Bis-Imidazolylidene Complexes
FULL PAPER
ligand, rather than having the pyrabim or benzimidazolyli-
dene ligand, and therefore discards the possibility that the
catalytic benefits provided by the bis-NHC ligand are
merely due to an increase in the stability.
Both catalytic reaction patterns that we have shown in
this study indicate that the presence of the dimetallic struc-
ture in the catalyst introduces some benefits to the catalytic
outcomes of the reactions. The results are clearer in the case
of the Suzuki–Miyaura coupling, for which both dimetallic
complexes unambiguously afford higher yields than their
monometallic analogue complexes under exactly the same
reaction conditions.
Experimental Section
General procedures: All manipulations were performed under a nitrogen
atmosphere using standard Schlenk techniques. Solvents and reagents are
commercially available and were used as received from commercial sup-
pliers. Bis(imidazolium)pyracene chloride (1),[6] acetanaphtoimidazolium
chloride,[12] and complex 5[13] were prepared according to previously re-
ported methods. NMR spectra were recorded using Varian Innova spec-
trometers operating at 300 and 500 MHz (1H NMR) and 75 or 125 MHz
(13C NMR), respectively, using CDCl3 and CD2Cl2 as solvents (Merck
and Aldrich). Electrospray mass spectra (ESI-MS) were recorded using a
Micromass Quattro LC instrument, and nitrogen was employed as the
drying and nebulizing gas.
Synthesis of compound [(m-pyrabim){PdACHTUNRGTNEUNG(allyl)Cl}2] (2): A 0.5m solution
We cannot attribute the reactivity differences to dispari-
ties in the stereoelectronic properties of the ligands, which
have been proven to be very similar,[6] and also to the cata-
lytic cooperativity between the two metals, which in our di-
metallic systems exceed by far the optimum M–M separa-
tion that has been proposed for this type of effect (3.6–
6 ꢄ).[17] With our results in hand, we believe that the catalyt-
ic enhancement shown by our dimetallic catalysts might be
either due to the effect of the higher local concentration of
the metal in the catalytic reaction (an effect that cannot be
replicated by monometallic catalysts), or to the presence of
an extended polyaromatic system, which might favor the in-
teraction between the catalyst and the aromatic substrates.
Although we are aware that a more detailed study is needed
to clarify the mechanisms that lead to this catalytic improve-
ment, we believe that our study introduces an interesting
proof of concept that might be worth developing in the near
future.
of KHMDS (1.36 mL, 0. 96 mmol) was added at À788C to a flame-dried
Schlenk tube that contained a salt precursor (0.30 g, 0.31 mmol), [{Pd-
AHCTUNTGRENN(GUN allyl)Cl}2] (0.112 g, 0.31 mmol), and a stirrer bar. After 10 min, diethyl
ether (7 mL) was added, the reaction mixture was maintained at À788C
for 10 min. Then it was allowed to warm to room temperature. After two
hours, the solvent was evaporated under vacuum. The crude mixture was
washed with a cannula first with hexane (2ꢅ10 mL), then diethyl ether
(2ꢅ10 mL). The product was extracted with a cannula with CH2Cl2 (2ꢅ
10 mL), and the solvent was removed under vacuum to yield a dark
green solid. Suitable crystals for X-ray analysis of 2 were obtained after
slow evaporation of a benzene solution at room temperature. Yield:
0.2854 g, 75%. 1H NMR (300 MHz, CD2Cl2, 303 K): d=7.36 (t, J=
8.4 Hz, 4H), 7.19 (d, J=8.4 Hz, 8H), 5.38 (br, 4H), 4.76 (m, 2H), 3.68 (d,
J=7.5 Hz, 2H), 3.19 (br, 4H), 3.11 (br, 4H), 2.65 (d, J=13.5 Hz, 2H),
1.64 (br, 2H), 1.25 (d, J=6.9 Hz, 24H), 1.12 (br, 2H), 1.01 ppm (d, J=
6.9 Hz, 24H); 13C{1H} NMR (300 MHz, 303 K): d=194.3, 146.3, 144.1,
135.9, 134.2, 130.7, 130.6, 124.6, 122.5, 114.9, 72.9, 51.4, 29.0, 25.6,
23.9 ppm; HRMS (ES+, 20 V): m/z calcd: 1227.4325 [MÀCl]+; found:
1227.4326.
Synthesis of compound [(m-pyrabim)
dine (4 mL) was added to a Schlenk tube that contained PdCl2 (0.109 g,
0.61 mmol), salt precursor (0.30 g, 0.31 mmol), K2CO3 (0.30 g,
ACHTUNGTNERNUG{PdCl2ACHTUNGTERN(NUGN 3-Clpyr)}2] (3): 3-Chloropyri-
a
0.31 mmol), and a stirring bar. The reaction mixture was heated with vig-
orous stirring at 908C for 24 h to give a brownish mixture. After cooling
to room temperature, the reaction mixture was diluted with CH2Cl2 and
passed through a short pad of silica gel covered with a pad of Celite elut-
ing with CH2Cl2 until the product was completely recovered. The pure
Pd–NHC complex was isolated after precipitation with pentane, decant-
ing off the supernatant, and drying under high vacuum to yield a dark
green solid. Crystals suitable for X-ray analysis of 3 were obtained by
slow evaporation of a mixture chloroform/pentane. Yield: 0.291 g, 64%.
1H NMR (300 MHz, CDCl3, 303 K): d=8.57 (s, 2H), 8.49 (d, 2H), 7.46
(ddd, 4H) 7.34 (d, 4H), 7.25 (t, 6H), 7.05 (m, 2H), 5.45 (br, 4H), 3.41
(m, 8H), 1.54 (d, 30H), 1.43 (d, 24H), 1.06 ppm (d, 24H); 13C{1H} NMR
(300 MHz, 303 K): d=160.4, 150.5, 149.4, 148.9, 147.5, 143.8, 137.4, 135.7,
133.0, 132.2, 130.2, 124.6, 124.3, 122.5, 28.8, 25.8, 24.2 ppm; HRMS (ES+,
20 V): m/z calcd: 1443.2959 [MÀCl]+; found: 1443.2957.
Conclusion
In summary, we have prepared and fully characterized two
new Pd-based complexes with our previously reported
ligand pyrabim. The molecular structures of both complexes
have been determined and compared with the molecular
structures of the related monometallic complexes (com-
plexes 4 and 5). The catalytic activity of all complexes has
been tested in the acylation of aryl halides with hydrocinna-
maldehyde and in the Suzuki–Miyaura coupling between
aryl halides and phenyl boronic acids. The catalytic results
show that the presence of a second metal in the catalyst in-
troduces some benefits into the catalytic reaction outcomes,
an effect that is clearer in the case of the Suzuki–Miyaura
coupling, for which both dimetallic catalysts display higher
activity than their monometallic counterparts. Although we
believe that the higher local concentration of the metal in
the dimetallic architectures might be the origin of this
effect, we are carrying out further studies to clarify this
point, and to extend the library of NHC-based rigid polytop-
ic ligands for the preparation of novel di- and trimetallic cat-
alysts.
Synthesis of [PdACHTUGNTERN(UNGN BIAN)AHCTNUTGREG(NNUN allyl)Cl] (4): A 0.5m solution of KHMDS
(0.4 mL, 0. 2 mmol) was added at À788C to a flame-dried Schlenk tube
that contained (imidazolium)acetonaphthene (BIAN) chloride (0.1 g,
0.18 mmol), [{PdACTHUNRGTNEUNG(allyl)Cl}2] (0.033 g, 0.09 mmol), and a stirrer bar. After
10 min, diethyl ether (7 mL) was added. The reaction mixture was main-
tained at À788C for 10 min, and then it was allowed to warm to room
temperature. After 2 h, the solvent was evaporated under vacuum. The
crude mixture was washed with a cannula with hexane (2ꢅ10 mL). The
product was extracted with a cannula with diethyl ether (3ꢅ10 mL). The
solvent was pumped off to yield a yellow solid. Suitable crystals for X-
ray analysis were obtained after slow evaporation of a CH2Cl2 solution at
room temperature. Yield: 0.126 g (92%). 1H NMR (300 MHz, CD2Cl2,
303 K): d=7.69 (d, J=9.3 Hz, 2H), 7.57 (t, J=15.6 Hz, 2H), 7.40 (d, J=
8.7 Hz, 4H), 7.32 (t, J=16.2 Hz, 2H), 6.82 (d, J=8.1 Hz, 2H), 4.93 (m,
1H), 3.82 (d, J=7.2 Hz, 1H), 3.30 (br, 2H), 3.09 (br, 2H), 2.82 (d, J=
13.5 Hz, 1H), 1.89 (d, J=13.5 Hz, 1H), 1.59 (br, 1H), 1.33 (d, J=6.9 Hz,
12H), 0.94 ppm (d, J=6.9, 12H); 13C{1H} NMR (300 MHz, 303 K): d=
Chem. Eur. J. 2013, 19, 10405 – 10411
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10409