.5 Hz, JHH = 0.6 mmol) in THF (5 mL) was added a solution of 4B (0.020 g, 0.044
3
Hz, 1H, CHpyr.), 6.84 (s, 2H, CHmes.), 6.60 (d, JHH = 1.7 Hz, 1H, mmol) in THF (5 mL) at -78 °C. A paramagnetic turquoise blue
3
CHimid.), 2.93 (sept, JHH = 6.9 Hz, 2H, CH(CH3)2), 2.20 (s, 3H, p- precipitate was formed immediately with very low solubility in
3
CH3 mes.), 2.18 (s, 6H, o-CH3 mes.), 1.23 (d, JHH = 6.9 Hz, 6H, THF. The supernatant was discarded and the resulting solid
3
CH(CH3)2), 1.11 (d, JHH = 6.9 Hz, 6H, CH(CH3)2). 13C{1H} NMR was dried under vacuum. The crude product was suspended in
(125.77 MHz, C6D6): 220.0 (CNHC), 163.4 (NCH), 154.1 (Carom.), Et2O (5 mL), cooled to -78 °C, and KC8 (0.006 g, 0.044 mmol)
153.3 (Carom.), 148.8 (Carom.), 146.2 (Carom.), 138.9 (p-CHpyr.), was added. Filtration of the solution and evaporation of the
138.7 (Carom.), 133.5 (Carom.), 129.4 (CHmes.), 129.3 (p-CHDiPP), solvent afforded an air-sensitive dark blue-green solid with
127.0 (Carom.), 123.8 (m-CHDiPP), 123.0 (CHimid.), 118.5 (CHpyr.), very good solubility in Et2O. No satisfactory elemental analysis
116.4 (CHpyr./imid.), 116.3 (CHimid./pyr.), 28.6 (CH(CH3)2), 24.5 could be obtained. Single crystals suitable for X-ray diffraction
(CH(CH3)2), 24.1 (CH(CH3)2), 20.9 (p-CH3 mes.), 18.5 (o-CH3 mes.).
studies were grown by slow evaporation of a pentane/Et2O
solution of the complex. Yield of the crystals: 0.007 g (7.3
Synthesis of and 4C. To a suspension of 5C (0.57 g, 1.0 mmol)
ꢀ
mol), 32% based on the metal. We report the yield based on
in Et2O (10 mL), precooled at -20 °C, was added a solution of the crystalline solid isolated since we cannot state how much
KN(SiMe3)2 (0.22 g, 1.1 mmol) in Et2O (5 mL). The resulting product was still present in the paramagnetic mother liquor.
mixture was allowed to reach rt and was stirred for 1 h, giving
a suspension of KBr in a light brown solution. Filtration and
X-ray Crystallography
evaporation of the solvent gave 4C as a cream-colored solid.
Yield: 0.22 g (0.45 mmol), 45%. (The relative low yield of 4C General methods. Summary of the crystal data, data collection
compared to that of 4B is probably due to the poor solubility and refinement for compounds 16C, 18 and 19 are given in
of 4C in Et2O.) Satisfactory elemental analysis data could not Table S1 (See ESI). The crystals were mounted on a glass fibre
be obtained but the purity of the compound can be assessed with grease, from Fomblin vacuum oil. Data sets for 18 and 19
1
from its NMR spectra (see ESI). H NMR (500.13 MHz, C6D6): δ were collected at 173(2) K on a Bruker APEX-II CCD Duo
3
8.75 (dd, JHH = 8.2 Hz, JHH = 0.7 Hz, 1H, CHpyr.), 8.43 (s, 1H, diffractometer (graphite-monochromated Mo-K
4
α radiation, λ
NCH), 8.24 (d, 3JHH = 1.6 Hz, 1H, CHimid.), 8.06 (dd, 3JHH = 7.5 Hz, = 0.71073 Å). Data sets for 16C were collected at 173(2) K on a
3
4JHH = 0.7 Hz, 1H, CHpyr.), 7.28 (dd, JHH = 8.0, 7.5 Hz, 1H, p- Kappa CCD diffractometer (graphite-monochromated Mo-K
α
3
CHDiPP), 7.21-7.14 (m, 5H, p-CHDiPP + 4 m-CHDiPP), 7.12 (t, JHH
=
radiation, λ = 0.71073 Å). Specific comments for each data set
7.9 Hz, 1H, CHpyr.), 6.57 (d, 3JHH = 1.6 Hz, 1H, CHimid.), 3.19 (sept, are given below. The cell parameters were determined using
3
3JHH = 6.9 Hz, 2H, CH(CH3)2), 2.91 (sept, JHH = 6.9 Hz, 2H, DENZO30 (Kappa) or APEX231 (APEX-II) softwares. The
3
CH(CH3)2), 1.23 (d, JHH = 6.9 Hz, 6H, CH(CH3)2), 1.21 (d, JHH
3
=
structures were solved by direct methods using the program
8 | J. Name., 2012, 00, 1-3
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ARTICLE
SHELXS-2013.32 The refinement and all further calculations
were carried out using SHELXL-2013.32b The H-atoms were
introduced into the geometrically calculated positions
(SHELXL-2013 procedure) unless stated otherwise and refined
riding on the corresponding parent atoms. The non-H atoms
were refined anisotropically, using weighted full-matrix least-
squares on F2.
8
9
J. M. Darmon, R. P. Yu, S. P. Semproni, Z. R. Turner, S. C. E.
Stieber, S. DeBeer and P. J. Chirik, Organometallics, 2014, 33
,
5423-5433.
(a) A. A. Danopoulos, D. Pugh, H. Smith and J.
Saßmannshausen, Chem. Eur. J., 2009, 15, 5491-5502; (b) T.
M. Baker, T. L. Mako, A. Vasilopoulos, B. Li, J. A. Byers and M.
L. Neidig, Organometallics, 2016, 35, 3692-3700.
10 (a) D. Peng, Y. Zhang, X. Du, L. Zhang, X. Leng, M. D. Walter
and Z. Huang, J. Am. Chem. Soc., 2013, 135, 19154-19166; (b)
T. Zell, P. Milko, K. L. Fillman, Y. Diskin-Posner, T. Bendikov,
M. A. Iron, G. Leitus, Y. Ben-David, M. L. Neidig and D.
Milstein, Chem. Eur. J., 2014, 20, 4403-4413; (c) B. Butschke,
K. L. Fillman, T. Bendikov, L. J. W. Shimon, Y. Diskin-Posner,
G. Leitus, S. I. Gorelsky, M. L. Neidig and D. Milstein, Inorg.
Chem., 2015, 54, 4909-4926; (d) M. Asay and D. Morales-
Morales, Dalton Trans., 2015, 44, 17432-17447; (e) T. Simler,
P. Braunstein and A. A. Danopoulos, Angew. Chem. Int. Ed.,
2015, 54, 13691-13695; (f) T. Simler, A. A. Danopoulos and P.
Braunstein, Chem. Commun., 2015, 51, 10699-10702; (g) R.
Gilbert-Wilson, W.-Y. Chu and T. B. Rauchfuss, Inorg. Chem.,
2015, 54, 5596-5603; (h) T. Simler, L. Karmazin, C. Bailly, P.
The following specific comments refer to the corresponding
crystal structures:
Complex 16C: SQUEEZE33 function was applied to eliminate
residual electron density corresponding to two molecules of
pentane. The complex crystallised in the trigonal space group
P31 2 1, which is chiral. The Flack parameter was refined as
0.012(12).
Complex 18: SQUEEZE33 function was applied to eliminate
residual electron density corresponding to 2.5 molecules of
THF.
Complex 19: SQUEEZE33 function was applied to eliminate
residual electron density corresponding to two molecules of
ether.
Braunstein and A. A. Danopoulos, Organometallics, 2016, 35
,
903-912; (i) Y. Wang, C. Qin, X. Jia, X. Leng and Z. Huang,
Angew. Chem. Int. Ed., 2017, 56, 1614-1618.
Acknowledgements
11 J. A. Wright, A. A. Danopoulos, W. B. Motherwell, R. J. Carroll
and S. Ellwood, J. Organomet. Chem., 2006, 691, 5204-5210.
12 F. He, A. A. Danopoulos and P. Braunstein, Organometallics,
2016, 35, 198-206.
The USIAS, CNRS, Unistra, Région Alsace and Communauté
Urbaine de Strasbourg are acknowledged for the award of
fellowships and a Gutenberg Excellence Chair (2010–11) to
AAD. We thank the CNRS and the MESR (Paris) for funding and
for a PhD grant to TS. We are grateful to Corinne Bailly and Dr.
Lydia Karmazin (Service de Radiocristallographie, Unistra) for
the determination of the crystal structures
13 A. Klapars and S. L. Buchwald, J. Am. Chem. Soc., 2002, 124
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14844-14845.
14 M. Frigerio, M. Santagostino, S. Sputore and G. Palmisano, J.
Org. Chem., 1995, 60, 7272-7276.
15 S. V. Ley, J. Norman, W. P. Griffith and S. P. Marsden,
Synthesis, 1994, 1994, 639-666.
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DOI: 10.1039/C7DT01014A
ZnII-promoted
N
synthesis
N
Br
N
DiPP
N
R
+ e-
- e-
+ e-
- e-
N
N
N
N
N
N
N
N
N
N
N
N
R
R
R
DiPP
DiPP
DiPP
Redox-active NiminNpyrCNHC Pincer Ligand when Coordinated to FeII
A ZnII-promoted modular synthesis allows access to new non-symmetrical, redox-active imino NHC
pyridine pincer ligands. Radical anionic and dianionic redox states of the ligand are involved in its
FeII complexes obtained from FeBr2/KC8.