Group 10 metal complexes with chelating macrocyclic dicarbene ligands bearing a 2,6-lutidinyl bridge: Synthesis, reactivity, and catalytic activity

Article abstract of DOI:10.1021/om401062p

Palladium(II) and platinum(II) complexes of the title ligands have been prepared; the two carbene moieties of the ligand coordinate to the metal in cis fashion, while the bridging pyridyl group remains outside the metal coordination sphere but close to the metal center. In this peculiar situation, the pyridyl group can assist the oxidation of the metal center to the +IV oxidation state upon coordination to the metal in the product. Furthermore, the pyridyl group is found to promote the catalytic role of the palladium(II) complexes in copper- and amine-free Sonogashira reactions.

Full text of DOI:10.1021/om401062p

Article
pubs.acs.org/Organometallics
Group 10 Metal Complexes with Chelating Macrocyclic Dicarbene
Ligands Bearing a 2,6-Lutidinyl Bridge: Synthesis, Reactivity, and
Catalytic Activity
Andrea Biffis,*,† Matteo Cipani,† Edoardo Bressan,† Cristina Tubaro,† Claudia Graiff,‡
and Alfonso Venzo§
†Dipartimento di Scienze Chimiche, Universita
̀
di Padova, Via Marzolo 1, 35131 Padova, Italy
‡Dipartimento di Chimica, Universita
̀
di Parma, Viale delle Scienze 17/A, 43100 Parma, Italy
§IENI-CNR, c/o Dipartimento di Scienze Chimiche, Universita
̀
di Padova, Via Marzolo 1, 35131 Padova, Italy
S
* Supporting Information
ABSTRACT: Palladium(II) and platinum(II) complexes of the title ligands have
been prepared; the two carbene moieties of the ligand coordinate to the metal in cis
fashion, while the bridging pyridyl group remains outside the metal coordination
sphere but close to the metal center. In this peculiar situation, the pyridyl group can
assist the oxidation of the metal center to the +IV oxidation state upon coordination
to the metal in the product. Furthermore, the pyridyl group is found to promote the
catalytic role of the palladium(II) complexes in copper- and amine-free Sonogashira
reactions.
Scheme 1. Nature of the Ligands Investigated in This Work
INTRODUCTION
■
Transition metal complexes in which the ligands are able not
only to influence the physicochemical properties, the reactivity,
and/or the stability of the metal center but also to exert a
function in their own right have nowadays come to the
forefront of organometallic and coordination chemistry
research. Such “non-innocent” ligands can for example act as
an electron relay toward the metal center (redox-active
ligands),1 take up an active role in a catalytic event promoted
by the metal complex (cooperative catalysis),2 or more simply
provide a handle for the construction of more complicated
structures such as higher nuclearity metal complexes and
clusters, supramolecular systems, or metal organic frameworks
(MOFs).
The development of ligands of this kind has however seldom
involved up to now the use of N-heterocyclic carbene (NHC)
moieties.3 In this context, we have become recently interested
in metal complexes with ligands bearing the structure outlined
in Scheme 1. A short linker between the two imidazolylidene
moieties should allow the preparation of mononuclear
complexes with d8 metal centers in which the carbene moieties
occupy two mutually cis coordination sites in a square planar
geometry, thus preventing the pyridyl group from coordinating
to the metal, while at the same time maintaining it close to the
metal; in such a situation, the pyridyl group should remain
available for working cooperatively with the metal in catalysis,
for example by acting as a base toward an incoming substrate or
as a stabilizing ligand toward the metal following its oxidation
(with a consequent change in coordination geometry to square
pyramidal or octahedral) in the course of the catalytic cycle. We
are intrigued by these possibilities, as it has been demonstrated
in recent years that complexes with monocarbene ligands
bearing pendant pyridyl groups can display remarkable catalytic
properties due to such a cooperative effect.4 Although one
example of a Pd(II) complex with a macrocyclic ligand of this
kind has been previously published,5 the presence of a pendant
pyridyl group in such kind of complexes has not been put to
advantage to any significant extent up to now.
Thus, in the frame of our continuing studies on the chemistry
of poly-NHC complexes of late transition metals,6 we present
in this contribution results concerning the preparation and
reactivity of complexes of macrocyclic dicarbene ligands of the
type highlighted above with group 10 metals in the +II
oxidation state. We show that the pyridyl group remains
uncoordinated in these complexes, but it can become
Received: October 31, 2013
Published: April 22, 2014
© 2014 American Chemical Society
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coordinated upon oxidation of the metal center to the +IV
oxidation state. Finally, we investigate on the application of Pd
complexes of this kind as catalysts for the copper- and amine-
free Sonogashira reaction, in which pendant basic groups close
to the metal center are expected to promote proton abstraction
from a π-coordinated alkyne and consequently to facilitate
catalysis.
of Ni(II)−carbene complexes of any kind, despite extensive
changes in the reaction conditions and also in the nature of the
employed nickel source; use of preformed silver(I) complexes
of the ligands9 as reagents for ligand transmetalation toward
nickel(II) centers also proved unsuccessful (see the Supporting
Information for details). These failed attempts are in contrast to
the easy and well-known preparation of CNC pincer complexes
of Ni(II) with open-chain ligands of this kind11 and lead us to
conclude that the geometric constraints imposed by the
employed short linker to the macrocyclic ligand structure do
not allow the preparation of stable Ni(II) complexes.
Consequently, we left aside Ni(II) as a metal center for this
chemistry and concentrated rather on Pd(II) and Pt(II).
We also attempted the preparation of tetracarbene complexes
of Pt(II) complexes bearing two macrocyclic ligand units per
metal; unfortunately, these synthetic attempts also failed, both
when the synthesis was performed upon reaction of Pt(acac)2
in the presence of 2 equiv of ligand precursor and NaOAc as
external base and when it was performed upon ligand
transmetalation from preformed silver(I) complexes toward
metal complexes bearing one macrocyclic ligand unit per metal
(see the Supporting Information for details). In the former case
only the 1:1 metal/macrocyclic ligand complex was obtained,
highlighting the instability of the corresponding 1:2 adducts.
Characterization of the Metal Complexes. The
elemental analyses of the Pd(II) and Pt(II) dicarbene
complexes are consistent with their proposed description as
mononuclear complexes with one ligand unit chelating the
metal center through the carbene moieties (Scheme 3). The
deprotonation of the diimidazolium salt is also confirmed by
the absence, in the 1H NMR spectra of the reaction products, of
the signal of the proton in position 2 of the imidazole rings.
Furthermore, upon coordination to the metal centers, the
dicarbene ligand is forced to adopt a rigid structure, with
consequent loss of magnetic equivalence for the benzyl protons
of the 2,6-lutidinyl group and the protons of the other bridge.
A closer inspection of the NMR spectra of these complexes
highlights however several untrivial features. Concerning the
RESULTS AND DISCUSSION
■
Synthesis of the Macrocyclic Ligand Precursors. The
macrocyclic precursors of dicarbene ligands employed in this
work are featured by a 2,6-lutidinyl bridge between the NHC
groups as well as by a linker 3 or 4 carbon atoms long (1,3-
propylene, o-xylylene, 1,4-butylene). The choice of such short
linkers was made to prevent the possibility of the ligands
forming complexes with the two NHC groups coordinated to
the metal in trans-fashion, which would also imply coordination
of the pyridyl nitrogen to yield CNC pincer complexes;
formation of complexes of this kind is indeed very common
with d8 metal centers and open-chain dicarbene ligands
connected by a 2,6-lutidinyl bridge,7 and the same holds in
the case of macrocyclic ligands containing long linkers.8 The
ligand precursors were prepared by a slight modification of
literature procedures, using dilution to ensure efficient
cyclization between the reagents 2,6-bis(bromomethyl)pyridine
and the various employed bis(imidazoles) (Scheme 2).9
Scheme 2. Synthetic Strategy for the Preparation of the
Diimidazolium Ligand Precursors
Synthesis of the Metal Complexes. The metal complexes
were synthesized directly from the diimidazolium salts upon
reaction with the corresponding metal acetylacetonate (Scheme
3). Use of Pt(acac)2 as reagent in the synthesis of Pt-NHC
1
Pt(II) complexes, it turns out that most of the H and 13C
signals of the complexes appear doubled in the spectra. This
feature is particularly evident for the benzylic protons of the
2,6-lutidinyl group (doubling of the signal noticeable already at
300 MHz), but it becomes apparent for many other signals
using higher frequency (400 or 600 MHz) NMR instruments.
For example, in the case of complex PtBr2(Lpropyl) doubling of
Scheme 3. Synthetic Strategy for the Preparation of Pd(II)
and Pt(II) Complexes with Macrocyclic Dicarbene Ligands
1
all the H signals but those due to the proton in position 4 of
the pyridyl group and the protons of the central methylene
group of the linker is evident at 600 MHz. Nevertheless, the
observation that at least some of the signals remain non-
doubled, together with the unicity of the 195Pt resonance
(recorded at −2245 ppm in complex PtBr2(Lpropyl)), suggests
an explanation in terms of an overall molecular asymmetry of
the complexes, rather than in terms of the copresence of two
geometrical or conformational isomers in slow chemical
exchange.
An even more complicated situation is apparent with the
Pd(II) complexes. In these cases, despite extensive purification
and satisfactory elemental analyses, the NMR spectra of all the
complexes invariably point out the presence of other minor sets
of signals besides the main one. Consequently, we started to
envisage the presence of some sort of equilibrium in solution, in
which the original complexes form other species to some
extent. In this connection, ESI-MS analysis of solutions of the
complexes was pioneered by the group of Strassner,10 and in
our hands it was very useful for preparing the Pt(II) complexes
of all three macrocyclic ligands. Furthermore, the method was
extended also to the preparation of Pd(II) complexes, as the
use of Pd(acac)2 gave better results in terms of yield and
product purity compared with the previously published route
employing Pd(OAc)2.5
We planned to extend the application of this synthetic
strategy to the preparation of Ni(II) complexes as well, but
unfortunately we never obtained any evidence of the formation
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complexes pointed out that, besides the expected mononuclear
compound described in ref 5. The Ccarbene−Pt−Ccarbene angle is
93.42(14)° and the Br−Pt−Br one is 91.128(14)°. The
dihedral angles between the PtBr2C2 coordination mean
plane and those of the carbene moieties are 71.58(2)° and
73.32(2)°, while the dihedral angle between the mean planes of
the carbene moieties is 78.32(2)°. Although the ligand is
potentially tridentate, the pyridine-nitrogen atom N5 is at a
distance of 2.949(3) Å from platinum, thus preventing any
bond interaction. The Pt−Br and Pt−Ccarbene bond distances
fall in the range for related complexes of the type cis-
[PtBr2(NHC)2].13
complex PdBr(L)+, significant amounts of dinuclear complexes
+
of general formula Pd2Br3(L)2 were also present. This
observation suggested the presence of an equilibrium of the
type highlighted in Scheme 4, described some years ago by
Albrecht for dicarbene palladium(II) complexes.12
Scheme 4. Mononuclear−Dinuclear Equilibrium Taking
Place with the Pd(II) Complexes
Reactivity of the Complexes with Bromine. There is a
steadily growing interest in the chemistry of organometallic
complexes of late transition metals in high oxidation state, both
from the fundamental and applied point of view; such interest is
driven in particular by the recognized formation of these
complexes as intermediates in several catalytic processes leading
to C−H activation and (oxidative) functionalization.14 The
macrocyclic ligands described herein appear particularly suited
for the preparation of Pt or Pd complexes in high oxidation
state: upon oxidation of the metal center to the +IV state, the
pyridyl group is expected to enter the coordination sphere of
the complex, with the ligand becoming altogether fac-
tricoordinated in an octahedral geometry. Such a coordination
set closely mimics that of analogous Pd complexes, which have
recently proven useful for C−H oxidative functionalization
processes.15 Furthermore, it has been suggested that fac-
tricoordinated complexes are particularly suited for carrying out
such processes.16
On the basis of the above, we have carried out a preliminary
evaluation of the possibility to prepare Pd or Pt complexes in
high oxidation state with our macrocyclic ligand systems.
Oxidation with elemental bromine was chosen as preparation
method, following a route already proven useful for related
complexes.17 It turned out that all Pt(II) complexes yielded
well-defined products in good yields after treatment with excess
bromine in DMSO (Scheme 5).
Further support for this interpretation is brought by the
study of the effect of the deliberate addition of bromide anions
to the solution (Supporting Information, Figure 1). As
expected, addition of LiBr switches the equilibrium to the
left, and consequently the mononuclear complex is the only
species present in solution. This is confirmed by the ESI-MS
spectra, which after LiBr addition show only the signal of the
mononuclear complexes.
Crystal Structure of Complex PtBr2(Lpropyl). The
structure of complex PtBr2(Lpropyl) has been further confirmed
by single-crystal X-ray analysis of a sample grown in DMF
solution. The complex crystallizes as a dimethylformamide
monosolvate. A view of the crystal structure of PtBr2(Lpropyl) is
shown in Figure 1, together with the atomic labeling scheme. A
Scheme 5. Synthetic Strategy for the Preparation of Pt(IV)
Complexes with Macrocyclic Dicarbene Ligands
Figure 1. ORTEP drawing of complex PtBr2(Lpropyl). Ellipsoids are
drawn at their 30% probability level. Hydrogen atoms and solvent
molecule of dimethylformamide have been omitted for clarity. Selected
bond distances (Å) and angles (deg): C1−Pt 1.977(4), C7−Pt
1.983(3), Br1−Pt 2.5003(4), Br2−Pt 2.4913(4), C1−N2 1.355(5),
C1−N1 1.378(5), C7−N4 1.359(4), C7−N3 1.366(4); C1−Pt−C7
93.42(14), C1−Pt−Br2 179.85(10), C7−Pt−Br2 86.71(10), C1−Pt−
Br1 88.74(10), C7−Pt−Br1 177.80(10), Br2−Pt−Br1 91.128(14),
N2−C1−N1 104.7(3), N2−C1−Pt 127.2(3), N1−C1−Pt 128.1(3),
N4−C7−N3 104.3(3), N4−C7−Pt 129.2(2), N3−C7−Pt 126.4(3).
The elemental analyses and ESI-MS characterization of the
Pt(IV) dicarbene complexes are consistent with their proposed
description as mononuclear complexes of general formula
PtBr4(L). For the complexes, we proposed a structure in which
the ligand is tricoordinated to the metal through the carbene
moieties and the pyridyl nitrogen; three bromide anions
complete the octahedral coordination set, and an additional
bromide serves as counteranion. The formation of such
complexes is also confirmed by the NMR spectra, which
show the expected changes upon oxidation of the metal center
and coordination of the pyridyl nitrogen. In particular, all
list of the most important bond distances and angles is also
reported in the figure caption. The Pt atom is disposed in a
quasi-square planar environment, comprising the two donors of
the carbene ligand mutually cis and the two halide donors
occupying the other pair of mutually cis sites. The metal atom is
coordinated outside the cavity of the macrocyclic ligand in an
analogous manner to that observed in the parent palladium
1
signals in the H NMR spectra of the complexes are shifted to
lower fields compared to the corresponding Pt(II) complexes,
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Table 1. Sonogashira Reaction Catalyzed by Pd(II) Complexes with Macrocyclic Dicarbene Ligands
a
b
1
Yield determined by H NMR analysis of the reaction mixture (CDCl3). Mixture of products.
suggesting ligand coordination to a more electron-withdrawing
metal center. Furthermore, formation of a more rigid complex
in the case of Pt(IV) is apparent from the much larger
difference in chemical shift recorded between the diastereotopic
protons of the methylene groups directly bound to the
imidazole nitrogens. In the 13C NMR spectra, the same shift
of the signals to lower field is evident, apart from the carbene
carbons, which are instead found at higher field compared to
the corresponding Pt(II) complexes; nevertheless, this
apparently anomalous shift was also expected, since in this
case the deshielding at the carbene carbon originated by the
increasingly electron-withdrawing metal center is overcompen-
sated by delocalization on the carbene carbon of electron
density from the imidazolylidene rings.17 Finally, also the
extensive downfield shift observed upon reaction in the 195Pt
NMR spectrum of the complexes (e.g., −253 ppm vs −2245
ppm in complex PtBr2(Lpropyl)) points toward oxidation of the
metal. Remarkably, no doubling of any NMR signal is observed
in the Pt(IV) complexes, in contrast to the corresponding
Pt(II) complexes, which is indicative of a less distorted
structure of the ligands in the Pt(IV) case.
Several attempts were made to crystallize the Pt(IV)
complexes from DMF or DMSO solutions, in order to obtain
the structure and consequently a conclusive proof of the
coordination of the pyridyl nitrogen to the metal center.
Unfortunately, all attempts performed up to now were
unsuccessful. Nevertheless, strong support toward coordination
of the pyridyl nitrogen in these complexes is provided by 15N
NMR spectroscopy. Whereas the signal of the pyridyl nitrogen
is recorded at almost the same chemical shift (312 ppm) in the
imidazolium precursor [H2Lpropyl]Br2 and in the Pt(II) complex
PtBr2(Lpropyl), indicating no coordination of the pyridyl
nitrogen to the metal, a considerable shift is observed on
going to the Pt(IV) complex, where the signal falls at 183 ppm.
Thus, it can be safely stated that the pyridyl nitrogen is indeed
coordinated to the Pt(IV) metal center. Additional support for
1
the proposed structure is provided by the invariance of the H
NMR spectra of the Pt(IV) complexes upon reaction with 1
equiv of AgNTf2 in DMSO despite the evident precipitation of
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AgBr: this proves that one of the bromides is not coordinated
to the metal and can be easily exchanged without affecting the
structure of the complex in solution.
(entries 4 and 5), whereas at 50 °C the activity becomes too
low to be practical (entry 6).
The effect of the nature of the employed inorganic base
(K2CO3 or Cs2CO3) was also briefly considered: Cs2CO3
ensured a slightly higher initial catalytic activity (compare for
example entries 7 and 8), but also a more rapid catalyst
deactivation (compare entries 10 and 11). In view of the much
lower cost of K2CO3 compared to Cs2CO3, the former was
considered as the preferred alternative.
A screening of different aryl halide reactants was then
performed using catalyst PdBr2(Lpropyl). Aryl bromides with
more electron-donating substituents such as 4-bromoanisole
reacted as expected more sluggishly compared to 4-
bromoacetophenone (entries 10 and 11), but good yields
were still reached at longer reaction time. In the case of 4-
bromotoluene (entry 9), the reaction was however not clean,
and besides the main Sonogashira product other minor
products were recorded, which await further characterization.
The effect of the nature of the halide was also considered: as
expected, aryl iodides were reactive substrates for the reaction
(entries 12 and 13), and it was even possible to achieve some
turnovers also with a less reactive aryl chloride such as 4-
chloroacetophenone (entry 16).
Reaction of excess bromine with the Pd(II) complexes does
not produce instead any Pd(IV) complex, and the starting
Pd(II) complexes are recovered unchanged after reaction.
Efforts are under way to prepare these complexes with stronger
oxidants, as it has been recently reported in the literature that
Pd(II) complexes with similar coordination sets can be oxidized
to Pd(IV) with, for example, chlorine instead of bromine.15
Catalytic Efficiency of the Pd(II) Complexes in
Copper-Free Sonogashira Coupling Reactions. The
Sonogashira reaction, i.e., the coupling of aryl halides with
terminal alkynes to yield arylalkynes, is one of the most widely
known cross-coupling reactions.18 Pd-based catalysts with
Cu(I) salts as cocatalysts are generally employed to promote
the reaction. Remarkably, whereas Pd-phosphine complexes
represent undoubtedly the most efficient catalysts known to
date, much less success has been reported with Pd-NHC
complexes, in spite of the extremely high levels of catalytic
efficiency achieved with these complexes in related cross-
coupling reactions, such as Heck or Suzuki couplings.19 This is
particularly true for the so-called copper-free Sonogashira
reaction, which is run without the Cu(I) cocatalyst. Efficient
catalysts for this variant of the Sonogashira reaction are much
sought after, since the Cu(I) cocatalyst speeds up the process
but on the other hand renders the reaction air-sensitive and
promotes other unwanted reactions such as the Glaser−Hay
coupling of the alkyne.20 An often employed trick to promote
the reactivity of the catalyst under copper-free conditions is to
use a large excess of an organic amine both as base and as
reaction promoter.18 In this connection, it is interesting to
remark that the most efficient Pd-NHC catalysts known to date
for the copper-free Sonogashira reaction contain NHC ligands
with pendant Lewis basic groups,21 which could possibly
promote the reaction by facilitating the deprotonation of an
alkyne π-bound to the metal and its consequent transformation
into a σ-bound alkynyl, without the intermediacy of a Cu(I)
cocatalyst and the need for the presence of excess base; the σ-
bound alkynyl is then reductively coupled with a metal-bound
aryl group to yield the product. Therefore, we chose to perform
a screening of our Pd(II) complexes as catalysts for the copper-
and amine-free Sonogashira reaction, under conditions
previously reported for other functionalized Pd-NHC com-
plexes. The results are reported in Table 1.
Initial tests were conducted with all three catalysts
PdBr2(Lpropyl), PdBr2(Lxylyl), and PdBr2(Lbutyl) using 4-bromoa-
cetophenone as substrate. Gratifyingly, all complexes gave good
yields after 1 h of reaction at 100 °C with 1 mol % catalyst
loading (67%, 65%, and 83%, respectively, entries 1−3), higher
than those obtained with other Pd-NHC complexes with
pendant Lewis basic groups reported in the literature (max.
43% at 3 mol % catalyst loading)21c and comparable to those
recently reported with a CNC pincer-type di(1,2,4-triazolin-5-
ylidene)Pd(II) complex.22
Looking at the evolution of the reaction yield with time, it is
apparent that all catalysts are mostly active in the first hour of
reaction, with the activity subsequently decreasing significantly
with time (see in particular entries 3 and 8). This is an
indication of catalyst deactivation, possibly due to catalyst
decomposition under the reaction conditions. Indeed, running
the reaction at 80 °C decreases the initial catalytic activity but
allows reaching complete conversions at longer reaction times
Finally, in order to establish if the presence of the bridging
pyridyl group indeed promotes catalytic activity, we compared
the behavior under the same reaction conditions of related
catalysts not containing the pyridyl group. Ideally, we intended
to employ as benchmark a complex bearing an m-xylylene
bridge instead of the 2,6-lutidinyl bridge. The synthesis of such
a complex was attempted starting from the macrocyclic ligand
precursor [H2LXpropyl]Br2 (Scheme 6), previously synthesized
Scheme 6. Attempted Synthesis of a Benchmark Pd(II)
Complex with a Macrocyclic Dicarbene Ligand without the
Pyridyl Group
by our group,9 but the reaction yielded a complex product
mixture, from which it proved impossible to isolate the desired
complex in pure form (see the Supporting Information for
details).
Therefore, we resolved to employ a related complex with an
open-chain dicarbene ligand and a bridging m-xylylene group,
such as complex PdBr2(LX) (Table 1, entry 15), which was
prepared following a procedure previously reported by Cavell et
al.23 Indeed, the catalytic activity of this complex proved much
poorer (entry 15) than that of the complexes bearing the
bridging pyridyl group. We additionally employed another
benchmark Pd-dicarbene catalyst such as PdBr2(diBIm) (Table
1, entry 16): the catalytic activity was improved with respect to
the previous benchmark, but it turned out to be again lower
than that of the complexes bearing the bridging pyridyl group.
Therefore, we conclude that indeed the presence of the
bridging pyridyl group seems to exert a positive effect on the
catalytic activity of this class of complexes.
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120.5 (NCH), 54.4 (NCH2Cpyr), 50.8 (NCH2Cxyl). ESI-MS (positive
ions, CH3CN): m/z 1134.8 (42%) [Pd2L2Br3]+, 605.8 (14%) [PdLBr2
− 2H]+, 568.7 (30%) [PdLBr(CH3CN)]+, 528.1 (100%) [PdLBr]+.
PdBr2(Lbutyl). Yield: 60%. Anal. Calcd for C17H19Br2N5Pd: C, 36.47;
H, 3.42; N, 12.52. Found: C, 36.71; H, 3.45; N, 12.63. The NMR
characterization data of the main species in solution matched those
previously reported for the same complex.2 ESI-MS (positive ions,
CONCLUSIONS
■
In conclusion, we have demonstrated that it is possible to tailor
macrocyclic di-NHC ligands containing 2,6-lutidinyl bridges in
a way that allows coordination to d8 metal centers of the
carbene moieties but not of the pyridyl group, which remains
close to the metal but outside its coordination sphere. We have
also shown that the pyridyl group may enter the coordination
sphere of the metal center upon metal oxidation, as well as that
the presence of the pyridyl group exerts a positive effect on the
catalytic efficiency of the complexes in standard Sonogashira
reactions. Efforts are under way in order to assess the role of
the pyridyl group in the catalytic cycle, as well as in order to
apply such complexes as catalysts in other reactions that
specifically involve the metal(II)/metal(IV) catalytic manifold.
CH3CN): m/z 1038.8 (90) [Pd2L2Br3]+, 557.7 (19%) [PdLBr2
−
2H]+, 479.9 (65%) [PdLBr]+, 398.1 (100%) [PdL]+.
PtBr2(Lpropyl). Yield: 76%. Anal. Calcd for C16H17Br2N5Pt: C, 30.30;
H, 2.70; N, 11.04. Found: C, 30.61; H, 2.61; N, 10.44. H NMR
(DMSO-d6): δ 7.91 (t, JHH = 7.5 Hz, 1H, Ar-Hpara), 7.52−7.47 (m,
1
3
4H, NCH), 7.41 (d, 3JHH = 7.5 Hz, 2H, Ar-Hmeta), 5.52−5.35 (m, 4H,
NCH2CAr), 4.91−4.79 (m, 2H, NCHH′CH2), 4.48−4.38 (m, 2H,
NCHH′CH2), 2.44−2.37 (m, 1H, NCH2CHH′), 1.94−1.88 (m, 1H,
NCH2CHH′). 13C NMR (DMSO-d6): δ 153.6, 153.4 (Ar-Cortho),
149.7 (NCN), 138.8 (Ar-Cpara), 124.0, 123.8 (NCH), 122.5, 122.2
(Ar-Cmeta), 121.1, 121.0 (NCH), 53.8, 53.5 (NCH2CAr), 52.8, 52.4
(NCH2CH2), 31.3 (NCH2CH2). 15N NMR (DMSO-d6): δ 312. 195Pt
NMR (DMSO-d6): δ −2245. Crystals of the compound suitable for X-
ray diffraction were grown upon slow diffusion of diethyl ether into a
solution of the complex in N,N-dimethylformamide.
EXPERIMENTAL SECTION
■
General Procedures. All manipulations were carried out using
standard Schlenk techniques under an atmosphere of dry argon or
dinitrogen. The reagents were purchased by Aldrich as high-purity
products and generally used as received. All solvents were technical
grade and used as received. The preparation of diimidazolium salts
[H2Lpropyl]Br2, [H2Lxylyl]Br2, [H2Lbutyl]Br2, and [H2LXpropyl]Br2 was
accomplished according to previously published procedures.9 The
complexes PdBr2(LX)23 and PdBr2(diBIm)24 were prepared following
literature procedures. NMR spectra were recorded on an Avance 300
PtBr2(Lxylyl). Yield: 40%. Anal. Calcd for C21H19Br2N5Pt: C, 36.20;
H, 2.75; N, 10.06. Found: C, 36.07; H, 2.80; N, 9.34. 1H NMR
(DMSO-d6): δ 7.99−7.89 (m, 3H, Arxyl-Hortho, Arpyr-Hpara), 7.87 (s,
2H, NCH), 7.63 (s, 2H, NCH), 7.55−7.46 (m, 4H, Arpyr-Hmeta, Arxyl-
2
Hmeta), 6.67 (m, 2H, NCHH′Cxyl), 5.57 (d, JHH = 13.8 Hz, 1H,
NCHH′Cpyr), 5.40−5.30 (m, 5H, NCHH′Cpyr, NCHH′Cxyl). 13C
NMR (DMSO-d6): δ 154.1, 153.8 (Arpyr-Cortho), 148.5 (NCN), 139.2
(Arpyr-Cpara), 134.8, 134.7 (Arxyl-Cipso), 131.4 (Arxyl-Cortho), 129.6
(Arxyl-Cmeta), 125.5, 125.4 (NCH), 121.5 (Arpyr-Cmeta), 120.9 (NCH),
54.6, 54.2 (NCH2Cpyr), 50.8, 50.6 (NCH2Cxyl). 195Pt NMR (DMSO-
d6): δ −2188.
1
MHz (300.1 MHz for H and 75.5 for 13C), on an Avance 400 MHz
(400.13 MHz for 1H and 100.61 for 13C), and on an Avance 600 MHz
1
(600.01 MHz for H and 150.07 for 13C); chemical shifts (δ) are
reported in units of ppm relative to the residual solvent signals for
1
both H and 13C, to aqueous 15NH4NO3 for 15N, and to aqueous
K2PtCl4 (adjusted to δ = −1628 ppm from Na2PtCl6) for 195Pt. ESI-
MS spectra were recorded on a Thermo LCQ-Duo ESI-MS.
Synthesis of the Pd(II) and Pt(II) Complexes. The syntheses
were performed following a literature procedure for related dicarbene
complexes.10
PtBr2(Lbutyl). Yield: 58%. Anal. Calcd for C17H19Br2N5Pt: C, 31.50;
H, 2.95; N, 10.80. Found: C, 31.91; H, 3.18, N, 10.81. 1H NMR
3
(DMSO-d6): δ 7.93 (t, JHH = 7.5 Hz, 1H, Ar-Hpara), 7.56−7.45 (m,
6H, NCHand Ar-Hmeta), 5.54−5.32 (m, 4H, NCH2CAr), 5.20−5.06
(m, 2H, NCH2CH2), 4.22−4.18 (m, 2H, NCH2CH2), 1.97 (bs, 2H,
NCH2CH2), 1.29−1.11 (m, 2H, NCH2CH2). 13C NMR (DMSO-d6):
δ 155.5 (Ar-Cortho), 149.7 (NCN), 139.0 (Ar-Cpara), 125.5 (Ar-Cmeta),
122.0 (NCHCHN), 54.5 (NCH2CAr), 47.0 (NCH2CH2), 25.0
(NCH2CH2).
General Procedure. A 0.160 mmol amount of Pd(acac)2 or
Pt(acac)2 and 0.160 mmol (1 equiv) of the diimidazolium dibromide
ligand precursor were placed in a Schlenk tube under an inert
atmosphere. The reagents were dissolved in 6 mL of DMSO, and the
resulting solution was heated under stirring to 60 °C for 2 h, 80 °C for
another 2 h, and finally 110 °C for 2 h. Heating was subsequently
turned off, and stirring was continued overnight. The solvent was
removed under reduced pressure, and the remaining solid was washed
three times with 2 mL aliquots of acetonitrile and finally dried under
reduced pressure.
Oxidation of the Pd(II) and Pt(II) Complexes with Bromine.
General Procedure. In a 50 mL round-bottomed flask were placed the
complex reagent (50−100 mg) and the minimum quantity of DMSO
necessary to dissolve it. Around 9 equiv of bromine was then added,
and the reaction mixture was stirred at room temperature overnight.
The addition of CH2Cl2 (10−20 mL) caused the precipitation of a
yellow solid, which was filtered, washed with 3 mL of CH2Cl2, and
dried at reduced pressure.
PdBr2(Lpropyl). Yield: 65%. Anal. Calcd for C16H17Br2N5Pd·
0.5CH3CN: C, 36.05; H, 3.29; N, 13.61. Found: C, 36.21; H, 3.65;
N, 13.66. 1H NMR (DMSO-d6) main species: δ 7.85 (t, 3JHH = 7.8 Hz,
PtBr4(Lpropyl). Yield: 25%. Anal. Calcd for C16H17Br4N5Pt·
0.8C2H6OS·0.6Br2: C, 22.17; H, 2.31; N, 7.35. Found: C, 21.68; H,
2.35; N, 7.38. 1H NMR (DMSO-d6): δ 8.36 (t, 3JHH = 7.5 Hz, 1H, Ar-
3
1H, Ar-Hpara), 7.44 (d, JHH = 7.5 Hz, 2H, Ar-Hmeta), 7.36 (s, 2H,
NCH), 7.26 (s, 2H, NCH), 5.70 (d, 2JHH = 14.1 Hz, 2H, NCHH′CAr),
3
3
2
Hpara), 7.89 (d, JHH = 7.5 Hz, 2H, Ar-Hmeta), 7.75 (d, JHH = 1.5 Hz,
5.26 (d, JHH = 14.4 Hz, 2H, NCHH′CAr), 4.99−4.90 (m, 2H,
3
2H, NCHCHN), 7.65 (d, JHH = 1.8 Hz, 2H, NCHCHN), 6.99 and
NCHH′CH2), 4.43−4.37 (m, 2H, NCHH′CH2), 2.45−2.35 (m, 1H,
NCH2CHH′), 1.88−1.78 (m, 1H, NCH2CHH′). 13C NMR (DMSO-
d6) main species: δ 162.4 (NCN), 154.0 (Ar-Cortho), 138.3 (Ar-Cpara),
123.6 (NCH), 121.7 (Ar-Cmeta), 120.1 (NCH), 53.7 (NCH2CAr), 52.6
(NCH2CH2), 31.0 (NCH2CH2). ESI-MS (positive ions, CH3CN): m/
z 1010.7 [Pd2L2Br3]+, 506.6 [PdLBr(CH3CN)]+, 466.0 [PdLBr]+.
PdBr2(Lxylyl). Yield: 68%. Anal. Calcd for C21H19Br2N5Pd: C, 41.51;
5.90 (AB system, 4H, NCH2CAr), 5.92−5.80 (m, 2H, NCH2CH2),
4.57−4.50 (m, 2H, NCH2CH2), 2.72−2.54 (m, 1H, NCH2CH2),
2.18−1.96 (m, 1H, NCH2CH2). 13C NMR (DMSO-d6): δ 156.9 (Ar-
Cortho), 143.2 (Ar-Cpara), 127.3 (NCHCHN), 126.6 (Ar-Cmeta), 123.4
(NCHCHN), 117.5 (NCN), 56.6 (NCH2CAr), 49.8 (NCH2CH2),
30.3 (NCH2CH2). 15N NMR (DMSO-d6): δ 183. 195Pt NMR
(DMSO-d6): δ −253. ESI-MS (positive ions, CH3CN): m/z 713.7
(100%) [PtLBr3]+, 634.8 (37%) [PtLBr2]+, 594.8 (44%) [PtLBr-
(CH3CN)]+, 571.6 (35%) [PtLBr(H2O)]+, 554.0 (24%) [PtLBr]+.
PtBr4(Lxylyl). Yield: 30%. Anal. Calcd for C21H19Br4N5Pt·1.3C2H6OS·
1.1Br2: C, 25.09; H, 2.39; N, 6.20. Found: C, 25.10; H, 2.38; N, 6.22.
1H NMR (DMSO-d6): δ 8.35−8.29 (m, 3H, Arpyr-Hpara and NCH),
8.00−7.96 (m, 2H, Arxyl-H), 7.85−7.77 (m, 4H, Arpyr-Hmeta and
1
H, 3.15; N, 11.53. Found: C, 40.72; H, 3.24; N, 11.14. H NMR
(DMSO-d6) main species: δ 7.93−7.92 (m, 2H, Arxyl-Hortho) 7.86−
7.81 (m, 1H, Arpyr-Hpara), 7.73 (s, 2H, NCH), 7.48−7.42 (m, 6H,
2
NCH, Arpyr-Hmeta, Arx2yl-Hmeta), 6.71 (d, JHH = 14.4 Hz, 2H,
NCHH′Cxyl), 5.75 (d, JHH = 13.5 Hz, 2H, NCHH′Cpyr), 5.26 (d,
2JHH = 13.5 Hz, 2H, NCHH′Cpyr), 5.19 (d, JHH = 14.4 Hz, 2H,
2
NCHH’Cxyl). 13C NMR (DMSO-d6) main species: δ 161.1 (NCN),
154.5 (Arpyr-Cortho), 138.7 (Arpyr-Cpara), 135.2 (Arxyl-Cipso), 131.1
(Arxyl-Cortho), 129.2 (Arxyl-Cmeta), 125.1 (NCH), 120.9 (Arpyr-Cmeta),
3
NCH), 7.43−7.40 (m, 2H, Arxyl-H), 7.30 (d, JHH = 15.3 Hz, 2H,
3
3
NCH2Cxyl), 6.91 (d, JHH = 18.3 Hz, 2H, NCH2Cpyr), 5.87 (d, JHH
=
2187
dx.doi.org/10.1021/om401062p | Organometallics 2014, 33, 2182−2188

Organometallics
Article
18.6 Hz, 2H, NCH2Cpyr), 5.46 (d, 3JHH = 15.0 Hz, 2H, NCH2Cxyl). 13
C
N.; Rominger, F.; Jakel, C.; Hashmi, A. S. K.; Limbach, M.
Organometallics 2010, 29, 4448.
̈
NMR (DMSO-d6): δ 156.3 (Arpyr-Cortho), 144.3 (Arpyr-Cpara), 136.5
(Arxyl-Cipso), 132.4 (Arxyl-C), 130.7 (Arxyl-C), 127.0 (NCH), 126.8
(NCH), 124.9 (Arpyr-Cmeta), 116.2 (NCN), 55.4 (NCH2Cpyr), 49.3
(NCH2Cxyl). 195Pt NMR (DMSO-d6): δ −186. ESI-MS (positive ions,
CH3CN): m/z 775.8 (6%) [PtLBr3]+, 693.9 (12%) [PtLBr2]+, 616.1
(34%) [PtLBr]+.
(5) Radloff, C.; Gong, H. Y.; Schulte to Brinke, C.; Pape, T.; Lynch,
V. M.; Sessler, J. L.; Hahn, F. E. Chem.Eur. J. 2010, 16, 13077.
(6) (a) Tubaro, C.; Baron, M.; Costante, M.; Basato, M.; Biffis, A.;
Gennaro, A.; Isse, A. A.; Graiff, C.; Accorsi, G. Dalton Trans. 2013, 42,
10952. (b) Marchenko, A. O.; Koidan, H. N.; Hurieva, A. N.; Gutov,
O. V.; Kostyuk, A. N.; Tubaro, C.; Lollo, S.; Lanza, A.; Nestola, F.;
Biffis, A. Organometallics 2013, 32, 718. (c) Tubaro, C.; Biffis, A.;
Gava, R.; Scattolin, E.; Volpe, A.; Basato, M.; Diaz-Requejo, M. M.;
Perez, P. J. Eur. J. Org. Chem. 2012, 1367. (d) Baron, M.; Tubaro, C.;
Basato, M.; Biffis, A.; Natile, M. M.; Graiff, C. Organometallics 2011,
30, 4607. (e) Buscemi, G.; Basato, M.; Biffis, A.; Gennaro, A.; Isse, A.
A.; Natile, M. M.; Tubaro, C. J. Organomet. Chem. 2010, 695, 2359.
(f) Biffis, A.; Gazzola, L.; Tubaro, C.; Basato, M. ChemSusChem 2010,
3, 834. (g) Biffis, A.; Gazzola, L.; Gobbo, P.; Buscemi, G.; Tubaro, C.;
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PtBr4(Lbutyl). Yield: 65%. Anal. Calcd for C17H19Br4N5Pt·0.4Br2: C,
1
23.52; H, 2.20; N, 8.08. Found: C, 23.86; H, 2.22; N, 7.92. H NMR
(DMSO-d6): δ 8.36 (t, 3JHH = 7.2 Hz, 1H, Ar-Hpara), 7.90−7.55 (d, Ar-
Hmeta and NCH), 6.92 and 5.92 (AB system, 4H, NCH2CAr), 5.80 (m,
2H, NCH2CH2), 4.38 (m, 2H, NCH2CH2), 2.00 (m, 1H, NCH2CH2),
1.41 (m, 1H, NCH2CH2). 13C NMR (DMSO-d6): δ 156.4 (Ar-Cortho),
144.1 (Ar-Cpara), 127.0 (Ar-Cmeta), 126.5 (NCH), 126.0 (NCH), 116.5
(NCN), 56.4 (NCH2CAr), 46.3 (NCH2CH2), 45.9 (NCH2CH2), 24.8
(NCH2CH2), 24.6 (NCH2CH2). 195Pt NMR (DMSO-d6): δ −141.
ESI-MS (positive ions, CH3CN): m/z 727.7 (100%) [PtLBr3]+, 646.9
(27%) [PtLBr2]+, 568.0 (67%) [PtLBr]+.
Catalytic Tests in the Sonogashira Reaction. The reaction
protocol was derived with some modification from a publication by
Samantaray et al.6 General procedure: In a Schlenck tube were
introduced under an inert atmosphere 1 equiv of aryl halide (around
0.600 mmol), 2 equiv of base (K2CO3 or Cs2CO3), and 1 mol %
catalyst. A 10 mL amount of a mixture DMF/H2O (3:1) was then
added, and the reaction mixture was left to stir for a few minutes.
Finally, 2 equiv of phenylacetylene was added, and the tube was then
placed in an oil bath thermostated at the reaction temperature (usually
100 °C). Samples of the reaction mixture (1 mL) were taken at
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1
intervals and analyzed by H NMR (CDCl3) after solvent removal.
ASSOCIATED CONTENT
* Supporting Information
■
S
Additional experimental procedures and X-ray crystallographic
details in cif format for complex PtBr2(Lpropyl). This material is
available free of charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
Corresponding Author
*Tel: +39-049-8275216. Fax: +39-049-8275223. E-mail:
andrea.biffis@unipd.it.
■
(15) McCall, A. S.; Kraft, S. Organometallics 2012, 31, 3527.
(16) Maleckis, A.; Sanford, M. S. Organometallics 2011, 30, 6617.
(17) Meyer, D.; Ahrens, S.; Strassner, T. Organometallics 2010, 29,
3392.
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
A.B. and C.T. thank the University of Padova for financial
support (Progetto Strategico HELIOS).
■
REFERENCES
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̈
In the version of this paper published on April 22, 2014, there
was an error in the reaction scheme given in Table 1. The
version of the paper that appears as of May 12, 2014, has the
correct reaction scheme.
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dx.doi.org/10.1021/om401062p | Organometallics 2014, 33, 2182−2188