Synthesis, reactivity, crystal structures and catalytic activity of new chelating bisimidazolium-carbene complexes of Rh

Article abstract of DOI:10.1002/ejic.200390157

A series of new bridging, chelating and pincer N-heterocyclic carbenes of Rh^I and Rh^III have been obtained under mild conditions. The compounds have been fully characterised and their crystal structures determined. The chelate-pincer

Full text of DOI:10.1002/ejic.200390157

FULL PAPER
Synthesis, Reactivity, Crystal Structures and Catalytic Activity of New
Chelating Bisimidazolium-Carbene Complexes of Rh
[a]
[a]
[a]
[b]
´
´
´
Macarena Poyatos, Elena Mas-Marza, Jose A. Mata, Mercedes Sanau, and
Eduardo Peris*[a]
Keywords: Heterocycles / Carbenes / Rhodium / Homogeneous catalysis
A series of new bridging, chelating and pincer N-heterocyc- plexes of Rh. The compounds have been tested in catalytic
lic carbenes of RhI and RhIII have been obtained under mild
conditions. The compounds have been fully characterised
and their crystal structures determined. The chelate-pincer
coordination of the ligands means that the stability of these
compounds is significantly greater than other carbene com-
reactions such as hydrogen transfer from alcohols to ketones,
and hydrosilylation of terminal olefins and alkynes; they
show a high activity for both processes.
( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2003)
Introduction
The precursor imidazolium salts are often easier to obtain
than phosphanes, but their coordination to the metal often
requires harsh preactivation conditions, in most cases need-
ing the addition of strong bases such as nBuLi.[5,6] Once
bound to the metal, however, monodentate carbenes are of-
ten reactive and undergo decomposition processes that can
reduce the catalytic efficiency at high temperatures.
In order to improve the stability of the carbene-imidazol-
ium compounds we[7Ϫ10] and others,[6,11Ϫ17] have reported
a series of compounds whose stability is entropically im-
proved by the chelate effect. In this sense, a series of bis-
chelate and pincer-bis-carbene compounds have been re-
ported, providing catalysts which resist higher temperatures
than other classical homogeneous catalysts.
Most of the tridentate pincer CNC bis-carbene ligands
reported so far are Pd-based and have been used in catalytic
C-C bond formation reactions (Heck, Suzuki, Sonoga-
shira).[9,10,15] Due to the high thermal stability of these cata-
lysts, traditionally inert bonds such as CϪCl are readily ac-
tivated providing the highest turnover numbers reported to
date in the Heck CϪC coupling.[9] Pincer CNC ligands have
also allowed the preparation of recyclable heterogeneous
catalysts which have also been effectively used in the Heck
CϪC coupling reaction.[18]
The number of synthetic applications of homogeneous
catalysts has increased enormously during the last three
decades. Highly active and selective catalysts have been pre-
pared to activate a large variety of bonds, thus providing
efficient methods to obtain new products with important
industrial and pharmaceutical applications. However,
homogeneous catalysts are far from being widely used in
industrial processes, mainly due to their low chemical and
thermal stability and to their low potential to provide re-
cyclable systems. With this in mind, recent research in this
area has focused mainly on the search for new methods for
the synthesis of stable, effective and recyclable catalysts,
since this would combine both economic and environ-
mental benefits.
Homogeneous organometallic catalysis has long de-
pended on phosphane ligands.[1,2] Despite their effectiveness
in controlling reactivity and selectivity, phosphane catalysts
require air-free handling to prevent the oxidation of the li-
gand and are subject to PϪC activation at elevated temper-
atures.[3] As an alternative to the use of phosphanes, N-het-
erocyclic carbenes are currently being used in the design of
new homogeneous catalysts.[4] The chemical versatility of
these compounds and their easy access provide methods to
obtain ligands with tunable steric and electronic properties.
Despite their obvious potential applications, the use of
bis-carbene chelate or pincer ligands with other metals is
limited to a few papers reporting Rh[7] and Ru[13] complexes
whose catalytic applications were envisaged.
Based on our previous findings, we now report the syn-
thesis and reactivity of a series of CNC-bis-carbene com-
plexes of RhI and RhIII. The crystal structures of several of
the reported compounds are described. We also report the
[a]
´
´
´
Departamento de Quımica Inorganica y Organica. Universitat
Jaume I,
´
12080 Castellon. Spain
Fax: (internat.) ϩ34-(0)96/472-8214
E-mail: eperis@qio.uji.es
[b]
´
´
`
Departamento de Quımica Inorganica, Universitat de Valencia,
c/Dr. Moliner 50, 46100 Valencia, Spain
Eur. J. Inorg. Chem. 2003, 1215Ϫ1221  2003 WILEY-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim 1434Ϫ1948/03/0306Ϫ1215 $ 20.00ϩ.50/0 1215

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M. Poyatos, E. Mas-Marza, J. A. Mata, M. Sanau, E. Peris
FULL PAPER
catalytic applications of our compounds toward the hydros- is recovered, together with the RhI complex 3, which can
ilylation of terminal olefins and alkynes, and hydrogen be considered as a reaction intermediate in the synthesis of
transfer between alcohols and ketones.
4. KBr is added to the reaction medium in order to favour
the formation of the perbrominated complex, although in
most occasions mixtures of the Cl and Br complexes were
still obtained. In the absence of KBr, mixtures of [Rh(CNC-
Bu2)Br3] and trans-[Rh(CNC-Bu2)Br2Cl] were obtained,
which we could not separate by conventional methods (i.e.
column chromatography, crystallization, etc.).
Results and Discussion
Synthesis and Reactivity of the Compounds
In order to obtain the new Rh compounds, we used the
The 1H NMR spectrum of 4 shows the typical pattern of
the bis-carbene ligand coordinated in the pincer form, very
similar to that of related Pd complexes.[9,10] In the 13C
NMR spectrum, the carbene signal is observed as a doublet
at δ ϭ 177.5 ppm with a coupling constant that is dia-
gnostic for Rh binding (1JRh,C ϭ 33.7 Hz).
[9]
ligand precursors 1-R2
(R ϭ Me, Bu) and 2-Me2. The
preactivation of the ligand (deprotonation) was performed
with NEt3, since we have previously found that this base
affords good results under mild reaction conditions. As
metal complex we used [RhCl(COD)]2.
Scheme 1 shows the general method for the synthesis of
complexes 3, 4 and 5. The reaction of [RhCl(COD)]2 with
1-R2 allowed the synthesis, in high yield, of two different
compounds, depending on the reaction conditions used.
Under inert conditions, using a Rh/1-R2 molar ratio of 1:1,
compound 3 was obtained (65Ϫ80% yield). The complex 3-
Bu2 was recently obtained and fully characterized in our
group,[19] and 3-Me2 has very similar spectroscopic fea-
tures[19] allowing for the obvious differences derived from
changing the two Bu wingtips by two methyl groups. The
more relevant NMR spectroscopic features arise from the
fact that the twofold symmetry of the ligand is maintained
upon coordination, and that the 13C NMR spectrum of the
complex shows RhϪC coupling (1JRh,C ϭ 49.2 Hz, δ ϭ
184.8 ppm), which confirms that metallation of the ligand
has occurred.
In order to confirm that 3 is in fact a reaction interme-
diate in the synthesis of 4, we reacted 3-Bu2 with an equim-
olar amount of 1-Bu2 in refluxing CH3CN, with the forma-
tion of 4 in almost quantitative yield (Scheme 1). We still
do not have a satisfactory explanation for the oxidation step
from 3-Bu2 to 4, although we have observed that aerated
solvents provide better yields. A possible explanation for
this process may be the oxidation of traces of BrϪ to Br2
by dioxygen, with the subsequent oxidative addition of Br2
to 3. Further studies on this matter are being carried out in
more detail.
All attempts to coordinate 1-Bu2 and 1-Me2 to RhI in
the pincer form were unsuccessful. Although all these at-
tempts were with [RhCl(COD)]2, we believe that any other
RhI starting compound would have given us similar results.
Reduction of the corresponding RhIII species was unsuc-
cessful, with decomposition of the compound and the
formation of unknown species.
We also wanted to see whether a chelating bis-carbene
coordination to RhI was possible, since all attempts made
so far have resulted in the oxidation to RhIII, both in the
results that we now present and in our previous work.[7]
With this aim in mind, we tried to coordinate the ligand
precursor 2-Me2, made from direct reaction of dichloro-o-
xylene and methylimidazole. The reaction of 2-Me2 with
[RhCl(COD)]2 in CH3CN in the presence of NEt3 yielded
compound 5 in high yield (85%). The synthesis of 5 occurs
without the formation of any RhIII species, which could
support the idea that prior oxidation of the halide (in this
case ClϪ to Cl2 obviously would be not possible) by air is
needed for the oxidation of RhI to RhIII. Evidence for the
chelate coordination of the bis-carbene ligand comes again
from NMR spectroscopy, which shows that the imidazole
rings are symmetry related. The metallation of the ligand is
confirmed by the presence of a doublet (δ ϭ 180.8 ppm;
Scheme 1
The synthesis of 4 can proceed both from [RhCl(COD)]2 2JRh,C ϭ 52.6 Hz) in the 13C NMR spectrum. The coor-
and from 3 (Scheme 1). The mechanism of this reaction is dination sphere of the rhodium atom is completed by the
not well understood, but it looks as if the oxidation of the presence of a molecule of 1,5-COD.
starting RhI complex is the first step in the reaction, which
The molecular structures of 3-Me2, 4 and 5 were unam-
is why aerating the initial CH3CN solution increases the biguously confirmed by single crystal X-ray diffraction.
reaction yields. In fact, when the reaction is carried out un- The molecular structure of 3-Me2 (Figure 1) is virtually
der a completely inert atmosphere, much lower yields are identical to that of complex 3-Bu2 which we have published
obtained, and most of the starting [RhCl(COD)]2 complex previously.[19] The compound consists of a dirhodium struc-
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Eur. J. Inorg. Chem. 2003, 1215Ϫ1221

Chelate Bisimidazolium-Carbene Complexes of Rh
FULL PAPER
ture bridged by 2,6-bis(1-methyl imidazolylidene-3-yl)pyrid-
ine, with the nitrogen atom of the pyridine unbound. The
geometry about each rhodium atom is pseudo-square-
planar, with RhϪCimid distances of 1.948(11) and 2.029(11)
˚
A suggesting that these bonds have a major σ contribution
with very little back-donation.
Figure 2. ORTEP diagram of 4 with 50% probability ellipsoids;
˚
hydrogen atoms omitted for clarity; selected bond lengths (A) and
angles (°): Rh(1)ϪN(3) 1.969(4), Rh(1)ϪC(21) 2.045(5),
Rh(1)ϪC(11) 2.055(5), Rh(1)ϪBr(1) 2.4382(12), Rh(1)ϪBr(3)
2.4922(12); N(3)ϪRh(1)ϪC(27) 78.7(2), N(3)ϪRh(1)ϪC(11)
78.9(2)
Figure 1. ORTEP diagram of 3-Me2 with 50% probability ellips-
˚
oids; hydrogen atoms omitted for clarity; selected bond lengths (A)
and angles (°): Rh(1)ϪC(3) 2.029(11), Rh(1)ϪC(15) 2.095(12),
Rh(1)ϪC(14) 2.105(12), Rh(1)ϪC(11) 2.200(12), Rh(1)ϪC(18)
2.220 (11), Rh(1)ϪBr(1) 2.5172(17), Rh(2)ϪC(30) 1.948(11),
Rh(2)ϪC(26) 2.101(11), Rh(2)ϪC(25) 2.157(12), Rh(2)ϪC(22)
2.189(11), Rh(2)ϪC(21) 2.216(11), Rh(2)ϪBr(2) 2.5169(18);
C(3)ϪRh(1)ϪBr(1) 87.7(3), C(30)ϪRh(2)ϪBr(2) 90.2(3)
Figure 2 shows the molecular structure of complex 4. The
crystal contains a mixture of [Rh(CNC-Bu2)Br3] and trans-
[Rh(CNC-Bu2)Br2Cl], and was refined with a 50% occu-
pancy for each. The molecular structure shows the Rh atom
in a distorted octahedral coordination, with a pincer-ligand
bite angle of 79.9°. The imidazole and pyridine rings are
virtually coplanar, and the two butyl groups point out of
˚
these planes. The RhϪC bond length (2.05 A) is slightly
longer than that obtained for a similar cis-chelate rhodiu-
[7]
˚
m() bis-carbene reported in our group (1.99 A), due to
the mutual trans influence.
Figure 3. ORTEP diagram of 5 with 50% probability ellipsoids;
˚
The molecular diagram of 5 is shown in Figure 3. The
structure shows the Rh atom in a distorted square-planar
coordination with a chelate bite angle of 90.55°. The RhϪC
hydrogen atoms omitted for clarity; selected bond lengths (A) and
angles (°): Rh(1)ϪC(12) 2.027(3), Rh(1)ϪC(1) 2.050(3),
Rh(1)ϪC(23) 2.193(3), Rh(1)ϪC(19) 2.199(3), Rh(1)ϪC(24)
2.222(3), Rh(1)ϪC(20) 2.225(3); C(12)ϪRh(1)ϪC(1) 90.55(11),
C(19)ϪRh(1)ϪC(24) 80.57(12), C(23)ϪRh(1)ϪC(20) 80.95(12)
˚
bond lengths of 2.03 and 2.05 A are similar to those of
compound 3, implying that the RhϪC bond has a major σ
contribution. To the best of our knowledge, this is the first
RhI chelate-bis-carbene complex reported so far.
creased over the last few years.[20,22,25Ϫ27] In view of the
compounds that we report in this paper, we thought that
catalytic hydrosilylation could be a good test of the catalytic
potential of these new complexes. Our RhI complexes (3-
Catalytic Hydrosilylation
Hydrosilylation of multiple bonds represents a useful Me2, 3-Bu2 and 5) display all the structural and chemical
class of catalytic processes for the functionalization of or- requirements for this catalytical system, and could throw
ganic molecules. Vinylsilanes, which are widely used inter- some light onto the reaction process.
mediates for organic synthesis, can be efficiently prepared
Table 1 shows the catalytic results for the hydrosilylation
by transition-metal-catalyzed addition of silanes to alkynes. of several terminal alkynes with HSi(OEt)3 and HSiMe2Ph.
Most of the recent efforts in the study of catalytic hydros- All the reactions were carried out without any special need
ilylation concern the design of new and efficient catalysts for inert conditions, since the catalysts used proved to be
that enable the preparation of both (Z)- and (E)-alkenylsil- fairly stable under oxygen-containing atmospheres, even at
anes independently.[20Ϫ25] For this reason, the number of high temperatures. The reactions were carried out in CDCl3
reports regarding mechanistic studies in order to rationalize at 60 °C, with 1 mol % catalyst. For all the reactions
the factors affecting the selectivity of this reaction have in- studied, all the catalysts used give a mixture of the β-(E),
Eur. J. Inorg. Chem. 2003, 1215Ϫ1221
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M. Poyatos, E. Mas-Marza, J. A. Mata, M. Sanau, E. Peris
FULL PAPER
As seen from Figure 4a, the reaction of phenylacetylene
with HSiMe2Ph using 3-Me2 as catalyst starts by showing
a higher preference for the β-(Z) isomer. As the reaction
proceeds, there is a conversion from the (Z)- to the (E)-
isomer, until total conversion into the (E)-isomer is
achieved. A similar behavior is observed for the nBuCϵCH/
HSiMe2Ph system with both neutral catalysts. These results
confirm that the rate of isomerization of the initially pre-
ferred (Z)-isomer to the thermodynamically more stable
(E)-isomer competes with the hydrosilylation process. The
fact that no (Z)-vinylsilane is observed at the end of the
reaction probably implies that the isomerization occurs via
a reversible addition of the (Z) product in the last step of
the reaction mechanism rather than a change in the selectiv-
ity of the catalyst due to a modification of its nature during
the reaction.[20,27] When HSi(OEt)3 is used, there is a clear
preference for the (E)-isomer from the start of the reaction,
and no (Z)-(E) conversion is observed with time (Fig-
ure 4b).
Table 1. Hydrosilylation of alkynes
Entry[a] RCϵCH
HSiR3
Catalyst Time E/Z/α
Yield
(%)
(h)
1
2
3
4
5
6
7
8
PhCϵCH HSiMe2Ph 3-Me2
PhCϵCH HSiMe2Ph 3-Bu2
PhCϵCH HSiMe2Ph 5
72
24
72
72
72
72
24
72
72
24
72
72
83/0/17 95
33/50/17 95
80/11/9 100
57/30/13 92
50/33/17 90
66/26/8 85
61/39/0 95
100
68/32/0 100
50/50/0 70
PhCϵCH HSi(OEt)3 3-Me2
PhCϵCH HSi(OEt)3 3-Bu2
PhCϵCH HSi(OEt)3 5
nBuCϵCH HSiMe2Ph 3-Me2
nBuCϵCH HSiMe2Ph 3-Bu2
nBuCϵCH HSiMe2Ph 5
64/35
9
10
11
12
nBuCϵCH HSi(OEt)3 3-Me2
nBuCϵCH HSi(OEt)3 3-Bu2
50/50
61/39
50
100
nBuCϵCH HSi(OEt)3
5
[a] 0.077 mmol of alkyne, 0.085 mmol of silane. and 1% mol catalyst
used in CDCl3. T ϭ 60 °C. Yields determined by 1H NMR spectro-
scopy.
For the cationic catalyst 5, the tendency to produce the
(E)-vinylsilane isomer is observed from the beginning of the
reaction. A higher selectivity for the (E)-isomer is obtained
when HSiMe2Ph is used instead of HSi(OEt)3.
β-(Z) and α-isomers, although the β-(E) isomer is preferred
in all cases where the process is carried out for long reaction
times (72 h). In most metal-catalyzed hydrosilylations, the
regioselectivity is affected by factors such as types of al-
kynes and silanes, catalyst, solvent or temperature. How-
ever, it has been reported that cationic Rh complexes cata-
lyze the hydrosilylation of alkynes to give the β-(E)-vinylsil-
anes as the major products, while neutral Rh complexes
show a higher preference to yield the β-(Z) ones.[20,28Ϫ30]
Since initially we did not see this tendency for the neutral
complexes, we decided to follow the reaction more carefully,
and study how the isomer ratio evolves with time (Figure 4).
These results are in agreement with other reports on
rhodium-catalyzed alkyne-hydrosilylation reactions, in
which
a
mixture of isomers is always obtained.[31]
[Rh(nbd)(dppe)2]PF6 gives no β-(Z) isomer, although some
α-isomer is formed.[22]
The relative efficiencies of these Rh complexes as olefin
hydrosilylation catalysts were compared by means of a
standard reaction of styrene with HSi(OEt)3 and HSi-
Me2Ph (Table 2). No induction periods were observed, and
the formation of the silylated compounds proceeded with-
out detectable by-products by NMR spectroscopy. As seen
from the results shown in Table 2, there is a predominant
tendency to yield the linear (terminal) phenethyl isomer.
The cationic complex 5 has a better ability to yield the men-
tioned isomer, displaying higher selectivities.
It is difficult to determine whether the neutral dimetallic
precatalysts 3-Me2 and 3-Bu2 remain dimetallic under the
reaction conditions of the hydrosilylation process. In fact,
we cannot exclude that the compounds evolve to a stable
RhIII species with the chelate carbene bound and a RhI
compound without carbene, which could be the true cata-
Table 2. Hydrosilylation of styrene
Entry[a] HSiR3
Catalyst Time (h) Linear/
branched
Yield (%)
1
2
3
4
5
6
HSiMe2Ph 3-Me2
HSiMe2Ph 3-Bu2
HSiMe2Ph
48
24
72
72
24
72
82/18
65/35
90/10
55/45
56/44
85/25
88
78
100
20
70
80
5
HSi(OEt)3 3-Me2
HSi(OEt)3 3-Bu2
HSi(OEt)3
5
Figure 4. Reaction profiles for the hydrosilylation reactions of
phenylacetylene by using 3-Me2 (1 mol % ratio) with: (a) HSi-
Me2Ph, and (b) HSi(OEt)3; percentage of isomer relative to molar
fraction (E ϩ Z ϩ α ϭ 100)
[a]
0.077 mmol of olefin, 0.085 mmol of silane and 1% mol catalyst
used in CDCl3. T ϭ 60 °C. Yields determined by 1H NMR spectro-
scopy.
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Eur. J. Inorg. Chem. 2003, 1215Ϫ1221

Chelate Bisimidazolium-Carbene Complexes of Rh
FULL PAPER
lytic species. However, according to our previous results on of the hydrogen transfer to the three ketones used. As seen
the hydroformylation of olefins,[19] we have observed that 3- in Figure 5, the reaction profile is a sigmoid curve for the
Me2 and 3-Bu2 do not decompose to mononuclear species acetophenone case, indicating that a preactivation period of
under the harsh conditions required for hydroformylation, 3Ϫ4 h is necessary only for this substrate. This result is also
thus suggesting that the complexes may maintain their di- observable for the reaction performed with 6 ϫ 10Ϫ3 mol
nuclear nature under the much milder conditions used in % catalyst (not shown in Figure 5). We still do not have a
hydrosilylation.
satisfactory explanation for this observation, although fur-
ther studies are being carried out.
Catalytic Hydrogen Transfer
Transfer hydrogenation to alkenes failed with our cata-
lyst, which in fact allows chemoselective reduction of CϭO
bonds in conjugated enones.
The pseudo-octahedral RhIII complex fulfills the elec-
tronic and structural requirements to catalyze the hydro-
genation of the CϭO groups of ketones via hydrogen trans-
fer from iPrOH/KOH. The reactions are slow at room tem-
perature, but proceed at very good rates at 80 °C. Again,
the high stability of 4 allowed us to run all the reactions
under non-inert conditions, with solvents used as purchased
and in the absence of any precautions concerning the pres-
ence of air or moisture. As seen from the data shown in
Table 3, the hydrogenation rates of the aromatic ketones are
significantly higher than those shown for the aliphatic ones.
The catalytic activity of this compound is similar to that
reported by us for another RhIII bis-carbene complex,[7] al-
though in that case aliphatic ketones were hydrogenated
faster than the aromatic ones.
Conclusion
We have obtained new bis-carbene complexes of RhI and
RhIII, and complex 4 is the first CNC-pincer complex of
Rh described to date. The synthetic methods that we have
described provide high yields under mild conditions, and
establish a new method of activating bis-imidazolium salts
in order to perform their coordination to metal complexes.
The complexes obtained have been tested in the catalytic
hydrosilylation of alkynes and olefins, and in the hydrogen
transfer from alcohols to ketones; they show high catalytic
activity. All in all, we have shown that a new family of Rh
bis-carbene complexes can be easily obtained, providing a
new and versatile class of catalysts whose properties deserve
to be exploited.
In order to check whether incubation of the catalyst is
important for this system we studied the reaction profiles
Table 3. Selected results for catalytic hydrogenation transfer of ke-
tones with 4 as catalyst
Entry[a] Substrate
Time (h) Catalyst TON
(mol %)
TOF
Experimental Section
General Remarks: NMR spectra were recorded on Varian Innova
1
2
3
4
5
6
7
8
9
10
Cyclohexanone
Cyclohexanone 24
7
0.06
0.06
0.06
0.06
0.006
0.06
0.06
0.006
0.006
0.006
1322
1322
500
1107
3500
1357
1357
3636
6450
189
55
71
300 MHz and 500 MHz spectrometers, with CDCl3 or [D6]DMSO
[7]
as solvents. The ligand precursors 1-R2 (R ϭ Me, nBu) and the
[19]
Acetophenone
Acetophenone
Acetophenone
Benzophenone
Benzophenone
Benzophenone
Benzophenone
Benzophenone
7
24
6
7
24
3
complex 3-Bu2
were prepared according to literature methods.
46
All other reagents are commercially available and were used as re-
ceived.
583
194
57
1212
921
Synthesis of [o-(CH3-imid-CH2)2Ph](Cl)2 (2): A mixture of α,αЈ-
dichloro-o-xylene (2 g, 11.2 mmol) and N-methylimidazole
(1.93 mL, 22.4 mmol) was heated at 150 °C for 12 h. The resulting
solid was then washed with CH2Cl2 and filtered, yielding pure 2
(95%). 1H NMR (CDCl3, 500 MHz): δ ϭ 9.61 (s, 2 H, NCHN),
7.92 (s, 2 H, imid), 7.84 (s, 2 H imid), 7.48 (2 H, Ph), 7.38 (2 H,
Ph), 5.82 (s, 4 H, CH2), 3.92 (s, 6 H, CH3) ppm. C16H20Cl2N4
7
24
10580 441
[a]
2 mmol of substrate, 10 mL 0.1  KOH in iPrOH T ϭ 80 °C.
Yields determined by 1H NMR spectroscopy. TON ϭ mol product/
mol catalyst. TOF ϭ mol product/(mol catalyst ϫ hour).
Figure 5. Reaction profile for the hydrogen-transfer reaction; 80 °C and 0.06 mol % catalyst (4)
Eur. J. Inorg. Chem. 2003, 1215Ϫ1221
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M. Poyatos, E. Mas-Marza, J. A. Mata, M. Sanau, E. Peris
FULL PAPER
(339.0): calcd. C 56.6, H 5.89, N 16.52; found C 56.2, H 5.67,
N 16.87.
1
[Rh(COD)(C∧C)]PF6 (5): H NMR (CDCl3, 300 MHz): δ ϭ 7.84
(m, 2 H, phenylene-H), 7.55 (d, 2 H, imidazole-H), 7.40 (m, 2 H,
phenylene-H), 7.19 (d, imidazole-H), 6.48 (d, 2 H, NCH2), 5.12 (d,
2 H, NCH2) ppm. 13C NMR ([D6]DMSO, 300 MHz): δ ϭ 180.8
Synthesis
of
[(COD)RhBr]2(µ-CNC-Me2)
(3-Me2)
and
2
[(COD)RhBr]2(µ-CNC-Bu2) (3-Bu2): A mixture of [RhCl(COD)]2
(200 mg, 0.41 mmol), 1-Me2 (327 mg, 0.82 mmol), KBr (300 mg)
and NEt3 (0.5 mL, 3.5 mmol) was heated at 45 °C for 12 h in
CH3CN. The reaction mixture was filtered and the solvent was
eliminated under vacuum. The resulting solid was redissolved in
CH2Cl2 and the solution was transferred to a chromatography col-
umn. Elution with CH2Cl2 separated a minor yellow band that
contained [RhCl(COD)]2. Further elution with a gradient of
CH2Cl2/acetone (10:1) allowed the separation of a major yellow
band that contained 3-Me2 (yield 80%). C29H37Br2N5Rh2 (820.9):
calcd. C 42.42, H 4.51, N 8.53; found C 42.1, H 4.49, N 8.59.
(d, JRh,C ϭ 52.6 Hz, NCN), 136.4 (Cortho, phenylene), 132.4 (C,
phenylene), 129.7 (C, phenylene), 124.3 (im-C), 121.9 (im-C), 90.2,
88.9 (COD), 51.3 (CH2), 39.2 (CH3), 31.0 (COD) ppm.
C24H30F6N4PRh (621.9): calcd. C 46.32, H 4.82, N 9.02; found C
46.7, H 4.73, N 8.82.
Hydrosilylation of 1-Alkynes and Olefins with Silanes. General Pro-
cedure: In
a 5 mm NMR tube, nBuCϵCH or PhCϵCH
(0.077 mmol), silane [HSi(OEt)3 or HSiMe2Ph, 0.085 mmol] and a
catalytic amount of 3-Me2, 3-Bu2 or 5 (1 mol %, 7.7 ϫ 10Ϫ5 mmol)
were dissolved in CDCl3 (0.5 mL). The mixture was kept at 60 °C
by immersion in a hot oil bath. The progress of the reaction was
1
1
[(COD)RhBr]2(CNC-Me2), (3-Me2): H NMR (CDCl3, 300 MHz):
monitored by H NMR spectroscopy, according to the data of the
δ ϭ 10.05 (d, 2JH,H ϭ 8.1 Hz, pyridine-H), 8.28 (t, 2JH,H ϭ 8.0 Hz,
pyridine-H), 7.80 (imidazole-H), 6.99 (imidazole-H), 4.21 (s, 6 H,
NCH3) ppm. 13C NMR ([D6]DMSO, 300 MHz): δ ϭ 184.8 (d,
2JRh-C ϭ 49.2 Hz, NCN), 149.8 (Cipso), 139.7 (Cortho), 123.1 (im-C),
123 (Cmeta), 116.2 (im-C), 98.8, 97.5, 70.2 (COD), 39.6 (CH3), 29.8,
29.3 (COD) ppm.
products obtained from the literature.[28,30]
Hydrogen-Transfer Catalysis: A mixture of the ketone (2 mmol),
KOH (10 mL, 0.2  in iPrOH) and a suspension of 4 (0.06% or
0.006% mol vs. substrate) in CH2Cl2 was heated to reflux. After
the desired reaction times, aliquots were extracted from the reaction
vessel and added to an NMR tube containing 0.5 mL of CDCl3.
1
Yields were determined by H NMR spectroscopy.
Synthesis of Rh(CNC-Bu2)Br3 (4): A mixture of [RhCl(COD)]2
(200 mg, 0.41 mmol), 1-Bu2 (400 mg, 0.82 mmol), KBr (300 mg)
and NEt3 (0.5 mL, 3.6 mmol) was refluxed in 10 mL of CH3CN
for 12 h. During this time the solvent was aerated several times by
bubbling a steam of air through the reaction medium. The reaction
mixture was filtered and the solvent of the filtrate was removed
under vacuum. The crude solid was redissolved in CH2Cl2 and the
solution was transferred to a column chromatography. Elution with
X-ray Diffraction Studies: Single crystals of 3-Me2, 4 and 5 were
mounted on a glass fiber in a random orientation. Crystal data
are summarized in Table 4. Data collection was performed at room
temperature on a Siemens Smart CCD diffractometer using graph-
˚
ite monochromated Mo-Kα radiation (λ ϭ 0.71073 A) with a nom-
inal crystal-to-detector distance of 4.0 cm. A hemisphere of data
was collected based on three ω-scan runs (starting with ω ϭ Ϫ28°)
at ϕ values of 0°, 90° and 180° with the detector at 2θ ϭ 28°. After
each of these runs, frames (606, 435 and 230) were collected at 0.3°
intervals and 30 s per frame. Space group assignment was based
on systematic absences, E statistics and successful refinement of the
structures. The structures were solved by direct methods with the
aid of successive difference Fourier maps and were refined using the
SHELXTL 5.1 software package.[32] All non-hydrogen were refined
anisotropically. Hydrogen atoms were assigned to ideal positions
and refined using a riding model. Details of the data collection,
cell dimensions and structure refinement are given in Table 4. The
diffraction frames were integrated using the SAINT[33] package and
corrected for absorption with SADABS.[34]
CH2Cl2 separated
a
minor yellow band that contained
[RhCl(COD)]2. Further elution with a gradient of CH2Cl2/acetone
(10:1) afforded the separation of an orange band containing 4
(yield 53%). Complex 4 can also be obtained from 3 as follows: a
mixture of 3-Bu2 (100 mg, 0.12 mmol), 1-Bu2 (59 mg, 0.12 mmol),
KBr (100 mg) and NEt3 (150 µL) was refluxed in CH3CN for 5 h.
After this time the workup was similar to that described above
(yield 62%).
1
Rh(CNC-Bu2)Br3 (4): H NMR ([D6]DMSO, 300 MHz): δ ϭ 8.54
3
3
(d, JH,H ϭ 1.5 Hz, 2 H, imidazoleϪH), 8.39 (t, JH,H ϭ 7.8 Hz, 1
3
H, pyridineϪH), 8.04 (d, JH,H ϭ 8.1 Hz, 2 H, pyridineϪH), 7.77
3
(d, JH,H
ϭ 1.5 Hz, 2 H, imidazoleϪH), 4.77 (t, 4 H,
NCH2CH2CH2CH3), 2.48 (quintet, 4 H, NCH2CH2CH2CH3),
CCDC-194088 (3), -194089 (4) and -194090 (5) contain the
supplementary crystallographic data for this paper. These data
can be obtained free of charge at www.ccdc.cam.ac.uk/conts/
retrieving.html [or from the Cambridge Crystallographic Data
Centre, 12, Union Road, Cambridge CB2 1EZ, UK; fax: (internat.)
ϩ44-1223/336-033; E-mail: deposit@ccdc.cam.ac.uk].
1.89 (sextet,
4
H, NCH2CH2CH2CH3), 0.88 (t,
6
H,
NCH2CH2CH2CH3) ppm. 13C NMR ([D6]DMSO, 300 MHz):
δ ϭ 177.5 (d, 1JRh,C ϭ 33.7 Hz, NCN), 152.5 (Cipso), 144.51 (Cortho),
124.97 (im-C), 118.54 (Cmeta), 108.44 (im-C), 50.39
(CH2CH2CH2CH3),
(CH2CH2CH2CH3),
33.70
14.68
(CH2CH2CH2CH3),
(CH2CH2CH2CH3)
20.02
ppm.
C19H25Br2.5Cl0.5N5Rh (643.85): calcd. C 35.42, H 3.88, N 10.87;
found C 35.42, H 4.11, N 10.07.
Acknowledgments
We gratefully acknowledge financial support from the MCYT
(MAT2002Ϫ04421Ϫ002Ϫ02) and Bancaixa (P1.1B2001Ϫ03) for
financial support. We also thank the Generalitat Valenciana for a
fellowship (J.A.M.). We would also like to thank Prof. Robert H.
Crabtree for his helpful comments.
Synthesis of [Rh(COD)(C∧C)]PF6 (5): A mixture of [RhCl(COD)]2
(200 mg, 0.41 mmol), 2-Me2 (275 mg, 0.82 mmol), KCl (400 mg)
and NEt3 (0.5 mL, 3.5 mmol) was heated at 45 °C for 12 h in
CH3CN. The reaction mixture was filtered and washed with
CH2Cl2, and the solvent was then eliminated under vacuum. The
crude solid was redissolved in CH2Cl2 and the solution was trans-
ferred to a chromatography column. Elution with CH2Cl2 separ-
ated a minor yellow band that contained [RhCl(COD)]2. The solu-
tion was eluted with a gradient of CH2Cl2/acetone/KPF6 allowing
the separation of a major yellow band that contained 5 (yield 85%).
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1220
Eur. J. Inorg. Chem. 2003, 1215Ϫ1221

Chelate Bisimidazolium-Carbene Complexes of Rh
FULL PAPER
Table 4. Crystallographic data
3-Me2
4
5
Empirical formula
Molecular mass
Wavelength
C29H37Br2N5Rh2
821.28
0.71073 A
Monoclinic
P21/c
C19H25Br2.50Cl0.50N5Rh
643.85
0.71073 A
Monoclinic
P21/c
C24H30F6N4PRh
622.40
0.71073 A
Monoclinic
P21/c
12.717
˚
˚
˚
Crystal system
Space group
˚
a (A)
14.635(5)
10.547(4)
19.842(7)
90°
12.777(4)
10.882(4)
16.432(5)
90°
˚
b (A)
13.849
14.887
90°
˚
b (A)
α
β
γ
99.015(9)°
90°
103.653(7)°
90°
100.097(5)°
90°
3
˚
V (A)
3024.8(19)
4
2220.2(12)
4
2581.4(12)
4
Z
Density (calculated)
Absorption coefficient
Reflections collected
Goodness-of-fit on F2
Final R indices [I Ͼ 2σ(I)]
1.803 Mg/m3
3.760 mmϪ1
24380
1.926 Mg/m3
5.344 mmϪ1
10540
1.602 Mg/m3
0.788 mmϪ1
20616
0.855
R1 ϭ 0.0677
wR2 ϭ 0.1863
1.039
R1 ϭ 0.0323,
wR2 ϭ 0.0753
1.058
R1 ϭ 0.0414,
wR2 ϭ 0.1026
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[I02533]
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